Ordering Dynamics of Compositionally Asymmetric Styrene−Isoprene Block Copolymers

Abstract

We investigate the development of mesophase order in compositionally asymmetric (13 wt % styrene) polystyrene−polyisoprene (SI) diblock and SIS triblock copolymers. The equilibrium ordered morphology of these polymers is a body-centered-cubic (bcc) lattice of spherical S microdomains. The ordering kinetics are measured by quenching the material from above the order−disorder transition temperature (T ODT) to a temperature where the ordered phase is stable and following the structure development by time-resolved small-angle X-ray scattering and dynamic oscillatory rheological measurements. The kinetics are significantly slower than those observed for nearly symmetric polymers of similar T ODT, resulting in a wider range of experimentally accessible quench depths. The temporal evolution of the storage modulus G‘ measured at constant frequency displays a two-step increase after a quench from above the ODT, for sufficiently low temperatures and/or high molecular weights. The long-time step corresponds to the development of a bcc lattice of spherical microdomains, while the first step results from the finite time required for large-amplitude composition fluctuations to develop fully.

Phase behavior and viscoelastic properties of entangled block copolymer gels

“…Whether the solutions are ordered [body-centered-cubic (BCC) lattice of spheres] or disordered is easily assessed rheologically, as they show the same signatures that we have described previously for the bulk materials. 16,20 For the tests described herein, all concentrated solutions remained in the metastable disordered state throughout the measurements, as evidenced by identical low-frequency moduli at the start and end of rheometric testing. These tests also confirm that no measurable degradation of polymer or evaporation of squalane occurred during the tests.…”

“…15,16 Table I summarizes the relevant molecular characteristics of these systems; the sample code identifies these polymers by their block chemistry and approximate weight-average molecular weight (M w ; kg/mol). Gel permeation chromatography (GPC) in toluene, referenced to narrow-distribution polystyrene standards, was employed recently to redetermine the true M w of the block copolymers, according to the method described by Sebastian and Register.…”

“…The microphase structure (ordered vs disordered) was assessed through small-angle X-ray scattering (SAXS), with the apparatus and procedures described elsewhere. 16,19 Data are presented as the desmeared absolute intensity I versus the scattering vector magnitude q ϭ (4 / )sin , where is the Cu K␣ radiation wavelength and 2 is the scattering angle. We also used SAXS to locate the order-disorder-transition temperatures (T ODT ) of the bulk polymers (listed in Table I) and for concentrated solutions capable of ordering (Ͼ13 wt % total polymer concentration), as described previously.…”

“…We also used SAXS to locate the order-disorder-transition temperatures (T ODT ) of the bulk polymers (listed in Table I) and for concentrated solutions capable of ordering (Ͼ13 wt % total polymer concentration), as described previously. 16,18,20,21 …”

Spin diffusion and spin-lattice relaxation in multiphase polymers

“…The small angle X-ray scattering measurement of the above sample, which was analyzed assuming a twodomain structure, indicated that PS forms spheres of 16.4 nm in a body-centered-cubic lattice. 16 The interspherical distance was found to be 30.2 nm. The above data can be converted to an equivalent cube of 13.2 nm for the PS domain and 26.7 nm for the long cubic period.…”