Biodegradable Superabsorbent Polymers Research Articles

Evaluating the Effectiveness of the Biodegradable Superabsorbent Polymer (Fasal Amrit) on Soil Hydrological Properties: A Laboratory Rainfall Simulation Study

Superabsorbent polymers (SAPs) are effective soil amendments that can control soil erosion by improving soil quality. However, many commercial SAPs face challenges including limited biodegradability, high costs, and adverse effects on soil hydrological properties, which can lead to increased water and soil loss. This study examined the potential of lower dosages of biodegradable SAPs to improve the hydrological properties of “Shimajiri-maji” (clay) soil. Three concentrations of biodegradable Fasal Amrit polymer (EFP) (P1: 0, P2: 3 g m−2, and P3: 6 g m−2) were evaluated under three simulated rainfall intensities (I1: 35; I2: 70 and I3: 110 mm h−1) and two gradients (7.5%, and 15%) during consecutive storms. The time to generate runoff, infiltration, runoff, soil loss, and water storage (WS) were quantified over one hour. The results show that runoff generation was delayed in EFP-treated soils compared to the control. Both polymer treatments enhanced infiltration (P2 > P3 > P1) and reduced runoff and soil loss (P2 < P3 < P1). Higher EFP rates improved water storage at surface depths (P3 > P2 > P1). EFP-treated soils exhibited lower interrill erodibility, suggesting greater resistance to soil erosion compared to the control. EFP treatments also significantly improved the soil’s physical properties (bulk density, porosity, organic matter, aggregate stability). EFPs can diminish runoff and soil loss as the EFP-treated plots exhibited greater aggregate stability than the control. It was concluded that low EFP concentrations can improve soil hydrological properties and mitigate soil erosion. Further investigations are needed to optimize the EFP concentrations for different soil types.

Synthesis and characterisation of microcrystalline cellulose-g-poly(Acrylamide) superporous absorbent composite using graft polymerisation methods

Superabsorbent polymer was synthesised by using graft polymerisation method incorporated with micro crystalline cellulose (MCC) as a filler and sodium bicarbonate (NaHCO3) as the foaming agent. The addition of organic filler and porosity generator produced a highly porous biodegradable superabsorbent polymer composite (HP-PAM-g-MCC), which improved the characteristics of the acquired products, in comparison with the conventional SAP. Determination of water absorbency was tested by using the tea bag method after immersing in distilled water. The effects on amount of MCC and sodium bicarbonate addition towards water absorbency were studied to determine the optimum condition of PAM-g-MCC SAPs composite. The maximum water absorbency of PAM-g-MCC composite was achieved at 1.0 wt% of MCC and 1 wt% NaHCO3, resulting in 74.01 g/g and 93.96 g/g of water absorbency, respectively. The Fourier transform infrared (FTIR) and scanning electron microscope (SEM) analyses data revealed the presence of chemical bonding and morphological characteristics corresponding to the water absorption capacity of HP-PAM-g-MCC.

Superabsorbent Polymer Sponge for Saliva Absorption Pad

Aim: Saliva is a significant hindrance to most dental procedures (e.g., root canal treatment) as the flow of saliva increases in patients undergoing dental treatment due to anxiety. Saliva absorption pads based on superabsorbent polymers can provide a dry oral environment to ease the dental treatment procedure and reduce the swallowing reflexes common during cotton use. This study focused on developing an indigenous saliva absorption pad using biodegradable superabsorbent polymer (BSAP) sponges. Materials and Methods: BSAP sponges were synthesized using carboxymethyl cellulose (CMC) as the base matrix. Different crosslinking mechanisms were implemented to prepare BSAP sponges, such as ionic crosslinking using aluminum ammonium sulfate (AlAS) and chemical crosslinking using methylene bisacrylamide. Three different BSAP sponges (Sap-2, SAP-PAA-1, and SAP-PAA-2) were characterized for their free swell capacity and thermal degradation kinetics along with other characterization techniques to optimize the composition for saliva absorption pad. One-way ANOVA was used for statistical evaluation. Results: SAP-2, synthesized using 10 wt.% AlAS showed the highest free swell capacity in water and saline (83.21 ± 3.8 g/g and 40.7 ± 3.4 g/g, respectively). However, the moisture content of the particular BSAP sponge was higher (~13%) compared to the standard limit (ISO 17190-4:2001(E)). It was observed that as the crosslinking density increases free swell capacity increases to a threshold point and decreases thereafter. As reported earlier, percentage swelling was controlled by multiple factors including crosslinking that opposes swelling and polymer/water interaction and Donnan pressure that promotes swelling. Conclusion: BSAP sponge based on crosslinked CMC matrix is highly advantageous in developing saliva absorption pad. Hydrophobic surface modification is recommended to reduce the moisture content to improve the storage stability of BSAP sponges.

