This work focuses on the linear viscoelastic behaviour at length scales shorter than an entangled strand and attempts to connect it to the nonlinear behaviour (from data in literature) in fast extensional flow where such short length scales play an important role. We have measured the linear viscoelasticity at high frequencies in the transition to glassy regime for a series of polystyrene (PS) melts and solutions. When the samples are compared at the same T ref -T g , where T ref is the reference temperature and T g is the glass transition temperature, their third crossover points (near the glassy regime) of G' G'' plots are located at the same frequency. While the unentangled styrene oligomer behaves as a solvent in the flow and rubber plateau regimes, it contributes the same as the PS melts in the transition regime, unless the chain length is close to a Kuhn segment. In fast extensional flow, using τ 0 as the time scale and G 0 as the modulus scale, where τ 0 and G 0 are both associated with the third crossover point, the normalized steady-state extensional viscosity seems to depend on the number of Kuhn segments per chain, but not on the PS fraction. This is probably due to the contribution from the styrene oligomer solvent. However, fracture may happen before the steady state if the stretched polymer chains are close to their theoretical maximum stretch ratio.