Factors driving the protonation of poly(N-vinylimidazole) hydrogels

Poly(N-vinylimidazole) hydrogels immersed in aqueous acid solutions produce an increment in the pH of the bath because of proton uptake by basic imidazole moieties, leading to hydrogel protonation. Both kinetic and equilibrium measurements of the pH of the bath have been performed under a variety of conditions and with different hydrogel samples. The kinetics of the xerogel protonation process (which includes solvent and titrant diffusion, the true protonation reaction or ion–dipole association, and the polymer relaxation to a new conformation) are mostly driven by the size of the hydrogel sample, whereas other magnitudes, such as the initial pH, the effective polymer concentration, and the network structure, governed by the crosslinker ratio and total comonomer concentration in the feeding, have a minor influence. pK_a changes with the degree of protonation (α), delimitating two different regions: (1) a broad α range in which pK_a decreases with increasing α but less pronouncedly with increasing ionic strength and (2) an α range close to α = 1 in which pK_a decreases abruptly, more markedly with sulfate than with chloride counteranions and with larger ionic strengths. In the first region, pK_a is determined by repulsive electrostatic interactions and so is larger for titration with H₂SO₄ than with HCl and increases as the effective polymer concentration and ionic strength increase. Two steps (i.e., two protonation sites) can be observed in the titration curves, the second one corresponding to abrupt changes in the basicity of the second pK_a-versus-α region. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2294–2307, 2004     

Salting-in effect of ionic liquids on poly(N-vinylimidazole) hydrogels

This work attempts to study the interaction of two ionic liquids (1-ethyl-3-methylimidazolium tosylate and 1-hexyl-3-methylimidazolium chloride) with chemically crosslinked poly(N-vinylimidazole) in aqueous media, and to compare it with that of NaCl, a typical salting-out electrolyte. The three salts show a salting-in effect whose intensity was measured on the basis of the decrease of the polymer–solvent interaction parameter and the increase of swelling with increasing ionic strength. It was thus found that the salting-in effect is the same for the two salts with the same anion (chloride), while the intensity of the salting-in effect exerted by the tosylate ionic liquid is larger. The coefficient of selective sorption, which expresses the salt excess inside the swollen gel with respect to the external solution was determined by comparing the initial and equilibrium compositions of the immersion bath. These results are discussed in terms of the hydrophobic character of the ions involved.     

Compression elastic modulus of neutral and protonated poly(N-vinylimidazole) hydrogels

The compression modulus of poly(N-vinylimidazole) (PVI) hydrogels synthesized by cross-linking polymerization in aqueous solution, was measured at room temperature in several related systems: i) just after polymerization, ii) swollen at equilibrium in deionized water, iii) swollen in HCl (aq) (pH=2.5), iv) swollen in HCl (pH=2.5) and 1 M NaCl (aq) solution and v) swollen in H₂SO₄ (pH=2.5) (aq) solution. Samples of the first and second groups are neutral whereas hydrogels of the other three groups are ionic because of protonation of basic imidazole groups. The experimental results were fitted with the Erman-Monnerie theory, applied to compression measurements for the first time, to determine the phantom modulus, [f_ph*], and the parameter κ_G which measures the constraining role of entanglements on the fluctuations of chains between knots.     

Determination of the parameters controlling swelling of chemically cross-linked pH-sensitive poly(N-vinylimidazole) hydrogels

The number of variables controlling the behavior of ionic gels is large and very often some of them are unknown. The aim of this work is to interpret quantitatively the swelling behavior of pH sensitive gels, with the minimum number of simplifying assumptions. With this purpose, the equilibrium degree of swelling (S) and protonation (alpha) of chemically cross-linked poly(N-vinylimidazole) (PVI) immersed in aqueous salt solutions were measured as a function of the ionic strength (mu), in the whole range of pH. In acid solutions with pH in the range 0 to 4, imidazole moieties become protonated, and PVI behaves as a polyelectrolyte gel: S decreases upon increasing mu both for NaCl and for CaCl(2), with HCl as protonating acid. In aqueous solutions with larger pH, between 4 and 12, the hydrogel is practically neutral, and S increases as mu rises, showing a salting-in effect. From the quantitative analysis of these results, the following facts emerged. Protonation induces chain stiffness (as measured by the non-Gaussian factor) and worsening of the solvent quality of the aqueous media (as measured by the polymer-solvent interaction parameter). For alpha below 33%, swelling seems to be governed by the excess of mobile counterions inside the gel with respect to the bath, with a minor but still significantly negative contribution of the osmotic swelling pressure due to polymer-solvent mixing. Above 33% protonation, it is necessary to consider Manning counterion condensation to get parameters with physical meaning. The crossover between polyelectrolyte and salting-in effects corresponds to alpha and mu values with the same ionic and mixing contributions to the osmotic swelling pressure. The formation of ionic nonpermanent cross-links, with H(2)SO(4) as the protonating acid, was discarded.     

