Equilibrium Degree Of Swelling Research Articles

Release of 5-fluorouracil from poly(acrylamide-co-monopropyl itaconate) hydrogels

The aim of this work was to test the application of copolymeric poly(acrylamide-co-monopropyl itaconate) (A-MPI) hydrogels on the release of 5-fluorouracil (5-FU). The equilibrium degree of swelling in saline solution was 83 ± 2%. 5-FU, as the sodium salt, was trapped in gels by placing it in the polymerization feed mixture. The diffusion coefficients for both swelling of the gels and the release of 5-FU were determined, in addition to the activation energies for both processes. To determine the applicability of these copolymers, A-MPI (75:25) gel was subcutaneously implanted in rats and the drug plasma concentration was determined by HPLC.

Biodegradable and pH sensitive hydrogels: synthesis by a polymer‐polymer reaction

AbstractpH sensitive hydrogels containing azoaromatic crosslinks were synthesized by a polymer‐polymer reaction. To minimize cyclization, a three step synthesis was employed. First, a polymeric precursor (P‐ONp) containing side‐chains terminated in reactive p‐nitrophenyl ester groups was synthesized by free radical precipitation copolymerization of N,N‐dimethylacrylamide, acrylic acid, N‐tert‐butylacrylamide, and N‐methacryloylglycylglycine p‐nitrophenyl ester. In the second step, the p‐nitrophenyl ester groups of P‐ONp were converted to NH2 groups (P‐NH2) by a polymer‐analogous reaction with an excess of a diamine, namely N,N′‐(ω‐aminocaproyl)‐4,4′‐diaminoazobenzene. The hydrogel was then prepared in the third step by a polymer‐polymer reaction of P‐ONp with P‐NH2. The linear copolymers were characterized by the content of functional groups and molecular weights. The hydrogels were characterized by equilibrium degree of swelling, modulus of elasticity in compression at pH 7.4, the network structure, and in vitro degradability. The swelling, mechanical properties, and rate of degradation depended on the crosslinking density. Although we were able to restrict cyclization, other side reactions such as unreacted pendent groups and hydrolysis need to be controlled. These hydrogels have potential for colon‐specific drug delivery.

Crossover of the concentration dependence of swelling and elastic properties for polysiloxane networks crosslinked in solution

We have investigated the preparation concentration dependence of the polymer volume fraction and the elastic modulus of the equilibrium swollen polydimethylsiloxane networks in toluene. The networks were prepared by the end-linking method in solution with a wide range of concentration. The network samples were attempted to be as small amount of structural defects as possible. The crossover has been observed for the preparation concentration dependence of the degree of equilibrium swelling and the elastic modulus. The crossover concentrations agree with each other, and are almost identical with the boundary concentration between the semidilute and the concentrated regime for the end-to-end distance of the polymer chain. The theoretical predictions which describe the experimental results have been derived as a function of the polymer volume fraction of preparation and swollen state, considering the effect of trapped entanglements on the elasticity and the concentration regime of preparation and swollen state. The good agreements of the theoretical predictions with the experimental results suggest that the trapped entanglements contribute to the elastic modulus together with the chemical crosslinks, and the crosslinking points moves affinely relative to the global polymer network on swelling.

Swelling of interpenetrating polymer networks

AbstractWe have studied the densities, kinetics, and equilibrium degree of swelling in a number of different solvents of poly(carbonate urethane)/poly(methyl methacrylate) and poly(carbonate urethane)/poly(vinyl pyridine) interpenetrating polymer networks (IPN's). The kinetics of solvent uptake are often anomalous. The equilibrium extent of swelling reflects, among other factors, the number of phases present. © 1993 John Wiley & Sons, Inc.

Role of maltodextrin in promoting structure formation in extruded soya isolate

The structure of maltodextrin soybean protein, and protein maltodextrin extrudates has been studied by optical microscopy, X-ray diffraction and FTIR-spectroscopy. Extrudates were obtained after the cooling viscous biopolymer systems using a long die to avoid vaporisation of water. X-ray diffraction pictures showed no dominant orientation of the biopolymers in the extrudates. IR-spectroscopy of the extrudate powders revealed no molecular protein-maltodextrin interactions. X-ray structural analysis and optical microscopy showed the texturates were multiphase systems. The composition and structure of extrudates have been observed to influence their equilibrium degree of swelling. It is concluded that the results cannot be explained by the orientation and cross-linking of biopolymeric molecules to form long aggregates. The results of this work indicate that the disperse particles of a multiphase water-plastified biopolymeric mass orientate in the flow during extrusion.

