Bottlebrush Polymers in the Melt and Polyelectrolytes in Solution Share Common Structural Features

Bottlebrush polymers in the melt and polyelectrolytes in solution share common structural features

Joel M. Sarapasa, Tyler B. Martina, Alexandros Chremosa, Jack F. Douglasa, and Kathryn L. Beersa,1

aMaterials Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899

Edited by Krzysztof Matyjaszewski, Carnegie Mellon University, Pittsburgh, PA, and approved January 29, 2020 (received for review September 24, 2019) Uncharged bottlebrush polymer melts and highly charged poly- the exponent λ ranges from 1/3 to 2/5 as backbone length is electrolytes in solution exhibit correlation peaks in scattering increased. A recent simulation study confirmed the finding, λ measurements and simulations. Given the striking superficial of 2/5, in the limit of long backbones (17). Interestingly, similar similarities of these scattering features, there may be a deeper characteristic features are also found in polyelectrolyte solu- structural interrelationship in these chemically different classes of tions and the nature of their origin has triggered an ongoing materials. Correspondingly, we constructed a library of isotopically theoretical discussion, including the origin and meaning of the labeled bottlebrush molecules and measured the bottlebrush so-called “polyelectrolyte peak” observed in scattering mea- correlation peak position q* = 2π=ξ by neutron scattering and in surements (28). The similarity of these two classes of materials simulations. We find that the correlation length scales with the has been noted, although largely in passing (29–31). ξ ∼ c−0.47 backbone concentration, BB , in striking accord with the scaling Here, we take advantage of a variety of polymerizations to of ξ with polymer concentration cP in semidilute polyelectrolyte so- access a library of materials with varying backbone chemistries, ξ ∼ c−1=2 lutions ( P ). The bottlebrush correlation peak broadens with backbone lengths, and sidechain lengths (Fig. 1). Samples were decreasing grafting density, similar to increasing salt concentration synthesized with deuterated sidechains to provide backbone ξ in polyelectrolyte solutions. also scales with sidechain length to a contrast by SANS and elucidate scaling relationships of ξ. The – power in the range of 0.35 0.44, suggesting that the sidechains are interpretation of our scattering experiments is aided by large- relatively collapsed in comparison to the bristlelike configurations scale molecular-dynamics (MD) simulations of coarse-grained often imagined for bottlebrush polymers. polymer model (32). We show that the scaling of ξ with sidechain length, backbone length, and backbone concentration all polymer chemistry | bottlebrush polymers | small-angle neutron display behavior surprisingly similar to polyelectrolyte solution scattering | polyelectrolytes scaling. These results add to our understanding of this important class of materials by providing a framework connecting archi- olymers with long, densely grafted sidechains, commonly tecture and bulk material structure. Pknown as bottlebrush polymers, have attracted significant interest in a variety of fields due to their unique properties (1–6). Results and Discussion Previous studies have established that bottlebrush polymers ex- Macroinitiator and Brush Polymer Synthesis. To access a wide va- hibit a lower propensity to entangle, a property derived from the riety of backbone chemistries, we employed grafting-from poly- relatively large size of the sidechains in comparison to the overall merizations to generate bottlebrush polymers. Atom transfer molecular dimensions (7–10). The discovery of this aspect of bottlebrush polymers has catalyzed the development of an entire Significance field centered on investigating ultrasoft, entanglement-free elastomers for soft robotics and biological tissue mimics (3, 11–13). However, the majority of the brush polymer literature focuses on Bottlebrush polymer materials show great transdisciplinary promise, from tissue engineering to photonics. Homopolymers solution properties and conformations, with interest in bulk exhibit exceptionally low moduli while block brushes segre- properties only gaining traction in the last 5 years (14–23). Even gate into micrometer-sized domains. Both effects are attrib- within these studies, there has been minimal attention given to the uted to the unique packing of brush polymers, a phenomenon potential effects of varying brush-backbone chemistry, despite the that is difficult to measure without careful experimental de- dramatic differences between the two most common brush back- sign. Through precision synthesis and deuterium labeling, we bones (polynorbornene and polyacrylate), both in terms of in- studied a library of brush polymers in the melt using neutron trinsic (ungrafted) rigidity and potential grafting density. There scattering to quantitatively assess backbone packing. We show have been attempts to define what features differentiate a bot- that bottlebrush polymers pack similarly to semidilute poly- tlebrush polymer from a comb polymer (15, 24, 25), but how these electrolyte solutions, and that decreasing grafting density is definitions translate to physical systems of varying backbone analogous to increasing salt concentration in polyelectrolytes. chemistry is not yet clear. These findings suggest hidden similarity in these different One can envision the packing of backbone chains (by ren- materials, one driven through sterics and the other through dering the sidechains invisible) similar to polymers in solutions, electrostatic repulsion. where the backbone chains interpenetrate to form a meshlike ξ structure characterized by a correlation length, . This correla- Author contributions: J.M.S., T.B.M., A.C., J.F.D., and K.L.B. designed research; A.C. de- tion length is typically probed by small-angle neutron and X-ray signed and performed the molecular-dynamics simulations; J.M.S., T.B.M., and A.C. per- scattering (SANS and SAXS, respectively) studies, where ξ is de- formed research; J.M.S. contributed new reagents/analytic tools; J.M.S., T.B.M., A.C., −1 J.F.D., and K.L.B. analyzed data; and J.M.S., T.B.M., A.C., J.F.D., and K.L.B. wrote the paper. fined by the primary peak of intermediate scattering, ξ ∼ qpeak . Early experimental studies suggested that the interbackbone dis- The authors declare no competing interest. tance scales linearly with the degree of polymerization of the This article is a PNAS Direct Submission. sidechains (26, 27), although follow-up simulations considering Published under the PNAS license. bottlebrush polymers with longer sidechains proposed a scaling of 1To whom correspondence may be addressed. Email: [email protected]. 1/2 (14). More recently, it was demonstrated by Chremos and This article contains supporting information online at https://www.pnas.org/lookup/suppl/ doi:10.1073/pnas.1916362117/-/DCSupplemental. Douglas (16) through simulations that the interbackboneλ spacing ~ ξ ∼ ~ scales primarily with the sidechain length NSC as NSC,where First published February 24, 2020.

