透明超高阻隔性高分子/石墨烯複合薄膜之研究

Study on transparent and super barrier film based on polymer/graphene nanocomposites

陳榮財(Jung-Tsai Chen)

指導教授 ： 胡蒨傑 ； 李魁然

中原大學/工學院/化學工程研究所/博士(2014年)

https://doi.org/10.6840/cycu201400063

摘要

目前工業應用對阻隔膜的需求與日俱增，高性能阻隔膜之開發備受注目，本研究利用不同技術製備透明、高阻隔性之高分子/石墨烯複合膜，研究成果大幅突破傳統高分子/黏土複合薄膜之性能。考量阻隔膜之性能與製程實用性，氧氣阻隔膜分別利用以下技術製備: (1) 層層自組裝(layer-by-layer self-assembly, LbL)技術製備分支狀聚乙烯亞胺(branched poly(ethylenimine, BPEI)/氧化石墨烯(graphene oxide, GO)複合膜；(2) 結合溶液混摻和恆溫再結晶技術製備聚乙烯醇(poly(vinyl alcohol), PVA)/GO複合膜；(3) 結合溶液混摻和前交聯法製備PVA/GO複合膜。水蒸氣阻隔膜則是先利用溶液混摻法製備環烯烴共聚物(cyclic olefin copolymer, COC)/熱還原石墨烯(thermally-reduced graphene oxide, TRG)複合膜，然後再進行熱處理和表面改質。
文獻指出利用LbL技術可製備出高阻隔和高透明之高分子/黏土薄膜，但卻需要40-70個雙層。為獲得高阻氣性並大幅降低雙層數，我們利用LbL技術製備BPEI/GO透明薄膜，並探討GO溶液pH值對薄膜微結構和氧氣阻隔性之影響。結果指出，在pH=3.5時，薄膜呈現非常緻密且規則之結構；於僅10個雙層條件下，展現出氧氣穿透率< 0.05 cc/m2/day，具有< 2.7 × 10-21 cm3 cm/cm2 /s/Pa的超低透過係數；此數值比傳統的SiOx鍍膜以及高分子-無機多層膜(PML)低了一個級數；比EVOH薄膜低了三個級數；比PVA/MMT混摻薄膜低了5個級數；也比其他LbL-高分子/黏土薄膜低了1~2個級數。此BPEI/GO薄膜展現出比其他複合材料優異許多的性能，可適用於電子產品，例如:液晶顯示器和太陽能電池模組等。
LbL製備之BPEI/GO複合膜雖然成功降低組裝雙層數，LbL製程仍較傳統溶液混摻技術複雜許多，為簡化製程，我們嘗試使用溶液摻混法製備PVA/GO複合膜，但由於溶液混摻薄膜的阻隔性較差。因此，我們發展出簡單結合溶液混摻和恆溫再結晶技術，製備具有超高阻氣性之PVA/GO複合膜。含0.1 wt% (0.07 vol%) GO之PVA/GO複合膜，經過6小時100C恆溫再結晶處理後，展現< 0.005 cc/m2/day的氧氣穿透率以及< 5.0  10-20 cm3 cm/cm2/s/Pa之氧氣透過係數，此數值低於其他高分子/無機粒子複合薄膜1~8個級數；顯示阻氧性遠優於其他混摻之高分子/無機粒子複合薄膜。