芴系環境應答型硬桿-柔軟嵌段共聚物:合成、形態與光物理性質之研究

Abstract

環境應答型嵌段共聚高分子因其具備對特定標的物之高靈敏度而受到廣泛應用。諸多研究中，更以共軛結構之硬桿-柔軟嵌段共聚高分子最受矚目，其可藉由外界環境微小變異有效操控材料之光電性質；且其自組裝能力亦因不同嵌段間之不互溶性及硬桿段π-π作用力影響，生成具新穎功能性之奈米構形。如此奈米構形進而對於材料之物理性質造成影響。然而環境應答型硬桿-柔軟嵌段共聚高分子結構與相型態對應於高分子光電性質之相關研究至今仍未趨完善。故本論文之研究目標著眼於合成環境應答型之芴系硬桿-柔軟嵌段共聚高分子，探討該材料高分子結構、微胞型態對於光電性質之影響。 本論文第一部份(第二章)為探討雙親性聚芴-聚甲基丙烯酸二甲基氨乙酯嵌段共聚高分子之合成、分子結構及其多功能感測特性。此新穎嵌段共聚物採原子轉移自由基聚合方式，製備不同長度之聚甲基丙烯酸二甲基氨乙酯鍊段(重複單元為31、45、93與185)，並藉溶劑組成、溫度與酸鹼值變化，探討微胞結構及其相應光電性質。其中柔軟鍊較短之共聚高分子因溶液水相比例提升，微胞型態由球狀轉為柱狀、束狀與環狀；但長距離規整性之微胞型態於柔軟鍊較長之高分子中則不覆見。於溫度變化之水相系統中，微胞型態則由束狀轉為球型呈可逆性變化，且於原子力顯微鏡與動態光散射儀鑑定下皆得到相似結果。對應於光電性質，提升水相比例將誘使螢光強度降低與吸收光譜藍移；於溫度變化時，螢光強度則與高分子低臨界溶液溫度有關。高分子螢光特性因受到溫度與酸鹼值影響，呈現指示劑之開/關特性：於溫度變化下具有開/關之可逆效果，進而搭配酸鹼值則可轉為關/開與開/關相互切換機制。 本論文第二部份(第三章)為探討雙親性聚芴-聚(氮-異丙基丙烯醯胺)嵌段共聚高分子之合成、分子結構及其熱感應特性。此具備不同聚芴(重複單元為7、15與25)與聚(氮-異丙基丙烯醯胺)鍊長之嵌段共聚物採用偶合反應與原子轉移自由基聚合方式合成，藉溶劑組成與溫度變化，探討微胞結構及其相應光電性質。水相比例提升下，微胞由球型轉為柱狀、蟲型、束狀與蜂窩狀結構。對於柔軟嵌段較長之嵌段共聚物，其微胞於低水相比例下生成球形、柱狀與束狀結構，並轉為小尺寸聚集於高水相環境中。相較於相似鍊長比例之雙嵌段與三嵌段高分子，三嵌段共聚物則較易於高水相環境下生成柱狀與束狀微胞結構。而於溫度變化之水相系統中，微胞型態則呈現可逆性變化。該變化於原子力顯微鏡、穿透式電子顯微鏡與動態光散射儀鑑定下皆可得到相似結果。對應於光電性質，雙嵌段共聚高分子提升水相比例可誘使螢光強度降低與吸收光譜藍移，促使聚芴鍊段產生H形態聚集；而三嵌段共聚高分子於水相比例提高下，則造成吸收與螢光光譜紅移，聚芴鍊段形成J形態聚集。 本論文第三部份(第四章)為探討硬桿-柔軟三嵌段共聚高分子，利用末端帶有疊氮官能基之聚(氮-異丙基丙烯醯胺)-聚(氮-羥甲丙烯醯胺)雙嵌段共聚高分子及具備碳-碳三鍵官能基之聚芴以速配接合反應製得。其中不同嵌段比之聚(氮-異丙基丙烯醯胺)-聚(氮-羥甲丙烯醯胺)嵌段共聚高分子乃藉由帶有疊氮官能基之起始劑以原子轉移自由基聚合方式進行製備；然而聚芴鍊段則由鈴木偶合反應並於末端接上碳-碳三鍵官能基。其中共聚高分子之低臨界溶液溫度伴隨親水性聚(氮-羥甲丙烯醯胺)鍊段長度提升而增加，因較長鍊段之聚(氮-羥甲丙烯醯胺)可促使共聚高分子伸展於高溫環境下。不同鍊長比之聚(氮-異丙基丙烯醯胺)-聚(氮-羥甲丙烯醯胺)於溫度變化下，微胞結構將由球型形變為聚集球體、液胞與蟲型微胞，此型態轉變歸因於聚(氮-異丙基丙烯醯胺)親水/親油相互交替性。相對於三嵌段共聚高分子而言，微胞型態則由柱狀轉換為束狀、中空柱狀，其所形成之條狀結構乃因芴鍊段π-π作用力所誘導。此因溫度變化所形成之微胞型態於原子力顯微鏡、穿透式電子顯微鏡與動態光散射儀鑑定下皆可得到相似之可逆性結果，且與高分子低臨界溶液溫度有關。對應於光電性質，高分子螢光特性則呈現指示劑之開/關特性：於較長聚(氮-異丙基丙烯醯胺)鍊長條件下，溫度提升具有開/關/開之特性，並於鍊長較短時呈現關/開之切換。 綜觀上述研究，共軛聚芴硬桿-柔軟嵌段共聚高分子能有效操控微胞型態於不同嵌段比例、溶劑、pH與溫度變化下，並明顯轉換其光物理性質。

Stimuli responsive block copolymers can provide a variety of applications due to their high sensitivity in detecting targeted species. Among of these studies, conjugated rod-coil block copolymers have attracted considerable attention because of their electronic and optoelectronic properties could be efficiently manipulated by minor perturbations of environmental stimuli. In addition, self-assembly of π-conjugated polymer based block copolymers lead to various nanoscale morphologies driven by inherent immiscibility between different blocks and the packing constrains imposed by the π-π interaction of the conjugated segment. Such morphological transformation can lead to the variation of their physical properties. Nevertheless, the effects of molecular architecture and phase behaviors on photophysical properties of rod-coil block copolymers with multifunctional sensory characteristics have rarely been explored. In this thesis, new fluorene based rod-coil block copolymers bearing stimuli responsive coil blocks were synthesized to explore the effects