“Plastic Deformation” Mechanism and Phase Transformation in a Shear-Induced Metastable Hexagonally Perforated Layer Phase of a Polystyrene-b-poly(ethylene oxide) Diblock Copolymer

Abstract

The hexagonally perforated layer (HPL) phase in a polystyrene-b-poly(ethylene oxide) (PS-b-PEO) diblock copolymer was systematically studied by small-angle X-ray scattering (SAXS) in reciprocal space and transmission electron microscopy (TEM) in real space. Detailed crystallographic analyses revealed that the shear-induced, “single-crystal”-like HPL phase in the PS-b-PEO diblock copolymer sample contained a mixture of trigonal twins (∼80%) and hexagonal (∼20%) structures. Both structures had the same orientation and the same in-plane unit cell parameters (a and α) but different out-of-plane (c-axis) dimensions, because the trigonal structure was constructed by three layers (ABC) and the hexagonal structure had two layers (AB). Computer-simulated diffraction and TEM results implied that the correlation length of the trigonal structure along the [001] direction was relatively large (at least over 20 layers, i.e., ≥∼400 nm) while that of the hexagonal structure was relatively small. The formation mechanism of the HPL phase was investigated by TEM. Various edge dislocations formed by “plastic deformation” under a large-amplitude mechanical shear were the cause of the trigonal twins and the hexagonal structure. Low-frequency rheological studies indicated that the HPL phase was metastable, and it transformed into a more stable double gyroid phase when the temperature exceeded 160 °C. The HPL structure reappeared after the sample was again subjected to the large-amplitude mechanical shear. Furthermore, the transformation from the HPL phase to the DG phase was also studied. It was found that the perforations of the hexagonal structure rearranged themselves first, followed by the perforations in the trigonal structure.

Monte Carlo phase diagram for diblock copolymer melts

“…Furthermore, the perforations are permanent and their positions are fixed to a lattice. The finite system size prevents hexagonal in-plane symmetry, but nevertheless the perfo- rations in adjacent layers are staggered as predicted by theory [9] and observed in experiment [27]. The fact that the two layer orientations led to such different populations of perforations suggests that their presence is very sensitive to the lamellar period.…”

Weak segregation theory and non-conventional morphologies in the ternary ABC triblock copolymers

“…We believe, however, that such an agreement or disagreement is to large degree occasional. First, the real density profiles 43,66,67 As a result, the structure and fluctuation contribution into the total scattering are comparable and the scattering curves I(q) observed in block copolymer ordered phases 21,78 are typically rather smooth as compared to the δ-function-like peaks predicted in refs 75, 76 and reveal only one big dominant peak and a few of relatively small and smooth secondary ones. Nevertheless, the common belief we also adopt is that it is the presence or absence of the secondary peaks, which distinguishes between the different morphologies even in the case of "smeared" scattering curves.…”

“…Thus, the G→G 2 phase transition could be identified by vanishing of the (4,0,0) and appearance of the (3,1,0) harmonics. iv) One more remark concerns the (3,2,1) reflections found 78 in the G phase for PS-PEO diblock copolymer and not observed 21 for PI-PS one. The relative weakness of this reflection as compared to that (4,0,0) observed in both cases 21,78 could be explained by the fact that these harmonics are generated by one and two pairs, respectively, of the first ones (see Table 2).…”

“…iv) One more remark concerns the (3,2,1) reflections found 78 in the G phase for PS-PEO diblock copolymer and not observed 21 for PI-PS one. The relative weakness of this reflection as compared to that (4,0,0) observed in both cases 21,78 could be explained by the fact that these harmonics are generated by one and two pairs, respectively, of the first ones (see Table 2). Therefore, the factor (3.37) is doubled and, accordingly, the scattering intensity (6.1) is about four times larger for the latter reflections than for that (3,2,1).…”

Mesoscopic crystallography of shear-aligned soft materials

“…Inside the ®rst ring are four weak re¯ections (AE57 with respect to the equator) that arise from perforations within the lamellae. The origin of these re¯ections has been discussed in detail elsewhere (Hamley et al, 1993(Hamley et al, , 1999Fo È rster et al, 1994;Vigild et al, 1998;Zhu et al, 2003), and is not reiterated here. The pattern obtained upon rotation to 60 (shown in Fig.…”