此優異的氧氣阻隔性能是由特殊之PVA結晶/GO奈米片混成結構所貢獻。在恆溫再結晶過程中，PVA結晶會圍繞著GO生成，並且填充在GO奈米片間的空隙，聯結GO形成可阻擋氧氣透過之超大不可透過區域。本論文所提出之混摻GO結合高分子再結晶技術，提供複合膜改善阻隔性突破性進展。
結合溶液混摻和恆溫再結晶技術相較於LbL而言已簡單許多，但是仍需要進行長時間(100C，6h)的後熱處理，對於工業量產此程序仍有再簡化之需求。因此，我們結合溶液混摻和前交聯法製備PVA/GO複合膜。本研究成功發展出一種控制交聯反應條件使鑄膜溶液適當交聯並且不發生膠化得以進行刮膜的技術，此法在實際應用面較再結晶法更適用於大規模量產程序。同時，此交聯之PVA/GO複合膜展現與再結晶之PVA/GO複合膜相同的超高氧氣阻隔性能(OTR< 0.005 cc/m2/day)。交聯劑所扮演的角色為: (1) 使高分子基質緻密化；(2) 使高分子鏈剛硬化；(3)提升GO和PVA間的界面黏著性；(4)因交聯網絡結構為氣體不可透過區域，他們可聯結GO形成超大之阻隔層，因此氧氣阻隔性能得以大幅提升。溶液混摻結合前交聯技術，可省略長時間的後處理程序，亦可提升大面積複合膜之均勻性並降低能源消耗、投資成本及所需空間；此技術應為目前量產高分子/石墨烯阻隔複合薄膜最可行之方法。
高性能阻隔膜需同時阻隔氧氣與水氣，親水性高分子/GO複合膜阻水性不足，因此我們嘗試混合超微量之TRG進入COC薄膜內製備水氣阻隔薄膜。TRG在0.06wt% (0.009vol%)之添加量下，COC/TRG複合膜有效降低了水蒸氣透過係數22%。基於相近的降低比例下，此含量低於其他高分子/黏土或是高分子/石墨烯複合膜添加量的10-100倍，顯示添加TRG是非常有效的方法。為了更進一步改善阻隔性能，針對COC/TRG-0.06wt%複合膜進行不同溫度的熱處理(annealing)，結果顯示，藉由熱處理方式可再大幅降低水蒸氣透過係數50%，這是由高分子鏈剛硬化以及界面黏著性獲得改善所貢獻的。最後，我們使用兩性高分子分別塗佈在COC和COC/TRG薄膜表面上，使其表面親水化。經表面塗布上一層親水層後，水氣的阻隔性可再獲得提升。對於親水層抑制水氣透過，我們首次提出水分子透過具有表面親水層之疏水阻隔膜之特殊傳輸機制。組合傳統與創新概念，本研究發展出阻水性優於所有高分子/無機物複合膜文獻值之高分子/石墨烯阻水氣複合薄膜。
本論文針對阻隔膜內的微結構與阻隔性能間的關聯性進行深入探討，並將氣體透過係數的實驗數據與Nielsen和Cussler模型進行比較，最終，我們對於如何製備具超高阻隔性之透明薄膜，提出深入且獨特的見解。本研究製備之氧氣和水氣阻隔膜皆展現優於其他文獻的性能，阻隔膜之性能遠優於食品與藥物應用之需求，並可適用於電子及能源產業。

