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The glass transition (Tg) of thin polystyrene films (ca. 3000 A˚) cast on silicon wafers was determined by a new technique. An ellipsometer was used to determine the refractive index and thickness of the polystyrene films. Tg was determined by measuring the temperature dependence of the refractive index. The change in thickness with temperature was used to calculate the linear and bulk thermal expansion coefficients of the material. A significant shift in Tg, possibly due to strains induced in the cooled films, was observed between heating and cooling for polystyrene films. © 1993 John Wiley & Sons, Inc.

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The persistence length of isotactic poly(hydroxy butyrate) was measured using small-angle neutron scattering. The value obtained from these measurements reflects a high degree of local chain persistence. If this local persistence is accounted for, scattering from these chains can be globally fit with Gaussian scaling. A global scattering function, the unified equation, is used, which decomposes the chain structure into two levels, one corresponding to the Gaussian regime and one to the persistence regime. The persistence length obtained using this global scattering function is compared to that obtained using the graphical approach of Kratky and Porod with good agreement. Additionally, the global fitting approach of Sharp and Bloomfield is also considered. The Kratky and the Sharp and Bloomfield approaches appear to yield different values for the persistence length. Additionally, the Sharp and Bloomfield function does not allow inspection of the component parts of the fit. One advantage of both global functions is that the level of statistical confidence in the persistence length can be determined in a least-squares fit. Another advantage is the removal of ambiguity concerning an apparent regime of non-Gaussian scaling between the persistence scaling regime and the Gaussian regime.

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Arylene-bridged polysilsesquioxanes are an interesting class of porous materials prepared by sol−gel processing of ethoxysilane monomers in which there are two or more trialkoxysilyl groups positioned about an arylene bridging group. The majority of these materials are highly porous with surface areas as high as 1880 m2/g. In an effort to understand the nature of porosity in these materials, small-angle X-ray and neutron scattering were employed to characterize phenylene-, biphenylene-, and terphenylene-bridged polysilsesquioxanes. Phenylene-bridged polysilsesquioxane xerogels and aerogels were also compared to understand the effect of drying protocol on pore structure. The effect of catalyst concentration is also reported for the base-catalyzed system. In all cases studied here, we find evidence for domains in the nanometer range with distinct fractal character. We associate these domains with porosity rather than microphase separation of organic and inorganic moieties. The nature of this porosity depends on the bridging group in a systematic way, but is only weakly dependent on other synthetic parameters such as catalyst type, catalyst concentration, and drying protocol.

Neutron scattering experiments were performed on three molecular weight pairs of symmetric, isotopic blends of poly(dimethylsiloxane) (PDMS) of near-critical composition. Scattering data covering close to 3 decades in size were globally fit using the random phase approximation (RPA) and the Debye function for Gaussian polymer coils using the interaction parameter, χ, and statistical segment length, b, as free parameters. These wide q range fits differ from the standard, narrow q range RPA fits in that the power-law scaling regime and exponential decay regimes, related to b, are accounted for. Values for χ showed a well-behaved linear dependence on inverse temperature. Critical temperatures were estimated from these data. Direct observations of the miscibility limit, through neutron cloud points, were made in several cases which agree to some extent with the extrapolated critical points. Monotonic dependencies in temperature of the coil expansion factor, α, as calculated from the statistical segment length, were observed. Under the assumption that the thermal dependence of α can be described in a Flory−Krigbaum form, this offers a second measure of the critical point in these blends. If coil expansion is accounted for in this way, the noncombinatorial entropic component of χ is observed to vanish in the high-molecular-weight limit in keeping with a Flory−Huggins/Hildebrand description of χ as B/T. The molecular weight dependence of χ supports the view that, after accounting for coil expansion, end-group effects are the sole source of noncombinatorial entropy in this model system.