Tetragonally Perforated Lamellae of Polybutadiene ... - ACS Publications

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Tetragonally Perforated Lamellae of Polybutadiene-block-poly(2vinylpyridine)-block-poly(tert-butyl methacrylate) (BVT) Triblock Terpolymers in the Bulk: Preparation, Cross-Linking, and Dissolution Felix H. Schacher,†,‡,* Hidekazu Sugimori,§ Song Hong,⊥ Hiroshi Jinnai,⊥,* and Axel H. E. Müller∥ †

Institut für Organische Chemie und Makromolekulare Chemie (IOMC), Friedrich-Schiller-Universität Jena, Humboldtstraße 10, D-07743 Jena, Germany ‡ Jena Center for Soft Matter (JCSM), Friedrich-Schiller-Universität Jena, Philosophenweg 7, D-07743 Jena, Germany § Department of Macromolecular Science and Engineering, Graduate School of Science and Engineering, Kyoto Institute of Technology, Kyoto 606-8585, Japan ⊥ Japan Science and Technology Agency (JST), ERATO, Takahara Soft Interfaces Project and Institute for Materials Chemistry, International Institute for Carbon Neutral Energy Research (WPI-I2CNER) and Engineering (IMCE), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan ∥ Makromolekulare Chemie II und Bayreuther Zentrum für Kolloide und Grenzflächen (BZKG), Universität Bayreuth, Universitätsstrasse 30, D-95440 Bayreuth, Germany ABSTRACT: We report on the formation of tetragonally perforated lamellae from a polybutadiene-block-poly(2-vinylpyridine)-block-poly(tert-butyl methacrylate) (BVT) triblock terpolymer, PB22P2VP29PtBMA49110, in the bulk state (the subscripts denote the volume fractions of the respective segment and the superscript the overall molecular weight in kg/mol). The perforations are formed by the protrusion of the block with the highest volume fraction, PtBMA, through both the central perforated PB lamella and the two adjacent perforated lamellae of P2VP. The morphological characterization is based on transmission electron microscopy (TEM) and three-dimensional reconstructions using transmission electron microtomography (TEMT) experiments. In addition, SAXS confirmed an overall lamellar pattern. The so generated structures can be used for the fabrication of well-defined porous sheets with tunable size, as demonstrated by cross-linking of the central perforated PB lamella, followed by sonication-assisted dissolution into micrometersized lamellar particles with tetragonally arranged pores in nonselective solvents like THF.

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morphologies become possible.6 In this particular case, structure formation is governed by several independent composition variables in addition to the chain topology: the volume fractions (ΦA, ΦB, ΦC), the three interaction parameters (χAB, χBC, χAC), and the respective interfacial tensions (γAB, γBC, γAC).7−9 Pioneering work on the phase diagram of polystyrene-block-polybutadiene-block-poly(methyl methacrylate) (SBM) triblock terpolymers was carried out by Stadler et al.: among the reported morphologies were lamellae, core−shell cylinders, and core−shell spheres but also more complex patterns featuring helical assemblies,10 “knitting pattern” morphologies,11 and a new structural motif comprising two different cylindrical domains instead of core−shell cylinders.12 A powerful method for the detailed three-dimensional investigation of block copolymer nanostructures is transmission electron microtomography (TEMT). Since its introduction in

INTRODUCTION Block copolymers are composed of two or more chemically distinct polymer segments linked together at one or more junction points.1 As a result of the inherent immiscibility of different polymeric materials, block copolymers undergo microphase separation in the bulk into different morphologies with periodicities on a length scale of 10−100 nm. The ability to form well-defined nanostructures with different morphologies and tunable periodicity or size is probably the main driving force for the intense research interest in this field over the past decades.2,3 The bulk morphology of block copolymers depends on several factors, including the materials architecture (linear diblock copolymers, triblock terpolymers, or miktoarm star terpolymers), the molecular weight, the respective volume fractions, the polydispersity, and any interactions of the constituting segments with the environment.4,5 For “simple” linear AB diblock copolymers, thermodynamically stable solid state morphologies include spherical, cylindrical, gyroid, and lamellar structures. If, on the other hand, linear ABC triblock terpolymers are investigated, a large number of complex © 2012 American Chemical Society

