Bio-inspired reversible electro-thermo-hygro reversible shape-changing materials by 4D printing

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Presentation of FFF process for pure thermoplastics and current limitations

Among the many 3D printing technologies, the most common method for printing polymers is Fused Deposition Modelling (FDM), also known as Fused Filament Fabrication (FFF), a trademarked material extrusion process patented by Stratasys in 1992 [6], [7]. In FFF, the thermoplastic filament is melted through a heated nozzle and the viscous polymer is deposited layer-by-layer onto a build plate where layers are fused together and, once solidified, create the final part [8]. This process is almost fully automatized, making this technology suitable for mass production. One of the main interest of FFF technology is its ability to precisely control the microstructure of the printed part and to locally control its properties (porosity, density and orientation) [9]. However, in order to shift FFF from the prototype stage to end products, a number of improvements are required [10]. These improvements are: a greater variety of available polymeric materials, a process improvement for better dimensional reliability, improvement in surface finish and finally, enhanced mechanical properties. As described in several articles, the mechanical performance of 3D printed parts is lower than their conventionally manufactured counterparts [11]–[13]. Masood et al. [12] observed that PolyCarbonate (PC) printed with FFF process exhibits tensile strength in the range of 70-90 % compared to injection molded PC parts . Similarly, Ahn et al. [11] studied Acrylonitrile Butadiene Styrene (ABS) and found that the tensile strength of FFF printed samples was 26 % lower than that of injection moulded samples.
Different strategies were investigated in the literature to improve the mechanical performance of FFF printed parts. The first one is the modification of the formulation of printable filaments through the addition of reinforcing fibers. This point will be discussed further in the section 3 of this chapter. The second strategy is to optimize printing and slicing parameters.
In 3D printed parts by FFF, the lower mechanical properties are mainly due to insufficient bonding strength between printed beads within and between printed layers, leading to poor interlayer strength as well as poor interbead cohesion and the creation of porosity. The adhesion between the printed layers is governed by the interdiffusion of polymer chains between the printed layers [14]. The formation of a bond between the printed layers and the adjacent beads in printed parts is similar to the process of welding polymer interfaces where similar physical mechanisms (i.e. reptation) are involved [15]. The formation of the bond interface is schematized in Figure 4 where the cross-section of the bead is idealized as circles.

Parametric studies on 3D printed parts for the improvement of mechanical properties

In the literature, numerous studies have focused on the influence of printing parameters on the mechanical properties of 3D printed thermoplastic parts, especially with regard to tensile tests. Popescu et al. [17] divided the parameters of the FFF process into two categories :
Printing parameters – extruder temperature, bed/plate temperature and environment temperature, flow rate, deposition speed.
Slicing parameters – raster angle, layer height, raster width, interfilament distance, infill percentage, infill pattern.
All these parameters are linked to the quality of bead bonding described above, which determines the mechanical performance of the parts printed by FFF [18]. Figure 5 describes the main printing and material parameters that govern the quality of the parts as well as the sintering of the beads during 3D printing process. The viscosity and surface tension of the material dictate the flow characteristics in 3D printing, which is also driven by the temperature conditions. For example, Zhao et al. stated that rheological behavior plays a significant role in the 3D printing process as low viscosities at high shear rates are required for easy low energy extrusion, but high zero-shear viscosities are also required for the extruded to retain its shape once being out of the nozzle and deposited [20]. The heat capacity and conductivity of the 3D printed polymer affect the temperature profile of the printed beads [19], [21]. The categories of slicing and printing parameters are discussed extensively in the following sections.

Influence of printing parameters

During the printing process, different heat exchanges occur, by convection with the environment and by conduction between adjacent beads, the printer bed and the nozzle that leads to various and complex thermal state [22]. The temperature and cooling conditions therefore influence the level of crystallinity of the polymer, the bonding of the beads and thus the mechanical properties [8]. For example, Aliheidari et al. [23] studied ABS and showed through Double Cantilever Beam tests, that higher extrusion temperature leads to a higher delamination resistance and therefore a better interlayer adhesion. Consequently, increasing the nozzle temperature enhances the mechanical performance of the part, but too high a nozzle temperature can lead to material degradation and deformation of the structure, leading to dimensional inaccuracy [24]. During cooling, shrinking and residual stresses develop in the printed layer because of the temperature gradient within the part, which may cause delamination or warping [25]. Sun et al. [10] studied the temperature profile of a printed ABS bead as a function of the number of printed layers (15 and 30 layers). On this purpose, the authors placed a thermocouple on the heated plate to record the temperature profile of the first layer. Thanks to this method, they studied the influence of nozzle temperature and environment temperature on the temperature profile. The results are shown in Figure 6.
The authors showed that the temperature of the first layer rises periodically with each additional layer printed and then decreases rapidly as the nozzle move away from the thermocouple. The study of the influence of the process parameters (Figure 6.b, c) shows a minimum temperature (lower limit) increase, which means that the first layer undergoes the thermal influence of the other printed layers. Moreover, the nozzle temperature seems to have less of an influence on the temperature profile than the environment profile. Indeed, the authors did not measure a significant effect of increasing nozzle temperature, whereas a 20 °C increase in environment temperature (50 °C to 70 °C) leads to an increase in the minimal temperature (average of the lowest temperature reached at each deposition pass) from 73 °C to 87 °C.

