Green Modification of Cellulose Nanocrystals and Their Reinforcement in Nanocomposites of Polylactic Acid

1 Introduction

During recent years, research of alternatives to petroleumbased materials has been a general trend to help alleviate the present energy shortage. Biocomposites may be regarded as an ideal type of product for the future, because they are biodegradable and recyclable.Polylactic acid (PLA) is among the most suitable matrix materials for such biocomposites because of its relatively low cost, biocompatibility, processability,and eco-friendly nature[1-2]. However, it has some drawbacks at the same time that limit its use in certain applications. Although its tensile strength and elastic modulus are comparable to poly(ethylene terephthalate) (PET), it is too brittle for most uses[3].Therefore, it is essential to find appropriate fillers to increase the toughness of PLA materials to expand their applications.

②局部性病变。主要包括：复杂的尿道和阴茎病变导致无法置入电切镜的患者；无法采用截石位的患者；合并巨大膀胱憩室，需开放手术一并处理者。合并体积较大的膀胱肿瘤,不宜与前列腺同时处理，应先切除肿瘤后再考虑TUPKP手术。PSA异常、MRI或肛门指检异常，怀疑前列腺癌的患者，应首先通过前列腺穿刺活检排除肿瘤；对于有神经系统疾病、脊髓外伤等相关病史的患者应进行尿动力学检查以排除神经源性膀胱。

数字矿山在绿色矿山的建设中具有必要的作用。数字化、信息化、自动化技术可以有效的变革矿山企业的生产工艺、管理模式，提供全面的信息资源，进而提高生产效率和水平。已经有学者提出了数字矿山是一种可视化平台[2]。除“数字矿山”之外，有的学者还提出了“智慧矿山”的概念，卢新明等(2010)[3]认为，智慧矿山是一套完整的数字化智慧体，针对矿山全部信息进行自动收集、传播、整合、展示；霍中刚等(2016)[4]认为智慧矿山是采矿技术伴随着计算机科学、信息传输、AI技术和RS、GPS、GIS技术发展融合的结果。

Cellulose nanocrystals (CNCs) have aroused much attention in the field of nanocomposites given their biocompatibility, biodegradability, light weight,nano-scale effects, low cost, high specific strength and modulus, unique morphology, and relatively reactive surface[4-7]. Nevertheless, there are two challenges to overcome in the application of CNCs in the field of nanocomposites. First, a large number of hydroxyl groups on the surface of CNCs make the nanoparticles present hydrophilic property, which limits the compatibility between hydrophilic CNCs and a hydrophobic polymeric matrix[8-10]. In addition, the preparation of CNCs based on sulfuric acid hydrolysis is considered the most main stream approach.Negatively charged sulfate esters are introduced on the surface of the obtained CNC particles when using the sulfuric acid as a hydrolyzing agent. The charged sulfate esters can promote dispersion of the CNCs in water because the like charges repel each other.However, the introduction of charged sulfate esters diminishes the thermostability of the nanoparticles because of the catalytic nature of the sulfate esters[11-12].The inferior thermostability of CNCs limits their use because most polymeric composites are processed at temperatures near 200℃ or above[5]. Therefore, these two challenges should be overcome in the preparation of nanocomposites reinforced by CNCs, i.e. inferior thermal stability and poor compatibility with non-polar materials.

进入磁场的等离子可能形成销层，出现电位差，椎角处大量损失高能电子、离子，体系减弱至到停止反应。这过程以毫秒计。

威利斯·威尔（Willis Ware）是一个政府委员会的主席，该委员会负责研究计算机技术应用于人类记录方面产生的问题，这项工作最终成为1974年《联邦隐私法案》的基础。

Part of the approach used in the present work was based upon the concept that poly(ethyelene oxide)(PEO) may be able to function as a compatibilizer within composites prepared using a hydrophobic PLA matrix and the much more hydrophilic CNCs,particularly after modification of the CNCs to remove sulfate ester groups. Although PEO is soluble in water[19], it has well known unique characteristics.In principle, by changing the details of its helical conformation in solution[12], PEO can exhibit a more hydrophilic or hydrophobic character. Evidence of this dual nature becomes apparent in the ability of phenolic resins or other co-factors to destabilize solutions of PEO[20]. Conformational changes also might explain why PEO can drop out of solution when the temperature is increased[21]. Accordingly, it is proposed in this work that PEO, by changing its conformation,might be able to compatibilize the CNC-PLA composite system, on account of PEO’s combination of polar and non-polar characteristics, depending on the details of its conformation.A combination of neutralization with sodium hydroxide and physical adsorption of PEO chains was innovatively adopted in this study to improve the thermal stability and reduce the hydrophilicity of CNCs. The period of the experiment was obviously shortened. Furthermore, PEO acted as a compatibilizing agent between the PLA matrix and CNCs.

