We have used photon correlation spectroscopy and quartz crystal microbalance techniques to examine the relaxation dynamics of ultrathin (hϽ400 Å) polystyrene films in both supported and freely standing geometries. These studies probe relaxation dynamics of polymer films in which the glass transition temperature (T g ) is reduced below the bulk value. Both the shape of the relaxation function and the dependence of relaxation time on temperature above the glass transition are remarkably similar to that of the bulk polymer, though the range of relaxation times is shifted according to the shift in T g . The results indicate that the microscopic relaxation dynamics of thin films remain similar to that of the bulk polymer even, in the extreme case in which the T g value is shifted more than 70 K below the bulk value. ͓S1063-651X͑98͒50508-9͔PACS number͑s͒: 68.60. Ϫp, 61.20.Lc, 62.80.ϩf, 64.70.Pf The increasing number of applications for polymer thin films has spurred a surge of activity aimed at increasing our understanding of the properties of these materials. Polymers in a thin film configuration may have physical properties different from those of the bulk material due to interfacial interactions and effects of molecular confinement. Of particular interest are anomalies in the glass transition temperature T g , which have been recently reported for thin polymer films. The T g values have been measured for polymer films supported by substrates ͓1-5͔ as well as for freely standing films ͓4,6͔. These experiments reveal that the T g values decrease for decreasing film thickness unless there is a strongly attractive polymer-substrate interaction ͓2͔. Films of polystyrene ͑PS͒ have been extensively studied on a number of different substrate materials encompassing both wetting ͓1,3,5͔ and nonwetting ͓4͔ systems. The measured T g values show only weak substrate dependent behavior. While the strength of the polymer substrate interaction does not strongly influence the measured T g for supported PS films, the simple presence of a substrate has been shown to alter dramatically the T g value in recent studies involving freely standing films ͓4,6͔.The glass transition temperature is intrinsically related to structural relaxation and thus in polymers to the segmental mobility. Structural relaxation dynamics of glass forming materials are generally well described by the stretched exponential functionwhere the stretching parameter ␤ describes the shape of the relaxation time distribution. The variation of the average relaxation time ͗͘ϭ͐(t)dt with temperature T generally obeys the empirical Vogel-Tammann-Fulcher ͑VTF͒ equationwhere the parameter 0 is a microscopic relaxation time, B describes the fragility of the glass former, and T 0 for polymers is generally T 0 ϳT g Ϫ50 K. For bulk polymers the relaxation behavior is well characterized by Eqs. ͑1͒ and ͑2͒. In order to help elucidate the origin of the large T g reductions observed for thin polymer films, a systematic study of the relaxation dynamics near T g must be performed. ...

The microscopic dynamics of the planar, multilamellar lipid bilayer system 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) has been investigated using quasielastic neutron scattering. The DMPC was hydrated to a level corresponding to approximately nine water molecules per lipid molecule. Selective deuteration has been used to separately extract the dynamics of the water, the acyl chains, and the polar head groups from the strong incoherent scattering of the remaining hydrogen atoms. Furthermore, the motions parallel and perpendicular to the bilayers were probed by using two different sample orientations relative to the incident neutron beam. For both sample orientations, the results showed an onset of water motions at 260 K on the experimental time scale of about 100 ps. From lack of wave-vector dependence of the onset temperature for water motions, it is evident that the observed water dynamics is of mainly rotational character at such low temperatures. At 290 K, i.e., slightly below the gel-to-liquid transition around 295 K, the nature of the water dynamics had changed to a more translational character, well described by a jump-diffusion model. On the limited experimental time and length (about 10 A) scales, this jump-diffusion process was isotropic, despite the very anisotropic system. The acyl chains exhibited a weak onset of anharmonic motions already at 120 K, probably due to conformational changes (trans-gauche and/or syn-anti) in the plane of the lipid bilayers. Other anharmonic motions were not observed on the experimental time scale until temperature had been reached above the gel-to-liquid transition around 295 K, where the acyl chains start to show more substantial motions.

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