Calorimetric Investigation of the Influence of Cross-Linker Concentration on the Volume Phase Transition of Poly(N-isopropylacrylamide) Colloidal Microgels

Abstract

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.

Soft Nanotechnology with Soft Nanoparticles

“…They observed an overall decrease in phasetransition enthalpy with increase in cross-linker density. [94] In addition to DSC they also used pulsed-gradient spin echo (PGSE) NMR spectroscopy to study the mobility of the solvent in the particle as a function of cross-linker density and observed that the diffusion of solvent is reduced with the increase in the cross-linker density, hence the particles are more rigid at high cross-linker density. Figure 8 depicts the change in relative diffusion coefficient of water in the microgels, measured by PGSE-NMR spectroscopy, for samples with different cross-linker densities.…”

Characterizing charge and crosslinker distributions in polyelectrolyte microgels

“…Electrophoresis can be used to measure mobility of a charged microgel within an electric field, enabling estimation of the microgel surface charge density [6]. Differential scanning calorimetry can be used to characterize the critical temperature as well as the magnitude of bulk phase transitions within thermally-responsive microgels [7][8][9]. Rheological analysis can be used to estimate the bulk water content within microgels or study the formation of colloidal crystals and other macroscopic assemblies of concentrated microgel suspensions [10][11][12].…”

Photon Force-Induced Phase Transition Dynamics of Single Hydrogel Nanoparticles in Water

“…These studies reveal that the PNIPAM gel particles dispersed in water are in a swollen state at room temperature due to hydrogen bonding between solvent water and amide groups, i.e. N -isopropylacrylamide (NIPAM) units and crosslinker methylenebisacrylamide (MBAM) units [21]. We have observed that PNIPAM microgel particles dispersed in water gradually shrank with elevating temperature (≈ 31…”