Fully Liquid-Crystalline ABA Triblock Copolymer of Fluorinated Side

Article pubs.acs.org/Macromolecules

Fully Liquid-Crystalline ABA Triblock Copolymer of Fluorinated Side-Chain Liquid-Crystalline A Block and Main-Chain Liquid-Crystalline B Block: Higher Order Structure in Bulk and Thin Film States Ryohei Ishige,*,† Noboru Ohta,§ Hiroki Ogawa,§ Masatoshi Tokita,∥ and Atsushi Takahara*,†,‡ †

Institute for Materials Chemistry and Engineering and ‡Graduate School of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan § Japan Synchrotron Radiation Research Institute (JASRI/SPring-8), Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan ∥ Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8552, Japan S Supporting Information *

ABSTRACT: Fully liquid-crystalline (LC) ABA-type triblock copolymers were synthesized by atom transfer radical polymerization; the A block was a fluorinated side-chain LC polymer, PFA-C8, and the B block was a main-chain LC polyester, BB-5(3-Me). The volume fraction of the A block (φA) was 0.11−0.70, and the B block had a constant molecular weight. Nanometer-scale segregated structures in the bulk and thin film states were investigated by synchrotron X-ray diffraction (XRD) in transmission and grazing-incidence (GI) geometries to examine the effect of competition between the LC orientation and polymer chain dimensions on the morphology. When φA is 0.11, matching of the mesogen orientation in the A and B blocks dominates the main-chain orientation, whereas when φA exceeds 0.28, matching of the lateral dimensions of the A and B blocks dominates the mesogen orientation, although all the polymers showed lamellar structure before isotropization of BB-5(3-Me). GI-XRD revealed that the lamellar structure in the thin film with φA = 0.70 was completely perpendicular to the Si substrate without surface modification or solvent annealing.

1. INTRODUCTION

Regarding the mechanism of segregation or microphase separation of BCPs, liquid crystallinity is expected to strongly affect the morphology through coupling of the orientation between mesogens and the main chain because a balance between the interfacial energy and conformational entropy dominates the morphology. If BCPs with a side-chain nematic LC polymer (SCLCP) block and an amorphous block form a segregated domain, the mesogen tends to be aligned along the interface in the nematic phase, and the segregated domain can be oriented along the nematic director.11,12,14,18 This phenomenon is known as the anchoring effect.11,14,18,19 When the SCLCP block in a BCP forms a smectic phase, two different orientations are observed depending on the degree of decoupling between the mobility of mesogens and the main-chain (backbone) conformation: main chains of the SCLCP are aligned perpendicular to the interface of the domains for a shorter spacer, that is, a lower degree of decoupling, whereas the main chains are aligned parallel to the interface for a longer spacer, that is, a higher degree of decoupling.20 If a main-chain LC polymer (MCLCP) is

Incorporation of liquid-crystalline (LC) components into a block copolymer (BCP) is not only useful for controlling the orientation of the nanometer-scale segregated structure but also scientifically interesting for understanding the effect of the main-chain orientation (conformational anisotropy) on the morphology of the segregated domain structure. Many LC-BCPs consisting of an LC block and an amorphous block have been reported, as introduced below. In terms of application of BCPs, the segregated structures (microphase-separated structures in the case of liquid−liquid phases) formed by BCPs have attracted much attention for use as a template in nanofabrication,1−3 in particular for lithography,4−6 and it is essential to control the orientation of the segregated structure. For lamellar and cylindrical structures, vertical orientation, in which the lamellar plane or cylinder axis is aligned perpendicular to the film surface, is preferable. Several techniques have been reported to control the orientation, for example, solvent annealing and surface modification of the substrate.7−10 In particular, LC-BCPs have been attractive because the LC segment can be aligned by an external field, for example, a magnetic or electric field, or shear flow.11−17 © XXXX American Chemical Society

Received: April 24, 2016 Revised: July 12, 2016

A

DOI: 10.1021/acs.macromol.6b00855 Macromolecules XXXX, XXX, XXX−XXX

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2. EXPERIMENTAL SECTION

incorporated into a BCP and segregated from an amorphous block, two different orientations are also observed depending on the type of LC phase. When the MCLCP forms a nematic phase, the main chain and mesogen are aligned parallel to the interface, in an effect similar to the anchoring effect in SCLCPs.19 When the MCLCP forms a smectic phase, a lamellar morphology is observed over a wide range of volume fractions, and the main chain is aligned perpendicular to the interface.21−24 Note that the distributions of the lamellar thickness and long period were very narrow, even though the MCLCP block was polymerized by a traditional polycondensation method and had a large polydispersity of close to 2. The mechanism of lamellar formation with a narrow distribution is perhaps explained as follows. First, the smectic layer grows epitaxially from the interface and is aligned parallel to the interface, resulting in the lamellar structure. Small-angle X-ray scattering (SAXS) analysis revealed that the main chain was folded in the lamellar domain.21−24 Folding is assumed to adjust the sequence length between folds and to decrease the defects in the smectic layer such as steps and kinks, and to decrease the energetic cost, resulting in a homogeneous and ordered lamellar structure with a narrow distribution. If a smectic MCLCP and a smectic SCLCP are covalently bound in a BCP and segregated into each domain, frustration will arise between the packing of the main chain in the domain and the orientation of the mesogens because SCLCPs usually adopt an oblate conformation, whereas MCLCPs adopt a prolate conformation. A few BCPs consisting of MCLCPs and SCLCPs have been reported, and those LC structures and phase transitions were individually observed by wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC), respectively. The results indicated segregation of the MCLCP and SCLCP blocks, although a clear segregated domain structure was not observed.25,26 In this work, an ABA-type LC-BCP was synthesized from an A block consisting of a fluorinated SCLCP, PFA-C8, and a B block consisting of an MCLCP, BB-5(3-Me), and the effect of the frustration on the morphology and LC structure is discussed on the basis of structural analysis by SAXS and WAXD using a synchrotron X-ray source. Five samples of the ABA triblock copolymer were prepared, where the volume fraction of the A block (PFA-C8), φA, varies, and the length of the B block [BB5(3-Me)] was constant. PFA-C8 consists of a polyacrylate backbone and perfluorooctyl (Rf) mesogens attached via a −CH2CH2− spacer and shows a tilted hexatic phase (Sm-F or Sm-I), where Rf mesogens form a bilayer structure.27−29 The Rf mesogen has two advantageous features for use in a BCP segment, which derive from the low surface free energy: immiscibility with nonfluorinated polymers and a surface orientation effect.30−33 Several BCPs bearing Rf groups have been reported to date, and the surface orientation of the Rf group was found to be effective to induce orientation of the segregated domain.34−38 On the other hand, BB-5(3-Me) consists of a bibenzoate (BB) mesogen and 3-methylpentane spacer and shows a Sm-CA phase.39−41 The 3-methylpentane spacer is predisposed to induce folding in the smectic structure.42−44 For these ABA triblock copolymers, DSC measurements, transmitted SAXS and WAXD at variable temperature, and grazing incidence (GI) SAXS and GI-WAXD of spin-coated thin films were performed to investigate the nanometer-scale segregated structure and LC structure. Competing effects between the main-chain dimensions and the mesogenic orientation in the domains are discussed on the basis of the structural analysis.

