Fast, Light-Responsive, Metal-Like Polymer Actuators Generating High Stresses at Low Strain

Abstract

Producing lightweight polymeric actuators able to generate high stresses typical of hard metals and/or ceramics remains challenging. The photo-mechanical responses of ultra-drawn ultrahigh molecular weight polyethylene (UHMWPE) actuators containing azobenzene photo-switches with symmetrically attached polyethylene (PE) side chains are reported. Long PE side chains promote dispersion within the apolar UHMWPE matrix, and the ultra-drawn films are highly aligned. The ultra-drawn azobenzenedoped UHMWPE films have high Young's moduli ($100 GPa) and are viscoelastic at room temperature at strains below 1%. The photo-mechanical response of the films is fast (<1 s), showing a high specific actuation stress response (>6 3 10 4 Pa (kg m À3 ) À1 ) to UV or visible light at a low strain ($0.06%). The actuator responds to rotating linearly polarized light, causing a photoinduced stress wave response. Such rapid, high-stress, low-strain, photo-mechanical responses are unique in soft polymer systems with physical values approaching hard metals/ceramics.

Stimuli-Responsive Shape Changing Commodity Polymer Composites and Bilayers

“… 11 Among the diverse stimuli exploited as triggers in actuators, we consider illumination as most favorable, as it is area selective, remotely controllable, and capable of initiating fast shape changes with possible additional selectivity based on dependence on light polarization, if desired. 72 Furthermore, as stimuli are often interconnected, 87 i.e. surrounding humidity is coupled with temperature, illumination can directly impact these conditions photothermally.…”

“…The addition of the stimuli-responsive molecules to these packages endows the final product with responsive parts shielded by the robust, encapsulating commodity polymer. An example of such a robust, anisotropic UHMWPE actuator is given in ref ( 72 ). This composite design contrasts bilayer-type actuators in which hydrogels, azobenzene-containing coatings, or LCN coatings are repeatedly exposed to daily life conditions, requiring top-coats to protect the relatively fragile materials.…”

“…In a similar way, azobenzene was embedded in ultrahigh molecular-weight polyethylene (UHMWPE), this time using direct casting. 72 Compatibility of the azobenzene with the apolar UHMWPE matrix was increased by attaching PE (C78) side chains to the azobenzene core. The as-cast composites were solid-state drawn to draw ratio 60, yielding high Young’s modulus (∼100 GPa) films with a high degree of chromophore alignment.…”

Transparent semi‐crystalline polymeric materials and their nanocomposites: A review

“…[142] Mechanical response of a 0.06 mol% C78-AZO-doped stretched UHMWPE film, C, utilizing a 365 nm LED source or, D, under increasing intensity of a 365 nm LED source. [83] E, Photo-thermal response observed upon UV exposure (0.9 A) of drawn 5 wt% BZT/UHMWPE film and, F, infrared images of 1 wt% BZT/UHMWPE film during a UV exposure cycle (10 seconds intervals) [84] [Color figure can be viewed at wileyonlinelibrary.com] fibrillary structures on the surface and in the bulk of the films after ultra-drawing will induce light scattering, resulting in a poor transparency in the visible light regime. [70,77] Thus, resultant opaque oriented polymer films have limited applicability in fields like built environment, impact resistant visors, screens, and automotive glazing where both high transparency and high mechanical properties are needed.…”

“…[150] F, Isomerization of spiropyran (SP) and merocyanine (MC) and photographs of drawn UHMWPE tapes containing SP before and after UV radiation, and, G, absorption spectra and corresponding pictures of drawn UHMWPE tapes containing SP irradiation with UV light at different radiation time [151] [Color figure can be viewed at wileyonlinelibrary.com] UHMWPE or HDPE films incorporating BZT can also lead to light-responsive polymer actuators with rapid response and exceptionally high actuation stress. [83][84][85] In case of a broad MWD (> 20), the drawn HDPE films revealed a high transparency (~90%) even without the incorporation of any additives at low draw ratios (~10) as shown in Figure 3B. [79] This is because here the low molecular-weight fraction could function as an additive with a relatively high refractive index that fills the voids.…”