From molecular mechanochemistry to stress-responsive materials

Abstract

Current activity in, and future prospects for, the incorporation of mechanochemically active functional groups (''mechanophores'') into polymers is reviewed. This area of research is treated in the context of two categories. The first category is the development of new chemistry in the service of material science, through the design and synthesis of mechanophores to provide stress-sensing and/or stress-responsive elements in materials. The second category is the reverse-the development of new material architectures that efficiently transmit macroscopic forces to targeted molecules in order to generate chemical reactivity that is inaccessible by other means. IntroductionThe mechanical forces typical of daily life have the potential to induce dramatic reactivity at the molecular level. The force between an infant's clenched finger and thumb, for example, is more than ten billion times that of the force between atoms in a carbon-carbon bond. Not only are macroscopic forces many orders of magnitude greater than atomic forces, they are also directional, and therefore differ from conventional forms of energy input such as heat and light. In the past four years, several studies have demonstrated that macroscopic mechanical forces can be harnessed at the molecular level, creating a new tool for the organic and materials chemist alike. Broadly, the opportunities in this area can be divided into two categories. First, there is the opportunity to develop new chemistry in the service of material science by designing and synthesizing mechanically activated functional groups (''mechanophores'') and incorporating them as stress-sensing and/or stress-responsive elements in materials. The second opportunity is the complement of the first-the development of new material architectures that efficiently transmit macroscopic forces to targeted molecules and, in so doing, open up a world of chemical reactivity that is inaccessible by other means. We will refer to these as ''chem / mat'' and ''mat / chem'' mechanochemistry, respectively (Fig. 1). The field of mechanochemistry therefore touches on materials chemistry from the point of view of each of its principle progenitors with potential utility in areas ranging from stoichiometric reactivity and catalysis to stress-responsive and selfhealing polymers.We see the greatest opportunities in mechanochemistry arising from situations in which the mechanical force is directly applied to the mechanophore, so that a directional coupling between the vector of applied force and the reaction of interest is possible. The focus of this paper, then, will be on the mechanochemistry of polymers under tension, where the mechanical coupling is most obvious and, therefore, most amenable to the chemist's intuition. Other aspects of mechanochemistry, such as those involved in the milling of crystals, metals, and alloys, 1 are well known and important fields, but they will not be discussed here. A comprehensive review of mechanochemistry in polymers has been published recently by Caruso et al., 2 and it is ...

Model studies of force-dependent kinetics of multi-barrier reactions

“…The compositions of all reaction mixtures were monitored for at least three half-lives by reverse-phase HPLC with aqueous CH 3 CN as eluent using an inert internal standard (see Supplementary Tables S3 and S4 for retention times and separation conditions). At mM concentrations of the cyclic disulphide, the reactions were first order in both phosphine and disulphide and yielded dithiol and phosphine oxides as the sole products, as confirmed independently by 1 The differential rate law for reduction of disulphides 1-6 is given by equations (1 and 2). To estimate the rate constants for the formation of the strain-free zwitterionic intermediates (Fig.…”

“…5). The variation of k 1 Z values across disulphides Z1-Z6 probably reflects the electronic influence of the linkers connecting the disulphide moiety to stiff stilbene (red, Fig. 3b).…”

“…Such force-dependent reactivity is thought to account for the behaviour of polymeric materials, melts and solutions under load, may be exploited to yield new stress-responsive, actuating and energy transducing materials [1][2][3][4][5][6][7][8][9][10] and may even allow characterization of transition states. 11 Considerable theoretical, 3,[12][13][14][15][16][17][18][19] computational 13,[20][21][22][23][24][25][26] and experimental [27][28][29][30][31][32][33][34] effort has been devoted to refine and validate a model of forcedependent kinetics for elementary (single-barrier) reactions.…”

Cephalopod-inspired design of electro-mechano-chemically responsive elastomers for on-demand fluorescent patterning

“…We then integrate the EMCR elastomer into a display panel that can be remotely controlled. Voltages applied to the display panel induce various patterns of large deformation on the surface of the EMCR elastomer, and the induced stresses in turn lead to a versatile range of fluorescent patterns including lines, circles and letters on demand [22][23][24][25] . The activation of EMCR elastomer and fluorescent patterning are reversible and repeatable over multiple cycles, in contrast to the irreversible plastic deformation or fracture required for activating most existing mechanoresponsive polymers 21,[26][27][28][29][30][31][32][33][34] .…”

Mechanochemistry in Polymers with Supramolecular Mechanophores

“…[3,9,12,13] Indeed, this emerging design approach has recently permitted the development a range of adaptive solid-state polymers in which pre-programmed functions can be triggered by external stimuli, including mechanical triggers. [14] While the general aspects of mechanochemistry with polymers have been the subject of several recent reviews, [15][16][17][18] this report summarizes the development of polymer mechanochemistry involving non-covalent mechanophores at the hand of pertinent examples from the recent literature. The materials are organized according to the types of noncovalent interactions exploited (pi-pi, metal-ligand, hydrogen-bonding).…”