Phase Behavior, Structure, and Properties of Regioselectively

Chapter 17

Phase Behavior, Structure, and Properties of Regioselectively Substituted Cellulose Derivatives in the Liquid-Crystalline State

Downloaded by COLUMBIA UNIV on March 18, 2013 | http://pubs.acs.org Publication Date: April 17, 1998 | doi: 10.1021/bk-1998-0688.ch017

Peter Zugenmaier and Christina Derleth Institute of Physical Chemistry, Technical University of Clausthal, D-38678 Clausthal, Zellerfeld, Germany

The helical twisting power, value and handedness, of the supermolecular helicoidal structure of liquid crystalline (lc) cellulose derivative / solvent systems strongly depends on the substituents introduced. Investigations on lc cellulosetrisphenylcarbamate and cellulosetris-3-chlorophenylcarbamate reveal different sign of the twisting power in the same solvent triethylene glycol monomethylether. The parameters which influence such a behavior have been studied. These are the site of phenylcarbamate substitution at the anhydroglucose unit (2, 3, 6), the site of substitution at the phenyl ring (3 or 4) including different substituents (hydrogen, chloro, methyl, fluoro) on regioselectively substituted chains in ethylene glycol monomethylether acetate and, for one case, a random substitution of two groups along the cellulose chain. A strong polymer solvent effect has been detected which is predominantly influenced by the substitution site at the anhydroglucose unit and at the phenyl ring. A study of the phase diagram supports the idea that clusters with bound solvent are formed. These have to be regarded as the structural units for the lyotropic cholesteric phase.

Lyotropic liquid crystalline cellulose derivatives formed by highly concentrated solutions belong to a special class of chiral materials (/). Right- and left-handed supermolecular hélicoïdal structures of chiral nematic, also termed cholesteric mesophases, are observed with positive or negative temperature and concentration gradients for the twisting power. Studies also show that not all cellulose derivatives exhibit lyotropic liquid crystals, rather some lead from the semi-dilute state, where already microgels appear, directly to the gel state, although the chain backbone of these structures has similar stiffness as compared to those derivatives which produce liquid crystalline (lc) phases. This behavior might depend on the chain length that is the molecular mass. Cellulose derivatives with high molecular mass normally omit the lc state and form gels ©1998 American Chemical Society In Cellulose Derivatives; Heinze, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

239


240 only. This behavior might also occur with short chain molecules depending on the substituents. Considering these observations, it seems that for the lyotropic lc cellulose derivative systems, the polymer solvent interaction plays an important role. Little is known about this interaction in cellulosic systems. From solvent built-in crystals, which are border line cases of liquid crystals, a fiber structure analysis shows that, depending on the solvent, different conformations of the cellulose backbone may occur, or only the side group conformation changes. In most cellulosics the molecular structure does not change at all, rather the packing of the chains accommodates for the solvent. The other border line case, the semi-dilute state, clearly exhibits for the fully substituted cellulose molecule (degree of substitution DS=3 for the anhydroglucose monomer unit) that reversible aggregates rather than molecular dispersed single molecules form the basic building blocks. This statement should also hold for the liquid crystalline state, since no dissolution of these blocks is observed when going to the liquid crystalline state. It has also been found that the solvent in these lyotropic systems, semi-dilute or liquid crystalline, is rather tightly bound to the polymer chain and not freely available for a crystallization process of the solvent by lowering the temperature. In the dilute state molecularly dispersed single molecules are observed for fully substituted cellulose derivatives not capable of hydrogen bonding. The driving force of the transition from the isotropic to the anisotropic phase of lyotropic systems for stiff chains is believed to lie in the structuring of the solvent. Studying semi-dilute solutions of cellulose derivatives, we have been able to show that most of the solvent is bound to the polymer that means little free solvent is left and that a structuring of the cellulose chains has to be considered as well forming reversible clusters (2, 3). Lyotropic liquid crystalline cellulose derivatives exhibit chirality at three levels. The chirality caused by the configuration of the molecule (here chiral centers), by the conformation of the macromolecules (here normally left-handed helices) and by supermolecular structures as the cholesteric hélicoïdal structure of various handedness. Chirality at these different levels should be reflected, e.g., by the twisting power of the cholesteric phase. At the present little is known about the correlation of chirality and twisting power except for the rare case of a thermotropic liquid crystal where two chiral centers are placed far apart in a molecule, and the conformation of the molecules with different configurations seems to be very similar. For this example additivity of the twisting power for the two centers was proven. The twisting power by mixing various configurations of these compounds has been described by the weighed twisting power of the different configurations with the molar fractions as weights (4). The ultimate goal of our investigations is to establish a relationship between chirality and the twisting power of cellulose derivatives knowing the twisting power of the various substituents at the different position of the anhydroglucose unit and the conformational helix. This would enable to predict the helix conformation in the lyotropic liquid crystalline state with the experimentally determined twisting power. Little is known about this correlation at the present time. There are two pathways to start such an investigation: Firstly, to substitute statistically with two different groups and changing the overall composition along the chain, preferential at the different possible sites forming a copolymer. Secondly, to substitute regio-selectively at the various positions 2, 3, 6 at the anhydroglucose unit uniformly along the chain. In this paper we will report results for various but very similar cellulose trisphenylcarbamates