Hydrogels for site‐specific oral delivery. Poly[(acrylic acid)‐co‐(butyl acrylate)] crosslinked with 4,4′‐bis(methacryloylamino)azobenzene

AbstractCopolymers of acrylic acid and butyl acrylate crosslinked with 4,4′‐bis(methacryloylamino)‐azobenzene were synthesized by radical copolymerization with different amounts of acrylic acid and crosslinker. They were characterized by equilibrium degree of swelling, modulus of elasticity in compression at pH 2 and 7, and effective network density. The acido‐basic properties of crosslinked copolymers were compared with linear (soluble) ones of the same composition. The study of the dependence of the apparent dissociation constant (pKapp) on the degree of dissociation has shown that conformational transition from the compact to the random conformation of the polymeric chains occurs only in crosslinked copolymers. The high values of pKapp obtained indicate that strong Donnan effects and low polarity of environment of carboxy groups are operative in acido‐basic equilibria.

Scaling pattern of swelling of polymer networks

Within the scaling theory a detailed analysis is made of the character of the swelling of network polymers obtained by branched polycondensation and crosslinking of linear chains via the terminal groups. The boundaries of the applicability of different theories describing the swelling of the networks have been established and the theoretical dependences of the degree of equilibrium swelling and osmotic pressure of the network on its density and the parameters characterizing its topology and the thermodynamic quality of the solvent have been found.

Quasielastic light scattering studies of swollen and stretched polymer gels

Cooperative diffusion D of polyacrylamide gels in water was investigated by quasielastic light scattering both under the isotropically swollen state and under the uniaxially stretched and swollen state. The concentration (Ce) dependence of D for the isotropic gels having equilibrium degrees of swelling was measured by systematically changing crosslinking density of gels. The results yielded D=(3.4±0.5)×10−6 C0.76±0.03e cm2/s, in accord with a scaling prediction. For each of the gels prepared and having a given Ce, the uniaxial stretching was applied, and the values D were investigated as a function of extension ratio α and direction β with respect to the stretching direction. The results gave D (α,Ce)=(3.4±0.5)×10−6 αx Cye cm2/s, where x=2/3 for β=0° and −1/4 for β=90°, and y=0.76, independent of α and β. The results clearly indicate that upon increasing α, D increases and decreases, respectively, in the direction parallel and perpendicular to the stretching direction, implying suppression and enhancement of the concentration fluctuations as a result of the network being stiffened and softened in the respective directions.

Release of macromolecules and daunomycin from hydrophilic gels containing enzymatically degradable bonds

The synthesis of hydrophilic gels based on N-(2-hydroxypropyl)methacrylamide copolymers crosslinked via degradable oligopeptide sequences is described. The influence of the conditions of preparation and of the gel structure on the equilibrium degree of swelling (network density) was determined. To evaluate the potential of such gels for controlled delivery of macromolecules and drugs, the release of FITC-dextrans of different molecular weights was studied and the rate of release was found to depend mainly on the equilibrium degree of swelling and not on the structure of the crosslinks. However, the degradation of the gels by a mixture of lysosomal enzymes isolated from rat liver (Tritosomes) or chymotrypsin was dependent on both the equilibrium degree of swelling and the structure of the crosslinks (length of the oligopeptide sequence and structure of the amino acid residues). The release of the anticancer drug daunomycin imbibed in gels was pH-dependent, the rate of release being higher at lower pH. In addition, a gel was synthesized which contained a pentapeptide in the crosslinks and daunomycin bound via a tetrapeptide side-chain, and in this case, incubation with Tritosomes led to degradation with simultaneous release of the drug.

