Tertiary ammonium counterions outperform quaternary ammonium counterions in ionic comb polymers: overcoming the trade-off between toughness and the elastic modulus | Polymer Journal

Jun 06, 2025

Polymer Journal (2025)Cite this article

257 Accesses

6 Altmetric

Metrics details

Ionic motifs incorporated into polymers, such as ionomers, polyelectrolytes, polyampholytes, and poly(ionic liquid)s (PILs), serve as physical crosslinks through ionic interactions, presenting an effective molecular design for improving both the toughness and elastic modulus of the resulting polymer. However, in glassy polymers, ionic motifs, particularly organic base counterions, seldom effectively contribute to toughening mechanisms, as observed in soft materials. Here, we investigate the influence of tertiary and quaternary ammonium counterions on the mechanical properties of ionic comb polymers with a focus on the counterion incorporation ratio. We discovered a specific ion content range where only tertiary ammonium counterions, such as triheptylamine, improved both the toughness and Young’s modulus of a precursor polymer containing carboxylic acid groups and oligo(ethylene glycol) side chains. Fourier transform infrared (FT-IR) analysis revealed the presence of neutral amines in the tertiary ammonium systems, as evidenced by slightly less intense carboxylate peaks compared with the quaternary ammonium system peak intensities. Furthermore, rheological analysis revealed that tertiary counterions induced plasticization and reduced relaxation times up to the rubbery region. This study demonstrated the distinct mechanical effects of organic bases on glassy polymers.

The development of multimaterial systems for use in lightweight transportation equipment requires innovative molecular design strategies to increase both the toughness and elastic modulus of the polymer materials [1]. Ionic motifs incorporated within polymer chains represent an attractive molecular design for modifying mechanical properties, as they act as physical crosslinks through ionic interactions [2,3,4]. The improvement in the mechanical properties caused by ionic motifs is influenced by the properties of the polymer matrix as well as the nature of the ions and counterions. In soft polymer media with high ion mobility, such as hydrogels [5,6,7,8], elastomeric ionomers [9,10,11,12,13,14,15,16], and poly(ionic liquid)s (PILs) [17,18,19], ionic crosslinks efficiently improve both the toughness and elastic modulus through ion dissociation by functioning as sacrificial bonds. However, in glassy polymers, ionic motifs rarely contribute effectively to toughening mechanisms, as observed in soft materials, with few exceptions [20, 21]; instead, these ionic motifs often result in hard and brittle polymers. Thus, developing a general molecular design for toughening glassy polymers through ionic interactions remains challenging.

One approach to promote ion mobility involves the use of larger ions to weaken electrostatic interactions, which in turn shortens the association lifetime of ionic motifs in the ionomer network [4, 22,23,24,25]. Chen et al. reported that the brittle-to-ductile transition in ionomer melts involves the continuous dissociation and reassociation of ionic motifs during material deformation by using sulfonated polystyrene ionomers with different association lifetimes on the basis of the size of the metal counterion (Na+, K+, Cs+, Rb+) [25, 26]. From the perspective of ion size control, bulky organic counterions are attractive because their charge screening ability can be adjusted through the molecular design of spacers (e.g., alkyl chains of different lengths) directly bonded to the charged groups [4]. For example, weakened ionic interactions with bulky organic ions can enable the development of water-free processable polyelectrolyte complexes (compleximers) [27] and lipophilic polyelectrolytes for use as superabsorbent polymers [28, 29]. Additionally, the organic spacers bonded to charged groups not only attenuate electrostatic interactions but can also form unique nanostructures [30,31,32,33] and function as plasticizers [34,35,36,37,38,39]. Compared with systems with glass transition temperatures below room temperature, ion dynamics in glassy polymers are further complicated by the restricted mobility of both ions and polymer chains. Matsumoto et al. reported that in PILs, while large counterions weaken electrostatic interactions, they also behave as solvent molecules, strengthening intramolecular interactions and consequently increasing backbone rigidity [40,41,42,43]. Moreover, Stacy and coworkers reported that the relationship between the ion diffusion activation energy and ion size in glassy polymers exhibits a nonmonotonic dependence with a minimum due to the competition between Coulombic and elastic forces [24]. Despite these advances in ion dynamics within polymers, understanding the impact of organic counterions on the mechanical properties of glassy bulk polymers remains limited owing to the complex interplay between backbone rigidity, ion mobility, and plasticizing effects.

In this context, we previously reported a unique toughening effect dependent on counterion size in glassy polynorbornene derivatives containing carboxylic acid groups. This toughening effect was observed exclusively with triheptylamine counterions of a specific size [44]. Although counterion size was initially considered the primary factor influencing mechanical properties, the ionic equilibrium of counterions may play an equally important role in governing ion dynamics within glassy polymers. A key consideration is that the dissociation constants in organic media are generally higher than those in water, although a precise prediction remains challenging because of the independent solvent effects on the pKa of acids and pKb of bases [45,46,47]. The complexity of ionic polymers is further amplified, as their pKa values differ from those of their low-molecular-weight analogs due to various factors, including spatial confinement, local concentration effects, and polymer dynamics [48,49,50]. To our knowledge, no detailed investigations have compared the bulk mechanical properties of glassy polymers containing tertiary and quaternary ammonium counterions.

In this study, we report the effects of tertiary and quaternary ammonium counterions on the mechanical properties of ionic comb polymers consisting of polynorbornene with triethylene glycol monomethyl ether (mOEG3) side chains and carboxylic acid groups in each monomer. We systematically investigated the contribution of ammonium counterions to toughening by varying the base/COOH ratio and the ionic size of the quaternary ammonium counterion. We found a specific window where only tertiary amines, not quaternary amines, improved both toughness and Young’s modulus. Furthermore, Fourier transform infrared (FT-IR) spectroscopy and rheological studies revealed that tertiary ammonium counterions toughen polymers through a combination of ionic crosslinking and plasticization effects due to the presence of partially neutral amines.