In Cellulose Derivatives; Heinze, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

241 also termed cellulosetricarbanilates (CTC cf. Scheme 1) belonging to both groups that will, on the other hand, establish equally well the importance of the polymer solvent interaction. ?


Experimental The cellulosetrisphenylcarbamate and cellulosetris(3-chlorophenylcarbamate) have been synthesized by well-established procedures as was the statistical copolymer with random distributed phenylcarbamate and 3-chlorophenylcarbamate side groups. These derivatives were characterized to establish their chemical constitution by elemental analysis, IR, N M R , etc. (5). The synthesis and characterization of regio-selective derivatives, always trisubstituted, have been described elsewhere (


0) Ό

-100 -200 Η -300 200 300 400

II (h

500

λ / nm

600 700 800

Figure 3. Optical rotatory dispersion (ORD) curves for the right-handed cholesteric mesophase C C H / E M M A c ; c = 0.70 g/ml; sample thickness 50μπι. The temperature for the various curves rises from left to right: 301 K , 305 K , 309 K,313K,317K.

100

750 Figure 4. Transmission curves in the visible spectral range (UV-VIS) for lc C C H / E M M A c ; c = 0.70 g/ml; sample thickness 50 μπι. The temperature for the various curves rises from left to right: 297 K , 301 K , 305 K , 309 K , 313 K .


247 for linearly polarized light according to theoretical considerations but is found to be only about 35 % or less at higher temperatures. The ORD curves can only be described with the general de Vries's equation when two additional factors are introduced. A dispersion term for the chromophores of the side groups that accounts for the rapid increase of the optical rotation at smaller wavelengths and a damping factor that reduces the optical rotation in the dispersion range. A perfect hélicoïdal structure should depict a singularity at the selective reflection wavelength λο. For a real structure the pitch Ρ or the twisting power P" are deduced from zero optical rotation in the anomalous dispersion region or the peak in the UV-VIS curves, which represent the selective reflection λο. With the knowledge of the mean refractive index η of a nematic sheet of the cholesteric structure, the pitch is calculated by Ρ = λ ο / η . The mean refractive index η was obtained by measuring the ordinary and extraordinary refractive index of the samples with an Abbé refractometer at the desired temperature. The temperature dependent measurements of the twisting power P" are represented for regio-selective lc cellulose derivative systems in Figure 5, and the pitch Ρ listed at T= 303 Κ in Table II. For some of the derivatives the selective reflection lies outside the instrumental spectral range. The pitch was then determined by the Grandjean-Cano technique.


1

1

Table IL Pitch Ρ and Handedness of Various Cellulose Urethane / Solvent Systems at Τ = 303 K, c = 0.7 g/ml and Slightly Varying Degree of Polymerization DP; Sample Thickness 50 μιη. Polymer/Solvent System

Pitch P/nm

DP°>

CCC/EMMAc C C M / EMMAc > MMC / EMMAc MMM/EMMAc

+ + + +

318 280 315 302

280 250 290 285

CCH/EMMAc

+ 413

210

CHC/EMMAc

+1140

270

HHC / E M M A c

-1015

280

-670 -522 -488 -517

245 245 110 100

b

HHH/EMMAc HHH / EMMAc HHF / E M M A c FFH / E M M A c

c)

c)

c)

a)

b)

c)

cf. réf. 6 c = 0.8 g/ml c = 0.9 g/ml


248


0,0045

0,0030

0,0015 Έ

c

^

0,0000 -

û. -0,0015 Η

-0,0030 -

-0,0045 -J 270

,

,

,

,

,

1

280

290

300

310

320

330

340

Τ/Κ 1

Figure 5. Temperature dependence of the twisting power P" of various left- and right-handed hélicoïdal structures of lc cellulose derivatives in E M M A c . C C M 0; M M M • , M M C V , C C C • , C C H A , H H H · , H H F Δ, HHF Ο (c = 0.80 g/ml); the concentrations for the other lc states are listed in Table Π.


