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The sensitivity of additive manufacturing (AM) to the variability of feedstock quality, machine calibration, and accuracy drives the need for frequent characterization of fabricated objects for a robust material process. The constant testing is fiscally and logistically intensive, often requiring coupons that are manufactured and tested in independent facilities. As a step toward integrating testing and characterization into the AM process while reducing cost, we propose the automated testing and characterization of AM (ATCAM). ATCAM is configured for fused deposition modeling (FDM) and introduces the concept of dynamic coupons to generate large quantities of basic AM samples. An in situ actuator is printed on the build surface to deploy coupons through impact, which is sensed by a load cell system utilizing machine learning (ML) to correlate AM data. We test ATCAM’s ability to distinguish the quality of three PLA feedstock at differing price points by generating and comparing 3000 dynamic coupons in 10 repetitions of 100 coupon cycles per material. ATCAM correlated the quality of each feedstock and visualized fatigue of in situ actuators over each testing cycle. Three ML algorithms were then compared, with Gradient Boost regression demonstrating a 71% correlation of dynamic coupons to their parent feedstock and provided confidence for the quality of AM data ATCAM generates.

Poly(propylene fumarate) (PPF) is a linear unsaturated polyester which has been widely investigated for tissue engineering due to its good biocompatibility and biodegradability. In order to extend the range of possible applications and enhance its mechanical properties, current approaches consist in the incorporation of various fillers or obtaining blends with other polymers. In the current study we designed a reinforcing agent based on carboxylated graphene oxide (GO-COOH) grafted with 2-hydroxyethyl methacrylate (GO@HEMA) for poly(propylene fumarate)/poly(ethylene glycol) dimethacrylate (PPF/PEGDMA), in order to enhance the nanofiller adhesion and compatibility with the polymer matrix, and in the same time to increase the crosslinking density. The covalent modification of GO-COOH was proved by Fourier-transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA) and Raman spectroscopy. The mechanical properties, water uptake capacity, morphology, biodegradability, mineralization and in vitro cytotoxicity of PPF/PEGDMA hybrid materials containing GO@HEMA were investigated. A 14-fold increase of the compressive modulus and a 2-fold improvement in compressive strength were observed after introduction of the nanofiller. Moreover, the decrease in sol fraction and solvent swelling in case of the hybrid materials containing GO@HEMA suggests an increase of the crosslinking density. SEM images illustrate an exfoliated structure at lower nanofiller content and a tendency for agglomeration at higher concentrations. Finally, the synthesized hybrid materials proved non-cytotoxic to murine pre-osteoblast cells and induced the formation of hydroxyapatite crystals under mineralization conditions.

Poly(3-hydroxybutyrate) (PHB) is a promising substitute to petroleum-based polymers in packaging and biomedical applications provided that its melt processability and degradability are improved. A new method to control the properties of PHB by using cheap calcium stearate (CS) as a lubricant and decomposition catalyst in melt-mixed PHB-CS compounds was first used. CS is composed of a metallic cation, which promotes PHB degradation, and a hydrophobic anion that improves the compatibility with PHB and processability. An environmentally friendly melt mixing technique was employed to obtain the PHB-CS compounds. Incorporation of 0.5 or 5 wt% CS reduced the melt viscosity and molecular weight of PHB, decreased the melting temperature with up to 5 °C, the crystallization temperature with more than 25 °C, and the degradation temperature with 15 and 40 °C, respectively. In small amounts (0.05 wt%), CS improved the processability and mechanical properties of PHB. In higher amount (0.5 wt%), CS slightly improved the Young's modulus, reduced the tensile strength and enhanced degradation. A better control of thermal and mechanical properties of PHB is, thus, possible by using different CS amount and processing conditions. These results are relevant for PHB application in the context of the global transition to biodegradable packaging.

In this study, composite hydrogels based on crosslinked polyethylene glycol diacrylate (PEGDA) and natural zeolite, with potential applications for the controlled release of phytoextracts, were synthesized by radical polymerization and characterized herein for the first time. The influence of various synthesis parameters, such as the use of distilled water or rose hip extract as reaction medium, or the percentage of the zeolite in the composite hydrogel upon the morphology, swelling kinetics and the rheological properties was investigated. By conducting polymerization of PEGDA in the rose hip extract, the crosslinking reaction yield was almost 100%, because of the chain transfer reactions of the polyphenols from the rose hip extract. Rose hip release tests were used to evaluate the release profiles of the natural bioactive substances from phytoextracts. The results showed that the hydrogel loaded with 3% zeolite displayed the lowest burst release and the slowest release rates of the extract.

Studies on the nitrate reduction reaction have shown that the bimetallic catalysts may be more efficient compared to the monometallic ones. To reduce nitrate, it is necessary to activate the precious metal by addition of a promoting second metal. Some studies have pointed out the relevance of the structure and geometry of the metal particles in the final mechanism of nitrate reduction. Well-defined nanoparticles could be used to catalyze structure-sensitive reactions with practical importance (pollution control). Our investigation aimed to make deeper insight into NO3- catalytic reduction and photocatalytic reduction mechanisms. Two types of Pt-Cu supported on TiO2 materials were employed in order to study the nitrate removal from aqueous phase and the ability of obtaining and using the in situ generated solar H2 as reducing agent. The prepared catalysts were characterized and tested in photocatalytic/catalytic reduction of nitrate in aqueous solution in water splitting reaction. A correlation between the catalytic/photocatalytic performances of the supported Pt-Cu catalysts and their associated physicochemical and adsorption properties has been made. The catalysts activity for the nitrate reduction can be directly related to the interactions between platinum and copper. Deposition with Pt nanoparticles was found to contribute to enhance the photocatalytic activity toward H2 production from aqueous solution. Our future research aims at optimizing the photocatalytic system having as future possible applications for hydrogen production using sunlight and water as the hydrogen source.

