Here we investigate the relationship between thermomechanical properties and chemical structure of well-characterized lignin-based epoxy resins. For this purpose, technical lignins from eucalyptus and spruce, obtained from the Kraft process, were used. The choice of lignins was based on the expected differences in molecular structure. The lignins were then refined by solvent fractionation, and three fractions with comparable molecular weights were selected to reduce effects of molar mass on the properties of the final thermoset resins. Consequently, any differences in thermomechanical properties are expected to correlate with molecular structure differences between the lignins. Oxirane moieties were selectively introduced to the refined fractions, and the resulting lignin epoxides were subsequently cross-linked with two commercially available polyether diamines (Mn = 2000 and 400) to obtain lignin-based epoxy resins. Molecular-scale characterization of the refined lignins and their derivatives were performed by 31P NMR, 2D-NMR, and DSC methods to obtain the detailed chemical structure of original and derivatized lignins. The thermosets were studied by DSC, DMA, and tensile tests and demonstrated diverse thermomechanical properties attributed to structural components in lignin and selected amine cross-linker. An epoxy resin with a lignin content of 66% showed a Tg of 79 °C from DMA, Young's modulus of 1.7 GPa, tensile strength of 66 MPa, and strain to failure of 8%. The effect of molecular lignin structure on thermomechanical properties was analyzed, finding significant differences between the rigid guaiacyl units in spruce lignin compared with sinapyl units in eucalyptus lignin. The methodology points toward rational design of molecularly tailored lignin-based thermosets.

中文翻译：

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木质素基环氧树脂：揭示结构与材料性能之间的关系。

在这里，我们研究了良好表征的木质素基环氧树脂的热机械性能与化学结构之间的关系。为此，使用了从卡夫工艺获得的来自桉树和云杉的工业木质素。木质素的选择基于预期的分子结构差异。然后通过溶剂分馏精制木质素，选择具有可比较分子量的三个馏分以减少摩尔质量对最终热固性树脂性能的影响。因此，预期热机械性质的任何差异都与木质素之间的分子结构差异相关。将环氧乙烷部分选择性地引入精制馏分中，然后将所得的木质素环氧化物与两种市售的聚醚二胺（Mn = 2000和400）交联，得到木质素基环氧树脂。通过31P NMR，2D-NMR和DSC方法对精制木质素及其衍生物进行分子规模表征，以获得原始木质素和衍生木质素的详细化学结构。通过DSC，DMA和拉伸测试研究了热固性塑料，并证明了木质素和选定的胺交联剂中的结构组分具有多种多样的热机械性能。木质素含量为66％的环氧树脂从DMA测得的Tg为79°C，杨氏模量为1.7 GPa，拉伸强度为66 MPa，断裂应变为8％。分析了分子木质素结构对热机械性能的影响，发现在云杉木质素中刚性愈创木瓜基单元与桉树木质素中芥子基单元之间存在显着差异。该方法论指向合理设计基于分子的木质素基热固性材料。