Experiment and molecular dynamics simulation of cellulose nanocrystals cement-based composites
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摘要: 探究纤维素纳米晶须(CNC)及其包覆聚乙烯(PE)纤维对砂浆性能的影响。采用XRD分析水化产物、核磁共振技术(NMR)测量试块孔隙率、SEM表征纤维水泥基复合材料界面、EDS测量水化硅酸钙(C-S-H)的钙硅比，使用分子动力学模拟(MD)分析两种官能化CNC和水泥基体间的吸附能、动力学特性和回转半径。 结果表明：CNC促进了水泥水化且增强了水化产物致密性，限制了基体的孔隙发育，提升了砂浆力学性能；不同官能团与水泥水化物之间的吸附存在差别，影响CNC的增强效果；CNC涂层可以增强纤维与水泥基体的界面粘结，提高复合材料协同工作性能。
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关键词:
- 水泥基复合材料 /
- 分子动力学模拟 /
- 官能化纤维素纳米晶须 /
- 吸附界面 /
- 纤维
Abstract: To explore the effects of cellulose nanocrystals (CNC) and coated CNC polyethylene (PE) fibers on the performance of mortar, XRD was employed to analyze the hydration products, and nuclear magnetic resonance (NMR) technique was taken to measure porosity. Calcium-silicon ratio of calcium silicate hydrate (C-S-H) was measured by EDS and PE/C-S-H interface was characterized by SEM. The adsorption energies, kinetic properties and gyration of radius between the two CNCs and C-S-H were analyzed by molecular dynamics simulations (MD). The needle-like CNC promotes hydration and affects the compactness of the hydration products, curbs the development of pores and improves the mechanical properties of the mortar. There are differences in the adsorption between different functional groups and cement matrix, which affects the reinforcement effect of cellulose nanocrystals. CNC coating improves the bonding between the fibers and the cement matrix, improving the synergistic performance of composite materials. -
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图 1 纤维素纳米晶须(CNC)、纤维材料及CNC包覆纤维的制备：(a) CNC凝胶；(b) 聚乙烯(PE)；(c) 聚乙烯醇(PVA)；(d) CNC纤维
Figure 1. Cellulose nanocrystal (CNC), fibers and fabrication of the CNC coated fiber: (a) CNC gel; (b) Polyethylene (PE); (c) Polyvinyl alcohol (PVA); (d) CNC fiber
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图 3 泥浆的XRD图谱 (a)、溶解峰 (b) 和水化热 (c) 结果；CNC-C (d)、CNC-H (e)、PE (f)、PVA (g) 和CNC纤维 (h) 的砂浆抗折强度
Figure 3. XRD patterns (a), dissolution peak (b) and hydration heat (c) of mortar; Flexural strength of CNC-C (d), CNC-H (e), PE (f), PVA (g) and CNC fiber (h)
CH—Calcium hydroxide; C3S—Tricalcium silicate; C2S—Dicalcium silicate
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图 4 CNC-C (a)、CNC-H (b) 砂浆的3天抗压强度；CNC-C (a)、CNC-H (b)、PE (c)、PVA (d) 和CNC纤维 (e) 的28天抗压强度及不同掺量的CNC-C和CNC-H砂浆 T2图谱 (f)
Figure 4. Compressive strength of CNC-C (a), CNC-H (b) mortar of 3 days; Compressive strength of CNC-C (a), CNC-H (b), PE (c), PVA (d) and CNC fiber (e) mortar of 28 days and T2 distribution spectra of different dosages CNC-C and CNC-H mortar (f)
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图 5 不同掺量CNC-C和CNC-H砂浆的核磁成像
Figure 5. NMR imaging of different dosages CNC-C and CNC-H mortar
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图 6 CNC砂浆的控制组 ((a), (c)) 和CNC-C ((b), (d)) 的SEM图像；控制组 (e)、CNC-H (f)、CNC-C (g) 的EDS分析
Figure 6. SEM images of control ((a), (c)), CNC-C ((b), (d)); EDS analysis of control (e), CNC-H (f), CNC-C (g)
