Enormously Wide Range Cylinder Phase of Liquid Crystalline PEO-b

Feb 17, 2014 - ... Graduate School of Engineering, Tokyo Metropolitan University, 1-1 Minamiosawa, Hachioji 192-0397, Japan ... In this paper, morphol...
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Enormously Wide Range Cylinder Phase of Liquid Crystalline PEO‑b‑PMA(Az) Block Copolymer Hideaki Komiyama,† Ryohei Sakai,† Shingo Hadano,†,⊥ Sadayuki Asaoka,†,∥ Kaori Kamata,† Tomokazu Iyoda,‡,† Motonori Komura,*,† Takeshi Yamada,§,# and Hirohisa Yoshida‡,§ †

Division of Integrated Molecular Engineering, Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa 226-8503 Japan ‡ Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Agency (JST) and §Department of Applied Chemistry, Graduate School of Engineering, Tokyo Metropolitan University, 1-1 Minamiosawa, Hachioji 192-0397, Japan S Supporting Information *

ABSTRACT: A series of amphiphilic liquid crystalline diblock copolymers, PEOm-b-PMA(Az)n, consisting of hydrophilic poly(ethylene oxide) and hydrophobic poly(methacrylate) moieties with side chains containing liquid crystalline (LC) azobenzene moieties, produced highly ordered microphaseseparated films with PEO cylinders aligned perpendicular to the smectic LC layer of azobenzene in the PMA(Az) matrix. In this paper, morphological phase diagrams of PEOm-b-PMA(Az)n diblock copolymers above and below the isotropic transition temperature of LC azobenzene (Tiso) are presented. The diagrams are based on small-angle X-ray scattering (SAXS) measurements of approximately 70 kinds of polymers with varying degrees of polymerization in each block. An asymmetric phase diagram described against the volume fraction of PEO ( f PEO) was obtained at temperatures above and below Tiso. The lamellar phase appears in the f PEO window 0.52 ≤ f PEO ≤ 0.78 above and below Tiso. Besides, the wide window, 0.087 ≤ f PEO < 0.52, allows the PEO cylinder phase to form below Tiso. In particular, the PEO sphere phase, observed above Tiso, was completely eliminated through an order− order transition (OOT) to the PEO cylinder phase in the window 0.087 ≤ f PEO ≤ 0.23. Such a large expansion in the PEOcylinder-phase window could be attributed to the main chain of LC PMA(Az) being shorter than that of the flexible PEO chain, and LC azobenzene forming a smectic layer in the microphase separated system.

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INTRODUCTION Block copolymers that consist of two or more polymer chains attached via a covalent bond are known to self-assemble into microphases with a variety of ordered morphologies, i.e. spherical, cylindrical, and lamellar structures. Microphaseseparated block copolymer films have received significant attention for their potential in nanotechnology applications, i.e., block copolymer lithography and templating processes. The films can spontaneously form ordered microdomains, tens of nanometers in size, with surface densities up to 1011 cm−2, and the microdomains possess inherent chemical and physical contrasts with the surrounding domains. Many experimental and theoretical studies have been performed to determine the morphological phase behavior of linear block copolymers that consist of conventional polymer blocks. These studies have been done in relation to controllable factors such as overall polymerization degree, N, volume fraction of the component block, f, and the Flory−Huggins interaction parameter, χ.1,2 Currently, amorphous−amorphous diblock copolymers are the best understood. The self-consistent mean-field theory (SCFT) provides a symmetric phase diagram for microphase-separated morphologies in diblock copolymers.3,4 The theoretical phase diagram is strikingly similar to the experimentally obtained © 2014 American Chemical Society

phase diagram for conventional diblock copolymers, such as poly(isoprene-b-styrene)5 and poly(ethylene oxide-b-isoprene),6 in terms of χ, N, and f. Since the side-chain liquid crystalline (SCLC) diblock copolymer was first reported by Adams et al.,7 SCLC block copolymers, in which the SCLC block is typically attached to an amorphous block, have attracted interest because they form hierarchical structures composed of microphase separation and liquid crystal structures. They also have the potential to form new thermoplastic LC elastomers, self-assembling disperse LC displays, and may show unique mechano-optic or electro-optic properties. A number of studies on the relationship between LC ordering and microphase separation have been reported,8−13 and the phase diagrams of microphase-separated SCLC block copolymers have been studied theoretically14,15 and experimentally.16−21 In this paper, we have designed and synthesized a series of amphiphilic liquid crystalline diblock copolymers, PEOm-bPMA(Az)n, consisting of hydrophilic poly(ethylene oxide) Received: November 14, 2013 Revised: January 27, 2014 Published: February 17, 2014 1777

