Effects of acrylic acid incorporation on the pressure–temperature behavior and the calorimetric properties of poly(N-isopropylacrylamide) in aqueous solutions

 We studied the effects of pH on the pressure–temperature dependence of coil–collapse transition for aqueous solutions of copolymers of N-isopropylacrylamide and acrylic acid (Ac). At low pressures, the transition temperature (T _tr) increased with pressure, but T _tr decrease with increasing pressure at pressures higher than 50–100 MPa. By increasing the pH, the transition contour shifted to a higher temperature. When the Ac content was increased, the effects of pH became more evident. From a calorimetric study at atmospheric pressure, ΔH _tr was found to become smaller by increasing the portion of the ionized residues in the copolymer. The ratio to the van't Hoff enthalpy changes became larger with an increase in pH, which indicated that the production of charge decreased the cooperative domain size. Received: 19 July 1999 /Accepted in revised form: 7 September 1999     

Synthesis and characterization of biodegradable pH-sensitive hydrogels based on poly(?-caprolactone), methacrylic acid, and poly(ethylene glycol)

In this work, a novel biodegradable pH-sensitive hydrogel based on poly(?-caprolactone) (PCL), methoxpoly(ethylene glycol) (MPEG) and methacrylic acid (MAA) was prepared by UV-initiated free radical polymerization. The resulting macromonomers and hydrogels were characterized by FTIR and/or ¹H NMR. Swelling behaviour and pH sensitivity of the hydrogels were studied in detail. With increase in pH of aqueous medium from 1.2 to 7.2, swelling ratio of the hydrogels increased accordingly. The hydrolytic degradation behaviour was also investigated. The prepared biodegradable pH-sensitive hydrogel based on PCL, MPEG, and MAA might have great potential application in smart drug delivery system.     

Effect of block lengths on the association behavior of poly(l-lactic acid)/poly(ethylene glycol) (PLA–PEG–PLA) micelles in aqueous solution

A series of poly(l-lactic acid)/poly(ethylene glycol) triblock copolymers with a PLA–PEG–PLA architecture were synthesized by a ring-opening polymerization (ROP) process. The copolymers were characterized by ¹H NMR and GPC. The total number average molecular weights were in the range of 4,700–50,000, whereas the degrees of polymerization of the PLA and PEG blocks varied from 15 to 359 and from 68 to 136, respectively. The self-association of these copolymers in aqueous environment was studied by emission fluorescence spectroscopy of anilinonaphthalene probe and the critical association concentration (CAC) of the copolymers was measured. It was found that the micellization process of these copolymers was mainly determined by the length of the hydrophobic LA block, while the length of the hydrophilic PEG block had little effect. Furthermore, the low CAC values of the copolymers suggest that the copolymers form stable supramolecular structures in aqueous solutions.     

Effect of poly(2-vinylpyridine) as a template for the radical polymerization of methacrylic acid—V: Influence of temperature

The influence of temperature on the polymerization of methacrylic acid along atactic and isotactic poly(2-vinylpyridine) templates was studied by means of isothermal and scanning (d.s.c.) calorimetric techniques. The overall energy of activation of both template polymerization systems was about 40 kJ/mol as compared to 80 kJ/mol for the blank. This large difference is caused by a high energy of activation of the cross-termination reaction (50 kJ/mol). The apparent energy of activation, obtained from scanning measurements, strongly depended on the scan speed because the steady-state assumption was not valid in the lower temperature region, i.e. the template radical concentration decreases with increasing scan speed causing a rise in the apparent energy of activation.     

Effect of halloysite nanotubes on the thermal degradation behaviour of poly(ε-caprolactone)/poly(lactic acid) microfibrillar composites

This paper reports on the thermal degradation behaviour and kinetics of halloysite nanotubes containing microfibrillated poly(ε-caprolactone) (PCL)/poly(lactic acid) (PLA) blends. It was found that the nanotubes probably catalyzed the PLA degradation, and that the free radicals formed during the PLA degradation initiated PCL degradation at lower temperatures, maybe in combination with halloysite nanotubes (HNT) catalysis. Drawing to form microfibrillated nanocomposites had little influence on the degradation behaviour of these materials, but pre-mixing of the HNT with PLA or PCL prior to melt-mixing and extrusion-drawing of the blends did influence the degradation behaviour, but in different ways. No evidence could be found that the presence and amount of HNT, or the mode of preparation, had an influence on the degradation mechanism, as evidenced through a Fourier-transform infrared (FTIR) analysis of the degradation products.     

