Relaxation Processes in Polymers

由于分子在玻璃态固体中运动而产生的亚玻璃弛豫过程，通常发生在非晶态聚合物和非晶态部分的半晶态聚合物中。它们被认为是由于局域构象运动与与玻璃化转变相关的较长距离的节段运动脱钩所致。在适当的情况下，柔性侧基的重新定位和主链上的局部构象迁移率都被提出作为机制。然而，几乎没有直接证据表明这些观念是正确的。本研究将利用最近收集的介电松弛数据和原子模拟在大块非晶体系的堆积和性质方面取得的最新进展，为柔性侧基运动所引起的亚玻璃松弛建立一个详细的分子模型。将在大体积多链环境中模拟侧链酯基的运动。对于局部主链类型的松弛，即聚乙烯中的伽马松弛，将进行类似的建模。以前建议的局部运动将在原子散体环境中建模，也将寻求其他局部运动。在其他研究中，将利用在更宽的频率或时间范围内进行介电测量的能力，以实验研究多种类型的驰豫过程的形状，并评估各种唯象方程表示形状的能力。由于研究的一个重要部分涉及数值密集型计算，因此还将在改进模拟技术方面做出努力。

Subglass relaxation processes, resulting from molecular motions in otherwise glassy solids, occur commonly in amorphous polymers and in the amorphous fraction of semicrystalline ones. They have been presumed to be due to localized conformational motions decoupled from the longer range segmental motions associated with the glass transition. In appropriate circumstances, both reorientation of flexible side groups and local conformational mobility in the main chain have been proposed as mechanisms. However there is little direct evidence that these notions are correct. The present research will take advantage of dielectric relaxation data recently collected on systems deliberately chosen to be good model systems for side group relaxation and of recent progress made in the atomistic simulation of the packing and properties of bulk amorphous systems in developing a detailed molecular model for subglass relaxation due to flexible side group motion. Motions of pendant ester groups will be modelled in a bulk multi-chain environment. Similar modelling will be carried out for a relaxation of the local main chain type, the gamma relaxation in polyethylene. Previously suggested local motions will be modelled in an atomistic bulk environment and other localized motions will be sought as well. In other research, advantage will be taken of the ability to make dielectric measurements over a broader frequency or time range to experimentally investigate the shapes of a number of types of relaxation processes and to assess the ability of various phenomenological equations to represent the shapes. Since an important part of the research involves numerically intensive computing, efforts will also be made in the area of improved simulation techniques.