Large Deformations in Oriented Polymer Glasses: Experimental Study and a New Glass-Melt Constitutive Model in Wiley InterScience (www.interscience.wiley.com)

ABSTRACT: An experimental study was made of the effects of prior molecular orientation on large tensile deformations of polystyrene in the glassy state. A new hybrid glass-melt constitutive model is proposed for describing and understanding the results, achieved by parallel coupling of the ROLIEPOLY molecularly-based melt model with a model previously proposed for polymer glasses. Monodisperse and polydisperse grades of polystyrene are considered. Comparisons between experimental results and simulations illustrate that the model captures characteristic features of both the melt and glassy states. Polystyrene was stretched in the melt state and quenched to below T g , and then tensile tested parallel to the orientation direction near the glass transition. The degree of strain-hardening was observed to increase with increasing prior stretch of molecules within their entanglement tubes, as predicted by the constitutive model. This was explored for varying temperature of stretching, degree of stretching, and dwell time before quenching. The model in its current form, however, lacks awareness of processes of subentanglement chain orientation. Therefore, it underpredicts the orientation-direction strain hardening and yield stress increase, when stretching occurs at the lowest temperatures and shortest times, where it is dominated by subentanglement orientation. INTRODUCTION As demonstrated by a wide range of polymer products, molecular orientation is one of the fundamental parameters that determine the mechanical response of a melt processed, thermoplastic polymer. 1 The degree of orientation present within a particular product is a complex function of its rheology and the process parameters employed. The rheological behavior itself is intrinsically linked to the molecular weight and its distribution, and the chain architecture, as well as the presence of any additives. Consequently, as industrial polymer processing engineers have long known, mechanical properties of practical importance such as Young's modulus, yield stress, and fracture toughness in any given direction are highly sensitive to the grade of polymer employed and the flow history encountered during processing. 2-9 Therefore, it is of great practical interest to understand better the development of frozen-in molecular orientation during melt processing, and the relationships between this orientation and resulting solid-state properties. In particular, there is an engineering need to achieve a predictive capability, for use in optimizing polymer products