Liquid-Crystalline Polymers - American Chemical Society

Chapter 13 Amorphous—Liquid-Crystalline Side-Chain AB Block Copolymers

Downloaded by CORNELL UNIV on September 21, 2016 | http://pubs.acs.org Publication Date: August 24, 1990 | doi: 10.1021/bk-1990-0435.ch013

Synthesis and Morphology Joerg Adams and Wolfram Gronski Institute of Macromolecular Chemistry, University of Freiburg, 7800 Freiburg, Federal Republic of Germany By a new synthetic method LC-block copolymers were prepared to investigate the effect of a restricted geometry on a liquid crystal. Monodisperse A-styrene-B-cholesteryl block copolymers were synthesized by anionic polymerization of the polymer backbone containing a polystyrene A-block and a 1,2-polybutadiene B -block. The olefin double bonds of the polybutadiene were converted into hydroxyl groups by hydroboration and in a second polymeranalogous reaction cholesteryl was connected to the alcohol group. The resulting polymer shows the same liquid -crystalline mesophase as the correspondent LC-homopolymer. In the case of a weight ratio of 1/1 between the two blocks a lamellar morphology was observed via electron microscopy. By small angle X-ray scattering the orientation of the mesogenes in the ultra thin layers of the liquid-crystal were investigated.

In polymer science two non-crystalline ordered types of polymers are known, liquidcrystalline polymers CL2) and block copolymers (3). It is obvious to combine the properties of the two polymer classes in an AB-block copolymer in which an amorphous A-block is connected to a liquid-crystalline B-block. If block copolymers with well defined block lengths can be synthesized, microphase separated systems with liquid-crystalline microphases may be realized, possessing long range ordered microdomain morphologies as conventional block copolymers. Two main problems can be investigated with these systems. One is the question how the size and type of the liquid-crystalline microphase, which can be controlled by the molecular weight and composition, affects the liquid-crystalline -> isotropic phase transition. This problem has recently been studied in the case of submicron size droplets of low molecular weight liquid crystals in a polymer matrix (4). The effect of a restricted geometry on the phase transition temperature can be treated by elementary thermodynamics (5). For a liquid crystalline sample enclosed between parallel planes an increase or decrease of the transition temperature with decreasing sample thickness is predicted depending on whether wetting or dewetting of the walls by the nematic phase occurs. 0097-6156/90/0435-0174$06.00/0 © 1990 American Chemical Society Weiss and Ober; Liquid-Crystalline Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1990.


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Amorphous-Liquid-CrystaUine Block Copolymers 175

A more exact treatment is available in the frame of the Landau - de Gennes theory (6). It was shown that the influence of an ordering contact potential at the boundary is analogous to the effect of a strong magnetic field leading to an increase of the transition temperature until the first order transition turns into a second order transition at a critical thickness. The critical point is predicted to occur at a thickness of the order of 100 A depending on the strength of the ordering force. The critical thicknesses are thus in the range of the dimensions of lamellar, cylindrical or spherical mesophases in block copolymers with ordered morphologies. The question is whether the phase boundary between the amorphous and the liquidcrystalline phase in a block copolymer will exert an ordering effect as assumed in the original theory or rather a disordering influence. The latter case and transitions between the two cases have also been treated recently by an extension of the theory (5). Therefore a theoretical framework exists, within which the transition behaviour of amorphous / liquid-crystalline block copolymers can be described. In addition to basic problems concerning the thermodynamic behaviour another question arises which is connected to the polymeric nature of the system. Unlike droplets of low molecular weight crystals in an amorphous matrix the liquid-crystalline and the amorphous phase at the interphase are coupled through polymer chains. Considering a block copolymer composed of an amorphous and a LC side chain polymer block which are the subject of this investigation, the question arises whether this coupling will have an effect on the director orientation with respect to the interphase by virtue of the fact that the polymer main chain of the LC block is preferentially oriented perpendicular to the interphase (7). If block copolymers with a long range ordered lamellar morphology are prepared, the director orientation with respect to the oriented lamellae may be influenced by the orientation of the polymer chains with respect to the interphase and the coupling of the mesogens to the polymer backbone. Different situations are expected for nematic and smectic systems and for different spacer lengths through which the mesogens are connected to the polymer backbone. It is not the intention of the present paper to answer all questions concerning the thermodynamic and orientational behaviour, but rather to demonstrate the synthetic methods by which block copolymers of ordered geometry can be prepared and to give a first description of the morphological and structural features of a particular system. To develop ordered morphologies it is necessary to polymerize block copolymers with a narrow molecular weight distribution which is usually realized by living anionic polymerization. For LC side chain polymers this polymerization is not suitable because most mesogens possess groups which react with the living anion. This imposes severe restrictions in the choice of the mesogenic group. In this paper a route will be shown how to synthesize monodisperse LC-homopolymers and block copolymers by two successive polymeranalogous reactions (8) with the possibility to use the broad variety of mesogenic units known up to now. The starting polymer is anionically polymerized polystyrene-block-1,2-polybutadien (PSPB), the molecular weight distribution of which will be conserved during the two reactions. In the first step the olefin double bonds of the PB-block are converted into alcohol groups by hydroboration with 9-borabicyclo[3.3.1]nonane (9-BBN) followed by an oxidation with H 0 (9). In the second step the mesogenic units are connected to the polystyrene-blockpolyalcohol polymer (PSPBOL) by an oxycarbonyl linkage to cholesteryl. 2

