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 symmetric stretch modes for the anions in poly(propylene glycol) (PPG) complexed with NaCF3SO3 and LiClO4 at an ether oxygen of alkali metal cation ratio of 16:1 have been studied as a function of the molecular weight of PPG and temperature using Raman spectroscopy. The splitting of these modes has been analyzed in terms of ‘‘free’’ anions and ion pairs. We have observed that the number of ion pairs increases with increasing molecular weight and increasing temperature. The effect is greater in the PPG–NaCF3SO3 complex than in the PPG–LiClO4 complex. We have found a simple exponential relationship for the number of free anions in a master-plot representation using a shift factor T* which increases with increasing molecular weight. These results are discussed in terms of the entropy and energetics of polymer–salt systems.

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Ion pairing effects in poly(propylene glycol)-salt complexes as a function of molecular weight and temperature: A Raman scattering study using NaCF3SO3 and LiClO4 J. Chem. Phys. 94, 6862 (1991); 10.1063/1.460265 On the ion association at low salt concentrations in polymer electrolytes; a Raman study of NaCF3SO3 and LiClO4 dissolved in poly(propylene oxide) J. Chem. Phys. 94, 6296 (1991); 10.1063/1.460418 A 7Li nuclear magnetic resonance study of LiCF3SO3 complexed in poly(propyleneglycol) J. Chem. Phys. 94, 1803 (1991); 10.1063/1.459954Elastic and dynamic properties of polymer electrolytes: A Brillouin scattering study of poly(propylene glycol)-NaCF3SO3 complexes Raman scattering measurements have been carried out on poly (propylene oxide) complexed with NaCF 3 S03 salt of concentration O:M = 30: 1 (where O:M is the PO:Na ratio) over a temperature range of 186-360 K in order to study ion-ion associations of the dopant salt and their temperature dependence. Splitting of the symmetric stretching mode of the CF 3 S03anion into a double band was observed and attributed to the existence of different environments of the anions. A two-component band analysis led to the identification of coexisting dissociated free ions and ion pairs, suggested to be in contact. Below the glass transition temperature, T g , the intensity of the mode corresponding to the free ions was more or less constant with temperature; the amount of free ions in the glassy state was found to be about 84% of the total salt concentration. Above Tg the amount of dissociated free ions decreased rapidly with temperature in an Arrhenius-type behavior. The resulting reduction of the number of charge carriers has little influence on the conductivity, which is reported to dramatically increase with increasing temperature. It is concluded that the major factor determining the temperature dependence of the conductivity is the mobility rather than the number of charge carriers.

Raman scattering of B203 has been performed from room temperature to 1273 K to study structural changes as the glass transforms into the melt via the supercooled regime. It is found that a structural model containing threefold six-member planar boroxol rings and chains of BO3 triangles can explain the experimental spectra. From the behavior of the internal vibrations of the boroxol rings and the BO3 triangles, we conclude that these molecular units do not change significantly as the temperature increases whereas the connectivity of the units is strongly affected. Still, network connectivity is observed in this "strong" liquid far above T, where the presence of transverse acoustic modes have also been reported.This supports the idea of a relation between structural properties and the dynamics of the liquid-glass transition as suggested in the "strong-fragile" classification scheme. The structural changes are demonstrated by the intensity profile of the spectra. It is shown that the strongest vibrational mode at 808 cm, attributed to the breathing mode of boroxol rings, decreases rapidly in intensity as the temperature is raised above the glass transition temperature Tg. The high-frequency multicomponent band at 1200-1600 cm also displays anomalous temperature behavior above Tg. A significant redistribution of the intensity from the two narrow lines at -1210 and -1260 cm ' into the broad band at -1325 cm is found. The observed effects are consistent with a gradual breakup of boroxol rings, which change into chains of BO3 triangles as the temperature increases above Tg. From a detailed analysis of the temperature dependence of the spectra, the structure is estimated to consist of about half of the number of atoms in boroxo1 rings at Tg. Heating the glass to the melting temperature leads to breaking of about -, ' of the boroxol rings.

The structural relaxation behavior of a strong glass former B 2 O 3 has been investigated over broad temperature ͑300-1275 K͒ and frequency ͑0.5 GHz-10 THz͒ ranges using depolarized light scattering. The spectra clearly show nonmonotonic temperature behavior with some dynamical crossover at about T c Ϸ800-900 K. Above T c the spectra develop qualitatively according to the general scenario predicted by the mode-coupling theory ͑MCT͒, including a fast ␤ process and a much slower ␣ process in addition to a vibrational contribution. However, there is disagreement between the observed functional form of the fast relaxational dynamics and that predicted by MCT. The disagreement seems to be related to the influence of low-lying vibrational contributions, the so-called boson peak, which generally seems to be more pronounced in strong glass formers. Below T c the spectra do not follow MCT predictions, not even qualitatively; the main signature is a decrease of the level of the fast relaxation spectrum. Analysis in terms of an alternative phenomenological approach, in which the fast relaxation contribution is related to the damping of the vibrational modes ͑giving rise to the boson peak͒, reveals some crossover of the damping rate at about the same temperature T c as the crossover of the fast relaxation dynamics itself, and with similar temperature dependence as that recently reported for the Brillouin linewidth. We suggest that these variations are related to the temperature dependence of the relative strength of the fast relaxation. We show that apart from differences in the vibrational contribution, strong and fragile glass formers differ concerning the temperature range of transition ͑between T c and T g ), being narrow for fragile systems (T c /T g Ϸ1.2) and broad for stronger ones (T c /T g Ϸ1.6 for B 2 O 3 ).

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