Dynamics of entangled H-polymers: Theory, rheology, and neutron-scattering

We present experiments and theory on the melt dynamics of monodisperse enta ngled polymers of H-shaped architecture. Frequency-dependent rheological da ta on a series of polyisoprene H-polymers are in good agreement with a tube model theory that combines path-length fluctuation (like that of star poly mer melts) at high frequency, with reptation of the self-entangled "cross-b ars" at low frequencies (like that of linear polymer melts). We account exp licitly for mild polydispersity. Nonlinear step-strain and transient data i n shear and extension confirm the presence of a relaxation time not seen in linear response, corresponding to the curvilinear stretch of the cross-bar s. This time is very sensitive to strain due to the exponential, dependence of the branch-point friction constants on the effective dangling path leng th. Strain-induced rearrangements of the branch points are confirmed by sma ll-angle neutron scattering (SANS) on stretched and quenched partially deut erated samples. We develop an extension of melt-scattering theory to deal w ith the presence of deformed tube variables to interpret the SANS data.