Acry1ic Acid Research Articles

Structural Study of Polystyrene-b-poly(acrylic acid) Micelles Complexed with Uranyl: A SAXS Core-Shell Model Analysis.

The interactions between natural colloidal organic matter and actinides in solutions are complex and not fully understood. In this work, a crew-cut polystyrene-b-poly(acry1ic acid) (PS-b-PAA) micelle is proposed as a model particle for humic acid (HA) colloid with the aim to better understand the sequestration, aggregation, and mobility of HA colloids in the presence of uranyl ions. The effects of uranyl ions on the structure of PS29k-b-PAA5k micelles in aqueous solution were mainly investigated by synchrotron small-angle X-ray scattering. A core-shell model, accounting for the thickness and contrast changes of the PAA corona induced by the adsorption of uranyl, was employed to analyze the scattering data. A combination of transmission electron microscopy, dynamic light scattering, and zetametry showed a strong affinity of uranyl ions to PAA segments in water at pH 4-5 that resulted in the shrinkage and improved contrast of the PAA corona, as well as colloidal destabilization at a high uranyl concentration.

Methods for Synthesis of Hydrogel Networks: A Review

Abstract Hydrogels are macromolecular networks that swell, but do not dissolve, in water. The ability of hydrogels to absorb water arises from hydrophilic functional groups attached to the polymeric backbone, while their resistance to dissolution arises from crosslinks between network chains. Many materials, both naturally occurring and synthetic, fit the definition of hydrogels. Crosslinked dextrans and collagens are examples of natural polymers that are modified to produce hydrogels. Classes of synthetic hydrogels include poly(hydroxyalkyl methacrylates), poly(acrylamide), poly(N-vinyl pyrrolidone), poly(acry1ic acid), and poly(vinyl alcohol).

Transport of ionizable drugs and proteins in crosslinked poly(acrylic acid) and poly(acrylic acid-co-2-hydroxyethyl methacrylate) hydrogels. I. Polymer characterization

SYNOPSIS The purpose of this investigation was to define the polymer structure and elucidate the swelling behavior of ionizable hydrophilic polymers (hydrogels) in water and buffered media. Poly(acry1ic acid) (PAA) and poly(acry1ic acid-co-2-hydroxyethyl methacrylate) [P(AA-coHEMA)] hydrogels were synthesized with varying degrees of hydrophilicity and crosslinking and were designed as potential bioadhesive controlled-release dosage forms. The thermal initiation procedure employed during polymerization was optimized to eliminate unreacted residuals. Equilibrium and dynamic swelling studies were undertaken to determine the polymer mesh size and molecular weight between crosslinks of the hydrogels in the ionized and nonionized states. The PAA hydrogel mesh sizes ranged from 100 to 400 8, over pH values of 3-7, whereas the P(AA-co-HEMA) hydrogel mesh sizes were between 13 and 140 A. These results demonstrated the significance of the swelling medium pH on the hydrated state of the polymers relative to crosslinking or copolymerization composition. 0 1996 John Wiley & Sons, Inc.

Phase Separation Phenomena and Viscosity Enhancements in Aqueous Mixtures of Poly(styrenesulfonate) with Poly(acrylic acid) at Different Degrees of Neutralization

Interactions between poly(styrenesu1fonate) (PSS) and poly(acry1ic acid) (PA) in aqueous solution have been studied, with and without added salt, and at various degrees of neutralization (a) of PA. Equilibrium phase diagrams have been determined, and the viscosities of monophasic mixtures have been measured. Both types of experiments reveal striking effects of a on the PA-PSS interactions. Salt-free mixtures with fully or partially neutralized poly(acry1ic acid) phase separate segregatively, except at very low a where, instead, an association between PA and PSS occurs. The association is evidenced by a dramatically increased viscosity, relative to solutions of PA or PSS alone, in semidilute mixtures. Addition of salt (1 M NaC1) results in an increased two-phase area at all a, and in the appearance of an associative phase separation for non-neutralized PA. The qualitative phase behavior observed in the presence of salt can be generated by calculations using the Flory-Huggins theory, if it is assumed that both the PSS-PA and the PA-solvent interactions change monotonically with a in a specified fashion. All experimental observations, and the theoretical modeling, suggest that the effective PA-PSS interaction changes (over a narrow interval of a) from an attraction at very low a to a repulsion at higher a. (Less)