2.1. Materials. The LC ABA triblock copolymer PFA-C8-b-BB-5(3Me)-b-PFA-C8 (Scheme 1) was synthesized by atom transfer radical

Scheme 1. Chemical Structure of Wholly Liquid-Crystalline ABA Triblock Copolymer PFA-C8-b-BB-5(3-Me)-b-PFA-C8

polymerization (ATRP) in hexafluoro-2-propanol solution, where 2-(perfluorooctyl)ethyl acrylate (FA-C8) monomer was polymerized from a BB-5(3-Me) macroinitiator. The synthetic procedure and characterization results are provided in the Supporting Information. The degree of polymerization (DP) of the BB-5(3-Me) initiator was evaluated as 31 from the 1H NMR spectrum in CDCl3 solution (AVANCE-III 400, Bruker Co.). The molar fraction of FA-C8 units in the triblock copolymer was evaluated from the ratio of the number of protons in −OCOCH2− (methylene protons neighboring the ester bond) in FA-C8 to that of the BB-5(3-Me) unit. 1H NMR measurements of the triblock copolymer were conducted using a mixed solvent of CDCl3 and AK-225 (Asahi Glass Co.) at a 1/1 weight ratio. Volume fractions of FA-C8, φA, were estimated from the molar fraction using the density of PFA-C8 homopolymer and BB-5(3-Me) homopolymer (1.23 g cm−3).22 The density of PFA-C8 was estimated at 1.84 g cm−3 from the lattice volume of the hexatic phase, where two repeating units were accommodated.29 In this paper, five samples in which the volume fraction of PFA-C8 ranged from 0.11 to 0.70 were prepared and are designated as P1, P2, P3, P4, and P5, as listed in Table 1.

Table 1. Thermodynamic Parameters of PFA-C8 and BB-5(3Me) in ABA Triblock Copolymer and Homopolymer sample

wAa

φAb

BB-5(3-Me) P1 P2 P3 P4 P5 PFA-C8

0 0.16 0.38 0.48 0.64 0.78 1

0.00 0.11 0.28 0.37 0.54 0.70 1.00

ΔHA ΔHB (kJ mol−1) (kJ mol−1) 0.00 0.31 2.08 2.89 4.29 4.95 7.4

3.50 2.87 2.02 1.64 1.09 0.61

Tgc (°C) 24.4 29.9 30.2 30.9 32 32

Ti,Ad (°C) 67.5 66.5 67.7 71.9 71.9 79

Ti,Be (°C) 149 150 153 153 149 149

a

Weight fraction of A block estimated by 1H NMR. bVolume fraction of A block estimated by 1H NMR. cGlass transition temperature of B-block. dIsotropization of A block. eIsotropization of B block. 2.2. Measurements. DSC measurements were conducted using PerkinElmer Pyris 1 DSC equipment at a scanning rate of 10 °C min−1 under a flow of dry nitrogen. Transmission electron microscopy (TEM) observations were conducted using a JEM-2010F microscope (JEOL, Ltd., Japan) operated at 200 kV. The samples were ultramicrotomed into 65−80 nm ultrathin films at −165 °C using an EM UC7 microtome with a cryoultramicrotomy chamber (EM FC7, Leica, Germany) and subsequently stained with RuO4 using a vacuum electron staining chamber (VSC1R1H, Filgen Co., Ltd., Japan). Simultaneous SAXS and WAXD measurements were performed at the BL40B2 beamline of SPring-8 (Japan Synchrotron Radiation Research Institute, Hyogo, Japan). Each two-dimensional (2-D) SAXS image was captured on a 3000 × 3000 pixel imaging plate (IP) with a pixel size of 100 × 100 μm2 (BAS-SR2025, Fujifilm Co., Tokyo, Japan). The IP was installed in an R-AXIS IV++ system (Rigaku Co., Tokyo, Japan). Each 2-D WAXD image was captured on a 1032 × 1032 pixel complementary metal−oxide−semiconductor flat panel detector (C9728DK, Hamamatsu Photonics K. K., Japan). The X-ray wavelength λ was 0.100 nm. The camera length was 2227 mm for SAXS and B


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Macromolecules 70.8 mm for WAXD. The camera length was calibrated using a silver behenate standard for SAXS and a CeO2 standard for WAXD. The sample was placed in a hot and cold stage with a poly(ether ether ketone) window (HCS302, Instec Inc.), and the temperature was controlled by an mk2000 temperature controller (Instec Inc., USA). The exposure time was 4.0 s for SAXS and 2.5 s for WAXD. The X-ray beam was ∼200 μm in diameter. GI-SAXS and GI-WAXD measurements of the thin films were performed at the BL03XU beamline in the SPring-8 facility. The X-ray wavelength λ was 0.100 nm. The camera length was 452 mm for GI-WAXD and 2218 mm for GI-SAXS measurements. The camera length was calibrated using a silver behenate standard. The 2-D diffraction images were captured with the same system as that for the transmitted SAXS at BL40B2. The exposure time for GI-SAXS was 1.0 s with X-rays through an attenuator of 30 μm Mo foil, and that for GI-WAXD was 10 s with X-rays through an attenuator of 10 μm Au foil. Thin films used for the measurements were prepared on disk-shaped silicon (111) wafers 25.4 mm in diameter. The Si disks were washed in an ultrasonic bath using acetone and then distilled water. Thin films were deposited on a Si wafer by spin-coating at 2000 rpm for 30 s from 1.0 wt % polymer solution in a mixed solvent of AK-225 and chloroform (volume ratio = 7/3). In this condition, the sample thickness is 100−200 nm. The spin-coated thin films were thermally annealed at 100 °C for 12 h. The samples were mounted in a homemade vacuum cell with a polyimide (Kapton) window to avoid sample oxidation and introduction of scattering effects from air. The X-ray beam size was 150 μm (horizontal) × 50 μm (vertical). The beam divergence is 12.3 μrad in horizontal and 1.1 μrad in vertical. The configuration of this beamline is presented in detail elsewhere.45,46

3. RESULTS AND DISCUSSION 3.1. Phase Transition Behavior of Each LiquidCrystalline Block. The phase transition behaviors of PFA-C8 and BB-5(3-Me) in the triblock copolymer are expected to depend on the solubility (degree of segregation) between the two blocks. The melting transition enthalpy (ΔH) of each LCP block was evaluated by DSC and compared with that of the homopolymer to evaluate the degree of liquid crystallinity (DLC). DSC thermograms of all five samples are presented in Figure 1 (DSC thermograms for the homopolymers are provided in the Supporting Information). All the samples clearly show a heat-flow step around 35 °C and two endothermic peaks around 70 and 150 °C during heating. The step and two endothermic peaks in the heating thermograms are assigned to the glass transition of the BB-5(3-Me) block and the melting points of the hexatic smectic phase of the PFA-C8 block and the Sm-CA phase of the BB-5(3-Me) block, respectively, in order of increasing temperature. In Table 1, the melting temperatures, Ti,A and Ti,B, and ΔHA and ΔHB are summarized with the weight fraction wA and volume fraction φA of the A block estimated from 1H NMR spectra. The weight fraction of the A block (wA_DSC) estimated as the ratio of the ΔH value of the BCP to that of the corresponding homopolymer is plotted versus wA in Figure 2.

