From molecules to supramolecular structure: Self assembling of wirelike poly(p-phenyleneethynylene)s

Supermolecular structures of lamellae folded into rodlike features were observed in thin films of dialkylpoly(p-phenyleneethynylene)s (R = 2-ethylhexyl, 2,5,5-trimethylhexyl, nonyl, dodecyl). The polymers have been melted onto oxidized silicon wafers and spontaneously wetted the wafers to form thin, micrometer thick films. These films were investigated by AFM microscopy and wide- and small-angle X-ray scattering. In all cases, spontaneous self-assembly of the PPEs occurs. The self-assembly process takes place on two length scales leading to lamellar ordering on the 2 nm scale and to rodlike structures on the 20-50 nm scale. The lamellar structure is observed by X-ray (all samples) where the large scale structure is detected by AFM. For high molecular weight PPE, the lamellae become rigid enough for the order to persist over larger domains, allowing the direct detection of the lamellae by AFM.