Styrene divinyl benzene copolymers. I. Texture of macroporous copolymers with ethyl-2-hexanoic acid in diluent

The texture and the pore size distribution of a set of styrene divinylbenzene copolymers, prepared by suspension polymerization with various compositions and various amounts of ethyl-2-hexanoic acid as diluent, have been studied using nitrogen adsorption-desorption isotherms, mercury porosimetry, and scanning-electron microscopy. It appears that pore volume and surface area may be well predicted in a large range of conditions (divinylbenzene contents between 20 and 60%, ethyl-2-hexanoic acid contents between 35% and 60%). In this range of conditions there are three structures: the elementary nuclei are spherical particles of about 100–200 Å in diameter. They are organized in microspheres of about 500–1000 Å, and the microspheres form aggregates (2500–5000 Å) more or less separated by large holes. The smaller pores (<40 Å) are in the microspheres, between the nuclei; they are present only if the amount of divinylbenzene is high enough. Intermediate pores (40–500 Å) are, in the aggregates, between the microspheres, although the larger pores between the aggregates range from about 500 Å to more than 10,000 Å, depending on the amount of diluent.     

Studies on Glutaraldehyde Crosslinked Chitosan Hydrogel Properties for Drug Delivery Systems

Chitosan was crosslinked with different amount of glutaraldehyde to prepare appropriate hydrogels to be used as drug delivery system. The swelling behavior of freeze-dried hydrogels in aqueous media at different temperature and pHs has been examined. The swelling, porosity and biocompatibility behavior of samples were investigated to check effects of polymer/polymer and polymer/drug interactions on these system characteristics. Obtained experimental results illustrates that with increasing crosslinking agent from 0.068 to 0.30, swelling of the prepared samples degrees from 1200% to 600% and pore diameters change from 100 to 500 µm. To investigate systems biocompatibility in gastric conditions, effects of crosslinker concentration on the pepsin enzyme activity have been studied using variation of relative viscosity of the system. Presented results also show that with increasing crosslinker agent concentration activity of enzyme reduces considerably and so crosslinker molar ratio to amine functional groups of chitosan must be less than 0.2.     

Swelling and elasticity of poly (N‐isopropylacrylamide‐co‐4‐vinyl benzene sulfonic acid sodium salt) hydrogels

The mechanical properties and the swelling behavior of hydrogels based on N‐isopropylacrylamide (NIPA) and 4‐vinylbenzenesulfonic acid sodium salt (styrene sulfonate, SSA) monomers were investigated. The mole fraction of SSA in the comonomer feed varied between 0 and 1, whereas the crosslinker ratio was fixed at 1/85. Both the swelling and the elasticity data of the hydrogels swollen in water show that they are in the non‐Gaussian regime. The exponents found for the charge density dependence of the equilibrium swelling ratio as well as for the volume dependence of the gel elastic modulus are in good agreement with the predicted course of the non‐Gaussian elasticity of swollen hydrogels. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 135–141, 2004     

Rapid pH‐dependent phase transition and elasticity of stimuli‐responsive cationic poly(N,N‐dimethylaminoethyl methacrylate) hydrogels prepared with a dimethacrylate crosslinker

Stimuli‐responsive hydrogels prepared from poly(N,N‐dimethylaminoethyl methacrylate) (PDMAEMA) and its copolymers have attracted much interest to serve in biomedical and pharmaceutical applications. To investigate pH‐dependent swelling and elasticity, a series of cationic hydrogels based on N,N‐dimethylaminoethyl methacrylate were prepared by free radical crosslinking copolymerization at 60 °C in the presence of tetraethylene glycol dimethacrylate as the crosslinker. The equilibrium swelling and the mechanical properties of the PDMAEMA hydrogels were investigated as a function of the gel preparation concentration. To explain the effect of pH on the equilibrium swelling of the hydrogels, pH‐dependent swelling studies were carried out in solutions of pH ranging from 2.1 to 10.7. It was found that the PDMAEMA hydrogels exhibit a rapid pH‐dependent phase transition in aqueous solutions; that is, the gels first remain in the swollen state at acidic pH then collapse in a very narrow range of pH. The results showed that the volume of PDMAEMA hydrogels in acidic conditions is about 10‐ to 40‐fold larger than that in the basic pH region. By using the Flory–Rehner theory, the characteristic network parameters of the PDMAEMA hydrogels were calculated and good agreement obtained between the swelling equilibria of hydrogels and their mechanical properties over the whole range of gel preparation concentration. © 2012 Society of Chemical Industry     

