Inside the Ionic Aggregates Constrained by Covalently Attached

Jun 20, 2019 - unit of the PS,. the required amount of. the. c. -. H. 2. SO. 4. against the PS. w. as determined. according to. a PS theoretical degre...
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Letter Cite This: ACS Macro Lett. 2019, 8, 841−845

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Inside the Ionic Aggregates Constrained by Covalently Attached Polymer Chain Segments: Order or Disorder? Zixin Yu,† Jie Wang,† Zhen Hu,† Chuanqun Hu,† Dachuan Ding,† Bin Yang,† Tao Hu,† Xinghou Gong,† Chonggang Wu,*,† and Masanori Hara‡ †

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Hubei Provincial Key Laboratory of Green Materials for Light Industry, Collaborative Innovation Center of Green Light-weight Materials and Processing, and School of Materials and Chemical Engineering, Hubei University of Technology, Wuhan, Hubei 430068, China ‡ Department of Chemical and Biochemical Engineering, Rutgers University, Piscataway, New Jersey 08854, United States S Supporting Information *

ABSTRACT: When a small-molecule ionic crystal is groupsubstituted with polymer chain-segments to form an ionomer, do its constrained ionic aggregates maintain ordered internal structures? This work presents, for a Na-salt sulfonated-polystyrene ionomer, reconciled TEM electron-diffraction schlieren textures and WAXS Bragg-type reflections from the ionic-aggregate nanodomains, which solidly prove the aggregates’ internal (mono)crystalline order. The observed DSC endotherm of the ionomer, identified by WAXS as an order−disorder transition interior to its aggregates, gradually becomes enhanced over a 3-month, roomtemperature physical aging process, indicating that the aggregates’ ordering is a slow relaxation process in which the degree of order increases with time. This work corroborates an uncommon form of order, i.e., polymer-bound small-molecule ionic (quasi)crystal, which is supplementary to the order phenomena in small molecules, polymers, and liquid crystals.

I

there are essentially ionic groups that coordinate counterions. The local coordination structure about a neutralizing counterion inside the aggregates, as revealed by X-ray absorption (EXAFS) spectroscopy, may vary with the ionic-group distribution along the backbone chains. For instance, each Zn2+ is coordinated to four O atoms from four bidentate bridging −COO− groups in the −2COO−−Zn2+ aggregates of poly(ethylene-ran-methacrylic acid) (E-MAA) Zn-salts,22 but from two bidentate chelating −COO− groups in those of Znsalt maleated poly(ethylene-ran-propylene) ionomers where every two −COO− groups neighbor each other.23 In the internal coordination of ionic aggregates, the presence of order has long been suspected.3,24−28 SAXS shows that the ionic aggregates of sulfonated polystyrene (SPS) Zn-salts have an electron density only ∼10% lower than Zn benzenesulfonate hexahydrate crystallites, suggesting an ordered structure within them.3 EXAFS studies of both random24 and telechelic25 carboxylate ionomers indicate that the local order around a neutralizing cation in the aggregates is very close to that in the crystallites of their metal acetate analogues. An EXAFS study of SPS Zn-salts also reveals the existence of order inside their aggregates.26 A crystalline order is assumed within the aggregates of E-MAA Zn-salts based on their differential

onomers, formed from group substitution of small-molecule ionic crystals with polymer chain segments, are ioncontaining polymers having a small amount (≤10−15 mol %) of pendant ionic groups covalently bound to nonionic backbone chains. Owing to their electrostatic attractions and incompatibility with the nonionic repeat units, the ionic groups partly are subject to self-assembly into aggregates microphaseseparated from the ionomer matrix. In the small-angle X-ray scattering (SAXS) profile from an ionomer, the ionic aggregates are characterized by an “ionic” peak at a scattering angle (2θ) of 2−5°,1,2 which, according to Yarusso−Cooper’s interaggregate interference model,3 interprets a liquid-like degree of order in their spatial arrangement as mathematically described by the Born−Green correlation function.4 Later, Eisenberg et al.5 proposed, in high ion-content ionomers, clustered (i.e., overlapped) (ionic aggregate)−(restricted chain mobility region) core−shell structures, which constitute a second, more rigid phase of higher aggregate concentration than the matrix phase. Down to the length scale of ionic aggregates themselves, while most people observed that the aggregates are spheroidal and on average 2−3 nm in diameter,6−15 others found they may be vesicles of ∼3 nm in shell thickness with diameters of 9−5516 and 5−2017 nm, nanoplatelets of ∼6 nm in thickness,18 rods,19 strings and large percolated networks,20 etc. Further interior to the ionic aggregates, although nonionic chain segments are assumed to participate,21 it is generally held that © 2019 American Chemical Society

