Rate- and Temperature-Dependent Strain Softening in Solid Polymers

It is demonstrated that a large number of solid poly- mers (PMMA, PLLA, iPP, PS) display a pronounced change in kinetics (strain-rate and temperature dependence) after yield. The phenomenon finds its origin in the fact that, in specific ranges of temperature and strain rate, two different molecular processes may contribute to the yield stress. Because of strain softening, the post-yield response is only controlled by one of the two, resulting in a strain-rate dependence of the yield drop. The universality of the phenomenon is discussed in connection to the alleged influ- ence of secondary transitions on the impact response of polymer glasses. A modification of the finite-strain elasto-viscoplastic EGP-model is proposed to enable an accurate description of the mechanical response of solid polymers in the transition range. The versatility of the model is demonstrated on the temperature and strain-rate dependence of the intrinsic mechanical behav- ior of PMMA, iPP, PS, and PLLA. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 50: 1757-1771, 2012 INTRODUCTION Structural failure in static or dynamic (impact) loading conditions is a major concern in the applica- tion of polymers in load-bearing components. 1 Hence, under- standing of the fundamental causes leading to that process, and, ideally, identification of the role of details in the molec- ular architecture, is of critical importance. Likewise, the need for quantitative predictive modeling tools for failure phenom- ena is of extreme relevance and importance in the design and optimization of reliable load-bearing polymer components. The short-term failure of polymers is known to originate in usually rapid development of localized irreversible (plastic) strain, 2, 3 as manifested in moderate localization in shear bands and/or necks, 4, 5 or in extreme localizations in crazes that lead to cracks. 6, 7 A loss of structural integrity of the product results, and product failure can therefore be either ductile, involving the development of large localized plastic deformation zones accompanied by (more stable) tearing phenomena, or brittle, which gives fragmentation of the part. Changing the tempera- ture or loading rate can cause the failure mode to switch: the so-called brittle-to-ductile (B-D) or ductile-to-brittle (D-B) transitions. 8, 9

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