This work aims at developing polyamide 1010 (PA1010) composites with improved fire behavior using a halogen-free flame-retardant system based on melamine (Me) and gallic acid (GA) complexes (MA). The MA complexes were formed by hydrogen bonding, starting from 1:2, 1:1, 2:1 Me:GA molar ratios. PA1010 composites were obtained by melt mixing, followed by compression molding. MA provided a plasticizing effect on the PA1010 matrix by decreasing the glass transition temperature. The influence of MA on PA1010 chain packaging was highlighted in the X-ray diffraction patterns, mainly in the amorphous phase, but affected also the &alpha, and &gamma, planes. This was reflected in the dynamic mechanical properties by the reduction of the storage modulus. H-bonds occurrence in MA complexes, improved the efficiency in the gaseous form during fire exposure, facilitating the gas formation and finally reflected in thermal stability, thermo-oxidative stability, LOI results, and vertical burning behavior results. PA1010 containing a higher amount of GA in the complex (MA12) displayed a limiting oxygen index (LOI) value of 33.6%, much higher when compared to neat PA1010 (25.8%). Vertical burning tests showed that all the composites can achieve the V-0 rating in contrast with neat PA1010 that has V-2 classification.

Eight different types of complexing agents were employed for the development of new biodegradable decontamination solutions that are able to form strippable coatings after they are dispersed and allowed to dry on a variety of surfaces contaminated with 60Co, 133Ba, 137Cs and 241Am radioactive isotopes. The new generation of eco-friendly chelators with superior biodegradability, utilized for the first time in such applications, can easily replace the non-biodegradable and carcinogenic complexing agents that are still in use today, due to their decontamination performances. Furthermore, besides the complexing action over the radionuclides, the solutions contain two types of clays, Bentonite (BT) and Saponite (SP), which have the capacity to adsorb specific ions, improving the decontamination efficiency of the solutions. Our research revealed that it is preferable to replace BT with SP, due to its better dispersibility, thermal stability, next to superior ability to gel and better thermal stability (Miles, 2011). The solutions showed a decontamination factor superior to 95% for 137Cs (on all surfaces), over 90% for 60Co and 133Ba, and more than 72% 241Am (on all surfaces), except for galvanized metal plates, where lower decontamination factors were obtained: over 70% for 133Ba, maximum 41.87% for 241Am and 43.19% for 60Co.

This study presents the design of novel composites nanogels, based on poly(ethylene glycol) diacrylate and natural zeolite particles, that are able to act as materials with controlled drug delivery properties. Natural zeolite–nanogels composite, with varying zeolite contents, were obtained by an inverse mini-emulsion technique and loaded with 5-fluorouracil, a widely used chemotherapeutic drug. Herein, the possibility of adjusting final properties by means of modifying the preparation conditions was investigated. The prepared composite nanogels are characterized by dynamic light scattering (DLS), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA). In light of this tunable drug-loading capability, swelling behaviour, and cytotoxicity, these composite nanogels could be highly attractive as drug reservoirs.

The present work is an attempt to improve the usseful properties of sulfonated polyphenylene oxide in order to obtain a proton exchange membrane (PEM) for proton exchange membrane fuel cells(PEMFC). Formation of siloxane compounds inside the polymer matrix through an in situ sol-gel process improves properties of the composite membrane: water retention, tensile strength and dimensional stability of the membrane. The presence of the silicone atoms inside the polymer matrix is highlited in the X-ray fluorescence spectra. Parameters related to water absorbtion and proton transport inside the membrane such as: water uptake, hydration number (λ), dimensional expansion by hydration, ion exchange capacity and sulfonation degree show an optimization of the composite membrane compared to the polymeric one. Furthermore, the tensile strength of the composite membranes is better than the polymeric one when both samples are fully hydrated.

A potentiostat is an essential piece of analytical equipment for studying electrochemical devices and reactions. As the design of electrochemical devices evolve applications for systems with multiple working electrodes have become more common. These applications drive a need for low-cost multi-channel potentiostat systems. We have developed a portable low-cost scalable system with a modular design that can support 8 to 64 channels at a cost as low as \$8 per channel. This design can replace the functionality of commercial potentiostats which cost upwards of \$10k for certain applications. Each channel in the multi-channel potentiostat has an independent adjustable voltage source with a built-in ammeter and switch, making the device flexible for various configurations. The multi-channel potentiostat is designed for low current applications (nA range), but its purpose can change by varying its shunt resistor value. The system can either function as a standalone device or remotely controlled. We demonstrate the functionality of this system for the control of a 24-channel bioelectronic ion pump for open- and closed- loop control of pH., Comment: 16 pages, 7 figures

