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

We utilize NMR field cycling relaxometry to study the crossover from glassy dynamics (t τα) through Rouse to reptation behavior in a series of monodisperse polybutadienes with molecular weights M=355 to 817000g/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 MR 500 and entanglement weight Me 2000; the Rouse spectrum saturates at Mmax 4000. The local order parameter S ≈ 0.11 is relatively large, indicating noticeable local packing already in the Rouse regime. The M dependence of the glass transition temperature Tg, obtained from dielectric relaxation spectra, shows distinctive kinks at MR and Me.