Building a phenomenological chain-level understanding of mechanics of semicrystalline polymers: 2. Conceptual

Abstract

To rationalize the comprehensive phenomenology in Paper 1 (this volume, p. 125878), we present a qualitative description for the mechanical behavior of semicrystalline polymers (SCPs) through a synthesis of prior understandings about the mechanical characteristics of glassy and semicrystalline polymers. Based on the central idea to regard an SCP as a crystalline chain network (CCN), in which a test chain can have crystalline and amorphous strands, we emphasize several key concepts: (a) crystallization determines the structure of the CCN, which is generally weaker than the pre-crystallization chain network in the molten state formed by interchain uncrossability, (b) ductility in SCPs is afforded by a sufficiently robust CCN, and (c) yielding and plastic deformation of SCPs require a successful structural transformation involving shape-change of crystalline phases through appreciable chain pull-out from the crystalline phases without breakdown of the CCN. In other words, it is the CCN that drives ductile SCPs to undergo yielding and subsequent large deformation through massive pull-out of load bearing strands (LBSs), which are the tie and entangling strands. Brittle fracture and lack of drawability occur in those SCPs where the CCN is too weak (due to a sparse population of LBSs) to cause sufficient meltdown of crystalline phases: There are not enough LBSs to undergo pull-out that is necessary for yielding and crystal transformation. Based on these concepts we can explore the processing-structure-property (P–S–P) relationship by demonstrating how pre-deformation in either crystalline or molten state produces more favorable structures for stronger mechanical characteristics.