Synthesis and Characterization of N-Isopropylacrylamide Microspheres as pH Sensors

Swellable polymer microspheres that respond to pH were prepared by free radical dispersion polymerization using N-isopropylacrylamide (NIPA), N,N′-methylenebisacrylamide (MBA), 2,2-dimethoxy-2-phenylacetylphenone, N-tert-butylacrylamide (NTBA), and a pH-sensitive functional comonomer (acrylic acid, methacrylic acid, ethacrylic acid, or propacrylic acid). The diameter of the microspheres was between 0.5 and 1.0 μm. These microspheres were cast into hydrogel membranes prepared by mixing the pH-sensitive swellable polymer particles with aqueous polyvinyl alcohol (PVA) solutions followed by crosslinking with glutaric dialdehyde for use as pH sensors. Large changes in the turbidity of the PVA membrane were observed as the pH of the buffer solution in contact with the membrane was varied. These changes were monitored by UV–visible absorbance spectroscopy. Polymer swelling of many NIPA copolymers was reversible and independent of the ionic strength of the buffer solution in contact with the membrane. Both the degree of swelling and the apparent pKa of the polymer microspheres increased with temperature. Furthermore, the apparent pKa of the polymer particles could be tuned to respond sharply to pH in a broad range (pH 4.0–7.0) by varying the amount of crosslinker (MBA) and transition temperature modifier (NTBA), and the amount, pKa, and hydrophobicity of the pH-sensitive functional comonomer (alkyl acrylic acid) used in the formulation. Potential applications of these polymer particles include fiber optic pH sensing where the pH-sensitive material can be immobilized on the distol end of an optical fiber.

Migration from acrylonitrile butadiene styrene (ABS) polymer: swelling effect of food simulants compared to real foods

Materials and articles made of acrylonitrile–butadiene–styrene (ABS) intended for contact with food must comply with the requirements of the European Plastic Regulation (EU) 10/2011, which lays down the food simulants and the time/temperature conditions to be applied for migration testing. Previous studies indicated that high concentrations of ethanol at temperatures above ambient may lead to swelling of ABS polymers resulting in increased migration. In this study migration kinetic data for a set of model substances at different temperatures were obtained using both food simulants stipulated in EU regulations and real food (milk, cream and olive oil). At the same time, the extent of polymer swelling was gravimetrically characterized after contact with simulants and different foods tested at several conditions to cover the majority of foreseeable applications of ABS. The obtained results confirmed that the use of high concentrations of ethanol–water, especially at high temperatures, causes the swelling of ABS polymers and results in significantly higher migration values compared to the tested foods as well as Tenax®. None of the real foods studied cause significant swelling of ABS. The widely used simulant 95% (v/v) aqueous ethanol proves not be suitable for compliance testing of ABS under the recommended conditions of Regulation (EU) 10/2011. Swelling of the polymer results in artificially higher diffusion coefficients or lower activation energies of diffusion. Migration prediction using polymer-specific diffusion parameters should therefore be considered to avoid over-conservative risk assessment for food contact materials and articles made of ABS.

A cosolvent surfactant mechanism affects polymer collapse in miscible good solvents

The coil–globule transition of aqueous polymers is of profound significance in understanding the structure and function of responsive soft matter. In particular, the remarkable effect of amphiphilic cosolvents (e.g., alcohols) that leads to both swelling and collapse of stimuli-responsive polymers has been hotly debated in the literature, often with contradictory mechanisms proposed. Using molecular dynamics simulations, we herein demonstrate that alcohols reduce the free energy cost of creating a repulsive polymer–solvent interface via a surfactant-like mechanism which surprisingly drives polymer collapse at low alcohol concentrations. This hitherto neglected role of interfacial solvation thermodynamics is common to all coil–globule transitions, and rationalizes the experimentally observed effects of higher alcohols and polymer molecular weight on the coil-to-globule transition of thermoresponsive polymers. Polymer–(co)solvent attractive interactions reinforce or compensate this mechanism and it is this interplay which drives polymer swelling or collapse.

COMPOSITE ION-EXCHANGES FOR THE RECYCLING OF LIQUID WASTE OF DAIRY INDUSTRY

The method of directed formation of particles of hydrated zirconium and titanium oxides into anion exchange resins has been developed. The approach based on the Ostwald-Freundlich thermodynamic equation is applied. Such approach, in particular, connects the size of particles with the solubility of the compound, volume and concentration of reagents. Less soluble zirconium dioxide is deposited as non-aggregated nanoparticles, the size of which does not exceed 10 nm. The composition of such composites is the most reproducible. In the case of more soluble titanium dioxide, aggregates of nanoparticles (up to 70 nm) are formed. When the concentration of the solution of metal salts in the polymer increases, the particles of micron size are deposited, the composition of this type of material is less reproduced. Non-aggregated nanoparticles increase the exchange capacity of the polymer. This leads to an increase in its electrical conductivity in 1.4-1.8 times. This is due to an increase in the concentration of mobile charge carriers in the polymer matrix that is caused by reducing its swelling. Other reason is a contribution of the counter-ions of the functional groups of inorganic component to ion transport. On the contrary, the aggregates of nanoparticles amplify the polymer swelling, resulting in a reduction of exchange capacity and electrical conductivity. Ion-exchangers were used for the ion exchange processing of nanofiltration permeate of milky whey, and for electromembrane desalination of protein concentrate.

Flexible Liquid Sensor Based on Rubber/Graphite Composite

Hydrocarbon liquids such as gasoline, diesel, jet fuel, and solvents are hazardous materials derived from petroleum. These materials can diffuse the rubber network structures and cause swelling in these polymers. Rubber materials containing conductive fillers exhibit a high conductivity at the higher percolation threshold. As direct effect of solvent on the polymer, swelling in the rubber composite alters the structure of the filler network and subsequently the composite properties change significantly. This Process can be considered as a signal for solvent or hydrocarbon fuel detector system and used conductive rubber composite as a flexible sensor. In this study, the nitrile/graphite composite samples were prepared containing different amounts of graphite particles. These samples have the ability to measure electrical resistance. The electrical resistance of rubber/graphite samples decreases with increasing content of graphite particles. The increase in the electrical resistance of the specimens was measured by using conductive composite swelling in toluene solvent and it was observed that all specimens eventually became electrical insulation. Samples with concentrations of 60, 70, and 80 phr are conductive after recovery and solvent removal. While their conductivity is less than that of the virgin samples. The incremental trend of electrical resistance against the conductive composite swelling was measured in toluene solvent and it was observed that all samples were eventually converted to electrical insulation. But there is little difference between the third swelling process and the second one. This phenomenon has occurred for all three samples, which reveals a good similarity with the Mullins effect.