The fiber-matrix interface in Ioncell cellulose fiber composites and its implications for the mechanical performance

Fiber-reinforced composites based on natural fibers are promising alternatives for materials made of metal or synthetic polymers. However, the inherent inhomogeneity of natural fibers limits the quality of the respective composites. Man-made cellulose fibers (MMCFs) prepared from cellulose solutions via wet or dry-jet wet spinning processes can overcome these limitations. Herein, MMCFs are used to prepare single fiber epoxy composites and UD composites with 20, 30, 40, and 60 wt% fiber loads. The mechanical properties increase gradually with fiber loading. Young's modulus is improved three times while tensile strength doubles at a loading of 60 wt%. Raman spectroscopy is employed to follow conformational changes of the cellulose chains within the fibers upon mechanical deformation of the composites. The shift of the characteristic Raman band under strain indicates the deformation mechanisms in the fiber. Provided stress transfer occurs through the interface, it is a direct measure of the fiber-matrix interaction, which is investigated herein. The shift rate of the 1095 cm⁻¹ band decreases in single fiber composites compared to the neat fibers and continues to decrease as the fiber loading increased.