Bulletin of Pure and Applied Sciences, Vol. 22C (No. 1) 2003; P. 1-8
SYNTHESIS OF CELLULOSE ACETATE WITH DIFFERENT DEGREES OF SUBSTITUTION AND STUDY OF THEIR VISCOMETRIC BEHAVIOUR IN DIFFERENT SOLVENTS M. A. Majid *, M. Y. Chowdhury and I. M. M. Rahman Department of Chemistry, University of Chittagong, Chittagong – 4331, Bangladesh ABSTRACT Cellulose acetate samples, CA–1 and CA–2 with different degrees of substitution, DS = 2.38 & 2.89, respectively have been prepared from α-cellulose isolated from rayon grade bamboo pulp by a single stage acetylation process followed by hydrolysis in the former case. The polymer samples have been characterized with respect to DS, viscosity average molar mass and solubility in different solvents. Viscometric behaviours of the polymer samples have been studied in appropriate solvents such as CH3COCH3, 1,4-dioxane, CHCl3, CH2Cl2 and CH2Cl2:EtOH (4:1, v/v) mixture at different temperatures. The behaviours have been discussed in the light of modified Fox-Flory treatment. Some abnormal viscosity behaviour of the highly substituted CA sample (DS = 2.89) has been observed in CH2Cl2 and CH2Cl2-EtOH mixture. Keywords: Cellulose acetate, α-cellulose, solubility, viscosity average molar mass, viscometric.
INTRODUCTION Since the first report of its synthesis, extensive research has been made on the synthetic methods, study of properties and various applications of CA. Of all the cellulose derivatives it has now acquired a unique position because of wide range of applications as textile fibres, plastics, film sheeting and lacquers. A good number of books (Young and Rowell, 1986; Kennedy et al, 1986; Rowell and Young, 1978; Novel and Zeronian, 1985) and research papers (Bergstrom et al, 1999; Ishigaki et al, 2000, Ishigaki et al, 2002) have been published on cellulose acetate and other cellulose derivatives. Research is still going on over development of new catalysts, synthesis of uniformly substituted products and properties in solution and in the solid state. In a previous investigation of our synthesis of CA in the University of Chittagong (Bhuiyan, 1996), we prepared CA with uniform substitution only after double-stage acetylation. In the present investigation, by improvement of pre-treatment stage, uniformly substituted product was prepared by a single-stage acetylation. CA has been found to show interesting solubility and viscometric behaviour. These aspects of the polymer synthesised in the present investigation have also been studied. EXPERIMENTAL Preparation of cellulose acetate A modified form of the procedure (Bhuiyan, 1996; Roy, 1963) followed before was used to prepare cellulose acetate. The procedure consisted of four stages – pretreatment, acetylation, hydrolysis and precipitation.
*
Corresponding author. E-mail:
[email protected]
M. A. Majid, M. Y. Chowdhury and I. M. M. Rahman
Pretreatment:α-cellulose was first treated with distilled water and then successively twice with glacial acetic acid for ensuring dewatering. Acetylation: The pretreated α-cellulose was subjected to acetylation first by stirring with a mixture of glacial acetic acid and a trace of conc. H2SO4 at room temperature, then stirring with acetic anhydride and a trace of conc. H2SO4 at 0°C. The formation of a clear viscous solution, free from fibre or grain but containing air bubbles indicated completion of acetylation. The temperature was thereafter raised to the ambient. Hydrolysis: Excess anhydride was destroyed using water and glacial acetic acid. The resulting solution, supposed to contain cellulose triacetate was hydrolysed by addition of conc. HCl and distilled water. Precipitation: The product was precipitated in 10 (ten) times its volume of distilled water, filtered, washed thoroughly, dispersed in water, and the remaining acetic acid was neutralized by Na2CO3. The product was filtered, washed thoroughly to remove any excess stabilizing agent and dried in a vacuum desiccator. The detailed procedure has been given elsewhere (Chowdhury, 1997). The CA−1 was prepared in this way. In preparation of sample CA−2, the hydrolysis step was omitted. Characterization DS: Modified Eberstadt method (Bhuiyan, 1996; Browning, 1967) was used for determination of DS. The DS of CA−1 was found to be 2.38 and that of CA−2, 2.89. Solubility: A qualitative investigation of solubility was made (Table–1). Table–1: Solubilitya behaviour of cellulose acetate with different DS in different solvents at room temperature (≈25°c) Solvent Acetone 1,4-Dioxane Pyridine Methanol CH2Cl2:C2H5OH (4:1 v/v)
CA–1 Soluble Soluble Insoluble Insoluble Soluble
CA–2 Insoluble Soluble Soluble Insoluble Soluble
Solvent Chloroform Methylene chloride Cyclohexane Ethanol CH2Cl2:CH3OH (4:1 v/v)
CA–1 Insoluble Insoluble Insoluble Insoluble Soluble
CA–2 Soluble Soluble Insoluble Insoluble Soluble
a
About 0.05 g of the sample was tested in about 5 ml solvent.