of polymer structure and morphology on photophysical properties. In the first part of this thesis (chapter 2), the synthesis, structures and multifunctional sensory properties of amphiphilic poly[2,7-(9,9-dihexylfluorene)]-block-poly[2-(dimethylamino)ethyl methacrylate] (PF-b-PDMAEMA) rod-coil diblock copolymers are reported. The new copolymers, with PDMAEMA coil lengths of 31, 45, 93 and 185 repeating units, were synthesized by atom transfer radical polymerization (ATRP). The surface structures and photophysical properties of the synthesized polymers were studied through the variation of solvent composition (THF/water), temperature, and pH. The PF7-b-PDMAEMA45 structure changed from spheres to separate cylinders, bundles of cylinders and spiral-shaped micelles as the solvent composition changed from 0 to 90 wt% water in THF. However, the long-range order structure of spiral-shaped loops was not observed at a long coil length. The micellar aggregates of PF7-b-PDMAEMA45 in water showed a reversible surface structure transformation from cylinder-bundles to spheres on heating from 25 to 75 oC. The variation of the micelle size with temperature was judged to be similar from both atomic force microscopy (AFM) and dynamic light scattering (DLS) measurements. The intermolecular PF aggregations led to fluorescence quenching and a blue-shift in the absorption spectra of the block copolymer as the water content increased. The photoluminescence (PL) intensity of PF7-b-PDMAEMA45 in water was thermoreversible based on its lower critical solution temperature. The PL characteristics suggested the new copolymers behave as an on/off fluorescence indicator of temperature or pH, with a reversible “on-off” profile at an elevated temperature in water: the pH-fluorescence intensity profile switched from “off-on” to “on-off” as the temperature increased. In the second part of this thesis (Chapter 3), the synthesis, structures and thermoresponsive properties of amphiphilic poly[2,7-(9,9-dihexylfluorene)]-b-poly(N-isopropylacryamide) (PF-b-PNIPAAm) rod-coil and coil-rod-coil block copolymers are reported. The new copolymers containing variant PF units of 7, 15 and 25 as the rigid segment and PNIPAAm as the flexible block were successfully synthesized via combining coupling reaction and ATRP. The surface structures and photophysical properties of the synthesized polymers were studied through the variation of solvent composition (THF/water), and temperature. Micellar structures with spherical, cylindrical, worm-like, bundles or honeycomb shape were particularly noticed. For the longer coil length of block copolymers, the structural transformation grew from sphere into cylinders and bundles at lower water compositions and changed into changed into smaller aggregates at higher water composition. A comparison between diblock