  In polymer melts, the interaction between segments are considered to be
screened and the ideal Gaussian chain statistics is recovered. The experimental
fact that linear viscoelasticity of unentangled polymers can be well described
by the Rouse model is naively considered as due to this screening effect.
Although various theoretical models are based on the screening effect and the
screening effect is believed to be reasonable, the screening effect cannot be
fully justified on a solid theoretical basis. In this work, we study the
screening effect by utilizing a simple dumbbell type model. We perform
simulations for dumbbell systems in which particles interact via the Gaussian
soft-core potential. We show that, if the density of dumbbells is high, the
Gaussian soft-core interaction is actually screened and the static structures
are well described by the ideal model without Gaussian soft-core interaction.
We also show that the relaxation moduli of interacting dumbbell systems
approximately coincide to those of the non-interacting dumbbell systems. In the
low density systems, we observe the deviations from the ideal non-interacting
systems. For example, the relaxation moduli become relatively broad. However,
the relaxation moduli of such systems can be decomposed into the relaxation
modes by the Gaussian soft-core interaction and the bond. The bond relaxation
mode can be successfully described by a single Maxwell relaxation with
effective relaxation strength and time. Our results support a naive use of the
Rouse model to analyze unentangled polymer melts.
