Graft Copolymerization of Ethyl Acrylate onto Cellulose Using Ceric

Jalma Maria Klein , Vanessa Silva de Lima , José Manoel Couto da Feira , Rosmary Nichele Brandalise , Maria Madalena de Camargo Forte. Journal of App...
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Biomacromolecules 2002, 3, 1087-1094

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Graft Copolymerization of Ethyl Acrylate onto Cellulose Using Ceric Ammonium Nitrate as Initiator in Aqueous Medium K. C. Gupta,* Sujata Sahoo, and Keerti Khandekar Polymer Research Laboratory, Department of Chemistry, Indian Institute of Technology, Roorkee, Roorkee - 247 667, U.A., India Received May 15, 2002; Revised Manuscript Received July 19, 2002

Ceric ammonium nitrate (CAN) in the presence of nitric acid has been used as efficient initiator for graft copolymerization of the ethyl acrylate onto cellulose at 35.0 ( 0.1 °C. Graft copolymerization of ethyl acrylate onto cellulose has taken place through the radical initiation process. The graft yield and other grafting parameters have been evaluated by varying concentration of ethyl acrylate from 2.5 × 10-1 to 15.0 × 10-1 mol dm-3 and ceric ammonium nitrate from 5.0 × 10-3 to 25.0 × 10-3 mol dm-3 at constant concentration of the nitric acid (8.0 × 10-2 mol dm-3). The rate of graft copolymerization has shown 1.5 order with respect to the concentration of the ceric ammonium nitrate. The graft copolymerization data obtained at different temperatures were used to calculate the energy of activation, which has been found to be 28.9 kJ mol-1 within the temperature range from 20 to 50 °C. The effect of addition of cationic and anionic surfactants on graft copolymerization has also been studied. On the basis of the experimental observations, reaction steps have been proposed and a suitable rate expression for graft copolymerization has been derived. Introduction Graft copolymerization is a useful technique for modifying the properties of the synthetic and natural polymers. Graft copolymers are finding their applications in development of selective permeable membranes1 and outstanding sorption agents2 and in fabrication of drug delivery systems.3 Graft copolymers have been used as stabilizers between grafted and ungrafted polymers.4 The properties of cellulose obtained by blending synthetic polymers do not last long due to the separation of blended synthetic polymers, whereas cellulose obtained by grafting of monomer gives rise to everlasting properties. Although cellulose has good properties, it has some undesirable ones such as low tensile strength, high moisture regain, and low strength against microbial attack, and hence, grafting of synthetic polymers on cellulose eliminates these drawbacks and allows the acquisition of additional properties of grafted polymers without destroying its own properties. The grafting of synthetic polymers may be accomplished by attaching preformed polymers onto cellulose, but the rate and extent of grafting by this method are usually low due to hindered diffusion of preformed polymers. The products obtained by preformed polymers or oligomers show inhomogeneous distribution of grafted chains onto cellulose in comparison to the graft copolymers obtained by copolymerization of monomers onto cellulose. The graft copolymerization of various monomers onto cellulose has been reported using radiation,5,6 redox systems,7,8 transition metal ions,9 and their chelates10 as initiating agencies. The * To whom correspondence may be addressed. E-mail: kcgptfcy@ iitr.ernet.in.

extent of grafting is also affected by the physical structure of cellulose.11,12 Although various monomers such as 4-vinylpyridene,13 alkylacrylates,14,15 styrene,16 and acrylamide17 have been used for cellulose modifications, studies on graft copolymerization of ethyl acrylate onto cellulose are not reported frequently.18 The grafting efficiency of ethyl acrylate onto carboxymethyl cellulose in comparison to other monomers has been reported, but extent of grafting has not been explained by determining other grafting parameters.19 The grafting of monomers onto cellulose has been carried out using different metal ions,9,10 but out of them ceric(IV) ions have shown their potential to form active sites onto cellulose by a single electron-transfer process,20 hence inhibiting the formation of homopolymer, which is usually found in the presence of other metal ions.9 The properties of grafted cellulose also depend on the weight of the grafted chains and their number onto the cellulose. The molecular weight of grafted chains could be controlled by experimental conditions and taking monomers having affinity for grafting onto cellulose; therefore, in these investigations an effort has been made to study the grafting of ethyl acrylate onto cellulose under various experimental conditions and characteristic grafting parameters have been determined in the presence of ceric ammonium nitrate as initiator. Experimental Section Chemicals Used. The cellulose powder (Loba Chemie, Mumbai, India) was washed with methanol, acetone, and deionized water to remove impurities and dried in a vacuum desiccator on calcium chloride. Ethyl acrylate (Central Drug House Pvt. Ltd., Mumbai, India) was washed with 5%