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Small-angle X-ray scattering (SAXS) measurements were carried out using a laboratory-scale SAXS instrument (Nano-Viewer, Rigaku corp., Japan) with a two-dimensional CCD camera (Rigaku Co. Ltd., Japan) or a two-dimensional semiconductor detector (PILATUS, Rigaku Co. Ltd., Japan). The X-ray wavelength was 1.541 Å. SAXS measurements were also performed at beamline 10C at the Photon Factory, High Energy Accelerator Research Organization, Tsukuba, Japan. Scattering X-ray was detected using a one-dimensional position-sensitive proportional counter (PSPC, 512 channels, Rigaku Co. Ltd., Japan). The wavelength of the X-ray was 1.488 Å. Pellet samples of the block copolymers, in the isotropic phase, were thermally annealed at 140 °C for 6 h. A hot stage (FP90/FP82HT, Mettler Toledo) was attached to the laboratory-scale SAXS system to control the temperature for the samples. The nonannealed pellet samples were heated to 140 °C on the hot stage. The SAXS measurements were carried out at 140, 100, and 30 °C during the first cooling process via natural cooling. Data acquisition time was typically 30 min. Transmission electron microscopy (TEM) observations were performed using a Hitachi H7000 operating at 100 kV. The solventcasted block copolymers were annealed at 140 °C for 6 h, and then the samples were stained with RuO4. Ultrathin sections of the block copolymers were obtained by cryo-ultramicrotoming (FC6, Leica). The ultrathin sections were stained with RuO4 again to enhance contrast for TEM images.

(PEO) and hydrophobic poly(methacrylate) (PMA) moieties with liquid crystalline azobenzene moieties in the side chains were designed and synthesized (Scheme 1).22,23 This PEOm-bScheme 1. Chemical Structure of Amphiphilic Liquid Crystalline PEOm-b-PMA(Az)n Diblock Copolymera

a

The subscript of m and n are polymerization degree of the blocks. The block copolymers used in this study have m of 114, 272, and 454 and various n values.

PMA(Az)n block copolymer forms well-ordered PEO nanocylinders, which are aligned perpendicular to various thin-film substrates, such as silicon wafer, glass, mica, and poly(ethylene terephthalate). The substrates have not been modified using treatments such as surface neutralization. The formation process of the nanostructure of the microphase-separated film was visualized by temperature controlled atomic force microscopy (AFM).24 The microphase-separated film with hydrophilic PEO cylinders can be easily applied to several block copolymer templating processes because of its amphiphilic properties. Such processes can be used to obtain anisotropic lithium-ion conductive membranes,25 permeable membranes with a high-density of straight channels for small molecules,26 and addressable template thin films for periodic and diameterdefined arrangements of gold or silver nanoparticles27,28 and Ge quantum nanodots.29 While perpendicular PEO nanocylinder structures have been the focus of such applications, other microphase-separated phases such as lamellar and spherical, have not been obtained despite the larger number of studies on PEOm-b-PMA(Az)n. Drawing a phase diagram for PEOm-bPMA(Az)n would be significant not only for understanding fundamental aspects of SCLC block copolymer phase behavior but also for developing applications based on microphase separation. PEOm-b-PMA(Az)n is suited for such applications, and to the best of our knowledge there are no reports of such applications with other SCLC block copolymers. In this study, we describe the phase diagram for amphiphilic liquid crystalline PEOm-b-PMA(Az)n block copolymers by controlling the molecular weight (Mw) and volume fraction of the PEO segment, f PEO. Determination of the morphological phases was achieved using small-angle X-ray scattering (SAXS) measurements, and transmission electron microscopy (TEM) was used to observe PEOm-b-PMA(Az)n block copolymers in their bulk state. We discuss the origin of the extremely asymmetric phase diagram by comparing results from below and above the isotropic transition temperature of the liquid crystal (Tiso). We also discuss the resulting wide cylinder-phase window.

