Challenges in quantifying how force affects bond formation have hindered the widespread adoption of mechanochemistry. We used parallel tip-based methods to determine reaction rates, activation energies, and activation volumes of force-accelerated [4+2] Diels-Alder cycloadditions between surface-immobilized anthracene and four dienophiles that differ in electronic and steric demand. The rate dependences on pressure were unexpectedly strong, and substantial differences were observed between the dienophiles. Multiscale modeling demonstrated that in proximity to a surface, mechanochemical trajectories ensued that were distinct from those observed solvothermally or under hydrostatic pressure. These results provide a framework for anticipating how experimental geometry, molecular confinement, and directed force contribute to mechanochemical kinetics.

The first documented example appears in the History of Herodotus, [2] which relates the story of an emissary who had a message tattooed into his head that was revealed upon shaving. Ever since, developing increasingly sophisticated steganographic methods has been a focus of statecraft and spycraft. One emphasis in this field has been on the development of "invisible inks" whose messages appear in response to an external stimulus. As late as World War II, fruit juices, urine, and vinegarinks that darken upon heating-were still used to transmit messages clandestinely. Modern invisible ink schemes invoke fluorescent chemicals, [3][4][5][6] metal-organic frameworks, [7] nanoparticles, [8,9] DNA, [10,11] proteins, [12,13] and even living organisms, [14] but in the cat-and-mouse world of steganography, there will always be a need for new inks that reveal hidden messages in response to an appropriate stimulus.Here we report new approaches to concealing information that rely upon creating microscale patterns composed of different polymer brushes, where at least one has an optically detectable response to external stimuli. In the first, thermoresponsive poly(N-isopropylacrylamide) (pNIPAM) brushes are patterned alongside nonresponsive poly(N,N-dimethylacrylamide) (pDMA) polymers of the same height (Figure 1A). Upon heating in water above their lower critical solution temperature (LCST) of 32 °C, [15][16][17] the pNIPAM brushes collapse, while the pDMA brushes remain unchanged (Figure 1B), thereby resulting in a change in contrast visible to the naked eye that reveals the hidden message. While such a steganographic system is conceptually simple, achieving it is not and requires addressing several major challenges in brush polymer and surface chemistries. Essentially, this application demands a printing method that can control five independent parameters of each pixel in the polymer brush pattern-the x and y position on the surface, the height of the brushes, their chemical composition, and their changes in time. By our recently coined terminology, [18] such a pattern would be designated a "5D hypersurface." The two major bottlenecks to creating these steganographic 5D hypersurface are: (i) finding conditions to print brushes of different chemical composition with the same heights, and (ii) printing these brushes into arbitrary patterns and with microscale dimensions. The former requires an indepth understanding of the polymerization kinetics of both A photochemical printer, equipped with a digital micromirror device (DMD), leads to the rapid elucidation of the kinetics of the surface-initiated atomtransfer radical photopolymerization of N,N-dimethylacrylamide (DMA) and N-isopropylacrylamide (NIPAM) monomers. This effort reveals conditions where polymer brushes of identical heights can be grown from each monomer. With these data, hidden images are created that appear upon heating the substrate above the lower critical solution temperature (LCST) of polyNIPAM. By introducing a third monomer, methacryloxyethyl thiocarbamoyl r...