Shifts of pNIPAM Lower Critical Solution Temperature in an Applied Electric Field

Abstract

Poly(N-isopropylacrylamide), or pNIPAM, is a free-radical polymer that is commonly studied for uses in surface coatings, tissue engineering, energy storage, biosensing, and more, due to its temperature responsiveness. Below its transition temperature, pNIPAM is dissolved in solution with a small particle size. Above the transition temperature, pNIPAM agglomerates and condenses out of solution. Dynamic Light Scattering (DLS) measures temperature induced agglomeration of pNIPAM by determining the transition temperature, which is typically 30-35oC for pNIPAM. We initially confirmed the lower critical solution temperature (LCST), or the point in which pNIPAM agglomerates, to be thermodynamically reversible and reproducible for both pNIPAM and an electrochemically active poly(N-isopropylacrylamide)-block-poly(ferrocenylmethyl methacrylate) block copolymer, or p(NIPAM-b-FMMA).Applying a potential across the DLS light path, we expected p(NIPAM-b-FMMA) to cause a shift in the transition temperature and agglomeration behavior. However, we unexpectedly observed a shift in the transition behavior of pNIPAM. At an applied electric field of 400 mV/cm and above, the transition temperature of pNIPAM started to shift negative. Further, upon cooling, the pNIPAM never re-dissolves, indicating an irreversible agglomeration in the presence of an applied.We have also started to investigate reasons on why this phenomenon might occur. We will discuss two primary hypotheses, including wetting behavior changes of the pNIPAM and further radicalization of the RAFT agents leading to cross-linking and irreversible behavior.Acknowledgements: The authors would like to acknowledge the financial support from NIH (P20 GM113131), NSF (CBET 1638896), the Collaborative Research Excellence Initiative Pilot Research Program at the University of New Hampshire, and the Hamel Center of Undergraduate Research at the University of New Hampshire. The authors also would like to acknowledge the assistance of Dr. Seitz’s laboratory and Dr. Halpern’s laboratory