Graft Copolymerization of Acrylonitrile and Ethyl Methacrylate

Biomacromolecules 2001, 2, 239-247

239

Graft Copolymerization of Acrylonitrile and Ethyl Methacrylate Comonomers on Cellulose Using Ceric Ions K. C. Gupta* and Sujata Sahoo Polymer Research Laboratory, Department of Chemistry, University of Roorkee, Roorkee-247667, U.P., India Received October 5, 2000; Revised Manuscript Received November 23, 2000

The graft copolymerization of acrylonitrile and ethyl methacrylate from their binary mixtures onto cellulose has been carried out in the presence of Ce(IV) ions in acidic media at 30 ( 0.1 °C. The graft copolymerization has shown an increasing trend on increasing the feed molarity of the comonomers and on increasing the reaction time. The analysis of grafted cellulose prepared with different feed compositions (fAN) has clearly indicated the existence of the synergistic effect of ethyl methacrylate on acrylonitrile which normally shows poor affinity for grafting on cellulose while used individully. The elemental analysis and IR data were used to determine the composition of grafted copolymers and reactivity ratios (r1, r2) of the monomers using the Mayo and Lewis method. The reactivity ratios for acrylontrile and ethyl methacrylate have been found to be 0.68 and 1.15, respectively, which supported the alternate arrangement of the monomer blocks of individual monomers in the grafted chains. The rate of grafting has shown a square dependence on the concentration of the comonomers within the studied range of feed molarity from 0.5 to 2.5 mol dm-3. The grafting data were used to calculate the number of grafted chains (Ng), frequency of grafting (GF) per molecule of the cellulose and efficiency of grafting (GE). The variation in these parameters as a function of feed molarity, feed composition, and ceric ion concentration has been determined. The rate of ceric ion disappearance as a function of feed molarity and reaction time has provided strong support to consider the participation of ceric ions in the formation of free radicals at the backbone of the cellulose. On the basis of the experinental observations, reaction steps for grafting of monomers on cellulose have been proposed. Introduction Modification of cellulose properties by graft copolymerization of vinyl monomers has been a subject of extensive research for the last 2 decades. Grafting provides a significant route to alter the physical and chemical properties of the cellulose for specific end uses.1-3 The grafting of various monomers has been investigated extensively using different techniques of graft copolymerization, but grafting of vinyl monomers from their mixtures on synthetic and natural polymers has been reported rarely,4-7 and in most cases, the concurrent formation of homopolymers and copolymers predominates over graft copolymerization. The grafting of monomers from their mixtures is an useful technique to introduce dual properties in the cellulose and influence the extent of grafting of monomer which show less tendency for grafting. The grafting from the mixtures of vinyl monomers is further important since chains of desired compositions and lengths can be introduced by manipulating the grafting conditions and selecting monomers of desired properties. Cellulose is a natural polymer with many useful properties; however, it lacks some properties exhibited by synthetic polymers. Various methods have been developed for the grafting of vinyl monomers onto cellulose.8-10 Radiation-induced grafting on cellulose fiber11 using binary mixtures of acrylic acid and styrene has been investigated, * Corresponding author. E-mail: [email protected]. Fax: +911332-73560. Telephone: +91-1332-85325 (O), -85279 (R).

and the effects of comonomer compositions and solvent are discussed. Matveera et al.,4 have studied the graft copolymerization on cellulose using binary mixtures of vinyl tetrazole in the presence of a Fe2+-H2O2-hydrazene redox system, but in these investigations, the extent of homopolymerization of the monomers is reported to be greater in comparison to the graft copolymerization. Therefore, grafting must be carried out with a initiator which is capable of creating active sites on the cellulose and inhibits the homocopolymerization and homopolymerization. From the literature survey, it is clear that ceric ions are capable of forming radicals easily at the backbone of the cellulose12 in the presence of a suitable amount of the acid.13 However, the grafting with ceric ions is reported14 to be less than five molecules of grafted polymer chain per molecule of the cellulose. Efforts have been made to increase the grafting frequency by removing the unabsorbed ceric ions from the polymerization mixture prior to the addition of the comonomers, but results of these studies have not shown a marked increase in the grafting frequency on cellulose backbone.15 It is generally realized that grafting depends on the physical state of the cellulose.16 The grafting mainly occurs in the amorphous region of the cellulose, which facilitates the diffusion of reactants to increase the extent of grafting. The grafting of monomer mixtures on cellulose is normally carried out in an aqueous medium, in which reactants partitioned easily between cellulose and continuous aqueous phase due to swelling of cellulose in water. Therefore, in the present investigation, an attempt has

10.1021/bm000102h CCC: $20.00 © 2001 American Chemical Society Published on Web 01/24/2001

