Preparation and characterization of Polyvinyl Alcohol grafted starch matrix
for controlled release
Abstract
Known biodegradability and gel forming nature of polyvinyl alcohol makes
it very attractive candidate for sustained release devices. Poly(methylacrylate)
provides mechanical strength and PVA gives it hydrophilic nature. Present
section describes the grafting of polyvinyl alcohol onto potato starch grafted
methyl
acrylate
surface
using
microwave
irradiation
technique.
The
representative graft copolymer was characterized by Fourier transform infrared
spectroscopy, scanning electron microscopy and thermal analysis. Swelling
properties of the prepared graft copolymers were also examined.
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Preparation and characterization of Polyvinyl Alcohol grafted starch matrix
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1. Introduction
To overcome the crises of environmental pollution, numerous efforts have
been made to develop eco-friendly biodegradable polymers in recent years [1-2].
Among the various naturally occurring polymers starch is gaining increasing
attention because of its inherent biodegradability, low cost and easy availability
[3]. But the poor mechanical properties and weak water resistance restricted its
spacious application. Grafting of starch with other synthetic polymers to produce
biodegradable composite materials with adequate mechanical strength could be
a responsive way to minimize the environmental issues [4].
Recently much effort has been made to graft starch with biodegradable
synthetic polymers [5-6]. Poly vinyl alcohol considered as biodegradable polymer
and its biodegradation in various microbial environments has been reported [78]. As a hydrophilic polymer, poly vinyl alcohol exhibits excellent water retention
properties [9]. Thus, polyvinyl alcohol has been reported to be an option to graft
onto starch.
To prepare a biodegradable polymer, PVA could be grafted onto starch
but the poor processibility and strong hydrophilicity are the main disadvantages
of starch graft PVA copolymer. Strongly hydrophilic polymeric materials are poor
in water resistance and are too weak to be used commercially after absorbing
water [10]. Obviously it is a key matter to decrease the water absorption property
of the high water swell able polymers for their broad applications.
Attempts to improve the water resistance and mechanical properties of
superabsorbent polymers are made by addition of plasticizer, cross linker or
grafting with other synthetic polymers [11-13]. For developing water soluble
packaging plastics, mixture of poly vinyl alcohol and starch was also used
previously [14]. Conventionally, vinyl monomers are grafted onto starch using
various redox systems [15-17]. Recently, microwave irradiation [18-19], is
emerging as efficient source of thermal energy and constitutes original procedure
for the heating of materials in a different way from the conventional ones.
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Preparation and characterization of Polyvinyl Alcohol grafted starch matrix
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Microwave energy can be directly and uniformly absorbed throughout the entire
volume and cause evenly and rapidly heating [20]. Recently microwave
irradiation has been exploited in graft copolymerization. Grafting of butylacrylate
[21], acrylic acid [22], acrylamide [23] and acrylonitrile [24] on to the starch,
grafting of acrylamide onto LLDPE films [25], grafting of butyl methacrylate onto
wool fibers [26], polymethylmethacrylate onto natural rubber [27] and grafting of
acrylic acid on to chitosan [28] have been studied recently using redox initiators
under microwave irradiation. The grafting of methylmethacrylate [29] and
acrylonitrile onto cassia seed gum [30], ethylacrylate [31], acrylamide [32] and
polyacrylonitrile [33] onto the guar gum, and synthesis of acrylamide were used
without initiators under microwave conditions [34].
In present work, a hydrophilic matrix with grafted copolymers of starch,
polyvinylalcohal and acrylate monomers were developed with wide range of
physicochemical properties which have demonstrated their versatility in field of
agriculture, as soil conditioners, controlled release devices and also in personal
care products, bio-sorbent, biomaterial and pharmaceutical applications
The objective of this work is to develop an economic and ecological starch
graft copolymers with good water resistance and thermal stability using green
technology for more and safe production within very short duration. Swelling
properties, thermal behavior and surface morphology were examined.
2. Experimental
2.1. Materials and method
LG Intellocook TM Model no MS-1947 C; 800W, Domestic microwave
oven having 2450 MHz microwave frequency and a power output from 0 to 800W
with continuous adjustment was used for all the experiments. All the reagents
used were of analytical grade. Distilled water was used throughout the study.
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Preparation and characterization of Polyvinyl Alcohol grafted starch matrix
for controlled release
Polyvinyl alcohol samples (mol. wt. 125000), was obtained from Sd fine
chem. Limited, Mumbai, India was used without further purification. K2S2O8 were
purchased from Merck, India.
St-g-PMA copolymers used in this synthesis as raw material were
previously prepared and characterized in chapter second. Two different
efficiencies of St-g-PMA- 50% and 97% were taken for further grafting.
2.2.
