Kinetics, Absorption, and Diffusion of Crosslinked Chitosan Hydrogels PDF

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See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/355437562 Kinetics, absorption and diffusion mechanism of crosslinked Chitosan Hydrogels Article in Indian Journal of Engineering and Materials Sciences · August 2021 CITATIONS READS 2 240 3 authors: Sakshi Nangia Deeksha Narula Katyal Indian Institute of Technology Guwahati Guru Gobind Singh Indraprastha University 6 PUBLICATIONS 65 CITATIONS 41 PUBLICATIONS 825 CITATIONS SEE PROFILE SEE PROFILE Sudhir Gopalrao Warkar Delhi Technological University 55 PUBLICATIONS 506 CITATIONS SEE PROFILE All content following this page was uploaded by Sakshi Nangia on 27 October 2021. The user has requested enhancement of the downloaded file.  Indian Journal of Engineering & Materials Sciences Vol. 28, August 2021, pp. 374-384 Kinetics, absorption and diffusion mechanism of crosslinked Chitosan Hydrogels Sakshi Nangiaa, Deeksha Katyala*, Sudhir Warkarb a University School of Environment Management, Guru Gobind Singh Indraprastha University, New Delhi 110 078, India b Department of Applied Chemistry, Delhi Technological University, New Delhi 110 042, India Received: 23 December 2020; Accepted: 11 July 2021 Green polymers are extremely useful for various environmental applications. One such biopolymer is Chitosan. In this study, crosslinked and physical Chitosan hydrogels were synthesized. The swelling of disc-shaped hydrogels crosslinked using different concentrations of Glutaraldehyde were compared with physical film and bead shaped hydrogels. Best swelling of around 3000% was observed in case of square film shaped hydrogels but they lacked rigidity and dissolved in mild acids. In case of crosslinked hydrogels, as the crosslinker concentration increased, the hydrogels entrapped less water but gained better mechanical strength. Characterization of synthesized crosslinked hydrogels was carried out using FTIR, TGA and DSC. Equilibrium swelling results indicated more water absorption at acidic pH (2.5). Simultaneously, increase in temperature led to enhancement of swelling degree. The hydrogels trapped more water leading to increased swelling, in case of lower molar salt concentrations. Second order kinetics was followed due to stress relaxation of polymeric chain. Diffusion was found to be anomalous since exponent values lied between 0.5 and 1. Peleg‟s and Exponential association model were used to carry out absorption modeling. The data was found to fit the Peleg‟s absorption model. Degree of swelling is a major factor for deciding a hydrogels utility. Swelling ability, biocompatibility and availability of lone pairs of oxygen and nitrogen on the surface of CS makes it ideal for applications in drug delivery, controlled release of fertilizers and adsorption of environmental contaminants. Keywords: Chitosan, Swelling kinetics, Peppas model, Peleg‟s and Exponential association model 1 Introduction toxicity are being researched upon for their utility in Hydrogels are 3-dimensional hydrophilic industrial and commercial applications7. polymeric networks in which water acts as the CS is a cationic biopolymer having chemical dispersion medium1. Superabsorbent polymers are the formula (1,4)2-amino-2-D-glucose. It is a hydrogels, which show a high swelling percentage biopolymer obtained from processing of chitin and is (>100) for water or other fluids2,3,4. The hydrophilicity known for its biocompatibility, biodegradability in these hydrogels/ superabsorbent polymers may be and abundance8. CS has hydrophilic functional attributed to presence of groups such as hydroxyl groups and can be used for synthesizing hydrogels. (-OH), amidic (-CONH-), carboxylic (-COOH) and The biggest challenge of working with CS is that it sulphonic (-SO3H), which are polar in nature5. lacks mechanical strength as compared to synthetic Hydrogels may be synthetic or natural polymer based polymers. For improving its mechanical strength and hydrophilic networks. Some of the synthetic polymers workability, it can be either blended with other used in hydrogel formation are acrylic acid, polymers like acrylamide9,10 or crosslinked with acrylamide, polyurethane and poly (ethylene glycol). agents such as glutaraldehyde and genipin11. Biopolymers used for synthesis of hydrogels are Hydrogels are known for their high swelling ability usually polysaccharides such as cellulose, dextran, and take several hours to achieve equilibrium or starch and chitosan (CS)6. maximum absorption capacity. The applicability in In most of the commercial applications such as areas such as drug delivery or as controlled release diapers, acrylate based hydrogels are employed fertilizers depends on the rate and degree of swelling that take several years to degrade. Therefore, of hydrogel5. Most dried hydrogels take time to superabsorbents with rapid biodegradability and less achieve equilibrium because the diffusion of water —————— into the glassy framework is a slow process12. *Corresponding author (E-mail: [email protected]) Mathematical modeling of swelling is an important  NANGIA et al.: KINECTIC, ADSORPTION & DIFFUSION MECHANISM OF CROSSLINKED CHITOSAN HYDROGELS 375 aspect for behavior and applicability of hydrogel 800000 Da (1526.464 g/mol) and degree of under different environmental conditions. This has deacetylation 90-98%, glutaraldehyde 25% solution gained a lot of attention in recent years. (Thomas Baker), acetic acid glacial (Fisher Scientific) The kinetics of hydrogel swelling has been were used as received. All the solutions were prepared described through various mathematical models. The using double distilled water. Wherever required, pH most commonly used is Fickian diffusion model that was adjusted using 0.1M HCl and 0.1M NaOH. describes solvent distribution into the gel matrix 2.2 Methods during its swelling or collapsing13. As per this model, the swelling fraction tends to increase linearly with 2.2.1 Synthesis of CS Hydrogels square root of time till a fraction of 0.4 and the Uncrosslinked CS beads swelling curve is never sigmoidal in shape till this CS (1 g) was added to 10mL 1% (v/v) acetic acid range. The second model is the collective diffusion dissolved in 50mL distilled water. The components model that relates the stress gradient with the swelling were mixed vigorously using a magnetic stirrer till a and network expansion of the gel14. Both of these uniform solution was obtained. After an hour, drops models fail to describe sigmoidal swelling curves of CS and acetic acid mixture were added to 0.1N resulting from large volume changes. Sigmoidal NaOH kept in another beaker. The NaOH solution swelling curves often tend to describe the non-Fickian acts as a precipitation bath for the formation of diffusion behavior. The Fickian diffusion describes chitosan beads19. The beads formed were left for 1 hr. the sigmoidal swelling behavior when the movement in NaOH for hardening. The beads were then taken of gel surface is considered correctly15. out, washed and dried and then placed in different pH Scarce literature is available on the swelling solutions for determining the swelling. properties and mathematical modeling of CS Uncrosslinked CS films hydrogels and also on the comparison of degree of The same procedure was followed for the swelling for different hydrogel shapes. There has synthesis; the only difference being that instead of been a growing interest in the area of shape pouring