Weiss and Ober; Liquid-Crystalline Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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LIQUID-CRYSTALLINE POLYMERS


Reaction Scheme

PSPBOL

Another possibility to link the mesogen to the polymer backbone is by polymeranalogous esterification with a carbonic acid. This method will be described in a subsequent communication. Experimental Part Starting Material. 1,2-polybutadiene (PB) and the A-polystyrene-B-1,2-poly-butadiene block copolymer (PSPB) were obtained by "IMng" anionic polymerisation under high-vacuum conditions at -78°C with sec-buthyllfthium as initiator. Polv(2-hvdroxvethvlethv1ene) (PBOU. Trie hydroboration of PB was carried out in the same way as described by Chung et al.(9). An excess of 9-bora-bicyclo[3.3.1 Jnonane (0.5 M THF solution)(Aidrich) was allowed to react with the polybutadiene for 4 h at -10°C under nitrogen in a solution of dry THF. Stoichiometric amounts of NaOH and H 0 were added and the resulting polyalcohol was purified by distillation with methanol to remove trace amounts of boric acid, and by precipitation from diethyl ether. Polvstvrene43lc 250

Figure 3. Heating curve of PCHOL at 40 °C/min. (Reproduced with permission from reference 8. Copyright 1989 Huthig and Wepf Verlag, Basel.)

50

100

150

200

T/°C

Figure 4. Heating and cooling curve of PSPCHOL I with a heating- (cooling-) rate of 20 °C/min. (Reproduced with permission from reference 8. Copyright 1989 Huthig and Wepf Verlag, Basel.)



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Amorphous-Liquid-Crystalline Block Copolymers 181

Corresponding changes of the transition temperature also predicted by the theory will be difficult to detect because of the small magnitude of the temperature shift (1 °C or less). The observed slight decrease of Tj of the block copolymers with respect to the PCHOL homopolymer (Table II) is explained by other effects. The main reason for this decrease is probably due to a small amount of residual cholesterol monomer which was not removed during the purification procedure. Another difficulty arises from the fact that small changes in transition temperature and enthalpy can also be caused by the thermal and mechanical history of the sample, e. g. by presence of internal stress. However, the large decrease of AH in the block copolymer with respect to the PCHOL homopolymer must have other causes. It is either a thermodynamic effect, as discussed above, or it may also be caused by a disordering of the mesogens at the phase boundary. Since the amorphous / LC phase boundary must be sharp the disordering cannot originate from an intermixture of styrene and mesogenic units in an interphase region. Although the phase boundary is sharp on a local scale the interphase may be of a very irregular structure which could oppose the ordering of the mesogens at the interface. In this explanation the decrease of AH occurs because a surface layer of PCHOL at the phase boundary is in the disordered state and only the material in the interior of the LC microphase takes part in the transition. Further systematic investigations are needed to clarify which of the two explanations are correct. Morpoloqy of the Block Copolymer. In order to get information on the morphology of the system, polymer films were cast from CHCI-solution and electron micrographs of these films were taken from ultra-thin sections stained with osmium tetroxide. Figure 5a shows an EM micrograph of the block copolymer with the higher molecular weight. The micrograph demonstrates the phase separation into the amorphous PS phase, the LC phase appears dark because of staining with Os0 . The basic morphology is the lamellar type as expected from the 1 :1 composition of the block copolymer. The long period of the lamellar is 510 A. Beside the lamellar structure a honeycomb structure with a larger periodicity of 820 A is also present. The block copolymer with M = 118 000 g/mol has been treated by oscillatory shear (10) above the glass transition of PS producing the well ordered lamellar morphology in Figure 5b with a repeat distance of 350 A. The result shows that at sufficiently low molecular weight thin films can be obtained from the melt by shear, possessing long range ordered lamellae oriented preferentially in the direction of shear. The oriented sample has been used to investigate the question whether a correlation does exist between the supramolecular lamellar order of the block copolymer and the liquid crystalline order in LC lamellar domains. Figure 6 shows the X-ray photograph of this film, the X-ray beam being directed normal to the shear direction. The inner reflection surrounding the beam stop originates from the long periodicity of the lamellar macrolattice. It is deformed into an ellipse with its long axis lying normal to the direction of shear, corresponding to a high degree of orientation of the lamellae as expected from the EM picture in Figure 5b. From the intensity maximum of this first order reflection the same period is obtained as from the EM micrograph. At large angle the reflection of the smectic layers within the LC lamellae can be seen. An enhancement of the intensity perpendicular to the lamellae normal is observed. This proves that the smectic layers are oriented preferentially normal to the polystyrene lamellae. Therefore, for the particular smectic system investigated, a defined correlation between lamellar order and LC order does exist. The situation is schematically shown in Figure 7 showing alternating amorphous and liquid-crystalline 3

