Alkyl Chain Length Dependence of Backbone-to-Backbone Distance

Article pubs.acs.org/Macromolecules

Alkyl Chain Length Dependence of Backbone-to-Backbone Distance in Polymerized Ionic Liquids: An Atomistic Simulation Perspective on Scattering Hongjun Liu and Stephen J. Paddison* Department of Chemical and Biomolecular Engineering, University of Tennessee, Knoxville, Tennessee 37996, United States S Supporting Information *

ABSTRACT: The backbone-to-backbone correlation length in polymerized ionic liquids (polyILs) manifested as the low-q peak in scattering profiles increases with increasing alkyl chain length, concomitant with ever-growing nonpolar domain size. Understanding the dependence of the correlation length on the pendant alkyl chain length is crucial to effectively designing polyILs for electrochemical applications. Herein, extensive atomistic MD simulations for a complete homologous series of poly(n-alkylvinylimidazolium bis(trifluoromethylsulfonyl)imide), poly(CnVim Tf2N) (n = 2−8), have been carried out to investigate the liquid structure with emphasis on scattering. Neutron scattering with isotopic labeling affords more versatile ways to vet the intrinsic structure, but different isotopically labeled neutron and X-ray scattering data do not necessarily lead to the same position of the low-q peaks. Such an unintended consequence has a nontrivial implication on exploitation of scattering experiments.

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INTRODUCTION Recent enthusiasm in polymerized ionic liquids (polyILs) with enhanced mechanical characteristics of polymers and unique physicochemical properties inherent in ionic liquids is fueled by their ability to selectively transport ions.1−9 Experiments suggest the morphology of polyILs has a significant effect on ion transport.10 Neutron and X-ray scattering techniques have long been used as a direct experimental tool to investigate the liquid structure. The determination of the structure of a liquid in terms of partial radial distribution functions is fundamental to a thorough understanding of many physicochemical properties. From a series of neutron diffraction measurements with isotopic substitution on a given system, one could determine the partial structure factors with limited accuracy and therefore the partial radial distribution functions by Fourier transformation. Such endeavors involving high fidelity data, absolute normalization, and delicate data manipulation are technically challenging and resource-intensive. The intrinsic complexity in the measured scattering profiles warrants an effective computational tool to disentangle the mystery and elucidate the individual components that contribute to the characteristic scattering peaks. Molecular simulations11−18 and the empirical potential structural refinement (EPSR) method19,20 have proven to be capable of tackling this challenge and play an indispensable role in calibrating our understanding of scattering experiments in ionic liquids, polymers, and ionomers. Despite significant interest in the morphology of polyILs, investigation by traditional experimental techniques remains elusive. Few X-ray scattering experiments yield structural information and provide little atomistic details.21−26 Neutron © XXXX American Chemical Society

scattering has not been done on this important class of materials. In previous work, we reported a comparison of results from atomistic simulations with X-ray scattering experiments on three homologous polyILs that strongly support the capability of simulations to provide fundamental understanding of the structure and morphology.27 Consistent with the prevailing mesoscopic picture of ionic liquids, the charged domains of polyILs form a continuous percolating ionic channel, while the nonpolar domains progressively grow with increasing alkyl chain length. For polyILs with short alkyl chains, small discrete nonpolar islands form first within the polar network. Nonpolar domains then grow beyond the percolation threshold and finally intertwine with the polar network into a bicontinuous spongelike nanostructure. Nanophase separation of incompatible components is a general phenomenon in amorphous side-chain polymers, ILs with long alkyl groups, and polyILs. Scattering experimental studies on various flexible and rigid polymers suggest that the size of the nonpolar nanodomains is mainly dictated by the number of alkyl carbon atoms per side chain.28 The X-ray scattering data for poly(n-alkyl acrylate) or poly(n-alkyl methacrylate) homopolymers and copolymers lead to a slope (a measurement of variation of backbone correlation length as a function of alkyl side chain length) of 1.05 Å/CH2 for n < 10.29 The majority of data of ILs are obtained from X-ray scattering, and the estimated slopes vary from 1.1 to 2.3 Å/CH2.19,30−35 Received: December 15, 2016 Revised: March 2, 2017

