以澱粉為基質製備pH應答型材料之研究

Abstract

本論文中，正電澱粉與不同的負電交聯分子將用以製備pH應答型材料。將分別介紹以正電澱粉為基質的薄膜、微凝膠與多層薄膜三種材料，並觀察三者在不同環境下(如、離子強度與pH)，因為靜電交聯調控的物性與結構變化。在製備薄膜與微凝膠試驗中，皆採用較低取代度(DS~0.12)的顆粒狀正電澱粉(gCS)。第三章中，探討不同正負電比例下添加的trisodium citrate (citrate)對於gCS薄膜膨潤能力的影響。實驗發現gCS薄膜的組成是由釋出的amylose形成連續相而未破損的膨潤顆粒分散其中，內部含有amylopectin。由成膜溶液的流變性質以及citrate-gCS薄膜的顯微外觀證實，citrate多累積在膨潤顆粒內，以靜電交聯調控gCS分子的pH應答膨潤行為。由小角度X光散射(SAXS)圖譜，進一步發現去質子化的citrate會導致分子鏈聚集而使SAXS圖譜low q範圍曲線強度上升，此聚集有助於增強薄膜的結構用以抵抗膨潤滲透壓，降低膨潤率。然而在裝載與釋放ampicillin的試驗中，因薄膜內的靜電交聯無法長時間穩定結構，使薄膜在5分鐘後開始溶出於溶液中。此一現象促使第四章中，決定引入交聯劑sodium trimetaphosphate (STMP)於部分糊化的gCS中穩定結構，用以製備球狀的正電澱粉微凝膠。實驗發現gCS經50°C部分糊化10 min，可以保存最多的完整膨潤顆粒，之後的STMP交聯也證實可增強微凝膠結構，長時間抵抗膨潤。此一微凝膠的膨潤體積能隨著交聯程度、離子強度與pH改變而改變。STMP交聯經鑑定後得知包含靜電交聯(unreacted STMP)以及化學交聯(distarch monophosphate)，且交聯度都在添加的STMP/CS重量比> 0.1時達到飽和。在膨潤行為上，靜電交聯幾乎沒有顯著性影響，微凝膠的膨潤體積主要由多數的正電基團與化學交聯主導。此外，微凝膠的可變形性在高度交聯的微凝膠仍被觀察到，再次說明STMP形成化學交聯的能力是受限的。雖然微凝膠無法在食品與人體內常見的pH範圍內嶄露pH應答行為，但是低度交聯的微凝膠具有良好的增稠能力，可作為一天然來源的增稠劑。第五章中，改使用高取代度(DS~0.6)的正電澱粉。由一般玉米澱粉或高直鏈玉米澱粉，製備出多分支(NCS)與多直鏈(HCS)的正電澱粉分別與負電的濃縮乳清蛋白(WPC)以逐層塗布法製備澱粉多層膜。因為WPC在pI前後淨電荷性翻轉，因此可以預期觀察到正電澱粉與WPC兩者相吸引之下的多層膜成長，以及兩者相排斥後多層膜表面崩解。首先發現，無論是多層膜的成長方式或崩解方式，皆隨著正電澱粉型態不同而不同。相較於多直鏈的HCS，多分支的NCS容易形成厚且多孔的結構，有助於留住大量的WPC。而實驗中，也的確在pH < pI (4.3)且高離子強度(i=0.1M)下，觀察到WPC與正電澱粉之間斥力產生而導致的結構崩解，只是兩種多層膜具有不同的崩解形式：以HCS為基質的多層膜，在很窄的pH範圍內即發生完整的崩解；NCS為基質的多層膜卻因為其交纏的網狀結構，導致結構崩解不完整。而本實驗也證實，經後裝載(post loading)進入多層膜的花青素釋放也是由WPC調控。綜合以上，本論文有助於設計與發展新的澱粉材料，拓展澱粉在更多樣領域上的應用。

In this thesis, pH-responsive materials were prepared by using cationic starch and different negatively charged cross-linkers. Three types of materials are presented: Casted thin film, microgel, and multilayer thin film. Their physical properties and microstructural change controlled by the ionic cross-linking upon varying environmental conditions (i.e., ionic strengths and pH) were investigated. For casted thin film and microgel, the granular cationic starch (gCS) with DS ~ 0.12 was adopted. In Chapter 3, the effects of trisodium citrate (citrate) on swelling property of citrate-gCS thin film across various (+/-) charge ratios were studied. The gCS thin film is composed of leached amylose and unbursted swollen granules mainly occupied by amylopectin. Proved by the rheological properties of citrate-gCS film forming solution and the microscopic images of citrate-gCS thin films, citrate governs the pH-dependent swelling behavior within the unbursted swollen granule via ionic cross-linking. The small angle X-ray scattering (SAXS) patterns of swollen citrate-gCS thin films are well fitted by employing the correlation length model. The rise of the exponent in low q region (n) is found to be correlated with the presence of deprotonated citrate induced aggregates, constructing a strong network against osmotic pressure, thus leading to a low swelling ratio of the thin film. However, during the release study of loaded ampicillin, the weak ionic cross-linking can’t keep citrate-gCS thin film from dissolving in solution for more than 5 min. It inspired the idea of preparing starch-based microgels where the chemical cross-linking was introduced. In Chapter 4, the spherical cationic starch microgels could be prepared by partially gelatinizing the gCS followed by stabilizing the swollen particles with sodium trimetaphosphate (STMP). It was found that the partial gelatinization process (50°C, 10 min) kept most of swollen granules intact. STMP cross-linking strengthens the structure of microgel against swelling, and was identified containingthe ionic (unreacted STMP) and chemical cross-linking (distarch monophosphate), while saturating as the STMP/CS > 0.1. Microgels swelling volume was subject to change by varying the cross-linking density, ionic strength and pH. Regarding the swelling behavior, the effect of ionic cross-linking is negligible, because the swelling volume of microgels is majorly governed by the excess cationic group and chemical cross-linking. A deformable character in microgels upon close-packing, was even found in a highly cross-linked one, explaining a limit reactivity of STMP to form chemical cross-linking. Although there is no significant pH dependent swelling in microgel observed within a practical pH range, a good thickening ability of a mildly cross-linked microgel is accessible. In Chapter 5, strongly charged cationic starch with DS ~ 0.6 was chosen. Highly branched (NCS) and linear cationic starches (HCS) were prepared to construct multilayers via a layer-by-layer (LbL) deposition with whey protein concentrate (WPC). The pH-dependent structure integrity of multilayer driven by the attraction/repulsion between WPC and cationic starch across its pI was expected and assessed. As a result, the build-up and break-down of multilayer structure differed with the conformation of cationic starch. The thick and porous NCS layer tended to retain more WPC than HCS did. A pH-dependent structural disruption was observed in both multilayers at pH < pI (4.3) under an ionic strength of 0.1 M. Unlike the HCS-based multilayer showing a complete structural dissociation within a narrow pH range, the entangled network in the NCS-based multilayer delayed the release of WPC. Also, the WPC driven release of the post-loaded anthocyanins from the NCS-based multilayer was confirmed. A potential of multilayer to apply as a pH-triggered active coating is foreseen. The findings of this work could have an impact on designing new starch-based pH-responsive materials and extended the application of starch to a broader field.