Figure 2. Weight fraction of (a) A block (wA_DSC) and (b) B block (wB_DSC) evaluated from isotropization transition enthalpies. (c) Degree of liquid crystallinity in entire sample.

From the dependence of wA_DSC on wA, the DLC of the PFA-C8 block is estimated as ∼80% in P2, P3, P4, and P5 and as 26% in P1. Further, the DLC of BB-5(3-Me) is larger than ∼85% in all samples. These result suggest that the PFA-C8 segment and BB-5(3-Me) segment are segregated from each LC domain, and the orientation of the main chain affects the DLC of PFA-C8, particularly in P1. Details of the structure are discussed in the next section.

Figure 1. DSC thermograms of P1, P2, P3, P4, and P5 during (a) second heating process and (b) second cooling process at a scanning rate of 10 °C min−1. Magnified thermograms of P1 are presented below and above in (a) and (b), respectively, for the visibility of the small signal. C


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Macromolecules 3.2. Periodic Lamellar Structure and Orientation of Smectic Structures in Each Segregated Domain. The morphologies of the segregated domain structure at ambient temperature in all the samples were observed using TEM. All the samples were pressed at 130 °C and annealed at 100 °C for 12 h before being ultramicrotomed. The TEM images are presented in Figure 3. In all the images, black stripe patterns are observed,

not large. The grain size and morphological features are discussed more quantitatively in section 3.3.1 using SAXS analysis. The relation between the orientations of the lamellar domain and the smectic structures of each block was investigated by SAXS/WAXD measurements. Figure 4 shows typical SAXS/ WAXD patterns of oriented samples of P1, P4, and P5 at 25 °C. The oriented fiber of P1 and the oriented film of P4 and P5 were spun and sheared, respectively, at 130 °C and annealed at 100 °C for 12 h (P4 and P5 were too low viscosity to be spun from the molten state). The spinning direction or velocity gradient direction, ∇v, for shearing direction was set to the vertical direction. All the samples showed sharp diffractions appearing at the same interval on the meridian in the SAXS pattern, indicating that the A and B blocks are strongly segregated to form periodic lamellar structure, as observed in the TEM images. The fiber of P1 shows higher orientation than the sheared sample of P4 and P5. The long periods of P1, P4, and P5 are 30.1, 32.6, and 28.9 nm, respectively (white indicators in the SAXS patterns). In the higher q region, a sharp layer diffraction of PFA-C8 (∼3.2 nm, green indicator, hereafter [100] diffraction) is observed on the equator for P4 and P5, whereas it appears on the meridian for P1. Sharp diffraction on the meridian in each WAXD pattern (0.81−0.84 nm, white indicators in lower row in Figure 4) is assigned to second-order layer diffraction of BB-5(3-Me), and broad scattering deriving from the distance between neighboring biphenyl mesogens in the smectic layer is observed around the equator (∼0.45 nm, white indicators). Particularly in the pattern of P1, the scattering is clearly split across the equator, which is characteristic of zigzag alignment of the mesogens in the Sm-CA structure.39−41 In the WAXD patterns of P4 and P5, third- and fifth-order layer diffractions of PFA-C8 appear around the equator (1.1 and 0.65 nm, [300] and [500], light green indicators),29 which is consistent with the direction of the first-order

Figure 3. TEM images of ultrathin films of P1, P2, P3, P4, and P5 stained with RuO4 at ambient temperature. Scale bar is 100 nm.

which correspond to BB-5(3-Me) domains stained with RuO4. These images are clear evidence that all the polymers form nanometer-scale lamellar domains. In these images, one can see that the lamellae exhibit intense undulation, and the correlation length in the lateral direction is short. Particularly for P5, it can be observed that separated lamellae of BB-5(3-Me) (black stripes) are surrounded by the PFA-C8 domain (white matrix). These TEM images suggest that the grain size of the lamellar structure is

Figure 4. Simultaneous SAXS patterns (upper row) and WAXD patterns (lower row) for oriented samples of (a) P1, (b) P4, and (c) P5 taken at 25 °C. Spinning direction or velocity gradient direction, ∇v, for shearing is along the meridian. The left and right side of SAXS image is log-scale and linear scale, respectively, in (a) and (b), and the left and right side is different linear scale in (c). Color scale bar is inset in each SAXS and WAXD pattern. White and light green indicators in SAXS and WAXD patterns represent the spacing of lamellar structure and smectic layer of PFA-C8 (light green) and that of BB-5(3-Me) (white). D


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3.3. Structural Analysis of Segregated Structure in Bulk State. 3.3.1. Evaluation of Domain Sizes by SAXS Intensity Analysis. SAXS intensity analysis of the powder samples was performed using the paracrystal model with a finite grain size, and the lamellar thickness of each domain at 25, 100, and 170 °C was evaluated as in the followings.47−50 All the samples were cooled from the isotropic melting point (170 °C) to 100 °C at 10 °C min−1 and then annealed at 100 °C for 12 h. The sectoraveraged SAXS intensity for a powder sample of the lamellar paracrystal, I(q), is calculated as

