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Recently, attention has been focusing on the development of smart materials, that is, materials which can change their mechanical properties such as stiffness, shape memory, actuation and self-healing, in response to several types of stimuli such as temperature or certain chemicals. These responsive materials can be used for a wide range of applications.
The effect of reducing the stiffness of a material by adding a component is bioinspired. For instance, hemicellulose composites can show a reduction of Young’s modulus by as much as three orders of magnitude when surrounded by water vapor. Another way to create the same effect is to cross-link large molecules and thus create a network which behaves quite differently from the molecules by themselves. In nature, such adaptive materials are created from a small deck of molecules arranged in different ways and layered in varying hierarchies to achieve different kinds of interactions between them, thus determining the ultimate properties of the built-up material.
The original inspiration for these is the sea cucumber, the dermis of which changes its mechanical properties in response to changes in the chemical properties of the water. Cellulose nanocomposites (CNCs) have been reported which soften when they are exposed to a watery environment at body temperature. These are linked to synthetic elastomers to create polymer nanocomposites. CNCs are single cellulose crystals, rigid and rodlike in shape, which are naturally stiff, strong and lightweight. Being biocompatible and sustainable, they also degrade readily, and have therefore been used in a host of applications such as coatings, cosmetics, drug delivery systems, and medical implants. Stiffening occurs when composites containing CNCs are exposed to water because of the transformation of the polymeric structure into a percolating network. This process can be reversed by altering the temperature, from 25 °C water to 37 °C.
The earliest work based upon the sea cucumber dermis was the combination of tunicate-derived CNCs (t-CNCs from the tunica of the sea cucumber) with a matrix formed of poly(ethylene oxide-co-epichlorohydrin) (EO-EPI). CNCs are being used in more and more nanocomposites designed for smart adaptations because of the abundant hydrogen bonding between hydrogen and carbon which may be prevented or promoted using another chemical. In this case the EO-EPI copolymer had a moderate uptake of water and a low modulus while remaining generally unreactive with the CNCs. The formation of a percolation network means above a certain limit the transfer of stress depends upon hydrogen bonds. Exposure to water then promotes hydrogen bonding within the CNCs and reduces the surface hydroxyl groups available for inter-CNC bonding, making the composite softer (wet-state modulus of 5 MPa). This polymer does swell up to 70% of its volume, however, with water.
Another innovation is the composite of polyvinyl acetate copolymer (PVAc) with CNC that can switch between wet and dry states, with a change in the mechanical properties, due to an induced plasticization of the matrix which is comprised of PVAc and solvation of the CNC water which results in switching off the CNC-CNC and CNC-matrix interactions. Experiments showed that changing the temperature shifted the glassy polymer state to a rubbery one, without disrupting the CNC network. Water exposure, however, switched off the interactions between the CNC molecules. These polymers could be used as reinforcement material for biological grafts as in craniofacial implants or guided bone regeneration, where the material to be grafted must be rigid initially but must soften soon after insertion so as to prevent inflammation. This was only 28% with the use of cotton-derived CNCs because of the difference in the surface sulfur groups, that is, the uptake of water was reduced significantly while the wet-state modulus was reduced quite comparably to that of PVAc/t-CNC at 12 MPa. With just 0.1% CNCs in a mixture of in situ PVAc/CNC and PLA, the modulus of elasticity, the yield strength, the break elongation was markedly improved.
Other researchers found that the wet-state modulus could be tuned by just changing the ratio of PBMA to PVAc because of the hydrophobic nature of the matrix which allowed only 15% water uptake.
Rubber-based CNC Nanocomposites
Another material is the rubber-based CNC nanocomposite which adapts to water by altering its mechanical behavior. Natural rubber was combined with epoxidized natural rubber to boost the sensitivity of the water and reversible mechanical switching to a much greater extent than was possible with natural rubber alone. The formation of hydrogen bonds disperses the filler more evenly and allows both a CNC-CNC filler network and a CNC-polymer network to form. The presence of both these networks makes these composites tunable with regards to their mechanical properties and water-responsiveness.
Other materials were synthesized to pinpoint the changing contribution of the polarity of the matrix and the glass transition temperature to the mechanical adaptive properties of the nanocomposite material using other rubber-based materials such as styrene butadiene rubber. These required more time (several days) for softening, but water was indeed taken up by being carried through the hydrophilic channels formed by the polar CNC molecules within the hydrophobic matrix.
Other approaches include the use of nanocrystals of chitin origin, or CNCs to which amino or carboxylic acid groups are added to incorporate into PVAc networks, to form networks which stiffened by gel formation at opposing pH values, which could mean the production of reversible mechanical adaptation of nanocomposites by switching off or on the CNC-CNC interactions .
The introduction of electrospun PVA mats marked a new type of plasticizer, after it was used as a filler for PVAc or EO-EPI matrices
LCST (lower critical solution temperature) polymers shift from rigid to soft on exposure to heat and water above the LCST as their chains swell. Nanocomposites have been synthesized using LCST polymers grafted on CNCs, and both are embedded in a matrix containing PVAc). The LCST polymer chains collapse to form a percolating network leading to stiffening when exposed to hot water. It can be dried at the same temperature to keep it in the same processed form, which means it may be used in biomedical procedures which require initial rigidity for placement but final rigidity when maneuvered into final position.
Thus this area is rapidly progressing in the search for better materials which can alter their stiffness on demand in response to water exposure.