Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
pubs.acs.org/Macromolecules
Dependence of Block Copolymer Domain Spacing and Morphology on the Cation Structure of Ionic Liquid Additives Thomas M. Bennett,†,‡ Lewis C. Chambers,† Kristofer J. Thurecht,†,‡,∥ Kevin S. Jack,§ and Idriss Blakey*,†,‡ Australian Institute for Bioengineering and Nanotechnology, ‡Centre for Advanced Imaging, §Centre for Microscopy and Microanalysis, and ∥ARC Centre of Excellence in Convergent Bio-Nano Science and Technology, The University of Queensland, Brisbane, QLD, Australia 4072
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†
S Supporting Information *
ABSTRACT: We find that the topology of phase diagrams for block copolymer/ionic liquid (IL) blends and the scaling of the phase-separated domains are significantly influenced by the cation structure of the IL. We have varied the cation structure of bis(trifluoromethane sulfonyl)imide ILs and investigated the influence of these changes on the thermodynamics of blends with polystyrene-b-poly(methyl methacrylate). Effective χ parameters (χeff) were determined and used to plot phase diagrams, where significant differences were observed, suggesting that the balance of Coulombic and van der Waals interactions was influencing the topology of the phase diagrams. The relationship between IL structure and scaling of domain spacing was also investigated. These important findings shed additional light on the thermodynamic-driven properties of IL-containing block copolymers and have practical implications for how they can be used in nanotemplating or block copolymer lithography applications.
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INTRODUCTION Block copolymers are intriguing because they autonomously assemble into numerous ordered mesoscopic morphologies, which are dictated by factors that include the degree of polymerization (N), volume fraction (f), and the Flory− Huggins polymer−polymer interaction parameter (χ).1,2 Linear A−B diblock copolymers typically exhibit classical microstructures such as hexagonally packed cylinders (HEX), lamellae (LAM), cocontinuous gyroid (GYR), and spheres with a body-centered cubic packing (SBCC).3 These morphologies have been investigated to be used in applications that include polyelectrolytes,4,5 membranes,6,7 templates for deposition of inorganic materials,8 block copolymer lithography,9,10 organic photovoltaics,11 and photonics.12,13 In many of these applications, the performance is influenced by the microstructure, so the ability to predict and tune block copolymer morphology is vital to achieve the best performance. Formation of blends with solvents is a facile method for tuning the morphology of block copolymers.14 When a selective solvent is used, a new effective block volume fraction (f ′A) and interaction parameter (χeff) can be calculated because most of the solvent partitions into one of the blocks.14−16 Progressively increasing the solvent content results in a series of morphological transitions. The volume fraction f ′A can then be calculated if it is assumed that ideal mixing occurs:16 f ′A = fϕP + (1 − ϕP)
Ionic liquids (ILs) are proving to be promising additives for tuning the morphology of block copolymers, which has allowed them to be applied to the fields of photonics17 and block copolymer lithography.18,19 ILs are usually composed of a bulky asymmetric organic cation and an anion with delocalized charge.20,21 These structural features result in most ILs having melting points lower than ambient temperature. In addition, by varying the anion/cation combination, properties that include ionic conductivity, hydrophobicity, and thermal stability can be tuned.20,21 A number of ILs are good solvents for polar polymers such as poly(ethylene oxide),22 poly(methyl methacrylate),23 and poly(2-vinylpyridine)24 but are poor solvents for polystyrene. This enables the ILs to selectively partition into the domains of these more polar polymers when paired with polystyrene.24,25 Incorporation of ionic moieties requires consideration of other interactions that are not present in conventional organic solvents, such as Coulombic forces. For example, Coulombic forces have been predicted to change the topology of block copolymer phase diagrams.26 We have previously constructed the phase diagram for blends of polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) and 1-ethyl-3-methylimidazolium bis(trifluoromethane sulfonyl)imide (EMIM Tf2N).27 A key finding was that EMIM Tf2N could drive low molar mass and disordered PS-b-PMMA to form ordered Received: September 10, 2018 Revised: October 18, 2018
