Modular Self-Assembly and Dynamics in Coordination Star Polymer

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Modular Self-assembly and Dynamics in Coordination Star Polymer Glasses: New Media for Ion Transport Sanjog S. Nagarkar, Masahiko Tsujimoto, Susumu Kitagawa, Nobuhiko Hosono, and Satoshi Horike Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b03481 • Publication Date (Web): 15 Nov 2018 Downloaded from http://pubs.acs.org on November 15, 2018

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Chemistry of Materials

Modular Self-assembly and Dynamics in Coordination Star Polymer Glasses: New Media for Ion Transport Sanjog S. Nagarkar,† Masahiko Tsujimoto,‡ Susumu Kitagawa,*,‡ Nobuhiko Hosono*,‡ and Satoshi Horike*,†,‡,§ †AIST-Kyoto

University Chemical Energy Materials Open Innovation Laboratory (ChEM-OIL), and ‡Institute for Integrated Cell-Material Sciences-Vidyasirimedhi Institute of Science and Technology Research Center, Institute for Advanced Study, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan §Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan ABSTRACT: We report modulation of self-assembled microstructure and dynamics of Coordination Star Polymer (CSP) materials and its utilization for ion transport. Series of CSPs with identical core but varying arm lengths were synthesized via convergent route. The assembled structure exhibit glassy nature with short range structural order while long range disorder with dynamic polymer arm domains. The hierarchical structure and mechanical properties of CSPs are tunable by controlling the size of the CSP corona. Glassy CPSs with dynamic polymer domain were employed as matrix for the transport of alkali metal cations, and the solvent free Li+ ion conductivities of the optimized glass materials were found to be comparable to the well know organic polymer electrolytes. Further we characterized that the ion transporting behavior in CSP glasses are strongly correlated to their structural ordering and local dynamics. The soft and dynamic nature of the CSPs was explored to improve solid-solid interface. The study demonstrates the potential of CSP glasses as new class of designable inorganic organic hybrid electrolyte for solid-state applications.

INTRODUCTION Coordination star polymers (CSPs) are a new class of inorganic-organic hybrid star shaped molecules with discrete metal-organic polyhedra (MOP) core decorated with radiating organic polymer arms.1-4 CSPs are unique class of compound, with synergistically combined advantages of metal-organic materials and polymers. Owing to compatibility with organic matrix and solubility, CSPs have been incorporated to the polymers for gas separation application,5-9 in another example they have been incorporated in to the lipids where it functions as ion channel.10, 11 CSPs have also been explored for catalysis,12, 13 drug delivery14-16 applications in solution. However, all the above applications benefit from the utilization of the intrinsic cavity within the MOP core, while the polymer arms primarily serve to improve the structure solubility in given a matrix. In bulk, owing to branched topology CSPs cannot pack well, but dynamic polymer chains self-assemble via entanglement and are held together by cohesive forces with distributed MOP cores.17 Few CSPs which could be crystallized under strict conditions, revealed highly disordered polymer arms even at low temperatures.3, 14, 1820 A large number of dangling end groups with conformational freedom provide CSPs with higher volume fractions and highly dynamic polymer chains in bulk, which is difficult to attain in polymer alone. This is

attractive as a media for mass transport, for example ion transport or gas diffusivity.21-26 Additionally, design of ion transport pathway in molecular solids having optimal mechanical property is of significant challenge toward various electronic devices.27, 28 Addition of plasticizer or nano-fillers like Al2O3, SiO2, TiO2 or Metal organic frameworks (MOFs), to polymer matrix to make nanocomposite is widely used method to reduce polymer packing and realize high ion transport.2931 However, homogeneity of composite is limited and these physical mixtures are rarely stable.32 Single component hyper-branched polymers are attractive, but accompany an undesirable features like poor control of molecular weight, broad molecular weight distributions and possible intramolecular cyclization, thus lack of control over structural uniformity.33-35 The microscopic and mesoscopic hierarchical structure of star shaped molecules and polymer chain dynamics depend on the core geometry, branching functionality (f), and polymer arm functionality.2, 36 In this regard, CSPs possess enormous compositional diversity of inorganic and organic components,37, 38 and allow facile control over branching functionality (f = 3-120),39 MOP size, geometry (anisotropy)40, 41 along with synthetic advances42, 43 provide wide opportunities to precisely tune the inherent structures and functions of CSPs over those of conventional star polymers.2, 44 However, the research about control of hierarchical structure of CSPs, its

