The effect of primary structural parameters of poly(methacrylic acid) xerogels on the kinetics of swelling

Xerogels of poly(methacrylic acid) (MAXG) with different primary structural parameters (PSP) [number average molar mass between the network crosslinks (M_c), the crosslink degree (ρ_c), and the distance between the macromolecular chains (d)] are successfully synthesized by crosslinking free‐radical polymerization. The isothermal swelling curves of synthesized MAXG are measured in distilled water in the temperature range of 25–40°C. The swelling conversion curves of all the MAXG may be described with kinetics model of phase boundary controlled reaction, i.e., contracting area (R2). The values of swelling kinetics parameters (E_a and lnA) linearly increase with the increase in M_c. This work presented a novel and quantum model of the mechanism of xerogel's swelling activation process based on resonant coupling of vibration oscillation of water molecules and vibration modes of xerogel. A quantum nature of E_a of swelling process is proved and explained the effects of PSP on the values of swelling kinetics parameters. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

The effects of reaction conditions on the electrical conductivity of PAAG hydrogels

The effects of reaction conditions: concentration of crosslinker, monomer, initiator and gelatin and neutralization degree (ND) of acrylic acid (AA), of a graft radical crosslinking polymerization of AA and gelatin, on the specific electrical conductivity and the primary structural parameters of synthesized poly(acrylic acid)-g-gelatin (PAAG) hydrogels were investigated. It was established that: (a) electrical conductivity of all of the investigated hydrogels is higher than of distilled water; (b) the increasing concentration of crosslinker, monomer and initiator leads to the two linear distinct increases of electrical conductivity with different slopes (c) the increasing ND of AA leads to linear decrease in electrical conductivity and (d) gelatin concentration does not significantly influence the electrical conductivity of hydrogel. An analyses of the primary structural parameters of synthesized PAAG xerogels reveals that: (a) values of molar mass between crosslinks and distance between polymer chains are power form function on crosslink density (ρ_c); (b) the critical value of ρ_c corresponds to the percolation threshold of H⁺/K⁺ ions through the hydrogels network and (c) electrical conductivity of PAAG hydrogels is a power form function on ρ_c. Fractal model of conductivity of hydrogel is suggested and explained.     

Influence of poly(acrylic acid) xerogel structure on swelling kinetics in distilled water

Summary The basic structural properties of xerogels of crosslinked poly(acrylic acid) were defined and determined: xerogel density (ρ_xg), xerogel volume fraction in the equilibrium-swollen state (v₂), the number average molar mass between network crosslinks , the crosslink density (ρ_c) and the distance between macromolecular chains (d). A crosslinking ratio (X) increase leads to a linear increase in the values for ρ_xg and ρ_c, while the values and d decrease. The isothermal swelling kinetic curves of four samples of structurally different poly(acrylic acid) xerogels in bidistilled water at different temperatures ranging from 25 to 45 °C were determined. It is shown that isothermal kinetic swelling curves could not be described with the model of first-order reaction kinetics in entire. It was found that these curves could be described by the Johanson-Mampel-Avrami (JMA) equation. For all of the investigated xerogel samples, the initial swelling rate (v_in), effective reaction rate constant (k) and equilibrium swelling degree increased with swelling temperature increase. Based on the determined values of the v_in and k, the activation energy (E_a) and pre-exponential factor (lnA) were determined. It was concluded that the activation energy linearly increased with increasing distance between macromolecular chains (d) and molar mass between the network crosslinks . The relationship between the activation energy changes with pre-exponential factor (compensation effect) caused by xerogel structural properties was established. Isothermal swelling kinetics could be completely described by the kinetics of phase transition of the xerogel transformation from glassy to rubbery state, i.e. with the JMA kinetic equation.     

Synthesis and properties of novel superabsorbent hydrogels with mechanically activated sugarcane bagasse and acrylic acid

The aim of this study was to develop a sugarcane bagasse (SCB)-based biodegradable superabsorbent hydrogels (SAH) with a good swelling. To this end, SCB was firstly mechanically activated by home-made high efficiency stirring mill to enhance its reactivity by breaking the lignin seal and decreasing crystallinity of cellulose in SCB. Then, the SAH were synthesized by graft copolymerization of acrylic acid (AA) onto SCB with different mechanical activation times (t _M) by using ammonium persulfate/sodium sulfite redox pair as an initiator in the presence of a crosslinker (N,N′-methylenebisacrylamide, MBAAm). The effect of t _M on the equilibrium swelling capacity (Q _eq), swelling kinetics of the SAH in deionized water, as well as the influences of pH, electrolytic media, and temperature on Q _eq were studied. In addition, the products were characterized by scanning electron microscopy and differential scanning calorimetry. The results showed that mechanical activation promoted the graft copolymerization and thereby altered the Q _eq of the SAH. The swelling process of the SAH exhibited anomalous swelling behavior and first-order dynamics, and the Q _eq of the SAH was pH, salt, and temperature dependent.     

