A general method to determine ionization constants of responsive polymer thin films

Area of Science:

• Polymer Science
• Surface Chemistry
• Analytical Chemistry

Background:

• Determining ionization constants of polymer thin films is crucial for understanding their behavior in various applications.
• Existing methods may have limitations in accuracy or applicability to thin film formats.

Purpose of the Study:

• To develop a general method for measuring ionization constants of polymer thin films.
• To utilize the stimuli-responsive properties of polymers for this determination.
• To validate the method using poly(4-vinylpyridine) (P4VP) as a model system.

Main Methods:

• Fabrication of robust polymer films on silicon wafers and gold slides using perfluorophenyl azide (PFPA) as a coupling agent.
• Measurement of ionization constants using ellipsometry, dynamic contact angle goniometry, and surface plasmon resonance imaging (SPRi).
• Utilizing the pH-responsive nature of P4VP thin films to assess ionization.

Main Results:

• Ellipsometry yielded a pK(a) of ~4.0, correlating with polymer film swelling in response to pH.
• Dynamic contact angle measurements gave a pK(a) of ~5.0, based on hydrophilicity changes during protonation/deprotonation.
• SPRi measured a pK(a) of ~4.9 by monitoring in situ changes in the polymer film's refractive index due to swelling.

Conclusions:

• A versatile method for determining polymer thin film ionization constants based on stimuli-responsiveness has been established.
• SPRi was successfully utilized for the first time to measure polymer ionization constants.
• The study demonstrates the utility of multiple techniques for characterizing polymer film properties.