Viscosity average molar mass, M v : M v of the CA−1 sample was estimated in CH3COCH3 at
25°C and that of CA−2 in CHCl3 at 30°C by measurement of the respective [η] and using Mark-Houwink equation, [η] = KM v a. Values of K & a for CA in CH3COCH3 at 25°C are
0.0244 dlg-1 & 0.0760 and in CHCl3 at 30°C are 4.5 × 10-3 dlg-1 & 0.900, respectively.
Viscometric measurement: CA solution of concentration 0.8% w/v (approximately) was prepared in a suitable solvent and used as stock solution. The other solutions were prepared by dilution with respective solvent. Reduced viscosities of five polymer solutions of different concentrations in each solvent were measured at different temperatures. Reduced viscosity, η sp / c vs concentration, c plot was extrapolated to zero concentration where possible to obtain limiting Bulletin of Pure and Applied Sciences, Vol. 22C (No. 1) 2003; P. 1-8 2
Synthesis of cellulose acetate with different degrees of substitution and study …
viscosity numbers, [η]. Thus [η] of CA−1 was obtained in CH3COCH3, 1,4-dioxane and CH2Cl2-EtOH (4:1, v/v). The sample was found insoluble in CHCl3 and CH2Cl2. [η] of CA−2 was also obtained in CHCl3 and 1,4-dioxane. [η] of this sample could not be obtained by extrapolation in CH2Cl2 and CH2Cl2:EtOH (4:1, v/v) mixture. The sample was found insoluble in CH3COCH3. RESULTS AND DISCUSSION Solubility behaviour The cellulose acetate, CA–1 (DS = 2.38) has been found highly soluble in CH3COCH3, 1,4-dioxane and CH2Cl2:EtOH (4:1, v/v) mixture but insoluble in CH2Cl2 and CHCl3 whereas CA–2 (DS = 2.89) has been found soluble in CHCl3, CH2Cl2 and CH2Cl2:EtOH (4:1, v/v) mixture but insoluble in CH3COCH3. Both the samples were found soluble in the non-polar solvent 1,4-dioxane. For a polar polymer like cellulose acetate, strong interaction with a polar solvent is very likely. However other polymer solvent interaction forces such as dispersion forces play dominant role in deciding solubility behaviour. Thus both the CA samples are soluble in non-polar solvent 1,4-dioxane but the sample CA–1 is insoluble in CHCl3 whereas the sample CA–2 is insoluble in CH3COCH3. Viscosity average molar mass, M v In CH3COCH3 at 25°C, M v of the CA−1 is found to be 1.63 × 104 and that of CA−2 in
CHCl3 at 30°C is 5.76 × 103. Both the CA samples have much lower values than the starting α-cellulose which has M v = 1.33 × 105 measured in cupramonium solution. The reduction in the values of M v and thus also in M v of the CA samples is possibly due to the substantial chain cleavage of cellulose during esterification. Viscosity behaviour in different solvents
ηsp / c vs c plots of CA – 1 in dilute solution in CH3COCH3 (Figure–1), 1,4-dioxane (Figure–2), CH2Cl2:EtOH (4:1, v/v) mixture (Figure–3) as expected are straight lines and the [η] could be obtained by extrapolation. A comparison of [η] in the three solvents indicate that these 0.70
(ηsp/c)/dl-g-1
0.65 0.60 0.55 0.50 0.45 0.40 0.00
0.25
0.50
0.75
1.00
-1
c/g-dl
Figure–1: Viscometric behaviour of CA−1 in CH3COCH3 at different temperatures. , 25°C; , 30°C; , 35°C.
