澱粉-黏土奈米複合材料之合成及性質研究

Abstract

高分子-黏土奈米複合材料以其優異的特性受到許多不同研究領域的重視。在低黏土添加量下，高分子-黏土奈米複合材料所具備的良好特性，包括機械強度、阻氣性、透光度等皆適合用於提高澱粉材料的用途。對於澱粉-黏土奈米複合材料而言，其合成上的挑戰是在奈米尺度下均勻分散黏土至澱粉基質之中。在本研究中，利用酸修飾澱粉結構及兩種合成方法製備良好分散之澱粉-黏土奈米複合材料。在酸修飾澱粉的過程中，鹽酸藉由溶劑攜帶先接觸到澱粉顆粒的表層、管狀通道周圍及中央空腔附近的區域，再由中央空腔向顆粒外圍擴散，水解澱粉顆粒內結構鬆散的非定晶區；隨著反應溫度及時間的提高與延長，半結晶區中的非定晶區澱粉分子也會被降解。經酸修飾處理的玉米澱粉，其分子量下降，支鏈澱粉長鏈段含量減少，短鏈段增加，且澱粉顆粒的表面粗糙，而其顆粒內的同心圓狀生長環結構會更明顯。透過改變酸水解的作用溫度及反應時間，可以製備得到不同分子結構的澱粉。 根據分子結構的測定結果，可能適合用於奈米複合材料製備之酸修飾澱粉為經鹽酸-甲醇於25C下處理48 h (25C48h) 及45C下處理6 h (45C6h) 和 12 h (45C12h) 的玉米澱粉；相較於天然玉米澱粉的高分子量 (Mw = 5.2×108)，其重量平均分子量分別為8.7×107、3.8×107和1.6×107。由於酸修飾玉米澱粉顆粒容易完全糊化，且其糊液的尖峰黏度和最終黏度較低，所以其澱粉膜的均勻度較天然玉米澱粉膜高。但是，過度的酸水解處理則會使得玉米澱粉鏈長過短而不足以形成良好的澱粉膜結構。在不同結構的澱粉中，以45C6h作為基質之澱粉膜同時具備較高的透光性、低水氣通透性及在高相對濕度 (97% RH) 下的低水分吸附性。而以水溶液作為反應環境，經鹽酸於45C下處理33 h (W45C33h)(鹽酸-水修飾玉米澱粉) 亦可以得到分子結構相似於45C6 h (鹽酸-甲醇修飾玉米澱粉) 之澱粉原料。 第一種合成澱粉-黏土奈米複合材料的方法為先在鹼性環境下快速地混合澱粉 (天然玉米澱粉或W45C33h) 及含有Mg(NO3)2、Al(NO3)3的鹽溶液，使得Mg2+和Al3+共沈澱產生LDH (layered double hydroxides) 晶核，再經濕熱處理使黏土晶核成長及澱粉溶出，所生成的LDH的側向大小為40-60 nm，而厚度為5-10 nm，其均勻地包埋於澱粉基質中，形成澱粉-黏土奈米複合材料。澱粉分子的分子量分佈會明顯地影響澱粉與黏土之交互作用。在酸修飾玉米澱粉基質中，LDH的分散度較高，但LDH則以較為聚集的型態存在於天然玉米澱粉膜內部。隨著LDH含量提高，酸修飾玉米澱粉基質的相分離程度越低；相反地，天然玉米澱粉基質的相分離程度則是越高。相較於不含LDH的酸修飾玉米澱粉膜之楊氏模數為2424 MPa，含10.47% LDH的酸修飾澱粉-黏土奈米複合材料有較高的楊氏模數 (3316 MPa)，其提高的幅度為37%。這是因為酸修飾玉米澱粉-黏土奈米複合材料的澱粉相分離程度較低，使得奈米複合材料具有較為均勻完整的材料結構；同時也是由於均勻分散的黏土能夠有效地承載奈米複合材料所受之應力，而達到強化澱粉材料剛性的功能。但是，可能是由於澱粉與LDH僅以物理性作用力如氫鍵及凡得瓦力交互作用，所以在高應力下，LDH並無法有效地轉移澱粉基質所承受之應力，造成添加LDH對於酸修飾玉米澱粉-LDH奈米複合材料之拉伸強度沒有顯著影響。酸修飾玉米澱粉-LDH奈米複合材料的透光度及等溫吸濕曲線並未隨著黏土添加量增加而提高，其具有與不含黏土之澱粉膜相近的透光度及親水性。 第二種合成澱粉-黏土奈米複合材料的方法則是在低濃度的摻合環境下將澱粉及黏土分散液均勻混合，再以酒精沈降澱粉，使良好分散的黏土與澱粉分子共同沈澱，最後加入塑化劑，得到澱粉-黏土奈米複合材料。蒙特石及合成鋰皂石皆能夠透過此合成方式均勻地包埋於澱粉材料之中。相較於不含黏土之澱粉膜的楊氏模數 (840 MPa)，此良好分散的澱粉-黏土奈米複合材料的楊氏模數為 ~ 1400 MPa，其提高幅度為60%。然而，此機械性質強化之現象僅在澱粉材料預先於43% RH下平衡後才能得到。這代表在適當的水分含量下，水分子扮演著澱粉與蒙特石交界面的相容劑角色，能夠幫助黏土強化澱粉材料之機械性質。對於澱粉-黏土奈米複合材料而言，其楊氏模數與蒙特石添加量之關係並非線性變化。在低蒙特石添加量 (1%) 下，奈米複合材料楊氏模數的提高幅度即可達到53%，且能夠改變澱粉材料於不同相對濕度下之機械性質反應。無論是透過酸修飾處理改變澱粉結構，進而改善澱粉及黏土之交互作用，或是以上述的兩種合成方法製備澱粉-黏土奈米複合材料，都能夠提高黏土分散度及澱粉基質均勻性，使得澱粉材料性質能夠提升，而達到澱粉-黏土奈米複合材料之合成目標。

