Browsing by Subject "mechanophore"

The coupling of mechanical stress fields in polymers to covalent chemistry (polymer mechanochemistry) has provided access to previously unattainable chemical reactions and polymer transformations. In the bulk, mechanochemical activation has been used as the basis for new classes of stress-responsive polymers that demonstrate stress/strain sensing, shear-induced intermolecular reactivity for molecular level remodeling and self-strengthening, and the release of acids and other small molecules that are potentially capable of triggering further chemical response. The potential utility of polymer mechanochemistry in functional materials is limited, however, by the fact that to date, all reported covalent activation in the bulk occurs in concert with plastic yield and deformation, so that the structure of the activated object is vastly different from its nascent form. Mechanochemically activated materials have thus been limited to “single use” demonstrations, rather than as multi-functional materials for structural and/or device applications. Here, we report that filled polydimethylsiloxane (PDMS) elastomers provide a robust elastic substrate into which mechanophores can be embedded and activated under conditions from which the sample regains its original shape and properties. Fabrication is straightforward and easily accessible, providing access for the first time to objects and devices that either release or reversibly activate chemical functionality over hundreds of loading cycles.

While the mechanically accelerated ring-opening reaction of spiropyran to merocyanine and associated color change provides a useful method by which to image the molecular scale stress/strain distribution within a polymer, the magnitude of the forces necessary for activation had yet to be quantified. Here, we report single molecule force spectroscopy studies of two spiropyran isomers. Ring opening on the timescale of tens of milliseconds is found to require forces of ~240 pN, well below that of previously characterized covalent mechanophores. The lower threshold force is a combination of a low force-free activation energy and the fact that the change in rate with force (activation length) of each isomer is greater than that inferred in other systems. Importantly, quantifying the magnitude of forces required to activate individual spiropyran-based force-probes enables the probe behave as a “scout” of molecular forces in materials; the observed behavior of which can be extrapolated to predict the reactivity of potential mechanophores within a given material and deformation.

We subsequently translated the design platform to existing dynamic soft technologies to fabricate the first mechanochemically responsive devices; first, by remotely inducing dielectric patterning of an elastic substrate to produce assorted fluorescent patterns in concert with topological changes; and second, by adopting a soft robotic platform to produce a color change from the strains inherent to pneumatically actuated robotic motion. Shown herein, covalent polymer mechanochemistry provides a viable mechanism to convert the same mechanical potential energy used for actuation into value-added, constructive covalent chemical responses. The color change associated with actuation suggests opportunities for not only new color changing or camouflaging strategies, but also the possibility for simultaneous activation of latent chemistry (e.g., release of small molecules, change in mechanical properties, activation of catalysts, etc.) in soft robots. In addition, mechanochromic stress mapping in a functional actuating device might provide a useful design and optimization tool, revealing spatial and temporal force evolution within the actuator in a way that might also be coupled to feedback loops that allow autonomous, self-regulation of activity.

In the future, both the specific material and the general approach should be useful in enriching the responsive functionality of soft elastomeric materials and devices. We anticipate the development of new mechanophores that, like the materials, are reversibly and repeatedly activated, expanding the capabilities of soft, active devices and further permitting dynamic control over chemical reactivity that is otherwise inaccessible, each in response to a single remote signal.

The insertion of force-sensitive motifs (mechanophores) into polymer backbones provides a mechanism to induce forbidden reactions, stabilize transition state, and build intrinsic stress-responsive materials. Although polymer mechanochemistry has provided the basis for a variety of stress-responsive materials (e.g., those that are mechanochromic, mechanoluminescent, and mechanocatalytic, or that release small molecules or generate novel chemical reactions), many desirable stress-responsive behaviors have yet to be realized. We applied molecular-level design and synthesis to engineer stress-responsive materials that address several gaps in the prior polymer mechanochemistry toolbox:

