Acoustic emission during polymer crystallization

Abstract

Ultrasonic emission was found to accompany several physical processes involving stress release, for example, microfracture in solids¹ or cavitation and bubble collapse in liquids². We here report the first observation of acoustic emission during crystallization. Acoustic emission was found to occur during the spherulitic crystallization of polymers from melt. The source of acoustic waves was identified as an abrupt stress release in regions of the melt occluded by spherulites during crystallization. The stress buildup in occluded areas is a result of density changes during crystallization. When the level of stress approaches the limit of the melt cohesion cavitation occurs, the stress is released and sound emitted. Because crystallization, in the form of aggregates growing simultaneously from many nuclei, always leads to the formation of occluded areas, and because the density of the melt or glass from which these aggregates grow is always lower than the crystallized matter, stress always arises. Depending on its level, the stress is either released by cavitation or remains frozen in the material. Thus acoustic emission is also expected to accompany crystallization in other, non-polymeric materials.