利用無乳化劑乳化共聚合法製備丙烯酸甲酯-甲基丙烯酸甲酯共聚物/蒙脫石奈米複合乳膠膜之結構及性質研究

Abstract

本論文主要研究蒙脫石在無乳化劑乳化聚合過程中之脫層機制，並藉由陰離子界面活性劑與蒙脫石吸附行為去解釋蒙脫石之脫層現象。此外，聚甲基丙烯酸甲酯乳液由於Tg太高無法直接成膜，因此我們加入低Tg的丙烯酸甲酯與甲基丙烯酸甲酯進行共聚合反應而製備蒙脫石/丙烯酸甲酯-甲基丙烯酸甲酯共聚物 [MMT/P(MA-co-MMA)] 奈米複合乳膠膜。此外，我們發現MMT/P(MA-co-MMA) 奈米複合乳膠膜具有特殊機械性質行為，因此將在以下研究中深入的去研究並討論。本研究分為三部份，第一部份探討十二烷基硫酸鈉 (SDS) 和溴化十六烷三甲基銨 (CTAB) 兩種離子界面活性劑在水中吸附於蒙脫石 (MMT) 上的機制，與乾燥後之混合物其界面活性劑與蒙脫石的插層結構。並藉由此現象去解釋蒙脫石於無乳化劑乳化聚合過程中，蒙脫石的脫層機制。第二部份探討脫層蒙脫石/丙烯酸甲酯-甲基丙烯酸甲酯共聚物奈米複合材料之特殊機械性質。最後，第三部份我們利用文獻方法設計一種測試剪切強度的裝置，利用此裝置測試蒙脫石/丙烯酸甲酯-甲基丙烯酸甲酯共聚物乳膠膜之剪切強度。 在第一部份，我們利用道南平衡 (Donnan equilibrium) 原理去解釋水溶液中SDS與蒙脫石的吸附行為，此行為導致SDS/MMT混合物呈現親水性質；然而CTAB吸附蒙脫石是利用庫倫引力 (Columbus attraction) 或陽離子交換 (cation exchange) 形成親油性的CTAB/MMT混合物。利用XRD和TEM分析探討後，結果發現SDS既不會增加蒙脫石的層間距也不會改變蒙脫石本身類似球狀的結構。另一方面， CTAB進入蒙脫石層間後使CTAB/MMT混合物會形成一個整體規則排列的層狀結構，同時蒙脫石的層間距也會增加。此外，蒙脫石在無乳化劑乳化聚合過程，水相中所形成之聚合離子自由基 (polymerizing ionic radicals) 會擴散進入蒙脫石層間，因此我們可以利用此現象推論出蒙脫石在無乳化劑乳化聚合之脫層機制。這些聚合離子自由基在蒙脫石層間會聚集形成盤狀的微胞結構，而在微胞化過程結束之前大部分的蒙脫石都已脫層完畢。蒙脫石脫層後，盤狀微胞會成為之後進行無乳化聚合反應的場所。相較於一般的無乳化劑乳化聚合反應，含有蒙脫石之無乳化聚合反應其盤狀微胞的量遠比球狀微胞的量要多很多，因此MMT/PMMA奈米複合材料乳液的聚合轉換率會比MMA乳液有顯著的提升。 在第二部份中，利用XRD和TEM可以發現在無乳化劑乳化聚合製備MMT/P(MA-co-MMA) 奈米複合乳膠膜時，在成膜過程中我們發現當蒙脫石達一定含量，脫層蒙脫石奈米矽片彼此間會傾向於再度堆疊的現象。當蒙脫石含量從0 wt% 增加至20 wt% 時，我們可以由DSC偵測到其Tg會從原來的19.2 0C些微的降低至17.2 0C。然而在動態機械分析儀測量下所得到的Tg反而會從22 0C增加至32 0C，這是由於高分子局部的鏈段被脫層蒙脫石奈米矽片給限制住。當添加1wt% 蒙脫石之P(MA-co-MMA) 奈米複合乳膠膜經由拉伸後會出現一些橢圓形的裂縫，此現象是由於脫層蒙脫石奈米矽片在高分子基材上的裂縫產生固著作用 (Pinning effect)。當蒙脫石添加量增加至10 wt% 以上時，MMT/P(MA-co-MMA) 奈米複合乳膠膜會從柔軟的狀態變成硬脆的性質，此現象是由於脫層蒙脫石奈米矽片重新堆疊導致其楊氏模數 (Young’s modulus) 和拉伸強度明顯增加所致。 在第三部份中，我們利用文獻方法設計一種測試剪切強度的裝置，先將試片固定在裝置中再控制試片剪切平面的角度，利用不同傾斜角度 (試片之剪切平面與裝置軸心之夾角) 所得到之破裂強度去推算出高分子乳膠膜之剪切強度。由於試片在裝置中呈現直立式的串聯模式，當一個試片破裂後，另一個試片會呈現剛要破裂的情形，因此我們可利用SEM去觀察高分子乳膠膜表面產生的剪切帶 (shear band)。當傾斜角度由300增加至600時，由於正向應力增加之故，剪切帶偏向集中在膜的中心位置。有趣的是，當角度由300增加至450時，垂直於破裂方向之相鄰兩條紋的距離會隨著角度增加而增加，而且出現平行於破裂方向的條紋。當角度增加至600時，我們可以發現破裂表面上會產生小島狀結構，而此小島狀結構是由平行和垂直交錯的破裂條紋所產生。然而，當試片被施予正向壓力時，剪切破裂被限制在試片的中心位置上，因此破裂面的平坦度會影響島嶼狀結構。另一方面，當添加蒙脫石時可發現MMT/P(MA-co-MMA) 奈米複合薄膜試片之破裂表面形貌與奈米複合薄膜所受之剪切力和正向應力有關。利用莫爾─庫倫準則 (Mohr-Coulomb criterion)，我們可以計算蒙脫石含量對於本質剪切強度 (τs，無正向應力之剪切強度) 之影響。當P(MA-co-MMA) 奈米複合薄膜添加1 wt% 蒙脫石時，τs 會從1.66增加至2.78 MPa。當蒙脫石含量增加至15 wt% 時，τs 會增加至5.32 MPa。此外，蒙脫石含量和傾斜角度也會影響奈米複合乳膠膜試片之破裂表面形貌。在低含量蒙脫石和低傾斜角的情況時，破裂表面會出現垂直於破裂方向的條紋。然而，蒙脫石含量或傾斜角度增加時，脫層蒙脫石奈米矽片與奈米複合薄膜彼此界面會出現微裂縫，使破裂表面變粗糙。蒙脫石含量和傾斜角度繼續增加時，被壓縮之脫層蒙脫石奈米矽片會在破裂表面上堆疊形成一群大薄片形貌。 總結，我們成功的利用無乳化劑無乳化聚合方法製備出脫層蒙脫石/高分子奈米複合乳液並且完整的探討藉由無乳化劑乳化聚合的方式來製備脫層蒙脫石的機制。因此利用此製程技術，未來將可以廣泛的應用在各種奈米結構之功能性材料上。

