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40596-AC7
Understanding Molecular Weight Scaling of Diffusion Coefficients in Entangled Polymer Solutions
Motivated by the surprising experimental discoveries made possible by application of particle-tracking velocimetry to shear flow of entangled polymers, we have embarked on a journey to explore new experimental and theoretical foundations of polymer rheology. Due to our intense efforts, a preliminary theoretical picture has emerged. Our new theoretical description not only explains the quantitative (time dependent) characters of both startup shear and extensional flows of entangled polymers but also predicts highly counterintuitive phenomena that are surprising according to the conventional wisdom. A theory is validated not because there are phenomena that are expected no matter what theory one has. A theory is more likely to have captured the right physics when it predicts behavior that is unexpected and has not been observed experimentally. Our new theoretical understanding fits the latter case. We have first predicted arrested wall slip and breakup of a stretched filament made of an entangled (SBR) melt. Then we went to our lab and discovered these intriguing phenomena.
Specifically, the available molecular mechanism for interfacial failure does not anticipate arrested wall slip. By “arrested wall slip”, we refer the following behavior: wall slip is not observed during a startup shear until the elapsed strain reaches γy; yet upon shear cessation at a strain of γs < γy wall slip is subsequently observed. Thus, disentanglement due to interfacial coil-stretch transition cannot be the correct mechanism because it is inconsistent with the observed arrested wall slip. What causes wall slip to take place after shear cessation? A theoretical understanding of why this counterintuitive behavior occurs should be regarded as an important advance. Our theoretical analysis indicates that retractive entropic force arising from chain deformation is capable of overcoming the entanglement cohesion, leading to chain disentanglement and subsequent wall slip.
More recently, we have embarked on a task to examine extensional deformation behavior of entangled polymers with the aim to search for a unified theoretical framework for nonlinear polymer rheology. There is a vast literature on the subject. Our approach differs from others because we envision an entangled melt to suffer structural failure at high extensions. We have identified the signature of yield and confirmed the theoretical prediction that a stretched sample would undergo yielding after a sudden extension.
Our fresh theoretical understanding has opened the door to the elusive world of nonlinear polymer rheology. We plan to focus onto obtaining a better understanding of extensional flow behavior of entangled polymers.
