Rheological Behavior of Partially Neutralized Oligomeric Sulfonated

Dec 27, 2016 - Dynamics of associative polymers. Zhijie Zhang , Quan Chen , Ralph H. Colby. Soft Matter 2018 14 (16), 2961-2977 ...
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Rheological Behavior of Partially Neutralized Oligomeric Sulfonated Polystyrene Ionomers Chongwen Huang,† Quan Chen,‡ and R. A. Weiss*,† †

Department of Polymer Engineering, University of Akron, Akron, Ohio 44325, United States State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China

‡

S Supporting Information *

ABSTRACT: The linear viscoelastic (LVE) behavior of partially neutralized oligomeric sulfonated polystyrene (SPS) ionomers with different degrees of sulfonation (p) and degrees of neutralization (x) was investigated. The ionic dissociation time, τs, obtained from the reversible gelation model [Chen et al. Macromolecules 2015, 48, 1221−1230] is mainly controlled by the neutralization degree, x, rather than the functional group (i.e., sulfonic acid and metal sulfonate) concentration, p. For a fixed p, increasing x significantly increases τs and the zero shear viscosity, η0, especially near complete neutralization. These results explain the observations reported by Lundberg et al. [Ions in Polymers; American Chemical Society: 1980; Vol. 187, pp 67−76] that the increase of the viscosity of SPS ionomers with neutralization undergoes a substantial increase between 90% and 100% neutralization of the sulfonic acid groups to metal salts. This rapid increase of τs and η0 is probably related to the decrease of sulfonic acid groups in the ionic aggregates with increasing x.

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INTRODUCTION Polymers containing small amounts, typically less than 15 mol %, of covalently attached ionic groups are often called ionomers. Those polar ionic groups tend to form nanometersized aggregates in a low dielectric constant medium because of their immiscibility with the nonpolar polymer phase and the strong dipolar or ionic interactions between the ionic species. The physical and mechanical properties of ionomers are significantly modified by the presence of ionic aggregates that behave as physical cross-links. Ionomers have applications in a wide variety of technologies, including polymer blend compatibilization, packaging films, molded thermoplastics, thermoplastic elastomers, organic viscosifiers, drilling fluids, adhesives, coatings, shape memory polymers, and self-healing materials.1−4 The dynamics of ionomers is strongly related to the dissociation of ion pairs from the ionic aggregates. The effects of the ionic content and cation type on the linear and nonlinear rheological behavior of ionomers were reviewed by Register and Prud’homme.5 However, studies of the effect of the extent of neutralization (i.e., conversion of the precursor acid groups to ionic species) on the dynamics of ionomers are limited.6−14 For instance, Bonotto et al.9 found that the mechanical properties of poly(ethylene-co-acrylic acid) ionomers (PEAA) depended mainly on the degree of neutralization rather than the type of metal cation. Increasing the degree of neutralization increased the modulus and decreased the melt index of all the metal salts of PEAA. Register et al.10−12 found that the terminal relaxation time and zero-shear viscosity of poly(ethylene-co© XXXX American Chemical Society

methacrylic acid) (PEMA) ionomers increased with the increasing neutralization degree for ionomers neutralized by monovalent and divalent metal cations. The unneutralized acid groups (free acid groups) exhibited a plasticization effect on the ionic interactions, significantly facilitating the ion-hopping process responsible for melt flow, especially for ionomers with low levels of neutralizations.12 As with PEMA ionomers, the melt viscosity of other ionomers, such as sulfonated polystyrene (SPS),8 carboxylated poly(methyl methacrylate),14 and poly(styrene-co-methacrylate)6,7 ionomers, increased with the degree of neutralization. One particularly interesting result by Lundberg et al.8 was that the melt viscosity of SPS ionomers increased monotonically with the degree of neutralization, but the increase became significantly faster above 90% neutralization. The reason for this transition of the melt viscosity has never been explained. For each of the studies discussed above, the ionomer chains were highly entangled, and one complication with studying entangled ionomers is the difficulty in separating the effects of entanglements, especially trapped entanglements, and the supramolecular ionic associations on the dynamics. That problem may be resolved by studying oligomeric ionomers with no chain entanglements. We have recently reported the effects of the degree of sulfonation, molecular weight, the choice of the metal cation, and blending two ionomers together Received: November 8, 2016 Revised: December 10, 2016

