Electric-Field-Induced OrderâOrder Transition from Hexagonally

Article pubs.acs.org/Macromolecules

Electric-Field-Induced Order−Order Transition from Hexagonally Perforated Lamellae to Lamellae Christian W. Pester,† Kristin Schmidt,‡ Markus Ruppel,∥ Heiko G. Schoberth,§ and Alexander Böker*,∥ †

Downloaded by UNIV OF CAMBRIDGE on September 11, 2015 | http://pubs.acs.org Publication Date (Web): August 19, 2015 | doi: 10.1021/acs.macromol.5b01336

Materials Research Laboratory & Department of Chemical Engineering, University of California, Santa Barbara, Santa Barbara, California 93106, United States ‡ Lehrstuhl für Physikalische Chemie II and §Lehrstuhl für Material- und Prozesssimulation, Universität Bayreuth, Universitätstrasse 30, 95447 Bayreuth, Germany ∥ Fraunhofer-Institut für Angewandte Polymerforschung, Lehrstuhl für Polymermaterialien und Polymertechnologie, Universität Potsdam, Geiselbergstraße 69, 14476 Potsdam-Golm, Germany

ABSTRACT: Block copolymers form a variety of microphase morphologies due to their ability to phase separate. The hexagonally perforated lamellar (HPL) morphology represents an unusually long-lived, nonequilibrium transient structure between lamellar and cylindrical phases. We present a detailed study of a concentrated, HPL-forming poly(styrene-b-isoprene) diblock copolymer solution in toluene in the presence of an electric field. We will show that this phase is readily aligned by a moderate electric field and provide experimental evidence for an electric-field-induced order−order transition toward the lamellar phase under sufficiently strong fields. This process is shown to be fully reversible as lamellar perforations reconnect immediately upon secession of the external stimulus, recovering highly aligned perforated lamellae.

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INTRODUCTION Block copolymers (BCPs) are an intriguing class of macromolecules. Chemically distinct homopolymers are covalently linked together, allowing a subtle balance between enthalpic and entropic contributions to their overall free energy to control their phase separation on a microscopic scale.1 Classical thermodynamically stable morphologies include spheres (S), cylinders (C), and lamellae (L), which already appeared in the very first calculations of diblock copolymer phase diagrams.1,2 Recent years have brought forth additional, nonclassical morphologies: the bicontinuous gyroid (G) structure, the Fddd (or O70) phase, and hexagonally perforated lamellae (HPL).3−5 The HPL phase retains the planar aspect of a lamellar phase. Layers comprising the minority domain contain closely packed protrusions (either cubic or hexagonal packing) through which the layers of the majority domain connect. HPL is often observed as an unusually long-lived nonequilibrium structure between L and C for stronger segregation and metastable against G.6,7 It is an important intermediary structure on the kinetic pathways between L, C, and G.8−13 Experimentally, the HPL morphology is most often found to be stable in symmetric ABA triblock copolymer thin films14,15 and © 2015 American Chemical Society

likewise as the equilibrium morphology in block copolymer/ homopolymer blends.5,16,17 There has been a considerable amount of research on the influence external stimuli pose on BCP morphologies including, but not limited to, alignment via shear,18 magnetic,19 or electric fields.20 External electric fields show a multitude of influences on BCPs: First, structures with a primary symmetry axis will align such that dielectric interfaces are oriented parallel to the direction of the applied field,21−28 also influencing lamellar domain spacings at high field strengths.29,30 Further, for BCPs, the phase-separated state may be destabilized, essentially shifting the order−disorder transition temperature, resulting in electric-field-induced mixing.31,32 Finally, highly symmetric morphologies (e.g., S and G) that cannot readily evade the electrostatic penalty through mere reorientation may ease their energetic frustration through recasting their crystal lattice to allow for a low-energy orientation with respect to the field direction.33 In the presence of an electric field, the equilibrium Received: June 19, 2015 Revised: July 23, 2015 Published: August 19, 2015 6206

DOI: 10.1021/acs.macromol.5b01336 Macromolecules 2015, 48, 6206−6213

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Macromolecules

Figure 1. (a) Radial integration of the two-dimensional small-angle X-ray scattering pattern in the inset. Scattering is from a 45 wt % solution of S67I3375 in toluene at ambient temperatures. The peak ratio confirms the presence of a hexagonally perforated lamellar (HPL) microstructure. Perforations are stacked in an ABCABC... type manner as depicted in (b). Lattice parameters c = 122.4 nm (= d003(HPL)) and a = 56.3 nm fully define the HPL morphology which is shown as a three-dimensional model from two viewing angles in (c).

