Homogeneous grafting of PMMA onto cellulose in presence of Ce 4+ as initiator

Transcription

1 Indian Journal of Chemical Technology Vol. 18, May 011, pp. - Homogeneous grafting of PMMA onto cellulose in presence of Ce + as initiator B Tosh* & C R Routray Department of Chemistry, Orissa Engineering College, Bhubaneswar , India Received 1 June 010; accepted March 011 Poly (methyl methacrylate) (PMMA) is successfully grafted onto cellulose in homogeneous medium in N,N-dimethyl acetamide/licl solvent system. The method is based upon ring opening reaction of cellulose with Ce + ion. Ceric ammonium nitrate (CAN) forms free radical on cellulose back bone in presence of DMSO via ring opening mechanism and PMMA is grafted through homogeneous breaking of the acrylic double bond. Methylene blue was used as an inhibitor to check the formation of homopolymer. The graft yield and grafting efficiency was studied by varying reaction time, temperature, and monomer concentration. The products are characterized by FTIR and 1 H-NMR analysis and a possible reaction mechanism is deduced. Thermal degradation of the grafted products is also studied by thermo-gravimetric analysis (TG). Keywords: Methyl methacrylate (MMA), Homogeneous medium, N,N Dimethyl Acetamide/LiCl, Ceric ammonium nitrate (CAN), DMSO, Grafting, Ring opening, FTIR, Grafting efficiency, Methylene blue, Inhibitor Cellulose is the most abundant naturally occurring polymer and its derivatives have many important applications in fiber, paper and paint industries. The polymeric material with desired properties is a current need of the society. To control the properties such as hydrophobicity, adhesivity, selectivity, drug delivery, wettability and thermosencitivity, graft copolymerization of suitable monomer is the versatile technique for cellulose modification 1. Although cellulose has good properties it has some undesirable ones such as low tensile strength, high moisture regain, and low strength against microbial attack and hence grafting of synthetic polymers on cellulose eliminates these drawbacks and allows the acquisition of additional properties of grafted polymers without destroying its own properties. Heterogeneous grafting of synthetic polymers onto cellulose back bone has been carried out by many researchers The discovery of new solvents for cellulose dissolution opened the possibility of performing derivatization and/or grafting reactions in homogeneous conditions, thus assuring important advantages, such as a better control of the degree of substitution 11, a more uniform distribution of substituents along the polymer and a higher conversion yield 1,1. During past few years a number of cellulose derivatives has already been synthesized under Corresponding author ( bntosh@yahoo.com) homogeneous conditions 11,1-1 and work on homogeneous grafting of vinyl monomers onto cellulose and some cellulose derivatives in nondegradable solvent systems such as DMSO/ PF 1,1,17,18, DMSO/PhMe 19,0 and DMAc/ LiCl 1,1,1, have been tried using different initiators like ammonium persulfate (APS), azobisisobutyronitrile(aibn), and benzoyl peroxide, but so far work on homogeneous graft copolymerization of cellulose dissolved in N,Ndimethyl acetamide/lithium chloride (DMAc/LiCl) solvent system 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 1- in heterogeneous medium but in homogeneous conditions this will produce gel confirming the regeneration of cellulose from DMAc/LiCl solvent system. It is only reported that CAN in presence of dimethyl sulfoxide (DMSO) can produce Ce + ion 1 and can be a suitable redox system to initiate graft copolymerization process, but hardly any work is reported on this system. It is also reported that presence of methylene blue in the reaction system reduces the formation of homopolymers in the graft copolymerization process. Therefore, in the present study we describe the homogeneous graft copolymerization of methylmethacrylate (MMA) onto cellulose in DMAc/LiCl solvent system taking CAN in presence

