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This is to certify that the

thesis entitled

THE MERE—ER CHARACTERISTICS
OF CoLAW/PQLYlMIDE A/AA/OCOMPOSITES

presented by

JiAjrAM Gamer/l GLL

has been accepted towards fulfillment
of the requirements for

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THE BARRIER CHARACTERISTICS OF CLAI/POLYIMIDE
NANOCOMPOSITES

BY

Jiajian (Georgia) Gu

A THESIS

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

MASTER OF SCIENCE

School of Packaging
1997

IUBSTTUUTP

THE BARRIER CHARACTERISTICS OF CLAY/POLYIMIDE
NANOCOMPOSITES

BY

Jiajian (Georgia) Gu

‘The permeability of water vapor, oxygen, carbon dioxide,
and ethyl acetate vapor through clay/polyimide nanocomposite
films was determined with the MOCON permeability testers.
Factors affecting the barrier properties of clay/polyimide
nanoconposites including: the clay loading level; the test
temperature; the organic vapor concentration; and the
relative humidity, were evaluated.

The incorporation of low loading levels of clay in the
polyimide hybrid, resulted in a significant reduction in the
permeability of the respective penetrants, as compared to the
pure polyimide film. The results showed a non—linear
,dependence of permeability on clay loading. The temperature
dependency of the transport processes, over the temperature
ranges studied, followed well the Arrhenius relationship. The
organic vapor concentration did not significantly affect the
permeability coefficient. When tested under humidified
conditions, the polyimide film showed a loss in barrier
property, while the clay/polyimide film showed an enhancement

in the barrier property to ethyl acetate.

To my dear family

iii

ACKNOWLEDGEMENTS

I would like to express my heartfelt thanks to my major
advisor Professor J. Giacin. His devoted and endless supports
enabled me to complete this research. I would also like to
thank Professor Hernandez and Professor Pinnavaia, for their
precious advice and discussion throughout the course of this
research. I also thank Dr. T. Lam and Dr. L. Rayas, for
helping me to get started at many stages during the research.
I also would thank people in MOCON (Modern Controls, Inc.),
for their help, as well as their kind donation of the
instrumentation. The research funding from CFMR (Center for
Fundamental Materials Research) is also highly appreciated.

Finally, my family deserves to have my deepest thanks

and love for their devoted supports during the research.

iv

TABLE OF CONTENTS

LIST OF TABLES O O O O O O O O O O O O O O O O O O O O O O O O O O O I O O O O O O O O 0 O O O O O O OViii
LIST OF FIGURES O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O 0 0x
INTRODUCTION. 0 O O O O O O O O O ..... O O ...... O O O O O O O O O O O O O O O O O O O O O O 1

LITERATURE REVIEW
A. Clay/Polymer Nanocomposites.............................7
1. Concepts of Composite and Nanocomposite Materials...7
2. Recent Approaches in Clay/Polymer Nanocomposites....9
3. The Barrier Properties of Clay/Polyimide
Nanocomposites.....................................13
B. General Theory Related to the Permeation Process.......17
C. Factors Affecting the Permeability Properties of
Polymers.........................................21
D. Models for the Permeability of Filled Polymers.........24
E. Permeability Measurement Methods.......................26
1. Gravimetric Method.................................26
2. Isostatic Method...................................27
3. Quasi—Isostatic Method.............................29
F. Commercial Permeability Test Systems...................31
1. Oxygen Transmission Rate Testers...................32
(1.1) MOCON Ox-Tran Testers..........................32
(1.2) MAS 500 Oxygen Diffusion System................33
2. Water Transmission Rate Testers....................33
(2.1) MOCON Permeatran WVTR Testers..................33
(2.2) MAS 1000 Moisture Permeation System.. ..... .....36
3. MOCON Permatran C-IV C02 Permeability
Testers..........................................36
4. Aromatran Organic Vapor Permeability Testers.......38
(4.1) MOCON Aromatran Organic Vapor Permeation
Series......................................38
(4.2) MAS 2000 Organic Vapor Permeability Test
System......................................41

MATERIALS AND METHODS
A. Materials For Synthesis of Clay/Polyimide
Nanocomposites........................................43
B. Penetrants for Permeability Tests.......................44
C. Synthesis of Clay/Polyimide Nanocomposite Film..........45
1. Preparation of Organically Modified
Montmorillonite................. ..... ............45

2. Preparation of 7% Polyamic Acid Solution...........45
3. Preparation of Clay/Polyimide Composite Films......46
D. Permeability Test Methods and Equipment................47
1. MOCON Permatran-W WVTR Tester......................48
2 . MOCON Permatran—IV C02 Permeability Tester. . . . . . . . . 49
3. MOCON Ox-Tran 200 Permeability Tester..............50
4. MOCON Aromatran-lA Organic Vapor Permeability
Tester............ ...... ....-....................51
(4.1) The Test Gas Delivery System and the Partial
Pressure of the Organic Vapor....................53
(4.2) Calibration Method..............................55
(4.3) Test Parameters.. ........ .......................55

RESULTS AND DISCUSSION
A. The Permeability Properties of Small Molecular
Penetrans........................................58
1. The Effect of Clay Loading on Water Vapor
Permeation......... ..... ................. ..... ...58
2. Oxygen Permeation..................................59
(2.1) The Effect of Clay Loading on Oxygen
Permeation .......... .............................59
(2.2) The Effect of Temperature on the Oxygen
Permeability Coefficients........................63
3. The Effect of Clay Loading on C02 Permeation through
Clay/Polyimide Composites........................65
4. The Estimation of the Aspect Ratio of the Clay
Fillers .......... . ...... .........................67
5. Discussion.... .......................... ...........72
(5.1) Comparison of the Barrier Properties of
Clay/Polyimide Nanocomposites to Literature
Values for Polyimides ........ ....... ...... .......72
(5.2) The Interpretation of the Barrier Property
Enhancement of Clay/Polyimide Nanocomposite
Films ................. . ......... .... ..... ........73

B. Organic Vapor Permeability Properties of Clay/Polyimide

Nanocomposite Films ......... . ..... . ......... ..........76
1. Permeability of Polyimide Films with and
without Clay Inclusion .......... .................77

2. The Effect of Test Temperature on the Ethyl Acetate
Permeability of Clay/Polyimide Nanocomposites....79
3. The Effect of Ethyl AcetateVapor Pressure on the
Permeability of Polyimide Films with and without
Clay Inclusion ............................... ...85
4. The Effect of Relative Humidity on the Permeability
of Ethyl Acetate through Polyimide Films with and
without Clay Inclusion.. .................. .......91
5. Estimation of Diffusion Coefficient Values for
Ethyl Acetate through Polyimide and 2.5%(v/v)
Clay Polyimide Films. ............. . ...... . ....... 95

ERROR ANALYSIS ........................................ ...99

Vi

SUMMARY AND CONCLUSIONOOOOO...OOOOOOOOOOOO0.0.00.000000103

APPENDIX
Appendix A: The Estimation of Diffusion Coefficient
Values.....................................106
Appendix B: Conversion Between SI Units and
ASTM Units.................................108

BIBLIOGRAPHIC......OOOOOOOOOOO0.00.00.00.00.0.0.0.0000000109

vii

Table
Table

Table

Table

Table

Table

Table

Table

Table

Table

Table

Table

Table

Table

Table

LIST OF TABLES

Clay Loading and H20 Permeability Coefficient.. . . .58
Clay Loading and Oz Permeability Coefficient. . . . . .63

Effect of Temperature on Oxygen Permeability
Coefficients of Clay/Polyimide Films.............64

Clay Loading and C02 Permeability Coefficient. . . .65

— Relative Permeability of Clay/Polyimide to
water vaporOOOOOOOOOOO ..... O. ..... 00.0.0000068

- Relative Permeability of Clay/Polyimide
to C02 eeeee eeeeeeeeeeee eeeee eeeeeeeeee eeeeee 68

— Relative Permeability of Clay/Polyimide
to 02..OO0...........OOOOOOOOOOOO... ....... .68

Comparison of the H20, 02 and C02 Permeabilty
Results with Literature Data.....................73

Ethyl Acetate Permeability Coefficients for 0% and
2.5% Clay/Polyimide Composite Films..............79

Ethyl Acetate Permeability Values Reported
in the Literature for Selected Commodity
Polymer Films..... ..................... ..........79

The Effect of Temperature on the Permeabilty of
Ethyl Acetate through Clay/Polyimide Films.. ..... 80

10 - Values of l/T and Ln P for Ethyl Acetate

PemeabilitYe O C OOOOOOOOOOOOOO C O O O O ......... O ..... 83

11 - Activation Energy Values for the Permeation of

Ethyl Acetate through Polymer Films Reported in
Literature .............. . ........ ... ............. 85

12 - Effect of Ethyl Acetate Vapor Pressure

on the Permeability of Clay/Polyimide
Composite Films ......... . ........................ 86

13 - Activation Energies at Different Vapor

viii

Table

Table

Table

Table

Pressures.. ................... . ..... ....... ...... 90

14 - Effect of Relative Humidity on Ethyl Acetate
Permeability of Clay/Polyimide Films............93

15.1 - Diffusion Coefficients for 0% Clay/Polyimide
FilmSeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee ........ 0.096

15.2 - Diffusion Coefficients for 2.5%(v/v)
Clay/Polyimide nanocomposite Films..... .......... 96

16 - Comparison of the Diffusion of Ethyl Acetate at
Low Vapor Activities in Polymers.................97

Figure

Figure

Figure

Figure

Figure

Figure
Figure
Figure

Figure

Figure

Figure

Figure

Figure

LIST OF FIGURES

1 - Structure Difference in Conventional Composite,
Intercalated and Exfoliated Clay/Polymer
NanocomPOSiteSeeeee ..... .......0..................9

2 - Water Vapor Permeability of Clay/Polyimide
Nanocomposite Films as a Function of Clay
Loadings.............0.0 ..... 0.......... ..... ....60

3 --C5 Permeability of Clay/Polyimide Nanocomposite
Films as a Function of Clay Loadings.............6l

4 - Arrennius Plot of 02 Permeabilty through
Clay/Polyimide Composite.........................62

5 - C02 Permeability of Clay/Polyimide Nanocomposite
Films as a Function of Clay Loadings.............66

6.1 - Pc/Pp (Hj3)'vs. Clay Loading (v/v%) ...... ......69
6.2 - Pc/Pp (C02) vs. Clay Loading (Vol%) ......... ...70
6.3 - Pc/Pp (02) vs. Clay Loading (Vol%).............71

7 - A Model for the Path of a Diffusion Gas Through
the Clay Polyimide Hybrid.......... ...... ........74

8 - Transmission Rate Profile Plots for Polyimide and
Clay/Polyimide Films...... ................ .......78

9 — Ethyl Acetate Transmission Rate Profile Curves for
Polyimide Films Samples as a Function of
Temperature ....................... . ....... .......81

10 — Ethyl Acetate Transmission Rate Profile Curves
for Clay/Polyimide Films Samples as a Function of
Temperature .................. ..... ............... 82

11 - Ethyl Acetate Permeability and Temperature
Relationship .......... .. ....... .... ............. 84

Figure

Figure

Figure

Figure

12

13

14

15

- The Effect of Organic Vapor Pressure and
Temperature on the Permeability of 0%
Clay/Polyimide Film.................. ......... ...87

- The Effect of Organic Vapor Pressure and
Temperature on the Permeability of 2.5%v/v
Clay/Polyimide Film......... ..... .......... ...... 88

- Arrhenius Plot of Ethyl Acetate Permeability
as a Function of Clay Loading and Permeant
Partial Pressure.................................89

- The Effect of Relative Humidity on the

Permeabilty of Ethyl Acetate through Polyimide
Films with and without Clay Inclusion............92

xi

INTRODUCTION

It has been proposed that a central problem to be solved
by packaging engineers in the next century will be cost
reduction (Packaging World Staff, PackExpo 96). Indeed, over
the last decade, numerous efforts have been made by packaging
professionals to reduce costs in the packaging industries.
The most significant change, with respect to the selection of
packaging materials, is the growing use of plastics, which
occupies 35.1% of the market share of packaging materials
(Boins,1994). The trend for plastics to replace older
packaging materials ensures a vigorous demand for plastic
packaging in many sectors, well into the next century. In
addition to economy, modern plastics brings many other
benifits to packaging, such as flexibility, light weight and
multiple functionality. On the other hand, with the.
development of high-tech industries, such as microelectronics
and space technology, packaging proposes new requirements for
modern plastics. Users expect new packaging materials with
high strength, light weight, high barrier, more
functionality, and low cost. Furthermore, environmental
concerns also require new packaging materials to meet source

reduction requirements and to be recyclable (Rattray, 1994).

2

One of the answers to the above demands is clay/polymer
nanocomposites. Due to their specific molecular textures,
nanocomposites usually exhibit significant enhancement in
mechanical, optical, conductive, nonlinear optical and
barrier properties, as compared to the single components and
conventional composites (Lan, 1994c).

Among various required properties, barrier properties
are especially important to polymeric materials used in the
packaging industry. In order to improve the barrier
properties of polymeric materials, various procedures have
been developed, to include: (I) co-polymerization, (II)
crosslinking, (III) metallizing, (IV) laminating (Ford,
1995), (V) biaxially orientation, (VI) blending (Watanabe,
1987.), and (VII) SiOx coating (Felts,1993.) (Brody,1993).
Normally, as barrier properties are increased, other
properties of the polymer may also be altered.

Clay/polymer nanocomposites are a promising class of
materials with potential applications in packaging.
Clay/polymer nanocomposites are usually stable after the
hybrid complexes are formed, and do not undergo phase
seperation. It can be produced in various forms, such as
films and bulk materials, without additional supporting
materials. In evaluating nanocomposites, because the
reinforcement from the organic layers will occur in two
rather than in one dimension, their mechanical properties are
potentially superior to the respective component materials or

even conventional fiber reinforced polymers (Krishnamoorti,

3

1996). In addition, the clay is dispersed to its molecular
layer level in the polymer matrix, so the barrier efficiency
of the clay is raised to its upper limit, which makes
clay/polymer nanocomposites a unique barrier material.

When nanocomposites are used as a packaging material,
the composite enhanced strength and barrier properties make
it possible to produce packages with a lower amount of
material, which is one way to achieve source reduction.

Also, clay resources abound in nature, and are cheap in cost.
Thus, the incorporation of low loading level of clay can
greatly improve the barrier properties of a polymer, and a
significant cost reduction can also be realized.

Furthermore, when using a nanocomposite barrier material, the
barrier properties are increased without requiring a
multipolymer layered design, allowing for recycling. Thus,
the clay/polymer nanocomposite is not only a low cost, high
performance material, but also is an environmentally friendly
material, which can lead to a new era of modern packaging
materials.

In industry, it is now of great interest to develop
technologies for processing clay/polymer nanocomposites.
According to the newest report (AMCOL web page, 1996), the
AMCOL International Corporation has announced that its newest
venture is in the field of nanocomposite technologies. The
Corporation is positioned to capitalize on a proprietary
technology that disperses nanoscale particles of chemically-

modified clays into plastic resins, thus improving the

4

polymer's strength, heat stability and barrier properties.
Nanocor, Inc., a wholly owned subsidiary of AMCOL
International Corp. (NASDAQ:ACOL), is conducting an intensive
feasibility study on locating its first commercial—scale
production facility in Aberdeen, Miss. The company currently
operates a pilot plant in Georgia, and expects to
commercialize its technology during the course of 1997.
Targeted applications being considered range from flexible
and rigid barrier packaging to structural components for
trucks and automobiles (Nanocor web page, 1996).' Some
successful clay/polymer nanocomposite systems with packaging
applications include polyethylene terephthalate (PET),
polypropylene (PP), and ethylene-vinyl alcohol
copolymer(EVOH), which involved the use of extruders( Lan,
1996).

