Unusual Nonlinearity and Nonexponentiality in Glass-Forming Polymers Governed by Ionic Interactions

Area of Science:

• Polymer Physics
• Materials Science
• Thermodynamics

Background:

• Glass transition in polymers exhibits complex dynamics, including non-Arrhenius, nonexponential, and nonlinear features.
• These dynamic parameters are often empirically correlated, but the underlying thermodynamic origins remain debated.
• Polyvinylpyrrolidone (PVP) and its salt complexes offer a model system to investigate these correlations.

Purpose of the Study:

• To systematically investigate the interrelationships between dynamic heterogeneity parameters and thermodynamic properties in PVP/salt complexes.
• To elucidate the role of ionic interactions in modulating the glass transition behavior.
• To establish a thermodynamic framework for understanding heterogeneous dynamics in polymer glasses.

Main Methods:

• Thermodynamic analysis of PVP/salt complexes using enthalpy relaxation parameters.
• Measurement of heat capacity jump (ΔCp) at the glass transition temperature (Tg) and enthalpy hysteresis (ΔHR).
• Application of the Adam-Gibbs model to correlate thermodynamic factors with dynamic parameters like fragility (m), nonexponentiality (β), and nonlinearity (x).

Main Results:

• Introduction of ionic interactions reduced dynamic fragility (m), nonexponentiality (β), and nonlinearity (x), disrupting established empirical correlations.
• Enhanced ΔCp indicated that thermodynamic factors are primary drivers for the concurrent decrease in x and m.
• An anomaly where x > β suggests physical inhomogeneities in retardation time distributions contribute to nonexponential dynamics, linked to lower activation energy from ionic interactions.

Conclusions:

• Thermodynamic properties, particularly ΔCp and ΔHR, are crucial for understanding dynamic heterogeneity and fragility in polymer glass formation.
• Ionic interactions significantly influence glass transition dynamics by altering thermodynamic and dynamic parameters.
• The study provides a novel thermodynamic framework for interpreting heterogeneous dynamics, reinforcing the link between thermodynamics and polymer dynamics.