Using nuclear reaction analysis, we have measured the rate of surface
segregation of an asymmetric diblock polyisoprene-polystyrene (PIPS) c
opolymer to the free surface of polystyrene (PS) homopolymers within w
hich it is incorporated at low bulk concentrations. We have also measu
red the tracer diffusion coefficient of PIPS in PS under similar condi
tions, and shown it to equal, within error, the self-diffusion coeffic
ient of PS of a similar length. The equilibrium surface excess Gamma o
f the diblock is low (<R(g), the radius of gyration of the PS block of
the copolymer), so that the diffusion rate is not expected to be sign
ificantly reduced by energy barrier effects. The rate of segregation a
ppears considerably slower than a diffusion-limited picture based on a
sink-boundary condition for the PIPS molecules at the PS surface woul
d suggest. A much better description of our data is provided by a simp
le kinetic model which assumes local equilibrium between unattached an
d tethered diblocks at the free PS surface.