Solid-state n.m.r. studies of crystalline phases in gel-spun ultrahigh molecular weight polyethylene

Abstract

Gel-spun ultrahigh molecular weight polyethylene (UHMWPE) was examined by advanced solid-state 13C n.m.r. techniques to extract detailed information on the morphology and nature of the crystalline phases. Two partially overlapping resonances separated by 1.5 ppm in the 13C magic angle spinning n.m.r. spectrum have been assigned to monoclinic and orthorhombic crystalline phases. To analyse better the small amount of monoclinic phase, a 13C chemical shift filter was employed to suppress partially the dominant orthorhombic signal. The monoclinic phase exhibits a slightly smaller chemical shielding anisotropy of 27 ppm with chemical shift principal tensor values of 50, 36.5 and 16 ppm as compared to a shielding anisotropy of 30 ppm and principal tensor values of 50, 35.5 and 13 ppm for the orthorhombic phase. Additionally, a minor fraction of interface was detected arising from heterogeneous molecular packing. Both crystalline phases reveal an extremely high degree of order, with 〈 p 2〉 around 0.94. Whereas the conformations of the chains in the non-crystalline regions of both gel-spun and melt-spun polyethylene are alike, the mobility of the former is largely reduced. The morphology of UHMWPE is compared with that of melt-spun polyethylene.