**3.2 Results and discussion**

Figure 4, shows the apparent water contact angle (WCA) of a drop on a flat photochromic polymer surface before and after pulsed UV and green laser irradiation. The WCA on the flat substrate, θY, is formed when the liquid is in contact with a solid surface in static equilibrium with its vapor, and is determined by the Young's equation:

$$
\gamma\_{\rm LV} \cos \theta\_{\rm Y} = \gamma\_{\rm SV} - \gamma\_{\rm SL} \tag{1}
$$

where γLV, γSV and γSL represent the interfacial tensions at the boundaries between the liquid (L), vapor (V), and solid (S).

Fig. 4. WCA of a water drop of volume 3 μl on flat photochromic polymer surface before and after UV-green laser irradiation. (5% wt of SP in PEMMA, λUV=308 nm, FUV=40 mJ cm-2, λgreen=532 nm, Fgreen=45 mJ cm-2) (Athanassiou et al 2006a)

As shown, before any irradiation the surface is hydrophilic with a WCA of 76°. After irradiation with enough UV laser pulses so as to reach the complete photoisomerization of all the non polar SP molecules to the polar MC form (50 pulses), the surface becomes more hydrophilic, with a WCA of 69°. The subsequent green irradiation with 500 laser pulses causes the reversible phenomenon, which is the conversion of the MC isomer to the SP form, and the increase of the WCA until it reaches its initial value. The maximum WCA difference measured on numerous flat surfaces upon UV irradiation was 7°±1°.

Fig. 5. Atomic force microscopy images of typical patterned surfaces of photochromic polymers. (Athanassiou et al 2006a)

In order to examine the effect of the photomechanical changes upon the same irradiation conditions, the aforementioned surfaces were microstructured using the SM technique. The replica used had a period α=1.3 μm, and by following the already described steps there were formed on the photochromic polymeric surface patterns of the same period (Figure 5). The WCA on the specific surface was found to be greatly affected by the patterning, showing an increased hydrophobicity with a value almost 30° greater than that of the flat

Figure 4, shows the apparent water contact angle (WCA) of a drop on a flat photochromic polymer surface before and after pulsed UV and green laser irradiation. The WCA on the

 γLV cosθY = γSV − γSL (1) where γLV, γSV and γSL represent the interfacial tensions at the boundaries between the

Fig. 4. WCA of a water drop of volume 3 μl on flat photochromic polymer surface before and after UV-green laser irradiation. (5% wt of SP in PEMMA, λUV=308 nm, FUV=40 mJ cm-2,

Fig. 5. Atomic force microscopy images of typical patterned surfaces of photochromic

In order to examine the effect of the photomechanical changes upon the same irradiation conditions, the aforementioned surfaces were microstructured using the SM technique. The replica used had a period α=1.3 μm, and by following the already described steps there were formed on the photochromic polymeric surface patterns of the same period (Figure 5). The WCA on the specific surface was found to be greatly affected by the patterning, showing an increased hydrophobicity with a value almost 30° greater than that of the flat

As shown, before any irradiation the surface is hydrophilic with a WCA of 76°. After irradiation with enough UV laser pulses so as to reach the complete photoisomerization of all the non polar SP molecules to the polar MC form (50 pulses), the surface becomes more hydrophilic, with a WCA of 69°. The subsequent green irradiation with 500 laser pulses causes the reversible phenomenon, which is the conversion of the MC isomer to the SP form, and the increase of the WCA until it reaches its initial value. The maximum WCA difference

equilibrium with its vapor, and is determined by the Young's equation:

λgreen=532 nm, Fgreen=45 mJ cm-2) (Athanassiou et al 2006a)

measured on numerous flat surfaces upon UV irradiation was 7°±1°.

