Effect of Chemical Crosslinking on the Swelling and Shrinking Properties of Thermal and pH-Responsive Chitosan Hydrogels (original) (raw)
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Polymer Bulletin, 2007
On heating, alkali chitin solutions undergo phase separation describing a characteristic "U-shaped" cloud point curve with a lower critical solution temperature (LCST) centered at ~30 °C. The process is accompanied by gelation of the polymer-rich phase. A different strategy to induce alkali chitin phase separation/gelation is by applying vacuum to the solution at room temperature during aprox. 72 h. Once washed to neutrality, chitin gels had a degree of acetylation of ~30-40 % (i.e. they were converted into chitosan). On cooling, these gels exhibit an exothermic peak in micro-DSC and a depression in G" and tan δ traces, evidencing a volume phase transition centered at ~20 °C. This transition is observed only within a narrow range of pH ~7.3 -7.6. Variation in the mechanical response as a result of cyclic stepwise changes in temperature between 50 and 0 °C at pH values from 7.3-7.6, revealed that the G' modulus of the gels increases on heating and decreases on cooling, a behavior that persists over at least four cycles of temperature change. Only marginal changes in G' at pH 8.0 and not at all at pH 12.0 are observed. By contrast, the variation of G" persists throughout the range of pH. This behavior is rationalized in terms of the existence of a fine balance between hydrophobic and hydrophilic interactions at varying temperature and pH, thus effectively controlling swelling and shrinking states of the gel network. The degree of swelling at pH 7.6 reaches a minimum at ~22-25 °C.
Effects of Cross-linker Variation on Swelling Behavior of Hydrogels
Asian Journal of Pharmaceutics, 2020
Introduction: The main objective of this study is to find the effects of cross-linker variation on swelling behavior of hydrogels at different temperatures, that is, 30 min–120 min. Hydrogels are three-dimensional cross-linked structural arrangement of the polymeric materials with the ability to absorb huge amounts of water while maintaining their dimensional stability. Materials and Methods: A solution of 20% glutaraldehyde (GA) was prepared in 100 ml standard flask by dissolving 20.0 g GA in a standard flask (of capacity 100 ml) and make up the volume up to the mark using demineralized water (DMW). The gelatin-polyethylene glycol (PEG) composite hydrogels were prepared by simultaneous method, in which all the constituent component networks are polymerized concurrently. Different combinations of gelatin-PEG composite hydrogels were prepared with methylene blue (MB) dye in it. Swelling was studied with the help of the following equation: dssdW-WI=×100W Results and Discussion: The ob...
European Polymer Journal, 2007
In this study, swelling behavior and mechanical properties of polyelectrolyte cationic hydrogels of poly((2-dimethylamino) ethyl methacrylate) (PDMAEMA), and poly((2-dimethylamino) ethyl methacrylate-co-butyl methacrylate) (P(DMAEMA-co-BMA)), were investigated. Hydrogels were prepared by free-radical solution copolymerization of DMA-EMA and BMA using ethylene glycol dimethacrylate (EGDMA) as the crosslinking agent. Compression-strain measurements were used to analyze the mechanical properties of the hydrogels. It was found that increasing the amount of BMA comonomer in the gel structure increases the compression modulus of the material. The results of mechanical measurements were used to characterize the network structure of the hydrogels, namely the effective crosslinking density (m à e Þ. It was found that m à e exceeds the theoretical crosslinking density (m t) calculated from the initial amount of EGDMA used for hydrogel synthesis. These hydrogels demonstrated dual sensitivity to both pH and temperature. It was shown that the pH-sensitive or temperature-sensitive phase transition behavior of the gels can be changed by changing the temperature or pH of the swelling medium at constant hydrogel composition. Increasing the temperature decreased the transition pH of the pH-sensitive phase transition. On the other hand, increasing the pH of the surrounding medium decreased the transition temperature of the temperature-sensitive phase transition. Incorporation of BMA in the gel structure has a significant effect on the transition point of the gel. Increasing the BMA content reduced the transition pH and temperature of the pHand temperature-sensitive phase transition, respectively. The similar effect of increasing temperature or BMA content can be explained by the role of hydrophobicity in the phase transition behavior of hydrogels. Finally, the results of equilibrium swelling and compression-strain measurements were used to calculate the polymer-solvent interaction parameters of these hydrogels using the Flory-Rehner equation of equilibrium swelling.
Chemical Industry
Hydrogels are synthesized by the method of radical polymerization of monomers: N-isopropylacrylamide (NIPAM) and acrylic acid (AA). Characterization of poly(N-isopropylacrylamide- co-acrylic acid) hydrogels, p(NIPAM/AA), has been performed by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and by determination of the swelling behaviour in aqueous solutions at different temperatures (25, 31 and 37?C) and pH values (2.2, 4.5, 6 and 6.8). After lyophilisation in the solution at pH 6 and temperature of 25?C, p(NIPAM/AA) hydrogels have rapidly reached equilibrium degree of swelling, ?e, in comparison to non-lyophilized samples. The mechanism of solvent transport within matrix in lyophilized samples corresponds to less Fickian diffusion, whereas Super case II diffusion is characteristic for non-lyophilized samples. p(NIPAM/AA) hydrogel with 1.5 mol% of ethylene glycol dimethacrylate (EGDM) at the temperature of 25?C and pH 6.8, has reached the highest swelling equi...
temperature sensitive hydrogels
A series of temperature and pH responsive hydrogels based on chitosan and poly(N-isopropyl acrylamide) (PNIPA) was prepared by redox polymerization. Effect of the composition on swelling behavior of the hydrogels and the release of 5-aminosaylcilic acid (5-ASA) at different temperatures and pHs have been investigated. Ammonium persulphate and TEMED were used as a redox pair at room temperature. As a cross linker, methacrylated chitosan was synthesized through the reaction of chitosan with glycidyl methacrylate (GMA). Introduction of the cross-linker provided the hydrogels with pH and temperature sensitivities. The phase transition temperatures of the hydrogels were determined by derivative differential scanning calorimeter (DDSC). Their phase transition temperatures were increased by chitosan content. Swelling behaviors and the release of 5-ASA varied significantly with pH, temperature and the gel composition. The release of 5-ASA from the hydrogels was followed by UV-Vis and fluorescence spectroscopy.