249

A negative helical twisting power, signifying a left-handed super-molecular helical structure of the lyotropic cholesteric phase, is found for cellulosetricarbanilate (HHH) and the 4-fluoro derivatives of the phenyl residue, HHF and FFH, as well as for HHC, the derivative of the tricarbanilate for which a 3-chloro substituted phenyl ring is placed at the 6 position of the anhydroglucose unit. All the other derivatives exhibit right-handed hélicoïdal super-molecular structures. The temperature gradients of the twisting power for all lc systems shown in Figure 5 are very similar in their absolute values. Nevertheless, they are positive for left-handed structures and negative for righthanded ones. From a broad study on lc behavior in ref. 3, it can be concluded that all meta substituted cellulosetricarbanilates (CTC) with F, CI, C H 0 , C F in a variety of solvents led to right-handed super-molecular structures as did all bis-substituted in 3, 4 position at the phenyl ring. All para substituted C T C (Cl, Br, F, C H 0 ) exhibit lefthanded ones. Only pure lc CTC changes handedness depending on solvent. Discussing the hélicoïdal structure in more detail, it is clear from Table II that replacing a 3-chlorophenyl by a 3-methylphenyl group at all sites of the anhydroglucose unit does not change the pitch. However, exchanging a 3-chloro or 3-methyl group by a hydrogen at the phenyl residue and placing this substituent in 2 or 3 position of the anhydroglucose unit drastically alters the pitch in size for CHC and in sign for HHC and HHH. Replacing a 3-chloro group by hydrogen at the phenyl substituent in 6 position slightly increases the pitch only. A compensated chiral nematic phase may be obtained for CHC and HHC by adjusting either the external parameters as temperature and concentration and/or by mixing the two compounds physically or statistically distributing the various groups along the molecular chain. These investigations of the helical twisting power reveal the most sensitive substitution sites for the supermolecular structure of lyotropic lc cellulose derivatives to be the 2 and 3 position at the anhydroglucose unit. Similar conclusions can be drawn for the helix conformation and packing of a single chain in the solid state for which the structure is predominantly determined by the substitution in 2 and 3 position at the anhydroglucose unit (10). The electronegativity of the substituent at the phenyl residue is of minor importance as compared with the substitution sites 3 or 4. The 4-fluoro derivatives HHF and F F H all lead to left-handed structures with similar pitch as H H H in contrast to the 3-chloro derivative C C H with a right-handed one. Since the pitch was found to depend on the molecular mass (77), the degree of polymerization DP is also listed in Table II. From former studies it can be concluded that a DP of > 150 is beyond any influence on the size of the pitch, but a small effect may be expected for the fluoro derivatives in comparison with all the other regio-selective cellulose derivatives. The temperature dependence of the twisting power P" of chiral nematic phases was investigated by Kimura et al. (72) and Equation 2 derived:


3

3

3

1

l

V

= Q(JJT-\)

(2)

Q is a factor that includes geometry and volume fraction of the polymer in solution; Q > 0 for right-handed super-molecular structures and Q < 0 for left-handed ones. T represents the inversion temperature for which the super-molecular structure changes the sign of the twisting power. T lies above the clearing temperature T for the systems considered. n

n


c

250


Two plots of the pitch for positive and negative Q values as a function of temperature are shown in Figure 6. Comparing the results for the regio-selectively substituted cellulose derivatives with those in Figure 6 leads to the following conclusions: Curve b (left plot) with an increasing pitch or decreasing twisting power best describes the experiments for right-handed hélicoïdal super-molecular structures (Q > 0). For such a behavior predominant polar and steric effects are responsible. The same argument holds for the left-handed structures. The decreasing size of twisting power with temperature for both types of structures also supports the idea that left-handed conformational helices produce a left-handed super-molecular structure and a right-handed conformational helix a right handed cholesteric structure.