Efficient preparation of stable colloidal suspensions of platinum nanoparticles was achieved by reduction of Pt precursor (H 2 PtCl 6 ·6H 2 O) in the presence of new thermosensitive polymers with triblock copolymer structure consisting of a poly(ethylene glycol) (PEG) central block and poly(N-isopropylacrylamide-co-N-t-butylacrylamide) statistical copolymer as side chains. The predominant shape and size of platinum nanoparticles was controlled by changing the N-t-butylacrylamide (TB)/N-isopropylacrylamide (NIPAM) mole ratio in the capping polymer. The triblock copolymer side chain composition was varied by increasing gradually the TB molar percentage from 0, 3 and 6 mol%. The molar ratio between the capping polymer, calculated based on the monomer unit, and platinum cation was in all cases 10:1. The morphology, shape and mean size of colloidal particles was determined from transmission electron microscopy (TEM) micrographs. The best morphological control was obtained using the thermosensitive polymer with 3 mol% TB within the side blocks, and the optimal reduction temperature for this polymer was 34 °C (below the critical temperature). The surface area of exposed platinum nanoparticles dispersed on TiO 2 was estimated by CO pulse chemisorption. The photocatalytic hydrogen generation over Pt/TiO 2 materials under solar irradiation was evaluated. The hydrogen production rate was found to be sensitive to the particle size.

Polymeric hydrogels are soft materials that can absorb large amounts of water or biological [...]

Microemulsions are thermodynamically stable and transparent systems composed of an aqueous phase, oil, a surfactant, and usually also contain a co-surfactant [1]. [...]

Future materials in the field of thermoplastics require a lower impact on the environment. Actual thermoplastic materials also require “greener” additives for thermal and fire resistance profile. In this work, some of the recent solutions for halogen-free flame retardants were discussed in the context of viable routes with economical and industrial relevance. The fire retardant mechanism was discussed in order to imagine new solutions for future additives. Some of the main (inorganic, organic or hybrid) flame retardants can be used as common platform for the next classes of thermoplastic composites (synthetic, natural, or bio-based). The new generation of flame retardants (based on the emerging solutions) will have to surpass two major barriers: the cost effectiveness and the need for high concentration (which usually affects other properties).

Biocompatible composites play a critical role as scaffolds in tissue engineering. Novel biocomposites made from poly(3-hydroxybutyrate) (PHB), polyhydroxyalkanoate (PHA) and bacterial cellulose (BC) in different concentrations were prepared by solution casting and their thermal and mechanical behavior as well as biocompatibility was characterized. BC addition increased the thermal stability of the polymer matrix as evidenced by thermogravimetric analysis. The crystallinity of PHB and the crystallization temperature decreased with the addition of BC and PHA, thus increasing the processing window. BC in small concentration determined an increase in the mechanical properties due to a concerted action of PHA and filler. Good cells attachment and proliferation were observed for all the biocomposites. By the addition of PHA (more hydrophobic than the matrix) and various amounts of BC (highly hydrophilic), surface properties and cell attachment can be controlled. Cytocompatibility studies using L929 cell line revealed that this material is suitable for biomedical applications. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 104A: 2576-2584, 2016.

Novel injectable thermosensitive hydrogel formulations with improved both water retention (WR) and drug release profile were prepared by adding dextran (DXT) to poly(N-isopropylacrylamide) (PNIPAM) aqueous solutions as a remedy against the demixing/syneresis phenomenon. The addition of the hydrophilic polysaccharide improved the WR of the hydrogel at 37°C from about 12% in the absence of DXT to about 40–55% out of the initial amount introduced, depending on both PNIPAM and DXT concentrations. Also, the 5-fluorouracil release experiments showed an appreciably reduced “burst effect” for the DXT-containing hydrogels. The shape of the release profiles revealed the presence of two stages, differing each other from the point of view of the drug release rate.

Translation between ionic currents and measurable electronic signals is essential for the integration of natural systems and artificial bioelectronic devices. Chloride ions (Cl−) play a pivotal role in bioelectricity, and they are involved in several brain pathologies, including epilepsy and disorders of the autistic spectra, as well as cancer and birth defects. As such, controlling [Cl−] in solution can actively influence biochemical processes and can be used in bioelectronic therapies. Here, we demonstrate a bioelectronic device that uses Ag/AgCl contacts to control [Cl−] in solution by electronic means. We do so by exploiting the potential dependence of the reversible reaction, Ag + Cl− ↔ AgCl + e−, at the contact/solution interface, which is at the basis of the well-known Ag/AgCl reference electrode. In short, a negative potential on the Ag/AgCl contact transfers Cl− from the contact to the solution with increasing [Cl−] and vice versa. With this strategy, we demonstrate precise spatiotemporal control of [Cl−] in solution that can be used to affect physiological processes that are dependent on [Cl−]. As proof-of-concept, we use [Cl−] control to influence the membrane voltage on human pluripotent stem cells.

The effect of hydrophilic bentonite addition on the thermogelation properties of aqueous solutions of poly(N-isopropylacrylamide)–poly(ethylene glycol)–poly(N-isopropylacrylamide) (PNIPAM-PEG-PNIPAM) triblock copolymers of various compositions and molecular weights was investigated. Dynamic rheometry and differential scanning calorimetry (DSC) measurements showed that increasing concentrations of clay added to 20 wt.% polymer aqueous solutions caused a decrease of the temperature at which the viscosity starts increasing, while the temperature corresponding to the maximum endothermic effect due to the PNIPAM chain dehydration remained practically unchanged. The storage modulus, G′, increased with clay concentration for shorter PNIPAM chain triblock copolymers, while an opposite situation occurred in the case of the block copolymer with the longest PNIPAM block. For bentonite concentrations above 1 wt.%, G′ was larger than the viscous modulus, G″, at temperatures higher than the phase separation temperature...

