Laser-Induced Reversible Volume Phase Transition of a Poly(N-isopropylacrylamide) Gel Explored by Raman Microspectroscopy

Abstract

A micro-rod of poly(N-isopropylacrylamide) (PNIPAM) in a D2O solvent undergoes reversible shrinkage/swelling induced by a focused near infrared (1064 nm) laser beam. Since the solvent is transparent at 1064 nm, the origin of the phenomenon is ascribed not to photothermal effects but completely to an interaction between the gel and the photon pressure generated by the laser beam. Features of this laser-induced shrinkage such as the dynamics and the effects of the combination of heat and photon pressure are clarified and compared with the normal thermo-responsive volume phase transition of a PNIPAM gel. The local structure of the rod of gel upon laser-induced shrinkage was investigated in detail by means of spatially-resolved micro-Raman spectroscopy. The Raman spectra obviously showed dehydration of the polymer chains around the shrunken area of the gel. As a result, the shrinkage was revealed to be a volume phase transition triggered by photon pressure. Furthermore, the fundamental mechanism for the laser-induced volume phase transition is discussed briefly on the basis of Flory-Rhener‘s model taking into account the χ parameter, which depends on the volume fraction of the polymer.