RESEARCH ARTICLE

Thermal and electrical properties of acrylic acid graftedonto starch by ceric ammonium nitrate and potassiumpermanganate initiator

Jaiswar Gautam & Manoj Kumar Pal & Beena Singh &

Urvi Bhatnagar

Received: 27 August 2011 /Accepted: 27 February 2012 /Published online: 20 March 2012# Central Institute of Plastics Engineering & Technology 2012

Abstract Graft copolymerization of acrylic acid (AA) onto corn starch (Starch) inaqueous media via a redox initiator system of ceric ammonium nitrate and potassiumpermanganate has been studied gravimetrically under nitrogen atmosphere. The grade ofgraft copolymers were synthesized under the effect of reaction variables such as redoxinitiator concentration, monomer concentration, reaction temperature and polymerizationtime on the grafting were studied in terms of percent grafting ratio (% G) and percentgrafting efficiency (% E). The graft copolymers obtained were characterized by fouriertransform infrared (FTIR), scanning electron microscope (SEM), and thermogravimetryanalysis (TGA). SEM morphology shows that the surface of the grafted starch is highlyrough in comparison with ungrafted starch which is attributed to the high graft density.The starch surface was dispersed heterogeneously among the polyacrylic matrix andrugged surface was observed that all fragments of these surfaces are rough, spherical, rodin shape. The FTIR shows characterized peak in the region 1722.28 cm−1 whichattributed to the shift in the position of C0O group deriving grafting of AA withstarch. TGA shows the graft copolymer degrades at higher temperature and electricalconductivity indicates conductivities lies between 10−7 and 10−5 S/cm.

Keywords Graft copolymer . Redox initiator . TGA . Electrical conductivity . Starch

Introduction

Graft copolymer is a natural extension of the concept of chain branching and involvesthe introduction of active centre, various natural products including starch, proteinand water soluble gums have been used as a basis for graft copolymer by theformation of active centres. There are number of ways of achieving active centres,many of which depend on an anionic or cationic mechanism.

Int J Plast Technol (December 2011) 15(2):188–198DOI 10.1007/s12588-012-9024-6

J. Gautam (*) :M. K. Pal : B. Singh :U. BhatnagarDepartment of Chemistry, Dr. Bhim Rao Ambedkar University, Agra 282002, Indiae-mail: gjaiswar@gmail.com

Graft copolymerization is considered to be one of the routes widely used to improvethe conditional and new properties of natural and synthetic polymer. Starch graftcopolymer has widely used as soil stabilizer in agriculture, biodegradable thermoplasticin plastic industries, thickeners and sizing agents in paper textile industries and superabsorbents in consumer items like diapers and sanitary napkins [1, 2]. Chemicalmodification of starch via graft copolymerization constitutes a powerful means ofimproving the starch properties such as sorbancy, ion exchange capabilities, elasticity,hydrophobicity, thermal resistance and resistance to microbial attacks [3–5]. Anumber of free radical initiating systems has been used to prepare graft copolymerand these generally divided into two broad categories chemical or photo irradiation,which will be greatly dependent on the types of monomer used. Among the severaltypes of chemical initiator used in graft copolymerization of starch with vinylicmonomers are hydrogen peroxide/ferrous ammonium sulphate, potassium permanga-nate in the presence of mineral acid, ceric ammonium nitrate and persulphate [6–11].

In the present paper attempts were made to use potassium permanganate and cericammonium nitrate for the preparation of acrylic acid/starch graft polymer, with thevariation of monomer, initiator concentration, temperature etc. were adapted andimportant properties like thermal and electrical conductivity were measured to eval-uate the usefulness of graft polymer in the field of semiconductor devices and alsokeeping in mind the thermal degradability of the final products.

Experimental

Materials

Corn starch was of AR grade dried at 110 °C to remove the absorbed water/moistureas it has been experimentally found to be an optimum time for drying to constantweight for starch and was subsequently stored over anhydrous CaCl2. The monomeracrylic acid (AA) was purchased from Alfa Aeasar (Germany). AA has washed by10 % NaOH solution before use and was kept in a cool and dry place specially keptunder refrigeration and was brought to room temperature before use. Ceric ammoni-um nitrate (CAN)/Potassium permanganate was dried at 100 °C and kept indescicator.

