Polymer Journal, Vol. 19, No. 12, pp 1377-1383 (1987)

A Further Study on Solvation in Cellulose Triacetate Solutions by Pulse-Fourier 1H and 13C NMR Method

Kenji KAMIDE, Masatoshi SAITO, Keisuke KOWSAKA, and Kunihiko OKAJIMA

Fundamental Research Laboratory of Fiber and Fiber-Forming Polymers, Asahi Chemical Industry Co., Ltd., 11-7, Hacchonawate Takatsuki, Osaka 569, Japan

(Received June 17, 1987)

ABSTRACT: An attempt was made by pulse-Fourier 1H and 13C NMR to measure the chemical shift b of the 0-acetyl-methyl (AMH) and -<:arbonyl carbon (ACC) of cellulose triacetate (CTA, the total degree of substitution = 2.92) in various deuterated solvents and to determine whether the possibility of the solvation depends on the position (C2 , C3 , or C6 ) of the 0-acetyl group or not and if so, to evaluate the positions of the 0-acetyl group, to which the solvent molecules are preferentially solvated. Among N,N-dimethylsulfoxide (DMSO), pyridine (Py), trichloromethane (TCM) and acetone, the spin-lattice relaxation time of AMH at C2 carbon and that of ACC at C2 , C3 , and C6 carbons were maximum in acetone, indicating that acetone molecules interact the most weakly with 0-acetyl group. The difference of b of AMH at each carbon position (C2 , C3 , and C6) between DMSO or Py or TCM and acetone was positive in DMSO and negative in Py, TCM. The similar difference of b of ACC was always positive, except the case of ACC at C2 and C3 positions in Py, where the difference was zero. Assuming no specific interaction between acetone and CTA we can conclude that DMSO and TCM molecules solvate equally with 0-acetyl group at C2 , C3 , and C6 positions, but the pyridine molecule solvates only with 0-acetyl group at C6 position. KEY WORDS Cellulose Triacetate I FT-NMR I Chemical Shift I Solvation I Methyl Proton I Carbonyl Carbon I

The earliest study of the interaction between that (1) there exists the solvation of solvent the cellulose derivative and the solvent mol• molecules to 0-acetyl and hydroxyl groups, ecule (i.e., the solvation) seems to have been (2) b becomes larger in polar solvents, and (3) that of Moore and his coworkers, 1 who in 1965 b of the hydroxyl proton attains maximum measured the adiabatic compressibility of so• at 2.5 in N,N-dimethylacetamide lutions of cellulose acetate (CA), cellulose ni• (DMAc). Successively Kamide and Saito2 trate, ethylcellulose and so on, estimating the measured the adiabatic compressibility of so• number of the solvent molecules solvated to a lutions of CA samples and concluded that sat glucopyranose ring, s. Unfortunately, as infinite dilution, s0 ( =lim s, cis the concentra-

Kamide and Saito2 pointed out later, the accuracy and preciseness of their data on the tion) depends on the solvent nature in the same sound velocity are not satisfactory. Recently manner as b. The strong correlationships of s0 Kamide et a/. 3 found from the measurements with the unperturbed chain dimension A and of 1 H NMR chemical shifts b of the hydroxyl• the partially draining parameter X4 - 8 were and 0-acetyl methyl protons of CA with the experimentally demonstrated for CA with total degree of substitution = 0.49-2.92 = 0.49-2.92 iii various solvents. These

