Grafting of polymethyl methacrylate onto cellulose acetate in homogeneous medium using ceric (IV) ion as initiator

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1 Indian Journal of Chemical Technology Vol. 0, May 01, pp 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 , India Received March 01; accepted 1 January 01 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 1 H-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 tow 1. 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 temperature,. 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 chains. 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 polymers. Graft polymerization proceeds by different mechanisms (free radical, anionic, cationic, ring opening, etc.) depending on the *Corresponding author. bntosh@yahoo.com 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 decades -18. The derivatization and/or grafting reactions in homogeneous conditions assure important advantages over heterogeneous system, like a better control of the degree of substitution 19, a more uniform distribution of substituents along the polymer and a higher conversion yield 0,. During past few years a number of cellulose derivatives and grafted products have already been synthesized under homogeneous conditions 19-1, 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 cellulose -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

2 ROUTRAY et al.: GRAFTING OF PMMA ONTO CELLULOSE ACETATE 0 presence of dimethyl sulfoxide (DMSO) can produce Ce + ion 0 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 process. 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 + 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 ( 1 H-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, 7.9 acetyl and M n ~ 0,000 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 aqueous NaOH, water and dried over Na SO and then under CaH at reduced pressure. The stabilizer free monomer was distilled under reduced pressure at 0 C and stored below C. Ceric ammonium nitrate (CAN) (E-Merck), dimethyl sulfoxide (DMSO) (Qualigen), methylene blue (E-Merck) were of reagent grade and used without further purification. N gas was passed through alkaline pyrogallol, sulfuric acid, and potassium hydroxide solution before it was passed into the reaction mixture. Grafting An amount of 1. g of cellulose acetate (7.0 mmol of the corresponding anhydroglucose unit by considering 7.9 acetyl content of CA) was dissolved in. ml DMSO taken in a three-necked round bottom flask equipped with a magnetic stirrer and temperature controlled oil bath to make a solution. To this, different amount of CAN ranging from 0. g to 0. g ( , mmol) was added followed by addition of 1..0 ml (.0 -., mmol) of MMA. All the reactions were carried out in a dry nitrogen atmosphere. The reactions were carried out for - h at varying temperature range of 0-80 C. The reaction was terminated by addition of hydroquinone. The polymerization mixture was poured into cold distilled water with vigorous stirring, kept overnight at C, filtered, washed thoroughly in cold distilled water, dried at 0 C and weighed. Then the products were soxhlet extracted with acetone for h to remove any adherent homopolymer. The extracted CA-grafted products were then dried at 0 C and stored over P O. 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. 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 relations : GY () = GE () = C - A A C - A B - A B - A TC() = D ( C - A) grafting = 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

3 0 INDIAN J. CHEM. TECHNOL., MAY 01 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 7 H SO to isolate PMMA. The intrinsic viscosities [η] (cm g -1 ) of isolated graft polymers were measured at C, taking acetone as a solvent to estimate the viscosity average molecular weight (M η ) by using the following Mark-Houwink-Sakurada equation : [η] Acetone = M η 0.7 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 cm -1, with a resolution of cm -1, using scans per sample. NMR analysis The 1 H-NMR spectra of the grafted products were collected on a Bruker WM 00 spectrometer operating at 00 MHz for proton. All the chemical shifts were reported in parts per million (ppm) using tetramethylsilane (TMS) as the internal standard and DMSO-d as the solvent for the samples. Thermal analysis Thermogravimetric (TG) and differential thermogravimetric (DTG) analyses of the grafted products were carried out using a Perkin Elmer simultaneous thermal analyzer (STA 000), in the temperature range 0 00 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, 19 mg of the samples were used. Results and Discussion Effect of reaction time Graft copolymerization of MMA onto CA in DMSO is carried out at 0, 0, 0, 0, 70 and 80 C with reaction time ranging from h to h with 1 h interval. The data on weight gain with respect to reaction time using 11.7 mmol MMA and 0. mmol CAN at different temperatures are shown in Table 1. Table represents the reaction at 0 C with 0. mmol CAN, and 1.0 and 18.7 mmol monomer respectively. Table shows the reactions at 0 C with 18.7 mmol MMA, and 0.7 and 0.91 mmol CAN 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 0 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 0 C. The conversion of monomer to graft copolymer (TC) reduces drastically and grafting efficiency (GE) increases comparatively at 0 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 0 C and keeping CAN concentration at 0. (0. mmol), the concentration of MMA is changed from.0 (11.7 mmol) (samples CA-g-PMMA-01 to CA-g- PMMA-0) (Table 1) to. (1.0 mmol) (samples CA-g-PMMA-1 to CA-g-PMMA-) (Table ) and. (18.7 mmol) (samples CA-g-PMMA- to CA-g-PMMA-0) (Table ). The GY and TC of the grafted products increase with the increase in reaction time and monomer concentration. Effect of initiator concentration Grafting reactions are carried out at 0 C and. (18.7 mmol) MMA. CAN concentration is varied from 0. (0. mmol) (samples CA-g-PMMA- to CA-g-PMMA-0) (Table ) to 0. (0.7 mmol) (samples CA-g-PMMA-1 to CA-g-PMMA-) (Table ) and 0.8 (0.91 mmol) (samples CA-g- PMMA- to CA-g-PMMA-0) (Table ). As evident from the tables, the GY and TC increase with the increase in the initiator concentration at a particular reaction time. 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.0 (11.7 mmol) MMA, 0. (0. mmol) CAN and 0.7 ml (1.09 ppm) methylene blue (Table ). 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 shows that the GY of the grafted products is little more as