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M. A. Majid, M. Y. Chowdhury and I. M. M. Rahman
0.70
(ηsp/c)/dl-g-1
0.65 0.60 0.55 0.50 0.45 0.40 0.00
0.25
0.50
0.75
1.00
c/g-dl-1
Figure–2: Viscometric behaviour of CA−1 in 1,4-dioxane at different temperatures. , 35°C; , 45°C; , 55°C; , 65°C
0.63
(ηsp/c)/dl-g-1
0.60 0.57 0.54 0.51 0.48 0.00
0.25
0.50
0.75
1.00
c/g-dl-1
Figure–3: Viscometric behaviour of CA−1 in CH2Cl2:EtOH (4:1, v/v) at different temperatures; , 25°C; , 30°C; , 35°C.
solvents have almost similar solvent power for the polymer. For a polar polymer like CA, it is expected that there should be better polymer solvent contacts in CH3COCH3 than in CH2Cl2:EtOH than in 1,4-dioxane. [η] should have the highest value in CH3COCH3 and the lowest in 1,4-dioxane. However similar values of [η] again indicate that apart from polymer solvent interaction there are other forces such as dispersion forces responsible for dissolution and viscosity behaviour.
η sp / c vs c plots of CA–2 in CHCl3 (Figure– 4) are straight lines as expected. However the plots in 1,4-dioxane (Figure–5), CH2Cl2 (Figure–6) and CH2Cl2: EtOH (4:1, v/v) mixture (Figure–7) show some unusual behaviour. In very dilute solutions (concentration < 0.4 g-dl-1). CA–2 showed almost linear behaviour in 1,4-dioxane and [η] could be obtained by extrapolation. [η] values thus obtained compared to [η] values in CHCl3. However in CH2Cl2 and CH2Cl2:EtOH (4:1, v/v), η sp / c increases with dilution, an unusual behaviour.
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Synthesis of cellulose acetate with different degrees of substitution and study …
0.14
(ηsp/c)/dl-g-1
0.13
0.11
0.10
0.08 0.00
0.25
0.50
0.75
1.00
c/g-dl-1
Figure–4: Viscometric behaviour of CA−2 in CHCl3 at different temperatures; , 30°C; , 35°C; , 40°C; , 45°C.
0.19
(ηsp/c)/dl-g-1
0.17 0.15 0.13 0.11 0.09 0.00
0.25
0.50
0.75
1.00
c/g-dl-1
Figure–5: Viscometric behaviour of CA−2 in 1,4-dioxane at different temperatures; , 30°C; , 40°C; , 50°C; , 60°C.
(ηsp/c)/dl-g-1
0.17
0.16
0.15
0.14 0.00
0.35
0.70
c/g-dl-1
Figure–6: Viscometric behaviour of CA−2 in CH2Cl2 at different temperatures; , 30°C.
Bulletin of Pure and Applied Sciences, Vol. 22C (No. 1) 2003; P. 1-8 5
M. A. Majid, M. Y. Chowdhury and I. M. M. Rahman
0.34
(ηsp/c)/dl-g-1
0.29 0.24 0.19 0.14 0.09 0.00
0.17
0.34
0.51
0.68
0.85
c/g-dl-1
Figure–7: Viscometric behaviour of CA−2 in CH2Cl2:EtOH (4:1 v/v) at different temperatures; , 30°C; , 40°C. Cellulose derivatives are stiff-chain polymers and their behaviour is different from that of flexible chain polymers. The solvent dependence of [η] for flexible chain polymers has been theoretically explained by Fox and Flory (Flory and Fox, 1951; Flory, 1953) with the help of the following equations. [η] = KM1/2α3 Where K = ϕ (r02 M ) 3 / 2 and α = (r 2 )1 / 2 (r02 )1 / 2 So, [η] = ϕ [r02 M ]3 / 2 M1/2α3 ‘ϕ’ is a universal constant and is equal to 2.1 × 1021 for flexible chains, (r02 M ) is a constant for a given polymer, so that ‘K’ is a constant for the polymer at a given temperature.
r 2 refers to the
mean square end-to-end distance for this polymer chains in the specified solvent and temperature and
r02 is
its value at theta (θ) condition. The solvent dependence of [η] is thus considered to
reflect a change in α3 and thus
(r02
r 2 for
flexible chain polymers. Flory et al (1958) found that
M ) is not a constant for stiff-chain polymers. For these cases, the dependence of [η] may
reflect changes in (r02 M ) rather than in α3, which results from specific interaction of polymer and solvent rather than the general polymer-solvent contacts. These polymers thus show some unusual viscometric behaviour, not shown by flexible chain polymers. [η] values of CA–1 show negative temperature co-efficient in all the solvents studied (Figure−8). CA–2 behaves differently in CHCl3 (Figure–9) where [η] first decreases and then increases with temperature. The polymer seems to behave as stiff-chain polymer upto 40°C and then as a flexible-chain polymer. Similar anomalies in the viscosity behaviour of stiff-chain polymers have been observed before (Kawai and Ueyama, 1960).