5168–5175 | PNAS | March 10, 2020 | vol. 117 | no. 10 www.pnas.org/cgi/doi/10.1073/pnas.1916362117 Downloaded by guest on September 29, 2021 Fig. 1. Schematic of bottlebrush polymers with relevant physical parameters labeled (A). General synthetic approach to three chemically distinct families of partially deuterated bottlebrush polymers (B).

radical polymerization is most commonly employed for grafting- from further measurements because the sidechain lengths are from reactions, where the key initiation moiety on the backbone not identical across backbone chemistries and lengths. Brush is often a bromo-isobutyryl ester group (Fig. 1B). While grafting- polymer samples were named based on their backbone chemistry from polymerizations may be considered a more sensitive route family followed by the backbone length letter (S, M, or L), back- to brush polymers, they provide unparalleled access to myriad bone degree of polymerization, graft length letter (S or L), and graft backbone chemistries. Grafting-through polymerizations, in con- length. For example, an acrylate-based brush polymer of backbone trast, allow for complex brush-block architectures but are generally length 80 and sidechain length 20 would be named AcL80S20. limitedtonorbornenebackbones, especially for materials with It is important to highlight that there have been recent dem- longer sidechains. onstrations of highly variable initiation efficiency from macro- Here, we present three macroinitiator families of varying initiators (33). Indeed, there are mixed results in the literature for CHEMISTRY backbone chemistry and grafting density: a flexible acrylate back- the efficiency and control of the grafting-from method. However, bone (Ac) with one sidechain per two backbone carbons, a single- there are a number of existing examples in the literature of care- arm rigid norbornene backbone (SNb) with one sidechain per five fully designed polymerization conditions which yield sidechains backbone carbons, and a double-arm rigid norbornene backbone with high efficiency and targeted molecular weights (34). We took (DNb) with two sidechains per five backbone carbons. The SNb those precautions here, and are confident in the efficiency of our and DNb families were accessed through ring-opening metath- initiation for two reasons: the reproducibility of the results, and the esis polymerization (ROMP), while their corresponding mono- clear and consistent trends that we see as a function of very small mers were synthesized in two steps (described or referenced in changes in the density of the sidechains, which would not be evi- Methods SI Appendix , full scheme available in , Scheme S1). dent in either loosely- or inconsistently grafted macromolecules. Norbornene monomers were polymerized with Grubbs’ third- This is further reinforced by the consistency of predicted bottle- generation catalyst to degrees of polymerization such that each brush molar mass values and those determined by light scattering. macroinitiator would have approximately 25, 45, or 80 grafts. As a result, the SNb family macroinitiators were twice the length of the DNb polymers. Quantitative conversion for both monomers Table 1. Degrees of polymerization, molecular masses, 1 was observed by H NMR spectroscopy after 20 min. The Ac dispersities, and correlation lengths family was polymerized by reversible addition fragmentation † † ξ‡ chain-transfer (RAFT) polymerization, using a trithiocarbonate as Sample NBB* NSC Mn ,kDa Ð* ,nm

the chain-transfer agent. AcS25S16 25 16 50 1.2 3.2

Grafting polymerizations in bulk styrene-d8 were run to 10% AcS25L34 25 34 98 1.1 4.2 conversion to ensure appropriate sidechain length and to suppress AcM45S9 45 9 56 1.2 2.7

the effects of interchain coupling more likely at higher conversions. AcM45L29 45 29 152 1.2 4.4