In the present study, a clay/polyimide nanocomposite
system was selected for investigation, and a comprehensive
study was conducted to characterize the barrier properties of
the composite. A polyimide structure was selected because
polyimides may have packaging applications such as in
electronics packaging, where barrier properties are of great
importance. Secondly, it is possible to produce films of
this kind of nanocomposite in laboratory conditions. The
preparation of film of this kind of nanocomposite material
has been successfully achieved by Lan (1994c).

To provide a better understanding of the barrier

properties of the clay—polyimide nanocomposite, mass

S

dioxide, water vapor, and ethyl acetate vapor. The selection
of the first three permeants is based on the fact that the
permeability of these permeants are of primary concern when a
polymer is used as a barrier material. Ethyl acetate was
selected based on its common use as a solvent in the
converting industry, and its presence as a residual from
laminating and printing processes (Huang, 1996). Factors
affecting the barrier properties of clay/polyimide
nanocomposites including the clay loading, the test
temperature, the organic vapor pressure and the relative
humidity was evaluated. The conclusions of this study are not
limited to this particular composite material but can be
extended to other commodity polymer packaging materials. The

specific objectives of this study include:

1. The preparation of clay/polyimide films composite

containing 0-7.4%(v/v) CH3(CH2)17NH3 montmorillonite clay.

2. Determine the permeability of the resultant
clay/polyimide composites to oxygen, C02 and water vapor as a
function of clay loading. The effect of temperature on the

oxygen permeability will also be evaluated.

3. The evaluation of the effect of clay inclusion, the
effect of temperature, ethyl acetate vapor partial pressure,

and relative humidity on the permeability of clay/polyimide

6

and relative humidity on the permeability of clay/polyimide
films to ethyl acetate vapor. The estimation of diffusion

coefficients.

LITERATURE REVIEW

A. Clay/Polymer Nanocomposites
1. Concepts of Composite and Nanocomposite Materials

A composite is a material which contains two or more
component materials, which often has properties superior to
each component. These superior properties come from the
combination of the desirable characters of the individual
materials. Composites have existed in nature for millions of
years. Some well-known examples are wood, bones and shells of
living creatures. With the development of synthetic polymer
and fibers in composite systems, it is now possible to
duplicate the structural advantages of natural materials,
which has resulted in the manufacture and application of the
high strength, light weight composites (Seferis,1986).

Generally, there are several distinct phases in a
composite. The continuous phase is called the matrix, and
the discontinuous phase is called the filler. According to
their structures, composites can be classified as
metal/metal, metal/ceramic, ceramic/ceramic, ceramic/
polymer, and polymer/polymer composites (Lan, 1994c).
Conventional composites are composed of fillers of the size

of microns and up.

8

A nanocomposite is a type of material containing
platelets that have at least one dimension in the size range
of a nanometer, 1 billionth of a meter, or 1/25 millionth of
an inch. Bentonite clay is one of the few materials that can
be separated into platelets of a nanometer scale (Nanocor,
1996). The tiny size of the fillers makes the nanocomposite
macroscopically homogeneous. So, the physical and mechanical
properties of a nanocomposite are determined not only by the
bulk properties of each component, but also by complex
interactions between the building blocks and their
interfaces. Nanocomposites usually exhibit a significant
enhancement in mechanical, optical, conductive, nonlinear
optical and barrier properties over the single component and
conventional composites (Lan, 1994c).

Nanocomposites can be devided into two general
categories, i.e. (I) intercalated nanocomposites and (II)
exfoliated nanocomposites. Fig 1. shows the structural
difference between conventional composites, intercalated and
exfoliated nanocomposites. In conventional clay/polymer
composites, which have been used for a number of years,
mechanically crushed clays are mixed with polymer matrices.
Clay tactoids exist in their original aggregated state and
act only as fillers. These clay components simply save on
polymer cost and play no functional role in determining the
performance properties of the final composites. In an
intercalated nanocomposite, organic polymer layers and

inorganic silicate layers are alternatively interstratified

9

at the molecular level. In an exfoliated nanocomposite,
the individual 10 A-thick clay layers are dispersed in a
continuous polymer matrix (Lan, 1994c). The average

distances depend on clay loading.

 

 

 

 

 

 

 

 

 

 

 

 

 

L l
{W
‘2 l ' [ ‘ S J
Conventional Intercalated Exfoliated
Composite Nanocomposite Nanocomposite

Figure 1. Structural Differences in Conventional,
Intercalated and Exfoliated Clay/Polymer
Composite/Nanocomposites (Lan, 1994c)

2. Recent Approaches in Clay/Polymer Nanocomposites

Although research on clay/organic polymer complexes has
been carried on since 1965 (Blumstein, 1965; Lagaly and
Beneke, 1975; Pinnavaia, 1983), world.wide interest in this
area was not realized until 1987, when Toyota researchers
developed several nanoscale polymer-clay hybrid composites.
These included systems based on the dispersion of an w~amino
acid and alkylammonium exchanged forms of montomorilonite
clay in semicrystalline nylon-6 (Fukushima and Inagaki,
1987), (Fukushima et al., 1988), (Usuki et al., 1995b),
amorphous epoxy (Usuki et al., 1989.), and polyimide (Yano et

al., 1993; Lan and Pinnavaia, 1994a). The resultant

10

composites exhibited properties which were far superior to
those of the pure polymer, or of conventional composite
materials. The Toyota researchers reported that the
clay/nylon-6 nanocomposites exhibited mechanical and thermal
properties which were remarkably superior to those of the
individual components. At a loading of 5.0 wt.% exfoliated
clay, the tensile strength of the composite increased by 50%
and the heat distortion temperature increased from 65 0C to
152 °C (Fukushima, 1987). These properties could not be
duplicated by conventional organic chemical modification of
the polymer. Transmission Electron Microscopy studies
revealed that the improved performance observed was due to
the formation of the nanoscopic composite. The
nanocomposites immediately found their use in the automobile
industry, as the structural material of bumpers (Okada, et
al., 1990). According to a recent report (Usuki, 1995b),
nylon 6-clay hybrid materials containing four different types
of clay minerals, montmorillonite, sponite, hectrite, and
synthetic mica were compared. It was found that the nylon 6-
clay hybrid using montmorillonite, was superior to the other
hybrids in mechanical properties. That was attributed to the
montmorillonite clayinteracting strongly with nylon 6 by
ionic interaction.

The intercalated clay/polymer nanocomposites are of
interest because organic polymer layers and inorganic
silicate layers are alternatively interstratified at the

molecular level. Two synthetic approaches have been reported

11

to prepare intercalated clay/polymer nanocomposites. The
first is preformed-polymer-intercalation. The preformed
polymer and clays are dissolved and dispersed in a solvent,
respectively, then mixed. Upon solvent removal, a
clay/polymer intercalated nanocomposite forms. Water soluble
polymers, such as polyethylene oxide (PEO)(Ruiz-Hitzky,1993),
ethylene—vinyl alcohol copolymer (EVOH)(Tohoh, 1992) and
polyvinyl alcohol (PVA)(Greenland, 1963) have been
intercalated into clay galleries by this approach. Molten
polymer can also be intercalated upon mixing with clays, such
as polystyrene into organo-montmorillonite clays (Vaia et
al., 1993). Limitations of this method are lack of
convenient solvents for some polymers and the high viscosity
of some molten polymers. The second method is the "in situ"
intercalative polymerization method. The desired monomers are
adsorbed or intercalated into the clay galleries. Then, the
intercalated monomers are polymerized inside the clay
galleries to form the clay/polymer nanocomposite. This method
has been used extensively and overcomes the disadvantages of
the first method. Various clay/polymer intercalated
nanocomposites have been prepared by using the second
approach. These include polyamide (nylon-6)(Kato et al.,
1979), polymethyl metharcylate (Blumstein, 1965), polyaniline
(Mehrotra and Giannelis, 1990), poly(e-caprolactone)
(Messersmith and Giannelis, 1993), polyacrylonitrile

(Blumstein, 1974), and polyfurfuryl alcohol (Bandosz, 1992).

12

These intercalated nanocomposites have exhibited interesting
electrical and optical properties.

At present, there are three major research groups world
wide which are focusing on clay/polymer nanocomposites, these
include Giannelis' group at Cornell University and
Pinnavaia's group at Michigan State University in the United
States, and the Toyota group in Japan. The most successful
nanocomposites are those which showed enhanced mechanical and
thermal mechanical properties, including those prepared with
poly(s-caprolactone) (Messersmith et al., 1993), epoxy resins
(Lan and Pinnavaia, 1994b; Messersmith et al., 1994),
polyimide (Lan and Pinnavaia,1994a) (Giannelis,1992), and.
polystyrene (Vaia et al., 1993). In addition, some reports
showed that organoclay/polymer nanocomposites not only
exhibited improved mechanical properties, but also showed"i
improved barrier properties (Yano et al., 1993; Lan and
Pinnavaia, 1994; Messersmith et al., 1995; Maul, 1995).

For example, Messersmith and Giannelis (1995) studied the
barrier properties of poly(e-caprolactone) -layered silicate
nanocomposites, and found that the water vapor permeability
of nanocomposites containing as low as 4.8% silicate by
volume, was reduced by nearly an order of magnitude as
compared to pure poly(e-caprolactone).

Based on these finding, it is now of great interest to
the industry to develop clay/nanocomposites processing
technologies, as well. For example, Nanocor, Inc. was

established by the AMCOL International Corporation,

13
specifically to develop clay-based technologies and products

for the plastics industry (Nanocor web page, 1996). The
company claims that their unique, chemically modified clays,
when added to plastic polymers form nanocomposites, an
exciting new category of materials. Compared to the base
polymer, a nanocomposite demonstrates improved heat
resistance as well as enhanced structural and barrier
properties. Nanocor has partnered with plastic resin
producers and users to develop the special technologies and
products suitable for several different polymer types.
Targeted applications being considered range from flexible
and rigid barrier packaging to structural components for
trucks and automobiles. Pilot plant facilities have been
established to advance process operations and produce trial
quantities of NANOMER”, nano-sized platelets. Initial
commercial production of some products is targeted for the
second half of 1997. Nanocor received its first patent
pertaining to the preparation of chemically modified clays
and nanocomposites in September 1996, with others pending
(Nanocor web page, 1996). Some successful polymer systems

include PET, PP, and EVOH (Lan, 1996).

3. The Barrier Properties of Clay/Polyimide Nanocomposites
Polyimides are one of the most stable and

environmentally resistant polymer systems currently available

for use at temperatures up to 300 oC. They are marketed as

film, moldings and molding powders, wire coating enamels,

14
adhesives and laminating resins (Critchley and Wright,1979).

For example, Imi-Tech® Corporation manufactures a range of
high performance, light weight, fire resistant SOLIMIDE®
polyimide foam products for defense, industrial and
commercial applications world wide. Those products are used
as thermal and acoustical insulation in aerospace, marine and
industrial markets, as well as for fire barriers, packaging
and other applications requiring either high temperature or
very low-temperature insulation in critical environments
(Imi—Tech web site, 1996). Sykes and St.Clair (1986) studied
the water vapor, oxygen and carbon dioxide transmission rates
for a series of polyimides derived from either 3,3',4,4'-
benzophenone tetracarboxylic dianhydride (BTDA) or
pyromellitic dianhydride (PMDA) with various diamines. Stern
et al.(1989) also studied the structure/ permeability
relationships of nine different polyimides to H2, N2, 02, CH4,
and C02, with specific interest focused on application to the
separation of gas mixtures.

Because of their heat resistance, chemical stability,
and superior electric properties, polyimides are largely used
for microelectronics and electronic packaging (Yano et al.,
1993). But if they are used for advanced electronics, some
properties are not sufficient. It is desired to reduce the
amount of moisture absorption, the thermal expansion
coefficient, and the dielectric constants of the polyimides.
To improve these properties, many types of polyimides, having

various chemical structures have been synthesized. One of the

15

examples included incorporating fluorine into the polymer
backbone to reduce moisture absorption and dielectric
constants for polyimides (Goff and Yuan, 1988; Labadie and
Hedrick, 1990). However, these fluorinated polymers had poor
mechanical properties. As a conventional method, clays are
also used as fillers to obtain increased barrier properties
(Muthy et al., 1986). Under such conditions, high levels of
clay loading(40%) are needed, due to the low aspect ratios of
the clay tactoids, which usually results in worsened
mechanical properties.

As montmorillonite is composed of stacked silicate
sheets, it has a low thermal expansion coefficient and high
barrier properties. Therefore, if the clay/polyimide hybrid
were synthesized, the above problems might be solved (Yano et
al., 1993). The achievement of the synthesis of the
clay/polymer nanocomposite led to a new and more efficient
way to improve the barrier properties of polyimide. Due to
their unique cation exchange properties, high aspect ratios
of individual clay layers can be achieved (Lan, 1994c), thus
only a small amount of organocation exchanged clay is needed
to achieve increased barrier properties, as well as
mechanical properties.

The earliest study on clay/polyimide nanocomposites, was
carried out by the Toyota group (Fukushimaand and Inagaki,
1987, 1988; Yano et al., 1993). The Toyota researchers found
that only 2 wt% addition of montmorillonite brought

permeability coefficients of various gases to values less

16
than half of those of ordinary polyimide. Furthermore, the

thermal expansion coefficients were reduced by 25% (Yano et
al., 1993). It has been suggested that nearly complete
dispersion (exfoliation) of the lo-A-thick clay layer is
needed to optimize the aspect ratio of the particles, which
in the case of montmorillonite can reach a value of 200.

Recently, experiments at Michigan State University
showed that similar composites, with an aspect ratio of 192,
have enhanced C02 barrier properties (Lan and Pinnavaia,
1994a). Further, X-ray diffraction (XRD) results showed that
the clay retained a crystallographically regular stacking
order with a monolayer of polymer intercalated between the
layers.

To describe the mechanism of the barrier property
enhancement of clay/polyimide nanocomposites, Lan (1994c)
compared the structural difference between exfoliated and
intercalated clay/polyimide nanocomposites. In the exfoliated
structure, the single lO—A-thick clay layers act as filler
elements. The aspect ratio of the individual plate and the
filler loading determine the barrier property improvement.
Whereas, in the intercalated structure, the clay fractal
aggregates, and not single layers, function as filler
elements (Lan, 1994c). The formation of the fractal clay
structure for natural clay minerals is due to a water
swelling effect. When the water molecules, which have been
pre-intercalated into the clay galleries evaporate from the

system, they tend to form a water domain region. This causes

17

the clay layers to slip and change their stacking pattern
from turbostratic to a staircase fashion. For the polyamic
acid and dimethylacetamide (DMAC) co-intercalated
organoclays, the DMAC upon heating (100 0C) will leave the
system. A rearrangement of the clay stacking then occurs to
form the fractal structure of the clay in the polyamic acid
matrix. After further heating at 300 0C, the clay fractal
structure will remain simply because of the low mobility of
the pre—formed polymer chains (Lan, 1994c).

While various researchers have proposed different
mechanisms to account for the barrier property enhancement of
clay/polyimide nanocomposites, the observed non-linear
dependence of the permeability as a function of clay loading
and the very low content of the inorganic phase, is crucial

in developing light-weight composites.

B. General Theory Rerlated to the Permeation Process
By definition, the permeability of a material is the
flux or rate at which a quantity of penetrant passes through
a unit surface area during unit time (Rogers, 1964).
There are several terms which have been used to describe
the steady state permeation rate of molecules through a
polymer film of surface area A and thickness 1 (Hernandez and

Giacin, 1997).

