Observing the relaxation of molecular orientation in sheared liquid crystalline polymer solutions

Abstract

Liquid crystalline polymers (solutions or melts) can be spun into fibers and films that have a higher axial strength and stiffness than conventionally processed polymers. These superior properties are due to the spontaneous molecular extension and alignment that is characteristic of liquid crystalline phases. Much of the effort in processing conventional polymers goes into extending and aligning the chains, while, in liquid crystalline polymer processing, the primary microstructural rearrangement involves converting local molecular alignment into global molecular alignment. Unfortunately, the global alignment introduced by processing relaxes quickly upon cessation of shear, and the molecular orientation develops a periodic misalignment relative to the shear direction. The axial strength and stiffness are reduced by this relaxation.Clearly there is a need to solidify the liquid crystalline state (i.e. remove heat or solvent) before significant relaxation occurs. Several researchers have observed this relaxation, mainly in solutions of hydroxypropyl cellulose (HPC) because they are lyotropic under ambient conditions.