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. How-ever, 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 a 0 -PP daughter lamellae that crystallized epitaxially on the a-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.

Step-Cycle Mechanical Processing of Gels of sPP-b-EPR-b-sPP Triblock Copolymer in Mineral Oil

“…Hiltner and co-workers observed quite different results for a PP copolymer with a larger content of ethylene (17%). 61 The initial monotonic nominal stress-strain curve showed soft elastomeric properties and modest strain hardening. Subsequent tensile stress-strain curves showed a much larger initial Young's modulus (24 MPa vs 6 MPa) and nearly linear strain hardening;the initial tensile strain produced a much stiffer material.…”

“…In our drawn block copolymer gels we observe only the scattering streak from the fibrils. Hiltner and co-workers observed quite different results for a PP copolymer with a larger content of ethylene (17%) . The initial monotonic nominal stress−strain curve showed soft elastomeric properties and modest strain hardening.…”

Tough and Elastic Thermoplastic Organogels and Elastomers Made of Semicrystalline Polyolefin-Based Block Copolymers

“…They are one the most versatile polymers in today’s market and are used in many applications like automotive parts, household goods, adhesives, footwear, and medical devices. The great advantages of thermoplastic elastomers are their ability to be designed to meet specific requirements for each application, for example in terms of elasticity, extensibility, toughness, heat resistance, and ease of processing. − Given the low cost and wide availability of olefinic monomers, much effort is now being directed toward the development of polyolefin TPEs. New developments in polyolefin synthesis give rise to polyolefin-based copolymers that combine the high melting crystallizable segments with elastomeric soft segments suitable for properties typical of the thermoplastic elastomers. ,− The present work is focused on triblock copolymers comprised of syndiotactic polypropylene (sPP) for the hard end-blocks and random amorphous poly(ethylene- co -propylene) (EPR) copolymer blocks for the soft midblock …”