Influence of structural and morphological factors on the mechanical properties of the polyethylenes

TLDR

Crystalline polymers cannot be understood solely by changes in crystallite structure, unlike small‑molecule crystals. Samples were prepared to isolate key structural variables, enabling assessment of each variable’s effect on deformation. Force‑length experiments revealed that partial melting‑recrystallization is important, molecular weight governs the modulus and ultimate strength of linear polymers through interlamellar structure, whereas copolymers and branched copolymers show invariant ultimate properties regardless of molecular weight, branching, or crystallinity, indicating that all basic structural regions contribute to deformation.

Abstract

Abstract Force‐length relations at ambient temperature have been determined for a set of polyethylenes which represent a wide range in molecular weight and molecular constitution. Taking advantage of previous work from this laboratory, samples have been prepared in such a manner that the important independent structural variables can be identified and isolated and their influence on the different aspects of the deformation process assessed. Partial melting‐recrystallization processes appear to play an important role. For the linear polymers there is a direct influence of molecular weight. The influence of molecular weight manifests itself in the structure of the interlamellar zone which has a major influence on the initial modulus as well as the ultimate properties. Copolymers and branched copolymers display quite different behavior. The most striking difference is the invariance of the ultimate properties with molecular weight, branching, and level of crystallinity. From the set of experimental results that are presented the molecular factors involved in the deformation process can be sorted out. It becomes quite evident that all of the basic structural regions, characteristic of semicrystalline polymers, contribute to the observations. Focus solely on the changes in the crystallite, in analogy to the deformation of small‐molecule crystalline systems, is inadequate in the case of crystalline polymers.