Volume 15, Issue 4

Developments in design and technology in the engineering and medical fields necessitate the use of smart and high-performance materials to meet higher engineering specifications. The general requirements of such materials include a combination of high stiffness and strength with significant weight savings, resistance to corrosion, chemical resistance, low maintenance, and reduced costs. Over the last three decades, it has been demonstrated that auxetic materials offer a huge potential for the fields of engineering, natural sciences, and biomedical engineering, and for many other industries, including the aerospace and defense industries, through their unique deformation mechanism and measured enhancements in mechanical properties. To meet future engineering challenges, auxetic materials are increasingly being recognized as integral components of smart and advanced materials. Although materials with a negative Poisson’s ratio have been known since the early 1900s, they did not capture researchers’ attention until the late 1980s. Since 1991, these materials have been known as auxetic materials. Since then, their benefits and applications have been expanded to all major classes of materials such as metals, ceramics, polymers, and composites, and they are also now being used in engineering applications. The goal of this review was to present the development of auxetic polymers, which were first fabricated in the form of polyurethane foam approximately three decades ago and are now used in the fabrication of non-woven nano/micropolymeric structures. This review could provide useful information for the future development of auxetic polymers.

The copolymers of 2-methylene-1,3-dioxepane (MDO) and N -phenyl maleimide (NPM) prepared by radical polymerization with high thermal stability, glass transition temperature and optical transparency are presented. The polymers made under specific reaction conditions, i.e., 120°C and high amounts of MDO, had degradable ester units, which were formed via radical ring-opening polymerization of MDO. The formation of charge-transfer complex between MDO and NPM also led to the formation of high-molar-mass copolymers by simple mixing and heating of monomers without the use of any initiator. Structural characterization of the copolymers including mechanistic studies was carried out using nuclear magnetic resonance spectroscopy, and their thermal properties were studied using differential scanning calorimetry and thermogravimetric analysis.

We demonstrate the synthesis of star-shaped poly(2-ethyl-2-oxazoline) featuring a porphyrin core starting from alkyne-functionalized porphyrin ([TPP-TB] 4 ) and azide-functionalized poly(2-ethyl-2-oxazoline) (PEtOx-N 3 ) via copper-catalyzed azide-alkyne cycloaddition (CuAAC). The porphyrin core was further utilized for the complexation of either copper or iron within the central cavity. The obtained materials were investigated using a combination of nuclear magnetic resonance spectroscopy, fourier transform infrared spectroscopy, ultraviolet-visible spectroscopy, size-exclusion chromatography, and matrix assisted laser desorption/ionization time-of-flight mass spectrometry. In the case of copper, the inclusion of the metal ion was achieved in a one-pot reaction during the CuAAC reaction for attaching the PEtOx-N 3 arms.

Chitosan/poly(vinyl alcohol) (CS/PVA) blend membranes were prepared using the casting method and their physiochemical properties were analyzed using Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD). FTIR and XRD demonstrated possible hydrogen bonds between CS and PVA. The addition of PVA to CS resulted in surface roughness as analyzed by SEM. The CS/PVA blend membrane exhibited high tensile properties (81.62%) and reduced water-holding capacity (53.8%) compared to a pure CS membrane (control). Cell viability and proliferation were assessed via an MTT assay with Vero cell culture. Associated with improved physicochemical properties, the CS/PVA blend membrane promotes cell proliferation of Vero cells with high specific growth rate (0.582 day -1 ). The results demonstrate that the blending of CS and PVA could significantly alter the surface rugosity, water-holding capacity and improve the mechanical and biological properties of the membrane. Interestingly, this concept can be extended for different anchorage-dependent cell lines, as an alternative carrier material.