Received: June 18, 2012 Revised: September 4, 2012 Published: September 24, 2012 7956

dx.doi.org/10.1021/ma3012398 | Macromolecules 2012, 45, 7956−7963

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columns (30 × 8 mm, 5 μm particle size) with 102, 103, 104 and 105 Å pore sizes using RI and UV detection (λ = 254 nm) at a flow rate of 1.0 mL/min (T = 40 °C). The calibration was based on 1,4polybutadiene standards. The polydispersity index (PDI) of PB22P2VP29PtBMA49110 was 1.02. Nuclear Magnetic Resonance Spectroscopy. 1H NMR measurements were performed on a 250 MHz Bruker AC spectrometer using CDCl3 or THF-d8 as solvent and tetramethylsilane (TMS) as internal standard. The molecular weights of the P2VP and the PtBMA block were calculated from the number-average molecular weight of the precursor obtained by MALDI−ToF mass spectrometry and the ratio of the characteristic NMR signals. The molecular characteristics of the BVT triblock terpolymer are summarized in Table 1.

1988,13 continuous improvements have been made for this technique, including the increase of the rotation range of the specimen, the overall sample thickness, or its combination with scattering techniques.14,15 In particular, if it comes to highly complex or bicontinuous morphologies,16 TEMT is unparalleled regarding the provided information on structural parameters on the nanometer-scale. For example, the double helical structure of achiral polystyrene-block-polybutadieneblock-poly(methyl methacrylate) (SBM) triblock terpolymers could be elucidated using TEMT, including the spatial arrangement of left- and right-handed helices.17 Further, TEMT was employed for an in-depth understanding of the epitaxial phase transition from the double gyroid (DG) to a hexagonal phase in polystyrene-block-polyisoprene (SI) diblock copolymers.18 We herein describe the microphase-separated bulk morphology of an ABC triblock terpolymer, polybutadiene-blockpoly(2-vinylpyridine)-block-poly(tert-butyl methacrylate) (BVT), with the composition PB22P2VP29PtBMA49110, where the subscripts denote the volume fraction of the corresponding block and the superscript the overall molecular weight in kg/ mol. The BVT system represents a very intriguing monomer combination, as each segment can be selectively addressed. This has been shown for the selective cross-linking of the PB domains in the bulk19 and in solution,20 for the formation of inorganic/organic hybrid structures by decorating the P2VP compartment with metal nanoparticles,21 or the generation of negatively charged micellar structures in aqueous solution via the hydrolysis of PtBMA to poly(methacrylic acid) (PMAA).22 For the composition reported here, tetragonally perforated lamellae are found via a combination of small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM), and TEMT. The central lamella consists of PB, located between two adjacent layers of P2VP and PtBMA. Both PB and P2VP are perforated by the third block, PtBMA, thereby connecting the two outer layers and forming an additional interface between PB and PtBMA. Moreover, the PB compartment could be selectively cross-linked and subsequent swelling in a nonselective solvent, followed by sonication-assisted dissolution, resulted in the formation of sheet-like particles still exhibiting tetragonally arranged perforations.

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Table 1. Molecular Characteristics of the Employed Triblock Terpolymer polymer