18

 

 

 

- Permeant Transmisson Rate,F'=' q (1)
At
q
- Permeance, R =-————- (2)
A.t.Ap
_ . q.1
Thickness Normal1zed Flow, N = (3)
A .t
. . q l
Permeability Constant, P = (4)
t AAP

where q = the quantity permeated during time, t

Ap = pz-pl, pressure drop across the film

The permeation of a substance through a polymer is
usually described by the permeability constant (or
coefficient) P, which is a measure of the permeation rate of
a gas or vapor through a polymer membrane at steady state.
It is related to two fundmental parameters, D and S, by the
expression:

13,:st (5)

where D is the diffusion coefficient which is a kinetic term
and describes how fast penetrant molecules move in a polymer
host. S is a thermodynamic term called the solubility
coefficient, which describes how many penetrant molecules
dissolve in a polymer host. The above expression is valid
when D is independent of permeant concentration and 8 follows
Henry's Law of solubility.

According to the permeability model, a simple permeation

process involves three basic steps: absorption, diffusion and

19

desorption. In the absorption step, penetrant molecules are
absorbed and dissolved into the polymer surface which is
exposed in a high penetrant concentration. In the diffusion
step, the dissolved penetrant molecules then diffuse through
the polymer membrane from the surface contacting the high
penetrant concentration to the surface where the
concentration is low. In the desorption step, penetrant
molecules evaporate from the low concentration surface.

The sorption and desorption steps are related to the
concentration of the penetrant in the polymer, and the
penetrant vapor pressure in equilibrium with the polymer.
The solubility of penetrants in polymers (especially polymers
above their glass transition temperature and penetrants at
low pressure) is described by the linear isotherm Henry's law
of solubility:

C: S * p (6)
where c is the concentration of gas in the polymer, p is the
partial pressure of penetrant at the interface, and S is the
Henry‘s law constant, which is the solubility coefficient.
It can be assumed that the partial pressure of the penetrant
is related to the penetrant concentration in the gas phase
through the ideal gas law, when the concentration of the
diffusant in the gas phase is very low (Hernandez et al.,
1986).

The diffusion step is described by Fick's first and

second laws of diffusion (Crank, 1975).

20

 

 

0C (7)
Fz-D
[ ..1
_Q_C_ :1) 02C (8)
at 02x

where F is the flux or the rate of transfer of penetrant per
unit area, c is the penetrant concentration in the polymer, x
is the direction of the diffusion process, t is time, and D
is the molecular diffusion coefficient.

Fick‘s first law is based on the hypothesis that the
rate of transfer of a diffusing substance through a unit area
of a section is proportional to the concentration gradient
measured normal to the section. Fick's second law shows that
the total change in the concentration across the slab bulk
phase with time is directly proportional to the change in the
concentration gradient with permeant penetration depth. The
equation is valid in this form only if D is constant and the
media is homogeneous. For most packaging applications, this
is the case (Giacin, 1994). When the diffuson coefficient
is independent of the penetrant concentration in the polymer,
it is assumed that the diffusion process is Fickian. If the
diffusion coefficient is a function of the penetrant
concentration in the polymer, the diffusion process is said

to be non-Fickian (Mears, 1965; Crank and Park, 1968).

21

C. Factors Affecting the Permeability Properties of

Polymers

There are several variables that can influence sorption,
diffusion, and consequently the permeability coefficient.
They include the nature of the polymer, the nature of the
penetrant, the penetrant concentration, temperature, and
relative humidity.

First, the nature of the polymer, such as the polymer
chemical composition, molecular structure and morphology can
influence the solubility and diffusivity of a specific
polymer/penetrant system. Typically, polymers with a high
level of crystallinity, hydrogen bonding, high Tg,
orientation, and low free volume, are likely to have low
permeability rates. For instance, polyolefins appear to have
good moisture barrier properties, since they are non—polar
polymers and have a low affinity to moisture. On the other
hand, polyamides (Nylons), polyvinylidienchloride (PVDC),
and ethylene-vinyl alcohol copolymer (EVOH) offer high gas
barrier properties, as well as solvent and odor barrier,
because of their polarity and/or hydrogen bonding property
(Dirienzo, 1993). Sykes and St.Clair (1986) studied the
effect of molecular structure on the gas transmission rate of
aromatic polyimides, and found that the molecular structure
had a strong influence on the gas transmission rates
obtained. The authors found that the barrier characteristics

of the films investigated varied by three orders of magnitude

22

for the respective polyimide films studied. In general, the
3,3',4,4'-benzophenone tetracarboxylic dianhydride (BTDA)
series of polyimide film had overall, lower gas transmission
rates than the pyromellitic dianhydride (PMDA) derived
series. Polyimides prepared with meta-oriented diamines
characteristically displayed lower gas transmission rates
than those prepared with para-oriented diamines.

Secondly, the nature of the penetrant will affect the
transport process. Small, non—polar and linear molecules,
such as oxygen, helium, and carbon dioxide, have little
interaction with polymer molecules, while permeating through
the polymer matrix. Organic vapor molecules are large, have
functional groups or have non-linear shapes, which makes the
diffusion process more complicated. DeLassus (1993) reported
that glassy polymers typicallyhave medium to high diffusion
coefficients for oxygen and very low diffusion coefficients
for flavor/aroma/solvent molecules at low concentrations.

Third, the temperature effect on permeability,
diffusion, and solubility coefficients values can be

described by the following Arrhenius relationship equations:

P(T) = P0 exp(—Ep/RT) (9)
D(T) = Do exp(-ED/RT) (10)
svr) = so exp(DHs/RT) (11)

where :Ep is the activation energy for permeation

ED is the activation energy for diffusion

DHsis the molar heat of sorption.

23

The above expressions are valid within a temperature range
which does not include the polymer glass transition
temperature. Usually, at temperatures above Tg, the
permeability coefficient is more temperature dependent, while
below Tg, it is less temperature dependent. DeLassus
(1985)reported that an increase in temperature resulted in
decreased solubility and increased diffusivity of limonene in
oriented polypropylene. Huang (1996) studied the temperature
dependent properties of the permeability coefficients of
selected organic vapor penetrants through a series of barrier
films, to include oriented polypropylene (OPP); saran coated
oriented polypropylene (Saran coated OPP); high density
polyethylene (HDPE); acrylic coated oriented polypropylene
(Acrylic coated OPP); glassine; and a metallized polyethylene
terephthalate (PET)/oriented polypropylene (Met PET/OPP)
structure. The temperature dependency of the transport
processes, over the temperature range studied, was found to
follow well the Arrhenius relationship.

Relative humidity will also have a significant
influence on hydrophilic polymers that are polar, or can form
inter and intramolecular hydrogen bonding and a lesser affect
on hydrophobic polymers such as polyolefins. For example,
the permeability of ethyl acetate vapor through an ethylene-
vinyl alcohol copolymer (EVAL-F film) (Sajiki, 1991) was
strongly affected by relative humidity. When the EVAL—F film
was tested under dry conditions, there was little observed

permeation. This can be attributed to the strong hydrogen

24

bonding between polymer chains, which restricts the diffusion
of ethyl acetate vapor. When moisture is present, sorbed
water plasticizes the hydrophilic barrier polymer which
results in an increase in the permeation rate for ethyl
acetate. On the other hand, Landois-Garza and Hotchikiss
(1988) reported that an increase in relative humidity
resulted in a decrease in the permeation of ethyl esters
through polyvinyl alcohol (PVOH).

Other factors affecting permeation include: permeant
concentration; plasticizers; and fillers. Concentration
dependent effects have been observed on the permeability of
organic compounds, such as aromas, flavors, and solvents by a
number of investigators (Meares, 1965; Hernandez et
al.,1986; Mohney et al., 1988). Some polymer systems
(polyethylene and nylon 6) when filled with 40%wt of mica
achieved a 60% decrease in H20 and Oz permeabilities (Murthy
et al., 1986). Recent studies on the permeability of
nanocomposites showed that a significant enhancement in
barrier properties was obtained with cation exchanged
organoclays as fillers (Yano et al., 1993; Lan and Pinnavaia,

1994a).
D. Models for the Permeability of Filled Polymers
Theories for the permeability of gases through polymers

filled with inorganic powders are quite limited. Nielsen

(1967) developed several simple models which show the trends

25

and limits to be expected for some of the effects of fillers
on the barrier properties of the resultant composite. These
models thus allowed interpretation of the observed
experimental data. One model was developed to predict the
maximum decrease in permeability that could be expected by
the addition of a filler to a polymer.

When a polymer is filled with inorganic particles, it
usually leads to a very tortuous pathway for molecules
diffusing through it. In addition to the tortuosity factor,
permeability rates are also reduced due to the fact that not
all of a given cross section of material is pure polymer. As
a first approximation, the following equation can be used

(Nielsen, 1967):
PF/Pu ... op / r (12)

where PF and Pu are the permeabilities of the filled and
unfilled polymer, dm,is the volume fractiOn of the polymer,
and ris the tortuosity factor. If it is assumed that the
filler particles are circular or rectangular plates and that
they are uniformly and completely dispersed in the polymer
and that the plates are oriented parallel to the polymer film

surface, it follows that:

t r: l + (L / 2W)<I>F (13)

26

where L is the length of a face of a filler particle, W the

thickness of the filler plates, and ¢T-the volume fraction

of filler. Combining Equation (12) and (13), the

permeability equation becomes:

<I>
PF = p (14)
pu 1 + (L / 2W) o,

 

Equation (14) shows that thin plates with a large aspect
ratio (L/W) can dramatically decrease the permeability of
filled polymers, if the particles can be oriented so that

their flat surfaces are parallel to the surfaces of the film.

E. Permeability Measurement Methods

Permeability, diffusion and solubility coefficient
values can be determined by the following procedures:
(1) Gravimetric Method:

In this procedure, permeation values are determined by
observing the change in weight of a test cell or intact
package system, as a result of the diffusion process. An
example is the method described in ASTM E-96 (ASTM, 1988),
where an aluminum dish containing desiccant is covered with a
packaging material, weighed and placed in a constant
temperature and humidity chamber. The samples are then
weighed periodically, and the water vapor transmission rate

is determined by the weight gain over time. Recently, an

27

electrobalance method has been used to measure the sorption
and diffusion of organic vapors by polymeric meterials
(Berens and Hopfenberg, 1982; Choy et al., 1984). The
experiments are carried out at equilibrium vapor pressure,
using an electrobalance which records continually the gain or
loss of weight by a test specimen as a function of time. The
gravimetric methods in general have low sensitivity,
especially for high barrier polymer films, and are applicable
over a limited vapor pressure range. However, application of
the electrobalance negated these limitations and provide for
high sensitivity and utility over a wide ranger of
temperature and sorbate vapor pressure (Hernandez et al.,

1986).

(2) Isostatic Method:

In the isostatic procedure, a test film is clamped in
a permeation cell which divides the cell into two chambers. A
penetrant (test gas) flows continually through the high
concentration cell chamber,and an inert carrier gas flows
through the low concontration cell chamber. The partical
pessure gradient of the penetrant, provides a driving force
for the penetrant permeating through to the low concentration
cell chamber, where the penetrant can be conveyed to a
detector for quantification. Steady state is reached when the
transmission rate, monitored continually, remains constant
under constant conditions of temperature and permeant vapor

pressure.

28

The permeability coefficient P is calculated from the

transmission rate at steady state by the expression:

= [C]. .fJ
P AAp ‘ (15)

 

where [C]. is steady state concentration of permeant conveyed
to the detector, in mass per unit volume; f is the rate of
carrier gas flow in the low concentration cell chamber, in
volume per unit of time; 1 is the film thickness, in length
unit; and A is the surface area, in area units. The diffusion

coefficient D is determined by the following expression:

12

D:
7.199. t“, (16)

 

where tbq is the time required to reach a transmission rate
value that is equal to half of that at the steady state, in
time unit.

Various detector devices have been used in isostatic
permeability studies. Thermal conductivity detectors and
thermistors were used to measure single permeants (Pasternak
et al., 1970; Ziegel et al., 1969; Yasuda and Rosengren,
1970). Flame ionization detectors (FID) have been used for
organic vapors and provide the advantage of not being
affected by the presence of the carrier gas and water vapor
(Zobel, 1982). When a complex mixture of organic permeants is
involved in the test, a gas chromatograph was interfaced with

an FID to separate individual permeant first before

29
performing quantification (Pye et al., 1976; DeLassus, 1985).

Other techniques, based on either a photoionization detector
or atmospheric pressure ionization mass spectrometer, were
employed by Caldecourt and Tou et al.(1985). Both detection
systems were found to be very sensitive and useful in the
characterization of the prmeabiity of barrier membranes to
organic penetrants. In some commercially available isostatic
permeability testers, IR sensors and coulumetric sensors are
also used for detecting water vapor and oxygen, respectively

(MOCON, 1982; 1989).

(3) Quasi-Isostatic Method:

The quasi-isostatic method is also called the
accumulation method. In this procedure, diffusion and
permeability coefficient values are obtained by quantifying
the amount of the penetrant gas or vapor that has passed
through the film and accumulated in the low concentration
cell chamber,as a function of time. A test film is mounted
betwen two cell chambers and the gas or vapor being tested is
allowed to flow through the high concentration cell chamber.
The partial pressure gradient provides the driving force for
the penetrant permeating into the the low concentraton cell
chamber. Periodically gas samples in the low concentration
chamber are removed and injected directly into a gas
chromatograph (GC), where a flame ionization detector is used

for quantification (Bauer et al., 1986)

30

In this procedure, as the quantity of permeant
accumulated in the low concentration cell chamber increases
over time, the relationship between quantity accumulated per
unit of time reaches a constant level, which means the
permeation process has reached its steady-state status. The
permeability coefficient can be determined from the flux at
steady state and test constants (Hernandez et al., 1986).

The permeability coefficient P is:

P: 92—1—— (17)

t.A1Ap

where Q is the total quantity of permeant that has
penetrated during unit time (t). A is the test area exposed
to the permeant, 1 is the film thickness and Ap is the
driving force, expressed as concentration or partial pressure
gradient. The duffusion coefficient (D) can also be
determined by the lag time method, and is expressed as
D = l2 /60, where 9 is the lag time and l is the film
thickness. The lag time is obtained from the intersection of
the projection of the steady state portion of the
transmission curve to the time axis (Hernandez et al., 1986).
A number of investigators (Gilbert et al., 1983;
Hatzidimitriu et a1, 1987; Mohney et a1, 1988; Sajiki, 1993)
have studied the concentration dependence of the permeation
procedure,employing a quasi-isostatic system as described
above, which allows measurement of the permeation rate for

organic vapors at any desired low concentration level.

31

F. Commercial Permeability Test Systems

Modern Controls, Inc. or MOCON (Minneapolis, MN) is a
leading developer, manufacturer, and marketer of high
technology instrumentation designed to test the barrier
properties of packages and packaging materials. MOCON's
testing instrumentation is used worldwide in research
laboratories, production environments and quality control
applications in the food, plastics, medical, and
pharmaceutical industries (MOCON web page, 1996). MOCON
permeability testers, which are commercially available, are
designed for measuring gas, water vapor, and organic vapor
permeability and are operatively friendly.

MAS Technologies, Inc. (Zumbrota, MN) is another
manufacture of permeability instrumentation. Its products
include the MAS 500 Isostatic Oxygen Diffusion System, the
MAS 100 Moisture Permeation System and the MAS 2000 Organic
Permeation Detector System.

The principle of both Modern Controls and MAS
Technologies' permeability instrumentation, is based on an
isostatic procedure. Typically, one film sample is mounted
between two cell chambers. One of the cell chambers holds
the test gas to be used as the permeant, while the other
chamber holds carrier gas that flows through the cell to the
detector. When the permeant diffuses from the high

concentration chamber into the low concentration chamber, it

32

is conveyed directly to the detector for quantitative

measurement .

1. Oxygen Transmission Rate Testers
(1.1) MOCON Ox-Tran Testers

At present, MOCON has developed several generations of
Oxygen Transmission Rate Testers, including the Ox-Tran 100,
Ox-Tran Twin, Ox- Tran 200, and Ox-Tran 2/20, which all meet
the requirements of the ASTM D3985-81 Standard (ASTM, 1988).

