Polymer Journal, Vol. 28, No. II, pp 937~941 (1996)

Characterization of Anisotropic Mesophase of Cellulose Acetate in Inorganic Solvent Mixture

Ikuya MIYAMOTO, Toshihiko MATSUI, Masatoshi SAITO, and Kenji KAMIDE*

Fundamental Research Laboratory of Natural and Synthetic Polymers, Asahi Chemical Industry Co., Ltd., I 1-7 Hacchonawate, Takatsuki, Osaka 569, Japan *Faculty of Education, Kumamoto University, 2--40 Kurokamicho, Kumamoto 860, Japan

(Received February 2, 1996)

ABSTRACT: An attempt was made to determine phase diagram of cellulose acetates (CA) with total degree of substitution <(F)) ranging from 0.52 to 2.86, in a mixture of sulfuric acid (SA), poly(phosphoric acid) (PPA), and water (W). In the mixture with very specific composition of SA/PPA/W ( =0.1/0.8/0.1, wt/wt/wt), in which cellulose has been reported to form liquid crystal by Kamide et al. [ Polym. J., 25, 453 (1993)], all of the cellulose acetates showed anisotropic behavior. Circular dichroism (CD) spectra of the cellulose acetate mesophase indicated that the texture is left handed cholesteric irrespective of <(F)). The helicoidal cholesteric pitch (P) determined by the CD method remarkably depends on <(F)), polymer concentration (Cp) and temperature (T). The pitch monotonically increases with an increase of <(F)) at constant temperature and polymer concentration. The reciprocal pitch of all samples is proportional to T and CP 3 • The coefficients of these relations indicated that the pitch of the sample with larger <(F)) tends to show less dependency on temperature and polymer concentration. KEY WORDS Anisotropic Mesophase / Celluloseacetate / Inorganic Solvents Mixture / Degree of Substitution / Cholesteric Pitch / Circular Dichroism /

