Preparation and Properties of a Degradable Interpenetrating Polymer

Networks Based on Starch with Water Retention, Amelioration of Soil,

and Slow Release of Nitrogen and Phosphorus Fertilizer
Shuping Jin, Yongsheng Wang, Jinfang He, Yan Yang, Xinghai Yu, Guoren Yue
Key Laboratory of Hexi Corridor Resources Utilization of Gansu Universities, Department of Chemistry, Hexi University,
Zhangye 734000, People’s Republic of China
Correspondence to: S. Jin (E-mail: zjxjsp@163.com)

ABSTRACT: In this study, we aimed to develop a degradable nitrogen and phosphorus (NP) fertilizer with properties of slow release, water
retention, and remediation of saline soil; the nitrogen and phosphorus was coated with starch/poly(acrylic acid-co-acrylamide) [poly(AA-co-
AM)] superabsorbent (SAAmF) by reverse suspension radical copolymerization. The variable influences on the water absorbency were inves-
tigated and optimized. The results of the structure and morphology characterization of SAAmF show that poly(AA-co-AM) was grafted
partly from the chain of starch, and the different contents of starch brought about a difference in the size of the three-dimensional net hole
of the coating polymer. The property of water retention, the behaviors of slow release of nutrient, and the degradation of the SAAmF were
evaluated, respectively, and the results revealed that the water transpiration ratio of soil with SAAmF was lower by approximately 8 percent-
age points than that of the blank test, about 60% nutrient was released from SAAmF by the 30th day, and 32 wt % of SAAmF with a content
of starch of 20% was degraded after 55 days. Moreover, a considerable decrease in the conductivity was observed, which revealed a sharp
reduction in the concentration of residual ions for the soil mixed with SAAmF. It may be inferred from these that the product seems to be a
promising vehicle for the management of soils, including saline soils. V C 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 000: 000–000, 2012

KEYWORDS: biodegradable; interpenetrating networks (IPN); radical polymerization

Received 29 October 2011; accepted 6 June 2012; published online

DOI: 10.1002/app.38162

INTRODUCTION soil by diffusion through the pores or the coatings’ erosion and
In the past 40 years, agrichemicals such as nitrogen and phos- degradation.4–6 Recently, superabsorbents (polymers with a
phorus fertilizer have largely been used to ensure the increase in crosslinked three-dimensional network structure and an appro-
foodstuffs;1 some serious environmental damages have been priate crosslinking degree) have been used as soil additives in
caused by the use of fertilizers, such as water entrofication and agricultural and horticultural industries for the improvement of
the destruction of near-shore marine ecosystems.2 soil’s physical properties, such as the water-holding capacity,
nutrient retention of sandy soils, permeability, density, and
Compared with common fertilizers in use, slow-release fertil- structure of soil.7–9 Superabsorbents improve the soil’s aeration,
izers (SRFs) have the advantages of decreasing a fertilizer’s loss
prevent soil from hardening, cracking, and crusting. Wu et al.10
rate, supplying nutrient sustainably, lowering the application
prepared a double-coated nitrogen, phosphate, and potash com-
frequency, and minimizing potential negative effects induced by
pound fertilizer; its inner and outer coatings were chitosan and
an overdose of common fertilizers. According to Shaviv and
poly(acrylic acid)/diatomite-containing urea, respectively. These
Mikkelsen,3 SRFs are classified into following four types: (1)
products have the properties of having good a slow release and
low-soluble inorganic materials, such as metal ammonium
water-retention capacity.
phosphates; (2) low-soluble and chemically or biologically
degradable materials, such as urea–formaldehyde; (3) relatively However, superabsorbents cannot be widely applied to the agri-
soluble materials that decompose gradually in soil; and (4) cultural and horticultural industries because of their poor
water-soluble fertilizers controlled by physical barriers (e.g., degradability and the generation of new pollution. Therefore,
coating and matrix formation).4–6 Coated fertilizers are pre- environmentally friendly, degradable superabsorbent materials
pared by the coating of conventional fertilizers with various are needed. Starch is a common polysaccharide that is used in
materials; this ensures the controlled release of nutrients to the many biomedical fields. Typically, it is considered to be

V
C 2012 Wiley Periodicals, Inc.

