Grafting of polymethyl methacrylate onto cellulose acetate in ... - NOPR

Indian Journal of Chemical Technology Vol. 20, May 2013, pp. 202-209

Grafting of polymethyl methacrylate onto cellulose acetate in homogeneous medium using ceric (IV) ion as initiator C R Routray, B Tosh* & N Nayak Department of Chemistry, Orissa Engineering College, Bhubaneswar 751 007, India Received 26 March 2012; accepted 1 January 2013 CA-g-PMMA copolymer has been synthesized in homogeneous medium of dimethyl sulfoxide (DMSO) using ceric ammonium nitrate (CAN) as initiator. Different grafting parameters, such as graft yield, grafting efficiency and total conversion of monomer to polymer are evaluated at different reaction conditions of temperature, time, monomer and initiator concentrations. It is observed that the graft yield decreases with increase in temperature of grafting. CAN in presence of DMSO forms free radical on cellulose acetate (CA) backbone via ring opening mechanism and polymethyl methacrylate (PMMA) is grafted through homogeneous breaking of the acrylic double bond. The role of methylene blue (MB) as a homopolymer inhibitor and its effect on grafting is also studied. In presence of MB the amount of PMMA homopolymer formation reduces and consequently graft yield increases. The viscosity average molecular weights of grafted PMMA and rate of grafting are also calculated. The products are characterized by FTIR and 1H-NMR analyses and a possible reaction mechanism is deduced. The thermal degradation of the grafted products is also studied by thermogravimetric analysis and differential thermogravimetry. Keywords: Ceric ammonium nitrate, Dimethyl sulfoxide, Grafting, Homogeneous medium, Homopolymer inhibitor, Methyl methacrylate, Ring opening, Methylene blue

Cellulose acetate (CA) is one of the many commercially important cellulose derivatives. It is a tough material with excellent optical clarity. Major applications are found in films, textile and cigarette tow1. It is well known that cellulose acetate has dimensional stability problem under high humidity and at elevated temperature. The other short comings are its high cost, very limited compatibility with other synthetic polymers and high processing temperature2,3. Grafting of synthetic vinyl monomers on to cellulose acetate offers the potential of preparing new materials where the properties of the derived graft copolymers may be tailored to meet certain specifications by controlling parameters such as the molecular weight of the grafted side chain, the number of the grafted side chains and the type of the grafted side chains4. The production of the graft copolymers can be classified by two reaction schemes, namely (i) graft polymerization of monomers initiated by the active sites on the polymer backbone and (ii) coupling of two reactive polymers5. Graft polymerization proceeds by different mechanisms (free radical, anionic, cationic, ring opening, etc.) depending on the —————— *Corresponding author. E-mail: [email protected]

nature of the active sites on the backbone and the type of monomer. Again depending on the reaction medium, the grafting reaction may be divided into heterogeneous and homogeneous grafting. Heterogeneous graft polymerization of synthetic polymers onto cellulose and cellulose acetate has been studied extensively over the past decades6-18. The derivatization and/or grafting reactions in homogeneous conditions assure important advantages over heterogeneous system, like a better control of the degree of substitution19, a more uniform distribution of substituents along the polymer and a higher conversion yield20,22 . During past few years a number of cellulose derivatives and grafted products have already been synthesized under homogeneous conditions19-31, but so far work on homogeneous graft copolymerization of cellulose acetate dissolved in dimethy sulfoxide (DMSO) and using ceric ammonium nitrate(CAN) has not been investigated. CAN in presence of nitric acid is an efficient initiator for graft copolymerization of vinyl monomers onto cellulose6-8 in heterogeneous medium but in homogeneous conditions this will produce gel confirming the regeneration of cellulose and also CA from the solution. It is only reported that CAN in

