1. INTRODUCTION:
During the past decades, most of the
pharmaceutical research activities have focused on the discovery or synthesis
of the novel drugs and drug administration systems. In this way, controlled
release drug delivery systems (DDS) have an outstanding place1,2. Among various kinds of polymeric systems, which
have been used as drug containers or release rate controlling barriers, hydrogels have gained considerable interest and reviewed
from different points of view. Hydrogels are a unique
class of macromolecular networks that may contain a large fraction of aqueous
solvent within their structure. They are particularly suitable for biomedical
and tissue engineering applications3-6 because of their ability to
simulate biological tissues and gained increased applications in bioseparations, agriculture and enhanced oil recovery7-9.
The hydrophilicity of the network is due to the
presence of chemical residues such as hydroxylic
(-OH), carboxylic (-COOH), amidic (-CONH-), primary amidic (-CONH2),
sulphonic (-SO3H), and others that can be
found within the polymer backbone or as lateral chains.
Nevertheless, it is also possible to
produce hydrogels containing a significant portion of
hydrophobic polymers, by blending or copolymerizing hydrophilic and hydrophobic
polymers. Biodegradable hydrogels play an important
role in controlled drug delivery10,11. They
are soft and rubbery, resembling the living tissue, exhibiting excellent
biocompatibility12. Among the numerous polymers that have been
proposed for the preparation of hydrogels,
polysaccharides are often preferred in comparison to synthetic polymers. This
is because of their non-toxicity, low cost, free availability and
biodegradability. However, natural polymers can be modified to overcome certain
drawbacks like uncontrolled rate of hydration, microbial contamination, drop in viscosity in storing, etc.
Starch, a natural biopolymer, is one of the
potential candidates for future biodegradable polymer products. Starch is
abundantly available. Global production of starch is 60 million ton per year in
2004 13. Starch is present in the body of many plants (tubers,
roots) as granules or cells with typical particle sizes between 1 and 100 lm.
The polymeric structure of starch consists of repeating anhydroglucose
units. There are two types of biopolymer in starch, amylose
(a linear polymer of anhydroglucoses with α-D-1,4-glucosidic bonds) and amylopectin
(a branched polymer with α -D-1,6-glucosidic bonds besides a-D-
1,4-glucosidic bonds). The content of amylose in
starches depends on the plant and typically varies between 18% and 28%. The
amylase – amylopectin ratio in native as well as
modified starches has a strong impact on the product properties. Starch films
are known to have good oxygen barrier properties.
However, as starch is highly hydrophilic,
it is water sensitive, and the mechanical properties of starch-based films are
generally inferior to those derived from synthetic polymers 14.
Starch modification is therefore needed to meet the product properties in a
number of application areas. Various modification strategies have been
explored, for instance grafting of monomers (like styrene and methyl methacrylate) to the starch backbone.
Here, we report on the synthesis of graft
copolymer hydrogels by free radical initiated
grafting of acrylic acid onto starch using ceric
ammonium nitrate as an initiator and N,N methylene bis acryl amide as a crosslinking agent. Use of ceric
ammonium nitrate as an initiator involves formation of radical sites onto
polysaccharide backbone and minimizes homopolymer
formation. The major objectives of the present work were to modify the starch
with acrylic acid and study the effect of pH of the swelling media and the
degree of cross linking on their swelling behavior.
2.
EXPERIMENT:
2.1
Materials:
Potato starch (s.d.
Fine chemicals, India) was used. Acrylic acid (Thomas Baker, India), was
freshly distilled under reduced pressure before use. N,N’
methylene bis acrylamide (MBAm) (s.d. fine Chemicals India), Ceric
ammonium nitrate (Qualigens, Germany) was dried at
110°C for 1 h. All other chemicals were used as such.
2.2.
Preparation of the hydrogels:
2 gm of potato starch powder was dispersed
in 75 ml distilled water at 80°C with constant stirring in N2 atmosphere. Then
required amount of initiator (CAN) was added over a 15 minutes time period
followed by monomer (AA) and cross linking agent (MBAm)
was added into solution with constant stirring. The concentration of CAN was
kept 0.005M 15. The reaction was completed after 3 hours. The
polymerization was indicated by the increase in the viscosity of the reaction
of reaction medium, the product was removed and washed with distilled water
(2-3 times) and finally with methanol, for removing homopolymer,
if any. The product was dried under vacuum oven at 500C this hydrogels were used for swelling studies at different pH of
buffer solution.
2.3
Infrared spectral analysis:
IR spectra of pure starch, hydrogels were taken on Perkin Elmer FTIR spectrum BS
spectrophotometer using KBr pellet technique.
2.4
Swelling studies:
The equilibrium swelling was measured
according to the conventional ‘‘tea bag’’ method. The completely dried preweighed graft copolymer was placed in 200 mL of various buffer solutions at 37°C, respectively. The
tea bag was taken out at regular time intervals, wiped superficially with filter
paper to remove surface water, weighed, and then placed in the same bath. The
mass measurements were continued until the equilibrium was attained. The
percentage mass swelling was determined using the following expression:
where Mo and Mt are the initial mass and mass at different time
intervals, respectively
3. RESULTS AND DISCUSSION:
3.1
Results:
A starch-based super-absorbent hydrogel was synthesized by free radical copolymerization
of acrylic acid onto the starch. As the primary objective was to study swelling
properties of the copolymer hydrogel, and as the
swelling characteristics are function of the degree of cross linking, the
copolymerization was carried out in the presence of varying amount, 0.5, 2.0
and 5.0 mole % of N,N– methylene bisacrylamide
as the cross linking agent.
