Determination of Carboxyl Dissociation Degree and pKa in Weak Polyelectrolyte Membranes via POT Titration and FTIR Analysis for Clean Technologies in Sustainability

Charged polymer membranes play a crucial role in clean technologies for sustainability. Previously, we have designed a series of weak polyelectrolyte membranes, i.e., acrylic acid–poly(ethylene glycol) diacrylate (AA-PEGDA) networks with a wide ion-exchange capacity range (IEC: 0–4 mequiv/g) and limited water swelling. An acrylic acid (AA) monomer was used to control the amount of dissociated charged (COO^–) groups versus pH. To systematically quantify the dissociated charged (COO^–) group content in the polymers, we reported the degree of ionization (α) and negative logarithm of acid dissociation constant (pK_a) of AA-PEGDA series at varied external pH via conventional potentiometric titration (POT titration) and widely available ATR-FTIR analysis. Contact angle measurement was also used to record the surface hydrophilicity vs pH to support the analysis. Overall, the degree of ionization (α) vs pH trends via three different methods well-align to each other, indicating that three methods which (1) measure pH changes in a solution phase (POT titration), (2) directly probe dissociated COO^– groups in a polymer phase (ATR-FTIR) and (3) record surface hydrophilicity (contact angle) reasonably describe the molecular-level, physical picture of dissociation process in the polymers. For all compositions, as external pH increases (pH = 5–12), the degree of ionization (α) also increases between 0 and 1, following the modified Henderson–Hasselbalch equation. As the maximum ion-exchange capacity (mIEC) increases (AA content increases and PEGDA cross-linker content decreases), the decreased cross-linking density and simultaneously increased water content dominantly reduce the overall electrostatic repulsion, showing decreased pK_a. Our system shows higher cross-linking density and was tested in dilute conditions, showing increased pK_a values compared with those of other polymers in the literature. To the best of our knowledge, this is the first time the charged group concentration (mIEC) and cross-linking density have been systematically changed to record the pK_a trend in thin film forms using a single polymer network system. Our AA-PEGDA series can provide a platform to advance our understanding of the dissociation process in weak polyelectrolyte polymers.