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RESULTS First, to determine the phase boundary between the cylinder and lamellar phases, a lamellar microdomain was obtained. Herein, we focused on the PEO272 series of the PEOm-bPMA(Az)n diblock copolymers with a large volume fraction of PEO (f PEO) and obtained PEO272-b-PMA(Az)15, PEO272-bPMA(Az)17, and PEO272-b-PMA(Az)23 with f PEO values of 0.59, 0.56, and 0.51, respectively. Characterization of the molecular weight (Mw), polydispersity (Mw/Mn), and f PEO for the PEO272b-PMA(Az)n diblock copolymers is shown in Table 1. SAXS Table 1. Characterization of PEO272-b-PMA(Az)n na

Mn (kg/mol)b

Mw/Mnc

f PEOd

15 17 23

19.4 20.4 23.3

1.10 1.12 1.10

0.59 0.56 0.51

a Polymerization degree of PMA(Az). bMolecular weight determined by NMR. cPolydispersity determined by GPC calibrated with polystyrene standards. dVolume fraction of PEO calculated by using molecular weight and density (PEO: 1.2 g/cm3 and PMA(Az): 1.1 g/ cm3) of each block.

measurements of the PEO272-b-PMA(Az)n block copolymers in the bulk state were performed to characterize the microphaseseparated nanostructures. Figure 1 shows the SAXS intensity profiles for PEO272-b-PMA(Az)n, n = 15, 17, and 23, in the bulk state after thermal annealing at 140 °C for 6 h. PEO272-bPMA(Az)23, with an f PEO of 0.51, exhibited a hexagonally packed structure with Bragg reflection peaks located at q*, √4q*, and √7q*. PEO272-b-PMA(Az)15 and PEO272-b-PMA(Az)17, with f PEO values of 0.59 and 0.56, respectively, exhibited a lamellar structure with Bragg reflection peaks located at q*, (2q*), and 3q*. A scattering peak located at q = 2.04 nm−1 was assigned to the smectic phase, SmX, of LC azobenzene, based on previous work.30 Figure 2 shows TEM images for the PEO microdomains of RuO4-stained PEO272-b-PMA(Az)15 and PEO272-b-PMA(Az)23. Lamellar and cylinder microdomains were observed in PEO272-b-PMA(Az)15 and PEO272-b-PMA(Az)23, respectively, which were in agreement with the SAXS

EXPERIMENTAL SECTION

A series of PEOm-b-PMA(Az)n diblock copolymers were systematically synthesized using atom transfer radical polymerization (ATRP) from a PEOm macroinitiator (typically m = 114, 272, and 454) based on our previous work.22 The molecular weight (Mw) and polydispersity (Mw/ Mn) were determined from 1H NMR and GPC measurements, respectively. The liquid crystalline azobenzene in the block copolymer system exhibited thermal phases that were smectic A (SmA), smectic C (SmC), and undefined smectic X (SmX) on decreasing temperature from the Tiso, which was around 120 °C. 1778

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was also observed in the PEO454 series of the PEOm-bPMA(Az)n diblock copolymers. SAXS measurements revealed that PEO454-b-PMA(Az)10 and PEO454-b-PMA(Az)34, with f PEO values of 0.78 and 0.52, respectively, exhibited the lamellar phase, whereas PEO454-b-PMA(Az)41 ( f PEO = 0.47) exhibited the cylinder phase (Figure S1). Thus, the phase boundary between the PEO cylinder and lamellar phases was located at an f PEO value of ∼0.52 in bulk-state PEOm-b-PMA(Az)n diblock copolymers. So far, approximately 70 types of PEOm-b-PMA(Az)n diblock copolymers with varying degrees of polymerization for each block, m (m = 114, 272, and 454) and n, have been synthesized in the cylinder phase and analyzed using SAXS measurements (characterization of these diblock copolymers are described in Table S1). Therefore, as shown in the bottom figure of Figure 4 and Figure S3, it is possible to draw a phase diagram for the observed microphase-separated nanostructures as a function of the volume fraction of PEO, f PEO. The PEOm-b-PMA(A)n diblock copolymers have an strongly asymmetric phase diagram, indicating that the phase boundaries for microphase separation would be greatly influenced by LC ordering. During the minimization of free energy, liquid crystalline ordering and microphase-separated nanostructure formation compete in the liquid crystal containing the block copolymer system. As mentioned above, the phase boundary for the cylinder-lamellar phase was around an f PEO value of 0.52. A PEO sphere structure was never observed at room temperature even though the f PEO was 0.087 for PEO114-b-PMA(Az)98. Moreover, PMA(Az) cylinders and spheres have not been observed for f PEO values up to 0.78. Larger f PEO values of the PEOm-bPMA(Az)n diblock copolymers have not been achieved because of the difficulty in limiting the degree of polymerization of PMA(Az)n to small values during its synthesis. Hammond et al. have reported the absence of LC cylinders and spheres in polystyrene-b-methacrylate-based side chain LC diblock copolymers.18 They indicated that the absence of LC cylinders and spheres could be caused by the lower stability of the smectic phase when the LC was confined within small curved domains. Meanwhile, Ober and Thomas have identified smectic LC cylinders at a volume fraction of 0.22 in an amorphous matrix of polystyrene-b-1,2-polyisoprene with azobenezenebased side chain diblock copolymers.17 However, in their case,

