| Chiral liquid crystalline polymers (LCPs) were a hotspot of LC research field at the present time. Due to the unique photo-electrical property, they have attracted more and more attention. Chiral LCPs with helical supermolecular structure can form cholesteric phase and chiral smectic C (SC*) phase. Although some LC copolymers containing non-mesogenic chiral menthyl units have been reported, to the best of our knowledge, research on chiral mesogenic monomers and their corresponding homopolymers 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-(4-(undec-10-enoyloxy)benzoyloxy)-ethoxy)biphenyl-4'-menthyloxyacetate (M1),4-(6-(4-(undec-10-enoyloxy)benzoyloxy)hexyloxy)-biphenyl-4'-menthyloxyacetate (M2),4-(2-(4-(undec-10-enoyloxy)benzoyloxy)ethoxy)biphenyl-4'-menthyloxyacetyloxybenzoate (M3),4-(6-(4-(undec-10-enoyloxy)benzoyloxy)hexyloxy)biphenyl-4'-menthyloxyacetyloxybenzoate (M4),4-(undec-10-enoyloxy)biphenyl-4'-allyloxybenzoate (Ni),4-(undec-10-enoyloxy)phenyl-4'-(undec-10-enoyloxy)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 and N1, and PMHS; or M2 and N2, and PMHS, respectively. The chemical structures of the monomers, crosslinking agents, homopolymers, and elastomers were characterized by FT-IR and'H-NMR. The special optical rotations were measured by a polarimeter. The selective reflection properties were studied with UV/VIS/IR. The mesomorphic properties and phase behavior were investigated polarizing optical microscopy (POM), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). 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 monomers Mi-M4 were levorotatory compounds, their specific rotation absolute values decreased with increasing rigidity. M1-M4 were thermotropic LC compounds, and showed cholesteric oily-streak texture and focal-conic texture on heating and cooling process. Moreover, the cube texture of blue phase is observed on the cooling process. The selective reflection shifted to short wavelength ("blue shift") with increasing temperature, and long wavelength ("red shift") with decreasing temperature at cholesteric phase. In addition, M2 and M4 also displayed broken fan-shaped textures of Sc phase besides cholesteric phase. The selective reflection shifted to long wavelength ("red shift") with increasing temperature, and short wavelength ("blue shift") with decreasing temperature at Sc* phase. With increasing the phenyl rings number, Tm and Ti of M1-M4 increased, and T; increased more than Tm. For example, compared with Tm and Ti of M1, those of M3 increased by 18.7℃and 107.2℃, respectively. In addition, Tm and Ti decreased, and Tm decreased more than Ti with increasing flexible spacer length (the number of methyene). For instance, compared with Tm and Ti of M1, those of M2 decreased by 72.8℃and 16.5℃, respectively.The crosslinking agents N1 and N2 showed enantiotropic fan-shaped texture of smetic A (SA) phase and schlieren or droplet textures of nemetic phase. In addition, N1 also exhibited monotropic broken fan-shaped texture of smectic C (SC) phase and mosaic texture of smectic B (SB) phase; while N2 only exhibited monotropic broken fan-shaped texture of Sc phase. With increasing the phenyl rings number and decreasing the end flexibility, Tm and Ti of N1 increased, and Ti increased more than Tm.Homopolymers P1-1-P1-4 were amorphous LCPs, and displayed cholesteric Grand-jean textures. The effect of the rigidity and flexible spacer in the mesogenic units on the glass transition temperature (Tg) and Ti is similar to the corresponding monomers. In addition, the mesophase temperature ranges of the homopolymers were greater than those of the corresponding monomers. The temperatures at which 5% weight loss occurred (Td) were greater than 300℃for P1-1-P1-4, which indicated that they have good thermal stability.LC elastomers P2 and P3 series displayed cholesteric Grand-jean textures. Effect of the content of crosslinking units on the phase transition temperatures and thermal property of LC elastomers has been discussed. With increasing the content of the crosslinking units in the elastomers, the crossponding Tg increased and Ti decreased. For example, when content of the crosslinking agent increased from 2 to 12 mol%, Tg increased from 14.6℃to 26.1℃, while Ti decreased from 128.2℃to 120.6℃for P2 series; Tg increased from 14.5℃to 16.2℃, while Ti decreased from 129.3℃to 112.8℃for P3 series.TGA showed that Td of all the elastomers were greater than 300℃. |
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