(361i) Novel Stimuli-Triggered Self-Healing and Strengthening Polymers

Authors: 
Gordon, M. B., University of Delaware
Kloxin, C. J., University of Delaware
In nature, all living organisms can sense and respond to environment cues, such as light, temperature or pH. Examples of stimuli-triggered processes include contraction of eyes in response to bright or healing after a cut. These stimuli-triggered processes found in nature motivate the design of â??smartâ? materials which can respond to environmental stimuli [1]. One such class of smart materials is self-healing polymers which have the capacity to recover their functionality after damage, thereby improving safety and increasing the service lifetime of the material. In particular, photo-active polymer networks are promising candidates for healing materials because the use of light offers spatiotemporal control and the response can be triggered remotely [2-4].

Here, we report a new class of photo-responsive polymer networks in which a dynamic bond is incorporated into the crosslinks of the network and undergoes a light-triggered, secondary polymerization which increases the modulus by two orders of magnitude while strengthening the network by over 100% [5]. Unlike traditional two-stage polymerization systems, in which the secondary polymerization is triggered by a leachable photoinitiator [6, 7], we incorporated a dynamic bond capable of initiating polymerization into the network backbone itself. By imparting functionality directly into the network architecture, our material is capable of initiating polymerization via the dissociation of its own crosslinks to become stronger in response to light. Specifically, a labile carbon-dithiocarbamate bond (or iniferter) was incorporated into the network backbone, which triggers a light-initiated, free-radical polymerization that increases the modulus by several orders of magnitude and approximately doubles the strength. The final modulus can be tuned a prioriby modifying the concentration of the polymerizable group in the formulation. By adjusting the concentration, the material properties post-cure can be tailored to meet a wide range of specifications. The dynamic polymer network is readily transferrable to several applications. Specifically, we have demonstrated three properties: 1) photo-induced healing, reforming, and strengthening of a specimen after it has been severed, 2) spatial confinement of bulk property changes via photopatterning and 3) photo-curing the film into any particular 3D configuration.

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[2] Habault, D., Zhang, H. J., and Zhao, Y., â??â??Light-triggered self-healing and shape-memory polymers,â??â?? Chemical Society Reviews 42, 7244-7256 (2013).

[3] Fiore, G. L., Rowan, S. J., and Weder, C., â??â??Optically healable polymers,â??â?? Chemical Society Reviews 42, 7278-7288 (2013).

[4] Chatani, S., Kloxin, C. J., and Bowman, C. N., â??â??The power of light in polymer science: photochemical processes to manipulate polymer formation, structure, and properties,â??â?? Polym. Chem. 5, 2187-2201 (2014).

[5] Gordon, M. B., French, J. M., Wagner, N. J., and Kloxin, C. J., â??â??Dynamic Bonds in Covalently Crosslinked Polymer Networks for Photoactivated Strengthening and Healing,â??â?? Adv Mater 27, 8007-10 (2015).

[6] Nair, D. P., Cramer, N. B., Gaipa, J. C., McBride, M. K., Matherly, E. M., McLeod, R. R., Shandas, R., and Bowman, C. N., â??â??Two-Stage Reactive Polymer Network Forming Systems,â??â?? Advanced Functional Materials 22, 1502-1510 (2012).

[7] Ma, S. J., Mannino, S. J., Wagner, N. J., and Kloxin, C. J., â??â??Photodirected Formation and Control of Wrinkles on a Thiol-ene Elastomer,â??â?? ACS Macro Letters 2, 474-477 (2013).

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