Synthesis and Investigation of Swelling Behavior of New Agar Based Superabsorbent Hydrogel as a Candidate for Agrochemical Delivery - Springer

9/1/2015 Synthesis and investigation of swelling behavior of new agar based superabsorbent hydrogel as a candidate for agrochemical delivery - Springer

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Journal of Polymer Research

© Springer Science+Business Media B.V. 200910.1007/s10965-009-9270-2

Original Paper

Synthesis and investigation of swellingbehavior of new agar based superabsorbenthydrogel as a candidate for agrochemicaldelivery

Ali Pourjavadi 1 , Bahareh Farhadpour 1 and Farzad Seidi 1

Polymer Research Laboratory, Department of Chemistry, Sharif University of Technology, P.O. Box 11365-9516, Tehran, Iran

Ali PourjavadiEmail: [email protected]

Received: 28 October 2008

Accepted: 27 January 2009

Published online: 25 February 2009

Abstract

In this investigation a new type of superabsorbent hydrogel based on agar was prepared, and

the effect of the feed ratio of some components (acrylic acid, MBA, APS and agar) on the

swelling capacity of the hydrogel was systematically studied. Maximum water absorbency of

the optimized final product was found to be 1,100 g/g in distilled water. The structure of the

hydrogel was characterized by FT-IR method and morphology of the samples was examined

by scanning electron microscopy (SEM). Swelling properties of optimized hydrogel sample in

different swelling mediums were investigated. The optimum hydrogel were also loaded with

potassium nitrate and its potential for controlled release of potassium was investigated by

measuring conductivity in various conditions.

Keywords Agar – Acrylic acid – Hydrogel – Swelling behavior – Potassium nitrate

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Introduction

Polymer networks consist of long macromolecular chains cross-linked with each other by

chemical or physical bonding. Polymer networks swollen in a liquid are called polymer gels.

Hydrogels are polymer gels, which swell extensively in water. The most common hydrogels are

polyelectrolyte gels: their high degree of swelling in water is due to the exerting osmotic

pressure of counter ions. Such gels can acquire up to several hundredweight parts of water per

one part of a dry polymer [1]. Considerable attention has been devoted to the preparation and

application of responsive hydrogels for medical and pharmaceutical fields particularly as

stimuli-sensitive drug delivery [2–4].

Stimuli-responsive hydrogels undergo a substantial volume change when an environmental

factor such as temperature, pH, or ionic strength is altered. The volume phase transition is

caused by a change in the hydrogel’s water content. Hydrogels have attracted attention

because of both potential and demonstrated utility of this behavior in a variety of applications

ranging from flow controllers in microfluidic devices to sorbents [5–7]. Conventional

application of agrochemicals results in ground water contamination. Thus, we need a more

controlled application of agrochemicals to reduce amounts of active ingredients without

diminishing efficiency. The replacement of conventional agrochemical formulations by

controlled release systems not only helps to avoid treatment with excess amounts of active

substances, but also offers the most suitable technical solution in special fields of application.

On the other hand, hydrogels of natural polymers, especially polysaccharides, have recently

found widespread applications because of their unique advantages. Polysaccharides are, in

general, non-toxic, biocompatible, biodegradable and abundant [8, 9].

Hydrogels can be prepared by simultaneous copolymerization and crosslinking of one or more

monofunctional and one multifunctional monomer or by crosslinking of a homopolymer or

copolymer in solution [10, 11]. Hydrogel properties depend strongly on the degree of

crosslinking, the chemical composition of the polymer chains, and the interactions of

network and surrounding liquid.

Agar (Scheme 1) is an alternating copolymer of 3-linked β-D-galactopyranose and 4-linked

3,6-anhydro-α-L-galactopyranose residues. Agar is the most hydrophobic polysaccharide

with the strongest tendency to gel formation that melts at about 40°C. Agar forms hydrogels

via intermolecular aggregations prompted by thermal changes [12]. Due to its

biocompatibility, it has generally been used as a cell immobilization matrix, drug delivery

vehicle, or dental impression material [13, 14].



Scheme 1

Proposed mechanistic pathway for synthesis of the agar-g-poly (AA-co-AMPS) hydrogel

The purpose of this paper is to synthesize agar-g-poly (acrylic acid-co-2-acrylamido-2-

methylpropanesulfonic acid) hydrogel. The effect of reaction variables affecting on water

absorbency of the composite and swelling behavior in various solvents, salt and pH solutions

was investigated. The absorbency under load (AUL) of optimized hydrogel was determined by

using an AUL tester [15] in various applied pressures. Dynamic swelling kinetics of the

hydrogel was also determined. Finally, we investigated the release of KNO3 from the optimum

hydrogel as models of ionic fertilizer.