Swelling of High Oil-Absorptive Network Based on 1-Octene and Isodecyl Acrylate Copolymers

Crosslinked 1-octene-isodecyl acrylate copolymers were synthesized and evaluated for oil-absorbency application. The copolymer was crosslinked at different concentrations of ethylene glycol diacrylate (EGDA) and ethylene glycol dimethacrylate (EGDMA) crosslinkers via catalytic initiation or by electron-beam irradiation at dose rate 80 kGy. The concentration of both crosslinkers was varied from 0.5% to 2%. The effect of crosslinking conditions, such as crosslinker concentration, method of polymerization and monomers concentration on conversion and gel fraction was examined through an oil-absorption test using petroleum crude oil. It was found that, the oil absorbency was influenced mainly by the degree of crosslinking and the hydrophobicity of the copolymer units. The final equilibrium oil content, volume fraction of polymer and swelling capacity were determined at 298 K. The effective crosslinking density V_e, the average molecular weight between the crosslinks M_c and the polymer-toluene interaction parameter were determined from stress–strain measurements. The crosslinking efficiencies of EGDA and EGDMA towards copolymers were determined.     

Preparation conditions and swelling equilibria of dextran hydrogels prepared by some crosslinking agents

Crosslinking reactions of Dextran (Dx) (M_n of 2.0×10⁶ g mol⁻¹) with some selective Cl-, P- and N-containing functional monomers such as epichlorohydrin (ECH), phosphorus oxychloride (POC1₃) and N,N′-methylenebisacrylamide (MBAM) were carried out in the basic aqueous solutions (2.8 N NaOH) at 25-50 °C. The optimum conditions of the effective swelling and crosslinking for the each system studied were found in copper (CuCl₂·2H₂O) solution. The percent swelling, equilibrium swelling, initial rate of swelling, swelling rate of constant, equilibrium water content, and diffusion type and constant values were evaluated for Dx/crosslinker (CL) systems at 1 mg/100 ml copper (CuCl₂·2H₂O) solution. A substantial difference of these parameters observed for the various Dx/CL systems was explained by the effect of nature of crosslinking agents on the mechanism of crosslinking and swelling processes. It was shown that S_eq and M_c values increase depending on the nature of CLs in the following order: ECH>MBAM>POCl₃. General scheme and proposed mechanism of crosslinking reactions in the Dx/CL systems were also described.     

Water Uptake Kinetics and Control Release of Agrochemical Fertilizers from Nanoclay-Assisted Semi-interpenetrating Sodium Acrylate-Based Hydrogel

Semi-interpenetrating composite hydrogels composed of poly(acrylic acid-co-sodium acrylate)/polyethylene glycol for the controlled release of fertilizers were synthesized by using nano-phyllosilicate. Hydrogel was characterized by infrared spectral analysis (Fourier transform infrared), X-ray diffraction, and scanning electron microscopy. The cross-link density (ρ_c), average molecular weight between cross-links (M_c), and the mesh size of the network (?) were also determined. The degree of neutralization of acrylic acid component has a positive impact in the swelling behavior of hydrogel, which was investigated through its swelling study and pH-oscillatory behavior. The dependency of synthesis parameters, pH and ionic strength on swelling characteristics were investigated. The type of the diffusion phenomena was investigated by calculating various kinetic parameters such as the diffusion coefficient, diffusion rate constant, swelling exponents, etc. The swelling ratio with varying synthetic parameters was recorded to investigate fourth-order factorial model. The model gives the idea of the effect of the independent parameters on the swelling ratio.     

Estimation of thermodynamic quality of the solvent from the concentration effect in gel permeation chromatography of polymers

Dependences of the elution volume, V_e, on the concentration of the injected polystyrene (PS), c, in gel chromatography with inorganic carriers were studied in various single and binary eluents. It was found that the dependence of V_e on c is approximately linear in the region of low concentrations and its slope depends on the thermodynamic quality of the eluent. This relationship is more pronounced for higher molecular weights. Correlation for the slope k with the limiting viscosity numbers [η] proposed earlier in the literature is not valid for thermodynamically poor eluents; however, a correlation was found for all the eluents employed between k and the product A₂M, where A₂ is the second virial coefficient of polymer in solution. This supports the interpretation of the concentration effect in gel chromatography of polymers as a consequence of the decreasing effective hydrodynamic volume of coils with increasing concentration. The link between the empirically found correlation of k with A₂M and the existing theory of the concentration effect in g.p.c. was examined. Comparison of the virial coefficients determined from the slope k for PS in various eluents with the literature data showed that gel chromatography could yield a rapid estimate of the value A₂. As has been shown by an analysis of the concentration effect on the distribution coefficient K the concentration effect leads to the formation of a non-linear isotherm and to the asymmetrical peak shapes. In the range of very low concentrations, deviations from the linear dependence of V_e on c can be expected as a result of both the sorption effect and the thermodynamic partition of the solute.     