240

Biomacromolecules, Vol. 2, No. 1, 2001

been made to study the grafting of ethyl methacrylateacrylonitrile comonomers on cellulose using ceric ammonium nitrate as an initiator. The ethyl methacrylate is reported to have antimicrobial properties, and grafting on cellulose in combination with acrylonitrile has not been reported in the literature. Experimental Section Ethyl methacrylate (Fluka, India) and acrylonitrile (E. Merck, India) were purified by extracting with aqueous sodium chloride-sodium hydroxide solution and dried over sodium sulfate. The stabilizer-free monomers were vacuum distilled and stored below 5 °C. The cellulose powder (Loba Chemie, India) was washed with methanol, acetone, and deionized water and vacuum-dried. Ceric ammonium nitrate (E. Merck, India) and nitric acid were of reagent grade and used without further purifications. The nitrogen gas was passed through alkaline pyragallol, sulfuric acid, and potassium hydroxide solution before it was passed into the reaction mixtures. Graft Copolymerization. Grafting of acrylonitrile and ethyl methacrylate onto cellulose was carried out by taking 1.0 g of purified cellulose sample (M h n ) 2.5 × 105) in a three necked round-bottom flask containing 50 mL of 10.0 × 10-2 mol dm-3 solution of nitric acid maintained at 30 ( 0.1 °C. A known amount of ceric ammonium nitrate (CAN) was added in the flask to keep a 12.0 × 10-3 mol dm-3 concentration of ceric ammonium nitrate in the reaction mixture. The solution was purged with nitrogen for about 20 min so that primary radicals formed at cellulose backbone by the interactions of ceric ions do not die out due to the atmospheric oxygen. After 20 min of ceric ammonium nitrate (CAN) addition, the comonomer mixture was added dropwise in the flask to maintain overall molarity of 1.5 mol dm-3 (7.50 g of acrylonitirle and 5.0 g of ethyl methacrylate) in the reaction mixture. The mole fraction of acrylonitrile (fAN) in the reaction mixture was maintained at 0.60. After addition of monomers, graft copolymerization was allowed to progress at constant speed of stirring with a continuous supply of nitrogen. At a specified time interval of grafting (120 min), the reaction was arrested by adding 2.0 mL of a 5.0% solution (w/v) of hydroquinone and poured into excess nonsolvent (methanol) to precipitate the grafted cellulose. The grafted crude thus obtained was filtered, washed repeatedly with hot and cold water to remove trapped unpolymerized monomers from the cellulose, and finally dried to constant weight in a vacuum oven at 50 °C. Extraction of Homopolymers (Hp). Coprecipitated ungrafted homopolymers were extracted with dimethylformamide and ethanol, in a Soxhlet apparatus, for about 30 h to extract polyacrylonitrile (PAN) and poly(ethyl methacrylate) (PEMA) from the grafted cellulose. The extracted homopolymers were precipitated with water-methanol mixture in the ratio of 1:1 (v/v) and dried to constant weight in a vacuum oven maintained at 50° C. Extraction of Homocopolymers (Hcp). The poly(ethyl methacrylate-co-acrylonitrile) homocopolymers (PEMAPAN) were extracted from the homopolymer-extracted

Gupta and Sahoo

cellulose with tetrahydrofuran (THF) in a Soxhlet apparatus for 30 h. The extracted homocopolymers were precipitated with a 1:1 water-methanol mixture (v/v) and washed repeatedly with distilled water. The homocopolymers were dried to constant weight in a vacuum oven maintained at 50 °C. Extraction of Ungrafted Cellulose. The ungrafted cellulose from the crude is extracted by keeping the grafted cellulose in cuoxam solution13 containing 1.2% NaHSO3 and stirring solution vigorously for about 6 h. The residue was filtered, washed with dilute acetic acid and deionized water, and finally dried to constant weight in a vacuum oven maintained at 50 °C. Extraction of Grafted Copolymers. Cellulose grafted copolymer chains were isolated by hydrolyzing the grafted cellulose in 72% sulfuric acid17 at 30 °C. After 6 h, the hydrolyzing mixture was diluted with water to 4% concentration of acid and heated under reflux for another 6 h to ensure complete hydrolysis of the grafted chains. The mixture was poured in acetone under vigorous stirring and resulted precipitate was filtered. After washing and drying of the precipitate, the hydrolyzed copolymer chains were extracted with tetrahydrofuran (THF) and the extract was precipitated in methanol. The hydrolyzed cellulose was also extracted with dimethylformamide and ethanol to isolate the grafted homopolymers of acrylonitrile and ethyl methacrylate. On pouring the extract into methanol, no precipitate was obtained, which has clearly confirmed that during grafting of vinyl monomers from their mixtures, the grafted chains of single monomer are hardly formed. Ceric Ion Consumption. The concentration variation of ceric ions during graft copolymerization has been determined by taking 5.0 mL of reaction mixture at different intervals of time and pouring it into a beaker containing excess ammonium sulfate solution. The consumed amount of ceric ions is estimated by back-titrating the excess ferrous ammonium sulfate with ceric sulfate using o-phenanthroline as an indicator. Characterization. To confirm the graft copolymerization of comonomers, FTIR spectra of homopolymers, homocopolymers, and grafted copolymers were recorded on a PerkinElmer 1600 spectrophotometer with KBr pallet. To determine the amount of homopolymers, the percent transmittances at 2240 and 1716 cm-1 corresponding to nitrile and ester groups are recorded and used to estimate the amount of the homopolymers formed with calibration curves drawn with known amount of the homopolymers. The composition of the homocopolymers and grafted copolymers was determined by the optical density ratio (ODR) as determined by following relationship and calibration curve prepared between the ODR and copolymers of known compositions: ODR )

log(base line % T/EMA % T ) log(base line % T/AN % T )

Elemental Analysis. The composition of homocopolymers and grafted copolymers is further verified with nitrogen content determined by a Perkin-Elmer-240C elemental analyzer.

Graft Copolymerization of Comonomers

Biomacromolecules, Vol. 2, No. 1, 2001 241

Molecular Weight of the Copolymers. The molecular weight of the copolymers were determined with gel permeation chromatography (Water Associate-440) fitted with Styragel columns (1000, 500, 100 Å) in series and a UV detector. Tetrahydrofuran was used as eluent at a flow rate of 1 mL/min, and a universal calibration curve was obtained with polystyrene standards. The molecular weight of homocopolymers were determined by measuring intrinsic viscosity with Ubbelohde type of viscometer and using the following relationships.18 [η]30°C ) 2.09 × 10-4M h ν0.75. PAN/DMF h ν0.50. PEMA/2-propanol [η]30°C ) 4.75 × 10-4M Grafting Parameters. Using the weight of grafted polymers, homopolymers, and consumed cellulose and the molecular weight of the polymers, different grafting parameters are calculated as defined below. Graft yield (G) has been taken as the ratio of grafted polymer to the original cellulose. True grafting (GT) has been taken as ratio of grafted polymer to true grafted cellulose. Graft conversion (Cg) has been taken as the fraction of monomer that graft copolymerized. Grafting efficiency (GE) has been taken as the ratio of grafted polymer to the total polymer. Grafting frequency (GF) has been taken as the number of grafted chains per cellulose chain. Total conversion (CT) has been taken as the fraction of the monomer that polymerized. Results and Discussions Acrylonitrile and its copolymers are of industrial importance; hence, the grafting of acrylonitrile with alkyl acrylate has prompted us to investigate grafting parameters of these monomers on cellulose in the presence of ceric ions, which create active centers at the cellulose backbone through single electron-transfer process.19 There are contradictory reports regarding the ceric ions initiated grafting of vinyl monomers. Some studies20,21 highlight that ceric ions promote homopolymer formation through facile reaction of ceric ions with monomers, but others considered it as a potential initiator to obtain optimum grafting under mild conditions. The formation of homopolymers during ceric ions initiated graft copolymerization has been attributed to the chain transfer from the growing graft copolymers to the monomer.19 From the reported studies on grafting of pure acrylonitrile and its binary mixtures on carboxymethyl cellulose, it has been reported that grafting of pure acrylonitrile occurs to a smaller extent even at high feed concentration of acrylonitrile,22 but from our investigations and from the reported studies, it has become evident that grafting of such monomers can be improved by optimizing the grafting conditions and using comonomers which can promote grafting through their synergistic effect.2,3,24 The grafting of individual monomer mainly depends on the molarity of the monomer in the reaction mixture, but in the presence of comonomer, the grafting not only depends on the molarity of the monomers

