Toughening Ionic Polymer Using Bulky Alkylammonium Counterions and Comb Architecture

Ionic interactions in ionic polymers, such as ionomers, polyelectrolytes, and polyampholytes, contribute to toughness in systems with high mobility and active ion dynamics, such as hydrogels and elastomers. However, it remains challenging to toughen rigid polymers through ionic interactions without lowering their elastic modulus through plasticization. Here, we present a strategy for toughening without sacrificing the elastic modulus by combining a comb polymer with bulky ammonium counterions. We designed and synthesized ionic comb polymers with oligoethylene glycol side chains and carboxylic acids in each monomer unit of the polynorbornene backbone, neutralized by trialkylamines, ranging from ethyl to octyl. The counterion size in ionic comb polymers influenced the mechanical properties of tensile testing—not the elongation at break and the elastic modulus but the ultimate strength and toughness. The ionic comb polymer containing heptylammonium counterions displayed the highest toughness of 77 MJ m–3. Tensile studies at various strain rates demonstrated a rate-dependent difference between heptyl- and octylammonium counterions. This result suggests that the heptylammonium counterion acted as a sacrificial bond by providing a moderate dissociation rate that was slightly slower than that of the octylammonium counterion, leading to toughening.

* sı Supporting Information ABSTRACT: Ionic interactions in ionic polymers, such as ionomers, polyelectrolytes, and polyampholytes, contribute to toughness in systems with high mobility and active ion dynamics, such as hydrogels and elastomers. However, it remains challenging to toughen rigid polymers through ionic interactions without lowering their elastic modulus through plasticization. Here, we present a strategy for toughening without sacrificing the elastic modulus by combining a comb polymer with bulky ammonium counterions. We designed and synthesized ionic comb polymers with oligoethylene glycol side chains and carboxylic acids in each monomer unit of the polynorbornene backbone, neutralized by trialkylamines, ranging from ethyl to octyl. The counterion size in ionic comb polymers influenced the mechanical properties of tensile testing�not the elongation at break and the elastic modulus but the ultimate strength and toughness. The ionic comb polymer containing heptylammonium counterions displayed the highest toughness of 77 MJ m −3 . Tensile studies at various strain rates demonstrated a rate-dependent difference between heptyl-and octylammonium counterions. This result suggests that the heptylammonium counterion acted as a sacrificial bond by providing a moderate dissociation rate that was slightly slower than that of the octylammonium counterion, leading to toughening. C ross-linked polymers have been widely used in industry due to their excellent mechanical properties and chemical stability. However, it remains challenging to achieve both high modulus and hardness. Sacrificial bonds, reported by Gong et al., emerged as an excellent concept for the design of toughening cross-linked hydrogels because they can dissipate fracture energy by preferentially breaking as weak bonds. 1,2 This idea has been extended in various weak bonds, such as ionic interactions, 3 hydrogen bonding, 4,5 metal coordination bonds, 6 and dynamic covalent bonds, 7 and is applicable to dry networks as well as to hydrogels. Among them, ionic interactions differentiate from other dissociative interactions in that the bulk properties of polymers are governed by the dynamics of the ionic network. 8 These dynamics are dependent on a variety of parameters including the quantity, nature, strength, and distance of ionic motifs as well as the chemical structure of the polymer backbone. 9−11 Notably, ionic soft polymers with low glass transition temperatures (T g ) allow for active ion dynamics, resulting in extraordinarily tough soft materials. 12,13 However, due to the limited mobility of polymers in rigid polymer networks with a high T g , it is difficult to develop a universal strategy for toughening using ionic interactions without a decrease in elastic modulus.
To address the invalidation of ionic interactions in rigid polymer systems, we present an ionic toughening strategy that facilitates ion exchange by combining bulky alkylammonium counterions with comb polymers composed of excess carboxylic acids and polar soft side chains. Because their alkyl chains partially mask electrostatic interactions, bulky alkylammonium counterions behave like plasticizers, preventing the formation of typical ionic aggregate structures 14 and thereby lowering T g , reducing melt viscosity, and softening mechanical properties. 15,16 Moreover, excess carboxylic acids and polar soft side chains also assist ion exchange by accelerating the rate of ion hopping 17,18 and preventing excessive ionic aggregation. 16 In this study, we synthesized comb polymers with carboxylic acid consisting of a polynorbornene backbone and oligo ethylene glycol (OEG) side chains and prepared ionic comb polymers by combining trialkylamines as counterions with different alkyl chain lengths. Surprisingly, trialkylammonium counterions with appropriate bulkiness toughened ionic comb polymers without compromising the elastic modulus or elongation at break.
