Segmental and chain dynamics of polyisoprene in block copolymer/homopolymer blends. A dielectric spectroscopy study

The local and global dynamics of polyisoprene (PI) have been studied in a w ell-defined restricted environment. The restriction is provided by the ther modynamic field of segregated poly(styrene-b-isoprene) (SI) diblock copolym ers. The copolymers were nearly symmetric in composition, and the amount of added PI's was kept below 20% to ensure the absence of macrophase separati on in the blends. Two studies have been made. In the first study, different PI's with molecular weights in the range from 1.2 x 10(3) to 5.5 x 10(4) h ave been added to a symmetric SI diblock copolymer. We found that both the homopolymer and PI block dynamics are modified in the blends, and the degre e of modification can be qualitatively accounted for by the reptation and a rm-retraction mechanisms, respectively. A quantitative comparison, however, would require an unrealistic increase of the entanglement molecular weight . In addition, for the higher molecular weight homopolymers there is an eff ect on the homopolymer chain dynamics due to the confinement within the cen ter of the lamellae. In a second study, the same PI was inserted in differe nt (symmetric) SI diblock copolymers with an increasing degree of segregati on. We found a dynamic signature of the homopolymer localization imposed by the thermodynamic field. Block copolymer/homopolymer blends are model syst ems for studying the effect of confinement on the local and global dynamics .