Shear‐induced textures and viscoelasticity in the nematic and isotropic phase of semiflexible thermotropic polymers. A rheo‐optical study

Abstract

The striking similarities in response to shear flow of nematic semi‐flexible thermotropic liquid crystalline polymers (LCPs) is reported. Insights into their complex flow was gained by in‐situ polarized optical microscopy (POM) whereas spatial correlations below optical resolution were characterized by small‐angle light scattering (SALS). The LCPs contained mesogenic units separated by ten (10) alkyl spacers, the polyether TPB10 of = 17 kDa and the polyester PSHQ10 of = 53 kDa. The quiescent nematic phase consisted of microdomains randomly oriented, that is, polydomain texture. Shear stretched and aligned the μm‐sized domains. After cessation of shear the texture sprang back and relaxed, PSHQ10 produced banded texture, whereas TPB10 only produced a disordered texture. Strikingly, high shear rate induced monodomains. Upon shear cessation the monodomain in PSHQ10 slowly relaxed to a polydomain, whereas TPB10 maintained a defect free texture. Shear reversal induced crisscross structure in both LCPs and SALS showed asymmetric “butterfly” patterns revealing a tilted microstructure. Shear rheometry in the nematic phase revealed that: (i) PSHQ10 displayed a terminal and rubbery regime, indicating the existence of entanglements; (ii) TPB10 exhibited only a liquid‐like behavior where G″ ≫ G′. The rubbery modulus Ge of PSHQ10 was ca. five times higher in the isotropic than in the nematic phase indicating that a change of conformation is involved in the nematic‐to‐isotropic transition. This study has resulted in correlations between rheological properties and shear‐induced microstructure thus developing new insights into their underlying deformation mechanisms.