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Free radical graft copolymerization of methacrylamide onto agar
PERGAMON
European Polymer Journal 35 (1999) 1237±1243
Free radical graft copolymerization of methacrylamide onto agar V.D. Athawale *, M.P. Padwaldesai Department of Chemistry, University of Mumbai, Vidyanagari, Mumbai 400 098, India Received 20 May 1998; accepted 9 July 1998
Abstract Graft copolymerization of methacrylamide (MAM) onto agar was studied in an aqueous solution using ceric ammonium nitrate, as an initiator. The grafting reaction was in¯uenced by the reaction time, concentrations of monomer and initiator and material-to-liquor ratio. The grafting reaction has also been studied in the presence of solvents. The maximum percentage add-on (17.28) has been observed at 308C for the concentration of monomer (0.69 mol/L), initiator (20 mmol/L), for the reaction time of 3 h. Acid hydrolysis and infrared (IR) spectroscopy were used for con®rmation of the graft copolymer formation. Further, thermogravimetric analyses (TGA) of agar and a typical graft copolymer were studied. The solubility/swellability and the gelatinization characteristics are reported. # 1999 Published by Elsevier Science Ltd. All rights reserved.
1. Introduction Today, natural polymers are modi®ed suitably such that their inherent properties are enhanced and are thus tailor-made for certain end-use requirements. Graft copolymerization is one of the techniques employed for modifying the chemical properties of a polymer. In the last decade in-depth study has been made [2±13] of the synthesis, characterization and applications of graft copolymers of starch and cellulose. However, very few authors [1] have studied agar graft copolymerization. Agar is the sulfuric acid ester of a linear galactan that is extracted from the seaweed of the Gelidium family. Agar is used as food by orientals, but its main use is as a gel forming agent in media for culturing microorganisms. Natural agar consists principally of the calcium salt of a sulfuric ester of a galactan in which about nine 1-3 links occur for each 1-4 linkage and in which there is one sulfate group for about 53 galactose units.
* Corresponding author.
The present report describes graft copolymerization of methacrylamide (MAM) onto agar, initiated by ceric ammonium nitrate. In this regard no detailed studies have been published so far. The eect of grafting on solubility/swellability, and the gelatinization of agar have also been studied. The eects of alcohols on the copolymerization were also investigated.
2. Experimental 2.1. Materials Agar, supplied by Loba chemie Pvt. Ltd, India was dried at 608C in vacuum and stored over anhydrous CaCl2. Methacrylamide [(LR), Sisco-chem industries India] having purity of 99.5% was distilled under reduced pressure prior to use. Ceric ammonium nitrate (CAN) (J.T. Baker chemical Co. NJ, USA) was dried at 1108C and then used in the form 0.1 M solution prepared in molar HNO3.
0014-3057/99/$ - see front matter # 1999 Published by Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 9 8 ) 0 0 2 0 0 - 6
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V.D. Athawale, M.P. Padwaldesai / European Polymer Journal 35 (1999) 1237±1243
Table 1 Eect of various solvents on the graft copolymerization of methacrylamide onto agara Solvent
% GE
% Add-on
Methanol Ethanol Propanol Butanol
8.46 8.06 7.53 7.30
20.83 19.48 18.43 17.96
a
Reaction conditions: Agar = 1 g; MAM = 0.69 mol/L; [CAN] = 20 mmol/L; time = 180 min; temperature = 308C
2.2. Graft copolymerization [9] 1 g of dried agar and 50 mL of distilled water were charged into a two necked round bottomed ¯ask, in a water bath at 308C. The contents of the ¯ask were stirred and bubbled with nitrogen gas for 10 min to remove the dissolved oxygen. Then, a predetermined amount of CAN was added to the reaction mixture and allowed to react for 10 min, followed by the addition of a known quantity of methacrylamide (MAM) with constant stirring. The nitrogen atmosphere was maintained throughout the reaction period. The experimental details are given in the footnote to Table 1 and Figs. 1±4. After the desired reaction period, the mixture was ®ltered through a Whatman ®lter paper no. 54 and washed repeatedly with water. The product
was dried at 608C in vacuum, for more than 24 h to constant weight. The product consisted of pure graft copolymer as the poly (MAM) formed during the grafting reaction, being water soluble, was removed during ®ltration. The reaction mechanism for the graft copolymerization of methacrylamide onto agar is given in the scheme [6]. While studying the solvent eect, the solvent:water ratio was maintained at 20:80 (v/v) in each case for which maximum grafting eciency was observed. 2.3. Infrared spectroscopy IR spectra of the grafted copolymers were recorded on Shimadzu FTIR-4200 dual beam spectrophotometer in the range 4000±400 cm ÿ 1 using KBr pellets. 2.4. Acid hydrolysis [9] In order to separate the grafted polymer from agar backbone, the agar±g±p(MAM) copolymer was subjected to acid hydrolysis. A clear solution was obtained on re¯uxing the graft copolymer with molar HCl for 1 h. The solution was cooled in an icebath and treated with excess of dimethylformamide (DMF) and the precipitate so obtained was further puri®ed by repeated precipitation from hot water into DMF and was ®nally dried in vacuum at 608C.