This paper investigates the frictionless two-dimensional contact problem of an inhomogeneously elastic material under a rigid punch. The inhomogeneous solid is deemed to comprise three distinct regions which represent a homogeneously elastic coating and substrate joined together by a functionally graded transition layer (interlayer) whose shear modulus depends exponentially on the vertical coordinate. We propose closed form solutions for the horizontal and vertical displacements of the solid which are analytic if the contact pressure is known exactly. These solutions are further used to derive a fast and efficient iterative algorithm from which the contact footprint resulting from the rigid stamp problem may be computed. A selection of numerical results are then presented using this method and it is found that our model compares well with those of other authors in the two particular limiting cases considered here. We then investigate the effects of material inhomogeneity and coating thickness on the cylindrical stamp problem and it is found that the maximum principal stress is highly dependent on the thickness and mechanical properties of the layer. In particular, it is found that the maximum principal stress that occurs in a material with a hard coating may be reduced by increasing the thickness of the transition layer whilst lower stresses are achieved in materials with soft coatings by decreasing interlayer thickness.

Novel triblock copolymers with poly(ethylene glycol) (PEG) middle chain and random copolymers of di(ethylene glycol) ethyl ether acrylate (DEGA) and oligo(ethylene glycol) methyl ether acrylate (OEGA) as side blocks were synthesized by ATRP. The thermal gelation behavior of their 20 wt% aqueous solutions was studied as a function of the molecular characteristics of the triblock copolymers. The results showed that both association and gelation temperatures decreased with increasing polymerization degree and decreasing OEGA monomer content of the polyacrylate block. Also, both temperatures increased with PEG length at smaller MW of the PEG chain, and decreased for higher MW PEG. Rheometrical measurements showed that the length of the PEG block is the most important factor influencing elastic modulus and even structure of the hydrogels formed at 37°C. [Supplementary materials are available for this article. Go to the publisher's online edition of Soft Materials for the following free supplemental resource(s)...

Accurate prediction of friction is crucial for design and efficient operation of many devices, comprising various contacts. In practice, contacting surfaces are rough and often wet. There are several mechanisms, which contribute to friction, including viscous shear of a coherent fluid film, as well as that of a thin adsorbed layer of boundary active molecular species. Additionally, adhesion and elastoplastic deformation of asperities on counterface surfaces may occur. Traditional friction models are based on statistical representation of surface topography as well as description of boundary shear films based on the theoretical lubricant film Eyring shear stress. The study reports a more realistic friction model than the traditional ones, which do not take into account the wet nature of the asperities. The fluid–surface interaction is a main contribution of the article, not hitherto reported in literature. It is shown that ignoring the effect of surface wetness can lead to the over-estimation of boundary friction and under-estimation of contact load-carrying capacity.

The present work describes the preparation of a set of new polyurea polymers suitable for ballistic protection applications by employing different aromatic diamines derivatives with different flexibility degrees. This modification would allow the synthesis of polymers with both enhanced and controllable mechanical properties through the combination of aliphatic and aromatic molecules. Besides the classical methylene diphenyl diisocyanate and polypropylene glycol bis(2-aminopropyl ether) (PPG), benzidine, 4,4′-diaminodiphenylmethane and 3,6-diamino carbazole were chosen as the third monomer and used in different ratios toward PPG, in order to optimize the polymer properties. The proportions between the polyurea components were selected as such to achieve the total consumption of the monomers. The different types of polyurea obtained have been characterized by Fourier transform infrared spectroscopy, thermal measurements (differential scanning calorimetry and thermogravimetric analysis), tensile tests, and scanning electron microscope. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43967.

The article proposes a mathematical model, which predicts the frictional performance of an internal combustion engine compression ring in the vicinity of the top dead center region. It accounts for the blow-by induced inter-ring pressures drop and for the cavitation region at the trailing edge of the contact. The model is used to predict the behaviour of both new and worn compression rings for wide open throttle operating conditions. It is shown that the wear of the ring profile increases the oil film thickness decrease the frictional losses and significantly reduces the extent of the cavitation region.

Kaolin–polymer hydrogel composites were synthesized via redox-initiated polymerization of acrylic acid (AA) in a concentrated aqueous dispersion of kaolin, using N,N′-methylene bisacrylamide as a crosslinker. The mechanical and thermal properties of the composites were investigated by systematic variation of the monomer feed composition. From rheological measurements, it was found that kaolin had a negative effect on the polymerization rate, which decreased the stiffness of the green bodies prepared at room temperature with increasing kaolin content. The gelation time was varied by changing the concentration of the redox initiator. On drying at 105 °C, the crosslinking continued to a higher degree. Larger kaolin contents yielded to stiffer composites, as determined from uniaxial compression measurements. The compressive strength increases with the AA concentration, up to AA concentrations > 25 mass% (relative to kaolin). The equilibrium swelling ratio of the fully-cured kaolin/poly(acrylic acid) composites in water increased as expected with increasing AA content. These particular composite systems formed porous kaolin ceramics with porosity > 60% after burning off the organic materials.