the mixture as drops in NaOH precipitation changing hydrogels in response to external stimuli 16. bath, the mixture was poured uniformly over a Few reports are available in literature on petridish and was allowed to dry completely at room crosslinking of CS with glutaraldehyde 17,1,18,11. The temperature. The beads form in a precipitation bath novelty in this synthesis is that the quantity of whereas spreading, drying and removing the layer can crosslinker involved is lower than that available in form the film. Once the film dried, a 2.5x2.5cm literature and different shapes of CS have been square was cut and placed in solutions of different pH compared along with their swelling kinetics in a to check the swelling. single manuscript. To the best of our knowledge, Disc shaped crosslinked CS Hydrogels absorption modeling has not been carried out for CS was mixed in 10mL 1% (v/v) acetic acid crosslinked CS hydrogels. and the solution was stirred in 100mL beaker20, 21. Henceforth, we have attempted to synthesize CS Glutaraldehyde solution that acts as crosslinker was hydrogels in the form of beads, discs and films and added before transferring the contents to test tubes subsequently investigated swelling characteristics of and was thoroughly mixed for about 5-10 minutes till synthesized hydrogels for its percent equilibrium the solution turned slightly viscous. The total content swelling and diffusion coefficient. Further the order of was kept as 50 mL by adding remaining quantity of kinetics was evaluated using two mathematical models, distilled water. The test tubes were retained at room which were not attempted earlier for estimating temperature and hydrogel formation was observed swelling kinetics parameters of CS hydrogels namely within an hour. The tubes were broken and hydrogels Peleg‟s model and Exponential association equation were removed and cut in the form of equal sized model. Best-fitted model was also yielded from cylindrical disks. The disks were thoroughly washed experimental data. with water and kept for drying in oven at 35C. The 2 Materials and Methods following compositions of hydrogel prepared along 2.1 Materials with their notations are stated in Table 1. CS or Poly(beta-(1,4)-D-glucosamine) (Acros The synthesis mechanism between the carbonyl Organics, USA) with molecular weight 600000- group of glutaraldehyde and amino groups of CS is 376 INDIAN J ENG MATER SCI, AUGUST 2021 Table 1 — Notations and Composition of Hydrogels formed 𝐸𝑆% = (𝑊 𝑠𝑤𝑊−𝑊 𝑑𝑟 ) ∗ 100 … (2) 𝑑𝑟 Notation of CS Glutaraldehyde State of Hydrogel Hydrogel formed (g) (mL) formation where, Wdr is weight of dry hydrogel and Wsw is CS1Gl0.2 1 0.2 Flaccid Hydrogel weight of swollen hydrogel. CS1Gl0.4 1 0.4 Firm Hydrogel with a good texture Equilibrium water content (EWC) CS1Gl0.6 1 0.6 Highly viscous In order to get the quantitative estimation of the Hydrogel amount of water absorbed by the hydrogel, EWC value is calculated using the equation: (𝑤 𝑠𝑤 −𝑤 𝑑𝑟 ) 𝐸𝑊𝐶 = × 100 … (3) 𝑤 𝑠𝑤 Swelling Kinetics Order of Kinetics The kinetic order and rate of swelling of CS hydrogels can be yielded through method prescribed by Druzynska, 201524. First-order kinetics: The rate of swelling of hydrogel at any time (t) can be expressed as: 𝑑𝑆 𝑑𝑡 = 𝑘1𝑟 (𝑆𝑒𝑞 − 𝑆) … (4) where, k1r is the rate constant of first order kinetics. Integrating Eq. 4 between the limits of S=0 to S at t from 0 to t, 𝑆𝑒𝑞 ln (𝑆 = 𝑘1𝑟 𝑡 … (5) 𝑒𝑞 −𝑆) Fig. 1 — Mechanism of synthesis of crosslinked CS hydrogels If the plot of ln(Seq/(Seq-S)) against t gives a straight line and R2 values approaching 1, the shown in Fig. 1. Due to the lone pair of electrons absorption by hydrogel shows first order kinetics. present on nitrogen part of amino group, there is a nucleophilic attack