1H NMR of thermoreversible polymers in solution and at interfaces: the influence of charged groups on the phase transition

The phase transition of thermoreversible polymers occurring at the lower critical solution temperature (LCST) is investigated by 1H NMR. Poly- N-isopropylacrylamide (PNIPAM) shows such a coil to globule transition at 32°C in aqueous solution. To study the effect of charged polymer segments on the phase transition, the temperature dependent properties of PNIPAM and of a charged PNIPAM-copolymer, containing 10% carboxylic groups, are investigated in solution. Experiments are performed by 1H spectra and PFG-NMR diffusion measurements at different polymer concentrations. The 1H liquid signal is sharply decreasing at the phase transition temperature. The transition is found to be equally sharp for the copolymer as for the homopolymer at concentrations below and around the overlap concentration, whereas the transition is broadened at higher concentrations. Diffusion measurements prove that the conformation of the polymer coils is maintained with increasing temperature until close to the phase transition, apart from a minor decrease of the hydrodynamic radius at about 2°C below the LCST. All data indicate identical phase transition properties of the copolymer as compared with the homopolymer. The introduction of charged groups (3% of monomers dissociated) has thus not altered the transition. Therefore, the copolymer is a suitable candidate for exhibiting a phase transition under electrostatic coupling conditions in layers. Both polymers are adsorbed to colloidal silica (Cab-O-Sil) and investigated by 1H NMR in order to monitor the phase transition in the restricted geometry of an adsorption layer. The liquid 1H intensities of both polymers are decreasing with temperature, this is interpreted as a phase transition of the loops and tails. The transition is substantially broader than in solution, especially at low surface coverage. Significant differences between the copolymer and the homopolymer are observed, since above the transition temperature a liquid signal from loops and tails of the copolymer is still observed. This is interpreted as a comparatively mobile arrangement of the copolymer layer, arising from electrostatic repulsion from the surface and between polymer segments, which is partly hindering globule formation.