Graft copolymerization acrylic acid onto starch

A 2.0 gm of dried starch was dissolved in 50 ml distilled water and was stirredmagnetically under N2 atmospheric pressure at 70 °C for 2 h until slurry was formed,then required amount of monomer acrylic acid was added simultaneously intoreaction flask and then stirred continuously for 60 min. The predetermined amountof redox initiator [(NH4)2Ce(NO3)6/KMnO4] was dissolved in 15 ml of distilled waterfinally this treatment was followed with drop wise addition of redox initiator for5 min interval to facilitate initiation on starch, the total volume was made 80 ml bydistilled water and the polymerization proceeded at 70 °C and for 3 h. The reactionwas stopped by the addition of 2 ml of 5 % (w/v) quinol solution to the reactionmixture. The mixture was poured into large excess of methanol with stirring to

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precipitate the polymer. The precipitate starch grafted poly acrylic acid (St-g-PAA)was washed with ethanol and the homopolymer of acrylic acid (PAA) was removedby exhaustive Soxhlet extraction with benzene for 12 h. The final copolymer wasthen dried under vacuum at 60 °C. The dried St-g-PAA was finally powdered andstored over anhydrous CaCl2.

Hydrolysis of the grafted copolymer

The grafted copolymer produced was hydrolyzed by adding 2 N NaOH to the productin a 100 ml flask immersed in thermostateted water bath fitted with magnetic stirrerand a reflux condenser. The hydrolysis was on for about 2 h at 70 °C. The pastymixture was allowed to cool to room temperature and neutralized to pH 8 by theaddition of 10 wt% aqueous acetic acid solution. The mixture was poured into excessmethanol to precipitates out. The precipitates were washed with ethyl alcohol/H2Omixture (80/20) during mechanical stirring to extract NaOAc and NaOH. The St-g-poly (acrylic acid) was dried under vacuum at 70 °C.

Grafting parameters

The percent grafting ratio (% G) and grafting efficiency (%E) were calculated fromthe increase in weight in the follow manner (11)

Grafting ratio %G½ � ¼ W1�W0W0

� 100

Grafting Efficiency %E½ � ¼ W1�W0W2

� 100

Where W1, W0 and W2 are the weight of starch-graft-poly(acrylic acid) (Starch-g-PAA), the weight of original starch and weight of acrylic acid used.

Characterization

Thermal analysis of graft copolymer was performed on (Perkin–Elmer Pyris–1 TGA)were heated up to 600 °C at a rate of 20.00 °C/min. FTIR spectra of the newlysynthesized graft copolymers were recorded on a Perkin Elmer spectrophotometerwith in the range of 450–4,000 cm−1 using resolution of 4 cm−1. The morphology ofcopolymers was studied by Scanning Electron Microscopy (Hitachi FM-SEM S4700). The electrical conductivity was measured by Precision LCR meter TH 2816B.

Results and discussion

Graft copolymerization of starch with acrylic acid monomer

Reaction mechanism

The prepared Starch-g-PAA copolymer appeared as yellowish white solid, partiallysoluble in water, Acetone, methanol, and has maximum swelling capacity than starch.The softening range has maximum up to 380 °C. The scheme of the possible

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polymerization reaction has been described in (Fig. 1). It indicate that when Potas-sium permanganate and ceric ammonium nitrate {KMnO4/(NH4)2 Ce(NO3)6} initi-ating redox systems are react in aqueous medium they decompose to Ce4+, Mn3+

ions. These generated Ce4+ abstract hydrogen atoms from starch molecules producingstarch free radical. The interaction between the produced starch and free radicals andmonomer molecules, which are close vicinity of the reaction sites, result in creation offree radical–graft copolymers. The following equation represents the reactionmechanism (Fig. 1)-

KMnO4 þ NH4ð Þ2Ce NO3ð Þ6� ��!Ce4þ þMn3þ

Effect of monomer concentration

The effect of acrylic acid concentration was studied in the range of 0.0694–0.6938 mol/L at fixed concentration of starch (2 gm), ceric ammonium nitrate/potassium permanganate (1 mmol), reaction temperature (70 °C), reaction time(3 h) and reaction volume (50 ml) (Fig. 2) shows the effect of concentration ofacrylic acid on the percentage graft yield (% G) and percentage graft efficiency (% E).In initial stage, both the grafting parameter increases with the increase concentration.The optimum value for % GY of 42.33 % and for % GE of 61.25 % was obtained at0.2775 mol/L of monomer respectively.

The initial increase in % E and % G may be due to the greater availability ofmonomer in the proximity of starch macro radical, however, the decrease in % G at alatter stage may be owing to the wastage of AA monomer in the formation of largeamount of homopolymers. This way evident from the increase in the viscosity of

Fig. 1 General mechanism for ceric-initiated graft copolymerization of a typical acrylic acid monomeronto starch

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reaction medium in which the homopolymers (PAA) was soluble and also from stickygrafted product unlike the granular product obtained at lower concentration of acrylicacid. These homopolymers lumps then progressively hinder the rate of diffusion ofmonomer molecules to the starch macroradicals, resulting in gradual decrease in % Eas well as % G.

