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Through this grant we were able to explore the degradation of polymers in transient elongational flows as well as during the polymers’ passage through packed porous media. As the former was reported last year, this report focuses on the latter.

Size-exclusion chromatography (SEC) is generally considered the premier method through which to obtain molar mass averages, distributions, and architectural information of natural and synthetic macromolecules. Ultra-high molar mass (M) polymers, such as certain polyolefins and poly(vinyl acetates) and natural polymers such as cellulose and amylopectin, experience large shear stresses during their passage through the packed chromatographic medium within an SEC column. For a typical SEC analysis, the shear stresses generated can be on the order of 10⁴ -10⁵ sec^-1,  comparable to the shear rates encountered during industrial processes such as gasoline engine lubrication, blade coating, or pigment milling.

We first examined the degradation of ultra-high M macromolecules during SEC analysis using narrow polydispersity polystyrene (PS) standards and columns of various pore sizes. Degradation was observed in the interstitial medium of the column (i.e., in the space in between the packing particles), for columns of pore size too small for penetration by the PS. A different set of degradation profiles was observed when using columns packed with the same size particles as in the preceding experiment, but with larger-sized pores which now allowed for permeation of the PS into the particle. As such, we can conclude that macromolecules can degrade both in the interstitial medium and at the pore, and do so by different mechanisms.

To determine if the flow fields responsible for degradation in the aforementioned scenarios were transient elongational in nature, we degraded the same PS standard ultrasonically for specific time intervals. The reason for this is that the flow fields created upon ultrasonic cavitational bubble collapse are transient elongational. The ultrasonically-degraded PS samples were examined by SEC at a flow rate at which degradation either does not occur or is negligible. The elution profiles of these ultrasonically-degraded samples did not resemble the profiles of the same sample when degraded either solely in the interstitial medium or in both the interstitial medium and at the pore. From this, we were able to conclude that degradation in SEC is not principally, if at all, due to transient elongational flows. It is most likely that degradation is due to steady-state elongational flow with, perhaps, a minor shear component (degradation of macromolecules in shear fields is highly unlikely, though not impossible).

The importance of this work was shown in a follow-up study which involved characterizing the molar mass averages and distribution of the polysaccharide alternan, as part of a collaboration with the US Department of Agriculture. Alternan is an ultra-high M polysaccharide (Mw in the range of 45-50 million g/mol) which possesses long-chain branching and two types of glycosidic linkages. It has great potential as a cost-effective domestic substitute for gum Arabic in applications such as food, paint, and ink binders and in coatings for time-released pharmaceuticals. To properly characterize alternan by multi-detector SEC, we performed experiments in both aqueous and organic media, at various temperatures, and using a multi-angle static light scattering (MALS) detector both on- and off-line. The latter experiments provided bulk averages of the polymer without chromatographic separation, i.e., provided benchmarks to which we could compare some of our chromatographic data. We first demonstrated that previous characterization of alternan by other groups, using aqueous flow field-flow fractionation, yielded results which were skewed high due to aggregation of the polysaccharide in water. We next showed that the polymer did not elute in so-called slalom chromatographic mode, by observing that polymeric size did not increase as a function of increasing elution volume. Lastly, we showed that characterization in dimethyl sulfoxide at 50 ^oC, at a flow rate of 0.2 mL/min, could provide what is to date the most accurate characterization of the molar mass averages and distribution of alternan. Lowe flow rates (i.e., lesser shear rates) did not improve the accuracy of the characterization, which still appeared to degrade the polymer to a very small but real degree. Current experiments in our lab are exploring the use of hydrodynamic chromatography to characterize ultra-high M polymers, as HDC is a much gentler technique than is SEC, albeit a technique with much lower chromatographic resolution.

In conclusion, during these last two years we have studied the effects of polymer architecture on the mechanochemical degradation of polymers, formulating a new concept for limiting molar mass and the first theory for a priori prediction of degradation in transient elongational flow (the “modified path theory,” which we continue to refine). We have also studied the flow-induced degradation of polymers during their passage through packed media, of consequence not only for chromatographic analysis but also in filtration, percolation, oil field work, etc. Finally, we applied our results to obtain the most accurate characterization to date of alternan, a macromolecule that can provide a renewable domestic resource for a number of applications.