Synthesis of St-g-PMA-g-PVA copolymer
5 %( w/v) polyvinyl alcohol solution was prepared by dissolving the
required amount of PVA in hot distill water (100ml). Starch graft PMA dispersion
in water was prepared by heating at 100 0C with constant stirring to get a
homogeneous mixture. To this mixture 0.02M K2S2O8 as initiator was added with
constant stirring. After vigorous stirring the flask was exposed to microwave
irradiation in domestic microwave oven for three minutes at 640 W. After
completion of the reaction, cooled mixture was precipitated with methanol: water
mixture (3:1 ratio). Obtained crude product was filtered and repeatedly washed
with water and dried at room temperature to a constant weight.
3. Characterization
3.1.
FTIR
Starch graft PMA, PVA and prepared hydrogel matrix were subjected to
FTIR Spectroscopy in range of 4000cm-1 to 400 cm-1. A Perkin elmeyer FTIR
Spectrophotometer was used for this study.
3.2.
Thermo gravimetric and Differential Scanning Calorimetric Analysis
EXSTAR TG/DTA 6300 was used to study the melting and crystallization
behavior of the polymeric samples. The melting studies were performed in the
temperature range of 25 0C to 800 0C at a heating rate of 20 0C/ min in oxygen
atmosphere.
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Preparation and characterization of Polyvinyl Alcohol grafted starch matrix
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3.3.
Surface morphology
Surface morphology of St-g-PMA matrix and prepared gel matrix was
evaluated with a model LEO 430 SEM after sputter coating of gold on the
specimen surface.
3.4.
Swelling behavior
The hydro gel matrix was directly immersed in distilled water at room
temperature for 8 h and the swollen product was dried at 37 0C under vacuum to
a constant weight. The equilibrium percentage of swelling was calculated using
following formula:
% swelling = (Wh-Wd) x100
Wd
Where W h is the weight at the swollen product after 8 h swelling, while
Wd is the weight of the dried product.
4. Result and Discussion
4.1.
FTIR Spectroscopy
The FTIR spectra of pure PVA (fig. 2) showed a broad peak around
3426.1 cm-1 indicating the presence of intramolecularly hydrogen bonded
hydroxyl groups in single bridge compounds and also showing peaks at 3022.1
and 1217.1 cm-1 due to C-H stretching and C-H bending indicating the presence
of hydrocarbon chromophore in PVA. St-g-PMA spectrum (fig 1) show peaks at
3450.6 and 1047.8 cm-1 which may be ascribed to the –OH stretching and
skeletal (C-O-C) vibrational stretching of starch in addition the band at 1746.1
cm-1 due to presence of carboxylic group (>C=O).
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Preparation and characterization of Polyvinyl Alcohol grafted starch matrix
for controlled release
Figure 1 : FTIR Spectra of St-g-PMA Matrix
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Preparation and characterization of Polyvinyl Alcohol grafted starch matrix
for controlled release
The spectrum of prepared hydrogel i.e. St-g-PMA-g-PVA matrix (fig 3)
showed a peak at 3434.3 cm-1, indicating the presence of intermolecular
hydrogen bonded hydroxyl groups having polymeric association. In addition to
the above peaks the hydrogel matrix is also showing peaks at 3025.1 and 1217.8
cm-1 indicating the presence of hydrocarbon chromophore. The absence of
characteristic peak of carboxylic group at 1746.1 cm -1 indicated absence of
>C=O groups from this. It can be inferred that all the carboxylic groups of St-gPMA has been used for grafting and favoured that PVA has been successfully
grafted onto St-g-PMA backbone.
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Preparation and characterization of Polyvinyl Alcohol grafted starch matrix
for controlled release
Figure 2: FTIR Spectra of Pure PVA
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Preparation and characterization of Polyvinyl Alcohol grafted starch matrix
for controlled release
Figure 3: FTIR Spectra of St-g-PMA-g-PVA matrix
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Preparation and characterization of Polyvinyl Alcohol grafted starch matrix
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4.2.
Thermo gravimetric Analysis
The thermal stability and degradation behavior of St-g-PMA and St-g-
PMA-g-PVA samples were evaluated by TGA under atmospheric conditions. The
results of thermo gravimetric analysis of St-g-PMA and St-g-PMA-g-PVA matrix
are shown in TGA /DTA curve [fig. 4, 5]. Thermogram of St-g-PMA (fig 4) shows
a weight loss in three stages. Initially up to 100 0C 3.7% of weight loss occurred
that may be ascribed to loss of physic-chemically bound moisture. The matrix
has decomposition temperatures at 200
0C,
300
0C
and 500
0C
with
corresponding weight losses of about 9.4, 41.8 and 86.2% respectively. TG data
were also supported with their corresponding derivative curve peaks (fig 5) at
290 0C, 437 0C and 465 0C with rate of decomposition 1.60, 0.77 and 0.73
mg/min respectively.
It is earlier reported that in pure PVA decomposition occurred in two
stages and it remains thermally stable up to 265 0C with a weight loss of around
15% [35-36]. The decomposition products of pure PVA were also reported
previously [37].
