Phase Separation of Aqueous Poly(vinyl methyl ether) Solutions Induced by the Photon Pressure of a Focused Near-Infrared Laser Beam

We demonstrate that phase separation of an aqueous poly(vinyl methyl ether) (PVME) solution can be triggered not only by a temperature change but also by photon pressure. A near-infrared continuous wave laser beam (λ=1064 nm) was focused through an objective lens into a heavy water (D2O) solution of PVME. A single PVME microparticle was produced and trapped at the focal point of the laser beam within several hundreds of seconds after switching on the laser. The origin of the microparticle formation (phase separation) is ascribed essentially to the photon force of the laser beam, and not to a rise in the local temperature, since heavy water is transparent at 1064 nm. The structures of the PVME microparticles produced by laser irradiation were studied in detail using confocal Raman microspectroscopy. Raman spectra of the microparticles and coiled/globular PVME were observed successfully over a wide wavenumber region. It was confirmed that the phase transition of the polymer chains from coiled to globular states proceeded during microparticle formation. The fundamental mechanism of photo-induced phase separation of PVME is discussed in terms of the interactions between the polymer and the photon force.