Synthesis of crosslinked poly(5-vinyltetrazole) and properties of hydrogels formed on its basis

Crosslinked polymer samples are synthesized through the alkylation of linear poly(5-vinyltetrazole) with di- and multifunctional compounds containing mobile halogen atoms or oxirane cycles. In the ionized state, the crosslinked poly(5-vinyltetrazole) shows limited swelling ability in water and forms hydrogels. It is demonstrated that tetrazole-containing hydrogels may serve as matrices for stabilization of nanosized metal particles.     

Porosity inherent to chemically crosslinked polymers. Poly(N-vinylimidazole) hydrogels

Swollen polymer networks exhibit multiscale pores filled with solvent. Such porosity, inherent to cross-linked polymers, determines some of their most relevant physical properties and applications. In this research, several samples of chemically crosslinked poly(N-vinylimidazole) were synthesized with the same permanent crosslinking density at two different conversions, and their inherent porosity was characterized on freeze-dried specimens by SEM, TEM and nitrogen physisorption. It was thus found that all of the samples showed pores, both on the nanometer and the micrometer scales, whose dimensions were mostly equal to or larger than the mesh size of the primary polymer network (22 nm) and whose volume and specific surface decreased with increasing conversion. Micropores have, in all cases, a very minor contribution. Samples synthesized with the largest comonomer concentrations show quasi-spherical mesopores (90 nm average diameter at any conversion) and macropores (from 5 to 10 microm with increasing conversion), whereas the mesopores of samples synthesized with the largest crosslinker ratios were channel-like (150 nm) and the macropores were interconnected contiguous voids (3 microm). Samples with intermediate compositions exhibit the lowest porosity due, mostly, to interconnected mesopores. The differences in shape were ascribed to the mechanism of phase separation, taking place during polymerization, even for samples that are transparent following polymerization. The inherent porosity is a significant source of spatial inhomogeneity, which contributes to the increase in turbidity. Light scattering decreases with increasing ionization when the degree of protonation is greater than 10%. An important consequence of the inherent porosity is that the degrees of swelling determined either gravimetrically or through size measurements are not equivalent.     

Synthesis and characterization of thermally reversible macroporous poly(N-isopropylacrylamide) hydrogels

Macroporous hydrogels are characterized by large pore sizes, high pore volumes, and high specific surface area. Besides these characteristics, macroporous hydrogels based on thermally reversible polymers respond to temperature changes much faster than hydrogels prepared by a conventional method. Crosslinked poly(N-isopropylacrylamide) (polyNIPAAm) forms a thermally reversible hydrogel which shows a lower critical solution temperature (LCST) ca. 33°C in aqueous solutions. We have synthesized thermally reversible polyNIPAAm hydrogels having macroporous structures by a new method. These macroporous hydrogels have large pore volumes, large average pore sizes, and faster macromolecule permeation rates in comparison to conventional polyNIPAAm hydrogels synthesized by a conventional method. Compared with conventional polyNIPAAm hydrogels, the macroporous polyNIPAAm hydrogels have higher swelling ratios at temperatures below the LCST and exhibit faster deswelling and reswelling rates. The deswelling rates are especially rapid. These thermally reversible macroporous hydrogels may be very useful in controlled active agent delivery and toxin removal, as well as dewatering of solutions. Peptides or proteins may behave as if they were in bulk solution within the large aqueous pores, and this may reduce their inactivation when such gels are used for their storage and later release. The gels may also be useful in microrobotic devices due to their fast response to temperature. © 1992 John Wiley & Sons, Inc.     

