Relaxation modes of molten polydimethylsiloxane

Although it is known that chain reptation with contour length fluctuations and tube constraint release are the relevant mechanisms of chain relaxation, only applications of the theory to specific polymers could allow one to determine the relative importance of these relaxation modes. With this purpose we study the relation between the shear relaxation modulus of entangled, linear and flexible polydimethylsiloxane (PDMS) samples with narrow molecular weight distribution (MWD) and some of their blends. The linear PDMS are synthesized by anionic polymerization. Binary blends having different concentrations of the linear PDMS with narrow MWD are also prepared. A theory developed by some of the authors has been proposed previously and is based on the double reptation mixing rule and a law for the relaxation time of the chains. Since, calculations in the theory must be carried out with rheometric data of linear viscoelasticity, the time-temperature superposition principle is used at a reference temperature of 20 degrees C to obtain the G' and G " master curves. We conclude that the PDMS chains in a matrix with Mw/Mn less than or equal to 1.5 relax predominantly by reptation while the tube renewal follows Rouse mode, which is proportional to chain molecular weight raised to the square power. It is also shown that dilution effects due to short chains start to be important in the binary blends of PDMS samples due to the presence of Zimm mode of relaxation, which is suggested by the theory for linear polymers of relatively high polydispersity.