Effect of Cross-Linker Length on the Absorption Characteristics of the Sodium Salt of Cross-Linked Polyaspartic Acid.

For the development of biodegradable superabsorbent polymers, the effect of the cross-linking length on the absorption characteristics of the Na salt of polyaspartic acid (PAspNa) was demonstrated using different concentrations of diamine cross-linking agents bearing carbon chains of different lengths, viz., ethylenediamine, 1,6-hexamethylenediamine, 1,8-diaminooctane, 1,10-diaminodecane, and 1,12-diaminododecane were used as cross-linking agents. The absorption of PAspNa was measured in deionised water and in a 0.9% aqueous NaCl solution. Under the conditions tested, when the alkyl chain of PAspNa was too short or too long, the absorbency was low and the cross-linking length was optimum. The success of the cross-linking reaction was confirmed by FT-IR spectroscopy. The degree of cross-linking was estimated and the ideal concentration for maximum water absorption was determined by elemental analysis. The sample obtained by cross-linking 1,8-diaminooctane at a concentration of 0.11 g/g polysuccinimide (PSI) showed the highest absorption. The thermal properties of each material were determined by dynamic scanning calorimetry. Therefore, the length of the cross-linking agent was found to strongly influence water absorption.

Nanoarchitectonics for Biodegradable Superabsorbent Based on Carboxymethyl Starch and Chitosan Cross-Linked with Vanillin.

Due to the growing demand for sustainable hygiene products (that will exhibit biodegradability and compostability properties), the challenge of developing a superabsorbent polymer that absorbs significant amounts of liquid has been raised so that it can be used in the hygiene sector in the future. The work covers the study of the swelling and dehydration kinetics of hydrogels formed by grafting polymerization of carboxymethyl starch (CMS) and chitosan (Ch). Vanillin (Van) was used as the crosslinking agent. The swelling and dehydration kinetics of the polymers were measured in various solutes including deionized water buffers with pH from 1 to 12 and in aqueous solutions of sodium chloride at 298 and 311 K. The surface morphology and texture properties of the analyzed hydrogels were observed by scanning electron microscopy (SEM). The influence of this structure on swelling and dehydration is discussed. Fourier transform infrared (FTIR) analyses confirmed the interaction between the carboxymethyl starch carbonyl groups and the chitosan amino groups in the resulting hydrogels. Additionally, spectroscopic analyses confirmed the formation of acetal crosslink bridges including vanillin molecules. The chemical dynamics studies revealed that new hydrogel dehydration kinetics strongly depend on the vanillin content. The main significance of the study concerns the positive results of the survey for the new superabsorbent polymer material, coupling high fluid absorbance with biodegradability. The studies on biodegradability indicated that resulting materials show good environmental degradability characteristics and can be considered true biodegradable superabsorbent polymers.

Cornstarch-based, Biodegradable Superabsorbent Polymer to Improve Water Retention, Reduce Nitrate Leaching, and Result in Improved Tomato Growth and Development