All grafting-from reactions proceeded smoothly and resulted in AcL80S11 80 11 116 1.4 2.8 well-controlled bottlebrush polymers. For all samples, bottlebrush AcL80L41 80 41 375 1.2 5.6

polymer dispersity, as assessed by differential refractive index de- DNbS12S19 12 19 55 1.1 3.3

tection, remained consistent with macroinitiator dispersity [gel DNbS12L46 12 46 122 1.1 4.7

permeation chromatography (GPC) traces are available in SI DNbM22S20 22 20 98 1.1 3.5 Appendix,Figs.S1–S3, samples tabulated in Table 1]. Light- DNbM22L41 22 41 191 1.1 4.8 scattering data also showed a consistent dispersity for Ac and DNbL40S21 40 21 178 1.3 3.3 DNb polymers, while some of the long-sidechain samples in the DNbL40L33 40 33 270 1.2 4.1 SNb family demonstrated small amounts of interchain coupling. SNbS25S21 25 21 60 1.2 3.3 Samples run to longer sidechain length also demonstrate small, SNbS25L38 25 38 101 1.2 4.0 low molar mass impurity peaks, likely derived from autopoly- SNbM45S15 45 15 74 1.2 3.0 merized styrene. Sidechain molar masses were estimated by SNbM45L28 45 28 131 1.2 3.9 subtracting backbone molar mass from the total brush molar SNbL80S18 80 18 148 1.2 3.2 mass determined by light scattering and dividing by the number SNbL80L26 80 26 213 1.2 3.9 of grafts for a given sample. These values were in relatively good *As determined by GPC-RI, calibrated against polystyrene standards. † agreement with our targeted degrees of polymerization and will Number average relative molar masses as determined by GPC-multiangle ultimately be used to determine brush polymer packing trends. light scattering detection. ‡ We note here that care must be taken in drawing conclusions As measured by SANS.

Sarapas et al. PNAS | March 10, 2020 | vol. 117 | no. 10 | 5169 Downloaded by guest on September 29, 2021 Structural Analysis. Fig. 2 shows the scattering results from the we expect and observe experimentally. To our knowledge, this bulk SANS experiments alongside the structure factors calculated backbone–backbone correlation has, up until this point, only been from MD simulations. The SANS data for all brush systems share observed using SAXS for short-sidechain (C18 and lower) poly- the same qualitative features: power-law scaling at low wave- olefin bottlebrushes and wedge polymers (36, 37). The use of number (momentum transfer), q, and a distinct, primary peak at SANS also allows us to unambiguously interpret the scattering as mid q. The mid-q peak is due to the interchain structure factor, i.e., the result of backbone–backbone correlations, whereas, in the spatial correlations between distinct brush chains. Under the as- SAXS analysis, the backbone correlations are convoluted with sumption of incompressibility, the fully hydrogenated backbone other density fluctuations. We emphasize that this SANS mea- and fully deuterated sidechains of our brush polymers allow us surement is only possible due to the synthesis of bottlebrush to equivalently interpret the SANS results as either the result polymers with isotopically labeled sidechains which, to our knowl- of sidechain–sidechain or backbone–backbone correlations, per edge, have not previously been reported in the literature. ’ – Focusing on the midangle scattering features observed in Fig. Babinet s principle (35). Here we focus on the backbone back- – bone interpretation and identify the high-q peak as arising from a 2 A C, a key trend is evident within each backbone family: longer sidechain grafts result in lower-q peaks, indicating a larger ξ regular backbone spacing in the bulk which we denote as the 2π p between brush backbones. This trend is consistent across both ξ = p q correlation length q ,where is the location of the peak. This backbone chemistry and length and is also reproduced in the – interpretation is corroborated by MD-calculated backbone back- backbone structure factor SBBðqÞ data from the MD simulations bone structure factors of molecules designed to match those in our (Fig. 2E). The primary peak broadens and moves to higher wave experiments, and shown in Fig. 2, which also demonstrate a cor- vectors as the sidechain’s length is decreased, effectively increasing responding primary peak at length scales that are similar to what the backbone polymer concentration, suggesting that the liquidlike

Fig. 2. SANS intensity vs. wavenumber (A–C), simulation snapshots (D), and simulated structure factor (E and F) for brush polymer systems. In A–C The SANS curves within each plot have been shifted vertically for clarity. Synthesized brush polymer samples were named based on their backbone chemistry family followed by the backbone length letter (S, M, or L), backbone degree of polymerization, graft length letter (S or L), and graft length. Simulated data are characterized by the ~ ~ ~ number of backbone statistical segments (beads) per chain NBB, sidechains per backbone bead f=NBB, and number of beads per sidechain NSC.

