Hydrogel diffraction grating as sensor: A tool for studying volume phase transition of thermo-responsive hydrogel

摘要

Hydrogel diffraction gratings were developed as a sensing tool to study volume phase transition of thermo-responsive hydrogels. The gratings were fabricated from hydrogels mainly consisted of N-isopropylacrylamide (NIPAAm) and acrylamide (AAm) as monomer units, in some cases, acrylic acid (AA) as well. The hydrogel diffraction gratings were prepared through photoinscribing, soft-lithographic duplication, micro-contact printing and photopolymerization/crosslinking of the monomers. Surface-relief-gratings (SRGs) on azo polymer films were inscribed by interfering Ar~+ laser beam irradiation and used as grating masters. Poly(dimethylsiloxane) (PDMS) elastomer stamps were prepared through replica molding by casting the prepolymer on the SRGs and curing. The diffraction grating sensors were obtained by contact printing and photopolymerization/crosslinking under UV light irradiation. The variation of the first order diffraction efficiency (DE_1) of the gratings was measured as a function of temperature to detect the transition temperatures of the hydrogels. The swelling ratios of the hydrogels were semi-quantitatively correlated with the DE_1/DE_(10) ratio. The methodology was used to study the effects of cross-linker content, monomer ratio and solution pH on transition temperature and swelling ratio. The hydrogel diffraction gratings show ability to sense the volume phase transition in a real-time way and can be used for studying thermo-responsive hydrogels.