A

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Macromolecules Here we revisit the poly(CnVim Tf2N) series and extend the analysis of its morphology to a complete homologous series of poly(CnVim Tf2N) with 2 ≤ n ≤ 8. Along with X-ray scattering where mostly carbons are highlighted without the possibility of distinguishing backbone and alkyl side chain contributions to the diffraction patterns, selective neutron scattering with isotopic labeling will be exploited to provide further physical insight into the morphology of poly(CnVim Tf2N). X-ray scattering of the system of interest offers only one kind of contrast between atoms and hence one particular representation (scattering profile) of the total structure, whereas neutron scattering through selective isotopic labeling can provide many different scattering profiles that emphasize different atoms, leading to a more complete picture of the total structure. The present investigation focuses on the alkyl chain length dependence of backbone-to-backbone distance, concomitant with nonpolar nanodomain size which has a profound effect on the ionic conductivity of polyILs. To our knowledge, this is the first extensive simulation study on neutron scattering of exhaustively isotopically labeled polyILs.

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Sαβ(q) = xαδαβ + xαxβρ0

w(r ) =

S X (q) =

1 ∑α xαfα2 (q)

N

S N (q) =

(3)

β

⎡

⎛ q ⎞2 ⎤ ⎟ ⎥ + c 4π ⎠ ⎦

⎣

(4)

1 ∑α xαbα 2

n

n

∑ ∑ bαbβSαβ(q) α

β

(5)

To pinpoint the peak positions, a peak deconvolution procedure was routinely applied to fit the scattering profiles up to 2 Å−1.

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RESULTS AND DISCUSSION Previously, our work reported the computed X-ray structure factors for three homologous poly(CnVim Tf2N) in favorable agreement with experiment.27 We have verified that both the direct and Fourier transform methods are equally effective in calculating the structure factors (see Figures S2 and S4). To facilitate precise location of the scattering peak positions, we present computed scattering profiles exclusively using the Fourier transform method. The structure factors of a complete homologous series of 7 poly(CnVim Tf2N) present a clear trend on the effect of alkyl chain length. Simulated X-ray structure factors of poly(CnVim Tf2N) as a function of alkyl chain length are compared with recent experiments in Figure 1. The homologous series of poly(CnVim Tf2N) shows three characteristic intermolecular features below 2 Å−1. The low-q peaks are taken to be the backbone-to-backbone peak (qb) or polarity peak, the intermediate-q peaks are denoted as the ionic peak (qi, correlation between anions) or charge peak, and the high-q peaks as the pendant-to-pendant peak (qp, correlation between pendant groups of the polycation) or adjacency peak.21,48 Three characteristic peaks are well reproduced by the atomistic simulations, and the trends are strikingly similar to the experimental results. Peaks above 2 Å−1 are generally intramolecular in nature and are therefore irrelevant to the

Nβ

∑ sin(q·rj) ∑ sin(q·rl)⟩ j=1

α

The tabulated coefficients ai, bi, and c are reported in Table 6.1.1.4 of the International Tables for Crystallography.46 For neutron scattering, fα(q) is replaced with the neutron scattering lengths, bα: −3.739, 5.803, 6.646, 9.36, 5.654, 6.671, and 2.847 fm for hydrogen, oxygen, carbon, nitrogen, fluorine, deuterium, and sulfur, respectively. Hence, the total neutron structure factor is

Nβ

Nα

n

∑ ai exp⎢−bi⎜⎝ i=1

1 α ⟨∑ ∑ e−iq·(rj − rl)⟩ N j=1 l=1 Nβ

n

∑ ∑ fα (q)fβ (q)Sαβ(q)

where fα(q) represents the X-ray atomic form factor of α and can be approximated with a series of Gaussian functions of q over the range of (0, 25) Å−1 according to fα (q) =