Precipitation of iron oxide filler particles into an elastomer

Samples of peroxide-cured butyl rubber were impregnated with anhydrous FeCl3, which was then hydrolyzed in a magnetic field to give ferric hydrous oxide particles. The filler thus formed in-situ was found to give good reinforcement of the elastomer. A relatively small but significant anisotropy was found for both the elongation modulus and the equilibrium degree of swelling.

Properties of syndiotacticity-rich poly(vinyl alcohol) thin film in water. IV. Elastic behavior of untreated thin film by repeated elongation and contraction in water

The activation energy ΔEa for diffusion of water molecules into untreated thin film of poly(vinyl alcohol) (PVAVTFA) derived from vinyl trifluoroacetate was estimated as 30.3 kcal/mol from the temperature dependence of the initiation time of swelling. The degree of equilibrium swelling increases with the increase in temperature in the range of 10–70°C, passing through the constant range between 30 and 50°C. Under the forced, repeated elongation and contraction for swollen PVAVTFA thin film in water, the perfect elastic behavior is kept up to higher elongation with the increase in temperature and especially up to about three times equilibrium swelling length at 70°C. Young's modulus of swollen PVAVTFA thin film in water was 6.47 × 106 to 1.60 × 106 dyn/cm2 at 25–70°C. The molecular weight between junctions Mc increased with the increase in temperature; Mc was nearly equal to about one-sixth of the molecular weight of raw polymer M at 25°C and about one-third of M at 70°C. The interaction parameter between PVAVTFA and water X1, was 0.493–0.497 at 25–70°C.

The Role of Chemical Bonding in the Adhesion of Elastomers

Abstract A review is given of several studies of the effect of interfacial bonding upon the mechanical strength of an adhesive joint. In the first, polybutadiene layers were crosslinked by a free radical process while in contact with silane-treated glass. A direct proportionality was found between the minimum peel strength of the joint, at high temperatures and low rates of peeling, and the vinyl content of the silane treatment liquid. Covalent bonding between the diene polymer and vinyl groups on the treated glass was inferred. When radioactively tagged silanes were employed, extensive combination with the glass substrates was demonstrated. Again, the greater the amount of vinyl silane found on the treated glass surface, the greater the mechanical strength of adhesion between the treated glass and a polybutadiene overlayer. In another series of experiments, two partially crosslinked sheets of polybutadiene were pressed together before the crosslinking was taken to completion. The additional crosslinking was determined from measurements of the elastic properties and of the degree of equilibrium swelling by a compatible liquid. Again, the mechanical strength of adhesion between the two sheets under threshold conditions was found to be directly proportional to the inferred degree of interfacial interlinking. Thus, at least at high temperatures and low rates of peel, there is substantial evidence for a direct correlation between the mechanical strength of a joint and the degree of interfacial chemical bonding. Moreover, the relationships established in these studies allow other bonding systems to be diagnosed as chemical or physical in nature. For example, a dramatic increase in the strength of adhesion between two crosslinked polybutadiene layers was observed if they were exposed to air or oxygen for periods of an hour or two before being pressed into contact. It is inferred that interfacial chemical bonds are formed as a consequence of rapid surface oxidation reactions.

Swelling and mechanical properties of cellulose hydrogels. I. Preparation, characterization, and swelling behavior

Cellulose hydrogels have been synthesized by reacting solutions of cellulose xanthate with different amounts of epichlorohydrin (0–24% w/w on cellulose) after which the cellulose was regenerated. The weight fraction of crystalline cellulose, determined by density measurements, decreases with the extent of chemical crosslinking and was estimated to vary between 30 and 42% for dry gels. The degree of equilibrium swelling in water of the prepared hydrogels varied between 3.05 and 6.33 g water/g dry gel (g/g). The degree of swelling decreases with increasing chemical crosslinking. As a result of the irreversible changes occurring during drying, the degree of swelling in water can be reduced down to 0.74 g/g. According to density gradient column measurements, the partial specific volume of water is 0.865 cm3/g at water contents below 0.13 g/g. It is suggested that water having this partial specific volume is the specific hydration water. At higher water contents, the partial specific volume of gel water equals the specific volume of bulk water. It is implicit in the interpretation of the density data in terms of a two state model of gel water that the crystallinity of cellulose is independent of the water content. Depending on the degree of swelling, heat treatment resulted in either an irreversible increase or decrease of the degree of swelling.

