Synthesis, Structures And Properties Of Liquid Crystalline Monomers And Elastomers Containing Menthyl Groups

In recent year, chiral liquid crystalline (LC) polymers have attracted more and more attention, and they not only have the unique optical-electrical property but also the potential application in the fields of nonlinear optics, fast light switch, microelectronics. 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 except for our research team.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 crosslinking agents were synthesized, which include4-(undec-10-enoyloxy)phenyl-4’-(2-(2-isopropyl-5-methylcyclohexyloxy)acetoxy)biphenyl-4-carboxylate (Mi),4’-(undec-10-enoyloxy)-biphenyl-4-yl-4-(4-(2-(2-isopropyi-5-menthylcyclohexyloxy)acetoxy)benzoyloxy)benzoate (M2),4’-(undec-10-enoyloxy)biphenyl-4-yl-4’-(2-(2-ispropyl-5-methylcyclohexyloxy)acetoxy)biphenyl-4-carboxylate (M3),4’-(4-(2-(2-isopropyi-5-menthylcyclohexyloxy)acetoxy)benzoyloxy) biphenyl-4-yl-4-(undec-10-enoyloxy)benzoate (M4),4-(4-(undec-10-enoyloxy) benzoyloxy) phenyl-4’-(2-(2-ispropyl-5-methylcyclohexyloxy)acetoxy)biphenyl-4-carboxylate (M5),4’-(alyloxy)-biphenyl-4-yl-4-(2-(isopropyl-5-menthlcyclohexyloxy)acetoxy) benzoate (M6),4-(4-allyloxy) benzoyloxy)phenyl-4’-(2-(2-isopropyl-5-menthylcyclohexyloxy)acetoxy)biphenyl-4-carboxylate (M7),4’-(allyloxy)biphenyl-4-yl-4’-(2-(2-isopropyl-5-menthylcyclohexyloxy)acetoxy)biphenyl-4-carboxylate (Mg),4-(undec-10-enoyloxy)phenyl-4-(2-(undec-10-enoyloxy)ethoxy)benzoate (N1) and4-(4-(undec-10-enoyloxy)benzoyloxy)phenyl-4-(2-(undec-10-enoyloxy)ethoxy)benzoate (N2). The homopolymers (Pn series) were prepared by graft polymerization with reacting from M1to Mg and PMHS, respectively. The two elastomers (P1series and P2series) were synthesized by one-step polymerization with reacting M5, N1, and PMHS; and M5, N2and PMHS, respectively. The chemical structures of the monomers, crosslinking agents, homopolymers, and elastomers were characterized by FT-IR or1H-NMR. The mesomorphic properties and phase behavior were investigated polarizing optical microscopy (POM) and differential scanning calorimetry (DSC). 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～M8were levorotatory compounds, their absolute values of specific rotation decreased with increasing rigidity or terminal flexible length. In addition, the monomers with near rigidity and same molecular weight, their values hardly changed.M1～M8were thermotropic LC compounds. M1～M5exhibited enantiotropic broken fan-shaped textures of chiral smectic C (S*) 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～M5shifted to long wavelength ("red shift") in Sc*phase, and short wavelength ("blue shift") in cholesteric phase. Whereas, M6～M8only displayed oily-streak textures and focal-conic textures of cholesteric phase becaues of shorter terminal spacer. In addition, M3, M4and M5also showed the cubic texture of blue phase on the cooling process. With increasing the phenyl rings number, the melting temperature (Tm) and clearing point (Ti) of the corresponding monomers increased, and the mesophase range widened because Ti increased more than Tm. For example, compare with M1, Tm of M3increased by29.5℃, while Ti increased by113.2℃In addition, with increasing terminal flexible length (the number of methyene), Tm and Ti decreased. For example, compare with M7, the methyl number of M5increased from1to8, Tm and Ti decreased by65.3℃and26.3℃, respectively.The crosslinking agent N1did not show obviously LC texture, and DSC thermographs only displayed a melting transition peak, so N1is a non-mesogenic compound. However, N2showed enantiotropic fan-shaped texture of smetic A (SA) phase and homeotropic textures or schlieren textures of nemetic phase.Homopolymers P1～P8were amorphous LC polymers, and displayed mesophase and cholesteric Grand-jean textures. The effect of mesogenic core and flexible spacer on glass transition temperature (Tg) and Ti of homopolymers is consistent with the corresponding monomers. Moreover, compared the monomers, the mesophase temperature range of the corresponding homopolymer widened, this indicates the polymerization can stabilize the mesophase.DSC curves of elastomers P1series and P2series only show a glass tansition, but POM results displayed mesophase and cholesteric Grand-jean textures. With increasing the content of the crosslinking units in the elastomers, the crossponding Tg and Ti decreased. When content of N1increased from Omol%to15%, Tg decreased from68.2℃of P1-0to41.8℃of P1-5, while Ti decreased from286.6℃of P1-0to238.5℃of P1-5, and Tg decreased from68.2℃of P2-0to48.2℃of P2-5, while Ti decreased from286.6℃of P2-0to248.0℃of P2-5.