On the banded texture and nematic elasticity in a thermotropic liquid crystalline polymer. In situ optical microscopy and light scattering

Abstract

Optical banded texture after cessation of shear was investigated in a semi‐flexible thermotropic liquid crystalline polymer (LCP) by in situ polarized optical microscopy (POM) and small‐angle light scattering (SALS). The LCP is a random copolymer consisting of mesogenic units separated by alkyl spacers. The quiescent state of the nematic phase exhibited a polydomain texture, with microdomains randomly oriented. Upon application of shear the texture aligned along the shear axis. In situ X‐ray scattering demonstrated that the texture alignment corresponded to macromolecular alignment along the shear axis. Above certain shear rate the aligned texture relaxed and produced a banded texture upon cessation of shear. Shearing at higher shear rate further aligned the texture to produce a monodomain. Stopping the shear spatially uncorrelated structures orthogonal to the shear direction were developed with incubation time in the 100 seconds of second. Hence, there is a threshold of texture alignment to produce banded texture. Shear rheometry showed that the liquid crystalline phase is viscoelastic with features akin to entangled linear polymer melts, for example, a minimum in mechanical damping tan δ [=G”/G']. Furthermore, the nematic phase exhibited considerable recoverable strain after creep. The elastic recoil after cessation of flow would be responsible to produce banded texture. Increasing the creep time reduced the recoverable strain, correlating with longer time to produced banded texture. In contrast, wholly aromatic LCPs exhibit much shorter recovery times after creep and shorter incubation time for banded texture formation.