The effect of substrate modification on the ordering and dewetting behavior of thin liquid-crystalline polymer films

Liquid-crystalline polymers with maleic acid anhydride moieties in the back bone and methoxybiphenyl groups in the side chain have been investigated wi th respect to the ordering in and stability of thin films on chemically mod ified silicon substrates. For unmodified silicon wafers, it is known that t he polymer-substrate interaction induces parallel layering in the film, res ulting in rather stable films. When the surface groups are changed, i.e., b y silylation, the interaction between the polymer and the surface can be tu ned. This results in unstable films with a dewetting behavior that is stron gly temperature and substrate dependent. At low temperatures in the mesopha se holes nucleate, which are encircled by unstable rims. Upon annealing at higher temperature the rim instability decreases and the dewetting velocity increases. This feature also occurs for another completely different side- chain liquid-crystalline polymer with a methacrylate backbone and cyanobiph enyl groups in the side chains. We ascribe the peculiar dewetting behavior to the presence of polycrystalline domains in the thin films. Especially th eir size and orientation and their ability to deform under shear are held r esponsible for the rim instabilities and, consequently, for the droplets re maining behind in the dry patches.