Article pubs.acs.org/Macromolecules
Macro- and Microphase Separation in Block Copolymer Supramolecular Assemblies Induced by Solvent Annealing Si Wu* and Christoph Bubeck Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany S Supporting Information *
ABSTRACT: We fabricated block copolymer (BCP) supramolecules by hydrogen bonding various carboxyl- and phenolcontaining azo compounds to the poly(4-vinylpyridine) blocks of polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP). Thin films of the BCP supramolecules were prepared by spincoating. Optical microscopy showed that all films of BCP supramolecules are macroscopically homogeneous immediately after spin-casting. To induce phase separation, all films were exposed to 1,4-dioxane vapor at room temperature. This solvent annealing caused always microphase separation between PS and P4VP-azo phases and sometimes also macrophase separation, i.e., azo compounds crystallized out of BCP matrices. The problem of macrophase separation in the BCP supramolecules is observed already at low concentrations of carboxyl-containing azo compounds. But phenol-containing azo compounds do not macrophase separate up to a molar ratio of azo compounds to repeat units of P4VP as large as 0.5. We conclude that self-associated hydrogen bonds of carboxylic groups and π−π stacking of azo chromophores are driving forces for macrophase separation.
1. INTRODUCTION The microphase separation of block copolymers (BCPs) can lead to various nanostructures which show applications in nanotechnology.1−7 BCP supramolecules are usually fabricated by bonding functional small molecules to BCPs via supramolecular interactions, which is a way to functionalize BCPs.4−6,8−17 BCP supramolecules show some advantages over the covalently linked analogues:4−6,8−17 (1) Synthesis of new BCPs can be avoided by using BCP supramolecules. (2) Morphology of BCP supramolecules is tunable by using the same BCP with different contents of small molecules. (3) Small molecules in BCP supramolecules can be selectively removed. A well-known BCP supramolecular system is polystyreneblock-poly(4-vinylpyridine) (PS-b-P4VP) with hydrogenbonded 3-n-pentadecylphenol (PDP).4,5,11,16,17 Thermal annealing or solvent vapor annealing can induce ordered microphase separated structures in PS-b-P4VP(PDP) supramolecules.4,5,11,16,17 Other functional small molecules besides PDP were also incorporated in BCPs to realize functional BCP supramolecular systems. However, annealing sometimes causes the problem of “macrophase separation” in BCP supramolecules, which means that small molecules crystallize out of BCP supramolecules.18−20 Ikkala and co-workers pointed out that extensive annealing at elevated temperatures can cause macrophase separation in their BCP supramolecules.20 Rancatore et al. showed that proper annealing temperature can avoid macrophase separation in BCP supramolecules.18 We expect that annealing-induced macrophase separation in BCP supramolecules might occur if the self-aggregation tendency of small molecules is larger than their binding strength to BCPs. © XXXX American Chemical Society
However, sufficient experimental verifications of this argument, such as how chemical structures and contents of functional small molecules affect annealing-induced macrophase separation in BCP supramolecules, are not presently available. The aim of our study is an improved understanding of the influence of chemical structures and contents of functional small molecules on annealing-induced macro- and microphase separation in BCP supramolecules. An improved understanding of macro- and microphase separation in BCP supramolecules would enable the design of BCP supramolecules without macrophase separation for a variety of applications in nanotechnology. We focus on azo compounds as prototype model molecules because they are photoresponsive small molecules which have been investigated in BCP systems already.14,21,22 We used various azo compounds substituted with carboxylic or phenolic groups which can form hydrogen bonds with the P4VP blocks of PS-b-P4VP (Figure 1). To study their solvent-annealinginduced phase separation, we used optical microscopy, SEM, AFM, UV−vis absorption spectroscopy, and FTIR spectroscopy. BCP supramolecules can undergo macro- or microphase separation depending on chemical structures and contents of azo compounds. We will show that self-associated hydrogen bonds among carboxylic groups and π−π stacking of azo chromophores can be considered as driving forces for annealing-induced macrophase separation. We will also Received: January 16, 2013 Revised: April 5, 2013
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available in the literature,25 and the solubility parameters of the other compounds are calculated by a group-contribution method (Supporting Information).26−28 The solubility parameters show that PS dissolves in 1,4-dioxane better than P4VP does (Supporting Information).
