# A Neutron Reflection Study of the Dissolution of Miscible Glassy Polymer Films over a Range of Temperature

## TL;DR

This study uses neutron reflection to investigate how glassy polymer films dissolve when in contact, revealing complex non-Fickian dynamics that resemble known polymer dissolution phenomena.

## Contribution

The paper introduces a novel experimental and modeling approach to study non-Fickian dissolution in glassy miscible polymer films.

## Key findings

- The initial interfacial mixing follows a fractional power-law growth with an exponent near 1/3.
- After an induction time, a front-like dissolution process occurs, resembling Case II non-Fickian diffusion.
- Phase field modeling helps explain the observed non-Fickian dynamics in the interfacial region.

## Abstract

Although the mixing of miscible liquids is generally
thought to
be relatively well-described by classical Fickian diffusion models,
it is well-known that the physics of glass-formation can greatly alter
the dissolution dynamics of polymer materials brought into contact
with solvents and other polymer materials. Despite the immense practical
importance of this phenomenon in many contexts where polymers are
“blended” (e.g., polymer recycling), models of the dissolution
dynamics of glassy polymer materials tend to be highly phenomenological
and to have limited general applicability. This situation is understandable
given the limited fundamental understanding generally of glass formation,
pointing to the need for high-resolution measurement methods on model
materials and an appropriate theoretical framework to elucidate the
fundamental nature of the polymer dissolution process under realistic
physical conditions. In the present work, we restrict our consideration
to the interfacial dynamics of a model system of this kind, composed
of two thermodynamically miscible, but glassy, polymer films formed
through spin-coating and film floating techniques. We utilize neutron
reflectivity (NR) to characterize the interfacial mixing dynamics
in these stacked glassy polymer films. The initial stage of the interfacial
mixing is found to be somewhat similar to Fickian interdiffusion of
miscible liquids, although we observe a fractional power-law growth
of the interfacial width in this transient regime with an exponent
near 1/3, as commonly observed in the coarsening of phase-separating
polymer blends. After an induction time t* over which
the interfacial concentration approaches that of the fully dispersed
polymer mixture, we generally observe a transition to a regime in
which the “mixed” polymer in the interfacial region
between the films invades both films in a front-like fashion. We adapt
phase field modeling of “solid” material dissolution
to qualitatively understand and characterize this evidently complex
non-Fickian dissolution process. The rates of the propagating composition
wave, as well as the evolution of the interfacial width separating
the relatively stable “uniformly mixed” polymer material
from the relatively unstable unmixed polymer material, are quantified.
Interestingly, this frontal dissolution process in glassy miscible
polymer films closely resembles the phenomenology of Case II non-Fickian
diffusion commonly observed in the dissolution of glassy polymers
by solvents, and we suggest that the latter corresponds to the situation
in which one of the layer components is not glassy so that the dissolution
wave propagates only in one direction.