【作者】 金磊；

【导师】 郑强； 上官勇刚；

【作者基本信息】 浙江大学 ， 高分子化学与物理， 2014， 博士

【摘要】 近几十年来,聚电解质材料已被广泛用作工程材料、日化添加剂、食品添加剂以及生物医用材料等,是国内外众多学者长期关注的研究热点之一。由于聚电解质材料常以溶液态使用,溶液黏弹行为是决定其性能的重要因素,因此流变学研究是聚电解质研究中最重要的内容之一。通常,聚电解质结构较为复杂。其中,缔合型聚电解质因含有大量缔合基团,可呈现出特殊的黏弹特性,而其分子相互作用更加复杂,导致缔合型聚电解质的相关研究相对滞后。因此,探明缔合型聚电解质溶液的流变行为与分子结构、缔合形态之间的构效关系,已成为目前聚电解质研究领域的重要课题。这一方面可为聚电解质材料的结构调控以及性能优化等提供理论指导,另一方面可为拓宽缔合型聚电解质材料的应用范围、提升使用功效积累实践经验。本论文针对应用最为广泛的聚电解质材料之一的聚丙烯酰胺(PAM)进行改性,选择壳聚糖(CS)作为主链,利用溶液聚合法制备了壳聚糖接枝聚丙烯酰胺(CS-g-PAM, GPAM)。本论文系统考察了GPAM的溶液性质、缔合形态与流变行为的内在关系,探索了剪切作用、温度、第二组分等外界因素对于GPAM溶液中的缔合结构演化及一系列特殊流变行为的影响,重点考察了剪切与热诱导增稠行为,揭示了GPAM溶液浓度与链结构等内在因素对其特殊流变行为的影响规律,初步建立了分子结构-缔合形态-内外界因素-特殊流变行为的关联机制。研究取得了以下主要结果：1.GPAM在稀溶液区即存在分子内缔合,在较低浓度下(C*附近)即可形成分子间缔合体,其溶液性质无法用经典的聚电解质理论来描述。透射电子显微镜(TEM)、动态光散射(DLS)等结果证明了缔合结构的存在,其黏度-浓度依赖性存在两个临界浓度(Co1,C02),分别对应缔合体的形成以及缔合体间作用的形成,这可作为分析缔合形态与流变行为内在关系的基础。2.缔合型聚电解质常出现多重剪切变稀、剪切增稠、负触变性等普通聚电解质所没有的特殊流变行为。GPAM溶液会呈现一种特殊的剪切诱导自增稠行为(Shear induced self-thickening,SIT),即浓度在C02以上的GPAM溶液在强剪切下会首先呈现剪切变稀；剪切停止后,体系黏度持续增加,其最终黏度远高于初始溶液,且其增稠效果不会自发回复。与传统的高分子剪切增稠相比较,该现象具有三个显著区别：1)增稠过程不同。该增稠过程由剪切变稀和停止高速剪切后的静态增稠两阶段组成,而传统的剪切增稠则是体系黏度随施加剪切而增大；2)增稠效果不同。该体系的增稠效果不会自发回复,而传统剪切增稠体系的黏度会在剪切停止后自动回复；3)增稠程度可控。其黏度增幅依赖于高速剪切条件,且呈现良好的重现性。GPAM的剪切变稀现象由三段剪切变稀构成,随着剪切速率增大溶液中逐步发生缔合体间作用、缔合体内作用以及缔合体瓦解后分子间相互作用的破坏过程。SIT行为可解释为强剪切作用之后,GPAM分子内氢键缔合逐渐转化为分子间氢键缔合,最终因GPAM缔合体规模增大导致溶液黏度增大。3.在适当浓度区间内的GPAM溶液能发生明显的热增稠。通常,由于富含氢键、疏水等基团,缔合高分子体系具有显著的温度响应性,在升温过程中,疏水基团(或LCST组分)引发的缔合甚至微相分离会导致黏度显著上升,发生热增稠。然而,不同于研究较多的疏水改性CS,GPAM引入的是亲水的PAM支链,这意味着GPAM的热增稠机理不同于常规的LCST组分驱动的热增稠。实验表明,升温前后GPAM溶液中发生了氢键主导缔合体向疏水作用主导缔合体的转变。这种缔合体的转变,使得缔合体数量显著增多,缔合体间作用得到增强,最终导致热增稠发生。同时,GPAM的热增稠行为受温度、加热时间、GPAM溶液浓度以及链形态等因素共同影响,且热增稠具备不可逆性,在降温过程中黏度不能回复至初始黏度。4.GPAM和CS均含有大量的氢键基团,可与富含羟基的β-环糊精(β-CD,CD)分子产生氢键复合效应。适当的CD浓度下,GPAM/CD与CS/CD体系均能表现出SIT行为以及大幅振荡之后模量增大的振荡硬化行为,这与强剪切或大幅振荡之后复合体系中大分子-CD-大分子之间氢键作用的均一化以及复合结构的加强有关。GPAM/CD与CS/CD体系的实验结果对比表明,PAM支链对复合溶液流变行为的剪切响应性有显著影响。更多还原