distance.42 In both cases, cylinders coarsen and straighten along the applied field direction at later stages. Additional work regarding OOTs of the HPL microstructure in particular is rather sparse. The HPL → C transition is the only published experimental order−order transition away from the HPL phase hitherto. In the following we present an experimental study of a transition toward the lamellar phase using an electric field as the external driving force.

morphology maximizes the polarization density along the applied field direction, allowing soft matter to readily adopt a more favorable symmetry under ambient conditions. This is the principal driving force, which generally promotes morphological transformations in an electric field, such as from S → C34 or G → C.33 Depending on the strength of both phase segregation and the applied field, field-induced disordering and field-induced order−order transitions (OOT) may occur simultaneously.20,35 The HPL morphology contains geometrical motifs pertaining to all neighboring phases. In particular, it reiterates the hexagonal aspect of the cylinder phase as well as the 3-fold junctions peculiar to the gyroidal geometry. HPL transforms into G under extended thermal equilibration and/or the application of shear.8 In thin films, perforated lamellae may persist as the equilibrium morphology depending on film thickness (confinement strength) and specific polymer− substrate interactions (surface field strength).15 Computer simulations indicate that HPL can be readily rearranged in the presence of an external electric field to furnish both cylindrical and lamellar morphologies. HPL also is a common transient structure for other order−order transitions.36,37 HPL → L and HPL → C order−order transformations are particularly appealing as they expand the range of BCP compositions suitable to template ordered arrays of conducting or magnetic nanowires.38 Ly et al. studied the HPL → C phase transition in thin films for both AB diblock copolymers39 and ABA triblock copolymers.40 In the former case, the electricfield-induced HPL → C transition much resembled the situation upon alteration of temperature or film thickness.41 In their study, originating from a defect-laden HPL phase, cylinders formed when 3-fold junctions disintegrated, which effectively lead to the fusion of the pores comprising the minority domain.39 Arms of 3-fold connections, ill-aligned with respect to the direction of the applied field, fractured. Undulating cylinders and perforated lamellar regions coexisted. Initial stages involved strongly distorted cylinders, which served as nuclei, through which the cylindrical morphology was able to propagate. The cylinders eventually straightened and coarsened along the field direction at later stages. For Ly et al.’s triblock studies,40 the wavelength of peristaltic modes equaled the final domain distance between cylinders, while the wavelength of the undulatory mode unveiled to be twice the final cylinder

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RESULTS AND DISCUSSION Figure 1a displays the small-angle X-ray scattering (SAXS) profile for a 45 wt % solution of a poly(styrene-b-isoprene) (S67I3375) diblock copolymer in toluene at room temperature obtained by radial integration of the two-dimensional scattering pattern displayed in the inset. Toluene was chosen for our studies due to its neutrality for SI diblocks.43 SAXS data were collected at the ID02 beamline of the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. At a total molecular weight of 75 kg mol−1 (Mw/Mn = 1.05) the polystyrene content of this anionically synthesized diblock copolymer amounted to 67 vol %, placing it well within the expected region for a HPL microstructure. HPL signifies multiple possible sequences of how individual perforated layers may be positioned with respect to one another. These sequences include, but are not limited to, ABAB... (hexagonal, space group P63/mmc), ABCABC... (trigonal, space group R3̅m), and combinations of the two.44−48 The √nq = 5:6:7:13:15:19:24:54 peak ratio of our experimental data demonstrates the presence of ABCABC... stacked HPL with R3̅m space group symmetry under ambient conditions (cf. Figure 1a and Table 1). This result is in coherence with SCFT predictions by Matsen, who showed that the ABCABC... stacking is the thermodynamically more stable when compared to the ABAB... HPL morphology.49 Figure 1b,c depicts both a geometrical model and a three-dimensional view of this R3̅m HPL morphology from two different angles. Full indexation and lattice parameters may be determined by solving50 1 dhkl 2 6207

=

4 ⎛ h2 + hk + k 2 ⎞ l2 + ⎟ ⎜ 3⎝ ⎠ c2 a2 DOI: 10.1021/acs.macromol.5b01336 Macromolecules 2015, 48, 6206−6213

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Macromolecules

A home-built capacitor setup with parallel gold electrodes allowed us to apply spatially homogeneous electric fields of up to E = 15 kV mm−1 with no detectible leakage currents ( 4 kV mm−1 the intensity of the primary perforation peak, q101, diminishes for all sectors and demarcates the experimental evidence for an electric-field-induced order−order transition from hexagonally perforated lamellae to a lamellar microstructure. Although the well-defined nature of the first-order Bragg reflection and the presence of higher-order peaks points toward a thermodynamically stable structure at E = 4 kV mm−1, we cannot fully exclude the occurrence of electric-field-induced disordering as previously reported.31,32,52 Upon increasing the electric field up to E = 8 kV mm−1, we indeed notice considerable broadening of Bragg reflections for all three sectors. The electric field acts as an additional disturbance to the free energy, increases lamellar fluctuations, and disintegrates microstructures. The resulting loss of long-range ordering leads to the observed increase in peak width, accompanied by an overall drop in scattering intensities (see Figure 3, 8 kV mm−1). 6209