Since Werbowyi-Gray, Maeno, and Pannar-Willcox the solvent used. independently reported 1 - 3 liquid crystal of cellulose de­ Very recently, Kamide et al. 9 found that cellulose in rivatives, such as hydroxylpropylcellulose and cellulose the mixture of sulfuric acid (SA), poly(phosphoric acid) esters and ethers in 1976, various kinds of cellulose de­ (PPA), and water (W) (SA/PPA/W = 1/8/1, w/w/w) rivatives including cellulose have been found to form generates lyotropic liquid crystal above some cellulose thermotropic and lyotropic liquid crystals. 4 •5 Concur­ concentration, depending on the degree of polymeriza­ rently a great deal of attention has been focused on tion (DP). This type of solvent mixture is advantageous the structure and properties of the cellulose derivatives with respect to low volatility, easiness to handle, and low mesophase from a desire to prepare high performance degree of decomposition of polymer in relatively low cellulosic materials as well as purely scientific interest. temperature range. Successively, cellulose acetates (CA) At present, several specific features of cellulose derivative with <(F)) ranging from 0.5 to 2.9 were found to be liquid crystals have been disclosed to exhibit: Almost all dissolved into the solvent mixture up to 30 wt% around the mesophases show that cholesteric nature and the which lyotropic mesophase with cholesteric texture structural parameters such as the helicoidal cholesteric forms. In this study an attempt was made to disclose pitch and sense are variations of temperature, polymer effects of <(F)) and temperature on the cholesteric concentration, chirality of the polymer and the solvent structure of the CA and cellulose mesophases. For this nature. 6 The total degree of substitution ( <(F))) de­ purpose, we prepared the mesophases of cellulose and pendence of these structural parameters is also suggest­ CA with various <(F)) (0.5-2.9) in the mixture of ed. Guo and Gray6 reported that the cholesteric pitch SA/PPA/W ( = 1/8/1) and determined the cholesteric (P) of mesophase of acetylethyl cellulose (AEC)-chloro­ pitch of the mesophases using circular dichroism method. form system, where AEC is prepared by acetylation of ethylcellulose with <(F)) ethyl= 2.5, monotonically changes EXPERIMENTAL with the acetyl content and the helicoidal sense reverses at <(F))acetyi -0.2, accordingly <(F))ethylacetyi = 2. 7. They Sample suggested that the minor change of the acetyl content An Alaska pulp with rx cellulose content of 90.1 wt% influences the chain conformation which may play a supplied from Alaska Pulp Co. (U.S.A.) was used for important role in the cholesteric properties. Gilbert 7 the cellulose sample (sample code CA-0). The viscosity­ pointed out that cellulose in trifluoroacetic acid (TFA) average molecular weight M v of the cellulose sample was and dichloroethane (DCE) mixture (TFA : DCE = 7 : 3) determined to be 5.36 x 104 by viscosity method. Here, forms mesophase with right handed cholesteric texture, viscosity equation used for determination is as follows 1°: while Ritchey et al. reported 8 that cellulose triacetate in [11]=3.85xl0- 2 Mw0 ·76 (cm 3 g- 1) (1) a similar solvent (TFA) shows left handed helicoidal sense. However, their studies should be criticized from where, [11] and Mw are the limiting-viscosity number several experimental points of view such as very limit­ in cadoxen (cadmium oxide/ethylenediamine/NaOH/ ed <(F)) range of the samples, and no information on H 20=5/28/l.4/166, w/w/w/w) and the weight-average gradual change of polymer concentrations and deg­ molecular weight, respectively. radation of polymers during preparation and optical Five kinds of CA samples with <(F)) = 0.66--2.46 measurements due to high volatility and high acidity of (sample code CA-l-CA-6) were obtained from cellulose 937 I. MIYAMOTO et al. triacetate with <(F)) = 2.89 by acid-hydrolysis method and cover glasses. The microscope observation was made using acetic acid as homogeneous reaction. 11 Mv of the with a Polarizing Microscope type 63227 (Nippon CA samples thus prepared was conveniently determined Kogaku, Japan) under crossed nicols. From the image through a series of viscosity equations established for of the sample under the shear the solution was classified CA with <(F))=0.5, 1.75, 2.46, and 2.92 in N,N'-di­ into three categories; optically isotropic solution which methyl acetamide (DMAc). 11 gives dark image even under the shear, flow orientation For estimation of total degree of substitution, 13C solution which shows bright image only in a few second, NMR measurement on CA in dimethyl sulfoxide and optically anisotropic mesophase which retains (DMSO) was carried out using GSX-400 FT NMR colored image during the observation. spectrometer (JEOL, Japan) with 100.2 MHz for 13C. From the intensity ratio of methyl carbon of acetyl Circular Dichroism Measurements groups to skeletal Cl carbon of cellulose in the NMR The mesophase samples for CD measurement were set spectrum of the sample, <(F)) was determined within between two quartz plates to be 5 µm thick. CD spectra an accuracy of 10%. The measuring conditions were were reordered with a JASCO J-720 type spectropola­ summarized as follows: Spectral width, 25000 Hz; Data rimeter equipped with thermocontroller. Stabilization of point, 32678; pulse mode, 1 H noise-decoupled mode the cholesteric structure was reached about 2 hours later without NOE (SGNNE); accumulation, l 5000-20000; from the set of the mesophase samples. The cholesteric flip angle, 45°; repetition time, 5s; internal standard, pitch (P) was calculated using an equation12 relating P tetramethylsilane (TMS). to the wavelength Ao of the reflection peak maximum in The Mv and <(F)) of the CA samples including cel­ CD spectra, as follows: lulose are listed in Table I. According to Kamide and (2) Saito 11 the degree of substitution at C2, C3, and C6 positions ( <(/2 )), <(/3 )), <(/6 )), respectively) is almost where ii is the average refractive index of the mesophase, equal for each CA prepared by above mentioned method. measured with an Abbe refractometer.

