Effect of Polymerized Ionic Liquid Structure and Morphology on

Letter Cite This: ACS Macro Lett. 2019, 8, 535−539

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Effect of Polymerized Ionic Liquid Structure and Morphology on Shockwave Energy Dissipation Jaejun Lee,†,§ Vivian M. Lau,‡,§ Yi Ren,‡,§ Christopher M. Evans,†,§ Jeffrey S. Moore,†,‡,§ and Nancy R. Sottos*,†,§ Department of Materials Science and Engineering, ‡Department of Chemistry, and §Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States

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†

S Supporting Information *

ABSTRACT: The ability of nanosegregated polymerized ionic liquids (PILs) to dissipate shockwave energy is investigated for a series of imidazolium-based PILs with varying alkyl spacer length. The PILs are designed to have similar glass transition temperatures but different structures. X-ray scattering analysis reveals that each of the amorphous PILs exhibit distinct nanoscale structural heterogeneity, depending on the length of the chain spacer. We find that a higher structural heterogeneity, determined from the intensity of the intercluster scattering peak, in the PILs with longer alkyl spacers results in greater shockwave energy dissipation. In addition, we observe the crystalline phase is less effective at dissipating shockwave energy than the amorphous phase due to the close packed morphology and slow kinetics.

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chemical reactions,16,17 and plastic deformations.18,19 Most of these strategies result in one-time irreversible changes and are unsuitable for dissipation of multiple shockwave events. An alternative approach for shockwave energy attenuation utilizes viscoelastic molecular relaxations.20 Recent investigations of polyurea layers emphasize the importance of microstructure on shockwave energy dissipation.21−23 Molecular dynamics simulations by Grujicic et al. predict that the deformation of soft domains associated with hard domain densification in nanosegregated polyurea contributes to energy dissipation.24 Arman et al. also predict that well-dispersed and easily pliable hard domains in microphase-separated polyurea play an important role for energy dissipation.21 Overall, though, there is a lack of systematic experimental investigation of polymer structure effects on shockwave energy dissipation. Polymeric ionic liquids (PILs), also known as ionenes, are a class of polymers containing ions chemically conjugated to backbone/side chains and have received considerable attention in the fields of materials science due to their distinct iontransport properties.25,26 Morphologies of PILs range from nanosegregated amorphous to crystalline depending on chemical composition and ionic strength.27−30 The role of structure on the electrochemical performance has been elucidated since ionic transport properties and morphologies are closely coupled.31 In this paper, we systematically vary the

nderstanding shockwave-induced physical and chemical changes of impact-absorbing materials is critical for the development of new materials for mitigating shockwave energy. Shockwaves are nonlinear, high amplitude acoustic pressure waves of extremely short duration, with a rise time from nanoseconds to milliseconds.1,2 Shockwave-induced traumatic brain injury (TBI) has become a serious issue for soldiers in recent years due to high energy explosives in modern warfare.3,4 Personal protective equipment is designed to physically shield soldiers from material penetration, but is vulnerable to shockwave impacts. There is an urgent need to develop materials that more effectively absorb low-intensity shockwaves. A common approach for shockwave mitigation is to utilize heterogeneous material structures that cause the shockwave to scatter at the interfaces, including porous materials,5,6 nanocomposites,7−9 and multilayered composites.10−12 Since the reflection of shockwaves is achieved by differences in impedance, the material design must consider the precise sequence of layers, the impedance/density of each layer, and porosity. In porous composites, Nesterenko reported that pores of insufficient size enhance the intensity of shockwave pressure.6 According to Grujicic et al., the selection and sequence of layered materials can lead to undesirable amplification of shockwave pressure compared to the input pressure.13 Several additional material-based energy absorbing strategies have been introduced to provide protection from high- and low-intensity shockwaves, including ordering transitions,14,15 © XXXX American Chemical Society

Received: February 22, 2019 Accepted: April 19, 2019

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DOI: 10.1021/acsmacrolett.9b00133 ACS Macro Lett. 2019, 8, 535−539

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ACS Macro Letters Scheme 1. Synthesis of Polymerized Ionic Liquids with Different Alkyl Chain Length Spacers

Figure 1. Amorphous and crystalline characteristics of PIL-C6. Optical images of (a) amorphous and (b) crystalline PIL-C6. (c) X-ray diffraction data of PIL-C6 indicates that the crystalline phase is formed after 24 h at 296 K.

