Selected studies on the thermal and mechanical responses of amorphous glassy polymers at different length scales
This thesis describes investigations into the mechanical and thermal characteristics of amorphous polymeric materials by structural changes on the molecular and the microscopic scale. On the molecular scale, the structure of a cross-linked polymeric material is controlled by changes in the molecular weight between cross-links, cross-link functionality, and chain stiffness. With control of the network structure, an expansive range of mechanical and thermal characteristics is possible. These properties range from intrinsic properties, such as the glass transition temperature, to performance properties, such as impact behavior. Relationships between the network structure and measured properties are established by the use of a variety of theories from rubber elasticity to free volume. Relationships are also established between the various measured properties through solid and fracture mechanics.
The introduction of soft rubbery particles on the microscopic scale into a glassy polymeric matrix is commonly employed to create a tougher material. Despite the prevalent use, the mechanisms and sequence of mechanisms of toughening are poorly understood. The mechanisms and sequence of mechanisms are elucidated in this investigation through the use of unique mechanical tests and materials with favorable properties. The unique mechanical tests in this investigation include tensile dilatometry and a multi-axial stress state test. The multi-axial stress state test, which allows independent control of the dilational and deviatoric stresses of a material between uniaxial compression and equal biaxial tension, consists of a uniaxially loaded and pressurized thin walled hollow cylinder. The materials include liquid rubber modified epoxies, voided epoxies, and core-shell rubber modified polyvinylchloride. The voided epoxy material separates the matrix contributions from those of the rubbery phase, while the core-shell rubber modified polyvinylchloride provides optical verification of rubber particle cavitation. By combined use of these materials and the unique mechanical tests, the mechanics of rubber toughening are evaluated.
0794: Materials science