45890-G7
Influence of Chain Conformation on the Mechanochemical Degradation of Polymers in Transient Elongational Flows
To date, funding through this grant has permitted us to determine the ultrasonic degradation of the near-rigid-rod form of poly(gamma-benzyl-L-glutamate) or PBLG, and has also permitted us to study the on-column, flow-induced degradation of polymers during size-exclusion chromatography (SEC) analysis. While the ultrasonic degradation of random coil polymers has been extensively studied, substantially less attention has been paid to limiting architectural cases such as rigid rods and hard spheres. To begin addressing this shortcoming, we studied the degradation of PBLG, a synthetic polypeptide which, under select solvent-temperature condition, is known to adopt a highly extended structure in dilute solution. Ultrasonically-degradaded PBLG was analyzed by multi-detector SEC including static multi-angle light scattering (MALS) and viscometric detectors. Results of these experiments showed that the limiting molar mass of PBLG, i.e., the molar mass beyond which this polymer does not further degrade for a given set of conditions, was approximately 114,000 g/mol. Additionally, the fractal dimension of PBLG, which exemplifies the highly extended state of the macromolecules, showed a virtual invariance as a function of chain degradation. Additional support for this invariance came from the dimensionless ratio of the different radii measured by MALS and viscometry. We concluded that, while the molar mass distribution, averages, and size of PBLG undergo great changes as a function of degradation, the macromolecule retains its extended structure throughout. Results from this study are being incorporated into a revised path theory for transient elongational flow degradation of polymers. Currently, researchers rely only on the molar mass and chemistry of a particular polymer for a posteriori explanation of the observed limiting molar mass. The path theory is meant to provide a priori prediction of the ratio of the limiting molar masses of various polymers, when degraded under identical conditions, based on the ratio of the persistence lengths of the macromolecules under consideration. Data from the PBLG experiments are consistent with previous degradation data from branched versus linear polymers and from various polysaccharides. Future ultrasonication experiments will focus on differences between the degradation of various types of copolymers of the same chemistry. Specifically, we intend to study the degradation of random, alternating, block, and gradient copolymers of styrene and methyl methacrylate. These experiments should further inform our knowledge of the influence of chain conformation of the mechanochemical degradation of polymers. We have also begun to study how ultra-high molar mass polymers degrade, due to flow, during their passage through SEC columns. This project is important not only from a chromatographic analysis point-of-view, but also from the perspective of those studying the behavior of polymers solutions and melts as they flow through porous media. To date, our studies appear to show that degradation occurs quite easily and that more care needs to be taken in analyzing ultra-high molar mass polymers than previously thought. We are currently exploring whether degradation in the interstitial medium is different from that occurring at the pore boundary, and whether either/both of these is transient elongational in nature.
