Biomass-based thermogelling copolymers consisting of lignin and grafted poly(N-isopropylacrylamide), poly(ethylene glycol), and poly(propylene glycol)

This paper reports the synthesis and characterization of a series of biomass-based thermogelling graft copolymers using atom transfer radical polymerization (ATRP) technique, which are composed of a lignin core and multiple arms of graft polymer chains, where each graft consists of a block of poly(N-isopropylacrylamide) (PNIPAAm) and a block of brush-like random copolymer of poly(ethylene glycol) (PEG) and poly(propylene glycol) (PPG). A few control copolymers with different chain architectures and/or compositions were also synthesized for comparison purposes. It was found that the lignin-based PNIPAAm-block-PEG/PPG copolymers were thermo-responsive in aqueous medium. The aqueous solutions of the copolymers displayed thermogelling behaviors, turning from a sol at low temperatures to a hydrogel at temperatures around 32-34 degrees C, and further to a dehydrated gel at higher temperatures. The thermogelling copolymers had very low critical gelation concentrations ranging from 1.3 to 2.5 wt%. The thermo-responsive rheological properties were investigated. The elastic modulus G' and viscous modulus G'' of the copolymer solutions were very low at low temperatures, and increased at higher temperatures, and then crossed over at their gelation temperatures, with G' dominating G" after the gel formation. The block architecture of the lignin-based PNIPAAm-block-PEG/PPG copolymers is key to their hydrogel formation, because the lignin-based PNIPAAm-random-PEG/PPG copolymer with the same monomer ratio, as a control polymer, could not form hydrogels at any temperature, whose solution only turned from transparent to turbid when the temperature was increased.