Although the newer models are upgraded by using a
computer and feature more functions, such as testing at
relative humidity, the basic principle is the same, (i.e. an
isostatic procedure). Also, the respective test systems all
use the same detector type, which is a coulometric cell. The
principle of which is based on well-known relationships
established by Faraday (MOCON,1989). It is a constant—current
generator, the output of which is a linear function of the
mass flow rate of oxygen entering the detector. In actual
practice, the cell current is displayed in terms of the DC
voltage developed across a fixed load resistance in the
coulometric cell circuit. In the Ox-Tran 200 system, the use
of a computer makes it possible to automatically select which
resistor is to be used. Also, the use of a temperature
control bath makes it convenient to test samples at sub-
ambient or elevated temperatures (5-50 oC). Further, the
newest generation, Ox-Tran 2/20 series have more functions

and on advanced sensor system. MOCON engineers have developed

33

a patented coulometric sensor (COULOX) with high performance,
which provides parts—per—billion sensitivity even in the
presence of water vapor (MOCON web page, 1996). There are
three models of this kind of high performance sensor for
testing films with different oxygen transmission rate range.
The blue sensor is used for testing ultra-low range, the red
sensor for mid-range, and the green one for ultra-high range.
In addition to the advanced sensor, the 2/20 series provides
the possibility for testing over a range of relative humidity

conditions.

(1.2) MAS 500m Oxygen Diffusion System

The MAS 500m Oxygen Diffusion System is designed to
measure the precise levels of oxygen diffusion and solubility
in packaging materials. The system incorporates precise
temperature, RH, and flow rate control, a broad sensitivity
range, high speed pentium-based computer control, and a
software application package. System benifits include
unmatched test throughput, solubility estimates for oxygen
scalping and gas flush applications, automated test
sequencing, and a full range of application analysis

including oxygen desorption, absorption, and transmittance.

2. Water Vapor Transmission Rate Testers
(2.1) MOCON Permatran WVTR Testers
The earliest standard test technique for measuring water

vapor transmission rates, was classified as a "gravimetric"

34

test, and was based on the weight gain or loss over a period
of time. This proceure met the requirements of the standard
ASTM E-96. In practice, this method showed to be labor
intensive, time consuming, and notoriously inaccurate
(Demorest, 1995). In 1990, a new test standard, ASTM
F1249—90, was adopted. This procedure is based on the
isostatic method, and combines solid state electronics with a
pulse-modulated infrared (PMIR) sensor, to detect down to one
ppm of water vapor. During the test, when water vapor
permeates through the film from the humidified side to the
dry side, the density and the pressure of the carrier gas
varies, and the absorption rate of infrared energy by water
vapor also changes. The infrared photodetector and an
amplifier senses the change in infrared energy, producing a
DC output which is directly proportional to the water vapor
in the exhaust of the test cell, and thus proportional to the
water vapor transmission of the barrier material. The MOCON
Permatran-W WVTR Tester, which was manufactured in the early
1980's, meets the requirement of this standard. In this
method, the driving force was generated by placing saturated
salt solutions on the humidified side. These salt solutions
tended to corrode the test equipment, were messy and very
operator dependent.

By later 1992, the Two-Pressure method of generating RH
had been developed, which enabled the operator to create the
desired relative humidity simply by adjusting the test gas

pressure as it passes over a distilled water humidifier. In

35

this case, salt solutions were no longer necessary. The Two-
Pressure method (MOCON, 1996) is based on the principle that
if pure water and a pressurized gas are confined in a
chamber, the gas will reach a relative humidity equilibrium
of 100%. If that gas is released to an area (or another
chamber) at a different (lower) pressure, the percent
relative humidity in the new are will be reduced. The amount
of reduction will be a ratio of the first and second
pressures. For example, if the gas pressure within the
humidifier is 29.4 psi and at 100% relative humidity, and is
released to an ambient pressure of 14.7 psi (at sea level),
its relative humidity will be 50%. The following formula can
be used to determine RH at a given pressure:

100 - [psiGauge / (ij-Gauge + 14.7)] x 100 = RH

Where psi - pounds per square inch.

This method has been applied to the newest designed
Permatran-W 3/31 WVTR Tester, as well as the Ox-Tran 2/20,
the Permatran C-4/40, and the Aromatran 1A, which can measure
gas or vapor permeabilities over a range of relative humidity
conditions. The sensor for detecting the RH is one of the
most advanced and reliable solid—state CMOS semiconductor
device, which exhibits excellent stability and repeatability
(MOCON, 1996). The Permatran—3/31 features a series of
technological enhancements over prior models. It is faster
and more stable, with increased upper and lower ranges, and

100 times greater sensitivity. It can perform tests directly

36

at the materials "real world" temperature and relative

humidity (MOCON web page, 1996).

(2.2) MAS 1000'”M Moisture Permeation System

The MAS 1000'm Moisture Permeation System, designed by
MAS Technologies, is another commercially available system
for testing the levels at which moisture will permeate.
Diffuse and solubility coefficient values for the packaging
material can also be determined. It employs mass transport
theory within a unique software package, which allows for
early prediction of steady state permeation values. This
highly sophisticated system incorporated precise temperature
and flow rate control, sensitivity in the 0.001 grams/m2/day
region, and a high speed 486DX computer with a very
accommodating software package. The test RH level is fully
controlled through software to assure consistent and
realistic measurements without the requirements of salt
solution preparation. MAS 1000m also incorporates a
multiple zone cell to provide the user with significant
improvement in test flexibility. This allows the user to
select the appropriate test sensitivity level. Measurements
requiring a high sensitivity may employ all zones while low
sensitivity measurements may be conducted using a single

zone .

3. C02 Permeation Testers

37

The most commonly used instrumentation for determining
the permeability of C02 includes the Permatran C-IV,
Permatran-C200, and the newest model the Permatran C-4/40
designed by MOCON. These systems are based on the isostatic
method, and use an infrared sensor with a closed loop system
to circulate the trapped carrier gas. For the Permatran C-W,
there are three methods which can be applied. The Static
Accumulation method is used for testing low transmitting
barriers, the Dynamic Accumulation method is used to measure
moderate barrier structures having transmission rates
generally greater than 50 cc/hfl/day. The Continuous Flow
method is a fast method to test barriers with moderate-to-
high transmission rates. The Permatran-C200 is interfaced to
an IBM compatible PC, so all the test functions are well
computer controlled. The system can test not only flat
barrier films, but also packages, such as filled carbonated
beverage containers. The PERMATRAN-C 4/40 unit performs
tests directly at "real world" temperature and C02 levels
(MOCON web page, 1996). The system's transmission rate
software will control up to 20 test cells in a system. In
addition, the software reduces the possibility of errors with
its ability to automatically determine equilibrium and
compensate for any flow changes. Pre-programmed test formats,
computer, and menu-driven screens make it easy to generate
and interpret test data. Like all MOCON permeation test
systems, the PERMATRAN-C 4/40 is modular. A flexible, dual

film test cell configuration and optional Package

38

Environmental Chamber allows for testing both flat films and

finished packages.

4. Organic Vapor Permeation Test Systems

Although studies on organic vapor permeation by
isostatic and quasi—isostatic test procedures have been
carried on for a period of time, there is no test standard
for such studies, since the measurement of organic vapor
permeation is quite complicated. Research workers usually
designed their own systems to conduct organic permeation
studies. The various procedures developed for quantifying the
rate of diffusion of organic penetrants through polymeric
membranes have been described by Hernandez, et al. (1986). In
responce to the industries demand for a convenient
instrumental system, MAS Technologies (Zumbrota, MN) first
developed, the TMI MAS 2000'm Organic Permeation test system
in 1994. In 1995, MOCON announced its competitive products,

Aromatran Series, i.e. Model 1A and Model 2.

(4.1) MOCON Aromatran Organic Vapor Permeation Series

The Aromatran 1A is a fully modular and automated system
designed to test single permeants under dry or at specified
relative humidity conditions, with a test temperature range
from 5 0C to 65 oC. The Aromatran Model 2 is an effective and
semiautomatic means of testing multiple permeants. The built—
in cryotrap also provides increased sensitivity for high

barrier materials.

39

The Aromatran series are also based on Fick's first and
second laws and Henry's law. The test results provide
permeability, diffusivity and solubility coefficient values
for test films, where the relationship of these three values
is P = S x D. Here, the assumption that the film
permeability follows Fickian behavior, is a prerequisite
condition for this system. The test system directly measures
the permeability of the film to organic vapor by utilizing a
calibrated FID detector. The approach of finding the
diffusion coefficient D when P is known, is the "half time"
method, which requires finding the time (thq) at half the
final permeation rate. When using tLQ, we can get D from the
following equation: 2

l

I):
7.199 . 11,, (16)

 

where l is the thickness of the film. When P and D are
known, S can be found by using S = P/D.

The half—time method requires the film to be outgassed
first, so the film is void of the permeant. Then the film is
challenged with a permeant on one side and the other side is
swept with a carrier gas to keep the permeant concentration
at zero. The technique involved here is called the
void/differential (VD) method. Another technique which can
be used is the Saturated/Differential (SD) method, which has
been found to be the fastest and most common test
today(MOCON, 1996). In the SD method, no outgassing is

necessary, and the film is "saturated" with the permeant at

40
the start of the test. The SD method is applied only when

permeability of the film structure is required. If the three
respective permeability parameters are required, the VD‘
method is the method of choice. The VD method is also
applied to the prediction function that is provided by the
Aromatran 1A.

The prediction method requires iteratives of an equation
with two unknowns. It can be described as following

(Pasternak et al., 1970):

F... _ 4 12 -12

 

 

 

 

— — —— e (18)
Ft [7? 4Dt ( 4Dt )
Let x, = —l—— (19)
4Dh
F F‘ (
co = 4 _ 20)
—— . x . ’31
(n:— F e
:D 12
: 21
4X1h ( )
s z i. (22)
I)

Fortunately, a value for x can be calculated for every value

of time (t) and thus the diffusion coefficient D, the

41

solubility coefficient S and the permeability coefficient Fm
(P) can be determined. One of the primary conditions of any
method or equation used to determine the diffusion
coefficient D, requires the permeant of interest to be
sufficiently outgassed from the polymer before the polymer is
challenged by the permeant. This is very important because
most polymers have retained residual organic volatiles, which

can be measured by the FID sensor.

(4.2) MAS 2000 Organic Vapor Permeability Test System

The permeation theory of the MAS 2000 System is based on
Fick‘s first and second laws, as well as Henry's Law. The
steady state permeability value, P, is assumed to be the
product of the material's solubility coefficient, S, and the
diffusion coefficient, D, as described by the expression
P = D x S. Based on the isostatic procedure, the apparatus
allows for the continuous collection and measurement of the
permeation rate of the organic vapor through a polymer film,
from the initial time zero to steady state conditions (Huang,
1996). Interfaced to an IBM 486$X computer, the MAS 2000
system demonstrates good temperature and flow rate control,
and it is convenient to perform on—sreen operations and
mornitoring. For each test, one film sample can be placed in
the diffusion cell. The test can be performed at dry
conditions only, with a test temperature range from room

temperature to 200 OC.

42
Recently, the MAS 2000 System was modified with a device

for trapping permeated organic vapors employing a dynamic
purge and trap technique (Chang, 1996), by MAS Technologies,
Inc. in collaboration with the School of Packaging (Michigan
State Uniiversity, East Lansing, MI). In this modified
configuration, the trapping system was designed to ensure
that the sample cell chamber is Continuously flushed with the
carrier gas and the permeated vapor is conveyed to the
trapping tube attached. The Dynamic Purge and Trap/Thermal
Desorption Procedure Showed an increase in sensitivity of
three to four orders of magnitude over the continuous flow

isostatic procedure (Chang, 1996).

MATERIALS AND METHODS

A. Materials for Synthesis of Clay/Polyimide

Nanocomposites

1. Natural montmorillonite
Source Clay Mineral Depository
University of Missouri, Columbia, Missouri

unit cell: Nao.86[M90.86A15.14(Sia.oo)020(0H)4]

2. 4,4'-diaminodiphenylether (4,4'—ODA)
Aldrich Chemical Co. (Milwaukee, WI)
Molecular Weight: 200.24
Melting Point: 190-192 0C

Purity: 99%

3. Pyromellitic dianhydride (PMDA)
Chriskev Company, Inc. ( Leawood, KS)
Molecular Weight: 218.12
Melting Point: 286.7 0C

Purity: 99%

43

44
4. Dimethylacetamide (DMAC)

Aldrich Chemical Co. (Milwaukee, WI)

Molecular Weight: 87.12

Melting Point: -20 °C ; Boiling Point: 164-166 °C

Purity: 95%

5. Octadecylamine (18A)
Aldrich Chemical Co. (Milwaukee, WI)
M.W. 269.52
Melting Point: 55-57 0C

Purity: 99%

B. Penetrants For Permeability Tests
1. Oxygen

LIQUID CARBONIC Specialty Gas Co. (Chicago, IL)

Purity: 2% 02, 98% nitrogen balance

2. Carbon Dioxide
AGA Gas, Inc. (Cleaveland, OH)

Purity: 99.9%

3. Ammonium Phosphate Monobasic (For making saturation
solution to generate 90%RH in the headspace)
Mallinckrodt Chemical Works (St. Louis, MO )

Purity: Analytical Reagent

45
4. Ethyl acetate (CH3COOC2H5)

J. T. Baker Chemical Co. (Phillipsburg, NJ)
Analytical Reagent grade

Molecular Weight: 88.11

Boiling Point: 77.2 +/- 0.5 °C

purity: 99.9%

C. Synthesis of Clay/Polyimide Nanocomposite Film

Clay/Polyimide nanocomposites film samples were prepared
following the procedure and formular discribed by Lan

(19940).

1. Preparation of Organically Modified Montmorillonite
Organclays were synthesized by an ion exchange reaction
(Lin, 1992) The cation exchange reaction was carried out by
adding 2.0 g of montmorillonite to 500 ml of 0.05 M
alkylammonium chloride ethanol:water (1:1) solution and the
solution stirred at 70 0C for 24 hours. The exchanged clay
was washed several times with ethanol:water (1:1), until no

chloride ions was detected with a 1.0M AgNO3 solution, and the

clay then was dried. Dried clay samples were ground and the

40-50 pm fraction was collected.

2. Preparation of 7% Polyamic Acid Solution.

46

Polyamic acid was synthesized by the condensation
reaction between 4,4'-diaminodiphenylether (4,4'-ODA) and
Pyromellitic dianhydride (PMDA) in Dimethylacetamide (DMAC).
A stoichiometric amount of PMDA.was slowly added to the 4,4'-
ODA/DMAC solution with vigorous stirring at 15-20 °C. The
mixture was then stirred an additional hour at room
tempeature. The final polyamic acid solution has a

concentration of approximately 7 wt%.

*1 H2 \ /""‘ O + D

(44-ODA) (PMDA)

HOOC COOH

—(——/=>——+ o O NH oc ICCU-Wei“

(Polyamic Acid)

 

3. Preparation of Clay/Polyimide Composite Films

A known amount of clay powder (the amount depends on the
desired clay loading in the resultant film) was added to a 20
gram solution of polyamic acid and the solution was stirred

vigorously at room temperature for 24 hours, followed by 2

47

hours to settle. The solution was then poured onto a glass
plate and dried at room temperature to form a self-
supporting film. The air-dried polyamic acid-clay films were
then heated at 2° C /min to 250° C and maintained at 250° C for

4 hours, to form cured polyimide—clay hybrid composites.

