Quantifying Changes in the High-Frequency Dynamics of Mixtures by Dielectric Spectroscopy

Abstract

Additives to polymeric materials can lead to appreciable changes in the rates of relaxation and reaction in these mixtures that can profoundly alter material properties and function. We develop a general theoretical framework for quantifying changes in the "high-frequency" relaxation dynamics of mixtures based on classical transition state theory, in conjunction with mathematical statements regarding the dependence of the entropy (S+) and enthalpy (E+) of activation of the high-frequency relaxation time on diluent mass fraction, X(w). Specifically, we deduce a general classification scheme for diluents based on a consideration of the sign of the differential change in S+ and E+ with x(w). Two of these classes of diluents exhibit a transition from plasticization to antiplasticization (defined specifically as a speeding up or slowing down of relaxation relative to the pure system, respectively) upon varying temperature through an "antiplasticization" temperature, T(anti). Extensive dielectric relaxation measurements on polycarbonate (PC) as a function of temperature and diluent (Aroclor) concentration are utilized to illustrate our theoretical model, and we focus particularly on the Arrhenius "beta" dielectric relaxation process of these mixtures. Many aspects of our scheme for quantifying changes in the high-frequency dynamics of mixtures are rationalized by our mixture model. In particular, we show that the dilution of PC by Aroclor is consistent with a theoretically predicted (one of the two antiplasticization mixture classes mentioned above) transition from antiplasticization to plasticization with decreasing temperature. We briefly compare our findings from dielectric measurements with those from elastic incoherent neutron scattering and dynamical-mechanical measurements, providing further evidence for the antiplasticization-to-plasticization transition phenomena that we observe in our high-frequency dielectric measurements.

Quantitative relations between cooperative motion, emergent elasticity, and free volume in model glass-forming polymer materials

“…This unusual type of kinetics has been observed in materials characterized by complex energy landscapes (50)(51)(52)(53), and negative activation parameters are observed in zeolite materials where confinement is very strong (54,55). The enthalpy-entropy compensation temperature, T comp (the slope of the linear relation) for the nanocomposites and thin films, is close to the estimated VFT temperature T 0 of their pure systems, as found in previous experimental studies of β-relaxation in GF liquids (56)(57)(58). Our data reduction in terms of the scale of collective motion is equally as compelling as the relation indicated above between τ and hu 2 i.…”

Stabilization of proteins in solid form

“…For example, Chang et al [84] reported that adding small amounts of an antiplasticizer (sorbitol or water) to sucrose-antibody formulations resulted in a monotonic decrease in T g and τ α , but optimal stabilization was obtained at intermediate levels of added sorbitol or water. In these experiments, ${\tau }_{{\beta }_{\mathrm{italicJG}}}$ changed with antiplastizer [86] in the same non-monotonic way as did stability, suggesting that these faster dynamic processes may be coupled directly to degradation rates.…”

Some tentative explanations for the enthalpy–entropy compensation effect in chemical kinetics: from experimental errors to the Hinshelwood-like model

“…The list of proposals considered hereafter does not try to be exhaustive, but only illustrative of the complexity of the problem analysed. Other plausible theoretical approximations have already been published by other authors [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21].…”