EVOLUTION OF MICROSTRUCTURE AND VISCOELASTICITY DURING FLOW-ALIGNMENTOF A LAMELLAR DIBLOCK COPOLYMER

The effects of flow alignment on the relaxation dynamics of a lamellar diblock copolymer melt and the dynamics of flow alignment itself are investigated using simultaneous measurements of shear stress and biref ringence in both the flow plane and the sample plane. The primary adva ntage of this rheo-optical approach in the context of flow alignment i s that it provides quantitative measurements of the evolution of the m acroscopic mechanical properties and the state of the microstructure i n real time, in situ as alignment occurs. Further, it provides informa tion on the molecular and microstructural dynamics that give rise to f low alignment. An entangled, nearly symmetric poly(ethylene-propylene) -poly(ethylethylene) of 50 kg/mol (PEP-PEE-2) is studied during flow a lignment under two different conditions, one that enhances alignment o f the lamellae parallel to the sample plane and another that induces a lignment perpendicular to the sample plane (lamellar normal along the vorticity axis). The results suggest that the flow process leading to parallel alignment PEP-PEE-2 is associated with inhomogeneous deformat ion such that the orientation of domains in the material undergoes irr eversible ''rocking'', while the process that produces perpendicular a lignment occurs under conditions in which the deformation is nearly ho mogeneous throughout the material.