Electronic Thesis and Dissertation Repository

Degree

Master of Science

Program

Physics

Supervisor

John R. de Bruyn

Abstract

The addition of a small amount of carbon nanotubes to a polymer matrix can result in large changes in its mechanical properties. In this work, we study the rheological behavior of solutions of polyisoprene (PI) in chloroform and suspensions of multi-walled carbon nanotubes (MWCNT) and single-walled carbon nanotubes (SWCNT) in these PI solutions using shear rheometry.

We perform rheological measurements to study the viscoelastic properties of solutions of trans-PI in chloroform for polymer concentrations ranging from 1.0 wt% to 10.0 wt%. Oscillatory strain sweep tests show a concentration-and-frequency independent critical strain of that defines the limit of the linear viscoelastic region. Frequency sweep tests performed in the linear viscoelastic regime give and for low concentrations, as predicted by the Maxwell model. The zero-shear viscosity of the PI solutions, determined by three methods in this work, has a dependence on polymer concentration c of c5, which implies that the polymer molecules in our solutions are entangled.

CNTs are dispersed in the PI solutions using a water-bath sonicator to obtain suspensions of MWCNTs and SWCNTs. Sonication times of many hours are required to obtain stable dispersions. We observe that the viscosity of the PI/CNT composites is a factor or 10 lower than that of the pure PI solution at the same polymer concentration, and independent of shear rate over the range studied. Films of PI/CNT composites are formed by evaporating the solvent from the suspensions. The well-dispersed state of CNTs in our PI/CNT systems is verified by scanning electron microscopy of the films and by the stability and reproducibility of our steady-state-shear flow measurements.

We discuss the mechanisms invoked in the literature to explain viscosity reductions observed in other systems and introduce a new mechanism that we believe that applies to our PI/CNT systems. We propose that the adsorption of polymer chains onto the CNT surface reduces the amount of polymer in solution and thus leads to a decrease in viscosity. Shear alignment of the CNTs may also enhance alignment of the PI chains remaining in solution, causing a further reduction in viscosity and eliminating shear-thinning behavior of the CNT-filled PI solutions.


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