Molecular dynamics simulation of local motion of polystyrene chain end - comparison with the fluorescence depolarization study

Molecular dynamics (MD) simulation of the local motion of a polystyrene (PS ) chain with anthryl group at the chain end surrounded by benzene molecules was performed and the results were compared with those obtained experiment ally by the fluorescence depolarization method. The molecular weight depend ence of the relaxation time of the probe obtained by the MD simulation was qualitatively in agreement with the results obtained by the fluorescence de polarization method. We also estimated the molecular weight dependence of t he relaxation time for the end-to-end vector. Below the degree of polymeriz ation (DP) less than or equal to 3, the mean relaxation time T-m for the en d-to-end vector was similar to that for the vector corresponding to the tra nsition moment of the probe. With the increase of DP, the T-m for the probe tended to reach an asymptotic value unlike that for the end-to-end vector, which monotonically increased with DP. This indicates that the entire moti on of a polymer coil contributes to the local motion to a lesser extent as the molecular weight increases. The MD simulations using artificial restrai nts showed that the rotational relaxation of the probe at the chain end for a dynamically stiff PS chain is realized by the cooperative rotation of th e main chain bonds. The internal modes which takes place below 5 monomer un its mainly led to the rotational relaxation of the probe at the PS chain en d. Finally, the change of T-m with the position along the PS main chain was examined. (C) 2000 Elsevier Science Ltd. All rights reserved.