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 ARTICLE

degradable and gelable in the presence of formaldehyde or bo- Scanning Electron Microscopy (SEM) Measurement
rax.11–13 Starch-based superabsorbents could be prepared by Two SAAmF samples with different contents of starch were swol-
graft polymerization with acrylic acid (AA) or/and acrylamide len completely in tap water at room temperature, and the coating
(AM) with the chain of starch or interpenetration and in situ si- polymer was peeled off carefully and freeze-dried with a Free-
multaneous crosslinking. Zone 2.0 freeze dry system (LABCONCO, USA) for 15 h after
the samples were frozen by liquid nitrogen to prevent the collapse
In this article, we report a novel and degradable coated slow-
of the porous structure. Then, the surface morphology of the xe-
release compound, in which the nutrient was entrapped in a
rogel was determined by a scanning electron microscope (SEM),
crosslinked starch matrix granule (SF), and the starch/poly(a-
JSM-5600LV SEM (JEOL Ltd., Japan).
crylic acid-co-acrylamide) [poly(AA-co-AM)] interpenetrating
polymer networks (IPNs) were used as an outer coating [starch/ Analysis of the Contents of Nitrogen and Phosphorus
poly(acrylic acid-co-acrylamide) superabsorbent (SAAmF)]. The in SAAmF
optimized coating conditions, moisture preservation, behaviors The content of nitrogen in SAAmF was determined by an elemen-
of nutrient release, electrical conductivity reduction of saline– tal analysis instrument, model 1106 (Germany Elemental Vario EL
sodic soils, and the coating polymer’s degradation were studied. Corp., Germany) and was determined to be 8.28%. The content of
phosphorus was determined by a spectrophotometer , model 722
(Third Analysis Instrument Corp. of Shanghai, China).11 One
EXPERIMENTAL
gram of SAAmF was added to a beaker and nitrified with 20.0 mL
Materials of concentrated nitric acid overnight. The solution was boiled until
AA was distilled at reduced pressure (boiling point ¼ 293–294 its volume was 10.0 mL, and then, its volume was adjusted to 250
K at 0.5 mmHg). Ammonium persulfate (APS) and N,N0 -meth- mL in a 250-mL volumetric flask. Finally, 2.50 mL of supernatant
ylene bisacrylamide (MBA) were recrystallized with water and was taken out and tested with the spectrophotometer. The results
95% ethanol, respectively. AM, corn starch, and diammonium show that the content of phosphorus in SAAmF was 9.03%.
phosphate were used without further purification. Test soil was
collected from garden topsoil (0–20 cm). The concentration of Water Absorbency (WA) Measurements
organic matter in the soil was 22.2 g/kg, the concentration of The SAAmF weighed previously was immersed in tap water
alkaline hydrolyzed N was 61 mg/kg, the concentration of avail- where the mass proportion of the sample to water was about 1 :
able P (P2O5) was 35 mg/kg, the available concentration of K 1000. Swelling continued until a constant weight was reached.
(K2O) was 112 mg/kg, the pH was 8.21, the volume weight was WA was calculated by the following equation:
1.33 g/cm3, and the porosity degree was 51.8%. WA ¼ M=M0  1 (1)
WA is expressed in grams of water retained in the swollen
Preparation of SFs SAAmF granules per gram of dried SAAmF and M and M0
First, 2 g of starch was added to a beaker, and then, 1.5 mL of denote the weights of the swollen and dry samples, respectively.
water saturated with borax was added and kept stirring at 80
C
until a stiff paste was formed. Subsequently, diammonium Release Behavior of SAAmF in Soil
phosphate (mass ratio of fertilizer to starch ¼ 5 : 1) was added The following experiment was carried out to study SAAmF’s
at 40
C. Finally, the paste was extruded, incised into SFs, and release behavior of nitrogen and phosphorus (P2O5) in soil. One
dried at 70
C. gram of SAAmF was mixed with 200 g of dry soil (<30 mesh) and
kept in a 250-mL glass beaker; then, 120 mL of distilled water was
added to SAAmF, and this mixture was incubated for different
Preparation of the NP Fertilizer Coated with
periods of time at room temperature. Proper amounts of diammo-
Starch/Poly(AA-co-AM) (SAAmF)
nium phosphate and urea were also mixed with the same amount
Five grams of SFs obtained previously was immersed into a
of soil and incubated for the same period at room temperature. In
starch paste for 15 min; this contained 5 mL of AA partially
the control tests, the total quantity of nutrition was kept same in 1
neutralized with ammonia and certain amounts of AM (the
g of SAAmF. The soil in the beaker was maintained at a 30%
mass ratios of starch to AA and AM was 15 wt %) and MBA,
water-holding capacity. The remaining granules in the soil were
and APS (shown later in Table II). Then, the mixture was
picked out and washed with distilled water and then dried over-
transferred into a flask equipped with a mechanical stirrer, a
night at room temperature after different incubation periods (1, 2,
condenser, and a thermometer. Cyclohexane (200 mL) and
3, 5, 8, 10, 15, 20, 25, and 30 days). The contents of nitrogen and
0.5 mL of sorbitan monooleate (Span 80) and poly(ethylene
phosphorus (P2O5) contained in the granules were determined by
glycol) sorbitan monostearate (Tween 60) were added to the
elemental analysis and a spectrophotometer mentioned previously,
flask, respectively, which was kept stirring for 2 h at 70
C.
respectively. The accumulated release percentage (RP) was calcu-
Cyclohexane was removed from the mixtures by filtration.
lated with the following expression:14
Then, the final product (SAAmF) was obtained.
RPð%Þ ¼ 250  Ci =1  C  100% (2)
Fourier Transform Infrared (FTIR) Spectroscopy
Starch and the coating polymer were pressed into a pellet with where Ci is the concentration of nitrogen or phosphorus in the
KBr and characterized by a Nicolet Nexus 670 FTIR spectrome- remaining granules at different times (g/mL) and C is the con-
ter (Nicolet Instrument Co., USA). tent of nitrogen or phosphorus in the primary sample.