2 TOSH & ROUTRAY: HOMOGENEOUS GRAFTING OF PMMA ONTO CELLULOSE of DMSO as initiator. The effect of varying in reaction time, temperature, concentration of initiators and monomer were studied to optimize the conditions under which grafting would occur most effectively. The effect of methylene blue on homopolymers formation is also studied. The grafted products obtained were 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 were determined. Finally, thermal degradation of the grafted products was studied by thermo-gravimetric (TG) analysis. Experimental Procedure Materials Cellulose powder from cotton linters, having viscosity 0-10 cp (Brookfield RTV, Spindle # 1, 0 rpm (lit)) was obtained from Sigma Aldrich Chemicals Pvt. Ltd. It was purified by washing with methanol, acetone, and de-ionized water and finally dried in an oven at 0 C for 7 days. The viscosity average molecular weight of cellulose was calculated by nitrating the sample and using Mark-Houwink- Sakurada equation and was found to be,00. Methyl methacrylate purchased from Sigma Aldrich was purified from the inhibitor (hydroquinone monomethyl ether) by extracting with aqueous sodium chloride sodium hydroxide solution and dried over sodium sulfate. The stabilizer free monomer was vacuum distilled and stored below C. DMAc (sd-fine chemicals) was distilled under CaH and stored on molecular sieve ( Å) under nitrogen atmosphere. Ceric ammonium nitrate and DMSO (E-Merck) were of reagent grade and used without further purification. Methylene blue was procured from E-Merck and is a 0.0 (w/v) aqueous solution. N gas was passed through alkaline pyrogallol, sulfuric acid and potassium hydroxide solution before it was passed into the reaction mixture. Preparation of cellulose solution A solution of cellulose was prepared by taking 8 g of cellulose in 00 ml of DMAc, heated at 10 C for min in a round bottom flask equipped with a short path condenser. Then 0 g LiCl was added and heated up to 1 C for 8 min. It was stirred overnight to get a clear solution 11,1. The solution was made 1 during grafting. Grafting ml of 1 cellulose solution (1. mmol of the corresponding anhydroglucose unit) was taken in a three necked round bottom flask, equipped with a magnetic stirrer and temperature controlled oil bath. To this different amount of CAN ranging from 0. to 0.7 g ( mmol) dissolved in 10 ml DMSO was added followed by addition of ml ( mmol) of MMA. All the reactions were carried out in a dry nitrogen atmosphere. The reaction was carried out at different temperatures between 0 and 80 C for - h. The reaction was terminated by addition of hydroquinone 1. The polymerization mixture was poured into cold distilled water with vigorous stirring and kept overnight at C and then filtered, washed thoroughly in cold distilled water and dried at 0 C and weighed. Then the products were soxhlet extracted with acetone for h to remove any adherent homopolymer. The extracted cellulose-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 monomer concentration 18.7 mmol and initiator concentration 1.8 mmol. The graft yield (GY), total conversion of monomer to polymer (TC), grafting efficiency (GE) and number of graft per cellulose chain were calculated on the basis of oven-dried weight of the cellulose from the increase in weight after grafting by using the following relations. C A GY () = 100 A C A GE () = 100 B A B A TC () = 100 D Number of grafts per cellulose chain = Molecular weight of Moleculr weight of cellulose GY grafted PMMA 100 Where A is the weight in grams of the original cellulose taken for the reaction; B is the weight in grams of the grafted cellulose before extraction; C is

