【作者】 许玉芝；

【导师】 储富祥；

【作者基本信息】 中国林业科学研究院 ， 林产化学加工工程， 2009， 博士

【摘要】 纤维素是由许多D-葡萄糖基通过β-1,4苷键连接而成的线性高分子,是自然界中最丰富的可再生天然资源。利用纤维素为原料,制取具有广阔应用前景的可生物降解纤维素酯,部分取代来源于石化产品的塑料,将为塑料工业开拓一条新的原料渠道。但商品化的短链纤维素酯或混合酯基本都要使用增塑剂来拓宽其加工温度,而纤维素内塑化材料是通过接枝或化学修饰的方法使其内部塑化,克服在塑化和使用过程中增塑剂的迁移和流失问题,从而提高材料的综合性能。合成出较长碳链的脂肪酸纤维素酯是一种较为可行的对纤维素进行内增塑改性的方法。本论文的基本思路是将纤维素溶液化,均相体系下合成出纤维素内塑化材料所需的长链脂肪酸纤维素酯,与可生物降解的聚己内酯复配增韧,并揭示其制备、性能与结构的关系,为开发生物质基可降解高分子新材料及纤维素的增值利用提供理论参考,其主要研究内容和结果如下:1.纤维素非衍生化离子溶液的制备纤维素具有的晶区和非晶区共存的复杂结构,使其在良溶剂氯化锂/二甲基乙酰胺(LiCl/DMAc)中溶解也需进行活化处理,实验采用超声波一步法制备了室温下稳定的纤维素非衍生化离子溶液。通过调节超声波的输出功率和处理时间确定制备溶液的优化工艺,研究了超声波处理对纤维素分子及形态结构的影响,并对该溶液的流变性能进行了考察。红外光谱(FT-IR)、X-ray衍射(XRD)结果表明,超声波对纤维素在LiCl/DMAc中的处理过程起到了物理活化的效果,只需对纤维素、氯化锂和二甲基乙酰胺混合物超声处理5～7 min,并于室温下搅拌3～5 h,即可获得纤维素均匀溶液;13C-NMR图谱中只出现了纤维素缩水葡萄糖苷结构单元碳原子的特征化学位移,证明了纤维素经此工艺条件制备的体系中并未形成新的衍生物,这有利于对其进行后续的功能化改性研究。2.微波法合成纤维素长链脂肪酸酯及结构表征在均相体系下运用微波法制备纤维素长链脂肪酸酯是快速获得系列纤维素内塑化材料的必备条件。反应采用上述方法制备的非衍生化溶液,高级脂肪酸(己酸、辛酸和月桂酸)/甲苯-磺酰氯作为纤维素酯化的共反应体系,借助微波辐射技术,获得了不同链长取代基和取代度的酯化产物,并主要研究了取代度大小、取代侧链长短对产物性质的影响。首先讨论了工艺条件对纤维素月桂酸酯收率及取代度的影响,发现微波辐射条件是影响其合成结果的关键性因素。为缩短酯化反应时间、减少纤维素在反应过程中引起的降解等因素,本文将微波功率设置在300 W,进行间歇辐射60～90 s。当采用共反应剂与羧酸相同摩尔配比,催化剂使用量与纤维素脱水葡萄糖苷单元摩尔数相等时,产物的收率和取代度均达到最大;为充分利用纤维素和制备所需取代度的产物,确定原料配比为1: 4: 4。所得产物的FI-IR图谱和1H-NMR结果均表明,纤维素发生了部分取代反应;在化学位移δ为174.1, 173.6和172.9处出现的谱峰分别对应于纤维素缩水葡萄糖苷上的C-6, C-3, C-2上的取代基碳原子,印证了酯化反应的进行,且被取代的优先次序依次为C-6,C-3和C-2位。X-ray衍射结果则证明了改性反应不仅发生在原料纤维素的无定型区,同时也在结晶结构中进行。而差示扫描量热分析结果显示酰化反应大大削弱了纤维素分子间及分子内的作用力,使产物在0～80℃之间均出现一个明显的玻璃化转变。且随着取代度的增大,纤维素长链脂肪酸酯的数均分子质量Mn、重均分子质量Mw均随之增大,热稳定性提高,但它们具有相近的分子质量分布和相似的热分解过程。取代度在2.1左右的纤维素月桂酸酯、纤维素辛酸酯、纤维素己酸酯均能在二甲基亚砜、氯仿和四氢呋喃中溶解。随着取代基链长的增加,相对分子质量随之增加,而分子质量分布相差不大。纤维素己酸酯没有表现出明显的玻璃化转变区间,但在0～80℃之间,纤维素辛酸酯和纤维素月桂酸酯均出现了玻璃化温度。随着取代侧链长度的增加,对纤维素分子间及分子内氢键的破坏越明显,这样纤维素月桂酸酯、纤维素辛酸酯比纤维素己酸酯的热稳定性要低,但纤维素月桂酸酯的稳定性又稍好于纤维素辛酸酯,这是因为此时链长的空间位阻对热稳定性的影响占了主导作用。3.纤维素塑化材料的结构与性能研究由于引入的较长酯基链减弱了纤维素分子内及分子间的次价键(即范德华力),增大了纤维素分子间的距离(自由体积)而形成一定的空间,加之改性后的纤维素其链结构变得不规则,内聚力减弱,结晶性降低,从而增加了纤维素分子键的移动性,改善了纤维素高级脂肪酸酯的可塑性和溶解性。实验采用溶液流延法制备薄膜,并对材料的耐水性能、热学性质、结晶形态及力学性能进行了研究,考查了不同取代度及不同侧链对纤维素衍生物塑化材料各项性能的影响。不同取代程度的月桂酸塑化纤维素材料的吸水率和接触角实验结果均表明,其表面均具有良好的疏水性,其接触角均在90°左右;而透射电镜(TEM)和XRD则证明了它们的相似结晶形态。动态热机械分析(DMA)测试的Tanδ温度谱显示出整个分子链段的运动出现在150℃左右的温度点上,而取代侧基的次级转变则在-30℃左右,正是由于此峰的存在才使得此类材料在玻璃化温度以下较宽的区域内(如常温)仍然保持柔软的特性。纤维素月桂酸酯的拉伸强度随着取代度的增加而减少,而断裂伸长率都达到了60%以上,但随着取代度的增加,长链基团增加到一定程度对材料的内增塑作用变得微弱而使应变的增加不明显。具有不同取代侧链的纤维素塑化材料表现出不同的性能特征。其中短链改性纤维素材料的吸水率均大于长链改性材料,相近取代链长改性材料则有相近的吸水性。引入的短链烷基使纤维素塑化材料具有较明显的结晶区,界限分明,而长链侧基改性后的纤维素材料结晶现象相像,但它们具有相似的热分解过程。商品化的醋酸纤维素、醋酸丙酸纤维素和醋酸丁酸纤维素均仅有20%左右的断裂伸长率。而采用长链烷基塑化纤维素,极大地优化了材料的柔性,改性材料最大伸长率可达70%。4.纤维素内塑基复合材料的性能与结构研究为了对单一聚合物的性能缺陷进行修复或改进,以某一种聚合物为主体,添加其它组分对主体聚合物进行改性已成为高分子材料科学及工程中最为活跃的领域之一。本实验通过加入聚己内酯(PCL)来进一步改善纤维素塑化材料的力学性能,并利用FT-IR、DMA、扫描电镜(SEM)和TEM等手段对PCL和纤维素月桂酸酯的相容性进行了研究。当加入10%质量比例的PCL时,对薄膜的改性效果最明显,拉伸强度从原来14 MPa几乎提高到了30 MPa,断裂伸长率也从70%增大到了103%;随着PCL加入到纤维素月桂酸酯内,复配薄膜的FT-IR谱图显示—OH和—CH—基团的伸缩振动均有蓝移现象,尤其是—C=O的吸收谱带比起复配改性前变化明显。这可能是由于C=O…O=C或者C=O…O—C之间的偶极作用以及酯基的强电子吸引效应造成的。运用DMA对不同复配比例的薄膜进行分析,其损耗因子Tanδ曲线和Tg数据表明纤维素月桂酸酯和聚己内酯的玻璃化温度彼此靠拢,说明这两种成分具有相互作用。扫描电镜和透射电镜的测试结果也显示,在聚己内酯加入量较低时,PCL和纤维素月桂酸酯具有一定的相容性。5.纤维素基材料的酶可降解性初探环境可降解高分子材料符合材料绿色化的趋势,并已成为世界各国的共识。对不同取代程度的纤维素月桂酸酯薄膜材料进行酶解处理,并运用高效液相色谱、凝胶渗透色谱(GPC)和SEM分别测定和观察了酶解过程产生的葡萄糖、降解前后的分子质量及薄膜表观形貌。随着取代度的降低,酶解液中的葡萄糖含量逐渐增加,葡萄糖得率也明显增加。所考察的纤维素衍生物在取代程度接近的情况下,短链的纤维素衍生物随着取代度的增加,其酶解液中的葡萄糖含量和葡萄糖得率并没有很明显的区别。而长链的纤维素衍生物其葡萄糖得率随着取代侧基链长的增加,酶解液中的葡萄糖含量逐渐减少。GPC分析结果显示,样品经过酶解处理后其Mn减少,Mw增大,说明该类材料的酶降解主要是小分子质量成分的瓦解和流失。另外,SEM图片中的薄膜表面均有斑点和凹洞出现。上述结果说明所得纤维素衍生物具有一定的酶可降解性。更多还原