5-Norbornene-2,3-dicarboxylic anhydride (NBC), triethylene glycol monomethyl ether (mOEG3), 4-dimethylaminopyridine (DMAP), ethyl vinyl ether, triheptylamine (Hep3N), tetrapropylammonium hydroxide (10% in water), and tetrabutylammonium hydroxide (10% in methanol) were purchased from Tokyo Chemical Industry Co., Ltd. Dichloro1,3-bis(2,4,6-trimethylphenyl)-2-imidazolidinylidenebis(3-bromopyridine)ruthenium(II) (3rd generation Grubbs Catalyst, M300), ion exchange resin (Amberlyst FPC3500), tetrahexylammonium hydroxide (40% in water), and dimethyl sulfoxide-d6 were purchased from Sigma Aldrich. N,N-Dimethylformamide (DMF; anhydrous), tetrahydrofuran (THF; anhydrous, stabilizer free), diethyl ether, and methanol (MeOH) were purchased from Wako Pure Chemical Industries. The calcium fluoride (CaF2) substrates were obtained from Pier Optics Co., Ltd. All the chemicals were used as received.

Ionic comb polymer (ICP-baseX) synthesis was performed according to our previously established protocols (Scheme S1) [44]. We used presynthesized CP-COOH as the precursor for ICP-baseX. To prepare the ICP-baseX samples, we performed neutralization in THF, MeOH, or H2O using a specific concentration of either trialkylamine or tetraalkylammonium hydroxide solution: THF for Hep3N; H2O for tetrapropylammonium (Pr4N) and tetrahexylammonium (Hex4N); and MeOH for tetrabutylammonium (Bu4N). Following overnight neutralization, we purified the resulting polymer through reprecipitation employing an excess of diethyl ether.

¹H NMR spectra were obtained using a JNM-AL400 spectrometer (JEOL) with dimethyl sulfoxide-d6 as the solvent. Fourier transform infrared (FT-IR) spectra were recorded using an FT-IR 600 instrument (JASCO). The samples for FT-IR analysis were prepared as spin-coated films on CaF₂ substrates by applying a polymer solution (approx. 0.02 g/mL) twice at 1000 rpm for 30 s. The spin-coated films were dried under vacuum at room temperature overnight to remove residual solvent. Thermogravimetry-differential thermal analysis (TG-DTA) was performed using a STA2500 Regulus instrument (NETZSCH) from room temperature to 500 °C at a heating rate of 5 °C/min under dry air. Size exclusion chromatography (SEC) was performed using DMF containing 0.1 mM LiBr as the eluent at 40 °C. The chromatography setup consisted of KF-805L columns (Shodex), a CO-2065 Plus Intelligent column oven, a PU-2080 Plus Intelligent HPLC pump, and an RI-2031 Plus Intelligent refractive index detector (JASCO). The system was calibrated using polystyrene standards with narrow molecular weight distributions (TOSOH).

Films for tensile tests and rheological studies were prepared using a casting method. We began the process by pouring the polymer THF solution into a PFA dish (Fig. S1). To prevent bubble formation, we employed a gradual solvent evaporation technique. This involved a three-step process: initial evaporation at room temperature to form a film, heating at 60 °C, and finally heating at 80 °C. After being cast, the films were subjected to a final treatment in a vacuum oven at 100 °C for 12 h to ensure complete solvent removal and optimal film properties.

Tensile tests were conducted using an A&D Co., Ltd. MCT-2150 instrument. The tests were performed at a constant crosshead speed of 10 mm/min and were repeated at least three times (n ≥ 3). Dog-bone-shaped test samples were prepared from the cast films using a JIS K6251 standard punching blade (Hagataya Co. Ltd.).

Rheological measurements were performed using an MCR302 modular compact rheometer (Anton Paar). Disk-shaped samples (diameter = 25 mm, thickness ~ 0.3 mm) were placed between parallel plates (diameter = 25 mm). Temperature sweep measurements were conducted at a strain amplitude of 0.1% and a frequency of 1 Hz from 30 °C to 150 °C. The same sample was subsequently used for dynamic frequency sweep analysis. Dynamic frequency sweep measurements were performed at 0.1% strain over a frequency range of 0.1–100 rad s⁻¹ at temperatures ranging from 60 °C to 140 °C in 10 °C increments. Master curves were constructed via time‒temperature superposition (tTs) at a reference temperature of 100 °C.

To investigate the impact of various tertiary or quaternary ammonium counterions on the mechanical properties of the resulting polymers, we synthesized ionic comb polymers with different base/COOH ratios (Fig. 1). In general, traditional hard ionomers containing metal ions exhibit varying mechanical properties depending on the degree of neutralization; for example, the Young’s modulus and tensile strength show a nonmonotonic increase with a local maximum value [51, 52]. We previously reported that among ionic comb polymers containing 20–30 mol% trialkylamine-neutralized carboxylic acid groups, the heptylamine (Hep3N) counterion was an optimal size for improving toughening effects [44]. To cover the ionic size range of the previously optimized Hep3N counterion, we selected Pr4N, Bu4N, and Hex4N as quaternary ammonium counterions. The samples were labeled as ICP-BaseX, where “Base” indicates the type of base and “X” represents the mole percentage of base to carboxylic acid groups (base/COOH).

