Synthesis And Properties Of Chiral Liquid Crystalline Monomers And Elastomers Containing Menthyl Groups

Chiral liquid crystalline (LC) polymers have attracted more and more attention, due to they have the unique optical-electrical property and the potential application in the fields of nonlinear optics, fast light switch, microelectronics. In recent years, the synthesis of chiral LC materials with new structure has been an important subject in LC research. Although chiral LC copolymers with non-mesogenic menthyl unit have been reported, but research on chiral LC monomers and homopolymers containing menthyl groups have not been reported. Therefore, a series of new LC monomers and the corresponding homopolymers containing menthyl groups will be studied.In this paper, eight chiral LC monomers containing menthyl groups and two LC crosslinking agents were synthesized, which include,4-(undec-10-enoyloxy)phenyl-4'-(4-(menthyloxyacetoxy)benzoyloxy)benzoate (M1),4-(4-(undec-10-enoyloxy)benzoyloxy) phenyl-4'-(menthyloxyacetoxy)benzoate (M2),4-(undec-10-enoyloxy)biphenyl-4'-(4-(menthyl-oxyacetoxy)benzoyloxy)benzoate (M3),4-(4-(undec-10-enoyloxy)benzoyloxy) biphenyl-4'-(menthyloxyacetoxy)benzoate (M4),4-(4-(undec-10-enoyloxy)benzoyloxy)phenyl-4'-(4-(menthyloxyacetoxy)benzoyloxy)benzoate (M5),4-allyloxybiphenyl-4'-(4-(menthyloxyacetoxy)-benzoyloxy)benzoate (M6),4-(4-allyloxybenzoyloxy)biphenyl-4'-(menthyloxyacetoxy)-benzoate (M7),4-(4-allyloxybenzoyloxy)phenyl-4'-(4-(menthyloxyacetoxy)benzoyloxy)benzoate (M8),4-(undec-10-enoyloxy)phenyl-4'-(undec-10-enoyloxy)benzoate (N1) and 4-(undec-10-enoyloxy)biphenyl-4'-(undec-10-enoyloxy)benzoate (N2). The homopolymers (Pn series) were prepared by graft polymerization with reacting M1～M7 and PMHS, respectively. The elastomers (P1 series and P2 series) were synthesized by one-step polymerization with reacting M3, N1, and PMHS; and M3, N2 and PMHS, respectively. The chemical structures of the monomers, crosslinking agents, homopolymers, and elastomers were characterized by FT-IR or 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 selective reflection properties of chiral LC monomers were studied with UV/Visible/NIR. The monomers M1～Mg were levorotatory compounds, their absolute values of specific rotation declined with increasing rigidity or terminal flexible length. In addition, the monomers with near rigidity and same molecular weight, their values hardly changed.M1～M8 were thermotropic LC compounds. M1～M5 exhibited enantiotropic broken fan-shaped textures of chiral smectic C (Sc) phase and oily-streak textures and focal-conic textures of cholesteric phase on heating and cooling cycles. With increasing temperature, the selective reflection of M1～M5 shifted to long wavelength "red shift" in Sc* phase, and short wavelength "blue shift" in cholesteric phase. Whereas, M6～Mg only displayed oily-streak textures and focal-conic textures of cholesteric phase. In addition, M1, M3 and M5 also showed the cubic texture of blue phase on the cooling process. With increasing the phenyl rings number, Tm and Ti of the corresponding monomers increased, and the mesophase range widened because Ti increased more than Tm. For example, compare with M1, Tm of M3 increased 35.1℃, While Ti increased 106.1℃In addition, with increasing terminal flexible length (the number of methyene),Tm and Ti decreased, For example, compare with M7, the methyl number of M4 increased from 2 to 8, Tm and Ti decreased 30.0℃and 21.5℃, respectively.Crosslinking agents N1 and N2 showed typical fan-shaped texture of smetic A phase and schlieren texture, droplets texture of nematic phase on heating and cooling process. In addition, N1 also exibited broken fan-shaped texture of smetic C phase on cooling process. With increasing the phenyl rings number, Tm and Ti of the corresponding crosslinking agent increased. Compared with N1, Tm and Ti of N2 increased 59.5℃and 109.6℃, respectively.Homopolymers P1～P7 were amorphous LCPs, and displayed cholesteric Grand-jean textures. The effect of mesogenic core and flexible spacer on Tg and Ti of homopolymers is consistent with the corresponding monomers. TGA showed that the temperatures at which 5% weight loss occurred (Td) were greater than 310℃for the homopolymerss,this indicated they have good thermal stability. LC elastomers P1 series and P2 series displayed cholesteric Grand-jean textures. With increasing the content of the crosslinking units in the elastomers, the crossponding Tg and Ti increased. When content of N1 increased from 2mol% to 12%, Tg of P1 series increased from 24.9℃to 32.2℃, while Ti increased from 231.1℃to 266.0℃; Tg of P2 series increased from 38.9℃to 57.5℃, while Ti increased from 276.2℃to 293.3℃TGA showed that Td were greater than 320℃for the elastomers.