Figure 1. Grafting yield (% G) as a function of [AN-EMA]. [Ce(IV)] ) 12.0 × 10-3 mol dm-3, [HNO3] ) 10.0 × 10-2 mol dm-3, fAN ) 0.6, [cell] ) 1.0 g, time ) 120 min, temperature ) 30 °C.

in the feed but also is affected significantly by interactions possibly occurring with available comonomer in the reaction mixture before grafting. The extent of these interactions between comonomers will further depend on the molarity of the comonomers, compositions, and the structures of selected monomers. These monomers form a donor-acceptor complexes as confirmed by UV25 and NMR26 studies which facilitate the transfer of monomer to the grafting sites at the cellulose and enhance the overall efficiency of grafting.27 The large π-electron monomer-monomer system assumed to be highly polarizable and highly reactive to active sites at the cellulose backbone. The increasing trend of graft yield in the present investigations has clearly suggested to us to assume the existence of the monomer-monomer complex prior to the formation of the grafted chains on the cellulose. These cross monomer-monomer interactions are also assumed to be responsible for preventing the formation of grafted chains purely of individual monomers as confirmed from the experimental data collected in the present investigations. The grafting parameters depend on various factors; hence, systematic investigations on these factors have been made by recording grafting yield as a function of molarity, feed composition, reaction time, and concentration of ceric ions. Effect of Monomer Concentration. The grafting pattern of the binary mixture of acrylonitrile and ethyl methacrylate monomers on cellulose has been investigated at different feed molarity of comonomers (AN + EMA) ranging from 0.5 to 4.0 mol dm-3 at a fixed mole fraction of acrylonitrile (fAN ) 0.6). From the data shown in Figure 1 and Table 1, it is apparent that on increasing the molarity of binary mixtures from 0.5 to 2.5 mol dm-3, the percent graft yield has increased appreciably but shows a decreasing trend on further increase in molarity of comonomers beyond 2.5 mol dm-3. Thus, the decreasing trend in percent graft yield may be assumed to be due to the increase in the viscosity of medium so that the rate of diffusion of comonomers, i.e, the donor-acceptor monomer complex, on active sites at cellulose decreases, and thereby, the growth of grafted chains has decreased substantially. During molarity variation, the composition of the comonomers in the binary mixture has

242


Gupta and Sahoo

Table 1. Effect of Comonomer Concentration on Grafting Parameters at 30 °Ca (a) [AN-EMA], mol dm-3

% GT

% Ct

% Cg

% Chc

% Cc

% GE

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

250 352 584 750 1071 968 838 708

18.6 20.8 31.2 40.5 54.0 48.8 44.7 42.1

9.6 10.8 16.6 21.5 28.9 20.7 14.2 9.1

8.5 9.0 13.5 17.5 23.0 26.0 28.3 30.5

40.0 42.0 45.0 52.0 60.0 58.0 55.0 50.0

51.6 51.9 52.6 53.0 53.6 42.5 31.8 21.6

(b) [AN-EMA], mol dm-3

GF

Ng × 106

Nhc × 105

M h n × 10-3 (Gp)

M h n × 10-3 (Hcp)

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

0.89 1.75 3.25 4.69 5.58 5.53 4.91 4.22

1.42 2.95 5.86 9.75 13.40 12.84 10.80 8.44

0.33 0.68 1.46 2.39 3.69 4.78 5.90 7.02

280 305 355 368 450 405 385 360

108 110 115 122 130 136 140 145

a [Ce(IV)] ) 12.0 × 10-3 mol dm-3, f -2 AN ) 0.6, [HNO3] ) 10.0 × 10 mol dm-3.

been kept constant (fAN ) 0.6), hence monomer-monomer interactions may be considered to be the same. The compositional analysis of the grafted chains obtained at different feed molarity has clearly indicated that the monomermonomer interactions are almost constant during feed molarity variation from 0.5 to 4.0 mol dm-3 and the decrease in percent conversion beyond 2.5 mol dm-3 is only because of variation in viscosity of the medium. The decreasing trend in percent grafting (% Cg) beyond 2.5 mol dm-3 of the feed molarity (Table 1a) has further confirmed the viscosity effect in retarding the rate of grafting of monomers onto cellulose and facilitating the formation of homopolymers. Although the extent of homopolymer formation has increased after 2.5 mol dm-3 molarity of the feed but overall conversion (% CT) of the monomers to produce polymers has decreased beyond this concentration (Table 1a). The decreasing trend in other grafting parameters such as cellulose conversion (% CC), grafting efficiency (% GE), and true grafting (% GT) has further confirmed the retarding effect of solution viscosity at high feed molarity. The analysis of the homocopolymers formed in the reaction mixture has shown a higher percentage of acrylonitrile than the copolymers grafted onto the cellulose. This variation in the composition of the homocopolymers and the grafted copolymers is due to different interactions of donor-acceptor monomer complex with the growing chains at the cellulose backbone and growing homocopolymer chains in the solution, but the composition of the homocopolymers remained almost constant (FAN ) 0.52) during variation in the feed molarity from 0.5 to 4.0 mol dm-3. The compositional analysis of the homocopolymers and grafted copolymers have been compared with those of samples collected at 120 min of grafting at different feed molarity ranging from 0.5 to 4.0 mol dm-3. However,

