How does a polymer chain pass through a cylindrical pore under an elongational flow field?

How a polymer chain translocates through a cylindrical pore with its pore size smaller than the chain size (ultrafiltration behavior) is a fundamental question in polymer physics. Answering this question can provide broader implications and lead to potential applications for many applicable processes, such as gene transfection, protein transportation, and separation of a mixture of polymer chains. In the process of an electroneutral polymer chain passing through a nanopore, generally, an external flow with a sufficient high shear stress is needed to apply at the entrance of pore to induce a conformation change from a coill-ike to a rod-like shape which is referred to as the "coil-to-stretch" transition, to squeeze into the pore. Up to last decade, many theoretical models have been built and carried out to predict how polymer chains pass through a nanopore. By contrast, rather limited experimental investigations have been performed to validate these theoretical predications, which is mainly because this kind of experimental study demands polymer chains with explicit topologies and nanopores with well-defined structures. Namely, 1) the polymer samples with narrow molecular-weight distributions, well-defined chain configurations, as well as hydrodynamic sizes larger than the pore radii; and 2) membranes with well-defined pore structure and isolated pore channels to prevent possible interaction between neighboring shearing flows. In recent years, the development of polymer synthetic technology and the improvement of membrane manufacturing technology have stimulated a mass of research work on understanding the ultrafiltration behavior of polymer chains under an elongational flow field. In this feature article, the authors would like to mainly focus on the ultrafiltration behavior of flexible polymer chains with various topologies in dilute solution. More specifically, we will elucidate how the structural parameters of a polymer chain are related to the critical volumetric flow rate and the shape of polymer retention curve. Further application of this ultrafiltration method to the separation of polymer chain mixture and the rapid transformation among various polymer chain aggregated structures will be discussed. It is hoped that this perspective can provide a better view in understanding the translocation behavior of (bio)macromolecules in various practical processes and offer some guidance for the design and reality of commercially available ultrafiltration separation apparatus in the future. (C) 2015 Elsevier Ltd. All rights reserved.