1 α ⟨∑ cos(q·rj) ∑ cos(q·rl) + N j=1 l=1

sin(qr ) w(r ) dr qr (2)

was applied to remedy the cutoff ripple artifact.44,45 For X-ray scattering, partial structure factors were weighted with the atomic form factors leading to the total X-ray structure factor:

The simulated polyILs systems each consist of 10 polymer molecules. Each polymer molecule consists of one 40-mer polycation and 40 Tf2N anions. The system size (number of atoms) ranges from 14 020 for poly(C2Vim Tf2N) to 21 220 for poly(C8Vim Tf2N). All molecular dynamics (MD) simulations were carried out with the GROMACS package36 using the general AMBER force field all-atom parameters following an established procedure.37,38 The GAFF parameters were derived via the ACPYPE tool.39 Initial configurations were generated by randomly inserting a prescribed number of polycations and anions into a large simulation box using the Packmol tool.40 A Bussi stochastic thermostat was used to control temperature,41 while a Berendsen barostat first and then a Parrinello−Rahman barostat were used to control pressure.42 NVT ensemble productions were performed at the desired density estimated by NPT ensemble simulations at 1 atm and 400 K, well beyond their glass transition temperatures (∼80 K above Tg).21 Cutoffs of 14 Å for the Lennard-Jones interactions with the appropriate tail corrections were utilized. Long-range electrostatic interactions were handled with the particle mesh Ewald (PME) method. The system was equilibrated for at least 50 ns, and trajectories of 50 ns were collected for structural analysis. Three independent runs were performed. Both the direct and Fourier transform methods were implemented to calculate the structure factors of the polyILs.43 In the former method, the partial structure factors, Sαβ(q), are computed from

=

4πr 2[gαβ (r ) − 1]

⎡ ⎤ 3 2πr cos(2πr /L)⎥ ⎢sin(2πr /L) − ⎦ L (2πr /L)3 ⎣

4

N

L /2

where δαβ is the Kronecker delta, xα is the corresponding mole fraction of α, and ρ0 is the total number density of the system. The revised Lorch window function

METHODS

Sαβ(q) ≡

∫0

l=1

(1)

where j loops over all Nα atoms of species α and l loops over all Nβ atoms of species β. In the latter method the partial structure factors are estimated by Fourier transformation of g(r), that is B

