Effects of composition and matrix polarity on network development in organogels of poly(ethylene glycol) and dibenzylidene sorbitol

Dibenzylidene sorbitol (DBS) is a low-molecular-weight organic molecule that can gel a variety of organic solvents and polymers by self-organizing into a three-dimensional nanofibrillar network through hydrogen-bonding and phenyl interactions. In this work, we investigate the composition dependence of such organogels prepared with poly(ethylene glycol) (PEG) and two PEG derivatives differing in methoxy end-group substitution, which serves to reduce matrix polarity. Transmission electron microscopy reveals that individual DBS nanofibrils measure from about 10 to 70 nm in diameter, with a primary nanofibrillar diameter of about 10 nm. Dynamic rheological measurements indicate that the rate by which the elastic modulus increases during gelation, the temperatures corresponding to gel formation and dissolution, and the magnitude of the elastic modulus are all sensitive to the DBS mass concentration (phi) and the matrix polarity. Hydroxy-end-capped PEG/DBS systems gel more slowly, but dissolve faster, than their methoxy-end-capped analogues at constant phi. The elastic modulus, however, is less dependent on matrix polarity and scales as phi(1.8) over the range of phi examined in PEG/DBS organogels. Time-temperature superposition analysis provides direct evidence for the activation energy of network evolution increasing linearly with (i) decreasing phi at constant matrix polarity and (ii) increasing matrix polarity at constant phi.