Zhang Siteng, Bension Yishayah, Shimizu Michael, Ge Ting
Department of Chemistry and Biochemistry, University of South Carolina, Columbia, SC 29208, USA.
Polymers (Basel). 2024 Dec 19;16(24):3552. doi: 10.3390/polym16243552.
The mechanical properties of lignin, an aromatic heteropolymer constituting 20-30% plant biomass, are important to the fabrication and processing of lignin-based sustainable polymeric materials. In this study, atomistic simulations are performed to provide microscopic insights into the mechanics of lignin. Representative samples of miscanthus, spruce, and birch lignin are studied. At room temperature below the glass transition temperature, the stress-strain curves for uniaxial compression and tensile loading are calculated and analyzed. The results show that lignin possesses rigidity with a Young's modulus in the order of GPa and exhibits strain hardening under strong compression. Meanwhile, lignin is brittle and fails through the microscopic mechanism of cavitation and chain pullout under local tensile loading. In addition to the three common lignin samples, minimalist model systems of monodisperse linear chains consisting of only guaiacyl units and β-O-4 linkages are simulated. Systematic variation of the model lignin chain length allows a focused examination of the molecular weight effects. The results show that the molecular weight does not affect the Young's modulus much, but higher molecular weight results in stronger strain hardening under compression. In the range of molecular weight studied, the lignin chains are not long enough to arrest the catastrophic chain pullout, explaining the brittleness of real lignin samples. This work demonstrates that the recently modified CHARMM force fields and the accompanying structural information of real lignin samples properly capture the mechanics of lignin, offering an in silico microscope to explore the atomistic details necessary for the valorizaiton of lignin.
木质素是一种构成20%-30%植物生物质的芳香族杂聚物,其机械性能对于木质素基可持续聚合物材料的制造和加工至关重要。在本研究中,进行了原子模拟以提供对木质素力学的微观见解。研究了芒草、云杉和桦木木素的代表性样品。在低于玻璃化转变温度的室温下,计算并分析了单轴压缩和拉伸加载的应力-应变曲线。结果表明,木质素具有刚性,杨氏模量在GPa量级,并且在强压缩下表现出应变硬化。同时,木质素是脆性的,在局部拉伸加载下通过空化和链拔出的微观机制失效。除了这三种常见的木质素样品外,还模拟了仅由愈创木基单元和β-O-4键组成的单分散线性链的极简模型系统。模型木质素链长度的系统变化允许聚焦研究分子量的影响。结果表明,分子量对杨氏模量影响不大,但较高的分子量在压缩下导致更强的应变硬化。在所研究的分子量范围内,木质素链不够长,无法阻止灾难性的链拔出,这解释了真实木质素样品的脆性。这项工作表明,最近修改的CHARMM力场以及真实木质素样品的伴随结构信息恰当地捕捉了木质素的力学性能,提供了一个计算机显微镜来探索木质素增值所需的原子细节。