Materials and methods

Materials

Agar was obtained from QUELAB/UK. Acrylic acid (AA, from Merck) distilled before use. N,

N'-methylene bisacrylamide (MBA, from Merck), ammonium persulfate (APS, from Merck),



2-acrylamido-2-methylpropanesulfonic acid (AMPS, from Fluka), were of analytical grade

and used without further purification. The solvents (all from Merck) were used as received. All

other chemicals were also analytical grades. Double distilled water was used for the hydrogel

preparation and swelling measurements.

Instrumental analysis

The infrared spectra of hydrogel were recorded by ABB Bomem MB-100 FTIR

spectrophotometer (Canada). The dried sample was ground with dried KBr powder and

compressed into a disc, and then was subjected to analysis. In order to prevent the

morphology of hydrogels, the dried samples were coated with gold under reduced pressure and

their scanning electron micrographs were obtained using a Philips (XL30) scanning electron

microscope.

Hydrogel synthesis

In a 200 mL flask, fitted with a mechanical stirrer (Heidolph RZR 2021, 200 rpm), is placed

40 mL of water and variable amounts (0.5–1.5 g) of agar and the mixture is stirred at 100°C

until a clear solution is obtained. Then the reaction flask is cooled until the temperature of

the mixture reaches to 85°C. The mixture is heated in a water bath preset at 85°C. Then,

appropriate amounts of 2-acrylamido-2-methylpropanesulfonic acid (AMPS, 0.5 g),

partially neutralized acrylic acid (AA, 4–7 g), and N, N'-methylene bisacrylamide (MBA,

0.03–0.09 g) in 10 mL H2O were added. After 5 min, ammonium persulfate (APS, 0.03–

0.09 g) in 5 mL H2O was added. After 10–15 min obtained gel was cooled at room

temperature and poured to excess non solvent ethanol (200 mL) and remained for 3 h to

dewater. Then, ethanol was decanted and the product cut into small pieces. Again, 200 mL

fresh ethanol was added and the composite remained for 48 h. Finally, the filtered gel was

dried in oven at 50°C for 24 h. After grinding, the powdered composite hydrogel was stored

away from moisture, heat and light.

Absorbency or swelling measurement

Absorbency of superabsorbent polymers are measured by the free swelling method and

expressed as a water retention value (WRV) calculated in grams of water per grams of dry

polymer. Thus, an accurately weighed quantity of the hydrogel (0.10 g) was immersed in

250 mL of distilled water and allowed to soak for 1 h at room temperature. The sample

particle sizes were 40–60 meshes (250–400 µm). The swollen hydrogel was then separated

from unabsorbed water by a tea bag (i.e. a 100 mesh nylon screen). The equilibrium swelling

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(ES) was calculated according to following equation:

Where, W 2 and W 1 are the weights of swollen hydrogel and dry hydrogel in grams,

respectively.

Swelling in various salt solutions

Hydrogel absorbency was evaluated in 0.15 M solutions of NaCl, CaCl2, and AlCl3 according

to the method described above. Moreover, water absorbency of the hydrogel was measured in

different concentration of NaCl salt solutions.

Swelling at various values of pH

Individual solutions with acidic and basic values of pH were prepared by dilution of NaOH

(pH = 14.0) and HCl (pH = 0.0) solutions to achieve pH ≥ 6.0 and pH ≤ 6.0, respectively.

Then, absorbency was measured according to Eq. (1).

pH sensitivity

pH-responsiveness of the hydrogel was investigated in terms of swelling and deswelling of the

final product at two basic (pH 8.0) and acidic (pH 2.0) solutions, respectively. Swelling

capacity of the hydrogels at each pH was measured according to Eq. (1) at consecutive time

intervals.

Swelling kinetics

Hydrogel sample (40–60 mesh, 0.10 g) was poured into a weighed tea bag and immersed in

250 mL distilled water. At consecutive time intervals, the water absorbency of the sample was

measured.