Polymer-clay nanocomposites with excellent properties have attracted considerable attention in various fields. They exhibit significant improvements in strength, barrier properties, and transparency at low clay contents. The main challenge for preparing high performance starch-clay nanocomposites is the dispersion of clays in nanometer scale. In this research, the acid-modified corn starch and two synthetic methods were employed for preparation of well-dispersed starch-clay nanocomposites. During acid modification, the acids primarily hydrolyzed the granular surface and the matrix surface surrounding cavity and channels, and then lateral diffused and hydrolyzed from cavity and channels throughout the amourphous regions in starch granules. The elevated reaction temperature and extended reaction time facilitated the hydrolysis of the amorphous regions in the semi-crystalline structure of starch granules. The molecular weight and the number of long-chain branches of acid-modified starch decreased with increasing in the degree of acid modification, but the number of short-chain branches increased. The acid-modified starch showed exocorrosion of starch granular surface and sharp growth rings. Starch molecules with different fine structures can be prepared at optimal combnations of acid-hydrolysis temperature and time. Based on the results of structural analysis, the HCl-methanol modified corn starches which modified at 25℃ for 48 h (25C48h), and at 45℃ for 6 h (45C6h) and 12 h (45C12h) might be suitable for the preparation of nanocomposites. Compared to the weight-averaged molecular weight (Mw) of native normal corn starch (Mw = 5.2×108), the Mw of 25C48h, 45C6h, and 45C12h were 8.7×107, 3.8×107, and 1.6×107, respectively. Acid modification decreased the ghost formation in gelatinized starch dispersions and the viscosity of starch film-forming dispersions. Thus, the homogeneity of acid-modified starch films was improved. Excess acid modification would dramatically degrade starch molecules and result in insufficient chain entanglement for film forming. Proper acid modification (45C6h) produced corn starch films with high transparency, low water vapor permeability and moisture absorption at 97% RH. The corn starch modified at 45℃ for 33 h in aqueous acid solution (W45C33h) had similar Mw to that of HCl-methanol modified corn starch (45C6h), and could be used for nanocomposites preparation. The first synthetic method for starch-clay nanocomposites involved a rapid mixing of starch (native normal corn starch or W45C33h) and salt solutions containing Mg(NO3)2 and Al(NO3)3 under alkaline conditions followed by controlled hydrothermal treatments that leached starch molecules from the granules and aged LDH (layered double hydroxides) nuclei. The LDH crystallites with 40-60 nm wide and 5-10 nm thick were embedded in starch matrix. The interaction between starch and LDH was significantly influenced by the molecular size profile of starch molecules. LDH crystallites dispersed homogeneously in an acid-modified starch (AMS) matrix, whereas they tended to aggregate in a native normal corn starch (NCS) matrix. The degree of phase separation in AMS matrix decreased with increasing in LDH contents, but not in NCS. AMS with 10.47% LDH had an enhanced Young’s modulus (37% increment) compared to the unfilled starch matrix. This is because the incorporation of the LDH reduced the phase separation of starch matrix and increased its stiffness. The well-dispersed clays transferred the stress across the nanocomposite components resulting in the increase of the stiffness of starch matrix. However, the tensile strength of AMS-LDH nanocomposites was affected differently by LDH concentration. This indicates that the interactions between starch and clay through hydrogen bonding and van der Waals force were too weak to increase the stiffness of the starch matrix at a high strain. The incorporation of LDH nanoparticles did not significantly reduce the transparency of AMS, and the hydrophilicity of both NCS and AMS matrix. The second synthetic method for starch-clay nanocomposite is synthesizing nanocomposites in diluted dispersions and followed by the coprecipitation of both starch molecules and nanoclays. The intercalation between plasticizers and clays was prevented when plasticizers were added after mixing clays in the starch matrix. The montmorillonite and laponite have been successfully incorporated into starch matrix. The well-dispersed starch-clay nanocomposites possessed enhanced Young’s modulus (~1400 MPa) compared to the unfilled starch materials (840 MPa). Such enhancement (60%) could only be obtained by conditioning starch nanocomposites at 43% RH before mechanical measurements. This indicates that small amount of water acted as an interfacial agent in starch systems for compatibility between polymeric chains and nanoclays. The improvement of modulus didn’t increase linearly with increasing in clay loading. The starch matrix with 1% montmorillonite had a 53% increment in Young’s modulus compared to the unfilled starch matrix. The incorporation of 1% montmorillonite not only improved the Young’s modulus of starch matrix, but also retarded the changes of mechanical properties in response to the environmental relative humidities. These results show that the well-dispersed starch-clay nanocomposites could be prepared by the two synthetic methods proposed in this study. The compatibility between starch and LDH was improved by using acid-modified corn starch. The high level of clay dispersion and homogeneous starch matrix resulted in the enhanced properties of starch-clay nanocomposites.