Edward S. Clark

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

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 α form is monoclinic, P21/c with a=9.05Å, b (fiber axis)=5.97Å, c=11.69Å, β=121.4° 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 β form is metrically monoclinic, with a=5.95Å, b=10.32Å, c (fiber axis)=4.94Å, β=101.3°. Examples of several classes of crystalline polymers demonstrating this Rule are presented.