Physics of strain hardening in glassy polymers

一些具有高分子量的玻璃态聚合物在大变形下表现出应变硬化状态。这一制度非常重要，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。该联合项目的法国合作者之一Long小组开发了一种塑性的微观理论，根据该理论，松弛通过约5 nm尺度的重组发生。在这个尺度上存储的弹性能量对应于能够实现塑性流动的自由能势垒的减少。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，可以计算在任何热机械历史的老化和再生过程中，松弛时间的分布及其在施加应变下的演变。它是基于一个动力学方程的张量序参数S的单体规模上的演变。取向增强了单体-单体相互作用，导致自由能垒的增加。单体尺度上的再取向动力学及其松弛似乎是描述应变硬化物理的关键。在这个项目中，联合实验和理论的方法将被开发来研究的局部取向和弛豫行为下的各种热机械载荷的历史，通过X射线散射，核磁共振和差示扫描量热法。固态核磁共振在这方面起着重要的作用，因为它将提供一个定量的洞察取向有序的分子水平。

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.