The objective of this work was to increase the interfacial strength between aramid fiber and epoxy matrix. This was achieved by functionalizing the aramid fiber followed by growth of a layer of ZnO nanowires on the fiber surface such that when embedded into the polymer, the load transfer and bonding area could be substantially enhanced. The functionalization procedure developed here created functional carboxylic acid surface groups that chemically interact with the ZnO and thus greatly enhance the strength of the interface between the fiber and the ZnO.
The matrix-ZnO interface is enhanced through increased surface area (>1,000 times), mechanical interlocking, and the creation of a functional gradient between the nanowires and matrix, which has been shown to improve the interface strength of a carbon fiber composite by well over 100 percent. The composite compressive strength, shear strength, shear modulus, interlaminar shear strength, and interfacial shear strength should all be enhanced because the graded interface reduces the stress concentration at the discrete fiber-to-matrix boundary.
The first milestone of the project was to develop the functionalization procedure to enhance the attachment of the ZnO nanowires to the aramid fiber. This was achieved with carboxylic acid groups that split the peptide bond, catalyzed by a strong base, and created a carboxylate and a primary amine functional group. Carboxylic acid groups are specifically chosen because they often discharge a proton leading to charge coordination between the negative oxygen atoms and the positive zinc ions. Furthermore, the bond angles of carboxylic acid functional groups are highly compatible with the zinc ion, making it the ideal functional group for bonding with zinc. Fourier Transform Infrared Spectroscopy (FTIR) was used for analyzing the absorbance frequencies and comparing to previously reported values, reactions, and bond structures for validation.
Single-fiber mechanical testing helped to determine the fiber tensile strength of the ZnO nanowire arrays. The results of the testing showed that there was no degradation of the fiber strength in spite of breaking enough surface bonds to create functional groups onto which to anchor the nanowires. It is critical that fiber strength is maintained during functionalization and growth, because the composite properties depend heavily on that fiber strength. Composite lamina mechanical testing was also employed. Because ZnO nanowire arrays have been shown to increase interfacial shear strength at the single fiber scale, improvements are expected in several properties at the composite scale. Interlaminar shear strength, laminar shear strength, and laminar shear modulus are expected to increase as a direct result of the functional gradient.