Solvent Interaction Parameter Research Articles

The Transient Response of the Swelling in Mixed Solvent for the Freeze/Thawed Polyacrylonitrile Gel Observed in Association with the Polymer Elution

The swelling properties of polyacrylonitrile gel prepared by the repeating cycle of freezing and thawing were examined, as its transient response toward the equilibrium swollen states varies with the variation of mixing ratio (r) of the immersion solvent which consisted of N, N–dimethylacetamide and benzene. Two experimental protocols were used; successive solvent exchanges were conducted in Protocol 1, and no solvent exchange but the varying of the volume of immersion solvent was conducted in Protocol 2. Also, the aging time was set between the end of the freezing and thawing cycle and the start of the swelling experiment. As a result, the equilibrium swelling ratio decreased with the increase in the aging time. Significant discrepancies for the transient responses were found for the immersion solvents between r ≤ 0.74 and r ≥ 0.78 both in Protocol 1 and 2, in which the elution of polyacrylonitrile molecules was related to the responses. A characteristic overshoot curve was observed in the swelling with r ≥ 0.78 in Protocol 2, which was also attributed to the polyacrylonitrile elution. The polymer solvent interaction parameter was calculated on the basis of the osmotic pressure equilibrium, which was associated with the interpretation of the transient response data.

β-Cyclodextrin/chitosan-based (polyvinyl alcohol-co-acrylic acid) interpenetrating hydrogels for oral drug delivery

Gallic acid is an important phenolic compound with extensive applications in the food and pharmaceutical industries due to its health-promoting properties. However, due to its poor solubility and bioavailability, it is rapidly excreted from the body. Therefore, β-cyclodextrin/chitosan-based (polyvinyl alcohol-co-acrylic acid) interpenetrating controlled release hydrogels were developed to improve its dissolution and bioavailability. pH, polymer ratios, dynamic and equilibrium swelling, porosity, sol-gel, FTIR, XRD, TGA, DSC, SEM and structural parameters like an average molecular weight between crosslinks, solvent interaction parameters, and diffusion coefficient affecting release behavior were investigated. The highest swelling and release were observed at pH 7.4. Furthermore, hydrogels showed good antioxidant and antibacterial properties. Hydrogels improved the bioavailability of gallic acid in a pharmacokinetics study in rabbits. In vitro biodegradation showed that hydrogels were more stable in blank PBS than lysozyme and collagenase. Hydrogels were safe for rabbits (3500 mg/kg) without causing hematological or histopathological changes. The hydrogels showed good biocompatibility, and no adverse reactions were observed. Moreover, the developed hydrogels can be used to improve the bioavailability of various other drugs.

Influences of material and processing conditions on the depolymerization speed of anhydride-cured epoxy during the solvent-assisted recycling

Thermosetting polymers with permanent crosslinking networks have been intensively used in high-performance applications requiring thermal stability, chemical resistance, and structural integrity. However, they are extremely hard to be reprocessed and recycled using conventional techniques. The recently developed solvent-assisted recycling technique is shown to depolymerize and recycle thermoset wastes using low-toxic solvents and mild processing conditions. The macromolecular polymer chains can be cleaved at target covalent bonds on the chain backbone through bond exchange reactions with solvent molecules. In this study, the influences of various material and processing conditions on the network depolymerization speed are studied from both experimental and theoretical points of view. A diffusion-reaction chemomechanics modeling framework is defined to link the physical properties of organic solvent and microscale reaction kinetics to the overall depolymerization speed of thermosets at various material and processing parameters. The study reveals how the temperature, network crosslinking density, and physical properties of the solvent, in particular, the solvent diffusivity and solvent/polymer interactions would affect the depolymerization speed. It enhances our understanding of the underlying fundamentals of thermoset recycling using organic solvents, which provides valuable insights for industrials to optimize the recycling conditions and develop appropriate strategies for waste management. • Influencing mechanisms of various material and processing parameters are revealed. • A theory is defined to link solvent properties to the depolymerization speed. • There is an optical content of good solvent to maximize the depolymerization speed. • There are threshold values for solvent hydroxyl-hydrocarbon ratio and interaction parameters.

