Unusual temperature-induced swelling of Q1 Q2 ionizable poly(N-isopropylacrylamide)-based microgels: experimental and theoretical insights into its molecular origin?

J.M. GIUSSI; M.I. VELASCO; G.S. LONGO; R.H. ACOSTA; O. AZZARONI

SOFT MATTER

ROYAL SOC CHEMISTRY

Lugar: CAMBRIDGE; Año: 2015 vol. 11 p. 8879 - 8886

1744-683X

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