Performance of glucose electrooxidation on Ni–Co composition dispersed on the poly(isonicotinic acid) (SDS) film

Poly(isonicotinic acid) (PINA) film was electrosynthesized on carbon paste electrode (CPE) by using the repeated potential cycling technique in aqueous solution containing isonicotinic acid (INA), sulfuric acid and sodium dodecyl sulfate (SDS). Then, nickel and cobalt ions were incorporated by immersion of CPE/PINA prepared in the presence of SDS (CPE/PINA(SDS)) in a solution with different proportions of nickel chloride and cobalt chloride. The electrochemical characterization of mixed hydroxides containing cobalt and nickel at the surface of the modified electrode is presented. The modified electrodes were successfully used in the electrocatalytic oxidation of glucose. Finally, the electrocatalytic oxidation peak currents of glucose exhibited a good linear dependence on concentration, and its quantification can be done easily. The good analytical performance, low cost and straightforward preparation method make this novel electrode material promising for the development of an effective glucose sensor.     

Role of the molecular weight and the composition on the hydrolysis kinetics of monolayers of poly(α-hydroxy acid)s

The hydrolysis kinetics of spread insoluble monolayers of poly(α-hydroxy acid)s with various molecular weights and lactic acid–glycolic acid molar ratios was followed by measuring simultaneously the decrease in the surface area at constant surface pressure and the evolution of the surface potential. The interfacial hydrolysis at alkaline pH leads to the progressive fragmentation of the polymer molecules and the appearance of charged insoluble products (detected by measuring the surface potential) and small soluble fragments (detected by measuring the decrease in the surface area). The data obtained by both approaches were interpreted in the framework of the random scission model. The rates of hydrolysis are larger for polymers with smaller initial polymerization numbers and increase with the decrease in the molar ratio of lactic acid units. Received: 7 December 1998 Accepted in revised form: 8 March 1999     

Thermal degradation of two classes of block copolymers based on poly(lactic-glycolic acid) and poly(ϵ-caprolactone) or poly(ethylene glycol)

Thermodegradative investigations of two classes of multi-block copolymers containing poly(D,L-lactic-glycolic acid) (PLGA) and either poly(ethylene glycol) (PEG) or poly(ϵ-caprolactone) diol-terminated (PCDT) segments were performed. In particular, the influence of the type and length of the segments as well as of the molar ratio between the D,L-lactic acid (LA) and glycolic acid (GA) residues was investigated at 180°C in air by viscometry, FT-IR analysis and isothermal thermogravimetry. The thermal oxidative degradation of these materials is largely affected by the LA/GA ratio, a higher LA content generally imparting higher stability. The FT-IR analysis suggests that, depending on the composition of the PLGA segments, degradative processes are triggered which can lead to a preferential degradation of the blocks.     

Effects of hyperbranched and linear architecture on properties of polymers composed of poly(ɛ-caprolactone) and hydroxycinnamic acid

Photoreactive and degradable polymers with linear and hyperbranched architectures which composed of poly(?-caprolactone) and hydroxycinnamic acid or its substituted derivatives were synthesized by thermal melt-polycondensation. The chemical structures of the polymers were confirmed by FTIR and ¹H NMR measurements. The polymers showed good photoreactivities and fluorescent properties, and the hyperbranched polymers showed higher photoreactive speed and weaker fluorescence properties. These polymers had excellent thermal stabilities due to the rigid conjugated structures and the π-π strcking interaction of the cinnamoyl group, especially for the hyperbranched polymer. Moreover, the hydrolysis experiments and the XRD results revealed that the hyperbranched and linear polymers are amorphous and crystalline, respectively, and the degradation rate of amorphous polymers are faster than crystalline samples.     