Re-Entrant Conformation Transition in Hydrogels.

Hydrogels are attractive materials not only for their tremendous applications but also for theoretical studies as they provide macroscopic monitoring of the conformation change of polymer chains. The pioneering theoretical work of Dusek predicting the discontinuous volume phase transition in gels followed by the experimental observation of Tanaka opened up a new area, called smart hydrogels, in the gel science. Many ionic hydrogels exhibit a discontinuous volume phase transition due to the change of the polymer–solvent interaction parameter χ depending on the external stimuli such as temperature, pH, composition of the solvent, etc. The observation of a discontinuous volume phase transition in nonionic hydrogels or organogels is still a challenging task as it requires a polymer–solvent system with a strong polymer concentration dependent χ parameter. Such an observation may open up the use of organogels as smart and hydrophobic soft materials. The re-entrant phenomenon first observed by Tanaka is another characteristic of stimuli responsive hydrogels in which they are frustrated between the swollen and collapsed states in a given solvent mixture. Thus, the hydrogel first collapses and then reswells if an environmental parameter is continuously increased. The re-entrant phenomenon of hydrogels in water–cosolvent mixtures is due to the competitive hydrogen-bonding and hydrophobic interactions leading to flow-in and flow-out of the cosolvent molecules through the hydrogel moving boundary as the composition of the solvent mixture is varied. The experimental results reviewed here show that a re-entrant conformation transition in hydrogels requires a hydrophobically modified hydrophilic network, and a moderate hydrogen-bonding cosolvent having competitive attractions with water and polymer. The re-entrant phenomenon may widen the applications of the hydrogels in mechanochemical transducers, switches, memories, and sensors.

Accounting for Cooperativity in the Thermotropic Volume Phase Transition of Smart Microgels.

A full quantitative description of the swelling of smart microgels is still problematic in many cases. The original approach of Flory and Huggins for the monomer–solvent interaction parameter cannot be applied to some microgels. The reason for this obviously is that the cross-linking enhances the cooperativity of the volume phase transitions, since all meshes of the network are mechanically coupled. This was ignored in previous approaches, arguing with distinct transition temperatures for different meshes to describe the continuous character of the transition of microgels. Here, we adjust the swelling curves of a series of smart microgels using the Flory–Rehner description, where the polymer–solvent interaction parameter is modeled by a Hill-like equation for a cooperative thermotropic transition. This leads to a very good description of all measured microgel swelling curves and yields the physically meaningful Hill parameter . A linear decrease of is found with increasing concentration of the cross-linker N,N-methylenebisacrylamide in the microgel particles p(NIPAM), p(NNPAM), and p(NIPMAM). The linearity suggests that the Hill parameter corresponds to the number of water molecules per network chain that cooperatively leave the chain at the volume phase transition. Driven by entropy, water molecules of the solvate become cooperatively “free” and leave the polymer network.

Gelatin/carboxymethyl cellulose based stimuli-responsive hydrogels for controlled delivery of 5-fluorouracil, development, in vitro characterization, in vivo safety and bioavailability evaluation