reaction by the carbonyl carbon of Second-order kinetics: If the hydrogels tend to glutaraldehyde. follow second order absorption kinetics25,1, the equation can be stated as: 2.2.2 Swelling studies 𝑑𝑆 Swelling ratio (SR) = 𝑘2𝑟 (𝑆𝑒𝑞 − 𝑆)2 … (6) 𝑑𝑡 Swelling ratio of the hydrogel was assessed using On integrating Eq. 6 from S=0 to S and t=0 to t, gravimetric measurement. The dried pellet form 2 𝑡 𝑘 2𝑟 𝑆𝑒𝑞 hydrogel sample was placed in a 100mL beaker filled 𝑆 = 1+𝑘 … (7) with 50mL distilled water at 25C. The swollen 2𝑟 𝑆𝑒𝑞 𝑡 hydrogel was regularly taken out from the beaker at On rearranging, different time slots and wiped with a blotting sheet. 𝑡 1 𝑡 Then it was weighed and kept back in the beaker22. =𝑘 2 +𝑆 … (8) 𝑆 2𝑟 𝑆𝑒𝑞 𝑒𝑞 The equation used for calculating SR was: If the plot of t/S against t gives a straight line with 𝑆𝑅 = 𝑊𝑒𝑖𝑔 𝑕 𝑡 𝑜𝑓 𝑆𝑤𝑜𝑙𝑙𝑒𝑛 𝑠𝑎𝑚𝑝𝑙𝑒 𝑊𝑒𝑖𝑔 𝑕 𝑡 𝑜𝑓 𝑑𝑟𝑦 𝑠𝑎𝑚𝑝𝑙𝑒 … (1) R2 values close to 1, hydrogel follows second order kinetics of absorption. Equilibrium swelling (ES%) The ES% of hydrogel was assessed by keeping Diffusion mechanism disc shaped hydrogel in distilled water at different Fick‟s law is used for obtaining the description of pH, temperatures and salt solutions23. The equation diffusion of water in polymeric network with respect used was: to different factors. Swelling kinetic parameters for  NANGIA et al.: KINECTIC, ADSORPTION & DIFFUSION MECHANISM OF CROSSLINKED CHITOSAN HYDROGELS 377 polymers can be obtained by theory given by Peppas 2.2.3 Characterization et al., 200026. Fourier Transform Infrared Spectroscopy of 𝑆𝑡 CS hydrogels 𝐹=𝑆 = 𝑘𝑡 𝑛 … (9) The Infrared Spectral analysis data for CS 𝑒𝑞 crosslinked with glutaraldehyde hydrogel was where, “F” represents the swelling fraction, “Seq” is collected by making KBr disc with its powdered the swelling seen in hydrogel when equilibrium is sample. Agilent Technologies Spectrophotometer was achieved, “n” is a solvent based diffusion parameter, used for this purpose in the region 4000-650cm-1 at a and k is the constant based on gel structure. Parameter resolution of 4cm-1. “n” helps us in identifying the type of diffusion. “n” values can be obtained from slope of ln F Thermogravimetric Analysis and Differential versus ln t graph by plotting the data till 60% solvent Scanning Calorimetry has entered the gel structure while “k” is obtained The decomposition of crosslinked CS hydrogels in from intercept values12. powdered form was performed under dynamic As per the Fick‟s second law of diffusion, for nitrogen flow with ramp 10C/min using TA cylindrical hydrogels, the diffusion coefficient can be Instruments, USA, Thermogravimetric Analyser represented using the following equation: (Q-500). The temperature range was kept between 30C and 500C. The thermal properties of 𝑘 1 𝐷 = 𝜋𝑟 2 (4 )𝑛 … (10) hydrogel were assessed using DSC unit Q-200, TA Instruments, USA while the temperature range was where, „D‟ represents the diffusion coefficient having kept between 30C and 500C with a heating rate of S.I. unit m2s-1 and „r‟ represents the radius of the swollen hydrogels having S.I. unit „m‟. 10C/ min under nitrogen atmosphere. Absorption Kinetics Modeling 3 Results and Discussion Water absorption has been studied and researched 3.1 Selection of hydrogel based on swelling ability upon by many researchers. They have proposed some Three different shapes of CS hydrogels were absorption models such as Peleg‟s model27 and prepared uncrosslinked CS beads, uncrosslinked CS Exponential association model28. films and CS disc shaped hydrogels crosslinked with Peleg‟s empirical equation involving two low concentrations of glutaraldehyde (as disc shaped parameters has been used for describing water hydrogels can be synthesized on addition of absorption by grains/ material29: crosslinker or using irradiation techniques). CS 𝑡 powder, passed through a particular mesh size 𝑆 = 𝑆𝑜 ± 𝑘 +𝑘 … (11) 1 2𝑡 was not used for comparison of swelling since where, So is the initial swelling value when t = 0 (g/g) CS does not dissolve in water but dissolves in mild S is the swelling value of hydrogel at time t (g/g) acids. The beads having a diameter of 0.3cm k1 is kinetic constant and k2 is characteristic displayed a swelling of about 100-200% at pH 7 and constant. even lower swelling in alkaline pH. CS has an In Eq. 11, ± becomes “+” if there is absorption effective pKa of 6-6.530. Above this value, CS is while “−” is used if desorption occurs. insoluble whereas below this value, CS is soluble. As per the Exponential association model, Since, the uncrosslinked CS materials dissolve in acidic solutions, the values for swelling in acidic 𝑆 = [1−exp(−𝑘𝑅2𝑡)] … (12) conditions is not stated. Swelling results for −1 where, 𝑘𝑅2 is the kinetic constant (h ). uncrosslinked CS beads of about 80% have been When a particular parameter is reported as a obtained in another study31 at pH 7 whereas this study function of time and shows either an exponential shows a slightly better swelling of about 100-200%. growth or decay, we can intend to use the The square film of size 2.5cm2.5cm,dissolved in Exponential association model. Converting Eq. 12 to acidic pH and displayed the best swelling at pH 7, a straight line equation i.e., y = mx+c, we can plot a which was about 3000%, but the handling was graph between log(S/Se) and time and then check difficult. The flat 2D shaped film, rolled and the curve fitting by estimating the correlation converted itself into a 3D hollow tubular shape by coefficient, R2 values. absorbing water. The absorption was rapid in this 378 INDIAN J ENG MATER SCI, AUGUST 2021 Fig. 2 — Different shapes/ geometries of prepared CS hydrogels. case. Since CS dissolves in mild acids, this hydrogel dissociated at low pH values. In another study on CS films, the swelling of uncrosslinked CS films was found to be about 1000%11. The slight differences in swelling in this study may be due to the differences in the molecular weight of CS or the degree of deacetylation. Figure 2 depicts the different shapes of prepared CS hydrogels. In terms of good swelling and ease of handling, crosslinked disc shaped hydrogels having diameter 1cm and thickness 0.4cm were chosen for further assessment. 3.2 Crosslinked CS disc shaped hydrogels - Swelling Results 3.2.1 Effect of pH The Equilibrium Swelling (ES%) was calculated using Eq. 2. The ES% results have been obtained in triplicate and the mean values have been plotted. Figure 3 depicts the effect of pH on the swelling of hydrogels with varying crosslinker concentrations. CS is a weak polybase/ polyelectrolyte with a pKa around 6.532. It can be observed that at low pH, i.e. in acidic range, the protonation of amino groups takes place. This leads to repulsion in polymer chains and rupture Fig. 3 — Effect of pH on equilibrium swelling (a) CS1Gl0.2, and (b) CS1Gl0.4. of intramolecular hydrogen bonding that allows more water to enter into the gel network. The swelling Thus, it can be understood that when the crosslinker decreases as the pH increases. Least swelling can be concentration is increased, the equilibrium swelling seen in case of alkaline pH (10.5) which could be gets reduced. When the crosslinker concentration was explained by the occurrence of deprotonation of increased to 0.6mL, it leads to the collapse of the amino groups and repulsion in polymer chains hydrogel matrix. The hydrogel formation with 0.6mL becoming significantly less that leads to shrinking. crosslinker occurred in the beaker during mixing and Therefore, lesser water tends to enter the network. swelling was also significantly low as compared to Crosslinking makes the network more compact1. the other presented compositions.  