From a simple liquid to a polymer melt:: NMR relaxometry study of polybutadiene

We utilize NMR field cycling relaxometry to study the crossover from glassy dynamics (t greater than or similar to tau(alpha)) through Rouse to reptation behavior in a series of monodisperse polybutadienes with molecular weights M=355 to 817 000 g/mol. We separate characteristic polymer dynamics from the total spectrum dominated by glassy dynamics. The polymer dynamics show typical Rouse relaxation features that grow with M and saturate at high M. Comparing to Rouse theory, we determine the Rouse unit size M(R)similar or equal to 500 and entanglement weight M(e)similar or equal to 2000; the Rouse spectrum saturates at M(max)similar or equal to 4000. The local order parameter S approximate to 0.11 is relatively large, indicating noticeable local packing already in the Rouse regime. The M dependence of the glass transition temperature T(g), obtained from dielectric relaxation spectra, shows distinctive kinks at M(R) and M(e).