Creating Hierarchical Structures in Renewable Composites by Attaching Bacterial Cellulose onto Sisal Fibers

摘要

The use of hierarchy is a well-established principle in structural engineering; consider, for example, the range of length scales in the Eiffel Tower. Nature, on the other hand, maximizes the efficiency of structural materials, such as bone and wood, by defining the arrangement of the constituents at every level from the molecular to the macroscopic; in marine sponge skeletons, at least seven separate levels of hierarchy have been identified. Microstructural control lies at the heart of materials science, but the potential to create new materials using hierarchy to improve physical properties as yet remains underexploited. This paper describes a new approach to producing hierarchical composite structures from completely renewable resources. Bacterial cellulose was introduced as a nanoscale reinforcement by attaching it to the surface of natural fibers, in this case sisal. The natural fibers have micrometer-scale diameters, and although they themselves have a hierarchical internal structure, the deliberate introduction of the nanofiller provides a new means of controlling their interaction with and the behavior of the matrix. The modified sisal fibers were incorporated into a bioderived polymer matrix, poly(L-lactic acid) (PLLA), to obtain a new class of hierarchical composite that is both derived from renewable resources and biodegradable. The attachment approach facilitates modification of the fiber surface and avoids the problems commonly associated with agglomeration and dispersion of nanofillers; the result is a rise in interfacial adhesion to the polymer matrix, and an associated improvement in the performance of the composite. The effects of modifying sisal fibers, both in their natural state (Sisal-N) and after extraction with acetone (Sisal-A), were assessed qualitatively by SEM, and quantitatively by single-fiber tensile and pull-out (interfacial shear strength) tests. The hierarchical-composite performance was determined using compression-molded samples loaded in tension, parallel (0°) and perpendicular (90°) to the primary reinforcing fibers; the response to water immersion was also explored.