Table
No: 1 Starch –acrylic acid graft copolymer hydrogelsa
|
Sr. No |
Polymer code |
Cross linker (MBAm) (mole%) |
SM at pH 1.2 |
SM at pH 7.4 |
|
1 |
St-PAA-1 |
0.5 |
149 |
281 |
|
2 |
St-PAA-2 |
2 |
130 |
246 |
|
3 |
St-PAA-3 |
5 |
124 |
175 |
a:Starch:2g, acrylic acid: 4 ml, initiator: ceric ammonium nitrate, (0.005M in 1M HNO3), 10
ml; medium: water, total volume: 100 ml, temperature: 37°C, time: 3h.
IR spectra of starch, St-PAA shown in
Figure 1. The IR
spectrum of starch showed absorption bands at 3409 (-OH stretching) and 971 cm –1
(skeletal vibration of C-O-C). IR spectra of St-PAA show peaks at 3425 and
980–1050 cm–1, which may be ascribed to the - OH stretching and
skeletal (C-O-C) vibration of starch in addition to the bands at 1725 cm–1
due to the carboxyl groups (>C=O stretching) of PAA, indicating that AA have
been successfully grafted onto starch. The IR spectra also shows a shoulder at
2179 cm-1, which is due to the presence of the –C–N– group of the crosslinking agent (N,N–methylene bisacrylamide). Since
the copolymers had already been extracted to remove the soluble contents, its
FTIR analysis proved that it is not a physical mixture but chemical linkages
have been formed during the free radical polymerization reaction.
Figure 1 IR spectra of graft copolymers (a)
ungrafted starch, (b) St-PAA hydrogel.
3.2
Swelling Characteristics:
The swelling characteristics of a cross
linked polymer depend not only on the hydrophilic-hydrophobic balance but also
on the degree of cross linking. The different graft copolymers prepared by
varying the mole percent of cross linking agent are listed in Table 1 along
with equilibrium swelling in media of pH 1.2 and 7.4. Also, the dynamic
swelling in pH 1.2 is shown graphically in Figure 2. Figure 2 shows water
uptake for all three copolymer hydrogels (ST-PAA-1,
ST-PAA-2 AND ST-PAA-3) which may be attributed to the high hydrophilicity
of both the components (ST and PAA) of the hydrogel.
The swelling was about 72 % in 30 min for
ST-PAA-1, whereas it was about 149% in 6 h. The extent of swelling was found to
decrease with increase in mole % of cross linker. Thus, the swelling was seen
to decrease from 149 to 124 % in 6 h, as the cross linker mole % increased from
0.5 to 5. The dynamic uptake of water by the hydrogels
at pH 7.4 is shown in Figure 3. The swelling was found to range from 121 to 83%
in 30 min for ST – PAA-1 to ST-PAA-3 series with the maximum swelling reaching
about 281 to 175% in 6 hr for the ST-PAA-1 to ST-PAA-3 series.
The pH of the swelling medium plays an
important role in influencing swelling behavior of hydrogels.
If the hydrogel contains some ionizable
groups, which can dissociate or get protonated at
some suitable pH of the swelling media, then the degree of swelling of hydrogels undergoes appreciable change with external pH. Figure 2 and 3 depicts the dynamic uptake of water by
the hydrogels in the buffer media of pH 1.2 and 7.4,
respectively at 37°C. The hydrogel exhibits minimum
swelling in the medium of pH 1.2, and as the pH becomes 7.4. The degree of
swelling increase with time until reaches equilibrium value. This can be
attributed to the fact that when the gel is allowed to swell in the media of pH
1.2, the -COOH groups present within the network remain almost nonionized, thus imparting almost nonpolyelectrolyte
type behavior to the gel. Moreover, there exits strong H-bonding interactions
between -COOH groups of acrylic acid and starch, which is present within the
network, thus resulting in a compact structure that does not permit much
movement of polymeric segments within the hydrogel.
However, in the medium of pH 7.4, the almost complete ionization of -COOH
group’s results in extensive chain relaxation due to repulsion among similarly
charged - COO– groups present along the macromolecular chains. Moreover, the
ionization also causes an increase in ion osmotic pressure. These two factors
are thus responsible for a higher degree of swelling in the medium of pH 7.4.
FIGURE
– 2 Dynamic uptake of water as a function of time for ST PAA graft copolymer hydrogels at pH 1.2. Values are mean ± SD of at
least three experiments
FIGURE
– 3 Dynamic uptake of water as a function of time for ST PAA graft copolymer hydrogels at pH 7.4. Values are mean ± SD of at
least three experiments
4. CONCLUSIONS:
The gel exhibit minimum swelling in the
medium at pH 1.2 and at pH 7.4, the degree of swelling at different time
intervals increases. At pH 7.4 ST – PAA-1 hydrogels
show swelling nearly about 281% where as at pH 1.2 it is 149% only similar result
were seen for hydrogel ST-PPA-2 were the swelling at
pH 7.4 was 246% and at pH 1.2 was 130% similarly ST-PPA-3 shows equilibrium
swelling at 175% at pH 7.4 and at pH 1.2
it was 124%. This can be attributed to the fact that when the gel allowed to swell
in the media of pH 1.2. The -COOH group present within network remains almost
unionized thus imparting almost nonpolyelectrolytic
type behaviour to gel, moreover there exist strong H- bonding interaction
between -COOH groups of acrylic acid and -CONH2 group of acryl amide which are
present within the network, thus resulting in movement of polymeric segment
within the hydrogel.
However, in the medium of pH 7.4 the most
complete ionization of COOH group result in extensive chain relaxation due to
repulsion among similarly charged –COO groups present in macromolecule chain.
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Received on
25.02.2014 Modified on 24.03.2014
Accepted on
09.04.2014 ©A&V Publications All right reserved
Research J.
Science and Tech. 6(2): April- June 2014; Page 75-78