Solvent RESULTS AND DISCUSSION Sulfuric acid of 97 wt% concn and polyphosphoric acid (H 3P 4 0 13) of 88 wt% concn and more were used An image of the polarizing microscope of the CA with as received from Wako Chemical Co. (Japan). Deionized <(F)) = 2.89 in a mixture of SA/PPA/W ( = 1/8/1, w/w/w) water with electroconductivity less than 0.3 µS cm - 1 was is representatively shown in Figure 1. This solution gives prepared using ion exchanger resin. The three solvents typical colored image peculiar to mesophase. Figure 2 were mixed at - 5°C and stored. shows phase diagrams determined by polarizing micro­ scopic observation for the CA-SA/PPA/W system at Solution constant polymer concentration (25 wt%). This figure The cellulose and CA samples were mixed with the also includes the result of the cellulose-SA/PPA/W solvent mixture of SA/PP A/W at the polymer con­ system (polymer concentration is 18 wt%) for compar­ centration (Cp) up to 30wt%. In order to avoid the ison. Here, 16 mixtures with different composition of decomposition of the polymer, the mixture of polymer SA/PPA/W, which did not give arise any decomposition and solvent was immediately transferred to refrigerator of the polymer for 10 hours at least, were chosen as controlled at - 5°C and then stirred sometimes by hand. solvent of the samples. With the decrease of <(F)), the The maximum time required for making clear solution region of optically anisotropic mesophase becomes was ca. 40 hours. The solution thus prepared served for narrower indicating that CA with higher <(F;) readily polarizing microscope observation and circular dichro­ tends to form liquid crystal in SA/PPA/W system. The ism (CD) measurement at 48 hours and 100 hours after solvent mixture with SA/PPA/W ( = 1/8/1, w/w/w) is a mixing, respectively. common solvent to form liquid crystal for cellulose and

Polarizing Microscope Observation The sample for the observation was prepared by sandwiching the solution between a microscope slide

Table I. Total degree of substitution «F)) and the viscosity­ average of molecular weight Mv of the samples

Sample No. MV X 10-4 (DP·)

CA-0 0.00 5.4 (330) CA-1 0.66 5.1 (300) CA-2 0.90 4.8 (240) CA-3 1.70 6.5 (350) CA-4 1.75 4.5 (190) CA-5 2.46 9.6 (360) CA-6 2.89 23.0 (810) Figure 1. Polarized photomicrograph of mesophase of CA-SA/ • Degree of polymerization. PPA/W system. «F)) = 2.89, Cr= 30%.

938 Polym. J., Vol. 28, No. 11, 1996 Characterization of Anisotropic Mesophase <> = 0.9 <> = 1.75 <> = 0 0 1.0 0 1.0 0.2 0.2 0.8 w 0.8 0 1.0 w /. 0.4 0.6 \ I 0.4 0.6 \ 0.2 0.8 0.6 0.6 w D D • o.4 SA D A e 0.4 SA 0.4 D A e D A e 0.8 0.8 0.6 \ A A • 0.2 A e e 0.2 la.6 A D D D o.4 SA 1.0 • • 1.0 0 D A o 0.2 0.4 o.s a.a to o,._--=o,...._2--0...._4--o--'.s---"'----"'o.8'----A-----"to 0 0.8 "' A "' "' 0.2 PPA--- PPA- A 1.0 A • ,.__ _,___..,__ _,__.,_-'-"--A--' 0 = 0 0.2 0.4 0.6 0.8 10 <> 2.46 <> = 2.89 PPA--- 0 1.0 0 1.0 0.2 0.8 0.2 0.8 w w 0.4 0.4 0.6 \ 0.6 \ ~6 A 0.4 SA la.6 • • 0.4 SA A • • • 0.8 • • A 0.2 QB • • • • • • • • 0.2 1.0,._ _ _,__ _ _,__ _ __,_..._=-....__...., 0 1.0 • • • 0 0.2 0.4 0.6 0.8 10 0 0 0.2 0.4 0.6 0.8 to PPA--- PPA --- Figure 2. Phase diagrams of CA-SA/PPA/W system. e, optically anisotropic solution; t:,, flow orientation solution; O, optically isotropic solution.

Here, x is able to be calculated from a persistence length 50 q and diameter d of the polymer chain. Kamide et al. reported16 that the q of CA in DMAc, determined by light scattering method becomes maximum at <(F)) = 2.5 due to specific polymer-solvent interaction (solvation). From this fact, total degree of substitution dependence 20 of CP * of CA-aq nitric system was reasonably explained in terms of chain rigidity of CA. The quite different

