Ian M. Hodge

Enthalpy relaxation and recovery in amorphous materials

Abstract The field of enthalpy relaxation is reviewed. Current phenomenologies for dealing with the non-linear and non-exponential character of enthalpy relaxation are presented, and their successes and shortcomings are discussed. Qualitative experimental data and quantitative parameterizations are summarized, and some directions for future research are suggested.

Physical aging in polymer glasses

Physical aging refers to structural relaxation of the glassy state toward the metastable equilibrium amorphous state, and it is accompanied by changes in almost all physical properties. These changes, which must be taken into account in the design, manufacture, and use of glassy polymer materials and devices, present a daunting challenge to theorists.

Strong and fragile liquids — a brief critique

The classification scheme of strong and fragile liquid relaxation behavior introduced by Angell is examined, and the relations between the different fragility parameters are derived and discussed. The relation between fragility and non-linear relaxation behavior within and below the glass transition temperature range is explored. It is suggested that the minimum value of the fragility parameter, m, is proportional to the quotient of the glassy state activation enthalpy and Tg, and implications of this interpretation are discussed. It is argued that TgΔCp(Tg) is an inappropriate measure of the thermodynamic contribution to the Angell strength parameter, D, because it contains the kinetic quantity, Tg.

ADAM-GIBBS FORMULATION OF ENTHALPY RELAXATION NEAR THE GLASS TRANSITION

The entropically based nonlinear Adam-Gibbs equation is discussed in the context of phenomenologies for nonlinear enthalpy relaxation within the glass transition temperature range. In many materials for which adequate data are available, the nonlinear Adam-Gibbs parameters are physically reasonable and agree with those obtained from linear relaxation data and thermodynamic extrapolations. Observed correlations between the traditional Tool-Narayanaswamy-Moynihan parameters are rationalized in terms of the Adam-Gibbs primary activation energy (Δμ) determining how close the kinetic glass transition temperature can get to the thermodynamic Kauzmann temperature. It is shown that increased nonlinearity in the glass transition temperature range is associated with greater fragility in the liquid/rubber state above Tg.

Adam-Gibbs formulation of non-linear enthalpy relaxation

Abstract The Adam-Gibbs (AG) expression linking relaxation times with configurational entropy gives a good account of the non-linearity observed in enthalpy relaxation of amorphous polymeric, inorganic, and simple molecular materials near and below Tg. Data analyses yield zero-entropy temperatures that are comparable with available Kauzmann values. The AG formulation, when coupled with a few plausible ancillary assumptions, predicts correlations between the Arrhenius parameters x, Δh ∗ , and the KWW exponent β that are observed experimentally. It is suggested that the AG primary activation energy, Δμ, is the single causative factor that generates these correlations.

Effects of heating rate on enthalpy recovery in polystyrene

Abstract Temperature gradients in polystyrene samples were measured at heating rates between 1.25 and 40 K min−1. Enthalpy recovery was measured at these heating rates and compared with phenomenological models that describe the heat capacity overshoot. At low heating rates, where gradients are shown to be negligible, the non-linearity parameter, x, and Kohlrausch-Williams-Watts (KWW) stretched exponential parameter, β, both change significantly with thermal history. In particular, both x and β decrease with increasing heating rate to an extent that cannot be completely accounted for by temperature gradients or thermal resistance effects. It is suggested that the Tool-Narayanaswamy and Kovacs-Aklonis-Hutchinson-Ramos (KAHR) formalisms are deficient for polymers.

Physical ageing and modelling of the glass-liquid transition of water and aqueous solutions imbibed in poly-(2-hydroxyethyl-methacrylate) and in the bulk state

Physical ageing of glassy water and aqueous solutions of 22 mol% ethylene glycol and 16 mol% lithium chloride imbibed in poly(2-hydroxyethyl-methacrylate) (PHEMA) and of the bulk solutions was studied by differential scanning calorimetry (DSC). The samples were isothermally annealed for a constant time at various temperatures below their glass → liquid transition temperature. The resulting DSC scans were analyzed in terms of the four-parameter model of Moynihan et al., which incorporates both the non-exponentiality of the relaxation function and the structure dependence of the retardation times. The non-exponentiality parameter, β , is smaller for the solutions imbibed in PHEMA than in the bulk, implying a broader distribution of retardation times and/or enhanced cooperativity of the relaxation process. The activation enthalpy and the parameter x , describing the structure dependence of relaxation, are on average the same whether the solutions are imbibed in PHEMA or in their bulk states. However, x varies systematically with thermal history, indicating that the phenomenology is not completely accurate. It is probable that the treatment of non-linearity by the model needs to be improved.

Reply to ‘Commentary on ‘Strong and fragile liquids - A brief critique’’ by C.M. Roland and K.L. Ngai

Abstract Some of the matters raised by Roland and Ngai in their Commentary (this issue) on the article ‘Strong and fragile liquids - A brief critique’ [J. Non-Cryst. Solids 202 (1996) 164] are addressed. Issues surrounding the Adam-Gibbs theory and an ancillary assumption relating cooperativity and non-exponentiality, and their relation to fragility, are discussed in further detail.