1. Introduction

Photosensitive liquid crystalline (LC) polymeric materials have been a subject of much research in the recent past [1–26] due to the large photomechanical deformations observed. Small changes in polymer network structure induced via photoisomerization processes are amplified into large macroscopic mechanical effects by coupling the photosensitive group to the order parameter of the LC network (LCN). Ikeda and coworkers [19] synthesized LCN containing side and main chain azobenzene mesogens which undergo bending and unbending deformations induced by changing the wavelength of a high pressure Hg lamp from UV (λ_exp = 365 nm) to visible (λ_exp > 540 nm). At temperatures above the glass transition temperature (85° C), the UV-induced trans-cis photoisomerization resulted in a decrease in the molecular length of the azobenzene moieties. Because of the strong absorption of trans azobenzene isomers at 365 nm and the scattering nature of the polydomain network, the process of trans-cis photoisomerization is localized in a thin surface layer of the film, of approximately 1 μm. Photomechanically, trans-cis isomerization results in a volumetric contractive strain caused mostly by a reduction in the order parameter of the azo-LCN but also by the change in molecular axis of azobenzene from 9 Å (trans) to 5.5 Å (cis). The orientation of the light-driven mechanical response is dependent on the polarization direction of the UV source. Molecules parallel to the polarization direction more efficiently undergo trans-cis isomerization causing the bending axis of the rectangular film to occur parallel to the polarization direction. UV exposure of uniaxially aligned monodomain or polydomain samples always causes bending towards the light source. Cis-trans isomerization caused by subsequent exposure of the azo-LCN to visible light in the green-red range restores the initial shape of the film. Other UV-driven photomechanical effects have recently been documented in similar materials that focus on differences in molecular architecture and liquid crystal network orientation [5–8,11,13].

Recently [12] we reported the laser-driven bidirectional bending of a polydomain azo-LCN. Both trans and cis isomers absorb the 457-514 nm output of an argon-ion laser (Ar⁺) causing the azobenzene moieties in the azo-LCN to undergo both trans-cis and cis-trans isomerization. The dichroic nature of azobenzene and the polarized state of the laser ultimately result in the reorientation of azobenzene mesogens normal to the light polarization [25]. Bending angles of as much as ±70° have been demonstrated with angular velocity of 170°/s (at a Ar⁺ intensity ~100 mW/cm²). The reorientation of the azobenzene mesogens causes either contraction or expansion of the surface depending on laser polarization enabling bending in either direction. If the polarization direction of the Ar⁺ is parallel to the long axis of the azo-LCN cantilever the film surface contracts directing bending towards the laser output; change of Ar⁺ polarization direction to orthogonal to cantilever long axis causes an expansion of the azo-LCN surface directing bending away from the laser output.

In almost all work reported to date, photomechanical effects observed in azo-LCN materials have been the result of the disordering trans-cis isomerization. This work reports on fast, large angular displacement and bidirectional photomechanical bending processes enabled by conditioning azo-LCN cantilevers to undergo cis-trans isomerization to restore order. The cantilevers examined here are based on azo-LCN polymers similar in chemistry to Ikeda [19] and our previous examination [12]. The photomechanical effects are distinguished by two-sided UV-pretreatment of the azo-LCN cantilever. UV exposure generates an approximately 1 μm-thick cis-isomer rich skin on both surfaces of the cantilever. In the work here, subsequent illumination of the UV-pretreated azo-LCN cantilever with an Ar⁺ laser operated in a multimode regime drives a two step photomechanical process. This work demonstrates that photomechanical effects are possible in UV-rich environments such as space. The ability to ‘pretreat’ the local surface environment also further extends the known photomechanical capabilities of azo-LCN particularly for remotely accessed, photoactuated bending.

2. Experimental results

The azobenzene liquid crystalline polymer network (azo-LCN) was prepared by thermal copolymerization of two azobenzene liquid crystal monomers, as previously reported [12, 19]. 4,4’-Di(6-acryloxyalkyloxy)azobenzene and the LC monomer 4-(6-acryloxy)hexyloxy-4’-ethoxyazobenzene were copolymerized at 93°C using 1,1’-azobis(cyclohexane-1-carbonitrile) (AIBN) as a thermal initiator. The concentration of the thermal initiator was 1 wt% and the mole concentration of crosslinker in the azo-LCN was 10 mol%. Polymerization was performed for 24 hours between two glass substrates, coated with poly(vinyl alcohol) and rubbed antiparallel, separated by 20 μm thick spacers. The polymer film was separated from the substrates and rectangular cantilevers of dimension 7 mm × 1 mm were cut for all experiments. The rubbing direction of the LCN was aligned along the long axis of the cantilever. Wide angle x-ray diffraction confirms the azo-LCN is polydomain. The films were pre-exposed to a UV lamp from both sides (λ_exp = 365 nm, I = 10 mW/cm²) for 10 minutes upon which a substantial fraction of the azobenzene moieties near the surface (1 μm) undergo trans-cis isomerization (see Fig. 1(a)).