10.1021/bm020060s CCC: $22.00 © 2002 American Chemical Society Published on Web 08/22/2002

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sodium hydroxide and stored below 5 °C after vacuum distillation. The ceric ammonium nitrate (CAN) and nitric acid were reagent grade chemicals (E. Merck, India) and used without further purification. Graft Copolymerization. The graft copolymerization of ethyl acrylate onto purified cellulose has been carried out by adding a calculated amount of cellulose (1.0 g) in a threenecked round-bottom flask containing a known amount of ceric ammonium nitrate in a 100 mL solution of nitric acid. The flask was fitted with an electrically operated stirrer and kept in a water bath maintained at 35.0 ( 0.1 °C. The solution was purged with nitrogen gas for about 30 min before adding ethyl acrylate monomer into the flask to initiate the graft copolymerization. The reaction mixture was stirred at a constant rate to avoid the adverse effect of stirring20 on graft copolymerization. At the end of the reaction, a 1.0% solution of hydroquinone was added in the reaction mixture to stop the graft copolymerization. Finally the reaction mixture was poured into a flask containing an excess amount of methanol, and precipitated crude was filtered with a sintered crucible. The crude was washed repeatedly with water to remove unpolymerized ethyl acrylate and other impurities, which were coprecipitated along with grafted crude. The purified grafted crude was dried under vacuum to constant weight at 50 °C. Extraction of Ungrafted Poly(ethyl acrylate). During graft copolymerization of ethyl acrylate onto cellulose a small amount of ungrafted poly(ethyl acrylate) was also formed which was coprecipitated with poly(ethyl acrylate) grafted cellulose on pouring the reaction mixture into methanol. The coprecipitated poly(ethyl acrylate) was not removed on washing the grafted crude with water; hence it was necessary to remove the ungrafted poly(ethyl acrylate) to obtain the cellulose samples containing only grafted poly(ethyl acrylate) chains. To separate the ungrafted poly(ethyl acrylate), the grafted cellulose crude was Soxhlet extracted for 24 h with benzene to ensure the complete removal of ungrafted poly(ethyl acrylate). The extract was poured in a 1:1 mixture of water in methanol (v/v) and precipitated poly(ethyl acrylate) was filtered. The separated poly(ethyl acrylate) was washed repeatedly with distilled water and dried to constant weight at 50 °C. The weight of poly(ethyl acrylate) thus obtained was used to calculate the total amount of polymer formed (% CT) and to determine the efficiency of grafting (% GE). Extraction of Ungrafted Cellulose. The grafting of ethyl acrylate onto cellulose was expressed as apparent graft yield (% G) and as true graft yield (%GT) by calculating the weight percent of grafted poly(ethyl acrylate) with respect to the amount of cellulose taken initially (1.0 g) in the reaction mixture for grafting and with amount of cellulose on which grafting of poly(ethyl acrylate) has actually been taken placed after graft copolymerization; hence to calculate the true grafting (% GT) and percent cellulose conversion (% CC), the separation of ungrafted cellulose was necessary to obtain these characteristic parameters of graft copolymerization. The ungrafted cellulose was extracted by keeping the grafted crude in cuoxam solution. After 2 h, the crude was filtered and washed with distilled water and finally dried at 60 °C

Gupta et al.

