Molecular assembling by the radiation pressure of a focused laser beam: Poly(N-isopropylacrylamide) in aqueous solution

Phase transitions in aqueous solutions of poly(N-isopropylacrylamide) (PNIPAM) with a molecular weight (<(M)over bar w>) of 63 000 were achieved by irradiating the solutions (0.2-3.6 wt %) with an IR laser beam (1064 nm) through an optical microscope. First, a microparticle with the size of the focused laser beam was formed (congruent to 1.5 mu m). This microparticle continuously grew and after prolonged irradiation (up to 10 min), a microparticle with a maximum size of 25 mu m was obtained. Upon further irradiation, the microparticle became unstable and finally disappeared. The importance of the optical alignment of the microscope/laser system is discussed. Particle formation was also found in D2O solutions of PNIPAM. These experimental results indicate that, besides a photothermal effect (heating up of the solution due to absorption of water at 1064 nm), there is influence of the ''radiation force'' upon particle formation and conformation properties of the polymer. The observations mentioned above are discussed in connection with the theory of the single beam gradient force optical trap for dielectric particles.