The opto-mechanical properties of a photo-responsive nematic polymer network (PRPN) are investigated using molecular dynamics simulation. For the implementation of the trans-to-cis isomerization of azo compounds, a switchable potential formalism for the N = N bond is applied to the crosslinked PRPN unit cell model. During the light switch-on and heating-up simulations at a wide range of temperatures, the scalar orientational order parameter for the mesogenic side group molecules, the effective photo-induced strain of the bulk polymer network, and the opto-mechanical properties are characterized. The correlation between the microstate which belongs to the molecular location and the macroscopically observed photostrain is identified according to the isomerization ratio of the diazene groups.

A liquid crystal network whose chromophores are functionalized by photochromic dye exhibits light-induced mechanical behaviour. As a result, the micro-scaled thermotropic traits of the network and the macroscopic phase behaviour are both influenced as light alternates the shape of the dyes. In this paper, we present an analysis of this photomechanical behaviour based on the proposed multiscale framework, which incorporates the molecular details of microstate evolution into a continuum-based understanding. The effects of trans-to-cis photoisomerization driven by actinic light irradiation are first examined using molecular dynamics simulations, and are compared against the predictions of the classical dilution model; this reveals certain characteristics of mesogenic interaction upon isomerization, followed by changes in the polymeric structure. We then upscale the thermotropic phase-related information with the aid of a nonlinear finite element analysis; macroscopic deflection with respect to the wide ranges of temperature and actinic light intensity are thereby examined, which reveals that the classical model underestimates the true deformation. This work therefore provides measures for analysing photomechanics in general by bridging the gap between the micro- and macro-scales.

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As a polymeric system incorporating rigid molecules within its structure, the liquid-crystal network (LCN) has been envisaged as a novel heterogeneous material. Under the influence of external stimuli, the orientational order of the liquid-crystalline phase becomes dilute and overall anisotropy is hence decreased; the actinic light absorbed by photochromic molecules, for example, induces the geometric isomerization and subsequently yields internal stress within the local network. In this study we investigate light- and temperature-induced spontaneous deformations of the LCN structure via a three-dimensional finite element model that incorporates geometric nonlinearity with a photomechanical constitutive model. We first examine the bending behavior and its nonlinearity and then parametrically study the various behaviors that stem from different origins ranging from the microscale to the macroscale: (i) the geometry of the LCN film, (ii) the macroscopic global order, (iii) the distorted mesogenic orientation due to the Fredericks distortion, and (iv) defect-induced instability. These interrelated behaviors demonstrate both the simulation capability and the necessity of the presenting framework. By employing a nonlinear consideration along with a microscopic shape parameter r the present approach facilitates further understanding of photomechanical physics such as the deconvolution of various stimuli and the deformed shape obtained due to snap-through instability. Furthermore, this study may offer insight into the design of light-sensitive actuation systems by deepening our knowledge and providing an efficient measure.

We investigated the optical and thermal actuation behavior of densely cross-linked photoresponsive polymer (PRP) and polymer nanocomposites containing gold nanoparticles (PRP/Au) using all-atom molecular dynamics (MD) simulations. The modeled molecular structures contain a large number of photoreactive mesogens with linear orientation. Flexible side chains are interconnected through covalent bonds under periodic boundary conditions. A switchable dihedral potential was applied on a diazene moiety to describe the photochemical trans-to-cis isomerization. To quantify the photoinduced molecular reorientation and its effect on the macroscopic actuation of the neat PRP and PRP/Au materials, we characterized the photostrain and other material properties including elastic stiffness and thermal stability according to the photoisomerization ratio of the reactive groups. We particularly examined the effect of nanoparticle size on the photothermal actuation by varying the diameter of the nanofiller (10-20 Å) under the same volume fraction of 1.62%. The results indicated that the insertion of the gold nanoparticles enlarges the photostrain of the material while enhancing its mechanical stiffness and thermal stability. When the diameter of the nanoparticle reaches a size similar to or smaller than the length of the mesogen, the interfacial energy between the nanofiller and the surrounding polymer matrix does not significantly affect the alignment of the mesogens, but rather the adsorption energy at the interface generates a stable interphase layer. Hence, these improvements were more effective as the size of the gold nanoparticle decreased. The present findings suggest a wider analysis of the nanofiller-reinforced PRP composites and could be a guide for the mechanical design of the PRP actuator system.

The photomechanical behavior of azobenzene-functionalized liquid-crystalline polymers (azo-LCPs) is closely related to UV light propagation. Here, we report the effect of light absorption by the LC host and the concerted isomerism of azobenzene on this property. In situ measurements of light absorptivity and computer simulations revealed that the light propagation of a liquid-crystalline polymer is affected by the UV absorption of the LC host under the same concerted isomerism of azobenzene. The lower UV light absorption of the LC host results in deeper UV light propagation and a sharper isomerization profile, which eventually induces faster bending when compared to the higher UV light absorption of the LC host. On the other hand, increasing the azobenzene concentration in the polymer greatly decreases the speed of light propagation, which is regarded as one of the factors inhibiting further increases in the bending speed of azo-LCP films.

We compare the actuation speeds of the azobenzene-doped nematic liquid crystal polymer (azo-LCP) by examining its activation mechanisms: photoisomerization and trans−cis−trans reorientation (TCTR) (also known as Weigert effect). Both experiments and modeling are carried out in this study to compare the speed of bending and contraction acting on azo-LCP. We identify that the TCTR-based photobending of azo-LCP generated less than half the photostrain in a unit cell than that in the photoisomerization case but involves a larger azo-LCP thickness in triggering photodeformation. Because photodeformation occurs in a deeper region than that in the photoisomerization case, the TCTR-based photobending of azo-LCP exhibits a faster bending speed than the photoisomerization-based photobending of azo-LCP, even though less strain is generated in the TCTR case.