Phase Transition from Disordered Sphere to Hex-Cylinder via Transient Ordering into Bcc-Sphere in SIS Triblock Copolymer

Abstract

We have investigated the ordering processes from the disordered-sphere phase (designated hereafter disordered sphere), where spherical microdomains exist but they have only a short-range liquidlike order, to the phase of hexagonally packed cylindrical microdomains (designated hereafter hex-cylinder), induced by the temperature drop (T-drop). The block copolymer studied is a compositionally asymmetric polystyrene-block-polyisoprene-block-polystyrene triblock copolymer and forms disordered sphere above the order−disorder transition temperature involving the lattice disordering−ordering transition temperature, T ODT or T LDOT, spheres in a body-centered-cubic lattice (designated hereafter bcc-sphere) below T LDOT but above the order−order transition (OOT) temperature, T OOT, and hex-cylinder below T OOT. The ordering processes induced by T-drop were explored in situ and at a real time by using time-resolved small-angle X-ray scattering, and the transient microdomain structures developed during the ordering processes were observed by a transmission electron microscope and a polarized optical microscope for specimens rapidly frozen below the glass transition temperature at particular times in the ordering processes. We confirmed that the freeze-in process does not change the long-range order of the system. When the specimen is deeply quenched into the temperature range below T OOT, hex-cylinder is directly formed from disordered sphere. However, when the specimen is shallowly quenched below T OOT, bcc-sphere is first formed from disordered sphere and fills the whole sample space. Then bcc-sphere is transformed into hex-cylinder. This striking ordering process first involves the ordering into a metastable structure followed by the OOT into a stable structure.

Process and kinetics of order–order transition from bcc-sphere to hex-cylinder in polystyrene-block-polyisoprene-block-polystyrene: Time-resolved SAXS and TEM studies

“…These profiles are not deliberately corrected for slit smearing to prevent any artifacts. As in the previous studies, [30,31] the freeze-in process in this study hardly changes the characteristics of the profiles at small q region (q! 0.4 nm K1 ).…”

“…Moreover, a broad scattering maximum at qZ 0.57 nm K1 marked by a thick arrow can be seen in the profile after freezing but not in that before freezing. This maximum indicates that the particle scattering from spheres, namely the scattering from form factor, appears because the interface between PS spheres and PI matrix is sharpened by freezing, as reported in the previous studies [30,31]. Point (2) in Fig.…”

“…Therefore, T OOT of the specimen is assessed to be 188 8C. This T OOT is slightly different from that described in the previous works [2,30,31] because the film specimen for this study are prepared newly and differ from those used previously. The lattice disordering/ordering transition (LDOT) [32,33] temperature, T LDOT , was assessed to be 215 8C.…”

“…The characteristics of this sample are as follows: the weight-averaged molecular weight, M w , is 1.4!10 5 ; the polydispersity index, M w /M n , where M n is the number-averaged molecular weight, is 1.11, and the weight fraction of polystyrene (PS) is 0.183 [2,30,31]. The block copolymer was dissolved into toluene with a small amount of antioxidant (Irganox 1010, Ciba-Geigy Group) and cast into film specimens from a 10 wt% solution by evaporating the solvent slowly in a fume hood for 1 week.…”

Order−Disorder Transition and Critical Micelle Temperature in Concentrated Block Copolymer Solutions

“…Some authors have suggested distinguishing this ODT from that observed for gyroid, cylinder, or lamellar samples, by terming it the lattice disordering transition. [27][28][29][30] If there are distinct micelles present above the ODT, then at sufficiently high-temperature such micelles should disperse in favor of free chains. Evidence for a "critical micelle temperature" (cmt) has been presented in bulk diblocks, although it should be recognized that such a cmt does not represent a true thermodynamic phase transition.…”