Synthesis and dynamic mechanical properties of heterogeneous network polymers from poly(L-glutamic acid) and poly(oxyethylene glycol)

Heterogeneous network polymers composed of rigid polypeptide chains and flexible polyether chains were synthesized. That is, poly(L -glutamic acid) (PLGA) was crosslinked with poly(oxyethylene glycol) (PEG) at various carboxy/hydroxyl mole ratios K. The solubility tests and hydrolysis of heterogeneous network polymers suggest that the crosslinking reaction proceeds by esterification. The dynamic mechanical properties of these polymers(100 Hz, ?100–200°C) are greatly influenced by the presence of a trace of water and the weight per cent of PLGA. In addition, some of these polymers show only one maximum in the temperature dispersion of dynamic loss modulus E″ and tan δ, although their shape is rather broad. The x-ray photographs of these polymers show an amorphous halo or weak Debye-Sherrer rings. These findings suggest that these polymers are not simple adducts; neverthless PLGA and/or PEG domains exist.     

Dynamic mechanical properties of poly(2-hydroxyethyl methacrylate) hydrogels

Homogeneous and heterogeneous poly(2-hydroxyethyl methacrylate) hydrogels were prepared by copolymerization of 2-hydroxyethyl methacrylate and small amounts of ethylene glycol dimethacrylate in the presence of water; concentrations of water (V₁) in the polymerization mixture and the volume fractions of water (v₁) of the gels swollen to equilibrium were 40, 50, 60, 70% by volume, and 0.475, 0.541, 0.669, 0.778, respectively. From the homogeneous (clear) hydrogel (V₁ = 40%) preparation, four hydrogels were prepared with v₁ = 0.434, 0.418, 0.378, 0.326. Tensile dynamic moduli were measured in the frequency range from 0.006 to 0.6 cps and the temperature range from 0 to 40°C. In these cases, the influence of swelling on the shape of the relaxation spectra and on the monomeric unit friction coefficient was studied. The dependence of the friction coefficient on water concentration was interpreted in terms of free volume. In the heterogeneous (opaque) gel preparations (V₁ = 50, 60, 70%), the effect of the aqueous phase in the system on the mechanical behavior was described by a modification of the blending law of Ninomiya. For the systems with V₁ = 60% and 70% the shapes of the storage and loss moduli in the main transition region and the friction coefficient were similar to those of the homogeneous gel with V₁ = 40%, except for the decrease in absolute value of the storage moduli. For the system with V₁ = 50% the shape of the relaxation spectrum changes appreciably and the wedge distribution does not hold.     

Synthesis and properties of poly(aniline-co-azidoaniline)

Copolymers of polyaniline and o-azidoaniline were synthesized by chemical oxidative polymerization. The copolymers were characterized by powder X-ray diffraction (XRD) and UV/Vis and FT-IR spectroscopy. Thermal activation of the azido chromophore in the copolymer caused it to react and cross-link into adjacent polymer chains. The cross-linking of the copolymers was indicated by the depletion of the azido band in the FT-IR spectrum. The effects of the cross-linking were studied by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and four-probe conductivity. The TGA showed that the thermal stability of the copolymers is improved due to cross-linking. However, the increased thermal stability is accompanied by a decrease in electrical conductivity due to the loss of conjugation detected by UV/Vis spectroscopy and a loss of crystallinity due to the azido substituents, which was demonstrated by XRD.     

NMR study of the structure and properties of poly(MMA-co-VP) crosslinked hydrogels

¹H- and ¹³C-NMR techniques were used to study the microscopic structure of NMA/VP copolymer hydrogels. Evidence was obtained for a plasticization effect of MMA chains by VP. An original ¹H-NMR approach revealed the existence of several types of water with various degree of bounding to the polymer network, a conclusion that is corroborated by a complementary ¹³C-NMR study. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 3619–3625, 1997     

Synthesis and properties of poly(dimethylxylylenylamides)

Poly(dimethylxylylenylamides) were synthesized from six different diphenylenediamenes and two dimethyl-substituted xylylenyldiacid chlorides by solution polycondensation at low temperature. The model compound 2,5-demethyl1,4-benzediacetanilide was synthesized and characterized by infrared (IR) and nuclear magnetic resonance (NMR) spectroscopy. The polyamides were characterized by IR and viscosity measurements. Some of the polyamides showed crystalline behavior in the x-ray diffractograms. The polyamdes have decomposition temperatures in the range 370–310°C in air and are soluble in all amide solvents. The effects of subsituents on crystallinity, thermal stability, and solubility of the polymides are also discussed.     