NANGIA et al.: KINECTIC, ADSORPTION & DIFFUSION MECHANISM OF CROSSLINKED CHITOSAN HYDROGELS 379 3.2.2 Effect of temperature concentration, the more would be the swelling. This The temperature dependent equilibrium swelling trend is clearly shown in Fig.5. pattern of CS hydrogels with varying crosslinker concentrations is shown in Fig. 4. The swelling was 3.3 FTIR of raw CS and CS hydrogel crosslinked with observed in deionized water with pH=7 at a Glutaraldehyde temperature range of 20-40C. It can be seen that as FTIR studies were performed with CS1Gl0.4 the temperature increased, the swelling also increased. crosslinked hydrogel as this showed a good texture A temperature responsive swelling behavior was and handling strength. The spectra of raw CS have observed which may be attributed to association also been taken into consideration in order to know (in case of low temperatures) or breakdown (at high the changes after crosslinking with glutaraldehyde. The FTIR spectrum forboth raw CS and CS1Gl0.4 temperatures) of hydrogen bonds connecting the amino groups11. crosslinked hydrogel presented in Fig. 6(a) shows a broad band between 3200-3400cm-1 signifying the 3.2.3 Effect of ionic strength of salts presence of hydroxyl group, which is responsible An increase in the ionic strength from 0.001M to for hydrophilicity of CS34. A sharp peak in the 0.1M NaCl, shows a significant decrease in the region around 1515-1570cm-1 depicts amide II equilibrium swelling. This might be due to the fact group35. The observed sharp peak around1400 cm-1 that osmotic pressure difference decreases between may be assigned to CH3 symmetrical deformation the polymer network and the external solution with mode36. A small peak at 1150cm-1 displays increase in ionic strength33. Moreover, CS is a antisymmetric stretching of C-O-C bonds. Bands cationic polyelectrolyte through protonation of amine between 1020-1080cm-1 (C-O stretching involving groups. This generally does not have the tendency to skeletal vibration) are characteristic of saccharide interact with sodium or potassium cation. The structure of CS37. A small peak can be observed in absorption of salt solutions would thus be less through case of CS1Gl0.4 at 1650cm-1that shows presence of these hydrogels. The lesser the molar salt amide I band which is a characteristic of CS and C=N Fig. 4 — Effect of temperature on equilibrium swelling (a) Fig. 5 — Effect of salts on equilibrium swelling (a) CS1Gl0.2, and CS1Gl0.2, and (b) CS1Gl0.4. (b) CS1Gl0.4. 380 INDIAN J ENG MATER SCI, AUGUST 2021 30C to 500C. During the first stage, loss of residual, bound or absorbed water present in the polymeric network is achieved by evaporation. By about 160C, around 12% loss of weight occurs38. The weight loss amount depends on factors such as hydrophilicity, crystallinity and structure of the polymer39. The maximum weight loss (about 43%) can be observed between 225 and 298C. This attributes to the second stage decomposition, which occurs due to splitting of monomeric units and conversion of complex molecules to simpler ones like acetic acid and lower fatty acids from the used materials. Chemical modification generally leads to a decrease in thermal stability of CS but is essential for improving its applicability in different fields39, 40. As per the study of Ostrowka-Czubenko, the thermal stability decreased when CS was modified with glutaraldehyde and sulfuric acid. But these agents were essential for improving the mechanical strength and acid stability of CS. In Fig. 6(c), two broad peaks could be observed. The first peak i.e. endothermic peak relates to evaporation of water bound to CS network and the functional groups of glutaraldehyde by hydrogen bonds or other interactions. The second peak that occurs at about 266C is the exothermic peak, which shows the degradation/melting of main chain of CS39. 