Thermo-responsive swelling and drug release switching of interpenetrating polymer networks composed of poly(acrylamide-co-butyl methacrylate) and poly (acrylic acid)

Abstract

The swelling behaviour of interpenetrating polymer networks (IPN) composed of poly (acrylic acid) (PAAc) and poly (acrylamide(AAm)-co-butyl methacrylate (BMA)) in water was studied and compared to crosslinked poly(AAm-co-BMA) and crosslinked poly(AAm-co-AAc-co-BMA). The IPNs were prepared by a sequential IPN method in which crosslinked PAAc chains were formed inside of three-dimensional networks ofpoly(AAm-co-BMA) as initial gels. Crosslinked poly(AAcco-AAm-co-BMA) gels (random gels) were also synthesized with the same monomer compositions as the IPNs to elucidate the effect of gel first-order structure on swelling behaviour. Positive swelling changes, i.e. lower swelling ratio at lower temperature and higher swelling ratio at higher temperature, were observed with temperature changes for both the IPNs and the random gels, while no change in swelling was observed for crosslinked poly(AAm-co-BMA) without AAc. In particular, IPN swelling showed a transition point at a certain temperature as a function of temperature change, while random gels showed simple and dull swelling changes with temperature change. The transition temperature of the IPNs shifts to higher temperature with increasing BMA content in IPNs. The positive temperature dependence is suggested to be linked to the formation and dissociation of hydrogen-bond complexes between AAc and AAm with temperature changes in IPNs and random gels. The observed transition in swelling changes in the IPN series is hypothesized to result from concurrent dissociation of continuous ladder-like polymer complexes between PAAc and PAAm matrices in the IPN. The shift of transition temperature induced by incorporation of BMA was due to the stabilization of the complex at higher temperature by increasing hydrophobic interactions. Reversible swelling changes, i.e. higher swelling ratio at high temperature (20°C, 30 °C, 40 °C) and lower swelling ratio at 10°C, in response to step-wise temperature changes between low and high temperature, were observed only for BMA-incorporated IPNs, while irreversible swelling was observed for IPNs without BMA. This result suggests that the presence of hydrophobic BMA had an important role in reversible swelling change of IPNs in response to temperature change. To evaluate the feasibility of IPNs for drug delivery systems, the release of ketoprofen from IPNs was performed with fluctuation of temperature between 10°C and 30 °C. The release rate of drug was reduced at 10°C, while it was increased at 30 °C. The difference of release rate between 10°C and 30 °C was considered to be due to different drug diffusivities between the temperatures induced by swelling changes of IPNs.