(170a) Invited Talk: Diffusion in Entangled Rod-Coil Block Copolymers

A large number of functional polymers of current interest have rodlike chain topologies, including main-chain semiconducting polymers and alpha-helical proteins.  The self-assembly of these functional materials using block copolymers provides an elegant route to the preparation of functional organic electronics or biomaterials.  However, little is known about the dynamics of these copolymers.  In particular, the mechanisms of molecular relaxation that enable modeling of diffusion, flow, and mechanical response necessary for understanding the rate of self-assembly and the processing of materials have yet to be elucidated.  Herein, rod-coil dynamics are studied in the entangled state, showing that dynamic behavior can be divided into two regimes with distinctly different relaxation mechanisms.  Using a combination of simulation, theory, and experiment, we demonstrate that rod-coil copolymers show significantly hindered diffusion when compared to either rod or coil homopolymers alone. 

In the short rod regime (rod length approximately equal to the diameter of the entanglement tube), the molecules continue to diffuse by a traditional reptation mechanism that is hindered by the rod rigidity.  Kremer-Grest simulations show a significant slowing of diffusion that is a function of only the ratio between the rod length and the tube entanglement radius.  A simple theory is developed that relates diffusion of the rod through the tortuous tube to Zwanzig’s theory of diffusion across a rough potential, consistent with the observations from simulation.  Experimental diffusion measurements on both rod-coil diblock and coil-rod-coil triblock copolymers are performed using forced Rayleigh scattering (FRS) to study model poly(ethylene oxide)/poly(alanine) copolymers diffusing through an entangled poly(ethylene oxide) solution.  These measurements confirm the predictions of theory and clearly illustrate that the slowing effect does not depend upon the specific molecular architecture of the block copolymer.

In the long rod regime (rod length much greater than the diameter of the entanglement tube), the molecules transition to an arm retraction mechanism, where the coil block must straighten to facilitate diffusion.  Simulations of diffusion through a fixed lattice of obstacles clearly demonstrate arm retraction scaling, and comparison of rod-coil and coil-rod-coil simulations shows that each coil block relaxes independently.  Experimental FRS measurements confirm the arm retraction scaling prediction at long rod lengths.