This paper describes a Fourier series based solution method for the displacements and sub-surface stresses within a graded elastic layered solid under pressure. The solid is assumed to be in a state of plane strain and thus the derived solution is valid for two-dimensional problems. Whilst this method provides a fully analytic solution when the contact pressure is known exactly, it may also be used when the contact pressure is only known numerically (see Section 4). The solution given in this paper is generic and easily utilised to solve real problems as it requires only known physical characteristics of the solid under study and an applied surface pressure.The solid consists of two distinct regions which are considered to be perfectly bonded. These comprise a graded elastic coating whose shear modulus varies exponentially with the depth coordinate and a homogeneously elastic substrate. As the stresses and displacements induced by the applied pressure decay very quickly outside of the contact region, the contact problem need only be solved in a small piece of the solid as the remainder is unaffected. It is found that accurate results are obtained when the contact problem is solved over a region of the solid 10 times larger than the contact region. This method as a result is computationally cheap to use as the number of Fourier modes needed to accurately capture the solution is small.

The study investigates the mechanism of ring-liner lubrication in the vicinity of the top and bottom dead centres of an internal combustion engine. Predicting lubricant transient behaviour is critical when the inlet reversal leads to thin films and inherent metal-to-metal interaction. It was found that the cavitation, which is located at the trailing edge of the contact before reversal, briefly survives after reversal as a confined bubble at the leading edge. This depletes the film promoting starvation. Several algorithms were compared. It is concluded that the lubricant film is thinner than initially thought.

Hydrogels from diepoxy-terminated poly(ethylene glycol)s (PEG) of approximate molecular weights 600, 1,000, 2,000, and 4,000 Da and aliphatic polyamines with different numbers of amine hydrogen atoms (ethylene diamine, diethylenetriamine, triethylenetetramine, tris(2-aminoethyl)amine) in various combinations were synthesized and several means to adjust their water absorption were investigated. It was shown that the equilibrium swelling degree (ESD) can be tuned by varying the cross-linking agent functionality and molecular weight (MW) of the PEG precursor, by either employing single components or mixtures of polyamines or PEG oligomers. The ESD was also influenced by the reaction time and concentration of oligomer in feed. Further investigation of the swelling characteristics and water diffusion in the hydrogels synthesized revealed that the semicrystalline character of the xerogels, influenced mainly by the functionality of the cross-linking agent, PEG molecular weight, and degree of perfection of the ne...

The hybrids 2-hydroxyethyl methacrylate – chlorometylstyrene – modified montmorillonite were synthesized by means of “in-situ” radical (co)polymerization. The composites were analysed by thermal analysis (DSC-TGA-MS), Fourier-Transform Infrared Spectroscopy (FT-IR), nuclear magnetic resonance (1H-NMR), X-ray diffraction, and scanning electron microscopy (SEM). The influence of the polymerization conditions and compositions on the thermal stability and morphology of the obtained composites is discussed.

New hydrogels were prepared from diepoxy-terminated poly(ethylene glycol)s of approximate molecular weights 600, 1000, 2000, and 4000 Da and aliphatic primary diamines with different numbers of carbon atoms (ethylenediamine, 1,4-diaminobutane, hexamethylenediamine, 1,8-octanediamine, 1,10-decanediamine, 1,12-dodecanediamine), in water or ethanol–water mixture, depending on the amine solubility. The swelling behavior of these gels was tested in distilled water/aqueous solution at constant temperature and the equilibrium swelling degree (ESD) was determined for structurally different hydrogels and under various environmental conditions. It was shown that ESD was influenced by the molecular weight of PEG oligomers, amine/epoxy groups mole ratio, amine chain length, temperature, pH, and concentration of salts present in the swelling medium. Higher ESDs were obtained for either longer-chain PEGs, non-stoichiometric amine/epoxy groups ratio, shorter amines, acidic pH, lower temperatures, or in the absence of salts. Copyright © 2009 John Wiley & Sons, Ltd.

Isothermal and thermoelastohydrodynamic lubrication (TEHL) analyses of grease lubricated bearings are presented. A grease plug flow is formed in the conjunction that, with no shear at the boundaries with the solid surfaces, adheres to them in the region of high pressures under isothermal conditions. The elastohydrodynamic lubrication grease pressure distribution conforms fairly closely to that of its base oil alone, with the exception of inlet trail and pressure spike regions. The dependency of film thickness on speed (rolling viscosity) and load parameters for the base oil agrees with previously reported findings of the research community. For grease there are subtle differences with the base oil film thickness load and speed dependencies. However, it is clear that extrapolated oil film thickness formulae for oils can be used reasonably for the prediction of grease films, at least as a first approximation. The results presented agree well with optical interferometric measurements reported in the literature for grease-lubricated contacts at low temperatures and low surface velocities. TEHL analysis shows breakdown of the plug flow and significant reduction in film thickness, which can lead to changes in the regime of lubrication to mixed or boundary conditions.

