Cooperative Molecular Motions in Polymer Glasses and Their Effects on Structure and Deformation Behavior

在通常的使用温度下-远低于聚合物软化温度的温度-粘弹性行为的起源被认为是冻结固体中的局部分子运动。 聚合物中二次弛豫的发生通常与延展行为有关。 为了理解这种复杂的关系，一系列的研究项目将旨在揭示分子结构和分子迁移率之间的一些基本关系。 这些研究中使用的基本材料BPA-聚碳酸酯因其众所周知的延展性而被选择。 合成了几个系列的BPA-聚碳酸酯同源共聚物，产生了以下独特而重要的观察结果：1。 BPA-聚碳酸酯中的二次弛豫是由于约7个重复单元的协同运动。 2.协同运动的程度可以通过插入某些化学键来系统地降低，例如， 对苯二甲酸酯，进入主链结构。 3. 协同运动的程度越大，聚合物越有可能表现出延展性行为（屈服）。 相反，限制合作会鼓励脆性行为（银纹）。 4. 基本单元沿着聚合物链的运动，例如，亚苯基环的旋转可以被沿着链和链之间的其它单元限制。 这样的约束也导致机械行为的深刻变化。 将合成具有各种二次松弛特性的共聚物，以获得具有不同尺度的协同运动以及具有不同程度的链内和链间约束的材料。 合作运动的规模将通过改变柔性对苯二甲酸酯键之间的双酚-A单元的嵌段长度来定制。 这些单元的作用是界定合作运动。 链内约束的强度将通过将庞大的四甲基双酚-A单元并入主链中而变化。 最后，通过将氟基团引入苯环上来控制经由偶联的链间约束的程度。 这些合成的材料，具有各种二次松弛特性，将用于应力松弛实验。 这项工作的最终目的是创建一个相对详细的了解聚合物分子的结构和运动如何影响材料的韧性行为和脆性行为之间的相互作用。 聚合物材料在先进的工程系统中发挥着越来越重要的作用。 然而，尽管有许多偶然的发现，聚合物的分子结构和它们的机械性能之间的关系仍然是一个谜。 要解释这种关系，需要了解聚合物分子如何聚集成固态，以及这种固态如何适应外部施加的应力及其时间依赖性。 ***

9422049 Yee At ordinary use temperatures - temperatures well below the softening temperature of polymers - the origin of the viscoelastic behavior is thought to be local molecular motions in the frozen solid. The occurrence of secondary relaxations in a polymer is often related to ductile behavior. To achieve an understanding of this complex relationship, a series of research projects will be aimed at uncovering some fundamental relationships between molecular structure and molecular mobilities. The basic material used in these studies, BPA-polycarbonate, has been chosen for its well-known ductility. The synthesis of several series of homologous copolymers derived from BPA-polycarbonate has generated the following unique and important observations: 1. The secondary relaxation in BPA-polycarbonate is due to the cooperative motion of about seven repeat units. 2. The extent of cooperative motion can be systematically reduced by the insertion of certain chemical linkages, e.g., terephthalate, into the backbone structure. 3. The greater the extent of cooperative motion, the more likely the polymer is to exhibit ductile behavior (yielding). Conversely, limiting the cooperation encourages brittle behavior (crazing). 4. Motions of elementary units along the polymer chain, e.g., the rotation of phenylene rings, can be constrained by other units along the chain and between chains. Such constraints also result in profound changes in the mechanical behavior. Copolymers with various secondary relaxation characteristics will be synthesized to obtain materials with different scales of cooperative motions, and with different degrees of intrachain and interchain constraints. The scale of cooperative motion will be tailored by changing the block lengths of the bisphenol-A units between flexible terephthalate linkages. These units act to delimit the cooperative motion. The strength of the intrachain constraint will be varied by incorporating bulky tetramethylbisphe nol-A units into the backbone. Finally, the degree of the interchain constraint via coupling will be controlled by the introduction of the fluoride groups onto the phenyl rings. These synthesized materials, with various secondary relaxation characteristics, will be used in stress relaxation experiments. %%% The ultimate purpose of this work is to create a relatively detailed understanding of how the structure and motions of polymer molecules can affect the interplay between ductile behavior and brittle behavior in the materials. Polymeric materials are increasingly playing a critical, enabling role in advanced engineering systems. Yet, despite many serendipitous discoveries, the relationship between the molecular structure of polymers and their mechanical properties remains an enigma. An explanation of this relationship requires an understanding of how polymer molecules aggregate into the solid state, and how this solid state adjusts to the externally imposed stress and its time dependence. ***