Viscoelastic study of the conformational transition of poly(methacrylic acid) in dilute aqueous solution

The dynamic viscosity of dilute aqueous solution of poly(methacrylic acid) (PMA) was measured over the frequency range 2–500 kHz for various degrees of ionization. The relaxation spectrum calculated from the viscosity exhibits a conformational relaxation spectrum in the short-time region and a rotational one in the long-time region. The former is fitted by the Zimm theory over the entire range of ionization, including the transition from a compact structure to an open one. The conformational relaxation time begins to increase at the midpoint of the transition region, but the rotational relaxation time increases very markedly in the initial stage of the transition. The present and previously reported experimental findings are interpreted in terms of the assumption that the compact structure is stabilized by short-range methyl–methyl bonding as well as “long-range” methyl–methyl bonding (bonding between methyl groups spatially close but remotely connected along the chain contour). In the initial stage of the transition, the latter primarily break up, resulting in expansion of overall chain dimensions, and subsequently the former are destroyed, resulting in the change in local chain conformation.     

Effect of urea on poly(methacrylic acid) and poly(acrylic acid) aqueous solutions

The effects of urea on aqueous solutions of both poly(methacrylic acid (PMA) and poly(acrylic acid) (PAA) have been investigated by using potentiometry, viscometry and study of the fluorescence of Auramine O, a cationic dye. The viscosity behaviour of unionized PMA obtained from direct dissolution of solid powder shows that the unneutralized macromolecules can be associated in water. The stability of such “aggregates” seems weak as indicated by their disappearance as soon as the charge density is very low. For PMA salt solution percolated through a cation (H⁺) exchange resin column, no association is observed. The pH-dependent conformational behaviour of PMA which, contrary to PAA, presents compact conformations in water at low charge density is discussed in terms of solvophobic/solvophilic interactions. It is shown that, even for urea concentration up to 8 M, the compact conformations of PMA are not completely destroyed. The formation of H⁺/urea complex is taken into account.     

Conformational transition of poly(methacrylic acid) in methanol–water mixtures

The effect of ionization on the potentiometric and viscometric behavior of both poly(methacrylic acid) (PMA) and poly(acrylic acid) (PAA) was investigated in methanol–water mixtures. The potentiometric and viscosity data revealed that the conformational change which takes place for PMA in water gradually disappears with increasing content of methanol in methanol–water mixtures. For mixtures containing 40 and 50% methanol, the behavior of PMA solutions becomes similar to that of PAA, i.e., the discontinuity versus the degree of ionization is no longer detectable and the molecule unfolds regularly upon ionization. The addition of methanol leads to the breakdown of the compact structure of PMA which is stabilized, at low degree of ionization in water, by nonelectrostatic interactions between the methyl groups of PMA.     

Dipole moment and conformational properties of poly(methacrylic acid) in solvents with hydrogen bonding

Dielectric polarization of solutions of un-ionized linear poly(methacrylic acid) in polar associated liquids is studied in the temperature range 20–50°C. The solutions are in methanol, with the molar fraction of polymer units x ₂ = (3 × 10^?3)?(1.5 × 10^?2), and in water, with x ₂ = (4 × 10^?5)?(4 × 10^?3). The permittivity ε₁₂ of the polyacid solutions in methanol is shown to be lower than the permittivity of the pure solvent ε₁; the permittivity of the polyacid solutions in water exceeds ε₁ of water in the concentration range x ₂ = (4 × 10^?5)?(2.13 × 10^?4) and becomes lower than ε₁ as the polymer concentration in the solution increases further. A procedure for estimating the dipole moment μ₂ per monomer unit of the polymer macromolecule in solution is proposed. The estimation is based on Buckingham’s statistical polarization theory for a two-component mixture of polar molecules under the conditions of infinite dilution. The μ₂ values amount to 2.76–2.14 D (x ₂ < 1.5 × 10^?2) in methanol at 20–50°C and to 11.4?3.8 D (x ₂ < 2.13 × 10^?4) in water at 20–40°C. The difference in the dipole moments of the polyacid and in the patterns of their temperature dependences in methanol and in water is due to the effects of the polyacid-solvent hydrogen bonding, to intramacromolecular hydrogen bonds, and to specificity of the local structure of the solvent. It is shown that the μ₂ value corresponds to the dipole moment of the solvates and decreases with temperature owing to changes in the stoichiometry of the solvates, to the formation of cyclic associates in the macromolecule, and to conformational changes in the chain.     

