Engström J, Jimenez A M, Malmström E
KTH Royal Institute of Technology, School of Engineering Sciences in Chemistry, Biotechnology and Health, Department of Fibre and Polymer Technology, Sweden.
Department of Chemical Engineering, Columbia University, 10027 New York, NY, USA.
Nanoscale. 2020 Mar 19;12(11):6462-6471. doi: 10.1039/c9nr10964a.
This study aims to describe and evaluate the mechanism for increased strain-at-break of composites made of cellulose nanofibrils (CNFs) reinforced with nanoscopic latex particles (<200 nm) stabilized by a cationic polyelectrolyte as corona. The applied latex nanoparticles (NPs), synthesized by polymerization-induced self-assembly (PISA), are composed of a neutral core polymer, either poly(butyl methacrylate) (PBMA) or poly(methyl methacrylate) (PMMA). At room temperature, PBMA is close to its glass transition (Tg), while PMMA is below its Tg. Nanocomposites with 75 wt% CNFs and 25 wt% NPs were analyzed using in situ small angle X-ray scattering during tensile testing, monitoring the structural evolution of the NPs under strain. The scattering of the spherical PMMA NPs, which do not coalesce like the PBMA NPs, shows changes to the organization of the NPs in the CNF-network. The observations are corroborated by cross-sectional transmission and scanning electron microscopy. No distinct change from spherical to ellipsoidal shape is evidenced for the PMMA NP cores during tensile strain. Changes in anisotropic scattering produced by the three-dimensional NP structure appear to be very different between nanocomposites loaded with PMMA or PBMA NPs, contrasting commonly described two-dimensional CNF networks. The discrete PMMA NPs can reorganize within the CNF-NP double network under strain, resulting in maintained strength and increased strain-at-break. Increasing the humidity (20, 50 and 80% RH) during in situ measurements further emphasizes this effect in the PMMA composite, relative to the PBMA composite and CNF reference films. The onset of deformation occurs at strain values beyond the fracture of the more brittle films, indicating the effect of secondary nanoscale interaction available only for the PMMA composite, extending the plastic deformation and increasing the ductility. These results provide key insights into the deformation mechanism occurring during tensile testing in the CNF composites loaded with PMMA NPs.
本研究旨在描述和评估由纤维素纳米纤维（CNF）制成的复合材料的断裂应变增加机制，该复合材料用阳离子聚电解质作为电晕稳定的纳米级乳胶颗粒（<200 nm）增强。通过聚合诱导自组装（PISA）合成的应用乳胶纳米颗粒（NP）由中性核聚合物组成，即聚（甲基丙烯酸丁酯）（PBMA）或聚（甲基丙烯酸甲酯）（PMMA）。在室温下，PBMA接近其玻璃化转变温度（Tg），而PMMA低于其Tg。使用拉伸测试期间的原位小角X射线散射分析了含有75 wt% CNF和25 wt% NP的纳米复合材料，监测了NP在应变下的结构演变。不像PBMA NP那样聚结的球形PMMA NP的散射显示了CNF网络中NP组织的变化。这些观察结果通过横截面透射和扫描电子显微镜得到证实。在拉伸应变期间，PMMA NP核没有明显从球形变为椭圆形的变化。由三维NP结构产生的各向异性散射变化在负载PMMA或PBMA NP的纳米复合材料之间似乎非常不同，这与通常描述的二维CNF网络形成对比。离散的PMMA NP在应变下可以在CNF-NP双网络内重新组织，从而保持强度并增加断裂应变。相对于PBMA复合材料和CNF参考膜，在原位测量期间增加湿度（20%、50%和80% RH）进一步强调了PMMA复合材料中的这种效果。变形的开始发生在比更脆的膜断裂更高的应变值处，表明仅对PMMA复合材料可用的二次纳米级相互作用的效果，扩展了塑性变形并增加了延展性。这些结果为负载PMMA NP的CNF复合材料在拉伸测试期间发生的变形机制提供了关键见解。