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increase in backbone-to-backbone separation and the concomitant growth of the nonpolar nanodomains. Though it appears poly(C2Vim Tf2N) is an outlier whose low-q peak position is somehow lower than expected, such a deviation is not observed in the various structure factors as demonstrated below. The low-q peak is insignificant compared with the other two scattering peaks in poly(C2Vim Tf2N) and becomes comparable in intensity when n = 4 or 5 and much more dominant in poly(CnVim Tf2N) with longer alkyl chains (n > 5). In contrast to qb, there is no substantial q shift observed in qi and qp: qi slightly shifts to lower q with weaker intensity, while there is very little shift in qp, but with stronger intensity as the alkyl chain becomes longer. The weaker correlation and slightly larger separation between anions with increasing alkyl chain length are because the ionic separation must increase to accommodate progressively larger nonpolar domains. The adjacency or direct contact distance is hardly affected by the nonpolar domains as there is no shift in the position of qp. Correlation lengths derived from peak positions in SX(q) of poly(CnVim Tf2N) as a function of alkyl chain length are presented in Figure 1. The overall agreement between simulation and experiment is satisfactory. The correlation length, lp, remains a constant 4.7 Å regardless of n, while li around 7.6 Å increases slightly with n. The simulated correlation length, lb, significantly overestimates the experimental scale, but both increase linearly with n at a similar rate of 1.19 Å/CH2 (simulation) vs 1.20 Å/CH2 (experiment). The direct manifestation of backbone-to-backbone correlation length through analysis of alkyl chain extension also leads to a slope of about 1.1 Å/CH2, corroborating the scattering findings (see Figure S6). These estimates are lower than the experimental rate of 1.5 Å/CH2 (n = 2, 4, and 8)21 and 1.3 Å/ CH2 (n = 2, 3, 6, and 8)22 and are higher than that of our recent simulation (1.0 Å/CH2 for n = 2, 5, and 8).27 The low-q peaks have been identified as a signature feature of nanoscale aggregation in both polymers and molecular ionic liquids. Our estimate is generally consistent with the experimental values ranging from 1.0 to 2.3 Å/CH2 in side-chain polymers and various ILs.19,29−35 The overestimated lb can be attributed to the downward shift in the low-q peaks at higher temperatures due to thermal expansion.21 Simulations were run at an elevated temperature (i.e., 400 K) to greatly improve the sampling quality of the configuration space, while the X-ray scattering experiments were measured at room temperature of 298 K. The effect of temperature on X-ray scattering is shown in Figure S8. Consistent with experimental findings,21 increasing temperature leads to a lower q position with stronger intensity for all three characteristic scattering peaks. Complementary to the X-ray scattering profiles, we also report simulated neutron scattering data for the series of poly(CnVim Tf2N) using hydrogen/deuterium substitution to further investigate the contributions of the various components to the total scattering. Selective isotopic labeling in neutron scattering provides a molecular level resolution and is essential for an in-depth understanding of the atomistic structure of polyILs. The total neutron scattering factors for the fully protiated, perdeuterated, backbone deuterated, and alkyl chain deuterated of 7 poly(CnVim Tf2N) are presented in Figure 2. The neutron scattering profiles markedly depend on the isotopic substitution pattern. The shapes and trends of the neutron scattering factors of the fully protiated poly(CnVim Tf2N) appear similar to those of the X-ray scattering factors. Three characteristic peaks are clearly evident: the low-q peaks

Figure 1. (a) Simulated total X-ray structure factors with an inset of the schematic structure of poly(CnVim Tf2N) (Cn is the pendant alkyl chain of length n) and (b) experimental X-ray scattering profiles of poly(CnVim Tf2N).47 (c) Correlation lengths derived from X-ray scattering peak positions of poly(CnVim Tf2N) as a function of alkyl chain length in comparison with previous experimental values21,22 and simulation.27 The solid lines are the least-squares fits of the simulated data, and the dashed lines are the experimental results. The error bars are smaller than the symbol size. lb, li, and lp are the backbone-tobackbone correlation length, ionic correlation length, and pendant-topendant correlation length, respectively.

discussion of correlation lengths. The most conspicuous feature in Figure 1 is that the low-q peak shifts to lower q with stronger intensity as the alkyl chain length increases, indicating an C

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Figure 2. Simulated total neutron structure factors of (a) the fully protiated, (b) perdeuterated, (c) backbone deuterated, and (d) alkyl chain deuterated poly(CnVim Tf2N).

