Adv.'ard i r£ il: Carr C 0 n C Cl cit C / OI ' Coe 0 3 c^p 3 o' OJ. Dec cor u! piA & l Phi losophy up&i:unac x o n , 3:00 P . M . , F e b . 11, 1954 R m , 221, P h y s i c s - M a t h B u ild in g U io io -r U i l c r ; O u tlin e oi I n cx j Cv i ■ Tire i u l ' 1 Lienee o i a ^ a ^ a c L i e 1 i e l d t h e r.iiero'.vave D i e l e c t r i c C o n s t a n t a u i f j u j.g C r y s t a l on oT s tu d ie s a u L) j o c t : i>'.atoe uia toes a.inor subject : l-a.o%i ■a p n c a l e t enis 19J30 Dorn, AuprcI in U naer-praciuate U T a C U .a U Jotasrurs , Vernont Si t u d i e s ,• i . i i c h i y a n O t Li. C. i o U ? frie :ic e : St ^ S ta te C o l l e 0 e, C. n "11] ;■,'i \ Te1i v 1y - :^ i o: . r _ ._r 1,e 1 C\ r ie n r ^ a n S ta te C o lle g e , 1939’ •-I-'-' lp S j — G r a d u a be A s s i s t a n t , " A i c h i .v —a3 n S t a t e C o l l e 19^3 -53 • S erved in th e U n ite d S ta re s s r 19U-S c c e n te r oi A m e ric a n P h y s ic a l S o c ie ty , and Si^m a Pi S ly ; THE IHFLUEHCE OF A l&AGHE'TIC 1 lELjj CO THE MlCR o I'YAVE DIELECTRIC CONSTANT OF A LIQUID CRYSTAL By Edward F . Carr A TRESIS Submitted to the School of Graduate Studies of Michigan State College of Agriculture and Applied Science in partial fulfillment of the requirements for the degree of DOCTOR OP' PHILOSOPHY Department of Physics 19Sh ProQuest Number: 10008461 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. uest ProQuest 10008461 Published by ProQuest LLC (2016). Copyright of the Dissertation is held by the Author. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code Microform Edition © ProQuest LLC. ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 4 8 1 0 6 - 1346 ThE li'JPLbmiGil OP A r.LAum-iLiI j.0 FIELD Om IVLe r»iLORu YvAV L DIELECTRIC CGdSIAdT OP1 A LIQUID CRYSTAL By Edward F. Carr Ad ABSTRACT Submitted to the School of Graduate Studies of Michigan State College of Agriculture and Applied Science in partial fulfillment of the requirements for the octree of DOCTOR OP PHILOSOPHY Department of Physics Year Approved _______ n a n m 195^1- . c m C£_ Edwarcl F. Carr The real and imaginary parts of the complex dielectric constant cl the liquid crystal and normal liquid phases of pazoxjanisole and p-azoxyphenetol© have been measured at a frequency of 1^J,0G Me . The measurements were made in the absence and presence of a static magnetic field which was applied parallel and perpendicular to the microwave elec­ tric field for p-azoxyanisole and parallel to the microwave electric field for p-azoxyphenetole 9 and at various tempera­ tures in the liquid crystal and normal liquid phases. The values of the complex dielectric constant were ob­ tained by measuring the power reflected from a cell contain­ ing the sample whose length was changed by a movable shortcircuiting plunger. The results for the Imaginary part of the dielectric constant of p-azoxyanisole were further checked by measuring the transmission through a section of sample. This was accomplished by replacing the plunger with a matched detector. This experiment showed that a magnetic field was capable of producing large changes in the observed values of the complex dielectric constant of compounds in the liquid crystal phase. A reorientation phenomena was observed which re ­ quired a length of time varying from a fraction of a minute to a few minutes to take place after the magnetic field was turned off. By using this phenomena it was shown that hjdv;a r d F • Gar r the magnetic moment responsible for the alignment in the liquid e r g t a l phase was induced and not pe rmanent. AG RhC W LLDuEjviIiiHT The author wishes to express his sincere thanks to Dr. R. D. Spence, under whose inspira­ tion, constant supervision, and unfailing interest this Investigatioxi was undertaken and to whom the results are herewith dedicated. TABLE OI1' CONTENTS •Page 1 INTRODUCTION i 3 Low Frequency Dielectric Constant £ !< Low Frequency Dielectric Loss 6 17 Optical Studies 19 EXPER IME N TAL 23 Reflection Method 23 Transmission Method 37 Microwave Components 37 Temperature Bath I4I Cell and. Micrometer Screw Preparation oi' P-azoxyanisole and P-azoxyphenetole RESULTS AND DISCUSSION 1+7 I Dielectric Constant Dielectric Loss £ £ Reorientation Phenomena BIBLIOGRAPHY 4.7 32 6l 63 DldT Oi. ' 1 i'T O'LXlXIjiO figure 1. 2. 3. iq. 5. 6. 7. 8 . 10. 11. 12. 13. i age Temperature dependence 01 the low frequency c*lei me­ tric constant in a magnetic field 7 Angular dependence ul tue low frequency dielectric constant in a magnetic field ...................... 7 Experimental set-up for measuring the low frequency dielectric constant .................................. 9 The effect oi a static electric field on the low frequency dielectric constant for p-azoxyanisole. . 10 The effect of a static electric field on the low frequency dielectric constant for p-azoxyphenetole . 11 Tne combined effect of an external electric and magnetic fie Id on the Low frequency dielectric constant of p-azoxy a n i s o l e ......... ............... 13 The combined effect of an external electric and magnetic field on the low frequency dielectric constant of p-azoxyphenetole......................... 14 The effect of the intensity of the oscillating field on the low frequency dielectric constant of p-azoxyanisole .................................... 15 P-azoxyanisole m o l e c u l e . 15 Dipole moment Dielectric' loss of p-azoxyanisole as a function of frequency .................................... 18 Critical frequencies as a function of temperature for p-azoxyanisole ............................. 18 Index of refraction as a function of temperature for p-azoxyanisole ............................. 20 Index of refraction as a function of temperature for p-azoxyphenetole ........................... 21 Figure Page li|. Photograph of experimental s e t - u p ........... . . . . 2.1+ 15. Block aiagram of experimental set-up It. Concept of multiple reflections 17 • Power reflected from a cell filled with liquid d i e l e c t r i c ................. .......................... 29 18 . Wave m e t e r ................... ....................... 39 19. Detector m o u n t ....................................... 