 

Fig. 1. Illustration of an azo-LCN cantilever after UV pretreatment (left), after initial Ar⁺ exposure (middle), and during trans-cis-trans reorientation (right). Two-sided, UV-pretreatment induces a strain (ε) on both sides of the material. Subsequent exposure to 457–514 nm light from an Ar⁺ laser initially reduces the strain on the exposed surface by driving cis-trans isomerization, before inducing an even larger strain through trans-cis-trans reorientation.

Download Full Size | PDF

 

Fig. 2. Experimental set-up: TC - twist cell; BE- beam expander, D - diaphragm, CL - cylindrical lens; AF - azo film; H - holder.

Download Full Size | PDF

The UV-pretreated polymer cantilever was irradiated with the linear-polarized beam of an argon-ion laser (Ar⁺) with 135 mW power operated in multi-mode regime (457, 488, 514 nm) in the geometry shown in Fig. 2. The laser beam, first expanded and collimated, was reduced in size by a 10 mm diaphragm and then focused with a cylindrical lens to 2.3 mm width. The power density of the laser beam was 430 mW/cm² and the cantilever was illuminated across the short axis. The direction of beam polarization E could be changed from vertical to horizontal position using a liquid crystal polarization rotator.

The photomechanical response of the UV-pretreated azo-LCN cantilever is shown pictorially here and in supplemental video Media 1 (E ∥ to long axis) and Media 2(E ∥ to short axis). Figure 3 compares images that detail the response dynamics of the UV-pretreated azo-LCN cantilever to Ar⁺ polarization parallel and orthogonal to the long axis of the cantilever. For either polarization, the UV-pre-irradiated cantilever bends away from the laser output to a large angle (~ 110°) over approximately 1.5 s. This initial response is caused by restoration of the LC order at the front surface of the cantilever which results in an expansion of this surface (relative to the bulk) which directs bending away from the source shown schematically in Fig. 1(b).

 

Fig. 3. Dynamics of bending of UV-pretreated azo-LCN cantilever to initial Ar⁺ exposure. Beam exposure time is: (1) 0 s, (2) 0.16 s, (3) 0.23 s, (4) 0.29 s, (5) 0.4 s, (6) 0.52 s, (7) 0.77 s, (8) 1.5. (Media 1 and Media 2)

Download Full Size | PDF

 

Fig. 4. (a) Dynamics of bending of the azo-LCN cantilever from position (Fig. 3, 8) with Ar⁺ laser polarized parallel to the long axis of cantilever. Exposure times are: (1) 1.3 s, (2) 3.06 s, (3) 3.97 s, (4) 4.54 s, (5) 4.93 s, (6) 8.56 s, (7) 34.6 s, (8) 57.1 s. Zero time corresponds to the time of position (Fig. 3, 8). See also (Media 1). (b) Dynamics of bending of the azo-LCN cantilever from position (Fig. 3, 8) with Ar⁺ laser polarized orthogonal to the long axis of cantilever. Exposure times are: (1) 1.04 s, (2) 4.26 s, (3) 7.97 s, (4) 10.57 s, (5) 12.62 s, (6) 16.06 s, (7) 22.55 s, (8) 39.82 s. Zero time corresponds to the time of position (Fig. 3, 8). See also (Media 2). Photos were taken with video camera using green cut-off filter. Arrow shows Ar⁺ direction.

Download Full Size | PDF

The second part of the photomechanical response of UV-pretreated azo-LCN cantilevers is shown in Fig. 4 which is continuation in time from the endpoints of Fig. 3. After the initial polarization independent behavior which saturates in only a second or so, the cantilever begins to bend in the opposite direction – moving towards the illumination laser to varying degrees depending on polarization. When the Ar⁺ is polarized parallel to the long axis of the cantilever, the film bends a total of ~170° from 110° to -60° over a minute in time (Fig. 4(a)). We have designated the positive angular deflection away from the laser source and negative angular deflection towards the laser source. Lesser bending angles are observed when the azo-LCN cantilever is exposed to Ar⁺ polarized orthogonal to the long axis of the cantilever (Fig. 4(b)). In both figures, the sign of the bending angle reverses with continued Ar⁺ exposure. The relationship between the long axis of the azo-LCN cantilever and the polarization direction of the Ar⁺ controls the overall deformation angle.

