微波辅助法合成淀粉醋酸酯及其理化性质研究

以玉米淀粉为实验材料,在微波辅助加热条件下,研究醋酸与醋酸酐体积比(酸酐比)对合成淀粉醋酸酯取代度的影响。分别采用红外光谱仪、扫描电镜、X-射线衍射仪、差示扫描量热仪、凝胶色谱等方法对合成淀粉醋酸酯的化学结构、颗粒形貌、结晶特性、热特性、分子质量分布进行表征;进而比较不同酸酐比条件下淀粉醋酸酯的理化性质变化。实验结果显示,酸酐比对于合成淀粉醋酸酯取代度有较大影响,随着醋酸酐用量增加,取代度先增大后减小,在酸酐体积比1∶1处取代度达到最大值2.92。此外,新合成淀粉醋酸酯的化学结构和物化性质相对于原淀粉发生较大变化。淀粉分子中的羟基比例减少,羰基比例增大;改性淀粉颗粒表面呈多孔状或崩解成许多细小颗粒;改性后,淀粉原有的晶形结构遭到破坏并发生重构,结晶度降低;淀粉醋酸酯的热特性也发生较大变化,主要表现为玻璃化温度和熔融温度降低,由此说明淀粉醋酸酯相对于原淀粉具有更好的热塑性;淀粉醋酸酯分子质量相对于原淀粉减小,分子质量分布变宽,表明淀粉分子链在酯化过程中发生一定程度断裂。

Study on the Synthesis and Physicochemical Characteristics of Starch Acetates Under Microwave Assistance

The effect of volume ratio between acetic acid and acetic anhydride on the degree of substitution(DS) was investigated for the synthesis of starch acetates by microwave assistance.Chemical structure、granule morphology、crystalline、thermal properties and molecular weight distribution of starch acetates were all determined separately by infrared spectrum(IR)、scanning electronic microscope(SEM)、X-Ray Diffractomer(XRD)、differential scanning calorimeter(DSC)and gel permeation chromatography(GPC).The physicochemical characteristics for starch acetates synthesized under different acetic acid to acetic anhydride ratio were also compared on the basis of those results.The results were obtained as follows:the ratio between acetic acid and acetic anhydride had a great effect on DS;DS was reached to the highest of 2.92 at the ratio of 1∶ 1(volumn ratio);and then,DS decreased along with the increase of acetic anhydride.Moreover,the chemical structures and physical-chemical properties had a great changes after the esterification.The hydroxyl groups of each glucose unit were replaced by acetyl groups gradually as the increase of DS.As it is shown from SEM results,the surface of starch acetate became rough and porous.Meanwhile some of the starch particles have been cracked into tiny parts;After modification,new crystal formed and old intrinsic crystalline structure destroyed.As a result of that,the degree of crystallization decreased.The thermal properties of starch acetates have also changed greatly with the decline of glass transition temperature(Tg) and melting temperature(Tm).Therefore,starch acetates are expected to exhibit better thermoplasticity than native starch.The molecular weight of starch acetates were decreased and its distribution became wider,this indicated that starch chain was broken during esterification.