Calorimetric investigation of the influence of cross-linker concentration on the volume phase transition of poly(N-isopropylacrylamide) colloidal microgels

A series of poly(N-isopropylacrylamide) microgels were prepared, by a surfactant-free emulsion polymerization method, incorporating varying cross-linker concentrations (0.25-30.0% of N,N'-methylenebisacrylamide). Turbidimetric, light scattering, and differential scanning calorimetric (DSC) analyses of aqueous dispersions of these microgels show that the temperature and the half-width of the overall volume phase transition (VPT) of the colloidal dispersions increase with increasing cross-linker concentration. DSC analysis also reveals a decrease in the overall calorimetric enthalpy of the VPTs with increasing cross-linker concentration. The DSC thermograms have been deconvoluted into two component transitions; one is asymmetric and attributed to aggregation of "free" N-isopropylacrylamide monomers/oligomers, and a second, two-state conformational transition, is associated with the cross-linked microgels. Pulsed-gradient spin-echo NMR diffusion measurements show that the diffusion of water within the particles can be estimated and this diffusion rate decreases with increasing cross-link concentration. Complementary fluorescence studies show an increased hydrophobicity as the cross-linker density increases.