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Table I. Experimental Control Factors (F1, F2, F3, F4, and F5) RESULTS AND DISCUSSION
and Their Levels Optimization of the Copolymerization Conditions
The Taguchi method, which is a powerful experimental design
Control factor Level 1 Level 2 Level 3 Level 4 tool developed by Taguchi, was used to optimize the variable
nAM/nAA (F1) 0.2 0.3 0.4 0.5 influences on WA of the SAAmF. The parameter design is the key
nAPS/nAAþAM (%; F2) 0.13 0.16 0.19 0.22
step in the Taguchi method in achieving high quality without an
increase in cost. This design of the Taguchi method generally
nMBA/nAAþAM (%; F3) 0.047 0.07 0.093 0.117
includes the following steps: (1) identification of the objective of
ND (%; F4) 60 65 70 75
the experiment, the quality characteristics (performance mea-
Temperature (
C; F5) 55 60 65 70 sure), and its measurement systems as well as the factors that
may influence the quality characteristics and their levels; (2)
Water Retention Measurement selection of the appropriate orthogonal array and assignment of
Two grams of SAAmF was mixed with 180 g of dry soil (<30 the factors at their levels to the orthogonal array; (3) conduction
mesh) in a glass beaker; then, it was covered with another 20 g of the test described by the trials in the orthogonal array; (4)
of dry soil, and 200 mL of tap water was slowly added to the analysis of the experimental data with the analysis of variance to
beaker. The beaker and its contents were weighed (W1). A con- see which factors are statistically significant and the determina-
trol experiment without SAAmF was also carried out. The tion of the optimum levels of the factors; and (5) verification of
beakers were placed in the laboratory at room temperature (in the optimal design parameters through a confirmation experi-
summer, the daily maximum temperature was 37
C, and the ment.15 The same process was adopted in this article.
temperature difference between day and night was up to 15
C)
and weighed every 3 days (Wi). Observation was done after a There existed a maximum WA that was dependent on the con-
period of 21 days. The water evaporation ratio [W (%)] in soil tent of AM (defined as the monomer unit molar ratio of AM to
was calculated with the following expression: AA, nAM/nAA), initiator APS (defined as the molar ratio of APS
to the comonomers unit of AA and AM, nAPS/nAAþAM), cross-
W ð%Þ ¼ ðW1  Wi Þ  100=200 (3) linker MBA (defined as the molar ratio of MBA to the mono-
mers of AA and AM, nMBA/nAAþAM), neutralization degree
Degradation of Starch/Poly(AA-co-AM) Superabsorbent in Soil (which was defined as the molar percentage of COO groups in
The degradation of the superabsorbent was monitored by the dry AA neutralized by ammonia), and temperature of reaction.16
weight loss. starch/poly(AA-co-AM) superabsorbents with differ- These parameters were varied at four levels, as shown in Table
ent contents of starch, which were the same as those used in the I, which were chosen on the basis of the preliminary experi-
previous experiment, were prepared in a tube. They were cut ments. The amount of starch (determined as the mass percent-
into disks and then dried. After weighing, the dried disks (ca. 4 age of starch in the mass of comonomers AA and AM, wstarch/
mm in diameter and 3 mm in thickness) were swollen completely wAAþAM) was chosen to be 15 wt %.
in tap water and buried 30 cm beneath the surface of the garden
soil at ambient temperature (in summer, the daily maximum An orthogonal array with four levels and five factors is shown
temperature was 37
C, and the day and night temperature differ- in Table II. Each row in the array represents a trial condition
ence was up to 15
C). The soil aeration was kept. After 5, 10, 15, with the factor levels, which are indicated by the numbers in
20, 25, 30, 35, 40, 45, 50, and 55 days, respectively, the disks were the row. The columns correspond to the factors specified in this
taken out, washed carefully with distilled water, and vacuum- study, and each column contains level 1, 2, 3, and 4 conditions.
dried to a constant weight. The percentage degradation (PD) of According to Table II, 16 groups of tests were carried out, WA
the superabsorbents was calculated from the following equation: was measured, respectively, and the results are shown in Table II
as well. The optimized circumstances and the contribution of
PD % ¼ ðW0  Wi Þ=W0  100 (4) each factor were obtained by analysis. It should be emphasized
that the interaction between the factors was neglected. The opti-
where W0 and Wi are the weights of the superabsorbent disks mized conditions for the highest WA, 81 g/g, were a tempera-
before and after degradation, respectively. ture of 60
C, a content of APS of 0.19%, a content of MBA of

