Molecular-Level Engineering of Stress-Responsive Materials
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.
single molecule force spectroscopy
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