In a multi-institutional collaboration, members of the Sheiko Group report in JACS on the synthesis and characterization of a complex polymeric architecture based on a block copolymer with a cylindrical brush block and a single-chain polymeric nanoparticle block folded due to strong intramolecular hydrogen-bonds. The self-assembly of these constructs on mica surfaces was studied with atomic force microscopy, corroborating the distinct presence of block copolymer architectures.
The article received attention in C&EN, where Stephen Ritter describes how advances in polymer synthetic techniques now allow better control over the size and shape of polymers, allowing researchers to think more like architects, dreaming up exotic new polymer designs. One goal of the work from the Sheiko Group is to create macromolecules with functional properties for drug delivery, catalysis, chemical sensing, and other applications.
The phenomenon of molecular "fatal adsorption," first reported by the Sheiko Group in 2006, is a unique mechanochemical process attributed to spontaneous scission of covalent bonds in branched macromolecules upon their adsorption onto a substrate. Self-destruction of synthetic macromolecules is caused by remarkably strong tension of the order of several nanoNewtons, which is developed in the polymer backbone due to steric repulsion between densely-grafted side chains. This unimolecular bond-scission process exhibits two distinct features. First, strong covalent bonds rupture spontaneously without applying any external force. Second, the rate constant of the bond-scission reaction exhibits extraordinary sensitivity to minute variations of the surface energy of the underlying substrate.
As published in PNAS, the Sheiko Group in collaboration with the Matyjaszewski Group at CMU has reported a third novel feature of the adsorption-induced mechanochemistry: an anti-Arrhenius decrease of the bond-scission rate with temperature. This behavior was caused by a decrease in the surface energy of the underlying substrate upon heating, which resulted in a corresponding decrease of bond tension in the adsorbed macromolecules. Even though the tension dropped minimally from 2.16 to 1.89 nN, this was sufficient to overpower the increase in the thermal energy (kBT) in the Arrhenius equation. The exponential variation of reaction rate constant with bond tension is vital for a wide range of applications ranging from lithography and self-healing materials to drug delivery and mechanocatalysis.