and the intermediate qi peaks monotonically shift to lower q with weaker intensity as n is increased, and the qp peak becomes weaker moving to higher q. In contrast, the total neutron scattering factor of perdeuterated poly(C2Vim Tf2N) has the most intense low-q peak. This result is rather counterintuitive and may be due to the improved contrast density between the polar and nonpolar domains of perdeuterated poly(C2Vim Tf2N). The intermediate and high-q peaks shift to lower q with stronger intensity, while the low-q peaks move to lower q, but with diminishing intensity. The low-q peaks reduce to just a shoulder for n = 7 or 8. The scattering lengths of the constituent elements in the perdeuterated samples are not that different and make all partial structure factors approximately equally weighted, which leads to the resultant total scattering as the true structure factor. The simulated true structure factors where all pair correlations are essentially equally weighted are presented in Figure S13 share considerable similarity with those of the perdeuterated. The most significant low-q peaks are found in the backbone deuterated samples in Figure 2c. The low-q peak contribution dominates the total scattering landscape even for poly(C2Vim Tf2N) with the shortest alkyl chain. The low-q peaks shift to lower q with significantly stronger intensity as the alkyl chain length is increased, while the reverse order is true for the high-q peaks, yet to a much lesser extent. The scattering intensity ratio between the low-q and high-q peak is about 3 for poly(C2Vim Tf2N) and rises to 10 for poly(C8Vim Tf2N). The intermediate peaks are absent or negligible relative to the other two peaks. The total neutron scattering factors of the alkyl chain deuterated poly(CnVim Tf2N) show all three peaks shift to lower q with stronger intensity (see Figure 2d). The

low-q peak of poly(C2Vim Tf2N) is a broad shoulder but is of too low an intensity to make a sizable contribution to the total scattering. Poly(C4Vim Tf2N) shows a low-q peak with an intensity greater than unity. The low-q peaks of poly(CnVim Tf2N) with n = 7 or 8 become comparable with the high-q peaks. Correlation lengths derived from neutron scattering peaks as a function of alkyl chain length are presented in Figures S9− S12. All correlation lengths of li from various neutron scattering implementations show a modest increase with alkyl chain length at rates varying from 0.21 to 0.31 Å/CH2, while those of lp remain almost intact around 4.7 Å with little variation. The growth rates of lb varied dramatically with different isotopic schemes: 1.29 Å/CH2 (protiated), 0.62 Å/CH2 (perdeuterated), 1.08 Å/CH2 (backbone deuterated), and 1.06 Å/CH2 (alkyl chain deuterated). Such variation suggests special attention is needed for an unambiguous explanation of the characteristic peak positions from different scattering implementations. Neutron scattering of five isotopic samples and X-ray scattering profiles of individual poly(CnVim Tf2N) (n = 3, 5, 7) are compared in Figure 3. The data as a whole are qualitatively consistent, yet it is important to notice that both peak position and intensity are significantly affected by the isotopic substitution patterns. For the fully protiated, perdeuterated, and alkyl chain deuterated samples, the low-q peak is relatively small, whereas in the backbone deuterated samples, it is much more intense and becomes the most dominant contribution to the overall scattering. The backbone deuterated polyILs samples should be among the best choices for experimentalists to exploit the capacity of SANS for quality D

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more consistent estimate of the low-q positions than that of its intermediate counterpart (C5). It is advised to pay extra heed to comparison of correlation lengths derived from different scattering implementations of the same compound. Several group contributions can be approximated using a common factor analysis of the aforementioned neutron scattering data. Figure 4 presents a few constructive examples

Figure 4. (a) Total neutron scattering differences of poly(CnVim Tf2N) showing the alkyl chain deuterated data minus fully protiated and the backbone deuterated minus fully protiated. (b) The perdeuterated minus alkyl chain deuterated and the perdeuterated minus apolar (including both backbones and alkyl chains) deuterated.

of such a manipulation. Subtraction of the scattering of a fully protiated sample from that of the alkyl chain deuterated or from that of the backbone deuterated represents the alkyl chain contribution or backbone contribution to the total scattering, respectively. The negative scattering length of the hydrogen atoms annihilates that of the bonded alkyl carbons and makes the entire alkyl chain almost invisible for neutron particles in the fully protiated sample when compared to the deuterated alkyl chain sample. The most pronounced feature is that the backbone correlation contributes significantly to the low-q peak, which substantiates the phenomenological assignment of the low-q peak as the backbone-to-backbone peak by Winey and co-workers,21 whereas the alkyl chain contribution is a relatively weak antipeak. Both correlations shift to lower q and intensify with increasing alkyl chain length. The other two sets of difference scattering spectra are from the scattering of a perdeuterated sample minus that of an alkyl chain deuterated