Fig. 1. Eect of monomer concentration on the graft copolymerization of methacrylamide onto agar. (W) % GE and (Q) % addon. Reaction conditions : Agar = 1 g; [CAN] = 10 mmol/L; water = 50 mL; time = 180 min, temperature = 308C.
V.D. Athawale, M.P. Padwaldesai / European Polymer Journal 35 (1999) 1237±1243
1239
Fig. 2. Eect of initiator concentration on the graft copolymerization of methacrylamide onto agar. (W) % GE and (Q) % add-on. Reaction conditions : Agar = 1 g; MAM = 0.69 mol/L; water = 50 mL; time = 180 min, temperature = 308C.
Fig. 3. Eect of time on the graft copolymerization of methacrylamide onto agar. (W) % GE and (Q) % add-on. Reaction conditions : Agar = 1 g; MAM = 0.69 mol/L; [CAN] = 20 mmol/L; water = 50 mL; temperature = 308C.
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V.D. Athawale, M.P. Padwaldesai / European Polymer Journal 35 (1999) 1237±1243
Fig. 4. Eect of material-to-liquor ratio on the graft copolymerization of methacrylamide onto agar. (W) % GE and (Q) % addon. Reaction conditions : Agar = 1 g; MAM = 3 g; [CAN] = 20 mmol/L; time = 180 min, temperature = 308C.
2.5. Thermogravimetric analysis The samples of pure agar and graft copolymer were tested on a Mettler TA 4000 TGA system, in the temperature range of 30 to 4508C with a rising rate of 108C/min.
3. Results and discussion 3.1. Evidence for grafting From the IR spectrum of agar±g±p(MAM) copolymer, direct evidence for grafting cannot be obtained because the characteristic band due to -NH stretching of the amide group (MAM) overlaps with that of the OH stretching band (3500±3000 cm ÿ 1) of agar and the C±N stretching band of the amide groups overlaps with the (CH2±O±CH2) etherlinkage (1100 cm ÿ 1) of the agar. However, indirectly the IR spectroscopy can be used for the con®rmation of graft copolymer formation by acid hydrolysis. The precipitate obtained in the acid hydrolysis study was analyzed using IR spectroscopy [2]. The IR spectrum of the dried precipitate was found to be identical with that reported for Poly(MAM). This result unam-
biguously (MAM).
indicated
the
formation
of
agar±g±p
3.2. Grafting parameters [9] The graft copolymerization parameters used in the present study are %GE %add on
weight of polymer grafted 100; weight of monomer charged weight of polymer grafted 100: weight of graft copolymer
The % add-on gives a clear idea about the extent of synthetic polymer present in the graft copolymer. Although the interpretation of the results is based on the % GE, it is equally applicable to % add-on. The agar was partly soluble in water [15]. The exact loss in weight of agar due to solubilization, during the grafting reaction, was measured by carrying out the control reactions thrice. A control reaction is the reaction in which all the experimental conditions are maintained identical for each set of the experiments except for the addition of monomer. The average loss in weight for all the controlled reactions was found to be 0.238 g/1.00 g of agar, which was added to the ®nal weight of the graft copolymer.
