Brownian motion in a single relaxation time maxwell fluid

A simple model of Brownian motion in a single relaxation time Maxwell fluid is described and compared to diffusing wave spectroscopy measurements of colloidal motion in representative viscoelastic fluids, namely, CTAB/KBr wormlike micelle solutions. The experimentally measured Brownian motion conforms to the model predictions at long times (low frequencies) and is an additional confirmation of the essentially Maxwellian stress relaxation behavior of wormlike micelle solutions at low frequencies. Surprisingly, the Maxwell model predicts a plateau onset time which, while capable of reducing the measured mean-square displacements to a master curve, also grossly underestimates the actual plateau onset time. The predicted rescaling is shown to be essentially that also predicted by the Doi-Edwards tube model for polymer solutions under good solvent (excluded volume) conditions where a more proper accounting of the short-time dynamics is made. This indicates that the success of the predicted Maxwell model plateau onset time rescaling is purely fortuitous.

Immobilizing a Fluorescent Dye Offers Potential to Investigate the Glass/Resin Interface.

Silane coupling agents are commonly applied to glass fibers to promote fiber/resin adhesion and enhance durability in composite parts. In this study, a coupling agent multilayer on glass was doped with trace levels of the dimethylaminonitrostilbene (DMANS) fluorophore. The fluorophore was immobilized on the glass surface by tethering the molecule to a triethoxy silane coupling agent, creating the DMANS/silane coupling agent molecule (DMSCA). DMSCA was then diluted with commonly used coupling agents and grafted to a glass microscope coverslip to create a model composite interface. A 53-nm blue shift in fluorescence from the immobilized DMSCA can be followed during cure of an epoxy resin overlayer, giving this technique potential to monitor the properties of the fiber/resin interface during composite processing. Contact angle measurements on these coupling agent layers were similar in the presence or absence of the DMSCA molecule, suggesting that trace levels of the fluorescent probe did not affect the structure of the layer. The immobilized DMSCA molecule behaved similarly to the DMANS precursor in solution. Both showed longer wavelength fluorescence in more polar environments. Copyright 2000 Academic Press.