Effect of pressure on dissociation of poly(methacrylic acid) in aqueous solution

The effect of pressure (up to 6300 atm) on the electrical conductivity of poly(methacrylic acid) (PMA) in aqueous solution was investigated in the concentration range 0.001M to 0.1M (in equivalents of carboxyl groups) at 30°C. The degree of ionization α calculated from the conductivity data increased linearly with pressure. From dilatometric measurements of volume changes when PMA was partially neutralized with NaOH, a conformational transition was confirmed to take place above α = 0.12, and the volume change accompanying the dissociation of the side chain groups (carboxyl groups) was found to be ? 12.5 ml/H⁺. It may be inferred from the values of α at each polymer concentration and from the dilatometric data that there is no conformational transition from a compact structure to the coiled form below at least 6,000 atm in the absence of Na⁺ counterions.     

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.     

The synthesis and characterization of monodisperse poly(acrylic acid) and poly(methacrylic acid)

A number of polyacrylic (PAA) and polymethacrylic (PMAA) acids have been synthesized by living anionic polymerization of the monomeric tert-butyl esters followed by subsequent hydrolysis of the corresponding polyesters. The necessary precautions were taken in order to assure good molecular weight control, as well as high yields in the polymerization reactions. The intermediate and final polymers were characterized by gel permeation chromatography and NMR-H¹ spectrometry.     

Calorimetric investigation of poly(methacrylic acid) and poly(acrylic acid) in aqueous solution

The enthalpy of dissociation of poly(acrylic acid) and of poly(methacrylic acid) in water and in 0.5N NaCl at 25°C has been measured over a wide range of degrees of neutralization of the polyelectrolytes. In the case of poly(methacrylic acid) the calorimetric data permit the direct evaluation of the enthalpy of conformational transition of the polymer. For this transition, with the aid of standard free energy data derived from potentiometric titrations, the change in entropy was also estimated. The relative accuracy of the thermodynamic data, and the possibility of deriving therefrom information on the mechanism of transitions of the type, globular coils → expanded coils for partially hydrophobic synthetic polyelectrolytes in aqueous solution are discussed.     

Binding of Cu(II) to poly(methacrylic acid)

The binding of Cu(II) ions to partly neutralized poly(methacrylic acid) (PMA) has been investigated by potentiometric titration and dialysis to determine the stoichiometry the Cu–PMA complexes formed. Partly ionized PMA was titrated with solutions of the metal ion to enable a large range of metal ion/polymer ratios to be studied. Combination of the results from these two techniques at ionic strength 0.1 indicates that at very low Cu(II)/polymer ratios, a 4:1 complex exists, but at higher ratios the complex breaks down to give a mainly 2:1 coordination with some 1:1 binding. Conductance titrations support these results. Viscometric titrations show strong interactions between the metal and polymer, preventing the full extension of the polyion at high degrees of ionization, and spectrophotometric titrations support the existence of at least two types of complexes in the solution.     

Diffusion of dextran within poly(methacrylic acid) hydrogels

We describe an investigation of fluorescence correlation spectroscopy into the diffusion of fluorescein‐tagged dextran (FDEX) in a poly(methacrylic acid) (PMAA) hydrogel. The temperature dependence of FDEX diffusion is shown to follow Zimm behavior in pure water, and the decrease in the diffusion coefficient when in the PMAA hydrogel has been modeled. The addition of acid and alkali (HCl and NaOH, respectively) not only control the swelling and collapse of the hydrogel but also reveal a strong pH dependence of the dextran diffusion coefficient, which shows a (nonmonatonic) increase with pH. The addition of NaCl and CaCl₂ salts similarly showed evidence of network swelling, most notably at low salt concentration, but also that the diffusion coefficient within the gel at these low concentrations is larger than that in the equivalent solution without the hydrogel, indicating that the combination of hydrogel and salt works to increase the diffusion coefficient above that in pure water. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Binding of a surfactant counterion to low-charge-density poly(acrylic acid) and poly(methacrylic acid)