Table II. Experimental Layout of an L16 Orthogonal Array According to Taguchi’s Suggestion and the Experimental Results for WA

Trial number
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
F1 1 1 1 1 2 2 2 2 3 3 3 3 4 4 4 4
F2 1 2 3 4 1 2 3 4 1 3 4 5 1 2 3 4
F3 1 2 3 4 2 1 4 3 3 4 1 2 4 3 2 1
F4 1 2 3 4 3 4 1 2 4 3 2 1 2 1 4 3
F5 1 2 3 4 4 3 2 1 2 1 4 3 3 4 1 2
WA 36 40 57 51 67 51 81 64 50 31 52 47 75 67 48 37

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sponded to the stretching vibration of OAH (carbons 2, 3, and

6). The band was wide and gentle because of free and associated
hydroxyl groups generated by hydrogen bonding. Copolymeriza-
tion brought groups of carboxylic and amide groups into the
polymer, which increased the intramolecular and intermolecular
hydrogen bonding, so the band became sharper, and there was a
positive shift from 3215 to 3247 cm1, which stemmed from a
formation of intermolecular hydrogen bonding between starch
and poly(AA-co-AM) and a negative shift from 3558 to about
3500 cm1 due to the decrease in content of free hydroxyl
groups, as shown in Figure 1(B). The band at about 3125.62
cm1, shown in Figure 1(B), was assigned to that of NAH and
confirmed the introduction of NH groups after copolymeriza-
tion.17 The peak observed at 2930.25 cm1 [Figure 1(A)] corre-
sponded to the stretching vibration of saturated CAH groups,
which shifted from 2930.25 to 2872.40 cm1 [Figure 1(B)] due
to the addition of grafting polymer chains. The characteristics
for the saccharide structure of starch were absorption bands
appearing at 1649.11 and 1159.07–992.53 cm1, as shown in
Figure 1(A), which could be assigned to symmetric and asym-
metric stretching vibrations of the CAOAC bridge, respectively.
It was evident that a stronger and wider peak appearing at
about 1658.70 cm1, shown in Figure 1(B), was due to the sym-
metric stretching vibration of the CAOAC bridge, which was
overlaid with the stretching vibration of C¼¼O (amide I). At the
same time, the multiplet observed at about 1117.44 cm1, as
shown in Figure 1(B), may have corresponded to the asymmet-
ric stretching vibration of the CAOAC group of carbon 6. This
fact, together with the new peak appearing at 1293.62 cm1,
corresponded to its symmetric stretching vibration and implied
a successful O-site grafting polymerization reaction.18 The bend-
ing vibration absorption peak of the CAOAC group, appearing
Scheme 1. Schematic diagrams of the copolymerization of AA and AM
(I) and the graft copolymerization of AA and AM from starch, in which
at about 541 cm1 further confirmed this. The absorption
R0 ¼ CO2H or CONH2.
bands at about 1562.63 and 1444.14 cm1, assigned to the
asymmetric and symmetric stretching vibrations of the COO,
respectively, signified that the starch/poly(AA-co-AM) should
have been an IPN.
0.117%, a content of AM of 0.3 (g/g), and a neutralization
degree of 60%, respectively. It was found that the effect of the A further process was carried out to confirm this conclusion. A
four factors on WA was similar to that reported in a previous similar free-radical polymerization of AA and AM was
article16 and so is not further explained here.