The paper presents an iterative method for obtaining footprint, pressure distribution, local deformation and sub-surface stress field for the contact between a rigid cylindrical indenter and an elastic flat substrate. The methodology is applicable for semi-infinite, as well as for thin or thick bonded elastic layered solids with high or low elastic moduli. All findings are in accord with the observed behaviour of hard wear resistant and soft solid lubricating coatings. It is shown that the decomposed contact pressure distribution into a series of harmonic waves induces sub-surface stress fields that decay into the depth of the solid according to their wavelengths. Consequently, conditions vis-a-vis fatigue spalling and adhesion performance may be predicted for given thickness of layered bonded elastic solids.

In nanoscale conjunctions, molecular interactions cannot be ignored, nor surface energy effects, adhesion of surface asperities, or their stiction by any wetting action of an intervening fluidic media. Many experimental micro-electromechanical systems (MEMS) are prone to failure or malfunction due to structural degradation (wear and damage) of their load-bearing and power-transmitting conjunctions. Remedial solutions have been attempted by surface treatments, such as the introduction of self-assembled monolayers (SAMs) that adhere to the surfaces, and which are hydrophobic in nature. The physics of many of the contributing phenomena, such as adhesion, meniscus action, or electrostatics, are reasonably well understood, but their interactions under small-scale impacting condition is less investigated. The problem has a transient nature, owing to inertial dynamics, surface topography, attractive surface energy of asperities, and formation of any condensates. The interplay between these kinetics, using the established models, can be quite complex, particularly with regard to prediction of contact area and pull-off force in rebound of impacting pairs. The behaviour can vary from near classical Hertzian to that dominated by adhesion. The repercussions can be very significant in power transmission or load bearing for these small devices. This paper attempts to contribute to the growing understanding of contact behaviour in smallscale.

According the American Cancer Society’s data, in 2013, an estimated 53,640 people developed head and neck cancers [1], which accounts for about 3% to 5% of all cancers in the United States. Removing head and neck malignant neoplasms is one of the first stages towards patient recovery. However, these types of invasive procedures often lead to disfiguring scars and resections with functional and aesthetical drawbacks (see Figure 1).Copyright © 2015 by ASME

Novel injectable thermoreversible hydrogel compositions with semi-interpenetrating network structure were prepared through the addition of sodium alginate (SA) to poly(N-isopropylacrylamide) (PNIPAM) aqueous solutions. The addition of the hydrophilic alginate strongly improved the stability against syneresis of the 15 wt% PNIPAM hydrogels formed at 37°C from less than 15 min in the absence of alginate to more than 7 days in the presence of 4 wt% SA. Besides the SA concentration, the hydrogel stability depended on the molecular weight and polydispersity of PNIPAM, being lower when a high molecular weight fraction was present. The phase transition temperature (Tph) of the PNIPAM aqueous solutions decreased with alginate concentration, while the dynamic viscosity and elastic modulus of the hydrogels increased. By decreasing the PNIPAM molecular weight and polydispersity, the dynamic viscosity and elastic modulus of the PNIPAM–alginate hydrogels formed above Tph diminished, while their viscoelastic behavior changed from predominantly elastic to predominantly viscous.

This paper investigates the physics of ultra-thin surface films in nano-scale counterformal conjunctions. A novel approach is proposed in which the transient behaviour of the fluid film is integrated with the contact mechanics of the approaching bodies. The method predicts film thickness and pressure distribution, as well as local elastic deflection and resulting sub-surface stress tensor. It is found that inertial dynamics of bodies, as determined by local squeeze film effect, prevents diminution of film below certain thickness and reduces the solvation effect. This approach has direct applications for micro-scale mechanisms, where prediction of thin surface adhered film thickness is required for tribological purposes, as well as structural in-service integrity of contacts of vanishing dimensions.

Poly(methyl methacrylate)-block-polysulfide-block-poly(methyl methacrylate) copolymers were synthesized for the first time through a new method involving the free radical polymerization of MMA in the presence of a thiocol oligomer as a chain transfer agent, followed by chemical oxidation of the remaining SH end-groups. The chain transfer constant of the SH end-groups of the thiocol was estimated from the rate of consumption of the thiol groups versus the rate of consumption of the monomer (CT=0.67). The triblock copolymers synthesized were characterized by SEC and 1H NMR measurements.

The paper investigates the tribological contact conditions between a polynomial automotive cam and a flat follower. A multi-body model of the system is presented to take into account the complex tappet motions. A set of such models is presented in order to predict the cam-tappet friction force, the tappet-tappet bore friction force and the resulting tappet spin. The tribological conditions giving rise to these kinetic actions include the elastohydrodynamic regime of lubrication in the concentrated cam-follower contact and a mix of hydrodynamic and boundary friction action in the tappet-tappet bore contact. While boundary friction dominates the tappet-tappet bore friction at low engine speeds, for high engine speeds the boundary friction dominates the closing event of the cam, and hydrodynamic and mixed lubrication dominate the opening event of the cam. The simplified friction models are experimentally validated on a single-cylinder diesel engine. The measuring techniques are based on a redesigned engine tappet equipped with four strain gauge bridges.

