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. SEM Images of the cellulosic fillers were taken on an Zeiss DSM 940 A at 15 kV. The samples were gold sputter coated beforehand.
Filament production
The filament production was carried out using a co-rotating twin-screw microcompounder (HAAKE MiniLab II, Thermo Scientific). The microcompounder used for the fabrication of filaments was equipped with a custom-made nozzle with a diameter of 1.5 mm to compensate for the die swelling at the nozzle outlet, which is caused by entropic effects. A constant extrusion rate was maintained by setting the screw speed to 50 rpm and the uptake with a bobbin facilitated the compensation of pulsation. Filaments with a diameter of 3 mm ± 0.1 mm were produced, which was a prerequisite for reliable filament feeding throughout the printing process. The average residence time of the fiber-matrix mixture in the extruder was about 30 s. Commercial black ABS was used as a passive layer.
Fused filament fabrication
Prior to the fabrication of the bilayers a digital model was created by using AutoCAD 2016 (Autodesk GmbH, Germany), Fig. 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 × 50 mm × 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.
Theoretical model
According to eqs. (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 (Fig. 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 × 2 mm × 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.
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), Fig. 2b.
Apparent cellulose orientation
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 40 kV and a current of 15 mA. The intensity distribution is measured by obtaining the intensity at a fixed Bragg angle or 2θ as a function of the angle relative to the fibre axis direction (azimuthal angle). The used method of fibre alignment is the full width of the (0,0,2) cellulosic plane azimuthal diffraction measured at one half of the maximium intensity. This measurement is usually designated as Z and given in degree. The measure represents the spread of the majority of the graphene planes. This value is the full spread of misalignment and should be thougt of as a cone angle because the alignment is in three dimensions. The value should be halved if one wishes to think of how far from the fibre axis the plans are misaligned [22, 23]. Thus, this method can give informations on the fiber orientation on the surface (limited by f.e. the excitation depth of the incident radiation) of the sample.
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 μm. 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.