layer diffraction in the SAXS patterns ([100], light green indicators). PFA-C8 exhibits [001] diffraction near the meridian in P4 and P5 but on the equator in P1, where it overlaps the scattering of BB-5(3-Me). The [001] diffraction corresponds to intermolecular lateral distance between Rf mesogens in the same layer. It is interesting that the orientation of the smectic layer of PFAC8 in the segregated domain depends on the fraction. On the other hand, that of BB-5(3-Me) in the lamellar domain shows the same orientation, in which the smectic layer is parallel to the interface of the two domains. In P1, the smectic layer of PFA-C8 is parallel to the interface between the two blocks and also to the smectic layer of BB-5(3-Me), and in the other polymers, it is perpendicular to the interface. The orientation in P1 is related to the low DLC estimated from ΔHA. The details are discussed in section 3.3.2. The second-order layer diffraction of BB-5(3-Me) in P1, P4, and P5 remains around the meridian at 100 °C, where the smectic phase of PFA-C8 melts into the isotropic phase, and only the broad scattering from the intermolecular distance of Rf mesogens is observed, as shown in Figure 5 (WAXD film patterns of P5 at 25 and 100 °C are provided in the Supporting Information). The orientation of the Sm-CA structure in the B block domain remains after isotropization of PFA-C8. The segregated lamellar structures also maintain the same orientation as in the SAXS patterns. At 170 °C, where both BB-5(3-Me) and PFA-C8 are in the isotropic phase, no sharp diffraction is observed in the WAXD region. In the SAXS patterns, the diffractions from segregated lamellar structure in P4 and P5 maintained their orientation (a SAXS azimuthal intensity profile of P5 is provided in the Supporting Information), whereas in P1, the diffraction that is concentrated on the meridian at 100 °C changes into an isotropic ring shape (Figure 6). Considering the SAXS result for P1 and the fact that the volume fraction of PFA-C8 in P1 is 0.11, a morphological transition from lamellar to spherical structure can be supposed (details are discussed in section 3.3.1).

Iobs(q) ∝ ⟨I(q)⟩av =

∫0

π /2

I(q)

2π sin β dβ 2π

I(q) ∝ ⟨f (q)2 ⟩ − ⟨f (q)⟩2 + ⟨f (q)⟩2 Z(q)

(1)

where ⟨...⟩ and ⟨...⟩av represent a statistic averaged value and an averaged value in whole solid angle, respectively, and the lattice factor Z(q) is given by Z(q) = Zx(q)Zy(q)Zz(q) ≈ Zz(q) N−1

=N+2

⎛ 1 2 2 ⎞ ⎜ k g (q·D)2 ⎟ ⎠ 2 zz

∑ (N − k) cos(k q·D) exp⎝− k=1

N−1

=N+2

⎛ 1 2 2 2 ⎞ ⎜ k g D (qzD cos β)2 ⎟ ⎠ 2

∑ (N − k) cos(kqzD cos β) exp⎝− k=1

2

2

gzz = g = Δzz 2 /D2 , gzx = gzy ≈ 0, Δzz 2 = D = (D sin β cos φ

D sin β sin φ

∫0

∞

hzz(z)(z − D)2 dz

D cos β)t , q = (0 0 qz)

(2)

Herein the value of β is the angle between the lamellar normal and z direction, and Zx(q), Zy(q), and Zz(q) are the lattice factor in x, y, and z directions, respectively. The z direction is parallel to the detector direction, which is arbitrary for the 2D detector.

Figure 5. Simultaneous SAXS patterns (upper row) and WAXD patterns (lower row) for oriented samples of (a) P1, (b) P4, and (c) P5 taken at 100 °C. The left and right side of SAXS image is log scale and linear scale, respectively, in (a) and (b), and the left and right side is different linear scale in (c). E


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Figure 6. Simultaneous SAXS patterns (upper row) and WAXD patterns (lower row) for oriented samples of (a) P1, (b) P4, and (c) P5 taken at 170 °C. The left and right side of each SAXS image is log scale and linear scale, respectively.

It also should be noted that the values of gzx and gzy are neglected because the second kind disorder in the lateral direction of the lamellar stacking is very small. The form factor, f(q), in eq 1 is given by f (q) = f (qz) ∝ L

sin(qzL cos β /2) qzL cos β /2

πR2

However, it is impossible to distinguish which lamellar thickness is evaluated from the intensity analysis because of the Babinet principle.51 The values of L listed in Table 2 correspond to the thickness of the lamella having smaller standard deviation (details are provided in the Supporting Information). The volume fraction values evaluated by SAXS analysis and 1H NMR spectra were compared each other to decide which lamella is A domain. Consequently, the volume fraction of the PFA-C8 domain was identified as either L/D or 1 − L/D, whichever is the nearer to φA presented by dotted lines in Figure 8. The value of φA is the volume fraction of PFA-C8 block evaluated by 1H NMR and listed in Table 1. The values of L/D (φL) and 1 − L/D (1 − φL) at 25, 100, and 170 °C are plotted versus φA in Figure 8. The values of φL and 1 − φL can be visibly compared with φA in Figure 8 (see dotted line), and the volume fraction of the A block evaluated by SAXS, φA_SAXS, can be identified. In Figure 8a,b, φL or 1 − φL deviates remarkably from the φA values determined by 1H NMR, except for P1, although either φL or 1 − φL is consistent with the dotted line for an ideal system. This phenomenon allows us to suppose that some PFA-C8 segments are excluded from a lamellar grain. In fact, TEM observation indicated that lamellae of the B block were partially separated and surrounded by the A block domain, as mentioned with respect to the TEM images in section 3.2. Consequently, the volume fraction of PFA-C8 estimated from Figure 8 (φA_SAXS) tends to be smaller than the φA value (dotted lines in Figure 8). If the lateral dimension of the PFA-C8 domain is mismatched with that of the BB-5(3-Me) domain, some of the PFA-C8 segments will be excluded from the lamellar grain to the lateral grain boundary, forming the periodic lamellar structure (Figure 9). In ABA triblock copolymers, the edge of the lamellar stacking, that is, the grain boundary, usually corresponds to the A domain. Consequently, the excluded chains (A block) from a grain are incorporated at the edge of the neighboring grain and surround the B-block domain. In this situation, the volume fraction of the

J1(qzR sin β) qzR sin β

(3)

Here, the lamellar normal direction is parallel to the z-axis. The parameters qz, N, and D in eq 2 are the z component of the scattering vector q (|q| = 4π sin θ/λ, where 2θ is the scattering angle), the number of lamellae in a domain, and the averaged lamellar period, respectively. Further, h(z) in eq 2 is a Gaussian function representing the distribution of the long period, D. In eq 3, L and R are the single lamellar thickness and radius of the A or B domain, respectively, and are assumed to have a Gaussian distribution with a standard deviation σL. The lateral domain size, R, is fixed at N × D for the calculation. The observed and simulated SAXS intensity profiles at these temperatures are presented in Figure 7. All the profiles except that of P1 at 170 °C were well simulated by the paracrystal lamellar model. Parameters used for the simulation are presented in Table 2. The effect of smearing by instrumental broadening was small enough to be neglected in the intensity simulation (detailed explanations are provided in the Supporting Information). The result that the g values become large at 170 °C for P2, P4, and P5 indicates that the statistical distribution of the lamellar thickness of BB-5(3-Me) increases dramatically in the isotropic phase (170 °C) in these polymers. It should be noted that the lateral domain size, R, does not significantly affect the intensity profile around the diffraction peaks, and almost the same peak shape is obtained for the simulation without R (infinite size in lateral direction). The paracrystal analysis gives a lamellar thickness as well as the long period and their standard deviations, as in Table 2. F


