Paper Number
SM35 

Session
Polymers Solutions, Melts, and Blends

Title
Determination of the number-average molecular weight of polyelectrolytes

Presentation Date and Time
October 13, 2021 (Wednesday) 1:55

Track / Room
Track 1 / Ballroom 5

Authors (Click on name to view author profile)

  1. Han, Aijie (Pennsylvania State University, Materials Science and Engineering)
  2. Uppala, Shravan (Virginia Tech, Department of Chemistry)
  3. Dixon, Benjamin J. (Pennsylvania State University, Materials Science and Engineering)
  4. Madsen, Louis A. (Virginia Tech, Department of Chemistry)
  5. Colby, Ralph H. (The Pennsylvania State University, Material Science and Engineering)

Author and Affiliation Lines (in printed abstract book)
Aijie Han1, Shravan Uppala2, Benjamin J. Dixon1, Louis A. Madsen2 and Ralph H. Colby1
1Material Science and Engineering, The Pennsylvania State University, University Park, PA 16802; 2Department of Chemistry, Virginia Tech, Blacksburg, VA 24061

Speaker / Presenter 
Han, Aijie

Keywords
polymer solutions

Text of Abstract
We have developed four methods using the chain dynamics in the semidilute unentangled regime to determine the number-average molecular weight (Mn) of polyelectrolytes based on the scaling model. Our team is collaborating to measure the correlation length (?) using X-ray scattering, the specific viscosity (?sp) and relaxation time (t) using rheometry, and the diffusion coefficient (D) using NMR diffusometry. Our methods show that combining the measured properties of polyelectrolyte solutions yields the number density of chains in solution which directly determines Mn. In this work, five nearly monodisperse cesium polystyrene sulfonate (CsPSS) solutions without salt are studied in water, anhydrous ethylene glycol (EG) and anhydrous glycerol to test each method. The molecular weights are also confirmed by means of static light scattering and intrinsic viscosity measurements in 0.1 M NaCl aqueous solutions. We found that all four methods work generally well for the highest molecular weight (N = 9150 chemical repeat units). For lower molecular weights, the method using ?sp is found to systematically underestimate Mn compared with the manufacturer’s reported value, while the method using D overestimates Mn. Both methods require prefactors which depend on degree of polymerization (N). This observation likely underlines the importance of non-uniform stretching along the chain of polyelectrolytes which disappears in the long chain limit. Additionally, using EG and glycerol as solvents slows down the relaxation dynamics significantly and enables us to obtain t for lower molecular weight CsPSS from the shear rate dependence of viscosity. The relaxation times of CsPSS in three different solvents can be reduced to a common curve in the semidilute unentangled regime using the scaling model. We found that the method using the combination of relaxation time and correlation length has similar N dependence of prefactors as the method using ?sp, and using the terminal modulus is the only method that does not require prefactors.