Size-dependent Thermosensitivity of Poly (N-isopropylacrylamide-co-acrylic acid) Hydrogel Microspheres Prepared by a Membrane Emulsification Method: Phase Transition Temperature Changes Depending on the Hydrogel Size.

Changes in the surface properties of poly (N-isopropylacrylamide-co-5mol% acrylic acid) microspheres depending on tempereture and the size of the microspheres were studied by measuring the electrophoretic mobility of the microspheres in electrolyte solutions at pH7.4 at 25, 30, 33, 35, 40, and 45°C. The hydrogel microspheres were monodisperse, since they were prepared from w/o emulsion using a membrane emulsification method. The size of the microspheres was dependent on the membrane pore size of the membrane. Membranes with four pore sizes ware used. Poly (N-isopropylacrylamide-co-5mol% acrylic acid) microspheres have shown negative electrophoretic mobility values. More megative values were obtainde with the smaller microspheres than the larger ones at each temperature. The surface became harder as temperature rose, independent of the microsphere size. This is caused by the shrinkage of poly (N-isopropylacrylamide) moiety, LCST of which is 33°C. It has been newly found that the smaller microspheres have a lower phase transition temperature (LCST) and exhibit the stronger dependence of surface charge density on the temperature than the larger ones. The surface charge density of the microspheres became higher only in the small microspheres as temperature rose, and it changed little in larger microspheres. That is, in the case of poly (N-isopropylacrylamide-co-5mol% acrylic acid) hydrogels, thermosensitive changes of their surface strcutures are dopendent on their size, because the hetrogeneity of the microsphere structural is different depending on their sizes. These observations suggest that the changes on structural heterogeneity of poly (N-isopropylacrylamide-co-5mol% acrylic acid) microgels affect their LCST and that the heterogeneous gel structure of microgels strongly affects their surface properties as comparing with those of bulk gels