Model networks of end‐linked polydimethylsiloxane chains. XII. Dependence of ultimate properties on dangling‐chain irregularities

AbstractElastomeric networks were prepared by end‐linking vinyl‐terminated polydimethylsiloxane (PDMS) chains having number‐average molecular weights of 11.3 × 103 g mol−1. The tetra‐functional end‐linking agent, Si[OSi(CH3)2H]4, was used in varying amounts smaller than that corresponding to a stoichiometric balance between its active hydrogen atoms and the chain vinyl groups. The number of dangling‐chain irregularities thus introduced into the networks was directly determined by iodometric titration for unreacted vinyl groups. The (unfilled) PDMS networks thus obtained were studied in elongation to their rupture points at 25°C (a temperature sufficiently high to prevent complications from strain‐induced crystallization), and in swelling equilibrium in benzene at room temperature. Small to moderately large proportions of dangling chains were found to have less of an effect on the elongation modulus than might be expected, and similarly a relatively small effect on the degree of equilibrium swelling. Most importantly, comparisons of constant values of the high deformation modulus show that dangling‐chain irregularities decrease both the maximum extensibility of a network and its ultimate strength.

The role of chemical bonding in the adhesion of elastomers

A review is given of several studies of the effect of interfacial bonding upon the mechanical strength of an adhesive joint. In the first, polybutadiene layers were crosslinked by a free radical process whilst in contact with silane-treated glass. A direct proportionality was found between the minimum peel strength of the joint, at high temperatures and low rates of peeling, and the vinyl content of the silane treatment liquid. Covalent bonding between the diene polymer and vinyl groups on the treated glass was inferred. When radioactively tagged silanes were employed, extensive combination with the glass substrates was demonstrated. Again, the greater the amount of vinyl silane found on the treated glass surface, the greater the mechanical strength of adhesion between the treated glass and a polybutadiene overlayer. In another series of experiments two partially-crosslinked sheets of polybutadiene were pressed together before the crosslinking was taken to completion. The additional crosslinking was determined from measurements of the elastic properties and of the degree of equilibrium swelling by a compatible liquid. Again, the mechanical strength of adhesion between the two sheets under threshold conditions was found to be directly proportional to the inferred degree of interfacial interlinking. Thus, at least at high temperatures and low rates of peel, there is substantial evidence for a direct correlation between the mechanical strength of a joint and the degree of interfacial chemical bonding. Moreover, the relationships established in these studies allow other bonding systems to be diagnosed as chemical or physical in nature. For example, a dramatic increase in the strength of adhesion between two crosslinked polybutadiene layers was observed if they were exposed to air or oxygen for periods of an hour or two before being pressed into contact. It is inferred that interfacial chemical bonds are formed as a consequence of rapid surface oxidation reactions.

Model networks of end‐linked polydimethylsiloxane chains. XI. Use of very short network chains to improve ultimate properties

AbstractElastomeric networks were prepared by tetrafunctionally end‐linking mixtures of various proportions of relatively long and very short polydimethylsiloxane (PDMS) chains. The former had a number‐average molecular weight of 18,500 and the latter either 660 or 220 g mole−1. The series of (unfilled) bimodal networks thus prepared were studied in elongation to the rupture point at 25°C, and in swelling equilibrium in benzene at room temperature. Elasticity constants characterizing the Gaussian regions of the stress–strain isotherms, and values of the degree of equilibrium swelling were used to evaluate the most recent molecular theories of rubberlike elasticity. The isotherms also gave values of the elongation at which the modulus begins to increase anomalously because of limited chain extensibility, and values of the elongation and nominal stress at the point of rupture. These results were interpreted in terms of the known configurational characteristics of the constituent PDMS chains. Values of the energy or work required for rupture were used as an overall measure of the “toughness” of the networks. The very short chains were found to give a marked increase in toughness, through an increase in ultimate strength without the usual corresponding decrease in maximum extensibility. A variety of additional experiments will be required in order to elucidate the molecular origins of this important effect.