To improve the thermal stability and weaken the hydrophilicity of CNCs, different chemical and/or physical modification strategies have been attempted.Chemical modification of the nanocrystal surface with polymer chains by “graft-onto” or “graft-from”procedures is the most common approach[13-17]. The“graft-onto” method is easier to complete compared to the “graft-from” method. However, it is difficult to achieve high grafting densities and control the length of the grafting chains[18]. The reaction process of “graft-from” is difficult to control, because it needs water- and oxygen-free conditions. An even more serious concern is that metal contamination may result because of the heavy metal catalyst. Compared to the chemical method, physical modification is much easier to complete and the environmental pollution can be greatly reduced.

2 Experimental

2.1 Materials

Medical adsorbent cotton, along with the analytical reagents sulfuric acid (98%) and sodium hydroxide(NaOH), were purchased from Sinopharm Chemical Reagent Co., Ltd. (SCRC). PEO with an average molecular weight (Mw) of 5×106 g/mol was supplied by Sigma-Aldrich and used for physical adsorption.PLA with a Mw=100,000 g/mol was purchased from Shanghai Yisheng Industry Ltd.

2.2 Preparation of CNCs

TEM was performed to observe the morphology and dimensions of CNCs, as shown in Fig.1. Fig.1(a) shows TEM photos of the obtained CNC, while Fig.1(c)shows the static size distributions of the length of the nanoparticles. Rod-like nanoparticles of a length of 240～300 nm and a width of approximately 10 nm were separated from cotton using sulfuric acid hydrolysis, in agreement with the previous literature[23]. Aggregation appeared because of the hydrogen bonding between the hydroxyl groups on the surface of the nanoparticles,but many particles independently existed on account of the repelling interaction between the negative charges introduced on the surface of the CNC during sulfuric acid hydrolysis. However, the thermostability of CNCs declined with the introduction of these negatively charged sulfate ester groups. Therefore, to improve the thermostability and weaken the hydrophilicity of CNCs, the surfaces of the nanoparticles were modified to remove the sulfate ester groups and the hydrophilic hydroxyl groups.

2.3 Physical adsorption of PEO on the nanocrystals

To analyze the compatibility between CNCs and PLA matrix, dynamic contact angle measurement, SEM,transmittance measurement, and differential scanning calorimetry (DSC) were performed. As shown in Fig.4,the dynamic contact angle measurement was conducted to show the hydrophilicity of CNCs before and after modification. The initial contact angle of the CNC modified with PEO increased approximately 25°in comparison to the original CNC. Furthermore, the water drop sharply decreased on the surface of the orginal CNC in 5 s because of the swelling property of the cellulose by water. However, the contact angle of the CNC modified with PEO was maintained at approximately 60° over 40 s, which was attributed to the shielding effect of the PEO layer to the surface of the CNCs. The results showed that the hydrophilicity of the modified CNCs was reduced by the PEO layer absorbed on the surface of the nanoparticles.

2.4 Preparation of PLA nanocomposites

PLA nanocomposites reinforced with pure CNCs(p-CNC), neutralized CNCs (n-CNC) and CNCs modified with PEO (CNC/PEO) were fabricated throughout the processing. The process was conducted at 170℃ for 10 min under 40 MPa of pressure. The p-CNC, n-CNC, and CNC/PEO were utilized to reinforce the PLA matrix, which were marked as PLA/p-CNC, PLA/n-CNC, and PLA/CNC/PEO, respectively.

2.5 Characterization

Transmission electron microscopy (TEM) was performed using a JEM-2100 electron microscope at an acceleration voltage of 200 kV to characterize the morphology and distribution of CNCs before and after modification.

