IMPROVING THE HYDROPHOBICITY OF POLYMERS THROUGH SURFACE TEXTURING

Abstract

Introduction of surface textures has long been used to improve the hydrophobicity of solid materials. This study focusses on understanding the effects of various micro-texture geometries on the hydrophobicity of textured polymer surfaces. Square pillar, cylindrical, hemispherical and conical surface features, both protrusion and cavity, are considered in this study for two polymers. Employing the well-known models, the study shows that introducing textures on polymer surfaces generally increases the contact angle and, therefore, improves the hydrophobicity of polymers. The effect of surface texture on hydrophobicity significantly varies with texture geometry and dimension. The study provides useful guidelines for improving hydrophobicity of polymers by introducing textures on the surface.

Introduction

Improvement in hydrophobic property of polymers has long been sought in various applications, such as in automotive, aerospace, solar cell, household appliances, and biomedical applications. Hydrophobic polymer surfaces can provide self-cleaning capability as well as improvement in long-term mechanical behavior. This improvement in long-term mechanical performance can significantly increase the lifespan of polymers used in various applications, such as the total joint replacement prostheses applications. The hydrophobicity of polymers, described by the ability to repel a mass of water, can be improved by introducing textures on polymer surfaces through chemical etching, ion beam etching, photolithography and laser texturing methods, among others [1]. The degree of hydrophobicity in solid surface generally depends on two factors: surface chemistry and surface topography [2].

Hydrophobicity of a surface is characterized by the contact angle (θ) between a water droplet and the surface. A hydrophobic surface displays strong repulsion towards water, whereas a hydrophilic surface displays strong affinity towards water. If the contact angle is less than 90°, the liquid is considered wetting the surface and the surface is deemed as hydrophilic [3]. On the other hand, if the contact angle is greater than 90°, the surface is considered hydrophobic [3]. The surface is considered superhydrophobic if the contact angle is greater than 150° [3].

Cassie-Baxter and Wenzel models are widely used to correlate hydrophobicity with the surface roughness of materials. According to the Wenzel model, the liquid completely fills the surface features of the solid surface. The contact angle for a rough surface, ${\theta }_{\mathrm{r}}^{\mathrm{W}}$is defined by the Wenzel model as [4]:

$$\cos {\theta }_{\mathrm{r}}^{\mathrm{W}}=[{\mathrm{r}}^{\mathrm{W}}(\mathrm{geo})]\cos {\theta }_{\text{flat}}^{\mathrm{e}}$$ (1)

Where, r^W(geo) is the roughness parameter of the surface, defined as the ratio of the actual area of the rough surface to its projected area, and ${\theta }_{\text{flat}}^{\mathrm{e}}$ is the equilibrium contact angle on the smooth or flat surface of the same material. Wenzel proposed that hydrophilic surface tends to become more hydrophilic and hydrophobic surface tends to become more hydrophobic with increase in surface roughness [3]. According to the Cassie-Baxter model, the liquid droplet sits on the top of the surface asperities. The equilibrium contact angle, ${\theta }_{\mathrm{r}}^{\text{CB}}$ is defined by the Cassie-Baxter model as [4]:

$$\cos {\theta }_{\mathrm{r}}^{\text{CB}}=[{\mathrm{f}}^{\text{CB}}(\mathrm{geo})](1+\cos {\theta }_{\text{flat}}^{\mathrm{e}})-1$$ (2)

Where, [f^CB(geo)] is the ratio of the liquid/solid contact area under the droplet to the total projected area of the drop basement, and ${\theta }_{\text{flat}}^{\mathrm{e}}$ is the equilibrium contact angle on the smooth or flat surface of the same material.

The present study focuses on understanding the correlation between various surface texture geometries and change in contact angle using both Wenzel and Cassie-Baxter models to investigate the feasibility of employing surface texture to improve the hydrophobicity of polymers. Ultrahigh molecular weight polyethylene (UHMWPE) and Polyetheretherketone (PEEK), which have been widely used in biomedical applications, is considered for that purpose. Square pillar, cylinder, hemisphere and conical surface textures (both protrusion and cavity) with variation in texture dimension are used to study the influence of these surface textures on contact angle, and, therefore, hydrophobicity of polymers.

Cases Considered

Figure 1 shows the various surface texture geometries considered in this study. The length (in case of square pillar) or diameter (in case of cylindrical, hemispherical and conical surface textures), L, the spacing between the surface features, S, and the height of the feature, H are all varied as 10, 20, 30, 40, 50, 100 and 150 μm, respectively. Both protrusion and cavity texture geometry are considered in this study. In case of hemispherical surface features, the diameter and height of the features are related as L=2H.