Figure 2. A log-log plot between Rp vs [AN-EMA]. [Ce(IV)] ) 12.0 × 10-3 mol dm-3, [HNO3] ) 10.0 × 10-2 mol dm-3, fAN ) 0.6, [cell] ) 1.0 g, time ) 120 min, temperature ) 30 °C.

homocopolymers and grafted copolymers obtained at later stages of grafting show different compositions, which is due to a variation in feed composition on account of the different reactivity ratios of the monomers used in graft copolymerization. It is interesting to note that the extent of compositional heterogeneity in grafted copolymers and homocopolymers is very small, hence suggesting to us to assume azeotropic types of interactions, among comonomers used for grafting. From these analyses, it has become apparent that the variation in the graft yield and other parameters of grafting on varying the feed molarity occurs only due to the variation in viscosity of the medium, which reduces the diffusion of monomer-monomer complex to the reactive sites at cellulose backbone and hence decreases graft yield but forms grafted chains of the same composition at the same reaction time. The retarding effect of viscosity is further confirmed by analyzing the increasing trend in number of homocopolymer chains (Nhc) and decreasing number of grafted copolymers chairs (Ng) and variation in molecular weight of the homocopolymers and grafted copolymers as shown in Table 1b. The ceric ions disappearance was also recorded as a function of the feed molarity (Figure 8B), which has shown no significant variation in the percent disappearance of ceric ions on varying the feed molarity. This has further confirmed the idea that ceric ions interact only with cellulose backbone to create free radicals and hardly interact with comonomers used in graft copolymerization; otherwise, the rate of ceric ion disappearance might have shown a significant variation on changing the feed molarity from 0.5 to 4.0 mol dm-3. The initial increase in the graft yield on increasing the feed molarity from 0.5 to 2.5 mol dm-3 can only be assumed to be due to a substantial increase in the rate of propagation of chains of various sizes and due to grafting on reactive sites which were initially underutilized due to an insufficient number of available comonomers, but as the feed molarity has increased, then every reactive site and growing chain receives sufficient comonomer to grow to its maximum degree of polymerization. However, after certain concentrations of the comonomer


Figure 3. Grafting yield (% G) as a function of the feed composition (fAN). [AN-EMA] ) 1.5 mol dm-3, [Ce(IV)] ) 12.0 × 10-3 mol dm-3, [HNO3] ) 10.0 × 10-2 mol dm-3, [cell] ) 1.0 g, time ) 120 min, temperature ) 30 °C.

Figure 4. Mayo and Lewis plot for reactivity ratios. [AN-EMA] ) 1.5 mol dm-3, [Ce(IV)] ) 12.0 × 10-3 mol dm-3, [HNO3] ) 10.0 × 10-2 mol dm-3, fAN ) 0.6, [cell] ) 1.0 g, time ) 120 min, temperature ) 30 °C.

Figure 5. Grafting yield (% G) as a function of [Ce(IV)]. [AN-EMA] ) 1.5 mol dm-3, [HNO3] ) 10.0 × 10-2 mol dm-3, fAN ) 0.6, [cell] ) 1.0 g, time ) 120 min, temperature ) 30 °C.

(beyond >2.5 mol dm-3), the rate of propagation has decreased due to the hindrances created by the grafted chains and due to an increase in the viscosity of the medium. The


Figure 6. A log-log plot between Rp vs [Ce(IV)]. [AN-EMA] ) 1.5 mol dm-3, [HNO3] ) 10.0 × 10-2 mol dm-3, fAN ) 0.6, [cell] ) 1.0 g, time ) 120 min, temperature ) 30 °C.

Figure 7. Grafting yield (% G) as a function of reaction time of graft copolymerization. [AN-EMA] ) 1.5 mol dm-3, [HNO3] ) 10.0 × 10-2 mol dm-3, fAN ) 0.6, [cell] ) 1.0 g, time ) 120 min, temperature ) 30 °C.

grafting rate has been found to be dependent on the square concentration of feed molarity (Figure 2) up to 2.5 mol dm-3 and shows a retarding trend beyond this concentration, which is due to the slow rate of diffusion of comonomers to the reactive sites at the cellulose backbones. Effect of Feed Composition. The grafting of the monomers from their binary mixtures on cellulose has also been studied by taking feeds with different mole fractions (fAN) of the comonomers at fixed molarity of comonomers (1.5 mol dm-3). These investigations are aimed to analyze the effect of monomer-monomer interactions on graft yield and the composition of the graft copolymers and the homocopolymers formed from the reaction mixture. During feed composition variation, the fixed molarity of the comonomers will not be affecting the viscosity of the medium as observed during feed molarity variation; hence, the variation in the graft yield and composition of the grafted chains and homocopolymer chains totally depend on the interactions operating between comonomers at different feed compositions. To understand the effect of feed composition, the mole

244


Gupta and Sahoo

Figure 8. Percent conversion of ceric ions as a function of reaction time (A) and feed molarity (B). [Ce(IV)] ) 12.0 × 10-3 mol dm-3 , [HNO3] ) 10.0 × 10-2 mol dm-3, fAN ) 0.6, [cell] ) 1.0 g, time ) 120 min (A), temperature ) 30 °C. Table 2. Effect of Feed Composition (fAN)a (a) On Grafting Parameters at 30 °C

fAN

% GT

% Ct

% Cg

% Chc

% Cc

% GE

0.0 0.2 0.4 0.6 0.8 1.0

770 423 516 584 598 366

52.8 19.1 25.7 31.7 36.8 31.5

26.8 9.6 13.2 16.6 18.8 8.4

21.5 9.0 11.5 13.5 16.0 21.0

50 42 43 45 48 44

53.2 50.4 51.4 52.6 51.0 26.7

(b) On Grafting Parameters at 30 °C

fAN

GF

0.0 0.2 0.4 0.6 0.8 1.0

4.13 2.43 3.00 3.25 3.18 2.09

Ng ×

106

8.27 3.93 5.15 5.86 6.10 3.68

Nhc ×

105

1.89 1.22 1.35 1.46 1.58 1.82

M h n × 10-3 (Gp)