This research is mainly to investigate the exfoliation mechanism of montmorillonite (MMT) in the soapless emulsion polymerization. Based on the adsorption of anionic surfactant by MMT in aqueous, we can explain that the exfoliation behavior of MMT. Furthermore, the fabricated MMT/PMMA nanocomposite latex particles are too rigid to be cast into a film. To lower the glass transition temperature of nanocomposites, we have added methacrylate monomers to copolymerize with methylmethylacrylate in the presence of MMT during soap-free emulsion polymerization. Therefore, we reported the unique mechanical properties of the exfoliated MMT/P(MA-co-MMA) nanocomposite films The study is divided into three parts; the first part of this research was to investigate the adsorption of sodium dodecylsulfate (SDS) and cetyltrimethylammonium bromide (CTAB) by MMT respectively in aqueous solution and their intercalation structures after removal of water. Based on the experimental observation, a justified exfoliation mechanism of MMT in the soapless emulsion polymerization under the presence of MMT was proposed. The second part of this research was to investigate the unique mechanical properties of exfoliated MMT/P(MA-co-MMA) nanocomposites. Finally, the third part of this research was to investigate the shear strength of fracture surface for MMT/P(MA-co-MMA) nanocomposite films utilized a unique test fixture. In the first part of this research, the adsorption of SDS by MMT in aqueous solution was associated with the Donnan equilibrium for ions leading to the hydrophilic nature of SDS/MMT hybrid, whereas the adsorption of CTAB resulted from the Coulomb attraction or so-called cation exchange leading to the hydrophobic nature of CTAB/MMT. X-ray scattering (XRD) and transmission electron microscopic (TEM) investigations on the dried SDS/MMT hybrids revealed that the adsorbed SDS neither substantially increased the interlayer distance nor changed the rough spherical texture of pristine MMT. On the other hand, the adsorbed cetyltrimethyl ammonium cations increased the interlayer distance of MMT, resulting in a well integrated CTAB/MMT lamellar structure. Therefore, we proposed a justified exfoliation mechanism of MMT during soapless emulsion polymerization based on the above experimental results as that the polymerizing ionic radicals in water phase were considered as a major component to diffuse into the gallery of MMT. They have been observed to organize into disk-like micelles in the interlayer regions to exfoliate MMT. The exfoliation of MMT was almost completed before micellization stage was over. After exfoliation, the disk-like micelles became a polymerization loci for monomers. Because the disk-like micelles in numbers were substantially over the commonly formed spherical micelles in the typical soap-free emulsion polymerization, the conversion rate