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Materials Characterization. Small-amplitude oscillatory shear measurements were conducted with a TA Instruments ARES-G2 rheometer. Dynamic frequency sweeps were run over a temperature range of 100−250 °C and a frequency range of 0.16−250 rad/s with either 25 or 8 mm parallel plates. All dynamic measurements were performed within the linear viscoelastic regime, as determined from the strain sweeps. Time−temperature superposition (TTS) master curves for all samples were constructed at the same reference temperature Tr = 140 °C. The difference in the glass transition temperature for the three SPS samples varied by only 4 °C, so using a common reference temperature for each introduces only a small error.

on the shear and extensional rheology of randomly sulfonated, oligomeric polystyrene ionomers (SPS).15−21 The gel point, pc, of SPS is reached when the sulfonation level, p, is on average one ionic group per chain for neat ionomers and ionomer blends.16,20 Below the gel point, p < pc, the ionomers exhibit a longer terminal relaxation time than the PS precursor due to increased chain friction, but no plateau occurs in the storage modulus.20,21 Close to the gel point, p ∼ pc, two power law regions corresponding to mean-field and critical percolations emerge.16,20,21 Above the gel point, p > pc, a clear plateau in the storage modulus occurs and the plateau modulus is independent of the cation type, but it increases with increasing p.16−18,20,21 A reversible gelation model16 adequately predicts the linear viscoelastic (LVE) behavior of neat ionomers16,21 and ionomer blends20 using only two parameters: the Rouse relaxation time of a Kuhn segment, τ0, and an ionic dissociation time, τs. The objective of this work described herein was to understand the effect of the extent of neutralization on the LVE behavior of SPS ionomers and blends of SPS ionomers. Three oligomeric SPS ionomers were studied, each prepared from the same PS precursor (Mw = 13.5K g/mol) and each with a sulfonation level above the gel point, i.e., p > pc.

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RESULTS AND DISCUSSION Linear Viscoelastic Behavior. The concentration (mol %) of the ionic groups at the gel point for a random ionomer is pc = 1/(N − 1) × 100%, where N is the degree of polymerization,16 so for SPS13.5 pc = 0.78 mol % sulfonate groups. Above the gel point (p > pc), a percolated ionic network (gel) forms and the fraction of gel increases with p until the system is completely gelled at p = 2pc.16 For the three SPS13.5 ionomers used in this investigation, the ionomers with p = 1.2 and 1.5 mol % are mixtures of sol and gel (pc < p < 2pc) and the third ionomer with p = 2.7 mol % is a complete gel (p > 2pc). Figure 1 shows the master curves of storage modulus, G′, and loss modulus G″, at Tr = 140 °C as a function of the degree of

