2 research outputs found
Liquid Crystalline Behavior of Graphene Oxide in the Formation and Deformation of Tough Nanocomposite Hydrogels
In this paper, we report the formation
and transformation of graphene
oxide (GO) liquid crystalline (LC) structures in the synthesis and
deformation of tough GO nanocomposite hydrogels. GO aqueous dispersions
form a nematic LC phase, while the addition of poly(<i>N</i>-vinylpyrrolidone) (PVP) and acrylamide (AAm), which are capable
of forming hydrogen bonding with GO nanosheets, shifts the isotropic/nematic
transition to a lower volume fraction of GO and enhances the formation
of nematic droplets. During the gelation process, a phase separation
of the polymers and GO nanosheets is accompanied by the directional
assembly of GO nanosheets, forming large LC tactoids with a radial
GO configuration. The shape of the large tactoids evolves from a sphere
to a toroid as the tactoids increase in size. Interestingly, during
cyclic uniaxial tensile deformation a reversible LC transition is
observed in the very tough hydrogels. The isolated birefringent domains
and the LC domains in the tactoids in the gels are highly oriented
under a high tensile strain
Tuning the Interfacial Mechanical Behaviors of Monolayer Graphene/PMMA Nanocomposites
The
van der Waals (vdW) force dominated interface between graphene and
polymer matrix creates weak points in the mechanical sense. Chemical
functionalization was expected to be an effective approach in transfer
of the outstanding performance of graphene across multiple length
scales up to the macroscopic level, due to possible improvements in
the interfacial adhesion. However, published works showed the contradiction
that improvements, insensitivity, or even worsening of macro-mechanical
performance have all been reported in graphene-based polymer nanocomposites.
Particularly central cause of such discrepancy is the variations in
graphene/polymer interfacial chemistry, which is critical in nanocomposites
with vast interfacial area. Herein, O<sub>3</sub>/H<sub>2</sub>O gaseous
mixture was utilized to oxidize monolayer graphene sheet with controlled
functionalization degrees. Hydrogen bonds (H bonds) are expected to
form between oxidized graphene sheet/poly(methyl methacrylate) (PMMA)
at the interface. On the basis of in situ tensile-micro Raman spectroscopy,
the impacts of bonding types (vdW and H-bonds) on both key interfacial
parameters (such as interfacial shear strength and critical length)
and failure modes of graphene/PMMA nanocomposite were clarified for
the first time at the microscopic level. Our results show that owing
to improved interfacial interaction via H bonds, the interface tends
to be stiffening and strengthening. Moreover, the mechanical properties
of the functionalized graphene/PMMA interface will be set by the competition
between the enhanced interfacial adhesion and the degraded elastic
modulus of graphene, which was caused by structural defects in the
graphene sheet during the functionalization process and could lead
to catastrophic failure of graphene sheets in our experimental observation.
Our results will be helpful to design various nanofiller-based nanocomposites
with high mechanical performance