39 20. Flange joint in tempers.tureb a t h .................... 63 21. Micrometer set-up ............. 26 .................... 27 .................................... 63 < 22. Temperature dependence oi £ in p-azoxyanisole at 15*3 K m c .......................................... 63 23. Temperature dependence of £ in p-azoxyphenetole at 15*3 K m c ................. ....................... 69 1« Temperature dependence of d in p-azoxyanisole ..........................................53 at X .3 Kmc 2i|. 1 25- Temperature dependence of 6 in p-azoxyphenetole at 15-3 K m c ................. ... ....................56 26 Temperature dependence of the quantity 'j where ^ is the v i s c o s i t y ................................... 5 8 27. Field dependence of the dielectric loss in p-azoxyanisole ...................................... 60 LIST OF TABLES Page Table I I. The Variation in the Dielectric Constant 6 as a Function of the Applied Magnetic Field II. III. IV. Maxima for Ref lee ted W a v e s .................... Vwavcmeter C a l i b r a t i o n ........... Microwave Dielectric Constant V. Microwave Dielectric Loss £ 5 .31 L}D ( £ fi . . - 15.3 Kmc} 50 p ( f = 15*3 K m c } . 55 INTRODUCTION The compounds para-azoxjanisole and para-azoxyohenetole are simple examples of substances which exhibit a liquid., crystal phase. As is the case with several other of the compounds which possess such a phase, their liquid crystal phases are characterized by the fact that they flow like ordinary liquids but in the presence oi' an electric or magnetic field they show the anisotropy commonly associated with the crystalline phase. liquid The temperature range of the crystal phase in p-azoxyanisole is 118-135.3° C and for para-azoxyphenetol I 3 8 -I 6 8 o C. The liquid crystal phase was first observed by Reinitzer^ in 1 §?8 8 . He noticed that cholesteryl benzoate melts sharply to form a turbid liquid and at a higher temperature it changes sharply to a normal liquid. Lehmann^ studied these phenomena in 1 8 8 9 * anc* discovered the fact that the turbid liquid is doubly refracting and gives interference patterns in polarized light. During the past sixty-five years many observers have studied substances which possess a liquid crystal phase in the solid, normal liquid phases. liquid crystals. liquid crystal, and These substances are usually called The liquid crystal phase has been studied in the absence and in the presence of external forces such as those arising from static electric and magnetic fields 2 and the walls of the container in which the compound is placed. Experiments have been carried out on dielectric constants, X-ray studies, electric and magnetic suscepti­ bility, viscosity, specific heat, and double refraction. As a result of a variety of experiments, many observers have attempted to construct a theory of the liquid crystal state. The most popular theory to date is the swarrn the­ ory, which is however not accepted by all workers in the field. Even among those scientists who accepted the swarm theory there appears to be no detailed agreement about the structure of a swarm* however,they agree that a swarm is a group of molecules that are somehow held together as a separate unit, and the number of molecules in a swarm may be the order of a million. According to the swarm theory, which was discussed by Ornstein and Kast^, the liquid crys­ tal phase is made up of such s w a r m s • In the remainder of this introduction, experiments on dielectric measurements will be discussed, which are close­ ly allied to the work reported in this thesis. The complex dielectric constant will be denoted by where c is the velocity of light and v is the velocity of electromagnetic wave in the medium to be studied and 6 represents the power absorbed. 3 Low Frequency Dielectric Constant C TJne low frequency dielectric constant of these com­ pounds has been studied by several investigators. Abegg and Seitz^ measured £ In 1399 in p-azoxyanisole and found that it varied from ip to if.3 * corresponding to a tempera­ ture change of 150° C down to the solid state,, state it decreased rapidly to about 2.3* in the solid 1901 Eichwald^ reported values of approximately 6 for €* in both pare-azoxyanisole and para-azoxyphenetole. Buhmer^ repeated the meas­ urements on both compounds, using the same method as Eichwald, and reported Ip*3 for the normal liquid state and Ip.9 for the liquid crystal state for both compounds. 8 In the year of 192ip Jezewski' and K a s 3 , independently, investigated the effect of an external magnetic field on the low frequency dielectric constant. Jezewski used the resonance method which consists of two oscillating circuits coupled together. of an inductance, In the primary circuit, which consisted and a condenser, oscillations are excited. This circuit was coupled to a secondary circuit, which con­ sisted of two inductances and a variable condenser. At resonance 2Tn/LC~ where n Is the frequency of the oscillations. Resonance is obtained by varying the capacity in that part of the k circuit, which includes the condenser that is filled with the liquid, to be studied. The external magnetic field was applied both parallel and perpendicular to the plates of the liquid-filled conde n s e r . A magnetic field of I4 OOO gauss parallel to the condenser plates produced no notice­ able change in the dielectric constant. perpendicular to the condenser plates cillating electric field) A magnetic field (parallel to the os­ produced a decrease in the di­ electric constant with increasing field, and the change de­ creased with increasing temperature. The measurements were carried out at a wave length of 720 meters ( »br2 Me). The results as given by Jezewski are shown in Table I where represents the decrease in the dielectric constant. Although the values of €* for para-azoxyanisole anci para- azorj'phenetole are not in complete agreement with other results, the variation which takes place with increasing field should be significant. Kast measurements were at a wave length of 2 0 0 meters, and his results were similar to Jezewski’s in that they showed a decrease