EXPERIMENTAL DETAILS

Materials. Oligomeric polystyrene (PS13.5), Mw = 13.5K g/mol, PDI < 1.06, was purchased from Pressure Chemical Co. (Pittsburgh, PA) and sulfonated in a 1,2-dichloroethane (DCE) solution using acetyl sulfate.22 Acetyl sulfate was prepared by adding concentrated sulfuric acid to a 60 mol % excess of acetic anhydride in DCE at 0 °C, and it was used almost immediately for the sulfonation reaction. The acetyl sulfate was added to an ∼10 wt % PS/DCE solution (∼200 mL) over a period of ∼1 min, at ∼50 °C with constant stirring. After 1 h the reaction was terminated by adding ∼2 mL of 2-propanol. Sulfonation to an aromatic ring is an electrophilic substitution reaction and the sulfonic acid product adds randomly, primarily at the para position of the styrene ring.23 The SPS was precipitated in boiling deionized (DI) water, washed three times with DI water, dried in air at 70 °C for 1 day, and finally dried at 120 °C in a vacuum oven for 1 week. The sulfonation level was determined by sulfur elemental analysis by Robertson Microlit Analysis (Madison, NJ). Three different SPS samples were prepared, with p = 1.2, 1.5, and 2.7 mol %, which correspond to an average of 1.6, 1.9, and 3.5 ionic groups per chain, respectively. The fractions of chains that have a specific number of ionic groups for each SPS is represented by a binomial distribution.24 Partially or fully neutralized SPS was prepared by adding a predetermined amount of a methanol solution of sodium hydroxide to an ∼15 wt % solution of SPS (acid derivative) in a 90/10 (v/v) mixture of toluene and methanol. After 30 min, the neutralized SPS was collected following the procedures as described above for SPS. For the completely neutralized SPS ionomers, a 50 mol % excess of sodium hydroxide was used to ensure complete neutralization. The percentage of acid groups that were neutralized, i.e., the degree of neutralization, was calculated from the sodium concentration, which was measured by sodium element analysis at Robertson Microlit Analysis (Madison, NJ). Binary blends used in this study were prepared by blending predetermined amounts of a fully neutralized SPS and an SPS acid derivative (i.e., no neutralization) in a mixed solvent of 90/10 (v/v) toluene and methanol. The blend samples were isolated using the same procedures for the neat ionomers. The sample nomenclature used hereafter is Nap-x(b), where Na denotes the cation, p represents the degree of sulfonation (mol %), x is degree of neutralization (%), and b denotes a blend. No b means the sample was a single ionomer. Fully neutralized SPS ionomer with p = 0.76 mol % (Na0.76-100) was obtained from previous studies of the rheology of SPS ionomers.16,20

Figure 1. Frequency dependence of storage and loss moduli, G′ and G″, master curves at Tr = 140 °C for Na1.2-x ionomers. The solid curves are the model predictions.

neutralization for the Na1.2-x ionomers (pc < p < 2pc). When x ≤ 76%, TTS worked well and no plateau in the storage modulus was observed, even though the Na−sulfonate concentration at 76% neutralization was pNa = 0.91 mol %, which was greater than pc. That result indicates that the presence of the extra unneutralized acid groups weaken the ionic associations and delay the appearance of the modulus plateau, which is similar to what Tierney and Register reported for PEMA ionomers.12 For the Na1.2-x ionomers with x ≤ B

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Macromolecules 76%, G′ and G″ exhibited Rouse-like behavior, where G′(ω) ∼ G″(ω) ∼ ω1/2 prior to the terminal relaxation at lower frequencies. For those ionomers increasing the degree of neutralization (between 0 and 76%) delayed the terminal relaxation and expanded the Rouse region to lower frequencies due to the stronger intermolecular associations of Na−sulfonate groups than the acid groups. When x ≥ 89%, a plateau in G′ became apparent (Figure 1), and the plateau became more pronounced with increasing x, though the plateau modulus, GN, was independent of the neutralization level. This result is consistent with previous results16−18,20 that GN for SPS was not sensitive to the energy of the interchain interactions. This finding also indicates that the hydrogen bonding of the sulfonic acid groups contributes to the network responsible for GN. However, no plateau in G′ was observed for the neat free-acid derivative and Na1.2-x with x ≤ 76%. The absence of the rubbery plateau may be a consequence of the near coincidence of the terminal relaxation time and the Rouse segmental relaxation time for the supramolecular Hbonding of the acid groups. If both relaxations occurred simultaneously or close together in time, the network would dissociate before the rubbery plateau was observed. If the acid groups did not participate in the formation of the gel network, GN would have been expected to increase with increasing neutralization due to the increased cross-linking density, which was not the case. Therefore, from the perspective of simply forming a supramolecular bond, the sulfonic acid groups did contribute to the network, but as will be discussed later in this paper, the presence of the acid groups also weakened the energy of the ionic or dipolar supramolecular bonds between the sodium sulfonate groups in the partially neutralized ionomers. For ionomers with x ≥ 89%, TTS worked well for G′ but failed on the high frequency side of the peak in G″ that represented the terminal process. The failure of TTS for G″ was due to the overlap of two relaxation processes, the ionic dissociation and the Rouse relaxation, with different temperature dependences.16−20 The success of TTS for G′ and not G″ is a consequence of the relative insensitivity of the storage modulus to the faster Rouse modes. Compared to PS13.5 precursor, the incorporation of sulfonic acid groups into the polymer shifted the terminal relaxation to lower frequency by a factor of 2−3 due to the H-bonding interactions between acid groups. The conversion of sulfonic acid to Na−sulfonate further delayed the terminal relaxation, but in this case by as much as 5 orders of magnitude for the fully neutralized ionomer (see Figure 1). The behavior of the Na1.5-x ionomers was similar to that of the Na1.2-x ionomers (Figures S1). The effect of the degree of neutralization on the master curves of Na2.7-x ionomers (p > 2pc) is shown in Figure 2. Similar to the Na1.2-x ionomers, the terminal relaxation shifted to lower frequency upon increasing the degree of neutralization, x. However, the shift is much greater for the Na2.7-x ionomers than for the Na1.2-x ionomers due to the higher sulfonation of the former. The presence of the unneutralized acid groups significantly delays the appearance of the plateau in G′. The modulus plateau began to appear at x = 50%, where the Na− sulfonate content pNa = 1.4 mol %, which was nearly twice that of concentration of sulfonate needed at the gel point, pc. As with the Na1.2-x and Na1.5-x ionomers, GN for the Na2.7-x ionomers was independent of the degree of neutralization. Those results are consistent with the conclusion discussed

