Preparation and characterization of pH-responsive poly(N,N-dimethyl acrylamide-co-methacryloyl sulfadimethoxine) hydrogels for application as food freshness indicators

A series of poly(N,N-dimethyl acrylamide-co-methacryloyl sulfadimethoxine) (poly(DMA-co-SDM)) hydrogels with different ratios of N,N-dimethylacrylamide (DMA) and methacryloyl sulfadimethoxine monomer (SDM) were synthesized by free radical polymerization. The pK(a) values of the monomer and hydrogels were measured to investigate the extent to which their pH-responsive properties were affected by the ionizable sulfonamide groups. Further, buffer solutions of pH 2.0-10.0, considered as the spoilage metabolites, were used to confirm the change in the pH-responsive properties. The composition ratios of DMA and SDM strongly affected the pK(a) values of the hydrogels, contributing to differences in the ionization around their pK(a) values. As a result, all the hydrogels showed different changes in their appearance, from opaque to transparent or vice versa at certain pH values. The change in the transparency of the as-prepared hydrogels is strongly dependent on both the change in the pH and the amount of buffer solution, which mainly occurs owing to the change in the quality of a product with time. Therefore, the synthesized pH-responsive poly(DMA-co-SDM) hydrogels can be potentially applied as transparency changing indicators in response to metabolites released owing to the change in the quality of the food products.