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

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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 side- chain 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 elas- tomers 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
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