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Figure 7. SAXS intensity profiles at 25 °C (blue), 100 °C (green), and 170 °C (red) are presented with calculated profiles (gray solid line): (a) P1 at 25 and 100 °C, (b) P1 at 170 °C, (c) P2, (d) P3, (e) P4, and (f) P5. Simulated profile for P1 at 170 °C was calculated using the PY model; other profiles were calculated using the lamellar paracrystal model presented in eqs 1−3. 1

H NMR. The mismatch of the interfacial area is caused mainly by the orientation mismatch between the Rf and BB mesogens. The mismatch will be resolved when PFA-C8 melts to the isotropic phase or the average number of folds in a chain of BB-5(3-Me), nf, increases. The fact that nf is ∼1 in P1, ∼2 in P2, P3, and P4, and ∼4 in P5 at 25 °C indicates that the mismatch is relatively large in P2, P3, and P4. The deviation between φA_SAXS and φA decreased after isotropization of PFA-C8 (at 100 °C) and became negligibly small at 170 °C, whereas the standard deviation of L, σL, increased dramatically, particularly in P2 and P5. For P1 at 170 °C, φA_SAXS was evaluated on the basis of a model where a spherical domain of PFA-C8 with a radius r is randomly distributed in the BB-5(3-Me) matrix. In this model, the profile was calculated as

Table 2. Parameters Used for SAXS Intensity Calculation in Figure 7 sample

φA

⟨N⟩a

Db (nm)

P1 P2 P3 P4 P5

0.11 0.28 0.37 0.54 0.70

2.7 5.4 5.8 4.0 3.9

30.0 24.2 26.5 29.8 26.1

P1 P2 P3 P4 P5

0.11 0.28 0.37 0.54 0.70

2.6 6.2 4.6 4.0 3.7

31.0 25.8 27.9 31.5 27.2

P1 P2 P3 P4 P5

0.11 0.28 0.37 0.54 0.70

8.0 4.4 4.0 3.4

25.2 25.6 27.8 31.5 32.7

Lc (nm) 25 °C 4.0 19.2 19.4 18.1 15.7 100 °C 4.2 19.1 7.9 18.2 16 170 °C 7.0 7.8 17.5 10.3

D−L (nm)

σLd (nm)

ge

φLf

26 5.0 7.1 11.7 10.4

2.6 1.0 1.0 1.0 0.9

0.24 0.057 0.060 0.075 0.042

0.13 0.79 0.73 0.61 0.60

26.8 6.7 20 13.3 11.2

2.0 0.9 1.5 1.0 1.0

0.240 0.055 0.053 0.063 0.044

0.13 0.74 0.28 0.58 0.59

18.6 20 14 22.4

1.3 1.5 2.0 2.2

0.135 0.055 0.085 0.140

0.098 0.27 0.28 0.55 0.31

I(q) = ⟨fs (q)2 ⟩ − ⟨fs (q)⟩2 + ⟨fs (q)⟩2 S(q)

(4)

The Percus−Yevick (PY) structure factor, S(q), is given by S(q) =

1 1 − ρC(q)

52,53

(5)

where ρ and q are the number density of the spheres and magnitude of the scattering vector |q|, respectively, and

a The number of lamellar stack in a grain. bLong period. cLamellar thickness. dStandard deviation of lamellar thickness. eSecond kind disorder of paracrystal model. fVolume fraction of either lamellar domain.

C(q) = − 4π α=

A block in a grain may be roughly approximated by (N + 1)/N × L/D. For P2, P3, P4, and P5 at 25 °C, the value of (N + 1)/N × L/D becomes 0.25, 0.32, 0.49, and 0.75, respectively, which are quite similar to φA = 0.28, 0.37, 0.54, and 0.70 evaluated by

∫0

2RHS

(1 + 2φ)2 , (1 − φ)4

(α + β(r /2RHS) + γ(r /2RHS)3 ) β = − 6φ

(1 + φ /2)2 , (1 − φ)4

γ=

sin(qr ) 2 r dr qr

φ (1 + 2φ)2 2 (1 − φ)2 (6)

where RHS and φ are the radius of the hard-sphere potential, which causes a repulsive force between spheres, and the G


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fraction of the PFA-C8 domain [≈ (4π/3)R3/(2RHS)3] was then evaluated as 9.8%, which is quite consistent with φA. 3.3.2. Chain Dimensions in Segregated Domains. The domain sizes (lamellar thicknesses or radii of a spherical domain) of the A and B blocks (LA and LB) at 25, 100, and 170 °C are plotted versus φA in parts a, b, and c of Figure 10, respectively.

Figure 8. Volume fractions of φL (red) and 1 − φL (blue) at 25, 100, and 170 °C evaluated by SAXS analyses versus φA values determined by 1H NMR.

Figure 10. Domain sizes LA (red) and LB (blue) versus φA at (a) 25, (b) 100, and (c) 170 °C. Extended chain length of PFA-C8 estimated from Mn is presented as closed triangles (gray solid line).

The SAXS/WAXD patterns of the oriented samples at 25 °C suggested that the main chains of both blocks are aligned perpendicular to the lamellar interface, except for P1. The layer of the smectic phase of PFA-C8 in P1 and the main chain of PFA-C8 are parallel to the interface. Then, except for P1, the chain dimensions can be directly compared to the evaluated lamellar thickness. The estimated LA values of P2, P3, P4, and P5 are slightly larger than the contour length of the PFA-C8 block estimated from the DP and unit length (DP = [10000 × wA/(1 − wA)]/518, extended unit length = 0.25 nm), even though the φA value implies that PFA-C8 chains are partially excluded from the lamellar domain. Therefore, the main chain of PFA-C8 in P2, P3, and P4 is extended (not folded) in the lamellar domain. The fact that LA is larger than the contour length can be attributed to the effect of polydispersity54,55 and orientation fluctuation at the end of the chain. For P5, LA is smaller than the contour length, implying that the main chain of PFA-C8 is partially folded back in the domain to increase the interfacial area. LA does not change greatly with temperature in P2, P3, and P4, whereas the domain size increases dramatically in P1 and P5 after Ti,B. In P1, this

Figure 9. Schematic model of grain boundary. Main chains of PFA-C8 near lateral grain boundary are excluded from a lamellar domain and incorporated into the edge of the neighboring grain.

volume fraction of a hypothetical hard sphere with radius RHS, respectively. Further, the form factor of a sphere, fs(q), is given by fs (q) = V

sin(qR ) − qR cos(qR ) (qR )3

(7)

where R and V are the averaged radius and volume of the spherical domain, respectively. In eq 4, the distribution of R is represented by a Gaussian function with a standard deviation σs. The parameters of the PY model, RHS, φ, R, and σR, are 12.6 nm, 0.37, 7.2 nm, and 1.6 nm, respectively. The volume H


