A quantitative study of the rapid shrinking kinetics of sub-millimetre N-isopropylacrylamide gels

Sub-millimetre spherical gels of composition 70:10:20 mol% N-isopropylacrylamide:N,N′-tert-butylacrylamide:N,N′-dimethylacrylamide were prepared by inverse suspension polymerisation. The shrinking kinetics of cylindrical and spherical gels of various sizes were determined by image analysis. We are able to describe the shrinking kinetics in the transition region, as, in the size range studied, the gels showed no shape distortion during temperature jumps (T-jumps) to temperatures in the transition region. We define T^* to be the temperature at which the gel is fully collapsed, and, for the gels studied, T^* is >38°C. All gels relaxed to their equilibrium size exponentially after T-jumps to temperatures below T^*(T_f<T^*), while the shrinking process consisted of two distinct regions if T_f>T^*. The relaxation times reported here for sub-millimetre gels are significantly faster than any previously reported, and approach the timescales required for invivo medical devices.