Influence of Molecular Architecture and Chain Flexibility on the Phase

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Influence of Molecular Architecture and Chain Flexibility on the Phase Map of Polystyrene-block-poly(dimethylsiloxane) Brush Block Copolymers Hua-Feng Fei, Benjamin M. Yavitt, Xiyu Hu, Gayathri Kopanati, Alexander Ribbe, and James J. Watkins*

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Department of Polymer Science and Engineering, University of Massachusetts Amherst, 120 Governors Drive, Amherst, Massachusetts 01003, United States S Supporting Information *

ABSTRACT: We report the microphase-separated morphologies of model bottlebrush block copolymers (BBCPs) over a wide range of architectural design parameters. Densely grafted polystyrene-block-poly(dimethylsiloxane) (PS-b-PDMS) BBCPs rapidly self-assemble into ordered lamellar, cylindrical, and deformed spherical morphologies depending on the volume fraction (f), side chain length (Nsc), and overall backbone length (Nbb). The microstructure was characterized by using electron microscopy and X-ray scattering. An experimental phase map is constructed, describing the dependence of morphologies and order−order transitions with respect to the design parameters. A lamellar morphology is primarily observed at symmetric f, while ordered cylindrical and deformed spherical morphologies appear at asymmetric f. The relative flexibility of the PS-b-PDMS backbone facilitates the accessibility of morphologies with curved interfaces and exceptionally large domain spacing. We also find that the breadth of the lamellar window decreases with increasing backbone length and side-chain asymmetry. These findings provide a comprehensive experimental description of the PS-b-PDMS BBCPs and provide insight into the rich phase behavior of this class of macromolecules.

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INTRODUCTION Bottlebrush block copolymers (BBCPs) are densely grafted macromolecules with polymeric side chains tethered to a backbone in discrete block segments.1−4 The branched architecture introduces several novel and useful physical properties.5−7 For example, coupling short side chains with high grafting density reduces the degree of chain entanglements, which enables rapid self-assembly into morphologies with large domain sizes (d0 > 100 nm) and well-developed long-range order (grain sizes on the order of 100 μm).5−13 The phase behavior of linear block copolymers (LBCPs) has been of considerable interest over the past several decades. The incompatibility between the chemical structures of two diblocks facilitates microphase separation into distinct domains. The specific morphology or ordered structure is dependent on the balance between minimizing the interfacial area and reducing chain stretching. As a result, microphaseseparated spheres, cylinders, lamellae, or gyroid structures are achieved by tuning the Flory−Huggins parameter (χ), the total degree of polymerization (N), and volume fraction of each block (f).14−16 Typically in an A−B diblock copolymer, f is dictated by the relative size of the respective blocks. The size of a specific polymeric block (A) is increased when increasing the block length NA. Therefore, f is practically tuned by controlling © XXXX American Chemical Society

the ratio of the block lengths NA/NB. In a BBCP architecture, there are two distinct variables that control f: the length of the block backbone (Nbb) and the length of the brush side chains (Nsc).17−19 Changing these two parameters results in diblock copolymers with the equivalent composition but dramatically different segment lengths and shapes due to the multidimensional nature of the bottlebrush molecular architecture. Changing the respective side chain length is analogous to changing the conformational asymmetry. Such effects are visualized by considering the bottlebrush molecule as a soft cylinder, for example in Scheme 1.10,12 While the phenomenon is understood conceptually, there remains a lack of systematic studies that describe the convoluted effects of variations in f, Nbb, and Nsc on the accessible phases and achievable morphologies of BBCPs. Several examples of well-ordered lamellar morphologies have been reported for a variety of BBCP systems with domain spacings as large as d0 = 160 nm, which are relevant for photonic applications.20−23 The tendency to form flat interfaces has been attributed to the semirigid nature of Received: April 24, 2019 Revised: August 8, 2019

