Non-Debye dielectric relaxation around the liquid-glass transition of a glass-forming polymer

Dynamics and thermodynamics of polymer glasses

The fate of matter when decreasing the temperature at constant pressure is that of passing from gas to liquid and, subsequently, from liquid to crystal. However, a class of materials can exist in an amorphous phase below the melting temperature. On cooling such materials, a glass is formed; that is, a material with the rigidity of a solid but exhibiting no long-range order. The study of the thermodynamics and dynamics of glass-forming systems is the subject of continuous research. Within the wide variety of glass formers, an important sub-class is represented by glass forming polymers. The presence of chain connectivity and, in some cases, conformational disorder are unfavourable factors from the point of view of crystallization. Furthermore, many of them, such as amorphous thermoplastics, thermosets and rubbers, are widely employed in many applications. In this review, the peculiarities of the thermodynamics and dynamics of glass-forming polymers are discussed, with particular emphasis on those topics currently the subject of debate. In particular, the following aspects will be reviewed in the present work: (i) the connection between the pronounced slowing down of glassy dynamics on cooling towards the glass transition temperature (Tg) and the thermodynamics; and, (ii) the fate of the dynamics and thermodynamics below Tg. Both aspects are reviewed in light of the possible presence of a singularity at a finite temperature with diverging relaxation time and zero configurational entropy. In this context, the specificity of glass-forming polymers is emphasized.

Scaling of the hysteresis in the glass transition of glycerol with the temperature scanning rate

By measuring the dependences of the temperature-dependent primary ("alpha") dielectric relaxation time behavior on the temperature scanning rate for the glass-forming glycerol, we study the scaling of hysteresis at the glass transition in glycerol. Based on the Vogel-Fulcher-Tammann (VFT) expression and the Angell's fragility concept, notable correlations of the systematic kinetic fragility, and of the hysteresis effect in the vitrification∕fusion "alpha"-relaxation process of glycerol, with the temperature scanning rate, were reasonably analyzed and discussed. It was observed that the kinetic fragility m and the apparent glass-transition temperature hysteresis width ΔT(g)(a), respectively, scaled the temperature scanning rate q as m ≈ α(m)q(-γ) and ΔT(g)(a) ≈ A(0) + αq(β), at which the exponents, γ and β, were suggested to be characteristic of the resistance to the structure change or fragility change of the system during the glass transition. The observed scaling laws are quite similar to the scaling power law for the thermal hysteresis in the first-order phase transition (FOPT) of solids, providing a significant insight into the hysteresis effect in the glass transition of the glass-forming liquids.

Dynamical Heterogeneity in Glassy o-Terphenyl: Manifestation of Environment Changes in Photoisomerization Kinetics of Probe Molecules

A new method for the investigation of dynamical heterogeneity in glassy matrixes is presented and illustrated by the example of o-terphenyl (OTP). UV-vis absorption spectroscopy has been used to monitor the cis-trans isomerization kinetics of probe molecules in glassy OTP. The dependence of isomerization quantum yield on light intensity has been established. This dependence is shown to be due to the change in the local environment of the probe molecules. The simple model is suggested to estimate the time required for the environment to change, tauex. The tauex values from 2.6 x 10(2) to 1.9 x 10(5) s have been obtained for environments of molecules of 1-naphthylazomethoxybenzene (NAMB) in OTP over a temperature range from 244 to 204 K (Tg+1 to Tg-39 K). The temperature dependence of exchange time has a non-Arrhenius character. As the temperature decreases, an increase in exchange time slows down. The activation energy of the relaxation process is 54 kJ/mol over the range of 224-239 K.

Theory of relaxation and elasticity in polymer glasses

The recently developed activated barrier hopping theory of deeply supercooled polymer melts [K. S. Schweizer and E. J. Saltzman, J. Chem. Phys. 121, 1984 (2004)] is extended to the nonequilibrium glass state. Below the kinetic glass temperature T(g), the exact statistical mechanical relation between the dimensionless amplitude of long wavelength density fluctuations, S(0), and the thermodynamic compressibility breaks down. Proper extension of the theory requires knowledge of the nonequilibrium S(0) which x-ray scattering experiments find to consist of a material specific and temperature-independent quenched disorder contribution plus a vibrational contribution which varies roughly linearly with temperature. Motivated by these experiments and general landscape concepts, a simple model is proposed for S(0)(T). Deep in the glass state the form of the temperature dependence of the segmental relaxation time is found to depend sensitively on the magnitude of frozen in density fluctuations. At the (modest) sub-T(g) temperatures typically probed in experiment, an effective Arrhenius behavior is generically predicted which is of nonequilibrium origin. The change in apparent activation energy across the glass transition is determined by the amplitude of frozen density fluctuations. For values of the latter consistent with experiment, the theory predicts a ratio of effective activation energies in the range of 3-6, in agreement with multiple measurements. Calculations of the shear modulus for atactic polymethylmethacrylate above and below the glass transition temperature have also been performed. The present work provides a foundation for the formulation of predictive theories of physical aging, the influence of deformation on the alpha relaxation process, and rate-dependent nonlinear mechanical properties of thermoplastics.

A simple model of entropy relaxation for explaining effective activation energy behavior below the glass transition temperature

Strong changes in relaxation rates observed at the glass transition region are frequently explained in terms of a physical singularity of the molecular motions. We show that the unexpected trends and values for activation energy and preexponential factor of the relaxation time tau, obtained at the glass transition from the analysis of the thermally stimulated current signal, result from the use of the Arrhenius law for treating the experimental data obtained in nonstationary experimental conditions. We then demonstrate that a simple model of structural relaxation based on a time dependent configurational entropy and Adam-Gibbs relaxation time is sufficient to explain the experimental behavior, without invoking a kinetic singularity at the glass transition region. The pronounced variation of the effective activation energy appears as a dynamic signature of entropy relaxation that governs the change of relaxation time in nonstationary conditions. A connection is demonstrated between the peak of apparent activation energy measured in nonequilibrium dielectric techniques, with the overshoot of the dynamic specific heat that is obtained in calorimetry techniques.

Thermal glass transition beyond the Vogel-Fulcher-Tammann behavior for glass forming diglycidylether of bisphenol A

For the low molecular weight fragile liquid diglycidyl ether of bisphenol A we report, based on Brillouin and dielectric spectroscopy, on a thermal glass transition where the relaxation time of the alpha process does not go to infinity. Instead, the structural alpha relaxation disappears spontaneously at the transition point. That discontinuity in relaxation time coincides with a kink in the longitudinal hypersonic velocity and determines unambiguously the transition from the liquid to the glassy state.