Reduction of (Meth) acrylate-Based Block Copolymers Provides

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Reduction of (Meth)acrylate-Based Block Copolymers Provides Access to Self-Assembled Materials with Ultrasmall Domains Wenxu Zhang,† Mingjun Huang,† Sarah al Abdullatif,† Mao Chen,† Yang Shao-Horn,‡ and Jeremiah A. Johnson*,† †

Department of Chemistry and ‡Department of Mechanical Engineering, Research Laboratory of Electronics, and Department of Materials Science and Engineering, Massachusetts Institution of Technology, Cambridge, Massachusetts 02143, United States

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S Supporting Information *

ABSTRACT: To enable future applications of block copolymer assemblies, it is critical to develop simple approaches to achieve ultrasmall domain spacings from readily available polymers. In this report, we demonstrate that reduction of polymethyl(meth)acrylate-containing block copolymers with LiAlH4 provides novel poly(hydroxyisobutylene)/poly(methallyl alcohol) and poly(hydroxypropylene)/poly(allyl alcohol))-based block copolymers that after thermal annealing display significantly enhanced microphase separation. The effective χ values for these polymers were found to be ≥0.3, and d-spacing values as small as 6.5−7.2 nm were obtained for various morphologies. This work establishes a simple and universal strategy for generation of high χ block copolymers from readily available precursors.

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INTRODUCTION Ordered block copolymers (BCPs) with well-defined periods and microdomain sizes have attracted significant research interest for several decades and have found applications as templates for the fabrication of nanowires1−3 and bit-patterned storage media4 as well as water filtration membranes.5−7 To further extend the applications of these materials, ultrasmall feature sizes/d spacings (d) are urgently needed. 8−10 Decreasing the degree of polymerization (N) is a common way to reduce the interdomain spacing of BCP materials; however, the extent to which N can be reduced is limited by the fact that χN, where χ is the Flory−Huggins interaction parameter, must be greater than a critical value (10.5 for diBCPs with conformational symmetry) to form ordered nanostructures.11 Thus, accessing ultrasmall d values with linear BCPs requires high χ−low N systems. (It is noted here as an alternative approach using polymer topology has also proven effective to some extent, which can be coupled with high χ block copolymers.12−19) One popular strategy to increase χ is to “dope” BCPs with strongly interacting additives. For example, inorganic salts have been used to enhance microphase separation in polystyrene-bpoly(ethylene oxide) (PS-b-PEO)20−24 and BCPs containing poly(vinylpyridine)s (PVPs).13 In addition, hydrogen bonding additives such as poly(acrylic acid) have been used to enhance the incompatibility of PEO-b-poly(propylene oxide)-b-PEO and provide well-ordered polymer blend melts with d values of ∼10 nm.25 Despite being a useful and effective strategy, additives can limit the processing conditions and application scope of BCPs. In search of BCPs that have intrinsically higher χ values, significant efforts have been made toward achieving sub-10 nm © XXXX American Chemical Society

spacing through careful pairing of traditional polymers. For example, poly(tert-butylstyrene) (PtBS)-b-P2VP and poly(dimethylsiloxane) (PDMS)-b-poly(methyl methacrylate) (PMMA) BCPs have produced lamellar d values as small as 9.6 and 8.7 nm, respectively.26,27 In addition, poly(lactic acid) (PLA)-b-PDMS BCPs have been shown to undergo strong microphase separation;28 analogous discrete PLA-b-PDMS cooligomers displayed d values as small as 6.8 and 6.5 nm for lamellar and cylindrical morphologies, respectively, after annealing at room temperature for 6 months.29 Novel monomers/polymers have also been investigated in pursuit high χ systems. For example, polystyrene can be converted to poly(cyclohexylethylene) (PCHE) by hydrogenation; after end-group modification and chain extension, PCHE-b-PMMA and PCHE-b-PEO BCPs displayed 9.0 and 7.9 nm lamellar spacings, respectively.30,31 BCPs with polyhydroxyl segments have shown promise as well. For example, poly(3-hydroxystyrene)-b-poly(tert-butylstyrene) (PtBS) BCPs displayed lamellar d as small as 8.8 nm32 while oligosaccharide-b-poly(p-trimethylsilylstyrene) BCP displayed a cylindrical d of 8.3 nm.33 In addition, poly(glycerol monomethacrylate)-b-PS34,35 and poly(3,4-dihydroxystyrene)b-PS36 have displayed remarkably small lamellar d values of 5.4 and 5.9 nm, respectively. Though the above systems are highly promising, their use of uncommon monomers/reagents and/or multistep chemical synthesis may limit their applications. Thus, we sought to develop a synthetic strategy that (1) uses readily available/ Received: July 24, 2018 Revised: August 8, 2018