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transition temperature (Tg) contrast (Tg of PS ∼100 °C, Tg of PDMS −125 °C).33,34 Bottlebrushes with PDMS side chains are used as a template for “supersoft elastomers” resulting from a combination of reduced chain entanglements and increased backbone flexibility.35,36 The overall stiffness of the BBCPs can be reduced by introducing PDMS as a flexible side chain. Our group recently reported the self-assembly of PS-b-PDMS BBCPs with short side chains (PS-Mn = 2.9 and 4.7 kg/mol, PDMS-Mn = 4.8 kg/mol) exhibiting exclusively lamellar morphologies.37 A comparison of the backbone contour length (L) and d0 revealed that d0 < 2L, confirming the BBCPs did not assemble into a rigid end-to-end bilayer arrangement, where d0 is equal to twice the backbone contour length (d0 = 2L). The lamellar assembly of the PS-b-PDMS system suggests that the BBCPs are less rigid than previously thought. The overall backbone conformation is driven by many factors such as excluded volume and steric repulsion from side chains, but the inclusion of short flexible PDMS side chains influences the enhanced backbone flexibility. The increase in flexibility could reduce energy penalties associated with the formation of the curved interface, providing more favorable access to additional morphologies. In this study, a new series of PS-b-PDMS BBCPs were prepared with similar side chain lengths. The BBCPs self-assemble into lamellar and cylindrical morphologies of various length scales (which are more commonly observed in the literature) as well as spherical morphologies with exceptionally large domain spacings (Scheme 1d).

Scheme 1. Schematic Representation of Proposed SelfAssembly of BBCPs with Different Side Chains Lengths (Nsc) and Volume Fractions ( f)

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EXPERIMENTAL SECTION

BBCPs were synthesized by sequential ring-opening metathesis polymerization (ROMP) with varying side chain length of PS (Mn = 3.1, 4.7, and 7.6 kg/mol) and PDMS (Mn = 4.8 kg/mol) at approximate volume fractions ranging from f PS = 10−90%, at either short or long backbone lengths (Nbb) (Scheme 2). A total of 54 samples were prepared. Samples in this study are denoted by sample code X-Y-Z%, where “X” represents the PS side chain’s molecular weight (in kg/mol), “Y” represents either “S” or “L” indicating a short (Nbb < 70) or long backbone length (Nbb > 70), respectively, and “Z” is the approximate PS volume fraction (in percent). For example, 3.1k-S-30%PS signifies a sample with a PS side chain length of 3.1 kg/ mol, a shorter overall backbone length, and a PS volume fraction of 30%. The molecular weight of PDMS side chain is 4.8 kg/mol for all the samples. Detailed molecular information about all BBCPs is presented in Table S2 of the Supporting Information. GPC-MALLS traces are presented for all samples in Figures S3 and S4. All samples display a monomodal peak with narrow distribution necessary for systematically investigating the self-assembly and phase behavior. Prior to characterization, all samples were dissolved in dichloromethane at a concentration of ∼10 mg/mL, drop-cast under controlled solvent evaporation until dry, then thermally annealed at T = 135 °C for 6 h before transmission electron microscopy (TEM)

molecular brush backbones (Scheme 1a).19 Rzayev et al. demonstrated that polystyrene-block-poly(lactide) (PS-b-PLA) BBCPs with long PS branches and short PLA branches attained a cylindrical morphology upon self-assembly in the melt (Scheme 1b).10 The long PS side chains induced a significant mismatch in the cross-sectional areas between the two blocks, which forced curvature at the interface.24,25 Additionally, the cylindrical morphology appeared to be aligned, after which the PLA domains were selectively etched to produce nanoporous materials with previously unattainable pore sizes. In this study, polystyrene-block-poly(dimethylsiloxane) (PSb-PDMS) BBCPs were selected as a model system to systematically study the impact of f, Nbb, and Nsc on the phase behavior and morphology transitions in BBCP materials. PS-b-PDMS is a highly relevant material system that is defined by high χ at room temperature (χ = 0.27), facilitating strong microphase segregation between each block.21,26−32 Additionally, PS and PDMS are amorphous and exhibit a high glass

