Preparation And Properties Of Chiral Liquid Crystalline Polymers Composite Materials Containing Carbon Nanotubes

At present, chiral liquid crystalline polymers (LCPs) are a hotspot of LC research domains, and they have attracted more and more attention due to the unique photo-electrical property. Chiral LCPs with helical supermolecular structure can form cholesteric phase and chiral smectic C (Sc*) phase. Carbon nanotubes (CNTs) not only have excellent mechanical properties, chemical and thermal stabilities, and electrical conductivity, but also have length-diameter ratio greater than1000, which make CNTs being a perfect reinforcement agent for polymer composites, and can give many new functions. In this study, new chiral LC monomers and the corresponding polymers based on menthol were synthesized and characterizated, and CNTs/chiral LCPs composite materials with differrent content of CNTs, those research has not been reported.In this paper, three chiral LC monomers were synthesized, which include4-(4-menthyloxyacetyloxybenzoyl)biphenyl-4’-(4-(2-(undec-10-enoyloxy)ethoxy)benzoyl)-benzoate (M1),4-(4-menthyloxyacetyloxybenzoyl)biphenyl-4’-(4-(6-(undec-10-enoyloxy) hexyloxy)benzoyl)benzoate (M2),4-(4-menthyloxyacetyloxybenzoyl)phenyl-4’-(4-(6-(undec-10-enoyloxy) hexyloxy)benzoyl)benzoate (M3). The homopolymers (P1and P2) were prepared by graft polymerization with reacting M1, M2and PMHS, respectively. In addition, CNTs/chiral LCP composite materials with differrent content of CNTs were obtained by reaction of M1and PMHS containing CNTs with hydroxyl group. The chemical structures of the monomers and homopolymers were characterized by FT-IR and1H-NMR. The mesomorphism and thermal behavior were investigated polarizing optical microscopy (POM), differential scanning calorimetry (DSC), X-ray diffraction (XRD), and thermogravimetric analysis (TGA). The special optical rotations were measured by a polarimeter. The selective reflection of light was investigated with ultraviolet/visible (UV/VIS). The influence of the mesogenic rigidity, spacer length, and menthyl steric effect on the optical rotations, phase transition temperatures, phase types and textures is discussed.The monomer M1-M3were levorotatory compounds, their absolute values of specific rotation declined with increasing rigidity and flexible spacer length. Moreover, by inserting a flexible spacer between the mesogenic core and the terminal menthyl groups, M1-M3are enantiotropic LC compounds, and exhibited break fan-shaped texture of a Sc phase, oily-streak texture and focal-conic texture of cholesteric phase on heating and cooling process. In addition, with increasing temperature, the selective reflection of light shifted to the long wavelength region (red shift) at the Sc phase range, and to the short wavelength region (blue shift) at the cholesteric range, respectively. With increasing the mesogenic rigidity or phenyl rings number, their melting temperature (Tm) and isotropic temperature (Ti) increased, and mesophase range (△T) widened because Ti increased more than Tm; With increasing the spacer length, the corresponding Tm decreased, while Ti increased, so△T widened.The homopolymers Pi and P2were amorphous LCPs, and optical textures were not typical. However, Pi and P2showed Sc phase and smectic A phase, which can be confirmed with XRD. When the measuring temperature increased from80to and160℃, the d-spacing of the first-order reflection gradually increased from38.4to40.2A, which gives strong evidence for the formation of a tilted Sc phase. The influence of the mesogenic rigidity on the phase behavior of P1and P2showed the same tendency as those described above for the monomers. Namely, with increasing the mesogenic rigidity, the glass transition temperature (Tg) and Ti all increased. This also indicated the polymerization could further stabilize and widen the mesophase range. TGA showed that the temperatures at which5%weight loss occurred (Td) were greater than310℃, this indicated that P1and P2had excellent thermal stability.For CNTs/P1polymer composites materials, the CNTs in LCPs did not change the chemical structure of LCPs matrix, but affected the mesophase transition temperature. They can not only improve Tg, Ti, and Td, but also widened the mesophase temperature range. In addition, the CNTs in LCPs did not obviously affect the texture and mesophase type. Moreover, a small quantity of CNTs was well dispersed in the LCP matrix and made the mesophase more stable. However, when the content of CNTs was greater than1%, the CNTs in LCPs could form agglomeration particle. SEM observation showed that CNTs was coated with liquid crystal polymer and polymer-coated CNTs in the composite gradually decreased with the increase of CNTs content, the amount of exposed CNTs gradually increased. In addition, the composite color deepened. Moreover, the distribution of CNTs in the liquid crystal polymer is homogeneous, and the CNTs from the nucleation role, so that the polymer structure refinement.