Temperature- and pH-Responsive Schizophrenic Copolymer Brush Coatings with Enhanced Temperature Response in Pure Water

Yana Shymborska; Yurij Stetsyshyn; Kamil Awsiuk; Joanna Raczkowska; Andrzej Bernasik; Natalia Janiszewska; Paweł Da Bczyński; Andrij Kostruba; Andrzej Budkowski

doi:10.1021/acsami.2c20395

Temperature- and pH-Responsive Schizophrenic Copolymer Brush Coatings with Enhanced Temperature Response in Pure Water

ACS Appl Mater Interfaces. 2023 Feb 15;15(6):8676-8690. doi: 10.1021/acsami.2c20395. Epub 2023 Feb 3.

Authors

Affiliations

¹ Smoluchowski Institute of Physics, Jagiellonian University, Łojasiewicza 11, 30-348 Kraków, Poland.
² Lviv Polytechnic National University, St. George's Square 2, 79013 Lviv, Ukraine.
³ Faculty of Physics and Applied Computer Science, AGH - University of Science and Technology, al. Mickiewicza 30, 30-049 Kraków, Poland.
⁴ Faculty of Food Technologies and Biotechnology, Stepan Gzhytskyi National University of Veterinary Medicine and Biotechnologies Lviv, Pekarska 50, 79000, Lviv, Ukraine.

Abstract

Novel brush coatings were fabricated with glass surface-grafted chains copolymerized using surface-initiated atom transfer radical polymerization (SI-ATRP) from 2-(2-methoxyethoxy)ethyl methacrylate (OEGMA188) and acrylamide (AAm), taken in different proportions. P(OEGMA188-co-AAm) brushes with AAm mole fraction >44% (determined with XPS and TOF-SIMS spectroscopy) and nearly constant with the depth copolymer composition (TOF-SIMS profiling) exhibit unusual temperature-induced transformations: The contact angle of water droplets on P(OEGMA188-co-AAm) coatings increases by ∼45° with temperature, compared to 17-18° for POEGMA188 and PAAm. The thickness of coatings immersed in water and the morphology of coatings imaged in air show a temperature response for POEGMA188 (using reflectance spectroscopy and AFM, respectively), but this response is weak for P(OEGMA188-co-AAm) and absent for PAAm. This suggests mechanisms more complex than a simple transition between hydrated loose coils and hydrophobic collapsed chains. For POEGMA188, the hydrogen bonds between the ether oxygens of poly(ethylene glycol) and water hydrogens are formed below the transition temperature T_c and disrupted above T_c when polymer-polymer interactions are favored. Different hydrogen bond structures of PAAm include free amide groups, cis-trans-multimers, and trans-multimers of amide groups. Here, hydrogen bonds between free amide groups and water dominate at T < T_c but structures favored at T > T_c, such as cis-trans-multimers and trans-multimers of amide groups, can still be hydrated. The enhanced temperature-dependent response of wettability for P(OEGMA188-co-AAm) with a high mole fraction of AAm suggests the formation at T_c of more hydrophobic structures, realized by hydrogen bonding between the ether oxygens of OEGMA188 and the amide fragments of AAm, where water molecules are caged. Furthermore, P(OEGMA188-co-AAm) coatings immersed in pH buffer solutions exhibit a 'schizophrenic' behavior in wettability, with transitions that mimic LCST and UCST for pH = 3, LCST for pH = 5 and 7, and any transition blocked for pH = 9.

Keywords: LCST; polymer brushes; thermoresponsive polymer; transfer radical polymerization; water structure; wettability; ‘schizophrenic’ copolymers.