  We report on the linear viscoelastic properties of mixtures comprising
multiarm star (as model soft colloids) and long linear chain homopolymers in a
good solvent. In contrast to earlier works, we investigated symmetric mixtures
(with a size ratio of 1) and showed that the polymeric and colloidal responses
can be decoupled. The adopted experimental protocol involved probing the linear
chain dynamics in different star environments. To this end, we studied mixtures
with different star mass fraction, which was kept constant while linear chains
were added and their entanglement plateau modulus ($G_p$) and terminal
relaxation time ($\tau_d$) were measured as functions of their concentration.
Two distinct scaling regimes were observed for both $G_p$ and $\tau_d$: at low
linear polymer concentrations, a weak concentration dependence was observed,
that became even weaker as the fraction of stars in the mixtures increased into
the star glassy regime. On the other hand, at higher linear polymer
concentrations, the classical entangled polymer scaling was recovered. Simple
scaling arguments show that the threshold crossover concentration between the
two regimes corresponds to the maximum osmotic star compression and signals the
transition from confined to bulk dynamics. These results provide the needed
ingredients to complete the state diagram of soft colloid-polymer mixtures and
investigate their dynamics at large polymer-colloid size ratios. They also
offer an alternative way to explore aspects of the colloidal glass transition
and the polymer dynamics in confinement. Finally, they provide a new avenue to
tailor the rheology of soft composites.
