Unusual temperature-induced swelling of ionizable poly(N-isopropylacrylamide)-based microgels: experimental and theoretical insights into its molecular origin.

摘要: In the traditional view of temperature-driven volume phase transitions in PNIPAM-based microgel solutions, a monotonic and sharp decrease particle size occurs upon heating solution to above transition temperature (VPTT). However, at sufficiently high concentrations under low salt conditions, our dynamic light scattering experiments reveal an unexpected non-monotonic evolution when increasing temperature. These findings show that poly(N-isopropylacrylamide-co-methacrylic acid) (P(NIPAM-co-MAA)) microgels swell range where NIPAM is water-soluble (i.e., below VPPT). Further leads collapse as typically observed temperatures VPTT. This novel behavior depends on concentration. We have expected temperature-response P(NIPAm-co-MAA) solutions density To gain insights into molecular origin unusual these we combined nuclear magnetic resonance studies molecular-level theoretical calculations system. A delicate balance between inter-particle steric compressions intra-microgel physical interactions chemical equilibria determines microgels. Both compression, due finite density, hydrogen bond formation interior favors more compact particle. On contrary, pH acid–base equilibrium constrains polymer charge increase, which swelling electrostatic repulsions. interplay occurring nanometer length-scale thermal-induced P(NIPAM-co-MAA)