M h n × 10-3 (Hcp)

405 305 320 355 385 285

142 92 106 115 126 144

(c) On Grafted Polymer Chains on Cellulose at 30 °C

fAN

FAN

% N2 in GP

m j M1

m j M2

R

PAN,AN

0.20 0.40 0.60 0.80

0.15 0.31 0.51 0.73

1.95 4.62 8.58 13.93

1.27 1.45 2.02 3.27

6.80 3.17 1.95 1.36

25.09 43.29 50.38 43.20

0.48 0.31 0.50 0.73

a [AN-EMA] ) 1.5 mol dm-3, [Ce(IV)] ) 12.0 × 10-3 mol dm-3, [HNO3] ) 10.0 × 10-2 mol dm-3. m j M1 and m j M2 are the average sequence lengths of AN and EMA monomers in grafted polymer (GP) chains. PAN,AN is the probability of an AN molecule to combine with a chain ending with an AN monomer. R is the average number of sequences per 200 monomer units.

fraction of the acrylonitrile (fAN) in the feed has been varied from 0.1 to 0.9, keeping a fixed molarity of 1.5 mol dm-3. The results are given in Table 2, parts a and b, and in Figure 3. The data on grafting of individual monomers on cellulose are also collected to compare the effect of comonomers on the trend of grafting parameters. The extent of grafting yield of pure acrylonitrile is very low (105%) in comparison to that of pure ethyl methacrylate (335%), which has indicated

that acrylonitrile alone has a lower affinity for grafting on cellulose than ethyl methacrylate. During feed composition variation, the grafting parameters have shown interesting trends at different fractions of acrylonitrile as clear from the data given in Table 2, parts a and b, and Figure 3A, which suggested to us to assume a significant variation in monomermonomer interactions on taking feeds of different mole fractions of the acrylonitrile (fAN). From the data shown in Figure 3 and Table 2, it is clear that as the acrylonitrile is added in the feed, the graft yield is decreased linearly. However, the grafted chains obtained were purely of ethyl methacrylate, and when the mole fraction of acrylonitrile in the feed reaches a certain value (fAN g 0.2), then the grafted chains were obtained with a small fraction of acrylonitrile (FAN ) 0.15) but the yield was minimum. With further increase in the mole fraction of acrylonitrile in the feed from 0.2 to 0.8, the graft yield has increased linearly and the mole fraction of the acrylonitrile has also increased. The mole fraction of the acrylonitrile in the grafted chains has been found to be less than the feed during feed composition variation from 0.2 to 0.8, but on further increase in the mole fraction of acrylonitrile (fAN) in the feed beyond 0.8, the graft yield has shown a decreasing trend and the grafted chains were observed to have a higher mole fraction of acrylonitrile (FAN ) 0.95) than the feed. The compositional analysis of the grafted chain and homocopolymer chains and the trend of grafting yields at different fractions of acrylonitrile in the feed have clearly suggested to assume a significant variation in intramonomer, intermonomer, and cellulose-comonomer interactions on taking feeds of different compositions. At a very low fraction of acrylonitrile in the feed (fAN e 0.1), the acrylonitrile molecules participate only in the formation of homopolymers and homocopolymers. In this state, the monomer-monomer interactions among ethyl methacrylate are stronger than the acrylonitrile-ethyl methacrylate; hence, acrylonitrile molecules do not copolymerize with ethyl methacrylate in grafted chains. At this concentration, these acrylonitrile molecules also participate in growing chains by chain transfer mechanism, and hence the yield and the molecular weight of the grafted chains are low up to this fraction of the acrylonitrile. On increasing the mole fraction of acrylonitrile above 0.1, the percent graft yield and other grafting parameters have shown an increasing trend. The fraction of the acrylonitrile in the grafted chains (FAN) has also increased from 0.15 to 0.73 on increasing the feed fraction (fAN) from 0.20 to 0.80. The increase in the graft yield and the fraction of the acrylonitrile in the grafted copolymers chains can be assumed to be due to the formation of the donor-acceptor monomer complex, which facilitates the transfer of acrylonitrile molecules to the growing chains on cellulose, which in turn facilitate the grafting of ethyl methacrylate molecules more than the acrylonitrile molecules. The interactions of the cellulose anchored acrylonitrile moiety seems to be stronger for ethyl methacrylate than for the acrylonitrile. Therefore, the acrylonitrile-containing growing chains at cellulose facilitate an easy transfer of ethyl methacrylate from the donor-acceptor monomers complex interacting with the growing chain, and hence the fraction of the ethyl methacrylate (FEMA) has been found to be higher