ROUTRAY et al.: GRAFTING OF PMMA ONTO CELLULOSE ACETATE

presence of dimethyl sulfoxide (DMSO) can produce Ce+4 ion20 and can be a suitable redox system to initiate graft copolymerization process, but no work has been carried out on this system. It is also reported that the presence of methylene blue in the reaction system reduces the formation of homopolymers in the graft copolymerization process32. Therefore, in the present study, cellulose acetate has been dissolved in DMSO and the grafting reaction has been carried out using CAN as an initiator. DMSO acts as a solvent for CA and also produces Ce+4 ion for initiation of the reaction. The effect of variation in reaction time, temperature, concentration of initiators and monomer is also studied to optimize the conditions under which grafting occurs most effectively. The effect of methylene blue on homopolymer formation is also studied. The grafted products obtained have been characterized by Fourier transformation infrared (FTIR) and proton nuclear magnetic resonance (1H-NMR) spectroscopy and their molecular weight and number of grafts per cellulose backbone are determined. Finally, thermal degradation of the grafted products is studied by thermogravimetric (TG) and differential thermogravimetric (DTG) analyses. Experimental Procedure Materials

Cellulose acetate (Sigma Aldrich, 37.9% acetyl and Mn ~ 50,000 by GPC) was purified by dissolution in tetrahydrofuran (THF), precipitation in diethyl ether, followed by Soxhlet extraction with the same solvent to remove any trace of low molecular alcohols and water. Thereafter it was thoroughly dried under vacuum. Methylmethacrylate (MMA) (Sigma Aldrich) was purified from the polymerization inhibitor (hydroquinone monomethylether) by extraction with 5% aqueous NaOH, water and dried over Na2SO4 and then under CaH2 at reduced pressure. The stabilizer free monomer was distilled under reduced pressure at 50°C and stored below 5°C. Ceric ammonium nitrate (CAN) (E-Merck), dimethyl sulfoxide (DMSO) (Qualigen), methylene blue (E-Merck) were of reagent grade and used without further purification. N2 gas was passed through alkaline pyrogallol, sulfuric acid, and potassium hydroxide solution before it was passed into the reaction mixture. Grafting

of

An amount of 1.25 g of cellulose acetate (7.0 mmol the corresponding anhydroglucose unit by

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considering 37.9% acetyl content of CA) was dissolved in 62.5 mL DMSO taken in a three-necked round bottom flask equipped with a magnetic stirrer and temperature controlled oil bath to make a 2% solution. To this, different amount of CAN ranging from 0.3 g to 0.5 g (0.5% - 0.8%, 0.55 – 0.916 mmol) was added followed by addition of 1.25 – 2.0 mL (2.0% - 3.2%, 11.7 – 18.7 mmol) of MMA. All the reactions were carried out in a dry nitrogen atmosphere. The reactions were carried out for 2 - 6 h at varying temperature range of 30-80°C. The reaction was terminated by addition of hydroquinone23. The polymerization mixture was poured into cold distilled water with vigorous stirring, kept overnight at 5°C, filtered, washed thoroughly in cold distilled water, dried at 50°C and weighed. Then the products were soxhlet extracted with acetone for 24 h to remove any adherent homopolymer. The extracted CA-grafted products were then dried at 50°C and stored over P2O5. A comparative study was also carried out to study the effect of methylene blue in the formation of homopolymer by adding 0.7 mL (1.09 ppm) of methylene blue to the reaction at 80°C having 11.7 mmol MMA and 0.55 mmol CAN. The graft yield (GY), total conversion of monomer to polymer (TC), grafting efficiency (GE), number of graft per cellulose acetate chain and the rate of grafting were calculated on the basis of oven-dried weight of the cellulose and the increase in weight after grafting by using the following relations33: GY (%) =

C-A × 100 A

GE (%) =

C- A × 100 B-A

TC (%) =

B- A × 100 D

Rate of grafting =

1000× (C - A) V× m×t

Number of grafts per CA chain = Molecular weight of CA GY × Molecular weight of grafted PMMA 100

where A is the weight (g) of the original CA taken for the reaction; B, the weight (g) of the grafted CA before extraction; C, the weight (g) of the grafted

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INDIAN J. CHEM. TECHNOL., MAY 2013

product after extraction; D, the weight (g) of monomer charged; V, the volume (L) of the reaction mixture; m, the molecular mass of the monomer; and t, the reaction time (s). Molecular weight