Mn [g/mol]b

PDIc

DPPBd

DPP2VPd

DPPtBMAd

PB22P2VP29PtBMA49a

110 000

1.02

420

280

390

a

Subscripts denote the weight fraction of the corresponding block. b Determined via a combination of MALDI−ToF mass spectrometry and 1 H NMR spectroscopy. c Determined by size exclusion chromatography using THF as eluent. dDetermined by 1H NMR spectroscopy. Cross-Linking. Films of PB22P2VP29PtBMA49110 were cocast with additional photoinitiator, Lucirin-TPO in this case. Typically, 5−10% photoinitiator with respect to the amount of PB (weight) were used. Cross-linking of the as-cast and annealed polymer films was carried out with a UV-lamp for 2 h (Hoehnle VG UVAHAND 250 GS, cutoff at 300 nm wavelength to avoid the depolymerization of the methacrylic compartment). Structural Characterization. For structural characterization, films of PB22P2VP29PtBMA49110 were cast from THF (the same structures were obtained if either CHCl3 or 1,4-dioxane were used as casting solvents) by slow evaporation of the solvent over several days. The films were afterward dried under vacuum at room temperature first for 24 h and at 50 °C for another 24 h. Small-Angle X-ray Scattering (SAXS). SAXS measurements on cross-linked films from PB22P2VP29PtBMA49110 were performed on a Bruker AXS Nanostar (Bruker, Karlsruhe, Germany), equipped with a microfocus X-ray source (Incoatec IμSCu E025, Incoatec, Geesthacht, Germany), operating at λ = 1.54 Å. A pinhole setup with 750, 400, and 1000 μm (in the order from source to sample) was used and the sample-to-detector distance was 107 cm. Samples (small pieces of cast triblock terpolymer bulk films) were mounted on a metal rack and fixed using tape. The scattering patterns were corrected for the beam stop and the background (Scotch tape) prior to evaluations. Ultrasound Sonication Treatment. This was performed in THF in small glass vials after swelling of the cross-linked polymer films for at least 12 h. The used instrument was a Branson Digital Sonifier equipped with a tungsten tip. The amplitude was kept at 20% and typical sonication intervals were 1 s, followed by a break of 4 s. Total sonication times are always the summarized seconds of real sonication without breaks. All samples were water cooled during the sonication procedure. Dynamic Light Scattering (DLS). DLS measurements were performed in sealed cylindrical scattering cells (d = 10 mm) at a scattering angle of 90° on an ALV DLS/SLS-SP 5022F equipment consisting of an ALV-SP 125 laser goniometer with an ALV 5000/E correlator and a He−Ne laser with the wavelength λ = 632.8 nm. The CONTIN algorithm was applied to analyze the obtained correlation functions. Apparent hydrodynamic radii were calculated according to the Stokes−Einstein equation. Transmission Electron Microscopy (TEM). Ultrathin (30−80 nm) samples for TEM analysis were cut from the as-cast (or crosslinked) polymer films with a Reichert-Jung Ultracut E equipped with a

EXPERIMENTAL PART

Materials. All solvents and reagents (Sigma-Aldrich, p.a. grade) were used without further purification. Lucirin-TPO (2,4,6-trimethylbenzoyldiphenylphosphine oxide), the UV-photoinitiator we used, was kindly provided by BASF and used as received. Synthesis. PB22P2VP29PtBMA49110 was synthesized via sequential living anionic polymerization of butadiene, 2-vinylpyridine, and tertbutyl methacrylate in THF at low temperatures using sec-butyl lithium as initiator. This has been described earlier.19 Briefly, in the presence of alkoxides (c ∼ 0.013 mol/L, typically this corresponds to a roughly 20fold excess compared to the concentration of growing polymer chains), 1,3-butadiene was initiated at −70 °C with sec-BuLi in THF and polymerized for 5 h at −10 °C. Subsequently, 2-vinylpyridine was added to the stirred tank reactor via an ampule. After polymerization of the second block (1 h), the living polybutadiene-block-poly(2vinylpyridine) chain ends were end-capped with 1,1-diphenylethylene (DPE) in order to attenuate the nucleophilicity of the propagating species for 1 h at −50 °C. In that way, transfer reactions upon addition of the third monomer, tBMA, due to too high nucleophilicity could be suppressed.23,24 The polymerization was terminated through the addition of degassed isopropanol. Size Exclusion Chromatography (SEC). SEC with THF as eluent was performed on an apparatus equipped with PSS SDVgel 7957