Scheme 2. Overview of PS-b-PDMS BBCPs Architectural Parameter Space

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Figure 1. TEM images of PS-b-PDMS BBCPs with short backbone length over a range of different side chain lengths and volume fractions. Contrast naturally arises between PS domains (light) and PDMS domains (dark). and small-angle X-ray scattering (SAXS) measurements. All the samples containing f PS ≥ 20% were measured by TEM. The domain spacing (d-spacing) was calculated by SAXS via the equation d0 = 2π/ q*, where q* is the primary scattering peak. All d-spacings are listed in Table S3.

Figures 1a,f,k show the morphologies of the samples with PS volume fraction of f PS = 30%. Figure 1a shows a BBCP with short PS side chains (sample 3.1k-S-30%PS) that exhibits a cylindrical morphology with PS cylinders hexagonally packed and embedded in the PDMS matrix. The well-packed hexagonal array is observed in addition to parallel alignment of the cylinders. Here, the volume fraction and the asymmetric side chain length produce a highly asymmetrical molecular architecture, as demonstrated in previous publications.17 There is a strong tendency for the interface to curve and form minority phase PS cylinders.24,38 The cylindrical morphology is consistent with SAXS analysis, where Bragg peak position ratios of q*:√7q* are resolved with a d-spacing of d0 = 46.2 nm (Figure 2a). Figure 1f shows the structure of sample 4.7kS-30%PS which has an increased PS side chain length while keeping f PS = 30%. PS cylinders embedded in the PDMS matrix are also identified, primarily as the perpendicular view

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RESULTS AND DISCUSSION TEM images of selected samples with backbone length Nbb ∼ 50−70 are presented in Figure 1. Each column displays a series of samples with a specific side chain length and increasing volume fraction (f PS) from top to bottom. (Figures 1a−e: PS Mn = 3.1k; Figures 1f−j: PS Mn = 4.7k; Figures 1k−o: PS Mn = 7.6k). Various lamellar, cylindrical, and spherical morphologies are observed across the wide range of f PS. Order−order and order−disorder transitions are resolved across each row and column, illustrating that the self-assembly of the PS-b-PDMS BBCPs is significantly affected by both f PS and Mn of the side chains.10,18,25 C

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Figure 2. Log−log SAXS spectra of PS-b-PDMS BBCPs with short backbone length at room temperature. (a) Mn = 3.1k PS side chain series with f PS ranging from 10 to 90%. (b) Mn = 4.7k PS side chain series with f PS ranging from 10 to 90%. (c) Mn = 7.6k PS side chain series with f PS ranging from 10 to 90%. Spectra are vertically shifted for clarity.

Lamellar structure is observed up to f PS = 60% in 3.1k-S60%PS and 4.7k-S-60%PS, which exhibit primarily asymmetric lamellar morphology where the thicknesses of the PS (light) and PDMS (dark) layers are not equal (Figures 1c,h). However, at the longest PS side chain length, 7.6k-S-60%PS exhibits a nonlamellar structure where the PDMS domain packs into an irregular hexagonal arrangement, while a parallel view of cylinders is observed as well (Figure 1m). SAXS results suggest a cylindrical structure with the peak position ratios of q*:√3q* (Figure 2c). The order−order transition with increasing PS side chain length is due to the significant increase in the asymmetry of the two block compositions despite the rather symmetric composition, in which the bulky PS domain curves over a collapsed PDMS cylindrical domain. The transition from lamellar to cylindrical morphology with increasing PS side chain length at constant f PS is also observed in samples with f PS = 70%. However, the onset is much earlier when increasing the PS side chain length compared f PS = 60%. At f PS = 60%, the cylindrical morphology only forms when the PS side chains are exceptionally long (7.6 kg/mol). 3.1k-S-70% PS exhibits highly asymmetrical lamellar morphology (Figure 1d), confirmed by SAXS result with peak position ratios of q*:2q*. However, 4.7k-S-70%PS and 7.6k-S-70%PS exhibit well-ordered cylindrical morphologies (Figures 1i,n). Figure 1i shows both perpendicular and parallel views of cylinders while Figure 1n shows the perpendicular view image (parallel view image is shown in Figure S8). Again, the SAXS shows the peak position ratios of q*:√7q* and a d-spacing of d0 = 46.5 nm (Figure 2b). As for 7.6k-S-70%PS, the peak position ratio is q*:√3q* and a d-spacing of d0 = 48.5 nm (Figure 2c). With majority PS composition, the contribution of both longer PS side chains and longer PS block backbones produces a significant asymmetric molecular architecture. The cooperative effect supports an earlier onset of the curved interface between the two domains forming PDMS cylinders embedded within the PS matrix.

