Linear viscoelastic and dielectric behavior was examined for blends of cis-polyisoprene (PI-20; M-PI = 19(.9) x 10(3)) and poly(4-tert-butylstyrene) (PtBS-16; M-PtBS = 16(.4) x 10(3)) with the PtBS concentration of W-PtBS = 30 and 50 wt %. At temperatures examined, T <= 120 degrees C, the PI/PtBS blends were in a statically homogeneous state. Since PI had the type-A dipole while PtBS did not, the dielectric loss of the blends measured at low angular frequencies omega (< 10(5) s(-1)) was attributed exclusively to the global motion of PI chains. At low T and/or high w(PtBS), the dielectric loss data of the PI chains did not obey the time-temperature superposition, despite the static homogeneity in the blends. Comparison of the viscoelastic and dielectric data indicated that in the blends PtBS relaxed more slowly than PI. Thus, PtBS possibly gave an effectively quenched, nonuniform frictional environment in the time scale of the global relaxation of PI, and changes of this frictional nonuniformity with T appeared to result in the failure of the time-temperature superposition for the PI relaxation. In contrast, the superposition held for the terminal viscoelastic data of the slow component, PtBS, possibly because the frictional nonuniformity in the time scale of PtBS relaxation was erased by the fast PI chains. Furthermore, WLF analysis of the viscoelastic data revealed that the PtBS relaxation was slower, by a factor of 16 (for W-PtBS = 30 wt %) and 5 (for W-PtBS = 50 wt %), in the blends than in an iso-frictional bulk state. This result suggested that bulky PtBS chains were subjected to motional constraint (not identical but somewhat similar to the entanglement in homopolymer systems) due to the flexible PI chains, despite the fact that M-PtBS was not large enough to allow the PtBS chains to be entangled by themselves even at W-PtBS = 100 wt % (in bulk state). This constraint for low-M PtBS chains was discussed in relation to the concept of packing length.