Figure 3. Simulated total neutron and X-ray structure factors of (a) poly(C3Vim Tf2N), (b) poly(C5Vim Tf2N), and (c) poly(C7Vim Tf2N).

signal in terms of the low-q scattering peak. Although all neutron or X-ray scattering of each poly(CnVim Tf2N) is vetting the very same intrinsic structure, complications from weighted individual partial structure factors by scattering lengths or form factors make identification of a unique peak nearly impossible. The X-ray scattering and distinct isotopic labeled neutron scattering data do not necessarily lead to the characteristic peaks with the same positions. Our simulations of the shorter alkyl chain (C3) or longer chain (C7) result in a E

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and the scattering of a perdeuterated sample minus that of an apolar (including both backbones and alkyl chains) deuterated, which approximate the contributions to the total scattering from the correlations of the cationic imidazolium ring plus backbone and imidazolium ring only, respectively. The scattering profiles for the apolar deuterated samples are presented in Figure S14, and the characteristic low-q peaks are either insignificant or completely missing. The contribution of the backbone plus imidazolium ring weakens, becoming negligible around C5 and C6, and then turns more negative with increasing n. The weaker contribution of the imidazolium ring with increasing n is also observed. The most dominant anion contribution to the low-q peak as demonstrated in our recent work27 is impossible to be identified without exploiting other isotopic substitution options (for instance, sulfur labeling 32 33 S/ S in the Tf2N anion). This difference scattering analysis provides a complementary experimental alternative to the detailed decomposition of the total structure factor afforded in molecular simulations or EPSR modeling.

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Ph +1 865-974-2026 (S.J.P.). ORCID

Stephen J. Paddison: 0000-0003-1064-8233 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. David Salas-de la Cruz and Prof. Karen Winey of UPenn for communicating the X-ray scattering data of polyILs in ref 21 and Dr. Ciprian Iacob and Prof. James Runt of Penn State for sharing the data in ref 47 before publication. Financial support of the U.S. Army Research Office under Contract W911NF-15-1-0501 is gratefully acknowledged. Computing resource was provided through XSEDE allocation DMR130078.