Enzymatic hydrolysis by Humicola lanuginosa lipase of poly(lactic acid)–poly(glycolic acid) monolayers

 The enzymatic hydrolysis by Humicola lanuginosa lipase (HLL) of spread insoluble monolayers of poly (α-hydroxy acid)s with various molecular weights and various lactic–glycolic molar ratios was studied using a barostat surface balance. The interfacial hydrolysis under enzyme action leads to the progressive fragmentation of the polymer molecules. The appearance at the interface of charged insoluble fragments was detected by measuring the surface potential, while the solubilization of the small soluble fragments was detected by measuring the decrease in the surface area. The data obtained were used to test the mode of fragmentation: either random or chain-end scission. The catalytic specific activity of HLL was estimated in the framework of the random scission model and compared with the activities obtained for the hydrolysis of simple molecules of di- and tri-glycerides organized as monolayers or emulsion. Received: 9 August 1999 Accepted: 4 January 2000     

Preparation and characterization of microspheres based on blend of poly(lactic acid) and poly(ɛ-caprolactone) with poly(vinyl alcohol) as emulsifier

In this paper, microspheres were prepared by oil-in-water (o/w) emulsion solvent evaporation method. Biodegradable polymer such as blend of poly (lactic acid) (PLA) and poly(?-caprolactone) (PCL) with certain compositions and characteristics was used to prepare the microspheres with poly(vinyl alcohol) (PVA) as an emulsifier. This study observed the microspheres particle’s size distribution at various concentrations of PVA (1%, 1.5%, 2%, and 2.5% PVA). The PVA volume variations effects during the process (50, 100, 150, 200, and 250 mL) were also observed. The blend of PLA and PCL is formed only by physical interaction between them. This can be seen from the FTIR spectrum which shows both PLA and PCL component. The microspheres physical size and appearance were observed by optical microscope (MO). The overall results of this study showed that the formula which used 50–150 mL of 2.5% polyvinyl alcohol produced the microspheres with the most uniform size distribution.     

The effects of the molecular weight and structure of poly(acrylic acid) on the formation of “blue silver”

Using UV-spectroscopy and transmittance electron microscopy it was shown that the structure of macromolecules and molecular weight of poly(acrylic acid) can significantly affect the synthesis and stabilization of so-called “blue silver.”     

Effects of fabrication conditions on the structure and function of membranes formed from poly(acrylonitrile–vinylchloride)

Phase-inversion membranes formed from poly(acrylonitril–co-vinylchloride) (PAN–PVC) have been utilized for encapsulating living cells for transplantation; however, a detailed analysis of the structure and function of the integral skin layer has not been reported. PAN–PVC membranes fabricated under different precipitation conditions were analyzed using microscopic techniques and several functional tests. Structural analysis with scanning atomic force microscopy (AFM) revealed the presence of nodular elements in the skin layer which changed as a function of precipitation conditions. In addition, membrane hydraulic permeabilities, sieving coefficients, and diffusive permeabilities also varied with precipitation conditions. Furthermore, changes in the functional properties could be related to the size of the nodular elements and their accompanying interstitial space. The results provide insight into the fundamental interrelationships that exist between membrane fabrication, the fine surface morphology of the skin layer, and membrane performance.     

One-pot method for the synthesis of sulfonated poly(1,3,4-oxadiazoles) based on 4,4’-oxydibenzoic acid

A series of new copolymers of poly(1,3,4-oxadiazole) that contain side protogenic sulfo groups are synthesized by the direct copolycondensation of terephthalic and 4,4’-oxydibenzoic acids and hydrazine sulfate in oleum. Variation in the polycondensation conditions makes it possible to synthesize copolymers containing phenylene moieties with either 4-sulfo-10,10-dioxophenoxatiine or 4,4’-oxybis(3-sulfophenyl) moieties or a combination of the three moieties. This approach permits easy control over the physicochemical properties of the copolymers and the amount of protogenic sulfo groups. It is shown that the introduction of sulfonated moieties into copolymer molecules makes it possible to obtain materials with high values of ionexchange capacity (1.5 mmol/g) and water absorption (110%), while the strength and thermal stability inherent in poly(p-phenylene 1,3,4-oxadiazoles) remain intact.     

Volumetric and compressibility behaviour of poly(propylene glycol) – Amino acid aqueous solutions at different temperatures

Precise density and sound velocity measurements have been carried out for aqueous solutions of PPG725 in the absence and presence of (0.2 and 0.5) mol · kg⁻¹ amino acids: alanine, glycine, serine and proline, and also for aqueous solutions of these amino acids in the absence and presence of 0.01 w/w PPG725 at T = (288.15, 293.15, 298.15, 303.15 and 308.15) K. From the experimental density and sound velocity values, the apparent molar volume and isentropic compressibility have been obtained and extrapolated to infinite dilution. The infinite dilution apparent molar properties for transfer of PPG from water to aqueous amino acids solutions and also those for transfer of amino acids from water to aqueous PPG solutions have been studied. Temperature dependency of the infinite dilution apparent molar volume was utilised to determine structure-breaker or structure-maker effects of the solutes. Hydration numbers of the amino acids in the investigated aqueous solutions have been evaluated from the volumetric and compressibility properties. All results are discussed based on the salting-out aptitude of the amino acids (hydrophilic + hydrophobic) interactions and (hydrophobic + hydrophobic) interactions occurred between PPG and the investigated amino acids.     

