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.

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

“…The well‐known banded texture was induced upon cessation of shear with relaxation times in the hundreds of seconds. These relaxation times are one to two orders of magnitude longer than in wholly aromatic LCPs 19‐22 . Above certain shear rate and ordering, the texture did not relax through a banded texture, but slowly returned to a polydomain.…”

“…This was the approach of Jackson and Khufuss at Eastman R&D Laboratories whom first reported the synthesis of a thermotropic LCP by the copolymerization of 1,4‐hydroxybenzoic acid (B) with ethylene glycol and terephthalic acid (ET) 16 . The thermotropic LCP synthesized at the Imperial Chemical Industries (ICI) R&D based on 50:50 mol% random units of chlorophenylene‐terephthalate and bis‐phenoxy ethane carboxylate (designated ClQT‐ClQG, where “T” = terephthalic acid, “Q” = hydroquinone, “ClQ” = chlorohydroquinone, and “G″ = bis[ p ‐carboxyphenoxy] ethane) also exhibit high‐performance physical properties due to the limited flexibility afforded by the incorporation of only two alkyl units in the backbone 17‐19 …”

“…A recent rheo‐optical study on ClQT‐ClQG showed that shear easily aligned its polydomain texture along the velocity axis and even induced monodomains 19 . The well‐known banded texture was induced upon cessation of shear with relaxation times in the hundreds of seconds.…”

“…Above certain shear rate and ordering, the texture did not relax through a banded texture, but slowly returned to a polydomain. Furthermore, the nematic phase of ClQT‐ClQG exhibited linear viscoelasticity, the signature of entanglements and significant elastic memory as measured by strain recovery after creep 19 …”

“…Thermotropic LCPs are complex fluids and their flow behavior is strongly influenced by previous thermal and mechanical history 1,3,19‐25 . The complexity of their flow behavior means that it is unlikely that only one experimental technique will provide the full picture.…”