Parameters characterizing the kinetics of the non-isothermal crystallization of poly(ethylene terephthalate) determined by d.s.c.

Field-Cycling NMR Relaxometry

The isothermal crystallization kinetics of polypropylene/montmorillonite (PP/MMT) nanocomposites synthesized via intercalation polymerization were investigated by using differential scanning calorimeter and polarizing optical microscope (POM). The crystallinity of the nanocomposites decreased with the increase of the montmorillonite content, indicating that the MMT layers dispersed in the PP matrices confined the PP chains and hindered the crystallization of the PP chains. The POM photographs showed that the spherulites of the PP/MMT nanocomposites were greatly decreased in size as MMT was introduced. On the other hand, the crystallization rate increased dramatically with the increasing of MMT content. The interfacial free-energy per unit area perpendicular to PP chains in PP/MMT nanocomposites decreased with increasing MMT content, suggesting that the MMT layers acted as heterogeneous nuclei in the nucleation of crystallization. The nucleus density increased with the increasing of MMT content, leading to a positive effect on the crystallization. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 1978–1985, 2002

Crystallization behaviors of polypropylene/montmorillonite nanocomposites

The thermal transitions of confined polymers are important for the application of polymers in molecular scale devices and advanced nanotechnology. However, thermal transitions of ultrathin polymer assemblies confined in subnanometre spaces are poorly understood. In this study, we show that incorporation of polyethylene glycol (PEG) into nanochannels of porous coordination polymers (PCPs) enabled observation of thermal transitions of the chain assemblies by differential scanning calorimetry. The pore size and surface functionality of PCPs can be tailored to study the transition behaviour of confined polymers. The transition temperature of PEG in PCPs was determined by manipulating the pore size and the pore-polymer interactions. It is also striking that the transition temperature of the confined PEG decreased as the molecular weight of PEG increased.

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Unveiling thermal transitions of polymers in subnanometre pores

The growth rate of isotactic polypropylene is deduced from microscopic observations during isothermal crystallizations. A change in the growth regime is observed at 138 ‡C and interpreted as a Regime III → Regime II transition, according to Hoffman's kinetic theory of polymer crystallization. A Regime II → Regime I transition is also theoretically predicted at 155 ‡C, i. e. at a temperature outside the investigated temperature range. The Regime III → Regime II transition is related to the positive to negative change in the spherulite birefringence, which is generally attributed to a change in the organization of crystalline lamellae: quadritic arrays of intercrossing lamellae atTc 138 ‡C (Regime II). It is suggested that such a morphological change could be interpreted using the concept of “non-adjacent re-entry” introduced in Hoffman's kinetic theory. This interpretation could also explain the interspherulitic ruptures observed in negative spherulites.

Growth transition and morphology change in polypropylene

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.

Kinetics of polymer crystallization from the melt (calorimetric approach)

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.

Effect of molecular weight on the crystallization and morphology of poly (ethylene oxide) fractions

This review is primarily concerned with mesomorphic behaviour of element-organic polymers, such as linear and cyclolinear polyorganosiloxanes and linear polyphosphazenes. These polymers can form thermotropic mesophases without any mesogens. Recent development in thermodynamics, kinetics, structure and morphology, elasticity and thermoelasticity and some other properties are considered. The influence of the molecular structure and molecular weight of linear macromolecules on the ability to form the mesophases and their temperature stability is discussed. The phase behaviour of cyclolinear polyorganosiloxanes is described with special emphases on the temperature stability of the mesophases as a function of molecular structure, tacticity, substituents and flexible spacers. The state of order of polyorganosiloxanes is analysed in comparison to mesophase states of linear and cyclic low molecular siloxanes. The lyotropic mesophase behaviour of the ladder polyorganosiloxanes is also examined briefly.

Thermotropic Mesophases in Element-Organic Polymers

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.

DSC studies of phase transitions in poly(diethylsiloxane)

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.

Yu. K. Godovsky

Papers

Kinetics of polymer crystallization from the melt (calorimetric approach)

Effect of molecular weight on the crystallization and morphology of poly (ethylene oxide) fractions

Thermotropic Mesophases in Element-Organic Polymers

DSC studies of phase transitions in poly(diethylsiloxane)

Structure and phase transitions in poly(diethylsiloxane)