V.D. Athawale, M.P. Padwaldesai / European Polymer Journal 35 (1999) 1237±1243
3.3. Eect of monomer concentration The eect of monomer concentration on % add-on and % GE was investigated at 10 mmol/L [CAN], time 3 h and 308C. The change of % add-on and % GE with monomer concentration is presented in Fig. 1. The % add-on increased up to 0.69 mol/L and remained almost constant with further increase in the MAM concentration. On the other hand, % GE decreased with increase in the MAM concentration. As the MAM concentration increases, the diusion of MAM molecules into the agar backbone increases, leading to a higher % add-on. The leveling o of the grafting after saturation could be associated with depletion in the available MAM concentration as well as reduction in the active sites on the agar backbone as the graft copolymerization proceeds. MAM has a high anity for its polymer substrate which means that the copolymerization would occur in the polymer phase to a large extent. Thus, it is likely that homopolymerization occurs, preferentially, over grafting and as a result a decrease in % GE is observed. 3.4. Eect of initiator concentration Graft copolymerization was studied at various initiator concentrations (2 mmol/L±40 mmol/L) at a ®xed monomer concentration (0.69 mol/L) and time 3 h. It has been observed that the % add-on and % GE increased initially on increasing the initiator concentration up to 20 mmol/L, but decreased later as shown in Fig. 2. The increase of % add-on and % GE with increasing initiator concentration may be ascribed to the increase of the active sites on the backbone of the agar arising from the attack of Ce + 4. The retarding eect of % add-on and % GE with initiator concentration beyond 20 mmol/L may be due to (1) predominance of homopolymerization over grafting; (2) termination of growing grafted chains by excess of primary radicals formed from the initiator. 3.5. Eect of time Grafting of agar was carried out at various polymerization times, keeping the monomer and initiator concentrations constant, as shown in Fig. 3. The % add-on and % GE increased gradually with increase in the polymerization time, and level o after 180 min, reaching saturation grafting value. The levelling o of grafting may be attributed to the saturation of the active agar backbone by MAM. 3.6. Eect of material-to-liquor ratio [8] Fig. 4 shows the dependence of the magnitude of grafting on the material-to-liquor ratio. The material-
1241
to-liquor ratio was varied by changing only the quantity of water and keeping the material amount constant [agar (1 g) + monomer (3 g) = 4 g]. The increase in the liquor amount from 5 to 12.5 brings about enhancement in grafting, whereas further increase in the liquor amount decreases the grafting. It is known that the volume of the reaction medium increases as the liquor amount increases. Meanwhile, the amount of the reactant, relatively, decreases and hence dilution occurs. This dilution decreases the collision probability of the reaction ingredients thereby decreasing grafting. Thus, the enhancement in grafting using a liquor amount up to 12.5, suggests that this particular liquor amount oers the most appropriate environment for the collision probability of the reaction ingredients. Higher ratio, on the other hand, decreases this probability and as a result decreased grafting is observed. 3.7. Solvent eect [12] The eects of organic solvents on % add-on and % GE have been studied and are illustrated in the Table 1. The solvents under study are alcohols such as methanol, ethanol, propanol and butanol. The ratio of alcohol:water is maintained at 20:80 (v/v) with ®xed concentration of MAM (0.69 mol/L) and CAN (20 mmol/L) and agar (1 g) at 308C for 3 h. The excellent solubility of alcohol in water enhanced the free radical formation, leading to more diusion and penetration of monomer into the active sites on the agar. The graft yields of MAM were higher for the case of the water±methanol system than for the water±ethanol, propanol, butanol systems. The lower alcohols are very soluble in water, and the solubility diminishes as the molecular weight increases. Their solubility in water is to be expected, since the oxygen atom of the hydroxyl group in the alcohols can form hydrogen bonds with the water molecules. In the lower alcohols, the hydroxyl group constitutes a large part of the molecule, whereas the hydrocarbon character increases with the molecular weight of the alcohol and hence the solubility in water decreases [14]. Therefore, a decreasing trend in the grafting eciency value was observed from methanol to butanol i.e. butanol < propanol < ethanol < methanol. 3.8. Solubility/swellability and gelatinization [2] The solubility of the graft copolymers in various polar and non-polar solvents was qualitatively studied. It can be noted that grafting did not alter the solubility of agar in various solvents under study except in glycerol. The agar swelled in glycerol to some extent though it did not solubilize in it; however, graft copo-
1242
V.D. Athawale, M.P. Padwaldesai / European Polymer Journal 35 (1999) 1237±1243
Fig. 5. The thermogravimetric analyses of agar and agar±g±p(MAM). (- - - - -) agar and (ÐÐ) agar±g±p(MAM).