0 2 3 dependence of CP * in SA/PPA/W system against <(F)) <> and lower values of CP of this system suggest that CA Figure 3. Lower critical concentration of mesophase solution of molecular chains are far rigid in the mixture than aq CA-SA/PPA/W system. e, CA-SA/PPA/W system; 0, CA-aq nitric nitric acid soln due to strong interaction between CA acid system. chains and SA/PPA/W molecules. The chain rigidity in the mixture may monotonically decrease with an increase CA with <(F)) ranging from 0.9 to 2.9. of <(F)). The lower critical concentration of the CA in the Figure 4 shows CD spectra of CA ( <(F)) = 0.9)-SA/ common solvent to form liquid crystal, CP *, is plotted PPA/W ( = 1/8/1, w/w/w) with CP=30wt% at l5°C. against <(F)) in Figure 3. In the figure the results of The lco for each spectrum means the time from set of the CA-aq nitric acid soln system13 are also depicted for mesophase into the cell of CD apparatus to start of the comparison. CP * of the CA-SA/PPA/W system, which CD measurement. The passage of the time brings about shows slight increase with <(F)), is for lower than that an increase of CD peak intensity and the shift of Ao to of the CA-aq nitric acid system. The effect of the mo­ higher side, indicating that the cholesteric texture which lecular weights of the CA samples must not be so serious is broken at the set of the sample is gradually rebuilt because Patel and Gilbert have already reported14 that in the CD cell. Peak intensity and )"0 does not change the molecular weight of CTA does not affect the critical above lco = 120 min, suggesting that cholesteric structure volume fraction for the A and B points in case of attains to the equilibrium state. CTA/TF A liquid crystal system. Figure 5 depicts CD spectra of the CA ( <(F)) = 0.66- Based on the lattice model, Flory proposed 15 a the­ 2.46) mesophase with CP = 30 wt% at 0°C in the steady oretical relationship between the lower critical volume state. All of the mesophase show positive dichroism in ( v2 ) of the polymer to generate anisotropic phase and an the wavelength range observed, indicating that the CA aspect ratio (x) of Kuhn statistical segment of the with <(F)) = 0.66-2.46 forms left handed cholesteric polymer; liquid crystal. A0 of the CA with <(F))=0.9-1.75 in­ creases monotonically with <(F)). The CD peak of the v2 = 8/x(l -2/x) (3) CA with <(F)) = 0, 0.66, and 2.46, 2.89 was out of detec- Polym. J., Vol. 28, No. II, 1996 939 I. MIYAMOTO et al.

Table II. Refractive index ii, peak wave length Ao in CD spectra and calculated pitch P for different CA samples in SA/PPA/W at 0°C

Sample No. <

I CA-0 0.00 67" - 1.0 CA-I 0.66 158" 8 CA-2 0.90 1.438 455 316 CA-3 1.70 1.440 547 380 CA-4 1.75 1.444 650 450 0 min CA-5 2.46 >800 >570 CA-6 2.89 >800 >570 600 700 400 • Estimated by the extrapolation method using the polymer

.,\/nm concentration dependence of A0 . Figure 4. CD spectra of mesophase of CA-SA/PPA/W system. «F))=0.9, CP=30wt%; measurement temperature, 15°C. 0.005 b

0

"O E a.. • 0 a u 500 -... -0.005 ,-

o -0.01 ~6:=::::::s::::s400 600 000 .l1nm

Figure 5. CD spectra of mesophase of CA-SA/PPA/W with various -0.015 «F» CP = 30 wt%, measurement temperature, 0°C. a, «F» = 0.66; b, 0 2 3 «F))=0.9; c, «F)) 1.70; d, «F))= 1.75; e, «F))=2.46. <

1.4

so.is 1.3 'E C I { - 1.2 0.13 au

,., 20 30 Cp(wt°io) Figure 9. (l/P)- 113 plot of mesophase of CA-SA/PPA/W system 1.0 against CP. e, {F»= 1.75; (), {F»=0.90; O, {F»=0.66; 0, {F»= 400 500 600 0.00; measurement temperature, 0°C. ..l./nm independent of CP. As shown in Figure 9, the exponent Figure 7. CD spectra of mesophase of CA-SA/PPA/W system x of these systems is about 3, in good accord with the O; CP = 20 wt%; a, 0°C; b, 18 measured at various temperature. {F» = result for cellulose/liq NH3/NH4 SCN. The coefficient 4°C; c, 7°C; d, 10°c; e, 13°C; f, 15°C; g, 17°C; h, 20°c. a' becomes smaller with larger <(F)) for possibly same reason as the case of temperature dependence of P.