5170 | www.pnas.org/cgi/doi/10.1073/pnas.1916362117 Sarapas et al. Downloaded by guest on September 29, 2021 order diminishes as the backbone concentration is increased. A significant broadening (see SI Appendix,Fig.S6as FWHM), Similar qualitative trends are observed in polyelectrolyte so- as well as a reduction in maximum peak intensity, is observed lutions, but no interpretation of this behavior has been pro- for the SNb family. These phenomena are qualitatively repro- posed (38–40). We interpret the observed trends in the height ducedinthesimulations(Fig.2D). The reduction in peak in- of the peak as deriving from a soft polymeric corona sur- tensity is likely not due to a loss in contrast; the scattering length − rounding the backbone (Fig. 2D), which effectively localizes ρ = × −6 2 densities of the three systems are similar: Ac 1.12 10 Å , the backbones at length scales associated with the peak loca- − − ρ = × −6 2 ρ = × −6 2 tion by repelling neighboring bottlebrush backbones (16). The DNb 1.12 10 Å ,and SNb 0.87 10 Å .FortheSNb primary peak shifts to smaller q values (larger real-space di- system, the increase in peak FWHM is likely related and indicates ~ an increase in the dispersity of ξ. We also note a trend of in- mensions) as the sidechain length NSC increases because this soft ~ creasing FWHM for increasing backbone carbons per sidechain, polymeric corona is controlled by NSC, which in turn influences the consistent across all families. Due to the broad distribution of average interchain distance. this peak, we posit that the packing of the SNb family is not Experimentally isolating the effect of backbone length NBB homogeneous and that indeed there are many polymer segments within each backbone family is challenging as none of the sys- with smaller and larger localized values of ξ. The distribution in tems have exactly the same sidechain degree of polymerization packing may be due to a number of factors, including interdig- (Table 1). However, within the DNb family, the short-sidechain ξ DNbS S DNbM S DNbL S itation of sidechains (which would decrease ), steric clustering samples ( 12 19, 22 20, and 40 21) have similar near interdigitated chains (which would increase ξ), greater measured sidechain degrees of polymerization (19, 20, and 21, ξ configurational freedom due to slightly larger spacing, or side- respectively). Within this series, we see almost no shift in (3.3, chains fanning out near brush end groups. Ultimately, the change 3.5, and 3.3 nm, respectively). This result is also qualitatively in peak shape observed in the SNb family clearly indicates a reproduced in the simulations (Fig. 2F) where the sidechain change in packing behavior while measured values of ξ are con- lengths and grafting densities are held constant and only the sistent across backbone families. We emphasize that there are backbone length is varied. This initially seems surprising, as one clear differences between single-arm norbornene brush poly- might anticipate shorter backbones of similar grafting density mers that are so prevalent across the literature and their more and sidechain length to yield smaller values of ξ due to the ’ densely grafted cousins, specifically in the nature and regularity sidechain s filling space near the bottlebrush end groups. How- of packing, and we hope this seminal finding catalyzes further ever, if we consider the bottlebrush backbone as a semidilute “ ” ξ comparative studies. linear polymer in a sidechain solution, it becomes clear that In large-scale simulations of bottlebrush polymer melts (16), it CHEMISTRY should not change with degree of polymerization (N ). The − = BB ∼ ~ 1 3 ~ correlation length of a linear polymer in semidilute solution was found that backbone peak scales as qpeak NSC when NBB ~ varies with polymer concentration but less so with molar mass is short relative to M at low grafting densities ðf=NBB < 2Þ, but for bottlebrush polymers; the backbone concentration is simply ~ −2=5 ~ there is cross-over to qpeak ∼ N when NBB becomes longer a function of grafting density and graft length and therefore, SC than N~ . Here, in Fig. 3, we expand these findings with the according to this analogy, backbone