Received: April 24, 2019 Accepted: June 14, 2019 Published: June 20, 2019 841

DOI: 10.1021/acsmacrolett.9b00296 ACS Macro Lett. 2019, 8, 841−845

Letter

ACS Macro Letters

Table 1. Assignments of the PS, SPS, and SPS−Na’s 1H NMR Peak Chemical Shift (δ) Values Shown in Figure 1a(1)−(3), Respectively, to Their H Atoms Numbered in Figure 1b(1)−(3)

scanning calorimetry (DSC) and thermal dilatometry investigations.27 A 23Na nuclear magnetic resonance (NMR) study of E-MAA Na-salts implies that their aggregates are made of closely packed − COO−−Na+ ion pairs.28 Nevertheless, no direct, solid evidence has ever been found to confirm the presence of order inside the ionomeric aggregates, which are constrained by covalently bound chain segments compared with those of small-molecule ionic crystals. For a Na-salt SPS ionomer (SPS−Na) that derives from the phenyl substitution of a Na−benzenesulfonate ionic crystal with polystyrene (PS) chain-segmental radicals, this work presents high-resolution transmission electron microscopy (HR-TEM) electron-diffraction schlieren textures and wideangle X-ray scattering (WAXS) Bragg features from the aggregates, as well as reconciled HR-TEM and WAXS results in the aggregate size and interplanar spacing, the three of which directly and solidly corroborate the internal order of the −SO3−−Na+ aggregates. Synthetically, the SPS−Na is prepared by 88.6 mol % of solution neutralization in methanol, with CH3COONa, of an SPS formed from 15.9 mol % of solution sulfonation in 1,2dichloroethane of a PS with propionyl hydrogen sulfate (Equations S1−S5). First, the peaks at chemical shift (δ) values of 1.43−1.52, 1.85−1.88, 6.58−6.60, and 7.08−7.09 ppm in the 1H NMR spectra (Figure 1a), respectively, are

δ (ppm)

a

H atom no.

PS

SPS

SPS−Na

1 2 3 4 5

1.43 1.85 6.60 7.09 a

1.52 1.88 6.58 7.09 7.58

1.50 1.86 6.58 7.08 7.58

These H atoms do not belong to the PS.

compared with the PS, which, obviously ascribed to H atom 5 adjacent to the −SO3− groups (Figure 1b (2) and (3)),30 is a sign of successful sulfonation of the PS. In the Fourier transform infrared (FTIR) spectra (Figure 1c), other than the PS (spectrum 1), the SPS (spectrum 2) and SPS−Na (spectrum 3) exhibit three new, strong absorption bands at 1214−1217, 1177−1182, and 1126 cm−1, the first two and last of which, respectively, are characteristic of the S−O antisymmetric and symmetric stretches of the −SO3− groups;31 this further confirms the success in the sulfonation. Subsequently, by means of inductively coupled plasma optical emission spectrometry (ICP-OES), while the Na contents of the PS and SPS are insignificant, that of the SPS−Na is detected to be 2.7 × 10−2 g g−1 (Table 2), from which its Table 2. Degrees of Sulfonation (DS’s), Na Contents, and Degrees of Neutralization (DN’s) for the PS, SPS, and SPS− Na

a

material

DS (mol %)

Na content (10−5 g g−1)

DN (mol %)

PS SPS SPS−Na

a 15.9 15.9

5.7 6.1 2700.3

a a 88.6

Not applicable.

degree of neutralization is estimated to be 88.6 mol % (Equation S5); this is evidence that the salinization of the SPS following the PS sulfonation is successful. Finally, in Figure 1d, a single broad peak, known as the “ionic” peak, arises at 2θ = 2.9° in the SAXS profile (profile 3) from the SPS−Na against that (profile 1) from the PS, indicating the presence of −SO3−−Na+ ionic aggregates within the SPS−Na matrix.1,2 Figure 2 shows the ionic-aggregate morphology (graphs a− c) and crystallography (graph d) by HR-TEM and WAXS, respectively, of the synthesized SPS−Na well dried and then physically aged at room temperature (RT) (i.e., ∼25 °C) in vacuo for 2 weeks. The aggregates (i.e., “dark spots”) are spheroids uniformly distributed in the SPS−Na matrix as observed from the bright-field HR-TEM image (Figure 2a), from which their mean size, D, is analyzed to be ∼2.3 nm in diameter with a narrow size distribution of mostly 1.5−3.0 nm (Figure 2b). Under higher magnifications (Figure 2c), interior to a typically chosen aggregate occurs a crystalline electrondiffraction pattern of distinct schlieren texture, from which a (minimum) interplanar spacing, d, of ∼2.1 Å is resolved as the feature of an unknown crystallographic plane of the seemingly monocrystalline aggregate. The other aggregates in Figure 2c, if further magnified to the upper-right magnification or so, all exhibit similar internal periodicity (i.e., d ∼ 2.1 Å) of different

