本研究主要探討PCBM在六甲基苯與耐綸晶相上的磊晶成長行為，以及在混摻薄膜中的分散聚集與結晶行為，如何影響耐綸晶相的Brill transition。不僅對不同晶相之間相互影響的方式進行討論，也希望瞭解混摻成份如何影響晶相結構的演變。
　　實驗觀察指出，六甲基苯晶板上雙晶成長所形成的鋸齒狀溝槽，可以調控PCBM的聚集行為，使PCBM沿著晶板表面溝槽聚集形成平行的陣列。藉由電子繞射與廣角度X光散射發現，此種Graphoepitaxy關係所引導的聚集行為，反而會阻礙PCBM的結晶。對Graphoepitaxy關係以及雙晶所帶來的效應來說，是一種新的認知與瞭解。
　　PCBM於耐綸分子晶相上的結晶，可以發展出真正的晶格對應關係，形成大範圍的單晶網路。但是這樣的晶格對應關係，藉由等溫即時記錄廣角度X光散射可得知，會使得耐綸66的Brill transition轉換溫度提升至220 oC，遠高於文獻所指出的轉換溫度範圍。且室溫相的 (100)晶面間距在轉換過程中幾乎保持不變，因此可知，藉由晶格對應關係，晶相間的接觸可以限制晶相內的結構轉變。推論Brill transition主要起使於晶相表面晶格的演變，而非於晶相內部開始發生。
　　PCBM分子與耐綸分子之間有很好的交互作用，因此在耐綸分子形成板晶時，少量混摻的PCBM分子會分散在板晶堆疊之間的非晶區 (interlamellar segregation) ，並限制非結晶耐綸分子鏈段的熱運動。於升溫過程記錄 (in-situ)的廣角X光繞射可知，耐綸晶相的Brill transition溫度都因混摻PCBM而上升，且上升的程度與混摻比例相關。這個實驗觀察指出，晶相結構的演變與非結晶鏈段熱運動的程度密切相關。此外，板晶堆疊的規則性與固態非晶層 (rigid amorphous layer)消長，也都會受到PCBM分子於非晶區中聚集與分散的影響。

關鍵詞： 耐綸、PCBM、磊晶成長、晶相結構的轉換

The influences of the interactions between disparate phases on the Brill transition of nylon crystals

Kun-Keng Li
Jr-Jeng Ruan
Department of Material Science and Engineering, National Cheng Kung University

SUMMARY

The aggregration of PCBM molecules can be adjusted by the surface saw-like ledges on HMB crystalline platelets. PCBM molecules can stack along surface ledges to form the oriented parallel arrays. Accroding to XRD and TEM observations, this kind of aggregration induced by Graphoepitaxy can hinder the crystallization of PCBM molecules.

PCBM molecules can epitaxially grow on nylon crystals by lattice matching, forming large-size single crystals. From in-situ WAXS results, such lattice matching makes Brill transition temperature (TB) of nylon 66 increased dramatically, and the d spacing of (100) plane hardly change during the whole heating and cooling processes. As a result, the contact between PCBM crystals and nylon crystals (lattice matching) can restrict Brill transition.

There is a great interaction between PCBM and nylon molecules. After blending into nylon, PCBM molecules will disperse in the amorphous region of nylon lamellae. The Brill transition temperature of nylon crystals enhances with the increasing PCBM blending ratio. Consequently, the development of Brill transition is related to the thermal motion of polymer chains in amorphous region, and the aggregation or dispersion of PCBM molecules in amorphous region will influence the development of rigid amorphous layer and the regularity of lamellar stacking process of nylon crystals.

Key words: Nylon、PCBM、Epitaxy、Brill transition

INTRODUCTION

As a derivative of C60, PCBM is the most commonly used electron-acceptor in organic photovoltaics. It is important to develop continuous PCBM single crystals for device performance, but PCBM tend to aggregrate irregularly and distribute uncontinuously in the film.

Epitaxy is an efficient method to control the crystallization of PCBM molecules. Due to the high melting point and crystallization temperature of PCBM, we choose nylon as substrate which can bare this high temperature epitaxy process to develop a large-size and continuous PCBM single crystals.

With the change of temperature, nylon will transform room temperature crystal structure into high temperature crystal structure, which is known as Brill transition in nylon materials. In this research, we develop continuous PCBM single crystals by epitaxy, and analyze the change of Brill transition and lamellar stacking process of nylon crystals caused by such kind of epitaxy and the blending effect

MATERIALS AND METHODS

Preparation of nylon and PCBM solutions
We use formic acid as the solvent for nylon, and chloroform for PCBM. Weighing nylon46, nylon66 and PCBM first, then add different solvents to these solutes, respectively. Make sure the solutes dissolved completely by ultrasonication. Then mix nylon and PCBM solutions together depending on different ratio.

