Haonan Wang

Orientation of carbon nanotubes in a sheared polymer melt

Optical measurements of the shear response of semidilute dispersions of polymer-dispersed multiwalled carbon nanotubes are presented. For a weakly elastic polymer melt, the data suggest that the semiflexible tubes orient along the direction of flow at low shear stress, with a transition to vorticity alignment above a critical shear stress, σc, corresponding to a critical Deborah number of approximately 0.15. In contrast, data for a highly elastic polymer solution suggest that the tubes orient with the flow field at high shear rates, in the limit of large Deborah number. The measurements are in qualitative agreement with previous experimental and theoretical studies of fiber orientation in elastic fluids under simple shear flow.

Optical measurements of structure and orientation in sheared carbon-nanotube suspensions

We describe an optical metrology for measuring shear-induced structure and orientation in dilute dispersions of multiwalled carbon nanotubes. Small-angle polarized light scattering and optical microscopy are combined in situ to quantify the structural anisotropy of multiwalled carbon nanotubes in semidilute, surfactant-stabilized aqueous suspensions under simple shear flow. Measurements performed as a function of the applied shear rate are used to demonstrate the capabilities and limitations of the experimental technique, which should be suitable for probing the shear response of polymer-nanotube melts and solutions.

Shear-induced structure in polymer blends with viscoelastic asymmetry

Light scattering and optical microscopy have been used to measure the morphology as a function of shear rate and composition in polymer blends with viscoelastic asymmetry in the melt components. The blends studied are immiscible mixtures of low-vinyl polybutadiene (PB) and high-vinyl polyisoprene (PI), where the vinyl content strongly influences the rheological properties of the melt. At the temperatures where the optical measurements described here were performed, the PI starts to exhibit an elastic response above a critical shear rate γ c , while the PB responds like a viscous fluid up to the highest shear rates of interest. The disparate rheology of the two fluids leads to a rich variety of domain patterns and orientations as the volume fraction of the more elastic component is varied.

High Frequency Loss Mechanism in Polymers Filled With Dielectric Modifiers

We analyzed the high frequency dielectric relaxation mechanism in high-k composite materials using film substrates made of low loss organic resin filled with ferroelectric ceramics and with single wall carbon nanotubes (SWNT). We performed broadband permittivity measurements of high-k film substrates at frequencies of 100 Hz to about 10 GHz. In order to analyze the effect of the dielectric thickness, dielectric constant, loss and conductive loss on the impedance characteristics, we used a High Frequency Structure Simulator to perform a full wave numerical analysis of several power planes. Small angle neutron scattering (SANS) was used to probe the dispersion of SWNTs in polymer matrices. It was found that organic-ceramic composites exhibit an intrinsic high frequency relaxation behavior that gives rise to frequency dependent dielectric loss. The highest frequency relaxation process dominates the overall loss characteristic. In the case of polymers modified with SWNTs, we observed that 2 % mass fraction of p-doped semi-conducting SWNTs increases the dielectric constant by 3 orders of magnitude, in apparent violation of the mixing-rule. The hybrid material appears to have preferential coupling within the dispersed phase. The experimental data and numerical simulation indicate that these materials can play a significant role as embedded passive devices with functional characteristics superior to that of discrete components.