Phase behaviors of ABAC tetrablock terpolymers

Abstract

The thesis focuses on the composition and the asymmetric effect on the phase behaviour of the tetrablock terpolymers using poly(styrene-b-isoprene-b-styrene-bethylene oxide) (SISO) as the model system, synthesized via a living anionic polymerization method and characterized with NMR, GPC, TEM, SAXS, DSC and DMS. In the compositional study, two series of the isoprene-rich and the styrene-rich symmetric SISO samples (the S blocks are divided equally) are studied in addition to the series with equal volume fractions of the S and I blocks. In the isoprene-rich series, the hexagonally ordered core-shell spherical and cylindrical morphologies (P6/mmm and P6/mm space group symmetries) have been identified with increasing O contents. The temperature dependence is examined using one specimen in the isoprene-rich series near the order-disorder boundary. A dodecagonal quasicrystalline (QC) phase has been identified to occur as an intermediate state between the simple hexagonal order (P6/mmm) and the Frank-Kasper σ-phase ( P /mnm 42 ) before the sample disorders, with THEX < TQC < Tσ < TODT. The TEM results reveal a QC morphology with a 12-fold rotational symmetry and the existence of the local dodecagonal quasicrystalline approximants that lack of any long-range translational order. The sequence of phases in the styrene-rich SISO series is the disordered state, spheres with a BCC lattice, liquid-like packed (LLP) spheres and the hexagonally (HEX) ordered cylinders. For SISO with fO = 11%, LLP, 12-fold QC after shorter annealing time of 30 minutes and Pm3n phase after longer annealing time of one day, σ-phase and the disordered states are formed with increasing temperatures; and for another SISO with fO v = 15%, the LLP spheres and HEX cylinders are formed. Changing the molecular architecture from the ISO triblocks to the SISO tetrablocks drives the preference from the hyperbolic interfaces inherent in the network morphologies to the higher, and zero Gauss, curvature surfaces present in the core-shell spheres and cylinders. In the asymmetric study, the asymmetric parameter τ = NS/(NS+NS’) = 0.21 – 0.70 is the only variable. The core-shell spheres on a hexagonal lattice and LLP have been observed at lower temperatures; the 12-fold QC phase with shorter annealing time of 30 minutes and the Pm3n phase after longer annealing time of one day have been observed at intermediate temperatures before the Frank-Kasper σ-phase is formed at the higher temperatures before the samples disorder. The phase behaviors of the symmetric SISO and the asymmetric SIS’O tetrablock terpolymers are nearly identical. Overall, six, and possibly more, sphere-forming phases have been identified in the SISO tetrablock terpolymers: BCC ( Im3m ), LLP, P6/mmm, QC, Pm3n and P /mnm 42 (σ-phase). These phases indicate the great design flexibility and opportunities for the novel phase formation with the ABA’C tetrablock molecular architecture at various molecular weights and compositions