Polymer nanocomposites (PNC) are composite materials consisting of inorganic nanoparticles dispersed within a polymer matrix. They offer enhanced material properties such as increased stiffness, thermal stability, increased fire barrier resistance and more. The classification of PNCs depends on the dimension of the nanoparticle, type of polymer, morphology of the nanoparticle-polymer matrix and other factors. PNCs are commonly prepared via in-situ polymerization, melt extrusion, solution dispersion. Other classifications of PNCs exist such as carbon-based, metal-based and ceramic-based nanoparticles. Depending on the preparation method, PNCs can be prepared following a bottom-up or top-down approach. The bottom-down approach involves creating the nanoparticle inside the polymer matrix or monomer solution. The top-down approach involves producing the nanoparticles first and then mixing them in the polymer matrix. Nanoparticles or nanostructures are better suited for polymer composites compared to macro- or micro-particles due to the greater interaction between the nanoparticles and the polymer matrix. With this greater interaction, PNCs offer enhanced properties of polymers by improving thermal, mechanical, barrier and optical properties. Although there are many advantages to PNCs, synthesis of PNCs can be challenging. Common challenges include achieving uniform dispersion of nanoparticles within polymer matrix, strong interfacial bonding to prevent accumulation of the nanoparticles within the polymer matrix and controlling the orientation of the nanoparticles within the polymer matrix. PNCs are used in various fields such as biomedical, pharmaceutical, engineering and more. Silver-embedded PNCs are used to for antibacterial activity. The biocompatible composite MXene, poly (2-(dimethylamino) ethyl methacrylate) (P(2(DMA)EMA), can be used in oral insulin drug delivery. PNCs with conductive nanoparticles such as carbon nanotubes can be used for glucose biosensors.
Synthesis of Polymer Nanocomposites
Sol-gel synthesis of polymer nanocomposites The sol-gel process involves the conversion of precursors to particles using hydrolysis and condensation. For PNCs, the nanoparticles are dispersed into a monomer solution and results in a network of polymer and nanoparticles interactions. Therefore, the polymer contributes to the nucleation and growth of the nanoparticles through the monomer solution. It has advantages with uniform nanoparticle distribution, control over particle size, can be done under low temperatures and can control porosity.
In-situ polymerization of polymer nanocomposites For in-situ polymerization of PNCs, the nanoparticles are dispersed within a monomer solution and polymerization starts with heat, UV-light exposure, initiator diffusion and/or by use of a catalyst. Therefore, the monomers polymerize with the nanoparticles in the mixture. This can form intercalated or exfoliated nanocomposites. Where intercalated nanocomposites means the polymer chains insert themselves within the layers of nanoparticles creating relatively thick layers of 20–80 Å. Exfoliated nanocomposites, however, form nanometer-scaled layers between the nanoparticles. This method has advantages of high dispersion of the nanoparticles, strong interfacial interaction (between the nanoparticles and the polymer) and versatile.
Electro-spinning of polymer nanocomposites In electro-spinning the formation of fibers is done by passing a polymer solution through a high-voltage syringe with a counter electrode (commonly an aluminum sheet) is placed underneath generating an electric field. By dispersing the nanoparticles within the polymer solution, the formation of PNCs is possible. This method has the advantages of control over the diameter of the fibers, of the morphology and of the composition.
Bio-hybrid polymer nanofibers Many technical applications of biological objects like proteins, viruses or bacteria such as chromatography, optical information technology, sensorics, catalysis and drug delivery require their immobilization. Carbon nanotubes, gold particles and synthetic polymers are used for this purpose. This immobilization has been achieved predominantly by adsorption or by chemical binding and to a lesser extent by incorporating these objects as guests in host matrices. In the guest host systems, an ideal method for the immobilization of biological objects and their integration into hierarchical architectures should be structured on a nanoscale to facilitate the interactions of biological nano-objects with their environment. Due to the large number of natural or synthetic polymers available and the advanced techniques developed to process such systems to nanofibres, rods, tubes etc. make polymers a good platform for the immobilization of biological objects.
Bio-hybrid nanofibres by electrospinning Polymer fibers are, in general, produced on a technical scale by extrusion, i.e., a polymer melt or a polymer solution is pumped through cylindrical dies and spun/drawn by a take-up device. The resulting fibers have diameters typically on the 10-μm scale or above. To come down in diameter into the range of several hundreds of nanometers or even down to a few nanometers, Electrospinning is today still the leading polymer processing technique available. A strong electric field of the order of 103 V/cm is applied to the polymer solution droplets emerging from a cylindrical die. The electric charges, which are accumulated on the surface of the droplet, cause droplet deformation along the field direction, even though the surface tension counteracts droplet evolution. In supercritical electric fields, the field strength overbears the surface tension and a fluid jet emanates from the droplet tip. The jet is accelerated towards the counter electrode. During this transport phase, the jet is subjected to strong electrically driven circular bending motions that cause a strong elongation and thinning of the jet, a solvent evaporation until, finally, the solid nanofibre is deposited on the counter electrode.
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