Absorbency under load (AUL)

A macro-porous sintered glass filter plate (porosity # 0, d = 80 mm, h = 7 mm) was placed in

a Petri dish (d = 118 mm, h = 12 mm), and a weighed, dried hydrogel (0.40 g) was uniformly

placed on the surface of a polyester gauze located on the sintered glass. A cylindrical solid

weight (Teflon, d = 60 mm, variable height) which could slip freely in a glass cylinder (d =

60 mm, h = 50 mm) was used to apply the desired load (applied pressure 0.3 and 0.6 psi

ES( / ) =gg

−W2 W1

W1

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(2,068 and 4,137 Pa)) to the dry hydrogel sample particles. Then, 0.9% NaCl solution was

added so that the liquid level was equal to the height of the sintered glass filter. Whole of the

set was covered to prevent surface evaporation and probable change in the saline

concentration. At consecutive time intervals, the water absorbency of the hydrogel was

measured. After 2 h, the swollen particles were weighed again, and AUL was calculated

according to Eq. (1).

Loading of KNO3

The dry gel was put into a solution of KNO3 and leave to swell for 2 h. The swollen gels were

dried at 50°C. The % loading was calculated by the following equation:

where m 1 and m 0 are the weights of loaded gel and dry gel, respectively.

Determination of KNO3 release

In a typical release experiment, the loaded gel with known weight (0.1 g, mesh 40–60) was

placed in measured volume (200 mL) of distilled water (release medium) and was stirred

mildly. The released amount of KNO3 at different time intervals (Mt) was determined by

measuring the conductivity of the release medium using a conductivity meter (Martini

Instruments, Mi 170, EC/TDS/NaCl/Temp Meter). This was related to the amount of KNO3

using a calibration plot.

Result and discussion

Synthesis and spectral characterization

The hydrogel was synthesized by simultaneous graft copolymerization of AA and AMPS on

to agar by using of APS as an initiator and MBA as a crosslinker. The mechanism for

crosslinking graft copolymerization shown in Scheme 1. The persulfate initiator is decomposed

under heating to generate sulfate anionradical. The radical abstracts hydrogen from the

hydroxyl group of the polysaccharide substrate to form alkoxy radicals on the substrate. These

macroradicals initiate AA/AMPS grafting onto agar backbone led to a graft copolymer. Since

a crosslinking agent, e.g. MBA, is presented in the system, the copolymer comprises a

crosslinked structure.

%Loading = ( − / ) × 100m1 m0 m0



FT-IR spectroscopy was used to confirm the chemical structure of hydrogel. The FT-IR

spectra of agar, poly (AA-co-AMPS) and synthesized hydrogel were shown in Fig. 1a–c. The

broad band at 3,200–3,500 cm−1 is due to stretching of these hydroxyl groups. FTIR

spectrum of the hydrogel [Fig. 1(c)] represents several new peaks. The bands observed at 512,

630 and 1,400 cm−1 can be attributed to sulfonate group of AMPS, bands at 1,545 and

1,710 cm−1 is due to stretching of carboxylate and carboxylic acid groups, respectively. The

small peak at ~930 cm−1 is due to the 3,6-anhydro-α-L-galactopyranose unites. The band

observed at ~890 cm−1 is due to the unsubstituted D-galactopyranose unites. Finally the

peak observed at ~1,646 cm−1 that is related to of agar is the characteristic absorption band of

polysaccharides.




Fig. 1

FTIR spectra of a agar, b poly (AA-co-AMPS), c synthesized hydrogel, d loaded hydrogel and e unloadedhydrogel

Also the IR transmittance spectra of unloaded (a) and loaded (b) hydrogels are presented in

Fig. 1d–e. The spectra confirm the presence of KNO3 as evident from vibrational frequency of

nitrate ion at 1,384 cm−1.

SEM images of hydrogel are shown in Fig. 2. It is obvious from this Figure; the hydrogel has a

porous structure. It is supposed that these pores are the regions of water permeation and

interaction sites of external stimuli with the hydrophilic groups of the graft copolymers.

Fig. 2

SEM photographs of the optimized superabsorbent hydrogel (Agar 1 g, NU 40%, AA 1.26 mol/L, AMPS0.044 mol/L, APS 0.0024 mol/L, and MBA 0.006 mol/L, 85°C). a 10,000× and b 20,000×

Optimization of the grafting conditions

Different variables affect the ultimate swelling capacity. In this research, we selected some of

these variables (i.e. concentration of AA, APS, and MBA, and polysaccharide). These

parameters were systematically varied to achieve maximum water absorbency.

