Here we report the significant enhancement effect of mesoscopic fillers in viscoelastic phase separation of dynamic asymmetric polymer blends. Mesoscopic fillers with their size much larger than the dimensions of the polymer chains, from nanometres to microns, are preferentially immersed into the slow dynamic phase and phase interface due to the entanglement with polymer chains. For sufficiently high volume fraction and fine dispersion, mesoscopic fillers conduce to the pronounced slowing down of the phase separation process, and result in refined structures with sharply decreased characteristic length scales. The pinning of the phase separation is attributed to the dramatic increase of dynamic asymmetry from the entanglement of polymer chains with mesoscopic fillers. The principal difference between mesoscopic fillers in classic and viscoelastic phase separation is whether there exists a filler enforced elastic-force balance condition. This suggests a general physical scenario of entanglement selection of the polymer chains under stress.

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We have investigated the effect of mesoscopic fillers on the polymerization induced viscoelastic phase separation of thermoplastic modified thermosets at near- and off-critical concentrations using optical microscopy, time-resolved light scattering, dynamic mechanical analyses, and rheological instrument. Mesoscopic fillers including sepiolite and nanosized silica showed a significant enhancement effect in viscoelastic phase separation, and resulted in pronounced differences in the phase structures at all concentrations of polyetherimide modified epoxy resins with dynamic asymmetry. For blends near critical concentration, the introduction of fillers led to much finer phase structure with smaller characteristic length scale. At off-critical composition (i.e., blends with low concentration of slow dynamic component), the strong polymer chain entanglement resulted in enwrapped mesoscopic fillers within a slow dynamic phase. The rheological behavior of the blends clearly demonstrated the significant enhancement effect of mesoscopic fillers in the viscoelastic phase separation. The apparent activation energy of polymer chain mobility obtained from dynamic mechanical study of glass transition reflected strong wrapping behavior of polymer chains on mesoscopic fillers, which were consistent with the rheological and light scattering study.