Mechanical actuation by responsive polyelectrolyte brushes and triblock gels

Progress in the development of actuating molecular devices based on responsive polymers is reviewed. The synthesis and characterization of grafted from brushes and triblock copolymers is reported. The responsive nature of polyelectrolyte brushes, grown by surface initiated atomic transfer radical polymerization (ATRP), has been characterized by scanning force microscopy, neutron reflectometry, and single molecule force measurements. The molecular response is measured directly for the brushes in terms of both the brush height and composition and the force generated by a single molecule. Triblock copolymers, based on hydrophobic end blocks and polyacid mid-block, have been used to produce polymer gels where the deformation of the molecules can be followed directly by small angle X-ray scattering (SAXS), and a correlation between molecular shape change and macroscopic deformation has been established. A Landolt pH-oscillator, based on bromate/sulfite/ferrocyanide, with a room temperature period of 20 min and a range of 3.1 < pH < 7.0, was used to drive periodic oscillations in volume in this pH responsive hydrogel. The triblock copolymers demonstrate that the individual response of the polyelectrolyte molecules scale affinely to produce the macroscopic response of the system in an oscillating chemical reaction.