Relaxation dynamics of entangled and unentangled multiarm polymer solutions: Experiment

Relaxation dynamics of several model A(3)-A-A(3) multiarm 1,4-polybutadiene melts and solutions are investigated experimentally using small amplitude oscillatory shear rheometry over a broad temperature range (-90 to +26 degreesC). Two rubbery plateaus are identified from loss G"(omega) minima at low frequencies. The storage modulus in the first plateau regime is of similar magnitude to the plateau modulus G(NO) of entangled linear 1,4-polybutadiene melts, and varies with multiarm solution concentration phi(pom) as G(N)(phi(pom)) = G(N0)phi(pom)(2.2+/-0.1). The second low-frequency plateau modulus G(N,II) increases with crossbar A volume fraction phi(cb) in nearly the manner expected for dynamics of crossbar segments in a network diluted by relaxed arms, A (i.e., G(N,II) approximate to G(N)(gamma(pom))phi(cb)(1+beta), with beta approximate to 1.3). Despite this, we find that arms exert a profound influence on crossbar dynamics well after they have relaxed. Specifically, at solution concentrations below the threshold for arm entanglement, the zero-shear viscosity of multiarm solutions vary with phi(pom) as eta(0) similar to phi(pom)(2.9+/-0.2), which is slightly weaker than expected for entangled linear polymers without contour length fluctuations. However, for these same materials the terminal time lambda is found to depend more strongly than expected on solution concentration, lambda similar to phi(pom)(2.6+/-0.3). At polymer concentrations above the threshold for arm entanglements, a transition to much stronger terminal property scalings with solution concentration and molecular weight are observed. In particular, for arm entanglement densities above two, the zero-shear viscosity and terminal time of multiarm solutions are exponential functions of overall polymer molecular weight and concentration.