Physics of strain hardening in glassy polymers

一些高相对分子质量的玻璃态聚合物在大变形时表现出应变硬化机制。这一机制非常重要，因为SH限制了聚合物玻璃的延展性，因此对其微观理解是一个重要的科学挑战。Long团队是这个联合项目的法国合作者之一，他们发展了一种关于塑性的微观理论，根据该理论，松弛发生在大约5纳米的尺度上的重组。在这个尺度上储存的弹性能对应于自由能垒的减少，这使得塑性流动成为可能。该小组开发了一种用于计算3D塑性流动的配套数值工具，其空间分辨率为5 nm尺度，允许计算任何热机械历史的老化和回春过程中松弛时间的分布及其在外加应变下的演变。它基于一个关于张量序参数S在单体尺度上演化的动力学方程。取向增强了单体-单体间的相互作用，导致自由能垒增大。单体尺度上的重取向动力学及其松弛似乎是描述应变硬化物理的关键。在本项目中，将发展一种实验和理论相结合的方法，利用X射线散射、核磁共振和差示扫描量热仪来研究不同热机械加载历史下的局部取向及其松弛行为。固体核磁共振在这方面发挥了重要作用，因为它将在分子水平上提供对取向有序性的定量洞察。

Some glassy polymers with high molecular weights exhibit a strain hardening regime at large deformation. This regime is very important because SH limits the ductility of polymer glasses, so its microscopic understanding is an important scientific challenge. The Long group, one of the French collaborators in this joint project, has developed a microscopic theory for plasticity, according to which relaxation takes place by reorganizations on a scale of about 5 nm. The stored elastic energy on this scale corresponds to a reduction of free energy barriers which enables plastic flow. This group has developed a companion numerical tool for calculating plastic flow in 3D, with a spatial resolution the 5 nm scale which allows for calculating the distribution of relaxation times and its evolution under applied strain, during ageing and rejuvenation for any thermo-mechanical history. It is based upon a dynamical equation regarding the evolution of a tensorial order parameter S on the monomer scale. The orientation enhances the monomer-monomer interactions which leads to an increase of the free energy barriers. The reorientation dynamics on the monomer scale and its relaxation appear to be key for describing the physics of strain hardening. In this project, a joint experimental and theoretical approach will be developed to study the local orientation and its relaxation behavior under various thermo-mechanical loading histories by X-ray scattering, NMR and Differential Scanning Calorimetry. Solid-state NMR plays an important role in this regard, as it will provide a quantitative insight into the orientational ordering at the molecular level.