3.2.2 患者及照顾者缺乏协调经验 患者及照顾者在安排预约、完成专科护理以及安排交通工具前往预约地点等方面，有不同程度的困难。有研究结果显示，尽管APN给予帮助，但患者在出院后约1周仍存在交通方面问题，最后导致后期和医生预约失败[17]。此外，一些患者及家庭照顾者最终拒绝APN协调，其中部分患者和家庭照顾者向APN报告会自行做出必要的安排，但未能如期执行。部分家庭照顾者允许APN陪同患者去医院就诊，但家庭照顾者并未与医院协调好，故导致患者治疗并不理想。此外，主要家庭照顾者未能及时参加APN TOC模式，APN从其他照顾者获取信息有很大出入，也干扰了APN进一步干预。

The thermostabilities of CNCs and PLA composites were analyzed using thermogravimetric analysis (TGA)and a differential scanning calorimeter (DSC). The thermal degradation of CNCs was analyzed using the thermal analyzer TGA/SDTA851e under nitrogen flow.Approximately 5 mg of dried samples were heated from 30℃ to 600℃ at a heating rate of 10℃/min. The thermal properties of the PLA composites were measured using a Pyris 1 DSC instrument. To eliminate the thermal history, the samples were scanned from 30℃ to 200℃at a heating rate of 10℃/min, and maintained at 200℃for 5 min, then cooled to 30℃ at a rate of 20℃/min and maintained for 5 min. Finally, the samples were scanned from 30℃ to 200℃ at a heating rate of 10℃/min.

X-ray diffraction (XRD) patterns of manufactured CNCs and modified CNCs were recorded using a Bruker Siemens D8 X-ray diffractometer operated at 3 kW with Cu Kα radiation (λ=0.154 nm) in the range of 2θ=3°～60°with a step of 0.02° and a scanning speed of 4°/min.

Contact angle measurement was performed to investigate the hydrophilicity of CNCs before and after modification, and was performed at room temperature using a DSA25S-Kruss contact angle measuring device. CNCs before and after modification were compacted under 20 MPa of pressure to obtain samples with smooth surfaces. A small drop of water (2 μL) was dropped on the surface of the samples. Then,the contact angle was calculated using a sessile drop contact angle system.

As shown in Fig.5, SEM images were recorded to show the dispersion of CNCs in the PLA composites.Compared to pure PLA, some nanoscale dark dots appeared in the PLA/p-CNC nanocomposites (Fig.5(b)and Fig.5(c)), which were ascribed to the carbonization and thermal decomposition of the CNCs during the hot-compaction process because of the inferior thermostability of the p-CNCs. There were no dots presenting the PLA composites reinforced with n-CNC(Fig.5(d), Fig.5(e), and Fig.5(f)) in comparison to those of the PLA/p-CNC, which showed that the thermostability of the n-CNC improved corresponding with the results of the TGA. However, serious agglomeration was observed for these composites. The agglomeration was weakened for the PLA composites filled with CNC/PEO (Fig.5(g), Fig.5(h), and Fig.5(i)),but microphase separation appeared in PLA/5% CNC/PEO, such as the hole in the cross section as indicated by the arrow in Fig.5(i). The results of SEM imaging showed that the compatibility between the modified CNCs and PLA matrix obviously improved.

As shown in Fig.3, the weight loss below 100℃ for all samples was attributed to the removal of moisture.The p-CNC presented a stepwise degradation behavior.The degradation from 160℃ to 280℃ was attributed to the degradation of more accessible and therefore more highly sulfated amorphous regions, whereas the higher temperature degradation process was related to the breakdown of the unsulfated crystalline domains.The results correspond to the conclusions previously reported[14]. The onset of the thermal degradation of n-CNC was improved by approximately 120℃ in comparison to the p-CNC as a result of the sulfate groups being removed from their surface[6]. The CNC/PEO also presented a stepwise degradation behavior.The degradation from 300℃ to 350℃ corresponded to the breakdown of CNCs, and the higher temperature degradation was attributed to the breakdown of the PEO chains. The thermal stability of the nanoparticles modified with the PEO chains was further improved compared to those of p-CNC and n-CNC, which was attributed to the shielding effect of the PEO layer to the sulfate esters on the surface of CNCs.

The mechanical properties of the PLA composites were investigated through tensile measurement using a universal material experiment machine. The samples were thin rectangular films with dimensions of approximately 100 mm×20 mm×0.5 mm. The drawing speed was 2 mm/min.

