Modifying the surface of cellulose nanofibrils (CNFs) produced by mechanical refinement with a variety of polymer functional groups in an entirely water‐based system is challenging because only surface hydroxyl groups are accessible. To address this limitation, an entirely water‐based, polymer modification scheme is developed. CNFs are functionalized with a reactive methacrylate functional group followed by subsequent grafting‐through polymerization. This modification worked with a variety of water‐soluble and water‐insoluble (meth)acrylates and (meth)acrylamides, grafting up to 45 wt% polymer on to the CNFs. The reaction conditions introducing the methacrylate functional group are adjusted to vary the degree of functionality. Soxhlet extraction of modified samples demonstrates that the reactive methacrylate group is necessary to facilitate polymer grafting. The degree of functionalization of the polymers is studied via quantitative transmission IR spectroscopy and the morphology of the resulting cellulose nanofibrils is studied via a combination of optical, scanning electron, and atomic force microscopy. High levels of polymer modification do not significantly affect the micrometer‐scale fibril morphology.
Cellulose nanofibrils (CNFs) are a promising reinforcement
for
biodegradable composite matrices such as poly(lactic acid) (PLA),
but they require commercially scalable drying methods that preserve
their fibrillar morphology along with improved interfacial interactions
with polymer matrices. In this work, a water-based grafting-through
polymerization scheme to modify CNFs improved spray drying behavior
and reinforcement capacity in PLA composites. All polymer modifications
yielded CNFs with a more fibrillar morphology after spray drying,
increasing specific surface area by up to 490% compared to unmodified
CNFs, values similar to conventional freeze drying. Polymer-grafted
CNFs in PLA composites improved the tensile strength by 16% at 20
wt % loading and stiffness by 22% at a 10 wt % loading with two different
graft-polymer chemistries compared to unmodified CNF composites. Surface
energy heterogeneity measurements of the reinforcements and PLA matrix
were employed to understand the improvements in composite properties.
Polymer modifications lowered the total surface energy of the CNFs,
and calculated ratios of work of adhesion to work of cohesion suggested
improved interfacial compatibility for four of the modified CNFs with
PLA. Rheological oscillatory shear studies of the composites correlated
solid-like melt behavior, as demonstrated by storage moduli dominance,
with higher tensile strength. Thermal analysis of the composites revealed
that excessive plasticization by the poly(oligoethylene glycol methyl
ether methacrylate)-grafted sample potentially offset mechanical property
improvements imparted by the more fibrillar morphology. This work
provides an opportunity for large-scale manufacturing of CNF/PLA composites
via an entirely aqueous modification scheme and industrially relevant
spray drying process.
A simple strategy was developed to synthesize polyimine-coated
cellulose nanofibrils (CNF) for effective CNF drying and composite
reinforcement. The polyimine was synthesized in an aqueous medium
using a selective hydrophilic and hydrophobic component that forces
the polyimine to precipitate, which prevents the reverse imine reaction.
The polyimine coating allowed the CNF to be easily oven-dried while
maintaining a fibrillar morphology to provide mechanical reinforcement
in poly(ethylene terephthalate glycol) (PETG) composites. In comparison,
poor mechanical performance and a heterogeneous fracture surface were
observed when the coated CNF were incorporated into poly(l-lactide) (PLA) composites. It is hypothesized that intermolecular
aromatic–aromatic interactions are formed at the interface
between fibers and the polymer matrix in the PETG system, while no
such phenomena occur in the PLA system. Overall, this facile strategy
to produce modified, easily dried CNF can be adapted to produce polyimine-coated
CNF with unique functionalities that are useful in a range of applications.
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