Rouse and reptation dynamics of linear polybutadiene chains studied by H-2NMR transverse relaxation

Deuterium NMR has been used to investigate two different types of dynamics of linear polybutadiene chains in the melt. The transverse relaxations of s hort Rouse chains of molecular weight 640-3000 were biexponential, which ha s been attributed to separate decays of the methylene and methine deuterons . Interpretation of the transverse relaxation rates using a model for Rouse dynamics, combined with molecular simulations, gave the shortest Rouse uni t as approximately 4.4 monomers and the shortest Rouse time as 8.3 x 10(-7) s. The reptation dynamics of higher molecular weight entangled chains were investigated using ABA isotopic triblock copolymers, of total molecular we ight 14000-135000, where A is protonated polybutadiene of molecular weight greater than the entanglement molecular weight and B is a deuterated block. These polymers were specifically synthesized so that the fast motion of th e Rouse-like chain ends should not complicate the signal. The fundamental p arameters found for the Rouse chain were used in the reptation model, assum ing fast dynamics, and gave an entanglement molecular weight, M-e, of 5380 or approximately 21 Rouse units. This M-e is more than twice the convention al value, obtained from rheology, and is more suggestive of the critical mo lecular weight M-c, consistent with previous NMR work. The theoretical anal ysis used in this work is based on the assumption that the chain dynamics a re fast on the time scale set by the NMR deuterium quadrupolar interaction. The highest molecular weight samples were found to not satisfy this criter ion and indicate the molecular weight at which a new theoretical approach i s needed.