in the dielectric constant with increas­ ing field, and also this effect decreased with increasing temperature. Ornstein^ published a theoretical discussion of the anistropy of liquid crystals. His theory gave expressions for the dielectric constant of a liquid crystal in a mag­ netic field based on (1 ) molecular theory, (2 ) a crystal TABLE THE V i J i l A T i ' O M IB Thr i D I L L s-CTH.IC C s ' 1ST A J'T 6* AS A r- JC P lO B Oi1' TliL APLL1LL LAG ilbT iC FI l LD ( Ttio re s u lts vvere taken i'rom Para-azox;yanisole T = 122° G Field (Gauss) a paper (£ 300 *0 L r-* cd 44> CQ C o o o ft -P O CD i —I Cl> *H Oj0 o c cd cd CVC cd o o 120 - 1000 Static electric field in volts/cm Fig, 6a. p-azoxyanisole (T = 118.6 °C, H = 6 000 g&uss) H I E 1000 Stacie oIccLric field in volts/cm. 0 Fig, 6b. CD O 500 P-azoxyanisole (T = 125 °C, H - 6-000 gauss) 0 ^ H l r~t cd -P CD •h CO o d a s cd O-lrO cd o o Fig. 6c. E •6.0 ■80 « 1000 .00 Static electric field in volts/cm P-azoxyanisole (T = 129.5 °C, H - 6-000 gauss) H X E d> a £ cd -P a) 120 ■H fjiO o a cu - cd H J E c\j O O o 500 1000 1500 Static electric field in volts/cm Fig, J a . P-azoxyphenetole (T = 1^0,6 °C, H = 5000 gauss) 0 (D O 0 -to * cd H 11 I -P CD •H C^O o £ cd cd ax; cd O o Fig, 7b* 11 = 0 ■00 i 0 ---- ------------ ----- I, -2 ' Static electric field in volts/cm P-azcxyphenetole (T = 165.3 °C, H = lj000 gauss) 15' 121.2 £ o C 123 . 9° c H It 120 id o Oscillating electric field in volts fig. (3. The low frequency dielectric constant of p- azoxjanisole as a function of the voltage of the os­ cillating electric field {*Y = .6l5 Me). The electric field was varied from 22 volts to 193 volts which cor­ responded to a variation of 37 volts/cm to 333 volts/cm. The ordinate represents a measure of the capacity for the liquid-filled condenser which is proportional to the dielectric constant. The results in this figure were . f Q - / eg ^ \a ^ taken from Maier. Fig. 9« P-azoxyanisole molecule. Dipole moment. rotationally symmetric particles of volume v, which possess the dielectric constant € t , in the direction of the axis of rotation and a larger dielectric con­ stant Ci in each of the perpendicular directions . The particles are assumed to execute independent Brownian motion in the absence of any field. Maier*1*? has investigated the dielectric constant anisotropy in compounds similar to p-azoxyanisole and p-azoxyphenetole and has related the sign of the aniso­ tropy to the dipole moments of the various groups found in the compounds. The dipole moment of the free p- azoxyanisole molecule has a magnitude of / A = 2 . 3 Debye units 18 , and the contributions to this dipole moment are indicated by the arrows in Figure 9- These contributions to the dipole moment are valid only for the free molecule. In molecular association the relative motions of different portions of the molecule with respect to each other must be taken into account. K r e u t z e r ^ has measured the endo- thermic heat of transition from the nematic to the normal liquid, and the resultant discontinuous increase in the specific heat from which he concludes that the assumed rotation of the O-GH^ groups begins at the transition from the liquid crystal to the normal liquid phase and not in the liquid crystal phase. 17 Low Frequenoy b ie 1 cetric Less £ Buhner ^1 in his measurements of the dielectric con­ stant also mentioned something about the loss. In p- azoxyanisole he reported a strong absorption for both the liquid crystal and normal liquid states, the solid state. but none for In p-azoxyphenetole he reported strong absorption for the liquid crystal state, the normal liquid state, somev;hat less for and none for the solid state. From X-ray studies of p-azoxyanisole in an oscillating 20 electric field hast predicted a relaxation frequency of £ about 10 cycles per second. A more detailed study of this \ relaxation frequency was reported by Ornsteih and K a s t ^ by measuring the dielectric loss in a condenser filled with p-azoxyanisole over a frequency range £ 0 0 0 to 3 ,0 0 0 , 0 0 0 cycles per second. Figure 10 shows the dielectric loss as a function of frequency for two different temperatures. The results show a strong temperature dependence, since a relaxation frequency is clearly shown for T = 122° C, but does appear over the frequencies covered for T = 12[j_° C. Tan / is defined as follows I (jo where HC CO is the angular frequency, R is the resistance of the circuit and C is the capacitance of the condenser. Figure 11 shows the critical frequency (relaxation frequency) o JI -u 4? d e-t 002 .001 I] . 1 0 Fig. 10. Dielectric loss of p-azox^anisole as a function of freq u e n c y . The results were taken from a paper1 by Ornstein and Kast. o d CD r—1 cr 1 - / Since this is a geometrical series, the sum may easily be evaluated and the square of the absolute value can be ex- 29 - wnore la 11'odue i n g th e a . id i ^ is t u e p a u s e angle iollcwin,.; nett-lions , A A J3o A, } where cl i, CO = />oX is the guide wave length in air, v/e nave that & + e - ) T F T 2 h "co,(zb'} - the coserved wave is propor­ tional to K r , the observed power is proportional to KE because of the square law characteristics ol the crystal detector. |^E C&n 06 reP resen^e^ as shown in the fol­ lowing. diagram Dielectric Col u r n * Fig. 17 30 where the numbers 1 - 6 indicate the points for the values or x corresponding; to the first six maxima. gives the values of 12 Table II at the different maxima for various values of the complex dielectric constant, and for two different frequencies which ere in the best oper­ ating range for this set-up. In general these values could not be used to interpret results on other liquids, but they could be an aid in analyzing other results in this region. In order to carry out the computations shown in Table II, it is necessary to obtain relations for and ^ . We consider the following illustration where 2i0 guide and represents the impedance in the empty wave 2* the impedance in the liquid dielectric Since we have a polarized wave, the boundary conditions at the air-dielectric interface can be expressed as fol­ lows Ei + Er ■-Et Hi + Hr -Hf- OJ A 3 A » A — A 3 • 3 3 A - -__ ^ ’- -J' O O -j" f"—O i~ s 'T' c O C Jv ■ *o or r'\ < 9 — i * * — ^ • '— *■ -■___ _ v C A 3 O s _ 3 O J O J * “ -C j 3 O J • 3 i>A • O ( 'S A] CO ■'j- LT\ O \ r> o — — j U\ (A CO Ov] OJ H • • « • » 3 cr> A • O -i 3 3 C O A_ i — 1 A * ‘ lo 3 03 H « O\ OJ OJ » «J — % __ -- AT O J 9 oj A CM 3 • * OJ 3 A A] • 3 ! A — 1 — • • m —1 r— i —i OJ • * O A O J • A A 3 A J 3 O • • -4 _ A o 3 C M # « ■ o ~ cr 03 3 C A 3* C3 3 A —O A — ( — I O J • • * * h r i l ^ L ^ C T R D w A V 3 3 i3 •H 3 -P C J f-i •A P -t 3 9 C D LA A A * A A A u \ C M A » 9 9 -± A Z .T 9 3 g A A O J iv iA A li.iA r'O R 3 3 A 3 • o\ LA C A O 3 ,A o^ LA 3 3 C A __r !'' CA * ') C\ "■■-4 rH OJ OJ 4 CT' 3 i— ( O CA rH • On l" — rH o OJ OJ • i —i CM CM • aCM * _j- o OA c\ -A LA cA O'' CA C'A • 1 —I CM o~ • AH p ■A♦ CAJ CA LA ♦ rH —i -H O r— c— -O" "J cd £ -rH 1 —1 (S CA I —1 4 -3 CO P > ■ Q pj E-* p !