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favorable for obtaining a perpendicular orientation of the lamellae in the film because the Rf mesogen has a strong tendency toward alignment perpendicular to the surface29−31 and the BB mesogen tends toward alignment parallel to the Si substrate (in-plane and out-of-plane GI-WAXD profiles for the BB-5(3-Me) homopolymer are provided in the Supporting Information). The orientation tendencies of the two mesogens are favorable to perpendicularly oriented lamellar structure, particularly in the thin film because of the large interface areas. First, the ABA triblock copolymer presents clear lamellar structure, and the main chains of both PFA-C8 and BB-5(3-Me) are perpendicularly oriented to the lamellar interface. Second, the strong orientation tendency of Rf and BB mesogens at the top surface and bottom interface of the substrate will cause the main chains to be aligned parallel to the surface, resulting in a perpendicular lamellar domain (Figure 11b). The effect of the orientation of the mesogens at the film/air and film/substrate interfaces on the domain orientation was investigated for thin films of P2, P3, and P5. Thin films were prepared by spin-coating on a Si wafer, and the segregated structure in these films was analyzed by GI-WAXD and GI-SAXS. Figure 12a,b shows GI-WAXD and GI-SAXS patterns taken with X-ray incident angles (αi) of 0.08° and 0.16°, respectively. These patterns were not corrected by the Lorentz factor. The former angle is smaller than the critical angle of the film, αc,f, and the X-ray beam proceeds along the surface as an evanescent wave, whereas the latter angle is larger than αc,f and close to the critical angle of the Si substrate, αc,Si, and the X-ray beam penetrates the film and is reflected at the film/Si interface. First, the results for αi = 0.16° are summarized (Figure 12b). In the small-angle region of the GI-SAXS patterns, a series of clear diffractions are observed on the meridian, and broad diffraction is observed on the horizontal line at α ∼ 0.12° for P2 and P3, whereas broad diffraction is observed only on the horizontal line for P5. If the film surface is smooth, the angle of 0.12° corresponds to the αc,f value of P2, P3, and P5. The d spacings of the meridional and equatorial diffractions are 20 and 25 nm in P2 and 22 and 24 nm in P3, respectively, and that of the equatorial diffraction for P5 is 28 nm. The d-spacing values are listed in Table 3. In the wide-angle region, strong diffraction assigned as the first-order layer diffraction, [100] diffraction, of PFA-C8 is observed on the meridian. The [100] and [200] diffractions of PFA-C8 are observed mainly on the meridian and appear weakly around the horizontal lines in the P2 and P3 films, whereas they are observed only on the meridian for P5. The orientation of the [001] diffraction (intermesogen distance in the lateral direction) of PFA-C8 in the wide-angle region is consistent with that of the layer diffraction. The layer diffraction of BB-5(3-Me) probably overlaps on the second-order layer diffraction of PFA-C8 and is difficult to distinguish for P2 and P3. On the other hand, it can be independently observed on the horizontal line for P5. These GI-SAXS and GI-WAXD results suggest that parallel and perpendicular lamellae coexist in the P2 and P3 films, whereas only perpendicular lamellae exist in the P5 film. Second, the results for αi = 0.08° are summarized (Figure 12a). In the GI-SAXS patterns, weak layer diffractions are observed in the out-of-plane direction for P2 and P3, although the intensity is much lower than those at αi = 0.16°. On the horizontal line at α ∼ 0.12°, which corresponds to αc,f, sharp diffractions are observed for all the films, and the spacings (23, 23, and 26 nm for P2, P3, and P5, respectively) differ slightly from those for αi = 0.16° (25, 24, and 28 nm for P2, P3, and P5, respectively). At αi = 0.08°, the penetration depth of X-rays with a wavelength

behavior is due to the morphological transition from the lamellar to spherical domain. For P5, the g value also increases at Ti,B, and the increase in LA and g will be related to the segments of PFA-C8 excluded from the lamellar domain. It can be supposed that the excluded chains are incorporated into the lamellae at 170 °C, increasing the LA and g values. On the other hand, the value of LB is less than half of the contour length of 51 nm (DP = 31, unit length = 1.64 nm) and does not show a clear temperature dependence. Furthermore, LB decreases linearly with φA. These results are explained by a folded chain model21−24 and imply that the effect of the conformational entropy of the MCLCP will be negligibly small in the smectic phase, although it is dominant in the nematic phase.56,57 The formation of folds allows the BB-5(3-Me) domain to match its cross section with that of the PFA-C8 domain. Then, as long as the main chain of PFA-C8 is fully extended, the main chain of BB-5(3-Me) does not need to make another fold. The value of nf is ∼1 in P1, ∼2 in P2, P3, and P4, and ∼4 in P5 at 25 °C. The increasing tendency of nf with φA is consistent with the chain conformation of PFA-C8, in which the main chain is fully extended in P2, P3, and P4 and partially folded in P5. As a result, the long period, D, monotonically increases up to P4 and then decreases in P5 below Ti,B. In P1 (small φA regime), the fact that the smectic layer of PFA-C8 is parallel to that of BB-5(3-Me) suggests that the tendency to match the orientations of these smectic layers exceeds the competing tendency to match the interfacial area of these domains. However, if all the main chains of PFA-C8 are aligned parallel to the interface, the interfacial area mismatch between PFA-C8 and BB-5(3-Me) will become too large to maintain the planar interface, even though the planar interface is energetically preferable to the Sm-CA structure. It can be assumed that the main chain of PFA-C8 is partially oriented to the lamellar normal direction to avoid the interfacial area mismatch. As a result, the DLC of PFA-C8 in P1 will be much smaller (31%) than that of the other samples. Schematic models of chain packing in the segregated structures are presented in Figure 11 to summarize the results of section 3.3.

Figure 11. Schematic model of polymer chain packing of each block in the lamellar structure for P1 (a) and P2, P3, P4, and P5 (b). Black ellipsoids and green undulating lines represent bibenzoate mesogen in BB-5(3-Me) block and perfluorooctyl mesogen in PFA-C8 block, respectively.