The binding of a cationic surfactant, dodecylpyridinium (C12Py) chloride, with a low-charge-density poly (methacrylic acid) (PMA) was investigated in buffer solutions under the condition of constant pH. The binding isotherms with PMA consisted of two and three steps at a pH lower and higher than 3.2, respectively. Bindings in the first step were independent of pH and this step was considered to correspond to the solubilization of the hydrocarbon chains of C12Py into the nonpolar region of the compact form of PMA. This is the indication of the compact form from the binding isotherm. At pH higher than 3.2, the second step was discriminated and it depended on the pH. In the third step, a sharp rise in the degree of binding (β) was observed accompanying the solubilization of the precipitates of the PMA–C12Py complex. The binding with poly(acrylic acid) (PAA) and PMA in conventional unbuffered NaCl solutions was also examined and the pH profile of the solution during the binding process was determined. In the case of unbuffered NaCl solutions, the binding with PAA took place cooperatively at the critical association concentration (cac). The binding isotherm consisted of two steps and the pH decreased with the increase in β. The binding isotherm of PMA, on the other hand, consisted of three steps: the pH decreased slightly in the first step and considerably in the second step with the increase in β but it increased with β in the third step, exhibiting a pH minimum around 3.2. The binding in the first step coincided with that obtained in the buffered solutions. Linear relationships between β and the pH were found for both polymers. In the case of PMA, no cac was observed in both buffered and unbuffered NaCl solutions. Received: 24 January 2001 Accepted: 23 May 2001     

pH-responsive poly(methacrylic acid)-grafted hollow silica vesicles

Poly(methacrylic acid)-grafted hollow silica vesicles (PMAA-g-hollow silica vesicles) were obtained through a grafting-from approach. PMAA brushes were formed by performing atom-transfer radical polymerisation of sodium methacrylate with an initiator attached to the hollow silica spheres. PMAA-g-hollow silica vesicles were characterised by using TEM, thermogravimetric analysis (TGA) and FTIR spectroscopy. pH-dependent ξ potential and (1)H NMR spectra of PMAA-g-hollow silica vesicles were measured, and the results indicated that MAA brushes in PMAA-g-hollow silica vesicles had a lower ionisation degree and low solubility in acidic aqueous solution, for example, pH 3.4, but a higher ionisation degree and high solubility when the pH was higher than 7. Also it was demonstrated that calcein blue and fluorescein isothiocyanate (FITC) labelled dextran (M(n):10 kDa) could be encapsulated in the interiors of the PMAA-g-hollow silica vesicles with a negligible amount in PMAA brushes at pH 2, and pH-triggered release of calcein blue and FITC-labelled dextran from PMAA-g-hollow silica vesicles was observed at pH 7.4.     

Investigation of conformational transition in poly(cyclohexyl methacrylate)

The variations of intrinsic viscosity and refractive index increment with temperature for poly(cyclohexyl methacrylate) in isobutyl methyl ketone have been studied. A conformational transition has been found in the range 10–30°C. The influence of the molar volume of the side-group on the conformational transition temperature has also been discussed with respect to results for related polymers.     

Plasma-induced graft copolymerization of poly(methacrylic acid) on electrospun poly(vinylidene fluoride) nanofiber membrane