-m P PH jJ CO rH * •H CO pa • -A CJ M •H PA oj CA 4 0 P -P> u o P oj OJ :■< O B o o OJ I —I o -nr H ' lA rA O "H 0 P *rH o * y P* .A 0 {A PH -H CA O AJ LA LA i —1 CM • » ™ « £ £ V , , cuici £ £ pepx'eseot the electric 1 leld 1 or Lne incident, r ei lec ted aria, transmitted. waves respectively, ana H j and , re pres eat the magne t i c i io 1 d for the inc ice.it„ r e 1lec tod , sac transmi t ted wave respectively , Us ing the lolloping rela t ions £■ /V, Z6 r _JV ' " Hr 2.‘ we have that ii_ = Ei + £ r Ht _3 »c + Hr 0 Et-- Er - 7 7' ° I ~ £/Et * K _ Z _ J _ K CO ° l - K« - Ao9 2 *-o -*t 2 ^-*o hor a Th-mode the wave i m p e d a n c e ^ 60 o A for the air iIIled guide and 7 Ei + E r 1 - a y 1 /> - <_/? Is given b;y 3k for the dielectric filled guide. The relations for K©© and the phase angle can be expressed as follows A, k - A “ - K m - ~ - J T + 0 / ]/ ' - > + * * * b “l - k b ’ / + b U + ti‘‘ + 2 b' = 4 i j 3 “------ + t * i,', t £ p - In order to derive an expression for £* , we start with the wave equation for the axial component of the m a g ­ netic field for the mode, - - o where v is the velocity in the medium. The solution of this equation is (2) He f(zy) - 0 1 ~ b udx ( (zirf = ft where l(/3B - 0 ) i) JZbr is the width of the wave guide. If we take to be equal to one for a dielectric and use the following relation for the velocity in the medium V = " Vue next make the following substitutions a no equate the real and imaginary p a r t s . /3 = / 3 ' - t /3 ■' £ . €' - * 6 " o> £• - ■ - & ( / * ’. + (4 ) £“ = where ys'i ZTyOki and Itel 1 and for jF “l - is the guide wave-length in the We can write equation (1 ) as follows dielectric. 1 ^ 1 Act £ - W “* i+ rki'C _ I + I X J e 96 , * - -Zb't co^'f**) / ^ e Z lK J C * Cos(2 i ' f large we make the following approximation - * 36 which may be written jKfj ~ |Koo| = - £ K » Co%(zb'} +■ where the left side of this equation represents the indicated in Figure 17* /9 S From this equation we obtain the relation that We now substitute for ft' ,5) <■ < - u ( ‘ and ft" in equations - (3 ) and (if) m (6 ) 1 The accuracy for £ / was checked for £ = if and ” £ = *if and the error was found to be much less than . 1 percent. The error increases by increasing - was a good approximation for enough for the final results. c 11 /-11 c . t/ \ Equation (6 ) , but was not accurate The final results were 37 obtained by using the computed values shown in Table II. Maxima for intermediate values of 6* and £** were approxi­ mated from the values given In the table. Transmission Method The results Tor the imaginary part of the dielectric constant for p-azoxyanisole were further checked by measur­ ing the transmission through a section of' sample. The ex­ perimental set-up was the same as shown in Figure 15, except the plunger was removed, and a matched detector was placed at the end of the c e l l . The relative power was measured for various temperatures and values of the magnetic field and for different orientations of the magnetic field rela­ tive to the electric f i e l d . Since the power measured in an electromagnetic wave Is proportional to the amplitude squared, it will be proportional to the square of the right side of the equation (2 ) which gives the relative values If we use the following relation 6 where \ 'i it is the wave length in air, the values of € can be obtained Microwave Components One of the directional couplers was purchased from Sperry Gyroscope Company, and the other was copied from it. The Sperry Coupler is a cross guide coupler with a 30 range of 12 .L\. - 17.0 Kmc, and its coupling is approxi­ mately 20 d b . The directivity is greater than 25 db at 15*2 Kmc and 1 5 db at either end of its range. The matching sections were slide screw tuners. One was purchased from Hewlett Packard Company and the other constructed similar to it. The variable attenuator was obtained from the Hewlett Packard Company and had a range of 0 - 20 d b . Figure 18 shows a side view of the wave meter. cavity of the wave meter was made of b r a s s • The The top and bottom were fastened to the cylinder of the wave meter by means of four screws in each. The cavity was coupled to the wave guide by means of two small holes as indicated in the diagram. The section of wave guide shown in the diagram was an integral part of the wave meter. The screw which moved the plunger was taken from a micrometer. In order to calibrate the wave meter, use was made of the relation I where d A. represents the wave length to be measured, the effective distance from the bottom of the wave meter, and Ac is the cut off wave length. mode in a cylindrical cavity For the 7£fl| fig# IB. Wave meter •crystal / short circuiting olunger U choke F i g • 19* Detector mount Ll O TABLE III WAVE METER CALIBRATION Frequency (Me) A T£012 TE011 I4OOO 2.1414 .6814 .1064 1/|100 2.1262 .6604 .0989 142OO 2.1112 .6518 .0916 14300 2.0964 .0845 14400 2.0819 .6376 .6240 14500 2 .Ob75 .6106 .0710 14600 .0583 140 00 2.0256 .5979 .5853 .5731 .0647 14700 2.0534 2.0394 14900 2.0120 .5612 .0463 15000 1.9986 .5497 O -2j~ O• 15100 15300 1 -9594 .5385 .5275 .5169 .0350 15200 1.9854 1.9723 15400 1.9467 .5065 .0190 15500 15800 1.8974 15900 1.8855 .4964 •4865 .4769 .4.675 .4583 .0139 15700 1.9341 1.9217 1.9095 16000 1.8737 .4492 -.0 0 9 7 16100 1.8621 .4406 -.0140 16200 1.8506 16300 1.8392 .4319 .4235 16400 1.8280 .4153 -.0 1 8 3 -.0 2 2 5 -. 