Segmental relaxation was measured for a series of polysytrenes (PS) of varying molecular weight. The normalized Arrhenius plots of segmental relaxation times (fragility curves) become less steep for shorter chain lengths. Since the shape of the relaxation function is invariant to molecular weight, such behavior is a deviation from the usual correlation between time and temperature dependencies. Although the dependence of relaxation times on Tg-normalized temperature is weaker (less fragile), lower molecular weight PS has a larger heat capacity change at Tg. This result is at odds with an interpretation of temperature dependencies in terms of the degrees of freedom available to the glass upon transition to the viscous liquid.

Steady-state photoluminescence, time-resolved photoluminescence, and x-ray photoelectron spectroscopy have been used to study the electrical and chemical properties of GaAs surfaces exposed to inorganic and organic sulfur donors. Despite a wide variation in S2-(aq) concentration, variation of the pH of aqueous HS-solutions had a small effect on the steady-state n-type GaAs photoluminescence intensity, with surfaces exposed to pH = 8, O.l-M HS -(aq) solutions displaying comparable luminescence intensity relative to those treated with pH = 14, 1.0-M Na2S*9H20( aq). Organic thiols (R-SH, where R = -CH2CH2SH or -C6H4C1) dissolved in nonaqueous solvents were found to effect increases in steady-state luminescence yields and in time-resolved luminescence decay lifetimes of ( lOO)-oriented GaAs. X-ray photoelectron spectroscopy showed that exposure of GaAs surfaces to these organic systems yielded thiols bound to the GaAs surface, but such exposure did not remove excess elemental As and did not form a detectable As2S, overlayer on the GaAs. These results imply that complete removal of As0 or formation of monolayers of AZ@, is not necessary to effect a reduction in the recombination rate at etched GaAs surfaces. Other compounds that do not contain sulfur but that are strong Lewis bases, such as methoxide ion, also improved the GaAs steady-state photoluminescence intensity. These results demonstrate that a general class of electrondonating reagents can be used to reduce nonradiative recombination at GaAs surfaces, and also imply that prior models focusing on the formation of monolayer coverages of As2Ss and Ga2S3 are not adequate to describe the passivating behavior of this class of reagents. The timeresolved, high level injection experiments clearly demonstrate that a shift in the equilibrium surface Fermi-level energy is not sufficient to explain the luminescence intensity changes, and confirm that HS -and thiol-based reagents induce substantial reductions in the surface recombination velocity through a change in the GaAs surface state recombination rate.

The segmental relaxation properties of a low molecular weight (4.6 kg/mol), cyclic polystyrene (PS) were characterized. The sample was obtained by fractionation using HPLC at the chromatographic critical condition, which yields a ring uncontaminated by its linear precursor. Both the glass temperature and the temperature dependence of the segmental relaxation times for the ring PS were equivalent to the high molecular weight limiting values for the linear polymer. These results are interpreted by considering the configurational mobility of a polymer lacking chain ends.

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The linear viscoelastic properties of 1,4-polyisoprene (PI; M W = 504 000) were measured by combined mechanical and dielectric spectroscopies. For local segmental motion, the respective mechanical and dielectric relaxation times, although differing substantially in magnitude, have identical temperature dependencies. The shape of the segmental relaxation function itself was sensitive to temperature. The data, obtained over a 160 degree range of temperatures, could be reduced to yield ostensibly satisfactory master curves; however, detailed analysis reveals the usual breakdown of time−temperature superpositioning in the glass transition zone. Given the near equivalence of the respective breadths of its terminal and segmental dispersions, thermorheological complexity in PI is unanticipated by theory. In the plateau zone, the mechanical spectrum of PI exhibits power law behavior, with an exponent (0.21) close to a proposed universal value.