To study the mechanical properties of ionic polymers with ionic groups and side chains, we first designed and synthesized ionic polynorbornene with OEG side chains and carboxylic acids in each monomer unit (Scheme 1). Due to the ease of ring-opening metathesis polymerization by Grubbs catalyst and the inertness of olefins to various chemical groups, the polynorbornene backbone has been the preferred choice for many reported graft polymers. For the OEG side length, we chose a methyl-terminated trimeric OEG (OEG 3 ) to provide typical plastic-like mechanical behavior to comb polymers containing carboxylic acids prior to neutralization ( Figure  S10). We synthesized ionic comb polymers by grafting OEG 3 onto polynorbornene-containing dicarboxylic anhydride to modify each monomer with OEG 3 side chains and carboxylic acids in equal quantities. Although norbornene dicarboxylic anhydride (NBC) is a low-cost and rational starting material for our synthetic strategy, there are few reports for homopolymerization due to the low reactivity, the poor solubility, and an extremely high glass transition temperature of its polymer. 19,20 We successfully produced poly NBC (pNBC) by reacting a high concentration of NBC with a thirdgeneration Grubbs catalyst at room temperature in DMF. 1 H NMR spectra demonstrated the polymerization of NBC by chemical shifts from ring-closed olefins of around 6.3 ppm to ring-opened olefins of around 5.3 and 5.5 ppm, as well as a broadening of the overall peak (Figures 1a and b). The subsequent esterification reaction of pNBC with mono alcohol OEG 3 catalyzed by DMAP was almost 100% efficient, as evidenced by the formation of a new methylene peak adjacent to the ester at around 4.0 ppm in the 1 H NMR spectrum ( Figure 1c). The progress of the pNBC esterification reaction is also confirmed by the presence of COOH in the FTIR spectrum and the shift of the C�O stretching vibration peak to lower wavenumbers ( Figure S2). In addition, the resultant comb polymer containing carboxylic acid (pNBC-g) had an average molecular weight of 1.22 × 10 5 Da estimated by the polystyrene equivalent with THF SEC ( Figure S1). Finally, the ionic comb polymers were produced by neutralizing a pNBC-g solution with trialkylamines ranging in size from Et 3 N to Oc 3 N. Figure 1d illustrates a typical 1 H NMR spectrum for pNBC-g-Et 3 N. Even after purification of the pNBC-g-Et 3 N, the presence of neutralized trialkylamine peaks provides evidence for the neutralization of carboxylic acids by Et 3 N, which is further confirmed by the presence of a carboxylate peak at 1564 cm −1 in the FTIR spectrum ( Figure S2). All pNBC-gbase samples were estimated to be 20−30% neutralized by H NMR and were kept in desiccators to minimize moisture absorption. Moreover, the T g of pNBC-g was determined to be 87°C using temperature-dependent rheological measurements, although differential scanning calorimetry was unable to identify it ( Figure S18). In contrast, the T g of the pNBC-gbase declined to 68°C with increasing counterion size, indicating plasticization, similar to previous studies involving alkylammonium counterions.

