羧甲基化对木聚糖结晶能力及水铸膜成膜性能的影响

Crystallization ability and water cast film formability of carboxymethylated xylan

木聚糖类半纤维素(简称木聚糖)是自然界中储量丰富的天然高分子,但是其在聚合物材料领域的应用尚未得到深入开发。木聚糖水铸膜的成膜性能参差不齐、成膜之后的力学性能较低等缺点阻碍了其在功能涂料及包覆材料中的应用,因此对木聚糖理化结构与其水铸膜成膜性能之间的构效关系进行研究十分重要。前期研究已发现,脱乙酰木聚糖的水合结晶是阻碍其水铸膜成膜的关键因素,而木聚糖结晶又受到其原生支链的调控,但是化学衍生化引入的支链对木聚糖结晶的影响还不清晰。笔者利用氯乙酸钠在碱性条件下对脱乙酰木聚糖进行羧甲基化改性,制备了羧甲基取代度为0.27~0.92的羧甲基木聚糖(CMH),并分析了羧甲基化改性对脱乙酰木聚糖结晶能力及水铸膜成膜性能的影响。结果表明:羧甲基的引入降低了CMH的结晶度以及玻璃化转变温度(T g),改善了木聚糖的水铸膜成膜性能;但是,羧甲基的引入不利于水铸膜力学性能的提升,取代度为0.58的木聚糖水铸膜在50%相对湿度下的抗拉强度为10.3 MPa,而取代度为0.92的在相同条件下的抗拉强度仅为0.26 MPa。

Xylan-type hemicellulose(xylan)is a type of very abundant natural polymers,but it has not been implemented to large-scale and high-valued applications in the polymeric material industries.Unpredictable film formability and poor mechanical strength of xylan water cast films have hindered their applications in functional coatings and encapsulant materials.Hence,it is very important to explore the structure-property relationship of xylans between their physical and chemical structures,film formability and film mechanical property.Original xylans in plants are in an amorphous state,but deacetylated xylans can form a crystalline structure with water molecules.Previous studies have found that the existence of xylan hydrate crystals is the key factor for preventing the formation of water cast film from deacetylated xylans,and the crystallization of xylan is regulated by its original branch chains,but the effects of the branch chains introduced by chemical derivations on the crystallization of xylan is not clear.In this paper,deacetylated xylan with high crystallinity and low degree of branching was selected.The xylans were carboxymethylated by sodium chloroacetate under alkaline conditions to prepare carboxymethyl xylans(CMH)with different degrees of substitution(DS)ranging from 0.27 to 0.92.The effects of xylan carboxymethylation on its water cast film formability and mechanical properties were analyzed.The results showed that the introduction of carboxymethyl groups greatly reduced the crystallinity of the deacetylated xylans;dynamic thermomechanical analysis(DMA)indicated that the glass transition temperature(T_(g))of the carboxymethylated xylans was also significantly lowered to 20-40℃.These changes led to the improvement of the water cast film formability of xylans;under relative humidity(RH)ranged from 11%to 75%,and unmodified xylan and CMH with DS 0.27 were not able to form integral films,while CMH with DS ranging from 0.58 to 0.92 were able to form integral films.However,excessive carboxymethylation would be detrimental to the mechanical properties of the xylan water cast films,since the tensile strength and elasticity modulus of the CMH film with DS 0.58 was 10.30 and 482.6 MPa at 50%RH,while the tensile strength and elasticity modulus of the CMH films with DS 0.85 were reduced to 6.50 and 254.8 MPa and the CMH films with DS 0.92 were reduced to 0.15 and 10.8 MPa,respectively,under the same conditions.The discovery in this study provides new principles to understand the crystallization ability and water cast film formability of xylans.