Synthesis and characterization of liquid crystalline elastomers bearing fluorinated mesogenic units and crosslinking mesogens

Several new side-chain liquid crystalline (LC) polysiloxanes and elastomers (IP-VIP) bearing fluorinated mesogenic units and crosslinking mesogens were synthesized by a one-step hydrosilylation reaction with poly(methylhydro-geno)siloxane, a fluorine-containing LC monomer 4'-undec-10-enoyloxy-biphenyl-4-yl 4-fluoro-benzoate and a crosslinking LC monomer 4'-(4-allyloxy-benzoxy)-biphenyl-4-yl 4-allyloxy-benzoate. The chemical structures and LC properties of the monomers and polymers were characterized by use of various experimental techniques such as FTIR, (1)H-NMR, EA, TGA, DSC, POM and XRD. The effect of crosslinking mesogens on mesomorphic properties of the fluorinated LC polymers was studied as well. The obtained polymers and elastomers were soluble in many solvents such as toluene, tetrahydrofuran, chloroform, and so forth. The temperatures at which 5% weight loss occurred (T(d)) were greater than 250 degrees C for all the polymers, and the weight of residue near 600 degrees C increased slightly with increase of the crosslinking mesogens in the fluorinated polymer systems. The samples IP, IIP, IIIP and IVP showed both smectic A and nematic phases when they were heated and cooled, but VP and VIP exhibited only a nematic mesophase. The glass transition temperature (T(g)) of polymers increased slightly with increase of crosslinking mesogens in the polymer systems, but the mesophase-isotropic phase transition temperature (T(i)) and smectic A-nematic mesophase transition temperature (T(S-N)) decreased slightly. It suggests that the temperature range of the mesophase became narrow with the increase of crosslinking mesogens for all the fluorinated polymers and elastomers. In XRD curves, the intensity of sharp reflections at low angle decreased with increase of crosslinking mesogens in the fluorinated polymers systems, indicating that the smectic order derived from fluorinated mesogenic units should be destroyed by introduction of more crosslinking mesogens. Copyright (C) 2008 John Wiley & Sons, Ltd.