1377 K. KAMIDE et a/. experimental facts indicate explicitly that the tube. For TCM and DMAc the dissolution solvation plays predominant roles in the flexi• was carried out at room temperature with bility of the unperturbed chain together with 20 min mixing and for pyridine and DMSO the the hydrodynamic interaction in CA solutions. tube was placed in an NMR cell and the In other words, it is considered that the solv• solution was prepared at 120oc by rotating the ated solvent molecule yields a significant ster• tube at a rate of 20 rpm. The mixture of the ic hindrance and an interaction between the CTA sample flake and acetone was quenched solvated and free solvent molecules results in to - 170°C, maintained at that temperature partial free drainage. It is natural to conceive for I h and the clean and transparent solution that the solvent molecules possibly solvate to of 3 wt% CTA was obtained by rasing the different extents with the 0-acetyl or hydroxyl temperature of the mixture very gradually to groups at three different carbons (i.e., C2 , C3 , room temperature. and C6 ) in a glucopyranose unit. J values of the 0-acetyl methyl proton If the above reasoning is unconditionally (AMH) and 0-acetyl carbonyl carbon (ACC) acceptable, the difference in the position of the were determined on a FX-200 JEOL FT• 0-acetyl and/or the hydroxyl group, to which NMR spectrometer (Japan Electron Optics the solvent molecules are solvated, is expected Laboratory Co., Tokyo) using tetramethyl• to cause a different effect on A and X. A recent silane (TMS) as a reference. 1H NMR spectra commerciallization of Fourier-transform were obtained under the following conditions: (FT)-NMR makes it possible to evaluate the Frequency 199.5 MHz, pulse width 45° (7 J.lS), intensity of the solvation. repetition time 8 s, data points 16384, accumu• In this article as an extension of our earlier lation 16 times. Only for CTA-acetone system, work3 we measure the chemical shift J and the 180-r-90° (r, pulse interval) pulse was used to relaxation time T1 by FT-1H and - 13C NMR eliminate peak of acetone having long spin• for cellulose triacetate (CTA) in various sol• lattice relaxation time T1 (in this case, r = 5 s, vents to disclose the solution structure of repetition time 30 s). 13C NMR spectra were CTA-solvent system. obtained under· the following conditions: Frequency 50.15 MHz, . pulse width 45° EXPERIMENTAL (6.5 J.lS), repetition time 3.2 s, complete de• coupling of 1H nuclei. For acetone, DMSO Polymer Sample and Solvents and pyridine the solvent peaks overflowed by ACTA whole polymer with {F)) =2.92 (the frequency domain accumulation. The repe• weight-average molecular weight Mw=2.32 x tition was ca. 1000-4000 times. T1 was mea• 10 5 and the number-average molecular weight sured by the inversion recovery method using M" = 5.85 x 104 ), employed in the previous I 80-r-90° pulse sequence. papers2 ·3 ·9 was used for NMR measure• ments. Deuterated trichloromethane (TCM)• RESULTS AND DISCUSSION d1, pyridine (Py)-d5 , acetone-d6 , N,N• dimethylsulfoxide (DMSO)-d6 , and DMAc-d9 Figure I shows the 1H NMR spectra of the were used as received from the manufacturer 0-acetyl methyl proton region of CTA in five (E. Merck, Darmstadt, FRG). deuterated solvents. Generally, three peaks were observed for the 0-acetyl methyl proton. 1 Hand 13C N M R Measurements J of these peaks varied over the wide range of The CTA sample, dried in vacuo at 40"C for 1.8-2.2 ppm. The intensity ratio in TCM, 24 h, was mixed with solvents except acetone calculated from the peak area, is 0.99: to give 7wt% solution in the NMR sample 1.04: 1.00 for three peaks at 1.95, 2.00,

1378 Polymer J., Vol. 19, No. 12, 1987 Solvation in Cellulose Triacetate Solutions

of these three peaks corresponds accurately to the degree of substitution of three hydrox•

yl groups at C3 , C2 , and C6 positions with the acetyl group (i.e., and and of the CTA sample employed here were determined using (=2.92), as 0.92, 1.01, and 0.99, respectively. These values are approximately the same as those (0.89, 1.02, 1.00) evaluated by Kamide and Okajima9 for similar CTA sample. The peak at the highest magnetic field (6 = 2.03 ppm) pyridine is extremely sharp, com• pared with the other two peaks. This suggests that the peak at y = 2.03 ppm has a long spin•

spin relaxation time T2 and may be due to low molecular weight compounds contaminated in e) TCM the solution. The peak intensity at 6 = 2.15 ppm is almost twice that at 6 = 2.04 ppm and we may assign the peak at 6 = 2.15 ppm to the

