Relationship of polymorphic crystalline phase texture to strain recovery and stiffness of a propylene-based elastomer

Abstract

A stretching process to enhance the stiffness of an elastomeric propylene-ethylene copolymer through orientation was examined. The tensile extension was performed at various temperatures within the unusually broad melting range of the copolymer. Stretching transformed the unmelted lamellar crystals into shishkebab fibers that acted as a scaffold for an elastomeric matrix of entangled, amorphous chains. Density measurements indicated that the process did not significantly affect the amount of crystallinity, which was about 23%. If the specimen was recrystallized by cooling after it recovered from the stretched state, the amount of orientation decreased with increasing stretching temperature. However, if recrystallization occurred in the stretched state, it led to the formation of a second crystalline network that prevented contraction of the oriented crystalline structure during strain recovery. It was suggested that the second network was anchored by α-PP daughter lamellae that crystallized epitaxially on the α-PP mother crystals of the extended fibrils. Although the manner in which the films were stretched and recrystallized strongly affected the modulus, good elasticity of the stretched films revealed the persistence of an elastomeric network. © 2009 Wiley Periodicals, Inc.