A

DOI: 10.1021/acs.macromol.8b01588 Macromolecules XXXX, XXX, XXX−XXX

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addition−fragmentation chain transfer radical polymerization (RAFT) techniques. To avoid undesired end-linking reactions following LiAlH4 reduction, the trithiocarbonate end-groups of BCPs prepared by RAFT were removed.40 The BCPs (reaction scales from 300 mg to 2 g were conducted) were dissolved in dry, degassed tetrahydrofuran (THF) and added dropwise to a slurry of LiAlH4 in THF. The reaction mixture was heated to reflux overnight and quenched via slow addition of excess water. To remove the byproducts, e.g., LiOH and Al(OH)3, aqueous HCl (ca. 3−5 M, 30 mL per gram of polymer) was added, and the crude mixture was heated to ca. 90 °C for 30 min. The insoluble polymer was collected and washed with an additional 30 mL of aqueous HCl and then deionized (DI) water (5 × 30 mL per gram of polymer). The material was then dried under vacuum at 70 °C for 24 h. Characterization data for the resulting polymerspoly(hydroxyisobutylene)/ poly(methallyl alcohol)-b-PS (PiBOH-b-PS), poly(hydroxypropylene)/poly(allyl alcohol)-b-PS (PPOH-b-PS), and PPOH-b-PtBSare summarized in Table 1. Fourier-transform infrared spectroscopy (FT-IR, Figures S1 and S2 in the Supporting Information) showed a complete disappearance of the carbonyl stretching peaks of PMMA and PMA (∼1730 cm−1). The absence of resonances at δ > 160 ppm in the 13C NMR spectra further suggested complete consumption of carbonyl groups. The low Đ values of the parent BCPs (as measured by gel permeation chromatography (GPC) with

accessible materials and (2) could be adopted by current largescale production methods. Herein, we report that reduction of PMMA- or poly(methyl acrylate) (PMA)-containing BCPs using LiAlH4,37,38 a reagent that is widely used for industrial synthesis,39 provides facile access to polyhydroxy-based BCPs with very high χ values (Scheme 1). This approach yields novel BCP materials with ultrasmall d values from readily available, low-cost monomers and reagents. Scheme 1. Synthesis of PiBOH- and PPOH-Based Block Copolymers

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RESULTS AND DISCUSSION We began with the synthesis of PMMA-b-PS, PMA-b-PS, and PMA-b-PtBS BCPs with varied molecular weights (MWs, from 1.9K to 30.8K) and narrow MW distributions (≤1.35) via atom transfer radical polymerization (ATRP) or reversible Table 1. Summary of Block Copolymers Investigated BCP

methoda

DPOHb

DPSb

PiBOH185-b-PS168 PiBOH185-b-PS145 PiBOH185-b-PS100 PiBOH120-b-PS122 PiBOH185-b-PS51 PiBOH84-b-PS51 PiBOH65-b-PS49 PiBOH65-b-PS43 PiBOH65-b-PS36.5 PiBOH65-b-PS27.5 PiBOH30-b-PS43 PiBOH30-b-PS28.5 PiBOH21-b-PS23.4 PiBOH21-b-PS14 PiBOH16-b-PS14 PiBOH16-b-PS13 PiBOH10.8-b-PS12.6 PPOH11.7-b-PS21 PPOH11.7-b-PS18.4 PPOH11.7-b-PS14.5 PPOH11.7-b-PtBS15.2 PPOH8.6-b-PtBS13.8 PPOH11.7-b-PtBS11.3 PPOH8-b-PtBS4.1

