(573g) Microphase Separation in Entangled Polymeric Solutions

The existence of a coil-stretch transition and a conformation hysteresis within a critical range of extension rates is a well-known phenomenon in dilute polymeric solutions. Recently, NEMD simulations of a well-entangled polyethylene melt revealed that within intermediate extensional rates, entangled melts could undergo a qualitatively similar coil-stretch transition, and exhibit bimodal configurational distributions with peaks corresponding to coiled and stretched configurations. Furthermore, it was shown that through a configurational microphase separation, the coiled molecules develop distinct domains surrounded by relatively stretched molecules.

On the other hand, various experimental studies have shown that the response of entangled polymeric melts to elongational flow fields could be very different from those of entangled solutions. Experiments also suggest that the solvent molecule architecture plays an important role in the dynamic behavior of entangled solutions. Such complexities bring up many questions about the coil-stretch transition in entangled polymeric liquids. Do entangled solutions undergo a coil-stretch transition? Do they experience any configurational or chemical microphase separation? How does the solvent molecular architecture affect this phenomenon?

To address these questions, we performed a series of NEMD simulations for entangled C₁₀₀₀H₂₀₀₂ polyethylene solutions in oligomeric C₁₆H₃₂ and benzene solvents. The solutions were subject to planar elongational flows within a wide range of extension rates of 0.05 < De < 20, where De is the extension rate made dimensionless using the Rouse relaxation time. The simulations revealed both similarities and differences between entangled melts and solutions that will be discussed.