Effect of crosslinker concentration

The effect of the extent of crosslinking on water absorbency of agar-g-poly (AA-co-AMPS)

hydrogel is shown in Fig. 3. The maximum absorbency (820 g/g) is achieved at




0.0060 mol/L of MBA. At the lower than this amount, the formation of very loosely

crosslinked networks, resulting in highly swollen hydrogels with very low gel strength. Higher

crosslinker concentration produces more crosslinked points in polymeric chains and increases

the extent of crosslinking of the polymer network, which results in less swelling when it is

brought into contact with the solvent.

Fig. 3

Dependency of swelling of the hydrogel vs. the crosslinker concentration employed in the polymerizationprocess. Reaction conditions: Agar 1 g, NU 40%, AA 1.26 mol/L, AMPS 0.044 mol/L, and APS0.0040 mol/L, 85°C

Effect of APS concentration

The relationship between the initiator concentration and water absorbency were studied by

varying the initiator concentration from 0.0012 to 0.0096 mol/L (Fig. 4). Figure 4 shows

that the absorbency is increased with increasing the APS concentration from 0.0012 up to

0.0024 mol/L and then, it is decreased considerably with a further increase in the amount of

initiator. The maximum absorbency (1,100 g/g) is achieved at concentration of 2.4 mmol/L

of the initiator. The initial increase in water absorbency may be ascribed to the increase in the

active sites on the backbone of the agar, arising from the attack of sulfate anion-radical.

Decrease in swelling is originated from an increase in terminating step reaction via bimolecular

collision which, in turn, causes to enhance crosslinking density. This possible phenomenon is

referred to as “self-crosslinking” by Chen and Zhao [16]. In addition, the free radical

degradation of polysaccharide backbones by sulfate radicalanions is an additional reason for

swelling-loss at higher APS concentration. A similar observation is recently reported in the

case of polysaccharides such as chitosan [17] and carrageenan [18].



Fig. 4

Dependency of swelling of the hydrogel vs. the initiator concentration employed in the polymerizationprocess. Reaction conditions: Agar 1 g, NU 40%, AA 1.26 mol/L, AMPS 0.044 mol/L, and MBA0.006 mol/L, 85 °C

Effect of AA concentration

The effect of AA concentration on the swelling capacity of the hydrogel was studied by

varying the AA concentration from 0.76 to 2.02 mol/L (Fig. 5). Maximum swelling

(1,100 g/g) was obtained at 1.26 mol/L of AA concentration. Enhanced acrylic acid

concentration increases the diffusion of AA molecules into the agar backbone that

consequently causes an increase in water absorbency. In addition, higher AA content

enhanced the hydrophilicity of the hydrogel that it caused a stronger affinity for more

absorption of water. The swelling decrease after the maximum may be attributed to (a)

preferential homopolymerization over graft copolymerization, (b) increase in viscosity of the

medium which hinders the movement of free radicals and monomer molecules, (c) the

enhanced chance of chain transfer to monomer molecules and (d) non-neutralized acid

groups of grafted and non-grafted polyacrylic acid chains.



Fig. 5

Dependency of swelling of the hydrogel vs. the monomer (AA) concentration employed in thepolymerization process. Reaction conditions: Agar 1 g, NU 40%, APS 0.0024 mol/L, AMPS 0.044 mol/L,and MBA 0.006 mol/L, 85°C

Effect of agar weight on swelling

The effect of agar weight on the swelling capacity of the hydrogel was studied by varying the

agar weight from 0.5 to 1.5 g (Fig. 6). Figure 6 shows that the water absorbency of the

hydrogel was increased by increasing agar content up to 1 g then decreased with any further

increase. As the agar weight was increased in the mixture feed (0.5 up to 1 g), the active sites

can react easily with monomers. Increasing agar content more than 1 g, results in a high

viscosity of the medium and a decrease in the diffusion of monomers to active sites to

produce crosslinked hydrogels.

Fig. 6



Dependency of swelling of the hydroegl vs. agar weight employed in the polymerization process. Reactionconditions: NU 40%, AA 1.26 mol/L, APS 0.0024 mol/L, AMPS 0.044 mol/L, and MBA 0.006 mol/L, 85°C

The environmental sensitivity

Salt effect

The swelling degree of the hydrogel in saline solutions was significantly decreased as compared

with the values measured in deionized water. This result ,commonly observed in the swelling

of ionic hydrogels [19], is often attributed to a charge screening effect of the additional

cations causing a non-perfect anion–anion electrostatic repulsion, led to a decreased osmotic

pressure (ionic pressure) difference between the hydrogel network and the external solution.

