Strain hardening of polymer melts in elongational flow is important for a variety of polymer processing operations such as thermoforming, foaming and blow molding. As such the measurement of transient extensional viscosity becomes important to understand the elongational behavior of the polymeric materials. It is well known that linear polypropylene; a widely used commodity polymer; does not strain harden in melt extensional flow and there arises a need to structurally modify this polymer so as to achieve strain hardening. In the present work, the effect of dispersing two different organoclays in linear maleated polypropylene on the transient uniaxial extensional viscosity and shear viscoelasticity (linear and nonlinear) has been investigated. The matrix polymer was a linear random copolymer of propylene and ethylene grafted with maleic anhydride (PP-g-MA). Two different grades of nanoclay organically modified with different ionic surfactants were used as fillers. The extensional rheology was carried out at different strain rates, on an Extensional Viscosity Fixture (EVF) mounted on a TA-ARES rheometer. The neat PP-g-MA matrix did not show any strain hardening at different extensional strain rates whereas the nanocomposites with different organoclays showed different extents of strain hardening. Strain hardening was quantified by calculating the ratio of transient extensional viscosity to the viscosity obtained from linear viscoelastic envelope, called as the strain hardening parameter (χ) which was around 3 at lower strain rates for one of the nanocomposite with 5wt% clay loading. Linear and non-linear shear rheology of these nanocomposites was also investigated to understand the reinforcement effects upon filler additions in the polymer matrix and to get an insight on the nanocomposite structure. From the combined observations in linear and nonlinear shear rheology along with nonlinear extensional rheology we were able to conclude that the polymer matrix chains physically coupled (mainly by hydrogen bonding) to the nanoclays and these particle attached polymer chains formed hindered chain entanglements with the free matrix chains. The formation of such entangled structure led to improvements in the reinforcement levels as well as strain hardening behavior in uniaxial extensional flow. The second part of this study is focused on understanding the effects of filler loadings on the strain hardening and non-linear viscoelasticity of these nanocomposites. This study showed that there lies an optimum level of nanoclay loading (2 vol%) with an effective aspect ratio and specific surface area that are necessary to form hindered chain entanglements necessary for strain hardening. For composites with higher nanoclay loadings, the strain hardening diminishes with decreasing effective aspect ratios.
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