of the hexagonal array (the parallel view of cylinders is shown in Figure S8). According to SAXS results, the peak position ratio q*:√3q*:√7q* also confirms a well-ordered cylindrical morphology with a d-spacing of d0 = 59.8 nm (Figure 2b). Therefore, cylindrical morphology is identified in a system with symmetric side chain lengths and asymmetric volume fraction as expected. However, a phase transition occurs with further increase in the molecular weight of PS side chain length to Mn = 7.6k (7.6k-S-30%PS) where in Figure 1k the dominating morphology is a disordered wormlike structure with a small region of lamellae. The morphology was assigned as “disordered” as most of the area observed from the TEM image is a wormlike state, additionally confirmed by SAXS where only one primary scattering peak is identified (Figure 2c). The ability of the BBCPs to order into a defined morphology decreases with increasing PS side chain Mn likely due to the increase in relative size of the bulky PS block, where the side chains are inherently stiffer than the more flexible, lowTg PDMS side chains. Figures 1b,j,l reveal the morphologies of the samples with slightly increased PS volume fraction, f PS = 40%. Here, all the samples (3.1k-S-40%PS, 4.7k-S-40%PS, and 7.6k-S-40%PS) exhibit a well-ordered lamellar morphology. The peak position ratios of q*:2q*:3q*:4q* for 3.1k-S-40%PS, q*:2q*:4q* for 4.7k-S-40%PS, and q*:2q*:4q* for 7.6k-S-40%PS confirm the well-ordered structure (Figures 2a, 2b, and 2c, respectively). The d-spacings calculated for the lamellae from the SAXS data for 3.1k-S-40%PS, 4.7k-S-40%PS, and 7.6k-S-40%PS samples are d0 = 52.3, 58.1, and 70.5nm, respectively (Table S3). We find an expected order−order transition from cylindrical to lamellar morphology with an increase from low to high f PS at constant side chain length. The phase transition is attributed in part to the composition becoming more symmetric, resulting in a flat interface between the two domains.18 However, the flat interface is accommodated within the limits of side chain lengths for both symmetric and highly asymmetric situations, which is unexpected. D