Chapter 2 presents four studies of structure-reactivity relationships. First, regiochemical effects on mechanophore reactivity is quantified in the context of three spiropyran (SP) derivatives that are incorporated into polydimethylsiloxane (PDMS) elastomers. Under thermodynamic control, we find that the relative activation of the regioisomers is well correlated with the extent of mechanochemical coupling between the equilibrium reaction coordinate and the applied force, as quantified by computational modeling. Second, subtle differences in stereochemistry between two gem-monochlorocyclopropane (gMCC) stereoisomers (i.e., syn and anti, relative to the polymer attachment points through which force is delivered) lead to dramatic differences in reactivity and reactivity outcomes. The two gMCCs were embedded along a polymer backbone and their mechanochemical reactivities were quantified using single molecule force spectroscopy (SMFS). The mechanical ring-opening of syn-gMCC proceeds along an anti-Woodward-Hoffmann-Depuy pathway and exhibits significantly lower reactivity than anti-gMCC. Further, under tension applied through ultrasonication, the syn-gMCC isomer generates about 0.25 equivalents of HCl per ring-opening event, whereas ring opening of the anti-gMCC led to no detectable HCl under identical conditions. Third, we report the dependence of the mechanical strength of a C-S bond on the oxidation state of the S atom (i.e., the relative mechanical strength of sulfide, sulfoxide and sulfone). Ultrasonication of gem-dichlorocyclopropane (gDCC) copolymers of each sulfur-containing group reveals that their relative mechanical strengths follow the order: polysulfide ~ polysulfone > polysulfoxide. Finally, we demonstrate the effect of cyclic polymer structure/architecture on mechanophore activation along a polymer backbone. A multi-mechanophore, cyclic gDCC copolymer was prepared via ring expansion metathesis polymerization (REMP), and its mechanochemical response to ultrasonication was compared to a linear analog prepared using ring opening metathesis polymerization (ROMP). The cyclic polymer experiences less gDCC activation per fragmentation along its backbone than does the linear analog. This observation suggests conformational memory effects in the nascent cyclic polymer during elongation and fragmentation.

Chapter 3 introduces two new mechanophores that convert mechanical input into potentially useful chemical signals, enriching the available toolkit of stress-responsive behaviors. The first mechanophore is a 1,2-diaitidinone (DAO) based four-member ring that generates reactive isocyanate upon mechanical activation via pulsed ultrasonication; evidence for the generation of isocyanate is acquired by 1H NMR analysis and trapping experiments. We anticipate that this latent reactive isocyanate might lead to materials that heal or strengthen in response to a mechanical load. A second mechanophore is a new thermally stable and nonscissile mechanoacid that is based on a methoxy-substituted gDCC and that overcomes drawbacks present in previously reported mechanoacids. The introduction of the methoxy substituent not only facilitates the release of HCl as a result of gDCC ring opening (0.58 equivalents per activation), it significantly lowers the force necessary to trigger rapid ring opening, as evidenced by SMFS studies. The utility of this new mechanoacid is demonstrated in PDMS elastomers, where its mechanical activation leads to a strain-triggered color change in a pH-sensitive dye prior to fracture of the elastomer. The post-activation kinetics of coloration are used to demonstrate a new concept in mechanochromism, namely not only a spectroscopic indicator of whether and where a mechanical event has occurred, but when it occurred.

Chapter 4 describes how the well-known photoswitch azobenzene, when embedded into PDMS elastomers, can be used as a mechanochromic probe of the molecular forces present in strained bulk materials. Specifically, the cis-to-trans isomerization of azobenzene is accelerated under uniaxial tension. The kinetics are cleanly described by a single exponential first-order process (k = 2.7 × 10-5 s-1) in the absence of tension, but they become multi-exponential under constant strains of 40-90%. The complex kinetics can be reasonably modeled as a two-component process. The majority (~92%) process is slower and occurs with a rate constant that is similar to that of the unstrained system (k = 2.3–2.7 × 10-5 s-1), whereas the rate constant of the minority (~8%) process increases from k = 10.1 × 10-5 s-1 at 40% strain to k = 21.3 × 10-5 s-1 at 90% strain. Simple models of expected force-rate relationships suggest that the average force of tension per strand in the minority component ranges from 28 pN to 44 pN across strains of 40-90%.

