酸水解法製備多醣奈米結晶粒子之研究

Abstract

本試驗中使用纖維素、幾丁質與澱粉為多醣原料，經酸水解法去除內部的非結晶區後得到奈米尺度的結晶區域，即為多醣奈米結晶粒子。纖維素的酸水解條件是以65 wt%硫酸於70℃下水解10分鐘。幾丁質奈米結晶粒子的製備則是利用3 N鹽酸於沸騰下，水解幾丁質粉末90分鐘；兩者水解後表面皆產生酸根，可在pH＞1的水溶液下解離帶電，形成穩定的奈米結晶粒子懸浮液。纖維素與幾丁質奈米結晶粒子在穿透式電子顯微鏡下皆呈桿狀外觀，直徑皆為20 nm左右，而纖維素奈米結晶粒子長度分布約200-400 nm，幾丁質則為200-500 nm。此外，當纖維素與幾丁質奈米結晶粒子懸浮液之濃度高於臨界濃度時，皆能排列出膽固醇型液晶相，可在偏光顯微鏡下觀察到其指紋狀紋理。本試驗中並以帶正電的幾丁聚醣和帶負電的纖維素奈米結晶粒子以旋轉塗佈法製備出以矽晶圓為基質的多層膜，而此多層膜在2-5雙層時會產生顏色變化，依序為藍色、黃色、及紫色。 由酸水解物產率曲線、澱粉粒外觀變化、X-ray圖譜之結晶度變化得知，以3.16 M硫酸在40℃下水解糯性玉米澱粉三天後，澱粉粒中的軟殼層已經不存在。而由不同酸水解天數澱粉之支鏈澱粉鏈長分布變化得知，澱粉經酸水解五天後，blocklets內部位於支鏈澱粉分支點部分的非結晶區已幾乎去除。因此，酸水解五天的澱粉水解物再經超音波震盪，是以酸水解法製備澱粉奈米結晶粒子的最適條件。然而在試驗中亦發現，澱粉奈米結晶粒子絮聚嚴重，無法形成穩定懸浮液，因此需尋找適當的分散劑以提供良好分散之澱粉奈米結晶粒子懸浮液。由視黏度和界面電位之測定結果得知，六偏磷酸鈉添加量為0.1 mM時，具有良好之澱粉奈米結晶粒子分散效果。在偏磷酸根吸附試驗上得知，偏磷酸根吸附於澱粉奈米結晶粒子之行為，符合Langmuir等溫吸附曲線，並計算出1.5%澱粉奈米結晶粒子之最大偏磷酸根吸附量為0.114 mole/kg。以0.1 mM六偏磷酸鈉分散的澱粉奈米結晶粒子懸浮液在沉降試驗中具有最好的穩定度，此外，以高濃度固形物與分散劑之澱粉奈米結晶粒子懸浮液方式貯存六天後，取出稀釋成含0.1 mM濃度分散劑之懸浮液，仍可維持長時間的穩定。澱粉奈米結晶粒子以六偏磷酸鈉分散後，可以以穿透式電子顯微鏡直接觀察其形態。觀察結果為澱粉奈米結晶粒子包括粒徑約10 nm的單一cluster，以及粒徑大於50 nm由多個clusters並排而成的片狀粒子。而動態光散射測得之粒徑數量百分比分布為40-200 nm，此測定值則包括澱粉奈米結晶粒子外部的電雙層，因此，較穿透式電子顯微鏡觀察量測值為大。

Cellulose, chitin and starch were the sources of polysaccharides in this research. When they submitted the acid hydrolysis, the amorphous regions were removed and finally got the nano-scale crystalline regions which were namely polysaccharide nanocrystals. The acid hydrolysis of cellulose was reacting at 70℃ for 10 mins with sulfuric acid of 65 wt% and chitin nanocrystals were prepared by hydrolyzing in 3N hydrochloric acid boiling for 90 mins. Both of them got acid groups on their surfaces after hydrolyzing which dissociated and charged as pH above 1 and thus they were able to form stable nanocrystals suspensions. Cellulose and chitin nanocrystals were rod-like shape with diameter around 20 nm and length distribution of 200-400 nm for cellulose nanocrystals, 200-500 nm for chitin ones according to transmission electronic microscope images. Besides, as the concentration of cellulose and chitin nanocrystals suspensions was beyond the critical concentration, both arranged the cholesteric liquid crystalline phase with special fingerprint texture could be observed under optical polarized microscope. In this research, the multilayers thin films composed of positive charged chitosan and negative charged cellulose nanocrystals were prepared by spin-coating on silicon wafers. The multilayers thin films showed blue, yellow and purple color in turn when bilayers increased form 2 to 5. According to the yield curve of hydrolysates, granular morphology variation and the crystallinity variation of X-ray patterns all concluded that the soft-shells were absent after hydrolyzing for three days with 3.16 M sulfuric acid at 40℃. From the chain length distribution of amylopectin in hydrolysates on different days, the amorphous regions within blocklets which were located the branching points of amylopectin were almost hydrolyzed. Therefore, it was the optimal condition to prepared starch nanocrystals with acid hydrolysis that the hydrolysates on the 5th day were submitted to sonication. However, during the experiment, it was observed that the aggregation of starch nanocrystals was so serious that they could not form stable suspensions. For this reason the suitable dispersant was required to disperse the starch nanocrystals suspension well. According to the results of apparent viscosity and zeta potential investigation, sodium hexametaphosphates with concentration of 0.1 mM showed excellent dispersing power. Besides, the adsorption test of hexametaphosphates groups showed that the adsorption behavior corresponded to Langmuir adsorption isotherm curve. Hence, the maximum adsorption amount of hexametaphosphates groups for 1.5% starch nanocrystals was calculated to be 0.114 mole/kg. The result of sedimentation test indicated that the best stability of starch nanocrystals suspension occurred while dispersed with 0.1 mM sodium hexametaphosphates. Moreover, even if the starch nanocrystals suspensions with condensed solid and dispersant content have been stored for 6 days, the diluted suspension containing 0.1 mM dispersant was still stable for a long time. When the starch nanocrystals were dispersed well, the morphology could be observed directly from images of electron transmission microscopy. The starch nanocrystals were composed of single cluster with diameter 10 nm and platelets arranged with several clusters with diameter above 50 nm. From detection with dynamic light scattering method, the particle size distribution of starch nanocrystals was 40-200 nm which included electrical double layers outside. Therefore, this particle size distribution of starch nanocrystals was larger than the observation on TEM images.