Heterogeneous styrene–divinylbenzene copolymers in collapsed and reexpanded states

The pore structure of styrene–divinylbenzene (DVB) copolymers formed by phase separation before or after gelation was compared using apparent densities and mercury porosimetry. The copolymers were prepared with di-2-ethylhexyl phthalate (DOP) as diluent. The pore structure of copolymers formed in homogeneous gelation can collapse upon drying in the rubbery state. The collapsed pores have a mean diameter of about 100–200 Å corresponding to the interstices between the microspheres. The collapsed microspheres reexpand again during the sulfonation or chloromethylation reactions, or during the solvent exchange. The pore structure of styrene–DVB networks formed in heterogeneous gelation do not collapse on drying in the swollen state, this being a stable and permanent porosity. The critical crosslink density for transition from homogeneous to the heterogeneous gelation represents a borderline between stable and unstable porosity. The drastical change of swelling and porosity values at the critical crosslink density is due to the collapse of unstable pores.     

Preparation and characterization of 2‐hydroxyethyl methacrylate‐based porous copolymeric particles

2‐Hydroxyethyl methacrylate was copolymerized with three different comonomers, methyl methacrylate (MMA), styrene (St), and N‐vinyl‐2‐pyrrolidone (NVP), respectively, to prepare porous particles crosslinked using ethylene glycol dimethacrylate (EGDMA) in the presence of an organic solvent, 1‐octanol (porogen), by means of suspension copolymerization in an aqueous phase initiated by 2,2‐azobisisobutyronitrile. Nano‐pores were observed in the particles. The pore size and the swelling properties of these particles can be controlled by changing comonomers or adjusting the crosslinker or porogen concentration. A lower crosslinker or porogen concentration favors generating smaller pores, whereas a higher concentration of a hydrophilic comonomer, higher concentration of crosslinker, and higher porogen volume ratio promote the generation of larger pores. In addition, the effects of the porous characteristics on the swelling properties were explored. The swelling capacity of the porous particles is reduced with the increase in the crosslinker concentration; however, there is a critical porogen volume ratio, in which the maximal swelling capacity is reached. Higher porosity in the particles and higher amount of hydrophilic comonomer favor a higher swelling capacity of the particles. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

USANS study of porosity and water content in sponge-like hydrogels

Ultra small-angle neutron scattering (USANS) methods and equilibrium swelling measurements are combined to characterize correlation length, pore volume fraction, and gel-phase water content in sponge-like hydrogels. USANS invariant analysis is applied to poly(hydroxyethylmethacrylate) gels prepared with 40-80 vol. % of an extractable porogen. These gels are strongly opaque in the swollen state, but become transparent upon drying due to shrinkage or disappearance of the pores. The correlation length associated with porosity is measured by fitting USANS data to an appropriate scattering model. By combining swelling measurements with USANS invariant analysis, it is possible to determine the equilibrium volume fraction of pores and the concentration of water in the gel phase without performing contrast variation measurements.     

Effect of synthesis composition on the swelling pressure of polymeric hydrogels

This paper focuses on the determination of the swelling pressure of polymeric hydrogels by variation of the composition of the monomer mixture used for synthesis via free radical polymerization. The method used is based on the principle of determining the swelling pressure at a given final volume of swelling. Data for different types of low swollen hydrogels based on weakly crosslinked poly(acrylic acid)/poly(sodium acrylate) copolymers are presented for swelling in deionised water. For polymer volume fractions ?_P between 0.03 and 0.30, swelling pressures p in the range of 0.20-4.23 MPa were obtained. No effect on the swelling pressure could be found by variation of monomer concentration and crosslinker content. In contrast, the content of sodium acrylate had an effect on the swelling pressure. The results are discussed based on the effect of counterion condensation.     