in a vacuum desiccator over phosphorus pentaoxide until constant weight. Extraction of Grafted Poly(ethyl acrylate). To characterize the grafted chains for their molecular weight, the grafted poly(ethyl acrylate) chains were separated by hydrolyzing the poly(ethyl acrylate) grafted cellulose in 72% solution of sulfuric acid at room temperature. At the end of 6 h, the cellulose residue was removed and washed repeatedly with distilled water and dried at 60 °C on phosphorus pentaoxide until constant weight. The sulfuric acid used for hydrolyzing the grafted poly(ethyl acrylate) from cellulose was able to hydrolyze the grafted chains of poly(ethyl acrylate) from the cellulose and did not participated in degradation of grafted poly(ethyl acrylate) chains. This was ensured by keeping poly(ethyl acrylate) homopolymer samples in 72% solution of sulfuric acid for 24 h and determining their molecular weights, which were same within experimental limit of variations. The weight difference between grafted cellulose and sulfuric acid hydrolyzed cellulose has been taken as weight of poly(ethyl acrylate) grafted onto cellulose and used to calculate percent graft conversion (% Cg), graft yield (% G), and other grafting parameters as defined below. Characteristic Grafting Parameters: percent graft yield (% G), weight percent of grafted polymer with respect to initial weight of cellulose; percent true grafting (% GT), weight percent of grafted polymer with respect to actual weight of the cellulose on which grafting has taken place; percent homopolymer (% Chp), weight percent of monomer converted into ungrafted homopolymer; percent conversion (% Cg), weight percent of the monomer converted into grafted polymer; percent cellulose conversion (% Cc), weight percent of the initial cellulose, which is grafted; percent efficiency (% GE), weight percent of total polymer, which is grafted onto the cellulose; grafted frequency (GF), number of grafted polymer chains (Ngp) per chain of the cellulose. Characterization: IR Spectra. The IR spectra of cellulose and grafted cellulose were recorded on KBr pallets using Perkin-Elmer 1600 Fourier transform infrared spectrophotometer. Thermal Studies. Thermal analysis of the cellulose and grafted cellulose has been carried out by recording thermogravimetric (TG) and differential thermogravimetric (DTG) curves using Perkin-Elmer 7.0 system at a heating rate of 10 °C/min under nitrogen atmosphere. The energy of activation for the decomposition of cellulose and poly(ethyl acrylate) grafted cellulose has been calculated with fraction (R) decomposed at different temperatures calculated from DTG curves obtained at a heating rate of 10 °C/min under nitrogen atmosphere and using the equation21 ln ln(1 - R)-1 )

Ea

100 θ+C T RTs f - Ti 2

(1)

where Ti and Tf are the initial and final decomposition temperatures and have been taken as points of deviation from baseline in the DTG curve above the dehydration temperature. The θ is the difference of decomposition temperature (T) and temperature of reference (Ts.). The plot between the

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Figure 2. TG and DTG curves of pure cellulose (;) and poly(ethyl acrylate) grafted cellulose (‚ ‚ ‚) at a heating rate of 10 °C/min.

Figure 1. FTIR spectra of ungrafted (A) and poly(ethyl acrylate) grafted (B) cellulose.

reciprocal of the double logarithmic (1 - R) vs temperature deference (T - Ts) enabled calculation of the value of activation energy of the sample. Molecular Weight Measurements. The molecular weights of the cellulose and poly(ethyl acrylate) grafted chains (PEA) have been estimated by viscometric method using the equations [η]25°C ) 38.5 × 103 cm3 g-1 M h v0.76

(2)

(for cellulose in cadoxen)22 [η]30°C ) 27.7 × 103 cm3 g-1 M h v0.67

(3)

(for PEA in benzene)23 Results and Discussion The graft copolymerization of various alkylacrylates onto cellulose using different initiating agents1-5 has been reported frequently, but investigations for grafting of ethyl acrylate monomer onto cellulose is limited.18 The high affinity of ethyl acrylate for grafting onto cellulose in comparison to methyl acrylate and methyl methacrylate has been attributed to the length of the ester alkyl group,24 which has increased its reactivity for grafting onto cellulose through electronpumping capacity of ester ethyl group. The formation of small amount of homopolymer during graft copolymerization of ethyl acrylate is an indication that ethyl acrylate has more affinity for grafting in comparison to the formation of homopolymer. The IR spectrum of the grafted cellulose (Figure 1B) shows an absorption band at 1750 cm-1 corresponding to an ester carbonyl group (>CO) of the ethyl

