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Master of Science in Chemical Engineering (M.S.Ch.E.)
Year Degree Awarded
Month Degree Awarded
Fibers in biological scaffolds like fibronectin stiffen when they experience forces between cells. It will expose binding sites under contractile forces and then form disulfide bonds. This on-demand strain-stiffening is a desirable property in synthetic materials. Tran et.al. (2017) mimicked the “cryptic” design of fibronectin by copolymerizing thiol crosslinking sites with monomers containing poly (ethylene glycol) chains. When the PEG chain increased from 350 to 950 g/mol, the strains-stiffening became on-demand while the curing process extended from 3 hours to 15 hours. Extra steric hindrance brought by longer PEG chains caused decreasing mass transfer rates of cryptic sites while the same level of strain rate was introduced. I proposed to use stronger ultrasound mechanical perturbation so that higher strain rate can be induced, and the shielding effect brought by the PEG chain can be overcome more easily. Utilizing ultrasound as a stimuli has the potential to improve the gelation speed or achieve high mechanical performance while retain the long shelf life of the “cryptic” materials.
To test this hypothesis, I synthesized “cryptic” polymer with aceto-acetoxy and primary amine as crosslinking sites such that, the only time limiting step is brought by the long PEG chain. This is because the bond formation reaction between these two reactive groups is rapid and spontaneous. When switching from weak to strong mechanical perturbation, the change in gelation speed owing to accelerated mass transfer between crosslinking sites can be easily compared. When the PEG chain is 300 Mw and 30 mol % crosslinking sites density, this “cryptic” polymer only showed strain-stiffening under ultrasound while strain under a rheometer was not able to overcome steric hindrance. Signs of chain scission appeared when the ultrasound amplitude was set at 75 %, but was counteracted by reducing amplitude mode over the time. The crosslinking was optimized by varying the ultrasound amplitude and intensity and a final mode of 1 hour 75 % amplitude, 0.5 hour 50 % amplitude and 3.5 hours 25% amplitude provided greatly improved on demand crosslinking. The estimated kinetic constant using this mode was two times higher than that of under simple shear strain. Through this study, I found that ultrasound can improve the curing time of this “cryptic” polymer system since it induces higher strain rate and expedite the mass transfer rate between crosslinking sites and optimizing the ultrasonic amplitude profile to limit chain scission provides improved crosslinking performance.
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Liu, Yinghong, "Ultrasound-Responsive Crosslinking with Temporal Control and Rheological Tunability" (2021). Masters Theses. 1132.