(1)
where ϕP is the block copolymer volume fraction. © XXXX American Chemical Society
A
DOI: 10.1021/acs.macromol.8b01953 Macromolecules XXXX, XXX, XXX−XXX
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morphologies and then undergo phase transitions where the χN, fA coordinates were substantially shifted when compared to neat diblock copolymer melts. EMIM Tf2N is one of an almost unlimited number of possible combinations of anions and cations. Interestingly, however, studies to date regarding the ability of ILs to induce lyotropic phase transitions when blended with block copolymers have mainly focused on varying the block copolymer and/or anion structures.22,28−30 Although this has proved to be an effective route for accessing a variety of morphologies and/or domain sizes in individual block copolymer samples, the ability to precisely control these variables was not demonstrated. This may be in part because the physicochemical properties of ILs are largely dictated by their anion identity;21 thus, varying the anion induces large thermodynamic (and by extension morphological) changes in the resulting block copolymer systems. In contrast, there have been no studies that investigate how varying the cation structure of an IL will alter the phase behavior of their blends with block copolymers, although this has been shown to change the relative strength of interactions of ILs with polymers grafted to silica colloids.23 Consequently, in this study we aim to investigate how the phase behavior and domain scaling of PS-b-PMMA block copolymers blended with ILs can be influenced by systematically varying the length of the alkyl chain and structure in the cation for bis(trifluoromethane sulfonyl)imide (Tf2N)-based ILs. We hypothesized that this could provide a simple means to further expand the versatility of morphology selection and finely control domain spacing. To this end we have mapped the phase behavior for low molecular weight PS-b-PMMA blended with a series of Tf2N-based ILs where the cations were 1-butyl-3-methylimidazolium (BMIM), 1-octyl-3-methylimidazolium (OMIM), and 1-butyl-1-methylpyrrolidinium (BMP) (Figure 1). The data were also compared with our previous findings for blends with 1-ethyl-3-methylimidazolium (EMIM Tf2N) (Figure 1).27 This allowed the effects of gradually increasing the hydrophobicity of the cations to be studied.
Article
EXPERIMENTAL SECTION
Block Copolymer. PS-b-PMMA diblock copolymers were purchased from Polymer Source Inc. The block copolymers were labeled as PSMMA(X−Y), where X and Y are the number-average molecular weights (Mn) of the PS and PMMA blocks, respectively. Gel permeation chromatography was performed to confirm the information provided by the supplier, as reported previously.27 These values are detailed in Table 1 and include Mn, molar mass dispersity (Đ), volume fraction of PS (f PS) (calculated using a density of 1.05 g/ cm3 for PS and 1.19 g/cm3 for PMMA31), N, and χN at 298 K (using χ = 0.0411).32 IL Preparation. BMIM Tf2N, OMIM Tf2N, and BMP Tf2N were synthesized via metathesis of BMIM Cl, OMIM Cl, and BMP Br, respectively, using a small excess of Li Tf2N (Sigma-Aldrich).33 Briefly, using BMIM Tf2N as a representative example, BMIM Cl (6.81 g, 0.039 mol) was dissolved in H2O (100 mL) and mixed with a solution of Li Tf2N (12.92 g, 0.045 mol) in H2O (100 mL). DCM (100 mL) was added, and the mixture was stirred at 25 °C for 16 h. The organic phase was separated, and the aqueous phase was extracted with DCM (3 × 50 mL). The organic phases were then combined and washed with H2O (3 × 50 mL). Following rotary evaporation, the product was placed under high vacuum for 7 days at 25 °C to give BMIM Tf2N as a colorless, viscous liquid (15.2 g, 93.2% yield). Proton nuclear magnetic resonance spectroscopy (Bruker Avance 400 high-resolution NMR spectrometer in DMSO-d6) was used to assess the structure and purity (see Figure S1 of the Supporting Information for the individual spectra) and compare the glass transition temperatures (Tg) with literature values. Differential scanning calorimetry of the ILs was performed on a Mettler Toledo 1 STARe DSC system, which involved two heating−cooling cycles between −100 and 100 °C using a heating rate of 10 °C/min. The Tg values from the second heating cycle are quoted. The Tg values for BMIM Tf2N, OMIM Tf2N, and BMP Tf2N were −88, −85, and −88 °C, respectively, which closely match the reported literature values of −86, −84, and −87 °C, respectively, for these ILs (see Figure S2 for DSC traces).34−36 Confocal Raman Microspectroscopy To Assess Swelling of PS