CONCLUSION Cellulose and cellulose acetates (CA) with various <(F)) (0.52-2.89) can form lyotropic liquid crystal in 10 the mixture of sulfuric acid (SA), polyphosphoric acid (PPA), and water (W) with specific composition (SA/ PPA/W = 1/8/1, w/w/w). Circular dichroism (CD) spec­ tra of the cellulose acetate mesophase indicated that the texture is left handed cholesteric irrespective of <(F)). 2.0 The cholesteric pitch determined by the CD method remarkably depends on polymer concentration and tem­ perature, and monotonically increases with an increase TIO 280 290 m of <(F)) at constant temperature and polymer concentra­ T(K) tion. 1 /P almost depends linearly on temperature and the cube of the polymer concentration. Figure 8. Reciprocal plot of cholesteric pitch of mesophase against temperature. e, SA/PPA/W system ({F»= 1.75); (), SA/PPA/W system ({F»=0.90); O, SA/PPA/W system ({F»=0.66); O, SA/ REFERENCES PPA/W system ({F»=0.00). I. R. S. Werbowyj and D. G. Gray, Mo/. Cryst. Liq/Cryst., 34, 97 Table III. Slope and TN in eq 4 for cellulose and (1976). celluloseacetate mesophase 2. J. Maeno, Japanese laid open patent, 52-96229 (1977). 3. M. Pannar and 0. B. Wilcox, Japan laid open patent, 53-96229 272 (1976). 4. Y. S. Chen and J. A. Cuculo, J. Polym. Sci., Polym. Chem. Ed., CA-0 0.00 4.55 361 1.64 24, 2075 (1986). CA-1 0.66 4.44 342 1.70 5. E. Bianchi, A. Cifferi, G. Conio, A. Cosani, and M. Terbojevich, CA-2 0.90 3.88 382 1.33 Macromolecules, 18, 646 (1985). CA-3 1.75 2.04 376 0.77 6. J. X. Guo and D. G. Gray, Macromolecules, 22, 2086 (1989). 7. D. L. Patel and R. D. Gilbert, J. Polym. Sci., Polym. Phys. Ed., 118, 1137 (1980). indicating that temperature dependence of the pitch 8. A. M. Ritcey, K. R. Holme, and D. G. Gray, Macromolecules, becomes smaller with <(F)). Guo and Gray have already 21, 2914 (1988). 9. K. Kamide, I. Miyamoto, and K. Okajima, Polym. J., 25, 453 reported that temperature dependence of the cholesteric (1993). pitch is originated to the change of the twist angle be­ 10. W. Brown and R. Wikstorm, Eur. Polym. J., 1, I (1965). tween the cholesteric layers. 6 We suggest that the high 11. K. Kamide and M. Saito, "Advanced in Polymer Science," steric hindrance of O-acetyl groups with high <(F)) Springer-Verlag, Berlin, Germany, 1987, p 30. sample cause the insensitiveness of the twisting power 12. H. de Vries, Acta Cystallogr., 4, 219 (1951). 13. K. Kamide and K. Okajima, unpublished data. between layers to temperature. 14. D. L. Patel and R. D. Gilbert, J. Polym. Sci., Polym. Phys. Ed., The pitch for the CA-SA/PPA/W system also depends 21, 1079 (1983). on the polymer concentration CP. For many lyotropic 15. P. J. Flory, Proc. R. Soc. London, Ser. A, 234, 73 (1956). liquid crystal systems, it is well-known that CP depen­ 16. K. Kamide and M. Saito, Eur. Polym. J., 20, 903 (1984). dence of P at constant temperature can be represented 17. A. M. Ritcey and D. G. Gray, Biopolymers, 27, 479 (1988). 18. G. V. Laivins and D. G. Gray, Polymer, 26, 1435 (1985). 18 as follows 19. K. S. Yang, M. H. Theil, and J. A. Cuculo, in "Polymer ACS Symposia Series, No. 384, M. (5) Association of Structures," 1/P=a'C/+b A. El-Norkaly, Ed., The American Chemical Society, Washing­ Here, coefficient a', exponent x, and constant b are ton, D.C., 1989, p 156. Polym. J., Vol. 28, No. 11, 1996 941