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图 7 纤维及砂浆的SEM图像：(a) PE；(b) CNC 纤维；(c) PE砂浆；(d) CNC纤维砂浆
Figure 7. SEM images of fibers and mortar: (a) PE; (b) CNC fiber; (c) PE mortar; (d) CNC fiber mortar
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图 8 CNC分子结构图：(a) CNC-C；(b) CNC-H
Figure 8. Molecular structure of CNC: (a) CNC-C; (b) CNC-H
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图 9 CNC/C-S-H计算单元：(a) CNC-C；(b) CNC-H
Figure 9. Calculation unit of CNC/C-S-H: (a) CNC-C; (b) CNC-H
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图 10 CNC-C/C-S-H (a) 和CNC-H/C-S-H (b) 界面原子的径向分布函数(RDF)；(c) CNC-C和CNC-H的时间相关函数(TCF)曲线
Figure 10. Radial of function(RDF) of the interface atoms of CNC-C (a) and CNC-H (b); (c) Time correlation function (TCF) of CNC-C and CNC-H results of CNC/C-S-H interface
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图 11 均方位移MSD计算结果：(a) 界面原子；(b) 分子
Figure 11. Mean square displacement MSD results: (a) Interface atoms; (b) Molecules
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图 12 CNC分子回转半径Rg (a) 和浓度剖面 (b)
Figure 12. Turning radius Rg (a) and concentration profile (b) results of CNC
表 1 PE及PVA纤维的物理及力学性能指标
Table 1 Physical and mechanical properties of PE and PVA fibers
Fiber Length/
mmDiameter/
μmElastic modulus/
GPaTensile strength/
MPaDensity/
(kg·m−3)PE 9 24 120 3000 970 PVA 9 20 40 1600 1300 
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表 2 普通硅酸盐水泥化学成分(P·O 42.5)
Table 2 Composition of ordinary portland cement (P·O 42.5)
Composition SiO2 Al2O3 Fe2O3 CaO MgO Loss Proportion
/%21.44 5.95 3.05 61.42 3.97 4.35
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表 3 不同实验组砂浆质量成分
Table 3 Mass composition of different experimental groups
g Notation Sand Cement Water Plain fiber CNC-C CNC-H CNC-C coated
polyethyleneCNC-H coated
polyethyleneA 1350 450 247.5 0 0 0 0 0 C-0.01 1350 450 247.5 0 0.045 0 0 0 C-0.05 1350 450 247.5 0 0.225 0 0 0 C-0.10 1350 450 247.5 0 0.450 0 0 0 C-0.15 1350 450 247.5 0 0.675 0 0 0 C-0.20 1350 450 247.5 0 0.900 0 0 0 H-0.01 1350 450 247.5 0 0 0.045 0 0 H-0.05 1350 450 247.5 0 0 0.225 0 0 H-0.10 1350 450 247.5 0 0 0.450 0 0 H-0.15 1350 450 247.5 0 0 0.675 0 0 H-0.20 1350 450 247.5 0 0 0.900 0 0 PE-0.10 1350 450 247.5 0.745 0 0 0 0 PE-0.20 1350 450 247.5 1.490 0 0 0 0 PE-0.30 1350 450 247.5 2.235 0 0 0 0 PE-0.40 1350 450 247.5 2.980 0 0 0 0 PE-0.50 1350 450 247.5 3.725 0 0 0 0 PVA-0.10 1350 450 247.5 0.998 0 0 0 0 PVA-0.20 1350 450 247.5 1.997 0 0 0 0 PVA-0.30 1350 450 247.5 2.995 0 0 0 0 PVA-0.40 1350 450 247.5 3.994 0 0 0 0 PVA-0.50 1350 450 247.5 4.992 0 0 0 0 PEC-0.30 1350 450 247.5 0 0 0 2.235 0 PEH-0.30 1350 450 247.5 0 0 0 0 2.235 Notes: CNC-C—Carboxylated CNC; CNC-H—Hydroxylated CNC; A—Control mortar sample; C—CNC-C mortar sample; H—CNC-H mortar sample; PEC—CNC-C coating fiber mortar sample; PEH—CNC-H coating fiber sample. Additionally, the number means the dosage of enhancement material. Such as C-0.01—Ratio of CNC-C is 0.01%; PE-0.1—Addition of PE fiber is 0.1%.