0-acetyl methyl proton at C2 and C6 positions

(AMH2 and AMH6 ) and the peak at 6 =2.04 2.0 1.6 ppm to the 0-acetyl methyl proton at ppm c3 position (AMH3 ). The intensity ratios of three 1 Figure 1. H NMR spectra of cellulose triacetate peaks, observed in TCM, acetone, and DMSO in various deuterated solvents in 0-acetyl

methyl proton region. a) DMAc-d9 ; b) DMSO-d6 ; c) over the range of 1.6-2.4 ppm, were almost acetone-d6 ; d) pyridine-d5 ; e) TCM-d1• independent of the solvents. The peaks from higher magnetic field can be attributed to the and 2.12 ppm. Up to now 1H NMR measure• 0-acetyl methyl protons at C3 , C2 , and C6 ments on CTA with >2.9 were without positions. An exception is DMAc, in which the exception carried out on 100 or less MHz peak intensity at 6 = 1.97 ppm (probably as•

NMR spectrometers, which have less resolved signed for AMH2) is about 10-15% smaller power for this purpose. Goodlett et a/. 10 as• than other two peak intensities, suggesting a signed, from a comparison of the reactivity small probability of resonance absorption by of three hydroxyl groups at C2, C3 , and AMH2 at 1.94 and 2.07 ppm, in addition to c6 positions with toluence-sulfonyl chloride, at 1.97 ppm. Here, we analyzed the spec• three peaks at 1.94, 1.99, and 2.09 ppm for trum in DMAc on the assumption that CTA in TCM to be the 0-acetyl methyl pro• each peak attributes to the 0-acetyl methyl tons at the C3 , C2 , and C6 position, respec• proton at C3 , C2 , and C6 positions (from tively. Shiraishi and his coworkers11 arrived higher magnetic field), respectively. at the same conclusion as Goodlett et a!. by Table I shows the chemical shifts of the 0- studying the correlations between the acidity acetyl methyl protons of CTA in various sol•

of 0-acetyl methyl groups at C2 , C3 , and vents. Figure 2 shows the 13C NMR spectra of c6 positions and their electron deshielding the 0-acetyl carbonyl carbon region of CTA. effect and by estimating the degree of shift of Until very recently, the peak assignment of the the methyl proton peak of CTA in TCM to carbonyl carbon region in 13C NMR spectra a lower magnetic field. The intensity ratio was an unsolved problem. Applying a low-

Polymer J., Vol. 19, No. 12, 1987 1379 K. KAMIDE et a/.

Table I. NMR chemical shift of 0-acetyl methyl proton and carbonyl carbon in cellulose triacetate solution at 40°C

Chemical shift•

Solvent 0-Acetyl methyl proton 0-Acetyl carbonyl carbon

c6b c2 c3 c6 Cz c3

DMAcc-d9 2.07 1.97 1.94 DMSOd-d6 2.06 1.94 1.88 169.9 169.0 168.7 acetone-d6 2.08' 1.96' 1.92' 170.7 169.9 169.5 pyridine-d5 2.15 2.15 2.04 170.5 169.9 169.5 TCMr-d1 2.12 2.00 1.95 170.0 169.5 169.1

a 0 ppm for tetramethylsilane. b Carbon position substituted by 0-acetyl methyl group. c N,N-Dimethylacetamide. d N,N-Dimethylsulfoxide. ' Measured by water eliminated Fourier transform method. r Trichloromethane.

able assignments. Three large peaks were as• a) l I signed to the acetyl groups at C6 , C3 , and C2 positions, from the low magnetic field side. J.J(" Table II summarizes T1 of the 0-acetyl methyl proton and 0-acetyl carbonyl carbon

in various solvents. Here, T1 for 0-acetyl il•"Woo methyl proton at C6 in TCM and DMSO and for 0-acetyl methyl proton at C2 in pyridine system could not be determined because of the overlapping of the peaks due to impurities in

these solutions. T1 of the 0-acetyl methyl proton at c3 position decreases in the follow• ing order; acetone> DMSO = TCM

d) TCM pyridine. No significant solvent depen• dence of T1 of the 0-acetyl methyl proton at C2 position was detected experimentally. T1 of the carbonyl carbon at Ci (i = 2, 3, 6) position varies depending on the carbon positions, to which the 0-acetyl group is ppm attached. T1 of the carbonyl carbon at c6 position attains. a maximum in acetone Figure 2. 13C NMR spectra of cellulose triacetate in various deuterated solvents in 0-acetyl. and minimum in DMSO. For the carbonyl carbonyl carbon region. a) DMSO-d6 ; b) acetone-d6 ; c) carbon at C2 , T1 decreases in the following pyridine-d5 ; d) TCM-d1. order: TCM >pyridine> D MSO and for the carbonyl carbon at C3 , power selective decoupling method to acetyl D MSO > TCM >pyridine. It is gen• methyl proton located at specific carbon po• erally considered that 13C nucleus relaxation sition, Kowsaka et a/. 12 made the most reason- is attributable most to the relaxation due to

