It is shown that covalent bonding between high density polyethylene (HDPE) and glass spheres can have a significant influence on the stress relaxation behaviour and the creep properties of the corresponding composites at room temperature. The bonding is obtained by reacting the glass spheres with an azide functional alkoxysilane which is capable of bonding to the HDPE-chain. The internal stress, evaluated from relaxation experiments, increased markedly as a result of this treatment, and it is suggested that the internal stress level reflects the properties of the interphase region between the filler and the bulk matrix and its effect on the viscoelastic properties.

The effect of filling high density polyethylene (HDPE) with calcium carbonate (up to 50% by weight) on the stress relaxation and the creep in uniaxial extension at room temperature was investigated. The addition of CaCO 3 was found to have a strong influence on the flow behaviour of HDPE. In particular, it was observed that the internal stress level, calculated from relaxation data, increased markedly with the filler content. The reduction in creep rate of the filled samples suggested that the CaCO3-particles induce a change in the structure of the HDPEinterphase close to the filler surface. This was supported by dynamic mechanical measurements performed at low temperatures on swollen HDPE-CaCO3 samples.

In many applications it is useful to be able to convert observed creep data of a material to corresponding stress relaxation data or vice versa. If the material exhibits non-linear viscoelasticity such a conversion can be rather difficult. In this paper two semi-empirical flow equations, the power law and the exponential law, are used to convert stress relaxation data into corresponding creep behaviour data. These two flow equations are often used to describe non-linear viscoelastic behaviour. The procedure adopted here is based on the assumption that the creep data during the experiment decrease due to an increase in the internal stress level, thus decreasing the effective stress for flow. The conversion method is applied to high density polyethylene and polycrystalline molybdenum at room temperature. In general predictions using the power law are in better agreement with the experimental results than predictions using the exponential formula. The concepts of secondary and ceasing creep are discussed in terms of build-up of internal stress during the creep process.

For many solid materials the stress relaxation process obeys the universal relation F = -(da/d In t)max = (0.1 -t-0.01) ((70 --0"i) , regardless of the structure of the material. Here a denotes the stress, t the time, a o the initial stress of the experiment and (7 i the internal stress. A cooperative model accounting for the similarity in relaxation behaviour between different materials was developed earlier. Since this model has a spectral character, the concepts of linear viscoelasticity are used here to evaluate the corresponding prediction of the dynamic mechanical properties, i.e. the frequency dependence of the storage E' (o)) and loss E" (o)) moduli. Useful numerical approximations of E'(~o) and E" (m) are also evaluated. It is noted that the universal relation in stress relaxation had a counterpart in the frequency dependence of E' (co). The theoretical prediction of the loss factor for high-density polyethylene is compared with experimental results. The agreement is good.

A number of theoretical models exist for describing the stress relaxation behaviour of solids with entirely different physical backgrounds. In this communication the potential of the spectral theory, the Williams-Watts function, the theory of stress dependent activation, the power law and a co-operative model to describe experimental relaxation curves are critically examined. The analysis is facilitated by the use of Li-curves, i. e. by plotting the derivative of the stress with regard to log (time) vs. the time dependent stress. The ability of the models to predict the linear character of the flow curves with regard to the initial stress of the relaxation experiments is also examined. The predictions of the theoretical models are compared with experimental results for high density polyethylene, polyisobutylene, indium and steel. From the analysis, it is concluded that the co-operative model appears to be the most useful for describing the stress relaxation behaviour of materials.