DSC studies of phase transitions in poly(diethylsiloxane)

Abstract

The differential scanning calorimetry studies have shown that high‐molecular linear poly(diethylsiloxane) can exist in two high‐temperature polymorphs which melt at 280 and 290 K. The heats of fusion of the high‐temperature polymorphs are 17 and 21 J/g, respectively. Each of the high‐temperature forms arises from the corresponding low‐temperature form the corresponding low‐temperature form when the polymer is heated: the first at 214 K (transition heat is 28 J/g) and the second at 206 K (transition heat is 26 J/g). The mesophase formed from the molten high‐temperature crystalline phases melts in a rather broad temperature range of 290 to 327 K, and the heat of this transition is 2.7 J/g. Crystallization of poly(diethylsiloxane) from the mesomorphic and the supercooled amorphous state is different. In the first case, apparently, the whole mesophase is converted to the crystalline phase and the samples have a crystallinity near 1. In the second case the crystallinity is only ca. 0.3. The temperature range in which the mesophase melts depends on the molecular weight of the polymer, presence of crosslinks and the conditions under which it has been formed, e.g., temperature.

Molecular Engineering of Liquid Crystalline Polymers

“…Therefore, liquid crystalline polymers display thermotropic and lyotropic mesophases which are similar to those exhibited by thermotropic and lyotropic low molar mass liquid crystals. In addition, rigidrod like main chain non-amphiphilic polymers like poly(phenyleneterephthalamide) (Kevlar®), 78 poly(p-phenylene-2,6-benzobisthiazole) and poly(p-phenylene-2,6-benzobisoxazole) 79 exhibit lyotropic nematic mesophases.…”

Structural characterization of poly(diethylsiloxane) in the crystalline, liquid crystalline and isotropic phases by solid‐state 17O NMR spectroscopy and ab initio MO calculations

“…A 1.16 g quantity of 17 O-enriched PDES (1.12 ð 10 2 mol) with a cloudy gum was acquired after the ethanol was removed by drying at 110°C for 12 h. 17 O-enriched PDES was the same as that of the nonunlabeled PDES reported previously. 5,11,16,18 From the DSC diagram, the 17 O-enriched PDES forms a low-temperaturě 1 crystalline phase at temperatures below 70°C, a hightemperatureˇ2 crystalline phase at temperatures of 60 to 0°C, a mesomorphic phase at temperatures of 10-25°C (actually the biphasic system consisting of the liquid crystalline and isotropic regions) and an isotropic phase at >40°C.…”

DTA measurements on polymers under high pressure

“…Each of the thermobatteries consists of forty differentially connected alumel-copel thermocouples (12) for measuring the heat flow. The temperature of the sample is measured by an alumel-chromel thermocouple (5). The thermocouple and thermobattery are electrically insulated from the sample holder (3) by a thin mica plate (13) (50 lxm thickness).…”

“…The phase transitions in PDES under AP has been investigated well enough [4][5][6][7] but there are no data for the phase transitions under HP.…”