HOOC COOH

( O—O+NH-OC O CO-N n 23305

(Polyamic Acid)
C) C)
/ \_ /=\ 2 H o
_(—‘ /, O—‘ /,—N I N4“. + n 2
C)
(Polyimide)

D. Permeability Test Methods and Equipment

In this study, the MOCON Permatran-W WVTR Tester, the
Permatran-IV COziPermeability Tester, the Ox-Tran 200
Permeability Tester and the Aromatran 1A Permeability Tester

were used. A general description of the test gas delivery

 

48

system, calibration method and test parameters is briefly

summarized below for the respective test units.

1. MOCON Permatran-W WVTR Tester

The Permatran-W WVTR Tester consists of two units,
namely the measuring system and the sample conditioning
system (MOCON, 1982). Usually, up to five film samples can be
conditioned while the sixth one is being tested. Each test
film is clamped in a difussion cell, with one side of the
film exposed to a continuous flow of dry air, the other side
is exposed to water vapor from the moistened pad in the
upstream cell cavity.

The carrier gas in the test chamber is connected to an
infrared sensor, which monitors the moisture amount permeated
through the film. The infrared response generates an
electrical signal that can be read from a volt meter.

Calibration of the sensor is done with a PET standard
film of known WVTR value, to determine the calibration
factor. The specific calculation is :

WVTR (sample) = WVTR (standard) x mv response(samp1e)

mv response (standard)
The WVTR is given in unit of g/(m?.day).

In this study,each film sample was mounted in a masking
device (Modern Controls, Inc) to reduce the test area, since
the polyimide has a high water vapor transmission rate. The
test parameters are:

Test temperature: 37.8 0C

49

Relative Humidity: 90%

Carrier gas flow: 150 cc/min

2. MOCON Permatran-IV C02 Permeability Tester

The Permatran C-IV is an instrument designed for
measuring the rate at which gaseous carbon dioxide diffuses
through a permeable barrier material, such as flat film, a
plastic bottle, and other flexible packaging systems. The
instrument incorporates a means for programmed operation in
which each of the samples are tested automatically, in
sequence, with minimum operator attention. Data are recorded
by a strip chart recorder (MOCON, 1983).

When testing a flat barrier material, each film is
clamped between the upper and lower halves of a diffusion
cell. Gaseous carbon dioxide is admitted into the upper
chamber, and pure nitrogen is flowed into the bottom chamber.
The bottom chamber is connected to the infrared sensor and
pump that creates a closed loop around which the trapped
carrier gas is circulated. The infrared sensor monitors the
increased C02 concentration in the carrier gas and generates

an electronic signal. The C02 transmission rate is then

derived from the slope of the trace recorded by the recorder.

SO

Calibration of this instrument is accomplished routinely
by injecting measured increments of C02 into the volume being
sensed and noting the voltage change produced.

In this study, due to the high CO; permeability of the
polyimide films, each film sample was mounted in a masking
device, which reduced the test area from 50-cm? to 5 cm2, so
measurable results could be obtained. Some parameters used
in the test include:

Carrier gas flow: 250 cc/min

C02 flow: 80 cc/min

Test temperature: 25 °C

3.MOCON Ox—Tran 200 Permeability Tester

The Ox-tran 200 is an extremely sensitive instrument
designed to measure the rate at which dry or humidified 02
gas passes through a barrier material mounted in the test
chambers of the diffusion cell. The system is computer-
controlled and has several options including packaging
testing. This system meets the requirements of ASTM D3985
(MOCON, 1989).

When operating with flat films, the test films are
mounted in the diffusion cells, the test temperature and the
flow rate of the gases are adjusted and the test parameters
are entered into the computer. Upon initiating the run, the

computer will commence testing the film specimens and

51

continue until the oxygen transmission rate reaches a
constant value. At that time, the computer will terminate
testing and store the data until printout.

Calibration of the Ox-tran 200 is carried out by testing
a PET standard film of known Oztransmission rate value, then
comparing the test value with the known value, and entering
the ratio of those two values into the computer as Gas Sensor
Gain. Any value of oxygen transmisson rate obtained from the
measurement of the film being tested is then multiplied by
the "gas senor gain" value. The Ox-Tran 200 reports values
of the oxygen transmission rate, in units of cc/(nfl.day).

In this study, to obtain measurable results, an aluminum
foil masking device was used to reduce the film area exposed
to the test gas. In addition, a gas cylinder with 2% Oz and
98% N2 balance was used as the test gas. In conducting the
test, the following operating parameters were used:

Carrier gas and test gas tank output: 30 psi

N2 flow rate: 30 cc/min

Test gas flow rate: 20 cc/min

Conditioning time: 10 hours
Examination time: 10 min (It specifies the duration of an
examination of any given cell for each cycle during the test

process.)

52
Test temperature: 23 °C for evaluating the effect of clay

loading; 0, 5, 15, 23, 30 0C for the effect of temperature.

4. MOCON Aromatran-1A Organic Vapor Permeability Tester

The Aromatran-1A Permeability Tester is specifically
designed for testing the permeation of aromas, flavors,
odors, and solvents under dry or at a specified relative
humidity. The two film test unit, incorporates an FID
detector, and is interfaced to a pentium IBM compatible
computer. The computer controls the entire test procedure,
undertakes data collection, and the final results
calculations. The build-in RH sensors, temperature
controller, barometer and flow meter, which are controlled
and monitored by the computer, make the testing system well
controlled under desired conditions. Furthermore, the
"generated RH" method (Demorest, 1995) is employed to
generate the precise desired humidity, so it is possible to
test samples at "real world" conditions. The final report
provides not only the results of permeability, but also the
values of diffusion and solubility for the tested
penetrant/polymer system, at the test temperature and vapor
partial pressure.

To test a flat film, the film sample is mounted in one
of the test cells. Testing takes place entirely inside the
test cell. The test temperature and humidity of the gases are

selected, and the test parameters and other test information

53

required are entered into the computer. Once the test is
initited, the computer tests the film as instructed. The
computer will terminate testing and store the data until

printout (MOCON, 1996).

(4.1) The Test Gas Delivery System and the Partial Pressure
of Organic Vapor

The test gas delivered can be a certified gas or
sparging gas. In this study, a sparging gas was used. The
permeant to be studied is placed into the sparger, and a
carrier gas is passed through the liquid in such a manner so
as to obtain a known and stable concentration of the
permeant in the test gas stream. The test gas stream is
connected directly to the test gas input port of the
Aromatran 1A. Test gas flows are normally set at 15 to 30
cc/min.

When the temperature controller bath is set at a
specified temperature, the organic liquid inside the sparger
generates a fixed and constant saturation vapor pressure at
that specific temperature. It is suggested that the sparging
system should be maintained at a temperature 15 °C below
ambient to insure that there is no condensaton within the
delivery system. In this study, the sparger was set at
either 0 0C or —10 oC depending upon the desired vapor
activity. The resultant organic vapor partial pressure in

the test cell is equal to the the saturation vapor pressure

54

generated in the sparger. The conclusion is based on the
following reasoning (Hernandez,1996):

The vapor pressure is generated at 0 °C or below zero
(point A), while the film permeability is evaluated with the
test cell maintained at a higher temperature (point B). The
parameters of partial pressure, temperature, concentration
and volume for the organic vapor generator and test
conditions are denoted as p1, p2, T1, T2, c1, c2, v1 and v2,
respectively.

Asssumptions:
(1) At points A and B, the mass flows are equal.

therefore, M1=M2=M, where M is the mass flow (mass/time)
(ii) The organic vapor pressure is usually very low, and
therefore the mass flow consists primary of air. For this
case, it can therefore be assumed that the organic vapor
behaves as an ideal gas.

therefore, F2 = F1 * Tz/Tl

and by definition: c = M/F, where F is gas flow rate

(volume/time)

So, the concentration,
at point A: c1 = Ml/Fl and
at point B: c; = MQ/Fz

Then, Cz/Cl =T1/T2

55

When expressing c or p by the ideal gas law, at point A:
p = nRT/v = R/MW * m/v * T = R/MW * c* T
Pz/pl = C2*T2/ C1*T1

P2 = P1 *1/01*T1 *01*T1/T2 * T2 = P1

So, at point A and B:
- Temperatures are different
- Mass flows are equal

Flow rates are different

- Partial pressures are equal

(4.2) Calibration Method

The calibration of the Aromatran 1A is based on the two
point method(Loebig,1996). In the Aromatran 1A, the
relationship between the FID response and the organic vapor
partical pressure is assumed to be linear between two points.
Therefore, only one point needs to be obtained
experimentally, since the second point is assumed to be zero.
The specific operation includes introducing the test gas
stream directly to the FID, and entering the information
about the molecular weight of the permeant, and the partial
pressure of the organic vapor to the computer. The
calibration test is initiated when the system shows a stable
baseline. This operation provides for determination of a
relationship between detector output and vapor partial

pressure. The relationship is computed and then stored in the

56

program memory. Following calibration, a permeability test
can be initiated. Here, the transmission rate profile is
monitored on the screen until a constant signal response is
observed. The permeability parameters are then directly

calculated by the software program.

(4.3) Test Parameters

In this study, permeability tests were carried on at
three temperatures, 15, 23, and 30 °C,respectively. At each
temperature, two ethyl acetate vapor partial pressures were
evaluated, i.e. 3205 Pa (24.36 mmHg) and 1693 Pa (12.87

mmHg). Other operation perameters include:

Gas tank outlet pressures:

N} (as sparging gas): 4 psi (dry test); 18 psi (wet test)
N2 (as carrier gas): 10 psi (dry test); 40 psi (wet test)
Air: 50 psi

H2: 50 p81

Test gas flow = 15 cc/min
Carrier gas flow = 15 cc/min
Hydrogen flow = 4 cc/min

Air flow = 132 cc/min

FID temperature = 140 0C
Sparger bath temperature:

O 0C (for generating 24.36 mmHg ethyl acetate vapor pressure)

S7
-10 °C (for 12.87 mmHg)

Exam Time (min) = 15 (for getting steady state permeability

values)

Rezero Frequency = 1 (for getting steady state permeability

values)

Individual Zero = No (for permeability values)

Cycle count = Infinite

Relative humidity = 0, or 50%

Film area = 50 cm2

RESULTS AND DISCUSSION

A. The Permeability Properties OF Small Molecular

Penetrants

1. The Effect of Clay Loading on Water Vapor

Permeation

The permeability of clay/polyimide nanocomposite films

to water vapor was determined with a MOCON Permatran-W WVTR

Tester. The test conditions were 37.8 °C, 90%RH.

During the

test, an aluminum foil mask was used for each film to reduce

the test area of the film samples, so the detector output

values fell within a measurable range. The WVTR values

Table 1. Clay_Loading and H20 Permeability Coefficient

Clay Loading

H20 Permeabilt

r Coefficient“)

 

 

 

 

 

 

 

 

 

(v/v%) [g.mil/(m2.day.mmag)](bl [10-15 kg.m/(m2.sec.Pa)]
(C)
0 8.62 i 0.21 19 i 0.5
1.25 4.06 i 0.34 9.1 i 0.8
2.5 3.43 i 0.27 7.6 i 0.6
5.0 2.43 i 0.13 5.4 i 0.3
7.5 1.45 i 0.10 3.2 i 0.2

 

(a) Average of replicate analyses
(b) Permeability coefficient expressed in ASTM standard units
(0) Permeability coefficient expressed in SI units

58

 

59

obtained were converted into permeability coefficients to
describe the moisture barrier characteristics of the test
films. Table 1 summarizes the effect of clay loading on the
water vapor permeability coefficients of the respective
clay/polyimide films. Each value is the average of replicate
runs and is expressed in both ASTM and SI units. The results
are also presented graphically in Figure 2, where the
permeabilty coefficient is plotted as a function of clay
loading.

As shown, compared with a pure polyimide film, the clay
/polyimide hybrid composites exhibited a significant
reduction in water vapor permeability. Further, there is a
non-linear dependence of H20 permeability, as a function of
clay loading. For example, the addition of only 1.25%(v/v)
clay resulted in approximately a 50% reduction in the
permeability coefficient, as compared to the permeability

coefficient of the pure polyimide.

2. Oxygen Permeation
(2.1) Permeability Coefficients of Oxygen Through
Clay/polyimide Films as a Function of Clay Loadings

The oxygen transmission rate studies were carried out on
an Ox-Tran 200 Oxygen Transmission Rate Tester. In order to
get measurable results, not only was a aluminum foil mask
used to reduce the sample surface area, but the upstream
oxygen concentration used was 2% (v/v), rather than the

standard pure oxygen or a 21% oxygen mixture. The 02

Permeability Coefficient x 10"

[ingmisecPafl

60

Figure 2. Water Vapor Permeability of
Clay/Polyimide Nanocomposite Films
as a Function of Clay Loading
Permatran- w wvm Tester, at 378°C. 90%RH

20 T I I I I I I I I I I f fi I I

1 ‘\
15

10 K

 

I I I I

 

I I I I

TIWF

 

TIII
IIJL

 

//

I I I I

 

 

 

 

 

 

 

Clay Loading (°/ov/v)

Permeability Coefficient x 1018

[m°.me2.eecPa)]

5.5

2.5

1.5

Figure 3. Oxygen Permeability of
Clay/Pelylmlde Nanocomposite Films

61

as a Function of Clay Loading

Ox-Tran 200 02TH Tester, 23°C, 0%FiH

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

__ I I I I I 1 I I f I 1 I I I _
: \ :
)- Ii
- -i
- .
- q
)- .J
: 1
P -
' '1
j- \ I!
l- d
h \ -
- \\\ e-
" I I 1 I I I I I I g I I J I 7
-2 0 2 4 8

Clay Loading (°/ov/v)

LnP

7.6 _ I I I I N I Fr I r I I I I I I I _
7.4 : :
I \ 0% Clay 2
7.2 ' ' ‘\ ‘
7 _ 6x \ j
r \
L 0
6.8 \
C \ 2
\
6.6 _ b\ I
: \ 2.5%v/vCIay _
6.4 ‘x
C \K :
6.2 ' - '
: ‘0 1
6 i' I I l I I J I I I I I I I I I L I I J I .
0.0032 0.0033 0.0034 0.0035 0.0036

62

Figure 4. Arrhenius Plot at 02 Permeability

Through Clay/Polyimide Composite
Pure Polyimide vs. Polyimide With 2.5%v/v Clay

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1N(1/k)

0.0037

63

permeability coefficient values for the polyimide films with
various clay loading levels are summarized in Table 2, and
are expressed in both ASTM and SI units. To provide a better
understanding of the effect of clay loading on the barrier
characteristics of the clay/polyimide nanocomposite films,
the permeability coefficient values are also plotted as a
function of clay loading (see Figure 3).

As shown, the incorporation of clay into the polyimide
hybrid resulted in a significant reduction in oxygen
permeabilty, with a 50% reduction in the Oszermeability

coefficient achieved at a 2.5% clay loading level.

Table 2. Clay_Loading and Oz Permeability Coefficient

 

 

 

 

 

 

 

 

 

Clay Loading 02 Permeabilit r Coefficient“)
(v/V%) [cc.mil/(m2.day.atm)](b) [10‘18 m3.m/(m2.sec.Pa)]
(C)
0 2321 i 552 6.8 i 1.6

1.25 1353 t 133 4.0 i .
2.5 1082 i 139 3.2 r
5.0 791 i 23 i

7.5 601 i 20. 1.8 i .

