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<p><a>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 </a>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. </p><p><br></p>
<p>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. </p>
<p><br></p><p>To study the effects of different types
and aspect ratios of CNCs on mechanical, thermal and morphological properties
of polyvinyl alcohol (PVA) composite <a>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. </a>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.</p><p><br></p>
<p>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. </p>
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| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/8039027 |
| Date | 11 June 2019 |
| Creators | Shikha Shrestha (6631748) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/Effect_of_nanocellulose_reinforcement_on_the_properties_of_polymer_composites/8039027 |
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