of MMA to MMT/PMMA nanocomposite latex was faster. In the second part of this research, the exfoliated montmorillonite (MMT) nanoplatelets tended to re-stack with each other after casting the MMT/ poly(methylacrylate-co-methylmethacrylate) P(MA-co-MMA) latex solutions fabricated by soap-free emulsion polymerization into films as revealed by X-ray diffraction and transmission electron microscopy. As the content of MMT was increased from 0 to 20 wt %, the Tg measured by differential scanning calorimetry was slightly decreased from 19.2 to 17.2 0C, whereas that measured by dynamic mechanical analysis was increased from 22 to 32 0C, indicating that the local motion of polymer segments has been retarded by MMT nanoplatelets. Besides, the elongated elliptical voids appeared during stretching of 1 wt % MMT/P(MA-co-MMA) film to cracking also illustrated the pinning effect provided by the exfoliated MMT. As the content of MMT was increased more than 10 wt %, the mechanical behavior of MMT/P(MA-co-MMA) nanocomposite films was changed from ductile to brittle nature with significant increase of Young’s modulus and tensile strength owing to the restacking of exfoliated MMT nanoplatelets. In the third part of this research, the shear strength of ductile polymer films was measured using a unique test fixture that is capable of controlling the normal to shear plane angle in a confined testing space. Two specimens were loaded in series in the test fixture such that as one fractured the other was in a state just prior to fracture and could be used for the investigation of shear bands by SEM. As the normal to shear plane angle was increased from 30 to 60°, the shear bands were more concentrated in the center region owing to the higher normal compressive stress. Interestingly, the fracture surface showed that the distance between two adjacent striations perpendicular to the fracture direction increased as the normal to shear plane angle was increased from 30 to 45° and the striations parallel to the fracture direction appeared. As the angle was further increased to 60°, small islands formed by cross-over between the parallel and perpendicular striations appeared in the fracture surface. The formation of islands was related to the flatness of the fracture surface because the shear fracture has been confined in the center of the specimens by high normal compressive stress. On the other hand, it was found that fracture of the films depended on both the shear and the normal stresses and could be described by Mohr-Coulomb criterion. The effects of MMT contents on the intrinsic shear strength (i.e., the shear strength at zero normal stress),