Polymer melt dynamics by NMR relaxation measurements

13C and 2H NMR measurements were performed on polymer melt systems including polyethylene, atactic polypropylene, polybutadiene, and poly(isoprene)/poly(vinylethylene) (PI/PVE) blend. Dynamics of polymer melts can be broken down into librational motions (sub picosecond), segmental motions (ns for polymer melt 50°C above its glass transition temperature T_g), and normal mode motions (hundreds of nanoseconds and beyond).; Segmental motions of polymer melts can be described effectively by a stretched exponential correlation function. Normal mode motions in the unentangled region can be described by Rouse model with modifications for very short modes. Entanglement effects include both the temporal effect of stretching the relaxation time for modes longer than the entanglement molecular weight and the spatial effect of fixing the end points of an entanglement on the time scale of entanglement time.; Unlike most polymer melts studied, the temperature dependence of segmental motions at temperature high above T_g in polyethylene differs from the temperature dependence of global motions (flow activation energy) of polyethylene. The lack of side groups in polyethylene allows conformational transitions to happen without substantial intermolecular cooperation. Thus, in contrast to many other polymers, conformational transitions are not the fundamental motion for flow in polyethylene. Branches as small as a methyl group introduce intermolecular cooperation into conformational transitions at the branch point and thus conformational transitions at the branch points have a stronger temperature dependence.; NMR relaxation measurements have synergistic relationship with molecular dynamics (MD) computer simulations. Data obtained from these experiments helped to make MD simulations more realistic. On the other hand, findings from MD simulations also helped us to analyze and understand our experimental results.