Fine structure of acetal resins and its effect on mechanical properties

Abstract

Stable, 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 properties.

Structure and Morphology of Polyoxymethylene

“…Th e width of the peak at 2θ = 34.6 ° is instead a measure of the length of the crystallite parallel to the molecular axis [25]. It is worth observing that the crystallite sizes obtainable from X-ray diff raction are of signifi cance only on a relative basis and are not correlated to the spherulite size in POM sample crystallized from melt, since Hammer et al, acting on crystallization temperature and on its rate of variation, prepared specimens with similar crystallite dimensions but very diff erent spherulites sizes [26].…”

Morphology of microinjection moulded polyoxymethylene

“…It has been suggested for conventional injection moulding 19,20 that polymers which form large spherulites upon crystallisation also exhibit higher crystallinity than those associated with smaller spherulites. However, the former tend to be more brittle due to a greater concentration of defects, including voids, at the edges of the large spherulites, which need more time to reach a high degree of perfection.…”

Physical Constants of Linear Homopolymers

“…The energy of mixing is the difference between these quantities, or (5) This equation is then recast in the form where (7) is a dimensionless quantity which characterizes the net interaction energy per solvent molecule divided by kT. The Helmholtz energy of mixing is obtained by combining equations (2) and ( 6):…”