than the feed (fEMA). Similarly growing chains having ethyl methacrylate monomers at the end facilitate the transfer of the acrylonitrile monomer from the donor-acceptor monomers complex in the solution; however, the extent of these interactions seems to be poorer than the interactions that occur between acrylonitrile bearing grafted chains and the donor-acceptor monomers complex in the feed. However, on comparing the extent of transfer of the acrylonitrile monomer from the feed to the cellulose in absence of comonomer, it has been found that the graft yield is higher and the presence of ethyl methacrylate facilitates the transfer of acrylonitrile on cellulose due to the reported synergistic effect on acrylonitrile. On further increasing the mole fraction of the acrylonitrile in the feed beyond 0.80, the graft yield and the mole fractions of the ethyl methacrylate in the grafted chains (FEMA) have shown a decreasing trend, which is due to variations in monomer-monomer interactions and the cross monomers interactions with growing chains at the cellulose at higher concentration of the acrylonitrile in the feed (fAN > 0.8). At this composition, the synergistic effect of comonomer has decreased, and hence the graft yield has shown a decreasing effect. The high mole fraction of the acrylonitrile also prevents ethyl methacrylate to interact with growing chains at cellulose hence decreasing mole fraction of ethyl methacrylate in grafted chains than the feed. The graft yield has decreased to a minimum when the mole fraction of the acrylonitrile (fAN) became unity, which is a clear indication for considering the low affinity of pure acrylonitrile to cellulose. These observations have clearly suggested that the synergistic effect of comonomer operates within a certain range of composition of the feed, and beyond this composition, the graft yield changes according to the affinity of the individual monomer. The graft yield did not follow a linear decreasing trend on varying the mole fraction of acrylonitrile as expected theoretically shown by dotted line (Figure 3B), which clearly suggested to us to consider the existence of monomer-monomer interactions and their variations at different mole fractions of the acrylonitrile in the feed. The other parameters of grafting such as grafting efficiency (% GE), grafting frequency (GF), and number of grafted chains (Ng) (Table 2b) have also shown consistency with the variation in the graft yield and variation in composition of the feed. To quantify the relative reactivity of the monomers used in the graft copolymerization, the reactivity ratios of acrylonitrile and ethyl methacrylate have been evaluated using the Mayo and Lewis method 28 (Figure 4). Using the mole fractions of monomers on the graft copolymers (Table 2c), the values of reactivity ratios (Figure 4) for the acrylonitrile (rAN) and ethyl methacrylate (rEMA) have been found to be 0.68 and 1.45, respectively. The high value of rEMA has clearly suggested that ethyl methacrylate has more affinity to transfer into grafted copolymer chains than the acrylonitrile. The product of the reactivity ratio (r1r2) is less than unity (0.98), which suggested to us to consider the alternate arrangement of the monomers in the grafted copolymers. The average sequence lengths (m j M) of the individual monomer have been calculated using the reactivity ratio and shown in Table 2c, which has also shown dependence on the composition of the feed. As the mole

Table 3. Effect of Ceric Ion Concentration on Grafting Parameters at 30 °Ca (a) [Ce(IV)] × mol dm-3

103

4.0 8.0 12.0 16.0 20.0 24.0 28.0 32.0

% GT

% Ct

% Cg

% Chc

% Cc

% GE

451 539 584 640 610 574 454 425

19.8 27.0 31.7 38.4 35.1 30.7 24.2 20.3

10.0 14.0 16.6 20.4 18.1 15.2 10.6 8.8

8.5 12.0 13.5 15.0 14.3 13.0 12.0 10.5

41.0 43.0 45.0 48.0 46.0 43.0 42.0 40.0

50.7 51.8 52.6 53.1 51.6 49.5 43.9 43.3

(b) [Ce(IV)] × 103 mol dm-3

GF

4.0 8.0 12.0 16.0 20.0 24.0 28.0 32.0

1.96 2.71 3.25 3.93 3.78 3.45 2.52 2.21

Ng × 106 Nhc × 105 3.21 4.66 5.86 7.54 6.96 5.93 4.23 3.54

0.85 1.27 1.46 1.69 1.72 1.80 1.92 2.02

M h n × 10-3 M h n × 10-3 (GP) (Hcp) 392 375 355 338 326 320 314 310

125 118 115 111 104 90 78 65

a [AN-EMA] ) 1.5 mol dm-3, f -2 mol AN ) 0.6, [HNO3] ) 10.0 × 10 dm-3, time ) 120 min.

fraction of acrylonitrile has increased in the feed, the average sequence length (m j M2) of ethyl methacrylate repeat units in the grafted chains has decreased, and the average sequence length of acrylonitrile (m j M1) has increased. This variation in the sequence length is due to variation in the probability of addition (P11) of the monomer to the radical ended with its own type of monomer. The average sequence length of the monomers has been further verified by the percent nitrogen content in the grafted chains. On increasing the mole fraction of the acrylonitrile in the feed, the run number (R) has shown a increasing trend up to 0.8 mole fraction of the acrylonitrile (fAN) in the feed but decreased on further increasing the mole fraction of acrylonitrile in the feed beyond 0.80, which is due to higher fraction of grafted chains with substantial sequence length of acrylonitrile (m j M1) than the ethyl methacrylate (m j M2). On the basis of these observations, the possible arrangement of the monomer sequences j M2) in the grafted chains is proposed as below (m j M1,m - (AN)mj M1 - (EMA)mj M2 - (AN)mj M1 - (EMA)mj M2 j M2) depend on the feed where values of (m j M1) and (m composition as shown in Table 2c. Effect of Ceric Ion Concentration. The grafting of acrylonitrile-ethyl methacrylate comonomers has also been studied on varying the concentration of ceric ammonium nitrate (CAN) at fixed molality (1.5 mol dm-3) and composition (fAN ) 0.6) of the feed and other conditions of graft copolymerization. The concentration of ceric ions has been varied from 4 to 32.0 × 10-3 mol dm-3 in the presence of excess concentration of the nitric acid (10.0 × 10-2 mol dm-3). The results are given in Table 3 and Figure 5. The graft yield and other grafting parameters have shown a linear

