Structure and chain orientation in thin films of side-chain liquid crystalline polymers

With the aim of investigating the effect of the side-chain length on the order and structure of thin films of side-chain liquid crystalline polymers, poly(oxyethylene)s containing various lengths of n-alkylsulfonylmethyl side chains (CH₂SO₂(CH₂)_m-1CH ₃; m = 5, 7, 10, and 16) were synthesized. The inner structure, surface structure, and adhesion behavior of thin films of these polymers were investigated. An X-ray reflectivity study of a polymer film spin-coated onto a silicon wafer showed that upon annealing above the glass transition temperature, the film has a lamellar structure in which the ordered layers are parallel to the substrate surface. When alkyl chain length in the side group is longer than the critical length, the layered structure is less perfect. Also, the air-polymer surface and the interface between layers are not smooth. This result implies that side chains are presumably loosely oriented. It can be expected that the critical length of the side chain is between m = 11 and m = 16. Near-edge X-ray absorption fine structure spectroscopy results also confirm that side chains near the top surface are loosely oriented when m is larger than the critical value. Force-displacement curves measured with atomic force microscopy also show an abrupt change in the adhesion force when the alkyl side chain length in the side group is longer than the critical length, which results from the side-chain conformation.