Figure 1.

Texture geometry considered: (a) square pillar (protrusion); (b) cylinder (protrusion); (c) hemisphere (protrusion); (d) cone (protrusion); (e) inverted cylinder (cavity); (f) inverted hemisphere (cavity); and (g) inverted cone (cavity).

To calculate the contact angle of textured surfaces using the Wenzel model, as described in equation 1, the surface roughness parameter, r^W(geo) for the various surface textures considered in this study are calculated using the equations (same for both protrusion and cavity) below:

$$\text{Square pillar}[4,5]:[{\mathrm{r}}^{\mathrm{W}}(\mathrm{geo})]=\frac{(L+S{)}^2+4LH}{(L+S{)}^2}$$ (3)

$$\text{Cylinder}[4]:[{\mathrm{r}}^{\mathrm{W}}(\mathrm{geo})]=\frac{(L+S{)}^2+\pi LH}{(L+S{)}^2}$$ (4)

$$\text{Hemisphere:}[{\mathrm{r}}^{\mathrm{W}}(\mathrm{geo})]=\frac{2\pi r^2+S^2+(L^2-\pi r^2)}{(L+S{)}^2}$$ (5)

Where, $r=\frac{L}{2}$.

$$\text{Cone:}[{\mathrm{r}}^{\mathrm{W}}(\mathrm{geo})]=\frac{\pi rl+S^2+(L^2-\pi r^2)}{(L+S{)}^2}$$ (6)

Where, $l=\sqrt{r^2+H^2}$, $r=\frac{L}{2}$.

To calculate the contact angle of textured surfaces using the Cassie-Baxter model, as described in equation 2, the [f^CB(geo)] for the various surface features considered in this study are calculated using the equations below:

$$\text{Square pillar}[4]:[{\mathrm{f}}^{\text{CB}}(\mathrm{geo})]=\frac{L^2}{(L+S{)}^2}$$ (7)

$$\text{Cylinder}[4]:[{\mathrm{f}}^{\text{CB}}(\mathrm{geo})]=\frac{\pi L^2}{4(L+S{)}^2}$$ (8)

$$\text{Hemisphere:}[{\mathrm{f}}^{\text{CB}}(\mathrm{geo})]=\frac{\pi r^2}{8(L+S{)}^2}$$ (9)

Where, $r=\frac{L}{2}$.

$$\text{Cone:}[{\mathrm{f}}^{\text{CB}}(\mathrm{geo})]=\frac{\pi rl}{16(L+S{)}^2}$$ (10)

Where, $l=\sqrt{r^2+H^2}$, $r=\frac{L}{2}$.

For cylindrical, hemispherical and conical cavity:

$$[{\mathrm{f}}^{\text{CB}}(\mathrm{geo})]=\frac{S^2+(L^2-\pi r^2)}{(L+S{)}^2}$$ (11)

Where, $r=\frac{L}{2}$.

To calculate the contact angle for the various micro-texture geometries considered in this study, the smooth or flat surface contact angle, ${\theta }_{\text{flat}}^{\mathrm{e}}$ for UHMWPE and PEEK are obtained from the literature. The ${\theta }_{\text{flat}}^{\mathrm{e}}$ for UHMWPE and PEEK are taken to be 87° [6] and 77.6° [7], respectively.

Results and Discussion

Figure 2 shows the percent change in contact angle (with respect to smooth or flat surface) with variation in hemispherical protruding surface texture spacing for UHMWPE and PEEK, using Wenzel and Cassie-Baxter models. As can be seen in the figure, both models show that improvement in hydrophobicity can be achieved by adding hemispherical protruding textures on the surface of both polymers (when the spacing is large enough, according to the Wenzel model), as evidenced by the increase in contact angle. However, the improvement is more pronounced in PEEK in comparison with UHMWPE. As shown in the figure, according to the Wenzel model, increase in spacing between the features generally increases the hydrophobicity of both polymers in the lower range. However, the spacing has negligible effect on hydrophobicity according to the Cassie-Baxter model. Furthermore, increase in hemispherical feature diameter (L) decreases the hydrophobicity of both polymers. Although the Wenzel model for hemispherical cavity shows the same findings, the Cassie-Baxter model for hemispherical surface cavity shows a different trend, as can be seen in Figure 3. As shown in Figure 3, according to the Cassie-Baxter model, increase in spacing between the hemispherical cavity significantly affects the hydrophobicity of both polymers at lower hemisphere diameter (L) range. The effect becomes negligible at higher hemisphere cavity diameter range. Furthermore, increase in hemisphere cavity diameter improves the hydrophobicity of both polymers, according to the Cassie-Baxter model.