Phase Behavior. The study of the phase behavior and properties at the phase transition represents a crucial test for theoretical models that have been developed for various kinds of liquid crystals. For lyotropic le CTC with predominant polar interactions, none of the existing models can explain the phase diagram exactly (2). Although the volume fraction for which the anisotropic-isotropic transition occurs might be predicted to some accuracy, the actual small biphasic region for a broad molecular mass distribution and the bending of the curves at higher temperatures cannot be described by any model. Figure 7 depicts the phase diagram for C C C / E M M A c taken from texture observations in the polarization microscope. A small biphasic region is detected and a bending of the curves occurs at higher temperatures, the same features as described above, although only steric mixed with polar interactions are detected, and instead of a left-handed super-molecular structure as above, a righthanded one was established. The same difficulties arise for the description of the CCC / E M M A c system adjusting to the theoretical models as for lc CTC, and only fair agreement is obtained with the model of Warner and Flory (73), although anisotropic interactions are taken into account. The bending of the curves may be explained by a variation of persistence length with temperature but the small biphasic region cannot be accounted for. It is questionable from the current knowledge of the structure of these systems that the models discussed may be suitable for a description of the isotropicanisotropic transition in cellulosics. Clearly cluster formation occurs and bound solvent is present in these systems. These effects may play an important role in considering phase transitions. Also a phase separation between lower and higher molecular masses may occur, since different kinds of clusters are observed depending on the size of the cellulosic molecules. At higher concentrations below the cholesteric phase, a columnar phase appears as in the CTC / diethylene glycol monoethylether system (14). In this case an oriented fiber could be produced, and the X-ray analysis revealed hexagonal packing of the molecules.

Acknowledgments. Part of the work reported was supported by a grant from Deutsche Forschungsgemeinschaft.



251

Figure 6. Pitch P versus temperature Τ according to Equation 2 for right-handed conformational helices Q > 0 (left plot) and left-handed ones Q < 0 (right plot): (a) predominant polar interactions; (b) polar and steric effects; (c) predominant steric effects. (Adapted from ref. 2).

380

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

c/(g/ml)

Figure 7. Phase diagram for the system CCC / E M M A c evaluated by texture observations.


252


Literature Cited 1. Gilbert, R. D. In Polymeric Materials Encyclopedia; Salamone, J. C., Ed.; CRC Press: Boca Raton, Florida, 1996, pp 118. 2. Haurand, P.; Zugenmaier, P. Polymer 1991, 32, 3026. 3. Klohr, E.; Zugenmaier, P. Cellulose 1995, 1, 259; Klohr, E. Dissertation, TU Clausthal, D-38678 Clausthal-Zellerfeld, 1995. 4. Dierking, I.; Gießelmann, F.; Zugenmaier, P. Mol. Cryst. Liq. Cryst. 1996, 281, 79. 5. San-Torcuato, A. Diploma Thesis, Institut für Physikalische Chemie der TU Clausthal, D-38678 Clausthal-Zellerfeld, 1989; Zugenmaier, P. Das Papier 1989, 43, 658. 6. Aust, N.; Derleth, C.; Zugenmaier, P. Macromol. Chem. Phys. 1997, 196, in press. 7. De Vries, H. Acta Cryst. 1951, 4, 219. 8. Grandjean, F. C. R. Acad. Sci. Fr. 1921, 172, 91; Cano, R. Bull. Soc. Fr. Minéral. Cristallogr. 1968, 91, 20. 9. Hartshorne, Ν. H. The Microscopy of Liquid Crystals, Microscope Publications Ltd., London, England, 1974, p 80. 10. Iwata, T.; Okamura, K.; Azuma, J.; Tanaka, F. Cellulose 1996, 3, 91 and 107; Möller, R. Diploma Thesis, Institut für Physikalische Chemie der TU Clausthal, D-38678 Clausthal-Zellerfeld, 1982. 11. Siekmeyer, M.; Zugenmaier, P. Makromol. Chem. Rapid Commun. 1987, 8, 511. 12. Kimura, H.; Hosino, M.; Nakano, H. J. Phys. (France) 1979, 40, C3-174 and J. Phys. Jpn. 1982, 51, 1584. 13. Warner, M.; Flory, P. J. J. Chem. Phys. 1980, 73, 6327. 14. Hildebrandt, F.-I. Diploma Thesis, Institut für Physikalische Chemie der TU Clausthal, D-38678 Clausthal-Zellerfeld, 1991; Zugenmaier, P. In Cellulosics: Chemical Biochemical and Material Aspects; Kennedy, J. F.; Phillips, G. O.; Williams, P. Α., Eds.; Ellis Horwood: New York, NY, 1993, pp 105.


Phase Behavior, Structure, and Properties of Regioselectively

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