acrylate which was initially absent in pure cellulose (Figure 1A). The presence of new absorption band at 1750 cm-1 has provided evidence for grafting of ethyl acrylate onto the cellulose. The thermal stability of the poly(ethyl acrylate) grafted cellulose has been found to be higher in comparison to the pure cellulose as clear from the TG and DTG curves recorded for pure cellulose and grafted cellulose (Figure 2). The initial decomposition temperature (Ti) of pure cellulose on grafting has shifted to 258 °C from its original decomposition temperature of 230 °C. The temperature of maximum decomposition rate (Tmax) in grafted cellulose has also shown a shift of 17 °C on the higher side in comparison to temperature of maximum decomposition rate (Tmax) in pure cellulose (340 °C). The final decomposition temperature (Tf) of the grafted cellulose has also increased to 434 °C from 410 °C decomposition temperature of pure cellulose. The decomposition temperatures (Td) for fixed weight loss (20%) of cellulose and grafted cellulose were determined by TG curves (Figure 2) which were found to be 302 °C and 315 °C, respectively, which has clearly indicated that the grafting of poly(ethyl acrylate) has increased the thermal stability of cellulose in comparison to pure cellulose. This has further verified from the energy of activation of decomposition of cellulose and grafted cellulose calculated with decomposed fraction (R) obtained from DTG curves (Figure 2) and drawing a plot between ln ln(1 - R)-1 vs T - Ts using eq 1 as shown in Figure 3. The energy of activation for the decomposition of poly(ethyl acrylate) grafted cellulose has been found to be higher (223.70 kJ mol-1) in comparison to pure cellulose (194.24 kJ mol-1). These thermal data have provided evidence for the grafting of poly(ethyl acrylate) and have also shown a increased thermal stability of cellulose after grafting. To optimize the experimental conditions for maximum graft yield, the graft copolymerization of the ethyl acrylate has been carried out as a function of concentration variation of monomer, initiator, and reaction temperature. The data collected during these experimental variations were used to explain the mechanism of graft copolymerization and

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Gupta et al. Table 1. a. Effect of [Ethyl Acrylate] on Grafting Parameters at 35 °Ca [EA] × 10 mol dm-3

% GT

% CT

% Cg

% Chp

% Cc

% GE

2.5 5.0 7.5 10.0 15.0

380 552 769 1034 1006

38.2 44.2 53.5 61.7 46.0

27.2 32.2 40.0 46.7 28.0

11.0 12.0 13.5 15.5 18.0

35.0 40.0 46.0 50.0 47.0

71.2 72.8 74.7 75.6 54.2

b. Effect of [Ethyl Acrylate] on Grafting Parameters at 35 °Ca [EA] × 10 mol dm-3

GF

Ngp × 106

Nhp × 106

2.5 5.0 7.5 10.0 15.0

1.47 2.87 4.43 5.93 5.65

2.06 4.60 8.15 11.90 10.60

2.25 4.61 7.13 10.00 15.80

M h v × 10-3 g mol-1 GP HP 330 350 368 392 395

122 130 142 150 170

a [CAN] ) 10.0 × 10-3 mol dm-3, [HNO ] ) 8.0 × 10-2 mol dm-3, [cell] 3 ) 1.0 g, and time ) 120 min.

Figure 3. Plots for determination of energy of activation of decomposition of cellulose (A) and poly(ethyl acrylate) grafted cellulose (B) using DTG data.

Figure 4. Effect of [EA] on graft yield (% G): [CAN] ) 10.0 × 10-3 mol dm-3; [HNO3] ) 8.0 × 10-2 mol dm-3; [cell] ) 1.0 g; time ) 120 min; temperature ) 35 °C.

to propose reaction steps to derive an equation showing dependence of rate of grafting on concentration of reactants used in the reaction mixture. Effect of Ethyl Acrylate Concentration. The graft copolymerization has been recorded at different concentrations of the ethyl acrylate ranging from 2.5 × 10-1 to 15.0 × 10-1 mol dm-3 at constant concentration of the ceric ammonium nitrate (10.0 × 10-3 mol dm-3), nitric acid (8.0 × 10-2 mol dm-3) at 35.0 ( 0.1 °C. The graft yield (% G) has shown a linear increase up to 10.0 × 10-1 mol dm-3 concentration of the ethyl acrylate and has shown a decreasing trend on increasing the concentration of ethyl acrylate beyond 10.0 × 10-1 mol dm-3 (Figure 4). Similar trends have been shown by true grafting (% GT), graft conversion (% Cg), and cellulose conversion (% Cc) as clear from the data shown in Table 1a. The efficiency (% GE) and frequency of grafting (GF) have also increased up to 10.0 × 10-1 mol dm-3 concentration of the ethyl acrylate (Table 1). The formation of homopolymer (% Chp) continued to increase beyond 10.0 × 10-1 mol dm-3 concentration of ethyl acrylate