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translation to bulk properties is still in its infancy and therefore systematic studies on structure-properties and their utilization for applications are highly demanded. There has been no report on the utilization of dynamic polymer domains in CSPs for optimized ion transport. In the present research, we focus on the dynamic properties of arms part of CSPs and linked hierarchical self-assembled structure for transport of ions. We synthesized three CSPs to investigate the role of arms for these purposes. Using SAXS, DMA, Indentation and thermal analysis techniques, influence of arms content on CSPs for assembling structure and dynamics, mechanical and thermal properties were investigated. The tailored self-assembled glassy structures with dynamic polymer arm domains were utilized as matrix for the transport of ions and to improve solid-solid interfaces and characterized by solid-state NMR and AC impedance analysis. Correlation of structural order in CSP glass and ionic movements is presented.

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revealed a single peak with a shorter elusion time for each CSPs compared to corresponding ligands (Figure 1b). The MALDI TOF-MS analysis of the elute yielded peaks at m/z = 15594 (CSP-1), 20035 (CSP-2) and 23231 (CSP-3) corresponding to 24 ligands and 24 Cu2+ from the 12 paddle-wheel clusters [calcd. m/z = 15708 (CSP-1), 20771 (CSP-2), 23291 (CSP-3)] (Figure S1-S3). The slight difference between the experimental and calculated mass, is due to the molecular weight distribution of the PEG chains. The 686 nm peak in the UV-visible spectra (Figure 1c), attributed to metal-ligand charge transfer, is a strong indication of the formation of dinuclear Cu2+ paddlewheel cluster in all the compounds.47, 48 This is further confirmed by the shift in the strong carbonyl vibration peak (νasymmetric: C=O; ~1720 cm−1) of the ligands, to lower wavenumbers (~1640 cm−1) in the CSPs (Figure S4).

RESULTS AND DISCUSSION A rhombicuboctahedral MOP with 12 Cu2+ paddle wheel centres and 24 isophthalate ligands (Cu24L24) was chosen as the CSP core (branching point, f = 24) to graft the polyethylene glycol (PEG) chains.45 PEG was chosen owing to its easy functionalization, high segmental mobility, relatively high dielectric constant (that facilitates charge separation), and solvating capability for a number of alkali metal salts.46 Figure 1 shows synthetic protocol for CSPs. The CSP-1, CSP-2, and CSP-3 with increasing PEG chain lengths grafted on identical MOP core were synthesized by reacting Cu(OAc)2·2H2O and ligands 1, 2, and 3 in THF at 25 °C respectively (Figure 1a). The formation of high-molecular weight CSPs without fragmentation was evidenced by size-exclusion chromatography (SEC), which

Figure 2. (a) Self-assembled structure of coordination star polymer (CSP). (b) SAXS profiles of CSP-1 (blue), CSP-2 (orange) and CSP-3 (olive). (c) TEM image of CSP-1; scale bar 5 nm.

Figure 1. (a) Schematic of the synthesis of CSPs via convergent route. (b) SEC traces of ligands 1, 2 and 3 (dashed lines) and corresponding CSP-1 (blue), CSP-2 (orange) and CSP-3 (olive) (solid lines) in THF. (c) UV-visible spectra of CSP-1 (blue), CSP-2 (orange) and CSP-3 (olive) in MeCN.

