2.1 Experimental Procedures

2.2 Kinetic Modeling

A kinetic model was developed for the sulfonation of the resins, taking into account the sulfonation and resin degradation reactions. Among the degradation effects involved in the sulfonation of styrene-based resins, polymer chain breakage into fragments can occur through various pathways, resulting in low molecular weight products. [¹⁵] Polymeric materials containing aromatic rings, such as copolymers derived from styrene, may undergo chain scission in acidic media, as illustrated by the mechanism presented in Fig. 1 [¹⁶].

Due to the large excess of sulfuric acid in the reaction system, its concentration was considered constant, leading to a pseudo-first-order assumption for the sulfonation reaction. Although the reaction order with respect to the polymer could be adjusted as a model parameter, doing so would increase model complexity and reduce robustness. In systems where the acid concentration is not constant, a second-order rate law involving both polymer and acid concentrations should be considered.

The total polymer mass in the reaction medium can be described by the following balance: (final mass of particles) = (mass of fed polymer) + (mass of attached sulfonic groups) − (mass of degraded material).

The mass of fed polymer and the final particle mass were measured gravimetrically, as detailed in Sect. 2.1. Accordingly, the model assumes that at the beginning of the reaction, m_ps =  0 and m_p = m_p0, where m_p0 is the mass of polymer initially fed, measured by weighing. At the end of the simulation, the sum m_p + m_ps corresponds to the final mass of the particles after sulfonation and drying, also determined by gravimetry.

3.1 Experimental Results

The analyses were conducted following the method outlined in the preceding section. The sulfonation reaction procedure was the same for both resins (DVB and TMPTA), but the reaction temperatures were different due to the presence of acrylate groups in TMPTA. These groups are susceptible to hydrolysis [¹⁷], which causes resin degradation at higher temperatures. In contrast, DVB exhibits less degradation under similar conditions. For this reason, different temperatures were selected for each resin. Based on literature, the temperature for the DVB resin was set at 90 °C [¹⁸], whereas for TMPTA, it was set at 57 °C [⁸] to reduce hydrolysis and degradation. Tab. 3 summarizes the results obtained under identical analytical conditions.

Based on the results presented, the resin from experiment D_ss1, synthesized with 6 % crosslinker (DVB) using batch copolymerization, exhibited the highest content of sulfonic groups, achieving an IEC of 3.18 mmol g⁻¹.

The IEC results for experiments D_ss4–D_ss6 averaged 2.79 mmol g⁻¹, whereas experiments D_ss7–D_ss9 showed an average of 3.08 mmol g⁻¹. In comparison, experiments D_ss1–D_ss3, synthesized with the lowest crosslinker content (6 %), had an average IEC of 2.96 mmol g⁻¹. Although these differences may suggest slight variations, they should be interpreted as a possible trend only.

Regarding the mass variation data, there appears to be an increasing trend with higher DVB content, with average mass increases of 1.80 (10 % DVB) and 2.13 (14 % DVB), compared to 0.33 for 6 % DVB. One possible interpretation is that the higher mass gains could relate to the incorporation of sulfonic groups of different types—such as SO₃H (81 g mol⁻¹) and SO₃SO₃H (161 g mol⁻¹)—as has been previously reported in the literature [¹⁹].

When compared to commercially available resins documented in the literature, the IEC of the synthesized resins was notably lower than the reported values for commercial resins (4.0–5.4 mmol g⁻¹), except for Amberlyst 70 (2.55 mmol g⁻¹). Moreover, it is recognized that the accessibility of a molecule to specific regions within the polymeric matrix depends on its size [¹¹]. As for the swelling index, no significant variation was observed based on the crosslinker percentage. These findings align closely with the swelling index of the commercial resin Amberlyst 36 (1.73) [¹⁵]. As observed in Tab. 3, despite the partial degradation of the resin during the sulfonation process, the mass variations were positive due to the accumulation of sulfonic groups, as previously noted.