In arid and semi-arid climates, water scarcity and nutrient availability are major constraints for food production. Excess fertilization to make up for the limited nutrient availability in dry soils leads to nitrogen runoff and groundwater contamination. Reducing nitrogen leaching into surface water while providing adequate nutrition remains a major challenge. Superabsorbent polymers (SAPs) can reduce water loss and improve nutrient retention and therefore minimize leaching and increase crop yields. SAPs are made from petroleum or natural products, but plant-based SAPs have been gaining popularity because they have fewer long-term effects on the environment. However, there is little known about how SAPs made from cornstarch effect plant growth and production in tomatoes. So, we evaluated total nitrogen and water retention in SAP-treated soils and evaluated their effects on growth and development of tomatoes (Solanum lycopersicum). Soils were amended with different rates of cornstarch-based SAP (i.e., 0 kg SAP, 0 kg SAP+N, 0.5 kg SAP+N, 1 kg SAP+N, 1.5 kg SAP+N, and 2 kg SAP+N). Results indicate that the mean volume of water and nitrates retained in the soils amended with cornstarch-based SAPs increased with increasing rate of SAP. The treatment containing the highest dose (i.e., 2 kg SAP) decreased the amount of leachate and nitrates from soil 79.34% and 93.11% at 3 days after fertilization (DAF) and 78.84% and 81.58% at 9 DAF in comparison with the soil-only and fertilizer-only treatments, respectively. The results also indicate cornstarch-based SAP significantly improved plant growth and yield parameters compared with the treatments without SAP. Furthermore, the greatest number of leaves, flowers, fruits, and dry matter production were found in the 1-kg SAP treatment. Therefore, application of cornstarch-based SAPs can improve tomato production in times of drought stress by retaining more water and nutrients in the active rooting zone and can reduce environmental pollution by reducing nitrogen runoff.

Performance characterization of γ-poly (glutamic acid) super absorbent polymer and its effect on soil water availability

ABSTRACT A novel biodegradable super absorbent polymer (SAP) was synthesized by γ-poly (glutamic acid) (γ-PGA) and poly (ethylene glycol) with the units of glycidyl ether as chain ends (PEGGE). The resultant γ-PGA SAPs with different PEGGE contents were characterized and their effects on soil total available water (TAW) were investigated. The γ-PGA SAP was synthesized at 45 °C for 48 h when γ-PGA was 160 g L−1 (pH = 4.9) and the minimum mass of PEGGE was 20% of the γ-PGA mass. When the PEGGE increased from 20% to 60%, the water absorption rate (WAR) of γ-PGA SAP in distilled water and 0.9% NaCl solution gradually decreased from 651.16 g g−1 to 302.91 g g−1 and from 44.83 g g−1 to 32.9 g g−1 respectively. The γ-PGA SAP with 40% PEGGE had the highest WAR after being reused six times. The field water capacity (FC) and TAW increased with the increasing γ-PGA SAP content; however, when the same amount of γ-PGA SAP was added to the soil, the FC and TAW in the soil gradually decreased with the increasing PEGGE in the γ-PGA SAP. The γ-PGA SAP with a proportion of 0.15% in the soil presented the largest soil WAR.

Swelling Properties of Biodegradable Superabsorbent Polymers

The size of the global market for biodegradable superabsorbent materials has been estimated at USD 120.64 billion. It is expected to register Compound Annual Growth Rate (CAGR) at 6.2% in 2018-2025. Superabsorbent polymers (SAP) are most frequently used in the hygiene products industry in the form of non-biodegradable poly (sodium acrylate). Most personal care products end up in landfills where decomposition times are estimated to be up to half a thousand years due to the synthetic polymers. Simple replacement of poly (sodium acrylate) with biodegradable superabsorbent polymer is a challenging task that includes several stages of scientific investigation. In this paper, the sorption of water and electrolyte solutions are discussed. Biodegradable superabsorbent polymers were obtained from polysaccharides, while a proportionally varying amount of the cross-linking agent was used. The absorption properties of deionized water and sodium salt solution were tested and the influence of polymer drying was discussed. The superabsorbent polymers were dried as follows: dM1-the sample was frozen at -20°C for 48 hours and was dried in vacuum (10-2Tr) at room temperature for 48 hours; dM2-the sample frozen to -200°C for 2 hours and was vacuum (10-2Tr) dried at room temperature for 48 hours; dM3-the sample was dried in a vacuum dryer (~10 Tr) at 50°C for 24 hours, dM4-the sample was frozen to -80°C for 24 hours and then freeze dried for 78 hours.

A self-powered insulin patch pump with a superabsorbent polymer as a biodegradable battery substitute.

Highly popular insulin patch pumps have in-built non-removable batteries. These batteries are routinely disposed of together with the used pumps as medical waste and end up in landfills. This is an environmental contamination conundrum by design. To address this issue, we proposed a self-powered patch pump that uses a biodegradable superabsorbent polymer (SAP) instead of a battery as a power source to drive the infusion. Continuous infusion rates from 6.1 μL min-1 to 49.1 μL min-1 were achieved. Together with valve throttling, basal and bolus infusion rates of ∼10 μL h-1 (1 U h-1) and 100 μL (10 U) in ∼11 min could also be implemented for glycemic control. The generated pressure at ∼0.7 psi is also adequate for infusion as it exceeded an adult's maximum peripheral venous pressure of 0.6 psi. Given the current number of patch pump users, the proposed design could prevent ∼100 000 used batteries from entering the medical waste stream and landfill daily. Most importantly, this work highlights the possibility of addressing environmental contamination without compromising on healthcare standards by using SAP as an alternative means of energy storage.

