Hydrodynamically induced crystallization of polymers from solution

Abstract

This paper primarily describes the large diversity in morphology of fibrillar polyethylene crystals precipitated from stirred solutions at elevated temperatures. Shish-kebabs crystallized at 112°C from stirred xylene solutions of linear polyethylene exhibit lamellar overgrowths having thicknesses of the order of 150 A from which it was concluded i. a. that at crystallization temperatures above 100°C only backbones are formed and that the surrounding polymer molecules deposit epitaxially on the substrate at lower temperatures. The variation in the appearance of the overgrowth and the inhomogeneity of the samples can be traced back to irregular stacking and to differences in washability of the fibrillar crystals, these differences being largely due to the non-uniform flow pattern along the various stirrer parts. Unusual lamellar shapes, interconnections, sheets between lamellae, the “veil” on the Shish-kebab noted byKeller andMachin, may well be explained byKeith, Padden andVadimskys mechanism of crystallization-induced orientation of macromolecules between the lamellae and the consequent formation of extendedchain crystals. The lamellar overgrowth appears to be arranged along the backbone according to a logarithmic normal distribution, and the average diameter of the lamellar crystals increases with the average spacing, both being approximately equal in size. The fibrillar crystals end in smooth tapered tails having lengths varying between 5 and 8 μ. These observed fibre ends agree best with the “unroll” model proposed for fibrillar crystal growth in flow fields if on the average each molecule introduces one chain fold and the chain ends are randomly assimilated by the crystal lattice. Linear polyethylene below molecular weight of 50×103 and low density polyethylene did not crystallize in fibrillar habit. whereas copolymers of ethylene and slight amounts of propylene and butylene proved capable of forming fibrillar crystals and isotactic polypropylene yielded smooth and overgrown fibres.