lymer became insoluble in glycerol. In other words, grafting hampers the swellability of agar in glycerol. It is well known that agar gelatinizes in water at about 458C; however, the aqueous slurries of graft copolymers of agar with MAM heated up to 1008C did not show any gelatinization tendency. It can be thus concluded that grafting of MAM aects the gelatinization behavior of agar. 3.9. Thermogravimetric analysis Fig. 5 shows the TGA plots of agar and agar±g± p(MAM) copolymer (% add-on = 17.28). The curves show that the thermal stability of the graft copolymer was similar to that of the agar. The TGA curves showed three distinct zones, an initial slight loss in weight (up to 120.08C) due to loss of water. Then a rather sharp break (temp range 180.0±400.08C) in each thermogram indicates the onset of a decomposition process involving rapid loss in weight and the ®nal stage (temp range 400.0±450.08C), shows slight curvature in the thermogram.
4. Conclusions Methacrylamide (MAM) can be easily graft copolymerized onto agar using a redox initiator e.g. Ce + 4 in an aqueous medium. The reaction variables such as
monomer, initiator concentration and time played key roles in the graft copolymerization of methacrylamide onto agar. However, in general, the grafting % of MAM onto agar was low, probably due to preferential homopolymerization. The rate of grafting depended upon the miscibility of the alcohols. The thermal properties of agar were less aected by grafting. The grafting of MAM onto agar negatively aected the solubility/swellability and gelatinization properties of agar.
References [1] Alves da silva M, Beddows CG, Gil MH, Guthrie JT, Guibmar AJ, Kotov S, Piedade AP. Radiat Phys Chem 1990;35(1±3):98±101. [2] Athawale VD, Rathi SC. J Polym Mater 1996;13:335±40. [3] Athawale VD, Rathi SC. J Appl Polym Sci 1997;66(7):1399±403. [4] Athawale VD, Rathi SC, Lele V. Eur Polym J 1998;34(2):159±61. [5] Athawale VD, Lele V. Carbohyd Polym. 1998;35:21±7. [6] Mino G, Kaizerman S. J Polym Sci 1960;31:242±3. [7] Giri G, Sahoo PK, Samal RK. J Appl Polym Sci 1990;40:471±83. [8] Mostafa Kh M. J Appl Polym Sci 1995;56:263±9. [9] Mishra BN, Dogra R, Kaur I, Sood D. J Polym Sci Polym Chem Edn. 1980;18:341±4.
V.D. Athawale, M.P. Padwaldesai / European Polymer Journal 35 (1999) 1237±1243 [10] Reyes Z, Rist CE, Russell CR. J Polym Sci Part A-1 1966;4(5):1031±43. [11] Khalil MI, Mostafa Kh M, Hebeish A. Starch/Starke 1990;42:107±11. [12] Samal RK, Dash S, Swain AK. Polym J 1989;21(10):821±9.
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[13] Hinojosa O, Ward TL, Benerito RR. ACS Symp Ser 1983;212:225±36. [14] Finar JL. The fundamental principles. In: Organic chemistry, Vol. 1, 6th ed. London: Longman, 1973. p. 180. [15] Chapman VJ. Seaweeds and their uses, 2nd ed. London: Methuen, 1970. p. 185.
European Polymer Journal 35 (1999) 1237±1243
Free radical graft copolymerization of methacrylamide onto agar V.D. Athawale *, M.P. Padwaldesai Department of Chemistry, University of Mumbai, Vidyanagari, Mumbai 400 098, India Received 20 May 1998; accepted 9 July 1998
Abstract Graft copolymerization of methacrylamide (MAM) onto agar was studied in an aqueous solution using ceric ammonium nitrate, as an initiator. The grafting reaction was in¯uenced by the reaction time, concentrations of monomer and initiator and material-to-liquor ratio. The grafting reaction has also been studied in the presence of solvents. The maximum percentage add-on (17.28) has been observed at 308C for the concentration of monomer (0.69 mol/L), initiator (20 mmol/L), for the reaction time of 3 h. Acid hydrolysis and infrared (IR) spectroscopy were used for con®rmation of the graft copolymer formation. Further, thermogravimetric analyses (TGA) of agar and a typical graft copolymer were studied. The solubility/swellability and the gelatinization characteristics are reported. # 1999 Published by Elsevier Science Ltd. All rights reserved.