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0.60
η/dl-g-1
0.55
0.50
0.45
0.40 20
30
40
50
60
70
Temperature/°C
Figure–8: Limiting Viscosity Number values obtained at different temperatures for CA−1 in different solvents; , CH3COCH3; , 1,4-dioxane; , CH2Cl2:EtOH (4:1 v/v).
For flexible chain polymers d [η ] dT is either positive or zero (Margerison and East, 1973). The factors affecting the variations have been explained by Fox-Flory treatment. The variation in [η] is usually small, of the order of 0.1% per °C (More, 1967). In contrast to flexible chain polymers, the temperature co-efficient of [η] for stiffer and more extended chains such as cellulose derivatives have been found to be negative and relatively large being of the order of 1% per °C. In the present investigation, it is found to be 0.5% or higher. 0.12
η/dl-g-1
0.11 0.10 0.09 0.08 25
30
35
40
45
50
55
60
65
Temperature/°C
Figure–9: Limiting Viscosity Number values obtained at different temperatures for CA−2 in different solvents; , CHCl3; , 1,4-dioxane. A modified form of the Fox-Flory treatment given by Flory et al (1958) has been used to explain the findings of the viscosity behaviours of the stiff-chain polymers as given by the equation
d log[η ] dT = d log{ϕ } dT + [3 / 2d log(r02 / M ] dT + d log α 3 dT
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where {ϕ} is equal to or less than the limiting value of ϕ as mentioned earlier. In these cases, the negative temperature co-efficients of [η] are primarily due to decrease in (r02 M ) as the temperature increases. Our viscosity results for cellulose acetate of CA−1 in CH3COCH3, 1,4-dioxane and CH2Cl2:EtOH (4:1, v/v) mixture are in conformity with the above observation.
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Bergstrom, L.; Stemme, S.; Dahlfors, T.; Arwin, H. O. (1999). Cellulose Commun., 6(1), p. 1−14.
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Bhuiyan, N. A. (1996). M. Sc. Thesis, Dept. of Chemistry, Ctg. Univ.
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Browning, B. L. (1967). Methods of Wood Chemistry, Interscience Publisher, N.Y., 2.
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Chowdhury, M. Y. (1997). M. Sc. Thesis, Dept. of Chemistry, Ctg. Univ.
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Flory, P. J. and Fox, T. G. (1951). J. Amer. Chem. Soc., 23, p. 1904.
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Flory, P. J. (1953). Principle of Polymer Chemistry, Cornell University Press, Ithaca, N. Y.
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Flory, P. J.; Spurr, O. K. and Carpenier, D. K. (1958). J. Polym. Sci., 27, p. 251.
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Ishigaki, T.; Sugano, W.; Ike, M. and Fujita, M. (2000). J. Biosci. & Bioeng., 90(4), p. 400−405.
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Ishigaki, T.; Sugano, W.; Ike, M.; Taniguchi, H.; Goto, T. and Fujita, M. (2002). Polym. Deg. & Stab., 78(3), p. 505−510.
10. Kawai, T. and Ueyama, M., J. (1960). Appl. Polym. Sci., 3, p. 227. 11. Kennedy, J. F. and others (1986). Cellulose and its Derivatives, Ellis Horwood Ltd. 12. Margerison, D. and East, G. C. (1973). An Introd. to Polym. Chem., Pergamon. Press, N.Y., p. 112. 13. More, W. R. (1967). Progress in Polym. Sci., Pergamon Press, N.Y., 1; p. 1−43. 14. Novell, T. P. and Zeronian, S. H. (1985). Cellulose Chemistry - its Application, Ellis Horwood Ltd. 15. Rowell, R. M. and Young, R. A. (1978). Modified Cellulose, Acad. Press. 16. Roy, L. W. (1963). Methods in Carbohydrate Chem. (Cellulose), Acad. Press, N.Y., 3; p. 194, 195, 200, 201. 17. Young, R. A. and Rowell, R. M. (1986). Cellulose: Structure, Modification and Hydrolysis, John Willey & Sons Inc.
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