Effect of starch concentration

The effect of starch concentration was studied in the range of 1.0–2.0 gm/50 ml atfixed concentration of ceric ammonium nitrate/potassium permanganate (1 mmol),reaction temperature (70 °C), acrylic acid (0.2775 mol/L), reaction time (3 h) andreaction volume (50 ml) (Fig. 3) shows the effect of concentration of starch on thepercentage graft efficiency (% E) and percentage graft yield. Both the % E and % Gincrease with increase in concentration of starch. The optimum value for % G of58.69 % and for % E of 111.36 % was obtained at 2 gm of starch respectively.

Fig. 2 Effects of acrylic acid concentration on % G and % E at [starch] 2 gm, [(NH4)2 Ce (NO3)6/KMnO4]1 mmol in 50 ml of distilled water; 70 °C for 3 h

Fig. 3 Effects of starch concentration on % G and % E at [Acrylic acid] 0.2775 mol/L, [(NH4)2 Ce (NO3)6/KMnO4] 1 mmol in 50 ml of distilled water; 70 °C for 3 h

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The increase in % G and % E may be due to the increase in the concentration ofstarch. The viscosity of the reaction medium causing to the fact that availability ofmore and more grafting sites at starch resulting in grafting parameters.

Effect of initiator concentration

The effect of initiator concentration was studied in the range of 0.5 mmol to 1 mmolat fixed concentration of starch (2 gm), acrylic acid (0.2775 mol/L), reaction temper-ature (70 °C), reaction time (3 h) and reaction volume (50 ml) (Fig. 4) shows theeffect of concentration of initiator on the percentage graft efficiency (% E) andpercentage graft yield (% G). Both the % E and % G increases with increase inconcentration of initiator. The optimum value for % GY of 60.62 % and for % E of76.32 % was obtained at 1 M of initiator respectively. The rise in % E and % G maybe attributed to the formation of increase in the number of free radical on the starchbackbone at which the monomer molecule was grafted.

Effect of reaction temperature

The effect of reaction temperature was studied in the range of 50–90 °C at fixedconcentration of starch (2 gm), acrylic acid (0.2775 mol/L), ceric ammonium nitrate/potassium permanganate (1 mmol), reaction time (3 h) and reaction volume (50 ml)(Fig. 5) shows the effect of reaction temperature on the percentage graft efficiency (%E) and percentage graft yield (% G). It can be noted that in initial stage, both % E and %G reaches a maximum at an optimum temperature of 70 °C and then decrease steadilywith further rise in temperature to reach at 80 °C. The optimum value for % G of82.44 % and for % E of 58.36 % was obtained at 70 °C of starch respectively.

The increase in %E upto 70 °C owing to the higher rate of diffusion of monomermolecule to starch macroradicals, however, the decline in % E beyond 70 °C can beexplained on the basis of increased rate of homopolymerization. This is also supportby visual observation that the yellow colour of starch slurry owing to CAN and

Fig. 4 Effects of ceric ammonium nitrate and potassium permanganate concentration on % G and % E at[starch] 2 gm, [Acrylic acid] 0.2775 mol/L in 50 ml of distilled water; 70 °C for 3 h

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KMnO4 gradually fades on addition of monomer and the rate of disappearance ofcolor increase sharply at higher polymerization temperature.

Effect of polymerization time

The effect of polymerization time was studied in the range of 60–300 min at fixedconcentration of starch (2 gm), acrylic acid (0.2775 mol/L), ceric ammonium nitrate/potassium permanganate (1 mmol), reaction temperature (70 °C), and reaction vol-ume (50 ml) (Fig. 6) shows the effect of reaction time on the percentage graftefficiency (% E) and percentage graft yield (% G). Both the % E and % G increasewith increase in time. The comparatively low yield upto 60 min indicate there existssome induction period upto which graft copolymerization of AA onto starch does notinitiate. The remarkable increase in rate of grafting is observed beyond 120 min,

Fig. 5 Effects of reaction temperature % G and % E at [starch] 2 gm, [Acrylic acid] 0.2775 mol/L, [(NH4)2Ce (NO3)6/KMnO4] 1 mmol in 50 ml of distilled water; for 3 h

Fig. 6 Effects of time duration % G and % E at [starch] 2 gm, [Acrylic acid] 0.2775 mol/L, [(NH4)2 Ce(NO3)6/KMnO4] 1 mmol in 50 ml of distilled water; 70 °C

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leading to an optimum value for % G of 52.69 % and for % E of 71.63 % wasobtained at 300 min respectively.