Figure 1. (a) 1H solution-state NMR spectra, (b) schematic chemical structures, and (c) FTIR absorption spectra of the (1) PS, (2) SPS, and (3) SPS−Na. (d) SAXS profiles from the (1) PS and (3) SPS− Na.

assigned to H atoms 1−4 numbered in the chemical structures (Figure 1b).29 Note that, in spectrum 1 (PS), there are 7.26 and 1.54 ppm peaks, respectively, of the residual CHCl3 and H2O in the chloroform-d solvent and that, in spectra 2 (SPS) and 3 (SPS−Na), 4.89−4.90 and 3.33 ppm peaks, respectively, for the residual CH3OH and H2O in the methanol-d4 solvent. As shown in Figure 1a and summarized in Table 1, the SPS and SPS−Na essentially produce a new peak at 7.58 ppm 842

DOI: 10.1021/acsmacrolett.9b00296 ACS Macro Lett. 2019, 8, 841−845

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ACS Macro Letters

Figure 2. (a) HR-TEM micrograph, (b) the mean size (i.e., diameter), D, and size distribution analyzed from (a), (c) another HR-TEM micrograph under higher magnifications showing crystalline electron-diffraction schlieren textures with a resolved interplanar spacing, d, of ∼2.1 Å, and (d) a WAXS pattern (curve 3), collected at RT, showing Bragg-type reflections for the ionic aggregates of the synthesized SPS−Na well vacuum-dried at 100 °C for at least 3 days and then physically aged at RT in vacuo for 2 weeks. For the HR-TEM, the aged, supposedly anhydrous SPS−Na sample is maximally ground with an agate pestle and mortar into a fine powder of ∼1 μm in diameter and then sonicated in anhydrous ethanol for 10−15 min as appropriate for forming a visually uniform dispersion, a drop of the supernatant of which is subsequently pipetted onto a 200-mesh copper-grid supported lacey carbon film, dried thoroughly with an infrared baking lamp, and finally subjected to observation using a microscope with point and lattice resolutions of 0.23 and 0.14 nm, respectively. For the WAXS, each of the PS and aged SPS−Na is fully ground with an agate pestle and mortar into a fine powder, which is then evenly spread onto a silicon-substrate sample holder for WAXS scan in the reflection mode.

that the expedient subtraction essentially makes a physical sense and that the WAXS pattern (curve 3) is indeed Braggtype reflections, from the internally monocrystalline aggregates, which constitute complementary evidence, still direct and strong, of the existence of order inside the chain-segment constrained −SO3−−Na+ aggregates. The observed aggregate monocrystallinity also is experimental evidence, against the reported physical simulations8,32 for crystalline ionomers, that there may be little PS segments present in the aggregates of the amorphous SPS−Na ionomer. To preclude the possibility that the HR-TEM and WAXS observed aggregates may be the residual CH3COONa remaining in the SPS−Na, their WAXS pattern is inspected against that of CH3COONa. As shown in Figure 3, the pattern (curve 3) of the aggregates is totally different in profile from those (curves 10 and 1) of CH3COONa and those (curves 20 and 2) of CH3COONa·3H2O, suggesting that the aggregates are not CH3COONa or its trihydrate but have necessarily to

extents and orientations from each other. These Figure 2c HRTEM observations constitute direct, strong evidence for the presence of crystalline order within the −SO 3 − −Na + aggregates constrained by connected PS chain segments. In Figure 2d, a weighted (i.e., 40%) subtraction of the scattered X-ray intensities (curve 1) of the PS from those (curve 2) of the SPS−Na resolves the WAXS pattern (curve 3) of the aggregates at RT; the scale factor of 40%, as well as any other ones below, is determined by a trial-and-error method according to, upon the subtraction, full repression of the “amorphous” peaks of the PS matrix as well as leveling-off of the declining baseline in the resolved WAXS pattern. From curve 3 the average diameter, D and a d of the aggregates’ crystallites, if spheroidal, are estimated to be ∼2.4 nm and ∼2.2 Å, respectively, using Scherrer’s equation (eq S7) and Bragg’s law (eq S8) at 2θ values of 16.8° and 41.4°. Therefore, the D and d derived from Figure 2d are reconciled with those given in Figure 2b,c (2.4 vs 2.3 nm and 2.2 vs 2.1 Å), confirming 843

