【文献解读】ACS Sustainable Chem. Eng. 3D打印可降解纳米纤维素复合材料

利用生物质可再生能源开发易于降解的高分子材料对于社会的可持续发展和环境保护都至关重要。聚羟基脂肪酸是最近兴起的一类生物可降解的高分子聚酯材料。这其中，聚（3-羟基丁酸-3-羟基己酸）PHBH由于其良好的热性质和可被细菌合成的特性，被广泛用于热固性高分子材料的共聚物。近日，来自意大利帕尔马大学的研究人员便利用PHBH的良好性质，将其与纳米纤维素进行混合，并通过熔融沉积成型（FDM）3D打印技术将其制备成不同的高分子材料。

Figure 1. Schematic of the acetylation process of CNC particles (a), of the chemical structures of reagents: cellulose (b); sulfuric acid (c) and acetic anhydride (d) and of the acetylated cellulose after the functionalization (e), with the representation of acetyl groups bonded to the cellulose structure, replacing the hydroxyl groups. TEM images show unmodified CNC (f) and acetylated CNC after 8 hours of acetylation (g), scale bars 150 nm.

首先，作者对纳米纤维素进行了表面修饰。如图1所示，通过乙酰化作用，作者在纳米纤维素表面引入了乙酰基。考虑到PHBH内包含大量不饱和的羰基，这些纳米纤维素表面引入的乙酰基可以与其很好地形成偶极-偶极相互作用，增强高分子材料的稳定性。

Figure 2. Chemical characterization by FT-IR analysis of the CNCs (a) and the absorbance ratio dependence over reaction time, as an indication of the degree of acetylation (b). Thermogravimetric analysis of pristine CNC and acetylated CNC at different reaction times (CNC_AC_X), where X corresponds to the reaction times expressed in hours (c). Zoom-in of the diagram for the evaluation of the effect of the acetylation process on the thermal stability of the powders, evaluated at 5 wt% of mass loss (d).

图2是纳米纤维素进行修饰后材料的红外表征数据。图2a可以看出，修饰后的材料中出现了乙酰基振动的三个特征峰（ 1745 cm-1[羰基拉伸C=O]， 1370 cm-1 [甲基在-O(C=O)-CH3中的平面弯曲]和 1230 cm-1[乙酰基C-O伸长]）。图2b表示随着反应时间的增加，C=O的比例逐渐增高并在8小时后保持稳定，这表明反应在8小时后完成。为了研究纳米纤维素表面羟基取代程度与其热性质的关系，作者还分别对不同反应时间后的纳米纤维素进行了热重分析。如图2c和d所示，随着反应时间的增加，T_5wt%的温度逐渐升高。乙酰基的引入可以不同程度的提高纳米纤维素的热稳定性。

Figure 3. Rheological properties of compounded materials with different acetylated CNC content (CNC_AC_8_XX), where XX represents the reinforcing agent content expressed as weight percent (5 wt% and 10 wt%). Shear viscosity curves of neat PHBH and PHBH+CNC_AC_8 composites at different CNC content are plotted. The fit of the viscosity curves was obtained using the Bird-Carreau-Yasuda (BCY) model.

为了确保3D打印过程中，修饰后的纳米纤维素材料的性能，作者还对其进行了流变性能分析。图3选取了反应8个小时的5%和10%乙酰基修饰的纳米纤维素的流变性能。结果表明，随着CNC含量的增加，由于纳米晶体与基体之间以及纳米晶体自身之间的相互作用，抑制了链的运动，阻碍了链的弛豫，高分子材料整体的粘度会增加。但是制得庆幸的是在实验期间进行的测量表明所有合成的化合物都是可打印的，因为所有化合物的粘度值都在FDM 3D打印可接受的范围内。

Figure 4. Thermal properties of PHBH and PHBH+CNC_AC_8 composites as a function of CNC content: 5 wt%, 10 wt%, 15 wt% and 20 wt%. Thermogravimetric analysis in air atmosphere (a). Thermal stability of composites evaluated at the temperature of 5 % of mass loss (T5wt%), which increases with the compounding of acetylated CNC, independently from its content (b).

Figure 5. Thermal behavior of the storage modulus evaluated at different CNC contents: 5 wt%, 10 wt%, 15 wt% and 20 wt% (a). Storage modulus values as a function of CNC content evaluated at two particular temperature conditions: at room temperature T = 25 °C (b) and at high temperature, T = 80 °C (c). Error bars show the standard deviation for three measurements.

随后，作者制备了一系列PHBH+CNC的高分子材料，并对其热力学性质，力学性质进行了测量。其结果如图4和图5所示，这两项测试均表明这种复合高分子材料具有很好的热性质和力学性质，为接下来的3D打印奠定了基础。

Figure 6. FDM 3D printed nanocellulose composites. Representative top view of simply shaped scaffolds printed by a filament of neat PHBH (a) and PHBH-acetylated CNC (10 wt%) composite (PHBH+CNC_AC_8_10) (b), with an alternation of 0-90° for directions of layers. Complex shaped object as a finger cast printed by a filament of PHBH composite with acetylated CNC at 10 wt% (PHBH+CNC_AC_8_10) (c). Example of use of a personalized medical device in case of finger dislocation (d).

如图6所示，作者利用FDM 3D打印方法，制备了多种不同形状的高分子材料。这其中还包括170层的高分子材料打印的可穿戴手指铸件，其上半部分轴线倾斜可达20度，显示了这种材料在医疗器械领域的广阔应用空间。

Figure 7. Disintegration of the PHBH bio-composites and of a traditional petroleum-based low-density polyethylene over time and under composting conditions in a laboratory-scale test. Plot of the degree of disintegration as a function of the time and CNC content (a). Visual appearance of the tested composites films (PHBH+CNC_AC) at starting moment, after 35 days and 78 days during the composting process (b). ESEM micrographs of the LDPE samples at different degradation times: starting moment (c), after 35 days (d) and 78 days (e). ESEM micrographs of the CNC_AC_8_15 samples, chosen as representative for the behavior of the bio-based composites, at different degradation times: starting moment (f), after 35 days (g) and 78 days (h).

这种生物基高分子材料相对于传统的石油基聚乙烯材料的最大优势就是其可降解性。如图7所示，作者最后对比了所合成的PHBH生物高分子材料与聚乙烯材料的降解效率。聚乙烯材料在放置78天之后仍然稳定存在。而合成的生物基高分子材料随着纳米纤维素含量的增加其降解效率也逐渐提高。含有20%纳米纤维素的高分子材料在78天之后便完全降解。

作者将纳米纤维素与PHBH相结合，通过FDM 3D打印技术制备了多种性能优越的高分子材料。这些高分子材料具有良好的热性质和力学性质，同时又能够在自然界中完全降解。展示了巨大的应用潜力。

https://pubs.acs.org/doi/10.1021/acssuschemeng.0c03385

ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/

acssuschemeng.0c03385 • Publication Date (Web): 16 Jun 2020

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