Understanding the dissociation process in weakly charged polymer membranes is essential to design innovative charged polymer membranes with desirable transport properties for broader applications in sustainability. Previously, we designed a library of weakly charged polymer membranes, i.e., cross-linked acrylic acid (AA)–poly(ethylene glycol) diacrylate (PEGDA) (AA–PEGDA) random copolymer networks with a wide ion-exchange capacity (IEC = 0–4 mequiv/g) range and limited water swelling. Here, we report the dissociation process of the weakly charged AA–PEGDA series in dilute (DI water) and concentrated (1 M NaCl (aq)) external salt solutions by probing dissociation in both 1) a solution phase (via potentiometric (POT) titration) and 2) a polymer phase (via ATR–FTIR analysis). Both POT titration and ATR–FTIR analysis describe the dissociation process well, showing similar dissociation parameters (degree of ionization (α), pKa). As the external pH increases (pH = 3–12), the degree of ionization (α) increases between 0 and 1, following the modified Henderson–Hasselbalch equation. The pre-titration analysis also confirms that our AA–PEGDA series shows a very small dissociation degree (α = 0–4%) before adding a strong base (NaOH). To determine the dominant molecular factors affecting the electrostatic interaction (thus, dissociation) in our system, we compare four relevant physical length scales, i.e., (1) the average charged group distance in a polymer (rc), (2) the average spacing between salt ions in an external solution (rion), and the respective (3) Bjerrum length (lB) and (4) Debye screening length (rD) in dilute and concentrated salt solutions. In the dilute solution (DI water), the enhanced electrostatic interaction (rc ≲ lB ≪ rD < rion) is the dominant factor for suppressing the dissociation and thus moving pKa to a higher range. The influence of varying polymer compositions (mIEC and PEGDA cross-linker length, n) has a relatively smaller impact on dissociation and pKa in dilute conditions. In contrast, in the concentrated salt solution (1 M NaCl (aq)), the screened electrostatic interaction (rD ≪ rc ≲ lB ≈ rion) via added external salts is the governing factor for promoting dissociation and thus substantially lowering pKa. Subsequently, changing polymer compositions (mIEC, n) shows greater influences on dissociation and pKa. If an external salt condition is fixed, varying polymer compositions (mIEC, n) in the AA–PEGDA series leads to different degrees of water swelling (ϕw) (high ϕw favors more dissociation), altering the dissociation trend and pKa. In our system, varying external salt conditions and water swelling (ϕw) ratios in the polymers are the major parameters for controlling dissociation behavior and pKa. Overall, our AA–PEGDA series follows a similar trend to that of other AA-based polymers in the literature, showing higher pKa values in dilute solutions and lower pKa range in concentrated salt solutions. To the best of our knowledge, this is the first time that the dissociation process has been systematically studied using a film form with systematically varied polymer compositions and external conditions. Our multiscale analysis of dissociation using the AA–PEGDA platform can help us understand the molecular-level physical picture of dissociation in weakly charged polymers and thus tune the dissociation of this material to achieve desired transport properties.
Kim et al. (Mon,) studied this question.
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