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

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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. ***