纤维素化学改性的研究进展

An Overview on Chemical Modification of Cellulose

纤维素作为自然界来源丰富的天然高分子材料,具有可再生、生物相容性好、可完全生物降解等优越性能,是最有潜力的绿色材料之一,对纤维素资源的有效利用已成为化学、化工和材料科学领域的研究热点。目前,纤维素的研究主要集中在以下三个方面:(1)寻找环境友好的溶剂体系,对纤维素溶液直接加工;(2)利用化学反应,减弱纤维素分子内及分子间氢键作用,制备可溶解在普通溶剂中的纤维素衍生物或接枝共聚物;(3)以纤维素衍生物为结构单元,通过引入特定基团或其他高聚物修饰来构建新型纤维素功能材料。然而,由于缺乏有效的纤维素溶剂,传统的纤维素改性均在多相介质中进行,存在工艺复杂、产品均一性差、结构控制困难及能耗高等问题,研究可发生化学反应的纤维素新溶剂体系成为解决这一问题的有效途径。在过去的几十年里,人们开发了多种新溶剂体系来溶解和加工纤维素,如N,N-二甲基乙酰胺/氯化锂(DMAc/LiCl)、聚甲醛/二甲基亚砜(PF/DMSO)、二甲基亚砜/三水合四丁基氟化铵(DMSO/TBAF·3H_2O)、N-甲基吗啉-N-氧化物(NMMO)、离子液体(ILs)、熔盐水合物、碱/尿素/水体系等。其中,DMAc/LiCl和ILs具有极强的化学稳定性,是纤维素均相酯化和接枝共聚的理想溶剂,碱/尿素/水体系由于碱的存在是纤维素进行均相醚化的优异介质。纤维素的均相化学改性为开发简单、经济、高效、高品质的纤维素基功能材料提供了新的途径。在上述体系中,以纤维素为原料已加工成不同类型的再生纤维素材料(再生纤维素纤维、膜、水凝胶、气凝胶、复合材料)及多种纤维素衍生物(纤维素酯、醚、接枝共聚物)。其中,TEMPO体系氧化反应、醛基-氨基的席夫碱反应、共价交联反应、叠氮-炔烃环加成反应、巯基-烯基的"点击化学"反应、活性/可控自由基聚合反应如原子转移自由基聚合(ATRP)、氮氧稳定的自由基聚合(NMP)及可逆加成断裂链转移聚合(RAFT),已成为纤维素基高聚物制备技术的发展态势,为纤维素的加工与功能化提供了新的多用途平台,在药物控释、污水净化、造纸行业、涂层材料等领域有广泛的应用。此外,纤维素衍生物除单独使用外,还可作为结构单元构筑具有新型结构与功能的纤维素材料,形成的接枝聚合物在特定条件下可表现出自组装行为。本文归纳了氧化纤维素的研究进展,综述了纤维素衍生物的最新制备方法及应用,介绍了基于交联纤维素构建纤维素水凝胶的发展现状,对比分析了传统自由基聚合、离子聚合、开环聚合及活性/可控自由基聚合制备纤维素接枝共聚物的原理与特点,最后探讨了未来纤维素的发展趋势。

Cellulose,as one of the most abundant natural polymer and the most promising green materials with renewability,biocompatibility and biodegradability,etc.,has become a hot spot for its effective utilization in the fields of chemistry,chemical engineering and materials science.The recent research of cellulose is mainly concentrated in the following three aspects:Ⅰ.the direct processing of the cellulose solution formed from environment-friendly solvent systems;Ⅱ.the preparation of cellulose derivative or graft copolymer which can be dissolved in common solvent by attenuating the intra-and inter-molecular hydrogen bonding interactions of cellulose via chemical modification;Ⅲ.the construction of novel cellulose functional materials by introducing specific chemical groups or other polymers into the cellulose derivatives structural units.On the other hand,the cellulose chemical modification reactions are conventionally carried out in multiphase medium,due to the lack of effective solvent for cellulose,resulting in complex process,poor product homogeneity,difficulty in structure control and high energy consumption,etc.Thus the development of new solvent systems for cellulose chemical reaction has been regarded as a plausible countermeasure.In the past few decades,there have emerged a variety of solvent systems which are more effective in dissolving cellulose,including N,N-dimethylacetamide/lithium chloride(DMAc/LiCl),polyoxymethylene/dimethyl sulfoxide(PF/DMSO),DMSO/tetrabutylammonium fluoride trihydrate(DMSO/TBAF·3H 2O),N-methylmorpholine oxide(NMMO),ionic liquids(ILs),some molten salt hydrates,alkali/urea aqueous systems,etc.Among them,DMAc/LiCl and ILs have displayed the potential as the ideal solvents for homogeneous esterification and graft copolymerization of cellulose,mainly for their high chemical stability.Alkali/urea aqueous systems are considered to be excellent medium for homogeneous etherification of cellulose due to the existence of alkali.The homogeneous chemical modification of cellulose opens a new way for the development of simple,economical,efficient and highly qualitative cellulose-based functional materials.By adopting the above mentioned systems,an extensive range of cellulose-based materials(e.g.,regenerated cellulose fibers,films,hydrogels,aerogels,composites,and cellulose esters,ethers,graft copolymers)have been fabricated successfully from cellulose as raw material.The TEMPO-mediated oxidation,aldehyde-amino Schiff reaction,covalent crosslinking,azide-alkyne cycloaddition,thiol-ene"click chemistry"reaction,living/controlled free radical polymerization such as atom transfer radical polymerization(ATRP),nitroxide-mediated living free-radical polymerizations(NMP)and reversible addition fragmentation chain transfer polymeri-zation(RAFT),have assumed future-representativeness in the preparation of cellulose based polymers.These revamped methods establish a new and versatile platform for cellulose processing and functionalization,providing satisfactory and widely applicable cellulose-based products for drug delivery and controlled release,sewage purification,papermaking,coating fabrication.In addition,cellulose derivatives can also be used as structural units to construct cellulose materials with novel structure or functions.The formed graft polymers are able to demonstrate self-assembly behavior under certain circumstances.This review makes a retrospection of the research efforts with respect to oxycellulose,and the homogeneous derivatization and graft copolymerization of cellulose through which can novel functionallized cellulose materials be obtained,then sketches out the advances in creating cellulose hydrogels by crosslinking of cellulose.We comparatively analyse the principles and characteristics of synthesizing cellulose graft copolymers by traditional free radical polymerization,ionic polymerization,ring-opening polymerization(ROP)and living/controlled free radical polymerization,and finally outline the future development trend.