026 b 16500 1.8169 .4073 -.0307 15600 .0777 .0522 .0295 .0242 .0090 .0042 -.0005 -.0051 til where a is the radius of the cavity. tance d for a certain reading on the micrometer screw was obtained by taking readings on the modes then subtracting. of readings. d ana * ^ 0 1 1 This was repeated for a number It was found that the effective distance for zero reading over the range covered ./as very con- sistant. The frequencies corresponding to the micrometer readings for the T E q h III. The effective dis­ and T E q ^ modes are given in Table The fifth digit is uncertain. The detector mount was constructed as shown in Figure 3 . The type of crystals used were 1N26 and 1 N 3 1 • Temperature Bath The dimensions of the temperature bath were 11+" by llj.,f by 9 ", and it was constructed by soldering copper sides to a quarter-inch brass plate. The temperature bath contained two stirrers which were run by small electric motors, an electromagnet which was capable of producing fields up to 2 0 0 0 gauss, a 5 0 0 watt immersion heater, an external heater. and The external heater was a piece of half-inch copper tubing coming out of the bottom and into the side of the bath. Part of the tube was wrapped with wet asbestos, then enough nicrome wire was wrapped around it to draw a current of 10 amperes at 115 v o l t s , The ni­ crome wire was then covered with a thick layer of wet as­ bestos and dried. ij2 The thermometer was placed near the bottom of the cell. The smallest division on the thermometer was 1/2 degree, and an attempt was made to calibrate it to *1 of a degree by comparison with a precision thermometer. How­ ever as a result of the open ends at the top and bottom of the cell the error in measuring the temperature was es­ timated to be. ♦ ,5 °C. One of the difficulties in carrying out the experi­ ment was obtaining a suitable liquid for the bath which had the desired properties of high flash point, cosity, and low vapor pressure. low vis­ Crisco was used first but turned rancid after being used a few times and the odor was very unpleasant. It also had the disadvantage of not being a liquid at room temperature. Gulf Grown E oil was tried next, but it was found to be much too viscous to provide the uniformity of temperature desired. Peanut oil was finally tried and proved to be veryr suitable in all r e s p e c t s . Cell and Micrometer Screw The cell was a continuation of the wave guide passing through the middle of the bath and about two inches from the bottom. A problem which gave considerable difficulty here was the construction of a flange joint at the bottom of the bath which would prevent hot oil from leaking In a n d which would at the same time minimize the microwave reflections. Rubber gaskets of various types were used i 3 m ica *U“ w indow washer - u -** c boke F ig , 20. P'lange . jo in t in tem perature M ic ro m e te r set-u p batb but only silicon rubber would stand the heat. The silicon rubber available at the time was too thick for the joint. Cork gaskets were tried but they dis­ solved in the hot o i l . to be satisfactory The final method which proved is shown in Figure 20. The flanges were soldered to the silver v/ave guide with pure lead using an acid flux. small screws The flanges were held together by then covered with a coat of gyptal and baked for a few hours with a heat lamp. Usually three or four coats were added in this manner to avoid a leak. Gyptal softens at high temperatures, but since it did not have to support any force, it acted like a thick glue and pre­ vented oil from leaking in. A teflon plug one-half wave length long was tried in place of the mica window, but it softened at high temperatures and came loose* Figure 21 shows the micrometer set-up for moving the plunger. three inches The plunger was made of brass and was about long with grooves milled in the sides for the liquid dielectric to move around the plunger. Preparation of P-azoxyanisole and P-azoxyphenetole The p-azoxyanisole was prepared by Mr. Caswell of the chemistry department of Michigan State College, and the following is his description of the method of prepara­ tion • k 5 jf1 -dimethoxyazoxybenzene (p-azoxyanisole) was p r e ­ pared by a mociirication of the raethod of Gatter-mann ana R i t s c h k e ^7 £ solution of sodium methoxide in methanol was prepared by dissolving 6 0 g. of sodium metal in 6 0 0 ml of commercial C. P. grade methanol in a two-liter roundbottom flask fitted with a reflux condenser topped with a calcium chloride tube. When all of the metal had dissolved, ana the solution had cooled to 3 5 -^ 0 °C, 1 0 0 g. (0 . 6 5 mole) of p-nitro&nisole solution, (Eastman C. P. grade) was added to the and the resulting mixture was allowed to stand, with occasional shaking, until a lemon-yellow color was attained. The mixture was then 'warmed slowrly on a Glas- Col mantle until it began to boil gently. (Rapid heating resulted in a violent and uncontrollable reaction.) the boiling point was reached, As the color of the mixture became a deep red; and after 5 - 1 0 minutes of gentle boil­ ing, a vigorous ebullition subsided gradually, accompanied by the deposition of a yellow, dense, crystalling precipi­ tate. The mixture was then gently refluxed for an additional six hours to ensure completion of the reaction. The reaction mixture was allowed to cool to room temperature, and was poured into a three-liter Erleumeyer flask and diluted with 1600 ml of distilled water to com­ plete the precipitation of the product. The resulting mixture was chilled in an ice bath and the product was recovered by filtration with suction. The cruoe product was recrystallized from about 3 -9 liters of methanol to which haa been added approximately 2 0 ml of concentrated tallization (9 2 7 7 £ 28 hydrochloric acid to promote crys- and to remove basic byproducts. percent) of a yellow, crystalline powder was obtained, m.p. 1 2 0 „ij.