Relationships between thermally induced residual stresses and architectureof epoxy-amine model networks

The capability of epoxy-amine resins to develop residual stresses was studi ed as a function of temperature and network. architecture. These residual s tresses were induced while cooling epoxy-glass bilayers from temperatures h igher than the network glass transition temperature, T-g. This behavior was the result of the marked differences (alpha(r) - alpha(g)), in Linear ther mal expansion coefficient of the two components, as evidenced by the measur ement of alpha(r) for the epoxy networks under study. Various network archi tectures were selected, resulting from variation of (1) the chemical nature of both epoxide and curing agent, (2) the nature and relative amount of th e chain-extensor agent, and (3) the stoichiometric ratio. Three ranges of c ooling temperature were observed systematically: first;, the range of tempe ratures above T-g, where no stress has been detected, then an intermediate temperature range (from T-g to T*), where stresses develop quite slowly, an d finally, the low temperature range (T < T*), where a linear increase in s tress accompanies the decrease of temperature. The two latter regimes were quantitatively characterized by the extent, T-g - T*, of the first one and by the slope, SDR, of the second one. T-g - T* values were shown to be gove rned by the T-g of the network: the higher the T-g, the larger the gap betw een T-g and T*. This result was interpreted by accounting for the variation of relaxation rate at T-g from one network to the other. It was also shown that a semiempirical relationship holds between SDR and T-g: SDR decreases monotonically as T-g increases. By inspecting the effects of network archi tecture in more details, it turned out that SDR is governed by the Young's moduli, E-r(T - T-g), of the epoxy resins in the glassy state: the lower E- r(T - T-g), the lower SDR in a series of homologous networks. As E-r(T - T- g) values are known to be related to the characteristics of the secondary r elaxation beta, which depends, in turn, on crosslink density, SDR values we re finally connected to the amplitude of the beta relaxation processes. Thi s finding was corroborated by the measurements on an antiplasticized dense network. Finally, data relative to thermoplastic-filled networks showed tha t the addition of thermoplastic reduces the development of residual stresse s, whatever the system, is homogeneous or biphasic. (C) 2000 John Wiley & S ons, Inc.