The dynamic mechanical loss tangent (tan delta) peak of polyisobutylene (PIB) reveals an asymmetrical broad structure with a maximum on the high-temperature side and a shoulder on the low-temperature side. By comparing with the literature results, it is suggested that the shoulder and the maximum originate from local segmental motion and Rouse modes, respectively. Blending polystyrene (PS) with PIB has two effects on the relaxation behavior of PIB. One effect is that the maximum and the shoulder are both suppressed, but the maximum is suppressed to a higher extent. After PS forms the continuous phase, the maximum becomes lower than the shoulder, and even almost disappears when the weight ratio of PIB/PS is under 20/80. The other effect is that, before PS forms the continuous phase, the temperature position of the maximum (T-s) and that of the shoulder (T-alpha) remains constant, but after PS forms the continuous phase, both of them are reduced with decreasing particle size of the PIB phase, in a way similar to nano-confinement effect on the depression of glass transition temperature. The depression amplitude of T-s is larger than that of T-alpha. The aforementioned two effects can be interpreted in terms of the limited expansion of free volume of the PIB phase exerted by the PS phase, which affects the maximum to a higher extent than the shoulder because Rouse modes are more sensitive to the free volume than local segments. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 2165-2172, 2010