length should not affect ξ. SC inclusion of higher grafting densities and compare them to ex- We appreciate that our samples explore a relatively limited perimental findings. The average uncertainty in the estimation of range of backbone degrees of polymerization. Even so, as the exponent was ±0.008, where the range corresponds to two discussed above we see very little change in scaling behavior SDs. In Fig. 3A, Inset we observe that the combined experimental between our particularly short- and intermediate-length sam- data scale as ξ ∼ N 2/5, similar to the scaling for the longer ples. Based on previous solution neutron studies of brush SC backbone length systems in the simulation. If we calculate sep- polymers, this is the exact regime where brush endgroup effects arate scaling exponents for each backbone family, as shown in would be expected to dominate (41). While we cannot de- Fig. 3A, we observe that the lower grafting density system displays cisively say that the trends shown in this work apply to longer a lower scaling exponent, which agrees with the simulated model. brush polymers, we can strongly suggest that scaling behaviors For the higher-density Ac and DNb, an exponent near 0.45 is at the longer degrees of polymerization have fully saturated, found, while for the lower grafting density SNb family we find an and longer degree of polymerization samples would likely not exponent near 0.35. Considering simulations that most closely vary dramatically. ~ ~ align with the SNb family (small values of NBB and f=NBB < 2), the Despite the differences in intrinsic (ungrafted) backbone stiff- = ξ ∼ −1 ∼ ~ 1 3 ness, comparisons between the highly grafted Ac and DNb families backbone peak scales as qpeak NSC , which corresponds to a (Fig. 2 A and B) yield few qualitative differences. This is quite an particle-like scaling (16). This means that the concentration of the important point, as there has been some contention as to what role backbone chains is sufficiently small that one can treat them as inherent backbone stiffness plays in brush polymer properties, and independent polymers in the melt. For longer backbone chains here it seems to impact molecular packing very little. Peak shape and higher grafting densities, the scaling exponent progressively as quantified by full width at half maximum (FWHM) (SI Ap- changes to a value λ ∼ 0.4. In figurative terms, the polymer chains pendix,Fig.S6)andξ-dependence on NSC are similar, although form a meshlike structure within the fluid (or matrix) with an slight peak tailing and increased FWHM can be observed for the average length scale, ξ. This type of cross-over is not surprising and long-graft DNb samples. We note that this finding does not nec- has been typically observed in polyelectrolyte solutions, where at essarily extend to other physical properties (e.g., modulus, glass low concentrations polyelectrolyte chains act as individual parti- transition temperature, or fracture energy) where the backbone cles, having a scaling exponent of 1/3, and as polymer concentra- chemistry may play a larger role in dictating the material proper- tion increases the scaling gradually changes to 1/2 (38, 39, 42–45). ties. The simulation snapshots in Fig. 2D show that, at high It is important to note that the scaling in bottlebrush polymers is a grafting density, the bottlebrush backbone becomes fully extended function of sidechain length, which effectively controls the back- even though the backbone itself has no intrinsic stiffness. In the bone concentration, while in polyelectrolyte solutions the scaling is real system, the intrinsic flexibility of the backbones in the Ac as a function of polymer concentration. series is likely diminished by the steric crowding of the sidechains In this discussion, we have shown that the packing of the to the point where it behaves like the unsaturated DNb family. bottlebrush backbones is analogous to charged polyelectrolyte Comparing either the DNb or Ac systems to the SNb family solutions. These arguments were invoked to explain the behavior provides stark contrast in correlation peak shape and intensity. of the correlation length ξ with NSC, NBB, and grafting density.

