Porous and bulk water‐responsive urethane‐based shape memory polymers (SMPs) containing poly(ethylene glycol) (PEG), poly(caprolactone), and poly(dimethylsiloxane) are fabricated. The copolymers are processed by electrospinning to achieve porous structures. Shape fixation and recovery are achieved via the solvation and recrystallization of the hydrophilic PEG switching segment. Mechanical testing is performed to determine the SMP functionality. Water uptake rate for porous SMP is found to be higher than bulk SMP partly due to higher surface area for water contact. This enables porous structure water‐responsive SMPs to recover faster compared to bulk SMPs. The water‐responsive SMP exhibits good extents of shape fixity and shape recovery when immersed in water (≈35 °C). Different actuation times can be achieved based on the total surface area and efficiency of water‐entry into the polymer.
Composites of ethylene‐vinyl acetate (EVA) reinforced with graphene platelets are fabricated. Morphological, thermal, mechanical, electrical properties as well as moisture absorption of the composites are characterized. Transmission electron microscopy shows a good dispersion of graphene platelets in the matrix. The unidirectional orientation of graphene platelets parallel to the surface of the composites is revealed by field emission scanning electron microscopy and is validated using the Halpin–Tsai model. Tensile strength and elongation of the composites are respectively improved by 109 and 83%, after the addition of 3 wt% graphene platelets. The incorporation of 5 wt% graphene platelets enhances the char residue of the composites from 0.544% for pure EVA to 6.63% for the composites. The electrical conductivity of the composite with 3 wt% graphene platelets is two orders of magnitude higher than that of pure EVA with 10−13 S cm–1 electrical conductivity.
The potential of polymerization in a dispersed system as an attractive technique for polymer/graphene composite synthesis is discussed. This overview is focused on the preparation of graphene/polymer composite materials by two methods: (i) emulsion mixing or blending of polymer and graphene aqueous dispersions, and (ii) in situ polymerization in a dispersed system (emulsion, miniemulsion, microemulsion, and Pickering‐stabilized emulsion). Various methods for the stabilization of graphene nanoplatelets (GNPs) prior to composite preparation are presented, and the established specific interactions between the filler and the matrix are discussed. The determination of the electrical conductivity and the opportunity offered by polymerization in a dispersed system for the formation of a segregated network of graphene filler in the frame of a polymer matrix are presented. The mechanical and thermal properties of the composites are also discussed. A short summary of the open questions regarding the synthesis of water‐borne polymer/graphene composites is presented.
This study introduces the concept of ion selective actuation in polymer metal composite actuators, employing crown ether bearing aromatic polyether materials. For this purpose, sulfonated poly(arylene ether ketone) (SPAEK) and crown ether containing SPAEK with molar masses suitable for membrane preparation are synthesized. The synthesized polymers are characterized using Nuclear magnetic resonance (NMR) and fourier transform infrared spectroscopy (FTIR) spectroscopy, thermal gravimetric analysis (TGA), and differential scanning calorimetry (DSC). Ionic polymer metal composite (IPMC) actuators are fabricated by electroless chemical deposition of a platinum (Pt) layer on both sides of SPAEK and crown‐ether containing SPAEK membranes, resulting in electrode layers of around 120 nm thickness. Actuation experiments demonstrate cation specific responses and bending degrees of the IPMC actuators. Incorporation of crown ether units in the polymer backbone results in an improved and ion‐selective bending displacement compared with SPAEK actuators. S(25)C(50)PAEK actuators show an increased bending displacement of 28% for Na+ and 20% for K+ ions.
Polystyrene (PS) commonly exhibits brittle behavior and poor mechanical properties due to the presence of structural heterogeneities promoting localized failure. The removal of this localized failure is shown here by processing PS into fibers with a range of diameters using electrospinning. Mechanical properties of individual electrospun fibers were quantified with atomic force microscopy based nanomechanical tensile testing. The resultant stress–strain behavior of PS fibers highlights considerable enhancement of mechanical properties when fiber diameter decreases below 600 nm such that polystyrene toughness increases significantly by over two orders of magnitude compared to the bulk. Consideration of the network properties of polystyrene is used to demonstrate the increase of draw ratio toward a theoretical limit and is potentially applicable to a range of glassy polymeric materials.
Recent years have witnessed a staggering escalation in the power density of modern electronic devices. Because increasingly high power density accumulates heat, efficient heat removal has become a critical limitation for the performance, reliability, and further development of modern electronic devices. Thermal interface materials (TIMs) are widely employed between the two solid contact surfaces of heat sources and heat sinks to increase heat removal for electric devices. Composites of graphene and matrix materials are expected to be the most promising TIMs because of the remarkable thermal conductivity of graphene. Here, the recent research on the thermal properties of graphene filled polymer composite TIMs is reviewed. First, the composition of graphene filled polymer composite TIMs is introduced. Then, the synthetic methods for graphene filled polymer composite TIMs are primarily described. This study focuses on introducing the methods for improving and characterizing the thermal properties of graphene filled polymer composite TIMs. Furthermore, the challenges facing graphene filled polymer composite TIMs for thermal management applications in the modern electronic industry and the further progress required in this field are discussed.
