**3.1 Introduction**

152 Advances in Unconventional Lithography

Except of the alteration in the optical properties, there are also other physical and chemical properties that change reversibly upon UV-visible irradiation even if the SP molecules are incorporated in the polymer matrix, such as dipole moment, surface energy, refractive index, and volume. Concerning the volume changes upon UV irradiation, occurs the formation of aggregates between different MC stereoisomers with zwitterionic character causing density fluctuations in the polymer matrix, reducing thus the MC partial molar volume. Consequently, takes place the short scale motion of the polymer chains in order to diminish the density fluctuations in the samples, and this leads to the macroscopic reduction of the dimensions of the matrix. This effect is reversible, since upon green irradiation, MC molecules return to the SP form, which does not form

For the microstructuring of the photochromic plastic films it is used the soft molding lithography (SM). It is actually based on the conformal contact between the material to be patterned and an elastomeric replica of a master structure, and it combines soft and nanoimprint lithography, using elastomeric elements and exploiting the glass transition of organic compounds. Particularly, an elastomeric mold is placed onto a polymeric film applying the pressure of its own weight, and consequently is heated up above the films' glass transition temperature, *Tg*. The subsequent cooling down, below *Tg*, freezes the pattern into the polymer, and the replica is peeled off. The micropatterns formation is based on the capillarity effect that drives the polymer to penetrate into the recessed features of the elastomeric replica. The SM presents various advantages compared to the nanoimprint lithography. Specifically, since penetration of the polymer into recessed features of the replica is driven by capillarity effects, SM is only slightly affected by problems caused by difficult polymer transport. Moreover, it does not need any pressing setup to ensure contact between the mold and the polymer. Finally, SM does not present pattern shrinkage and distortion due to the solvents employed by other soft lithography techniques. (Pisignano et

In Figure 3 it is represented the process followed for the SM. Initially, the original master structures are fabricated onto glass or Si by both photo- and electron-beam lithography. The realized masters are used as templates on which elastomeric replicas were realized using polydimethylsiloxane (PDMS, Sylgard 184, Dow Corning, Midland, MI) according to a standard replica molding procedure, and placed onto polymer films under their own

In order to form the microstructured photochromic polymer substrates for the wettability study, elastomeric molds of PDMS having periods α= 1.3, 28.0, and 180*.*0 μm were placed onto flat spin-cast films. Then the system was heated at 50 ºC, a temperature higher than the Tg of the PEMMA films (Tg = 48 °C). After the thermal cycle, the replica was easily peeled off from the photochromic polymer substrates, on which the patterns are transferred. For the preparation of the gratings on the films for the diffraction efficiency study, the substrates were placed on a hot plate and heated until they reach a temperature Tgrating of 65 °C. Then an elastomeric mold with α=4 μm was placed on the substrate for 10 min, resulting in the formation of the gratings. In both cases the SM procedure was carried out in nitrogen atmosphere to avoid the deterioration of the photochromic

aggregates (Athanassiou et al. 2005)

al 2004)

weight. (Pisignano et al 2004)

molecules upon heating.

**2.2 Substrates microstructuring: soft molding** 

As abovementioned, the modification of the wetting characteristics of photochromic surfaces depends mainly on the photochemical processes which modify the surface tension and are caused by the UV-visible irradiation cycle. However, studies on patterned surfaces have demonstrated that the surface roughness affects significantly the wettability properties (Patankar 2003). Here it is studied the combined effect of the two aforesaid factors on the wettability properties of patterned photochromic polymeric surfaces. The wettability changes induced by photomechanical and photochemical changes are reversible upon UV-visible light irradiation, resulting in reversible changes of the wetting properties of the surfaces.

In particular, it is shown that the hydrophilicity of the photochromic polymeric surfaces is increased upon UV laser irradiation due to the polarity change caused by the photoisomerization, while the process is reversed upon green laser irradiation. The microstructuring of the surfaces enhances significantly the hydrophobicity of the system due to the increased surface roughness, and the light-induced wettability variations of the structured surfaces are enhanced by a factor of 3 compared to those on the flat surfaces. (Athanassiou et al 2006a, 2006b) In addition, by changing the topological parameters of the introduced pattern (e.g. by decreasing the period), are achieved higher differences in the surface wetting properties (Lygeraki et al 2008).

Photocontrolled Reversible Dimensional Changes of Microstructured Photochromic Polymers 155

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

Fig. 6. WCA images obtained on patterned surfaces before and after laser irradiation. (5% wt

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)

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,

cosθr=-1 + fs(1 + cosθY) (2)

θ

*<sup>Y</sup>*) is given by the following equation:

of SP in PEMMA, λUV=308 nm, FUV=40 mJ cm-2, λgreen=532 nm, Fgreen=45 mJ cm-2)

reversible photoinduced wettability changes of the surfaces.

(Athanassiou et al 2006a)

and to Wenzel model (b).

where the relation with the WCA of the flat surface (
