Dual laser-induced local phase transition of poly(N-isopropylacrylamide) solution

• Main Authors: Lin, Po-Yu, 林柏宇
• Other Authors: Hiroshi Masuhara
• Format: Others
• Language: en_US
• Published: 2014
• Online Access: http://ndltd.ncl.edu.tw/handle/33012420980395616162

碩士 === 國立交通大學 === 應用化學系碩博士班 === 102 === Laser trapping is a powerful and useful method for controlling and manipulating the small particles and cluster of molecules. It not only can trap the small particle but also induce molecules assembly. We applied this technique to study a phase transition of poly(N-isopropylacrylamide) (PNIPAM). PNIPAM is a thermoresponsive polymer which shows phase transition from coil to globule state where hydrated water molecules are excluded from the polymer matrix when temperature is above lower critical solution temperature (LCST, 32oC). We found an unusual phase transition behavior by irradiating trapping laser (near-infrared 1064 nm) and additional weak UV laser (325 nm). A spherical particle formed by trapping laser irradiation shows an expansion of its diameter when weak 325 nm laser is introduced simultaneously and only 325 nm laser can induce such phenomenon. We proposed two mechanisms for this particle expansion behavior: one is resonance effect and the other is cyclic photothermal heating effect. For resonance effect, by irradiating weak 325 nm laser, it may improve the trapping efficiency by resonance between PNIPAM absorption and laser wavelength. For cyclic photothermal heating effect, weak 325 nm excites the PNIPAM molecules and induce the heating at the excited state by NIR trapping laser. More interestingly, we discovered particle expansion will be enhanced by repeating dual laser-induced particle expansion experiments. We name this phenomenon “accumulation effect”. We consider accumulation effect is due to the diminished diffusion of strongly interacted PNIPAM molecules which is gathered and formed by prior dual laser-induced particle expansion experiments. It will improve the efficiency of phase transition behavior. Proposed reason may strongly relate to above mentioned two mechanisms. Another unusual point in accumulation effect is color dot formation in the PNIPAM matrix. Usually PNIPAM does not show the color, so we consider observed color is a structural color formed by assembled small particles. By adding different dye molecules into PNIPAM solution, it also shows intensive particle expansion by dual laser irradiation.