Poly(ethylene oxide)-block-polystyrene (PEO–PSt) block copolymers were prepared by radical polymerization of styrene in the presence of iodoacetate—terminated PEO (PEO-I) as a macromolecular chain-transfer agent. PEO-I was synthesized by successively converting the OH end-group of α-methoxy ω-hydroxy PEO to chloroacetate and then to the iodoacetate. The chain-transfer constant of PEO-I was estimated from the rate of consumption of the transfer agent versus the rate of consumption of the monomer (Ctr, PEO-I = 0.23). Due to the involvement of degenerative transfer, styrene polymerization in the presence of PEO-I displayed some of the characteristics of a controlled/‘living’ process, namely an increase in the molecular weight and decrease of polydispersity with monomer conversion. However, because of the slow consumption of PEO-I due to its low chain-transfer constant, this process was not a fully controlled one, as indicated by the polydispersity being higher than in a controlled polymerization process (1.65 versus < 1.5). The formation of PEO–PSt block copolymers was confirmed by the use of size-exclusion chromatography and 1H NMR spectroscopy. Copyright © 2004 Society of Chemical Industry

Three diblock copolymers of poly[ N -(2-hydroxypropyl)methacrylamide] (poly(HPMA)) and poly( n -butyl acrylate) (poly(BA)) with varying lengths of blocks were prepared by atom transfer radical polymerization. All copolymers were found to be soluble in dimethylformamide (DMF) and poorly soluble or insoluble in water. In water and mixed DMF/H 2 O solvents, the copolymers were dispersed in micellar form by controlled addition of water to DMF solutions of copolymers under continuous intensive stirring. The micellar solutions in water were prepared by dialysis of solutions in DMF/H 2 O (95 vol% of H 2 O) against water. Solution properties of diblock copolymers of poly(HPMA) and poly(BA) were studied using static and dynamic laser light scattering to characterize the behavior of the copolymers at the supramolecular level. The effects of preparation mode, organic solvent (DMF) and copolymer chemical composition on the formation of micelles were studied. While a slower mixing procedure was optimal for copolymers with short poly(HPMA) blocks, a faster mixing was more suitable for copolymers having longer poly(HPMA) blocks. Finally, the dimensions of micelles in water were evaluated. The most compact micelles were prepared from copolymers having short hydrophilic poly(HPMA) blocks. On the other hand, the copolymer with the longest poly(HPMA) block formed micelles with the smallest size and the lowest density.

Polyurea nanocomposites represent a promising option in the development of advanced materials for applications that require high mechanical resistance. This article describes an optimized synthetic route for obtaining polyurea nanocomposites with enhanced mechanical properties by employing epoxy-functionalized multiwalled carbon nanotubes (MWCNTs) as reinforcing agent. The experimental measurements revealed that these functionalized nanofillers have a positive effect on the properties of the composite only until they reach a certain concentration; therefore, the optimal composition was reported (the samples containing 0.2 weight % functionalized MWCNTs). The functionalization of the MWCNTs was confirmed through RAMAN, X-ray photoelectron spectroscopy, scanning electron microscopy and thermogravimetric analysis, while the polyurea nanocomposites obtained have been characterized by thermal (differential scanning calorimetry and TGA) and mechanical (dynamic mechanical analysis and tensile tests) analyses. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 45061.

This paper proposes a hybrid (semi-analytic) solution for determining the contact footprint and subsurface stress field in a two-dimensional adhesive problem involving a multi-layered elastic solid loaded normally by a rigid indenter. The subsurface stress field is determined using a semi-analytic solution and the footprint using a fast converging iterative algorithm. The solid to be indented consists of a graded elasticity coating with exponential increase of decay of its shear modulus bonded on a homogeneously elastic substrate. By applying the Fourier Transform to the governing boundary value problem, we formulate expressions for the stresses and displacements induced by the application of line forces acting both normally and tangentially at the origin. The superposition principle is then used to generalize these expressions to the case of distributed normal pressure acting on the solid surface. A pair of coupled integral equations are further derived for the parabolic stamp problem which are easily solved using collocation methods.Copyright © 2014 by ASME

Recent advances in polymer technology together with the growing need of smaller and lighter electronic components and biomedical equipment led to the development of new applications of polymers at micro scale. However, unlike traditional materials (e.g., metal silicon), polymers exhibit a significant time dependency in their response to load (e.g., viscoelastic, hyperelastic). Therefore, predicting the behavior of such polymer components at small scale requires accurate simulations of the effects of creep and relaxation within the systems. The current study uses a mesh-free particle method (smoothed particle hydrodynamics) to predict time-dependent mechanical response of polymers. As a first step towards investigating the response of a polymer microstructure under load, we simulate the behavior of a slender polymer rod compressed and tangentially dragged against a smooth glass surface. It is shown that although accurate prediction of polymer deformation cannot be achieved with a fully analytic model, a simplified generalized Kelvin model could calibrated to capture most of the characteristics of the fully numerical model. This cold be used for predicting the behavior under load of a passive subsystem of imbedded in a control algorithm to extend the measuring domain of a possible sensor or prevent potentially dangerous operating conditions.

Ternary diagrams for the methyl methacrylate (MMA) (methanol (MeOH)/water and tetraethoxysilane (TEOS)/MeOH/water systems were studied. The composition domains, where microemulsions form owing to the nanostructures developed by the MeOH–water system, were determined. Refractometric and conductometric studies showed that water-in-oil (w/o), bicontinuous or oil-in-water )(o/w) microemulsions may form. Hybrid materials were obtained by the sol–gel reaction of TEOS combined with the free-radical copolymerization of MMA and methacryloxypropyltrimethoxysilane (MPS). MPS was added in order to avoid phase separation. The change of the glass transition temperature and of the thermal stability of the organic network in the presence of the inorganic one, and the similar forming mechanisms and rates proved the formation of simultaneous interpenetrating polymer-inorganic networks.

