Molecular basis creep

In this section we are going to examine such viscoelastic properties in some detail and we will start by examining in turn three important mechanical methods of measurement creep, stress relaxation, and dynamic mechanical analysis. This will lead us to interesting things like time-temperature equivalence and a discussion of the molecular basis of what we have referred to as relaxation behavior. 

Wilding and Ward showed that the creep and recovery behaviour of the low molecular weight samples could be represented to a good approximation by the model representation shown in Fig. 35(b), which consists of a Maxwell and Voigt element in series, on the basis that the parameters E, E, r and r), are dependent on the stress level. Data for the creep response of the samples under discussion at a constant applied stress Op were therefore fitted to the equation... 

Their molecular assignments could be safely made, and separate contributions of the PCN network and LPU to the relaxation spectra were estimated. On this basis, first of all, owing to the analysis of creep rate spectra, besides segmental dynamics heterogeneity, also some information concerning the nanoscale heterogeneity of network cross-linking was obtained. 

Example 15.2 Using the four-parameter model as a basis, qualitatively sketch the effects of (a) increasing molecular weight and (b) increasing degrees of crosslinking on the creep response of a linear, amorphous polymer. 

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