Figure 1. SAXS profiles of (a) PEO272-b-PMA(Az)15 ( f PEO of 0.59), (b) PEO272-b-PMA(Az)17 ( f PEO of 0.56), and (c) PEO272-b-PMA(Az)23 ( f PEO of 0.51) after thermal annealing at 140 °C for 6 h. The measurements were performed at room temperature.

Figure 2. TEM images of (a) PEO272-b-PMA(Az)15 (lamellae) and (b) PEO272-b-PMA(Az)23 (cylinder) in bulk state. The diblock copolymer was annealed at 140 °C for 6 h. The specimen was prepared with cryomicrotome cutting method and stained by RuO4 before and after the cutting.

results. Lamellar microdomains in the PEOm-b-PMA(Az)n diblock copolymers were successfully obtained based on the measurements of real and reciprocal spaces. The lamellar phase

Figure 3. SAXS profiles of (a) PEO114-b-PMA(Az)74 ( f PEO = 0.11), (b) PEO272-b-PMA(Az)53 ( f PEO = 0.30), (c) PEO272-b-PMA(Az)15 ( f PEO = 0.59), and (d) PEO454-b-PMA(Az)10 ( f PEO = 0.78) at temperatures of 140 °C (red plots, in isotropic phase for liquid crystal), 100 °C (black plots, in smectic A phase), and 30 °C (blue plots, in smectic X phase). 1779

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isotropic transition of this sample occurred because of the low polymerization degree of the PMA(Az)10 block. The phase behaviors of the PEOm-b-PMA(Az)n diblock copolymers (m = 114, 272, and 454) above and below Tiso are summarized in Figure 4 as a function of f PEO. Phase boundaries

