COLLOIDAL DISPERSIONS IN POLYMERIC MEDIA: INTERPARTICLE FORCES, MICROSTRUCTURE AND RHEOLOGY
Series: Final Public Oral Examinations
Location: Bowen Hall Auditorium
Date/Time: Friday, January 20, 2012, 10:30 a.m. - 12:00 p.m.
To enhance properties of the ultimate materials, melt processed polymers are commonly filled with colloidal particles, such as inorganic oxides. Dispersing such particles in a melt is generally difficult due to the strong van der Waals attractions. These attractive forces can be modulated through surface modifications such as polymer grafting. Indeed, the relative viscosity of 430 nm Al2O3 particles stabilized by end-tethered poly (dimethylsiloxane) (PDMS) in PDMS melts decreases with increasing graft density and molecular weight as expected, but also with increasing molecular weight of the melt, contrary to well established theories. The relative steady shear viscosity exhibits neither a low shear limit nor a yield stress, but follows a power law characterized by relative high shear viscosity (η∞/μ) and a structural relaxation time (τ ). The measured structural time can be correlated reasonably well with a characteristic relaxation time, τo, estimated by equating the viscous resistance with the maximum attractive force. We further explored the significance of this power law with TiO2 nanoparticles in PDMS melts with a reduction in size and an increase in Hamaker constant. Bare octadecyl-coated, and 9k PDMS grafted TiO2 particles dispersed in neat and binary PDMS melts revealed behavior similar to that of the large alumina particles, as the increased strength of van der Waals forces offset the reduction in size. To complete the study ZrO2 nanoparticles were dispersed in solution of associative polymers and characterized by small amplitude oscillatory shear. The data exhibits two relaxation modes: Maxwellian behavior at high frequency imparted by the associating polymers and a power law spectrum at low frequency from the particles. The timescales and volume fraction dependence reflect weak attractions between particles with adsorbed polymer layers dispersed in a percolated network of associative polymers. Together these studies demonstrate the range and origin of the rheology possible with particles dispersed in polymeric media.