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Cudjoe E
,
Khani S
,
Way AE
,
Hore MJA
,
Maia J
,
Rowan SJ
.
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Inspired by the ability of the sea cucumber to (reversibly) increase the stiffness of its dermis upon exposure to a stimulus, we herein report a stimuli-responsive nanocomposite that can reversibly increase its stiffness upon exposure to warm water. Nanocomposites composed of cellulose nanocrystals (CNCs) that are grafted with a lower critical solution temperature (LCST) polymer embedded within a poly(vinyl acetate) (PVAc) matrix show a dramatic increase in modulus, for example, from 1 to 350 MPa upon exposure to warm water, the hypothesis being that grafting the polymers from the CNCs disrupts the interactions between the nanofibers and minimizes the mechanical reinforcement of the film. However, exposure to water above the LCST leads to the collapse of the polymer chains and subsequent stiffening of the nanocomposite as a result of the enhanced CNC interactions. Backing up this hypothesis are energy conserving dissipative particle dynamics (EDPD) simulations which show that the attractive interactions between CNCs are switched on upon the temperature-induced collapse of the grafted polymer chains, resulting in the formation of a percolating reinforcing network.
Figure 1. Schematic showing the concept of a reversible,
thermally stiffening
water swollen composite below the LCST and a stiff reinforced composite
above the LCST. The brown rods represent the CNCs, blue attachments
represent the LCST polymers, and clear bubbles represent water. Below
the LCST, polymer chains prevent CNC interactions; however, above
LCST, polymer chains collapse, allowing interactions between the CNC.
Scheme 1. Synthesis of the t-CNC-g-POEGy(M)A
Figure 2. Characterization
of t-CNC-g-POEG3A. (a)
AFM height images (line in AFM height images corresponds
to cross sections used to calculate the area and height of the t-CNCs). (b) Averaged height profiles of t-CNC–COOH and t-CNC-g-POEG3A. (c) Cloud point test as a function of temperature showing
the LCST behavior of an aqueous dispersion of t-CNC-POEG3A. (d) Small angle neutron scattering profile of t-CNC-g-POEG3A as a function of temperature.
(e) Kratky plot showing the difference in the slope of the scattering
profile at different temperatures (linear fit is inserted to help
guide the eye). (f) Simulation of the mean square radius of gyration
of grafted polymer on nanorods with respect to temperature (kBT set at 1 for LCST); blue
dots are the simulated radius of gyration, red dotted line is a guide
for the reader’s eye, and errors bars represent the standard
deviations of the simulated data.
Figure 3. Thermomechanical properties and simulation results of t-CNC-g-POEG3A/PVAc nanocomposite
above
the LCST. (a) Tensile storage modulus (E′) of t-CNC-g-POEG3A/PVAc composites dry as-processed
(1DAP, black squares), soaked
in water (25 °C) below the LCST for 3 days (1WBL, red circles), placed in water (60 °C)
above LCST for 1 h (1WAL, blue
triangles), and redried above 60 °C (1RDAL, green triangles). (b) Percent of water uptake of
the t-CNC-g-POEG3A/PVAc
composites below and above LCST. (c) Solubility parameter of the simulated
system as a function of temperature (kBT set at 1 for LCST). (d) Storage modulus of 1DAP and 1RDAL above glass transition temperature of the t-CNC-g-POEG3A/PVAc nanocomposites
(75 °C).
Figure 4. Network formation (experimental
and simulation) of t-CNC-g-POEG3A/PVAc nanocomposite above
the LCST. (a) Dry tensile storage modulus (E′)
at 75 °C of the t-CNC-g-POEG3A/PVAc films with the films exposed to 60 °C water and
dried (1RDAL) versus different
volume fractions of t-CNC component (not including
the grafted polymer); the dotted line represents the modulus predicted
by the percolation model. (b) Simulated average number of contacts
on each rod with respect to the system’s temperature. (c) Simulation
snapshots at various time points. A color coding algorithm is used
to identify the number of contacts between neighboring rods. Blue
color represents rods without any contact, and as the number of contacts
increases, the rod turns yellow.
Figure 5. Stiffness demonstration, kinetics, and reversibility
(experimental
and simulated) properties of t-CNC-g-POEG3A/PVAc nanocomposites. (a) Images of 30 wt % t-CNC-g-POEG3A/PVAc composite
placed in 25 °C water, removed from water but unable to penetrate
gelatin due to low mechanical strength, placed in 60 °C water
for 2 min, and then removed from water but now strong enough to penetrate
gelatin. (b) Tensile storage modulus (E′)
of the 30 wt % t-CNC-g-POEG3A/PVAc composite cycled 4 times between 25 and 60 °C
water. (c) EDPD simulation demonstrating the reversibility of the
temperature responsive system. (d) Stiffness versus time of 30 and
15 wt % t-CNC-g-POEG3A/PVAc composites examined using DMA. Experiments start with the
wet swollen films (1WBL), which
are then exposed to 60 °C water (1WAL). (e) EDPD kinetic simulation of network formation as a function
of time for different filler content.
Figure 6. Properties and demonstration of the mechanical
stiffening of the
nanocomposites under biologically relevant conditions. (a) Comparison
of the tensile storage modulus (E′) of 30
wt % t-CNC-g-POEG3A/PVAc
(1) and t-CNC-g-POEG2MA/PVAc (2) nanocomposite films in water at different temperatures.
(b) Images of a wet 30 wt % t-CNC-g-POEG2MA/PVAc composite stiffened enough to retain the
shape of a human nose upon exposure to a warm hand.