Although graphene‐based materials have been used as fillers in polymer nanocomposites, a deleterious trade‐off in mechanical strength and ductility is typically observed with increasing graphene loading, resulting in strong but brittle polymer nanocomposite materials. To provide outstanding compatibility with a standard high strength polymer, thermoplastic polyurethane (TPU), the use of a simple and mild sol‐gel reaction to chemically attached silica nanoparticles to graphene oxide (GO) basal plane is reported. The silica modification imparts a highly porous GO surface structure, providing noncovalent attachment sites that improve physical entanglement between the GO and TPU. Furthermore, the silica modification enhances surface polarity, which imparts chemical affinity between the silica/GO nanocomposite and TPU. As a result, the prepared polymer nanocomposites exhibit significantly improved Young's modulus and tensile strength with only a small reduction of elongation at break over the neat polymer.
This study investigates the fabrication and performance of broadband and omnidirectional antireflective polymer foils, in the visible spectrum (400–800 nm), consisting of subwavelength inverted moth‐eye structures. The foils are fabricated by a high throughput roll‐to‐roll extrusion coating process allowing structuring on both sides at a rate of 60 m min−1, with web width 45 cm. The highest average transmittance obtained in the visible spectrum is (98 ± 1) %; compared with (92 ± 1) % for the unstructured foil. The antireflective foil shows no significant difference in transmittance between normal incidence and incidence up to at least 60°. The foil performance is also investigated for different depths (Dp ) and shapes of structures. The transmittance initially increases with Dp and reaches a maximum at Dp ≈ 120 nm. For process parameters yielding greater depths, other shape factors also play a critical role in the foil's antireflective properties.
The previously introduced process for enzyme‐mediated in situ synthesis and deposition of eumelanin is investigated with covalent immobilization of the tyrosinase. It results in a monolayer structure of non‐coalesced melanin particles, with a film thickness of 5–8 nm. The reaction is self‐terminating due to overlay of the enzymes with particles. The melanin particles are rodlike with lengths down to 6 nm. Isolated melanin structures of such small size have not been observed before and might be a kind of protoparticle in the supramolecular buildup of melanin oligomers. Utilization of melanin particles with such small size can enable nanotechnological applications in the areas of bioelectronics and biosensors.
Many connective tissue diseases and defects are associated with poor synthesis or excessive degradation of collagen. The modern tissue engineering approach is to replace the defective site via the implantation of a biocompatible scaffold which serves as a carrier for cell incorporation, proliferation, and growth. Collagen is widely used in the field of clinical medicine in connection with both hard and soft tissue applications. However, certain collagen properties such as poor dimensional stability, poor in vivo mechanical strength, low degree of elasticity, variable nature in terms of enzymatic degradation, crosslinking density, fiber size, trace impurities, and side effects frequently limit both its analysis and application. This review focuses particularly on the processing and modification of collagen type I with respect to its biological and mechanical properties. The processing of collagen into scaffolds is crucial to mimic successfully the extracellular matrices. Moreover, the review suggests several ways in which the most common problems related to the isolation, handling, electrospinning, and crosslinking of collagen can be overcome while maintaining its native character as much as possible. Further, the review provides a summary of the analytical methods available for the physicochemical characterization of collagen with respect to both its molecular and submolecular structure.
Conductive textiles with exceptional electrical properties have been prepared by coating the conjugated polymer, poly(3,4‐ethylenedioxyphiophene)‐polystyrenesulfonate(PEDOT‐PSS), on polyethylene terephthalate (PET) nonwoven fabrics. Phase segregation from covalent bond formation to surface silica particles generates PEDOT‐PSS coated textiles that hold potential for wearable electronics due to the breathability of the fabric, low toxicity, easy processing and lightweight with high current carrying capacity. The conductive textiles were demonstrated for applications such as electrical connections and resistive heating.
Visible light curing of photopolymers has gained increasing interest in recent years. Dental materials are one of the important areas of application, where the bimolecular camphorquinone/amine initiator system is currently state of the art initiator. In this study, the authors describe the synthesis and photochemistry of tetrakis(2,4,6‐trimethylbenzoyl)silane, as cleavable Type I visible light photoinitiator. Besides excellent photobleaching behavior, this photoinitiator can well compete with up to now used long wavelength initiators.
Additive manufacturing (AM) is still underutilized as an industrial process, but is quickly gaining momentum with the development of innovative techniques and materials for various applications. In particular, stereolithography (SLA) is now shifting from rapid prototyping to rapid manufacturing, but is facing challenges in parts performance and printing speed, among others. This review discusses the application of SLA for polymer nanocomposites fabrication to show the technology's potential in increasing the applicability of current SLA‐printed parts. Photopolymerization chemistry, nanocomposite preparation, and applications in various industries are also explained to provide a comprehensive picture of the current and future capabilities of the technique and materials involved.