FTIR Analysis of the SAAmF

The superabsorbent consisted of the rigid framework of starch
together with the flexible copolymer chains of poly(AA-co-AM)
and possessed the characteristic of absorbing a large amount of
water, which was hardly removed, even under pressure. The ini-
tiator APS was used to initiate the copolymerization of AA and
AM. Graft copolymerization was triggered from the chain of
starch simultaneously, and the starch-g-poly(AA-co-AM) graft
copolymer was generated. The mechanisms of the random and
graft copolymerization reactions are shown in Scheme 1. For
the reason that SAAmF may have been an IPN composed of
crosslinked starch, poly(AA-co-AM), and starch-graft-poly(AA-
co-AM), this endowed the coating polymer with a high strength
and partial degradability.
Figure 1(A) shows the infrared spectrum of starch with a char- Figure 1. FTIR spectra of the (A) starch and (B) starch/poly(AA-co-AM)
acteristic absorption band at 3215–3558 cm1, which corre- IPN.

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Effect of the Starch Amount on WA

Figure 2 describes the starch dependence of WA of coated slow-
release SAAmF in tap water. WA increased as the amount of
starch (determined as the mass percentage of starch in the
monomers, wstarch/wAAþAM) rose from 0 to 15 wt %. This may
have been due to the stiff structure of starch and the hydrogen
bond, which endowed the coating polymer with large amounts
of effective net chain and hydrophilicity. However, WA decreased
considerably with further increases in the amount of starch. This
may have been due to the formation of excess hydrogen bonds
and the physical entanglement among polymer chains, which
would have led to a high crosslinking density, restricted the
relaxation of the polymer chain, and brought about a rigid
structure of the polymer matrix. The other important reason
was that hydrophilic poly(AA-co-AM) was less with increasing
amount of starch, which would have led to a reduced WA.
Figure 2. Mass ratio of starch to AA and AM dependence of WA of
SAAmF in tap water. Morphology of SAAmF
Figure 3(A,B) shows the photographs of the dry SAAmF and the
one swollen in tap water. Its core was the crosslinked starch matrix
with trapped nutrients in it, and the outer coating consisted of
performed in the presence of starch but without MBA. The
starch-g-poly(AA-co-AM) and poly(AA-co-AM). The average
product was leached with anhydrous methanol followed by pre-
thickness of the coating of the swollen SAAmF was about 2–3 mm.
cipitation into anhydrous diethyl ether. Then, it was verified by
FTIR spectroscopy that the precipitate was poly(AA-co-AM) af- The cross-sectional morphology of SAAmF with 40 and 15 wt
ter thorough drying. % starch, which were swollen completely in tap water, are

Figure 3. Photographs of (A) dry and (B) swollen SAAmF granules, (C) SEM section micrographs of SAAmF with 40 wt % starch, and (D) 15 wt %
starch after equilibrium swelling in tap water.

