Relating the dynamics of hydrated poly(vinyl pyrrolidone) to the dynamics of highly asymmetric mixtures and polymer blends

Abstract

Linking the dynamics of mixtures of water with hydrophilic molecules and polymers and hydrated proteins to those of glass-forming substances is a common practice in the research community to gain better understanding of the former from the latter. The glass transition temperature Tg of water is 136 K while the Tg of most neat molecules, polymers, and proteins are higher. Among hydrophilic polymers, poly(vinyl pyrrolidone) stands out in having exceptionally high Tg ranging typically from 396 to 445 K depending on the molecular weight. Thus the dynamics of poly(vinyl pyrrolidone)-water mixtures can be expected to resemble that of the highly asymmetric mixtures (HAM) and highly asymmetric polymer blends (HAPB) where the difference in the Tg of the two components ΔTg is large. Multiple experimental studies in recent years have revealed novel component dynamics in HAM and HAPB not commonly found in pure glass-formers. Theoretical investigations have uncovered new physics governing the interrelated dynamics of the two components. Since poly(vinyl pyrrolidone)-water mixtures are analogues of HAM and HAPB, it is worthwhile to compare the dynamics of the two systems. The comparison has the potential of discovering properties in poly(vinyl pyrrolidone)-water mixtures that are either new or unnoticed before. In this paper we report dielectric relaxation measurements of mixtures with 60, 65, and 70 wt% poly(vinyl pyrrolidone) more extensive than done before, together with detailed analysis and theoretical explanation of the data. The properties of the dynamics of poly(vinyl pyrrolidone)-water mixtures are found similar to HAM and HAPB and hence both are governed by the same physics. The results from this study broaden the knowledge of the physics behind the dynamics of processes in polymer-water mixtures, and may also apply to hydrated proteins.