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Figure 4. Time evolution of azimuthal intensity distributions (Δq < 0.007 nm−1) of the first-order lamellar Bragg peak (a, q003(HPL) = 0.154 nm−1) and hexagonally ordered cylindrical perforations (c, q101 = 0.139 nm−1). Individual integration areas are shown as a magenta shading in exemplary scattering patterns (b and d). Vertical dashed lines indicate the time points when the magnitude of the external stimulus was stepwise increased to a value printed at the top of the figure.

inception of the electric field.23,24,52,53 Reorientation of grains manifests itself by distinct shift of azimuthal intensity along the first-order Debye−Scherrer ring. In contrast, the often faster nucleation and growth mechanism is characterized by disappearance of azimuthal maxima, which subsequently re-form at a more favorable angle with respect to E. Figure 4 shows the time evolution of azimuthal intensity distributions for the Debye−Scherrer ring corresponding to the lamellar stacks (Figure 4a, q003(HPL) = 0.154 nm−1) and the hexagonally packed perforations (Figure 4c, q101 = 0.139 nm−1). Integration about q003 also allows us to map the time evolution of the order parameter, P2, to support the following mechanistic discussion. Previously, the order parameter, and its evolution with time, has been extensively used to study alignment kinetics,55 and the influence of initial lamellar alignment24 on the underlying mechanistic reorientation pathway lamellae follows while minimizing their free energy. The order parameter is commonly defined as56

Upon secession of the external stimulus, peaks from perforations recover, primarily in the φ = 0° sector. This resembles a highly aligned HPL structure with lamellar interfaces perpendicular to the gold electrode surfaces (parallel to the previously applied electric field). Hexagonally packed perforations are re-formed perpendicular to the lamellar stacks and parallel to the electrodes. This indicates full thermodynamic relaxation of the electric field-induced lamellar microstructure toward its initial HPL morphology and complete reversal of the HPL → L order−order transition. Figure 3a also illustrates how the application of strong electric fields can compress lamellar spacings in lamellar diblock copolymers. This is indicated by a shift of the principal Bragg maximum of the lamellae which shifts in φ = 90° direction toward larger q vectors. This corresponds to an electric-fieldinduced decrease of the lamellar spacings. Our group elaborated on this effect and its temperature dependence in great detail in two previous publications.29,30 Similar to purely lamellar morphologies, the stacks of hexagonally perforated lamellae also show distinct, anisotropic behavior regarding how their lamellar spacings are influenced by the electric field. Lamellar stacks with interfaces parallel to E (φ = 90°) are compressed; the spacing of those perpendicular to E (φ = 0°) increases. We will now shift our attention to the mechanistic pathway for HPL reorientation. There are three dominant pathways for lamellar morphologies to align and lower their free energy in the presence of an electric field: reorientation of grains by defect movement, nucleation and growth, or selective disordering.22−24,52−54 The former two show noticeable differences in the temporal evolution of azimuthal scattering intensities upon

P2 =

1 (3⟨cos2 φ⟩ − 1) 2

with 2π

2

⟨cos φ⟩ =

∫0 (Iq(φ) cos2 φ|sin φ|) dφ 2π

∫0 (Iq(φ)|sin φ|) dφ For our definition of φ (cf. Figure 2a), lamellar alignment of interfaces perpendicular (φmax = 0/180°) or along (φmax = 90/ 270°) the electric field yields positive (0 < P2 < 1) or negative 6210


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Figure 5. (a) Time evolution of the order parameter, P2, for azimuthal integration of a small q range (Δq < 0.007 nm−1) around the first-order lamellar Bragg peak (q003 = 0.154 nm−1). Positive and negative P2 values correspond to (hexagonally perforated) lamellar interfaces aligned perpendicular and parallel toward the electric field vector, respectively. Arrows and numbers above the plot indicate the time points at which the electric field was altered. Insets are 2D SAXS scattering patterns at 0 kV mm−1 (left) and 4 kV mm−1 (right). (b) Scattering pattern after secession of the electric field, which leads to L → HPL recasting of highly aligned HPL. The inset illustrates the relative alignment of the domains with respect to the detector image and the electric field vector, E.

field-induced alignment.23,52,53 The low degree of initial ordering (P2,0 = −0.3) and high degree of phase separation as present in our sample are indeed expected to favor this mechanistic pathway.23,24 The azimuthal intensity evolution of q101 perforation reflections reinforces this (cf. Figure 4c). Slow and steady reorientation toward the electrothermodynamically more favorable orientation shows the fingerprint of a reorientation of grains. Upon completion of lamellar orientation, cylindrical perforations align perpendicular to both lamellar stacks and E (φ = 0°). Figure 4c also demarcates the onset of the HPL → L order− order transition at E = 4 kV mm−1 (t = 1000 s). Remaining illaligned perforations persist, but their intensities for φ = 0° drastically decrease until they fully vanish at stronger fields, completing the HPL → L order−order transition.