【Abstract】 Cellulose, a linear homo-polymer composed of D-anhydroglucopyranose units (AGU), linked together byβ(1→4)-glycosidic bonds, is the most abundant polymer raw material available today worldwide. Esterification of cellulose represents one of the most versatile transformations as it provides access to a variety of bio-based materials with valuable properties. The internal plasticization of cellulose through grafting or chemical modification is an effective way to eliminate the change of material performance resulted from exudation or volatilization of low molecular weight plasticizers in processing and application. One potentially feasible solution would be the synthesis of esters of cellulose with longer chain acids than the currently commercial acetate, propionate and butyrate.The idea of the present paper is to synthesize long-chain cellulose esters in a homogeneous phase by microwave irradiation for preparing internally plasticized materials, which are modified by blending with other polymers, and to discover the correlation of its preparation, properties and structure, and as well as to provide access to a variety of cellulose-based materials with valuable properties. The main research works and results are summarized as follows:1. Preparation of cellulose solution by ultrasonic treatmentThe insolubility of cellulose in water and in most organic solvents was caused by areas of both high order (crystalline) and low order (amorphous). And the mixed solvent N,N’-dimethyl acetamide with lithium chloride (LiCl/DMAc) was used to dissolve cellulose after being activated. One-step procedure of activation and dissolution of cellulose in LiCl/DMAc irradiated by ultrasonic was studied. The effect of the ultrasonic on the supermolecular structure of cellulose was assessed by means of FT-IR and Wide-angle X-ray diffractometry, respectively. And ultrasonic irradiation exerted a clear influence on the morphological character of cellulose measured by scanning electron microscopy. The results of analysis showed that ultrasonic treatment affected the crystallization of the cellulose examined. The multiple parameters during the solvation process such as ultrasonic power, treatment time and degree of polymerization shown in the paper were evaluated. It was found that cellulose could be easily solubilized in LiCl/DMAc after being treated for 7 min by ultrasound at 400 w, with no degradation of the cellulose chain during the process. A study of the rheological property of the cellulose solution indicated that it was non-Newtonian fluids.13C-NMR