Synthesis and characterization of dextran hydrogels prepared with chlor‐ and nitrogen‐containing crosslinkers

In this study, we have synthesized dextran hydrogels by the crosslinking reactions of dextran with some selective Cl‐, and N‐containing functional monomers, such as epichlorohydrin (ECH), N,N′‐methylenebisacrylamide (MBAm), and glutaraldehyde (GA). Crosslinking reactions were carried out in the basic aqueous solutions (2.8NNaOH) at 25–50°C. The optimum conditions for effective crosslinking, i.e., temperature, crosslinking time, and amount of crosslinker, were determined for each system. The hydrogel discs of 3 mm diameter and 1.5 mm thickness were subjected to a number of Tris‐buffer solutions of desired pH (2.0–9.0) at 37°C. Swelling kinetics of the hydrogels were evaluated with second–order swelling model. The pH‐dependent swelling of hydrogels was strongly influenced by the functional group of crosslinker and crosslinker content. While the hydrogels prepared with ECH and MBAm shows higher swelling ability at basic medium than that of acidic medium, GA‐containing hydrogels exhibited just the opposite behavior. Mesh sizes (ξ) and average molecular weights between crosslinks (M_c) were estimated from swelling data using the Flory‐Rehner theory. Characterization studies were completed by Fourier transform infrared spectroscopy and thermal gravimetric analysis. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102:4213–4221, 2006     

Development and evaluation of pH-dependent interpenetrating network of acrylic acid/polyvinyl alcohol

The objective of the present work was to synthesize interpenetrating networks (IPNs) of acrylic acid/polyvinyl alcohol (AA/PVA) by free radical polymerization using N,N-methylenebisacrylamide (MBAAm) and glutaraldehyde as cross-linkers. The IPNs were evaluated for swelling, diffusion coefficient and network parameters by using Flory–Huggins theory, i.e., the average molecular weight between cross-links (M _c), polymer volume fraction in swollen state (V _2,s), number of repeating units between cross-links (M _r) and cross-linking density (N). It was found that the degree of swelling of AA/PVA interpenetrating network increases greatly within the pH range 5–7 depending on composition. The gel fraction and porosity increased by increasing the concentration of AA or PVA, while by increasing the degree of cross-linking, porosity decreased and gel fraction increased. Selected samples were loaded with chlorpheniramine maleate as a model drug. Drug release was studied in USP, hydrochloric acid buffer solution of pH 1.2 and phosphate buffer solutions of pH 5.5 and 7.5. Drug release data were fitted into various kinetics models, e.g., zero-order, first-order, Higuchi and Peppas models. The results of the kinetics investigation showed that the drug release from IPNs followed non-Fickian diffusion. Fourier transform infrared spectra confirmed the formation of cross-linked IPNs as there was a shifting to lower frequency of 1,713–1,718 cm^?1 with reduced intensity, while scanning electron microscopy revealed uniform distribution of drug in IPNs.     

Synthesis and characterization of oil sorbers based on docosanyl acrylate and methacrylates copolymers

Docosanyl acrylate (DCA) monomer was copolymerized with different monomer feed ratios of cinnamoyloxy ethyl methacrylate (CEMA) or methyl methacrylate (MMA) monomer to produce different compositions for DCA/CEMA or DCA/MMA copolymer with low conversions.¹H NMR spectroscopy was used to confirm the copolymer structure. DCA was crosslinked with different mol % of CEMA or MMA using dibenzoyl peroxide as initiator and various weight percentages of either 1,1,1‐trimethylolpropane triacrylates or 1,1,1‐trimethylolpropane trimethacrylates crosslinkers. The effects of monomer feed composition, crosslinker concentration, and the hydrophobicity of the copolymer units on swelling properties of the crosslinked polymers were studied through the oil absorbency tests. The network parameters, such as polymer solvent interaction (χ), effective crosslink density (υ_e), equilibrium modulus of elasticity (G_T), and average molecular weight between crosslinks (M_c), were determined and correlated with the structure of the synthesized copolymers. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Ionizable interpenetrating polymer networks of carboxymethyl cellulose and polyacrylic acid: Evaluation of water uptake