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diamond knife. TEM micrographs were taken on either a Zeiss CEM 902 operating at 80 kV or a Zeiss 922 OMEGA operating at 200 kV, both in bright field mode. In order to selectively enhance the electron density in one of the three compartments and therefore improve the contrast, the samples were stained with either OsO4 (staining of polybutadiene domains25) or iodine (staining of poly(2-vinylpyridine) domains26). From solutions, samples were prepared through deposition of a drop of the corresponding solution (concentration typically 0.1 g/L) onto the TEM grid (Cu, 200 mesh). Afterward the remaining solvent was removed with a filter paper. Transmission Electron Microtomography (TEMT). For TEMT measurements, samples (∼200 nm thickness) were microtome cut onto Cu mesh grids with a carbon film and an additional underlying polyvinylformal coating. Prior to TEMT, samples were stained using OsO4 vapor (PB) or iodine (P2VP), and Au nanoparticles with a diameter of approximately 5 nm (BB International Ltd., U.K.) were deposited onto the triblock terpolymer films. TEMT measurements were performed on a JEM-2200FS (JEOL Co., Ltd., Japan) at an accelerating voltage of 200 kV and equipped with a slow-scan CCD camera (GATAN USC4000, Gatan Inc., USA). The TEM micrographs were obtained at 1° increment between −65° and +65° tilt angle. The image set was processed according to the same protocol described elsewhere.15 Subsequently, the tilt series of the TEM images were aligned by using the previously deposited Au nanoparticles as fiducial markers and then reconstructed on the basis of the filtered-backprojection (FBP) method.14 The reconstructed images were further visualized using the software platform Avizo (Visualization Sciences Group, http://www.vsg3d.com).

Scheme 1. Structure of the Employed BVT Triblock Terpolymers PB22P2VP29PtBMA49110

terpolymer at ambient temperatures.19 TEM micrographs of the resulting slices are shown in Figure 1.

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RESULTS AND DISCUSSION Over the last few years we have demonstrated that polybutadiene-block-poly(2-vinylpyridine)-block-poly(tert-butyl methacrylate) (BVT) triblock terpolymers represent a versatile platform for the preparation of functional nanostructured systems in diverse microenvironments. The straightforward combination of a soft hydrophobic (PB), a nitrogen-containing weak polyelectrolyte (P2VP), and a third block, which can either be selectively degraded via UV or transformed into poly(methacrylic acid) (PMAA) by ester hydrolysis, could be realized by sequential living anionic polymerization in THF at low temperatures.19 Subsequent self-assembly has led to a variety of complex and compartmentalized morphologies, including hexagonally packed core−shell cylinders in the bulk19 and in thin films,27 or multicompartment micelles in organic media.20 Probably the most remarkable feature of the BVT system is that each block can be selectively addressed using suitable protocols, demonstrated through the crosslinking or the hydroxylation of the PB compartment28 or the generation of ampholytic systems by the quaternization of P2VP to P2VPq, followed by hydrolysis of PtBMA to PMAA.22 Morphological Characterization of PB22P2VP29PtBMA49110. Here we describe the bulk morphology of PB22P2VP29PtBMA49110. The molecular weight is 110 000 g/mol (superscript) and the degrees of polymerization are 420 (PB), 280 (P2VP), and 390 (PtBMA), as determined by a combination of mass spectrometry (PB) and 1H NMR measurements (Table 1). The chemical structure of the terpolymer is shown in Scheme 1. Bulk films of PB22P2VP29PtBMA49110 were slowly cast from THF solutions (during approximately one week) and subsequently annealed in a vacuum oven for further 48 h (24 h at room temperature, followed by 24 h at 50 °C). Ultrathin (30−80 nm) samples for transmission electron microscopy were cut with a microtome, the content of polybutadiene (22%) still being low enough to allow us to microtome the

Figure 1. TEM micrographs of PB22P2VP29PtBMA49110 bulk slices without additional staining (A), and after treatment with OsO4 (B, D), and iodine (C). Both bars and circles in parts A, C, and D correspond to the respective domain sizes discussed in the text.

Figure 1A shows the bulk morphology of PB22P2VP29PtBMA49110 without additional staining: at first glance, the structure seems to be of lamellar type, although the domains exhibiting the highest electron density appear undulated. According to the terpolymer composition, the dark domains should consist of P2VP whereas PB is expected to appear lighter.19 The block with the largest volume fraction, PtBMA (49%), is expected to suffer from radiation damage and, thereby, volume shrinkage.29 According to TEM, an interlamellar distance of 87 ± 8 nm (a white bar is added in Figure 1A) can be determined (Table 2). Treatment of the triblock terpolymer bulk sample with OsO4 results in the preferential deposition of heavy metal atoms in the PB compartments.25 The corresponding TEM micrographs are depicted in Figure 1, parts B and D, the latter showing a higher magnification image. In this case, a lamellar morphology is likewise observed, but the dark PB lamellae now exhibit regular perforations with an average radius of 11 ± 2 nm (highlighted by three black circles 7958


