Side-Chain Engineering of Polyphenylacetylene Derivatives for Tuning the Self-Assembly and Gas Permeability Properties

The self-assembly and gas permeability properties of polyphenylacetylene derivatives were investigated as a function of the side-chain structures. When 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) units were introduced into the side chain of poly(4-substituted phenylacetylene) derivatives by the [2+2] cycloaddition-retroelectrocyclization reaction, a remarkable morphology change was observed in the atomic force microscopy (AFM) images of their cast films. The precursor polyphenylacetylene showed an unstructured homogeneous surface image, while the 50 mol% TCBD-adducted polymer formed fibrous nanoobjects. Furthermore, the fully TCBD-adducted polymer exhibited a spherical morphology. The X-ray diffraction (XRD) measurements of these thin films suggested peaks ascribed to the side chain lengths. The gas permeability and diffusion coefficients of O2 and N2 were measured using the self-standing polymer membranes. The O2 permeability (PO2) was 2.6-2.9 times greater than the corresponding N2 permeability (PN2) irrespective of the side-chain moieties. However, the diffusion coefficients were dramatically reduced by formation of the TCBD units, probably due to the decrease in the void spaces caused by the interactions with the nitrile groups. The different substitution patterns of the side-chain phenyl group also caused different properties.