A Rule Governing Crystallization of Configurationally Directional Polymers: The Crystal Structure of the α and β Forms of Polypivalolactone

Abstract

A "Rule" is proposed for incorporation of polymer chains having directional configuration, e.g. A-B-C-A-B-C, into a crystal. Crystallization into a lamella morphology, as in slow crystallization from the melt, will incorporate antiparallel sequences ("#"#"#). Formation of a fiber by drawing the lamellar morphology must produce a different crystal structure containing parallel directional sequences. The drawn fiber must be polymorphic with a disordered aggregation of antiparallel and parallel crystal polymorphs. An example of this rule is found in the crystal structure of polypivalolactone. The melt crystallized a form is monoclinic, P2 1 /c with a ¼ 9.05Å , b (fiber axis) ¼ 5.97Å , c ¼ 11.69Å , b ¼ 121.48 and consists of planar antiparallel sequences. The molecular conformation is a folded zig-zag arrangement. On drawing a fiber, a disordered second phase of parallel plus antiparallel sequences is created. The chain conformation is a slightly distorted extended zigzag. The crystal structure of the directionally disordered b form is metrically monoclinic, with a ¼ 5.95Å , b ¼ 10.32Å , c (fiber axis) ¼ 4.94Å , b ¼ 101.38. Examples of several classes of crystalline polymers demonstrating this Rule are presented.