【Abstract】 In the past decades, polyelectrolyte has been widely used in industry, biomedical, daily chemicals and food fields, etc., and polyelectrolyte has attracted increasing attention in scientific research. Polyelectrolyte is mainly used in solution state, and its viscoelasticity usually determines the performance of polyelectrolyte materials. It is noted that rheology has become one of the most important methods in polyelectrolyte research. Among the polyelectrolytes with complex structures, the one containing some associative groups can be named ’associative polyelectrolyte’. Associative polyelectrolytes present some unique viscoelastic properties which are suitable for industry applications. However, systematic rheological research has rarely been carried out up to now. In fact, the aggregates or associative structure in polyelectrolyte solutions significantly affect their rheological behavior. To achieve better structure modulations and performance optimizations, it is necessary to investigate the relationship between the molecular structures or associative states and the rheological behavior. Furthermore, the rheological results of associative polyelectrolytes will extend the knowledge field of polyelectrolytes.In this thesis, polyacrylamide-g-chitosan (CS-g-PAM, GPAM) by graft modifying PAM (one of the most used polyelectrolyte materials in industry) with CS as the backbone was prepared through solution polymerization.The solution properties, associative states and basic rheological behavior of GPAM were investigated. Then we focused on the influences of shear, temperature and second component on the associative state and the complex rheological behavior. Based on the influences of the concentration of GPAM and the structural information of GPAM chain, the special rheological behavior was discussed in detail. Based on the above experimental conclusions, the correlation among molecular structure, associative state, internal/external conditions and special rheological behavior was established. The main conclusions are as following: The results reveal that aggregate forms even in GPAM dilute solution with the concentration near C*, and the classical theory of polyelectrolyte solution is fail to describe the solution properties of GPAM. The observations of transmission electron microscope (TEM) and dynamic light scattering (DLS) confirm the formation of GPAM aggregate. Besides, the scaling relationship between viscosity and concentration gives two critical concentrations (C01and C02) for GPAM solutions, indicating the initial concentrations for the formations of single aggregate and the interaction among these aggregates, respectively. It is believed that, C01and C02may help to analyse the mechanisms of these special rheology behavior.It is noted that associative polymers are sensitive to shear effect and can present some special responses like multistage shear thinning, shear thickening and negative thixotropy, etc. GPAM is an associative polyelectrolyte presenting more complex structure, and GPAM appears three-region shear thinning, and shear-induced self-thickening (SIT). Different from previously reported rheopexy and shear-thickening, SIT presents three distinct features:i) different thickening process, in which this thickening behavior consists of an initial shear-thinning and a subsequent thickening region after removing strong shear, while the thickening of conventional shear-thickening systems only happens when undergoing shear; ii) maintainable thickening effect, with which the resulting thickening could be hold until undergoing strong shear once again while for the conventional shear-thickening most of the increased viscosity may spontaneously recover; iii) controllable thickening extent, i.e., the viscosity increment of SIT is related to the shear conditions and could repeat well. The distinct three-region shear thinning reveals that there exist complex structures or interactions in GPAM solution which can be distinguished by shear rate. SIT is attributed to the transformation from the intramolecular aggregate to the intermolecular aggregate during the rebuilding process of GPAM H-bonding aggregate after the strong shear, which results in the enhanced GPAM aggregation and increased viscosity.Furthermore, in a proper concentration range, GPAM solution displays obvious thermo-thickening. Thermo-thickening is one of the most important thermo-responsive behavior, in which the hydrophobic groups collapse and form aggregates, resulting in a sharp increase in viscosity upon heating. Different from the widely reported thermo-thickening in hydrophobic modified CS, GPAM is a hydrophilically modified one containing lots of PAM side chains. Hence, the mechanism of GPAM thermo-thickening is unusual, and it is found that the aggregate transformation from hydrophobic aggregate to H-bonding aggregate dominates the thickening process. Furthermore, the thickening of GPAM is influenced by temperature, heating time, GPAM concentration, and the structure of GPAM chain. Besides, the thermo-thickening is irreversible, i.e., the increased viscosity upon heating would not recover when the temperature drops to room temperature.Owing to lots of H-bonding groups, GPAM and CS can compound with β-cyclodextrin (β-CD, CD) through intermolecular H-bonds. With appropriate CD concentration, both GPAM/CD and CS/CD solutions present SIT and a novel rheological behavior named hardening after large amplitude oscillation, which may be related to the increased amount of polymer-CD-polymer H-bonds and the enhanced H-bonding network structure. According to the comparison between GPAM/CD and CS/CD, PAM side chain is considered as an important influence factor to the shear-response of the complex solutions.更多还原