Constraint Release in Star/Star Blends and Partial Tube Dilation in Monodisperse Star Systems

The molecular picture of partial dynamic tube dilation (partial-DTD) was tested for monodisperse high- M star polyisoprenes (PI) with the number of entanglements per arm, N a = 12 and 16. To achieve this test on an experimental basis, linear viscoelastic measurements were made for entangled binary blends of these high- M star PI in matrices of lower- M star PI having N a = 2−8. In the blends, the high- M star PI was dilute and entangled only with the lower- M matrix stars. Constraint release (CR) relaxation process was viscoelastically detected for those dilute high- M stars. The terminal CR mode was close to that expected for the Rouse relaxation of a tethered chain, and the terminal CR relaxation time at 40 °C was described by an empirical equation, 〈 〉/s ≅ 4.0 × 10 -5 N a2 2 exp(0.71 N a1 ) with N a1 and N a2 being the number of entanglement segments per arm of the matrix and probe chains. These 〈 〉 data were extrapolated to the monodisperse systems of the high- M stars (where N a1 = N a2 ) to give the longest CR relaxation time in these systems, and the data were utilized to estimate the maximum possible number β*( t ) of entanglement segments at time t that were mutually equilibrated through the Rouse−CR mechanism. The corresponding normalized viscoelastic relaxation function for the partial-DTD process, μ p - DTD ( t ) = φ‘( t )/β*( t ) with φ‘( t ) being the dielectrically evaluated survival fraction of the dilated tube, was close to the μ( t ) data, suggesting that the partial-DTD picture serves as a good starting point for describing the entanglement dynamics of monodisperse stars.