Water-soluble copolymers and their hydrogels with pH-tunable diverse thermoresponsive behaviors enabled by hydrogen bonding

Abstract

Water-soluble copolymers of poly(acrylic acid-co-N-vinylcaprolactam) (PAN) and poly(acrylic acid-co-N-vinylcaprolactam-co-dimethyl acrylamide) (PAND) were synthesized and found to exhibit opposite, and pH-tunable, UCST (upper critical solution temperature) and LCST (lower critical solution temperature) thermosensitive solubility in aqueous solution, respectively. For PAN (UCST), the insoluble state is determined by hydrogen bonding between comonomer units (AA–NVCL and AA–AA), and the solubility is obtained on heating as these intra- and inter-polymer H-bonds are weakened, promoting the solubilization of polymer chains by water molecules. For PAND (LCST), the hydrogen bonding between comonomer units still plays the determining role, for which the hydrophobic H-bonded “complex” of AA–NVCL and AA–AA exerts a similar effect as a hydrophobic comonomer on the LCST of poly(dimethyl acrylamide) (PDMA), causing the hydration–dehydration transition upon heating of DMA segments in PAND. UCST and LCST hydrogels were prepared using PAN and PAND, respectively, and used to demonstrate pH sensibility-enabled information recording and opposite encryption/decryption processes through temperature change.