A novel method of preparing skinned asymmetric membranes with two distinctive layers is described: a top layer composed of chemically cross‐linked polymer chains (dense layer) and a bottom layer of non‐cross‐linked polymer chains (porous substructure). The method consists of two simple steps that are compatible with industrial membrane fabrication facilities. Unlike conventional processes to prepare asymmetric membranes, with this approach it is possible to finely control the structure and functionalities of the final membrane. The thickness of the dense layer can be easily controlled over several orders of magnitude and targeted functional groups can be readily incorporated in it.
Assembly of anisotropic nanoparticles on polymeric templates has attracted much attention recently because of the potential useful applications. In this work, 3D electrospun nanofiber membrane is used as template for the assembly of ex situ synthesized palladium nanocubes. The assembly process is achieved by simple immersion step, in which the electrostatic assembly of the nanocubes occurs. The dense nanocubes on the nanofibers as well as the fibrous nanostructure render the nanocomposite membrane excellent catalytic activity. Moreover, the catalytic membrane can be recycled for at least six times, which makes it possible for practical usage.
Different softwood lignin O‐acyl derivatives, i.e., methacrylated, hexanoylated, benzoylated, methoxybenzoylated, and cinnamoylated lignin are synthesized and subjected to melt spinning. In the presence of spinning aids such as vanillin and ethylene glycol dimethacrylate, multifilament melt spinning is accomplished with spinning speeds up to 500 m min−1, which allowed for realizing uniform precursor fibers 17 μm in diameter. Out of all acyl‐derivatives of softwood lignin investigated, cinnamoylated softwood lignin (CL) turned out to be superior in terms of processability. CL‐derived precursor fibers are oxidatively thermostabilized and then carbonized applying carbonization temperatures up to 2200 °C. Carbon fiber structure formation is followed in detail by wide‐angle X‐ray scattering and Raman spectroscopy. An orientation ≤53% and a d 002 spacing of 0.353 nm is achieved. According to small angle X‐ray scattering, carbon fibers have a porosity of ≈38%. CL‐derived carbon fibers are also characterized in terms of mechanical properties. Tensile strengths up to 0.93 GPa (average 0.75 GPa) are obtained and follow Weibull statistics. Elastic moduli are ≤66.5 GPa (average 41.1 GPa).
Photo‐reversible polyurethane (PU) coatings based on coumarin diol (CD) are obtained. Initially, pre‐polymers based on different amounts of coumarin (5, 15, and 25 mol%) and 1,6‐hexamethylene diisocyanate are prepared to obtain PUs with a large incorporation of CD and high molecular weight. The pre‐polymer is posterior reacted with poly(ε‐caprolactone) diol (PCL‐diol), either with molecular weight = 530 or 2000 g mol–1. The thermal stabilities of the PUs are studied using thermogravimetric analysis. Polymers with a higher content of CD present higher stability. The thermal transitions and the mechanical response are analyzed using differential scanning calorimetry and strain‐stress tests, respectively. Moreover, the photo‐reversibility of CD‐based PUs is followed by UV absorption. In general, photo‐dimerization induces better mechanical properties of the final PUs. Materials obtained with short PCL‐diol ( = 530 g mol–1) and the highest amount of CD present higher reversibility processes. Therefore, these polymers are promising for application as coating systems.
A chemical strategy is attempted to modify graphene for its facilitated dispersion in poly(ε‐caprolactone) (PCL) matrix. Herein, graphite oxide is subjected to sequential treatment with phenyl isocyanate and vitamin C (VC) to yield graphene nanosheets (iG‐VC). It is noteworthy that following the reduction treatment, iG‐VC graphene sheets exfoliate within the PCL matrix and show appreciable interfacial compatibility with PCL matrix in organic solvent by virtue of improved polarity from isocyanate treatment. The tensile yield strength and Young's modulus of the PCL/iG‐VC composite exhibit pronounced enhancement as compared to neat PCL, despite of mere composition of graphene sheets. The tensile yield stress of composite is increased notably to reach 18.6 MPa at 3 wt% graphene sheets as compared to neat PCL. Likewise, Young's modulus of composite is observed to increase from 370 to 470 MPa at 5 wt% graphene sheets. Moreover, the crystallization temperature (T c) and crystallinity of PCL increase significantly upon incorporation of small amount of iG‐VC. Ultimately, functional role of iG‐VC graphene sheets is demonstrated in enhancing electrical conductivity of PCL‐based nanocomposites. The plausible mechanisms are also proposed to explain the increased T c, improved mechanical property, and improved electrical conductivity of PCL/iG‐VC composite.
The direct injection of a drug into a joint can relieve osteoarthritic pain for a short period of time. The problem is that the drug will not stay at the allocated location. Therefore, a proof‐of‐concept in situ is designed forming hydrogel containing liposomes that are covalently linked to the hydrogel network. When the liposomes are filled with a cargo, the formed hydrogel is thus loaded with this cargo, too. Due to the link between the hydrogel and the liposomes, a compression or other mechanical force applied to the hydrogel will rupture the liposomes and release a small percentage of the cargo. Overall, a long‐term intra‐articular drug release is feasible.