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swelled, there was an increase in the pore size of the three-

dimensional network, which benefitted the diffusion of the fer-
tilizer solution. SF in the core of SAAmF swelled slowly because
of the breakage of the interaction of hydroxyl groups and boron
ions in water absorbed in hydrogel network, and the entrapped
molecules dissolved slowly.20 There existed a dynamic exchange
between the free water in the hydrogel and the one in soil,21,22
and then, the nutrient was released slowly into the soil through
the grids with dynamic exchange.14,23
The influence of WA on the release behavior of nitrogen from
SAAmF in soil is shown in Figure 5, in which the contents of
starch were 15, 20, and 30 wt % and the corresponding WAs
were about 81, 51, and 28 g/g, respectively. From this figure, we
can see that all of the curves were similar, and the higher the
WA was, the higher the release rate was. The reason was that
the apertures in the three-dimensional network of the swollen
hydrogel were bigger with higher WAs. As a result, the exchange
Figure 4. (*, l) Nitrogen and (h, n) phosphorus (P2O5) release behav- of free water between the solution and the network and the nu-
iors from untreated fertilizer (solid) and SAAmF (hollow) in soil. trient released through it was easier. Furthermore, this resulted
in an increase in the concentration gradient between the coating
shown in Figure 3(C,D), respectively. Two peculiar features were polymer and the core, which was the driving force of nutrient
found through comparison with the morphology of SAAmF release from the core. Consequently, this resulted in an increase
shown in Figure 3(C,D). One was a difference in the diameter in the release rate.24 For SAAmF with a content of starch of 30
of the macropores of SAAmF. Figure 3(C) shows the smaller wt %, the release rate of nitrogen was lower than that with a
three-dimensional net hole due to the rather compact structure starch content of 20 wt % by the 15th day but was close to that
of the coating polymer with 40 wt % starch. Figure 3(D) shows after it. This may have been due to a cooperative effect of WA
the larger net hole of the coating polymer with 15 wt % starch. and the degradation of SAAmF. During the period investigated
From these, it was easy for us to understand why the SAAmF’s after 15 days, the sample with 30 wt % starch lost weight faster
with different contents of starch had different swelling capaci- than the others, so the rate of release of nitrogen was faster and
ties. The other feature was the stronger wall of SAAmF with 40 close to that with 20 wt % starch. The influence of WA on the
wt % starch because of the stronger hydrogen bonding and release behavior of phosphorus from SAAmF in soil was also
rather orderly aggregates of polymer chain segments. established, and the results were very similar to those of nitrogen.

Nutrient Release Behavior of SAAmF in Soil Water Retention Behavior of SAAmF in Soil
One of the most important characteristics of SAAmF was its Besides its slow-release properties, as discussed previously,
slow-release properties. The rate of NP release as a function of another important property of SAAmF was its water retention
time (days) was investigated for SAAmF’s with different con- characteristics in soil. The coating polymer could absorb water
tents of starch. Figure 4 shows plots of the RP of nitrogen and
phosphorus (P2O5) for untreated fertilizer and SAAmF with a
content of 20 wt % starch in soil. More than 93% of the nitro-
gen and 76% of the phosphorus were released from the
untreated fertilizer by the second day, as shown in the solid
curves of Figure 4. Comparatively, the release rate of nutrient
from SAAmF decreased sharply. About 7, 17, 45, and 60% of
nitrogen were released from SAAmF by the 2nd, 5th, 15th, and
30th days, respectively. These results indicate that SAAmF had
excellent slow-release properties, which agreed with the standard
of SRFs of the Committee of European Normalization.19
It is well known that diammonium phosphate and urea dissolve
quickly in water after being added to soil, and the nutrient
released out with that. The SF swelled slowly because of hydro-
gen bonding and polymer–boron ion complex formation with
the crosslinking reaction at the OAH site of starch, as revealed
by FTIR.20 This restricted the relaxation of polymer chains and
brought about a decrease in the diffusion of water molecules
and a subsequent reduction in the solution of phosphate. After Figure 5. Nitrogen-release behaviors from SAAmF with different WAs:
the coating polymer slowly absorbed the water in the soil and (*) 81, (h) 51, and (~) 28 g/g in soil.

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adsorption ratio) of soil.25,26 A study revealed that starch

