Recently, some arguments were published that cast doubt on the validity of the Gibbs adsorption isotherm. The doubt was on whether the often visible linearly declining part in the surface tension ...versus logarithm of concentration plot of a surfactant solution, just before the critical micelle concentration, really represents a situation of constant adsorption. Those published arguments are partly of a conceptual nature and partly based on experimental evidence. The conceptual arguments appear to be based on a misunderstanding of the theory, while the arguments based on experimental evidence stem from an inaccurate treatment of these data. Our conclusion is that none of the relevant arguments put forward are valid. The experimental evidence, if properly treated, is in line with the Gibbs theory.
A liquid cross-linker, epoxidized linseed oil (ELO), was encapsulated in a plastic with a high glass transition temperature (poly(N-vinyl-2-pyrrolidone); PVP). The process parameters of the ...spray-drying employed were optimized by a Design-of-Experiments (DoE) approach. Three factors concerning both the dispersion feed (total concentration of additive and core to encapsulant ratio) and the spray-drying processes (spray flow of the spray-drier) were chosen. A full 23 factorial Design of Experiments with center point was carried out in duplicate. The aim of the Design of Experiments was to understand and to optimize the encapsulation process in terms of total amount of epoxidized linseed oil in the powder (payload) and amount of epoxidized linseed oil enclosed in the polyvinylpyrrolidone (encapsulation efficiency). The results enabled the production of a free-flowing spray-dried powder of ELO encapsulated in PVP with an encapsulation efficiency of ∼85%.
A range of copolymers of methacrylic acid (MAA) macromonomers prepared by cobalt-catalyzed chain transfer and methyl methacrylate (MMA) and/or butyl acrylate (BA) was synthesized and used as a ...stabilizer in the emulsion polymerization of MMA. Although clear differences were observed in polymerization rates using the different MAA x -MMA y stabilizers, these differences were not as clearly reflected in the particle sizes nor in the rates per particle. However, a clear difference between these systems and those stabilized by MAA x -BA y was observed. The latter systems were all characterized by much smaller particle sizes and corresponding higher rates of polymerization. In addition, the molar masses in the latter systems were all significantly larger than those obtained in the MAA x -MMA y stabilized system, in which the stabilizers act as “sulfur-free” RAFT agents. Interestingly, the prepared latexes showed a range of appearances varying from “milky” to “gel-like” depending on the used stabilizer. The MAA x -BA y stabilized latexes had, in general, a lower viscosity and a significantly smaller (if any) yield stress than the MAA x -MMA y stabilized latexes, and in the latter case the rheological behavior was found to depend on the block lengths in and concentration of the stabilizer.
The mechanical failure and internal strain when bending a brittle thin film on a flexible substrate was analyzed using an electro-mechanical method and optical microscopy. Bending was realized by ...clamping the ends of the sample and rotating the clamps in a two-point rotation (2PR) device, as elaborated in the paper. This test allows to impose both tensile and compressive strains on the brittle layer, in a single test, without remounting the sample. With the electro-mechanical method the real-time electrical resistance is measured in a very thin, conductive coating deposited on the brittle layer in which cracks develop across the width of the film. Simultaneously, the propagation of individual cracks is observed by optical microscopy. Real-time combination of both analyses in principle enables linking the fracture behavior (size, number and pattern of cracks) with the electrical resistance at a well-defined imposed strain.
With this 2PR method, the subcritical mechanical failure of a 150nm thick SiNx barrier layer was analyzed, as deposited on a 125μm thick polyethylene naphtalate sheet (system 1). Its characteristic static failure time, as obtained from a Weibull analysis, was 0.10s at a strain of 0.84% and 2.2s at 0.77%. From the crack opening on reloading tests, an average internal compressive strain of 0.38% was deduced. In order to validate the results of the 2PR device, system 1 was also tested using two-plate bending (2PB), as described earlier. Results on characteristic failure strains as obtained with both methods for bending match very well and have similar accuracy. However, the option to assess tensile internal strains, together with the aforementioned advantages, renders the 2PR test the method of choice for testing brittle layers on polymer substrates. Comparison of system 1 with another barrier system (2), which only differs in processing conditions, indicates a considerable difference in time dependence of the characteristic failure strain, indicating the sensitivity of the resulting material properties on processing conditions.
•A 2-point Rotation (2PR) device was built to assess failure in thin brittle films.•Electrical measurements were performed using the 2PR and 2-point Bending (2PB) method.•The characteristic failure strain was determined by these two methods.•The 2PR test yields the same accuracy as the conventional 2PB.•Both tensile and compressive internal strain can be determined using the 2PR method.
The influence of the conditions of artificial degradation experiments on the photodegradation process of polyester–urethane clearcoats has been studied by comparing three types of exposure ...experiments with different conditions regarding the spectral power distribution (SPD), the exposure atmosphere (aerobic and anaerobic) and the presence of water. The presence of short wavelengths (λ < 295 nm) in the SPD largely influences the depth-inhomogeneity of degradation with respect to the optical properties and the chemical composition of the coating. The availability of oxygen in the exposure atmosphere determines the degradation pathway that is followed, such as to what extent the photo-oxidative breakage of urethane bonds or the formation of yellow chromophores due to aromatic crosslinking reactions occurs. Indentations at the surfaces of virgin and degraded coatings showed an increase in Young's modulus and hardness when degraded under aerobic conditions, while degradation under anaerobic conditions did not lead to significant changes. The presence of water is responsible for increasing the surface roughness of the coating during degradation, which directly influences the coating's gloss retention. Several time-independent correlations between the changes in chemical, optical and mechanical properties of coatings resulting from different exposure experiments have been established.
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•Highly lubricious polymer coatings with minimal bulk swelling.•Systematic study on network structure parameter/friction property relations.•Large increase in lubricating properties ...by addition of hydrophilic precursors.•Extreme synergistic effect between hydrophobic and hydrophilic precursors.
Bio-lubricated surfaces found in nature have inspired the design of low friction polymer coatings for biomedical applications. This work presents a systematic study of the relation between the network structure parameters and the macroscopic friction properties of highly lubricious dual hydrophobic/hydrophilic polyurethane (PU) coatings in an aqueous environment. Chemically cross-linked PU coatings were prepared by adding poly(ethylene glycol) mono-methyl ether (mPEG) as hydrophilic dangling chains, or poly(ethylene glycol) (PEG)-diol as hydrophilic elastically active network chains, to poly(propylene glycol) (PPG)-PU coating formulations. The friction behaviour of the water swollen coatings was measured using a custom-made water immersed tribology setup. Addition of the PEG segments or mPEG dangling chains to hydrophobic PPG coatings greatly enhances the lubricious properties of the coatings. These dual hydrophobic/hydrophilic diol PU network exhibit a surface with a lower coefficient of friction compared to reference coatings from either individual precursors, demonstrating a large synergistic effect between the hydrophobic PPG and the hydrophilic PEG in the coatings. Based on network structure and surface chain considerations it is hypothesized that the existence of a thin and softer hydrated surface layer on top of a less hydrated, more rigid, coating bulk layer gives rise to the observed enhanced lubricious properties, hereby mimicking to some extent bio-lubricated systems, such as cartilage.
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