Photomechanical bending mechanics of polydomain azobenzene liquid crystal polymer network films

Abstract

Glassy, polydomain azobenzene liquid crystal polymer networks (azo-LCNs) have been synthesized, characterized, and modeled to understand composition dependence on large amplitude, bidirectional bending, and twisting deformation upon irradiation with linearly polarized blue-green (440–514 nm) light. These materials exhibit interesting properties for adaptive structure applications in which the shape of the photoresponsive material can be rapidly reconfigured with light. The basis for the photomechanical output observed in these materials is absorption of actinic light by azobenzene, which upon photoisomerization dictates an internal stress within the local polymer network. The photoinduced evolution of the underlying liquid crystal microstructure is manifested as macroscopic deformation of the glassy polymer film. Accordingly, this work examines the polarization-controlled bidirectional bending of highly concentrated azo-LCN materials and correlates the macroscopic output (observed as bending) to measured blocked stresses upon irradiation with blue-green light of varying polarization. The resulting photomechanical output is highly dependent on the concentration of crosslinked azobenzene mesogens employed in the formulation. Experiments that quantify photomechanical bending and photogenerated stress are compared to a large deformation photomechanical shell model to quantify the effect of polarized light interactions with the material during static and dynamic polarized light induced deformation. The model comparisons illustrate differences in internal photostrain and deformation rates as a function of composition and external mechanical constraints.

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“…Azo-OPDI-25 was synthesized from OPDA (0.6204 g, 2.000 mmol), APB (0.3654 g, 1.250 mmol), tris(azobenzene-amine) crosslinker (0.4460 g, 0.500 mmol), and DMAc (10 mL) using the same procedure as that used for azo-PMDI- 25 . This procedure was followed to prepare a subset of polymers referred to as azo-OPDI-xx where xx refers to the concentration of the tris(azobenzenene-amine) crosslinker in mol%.…”

“…The strain gradient through the sample thickness is dictated by the strong attenuation of light due to the absorbance of azobenzene (or other materials) that localizes the strain on the exposed surface [5,21e23]. As visualized experimentally and described computationally [24,25], the directionality of the strain dictates the magnitude of the bend or twist. If the strain is contractile along the length of the cantilever (long axis), the cantilever will deflect towards the light source.…”

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“…2 The polarization dependence is antisymmetric with respect to 45 • . This shows that photostrain can be reversed by changing polarization.…”

“…The average strain of the polymer in the x, y, and xy (shear) direction is governed bȳ ε ps xx = −ε ps cos(2θ) ε ps yy =ε ps cos(2θ) (2) ε ps xy =ε ps sin(2θ), respectively, where θ is the light polarization angle that ranges from 0 • to 90 • . 2 The polarization dependence is antisymmetric with respect to 45 • .…”

“…2 The molecular structure of azo-LCNs is illustrated in Figure 2. Light causes the molecular structure to kink and this is known as the cis-state (Figure 2(b)).…”