4 ROUTRAY et al.: GRAFTING OF PMMA ONTO CELLULOSE ACETATE 0 Table 1 Graft copolymerization of PMMA onto cellulose acetate at different temperatures using 11.7 mmol MMA and 0. mmol CAN Reaction temp. C Sample code Reaction time, h GY GE TC Mw of PMMA No. of grafts/ CA chain grafting 0 CA-g-PMMA-01 CA-g-PMMA-0 CA-g-PMMA-0 CA-g-PMMA-0 CA-g-PMMA CA-g-PMMA-0 CA-g-PMMA-07 CA-g-PMMA-08 CA-g-PMMA-09 CA-g-PMMA CA-g-PMMA-11 CA-g-PMMA-1 CA-g-PMMA-1 CA-g-PMMA-1 CA-g-PMMA CA-g-PMMA-1 CA-g-PMMA-17 CA-g-PMMA-18 CA-g-PMMA-19 CA-g-PMMA CA-g-PMMA-1 CA-g-PMMA- CA-g-PMMA- CA-g-PMMA- CA-g-PMMA CA-g-PMMA- CA-g-PMMA-7 CA-g-PMMA-8 CA-g-PMMA-9 CA-g-PMMA GY Graft yield, GE Grafting efficiency, TC Total conversion of monomer to polymer. Table Graft copolymerization of PMMA onto cellulose acetate at 0 C with different MMA concentrations and 0. mmol CAN MMA conc. mmol Sample code 1.0 CA-g-PMMA-1 CA-g-PMMA- CA-g-PMMA- CA-g-PMMA- CA-g-PMMA- Reaction time, h GY 0 8 GE TC Mw of PMMA No. of grafts/ CA chain grafting CA-g-PMMA- CA-g-PMMA-7 CA-g-PMMA-8 CA-g-PMMA-9 CA-g-PMMA

