Synthesis And Properties Of New Chiral Liquid Crystalline Monomers And Network Polymers

Chiral liquid crystalline polymers (LCPs) were a hotspot of LC research domains at the present time. Due to the unique photo-electrical property, they have obtained more and more attention. LCPs with helical supermolecular structure can form cholesteric phase and chiral smectic C (Sc) phase. To the best of our knowledge, research on chiral LC monomers and network polymers containing menthyl groups have not been reported.In this paper, four chiral LC monomers containing menthyl groups and two LC crosslinking agents were synthesized, which include 4-(2-(undec-10-enoyloxy)ethoxy) biphenyl-4'-menthyloxyacetate (M1),4-(6-(undec-10-enoyloxy)hexyloxy)biphenyl-4'-menthyloxyacetate (M2),4-(2-(undec-10-enoyloxy)ethoxy) biphenyl - 4'-menthyloxyacetyloxybenzoate (M3), 4-(2-(undec-10-enoyloxy)hexyloxy) biphenyl - 4'-menthyloxyacetyloxybenzoate (M4), 4-(undec-10-enoyloxy)biphenyl -4'-(2-(undec-10-enoyloxy)ethoxy)benzoate (N1), 4-(undec-10-enoyloxy)biphenyl-4'-(6-(undec-10-enoyloxy)hexyloxy)benzoate(N2). The homopolymers (P1 series) were prepared by graft polymerization with reacting M1～M4 and PMHS, respectively. The elastomers (P2 and P3 series) were synthesized by one-step polymerization with reacting M2, N1, and PMHS; and M2, N2 and PMHS, respectively. The chemical structures of the monomers, crosslinking agents, homopolymers, and elastomers were characterized by FT-IR and 1H-NMR. The mesomorphic properties and phase behavior were investigated polarizing optical microscopy (POM), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). The special optical rotations were measured by a polarimeter. The effect of molecular structures on the optical rotations, phase transition temperatures, phase types and textures is discussed. Moreover, the influence of the crosslinking units on the phase behavior of elastomers is also discussed.The monomer M1～M4 were levorotatory compounds, their absolute values of specific rotation declined with increasing rigidity and also declined with increasing flexible chain segments. All the monomers and crosslinking agents are thermotropic LC compounds except the M1 which is monotropic liquid crystal. The M1 displayed cholesteric focal-conic textures on cooling process. The M2 displayed cholesteric focal-conic textures on heating and cooling process. Selective reflection shifted to short wavelength ("blue shift") in cholesteric phase with increasing temperature. For the M3 and M4, they displayed break fan-shaped textures of Sc phase, cholesteric oily-streak textures and focal-conic textures on heating and cooling process. Moreover, the cube texture of blue phase is observed on the cooling process. Selective reflection properties appeared in Sc* phase, the selective reflection shifted to long wavelength ("red shift"), and short wavelength ("blue shift") in cholesteric phase with increasing temperature. Crosslinking agents N1 and N2 showed typical fan-shaped texture of smetic A (SA) phase and break fan-shaped texture of smetic C (Sc) phase on heating and cooling process. With increasing the phenyl rings number, Tm and Ti of M1～M4 increased, and Ti increased more than Tm. With increasing flexible chain segments, Tm of M1～M4 decreased. For N1 and N2, with increasing flexible chain segments, Tm decreased and Ti increased slightly.Homopolymers P1-1～P1-4 were amorphous LCPs, and displayed cholesteric Grand-jean textures. The trend of Tg and Ti corresponds to changes in trend of monomers. LC elastomers P2 and P3 series displayed cholesteric Grand-jean textures. Effect of the content of crosslinking units on LC phase transition temperatures and thermal property of LC elastomers has been discussed. For the P2 series, with increasing the content of the crosslinking units in the elastomers, the Tg increased and Ti decreased; For the P3 series, with increasing the content of the crosslinking units in the elastomers, the Tg decreased and Ti did not change greatly. But compared to homopolymer, Ti of the P3 series decreased.TGA showed that the temperatures at which 5% weight loss occurred (Td) were greater than 310℃for the homopolymers P1 series and elastomers P2,P3 series, this indicated these elastomers have good thermal stability.