Preparation of a biodegradable superabsorbent polymer and measurements of changes in absorption properties depending on the type of surface‐crosslinker

A superabsorbent polymer (SAP) is a special polymer material that can absorb up to 500 times its own weight of pure water, but has a problem that it does not biodegrade itself and cause environmental pollution. Therefore, we aim to prepare a biodegradable SAP by using biomass‐based IA. The SAP must be able to retain absorbed water and absorb water under a given pressure. We have carried out studies to improve the surface hardness of the SAP to enhance absorption of water under a given pressure by surface‐crosslinking. Four types of surface‐crosslinkers, ethylene glycol diglycidyl ether (EGDGE), ethylene carbonate (EC), 1,4‐butanediol (BD), or glycerol, were used. We confirmed the water absorption capacity of the SAP by measuring its centrifuge retention capacity (CRC) and absorbency under load (AUL). The structural characteristics of the SAP were confirmed by attenuated total reflection (ATR) and X‐ray photoelectron spectroscopy (XPS), and the surface characteristics were confirmed by scanning electron microscopy (SEM).

Conditioning effects of biodegradable superabsorbent polymer and vermi-products on media properties and growth of gerbera

With the objective of identifying an ecologically sustainable substrate for commercial gerbera production, we conducted an experiment to assess the effects of super absorbent polymer and organic vermi-products on the physico-chemical properties of the growing media, and plant growth and flower yield in gerbera cv. Yosemite. CPV (Cocopeat: Perlite: Vermiculite) and soil amended with vermicompost (VC, 20%), Pusa hydrogel (PHG, 0.25%) and horn bio-manure (HBM, 1%) had significant positive effects on growth and flowering of gerbera. The lowest (0.29 ± 0.02 g cm−3) bulk density was recorded in CPV + PHG + HBM while the maximum (59.75 ± 0.63%) water holding capacity was observed in CPV + PHG + VC substrate. The maximum electric conductivity (0.99 ± 0.06 dS m−1) and pH (7.90 ± 0.04) were recorded in Soil + PHG + VC. Plants grown on CPV + PHG + VC media and sprayed with 20% of vermiwash produced the highest number of leaves during the entire period (31 ± 0.58), maximum leaf length (30.18 ± 0.18 cm) and width (8.30 ± 0.16 cm), maximum number of primary roots (42 ± 1.53), highest flowers per plant (19.67 ± 0.33), stalk length (41.08 ± 0.10 cm) and flower head diameter (10.11 ± 0.03 cm). Flowers produced on this medium also had an extended attractive appearance (20.33 ± 0.33 days) and longer vase life (14.33 ± 0.33 days). Water requirement of gerbera (litre/crop season) on CPV + PHG + VC medium, with or without Vermiwash spray was also considerably lower than other treatments. Overall results suggest that CPV amended with PHG and VC coupled with Vermiwash spray provide congenial conditions for gerbera plant growth and flower production while also simultaneously reducing irrigation water use considerably.

Preparation of Superabsorbent Polymer from Sugarcane Bagasse via Extrusion Process

The aim of this research study is to develop a biodegradable superabsorbent polymer, based on cellulose from sugarcane bagasse (SCB), for water resource management, horticulture, and other products in agricultural applications. A SCB superabsorbent polymer was polymerized from SCB and acrylic acid (AA) monomer in the presence of N, N-methylene-bisacrylamide as a cross-linker by using corotating twin-screw extruder. The ratios of SCB and AA were varied in order to enhance the swelling ability of the absorbent polymer. Polymerization of SCB superabsorbent polymer was initiated by using ammonium persulfate as an initiator in a normal system and ammonium persulfate/sodium sulfite as a couple initiator in redox system. The results indicated that the swelling ability of superabsorbent polymer prepared by ammonium persulfate/sodium sulfite as a couple initiator was higher than that of the polymer prepared by ammonium persulfate as the initiator. The SCB superabsorbent polymer prepared under optimized polymerization conditions was mixed in soil, and the water-retaining ability of the soil was subsequently investigated. Soil water retention was improved in the presence of the SCB superabsorbent polymer. These results confirmed that the superabsorbent polymer prepared from polymerization of SCB and AA in the presence of N, N-methylene-bisacrylamide as a cross-linker and ammonium persulfate as an initiator by using corotating twin-screw extruder could potentially be used as an agricultural water resource.