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Table 2. Long Period and Lamellae Thickness As Determined by Different Characterization Methods d [nm] dPB [nm] dP2VP [nm] dPtBMA [nm] RPerforations [nm] DPerforations [nm]

TEMa

TEM [OsO4 staining]

TEM [I2 staining]

87 ± 8 − − 12c − 50−60

89 ± 9 45 ± 4 − − 11 ± 2 −

86 ± 8 − 16 ± 2 − − −

TEMT [OsO4 staining] 59 22 − − 10 65

±5 ±2

±1 ±6

TEMT [I2 staining]

SAXSb

62 ± 5 20 ± 5 12 ± 3 18c − −

80 ± 5 − − − − −

a

The error from determining the long period by TEM or TEMT was estimated to be 10%. bThe error resulting from the SAXS measurements depends on the width of the peak and was estimated to be 5%. cCalculated according to the overall d-spacing taking into account the thickness of both PB and P2VP lamellae.

Figure 2. TEMT reconstructed images for bulk samples of PB22P2VP29PtBMA49110 after staining with OsO4 (A, C, D, only the PB domain shown, A provides an overview; the projection in C allows for an accurate determination of the distance between two individual perforations), iodine (B, only the P2VP phase shown, the two solid red lines depict two adjacent P2VP lamellae, the PB lamella is located in between), and a combination of OsO4/iodine (E, both PB and P2VP domains shown).

in Figure 1D). The interlamellar distance is 89 ± 9 nm, which is comparable to the value deduced from Figure 1A (black bar #1). In addition, an average distance between individual features of ∼50−60 nm can be observed (black bar #2). The thickness of the central PB lamellae according to Figure 1D is 45 ± 4 nm (black bar #3). If the sample is treated with iodine, preferentially staining the P2VP domains via the formation of charge-transfer complexes, the lamellar structure can be confirmed. Two thin P2VP lamellae with a thickness of 16 ± 2 nm can be seen (Figure 1C, black bar #1 highlights the undulated P2VP lamella), and the overall long period was determined to be 86 ± 8 nm (black bar #2). Also here, the lamellae appear undulated and perforations are visible. Compared to our earlier studies on BVT triblock terpolymer bulk structures of different composition,19 an overall long period of roughly 90 nm in TEM seems appropriate, considering that high molecular weight materials (>100 000 g/mol) are studied. Further, it has to be taken into account that the films experience a certain distortion during the microtome cutting, leading to higher values for the corresponding domain sizes deduced from these micrographs. So far, the bulk morphology of PB22P2VP29PtBMA49110 is best described as a lamellar (ll) structure, with a central lamella of PB, and, in accordance with the block sequence, two surrounding layers of P2VP and PtBMA. However, both the PB and the P2VP

domains exhibit regular perforations which, from the staining experiments, are expected to be formed by PtBMA, the block with the highest volume fraction. To confirm the above-mentioned observations, TEMT experiments were performed. PB22P2VP29PtBMA49110 samples after staining with OsO4, iodine, and OsO4/iodine were investigated. The corresponding reconstructed TEMT images are shown in Figure 2. Figure 2A displays a TEMT reconstruction of an OsO4treated sample of PB22P2VP29PtBMA49110. Only the PB phase is shown, composed of perforated lamellae with a thickness of 22 ± 2 nm (Table 2, determined from Figure 2D). Again, regular perforations with a tetragonal arrangement and a radius of 10 ± 1 nm are found, and a different projection in Figure 2C allows for an accurate determination of the distance between individual features (65 ± 6 nm), confirming the size observed in the TEM micrographs in Figure 1 (50−60 nm) within the experimental error. A schematic depiction of the proposed bulk morphology is also shown in Figure 3. The overall long period of the lamellar structure according to these reconstructions is 59 ± 5 nm. The fact that the in-plane spacing (distance between perforations) is larger if compared to the spacing in between the PB layers has already been observed for a hexagonally perforated lamellae (HPL) phase in polystyreneblock-polyisoprene (PS-b-PI) diblock copolymers and was 7959


