Polymer film with optically controlled form and actuation

A low power laser beam is used to induce large and fast variations in the shape of a polymer film due to photoinduced contraction and expansion of the polymer film surface subject to the beam. The direction of the photoinduced bend or twist of the polymer can be reversed by changing the polarization of the beam. Thus the film orientation could be varied within +/-70 masculine. The phenomenon is a result of optically induced reorientation of azobenzene moieties in the polymer network.


ABSTRACT
Polymer networks containing azobenzene liquid-crystalline (azo LC) moieties are capable of changing their macroscopic shape when influenced by light. Two distinct processes take place in azo LCs due to the photoisomerization of the azobenzene chromophores. Trans-cis isomerization and thus a reduced order parameter is dominant at UV wavelengths whereas trans-cis-trans isomerization processes are dominant at visible wavelengths resulting in orientation of the molecules perpendicular to the beam polarization. Polymer networks containing azobenzene liquid-crystalline (azo LC) moieties are capable of changing their macroscopic shape when influenced by light [1][2][3][4]. Two distinct processes take place in azo LCs due to the photoisomerization of the azobenzene chromophores. Trans-cis isomerization and thus a reduced order parameter is dominant at UV wavelengths whereas trans-Gis-trans isomerization processes are dominant at visible wavelengths resulting in orientation of the molecules perpendicular to the beam polarization [5].

SUBJECT
Bending of LC network films containing azobenzene chromophore was first demonstrated for UV light [1]. Bending of the film (towards the radiation source only), heated to 85°C, was induced by radiation of 1= 366 nm wavelength. The initial shape could be restored with visible radiation (1 = 540 nm) within several seconds. Two orders of magnitude faster . photomechanical response was observed in a LC elastomer doped with an azo chromophore using argon-ion laser radiation [2] where a bending angle of~67°(towards radiation source) in a highly absorbing 320 JLI11-thick material was achieved with 1.3 W power. We recently reported the laser-induced photomechanical actuation of azo LC polymer films characterized by: 1) single wavelength operation; 2) reversible bi-directional bending (70°< a < 70°); 3) high speed of photoinduced deformation (1700/s); and 4) room temperature operation. These results were obtained for thin samples (10-50 JLI11) controlled with low power density radiation (~0.1 W/cm2) [3].
Two monomers, 4,4'-Di(6-acryloxyalkyloxy)azobenzene and the LC monomer 4-(6acryloxy)hexyloxy-4'-ethoxyazobenzene were copolymerized according to [1]. Polymerization was performed between two glass substrates (10-50 Inn) coated with poly(vinyl alcohol) and rubbed to create an easy-axis for the azo LC moieties. Rectangular slices of the polymer (removed from the substrates), a lower edge fixed to a platform, were exposed to a linearly polarized beam of a multimode cw argon-ion laser, expanded to the film size. The polymer film bends away or towards the laser for polarization perpendicular or parallel to the easy-axis, respectively. Figure 1 shows the magnitude of the bend angle for both polarizations as a function of power density and the deformation dynamics. Films could be reversibly bent towards and away from the incoming laser by switching the beam polarization. The complete oscillation for the extremes of bend angles (70°< a < 70°) was accomplished in~1.3 s.
The polarization dependence of the deformation sign results from the optically-induced realignment of LC chromophores which shrinks the volume of the polymer along the polarization direction and expands it along the direction perpendicular to it. Due to light attenuation caused by absorption and scattering, this effect is more efficient at the input surface of the incident beam causing the bending.
In summary, we report on the large and fast photomechanical actuation of a LC polymer film, operational at room temperature with bi-directionality of the mechanical response using a low power, single wavelength laser beam. This demonstration opens up interesting practical opportunities for controlling light beams, for adaptive optics, nonlinear optics, and lays the groundwork for enabling components for the next generation of Micro-Opto-Mechanical Systems.