NEAR-SURFACE ALIGNMENT OF POLYMERS IN RUBBED FILMS

RUBBED polymer films (generally polyimides) are used in flat-panel dis plays to control the alignment of liquid crystals in contact with the polymer(1-8), a phenomenon first discovered by Maugin(1) in 1911. Buff ing the film with a cloth produces liquid-crystal alignment in the rub bing direction. Several mechanisms have been proposed to explain this effect, The generation of microgrooves or scratches on the polymer sur face during rubbing has led to the suggestion that alignment is the re sult of long-range elastic effects induced by these surface features(3 -5). Others have suggested that the polymer chains near the surface ar e aligned during rubbing and that these then serve as templates for li quid-crystal alignment(6-13). Other studies(10-12) have implied that b oth mechanisms might be operative. Here we present X-ray scattering me asurements which show unambiguously that rubbing a polyimide film caus es nearsurface alignment of the polymer molecules. For a film 200 nm t hick, most of the polymer chains within a thin surface region (about 5 nm thick) are aligned in the rubbing direction; for a 6-nm film essen tially all of the chains are aligned within 20 degrees of the rubbing direction. This marked orientation of the near-surface chains at tempe ratures far below the bulk glass transition temperature shows that the mechanical properties of the near-surface region differ significantly from those of the bulk polymer.