Langmuir, Vol.23, No.14, 7715-7723, 2007
Structure-property relationship in stimulus-responsive bolaamphiphile hydrogels
The formation of temperature-, concentration-, and pH-responsive hydrogels composed of the symmetric long-chain bolaamphiphile dotriacontane-1,1'-diyl bis[[2-(dimethylammonio) ethyl] phosphate] (Me2PE-C32-Me2PE) was investigated by rheological, scattering, and spectroscopic techniques. At pH 5, this bolaamphiphile is known to form a dense network of helically structured nanofibers (Kohler et al. Soft Matter 2006, 2, 77-86). Rheological measurements and dynamic light scattering were used to describe the macroscopic behavior of the hydrogels. Small-angle neutron scattering (SANS) and time-resolved static light scattering were applied to get information about the morphology of the self-assembled aggregates. Finally, solid-state P-31 NMR spectroscopy was used to gain insight into the mobility of the bolaamphiphile molecules within the fiber aggregates. In comparison with the previously examined trimethylammonio analogue PC-C32-PC, which forms temperature-dependent hydrogels, Me2PE-C32-Me2PE exhibits additional concentration- and pH-dependent gelling properties. The significantly higher stability of the Me2PE-C32-Me2PE hydrogel is supported by the SANS data, which indicate the presence of fiber aggregates up to 50 degrees C.