A New Route To Stabilize the Smectic C Phase in a Series of Laterally Attached Side-Chain Liquid Crystalline Polynorbornenes with a One-Carbon Spacer Gue-Hyun Kim, Coleen Pugh,* and Stephen Z. D. Cheng* Maurice Morton Institute and Department of Polymer Science, The University of Akron, Akron, Ohio 44325-3909 Received November 3, 1999; Revised Manuscript Received August 29, 2000 ABSTRACT: A series of polynorbornenes (PNBEs) with 1,4-bis[(3′-fluoro-4′-n-alkoxyphenyl)ethynyl]- benzene mesogens (n ) 9-12) laterally attached to the polymer backbone through a one-carbon spacer were previously synthesized by ring-opening metathesis polymerization of the corresponding norbornene- based monomers. Wide-angle X-ray diffraction (WAXD) experiments demonstrate that the mesogens organize into the tilted layer structure of a smectic C (S C) phase at room temperature, and polarized light microscopy demonstrates that the highest temperature ordered phase is a nematic (N) phase. Upon heating above room temperature, the tilt angle of the SC phase of all of the PNBEs (n ) 9-12) continuously decreases, especially at temperatures above 70 °C for the PNBEs (n ) 9-11) and 60 °C for the PNBE (n ) 12). However, the SC phase of the PNBEs (n ) 9-11) transforms to a N phase before the tilt angle reaches zero. That is, the SA phase is never actually achieved in the PNBEs (n ) 9-11), although the decrease in the tilt angles corresponds to a broad endotherm in the differential scanning calorimetry (DSC) experiments in the same temperature region. This speculation is proven by the stable SA phase of PNBE (n ) 12) observed by WAXD at temperatures above 83 °C. The transition between the SC and SA phases is not a first-order transition according to DSC measurements and is instead a second-order transition based on the gradual decrease to 0° in the tilt angle, which serves as an order parameter. Introduction Traditional side-chain liquid crystalline (LC) polymers are synthesized with the intention of retaining the LC behavior of low molecular mass liquid crystals, while trying to improve their mechanical and phase stability. The typical molecular design attaches the mesogens terminally to the polymer backbone. One disadvantage of using side-chain LC polymers for electrooptical display applications is their relatively high viscosity, which slows their response toward external fields. 1 Recently, interest in LCs that generate a smectic C (S C ) phase has increased because these molecules can be either functionalized with a chiral substituent or doped with chiral compounds to generate a S C * phase, which spontaneously polarizes if the helix is unwound. 2 This decreases the switching time from one anisotropic orientation to the opposite orientation in side-chain LC polymers. Although many terminally attached side-chain LC polymers have exhibited the S C phase, 5 we 1,4 proposed that a more appropriate design for LC polymers that exhibit S C phase behavior is to laterally attach extended mesogens to the polymer backbone. This is because low molecular mass liquid crystals tend to exhibit the S C phase when they are based on symmetrically disubsti- tuted extended mesogens with long n-alkoxy substitu- ents, 3 and lateral attachment maintains the symmetric disubstitution of the extended mesogens. However, almost all laterally attached side-chain LC polymers synthesized so far have exhibited only the nematic (N) phase. 6-17 Since the laterally attached mesogens are generally much larger in size than the repeat units of the polymer backbone, the mesogens force the backbone into a somewhat extended helical conformation, thereby allowing the mesogens in the side chains to wrap around it to form a cylindrical mesogen jacket. 8,13,15,17 In this case, it should be difficult to form a smectic phase. Nevertheless, side-chain LC polymers with laterally attached 1,4-bis[(4′-n-alkoxybenzoyl)oxy]benzene me- sogens can be forced to order into S C layers by terminat- ing their n-alkoxy substituents with either immiscible fluorocarbon 18 or siloxane 19 segments. However, the question still remains whether laterally attached side-chain LC polymers can exhibit S C phase behavior without introducing immiscible components. For example, laterally attached side-chain LC polynor- bornenes (PNBEs) with symmetrically disubstituted extended mesogens and an 11-carbon spacer apparently exhibit only a N phase, 20 although the corresponding low molecular mass 1,4-bis[(3′-fluoro-4′-n-alkoxyphenyl)- ethynyl]benzenes with n ) 6-12 exhibit a stable S C phase over a relatively wide temperature range. 21,22 This is also in spite of the fact that long spacers generally favor smectic mesophases in terminally attached side- chain LC polymers. 23 Therefore, the lateral polymeric substituent apparently destabilizes the S C phase. To determine the extent to which lateral attachment destabilizes the S C phase, this study investigates the phase behavior of a series of polynorbornenes (PNBEs) 22 with 1,4-bis[(3′-fluoro-4′-n-alkoxyphenyl)ethynyl]ben- zene mesogens (n ) 9-12) laterally attached to the polymer backbone through a one-carbon spacer. Since the S C phase is generally stabilized by increasing the number of methylenic units in the flexible substitu- ents, 23 the effect of the lateral attachment may vary with the length of the n-alkoxy substituents. In addition to the glass transition, these polymers were previously reported to exhibit only a N phase during the time scale of a DSC experiment. 22 * To whom correspondence should be addressed. 8983 Macromolecules 2000, 33, 8983-8991 10.1021/ma9918607 CCC: $19.00 © 2000 American Chemical Society Published on Web 11/10/2000