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= 116.3 nm. Here, an impressive cylindrical morphology with a large d-spacing over 100 nm is obtained via the self-assembly of BBCPs. The ultra-large spacing bears tremendous potential for an array of applications in filtration and templating. Additionally, an order−order transition is resolved when increasing Nbb, as 3.1k-S-40%PS with Nbb = 60 has a lamellar morphology despite the equivalent side chain lengths and volume fraction (Figure 1b). The phase transition could be attributed to either an increase in the overall segregation strength (χN) or the longer backbone, which has a tendency to accommodate additional conformations as it extends away from the domain interface. The additional degrees of freedom may facilitate curvature at the interface. In extreme cases, a BBCP backbone may be flexible enough to adopt a looping conformation, as discussed by Chang et al. and Sunday et al. in ABC triblock bottlebrush systems.40,43 As previously noted, the lamellar periods of the PS-b-PDMS BBCPs do not follow the extended bilayer motif, suggesting similar flexibility (Figure S5). Understanding the relationship between the backbone persistence length (lp) and backbone contour length (L) is also important for describing the order−order transition. We assume the lp of a densely grafted BBCP is approximately on the order of the side chain size ⟨Rsc⟩.1 Here, we estimate each side chain size according to the packing length concept described by Fetters et al.41,42 For the PS side chains, ⟨Rsc⟩ = 3.7, 4.5, and 5.7 nm for the 3.1k, 4.7k, and 7.6k. For the 4.8k PDMS, ⟨Rsc⟩ = 4.7 nm (see the Supporting Information for calculation).41−43 Because of the diblock architecture of the BBCPs, the backbone on each side of the interface exhibits a unique lp that is dependent on the specific side chain configuration and corresponding excluded volume effects. The total backbone contour length (L) is equal to Nbb·L0, where L0 is the length of a poly(norbornene) repeat unit (L0 = 0.62 nm).21,39,44 L increases from 37.2 to 95.5 nm in the 3.1kS-40%PS to 3.1k-L-40%PS samples, respectively. Therefore, L ≫ lp for the longer BBCP (3.1k-L-40%PS), indicating both the PS and PDMS blocks are composed of multiple persistence segments. L and lp are calculated for all samples and are listed in Table S5. In general, when ratio L/lp > 10 for a respective block, the morphology tends to transition to a nonlamellar or disordered state. The observed structure is highly dependent on the volume fraction as well, so the direct impact of the backbone bending becomes convoluted with additional design parameters. For example, 4.7k-L-40%PS exhibits lamellar morphology (Figure 4g) despite the rather large backbone length of Nbb = 119 and L/lp ∼ 10. Here, the PS side chains (Mn = 4.7 kg/mol) and PDMS side chains (Mn = 4.8 kg/mol) are equivalent, and the conformational volume occupied by both blocks is relatively equal. In the ROMP-based BBCPs, the backbone stiffness is due to the inherent stiffness of the backbone (poly(norbornene)) as well as crowding and steric repulsion in the densely grafted side chains. We find that ⟨Rsc⟩ is comparable to the lp of PS-b-PLA BBCPs determined by selfconsistent field theory by Dalsin et al., where lp is ∼5 backbone repeat units (5L0 ∼ 3.1 nm).21 The direct measurement of lp is desired to understand the relative contributions from the backbone and side chains. Additional simulations and modeling studies are necessary to further elucidate the complex relationship between backbone flexibility and block conformation volume and support the experimental findings. Order−order transitions through increased PS side chain length are also apparent at f PS = 60% in the large Nbb series of BBCPs. 3.1k-L-60%PS exhibits lamellar morphology while

Figure 3. Deformed spherical morphology depicted by 3D reconstruction of HAADF-STEM tomography for sample 7.6k-S80%PS. Gray spheres are the minor PDMS domain.

