Paper Number
ET6 

Session
Advanced Experimental Techniques/Methods in Rheology

Title
Using extensional viscosity to determine the length (or molecular weight) of rod-like molecules, and its importance in studying the phase behavior of carbon nanotube solutions

Presentation Date and Time
October 15, 2018 (Monday) 1:30

Track / Room
Track 5 / San Felipe Room

Authors (Click on name to view author profile)

1. Pasquali, Matteo (Rice University, Department of Chemical & Biomolecular Engineering)
2. Tsentalovich, Dmitri E. (Rice University)
3. Ma, Anson (University of Connecticut, Chemical and Biomolecular Engineering)
4. Lee, Jun A. (Rice University)
5. Behabtu, Natnael (Rice University)
6. Bengio, Elie A. (Rice University, Department of Chemical & Biomolecular Engineering)
7. Headrick, Robert J. (Rice University)
8. Green, Micah J. (Texas A&M University, Chemical Engineering)
9. Talmon, Ishi (Technion Israel Institute of Technology, Chemical Engineering)

Author and Affiliation Lines (in printed abstract book)
Matteo Pasquali¹, Dmitri E. Tsentalovich¹, Anson Ma², Jun A. Lee¹, Natnael Behabtu¹, Elie A. Bengio¹, Robert J. Headrick¹, Micah J. Green³, and Ishi Talmon^4
1Department of Chemical & Biomolecular Engineering, Rice University, Houston, TX 77005; ²Chemical and Biomolecular Engineering, University of Connecticut, Storrs, CT 06268; ³Chemical Engineering, Texas A&M University, College Station, TX 77843; ⁴Chemical Engineering, Technion Israel Institute of Technology, Haifa, Israel

Speaker / Presenter 
Pasquali, Matteo

Text of Abstract
Intrinsic viscosity is the oldest, simplest method for determining the molecular weight of a polymer. For polymer coils, the method is based on measuring the shear viscosity of a dilute polymer solution at low enough shear rate that the polymer conformation is unaffected by the viscous forces. The method fails for progressively longer, rod-like molecules, as the longest relaxation time scales with the cube of molecular weight (length) in rods (vs. the 1.5 to 1.8 power for coils in theta or good solvents). We develop a new method for measuring the length of rod-like molecules in extensional flows. The method is based on measuring the extensional viscosity of solutions of rod-like polymers with a customized capillary thinning rheometer. It relies on the relative ease of aligning rods in extensional flow, combined with the theoretical relation between extensional viscosity and aspect ratio in semidilute solutions of aligned rods. We demonstrate that this method can determine the aspect ratio of carbon nanotubes (CNTs) simply and accurately. We measure CNT diameter d by transmission electron microscopy (TEM) and arrive at CNT length L. By studying CNT samples synthesized by different methods, we show that the method works well for CNT lengths ranging from 0.4 to at least 20 µm, a wider range than for previous techniques. Moreover, we measure the isotropic-to-nematic transition concentration (i.e., isotropic cloud point) f* of CNT solutions and show that this transition follows Onsager-like scaling f* ~ d/L. We characterize the length distributions of CNT samples by combining the measurements of extensional viscosity and transition concentration and show that the resulting length distributions closely match distributions obtained by cryo-TEM measurements. Interestingly, CNTs appear to have relatively low polydispersity compared to polymers and high polydispersity compared to colloidal particles.