 

 

 

(a) Average of replicate runs
(b) Permeability coefficient expressed in ASTM standard units
(c) Permeability coefficient expressd in SI units

(2.2) The Effect of Temperature on the Oxygen Permeability
Coefficients

The temperature dependency for 02 permeability, over the
temperature range 0-30 oC,‘was also determined for the

clay/polyimide hybrids. Table 3. summarizes the results

 

64

obtained for the pure polyimide film and the polyimide film

with 2.5%v/v clay loading. Values are the average of

replicate runs and are reported in both ASTM and SI units.
From Table 3, it becomes evident that the permeability

coefficient exhibits a temperature dependence at a constant

Table 3. Effect of Temperature on Oxygen Permeability
Coefficients of Clay/Polyimide Films

 

 

 

 

 

 

 

 

 

 

 

 

Temperature Permeability Permeability
Coefficientia) Coefficientia)
(°C) [cc.mil/(m2.day.atm)](b) [10"18 m3.m/(m2.sec.Pa)]
(C)
Clay Loading Clay Loading
0%v/v 2.5%v/v 0%v/v 2.5%v/vfifi4
942 463 2.75 1.36
1074 540 3.16 1.59
15 1371 703 4.03 2.07
23 1724 943 5.07 2.77
30 1934 1059 5.69 3.11 1;:

 

 

 

(a) Average of replicate runs
(b) Permeability coefficient expressed in ASTM standard units
(c) permeability coefficient expressed in SI units

permeant concentration level. This is illustrated in Figure
4, where the permeability coefficient is plotted as a
function of temperature [l/T (9K4)], for the studies carried
out. As can be seen, the temperature dependency of the
transport process, over the temperature range studied (0-30
OC), follows well the(Arrhenius relatioship. From the slopes
of the Arrhenius plots, the activation energy for the
permeation process (Ea) was determined for the respective _

film samples and the following values obtained:

65
0% clay/polyimide: Ea = 16.8 KJol/mole (4.038 KCal/mole)

2.5%(v/v) clay/polyimide composite: Ea = 19.6 KJol/mole
(4.692 KCal/mole)

The Arrhenius expression used to describe the
permeability coefficient as a function of temperature is
typically applied over a temperature range above or below the
glass transition temperature (T9) of the polymer, but not
within a temperature range which includes Tg. A straight line
extrapolation typically cannot be made through T9 and
graphical analysis is expected to show a change in slope at
the Tg. This was assumed not to be a concern for the
polyimide based structures, since the T9 of the polyimide (T9
= 399 °C)(Sykes, 1986) is well above the maximum temperature

evaluated in the present study.

3. The Effect of Clay Loading on Carbon Dioxide

Permeation through Clay/Polyimide Composites

Table 4. Clay Loading and COgPermeability Coefficient

 

 

 

 

 

 

 

 

Clay Loading Permeability Coefficientflfl
(v/V%) [cc. mil/(m2.day.atm)] [10"17 m3.m/(m2.sec.Pa)]
0 10516 (b) 3.1 (b)
1.25 6519 i 24 1.9 i 0.01
2.5 5154 i 983 1.5 i 0.3
5.0 3812 i 132 1.1 i 0.03
7.5 1852 i 257 0.54 i 0.08

 

 

 

(a) Average of replicate runs
(b) No measurable deviation from mean was calculated for replicate runs

 

Permeability Coetiiclent x 10‘7

[m’.ml(m2.saapa)1

3.5

2.5

1.5

66

Figure 5. Carbon Dioxide Permeability of

Clay/Polyimide Nanocomposite Films

as a Function of Clay Loading

Permatran C-IV 002m Tester, 25°C, 0%RH

 

 

 

 

 

 

 

 

 

 

 

 

 

 

_fIiI—I I I I I I r I I I TI I4
h g d
p 3 d
i- -(
:- uni
_ Ci
D G
P u
)- u
i- ..
i. -
. K .
h u
k -
P K d
I “\N Z
P \-
. 1
h I
- 1
7 14 I II 1 IJ I I I I I I I "
-2 0 2 4 6 8

Clay Loading (%v/v)

67

The CO; transmission rate studies were carried out on
the Permatran C-IV C02 Permeability Tester. The permeability
coefficient values calculated are summarized in Table 4.
Figure 5 shows graphically the relationship between CO;
permeability coefficient and the clay loading level.

From the results presented above, it appears that there
is a non—linear dependence of COszermeability, as a function
of clay loading, with the addition of 2.5%(v/v) clay
resulting in a 50% reduction in the C02 permeabilty
coefficient. This is supported by the results of Lan (1994c),
who studied the permeation of C02 through clay/polyimide
nanocomposite films and reported that there was a nonlinear
dependence of C02 permeability on clay loading for a series of

composite films containing 0-7.4% montmorillonite clay.

4. The Estimation of the Aspect Ratio Of Clay Fillers
According to Nielsen(1967), the dependence of
permeability on filler (i.e. c1ay)loading can be estimated

from the equation:

P (DP .
____F = (14)
Pu 1 + (L / 2W) (1)):

where PF and PU are the permeability values for the composite
film and that of the unfilled control film, respectively. 45,
and‘DF are the volume fractions of polymer and filler, and L/W

is the width-to-thickness ratio, or the aspect ratio, of the

filler. Based on this expression, the aspect ratio can be

estimated from relative permeablity data (PF/PU).

68

Tables 5.1,

5.2 and 5.3 summarize the relative permeability data for

Table 5.1. Relative Permeability of Clay/Polyimide

to Water VEPOI'

 

 

 

 

 

 

 

 

Clay Loading Relatiie “
Permeability
(v/v%) (Pp/Pu) (I
0 1
1.25 0.47
2.5 0.39
5.0 0.28
7.5 0.17 “

 

 

Table 5.2. Relative Permeabilty of Clay/Polyimide to C02

 

 

 

 

 

 

 

 

Clay Loading Relative
Permeability
(V/V%) (Pp/Pu)
0 1

1.25 0.62

2.5 0.54

5.0 0.36

7.5 0.18

 

 

 

Table 5.3. Relative Permeability of Clay/Polyimide to Oz

 

 

 

 

 

 

 

 

 

Clay Loading Relative
Permeability
(v/v%) (Pp/PU)
0 1

1.25 0.58

2.5 0.47

5.0 0.32

7.5 0.26

 

 

 

69

Figure 6.1. P'_jPU (H20) vs. Clay Loading (vlv%)

 

1.2 I I I j I I I I I I I I l I I IJ
...—..-W/TZZO

 

 

1

 

/¢
/
I
3:
§

0.8

 

0.6 _

 

PFPU
V/ "
,

’ I
I
I
I
i
I
I

 

 

 

 

 

 

 

 

0.4
' \ 0 1
0'2 . e KM
'- e, A
O b I I I 14 J -;.I.I. .-I.-I.i1.l.1.l ..

 

-0.02 O 0.02 0.04 0.06 0.08
Clay Loading (%v/v)

Estimated Aspect Ratio: 132

 

y - (1-m0)/(1+(m1/2)*m0)

 

 

 

 

 

 

 

 

Value Error

m1 132.39 16.035
Chisq 0.009241 2 NA
R 0.98875 NA

 

70

Figure 6.2. PF/PU (C02) vs. Clay Loading (vlv%)

 

1.2 rwr I fl T f I I I I I I I r I I1

 

 

 

 

 

 

 

 

 

 

 

1 " —-—--er20

C \\ ----- W/T21000
- I “. —e—002

0.8 P : '\.- .4
E: 06 : ' I 7 ‘ :
n. ' - '. \ I - - -
; . \Q\ 1
0.4 _ 'I \\\o\ ..
: .e \:
0.2 _ 0.
O I I I I I l l. .I.I..‘.I.I. 'I' i'l7l'l ‘

 

 

 

-0.02 0 0.02 0.04 0.06 0.08
Clay Loading (%v/v)
Estimated Aspect Ratio: 83

 

y = (1-mO)/(1+(m1/2)*m0)

 

 

 

 

Value Error

m1 83.426 7.3816
Chisq 0.0056643 NA
R 0.99256 NA

 

 

 

 

 

71

Figure 6.3. PF/PU (02) vs. Clay Loading (v/v%)

 

 

 

 

 

 

 

 

 

 

 

1.2 _ I I I I fiI I I T
1 I \
0.8 l '- -g

t I \ 5‘ a
3 I \ -l
I s \ L q
5 0.6 (- 1‘ °\ C ..... u
04 r |\ \ "
. b ‘ \x .-
I- \ u
02 - \ N1
0 .4 I I I I I “1‘7-5‘- 'E’i'T-'g-r-I .1

 

 

 

-0.02 0 0.02 0.04 0.06 0.08
Clay Loading (%v/v)

Estimated Aspect Ratio: 87

 

y =- (1-m0)/(1+(m1/2)*m0)

 

 

 

 

 

 

 

 

Value Error

ml 87.684 7.9146
Chisq 0.0058398 NA
R 0.99147 NA

 

72

water vapor, C02 and oxygen, respectively. Figures 6.1 , 6.2
and 6.3 present graphically the results obtained by
application of equation 14 and the experimental data. The
solid curve represents the best fit of the calculated values
from equation 14 to the experimental data. The dashed curves
are calculated for fillers with aspect ratios of 20 and 1000,
which are representatives of nonintercalated and completely
exfoliated clay particals, respectively. The best fit curve
yields an apparent clay aspect ratio of approximately 132
from the water vapor relative permeability data, 83 when
using the C02 relative permeability data, and an aspect ratio

of 87 from the 02 relative permeability data.

5. Discussion
(5.1) Comparison of the Barrier Properties of
Clay/Polyimide Nanocomposite to Literature Values for
Polyimides

Sykes and St. Clair (1986) determined the water vapor,
oxygen and carbon dioxide barrier properties for a series of
polyimides. Table 6. summarizes both the permeability
coefficient values of the polyimide based samples evaluated
in this study and the values reported by Sykes and St. Clair
(1986). The polyimide films in both studies were synthesized
from the same materials. Data are reported in both ASTM and
SI units.

As shown in Table 6, the results obtained in this study

were in good agreement with the literature values, for the

73

simple polyimide samples. This indicates that the films

prepared in the present study were of good quality (i.e.

level of pinholes, uniform thickness) and the permeability

values determined were representative of the polyimide

structure .

Table 6. Comparison of the H20, 02, C02 Permeability Results

with Literature Data

 

Permeability Coefficient H

 

 

 

 

 

 

 

Literature”) This study II
0% Clay 2.5% Clay
Polyimide Polyimide Polyimide
P320 at 37.8°C
kg.m/(m2.sec.Pa)(b) 1.6 x 10‘14 1.9x 10‘14 0.76 x10"14
g.mil/(m2,day.mfig)(c) 6.94 8.62 3.43
PC02 at 23 °C
m3.m/(m2.sec.Pa)(b) 4.0 x 10’17 3.1 x 10"17 1.5 x 10‘17
cc.mil/(m2.day.atm)(°) 13551 10516 5154
P02 at 23 °C
m3.m/(m2.sec.Pa)(b) 1.2 x 10'17 0.68 x10"17 0.32 x 10"17
cc.mil/(m2.day.atm)(c) 3975 2321 1082

 

 

 

 

(a) Data are converted from original Transmission Rate value (Sykes &

St. Clair, 1986)

(b) Permeability coefficient expresed in SI units
(c) Permeability coefficient expressed in ASTM standard units

(5.2) The Interpretation of the Barrier Property

Enhancement of Clay/Polyimide Nanocomposite Films

Permeation studies showed that for 02,(Ib and water

vapor as penetrants, the incorporation of low loading levels

of clay (i.e. 2.5%v/v) dramatically reduced the permeability

74
of polyimide films. The dramatic lowering of the

permeability characteristics of the nanocomposites is
attributed to the presence of dispersed, large aspect ratio,
clay layers in the polymer matrix. Previous work (Bissot,
1989) on the barrier properties of polymer films containing
conventional clay—type fillers showed that a large amount of
clay loading (40%wt) is required to reduce the permeability
of the clay filled structure, owing primarily to the low
aspect ratio (less than 20) of the particles. The method
used for preparing the film in the present study can result
in, in-plane orientation of the clay layers, which are
impermeable to penetrant molecules. This forces penetrant
molecules permeating through the film to follow a tortuous
path through the polymer matrix surrounding the clay
particles, thereby increasing the effective path length for

diffusion. Figure 7. shows such a model.

 

 

 

 

 

 

 

 

 

Figure 7. A.Model for the Path of a Diffusing Gas Through
the Clay/Polyimide Hybrid (Yano et al., 1993)

75

Previous studies on the montmorillonite clay/polyimide
system, described by Yano et al.(1993), showed that the clay
was dispersed homogeneously within the polyimide matrix and
was oriented parallel to the film surface. So the total path
of the diffusing gas was therefore increased. In this hybrid,
the clay consisted of stacked silicate sheets about 2000 A in
length, 10 A in thickness (Yano et al., 1993), to give an
aspect ratio of 200.

The X-ray crystallography results descibed by Lan
(1994c) for a montmorillonite clay/polyimide nanocomposite
showed that the clay retained a crystallographically regular
layer stacking order, with a monolayer of polymer
intercalated between the layers. This is consistent with a
possible self-similar clay aggregation mechanism, where the
face-face associated layers are slipped in a staircase-like
fashion. In order to more fully characterize the particle
texture, Transmission Electron Microscope (TEM) images were
obtained on thin sections of the clay-polyimide hybrid and
two characteristic domains were observed. There were domains
of highly exfoliated clay layers, as well as domains of
regularly intercalated layers. The aspect ratio reported by
Lan et al.(1994a) was 192.

The aspect ratio determined in the present study ranged
from 132 to 83 and is within the range of 200 to 70, which
was reported by Yano et al.(1993), Lan et al.(1994 a,c) and
Messersmith et al.(1995). It was assumed that the clay

texture in the nanocomposite films evaluated in the present

. 76
study would be similar to that obtained by Lan (1994c), since

the chemistry and methodologies employed in both studies were
similar. The aspect ratios reported in this study were
estimated from the permeability data and the solution of
Equation 14, and are therefore assumed to be a bulk property
of the clay/polyimide hybrid with both exfoliated and
intercalated domains contributing to the resultant
permeability values. The exfoliated state of the clay was
considered to be important to the barrier property of the
composite films (Lan, 1994c). Also, it should be pointed out
that the diffusion path model is based on the assumption that
the impermeable clay layers are perfectly oriented and evenly
spaced within the plane of the film, a situation not likely
to be realized in actual composite film structures
(Messersmith et al., 1995).

The observed enhancement in the barrier properties is of
great importance in evaluating clay/polymer nanocomposites
for use in packaging and other applications where efficient
polymeric barriers are needed. For these applications, a
significant reduction in permeability can result from either
increased barrier efficiency, or reduced thickness of the
barrier layer for the same efficiency (Messersmith et al.,

1995).

B. Organic Vapor Permeability Properties of

Clay/Polyimide Nanocomposite Films

77

To develop a better understanding of the mass transfer
properties of nanocomposites films, with particular emphasis
on organic permeants, studies to determine the permeability
of clay-polyimide nanaocomposite films to ethyl acetate were
carried out with the MOCON Aromatran-1A Permeability Tester.
This test system is based on an isostatic test procedure.

Permeability studies were carried out on polyimide film
with 2.5%(v/v) clay loading and on film without clay added.
The effect of the temperature, organic vapor activity and
relative humidity, on the barrier characteristics of the test

polyimide films, was evaluated.

1. Permeability of Polyimide Films with and without
Clay Inclusion

To evaluate the effect of clay loading on the barrier
characteristics of the nanocomposite films, tests were
carried out at constant tempeature (23 OC) and permeant vapor
pressure (Ap = 3.2 KPa). Replicate runs were carried out and
the mean permeability coefficient values reported. The
results are summarized in Table 7.

As shown in Table 7, there was an 85% reduction in the
permeability coefficient for the clay/polyimide film,when
compared to the polyimide film without clay loading. Fig 8.
presents the transmission rate profile curves for the test
polyimide films (i.e. with and without clay), and illustrates
graphically the significant reduction in the transmission

rate of the polyimide film with clay inclusion. Similar

78

Fig 8. Transmission Rate Profile Plots
for Polylmlde Composite Films
0% vs. 2.5% clay loading at 23 °C. 3205 Pa“ ethyl acetate vapor pressure

1 I I I

 

  
 

01
O
m IIT

 

 

I 1L1] III!

