Rheological properties of dibenzylidene sorbitol networks in poly(propylene oxide) matrices

We have systematically investigated gels of dibenzylidene sorbitol (DBS) in amorphous poly(propylene oxide) matrices by temperature, strain, and frequency dependent rheological measurements. The concentration and the substitution pattern of DBS as well as the molecular weight of the matrix have been varied. Even very small amounts below 0.3 wt % of the low molecular weight gelators lead to a significant increase in the viscosity and elasticity as compared with the neat matrix. This reinforcing effect has been attributed to the formation of an internal DBS network, also proven by our morphological investigations. By means of temperature dependent tests, the temperature $Tf$ of the thermoreversible gel formation has been detected. Strain dependent measurements establish a comparatively small range of linear viscoelastic response and show the extreme deformation sensibility of the gels above a critical strain. Because the principle of time temperature superposition is unexpectedly valid for these heterogeneous systems, master curves can be constructed from frequency dependent measurements at temperatures well below $Tf.$ In the case of higher DBS concentrations above 0.4 wt %, a power law behavior with a small exponent instead of an equilibrium modulus $Ge$ has been observed in the low frequency range of the master curves. We relate this peculiar feature to the dynamic break and reformation of the weak network junctions.