Current work focuses on the synthesis and characterization of novel pH-sensitive biocompatible gelatin/carboxymethyl cellulose based hydrogels by free radical polymerization technique cross-linked with glutaraldehyde. Effect of pH, polymer ratio and variable crosslinking concentrations on dynamic swelling, equilibrium swelling, porosity, sol-gel analysis and in vitro release pattern was investigated. Hydrogel structure was confirmed by FTIR, XRD, and DSC. Moreover scanning electron microscopy confirmed the porous structure of gel network. Various structure property relationships like average molecular weight between crosslinks (Mc), solvent interaction parameters, volume fraction of polymer (V2,s) and diffusion coefficient (D) that affect the release behaviour were determined. Results showed that maximum swelling and highest release of drug occurred at pH 1.2. Porosity and gel fraction increased by increasing polymer load. The invivo absorption and pharmacokinetics evaluation in rabbit’s models revealed the controlled nature of hydrogels. MTT assay confirmed the biocompatible nature of blank hydrogels against Vero cell lines and cytotoxic potential against HeLa cell lines. The preliminary safety evaluation and oral tolerability revealed that the hydrogel solution is safe up to 4000 mg/kg body weight without causing any hematological or histopathological changes in rabbits.

Determination of Binary Interaction Parameters for Ternary Polymer–Polymer–Solvent Systems Using Raman Spectroscopy

AbstractMultilayer coatings and drying of polymer films are widely used to produce different applications like organic electronics or barrier foils. If the product quality requires separated layers, partial or immiscible systems can be used to avoid interdiffusion between the layers. For the prediction of a possible phase separation of a ternary polymer–polymer–solvent system, it is required to measure or obtain from a data bank the three binary interaction parameters of the system to describe its thermodynamic behavior. This poses a challenge due to the scarce data and measurement techniques for polymer–solvent interaction parameters, but even more for polymer–polymer interaction parameters. In this work, a numerical routine is developed to predict the phase separation for given interaction parameters and is validated with literature results. Subsequently, different ternary polystyrene–poly(methyl methacrylate)–toluene mixtures are prepared and the resulting composition of the phase (or phases) is measured via Raman spectroscopy. The determined equilibrium data are used to fit the binary interaction parameters, which can be used afterward as input parameters for the numerical routine in order to predict the ternary phase diagram of the system.

Phase Diagrams of n-Type Low Bandgap Naphthalenediimide-Bithiophene Copolymer Solutions and Blends.

Phase diagrams of n-type low bandgap poly{(N,N′-bis(2-octyldodecyl)naphthalene -1,4,5,8-bis(dicarboximide)-2,6-diyl)-alt-5,5′,-(2,2′-bithiophene)} (P(NDI2OD-T2)) solutions and blends were constructed. To this end, we employed the Flory–Huggins (FH) lattice theory for qualitatively understanding the phase behavior of P(NDI2OD-T2) solutions as a function of solvent, chlorobenzene, chloroform, and p-xylene. Herein, the polymer–solvent interaction parameter (χ) was obtained from a water contact angle measurement, leading to the solubility parameter. The phase behavior of these P(NDI2OD-T2) solutions showed both liquid–liquid (L–L) and liquid–solid (L–S) phase transitions. However, depending on the solvent, the relative position of the liquid–liquid phase equilibria (LLE) and solid–liquid phase equilibria (SLE) (i.e., two-phase co-existence curves) could be changed drastically, i.e., LLE > SLE, LLE ≈ SLE, and SLE > LLE. Finally, we studied the phase behavior of the polymer–polymer mixture composed of P(NDI2OD-T2) and regioregular poly(3-hexylthiophene-2,5-dyil) (r-reg P3HT), in which the melting transition curve was compared with the theory of melting point depression combined with the FH model. The FH theory describes excellently the melting temperature of the r-reg P3HT/P(NDI2OD-T2) mixture when the entropic contribution to the polymer–polymer interaction parameter (χ = 116.8 K/T − 0.185, dimensionless) was properly accounted for, indicating an increase of entropy by forming a new contact between two different polymer segments. Understanding the phase behavior of the polymer solutions and blends affecting morphologies plays an integral role towards developing polymer optoelectronic devices.