1380 Polymer J., Vol. 19, No. 12, 1987 Solvation in Cellulose Triacetate Solutions

Table II. Spin-lattice relaxation time (T1) of the 0-acetyl methyl proton and 0-acetyl carbonyl carbon in cellulose triacetate solution

Solvent 0-Acetyl methyl proton 0-Acetyl carbonyl carbon

c·0 c3 cl Co c3 cl

DMSOb-d0 - ' 0.80 0.77 2.25 2.33 2.41 pyridine-d5 0.77d 0.77d - ' 2.56 2.26 2.46 TCM•-d, - ' 0.80 0.77 2.83 2.28 2.49 _f acetone-d0 0.85 0.77 3.27 3.36 2.61

' Carbon position substituted by 0-acetyl group. b N,N-Dimethylsulfoxide. ' Overlapping with the peak of impurity. d Overlapping with neighboring CTA peaks. • Trichloromethane. r Overlapping with solvent peak. dipole-dipole interaction between 13C nucleus methyl proton and the 0-acetyl carbonyl car• and 1 H nucleus (its relaxation time hereafter bon of the CTA in five solvents. The number in 0 referred to as T1 ° ). In the case of CTA, there the figures means the difference (ppm) from c5 is no 1 H nucleus directly combined with the 0- in acetone. When acetone is taken as the acetyl carbonyl carbon and a possible in• standard, the peaks of the methyl protons at teraction of the carbonyl carbon with 1 H three carbon positions are shifted downfield in nucleus attached to glucopyranose ring (skele• pyridine and TCM and to upfield in DMSO. ton) carbon and 1 H nucleus in the 0-acetyl In DMAc the proton peak at C6 position methyl group may be negligible because a slightly shifts upfield, but the proton peaks at 0 reciprocal of T1 ° decreases rapidly in reverse C2 and C3 positions downfield. On the other proportion to the sixth power of the distance hand, all the peaks of the carbonyl carbons, of 13C and 1 H nuclei. Therefore, we can con• except for those at c2 and c3 positions in clude that the difference in T1 of the carbonyl pyridine, shift more than 0.2 ppm upfield. carbon reflects a difference of the interaction Based on the above experimental results, we between the 0-acetyl group and the solvent: can consider that in CTA-solvent systems, shorter T1 for stronger interaction. T1 was the there exist some interactions between the 0- longest in acetone for all carbon positions. acetyl group and the solvent molecules. This means that the interaction of acetone molecule with the 0-acetyl carbonyl carbon is (1) TC M and Pyridine the weakest among the four solvents exam• The chlorine atom in TCM molecule has a ined, regardless of the carbon position. T1 of strong electron donating property to act with the carbonyl carbons at three positions de• 0-acetyl carbonyl carbons located at C2 , C3 , creases in the following order in each solvent: and C6 (Figure 4c), resulting in shielding of the 13 in D MSO C2 > C3 > C6; in pyridine 0-acetyl carbonyl carbons, whose C NMR C6 > C2 > C3; in TCM C6 > C2 > C3; in acetone peaks shift to a higher magnetic field than those of the carbonyl carbon of acetone. For Figures 3 and 4 show schematic represen• this reason, weakening of the double bond tation of the NMR spectra of the 0-acetyl nature of the carbonyl group brings about

Polymer J., Vol. 19, No. 12, 1987 1381 K. KAMIDE et a/.

a)OMAc

b)DMSO

2.2 2.1 2.0 1.9 8/ppm

13 Figure 3. I H-NMR chemical shift c5 (in ppm) of a• Figure 4. C NMR chemical shift c5 (in ppm) of a• acetyl methyl proton of CTA molecules in various acetyl carbonyl carbon of CTA molecules in various deuterated solvents. Number of each bar denotes the deuterated solvents. Number of each bar denotes the difference of the c5 value between that in acetone and in difference of the o value between that in acetone and in other solvents. other solvents.