CA grafted with PMMA was hydrolyzed with 72% H2SO4 to isolate PMMA23. The intrinsic viscosities [η] (cm3g-1) of isolated graft polymers were measured at 25°C, taking acetone as a solvent to estimate the viscosity average molecular weight (Mη) by using the following Mark-Houwink-Sakurada equation23: [η]Acetone = 5.3 × 10-3 Mη0.73 FTIR analysis

IR spectra of the grafted and ungrafted CA samples were recorded on a Perkin Elmer spectrophotometer (Spectrum RX1, Perkin Elmer, Singapore) using KBr pellet technique, in the range 4000-400 cm-1, with a resolution of 2 cm-1, using 4 scans per sample. NMR analysis

The 1H-NMR spectra of the grafted products were collected on a Bruker WM–400 spectrometer operating at 300 MHz for proton. All the chemical shifts were reported in parts per million (ppm) using tetramethylsilane (TMS) as the internal standard and DMSO-d6 as the solvent for the samples.

respectively. It is observed that the %GY and %TC increase with increase in reaction time. Effect of temperature

Grafting reactions are carried out by varying the temperature from 30°C to 80°C and the data of weight gain are given in Table 1. It is observed that at a particular reaction time the %GY of the MMA grafted products decreases with increase in reaction temperature, except at 60°C. The conversion of monomer to graft copolymer (%TC) reduces drastically and grafting efficiency (%GE) increases comparatively at 60°C. This may be due to the more tendencies for increasing the chain length of the graft copolymer rather than increasing the reaction site, thereby decreasing the number of grafts per CA chain (Table 1). Effect of monomer concentration

At a reaction temperature of 30°C and keeping CAN concentration at 0.5% (0.55 mmol), the concentration of MMA is changed from 2.0% (11.7 mmol) (samples CA-g-PMMA-01 to CA-gPMMA-05) (Table 1) to 2.4% (14.04 mmol) (samples CA-g-PMMA-31 to CA-g-PMMA-35) (Table 2) and 3.2% (18.72 mmol) (samples CA-g-PMMA-36 to CA-g-PMMA-40) (Table 2). The %GY and %TC of the grafted products increase with the increase in reaction time and monomer concentration.

Thermal analysis

Thermogravimetric (TG) and differential thermogravimetric (DTG) analyses of the grafted products were carried out using a Perkin Elmer simultaneous thermal analyzer (STA 6000), in the temperature range 50 – 600°C at a heating rate of 10°C min-1, under the nitrogen atmosphere. Indium was used as reference material for the study. For the analyses, 6 – 19 mg of the samples were used. Results and Discussion

Effect of initiator concentration

Grafting reactions are carried out at 30°C and 3.2% (18.72 mmol) MMA. CAN concentration is varied from 0.5% (0.55 mmol) (samples CA-g-PMMA-36 to CA-g-PMMA-40) (Table 2) to 0.64% (0.73 mmol) (samples CA-g-PMMA-41 to CA-g-PMMA-45) (Table 3) and 0.8% (0.916 mmol) (samples CA-gPMMA-46 to CA-g-PMMA-50) (Table 3). As evident from the tables, the %GY and %TC increase with the increase in the initiator concentration at a particular reaction time.

Effect of reaction time

Graft copolymerization of MMA onto CA in DMSO is carried out at 30, 40, 50, 60, 70 and 80°C with reaction time ranging from 2 h to 6 h with 1 h interval. The data on weight gain with respect to reaction time using 11.7 mmol MMA and 0.55 mmol CAN at different temperatures are shown in Table 1. Table 2 represents the reaction at 30°C with 0.55 mmol CAN, and 14.04 and 18.72 mmol monomer respectively. Table 3 shows the reactions at 30°C with 18.72 mmol MMA, and 0.73 and 0.916 mmol CAN

Effect of inhibitor

To study the effect of methylene blue as the inhibitor for the formation of homopolymer, the reaction is carried out at 80°C with 2.0% (11.7 mmol) MMA, 0.5% (0.55 mmol) CAN and 0.7 mL (1.09 ppm) methylene blue (Table 4). As the molecular weight of the extracted polymer is not uniform at 80°C (Table 1), the same condition is followed for this comparative study. Table 4 shows that the %GY of the grafted products is little more as


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Table 1—Graft copolymerization of PMMA onto cellulose acetate at different temperatures using 11.7 mmol MMA and 0.55 mmol CAN Reaction temp.