Figure 7 showed the change of the swelling ratio as a function of NaCl concentration. It

showed that initially the equilibrium swelling ratio dropped down quickly as NaCl added, then

flattened out after NaCl concentration exceeded 0.15 mol/L.

Fig. 7

Swelling capacity variation of the optimized hydrogel in saline solutions with various concentrations

The effect of cation type (cations with different charge) on swelling behavior is shown in

Fig. 8. In the case of salt solutions with multivalent cations, ‘ionic crosslinking’ causes an

appreciable decrease in swelling capacity. Therefore, the absorbency for the hydrogel in the

studied salt solutions is in the order of monovalent > divalent > trivalent cations. Similar results

have been reported in similar previous studies [20, 21].



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Fig. 8

Swelling capacity of the optimized hydrogel in different chloride salt solutions (0.15 M)

Solvent-induced phase transition

We examined the swelling changes of the optimized hydrogel in different water-solvent

systems. The swelling-loss in these mixtures related to decrease of slovatation of ionic groups

in water-solvent systems. Sodium carboxylate and solfonate groups are easily solvated by

water molecules. However, it is widely restricted in these organic solvent-water systems

because the organic solvent molecules (ethanol and isopropanol) cannot solvate these ionic

groups. As a consequence, the swelling capacities are considerably decreased.

Figure 9 shows that in a fixed ratio of solvent–water (e.g., a 30:70 w/w solvent-water

mixture), the swelling is decreased in order of ethanol < 2-propanol. This can be explained by

the Hildebrand eq. (3) [22]:

where ΔH m is the enthalpy change on mixing of a polymer and a solvent, V is the whole

volume of the solution, Φ 1 and Φ 2 are the volume fractions for the solvent and the polymer,

δ 1 and δ 2 are the solubility parameters for the solvent and the polymer, respectively.

ΔHm/(V ) =Φ1Φ2 ( − )δ1 δ22



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Fig. 9

Sensitivity of the optimized hydrogel swelled in different solvent–water media with varied composition

It can be concluded from this equation that to dissolve a polymer in a solvent, the δ values

must be close to each other. As swelling absorbency of the superabsorbent hydrogels in water

is maximum, the δ value of water (23.4 (cal/cm3)1/2) can be regarded as the solubility

parameter of it. The solubility parameter for solvent-water mixtures (δ mix ) can be calculated

using following eq. (4)

where Φ 1 and Φ 2 are the volume fraction, and δ 1 and δ 2 are the solubility parameters of the

two solvents.

It can be seen from Table 1 that with increasing of δ mix values toward 23.4, the hydrogel can

be highly swollen as in pure water. In other words, the swelling capacity of the hydrogel in the

solvent–water mixture will be close to that in pure water if δ mix is close to δ water . A similar

observation is recently reported by Zohuriaan-Mehr and co-workers [23].

= +δmix δ1Φ1 δ2Φ2



Table 1

Solubility parameters for solvents [22]. Symbols δ and δ mix [(cal/cm3)1/2] are the solubility parameters forthe solvent and the solvent–water mixture, respectively

Solvent δ (or δmix)a Swelling capacity (g/g)

Acetone 9.9 -

2-propanol 11.5 -

Ethanol 12.7 -

Water 23.4 1,100

Ethanol- water (30:70) 20.19 571

2-propanol- water (30:70) 19.83 495

Equilibrium swelling at various pH solutions

Ionic superabsorbent hydrogels exhibit swelling changes at a wide range of pH values. Since the

swelling capacity of all “ionic” hydrogels is appreciably decreased by addition of counter ions

to the swelling medium, no buffer solutions were used. Therefore, stock NaOH (pH 14.0) and

HCl (0.0) solutions were diluted with distilled water to reach desired basic and acidic pH

values, respectively.

