Architecture of synthetic polymers has become increasingly complex and difficult to fully characterize making it challenging to determine structure-function relationships. While nuclear magnetic resonance (NMR) has been a reliable tool to determine architecture, it relies on unique signatures that can be obscured by complex molecular environments and is not capable of providing information about distributions. Highly branched polymers with large molar masses (>1 MDa) often exhibit non-ideal chromatography behavior that further obfuscate characterization. Thermal field-flow fractionation (ThFFF) has shown promise as a novel characterization method which resolves some of these challenges.
A series of poly(2-(2-bromoisobutyryloxy) ethyl methacrylate)-graft-poly (n-butyl acrylate) bottlebrushes with 100% grafting density and varied degrees of polymerization (DP) in backbone and sidechain were analyzed by ThFFF, NMR and SEC. Backbone DP ranged from 20-400 with sidechain DP from 20-120, which resulted in a range of molar masses from 10⁵-10⁷ Da. An additional series of bottlebrushes with 30% grafting density and a backbone DP of 100 were also analyzed. Molar masses determined by NMR and ThFFF agreed within 20% except for brushes whose molar mass exceeded 3 MDa. SEC molar mass results by calibration yielded values up to 600% lower than NMR and ThFFF. It was demonstrated that the Soret coefficient (S_T) exhibits a linear relationship with the DP of both sidechains and backbone. A contraction ratio of the Soret coefficient (g”) was calculated by assuming a linear analog composed solely of the sidechain. A trend of increasing contraction with the number of chain ends was observed. A comparison of reported g” values with those from the current work suggests specific ranges of g” for different architectures. While the mechanism of architectural sensitivity is not fully understood, this work demonstrates the immediate usefulness of this method.

A graph of the log of the Soret contraction factor (g") versus the number of chain ends illustrates the potential of this analytical method to differentiate between architectures.

James Oliver

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