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.

The structure changes accompanying phase transitions in poly(diethylsiloxane) (PDES) have been studied by WAXS and SAXS techniques using oriented and isotropic samples. PDES may exist in two low‐temperature modifications (the monoclinic α1‐form and presumably the “tetragonal” β1‐form) and two high‐temperature modifications (the monoclinic α2‐form and the “tetragonal” β2‐form). In linear PDES the crystal ‐ crystal transitions α1–α2 and β1–β2 occur near 214 and 206 K, respectively. At higher temperatures α2 (280 K) and β2 (290 K) forms transform into the mesomorphic phase αm that gradually melts at 280–300 K giving an amorphous phase. According to x‐ray and density data, αm phase is also characterized by monoclinic structure slightly different from hexagonal packing.

A c d m y of Science of the USSR, Moscow, USSR SynopsisOptical and calorimetric studies of the kinetics of mesophase formation and crystallization in poly(diethylsi1oxane) have been conducted. The mesomorphic phase is found to grow from the isotropic melt in the form of lamellar domains about 2 pm thick in the temperature range 293-307 K. According to birefringence data, macromolecules in the mesomorphic lamellae are perpendicular t o the end faces. The kinetics of mesophase formation obey the Avrami equation with the morphological parameter n close to 2 (It is equal to 1.75 +_ 0.05). which corresponds to t h e two-dimensional growth of the mesomorphic phase from athermal nuclei. The limiting conversion of t h e isotropic melt w a s shown to bc temperature-dependent. This is likely to he connected with a change in the number of nuclei with temperature. T h e crystallization of polymer from t h e mesomorphic state occurs with retention of the optical texture of the .sample. T h e process proceeds not as a sporadic crystallization of individual mesomorphic lamellac but a s a growth of the nucleated crystalline regions via a consecutive incorporation of adjacent crystalliz. ing lamellae.

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