A semi interpenetrating polymer network (IPN) of carboxymethyl cellulose (CMC) and crosslinked polyacrylic acid (PAA) has been prepared and its water‐sorption capacity has been evaluated as a function of chemical architecture of the IPN, pH, and temperature of the swelling medium. The water uptake potential of the IPNs has also been investigated in inorganic salt containing aqueous solutions and simulated biological fluids. The IPN was characterized by IR spectral analysis, and the network parameters such as average molecular weight between crosslinks (M_c), crosslink density (q), and number of elastically effective chains (V_e) have been evaluated by water‐sorption measurements. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 2054–2065, 2004     

Effect of crosslinkers on size and temperature sensitivity of poly(N-isopropylacrylamide) microgels

In this work, we study the effect of crosslinkers on the size and swelling properties of temperature sensitive N-isopropylacrylamide (NIPAAm) microgels produced by dispersion polymerization. The crosslinkers studied were N,N′-methylenebisacrylamide (MBA), ethylene glycol dimethacrylate (EGDMA) an 3,9-divinyl-2,4,8,10-tetra-oxaspiro[5.5] undecane (DVA). The type of crosslinker had a major impact on the size and swelling behavior, although the proportion of crosslinker used in each case was low (maximum 5 mol%). The effect can be related to the hydrophilic/hydrophobic characteristics of the crosslinkers. DVA produces smaller hydrogels with reduced swelling ratio, MBA produces bigger microgels with higher swelling ratio, while EGDMA results in an intermediate behavior. With increasing amount of crosslinker used in the synthesis, the extent of the swelling ratio decreases for DVA and EGDMA crosslinked microgels, while increases for MBA crosslinked microgels. There is also a slight effect on the critical transition temperature (T _c) of the microgels from 30 °C (DVA) to 34 °C (MBA) as observed in copolymers of NIPAAm with hydrophilic or hydrophobic comonomers.     

Controlled release of aminophylline from poly (N‐isopropylacrylamide‐co‐itaconic acid) hydrogels

N‐Isopropylacrylamide (NIPA) has been copolymerized with itaconic acid (IA) in the presence of N,N‐methylenebisacrylamide (BIS) as crosslinker. The swelling capacity and the release rate of aminophylline at 37 °C are reported. Maximum equilibrium swelling increases as the itaconic acid content in the hydrogel increases. The experimental data suggest clearly that the swelling process obeys second‐order kinetics. According to this, the kinetic constant, k, and the maximum equilibrium swelling, W_∞, have been calculated. Drug release from fully swollen hydrogels follows Fick's law closely, but deviates from it for xerogels. © 2001 Society of Chemical Industry     

Concentration dependence of chain dimensions and its role in gel chromatography

A theoretical model has been developed for the concentration effect in gel chromatography, i.e. the dependence of the elution volume V_e on the concentration of injected polymer c. On the basis of the theoretical relations of Yamakawa and Eizner for coil shrinkage with increasing concentration in the range of dilute polymer solutions, relations predicting the extent of the concentration effect have been derived. A comparison of the calculated and experimental data for polystyrene in tetrahydrofuran and toluene has shown that both theories slightly underestimate the extent of the concentration effect but qualitatively correctly describe its dependence on molecular mass M and on thermodynamic quality of an eluent given by the product A₂M, where A₂ is the second virial coefficient of the polymer-eluent system. The proposed model explains the recently established correlation between the slope of the concentration dependence of V_e and the thermodynamic quality of the eluent and theoretically accounts for the method for estimating the coefficient A₂ from gel chromatographic measurements. The possibility of using the measurements of concentration effects for examining the reliability of the theoretical relations for coil shrinkage with concentration in dilute polymer solution as well as for eventual semiempirical modification of these relations is examined.     

Experimental evidence for trapped chain entanglements: their influence on macroscopic behaviour of networks

The swelling and elastic properties of poly(dimethyl siloxane) networks prepared by end-linking and subsequently swollen in heptane and toluene have been investigated as a function of the volume fraction, v_c, at which networks are generated. Increases in both swelling degree and shear modulus with v_c demonstrate the increase in number of trapped entanglements with v_c. The results may be described by a simple scaling law approach.     