Figure 2. Frequency dependence of storage and loss moduli, G′ and G″, master curves at Tr = 140 °C for Na2.7-x ionomers. The solid curves are the model predictions.

above that the H-bonding of the sulfonic acid groups contribute to the supramolecular network. When the Na−sulfonate concentration increased, the modulus plateau persisted over a greater range of frequencies. TTS worked well for Na2.7-x ionomers with x ≤ 40% but failed again on the high frequency side of the terminal relaxation function when x ≥ 50% due to the overlap of ionic dissociation and Rouse relaxation.16−20 The terminal relaxation time and zero shear viscosity for three Nap-x ionomers were calculated from the terminal regions where G′ ∝ ω2 and G″ ∝ ω1 in the master curves (see eqs 1 and 2). The values are summarized in Figure 3. ⎡ G′(ω) ⎤ τ = lim ⎢ ⎥ ω→ 0⎣ ωG″(ω) ⎦

(1)

⎡ G″(ω) ⎤ η0 = lim ⎢ ⎥ ω→ 0⎣ ω ⎦

(2)

For a fixed degree of neutralization, x, the terminal relaxation time, τ, and zero shear viscosity, η0, increased with increasing p due to the enhanced ionic interaction. In addition, the increase of τ and η0 was strongly dependent on x. For Na1.2-x ionomers, there was a transition at about x = 89% where the dependence of the τ on the x became much more sensitive. For example, for 0 < x < 89%, the terminal relaxation time, τ, increased about 2 orders of magnitude, while for 89 < x < 100%, another 3 orders of magnitude increase of τ was achieved. As with the lower sulfonated ionomers (Na1.2-x), the increase in the τ of Na2.7-x increased much faster at the higher neutralization levels. From 0 ≤ x < 81%, the relaxation time increased by 3 orders of magnitude, but another 3 orders of magnitude increase was achieved for 81 ≤ x ≤ 100%. Similar transition also occurred in the terminal relaxation time, τ, of Na1.5-x ionomers and in the zero shear viscosity, η0, for all Nap-x ionomers. C