Figure 10 shows the effect of pH on the swelling ratio of hydrogel. The results clearly indicate

that the hydrogel exhibits extremely low degree of swelling in the media with low pH (pH <

2), whereas the gel demonstrates extensive swelling in the swelling media of pH ~ 6. The low

swelling in the media with low pH may be attributed to the fact that the —COOH and -

SO3H groups present along the macromolecular chains in the matrix remain almost unionized

(since pKa of the acrylic acid is 5.4), thus resulting into almost nil osmotic swelling pressure as

there are no mobile/counter ions present inside the gel matrix [17]. In addition, there occur

H-bonding interactions among the carboxylic groups within the matrix, thus providing a

compact H-bonded structure to the hydrogel [18], which ultimately restricts the movements

of polymeric segments and highly discourage the solvent entrance. However, when the gel is

put in the medium of pH 6, the ionization of —COOH and -SO3H groups not only increases

the osmotic swelling pressure but it also results in relaxation of polymeric chains due to

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repulsion among similarly charged —COO- and -SO 3 - groups along the macromolecular

chains. This causes extensive swelling of the hydrogel as indicated by higher water uptake of

value of the gel. The swelling-loss in the highly basic solutions may be attributed to the

“charge screening effect” of excess Na+ in the swelling media, which, in turn, shields the

carboxylate and sulfonate anions and prevents effective anion-anion repulsion.

Fig. 10

Effect of pH of solutions on swelling capacity of the hydrogel

pH-responsiveness behavior of the hydrogel

The pH-dependent swelling reversibility of the hydrogels was examined by immersing the

hydrogel in solutions with pH 2 and pH 8 and the swelling ratio was measured at relevant

intervals (Fig. 11). Since the pH of the working solutions may be changed by protonation–

deprotonation process of superabsorbent hydrogel, a fresh solution was used in each cycle. At

pH 8, the hydrogel swells up to 1,080 g/g due to anion–anion repulsive electrostatic forces,

while at pH 2, it shrinks within a few minutes due to protonation of carboxylate groups. As

illustrated in Fig. 11 the hydrogel sample exhibited an excellent reversibility. The swelling rate

and swelling capacity was almost no losing even after many times of oscillation. Such on-off

switching behavior as reversible swelling and deswelling has been reported for other ionic

hydrogels [24–27].



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Fig. 11

On–off switching behavior as reversible pulsatile swelling (pH 8) and deswelling (pH 2) of the optimizedhydrogel

Swelling kinetics

Figure 12(a) represented the dynamic swelling behavior of optimized hydrogel sample with

certain particle sizes (40–60 mesh) in water. The samples swelled rapidly and reached

equilibrium within about 25 min. Power law behaviors were obvious from Fig. 12(a). The

initial swelling rate can be calculated using Voigt-based equation (Eq. (5)) [28].

where S t (g/g) is swelling at time t, S e (g/g) is equilibrium swelling; t(min) is time for

swelling S t , and τ (min) stand for the "rate parameter". To find the rate parameter for

superabsorbent sample, Ln[1-S t /S e ] versus time (t) was plotted [Fig. 12(b)]. The slope of

the line passing the point zero and the point of 60% fractional swelling was determined (slope

= −l/τ, gives the rate parameter). Therefore, the rate parameter for the superabsorbent

composite was found to be 2.1 min in distilled water.

= (1 − )St Se e−t/τ



Fig. 12

Representative swelling kinetics of the optimized hydrogel with certain particle sizes (40–60 mesh) (a), andthe rate parameter (s) calculating graph (b)

Absorbency under load (AUL)

The AUL is an effective factor to investigate the swollen gel strength in the practical

application of superabsorbents, which is usually given in the patent literature and technical

data sheets offered by industrial hydrogel manufacturers. The AUL of hydrogel samples is

determined by using an AUL tester according to a procedure reported earlier [29]. Figure 13

represented the AUL of the hydrogel in saline solution (NaCl 0.9%) at r.t. It is obvious that

the minimum time needed for the highest AUL in the case of each load was determined to be

20 min. After this time, the swelling values under load were approximately unchanged.

Fig. 13



Time dependence of the AUL values for optimized sample

Release study of KNO3

The release of an active agent from swellable polymeric matrix is an important aspect of

hydrogel. The release of solute from loaded hydrogel involves the sorption of water into the

matrix and simultaneous release of solute via diffusion. The concentration difference of solute

between medium and hydrogels drives the solute out of the loaded hydrogel, but the special

space arrangements of function groups constructed by hydrogen bonds hinder solute to

migrate out. The release kinetics of a loaded hydrogel is closely related to its water sorption

kinetics, that is, initially; the rate of release sharply increases and then begins to level off. As a

result a highly swelling hydrogel should release a greater amount of solute entrapped within

the gel. (The results for all of the conductivities is fitted to 1 gr of loaded or unloaded

hydrogel)

