Molecular visualization of brush-like macromolecules on surfaces: Molecular conformation and spreading dynamics.

摘要

Polymer molecules change their conformation in response to variations of the substrate's surface energy, vapor pressure of the surrounding environment, and under lateral compression. As such, visualization of brush-like macromolecules as they change their conformation on substrates opens intriguing opportunities for quantitative studies of various interfacial phenomena with molecular (sub-100 nm) resolution. Atomic force microscopy was used to study the conformational switching due to variations in the surface energy, molecular swelling and collapse in asymmetric polymer mixtures, pressure gradient and frictional coefficient within a spreading film, and adsorption-induced scission of covalent bonds. The responsiveness of the molecular brushes allowed us to use them as miniature probes to measure both the pressure gradient and frictional coefficient on the substrate. In addition to conformational changes, we discovered that brush-like macromolecules undergo scission of the covalent bonds that make up their backbone due to physical interaction with the substrate. This significantly expands the range of future applications of branched macromolecules as environmental sensors, mechanical actuators, and reaction accelerators. The process of adsorption-induced bond scission is of first order kinetics and the corresponding rate constant was determined.