Figure 2.

Percent change in contact angle with variation in hemispherical surface feature spacing for: (a) UHMWPE, using Wenzel model; (b) UHMWPE, using Cassie-Baxter model; (c) PEEK, using Wenzel model; and (d) PEEK, using Cassie-Baxter model. (L is in μm).

Figure 3.

Percent change in contact angle with variation in hemispherical cavity spacing for: (a) UHMWPE; and (b) PEEK; using Cassie-Baxter model. (L is in μm).

Figure 4 shows the percent change in contact angle (with respect to smooth or flat surface) with variation in square pillar height for a constant pillar length of 30 μm, for UHMWPE and PEEK, using Wenzel and Cassie-Baxter models. As can be seen in the figure, according to the Wenzel model, introduction of square pillar decreases the hydrophobicity of both polymers. Increase in pillar height decreases the hydrophobicity, however, increase in pillar spacing seems to lessen the effect. Same conclusion can be drawn for other pillar lengths (not shown to avoid redundancy), although the effect of spacing on hydrophobicity is lesser with increase in pillar length. According to the Cassie-Baxter model, hydrophobicity of both polymers improves with introduction of square pillars, with pillar height has no influence on hydrophobicity. However, increase in spacing improves the hydrophobicity of both polymers. Same conclusion can be drawn for other pillar lengths which are not shown to avoid redundancy. Cylindrical protruding surface features also show similar trend as square pillars.

Figure 4.

Percent change in contact angle with variation in square pillar height for a constant pillar length of 30 μm, for: (a) UHMWPE, using Wenzel model; (b) UHMWPE, using Cassie-Baxter model; (c) PEEK, using Wenzel model; and (d) PEEK, using Cassie-Baxter model. (S is in μm).

Figure 5 shows the percent change in contact angle (with respect to smooth or flat surface) with variation in conical protruding surface feature height for a constant cone diameter of 30 μm for UHMWPE and PEEK, using Wenzel and Cassie-Baxter models. According to the Wenzel model, the effects of conical surface texture height, spacing and diameter on hydrophobicity are similar as square pillar as discussed earlier. According to the Cassie-Baxter model, hydrophobicity of both polymers improves with introduction of conical protrusion, with increase in cone height decreases the hydrophobicity. However, increase in cone spacing seems to lessen the effect. Same conclusion can be drawn for other cone diameters (not shown to avoid redundancy), although the effect of spacing on hydrophobicity is lesser with increase in cone diameter.

Figure 5.

Percent change in contact angle with variation in conical surface feature height for a constant cone diameter of 30 μm, for: (a) UHMWPE, using Wenzel model; (b) UHMWPE, using Cassie-Baxter model; (c) PEEK, using Wenzel model; and (d) PEEK, using Cassie-Baxter model. (S is in μm).

Figure 6 shows the percent change in contact angle (with respect to smooth or flat surface) with variation in cavity height or depth for a constant inverted cone or cylinder diameter of 30 μm for UHMWPE and PEEK, using Cassie-Baxter model since Wenzel model shows same findings as corresponding protruding surface features. As shown in the figure, according to the Cassie-Baxter model, cylindrical or conical cavity is expected to improve the hydrophobicity of both polymers. The cavity depth (or height) has no influence on hydrophobicity, however, increase in spacing between the cavity decreases the hydrophobicity of both polymers. Same conclusion can be drawn for other cavity diameters (not shown to avoid redundancy), although the effect of spacing on hydrophobicity is lesser with increase in cavity diameter.

Figure 6.

Percent change in contact angle with variation in cavity height/depth for a constant inverted cone/cylinder diameter of 30 μm, for: (a) UHMWPE; and (b) PEEK; using Cassie-Baxter model. (S is in μm).

It should be noted that both Wenzel and Cassie-Baxter models have some limitations to accurately predict the contact angle on textured surfaces. Therefore, caution should be taken when applying the models to predict the hydrophobicity of polymers. Nevertheless, both models provide useful suggestion on the effect of texture geometry on hydrophobicity. It is expected that the experimental results and comparison with the model findings will be presented.

Conclusions

In this study, effects of various surface texture geometries, such as square pillar, cylinder, hemisphere and cone (both protrusion and cavity), on the contact angle of UHMWPE and PEEK have been investigated using Wenzel and Cassie-Baxter models. The results show that introducing surface textures on polymer surfaces in general expected to improve hydrophobicity. However, the effects of texture length or diameter, height or depth, and spacing on hydrophobicity varies with texture geometry. The study provides useful guidelines to improve hydrophobicity of polymers by correlating texture geometry with hydrophobicity.