Liu Yinghai, Liu Zhanjun, Liu
Xiaohui, Deng Kuilin # Received on Nov.1, 2002. Abstract Graft cellulose- polymethylacrylate copolymers have been obtained in aqueous alkaline medium. The copolymerization was carried out using a redox system-potassium diperiodatocuprate (III) [Cu (III)]- cellulose system as an initiator. The various reaction conditions were investigated at different initiator concentration and varying pH of the reaction system, the ratio of monomer to cellulose, the temperature and the length of time of copolymerization. It was found that the grafting percentage and grafting efficiency were relatively higher than those of other methods, indicating that [Cu (III)]- cellulose system is an efficient redox initiating system for cellulose graft copolymerization. The optimum conditions for affording maximum grafting have been evaluated. The structures and the thermal stability of cellulose and cellulose- g- PMA were individually characterized by infrared spectroscopy (IR) and thermogravimetric analysis (TGA). The scanning electron microscope (SEM) photographs and X- ray diffraction spectra indicate that the graft copolymer has been largely modified in the microstructure compared with the cellulose. A mechanism is proposed to explain the generation of radicals and the initiation.Keywords potassium diperiodatocuprate (III) [Cu (III)]; cellulose; methyl acrylate; graft copolymerization 1 INTRODUCTION
2 EXPERIMENTAL 2.2 Graft Copolymerization and Treatment of Copolymer Graft copolymerization was carried out in a 50 mL four- necked flask equipped with thermometer, condenser, stirrer and gas inlet. In a typical reaction, 0.3g cellulose and distilled water was added with constant stirring under nitrogen. The required amount of monomer MA was added, followed by Cu (III) aqueous solution and the total volume was made up to 20 mL. The graft copolymerization was performed on the different conditions of initiator concentration, monomer concentration, different pH, temperature and the span of time. After the completion of reaction, the reactant was cooled and neutralized by aqueous hydrochloric acid solution. Then it was filtered through weighed sintered glass funnel, washed to neutral and dried to a constant weight under vacuum at 70ºC. The total weight of PMA, including the weight of homopolymer of MA and PMA grafted on Cellulose, was calculated from the weights of the product after drying and the cellulose charged before reaction. The homopolymer of methyl acrylate was removed from the crude graft copolymer by exhaustive Soxhlet extraction with acetone for 48 hours. The final copolymer was then dried to a constant weight under vacuum. 2.3 Measurements Cellulose- g- PMA was characterized, after exhaustive Soxhlet extraction to remove PMA, by IR analysis using an FTS- 40 spectrophotometer (BIO-RAD Co. U.S.A.) in the potassium bromide pellets. The TGA of cellulose (2.75 mg) and the copolymers (2.58 mg) were carried out on a Shimadzu apparatus DGC-40 DTA-TG in nitrogen atmosphere at a heating rate of 10ºC/min. A scanning electron microscope, AMKAY- 1000B, and a Yaa 900 X- ray diffractometer was used to observe the morphologies of cellulose fibers and cellulose- g- PMA fibers. 3 RESULTS AND DISCUSSION The grafting parameters, such as total conversion percentage (C %), grafting efficiency (E %) and grafting percentage (G %) were defined and calculated as follows: C % = (total weight of PMA/ weight of MA charged) กม100 % E % = (weight of PMA grafted/ total weight of PMA) กม100 % G % = (weight of PMA grafted/ weight of cellulose) กม100 % 3.1 Effect of the concentration of Cu (III)
As shown in Fig.2., the grafting parameters were evaluated under the conditions of different pH. The required amount of aqueous hydrochloric acid solution or aqueous potassium hydroxide solution was added to the reaction system before Cu (III) was injected to make pH vary, so the graft copolymerization could be carried out at different pH. The variation of pH causes the change of C %, E% and G % because of existence of different complex forms of Cu (III) at different pH. In alkali aqueous solution, the ratio of the concentration of H3IO62- to H2IO63- changes with pH [14, 15], which leads to the different activity of the Cu (III) complexes and directly influences the amount of radicals in the reaction system. The optimum pH for maximum grafting MA onto cellulose is at 11.6. Fig. 1 Effect of [Cu (III)] on graft parameters. MA/cellulose: 9.5; pH: 11.6; 35ºC; 1 h. Fig. 2 Effect of pH on graft parameters. MA/cellulose: 9.5; [Cu(III)]: 2.5กม10 -3 M ;35ºC; 1 h. 3.3 Effect of the ratio of MA to cellulose
