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Title:Multiscale characterization of mechanochemical reactions in mechanophore-crosslinked polymers under shear loading
Author(s):Kingsbury, Cassandra
Director of Research:Sottos, Nancy R.
Doctoral Committee Chair(s):Sottos, Nancy R.
Doctoral Committee Member(s):White, Scott R.; Braun, Paul V.; Schweizer, Kenneth S.; Freund, Lambert B.
Department / Program:Materials Science & Engineerng
Discipline:Materials Science & Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Shear deformation of polymers
Interfacial shear stress
Abstract:Inspired by mechanotransduction, new strategies are employed to impart productive mechanochemical response to polymeric materials. These mechanoresponsive materials possess the ability to respond to an applied global force through a local chemical reaction. In synthetic polymers, mechanochemical response is achieved through the use of a mechanophore, a single molecule with a detectable, force-driven chemical response, linked to the polymer backbone. In this dissertation, evidence of a mechanically-induced local chemical reaction (an electrocyclic ring-opening) is provided by a color- and fluorescence-generating spiropyran mechanophore. The opening of spiropyran (SP) into merocyanine (MC) is driven by mechanical force, UV light, or heat, and is reversible. The focus of this dissertation is to evaluate the transformation of SP to MC under shear loading conditions in bulk polymers and at the interface between a glass fiber and a polymer matrix. Spiropyran is successfully incorporated as a co-crosslinker into poly(methyl methacrylate) and the mechanical transformation of SP to MC, or activation of SP, in bulk mechanophore-crosslinked polymers is studied under monotonic torsion. In situ full field fluorescence imaging is used to determine the threshold stress and strain required for activation as a function of strain rate and polymer architecture, both of which have a significant effect on mechanochemical activity of spiropyran in the bulk polymer. Increasing the strain rate leads to an increase in activation stress, similar to the yield stress in polymers. Increasing the length of the primary crosslinker with respect to the spiropyran leads to a decrease in activation stress, while the activation strain becomes more strain rate dependent with longer primary crosslinkers. These findings show that the molecular details of the network architecture can be altered to tune the mechanochemical response. To further investigate the role of the polymer network effects on the mechanical activation of spiropyran, torsional creep experiments are carried out with previously established imaging and analysis methods for monotonic loading. In contrast to constant strain rate torsion results, mechanical activation of spiropyran is achieved at stress levels less than the yield stress of the bulk polymer, given sufficient time. Lower levels of creep stress require a longer time but smaller threshold strain for activation. Additionally, a peak in the creep strain rate corresponds closely to the measured optical activation of the spiropyran, indicating mechanophore activation occurs near the onset of strain hardening, when polymer mobility is highest. A strong correlation between the mechanical activation of spiropyran and the rate of deformation is also found, and holds for both monotonic torsion and torsional creep. Finally, localized activation of a mechanophore-functionalized interface is demonstrated. E-glass fibers are surface functionalized with SP and embedded in a polymer matrix. Interfacial shear stress is imparted to the SP by a single fiber microbond testing protocol. In situ detection of SP activation at the interface is monitored by fluorescence spectroscopy. Unlike previous studies for bulk polymers, SP activation is achieved and detected at very low levels of applied shear stress. By linking SP at the glass-polymer interface and transferring load directly to the interface, a more efficient mechanism for eliciting SP response is achieved. The combined experimental, imaging and analysis techniques developed in this dissertation quantify activation of a SP mechanophore at different size scales and facilitate the understanding of the mechanisms of mechanochemical activation in polymeric materials.
Issue Date:2013-02-03
Rights Information:Copyright 2012 Cassandra Kingsbury
Date Available in IDEALS:2013-02-03
Date Deposited:2012-12

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