The X-ray powder diffraction (XRPD) measurements reveal the amorphous nature of the self-assembled structure (Figure 2a and S5). Hence, the three dimensional (3D) microstructures of dried CSPs were investigated using small-angle X-ray scattering (SAXS) measurements. A single broad Bragg reflection peak was observed for all the samples as seen in Figure 2b, confirming short range structural order in 3D.49 The extracted d-spacing values, which correspond to the center to center distance between adjacent CSPs, are found to increase with polymer chain lengths from 2.9 nm in CSP-1 to 3.6 nm in CSP-2 and 4.1 nm in CSP-3. Although, these distances are longer than those expected for the MOP core (~2.5 nm), however are much smaller than would be expected for fully flexed chains, suggests that the PEG chains from adjacent CSPs are rather entangled than interdigitated.19 Owing to the highly flexible nature of the PEG chains, the well separated


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Chemistry of Materials branching positions and large enough aperture size of the MOP core (8 and 12 Å characterized from the crystal structures),1, 19 the possibility of the interpenetration of the arms in adjacent MOP cores and/or self-inclusion of PEG chains in MOP core cannot be ruled out. Thus CSP form a densely packed structure with short range order. Transmission electron microscopy (TEM) provided information on the size, shape and assembly of CSPs in 2D. Although, the direct visualization of CSPs by TEM is difficult due to sensitivity of the MOP core to high energy electron beam.14 We observed, a uniform distribution of spherical CSP particles of ~2 nm for all CSP samples corresponding the Cu paddle wheel shell (Figure 2c and S6). A higher glass transition temperature (Tg) is recorded for CSP-1 (−20.9 °C) compared to ligand 1 (−52.8 °C), whereas no significant differences between the Tg of CSP2 and CSP-3 and those of the respective ligands are observed (Figure S7-S8, Table S1). These data are indicative stronger attractive forces between the polymer chains and core, dictating the improved packing effect and density in CSP-1. In contrast, the Tg values of CSP-2 and CSP-3 are mainly governed by the packing of polymer component.50, 51 The strength of intermolecular interactions and cooperative dynamic movements are of particular interest because of their key role in the material properties such as stiffness, diffusion, and responsiveness. In this regard, dynamical mechanical analysis (DMA) provides insight into the transitions caused by molecular motions and free volume changes in the material. As seen in figure 3a, by changing the polymer chain length, the volume fractions of the CSPs and associated mechanical modulus could be readily altered (Table S1). For all core fraction, both the G' and G" showed comparable magnitude and with increasing frequency, a fluid-like (G' < G") to solid-like (G' > G") transition was observed. However, the CSPs are still stiffer than their respective constituent ligands and thus, the behavior is different from ligands. Similar behavior was observed in the literature.52 The G'

Figure 3. (a) Frequency sweep rheological properties of CSP1 (blue), CSP-2 (orange), and CSP-3 (olive) at 25 °C; G' (solid)

and G" (open). (b) Variable temperature rheological properties of CSP-1 (blue), CSP-2 (orange) and CSP-3 (olive) at 1 Hz; G' (solid) and G" (open). (c) Transparent CSP-1 pellet broken into pieces and then re-mended by gentle pressure above 40 °C. (d) Elastic moduli (E) of CSP-1 as a function of indentation depth (error bars representing the standard deviation of 10 measurements)

became significantly higher than G" in case of CSP-1, while for CSP-2 and CSP-3 both the moduli remained comparable. The absolute G' values decrease with increasing polymer arm content (CSP-1 > CSP-2 > CSP-3) with CSP-1 being the stiffest. As the polymer chain length increases, the chains experience less steric crowding and poor packing because of the star shape. Thus, the decreasing stiffness of CSP with longer arms can be ascribed to the increased thickness of the deformable corona.53 The G' values for CSP-2 (27.2 kPa) and CSP-3 (11.3 kPa) are comparable to those of reported for PMMAPBA based block copolymer CSP,38 whereas that of CSP-1 (1236.1 kPa) is significantly higher (at 0.1 rad s−1). All the samples showed solid-like (G' > G") to fluid-like (G' < G") transition, when heated above 45 °C, demonstrating thermoplastic behavior, while retaining large storage modulus of ≥105 Pa up to 50 °C (Figure 3b). Over the studied temperature range, CSP core remains stable (>50 °C) due to positive chelate cooperativity,18 there by indicating that the solid-like to fluid-like transition is facilitated by the dynamic PEG chains. The dynamic nature of CSPs was used to correct the grain boundaries formed in the system. A transparent pellet of CSP-1 was obtained by pressing, which indicate the absence of internal grain boundaries scattering the incident light (Figure 3c and S9). The scanning electron microscopy (SEM) analysis of the pellet also confirm grain boundary free smooth surface (Figure S10). Moreover, when broken into pieces, the pellet could be re-mended by the application of gentle pressure above 40 °C while maintaining transparency (Figure 3c and S9). Nanoindentation is an important technique to understand the mechanical behavior of the material on nanoscale.54 Figure 3d shows evolution of Elastic moduli (E) as function of indentation depth. For CSP-1, E was found to decrease rapidly with indentation depth up to 500 nm (hth), whereas it becomes constant above this value (Figure 3d). The hth observed for CSP-1 is higher than those reported for inorganic-organic framework materials (~100 nm),55 however is comparable to organic star polymers (~400-650 nm).56 A mean value of E = 687.902 ± 23.903 MPa was calculated from 10 indentation experiments and depths from 500 to 1200 nm. The consistency of E over the indentation depth confirms the homogeneity and robustness of the CSP. The observed E is one to two orders of magnitude lower than that reported for brittle metal-organic framework (MOF) single crystals and is one order higher than that of a hydrated thin film of MIL-101(Cr) nanoparticles (40 ± 10 MPa).57 The average hardness (H) for CSP-1 was calculated to be 27.347 ± 2.486 MPa (Figure S11), which is higher than that of poly-lactic acid functionalized inorganic organic hybrid polyhedral oligomeric