Chemical structures of CP-COOH and ICP-baseX

The ICP-base samples were prepared by neutralizing the carboxylic acid groups in the precursor CP-COOH, polynorbornene with triethylene glycol monomethyl ether (mOEG3) side chains and carboxylic acids on each monomer, following our previously reported procedure [44]. CP-COOH exhibited a number-average molecular weight (Mn) of 1.1 × 10⁶ and dispersity (Đ) of 3.22, as determined by SEC calibrated with polystyrene standards using LiBr/DMF (0.1 mM) as the eluent (Fig. S29). For the quaternary ammonium counterions, we utilized solutions of quaternary ammonium hydroxide (aqueous solutions for Pr4N and Hex4N and a methanol solution for Bu4N). The incorporation of counterions into the comb polymer was characterized using 1H NMR and FT-IR spectroscopy, as detailed in the Supporting Information. The ICP-base films were prepared through solvent casting from a THF solution and stored in a desiccator. For the casting process, a drying temperature of 100 °C under vacuum was carefully selected to exceed the glass transition temperature (Tg) of all the samples while avoiding undesirable side reactions. TG-DTA confirmed that virtually no solvent remained in the films prepared using this method (Fig. S19).

To investigate ionic dissociation in the ionic comb polymer films, we compared the FT-IR spectra of ICP-Hep3N and ICP-Hex4N with base/COOH ratios ranging from 0% to 40%. Although the addition of the organic base caused various peak intensities to increase, comparisons among different base/COOH ratios were possible by normalizing the peaks on the basis of the C–O stretching vibration at 1141 cm−1 from the ether bond on the side chain (Fig. S3). Upon increasing the base/COOH ratio in both the Hep3N and Hex4N systems, we observed common spectral changes: a decrease in the C = O stretching peak (ν(C = O)) at 1731 cm−1, an increase in the carboxylate asymmetric stretching peak (νas(COO⁻)) at 1561 cm−1, and a decrease in the O–H stretching peak at 3200 cm−1. Thus, the carboxylic acid groups in both the ICP-Hep3N and ICP-Hex4N films were neutralized by their respective counterions to form carboxylate groups.

Figure 2a presents the FT-IR spectra, in which the carboxylic acid and carboxylate regions (1400–1800 cm−1) are expanded, for the ICP-Hep3N and ICP-Hex4N films with base/COOH ratios ranging from 0 to 40%. In response to an increasing base/COOH ratio, multiple peaks attributed to COO− were observed in the range of 1440–1660 cm−1, which were not present in the spectrum of the base alone. Guo et al. reported that the position of the COO− peaks varies depending on the coordination geometries of the metal‒ligand bonds; that is, the unidentate mode appears at 1638 cm−1, the bridging and bidentate modes appear at 1560 cm−1, and the ionic mode appears at 1460 and 1436 cm−1 [53]. Moreover, Peng et al. reported similar multiple FT-IR peaks at 1600 and 1560 cm⁻¹ with combinations of tertiary amines and carboxylate ions as the neutralization reaction progressed [54]. In the present ICP series, the COO− peaks exhibited patterns similar to those of the metal‒ligand coordination bond modes. Therefore, we assigned the COO− interaction modes according to the literature [53, 55,56,57,58] as follows: unidentate-like mode at 1648 cm−1, bridging-like mode at 1600 cm−1, bidentate-like mode at 1561 cm−1, and ionic mode at 1456 cm−1. Notably, these vibration mode assignments reference metal‒ligand coordination bonds; therefore, the precise assignment of the vibration modes upon carboxylic acid‒amine neutralization requires further investigation.

a FT-IR spectra, expanded in the carboxylic acid and carboxylate regions (1400–1800 cm−¹), of the ICP-Hep3N and ICP-Hex4N films on CaF2 substrates with various base/COOH ratios (0–40%). Peak intensities normalized to A1211 as a function of the base/COOH molar ratio for b ν(C = O) at 1731 cm−1, (c) νs(COO−) at 1456 cm−1, (d) νas(COO−) in bidentate mode at 1561 cm−1, and (e) νas(COO−) in bridging mode at 1600 cm−1

To compare neutralization of the carboxylic acid groups by tertiary and quaternary ammonium counterions, we analyzed changes in peak intensities in the ICP-Hep3N and ICP-Hex4N spectra as a function of the base/COOH ratio. The ν(C = O) peak at 1731 cm−1, which overlapped with the ester and carboxylic acid peaks, exhibited a slightly greater decrease in intensity in the ICP-Hex4N spectrum than in the ICP-Hep3N spectrum as the base/COOH ratio increased (Fig. 2b). Correspondingly, the symmetric carboxylate stretching vibration (νs(COO⁻)) at 1456 cm−1 increased with increasing base/COOH ratio, with ICP-Hex4N showing a slightly greater rate of increase than ICP-Hep3N (Fig. 2c). Thus, the tertiary Hep3N counterion exists partially in its neutral amine form within the ionic comb polymer films.

Interestingly, the peak at 1561 cm−1, attributed to the bidentate-like mode, exhibited behavior distinct from the ν(C = O) and νas(COO−) peaks. The peak at 1561 cm−1 in both ICP-Hep3N and ICP-Hex4N spectra displayed similar rates of increase in the base/COOH ratio range of 0–20%; however, the intensity of this peak decreased only in the ICP-Hex4N spectrum when the ratio exceeded 25% (Fig. 2d). In contrast, the intensity of the bridging-like mode peak at 1600 cm−1 did not significantly differ between the ICP-Hex3N and ICP-Hex4N spectra (Fig. 2e). These results suggest that the quaternary Hex4N counterion induces the structure to change from the bidentate mode at a base/COOH ratio of approximately 30%.