-1 2 0 ,7 °C (corrected); point, 1 3 5 *2 -1 3 5 •0 °C A yield of transition (corrected). The p-azoxyphenetole was prepared by Mr. L. V. Patel of the chemical engineering department of Michigan State College, and the following Is his description of the method of preparation. The method was the nreduction of p-nitrophenol alkyl ester ^0 Qne m o x e alkyl ester was put in a three­ necked flask and 1 . 5 moles of a 25 percent solution of NaOH was added while stirring. The temperature was raised to 80°C and 1.5 moles of glucose was added slowly while maintaining the temperature at 8 0 °C for i\S minutes . The X mixture was filtered and washed with water. The precipi­ tates were distilled to remove impurities, such as unreacted nitrophenyl alkyl ester or any side products. The product was purified further by recrystallization from alcohol and benzene. RESULTS AND D I S C U S S I O N Dielectric Uons ba 11D — £ Figures 22 ana 23 show the temperature dependence of the real part of the complex dielectric constant for paraazoxyanisole ana para-azoxyphenetole in zero magnetic field and in a field of 2 0 0 0 gauss parallel to the microwave elec­ tric field , Table IV gives the computed values which are plotted in Figures 22 and 23• tain. The fourth digits are uncer­ The estimated error is about 0.1 percent for the case and 0.2 percent for the other two cases. In para- azoxyanisole the results for a field of 2 0 0 0 gauss perpendi­ cular to the electric field were about 0 . 2 those obtained in a zero field . percent lower than Thus in this case the dielec­ tric constant in a parallel magnetic field exceeds perpendicular field. that in a This situation is exactly opposite that found by Jezewski"^ in his measurements of the static dielec­ tric constant. Maier^? has interpreted Jezewski’s results as indicating that the contribution to the static dielectric constant from the permanent electric dipole moment, which is perpendicular to the axis of the molecule, is greater than the contribution from the induced polarization along the axis of the molecule. Evidently at microwave frequencies the in­ duced polarization yields the predominant effect. 48 H,. =2000 3.5 U _______ J______ l_______ I_______i_______ l__ 120 124 128 132 136 TEMPERATURE °C 140 FIG-. 22 , Temperature dependence of the real part of the complex dielectric constant in a magnetic field of 2000 gauss parallel to the microwave electric field and in zero field for para-azoxy-anisole. Results for with a magnetic field of 2000 gauss perpendicular to the microwave electric field were found tgr be almost the same as for zero field. The liquid crystal range is 118 - 135 .8 °C. k9 3.7 Hu*2000 3.6 33 142 150 158 166 174 TEMPERATURE °C 182 FIG-* 23 . Temperature dependence of the real part of the complex dielectric constant in a magnetic field of 2000 gauss parallel to the microwave electric field and in zero field for para-azoxy-phenetole* liquid crystal range is 138 ~ 168 °C* The 30 TAbLh i.*iCK0aA\33 Hj_= 2000 Gauss nli - 200 Gauss 3* 6 09 about 0 . 2 percent lower 3 >305 3 .780 3.779 than lor II = 0 36.1s 3' , ,3 3 3.772 3 .619 3 .638 P-azoxypheaetole Tenperat ure in °C II = 0 = 2 0 0 C Gaus s 1/|2 3*289 3 *bii-5 150 3.334 3.631 160 3.372 3 *6.13 167 3 .4071- 3.602 170 3 .962 3 .662 130 3 .967 3*967 b± At present there appears to be no satisfactory theory for dealing with the dynamic dielectric constant of an aniso­ tropic liquid such as we are considering here. By borrowing from the theory7- of the complete dielectric constant of con­ ventional liquids we may write down the following tentative expressions for the parallel and perpendicular cases. These expressions consist of separate dipolar terms for the parallel and perpendicular cases with 6 * added to the parallel case to represent the contribution from the induced moment. For the dipolar terms the 7^ s , and Ai s denote the relaxation time and the change in the dipolar contribution to the real part of the dielectric constant in passing from I to I . butions for / . The 6 ^$ represent the dipolar contri­ To these equations we must add the assump­ tions that for this frequency of observation (u)V» / , and that temperature. 6* decreases rather markedly with The plausibility of the last assumption follows from the disorienting effect of thermal motions. As in most liquids we should expect the with increasing temperature. justified in the next section. f 5 to decrease This supposition will be further Therefore, for the perpendicular £2 case the real part of' £■he dielectric constant should in­ crease wit h increasing temperature. lor the parallel case the contribution to the real part from the dipolar term is small and its temperature dependence is unimportant com* pared to the decreasing contribution from 6* as the tempera­ ture increases. * Dielectric Loss - 6 *i As is shown in Figure 2ip, the dielectric loss of the liquid crystal phase of para-azoxyanisole decreases in a static magnetic field parallel to the microwave electric field and increases if the magnetic field is perpendicular to the electric lield. A similar situation prevails for the liquid crystal phase of para-azoi^p hen eto le. Figure 2£ shows the decrease in loss in a parallel magnetic field lor this case. Table V gives the computed values which are'plotted in Figures 2ip and 2 £ . The estimated error is about two perii cent in the measurement of c except m p-azoxyphenetole for the case where H = 0 where it is about five percent. From Figures 2.1f and 2£ it is clear that regardless of the magnitude or orientation of the applied magnetic field, £** increases as the temperature of the liquid crystal phase increases. The total increase throughout the liquid crystal phase is approximately the same for corresponding values and orientations of the magnetic field for the two compounds. In the case H * 0 the values of fluctuate in time and the results plotted for this case represent 53 H= 0 0.8 it 0.7 0 3 UO I------- *------- *------- ■------- ■------ ■— 120 124 128 132 136 TEMPERATURE ° C 140 FIGr.24. The temperature dependence of the imaginary part of the complex dielectric constant in a magnetic field of 2000 gauss parallel and perpendicular to the microwave electric field, and in zero field, for para-azoxyanisole. 