Polymer nanocomposite sub-micron thin films have promising applications such as in flexible electronics, photovoltaic devices and biomedical engineering. However, there is insufficient understanding on their structures and properties, partially due to the difficulty in sample fabrication and characterization caused by the confined geometries. The present research aimed at overcoming this difficulty and investigating structures and properties of polymer nanocomposite sub-micron thin films. Three representative nanocomposite systems were chosen, samples were prepared by solution mixing followed by casting techniques, structures and properties were characterized using conventional and novel experimental methods including SIEBIMM for elastic modulus measurements, and results were compared with previous studies on bulk polymer nanocomposites.
Firstly, bio-derived polymer / clay nanocomposite thin films were prepared using poly(lactic acid) (PLA) and polycaprolactone (PCL) matrices and Cloisite 30B (C30B) nanoparticles. X-ray scattering and TEM results suggested that C30B nanoparticles were partially intercalated and partially aggregated in PLA, but almost completely intercalated in PCL. The dispersion of C30B also improved continuously with increasing PCL content in PLA:PCL blends. In addition, C30B nanoparticles were found oriented parallel to the film surface, a morphology useful for barrier applications. The modulus enhancement by C30B was observed in both PLA and PCL matrices with only 2 wt% addition. At higher loadings of C30B, the elastic modulus of PLA/C30B remained stable, while that of PCL/C30B increased continuously due to improved dispersion of C30B.
Secondly, magnetic polymer nanocomposite thin films were prepared using the polystyrene (PS) matrix and PS-coated cobalt (Co) nanoparticles. Co nanoparticles were either randomly distributed or aligned to form 1-D chains as directed by an external magnetic field. TEM and GISAXS results revealed that the 1-D chains consisted of individual Co nanoparticles attached one after another. The flow-coated nanocomposite thin films retained ferromagnetism from the Co nanoparticles. More importantly, the elastic modulus of nanocomposite thin films was dependent on the alignment of Co nanoparticles. Spherical Co was found incapable of improving PS modulus, possibly due to the isotropic geometry. In contrast, cylindrical Co enhanced the modulus of PS by up to 10% when aligned perpendicular to buckling.
Lastly, polymer nanocomposite thin and ultrathin films (thickness below 50 nm) with carbon-based nanoparticles were studied. C60 was found to enhance the PLA and poly(methyl methacrylate) (PMMA) matrices, but not PS or polysulfone (PSU). When added to the as cast block copolymer PS-b-PMMA matrix, no modulus enhancement was observed, possibly concealed by the PS block. The moduli of PS/C60 and PMMA/C60 ultrathin films were found to decrease rapidly with decreasing film thickness, similar to previous studies on homopolymer ultrathin films. The dispersion of graphene oxide was found to be poor in PS and moderate in PMMA due to hydrogen bonding, and was intermediate in the PS-b-PMMA block copolymer. Finally, PS/SWCNT formed a microcomposite instead of a nanocomposite due to the extremely strong interaction between individual nanotubes.