Effects of Thermal Activation on the Electrocatalysis of the Ru(0001) Surface

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Poly(2-vinylpyridine) as a template for the radical polymerization of methacrylic acid—VI. Influence of the solvent

The template polymerization (TP) of methacrylic acid (MAA) along poly(2-vinylpyridine) (P2VP) has been investigated to study effects of solvent polarity upon the kinetics. The solvents were DMSO, DMF and dioxane. In DMSO, there is no rate enhancement due to the presence of the template; this is consistent with the fact that poly(methacrylic acid) (PMAA) and P2VP do not form complexes in this strongly solvating medium. The rate enhancement (template effect) in DMF is primarily due to termination retardation of P2VP-bound PMAA radicals. The largest template effect was found in 1,4-dioxane (about three times that in DMF). The cause may be the faster complexation of PMAA-radicals to the template (ca 10 times that in DMF). The non-steady state conditions for the template radical concentration, which is an essential aspect for the estimation of the various rate coefficients of the proposed kinetic model for TP, could be confirmed with EPR-measurements.     

Effect of water-soluble cross-linker on the growth and properties of ethyl acrylate–methacrylic acid emulsion opolymer particles

Model ethyl acrylate–methacrylic acid copolymer latices and latices of particles cross-linked by copolymerizing small amounts of water-soluble N,N′-methylenebisacrylamide were prepared by nonseeded semicontinuous emulsion copolymerization. Dynamic and static light scattering measurements indicated a slightly higher degree of polydispersity in the case of cross-linked particles, especially in the initial stages of polymerization. The hydrodynamic volume of the alkalinized particles controlling the viscosity properties of the dispersions decreased with the time of polymerization and in the case of cross-linked copolymer almost reached a constant value at about 1 h. The different character of the particle structure was confirmed by differences in particle disintegration after alkali addition or in the presence of methanol. Received: 2 February 1999/Accepted: 28 June 2000     

Effect of γ-irradiation on the properties of poly(ester urethane) elastomer

The influence of irradiation with ⁶⁰Co γ-rays on gelation, change in the mechanical characteristics, and the formation of supermolecular structure in a poly(ester urethane) elastomer synthesized from oligoether adipate and diisocyanate, as well as a diol with or without double carbon-carbon bonds, was studied. It was found that the presence of double carbon-carbon bonds in the structure of the elastomer ensures the predominance of crosslinking over degradation processes. It was shown that γ-irradiation at a dose of 380 kGy is accompanied by the formation of a microglobular structure of ～1 μm size. The specifics of changes in the mechanical characteristics depending on the absorbed dose are discussed.     

Synthesis of organo-modified α-zirconium phosphate and its effect on the flame retardancy of IFR poly(lactic acid) systems

Novel intumescent flame-retardant poly(lactic acid) (PLA/IFR)/organo-modified α-zirconium phosphate(OZrP) nanocomposites were prepared via incorporation of charring agent (CA), ammonium polyphosphate (APP) and OZrP into PLA. OZrP was synthesized directly by a solvent thermal method. The morphological characterization of PLA/IFR/OZrP nanocomposites was conducted by wide angle X-ray diffraction (WXRD) and transmission electron microscopy (TEM). The effect of the OZrP on flame retardancy and the thermal stability of PLA/IFR composites were studied by thermogravimetric analysis (TGA), limiting oxygen index (LOI), vertical burning test (UL-94) and cone calorimeter test. The TGA data illustrated that the OZrP could increase the residue and significantly improve the flame retardancy of PLA/IFR/OZrP nanocomposites showing an excellent synergistic effect. The addition of OZrP to the flame-retardant PLA increases the LOI and enhances the UL-94 rating. Cone calorimeter tests gave clear evidence that the incorporation of OZrP into PLA/IFR composites resulted in the significant reduction of the heat release rate (HRR), low total heat release (THR) and high amount of char residues during combustion. The flame-retardant mechanism of PLA/IFR/OZrP nanocomposites may correspond to the intumescent flame-retardant mechanism and catalyzed carbonization mechanism caused by OZrP.