Next, we investigated the impacts of tertiary and quaternary ammonium counterions on the mechanical properties of the ICP-base films. The films were prepared via the solvent casting method and stored under dry conditions until immediately before tensile testing (Fig. S1). The mechanical properties derived from all the stress‒strain curves are summarized in the Supporting Information (Table S1). Figure 3a presents the stress‒strain curves of the ICP-Hep3N samples with various base/COOH ratios, which show a nonmonotonic trend. Figure 3b, c present a summary of the mechanical properties of ICP-Hep3N, including the tensile strength (σb), yield stress (σy), strain at break (εb), work-of-fracture (Ut), and Young’s modulus (E), as obtained from the stress‒strain curves. At a low neutralization ratio ( ≤ 10 mol%), ICP-Hep3N5 and ICP-Hep3N10 showed typical mechanical strengthening behavior compared with CP-COOH, exhibiting increases in the yielding stress (σy ≥ 40 MPa) and Young’s modulus (E ≥ 0.8 GPa), albeit at the sacrifice of ductility (εb = approx. 200%). Notably, within the optimal neutralization range (10–30 mol%), ICP-Hep3N improved both the mechanical strength ( ≥ 30 MPa) and ductility ( ≥ 380%), with the toughness reaching its maximum value of 94 MJ/m³ at a 15% base/COOH ratio. However, beyond the 30 mol% neutralization threshold, ICP-Hep3N became brittle, exhibiting decreased mechanical strength and ductility, with the toughness significantly reduced to less than 10 MJ/m³. Thus, ICP-Hep3N was found to have a specific window in which the base/COOH ratio effectively enhanced toughening, with work-of-fracture values dramatically increasing from 47 MJ/m³ in CP-COOH to 94 MJ/m³, double the original value. Similarly, the Young’s modulus also increased from 471 MPa to 1016 MPa within this toughening window, exceeding that of CP-COOH. Importantly, both the work-of-fracture and Young’s modulus, which typically exhibit a trade-off relationship, display a nonmonotonic dependence on the base/COOH ratio with distinct local maxima.

Stress‒strain curves of (a) ICP-Hep3N, (d) ICP-Pr4N, (e) ICP-Bu4N, and (f) ICP-Hex4N with varying base/COOH molar ratios and (g) comparisons of the ionic sizes of different quaternary ammonium counterions (ICP-Pr4N15, ICP-Bu4N10, and ICP-Hex4N12). Mechanical properties of ICP-Hep3N as a function of the base/COOH molar ratio: (b) tensile strength (σb) and strain at break (εb); and (c) work-of-fracture (Ut) and Young’s modulus (E). Comparison of the (h) Ut and (i) E values for ICP-Hep3N, ICP-Pr4N, ICP-Bu4N, and ICP-Hex4N with varying base/COOH molar ratios. The asterisk represents the mechanical properties extracted from our previous work described in ref. 44

In contrast, the stress‒strain curves of the ICP‒base polymers containing quaternary ammonium counterions markedly differed with increasing base/COOH ratios. Specifically, ICP-Pr4N, ICP-Bu4N, and ICP-Hex4N commonly exhibited a clear transition from brittle to ductile behavior by sacrificing either elongation at break or tensile strength without showing a toughening window across varying base/COOH ratios. Figure 3d–f presents the stress‒strain curves of ICP-Pr4N, ICP-Bu4N, and ICP-Hex4N with various base/COOH ratios. At moderate base/COOH ratios ( ≤ 45 mol%), both ICP-Pr4N and ICP-Bu4N presented high yield stress (σy ≥ 35 MPa) and brittle mechanical behavior with minimal ductility (εb ≤ 50%), whereas excessive base content ( ≥ 60 mol%) resulted in ductile behavior (Fig. 3d, e). Furthermore, ICP-Hex4N exhibited material softening and increased ductility compared with the smaller quaternary counterions (Pr4N and Bu4N), similar to the reported plasticization effect of tetrakis(decyl)ammonium counterions in polyurethane [36]. When comparing the effects of ionic size at similar counterion contents, ICP-Pr4N15, ICP-Bu4N10, and ICP-Hex4N12 showed increasingly softer mechanical properties as the size of the counterion increased, demonstrating a plasticization effect typical of alkyl chains (Fig. 3f).

Importantly, while the quaternary ammonium counterions increased either the ductility or mechanical strength of the ICP-base polymers, they did not improve the work-of-fracture (Ut), which differs from the behavior observed with the ICP-Hep3N series. These distinct effects of tertiary and quaternary ammonium counterions on the mechanical properties of ionic comb polymers are highlighted in plots of the Young’s modulus and work-of-fracture versus the base/COOH ratio (Fig. 3g, h). In the plot of the Ut versus base/COOH ratio, the quaternary ammonium counterions (ICP-Pr4N, ICP-Bu4N, and ICP-Hex4N) did not exceed the toughness of the original CP-COOH across all base/COOH ratios (Fig. 3g). The Ut values for ICP-base polymers containing quaternary ammonium counterions exhibited a local minimum across base/COOH ratios, with this minimum shifting toward lower base/COOH ratios as the ionic size increased. In contrast, ICP-Hep3N improved the Ut and exhibited a maximum value as the base/COOH ratio increased, demonstrating enhanced toughening, except when the base/COOH ratio exceeded 40%. All the ICP-base polymers, including ICP-Hep3N, ICP-Et4N, ICP-Bu4N, and ICP-Hex4N, exhibited increased Young’s moduli (E values) relative to those of CP-COOH, indicating a nonmonotonic trend with local maxima. The local maxima of the E values in the ICP-base polymers with quaternary counterions were inversely related to the Ut trend, indicating a typical trade-off between stiffness and toughness. The decrease in toughness, in response to the increases in the Young’s modulus with increasing neutralization ratio, is a trend typically observed in relatively hard ionic polymer films containing metal counterions that exhibit a yield point [51, 52, 59].

Therefore, tertiary ammonium counterions were found to overcome this trade-off and simultaneously improve both the toughness and Young’s modulus, which cannot be achieved by quaternary ammonium or metal counterions.