Sarapas et al. PNAS | March 10, 2020 | vol. 117 | no. 10 | 5171 Downloaded by guest on September 29, 2021 Fig. 3. Scaling of the backbone–backbone correlation length with sidechain length for the experiments (A) and simulations (B). (A, Inset) represents the overall scaling of the data with the main figure showing the scaling for each backbone chemistry. Separate notations for the sidechain lengths in the ex- ~ periments and simulations are used in order to highlight that NSC is a sidechain degree of polymerization while NSC corresponds to the number of sidechain statistical segments in the simulation.

We can further quantify this analogy by calculating the backbone density. Leveraging this synthetic library, we analyzed the backbone concentration in the melt as —backbone correlations of bulk brush polymers using SANS and molecular simulation. We found that backbone correlations, ξ,in V c = BB , the bulk material vary systematically with sidechain length and the BB + ð Þ ξ ∝ λ λ = VBB f VSC scaling exponent with grafting density ( NSC, 0.35 to 0.44), but showed that ξ is mostly invariant in backbone length. Peak where cBB is backbone concentration, VBB is the backbone repeat breadth characterized by FWHM increased as grafting density unit molar volume, VSC is the sidechain molar volume as esti- decreased, suggesting important packing disparities between high mated by NSC plus one bromine atom, and f is the number of grafting density acrylate systems and lower grafting density nor- grafts per backbone repeat unit. Note that in this calculation bornene systems. Interestingly, the double-arm norbornene and VBB and VSC are defined by polymer portions providing hydro- acrylate systems showed few differences, implying that intrinsic gen and deuterium contrast, respectively. For neutral polymers μ backbone stiffness is not especially impactful in packing. Most in semidilute conditions ξ has been found to scale as ξ ∼ c- surprisingly these trends are similar to those found in poly- with an exponent of μ = 3/4 and polyelectrolytes with an expo- electrolyte solutions, despite our brush polymers being un- nent of μ = 1/2. In Fig. 4, plotting ξ as a function of cBB provides a clean trend with a power-law scaling of μ = 0.47, in quanti- charged. We validated the polyelectrolyte analogy by showing that ξ ξ ∼ −0.47 tative agreement with polyelectrolyte scaling in semidilute so- scales with the backbone monomer concentration as cBB , lution. Based on previous simulations that demonstrate radius which is in near-quantitative agreement with the observed scaling 0.7 −1=2 of gyration of bottlebrush backbones RgBB scales as NBB ,we for polyelectrolyte solutions, ξ ∼ c . can estimate an overlap concentration c* between 0.05 and Brush polymers offer promise as novel materials for applica- 0.01 for our samples, suggesting that our backbones are well tions ranging from photonics to adhesives to biomaterials but a above c*, indicating semidilute conditions. Combining this with our previous results, our data strongly indicate that our uncharged brush polymers display structural scaling more similar to polyelectrolytes than uncharged linear polymers. We illus- trate the mesh formed both by bottlebrush polymers and polyelectrolyte solutions in Fig. 5. We hypothesize that brush polymer backbones repel similarly to polyelectrolyte backbones, but with a long-range repulsion (as illustrated by the sidechain “shell”) derived from excluded volume rather than Coulombic interactions. We take a moment here to also ad- dress the low-q scattering feature observed in our samples that is so often seen in polyelectrolyte solutions (28). Here, ultrasmall- angle X-ray scattering analysis (SI Appendix,Fig.S7)showsthe same strong low-q scattering feature in our samples, which due to strong scattering contrast indicates the feature originates from voids. Such voids may be introduced through sample preparation, but stable void regions (and clustering) are a characteristic feature of charged colloid solutions, polymer-grafted nanoparticles, and plasmas (46–48). At present, we may only conclude that the low- q scattering of bottlebrush melts resembles polyelectrolyte solutions, regardless of the interpretation of this feature. Conclusion We have synthesized a diverse library of brush polymers including both acrylate- and norbornene-based backbone chemistries with Fig. 4. Scaling of the experimental backbone–backbone correlation length varying sidechain length, backbone length, and sidechain grafting with backbone concentration cBB.

5172 | www.pnas.org/cgi/doi/10.1073/pnas.1916362117 Sarapas et al. Downloaded by guest on September 29, 2021 Fig. 5. Schematic depiction of the packing of bulk brush polymers (A) and polyelectrolyte solutions (B).

