Communications to the Editor Outer-Surface-Induced Crystallization of Semirigid Polymer Films P. Damman,* M. G. Zolotukhin, and D. Villers Laboratoire de Physicochimie des Polyme ` res, Universite ´ de Mons-Hainaut, 20, Place du Parc, B-7000 Mons, Belgium V. M. Geskin and R. Lazzaroni Service de Chimie des Mate ´ riaux Nouveaux, Universite ´ de Mons-Hainaut, 20, Place du Parc, B-7000 Mons, Belgium Received July 19, 2001 Revised Manuscript Received October 9, 2001 Introduction. Rubbed polymer films are widely used in flat-panel liquid crystals (LC) displays; they are used to induce the alignment of LC molecules parallel to the sliding direction. From a large number of studies, it is now clear that the polymer chains near the surface are aligned during rubbing and act as a template that induces order within the LC layer via intermolecular interactions. This “epitaxial” model was confirmed by a wide set of experiments based on direct optical retardation measurements, 1,2 surface second harmonic generation, 2,3 grazing incidence X-ray diffraction (GIXD), 4 and near-edge X-ray absorption fine structure. 5 From optical retardation measurements, Van Aerle et al. proposed that the penetration depth of rubbing varies from 10 to 60 nm depending on the experimental conditions. 2 This proposal was further confirmed by GIXD experiments, which allows thickness mapping by varying the incidence angle of the X-ray beam. 4 Studies of rubbed polymer thin films also reveal a relaxation phenomenon around the glass transition. In fact, when heated close to or slightly above the glass transition temperature (T g ), rubbed amorphous poly- styrene (PS) films exhibit a drastic decrease of the surface roughness and a complete vanishing of the optical retardation. 6-8 From the thermal dependence of the surface relaxation, T g was found to vary with film thickness. Furthermore, the study of PS thick films showed that molecules closer to the polymer-air inter- face relax more easily than bulk molecules. 6 In fact, there is a consensus to agree that, for free-standing and supported thin films, a decrease of T g is associated with a decrease of the film thickness. 9-11 For instance, a decrease of T g by about 40 deg was reported for supported PS thin films 10 nm thick. 11 In this paper, we report on the influence of rubbing and surface relaxation of amorphous polymers on the cold-crystallization process. We selected polymers with relatively low T g and able to partially crystallize from the glassy state: poly(ethylene terephthalate), PET, and poly(aryl ether ketone ketone), PEKK. 12 The surface relaxation and subsequent crystallization were inves- tigated by FTIR spectroscopy, atomic force microscopy (AFM), and optical microscopy (OM) observations. Experimental Section. a. Materials. PET samples were purchased from Aldrich and used without further purification. The PEKK samples were obtained by precipitation electrophilic polycondensation according to ref 12 and correspond to the following repeat unit: The T g of PET and PEKK samples are close to 80 and 160 °C, respectively. The polymer films were obtained by casting from dichloroacetic acid solutions onto glass slides and slow evaporation of the solvent at 100 °C overnight (these samples are called CAST). Before their use, the cast films were melted for 60 s and quenched in water (these amorphous samples were hereafter * To whom correspondence should be addressed. E-mail: damman@umh.ac.be. Figure 1. Optical micrographs, between crossed polars, of rubbed (a) and cold-crystallized (b) PET thin films. The rubbing conditions were the following: pressure ) 20 g/cm 2 , transport velocity ) 1 cm/s, and rubbing distance ) 60 cm. The crystallization was performed by heating the RUB films from 30 to 166 °C at 10 °C/min. 2 Macromolecules 2002, 35, 2-5 10.1021/ma011278u CCC: $22.00 © 2002 American Chemical Society Published on Web 11/28/2001