The sub-Tg relaxations in pure and antiplasticized model epoxy networks as studied by high resolution creep rate spectroscopy

An original laser-interferometric creep rate spectroscopy method was used for studying the molecular mobility in the glassy state of a series of 'model' epoxy-amine networks varying in cross-link density and rigidity. The experiments were carried out over the temperature range 115-300 K, which covers the regions where the secondary relaxations are expected to occur. The creep rate spectra (CRS) obtained at low compressive stresses exhibited systematically multiple creep rate (epsilon)over dot peaks, irrespective of the applied stress and the experimental time in the ranges under consideration, namely 20-40 MPa and 10-40 s, respectively. Data analysis was based on the inspection of the major peaks of the CRS without entering much into the details of the fine structure. The peaks assigned to the beta relaxation were shown to depend on the changes in network cross-link density and to the addition of antiplasticizing molecules in the same way as reported earlier from dynamic mechanical analysis (DMA) and C-13 nuclear magnetic resonance (NMR) studies. In the case of the resins cured with aromatic amines, for which no gamma relaxation is evidenced by DMA, creep rate spectroscopy allowed the detection of an additional relaxation, so called gamma', at temperatures well below the beta process. It was tentatively assigned to DMA-inactive motions of the aromatic amine residues, i.e. to the phenyl ring flips of the diphenylmethane units. (C) 1999 Elsevier Science Ltd. All rights reserved.