(−0.5 < P2 < 0) values for the order parameter, respectively. Both a fully disordered phase and a state where phasesegregated domains are uniformly and randomly distributed are quantified by P2 = 0. We observed q101 and q003 separately by choosing a very small shift in momentum transfer (Δqhkl < 0.007 nm−1) for the individual integrations (Figure 4b,d); that is, their intensity distributions do not overlap at relatively low electric fields. At higher fields the above-mentioned electric-field-induced distortion of lamellar spacings29,30 leads to a shift of the q003 peak to higher q values, which in turn also influences absolute intensities of the q101 reflection and hinders interpretation. To circumvent this issue, we focus the following discussion on the time evolution of scattering patterns up to 4 kV mm−1. Until this field strength the influence of the external field on the morphology’s periodicities may be neglected. Before inception of the electric field (Figure 4, t = 0 s), perforated lamellar stacks (i.e., the (003) plane) are predominantly aligned perpendicular with respect to E.23,24 This is apparent from higher scattering intensities in φ = 0° direction and well-corroborated by a consequently positive P2 = 0.3 value (see Figure 5). At 2 kV mm−1, there is a slight trend for alignment along E (cf. Figure 4a, t = 320−756 s). This is evident from a small shift in maximum azimuthal intensity from φmax = 0/180° (n||E) toward φmax = 90/270°. At a field strength of E = 3 kV mm−1, reorientation properly commences (Figure 4a, t = 756 s). Azimuthal intensity clearly shifts toward φmax = 90°, and lamellar interfaces align along E. Accordingly, P2 shifts from P2(t=0) = 0.3 to negative values and eventually plateaus at P2 = −0.3 (see Figure 5), indicating successful electric-field-induced alignment of the HPL’s lamellae parallel to E from initial alignment predominantly parallel to the electrodes. Upon removal of the electric field, P2 remains at negative values, indicating thermal stability of the now highly aligned HPL morphology. Previous studies found that purely lamellar SI diblock copolymers, of comparable molecular weight and concentration, readily reorient at fields as low as E = 1 kV mm−1,23 which indicates that the HPL’s perforations increase the critical field strength required for electric-field-induced alignment. The time scale required for this slow shift in maximum azimuthal scattering intensities along the Debye−Scherrer ring from φmax = 0° to φmax = 90° (cf. Figure 4a) is indicative of the reorientation of grains by defect movement mechanism for electric-

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CONCLUSION This contribution presented experimental evidence for a fully reversible, electric-field-induced order−order transition from hexagonally perforated lamellae (HPL) to lamellae (L). Our concentrated solution of poly(styrene-b-isoprene) in toluene formed a stable hexagonally perforated lamellar microstructure with perforations stacked in an ABC-type manner. Successful alignment following a reorientation of grains by defect movement mechanism was achieved above electric fields of 3 kV mm−1. Stronger fields subsequently shifted the thermodynamic equilibrium toward lamellae, removing hexagonal perforations and yielding a highly aligned lamellar microstructure with interfaces coinciding with the direction of the external stimulus. Removal of the electric field allows perforations to recover, reforming an aligned hexagonally perforated lamellar structure.

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EXPERIMENTAL SECTION

Synthesis. Poly(styrene-b-isoprene) diblock copolymers were synthesized by sequential living anionic polymerization according to standard methods.57 Molecular weight, polydispersity, and volume fractions were determined by gel permeation chromatography (GPC) and 1H NMR. The volume fraction of the polystyrene block amounts to 67 wt % at a total molecular weight of 75 kg mol−1. SAXS confirmed hexagonally perforated lamellar morphologies in concentrated solutions of toluene (45 wt %) above the order−disorder concentration and at ambient temperatures. 6211


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Setup. Experiments were performed in dedicated capacitors with parallel gold electrode geometry which allowed application of electric fields of up to 15 kV mm−1 without measurable leakage currents (

Electric-Field-Induced OrderâOrder Transition from Hexagonally

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Electric-Field-Induced OrderâOrder Transition from Hexagonally