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Next, we move to investigate the tensile mechanical properties of the pNBC-g base samples neutralized by varying ammonium counterion sizes. Figure 2a depicts the stress− strain curves of the pNBC-g base films with trialkylammonium of various alkyl chain lengths from ethyl to octyl. Interestingly, even though larger ammonium counterions generally plasticize the polymer, 21 the elongation at break was almost the same independent of counterion sizes, with only the stress differing. Figure 2b summarizes the ultimate strength (σ b ) and elongation at break (ε b ) as a function of trialkylammonium counterion sizes. Depending on the counterion size, the neutralization of carboxylic-acid-containing comb polymers by alkylamines decreased, remained constant, or increased the σ b . For pNBC-g-Oc 3 N, low stress level deformation appears with a small yielding stress (σ y ) of about 11−13 MPa and eventually breaks at about 10−15 MPa. pNBC-g base with moderate counterion sizes in the range of Et 3 N to Hex 3 N had almost the same σ y and σ b of 15−25 MPa as comb norbornene without ion neutralization. Remarkably, pNBC-g-Hep 3 N deformed while maintaining a high level of stress, demonstrating σ y and σ b of 25−30 MPa. This mechanical strengthening effect of the heptylammonium counterion while maintaining its ductile properties also resulted in an increase in toughness. Figure 2c shows the toughness (U t ) calculated from the area of the stress−strain curve as a function of counterion sizes. Toughness follows a nonlinear trend, with pNBC-g-Hep 3 N exhibiting the highest U t (77 MJ m −3 ), comparable with commercial polycarbonate and previously reported toughened  (Figure 2c). Incidentally, the neutralization ratio had a significant effect on the mechanical properties; when the Hep 3 N fraction exceeded 40%, the toughness decreased dramatically ( Figure S14). In addition, the elastic modulus of a series of pNBC-g bases was comparable with that of carboxylic comb norbornene, except for pNBC-g-Oc 3 N (Figure 2d). Thus, the appropriate ion size enables the toughening of ionic comb polymers by increasing only the stress level, without reducing the elastic modulus and ductility. Moreover, to understand the contribution of OEG side chains to toughening, we synthesized a model polymer without side chains, called Me, in which the OEG3 side chain was substituted by a methyl ester. Figure 2e shows the stress− strain curves of pNBC-g, pNBC-g-Hep 3 N, polynorbornene (pNB), and Me. pNBC-g-Hep 3 N combines high σ b and ε b in comparison to pNB, pNBC-g, and Me, improving the U t of pNB without side chains and counterions by about 192 times. The toughened pNBC-g-Hep 3 N displayed transparent deformation up to 200% elongation beyond yielding in the plateau region of the stress−strain curve, followed by whitening, indicating the production of crazes during strain hardening (Figure 2f). Additionally, using Me-Hex 3 N (Me neutralized with Hex 3 N), we also examined whether the combination of a bulky counterion and a side chain is essential for toughening. Figure 2g represents the stress−strain curves of Me, Me-Hex 3 N, pNBC-g, and pNBC-g-Hex 3 N. Despite the similar tensile behavior of pNBC-g and Me in the absence of counterions, the presence of counterions had opposing effects on mechanical properties. Despite possessing counterions with long alkyl chains, Me-Hex 3 N exhibited brittle mechanical behavior, whereas pNBC-g-Hex 3 N exhibited tensile behavior similar to pNBC-g. The U t calculated from these stress−strain curves is compared in Figure 2h. Despite pNBC-g-Hex 3 N displaying a similar U t value to pNBC-g, the neutralization of Hex 3 N to Me without side chains decreased U t from 37 to 1 MJ m −3 . These results suggest that the presence of side chains promotes the dissociation of bulky alkylammonium from the rigid norbornene backbone, allowing for the formation of sacrificial bonds.
To comprehend the highest strength and toughness of the heptyl counterion, we studied ion exchange rates as estimated by tensile testing at different crosshead speeds and rheological studies. According to Leibler et al., the mechanical behavior of dynamically cross-linked polymers is strain rate dependent because dynamic cross-links behave as cross-linking points on time scales shorter than their lifetime and become viscoelastic on longer time scales. 22 Figure 3a displays plots of normalized yielding strength based on 1 mm/min as a function of strain rate for pNBC-g base with various counterions. A higher normalized yield strength indicates a larger increase in yield strength when deformed at a faster rate than 1 mm/min. Based on Leibler's theory, 22 the higher the normalized yield strength, the faster the exchange rate of reversible bonds, which in this study means a faster ion exchange rate. Comparing triethylamine, trihexylamine, and heptylamine, the strain rate dependency of the normalized yield strength showed little