1. Introduction Today, natural polymers are modi®ed suitably such that their inherent properties are enhanced and are thus tailor-made for certain end-use requirements. Graft copolymerization is one of the techniques employed for modifying the chemical properties of a polymer. In the last decade in-depth study has been made [2±13] of the synthesis, characterization and applications of graft copolymers of starch and cellulose. However, very few authors [1] have studied agar graft copolymerization. Agar is the sulfuric acid ester of a linear galactan that is extracted from the seaweed of the Gelidium family. Agar is used as food by orientals, but its main use is as a gel forming agent in media for culturing microorganisms. Natural agar consists principally of the calcium salt of a sulfuric ester of a galactan in which about nine 1-3 links occur for each 1-4 linkage and in which there is one sulfate group for about 53 galactose units.
* Corresponding author.
The present report describes graft copolymerization of methacrylamide (MAM) onto agar, initiated by ceric ammonium nitrate. In this regard no detailed studies have been published so far. The eect of grafting on solubility/swellability, and the gelatinization of agar have also been studied. The eects of alcohols on the copolymerization were also investigated.
2. Experimental 2.1. Materials Agar, supplied by Loba chemie Pvt. Ltd, India was dried at 608C in vacuum and stored over anhydrous CaCl2. Methacrylamide [(LR), Sisco-chem industries India] having purity of 99.5% was distilled under reduced pressure prior to use. Ceric ammonium nitrate (CAN) (J.T. Baker chemical Co. NJ, USA) was dried at 1108C and then used in the form 0.1 M solution prepared in molar HNO3.
0014-3057/99/$ - see front matter # 1999 Published by Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 9 8 ) 0 0 2 0 0 - 6
1238
V.D. Athawale, M.P. Padwaldesai / European Polymer Journal 35 (1999) 1237±1243
Table 1 Eect of various solvents on the graft copolymerization of methacrylamide onto agara Solvent
% GE
% Add-on
Methanol Ethanol Propanol Butanol
8.46 8.06 7.53 7.30
20.83 19.48 18.43 17.96
a
Reaction conditions: Agar = 1 g; MAM = 0.69 mol/L; [CAN] = 20 mmol/L; time = 180 min; temperature = 308C
2.2. Graft copolymerization [9] 1 g of dried agar and 50 mL of distilled water were charged into a two necked round bottomed ¯ask, in a water bath at 308C. The contents of the ¯ask were stirred and bubbled with nitrogen gas for 10 min to remove the dissolved oxygen. Then, a predetermined amount of CAN was added to the reaction mixture and allowed to react for 10 min, followed by the addition of a known quantity of methacrylamide (MAM) with constant stirring. The nitrogen atmosphere was maintained throughout the reaction period. The experimental details are given in the footnote to Table 1 and Figs. 1±4. After the desired reaction period, the mixture was ®ltered through a Whatman ®lter paper no. 54 and washed repeatedly with water. The product
was dried at 608C in vacuum, for more than 24 h to constant weight. The product consisted of pure graft copolymer as the poly (MAM) formed during the grafting reaction, being water soluble, was removed during ®ltration. The reaction mechanism for the graft copolymerization of methacrylamide onto agar is given in the scheme [6]. While studying the solvent eect, the solvent:water ratio was maintained at 20:80 (v/v) in each case for which maximum grafting eciency was observed. 2.3. Infrared spectroscopy IR spectra of the grafted copolymers were recorded on Shimadzu FTIR-4200 dual beam spectrophotometer in the range 4000±400 cm ÿ 1 using KBr pellets. 2.4. Acid hydrolysis [9] In order to separate the grafted polymer from agar backbone, the agar±g±p(MAM) copolymer was subjected to acid hydrolysis. A clear solution was obtained on re¯uxing the graft copolymer with molar HCl for 1 h. The solution was cooled in an icebath and treated with excess of dimethylformamide (DMF) and the precipitate so obtained was further puri®ed by repeated precipitation from hot water into DMF and was ®nally dried in vacuum at 608C.
Fig. 1. Eect of monomer concentration on the graft copolymerization of methacrylamide onto agar. (W) % GE and (Q) % addon. Reaction conditions : Agar = 1 g; [CAN] = 10 mmol/L; water = 50 mL; time = 180 min, temperature = 308C.
V.D. Athawale, M.P. Padwaldesai / European Polymer Journal 35 (1999) 1237±1243
1239
Fig. 2. Eect of initiator concentration on the graft copolymerization of methacrylamide onto agar. (W) % GE and (Q) % add-on. Reaction conditions : Agar = 1 g; MAM = 0.69 mol/L; water = 50 mL; time = 180 min, temperature = 308C.
