# Understanding Solvent-Induced Glass Transition in Polymer Thin Films Using Absorption–Desorption Isotherms

## TL;DR

This paper explores how solvents affect the glass transition in polymer thin films using absorption and desorption isotherms, revealing new insights into solvent-polymer interactions.

## Contribution

The study identifies four distinct solvent-polymer interaction regimes and links solvent-induced glass transition to physical aging and plasticization effects.

## Key findings

- Glass transition trends are more robust in absorption than desorption, contrary to prior reports.
- Excess osmotic pressure analysis quantifies physical aging in thin films below the glass transition.
- Solvent annealing effects are strongly influenced by solvent viscosity and mass transport.

## Abstract

The fundamental thermodynamic and mechanical underpinnings
of polymer
thin films exposed to solvent vapor are critical for the development
of advanced nanolithography and high-performance coatings. This work
investigates the solvent–polymer interactions of glassy thin
films by using the solvent absorption–desorption isotherms.
An analogous relationship to the Flory–Fox equation was observed
between solvent–induced glass transition, swelling, Flory–Huggins
interaction parameter, and molecular weight. Isothermal swelling measurements
revealed that the glass transition trends are more robust in the absorption
curve compared to desorption, contrary to previous reports. Excess
osmotic pressure analysis of the isotherm provides a measure of the
degree of physical aging in thin films annealed below the glass transition.
This is further validated in the ordering of block copolymer (BCP)
films annealed at low solvent activity. In agreement with the thermal
analysis, free-surface plasticization effects become the most prominent
below 100 nm. However, solvent annealing is largely dependent on solvent
mass transport, as made evident by the strong dependence on solvent
viscosity. From these observations, four general types of isotherms
are identified that graphically capture distinct solvent–polymer
interaction regimes. More broadly, these results inform solvent vapor
annealing-induced self-assembly, sequential infiltration synthesis,
membrane-based separations, adsorptive processes, and swelling-based
responsive materials design.