介质和力场协同作用对纳米纤维素形貌结构的调控

纤维素是一种由直链多聚糖通过糖苷键连接而成的巨型线性高分子,纤维素分子链通过氢键紧密排列形成纤维素晶体.由于纤维素晶体具有优良的化学可修饰性和机械性能等优点,纳米化加工的纤维素可广泛应用于日常生活和工业生产的各个领域.本文主要介绍了本课题组在机械剪切力作用下,实现纤维素纳米化并同时进行亲水或疏水改性的研究进展,重点介绍了介质极性对纤维素分子链之间相互作用的影响,并通过改变分子链之间的相互作用来调控纳米化纤维素的形貌和亲、疏水性.提出机械外力和环境极性协同作用下,晶面导向剥离纤维素的理论.

Influence of Medium Polarity and Mechanical Force on Morphology and Structure of Nanocellulose

The crystallite of native cellulose has hydrophilic (110)/(1(1)0) facets with abundant hydroxyl groups and hydrophobic (200) facets with C-H bonds.The cellulose chains form crystal via intra-layer hydrogen bonds along (200) plane and van der Waals stacking along (110) and (1(1)0) planes.Inspired by these anisotropic interactions,we propose new concepts for manipulating themorphology and properties of nanocellulose.The progress along this line in our group in the last five years is reviewed and the influence of medium polarity on the way of cellulose disintegration is emphasized.Dry (no liquid) milling of cellulose causes its decrystallization,while the presence of a liquid changes the behavior significantly.Addition of nonpolar liquids slows down the decrystallization process,and a new form of cellulose is generated in this process as an intermediate product,namely "cellulose nanosheet",which has a form of flat sheets with basic unit of 2-4 nm thickness,apparently formed by coalescence of elementary fibrils.Addition of polar liquids basically prevents decrystallization,leading to individualization of nanofiber into elementary fibrils or their aggregate bundles.Addition of esterifying agents to cellulose-organic solvent system promotes nanofiber dispersion via esterification of the surface hydroxyls of cellulose.Here,a characteristic difference is found between polar and nonpolar solvents as reaction medium,suggesting the involvement of specific interaction between solvent molecules and crystallographic planes of the cellulose.One unique discovery is the involvement of pot material in cellulose milling:dry milling of cellulose in PTFE pot was found to result in nano-coating of cellulose particles by PTFE.This phenomenon is interpreted as an instance of friction transfer,which has been observed for PTFE under controlled setups.Its occurrence in ball milling of cellulose may lead to a new class of cellulose-based materials.As a whole,physicochemical modification of crystalline cellulose was found to involve interactions with environments of polar/nonpolar natures.Exploration of this concept may lead to various innovative technologies in cellulose processing.