Model networks of end-linked polydimethylsiloxane chains. IX. Gaussian, non-Gaussian, and ultimate properties of the trifunctional networks

Model elastomeric networks were prepared by end-linking hydroxyl-terminated polydimethylsiloxane chains with a trialkoxy silane. Mixtures of various proportions of relatively long and very short chains were employed, since the resulting ’’bimodal’’ networks are of unique importance in characterizing non-Gaussian effects related to limited chain extensibility. Stress–strain isotherms of these trifunctional networks gave values of the elongation at which the modulus begins to increase anomalously, and values of the elongation and modulus at the point of rupture. These results were interpreted in terms of calculated values of the maximum extensibilities of the chains, and were compared with previously reported results on the corresponding tetrafunctional networks in order to characterize the effects of crosslink functionality on these properties. In addition, the elasticity constants characterizing the Gaussian regions of the isotherms, and values of the degree of equilibrium swelling were used to evaluate the most recent molecular theories of rubberlike elasticity, particularly with regard to how the elastic effectiveness of the network chains depends on chain length and extent of deformation.

Model networks of end-linked polydimethylsiloxane chains. VII. Networks designed to demonstrate non-Gaussian effects related to limited chain extensibility

End-linking polymer chains by means of a multifunctional cross-linking agent provides an ideal way for obtaining elastomeric networks of any desired distribution of chain lengths. In the present investigation, this technique was employed to give polydimethylsiloxane networks consisting of various proportions of relatively long and very short network chains. The stress–strain isotherms of these networks generally showed an anomalous increase in the modulus at high elongation, and the increase persisted at high temperatures and high degrees of swelling. This non-Gaussian effect was quantitatively correlated with the limited extensibility of the network chains; specifcally, the increase in modulus was found to begin at approximately 60%–70% of the maximum extensibility of the network chains, and network rupture at 80%–90%. The elongation at which the increase becomes evident increases with decrease in the proportion of the very short chains, thus verifying the nonaffine nature of the deformation at high elongation. In addition, the elasticity constants characterizing the Gaussian regions of the isotherms, and the values of the degree of equilibrium swelling were used to evaluate the most recent molecular theories of rubberlike elasticity, particularly with regard to the structure factor relating the modulus of an elastomer to the average length or molecular weight of the network chains.

Model networks of end-linked polydimethylsiloxane chains. IV. Elastomeric properties of the tetrafunctional networks prepared at different degrees of dilution

Highly elastomeric ’’model’’ networks were prepared by end-linking polydimethylsiloxane chains of known molecular weight, using a tetrafunctional orthosilicate. The reactions were carried out in both the undiluted state and in solution, with n-dodecane, cyclic dimethylsiloxane oligomer, and linear dimethylsiloxane oligomer as diluents. The networks thus prepared were studied with regard to their stress–strain isotherms in the unswollen state, in elongation at 25° C, and with regard to their equilibrium swelling in benzene at room temperature. The results obtained did not depend significantly on the amount of diluent present during the preparation of the networks, which indicates that the end-linking reaction employed must be very nearly complete even in the undiluted state. Values of the modulus in the reaction employed must be very nearly complete even in the undiluted state. Values of the modulus in the limits of large and small elongation were obtained from the stress–strain isotherms and were found to be in satisfactory agreement with theoretical predictions for nonaffine and affine deformations, respectively. The results from the swelling equilibrium measurements were found to fall between these two extremes. They are in excellent agreement, however, with the results of a very recent theory of network swelling in which the degree of nonaffiness depends on both the network structure and the equilibrium degree of swelling. The relatively good agreement between experiment and theory suggests that interchain entanglements do not make a large contribution to the equilibrium elastomeric properties of a polymer network.

Investigation of the relaxation properties of butadiene-styrene block copolymers

Stress relaxation has been investigated in films of block copolymers of polybutadiene and styrene (70:30). Films were prepared from the solvent mixed with a selective precipitant for polystyrene, varying the composition of the mixtures. It was found that a rapid acceleration of relazation processes accompanied by simultaneously reduced equilibrium stress occurs on adding the precipitant to the system right to the point when dispersion is obtained. The swelling of films prepared from a good solvent and from the dispersions was also investigated. The results show the relationship between the rate and equilibrium degree of swelling and the kinetics of the relaxation processes.