Finally, in Chapter 5, polymer mechanochemistry is integrated into two degradable polymers to demonstrate new concepts in mechanically coupled degradable polymers. The unintentional scission of chemical degradable functionalities on the polymer backbone can diminish polymer properties, and we report a strategy that combats unintended degradation in polymers by combining two common degradation stimuli—mechanical and acid triggers—in an “AND gate” fashion. A cyclobutane (CB) mechanophore is used as a mechanical gate to regulate an acid-sensitive ketal that has been widely employed in acid degradable polymers. This gated ketal is further incorporated into the polymer backbone. In the presence of acid trigger alone, the pristine polymer retains its backbone integrity, and delivering high mechanical forces alone by ultrasonication degrades the polymer to an apparent limiting molecular weight of 28 kDa. The sequential treatment of ultrasonication followed by acid, however, leads to a further 11-fold decrease in molecular weight to 2.5 kDa. Experimental and computational evidence further indicate that the ungated ketal possesses mechanical strength that is commensurate with the conventional polymer backbones. Single molecule force spectroscopy (SMFS) reveals that the force necessary to activate the CB molecular gate on the timescale of 100 ms is approximately 2 nN. With this success in hand, we noted that mechanical-only polymer degradation is intrinsically limited to one chain scission per stretching event. To overcome this drawback, we integrate multiple copies of a [4.2.0]bicyclooctene (BCOE) based mechanophore into the polymer backbone. Mechanochemical remodeling of the polymer backbone occurs through the force-promoted forbidden ring-opening of BCOE, the product of which undergoes a subsequent, slower cascade lactonization that leads to a spontaneous, force-free decrease in average molecular weight to 4.4 kDa (from an initial molecular weight of over 120 kDa) over the course of 9 days.

Toughness, the measure of how much energy a material can absorb before rupture, is an important property of materials. It has been demonstrated that the toughness of a single polymer chain of gem-dihalocyclopropane (gDHC) functionalized polybutadiene (PB) is increased dramatically over PB alone, due to the mechanically triggered electrocyclic ring opening reaction of gDHC into 2,3-dibromoalkenes. This thesis explores whether this molecular mechanical property can also be manifested in bulk material properties. Crosslinked gem-dichlorocyclopropane (gDCC) embedded PB polymers were swollen in various solvents, and the resulting gels were mechanically deformed under tensile stress. Young's modulus and fracture toughness were compared among PBs with gDCC incorporated in the backbones and/or crosslinking positions. The results showed that the incorporation of gDCC does not measurably increase the fracture toughness of the crosslinked polymer gels. Neither NMR nor FT-IR characterization of the post-test samples revealed detectable activation of the gDCC in the crosslinked PB. Further experiments will be focused on optimizing the polymer structure and testing methods to more effectively transfer the macroscopic force to the mechanophore in the material and continuing exploring the correlation between molecular responses and changes in macroscopic properties.

Materials often fail as a result of the mechanical loads they experience during use. On the molecular level, forces within polymers are distributed unevenly throughout the material, and some polymer subchains experience greater stress than others. In some cases, the forces experienced by these overstressed subchains can trigger chain scission events. Chain scission in turn might nucleate the formation of a microcrack that subsequently propagates, ultimately leading to material failure. In recent years, force reactive functional groups, or mechanophores, have emerged as the basis of a potential strategy for combatting this destructive cascade. The strategy, known as activated remodeling via mechanochemistry (ARM), comprises embedding mechanophores along the polymer backbone or within cross-­‐‑links, so that otherwise destructive force within an overstressed subchain triggers a constructive, rather than a destructive, response. ARM functions in both solution and bulk to form remodeled polymer networks where the number of bonds formed exceeds the number of bonds broken under typically destructive mechanical conditions. It requires no additional external stimulus or energy input beyond the imposed shear and results in orders-­‐‑of-­‐‑magnitude increases in bulk moduli.