Ammonium nitrogen adsorption from aqueous solution by poly(sodium acrylate)s: Effect on the amount of crosslinker and initiator

In this work, we discuss the ammonium nitrogen adsorption and reusability from aqueous solution by using poly(sodium acrylate) (PANa) hydrogels (Polymers 1–6 ) under different amount of crosslinker and initiator. The PANa hydrogels were synthesized from the neutralized acrylic acid (AA) monomer via free radical thermal polymerization by using ammonium persulfate (APS) as an initiator and N,N′-methylene-bisacrylamide (MBA) as a crosslinker. These polymers exhibited glass transition temperatures (T_g) of 68–88°C and T_d values (5% weight loss temperature) in the range of 190–221°C under nitrogen atmosphere. The PANa hydrogels had swollen ratios ranging from 387 to 4,063%, related to the crosslinking density. The final equilibrium adsorption capacity of the polymers was in the range of 20–39 ppm with an initial ammonium nitrogen concentration of 100 ppm. Among them, Polymer 3 without MBA crosslinker displayed the highest swollen characteristic along with the most efficient adsorption capacity. In comparison, the as-prepared high crosslinking density hydrogels showed relative lower adsorption capability but higher reusability. The polymer composition in this work determines the ability to absorb and desorb ammonium nitrogen compound.     

Freezing as a path to build macroporous structures: Superfast responsive polyacrylamide hydrogels

Macroporous polyacrylamide (PAAm) hydrogels were prepared from acrylamide monomer and N,N′-methylene(bis)acrylamide (BAAm) crosslinker in frozen aqueous solutions. It was found that the swelling properties and the elastic behavior of the hydrogels drastically change at a gel preparation temperature of −6 °C. The hydrogels prepared below −6 °C exhibit a heterogeneous morphology consisting of pores of sizes 10–70 μm, while those formed at higher temperatures have a non-porous structure. PAAm networks with largest pores were obtained at −18 °C. The pore size of the networks increased while the thickness of the pore walls decreased by decreasing the monomer concentration. The hydrogels formed below −6 °C exhibit superfast swelling and deswelling properties as well as reversible swelling–deswelling cycles in water and in acetone, respectively.     

Rheological properties of highly swollen hydrogel suspensions

The shear rheology of diluted to concentrated suspensions of weakly cross-linked sodium polyacrylate hydrogels were investigated by concentric cylinder rheometry. The size of the swollen gel particles is dependent on the initial size of the dry particle and on the nature of the added salt. Polyvalent salts are more efficient for contracting the swollen particles than monovalent ones. When suspended in water or in monovalent salt solutions, the viscosity at low concentrations shows a behaviour very similar to the one of flexible polymer solutions. Below the critical overlap concentration, the viscosity is independent of shear rate. The intrinsic viscosity is very large, on the order of 10–15 dL/g. This is in agreement with the large size of these swollen particles. Suspensions of such swollen gel particles are thus a good model for extremely high molecular weight polymer solutions. In polyvalent salt solutions different behaviour of the viscosity curves versus concentration was observed. This effect is caused by chemical cross-linking of the hydrogel by polyvalent ions. © 1994 John Wiley & Sons, Inc.     

Studies on metal impregnated activated carbon: Complete pore structure analysis

Coconut shell activated carbon was impregnated with 2.08, 3.01, 6.41, 8.24 and 9.39% (w/w) copper. Mercury porosimetry was used to analyse pores ranging between 50 and 75,000 Å. Benzene adsorption isotherm and the Kelvin equation were used to analyse pores of radii 15–50 Å. Pores below 15 Å radius (micropore region) were analysed employing Kadlec's method and the Dubinin-Astakhov equation. Impregnants affect micro and transitional pores reducing the pore volume but without altering the basic structure.     