Electrospun nanofibrous membranes (ENM) which have a porous structure have a huge potential for various liquid filtration applications. In this paper, we explore the viability of using plasma-induced graft copolymerization to reduce the pore sizes of ENMs. Poly(vinylidene) fluoride (PVDF) was electrospun to produce a nonwoven membrane, comprised of nanofibers with diameters in the range of 200-600 nm. The surface of the ENM was exposed to argon plasma and subsequently graft-copolymerized with methacrylic acid. The effect of plasma exposure time on grafting was studied for both the ENM and a commercial hydrophobic PVDF (HVHP) membrane. The grafting density was quantitatively measured with toluidine blue-O. The degree of grafting increased steeply with an increase in plasma exposure time for the ENM, attaining a maximum of 180 nmol/mg after 120 s of plasma treatment. However, the increase in the grafting density on the surface of the HVHP membrane was not as drastic, reaching a plateau of 65 nmol/mg after 60 s. The liquid entry permeation of water dropped extensively for both membranes, indicating a change in surface properties. Field emission scanning electron microscopy micrographs revealed an alteration in the surface pore structure for both membranes after grafting. Bubble point measurements of the ENM reduced from 3.6 to 0.9 um after grafting. The pore-size distribution obtained using the capillary flow porometer for the grafted ENM revealed that it had a similar profile to that of a commercial hydrophilic commercial PVDF (HVLP) membrane. More significantly, water filtration studies revealed that the grafted ENM had a better flux throughput than the HVLP membrane. This suggests that ENMs can be successfully engineered through surface modification to achieve smaller pores while retaining their high flux performance.     

Activity of counterions in hydrogels based on poly(acrylic acid) and poly(methacrylic acid): Potentiometric measurements

The behavior of partially ionized weakly crosslinked gels based on poly(acrylic acid) and poly(methacrylic acid) undergoing contraction in the presence of a low-molecular-mass salt is studied experimentally. The concentration dependences of the enthalpies of swelling of gels in water and aqueous solutions with potassium chloride concentrations of 1, 10, and 100 mmol/L are determined via the method of isothermal calorimetry. On the basis of the obtained data, the enthalpy parameter of interaction between a polymer network and a medium is estimated as a function of the amount of the salt. This parameter does not exceed 0.3 and monotonically decreases with an increase in the concentration of the salt. The Donnan potential of gels depending on the concentration of KCl in the external solution is measured via the method of potentiometry with the use of capillary electrodes, and the activity of potassium counterions in the medium of hydrogel is calculated. The main factor that causes contraction of polyelectrolyte gels in a solution of a low-molecular-mass salt is a decline in the activity of counterions that leads to a decrease in the osmotic pressure inside the gel. A decline in activity may be associated with both a reduction in the activity coefficient and ionic association processes in the hydrogel.     

Molecular structure and dynamics of poly(methacrylic acid) and poly(acrylic acid) complexes with dodecyl-substituted poly(ethylene glycol)

The molecular dynamics and the structure of molecular complexes formed by micelles of dodecyl-substituted poly(ethylene glycol) with poly(methacrylic acid) and poly(acrylic acid) in aqueous solutions were studied by viscosimetry, pH measurement, and electron spin resonance spin-probe techniques. At low surfactant concentrations, the conformation of the complex is a compact globule. The local mobility of surfactant molecules in such a complex is much slower than that in the free micelle. At high surfactant concentration, the nonionic micelles and polyacids form hydrophilic associates. The associates have the conformation of extended coils. In an associate, a major part of the micellar poly(ethylene glycol) groups is free. The local mobility of the micellar phase depends on the number of micelles involved in an associate. The mobility of surfactant molecules is slower in the complexes of poly(methacrylic acid) than in the complexes of poly(acrylic acid).     

Influence of stereoregularity on binding of counterions by poly(methacrylic acid)

The activity coefficient γ, of sodium ion, in aqueous solutions of both isotactic and conventional poly(methacrylic acid) (PMA) has been studies throughout the whole range of degree of neutralization α with a Beckman Na electrode and membrane electrodes. The results show that the fraction of sodium ions bound to isotactic PMA is higher than that bound to conventional (or syndiotactic) PMA over the entire range of α. The binding of divalent counterions (Mg⁺², Cu⁺², Co⁺², and Ni⁺²) by the two forms of PMA (at α = 0.95) has been evaluated from the release of Na⁺ ions with the Beckman Na electrode. Turbidimetric determinations of the critical amount of various divalent cation chlorides necessary to bring about precipitation of both isotactic and conventional PMA (at α = 0.95) have also been carried out.