共軛高分子奈米薄膜在溶劑蒸氣內之機械穩定度與光電行為變化

The instability of conjugated polymer ultrathin films in solvent vapor and the induced changes in optoelectronic properties

劉奇京

指導教授 ： 楊長謀

國立清華大學/工學院/材料科學工程學系/碩士(2013年)

摘要

本論文探討共軛高分子MEH-PPV /PS混合薄膜於室溫經溶劑退火後薄膜之機械穩定度與螢光性質之改變。MEH-PPV濃度(c)為5％時薄膜將進行除潤過程，此時因分子剪切流變造成高分子間、高分子與基材間劇烈拉扯，抑制電子聲子交互作用而在殘留薄膜產生10倍螢光增益。除潤時螢光光譜先出現藍位移再紅位移，表示除潤過程中高分子的共軛結構不斷改變。使用貧溶劑除潤後初期暫態藍位移量明顯下降，此外用高分子量PS進行除潤藍位移量上昇，以上觀察皆表示於除潤溶劑與高分子間在分子尺度因溶解度、分子交纏之差異改變除潤後MEH-PPV之共軛結構。經溶劑退火後，溶劑分子擴散進入薄膜中並累積在基材之上，造成薄膜底層密度異常鬆散，之後薄膜之機械不穩定行為依靠此鬆散層運動。
c為1%時進行溶劑除潤，分子流變能有效作用在各個MEH-PPV分子，產生螢光光譜約30 nm的藍位移。然而當c為25%之共軛高分子薄膜經溶劑退火後，因MEH-PPV分子間形成有效的交聯網絡阻止除潤進行，表面只會出現些微起伏來降低系統自由能。此時因為MEH-PPV分子間作用提昇使得螢光光譜紅位移及螢光效率降低。溶劑退火後的共軛高分子薄膜經長時間時效觀察，發現薄膜出現約50 nm起伏之皺折結構，同時伴隨光譜藍位移。

關鍵字

共軛高分子 ； 溶劑退火 ； 除潤 ； 螢光效率 ； 暫態交纏點 ； 高分子組態

並列摘要

Solvent annealing induces the instability of conjugated polymer ultrathin films and the changes in optoelectronic properties at room temperature was investigated in this report. At first, we dispersed MEH-PPV molecules in inert polystyrene (PS) in the concentration (c) of 5%. Solvent annealing-driven dewetting created massive PL enhancement when polymers flowed into dewetting droplets and residual layer. This molecular shear flow suppressed electron-phonon interaction in polymer chains and the PL enhancement is up to ~ 10 folds at the residual layer. The blue shift accompanying with dewetting was recorded. As annealing continued on, the blue shift reversed, illustrating the increase of conjugation length during solvent annealing. The transient blue shift became smaller by poor solvent dewetting and larger as we used entangling long chains PS matrix. These results indicated solvent-polymer interactions and alteration of inter-segmental chain entanglements influences up to the stage of dewetting. Before dewetting, solvent molecules penetrated into the polymer film and saturated the underlayer. All the film instability process too place on the loosen underlayer.
As c decreases, molecular shear stress could effectively transferred to individual MEH-PPV molecule and created huge blue shift (~ 30 nm). However, high MEH-PPV concentration film (c = 25%) wouldn’t dewetting during solvent annealing due to sufficient percolation of MEH-PPV. As a result, only few bumps emergence on the surface in order to decrease system free energy. The PL spectrum red shift and the intensity drop after solvent annealing. Interestingly, solvent-annealed polymer films transformed to wrinkle structure after aging in the room temperature and accompanying with blue shift.

參考文獻

31. 楊志偉, 清華大學材料系碩士論文 2006, 利用除潤與薄膜拉伸測試研究分子鏈運動對共軛高分子發光之影響.

11. Collini, E.; Scholes, G. D., Science 2009, 323, 369-373.

13. Tung, K. P.; Chen, C. C.; Lee, P. W.; Liu, Y. W.; Hong, T. M.; Hwang, K. C.; Hsu, J. H.; White, J. D.; Yang, A. C.-M., ACS Nano 2011, 5, 7296-7302.

15. Yang, M. H.; Hou, S. Y.; Chang, Y. L.; Yang, A. C.-M., Physical Review Letters 2006, 96, 066105.

17. Reiter, G., Langmuir 1993, 9, 1344-1351.