Sample code

°C

Reaction time, h

% GY

% GE

% TC

Mw of PMMA

No. of grafts/ CA chain

Rate of grafting

30

CA-g-PMMA-01 CA-g-PMMA-02 CA-g-PMMA-03 CA-g-PMMA-04 CA-g-PMMA-05

2 3 4 5 6

32 36 44 48 52

14.2 15.0 18.3 16.2 16.3

47.8 51.2 52.1 61.5 66.6

9434 11991 14662 18273 22059

1.7 1.5 1.5 1.3 1.2

38400 64800 105600 144000 187200

40


2 3 4 5 6

32 32 36 48 52

11.2 22.3 20.4 16.9 27.3

29.8 29.9 34.2 35.7 41.8

9434 7608 8046 9894 11803

1.7 2.1 2.2 2.4 2.2

38400 57600 86400 144000 187200

50


2 3 4 5 6

28 32 36 40 48

20.1 21.8 18.4 29.2 25.2

29.9 32.7 32.4 35.7 34.9

18029 20338 18030 14662 10291

0.8 0.8 1.0 1.4 2.3

33600 57600 86400 120000 172800

60


2 3 4 5 6

28 40 44 48 68

46.8 55.3 57.1 38.1 65.6

12.2 15.6 16.3 17.1 22.3

22766 28027 22945 24364 25249

0.6 0.7 1.0 1.0 1.3

33600 72000 105600 144000 224800

70


2 3 4 5 6

16 24 28 36 44

40.0 54.5 46.6 60.0 68.7

8.52 9.40 12.8 12.9 13.6

13161 11207 14662 10330 11710

0.6 1.1 1.0 1.7 1.9

19200 43200 67200 108000 158400

80


2 3 4 5 6

08 12 16 16 20

40.0 42.8 40.0 66.6 50.0

4.27 5.98 8.54 5.12 8.77

4891 11446 12105 19023 23423

0.8 0.5 0.7 0.4 0.4

9600 21600 38400 48000 187200

GY – Graft yield, GE – Grafting efficiency, TC – Total conversion of monomer to polymer. Table 2—Graft copolymerization of PMMA onto cellulose acetate at 30°C with different MMA concentrations and 0.55 mmol CAN MMA conc. mmol

Sample code

Reaction time, h

% GY

% GE

% TC

Mw of PMMA


Rate of grafting

14.04


2 3 4 5 6

36 40 48 52 64

25.0 37.03 46.15 43.33 50.00

25.89 19.22 18.51 21.36 25.25

3492 4707 5642 6480 6209

5.2 4.2 4.3 4.0 5.2

43200 72000 115200 156000 230400

18.72


2 3 4 5 6

40 44 48 56 72

45.40 47.00 41.66 43.80 48.07

11.76 12.55 15.38 15.63 17.77

5642 6575 6391 6945 5042

3.5 3.3 3.8 4.0 7.1

48000 79200 115200 168000 259200


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Table 3—Graft copolymerization of PMMA onto CA at 30°C with 18.72 mmol MMA and different concentrations of CAN CAN conc. mmol

Sample code

Reaction time, h

% GY

% GE

% TC

Mw of PMMA


Rate of grafting

0.73


2 3 4 5 6

40 48 56 68 80

25.64 23.52 24.13 29.31 26.66

20.83 27.24 30.98 28.31 40.06

5745 6676 6391 7240 5042

3.5 3.6 4.4 4.7 7.9

48000 86400 134400 204000 288000

0.916


2 3 4 5 6

36 48 52 60 92

11.34 14.63 15.11 19.73 17.16

42.20 43.80 45.94 40.59 71.58

6147 6391 3492 6738 6106

2.9 3.8 7.4 4.5 7.5

43200 86400 124800 180000 331200

Table 4—Graft copolymerization of PMMA onto cellulose acetate at 80°C with 11.7 mmol MMA, 0.55 mmol CAN and 0.7 mL (1.09 ppm) methylene blue Sample code