FT-IR analysis

FTIR spectra of starch, polyacrylic acid and St-g-PAA are shown in Fig. 7 which giveinformation regarding grafting of AA on starch. The FTIR spectrum of poly (AA)showed (Fig. 7b) a strong band at 1,730 cm−1 corresponding to the vibrationstretching of carboxylic acid groups (C0O) and the FTIR of St-g-PAA showed(Fig. 7c) the formation of peak of C0O ester group 1,722 cm−1 indicatecondensation between the –COOH group of acrylic acid and –OH group of starchand formation of -COO group shows clear indication of grafting of acrylic acid withstarch. The medium peak at 2,930 cm−1 is due to C-H stretching of CH2 group and1,656 cm−1 are assigned to C-O-C glucosidic linkage and at 1,121 cm−1 correspondsto the stretching vibration of –C-O group respectively.

Thermal analysis

Thermo gravimetric analysis (TGA) is a simple and accurate method for studying thedecomposition pattern and the thermal stability of polymers. Figure 8 shows threestep thermograms. The first stage occurs due to loss of moisture [12] between 50 °Cto 150 °C and displays about 10.6, 16.9 and 9.3 % weight loss. The second partrepresents the maximum weight loss due to thermal degradation of the sample and itoccurs with in temperature ranging from 200 °C to 350 °C with weight loss of 43.9,31.9 and 49.5 % respectively. Third part represents the final stage of decompositionwhich is due to the formation and evaporation of some volatile compounds with intemperature ranging from 500 °C with 20.5 and 17.6 % weight loss in Fig. 8b and cbut in Fig. 8a at 400 °C with 6.4 % weight loss was obtained. At 550 °C char yield of16.8 and 22.7 % was obtained in Fig. 8b and c but in Fig. 8a at 450 °C char yield

Fig. 7 FTIR spectra of (a) starch (b) polyacrylic acid (c) starch-g-PAA

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26.1 % was obtained. It shows that the graft polymerization reaction is generallyreducing the crystalline regions of starch and increase the amorphous region [13] andthe crystalline structures are thermally degraded at higher temperature compared toamorphous structure [14, 15].

Morphological study

Figure 9 shows starch surface was dispersed heterogeously among the polyacrylicmatrix and rugged surface was observed that all fragments of these surfaces are

Fig. 8 TGA graphs of starch-g-PAA

Fig. 9 SEM image of starch-g-poly(AA)

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rough, spherical, rod in shape. The globular structure of polyacrylic acid leading tomodification of starch surface can be clearly distinguished. Researcher [16, 17] haveproved that the grafted polyacrylic acid layer give rise to a rougher starch surface dueto an increase surface area for bonding and mechanical interlocking. It was observedthat the surface of the grafted starch is highly rough in comparison with ungraftedstarch which is attributed to the high graft density. The highly rough surface could beused for the interfacial interaction with various monomers like ions, enzymes,biologically active surface.

Electrical conductivity measurement

The electrical conductivity of the graft copolymer was determined by four point probemethod [18] where resistively was measured by varying the frequency. The graph ofconductivity versus frequency were evaluated and presented in the graph which isshown in Fig. 10. The observation show that graft copolymer is mild conducting innature with the conductivity values lies between 10−5 and 10−7 S/cm for all the graftcopolymer.

Conclusion

The graft copolymers were prepared with the help of acrylic acid onto starch in thepresence of ceric ammonium nitrate and potassium permanganate as redox initiatorhas been found to have physico-chemical as will as morphological impact. Differentreaction conditions such as starch, reaction time, temperature, initiator and themonomer concentration was varied and found to be a great influence on graftcopolymerization. In the present work graft copolymers were synthesized by varyingthe concentration of monomers, starch, redox initiator, temperature and period ofpolymerization. The swelling properties of graft copolymer in distilled and salinewater were evaluated which shows that the polymer increases its weight to about448.8 % when checked with distilled water but its percentage decrease upto 90.9 % insaline water absorption. In FTIR, the peak nearly at 1,730 cm−1 shows the formation

Fig. 10 Electrical conductivity of starch-g-PAA

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of ester linkage of starch with polyacrylic acid and broad band of carboxylic group at3,500–2,500 cm−1 indication the formation graft copolymer. The SEM image showsthe heterogeneous appearances of two phases, the rod/spherical shaped object is dueto starch grafted on the homogeneous poly acrylic acid surface as some granularformation. TGA results show that graft copolymers are more stable than purepolymer. Electrical conductivity results show that all graft copolymer aresemiconducting in nature.

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