(363h) Atomistic Simulations of Thermoresponsive Poly(N-isopropylacrylamide) Polymers

Authors: 
Abbott, L. J., Pennsylvania State University
Stevens, M. J., Sandia National Laboratories

Poly(N-isopropylacrylamide)  (PNIPAM) is a thermoresponsive polymer that undergoes a coil-to-globule transition in water around 32 °C. Because its structure is sensitive to external stimuli, PNIPAM-based systems (e.g., hydrogels, micelles, brushes) are of interest for applications like drug delivery, biosensors, and tissue engineering. The transition of the polymer from an extended coil to a collapsed globule conformation at the lower critical solution temperature (LCST) results from the interplay of polymer–polymer and polymer–water interactions. While it is suggested that the polymer dehydrates upon heating above the LCST, a full understanding of the structural changes in the polymer is still elusive. In this regard, we perform atomistic molecular dynamics simulations to study single oligomers of PNIPAM solvated in water at temperatures below and above the LCST. In addition, we treat a surfactant molecule that is a diblock of PNIPAM and an alkane chain. At higher temperatures, we observed a collapse of the PNIPAM structure in accordance with a decrease in polymer–water hydrogen bonds, an increase in the intramolecular polymer–polymer hydrogen bonds, and a decrease in the hydration number. These results are in agreement with experimental data. Additional insight is provided into the nature of the water around the polymer, such as the formation of a hydrogen bond network below the LCST. Lastly, we explore the formation of micelles from PNIPAM-based surfactants with a temperature-dependent hydrophilicity.

Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.