the cylinder curvature (diameter of 38 nm) was sufficiently large to stabilize the LC cylinders. It is interesting to note that the PEOm-b-PMA(Az)n diblock copolymer forms PEO cylinders over a very wide range of f PEO values (referred to as f PEO window hereafter), from 0.087 to 0.52, compared to other block copolymers. On the basis of the phase diagram well-studied and theoretically obtained by selfconsistent mean-field theory (SCFT), the cylinder phase in conventional amorphous−amorphous diblock copolymers was produced only when the volume fraction of the minor segment was between 0.2 and 0.35.3,4 In the case of side-chain liquid crystalline (SCLC) block copolymers, the range of f PEO values that form an amorphous cylinder is wider than that of conventional amorphous−amorphous diblock copolymers since the formation of a continuous liquid crystalline phase can stabilize the cylindrical structure.17−20 Hammond et al. reported that an amorphous cylinder could be obtained when the volume fraction of the amorphous segment was between 0.15 and 0.45.14 The f PEO window that affords PEO cylinders is much wider for the PEOm-b-PMA(Az)n diblock copolymer when compared to that of conventional amorphous− amorphous and SCLC block copolymers. The f PEO window that results in PEO cylinders is further expanded, particularly toward the sphere region, for the PEOmb-PMA(Az)n copolymers. The expansion can be attributed to two effects: architectural asymmetry of the block molecules with small PEO segments and large PMA(Az) side chain segments and the one-directional structure of LC ordering. The effects will be discussed in detail after the temperature dependence of the phases is shown, especially the isotropic condition phase diagram. SAXS measurements were also performed during the first thermal annealing, with the temperature decreasing from 140 to 30 °C. Figure 3 contains typical SAXS intensity profiles for PEO114-b-PMA(Az)74 ( f PEO = 0.11), PEO272-b-PMA(Az)53 (f PEO = 0.30), and PEO272-b-PMA(Az)15 (f PEO = 0.59) at 140, 100, and 30 °C, where the LC azobenzene mesogens in the PMA(Az) segments are in the isotropic phases, SmA and SmX, respectively. DSC profiles of these polymers during the first cooling process from 140 °C are shown in Figure S2. On the basis of our previous report,22 the LC layer peaks around q = 2 nm−1 at 100 and 30 °C were assigned to the SmA and SmX phases, respectively, for all the samples. The LC layer peaks completely disappeared at 140 °C (iso) in Figure 3. PEO114-bPMA(Az)74 ( f PEO = 0.11) showed typical Bragg reflection peaks at q*, √3q*, √4q*, and √7q* at 100 (SmA) and 30 °C (SmX), indicating a hexagonal cylinder structure. However, PEO114-b-PMA(Az)74 at 140 °C (iso) showed main peaks located at q*, √2q*, and √3q*, indicating a spherical structure (Figure 3a). It should be noted that order−order transition (OOT) was induced by liquid crystallization. Once the smectic layer of azobenzene forms, the morphological phase does not change. On the other hand, PEO272-b-PMA(Az)53, with an f PEO value of 0.30 (almost in the center of the f PEO window that allows the cylinder phase at room temperature), maintained the cylindrical phase at all temperatures, according to SAXS measurements (Figure 3(b)). PEO272-b-PMA(Az)15, with an f PEO of 0.59, maintained the lamellar phase at all temperatures (Figure 3(c)). While PEO454-b-PMA(Az)10, f PEO of 0.78, exhibited the lamellar phase at 30 °C, the main peaks at 140 °C were located at q1*, √2q1*, √3q1*, and √5q1*, which indicated the PMA(Az) spherical phase. A mixed profile of lamellar and spherical phases appeared at 100 °C, where the

Figure 4. Temperature dependence of phase diagrams for PEOm-bPMA(Az)n diblock copolymers. Top and bottom are phase diagrams at 140 °C (above Tiso) and 30 °C, respectively. Plots on squares, circles, and triangles mean a series of PEOm-b-PMA(Az)n diblock copolymers with different polymerization degree of PEO, m, for PEO114-bPMA(Az)n, PEO272-b-PMA(Az)n, and PEO454-b-PMA(Az)n diblock copolymers. Red, blue, green, and orange plots represent PEO sphere, PEO cylinder, lamellar, and PMA(Az) sphere phases. Gray plots indicate no phase assignment due to only first order peak acquisition in SAXS profiles.

for sphere-cylinder and cylinder-lamellar transitions were drawn at f PEO values of 0.23 and 0.52, respectively, although phases of some PEOm-b-PMA(Az)n samples at 140 °C (iso) were not assigned because only first order peak acquisition was performed. This phase diagram shows that OOT from sphere to cylinder, induced by liquid crystallization, was observed for PEO114-b-PMA(Az)n diblock copolymers with f PEO values between 0.087 and 0.23 and PEO272-b-PMA(Az)116 diblock copolymers with an f PEO value of 0.16. We recognized that the sphere to cylinder OOT, at temperatures below the Tiso, occurred in the range 0.087 ≤ f PEO ≤ 0.23 for the PEOm-bPMA(Az)n copolymer. On the other hand, no apparent OOT from the cylinder to lamellar phase was seen in Figure 4.

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DISCUSSION The resulting phase diagram of PEOm-b-PMA(Az)n, with sphere to cylinder OOT, will be discussed within the scope of the phase diagrams based on a scaling-based free energy model for SCLC block copolymers, reported by Hammond et al.14 The phase diagram of PEOm-b-PMA(Az)n showed asymmetry in the liquid crystalline and isotropic phases, which indicates that the asymmetry cannot simply be explained by the effect of liquid crystalline ordering. The phase diagram asymmetry in the isotropic phase without LC ordering is caused by the architectural asymmetry of the PEOm-b-PMA(Az)n blocks. Because the PMA(Az) block has a shorter backbone contour length per unit volume, the PMA(Az) segments cannot exist away from the interface of microphase separation (Figure 5a). Therefore, because the PMA(Az) segments are near the interface and the linear PEO segments naturally spread, the balance of the interface curvature becomes skewed. Accordingly, the PMA(Az) segments then tend to form an outer domain of microphase separation, such as matrix domains of sphere or cylinder phases with a higher interface curvature. As a result, the phase diagram shows asymmetry. Molecular architectural asymmetry is also seen in other branched polymers such as AnB miktoarm star copolymers and dendritic block copolymers.31−33 Additionally, the phase boundaries of sphere to cylinder and cylinder to lamellar phases at f PEO values of 0.23 and 0.52 in the isotropic phase of PEOm-b-PMA(Az)n, 1780