At the conclusion of each synthesis step, which included copolymerization followed by the sulfonation reaction, measurements of mass variation (Δm), IEC, and swelling index (S_i) were recorded. Detailed results can be found in Tab. 4.

Based on the data in Tab. 4, the resins showed moderate mass variation ranging from 10 % to 20 %. However, this may suggest some level of degradation, especially in resins containing acrylate groups that are prone to hydrolysis under sulfonation conditions [^{15, 20}]. The hypothesis that hydrolysis may have occurred due to water present from the sulfuric acid equilibrium [²¹] remains a plausible explanation, although it cannot be confirmed based on the current data.

The IECs observed ranged from 0.36 to 0.64 mmol g⁻¹, notably lower than those typically reported for commercial DVB-based resins, such as Amberlyst 36 (IEC = 5.45 mmol g⁻¹) [²²]. Although degradation during sulfonation is one possible contributing factor to the lower IEC values, no consistent effect of the factors studied could be identified. It is also worth noting that the limited number of experiments may restrict the ability to fully assess potential influences on the results.

To address the challenge of negative mass variation, a new set of experiments was proposed and conducted. The temperature of 30 °C was selected for these additional experiments with the aim of preventing resin degradation, while also examining the impact of sulfonation time on the IEC, in an effort to increase it without compromising the integrity of the resins. The conditions and subsequent results of this new approach are detailed in Tab. 5.

The results in Tab. 5 demonstrate an apparent increase in IEC values compared to those in Tab. 4. This improvement is more pronounced in experiments with shorter sulfonation times (60 min), which also show reduced mass loss in most cases. However, these observations are based on visual inspection of the data. Experiment T_ss9, for example, presented a high IEC along with considerable mass variation, further illustrating the variability in the results. Although the resin swelling procedure carried out prior to sulfonation may have allowed greater acid diffusion and more extensive sulfonation in certain cases, a causal relationship cannot be confirmed based on the available data. The suggestion that improved acid accessibility may have contributed to the observed increase in IEC is a working hypothesis consistent with findings reported in the literature [²³].

A broader look at the literature (Tab. 6) shows that a wide range of IEC values can be achieved depending on the resin structure and synthesis conditions. Although DVB-based systems often show higher IECs, various formulations have been used effectively depending on the application.

Among the resins presented in Tab. 6, it is noticeable that those crosslinked with DVB generally demonstrate higher IECs compared to the other types of resins. In this study specifically, the mean IEC for a 14 % crosslinker concentration was 3.08 mmol g⁻¹. In a study by Torquato et al. [²⁶], comparisons among sulfonic DVB resins indicated that those derived from hypercrosslinked resins demonstrated even greater IECs, reaching 5.06 mmol g⁻¹. This study similarly shows that resins with higher crosslinker concentrations had higher average IEC values.

Literature on swelling indices underscores the influence of different crosslinkers and their concentrations on resin swelling behavior, depending on the solvent used. For instance, Rana et al. [²⁸] conducted swelling experiments on 11 distinct DVB crosslinked polystyrene resins across a wide array of solvents. Their data demonstrated that less crosslinked materials exhibited higher degrees of swelling. For example, when using CHCl₃ as a solvent, a resin with a 3 % DVB exhibited up to nine times greater swelling compared to a resin with a 6 % DVB. This trend suggests that reduced crosslinking results in increased swelling compared to highly crosslinked resins.

3.2 Kinetic Modeling Results

The variables percentage of mass variation (Δm_%) and IEC were predicted by the model through Eqs. (8) and (9).

It is noteworthy that Y_DVB and T_DVB are variables related to the copolymerization step, which defines the structure of the polymer prior to sulfonation. Although these variables are not explicitly present in Eqs. (8) and (9), which calculate IEC and Δm_%, they influence the model outcomes by modifying the polymer characteristics. In particular, the degree of crosslinking depends on Y_DVB and T_DVB, and this, in turn, affects the values of the kinetic parameters k_d and k_s. As a result, the effects of Y_DVB and T_DVB on IEC and Δm_% are indirectly captured through their influence on k_d and k_s. The same reasoning applies to the crosslinker TMPTA, which also affects the polymer structure and, consequently, the model parameters.