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REFERENCES

(1) Matsumi, N.; Sugai, K.; Miyake, M.; Ohno, H. Polymerized ionic liquids via hydroboration polymerization as single ion conductive polymer electrolytes. Macromolecules 2006, 39, 6924−6927. (2) Armand, M.; Endres, F.; MacFarlane, D. R.; Ohno, H.; Scrosati, B. Ionic-liquid materials for the electrochemical challenges of the future. Nat. Mater. 2009, 8, 621−629. (3) Green, O.; Grubjesic, S.; Lee, S. W.; Firestone, M. A. The Design of Polymeric Ionic Liquids for the Preparation of Functional Materials. Polym. Rev. 2009, 49, 339−360. (4) Weber, R. L.; Ye, Y. S.; Schmitt, A. L.; Banik, S. M.; Elabd, Y. A.; Mahanthappa, M. K. Effect of Nanoscale Morphology on the Conductivity of Polymerized Ionic Liquid Block Copolymers. Macromolecules 2011, 44, 5727−5735. (5) Lee, M.; Choi, U. H.; Colby, R. H.; Gibson, H. W. Ion Conduction in Imidazolium Acrylate Ionic Liquids and their Polymers. Chem. Mater. 2010, 22, 5814−5822. (6) Hoshino, K.; Yoshio, M.; Mukai, T.; Kishimoto, K.; Ohno, H.; Kato, T. Nanostructured ion-conductive films: Layered assembly of a side-chain liquid-crystalline polymer with an imidazolium ionic moiety. J. Polym. Sci., Part A: Polym. Chem. 2003, 41, 3486−3492. (7) Chen, H.; Choi, J. H.; Salas-de La Cruz, D.; Winey, K. I.; Elabd, Y. A. Polymerized Ionic Liquids: The Effect of Random Copolymer Composition on Ion Conduction. Macromolecules 2009, 42, 4809− 4816. (8) Yuan, J. Y.; Mecerreyes, D.; Antonietti, M. Poly(ionic liquid)s: An update. Prog. Polym. Sci. 2013, 38, 1009−1036. (9) Shaplov, A. S.; Vlasov, P. S.; Lozinskaya, E. I.; Ponkratov, D. O.; Malyshkina, I. A.; Vidal, F.; Okatova, O. V.; Pavlov, G. M.; Wandrey, C.; Bhide, A.; et al. Polymeric Ionic Liquids: Comparison of Polycations and Polyanions. Macromolecules 2011, 44, 9792−9803. (10) Mecerreyes, D. Polymeric ionic liquids: Broadening the properties and applications of polyelectrolytes. Prog. Polym. Sci. 2011, 36, 1629−1648. (11) Kashyap, H. K.; Santos, C. S.; Annapureddy, H. V. R.; Murthy, N. S.; Margulis, C. J.; Castner, E. W., Jr. Temperature-dependent structure of ionic liquids: X-ray scattering and simulations. Faraday Discuss. 2012, 154, 133−143. (12) Song, X.; Hamano, H.; Minofar, B.; Kanzaki, R.; Fujii, K.; Kameda, Y.; Kohara, S.; Watanabe, M.; Ishiguro, S.-i.; Umebayashi, Y. Structural Heterogeneity and Unique Distorted Hydrogen Bonding in Primary Ammonium Nitrate Ionic Liquids Studied by High-Energy Xray Diffraction Experiments and MD Simulations. J. Phys. Chem. B 2012, 116, 2801−2813. (13) Pereiro, A. B.; Pastoriza-Gallego, M. J.; Shimizu, K.; Marrucho, I. M.; Lopes, J. N. C.; Pineiro, M. M.; Rebelo, L. P. N. On the Formation of a Third, Nanostructured Domain in Ionic Liquids. J. Phys. Chem. B 2013, 117, 10826−10833.

CONCLUSIONS In summary, we have carried out extensive atomistic MD simulations for a complete homologous series of poly(CnVim Tf2N) (n = 2−8) to investigate their molecular structure with emphasis on X-ray and neutron scattering. We observed excellent agreement between the atomistic simulations and experimental X-ray scattering results in terms of the general trend. The backbone-to-backbone correlation length derived from the low-q peak position grows with increasing alkyl chain length at a rate of 1.2 Å/CH2, in accord with experiments. More importantly, we exploited the selective labeling technique of neutron scattering using hydrogen/deuterium substitution to afford further insight into the liquid structure of poly(CnVim Tf2N). The neutron scattering profiles markedly depend on the isotopic substitution pattern. The total neutron structure factors of the backbone deuterated samples reveal the most noticeable low-q peak and are much more intense than those observed in single isotope neutron scattering. Our results strongly suggest that the X-ray scattering and distinct isotopic labeled neutron scattering data do not necessarily give rise to the characteristic correlation peaks with the same peak position. Therefore, the estimated slopes of the backbone-to-backbone distance versus the alkyl chain length based on various scattering implementations will vary significantly. The experimentally accessible difference scattering spectra based on common factor analysis offer a complementary view to the detailed decomposition of the total structure factor provided in molecular simulation and ESPR modeling. We hope the current work might stimulate experimentalists to explore polyILs using selective neutron scattering.

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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.6b02708. Methodology, calculation of structure factors, conformation of alkyl side chains, temperature effect on X-ray scattering, and neutron scattering of isotopically labeled polyILs (PDF) F

DOI: 10.1021/acs.macromol.6b02708 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.6b02708 Macromolecules XXXX, XXX, XXX−XXX