3 Results and discussion

3.1 Morphology and structure of the nanoparticles

According to our previous research[22], CNCs were fabricated from cotton using sulfuric acid hydrolysis.In brief, approximately 10 g of cotton was mixed with 200 mL of sulfuric acid aqueous solution (64 wt%)in a three-neck flask, which was set with a stirrer,thermometer, and condenser. The reactive mixture was continuously stirred for 1 h at 45℃ for hydrolysis, then approximately 200 mL of cold water (approximately 0℃) was poured into the obtained suspension to stop the reaction. Subsequently, the suspension was centrifuged at 10,000 r/min until it had no obvious stratification and existed as a type of transparent dispersion. To remove the sulfate groups on the surface of CNCs, the mixture was neutralized using sodium hydroxide aqueous solution (1 wt%). Nanoparticles with a sulfate group content nSOsH=0.26 mol/kg were obtained.

Fig.1 TEM photographs of CNCs (a), CNC/PEO (b) and distributions of the particle length of CNCs (c) and CNC/PEO (d)

The crystalline structures of CNCs before and after modification are shown in Fig.2. The crystallinity index (Ic) of the cellulose was approximately 87.3%for p-CNC, 87.1% for n-CNC, and 86.7% for CNC/PEO. Compared to p-CNC, the Ic of n-CNC was nearly unchanged, which showed that the neutralization with the sodium hydroxide solution did not affect the crystalline structure of CNCs. A new diffraction peak at 2θ=19.2°indicated the presence of PEO chains as it was attributed to the diffraction peak of PEO. The Ic of CNC/PEO declined because of the amorphous PEO chains, which corresponded to the emergence of a new diffraction peak at 2θ=19.2°. Nevertheless, the crystalline structures of all samples were not destroyed,because the diffraction peaks at 2θ of 14.8°, 16.4°,22.5°, and 34.5° appeared in the XRD spectra of all CNC samples, representing the crystalline structure of CNC[24].

Fig.2 XRD patterns of the p-CNC, n-CNC,CNC/PEO, and PEO

3.2 Thermal stability of the CNCs

The thermal degradation of PEO and CNCs before and after modification are shown in Fig.3.

Fig.3 TGA thermograms for p-CNC, n-CNC,CNC/PEO, and PEO

本刊讯 2016年6月13日，北京市“东四——朝阳门——建国门”学区“读红色经典 铸理想信念”——纪念建党95周年诵读活动在北京史家小学高年级部礼堂举行。北京市东城区委宣传部副部长王铁峰、东城区委教育工委书记冯洪荣、东城区教委主任周玉玲、东城区政府教育督导室主任付葵、东城区委教工委副书记刘藻、东城区教育纪工委书记刘宏明、东城区教委副主任尤娜、东城区政府教育督导室副主任段勇、东城区教委副处级调研员王守仙出席活动。来自北京市东城区委宣传部理论科，东城区委教育工委、东城区教委的领导以及东朝建学区内的16个单位的党员教师400余人参加活动。

3.3 Compatibility between CNCs and PLA matrix

PEO was added into the distilled water, followed by stirring at 500 r/min for 4 days at an ambient temperature under protection against light by aluminum foil. A specified amount of neutralized CNCs was added into the PEO solution. The mixture was freezedried after stirring for 5 h to obtain the CNC powder modified with PEO. The quality fraction of the PEO on the CNC surface was controlled at 35%.

Fig.4 Dynamic contact angle for the CNC and CNC/PEO

Fig.5 SEM images of PLA nanocomposites reinforced with CNCs: (a) PLA; (b) PLA/1% p-CNC; (c) PLA/2%p-CNC; (d) PLA/1% n-CNC; (e) PLA/3% n-CNC; (f) PLA/5% n-CNC; (g) PLA/1% CNC/PEO; (h) PLA/3%CNC/PEO and (i) PLA/5% CNC/PEO

The homogeneity of the PLA composites was observed using scanning electron microscopy(SEM) with a su1510 device(Hitachi Zosen Corporation) at 30 kV.

