Thermo-Mechanics of PNIPAM Gels: from a Single Chain to a Network Response

Poly(N-isopropylacrylamide) (PNIPAM) is a temperature-responsive polymer that exhibits a lower critical solution temperature (LCST) phase transition. On the chain level, this behavior stems from a coil-to-globule configurational transition at a critical temperature. On the macroscopic level, copolymerization or cross-linking of PNIPAM results in a hydrogel that decreases its volume significantly at the volume phase transition temperature (VPTT). This behavior is advantageous in a wide range of applications, including tissue engineering, drug delivery systems, and soft robotics. To fully exploit the unique properties of PNIPAM, it is important to understand the underlying mechanisms that govern its thermo-mechanical response. In this work we present a microscopically motivated energy-based model to explain the coil-to-globule transition in PNIPAM networks. We begin by considering a single chain below and above the LCST and employ tools from polymer physics to capture the water-polymer interactions and the role of water cages on the entropy. We present physically motivated parameters to describe the transition of a PNIPAM chain in response to temperature and an external force and validate our model against nanofishing experiments on single PNIPAM chains. To determine the macroscopic response, we employ the chain model and integrate from the chain to the network level. Our model illustrates the influence of the coil-to-globule transition on the decrease in volume of a PNIPAM gel as the temperature increases. As opposed to the classical approaches, in which the interaction parameter χ is taken as a function of temperature, the proposed model captures the transition at the chain level, which directly affects the macroscopic response. To demonstrate the merit of the model, we compare its predictions to experimental data on the volumetric deformations and the stress of traction free and of constrained PNIPAM networks. The findings from this work provide valuable insights into the molecular mechanisms that dominate the response of PNIPAM gels and provide fundamental tools for the design and optimization of PNIPAM-based gels for various applications.