Most polymers solidify into a glassy amorphous state, accompanied by a rapid increase in the viscosity when cooled below the glass transition temperature (T(g)). There is an ongoing debate on whether the T(g) changes with decreasing polymer film thickness and on the origin of the changes. We measured the viscosity of unentangled, short-chain polystyrene films on silicon at different temperatures and found that the transition temperature for the viscosity decreases with decreasing film thickness, consistent with the changes in the T(g) of the films observed before. By applying the hydrodynamic equations to the films, the data can be explained by the presence of a highly mobile surface liquid layer, which follows an Arrhenius dynamic and is able to dominate the flow in the thinnest films studied.

A uniform alignment of liquid crystal (LC) with finite pretilt was observed on microtextured substrates that were lithographically fabricated with alternating horizontal and vertical corrugations. As the period of alternation was decreased toward 0:8 m, the nematic LC alignment on these substrates changed from inhomogeneous in plane, copying the substrate corrugations, to a uniform configuration with a large pretilt of 40 . This transition is pertinent to a frustrated boundary wherein a lowering in the LC elastic energy due to spatial variation in the LC orientation compromises an increase in the surface anchoring energy. A model based on this idea demonstrates good agreement with the experiment. This result may open up a new arena for tailoring substrate characteristics for LC alignment. DOI: 10.1103/PhysRevLett.91.215501 PACS numbers: 61.30.Hn, 61.30.Dk, 61.30.Eb Substrate conditioning for the alignment of liquid crystal (LC) molecules is essential to the operation of LC displays. Most existing techniques used to produce the alignment effect involve uniform treatment of the substrate surface. An unconventional approach to uniform LC alignment, based on inhomogeneous patterning of the substrate, was proposed some time ago [1,2]. The idea is to create a situation wherein the LC director, in coping with the inhomogeneous substrate pattern, acquires so large an elastic energy that the LC has to compromise with a uniform configuration. In general, this frustrated boundary condition can be achieved by making the pattern period small enough. A few versions of this idea, including the use of inhomogeneous surfactant treatment [1,3] and inhomogeneous rubbing [4], have been reported successful. More recently, an interesting application of these substrates in multistable display, which may potentially lead to lower energy consumption, was demonstrated [5,6]. In these demonstrations, the LC director makes zero pretilt with the substrate, and the switching between different stable LC states was achieved by varying an in-plane electric field. However, out-of-plane switching is preferred since the electric field can be more uniform. But that requires the LC pretilt to be finite [7], which has not been found in any reported embodiments [1,3,4]. In yet some designs, a large LC pretilt is crucial as it widens the range of LC elastic constants usable, which would otherwise be too narrow to be practical [8]. Here we show that on lithographically made substrates with alternating horizontal and vertical corrugations, the nematic alignment undergoes an orientational transition that results in a large pretilt angle (40 ). A simple model used to simulate the transition not only showed good agreement with the experiment but also revealed that the polar surface energies of the frustrated boundary are only 1=10 of typical values. Weak surface anchoring energies are virtuous for lowering device energy consumption and response time [9]. Figure 1 shows a subset of the microtextured substrates employed in this study. They consist of arra...

Spin-coating is a common method of making thin polymer films. Recent experiments show that polymer films produced by this method are highly nonequilibrated. By monitoring the temporal evolution of the surface structure of freshly spin-cast polystyrene films on Si with molecular weights, 2.3 ≤ M w ≤ 393 kg/mol, we find that the relaxations can be fully accounted for by thermal excitations of surface capillary waves on the film surface. Modeling of the data based on this relaxation scheme leads to excellent agreement between the viscosity of the films and that of the bulk polymers. Our results provide compelling evidence that thickness uniformity is the major cause of the nonequilibration of the films.