Linear Melt Rheology and Small-Angle X-ray Scattering of AB Diblocks vs A2B2 Four Arm Star Block Copolymers

Abstract

The frequency dependent viscoelastic properties and lamellar spacing of three symmetric styrene−isoprene (PS−PI) diblock copolymers are compared to those of their hetero-four-arm star counterparts. The PS and PI arm molecular weights of the three linear and three star samples are 10, 20, and 60 kg/mol, respectively. All six samples were unoriented and had lamellar morphology for temperatures less than TODT, the order−disorder temperature for each molecular weight. The lamellar spacing D at the same temperature was found to scale with overall molecular weight N according to D ∼ Nδ, with δ ≈ 0.7 for both linear and stars. However, the star chains were consistently 5−10% more strongly stretched compared to their linear counterparts. For the 10K arm materials, the critical frequency for the onset of mesophase relaxations (ωc) for the stars was found to be about 20 times smaller compared to the linears. This difference correlated very well with quantitative estimates of the inverse layer hopping time of the ch...