Reaction time, h

% GY

% GE

% TC

Mw of PMMA

2 3 4 5 6

20 20 24 24 28

50.0 49.15 47.27 47.08 45.19

0.08 1.70 1.79 1.82 5.1

14288 26388 29655 31513 27503


compared to that when the reaction is carried out without the inhibitor (Table 1). The molecular weight of PMMA is more uniform at higher reaction time. Molecular weight per CA chain

of

PMMA

and

number

of

grafts

The molecular weights of PMMA extracted from the grafted samples prepared under different reaction conditions are determined. As seen in Table 1, the grafting reactions with 11.7 mmol monomer and 0.55 mmol CAN, at six temperature conditions give the grafted products having well control on the molecular weight of the PMMA and number of grafts per CA chain. At monomer concentration of 14.04 and 18.72 mmol (Table 2), the molecular weight of PMMA goes on increasing with increase in time up to 5 h and then decreases. The number of grafts per CA chain does not show much variation except at 6 h reaction time for the sample CA-g-PMMA-40, for which it has the maximum value of 7.1. The same trend is also observed at a monomer concentration of 18.72 mmol with CAN concentration of 0.73 and 0.916 mmol (Table 3) having a maximum value at the reaction time of 6 h (samples CA-g-PMMA-45 and CA-g-PMMA-50). For the reactions with methylene blue (Table 4), there is almost no change in the extracted polymer molecular weight and the number

No. of grafts/CA chain 0.7 0.4 0.4 0.4 0.5

Rate of grafting 24000 36000 57600 72000 100800

of grafts per CA for the reaction time 3–6 h. This shows that methylene blue acts as a good inhibitor for the formation of homopolymer, thereby increasing the molecular weight of the graft copolymer and decreasing the grafting sites in the CA chain (Table 4). Moreover, Tables 1–3 show that the conditions for getting the sample CA-g-PMMA-40 are considered as the optimum conditions, such as grafting temperature 30°C, reaction time 6 h, monomer concentration 18.72 mmol and initiator concentration 0.55 mmol. For this sample, the %GY is 72, % GE is 48.07, molecular weight of the homopolymer is 5042 and number of grafts per CA chain is 7.1. Therefore, under these reaction conditions, the molecular weight of the grafted side chain can be increased by using the inhibitor (Table 4). FTIR studies

FTIR spectra of the CA and grafted cellulose acetate in the absence of methylene blue (CA-g-PMMA-50) and in presence of methylene blue (CA-g-PMMA-55) show identical peaks at 3439 cm-1 (OH str of CA), 2998 cm-1 (-CH3 and –CH2- of PMMA and CA), 1725 cm-1 (>C=O str. of PMMA and CA), 1635 cm-1 (C-C str), 1481 cm-1 (OH bending of CA), 1443 cm-1 (-CH- bending), 1270 and 1240 cm-1 (–C-O-C- bending of PMMA), 1190 and


1144 cm-1 (C-C str of CA and PMMA), 1060 cm-1 (–CH2- wagging of CA), and 745 cm-1 (CH rocking vibrations of CA and PMMA), thereby indicating the formation of MMA-grafted CA. NMR studies 1

H-NMR spectra of the grafted cellulose acetate with PMMA (CA-g-PMMA-50 and CA-g-PMMA-55) show identical peaks and the peak at 3.36 ppm is due to the –O–CH3 group of the grafted polymer. The –CH2- group shows peaks at 2.07, 1.94 and 1.87 ppm and the peak at 1.18 ppm is for the –CH3 group34. The peak at 5.06 ppm is due to the –OH group of the cellulose chain35 and the peaks at 4.53 and 2.5 ppm are due to the –CH2-O-CO-CH3 of CA. Mechanism of polymerization

It is known that metallic cations form complexes with carbon hydrates. After complexation with CA, ceric ion is reduced to cerous ion, the bond between C2 and C3 is broken and a free radical appears on C2 or C336. Then this free radical initiates the monomer grafting and the polymerization reaction of MMA. The FTIR and 1H-NMR spectra of the grafted products also show the peacks for the –OH group which proves that the grafting occurs by breaking of a C-C bond and not at the –OH group. The reaction mechanism is shown in Scheme 1.