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silsesquioxane (POSS) star polymers.58 The nanomechanical properties of CSP-1 lie between those of MOFs and organic polymers, still in the region of hybrid framework materials.55 Considering results obtained from SAXS and mechanical property analysis, the microstructure of the CSPs can be regarded as soft nanophases of PEG with homogeneously distributed hard nanophases of MOP, similar to that of block copolymer systems employed for improving mechanical properties of organic polymeric materials.59 Mechanically robust glassy CSPs with dynamic nature of arms would be suitable to study ion transport behavior above Tg. There are many examples of ion conducting solids but the main issue associated with them is the optimization of the interface of the materials to be compatible with electrode.60, 61 In this regard, material with soft behavior and efficient ion transporting properties are advantageous to improve solid-solid interface.62 Li+ ion transport in solids has been studied for batteries and other energy devices.63 Also PEG is known to solvate and transport Li+ efficiently. We doped CSP-1 with Lithium bis- (trifluoromethane)sulfonimide (LiTFSI) salt maintaining ethylene oxide (EO) to Li+ [EO:Li] ratio of 20:1, 10:1, 5:1. LiTFSI salt was chosen due to its high dissociating ability with desired thermal and electronic stabilities. The integrity of the CSP core in doped samples was determined by X-ray absorption spectroscopy (XAS) analysis at the Cu K-edge. For all samples, a shoulder peak at 8985.3 eV (1s-4p dipolar shake down transition) in X-ray absorption near edge spectroscopy (XANES) and a weak peak at 8976.6 eV (1s-3d transition) in the pre-edge region corresponding to Cu2+ are observed (Figure 4a).64 The

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Cu2+. Moreover, the preservation of the dinuclear Cu2+ cluster was confirmed by the good fit of the radial distribution function (RDF) profiles with model paddlewheel compound Cu2(CH3COO)4·2H2O (Figure 4b). Unaltered 686 nm peak in UV-visible profile along with identical SEC retention time also confirmed the integrity of CSPs on LiTFSI doping (Figure S12-S13). No salt aggregation is observed in the doped sample according to PXRD profiles (Figure S14). Absence of solvent molecules in doped CSPs was ascertained by FT-IR or thermal analysis (Figure S15-S16). All the samples retained the selfassembled structure on short range as that of CSP-1 with gradual increase in d-spacing with increased doping amount as (Figure 4c). Both PXRD and SAXS measurements imply that the salts occupied the PEG domains leading to increase in d-spacing (Figure 4c). Solid-state 7Li NMR of the 10:1 sample exhibit two peaks at −1.7 ppm and −3.0 ppm, clearly distinguishable from that of pure LiTFSI at −1.3 ppm (Figure 4d).65, 66 This suggests that the Li+ in the 10:1 sample are well separated and solvated by the polymer matrix in two different environments.

