Dependence of temperature-sensitivity of poly(N-isopropylacrylamide-co-acrylic acid) hydrogel microspheres upon their sizes

Monodisperse poly(N-isopropylacrylamide-co-acrylic acid) hydrogel microsphe res were prepared by a membrane emulsification method using membranes of po re diameters of 0.33, 0.73, 1.15, and 1.70 mu m. The hydrogels were synthes ized by polymerization of 3.6 M N-isopropylacrylamide (N-IPAAm or NIPAM) an d 0.4M acrylic acid (AAc). Their surface properties were studied by measuri ng the electrophoretic mobility of the microspheres in electrolyte solution s at pH 7.4 at 25, 30, 33, 35, 40, and 45 degrees C. Poly(N-IPAAm-co-AAc) m icrospheres have shown negative mobility. More negative values of electroph oretic mobility were obtained with the smaller microspheres than the larger ones at each temperature. The surface charge density of the microspheres i ncreased and their surfaces became harder above 35 degrees C, since the mic rospheres contained thermosensitive poly(N-IPAAm) moiety and LCST increased by the addition of AAc, while that of poly(N-IPAAm) was 33 degrees C. It h as recently been found that the smaller microspheres exhibit the stronger d ependence of both surface charge density and softness on the temperature. ( C) 2000 Academic Press.