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

Abstract

A general method has been developed to determine the ionization constants of polymer thin films based on the stimuli-responsiveness of the polymer. Robust polymer films were fabricated on silicon wafers and gold slides using perfluorophenyl azide (PFPA) as the coupling agent. The ionization constants were measured by a number of techniques including ellipsometry, dynamic contact angle goniometry, and surface plasmon resonance imaging (SPRi). Using poly(4-vinylpyridine) (P4VP) as the model system, P4VP thin films were fabricated and the ionization constants of the films were measured taking advantage of the pH responsive property of the polymer. The pKa determined by ellipsometry, ~4.0, reflects the swelling of the polymer film in response to pH. The pKa value calculated from the dynamic contact angle measurements, ~5.0, relies on the change in hydrophilicity/hydrophobicity of the films as the polymer undergoes protonation/deprotonation. The pKa value measured by SPRi, ~4.9, monitors in situ the change of refractive index of the polymer thin film as it swells upon protonation. This was the first example where SPRi was used to measure the ionization constant of polymers.

Highly swellable ultrathin poly(4-vinylpyridine) multilayer hydrogels with pH-triggered surface wettability

“…Other types of PVP hydrogels known to date, e.g., obtained by UV or chemical cross-linking, demonstrate swelling ratios in the range of 1.5 to 3. 42,70 In another example, the nanocomposites obtained from cross-linked PVP-SiO 2 showed 2.7-fold swelling at pH ¼ 2, 49 while the cross-linked PVP capsules synthesized from coprecipitation of the poly(4-vinylpyridine) hydrogel onto the silica particles showed only 1.4-fold swelling. 51 Our results suggest that removal of PMAA from the cross-linked multilayer provides a large free volume which allows the lm to drastically swell or collapse unlike limited volume transitions in bulk and surface networks.…”

Acidity of the methyne group of poly(4-vinylpyridine) leads to side-chain protonation in pyridine

“…1,2 Because of the lone pair of sp 2 electrons, P4VP demonstrates interesting photochemical properties leading to various applications such as coatings that respond to changes in pH. 3 When P4VP is dissolved in dry liquid pyridine and allowed to gel, unique opto-and thermo-electronic behavior is observed. These properties have been studied extensively during the last few years, both experimentally [4][5][6] and by using computational methods.…”

Confinement of Candida Antarctica Lipase B in a Multifunctional Cyclodextrin-Derived Silicified Hydrogel and Its Application as Enzymatic Nanoreactor

“…We therefore opted for covering our biocatalyst with a pH-sensitive polymer, the poly­(4-vinylpyridine) (P4VP). P4VP has a p K a value of approximately 4.5 and is able to swell upon protonation . Under the immobilization conditions employed (acetate buffer, pH 4.6), P4VP is near neutral, and therefore, no electrostatic interactions may occur between the polymer and the enzyme.…”

“…P4VP has a pK a value of approximately 4.5 and is able to swell upon protonation. 69 Under the immobilization conditions employed (acetate buffer, pH 4.6), P4VP is near neutral, and therefore, no electrostatic interactions may occur between the polymer and the enzyme. However, because of the presence of pyridine units on its structure, P4VP can readily serve as a hydrogen bond donor or acceptor.…”