5 0 INDIAN J. CHEM. TECHNOL., MAY 01 Table Graft copolymerization of PMMA onto CA at 0 C with 18.7 mmol MMA and different concentrations of CAN CAN conc. mmol Sample code 0.7 CA-g-PMMA-1 CA-g-PMMA- CA-g-PMMA- CA-g-PMMA- CA-g-PMMA- Reaction time, h GY GE TC Mw of PMMA No. of grafts/ CA chain grafting CA-g-PMMA- CA-g-PMMA-7 CA-g-PMMA-8 CA-g-PMMA-9 CA-g-PMMA Table Graft copolymerization of PMMA onto cellulose acetate at 80 C with 11.7 mmol MMA, 0. mmol CAN and 0.7 ml (1.09 ppm) methylene blue Sample code CA-g-PMMA-1 CA-g-PMMA- CA-g-PMMA- CA-g-PMMA- CA-g-PMMA- Reaction time, h GY GE TC Mw of PMMA No. of grafts/ca chain grafting 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 of PMMA and number of grafts per CA chain 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. 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 1.0 and 18.7 mmol (Table ), the molecular weight of PMMA goes on increasing with increase in time up to h and then decreases. The number of grafts per CA chain does not show much variation except at h reaction time for the sample CA-g-PMMA-0, for which it has the maximum value of 7.1. The same trend is also observed at a monomer concentration of 18.7 mmol with CAN concentration of 0.7 and 0.91 mmol (Table ) having a maximum value at the reaction time of h (samples CA-g-PMMA- and CA-g-PMMA-0). For the reactions with methylene blue (Table ), there is almost no change in the extracted polymer molecular weight and the number of grafts per CA for the reaction time 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 ). Moreover, Tables 1 show that the conditions for getting the sample CA-g-PMMA-0 are considered as the optimum conditions, such as grafting temperature 0 C, reaction time h, monomer concentration 18.7 mmol and initiator concentration 0. mmol. For this sample, the GY is 7, GE is 8.07, molecular weight of the homopolymer is 0 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 ). FTIR studies FTIR spectra of the CA and grafted cellulose acetate in the absence of methylene blue (CA-g-PMMA-0) and in presence of methylene blue (CA-g-PMMA-) show identical peaks at 9 cm -1 (OH str of CA), 998 cm -1 (-CH and CH - of PMMA and CA), 17 cm -1 (>C=O str. of PMMA and CA), 1 cm -1 (C-C str), 181 cm -1 (OH bending of CA), 1 cm -1 (-CH- bending), 170 and 10 cm -1 ( C-O-C- bending of PMMA), 1190 and

6 ROUTRAY et al.: GRAFTING OF PMMA ONTO CELLULOSE ACETATE cm -1 (C-C str of CA and PMMA), 100 cm -1 ( CH - wagging of CA), and 7 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-0 and CA-g-PMMA-) show identical peaks and the peak at. ppm is due to the O CH group of the grafted polymer. The CH - group shows peaks at.07, 1.9 and 1.87 ppm and the peak at 1.18 ppm is for the CH group. The peak at.0 ppm is due to the OH group of the cellulose chain and the peaks at. and. ppm are due to the CH -O-CO-CH 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 C and C is broken and a free radical appears on C or C. Then this free radical initiates the monomer grafting and the polymerization reaction of MMA. The FTIR and 1 H-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. Methylene blue (tetramethyl thionine chloride, C 1 H 18 ClN S) (Scheme ) 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 ions 7. Hence, the hydrogen ion generated in the initiation step (Scheme 1) reduces MB to leuco methylene blue (LMB) and by this Ce +, again converted to Ce +. Since the dye affects the termination step 8, 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 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

7 08 INDIAN J. CHEM. TECHNOL., MAY 01 Scheme Effect of methylene blue in the initiation and termination step Table 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, CA CA-g-PMMA-0 CA-g-PMMA C 0 C 00 C 0 C 00 C 0 C 00 C 0 C IDT Initial decomposition temperature, ADT Active decomposition temperature carbonization 9. The percentage weight loss of these samples with temperature are given in Table. The initial decomposition for CA-g-PMMA-0 starts at C, whereas that for CA-g-PMMA- starts at 1 C which is less than CA (7 C). The DTG curves show the temperature of active decomposition (ADT), which is 7 C for CA and goes on decreasing to 1 C for CA-g-PMMA-0 and 8 C for CA-g-PMMA-. 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 grafting 0 and molecular weight of the grafted polymer. The thermal degradation of CA-g-PMMA-0 is less in comparison to Cell-g-PMMA- (Table ) which may be due to the high molecular weight of the grafted PMMA chain. 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 1 H-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 + proceeds through ring opening of cellulose and formation of 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. Acknowledgement The authors are thankful to the Department of Science and Technology, New Delhi for financial support to carry out the work. References 1 Reveley A, Cellulose and its derivatives: Chemistry, Biochemistry and Applications 1. Cellulose, edited by J F Kennedy (Ellis Horwood Limited, Chichester), 198. Brewer R J & Bogan R T, Encyclopedia of Polymer Science and Engineering, Vol, nd edn, edited by Mark H F & Bikales N M (Wiley Interscience, New York), 198. Buchanan C M, Gedon S C, Pearcy B G, White A W & Wood M D, Macromol, (199) 70. Biermann C J, Chung J B & Narayan R, Macromol, 0 (1987) 9. Nie L & Narayan R, J Appl Polym Sci, (199) 01. Gupta K C & Khandekar K, Biomacromol, (00) Gupta K C, Sahoo S & Khandekar K, Biomacromol, (00) 1087.

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