Thermoresponsive Poly(N-isopropylacrylamide) Copolymers:  Contact Angles and Surface Energies of Polymer Films

Abstract

Surface properties of poly(N-isopropylacrylamide) (PNIPAM) copolymer films were studied by contact angle measurements and optical and atomic force microscopy. We prepared a series of copolymers of N-isopropylacrylamide with N-tert-butylacrylamide (NtBA) in order of increasing hydrophobicity. The measurements of the advancing contact angle of water at 37 degrees C were hampered by the observation of a distinct stick/slip pattern on all polymers in the series with the exception of poly(NtBA) (PNtBA). We attributed this behavior to the film deformation by the vertical component of liquid surface tension leading to the pinning of the moving contact line. This was confirmed by the observation of a ridge formed at the pinned contact line by optical microscopy. However, meaningful contact (without the stick/slip pattern and with a time-independent advancing contact angle) angles for this thermoresponsive polymer series could be obtained with carefully selected organic liquids. We used the Li and Neumann equation of state to calculate the surface energy and contact angles of water for all polymers in the series of copolymers and van Oss, Chaudhury, and Good (vOCG) acid-base theory for PNtBA. The surface energies of the thermoresponsive polymers were in the range of 38.9 mJ/m2 (PNIPAM) to 31 mJ/m2 (PNtBA) from the equation of state approach. The surface energy of PNtBA calculated using vOCG theory was 29.0 mJ/m2. The calculated contact angle for PNIPAM (74.5 +/- 0.2 degrees ) is compared with previously reported contact angles obtained for PNIPAM-modified surfaces.

Polarity Assessment of Thermoresponsive Poly(NIPAM-co-NtBA) Copolymer Films Using Fluorescence Methods

“…All solvents were reagent grade and were purified before use [20]. A series of copolymers of N-isopropylacrylamide (NIPAM) and N-tert-butylacrylamide (NtBA) with ratios (w/w) of 100:0, 85:15 (P85), 65:35 (P65), 50:50 (P50), and 0:100 were synthesized as described previously [20].…”

“…The LCST of these poly-N-isopropylacrylamide -N-tert-butylacrylamide (pNIPAM-co-NtBA) thermoresponsive copolymers can be decreased by increasing the NtBA fraction. The importance of this study lies in the fact that these pNIPAM-co-NtBA copolymers are currently being investigated as potential drug elution matrices and for cell culture applications [16][17][18][19][20]. Because these copolymers are relatively hydrophilic and will uptake water, it can be difficult to characterise the materials using conventional techniques like Contact Angle Measurements.…”

Measuring the Micro-Polarity and Hydrogen-Bond Donor/Acceptor Ability of Thermoresponsive N-Isopropylacrylamide/N-tert-Butylacrylamide Copolymer Films Using Solvatochromic Indicators

“…All solvents were reagent grade and were purified before use. 28 A series of copolymers of Nisopropylacrylamide (NIPAM) and N-tert-butylacrylamide (NtBA) with ratios (w/w) of 100:0, 85:15, 65:35, and 0:100 were synthesized as described previously. 28 The chemical structure of the copolymers is shown in Figure 1.…”

Nanostructured ZnO‐Based Surface with Reversible Electrochemically Adjustable Wettability

“…Several solutions have been proposed in the literature, with many papers pointing out WCA changes induced by UV-light irradiation, storage in the dark, or thermal treatment. [11][12][13][14][15][16][17][18][19][20] For instance, roughness-enhanced thermal responsive wettability of a polymer has been reported in poly(N-isopropylacrylamide). [20] Reversible WCA changes on various oxide surfaces have also been described for TiO 2 , [12][13][14] SnO 2 , [15] and ZnO.…”

“…[11][12][13][14][15][16][17][18][19][20] For instance, roughness-enhanced thermal responsive wettability of a polymer has been reported in poly(N-isopropylacrylamide). [20] Reversible WCA changes on various oxide surfaces have also been described for TiO 2 , [12][13][14] SnO 2 , [15] and ZnO. [16][17][18][19] The WCA drops to a low value after several hours of UV-light irradiation and the initial high WCA is recovered after storage in the dark for more than a week.…”