An investigation of the relaxation phenomena of polymeric and small organic glass formers in the glass transition region.

摘要

A series of poly(sulfone) model compounds, which have a common structure of diphenyl sulfone with a variety of end groups, were synthesized in order to study the relaxation phenomena in the glass transition region. The effects of annealing time on enthalpy relaxations during heating through the glass transition region were characterized by DSC, and the experimental results were fit with the Narayanaswamy model with both the traditional linearized shift function and Adam-Gibbs form of the shift function. Both models can qualitatively describe the enthalpy relaxations; however a quantitative description was only achieved at short annealing times, and when a different parameter set was used for each thermal history. Dielectric relaxation of sulfone molecules was examined in the glass transition region in a frequency range of ;The large scale segmental motions that are associated with the glass transition were measured using a specifically selectively deuterated sulfone molecule. Specifically, only proton-proton dipole on one of the inner phenyl ring is NMR active; consequently proton NMR spectra will not be sensitive to motions of the outer phenyl groups nor to phenyl flip motions depending only reorientation of the whole molecule. The results of proton variable temperature experiments and a modified spin alignment experiment indicate that (i) the motional heterogeneity exists in the glass transition region with both slow and fast components and (ii) the fraction of the fast component increases with temperature. The nonexponential loss of correlation was examined in sulfone-d16 using a reduced 4-dimensional exchange NMR experiment. The results indicate that for sulfone-d16 at 291K the selected slow relaxation component remains slow for times less than 10ms while for longer times the slowly relaxing region will begin to move more rapidly. These NMR results support the postulate that amorphous materials are motionally and spatially heterogeneous, and the heterogeneity is dynamic. The implications of a dynamically evolving spatially heterogeneous mobility domain on models for developing macroscopy structural relaxation in glasses are discussed.