The sub-T-g 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 f or studying the molecular mobility in the glassy state of a series of 'mode l' epoxy-amine networks varying in cross-link density and rigidity. The exp eriments were carried out over the temperature range 115-300 K, which cover s the regions where the secondary relaxations are expected to occur. The cr eep rate spectra (CRS) obtained at low compressive stresses exhibited syste matically 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 de tails 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 ad dition of antiplasticizing molecules in the same way as reported earlier fr om dynamic mechanical analysis (DMA) and C-13 nuclear magnetic resonance (N MR) studies. In the case of the resins cured with aromatic amines, for whic h no gamma relaxation is evidenced by DMA, creep rate spectroscopy allowed the detection of an additional relaxation, so called gamma', at temperature s 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 th e diphenylmethane units. (C) 1999 Elsevier Science Ltd. All rights reserved .