Polymer chain dynamics in epoxy based composites as investigated by broadband dielectric spectroscopy

Epoxy networks of the diglycidyl ether of bisphenol A (DGEBA) were prepared using 3,3'-and 4,4'-diaminodiphenyl sulfone isomer crosslinkers. Secondary relaxations and the glass transitions of resultant networks were probed using broadband dielectric spectroscopy (BDS). A sub-T-g gamma relaxation peak for both networks shifts to higher frequencies (f) with increasing temperature in Arrhenius fashion, both processes having the same activation energy and being assigned to phenyl ring flipping in DGEBA chains. A beta relaxation is assigned to local motions of dipoles that were created during crosslinking reactions. 4,4'-based networks exhibited higher T-g relative to 3,3'-based networks as per dynamic mechanical as well as BDS analyses. The Vogel-Fulcher-Tam mann-Hesse equation fitted well to relaxation time vs. temperature data and comparison of Vogel temperatures suggests lower free volume per mass for the 3,3'-based network. The Kramers-Kronig transformation was used to directly calculate dc-free epsilon '' vs. f data from experimental epsilon' vs. f data. Distribution of relaxation times (DRT) curves are bi-modal for the 3,3'-crosslinked resin suggesting large-scale microstructural heterogeneity as opposed to homogeneity for the 4,4'-based network whose DRT consists of a single peak. (C) 2015 The Authors. Production and hosting by Elsevier B.V. on behalf of King Saud University.