29 research outputs found
Stress-transfer in anisotropic and environmentally adaptive cellulose whisker nanocomposites
Quantitative insights into the stress-transfer mechanisms that determine the mechanical properties of tunicate cellulose whisker/poly(vinyl acetate) nanocomposites were gained by Raman spectroscopy. The extent of stress-transfer is influenced by local orientation (or anisotropy) of the whiskers, which in turn is governed by the processing conditions used to fabricate the nanocomposites. Solution-cast materials display no microscopic anisotropy, while samples that were cast and subsequently compression molded contain both isotropic regions as well as domains of locally oriented whiskers. Polarized optical microscopy showed these regions to have dimensions in the hundreds of Ī¼m. Polarized Raman spectroscopy of the 1095 cmā»Ā¹ Raman band, associated with CāO ring stretching of the cellulose backbone, was used to quantify the local orientation of the cellulose whiskers. Clear and discernible shifts of this Raman band upon uniaxial deformation of nanocomposite films were further used to determine the level of stress experienced by the cellulose whiskers, ultimately reflecting the levels of stress-transfer predominantly between the poly(vinyl acetate) matrix and the tunicate whiskers, but also between the whiskers within the network. In the isotropic regions, where whiskers form a percolating network, the observed Raman shift rate with respect to strain is smaller than in the regions where the whiskers are uniaxially orientated. The Raman shift is strongly affected by the presence of water, leading to a lack of stress-transfer when the samples are fully hydrated, which is clearly detected by the Raman technique. Heating of the nanocomposites above the glass transition temperature of the poly(vinyl acetate) matrix also reduces the stress experienced by the individual whiskers
Bio-inspired mechanically-adaptive nanocomposites derived from cotton cellulose whiskers
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Photopolymerizable compositions for solventless fiber spinning
Disclosed are methods of fiber spinning and polymer fibers that utilize multifunctional thiol and enes compounds. Also, the subject matter disclosed herein relates to uses of polymer fibers and articles prepared from such fibers.Board of Regents, University of Texas Syste
Biomimetic mechanically adaptive nanocomposites
The development of a new class of mechanically adaptive nanocomposites has been inspired by biological creatures such as sea cucumbers, which have the ability to reversibly change the stiffness of their dermis. Several recent studies have related this dynamic mechanical behaviour to the distinctive nanocomposite architecture of the collagenous tissue, in which interactions among rigid collagen fibrils, embedded in a viscoelastic matrix of fibrillin microfibrils, are regulated by neurosecretory proteins. Here we review the development of a new family of artificial polymer nanocomposites that mimic the architecture and the mechanic adaptability of the sea cucumber dermis. The new materials are based on low-modulus matrix polymers that are reinforced with a percolating cellulose nanofiber network. Owing to the abundance of surface hydroxyl groups, the cellulose nanofibers display strong interactions between themselves, causing the evenly dispersed percolating nanocomposites to display a high stiffness. The nanofiberānanofiber interactions can be largely switched off by the introduction of a chemical regulator that allows for competitive hydrogen bonding, resulting in a significant decrease in the stiffness of the material
Fire-Retardant, Self-Extinguishing Inorganic/Polymer Composite Memory Foams
Polymeric
foams used in furniture and automotive and aircraft seating applications
rely on the incorporation of environmentally hazardous fire-retardant
additives to meet fire safety norms. This has occasioned significant
interest in novel approaches to the elimination of fire-retardant
additives. Foams based on polymer nanocomposites or based on fire-retardant
coatings show compromised mechanical performance and require additional
processing steps. Here, we demonstrate a one-step preparation of a
fire-retardant ice-templated inorganic/polymer hybrid that does not
incorporate fire-retardant additives. The hybrid foams exhibit excellent
mechanical properties. They are elastic to large compressional strain,
despite the high inorganic content. They also exhibit tunable mechanical
recovery, including viscoelastic āmemoryā. These hybrid
foams are prepared using ice-templating that relies on a green solvent,
water, as a porogen. Because these foams are predominantly comprised
of inorganic components, they exhibit exceptional fire retardance
in torch burn tests and are self-extinguishing. After being subjected
to a flame, the foam retains its porous structure and does not drip
or collapse. In micro-combustion calorimetry, the hybrid foams show
a peak heat release rate that is only 25% that of a commercial fire-retardant
polyurethanes. Finally, we demonstrate that we can use ice-templating
to prepare hybrid foams with different inorganic colloids, including
cheap commercial materials. We also demonstrate that ice-templating
is amenable to scale up, without loss of mechanical performance or
fire-retardant properties
Biologically inspired hierarchical design of nanocomposites based on poly(ethylene oxide) and cellulose nanofibers