207

Methylene blue (tetramethyl thionine chloride, C16H18ClN3S) (Scheme 2) is a heterocyclic aromatic dye, a member of thiazine dyes. The redox properties of MB are provided by an ability to accept or donate hydrogen ions37. Hence, the hydrogen ion generated in the initiation step (Scheme 1) reduces MB to leuco methylene blue (LMB) and by this Ce3+, again converted to Ce4+. Since the dye affects the termination step38, in this step the chloride ion of MB can easily react with active free radical of the monomer to generate semi-reduced MB radical, which can readily be recombined with another radical to terminate the step and hinders the homopolymer formation. This is also evident from Table 4 that all the samples prepared in the presence of methylene blue show uniformity in the homopolymer molecular weight. Thermogravimetric analysis

Dynamic thermogravimetric curves of CA and CA grafted products show three different weight loss zones. An initial zone of slight loss in weight is due to evaporation of water. Then the break in each thermogram indicates the onset of the decomposition process involving rapid loss in weight. At the end of this break a slight curvature is formed which might be due to the formation and evaporation of some volatile compounds. Finally, the decomposition rate decreases gradually to a constant weight representing

Scheme 1—Mechanism of grafting of PMMA onto CA by CAN


208

Scheme 2—Effect of methylene blue in the initiation and termination step Table 5—Thermal stability of CA and CA grafted products in nitrogen atmosphere at heating rate 10°C min-1 Sample code

IDT, °C

ADT, °C

Wt loss, %

200°C 250°C 300°C 350°C CA 337 367 2.3 2.6 3.9 22.4 CA-g-PMMA-50 325 361 4.6 6.2 11.4 42.0 CA-g-PMMA-55 316 358 13.9 17.1 25.3 51.2 IDT – Initial decomposition temperature, ADT – Active decomposition temperature.

400°C 83.1 84.0 87.6

450°C 85.7 86.7 93.0

500°C 88.2 90.4 97.4

550°C 92.2 96.6 100.0

carbonization39. The percentage weight loss of these samples with temperature are given in Table 5. The initial decomposition for CA-g-PMMA-50 starts at 325°C, whereas that for CA-g-PMMA-55 starts at 316°C which is less than CA (337°C). The DTG curves show the temperature of active decomposition (ADT), which is 367°C for CA and goes on decreasing to 361°C for CA-g-PMMA-50 and 358°C for CA-g-PMMA-55. This may be due to the increase in the percentage of grafting and molecular weight of the homo polymer for the grafted products. The results reveal a decrease in the thermal stability with an increase in the percentage of grafting40 and molecular weight of the grafted polymer. The thermal degradation of CA-g-PMMA-50 is less in comparison to Cell-g-PMMA-55 (Table 5) which may be due to the high molecular weight of the grafted PMMA chain.

free radical in the CA chain, which initiates the polymerization by free radical mechanism. It is concluded that in presence of methylene blue as the inhibitor for homopolymer formation, the grafted products show uniform molecular weight of the grafted chain. It is also concluded that the increase in the percentage of grafting and molecular weight of the homopolymer decreases the thermal stability of the compound.

Conclusion Homogeneous graft copolymerization of MMA onto CA in DMSO solvent system can be carried out by using CAN as an initiator. The formation of grafted products is confirmed by FTIR and 1H-NMR spectroscopy. The effect of reaction time, reaction temperature, monomer and initiator concentration on the %GY, %GE and %TC is evaluated. Graft copolymerization of CA in presence of Ce+4 proceeds through ring opening of cellulose and formation of

2

Acknowledgement The authors are thankful to the Department of Science and Technology, New Delhi for financial support to carry out the work. References 1

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