Article pubs.acs.org/Macromolecules
Epitaxial Phase Transition between Double Gyroid and Cylinder Phase in Diblock Copolymer Thin Film Jueun Jung,† Junyoung Lee,† Hae-Woong Park,† Taihyun Chang,*,† Hidekazu Sugimori,‡ and Hiroshi Jinnai‡,§ †
Division of Advanced Materials Science and Department of Chemistry, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea ‡ Department of Macromolecular Science and Engineering, Graduate School of Science and Engineering, Kyoto Institute of Technology, Kyoto 606-8585, Japan § Institute for Materials Chemistry and Engineering (IMCE), CE80, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan S Supporting Information *
ABSTRACT: The epitaxial relationship in the thermal phase transition between double gyroid (DG) and hexagonally packed cylinder (HEX) phases in polystyrene-block-polyisoprene thin films on Si wafer was investigated using transmission electron microtomography and grazing incidence small-angle X-ray scattering. Two different types of epitaxial transitions were observed, and they appeared to be selectively favored depending on the transition direction. One type of epitaxial relationship prevails in the phase transition from DG to HEX upon heating in which {121}DG, {111}DG, and {220}DG are converted to {100}HEX, {110}HEX, and {001}HEX, respectively. The interphase planes are {220}DG and {001}HEX, and the cylinders meet the {220}DG plane perpendicularly (head-on, Type A) at the grain boundary between DG and HEX. Although there are small dimensional mismatch and distortion in the location of the cylinders in this epitaxial relationship, all cylinders are formed along the topologically equivalent DG skeletal path. On the other hand, in the transition from HEX to DG upon cooling, another epitaxial relationship as well as the head-on type epitaxy was observed, in which {100}HEX, {110}HEX, and {001}HEX are converted to {121}DG, {220}DG, and {111}DG, respectively. The interphase planes are {220}DG and {110}HEX, and the cylinders meet the {220}DG plane in parallel (side-on, Type B) at the grain boundary between HEX and DG. The domain spacing and the symmetry of the two phases are matched near perfectly, but cylinders are converted to two different DG skeletal paths. The Type B epitaxy is hardly observed in the transition from DG to HEX.
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sample preparation method.22,23,25,26 There is a general consensus that the phase transition between HEX and DG has an epitaxial relationship between the {121}DG plane and {100}HEX plane and between the ⟨111⟩DG direction and the HEX cylinder axis.8,14,17,20,25,27,28 In this type of epitaxial transition, DG and HEX phases are well-matched with respect to the spatial position as well as the domain spacing. On the basis of self-consistent field theory, Matsen proposed this epitaxy in the phase transition between DG and HEX, and it was further supported by other studies.17,24−26,28 In a previous study, we found a new epitaxial phase transition from DG to HEX in polystyrene-block-polyisoprene (PS-b-PI) thin film upon heating.22 This transition showed the following epitaxial relationship: {121}DG → {100}HEX, {111}DG → {110}HEX, and {220}DG → {001}HEX. The cylinders meet the
INTRODUCTION
Diblock copolymers exhibit self-assembled microphase-separated morphologies such as lamellar (LAM), hexagonally perforated lamellar (HPL), Fddd, double gyroid (DG), hexagonally packed cylinder (HEX), and body-centered cubic (BCC).1−4 The crystal-like periodic structures of the block copolymer ordered phases and the phase transition between the ordered phases have attracted much interest in the past few decades.5−16 Among the various order-to-order phase transitions of diblock copolymers, the transitions involving DG phase are intriguing in particular since DG has a complex but highly symmetric Ia3d structure.8,16−18 The DG phase has a cubic unit cell in which the minor block forms two interpenetrating networks of tripodal skeletons,8,19 and DG phase can be transformed to other phases by changing temperature6,8,10,11,13,14,17,20−22 or by applying external field such as shear flow23,24 or electric field.24−26 The phase transition occurs epitaxially, but the reported epitaxial relationship varies depending on the direction of the external force or © 2014 American Chemical Society
Received: October 1, 2014 Revised: November 17, 2014 Published: December 3, 2014 8761
dx.doi.org/10.1021/ma5020275 | Macromolecules 2014, 47, 8761−8767
Macromolecules
Article
{220}DG plane perpendicularly at the grain boundary of {220}DG and {001}HEX planes. All cylinders are formed through a topologically equivalent DG skeletal path although there are small dimensional mismatch and distortion in the location of the cylinders. It was a surprising result since the new type of phase transition was not only an unexpected one but also the expected epitaxial transition was not observed at all. In this paper, we extended the previous study to investigate the morphology of PS-b-PI in thin film at the phase transition between DG and HEX upon both heating and cooling by grazing incidence small-angle X-ray scattering (GISAXS) and transmission electron microtomography (TEMT).
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EXPERIMENTAL SECTION
Sample Preparation. Two PS-b-PIs were synthesized by sequential anionic polymerization in cyclohexane at 40 °C. The number-average molecular weight (Mn) and the dispersity (Mw/Mn) of each polymer were measured by size exclusion chromatography equipped with a light scattering detector (Wyatt, WTREOS-05). The two PS-b-PIs are coded as SI-1 (Mn = 32 300, Mw/Mn = 1.01, f PI = 0.670) and SI-2 (Mn = 36 300, Mw/Mn = 1.01, f PI = 0.680). The detailed synthesis and characterization methods were described elsewhere.29,30 Thin films of the PS-b-PIs were prepared from 10 wt % toluene solution with a small amount (
