End-to-End Fluctuation of cis-Poly(isoprene) under Constraints from Slow Poly(tert-butyl styrene)

Abstract

The properties of miscible polymer blends are strongly affected by the dynamic asymmetry of the components reflecting a contrast of their bulk Tg. This study focused on a blend of unentangled polyisoprene (PI3; M = 3.0 kg/mol) and poly(4-tert-butyl)styrene (PtBS27; M = 27.2 kg/mol) with a PI content of wPI = 50 wt % to examine this effect on viscoelastic and dielectric properties. The dielectric loss (ε″) peak characterizing the end-to-end fluctuation of PI chains emerged at an angular frequency ω much higher than that of the terminal viscoelastic relaxation, confirming that PI3 and PtBS27 were the fast and slow components in the blend. At low temperatures T, PI3 exhibited a very broad ε″ peak and its terminal relaxation frequency, much lower than the peak frequency, was close to the Rouse relaxation frequency of PtBS27 over a length scale identical to the end-to-end distance RPI of the PI3 chain. This dielectric relaxation narrowed at higher T, namely, on the reduction of the dynamic asymmetry between PI3 and PtBS27. These observations suggested a molecular scenario that the dynamic concentration heterogeneity of the PtBS chains was essentially frozen during the relaxation of PI3 and the PI3 chains experienced broad distributions in both friction and motional constraints to exhibit broadened and retarded relaxation at low T. This scenario was in harmony with the molecular dynamics simulation in both atomistic and coarse-grained (cg) levels. Specifically, the end-to-end vector autocorrelation function of PI obtained from the cg simulation showed stronger broadening in its higher-order modes, which cannot be attained without the distributions of the friction and motional constraint for PI. This result demonstrated the importance of the dynamic concentration heterogeneity of PtBS giving those distributions during the PI relaxation.