Synthesis, characterisation and solution thermal behaviour of a family of poly (N-isopropyl acrylamide-co-N-hydroxymethyl acrylamide) copolymers

Thermo-responsive copolymers of poly (N-isopropyl acrylamide-co-N-hydroxymethyl acrylamide)p(NIPAAm-co-HMAAm) with a range of content of hydroxy groups have been synthesised by free radical polymerisation. The polymers were characterised by NMR, FTIR and GPC. A detailed study of the solution thermal properties showed that polymers with up to about 50 wt.% hydroxy monomer show lower critical solution temperature (LCST) properties in water. Polymers with higher hydroxy monomer content are fully soluble at all temperatures up to 100 °C. The effect of pH, salts and solvent additives on the solution thermal behaviour of the copolymers was investigated, showing that “salting-out” salts lowered the LCST and “salting-in” salts caused an initial increase in LCST at low concentrations, but reduced LCST at higher concentrations, in line with the Hoffmeister series. The LCST of any copolymer composition from this family in pure water can be predicted from the empirical equation; LCST = 0.015x2 + 0.25x + 31.76, where x is the fraction of the hydroxy monomer. Due to differences in polarity and the length of the carbon chain, methanol and ethanol altered LCST of p(NIPAAm) and its hydroxyl copolymers in different manners, showing a transition from cononsolvency to cosolvency as the hydroxyl content of the copolymer increased. A more complex polyhydroxy compound, sucrose, had very little effect on LCST for either p(NIPAAm) or its hydroxyl copolymers.