Viscometric behavior of hydroxyethyl cellulose in aqueous solutions of some imidazolium ionic liquids

In this work, viscosity of hydroxyethyl cellulose in aqueous solutions of ionic liquids, 1-butyl-3-methyl imidazolium bromide, 1-hexyl-3-methyl imidazolium bromide and 1-octyl-3-methyl imidazolium bromide were measured at temperature range of T = 288.15–308.15 K. Some hydrodynamic and thermodynamic parameters such as intrinsic viscosity, polymer–solvent interaction parameter and second virial coefficient were calculated using viscosity data. Obtained results reveal that thermodynamic quality of aqueous solutions of ionic liquids for hydroxyethyl cellulose decreases with increasing temperature and addition of ionic liquids. The flow activation energy was found from the linearization of Arrhenius equation and correlated in terms of polymer concentration. From sign of the initial slope of the activation energy versus polymer concentration at zero concentration, it is concluded that thermodynamic quality of aqueous solutions of ionic liquids are reduced by increasing temperature.

Effect of Ethanol and Temperature on Partition Coefficients of Ethyl Acetate, Isoamyl Acetate, and Isoamyl Alcohol: Instrumental and Predictive Investigation

For alcoholic beverages such as beer, downstream processing for either dealcoholization or off-flavor removal requires both quantitative data and suitable predictive methods. Along with experimental investigations, we use a method initially developed for studying the solubility of gases in two or more miscible liquid solvents to monitor the effect of ethanol on air–water partition coefficients of three major flavors found in beer, namely, isoamyl alcohol, ethyl acetate, and isoamyl acetate. In the ethanol concentration range between 0 and 0.1 mole fraction, a slight, rather linear increase in the Henry’s solubility coefficient was observed. This overall behavior can be captured well using Henry coefficients for aqueous binary and ternary systems together with the Wohl expansion for excess Gibbs free energy coupled with the one-parameter Margules equation. Based on the developed model, the Wohl’s expansion parameter for ethanol–water is introduced as the solvent–solvent interaction parameter. The van ’t Hoff parameters for temperature dependence of Henry coefficients for binary water–flavor solutions are determined in the range of 30 to 60 °C.

Sequential Deposition of Organic Films with Eco-Compatible Solvents Improves Performance and Enables Over 12%-Efficiency Nonfullerene Solar Cells.

Casting of a donor:acceptor bulk-heterojunction structure from a single ink has been the predominant fabrication method of organic photovoltaics (OPVs). Despite the success of such bulk heterojunctions, the task ofcontrolling the microstructure in a single casting process has been arduous and alternative approaches are desired. To achieve OPVs with a desirable microstructure, a facile and eco-compatible sequential deposition approach is demonstrated for polymer/small-molecule pairs. Using a nominally amorphous polymer as the model material, the profound influence of casting solvent is shown on the molecular ordering of the film, and thus the device performance and mesoscale morphology of sequentially deposited OPVs can be tuned. Static and in situ X-ray scattering indicate that applying (R)-(+)-limonene is able to greatly promote the molecular order of weakly crystalline polymers and form the largest domain spacing exclusively, which correlates well with the best efficiency of 12.5% in sequentially deposited devices. The sequentially cast device generally outperforms its control device based on traditional single-ink bulk-heterojunction structure. More crucially, a simple polymer:solvent interaction parameter χ is positively correlated with domain spacing in these sequentially deposited devices. These findings shed light on innovative approaches to rationally create environmentally friendly and highly efficient electronics.

Preparation, characterization, dielectric properties and solvent-imbibing behavior of styrene–butadiene rubber/zinc sulfide nanocomposites

This work focuses on the preparation of styrene–butadiene rubber (SBR)/zinc sulfide (ZnS) nanocomposites using a simple and inexpensive two-roll mill mixing technique. The structural, morphological and thermal properties of the nanocomposites were analyzed by FT-IR, UV–visible spectroscopy, XRD, SEM and DSC measurements. The results of the spectroscopic studies revealed the interaction of ZnS nanoparticles with SBR through the shift in the peaks of UV and FT-IR spectra. The XRD and SEM studies showed that the nanoparticles were well inserted into the macromolecular structure of SBR. The glass transition temperature of the composites increased with the addition of nanoparticles. The dielectric constant of nanocomposite with different loading of nanoparticles was carried out in the frequency range 100–106 Hz. The dielectric properties of nanocomposites were higher than that of pure SBR. In addition, the diffusion and transport properties of organic solvents through SBR/ZnS nanocomposites were studied in detail as a function of ZnS content. The polymer–solvent interaction parameter, enthalpy and entropy of sorption data were estimated from the transport properties. The high dielectric constant of these nanocomposites infers that ZnS is one of the ideal nanofiller for the development of high-k materials for flexible energy storage applications.