3.4. Structural Analysis of Thin Films Based on GI-WAXD and GI-SAXS. 3.4.1. Orientation of Smectic Structure and Segregated Domains in Thin Films. The orientation of Rf and p,p′-bibenzoate (BB) mesogens in the lamellar structure is I


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Figure 12. GI-SAXS and GI-WAXD patterns with X-ray incident angles (αi) of (a) 0.08° and (b) 0.16°. GI-SAXS patterns in the upper rows of (a) and (b). GI-WAXD patterns in the lower rows of (a) and (b) are presented in linear-scale. Color scale bar is inset in each pattern are presented in log-scale. The [hkl] indices of PFA-C8 are inset in each GI-WAXD pattern.

of 0.100 nm is estimated as 5−7 nm for a polymer film with a density of 1−2 g cm−3. The periodic scatterings in the meridional direction are possibly due to several effects: X-ray penetration from the edge of the film, continuous attenuation of the X-ray evanescent wave, surface roughness, etc. The footprint of the X-ray beam (vertical size of 50 μm) reaches 36 mm at αi = 0.08° and exceeds the film size (25.4 mm), while the beam divergence is negligibly small. Also, the intensity of X-ray evanescent wave at a depth of 20 nm is still ca. 2−5% of that at the outermost surface. As a result, a small part of the X-ray beam penetrated the interior film, and periodic scattering from the lamellar structure in the film appeared weakly on the meridian. Nevertheless, considering the large difference between the patterns for αi = 0.08°

and 0.16°, these GI-SAXS patterns are expected to contain mainly scattering from the top surface of the film. In the GI-WAXD patterns at αi = 0.08°, only the layer diffractions of PFA-C8 are clearly observed. The patterns at αi = 0.08° for P2 and P5 are similar to that observed at αi = 0.16°, whereas P3 shows different orientation patterns. The geometry of the (001) diffractions of the PFA-C8 block of P3 in the GI-WAXD patterns at αi = 0.08° and 0.16° suggests that most of the Rf mesogens are aligned vertically at the outermost surface, whereas parallel and perpendicular orientations coexist equally in the film interior. 3.4.2. Chain Packing in Lamellar Domain and Orientation Mechanism. GI-SAXS intensity analysis based on one-dimensional paracrystal model was conducted to evaluate the lamellar J


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is, the volume fraction of the lamella, φL = L/D, and the volume fraction of the A domain, φA_GI = φL or 1 − φL, are summarized in Table 3. For P2, the thickness (LA) and volume fraction (φA_GI) of the parallel and perpendicular lamellae of PFA-C8 inside the film are 1.1 and 4.0 nm and 0.055 and 0.18, respectively, whereas φA in P2 evaluated from 1H NMR is 0.28. Furthermore, the value of LA in the parallel lamellar structure is much smaller than that in the bulk sample, whereas LB (∼20.1 nm) is almost the same as that in the bulk sample (19.2 nm). At the outermost surface of the film, the LA and φA_GI values of the perpendicular lamellae are close to those in the bulk. The reason that the φA_GI value of the parallel lamellae (0.18) is smaller than φA (0.28) is probably attributable to a grain boundary effect. On the basis of the similar discussion in section 3.3.1, the volume fraction of the A block in a grain is expected to be (N + 1)/N × L/D = 0.25 for N = 3, which is consistent with φA = 0.28. In the parallel lamellae, the φA_GI value of 0.055, which is much smaller than 0.28, implies that most of the PFA-C8 segment should be excluded from the lamellar structure to the lateral grain boundary. Note that most of the Rf mesogens are aligned parallel to the surface not only in the film interior but also in the surface region because [001] diffraction of PFA-C8 is concentrated around the meridian in the GI-WAXD pattern of P2. These parallel-oriented Rf mesogens are not incorporated into the perpendicular lamellae because Rf mesogens in the perpendicular lamellar domain should be aligned perpendicular to the surface. Considering the matching of the orientation between Rf and BB mesogens, the Rf mesogens excluded from the perpendicular lamellae can be assumed to be aligned parallel to the BB mesogens in the perpendicular lamellar domain. For P3, parallel and perpendicular lamellae coexist, similar to the case of P2, but the thicknesses of these lamellae are almost the same, whereas they are different in P2. In the film interior, the LA and φA_GI values of the parallel and perpendicular lamellae of PFA-C8 are 4.9 and 5.0 nm and 0.22 and 0.21, respectively. The values of LB in the parallel and perpendicular lamellar structures are 17.6 and 18.5 nm, respectively, which are slightly smaller than the 19.4 nm observed in the bulk sample. On the basis of the similar discussion for P2, (N + 1)/N × L/D = 0.32 and 0.31, which is comparable with φA = 0.37. At the outermost surface of the P2 film, the LA and φA_GI values for the perpendicular lamellae are 3.6 nm and 0.16, respectively, which are much smaller than those in the film interior, and LB is the same as that observed in bulk (19.4 nm), even though N is almost the same at the surface and inside of the film. The GI-WAXD and GI-SAXS patterns at αi = 0.08° suggest that most of the smectic layers of PFA-C8 are aligned parallel to the surface, and the lamellar structure is aligned mainly perpendicular to the surface in the surface region. The large deviation of φA_GI from φA in the surface region implies that PFA-C8 segments are excluded from the perpendicular lamellae and cover the surface owing to the low surface energy of Rf mesogens. For P5, only the perpendicular lamellar structure was observed, not only at the surface but also in the film interior. However, the values of LA and φA_GI differ slightly between the surface and the inside of the film. Inside the film, LA and φA_GI are 18.5 nm and 0.67, and the latter is consistent with φA = 0.70. On the other hand, at the surface of the film, LA and φA_GI are 19.2 nm and 0.75, and the latter is slightly larger than φA. These results indicate that PFA-C8 segments are likely to be relatively concentrated near the surface, whereas BB-5(3-Me) segments are relatively concentrated near the substrate. This concentration

Table 3. Parameters Used for GI-SAXS Intensity Calculation φA 0.28 0.37 0.70 0.28 0.37 0.70 0.28 0.37 0.70

sample

⟨N⟩a

Db (nm)

Lc (nm)

D−L (nm)

σLd (nm)

ge

out-of-plane, αi = 0.16° (parallel lamella inside film) P2 2.6 20.1 1.1 19.0 0.4 0.03 P3 3.2 22.5 4.9 17.6 0.3 0.02 P5 in-plane, αi = 0.16° (perpendicular lamella inside film) P2 4.4 25 20.5 4.5 2.0 0.12 P3 2.3 23.5 5.0 18.5 1.0 0.10 P5 3.0 27.5 9.0 18.5 2.5 0.15 in-plane, αi = 0.08° (perpendicular lamella at film surface) P2 3.2 22.5 4.0 18.5 2.0 0.13 P3 2.4 23.0 19.4 3.6 2.0 0.15 P5 2.3 25.5 19.2 6.3 4.5 0.13

φA_GIf 0.055 0.22

0.18 0.21 0.67 0.18 0.16 0.75

a

The number of lamellar stack in a grain. bLong period. cLamellar thickness. dStandard deviation of lamellar thickness. eSecond kind disorder of paracrystal model. fVolume fraction of A-domain.

thickness of the parallel and perpendicular lamellar structure (Figure 13). The effect of lateral size is neglected for the analysis

Figure 13. GI-SAXS intensity profiles of P2 (blue), P3 (green), and P5 (orange) at 25 °C: in-plane direction with (a) αi = 0.08° and (b) 0.16° and out-of-plane direction with (c) αi = 0.08° and (d) 0.16° (d). Gray solid line represents calculated profiles.