(Table 1a). The decreasing trend in graft yield (% G) and other grafting parameters at high concentration of ethyl acrylate (>10.0 × 10-1 mol dm-3) has been attributed to the increase in viscosity of the medium which has retarded the rate of diffusion of ethyl acrylate molecules onto the cellulose surface; hence the graft yield has decreased. The steric hindrance created by grafted chains is another factor, which caused the retardation in rate of graft copolymerization at high concentration of ethyl acrylate. The decreasing trend in cellulose conversion (% Cc) and frequency of grafting (GF) at high concentration of ethyl acrylate (>10.0 × 10-1 mol dm-3) is evidence for steric hindrances created by the grafted chains and viscosity of the medium which prevented the monomer molecules to reach the reactive sites to create new grafted chains onto cellulose. The high concentration of ethyl acrylate has also reduced the degree of swelling of the cellulose; hence grafting has taken placed more at the cellulose surface than inner inside of the cellulose matrix. At high concentration of monomer (>10.0 × 10-1 mol dm-3), the rate of grafting decreased, monomer molecules were consumed more in the formation of homopolymer (% Chp) than in formation of grafted chains, and the percentage of homopolymer continued to increase beyond 10.0 × 10-1 mol dm-3 concentration of ethyl acrylate. The molecular weights of grafted and ungrafted chains continued to increased beyond 10.0 × 10-1 mol dm-3 concentration of ethyl acrylate, which has indicated that the frequency of chains termination was not increased on increasing the concentration of the monomer in the reaction mixture (Table 1b). The rate of grafting (Rp) at different concentrations of the ethyl acrylate has been used to determine the order of reaction by drawing a log-log plot (Figure 5) between rate of graft copolymerization (Rp) and concentration of ethyl acrylate. The slope of plot (Figure 5) has been found to be 1.54 indicating a different mechanism of graft polymerization in comparison to routine mechanism in which rate of graft copolymerization is usually proportional to the concentration of the monomer molecules. Effect of Ceric(IV) Ion Concentration. The graft copolymerization of ethyl acrylate onto cellulose has been studied

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a. Effect of [Ceric Ammonium Nitrate] on Grafting Parameters at 35 °Ca [CAN] × 103 mol dm-3

% GT

% CT

% Cg

% Chp

% Cc

% GE

5.0 10.0 15.0 20.0 25.0

486 552 609 636 597

32.2 44.2 53.4 61.4 57.0

22.2 32.2 39.6 46.4 40.0

10.0 12.0 13.8 15.4 17.0

36 40 42 45 43

68.9 72.8 74.2 75.1 70.2

b. Effect of [Ceric Ammonium Nitrate] on Grafting Parameters at 35 °Ca [CAN] × mol dm-3

103

Figure 5. Double logarithmic plot between Rp vs [EA]: [CAN] ) 10.0 × 10-3 mol dm-3; [HNO3] ) 8.0 × 10-2 mol dm-3; [cell] ) 1.0 g; time ) 120 min; temperature ) 35 °C.

5.0 10.0 15.0 20.0 25.0

GF

106Ngp

106Nhp

2.11 2.87 3.45 4.22 3.85

3.04 4.60 5.80 7.60 6.62

3.73 4.61 5.31 6.00 7.08

M h v × 10-3 g mol-1 GP HP 365 350 341 305 302

134 130 127 125 120

a [EA] ) 5.0 × 10-1 mol dm-3, [HNO ] ) 8.0 × 10-2, [cell] ) 1.0 g, 3 time ) 120 min.

Figure 6. Effect of [CAN] on graft yield (% G): [EA] ) 5.0 × 10-1 mol dm-3; [HNO3] ) 8.0 × 102 mol dm-3; [cell] ) 1.0 g; time ) 120 min; temperature ) 35 °C.