These demonstrations have spurred a range of important and fundamental questions about stress-­‐‑responsive remodeling, including how to dissect the complex interplay between material deformation, mechanophore activation, nascent cross-­‐‑link rupture, mechanochemically triggered cross-­‐‑link formation, and the impact of various stages of each on the mechanical properties and eventual failure of the material. The answers to these questions require new mechanophores that not only activate and then cross-­‐‑link efficiently, but that give clear spectroscopic signatures of their state so that the levels of both activation and cross-­‐‑linking can be measured in situ and in real time.

In this dissertation, we design and explore two families of mechanophores for use in the context of the ARM concept. The first family is based on a substituted cyclobutene scaffold, which undergoes a force-­‐‑induced electrocyclic ring-­‐‑opening reaction to unveil butadiene. In Chapter 2, we investigate the intrinsic stability of a variety of substituted cyclobutenes, and then utilize pulsed ultrasound to change electronic distributions and spectroscopic signatures via mechanically unveiling latent conjugation pathways. Furthermore, we show the potential ARM-­‐‑type utility of the cyclobutene mechanophore by using click chemistry to react the activated butadiene with 4-­‐‑phenyl-­‐‑1,2,4-­‐‑triazoline-­‐‑3,5-­‐‑dione (PTAD).

These studies motivated quantification of the mechanical reactivity of the cyclobutene system as a function of substitution. In Chapters 3 and 4, we use single-­‐‑ molecule force spectroscopy (SMFS) to pull individual polymers comprised of cyclobutene mechanophore repeating units, and measure the force required to mechanically induce the ring-­‐‑opening reaction on the time scale of several hundred

milliseconds. We show that changes in polymer attachment near a reacting benzocyclobutene mechanophore can have dramatic effects when pulling from cis handles, but not when pulling from trans handles. Additionally, we provide evidence that electronic effects further away from the cyclobutene ring can be tuned without significantly altering the force at which CBE mechanically ring-­‐‑opens. As demonstrated in Chapter 2, these electronic effects can still have substantial effects for altering conjugation pathways and unveiled reactivity in the mechanically ring-­‐‑opened butadiene product.

The second family of mechanophore investigated in this dissertation is based on the ring-­‐‑opening of an oxabicyclo[2.1.0’pentane (OBP) to reversibly generate a highly colored carbonyl ylide. In Chapter 5, we synthesize a dibromoaryl substituted OBP and characterize the carbonyl ylide generated from application of UV light or heating above 100 ̊C. The carbonyl ylide is highly reactive with dipolarophiles or in the presence of oxygen. Unfortunately, most derivatives are highly sensitive to trace amounts of acid and we were unable to incorporate the putative mechanophore in a polymer. Through our efforts, however, we were able to identify two stable, sulfur-­‐‑based OBPs that we utilize in Chapter 6 in single-­‐‑molecule conductance experiments. In these experiments, we observe no evidence of mechanophore activation as a function of break-­‐‑junction elongation, which suggests that the guiding principles used to understand force-­‐‑induced reactivity may not hold in systems of high confinement.

The final chapter of this dissertation describes an easy-­‐‑to-­‐‑implement science outreach demonstration featuring a mechanically and photochemically color-­‐‑changing polymer. The active polymeric material is a filled poly(dimethylsiloxane) (PDMS) elastomer that is covalently functionalized with spiropyran (SP), which is both a photochemical and mechanochemical switch. The material can be reversibly changed from colorless to dark purple by exposing to light from a blue laser pointer or providing a mechanical stimulus such as hitting the polymer with a hammer or dragging a blunt object across the surface. The keynote demonstration is a PDMS chemical-­‐‑drawing board that allows children to literally ‘write without ink’ using a laser pointer or a blunt stylus. Collectively, these demonstrations are suitable for various student groups, and encompass concepts in polymer and materials chemistry, photochemistry, and mechanochemistry. This demonstration has been successfully employed dozens of times in multiple universities across North America.