Effects of amylose/amylopectin starch on starch‐based superabsorbent polymers prepared by γ‐radiation

Biodegradable superabsorbent polymers (SAPs) were synthesized in this study by graft copolymerization of acrylic acid onto amylose and amylopectin using N,N′‐methylenebisacrylamide as a cross‐linker, initiated by an initiator (potassium persulfate) and γ‐ray radiation, respectively. The structures of the resultant amylose‐graft‐poly(acrylic acid) (amylose‐g‐PAA) and amylopectin‐graft‐poly(acrylic acid) (amylopectin‐g‐PAA) SAPs were characterized by Fourier transform infrared (FTIR) spectroscopy, solid‐state 13C nuclear magnetic resonance (NMR) spectroscopy, and thermogravimetric analysis (TGA). The main purpose of this study is to investigate the effects of the two initiation modes and chain structures of starches on the grafting ratios (GR) and properties of these amylose‐g‐PAA and amylopectin‐g‐PAA SAPs. The results show that SAPs prepared by γ‐radiation have higher water absorption and GR than SAPs prepared by the initiator. In addition, amylose‐g‐PAA SAPs have higher water absorption and GR than amylopectin‐g‐PAA SAPs. The optimum water absorption is 530 g/g for amylose‐g‐PAA SAPs and 355 g/g for amylopectin‐g‐PAA SAPs, respectively. It is concluded that γ‐radiation is more time‐saving, energy‐saving, and efficient than the initiator.

Effects of chain structures of corn starches on starch‐based superabsorbent polymers

Biodegradable superabsorbent polymers (SAPs) were synthesized in this study by graft copolymerization of monomers (acrylic acid or acrylamide) onto amylose and amylopectin, respectively, using potassium persulfate as an initiator and N,N′‐methylenebisacrylamide as a cross‐linker. The main purpose of this study is to investigate the effects of the chain structures of corn starches on the grafting ratio (GR) and physicochemical properties of starch‐based SAPs including starch‐graft‐polyacrylic acid and starch‐graft‐polyacrylamide SAPs. The structure of these SAPs was characterized by FTIR, 13C nuclear magnetic resonance (13C‐NMR), and thermogravimetric analysis (TGA), and the GR was evaluated by acid hydrolysis. We also investigated their swelling kinetics and reswelling capacity. The results show that the water absorption capacity and GR of amylose graft copolymers are superior to those of amylopectin graft copolymers; this may be due to the differences in structures and physicochemical properties between amylose and amylopectin.

Photopolymerisation and characterization of maleylatedcellulose-g-poly(acrylic acid) superabsorbent polymer

A novel biodegradable superabsorbent polymer has been prepared from maleylated cotton stalk cellulose (MCSC) crosslinker and acrylic acid (AA) by ultraviolet (UV) photopolymerization in aqueous solution at room temperature, and irgacure 651 as a photoinitiator. The resulting superabsorbent was characterized by FT-IR, 1H NMR, SEM and TGA. The effects of preparation conditions such as degree of substitution (DS), amount of maleylated cotton stalk cellulose, exposed time, photoinitiator amount and monomer concentration on the water absorbency and the monomer conversion in graft were evaluated. The swelling kinetics, salt-resistance, water retention capacity and biodegradability of the MCSC-g-PAA superabsorbent were investigated. It was found that, the obtained superabsorbent have good swelling degree that greatly affected by its composition and preparation conditions. Owing to its considerable good water retention capacity, being economical and environment-friendly, it might be useful for its application in agriculture field.

