  Processes on different length scales affect the dynamics of chain molecules.
In this work, we focus on structures on the scale of a monomer and investigate
polyolefins, i.e. hydrocarbon chains with different small scale architectures.
We present an exact enumeration scheme for the simulation of interactions and
relative motion of two short chain sections on a lattice and employ it to
deduce the probability for segmental motion for polymers of four different
architectures in the melt. The probability for segmental motion is inversely
proportional to the monomeric friction coefficient and hence the viscosity of a
polymer. Combining our simulation results with an equation of state for the
thermodynamic properties of the polymers, we are able to make predictions about
the variation of the friction coefficient with temperature, pressure, and small
scale architecture. To compare our results with experimental data, we have
determined monomeric friction coefficients from experimental viscosity data for
the four polyolefins considered in this work. For temperatures well above the
glass transition temperature, we find that our simple approach gives a good
qualitative representation of the variation of the friction coefficient with
chain architecture, temperature and pressure.
