Relationship between Tranlational and Rotational Diffusions of Polymer Chains

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Measurements of the translational diffusion coefficient D and the relaxation time tau for reorientation of the end-to-end vector of polyisoprene (PI) were carried by means of forced Rayleigh scattering and dielectric methods, respectively. From the dielectric relaxation strength we also determined the mean square end-to-end distance <r^2>. From the data of D, tau, and <r^2> over wide range of molecular weight M, the values of a dimensionless quantity Dtau/<r^2> were determined and compared with the Rouse theory and tube model.In monodisperse polyisoprene, D decreased with increasing M in proportion to M^<-2.8> in the range of 10000 < M < 40000 but the exponent decreased slight in 40000 < M.This behavior does not agree with the tube model which predicts D * M^<-2.0>. In a binary blends in which a trace amount of labeled PI is dissolved in a matrix of non-labeled PIs, D depended strongly on the M of the matrix PI,i.e., D * M^<-1.0> when the M of the diffusant is much higher than that of matrix and D * M^<-2.5> wehn the matrix has much higher M than the diffusant. The former behavior agrees with the Rousse theory. The value of Dtau/<r^2> in the monodisperse system is calpha one order smaller than the prediction of the Rouse and tube models both of which predict Dtau/<r^2> = 1/3pi^2. On the other hand, in the binary blends of low molecular weight matrix, Dtau/<^2> agreed well with the Rouse theory.