Starch modification using a twin‐roll mixer as a reactor

AbstractA starch modification reactor has been established using a HAAKE rheometer incorporating a twin‐roll mixer, which has been modified to improve its sealing and feeding, and to establish an oxygen‐free environmental. The advantages of the system include: (i) starch modification can be carried out at high starch concentration (up to 70%); (ii) combining the steps of starch gelatinization and chemical modification; (iii) controllable reaction time; and (iv) smaller sample size (60 g) requirement. The effects of water content, rpm, time, and temperature on the reactive system was systematically investigated. Corn starch (60% starch concentration) was successfully grafted with acrylamide then crosslinked by N,N′‐methylene‐bisacrylamide to produce biodegradable superabsorbent polymers using this reactor.

Effects of amylose/amylopectin ratio on starch-based superabsorbent polymers

Abstract Biodegradable superabsorbent polymers (SAPs) were prepared by grafting acrylamide onto starches then crosslinking with N,N′-methylene-bisacrylamide. This work focused on the effects of the amylose/amylopectin ratio of starches from the same source (corn) on the grafting reactions and performance of the resultant starch-based SAPs. To characterise each SAP, the acrylamide groups grafted onto the starch were detected by FTIR; grafting ratio and grafting efficiency were evaluated by a gravimetric method; and graft position and the length of the grafted segment were investigated by NMR. The relationships between the microstructures of the starches, and the graft reactions and performance of the SAPs were studied based on the amylose content in the starches. It was found that under the same reaction conditions, the grafting ratio and efficiency increased with increasing amylose content, which corresponds with water absorption ratio. NMR results indicated that the acrylamide group mainly grafted onto C6, and that the length of the grafted segment decreased with increasing amylopectin content in general, and in particular for waxy starch. The high molecular weight and branched structure of amylopectin reduced the mobility of the polymer chains and increased viscosity, which could explain the graft reactions and performance of the starch-based SAPs.

Synthesis and Characterization of Silk Sericin / Acrylic Acid / Acrylamide Superabsorbent Polymer

A composite superabsorbent material was synthesized based on silk sericin, acrylic acid and acrylamide by using N, N-methylenebisacrylamide (MBA) as the crosslinker and ammonium persulfate (APS) as the initiator reagent. The water absorbency and water retaining properties of the composite under different preparation conditions were investigated. The absorbency of silk sericin / acrylic acid / acrylamide (SS/AA/AM) superabsorbent polymer polymer in deionized water, tap water and 0.9% NaCl solution were 285337 g/g, 191220 g/g, 3643g/g, respectively. SS/AA/AM material has excellent re-absorbent abilities and can be reused. The product showed different water absorbency in aqueous chloride salt solutions with the order of Na+ > Ca2+ > Mg2+ > Al3+. FTIR spectra results indicated that silk sericin was successfully grafted with acrylic acid and acrylamide molecules. The study on the absorbent material based on sericin is useful for further research on biodegradable superabsorbent polymer.

Water Sorption Isotherms and Textural Properties of Biodegradable Starch-Based Superabsorbent Polymers

A series of biodegradable superabsorbent polymers (SAP) based on starch grafted with acrylic acid (AA) and crosslinked with N,N’-methylenebisacrylamide (MBIS) were obtained. The parameters that define SAPs properties, such as starch type, amount of initiator, acrylic acid and crosslinker concentrations, and degree of neutralization were varied according to an experimental design. The swelling rate, the degree of swelling, the water sorption isotherms and the texture profile of the new materials were determined. A new method was used to measure the swelling rate. The water sorption results were correlated using various isothermal models as the BET (Brunauer-Emmett-Teller) and GAB (Guggenheim-Andersson-DeBoer) models. Hydrogels obtained from regular cornstarch presented better absorption capacities than those from waxy cornstarch. Maximum swelling was observed for 70% neutralization of carboxylic acid groups. More rigid SAPs presented lower swelling rates, but not necessarily lower swelling degrees.

Dynamic and Equilibrium Swelling of Biodegradable Starch-Based Superabsorbent Polymers

A series of biodegradable superabsorbent polymers (SAP’s) based on starch grafted with acrylic acid (AA) and crosslinked with N,N’-methylenebisacrylamide (MBIS) were obtained. The parameters that define SAP’s properties, such as starch type, amount of initiator, acrylic acid and crosslinker concentrations, and degree of neutralization were varied. The dynamic swelling was interpreted in terms of both a simple power of time equation and a more detailed model based on a coupled diffusion–relaxation mechanism. The diffusivity coefficients calculated from diffusion–relaxation model were compared with the microimages obtained by optical microscopic.