at different concentrations of the ceric ammonium nitrate ranging from 5.0 × 10-3 to 25.0 × 10-3 mol dm-3 at constant concentration of nitric acid (8.0 × 10-2 mol dm-3) and ethyl acrylate (5.0 × 10-1 mol dm-3). On varying the concentration of ceric ammonium nitrate, the graft yield (% G) has shown an increasing trend up to 20.0 × 10-3 mol dm-3 and then a sharp decrease on further increasing the concentration of ceric ammonium nitrate beyond 20.0 × 10-3mol dm-3 (Figure 6). A similar trend has been shown by true grafting (% GT), graft conversion (% Cg), and cellulose conversion (% Cc) as shown in Table 2a.The efficiency (% GE) and frequency of grafting (GF) have also shown agreement with other grafting parameters (Table 2b). The increasing trends in grafting parameters up to 20.0 × 10-3 mol dm-3 concentration of ceric ammonium nitrate has been attributed to the formation of active ceric(IV) ions in the presence of excess nitric acid25 in the reaction mixture (8.0 × 10-3 mol dm-3) in comparison to ceric ammonium nitrate. The produced ceric(IV) ions were consumed in the formation of active sites onto cellulose, which is apparent from the increasing trend in the cellulose conversion (% Cc) and the frequency of grafting (GF). The decreasing trend in grafting parameters at higher concentration of ceric ammonium nitrate (>20.0 × 10-3 mol dm-3)

at constant concentration of nitric acid (8.0 × 10-2 mol dm-3) has been due to the decrease in ratio of nitric acid to ceric ammonium nitrate; hence a hydrated form of ceric(IV) ions were produced, which were not able to produce active sites onto the cellulose backbone. This has resulted in an appreciable decrease in graft conversion (% Cg) and other grafting parameters (Table 2). The percent increase in homopolymer (% Chp) at relatively high concentration of ceric ammonium nitrate (>20.0 × 10-3 mol dm-3) is an indication that hydrated ceric(IV) ions participate preferably in formation of homopolymer than in formation of graft polymer (Table 2a). The decreasing trend in molecular weight of grafted and ungrafted poly(ethyl acrylate) chains on increasing the concentration of ceric ammonium nitrate has suggested the ceric(IV) ion participation in a chain-termination process (Table 2b).This premature termination of growing chains has resulted in increase in number of ungrafted chains (Nhp) as shown in Table 2b. These data have suggested that the extent of grafting would be high only at high ratio of nitric acid to ceric ammonium nitrate otherwise, grafting of ethyl acrylate onto cellulose would be low. The rate of graft copolymerization (RP) calculated at different concentrations of ceric ammonium nitrate was used to determine the order of reaction with respect to the concentration of ceric ammonium nitrate as shown in Figure 7. The slope of log-log plot drawn between rate of graft copolymerization (RP) vs concentration of ceric ammonium nitrate has been found to be 0.55, indicating half order with respect to the concentration of ceric ammonium nitrate. This half order dependence on concentration of ceric ammonium nitrate has suggested the termination of grafted and ungrafted chains by bimolecular coupling of two growing chains. Effect of Temperature. The graft copolymerization of ethyl acrylate onto cellulose has also been studied by varying the reaction temperature from 20 °C to 60 °C at constant concentration of ethyl acrylate (5.0 × 10-1 mol dm-3), ceric ammonium nitrate (10.0 × 10-3 mol dm-3), and nitric acid

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Gupta et al. Table 3. a. Effect of Temperature on Grafting Parametersa temp (°C)

% GT

% CT

% Cg

% Chp

% Cc

% GE

20 30 35 40 50 60

500 550 552 581 568 564

28.4 38.0 44.2 54.0 51.8 48.2

18.4 26.8 32.2 40.0 34.8 29.2

10.0 11.2 12.0 14.0 17.0 19.0

32 36 40 44 41 37

64.8 70.5 72.8 74.0 67.1 60.6

b. Effect of Temperature on Grafting Parametersa

Figure 7. Double logarithmic plot between Rp vs [CAN]: [EA] ) 5.0 × 10-1 mol dm-3; [HNO3] ) 8.0 × 10-2 mol dm-3; [cell] ) 1.0 g; time ) 120 min; temperature ) 35 °C.

temp (°C) 20 30 35 40 50 60

GF

106Ngp

106Nhp

1.98 2.61 2.87 3.32 3.14 2.94

2.54 3.76 4.60 5.84 5.15 4.35

3.57 4.18 4.61 5.51 6.91 7.92

M h v × 10-3 g mol-1 GP HP 362 356 350 342 338 335

140 134 130 127 123 120

a [EA] ) 5.0 × 10-1 mol dm-3, [CAN] ) 10.0 × 10-3 mol dm-3, [HNO ] 3 ) 8.0 × 10-2 mol dm-3, [cell] ) 1.0 g, time ) 120 min.