DOI: 10.1021/acsmacrolett.9b00296 ACS Macro Lett. 2019, 8, 841−845

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ACS Macro Letters

traction, displays no Bragg reflection at all suggestive of their internal disordered structure. This contrast infers that the DSC endotherm characterizes an order−disorder (i.e., melting) transition within the aggregates of the 3-month aged SPS−Na. Interestingly, with prolonging aging time of the SPS−Na steadily from 0 h until 3 months (in the order of trace 1 to 4 in Figure 4a), its DSC endotherm is monotonously increased in both temperature and intensity. This reveals that the RT ordering inside the aggregates is a slow relaxation process in which the degree of order improves with time. Also observed from Figure 4a is a monotonic increase in the SPS−Na matrix Tg from 205.3 to 218.5 °C during its 3-month aging process, possibly due to an enhancement of ionic cross-linking. It is worth noting that the degree of crystalline order inside the aggregates may remarkably be lower than small-molecule ionic crystallites presumably due to a large constraining effect of their covalently bound PS segments. And, in the SPS−Na DSC samples, the aggregates are a very minor nanophase of ∼2.3 nm in diameter accounting for only ∼3 vol %,33 so a smallvolume-fraction effect that considerably reduces their melting enthalpy combined with a strong nano effect 34 that significantly decreases their melting temperature has to be taken into account. These three effects, respectively, may make the SPS−Na’s DSC endotherm broad, weak, and occur at a low temperature (80−110 °C). In conclusion, reconciled HR-TEM electron-diffraction schlieren textures and WAXS Bragg-type reflections from the −SO3−−Na+ ionic aggregates identified within the matrix of an SPS−Na ionomer directly and strongly evidence that they are single (quasi)crystals with different degrees of internal order. Accordingly, the observed low-temperature (80−110 °C) DSC endotherm of the SPS−Na may originate probably from an order−disorder transition interior to its aggregates as revealed by WAXS. Further, the endotherm temperature and -intensity are both enhanced steadily as a 3-month physical aging of the SPS−Na proceeds at RT, indicating that the development of crystalline order within the aggregates is a slow relaxation process in which the degree of order increases with time. By means of the SPS−Na, this work argues that, although substituted and thus constrained by PS chain segments, the −SO3−−Na+ aggregates, like those of a Na−benzenesulfonate ionic crystal, still retain their internal crystalline order to an as large extent as can be observed by HR-TEM and WAXS. The universality of this argument is to be addressed in our future work by systematic investigations of other typical ionomers.

Figure 3. WAXS patterns from (10) CH3COONa (PDF standard card), (1) CH3COONa (experimental), (20) CH3COONa·3H2O (PDF standard card), (2) CH3COONa·3H2O (experimental), and (3) the ionic aggregates of the SPS−Na aged at RT in vacuo for 2 weeks.

be the intrinsic, covalently attached −SO3−−Na+ aggregates of the SPS−Na. Actually, the excess CH3COONa is subject to thorough removal during precipitation, filtration, and washing of the SPS−Na (the Salinization of the SPS with Na+ Ions section, Supporting Information). Observed from the DSC thermogram (trace 4, Figure 4a) of the SPS−Na physically aged at RT in vacuo for 3 months is its

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Figure 4. (a) DSC thermograms at a heating rate of 40 °C/min for the SPS−Na samples physically aged at RT in vacuo for (1) 0 h (i.e., unaged), (2) 3 days, (3) 2 weeks, and (4) 3 months. WAXS patterns (curves 3), collected at (b) RT and (c) 190 °C, from the ionic aggregates of the SPS−Na physically aged at RT in vacuo for 3 months.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.9b00296. Experimental details, i.e., materials, preparation, and characterizations of the Na-salt sulfonated polystyrene ionomer (PDF)

strong endothermic peak at 103.6 °C followed by matrix glasstransition temperature (Tg)5 at 218.5 °C. At RT before the onset of the DSC endotherm (Figure 4b), the WAXS pattern (curve 3) of the SPS−Na aggregates, resolved by a weighted (50%) subtraction of the PS pattern (curve 1) from the SPS− Na pattern (curve 2), exhibits a major Bragg reflection at a 2θ of ∼17.0° indicative of their internal crystalline order (cf. the inset of Figure 2d). Nevertheless, at 190 °C after the end of the DSC endotherm (Figure 4c), the WAXS (curve 3) of the aggregates, resolved likewise by a weighted (110%) sub-

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Chonggang Wu: 0000-0002-5087-7834 Notes

The authors declare no competing financial interest. 844

DOI: 10.1021/acsmacrolett.9b00296 ACS Macro Lett. 2019, 8, 841−845

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ACS Macro Letters

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ACKNOWLEDGMENTS This work was supported by the Select Overseas Chinese Scholars Science and Technology Activities Foundation of the Ministry of Human Resources and Social Security of China ([2013]277), as well as by the Natural Science Foundation of the Hubei Province of China (2014CFA094), the Overseas High-level Talents Scientific-research Starting Fund of Hubei University of Technology, China (HBUT-science-[2005]2), and the National Natural Science Foundation of China (51703053).

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DOI: 10.1021/acsmacrolett.9b00296 ACS Macro Lett. 2019, 8, 841−845