Structural studies of Poly(N-isopropylacrylamide) microgels:: Effect of SDS surfactant concentration in the microgel synthesis

Thermoresponsive colloidal microgels were prepared by polymerization of N-isopropylacrylamide (NIPAM) in the presence of a crosslinking monomer, N,N-methylenebisacrylamide, in water with varying concentrations (< CMC) of an anionic surfactant, sodium dodecylsulphate (SDS). Volume phase transitions of the prepared microgels were studied in D2O by H-1 NMR spectroscopy including the measurements of spin-lattice (T-1) and spin-spin (T-2) relaxation times for the protons of poly(N-isopropylacrylamide) (PNIPAM) at temperature range 22-50 degrees C. In addition, microcalorimetry, turbidometry, dynamic light scattering, and electrophoretic mobility measurements were used to characterize the aqueous microgels. As expected, increasing SDS concentration in the polymerization batch decreased the hydrodynamic size of an aqueous microgel. Structures with high mobilities at temperatures above the LCST of PNIPAM were observed in the microgels prepared with small amount of SDS, as indicated by the relaxation times of different PNIPAM protons. It was concluded that the high mobility at high temperatures is in connection to a mobile surface layer with polyelectrolyte nature and with high local LCST. High SDS concentration in the synthesis was observed to prevent the formation of permanent, solid PNIPAM particles. The results from different characterization methods indicated that PNIPAM microgels prepared in high SDS concentrations appear to be more homogeneously structured than their correspondences prepared in low SDS concentration. (c) 2006 Wiley Periodicals, Inc.