A deuterium NMR study of selectively labeled polybutadiene star polymers

From a combination of tailored synthesis and deuterium NMR free induction decay (FID) measurements, the molecular dynamics of various entanglement sections in four-arm star polybutadienes has been studied. Each arm of the polybutadiene stars has a monodisperse molecular weight of 30 000 g mol(-1), and 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 and at the core. Although the FIDs from the core and near the free end are always distinctly different, the FIDs from the other sections are only clearly distinguishable above ca. 40 degrees C. This is interpreted in terms of the molecular weight corresponding to the longest Rouse mode detectable on the NMR time scale at various temperatures. The deuterium FIDs have been analyzed according to an exact solution for the transverse relaxation, which produces an effective correlation time that increases toward the core of the star and with decreasing temperature.