The predictions of IEC and Δm_% were compared to the experimental data, first, considering M_s =  81 g mol⁻¹, which provided the fittings depicted in Fig. 2a,b. Kucera [¹⁹] states that the sulfonation of aromatic rings can yield different groups attached to them, such as SO₃H (81 g mol⁻¹) and SO₃SO₃H (161 g mol⁻¹). It can be observed that Fig. 2b shows a reasonable fitting for IEC. In spite of being related to the IEC, most predicted values of Δm_% are below the respective experimental values for M_s =  81 g mol⁻¹, as illustrated in Fig. 2a. It can be noted that, a way of increasing Δm_% with constant IEC is by considering a higher molar mass of the attached groups. In this sense, the authors repeated the simulation with M_s =  161 g mol⁻¹, found by Kucera [¹⁹]. The results are illustrated in Fig. 2c,d.

As discussed in the literature, sulfonation may lead to the incorporation of both ─SO₃H and ─SO₃–SO₃H groups into the polymer structure [¹⁹]. In the present work, the model was used to evaluate the impact of two limiting values of molar mass: 81 g mol⁻¹, corresponding to one ─SO₃H group per site, and 161 g mol⁻¹, associated with the ─SO₃–SO₃H structure. The best agreement between model and experimental data was obtained using 161 g mol⁻¹, suggesting a predominance of ─SO₃–SO₃H units for the conditions studied herein, in-line with the mechanism proposed by Kucera and Jancar [¹⁹]. Future investigations may focus on intermediate average values or alternative reaction pathways, including the formation of sulfone-type crosslinks (─SO₂─), as discussed by Yu [²⁹].

The modeling results of sulfonation of styrene-TMPTA resins are illustrated in Fig. 3.

The fittings shown in Fig. 3 indicate a similar behavior than that observed in Fig. 2 for styrene-DVB resins. It suggests that, for styrene-TMPTA resins, the sulfonated rings may predominantly feature SO₃–SO₃H groups attached to them. The modeling of the sulfonation of styrene-co-TMPTA resins swollen in 1,2-dichloroethane also provided better fittings for M_s =  161 g mol⁻¹ (R² = 0.9941 for IEC and R² ≈ 1 for Δm). The fitted parameters for all the simulations considering M_s =  161 g mol⁻¹ are related in Tab. 7.

It can be observed in Tab. 7 that the sulfonation of STY/DVB at 90 °C and STY/TMPTA at 57 °C without DCE yields quite similar k_s values, despite the difference in temperatures. This result can be attributed to the natural swelling of STY/TMPTA in the reaction medium, which is more pronounced in comparison to STY/DVB resins. When STY/TMPTA is previously swollen in DCE, the sulfonation reactivities increase considerably, even conducting the reaction at 30 °C, as can be observed in Tab. 7.

In terms of degradation, it can be observed that styrene-DVB resins are way more resistant chemically in comparison to styrene-TMPTA resins. Even when sulfonation is carried out at 90 °C, the average k_d value is considerably lower for STY/DVB than for STY/TMPTA processed at lower temperatures. These findings validate the experimental observations and suggest that TMPTA resins possess the benefit of enhanced swelling (improved accessibility). However, they also demonstrate a drawback, namely, a tendency toward easier degradation, likely attributed to the consumption of their acrylate groups during the sulfonation process.

As per literature, the stability of a resin is inversely related to its swelling properties. Consequently, TMPTA, known for its favorable swelling properties, tends to be more susceptible to degradation [³⁰]. Similarly, Theodoro [⁸] observed that larger chains, such as sulfonated EGDMA and TEGDMA, exhibit increased vulnerability to degradation. Furthermore, degradation might also occur due to oxygen presence during copolymerization, resulting in the formation of peroxide bridges that weaken structural integrity [³⁰].