SynopsisPrecise melting and crystallization temperatures of extended-chain and folded-chain crystals of form I and folded-chain crystals of form I1 poly(viny1idene fluoride) under high pressure have been obtained by microdifferential thermal analysis (DTA). Upon heating at pressures above 4000 kg/cm2, the micro-DTA thermogram of form I1 crystallized from the melt a t atmospheric pressure shows melting of the form I1 structure and the melting of the folded-chain and extended-chain crystals of form I, formed through recrystallization processes. These features were clarified by supplemental methods. The bandwidth seen in electron micrographs of the extended-chain crystal of form I obtained by crystallization under high pressure was in the range of 1500 to 2000 A. At atmospheric pressure, the extended-chain crystal of form I melted at 207"C, approximately 17OC higher than the folded-chain crystal of form I and 31°C higher than the folded-chain crystal of form 11.
The phase diagram, growth processes of extended-chain crystal and high pressure phase, and the temperature dependency of lattice parameter in polyethylene have been studied under high pressure up to 6000 kg/cm2 using a new high pressure X-ray diffraction apparatus. The high pressure phase diagram obtained by micro-DTA measurement agrees with that by X-ray measurement. The crystallization process of the high pressure phase seems to be a homogeneous and diffusion controlled two dimensional growth. The growth process of the extended-chain crystal is very rapid, and the direct formation of the extended-chain crystal without passing through the high pressure phase is clarified. The temperature dependency of the lattice parameters of the extended-chain crystal and the high pressure phase at 5000 kg/cm2 were obtained, and the shrinkage of the c-axis in the high pressure phase which shows only a sharp (100) diffraction pattern is discussed with the aid of dilatometric data.
ABSTRACT:Melting and crystallization temperatures of extended-and folded-chain crystals under high pressure are precisely obtained by a microdifferential thermal analysis. In the cooling process from the fused state the crystallizations of extended and folded chains occur successively by two steps. The growth rate of extended-chain crystals is fairly large at the crystallization temperature of the extended chain, and is accelerated by raising pressure. The extended-chain crystal grows directly at its crystallization temperature under high pressure and it does not need long duration to form a complete extended-chain crystal. When the sample is quenched to the crystallization temperature of the folded-chain crystal, the folded-chain lamellae grow for the first time, and gradually thicken into an extended-chain crystal for the reason that the extended chain has a lower free energy than the folded chain under high pressure. These facts are clarified by the use of morphological study.
Raman spectroscopic investigations of the high-pressure phase in polyethylene (PE) and each phase in poly(tetrafluoroethylene) (PTFE) under high pressure were performed using a hydrostatic high-pressure optical vessel. It was found that the molecular chain in the high-pressure phase of PE contains many gauche bonds similarly to the liquid phase, and it was confirmed that this phase is like a liquid crystal, which is consistent wih the other results of X-ray, optical microscopic, and ultrasonic experiments. In the case of PTFE, the behavior of the two weak bands at 581 and 601 em-1 , whose assignment has been the subject of discussion for some time in previous literatures, was investigated in detail throughout the whole range of PTFE phase diagram. It is suggested that the bands at 581 and 60 I em-1 should be attributed to 136 helical and transplanar conformations, respectively. Furthermore, the mechanism of untwisting in the PTFE molecular chain under high pressure is proposed. KEY WORDS Polyethylene I Poly(tetrafluoroethylene) I High-Pressure Phase I Raman Spectroscopy I High-Pressure Optical Vessel I Liquid Crystal-Like Phase I Molecular Conformation I melt T or tho.
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