ATRP ATRP ATRP ATRP ATRP ATRP RAFT-ER RAFT-ER RAFT-ER RAFT-ER RAFT-ER RAFT-ER RAFT-ER RAFT-ER RAFT-ER RAFT-ER RAFT-ER ATRP ATRP ATRP ATRP ATRP ATRP ATRP

185 185 185 120 185 84 65 65 65 65 30 30 21 21 16 16 10.8 11.7 11.7 11.7 11.7 8.6 11.7 8

168 145 100 122 51 51 49 43 36.5 27.5 43 28.5 23.4 14 14 13 12.6 21 18.4 14.5 15.2 13.8 11.3 4.1

Đc 1.16 1.35 1.31 1.28 1.31 1.24 1.28 1.22 1.24 1.22 1.19 1.25 1.16 1.18 1.31 1.25 1.21 1.08 1.06 1.08 1.06 1.06 1.06 1.05

(1.29) (1.37) (1.33) (1.24) (1.35) (1.11) (1.29) (1.30) (1.22) (1.22) (1.10) (1.26) (1.24) (1.15) (1.20) (1.15) (1.15) (1.05) (1.04) (1.06) (1.02) (1.03) (1.02) (1.01)

Nd

f OHe (%)

phasef

dg (nm)

401.9 369.5 306.0 279.7 237.0 147.9 128.3 119.8 110.7 98.0 88.9 68.5 53.4 40.1 35.7 34.3 29.1 38.1 34.4 28.9 44.6 39.2 35.3 25.8

40.7 44.3 53.5 38.0 71.7 50.5 45.0 48.2 52.3 59.1 30.1 39.2 35.4 47.3 40.6 46.2 37.5 22.3 24.7 29.4 19.1 16.4 24.1 26.3

LAM LAM LAM LAM HEX LAM LAM LAM LAM LAM HEX LAM HEX LAM LAM DIS DIS HEX HEX HEX BCC ODT HEX DIS

41.9 (41.9) 46.2 35.7 42.7 26.1 22.5 21.1 (20.8) 19.3 18.4 17.7 12.6 12.3 9.2 7.7 7.2 (N.A.) N.A. N.A. 7.5 (7.5) 7.4 (7.4) 7.3 (7.3) (7.1) (6.7) (6.5) N.A.

a

The technique utilized for polymer synthesis before LiAlH4 reduction, either ATRP or RAFT-ER (RAFT with the end-group removal). bDPOH and DPs are degrees of polymerization for the polyhydroxy and polystyrenic blocks, respectively, which are calculated by end-group analysis using 1 H NMR spectra. cMeasured by GPC. The values in the parentheses are for the polymer samples before LiAlH4 reduction (for RAFT-ER synthesized BCPs, the values were obtained after the removal of trithiocarbonate groups). dCalculated using a reference volume of 118 Å3, based on the densities of PiBOH, PS, and PtBS being 1.15, 1.04, and 0.95 g/cm3, respectively, while the density of PPOH was estimated to be 1.22 g/cm3. e Volume fraction of polyhydroxy block domains. fMorphologies observed upon thermal annealing. HEX denotes hexagonally packed cylinders, LAM denotes lamellae, BCC is for spheres with body-centered cubic packing, DIS stands for disordered phase, while ODT means the sample was in order−disorder transition state. gThe d-spacing achieved by thermal annealing at 134 ± 1 °C; the values in parentheses are for samples annealed at 179 ± 1 °C. B

