Synthesis And Liquid Crystalline Properties Of(Meth)Acrylate Monomers And Polymers Containing Menthyl Groups

In recent year, chiral liquid crystalline (LC) materials have been attracted more and more attention because of their unique optical-electrical property and the potential application in the fields of nonlinear optics, fast light switch, microelectronics. Although chiral LC copolymers containing non-mesogenic menthyl groups has been reported, research on chiral LC monomers and homopolymers containing menthyl groups has not been reported by other research teams. By inserting a flexible spacer was between the rigid mesogenic core and the terminal menthyl groups by reducing the steric effect, the LC property can be realized and different mesophase such as a smectic C (Sc) phase, cholesteric pahse and blue phase etc can obtained. Therefore, research on new LC materials based on menthol holds not only important theoretical purpose, but also potential applications in numerous areas, especially in the fields of piezoelectric materials and ferroelecteric etc.In this paper, the synthesis of seven chiral monomers containing menthyl groups and a crosslinking agent is presented, which include4-(6-(acryloyloxy)hexyloxy)biphenyl-4’-menthyloxyace-tate (M1),4-(4-(methacryloyloxy)butoxy)biphenyl-4’-menthyloxyacetate (M2),4-(4-(methacryloylo-xy)hexyloxy)biphenyl-4’-menthyloxyacetate (M3),4-(4-(acryloyloxy)-buto xy)bipheny1-4’-menthyl-oxyacetyloxybenzoate (M4),4-(4-(methacryloyloxy)butoxy)-biphenyl-4’-menthyloxyacetyloxyben-zoate (M5),4-(6-(acryloyloxy)hexyloxy)biphenyl-4’-menthyloxyacetyloxybenzoate (M6),4-(6-(met-hacryloyloxy)hexyloxy)biphenyl-4’-menthyl-oxyacetyloxybenzoate (M7) and4-(6-(methacryloyloxy)hexyloxy)biphenyl-4’-(2-(acryloyloxy)ethoxy)benzoate (N). The homopolymers (Pn series) were prepared by radical polymerization with M4-M7, respectively. In addition, the elastomers (En series) were synthesized by one-step polymerization with reacting M6and N. 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 by 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 structure-property relationships of M1～M7are discussed. The effect of the content of crosslinking agent on phase behavior of elastomers is studied.The monomers M1-M7were levorotatory compounds, and their absolute values of specific rotation decreased with increasing rigidity or flexible spacer length.M1～M3were non-mesogenic compounds because of their weaker rigidity of mesogenic core, while M4～M7were thermotropic LC compounds. Moreover, M4～M6exhibited typical enantiotropic brokenfan-shaped textures of Sc*phase and oily-streak texture and focal-conic texture of cholesteric phase on heating and cooling cycles. M7only exhibited cholesteric oily-streak texture and focal-conic texture. In addition, the selective reflection of M5shifted to short wavelength ("blue shift") in cholesteric phase with increasing temperature. The melting temperature (Tm) and clearing temperature (Ti) of the chiral monomers increased with the phenyl rings number or the rigidity of the mesogenic core. For example, compared with Tm and Ti of M3, those of M7increased by26.3℃and111.5～, respectively. With increasing the flexible spacer length (the number of methylene increased from4to6), the corresponding Tm and Ti decreased. For example, compare with M4, Tm and Ti of M6with longer spacer (six methylene) decreased by17.1℃and8.2℃, respectively. Tm of the methacrylate monomers was less than that of the acrylate monomers with same mesogenic core, spacer and terminal group. The corsslinking agent N exhibited nematic shilerent texture and droplet texture on heating and cooling cycles.Homopolymers P4-P7were amorphous LC polymers, and displayed cholesteric Grandjean texture. 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 with monomer, the mesophase temperature range of the corresponding homopolymer widened, this indicates the polymerization can stabilizes the mesophase. LC elastomers En series exhibited color texture. With increasing the content of the crosslinking units in the elastomers, the crossponding Tg firstly decreased and then increased, and Ti increased. When content of the crosslinking agent increased from6to15mol%, Tg decreased from45.2℃for E1to40.0℃for E2, and increased from40.0℃for E2to71.1℃for E3, while Ti increased from157.3℃for E1to181.3℃for E3.