Attempts to create hierarchically structured, uniaxially oriented nanocomposites comprising cellulose nanowhiskers (CNWs), which promise anisotropic mechanical properties, are exceedingly rare. We report here the fabrication of uniaxially-oriented arrays of microfibers based on poly(ethylene oxide) (PEO) and CNWs by electrospinning. Compared with the neat PEO fibers, the incorporation of CNWs within the fibers increased the storage modulus (Eā²) of arrays along the fiber axis of the PEO/CNW nanocomposite fibers. Successful incorporation of the CNWs within each of the as-spun PEO/CNW nanocomposite fibers in the direction parallel to the fiber axis was verified by both scanning and transmission electron microscopy
Stimuli-responsive mechanically adaptive polymer nanocomposites
A new series of biomimetic stimuli-responsive nanocomposites, which change their mechanical properties upon exposure to physiological conditions, was prepared and investigated. The materials were produced by introducing percolating networks of cellulose nanofibers or āwhiskersā derived from tunicates into poly(vinyl acetate) (PVAc), poly(butyl methacrylate) (PBMA), and blends of these polymers, with the objective of determining how the hydrophobicity and glass-transition temperature (Tg) of the polymer matrix affect the water-induced mechanically dynamic behavior. Below the Tg (ā¼60ā70 Ā°C), the incorporation of whiskers (15.1ā16.5% v/v) modestly increased the tensile storage moduli (Eā²) of the neat polymers from 0.6 to 3.8 GPa (PBMA) and from 2 to 5.2 GPa (PVAc). The reinforcement was much more dramatic above Tg, where Eā² increased from 1.2 to 690 MPa (PVAc) and ā¼1 MPa to 1.1 GPa (PBMA). Upon exposure to physiological conditions (immersion in artificial cerebrospinal fluid, ACSF, at 37 Ā°C) all materials displayed a decrease in Eā². The most significant contrast was seen in PVAc; for example, the Eā² of a 16.5% v/v PVAc/whisker nanocomposite decreased from 5.2 GPa to 12.7 MPa. Only a modest modulus decrease was measured for PBMA/whisker nanocomposite; here the Eā² of a 15.1% v/v PBMA/whisker nanocomposite decreased from 3.8 to 1.2 GPa. A systematic investigation revealed that the magnitude of the mechanical contrast was related to the degree of swelling with ACSF, which was shown to increase with whisker content, temperature, and polarity of the matrix (PVAc > PBMA). The mechanical morphing of the new materials can be described in the framework of both the percolation and HalpināKardos models for nanocomposite reinforcement, and is the result of changing interactions among the nanoparticles and plasticization of the matrix upon swelling
Bio-inspired mechanically-adaptive nanocomposites derived from cotton cellulose whiskers
A new series of biomimetic, stimuli-responsive nanocomposites, which change their mechanical properties upon exposure to physiological conditions, was investigated. The materials were produced by introducing percolating networks of cellulose whiskers isolated from cotton into poly(vinyl acetate). Below the glass-transition temperature (Tgā¼ 63 Ā°C), the tensile storage moduli (Eā²) of the dry nanocomposites increased two fold, from 2 GPa for the neat polymer to 4 GPa for a nanocomposite with 16.5% v/v whiskers. The relative reinforcement was more significant above Tg, where Eā² was increased nearly 40 fold, from ā¼1.2 MPa to ā¼45 MPa. Upon exposure to emulated physiological conditions (immersion in artificial cerebrospinal fluid at 37 Ā°C) all nanocomposites showed a pronounced decrease in Eā², for example to 5 MPa for the 16.5% v/v whisker nanocomposites with only about 28% w/w swelling. This is a significant reduction in the amount of swelling required to decrease the Eā², compared to earlier material versions based on cellulose whiskers with higher surface charge density; the decreased swelling may be a considerable advantage for the intended use of these materials as adaptive substrates for intracortical electrodes and other biomedical applications
Fire-Retardant, Self-Extinguishing Inorganic/Polymer Composite Memory Foams
Polymeric
foams used in furniture and automotive and aircraft seating applications
rely on the incorporation of environmentally hazardous fire-retardant
additives to meet fire safety norms. This has occasioned significant
interest in novel approaches to the elimination of fire-retardant
additives. Foams based on polymer nanocomposites or based on fire-retardant
coatings show compromised mechanical performance and require additional
processing steps. Here, we demonstrate a one-step preparation of a
fire-retardant ice-templated inorganic/polymer hybrid that does not
incorporate fire-retardant additives. The hybrid foams exhibit excellent
mechanical properties. They are elastic to large compressional strain,
despite the high inorganic content. They also exhibit tunable mechanical
recovery, including viscoelastic āmemoryā. These hybrid
foams are prepared using ice-templating that relies on a green solvent,
water, as a porogen. Because these foams are predominantly comprised
of inorganic components, they exhibit exceptional fire retardance
in torch burn tests and are self-extinguishing. After being subjected
to a flame, the foam retains its porous structure and does not drip
or collapse. In micro-combustion calorimetry, the hybrid foams show
a peak heat release rate that is only 25% that of a commercial fire-retardant
polyurethanes. Finally, we demonstrate that we can use ice-templating
to prepare hybrid foams with different inorganic colloids, including
cheap commercial materials. We also demonstrate that ice-templating
is amenable to scale up, without loss of mechanical performance or
fire-retardant properties