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perforated lamellae of P2VP (12 nm) and a not-perforated lamella of PtBMA (18 nm, the values were deduced from TEMT results in Figure 2). Both the PB and the P2VP domains show perforations of 10 nm radius with an overall tetragonal packing. These structural features are formed by the block with the highest volume fraction, PtBMA, which protrudes through both the other compartments, thereby connecting different PtBMA phases. The long period of the bulk structure was determined to be around 60 nm and the average distance between neighboring perforations is 65 nm (top row, Figure 3). According to our structural model (TEMT), the PtBMA compartment has a volume fraction of 41 ± 8%, which is in relatively good agreement with the data obtained via 1H NMR (49%). Figure 3 (lower part) also shows a side-view of the PB22P2VP29PtBMA49110 bulk structure: the PtBMA perforations lead to the formation of an additional PB-PtBMA interface, which, according to the block sequence (BVT), is not necessarily formed. This can be tentatively explained by the incompatibilities between the three different segments: here, the interaction parameter for the PB-P2VP interface is with χBV = 0.32 relatively large if compared to P2VP-PtBMA (χVT = 0.13−0.26) and PB-PtBMA (χBT = 0.007).31,32 Hence, the generation of a PB-PtBMA interface is likely to be more favorable than an alternative scenario, where the perforations protruding through the PB lamella would consist of a PtBMA “core” and a P2VP “shell” (although a definitive conclusion cannot be drawn from the TEMT data presented here). In that way, a noncontinuous “coating” of the PB lamellae with P2VP would be formed, which we already found for cylindrical morphologies of different BVT compositions, where PB formed a cylindrical “core” and the P2VP domains existed either as localized spheres with ∼10 nm size or as double-helical strands.19 The formation of an additional interface between A and C block of ABC triblock terpolymers has also been reported for polystyrene-block-polybutadiene-block-poly(methyl methacrylate) (SBM), where the middle block, PB, formed a helix around the central PS cylinder, thus generating an additional PS−PMMA interface.17 Further, this has also been observed for the solution self-assembly of BVT triblock terpolymers in acetone, a nonsolvent for the PB block.20 Here, multicompartment-core micelles adopting a spheres-onsphere morphology were found, although P2VP was expected to be molecularly soluble in acetone. The SAXS pattern from the corresponding PB22P2VP29PtBMA49110 films is shown in Figure 4, displaying a lamellar pattern (the inset depicts the scattering pattern observed at the detector). We assigned the following reflections, using the black numbers in Figure 4 with the relative positions 1: 2: 3: 4 for the [100], [200], [300], [400], and [500] reflections of a lamellar pattern. The respective qvalues are 0.079 nm−1 [100], 0.156 nm−1 [200], 0.228 nm−1 [300], 0.289 nm−1 [400] (very weak), and 0.355 nm−1 [500]. According to the position of the [200] signal, a d-spacing of 80 ± 5 nm could be calculated for the lamellar bulk morphology, being larger than the thickness as observed by TEMT (∼60 nm) and slightly smaller if compared to that of TEM (87 nm). Although the scattering pattern obtained shows an anisotropy, comparable values for the d-spacing were obtained from different sectors according to the [200] reflection (e.g., lower right and upper left quadrant). Similar bulk morphologies (hexagonally perforated lamellae (HPL)) have been reported for polyisoprene-block-polystyrene

Figure 3. proposed bulk morphology for PB22P2VP29PtBMA49110, according to TEMT measurements (top); side-view of the bulk structure, highlighting the additional PB-PtBMA interface which is created and showing two possible triblock terpolymer chain arrangements (bottom).