Poly(1-vinylimidazole) (PVIm)/silica nanocomposite particles were prepared via free radical polymerization of VIm in the presence of unmodified silica and crosslinker N,N′-methylenebisacrylamide without any surfactant by a one-pot route in supercritical carbon dioxide. The strong acid-base interaction between the hydroxyl groups (acidic) of silica surfaces and the imidazole groups (basic) of 1-VIm was strong enough to promote the formation of long-stable PVIm/silica nanocomposite particles. Transmission electron microscopy and scanning electron microscopy results showed that the silica nanoparticles were encapsulated into the polymer and with a specific pore structure. Fourier transform infrared spectroscopy results suggested the strong interaction between PVIm and silica. The nanocomposites were shown to possess higher thermal stability than PVIm. A maximum surface area of 60.76 m 2 /g was obtained via standard nitrogen adsorption analysis. The property of the composite materials in terms of heavy metal ion removal was investigated. The maximum adsorption capacity of Cr (VI) is 331.5 mg/g in the experimental range.

Chitosan and poly(vinyl alcohol) were blended and cross-linked with tetraethoxysilane and showed conductive properties. Impedance spectroscopy was used to study the influence of temperature on the electrical properties of the membranes. The conductivity of the membranes was increased with an increase in temperature. Free water was decreased and bound water was increased with an increase in cross-linker contents. The tensile strength improved, whereas elongation at break decreased by increasing the amount of cross-linker contents. The water contact angle of the membranes lowered with time, exhibiting the hydrophilic nature of the membranes. The novel characteristics of biocompatible membranes can be used in biomedical applications including biological schemes that require smaller charge in medicinal apparatus, bioelectrode coatings, etc.

Cross-linked cationic polystyrene (CCPS) nanospheres with monodispersity were successfully generated by soap-free miniemulsion polymerization of styrene and divinylbenzene with 2,2′-azobis (2-amidinopropane) dihydrochloride as a water-soluble initiator. The effects of output power of the homogenizer and the concentration of the initiator and cross-linking agent on the particle size and coefficient of variation of CCPS nanospheres were systematically investigated and discussed. The weight percent of N element on the particle surface and the ζ potential were determined as different initiator concentrations were employed for the preparation of CCPS nanospheres. Furthermore, the technique of nitrogen adsorption/desorption was applied to characterize the specific surface area and the total pore volume of the CCPS nanospheres, which were generated from different amounts of divinylbenzene. Overall, the present study provides a simple route for producing cross-linked polymer nanospheres with monodispersity.

A tough double-network (DN) superabsorbent was synthesized by a two-step method using N , N -methylenebisacrylamide as a covalent cross-linker for one monomer [acrylic acid (AA)], Ca 2+ (CaCl 2 ) as an ionic cross-linker for the other monomer (sodium alginate [SA]) and ammonium peroxodisulfate as the redox initiator. The optimized experimental conditions for the absorbency in deionized water were determined according to orthogonal experiments. Unlike conventional chemical cross-linked single-network superabsorbents, SA/poly(acrylic acid) (PAA) DN superabsorbents exhibit superb mechanical properties. Compared with the tensile strength of PAA-only superabsorbents, that of SA/PAA DN superabsorbents showed an approximately 371.9% increase with increasing amount of 6 wt.% SA. We also investigated the capacity of SA/PAA DN superabsorbents to remove heavy metal ions. It was found that the addition of SA can truly increase the metal ion removal capacity of the PAA superabsorbents and that the affinity order was Pb 2+ >Zn 2+ .

It has been proved that alcohol molecules exist as cyclic and chain aggregates of different sizes in pure or mixed solvent systems. Here, it will be shown that these aggregates can radically change the rheological properties of a concentrated polymer solution containing self-assembled chains. In a previous study by the same authors, the existence of self-assembled structures in dilute solution of poly(dimethylaminoethyl methacrylate- co -methyl methacrylate- co -butyl methacrylate) in an alcoholic solvent mixture was shown according to small angle X-ray scattering results, showing that these structure were much more compact than those in acetone. This finding is based on the role of alcohol aggregates as physical cross-linkers. Here, the existence of self-assembled structures in concentrated solutions of the same terpolymer was confirmed by atomic force microscopy and rheology results both in acetone (a good solvent) and in a solvent mixture composed of acetone, ethanol and 1-propanol. For the terpolymer solutions in the solvent mixture, very little decrease in complex viscosity and shear thickening were observed at high strains and frequencies, respectively. It can be concluded that the alcohol aggregates can cause the formation of strong self-assembled structures that can even resist high shear forces or strains.