Polymers2014, 6(12), 2896-2911; doi:10.3390/polym6122896 - published 26 November 2014 Show/Hide Abstract
Abstract: Silica in industrial production processes is precipitated by mixing an acid and an inorganic precursor. In this aqueous solution, silica particles form due to a polymerization reaction and agglomeration and, finally, build a gel. Thereafter, the reaction continues, and the gel network shrinks with the expulsion of the enclosed pore liquid. This slow process is known as “natural syneresis” and strongly affects the product properties, such as the agglomerate size, specific surface or porosity of the silica produced. In order to investigate the influence of process parameters, such as temperature, pH or ionic strength, on the shrinkage in shorter time-scales, we propose an acceleration of this process and define it as “enforced syneresis”. The acceleration is performed by applying a mechanical external force to the gel by means of a plunger and measuring the shrinkage behavior under these conditions. Thereby, the conceptual idea is the prediction of the shrinkage due to natural syneresis based on the results of enforced syneresis. We are now able to predict the natural syneresis behavior from enforced syneresis data by the development of a correlative model. Using this prediction model, we can show the influence of temperature on the maximum shrinkage and on its rate in a significantly shorter time of about 12 h instead of several days.
Polymers2014, 6(11), 2875-2895; doi:10.3390/polym6112875 - published 20 November 2014 Show/Hide Abstract
Abstract: Thermally reduced graphite oxide (TRGO), containing only four single carbon layers on average, was combined with ammonium polyphosphate (APP) and magnesium hydroxide (MH), respectively, in polypropylene (PP). The nanoparticle’s influence on different flame-retarding systems and possible synergisms in pyrolysis, reaction to small flame, fire behavior and mechanical properties were determined. TRGO has a positive effect on the yield stress, which is decreased by both flame-retardants and acts as a synergist with regard to Young’s modulus. The applicability and effects of TRGO as an adjuvant in combination with conventional flame-retardants depends strongly on the particular flame-retardancy mechanism. In the intumescent system, even small concentrations of TRGO change the viscosity of the pyrolysing melt crucially. In case of oxygen index (OI) and UL 94 test, the addition of increasing amounts of TRGO to PP/APP had a negative impact on the oxygen index and the UL 94 classification. Nevertheless, systems with only low amounts (≤1 wt%) of TRGO achieved V-0 classification in the UL 94 test and high oxygen indices (>31 vol%). TRGO strengthens the residue structure of MH and therefore functions as a strong synergist in terms of OI and UL 94 classification (from HB to V-0).
Polymers2014, 6(11), 2862-2874; doi:10.3390/polym6112862 - published 14 November 2014 Show/Hide Abstract
Abstract: Photochemically mediated reversible-deactivation radical polymerization of methyl methacrylate was successfully performed using 50–400 ppm of various copper compounds such as CuSO4·5H2O, copper acetate, copper triflate and copper acetylacetonate as catalysts. The copper catalysts were reduced in situ by irradiation at wavelengths of 366–546 nm, without using any additional reducing agent. Bromopropionitrile was used as an initiator. The effects of various solvents and the concentration and structure of ligands were investigated. Well-defined polymers were obtained when at least 100 or 200 ppm of any catalyst complexed with excess tris(2-pyridylmethyl)amine as a ligand was used in dimethyl sulfoxide as a solvent.
Polymers2014, 6(11), 2845-2861; doi:10.3390/polym6112845 - published 14 November 2014 Show/Hide Abstract
Abstract: The generation of nano-microstructured surfaces is a current challenge in polymer science. The fabrication of such surfaces has been accomplished mainly following two different alternatives i.e., by adapting techniques, such as molding (embossing) or nano/microimprinting, or by developing novel techniques including laser ablation, soft lithography or laser scanning. Surface instabilities have been recently highlighted as a promising alternative to induce surface features. In particular, wrinkles have been extensively explored for this purpose. Herein, we describe the preparation of wrinkled interfaces by confining a photosensitive monomeric mixture composed of monofunctional monomer and a crosslinking agent within a substrate and a cover. The wrinkle characteristics can be controlled by the monomer mixture and the experimental conditions employed for the photopolymerization. More interestingly, incorporation within the material of a functional copolymer allowed us to vary the surface chemical composition while maintaining the surface structure. For that purpose we incorporated either a fluorinated copolymer that enhanced the surface hydrophobicity of the wrinkled interface or an acrylic acid containing copolymer that increased the hydrophilicity of the wrinkled surface. Finally, the role of the hydrophobicity on the bacterial surface adhesion will be tested by using Staphylococcus aureus.
Polymers2014, 6(11), 2832-2844; doi:10.3390/polym6112832 - published 13 November 2014 Show/Hide Abstract
Abstract: Water-based polymer nanoparticle dispersions (solar paint) offer the prospect of addressing two of the main challenges associated with printing large area organic photovoltaic devices; namely, how to control the nanoscale architecture of the active layer and eliminate the need for hazardous organic solvents during device fabrication. In this paper, we review progress in the field of nanoparticulate organic photovoltaic (NPOPV) devices and future prospects for large-scale manufacturing of solar cells based on this technology.
Polymers2014, 6(11), 2819-2831; doi:10.3390/polym6112819 - published 12 November 2014 Show/Hide Abstract
Abstract: Magnesium phosphate cement (MPC) has been proven to be a very good repair material for deteriorated concrete structures. It has excellent adhesion performance, leading to high bonding strength with old concrete substrates. This paper presents an experimental study into the properties of MPC binder as the matrix of carbon fiber sheets to form fiber-reinforced inorganic polymer (FRIP) composites. The physical and mechanical performance of the fresh mixed and the hardened MPC paste, the bond strength of carbon fiber sheets in the MPC matrix, the tensile strength of the carbon FRIP composites and the microstructure of the MPC matrix and fiber-reinforced MPC composites were investigated. The test results showed that the improved MPC binder is well suited for developing FRIP composites, which can be a promising alternative to externally-bonded fiber-reinforced polymer (FRP) composites for the strengthening of concrete structures. Through the present study, an in-depth understanding of the behavior of fiber-reinforced inorganic MPC composites has been achieved.