Glucose-sensitive insulin-releasing molecular systems

Abstract

Phosphorescence investigation of the conformation of a bromonaphthalene-labeled poly(acrylic acid)

A copolymer of 4-bromo-1-naphthyl vinyl ketone and acrylic acid (BNPAA) has been prepared by free radical polymerization. The pendant bromonaphthalene (BN) groups endow the polymer with the unusual property of readily observable phosphorescence in solution at room temperature. The phosphorescence of BNPAA was investigated systematically in solution at room temperature as a function of polymer concentration, salt concentration, solvent structure, and other variables and shows that phosphorescence can serve as a probe of polymer conformation. By assuming that self-quenching of a triplet BN moiety by a pendant ground-state BN moiety on the same polymer chain determines the triplet lifetime, the phosphorescence lifetime of the labeled polymer at room temperature can be correlated to the chain conformation of the polymer BNPAA in dilute solution in the following manner: collapsed, globular conformations of the polymer correlate with significant self-quenching and relatively short phosphorescence lifetimes, and extended conformations of the polymer correlate with inhibition of self-quenching and relatively long phosphorescence lifetimes. Employing the phosphorescence lifetime criterion as a probe, the conformation of BNPAA was studied under a variety of conditions. The effect of different solvents on the polymer conformation was investigated and the strength of solvent-polymer interactions was found to decrease in the order water > methanol > dioxane. These results are consistent with those involving a pyrene-labeled poly(acry1ic acid), PyPAA, for which the value of the monomer to excimer fluorescence emission ratio is correlated with the same solvent-dependence of polymer conformation. The influence of salt concentration was investigated, and the study of phosphorescence lifetimes as a function of added electrolytes allowed for the determination of the radius of gyration of the polymer in salt solution. The influence of solvent viscosity on polymer mobility and conformation was also studied by the phosphorescence probe technique. Quenching of phosphorescence with an aqueous phase quencher, NaN02, elucidated the role of polymer conformation on the quenching process.

Spin-polarized xenon-129 NMR study of a polymer surface

A method for NMR studies of low surface area materials using optically pumped xenon gas is described. The method has been used to investigate spin-polarized xenon adsorbed onto poly(acry1ic acid). The temperature dependence of the xenon-surface interaction, as measured by the xenon chemical shifts extrapolated to zero pressure, is shown to be consistent with a simple model of chemical exchange between the gas and adsorbed phases. The magnitude of the surface contribution to the 129Xe chemical shift indicates a relatively strong interaction between xenon and poly(acry1ic acid), possibly due to the polar carboxylic acid functional groups at the polymer surface. From the pressure dependence of the It9Xe shift, the diffusion coefficient of xenon on poly(acry1ic acid) is estimated to be 3.3 X lo-' cm2/s. I. Iatroduction The surface and interfacial characteristics of solid systems, such as inorganic solids, polymers, and thin films, are important to many of their useful material properties. The task of relating the microscopic structure to their macroscopic physical properties, however, has been hindered by the lack of suitable techniques capableof probing these materials. Due to their instability under ultrahigh-vacuum conditions, organic solids and catalytic materials such as metal oxides are difficult to study by the surface science techniques that have been successfully employed for singlecrystal studies. As a nonintrusive experimental method capable of giving dynamic and structural information at an atomic level, nuclear magnetic resonance spectroscopy should be an attractive probeof heterogeneous surfaces. Indeed, through thedevelopment of high-resolution techniques, NMR has contributed greatly toward the chemical and physical understanding of bulk so1ids.l The main obstacle to the successful application of NMR to surface studies is, of course, its low sensitivity compared to other spectroscopic methods as well as the problem of distinguishing between the surface and bulk spins. Various approaches to increasing the NMR signal have been pursued, such as isotopic enrichment, high applied magnetic fields, millikelvin NMR? and cross polarization.3 These experiments, which are based on increasing the quilibrium magnetization, do not, in general, give the signal enhancements rquired to detect the surface spins of materials with surface areas lower than - 10 m2/g. Another approach toward enhancing the magnetic resonance signal is to produce a large nonquilibrium magnetization by optical p~mping.~ Large nuclear spin polarization of xenon and other noble gases can be created via spin exchange collisions with Optically pumped alkali metal vapor. These experiments have long been employed to measure fundamental physical quantities and to study spin relaxation processes in the gas phase.5 Due to the formation of relatively long lived van der Waals molecules, xenon can be efficiently spin polarized through collisions with optically pumped rubidium vapor: allowing the investigation of surface interactions between i3lXe gas and glass

