1H-NMR-Investigation of the Phase Transition of Thermo-Reversible Polymers in Solution and at Interfaces

Abstract

The thermoreversible polymer Poly-N-isopropylacrylamide (PNIPAM) shows a coil to globule transition at 32°C in solution. We address the question of whether charged groups in the polymer are affecting the phase transition, and study their influence in solution and at the interface. IH spectra under liquids conditions and PFG-NMR diffusion coefficients are measured at various concentrations in solution. A decrease of the IH liquid signal at the phase transition temperature monitors the formation of solid globules. The transition is sharp and for the copolymer no significant influence of the charges on the transition is found. PFG­ NMR diffusion coefficients prove that the size of the polymer coils is maintained until close to the transition. Both polymers are adsorbed to colloidal silica (Cab-O-Sil) to monitor the phase transition behaviour in the restricted geometry of an adsorption layer. A liquid IH signal from loops and tails is observed, the intensities are decreasing with temperature, which is interpreted as a phase transition of the loops and tails. The phase transition is substantially broader than in solution, especially at low surface coverage. For the copolymer, the transition is further broadened, and mobile segments remain even above the phase transition. We attribute this to a comparatively mobile arrangement of the copolymer layer: Due to the electrostatic repulsion from the surface and between polymer segments, the copolymer layer is probably confined in a configuration extending further from the interface. Thus, while the copolymer charges do not affect the phase transition in solution, they determine the phase transition properties at a charged interface.