The local dynamics of polymer chains in concentrated and semi-dilute solutions are investigated using synchroton-excited fluorescence anisotropy decay. A small amount (less than 1%) of monodisperse polystyrene labelled with anthracene in the middle of the chain is embedded in a mixture of monodisperse unlabelled chains and toluene. Two different labelled chains (with molecular weights respectively 30000 and 300000) and unlabelled chains of various molecular weights (ranging from 23000 to 1300000) were used, and the polymer weight fraction of the mixtures was varied from 0.01 to 0.51. It is shown that segmental motions are affected by the polymer weight fraction when it exceeds 0.15, but remains unaffected by the molecular weight of the labelled chain and that of the matrix. In the whole range of concentrations, the orientation autocorrelation function obeys the same generalized diffusion and loss model as in dilute solution. The evolution of the characteristic time for segmental motions as a function of the concentration should provide rather direct access to the evolution of the friction coefficient. © 1986.
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