Colin J. Crook

Chemically induced oscillations in a pH-responsive hydrogel

A Landolt pH-oscillator based on a bromate/sulfite/ferrocyanide reaction has been developed with a room temperature period of 20 min and a range of 3.1 ≤ pH ≤ 7.0. This system has been used to drive periodic oscillations in volume in a pH responsive hydrogel. The gel is coupled to the reaction and changes volume by a factor of at least 6. A continuously stirred, constant volume, tank reactor was set-up on an optical microscope and the reaction pH and gel size monitored.

Direct visualization of the real time swelling and collapse of a poly(methacrylic acid) brush using atomic force microscopy

The reversible height change dynamics of a poly(methacrylic acid) brush, a weak polyelectrolyte in response to pH changes, were observed using atomic force microscopy. The brush thickness could be repeatably and reversibly switched between 40 nm at pH 3 and 120 nm at pH 10.5. The swelling and collapse transitions took 6 s in the AFM measurements, but high resolution force spectroscopy measurements showed that the collapse can be made to happen in less than a second. We conclude that the response time of these switches is limited in practise by the physical time taken to exchange solvents and by the intrinsic slow dynamics in the brush.

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. Dedicated to Professor John L. Stanford on the occasion of his 60th birthday.