To understand the unique toughening phenomenon observed only with the ICP-Hep3N samples, we investigated the rheological properties of ionic comb polymers containing different counterions. Figure 4a–e presents the master curves of the storage modulus (G′) and loss modulus (G″) versus frequency at a reference temperature (Tr) of 100 °C for CP-COOH, ICP-Hep3N15, ICP-Pr4N15, ICP-Bu4N15, and ICP-Hex4N15. The time‒temperature superposition principle using tanδ was successfully applied across the entire frequency range (from glassy to rubbery regimes) for all ionic comb polymers, which is consistent with the observations with PILs [43]. The viscoelastic shift factor (aT) of all the samples followed the Williams–Landel–Ferry equation (Fig. 4f):

where C1 and C2 are constants and Tr is the reference temperature, which was set to 100 °C for this study. The C1 and C2 values, determined by linear approximation, are summarized in Table 1 (Fig. S26).

Master curves of (a) CP-COOH, (b) ICP-Hep3N15, (c) ICP-Pr4N15, (d) ICP-Bu4N15, and (e) ICP-Hex4N15 at a reference temperature (Tr) of 100 °C. f The dotted lines represent the curves fitted to the Williams-Landel-Ferry (WLF) equation: log aT = −C1(T − Tr)/(C2 + T − Tr), where C1 and C2 are constants provided in Table 1

The master curves for all the CP-COOH and ICP-base samples exhibit two crossovers (ω0 and ωe), corresponding to the reciprocals of the segmental relaxation (τ0) and entanglement relaxation (τe) times, respectively (Table 1). The transition region between ω₀ and ωₑ was broader for the ICP-base samples than for the CP-COOH samples. Matsumoto and Inoue et al. reported that a broader transition region in PILs indicates increased polymer chain rigidity, which increases with counterion volume [40, 43]. In addition, τₑ corresponds to the Rouse relaxation time between entangled strands [60,61,62]. The τₑ of ICP-Hep3N was lower than that of ICP-base polymers containing quaternary ammonium counterions, indicating accelerated Rouse relaxation at room temperature. In other words, although the polymer chains are more rigid than CP-COOH due to the bulky counterions, ICP-Hep3N may maintain active polymer dynamics during deformation. Note that more rigorous analyses of the effects of counterions on relaxation times require further validation via the use of appropriate model polymers, model equations, and reference temperatures, as both the glass transition temperatures and the molecular weights of the entanglements differ.

In the rubbery region (ωaT < ωe), all samples exhibited a rubbery plateau, indicating network formation through both entanglements and ionic crosslinks. The plateau modulus (GN0) was determined from the minima in the van Gurp-Palmen plots (Fig. S28) [63, 64]. Compared with the plateau modulus (GN0 = 2.6 × 106 Pa) of CP-COOH, all the ICP-base samples presented higher GN0 values, suggesting network formation through ionic bonding (Table 1). Interestingly, the plateau modulus (GN0) values showed a stronger dependence on ionic size than on the tertiary or quaternary nature of the counterion, with GN0 decreasing as the ionic size increased, with the highest GN0 observed for ICP-Pr4N15. In other words, despite their slightly lower ionization rate, as observed via FT-IR analysis, tertiary ammonium counterions can form ionic crosslinks at levels comparable to those of quaternary ammonium counterions. In addition, the GN0 of 8.4 × 104 Pa for ICP-Hep3N15 indicates a lower network density than that of well-known tough polymers such as polyethylene and polycarbonate, which typically have GN0 values of approximately 106 Pa.

Because the glass transition temperature (Tg) could not be determined by DSC, we used the peak temperature of tanδ from the temperature sweep rheological measurements to estimate the Tg value (Fig. S27). Compared with those of CP-COOH, the Tg of the ICP-base samples containing quaternary ammonium counterions increased by more than 10 °C due to their strong ionic interactions. When comparing the Tg values of quaternary ammonium counterions of different sizes, we observed a slight decrease in the Tg with increasing ionic size, likely due to the plasticizing effect of the alkyl chains. This trend corresponds to our previous reports on tertiary ammonium counterions [44]. In contrast, ICP-Hep3N15 did not increase the Tg of CP-COOH, suggesting competition between increasing Tg due to ionic interactions and decreasing Tg due to the plasticizing effect. The Tg of ICP-Hep3N15 was lower than that of ICP-Hex4N15, despite their similar counterion sizes, because neutral tertiary amines behave as plasticizers, as indicated by FT-IR analysis.

Therefore, the tertiary Hep3N counterion contributes to ionic crosslinking while also enhancing polymer dynamics due to the plasticizing effect of its partial neutrality. In contrast, quaternary ammonium counterions demonstrate typical ionic crosslinking effects, which increase the Tg and extend the relaxation time.

In summary, we comprehensively investigated the effects of tertiary and quaternary ammonium counterions on the mechanical properties of ionic comb polymers with varying base/COOH ratios through tensile tests. We incorporated Hep3N as a tertiary ammonium counterion and Pr4N, Bu4N, and Hex4N as quaternary ammonium counterions into the precursor polymer CP-COOH. Notably, only the tertiary ammonium counterion Hep3N simultaneously enhanced both the toughness and Young’s modulus of CP-COOH at base/COOH ratios of 5–30%, achieving a maximum toughness of 94 MJ/m3. In contrast, the quaternary ammonium counterions exhibited a consistent trade-off between toughness and the Young’s modulus relative to the base/COOH ratio. FT-IR analyses of ICP-Hep3N and ICP-Hex4N revealed that the rate of increase in the intensity of the COO- ion peak as a function of the base/COOH ratio was slightly lower for Hep3N than for Hex4N, suggesting the presence of neutral amines. Rheological studies of the ICP-base15 samples demonstrated that while quaternary ammonium counterions increased the Tg of CP-COOH, the Tg with Hep3N counterions decreased slightly via plasticization. Nevertheless, the plateau modulus increased from GN0 = 2.6 × 106 Pa for CP-COOH to GN0 = 8.4 × 104 Pa for ICP-Hep3N15, indicating network densification. These rheological results demonstrate that tertiary ammonium counterions can simultaneously facilitate crosslink formation and enhance polymer chain dynamics.

