New techniques for NMR spectroscopy of monomer-thin liquid polymer films

摘要

Polymer chains confined between two closely spaced planar interfaces are found in many practical applications and they have been the subjects of numerous theoretical, numerical and experimental studies. Despite this body of work, very few measurements have been made that directly probe the orientational order and dynamics of individual chain segments in such ultrathin films. It is well known that anisotropic NMR interactions can reveal this type of information but application of NMR to ultrathin films has been hampered by low sensitivity. With one exception, NMR studies have been limited to "films" deposited either on the external surfaces of randomly oriented particles or on the internal surfaces of random or partially oriented pores. In the case of very high surface area powders, such as silica gel or activated carbon, both types of surfaces may exist and the precise nature of the confining environment may be difficult to determine. In one recent study, thin films of perdeuterated polydimethylsiloxane PDMS(D) were prepared on planar surface composed of polystyrene chains end-grafted onto silicon wafer substrates. Deuterium NMR spectroscopy revealed rapid and anisotropic segmental motions with uniaxial symmetry about the surface normal. Unfortunately, sensitivity issues limited the minimum film thickness that could be measured to approximately 6 nm, a dimension that is many times greater than the persistence length of PDMS chains. Not surprisingly, the maximum order parameter |S| measured in this study was only on the order of 10~~(-3).