生物质基温敏智能材料的研究进展

A Survey on the Study of Biomass-based Thermosensitive Smart Materials

温敏材料是重要的智能材料之一。虽然温敏均聚物具有良好的环境敏感性能,但其力学性能无法满足使用要求,且部分温敏均聚物的最低临界溶解温度(Lower critical solution temperature,LCST)难以改变,从而限制了其应用领域。当温敏聚合物与其他基材复合或接枝共聚时,可以有效提升温敏材料的力学性能,同时通过改变物料组成及配比可以调节温敏材料的临界温度,拓展其应用范围。制备温敏智能材料的原料大多来源于不可再生的石油资源,随着石油资源日渐匮乏,人们逐渐将目光转移到其他资源。生物质作为可再生资源,广泛存在于自然界中,具有资源丰富、可持续利用的优点,特别是其含有羟基、胺基、醚键和羧基等活性官能团,可以提供多种活性位点,与温敏单体接枝共聚来制备温敏材料,是一种很好的温敏材料基材。已成功应用在生物质温敏智能材料中的生物质原料包括纤维素、纤维素醚、半纤维素、木质素、壳聚糖等。然而,制备生物质基温敏智能材料的接枝共聚方法单一,传统的自由基共聚制备的温敏材料存在温度响应范围窄、产生温敏均聚物较多且难分离以及制备的材料形态单一等问题。生物质温敏材料的接枝共聚方法已经从最初的以引发剂引发的普通自由基聚合发展到可控性较强的光引发自由基聚合、原子转移自由基聚合(Atom transfer radical polymerization,ATRP)、单电子转移活性自由基聚合(Single election transfer living radical polymerization,SET-LRP)、可逆加成-断裂链转移法(Reversible addition-fragmentation chain transfer,RAFT)等接枝共聚方法。温敏接枝单体较多,其中研究最多的为N-异丙基丙烯酰胺(N-isopropylacrylamide,NIPAM),其具有明确的临界溶解温度,且最低临界溶解温度与人体温度相差不大,它与生物质材料一起制备的温敏性膜、温敏性水凝胶和温敏性微球等在药物释放、组织工程和工农业等方面具有广泛的应用。本文详细归纳了生物质大分子制备温敏材料的方法,对这些接枝共聚方法的特点进行总结,同时介绍了制备温敏材料涉及的温敏物质、温度响应机理以及生物质基温敏智能材料的应用,最后总结了现阶段生物质基温敏智能材料制备及应用中存在的难点,并对未来的技术发展进行了展望。

Thermosensitive materials are recognized as one of the most significant smart materials at present.Thermosensitive homopolymers show superior environmental sensitivity,nevertheless they suffer from poor mechanical properties,which cannot meet the applied requirements.Furthermore,it is difficult to alter the lower critical solution temperature (LCST) of some thermosensitive polymers,which blocks their widespread application.The combination or graft copolymerization of the thermosensitive materials with other substrates will greatly contribute to the properties of thermosensitive polymer.Meanwhile,the critical temperature of some thermosensitive materials can be adjusted by changing the material composition or ratio,thus their application field can be expanded. The majority of the raw materials for preparing the thermosensitive materials are derived from non-renewable petroleum resources.The growing deficits of petroleum resources has given impetus to seek other alternative resources.Biomass,as a renewable resource,is widely distributed in nature,showing the advantages of abundant reserves and sustainable utilization.Especially,their active functional groups,including hydroxyl,amine,ether and carboxyl groups,can provide a variety of active sites,which is an ideal substrate for preparing thermosensitive materials by graft copolymerization with thermosensitive monomers.Biomass materials that have been successfully applied in biomass thermo-sensitive smart materials include cellulose,cellulose ether,hemicellulose,lignin,chitosan,etc.However,the graft copolymerization method for preparing biomass-based temperature-sensitive smart materials is short of diversity.The temperature-sensitive materials prepared by conventional free radical copolymerization bear the narrow temperature response range,large generation of homopolymers difficult to separate,and single morphology of mate- rial.The approach of graft copolymerization of biomass thermosensitive materials has developed from common free radical polymerization by initiators to highly controllable free radical polymerization for photoinitiated free radical polymerization,atom transfer radical polymerization(ATRP),single electron transfer mediated living radical polymerization (SET-LRP),reversible addition-fragmentation chain transfer (RAFT).A variety of thermosensitive monomers can be selected,among them,N-isopropylacrylamide (NIPAM) received most research attentions.It possesses a clear critical solution temperature,a close LCST with human body temperature.The thermosensitive membranes,thermo-sensitive hydrogels and thermo-sensitive microspheres prepared by NIPAM together with biomaterials exhibit extensive application in drug release,tissue engineering,industrial and agricultural,etc. In this article,we present a detailed overview of the approaches for preparing temperature sensitive materials by biomass macromolecules,summarize the character of the grafting copolymerization,and introduce the temperature-sensitive substances involved in the preparation of thermo-sensitive materials,mechanism of temperature response and the application of biomass thermosensitive materials.Finally,we point out the difficulties existing in the preparation and application of biomass thermosensitive smart materials,and the future development of technology.