2.1 Materials
Polylactic acid (PLA, Ingeo 3251, NatureWorks LLC) and vinylacetate-vinylversatate-ethylene (Vac-VV-E, Vinnex LL 2505, Wacker Chemie AG) were used as received. The acrylonitrile-butadiene-styrene (ABS) filament (ABS black, Material 4 Print GmbH) has a mean diameter of 3 mm. As determined by microscopic image analysis microcrystalline cellulose (MCC, Alfa Aesar GmbH & Co. KG) exhibit an average particle size of 27 μm, whereas Arbocel type F 140 K (AC, Rettenmaier & Söhne GmbH & Co. KG) have an average fiber length of 62 μm and were used as received.
2.2 Filament production
The filament production was carried out using a co-rotating twin-screw microcompounder (HAAKE MiniLab II, Thermo Scientific) equipped with a custom-made nozzle with a diameter of 1.5 mm. Filament extrusion was carried out at 190 °C at a screw speed of 50 rpm. The average residence time of the fiber-matrix mixture in the extruder was about 30 seconds. Commercial black ABS was used as a passive layer.
2.3 Fused Filament Fabrication (FFF)
Prior to the fabrication of the bilayers a digital model was created by using AutoCAD 2016 (Autodesk GmbH, Germany), figure 1 a. The model consisted of a passive layer of two perpendicularly oriented extrudate arrays of ABS, and of an active layer of PLA-cellulose compound. The bilayer model had the dimensions of 50 mm x 50 mm x 0.75 mm, with an overall thickness of the passive layer and the active layer of 0.5 mm and 0.25 mm, respectively. For the exact determination of the actuation the protrusion of the passive layer was removed. Models created in AutoCAD 2016 further processed with the AXON2 program (3D systems Inc.). A 3D-printer operating according to the FFF principle (3D Touch, 3D systems Inc.) was used to print the bilayers. The diameter of the nozzle opening was 0.25 mm. ABS and the active layer containing the PLA-cellulose composite were successively formed at 260 °C and 210 C, respectively. Each bilayer formulation was printed and characterized in duplicate
2.4 Theoretical model
According to equations (1) and (2), the determination of the elastic moduli of the individual layers Ea and Ep and the hygrometric coefficients of linear expansion αa and αp is needed. The elastic moduli of the individual layers were determined according to DIN EN 527 on six printed tensile bars (figure 1b) using a universal testing machine (smarTens, Karg Industrietechnik, Krailling, Germany). The results are given as the averages of six identically fabricated and tested specimens. The hygrometric coefficients of linear expansion were determined after water-immersion of printed specimens (100 mm x 2 mm x 0.5 mm) at room temperature for 1, 2, 3, 20 h and measuring the dimensional change using a digital caliper with an accuracy of 0.02 mm.
2.5 Determination of the actuation
The actuation (i.e. bending of the bilayer upon change in humidity) was followed by placing printed bilayers into a home-made climate chamber and subsequent video analysis. The humidity was established by placing dishes filled with water or different saturated salt solutions into the chamber. Humidity and temperature were measured constantly by a hygrometer 6100 (Electronic Temperature Instruments Ltd, Easting Close, UK). A tripod-mounted camera (Canon EOS 550D, Canon Germany GmbH, Krefeld, Germany) was connected to a PC. Images were taken at time intervals of 120 s with a total experimental time of 7 h (3.5 h for deflection and 3.5 h for provision). Each sample was determined in duplicate, and the results are given as averages. The camera operated with following settings: shutter speed (Tv) 1/25, aperture (Av) 12, sensitivity of the image sensor ISO 400. Subsequently, the resulting 210 pictures were converted to an .avi video file with 30 frames per second. Finally, the videos were evaluated using the video analysis software Tracker (Douglas Brown, www.opensourcephysics.org), figure 2b.
2.6 Apparent cellulose orientation
2.6.1 Cellulose orientation close to the surface
A Rigaku MniFlex 600 (Rigaku Corporation, Tokyo, Japan) X-ray diffractometer was used for the measurement. The sample was turned in steps of 10° after each measurement in order to determine a possible orientation of the fibres. An irradiation angle 2θ of 5° to 40° was investigated. The step size was 0.1° at a speed of 10° per min at a voltage of 40kV and a current of 15mA.
2.6.2 Cross-sectional cellulose orientation
In order to investigate the orientation of individual fibers throughout the entire active layer, the samples were subsequently analyzed by micro computed tomography (μ-CT). The μ-CT measurement was carried out at the Fraunhofer Institute for Integrated Circuits IIS (Application Center CT in measurement technology (CTMT), Deggendorf). The scans were performed with a TomoScope HV 500 (Werth Messtechnik GmbH, Gießen Germany) tomograph. The parameters used for the measurements were: current 80 A, voltage 180 kV, 1600 steps for a 360° rotation and a total measurement time of 33 min. The resolution of the images was dependent on the measurement and ranged from 8 to 12.5 microns. Scans were conducted on AC bilayer specimens with fiber contents of 25% and 50% to investigate a possible re-orientation of the fibers by the printing process and a 10 cm long piece of filament with 50% Arbocel to examine the distribution of the fibers in the filament over the cross section prior to the printing process. Furthermore, a cylindrical shaped printed specimen was investigated, to compare the results with XRD measurements.