and triblock copolymer, the micellar aggregates of triblock copolymer developed into cylinders and bundle micelles at higher water composition than the diblock copolymers with similar rod/coil ratio. The micellar aggregates in water also showed a reversible surface structure transformation on heating from 20 to 40 oC. The transformation of the micellar structure with different solution compositions and temperatures was judged to be similar from AFM, transmission electron microscopy (TEM) and DLS measurements. In addition, the intermolecular PF aggregations of the diblock copolymer led to fluorescence quenching and a blue-shift in the absorption spectra and PL spectra as the water content increased, suggesting an H-type aggregation. However, triblock copolymer exhibited a red shift in both absorption and PL spectra by increasing the water content, which reflected the J-type aggregation. In the third part of this thesis (Chapter 4), new thermoresponsive conjugated rod-coil-coil triblock copolymers were successfully synthesized from terminal azido functionalized poly(N-isopropylacrylamide)-b-poly(N-hydroxyethylacrylamide) (PNIPAAm-b-PHEAA) and alkynyl functionalized PF via click reaction. The azido functionalized PNIPAAm-b-PHEAA copolymers with different block ratio was prepared by atom transfer radical polymerization from an initiator bearing the azide group, whereas alkynyl functionalized PF was synthesized by Suzuki coupling reaction. The lower critical solution temperature (LCST) of the block copolymers increased with an enhanced hydrophilic PHEAA block ratio, since the longer PHEAA segment facilitated the copolymer chains to stretch at an elevated temperature. The micelles of PNIPAAm-b-PHEAA with different block ratio changed into spheres, aggregate spheres, vesicles, and wormlike micelles as the temperature was increased, due to the variation on the hydrophilic/hydrophobic characteristic of PNIPAAm. However, the micellar morphologies became cylinders, bundles, and hollow cylinders in the triblock PF-b-PNIPAAm-b-PHEAA, which were probably induced by the π-π interaction among the fluorene segments. The variation of the micelle morphology with temperature was consistent from the results of TEM, AFM, and DLS. Also, the micelle morphologies of PF-b-PNIPAAm-b-PHEAA showed a thermoreversible property based on its LCST. The PL characteristics behaved as an on/off fluorescence indicator of temperature, showing an “on-off-on” profile at an elevated temperature in water at a higher block ratio of PNIPAAm and switching to “on-off” as the block ratio of PNIPAAm decreased.The above results suggested that various morphologies of conjugated fluorene-coil block copolymers could be efficiently manipulated by block ratio, solvent, pH, or temperature and lead to significantly tuning on the photophysical properties.