Phosphorescence from a bromonaphthalene lumophore as a photophysical probe of polymer conformation and interpolymer interactions

A bromonaphthalene-labeled poly(acry1ic acid), BNPAA, was synthesized and found to be phosphorescent in solution at room temperature. The phosphorescence of the polymer was employed to investigate (1) the influence of pH on the conformations of the polymer in aqueous solutions, (2) the interactions of BNPAA with complementary polymers (poly( 1-vinyl-2-pyrrolidinone) and poly(ethy1ene oxide)), and (3) conformations of the polymer adsorbed on a solid-liquid interface. The phosphorescence lifetimes of BN- PAA provide a probe of changes in polymer conformations that result from inter- and intrapolymer interactions. The results suggest an unusual pH dependence for BNPAA conformations in dilute solutions; e.g., significant breaks in the lifetime/pH profile are found at pH 4 and pH 10. These unexpected results were confirmed by the observation of the same two breaks at pH 4 and pH 10 with the use of two independent photolu- minescence probes, a pyrene-labeled poly(acry1ic acid), PyPAA, and a ruthenium complex, Ru(byp)gZ+. The two breaks in the conformation/pH profile are explained in terms of combination of hydrogen bonding and a hydrophobic substituent effect at low pH and charge screening at high pH. Phosphorescence lifetime measurements are also used to probe the interactions of the BNPAA with complementary macromolecules such as poly( 1-vinyl-2-pyrrolidinone) and poly(ethy1ene oxide) and with a solid alumina surface. Finally, the interaction of BNPAA with a low molecular weight substance, the cationic surfactant dodecyltrimethylam- monium bromide, was also investigated by employing the phosphorescent probe.

Photophysical investigations of the interactions of phospholipids and a pyrene-substituted poly(acrylic acid) in dilute solutions

Fluorescence spectra of a pyrene-substituted poly(acry1ic acid) (1) were measured in the presence of dipalmitoylphosphatidylglycerol (DPPG) at pH values of 4 and 7 to investigate the interactions between 1 and phospholipids. These photophysical investigations showed that at pH 4 mixed micelles of 1 and phospholipids were formed with the incorporation of 1 in the hydrophobic core of the mixed micelle. The measurements of fluorescence decay and quenching of fluorescence by nitromethane in solutions of 1 and DPPG also supported this model for the interactions of 1 and phospholipids. These results were in agreement with a calorimetric investigation of the interactions between phospholipids and poly(acry1ic acid) derivatives (Tirrell, D. A.; Takigawa, D. Y.; Seki, K. Ann. N.Y. Acad. Sci. 1985, 446, 237). The effects of head group and temperature on the interactions of 1 and phospholipids were also investigated. Investigations of the interactions of phospholipids and synthetic polymers could be expected to play important roles in the pursuit of scientific knowledge of the structures and functions of biological macromolecules and mem- branes. These investigations are also of importance for the synthesis of novel functional polymers for reactions in organized media and to mimic the functions of biological molecules. Tirrell and co-workers1,2 have investigated the interactions of a variety of synthetic polymers and phos- pholipids in aqueous solutions. Photophysical techniques have been extensively used in the investigations of the structure and functions of synthetic and biological mac- romolecule~.~ Photophysical properties of the pyrene chromophore covalently bound to poly(acry1ic acid) (PAA) have been used by Turro and Arora4 to investigate the inter- and intrapolymer interactions in dilute aqueous solutions of PAA and complementary polymers. Intra- chain interactions of pyrene groups in pyrene-substituted PAA (1) investigated by measurements of fluorescence spectra, fluorescence lifetimes, and quenching of fluores- cence were found to be useful probes for inter- and in-