Figure 2. Microstructural and thermal analysis of PILs. (a) X-ray diffraction data of a series of PILs shows each PIL has distinct microstructure. (b) Differential scanning calorimetry (DSC) measurements of PILs indicate all PILs have a similar glass transition temperature around 237 K.

shockwave absorption performance. The glass transition temperatures of the NILs also varied with alkyl chain length, indicating that the molecular structure influences the relaxation behavior. For NILs, measuring the contribution of structure to energy dissipation is complex since the dynamics of segmental motion in materials are an important factor in determining energy absorbing performance.37−39 In this work, we systematically explore the effects of polymer structure on shockwave energy dissipation through the selection of PILs that exhibit similar dynamics, regardless of chain size. Charged domains and nonpolar domains in PILs are also nanosegregated and the morphology is tuned by the molecular structure of the polymer.40,41 A series of polymerized ionic liquids was synthesized via simple step-growth polymerization between alkyl bis(imidazole) and dibromide starting materials (Scheme 1). A stoichiometric mixture of bis(imidazole) and dibromide was dissolved in DMF and heated to 80 °C for 48 h to yield the

structure of PILs and characterize the corresponding change in energy dissipation, providing insight into mechanisms for shock wave absorption. Schroder et al. first proposed a nanosegregated morphology of imidazolium-based ionic liquids consisting of polar and nonpolar regions.32 Subsequent studies explored the diverse microstructures of ionic liquids depending on cations, anions, and nonpolar moieties.33−36 Recently, Yang et al. reported a series of network-forming ionic liquids (NILs) that possessed intriguing shockwave absorption properties.14 The nanosegregated microstructure of quaternary ammonium networkforming ionic liquids (NILs) depends on the length of the side alkyl chains. The soft alkyl domain in NILs played an important role in absorbing shockwaves. A shock-induced ordering in the NIL with the longest (hexyl) side chain was observed as an increase in the X-ray scattering intensity for intercluster correlations, indicating that both nanosegregated structure and shock-induced ordering contributed to the NIL 536

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ACS Macro Letters

Figure 3. Shockwave energy dissipation properties of PILs. (a) Schematic depiction of laser-induced shockwave test setup and specimen structure for measuring shockwave energy dissipation. (b) Average peak pressures at different input laser fluences for PILs.

corresponding PIL with Br− as the counterion. The bromide anion was exchanged for bis(triflimide) (NTf2-) by stirring the resulting PILs in methanol with excess lithium bis(triflimide) for 24 h, then precipitated into deionized water, and freezedried to yield thick, syrup-like polymers. The alkyl chain methylene unit number, X, designates a name of each PIL as PIL-CX. The PILs were characterized by 1H NMR spectroscopy and the Mn values for PIL-C6, C8, C12 and C16 were 15.8, 16.7, 17.3, and 15.9 kg/mol, respectively as determined by size exclusion chromatography with multiangle laser light scattering. Details of the 1H NMR spectroscopy, Mn values and viscosity data are provided in the Supporting Information. The melting temperature of PIL-C6 measured by DSC trace is 321 K (see Supporting Information, Figure S1). Solid PILC6 melts upon heating, and subsequent cooling of the molten PIL-C6 results in an amorphous phase, as shown in Figure 1a. Nucleation starts within a few hours, followed by transforming into a crystalline phase (Figure 1b). X-ray diffraction data supports the amorphous-to-crystalline phase transformation at room temperature (296 K). A slow crystallization process with a few nuclei creates large grains, which minimizes shockwave energy dissipation from grain boundaries, and allows us to examine shockwave energy dissipation properties for each phase individually. At 296 K, the PILs with a shorter alkyl chain favor a crystalline microstructure, while PILs with a longer alkyl chain have a random morphology. Both PIL-C4 and -C6 favor a crystalline phase at 296 K with a high melting temperature (Figure S1). In contrast, the PILs with longer alkyl chain spacers than PIL-C6 form an amorphous phase at 296 K. Melting temperatures of PIL-C8, -C12, and -C16 are not even found in the temperature range 173.15−373.15 K by differential scanning calorimeter. The microstructural features of PILs were analyzed using powder X-ray diffraction (PXRD) and correlated with stress wave energy dissipation. Figure 2a shows the X-ray scattering intensity as a function of Q for amorphous PIL-C6, -C8, -C12, and -C16, indicating three main peaks, peak I at Q ≈ 0.3−0.5 Å−1, peak II at Q ≈ 0.8−0.9 Å−1, and peak III at Q ≈ 1.4 Å−1. As the alkyl backbone spacer length increases, peak II slightly shifts to a lower Q with weaker intensity, representing a somewhat larger separation between anions.42 Peak III, the amorphous halo, remains unchanged regardless of the length of the alkyl spacer. Only two peaks are observed in PIL-C6, while for PIL-C8, an additional small peak appears on the lower Q

