Water-Responsive
Mechanically Adaptive Nanocomposites Based on Styrene–Butadiene
Rubber and Cellulose NanocrystalsProcessing Matters
- Publication date
- Publisher
Abstract
Biomimetic, stimuli-responsive polymer
nanocomposites based on a hydrophobic styrene–butadiene rubber
(SBR) matrix and rigid, rod-like cellulose nanocrystals (CNCs) isolated
from cotton were prepared by three different approaches, and their
properties were studied and related to the composition, processing
history, and exposure to water as a stimulus. The first processing
approach involved mixing an aqueous SBR latex with aqueous CNC dispersions,
and films were subsequently formed by solution-casting. The second
method utilized the first protocol, but films were additionally compression-molded.
The third method involved the formation of a CNC organogel via a solvent
exchange with acetone, followed by infusing this gel, in which the
CNCs form a percolating network with solutions of SBR in tetrahydrofuran.
The thermomechanical properties of the materials were established
by dynamic mechanical thermal analysis (DMTA). In the dry state, all
nanocomposites show much higher tensile storage moduli, <i>E</i>′, than the neat SBR or the SBR latex. <i>E</i>′
increases with the CNC content and depends strongly on the processing
method, which appears to influence the morphology of the SBR nanocomposites
produced. The highest <i>E</i>′ values were observed
for the solution cast samples involving an SBR latex, where <i>E</i>′ increased from 3 MPa for the neat SBR to ca. 740
MPa for the nanocomposite containing 20% v/v CNCs. Upon submersion
in deionized water, a dramatic reduction of <i>E</i>′
was observed, for example from 740 to 5 MPa for the solution-cast
nanocomposite containing 20% v/v CNCs. This change is interpreted
as a disengagement of the percolating CNC network, on account of modest
aqueous swelling and competitive hydrogen bonding of water molecules
with the CNCs. It is shown that the method of preparation also influenced
the swelling behavior and kinetics of modulus switching, consistent
with different arrangements of the CNCs, which serve as channels for
water absorption and transport within the hydrophobic SBR matrix