Fine structure of acetal resins and its effect on mechanical properties

摘要

AbstractStable, high molecular‐weight polyoxymethylene resins show the room temperature x‐ray crystalline patterns expected from early work on unstable or low molecular weight polyoxymethylene resins. High‐temperature studies show the specific features of this resin to be: (1) disappearance of crystalline peaks at 180°C., and (2) a super‐cooling of 15°C. (i.e., nucleation periods much greater than 15 minutes for temperatures above 165°C.). Studies utilizing optical birefringence show similar melting and freezing behavior. Birefringence studies show effect of quench temperature and melt time and temperature on the average spherulite size and appearance. Under suitable conditions, very large (1 mm. diameter) spherulites can be grown. The effect of variations in the fine structure (spherulite size, crystallite size, and degree of crystallinity) on mechanical properties of thin sections shows that: (1) an increase in crystallinity (by specific volume) gives higher moduli, higher yield strengths, and lower values of elongation to break; (2) smaller spherulites increase the ductility, or ability to retain ductility as the rate of strain of the test is increased; (3) changes in crystallite size over a twofold range have lesser effects on these