Figure 8. Effect of temperature on graft yield (% G): [EA] ) 5.0 × 10-1 mol dm-3; [CAN] ) 10.0 × 10-3 mol dm-3; [HNO3] ) 8.0 × 10-2 mol dm-3; [cell] ) 1.0 g; time ) 120 min.

(8.0 × 10-2 mol dm-3). To compare the effect of temperature, the graft yield (% G) recorded at different temperatures was used to calculate grafting parameters as shown in Figure 8 and Table 3. The graft yield (% G), true grafting (% GT), graft conversion (% Cg), and efficiency of grafting (% GE) have shown an increasing trend up to 40 °C as clear from the data shown in Table 3a and Figure 8. The cellulose conversion (% Cc) as a function of temperature has shown a complete agreement with number (Ngp) and frequency (GF) of grafted chains as clear from the data shown in Table 3. On increase of the temperature, the kinetic energy of monomer molecules has increased which ultimately has increased the concentration of monomer molecules nearby to the active sites onto the cellulose due to the enhanced rate of diffusion of monomer molecules from the reaction mixture to the cellulose. This positive effect of temperature has decreased on further increasing the reaction temperature beyond 40 °C due to the substantial increase in the rate of chain transfer and chain termination reactions between grafted chains and monomer molecules. This is evident from the observed increasing trend of number of homopolymer chains (Nhp) beyond 40 °C and decreasing trend in molecular weight of grafted chains as well as ungrafted poly(ethyl

Figure 9. Arrhenius plot between log kp vs 1/T: [EA] ) 5.0 × 10-1 mol dm-3; [CAN] ) 10.0 × 10-3 mol dm-3; [HNO3] ) 8.0 × 10-2 mol dm-3; [cell] ) 1.0 g; time ) 120 min.

acrylate) chains (Table 3b). The energy of activation of graft copolymerization (∆Ea) has been determined from an Arrhenius plot (Figure 9) drawn between the rate constant of graft copolymerization (kp) and reaction temperature, which has been found to be 28.9 kJ mol-1. The low energy of activation of graft copolymerization of ethyl acrylate onto cellulose is an indication for high affinity of ethyl acrylate for grafting onto cellulose. Effect of Additives. The graft copolymerization of ethyl acrylate has also been studied in the presence of different additives as shown in Figure 10. The effect of additives on graft copolymerization has been compared with a controlled experiment as shown in Figure 10. The addition of sodium lauryl sulfate (NaLS) in the reaction mixture has shown a decreasing effect on rate of grafting and maximum graft yield. On addition of sodium lauryl sulfate (NaLS) in the reaction mixture, a layer of lauryl sulfate anion was formed onto the cellulose surface, which has prevented the

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Reaction Mechanism. The reaction mechanism is as follows: radical formation: K

Cell + Ce(IV) 798 Complex

(4)

k4

Complex + EA 98 Cell• + Ce(III) + EAH+

(5)

initiation: k1

Cell• + EA 98 Cell-EA•-

(6)

propagation: kp

Cell-EA• + EA 98 Cell-EA2•10-1

dm-3;

Figure 10. Effect of additives: control [EA] ) 5.0 × mol [CAN] ) 10.0 × 10-3 mol dm-3; [HNO3] ) 8.0 × 10-2 mol dm-3; [cell] ) 1.0 g; temperature ) 35 °C; [NaLS] ) 8.0 × 10-3 mol dm-3; [CTAB] ) 2.0 × 10-3 mol dm-3; [MeOH] ) 5% (v/v).

