Crystallization and fibril formation in polymers

Abstract

Solutions of crystallizable polymers subjected to orientation crystallize in a fibrillar morphology if the polymer is solidified rapidly. The central core of the polymer fibers consists of thin, extended chain crystallites interspersed with disordered regions. The extended chain crystals are small fibrils having diameters generally less than a few hundred angstroms, while the lengths are usually not more than 5000 A. These dimensions are limited by an unknown mechanism, because even in a saturated solution, the fibril crystal size is limited. Hoffman's theory of flow-induced crystallization predicts that cumulative strain limits the growth of the central core fibril such that the diameter and length of the fibril are an inverse function of undercooling. This study was undertaken to obtain data to test the theory. The dependence of the fibril dimensions on undercooling at the time of orientation has been studied. Polyethylene fibrils were made by shearing a dilute solution between two slides under isothermal conditions at an elevated temperature, and the dimensions of the resulting fibrils were measured with transmission electron microscopy. The fibril diameter appeared to be a function of undercooling, while the fibril length was constant and not a function of undercooling.