Y, is formed when the liquid is in contact with a solid surface in static

**3.2 Results and discussion** 

θ

liquid (L), vapor (V), and solid (S).

polymers. (Athanassiou et al 2006a)

flat substrate,

surfaces (Figure 6). Moreover, always in comparison with the flat surfaces, the light induced WCA changes due to the photoisomerization effect are enhanced by a factor of 3, since the WCA change before and after UV irradiation is ca. 20° (WCA change on the flat surface, ca. 7°). This guides to the conclusion that the microstructuring affects significantly the reversible photoinduced wettability changes of the surfaces.

Fig. 6. WCA images obtained on patterned surfaces before and after laser irradiation. (5% wt of SP in PEMMA, λUV=308 nm, FUV=40 mJ cm-2, λgreen=532 nm, Fgreen=45 mJ cm-2) (Athanassiou et al 2006a)

In order to explain the effect of roughness on the wetting characteristics of a surface, there are proposed two theories. The first is referred to as the Cassie-Baxter model (Cassie and Baxter 1944) (Figure 7a), and describes the wettability of rough surfaces, where only partial wetting may occur due to the trapping of air underneath the drop at the recessed regions of the surfaces. Since the drop is situated partially on air, the surface exhibits an enhanced hydrophobic behavior. The second one is the Wenzel model (Wenzel 1936), and it proposes that roughness increases the liquid-solid interfacial area, and thus hydrophilic surfaces (θ<90°) become more hydrophilic, and hydrophobic (θ>90°) more hydrophobic (Figure 7b).

Fig. 7. Representation of a drop on a patterned surface, according to Cassie-Baxter model (a) and to Wenzel model (b).

In the presented cases the WCA of the drop on the flat surface is 76° thus hydrophilic (<90°) and according to the Wenzel model the patterning should increase its hydrophilicity. However, the experimental results presented above show that the WCA after the patterning is significantly increased, reaching a maximum value of 104°, indicating that the surface became hydrophobic (>90°). Thus, the presented surfaces follow the Cassie-Baxter model, where the relation with the WCA of the flat surface (θ*<sup>Y</sup>*) is given by the following equation:

$$
\cos \theta\_{\mathbf{r}} \mathbf{\bar{r}} = \mathbf{1} + \mathbf{f}\_s (1 + \cos \theta\_{\mathbf{Y}}) \tag{2}
$$

Photocontrolled Reversible Dimensional Changes of Microstructured Photochromic Polymers 157

larger than the size of each feature projected on the plane of the interface in the case of the patterns with a 180 μm period, about 140 times greater in the case of the patterns with a 28 μm period, and finally more than 2950 times greater in the case of the patterns with a 1*.*3 μm period. Accordingly, as shown at the table, for the pattern with period of 180 μm there is a slight change of the *f* upon UV-green irradiation. This is not the case for the smaller pattern where the change upon UV-green irradiation is almost 15%, while for the pattern of period

Fig. 9. (a) Mean WCA values of patterned surfaces of 10% wt SP in PEMMA for each pattern, for the flat surface and for the PEMMA surface. The media is taken after studying 10 samples of each case. (b) WCA difference (Δθ) from the initial one after UV irradiation. (λUV=308 nm, FUV=20 mJ cm-2, λgreen=532 nm, Fgreen=25 mJ cm-2) (Lygeraki et al 2008)

Period 1.3μm 28μm 180μm Initial 0.68 0.88 0.94 UV1 0.80 0.94 0.96 Green1 0.66 0.89 0.91 UV2 0.81 0.94 0.95 Green2 0.68 0.91 0.94

Table 1. Factor *f* calculated by the Cassie–Baxter model under different irradiation

conditions.