This study paves the way for a novel molecular design strategy to simultaneously enhance the toughness and elastic modulus of glassy polymers using organic base counterions. In the future, detailed studies, including rheological analyses with nonentangled model polymers, dielectric relaxation analyses to assess ion mobility in the glassy state, and nanostructure characterization using SAXS/WAXS, are needed to elucidate the underlying mechanisms involved.

Naskar AK, Keum JK, Boeman RG. Polymer matrix nanocomposites for automotive structural components. Nat Nanotechnol. 2016;11:1026–30.

Article CAS PubMed Google Scholar

Zhang L, Brostowitz NR, Cavicchi KA, Weiss RA. Perspective: Ionomer research and applications: Perspective: Ionomer research and applications. Macromol React Eng. 2014;8:81–99.

Article Google Scholar

Middleton LR, Winey K. Nanoscale aggregation in acid- and ion-containing polymers. Annu Rev Chem Biomol Eng. 2017;8:499–523.

Article CAS PubMed Google Scholar

Potaufeux J-E, Odent J, Notta-Cuvier D, Lauro F, Raquez J-M. A comprehensive review of the structures and properties of ionic polymeric materials. Polym Chem. 2020;11:5914–36.

Article CAS Google Scholar

Sun J-Y, Zhao X, Illeperuma WRK, Chaudhuri O, Oh KH, Mooney DJ, et al. Highly stretchable and tough hydrogels. Nature. 2012;489:133–6.

Article CAS PubMed PubMed Central Google Scholar

Sun TL, Kurokawa T, Kuroda S, Ihsan AB, Akasaki T, Sato K, et al. Physical hydrogels composed of polyampholytes demonstrate high toughness and viscoelasticity. Nat Mater. 2013;12:932–7.

Article CAS PubMed Google Scholar

Yu HC, Zheng SY, Fang L, Ying Z, Du M, Wang J, et al. Reversibly transforming a highly swollen polyelectrolyte hydrogel to an extremely tough one and its application as a tubular grasper. Adv Mater. 2020;32:e2005171.

Article PubMed Google Scholar

Cui W, Zheng Y, Zhu R, Mu Q, Wang X, Wang Z, et al. Strong tough conductive hydrogels via the synergy of ion-induced cross-linking and salting-out. Adv Funct Mater. 2022;32:2204823.

Article CAS Google Scholar

Filippidi E, Cristiani T, Eisenbach CD, Eisenbach CD, Waite J, Israelachvili J, et al. Toughening elastomers using mussel-inspired iron-catechol complexes. Science. 2017;358:502–5.

Article CAS PubMed PubMed Central Google Scholar

Miwa Y, Kurachi J, Kohbara Y, Kutsumizu S. Dynamic ionic crosslinks enable high strength and ultrastretchability in a single elastomer. Commun Chem. 2018;1:5.

Miwa Y, Taira K, Kurachi J, Udagawa T, Kutsumizu S. A gas-plastic elastomer that quickly self-heals damage with the aid of CO2 gas. Nat Commun. 2019;10:1828.

Article PubMed PubMed Central Google Scholar

Kajita T, Noro A, Oda R, Hashimoto S. Highly impact-resistant block polymer-based thermoplastic elastomers with an ionically functionalized rubber phase. ACS Omega. 2022;7:2821–30.

Article CAS PubMed Google Scholar

Das A, Sallat A, Böhme F, Suckow M, Basu D, Wiessner S, et al. Ionic modification turns commercial rubber into a self-healing material. ACS Appl Mater Interfaces. 2015;7:20623–30.

Article CAS PubMed Google Scholar

Gregory GL, Williams CK. Exploiting sodium coordination in alternating monomer sequences to toughen degradable block polyester thermoplastic elastomers. Macromolecules. 2022;55:2290–9.

Article CAS PubMed PubMed Central Google Scholar

Zhang J, Mao X, Ma Z, Pan L, Wang B, Li Y. High-performance polyethylene-ionomer-based thermoplastic elastomers exhibiting counteranion-mediated mechanical properties. Macromolecules. 2023;56:4219–30.

Article CAS Google Scholar

Gregory GL, Sulley GS, Kimpel J, Łagodzińska M, Häfele L, Carrodeguas LP, et al. Block poly(carbonate-ester) ionomers as high-performance and recyclable thermoplastic elastomers. Angew Chem Int Ed Engl. 2022;61:e202210748.

Article CAS PubMed PubMed Central Google Scholar

Li L, Wang X, Gao S, Zheng S, Zou X, Xiong J, et al. High-toughness and high-strength solvent-free linear poly(ionic liquid) elastomers. Adv Mater. 2024;36:e2308547.

Article PubMed Google Scholar

Sun S, Liu J, Yue Q, Lv J, Wang S, Wei Y. Charge delocalization for electrically detachable poly(ionic liquids) ionoadhesives with ultrahigh mechanical robustness. Macromolecules. 2024;57:9355–66.

Article CAS Google Scholar

Yu L, Huang C, Gong Y, Zheng S, Zhou P, Zhang X, et al. Ultrastretchable and tough poly(ionic liquid) elastomer with strain-stiffening ability enabled by strong/weak ionic interactions. Macromolecules. 2024;57:2339–50.

Article CAS Google Scholar

Shao J, Dong X, Wang D. Stretchable self-healing plastic polyurethane with super-high modulus by local phase-lock strategy. Macromol Rapid Commun. 2023;44:e2200299.

Article PubMed Google Scholar

Zhang M, Yuan X, Wang L, Chung TCM, Huang T, deGroot W. Synthesis and characterization of well-controlled isotactic polypropylene ionomers containing ammonium ion groups. Macromolecules. 2014;47:571–81.

