EFFECT OF HEATING RATE ON THERMO-INDUCED AGGREGATION OF POLY(ETHYLENE OXIDE)-b-POLY(N-ISOPROPYLACRYLAMIDE) IN AQUEOUS SOLUTIONS

The effects of heating rate on the aggregate behavior of poly（ethylene oxide）-b-poly（N-isopropylacrylamide） in aqueous solutions were investigated in detail by laser light scattering and TEM.By employing two separate heating protocols,step-by-step heating at<5 K/step and one-step jump,to heat the sample from 15℃to the selected temperature, we found that the heating rate only showed significant effect on the aggregates above the cloud point.The aggregate formed by step-by-step heating exhibited a much larger size and a broader size distribution than those formed by one-step jump heating.Moreover,neither of the aggregates were ideal micellar structures as indicated by the size and the R;/R;values.On the contrary,at temperatures below the cloud point where the block copolymer formed core-shelled micelles,the heating rate showed negligible effect on the size and size distribution of the micelles.Since the system underwent a phase separation above the cloud point,the heating rate effect could be reasonably explained by the phase separation mechanisms:the nucleation-and-growth mechanism in the metastable region and the spinodal decomposition mechanism in the unstable region.