The temperature dependence of the isothermal growth rate of spherulites and overall crystallization kinetics of different fractions of poly (ethylene oxide) over the range of molecular weight from 300 to 20,000 were studied by calorimetry, dilatometry, and polarized light microscopy, in order to establish the dependence of the kinetic parameters upon molecular weight. The morphology of crystallites was studied by small‐angle x‐ray scattering. In the course of this study it has been established that the growth rates of spherulites and the overall crystallization rates at any fixed supercooling essentially depend upon molecular weight. The minimum in the crystallization rate which occurs at molecular weight 4000 is a consequence of transition from extended‐chain crystallization to folded‐chain crystallization. Small‐angle x‐ray scattering study of the morphological features confirmed this conclusion. This minimum depends very strongly on the degree of supercooling, increasing rather sharply at small supercooling. The temperature and molecular weight dependence of the growth rates of spherulites and the overall crystallization rates has been analyzed by means of the theoretical expression given by Hoffman and Lauritzen. The product of the lateral and basal surface free energies of the crystallites σσe increases with molecular weight and reaches a limiting value at molecular weight 6000. Such behavior is in agreement with morphological transformations.

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.

Crystallization kinetics for 12 polymers including polyolefins, polyesters, polyurethanes, polysiloxanes was measured by the evolution of heat in a modified Calvet‐type calorimeter over wide temperature ranges. The results are analyzed in terms of the Avrami equation and a comparison between calorimetric and dilatometric results is carried out. It is concluded that, although in the majority of cases experimental results do not obey the Avrami equation, for some polymers the agreement is rather good. The Avrami parameter obtained, however, depends on the experimental technique. Possible reasons for this disagreement are discussed. Analysis of the calorimetric crystallization rate in the vicinity of the melting point by using the kinetic theory of crystallization shows that the growth is controlled by surface (two‐dimentional) nucleation. Energy parameters for the crystallites were determined and it is shown that the surface energy of the crystallites depends on the molecular structure of the polymer. Temperature dependence of the calorimetric crystallization rate of the polymers for which crystallization rates could be determined above and below the maximum rate are analyzed using a kinetic equation with common approximations for the transport term. The influence of melting conditions on the crystallization rate was studied. The results indicate heterogeneous nucleation in the polymer melt. It is concluded that this may be due both to impurities and to high regularity of macromolecules in the polymer melt.

Experimental partPoly(dipropylsiloxane)a) (PDPS) was obtained by anionic polymerization of hexapropylcyclotrisiloxane in the presence of KOH under an argon flux at 115°C. The polymer obtained was fractionated from a toluene/methanol (1 : 99 by vol.) solution at 25 "C. Fractions were taken 24 h after reaching equilibrium conditions. The molecular weights were determined by light scattering in toluene at 25 "C. In our study the fraction with an intrinsic viscosity [q] = 0,26 dl/g in toluene at 25 "C and a weight-average molecular weight M, = 82000 was used. Calorimetric and X-ray studies were carried out on the sample which after fractionation was heated to 500 K and cooled slowly to room temperature.Systematic name: poly(oxydipropylsily1ene).