a) TC M b) pyridine

Figure 5. Schematic representation of the interaction between a CTA molecule and various solvents. deshielding of the 0-acetyl methyl carbon, tween the CTA molecule and the TCM mol• shielding before and a downfield shift of the 0- ecules. Tertiary amine in pyridine also acts as acetyl methyl proton peak. Figure Sa shows a an electron doner and seems, judging from schematic representation of the interaction be- Figure 4b, to interact only with the 0-acetyl

1382 Polymer J., Vol. 19, No. i2, 1987 Solvation in Cellulose Triacetate Solutions

carbonyl carbon at c6 position as demon• expected to be small and that there occurs an

strated in Figure 5b. Unfortunately, it is not interaction between 0-acetyl group at C2 and

understood why the large shift of b of 0-acetyl C3 and DMAc and the interaction at C3

methyl protons at C2 and C3 positions toward position may be a little stronger than that at C2 lower magnetic field side occurs and why the position. This can be explained by the ampho• tertiary amine has an interaction only to 0- teric nature of the DMAc molecule. Figure 5d acetyl carbonyl carbon at C6 position. schematically illustrates the interaction of DMAc with the CTA molecule. {2) DMSO Summarizing, it can be concluded that the The oxygen atom in the DMSO molecule solvent molecule interacts significantly with donates electrons to 0-acetyl carbonyl carbons the 0-acetyl groups of cellulose triacetate and at C2 , C3 , and C6 positions, resulting in a that the extent of solvation varies depending significant shift of the acetyl carbonyl carbon on the positions at which the 0-acetyl group is peak to upfield. This also brings about located and on the solvent nature. weakening of the shielding arect, originally existing between the 0-acetyl carbonyl carbon REFERENCES and carbonyl oxygen. As a result, the peaks of 0-acetyl methyl protons are expected to shift I. See, for example, W. R. Moore, and R. M. Tidwell, Makrornol. Chern., 81, I (1965); W. R. Moore, J. to a lower magnetic field. However, the exper• Polyrn. Sci., C, No. 16, 571 (1967); W. R. Moore, imental data show the shift of the peaks to a International Symposium on Solution Properties of higher magnetic field, suggesting electrostatic Natural Polymers, Edinburgh, 1967. interaction between the oxygen atom in 2. K. Kamide and M. Saito, Eur. Polyrn. J., 20, 903 (1984). DMSO and 0-acetyl methyl protons. Figure 3. K. Kamide, K. Okajima, and M. Saito, Polyrn. J., 13, 5c illustrates the solvent effect of DMSO on 115(1981). the CTA molecule. Note that, although 4. K. Kamide, T. Terakawa, andY. Miyazaki, Polyrn. DMSO has a similar chemical structure to ]., 11, 285 (1979). 5. K. Kamide, Y. Miyazaki, and T. Abe, Polyrn. J., 11, acetone, the former strongly interacts with the 523 (1979). 0-acetyl group as compared with the latter. 6. K. Kamide, M. Saito, and T. Abe, Polyrn. J., 13, 421 This conclusion was deduced from the chemi• (1981). cal shifts of 0-acetyl methyl proton and car• 7. M. Saito, Polyrn. J., 15, 249 (1983). 8. K. Kamide and M. Saito, Polyrn. J., 14, 517 (1982). bonyl carbon and from T1 of the 0-acetyl 9. K. Kamide and K. Okajima, Polyrn. J., 13, 127 carbonyl carbon, and is quite consistent with (1981). the higher polarity of sulfonyl group than that 10. V. W. Goodlett, T. J. Dougherty, and H. W. Patton, of the carbonyl group. J. Polyrn. Sci., A-1, 9, 155 (1971). II. N. Shiraishi, T. Katayama, andY. Yokota, Cellulose Chern. Tech., 12, 429 (1978). (3) DMAc 12. K. Kowsaka, K. Okajima, and K. Kamide, Polyrn. Figure 3a shows that an interaction between J.' 1.8. 843 (1986).

0-acetyl group at C6 position and DMAc is

Polymer J., Vol. 19, No. 12, 1987 1383