Unusual Temperature-Induced Swelling of Ionizable Poly(N-isopropylacrylamide)-Based Microgels: Experimental and Theoretical Insights into its Molecular Origin

Abstract

In the traditional view of temperature-driven volume phase transitions in PNIPAm-based microgelsolutions, a monotonic and sharp decrease in particle size occurs upon heating the solution abovethe volume phase transition temperature (VPTT). However, at sufficiently high microgelconcentrations and low salt conditions, our dynamic light scattering experiments reveal anunexpected non-monotonic evolution of particle size when increasing the solution temperature.These findings show that poly(N-isopropylacrylamide-co-methacrylic acid) (P(NIPAm-co-MAA))microgels swell upon heating the solution in the temperature range where NIPAM is water-soluble(i.e., below the VPPT). Further heating the microgel solution leads to microgel collapse as typicallyobserved at temperatures above the VPTT. This novel behavior depends on particle and saltconcentration. We have observed the expected monotonic temperature-response of P(NIPAm-co-MAA) microgel solutions at low particle density and high salt concentration. To gain insights intothe molecular origin of the unusual behavior of these microgel solutions, we have combined nuclearmagnetic resonance studies and molecular-level theoretical calculations of the system. A delicatebalance between inter-particle steric compressions and intra-microgel physical interactions andchemical equilibria determines the size of these microgels. Both steric compression, due to finitedensity, and hydrogen bond formation in the interior of the microgels favor a more compactparticle. On the contrary, at the pH of the experiments the acid-base equilibrium constraints thepolymer charge to increase, which favors particle swelling due to intra-microgel electrostaticrepulsions. This interplay between physical interactions and chemical equilibria occurring at thenanometer length-scale determines the unusual thermal-induced swelling of P(NIPAm-co-MAA)