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Research Papers

Mechanochemically Responsive Viscoelastic Elastomers

[+] Author and Article Information
Mahdi Takaffoli, Teng Zhang, David Parks

Department of Mechanical Engineering,
Massachusetts Institute of Technology,
Cambridge, MA 02139

Xuanhe Zhao

Department of Mechanical Engineering,
Massachusetts Institute of Technology,
Cambridge, MA 02139;
Department of Civil and
Environmental Engineering,
Massachusetts Institute of Technology,
Cambridge, MA 02139
e-mail: zhaox@mit.edu

1Present address: Department of Mechanical and Aerospace Engineering, Syracuse University, Syracuse, NY 13244.

2Corresponding author.

Contributed by the Applied Mechanics Division of ASME for publication in the JOURNAL OF APPLIED MECHANICS. Manuscript received March 6, 2016; final manuscript received April 12, 2016; published online May 9, 2016. Editor: Yonggang Huang.

J. Appl. Mech 83(7), 071007 (May 09, 2016) (10 pages) Paper No: JAM-16-1121; doi: 10.1115/1.4033431 History: Received March 06, 2016; Revised April 12, 2016

Mechanochemically responsive (MCR) polymers have been designed to possess unconventional properties such as changing colors, self-healing, and releasing catalysts under deformation. These properties of MCR polymers stem from a class of molecules, referred to as mechanophores, whose chemical reactions can be controlled by mechanical forces. Although extensive studies have been devoted to the syntheses of MCR polymers by incorporating various mechanophores into polymer networks, the intricate interactions between mechanical forces and chemical reactions in MCR polymers across multiple length and time scales are still not well understood. In this paper, we focus on mechanochemical responses in viscoelastic elastomers and develop a theoretical model to characterize the coupling between viscoelasticity and chemical reactions of MCR elastomers. We show that the kinetics of viscoelasticity and mechanophore reactions introduce different time scales into the MCR elastomers. The model can consistently represent experimental data on both mechanical properties and chemical reactions of MCR viscoelastic elastomers. In particular, we explain recent experimental observations on the increasing chemical activation during stress relaxation of MCR elastomers, which cannot be explained with existing models. The proposed model provides a theoretical foundation for the design of future MCR polymers with desirable properties.

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Copyright © 2016 by ASME
Topics: Elastomers , Stress
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Figures

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Fig. 1

A piece of a viscoelastic elastomer in the undeformed reference state (a) and in the deformed current state (b). Schematics of the polymer network of a MCR viscoelastic elastomer in the reference (c) and current (d) states. The elastomer consists of pure elastic (black) and relaxable chains (gray). (e) A schematic of a generalized Maxwell model coupled with modules of mechanophores.

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Fig. 2

Mechanochemical transformation of a mechanophore molecule through a reversible ring-opening reaction from the spiropyran state to the merocyanine state (a). Potential energy landscapes of the mechanophore under zero (b) and nonzero (c) applied force.

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Fig. 3

A schematic of a mechanophore-coupled Maxwell element (a). In response to a constant stretch (b), the model illustrated in (a) varies its chain force (c) and activation efficiency (d) over time. A schematic of a mechanophore-coupled viscoelastic elastomer model (e). In response to a constant stretch (f), the model illustrated in (e) varies its chain force (g) and activation efficiency (h) over time.

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Fig. 4

Comparisons of the model's predictions with experimental results: (a) and (b) monotonic loadings at various stretch rates, (c) and (d) a monotonic loading at a stretch rate of 0.02 s−1 accompanied by a constant stretch of 8 held over 300 s. Time evolution of stress in networks A and B (e) and contribution of each network to the total activation efficiency (f) during the loading–relaxation process.

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