fundamental understanding of their structure–property behavior organic phase was dried over Na2SO4 and concentrated. The residue was is essential toward their rational design. While many previous purified by silica flash chromatography (hexanes: ethyl acetate:: 4: 1) to yield 1 δ δ studies have been conducted on these materials, the present pure monomer 1 as a clear oil: H NMR (600 MHz, CDCl3): 6.15 (2H, m), 4.36–4.14 (8H, m), δ 3.05 (2H, m), δ 2.57 (2H, d, J = 1.8 Hz), δ 1.98 (1H, m), study highlights that bulk bottlebrush polymers pack analogously 13 δ 1.87 (12H, s), δ 1.42 (1H, m); C NMR (150 MHz, CDCl3): δ 173.2, δ, 171.5, δ to polyelectrolytes. We emphasize that this result was only at- 137.9, δ 63.6, δ 62.0, δ 55.5, δ 47.2, δ 45.8, δ 45.4, δ 30.7. tainable via a diverse library of isotopically labeled brush polymers including both chemical and architectural variation combined with Synthesis of Bottlebrush Backbone by RAFT Polymerization. In a 20-mL Schlenk tube, 2-(2- neutron scattering and carefully matched MD simulations. bromoisobutyryloxy)ethyl acrylate (400 mg, 1.51 mmol), 4-cyano-4-[(dodecylsulfanyl- thiocarbonyl)sulfanyl]pentanoic acid (3 mg, 7.43 μmol), and AIBN (0.25 mg, 1.52 μmol) Methods were dissolved in toluene (0.6 mL). The solution was degassed by freeze pump thaw General. Air- and water-sensitive procedures were carried out under an inert five times, backfilled with argon, and stirred at 75 °C for 4 h. Aliquots were taken every argon atmosphere, while all ROMP polymerizations and network formations hour to ensure conversion did not exceed 50%, and the reaction was stopped after 4 h CHEMISTRY were conducted in an argon glovebox. An Avance II 600-MHz Bruker spec- by submerging it in ice. The polymer was precipitated into cold hexanes three times trometer equipped with a broadband inverse room-temperature probe was from acetone and dried in vacuo to yield pure macroinitiator. † used to record all NMR spectra. GPC measurements were conducted on a Tosoh EcoSEC system with differential refractive index (RI) detection coupled Synthesis of Bottlebrush Backbone by ROMP Polymerization. In an argon to a Wyatt Dawn Heleos II multiangle light-scattering detector (18 angles) glovebox, either monomer 1 or 2 (0.176 mmol) was dissolved in dry THF and a Wyatt Viscostar III differential viscometer. The separation used tetra- (1.65 mL) in a 20-mL scintillation vial. A stock solution of G3 was prepared in hydrofuran (THF) as the eluent at 35 °C and the stationary phase was a set of dry THF and added to the stirring monomer solution such that the final two Tosoh mixed-pore columns (2 x TSK Gel GMHHR-H). Macroinitiator concentration was 0.1 mol/L, and that the molar ratio of monomer and molar mass was determined relative to polystyrene standards and checked catalyst was the target degree of polymerization. The reaction was stirred against theoretical degree of polymerization based on percent conversion. for 20 min, removed from the glovebox, and quenched with 0.25 mL ethyl Bottlebrush polymer data were collected using Astra 7 and molar masses vinyl ether. The quenched reaction was stirred for 1 h, after which solvent were determined based on light-scattering data fit to a linear Zimm for- was removed under reduced pressure. The polymer was precipitated into a malism. The differential RI increment (dn/dc) values used for each polymer 2:1 hexanes:ether solution and dried in vacuo to yield pure macroinitiator. were estimated from the integration of the RI detector signal, assuming 100% recovery of the injected mass. Chain Extension of Macroinitiators. Anisole (0.15 mL), N,N,N′,N″,N″-penta-

methyldiethylenetriamine (30 μL, 0.21 mmol), and styrene-d8 (1 mL) were Materials. All chemicals were purchased from either Sigma-Aldrich, Fisher added to a small conical round-bottom flask with a stir bar and degassed for Scientific, TCI America, or Cambridge Isotopes and used without further 15 min by sparging with argon. Copper (I) bromide (6 mg, 0.042 mmol) was

purification unless otherwise noted. Styrene-d8 was passed over basic alu- quickly added, and the solution was sparged an additional 5 min before being mina before use to remove inhibitors. THF and dichloromethane (DCM) were stirred under argon for 30 min. In a Schlenk tube, macroinitiator (10 mg, 0.038 purified using a solvent purifier apparatus. Copper (I) bromide was purified mmol Br) was dissolved in styrene-d8 (0.7 mL) and sparged for 15 min with by alternating washes of acetic acid and ethanol. The 2-bromopyridine argon. The copper solution was cannulated into the macroinitiator solution, version of Grubbs’ third-generation catalyst (G3) (49), exo-5-norbornene and the reaction was stirred at 80 °C until roughly 10% conversion was methanol (50), exo-5-norbornene methyl 2-bromoisobutyrate (51), 2-hydroxyethyl observed. For shorter sidechains, this was usually on the order of 2 to 4 h, 2-bromoisobutyrate (52), and 2-(2-bromoisobutyryloxy)ethyl acrylate (53) while for longer sidechains this would often take 12 to 24 h. The polymer were prepared according to previous procedures. was precipitated once into hexanes, filtered, and redissolved in DCM. The solution was passed through a basic alumina plug to remove residual Synthesis of Monomer 1. To a 50-mL round-bottom flask, cis-5-norbornene- copper and dried in vacuo to yield pure bottlebrush polymer. exo-2,3-dicarboxylic anhydride (222 mg, 1.35 mmol), 2-hydroxyethyl 2-bromoisobutyrate (600 mg, 2.84 mmol), and 4-dimethylaminopyridine (35 mg, Preparation of Films for SANS. Dry toluene was filtered through a 0.2-μm 0.287 mmol) were added. The solids were dissolved in 9 mL dry DCM, to a poly(tetrafluoroethylene) (PTFE) syringe filter and used to rinse all glass vials final concentration of anhydride of 0.3 mol/L. The flask was submerged in ice used in sample preparation. Bottlebrush polymers were dissolved in DCM, water, and N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride filtered through a 0.2-μm PTFE syringe filter into a rinsed glass vial, con- (326 mg, 1.70 mmol) was added under vigorous stirring. The reaction was centrated, and redissolved in filtered toluene to a concentration of warmed to room temperature and stirred under argon overnight. The re- 100 mg/mL The solution was then pipetted on a cleaned quartz window, air-dried action was then diluted with 50 mL DCM, washed with 100 mL 0.1 mol/L HCl, under aluminum foil for 2 h, then dried in a vacuum oven at 50 °C for 24 h.

twice with 100 mL saturated NaHCO3, and once with 100 mL brine. The Samples were assembled with a second quartz window in a zero-millimeter-gap SANS cell.