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Letter difference within the measured range except for the initial slope magnitude. In contrast, from heptyl to octyl, the normalized yield strength became significantly more sensitive to strain rate and increased dramatically with exponential decay. This tendency indicates that the ion exchange rate of octyl is significantly more active than that of other alkylamines shorter than heptyl, which is supported by the softer stress− strain behavior of pNBC-g-Oc 3 N (Figure 2a). Interestingly, our pNBC-g-Hep 3 N, which has one less carbon atom than this softening pNBC-g-Oc 3 N, offers the best strength and toughness without compromising elastic modulus and elongation at break. Note that the effective cross-link density in relation to the mechanical properties should be nearly identical from propyl to heptyl, as the swelling ratio was almost the same ( Figure S15). Even though the cross-link densities were almost the same, only pNBC-g-Hep 3 N grew tougher. This is most likely due to the cross-linking points dissociating at the appropriate rate, allowing them to behave as sacrificial bonds that break preferentially during deformation. Moreover, we evaluated the variation in ion exchange rate with counterion size by analyzing the frequency-dependent dynamics in rheological measurements. Miwa et al. explained the softening of mechanical behavior at high ion exchange rates estimated by relaxation times of network rearrangement using polymers, whose ion exchange rate varies depending on a gas atmosphere. 23 Figure 3b exhibits a representative master curve of the storage and loss modulus for pNBC-g-Hex 3 N at T r = 60°C . Time−temperature superposition (tTs) was successfully applied to all samples, including pNBC-g-Et 3 N, pNBC-g-Hex 3 N, pNBC-g-Hep 3 N, and pNBC-g-Oc 3 N, within the temperature range of 60−120°C ( Figure S17). The reversible gelation model by Chen et al. 24 in the terminal flow with G′ ∝ ω 2 and G′′ ∝ ω can be used to determine the ionic dissociation time in the rheological studies. Unfortunately, even at high temperatures up to 200°C, the pNBC-g base samples remained in the rubber region (G′ > G″), presumably due to the long termination relaxation times caused by the high concentration of COOH content in each pNBC monomer. Notably, the regions where G′ and G″ values were almost the same indicate the presence of an equilibrium of trapped strands, which is a characteristic of supramolecular polymers with typical sticky side groups. 25 Because the pNBC-g base samples behave as supramolecular polymers, we attempted to calculate the apparent activation energy (E a app ) of reversible gels with strong association using the Rubinstein−Semenov theory 26 in order to assess the ion exchange rate. The E a app was extracted by Arrhenius plots of the shift factor at 60−120°C when the tTs is valid (Figure 3c). 27 The resultant E a app was nearly the same for Et 3 N and Hex 3 N, at 86 kJ/mol, but dramatically decreased for Hep 3 N and Oc 3 N, at 78 and 65 kJ/ mol, respectively (Figure 3d). The consistent E a app values for Et 3 N and Hex 3 N support the nearly identical stress−strain curve trends of pNBC-g-Et 3 N and pNBC-g-Hex 3 N shown in Figure 2a. Because larger counterions have faster molecular dynamics, 14 there probably exists a threshold between counterion sizes of Hex 3 N and Hep 3 N like a critical chain length effect, at which counterion exchange becomes active. Oc 3 N exhibited significantly lower E a app than Hep 3 N, indicating a faster dissociation rate compared to the other shorter counterion. The active exchange of counterions in pNBC-g-Oc 3 N is primarily responsible for its softness, leading to low stress levels and a strong dependence on strain rate in tensile testing. The significant contrast in mechanical proper-ties between Hep 3 N and Oc 3 N counterions necessitates an appropriate counterion exchange rate for effective toughening through ionic interactions in a stiff polymer with a high T g (Figure 3e). Such an exchange rate can facilitate sacrificial bonding in the material, dissipating fracture energy and thereby enhancing toughness. Therefore, even rigid polynorbornene backbones can be efficiently toughened by combining bulky counterions and comb architecture to enhance ion dissociation rates.
In conclusion, combining bulky alkylammonium counterions with a comb architecture has enabled the successful toughening of a rigid polynorbornene derivative while maintaining their high elastic modulus, based on the notion that high counterion mobility promotes toughness even in high T g polymers. We designed and synthesized pNBC-g base, an ionic comb polymer incorporating alkylammonium counterions with alkyl chain lengths ranging from ethyl to octyl. The neutralization ratios of these synthesized pNBC-g bases were determined to be 20−30% using 1 H NMR. In the stress−strain curves for the pNBC-g bases, the elongation at break and elastic modulus were nearly constant regardless of ion size; however, the overall stress level and toughness changed with ion size. Remarkably, pNBC-g-Hep 3 N displayed a toughness of 77 MJ m −3 , which is 1.7 times that of pNBC-g and 192 times that of a typical norbornene pNB. To explain this toughening, the rate dependence of stress associated with the dissociation rate of dynamic bonds was studied. Octylammonium counterions displayed a greater dependence on rate than other counterions, indicating a fast dissociation. The heptylamine, which is only one carbon shorter than octylamine, was toughened because it works as a sacrificial bond because its dissociation rate was not extremely rapid like octyl's, but rather appropriate. This research has paved the way for toughening even rigid polymers without compromising their elastic modulus by promoting ionic interaction dissociation rates. If the critical factors for toughness can be quantified in conjunction with rheology and X-ray spectroscopy, this work will contribute to a more universal strategy for toughness for a variety of rigid polymer network materials, not just bulky counterion and comb architectural combinations. ■ ASSOCIATED CONTENT https://pubs.acs.org/10.1021/acsmacrolett.2c00737

Notes
The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS
This work was supported by the Iketani Foundation.