controlled release of urea from biodegradable starch/ polyvinyl

Controlled release of urea from biodegradable starch/ Polyvinyl Alcohol matrix

Abstract

To minimize the agro-environmental pollution, completely biodegradable

potato starch graft polymethylacrylate co graft polyvinyl alcohol (St-g-PMA-g-

PVA) based formulation has been prepared under microwave irradiation.

Successful grafting of poly(vinylalcohol) onto potato starch graft

poly(methylacrylate) backbone was confirmed with FTIR, SEM, TGA and DSC.

Present study deals with formulation of St-g-PMA-g-PVA based agrochemical

(urea) delivery system for their controlled release. Formulation characteristics like

entrapped efficiency of urea, equilibrium water absorbency and diffusion rate of

urea loaded hydro gel was studied. The introduction of hydrophilic PMA-PVA

content increase the swell ability of starch matrix and the release rate of urea

from loaded hydro gels could be controlled by adjusting the graft efficiency.

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1. Introduction

Controlled drug delivery technology has now emerged as a truly

interdisciplinary science aimed at improving human health and received attention

in expression of a growing awareness for that substances ranging from drugs to

agricultural chemicals which are often excessively toxic and sometimes futile

when applied by conventional method, thereby preventing any adverse effect

associated with traditional drug administration and deliver the drug to specific

sites for a preferred extent.

Over the past decades carbohydrates biodegradable graft copolymers

have been explored to convince the necessities of particular sector of polymer

industry for drug delivery [1].

Despite of several polymers being used in the preparation of drug deliver,

natural polymers give the impression of obvious choice due to their excellent

biocompatibility low toxicity high enzymatic degradability and unique mechanical

properties [2-5]. On the other hand synthetic polymers provide many desired

advantageous properties with wide choice availability thus the combination of

natural and synthetic polymers may provide mechanical stability and biological

acceptability, acquiring from synergistic properties of both materials for controlled

drug delivery [6-9]. In order to achieve this, the properties of natural and synthetic

polymers have been modified by grafting, blending and other means [10-13]. Grafting of vinyl monomers onto natural polymers has been widely accepted [14-16]. Among the various chemical and physical combination method, grafting have

practical and academic interest for CR of drugs because they provide a

convenient route for the modification of properties to meet specific needs.

Recently hydrophilic starch graft copolymers with high swell ability have

been broadly used to formulate a CR device for highly water soluble drugs to

slow down the release of agrochemical and nutrients in agricultural application

[17-20]. Such graft copolymers showed superior performance over the

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conventional individual polymers for CR device and consequently the range of

application has grown rapidly for such class of materials.

Among the various agrochemicals nitrogen is the most vital nutrient for

crops. Urea is widely used because of its high nitrogen content and low cost

production but, the high solubility of urea causes large economic and resource

losses along with environmental pollution [21-24]. Controlled release technique

could effectively resolve these problems. The delivery of agrochemicals by CR

formulation offers with economical advantage. A large amount of work has been

done and method encapsulation various agrochemicals with in modified starch

matrices [25-26].

Early report showed that controlled release of various herbicides such as

simazine, 2, 4, 5 trichlorophenoxy acetic acid, triazine and furan by using

polymeric matrices have been studied [27-30]. Polymeric metrics such as corn

starch, St-g-poly butyl acrylate [6] and alginate gel have been used for thiram

release [31]; St-g-polylactide, [7] and St-g-polyacrylic acid [17] have been used

for urea release. Thus, grafting of synthetic monomers onto Starch backbone

constitutes a powerful means for improving starch properties.

In the present work an attempt of grafting polyvinyl alcohol onto St-g-PMA

matrix made to develop a delivery system to enhance the mechanical strength of

natural graft copolymers and to overcome the biological draw backs of synthetic

polymers. These composites were formulated into hydrogel beads and

subsequently loaded with agrochemicals.

2. Experimental

2.1. Material and Method

Domestic microwave oven model no LG Intellocook TM MS-1947 C was

used for the synthesis having 2450 MHz microwave frequency. Distilled water

was used throughout the study. Urea used in this experiment purchased from

Merck, India, is of industrial grade. P-dimithylamino benzaldehyde, hydrochloric

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acid and other solvents were of analytical grade. The St-g-PMA was previously

prepared as described in chapter second used as raw material for the synthesis

of St-g-PMA-g-PVA matrix.

The encapsulating matrix, St-g-PMA-g-PVA was synthesized and

characterized as describe in chapter fourth. The starch surface was firstly

modified by reacting the –OH groups on the starch with poly(methylacrylate) and

then the further grafting of poly(vinyl alcohol) onto St-g-PMA-g-PVA surface in

presence of K2S2O8 as initiator under influence of microwave irradiation. After

completion of the reaction the untreated PVA was washed with water and dried

till the constant weight under vacuums ovens at 40 0C. Small rounded beads

were cut from this matrix using a circular whole cutter.

These rounded beads were used for the controlled release of agrochemical

encapsulated within the matrix.

2.2. Swelling Equilibrium

A weighed quantity of the composite matrix was immersed in distilled water at

room temperature to reach equilibrium. The swelled samples were taken out from

the water and wiped with filter paper to remove the excess water. The equilibrium

water absorbency (Qev) of matrix was determined by weighing the swollen

samples. The Qev of matrix was calculated using following equation:

Qev (g/g) = M2-M1

M1

Where M2 is the weight of swelled samples and M1 is the weight of dried

samples. Qev expressed in gram/g.