DOI: 10.1021/acs.macromol.8b01588 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules 0.025 M LiBr in DMF as the eluent) were retained after LiAlH4 reduction, which suggests minimal backbone degradation. The GPC traces (Figure 1) showed nearly monomodal

Figure 1. Representative GPC traces of BCPs prepared in this work (traces stacked vertically for clarity). Dashed and solid traces are from BCP samples before and after LiAlH4 reduction, respectively. The top two samples were initially prepared by ATRP, while the bottom sample was prepared by RAFT-ER. The high-molecular-weight shoulder in the ATRP-based polymers is attributed to a small amount of radical coupling during the LiAlH4 reduction process (see Supporting Information section S11).

MW distributions and symmetric shapes; however, high-MW shoulders were observed in cases of polymers prepared by ATRP. These high-MW shoulders are attributed to a small amount of biradical coupling resulting from LiAlH4 reduction of the bromine chain end of these polymers (see the Supporting Information section S11 for a detailed discussion and spectroscopic evidence); they are expected to have little effect on their bulk morphological behavior.41 Elemental analysis of representative BCP samples revealed only trace amounts of chloride salts (60, we expect that residual salts should have a negligible effect on the segregation strength of this BCP. The suppression of the second-order peak in PiBOH65-b-PS49 is likely a consequence of the symmetry in volume fractions. Altogether, these data suggest that PiBOH-b-PS has a significantly enhanced χ compared to PMMA-b-PS. The morphologies of a series of PiBOH-b-PS samples were investigated. Most of these BCPs have volume fractions close to 50% (Table 1); they displayed lamellar morphologies as confirmed by SAXS. For example, the SAXS data for PiBOH84b-PS51 (Figure 3a) revealed ordered lamellae (d = 22.5 nm). Transmission electron microscopy (TEM) images of this sample (Figure 3a, bottom inset) showed layered structures with an average center-to-center distance of ∼20 nm, which C

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at 150 °C and the peak broadened significantly, which was a sign of entering a disordered phase (Figure 3b). Increasing the annealing temperature led to further broadening. The BCP passed the order−disorder transition (ODT) between 134 and 150 °C. Thus, the χeff value at 134 °C is only slightly higher than 0.29, where χeff is the effective χ value combining all thermodynamic factors assuming that (χeffN)critical = 10.5 at ODT.10,45 In contrast, the polymer with N = 34.3 (PiBOH16-b-PS13) showed only a broad peak in the SAXS pattern (Figure S14). These observations suggest that the χeffN value crossed the critical point of 10.5 when N changed from 35.7 to 34.3. Thus, we can estimate the χeff parameter to be 0.3 at 134 °C, which provides a χeffN value of 10.7 for PiBOH16-b-PS14, which is consistent with the observed ODT. In contrast, PMMA-b-PS is known to have χ = 0.03 with a reference volume of 118 Å3 at 150 °C (determined by small-angle neutron scattering).46 Thus, remarkably, our results show that χeff can be increased by 1 order of magnitude using a simple organic transformation (LiAlH4 reduction) of a commodity polymer (PMMA) substrate. This large increase in χeff is likely a consequence of both the increased polarity of the PiBOH block as well as hydrogen bonding interactions that were not present in the PMMA precursor. To further elucidate the significance of the obtained χeff and place our results in context, we compared our data to other high χeff values reported in the literature that were determined using ODT. At 150 °C and with a reference volume of 118 Å3, PtBS-b-P2VP, PS-b-PDMS, PMMA-bPDMS, PCHE-b-PMMA, PCHE-b-PEO, and poly(glycerol monomethacrylate)-b-PS were found to possess χeff values of 0.11, 0.11, 0.24, 0.18, 0.22, and 0.40, respectively.10,26,27,31,35,47,48 Various χ values were reported using different testing methods for the P4VP/PS system. From the work of Chang Dae Han and co-workers,49 we estimate that the χeff value is lower than 0.34 using ODT, when the temperature and reference volume are 160 °C and 118 Å3, respectively. Similarly, we reanalyze the data in the literature and find 0.37 < χeff < 0.50 for poly(3,4-dihydroxystyrene)-b-PS (170 °C),36 0.1 < χeff < 0.20 for poly(trimethylsilylstyrene)-bPLA (140 °C),50 and χeff ∼ 0.2 for PDMS-b-PLA (from discrete samples, 150 °C).29 To better understand how d is affected by N, d values of lamellar samples were plotted versus N in a log−log manner (Figure 4). Linear fitting provides a scaling relationship of d ∼