3. RESULTS AND DISCUSSION 3.1. Phase Separation in PS-b-P4VP with CarboxylContaining Azo Compounds. We prepared thin films of PSb-P4VP(An)x (n = 1, 2, and 3 and 0.1 ≤ x ≤ 0.5) by spincoating. Before solvent annealing, all films were macroscopically homogeneous. No macrophase separation was detected by optical microscopy. The image of PS-b-P4VP(A1)0.5 before annealing is shown in Figure 2a as an example. Other samples before annealing are shown in the Supporting Information (Figure S1). Figure 1. Chemical structures, abbreviations, and hydrogen bonding of block copolymer (BCP) supramolecules.
demonstrate that a significant amount of phenol-containing azo compounds can be incorporated homogeneously into P4VP blocks of PS-b-P4VP without macrophase separation.
2. EXPERIMENTAL SECTION 2.1. Materials. The chemical structures of PS-b-P4VP and azo compounds are shown in Figure 1. PS-b-P4VP with Mn = 330-b-125 kg/mol and PDI = 1.18 was purchased from Polymer Source (Dorval). As shown in Figure 1, the azo compounds are divided into carboxylcontaining Type A compounds (A1, A2, and A3) and phenolcontaining Type B compounds (B1, B2, and B3). The azo compounds A1 and B1 were purchased from Sigma-Aldrich. A2, B2, and B3 were synthesized according to our previous work.14,23 A3 was synthesized according to the literature.24 All other chemicals were purchased from Sigma-Aldrich. 2.2. Methods. Optical microscopy images were recorded on a Zeiss Axiophot microscope. Scanning electron microscopy (SEM) images were obtained with a LEO Gemini 1530 system. Atomic force microscopy (AFM) images were obtained on a Dimension 3100 system using tapping mode. Fourier transfer infrared (FTIR) spectra were obtained using a Nicolet 850 spectrometer. UV−vis absorption spectra were measured on a PerkinElmer Lambda 900 UV−vis spectrometer. Film thicknesses were measured by a Tencor P-10 step profiler. 2.3. Preparation of Thin Films of BCP Supramolecules. PS-bP4VP and azo compounds were dissolved in cyclopentanone separately. The solutions were combined leading to BCP supramolecules which are denoted as PS-b-P4VP(An)x or PS-b-P4VP(Bn)x (n = 1, 2, and 3 and 0.05 ≤ x ≤ 1.0), where x refers to the molar ratio of azo compounds to repeat units of P4VP. Mixed solutions of PS-bP4VP and azo compounds were stirred overnight and filtered through a 0.2 μm filter before use. Thin films of BCP supramolecules were prepared by spin-coating. For UV−vis spectra measurements, fused silica slides were used as substrates. For other measurements, we used silicon wafers. The film thickness was typically ∼200 nm, which was adjusted by spinning speed and concentration of solutions. The spincast films were dried in an oven under vacuum at room temperature overnight. 2.4. Solvent Annealing. The dried films were exposed to saturated 1,4-dioxane vapor in a sealed glass container for 2 days at room temperature. The films were removed from the container after annealing, and the solvent in the films evaporated at ambient laboratory conditions. The solvent vapor can swell the polymers and increase the mobility of polymer chains.2,5 The reason for using 1,4dioxane is that earlier reports showed that this solvent can induce microphase-separated cylinders, which are oriented perpendicularly to substrates.6,8 The solubility parameters of PS and 1,4-dioxane are
Figure 2. Optical microscopy images of spin-cast films of PS-bP4VP(A1)0.5 before (a) and after (b) solvent annealing. Insets are polarized optical microscopy images. (c, d) Scanning electron microscopy (SEM) images of PS-b-P4VP(A1)0.5 after solvent annealing. (e, f) SEM images of PS-b-P4VP(A1)0.5 after solvent annealing and subsequent rinsing in ethanol. The indentations in (e) and (f) show shapes of the crystals which are washed away. The microphase separation of the polymer matrix is visible at higher magnification in (d) and (f).
