(670c) Study on Linear Viscoelastic Relaxation of Polymers Near and Above Glass Transition

The linear viscoelastic G’ and G” response has been measured for a series of crosslinked polymers. Shear moduli isotherms above the glass transition temperature were superposed and the associated relaxation spectrum was determined via Tikhonov regularization. The relaxation spectrum was determined in the α+ region, which is defined as relaxation processes that are longer than the α relaxation (i.e. glass transition region) and shorter than the entanglement relaxation, terminal relaxation response or rubbery plateau. The α+ region spans between 2 to 4 logarithmic decades in relaxation time. With finer discretization in the regularization procedure, the α+ peak(s) in the relaxation spectrum evolves from a single-broad peak or a convoluted shoulder on the α peak, to one or more clearly separated, single relaxation time, Debye peaks.

The presence of discrete Debye processes in the α+ region is observed for variety of materials, including crosslinked epoxy resins that were experimentally measured in this study, linear polymers with and without entanglements, oligomers and some glass-forming small molecules. The α+ Debye peaks were observed in dynamic mechanical and dielectric data. Several hypotheses have been previously proposed to describe relaxation processes in the α+ region, including network relaxation, entanglements and secondary molecular interactions like a hydrogen bonding network; however, none of these mechanisms provide a unified explanation of the origin α+ peak for this diverse range of materials. The postulate that the relaxation processes are just an extension of the relaxation processes in the glass transition region is being investigated, where the objective is a unified origin of the α+ relaxation process.

KEY WORDS: dynamic mechanical analysis, relaxation spectrum, glass transition