Temperature-responsive shrinking kinetics of poly ( N-isopropylacrylamide) copolymer gels with hydrophilic and hydrophobic comonomers

Abstract

Temperature-responsive poly( N-isopropylacrylamide) and its copolymer gels with the hydrophilic comonomer, acrylamide and the hydrophobic comonomer, butyl methacrylate, all exhibit swelling-deswelling changes in response to external temperature changes. These hydrogels show negative swelling thermosensitivities, particularly swelling at lower temperature and complete deswelling over specific phase transition temperatures ( T p). Shrinking kinetics of these gels from swollen to deswollen states at several different temperature changes have been investigated. When temperature changes are performed entirely below T p, the shrinking process is dominated by polymer network diffusion. On the other hand, shrinking kinetics for temperature changes from below to above T p, are dramatically influenced by gel surface structural changes and formation of a collapsed polymer skin layer. This surface skin formation prompts the accumulation of internal hydrodynamic pressure inside the gels upon shrinking by blocking the outflux of entrapped water. Both rate and magnitude of internal hydrodynamic pressure are modulated by the gel volume, the hydrophobicity of the gel polymer chains and the degree of external temperature changes. This internal pressure eventually causes convective outflow of water from the gel interior. Hydrodynamic internal pressure affects the drug release on-off pattern during gel shrinking.