After solvent annealing for 2 days, some crystals appeared on film surfaces of all PS-b-P4VP(An)x, even when x is as small as 0.1 as observed by optical microscopy (see Figure 2b and Figure S2). We deduce that the crystals are azo compounds because only azo compounds can form crystals whereas the other compositions are amorphous. We will confirm this point by the SEM investigations described below. Figures 2c,d show SEM images of PS-b-P4VP(A1)0.5 after solvent annealing. PS-b-P4VP(A1)0.5 forms macrophase-separated structures; i.e., micrometer-sized crystals of A1 are visible at the surface. Phase-separated nanostructures of the polymer matrix appear with a period of ∼75−120 nm (Figure 2d,f). B
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formation of hydrogen bonding between carboxylic groups and pyridine groups.20,31−33 After solvent annealing of PS-b-P4VP(A1)0.5, the characteristic broad bands of the hydrogen bonding between carboxylic groups and pyridine groups at 2480 and 1920 cm−1 disappear. On the other hand, the characteristic bands of self-associated carboxylic groups in pure A1 at 2673, 2550, 1680, and 1695 cm−1 appear in the spectrum of PS-b-P4VP(A1)0.5 after annealing. These spectral changes provide evidence that hydrogen bonds between carboxylic groups and pyridine groups are broken, and new hydrogen bonds among carboxylic groups of A1 are formed. Complete band assignments of the FTIR spectra are listed in Table S1. A model of solvent-annealing-induced macrophase separation in PS-b-P4VP(An)x is shown in Figure 4a. Solvent
Considering the higher weight fraction of PS, we expect that PS should be the continuous phase, and P4VP with azo compounds should be the dispersed phase. To confirm this point experimentally, PS-b-P4VP(A1)0.5 was rinsed in ethanol for 30 min, dried by a gentle flow of nitrogen, and observed subsequently by SEM. Ethanol is a poor solvent for PS blocks but a good solvent for azo compounds and P4VP blocks. So, ethanol can selectively dissolve azo compounds and swell the P4VP blocks, which cause reconstruction of the phaseseparated structures.6,8,29,30 SEM images of PS-b-P4VP(A1)0.5 after rinsing in ethanol are shown in Figure 2e,f. The crystals on PS-b-P4VP(A1)0.5 are selectively removed but leave indentations on the surface of the film, which indicates that the crystals are composed of A1. Dispersed phases in PS-bP4VP(A1)0.5 change to porous structures, indicating dispersed phases consist of P4VP. The above results show that A1 molecules diffuse out of the BCP matrix and undergo macrophase separation. Similar to PS-b-P4VP(A1)0.5, all other BCP supramolecules with Type A compounds form macrophase-separated structures even at concentrations as low as x = 0.1 (see, for example, Figures S2−S4). We studied the mechanism of macrophase separation also by FTIR spectroscopy. Figure 3 shows FTIR spectra of A1, PS-b-
Figure 4. Model of solvent-annealing-induced macrophase separation in BCP suparmolecules with carboxyl-containing (Type A) azo compounds PS-b-P4VP(An)x (n = 1, 2, 3 and 0.1 ≤ x ≤ 0.5). The black arrows in (a) indicate various diffusion pathways of Type A azo molecules from the polymer matrix to the film surface to form crystals.
annealing initiates Type A azo molecules to diffuse out of P4VP and crystallize at the film surface. The major rearrangements of hydrogen bonds are shown in Figure 4b. Hydrogen bonds between carboxylic groups and pyridine groups are broken, and new hydrogen bonds among carboxylic groups of A1 are formed. 3.2. Phase Separation in PS-b-P4VP with PhenolContaining Azo Compounds. We prepared thin films of PSb-P4VP(Bn)x (n = 1, 2, and 3 and x = 0.5 and 1). All thin films of PS-b-P4VP(Bn)x are macroscopically homogeneous before annealing. The optical microscopy images look similar to Figure 2a. After solvent annealing, we studied the morphology of PS-bP4VP(Bn)x (n = 1, 2, and 3 and x = 0.5 and 1) again by optical microscopy. Unlike the samples of PS-b-P4VP with Type A compounds, PS-b-P4VP(Bn)0.5 (n = 1, 2, and 3) keeps
Figure 3. FTIR spectra of A1, PS-b-P4VP, and PS-b-P4VP(A1)0.5 before and after solvent annealing in the 3300−1780 cm−1 region (a) and 1780−1630 cm−1 region (b). The particular broad band at 1920 cm−1 in (a) is indicated by the dotted curve under the spectrum.