shoulder of the peak II. For monomeric ionic liquids, peak I is generally considered to correspond to the separation between charged aggregates surrounded by nonpolar regions.33,42 Similarly for PILs, nanoscale structural heterogeneity is also associated with peak I.43,44 As the number of methylene groups in the alkyl chain spacer increases up to 16, the backbone-tobackbone separation increases, causing growth of nonpolar domains along with the development of structural heterogeneity. Lopes et al. reported that for imidazolium-based ionic liquids, homogeneous domains are formed with a short alkyl chain.35 As the length of the alkyl chain increases, nonpolar domains form and grow due to aggregation of the alkyl chains, resulting in a phase separated nanostructure with charged clusters. Based on the intensity of peak I for PIL-C12, the nonpolar domains start to separate the charged aggregates. As shown in Figure 2a for PIL-C16, there is a corresponding increase in intensity and a large shift of peak I toward lower Q.35,45 Comparing the peak I of PIL-C12 and PIL-C16, the correlation length shifts from 13.6 to 17.4 Å with a significant increase in the intensity, supporting not only the formation of more charged clusters, but also a larger separation. The glass transition temperatures (Tg) of the PILs were measured by differential scanning calorimetry (DSC; Figure 2b). In contrast to the NILs reported previously by Yang et al.,46 the PILs have similar glass transition temperatures of approximately 237 K, independent of the alkyl spacer length. Linear polyethylene has a Tg = 231 ± 9 K,47 which is comparable to that of the PILs in this study. Since Tg is associated with the α process characteristic relaxation time, we infer that the dynamics of the PILs are strongly influenced by the α relaxation dynamics of the polyethylene-like chain spacers.48 The dynamics of the NILs studied previously were more strongly influenced by the ionic strength and van der Waals force since Tg decreased with a decrease in the charge density and an increase in the length of the alkyl chain spacer. The shockwave energy dissipation performance of the PILs was examined using a laser-induced shockwave protocol developed previously14 and described in the Supporting Information. The experimental setup and specimen geometry are shown in Figure 3a. The PILs are sandwiched between two glass substrates, and a short duration laser pulse impinges on one side of the specimen to create a high amplitude, short duration shockwave. By incrementally increasing the laser fluence, shockwaves with corresponding increases in peak pressure are generated. The in situ displacement of the 537

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ACS Macro Letters opposite surface of the specimen is measured interferometrically and used to calculate the peak pressure (Figure 3b) and the energy transferred (Supporting Information, Figure S3) for each PIL specimen. A lower peak pressure and total energy transferred correspond to more effective shockwave energy dissipation. As shown in Figure 3b, amorphous PIL-C6 attenuates more pressure than crystalline PIL-C6 across all input laser fluences. The amorphous phase consisting of entangled chains with weak interchain van der Waals forces is more easily densified by the shockwave, converting shock wave energy to irreversible heat.49 We hypothesize that the difference in energy dissipation between the amorphous and crystalline morphologies is due to the difference in the mobility of the alkyl chain segments. The amorphous region with higher atomic-scale mobility can have more degrees of freedom to attenuate shockwave energy via various molecular motions.50 Amorphous PIL-C6 and PIL-C8 dissipate almost identical amounts of pressure at all fluences (Figure 3b). For both PILC6 and PIL-C8, no peak I is observed in the XRD pattern (Figure 2a), indicating that the structural heterogeneity is not well developed. Although the shorter alkyl spacers in PIL-C6 should result in a stronger correlation between anions due to a stronger ionic strength than PIL-C8, the anion−anion correlations in PILs do not appear to have a strong effect on energy dissipation. As the length of the alkyl chain spacer increases for PIL-C12 and PIL-C16, shockwave energy dissipation also improves dramatically (Figure 3b). At the highest fluence, PIL-C16 shows a 25% decrease in peak pressure and 42% decrease in energy transferred over amorphous PIL-C6. The higher intensity of the XRD peak I for the PILs (Figure 2a) is strongly correlated to greater energy dissipation and lower average peak pressures at all input laser fluences. Hence, more developed structural heterogeneity and charged clusters lead to enhanced shockwave energy dissipation properties. This result is consistent with a prior simulation of shockwave attenuation in polyurea with a nanosegregated microstructure that suggests dissociation of hard domains under high pressure absorbs shockwave energy.21 Similarly in the PIL materials, a more charged cluster absorbs more energy. In summary, nanoscale structure and ordering have a significant effect on the shockwave energy dissipation of PILs. As the length of the alkyl chain spacer between cationic imidazolium groups increases, the degree of microstructural heterogeneity increases. However, the dynamics of the PILs, as characterized by Tg, remain similar for the different spacer lengths. We conclude that increased order of charged clusters in amorphous PILs favor effective energy dissipation, yet the crystalline phase is much less effective at dissipating energy. Hence, the structure and morphology are critical for designing soft materials for effective dissipation of shockwave loading.

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specimens; Laser-induced shockwave test protocols and energy transferred per area data for PILs (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yi Ren: 0000-0002-6952-6620 Christopher M. Evans: 0000-0003-0668-2500 Nancy R. Sottos: 0000-0002-5818-520X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to acknowledge financial support from the Department of the Navy, Office of Naval Research, Grant No. 0014-12-1-0828.

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REFERENCES

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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/acsmacrolett.9b00133. Materials; Synthesis of PILs, which includes NMR spectra; X-ray scattering; Differential scanning calorimeter (DSC) data; Rheometric data for the viscosity of the PIL; Preparation of PIL shockwave impact test 538

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