Stimuli-responsive surfaces with switchable wettability assumed significance in drug delivery, biomedical engineering, sensors and bio-fuel cells. Most of the responsive surfaces show intrinsic responsive wettability and it is difficult to tune chemical structures with controlled wetting characteristics.
Researchers at the Surface Engineering Laboratory, School of Materials Science and Engineering, Dalian University of Technology, China have demonstrated that it would be possible to fabricate smart surfaces with tunable wettability and reversibly switchable pH-responsiveness using plasma copolymerization technique (Reference: Iqbal Muzammil et al., Plasma Processes and Polymers, 14 (10) (2017), DOI: 10.1002/ppap.201700053).
Plasma copolymerization is an efficient one-step process to fabricate new surfaces. It is a clean, dry, and environmentally benign process that enables conformal deposition of coatings over surfaces with complex geometries. In this perspective, the Chinese researchers have deposited a series of plasma copolymer coatings with various carboxylic acid and fluorocarbon group ratio on nanotextured low-density polyethylene (LDPE) surfaces via capacitively coupled radio frequency plasma (CCP) polymerization technique.
Acrylic acid (AA) and octafluorocyclobutane (C4F8) were used as monomers. Low density polyethylene (LDPE) was chosen as the substrate and it was oxygen plasma etched at 200W for 30 min with an oxygen flow rate of 50 sccm to develop a nanotextured surface. The C4F8-co-AA plasma polymer coatings was deposited on flat nanotextured LDPE surfaces by radio frequency (RF) capacitively coupled plasma reactor (CCP) mode at 50W for 1 min. The C4F8 monomer flow rate was fixed for 40 sccm while the AA monomer flow rates was changed from 5 to 40 sccm.
C4F8 plasma polymer coating deposited on LDPE surface shows a static water contact angle (SWCA) of 119°. As the carboxylic acid group concentration increases, the SWCA of C4F8-co-AA plasma polymer coatings is decreased. For C4F8-co-AA (40:5) plasma polymer coatings, the SWCA is decreased to 97°. C4F8-co-AA (40:40) plasma polymer coatings show a rapid decrease in SWCA leading to a lower hysteresis. An increase in AA feed ratio increases the ratio of carboxylic acid group to CFx group, leading to a lower SWCA. The hydrophilic carboxylic acid group controls the wetting state since the polar carboxylic acid group allows permeation of polar water molecules into plasma copolymer coatings. In contrast, the hydrophobic fluorocarbon group controls the dewetting state since the nonpolar groups like CF2 and CF3 of low surface energy repel water.
The oxygen plasma etching treatment enables the formation forest like nano-filaments on the surface of LDPE (Fig. 1(a)). Subsequent deposition of either C4F8 (Fig. 1(b)) as well as C4F8-co-AA plasma polymer coatings (Figs. 1 (c) and 1(d)) has no significant effect on the surface nanotexture. Nevertheless, the surface nanotextures amplify the surface wettability.
Fig. 1 Scanning electron micrographs (a) nanotextured surface of LDPE after oxygen plasma etching treatment; (b) C4F8 plasma polymer coating; (c) C4F8-co-AA (40:15) plasma polymer coating; and (d) C4F8-co-AA (40:25) plasma polymer coating on nanotextured LDPE surfaces
C4F8 plasma polymer coating deposited over nanotextured LDPE surface became superhydrophobic with a SWCA of ~163° and low apparent hysteresis of < 1°. This high SWCA with low hysteresis developed over the nanotextured surface can be explained by the Cassie model. Accordingly, water droplet cannot penetrate into cavities of the nanotextured surface as air is trapped at the interface between the water droplet and sharp corners of nanotextured surface.
C4F8 plasma polymer coatings show no significant SWCA change with a change in pH, suggesting the absence of pH-responsive behaviour. In contrast, the C4F8-co-AA plasma polymer coatings start to a exhibit pH-responsive behaviour with sufficient increase in pH-sensitive carboxylic acid groups. C4F8-co-AA (40:5) plasma polymer coatings in different pH solutions of 1, 4, 9, and 13 for 10 min shows a SWCA of 80, 76, 70, and 65°, respectively. C4F8-co-AA (40:10) and (40:15) plasma polymer coatings show a SWCA 72 and 70° at pH 1, 67 and 64° at pH 4, 61 and 58° at pH 9, 54 and 49° at pH 13, respectively.
An increase in concentration of carboxylic acid groups as well as pH leads to a decrease in SWCA. This phenomenon can be explained by the protonation and deprotonation of dangling carboxylic acid groups. These carboxylic acid groups become uncharged due to protonation at low pH. It shrinks and gives an additional surface to fluorinated part of the copolymer thus higher SWCA is observed. Correspondingly, at higher pH’s due to deprotonation of the carboxylic acid groups, it becomes charged and the charged state increases the polarity of the polymeric coatings leading to a lower SWCA.
Fig. 2 Schematic illustration of the change in static water contact angle with pH demonstrating the development of C4F8-co-AA plasma polymer coatings with tunable wettability and reversibly switchable pH-responsiveness using plasma copolymerization technique
This methodology opens up a potential door for the fabrication of smart surfaces.
T.S.N. Sankara Narayanan
For more information, the reader may kindly refer: Iqbal Muzammil et al., Tunable wettability and pH-responsiveness of plasma copolymers of acrylic acid and octafluorocyclobutane, Plasma Processes and Polymers, 14 (10) (2017), DOI: 10.1002/ppap.201700053
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