Formation and Thermal Behavior of Polystyrene and Polystyrene/Clay Gels

Nanoconfined gelation of polyacrylonitrile, poly(vinyl alcohol), and isotactic polypropylene probed by calorimetry

Differential scanning calorimetry is used to obtain insights into the kinetics and thermodynamics of nanoconfined gelation. Gels of polyacrylonitrile in propylene carbonate, poly(vinyl alcohol) in ethylene glycol, and isotactic polypropylene in o-dichlorobenzene and decalin are studied in silica nanopores. Two major effects are observed for nanoconfined gels: a decrease in the heat of gelation and an increase in the gelation temperature. The smaller heat indicates that nanoconfinement of polymer chains results in the formation of fewer ordered crosslinks. The increased gelation temperature suggests acceleration of the gelation kinetics. The kinetics has been treated by an advanced isoconversional method and interpreted in terms of the Fisher-Turnbull model. It is found that acceleration of gelation in nanopores is associated with a decrease in the free energy barrier to nucleation, as one would expect for a change in the process mechanism from homogenous to heterogenous nucleation.

Gelation of Poly(Vinylidene Fluoride) Solutions in Native and Organically Modified Silica Nanopores

The purpose of this study is to highlight the surface and size effects of the nanopores on the thermodynamics and kinetics of gelation. The effects have been probed by applying differential scanning calorimetry to poly(vinylidene fluoride) solutions in tetraethylene glycol dimethyl ether (tetraglyme) and γ-butyrolactone. Nanoconfinement has been accomplished by introducing gels into native and organically modified silica nanopores (4–30 nm). Nanoconfinement has produced two major effects. First, the heat of gelation has decreased three to four times compared to that for the bulk systems. Second, the temperature of gelation has increased by ~40 °C (tetraglyme based systems) and ~70 °C (γ-butyrolactone based systems), the increase being stronger in native nanopores. The effects are discussed in terms of acceleration of gelation due to heterogeneous nucleation at the confining surface, and retardation of gelation due to constricted polymer chain mobility in the middle of the pore volume. Calorimetric data have been subjected to isoconversional kinetics analysis. The obtained temperature dependencies of the activation energies of gelation have been interpreted in the frameworks of the nucleation model of Turnbull and Fisher. The results suggest that nanoconfinement leads to a lowering of both the free energy of nucleation and activation energy of diffusion.

Melting kinetics of superheated crystals of glucose and fructose

Glucose and fructose crystals undergo significant superheating during melting that allows one to study the kinetics of this process. Melting of both compounds has been studied by differential scanning calorimetry (DSC). The obtained data have been subjected to isoconversional kinetic analysis. The process has been determined to have unusually large values of the activation energy and preexponential factor that indicate that melting occurs by cooperatively breaking multiple hydrogen bonds. The experimentally determined activation energy of melting demonstrates a decrease with increasing temperature. The use of the nucleation and growth models has permitted deriving theoretical dependencies of the activation energy on temperature. Testing the theoretical dependencies against the experimental ones suggests that from either the statistical or physical viewpoint the melting kinetics should be parameterized by means of the growth model. This suggests that the mechanism of melting involves the growth of the stable melt nuclei that exist on the crystal surface below the equilibrium melting temperature.

Melting of gelatin gels confined to silica nanopores

Nanoconfinement is a way to create materials whose properties differ from the bulk. For the first time, this research explores the effect of nanoconfinement on the thermodynamics and kinetics of gel melting. Differential scanning calorimetry has been employed to study gelatin gels prepared inside 4, 6, 15, and 30 nm pores of a silica matrix. It has been found that with decreasing the pore size the heat of melting decreases from 3.5 J g^-1 in bulk to 0.6 J g^-1 in 6 nm pores, which is linked to a decrease in crosslinks formed via hydrogen bonding. Despite decreases in crosslink formation, the melting temperature for gels confined to 6 nm pores increased nearly 10 °C compared to bulk gel. In 4 nm pores, no gel melting was observed. Isoconversional kinetic analysis of the melting data has revealed that the increase in thermal stability is associated with a decrease in the pre-exponential factor that occurs upon nanoconfinement. The origins of the effect have been linked to diminished molecular mobility of the gelatin chains confined inside the nanopores, which leads to enhanced restoration of broken crosslinks.

Isoconversional Kinetics of Polymers: The Decade Past

This article surveys the decade of progress accomplished in the application of isoconversional methods to thermally stimulated processes in polymers. The processes of interest include: crystallization and melting of polymers, gelation of polymer solutions and gel melting, denaturation (unfolding) of proteins, glass transition, polymerization and crosslinking (curing), and thermal and thermo-oxidative degradation. Special attention is paid to the kinetics of polymeric nanomaterials. The article discusses basic principles for understanding the variations in the activation energy and emphasizes the possibility of using models for linking such variations to the parameters of individual kinetic steps. It is stressed that many kinetic effects are not linked to a change in the activation energy alone and may arise from changes in the preexponential factor and reaction model. Also noted is that some isoconversional methods are inapplicable to processes taking place on cooling and cannot be used to study such processes as the melt crystallization.