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 well-defined restricted environment. The restriction is provided by the thermodynamic field of segregated poly(styrene6-isoprene) (SI) diblock copolymers. The copolymers were nearly symmetric in composition, and the amount of added Pi's was kept below 20% to ensure the absence of macrophase separation in the blends. Two studies have been made. In the first study, different Pi's with molecular weights in the range from 1.2 × 10³ to 5.5 × 10⁴ have 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 degree of modification can be qualitatively accounted for by the reptation and arm-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 effect on the homopolymer chain dynamics due to the confinement within the center of the lamellae. In a second study, the same PI was inserted in different (symmetric) SI diblock copolymers with an increasing degree of segregation. We found a dynamic signature of the homopolymer localization imposed by the thermodynamic field. Block copolymer/homopolymer blends are model systems for studying the effect of confinement on the local and global dynamics.