28 μm there is an intermediate change of ca. 6%.

with *fs* the solid fraction of the surface in contact with the liquid. According to this equation, the WCA decreases when the *fs* is increasing. Using the WCA (θ*<sup>Y</sup>*) measured on the flat surface and the one of the patterned in all three cases, before, after UV, and after green irradiation, the *fs* is calculated. The results show that the fraction of the patterned surface in contact with the liquid is increased after UV irradiation (*fs*=0.8) compared to the no irradiated sample (*fs*=0.6). AFM microscopy studies of the topological changes of the gratings after UV irradiation, showed that the average volume decrease of each nanoimprinted stripe is ca. 30 nm. However, since the *fs* is higher, and the stripes of the pattern are narrower, it is concluded that the water drop penetrates deeper into the channels of the UV irradiated pattern (partial wetting-Figure 8b), decreasing thus the WCA value. After irradiation with green laser pulses the MC molecules return to their SP isomers, the stripes recover their previous volume, and thus the drop returns to its previous condition, demonstrating once more that the wetting behavior is greatly influenced by both, the changes in the surface polarity and the volume of the stripes (Athanassiou et al 2006a).

Fig. 8. Representation of a drop on a patterned surface, according to Cassie-Baxter model, before (a) and after (b) UV irradiation.

A further demonstration of the abovementioned statement is the study of the wetting properties of patterned surfaces with different topological parameters. In particular, the periods of the elastomeric replicas used for the formation of the patterns on the photochromic polymer surfaces (10% wt of SP in PEMMA) were 1.3, 28.0, and 180*.*0 μm. Figure 9a demonstrates the WCA changes upon three UV-green irradiation cycles for each of the patterned substrates and the comparison with the flat surface. For comparison reasons it is presented also the WCA of the pure polymeric surface, without any addition of photochromic molecules, which is not affected by the irradiation as expected. As shown, the patterning of the surfaces increases the initial WCA in all three cases. However, the smaller the period of the pattern the more hydrophobic is its behavior compared to the flat surface. After UV irradiation, all substrates become more hydrophilic, but again the WCA difference from the initial one is greater at the pattern with the smaller period, and becomes smaller as the period is increasing (Figure 9b).

The observed difference between the various patterns cannot be attributed to the surface chemistry changes upon UV-green irradiation, since this is the same in all samples. Additionally, if this was happening, it should be observed the inverse phenomenon, thus the greater change for the pattern with the greater period, since in these cases the drops are in contact with a greater percentage of the solid surface, as seen from the calculated value of *f*, at Table 1. Thus, the reported differences can be attributed to the volume shrinkage of the patterns upon UV irradiation. This reduction of the patterns volume affects much more the contact angle of the samples with patterns of smaller period, since the drop lies on a greater number of patterned features. Indeed it is calculated that the radius of the drop is 20 times

with *fs* the solid fraction of the surface in contact with the liquid. According to this equation,

surface and the one of the patterned in all three cases, before, after UV, and after green irradiation, the *fs* is calculated. The results show that the fraction of the patterned surface in contact with the liquid is increased after UV irradiation (*fs*=0.8) compared to the no irradiated sample (*fs*=0.6). AFM microscopy studies of the topological changes of the gratings after UV irradiation, showed that the average volume decrease of each nanoimprinted stripe is ca. 30 nm. However, since the *fs* is higher, and the stripes of the pattern are narrower, it is concluded that the water drop penetrates deeper into the channels of the UV irradiated pattern (partial wetting-Figure 8b), decreasing thus the WCA value. After irradiation with green laser pulses the MC molecules return to their SP isomers, the stripes recover their previous volume, and thus the drop returns to its previous condition, demonstrating once more that the wetting behavior is greatly influenced by both, the changes in the surface polarity and the volume of the stripes (Athanassiou et al 2006a).

Fig. 8. Representation of a drop on a patterned surface, according to Cassie-Baxter model,

A further demonstration of the abovementioned statement is the study of the wetting properties of patterned surfaces with different topological parameters. In particular, the periods of the elastomeric replicas used for the formation of the patterns on the photochromic polymer surfaces (10% wt of SP in PEMMA) were 1.3, 28.0, and 180*.*0 μm. Figure 9a demonstrates the WCA changes upon three UV-green irradiation cycles for each of the patterned substrates and the comparison with the flat surface. For comparison reasons it is presented also the WCA of the pure polymeric surface, without any addition of photochromic molecules, which is not affected by the irradiation as expected. As shown, the patterning of the surfaces increases the initial WCA in all three cases. However, the smaller the period of the pattern the more hydrophobic is its behavior compared to the flat surface. After UV irradiation, all substrates become more hydrophilic, but again the WCA difference from the initial one is greater at the pattern with the smaller period, and becomes smaller as