The Structural, Crystallinity and Thermal Properties of pH responsive Interpenetrating Gelatin/Sodium Alginate Based Polymeric Composites for the Controlled Delivery of Cetirizine HCl

The present work aimed to design and synthesize pH-sensitive cross-linked Ge/SA hydrogels using different ratios of each polymer, and to investigate the effect of each polymer on dynamic, equilibrium swelling, and in vitro release pattern of cetirizine hydrochloride, which was selected as a model drug. These gelatin and sodium alginate hydrogels were prepared at room temperature through free radical polymerization using glutaraldehyde as a crosslinker. These polymeric composites were used as model systems to envisage various important characterizations. The in vitro release pattern of drug was investigated in three different mediums (phosphate buffer solution of pH 1.2, 5.5, 7.5 whose ionic strength was kept constant). Various structure property relationships that affect its release behavior were determined such as swelling analysis, porosity, sol-gel analysis, average molecular weight between crosslinks (Mc), solvent interaction parameter (χ), volume fraction of polymer (V2,s) and diffusion coefficient. The structural, crystallinity, and thermal stability were confirmed using FTIR, XRD, and DSC analysis. These hydrogels showed maximum swelling at pH 1.2. Zero-order, first-order, Higuchi, and Peppas models were applied to demonstrate the release pattern of drug. The release of drug occurred through non-Fickian diffusion or anomalous mechanism. Porosity was found increased with an increase in concentration of both polymers, and porosity decreased when the concentration of the crosslinker was increased. Gel fraction increased with an increase in concentration of SA, Ge, and glutaraldehyde. The prepared pH sensitive hydrogels can be used as a potential carrier for the sustained delivery of cetirizine hydrochloride.

Biocompatible hydrogels for the controlled delivery of anti-hypertensive agent: development, characterization and in vitro evaluation

The aim of the present exploration was to develop novel pH-sensitive cross-linked Gelatin/Polyvinyl pyrrolidone hydrogels using different ratios of both the polymers and to investigate the effect of polymers and degree of crosslinking on dynamic, equilibrium swelling and invitro release pattern of the model drug (captopril). Grafting polymerization technique was used for the preparation of these hydrogels using glutaraldehyde as crosslinking agent. These polymeric materials were then used as model systems to envisage various important characterizations like FTIR (Fourier transform infrared spectroscopy), XRD (X-ray diffraction) and scanning electron microscopy (SEM). Phosphate buffers of pH 1.2, 6.5 and 7.5 were used for swelling and invitro drug release profile investigation. Different parameters like swelling analysis, porosity, sol-gel analysis, average molecular weight between crosslinks (Mc), solvent interaction parameter (χ), volume fraction of polymer (V2,s) and diffusion coefficient that affects the drug release behavior were also determined. Higher swelling and release was observed at lower pH values. FTIR spectra showed interaction between gelatin and polyvinyl pyrrolidone and successful formation of cross-linked structure. Pulsatile drug release study showed the controlled delivery of model drug. The release of drug occurred through non-fickian diffusion or anomalous mechanism. Aforementioned characterizations reveal successful formation of copolymer. pH sensitive swelling ability and drug release behavior suggest that the rate of polymer chain relaxation and the rate of drug diffusion from these hydrogels are comparable which also predicts their possible use for site specific captopril delivery.