A
O

 

 

 

 

 

GD
0

 

 

N x 10“
[mm I (NZ-890)] '

N

O

_I.
O

 

O

 

IIIIIIIlr [III IIII IIIY

 

 

 

 

 

 

llll llll JILL llil

.1.
o
..
.
_
.

 

-1O 0 10 20 30 40

Time (min)

' Normalized Transmission rate: Data are thickness normalized
" Partial pressure oi ethyl acetate In test gas

79

results were obtained for studies carried out at different
test temperatures and organic vapor concentrations (see Table
9 and 12).

When the ethyl acetate permeabiity coefficient values
were compared to these obtained for selected commodity films
(see Table 8), it was found that polyimide films,and in
particular the clay/polyimide composite films, had much

better barrier properties to ethyl acetate.

Table 7. Ethyl Acetate Permeability Coefficients for 0% and

 

 

 

2.5% v/v Cla./£g;ybmide Com site Films
=L=l=£tc1ay Loading No. pmgabfim
(%v/v) gikg.m/( m?.sec.Pa)]
1 1.22 1'
0 2 1.27
Avg. .1.24 i:().03
2.5 1 0.186
2 0.189
Avg _ 0.188 t 0.002

 

 

 

 

 

an Partial pressuretof ethyl aEEtate = 3.2 KPa
an Temperature of test = 23 W2

Table 8. Ethyl Acetate Permeability Values Reported in the
Literature for Selected Commodity Films

 

Polymer Structure

 

Oriented Polypropylene

 

 

 

 

 

 

 

Glassine

High Density Polyethylene 40 “n H
0% Clay/Polyimide o . 12 (b) ll
2.5%(y/v) Clay/Polyimide 0.019 an

 

(a) Determined at 30 °C (Huang, 1996)
(b) Determined at 30 °C (This study)

80
2. The Effect of Test Temperature on the ethyl Acetate

Permeability of Clay/Polyimide Nanocomposites

Both the pure polyimide and the 2.5%(v/v) clay/polyimide
nanocomposite films were tested at 15°C, 23°C, and 30°C,
respectively. Tests were performed at dry conditions, with a
penetrant vapor partial pressure of 3.2 KPa (24.36 mmHg).

The results are summarized in Table 9.

Table 9. The Effect of Temperature on the Permeability of
Ethyl Acetate Throu h Clay/Polyimide Films

 

Temperature No . Permeabil ity Coef f ic ient x 1 0 19

 

 

 

 

 

 

(°C) [kg.m/(m?.sec.Pa)1
0% Clay/Polyimide 2.5%Clay/Polyimide
Films Films
15 l 1.19 0.178
2 1.17 0.173
Avg. 1.18 i 0.01 0.175 1 0.003
23 1 1.22 0.186
2 1.27 0.189
Avg. 1.24 i 0.03 0.188 t 0.002
30 1 1.34 0.189
2 1.39 0.190 I
Avg. 1.37 i 0.03 0.190 t 0.001

 

 

 

From the above results, it appears that the test

temperature does not markedly affect the permeability of
ethyl acetate through the polyimide and clay loaded polyimide
films, over the temperature range studied. However, as shown,
there is a slight increase in the permeation rates for both
film structures as a function of temperature. Typical

transmission rate profile curves, as a function of

Trananlaslon Rate 1:104
(What!)

3.5

0.5

-0.5

81

Figure 9. Ethyl Acetate Transmission Rate
Profile Curves for Polylmlde Film Samples

as a Function of Temperature
0% Clay Polylmlde Film at 15, 23. 30 °C. 0%RH. 3.2 KPa Ethyl Acetate

 

 

 
 

 

   

 

 

 

 

 

 

 

 

 

 

 

 

   

 

 

 

. 1 I I I I Ifi I I I I T ... v. '.'..‘..
i- ‘ ..
E s
. Ago .
; no :
. 4° 1

‘35 1

E % :
E 0 a
" A
E a 5
: an - , :
: 141 l l l l 1 1 l l l J L l l_l 1 I I l l :

-10 1o 20 30

Time (min)

82

Figure 10. Ethyl Acetate Transmission Rate
Profile Curves for 2.5%v/v Clay/Polylmlde
Film Samples as a Function of Temperature
35 25°ka Clay/Polyimide'Film'at115,I2€'3,303C‘0‘1'6F1H,3.2KPa Eth Acetate

 

    

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

5 . - a

3 _ . H .-. H‘ .......... ”...—...

.0 E f

x E . ;

g 5 2 : 00 ° :

: o :

8 N 1 5 : ;e o :

'3 g : :
'- ‘I 9

g E 0 o lSdegreeC E

i- 0.5 _ 0 0 23degreeC -

5 0 A 30degreeC E

0 ,. ‘ d

-05 _ I I I I I I I4 I I 14 I I I I i I I I I I

-10 0 10 20 30 40

Time (min)

83

temperature, for both test films are presented in Figures 9
and 10, respectively.

Table 10. summarizes the values of Ln P and 1/T,
converted from the above data. The Arrhenius plot of the
permeability data, where Ln P is plotted as a function of T4-
(Kfl), is presented in Figure 11.

Table 10. Values of l/T and Ln P for Ethyl Acetate
Permeability

 

 

 

 

 

Temperature l/T Ln P1 Ln P2
(0C) (°Kf1) (0% Clay) (2.5% Clay)
15 0.003472 -41.281 -43.190
23 0.003378 —41.231 -43.118
30 0.0033 —41.132 —43.107

 

 

 

 

The relationship between Ln P and 1/T for both films is
described by following expressions:
(1) 0% Clay polyimide
Ln P = -38.317-856.5/T, R = 0.97013
(2) 2.5% Clay polyimide
Ln P = -41.488-487.75/T, R = 0.93888
The calculated activation energies are, 7.118 KJol/mol (1.702
KCal/mol) for the 0% clay loading film, and 4.053 KJol/mol
(0.97 KCal/mol) for the 2.5% clay loading film, respectively.
The activation energy values for ethyl acetate
permeability were quite low, as compared with activation
energy values determined for ethyl acetate permeabilty

through selected commodity polymers (see Table 11),

 

LnP

84

Figure 11. Ethyl Acetate Permeability
and Temperature Relationship

 

'41 rII II—I1 IIIIfIIIfiIfiI
------

.- ---v-ui ------ P-O

I
Illl

T

 

 

 

IIII

1 .
N Is
01 N
L

 

- -e- - O°/ocla
-a— 2.5%Clay

 

IT'r IIII

 

rs— ~9—

 

~13

 

 

 

 

 

 

I 144

fi‘l F‘

-435 4I4 _IIII IIII IIII III

0.00325 0.0033 0.00335 0.0034 0.00345 0.0035
1mk")

 

85

illustrating the lack of a significant temperature dependence
of the mass transfer process over the temperature range
evaluated. As discussed above, this behavior is typical of
glassy polymers. In this case, the test temperatures were

Table 11. Activation Energy Values for the Permeation of
Ethyl Acetate through Polymer Films Reported in Literature“”

 

 

 

 

 

 

 

 

 

Polymer Membranes Ea (KJol/mole)
HDPE 41.50

OPP 83.38

Saran Coated OPP 102.84
Acrylic Coated OPP 100.57
Glassine 69.19

0% Clay/Polyimide 7.10”
2.5%(v/v) Clay/Polyimde 4.10”

 

 

 

(a)Determineded at an ethyl acetate vapor activity of 0.095 (Huang,1996)
(b)Determined in this study

far below the Tg of the polyimide, which is 399 °C (Sykes and
St.Clair,1986). When the polymer is at a temperature well
below its Tg, polymer chains are less flexible, and the free
volume paths available for permeant diffusion are fixed. So
the permeability coefficient is quite constant at test

temperatures below T9.

3. The Effect of Ethyl Acetate Vapor Pressure on the
Permeability of Polyimide Film with and without Clay

Inclusion

86

The effect of organic vapor concentration on the
permeability of polyimide films with and.without clay loading
was evaluated at two penetrant partial pressure levels. For
each partial pressure value, studies were carried out at
three temperatures. The results are summarized in Table 12.

Table 12. Effect of Ethyl Acetate Vapor Pressure on the
Permeability of Clay/Polyimide Composite Films

 

 

 

 

 

 

 

 

 

 

Ethyl Acetate Test Permeability Coefficient
Partial Temperature x 1018
Pressure (°C) [kg.m./ (m?.sec.Pa )](3>
(KPa)
0% v/v Clay, 2.5%v/v Clay
3.2 15 1.18 0.175 II
(24.36 mmHg) 23 1.24 0.188 H
30 1.37 0.190 ]|
1.7 15 1.12 0.134 H
(12.87 mmHg) 23 1.14 0.127 H
30 1.20 0.148 H

 

 

 

 

(a) Each Value is the average of replicate analyses

The above results show that the organic vapor
concentration did not significantly affect the permeability
coefficient values for the respective polyimide films. From
Table 12, it is apparant that when the organic vapor
concentration is reduced from 3.2 KPa to 1.6 KPa, there is
very little observed change in the permeability coefficient
values of either film. The effect of temperature and vapor
pressure on the barrier characteristics of the nanocomposite
films is illustrated in the histograms presented in Figures

12. and 13, respectively.

Permeability Coefficient x 10 ‘°

[kg.ml(m2.sec.Pa)]

 

87

Figure 12. The Effect of Organic Vapor Pressure
and Temperature on the Permeability of
0% Clay Polyimide Film

32

1.7KPa

1 5 23 30 1 5 23 30
Temperature (°C)

88

Figure 13. The Effect of Organic Vapor Pressure

0'2 l l I l l

Permeability Coefficient x 10"
[kg me secPa)]
O O
‘o ‘3 '_.
U1 -* 01

O

and Temperature on the Permeability
of 2.5%v/v Clay/Polylmlde Fllm

' ' I 3205Pa
1693Pa
‘——
......

 

 

  

 

 

 

 

 

 

 

 

15 23

3 0 1 5
Temperature (°C)

LnP

89

Figure 14. Arrhenius Plot of Ethyl Acetate
Permeability as a Function of Clay Loading
and permeant partial pressure

 

 

 

 

 

 

 

 

 

 

 

‘41 I I 1 I I r r I I I I I I I fl I I I I
; [b— L _ .. __ _ 1
: CT 5 7 7 T “'43 1
-41.5
t 3
-42 ’
'F' —6— 0%clay (1693Pa)
42 5 _ — El- -0% Clay (3205Pa)
' ' . --x--2.5%clay(1693Pa)
I -- + -- 2.5%Clay (3205 Pa)
-43 ,
: iii-1+ j
: x ........... ‘
-43.5 _ x “““““ "’4‘ j
I 1
_44 I I I I I L I I I I I I I I I I I I I I

 

 

 

 

 

 

 

0.00325 0.0033 0.00335 0.0034 0.00345 0.0035
1/T(1/k°)

90

The effect of both organic vapor pressure and
temperature on the permeability of ethyl acetate through
clay/polyimide films is illustrated in Figure 14, where the
permeability coefficient is plotted as a function of
temperature [1/T(°K)] for studies carried out at.Ap = 1.7 and
3.2 KPa. As can be seen, the temperature dependency of the
transport values, over the temperature range studied, follows
the Arrhenius relationship. From the slopes of the Arrhenius
plots, the activation energy values at the test permeant
vapor pressure (Ep) level were determined for the respective
permeant/polymer systems. The values are summarized in Table

13.

Table 13. Activation Ener ies at Different Va or Pressures

 

 
   

 

 

 

 

Organic Vapor Activation Energy
Pressure (KPa) (KJol/mol)
0% v/v Clay 2.5%v/v Clay
3.2 7.1 4.1
1.7 3.3 4.5

 

 

 

The results presented in Table 13, indicate that the
activation energy for the clay/polyimide composite film was
not affected by the change of penetrant vapor pressure, while
the activation energy of the pure polyimide showed a minimal
dependency on penetrant concentration.

When tested under dry conditons, there was no observed
effect of repeated exposure to the organic vapor on the
barrier properties of the respective polyimide and

clay/polyimide structures. Thus, no apparant morphological

91

changes were indicated as a result of sorption of ethyl
acetate by the test films. The sorbed ethyl acetate can thus
be totally outgassed and similar permeability coefficient
values obtained on repeated analysis of the same film. The
relatively constant activation energy values exhibited by the
two structures, to different organic vapor pressures, provide

further evidence of their chemical resistance.

4. The Effect of Relative Humidity on the Permeability
of Ethyl Acetate Through Polyimide Films with and

without Clay Inclusion

The effect of sorbed water on the permeability of ethyl
acetate through polyimide-clay films was measureed by setting
the relative humidity of the permeant vapor at 50%RH and
performing permeability studies at 23 °C and an organic vapor
pressure of 3.2 KPa. In order to minimize the variation of
the transmission rate due to the fluctuation of the FID
signal under wet test conditions, six measurements were
carried out for each structure. The results are reported as
averages and the standard deviations, and are summarized in
Table 14. As shown, an increase in the permeability
coefficient for the pure polyimide film.was observed with an
increase in relative humidity, while a significant reduction
in the permeation rate was observed for the film with 2.5%
clay loading, as compared to the results determined at dry

condition. A graphical presentation of the permeability

PatneulltyCoefficientx10a

[ "OW (ma-994F811

92

Figure 15. Effect of Relative Humidity
on the Permeability of Ethyl Acetate through

Clay/Polyimide Nanocomposite Films
0% vs. 2.5%vlv Clay-polyimide Film at 23°C. 3205 Pa Ethyl Acetate

 

 

 

 

 

 

 

 

 

 

 

 

 

   

 

 

 

 

 

 

 

3 r I I I I 1 I I I I I I I I 1 I ..
‘ :
2.5 _ -
L: I 0%Clay
2 h I 2.5%Clay
i-
: I
1.5
1 ....................... 1
C .
0.5 . n ........... 1
i- -
‘V .i
O l l l l-_ --i

Relative Humidity (%)

93

values obtained under dry and humidified conditions is shown
in Figure 15.

The observed increase in the permeability values
observed for the film without clay may result from the
interaction of water vapor with the polymer. Previous
permeability studies carried out on the polyimide films
showed a very high WVTR value for this film (Table 1). It is
therefore, considered a poor moisture barrier.

Table 14. Effect of Relative Humidity on Ethyl Acetate
Permeability of Cla /Polyimide Films

 

 

 

 

 

Relative Humidity Permeability Coefficient x 1018(4) H
% [kg.m / (m?.sec.Pa )]
0%v/v Clay 2.5% v/v Clay, I
0 Avg. 1.24 i 0.03 Avg. 0.19 i 0.002
50 Avg.(b) 2.89 Avg.(b) 0.09
Std.dev. 0.92 Std.dev. 0.03 h

 

 

 

 

(a) Tested at 23 °C, 3.2 KPa ethyl acetate vapor pressure
(b) Average of six replicate analyses

Although the polyimide film exhibited a low permeability

to ethyl acetate, when compared to the permeability of a
commodity film such as polypropylene [P = 2.0 x 10 4”
kg.m/(m?.sec.Pa )](Huang, 1996), the sorbed water may
plasticize the film, which can lead to a pathway that allows
for a higher rate of diffusion for organic molecules.
Sorption of moisture by the polyimide film, can thus account
for the observed increase in the permeation rate of the
organic vapor under humidified conditions. Such an effect of
relative humidity on the barrier properties of the polyimide

structure studied is not unexpected and precedence for such

94

behaviour in polymers with an affinity for water is found in
the literature (Barrie, J.A., 1968; Meyer, J.A.et al., 1957;
Ito, 1961; Petrak, 1980; Long, 1953)

Reduction in the organic vapor permeability of the film
containing clay is not fully understood, but may result from
a reduction in the solubility or diffusivity of the organic
vapor in the clay/polyimide nanocomposites. In the clay-
polyimide structure, there are a number of Si-O and Al-O
groups, which can readily interact with sorbed water vapor.‘
Assuming there is a competitive sorption between H30 and ethyl
acetate for active sorption sites, when more active sites are
occupied by H20, the solubility of organic vapor will
decrease.