Swelling properties of copolymeric hydrogels prepared by gamma irradiation

Crosslinked xerogels in the form of rods have been prepared to > 99.9% conversion by the γ-ray initiated copolymerization in vacuo ob n-butyl acrylate (BA) with N-vinyl-2-pyrrolidone (VP) in the presence and absence of a crosslinking agent. Thin (about 0.5 mm) disks were swollen in water at 21°C, the final equilibrium swelling being the true water content only for copolymers of high BA content. At low-medium BA content, swelling was accompanied by the release of a small water-soluble fraction, leading to a slightly reduced apparent value for the water content. True water contents could be obtained more rapidly by prior Soxhlet extraction with boiling water. For xerogels containing > about 75% VP by weight, the resultant hydrogels had water contents > about 80%. The influences of dose, dose rate copolymer composition, and concentration of crosslinker on the water content, sol fraction, and extension ratio were investigated. A high level of reproducibility in properties of the hydrogels was obtained by numerous replicate experiments. The feasibility of scaling up the preparation to produce long (60 cm) xerogel rods of uniform properties has been demonstrated. For these copolymers, swelling in several common organic solvents generally exceeds that in water.     

Particle structure change of CaO during high temperature sulphatization

The effects of particle size (layer thickness), BET surface area and pore size distribution on the reactivity of CaO derived from limestone were studied by using a pH-stat and the BET method. Visual analysis was obtained with SEM, and X-ray diffraction was used to determine the crystalline components present. It was found that the pore size distribution, including pore volume and pore surface distribution, affected the sulphatization behaviour. The larger pores (D > 50 Å) with their pore volume and pore surface were to some extent responsible for both initial reactivity and ultimate capacity of CaO.     

Novel preparative method for porous hydrogels using overrun process

Porous poly(vinyl alcohol) (PVA) hydrogels were prepared using the overrun process which is usually used in manufacturing ice cream. The pores in the hydrogel formed exhibit dual‐pore structure due to the injection of air bubbles and ice recrystallization. Morphological investigation revealed that overrun‐processed hydrogels had closed pore structures and that their pore size and size distribution had been influenced by the impeller rate and concentration of polymer solution. The closed‐pore structure was reformed into interconnected open‐pore structure at lower concentrations of the solution. The freeze–thawing process, which takes place in PVA cross‐linking, has no effect on the bubble structure, but removes the small pores formed during ice recrystallization. Besides the swelling ratio of overrun‐processed PVA hydrogels is increased tenfold in comparison with non‐porous hydrogels. Overrun‐processed hydrogels showed more rapid swelling kinetics than freeze‐dried and film‐like hydrogels due to their larger surface area. In the future, the overrun process can be applied to prepare porous scaffolds containing proteins, such as growth factors and other cytokines, without denaturation, because it operates at a low temperature. Copyright © 2004 Society of Chemical Industry     

Semi‐IPN carrageenan‐based nanocomposite hydrogels: Synthesis and swelling behavior

Inclusion of nano‐clays into hydrogels is an efficient approach to produce nanocomposite hydrogels. The introduction of nano‐clay into hydrogels causes an increase in water absorbency. In the present work, Nanocomposite hydrogels based on kappa‐carrageenan were synthesized using sodium montmorillonite as nano‐clay. Acrylamide and methylenebisacrylamide were used as monomer and crosslinker, respectively. The structure of nanocomposite hydrogels was investigated by XRD and SEM techniques. Swelling behavior of nanocomposite hydrogels was studied by varying clay and carrageenan contents as well as methylenebisacrylamide concentration. An optimum swelling capacity was achieved at 12% of sodium montmorilonite. The swollen nanocomposite hydrogels were used to study water retention capacity (WRC) under heating. The results revealed an increase in WRC due to inclusion of sodium montmorilonite clay. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Reentrant phase transition and fast responsive behaviors of poly{N‐[3‐(dimethylaminopropyl)] methacrylamide} hydrogels prepared in poly(ethylene glycol) solutions

