A basic understanding of the properties of thin polymer films is of fundamental importance for developing applications in nanotechnology. Results of energy and angle dispersive x-ray reflectivity measurements on polymer thin films as a function of temperature exhibit reversible negative thermal expansion below the glass transition temperature T(g). Above T(g), the thickness expansion becomes almost equal to the expected bulk volume expansion. These results could be explained on the basis of evolution of disorder with temperature at the interfaces, chain entanglement and associated entropy changes.

The double-layer structure of Langmuir−Blodgett (LB) multilayer assemblies formed by a thermotropic polyacrylate with 2-hydroxyethyl and [[[[(2-methylbutyl)oxy]phenyl]azo]phenoxy]hexyl side groups is modified by annealing and on ultraviolet (UV) irradiation. Both procedures cause order−disorder transitions. The interaction with linearly polarized visible light results in a reversible reorientation of the azobenzene side groups maintaining the layered structure. In this way the optical anisotropy of the LB films is modified reversibly and continuously. In the original multilayer the photoreorientation process is restricted by the dense packing of the side groups. But the process takes place after weakening the intermolecular interactions. This has been achieved by a suitable preparation of the films, by annealing of the thermotropic polymer, or by a photochemically induced decrease of the supramolecular order. The photogeneration of anisotropy is governed by the molecular aggregation, the orientational order, the thermotropic ordering tendency of the polymer to establish a homeotropic alignment, and the irradiation conditions (wavelength, power density, polarization direction). Continued irradiation or annealing above the glass transition temperature (T g) results in a decrease of absorbance indicating smaller tilt angles. The photoinduced dichroism is long-term stable but can be erased by irradiation and heating.

The glass transition process gets affected in ultrathin films having thickness comparable to the size of the molecules. We observe systematic broadening of the glass transition temperature (T(g)) as the thickness of an ultrathin polymer film reduces below the radius of gyration but the change in the average T(g) was found to be very small. The existence of reversible negative and positive thermal expansion below and above T(g) increased the sensitivity of our thickness measurements performed using energy-dispersive x-ray reflectivity. A simple model of the T(g) variation as a function of depth expected from sliding motion could explain the results.

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