Behavior of Other Model Elastomers

The viscoelastic properties of a series of fully cured epoxy resins with different crosslink densities and their trends have been discussed in some details in the previous section. Qualitatively these properties are shared by all crosslinked elastomers, although quantitatively they depend on molecular architecture and the chemical type of the network polymer and the crosslinking agent. Hence it is instructive to show the viscoelastic behavior of other model network systems and compare them. 

FIGURE 11 Logarithmic plot of the reduced shear creep compliance curves Jp t) (in cmVdyne = lOPa ) against the reduced time I/oy (in seconds).The reference temperature of reduction. To, is -20.0 C for all curves. Data points are shown only for Sample 11-A. 

The horizontal logarithmic time scale shifts that are required to superpose the data obtained at different temperature are the logarithms of the Ut shift factors. The Uj values thus reflect the principal temperature dependence of the viscoelastic process. It was possible to represent the time-scale temperature dependences of the three samples with a single VFTH Eq. (33) in which only one parameter T , which reflects the change in Tg, varies with the level of crosslinking. The fit achieved is shown in Fig. 12. The atmosphere in which the measurements were made is important since samples measured in air contain the moisture absorbed under ambient conditions, whereas those measured in rough vacuum (use about symbol - lO torr = 1.3 Pa) are at least partially 

FIGURE 12 Logarithmic plot of the time-scale shift factors against temperature differences. The atmosphere in which the measurements were made is either in air containing the moisture absorbed under ambient condition or in rough vacuum, which is partially dry. T. = -57, -54, -50, -48°C, respectively, for Samples 11-A (air), 10-A (vacuum), 10-B (air), and 10-B (vacuum). 

Retardation spectra Lp were determined from the Jp(t) curves of Fig. 10. The results reduced to To = -20°C are shown in Fig. 13, where the principal maximum concentration of retardation mechanisms is shown to be at correspondingly longer times as Fg is increased with additional crosslinking. The 

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