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We investigate the effect of short-chain poly(ethylene glycol) (PEG) graft density on the formation of methylcellulose (MC) fibrils at elevated temperatures. Thiol-ene click chemistry was used to systematically graft 800 and 2000 g/mol PEG onto the backbone of allylated MC, with a wide range of grafting densities from 0.7% to 33%. As determined from light scattering, grafting leads to an increase in the persistence length of this semiflexible copolymer, by as much as a factor of 10. Upon heating, SAXS and AFM studies show that fibril formation is suppressed at around 10% grafting density for shorter PEG grafts, corresponding to persistence lengths about ∼22 nm. For longer grafts fibril formation is suppressed at 7% grafting density, at around the same ∼22 nm persistence length. The radius of the fibrils increases with the square root of the persistence length of the chains, which is consistent with a theory for the radius of twisted chains. The ability to form networks at 80 °C is highly correlated to the ability to form fibrils, and accordingly the modulus systematically decreases with grafting density. When the fibril formation is suppressed, MC solutions no longer form solid networks. Therefore, grafting modulates the molecular architecture and gelation properties of MC and also provides new insight into the structure of MC fibrils.
Bibliographical noteFunding Information:
This work was supported primarily by the National Science Foundation through the University of Minnesota MRSEC under Award DMR-1420013. We thank the Dow Chemical Company for generously providing the MC samples. The SAXS measurements were taken at the DuPont-Northwestern-Dow Collaborative Access Team (DND-CAT) located at Sector 5 of the Advanced Photon Source (APS). DND-CAT is supported by Northwestern University, E.I. DuPont de Nemours & Co., and The Dow Chemical Company. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract DE-AC02-06CH11357. The AFM images were collected at the Characterization Facility, University of Minnesota, which receives partial support from the NSF through the MRSEC program. We thank Douglas Hall, Dr. S. Piril Ertem, and Professor Peter Olmsted for helpful discussions.
© 2018 American Chemical Society.
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Reporting period for MRSEC
- Period 5