Large deformations in oriented polymer glasses: Experimental study and a new glass‐melt constitutive model

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. INTRODUCTIONAs 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.

Viscoplasticity and large-scale chain relaxation in glassy-polymeric strain hardening

“…Recent experiments and modeling [47,53,57,64] have provided strong support to the notion that entanglements play only a secondary role in glassy-polymeric strain hardening, at least for the majority of synthetic polymers and in the weak hardening regime. Refs.…”

Targeted deletion of Dicer disrupts lens morphogenesis, corneal epithelium stratification, and whole eye development

“…Initially, this negative temperature dependence was interpreted in terms of a viscoelastic stress contribution originating from temperature–activated relaxation of the entanglement network through chain slip,34, 39 a view consistent with the observed molecular weight dependence of strain hardening 39. The idea was elegantly put to the test by De Focatiis et al, [40–42 who combined the OGR model with the well‐known RoliePoly conformational melt model43 in an attempt to capture the effect of melt orientation on strain hardening. However, in its current form, the model only captures chain orientation on the entanglement length scale, and it, therefore, underpredicts strain hardening at temperatures well below T g , where it is dominated by sub‐entanglement chain orientation.…”

A modelling study of the visco-elastic behaviour of polyester-based coil coatings

“…This is why the comparison between the model and the experimental data is poor at all temperatures. A model capable of capturing the behaviour of ductile polymers has been proposed by Buckley and co-workers (see for example [37][38][39]). Future work will focus on the application of such models to the materials at hand, in order to obtain a suitable description of the paint mechanical response throughout the entire temperature range.…”