showed a significant adsorption capacity for Cu2þ ions, which
bind to this material to form chelated compounds.27 In this
investigation, the electrical conductivity (j) of 185 mL of dilute
leaching liquor of a mixture of 20 g of saline soil and 2 g of
SAAmF (content of starch ¼ 20 wt %) was measured. It was
incubated for different periods at ambient temperature (in
summer, the daily maximum temperature was 37
C, and the
day and night temperature difference was up to 15
C) before
leaching. The tested saline soil was collected from Zhangye Pre-
fecture in China. The results are represented in Table III. From
them, we can see the different tendencies of conductivity when
SAAmF and commercial fertilizer were used, respectively. This
fact verified that SAAmF may have contributed to the reclama-
tion of saline–alkaline soil because a dramatical decrease in the
conductivity was observed, which revealed a sharp decrease in
the concentration of residual ions for the soil. This was attrib-
Figure 6. Water retention behavior of SAAmF in soil.
uted to the polymer matrix used in the core and the coating.
during rainfall and irrigation and was released slowly into the That is, cation adsorption on the surface of the coating polymer
soil in dry times. This was especially important in drought- was assumed to occur through ion exchange and chelation
prone areas, where the availability of water was insufficient. between positively charged ions, such as Naþ or Ca2þ, and car-
Photographs of the swollen samples, presented in Figure 3, boxylic groups or hydroxyl groups within the polymer, which
show that SAAmF was capable of absorbing water into the are schematically presented in the following equations:
hydrogel matrix. This indicated that SAAmF could effectively
store rainwater or irrigation water and could improve the utili-
zation of water resources.
Figure 6 shows that the water retention behavior of soil with
SAAmF that contained 20 wt % starch was greater than that
without it. The water transpiration ratios of soil without
SAAmF reached 55.65 and 95.75% on the 10th and 21th days,
respectively, whereas those of the soil with SAAmF were 48.65
and 89.32%, respectively. This property resulted from the coat- The adsorption mechanism could be given evidence by three
ing polymer, which absorbed and stored a large quantity of changes in the characteristic absorption bands of starch/pol-
water and allowed it to release slowly when the soil moisture y(AA-co-AM), shown in Figure 7; this was made a process of
decreased. The swollen coating was just like a reservoir for the absorption in 0.02 mol/L CaCl2 aqueous solution followed by
plant–soil system. Consequently, it would prolong irrigation drying. We know that the hydroxyl group of starch/poly(AA-co-
cycles, reduce irrigation frequencies, and strengthen the ability AM) exhibited stretching vibration peaks around 3247 and 3500
of plants to fight against drought. It was also noted that many cm1 [Figure 1(B)]. The strong absorption bands of 3247 cm1
granular structures formed in the soil with SAAmF; these gran- shifted to higher wave numbers of about 3416 cm1. This may
ular structures would contribute to great improvements in aera- have been due to the chelation between Ca2þ and OH groups,
tion and permeability and prevent the soil from hardening, which certainly weakened the hydrogen-bond interaction of
cracking, and crusting.9 hydroxyl groups. The COO groups on the poly(AA-co-AM)
chain exhibited two peaks at approximately 1629.89 and
Effect of SAAmF on the Electrical Conductivity of the Soil 1444.13 cm1, which were assigned to asymmetric and symmet-
It is also worth noting that the salinization of soil is more and ric stretching vibrations, respectively. In this study, the intensity
more serious; in other words, there has been an increase in the of these bands increased after the ion exchange of Ca2þ and

salinity parameters (electrical conductivity, pH, and sodium NHþ 4 because of an increase in the bonding strength of COO

Table III. Electrical Conductivity (j; ls/cm) of the Saline Soil

Time (days)
0 1 2 5 10 15 20 25 30
a
j (ls/cm) 0.900 0.880 0.840 0.802 0.810 0.779 0.705 0.700 0.699
j (ls/cm)b 0.900 0.740 0.620 0.580 0.582 0.558 0.430 0.390 0.378
a
After being mixed with commercial nitrogen fertilizer for different periods at ambient temperature.
b
After being mixed with SAAmF for different periods at ambient temperature.