(details are provided in the Supporting Information). The analyses in the meridional direction were performed only for the profiles of P2 and P3 with αi = 0.16° because the profiles at αi = 0.08° did not contain essentially different information from those at αi = 0.16°, and the profiles of the P5 film did not exhibit a clear long period in the meridional direction. In the intensity analysis, only the contribution from the reflected X-ray beam was taken into account in calculating the in-plane intensity profile, and the contributions from both the reflected and transmitted X-ray beams were taken into account to calculate the out-ofplane intensity (details are provided in the Supporting Information).58−61 The parameters used for the analysis, that K


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Macromolecules gradient is probably responsible for the mismatch between φA_GI and φA.3 Two key factors for lamellar orientation during film formation are considered: shear flow during spin-coating and the orientation of Rf mesogens in the PFA-C8 block. The former factor is considered to promote the parallel lamellar structure. During spin-coating, the lamellar structures probably emerge in the swollen film and are subjected to shear flow. The shear flow promotes the parallel lamellar structure if the relaxation time of the layer fluctuation is sufficiently small compared with the reciprocal of the shear rate.62 The parallel lamellae, once formed during spin-coating, are probably difficult to rearrange into perpendicular lamellae by thermal annealing. The latter factor promotes the perpendicular lamellar structure because Rf mesogens are parallel to the lamellar interface and have a strong tendency to be aligned perpendicular to the surface. In the small φA region (P2, P3), the effect of the shear flow competes with the effect of the orientation of Rf mesogens, whereas in the large φA region (P5), the orientation of Rf dominates the lamellar orientation. Note that the tendency of PFA-C8 to be concentrated near the film surface owing to the low surface free energy is probably not the dominant factor affecting the parallel lamellar structure because parallel lamellae were not observed in the P5 film, where the PFA-C8 block is most concentrated at the top surface among the three samples. Consequently, Rf mesogens, which are aligned perpendicular to the main chain of PFA-C8 and the surface, are considered to be the most important factor in fabricating the perpendicular lamellar structure.

and it is difficult to change the lamellar orientation by the subsequent thermal annealing process. On the other hand, the latter promotes the perpendicular orientation because Rf mesogens were oriented parallel to the lamellar interface when φA was larger than 0.28 and tended strongly to be aligned perpendicular to the surface, resulting in the perpendicular lamellar structure. Consequently, when φA was close to 0.7, only the perpendicular lamellar structure was successfully obtained in the thin film without any surface modification of the substrate, although when φA was smaller than 0.4, the parallel lamellar structure coexisted with the perpendicular lamellar structure.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b00855. Synthetic procedure of PFA-C8-b-BB-5(3-Me)-b-PFA-C8; characterization of PFA-C8-b-BB-5(3-Me)-b-PFA-C8; DSC thermograms for homopolymers; temperature dependence of LC and lamellar orientations in P5; smearing effect on SAXS intensity analysis; relation between paracrystal model and Babinet’s principle; GI-WAXD intensity profiles of BB-5(3-Me) thin film; effect of lateral domain size on GI-SAXS; and GI-SAXS intensity calculation in DWBA (distorted-wave Born approximation) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

4. CONCLUSION ABA-type triblock copolymers consisting of fluorinated sidechain LC A blocks and a main-chain LC B block were synthesized by ATRP from the same BB-5(3-Me) macroinitiator. For all five BCPs, P1−P5, with different volume fractions of the A block (φA = 0.11−0.70), lamellar segregated structure was found in the bulk and the thin film by SAXS, WAXD, GI-SAXS, and GI-WAXD using a synchrotron radiation X-ray source. First, the conclusions for the bulk samples are summarized. When φA was larger than 0.28, Rf mesogens in the A blocks were aligned parallel to the lamellar interface, whereas BB in the B block was aligned perpendicular to the interface; that is, the main chains of both PFA-C8 and BB-5(3-Me) were aligned perpendicular to the interface. When φA was small (∼0.10), both the mesogens were aligned in the same direction, that is, perpendicular to the interface. In all the polymers, the main chain of BB-5(3-Me) was perpendicular to the interface. Considering that the A and B blocks were strongly segregated even in the high-temperature region where both LC blocks were in the isotropic phase, the smectic layer of BB-5(3-Me) probably grew epitaxially along the interface; the main chain was aligned perpendicular to the interface, and the lamellar structure formed. The orientation of the main chain of PFA-C8 was affected by not only the orientational matching between Rf and BB mesogens but also the matching of the interfacial area of the lamellar domain. When φA was small (∼0.10), the orientation matching dominated the orientation of Rf in the lamellar domain. On the other hand, when φA was larger than 0.28, the matching of the interfacial area dominated the orientation of the main chain of PFA-C8. Second, the conclusions for the thin films are summarized. In this case, two competing factors, shear flow during spin-coating and the orientation of Rf mesogens, probably dominate the orientation of the lamellar structure. The former factor promotes the parallel orientation because the lamellae slip mutually in the shear flow,

*(R.I.) E-mail: [email protected]. *(A.T.) E-mail: [email protected]. Present Address

R.I.: Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8552, Japan. H.O.: Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by JSPS KAKENHI [Grant-inAid for Young Scientists (B), Grant No. 24750223]. Simultaneous SAXS and WAXD and GI-SAXS and GI-WAXD measurements using a synchrotron X-ray source were conducted at the BL40B2 and BL03XU beamlines in SPring-8, respectively, with the approval of the Japan Synchrotron Radiation Research Institute (JASRI). The proposal numbers were 2012B1218 and 2013A1470 for BL40B2. We gratefully thank Dr. Jun-ichiro Koike and Dr. Masahiko Asada (DIC Cooperation, Chiba, Japan) for kindly providing the opportunity for the GI-SAXS and GI-WAXD measurements in BL03XU. We gratefully acknowledge Mr. Jun Koki (Technical Department, Tokyo Institute of Technology) for the TEM observations.

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REFERENCES

(1) Kim, D. H.; Kim, S. H.; Lavery, K.; Russell, T. P. Inorganic Nanodots from Thin Films of Block Copolymers. Nano Lett. 2004, 4, 1841−1844. (2) Hashimoto, T.; Fukunaga, K. Nanofabrication of Block Copolymer Bulk and Thin Films: Microdomain Structures as Templates. In Nanostructured Soft Matter: Experiment, Theory, Simulation and

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Fully Liquid-Crystalline ABA Triblock Copolymer of Fluorinated Side

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