All issues / Volume 12 (2018) / Issue 11 (November)
This is an editorial article. It has no abstract.
Electrorheological fluids based on polydimethylsiloxane filled with halloysite nanotubes were studied. The filler structure was characterized by TEM, SEM, and X-ray diffraction. When an electric field is applied to suspensions, their rheological behavior changes – the contribution of the elastic component becomes significant and samples behave like a solid body. The effect of the electric field and filler concentration on the electrorheological behavior was investigated. The influence of water content on the filler structure, as well as on electrorheological and electrophysical properties of suspensions, was considered. Electrorheological fluids filled by halloysite with small water content exhibit slightly higher rheological characteristics under an electric field than dried ones. This study shows the prospects of using halloysite nanotubes as a dispersed phase for electrorheological fluids.
A set of commercial (meth)acrylic resins was photopolymerized under identical irradiation conditions and evolution of their refractive index was monitored as a function of double bond conversion. Initial refractive index values ranged from 1.4445 to 1.5454 and then linearly increased with conversion as long as the material was not in the glassy state. This increase was related to an increase of the material density arising during polymerization. Final refractive index values ranged from 1.4804 to 1.5632. The knowledge of the refractive index and of its evolution during the photocuring is indispensable, in particularly to elaborate composite materials (polymer matrix + filler) with well controlled optical properties.
A study is presented on the plasma treatment of two different ultra-high molecular weight polyethylene (UHMWPE) and polypropylene (PP) surfaces by using diffuse coplanar surface barrier discharges under atmospheric air conditions. The plasma-treated polymer surfaces are characterized in terms of wettability, surface chemistry, topography, adhesion and tribology. It is found that plasma treatment significantly improves wettability through formation of various oxygen containing functionalities at the surface. The plasma treatment consequently improves adhesion for either polymer/polymer or polymer/steel joints. Under dry sliding conditions the coefficient of friction slightly increases after plasma treatment in parallel with the higher adhesion. In contrast, the coefficient of friction significantly decreases under oil lubrication conditions and it remains low even after cleaning the oil likely due to the improved oil retention capability of the plasma-treated surface. The observed tendencies in adhesion and friction are further analyzed in terms of surface chemistry after plasma treatment and after sliding by use of spectroscopic methods and chemical imaging.
Geraniol was successfully micro-encapsulated with zein in a one-step phase separation process in which the zein and geraniol were both initially dissolved in a water-ethanol mixture. Microcapsules were only formed at geraniol-to-zein mass ratios above unity. This is ascribed to the relative solubility of the oil and protein in the solvent at the point of incipient phase separation: The geraniol must phase separate first before zein precipitation commences. The resultant oil droplets subsequently provide the interfacial surfaces that act as nucleation sites for the precipitation of the amphoteric zein molecules on continued dilution with water. The near spherical microcapsules formed under these conditions, comprised a foamed closed-cell wall that enclose large internal cavities in which the geraniol was trapped. The highest particle yield, 85.3%, and oil encapsulation efficiency, 83.5%, were obtained at a geraniol to zein mass ratio of 3:1. The particle size distribution for this system was log-normal with d10, d50 and d90 values of 10, 32 and 88 μm respectively. The temperature dependent geraniol release rate from these capsules followed power law kinetics with a release exponent of 0.80.
Nanocomposite hydrogels (NC gels) with polymer–clay network assemblies are useful in a number of applications because of their unique properties and characteristics. Herein, we describe a distinctive strategy for the preparation of discrete monometallic (Ag, Au, and Pd) and bimetallic (Pt-Pd, Au-Pd) nanoparticles that uses a nanocomposite hydrogel composed of a polymer–clay network. Thermoresponsive NC gels were synthesized by the in-situ free-radical polymerization of N-isopropylacrylamide in the presence of clay (synthetic hectorite) nanosheets (CNSs). Since CNSs have strong affinities for metals ions that facilitate the concentration of metal precursors around them, the reduction of metal ions by ascorbic acid in NC gels provides well-dispersed, non-aggregated spherical monometallic and bimetallic nanoparticles (NPs) that are strongly immobilized within the polymer-clay network. The resulting hybrid NP-NC gels, which contain monometallic or bimetallic NPs, exhibit high catalytic activities for the hydrogenation of nitrophenol to aminophenol. The combination of well-defined metal NPs and mechanically tough NC gels opens up new possibilities for the design of environmentally friendly and sustainable functional NP-NC-gel materials.
Smart polymer gels of microscale size are attracting a lot of attention since their properties, such as fast response to various external stimuli, make them suitable candidates for many potential applications including drug delivery, nanoreactors, or separation techniques. In this research, thermosensitive microgels of poly(N-isopropylacrylamide), poly(N-vinylcaprolactam), poly(N-isopropylacrylamide-co-sodium vinylsulfonate) and poly(N-vinylcaprolactam-co-1-vinylimidazole) were prepared via aqueous free-radical precipitation polymerization. The thermo-induced collapse of the microgels and subchain mobility were thoroughly investigated by means of proton nuclear magnetic resonance (1H NMR) spectroscopy, as well as scanning electron microscopy and dynamic light scattering. It was found that in copolymer microgels a part of the thermosensitive monomer units does not collapse upon heating, and the critical temperature is not affected strongly by the addition of ionogenic groups. That could be explained by non-uniform monomer unit distribution, leading to the appearance of thermosensitive core-charged corona structure of the microgel particle. Сore-corona architecture of microgels results in film formation with the ordering of the self-assembly structure.
Polylactic acid (PLA)/starch and PLA/starch-glycerol composites with different glycerol contents were prepared in a wide composition range, in order to study their applicability as packaging materials for dry products. Water uptake was determined at a temperature of 23 °C and different relative humidities. Structure and mechanical properties were also investigated. PLA/ unplasticized starch composites could absorb a considerable amount of water. As a result, they may be adequate as a biodegradable inner container in dry packaging. The absorption capacity of the composites increased significantly with increasing starch content and relative humidity, respectively. Unplasticized starch exhibited not only a desiccant but also a reinforcing effect in PLA, thus both stiffness and strength increased with increasing starch loading. The nfluence of glycerol content on the water uptake was difficult to reveal due to the migration of glycerol from the bulk to the surface. Furthermore, glycerol weakened the PLA/starch adhesion and softened starch particles.