3.5 Swelling Kinetics 3.5.1 Order First Order Kinetics Plot of „ln(Seq/(Seq-S))‟ against „t‟ for the crosslinked CS hydrogels represents graph of First order kinetics. If the swelling is diffusion controlled and obeys Fig. 6 — (a) FTIR, (b) TGA, and (c) DSC of CS crosslinked with glutaraldehyde. Fick‟s laws, the polymers tend to follow first order kinetics41. But the R2 values for First Order Kinetics stretching band of Schiff‟s base35. This new peak (presented in Table 2) of these hydrogels need to be confirms reaction between CS and Glutaraldehyde compared with the second order R2 values in order to resulting in formation of imines that are Schiff bases. find out the order of swelling kinetics. So, covalent crosslinking can be observed in case of The R2values are the deciding factors for predicting CS and Glutaraldehyde. the order of kinetics. The closer the values are to 1, the better it follows that order. 3.4 Thermogravimetric analysis and Differential Scanning Calorimetry Second Order Kinetics CS crosslinked with glutaraldehyde tends to follow Second order Kinetics plots have been depicted for a two-stage decomposition process with a minor different Environmental conditions in Fig. 7. During first stage and major second stage decomposition extensive swelling studies, the polymer swelling is (Fig. 6(b)). Based on available literature, the not only based on diffusion based on Fick‟s laws but decomposition temperature range was varied from there is some stress relaxation also by the amorphous  NANGIA et al.: KINECTIC, ADSORPTION & DIFFUSION MECHANISM OF CROSSLINKED CHITOSAN HYDROGELS 381 Table 2 — First orderR2 values for CS hydrogels Table 3 — Second order kinetics parameters for CS hydrogels First Order Kinetics Second Order Kinetics for CS1Gl0.2 CS1Glut0.2 CS1Glut0.4 Parameter Seq(obtained) Seq (calculated) K2 R2 -4 Condition R2 R2 pH 2.5 1081.17 1428.57 2.04 × 10 0.9899 pH 2.5 0.9894 0.9196 pH 7 1006.74 1428.57 1.44 × 10-4 0.9924 pH 7 0.9971 0.9866 pH 10.5 443.9 500 1.05 × 10-4 0.9976 pH 10.5 0.9668 0.9789 Temp. 20°C 506.63 588.23 5.78 × 10-4 0.9974 Temp 20°C 0.9761 0.9848 Temp. 30°C 965.67 1111.11 4.05 × 10-4 0.9964 Temp 30°C 0.9826 0.9989 Temp. 40°C 1283.13 1666.66 1.63 × 10-4 0.9938 Temp 40°C 0.9957 0.9968 0.1M NaCl 371.7 500 4.39 × 10-4 0.9969 0.1M NaCl 0.9946 0.9759 0.01M NaCl 498.06 625 4.19 × 10-4 0.9978 0.01M NaCl 0.9946 0.972 0.001M NaCl 587.76 714.28 3.92 × 10-4 0.9961 0.001M NaCl 0.9898 0.9646 0.1M CaCl2 352.49 434.78 5.23 × 10-4 0.9941 0.1M CaCl2 0.9937 0.9581 Second Order Kinetics for CS1Gl0.4 Parameter Seq(obtained) Seq (calculated) K2 R2 pH 2.5 787.22 909.09 5.04 × 10-4 0.9875 pH 7 642 714.28 5.29 × 10-4 0.9994 pH 10.5 360.38 434.78 6.61 × 10-4 0.99 Temp. 20°C 842.63 2000 2.84 × 10-5 0.9173 Temp. 30°C 939.9 1666.66 5.80 × 10-5 0.9739 Temp. 40°C 1010.21 1666.66 6.66 × 10-5 0.9073 0.1M NaCl 263.98 285.71 2 × 10-3 0.9994 0.01M NaCl 343.98 384.61 1.18 × 10-3 0.9975 0.001M NaCl 387.79 434.78 1.03 × 10-3 0.9977 0.1M CaCl2 259.52 285.71 2.11 × 10-3 0.9973 3.5.2 Diffusion of Water The Diffusion exponent values for different parameters are given in Table 4. Alfrey et al.41 described three cases of solvent diffusion into the polymeric network. Case I- is Fickian diffusion (n=0.5) in which the polymer chains relaxation rate is faster than the diffusion of solvent inside the network. Case II- is non-Fickian diffusion (n=1) in which the diffusion of solvent is faster than polymer chains relaxation rate. Case III- is Anomalous diffusion (0.5