Homopolymer. Atactic polystyrene pellets (BASF) were immersed in BMIM TF2N or OMIM TF2N and then annealed at 175 °C for 72 h. The polymer and liquid phases were then analyzed using confocal Raman microspectroscopy (Thermo Almega). Spectra were collected using the 50× objective and the 785 nm laser to give a nominal X−Y spatial resolution of 1 μm × 1 μm. Spectra were collected between 90 and 3900 cm−1 using a single grating, averaging 32 or more scans using an acquisition time of 1 s. Block Copolymer/IL Mixture Preparation. Blends of the ILs BMIM Tf2N, OMIM Tf2N, and BMP Tf2N were prepared with the five block copolymers shown in Table 1 for ϕP = 1.0−0.5, where ϕP is the volume fraction of copolymer. This is described in the Supporting Information. Characterization of Morphology. The microstructure of the block copolymer/IL blends was assessed using small-angle X-ray scattering (SAXS) at the Australian Synchrotron. Samples dissolved in THF were drop cast into aluminum plates drilled with a 12 × 8 array of holes and coated with a Kapton window. The plates were heated at 50 °C for 24 h to evaporate solvent and then were heated in a nitrogen atmosphere at 180 °C for 24 h. After cooling, the plates were sealed with a second Kapton window. To ensure that the data were representative of the sample, profiles were collected from at least three positions within each well. The X-ray wavelength was 1.512 Å, and the beam size was 235 × 140 μm2. The scattering images were collected using a Dectris Pilatus 1M detector where the distance between sample and detector was 3412 mm. The data were processed by integrating the images azimuthally to give one-dimensional scattering plots.
Figure 1. Structures of (a) EMIM, (b) BMIM, (c) OMIM, (d) BMP, and (e) Tf2N. (f) General structure of the PS-b-PMMA. B
DOI: 10.1021/acs.macromol.8b01953 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Table 1. Characteristics of the PS-b-PMMA Block Copolymers
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polymer no.
polymer
total Mn (kDa)
Đ
f PS
N
χN298 K
1 2 3 4 5
PSMMA(5.0−5.0) PSMMA(9.0−3.3) PSMMA(8.5−9.5) PSMMA(7.1−11.5) PSMMA(10.0−10.0)
10 12.3 18 18.6 20
1.18 1.11 1.06 1.06 1.05
0.529 0.754 0.501 0.410 0.529
98 119 177 183 196
4.0 4.9 7.3 7.5 8.1
coupled with the S−N stretching mode νs(S−N−S) of the TF2N anion37 (green highlight). The 1000 cm−1 band from PS was not observed in the spectra of either of the two ILs phases, and the 744 cm−1 band from the IL is not present in the PS phase. In agreement with previous studies for similar ILs, we suggest that these results confirm that PS is not significantly swollen by these ILs (or partially soluble in them). Influence of IL Structure on the Disorder-to-Order Transition. The five block copolymers have χN values ranging from 4.0 to 8.1 (Table 1), which results in disordered systems when ϕP = 1. This was confirmed by SAXS, where only a broad q* peak was observed for the pure block copolymers (see Figure S3). Addition of IL resulted in significant narrowing of q* at specific concentrations, as can be observed in Figure S3. Narrowing of q* is due to the system transitioning from a disordered to an ordered structure, where the local composition varies in a sinusoidal fashion. Quantitative examples are shown in Figure 3a, which plots the full width at half-maximum of q* versus ϕP for blends of PSMMA(9.0−
RESULTS AND DISCUSSION Selectivity. Ueno et al.23 have shown that EMIM, BMIM, OMIM, and BMP TF2 N can solvate PMMA chains. Furthermore, DSC studies of blends of EMIM TF2N with PS-b-PMMA and both EMIM TF2N and BMP Tf2N with PS homopolymer have shown that there is not significant incorporation of IL in the PS phase.19,25 To determine whether butyl- and octyl-functionalized imidazolium ILs can swell PS, PS homopolymer was annealed in the presence of BMIM TF2N and OMIM TF2N at 175 °C for 72 h, and then after cooling the polymer and IL phases were analyzed by Raman spectroscopy (Figure 2). The most intense peak for the PS was the ring breathing mode at 1000 cm−1 (blue highlight), while for both the ILs the most intense peak was at 744 cm−1, which is assigned to the CF3 bending vibration δs(CF3)
Figure 2. Raman spectra of (a) pure PS control, (b) the polymer phase from annealing PS with BMIM TF2N, (c) the IL phase from annealing PS with BMIM TF2N, (d) polymer phase from annealing PS with OMIM TF2N, and (e) the IL phase from annealing PS with OMIM TF2N. The highlights indicate the positions of the ring breathing mode for PS (blue) and the CF3 bending vibration δs(CF3) coupled with the S−N stretching mode νs(S−N−S) of the TF2N anion (green).
