  Local chain structure and local environment play an important role in the
dynamics of polymer chains in miscible blends. In general, the friction
coefficients that describe the segmental dynamics of the two components in a
blend differ from each other and from those of the pure melts. In this work, we
investigate polymer blend dynamics with Monte Carlo simulations of a
generalized bond-fluctuation model, where differences in the interaction
energies between non-bonded nearest neighbors distinguish the two components of
a blend. Simulations employing only local moves and respecting a non-bond
crossing condition were carried out for blends with a range of compositions,
densities, and chain lengths. The blends investigated here have long-chain
dynamics in the crossover region between Rouse and entangled behavior. In order
to investigate the scaling of the self-diffusion coefficients, characteristic
chain lengths $N_\mathrm{c}$ are calculated from the packing length of the
chains. These are combined with a local mobility $\mu$ determined from the
acceptance rate and the effective bond length to yield characteristic
self-diffusion coefficients $D_\mathrm{c}=\mu/N_\mathrm{c}$. We find that the
data for both melts and blends collapse onto a common line in a graph of
reduced diffusion coefficients $D/D_\mathrm{c}$ as a function of reduced chain
length $N/N_\mathrm{c}$. The composition dependence of dynamic properties is
investigated in detail for melts and blends with chains of length twenty at
three different densities. For these blends, we calculate friction coefficients
from the local mobilities and consider their composition and pressure
dependence. The friction coefficients determined in this way show many of the
characteristics observed in experiments on miscible blends.