The nanofibrillar array of a Gecko inspired Synthetic Adhesive (GSA) adheres to a surfaces when fibers undergo deformations of both the stems and the tip. The GSA’s show interesting changes in effectiveness dependent on use patterns, preloads, and material types, amongst other parameters. The polymers fibers also, display plastic creep even at relatively low strain rates and stresses below plastic yield. Therefore, a suitable numerical solution, which predicts the fiber geometry, must consider not only the initial shape of the fiber, but also the fiber progressive deformation (local and global) and the influence this has on the local mechanical properties (elastic, viscoelastic, strain hardening/softening and plastic flow). The localized mechanical properties are difficult to calculate using traditional methods because of the nonlinearities associated with viscoelastic effects, the large deformations, and the variable boundary conditions. However, the variable boundary conditions make a mesh free modeling method ideal. Smooth Particle Hydrodynamics (SPH) is one of the most prominent mesh free Lagrange method, which takes a set particle and uses particle kinematics, density gradients, and material properties to determine the interaction between particles. As a first step towards modeling the behavior of a fibrillar adhesive surface, this paper focuses on the modeling of a single polymer fiber. The single micro fiber will be subjected to similar conditions to what it would see as part of an array. This will allow the SPH method of simulation to be critiqued for its further use in simulating polymer microfiber. While the localized mechanical properties of the polymer, which depend on viscoelastic effect and other nonlinear phenomena, are difficult to determine analytically. The modeling technique can be compared to standard analytical methods for global parameters. It was found that the SPH method was able to appropriately model the effect of various scenarios on the mechanical deformation and resonance of a polymer microfiber. Further more the friction force for the fiber on glass was calculated as were the localized fiber velocities and stresses.

The current paper proposes a semi-analytical solution method which predicts both the sub-surface stress fields and pressures that result from adhesive contact between a multi-layered functionally graded material (FGM) and a rigid punch. The FGM is deemed to consist of a homogeneously elastic coating and substrate joined together by a transition layer whose shear modulus depends on the depth coordinate in some predetermined way. It is further assumed that the FGM is in a state of plane strain so that we may perform a two-dimensional analysis. We present analytical solutions for the horizontal and vertical displacements within the solid which hold when the applied surface pressure is known and then discuss how we may use these solutions to solve the contact problem. A selection of numerical results are then presented using this model and it is found that the algorithm proposed in this work is both accurate and computationally cheap to use.

This work is concerned with the derivation of an iterative solver which allows the accurate estimation of both the contact half-width and contact pressure when an inhomogeneouly elastic solid comprising a homogeneous coating and substrate joined by a graded layer is indented by a rigid punch. A selection of numerical results are then presented illustrating the accuracy of this model.Copyright © 2012 by ASME

Unless protected by an inert gas atmosphere, micro-scale conjunctions are often separated by molecularly-thin adhered films. Therefore, predicting contact load, friction or adhesion, must consider the contribution of this layer to the overall contact problem. The contribution of an adhered layer can be accounted for using a simplified solution (e.g. an adjustment to the energy of adhesion to account for the liquid film). However, these methods cannot account for layers consisting of multiple species of molecules. The most common approach, which accounts for inter-molecular forces between molecules of various species, is a molecular dynamics simulation. However, this is time consuming, and therefore, often limited for small volumes of fluid and small scale contacts. The current paper proposes an alternative approach, where the pressure and shear between two smooth surfaces separated by an ultra-thin film is predicted using a statistical mechanics based model. This method accounts for the chemical structure of each species of molecules comprising the ultra-thin film, their concentration, intermolecular forces and adsorption to the wall. This approach is very fast, therefore, it can be easily included in a larger scale code predicting the behavior of the entire micro-scale mechanism. It was found that for a specified material of the solid boundary the model can predict the optimal concentration of each species of molecule in the intervening ultra-thin film, to minimize friction or adhesion.Copyright © 2012 by ASME

Crosslinked hydrogels with well-defined chemical structures and characteristics were prepared through the reaction between diepoxy-terminated poly(ethylene glycol)s of various molecular weights and aliphatic polyamines of different hydrocarbon chain length and functionalities, and the influence of some network parameters (molecular weight between crosslinking points, crosslinking degree, hydrophobic character) upon the absorption and release of drugs of different capacity to interact with the polymer chains was comparatively investigated. Diclofenac sodium (DCFNa) and 5-fluorouracil (5FU) were used as model drugs, based on their dissimilar hydrophobic character and ability of DCFNa to form crown ether-like complexes with PEG chains through the sodium cation. The experiments showed that the most important interactions occurring in these systems were mainly the hydrophobic ones and to a lesser extent the complexation of the Na(+) ion by the PEG chains. Both of them were in favor of DCFNa, resulting in a larger incorporation and a slower release of this one in comparison with 5FU. For both drugs, loading was larger for hydrogels with shorter PEG chains and/or crosslinked with amines with longer hydrocarbon chain or higher functionality. Drug release tests showed a lower rate for stronger drug-network interactions in agreement with the absorption experiments.