Characteristic smectic structures of main-chain liquid-crystalline polyimides driven by a microphase separation between aromatic imide mesogen and a siloxane spacer

Abstract

We investigated the phase transition behavior and liquid-crystalline structure for main-chain liquidcrystalline polyimides with aromatic imide cores and alkyl and dimethyl siloxane spacers, BPDA/ 3SiO4-3F. They formed crystal, smectic C (S C ), smectic A (S A ), and isotropic phases in order of increasing temperature. The characteristic structures of the S A and S C phases were extracted with wideangle X-ray diffraction (WAXD) and Fourier transform infrared (FTIR) spectroscopy measurements. In the S C phase, the mesogenic groups were tilted by approximately 40 to the smectic layer and the unusual quasi two-dimensional structural order exists irrespective of the liquid-like packing of aromatic cores within a layer. On heating, the S C phase transformed into the S A phase with a nature of first-order transition. The resulting S A structure was also unusual. The orientational order parameter was 0.43, which was lower than 0.71 in the S C phase. This indicates that a ''de Vries''-type of smectic A is formed in which the mesogenic groups are tilted away from the optic axis, but in azimuthally random directions. The average tilt angle was estimated to be 42 . These characteristics were produced because of the strong segregation effect of the dimethyl siloxane spacer groups with a relatively larger molecular area compared to alkyl chain and aromatic imide cores.BPDA/3SiO4-3F, which has an F-substituted BPDA mesogenic unit. In this paper, we report the nature of the S A -S

Spontaneous Chain Orientation of Aromatic Polyimides Evolved during Thermal Imidization from Shear-Oriented Glassy Liquid Crystalline Precursors

“…where θ is the angle between the averaged molecular chain axis (the long axis of mesogens 35 ) and averaged orientation direction, i.e., the shear flow direction in this case. 68−70 The distribution of θ is related to the azimuthal intensity distribution of the [hk0] diffractions on the equator.…”

“…Structure and orientation evolution of the solid film during heating are now quantitatively discussed using the uniaxial orientation order parameter, S , defined by where θ is the angle between the averaged molecular chain axis (the long axis of mesogens) and averaged orientation direction, i.e., the shear flow direction in this case. − The distribution of θ is related to the azimuthal intensity distribution of the [ hk 0] diffractions on the equator. The statistically averaged value of ⟨cos 2 θ⟩ was evaluated using eqs and , where β is the azimuthal angle in the diffraction pattern and θ B is the Bragg angle at the observed equatorial diffraction , As displayed in Figure , the SR-WAXD patterns of PM6 dried film (amount of residual NMP < 10 wt %) and its resultant PI measured with the X-ray beam parallel to the film surface and perpendicular to the shear direction (“edge” direction) presented the same characteristics as the through patterns in Figure .…”

“…As seen in the wet spinning processes of aromatic polyamides, LC phases are useful to obtain highly oriented polymer materials, owing to the good fluidity and high susceptibility of the LC phases to external fields such as shear flow and electric and magnetic fields. A few thermotropic PIs based on semiflexible structures − and lyotropic PIs based on bulky aromatic dianhydrides and diamines having soluble side groups − have also been reported. However, molecular designs of PIs toward LC properties reduce the thermal and chemical stabilities of the PIs.…”

Cyanostilbene‐Based AIEgen Smart Film: Optical Switching by Engineering Molecular Packing Structure and Molecular Conformation Through Thermal and Photoinduced Monotropic Phase Transition

“…For this reason, various siloxane groups are chosen as chemical connectors, end chains and polymer backbones to tune molecular packing structures as well as to program phase transitions. [ 32–38 ] In particular, it is notable that cyclosiloxanes have advantages in view of geometry and dimensional diversity, multifunctionalization, relatively flexible building unit for meso‐nanostructure, organic–inorganic hybrids, and functional amphiphiles. [ 39–45 ] In this context, there have been several reports about the synthesis and application of cyclosiloxane‐based molecules, but the thermodynamic and kinetic relationships between the chemical structure, molecular packing structure, and resulting material properties of cyclosiloxane‐based functional molecules need to be further studied.…”

“…For this reason, various siloxane groups are chosen as chemical connectors, end chains and polymer backbones to tune molecular packing structures as well as to program phase transitions. [32][33][34][35][36][37][38] In particular, it is notable that cyclosiloxanes have advantages in view of geometry and dimensional diversity, multifunctionalization, relatively flexible building unit…”