# Phase Coexistence in Thermoresponsive PNIPAM Hydrogels Triggered by Mechanical Forces

## TL;DR

This paper explores how mechanical forces can control the phase transition of PNIPAM hydrogels, offering a new way to tune their behavior.

## Contribution

A statistical-mechanics framework is proposed to model phase coexistence in PNIPAM hydrogels under mechanical constraints.

## Key findings

- Collapsed domains form in a fixed swollen rod over time near the VPTT.
- Swollen domains can nucleate in a collapsed rod under uniaxial extension.
- Mechanical constraints can actively tailor the VPTT of PNIPAM hydrogels.

## Abstract

Poly­(N-isopropylacrylamide) (PNIPAM)
is a temperature-responsive
polymer that undergoes large volumetric deformations through a transition
from a swollen to a collapsed state at a volume phase transition temperature
(VPTT). Locally, these deformations stem from the coil-to-globule
transition of individual chains. In this contribution, I revisit the
study of

SuzukiA.,
;
IshiiT.,

[J. Chem. Phys.
1999, 110, 2289–2296
], which demonstrated
that a PNIPAM rod can exhibit phase coexistence (i.e., comprise swollen
and collapsed domains simultaneously) near the VPTT when subjected
to mechanical constraints. Specifically, that paper showed that (1)
collapsed domains gradually form in a fixed swollen rod with time
and (2) swollen domains can nucleate in a collapsed rod under uniaxial
extension. These behaviors originate from the local thermo-mechanical
response of the chains, which transition between states in response
to the applied mechanical loading. Here, I develop a statistical-mechanics
based framework that captures the behavior of individual chains below
and above the VPTT and propose a probabilistic model based on the
local chain response that sheds light on the underlying mechanisms
governing phase nucleation and growth. The model is validated through
comparison with experimental data. The findings from this work suggest
that in addition to the classical approaches, in which the VPTT is
programmed through chemical composition and network topology, the
transition can be tuned by mechanical constraints. Furthermore, the
proposed framework offers a pathway to actively tailor the VPTT through
the exertion of mechanical forces, enabling improved control and performance
of PNIPAM hydrogels in modern applications.