Self-assembly of montmorillonite-cellulose nanocrystal for enhancing mechanical properties of poly(vinyl alcohol) films
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摘要: 自然生物体由于多级有序的结构、复杂的有机/无机界面相互作用使其具有优异的力学性能，而合成材料仅通过组分和结构的模拟通常无法实现强度、韧性、断裂伸长率的同步提升。基于此，本文通过静电自组装结合溶剂蒸发获得了蒙脱土-纤维素纳米晶/聚乙烯醇(MMT-PCNC/PVA)层状结构复合膜，利用TEM跟踪组装体的形成过程，结合FTIR图谱结果显示复合膜中存在静电、氢键等多重弱相互作用。系统研究了组装体中MMT与聚乙烯亚胺(PEI)修饰的PCNC之间的质量比例对PVA膜力学性能的影响。结果表明，相较纯的PVA膜，不同MMT、PCNC质量比例的组装体对PVA膜的力学性能均有提升作用。其中二者质量比例为1∶1、1∶2时最为明显，1MMT-1PCNC/PVA复合膜的拉伸强度提升了196%；1MMT-2PCNC/PVA复合膜的断裂伸长率、韧性分别提升了175%和900%。这些均得益于复合膜内部的弱相互作用，给应力的传递提供了有效的途径，同时引起裂纹偏转，达到消耗能量的目的，使其在拉伸强度提高的同时韧性也得到提升。Abstract: Natural organisms exhibit excellent mechanical properties due to hierarchical ordered structure and complex organic/inorganic interface interaction. However, the synthetic materials have proven to be difficult to achieve the synchronous improvement of strength, toughness and elongation at break, by merely mimicking their component and structural. Herein, the hybrid films of montmorillonite-cellulose nanocrystal/poly(vinyl alcohol) (MMT-PCNC/PVA) were prepared by electrostatic self-assembly and solvent evaporation. TEM was used to track the formation process. The results of FTIR spectra show that there are multiple weak interactions such as electrostatic and hydrogen bonds in the composite films. The effect of the proportion of MMT and polyethyleneimine (PEI) modified PCNC on the mechanical properties of the composite films was studied. As a result, it is most obvious when the mass ratio is 1∶1 and 1∶2 of MMT and PCNC can drastically enhance the tensile strength (196% for 1MMT-1PCNC/PVA), elongation at break (175% for 1MMT-2PCNC/PVA), and toughness (900% for 1MMT-2PCNC/PVA), which are superior to the pure PVA film. The multiple types of interactions between nanoscale building blocks improve the efficiency of load transfer and cause crack deflection, which result in a highly energy consumption and excellent mechanical properties (such as strength, toughness).
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图 1 显示悬浮液稳定性的光学照片: (a) 晶态纳米纤维素(CNC)；(b) PCNC；(c) MMT；(d) 相应悬浮液的Zeta电势值
Figure 1. Photographs showing the stability of suspensions: (a) Cellulose nanocrystals (CNC); (b) PCNC; (c) MMT; (d) Zeta potentials of corresponding suspensions
图 2 CNC修饰前后的形貌表征: CNC (a)和PCNC (b)的TEM图像
Figure 2. Morphology before and after CNC modification: TEM images of pristine CNC (a) and PCNC (b)
图 3 CNC (a)和PCNC (b)的长度分布图; CNC (c)和PCNC (d)的直径分布图
Figure 3. Diameter distributions of CNC (a) and PCNC (b); Length distributions of CNC (c) and PCNC (d)
图 4 MMT-PCNC组装体的形成过程: MMT (a)、PCNC (b)和1MMT-2PCNC组装体 (c)的TEM图像
Figure 4. Tracking the formation of MMT-PCNC: TEM images of MMT (a), PCNC (b) and supramolecular ensemble of 1MMT-2PCNC (c)
图 5 不同MMT-PCNC/PVA复合膜的SEM图像: (a) 纯PVA薄膜；(b) 1MMT-1PCNC/PVA；(c) 1MMT-2PCNC/PVA；(d) 1MMT-4PCNC/PVA
Figure 5. SEM images of MMT-PCNC/PVA: (a) Pure PVA film; (b) 1MMT-1PCNC/PVA; (c) 1MMT-2PCNC/PVA; (d) 1MMT-4PCNC/PVA
图 6 不同MMT和PCNC质量比的组装体复合膜力学性能
Figure 6. Mechanical properties of hybrid films with varying weight ratio of MMT and PCNC
图 7 通过纯PVA、CNC以及1MMT-2PCNC/PVA复合膜的FTIR图谱显示1MMT-2PCNC和PVA之间氢键的形成
Figure 7. FTIR spectra of CNC, PVA and hybrid films of 1MMT-2PCNC/PVA showing multiple hydrogen bonds between 1MMT-2PCNC and PVA
图 8 1MMT-2PCNC/PVA复合膜断裂形态的SEM图像和断裂示意图：(a) 断裂过程中裂纹扩展的SEM图像(白色折线表示裂纹偏转)；(b) 断裂模型示意图
Figure 8. SEM image of fracture morphologies of 1MMT-2PCNC/PVA hybrid films and the proposed fracture model: (a) SEM image of crack propagation occurred during fracture (White line indicate crack propagation); (b) Schematic illustration of the fracture model
表 1 蒙脱土-纤维素纳米晶(MMT-PCNC)组装体的组成
Table 1. Compositions of the montmorillonite-cellulose nanocrystal (MMT-PCNC) assemblies
Sample 1MMT-1PCNC 1MMT-2PCNC 1MMT-4PCNC Composition Mass fraction Mass ratio Mass ratio Mass ratio MMT 0.2% 1 1 1 PCNC 1.0% 1 2 4 
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表 2 MMT-PCNC/PVA复合膜的力学性能
Table 2. Mechanical properties of the MMT-PCNC/PVA hybrid films
Hybrid films of filler/PVA Elongation-at-break
/%Tensile strength
/MPaYoung’s modulus
/MPaToughness
/(MJ·m−3)PVA 28±3 26±3 101±13 3±1 1MMT-1PCNC/PVA 50±4 77±7 97±6 27±6 1MMT-2PCNC/PVA 77±4 63±8 100±3 30±9 1MMT-4PCNC/PVA 42±2 54±9 92±7 13±2
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