Thermoreversible polymers adsorbed to colloidal silica:: A 1H NMR and DSC study of the phase transition in confined geometry

The phase transition of the thermoreversible polymer PNIPAM, which shows a coil-to-globule transition due to the lower critical solution temperature (LCST) behavior in aqueous solution, is investigated in the restricted geometry of an adsorption layer. Furthermore, a charged copolymer is studied to investigate the influence of charges on the phase transition. Both polymers are adsorbed to colloidal silica and studied by H-1 NMR and differential scanning calorimetric (DSC) experiments. In solid state 1H NMR relaxation experiments the signals of solid spins in trains and of liquid spins in tails and loops can be identified. 1H liquid state spectra detect the phase transition of the loops and tails into immobile segments with increasing temperature. The transition is broadened as compared to the polymer in solution, especially at low surface coverage. For the copolymer, the transition is incomplete, since mobile segments remain even at high temperature. They are attributed to electrostatic repulsion from the surface, leading to a mobile arrangement of the copolymer layer. Micro-DSC experiments confirm the finding of an increased width of the transition at the interface, which depends on the surface coverage. Furthermore, an increase of the transition temperature with decreasing polymer amount is observed, which is most pronounced for the copolymer, and is a further indication of an electrostatically hindered phase transition. In conclusion, despite a negligible influence of a low fraction of charges on the phase transition in solution, the phase transition in adsorption layers is very sensitive to charged segments. Combining NMR and DSC methods, local information on the transition behavior of different polymer segments at the interface, such as loops and tails, or charged monomers, can be obtained.