Crosslinked poly(sodium acrylate) hydrogels

Samples of poly(acry1ic acid) (PAA) neutralized to varying degrees with sodium hydroxide were subjected to γ-irradiation to yield insoluble crosslinked polyacrylate hydrogels. The efficiency of crosslinking was found to be dependent on the degree of neutralization of the sample and the concentration of the irradiated polymer solution. The extent of water uptake of the gel was shown to be related to the degree of neutralization by titration of acid groups within the gel subsequent to cross-linking. Lightly crosslinked gels exhibited a maximum degree of swelling at a particular neutralization level. This was ascribed to a maximum in the long-range forces exerted cooperatively by the polymeric charges. The presence of simple electrolyte ions reduces swelling by screening these forces.

Dehydration kinetics and glass transition of poly(acrylic acid)

AbstractThe kinetics of dehydration and decarboxylation as well as the glass transition temperature as a function of anhydride content were measured for poly(acry1ic acid). It was found that the glass transition of PAA is of the order of 103°C and increases with increasing anhydride content, reaching an extrapolated value of 140°C for the pure linear anhydride. Anhydride formation is a firstsrder reaction, as is also decarboxylation, the latter being much slower than the former. The rate constants are for dehydration, ka = 2.5 × 109 exp {−26000/RT}; for decarboxylation, kd = 2.9 × 108 exp {−27000/RT}. Anhydride formation occurs primarily by an intramolecular process.

An ion-exchange process with thermal regeneration. III. Properties of weakly acidic ion-exchange resins

A series of cross-linked poly(acrylic acid) and poly(methacrylic acid) resins has been synthesized. Their titration curves, and those of similar commercial resins, show that at c. 20� the pH value at half neutralization may be varied within about one unit by using acrylic or methacrylic acid monomers, or by changing the degree of cross-linking. The titration curves of such resins do not have a pronounced plateau although the acidity is due only to carboxyl groups. Resins with flatter titration curves can be made by copolymerizing acrylic or methacrylic acids with methyl methacrylate. Copolymerization reduces the effective acid strength of carboxylic acid resins but does not always produce a resin with a flatter titration curve. The effects of salt concentration, calcium ions, and temperature on the titration curves of a poly(acry1ic acid) resin have been studied. Heating the resins in a salt solution from c. 20 to 80� decreases their effective acidity slightly. A bigger reduction in acidity with heating is observed with several polymers in which a "snake" polymer, such as a poly(ethy1ene oxide), is incorporated within a cross-linked poly(acrylic acid) resin cage.

An ion-exchange process with thermal regeneration. IV. Equilibria in a mixed bed of weak-electrolyte resins

The titration curve data at c. 20� and 80� for weakly basic amine and carboxylic acid resins suggest that if a mixed bed of such resins is in equilibrium with a salt solution, then when the temperature of the system is raised, protons will be transferred from the base to the acid resin to an extent determined by the overlap of the titration curves of the resins. Maximum overlap would occur with two resins of appropriate acid and basic strength and which possess plateau-type titration curves, but such acid resins are not available. The theory has been confirmed by experiment. The change in salt adsorption on heating (effective capacity) depends critically on pH and may be enhanced for amine resins of homo-functional structure by increasing the ratio of acid to base resin. It is shown that to achieve the maximum effective capacity for each resin combination there is an optimum pH value and resin ratio. A screening procedure, which uses titration curve data at c. 20� and 80� and allows for the effect of pH and resin ratio, has been devised for selecting resin combinations. In a 0.030M salt solution the effective capacity of the commercial resins studied is greatest, for a poly(vinylbenzyldiethy1amine) resin in combination with a poly(acry1ic acid) resin. A-rigorous procedure has been devised for obtaining equilibrium data, which are presented for this resin combination at c. 20� and 80� in sodium chloride solutions with concentrations ranging from 0.0017 to 0.13M.