From the TG curve of St-g-PMA-g-PVA matrix (Fig 4), the gel matrix
shows a gradual weight loss up to 290 0C with 1.3% weight loss of residual water
on surface, beyond that a rapid decrease in weight has been observed. The
decomposition temperatures are 200 0C, 300 0C, 400 0C, 500 0C and 600 0C with
weight losses of about 7.3, 33.3, 48.2, 69.6 and 89.0% respectively. The data
supported with corresponding derivative curve peaks (fig 5) at 290 0C, 430 0C
and 536 0C with rate of decomposition 1.61, 0.76 and 0.63 mg/min respectively.
Comparative thermal study shows that prepared gel matrix i.e. St-g-PMA-g-PVA
shows better thermal resistance up to 600 0C.
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Preparation and characterization of Polyvinyl Alcohol grafted starch matrix
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ST-g-PMA-g-PVA
120
STARCH
100
TGA %
80
60
40
20
0
-20
100
200
300
400
500
600
700
800
900
TEMP. CELC
Figure 4: Comparative TGA graph of St-g-PMA and St-g-PMA-g-PVA
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Preparation and characterization of Polyvinyl Alcohol grafted starch matrix
for controlled release
St-g-PMA-g-PVA
2.5
St-g-PMA
2.0
DTG mg/min
1.5
1.0
0.5
0.0
0
100
200
300
400
500
600
700
800
900
Temp cel
Figure 5: DTG Curves of St-g-PMA and St-g-PMA-g-PVA
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Preparation and characterization of Polyvinyl Alcohol grafted starch matrix
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4.3.
DSC
The DSC thermogram of St-g-PMA (fig 6) showing an exothermic peak at
temperature ranging 358 0C (71mw) to 469 0C (158 mw) with (-∆H) 4390 mJ/mg
concurrent with significant weight loss. While in St-g-PMA-g-PVA gel matrix
thermogram an exothermic peak is obtained at 550 0C (158 mw) of (-∆H) 3490
mJ/mg indicating the improved thermal resistance with lower thermal degradation
of this material.
St-g-PMA-g-PVA
StARCH
200
180
160
140
DSC mw
120
100
80
60
40
20
0
-20
0
100 200 300 400 500 600 700 800 900
Temp cel
Figure 6: DSC Curve of St-g-PMA and St-g-PMA-g-PVA
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Preparation and characterization of Polyvinyl Alcohol grafted starch matrix
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4.4.
Surface morphology
The significant changes in the surface morphology of PMA grafted starch
and PVA/PMA grafted starch composite was clearly indicated in SEM
micrographs. Grafted starch matrix shows rather a coarse surface, which implies
that the amorphous starch is partially miscible with poly(methylacrylate) (fig 7a).
In figure (7b) polyvinyl alcohol coarsely disperse in St-g-PMA matrix.
Surface appears smooth, containing voids as shown in fig. when compared to
agglomerated surface of St-g-PMA (fig. 7a).
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Preparation and characterization of Polyvinyl Alcohol grafted starch matrix
for controlled release
Figure 7a: St-g-PMA matrix
Figure 7 b: St-g-PMA-g-PVA matrix
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Preparation and characterization of Polyvinyl Alcohol grafted starch matrix
for controlled release
4.5.
Swelling studies
The equilibrium swelling of product matrix was attained after eight hours
and remained almost constant up to 250 h. The obtained gel matrix showed an
equilibrium swelling of 364% which may be decreased with increasing contents
of poly(methylacrylate). The swell ability of the St-g-PMA matrix was obviously
reduced by graft copolymerization with synthetic hydrophobic monomers.
On the other hand the water absorbency increased with increasing PVA
content. Due to free hydroxyl groups, the content of hydrophilic groups is greater
and the water absorbency than gradually increased.
Swelling equilibrium of St-g-PMA and St-g-PMA-g-PVA copolymer were
comparatively examined and given in table 1. Maximum swelling equilibrium was
attained when both contents (St-g-PMA and PVA) are in equal ratio.
Table 1
St-g-PMA-g-PVA Copolymer matrix
St-g-PMA
Copolymer
Ratio of St-g-PMA: 25:75
PVA Content
50:50
75:25
100
Equilibrium
(%)
364
186
150
swelling 286
Conclusion
St-g-PMA-g-PVA matrix was successfully synthesized under microwave
irradiation, using very small conc. of potassium persulfate. A very high yield was
obtained. Hydrophobic nature of poly(methylacrylate) reduces the swelling
capacity of the gel matrix with improved water resistance. Introduction of PMA
had proven efficiently to improve the mechanical properties of the compatible
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Preparation and characterization of Polyvinyl Alcohol grafted starch matrix
for controlled release
composite along with biodegradation. Grafting of PVA onto St-g-PMA improved
the swelling efficiency as well as thermal behaviors.
The prepared gel matrix can be one of the good candidates in potential
application as a superabsorbent or a drug delivery carrier in heavy water
environments.
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Preparation and characterization of Polyvinyl Alcohol grafted starch matrix
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