Figure 3. (a) Plots of fwhm versus ϕP for PSMMA(9.0−3.3)/IL blends as indicated in the figure legend. The dotted lines show the positions of the DOTs. (b) χeff,DOT versus ϕIL plots for each block copolymer−IL series. In all cases the data for the blends with EMIM Tf2N have been reproduced from Bennett et al.27 The solid lines are linear regression fits of the data. C
DOI: 10.1021/acs.macromol.8b01953 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Table 2. Effect of the ILs on the DOT and Related Thermodynamic Properties
EMIM BMIM OMIM BMP
0.93 0.68 0.53 0.76
± ± ± ±
molar volume (mL/mol)
χ0
m
cation
0.06 0.07 0.04 0.06
(4.2 (4.4 (4.3 (4.3
± ± ± ±
0.2) 0.3) 0.3) 0.2)
× × × ×
10−2 10−2 10−2 10−2
ΔHvap (kJ/mol, T = 298 K)
258.5 292.8 361.6 303.9
136 155 192 152
± ± ± ±
6a 6a 6a 3b
simulated cohesive energiesc (kJ/mol)
δNPd (MPa0.5)
157 (91 + 62)a 171 (100 + 70)a 198 (120 + 69)a
22.55 21.73 20.74
a
From Santos et al.41 bFrom Deyko et al.42 cValues in parentheses are for contributions from van der Waals and Coulombic interactions, respectively. dFrom Batista and co-workers.40
more data are required before this can be conclusively inferred for IL systems based on other cation structures (e.g., pyridinium, sulfonium, etc.). Influence of IL Structure on the Scaling of Lamellar Domain Spacing. Further insight into the behavior of the blends can be gained by analyzing how the phase-separated domains scale with IL content because concentration-dependent stretching or contraction of the chains can occur depending on the selectivity of the solvent.16,43−45 For a diblock copolymer the following relationship holds:
3.3) with each IL. The point of inflection in these plots was taken as the ϕP at the disorder-to-order transition (ϕP,DOT), and Table S1 lists the ϕP,DOT for each block copolymer/IL combination. It can be seen from this data that the addition of each of the ILs is equivalent to increasing the χeff of the system. χeff,DOT for each blend was determined using the equation1 SDOT = χeff,DOT N
(2)
where SDOT is the value of χN at the ODT (or DOT) as a function of f PS as predicted by Leibler; for example, for symmetrical copolymers (f PS = 0.5), SDOT = 10.5 and N is the volume adjusted degree of polymerization. However, here we substitute f PS with the effective PS volume fraction (f ′PS) when blended with IL, taken as 1 − f′PMMA (calculated from eq 1), based on the evidence that each IL completely segregates into the PMMA domains of each blend (see above).19,23,25 Figure 3b plots the χeff values for the block copolymer/ILs versus the IL volume fraction. The data were fitted to a linear function, in an analogous treatment to the relationship between χeff and the concentration of doped lithium salts:38,39 χeff = mϕIL + χ0
d ∼ ϕP α
(4)
where the scaling parameter (α) quantifies how domain spacing (d) is related to the solvent content. Figure 4 and
(3)