† SANS. SANS measurements were collected at the National Institute of The equipment, instruments, materials, and software are identified in the paper in order to adequately specify the experimental details. Such identification does not imply rec- Standards and Technology (NIST) Center for Neutron Research (NCNR) on the ommendation by NIST, nor does it imply the materials are necessarily the best available nSoft 10 m SANS instrument. Data were collected in three configurations at for the purpose. 25 °C over two wavelengths (λ = 12 and 5 Å) and two sample-to-detector

Sarapas et al. PNAS | March 10, 2020 | vol. 117 | no. 10 | 5173 Downloaded by guest on September 29, 2021 distances (1.15 and 4.65 m) to achieve a combined momentum transfer units of real polymer chains, the beads should be identified with statistical −1 segments of flexible polymer having a typical scale of σ = 1nmto2nm. q = 4π sinðθÞ range of ∼0.003 to 0.5 Å . All measurements were conducted λ Simulations were performed in a cubic box with length L; periodic with a wavelength distribution of Δλ=λ = 0.14. The collected raw data were boundary conditions were applied in all three directions. We utilized the reduced and azimuthally averaged using the NCNR SANS reduction macros large-scale atomic/molecular massively parallel simulator (LAMMPS). Simu- and reduced data were analyzed using the SasView software package (54). lations were performed in the NVT ensemble after equilibration in the NPT ensemble at the desired temperature. Time averaging was conducted for ~ 8 Model and Computational Methods. Our system consists of NP polymers (16). O(10 ) time steps after equilibration. The time step was set δt = 0.005τ, We note that all structural simulation variables are denoted with a tilde in where τ = σ (m/e)1/2 is the unit of time. Temperature and pressure are 3 order to avoid confusion with the experimental parameters. A bottlebrush measured in units of e/kB and σ /e, respectively. Simulations were performed polymer has two main features, namely a linear chain backbone and side- at temperature T = 0.8, and P ≈ 0.1 in reduced units. Each polymer segment ~ σ = chains. The backbone is composed of NBB segments and the sidechains each corresponds to a Kuhn segment, having a typical scale of 1 to 2 nm; for ~ σ = composed of NSC segments. Each bottlebrush polymer has f sidechains, polystyrene 1.8 nm (for more details see ref. 55). where one of their free ends is grafted along the backbone chain in a uni- In the analysis of the simulations, the static structure factor of the back- form fashion. Thus, the total number of interaction centers per bottlebrush bone chain segments SBB(q) is calculated as X X ~ = ~ + ~ 1 N N − q·ðr −r Þ polymer is Nt fNSC NBB. S ðqÞ = e i j k , BB Æ j=1 k=1 æ The main focus of the current study is on the following set of molecular pffiffiffiffiffiffi N ~ = − = jqj r parameters: arm lengths of NSC = 2, 5, 10, and 20 segments, backbone lengths where i 1, q is the wavenumber, j is the position of particle j,and ~ ~ N is the total number of backbone segments defined as N = N~ N~ . of NBB = 5, 10, 20, and 40 segments, and grafting densities f=NBB = 1and2. P BB This particular set of parameters leads to a wide distribution of molecular ~ Data Availability. Data are available at the request of the corresponding masses ranging from Nt = 15 to 1,640. A schematic that illustrates the bottlebrush molecular architecture and typical molecular conformations for dif- author, K.L.B. ferent bottlebrush polymers is presented in Fig. 1. The interactions between all types of segments are described by the cut-and-shifted Lennard-Jones poten- ACKNOWLEDGMENTS. This work was supported by the NIST nSoft consor- tial where e and σ define the units of energy and length, and a cutoff distance tium. This work benefited from the use of the SasView application, orig- inally developed under NSF Award DMR-0520547. SasView contains code r = 2.5 σ. The segments along a chain are connected with their neighbors via a c developed with funding from the European Union’s Horizon 2020 re- ð Þ = ð − Þ2 = σ stiff harmonic spring, VH r k r l0 ,wherel0 0.99 is the equilibrium search and innovation programme under the SINE2020 project, Grant length of the spring, and k = 2,500 «=σ2 is the spring constant. In terms of the Agreement 654000.

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