P4VP, and PS-b-P4VP(A1)0.5 before and after solvent annealing. Before annealing, PS-b-P4VP(A1)0.5 shows two particularly broad bands at around 2480 and 1920 cm−1, which are the characteristic bands of hydrogen bonding between carboxylic groups and pyridine groups.31−33 The CO band shifts from 1680 and 1695 cm−1 in pure A1 to 1703 cm−1 in PS-b-P4VP(A1)0.5, which also indicates the C
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fraction of P4VP(B2)0.5 is as high as 43.8% in PS-bP4VP(B2)0.5. It is well-known that solvent annealing can induce different phases and morphologies in the same BCP at different solvent annealing conditions.2,5 BCPs after solvent annealing usually contain some solvents which may affect their morphology. So, solvent-annealing-induced microphase-separated structures might be different from thermal-annealing-induced structures. In our case, PS-b-P4VP(B2)0.5 is solvent annealed for 2 days. Obviously, PS-b-P4VP(B2)0.5 preferably forms cylinder-like structures at the film surface at the current solvent annealing condition. Additionally, different from phase diagrams of normal BCPs such as the phase diagram of PS-b-PMMA, the phase diagrams of both BCP supramolecules and azobenzenecontaining BCPs are strongly asymmetric.34,35 Our results show unambiguously that solvent annealing can induce cylinder-like structures at the film surfaces in PS-bP4VP(B2)x in a wide range of weight fractions of P4VP(B2) (from 27.5% to 43.8%). But we cannot specify a detailed reason why such cylinder-like structures are formed at the film surfaces. We used FTIR to study phase separation of PS-b-P4VP(B2)x with different concentrations of B2 and present the spectra in Figure 6. Pure PS-b-P4VP has a band of free pyridine groups at 993 cm−1.9,11,14,36,37 Hydrogen-bonded pyridine groups show a band at 1011 cm−1.9,11,14,36,37
macroscopically homogeneous after annealing (Figure 5a and Figure S5). However, samples with higher concentrations (x >
Figure 5. Optical microscopy images of PS-b-P4VP(B2)0.5 (a) and PSb-P4VP(B2)1.0 (b) after solvent annealing. Insets are polarized optical microscopy images. (c) AFM height image of PS-b-P4VP(B2)0.5 after solvent annealing. (d) Center-to-center distance (period) of microphase-separated nanostructures in PS-b-P4VP(B2)x as a function of x.
0.5), such as PS-b-P4VP(Bn)1.0 (n = 1, 2, and 3), show that crystals appear on the sample surfaces after annealing (Figure 5b and Figure S6), which indicates macrophase separation. Obviously, the macrophase separation of PS-b-P4VP(Bn)x is strongly dependent on the concentration of Type B compounds. In the case of PS-b-P4VP(B2)x, we prepared thin films with six different concentrations of B2 (0.05 ≤ x ≤ 1) and studied the concentration-dependent macrophase separation of PS-b-P4VP(B2)x. After solvent annealing, PS-bP4VP(B2)x with x ≤ 0.5 keeps macroscopically homogeneous, and PS-b-P4VP(B2)x with x ≥ 0.7 macrophase separates (Figure 5 and Figure S7). It is important to note that macrophase separation after solvent annealing does not happen with Type B azo compounds in PS-b-P4VP(Bn)x with concentrations at least up to x = 0.5. We used AFM to observe microphase separation in PS-bP4VP(B2)x (x ≤ 0.5). We observed that PS-b-P4VP(B2)x (x ≤ 0.5) form quasi-hexagonally packed cylinder-like structures at the film surfaces (Figure 5c and Figure S8). We confirm that the cylinder-like structures, which are perpendicular to substrates, are P4VP(B2)x, and the continuous phase is PS by rinsing the samples in ethanol, similar to the procedure shown in Figure 2e,f. After rinsing in ethanol, the penetration depth of the pores measured by AFM is ∼50 nm. It is difficult to judge what kind of nanostructures are inside the film by crosssectional SEM (Figure S11). The ordering front at the film surface may not go deep into the film at the solvent annealing process. So, the cylinder-like structures may be only at the film surface. The weight fraction of P4VP in pure PS-b-P4VP is 27.5%, and it forms cylinder-like structures at the film surface (Figure S8a). As x increases, the weight fraction of P4VP(B2)x increases and the center-to-center distance of cylinder-like structures increases (Figure 5d). But we do not observe a possible cylinder-to-lamella transition even when the weight
Figure 6. FTIR spectra of B2, PS-b-P4VP, and PS-b-P4VP(B2)x before (solid lines) and after (dashed lines) solvent annealing.