3 INDIAN J. CHEM TECHNOL., MAY 011 the weight in grams of the grafted product after extraction and D is the weight in grams of monomer charged. Molecular weight Cellulose grafted with PMMA was hydrolyzed with 7 H SO to isolate PMMA 1. 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 by using the following Mark-Houwink-Sakurada equation 1. [η] Acetone = M 0.7 FTIR analysis IR spectra of the grafted and ungrafted cellulose samples were recorded on a PerkinElmer spectrometer (Spectrum RX1, PerkinElmer, 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 CDCl as the solvent for the samples. Thermal analysis Thermo gravimetric (TG) analysis of the grafted products was carried out using a Simadzu thermal analyzer 0, in the temperature ranges from C at a heating rate 10 C.min -1, in air atmosphere. Alphaalumina was used as reference material for the study. 7-9 mg of the samples were taken for analysis. Results and Discussion Effect of reaction time The graft co-polymerization of MMA onto cellulose is carried out at 0, 70, and 80 C with reaction time ranging from - h with 1 h interval. The data on weight gain with respect to reaction time at different temperatures are shown in Tables 1-. Figures 1- represent the percentage weight gain versus reaction time at temperatures 0, 70 and 80 C, respectively with MMA concentration 11.7 mmol and CAN concentration 0.91 mmol. Figures and represent the reactions at 80 C with CAN concentration 0.91 mmol and monomer concentrations 1.0 and 18.7 mmol. Figures 7 and 8 show the percentage weight gain versus reaction time at 80 C with MMA concentration 18.7 mmol and CAN concentration 1.1 and 1.8 mmol. It is observed that the GY and TC is increased with increase in reaction time in all the cases. Effect of temperature Grafting reactions are carried out by varying the temperature from 0 to 80 C and the data of weight Table 1 Graft co-polymerization of MMA onto cellulose at different temperatures with MMA concentration (11.7 mmol) and CAN concentration (0.91 mmol) in 10 ml DMSO Reaction temperature ( C) Sample code Reaction time (h) GY GE TC Mw of PMMA No of grafts/cellulose chain 0 Cell-g-PMMA-01 Cell-g-PMMA-0 Cell-g-PMMA-0 Cell-g-PMMA-0 Cell-g-PMMA Cell-g-PMMA-0 Cell-g-PMMA-07 Cell-g-PMMA-08 Cell-g-PMMA-09 Cell-g-PMMA Cell-g-PMMA-11 Cell-g-PMMA-1 Cell-g-PMMA-1 Cell-g-PMMA-1 Cell-g-PMMA

4 TOSH & ROUTRAY: HOMOGENEOUS GRAFTING OF PMMA ONTO CELLULOSE 7 Table Graft co-polymerization of MMA onto cellulose with different monomer concentration at 80 C and CAN concentration (0.91 mmol) in 10 ml DMSO Monomer concentration Sample code Reaction time (h) GY GE TC Mw of PMMA No of grafts/cellulose chain (1.0 mmol) Cell-g-PMMA-1 Cell-g-PMMA-17 Cell-g-PMMA-18 Cell-g-PMMA-19 Cell-g-PMMA (18.7 mmol) Cell-g-PMMA-1 Cell-g-PMMA- Cell-g-PMMA- Cell-g-PMMA- Cell-g-PMMA- 0 Table Graft co-polymerization of MMA onto cellulose with different initiator concentration at 80 C and MMA concentration 8 (18.7 mmol) Initiator concentration Sample code Reaction time (h) GY GE TC Mw of PMMA No of grafts/cellulose chain (1.1 mmol) Cell-g-PMMA- Cell-g-PMMA-7 Cell-g-PMMA-8 Cell-g-PMMA-9 Cell-g-PMMA (1.8 mmol) Cell-g-PMMA-1 Cell-g-PMMA- Cell-g-PMMA- Cell-g-PMMA- Cell-g-PMMA- gain are given in Table 1 and Figures 1-. It is observed that at a particular reaction time the GY of the MMA grafted product is also increased with increase in reaction temperature. Effect of monomer concentration At a reaction temperature of 80 C, keeping the CAN concentration at (0.91 mmol) the concentration of MMA is changed from (11.7 mmol) (Samples; Cell-g-PMMA-11 to Cell-g- PMMA-1; Table 1) to (1.0 mmol) (Samples; Cell-g-PMMA-1 to Cell-g-PMMA-0; Table ) 8 0 Table Graft co-polymerization of MMA onto cellulose at 80 C with CAN concentration 7 (1.8 mmol) and MMA concentration 8 (18.7 mmol) and methylene blue 0.7 ml (1.09 ppm). Sample code Cell-g-PMMA- Cell-g-PMMA-7 Cell-g-PMMA-8 Cell-g-PMMA-9 Cell-g-PMMA-0 Reaction time (h) GY 8 0 GE TC Mw of PMMA No of grafts/cellulose chain and 8 (18.7 mmol) (Samples; Cell-g-PMMA-1 to Cell-g-PMMA-; Table ). Figures - show the GY, GE and TC of the grafted products. GY and TC increase with increase in reaction time and monomer concentration. Effect of initiator concentration Grafting reactions are carried out at 80 C at MMA concentration 8 (18.7 mmol) and CAN concentration is changed from (0.91 mmol) (Samples; Cell-g-PMMA-1 to Cell-g-PMMA-; Table ) to (1.1 mmol) (Samples; Cell-g-PMMA-