246


increasing trend up to 16.0 × 10-3 mol dm-3 concentration of the ceric ammonium nitrate, but on further increase in concentration of ceric ammonium nitrate (CAN) beyond 16.0 × 10-3 mol dm-3, a sharp decreasing trend is observed. The percent cellulose conversion (% Cc) increases up to 16.0 × 10-3 mol dm-3 concentration of the ceric ammonium nitrate (CAN) and shows a decreasing trend beyond this concentration of ceric ammonium nitrate (CAN), which clearly indicates that ceric ions are not being used in the formation of the active sites at the cellulose, which may be due to the hindrances created by the growing chains at the cellulose surface and may be due to the nonavailability of the labile protons at the cellulose to react with ceric ions to form radical by one electron-transfer process. The decreasing trend in the cellulose percent conversion (% Cc), on taking ceric ammonium nitrate concentration beyond 16.0 × 10-3 mol dm-3, is due to the decrease in the accessibility of the ceric ions to the labile protons at the cellulose surface but is not due to the nonavailability of the hydroxyl groups at the cellulose; otherwise, the percent cellulose conversion (% Cc) would have been constant beyond the 16.0 × 10-3 mol dm-3 concentration of the ceric ammonium nitrate. Thus, on higher concentration of the ceric ammonium nitrate beyond 16.0 × 10-3 mol dm-3, the ceric ions start participating in oxidative termination of the growing grafted chains.29 The decreasing trend in the percent homocopolymer formation (% Hcp) has further confirmed that ceric ions do not participate in the formation of active sites at the monomers molecules otherwise the molecular weight of the homocopolymer might have increased (Table 3b). The decreasing trend in the number of grafted chains (Ng) and grafting frequency (GF) also supported the idea that the ceric ions are unable to participate in reactive site formation beyond 16.0 × 10-3 mol dm-3 concentration of ceric ammonium nitrate; otherwise, the frequency of grafting (GF) and number of grafted chains (Ng) per glucose unit might have increased. These investigations have clearly suggested that percent graft yield or the efficiency of grafting (% GE) can be increased up to 16.0 × 10-3 mol dm-3 concentration of the ceric ions at 10.0 × 10-2 mol dm-3 concentration of nitric acid. On taking a higher concentration of nitric acid, the rate of grafting has increased but the overall graft yield has decreased, which may be due to the hydrolysis of the cellulose and due to the increase in the crystallinity of cellulose at higher concentration of the acid. The rate of grafting (Rp) as a function of ceric ions concentration has been used to calculate the order of reaction with respect to the ceric ions. The log-log plot (Figure 6) between rate of grafting (Rp) vs ceric ions concentration has given a slope of 0.51 within the concentration range of (4.0-16.0) × 10-3 mol dm-3 of ceric ammonium nitrite but is found to deviate on higher concentration (beyond 16.0 × 10-3 mol dm-3), suggesting consideration of ceric ions participation in reactions other than radical formation on cellulose backbones. Effect of Reaction Time. The extent of grafting of monomers on cellulose has also studied at different intervals of time at fixed molarity (1.5 mol dm-3) and ceric ions concentration (12.0 × 10-3 mol dm-3) in the reaction mixture. The data of graft yield are recorded in Table 4a

Gupta and Sahoo Table 4. Effect of Reaction Time on Grafting Parameters at 30 °Ca (a) time of grafting (min)

% GT

% Ct

% Cg

% Chc

% Cc

% GE

60 90 120 150 180 240 360 480 600 1440

310 519 584 639 751 806 872 875 881 889

14.6 24.3 31.7 36.0 45.0 51.5 59.2 61.0 62.0 62.7

7.6 12.8 16.6 19.2 24.0 27.5 31.7 33.0 33.3 33.9

6.0 10.2 13.5 15.0 18.9 22.0 25.0 25.5 25.9 26.0

40.0 42.0 45.0 46.0 47.0 49.0 51.0 52.5 53.0 53.0

52.0 52.4 52.6 53.3 53.4 53.5 53.7 54.0 54.1 54.3

(b) time of grafting (min)

GF

60 90 120 150 180 240 360 480 600 1440

2.12 2.97 3.25 3.41 3.94 3.97 4.03 4.04 4.07 4.08

Ng × 106 Nhc × 105 3.39 5.00 5.86 6.28 7.40 7.78 8.23 8.49 8.62 8.65

0.71 1.15 1.46 1.59 1.96 2.08 2.18 2.19 2.20 2.20

M h n × 10-3 M h n × 10-3 (GP) (Hcp) 280 320 355 382 405 442 481 485 487 490

105 111 115 118 120 132 143 145 147 490

a [AN-EMA] ) 1.5 mol dm-3, f -2 mol AN ) 0.6, [HNO3] ) 10.0 × 10 dm-3. [Ce(IV)] ) 12.0 × 10-3 mol dm-3.

and in Figure 7. The observation of the grafting trend of the comonomers onto cellulose as a function of time has clearly indicated that graft yield increases sharply during the initial 3 h of the grafting but starts leveling off beyond 3 h and finally becomes almost constant after 8 h. This slow increasing trend in the graft yield and other parameters of the grafting beyond 3 h can be assumed to be due to the depletion of the monomers in reaction mixture and due to the steric hindrance offered by the grafted chains of sufficient length which retards17 the interactions of the comonomers complex with growing chains. Hence, the rate of transfer of monomer to the growing chains is decreased but is allowed to diffuse monomers and ceric ions at a slower rate, which is clear from the grafting parameters as given in Table 4. The increasing trend in grafting frequency (GF) and the number of grafted chains (Ng) as a function of reaction time has clearly suggested that the process of formation of new reactive sites on cellulose continued even after 3 h, but at a slow rate. This is further confirmed by comparing the percent disappearance of ceric ions as a function of time as shown in Figure 8A. The rate of ceric ions disappearance is very fast in the initial 2 h of graft copolymerization and decreased beyond 2 h. At the end of 3 h of graft copolymerization, the rate of disappearance of ceric ions became almost constant as the maximum amount of ceric ions had been exhausted, either in formation of the reactive sites on the cellulose or in other side reactions taking place in the reaction mixture. These investigations have clearly indicated that the optimum time for grafting of acrylonitrile-ethyl methacrylate comonomers from their binary mixture is 2 h, where the efficiency



of ceric ions is a maximum and the participation of comonomers in grafting is a maximum. On the basis of the experimental data collected as a function of the feed molarity and reaction time, the following steps are proposed for the grafting of acrylonitrile (AN) and ethyl methacrylate (EMA) on to cellulose in the presence of the ceric ions. Radical formation: K

cell-H + Ce(IV) 98 cell• + Ce(III) + H+ (R•)

(1)

Initiation: k1

R• + AN 98 R-AN•

(2)

k1

R• + EMA 98 R-EMA•

(3)

