Storage modulus frequency-dependent increase

The frequency-dependent increase in storage modulus to 1 GPa and the frequency-dependent loss peaks signify vitrification during cure. Vitrification, as expected, is frequency-dependent since it is the cure-induced glass transition. Gelation, however, is an event that is independent of frequency. It is important to note that the shear sandwich experiment is useful primarily for B-staged resins whose viscosity is sufficient so that the resin does not flow out of the gap between the parallel-plate shear sandwich fixtures. 

Now these expressions describe the frequency dependence of the stress with respect to the strain. It is normal to represent these as two moduli which determine the component of stress in phase with the applied strain (storage modulus) and the component out of phase by 90°. The functions have some identifying features. As the frequency increases, the loss modulus at first increases from zero to G/2 and then reduces to zero giving the bell-shaped curve in Figure 4.7. The maximum in the curve and crossover point between storage and loss moduli occurs at im. 

Figure 4-30 shows the dynamic properties, storage modulus (G, Pa) and loss modulus G", Pa) versus frequency o> (rad s ) of gelatinized cross-linked waxy maize (CLW) starch dispersions of 3, 4, and 5% solids. In all dispersions, G was much higher than G", that is, the dispersions exhibited gel-like behavior over the studied range of frequencies. Both G and G" increased with concentration and showed a weak dependence on frequency at low values. This dependency was more pronounced in dispersions of higher concentration. 

The Influence of Temperature on the Viscoelastic Properties. The viscoelastic properties of the dilute surfactant systems depend on the temperature of the solutions strongly. Figure 8 shows the values for the storage modulus G as a function of the angular frequency at different temperatures for a 20 mM solution of CPySal. The elastic properties of the surfactant solutions decrease with increasing temperature. The solution equilibrated at 35 C shows only little elasticity in the frequency range below 1 Hz and at temperatures of 50 C the solutions behave as Newtonian fluids. The supermolecular structures which are present in these solutions and which are responsible for the viscoelastic properties seem to be completely destroyed under these experimental conditions. 

Fig. 26 Viscoelastic properties of electropolymerized poly(3-hexylthiophene). Fitted (a) storage modulus (G ) and (b) loss modulus (C") as functions of charge and frequency. Frequency dependence obtained by use of harmonics of polished 10 MHz fundamental Au-coated TSM resonator open (filled) symbols for increasing (decreasing) potential measurement sequence (lines are merely a guide to the eye).

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