Rheology of lightly-cured polydimethylsiloxane liquid blends

The rheology of liquid blends based on PDMS has been studied by varying the components molecular weight, the temperature and the concentration of the curing agent, at values well below the gelation threshold. The formulated lightly-cured blends showed interesting unusual behaviors. An interpretative model based on the nanocomposite polymer, the relaxation of dangling chains and the free-volume variation has been proposed. At low temperatures both the M/H (Medium-in-High molecular weights) and L/H (Low-in-High) blends confirm the previously observed anomalous rheology weakening at very low crosslinker concentrations. This is explained by means of the nanosized random coils of the grafted prepolymer nucleated in situ. As the concentration of curing agent increases, crosslinked isolated domains will be sufficiently wide to give the expected increase in rheological properties. At the temperature of 70 degrees C, the M/H blend shows a reverse creep behavior: the most crosslinked material exhibits an increase in compliance and the less one a decrease. This behavior has been explained taking into account the entanglement onset with a free-volume reduction (Free-Volume Change model); in contrast, the shorter chains of the micro-domains due to thermal agitation would expand the free-volume interface, with the drop of rheological performance. Moving from the M/H blend to the L/H one, at low temperatures we observe a strong decrease in compliance due to the trapping of the short chains on the entanglement sites, with the increase of the entanglement stability (Entanglement Swelling Tube model). The creep experiments show an unexpected shrinkage phenomenon for liquid blends at higher crosslinker concentration: the crosslinked domains enough extended should aggregate by non-bonding interactions giving a prestressed Bingham fluid. The rheology in rotational mode are confirmed in oscillating regime by the dissipation tans maximum at high frequency and the peak broadening with the curing agent concentration.