计算表明，边坡上的潜在危险滑面主要分布在居民区305省道的公路路基部位，且其潜在危险滑面在暴雨工况下安全系数较低，公路路基挡墙部位不满足建筑边坡工程Ⅱ级边坡最小抗滑稳定安全系数，与挡墙部位出现拉裂外倾、路面塌陷及前缘坡脚开裂情况相符，说明边坡前缘居民区内局部土质边坡处于临界稳定或欠稳定状态。

Transmittance measurements were performed to elaborate upon the dispersion of the CNCs in the PLA composites. As shown in Fig.6, the transmittance of the p-PLA matrix was near 75%, and sharply decreased with the addition of cellulose nanoparticles, which was ascribed to the uneven dispersion and degradation of the CNCs during hotpressing. The transmittance of PLA/CNC/PEO increased compared to that of PLA/n-CNC.To visually present the transparency of the composites,photographs of different types of PLA composites are shown in Fig.7. It is obvious that a large number of dark dots appeared in the PLA/p-CNC composites,which was attributed to the reduction in thermostability caused by the sulfate groups on the surface of the CNCs. Although dark dots were not observed in the composites filled with n-CNCs, the color of the composites gradually darkened with the increase in n-CNC content, which was ascribed to the aggregation of nanoparticles in the polymeric matrix. Compared to these composites, the transparency of the PLA/CNC/PEO composites obviously improved, which was attributed to the improvement in thermostability and the decrease in hydrophilicity of the CNCs modified with PEO chains.

Fig.6 UV-vis transmittance spectra for PLA composites filled with n-CNC and CNC/PEO

Fig.7 Images of PLA, PLA/p-CNC, PLA/n-CNC,and PLA/CNC/PEO

As shown in the DSC curves in Fig.8, the glass transition temperature (Tg) of the composites nearly remained stable with the addition of the nanoparticles,which was attributed to the favorable compatibility of the nanofillers with the PLA matrix. The cold crystallization temperature (Tcc) of PLA/CNC/PEO slightly decreased, which indicated faster crystallization induced by the CNCs which acted as nucleating agents for the PLA[25]. CNCs modified with PEO chains, as nucleating agents, helped lower the free energy barrier,thus favoring faster nucleation[26-27].

Fig.8 DSC curves for PLA composites reinforced with n-CNC and CNC/PEO

The breaking strength and elongation of the films were tested to characterize the mechanical properties of the nanocomposites. As shown in Fig.9, the elongation of the pure PLA was 1.93%, indicating the obvious brittleness of the PLA matrix. The breaking strength of the PLA composites increased because of the intrinsic rigidness of the nanofillers. The elongation of the PLA composites decreased with the addition of unmodified CNCs because of the poor compatibility between the CNC controls and the PLA matrix. The elongation at the break of the PLA composites reinforced with CNCs modified with PEO chains improved in comparison to the unmodified CNCs, which was attributed to the cohesion at the phase interface produced by the addition of the modified CNCs. In contrast, the mechanical properties of the composites decreased with the addition of n-CNC because of the CNC aggregation,which generated points of stress concentration, leading to weakness.

Fig.9 Mechanical properties of PLA/n-CNC nanocomposites (a)and PLA/CNC/PEO nanocomposites (b)

4 Conclusions

Neutralization with alkaline and physical adsorption with PEO chains were adopted to improve the thermal stability and reduce the hydrophilicity of CNCs.The results of the characterization indicated that the degradation temperature of the modified CNCs increased by approximately 120℃ and the contact angle of the modified CNCs increased by approximately 25°in comparison to those of the original CNCs, which was attributed to the removal of sulfate ester groups and the PEO layers covering the surface of the CNCs.The mechanical properties of the PLA composites improved because of the reinforcement of the modified CNCs, which showed that the compatibility between the PLA matrix and the modified CNCs improved.However, the effects on the mechanical properties were not statistically significant, which may be ascribed to the slippage of the PEO chains on the surface of the CNCs because of the weak interaction between the PEO chains and CNC macromolecules. Therefore, it is important to research further modification methods to improve upon the effects achieved in the present work.These studies are in progress and will be presented in future publications.

Acknowledgments

The authors are grateful to the National Natural Science Foundation of China (grant Nos. 31570578 and 31270632), the Fundamental Research Funds for the Central Universities (grant No. JUSRP51622A) and the State Key Laboratory of Pulp and Paper Engineering(grant No. 201809).

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