MOLECULAR-DYNAMICS IN LINEAR AND MULTIARMED STAR POLYMERS OF CIS-POLYISOPRENE AS STUDIED BY BROAD-BAND DIELECTRIC-SPECTROSCOPY

Broadband dielectric spectroscopy (10(-1)-10(9) Hz) was employed to investigate bulk amorphous multi-armed stars of cis-polyisoprene and their linear counterparts, both having narrow molecular weight distributions. The star polymers resemble the linear polymer in showing two distinct regions of dielectric dispersion: a molecular weight independent segmental mode process and a normal mode process which depends strongly on the (arm) molecular weight. The segmental mode is nearly identical for linear and multi-armed star polymers. By contrast, the normal mode is shifted to longer relaxation times if one compares star polymers with linear systems having the same (arm) molecular weight. The relaxation processes are analyzed in terms of the beta-parameter ('stretched exponential') of the autocorrelation function. Below a critical molecular weight, M(c) (almost-equal-to 10(4)), the dynamics of the normal mode can be described by the Rouse theory. Above M(c), the relaxation time follows the 3.7 power of the (arm) molecular weight wchic is characteristic for entangled macromolecules. The relaxation for the star polymers can be quantitatively interpreted by conformational fluctuations of a tethered chain.