Metal ion promoted hydrogels for bovine serum albumin adsorption: Cu(II) and Co(II) chelated poly[(N-vinylimidazole)-maleic acid

Poly[(N-vinylimidazole)-maleic acid] (poly(VIm-MA)), copolymeric hydrogels were prepared by gamma-irradiating ternary mixtures of N-vinylimidazole-maleic acid-water in a (60)Co-gamma source. Cu(II) and Co(II) ions were chelated within the gels at pH=5.0. The maximum adsorption capacity of the gels were 3.71 mmol/g dry gel for Cu(II) and 1.25 mmol/g dry gel for Co(II) at pH=5.0. The swelling ratios of the gels were 1200% for poly(VIm-MA), 60 and 45% for Cu(II) and Co(II)-chelated poly(VIm-MA) gels at pH=5.0 in acetate buffer solution. These affinity gels with different swelling ratios for plain poly(VIm-MA), Cu(II)-, and Co(II)-chelated poly(VIm-MA), in acetate and phosphate buffers were used in the bovine serum albumin (BSA) adsorption/desorption studies in batch reactor. The maximum BSA adsorption capacities of the gels were 0.38 g/g dry gel for plain, 0.88 g/g dry gel for Cu(II)-chelated poly(VIm-MA) and 1.05 g/g dry gel for Co(II)-chelated poly(VIm-MA) gels. Adsorption capacity of BSA by the gels was reduced dramatically by increasing the ionic strength adjusted with NaCl. More than 95% of BSA were desorbed in 10 h in desorption medium containing 0.1M of EDTA for metal ion-chelated gels at pH=4.7.     

Synthesis and characterization of poly(methacrylic acid) hydrogels for metoclopramide delivery

Polymeric hydrogels based on biocompatible materials, methacrylic acid (MAA), were designed and synthesized. Synthesis was carried out by free-radical copolymerization using potassium persulfate as initiator and N,N^′-methylenebisacrylamide as crosslinker. Hydrogels were also characterized by Fourier transform infrared spectroscopy (FTIR), proton nuclear magnetic resonance spectroscopy (¹H NMR) and differential scanning calorimetry (DSC). DSC was used for the quantitive determination of the amounts of freezing and non-freezing water of the hydrogels with 0.5% of N,N^′-methylenebisacrylamide. Equilibrium swelling of hydrogels was studied in phosphate buffer of physiological pH (1.0, 4.0, 7.4 and 8.5) at 37 °C. The swelling kinetic of the hydrogels were studied and the kinetic characteristic constant of copolymeric systems, k, and the exponent which characterizes the mechanism of water transport at short times, n, were obtained. Metoclopramide hydrochloride was entrapped into the hydrogels by sorption and the “in vitro” release profile of this drug was established in simulated gastric fluid (SGF) and simulated intestinal fluid (SIF). It was observed that the drug release mechanism was non-Fickian.     

Structure and mechanical properties of superabsorbent poly(acrylamide)-montmorillonite composite hydrogels

The structure of superabsorbent poly(acrylamide)-clay-based composite (SAPC) was studied by means of XRD, SEM, NMR, FTIR, TGA and DSC. The study showed that the acrylamide was intercalated in the lamina of clay in bimolecular layers. The hydrogels exhibited typical viscoelastic properties. The composite showed a strong response to changes in ionic environment (pH) and electric field strength, as well as chemomechanical properties.     