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not theoretically expected, which would indicate that the high bending elasticity of PMA(Az) LC ordering and the flexibility of the PEO chain can afford an asymmetric phase diagram with an anomalously wide PEO-cylinder window. The main chain of PMA(Az) is stretched away from the interface of microphase separation by liquid crystallization and has a larger persistent length because the PMA(Az) block is confined and compressed in the two-dimensional LC layer as shown in the bottom figure of Figure 5b. PMA(Az) main-chain stretching may induce sphere to cylinder or cylinder to lamellar OOTs because the short contour length of the PMA(Az) main chain affords the asymmetric phase diagram above Tiso. Additionally, the stretching would decrease the interface curvature of microphase separation, resulting in recovery of the asymmetry of the phase diagram. Although this effect may partly contribute to the sphere to cylinder OOT, the LC ordering explained above would mainly induce the OOT because there were no obvious cylinder to lamellar OOTs. If there were, it would indicate that the effect is not so strong as to eliminate the whole sphere window, even if the region of the OOT has a higher fraction of PMA(Az). The stretching effect of the PMA(Az) main chain can be clearly seen in the regions without OOTs in Figure 3, parts b and c. The q-values for the first order peaks at 30 °C (SmX) were slightly smaller than those at 140 °C (iso), indicating that the d-spaces at 30 °C (SmX) were larger than at 140 °C (iso). At 140 °C (iso), the persistent length of the backbone in the LC-containing segment decreased, resulting from higher energies required to stretch the segment (Figure 5b). Below Tiso, the block copolymer domains are expanded by rearrangement of the LC mesogens to form the energetically favored smectic layers, as observed with the behavior of LC layers in SAXS results. Additionally PEO454-b-PMA(Az)10 showed the OOT from the PMA(Az) sphere to lamellar phase in Figure 3d. This peculiar phenomenon of the OOT skipping the cylinder phase cannot be explained by the mechanism described above and will require further investigation. As previously mentioned, the OOT from the sphere to the cylinder phase was induced in the f PEO window from 0.087 to 0.23 at Tiso in the liquid crystal, because of liquid crystal ordering. A similar phase transition was reported by the Stühn group.13 PS-block-poly(1,2-butadiene) with cyanobiphenyl mesogen-block-PS triblock copolymers (12 vol % in PS block) showed PS spherical microdomains above the nematic−isotropic phase transition temperature, whereas below that temperature, it showed PS cylindrical microdomains. In the isotropic state, the spherical (body-centered cubic (bcc)) microdomains minimize the free energy derived from the contact enthalpy between the microdomains and entropy loss associated with extended chain configurations. Interestingly, the transition of PS cylinders was induced by the coalescence of the PS spheres along the ⟨111⟩ direction of the bcc lattice on transition of the LC to the nematic phase. The OOT of the PEOm-b-PMA(Az)n diblock copolymers would cause a similar mechanism in the bulk state, though the PEO cylinders grow along the ⟨110⟩ direction of the bcc lattice in the film state due to the homeotropic orientation of the liquid crystalline side chains.24 The OOT was not observed when the f PEO value was in the center of the phase window for cylinder or lamellar phases in the phase diagram for the PEOm-b-PMA(Az)n diblock copolymer. Moreover, the order−disorder transition (ODT)

Figure 5. Schematic illustrations for explanation of the phase behavior of PEO-b-PMA(Az). (a) The asymmetric molecular structure induces the PMA(Az) block with a shorter backbone to form an outer domain of microphase separation. (b) Formation of the liquid crystalline smectic layers expands periodicities of cylinder or lamellar microphase separations.

