Aggregation behavior of poly(N-isopropylacrylamide) microspheres

Electrophoretic mobility and aggregation in suspensions of three types of microspheres (Ms 1. Ms 2 and Ms 3) are studied at different pH, ionic strengths and temperatures of the medium. Here Ms 1 is a core particle composed of poly(N-isopropylacrylamide-co-styrene). Ms 2 is a core-shell microsphere consisting of Ms 1 as the particle core covered with a surface layer of poly(N-isopropylacrylamide) hydrogel. Ms 3 is also a core-shell microsphere composed of MS-1 covered with a surface layer of poly(N-isopropylacrylamide-co-acrylic acid) hydrogel. The charge density =N and the softness parameter 1/lambda of the microspheres were obtained from the electrophoretic mobility data on the basis of an electrokinetic theory of soft particles. It is shown that when =N is large, suspensions of microspheres are always stable, showing no aggregation. When =N is small, the suspensions are stable for large 1/lambda. but show strong aggregation for small 1/lambda. (C) 2001 Elsevier Science B.V. All rights reserved.