Stimuli‐sensitivities of hydrogels containing phosphate groups

Abstract The effect of an introduction of phosphate groups into hydrogels on their stimuli‐sensitivities has been investigated. Several hydrogels containing various phosphate group content were synthesized via solution copolymerization of 2‐(methacryloyloxy)ethyl dihydrogen phosphate (Phosmer) and 2‐hydroxyethyl methacrylate (HEMA) in the presence of 2,2′‐azoisobutyronitrile (AIBN) in methanol at 70°C for 4 h. The content of phosphate groups incorporated into the resulting hydrogels ranged from 1 to 13 mol‐%. The swelling ratio of these hydrogels was found to be strongly affected by pH, solvent, temperature and phosphate group content. The polymersolvent interaction parameter χ of the hydrogel was determined from the Flory‐Rehner equation, using the swelling ratio and the crosslinking density derived from the compression modulus. Furthermore, χ was divided into its enthalpic (χ H ) and entropic component (χ S ). Using the interaction parameter, the stimuli‐sensitivity of the hydrogel is discussed in detail based on thermodynamic aspects.