Fabrication of PAA-co-PNIPAm brushes via surface-initiated-PET-RAFT polymerization and investigation of their dual responsive behavior

Abstract

Stimuli-responsive polymers have attracted considerable attention because they can alter their chemical structures or physical properties in response to external triggers. Such smart polymers have found applications in various fields, including sensors, drug delivery, water purification, recyclable catalysis, separation, and more. Polymers exhibiting stimuli responsive behavior have been synthesized in response to various stimuli to date. In this study, the relatively new PET-RAFT polymerization technique was employed to synthesize and characterize both homo- and copolymer brushes on silicon disc surfaces. Brushes consisting of a single polymer segment (poly(acrylic acid) (PAA), poly(N-isopropylacrylamide) (PNIPAm)) and two polymer segments ((PAA-co-PNIPAm), denoted as P1, P2 and P3) were prepared using SI-PET-RAFT polymerization to analyze their stimuli responsive behavior for the first time. Water contact angle (WCA) measurements revealed pH- and temperature-triggered transitions. PAA brushes showed a distinct transition around pH 5–6, with the WCA decreasing by approximately 35° as pH increased from 4 to 6. PNIPAm brushes exhibited a thermal transition near 30 °C, with the WCA increasing by about 10° as the temperature increased from 28 °C to 32 °C. The copolymer brushes displayed composition-dependent tunable responses. P1 (PAA-rich) presented a sharp pH transition around pH 5, with the WCA decreasing by approximately 46° as pH increased from 4 to 6. P2 (equimolar PAA/PNIPAm) showed a slightly less sharp pH transition around pH 5–6, with WCA decreasing by around 39° over pH 4–6. P3 (PNIPAm-rich) exhibited a transition around pH 5, with WCA decreasing by roughly 36° over the same range. Although the sharpness of the pH-induced transition slightly decreased from P1 to P2 and P3 as the PAA fraction decreased and PNIPAm fraction increased, a clear pH response was retained. Thermally, P1 and P2 underwent transitions around 29 °C, with WCA increasing by about 10° and 7.5° respectively, as the temperature increased from 28 °C to 32 °C. P3 showed a slightly shifted transition around 31 °C, with WCA increasing by about 10.5° over the same range. These findings demonstrate that dual responsive behavior can be tuned by monomer composition, thereby offering potential for controlled drug delivery and other bio-related applications.