spectrum proved that no derivatives of cellulose were formed during the solvation process.2. Esterification of cellulose with long-chain fatty acid irradiated by ?microwaveAcylation of cellulose in a homogeneous phase by microwave irradiation was essential to get internally plasticized cellulose derivatives. The reaction system was cellulose, fatty acid and LiCl/DMAc with p-toluenesulfonyl chloride as an activating reagent. The cellulose esters of different degree of substitution (DS) were directly synthesized, and the correlation of properties, structure and DS, and as well substituents were investigated.The effect of the reaction conditions on esterification was evaluated and the results showed that microwave power and irradiation time were critical factors. For reducing reaction time and utilizing the cellulose sufficiently, the reaction time of 60-90s at 300 w was chosen in the experiment. The reagent molar ratio has significant impact on weight yields (Rmass ), molar yields(Rmol), and the DS, when irradiation power and time was kept constant. The feasible molar ratio of reagent was 1:4:4 to obtain products with desired DS.The—OH band in FT-IR spectra of cellulose laureates with different DS did not disappear thoroughly due to partial substitution and the remaining hydroxyl groups from crystalline regions of the cellulose, which were in good agreement with those observation from 1H-NMR for all DS < 3. Concerning 13C-NMR data of cellulose laurate with 2.6, it was observed that C-6 position was substituted, preferentially to C-3 and then to C-2 in the study. And the changes in the WAXD pattern of starting cellulose, caused by esterification reaction, were attributed to the long-chain ester bonded to cellulose. These cellulose esters had glass-transition temperature (Tg) in the 0-80°C range. However, further increase in molecular weight and DS had very little effect on Tg although the Mn and Mw of cellulose lauroyl esters increased with the DS increased.Cellulose laurate, cellulose caprylate, and cellulose caproate are all soluble in DMSO, CHCl3 and THF when the DS was around 2.1. At cellulose caproate,the endotherm was barely visible in the DSC thermograms, while a clear glass transition was found for cellulose caprylate and cellulose laurate in the 0-80°C range. The Mn and Mw of these cellulose esters increased with the ester chain length increasing, but thermal stability was decreased because of the weakening of extensive hydrogen bond.3. Structure and properties of cellulose ester plasticsThese longer chain esters (LCCEs) show promise as internally plasticized cellulose esters because the supermolecular structure of cellulose was significantly changed with the introduction of longer substituents. The internally plasticized cellulose films were obtained by casting and the influences of the DS and substitutents on the properties of these films, such as water resistance, thermal stability, glass transition temperature, tensile