Phase coexistence in thermo-responsive PNIPAM hydrogels triggered by mechanical forces

TL;DR

This work addresses how PNIPAM hydrogels can exhibit coexisting swollen and collapsed phases near the volume phase transition temperature $T_{VPTT}$ when subjected to mechanical constraints. It develops a statistical-mechanics framework that connects chain-level coil-to-globule transitions to network-scale phase nucleation using a probabilistic transition model $P(\rho_m)$ and a micro-sphere network integration to compute macroscopic stresses. The model provides energy descriptions for swollen and collapsed states, mechanical and chemical equilibrium conditions, and deformation fields for coexisting phases, and it is validated against stretch-induced and relaxation-induced experiments by Suzuki and Ishii. The results reveal that mechanical loading can tune $T_{VPTT}$ and control phase evolution, offering a pathway to actively tailor PNIPAM hydrogels for soft robotics, drug delivery, and other responsive applications.

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 the 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 Suzuki and Ishii ("Phase coexistence of neutral polymer gels under mechanical constraint"), which demonstrated that a PNIPAM rod can exhibit phase coexistence (i.e. comprise swollen and collapsed domains) 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 that 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.

Figures (9)

• Figure 1: The swollen phase, phase coexistence, and the collapsed phase of a PNIPAM rod.
• Figure 2: The stretching of a “ collapsed” chain to induce an extended chain by breaking the intramolecular bonds at temperature $T>T_{VPTT}$.
• Figure 3: The force $f_{g}$ exerted on a collapsed chain as a function of the ratio $\rho_{m}$.
• Figure 4: A schematic illustration of the stretching of a collapsed PNIPAM chain: a collapsed chain is stretched from the reference configuration by $\lambda<\lambda_{1}$. Further extension ($\lambda_{1}<\lambda<\lambda_{2}$) leads to the coexistence of phases - the rod is stretched from a fictitious to a deformed state comprising collapsed and swollen domains. At $\lambda=\lambda_{2}$ the rod transitions completely to the swollen state, and further stretch ($\lambda>\lambda_{2}$) leads to the elongation of a swollen rod.
• Figure 5: Illustration of the probability $P$ as a function of the ratio $\rho_{m}$ that a chain transitions from globule to coil.