STIMULI-SENSITIVITIES OF HYDROGELS CONTAINING PHOSPHATE GROUPS

The effect of an introduction of phosphate groups into hydrogels on th eir stimuli-sensitivities has been investigated. Several hydrogels con taining various phosphate group content were synthesized via solution copolymerization of 2-(methacryloyloxy)ethyl dihydrogen phosphate (Pho smer) and 2-hydroxyethyl methacrylate (HEMA) in the presence of 2,2'-a zoisobutyronitrile (AIBN) in methanol at 70-degrees-C for 4 h. The con tent of phosphate groups incorporated into the resulting hydrogels ran ged from 1 to 13 mol-%. The swelling ratio of these hydrogels was foun d to be strongly affected by pH, solvent, temperature and phosphate gr oup content. The polymer-solvent interaction parameter chi of the hydr ogel was determined from the Flory-Rehner equation, using the swelling ratio and the crosslinking density derived from the compression modul us. Furthermore, chi was divided into its enthalpic (chi(H)) and entro pic component (chi(S)). Using the interaction parameter, the stimuli-s ensitivity of the hydrogel is discussed in detail based on thermodynam ic aspects.