Article CAS Google Scholar

Chen Q, Huang C, Weiss RA, Colby RH. Viscoelasticity of reversible gelation for ionomers. Macromolecules. 2015;48:1221–30.

Article CAS Google Scholar

Huang C, Chen Q, Weiss RA. Rheological behavior of partially neutralized oligomeric sulfonated polystyrene ionomers. Macromolecules. 2017;50:424–31.

Article CAS Google Scholar

Stacy EW, Gainaru CP, Gobet M, Wojnarowska Z, Bocharova V, Greenbaum SG, et al. Fundamental limitations of ionic conductivity in polymerized ionic liquids. Macromolecules. 2018;51:8637–45.

Article CAS Google Scholar

Liu S, Cao X, Huang C, Weiss RA, Zhang Z, Chen Q. Brittle-to-ductile transition of sulfonated polystyrene ionomers. ACS Macro Lett. 2021;10:503–9.

Article CAS PubMed Google Scholar

Cao X, Peng L, Huang X, Chen Q. A trade-off between hardness and stretchability of associative networks during the sol-to-gel transition. J Rheol (N. Y N. Y). 2023;67:1119–28.

Article CAS Google Scholar

van Lange SGM, Te Brake DW, Portale G, Palanisamy A, Sprakel J, van der Gucht J. Moderated ionic bonding for water-free recyclable polyelectrolyte complex materials. Sci Adv. 2024;10:eadi3606.

Article PubMed PubMed Central Google Scholar

Ono T, Sugimoto T, Shinkai S, Sada K. Lipophilic polyelectrolyte gels as super-absorbent polymers for nonpolar organic solvents. Nat Mater. 2007;6:429–33.

Article CAS PubMed Google Scholar

Ono T, Sugimoto T, Shinkai S, Sada K. Molecular design of superabsorbent polymers for organic solvents by crosslinked lipophilic polyelectrolytes. Adv Funct Mater. 2008;18:3936–40.

Article CAS Google Scholar

Hara M, Iijima Y, Nagano S, Seki T. Simple linear ionic polysiloxane showing unexpected nanostructure and mechanical properties. Sci Rep.2021;11:17683.

Article CAS PubMed PubMed Central Google Scholar

Hara M, Kodama A, Washiyama S, Fujii Y, Nagano S, Seki T. Humidity-induced self-assembled nanostructures via ion aggregation in ionic linear polysiloxanes. Macromolecules. 2022;55:4313–9.

Article CAS Google Scholar

Sujita R, Imai S, Ouchi M, Aoki H, Terashima T. Microphase separation of cationic homopolymers bearing alkyl ammonium salts into sub-4 nm lamellar materials with water intercalation channels. Macromolecules. 2023. https://doi.org/10.1021/acs.macromol.3c01722.

Article Google Scholar

Sujita R, Aoki H, Takenaka M, Ouchi M, Terashima T. Universal access to water-compatible and nanostructured materials via the self-assembly of cationic alternating copolymers. ACS Macro Lett. 2024;13:747–53.

Article CAS PubMed Google Scholar

Tudryn GJ, Liu W, Wang S-W, Colby RH. Counterion dynamics in Polyester−Sulfonate ionomers with ionic liquid counterions. Macromolecules. 2011;44:3572–82.

Article CAS Google Scholar

Cavicchi KA. Synthesis and polymerization of substituted ammonium sulfonate monomers for advanced materials applications. ACS Appl Mater Interfaces. 2012;4:518–26.

Article CAS PubMed Google Scholar

Zander ZK, Wang F, Becker ML, Weiss RA. Ionomers for tunable softening of thermoplastic polyurethane. Macromolecules. 2016;49:926–34.

Article CAS Google Scholar

Enokida JS, Tanna VA, Winter HH, Coughlin EB. Progression of the morphology in random ionomers containing bulky ammonium counterions. Macromolecules. 2018;51:7377–85.

Article CAS Google Scholar

Enokida JS, Hu W, Fang H, Morgan BF, Beyer FL, Winter HH, et al. Modifying the structure and dynamics of ionomers through counterion sterics. Macromolecules. 2020;53:1767–76.

Article CAS Google Scholar

Wang W, Madsen J, Genina N, Hassager O, Skov AL, Huang Q. Toward a design for flowable and extensible ionomers: An example of diamine-neutralized entangled poly(styrene-co-4-vinylbenzoic acid) ionomer melts. Macromolecules. 2021;54:2306–15.

Article CAS Google Scholar

Inoue T, Matsumoto A, Nakamura K. Dynamic viscoelasticity and birefringence of poly(ionic liquids) in the vicinity of glass transition zone. Macromolecules. 2013;46:6104–9.

Article CAS Google Scholar

Matsumoto A, Inoue T. Detailed Analysis of sub-Rouse Mode Observed in Polymerized Ionic Liquids with dynamic birefringence measurements. Journal Soc Rheol, Jpn. 2014;42:227–33.

Article CAS Google Scholar

Iacob C, Matsumoto A, Brennan M, Liu H, Paddison SJ, Urakawa O, et al. Polymerized ionic liquids: Correlation of ionic conductivity with nanoscale morphology and counterion volume. ACS Macro Lett. 2017;6:941–6.

Article CAS PubMed Google Scholar

Matsumoto A, Iacob C, Noda T, Urakawa O, Runt J, Inoue T. Introducing large counteranions enhances the elastic modulus of imidazolium-based polymerized ionic liquids. Macromolecules. 2018;51:4129–42.

Article CAS Google Scholar

Aoki D, Yasuda K, Arimitsu K. Toughening ionic polymer using bulky alkylammonium counterions and comb architecture. ACS Macro Lett. 2023;12:462–7.

Article CAS PubMed PubMed Central Google Scholar

Cox BG. Acids, Bases, and Salts in Mixed-Aqueous Solvents. Org Process Res Dev. 2015;19:1800–8.