Fig. 3. Eect of time on the graft copolymerization of methacrylamide onto agar. (W) % GE and (Q) % add-on. Reaction conditions : Agar = 1 g; MAM = 0.69 mol/L; [CAN] = 20 mmol/L; water = 50 mL; temperature = 308C.
1240
V.D. Athawale, M.P. Padwaldesai / European Polymer Journal 35 (1999) 1237±1243
Fig. 4. Eect of material-to-liquor ratio on the graft copolymerization of methacrylamide onto agar. (W) % GE and (Q) % addon. Reaction conditions : Agar = 1 g; MAM = 3 g; [CAN] = 20 mmol/L; time = 180 min, temperature = 308C.
2.5. Thermogravimetric analysis The samples of pure agar and graft copolymer were tested on a Mettler TA 4000 TGA system, in the temperature range of 30 to 4508C with a rising rate of 108C/min.
3. Results and discussion 3.1. Evidence for grafting From the IR spectrum of agar±g±p(MAM) copolymer, direct evidence for grafting cannot be obtained because the characteristic band due to -NH stretching of the amide group (MAM) overlaps with that of the OH stretching band (3500±3000 cm ÿ 1) of agar and the C±N stretching band of the amide groups overlaps with the (CH2±O±CH2) etherlinkage (1100 cm ÿ 1) of the agar. However, indirectly the IR spectroscopy can be used for the con®rmation of graft copolymer formation by acid hydrolysis. The precipitate obtained in the acid hydrolysis study was analyzed using IR spectroscopy [2]. The IR spectrum of the dried precipitate was found to be identical with that reported for Poly(MAM). This result unam-
biguously (MAM).
indicated
the
formation
of
agar±g±p
3.2. Grafting parameters [9] The graft copolymerization parameters used in the present study are %GE %add on
weight of polymer grafted 100; weight of monomer charged weight of polymer grafted 100: weight of graft copolymer
The % add-on gives a clear idea about the extent of synthetic polymer present in the graft copolymer. Although the interpretation of the results is based on the % GE, it is equally applicable to % add-on. The agar was partly soluble in water [15]. The exact loss in weight of agar due to solubilization, during the grafting reaction, was measured by carrying out the control reactions thrice. A control reaction is the reaction in which all the experimental conditions are maintained identical for each set of the experiments except for the addition of monomer. The average loss in weight for all the controlled reactions was found to be 0.238 g/1.00 g of agar, which was added to the ®nal weight of the graft copolymer.
V.D. Athawale, M.P. Padwaldesai / European Polymer Journal 35 (1999) 1237±1243
3.3. Eect of monomer concentration The eect of monomer concentration on % add-on and % GE was investigated at 10 mmol/L [CAN], time 3 h and 308C. The change of % add-on and % GE with monomer concentration is presented in Fig. 1. The % add-on increased up to 0.69 mol/L and remained almost constant with further increase in the MAM concentration. On the other hand, % GE decreased with increase in the MAM concentration. As the MAM concentration increases, the diusion of MAM molecules into the agar backbone increases, leading to a higher % add-on. The leveling o of the grafting after saturation could be associated with depletion in the available MAM concentration as well as reduction in the active sites on the agar backbone as the graft copolymerization proceeds. MAM has a high anity for its polymer substrate which means that the copolymerization would occur in the polymer phase to a large extent. Thus, it is likely that homopolymerization occurs, preferentially, over grafting and as a result a decrease in % GE is observed. 3.4. Eect of initiator concentration Graft copolymerization was studied at various initiator concentrations (2 mmol/L±40 mmol/L) at a ®xed monomer concentration (0.69 mol/L) and time 3 h. It has been observed that the % add-on and % GE increased initially on increasing the initiator concentration up to 20 mmol/L, but decreased later as shown in Fig. 2. The increase of % add-on and % GE with increasing initiator concentration may be ascribed to the increase of the active sites on the backbone of the agar arising from the attack of Ce + 4. The retarding eect of % add-on and % GE with initiator concentration beyond 20 mmol/L may be due to (1) predominance of homopolymerization over grafting; (2) termination of growing grafted chains by excess of primary radicals formed from the initiator. 3.5. Eect of time Grafting of agar was carried out at various polymerization times, keeping the monomer and initiator concentrations constant, as shown in Fig. 3. The % add-on and % GE increased gradually with increase in the polymerization time, and level o after 180 min, reaching saturation grafting value. The levelling o of grafting may be attributed to the saturation of the active agar backbone by MAM. 3.6. Eect of material-to-liquor ratio [8] Fig. 4 shows the dependence of the magnitude of grafting on the material-to-liquor ratio. The material-
