Reversible phase transitions in polymer gels induced by radiation forces

Many polymer gels undergo reversible, discontinuous volume changes in respo nse to changes in the balance between repulsive intermolecular forces that act to expand the polymer network and attractive forces that act to shrink it. Repulsive forces are usually electrostatic or hydrophobic in nature, wh ereas attraction is mediated by hydrogen bonding or van der Waals interacti ons. The competition between these counteracting forces, and hence the gel volume(1-3), can thus be controlled by subtle changes in parameters such as pH (ref. 4), temperature(5), solvent composition(6) or gel composition(7). Here we describe a more direct influence on this balance of forces, by sho wing that the radiation force generated by a focused laser beam induces rev ersible shrinkage in polymer gels. Control experiments confirm that the las er-induced volume phase transitions are due to radiation forces, rather tha n local heating, modifying the weak interactions in the gels, in agreement with previous observations of light-induced chain association in polymer so lutions(8,9). We rnd that, owing to shear-relaxation processes(10), gel shr inkage occurs up to several tens of micrometres away from the irradiation s pot, raising the prospect that the combination of stimuli-responsive polyme r gels and laser light might lead to new gel-based systems for applications such as actuating or sensing.