accumulation of ethyl acrylate molecules from reaching active sites on cellulose; hence the rate of graft copolymerization and maximum graft yield have shown decreasing trends. The addition of cetyltrimethylammoniumbromide (CTAB) has shown an increasing effect on graft copolymerization due to the formation of cationic layer at cellulose surface which has facilitated the accumulation of ethyl acrylate molecules nearby to active sites onto cellulose; hence the rate of grafting and maximum yield have increased as shown in Figure 10. Thus on addition of surfactants, the Gouy and Chapman type of double layer at the cellulose surface has formed either with anions of sodium lauryl sulfate or with cations of cetyltrimethylammoniumbromide which has controlled the concentration of ethyl acrylate molecules in diffused portion of the electrical double; hence the rate of graft copolymerization was controlled by the time average potential of the electrical double layer.26 The addition of methanol in the reaction mixture has shown a decreasing trend in the graft copolymerization of the ethyl acrylate onto the cellulose. The decreasing effect of addition of methanol on graft copolymerization has been attributed to the formation of the inactive radicals by the interactions of ceric(IV) ions with methanol which were incapable of generating active sites onto cellulose; hence the rate of graft copolymerization and maximum graft yield have shown a decreasing trend in the presence of methanol. The decreasing effect of methanol is also attributed to the chain transfer reactions that occurred between growing chains and methanol molecules. The methoxy radicals generated from the chain transfer reactions with added methanol were incapable of propagating polymer chains or of producing active sites onto the cellulose; hence the extent of grafting was decreased. The addition of methanol has also decreased the degree of swelling of cellulose, which is also a significant reason for the decreasing effect of methanol on graft copolymerization of ethyl acrylate onto cellulose. Thus on the basis of the observed experimental data for graft copolymerization of ethyl acrylate onto cellulose in the presence of ceric ammonium nitrate, the following reaction steps have been proposed.

(7)

kp

Cell-(EA)n-1-EA• + EA 98 Cell-(EA)n-EA•- (8) termination: kκ

Cell-(EA)•n- + -m•(EA)-Cell 98 Grafted cellulose

(9)

Considering reaction steps 4-9 involved in graft copolymerization of ethyl acrylate onto cellulose, the following rate expression for the overall rate of graft copolymerization (Rp) has been derived

( )

RP ) kP

kdK 2ktc

1/2

[EA]3/2[cell]1/2[CAN]1/2

(10)

where K and kd are the equilibrium constant for the complex formation between cellulose and ceric ions and rate constant for the decomposition of the complex. ki, kp, and ktc are the rate constants for the initiation, propagation, and termination reactions taking placed during graft copolymerization of ethyl acrylate onto cellulose in the presence of ceric ammonium nitrate. The rate expression (eq 10) explains the dependence of rate of graft copolymerization on concentration of ethyl acrylate, ceric ammonium nitrate in agreement with experimental observations; hence it has provided strong support for proposed steps for graft copolymerization of ethyl acrylate onto cellulose. Conclusion The ceric ammonium nitrate has been found to be an efficient initiator for graft copolymerization of ethyl acrylate onto cellulose and has reduced sufficiently the formation of ungrafted poly(ethyl acrylate) by forming active sites onto cellulose only. The formation of ungrafted homopolymer in the reaction mixture was due to the transfer reactions operating between growing chains and monomer molecules. The poly(ethyl acrylate) grafted cellulose has been found to be thermally more stable than pure cellulose as clear from the thermal decomposition temperatures and energy of activation of decomposition of grafted samples determined by DTG data. The extent of grafting of ethyl acrylate onto cellulose has shown dependence on feed molarity, concentration of ceric ammonium nitrate, and temperature of graft

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copolymerization. The low energy of graft copolymerization of ethyl acrylate onto cellulose and formation of small amount of homopolymer were indications of high affinity of ethyl acrylate monomer for grafting onto the cellulose under studied conditions of graft copolymerization. Reaction steps for graft copolymerization were proposed and rate expression was derived which was in agreement to the experimental results. The effect of addition of surfactants on graft copolymerization has also been studied, and cationic surfactant has been shown to have a positive effect which has further provided support for the electron pumping behavior of ester ethyl groups of ethyl acrylate producing negative polarity on monomers to facilitate its accumulation nearby to the surface of cellulose to increase the extent of grafting. Acknowledgment. The financial assistance from UGC, New Delhi, is thankfully acknowledged. Ms. Sujata Sahoo is thankful to CSIR, New Delhi, for the award of the SRF. The authors are also thankful to I.I.T. Roorkee for providing facilities to complete this work. References and Notes (1) Sato, T.; Nambu, Y.; Endo, T. J. Polym. Sci., Part C: Polym. Lett. 1988, 26, 341. (2) Suzuki, M. J. Controlled Release 1991, 17, 259. (3) Sato, T. J. Appl. Polym. Sci. 1991, 45, 259. (4) Nishioka, N.; Tabata, M.; Saito, M.; Kishigami, N.; Iwamoto, M.; Uno, M. Polym. J. 1997, 29, 508. (5) Jianquin, L.; Maolin, Z.; Hongfei, H. Radiat. Phys. Chem. 1999, 55, 35.

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