Photopolymerisation and characterisation of negative temperature sensitive hydrogels based on N,N-diethylacrylamide

Despite the many advantages of photopolymerisation in the fabrication of hydrogels, studies on the synthesis of poly(N,N-diethylacrylamide) (PDEAAm) using this technique have received limited attention in the literature. A series of temperature sensitive hydrogels were prepared by free-radical crosslinking copolymerisation of N,N-diethylacrylamide (DEAAm) with 1-vinyl-2-pyrrolidinone (NVP) and N,N-dimethylacrylamide (DMAAm), respectively. Two ultraviolet (UV) light sensitive initiators were trialled in the synthesis, namely 1-hydroxycyclohexylphenylketone and 2-hydroxy-1-[4-(hydroxy-ethoxy)phenyl]-2- methyl-1-propanone, with poly(ethylene glycol)dimethacrylate being used as the crosslinking agent. The lower critical solution temperatures (LCSTs) of the hydrogels synthesised were shown to be close to body temperature using cloud point measurement and modulated differential scanning calorimetry, which is favourable particularly for 'smart' drug delivery applications. The swelling behaviour of the samples was investigated upon stepwise temperature change revealing that the hydrogels underwent reproducible pulsatile swelling behaviour. Oscillatory rheological studies showed that increasing the ratio of crosslinking agent could be used as a means of improving the mechanical properties of the photopolymerised temperature sensitive hydrogels.