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

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 polymers containing menthyl groups have not been reported.In this paper, five new chiral LC monomers containing menthyl groups and three LC crosslinking agents were synthesized, which include,4-(10-undecylen-1-yloxy)biphenyl-4'-menthyloxyacetate (M1),4-(10-undecylen-1-yloxybenzoyloxy)biphenyl-4'-menthyloxyacetate (M2),4-(10-undecylen-1-yloxy)biphenyl-4'-menthyloxyacetyloxybenzoate (M3),4-(10-undecylen-1-yloxybenzoyloxy)phenyl-4'-menthyloxyacetyloxybenzoate (M4),4-(10-undecylen-1-yloxy-benzoyloxy)biphenyl-4'-menthyloxyacetyloxybenzoate (M5),4-(10-undecylen-1-yloxy)phenyl-4'-allyloxybenzoate (N1),4-(10-undecylen-1-yloxy)biphenyl-4'-allyloxybenzoate (N2), 4-allyloxybiphenyl-4'-(10-undecylen-1-yloxy)benzoate (N3). The homopolymers (P1 series) were prepared by graft polymerization with reacting M1～M5 and PMHS, respectively. The elastomers (P2 and P3 series) were synthesized by one-step polymerization with reacting M2, 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 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～M5 were levorotatory compounds, their absolute values of specific rotation declined with increasing rigidity, but for the compounds with near rigidity and same molecular weight, their values hardly changed. Besides M1, M2～M5 were thermotropic LC compounds. They displayed break fan-shaped textures of Sc* phase, cholesteric oily-streak textures and focal-conic textures on heating and 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～N3 showed typical nematic threaded texture and schlieren texture. In addition, N2 also exibited fan-shaped texture of smetic A (SA) phase, break fan-shaped texture of smetic C (Sc) phase and mosaic texture of smetic B (SB) phase on cooling cycle. N3 displayed fan-shaped texture of SA phase. With increasing the phenyl rings number, Tm and Ti of M2-M5 incresed, and Ti increased more than Tm. Compared M2 with M3, N2 with N3, they have same molecular weight, synthetic method and raw material were same, and M2 and M3 had a little different in phase transition temperatures, but phase types and textures have not changed. However, the phase types and textures of N2 and N3have changed.Homopolymers P1₁-P1-5 were amorphous LCPs, and displayed cholesteric Grand-jean textures. Although M1 did not show any mesmorphic property, the corresponding P1-1 showed cholesteric phase. 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. With increasing the content of the crosslinking units in the elastomers, the crossponding Tg increased and Ti decreased. For the P2 series, when content of the crosslinking agent increased from 4mol% to 20%, Tg increased from 27.2℃to 33.4℃, but Ti decreased from 141.7℃to 131.4℃. TGA showed that the temperatures at which 5% weight loss occurred (Td) were greater than 340℃for the elastomers P2 and P3 series, this indicated these elastomers have good thermal stability.