Effect of sizing agent on interfacial properties of aramid knitted structural composites

Tensile tests have been carried out on aramid knitted fabrics/epoxy resin composites in which the aramid knitted fabrics are treated with different sizing agents. Two kinds of surface treatment are performed; one uses an epoxy sizing agent and the other uses a polyethylene sizing agent. Tensile modulus and strength of epoxy-sized composites are higher than those of polyethylene-sized composites. The fracture process is different between epoxy- and polyethylene-sized materials. This difference in fracture process is caused by the different interphase made from either epoxy or polyethylene sizing treatments, resulting in the different tensile performance. Moreover, the tensile properties of the wale specimen are more affected than those of the course specimen by the interphase.     

Application of model-fitting and model-free kinetics to the study of non-isothermal dehydration of equilibrium swollen poly (acrylic acid) hydrogel: Thermogravimetric analysis

The dehydration kinetics of equilibrium swollen poly (acrylic acid) hydrogel is analyzed by both model-fitting and model-free approaches. The conventional model-fitting approach assuming a fixed mechanism throughout the reaction and extract a single values of the apparent activation energy (E_a) and pre-exponential factor (A) and was found to be too simplistic. The values of Arrhenius parameters obtained in such a way are in fact an average that does not reflect changes in the reaction mechanism and kinetics with the extent of conversion. The model-free approach allows for a change of mechanism and activation energy, E_a, during the course of a reaction and is therefore more realistic. The complexity of the dehydration of poly (acrylic acid) hydrogel is illustrated by the dependence of E_a and A on the extent of conversion, α (0.05 ≤ α ≤ 0.98). Under non-isothermal conditions, E_a decreases with α for 0 ≤ α ≤ 0.50, followed by an approximately constant value of E_a during further dehydration. For 0 ≤ α ≤ 0.50, dehydration is complex, which probably involving a combination of several processes. In the constant-E_a region, non-isothermal dehydration follows the three-dimensional phase boundary model (R3). The complex hydrogen-bond pattern in poly (acrylic acid) hydrogel is probably responsible for the observed dehydration behavior. An existence of compensation effect is accepted and explanation of compensation effect appearance during the hydrogel dehydration is suggested.     

Electrosteric stability of styrene/acrylic acid copolymer latices under emulsion polymerization reaction conditions

Acrylic acid (AA) is used in many emulsion polymerization formulations to improve the colloidal stability during and after the production of latex products. Theoretically, the improved stability originates from electrostatic repulsion complemented with steric repulsion. The objective of this work was to study the contribution of AA to the colloidal stability of polystyrene and styrene/AA copolymer latices under simulated reaction conditions. The strength of electrostatic and steric repulsion forces as a function of the electrolyte concentration, pH, and temperature was investigated via coagulation experiments with monomer‐swollen latices in stirred tank reactors. Transmission electron microscopy pictures and dynamic light scattering measurements provided an understanding of the conditions and mechanisms leading to coagulation. The experiments demonstrated that the presence of surface‐bound carboxylic groups only improved the colloidal stability if the carboxylic groups were charged, that is, at a high pH. At a low pH, the copolymer latices were even less stable than the homopolymer latex, and this indicated that the addition of AA did not improve the colloidal stability of a growing polystyrene latex. With respect to emulsion polymerization process operations, insufficient mixing and a highly concentrated electrolyte feed were found to be sources of fouling and enhanced macroscopic coagulation. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 726–732, 2005     

Electrical conductivity of aqueous polymer solutions

 Theoretical equations were proposed to adequately simulate the electrical conductivity behavior of aqueous solutions of both charged and uncharged polymers. The theory, based on the mixture equation of Boned and Peyrelasse, was experimentally verified on poly(acrylic acid) (PAA) in water and poly(ethylene oxide) (PEO) in aqueous electrolyte solutions. The data analysis suggested that both the polymer coils may be depicted as oblate ellipsoids. Subsequently, the semiaxes values of the polymer coils were determined, and they were in good agreement with the results reported in the literature. Received: 25 June 1996 Accepted: 2 October 1996     

Synthesis and characterization of polyethylene film grafted with acrylic acid and diethyleneglycol-dimethacrylate

Acrylic acid (AA) and diethyleneglycol-dimethacrylate (DEGDM) are grafted onto 25 μm low-density radio-peroxided polyethylene. Monomer content is determined by exchange capacity measurements. Studies of electrical resistance and zero-current membrane potential allow for the determination of the influence of DEGDM on both the dissociation of grafted AA and the permselectivity of these so-obtained cation-exchange membranes. DEGDM, acting as a crosslinking agent, limits the AA degree of grafting, slightly increases the electrical resistance, but contributes to enhance the permselectivity. For example, when these grafted films are immersed in 3M NaCl aqueous solutions, the addition of DEGDM leads to an increase of the transport number of Na⁺ close to 20%. © 1993 John Wiley & Sons, Inc.     