m is a constant that describes how χeff varies with IL content, ϕIL, and χ0 is the interaction parameter for the neat block copolymer. Within experimental error the y-intercept for each plot converges on values of χ0 that agree with the literature value of 0.0411.32 The formation of block copolymer/IL blends will be driven by the relative polymer−IL, polymer−polymer, and IL−IL interactions. Hence, the differences in the m values for different ILs can be understood by considering the differences in their molecular structures and corresponding chemical properties. The van der Waals or dispersive component of these interactions can be estimated from nonpolar solubility parameters (δNP), and m can be seen to scale well with the literature values of δNP in Table 2.40 The overall interactions or cohesive forces have been estimated from the molar enthalpies of vaporization (ΔHvap). For example, Santos and co-workers41 have measured ΔHvap for a homologous series of methylimidazolium Tf2N ILs, where ΔHvap increases with the number of carbons in the alkyl chain. The values relevant to the ILs in this study are listed in Table 2. These values also inversely scale with the m values, and this is consistent with stronger IL−IL interactions for the longer alkyl chains, resulting in lower χeff values for a given IL concentration. Overall, these results highlight that the trends describing the differences in the DOT concentrations of the blends (and thus their corresponding thermodynamic properties) follow the trends in the IL properties related to solubility, namely, the cohesive energy and δNP. Furthermore, the trends also appear to be preserved when the cation is changed from imidazolium to pyrrolidinium when keeping the two alkyl groups constant, but
Figure 4. (a) Variation of d of lamellae for PSMMA(10.0−10.0) and PSMMA(9.0−3.3) for blends with EMIM Tf2N (red), BMIM Tf2N (blue), OMIM Tf2N (green), and BMP Tf2N (yellow). The data for EMIM Tf2N has been previously published and is included for comparison.27 The solid lines are power law fits according to eq 4.
Figure S4 show how the domain spacing of the lamellae varies as a function of ϕP, and corresponding values of α are shown in Table 3. The lamellar phase was selected because 19/20 of the block copolymer−IL pairs exhibited this phase. If we first consider the PSMMA(9.0−3.3) blends in Figure 4, it can be observed that each IL swells the domains to a different extent. For example, at a ϕP of 0.70 the domain spacing for the OMIM, BMIM, BMP, and EMIM blends are 17.9, 19.7, 20.2, and 21.2 nm, respectively, indicating that the swelling is not simply volumetric in terms of IL content. This is also reflected in the values of α which are −0.21, −0.31, −0.43, and −0.63 for the same series (Table 3). Because all the ILs are strongly selective toward the PMMA block, the magnitude of α allows us to quantify how good a solvent the IL is for PMMA, with more negative α values indicating better solvents. Considering the methylimidazolium series, the α values scale inversely with the cohesive energies listed in Table 2, where D
DOI: 10.1021/acs.macromol.8b01953 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Table 3. α Values as the IL Cation Is Varied α values polymer
N
EMIM
BMIM
OMIM
BMP
BMIM−EMIM
PSMMA(5.0−5.0) PSMMA(9.0−3.3) PSMMA(8.5−9.5) PSMMA(7.1−11.5) PSMMA(10.0−10.0)
98.0 119.4 176.5 183.0 195.9
−0.68 −0.63 −1.37 −1.63 −1.69
−0.50 −0.31 −1.18 −1.16 −1.17
−a −0.21 −1.05 −0.93b −1.10
−0.50 −0.43 −1.25 −1.28 −1.38
0.18 0.32 0.19 0.47 0.52
Lamellar morphology not observed. bOnly three data points available to fit.
