Lamellar characteristics controlled by prior polymer concentration for solution-crystallized ultra-high molecular weight polyethylene

Abstract

The effect of prior polymer concentration on solution-crystallized morphologies in ultra-high molecular weight polyethylene has been characterized by transmission electron microscopy (TEM) and X-ray analysis. Molecular motion in crystalline and amorphous phases was evaluated by nuclear magnetic resonance (NMR) techniques. The series of solution-crystallized mats was prepared from different polymer concentrations. TEM results indicate these samples consist of lamellae aligned parallel to the mat surface. The observed high order scattering of small-angle X-ray scattering (SAXS) patterns indicate a regular stacking of lamellae. The long period was ∼11nm, independent of prior polymer concentration. Wide-angle X-ray diffraction analysis also revealed the same level of crystallinity for these solution-crystallized samples, well coincident with 1H-NMR results. These indicate that the thickness of crystal/amorphous layers was constant, which was also confirmed by layer thickness distribution analysis for this series of samples, based on direct TEM observation. However, the SAXS peak gradually grew and sharpened with increasing prior polymer concentration. 1H-NMR results also indicate amorphous chain mobility was restricted for the sample prepared from lower polymer concentration in spite of unchanged crystallinity and relaxation characteristics for crystalline chains. Morphological and relaxation characteristics information demonstrates that lower prior polymer concentration might prefer formation of tie molecules which connect tightly crystalline layers. In contrast, the higher concentration could trap loosely entangled chains in amorphous layers, providing for an ease of chain relaxation estimated by 1H-NMR analysis.