Cellulose Nanofiber Composite Polymeric Materials with Reversible and Movable Cross-links and Evaluation of their Mechanical Properties

Improvement in the elasticity and toughness of polymeric materials is an important issue in the fabrication of polymeric materials. However, solving this issue is challenging because of the trade-off between toughness and elasticity. Herein, citric acid-modified cellulose (CAC) was introduced as a filler into movable cross-linked polymeric materials, in which cyclic molecules move along the polymer chain. The resulting CAC/polymer composite materials contained both reversible and movable cross-links. The reversible cross-links were hydrogen bonds formed between the carboxyl group of acrylic acid units (AA; in the polymer chain) and the CAC filler. The movable cross-links were formed by a cyclic molecule along the polymer chain. This combination of reversible and movable cross-links resulted in the high toughness of the composite materials against an applied stress. The CAC/polymer composite materials with 3 wt % CAC increased the Young's modulus by 1.6 times and maintained the toughness compared to the original elastomer without the CAC. The CAC/polymer composite materials without CAC or the AA unit did not show this increase. Dynamic viscoelasticity measurements revealed the relationship between the relaxation modes and the toughness of the CAC/polymer composite materials. Upon combining the appropriate unit ratio of movable and reversible cross-links, the CAC/polymer composite materials exhibited the highest Young's modulus and toughness. The design of the CAC/polymer composite materials improved the toughness with a high Young's modulus.