Figure 5 plots the bending angle as a function of exposure time for the entire response of UV-pretreated azo-LCN cantilevers to Ar⁺ exposure of parallel and orthogonal polarization. The response is independent of laser polarization for approximately the first 5 s of illumination. The final bending angle of the cantilever reaches a maximum angle of -76° for light polarized parallel to the cantilever axis while only -12° for light perpendicular to the cantilever axis.

 

Fig. 5. Bending angle of UV-pretreated azo-LCN cantilevers as a function of time for Ar⁺ light polarized parallel to the cantilever long axis () and for Ar⁺ light polarized perpendicular to the cantilever long axis (). The Ar⁺ is lasing 457, 488, and 514 nm light. The bending angle was calculated from initial state of the film.

Download Full Size | PDF

3. Discussion

The initial treatment of the azo-LCN cantilever to 365 nm light drives trans-cis isomerization. However, the strong absorbance of 365 nm by azobenzene and the scattering nature of the polydomain network structure limit the interaction length of the UV light to approximately 1 μm. As depicted in Fig. 1(a), this forms a structure of two ~1 μm thick skins (cis-rich) and an ~18 μm bulk (trans-rich) for the 20 μm thick cantilever. As has been demonstrated in the literature, typically UV exposure will cause a deformation towards the light source as the cis-rich surface skin generates a contractile force relative to the bulk of the material. We however, do not see deformation to the UV light because we expose the azo-LCN cantilever from both sides, in essence generating counterbalancing contractile forces that maintain the initial flat shape of the cantilever. Exposure of this bimorph structure to longer wavelength light, such as the 457–514 nm output of the Ar⁺ used here, results in the reverse cis-trans isomerization on only the exposed surface. The cis-trans isomerization at the exposed surface restores the liquid crystalline order at the surface which removes the contractile strain on the front surface relative to the bulk. There is no preferential orientation of the cis-isomers with respect to the cantilever axis and, as expected, this photomechanical response is independent of polarization. The result of the removal of the exposed surface contractile force is the cantilever bending away from the laser source (Fig. 1(b)). This initial movement is caused mostly by the contractile force of the unexposed back surface.

As shown in Fig. 4, a second process occurs upon generation of sufficient number of trans isomers the front surface, the orientation of the trans-azobenzene molecules normal to the beam polarization as a result of repetitive trans-cis and cis-trans isomerization processes. This process is similar to our previous work, in that the Ar⁺ laser is used to reorient the azobenzene mesogens in the LCN network structure. Reorientation of these chromophores results in a surface contraction or surface expansion, depending on the relationship between the long axis of the cantilever and the polarization direction of the Ar⁺ source. In our previous work, this effect resulted in bidirectional bending of a cantilever by simple adjustment of the polarization of the Ar⁺ beam. In this work, the cantilever deflects towards the laser in both polarization directions examined (Fig. 1(c)). Cis-rich back skin of the azo LCN is the main source of the different photomechanical response observed in this work with respect to our earlier work [12]. As the effect of trans-cis-trans begins to take form, the contraction occurring again on the front surface overpowers the contraction on the back surface causing deflection towards the laser source.

From a wider perspective, this work demonstrates photomechanical effects can be generated in UV-rich environments such as space. This means that potential applications relevant in space, such as adaptive optics or photomechanical actuation, can leverage the photomechanical deformation described here. Furthermore, this effect is a new approach to generation of photomechanical work. One can imagine using the reversal in bending angle as a photomechanical trigger or gating device. Future work will explore the utility of UV-pretreatment with recently observed photodriven oscillations in monodomain azo-LCN [1,5] analogues. Additionally, the potential exists to use UV-pretreatment to sensitize these materials to red light, as recently demonstrated in azobenzene cholesteric liquid crystals [27–29].

4. Conclusion

This work examines the impact of UV-pretreatment of the surface structure of thin photosensitive cantilevers on their photomechanical response to laser light capable of driving trans-cis-trans reorientation. It is shown here that the response of these materials occurs in two parts, first bending 110° away from the laser source then bending towards the laser source to angles ranging from 12–76°. The initial polarization independent response of the cantilever to the Ar⁺ laser is caused by cis-trans isomerization at the exposed surface which removes the counterbalancing contractile force resulting in the as much as 110° bend away from the laser course. Thereafter, the Ar⁺ laser interacts with the exposed surface causing a distinct photomechanical effect that results in deformation of the cantilever towards the laser source. We show this second response to be polarization controlled, with light polarized parallel to the long axis of the cantilever causing 76° bending and light polarized orthogonal to the cantilever long axis causing 12° bending.