Evaluation of cellulose nanocrystal alignment in oriented electrospun fibers

Abstract: Electrospinning is a fiber production method that has gained a special attention due to the simple setup and potential for the industry scale up to produce nanoregime polymer fibers. However, electrospun fibers have relatively poor mechanical properties, which could be improved by introducing reinforcing agents. Cellulose nanocrystals (CNCs) are a promising candidate for use as such fiber reinforcing phase due to nanoscale dimensions, excellent mechanical properties, abundance in nature, biocompatibility and renewability. The mechanical properties of reinforced fibers can be further improved by aligning them uniaxially. There are several reports available on aligning electrospun fibers and reinforcing them with CNCs. However, alignment of the reinforcing phase, such as CNCs, inside matrix is not studied extensively. The importance of aligning arises from different mechanical properties exhibited by the CNCs in longitudinal and transverse directions due to the high aspect ratio. This anisotropic nature of CNCs could be employed in nanocomposite fibers by aligning crystals along the fiber direction. In this study, the effect of the electric field modification on the alignment of CNCs in poly(vinyl) alcohol fibers was investigated. Fibers were collected using four different collector types, which also gave four different electric field configurations. Alignment of the reinforcing crystals in fibers with different degree of macroscopic orientation was studied using 2D XRD and polarized FT-IR. These studies confirm the alignment of both CNCs and PVA in uniaxially aligned fibers. Mechanical testing showed that improvement in alignment is directly related to the increase of the strength of the material. Aligned PVA-CNCs fibers had more than 100 times higher elastic modulus compared to non-aligned fibers. The rule of mixtures, Halpin-Tsai equation and orientation modified Cox’s equation were used to calculate theoretical values of elastic modulus and compare with experimental values. The comparison of between experimentally observed alignment of CNCs and theoretically predicted values shows that there is a potential for further improvement. The demonstrated improvements in the alignment of reinforcing phase could find applications, where well-aligned architectures are required, for example in uses as tissue engineering, scaffolds, membranes, microelectronic devices etc.