Self-Diffusion and Constraint Release in Isotropic
Entangled Rod–Coil Block Copolymers
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Abstract
Understanding dynamic relaxation
mechanisms in self-diffusion and
constraint release processes of rod–coil block copolymers is
important for many technological applications that employ neat melts
or concentrated solutions. Using a model system composed of poly(alkoxyphenylenevinylene)
rods and polyisoprene coils, reptation theories of entangled rod–coil
block copolymers are investigated in the isotropic melt state. Self-diffusion
was measured by forced Rayleigh scattering using a red laser line
and a new blue photoswitchable dye that allow operation above the
bandgap of most semiconducting polymers. In contrast to previous tracer
studies where the diffusion of rod–coils through a coil homopolymer
matrix is slowed relative to coil homopolymers because of a mismatch
in the curvature of the rod and coil entanglement tubes, slowed diffusion
is only present in self-diffusion measurements above a critical molecular
weight. An activated reptation mechanism with constraint release is
proposed as a modification to the description of entangled rod–coil
block copolymer dynamics, where the slowing occurs when the time scale
of rod block reptation is faster than the reorganization of the surrounding
entanglement tube. This mechanism is supported by additional tracer
diffusion experiments on polyalanine-<i>b</i>-poly(ethylene
oxide) diblocks in aqueous entangled poly(ethylene oxide) matrix solutions
and Kremer–Grest simulations where the matrix molecular weight
is varied. The slowing of tracer diffusion in rod–coil block
copolymers relative to coil homopolymers is significantly weaker for
smaller matrix polymers, confirming the proposed constraint release
effects