Effect of nanocellulose reinforcement on the properties of polymer composites
Polymer nanocomposites are envisioned for use in many advanced applications, such as structural industries, aerospace, automotive technology and electronic materials, due to the improved properties like mechanical strengthening, thermal and chemical stability, easy bulk processing, and/or light-weight instigated by the filler-matrix combination compared to the neat matrix. In recent years, due to increasing environmental concerns, many industries are inclining towards developing sustainable and renewable polymer nanocomposites. Cellulose nanomaterials (CNs), including cellulose nanocrystals (CNCs) and cellulose nanofibrils (CNFs), have gained popularity due to their excellent mechanical properties and eco-friendliness (extracted from trees, algae, plants etc.). However, to develop CN-reinforced nanocomposites with industrial applications it is necessary to understand impact of hygroscopic swelling (which has very limited quantitative study at present), aspect ratio, orientation, and content of CNs on the overall performance of nanocomposites; and overcome the low dispersibility of CNs and improve their compatibility with hydrophobic matrix. In this work, we attempt to understand the influence of single nanocrystals in the hygroscopic and optical response exhibited by nanostructured films; effect of CNCs on the properties of PVA/CNC fibers by experimental evidence with mathematical modeling predictions; and hydrophobized CNFs using a facile, aqueous surface modification to improve interfacial compatibility with epoxy.
To evaluate the effect of CNC alignment in the bulk response to hygroscopic expansion, self-organized and shear-oriented CNC films were prepared under two different mechanisms. The coefficient of hygroscopic swelling (CHS) of these films was determined by using a new contact-free method of Contrast Enhanced Microscopy Digital Image Correlation (CEMDIC) that enabled the characterization of dimensional changes induced by hygroscopic swelling of the films. This method can be readily used for other soft materials to accurately measure hygroscopic strain in a non-destructive way. By calculating the CHS values of CNC films, it was determined that hygroscopic swelling is highly dependent on the alignment of nanocrystals within the films, with aligned CNC films showing dramatically reduced hygroscopic expansion than randomly oriented films. Finite element analysis was used to simulate moisture sorption and kinetics profile which further predicted moisture diffusion as the predominant mechanism for swelling of CNC films.
To study the effects of different types and aspect ratios of CNCs on mechanical, thermal and morphological properties of polyvinyl alcohol (PVA) composite fibers, CNCs extracted from wood pulp and cotton were reinforced into PVA to produce fibers by dry-jet-wet spinning. The fibers were collected as-spun and with first stage drawing up to draw ratio 2. The elastic modulus and tensile strength of the fibers improved with increasing CNC content (5 – 15 wt. %) at the expense of their strain-to-failure. The mechanical properties of fibers with cotton CNC were higher than the fibers with wood CNC when the same amount of CNCs were added due to their higher aspect ratio. The degree of orientation along the spun fiber axis was quantified by 2D X-ray diffraction. As expected, the CNC orientation correlates to the mechanical properties of the composite fibers. Micromechanical models were used to predict the fiber performance and compare with experimental results. Finally, surface and cross-sectional morphologies of fibers were analyzed by scanning electron microscopy and optical microscopy.
To improve the dispersibility and compatibility of CNFs with epoxy, CNFs were modified by using a two-step water-based method where tannic acid (TA) acts as a primer with CNF suspension and reacts with hexadecylamine (HDA), forming the modified product as CNF-TA-HDA. The modified (-m) and unmodified (-um) CNFs were filled into hydrophobic epoxy resin with a co-solvent (acetone), which was subsequently removed to form a solvent-free two component epoxy system, followed by addition of hardener to cure the resin. Better dispersion and stronger adhesion between fillers and epoxy were obtained for m-CNF than the um-CNF, resulting in better mechanical properties of nanocomposites at the same loading. Thermal stability and the degradation temperature of m-CNF/epoxy improved when compared to neat epoxy.