**2. Materials and methods**

### **2.1 Photochromic polymers and properties**

Photochromic doped polymer films were prepared by incorporating photochromic molecules into polymer matrices. In particular, solutions of the polyethylmethacrylate-comethylacrylate copolymer (PEMMA) (average molecular weight, Mw=100,000) (Aldrich) mixed with the photochromic molecule 1',3'-dihydro-1',3',3'- trimethyl- 6-nitrospiro[2H-1 benzopyran-2,2'-(2H)-indole] or (6-NO2 BIPS) (Aldrich) (weight ratio 90/10 respectively or 95/5) were prepared in toluene. Consequently a certain volume of this solution was spincoated onto a glass substrate.

The photochromic dopant 6-NO2 BIPS, belongs to the family of spiropyrans (SP) which have been extensively studied in the past decades (Görner 1998). Initially it has a 3D structure, and exists predominately in its non polar form (Figure 1). It is colorless, denoting that it is transparent in the visible range of the spectrum, but absorbs in the ultraviolet (UV). Upon irradiation with UV light, it is converted to its isomeric form, merocyanine (MC), through the photochemical cleavage of its carbon–oxygen (Cspiro-O) bond. The MC has a planar structure, it is colored and polar, and has a new absorption band in the visible range of the spectrum. MC can revert back to the SP form photochemically, using visible-light irradiation.

Fig. 1. The photochromic dopant spiropyran (SP) and the stable form of its isomer merocyanine (MC).

the performance of these two different type of applications, namely the microfluidic devices and the diffraction gratings. In both cases the lithographic technique used for the microstructuring of the photochromic doped plastic films is the soft molding. Concerning the microfluidics applications, the presented microstructured photochromic plastic films exhibit a significant improvement on the reversible wetting characteristics compared to those on the flat surfaces. This improvement is due to the combination of the changes in the surface polarity and thus in the wetting properties with the modified surface conformation, both provoked by the light induced changes of the photochromic molecules. Moreover, regarding the optical gratings, we present a different approach in where the control of their diffraction efficiency relies on the dimensional variations of the gratings upon laser irradiation. Following this approach, the efficiency of the gratings is significantly improved

Such findings open the way for the production of optically switchable gratings based on reversible dimensional changes, and can be of great importance in all-optical signal processing systems. Moreover, the ability to control the wettability of surfaces by microstructuring and to tune it by using photochromic molecules, permits the application of these lithographically formed structures to all-optically controlled switches capable of

Photochromic doped polymer films were prepared by incorporating photochromic molecules into polymer matrices. In particular, solutions of the polyethylmethacrylate-comethylacrylate copolymer (PEMMA) (average molecular weight, Mw=100,000) (Aldrich) mixed with the photochromic molecule 1',3'-dihydro-1',3',3'- trimethyl- 6-nitrospiro[2H-1 benzopyran-2,2'-(2H)-indole] or (6-NO2 BIPS) (Aldrich) (weight ratio 90/10 respectively or 95/5) were prepared in toluene. Consequently a certain volume of this solution was spin-

The photochromic dopant 6-NO2 BIPS, belongs to the family of spiropyrans (SP) which have been extensively studied in the past decades (Görner 1998). Initially it has a 3D structure, and exists predominately in its non polar form (Figure 1). It is colorless, denoting that it is transparent in the visible range of the spectrum, but absorbs in the ultraviolet (UV). Upon irradiation with UV light, it is converted to its isomeric form, merocyanine (MC), through the photochemical cleavage of its carbon–oxygen (Cspiro-O) bond. The MC has a planar structure, it is colored and polar, and has a new absorption band in the visible range of the spectrum. MC

can revert back to the SP form photochemically, using visible-light irradiation.

Fig. 1. The photochromic dopant spiropyran (SP) and the stable form of its isomer

with respect to previous works.

**2. Materials and methods** 

coated onto a glass substrate.

merocyanine (MC).

**2.1 Photochromic polymers and properties** 

operating with tunable speed, and to microfluidic actuation.

Briefly, UV and visible irradiation causes the reversible transformation of these chemical species, between two states (isomers) that have light absorption bands in characteristic spectral regions. This property is retained when the photochromic molecules are incorporated in polymer matrices, where they are homogeneously dispersed forming miscible systems. Specifically, the absorption properties of the photochromic polymer films prepared as described above change reversely upon UV-visible irradiation as shown at Figure 2. Initially the system is transparent at the visible range of the spectrum. Upon pulsed UV laser irradiation the SP is slowly converting to the MC isomer, fact indicated by the new absorption band in the visible region of the spectrum (ca. 565 nm). The intensity of the peak increases with the number of UV pulses until a plateau is reached, which suggests that the photoisomerization is completed and that the system has reached the equilibrium. The subsequent irradiation with green laser light, causes the decrease of the intensity of the previously formed MC peak, while after a certain number of pulses the spectrum reaches its initial form, indicating that MC reverts fully to the SP isomer. These data confirm that under the irradiation conditions mentioned in the figure caption of Figure 2, the reversible properties of SP are retained in the host polymer matrix. Depending on the irradiation conditions and the weight percentage of the photochromic molecules in the polymer matrix (usually ≤10%), it has been shown that typically about 4-10 irradiation cycles can be performed, while further irradiation causes the degradative photooxidation of the photochromic molecules, restricting thus the lifetime of the system (Athanassiou et al 2006c). Additionally, the degradative phenomena start to be evident usually after the third cycle. In order to exclude this parameter from the following study, results derived by the first three irradiation cycles are presented.

Fig. 2. Absorption spectra of the PEMMA/SP 10% wt upon UV and visible irradiation. For the specific study, the irradiation conditions used are: λUV=355 nm, fluence FUV=20 mJ cm-2, λvis=532 nm, Fvis=35 mJ cm-2.

Photocontrolled Reversible Dimensional Changes of Microstructured Photochromic Polymers 153

Fig. 3. Schematic diagram of the process of master replication and soft molding.

irradiation, resulting in reversible changes of the wetting properties of the surfaces.

surface wetting properties (Lygeraki et al 2008).

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

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

**3. Light induced wettability changes of patterned substrates** 

**3.1 Introduction** 

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 aggregates (Athanassiou et al. 2005)

## **2.2 Substrates microstructuring: soft molding**

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 al 2004)

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 weight. (Pisignano et al 2004)

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 molecules upon heating.

Fig. 3. Schematic diagram of the process of master replication and soft molding.