strength and elongation, were investigated.Contact angle of cellulose laurate films with different DS was nearly 90°. TEM images and XRD diffraction patterns all showed that their crystalline state was similar. DMTA in tension mode revealed separate relaxations, denotedαandβ, corresponding to the glass transition and side-chain melting processes, respectively. For the three cellulose laurate samples, two main peaks separated were readily distinguished in the tanδcurves. The relaxation at the higher temperature (αransition), which varied between 120°C and 200°C, was accompanied by the most important decrease of G′, and this indicated that it corresponded to the glass transition of the polymer. At a lower temperature, the peak corresponded toβtransition, accompanying lateral substituent melting in the vicinity of -30°C.The films properties of long-chain cellulose esters were also compared to those of cellulose acetate, cellulose acetate propionate and cellulose acetate butyrate. The results showed that the water absorption and tensile strength of short-chain cellulose ester plastic films were higher than the long-chain ones with preferable failure strain level. In addition to this, thermal decomposition process was similar.4. Compatibility Characterization of cellulose ester matrixBlending is a practical way to improve the performance of a single polymer. Cellulose laurate(CE) was blended with polycaprolactone (PCL) in order to improve its mechanical property. The result indicated that the elongation of the CE was improved significantly through blending with PCL, and a synergism was observed for certain compositions (CE/PCL = 90/10). The tensile strength rose from around 12MPa to 30MPa, and tensile strain increased from 70% to 103%. All the blend films showed better water resistibility regardless of mass ratio of CE and PCL. FT-IR spectroscopy revealed that there were several specific interactions between the chains of CE and PCL, such as the interaction between C—H and O=C—and C=O…O=C or C=O…O—C dipole–dipole interactions. TEM and SEM images showed that there was somewhat compatibility between CE and PCL.5. Elementary investigation on the enzyme degradation of cellulose ester filmsBiodegradable polymers featuring ecological advantages for sustainable development have attracted great commercial interest because of growing environmental concerns. A different assay was developed based on the utilization of ester enzyme and cellulolytic enzymes rather than dynamic mixed cultures found in the natural environment.The extent of biodegradation was followed by monitoring glucose production using HPLC. As the DS and the acyl chain length increased, the rate of biodegradation rates decreased. Preferential attack of the microorganisms was observed on the face of the cellulose laurate film in SEM images. Collectively, this work indicated that the cellulose ester films with lower DS and shorter side-chain showed better biodegradability.更多还原