European Polymer Journal, 2001
Hydrogels were prepared by free radical polymerisation in aqueous solution of N-isopropylacrylamide (NIPA) and of NIPA with di-npropylacrylamide (DPAM), di-n-octylacrylamide (DOAM) or di-dodecylacrylamide (DDAM) as hydrophobic comonomer. N,N-methylene bisacrylamide (BIS) and glyoxal bis(diallyacetal) (GLY) were used as crosslinkers. A series of copolymers with three different comonomer contents was synthesised and for some polymers three different crosslinker concentrations were employed. The swelling equilibrium of these hydrogels was studied as a function of temperature, hydrophobic comonomer species and content in aqueous solutions of the anionic surfactant sodium dodecyl sulfate (SDS). In pure water the gels showed a discontinuous volume phase transition at 33 and 30 8C for PNIPA and hydrophobically modi®ed PNIPA copolymeric hydrogels, respectively. The swelling ratio r and the transition temperature (LCST) increased at low temperatures with the addition of SDS, this is ascribed to the conversion of non-ionic PNIPA gels into polyelectrolyte gels through the binding of SDS. At SDS concentration below 0.5 wt%, gels exhibited a single discontinuous volume transition at 36± 38 8C. However, for SDS concentration above 0.5 wt%, two discontinuous volume transitions at 36±40 and 70 8C were observed. Additionally, the replacement of BIS by the novel octafunctional crosslinker glyoxal bis(diallylacetal) (GLY) yielded an increase in the swelling ratio. q
Dynamic swelling behavior of interpenetrating polymer networks in response to temperature and pH
Journal of Applied Polymer Science, 2015
Temperature responsive hydrogels based on ionic polymers exhibit swelling transitions in aqueous solutions as a function of shifting pH and ionic strength, in addition to temperature. Applying these hydrogels to useful applications, particularly for biomedical purposes such as drug delivery and regenerative medicine, is critically dependent on understanding the hydrogel solution responses as a function of all three parameters together. In this work, interpenetrating polymer network (IPN) hydrogels of polyacrylamide and poly(acrylic acid) were formulated over a broad range of synthesis variables using a fractional factorial design, and were examined for equilibrium temperature responsive swelling in a variety of solution conditions. Due to the acidic nature of these IPN hydrogels, usable upper critical solution temperature (UCST) responses for this system occur in mildly acidic environments. Responses were characterized in terms of maximum equilibrium swelling and temperature-triggered swelling using turbidity and gravimetric measurements. Additionally, synthesis parameters critical to achieving optimal overall swelling, temperature-triggered swelling, and sigmoidal temperature transitions for this IPN system were analyzed based on the fractional factorial design used to formulate these hydrogels.
In the present work, a series of cross-linked LVCS/PVA hydrogels with various feed compositions were prepared using glutaraldehyde as cross-linking agent. The prepared hydrogels were used for dynamic and equilibrium swelling studies. The swelling behavior of these hydrogels was investigated as functions of effect of pH, polymeric compositions and degree of cross-linking. Swelling studies were performed in 0.05 M USP phosphate buffer solutions of varying pH 1.2, 5.5, 6.5 and 7.5. Results showed that swelling increased by increasing PVA contents in the structure of hydrogels in solutions of higher pH values. This is due to the presence of more hydroxyl groups (-OH) in the PVA structure. On the other hand, by increasing LVCS contents, swelling increased in a solution of acidic pH and it is due to ionization of amino groups (-NH 2 ), but this swelling was not significant. Swelling of hydrogels was decreased with increase in cross-linking ratio due to tighter hydrogel structure. Porosity and sol-gel fraction were also investigated. It was found that with increase in LVCS and PVA contents porosity and gel fraction increased, whereas by increasing glutaraldehyde content gel fraction increased and porosity decreased. Diffusion coefficient (D) and network parameters, i.e., the average molecular weight between cross-links (M C ), solvent interaction parameters (v), polymer volume fraction in swollen state (V 2S ) and cross-linked density (q) were calculated using Flory-Rehner theory. Selected samples were loaded with model drug diphenhydramine HCl. The release of diphenhydramine HCl was studied for 12 h period in 0.05 M USP phosphate buffer solutions of varying pH 1.2, 5.5 and 7.5. It was observed that drug release increased with increasing PVA contents in the hydrogels, while release of drug decreased as the ratio of crosslinking agent increased in the hydrogel structure owing to strong physical Author's personal copy entanglements between polymers. The release mechanisms were studied by fitting experimental data to model equations like zero order, first order, Higuchi and Peppas. Results showed that the kinetics of drug release from hydrogels in buffer solutions of pH 1.2, 5.5 and 7.5 was mainly non-fickian diffusion. Hydrogels were characterized by Fourier transform infrared and X-ray diffraction to confirm the structure and study the crystallinity of hydrogel, respectively.