Abstract
Thermal expansion behavior of polymers is discussed relating to chemical structures. Polymers having flexible linkages and bent bonds to give a variety of conformation show larger values of thermal expansion coefficient (TEC) than the rod-like polymers composed of rigid segment and extended bond. The latters show anisotropic properties more considerably than the formers. Highly extended and oriented polymers exhibit negative thermal expansion (thermal contraction) along the chain axes and positive thermal expansion lateral to the chain axes. Such behavior is ascribed to the thermal fluctuation of a molecular chain moving perpendicular to the chain axis. As for the three dimensional network polymers, when the crosslinking density of cured thermosets increases, its TEC decreases in rubbery state but increases in glassy state. This can be interpreted as that the crosslinking points restrain the micro-Brownian motion of chains in the rubbery state, and cause an increase of free volume in the glassy state. The lowering of the TEC for polymers results from the suppression of low temperature relaxation.