Conclusion The grafting of acrylonitrile and ethyl methacrylate from their binary mixtures has been carried out successfully in the presence of the ceric ammonium nitrate as initiator. The product of reactivity ratios of the acrylonitrile and ethyl methacrylate has been found to be less than unity, which suggested an alternate sequence of the monomer blocks in the grafted copolymer chains. The elemental analysis has further supported the proposed alternate arrangement of monomer blocks in the grafted copolymer chains. These investigations have clearly suggested that the presence of the comonomer has increased the grafting of acrylonitrile on cellulose due to synergistic effect of ethyl methacrylate. Acknowledgment. S.S. is thankful to U. G. C., New Delhi, India, for the award of a fellowship. The authors thank the University of Roorkee for providing facilities to carry out these investigations. References and Notes

Propagation: kp

R-(AN)•n-1- - - + AN 98 R-(AN)•n- - - -

(4)

kp

R-(EMA)•m-1- - - - + EMA 98 R-(EMA)•m- - - - (5) kp

R-(AN)•n- - + EMA 98 R-(AN)n-EMA•- kp

R-(EMA)•m- - + AN 98 R-(EMA)m-AN•- -

(6) (7)

Termination: ktc

R-(AN)n-EMA• + •EMA-(AN)n-R 98 R-(AN)n- -(EMA)m-(AN)n-R (8) ktc

R-(EMA)m-AN• + •AN-(EMA)m-R 98 R-(EMA)m-(AN)n- -(EMA)m-R (9) ktc

R-(AN)n-EMA• + •AN-(EMA)m-R 98 R-(AN)n- EMA-AN-(EMA)m-R (10) Oxidative termination: ko

R + Ce(IV) 98 oxidation product + Ce(III) + H+ (11) where n and m are the average sequence lengths of the j M2) blocks in acrylonitrile (m j M1) and ethyl methacrylate (m the grafted chains. The values of n and m varied from 1.27 to 3.27 and from 1.36 to 6.80 during grafting as given in Table 3c.

(1) Gawish, S. W.; Kantouch, A.; El-Nagar, A. M. A.; Moslen, S. M. J. Appl. Polym. Sci. 1995, 57, 45. (2) Dessarki, A. M.; Toher, N. H.; El-Arnauty, M. B. Polym. Int. 1998, 45, 67. (3) Gupta, K. C.; Sujata Sahoo. J. Appl. Polym. Sci. 2000, 76, 906. (4) Matveera, D. N.; Galbraikh, L. S.; Kalmykova, T. K.; Vysatskaya, E. P. Khim. D. ReV. 1983, 747, 01. (5) McCornic, C. L.; Park, L. S. J. Appl. Polym. Sci. 1985, 30, 45. (6) Yamauchi, J.; Okamura, T. Jpn. Pat. 7,219,421, 1972; Chem. Abstr. 1973, 78, 17519. (7) Malda, S. Jpn. Pat. 7,244,541, 1972; Chem. Abstr. 1973, 79, 137881. (8) Hebeish, A.; Gutherie, J. T. The Chemistry and Technology of Cellulose Copolymers; Springler-Verlag: New York, 1981. (9) McDowell, D. J.; Gupta, B. S.; Stanett, V. T. Prog. Polym. Sci. 1985, 11, 01. (10) Hebeish, A.; El-Hilw, Z. H. J. Appl. Polym. Sci. 1998, 67, 739. (11) El-Salmawi, K.; El-Nagar, A. M.; Altia, S. E. Polym. Int. 1997, 44, 181. (12) Mansour, O. Y.; Wagaty, A. Prog. Polym. Sci. 1985, 11, 91. (13) Jayams, G.; Lang, F. In Methods in Carbohydrate Chemistry; Whitler, R. L., Ed.; Academic Press: New York, 1969; Vol. 3, p 77. (14) Okiemen, F. E.; Ebhoaye, J. E. J. Appl. Poym. Sci. 1986, 31, 1275. (15) Bazuaye, A.; Okiemen, F. E.; Sai, O. B. Acta Polym. 1989, 40, 126. (16) Hori, Y.; Takahashi, T.; Nagata, M.; Sats, T. J. Soc. Fiber Sci., Technol. Jpn. Sen. Gakkaishi 1996, 22 (10), 443. (17) Nishioka, N.; Matsumoto, Y.; Yumen, K.; Monmal, K.; Kosai, K. Polym. J. 1988, 18, 323. (18) Kurata, M.; Tsunashina, Y.; Iwana, M.; Kamata, K. Polymer Hand Book; 2nd ed. 1975; p 01. (19) Mino, G.; Kaiserman, S. J. Polym. Sci. 1958, 31, 54. (20) Pledger, H., Jr.; Wu, G. S.; Young, T. S.; Hogen-Esch, T. E.; Butler, G. B. J. Makromol. Sci. Chem. 1985, A22, 1297. (21) Graczyk, T.; Hornof, V. J. Makromol. Sci., Chem. 1988, A25, 1633. (22) Okiemen, F. E.; Ogbeifun, D. E. J. Appl. Polym. Sci. 1996, 59, 981. (23) Iwatsuki, S.; Yamashita, Y. Makromol. Chem. 1965, 89, 205. (24) Tsuchida, S.; Tomoro, T. Makromol. Chem. 1971, 141, 265. (25) Kaku, U.; Toshinmaka, N.; Yushimaka, K.; Niro, M. J. Chem. Soc. Jpn. Ind. Chem. 1967, 70, 1945. (26) Maslinka-Solich, J. J. Polym. Sci. Polym. Symp. 1973, 42, 411. (27) Fanta, G. F.; Bur, R. C.; Doanne, W. M.; Russel, C. R. J. Polym. Sci. 1979, 23, 229. (28) Mayo, F. R.; Lewis, M. J. J. Am. Chem. Soc. 1944, 66, 1594. (29) Arthur, J. C.; Baug, P. J.; Hinojosa, O. J. J. Appl. Polym. Sci. 1966, 10, 1591.

BM000102H

Graft Copolymerization of Acrylonitrile and Ethyl Methacrylate

Recommend Documents