Influence of loading on the release

One of the primary factor in the use of hydrogels as the carrier of active compounds is the

effect of percent loading on the rate of solute release. For this purpose, the optimum hydrogel

sample was equilibrated with KNO3 solution of varying concentrations (0.1, 0.5 and 1 M) for

2 h. The dry loaded hydrogel was mildly stirred in release medium and the progress of the

release process was monitored conductometrically. Also the conductivity of swelling medium

for 0.1 g of unloaded optimum hydrogel was determined. At consective time intervals, the

conductivity of solution was measured and these values were detracted from the conductivity

values of the release medium of loaded hydrogel at each time; After 2 h the increment in

conductivity of this solution was only 4 μs (equals to 1.44 mg KNO3). The results are

represented in Fig. 14 and Table 2. The release results indicate that the amounts of KNO3

released increases with increasing percent loading of the hydrogel. The results are as expected

because larger the initial load, the faster is the movement of the solvent front penetrating the

surface of the loaded hydrogel [30]. A larger loading of the hydrogel may also facilitate the

relaxation of macromolecular chains.



Fig. 14

Influence of loading on the release rate of KNO3

Table 2

Loading condition, loading percent and release percent after 24 h

Entry Morality of KNO3 in loading medium % load % release after 24 h

1 0.1 M 44.6% 78%

2 0.5 M 83.7% 67%

3 1 M 132.6% 47%

Influence of pH on the release

Since pH of the swelling medium is a key factor in the application of hydrogels in agriculture

field, the investigation of this parameter is of prime significance. For this purpose, the

optimum hydrogel sample was equilibrated with 1 M KNO3 solution for 2 h. Then the

amount of released KNO3 in phosphate buffer solutions with various pH (concentration of

phosphate buffer was 0.05 mM) was determined by measuring the conductivity of the release

medium. The results are shown in Fig. 15. There is a burst release initially for the first hour

both in acidic and basic medium, followed by an almost constant release of KNO3 from the

hydrogel for the studied period of time. The initial burst release may be attributed to the



release of potassium nitrates loaded near the surfaces of the hydrogel. This figure clearly

indicates that the release in highly basic or acidic solutions is low but in pH = 5.7 maximum of

released is observed. This can be explained by this fact that the active agent in the hydrogel

could be released as a result of the hydrogel volume change. According to the Fig. 10, the

maximum swelling of hydrogel is in pH ~ 6 and according to the Fig. 15, maximum release is

in pH = 5.7. This result indicates that the higher swelling ratios of the hydrogel create larger

surface areas to diffuse the KNO3.

Fig. 15

Effect of pH on KNO3 release

Conclusion

In the present study, we prepared a novel superabsorbent hydrogel by simultaneous graft

copolymerization of AA and AMPS onto agar. Crosslinking graft copolymerization of AA

and AMPS onto agar was performed in an aqueous medium using a persulfate initiator and a

bifunctional hydrophilic crosslinker. Variations in the reaction parameters affecting the

ultimate swelling capacity of the final product optimized the synthesis of superabsorbent

hydrogel. The maximum water absorbency (1,100 g/g) was achieved under the optimum

conditions, which were found to be AA 1.26 mol/L, AMPS 0.044 mol/L, MBA

0.0060 mol/L, APS 0.0024 mol/L, neutralization percent 40%,agar weight 0.5 g, and at a

reaction temperature of 85°C. Swelling measurement of the optimized hydrogel in different

salt solutions showed appreciable swelling capacity in comparison with our previous works.

Moreover, the synthesized hydrogel exhibited high sensitivity to pH. Furthermore, the

absorbency under load (AUL) of optimized hydrogel was investigated at various applied



1.

2.

3.

4.

5.

6.

7.

pressures. Dynamic swelling kinetics of the hydrogel was also determined. Therefore, this

hydrogel may be considered as an excellent candidate for different applications in the future.

Finally, the dry hydrogel loaded with potassium nitrate swells in aqueous medium releasing

the salt entrapped within the matrix. The release rate was related to pH and KNO3 percent

loading of hydrogels. Therefore, this hydrogel may be considered as a candidate for fertilizer

delivery in agriculture.

We express our appreciation to Hamid Salimi (Ph.D student of Sharif University) for his

patient and unflagging assistance.

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Synthesis and Investigation of Swelling Behavior of New Agar Based Superabsorbent Hydrogel as a Candidate for Agrochemical Delivery - Springer