Superabsorbent polymeric materials. IV. Swelling behavior of crosslinked poly [sodium acrylate-co-N,N-dimethyl (acrylamidopropyl) ammonium propane sulfonate] in aqueous salt solution

A series of xerogels based on sodium acrylate (SA), N,N-dimethyl(acrylamidopropyl)ammonium propane sulfonate (DMAAPS), and N,N′-methylene bisacrylamide (NMBA) were prepared by inverse suspension polymerization. The water absorbency or swelling behavior for these xerogels in water or various saline solutions was investigated. Results obtained from this study revealed a water absorbency of 721 g H₂O/g sample in deionized water and 83 g H₂O/g sample in 0.9 wt % NaCl solution for a gel containing a 1.50 × 10 ⁻² molar fraction of DMAAPS. The absorbency in the chloride salt solutions decreased with an increase in the ionic strength of the salt. For the same ionic strength of various salt solutions, the swelling amount had the following tendency: Co ²⁺ > Ni ²⁺ > Cu ²⁺ for the higher ionic strength of 2.44 × 10 ⁻⁵–1.8 × 10 ⁻² M. The Co ²⁺, Ni²⁺, and Cu ²⁺ solutions induce approximately the same degree of swelling at the lower ionic strength of <2.44 × 10 ^minus;5 M. The pH effect on the water absorbency for these xerogels was also investigated. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66:499–507, 1997     

Swelling and Network Parameters of Oil Sorbers Based on Alkyl Acrylates and Cinnamoyloxy Ethyl Methacrylate Copolymers

Cinnamoyloxy ethyl methacrylate (CEMA) monomer was copolymerized with different monomer feed ratios of alkyl acrylate, such as dodecyl and octadecyl acrylate DDA and ODA, respectively. The monomers were copolymerized with different mole% to produce different compositions for each CEMA/alkylacrylate copolymer with low conversion. ¹HNMR was used to determine the copolymer compositions. The monomer reactivity ratios of each CEMA/alkylacrylate copolymer were determined using Fineman-Ross and Kelen-Tudos methods. CEMA was copolymerized with DDA or ODA and crosslinked using azobis isobutyronitrile (AIBN) as the initiator and 1% weight content of either 1,1,1-trimethylolpropane triacrylate (TPT) or 1,1,1-trimethylolpropane trimethacrylate (TPT_m) crosslinkers. The swelling parameters, such as the maximum oil absorbency (Q _max), characteristic oil sorbency (Q), characteristic swelling time (T), and swelling rate constant (k), were evaluated for the synthesized sorbers. The network parameters, such as the polymer solvent interaction (χ), effective crosslink density (υ _e), equilibrium modulus of elasticity (G _T), average molecular weight between crosslinks (M _c), and the theoretical crosslink density (υ _t), were determined and correlated with the structure of the synthesized sorbers.     

Elastic behavior of concentrated solutions of acrylonitrile copolymers

The elastic behavior of concentrated solution of acrylonitrile copolymer was investigated by the capillary end correction method. The results were as follows. (1) The shear stress is proportional to recoverable shear strain in accordance with Hooke's law below critical concentration; above a critical concentration, however, the shear modulus depends on shear stress. (2) The log–log plots of zero shear modulus against polymer concentration and molecular weight fall on two straight lines with different slopes. The intersection of lines is considered to be the onset of elastically deformable entanglement network. We denote this inflection point as (C_c)_e or (M_c)_e. (3) The log–log plot of viscosity against polymer concentration does not show a change of slope at the critical concentration (C_c)_e. (4) By the application of the kinetic theory of rubberlike elasticity to the pseudo-network structure of concentrated polymer solution, in the range of C_c < C < (C_c)_e or M_c < M < (M_c)_e, the number of chain entanglement per molecule is kept one; moreover, in the range of C > (C_c)_e, or M > (M_c)_e, the number of chain entanglement increases to three.     

Preparation and characterization of pH-sensitive,interpenetrating networks of poly(vinyl alcohol) and poly(acrylic acid)

Hydrogels with varying cross-linking ratio and ionic content were prepared from interpenetrating networks of poly(vinyl alcohol) and poly(acrylic acid). Equilibrium swelling studies were conducted and the average molecular weight between cross-links, M _c, and mesh size were determined. Hydrogels with large M _c, values were found to swell to a greater extent than those with small M _c values. It was also observed that an increase in M _c yielded faster swelling and deswelling rates, as the rates for membranes with M _c = 18,000 were about twice as fast as were the rates for membranes with M _c = 34,000. Oscillatory swelling behavior was investigated in response to changes in the pH and ionic strength of the swelling medium. A change in pH from 3 to 6 caused an ionization of the hydrogels and an increase in the weight swelling ratio, with a greater increase exhibited by IPNs with a higher ionic content. Increase in pH also caused an increase in the average mesh size. © 1995 John Wiley & Sons, Inc.