(250i) Diffusion of Entangled Rod-Coil Block Copolymers

Polymer science is exploring advanced materials which combine functional domains such as proteins and semiconducting polymers with traditional flexible polymers onto the same molecule. While thermodynamic assemblies of different geometries introduce many interesting new phenomena such as entropic packing and liquid crystalline interactions, dynamic effects are also important to understand for optimal design of material mechanics, processing kinetics, and because of the new physics that directly arises from the motion of multiple domains of dissimilar geometries. 

In this talk, we describe a reptation theory for entangled rod-coil block copolymers as a model for this wider class of functional polymeric materials. The large geometrical differences between rigid rods and Gaussian coils cause significant nonlinearities in dynamical behavior as these two motifs are combined on the same molecule. In particular, our theory hypothesizes that the motion of rod-coils is slowed relative to rod and coil homopolymers because of a mismatch between the curvature of the rod and coil entanglement tubes. In the small rod limit where the rod is a perturbation on coil motion, the randomly varying curvature of the coil’s tube presents entropic barriers to the reptation of the rod, modifying the unhindered motion of the coil along its tube into an activated reptation process. In the large rod limit where the coil is a perturbation on rod motion, the long rod cannot rotate around the surrounding entanglements so motion is only possible when the coil moves into a straightened entanglement tube in an arm retraction process. These mechanisms were verified by tracer diffusion measurements using molecular dynamics simulation and forced Rayleigh scattering experiments. Additional simulations using a coarse-grained slip-spring model provide direct evidence for the activated reptation mechanism.