Influence of the porosity on the photoresponse of a liquid crystal elastomer

Azobenzene containing liquid crystal elastomers (LCEs) are among of the most prominent photoresponsive polymers due to their fast and reversible response to different light stimuli. To bring new functions into the present framework, novel modifications in bulk material morphology are required. Therefore, we produced azobenzene LCE free-standing films with different porosities. While the porosity provided macroscopic morphological changes, at the same time, it induced modifications in alignment of liquid crystal azobenzene units in the films. We found that a high porosity increased the photoresponse of the LCE in terms of bending angle with high significance. Moreover, the porous LCE films showed similar bending forces to those of pore-free LCE films.


Reagents / Reactants
If not noted otherwise, all reagents were used as received.

Reagent Supplier Purity
Acroylchloride Alfa Aesar Inc. 99% Ultra high resolution mass spectra were recorded on a JEOL ACCUTOF GCV JMS-T100GCVat 70 eV ionisation energy. IR spectra were recorded on a Perkin Elmer Paragon 1000 FT-IR spectrometer with a A531-G Golden-Gate-ATR-unit. Dynamic scanning calorimetry was performed on a Perkin Elmer Pyris with a temperature ramp of 10 K/minute. All melting points were recorded on an electrothermal melting point apparatus LG 1586 and are uncorrected.

Synthetic and Purification Equipment
Polymerizations were performed on a Linkam LTS-420 heating stage in between PTFE coated glass slides (see description below). Thin layer chromatography (TLC) was performed with pre-coated TLC-sheets from Macherey-Nagel GmbH & Co. KG with silica 60 and fluorescent indicator UV254. Column chromatography was performed with silica gel 60M from Macherey-Nagel GmbH & Co. KG with a size of 0.040 -0.063 mm.
The porosities of the samples were calculated with the image analysis software ImageJ 1.47v (NIH, USA). The colours of the image were separated into blue, green and red channels. The blue channel was used in the colour threshold analysis of the software, which allowed the calculation of the area of the pores of the apparent surface area of the film ( Figure S1). The ratio of the calculated area to the total area was then used to estimate the porosity of the film.

Bending and Force Measurements
In the bending and force measurements, a UV LED source with λ = 365 nm (Hoenle LED UV Pen 2.0, approx. 5 W), and a homemade LED visible light source (approx. 3 W) with λ = 455nm, were used for illumination. For the photoactuation, the free standing film samples were fixed with tweezers while the lateral side of the films was illuminated with the light source.
The different light intensities were obtained by varying the distances of sample to the UV light source.
The distances to the film substrate were 1.2 cm, 1.5 cm and 2 cm. At a distance of 1.2 cm, the light was in focus and light intensity was 7.5 W/cm 2 at the center of the Gaussian beam (beam diameter= ̴ 4 mm). At 1.5 cm and 2 cm, the light intensities were 80% (6 W/cm 2 ) and 40% (3 W/cm 2 ) at the center of light beam, respectively i .The c d 1mm Figure S2 shows the measurement of forces during photoisomerisation of LCE film with 67% porosity with a size 5 x 2 x 0.1 mm. To measure forces exerted by the azobenzene LCE films during photoisomerisation, a tube-shaped glass cantilever with a diameter of 20 µm was used as force probe. The glass cantilever was marked with paint to obtain a point of reference to measure the deflection of the cantilever during the experiments. In order to calibrate the glass cantilever, a highly precise ultra micro balance (MT-SICS-UMX2, Mettler Toledo GmbH, Greifensee, Switzerland) and a motor driven micromanipulator (DC3001R with controller MS314, World Precision Instruments Inc., Sarasota, FL, USA) were used. The glass cantilever was driven by the micromanipulator onto a vertically mounted razor blade on the ultra micro balance. For each 10 µm distance driven, the weight was recorded and converted to the corresponding forces. The LCE films were brought into contact with the cantilever, so that the reference point was visible. Using a monochrome high-speed digital  were dissolved in dichloromethane. The solvent was evaporated and the solid mixture was introduced into a homemade LC cell on a heating stage (see description below). The mixture was melted at 100 °C. Then, the temperature was decreased to 91 °C over the course of 120 min. This temperature was held for 12 h (temperature stability < 0.1 °C). During the polymerization process, the gas evolution due to the decomposition of the initiator 3 produced bubbles in the polymer film. For the extraction of the polymer film from the LC cell, it was introduced in a bath of chloroform. Unreacted monomers dissolved, while the polymer film delaminated from the substrate. The resulting polymer film was extracted by soxleth extraction with chloroform for 10 h. The film was dried in a vacuum oven at 50 °C at a pressure of 10 mbar for 24 h.

Glass Cell and Polymerization Methodology
a) Fabrication of the glass cell: Supplementary Figure S3. A glass slide was heated on a magnetic stirrer to approx. 250 °C. A bulk PTFE stick was rubbed with a speed of approx. 7 cm / 30s over the surface of the glass slide. The method used is consistent with the previous work to produce homeotropic alignments.
ii b) Using the glass slides as polymerization reactors for the LCE films.
Supplementary Figure S4. A PTFE coated glass slide was placed on the heating stage. The mixture of monomers and initiator (see experimental procedure) was placed on the glass slide (a).The mixture was covered with another glass PTFE coated glass slide (b) and subsequently heated to 100 °C (at this temperature, the mixture gives an isotropic melt) (c). The stage was closed and cooled down to 94 °C over the course of 60 min. (d) The temperature was hold for 18 h while the mixture is polymerizing.

Thermal Recovery
The thermal back-isomerisation from cis to trans was completed in 24 h which is consistent with original characterisation of by Ikeda and co-workers since we used similar compounds and polymerization technique iii b a c d

Syntheses
The analysis of all compounds is consistent with the original characterization by Ikeda and co-workers. iv

4-(9-Hydroxynonanyloxy)nitrobenzene (1)
The procedure by Ikeda and coworkers was performed under Schlenk conditions in an atmosphere of dry nitrogen and modified as follows: Sodium hydride (8.96 mmol, 215 mg) was suspended in a solution of 4-nitrophenol (8.96 mmol, 1.25 g) in DMF (50,0 mL).The reaction mixture was cooled to 0 °C and a solution of 9-bromo-1-nonanol (8.96 mmol, 2.00 g) in DMF (10 mL) was added via syringe over the course of 5 min. The reaction mixture was heated at 120 °C for 90 h. After cooling to 20 °C, the residue was filtered. The solvent was evaporated in vacuo.
The crude product was purified by column chromatography (

4-Hydroxy-4'-(9-hydroxynonanyloxy)azobenzene (3)
The original synthesis by Ikeda and co-workers was modified as follows: A solution of 4-(9hydroxynonanyloxy)aniline (2) (8.46 g, 33.7 mmol) in hydrochloric acid (80.0 mL, 6 M) was cooled to -8 °C. A solution of sodium nitrite (2.51 g, 36.4 mmol) in water (25 ml) was cooled to 0 °C and added dropwise to the reaction mixture over the course of 15 min. A solution of phenol (3.17 g, 33.7 mmol) in an aqueous sodium hydroxide solution (100 mL, 0.75 M), cooled to -5 °C was added dropwise over the course of 90 min. The temperature of the reaction mixture was held at -5 °C for 30 min without stirring while the product precipitates.
The precipitate was filtered and washed with water (3x 100 mL). The product was recrystallized from ethanol. A brown solid was obtained in a yield of 89 % (10.7 g) (Lit.: 59%).