ascribed to an ABC stacking sequence of the perforated layers.30 In the case reported here, however, Figure 2C hints toward a stacking of the PB layers so that the positions of the PtBMA perforations match. The thickness of the perforated P2VP lamellae (12 ± 3 nm) is revealed in Figure 2B, displaying a reconstructed TEMT image from PB22P2VP29PtBMA49110 after staining with iodine (only the P2VP phase is shown). The two red lines in the figure highlight two P2VP domains, where the PB lamella is supposed to be located in between. The thickness deduced for the PB compartment, as determined from Figure 2B (20 ± 5 nm, the rather large error here can mainly be ascribed to the rather irregular surface of the P2VP lamellae as well as to a certain distortion occurring during the microtoming process), is in good agreement with the domain size deduced from OsO4 staining (22 ± 2 nm). This also clearly demonstrates that the structural parameters observed in the TEM micrographs (see Figure 1) from very thin (30−80 nm) specimens are too large, most probably due to the distortion of the microdomains during sample preparation. The films used in the TEMT measurements, on the other hand, typically exhibit a thickness of 200 nm. The thickness of the PtBMA domains (18 nm) can be calculated using the values for PB and P2VP and for the long period according to Figure 2B, 62 ± 5 nm. The relatively thin PtBMA lamellae, if compared to the volume fraction (49%), can be explained in two following ways: first, the observed 18 nm do not include the PtBMA volume which is lost through the perforation of both the PB and the P2VP phase and, second, it is known that polymethacrylates undergo significant volume shrinkage during electron microscopy.29 TEMT clearly confirms the presence of a perforated lamellar bulk structure in case of PB22P2VP29PtBMA49110 and we assume a tetragonal pattern formed by the individual perforations according to Figure 2A and 2C. If both PB (OsO4) and P2VP (iodine) phases are stained, the lamellae in TEMT should appear thicker (i.e., the combined thickness of the PB and the P2VP domains), as depicted in Figure 2E. We therefore propose the following bulk morphology according to the presented TEMT results (Figure 3): PB22P2VP29PtBMA49110 exhibits a lamellar structure (ll), comprising a central perforated PB lamella of 20 nm thickness, followed by two adjacent 7960


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render cross-linked perforated lamellar sheets with a thickness of roughly 20 nm and tetragonally arranged perforations. The whole particles would be covered by a thin layer of P2VP, possibly providing solubility in aqueous media at low pHvalues. The interior of the perforations, on the other hand, would be covered by PtBMA. After hydrolysis to PMAA, this could lead to a pH-dependent permeability of the porous sheets, turning them into (eventually) nanometer-sized supramolecular membrane layers. We therefore cocasted films of PB22P2VP29PtBMA49110 with additional 10% of cross-linker, as has been described earlier for different samples of the BVT series.19 The addition of TPO of up to 20% (respective to the weight fraction of the PB domains) did not show any influence on the bulk morphology. After film casting and annealing, the sample was subjected to UV induced cross-linking for 1 h. Afterward, swelling of the astreated samples in nonselective solvents like THF or dioxane indicated a successful cross-linking. After at least 12 h of swelling in THF, the film was subjected to sonication to break up the cross-linked PB lamellae into soluble fragments. Typically, the sample vial was immersed in water to prevent heating up of the dispersion and was sonicated at 20% amplitude for one second, followed by a break of four seconds. The whole process is depicted in Scheme 2. After different sonication times, samples were taken with a syringe for dynamic light scattering (DLS) and TEM experiments. The corresponding TEM micrographs and DLS CONTIN plots for 5 and 25 min sonication time are shown in Figure 5. After 60 s of sonication (60 × 1 s) the resulting particles, visible by the naked eye, immediately sediment, if the sample is not continuously stirred. If the sonication time is increased to 5 min, temporarily stable dispersions are found, with a bimodal size distribution in DLS (Rh,app = 173 + 976 nm, −○−). When the corresponding particles were deposited onto carboncoated TEM grids and imaged, perforated sheets of several μm length and/or width can be seen (Figure 5B). The tetragonal packing of the perforations within the PB lamellae is evident. The P2VP shell covers the perforated PB lamellae whereas the PtBMA corona is not visible due to degradation through the incident electron beam. Rarely, some cylindrical structures can be also found, which we tentatively attribute to morphological defects at, e.g., the sample edges (Figure 5C). For even longer sonication times (25 min), stable dispersions were obtained with hydrodynamic radii of Rh,app = 105 + 473 nm (−□−). The corresponding TEM micrograph is shown in Figure 5D. Clearly, smaller fragments of the cross-linked perforated PB lamellae with dimensions on the order of 200−

Figure 4. SAXS pattern of a bulk specimen of PB22P2VP29PtBMA49110, the relative reflex positions are indicated by integer numbers.

