Cooperative strings in glassy nanoparticles

TL;DR

This paper extends a cooperative-string model to spherical nanoparticles, predicting how confinement affects the glass transition, including a critical radius below which vitrification is impossible, aligning with recent experimental findings.

Contribution

It introduces a minimal theoretical framework for glass transition in spherical confinement, successfully predicting nanoparticle size effects on vitrification.

Findings

Prediction of mobile-layer thickness as a function of temperature

Identification of a critical nanoparticle radius for vitrification

Agreement with experimental data on polystyrene nanoparticles

Abstract

Motivated by recent experimental results on glassy polymer nanoparticles, we develop a minimal theoretical framework for the glass transition in spherical confinement. This is accomplished using our cooperative-string model for supercooled dynamics, that was successful at recovering the bulk phenomenology and describing the thin-film anomalies. In particular, we obtain predictions for the mobile-layer thickness as a function of temperature, and for the effective glass-transition temperature as a function of the radius of the spherical nanoparticle - including the existence of a critical particle radius below which vitrification never occurs. Finally, we compare the theoretical results to experimental data on polystyrene from the recent literature, and we discuss the latter.