almost agree with the theoretical prediction by Hammond et al.14 Additionally, it is noteworthy that gyroid phase does not appear in the block copolymer system even in the isotropic condition of the liquid crystal. It indicates that the block copolymer would has a high χ-parameter to disappear gyroid phase. Next, we discuss the effect of LC ordering of PMA(Az) on the phase diagram. As shown in Figure 4, the sphere phase disappears, and the range of f PEO values, which allow the cylinder phase, expands to 0.087, and the boundary between the cylinder and lamellar phases did not change. This phenomenon can be simply explained by the one-directional structure of LC ordering although main-chain stretching of PMA(Az) in the LC state, which will be discussed in the next paragraph, should also be considered. The one-directional structure of LC ordering is expected to favor a straight interface with zero gauss curvature, found in cylinder or lamellar phases, and destabilize the formation of a sphere phase with a positive gauss curvature. There would be competition between the bending elasticity of the PMA(Az) LC ordering and conservation of the microphase separation equilibrium state, which would be determined by volume fraction and molecular architecture. In the resulting sphere to cylinder OOT, occurring over the entire range of temperatures above Tiso that exhibit the sphere phase (referred to as sphere window hereafter), significant points would not only be the high bending elasticity of PMA(Az) LC ordering but also the high flexibility of the PEO chain at 140 °C (iso), which is much higher than the PEO glass transition temperature. Although the PEO chains were more confined along the radial direction of the cylinder domain than the sphere domain, flexible PEO chains having inherently lower compression energies can tolerate the confinement. Furthermore, complete elimination of the sphere window was 1781

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no longer occurred in PEOm-b-PMA(Az)n diblock copolymers between 140 °C (iso) and 30 °C (SmX). The ODT is normally preferred for relatively low molecular weight block copolymers, since the driving force for microphase separation is smaller compared to those with high molecular weight because of the small value of χN. There are several reports about LC-induced ODT at relatively small molecular weight in LC-containing block copolymers.34,35 ODT was not identified in the PEOm-bPMA(Az)n diblock copolymers because they have sufficiently large molecular weights.

CONCLUSIONS In this paper, an asymmetric phase diagram with a wide PEOcylinder-phase window was observed for the amphiphilic liquid crystalline PEOm-b-PMA(Az)n diblock copolymers. The asymmetry was found not only in the liquid crystalline or semicrystalline phases but also in the isotropic phase of PMA(Az). The huge expansion in the cylinder-phase window could therefore be attributed to two effects: (i) architectural asymmetry between the flexible PEO segment and the LC PMA(Az) segment, whose main chain is much shorter than that of PEO and (ii) the LC azobenzene ordering to form smectic layers in the microphase-separated system. The wide f PEO window for the PEO cylinder phase offers control over the center-to-center distance between cylinders under a constant cylinder diameter. For templating processes using PEOm-bPMA(Az)n in its thin-film state, such a specific feature is of particular interest to systematically investigate the periodic property of nanomaterials fabricated using the PEOm-bPMA(Az)n thin film as a template. ASSOCIATED CONTENT

S Supporting Information *

Additional SAXS profiles, DSC curves and characterization of the PEOm-b-PMA(Az)n diblock copolymers. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(M.K.) E-mail: [email protected]. Telephone: +81 045 924 5246. Fax: +81 045 924 5247. Present Addresses ⊥

(S.H.) Department of Applied Science, Faculty of Science, Kochi University, 2-5-1 Akebono-cho, Kochi 780-8520, Japan. ∥ (S.A.) Department of Biomolecular Engineering, Kyoto Institute of Technology, Matsugasaki Goshokaido-cho, Sakyoku, Kyoto 606-0962, Japan. # (T.Y.) Comprehensive Research Organization for Science and Society, Research Center for Neutron Science and Technology, 162-1 Shirakata, Tokai, Naka, Ibaraki 319-1106, Japan Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank Ms. K. Ogawa, Ms. H. Tokimori, and Mr. H. Nakajima for synthetic assistance. H. K. thanks the Japan Society for the Promotion of Science for a research fellowship for young scientists. The present work was partly supported by a Grand-in-Aid for Scientific Research (S) (No. 18101005) form the Ministry of Education, Culture, Sports, Science, and Technology, Japan. 1782

dx.doi.org/10.1021/ma402356z | Macromolecules 2014, 47, 1777−1782