Figure 3. (a) SAXS profiles (stacked vertically for clarity) for PiBOH84-b-PS51 and PiBOH21-b-PS14 annealed at 134 °C. Insets: representative TEM images, where the dark area is the PiBOH domain. (b) SAXS patterns for PiBOH16-b-PS14 at various T. Asterisk denotes the scattering peak from Kapton tape.

was consistent with the SAXS measurement. The SAXS curve for a sample of lower N, PiBOH21-b-PS14 (N = 40.1), revealed a d value of 7.7 nm (Figure 3a), while the high-order peaks were suppressed. The lamellar morphology was confirmed by TEM, where the average center-to-center distance (∼7.6 nm) was in excellent agreement with SAXS (Figure 3a, top inset). The volume fraction was also tuned to achieve other morphologies. For example, PiBOH21-b-PS23.4 (volume fraction of PiBOH, f OH, of 38.6%) showed peaks at a position ratio of 1:√3:√4, which is indicative of hexagonally packed PiBOH cylinders (Figure S12). In contrast, PiBOH185-b-PS51 ( f OH = 71.7%) exhibited peak positions of 1:√4:√7:√12, which is likely the inverse cylindrical morphology (Figure S13). Next, we investigated the morphologies of BCPs with various N values. The shortest PiBOH-b-PS sample that showed a sharp peak (Figure 3b) at 134 °C is PiBOH16-bPS14(N = 35.7); this sample displayed d = 7.2 nm. On the basis of its volume fractions, we expect a lamellar morphology for this sample. The nearly identical volume fractions lead to a suppression of the higher order SAXS peaks. To confirm the ordered microphase separation, PiBOH16-b-PS14 was annealed

Figure 4. Logarithm plot of d versus N for PiBOH-b-PS samples. D

DOI: 10.1021/acs.macromol.8b01588 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Nδ with δ = 0.77 (R2 = 0.98). This value of δ is greater than the value (2/3) predicted by strong segregation theory,51,52 which suggests that these BCP chains adopt a relatively stretched conformation. A similar phenomenon has been reported in a few cases, where by either theoretical or experimental work, a relatively small window of such a segregation regime has been identified.53−55 However, it is surprising here that this regime of stretched chains persists for a large range of N (from ∼30 to ∼400). Next, PPOH-b-PS samples were examined. Upon annealing at 134 °C, PPOH11.7-b-PS21 and PPOH11.7-b-PS18.4 exhibited peaks at position ratios of 1:√3:√5, indicating hexagonal cylinder morphologies with d values of 7.5 and 7.4 nm, respectively (Figure S15). With an N value of 28.9, PPOH11.7b-PS14.5 also displayed a hexagonal morphology (d = 7.3 nm), as confirmed by both SAXS and TEM (Figure 5, inset). These data indicate that PPOH-b-PS has a χeff at least as high as 0.37.

