培養條件對細菌纖維素之乾重、含水量、機械性質的影響

Abstract

由於細菌纖維素是由β-1,4-葡聚糖鏈組成的3D網狀奈米結構，其內部孔隙度高、表面積高，且鏈上具有許多親水的氫氧基，細菌纖維素因此具有很高的含水量。而數條葡聚糖鏈聚集形成50~80 nm的纖維束，肉眼可見的細菌纖維素則是由非常多的纖維束交織纏繞而成，造就細菌纖維素較高的楊氏模數。 本研究將細菌纖維素的乾重、含水量與機械性質連結。分別以葡萄糖、甘油、乙醇、葡萄糖加甘油作為碳源，培養基起始pH值調整為4.0、5.5、7.0，培養合成出不同產量的細菌纖維素，以先前實驗室訂立的瀝乾法及瀝乾準則測量含水量，再以拉伸機對細菌纖維素薄膜做拉伸測試得到不同乾重、含水量的應力應變曲線。 研究結果發現乙醇做為碳源所合成的纖維素乾重較低，約只有1.0 g/L，pH4-gly2、pH7-glu1+gly1、pH5.5-glu0.5+gly1.5、pH7-glu0.5+gly1.5四組乾重較大，約為3.0 g/L。以上四組乾重較大的纖維素含水量較小，平均為68.9 g H2O/g dry BC，而乙醇作為碳源合成的纖維素含水量最高，為214.1 g H2O/g dry BC。含水量與乾重的反向關係顯示乾重愈小含水量愈大，因為乾重少的纖維素其纖維束合成的較少，結構較疏鬆，因此含水量較大。 此外，由應力應變曲線可以得知細菌纖維素的機械性質，含水量60 g H2O/g dry BC左右的纖維素楊氏模數約為 30 MPa，含水量180 g H2O/g dry BC左右的則為3 MPa。含水量愈大的纖維素，因為結構疏鬆所以降伏強度愈小、降伏形變愈大、楊氏模數愈小。根據此項關係以及應力應變曲線數學模型，即可以根據纖維素的機械性質推估其含水量，也能預測已知含水量的細菌纖維素經拉伸的應力應變曲線。

Bacterial cellulose (BC) is composed of β-1,4-glucan chains. The special 3D nano-network structure and hydrogen bonds between cellulose and water makes BC a high porosity, high surface area, and high hydrophilicity material. These properties result in high water holding capacity (WHC) of BC. Several glucan chains consists of a 50~80 nm cellulosic ribbon, and several ribbons intertwine to form BC pellicles. Therefore, BC has high Young’s modulus. In this study, we aim to find the relation between WHC and mechanical properties of BC. Glucose, glycerol, ethanol, and mixture of glucose and glycerol are used as the cabon source. The initial pH value is set at 4.0, 5.5, and 7.0. To measure WHC of different dry weight of BC pellicles, draining method invented previously is implemented in this experiment. On the other hand, stress-strain curves of different dry weight and WHC of BC pellicles are obtained by using the tensile testing machine. Experiment results suggests ethanol can only yield 1.0 g/L of BC, WHC of which is 214.1 g H2O/g dry BC. In contrast, group pH4-gly2, pH7-gly1+gly1, pH5.5-glu0.5+gly1.5, and pH7-glu0.5+gly1.5 can yield more BC, which is approximately 3.0 g/L BC, the average WHC of which is 68.9 g H2O/g dry BC. The reverse relation between BC dry weight and WHC indicates that fewer dry weight means fewer cellulosic ribbons are produced in each BC pellicle, resulting in looser structure. Thus, the fewer BC dry weight is, the larger WHC is. Mechanical properties can be calculated from stress-strain curves of BC pellicles. The BC pellicle whose WHC is 60 g H2O/g dry BC has the highest Young’s modulus of 30 MPa. In contrast, the BC pellicle with high WHC of 180 g H2O/g dry BC has the lowest Young’s modulus of 3 MPa. In other words, BC pellicles with less compact structure and higher WHC consequently have lower yield strength (σ_Y), higher yield strain (ε_Y), and lower Young’s modulus (E_max). Furthermore, according to this relation and the stress-strain model deduced by experiment datas, WHC can be estimated by measuring σ_Y, ε_Y, or E_max of BC via tensile test, and the stress-strain curves of BC pellicles with known WHC become predictable.