The observed difference between the various patterns cannot be attributed to the surface chemistry changes upon UV-green irradiation, since this is the same in all samples. Additionally, if this was happening, it should be observed the inverse phenomenon, thus the greater change for the pattern with the greater period, since in these cases the drops are in contact with a greater percentage of the solid surface, as seen from the calculated value of *f*, at Table 1. Thus, the reported differences can be attributed to the volume shrinkage of the patterns upon UV irradiation. This reduction of the patterns volume affects much more the contact angle of the samples with patterns of smaller period, since the drop lies on a greater number of patterned features. Indeed it is calculated that the radius of the drop is 20 times

before (a) and after (b) UV irradiation.

the period is increasing (Figure 9b).

θ

*<sup>Y</sup>*) measured on the flat

the WCA decreases when the *fs* is increasing. Using the WCA (

larger than the size of each feature projected on the plane of the interface in the case of the patterns with a 180 μm period, about 140 times greater in the case of the patterns with a 28 μm period, and finally more than 2950 times greater in the case of the patterns with a 1*.*3 μm period. Accordingly, as shown at the table, for the pattern with period of 180 μm there is a slight change of the *f* upon UV-green irradiation. This is not the case for the smaller pattern where the change upon UV-green irradiation is almost 15%, while for the pattern of period 28 μm there is an intermediate change of ca. 6%.

Fig. 9. (a) Mean WCA values of patterned surfaces of 10% wt SP in PEMMA for each pattern, for the flat surface and for the PEMMA surface. The media is taken after studying 10 samples of each case. (b) WCA difference (Δθ) from the initial one after UV irradiation. (λUV=308 nm, FUV=20 mJ cm-2, λgreen=532 nm, Fgreen=25 mJ cm-2) (Lygeraki et al 2008)


Table 1. Factor *f* calculated by the Cassie–Baxter model under different irradiation conditions.

Photocontrolled Reversible Dimensional Changes of Microstructured Photochromic Polymers 159

caused the DE to recover close to its initial value. It is worth noticing that the relative changes of the DE during the first irradiation cycle exhibit big variations between the various examined samples, in contrary with the following irradiation cycles, where the changes are similar for all the examined gratings. This is mainly attributed to internal stresses of the polymer matrix, produced during the preparation of the gratings, that are released in a random way upon irradiation (Liang et al 2007). Nevertheless, at the second cycle during UV irradiation the DE increases with increasing number of pulses, until it stabilizes to an average value of approximately 7.4±3.0% with respect to its initial value. After green irradiation the DE is slowly reaching its initial value with increasing number of

Fig. 10. Experimental setup for the measurement of the diffraction efficiency of the photochromic gratings (10% wt. SP in PEMMA) (λUV=355nm, FUV=20 mJ cm-2, λgreen=532

Fig. 11. Diffraction efficiency changes of the grating upon UV-green irradiation. (Fragouli et

In order to examine the effect of the refractive index (*n*) change of the photochromic polymer sample upon UV-green irradiation on the observed change to the DE, ellipsometric

pulses. This behavior is repeated also at the third cycle.

nm, Fgreen=35 mJ cm-2, λreading beam=822 nm.)

al 2008)

In conclusion, at this section it is presented the possibility to create both hydrophobic and hydrophilic surfaces starting from the same photochromic polymeric sample by changing the topological parameters of its surface features using soft molding lithography. Due to the photochromic transformations taking place upon alternating UV and green irradiation, these surfaces can reversibly change their wettability. By careful control of the surface topology these changes can be fully controlled and tuned, in such a way that the surfaces can be wetted in a reversible manner.