1241
to-liquor ratio was varied by changing only the quantity of water and keeping the material amount constant [agar (1 g) + monomer (3 g) = 4 g]. The increase in the liquor amount from 5 to 12.5 brings about enhancement in grafting, whereas further increase in the liquor amount decreases the grafting. It is known that the volume of the reaction medium increases as the liquor amount increases. Meanwhile, the amount of the reactant, relatively, decreases and hence dilution occurs. This dilution decreases the collision probability of the reaction ingredients thereby decreasing grafting. Thus, the enhancement in grafting using a liquor amount up to 12.5, suggests that this particular liquor amount oers the most appropriate environment for the collision probability of the reaction ingredients. Higher ratio, on the other hand, decreases this probability and as a result decreased grafting is observed. 3.7. Solvent eect [12] The eects of organic solvents on % add-on and % GE have been studied and are illustrated in the Table 1. The solvents under study are alcohols such as methanol, ethanol, propanol and butanol. The ratio of alcohol:water is maintained at 20:80 (v/v) with ®xed concentration of MAM (0.69 mol/L) and CAN (20 mmol/L) and agar (1 g) at 308C for 3 h. The excellent solubility of alcohol in water enhanced the free radical formation, leading to more diusion and penetration of monomer into the active sites on the agar. The graft yields of MAM were higher for the case of the water±methanol system than for the water±ethanol, propanol, butanol systems. The lower alcohols are very soluble in water, and the solubility diminishes as the molecular weight increases. Their solubility in water is to be expected, since the oxygen atom of the hydroxyl group in the alcohols can form hydrogen bonds with the water molecules. In the lower alcohols, the hydroxyl group constitutes a large part of the molecule, whereas the hydrocarbon character increases with the molecular weight of the alcohol and hence the solubility in water decreases [14]. Therefore, a decreasing trend in the grafting eciency value was observed from methanol to butanol i.e. butanol < propanol < ethanol < methanol. 3.8. Solubility/swellability and gelatinization [2] The solubility of the graft copolymers in various polar and non-polar solvents was qualitatively studied. It can be noted that grafting did not alter the solubility of agar in various solvents under study except in glycerol. The agar swelled in glycerol to some extent though it did not solubilize in it; however, graft copo-
1242
V.D. Athawale, M.P. Padwaldesai / European Polymer Journal 35 (1999) 1237±1243
Fig. 5. The thermogravimetric analyses of agar and agar±g±p(MAM). (- - - - -) agar and (ÐÐ) agar±g±p(MAM).
lymer became insoluble in glycerol. In other words, grafting hampers the swellability of agar in glycerol. It is well known that agar gelatinizes in water at about 458C; however, the aqueous slurries of graft copolymers of agar with MAM heated up to 1008C did not show any gelatinization tendency. It can be thus concluded that grafting of MAM aects the gelatinization behavior of agar. 3.9. Thermogravimetric analysis Fig. 5 shows the TGA plots of agar and agar±g± p(MAM) copolymer (% add-on = 17.28). The curves show that the thermal stability of the graft copolymer was similar to that of the agar. The TGA curves showed three distinct zones, an initial slight loss in weight (up to 120.08C) due to loss of water. Then a rather sharp break (temp range 180.0±400.08C) in each thermogram indicates the onset of a decomposition process involving rapid loss in weight and the ®nal stage (temp range 400.0±450.08C), shows slight curvature in the thermogram.
4. Conclusions Methacrylamide (MAM) can be easily graft copolymerized onto agar using a redox initiator e.g. Ce + 4 in an aqueous medium. The reaction variables such as
monomer, initiator concentration and time played key roles in the graft copolymerization of methacrylamide onto agar. However, in general, the grafting % of MAM onto agar was low, probably due to preferential homopolymerization. The rate of grafting depended upon the miscibility of the alcohols. The thermal properties of agar were less aected by grafting. The grafting of MAM onto agar negatively aected the solubility/swellability and gelatinization properties of agar.
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