Fig. 3 Effect of MA/cellulose on graft parameters. [Cu (III)]: 2.5กม10 -3 M; pH: 11.6; 35 กใ C; 1.5 h. Fig. 4 Effect of
temperature on graft parameters. Fig. 5 illustrates the influence of the reaction time on graft parameters. As expected, both the grafting percentage and the grafting efficiency increase in value with increasing the reaction time first. However, the prolonged reaction time does not improve them after 1.5 hours. The reason can be attributed to the following fact that the reaction is carried out in alkali medium, the amount of the hydrolysis of carbonyl group increases with the prolonged time and leads to the decrease of pH. So, prolonging the reaction time obtains the similar results in contrast with the reaction temperature. Fig. 6 Infrared spectra of cellulose-g-MA (1) and cellulose (2) 3.6 IR spectral studies The grafting was confirmed by comparing the IR spectrum of cellulose with that of the grafted product and the results obtained are shown in Fig.6. The main difference observed is the appearance of a carbonyl absorption band at 1730 cm-1 corresponding to the carbonyl group of PMA chains. Absorption bands at 839 cm-1 and 751 cm-1 are observed due to the rocking absorption of methylene groups in PMA. All these bands are absent in the IR spectrum of pure cellulose. Owing to the results above, it could be proposed that Cu (III) may react with -OH group in cellulose to originate macroradicals first and then initiate MA grafting polymerization. Obviously, it is demonstrated that the final product is a graft copolymer of cellulose and MA. 3.7 Thermal analysis Thermogravimetric analysis (TGA) of pure cellulose and the grafted copolymer is shown in Fig. 7. The TGA of cellulose shows a weight loss in two stages. The first stage ranges from 20ºC to 90ºC and shows about 4.09 % loss in weight. This may be assigned to the loss of adsorbed and bound water. The second stage of weight loss starts at 258.6ºC and continues up to 501.6ºC, during which there was 81.13 % of weight loss due to the degradation of cellulose. The TGA of the grafted product is different from that of cellulose. It is observed that the grafted copolymer only has one stage of distinct weight loss between 260.1ºC and 401.1 ºC with about 90.11 % of the weight loss, which is attributed to the degradation of grafted polymer. It is evident that grafting MA onto cellulose can improve the thermal stability of pure cellulose. Due to the presence of PMA, the copolymer enhances hydrophobic character compared with that of pure cellulose. Fig. 7 Thermograms of cellulose (2) and cellulose-g-PMA (1) 3.8 Study of scanning electron microscope
Fig. 8 SEM micrographs of cellulose-g-MA (1) and cellulose (2) 3.9 Analysis X- ray diffraction 3.10 The initiation mechanism of grafting
reaction 4 CONCLUSION The feasibility of grafting MA onto cellulose by using Cu (III) as the redox initiator has been demonstrated by this work. The thermal stability of grafted product has been improved greatly and the microstructure changes of graft copolymers were very obvious. The conclusions are obtained as follows: First, Cu (III) obtained from CuSO4·5H2O is cheaper than other initiators; hence, it is suitable to popularize its application. Second, graft copolymers with high grafting efficiency and grafting percentage have been produced. Cu (III) is concluded to be an efficient redox initiator for the graft copolymerization of cellulose and MA. Moreover, because the activation energy of the reaction employing Cu (III) as initiator is low so that the graft copolymerization is able to be carried out at a mild temperature (35ºC) and in alkali aqueous medium, which is superior to other initiators. So, Cu (III) used as initiator is thought to be practical and has a good foreground. ACKNOWLEDGEMENTS REFERENCES
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