Landois—Garza and Hotchkiss (1988) reported an example
of an increase in the effectiveness as a barrier of a
polyvinyl alcohol (PVOH) film with increasing relative
humidity for the penetrant, ethyl propionate. The authors
explained the observed decrease in permeation, as relative
humidity increases, in terms of the change in diffusion (D)
and solubility (S) coefficient values as a function of
relative humidity. For this system, diffusion coefficient
values remained relatively constant, but at high relative
humidity showed a slightly downward trend. Solubility
coefficient, however, showed a clear trend toward lower
solubility at high relative humidity. The authors thus

concluded that the decrease in P as relative humidity

95

increases was mainly the result of the decrease in penetrant
solubility.

Hernandez (1994) studied the effect of sorbed moisture
on the permeability coefficient of oxygen through an
amorphous Nylon (Nylon 6I/6T). Values of the oxygen
permeability decreased sharply in the range of water activity
from O to 0.20 at 11.9 °C and 22 °C, and from 0 to near 0.45
at 40.3 °C. Further studies showed that the diffusion
coefficient tended to increase as the moisture content in
Nylon 6I/6T increased, and there was a rapid decrease in
solubility values when the water activity range was between 0
to 0.2. Based on these findings, the author proposed that the
combined effect of sorbed water on both the diffusion and
solubility coefficients must account for the net decrease in
the permeability of oxygen as a function of sorbed water

vapor.

5. Estimation of Diffusion Coefficient Values for
Ethyl Acetate Through Polyimide and 2.5%(v/v) Clay

Polyimide Films

In addition to the permeability coefficient, the
isostatic permeability technique also allows determination of
diffusion coefficient values. Tables 15.1 and 15.2 summarize
the diffusion coefficient values obtained for both non-clay
loading polyimide and 2,5%(v/v) clay/polyimide nanocomposite

films. The procedure followed for determining the diffusion

96

coefficient values is discribed in detail in Appendix A.

Further, inherent sources of error associated with the

Table 15.1. Diffusion Coefficients for 0% Clay/Polyimide
Films

 

 

 

 

 

 

Ethyl Acetate Test Diffusion
Vapor Temperature Coefficient x 1014
Pressure (°C) (m?/sec)iaHb)
(KPa)
3.2 15 6.():t 2.2
23 3.1 i 0.2
30 2.8 (c)
1.7 15 6.1 i 2.2
23 5.7 i 1.3
30 2.9 (d) _

 

 

 

(a) Average of replicate runs

(b) Determined by the baseline subtracting method (See Appendix A)
(c) One measurement

(d) At two significant figures, no measurable deviation from mean was
calculated for replicate analyses

Table 15.2. Diffusion Coefficients for 2.5%(v/v)
'Clay/Polyimide Nanocomposite Films

 

 

 

 

Ethyl Acetate Test Diffusion
Vapor Temperature Coefficient x 1013
Pressure (°C) (m?/sec)laHb)
(KPa)
3.2 15 1.8 i .
23 2.5 i
30 2.4 i .
1.7 15 i .
23 1.4 i .
30 2.2 i .

 

 

 

(a) Average of replicate runs
(b) Determined by the baseline subtraction method (See Appendix A)

 

97

diffusion coefficients determined for high barrier
structures, such as the clay/polyimde nanocomposites, are
described in the Error Analysis Section.

DeLassus (1993) proposed that the high barrier
properties of glassy polymers to organic penetrants such as
flavor, aroma and solvent molecules can be attributed to the
low diffusion coefficient values for these penetrant/polymer
systems. Typically, these values are too low to be measured
by standard analytical procedures and are less than 10-317
nfilsec, or possibly orders of magnitude lower. The diffusion
coefficient determines the dynamics of the permeation process
and thus the time to reach steady state, which accounts for
glassy polymers exhibiting high barrier characteristics to
organic permeants. Polyolefins, being well above their glass
transition temperature, are non-glassy polymers and have high
diffusion coefficients for organic permeants, and steady
state permeation is established quickly in such structures.

Table 16. Comparison of the Diffusion of Ethyl Acetate at
Low Vapor Avtivities in Polymers

 

 

 

 

 

 

 

Polymer Structure Diffusion Coefficient x 1014
1 ( mZ/sec)

High Density Polyethylenefifi 4.0

Oriented Polypropylenefifl 3.0

Saran Coated OPPia) 0.1603)

Polyimidekfl 2.9

2.5%(v/v) Clay/Polyimidefifi 22

 

 

 

(a) Tested at 30 0C (Huang, 1996)
(b) Apparent diffusion coefficient, representative of the structure
(0) Tested at 30 °C (This study)

98
Table 16 lists the diffusion coefficients for ethyl

acetate in several commodity polymers. Table 16 also
contains the diffusion coefficient values for the clay and
non—clay polyimide films investigated in the present study.
The polyolefin films show high diffusion coefficients, which
is consistent with their high permeation rates to ethyl
acetate vapor (Huang, 1996). The Saran coated OPP structure
has the lowest diffusion coefficient, which is consistent
with its low ethyl acetate permeability (Huang, 1996).

As described above, glassy polymers typically have low
diffusion and permeability coefficients for organic
penetrants at low vapor concentrations. However, from Table
15.1 and 15.2, it becomes evident that the polyimide films,
with and without clay loading, behave atypically in that
these films exhibit high diffusion coefficients, comparable
to the polyolefins, and low organic vapor permeability rates.

Assuming the relationship given by the Equation:

P = D.S (5)
is valid, a possible explanation for the low permeability
coefficient and high diffusion coefficient values obtained
for the respective ethyl acetate/polyimide systems may lie
with the low solubility of ethyl acetate in the polyimide

film structures.

ERROR ANALYSIS

The errors in the experiments of this study can come
from several sources, which include the uniformity of the
test films which is related to the film processing, as well
as measurement errors and uncertainties from the test
procedures.

First, the uniformity of the test films introduced the
most variation in this study. Each sample of film was
prepared by an individual batch procedure. Even for the pure
polyimide films, variations may occur when using polymer
solutions from a different condensation polymerization
series. In addition, when preparing clay/polyimide
composites, variation in stirring efficiencies in the
reaction vials may result in different aspect ratios for the
clay particles. Finally, each film was cast on individual
glass plates. Differences in surface properties of the
respective glass plates may result in variation of film
thickness. All of these variables can contribute to
differences in the individual film samples in terms of the
film thickness (estimated 10%), and aspect ratio of the clay
particles (hard to be estimated), and can account for the
variation observed for the permeability values and the

apparant aspect ratio values.

99

100

Secondly, the instrumental measurements can introduce
specific errors. For the WVTR testing, the errors and
uncertainty can come from: (1) the purity of the salt used to
prepare the saturated salt solutions, which may affect the
headspace driving force (estimated 1%); (2) The variation of
the steady state transmission rate values (estimated 3%); (3)
The precision of the voltmeter (1%); The thickness variation
(10%). The estimated compound error of the permeability
coefficient value was approximatly 15%.

For oxygen transmission rate testing, since instrument
parameters and detector signal output are controlled by a
computer and the temperature control bath is well controlled,
variations from steady state transmission rate is limited
(estimated 1%). The thickness variation was the same as
above(10%). In this specific study, an additional concern may
be the concentration of the test gas, which contains 2%
oxygen (estimated error 5%). At such a low concentration
level, any inaccuracy would have a significant influnce on
the permeability. The estimated compound error was
approxmatly 16%.

For C02 testing, variation in test temperatures could be
an issue since the test were carried out at room
temperature(i.e. 25:3 °C). Any variation in room temperature
would result in variation of the transmition rate (estimated
3%). The graphic calculation is estimated to introduce an

error of 1%. The thickness variation was the same as

101

described above. The compound error is therefore estimated
at 14%.

For ethyl acetate vapor permeability testing, there are
several sources which may result in errors. First, the
actual partial pressure of ethyl acetate in the test gas may
not be exactly equal to the assumed value, which is the
saturation vapor pressure generated in the sparger. If there
is a variation between the assumed and actual vapor pressure,
the actual vapor pressure will be somewhat lower than the
assumed value. It is difficult to estimate quantitatively the
variation between the assumed and actual vapor pressure
values, without an external calibration. The temperature
variation of the sparger bath (:1 °C) can affect the actual
organic vapor pressure (estimated 4%). Secondly, the steady
state transmission rate values showed a variation of about
3% for testing at dry conditions and 5% for testing at wet
conditions. Thickness variation was the same as described
above (estimated 10%). The estimated compound error for the
permeability coefficients was about 17% for the dry test and
19% for the wet test, when using the Saturated/Differential
(S/D) mode.

To determine the diffusion coefficient from a
permeability experiment with the Aromatran 1A system, the
instrument must be operated in the Void/Differential (VD)
mode (see Literature Review). This requires the outgassing of
any pe-absorbed residual organic volatiles before the test

run is initiated. In practice, when the system was switched

102

from outgassing to testing status, there was a shift in the
instrument baseline (i.e. increase in detector output
signal), which was included in the detector signal output
values used by the instrument software to determine both the
permeability and diffusion coefficient values. For poor
barrier films, exhibiting high permeation rates, this shift
or "jump" in the baseline would be minimal and would not
introduce significant error to the determined permeability
and diffusion coefficient values. However, for films with
very low permeability values, such as the clay/polyimide
composite film, such a shift in the baseline could greatly
affect the accuracy of the.test results. Since determination
of the diffusion coefficient is based on the "half-time"
method, this shift in the baseline can result in an over
estimation of the diffusion coefficient value, as well as
contribute to error in the permeability coefficient values
determined, partically for high barrier structures. Because
of these considerations, in the present study, the
Saturated/Differential (SD) procedure was used for accurate
determination of permeability coefficient values, while the
Void/Differential (VD) procedure was used to estimate the
diffusion coefficients. As described in Appendix A, the
detector output signal was corrected from the shift in
baseline and the diffusion coefficient values were then
determined. An estimated compound error for the diffusion
coefficient values was about 17% for the studies carried out

under non-humidified conditions.

SUMMARY AND CONCLUSIONS

The permeability of water vapor, oxygen, carbon dioxide,
and ethyl acetate vapor through clay/polyimide nanocomposite
films was determined with the MOCON Permeatran-W, Ox-Tran
200, Permatran C-IV, and the Aromatran 1A permeability
testers, respectively. Factors affecting the barrier
properties of clay/polyimide nanoconposites including the
following: (i) the clay loading level; (ii) the test
temperature; (iii) the organic vapor concentration ; and (iv)
the relative humidity, were evaluated.

Based on the results presented in the previous sections,
the conclusions which can be drawn from the study are
summarized below: '

1. The mass transfer process for the respective
penetrants, i.e. H20, C02 and 02, is highly dependent on the
clay loading level and exhibits a non-linear dependency with
respect to the decrease in permeability. With a clay loading
level of 2.5%(v/v), the permeability coefficients to H20, 02
and C02*were reduced to 39%, 47% and 54% of the permeability
value of the pure polyimide.

2. The temperature dependency of the transport process
for oxygen, over the temperature range studied (0-30 °C),

followed well the Arrhenius relationship. The obtained

103

104-

activation energies for the pure polyimide and the 2.5%v/v
clay/polyimide are 16.9 KJol/mol and 19.6 KJol/mol,
respectively.

3. The width-to-thickness ratios or the apparent aspect
ratios for the clay particles were estimated to be within the
range of 83-132, which increased the total path of the
diffusing gas and resulted in a non-linear decrease in
permeability values.

4. For ethyl acetate as penetrant, the following results
were obtained:

(a) When tested at dry conditions, the polyimide with
the 2.5%v/v clay loading exhibited enhanced barrier
properties to ethyl acetate vapor when compared to the pure
polyimide film. The clay/polyimide'nanocomposite film showed
an 85% reduction in the permeability coefficient, when
compared to the polyimide film without clay loading.

(b) Within the test temperature range from 15°C to 30 RE,
the effect of temperature on the permeability coefficients is
minimal. When tested at 3.2 KPa ethyl acetate vapor pressure,
the activation energies for the pure polyimide and the
2.5%v/v clay/polyimide are 7.1 KJol/mol and 4.1 KJol/mol ,
respectively.

(c) Over an ethyl acetate vapor pressure range from 1.7
KPa to 3.2 KPa, the permeability coefficient appears to be
independent of vapor concentration. There was no indicated
effect of penetrant vapor pressure on the activation energies

for the clay/polyimide composite film, while the activation

105

energy of the pure polyimide showed a minimal dependency on
penetrant concentration.

(d) When moisture (50% RH) was introduced to the test
system, the permeability coefficient value of the pure
polyimide film increased by one hundred percent as compared
to the permeability coefficent value determined under dry
conditions. However, the permeability coefficient for the
clay/polyimide nanocomposite (2.5%v/v) film showed a decrease
of approximately 50% when determined under humidified
conditions.

(e) The low permeability but high diffusivity makes the

polyimide and clay polyimide films a unique property.

APPENDICES

APPENDIX A

APPENDIX A

Estimation of the Diffusion Coefficient Values

In order to obtain the diffusion profiles of both
polyimide and clay/polyimide films to ethyl acetate, it was
found necessary to skip the "rezero" setting during the test
and to operate the system in the Void/Differential mode. This
allowed for acquisition of a sufficient number of data points
before the permeation rate reached the equilibrium or steady
state level. Since no correction was made for shifting of the
baseline, the steady state transmission rate values obtained
would not be accurate for films with very low permeability
values (see Error Analysis), such as the clay/polyimide
composite material. Also, it would introduce errors to the
diffusion coefficient values when using the "Half Time
Method", since the determination of the half time (tLQ) was
related to the equilibrium transmission rate value. It should
be pointed out that by operating the system in the
Saturated/Differential mode, the "rezero" setting can be
applied since in this mode only the steady state permeation
rate value, and not the total diffusion profile curve, is to
be determined. Thus, permeation rate values obtained in the
Saturated/Differential mode, with the "rezero" setting, are

assumed to be accurate.

106

107

To estimate the diffusion coefficient values more
accurately, the following calculations were performed, based
on the assumption that the detector output contributed by the
shifting of the detector baseline, was a fairly consistent
value [around 2000 ug/(m?.day)]:

(i) Subtract the transmission rate value associated with the
baseline shift from all the data points obtained to generate
the diffusion profile curve.

(ii) Re-plot the diffusion profile curve.

(iii) Find the value of thql,where the transmission rate
value is half of the equilibrium transmission rate value,
from the new curve.

(iv) Use the following formula to get D:

12

D: ‘
7.199.t1,2 (16)

 

where l is the thickness of the test film.

Althernatively, the tlngvalue can be determined as
follows: i.e. find the t1“; from the primary curve where the
transmission rate Ema”) is:

F(half) = [(Foo - Fs)/2 ] + F5
where Fm is the transmission rate value at equilibrium state,
F5 is the transmission rate value associated with the

baseline shift.

APPENDIX B

APPENDIX B

Conversion Between ASTM Units and SI Units

In this study, most results are reported in SI units. In
some case, ASTM units are also reported so that we can
compare those results with some literature data. The
conversion between ASTM units and SI units in terms of
permeability coefficients for both gases and vapors are as

followings:

For gases:

1 cc.mil/(m2.day.atm) 2.94 x 10'21 m3.m/(m2.sec.Pa)

For vapors:

1 g.mil/(m?.day.mmHg) 2.23 x 10'15 kg.m/(m?.sec.Pa)

108

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