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目的
纤维素纳米晶须(CNC)应用在水泥基复合材料时拥有“填充效应”、“成核效应”和“传递荷载”等特性，可以促进水泥水化进程，提升样品力学性能。这些特性和二者的界面吸附性能相关。定量的分析纳米尺寸的界面特性十分困难，本文利用分子动力学模拟(MD)技术，从原子尺度计算CNC/C-S-H吸附界面的参数，探索CNC表面官能团对吸附界面的影响。
方法通过XRD和水化热测试来分析两种CNC对水泥水化进程的影响，使用SEM观察CNC的形貌，并通过EDS(能谱)分析钙硅比来表征水产物致密度，最后通过核磁共振技术来测量样品的孔隙率。此外，将两种CNC作为偶联剂，改性聚乙烯纤维表面极性和粗糙度，通过微观形貌和力学性能来分析两种CNC对纤维表面的改性程度。利用分子动力学模拟计算两种CNC/C-S-H吸附界面处的吸附能、界面氢键生成情况、氢键寿命、界面原子运动情况、CNC链吸附形态等吸附参数。将纳米尺度的界面吸附特性和促进水化及增强样品力学性能联系起来，建立了从宏观性能到微观形貌再到纳观吸附的多维度分析机理。
结果1. CNC会降低砂浆流动度，由XRD结果可知，CNC促进了未水化的CS和CS熟料向CaOH的转化，此外，水化热测试表明，CNC加速了水泥的溶解，缩短水泥的凝结时间，增加了水化放热量，促进了水泥水化。且羧基化的纤维素纳米晶须(CNC-C)对水化的促进效果强于羟基化纤维素纳米晶须(CNC-H)。2. CNC可以提高砂浆力学性能，CNC-C样品的抗折强度和抗压强度最高可提升27.27%和22.06%，CNC-H砂浆的抗折和抗压强度分别可提升20.45和19.22%。3. EDS结果表明，CNC降低了水化产物的钙硅比，细化了水化晶体的尺寸，增强了基体的致密性。此外，核磁共振T2图谱和核磁成像表明，CNC可以有效填充孔隙，降低基体孔隙率，且CNC-C的填充效果强于CNC-H。但过量的CNC会形成团聚体，引入毛细孔和空气夹层，增加样品孔隙率，降低CNC的增强效果。4. CNC涂层可以增强聚乙烯纤维表面粗糙度，改性纤维表面极性，给纤维表面引入活性基团，形成纤维-CNC涂层-水泥基体结构，改善纤维和水泥基体吸附界面的粘结情况，增强样品的力学性能，CNC涂层纤维样品的抗折强度，强于单独使用CNC和纤维的样品组。5.分子动力学计算得CNC-C/C-S-H界面吸附能是CNC-H/C-S-H的1.75倍。CNC-C/C-S-H界面的氧原子和氢原子间的空间相关性强于CNC-H/C-S-H界面，CNC-C/C-S-H界面更容易形成数量更多且更稳定的氢键。CNC-C/C-S-H界面原子受到界面的限制更大，界面原子的运动微弱，界面结构更稳定, CNC-C链在吸附过程中可以平直的吸附在C-S-H表面，而CNC-H链和C-S-H的吸附较弱，会发生扭曲变形，减少吸附界面接触面积，降低界面吸附强度。
结论两种CNC和C-S-H间的性能存在较大差异，界面吸附性能决定CNC传输水分子的能力，影响水化进程。界面吸附强度还会影响CNC可以帮水泥基体转移的最大荷载，决定了样品的力学强度。有效官能化纳米表面是提高纳米材料增强水泥基复合材料效果的关键。此外，纳米材料还能改性其他水泥基材料和水泥基体的粘结界面，不仅可以缩减工程成本，也能提升样品性能。
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纤维素纳米晶须(CNC)是由C6H10O5单体通过β糖苷键结合起来的高分子纳米材料，具有较大的比表面积和反应活性。它会促进水泥水化，提高水泥基材料的力学和耐久性能。聚乙烯纤维(PE)高强、高模、耐冲击、耐腐蚀、耐辐射等优点，在相对较低的体积分数下可以有效控制混凝土的塑性收缩和开裂。但PE纤维与水泥基体间的粘结较差，严重阻碍了它的实际应用。
本文利用纤维素纳米晶须作为偶联剂，将其作为涂层吸附在PE纤维表面，增强了纤维表面粗糙度且改性纤维表面极性，改善了纤维/基体界面处的键合。与直接使用纤维素纳米晶须相比，用作偶联剂时，纤维素纳米晶须的用量大大减少，且改性PE纤维对砂浆抗折强度的提升效果强于单独使用纤维和纤维素纳米晶须时的效果，这不仅显著减低了生产成本，还提升了PE纤维的应用价值。本文首先通过宏观力学性能及微观形貌表征纤维和水泥基材料间的粘结情况，然后通过分子动力学模拟分析了两种官能化纤维素纳米晶须和水化硅酸钙的界面吸附情况，建立了从宏观到微观最后到纳观的多维度、多尺度分析体系。
纤维素纳米晶须涂层PE纤维示意图
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