2.3. Encapsulation of hydro gel beads

Encapsulation of urea was carried out by immersing accurately weighed

quantity of prepared beads (on dry basis) in saturated solution of urea at room

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temperature to reach swelling equilibrium. After attaining equilibrium, the swollen

beads were taken out and the liquid on the samples surface was absorbed by a

filter paper. The water was evaporated slowly at 40 0 C over three days. Before

the release experiment the samples were washed with water rapidly in order to

remove the urea exposed onto surface of beads.

2.4. Encapsulation Efficiency

Total weight percentage of encapsulated material within the matrix

represents the encapsulation efficiency of the matrix. To determine the actually

encapsulated amount of urea, the samples were weighed and wash with 20 ml of

water to remove the excessively surface adhered urea. Then urea content in

water was measured spectrophotometrically at 420 nm [32]. The encapsulation

efficiency was calculated according to the given equation:

EE (%) = [1 - W2 ] X100% [W0xC]

Where W0 is weight of loaded urea samples, W2 is the urea exposed on

surface of the hydrogel and C is the urea content of the hydrogel calculated from

the feed composition.

2.5. Urea Release study

In vitro release of urea from St-g-PMA-g-PVA hydrogels were studied by

keeping dried and loaded 200 mg of samples in 500 ml distill water at 25 0C. Two

ml of solution were withdrawn at regular interval and same volume of water was

added to make the volume constant. The amount of urea released was

measured by UV-spectrophotometer at 420 nm [32].

2.6. Surface Morphology

The surfaces of the polymeric hydrogels prier and after loading with urea

were observed with SEM. Fig (2) shows the SEM pictures of the urea loaded gel

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matrix. It is obvious from this fig (1) that grafting of polyvinyl alcohol was

uniformly onto grafted starch backbone. SEM of St-g-PMA-g-PVA gel revealed

that grafting of PVA and PMA led to physical and chemical cross linking; as well

defined pores are visible in these micrographs. 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.

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Fig 1: SEM of St-g-PMA-g-PVA matrix

Fig 2: Urea encapsulated St-g-PMA-g-PVA matrix

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3. Result and Discussion

St-g-PMA used for grafting reaction was previously prepared in our

laboratory and characterized. After drying the St-g-PMA, the free carboxylic

groups (>C=O) of PMA reacts with OH groups of PVA to obtains the graft

copolymer under influence of microwave irradiation and grafting reaction

produces very low quantity of homopolymer. The grafted product and PVA could

be easily separated by cold water treatment in which PVA is soluble.

St-g-PMA-g-PVA gel matrix shows (table 1) better water holding

capacities, which was obviously greater than that of poly(methylacrylate) grafted

starch matrix. Poly vinyl alcohol is a hydrophilic polymer; responsible for higher

water holding capacity. Maximum urea loading was obtained when the St-g-PMA

and PVA contents are in equal ratios.

Table- 1

S. No St-g-PMA: PVA (25: 75)

St-g-PMA: PVA (50:50)

St-g- PMA:PVA (75: 25)

St-g-PMA

Water Loding %

6.1 4.2 2.5 1.5

Urea Loding %

43.70 69.98 32.1 14

The gel matrix encapsulates urea to larger extent due to presence of voids

in gel matrix. Fig 2 shows typical SEM micrographs of urea loaded hydro gels, in

which voids are clearly seen. Urea loading capacities were increased with the

increase of PVA contents and maximal loading capacity was reached up to 50%,

when grafting efficiency of PVA was 50%.

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The influences of the graft modification and grafting ratio on the release

rate of urea from gel matrix are shown in table 1. The urea release was generally

reduced from the obtained gel matrix compared with St-g-PMA matrix. The

following equation was used to predict the diffusion nature of urea into gel matrix.

F = MT/M0 = Ktn

Where MT/M∞ is the fractional release of urea in time t, k is the constant

related to the structure of the network and exponent n is diffusion exponent

characteristic of the release mechanism. For normal fickion diffusion the value of

n=0.5, case II diffusion n=1.0 and non fickion n=0.5-1.0.

Diffusion exponent n and gel characteristic constant K for the release of

urea have been calculated from the slope and intercept of the straight line

obtained by the plots of ln F versus ln t. From the fig 3 it can be clearly seen that

values of n ranges between 0.8 to 0.9 and urea release from gel matrix was

assumed to be Non fickion in diffusion character.

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Figure 3: Kinetic release of urea

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Figure 4: Urea released %

The maximum release of urea from St-g-PMA-g-PVA gel matrix was about

98.38 % after 26 hours (fig 5). In early stage release rate is very fast and attained

almost maximum values after a time of about 6-7 hours. The observation of plot

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reveals that in early six hour time period the release rate has a linear behavior

and then after release rate becomes slower attaining almost complete release in

ten hours time period.

The gel matrix releases the entirely encapsulated urea in a very controlled

and sustained manner, which is the primary requisite for the use of agro-

chemicals to control the environment and health hazard.

4. Conclusion Starch based semi interpenetrating polymer networks are synthesized in

presence of monomers of methyl acrylate and poly vinyl alcohol. Swelling rates

were measured. The amount of release of the urea is studied by

sphetrophotometric method. Although increasing the amount of initial St-g-PMA

copolymer leads to a stronger structure. It results in a decrease in the swelling

rate and also releases rates. As the conc. of PVA increases, the mechanical

strength of the network increases and releases rate of urea was reduced up to

lower extent.

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controlled release of urea from biodegradable starch/ polyvinyl

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