5 8 INDIAN J. CHEM TECHNOL., MAY 011 Fig. 1 Effect of reaction time on grafting of MMA onto cellulose in presence of CAN at 0 C: cellulose, 1 (1. mmol); MMA, 11.7 mmol; CAN, 0.91 mmol Fig. Effect of reaction time on grafting of MMA onto cellulose in presence of CAN at 80 C: cellulose, 1 (1. mmol); MMA, 1.0 mmol; CAN, 0.91 mmol Fig. Effect of reaction time on grafting of MMA onto cellulose in presence of CAN at 70 C: cellulose, 1 (1. mmol); MMA, 11.7 mmol; CAN, 0.91 mmol Fig. Effect of reaction time on grafting of MMA onto cellulose in presence of CAN at 80 C: cellulose, 1 (1. mmol); MMA, 11.7 mmol; CAN, 0.91 mmol Fig. Effect of reaction time on grafting of MMA onto cellulose in presence of CAN at 80 C: cellulose, 1 (1. mmol); MMA, 18.7 mmol; CAN, 0.91 mmol to Cell-g-PMMA-0; Table ) and 7 (1.8 mmol) (Samples; Cell-g-PMMA-1 to Cell-g-PMMA-; (Table ). Figures -7 indicate the effect of CAN concentration on the weight gain with respect to reaction time. As evident from the tables and figures, the GY and TC increase with 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 by taking ml of 1 cellulose solution, CAN concentration 7 (1.8 mmol), MMA concentration 8 (18.7 mmol) at 80 C, with addition of 0.7 ml (1.09 ppm) methylene blue and the data are shown in Table and Fig. 8.A comparative study of Table and shows that, the GY of the grafted product remains the same but the TC of the grafted product without methylene

6 TOSH & ROUTRAY: HOMOGENEOUS GRAFTING OF PMMA ONTO CELLULOSE 9 blue goes on increasing rapidly with reaction time (Table ), where as in the presence of the inhibitor the TC increases very slowly. There is not much more variation in the GE in the later case (Table ). Fig. Effect of reaction time on grafting of MMA onto cellulose in presence of CAN at 80 C: cellulose, 1 (1. mmol); MMA, 18.7 mmol; CAN, 1.1 mmol Fig. 7 Effect of reaction time on grafting of MMA onto cellulose in presence of CAN at 80 C: cellulose, 1 (1. mmol); MMA, 18.7 mmol; CAN, 1.8 mmol Fig. 8 Effect of reaction time on grafting of MMA onto cellulose in presence of CAN at 80 C: cellulose, 1 (1. mmol); MMA, 18.7 mmol; CAN, 1.8 mmol; Methylene blue, 0.7 ml (1.09 ppm) Molecular weight of PMMA and number of grafts per cellulose chain The molecular weight of PMMA extracted from the grafted samples prepared under different reaction conditions were determined and reported in the respective tables. As seen in Table 1, the grafting reactions with monomer concentration 11.7 mmol and CAN concentration 0.91 mmol, in the three temperature conditions give the grafted product having well control on the molecular weight of the PMMA and number of grafts per cellulose chain. At monomer concentration 1.0 and 18.7 mmol (Table ) the molecular weight of PMMA goes on increasing up to a reaction time of - h and then decreases slowly. But the number of grafts per cellulose chain goes on increasing with increase in reaction time. The same trend is also observed at a monomer concentration of 18.7 mmol with MMA concentration 1.1 and 1.8 mmol (Table ). For the reactions with methylene blue as the data show (Table ), there is almost no change in the extracted polymer molecular weight and the number of grafts per cellulose chain increases slowly in comparison to other conditions. This shows that, methylene blue acts as a good inhibitor for the formation of homopolymer and controls the molecular weight of the graft copolymer and increases the grafting sites in the cellulose chain as evident from Table. Moreover, as seen from the Tables 1-, the grafting efficiency is maximum of 77.8 for sample Cell-g- PMMA-09, but for this sample the GE is only 8 and also the molecular weight of the homo polymer is 8. The samples prepared in the presence of methylene blue as inhibitor (Table ) give better result than the former. By observing the tables, the conditions for getting the sample Cell-g-PMMA-9 is considered as the optimum conditions, which is grafting at 80 C for h with a monomer concentration 18.7 mmol, initiator concentration 1.8 mmol and inhibitor concentration 1.09 ppm. For this sample the GE is, GE is., molecular weight of the homo polymer is 0 and number of grafts per cellulose chain is.7. FTIR studies FTIR spectra of the grafted cellulose in the absence of methylene blue (Cell-g-PMMA-) and presence

