A partially miscible polymer blend can be homogeneous or heterogeneous depending on its composition and temperature.The phaseseparated blend and its homogeneous counterpart have different internal structures and thus show different viscoelastic properties

which have received considerable attention in the past few decades.It has been found that the homogeneous blends

as their constituents

typically exhibit simple thermorheological behavior

and their rheological functions at different temperatures can fall on the same curve using time-temperature superposition (tTS); on the other hand

the phase-separated blends display pronounced elasticity in the terminal region

which makes the tTS principle invalid for them

and show thermorheological complexity.Such enhanced elasticity causes a discontinuity in the temperature dependence of the storage modulus at low frequencies.These characteristic fingerprints

in turn

have been used not only for characterizing the miscibility of partially miscible polymer blends but also even for quantitatively estimating the phase boundary.The phase separation (binodal) temperatures were marked by a change in the slope of the curve of the dynamic storage modulus versus temperature.However

the apparent phase separation temperatures obtained by rheological tests are frequency dependent

and lower or higher than the binodal temperatures detected by other methods such as turbidity method when lower and higher frequency is applied

respectively.There is still no general knowledge on what causes these divergences.In addition

it is unknown that what size of domains the rheological method can resolve up to now.Experiments and discussion in this paper tries to overcome these problems.The phase separation temperatures of partially miscible poly(styrene-co-maleic anhydride)/poly(methyl methacrylate) blends have been determined using small amplitude oscillatory shear (SAOS) test by dynamic frequency sweep mode and using small angle light scattering (SALS) method.It has been found that

as reported in the literatures

the phase separation temperatures obtained by rheological methods are frequency dependent and lower or higher than those obtained by SALS measurements when a frequency of 0.1 or 0.4 rad/s is applied.The relative increases of the storage moduli of the phaseseparated blends at different frequencies are compared at the same temperature

where the dynamic moduli of the phase-separated blends were obtained by rheological measurements

and those of their homogeneous counterparts were evaluated by the double reptation model.The results indicate the enhanced storage modulus caused by the phase separation at lower frequency is more sensitive than that at higher frequency.In order to probe resolution scale of rheological method for determining phase separation temperature

the Parlierne model was applied to evaluate the size of droplets.The results indicate that

in the present case

rheological measurements at 0.1 rad/s and 0.4 rad/s frequencies can resolve the size of domains over a 100 nm and 700 nm scales

respectively.According to these results

we argue that the divergence between the rheological binodal curves at different frequencies is attributed to the different sensitivity of rheological measurements at different frequencies

and compared with the size of domains over a 500 nm scale resolved by small angle light scattering

the rheological method can be more sensitive when the lower frequency is applied.