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1.2 mol % and Na2.7-50 with pNa = 1.4 mol %). For pNa ∼ pc, there was a significant difference in the LVE behavior of partially neutralized ionomers and fully neutralized ionomer. The dynamic modulus of Na0.76-100 ionomer showed critical percolation behavior (G′ ∼ G″∼ ω2/3) but that power law behavior was not observed for the partially neutralized ionomers, Na1.5-56 and Na2.7-30. The major difference in the materials was the presence of unneutralized sulfonic acid groups in the latter two ionomers. Compared with the fully neutralized Na0.76-100 ionomers, the two partially neutralized ionomers had slightly higher dynamic modulus in the Rouse region (G′ ∼ G″ ∼ ω1/2) and lower modulus in the terminal region. The Rouse region modulus increased and the modulus in the terminal region decreased with increasing sulfonation (p) even though the concentrations of Na−sulfonate in all three materials were similar. The concentration of free acid groups in the three ionomers increased from 0 for the fully neutralized ionomer to 0.7 and 1.9 mol % for the Na1.5-56 and Na2.7-30 ionomers, respectively. The slightly greater values of G′ and G″ for the partially neutralized ionomers compared with the fully neutralized ionomers in Figure 4 is a consequence of the higher sulfonation level and is consistent with the previously stated conclusion that hydrogen bonding of the sulfonic acid groups contributes to the supramolecular network in the Rouse region. Note, however, that the presence of the sulfonic acid groups also weakened (plasticized) the ionic interactions in the terminal region, which accounts for the increased frequency of the onset of terminal behavior. For the ionomers with pc < pNa < 2pc (Figure 4b), the partially neutralized ionomer (Na2.7-50) exhibited very weak plateau-like behavior for G′, while the fully neutralized ionomer (Na1.2-100) exhibited a well-defined plateau that persisted for more than 3 orders of frequency, even though the Na− sulfonate concentration was lower than the partially neutralized ionomer (1.2 mol % vs 1.4 mol %). The terminal relaxation time for the fully neutralized ionomer was almost 4 orders of magnitude greater than for the partially neutralized ionomer, which in addition to 1.4 mol % Na−sulfonate also contained ∼1.4 mol % sulfonic acid groups. Those results support the hypothesis described in the previous paragraph based on the data shown in Figure 4a that plasticization of the ionic interactions by the acid derivative decreased the terminal relaxation time, τ, and shortened the modulus plateau. One may

Figure 3. Effect of the degree of neutralization, x, on the terminal relaxation time and zero shear viscosity of Nap-x ionomers at Tr = 140 °C.

For each Nap-x ionomers, the terminal relaxation time and complex viscosity increased rather rapidly when x > 80%. This result agrees with the report by Lundberg et al.8 that the slope of melt viscosity versus the extent of neutralization for entangled SPS ionomers increased abruptly at x = 90%. No similar observation was reported by Tierney et al.11,12 for PEMA ionomer system, which could be a consequence of the weaker supramolecular bond between metal carboxylates compared with metal sulfonates or simply because their experimental data was restricted to x ≤ 90%. Figure 4 compares the LVE behavior of partially neutralized ionomers and the fully neutralized ionomer containing a similar Na−sulfonate concentration. Figure 4a shows the LVE behavior of ionomers with pNa ∼ pc = 0.78 mol % (Na0.76-100 with pNa = 0.76 mol %, Na1.5-56 with pNa = 0.84 mol %, and Na2.7-30 with pNa = 0.81 mol %), and Figure 4b compares the LVE behavior of ionomers with pc < pNa < 2pc (Na1.2-100 with pNa =

Figure 4. Comparison of LVE behavior at 140 °C for partially neutralized ionomers and fully neutralized ionomers with similar Na−sulfonate concentrations: (a) ionomers with pNa ∼ pc = 0.78 mol %: Na0.76-100 (pNa = 0.76 mol %), Na1.5-56 (pNa = 0.84 mol %), and Na2.7-30 (pNa = 0.81 mol %); (b) ionomers with pc < pNa < 2pc: Na1.2-100 (pNa = 1.2 mol %) and Na2.7-50 (pNa = 1.4 mol %). The numbers in parentheses of the legend denote the values of pNa. D