a
Figure 5. χeffN versus f ′PS empirical phase diagrams for blends of PS-b-PMMA with (a) EMIM Tf2N (reproduced from Bennett et al.27), (b) BMIM Tf2N, (c) OMIM Tf2N, and (d) BMP Tf2N. The red dotted lines show the mean-field theory phase boundaries for diblock copolymer melts.2
and is consistent with the results of Lai and co-workers for blends of poly(styrene-b-isoprene) block copolymers with selective conventional solvents.44 A practical implication of these findings is that the spacing can be tuned by changing the structure of the cation. For example, blends using PSMMA(9.0−3.3) can achieve lamellar domain spacing’s ranging from 17.2 up to 23.8 nm by varying the structure of the cation and the concentration of the IL. However, it is important to note that the dependence of α on factors such as N and f PS means that the degree of volumetric swelling cannot be simply predicted from the IL content, and the system must considered as a whole. Influence of IL Structure on the Phase Diagram. Systematically increasing the volume fraction of IL in the blends caused a series of order-to-order transitions that were evident in the SAXS profiles. Representative SAXS profiles are shown in Figure S3, which illustrate how the morphologies varied as a function of ϕP. All the phase transitions are detailed in Tables S2−S6. Figure 5 collates this data into empirical χeffN versus f ′PS phase diagrams for blends of PS-b-PMMA with BMIM Tf2N (Figure 5b), OMIM Tf2N (Figure 5c), and BMP Tf2N (Figure 5d). The results for EMIM Tf2N from our
the stronger IL−IL interactions that result from increasing the alkyl chain length are reducing the ability of the IL to swell the PMMA domains. Similar overall trends can be observed for the blends; however, the magnitude of α varied significantly and is dependent on both N and f PS. For the two symmetric block copolymers it can be seen that α becomes less negative going from PSMMA(10.0−10.0) to PSMMA(5.0−5.0), and the spread of the values as a function of solvent structure becomes smaller. For example, the difference between the α values for the BMIM and EMIM blends decreases from 0.52 to 0.18 (Table 3). This can be attributed to the PS−PMMA interactions contributing more to the systems thermodynamics for shorter polymers (i.e., the interfacial width of block copolymers relative to their domain size increases in the weak segregation regime).27 In terms of the composition, the asymmetric PSMMA(9.0−3.3) has less negative α values than the symmetric PSMMA(5.0−5.0), but the relative change in the values is larger; that is, α decreases by 50% going from EMIM to the BMIM PSMMA(9.0−3.3) blends, while the corresponding decrease is 26% for the PSMMA(5.0−5.0) blends. This can be attributed to the high IL content required to swell the PSMMA(9.0−3.3) to form a lamellar morphology E
DOI: 10.1021/acs.macromol.8b01953 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
The ability to shift the phase boundaries of the blends by varying only the cation structure shows that certain microstructures can be targeted at a range of different volume fractions of ionic liquid. In particular, we have shown the ability to target the gyroid phase for a range of blends, which is often a difficult phase to access in neat diblock copolymer systems. These are exciting prospects because certain properties of ILs are primarily determined by the anion, which means that if the anion structure is kept constant, then those properties can be maintained, while the microstructure of the blends can be tuned by varying the cation to suit a broader range of uses.20
previous publication (Figure 5a) are also shown to allow easy comparison.27 The mean field theory phase diagram for diblock copolymer melts2 is also overlaid for comparison. In a similar way to other studies of polymer−solvent blends, the value of N took into consideration the volumetric swelling using eq 5.25,27,30,46 N = NPS + NPMMA +
VIL VM,PMMA
(5)
NPS and NPMMA are the degrees of polymerization, VIL is the volume of IL per polymer chain, and VM,PMMA is the molar volume of a PMMA repeat unit. f ′PS > 0.5. For values of f ′PS >0.5 the phase diagrams have a similar topology to those predicted by mean-field theory. For example, blends of PSMMA(9.0−3.3) with the ILs start their trajectory through the phase diagram in the disordered state at a f ′PS of 0.754 and a χeffN of 4.91. Increasing the IL concentration results in the blends traversing to higher χeffN and lower f ′PS values and initially undergoing a DIS-toHEXPMMA transition, followed by a HEXPMMA-to-LAM transition. Here HEXPMMA refers to a hexagonal morphology where PMMA forms the cylinders in a PS matrix. The primary difference caused by changing the length of the alkyl groups in this region is that the HEXPMMA window narrows for OMIM TF2N. No HEXPMMA-to-GYR transition was observed for any of the blends, which indicates that the windows for the PS-rich GYR phase are narrower than 1 vol %. For the more symmetrical block copolymers, transitions from the DIS phase to LAM are observed in the f ′PS > 0.5 region, which is expected. f ′PS ≪ 0.5. When f ′PS is