5k H =0 Hu*2000 0.2 142 FIG* 25 * 150 158 166 174 TEMPERATURE °C The temperature dependence of the imaginary part of the complex dielectric constant in a magnetic field of 2000 gauss parallel to the microwave electric field and in zero field for para-azoxyphenetole. V 'i c i/;c CJ. ,6 O e.upei-e ti i-e in ° C _ Ig O ~ C V . v .- . *sole 1l l ~ £LO' O G'CV. 11S S 76 .61 .3 5 126 126 79 .1 7 ■J0 136 135 .51 .31 »3 6 / -■ G/ ■» x - 7 0' •"i '• 1 37 .1 2 j 60 . 60 „ U P “ d . z c x \ TeGipcrs ture in °C i') 1 1 e n e t o 1 e Hll 162 .X0_^ 66 ■ 15C . C G' 1Go .c6 1g 7 .70 173 .66 :ooo Jr- ‘J a time average of such values. This fluctuation will be rel exu'ed. to again later in this thesis. Since this frequency of 15,3^0 Me was 15,000 times that • A at which Kast‘-U reported firming an absorption maxima in para-azox yan iso le, it seems unlikely that the microwave ab­ sorption which was observed can be regarded as merely a high frequency manifestation of the absorption which he found. It appears that these compounds possess another relaxation frequency higher than that reported by Kast. We have previously assumed that our frequency of observation is considerably greater than of the postulated absorption. '/t where is the relaxation In the simple Debye theory the relaxation time should be proportional to is the viscosity and T* i/t where ^ is the absolute temperature. Since in equations 1 and 2 , we have assumed the absorption due to the induced moment to be negligible, to t/r or t/i for w f / r . t Is proportional The viscosities of para- azoxyanisole and para-azoxyphenetole in the presence of a static magnetic field have been measured by Miesowicz31 by observing the decrement of the vertical oscillations of a thin glass plate dipping into the liquid. From such obser­ vations he obtained the viscosity for various temperatures, for various values of the magnetic field intensity and for different orientations of the magnetic field in respect to the measuring plate. Before we can apply his data to this problem we must form some picture of the relation between 57 the molecular motion induced by dipping the plate at a given angle with respect to the magnetic field, and that produced by the microwave electric field applied at a given angle with respect to the magnetic field. Since the long axis of para- azoxyanisole and para-azoxyphenetole molecules lease diamagnetic susceptibility, is the axis of they tend to orient them­ selves with this axis parallel to the magnetic field. Ivlaier has shown that the permanent electric dipole moment lor liquid crystals with a central azoxy group is transverse to the long axis of the molecule, and therefore the application of a m i c r o ­ wave electric field parallel to the magnetic field tends to produce a rotation about an axis perpendicular to the plane of the permanent electric dipole moment and the magnetic field. The appropriate viscosity for the case in which the electric field is applied parallel to the magnetic field is -that ob ­ served when the dipping plate is perpendicular to the magnetic field and therefore to the long axis of the m o l e c u l e s . In this case the motion induced in the molecules by the motion of the plate is also about an axis perpendicular to the m a g ­ netic field. Figure 26 shows the plotted values of for para-azosyanisole calculated from M i e s o w i c z 's data as a function of temperature in zero field and In a field of 3 8 OO gauss perpendicular to the dipping plate. Althoug a detailed comparison of Figures 2lp and 26 reveals certain numerical discrepancies, there is sufficient agreement in the general form of the two plots to suggest that our simple model is 58 H *0 8000 H =3800 J- TO PLATE 120 FIG-. 26 . where T 124 128 132 136 TEMPERATURE °C 140 Temperature dependence of the quantity is the absolute temperature and viscosity from Miesowicz*s data. ^ VI is the 59 essentially correct. Suclri differences as do appear may p o s s i ­ bly be explained by a) tiie fact that the measured viscosity is a macroscopic rather than a microscopic viscosity implied by the Debye theory, b) the constants of proportionality between T/ and *j/r are different for the case H = 0 and H parallel to the electric field, c) a small temperature and field dependence of the quanti­ ties and £** The order of magnitude of € ii in the liquid crystal phase is the same as for the normal liquid, and this indicates that the loss is primarily due to a rotation of the molecules rather than a rotation of s w a r m s . Figure 27 shows the field dependence of the .imaginary part of the complex dielectric at a temperature of 12ip°C. constant for para-azoxyanisole In para-azoxypheuetole the results of the field dependence were quite similar to those of paraazoxyTa n i s o l e . The general shape ol1 this curve is very much like that observed by Miesowicz, of the viscosity. for t h e field dependence Miesowicz's results show the region of saturation to be at a higher field than shown by our results, but this could be expected on the basis of the orientation phenomena to be discussed next* 6o 200 400 600 800 1000 H* (GAUSS) FIG-.27* Field dependence of the Imaginary part of the complex dielectric constant for para-azoxyanisole at a temperature of 124 °C. Reorientation Phenomena It was shown earlier that the microwave dielectric loss in the liquid crystal phase ol‘ para-azoxyanisole and para-azoxyphenetole is decreased by the application of a magnetic field parallel to the microwave electric field. If the magnetic field, which produces the anisotropy of the liquid crystal phase, is suddenly turned oil, a length of time vary ing from a fraction of a minute to a few minutes is required lor the liquid crystal to return to the zero field distribution. This can be shown by measuring the change of the dielectric loss after the external magnetic field is turned off. The time required to return to the zero field distribution is dependent upon the temperature, as the temperature is increased. and decreases for any given temperature there is some variation in the 11orientation 11 time. This variation appears to be related to the