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τs increased by about 6 orders of magnitude when the degree of neutralization was increased from 0% to 100%. Also, note the sharp upturn in the values of τs above x ∼ 80%, which is consistent with the increase in τ and η0 data and the report of Lundberg et al.8 for the large increase in viscosity in this same neutralization range. The LVE behaviors of partially neutralized neat ionomers and blends of the unneutralized (i.e., acid derivative) and fully neutralized ionomer with the same Na−sulfonate concentrations are compared in Figure 6. For the partially neutralized

even argue that there is no real modulus plateau in the LVE behavior of Na2.7-50, which looks more like that of an entangled polymer melt than one with a cross-linked network. However, these oligomeric ionomers are below their entanglement molecular weight. Therefore, the plateau-like behavior is attributed to the formation of weak network due to the ionic interaction. The plasticization effect of free acid is consistent with the study of Navratil and Eisenberg,6 who compared relaxation modulus of partially neutralized poly(styrene-comethacrylate) ionomers containing equal amounts of Na salts but different concentrations of free acrylate acid groups and discovered that extra acid groups did accelerate the stress relaxation, though the effect was less significant than current study. The LVE behavior of the partially neutralized ionomers were fit with the reversible gelation model from ref 16. That model has two fitting parameters: the Rouse relaxation time of a Kuhn segment, τ0, and a dissociation time, τs. The dependences of the values of the two relaxation times on x are shown in Figure 5,

Figure 6. Comparison of LVE behavior at Tr = 140 °C for partially neutralized ionomers and binary blends of fully neutralized and unneutralized ionomers having similar Na−sulfonate concentrations.

ionomers, one expects that the Na−sulfonate groups and sulfonic acid groups were randomly distributed. That is, the neutralization reaction, which was conducted in solution, should be random, and thus each chain is expected to have Na−sulfonate and sulfonic acid groups. In contrast, for the blends some chains contained only Na−sulfonate groups and the other chains contained only sulfonic acid groups, assuming the absence of ion exchange. For each Na−sulfonate concentration (pNa = 1.1, 1.9, or 2.6 mol %, i.e., x = 40, 71, or 95%), the ionomer blend exhibited almost the same LVE behavior with that of partially neutralized ionomer. That result is consistent with a previous study of PEAA ionomers that the partially neutralized PEAA ionomers showed similar melt indices, infrared spectra, and mechanical properties as blends of highly neutralized and unneutralized PEAA ionomers.9 Diffusion experiments of fully neutralized SPS ionomers25 and partially neutralized PEMA ionomers11,12 showed that the diffusion coefficients of the counterions are 3−5 orders of magnitude faster than those of the polymer chains. Therefore, compared to the ion-hopping process by which ionomers are thought to flow, the cations diffuse rather quickly within the supramolecular aggregates, which would favor ion exchange of the metal cations with the free acid groups.11,25 Thus, one might expect that when the acid and metal salt derivatives were mixed, they quickly equilibrated in the melt, and maybe even during the solution mixing process, to achieve a random distribution of both species among the polymer chains. That result would explain the absence of any differences in the LVE behavior of the partially neutralized ionomer and a blend of the acid and fully neutralized derivatives with the same concentration of sulfonic acid and Na−sulfonate groups (Figure 6). In addition, the similar LVE behavior for the partially neutralized ionomers and a blend with the same overall

Figure 5. Effect of neutralization degree on the segment relaxation time τ0 and ionic dissociation time τs determined from the reversible gelation model. In panel b, the dashed lines show the predictions of eq 3, and the solid line shows the prediction from eq 4.

and the model fits are shown by the solid lines in Figures 1 and 2 as well as Figure S1. For all the ionomer samples, the model captured the main features of G′ and G″, but it overpredicted the plateau modulus, GN, because the model assumes an ideal network, i.e., defects in the ionic network, such as dangling chain-ends and loops that do not support stress are not considered by the model.16,20 For each of the Nap-x ionomers, τ0 increased exponentially with the degree of neutralization (x) (Figure 5a) due to the decreasing mobility of the polymer chain as the concentration of Na−sulfonate interactions increased. In all cases, the difference of τ0 between 0% and 100% neutralization was less than an order of magnitude. The ionic dissociation time τs also increased with x because of the increased strength of the intermolecular interactions, and the increase of τs with x was much stronger than τ0 (Figure 5b). E

