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PFG-NMR Studies of Linear and Dendritic Polymers

James E. Hanson1,, Sibel Alkan1, Hershel Lackey2, 3, Judith B. Cain2

1Department of Chemistry and Biochemistry, Seton Hall University, South Orange, NJ, USA

2Chemistry Department, Virginia Military Institute, Lexington, VA, USA

3Department of Chemistry, University of Utah, Salt Lake City UT, USA

Journal of Polymer and Biopolymer Physics Chemistry. 2018, 6(1), 26-30. DOI: 10.12691/jpbpc-6-1-3
Received August 12, 2018; Revised September 13, 2018; Accepted October 10, 2018

Abstract

Diffusion coefficients were measured by pulsed-field gradient NMR for low molecular weight linear polystyrenes in THF and for a broader molecular weight range of linear polystyrenes in chloroform and for PAMAM dendrimers up to generation methanol. Radii were calculated from the measured diffusion coefficients using the Stokes-Einstein relationship. The linear polystyrenes displayed a relationship between radius and molecular weight that followed the expected power law. From simple theoretical considerations, the dendritic polymers were expected to follow a logarithmic relationship between radius and molecular weight. The PAMAM dendrimers gave reasonable fits to both a power law and a logarithmic relationship from generation 0 to generation 3 (the power law gave a slightly better fit), but displayed a turnover with generation 4, which gave a smaller Stokes radius than generation 3. These results were compared with earlier results from poly (aryl ether) monodendrons, where the relationship was ambiguous between a power law and a logarithmic relationship.

Keywords:

dendrimers, pulsed field gradient NMR, diffusion coefficients, scaling law, hydrodynamic radii
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