雙雷射誘發聚(N-異丙基丙烯醯胺)溶液之相轉變

Abstract

雷射捕捉是一種用來控制微小粒子或是分子的方法，它不僅可以捕捉到小粒子還可以誘發分子的聚集。我們用這個方法來研究聚(N-異丙基丙烯醯胺)(PNIPAM)的相轉變過程。PNIPAM是一個熱感性聚合物，當溫度超過低臨界溶液溫度(LCST, 32oC)時，相轉變會從線圈狀態變成球狀態，同時發生脫水現象。 在同時照射捕捉雷射(近紅外光1064 奈米)和另一個弱的紫外光雷射(325 奈米)情況下，我們發現了一個非常特別的相轉變行為。照射捕捉雷射時，球型粒子的形成代表相轉變已經發生，再加入325 奈米雷射和捕捉雷射同時照射下，會引發球型粒子的擴大，而且只有加入325奈米的雷射才會有這個現象。對於這樣的現象，我們提出了兩個機制: 共振效應和光轉熱能循環效應。因為PNIPAM會吸收325奈米的光，可以和325奈米的雷射產生共振效應，因此可能可以增進PNIPAM被捕捉的效率。325奈米的光可以激發PNIPAM分子(S0→S1)，而1064奈米的捕捉雷射可以進一步激發PNIPAM到更高的激發態(S1→Sn)，當回到最低激發態(S1)時，能量會轉換成熱能，捕捉雷射持續的照射下，PNIPAM分子不斷的在激發態之間循環，導致光轉熱能循環效應。 在重複使用同一個PNIPAM溶液做雙雷射誘發PNIPAM相轉變的實驗，發現粒子的擴展效率會越來越高，我們稱之為”累積效應”。已經聚集的PNIPAM分子在關掉雷射後並不會擴散進而回到原始的溶液狀態，導致在之後的實驗會增進相轉變的效率，這樣的現象也和前面所述的兩種可能的機制有很大的關係。在”累積效應”，我們還發現了結構性顏色(structural color)的產生。如果PNIPAM混合不同的染料分子，強烈的粒子擴展現象也會發生在雙雷射的照射下。

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.