Synthesis of mesoporous alumina through photo cross-linked poly(dimethylacrylamide) hydrogels

Catalysis plays a central role in many fields of life, e.g., in biochemical processes, to reduce energy costs and resources in chemical industry and to decrease or even avoid environmental pollution and in energy management. Porous alumina (Al₂O₃) is an essential material in various applications, especially as a support material for catalysts. It is often prepared by nanocasting using porous carbon materials that serve as rigid structure matrices. In this work, an alternative way to synthesize mesoporous Al₂O₃ by using hydrogels as porogenic material is presented. Hydrogels can easily be patterned by light and used to imprint their structure onto alumina opening a new approach to fabricate patterned Al₂O₃. The hydrogels used in this work are based on poly(dimethylacrylamide) and were photo-chemically cross-linked. Followed by a nanocasting process, mesoporous alumina samples were synthesized and characterized by N₂ physisorption and X-ray diffraction. The cross-linker amount in the polymer network was varied and the influence on the properties of the Al₂O₃ is analyzed.     

Water sorption properties of poly(ethyl acrylate-co-hydroxyethyl methacrylate) macroporous hydrogels

Synthetic porous hydrogels are becoming more and more important in the field of biomaterials. Different studies demonstrate that the porous structure promotes the colonisation of living cells and improves the biocompatibility of the implants. The macroporous structure allows not only the control of cellular ingrowth morphology but also the mechanical integration and the regulation of nutrient and hydraulic flow in the hydrogel. In this work poly(ethyl acrylate-co-hydroxyethyl methacrylate) (PEA/PHEMA) copolymers were polymerized using 2% of ethylene glycol dimethacrylate as cross-linking agent and azoiso-botyronitrile as initiator. Five samples were prepared with the EA/HEMA weight ratios of 75/25, 50/50, 25/75 and pure PEA and PHEMA polymers, obtaining different degrees of hydrophilicity. The macroporous structure was obtained by adding poly(acrylonitrile) fibres to the monomers. After polymerization the fibres were eliminated by dissolution in dimethyl formamide. The holes are cylinders of approximately 40μm diameter and are all, more or less, in the same direction, although they are not uniformly distributed. Water sorption isotherms and diffusion properties of the macroporous samples are compared with the samples without holes.     

pH-thermoreversible hydrogels. I. Synthesis and characterization of poly(N-isopropylacrylamide/maleic acid) copolymeric hydrogels

N-isopropylacrylamide (NIPAAM)/maleic acid (MA) copolymeric hydrogels were prepared by irradiating the ternary mixtures of NIPAAM/MA/Water by γ-rays at ambient temperature. The influence of external stimuli such as pH and temperature of the swelling media on the equilibrium swelling properties was investigated. The hydrogels showed both temperature and pH responses. The effect of comonomer concentration and irradiation dose on the swelling equilibria and phase transition was studied. For the characterization of these hydrogels, the diffusion behaviour and molecular weight between crosslinks were investigated.     

Synthesis and properties of poly(dipropargyl sulfoxide)

The preparation and cyclopolymerization of dipropargyl sulfoxide were studied. The polymerization of dipropargyl sulfoxide was carried out by various transition metal catalysts. WCl₆–EtAlCl₂, MoCl₅, and PdCl₂ catalyst systems were very effective. The resulting poly(dipropargyl sulfoxide) structures were characterized by NMR (¹H and ¹³C), IR, and elemental analysis to have conjugated polyene units. Poly(dipropargyl sulfoxide) prepared by PdCl₂ was mostly soluble in organic solvents such as DMF and DMSO. Thermal and oxidative properties of poly(dipropargyl sulfoxide) were also studied. The electrical conductivity of iodine-doped poly(dipropargyl sulfoxide) was 5.2 × 10^?2 Ω^?1 cm^?1. Comparisons of poly(dipropargyl sulfoxide) properties with other similar polymers from dipropargyl sulfur derivatives such as dipropargyl sulfide and dipropargyl sulfone were also carried out. © 1993 John Wiley & Sons, Inc.     

Synthesis and properties of poly(silylenephenylenevinylene)s

Polymerization of o-, m-, and p-(dimethylsilyl)phenylacetylene by chloroplatinic acid-catalyzed hydrosilylation gave the corresponding poly(dimethylsilylenephenylenevinylene)s. The monomer reactivity and polymer structure were very much dependent upon the substituent position. Interesting optical behavior and thermal properties were observed which suggested the polymers to be useful as preceramic materials. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 2263–2273, 1999