Before annealing, as the concentration of B2 increases, the band of free pyridine groups at 993 cm−1 gradually decreases and the band of hydrogen-bonded pyridine groups at 1011 cm−1 gradually increases. For PS-b-P4VP(B2)1.0, the band of free pyridine groups at 993 cm−1 almost completely disappears, suggesting that B2 can be bonded to nearly all repeat units of P4VP.37 After annealing, the spectra of PS-b-P4VP(B2)x with x ≤ 0.5 do not change, indicating that annealing does not break hydrogen bonds between B2 and pyridine groups. However, the spectra of PS-b-P4VP(B2)x with x ≥ 0.7 change significantly after annealing. The band of hydrogen-bonded pyridine groups at 1011 cm−1 decreases, and the band of free pyridine groups at 993 cm−1 increases. This result indicates that part of the hydrogen bonds between B2 and pyridine groups is broken. Obviously, solvent annealing can break hydrogen bonds between B2 and pyridine groups when the concentration D
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Figure 7. UV−vis absorption spectra of PS-b-P4VP(B2)x before (a) and after (b) solvent annealing. Wavelength of absorption maximum λmax (c) and absorption coefficient at wavelength of absorption maximum αmax (d) of PS-b-P4VP(B2)x before and after solvent annealing. The dashed lines in (a) and (b) indicate λmax of PS-b-P4VP(B2)0.05.
of B2 is too high (x ≥ 0.7). Clearly, P4VP blocks have a limited capacity to incorporate Type B azo compounds. We also studied the mechanism of solvent-induced macroand microphase separation in PS-b-P4VP(B2)x by UV−vis absorption spectroscopy. Figure 7a shows spectra of PS-bP4VP(B2)x before annealing. As x increases, the wavelength of the absorption maximum (λmax) is blue-shifted and the absorption coefficient at λmax (αmax) increases (see Figures 7c and 7d, respectively). The blue-shift of λmax indicates that B2 molecules form H-aggregates, which is a special type of π−π stacking.38−41 Figure 7b shows UV−vis absorption spectra of PS-bP4VP(B2)x after annealing. At concentrations x ≤ 0.5, λmax and αmax of PS-b-P4VP(B2)x do not change (Figure 7c,d). This fact shows that the microscopic environment of the azo chromophores does not change by solvent annealing of PS-bP4VP(B2)x at concentrations x ≤ 0.5. However, spectra of PSb-P4VP(B2)x with x ≥ 0.7 change significantly after annealing. At x ≥ 0.7, we observe a red-shift of λmax after annealing (Figure 7c). At x = 1.0, an apparent tail at λ > 450 nm appears additionally in the spectrum of PS-b-P4VP(B2)1.0 (Figure 7b). We interpret this tail with light scattering by objects with sizes in the order of the wavelength of light, presumably crystals of B2 shown in Figure 5b. We also observed in Figure 7d a sudden decrease of absorption coefficients in PS-b-P4VP(B2)x with x ≥ 0.7. This result implies that macrophase separation causes aggregates of chromophores which are known to result in deviations from the Beer−Lambert law. Schematic views of hydrogen-bonding structures in PS-bP4VP(B2)x are shown in Figure 8. In PS-b-P4VP(B2)x with x ≤ 0.5, solvent annealing does induce neither changes of hydrogen bonds nor macrophase separation. However, in PS-b-P4VP(B2)x with x ≥ 0.7, B2 molecules are getting aggregated, solvent annealing can induce significant changes of the
Figure 8. Model of solvent annealing of PS-b-P4VP(Bn)x with phenolcontaining (Type B) azo compounds. Low concentration region x ≤ 0.5 (a) and higher concentration region x ≥ 0.7 (b) of B2-containing BCP supramolecules.
