Structural Investigation of Orthorhombic-to-Hexagonal Phase Transition in Polyethylene Crystal:  The Experimental Confirmation of the Conformationally Disordered Structure by X-ray Diffraction and Infrared/Raman Spectroscopic Measurements

Abstract

Structural changes in the orthorhombic-to-hexagonal phase transition of polyethylene crystal has been investigated by measuring the differential scanning calorimetry, X-ray diffraction, and infrared and Raman spectra for the geometrically-constrained ultradrawn samples in the course of heating up to the melting temperature. The infrared and Raman spectral patterns characteristic of the hexagonal phase have been confirmed. In particular, the bands characteristic of the disordered short trans segments (shorter than five CH2 units) and the bands of the kink or double gauche linkages have been detected definitely. The degree of orientation of the averaged chain axis, as detected from the temperature dependence of the X-ray fiber diagram, was reserved well enough even in the hexagonal phase, while the orientational degree of the methylene units, as detected from the polarized infrared spectral measurement, was found to be lowered appreciably. This finding, combined with the observation of the trans and gauche bands, has confirmed experimentally and definitely the existence of the conformationally disordered chains in the hexagonal phase.

Microscopically-Viewed Structural Change of Polyethylene during Isothermal Crystallization from the Melt I. Time-Resolved FT-IR Spectral Measurements

“…32 The band component was assumed to be a weighted mixture of the Lorentzian and Gaussian functions and the variables (the relative weight of these two functions, the height, the width, and the peak position) were adjusted by a nonlinear least-squares method so that the totally summed spectral profile was fitted to the observed pattern as well as possible. The program installed in the Bio-Rad FTS-60A/896 FT-IR spectrometer was used.…”

“…Recently we succeeded in obtaining the detailed structural information on the orthorhombic-to-hexagonal transition occurring in the narrow temperature region just below the melting point of PE sample by carrying out the time-resolved X-ray diffraction and the FT-IR and FT-Raman spectral measurements. 32 In particular, IR bands characteristic of the hexagonal phase or the conformationally-disordered (CONDIS) phase were revealed experimentally for the first time. This allowed us to speculate that the conformationally disordered trans chain segments might appear during the transition from the melt to the regular orthorhombic phase.…”

Temperature window effect and its application in extrusion of ultrahigh molecular weight polyethylene

“…Keller and coworkers [15][16][17][18][19][20][21][22], Cheng [23] and Somani et al [25] independently or collaboratively studied the mechanisms of unexpected melt flow behavior, and attributed reduced flow resistance to the transient mobile hexagonal mesophase by flow-induced chain alignment critical to strain rate and molecular weight. This hypothesis was confirmed by the observation of the orthorhombic transformation to the hexagonal phase through in situ wide angle X-ray diffraction [25][26][27]. These new findings have potential significance for polymer processing at low pressures.…”

“…The elastic energy stored in the melt during flow in the convergent entry of the die is released abruptly when the melt enters the capillary, thereby reducing extrusion pressure. The melt flows steadily at a constant extrusion speed -a phenomenon that can be explained from two aspects: (1) chain mobility is enhanced at temperatures 10°C higher than the theoretical equilibrium melting point of orthorhombic polyethylene crystals [30]; and (2) chains are extended at the orifice and aligned along the flow direction, producing a chain-extended structure similar to the hexagonal crystal phase [26]. This is the so-called elongational flow-induced crystallization [11].…”

“…Under equilibrium conditions, the quiescent melting point of nascent UHMWPE is around 142-143°C, very close to the theoretical equilibrium melting point of orthorhombic polyethylene crystals. After erasing the thermal history, the quiescent melting point decreases to 132-135°C [26]. In the flow state, the elongational flow in the die causes chain extension and induces the formation of extended chain crystals.…”