Quantitative determination of the monoclinic crystalline phase content in polyethylene by 13C n.m.r.

Solid-state ¹³C n.m.r. spectroscopy involving the techniques of cross-polarization (CP), magic angle spinning (MAS), and high power proton decoupling, has been used to determine quantitatively the ratio of monoclinic to orthorhombic crystalline phases in compression moalded ultra-high molecular weight polyethylene (UHMWPE) sheet which had been stretched uniaxially. Criteria for expecting quantitative relative intensities in ¹³C CP-MAS spectra are discussed. Attenuation of the non-crystalline (NC) signals relative to crystalline signals was observed. Experiments were therefore carried out to ascertain whether measurable relative intensity distortions exist between the monoclinic crystalline phase (MCP) and the orthorhombic crystalline phase (OCP) resonances due to possible differences in proton ‘spin diffusion’ between the NC and the two crystalline phases during cross-polarization. No relative intensity distortions were detected. This result, coupled with experiments in which spin diffusion was monitored at times longer than those used for cross-polarization, suggests that the average distance from the protons in a given crystalline phase to the nearest protons in the NC regions is the same for the MCP and the OCP. Finally, non-spinning ¹³C spectra of the deformed polyethylene were recorded to determine the orientation of the chains in the crystalline and NC regions. The Hermans orientation function, F_c, was determined independently for the crystalline (combined OCP and MCP) and NC regions, and found to be 0.66 + 0.06 and 0.23 + 0.04 respectively. The occurrence of orientation in the NC regions may be evidence for internal stresses, which, it is suggested, also stabilize the metastable MCP in the stretched sample.