The redispersibility and physico-chemical properties of freeze-dried colloidal microgels

Monodisperse colloidal microgels of poly(N-isopropylacrylamide) having different surface charge groups have been prepared by a surfactant-free emulsion polymerisation technique. A co-polymer microgel of poly(N-isopropylacrylamide/acrylic acid) was also prepared by the same method. Turbidimetric and calorimetric analyses showed the microgels to undergo a thermoreversible phase transition when dispersed in water. This was found to occur at 34 degrees C for the homopolymer microgels and to be variable for the co-polymer, depending on the pH of the dispersion environment. Each microgel sample was freeze-dried and the physico-chemical properties were measured before and after the lyophilisation procedure. Turbidimetric and calorimetric analyses showed that the freeze-drying procedure did not appear to have a detrimental effect on the physico-chemical properties of the microgel particles. The lyophilised products were studied with respect to their moisture absorption characteristics; the absorption process was found to be reversible with respect to the relative humidity of their environment. The incorporation of hydrophilic acrylic acid residues into the co-polymer appeared to enhance the moisture absorption capacity of the microgels. (C) 1998 Elsevier Science B.V.