| 摘要: | 本研究主題為利用旋鍍方式，製備聚苯乙烯聚氧化乙烯(poly(styrene-block-ethylene oxide), P(S-b-EO))團鏈共聚合物薄膜，探討其共溶劑退火下奈米結構型態。首先將先探討事前利用水氣與醇類進行pre-annealing的步驟，對於團鏈共聚物中兩鏈段之影響差異。由於極性溶劑先膨潤PEO鏈段，增強兩鏈段間的分離強度(因聚苯乙烯為一疏水性鏈段，不會受水氣膨潤而維持在玻璃態，反之，親水性的PEO鏈段將受到水氣膨潤，大幅降低兩鏈段間的相容性，意味著增強了兩鏈段間的分離強度)，此後在恆溫17 oC下進行有機溶劑退火程序，以非極性溶劑—苯為例，加入pre-annealing步驟的實驗組可得到長程有序的球相結構薄膜，反觀之，直接進行苯蒸氣退火程序的對照組卻為一紊亂無序的球相結構薄膜，後者原因在於退火過程中，苯蒸氣仍會輕微膨潤PEO鏈段，造成團鏈共聚物的分離強度下降，因此得到無序球陣列結構。
隨後將介紹共溶劑退火之影響系統，同樣先利用極性溶劑膨潤薄膜後(如同上述，藉由水氣預先膨潤的步驟以利提高鏈段間的分離強度後)，於恆溫17 oC下，採用非極性(苯、四氫?喃)搭配極性(水、醇類)溶劑之數種組合進行共溶劑退火程序，在四氫?喃系統中，與低碳數醇類溶劑(甲醇、乙醇)進行共溶劑退火的薄膜奈米結構，由於低碳數的極性溶劑具有較高的蒸氣壓，於高膨潤程度的共溶劑系統下，結構從無序球相結構直接變為無序圓柱結構；在高碳數醇類溶劑系統中(丙醇、丁醇、正己醇)，經由低膨潤效果的退火程序，發現不易直接轉變成圓柱相，以能量觀點而言，推測此轉變過程中衍生一有序的球相態(meta-stable state)，須越過此能障方能轉變為有序的平行圓柱相(stable state)。相較於四氫?喃系統，苯具有較差的膨潤效果(Psat,benzene < Psat,THF)，與低碳數醇類溶劑(甲醇、乙醇)會形成無序圓柱相，而與高碳數醇類溶劑(丙醇、丁醇、正己醇)則會形成有序球相。
此外，我們發現當極性溶劑為水與時，在17 oC下與非極性溶劑退火皆得到球相結構。我們嘗試將溫度下降於12oC下，發現雖然在低溫時χ值增加，不過仍得不到球相結構。因此我們利用非極性溶劑為四氫?喃，極性溶劑為丙醇共溶劑退火，我們發現在20oC下三小時，結構為球與圓柱共存，不過當共溶劑退火到五小時，發現結構變為球相結構，因此，證實說在高溫，雖然蒸氣壓增加，不過因為χ值下降，因此結構變為球相結構，也證實說當極性溶劑為水的系統，因χ值低而得不到圓柱結構。而我們利用相同體積分率，分子量為兩倍的PS-b-PEO，在四氫?喃與水退火三小時得到平行無序圓柱結構，證實為分離強度的影響。
In this study, I have investigated the micro-phase separated structures of solvent annealed PS-b-PEO thin films in mixed vapors of binary nonpolar/polar solvents. Before solvent annealing, the surface morphology of as-spun films was dominated by disordered spheres. First, polar-solvents were used to preferentially swell PEO chains, by which the segregation strength between the two segments can be enhanced. This stage is called as a “pre-annealing” step. Then the vapor of nonpolar solvents (benzene or THF) was used for annealing thin films at 17 °C. After solvent annealing, the surface morphology was dominated by hexagonal arrays of spheres. By contrast, if thin films were directly exposed to the vapor of benzene or THF without undergoing the pre-annealing step, the surface morphology was dominated by disordered spheres. The reason is that benzene or THF vapor also swelled the PEO domain in addition to swelling the PS domain. This reduced the segregation strength between the two segments.
Next, thin films were exposed to non-polar/polar co-solvent vapors at 17 °C. The non-polar solvents used were, respectively, toluene, benzene and THF, and polar-solvents were water, methanol, ethanol, propanol, butanol, and hexanol, respectively. In systems of solvent annealing in THF/alcohol co-solvent vapors, both methanol and ethanol have high vapor pressure. Upon exposing to the vapor of THF/methanol and THF/ethanol, the surface morphology of solvent-annealed films revealed disordered nanocylinders. By contrast, upon exposing to solvent vapors of THF mixed with propanol, butanol or hexanol which have lower vapor pressure than that of methanol and ethanol, parallel-oriented nanocylinders with little density of defects can be obtained through transitions from disordered spheres to hexagonal packed ones and then to parallel nanocylinders with long-range order. The reason is that the vapor pressure of propanol, butanol and hexanol is lower than that of methanol and ethanol. As a result, the energy barrier for the transformation directly from disordered spheres to lying cylinders with long-range order was high. I speculate that the disordered spheres transform into lying nanocylinders with long-range order proceeding through an intermediate stage of hexagonal-packed spheres.
Furthermore, only spheres were present in thin films with solvent annealing in vapor of THF/water at 17 °C. As the temperature was decreased to 12 °C, the ordering of nanospheres can be improved. Such morphology was also obtained for solvent annealing in vapor of THF/propanol at 20 °C (5h). In the final part of the thesis, I demonstrate that switchable phase transitions can be induced upon solvent annealing at different temperatures or in vapors of different co-solvent mixtures. |
