A deuterium NMR study of selectively labeled polybutadiene star polymers

From a combination of tailored synthesis and deuterium NMR free induction d ecay (FID) measurements, the molecular dynamics of various entanglement sec tions in four-arm star polybutadienes has been studied. Each arm of the pol ybutadiene stars has a monodisperse molecular weight of 30 000 g mol(-1), a nd each has a deuterated sequence of molecular weight 2000 g mol(-1). This sequence, which corresponds to an entanglement length, has been located at different distances from each arm end, namely one entanglement length from the free end, 1/4, 1/2, and 3/4 Of the way down the arm to the star core an d at the core. Although the FIDs from the core and near the free end are al ways distinctly different, the FIDs from the other sections are only clearl y distinguishable above ca. 40 degrees C. This is interpreted in terms of t he molecular weight corresponding to the longest Rouse mode detectable on t he NMR time scale at various temperatures. The deuterium FIDs have been ana lyzed according to an exact solution for the transverse relaxation, which p roduces an effective correlation time that increases toward the core of the star and with decreasing temperature.