Thermoreversible Cylinder−Sphere Transition of Polystyrene-block-polyisoprene Diblock Copolymers in Dioctyl Phthalate Solutions

Abstract

We present experimental results of a thermoreversible morphological transition between spheres and cylinders for polystyrene-block-polyisoprene diblock copolymers (SI) in dioctyl phthalate solutions. Morphologies were characterized using small-angle X-ray scattering (SAXS), and spherical and cylindrical states were found above and below the order-order transition (OOT) temperature, TOOT, respectively. TOOT increased with an increase of polymer concentration for the range covered in this study (higher than 70 wt % polymer concentration). The concentration dependence of TOOT was found to be given by the relationship of ( effrC)OOT/φprC ) A + B/TOOT, where ( effrC)OOT denotes the critical value of the product of eff and rC at the cylinder-sphere OOT, eff is the effective interaction parameter between styrene and isoprene segments in the presence of solvent, rC is the reduced degree of polymerization of an entire block copolymer, and φp is the volume fraction of the block copolymer in the solution. A and B are constants characterizing the temperature dependence of the segmental interaction parameter SI in bulk, where A ) -0.0258 and B ) 27.9 were evaluated by analyzing SAXS profiles from the disordered state. The critical values( effrC)OOT ) 38.4 for the cylinder-sphere transition and 30.5 e ( effrC)ODT e 32.3 for the order-disorder transition (ODT) were determined from our experimental result on the φp dependence of TOOT and that of TODT, respectively. These critical values were compared to the results of some theoretical studies. The SAXS measurements revealed thermoreversibility of the cylinder-sphere transition between 110 and 120°C for an 80 wt % solution.

Phase Behavior of a Block Copolymer in Solvents of Varying Selectivity

“…We therefore define as "normal" (subscript 1) a structure in which the longer block occupies the major domains and as "inverted" (subscript 2) a phase where the shorter block is located in the major domains. For example, the C 2 phase of SI (11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21) denotes PI + solvent cylinders in a matrix of PS + solvent. Since SAXS does not readily distinguish normal and inverted structures, the phase assignments were based on the estimated volume fractions of the PS and PI domains.…”

“…Decreasing the alkyl chain length of dialkyl phthalates increases the selectivity toward styrene, and therefore DEP should be more styrene-selective than DBP. The phase diagram of SI- (11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21) in DEP is presented in Figure 1. Qualitatively, the results are similar to SI(11-21) in DBP, but there are several significant differences.…”

“…[14][15][16][17][18] The effect of added neutral solvent is to simply "dilute" the monomer-monomer contacts, thereby reducing the effective segregation; this is analogous to increasing the temperature in the melt and consequently corresponds to a vertical trajectory across the phase map (i.e., decreasing eff N at constant f ′ PS ). However, this is not exactly the case with SI (11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21) in DOP, as the C 1 phase is diminished upon dilution due to competing concentration scalings of eff at the OOT ( OOT ∼ φ -1 ) and ODT ( ODT ∼ φ -1.4 ).…”

“…This is illustrated in Figure 11, where six trajectories associated with the expected phase behavior in each of the investigated solvents are superimposed onto the melt phase map, presented in terms of an effective PS volume fraction (f ′ PS ) and degree of segregation eff N. The melt phase map is drawn based on extensive results for SI melts, 45,46 with the polystyrene composition as the independent variable, a PS reference volume for N, and eff from eq 2. As a common reference, the trajectories originate at the estimated segregation of neat SI (11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21) at 0°C.…”

“…Accordingly, addition of a selective solvent corresponds to a trajectory with a substantial horizontal component to 45 (circles) and Ryu et al 46 (squares) for SI copolymers, with the dashed lines marking the estimated phase boundaries. The trajectories start at the estimated segregation of neat SI (11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21) at 0°C.…”