(PI-b-PS) diblock copolymers in the bulk30 or for PS-b-P2VPb-PtBMA triblock terpolymers in thin films.33 In both cases, the perforations exhibited a hexagonal arrangement and in the case of PS-b-P2VP-b-PtBMA, the formation of the HPL phase was explained through differences in the wetting behavior of the three blocks in combination with long-term solvent annealing of the films. Tetragonally perforated structures, on the other hand, have already been observed for the self-assembly of poly(propylene oxide)-polyphenylene (AB)34 or polyethylene− oligophenylene−poly(ethylene oxide) (ABC)35 rod−coil molecules in the bulk state, as evidenced by a combination of X-ray scattering and differential scanning calorimetry. Preparation of Core-Cross-Linked Perforated Sheets. The selective cross-linking of individual domains within block copolymer nanostructures, followed by sonication-assisted dissolution in nonselective solvents represents a facile and attractive approach for the preparation of soft, core-cross-linked nanoparticles of different shapes. This has been demonstrated for Janus particles of different morphologies36,37 and also for compartmentalized cylinders from BVT triblock terpolymers.21 In the latter case, Lucirin TPO, an UV-photoinitiator, has been employed that decomposes at wavelengths of 350−370 nm. This prevents any depolymerization of the PtBMA compartment, which would occur at lower wavelengths. For PB22P2VP29PtBMA49110, this approach seemed quite attractive, as a selective cross-linking of the PB domain would

Scheme 2. Cross-Linking of Bulk Films of PB22P2VP29PtBMA49110 Using Lucirin TPO as an UV-Photoinitiator, Followed by Sonication-Assisted Dissolution into Core-Cross-Linked Sheets

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AUTHOR INFORMATION

Corresponding Author

*E-mail: (F.H.S.) [email protected]; (H.J.) hjinnai@ cstf.kyushu-u.ac.jp. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Volker Abetz for fruitful discussions and Dr. Holger Schmalz for help during the polymer synthesis. We also thank Sabine Wunder for performing SEC measurements, Dr. Jiayin Yuan for some TEM measurements, and Melanie Förtsch for help with the microtome cutting of the samples. The crosslinking agent, Lucirin-TPO, was kindly provided by BASF. FHS is grateful for a fellowship from the Verband der Chemischen Industrie (VCI) and to the Thuringian Ministry for Education, Science and Culture (TMBWK, Grant No. B515-10065, ChaPoNano, Grant No. B514-09051, NanoConSens, and Grant No. B515-11028, SWAXS-JCSM) for financial support. H.J. is grateful to the Ministry of Education, Science, Sports, and Culture through Grants-in-Aid No. 24310092.

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Figure 5. DLS CONTIN plots for core-cross-linked PB22P2VP29PtBMA49110 particles after 5 min (Rh,app = 173 + 976 nm, −○−), and 25 min of sonication time (Rh,app = 105 + 473 nm, −□−, A); TEM micrographs of core-cross-linked perforated lamellae after 5 min (B, C) and 25 min sonication time (D).

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500 nm are present. Even more, a thin dark ring can be observed inside the tetragonally arranged pores, which we tentatively assign to a thin P2VP shell of the structures.

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REFERENCES

CONCLUSION

An intriguing new bulk morphology was found for a PB22P2VP29PtBMA49110 triblock terpolymer: a tetragonally perforated lamellar structure, comprising a central perforated PB lamella, flanked by two perforated P2VP layers, and finally (not perforated) lamellae of PtBMA. The latter segment protrudes through both the PB and the P2VP domains and forms the observed perforations with a diameter of roughly 20 nm. In that way, an additional interface is generated between PB and PtBMA, presumably due to the high incompatibility between PB and P2VP as well as the rather low interaction parameter for the newly formed PB/PtBMA boundary. The nature of the bulk morphology was verified using a combination of SAXS, TEM, and transmission electron microtomography (TEMT). Further, the tetragonal ordering of the perforations was verified by first studies on the cross-linking of the central PB lamellae and the subsequent fabrication of core-cross-linked perforated sheets via sonication-assisted dissolution in THF. The so-generated porous particles represent very interesting candidates for interfacially active and soft multicompartment particles. Therefore, future studies will deal with the transformation of the PtBMA corona into pH-sensitive poly(methacrylic acid) (PMAA), in an analogue way to recently published studies on core−shell-corona micelles.22 In addition, the control over the final particle size (and, alongside, the particle size distribution) needs to be optimized. 7962


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Tetragonally Perforated Lamellae of Polybutadiene ... - ACS Publications

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