after 3-D reconstruction generated by HAADF-STEM tomography for 7.6k-S-80%PS. The minor phase PDMS domains appear as oblate, elongated spheres, suggesting the morphology is a “deformed spherical” structure. The weak packing and nonuniform interface could be due to the increased frustration of the longer PS block compressing the much shorter PDMS block. The overall degree of polymerization of the BBCP is also expected to impact the phase behavior due to direct relationship with the segregation strength (χN). Therefore, a separate series of BBCPs with increased Nbb were prepared (Table S2), denoted by sample codes X-L-Z%PS. Figure 4 shows TEM images of selected samples with the longer backbones arranged in a similar configuration as presented in Figure 1. Figures 4a,f,k show the morphologies of the samples with f PS = 30% PS. Here, the backbone lengths are now Nbb = 114, 111, and 122 repeat units for 3.1k-L-30%PS, 4.7k-L-30% PS, and 7.6k-L-30%PS, respectively, compared to Nbb = 56, 65, and 58 for their X-S-Z%PS counterparts. All three samples exhibit disordered wormlike structure. The SAXS spectra confirm the poor long-range order as the primary scattering peak is broad and weak for all three samples (Figures 5a−c). The observation of a disordered structure is surprising in comparison to the shorter Nbb samples, which exhibit cylindrical morphologies at equivalent Nsc and f ps. One would expect the increase in Nbb to also increase the segregation strength. Figures 4b,g,l show the morphologies of the samples with increasing PS content to f PS = 40%. 3.1k-L40%PS shows an ordered cylindrical morphology at a backbone length of Nbb = 154. Figure 4b shows both the perpendicular and parallel orientation of the minor phase PS cylinders. The morphology was also verified by the SAXS data with the peak position ratio of q*:√3q* with a d-spacing of d0 E

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Figure 4. Bright-field TEM images of PS-b-PDMS BBCPs with long backbone length over a range of different side chain lengths and volume fractions. Contrast naturally arises between PS domains (light) and PDMS domains (dark).

sample 4.7k-L-60%PS transitions directly to the deformed spherical morphology (Figure 4h). The d-spacing for the spherical structure is d0 = 103 nm. Similar to the large cylindrical array, a spherical morphology with a large d-spacing of >100 nm is impressive compared to that typically obtained with linear block copolymers. Such materials have potential in templating and preparation of large pore scaffolds for application in photonic and electronic devices.10,11 7.6k-L60%PS (Figure 4m) and all samples with f PS = 70% (Figures 4d,i,n) and f PS = 80% (Figures 4e,j,o) are clearly identified as the deformed spherical morphology from the TEM images. This suggests that the BBCPs with long backbone have a

stronger tendency to assemble into deformed spherical microstructures over a wider window of f PS and side chain Mn compared to the shorter backbone specimens. The samples with f PS = 10%, irrespective of the short or long backbone length, could not be prepared for TEM by cryomicrotome as they are in a liquidlike state at room temperature as well as at cryogenic temperature due to the low Tg of the PDMS domain. Therefore, the morphology was assessed from SAXS exclusively (Figures 2 and 5). From the SAXS data, all the samples with f PS = 10% exhibit weak microphase separation (broad primary peak), indicating that they most likely exhibit a disordered morphology. We note that F

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Figure 5. Log−log SAXS profiles of PS-b-PDMS BBCPs with long backbone length at room temperature. (a) Mn = 3.1k PS side chain series with f PS ranging from 10 to 90%. (b) Mn = 4.7k PS side chain series with f PS ranging from 10 to 90%. (c) Mn = 7.6k PS side chain series with f PS ranging from 10 to 90%. Spectra are vertically shifted for clarity.

Figure 6. Experimental phase maps of morphologies found in microphase separated PS-b-PDMS BBCP. (a) Structure for short backbone length BBCPs with respect to side chain asymmetry and volume fraction. (b) Structure of BBCPs with PS side chain Mn = 3.1 kg/mol and PDMS side chain Mn = 4.8 kg/mol with respect to backbone length and volume fraction. (c) 3D phase map of all the BBCPs.

transition with varying f PS and C for the short backbone samples X-S-Z. All the samples with f PS < 20% hardly display any well-ordered morphology, i.e., the disordered wormlike structure. The volume fraction window (range of f PS across which the structure is observed) of the disordered morphology increases as C increases, suggesting the longer PS side chains disrupt the formation of an equalized interfacial area and longrange assembly. As a consequence, the lamellar window decreases with increasing C. For example, at C = 0.64, lamellae are observed from f PS = 40−70%, but the window decreases to f PS = 40−50% when C = 1.58, suggesting the prevalence of lamellae is strongly dependent on the side chain length and the limits are skewed to the PS side chains. Such a phenomenon is expected due to the necessary increase in curvature forced by the longer PS side chains. However, it is surprising that such an effect is less prevalent at lower values of C, where the PDMS side chains are longer than the PS side chains. The flexible PDMS side chains can accommodate the formation of a flat lamellar interface better than longer, stiffer PS side chains.

