Additive manufacturing with a flex activated mechanophore for nondestructive assessment of mechanochemical reactivity in complex object geometries
Graphical abstract
Introduction
Mechanophores are molecular moieties that respond chemically to external mechanical stimuli [1]. In materials that contain mechanophores, macroscopic strains are translated into molecular-scale deformations at the mechanophore, which augment the potential energy surface toward specific chemical reactivity. To date, mechanophores have been demonstrated in polymers and network materials to achieve behaviors such as color change (mechanochromism) [[2], [3], [4]], enhanced luminescence [5], fluorescence [6], catalysis [7], self-reinforcement [8], polymer backbone structural reconfiguration [9], and small molecule release [10]. Besides molecularly designing new mechanophores that undergo force-induced bond cleavage [11], cycloreversion [12], or electrocyclic ring opening [13], efforts have been made to bridge functional mechanophores with engineering applications. For example, Zhao and coworkers fabricated electro-mechano-chemically responsive elastomeric display systems that were able to generate fluorescent patterns by coupling the mechanochemical isomerization of spiropyran with macroscopic shape deformation caused by applied electric fields [14]. In a demonstration of autonomously self-reinforcing materials, Craig and coworkers explored polymeric materials that contained gem-dibromocyclopropane mechanophores, wherein spontaneous nucleophilic substitution resulted in crosslinking and strengthening of the material under otherwise destructive forces [8].
As polymer mechanochemistry continues to develop as an avenue for designing stimuli-responsive materials, one can begin to consider opportunities at the interfaces of mechanochemistry and manufacturing. One of the most rapidly developing manufacturing techniques is additive manufacturing (AM), commonly referred to as “3D printing,” which has enabled the production of complex three-dimensional (3D) structures with relative ease [15]. Recently, the fabrication of rapidly customizable mechanochromic devices was achieved through the integration of custom mechanochromic filaments with melt material extrusion AM [16,17]. These reports signified a potentially exciting opportunity to pair molecular-level strain sensitivity with custom macroscopic object geometries. As an early step toward realizing the potential of AM with mechanochemically responsive build materials, we sought a mechanophore that could quantitatively “report” activation throughout a geometrically complex object.
In many cases, mechanophore activation is assessed through spectroscopic means, which involves either fabrication of test samples that are specific to the method of analysis, or processing of the bulk material in a way that destroys the test specimen (e.g., dissolving the material for solution-based analyses). Mechanochromic responses can help address this challenge, but quantitative assessment is often limited to smooth, flat surfaces that are optically accessible. For instance, lattice and microstructed materials would each present formidable challenge to comprehensive assessment of mechanochromism. We hypothesized that mechanophores capable of releasing extractable, small molecules could provide a means toward quantifying mechanophore reactivity throughout the entire volume of a test specimen without requiring dissolution, digestion, or otherwise destroying the specimen. Flex activated mechanophores, such as oxanorbornadiene (OND), are potentially good candidates for such applications [18]. The OND mechanophore was previously demonstrated in bulk thermoplastics and polyurethane elastomers to release a small molecule (benzyl furfuryl ether) that could be extracted and quantified after mechanical activation (Fig. 1) [10,18]. One notable caveat is that the efficiency of extraction would be dependent upon the swelling characteristics of the bulk material and the dimensions of the test specimen. In this study, we present the first demonstration, and characterization, of 3D structures with complex object geometries fabricated via AM, which incorporate a flex activated mechanophore that enables the quantitative assessment of mechanophore activation within mechanically reversible regimes.
Section snippets
Results and discussion
We first selected an appropriate AM method based on the intended target geometries of our test specimens (e.g. lattice materials) and the inherent reactivity of the OND mechanophore. The desire to have overhangs and void spaces within lattice designs prompted the use of vat photopolymerization [19]. Moreover, the general thermal instability of OND substrates was likely to preclude melt material extrusion. As a subset of vat photopolymerization, we applied digital light processing additive
Conclusions
In summary, we have confirmed the compatibility of a flex activated OND mechanophore with DLP-AM techniques. To our knowledge, this is the first demonstration of AM with a flex activated mechanophore. The OND mechanophore was incorporated into an elastomer photocurable resin that resulted in good printing characteristics for a series of microstructured test specimens. This enabled investigation of the force-induced small molecule release characteristics of the printed material by bulk
Acknowledgements
We gratefully acknowledge financial support from the Army Research Office (Grant No. W911NF-15-1-0139 and W911NF-17-1-0595). We thank Professors Duane Storti (University of Washington), Mark Ganter (University of Washington), Alshakim Nelson (University of Washington), and Stephen Craig (Duke University) for helpful discussions.
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