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to a dissociation/association process of the aggregated ionic/ acid groups. In particular, the intensity of the high-ω α process increased with increasing concentration of sulfonic acid.32 A dielectric α2 process occurred at lower frequency part upon neutralization due to restricted motion of the segments near the ionic aggregates. The intensity of α2 process increased with increasing neutralization of the acid groups as the ionic aggregates contained more metal sulfonated groups and the chain mobility was reduced.26 In addition, increasing the degree of neutralization increased the dielectric strength of α2 process and decreased the dielectric strength of the α process due to the conversion of free acid groups to metal sulfonate groups.26 In that work, the change of dielectric strength of two α relaxations correlated well with the variation of ionic dissociation time. Measurements of dielectric relaxation behavior of the partially neutralized SPS ionomers used in the present investigation agreed with the conclusions of Runt and coworkers26,32,33 (cf. Figures S2−S4). With the help of the Onsager equation,34 a correlation between the degree of neutralization, x, and the localized dielectric constant, εs, within the ionic aggregates was established (cf. Figures S5 and S6 and eq S7). Since the ionic dissociation time τs is related to the Rouse relaxation time, τ0, by τs = τ0 exp(Ea/kT)16 and the activation energy Ea ∼ 1/εs,35,36 the ionic dissociation time, τs, of the partially neutralized ionomer with the neutralization degree of x can then be obtained from eq 4:

composition strongly suggests that the ionic aggregate morphology in both are similar. As such, it seems to be a reasonable assumption that the sulfonic acid groups and the Na−sulfonate groups coexist in the ionic aggregate structure of an ionomer. That would not only facilitate ion exchange, but it would also explain the plasticization effect of the acid groups on the Na−sulfonate interactions. That supposition is supported by SAXS and dielectric studies of SPS ionomers that indicated that the size of the ionic aggregates was independent of the degree of neutralization, x, and that more metal sulfonates groups were incorporated into the ionic aggregates upon increasing x.26 A previous study of the LVE behavior of binary blends of fully neutralized SPS with different alkali metal cations showed that the average association lifetime of the ionic interactions in the blend, τs̅ b, could be predicted from the ionic lifetimes of the two components, τs1 and τs2 ϕ ϕ 1 = 1 + 2 τsb̅ τs1 τs2

(3)

where ϕ1 and ϕ2 are the mole fractions of cations 1 and 2. However, eq 3 did not work well at predicting the average ionic dissociation time for the SPS acid and salt blends, as demonstrated by the dashed curves in Figure 5b. The problem with applying eq 3 to the data in Figure 5b is that eq 3 assumes that the dissociation frequency (i.e., the reciprocal of ionic dissociation time) of one alkali sulfonate group is not strongly affected by the existence of a different kind of alkali sulfonate group. However, as discussed earlier in this paper, the acid groups tend to plasticize the ionic interactions or the Na−sulfonate groups. One might consider the effect of the sulfonic acid groups in the blends and partially neutralized SPS ionomers to be similar to the plasticization of polystyrene ionomers with a polar plasticizer, such as glycerol,27,28 which tends to dissolve the ionic aggregate nanodomains and eliminate the long-time relaxations due to the ionic interactions. Therefore, the assumption for eq 3 that the dissociation frequency of an alkali metal sulfonate group is independent of the presence of other sulfonate or sulfonic acid species is not valid, and the interaction between acid groups and Na−sulfonate needs to be considered. Figure 5b shows that the ionic dissociation time, τs, for the partially neutralized Nap-x ionomers is controlled by the degree of neutralization, x, rather than the sulfonation level, p. However, for a given x, the relative concentrations of acid and salt groups in the ionomer are the same, so it is probably the ratio of metal sulfonate groups and sulfonic acid that is the important dependent variable. Since the sulfonic acid groups have a higher dielectric constant than the Na−sulfonate groups,29,30 the increase of τs with increasing x is most likely a consequence of the decrease in the dielectric constant within the supramolecular aggregates, which is expected to increase the energy of the supramolecular interactions.31 The mechanism by which sulfonic acid groups weaken the ionic interactions in the partially neutralized SPS ionomers was previously studied by dielectric spectroscopy by Runt and coworkers.26,32,33 They observed three dielectric processes for the SPS samples: (1) a high-ω α dielectric process attributable to the segment relaxation, (2) an intermediate-ω dielectric process attributable to Maxwell−Wagner−Sillars (MWS) interfacial polarization at the interface of the ionic/acid aggregates and surrounding medium, and (3) a low-ω α* process attributable