Tunable elasticity and thermodynamic parameters of hydroxypropyl methacrylate‐based gels with varying extents of monomer concentration: Statistical mechanics treatments of physical observations

ABSTRACTThe present article reports the scaling laws relating the preparation conditions with the equilibrium swelling degree and the crosslinking density of a new family of hydroxyl functional hydrogels (Hgs) prepared by free radical crosslinking polymerization of hydroxypropyl methacrylate (HPMA) in aqueous solution with tetraethylene glycol dimethacrylate (TEGDMA) as crosslinker. For comparison of physical properties, chemical and mechanical stability, HPMA cryogels (Cgs) were also prepared using the cryotropic gelation technique by conducting the gelation reactions at subzero temperatures. The measurements of the equilibrium swelling degree and the elastic properties of poly(hydroxypropyl methacrylate) (PHPMA) gel samples having different initial monomer concentration were treated according to the scaling theory to explain the rubbery behavior of the resulting Hgs. The obtained scaling relation between the reduced modulus and the swelling degree indicated limiting chain extensibility which is neglected in the Flory–Rehner theory. The effective crosslink density and polymer–solvent interaction parameter of PHPMA Hgs were used to interpret the observed deviation. Results showed that the properties of PHPMA Hgs and Cgs can be controlled by varying the HPMA and TEGDMA content. Increase of total monomer concentrations causes a pronounced decrease in the equilibrium swelling degree, and an increase in the elastic response of PHPMA Hgs and Cgs. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 45889.

Synthesis and evaluation of pH dependent polyethylene glycol-co-acrylic acid hydrogels for controlled release of venlafaxine HCl

pH dependent polyethylene glycol/acrylic acid(AA/PEG) hydrogels of an anti-depressant, venlafaxine hydrochloride were prepared by free-radical co-polymerization method, using benzyl peroxide (BPO) as initiator and methylene bisacrylamide (MBA) as cross-linker. Different formulations were prepared with varying concentration of polymer, monomer and cross-linker to study their effect on gel swelling, diffusion characteristics and subsequent drug release. Structural parameters of AA/PEG hydrogels were investigated by measuring diffusion co-efficient, volume fraction of polymer in the swollen state, effective molecular weight of polymer chain between cross-linking points, the number of links between two crosslink and solvent interaction parameters. Swelling coefficients were found to be increased with increase in pH i.e. minimum at pH 1.2 and maximum at pH 7.5. Increase in acrylic acid concentration increases swelling while increase in cross-linker concentration has an opposite effect due to denser structure. AA/PEG hydrogels were characterized by fourier-transform infrared spectroscopy (FT-IR), scanning electron microscope (SEM), X-Ray diffraction (XRD), thermo gravimetric analysis (TGA) to determine the polymer structure, its morphology and strength. Drug release studies performed at pH 1.2, 5.5 and 7.5 shows higher release at higher pH. Drug release was increased by increasing acrylic acid concentration and decreasing cross-linker concentration.

Curing of polyfurfuryl alcohol resin catalyzed by a homologous series of dicarboxylic acid catalysts. II. Swelling behavior and thermal properties

ABSTRACTPolyfurfuryl alcohol (PFA), as a bio‐based resin made from lignocellulosic materials, was crosslinked using a homologous series of dicarboxylic acid catalysts consisting of oxalic, succinic, and adipic acids, which are different in their dissociation constants. Swelling behavior, thermal stability, and non‐oxidative char residue of the resulting networks were characterized as a function of the acids strength and their concentration. Swelling of the networks were investigated at room temperature in eight solvents differing in molecular size and solubility parameter. Using acetonitrile as a solvent, the swelling mechanism was explored by applying kinetic models to the swelling data. Dynamic swelling studies during 16–30 days supported non‐Fickian and anomalous diffusion mechanism in highly crosslinked samples supporting PFA chain rigidity and high crosslinking density of the networks. Polymer–solvent interaction parameter, molecular weight between crosslinks and crosslinking densities were also determined. According to the results, the extent of PFA crosslinking and non‐Fickian behavior of the swelling solvent diffusion through the networks are strongly dependent on the concentration and dissociation characteristics of the catalysts used. Thermal stability studies showed no significant differences between the compositions, up to 900 °C. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 45770.