Figure 5. Conductivity as a function of inverse of temperature for CSP-1; 20:1 (red), 10:1 (blue), 5:1 (olive), CSP-2 10:1 (orange), CSP-3 10:1 (violet). Solid lines are respective fits obtained using VFT expression.

Figure 4. (a) XANES profiles, (b) RDF profiles (solid lines) fitted profile (dotted line) (Inset: Cu2(CH3COO)4·2H2O model structure), (c) SAXS profiles of CSP-1 (blue), 20:1 (orange), 10:1 (olive) and 5:1 (red). (d) Solid-state 7Li NMR spectra of LiTFSI salt (black) and 10:1 (olive).

identical white line position at 8996.8 eV for all samples suggests that they have same site symmetry for

Figure 5 shows, temperature dependence of ionic conductivity for Li+ doped CSP samples. For the lowest Li+ doped sample (20:1), an ionic conductivity of 2.9 × 10−7 S·cm−1 is determined at 30 °C in the absence of solvent, which linearly increased with the temperature up to 60 °C to a value of 1.1 × 10−5 S·cm−1. Although the doped samples remained stable up to 250 °C, the conductivity could not be determined precisely above 60 °C due to softening of material. This observation is consistent with the DMA results. A further increase in the Li+ concentration to 10:1 lead to negligible improvement in the Li+ ion conductivity (2.8 × 10−7 and 1.2 × 10−5 S·cm−1 at 30 °C and 60 °C, respectively). Interestingly, for the 5:1 sample, a significant drop in ionic conductivity is observed over the studied temperature range (Figure S17). The conductivity values of 6.3 × 10−8 S·cm−1 and 5.9 × 10−6 S·cm−1 are recorded at 30 and 60 °C, respectively. The influence of doping in structural dynamics is also evidenced by the Tg values, which are lower than or comparable to neat CSP-1 for the 20:1 and 10:1 samples (−20.0 °C, −35.1 °C and −20.4


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Chemistry of Materials °C, respectively), whereas it increases to −12.5 °C for the 5:1 sample, indicating a reduced chain motion in the latter (Figure S18). Significant decrease in Li+ ion conductivity for 5:1 sample is ascribed to decrese in number of mobile Li+ ions owing to the stoichiometric complex formation, as observed in vast literature of PEG/LiTFSI mixture.67 While at lower concentrations, although the Tg is lower, the number of mobile ions are insufficient to provide maximum conductivity.68 Similar behavior also is observed for neat PEG. Vogel−Fulcher−Tammann (VFT) model, σ = A exp(B(T−To)); where A is the prefactor, B is the apparent activation energy for ion transport, and To is the shift temperature used to fit the conductivity data.68 Good fit of conductivity data with VFT model suggest that the ion migration is dominated by segmental motion of PEG chains (Table S2). Although, DMA data which shows temperature induced softening, no jump in conductivity was observed on heating. This is expected due to the thermal jamming effect known for similar class of materials, which increase physical crosslinking of polymer arms with temperature, while negligibly affecting ion conductivity.69 To probe the effect of volume fraction of conducting phase on the conductivity CSP-2 and CSP-3 samples were doped with LiTFSI maintaining EO:Li+ ratio to be 10:1. The conductivity was found to increase with increasing volume fraction of conductive phase (Figure 5). The conductivity values for CSP-2 10:1 sample were found to be 1.0 × 10−6 and 32.2 × 10−5 S·cm−1 at 30 °C and 60 °C; while for CSP-3 10:1 sample were found to be 1.8 × 10−6 and 3.7 × 10−5 S·cm−1 at 30 °C and 60 °C, respectively. Interestingly, for CSP-2 and CSP-3 samples, the d-spacing values decreased on Li+ doping when compared to pristine once (Figure S20). This implies that the Li+ is associated with the polymer chain with maximum dissociation of ion pairs and can move efficiently conducting phase with larger volume fraction thus increasing the conductivity.62 The dry state Li+ conductivities of 10:1 samples are comparable to that of Li+ salt doped PEG functionalized hyperbranched polymers,33 PEG doped with a plasticizer,70 PEG/Polymer blends,71 MOF doped PEG electrolytes30 and Li+ doped nanoscale organic hybrid electrolytes.52 As mentioned previously, the homogenous distribution of nano-fillers is an issue with above composite materials. The single component inorganicorganic hybrid CSP molecular solid electrolyte is highly advantageous in this regards to mitigate the issue.

