Spreading of diblock copolymer droplets: a probe of polymer micro-rheology

We present an experimental study of the spreading dynamics of symmetric diblock copolymer droplets above and below the order-disorder transition. Disordered diblock droplets are found to spread as a homopolymer and follow Tanner's law (the radius grows as R approximately t(m), where t is time and m = 1/10). However, droplets that are in the ordered phase are found to be frustrated by the imposed lamellar microstructure. This frustration is likely at the root of the observed deviation from Tanner's law: droplet spreading has a much slower power law (m approximately 0.05+/-0.01). We show that the different spreading dynamics can be reconciled with conventional theory if a strain-rate-dependent viscosity is taken into account.