Preparation of in-situ WAXS, SAXS and IR samples
Drop the nylon and PCBM mixing solution on glass at 70 oC, make solution evaporized rapidly to avoid the phase separation. Then collect the film to do in-situ WAXS and SAXS observations. Cut the film into powder and mix with KBr, pressed into the standard sample to do in-situ IR experiment.

Preparation of TEM samples
PCBM solution is spin coated on directional nylon substrates with the two-step method. The first step speed is 500 r.p.m. for 20 seconds to control the film thickness, the second step speed is 2000 r.p.m. for 30 seconds to evaporize the solvents. After the spin coating process, we anneal samples at high temperature to develop epitaxial crystallization of PCBM on nylon substrates, then coat a carbon layer on the samples. Use PAA sticking on the film then standing for 1 day, then remove steam in vacuum oven. Finally pull down PAA and place into deionized water upside down for 1 day to make PAA dissolved, then scoop up the film by cupper grids and remove steam again to do TEM observation.

RESULTS AND DISCUSSION

The interaction between two crystalline phases
The melting state of HMB can serve as solvent for PCBM, during cooling process, HMB crystalized first as substrate. Subsequently annealed at 140 oC for 1 hour, PCBM molecules can stack along surface ledges on HMB crystalline platelets to form the oriented parallel arrays. Accroding to XRD and TEM observations, the crystallinity of PCBM becomes weaker after grew on HMB. In general, most epitaxy can help to develop the crystallization, but this kind of aggregration induced by Graphoepitaxy hinder the crystallization of PCBM molecules. We speculate that the oblique crystalline planes of HMB surface ladges will effect the aggregration of PCBM molecules.

PCBM solution is spin coated on directional nylon substrates, and the consequent high temperature annealing makes PCBM epitaxially grow on nylon to form directional PCBM single crystals. The ab plane of PCBM crystals serve as contact plane with ac plane of nylon crystals. Packing of PCBM along the b-axis matches with interchain distance along the a-axis of nylon crystals, which generates very small mismatch ratio of 2.3% in crystallographic b-axis of PCBM crystals on nylon46, although a somewhat large mismatch of 3.4% in b-axis
is obtained on nylon66.

After the epitaxial growth of PCBM crystals on nylon66 crystals, the Brill transition temperature of nylon66 dramatically increased to 220 oC. In addition, the d spacing of (100) plane barely change during the whole heating and cooling processes, the lattice dimension along the a-axis remains constant. However, the (010)/(110) peak shifts to lower angle and disappears, the expansion of lattice dimension along the b-axis accompanied with the loss of periodicity. We speculate that without lattice interactions, the d(100) is decreased upon heating, which restricts the thermal motion of CH2 segments and maintains the regular stacking of ac plane. With the presence of lattice interactions, on the other hand, the d(100) is fixed upon heating and not able to decrease, and therefore a larger space is available for the thermal motion of CH2 segments. In this case, the continuous increase of segmental thermal motion can cause the stacking of ac plane less regular along the b-axis.

The effect of the interaction between crystalline phase and amorphous phase on Brill transition
We are also interested in the blending effect of PCBM. After blend with PCBM, the Brill transition temperature of both nylon46 and nylon66 increase, changing with the blending ratio. We exclude the possibility of intercalation of PCBM molecules into nylon crystals according to in-situ IR and WAXS observation, and confirm such blending effect is caused by the dispersion or aggregration of PCBM molecules in amorphous region of nylon. On the other hand, the aggregation or dispersion of PCBM molecules in amorphous region will influence the development of rigid amorphous layer and the regularity of lamellar stacking process of nylon crystals. The lamellar stacking of nylon crystals change from reversible process of pure nylon to irreversible process of the blending systems.

CONCLUSION

PCBM molecules can stack along surface ledges to form the oriented parallel arrays. Accroding to XRD and TEM observations, this kind of aggregration induced by Graphoepitaxy can hinder the crystallization of PCBM molecules.

PCBM molecules can epitaxially grow on nylon crystals by lattice matching, forming large-size single crystals, and such lattice matching makes Brill transition temperature (TB) of nylon 66 increased dramatically, and the d spacing of (100) plane hardly change during the whole heating and cooling processes.

The Brill transition temperature of nylon crystals enhances with the increasing PCBM blending ratio, and the aggregation or dispersion of PCBM molecules in amorphous region will influence the development of rigid amorphous layer and the regularity of lamellar stacking process of nylon crystals.

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