Figure 6. Nyquist plots of the symmetrical Li/10:1/Li cell, before (Solid) and after annealing (open) at 30 °C. (Inset: Schematic of the symmetric cell).

The ordered but relatively free domain of the PEG chains in 10:1 can used to modify the solid-solid interface resistivity by annealing.72, 73 A symmetrical Li/[10:1]/Li coin cell was fabricated and the interfacial resistance was tested before and after annealing for 2 h at 40 °C by AC impedance spectroscopy. Figure 6 represents the Nyquist plot for both the symmetrical coin cells, which showed two complete semicircles without tails, demonstrating the transport of Li+ ions. Following the fitting of the Nyquist plot, the high and low frequency semicircles were assigned to the bulk+grain boundary and interface resistance, respectively.72, 74 The reduction in interfacial resistance upon annealing suggest improved electrodeelectrolyte interface. This is feasible because of the thermoplastic behavior of CSP with tuned mechanical property and arm alignment. As the electrolytes was fabricated as pressed pellet, a simultaneous decrease in bulk + grain boundary resistance is expected. This highlights advantage of dynamic polymer arms in CSP in improving solid-solid interfaces. Although in the present work we have reported an example of a CSP with f = 24, by the judicious selection of MOP core (size, shape, geometry) and/or polymer arms components or mixing of different CSPs we can incorporate and fine tune the desired material properties and expand the chemistry of the CSP materials.

CONCLUSION In conclusion, we present a novel class of coordination star polymer glass that features MOP core with dynamic organic polymer arm. We demonstrate that the hierarchical self-assembled CSP structure and associated mechanical properties are tunable by controlling the polymer arm content. The mechanical properties CSPs lie between inorganic and organic materials and are suitable for real device integrations. For the first time, the dynamic polymer domains in the CSP glasses are utilized as media for the transport of alkali metal cation. Li+ conductivity of CSP glass under solvent free conditions, is comparable to well know organic polymer electrolytes. We demonstrated that the ion transport through CSP glass is highly influenced by the hierarchical structural


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ordering and optimal arm packing. The soft behavior of glass was utilized to correct solid-solid interfaces. This research was driven by the desire to further manipulate and/or discover novel physical and mechanical properties of CSPs. Strategies like mikto arm CSPs (chemically different arms radiating from same core),75 mixture of CSPs (having enantiomeric arms for optical properties),58 responsive building units are available for the further modulation of CSPs structure-properties. Thus, with the available pool of molecular synthons and topological variations, we believe that the research promises to broaden the scope of CSPs as a platform of solid material with modular thermal, mechanical property and intriguing functions.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Experimental details and additional characterizations (MALDI-TOF, FT-IR, PXRD, TGA, DSC, SEM, TEM, UV-VIS, SEC)

AUTHOR INFORMATION Corresponding Author *(S.K.) E-mail: [email protected]. *(N.H.) E-mail: [email protected]. *(S.H.) E-mail: [email protected].

Author Contributions The manuscript was written through contributions of all authors.

Funding Sources The work was supported by the Japan Society of the Promotion of Science (JSPS) for a Grant-in-Aid for Scientific Research (B) (JP 18H02032), a Grant-in-Aid for Specially Promoted Research (JP25000007), a Grant-in-Aid for Young Scientists (B) (JP16K17959) from the Ministry of Education, Culture, Sports, Science and Technology, Japan, and Strategic International Collaborative Research Program (SICORP) and Adaptable and Seamless Technology Transfer Program through Target-driven R&D (A-STEP) from the Japan Science and Technology, Japan.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT We thank Ms. Nanae Shimanaka, Ms. Mika Gochomori, Mr. Wenbo Guo, Ms. Kayako Honjo and Dr. Kenichiro Omoto for their technical synthetic and analysis supports.

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Chemistry of Materials

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Modular Self-Assembly and Dynamics in Coordination Star Polymer

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