⎛ E (100) × εs(100) ⎞ ⎛ E (x) ⎞ τs(x) = τ0(x) exp⎜ a ⎟ = τ0(x) exp⎜ a ⎟ ⎝ kT ⎠ εs(x)kT ⎝ ⎠ (4)

where Ea(100) is the activation energy of the fully neutralized ionomer (x = 100%) and εs(x) is the localized dielectric constant within the aggregates for ionomer with a neutralization degree of x. Based on the values of τ0 and τs (shown in Figures 5a and 5b), Ea (100) was determined to be 17.6, 17.5, and 16.1 kT for Na1.2-100, Na1.5-100, and Na2.7-100 ionomers, respectively. For simplicity, Ea(100) was fixed as 17.1 kT, the average activation energy of three Nap-100 ionomers, and τ0(x) was fixed as 130 μs, the average τ0 for the six Nap-x ionomers with p = 1.2, 1.5, and 2.7 and x = 0 and 100%. Figure 5b shows the fit of eq 4 to the data for τs(x). Although eq 4 overpredicted τs and did not predict the more rapid increase in the relaxation time at the higher neutralization levels, the fit is not bad considering there were no fitting parameter used. This agreement between dielectric prediction and rheological data confirms our previous concept that the acid groups increase the localized dielectric constant within the ionic aggregates, thus plasticizing the ionic interaction of Na− sulfonates and accelerating the ionic dissociation.

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CONCLUSIONS The LVE behavior of oligomeric SPS ionomers with varying degrees of neutralization was fit well with the reversible gelation model proposed by Chen et al.16 Intermolecular interactions between sulfonic acid groups and between Na−sulfonate groups contribute similarly to the network structure, as demonstrated by the independence of the plateau modulus on the degree of neutralization x, but the presence of sulfonic acid groups weakens the network structure. The ionic dissociation time, τs, of partially neutralized SPS ionomers is F

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Macromolecules

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essentially determined by the degree of neutralization, x. The presence of unneutralized sulfonic acid groups in the ionomer plasticizes the ionic interactions and decreases τs and the zero shear viscosity, η0. Increasing x significantly increases τs and η0, which is consistent with the earlier observations reported by Lundberg et al.8 that the increase of those properties with x significantly accelerate as x approaches 100%. The plasticization effect of the sulfonic acid groups is due to the weakening of the supramolecular cross-links as a result of the increased dielectric constant within the ionic aggregates. That effect also explained the rapid acceleration of τs and η0 as the sulfonic acid groups are eliminated from the ionic aggregates. The significance of these findings is the demonstration of the fundamental link between the effect of the polar acid groups on the local environment of the physical cross-links that control the rheological and mechanical properties of ionomers. That should also have implications regarding how polar and nonpolar additives affect the dynamics and mechanical behavior of ionomers, which are important issues with regard to their commercial applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b02423. Figures S1−S6 and eqs S1−S7 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (R.A.W.). ORCID

Chongwen Huang: 0000-0003-0726-3471 Quan Chen: 0000-0002-7771-5050 R. A. Weiss: 0000-0002-5700-6871 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a grant from the Polymers Program of the Division of Materials Research at the National Science Foundation (Grant DMR-1309853). Q.C. also thanks the Natural Science Foundation of China (21674117) and Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, for support.

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DOI: 10.1021/acs.macromol.6b02423 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.6b02423 Macromolecules XXXX, XXX, XXX−XXX