Article CAS Google Scholar

Busch M, Ahlberg E, Laasonen K. Universal Trends between Acid Dissociation Constants in Protic and Aprotic Solvents. Chem Eur J. 2022;28:e202201667.

Article CAS PubMed Google Scholar

Ding F, Smith JM, Wang H. First-Principles Calculation of PKa Values for Organic Acids in Nonaqueous Solution. J Org Chem. 2009;74:2679–91.

Article CAS PubMed Google Scholar

Yekymov E, Attia D, Levi-Kalisman Y, Bitton R, Yerushalmi-Rozen R. Effects of Non-Ionic Micelles on the Acid-Base Equilibria of a Weak Polyelectrolyte. Polymers. 2022;14:1926.

Article CAS PubMed PubMed Central Google Scholar

Dong H, Du H, Wickramasinghe SR, Qian X. The Effects of Chemical Substitution and Polymerization on the PKa Values of Sulfonic Acids. J Phys Chem B. 2009;113:14094–101.

Article CAS PubMed Google Scholar

Brilmayer R, Kübelbeck S, Khalil A, Brodrecht M, Kunz U, Kleebe H-J, et al. Influence of Nanoconfinement on the PKa of Polyelectrolyte Functionalized Silica Mesopores. Adv Mater Interfaces. 2020;7:1901914.

Article CAS Google Scholar

Pregi E, Kun D, Wacha A, Pukánszky B. The role of ionic clusters in the determination of the properties of partially neutralized ethylene-acrylic acid ionomers. Eur Polym J. 2021;142:110110.

Article CAS Google Scholar

Spencer MW, Wetzel MD, Troeltzsch C, Paul DR. Effects of acid neutralization on the properties of K+ and Na+poly(ethylene-co-methacrylic acid) ionomers. Polymer. 2012;53:569–80.

Article CAS Google Scholar

Guo X, Nakagawa S, Yoshie N. Tunable mechanical properties in microphase-separated thermoplastic elastomers via metal–ligand coordination. Macromolecules. 2024;57:2351–62.

Article CAS Google Scholar

Peng Y, Zhao L, Yang C, Yang Y, Song C, Wu Q, et al. Super tough and strong self-healing elastomers based on polyampholytes. J Mater Chem A Mater Energy Sustain. 2018;6:19066–74.

Article CAS Google Scholar

Bronstein LM, Huang X, Retrum J, Schmucker A, Pink M, Stein BD, et al. Influence of iron oleate complex structure on iron oxide nanoparticle formation. Chem Mater. 2007;19:3624–32.

Article CAS Google Scholar

Söderlind F, Pedersen H, Petoral RM Jr, Käll P-O, Uvdal K. Synthesis and characterisation of Gd2O3 nanocrystals functionalised by organic acids. J Colloid Interface Sci. 2005;288:140–8.

Article PubMed Google Scholar

Ibrahim M, Nada A, Kamal DE. Density functional theory and FTIR spectroscopic study of carboxyl group. Indian J Pure Appl Phys. 2005;43:911–7.

CAS Google Scholar

Max J-J, Chapados C. Infrared spectroscopy of aqueous carboxylic acids: Comparison between different acids and their salts. J Phys Chem A. 2004;108:3324–37.

Article CAS Google Scholar

Middleton LR, Trigg EB, Yan L, Winey KI. Deformation-induced morphology evolution of precise polyethylene ionomers. Polymer (Guildf). 2018;144:184–91.

Article CAS Google Scholar

Liu G, Larson RG, He X, Dan Y, Niu Y, Li G. Multiple relaxations and entanglement behaviors of polymerized ionic liquids with various anions/cations. Macromolecules. 2025;58:627–38.

Article CAS Google Scholar

Szántó L, Vogt R, Meier J, Auhl D, Van Ruymbeke E, Friedrich C. Entanglement relaxation time of polyethylene melts from high-frequency rheometry in the mega-hertz range. J Rheol. 2017;61:1023–33.

Ramos J, Vega JF, Theodorou DN, Martinez-Salazar J. Entanglement relaxation time in polyethylene: Simulation versus experimental data. Macromolecules. 2008;41:2959–62.

Article CAS Google Scholar

Gurp M, Palmen J. Time-temperature superposition for polymeric blends. 1998;67:5–8.

Trinkle S, Friedrich C. Van Gurp-Palmen-plot: a way to characterize polydispersity of linear polymers. Rheologica Acta. 2001;40:322–8.

Article CAS Google Scholar

Download references

This research was financially supported by the Iketani Foundation (grant number 0341149-A), JSPS KAKENHI (grant number JP23K13802, Early Career Scientists), JST ACT-X (grant number JPMJAX24D1), and the Tokyo University of Science.

Open Access funding provided by Tokyo University of Science.

Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science, Chiba, Japan

Daisuke Aoki, Kento Yasuda, Kotaro Uchiyama & Koji Arimitsu

You can also search for this author inPubMed Google Scholar

You can also search for this author inPubMed Google Scholar

You can also search for this author inPubMed Google Scholar

You can also search for this author inPubMed Google Scholar

Correspondence to Daisuke Aoki or Koji Arimitsu.

The authors declare no competing interests.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

Reprints and permissions

Aoki, D., Yasuda, K., Uchiyama, K. et al. Tertiary ammonium counterions outperform quaternary ammonium counterions in ionic comb polymers: overcoming the trade-off between toughness and the elastic modulus. Polym J (2025). https://doi.org/10.1038/s41428-025-01061-5

Download citation

Received: 06 March 2025

Revised: 24 April 2025

Accepted: 08 May 2025

Published: 03 June 2025

DOI: https://doi.org/10.1038/s41428-025-01061-5

Anyone you share the following link with will be able to read this content:

Sorry, a shareable link is not currently available for this article.

Provided by the Springer Nature SharedIt content-sharing initiative