Molecular dynamics of adsorbed polymer thin films using NMR field cycling relaxometry

摘要

NMR Field cycling relaxometry was used to study polymer thin films formed on porous metal oxides surfaces. The thickness of the film was varied in the range from several monolayers to sub-monolayer nominally. In all cases, a slowing down of motion as reflected by lower relaxation times compared to bulk was observed, and a qualitative deviation from bulk occurred in cases of strong confinements. Two different polymers, poly(dimethylsiloxane) (PDMS) and polybutadiene (PB) were used. As confinements, porous alumina membranes (Anopore) and porous silica rods (Vycor) were used. The substrates differed in both size as well as surface interaction potential. The results from relaxation dispersions were analysed based on model theories available for melts and completely confined systems. The different polymer-substrate systems showed different effects. PB in Anopore showed a simple reduction in relaxation times with the underlying dynamics being similar to the bulk. As the layer thickness decreased, slowing down of motion was seen from the decreasing relaxation times. Poor wetting and weak interactions resulting in a bulk-like layer is discussed as the reason for this behaviour. PB in Vycor showed stronger motional restrictions. PB completely filling in Vycor pores showed a tendency to follow limit II of the reptation model, while partially filled samples showed a much weaker slope compared to the completely filled sample. Although it cannot be conclusively stated, the void space, the surface interactions and a change in characteristic times are discussed as some of the potential reasons. PDMS with differet molecular weights were studied in Anopore membranes. The mobility of all thin films of PDMS in Anopore were considerably reduced pending on the thickness. Relaxation dispersion of films below a certain thickness determined by the chain size showed deviation from bulk while thicker films only showed a reduction in relaxation times but followed bulk dynamics. The deviations occur due to a different weight of chain modes to the relaxation dispersion that can arrive from a change in the 3D topology as well as a change in chain conformations. The results are discussed on the basis of a model dividing the layer into two zones with different characteristics. The different effects arrive due to different contributions from the two zones and the dominance of one of them. Study on PDMS chain conformations were carried out using paramagnetic relaxation enhancement agents. The study clearly showed that a major part of the chain exists on the surface however did not conclusively give proof to flattened chain conformations at submonolayer coverages. Uneven distribution of the paramagnetic relaxation agent and the weak adsorption of the chain to the surface resulting in exchange dynamics are discussed as potential factors influencing the results.