Macroporous temperature‐sensitive poly {N‐[3‐(dimethylaminopropyl)] methacrylamide} hydrogels were synthesized by free‐radical crosslinking polymerization of the monomer N‐[3‐(dimethylaminopropyl)] methacrylamide and the crosslinker N,N′‐methylenebisacrylamide in aqueous solutions at 22°C. Poly(ethylene glycol) (PEG) with a molecular weight of 1000 g/mol was used as the pore‐forming agent during the polymerization reaction. The concentration of PEG in the polymerization solutions was varied between 0 and 18 wt %, whereas the crosslinker (N,N′‐methylenebisacrylamide) concentration was fixed at 2 wt % (with respect to the monomer). The effects of the PEG concentration on the thermo‐induced phase‐transition behavior and the chemical structure, interior morphology, and swelling/deswelling kinetics were investigated. Normal‐type hydrogels were also prepared under the same conditions without PEG. An interesting feature of the swelling behavior of both the normal‐type and macroporous hydrogels was the reentrant phase transition, in which the hydrogels collapsed once and reswelled as the temperature was continuously increased. Scanning electron micrographs revealed that the interior network structure of the hydrogels prepared in PEG solutions became more porous with an increase in the PEG concentration in the polymerization solution. This more porous matrix provided numerous water channels for water diffusion in or out of the matrix and, therefore, an improved responsive rate to external temperature changes during the deswelling and swelling processes. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Faserstruktur und Färbeeigenschaften von Cellulosefasern, 1. Zusammenhänge zwischen Veränderungen der Faserstruktur und der Farbstoffadsorption bei Baumwolle durch Mercerisation und Flüssigammoniak-Behandlungen

Changes of fiber structure and the adsorption of direct dyes in cotton subjected to different kinds of mercerisation and liquid ammonia treatment have been studied. Therefrom a new general relationship between dye adsorption and fiber structure of cellulosic fibres has been given as a result. The fiber structure in the water swollen state was characterized by measurements of specific pore volume, specific pore surface and pore size distribution. The dyeing behaviour of the different treated cotton was determined by adsorption isotherms. The results revealed that for equilibrium dye adsorption from aqueous solution a transitional pore region with pore diameters of 20–60 Å has a special importance. Changes of these pores by the different swelling treatments correlated directly with the dye uptake. This pore region is found in cotton between the elementary fibrils.     

Studies of porous polymer gels. II. Structure and porosity of moderately crosslinked poly(methacrylic acid) gels

The porosity of suspension copolymers of methacrylic acid (MA) and divinylbenzene (DVB) has been studied. The copolymers were prepared both directly from monomer blends and with toluene or n-octane as inert diluents. The pore volume and radii of pores were larger for copolymers containing 5.0% of DVB than those for copolymers containing 9.1% of DVB. The volume and radii of pores increased with increasing diluent content, but the effect of n-octane was more pronounced than that of toluene. Based on considerations of monomer reactivities and copolymer-diluent and copolymer-unreacted monomer interactions, the following model of structure of MA and DVB copolymers has been proposed: The copolymers are composed of microgels interconnected into a rigid skeletonlike structure, surrounded by lightly crosslinked and unentangled poly(methacrylic acid) chains.     

Porosimetrie gequollener makroretikularer Ionenaustauscher

The relative micro and macro pore volumes and the macro pore size distribution were used to characterize the pore structure of ion exchangers. The evaluation of these from density measurements and mercury penetration measurements was described for a sulphonated macro reticular ion exchange resin in the swollen state with different contents of swelling agent. On taking up water the dry volume of the exchange resin increased (swelling) until a water content of about 33 wt.-% was reached. Additional water filled up the macro pores to a final content of 58 wt.-%. With the specific volume of the exchange resin in the swollen state in addition to the specific volume of the gel phase, the relative macro pore volume of the fully swollen state was evaluated. The results of the mercury penetration method agreed with those of the density measurements taking into account the compressibility of the mercury itself and those of the swelling agent and the matrix. Corresponding correction factors could be evaluated from measurements on micro reticular ion exchange resins. The mercury penetration measurements provided additional information on the macro pore size distribution of the swollen state. For the examined ion exchange resins a logarithmic standard distribution with an arithmetically weighed mean pore radius of about 1 100 Å was evaluated. The relative over-all pore volume of 0,65 was not identical with the value of 0,61 for the partition coefficient of ethylene glycol from dilute aqueous solutions, as only that part of the water, which is bound to the sulphonic groups (hydrated water), could be exchanged by ethylene glycol. Pure ethylene glycol substituted the hydrated water completely. The specific volume of the exchange resin thereby decreased in comparison to that of the water-swollen state.