關鍵字

透明阻隔膜 ； 石墨烯 ； 複合膜

並列摘要

The demands for barrier films in industrial applications increase at present; therefore, developing high-performance barrier films has been considered by many researchers. In this study, transparent and high-barrier polymer/graphene composite films were prepared by different techniques. Their performance is superior to other polymer/clay composites. With both the barrier film performance and the process practicability considered, oxygen barrier films were prepared using different techniques: (1) branched poly(ethylenimine) (BPEI)/graphene oxide (GO) composites fabricated by the method of layer-by-layer (LbL) self-assembly; (2) poly(vinyl alcohol) (PVA)/GO composites by the combined methods of solution blending and isothermal crystallization; (3) PVA/GO composites by the combined methods of solution blending and pre-crosslinking. Moreover, water vapor barrier films of cyclic olefin copolymer (COC)/thermally-reduced graphene oxide (TRG) composites were fabricated by solution blending and followed by annealing and surface modification.
Previous research indicated that transparent and high-barrier films, assembled from 40 bilayers to 70 bilayers of polymer/clay, could be prepared by an LbL technique. To obtain ultra-high barrier films with much lower number of bilayers, we used an LbL technique to fabricate BPEI/GO films and to discuss the effect of the GO suspension pH on the nanostructure and the oxygen barrier properties of LbL-BPEI/GO films. Film assemblies with only 10 bilayers, which were prepared at a pH of 3.5, exhibited a very dense and ordered structure and delivered very low oxygen transmission rates (< 0.05 cc/m2/day) (GO/BPEI-3.5)10 and an oxygen permeability of ~2.7 × 10-21 cm3 cm/cm2/s/Pa. This permeability is one order of magnitude lower compared with a typical SiOx nanocoating and polymer multilayer (PML) coatings, three orders of magnitude lower relative to EVOH films, five orders of magnitude lower to a PVA/MMT composite, and two orders of magnitude lower to polymer/clay assemblies. These data indicated that such films had a far superior performance compared with other films; they can be applied for electronics, such as liquid crystal display and photovoltaic modules.
Although a high-barrier LbL film with relatively much lower number of bilayers was prepared successfully, the LbL method is still more complex than the solution blending; hence, we tried to use the solution blending process to make PVA/GO composite films. However, the gas barrier performance of the PVA/GO blend film was insufficient for electronics. Therefore, we developed a simple method that combined solution blending and isothermal crystallization to fabricate PVA/GO composite films with super gas barrier properties. These films with only 0.1 wt% (0.07 vol%) GO, isothermally crystallized at 100C for 6 h, gave an O2 transmission rate < 0.005 cc/m2/day and an O2 permeability < 5.0  10-20 cm3 cm/cm2/Pa/s; they are far superior to other blend polymer/inorganic composites. The excellent O2 barrier properties are attributed to a unique hybrid of PVA crystals and GO nanoplatelets. PVA crystals form around the GO during the isothermal crystallization; the crystals fill in the spaces between the GO nanoplatelets, and together they become ultra-large impermeable regions, which can prevent the passage of O2. The combined methods of blending GO and polymer recrystallization technique proposed in this study, demonstrate a breakthrough in improving barrier properties of barrier films.
The combination of solution blending and isothermal crystallization is much simpler than the LbL method. But these combined methods require a long post-heat treatment (100C for 6h), which is impractical for industrial production processes. Therefore, we develop a method of combining solution blending and in-situ pre-crosslinking to fabricate crosslinked PVA/GO films. We controlled the crosslinking reaction conditions in PVA/GO solutions to avoid the occurrence of gelation. The PVA/GO films also exhibited the same oxygen barrier properties (OTR < 0.005 cc/m2/day) as the crystallized PVA/GO films did. The crosslinker functions were to: (1) densify the polymer matrix; (2) rigidify the polymer chain; (3) improve the interfacial adhesion between GO and PVA; (4) connect the GO sheets to each other to form an ultra-large impermeable region. As a result, the oxygen barrier properties were highly enhanced. The combined methods of solution blending and pre-crosslinking can skip the long post-heat treatment and it can also improve the uniformity of producing large-area composite film and reduce the energy consumption, cost and spaces. This is the most feasible technique for the mass production of polymer/graphene composite film at present.
High-performance barrier films hinder oxygen and water vapor from passing through at the same time, but the hydrophilic polymer/GO composite films lack water vapor barrier performance. Therefore, we prepared a water vapor barrier film by incorporating an ultra-low content of TRG into a hydrophobic COC. This film with only 0.06 wt% (0.009 vol%) TRG reduced the water vapor permeability by 22%. This TRG content is 10 to 100 times lower than that of other polymer/clay or polymer/graphene composites that reduce water vapor permeability at the same percentage. To further improve the barrier performance of the COC/TRG-0.06wt% composite film, it was annealed at different temperatures. Results showed that because of annealing, the permeabilities were greatly reduced; the treatment caused to rigidify the polymer chain and to improve the interfacial adhesion. An amphiphilic polymer was used to modify COC and COC/TRG films to make their surface hydrophilic. This study is the first to propose a special transport mechanism by which water passes through the barrier film with a hydrophilic surface layer. Combining the traditional and the novel aspects, this study develops a polymer/graphene composite film with barrier properties that is superior to other polymer/inorganic composite film.
The relationship between the barrier properties and the barrier film microstructure was thoroughly investigated. Experimental data on gas permeabilities were compared with Nielsen and Cussler models. As shown from analyzing the experimental results, we have given insights into how a transparent film with super gas barrier properties was prepared. The transparent composite film prepared in this study exhibited outstanding barrier performance far superior to other films. These films exhibit high performances that are much better than the requirements of the food packaging and the pharmaceutical; they can be applied to the applications of the electronics and the energy industry.

並列關鍵字

transparent barrier film ； composite film ； graphene

參考文獻

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