PtBS11.3 (N = 35.3, f OH = 24.1%). This sample displayed a series of sharp peaks at a position ratio of 1:√3:√7 (the peak at √4q* is canceled by the form factor), which suggests a hexagonal cylinder morphology (as supported by TEM, Figure 5). We estimate that the cylinder diameter is ∼4.1 nm. These values are comparable with the smallest d (6.5 nm) and feature size (∼4 nm) reported for the hexagonal cylinder morphology.29 The χeff of PPOH-b-PtBS is calculated to be at least 0.3. It is also observed that when blended with LiTFSI PPOH-bPtBS can provide ordered lamellar morphologies with d values as small as 5.6 nm (Figure S16). In conclusion, this work demonstrates a facile methodology using nonexotic monomers and readily accessible chemistry to obtain high χ BCPs. We show that LiAlH4 can be used to reduce BCPs of PMMA or PMA with little impact on the MW distribution. This change in chemical structure endows the new BCPs with a greatly enhanced χeff, which allows them to form well-ordered morphologies at very short chain lengths, i.e., N = 28.5 in this work. The smallest d values achieved in this report are 7.2 nm for lamellae, 6.5 nm for hexagonally packed cylinders, and 7.1 nm for body-centered cubic. We envision that this method can be easily applied to other (co)polymers/oligomers, such as polyacrylamides, poly(vinyl ester)s, and poly(styrene-co-maleic anhydride)s, especially considering that recent simulation work has suggested that hydroxyl-group based block oligomers can lead to extremely small domain sizes.56 When coupled with other nonpolar polymers, for example, PDMS, polybutadiene, and poly(trimethylsilylstyrene), it is possible that BCPs with even higher χeff values can be prepared. These copolymers can also be complexed with additives that interact strongly to further enhance the χeff values, since the hydroxyl/amine groups are good hydrogen bond donors/acceptors as well as potential ligands for inorganic salts. This report lays out a convenient pathway to accessing ultrasmall spacing block copolymers.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b01588. Experimental details, polymer characterization (1H NMR, Figure 5. SAXS profiles (stacked vertically for clarity) of PPOH11.7-bPS14.5, PPOH11.7-b-PtBS15.2, PPOH8.6-b-PtBS13.8, and PPOH11.7-bPtBS11.3 at 179 °C. Asterisks mark the scattering peak from Kapton tape. Insets: TEM images, where the dark area is the PPOH domain. Top set of TEM images: left [110] plane; right [001] plane.

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C NMR, FT-IR, elemental analysis, and

DSC), and additional SAXS data (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.A.J.).

Similarly, the PPOH-b-PtBS samples were thermally annealed (at 179 °C due to the Tg of PtBS). PPOH11.7-bPtBS15.2 (N = 44.6 and f OH = 19.1%) displayed a set of peaks with a position ratio of 1:√2:√3:√4:√5 (Figure 5), which indicates a body-centered cubic packing of spherical morphology with d = 7.1 nm. TEM supported this result (Figure 5, inset shows [110] face), which is among the smallest d ever reported for a BCP spherical morphology. At 179 °C, the ODT was detected for PPOH8.6-b-PtBS13.8 (N = 39.2, f OH = 16.4%) since a coexistence of a broad peak and a sharp firstorder peak was observed (Figure 5). The smallest d achieved in this work is 6.5 nm, which was observed for PPOH11.7-b-

ORCID

Mao Chen: 0000-0002-5504-3775 Yang Shao-Horn: 0000-0001-8714-2121 Jeremiah A. Johnson: 0000-0001-9157-6491 Author Contributions

W.Z. and J.A.J. conceived the concept and designed the experiments. W.Z., S.a.A., and M.C. performed the synthesis. W.Z. and M.H. investigated the morphologies. Notes

The authors declare no competing financial interest. E

DOI: 10.1021/acs.macromol.8b01588 Macromolecules XXXX, XXX, XXX−XXX

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ACKNOWLEDGMENTS We thank Samsung Advanced Institute of Technology (SAIT) for support of this work. X-ray scattering experiments were performed at the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory (DE-AC0206CH11357). We thank Prof. T. Swager for the use of thermal analysis instrumentation, Prof. S. Z. D. Cheng and Jiahao Huang for TEM imaging, and Prof. X. Gu, Dr. Z. Sun, and Y. Gu for helpful discussions.

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DOI: 10.1021/acs.macromol.8b01588 Macromolecules XXXX, XXX, XXX−XXX