7 0 INDIAN J. CHEM TECHNOL., MAY 011 of methylene blue (Cell-g-PMMA-9) are shown in Fig. 9. Both the products show identical peaks at cm -1 (OH str of cellulose), 98 cm -1 ( CH - of PMMA), 178cm -1 (>C=O str. of PMMA), 1 cm -1 (C-C str of PMMA), 18 cm -1 (OH bending of cellulose), 17 cm -1 (CH bending of cellulose), 197 cm-1 (CH deformation of cellulose), 11 cm -1 ( C-O-C- str of PMMA), 1190 and 117 cm -1 (C-C str of cellulose and PMMA), 101 cm -1 ( CH - wagging of cellulose), 801 and 7 cm -1 (CH rocking vibrations of cellulose and PMMA), thereby indicating the formation of MMA-grafted cellulose. NMR studies 1H-NMR spectra of the grafted cellulose with PMMA (Cell-g-PMMA- and Cell-g-PMMA-9) are shown in Fig. 10. Both the products 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.0, 1.8 and 1.7 ppm and the peaks at 1.18, 0.9 and 0.78 ppm are for the CH group. The peak at.0 ppm is due to the OH group of the cellulose chain 7. Mechanism of polymerization It is known that metallic cations form complexes with carbon hydrates. After complexation with cellulose, ceric ion is reduced to cerous ion, the bond between C and C is broken and a free radical appears on C or C 8. Then this free radical initiates the monomer grafting and the polymerization reaction of MMA. The FTIR and 1 H-NMR spectra of the Fig. 9 FTIR spectra of PMMA grafted cellulose (a) in presence of methylene blue and (B)

8 TOSH & ROUTRAY: HOMOGENEOUS GRAFTING OF PMMA ONTO CELLULOSE 1 grafted products also shows 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. Hence the mechanism is the one which is shown in Scheme 1. Methylene blue acts as inhibitor and affects the termination step and the rate of the reaction will be affected 9. This is also evident from Table that all the samples prepared in the presence of methylene blue show uniformity in the homo polymer molecular weight. Fig H-NMR spectra of (a) PMMA grafted cellulose and (b) PMMA grafted cellulose prepared in presence of methylene blue

9 INDIAN J. CHEM TECHNOL., MAY 011 Scheme 1 Mechanism of grafting of PMMA onto cellulose by Ce + ion Table Thermal stability of cellulose grafted products in air at heating rate 10 C.min -1. Sample code Cell-g-PMMA- Cell-g-PMMA-9 IDT a ( C) a IDT is the initial decomposition temperature. Wt loss () 00 C 0 C 00 C 0 C 00 C 0 C 00 C Thermogravimetric analysis Figure 11 shows the TG curves of grafted samples Cell-g-PMMA- and Cell-g-PMMA-9. The percentage weight loss of these samples with temperature are given in Table. The initial decomposition for Cell-g-PMMA- started at 170 C where as that for Cell-g-PMMA-9 is 10 C. This may be due to the increase in the percentage of grafting and molecular weight of the homopolymer for the former case. 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. Up to a temperature 0 C the thermal degradation of Cell-g-PMMA-9 is less in comparison to Cell-g-PMMA- (Table ) and from Fig. 11 Thermogravimetric (TG) analysis of (a) PMMA grafted cellulose and (b) PMMA grafted cellulose prepared in presence of methylene blue

