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Title:Mechanical and optical characterization of force induced chemical reactions in solid state linear polymers
Author(s):Beiermann, Brett
Director of Research:Sottos, Nancy R.
Doctoral Committee Chair(s):Sottos, Nancy R.
Doctoral Committee Member(s):White, Scott R.; Braun, Paul V.; Moore, Jeffrey S.; Cheng, Jianjun
Department / Program:Materials Science & Engineerng
Discipline:Materials Science & Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Polymer Mechanics
Abstract:Traditionally, chemical reactions are driven by thermal, chemical, or electrical potential. By linking force-sensitive chemical species (mechanophores) into polymer backbones, mechanical force can drive chemical reactions. Mechanophores have been developed with potential as damage sensing, self-healing, and self-reinforcing materials. This research investigates the conditions for promoting mechanophore activation in bulk, linear polymers. An optically active mechanophore is studied. The mechanophore, spiropyran (SP), reacts to a merocyanine (MC) form under tensile force when linked into a polymer backbone. This reaction is reversible and can be driven toward either SP or MC photochemically. Reaction of SP to MC (activation) is accompanied by the emergence of a strong color change and fluorescence signal. SP is incorporated into a polymer backbone by using the mechanophore as an initiator for a living radical polymerization and growing polymer chains at two sites across the SP molecule, thereby covalently bonding the mechanophore in the center of a polymer chain. Polymer mechanics and mechanophore activation are characterized in both glassy and elastomeric polymers. An experimental set-up is designed and implemented to simultaneously measure stress, strain, fluorescence, and birefringence during tensile deformation of SP-linked polymer samples. By varying the loading conditions and polymer mechanical properties, mechanophore activation is examined as a function of the stress, polymer mobility, structure and orientation of polymer chains. In an elastomeric polymer, poly(methyl acrylate) (PMA), higher macroscopic stress leads to higher degrees of SP activation at lower levels of deformation. By changing the polymer architecture - increasing the number of polymer chains attached to the mechanophore - increased activation is demonstrated at relatively slow deformation rates. Activation energy for the SP↔MC conversion is quantified for an elastomeric polymer based on the kinetics of the reaction. The effect of varying stress on reaction rates and energy barriers is determined using a combined experimental and theoretical approach. Tensile deformation of SP-linked glassy polymers at room temperature (RT) does not lead to detectable mechanophore activation. Increasing polymer chain mobility, either using a plasticizing solvent or varying test temperature, leads to a range of thermomechanical properties in which glassy SP-linked polymers can be activated by tensile deformation. Within this favorable activation window, the strain to activation varies based on the stiffness of the polymer. The minimum observed strain to activation is approximately 5%, coincident with the onset of polymer yield. The role of polymer chain alignment and mechanophore orientation are studied using optical techniques. Polymer chain alignment is determined by measurement of birefringence. Activation of mechanophores occurred when polymer chains reached a maximum alignment implying that energy is most efficiently transferred to SP when the polymer chains are aligned in the direction of force. Additionally, mechanophore orientation within the polymer backbone is measured by polarized fluorescence measurements. Mechanophores oriented in the direction of force activate preferentially when compared to those unaligned with the loading direction. Polarized fluorescence measurements also provide insight on polymer mechanics and force on polymer chains. The force driven reaction of spiropyran mechanophores investigated in this dissertation provides useful guidelines for development and characterization of future mechanochemically active material systems. Polymer architecture, mobility and molecular force transfer are critical variables that control mechanophore activity in bulk polymers. 
Issue Date:2013-05-24
Rights Information:Copyright 2013 Brett Beiermann
Date Available in IDEALS:2013-05-24
Date Deposited:2013-05

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