Linear viscoelastic relaxation in the α and α plus regions of linear polymers, crosslinked polymers and small molecules

The dynamic linear viscoelastic behavior in the region above T-g was studied for a series of diglycidyl ether of bisphenol-A (DGEBA) based epoxy resins cured with 4,4'-methylenedianiline (MDA) and 4,4'-diaminodiphenyl sulfone (DDS) as well as a linear DGEBA-based polymer - phenoxy. Master curves were constructed from the frequency dependent storage and loss isotherms. The relaxation spectra were determined for the MDA and DDS cured systems and also, using the reported dielectric relaxation data (Casalini et al. Phys. Rev. B, 1997), for the DGEBA monomer. For the tighter DGEBA networks, a single broad relaxation peak was observed vs looser DGEBA networks and a phenoxy thermoplastic where two relaxation processes were observed: the main alpha-process and an alpha+-process that occurs at longer relaxation times and higher temperatures. The spectrum for DGEBA monomer exhibited only an alpha-process with a very abrupt end. The temperature dependence of loga T follows one of the two distinct branches when adjusted for T-g: the tight DGEBA networks all exhibit a steep response; in contrast, the DGEBA monomer, phenoxy and looser DGEBA networks exhibit a less steep response. Relaxation spectrum for the loosely crosslinked Epon1009-MDA system consists of alpha and alpha+ components; intriguingly, loga(T) for the alpha-process follows the steep branch vs the alpha+-process following the less steep branch.