Synthesis and properties of chitosan‐based thermo‐ and pH‐responsive nanoparticles and application in drug release

In this research, thermo‐ and pH‐responsive nanoparticles with an average diameter of about 50–200 nm were synthesized via the surfactant‐free emulsion polymerization. The thermal/pH dual responsive properties of these nanoparticles were designed by the addition of a pH sensitive monomer, acrylic acid (AA), to be copolymerized with N‐isopropylacrylamide (NIPAAm) in a chitosan (CS) solution. The molar ratio of CS/AA/NIPAAm in the feed was changed to investigate its effect on structure, morphology, thermal‐ and pH‐responsive properties of the nanoparticles. It was found that CS‐PAA‐PNIPAAm nanoparticles could be well dispersed in the aqueous solution and carried positive charges on the surface. The addition of thermal‐sensitive NIPAAm monomer affected the polymerization mechanism and interactions between CS and AA. The particle size of the nanoparticles was found to be varied with the composition of NIPAAm monomer in the feed. The synthesized nanoparticles exhibited stimuli‐responsive properties, and their mean diameter thus could be manipulated by changing pH value and temperature of the environment. The nanoparticles showed a continuous release of the encapsulated doxycycline hyclate up to 10 days during an in vitro release experiment. The environmentally responsive nanoparticles are expected to be used in many fields such as drug delivery system. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 2798–2810, 2009     

Molecular modeling of polymers. 10. Characterization of conformational transitions of poly(acrylic acid) and poly(methacrylic acid) using dyad structures

The conformational profiles of nearest side-chain neighbors, methylene-dyad structures, of poly(acrylic acid), PAA, and poly(methacrylic acid), PMA, were determined as a function of tacticity, extent of ionization, and presence of counterion. The dominant backbone conformer states are quite similar for both isotactic and syndiotactic diads in a common charge state. Thus, the overall dimensional properties of isotactic syndiotactic and atactic chains of PAA or PMA, based upon dyad interactions, are predicted to be alike for a given charge state. Significant deviations from precise t, g⁺, and g^? states are found for the dyad minimum energy conformations. The rod-to-coil and coil-to-rod transitions observed in PAA and PMA, respectively, as a function of increasing counterion concentration can be explained, to a large extent, by the conformational profiles of the corresponding dyad model structures. © 1994 John Wiley & Sons, Inc.     

Changes in the properties of HDPE fibers upon radiation grafting with acrylic acid

The morphology and the physical and mechanical properties of graft-modified polyethylene fibers have been studied. Two types of fibers, with the diameters of 10 μm (1.1 dtex) and 40 μm (7.5 dtex), were modified by radiation-induced grafting with acrylic acid. The extent of grafting was determined gravimetrically. Confirmation of gravimetrically obtained values was achieved using conductometric titration. The fibers were hydrated at pH 2 and pH 7. The degree of swelling was 120% at pH 2 and 200% at pH 7. The transversal distribution of polyacrylic acid in the fibers was determined. Fibers were stained and observed with an optical microscope. The diffusion of the monomer into the bulk was found to be rather fast. The changes in the total crystalline content and the lamellar thickness distributions in consequence of irradiation and grafting were determined by differential scanning calorimetry analysis. The measurements showed no effects of irradiation on the crystallinity in either type of fiber, whereas a decreasing crystallinity caused by grafting was noticed in the 40 μm fibers. The lamellar thickness distributions narrowed upon irradiation, indicating recrystallization as a result of chain scission. Wide angle x-ray scattering and Raman analysis of dry and hydrated fibers were conducted to study the behavior of the fibers in an aqueous environment. These results both showed a decreasing crystalline content caused by fiber hydration. Tensile tests were carried out to evaluate how grafting, hydration and Ca²⁺-crosslinking of grafts affected the fiber strength. Grafting and Ca²⁺-crosslinking, as well as hydration, resulted in a decreasing E-modulus for the 40 μm fibers, whereas no significant change could be noticed in the 10 μm fibers. © 1995 John Wiley & Sons, Inc.     

Development of monolithic enzymatic reactors in glass microchips for the quantitative determination of enzyme substrates using the example of glucose determination via immobilized glucose oxidase

A one-step procedure for the immobilization of glucose oxidase in fused-silica capillaries and in glass microchips was developed based on enzyme entrapment in a polyacrylamide-based monolithic column. The inner capillary surface was silanized with gamma-methacryloxypropyltrimethoxysilane (gamma-MAPS) to allow covalent binding of the gel to the surface. The composition of the polymer was optimized to prevent the formation of bubbles, allow liquid transportation by electroosmotic flow and to maintain the enzymatic activity. These requirements resulted in the addition of polyethylene glycol and poly(acrylic acid) to the acrylamide mixture. The gel containing the enzyme was formed in situ in the capillaries, respectively, in one channel of the microchip. In the microchip, it was limited to the sample injection channel by accordingly controlled silanization of the inner capillary surface. Glucose was detected via the amperometric determination of hydrogen peroxide. A linear correlation between signals and glucose concentration was observed from 0.05 to 1.1 mM glucose with a correlation coefficient of 0.999. The enzymatic monolithic microreactor showed no loss of activity during 8 h of continuous use and during storage in the running buffer at 4 degrees C for about 2 months. Interferents, such as ascorbic acid, were separated from the analyte electrophoretically, so that glucose could be quantified in diluted juices.     

Polymeric photoinitiators based on side-chain benzoin methyl ether and tertiary amine moieties for fast UV curable coatings

The synthesis is reported of a new series of polymeric photoinitiators obtained by copolymerization of a-methylolbenzoin methyl ether acrylate (MBA) with different N,N-dialkylamino alkyl acrylates. The copolymers have been fully characterized and employed in the photoinitiated polymerization and crosslinking reactions of a standard acrylic formulation for clear curable coatings. The photoinitiation activity of the above systems has been measured by using microwave dielectrometry and differential photocalorimetry, and compared with that found for the corresponding low-molecular-weight models. The results clearly indicate that the copolymeric systems display a remarkable decrease of the induction period and an improved overall activity in the UV curing of the acrylic coatings. An interpretation of the experimental findings is also reported.