半硬式共軛高分子於溶液態構形及聚集行為之研究

Abstract

共軛高分子由於具有優越的電學及光學半導體性質，因此在學界及產業界均投入廣泛地研究。然而，許多文獻利用光譜証明共軛高分子既使於稀薄溶液中亦會形成分子鏈間的聚集體，進而影響其發光性質。然而，此聚集體之形成之原因及構形則鮮少有文獻報導。在此篇研究中，我們將研究共軛高分子poly(diphenyl phenylenevinylene) (DP-PPV) 高分子於溶液態之分子構形及其分子間聚集行為。 Poly(2,3-diphenyl-5-hexyl-1,4-phenylenevinylene) (DP6-PPV) 分子鍵於不同溶液性質中(氯仿、甲苯)，均具有聚集的情況發生，且聚集體之碎形尺度(mass fractal dimension)大約於 2.2 ~ 2.7之間。此聚集體中於氯仿溶液中形成較為鬆散結構但在甲苯溶液中變的較為緊密的結構，乃由於DP6-PPV分子鍵在甲苯溶液中，高分子鏈間之作用力所致。於甲苯溶液中，此種較弱的分子鏈間作用力可藉由增加系統溫度而去除之。然而，加熱至高溫時，依然有許多的分子鏈間作用力無法被完全消除，因此，在分子鏈間存在著另一種具有高穩定性之作用力。此種高穩定性的分子鏈作用力存在於氯仿及甲苯溶液之中。此種高穩定性之作用力形成原因為π-π complex 存在於DP6-PPV粉末中。高分子鏈被此作用力緊緊地綁在一起並於溶劑中形成了網狀結構，因此降低了高分子之溶解度。 另一方面Poly(2,3-diphenyl-5-decxyl-1,4-phenylenevinylene) (DP10-PPV)為具有較長側鍵之高分子於溶液態之構形亦於本研究討論之。其實驗結果發現，於氯仿溶液中，DP10-PPV分子鍵具有較佳之分散行為，推論為DP10-PPV之長側鍵可有效的抑制π-π complex於DP10-PPV粉末的產生，因此分子間之作用力較DP6-PPV來的弱。所以DP10-PPV高分子可均勻分佈於氯仿溶液中。而於低濃度時，DP10-PPV分子鍵承呈現worm-like chain行為，且其抗彎長度(persistence length)與濃度成正比。另一方面於甲苯溶液中，其分子間作用力變強，因此，DP10-PPV分子鍵形成碎形之聚集體。其聚集體之結構隨濃度增高而更緊密，此種聚集體具disklike之行為，而其厚度可藉由Kratky-Porod方程式來求得其disklike之厚度(T ~ 20Å)。由此結果可推論DP10-PPV於甲苯溶液中所形成之聚集體為其分子鍵互相交錯而成。

Conjugated polymers constitute a family of semi-rigid polymer which have found great potential in opto-electronic applications due to their unique semiconducting properties and solution processability. The solutions of light-emitting polymers are usually homogeneous in appearance; however, their conjugated segments may exhibit some extent of aggregation in the solution and such an aggregation may exert dramatic impact on the photophysical properties of the polymer in the solutions. Previous studies based primarily on absorption and photoluminescence spectroscopies have indicated that these polymers underwent inter-chain aggregation in the solution state even at large dilution; however, the origin of this event and the structure of the resultant aggregates remain the crucial issues to be resolved. This thesis centers on the studies of the conformational structures and aggregation behavior of two poly(diphenyl phenylenevinylene)s (DP-PPVs) bearing different side chains in the solutions states. We reveal that the inter-chain aggregation of the conjugated polymer, poly(2,3-diphenyl-5-hexyl-1,4- phenylenevinylene) (DP6-PPV) composing hexyl side chains, in the solutions with chloroform and toluene generates network aggregates with the hydrodynamic radii of several □m. Small angle neutron scattering (SANS) demonstrates that the internal structure of these aggregates can be characterized by the mass fractal dimensions of 2.2 ~ 2.7. The networks are looser in chloroform but become highly compact in the poorer toluene solvent due to severe segmental association. Increasing the temperature alleviates the segmental association in toluene while largely retaining the mass fractal dimension of the aggregates. However, the inter-chain aggregation is never completely dissipated by the heating, suggesting the existence of two types of segmental association with distinct stability. The highly stable segmental association that can neither be solvated by chloroform nor be disrupted thermally in toluene is attributed to the □-□ complex already present in the DP6-PPV powder used for the solution preparation. The chains tied firmly by this complex form network aggregates in the solution and hence reduce the entropy of mixing of the polymer. In the poorer toluene solvent further segmental association takes place within the pre-existing aggregates, making the networks more compact. This type of segmental association can be disrupted by moderate heating and its occurrence is ascribed to the poor affinity of the aliphatic side chains of DP6-PPV to toluene. The conformational structure and the aggregation behavior of another hairy-rod conjugated polymer, poly(2,3-diphenyl-5-decyl-1,4-phenylenevinylene) (DP10-PPV) bearing decyl side chains, in the solutions with chloroform and toluene have also been investigated by means of SANS and DLS. The results are systematically compared with those of DP6-PPV to reveal the effect of side chain length on the aggregation behavior. The DP10-PPV chains well dispersed in chloroform are found to exhibit the expanded wormlike chain conformation with the persistent length increasing with overall polymer concentration. This molecular level dissolution is in contrast with the colloidal dissolution of DP6-PPV in chloroform, indicating that increasing the length of the side chain tends to suppress the segmental aggregation of DP-PPVs. DP10-PPV however aggregates significantly to form aggregates of several □m in size in toluene. The internal structure of the aggregates is characterized by a fractal dimension of 1.3 at the overall polymer concentration of 0.1 wt%, indicating that the aggregates are loose networks. Compact disklike domains develop within the aggregates as the concentration increases to 0.5 wt% and 1.0 wt%. The thickness of the disk domains determined from the Kratky-Porod approximation is ca. 20 Å, implying that the DP10-PPV chains formed a bilayer structure in the domains. The formation of the layer like structure is assisted by the longer side chains attached to DP10-PPV compared with DP6-PPV.