Synthesis And Properties Of Crystalline Monomers And Elastomers Based On L-Isoleucine Derivative

In recent year, chiral liquid crystalline polymers (LCPs) have attracted more and more attention due to their unique optical-electrical property and the potential application in the fields of nonlinear optics, fast light switch, microelectronics. The synthesis of chiral LC materials with new structure has been a very important subject in LC research. However, to the best of our knowledge, little research on LC compounds based on L-isoleucine derivative is reported. Therefore, it is necessary to study the new chiral LC materials containing L-isoleucine derivative, which are not only affluent in the content of LCPs, but also provide theoretical basis and novel thought to synthesize chiral LCEs with special optical-electricl properties. Moreover, they provide necessary experimental data and technology to explore their application.In this paper, the synthesis of four chiral LC monomers containing L-isoleucine derivative and two mesogenic crosslinking agents is described, which include4-(2-chloro-3-methylpentanoy-loxy)biphenyl-4’-undec-10-enoate (Mi),4-(undec-10-enoyloxy)-biphenyl-4’-(2-chloro-3-methy-lpentanoyloxy)benzoate (M2),4-(2-chloro-3-methylpentanoyloxy)-biphenyl-4’-(undec-10-enoy-loxy)benzoate (M3),4-(undec-10-enoyloxybenzoyloxy)biphenyl-4’-(2-chloro-3-methylpen-tan-oyloxy)benzoate (M4),4-(undec-10-enoyloxy)biphenyl-4’-(6-(undec-10-enoyloxy)hexyloxy) be-nzoate (N1) and4-(4-undec-10-enoyloxy)benzoyloxy)-phenyl-4’-(6-(undec-10-enoyloxy) hexy-loxy)benzoate (N2). The homopolymers (Pn series) were prepared by graft polymerization with reacting M1-M4and PMHS, respectively. The two elastomers (E1series and E2series) were synthesized by one-step polymerization with reacting M3, N1and PMHS; and M3, N2and PMHS, respectively. Their chemical structures 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-M4were dextral compounds, their values of specific rotation decreased with increasing rigidity. In addition, the monomers with near rigidity and same terminal groups, their specific rotation values hardly changed.M1-M4were thermotropic LC compounds, and M1showed broken fan-shaped texture of chiral smectic C (Sc) phase; M2and M3revealed broken fan-shaped texture of Sc-phase and oily-streak texture and focal-conic texture of cholesteric phase on heating and cooling cycles; M4exhibited cholesteric oily-streak texture and focal-conic texture. The selective reflection shifted to long wavelength ("red shift") at Sc phase, and short wavelength ("blue shift") at cholesteric phase with increasing temperature. For M1～M4, with increasing the number of phenyl ring, the melting temperature (Tm) and clearting temperature (Ti) of the corresponding monomers increased. For example, compared with Tm and Ti of Mi, those of M3increased by36.7℃and93.1℃, respectively. For M2and M3, they have same molecular weight and termina groups, moreover, synthetic method and raw material were same, but the arrangement way is different, their phase transition temperatures had a little different, but phase types and textures have not changed.The crosslinking agents N1and N2were enantiotropic mesogenic compounds, N1showed typical fan-shaped texture of smetic A (SA) phase and schlieren texture of nematic phase on heating and cooling cycles. N2exhibited masaic texture of smetic B (SB), broken fan-shaped texture of smectic C (Sc) phase, fan-shaped texture of smetic A (SA) phase and schlieren texture of nematic phase. The ester linkage bond in the mesogenic core also had an influence on the phase transition temperatures because the conjugation effect of the ester linkage bond, this can make Tm and Ti increase.The homopolymers P1～P4were amorphous LCPs with a glass transition, and exhbited non-typical color texture. The effect of the rigidity of 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. This indicates the polymerization can stabilize the LC phase, and widen the mesophase temperature ranges.LC elastomers E1and E2series displayed cholesteric Grandjean textures. The effect of the content of crosslinking units on the phase transition temperatures of LC elastomers is 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 from3mol%to15mol%, Tg increased from11.2℃to25.1℃, while Ti decreased from180.3℃to148.4℃for E1series; Tg increased from13.5℃to14.7℃, while T\decreased from199.3℃to183.1℃for E2series.