WWW.MATERIALSVIEWS.COM WILEYONLINELIBRARY.COM/APP J. APPL. POLYM. SCI. 2012, DOI: 10.1002/APP.38162 7

 ARTICLE

copolymerization constitutes the most important field for affording

hydrogels with biodegradability and biocompatibility.27
In this study, the degradation of starch/poly(AA-co-AM) super-
absorbent was monitored by the examination of the weight loss
of the polymer with incubation time in soil at ambient temper-
ature. Figure 8 shows the PD of starch/poly(AA-co-AM) super-
absorbent with different contents of starch versus the incubation
time and its section micrograph with 50 wt % starch after 50
days of degradation. A decrease in weight demonstrated the
degradability of the superabsorbent. Kelner and Schacht34
reported that the type of degradable link and the structure of
the network play important roles in the control of the degrada-
tion behavior. Scheme 1 indicates that the connectivity of the
starch/poly(AA-co-AM) hydrogel network was maintained by
the glycosidic bonds of the starch molecules, amide links
(ACOANHA) from the crosslinker, ether links (AOA) of
grafted poly(AA-co-AM), and alkyl linkages formed via radical
copolymerization. Therefore, the degradation of the superab-
sorbent was caused by the breakage of glycosidic bonds, amide
Figure 7. FTIR spectra of the starch/poly(AA-co-AM) IPN (A) before and links, and ether bonds. Soil is a complex ecosystem, and there
(B) after absorption in a 0.02 mol/L CaCl2 aqueous solution. are different bacterial communities. The starch was invaded by
and Ca2þ. What is more, a strong absorption band appearing at bacteria, fungi, and other microorganisms under the appropri-
547 cm1 may have been due to O¼ ¼CACl, which further con- ate temperature and moderate conditions when SAAmF was
firmed the chelation between COO and Ca2þ. incubated in soil. Simultaneously, amide links and ether links
were broken through hydrolysis. Finally, these actions were suf-
All the information suggested that along with the shift or ficient to degrade the polymer into carbon dioxide and water.35
increases in the intensity of some characteristic adsorption
peaks, ion exchange and chelation occurred between the adsorb- PD increased with increasing starch content. During the period
ent and adsorbate.27 we investigated, the sample with 50 wt % starch lost weight faster
than the others, but the rate of weight loss of pure poly(AA-co-
Degradation of the Starch/Poly(AA-co-AM) Superabsorbent AM) was the slowest. As shown in Figure 8, after 55 days, the PD
With the development of polymer hydrogels, researchers have focused values were 41.7, 36.8, 32, and 18.1% for samples with starch con-
on the incorporation of biodegradable, environmentally friendly tents of 50, 35, 20, and 0 wt %, respectively. These results indicate
polysaccharides, such as starch,27–29 cellulose,30,31 and chitosan,32,33 that starch/poly(AA-co-AM) superabsorbent was partially
into the hydrogel. Starch is an abundant, inexpensive, renewable, and degraded and could be used as a coating for fertilizers to alleviate
fully biodegradable natural raw material. Modified starch via vinyl the environmental pollution.2,16

Figure 8. Degradation behavior of SAAmF with different contents of starch [(l) 0, () 20, (*) 35, and () 50 wt %] versus incubation time and SEM
micrograph of the superabsorbent with starch 50 wt % after degradation for 50 days.

8 J. APPL. POLYM. SCI. 2012, DOI: 10.1002/APP.38162 WILEYONLINELIBRARY.COM/APP

 ARTICLE

CONCLUSIONS 15. Guo, M. Y.; Liu, M. Z.; Zhan, F. L.; Wu, L. Ind. Eng. Chem.
Res. 2005, 44, 4206.
A slow-release and partially degradable NP fertilizer coated with
starch/poly(AA-co-AM) superabsorbent proved to be an effi- 16. Pourjavadi, A.; Ayyari, M.; Amini-Fazl, M. S. Eur. Polym. J.
cient and superior source of nitrogen and phosphorus and 2008, 44, 1209.
could be applied to improve the utilization efficiency of fertil- 17. Teli, M. D.; Waghmare, N. G. Carbohydr. Polym. 2009, 78,
izer and water, and it has potential practical applications in the 492.
remediation of saline soil. 18. Liu, Z. X.; Miao, Y. G.; Wang, Z. Y.; Yin, G. H. Carbohydr.
Polym. 2009, 77, 131.
ACKNOWLEDGMENTS
19. Trenkel, M. E. International Fertilizer Industry Association.
The authors thank Biao Lü for his assistance in the characterization Stratospheric Ozone; HMSO; London, 1997; p 11.
of the soil. This work was supported by Surface Project Funds of 20. Kale, S. N.; Mona, J.; Dhobale, S.; Thite, T.; Laware, S. L.
the Key Laboratory of Hexi Corridor Resources Utilization of J. Appl. Polym. Sci. 2011, 121, 2450.
Gansu Universities under contract grant number XZ0801 and col-
21. Hu, D. S. G.; Lin, M. T. S. Polymer 1994, 35, 4416.
lege tutor research projects in Gansu Province 110903.
22. Smyth, G.; Quinn, F. X.; McBrierty, V. J. Macromolecules
1988, 21, 3198.
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