How macromolecules penetrate narrow pores*

Herein we address the question of how macromolecules penetrate narrow pores relevant in several applications. In particular, we follow the polymer dynamics in situ at the chain length scale by studying the evolution of normal modes in the type A polymer polyisoprene (PI) during imbibition in nanopores. The specific scientific question we address is, if and how, adsorption affects the imbibition kinetics of polymers. This question, despite of importance in the design of membranes/devices, has not been addressed so far. It requires precise measurements of polymer dynamics during flow. Herein we follow the details of polymer imbibition in situ by employing the nanofluidic method. The latter provides simultaneous access to (i) the kinetics of imbibition, (ii) the molecular dynamics and (iii) the viscosity experienced by the polymer during flow. Results show that polymer imbibition proceeds in two time-regimes with higher effective viscosity than in bulk. We discuss this finding with the help of a microscopic picture that considers the competition from an increasing number of chains entering the pores and a decreasing number of fluctuating chains with time. The latter provide unambiguous evidence for increasing adsorption sites during flow. In a second step we investigate the imbibition of PI blends in the same nanopores. We show that a homogeneous mixture composed of long and short PI chains phase separates on entering the pores by the different imbibition speeds of the respective chains and evaluate the dynamics at the chain length scale.
The work is a collaboration with the Chien-Hua Tu and Hans-Juergen Butt at the Max Planck Institute for Polymer Research, Mainz.