Poly(N-isopropylacrylamide) microparticles produced by radiation pressure of a focused laser beam: A structural analysis by confocal Raman microspectroscopy combined with a laser-trapping technique

We developed a confocal Raman microspectroscopy system combined with a laser trapping technique and applied it to aqueous solutions (H2O and D2O) Of poly(N-isopropylacrylamide) (PNIPA), which is well-known as a representative thermo-responsive polymer, i.e., phase transition/separation between coiled and globular states. By introducing a near-infrared (1064 nm) laser beam into a microscope, PNIPA microparticles were produced at the focused spot of the laser beam, both in H2O and D2O. By using the present system, we succeeded in obtaining the Raman spectra of PNIPA in the coiled and globular states over a wide wavenurnber region (800-3500 cm(-1)) for the first time. For the D2O solutions (in which the photothermal effect is negligible and hence the microparticles should be produced purely by the effect of radiation pressure), some significant differences were observed in the Raman spectra for the coiled state, in the globular state, and for laser induced microparticles. By analyzing these spectra in detail, we revealed that the structure of the laser-induced microparticles was analogous to that in the globular state. We also discuss the fundamental mechanism underlying the transformation of the higher order structure of a polymer by radiation pressure.