Electric Double Layer Composed of an Antagonistic Salt in an Aqueous Mixture: Local Charge Separation and Surface Phase Transition.

We examine an electric double layer containing an antagonistic salt in an aqueous mixture, where the cations are small and hydrophilic but the anions are large and hydrophobic. In this situation, a strong coupling arises between the charge density and the solvent composition. As a result, the anions are trapped in an oil-rich adsorption layer on a hydrophobic wall. We then vary the surface charge density σ on the wall. For σ>0 the anions remain accumulated, but for σ<0 the cations are attracted to the wall with increasing |σ|. Furthermore, the electric potential drop Ψ(σ) is nonmonotonic when the solvent interaction parameter χ(T) exceeds a critical value χ_{c} determined by the composition and the ion density in the bulk. This leads to a first-order phase transition between two kinds of electric double layers with different σ and common Ψ. In equilibrium such two-layer regions can coexist. The steric effect due to finite ion sizes is crucial in these phenomena.

Salt-Induced Swelling and Volume Phase Transition of Polyelectrolyte Gels

A theoretical model of polyelectrolyte gels is presented to study continuous and discontinuous volume phase transitions induced by changing salt concentration in the external solution. Phase diagrams are constructed in terms of the polymer–solvent interaction parameters, external salt concentration, and concentration of fixed charges. Comparisons with previous experiments for an ionized acrylamide gel in mixed water–acetone solvents are made with good quantitative agreement for a monovalent salt (NaCl) but fair qualitative agreement for a divalent salt (MgCl2), using a simple set of parameters for both cases. The effective polymer–solvent interactions vary with the volume fraction of acetone in the mixed solvent, leading to either continuous or discontinuous volume transitions. The presence of divalent ions (Mg2+) in addition to monovalent ions in the external solution reduces the critical salt concentration for the discontinuous transition by several orders of magnitude. Moreover, a secondary continuous transition is predicted between two highly swollen states for the case of a divalent salt. The present model may be further extended to study volume phase transitions of polyelectrolyte gels in response to other stimuli such as temperature, pH and electrical field.

PH responsive cross-linked polymeric matrices based on natural polymers: effect of process variables on swelling characterization and drug delivery properties.

Introduction: The current work was aimed to design and synthesize novel crosslinked pH-sensitive gelatin/pectin (Ge/Pec) hydrogels using different polymeric ratios and to explore the effect of polymers and degree of crosslinking on dynamic, equilibrium swelling and in vitro release behavior of the model drug (Mannitol). Methods: The Ge/Pec based hydrogels were prepared using glutaraldehyde as the crosslinker. Various structural parameters that affect their release behavior were determined, including swelling study, porosity, sol-gel analysis, average molecular weight between crosslinks (Mc), volume fraction of polymer (V2,s), solvent interaction parameter (χ) and diffusion coefficient. The synthesized hydrogels were subjected to various characterization tools like Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and DSC differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). Results:The hydrogels show highest water uptake and release at lower pH values. The FTIR spectra showed an interaction between Ge and Pec, and the drug-loaded samples also showed the drug-related peaks, indicating proper loading of the drug. DSC and TGA studies confirmed the thermal stability of hydrogel samples, while SEM showed the porous nature of hydrogels. The drug release followed non-Fickian diffusion or anomalous mechanism. Conclusion: Aforementioned characterizations reveal the successful formation of copolymer hydrogels. The pH-sensitive swelling ability and drug release behavior suggest that the rate of polymer chain relaxation and drug diffusion from these hydrogels are comparable which also predicts their possible use for site-specific drug delivery.