Structure and Properties of Tapered Block Polymers of Styrene and Isoprene II. Dynamic Mechanical Responses and Their Structural Interpretations

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Abstract

The dynamic mechanical properties of “tapered” block polymers consisting of styrene and isoprene were studied in relation to the unique microdomain structure of these polymers. It was found from morphological observations that considerable mixing of the two monomers occurs in the primary structure, giving rise to a class of block polymers consisting approximately of “styrene-rich” and “isoprene-rich” block chains. Microphase-separated domain structures existed in the tapered block polymers prepared by sequential living anionic polymerization. Each domain, however, contained a substantial amount of dissimilar monomeric units, thus having an order-to-disorder transition temperature Tc (the temperature at which the microdomains are dissolved into a homogeneous mixture) lower than that of the ideal block. This lowering in Tc should have a substantial influence on flow behavior. The tapered block polymers gave essentially a single broad primary dispersion. This dispersion is related to the microbrownian motion of segments with a relaxation time between those of the parent homopolymers. The temperature dependence of the shift factor aT obeyed the WLF equation, in spite of the heterophase structure. The mechanical behavior was interpreted using a model in which two incompatible units undergo extensive mixing in each domain, leading to an increase in the interfacial volume fraction.

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Keywords

  • Block Polymer
  • Tapered Block Polymer
  • Microdomain Structure
  • Dynamic Mechanical Properties
  • WLF Equation
  • Microphase Separation
  • Order-Disorder Transition
  • Critical Temperature
  • Small-Angle X-Ray Scattering
  • Segmental Mixing

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