fluctuations observed in the measurements of in zero field. These may have their source in thermal currents in the liquid crystal phase. By suddenly reversing an externally applied magnetic field of about 2 0 0 gauss, it can be shown that the aligning effect of the magnetic field is entirely due to the diamag­ netic nature of these compou nds . At the field and temperature at which the observations were rnac.e, about one minute was necessary to produce the alignment in the liquid crystal, when the field was turned on. If the alignment were due to a permanent magnetic moment the molecules or swarms would 62 have turned over when the H e l d was reversed and while turning over they would pass through a state ol' random orientation. Due to the long time required lor the align­ ment to take place, this random distribution would have caused a change in the dielectric loss as the magnetic field was reversed. Reversing the magnetic field showed that there was no turning over of the particles which im­ plies that the magnetic moment in para-azoxyanisole is induced. It is interesting to consider the effect of this re­ orientation relaxation process on other m e a s u r e m e n t s . For example one might question whether viscosities measured by an oscillating plate such as used by Miesowicz can ac­ tually be considered as the true viscosities if the period of oscillation is much less than the time for*the disturbed liq­ uid to return to its original orientation. B IB LIO ltK a F ii.1 k e in itz e r, I'. Beitrage z u r K e n n t n i s c e s G L o l e s t e r i n s . Monatshefte fur Chemie (Wien) 9 s^-21 (1883) „ a L e h m a n . : C . sccic :J iessende Krystable. Z. 1 , Chem., >.8 :129 (1309) . O r a s t e i n , L . and. h a s t , W, New A i v u tie a t a f o r t i e Sw ar m T h e o r y o l L i a u i d Cry s t a l e . Farauay Boc. T r a n s ., 29: 931 -9 44 (1 9 3 3 ). »I Abegg, A. a n d B e l t s , W. Uber das d i e l e k t r i s c h © V e r b a l t e n e in e r (< ris ta llin is c iie n l l u s s i g k e i t . 2 * 1 . piiys . C h e m ., 29:It-91 ( 1 8 9 9 ) . E ich w a lc ., L . N e u e U n t e r s u c i i u n g e n u s e r ctie k r l s t a l l . i n i s ciien I ' T u s s i g k e i t e n . I n a u g u r a l d i s s e r t a t i o n . m arburg (19CI>) Schenck. 157 . K ris ta llin is c h e PP. J e ze w s k i, k . D ie le c tric i n a M ag n etic F i e l d . 6 : 5 9 “ 8i-j ( 1 9 2 5 ) . K a s t , W. D ie le n o i-is c h e ilu s s ig e n K r is t a lle . F lu ss i g k e i t e n . L e ip z ig (1903 ) - C o n s t a n t ol' L i q u i d C r y s t a l s J . de P h y s i q u e e t l e h a c i u i n uni Ann a tu 1agS s i l l S O >3* 0 d i e d e r d * p bye '’Lk, 7 3 i • ; .- 1 6 0 IV 1 U£ ige r m i s :.S ; 1 e 11 u T 0 i t a t s kon s :■i a n t e n ;k e l t 8ii « Sj « r m x cj t * > 79 1 0 i e 1 fl !' J -L - ii £l 9 2 k ) J. *7 U. J e ze w s k i, is c h e n U b e i 1 e l e k t r i s c b e A n i s o t r o p i e dei> k c - i s t a l l i n F liiss ig k e i t e n . Z . P h y s ik , 4 0 : l 5 3 " l t 0 ( 1 9 2 7 ) . h a s t , W. L i e l e k t r i s e h e Un t e r s u c i i u n g e n a n d e n a n i s o t r o o o - n Zu s t a nc e es p a r a. - A z o »y £111 Ls o 1 s . A n n . cl . F h v s i k . 3 1 / ^ • c n „ ' 17 Mo, ok 1 J e z e w s k i, M. Uber d i e l e k t r i s c l i e l l u s s i g k e i t e n im m a g n e t i s c h e n k 2 t>8-2 7 9 -■ > A n is o tr o p ie der n em atisch e F e ld . Z . Play s i k , 3 2 : (1928 ). J e ze w s k i, M • U b e r d en L i n f l u s s des o l e k t r o s t a t i s c l i e n F e le e s a u f d ie D ie le k t r iz i t a t s k o n s t a n t e der K orper in n e m a t i s c h e r P h a s e ( l ’l ’f i s s i g e K r i s t a l l e ) . Z. P h y s ik , 51; 139-lb^ (j-928). • 66 19. 13. 16 . 17. 16 . 19. it Jezewski, M. liber die dielektrischen Ligenschaften der nem&tischen Fltissigkei tee jm rleichzeitigen ma.!;:neto-und elektrostatisehen F::luc . Z» P h y - ’h, 02:67 6 • V';i ) . Maier, W. Die Felc s tarkeabh&ngigke it der DielektrizIt 6 tskonstante dee p-Azoxyanisoi s. Ana a. Physik, 3 3 • 2 1 0 - 2 2 5 (1 9 3 6 ). Freedericksz, V., end Rea lew a, A> Theoretisches unci w i ­ per ime.n be .Lies zur Fi age naeh der ilatur aniso tropen Fills s ig'ke I t e n . t 2 . 1 . rnysik, 4 2 • ( :t>d2 -p 4 o V192? j # lViaier ^ W . Zur Frage der molekularen Struktur krista.lliner Flussigkeiten: II Die dielektrische Anisotropie geordneter k-i•istalliner Fluesigkeiten vom Typ des p .p 1-Azoxyanisols . 2. haturForsch ., 2a:456-ij63 (194-7) • Err era, J. El ektr is ct.es foment des p-Azoxyanisol$ . Pays Zeits. 29:1)20-629 (1 9 2 8 ). 't Kreutzer, C. Kalorimetrlsche Messungen beim Ubergang von der aniso tropen zur isotropen Fldfssigen Phase. Ann. d. Physik, g3;192-209 (1938). 2 0 . Kast, V'y. Anisotrope Plussigkeiten (Flussige Kristalle)i in elek tr ischen Feld e . Z. Phy sik, 7 1 0 9 (1931)* 21. 22 . Pellet and Chatelain. Sur les' indices des cristaux liquides mesure par la method© du prisme ot etude theorique. Bull, s o c . F r a n c . mineral, 73;154~172. (1950). Freedericksz, V., and Zonlina, V. Forces Causing the Orientation oF an Anisotropic Liquid. Faraday Soc. Trans. 29:919-930 (1933)* 23. B jorns balii , 1. Untersuchungen.fiber aaisotrope Flussig­ keiten. Ann. c. Physik, 56:161-207 (19189. 2 6-. Z w e t k o f F • streuung ues Li.ch.tes dur ch anisotrope Flussig­ keiten Parts I and ll. Acta piny s i^ och imio a . 9.1: 111- la 0 * (1 9 3 U ) . Kamo and Wininner y . Fields and Waves in Modern Radio, John Wiley and Sons. (1964-)* ■ 25- 26. Gattermann and Ritschke, 27. Vorlander, Lerichte, 26 . Blatt, A. Ii. "Organic Syntheses," Coll. Vol. II, Pp. 57-56 . John Wiley and Sons., Inc., (1963). 1423 Berichte, 23, 1739 (1890). (1907). 65 Dauar*^ , H . , and r „ e i i » s b e & c , r . t . chlcronitrooenm. Do x tr o s e a ad u \ So alum Ar s e a ite . Can. J . neso a 27^:6906 ( 1 9 6 9 ) . 30. dalbraith, 11. w ., Degerinc,, ii. 1 . and Hitch, D. 1-• J , Am • Che in . Soc ., 73*13^3 (19h 1) • 31. luiecowicz, in. Der mini luss des ma^netIschen l olde aur die Viskositat der F!1 $ssigkeiten in der nematiseheri Phase. Dull. Acad, polca d . Scicnc Letoics (Serie A) :226-21.7 (1930).