samples containing f PS = 20% always tend to form disordered wormlike morphology (Figures S6 and S7). Although the SAXS data show that some samples express higher order scattering peaks, the peaks are consistently broad and weak, and the ratio of peak position does not correspond to an ordered morphology. All samples with f PS = 90% exhibit spherical morphologies (Figures S6 and S7) due to the highly asymmetric composition where the side chains force a highly curved interface of the two domains, demonstrating the extreme limits of the molecular asymmetry. The experimentally determined phase transitions are summarized in a series of phase maps presented in Figure 6. A typical linear BCP “phase diagram” is plotted as the segregation strength χN versus volume fraction f. However, considering the complexity of the structural parameters that define the BBCP architectural, phase maps were constructed based on the side chain Mn ratio (termed a side chain asymmetry constant C = PS Mn/PDMS Mn) and the backbone length (Nbb) rather than χN. Figure 6a shows the morphology G

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Samples with highly asymmetric composition (PS major domain) exhibit deformed spherical morphology for all side chain lengths. For the samples with highly asymmetric f PS, the impact of the volume fraction is far greater that from backbone flexibility or side chain conformation. Figure 6b displays the phase transitions with respect to backbone length Nbb. Here, we show the trends at a constant side chain length with C = 0.64 (PS-3.1k and PDMS-4.8k). The disordered morphology window at low f PS also increases as Nbb increases. Meanwhile, the lamellar window becomes much smaller with increasing Nbb (f PS = 50−60%). Again, samples with highly asymmetric volume fraction (PS major, f PS > 80%) would exhibit deformed spherical morphology irrespective of the backbone length. Finally, a three-dimensional phase map was constructed (Figure 6c) summarizing all order−order and order−disorder transitions observed across the series of PS-b-PDMS BBCPs. Those BBCPs with highly asymmetric molecular architecture (toward a PDMS major domain and long backbone length) always self-assemble into a disordered morphology with poor long-range order. A narrow window for cylindrical morphology at f PS = 30% and f PS = 70% was observed. As expected, rather symmetric compositions ( f PS ∼ 50%) self-assemble into lamellar structure at all levels of side chain Mn and Nbb. However, the window shrinks significantly with increasing PS side chain length and overall backbone length. BBCPs with high asymmetry of volume fraction (f PS > 80%) have a strong tendency to self-assemble into deformed spherical morphology.

AUTHOR INFORMATION

Corresponding Author

*(J.J.W.) E-mail: [email protected]. ORCID

Hua-Feng Fei: 0000-0002-9983-2725 Benjamin M. Yavitt: 0000-0001-9308-7472 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the NSF Center for Hierarchical Manufacturing at the University of Massachusetts Amherst (CMMI-1025020). The Department of Polymer Science and Engineering at the University of Massachusetts Amherst supported facilities used in this work. This research used resources of the CMS beamline (11-BM) of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract DE-SC0012704. The authors thank Drs. M. Fukuto and R. Li of CMS for their assistance with X-ray scattering studies.

