Solution thermal properties of a family of thermo-responsive N-isopropyl acrylamide-co-N-hydroxymethyl acrylamide copolymers - Aspects intrinsic to the polymers

A family of (N-isopropyl acrylamide-co-N-hydroxymethyl acrylamide) copolymers, p(NIPAAm-co-HMAAm), was synthesised and characterised with respect to solution thermal behaviour. A notable increase in cloud point (CP) was observed as the concentration of the polymer solutions was lowered, though the effect of concentration on CP of pure p(NIPAAm), was relatively small. In all cases, the elevation of CP was most evident at low concentration between 0.1 wt.% and 0.01 wt.%. Both at fixed and decreasing concentrations of p(NIPAAm) solutions, a direct relationship between molecular weight and CP was observed, where CP shifted to higher temperatures as the molecular weight increased. The polymer coil size in solution below the CP in the concentration range studied was between 4 and 11 nm for all the polymers in this study, irrespective of the concentration and of the fraction of hydroxy monomer incorporated, but increased dramatically above the CP. CPs obtained by Dynamic Light Scattering (DLS) were lower than those obtained by UV absorbance as the latter is less sensitive. Copolymer compositions with more than 13% hydroxy monomer showed no thermal hysteresis, in contrast to pure p(NIPAAm). The rate of heating/cooling had almost no effect on the CP values recorded.