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Figure 5.

Electron micrographs of the block copolymers stained with Os0 , a: PSPCHOL II, b: PSPCHOL I


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Amorphous-Liquid-Crystalline Block Copolymers 183

Figure 6. X-ray diffraction pattern of an oriented PSPCHOL I film

Figure 7.

Model of the orientation in the smectic liquid crystalline AB-block copolymer


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lamellae, the layers within the lattice directed normal to the amorphous lamellae. Of course, this is an oversimplified representation. Not shown is the distribution of the smectic director in the lamellar plane. Conclusion


The synthesis described in this paper renders possible the preparation of block copolymers of uniform molecular weight composed of amorphous and LC side chain blocks. Beside the specific cholesterol mesogen introduced by carbonate linkages leading to a smectic system other mesogens with various spacer lengths can be introduced, e.g. by esterification. This opens the possibility to tailor block copolymers with a wide variety of LC phases and phase transition temperatures. A interesting possibility is the preparation of thermoplastic LC elastomers of the ABA-type with amorphous A-blocks having a high T and an elastomeric LC B-block with lowT. An uniform director orientation can be achieved in these systems by stress as shown recently for chemically crosslinked elastomers (12). Various applications of these systems in which optical uniaxiality and transparancy are induced by strain can be envisaged. g

g

Acknowledgments The authors thank Prof. H. Finkelmann and Dr. IV. Gleim for valuable suggestions and discussions. The work was carried out in the SFB 60 of the Deutsche Forschungs-gemeinschaft.

Literature cited 1.

Shibaev, V. P.; Platé, N. A. Polym. Sci. USSR (engl. Trans.) 1978, 19, 1065; Vysokomol.Soedin.,Ser. A 1977, 19, 923. 2. Finkelmann, H.; Ringsdorf, H.; Wendorff, J. H. Makromol. Chem. 1978, 179, 273 3. Gallot, B. Adv. Polym. Sci. 1978, 29, 85 4. Golemme, A.; Zumer, S.; Allender, D. W.; Doane, J. W. Phys. Rev. Lett. 1988,61,2937 5. Poniewierski, A.; Sluckin, T. J. Liquid Cryst. 1987, 2, 281 6. Sheng, P. Phys. Rev. A 1982, 26, 1610 7. Hasegawa, H.; Hashimoto, T.; Kawai, H.; Lodge, T. P.; Amis, E. J.; Glinka, C. J.; Han, C. C. Macromolecules 1985, 18, 67 8. Adams, J.; Gronski, W. Makromol. Chem., Rapid Commun. 1989, 10, 553 9. Chung, T. C.; Raate, M.; Berluche, E.; Schulz, D. N. Macromolecules 1988, 21, 1903 10. Hadziloannou, G.; Mathis, A.; Skoulios, A. Colloid & Polyme. Sci. 1979, 257, 136 11. Shibaev, V. P.; Platé, N. A.; Freidzon, YA. S. J. Polm. Sci., Polym. Chem. Ed. 1979, 17, 1655 12. Finkelmann, H.; Kock, H.; Rehage, G. Makromol.Chem.,Rapid Commun. 1981, 2, 317 RECEIVED May 9, 1990


Liquid-Crystalline Polymers - American Chemical Society

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