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REFERENCES

(1) Paturej, J.; Sheiko, S. S.; Panyukov, S.; Rubinstein, M. Molecular Structure of Bottlebrush Polymers in Melts. Sci. Adv. 2016, 2 (11), No. e1601478. (2) Verduzco, R.; Li, X.; Pesek, S. L.; Stein, G. E. Structure, Function, Self-Assembly, and Applications of Bottlebrush Copolymers. Chem. Soc. Rev. 2015, 44, 2405−2420. (3) Xia, Y.; Olsen, B. D.; Kornfield, J. A.; Grubbs, R. H. Efficient Synthesis of Narrowly Dispersed Brush Copolymers and Study of Their Assemblies: The Importance of Side Chain Arrangement. J. Am. Chem. Soc. 2009, 131 (51), 18525−18532. (4) Xia, Y.; Kornfield, J. A.; Grubbs, R. H. Efficient Synthesis of Narrowly Dispersed Brush Polymers via Living Ring-Opening Metathesis Polymerization of Macromonomers. Macromolecules 2009, 42 (11), 3761−3766. (5) Sveinbjornsson, B. R.; Weitekamp, R. A.; Miyake, G. M.; Xia, Y.; Atwater, H. A.; Grubbs, R. H. Rapid Self-Assembly of Brush Block Copolymers to Photonic Crystals. Proc. Natl. Acad. Sci. U. S. A. 2012, 109 (36), 14332−14336. (6) Liberman-Martin, A. L.; Chu, C. K.; Grubbs, R. H. Application of Bottlebrush Block Copolymers as Photonic Crystals. Macromol. Rapid Commun. 2017, 38, 1700058. (7) Miyake, G. M.; Piunova, V. A.; Weitekamp, R. A.; Grubbs, R. H. Precisely Tunable Photonic Crystals from Rapidly Self-Assembling Brush Block Copolymer Blends. Angew. Chem., Int. Ed. 2012, 51 (45), 11246−11248. (8) Song, D.-P.; Jacucci, G.; Dundar, F.; Naik, A.; Fei, H.-F.; Vignolini, S.; Watkins, J. J. Photonic Resins: Designing Optical Appearance via Block Copolymer Self-Assembly. Macromolecules 2018, 51 (6), 2395−2400. (9) Song, D.-P.; Li, C.; Li, W.; Watkins, J. J. Block Copolymer Nanocomposites with High Refractive Index Contrast for One-Step Photonics. ACS Nano 2016, 10 (1), 1216−1223. (10) Bolton, J.; Bailey, T. S.; Rzayev, J. Large Pore Size Nanoporous Materials from the Self-Assembly of Asymmetric Bottlebrush Block Copolymers. Nano Lett. 2011, 11 (3), 998−1001. (11) Rzayev, J. Synthesis of Polystyrene-Polylactide Bottlebrush Block Copolymers and Their Melt Self-Assembly into Large Domain Nanostructures. Macromolecules 2009, 42 (6), 2135−2141. (12) Song, D.-P.; Li, C.; Colella, N. S.; Xie, W.; Li, S.; Lu, X.; Gido, S. P.; Lee, J.-H.; Watkins, J. J. Large-Volume Self-Organization of Polymer/Nanoparticle Hybrids with Millimeter-Scale Grain Sizes

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CONCLUSION In summary, the self-assembly of PS-b-PDMS BBCPs was resolved over a wide range of different volume fractions, side chain lengths, and backbone lengths. Comprehensive phase maps were constructed based on structural analysis from TEM images and SAXS spectra. Several general trends were revealed, such as the tendency of the breadth of the lamellar window to decrease with increasing asymmetry of side chains and increasing backbone length. Microstructural diversity, such as the formation of cylindrical and deformed spherical morphology, could be attained via tuning the volume fraction and side chain asymmetry as well as the backbone length. In addition to acquiring lamellar microstructure with d0 > 100 nm, the cylindrical and deformed spherical morphology with large dspacing (over 100 nm) were also obtained via rapid selfassembly of BBCPs. We believe the experimental findings presented in this study will support future work directed toward understanding the complex relationship between the BBCP architectures and the resulting morphologies as well as provide a more comprehensive framework for engineering specific microstructure with large domain spacings for advanced applications.

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H NMR and GPC traces of PS-b-PDMS BBCPs; detailed information about macromonomers and BBCPs; additional TEM images (PDF) H

DOI: 10.1021/acs.macromol.9b00843 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.9b00843 Macromolecules XXXX, XXX, XXX−XXX