aggregation state, and furthermore macrophase separation between B2 and PS-b-P4VP occurs. 3.3. Mechanisms of Macrophase Separation Induced by Solvent Vapor Annealing. Our results show that solvent annealing can induce macrophase separation in both types of samples, which are composed of carboxyl-containing (Type A) or phenol-containing (Type B) azo compounds embedded in E
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PS-b-P4VP. Macrophase separation is established by appearance of crystals of azo compounds, which occurs always at the surfaces of the films. The crystal growth at the surface is interpreted with solvent annealing, which swells the surface region first, causes increased mobility of azo compounds, and facilitates their nucleation at the surface. This nucleation at the surface is in line with the indentations observed in Figure 2f. The main difference between macrophase separation in PS-bP4VP(An)x and PS-b-P4VP(Bn)x samples is the critical concentration above which crystal growth begins to start. We recall that Type A azo compounds already form crystals at the lowest concentration used (x = 0.1), whereas Type B azo compounds start to show macrophase separation at significantly larger concentrations x > 0.5. We interpret the different behaviors by the following consideration. During solvent vapor annealing, there is an increased mobility of azo compounds within the matrix of PS-b-P4VP. There is a competition between the hydrogen bonding of azo compounds to the pyridine groups and the self-aggregation of azo compounds, which gains energy from π−π stacking of azo chromophores and self-associated hydrogen bonds. We assume that the π−π stacking energy is rather similar in Type A and Type B azo compounds. But the hydrogen bonding among carboxylic groups (see Figure 4b) could result in a much stronger gain of packing energy as compared to phenolcontaining azo compounds. Therefore, the main reason for the different phase separation behaviors is different types of the hydrogen bonding (i) between pyridine groups and phenolic groups or carboxylic groups and (ii) within the different types of azo crystals. In this respect, the type of hydrogen bonding is the decisive criterion for the occurrence of macrophase separation. Furthermore, in BCP supramolecules with high concentrations of azo compounds (x ≥ 0.7), the π−π stacking energy of H-aggregates becomes sufficiently strong to initiate macrophase separation as sketched in Figure 8b. Therefore, the driving forces for macrophase separation results from energy gain of aggregates by (i) self-associated hydrogen bonds and (ii) π−π stacking of the azo chromophores.
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ASSOCIATED CONTENT
S Supporting Information *
Optical microscopy images, SEM images, UV−vis spectra, AFM images, and detailed analyses of FTIR spectra. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (S.W.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS S.W. was supported by the joint program of the Max Planck Society and the Chinese Academy of Sciences. REFERENCES
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4. CONCLUSION We studied solvent-annealing-induced macro- and microphase separation in PS-b-P4VP with hydrogen-bonded azo compounds. Macrophase separation is identified by crystallization of azo compounds on the sample surfaces. The macrophase separation is observed in PS-b-P4VP with carboxyl-containing azo compounds even when the molar ratio of azo compounds to pyridine groups (x) is as small as 0.1. In contrast to carboxylcontaining azo compounds, phenol-containing azo compounds can be incorporated homogeneously into P4VP phases at a molar ratio x up to 0.5. The use of phenolic groups as hydrogen-bonding donors instead of using carboxylic groups can efficiently suppress macrophase separation because carboxylic groups self-associate more strongly than phenolic groups. Additionally, reducing the π−π stacking of small molecules can also suppress macrophase separation because π−π stacking is another enabling factor for macrophase separation. The key factor to avoid macrophase separation in BCP supramolecules is to reduce self-associations of small molecules and to increase attractions between small molecules and BCPs. Our results demonstrate a strategy for the design of functional BCP supramolecules which avoid the problem of macrophase separation. F
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dx.doi.org/10.1021/ma400104d | Macromolecules XXXX, XXX, XXX−XXX