10 TOSH & ROUTRAY: HOMOGENEOUS GRAFTING OF PMMA ONTO CELLULOSE 00 C and above both samples show nearly equal degradation profile. Conclusions Homogeneous graft copolymerization of MMA onto cellulose in DMAc/LiCl solvent system was carried out by using CAN as an initiator in presence of DMSO. The formation of grafted products is confirmed by FTIR spectroscopy. The effect of reaction time and temperature, monomer and initiator concentration on the GY, GE and TC is evaluated. Graft copolymerization of cellulose in presence of Ce + proceeds through ring opening of cellulose and formation of free radical in the cellulose 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 shows 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 decrease 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 and to the Principal and President G.B., Orissa Engineering College, Bhubaneswar, India, for their encouragement and support. References 1 Gupta K C & Khandekar K, Biomacromol, (00) 78. Gupta K C, Sahoo S & Khandekar K, Biomacromol, (00) Gupta K C & Sahoo S, Biomacromol, (001) 9. Princi E, Vicini S, Pedemonte E, Mulas A, Franceschi E, Luciano G & Trafiletti V, Thermochim Acta, (00) 17. Sabba M W & Moktar S M, Polym Testing, 1 (00) 7. Lomberg H, Zhow Q, Brumer H, Teeri T T, Malmstrom E & Hult A, Biomacromol, 7 (00) Carl mark A & Malmstrom E E, Biomacromol, (00) Halab-kessira L & Ricard A, Eu Polym J, (1999) Saikia C N & Ali F, Bioresour Technol, 8 (1999) Princi E, Vicini S, Proietti N & Capitani D, Eu Polym J, 1 (00) Tosh B, Saikia C N & Dass N N, Carbohydr Res, 7 (000). 1 Bianchi E, Marsano E, Ricco L & Russo S, Carbohydr Polym, (1998) 1. 1 Das P & Saikia C N, J Appl Polym Sci, 89 (00) 0. 1 Nishioka N & Kosai K, Polym J, 1 (1981) Nishioka N, Matsumoto K & Kosai K, Polym J, 1 (198) 1. 1 Saikia C N, Tosh B N, Goswami T & Ghosh A C, Indian J Chem Technol, (199). 17 Nishioka N, Minami K & Kosai K, Polym J, 1 (198) Nishioka N, Matsumoto K, Yumen T, Monmac K & Kosai K, Polym J, 18 (198). 19 Abdel-Razik E A, Polym J, 1 ( Abdel-Razik E A, Polym J, (199) 0. 1 Das P, Saikia C N & Dass N N, J Appl Polym Sci, 9 (00) 71. Bianchi E, Bonazza A, Marsano E & Russo S, Carbohydr Polym, 1 (000) 7. Molenberg A, US Pat. 7088, 00. Tosh B N & Saikia C N, Indian J Chem Technol, (1997) 7. Fernandez M J, Casinos I & Guzman G M, J Polym Sci Part A: Polym Chem, 8 (1990) 7. Kriz J, Masar B, Pospisil H, Plestil J, Tuzar Z & Kiselev M A, Macromol, 9 (199) Tosh B, Studies on the kinetics of homogeneous esterification of prepolymers like fractionated cellulose and polyvinyl alcohol of different molecular weights, Ph.D. Thesis, Dibrugarh University, Assam, Halab-Kessira H & Ricard A, Eu Polym J, (1999) Srivastava A K, Misra P K & Mathur G N, Indian J Chem, 1 A (198) 0. 0 Sabba M W & Mokhtar S M, Polym Testing, 1 (00) 7.