3D printing of fiber reinforced composite materials

The addition of reinforcing fibers in printable filaments follows two strategies. The first one is the addition of discontinuous fibers to improve mechanical performance, reduce warping during printing and increase dimensional stability [14]. The second is the addition of continuous fibers used to greatly improve mechanical performance. In the following section, both strategies and the respective studies are presented and discussed.

3D printing of discontinuous fiber-reinforced composites

To the best of our knowledge, the first literature research which aimed at reinforcing FFF printed parts with discontinuous fibers was conducted by Zhong et al. in 2001 [44]. In this research, the authors aimed to develop a printable filament made of ABS reinforced with short glass fibers using a twin-screw extruder. The authors observed an improvement of mechanical performance with the addition of glass fibers (+140 % between pure ABS feedstock and ABS reinforced with 11.4 % vol. of short-glass fibers) but also a strain reduction. Strain was improved thanks to addition of plasticizer and compatibilizer. Following this research, several teams have worked on the addition of fibrous reinforcement such as carbon nanotubes [45], [46], graphene [47], [48] or carbon/glass short fibers [31], [49], [50].
Actually, 3D printing of discontinuous fiber-reinforced polymer composites shows several advantages compared to neat thermoplastic 3D printed parts. First, mechanical performances are improved by the addition of short fibers. Ning et al. [49] showed that the addition of 4.5 % vol. of short carbon fibers improved the tensile strength (+24 %) and the modulus (+27 %) compared to neat ABS. Love et al. [51] showed that the addition of 8 % vol. of short carbon fibers to ABS leads to an increase in tensile modulus (+630 %) and tensile strength (+144 %). Jiang et al. [52] investigated the addition of carbon fibers to three different matrices (ABS, polylactic acid (PLA) and polyethylene glycol (PETG)). The authors investigated three fibers fraction, i.e. 10.5 % vol.; 9.4 % vol. and 14.3 % vol. for ABS, PLA and PETG; respectively. The authors measured a strong increase in tensile strength (+211 % for ABS-based composites, +164 % for PLA and +314 % for PETG) and tensile strength (+33 % for ABS-based composites, +14 % for PLA and +48 % for PLA). Carneiro et al. [53] showed that addition of glass fibers to polypropylene (PP) leads to an increase of 30 % and 40 % for the modulus and the ultimate tensile strength, respectively. These results are displayed in Figure 9. Second, the introduction of fibers leads to a lower coefficient of thermal expansion which reduces the thermal strain gradient during 3D printing and therefore limits warping during the process [51], [54]. Finally, short fibers made it possible to achieve greater geometric precision and subsequently improve the surface appearance of the printed structures [51]. Greater dimensional stability allows to develop new fields of application such the Big Area Additive Manufacturing (BAAM) [55], [56] where large scale structures are printed like a boat at the university of Maine [57].

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Table of contents :

Glossary
General introduction
Chapter 1: State of the art in 3D/4D printing of continuous fiberreinforced (bio)composites: achievements and future outlooks .
1. General presentation of 3D printing and Fused Filament Fabrication
1.1. Principle of 3D Printing
1.2. Glossary and definitions of 3D printing
2. Overview of current research on 3D printing of thermoplastic materials
with Fused Filament Fabrication (FFF)
2.1. Presentation of FFF process for pure thermoplastics and current limitations
2.2. Parametric studies on 3D printed parts for the improvement of mechanical properties
2.2.1. Influence of printing parameters
2.2.2. Influence of slicing parameters
3. 3D printing of fiber reinforced composite materials
3.1. 3D printing of discontinuous fiber-reinforced composites
3.2. 3D printing of continuous fiber reinforced composites
3.2.1. Overview of the mechanical characterization of 3D printed continuous fiber reinforced composites
3.2.2. Current limitations in 3D printing of continuous fiber composites
3.2.3. Future outlook on the optimization of 3D printing of continuous fiber reinforced composites
4. 4D Printing of synthetic materials
4.1. 4D printing of polymer and composite materials
4.2. Humidity as a motor of actuation
4.3. Future outlook on the optimization of 4D printing of fiber reinforced composites
5. 3D and 4D printing of natural fiber reinforced composites
5.1. Natural fiber specificities for 3D printing
5.2. Filament production
5.3. 3D printing of natural fiber composites
5.4. Future outlook on the optimization of 3D printing of natural fiber reinforced composites
5.5. 4D printing of natural fiber reinforced composites
5.6. Future outlook on the optimization of 4D printing hygromorphic (bio)composites
6. Bibliography summary
Chapter 2: Materials and Methods
1. Material selection
1.1. Synthetic continuous fiber reinforced composites
1.2. Flax continuous fiber reinforced biopolymer composites
2. Manufacturing by 3D printing
2.1. Synthetic continuous fiber composites
2.2. Printing of continuous flax fiber reinforced composites
3. Mechanical characterization
3.1. Tensile tests on single fiber and composites
3.1.1. Tensile tests on synthetic fiber
3.1.2. Tensile tests on synthetic-fiber reinforced composites
3.1.3. Tensile tests on flax-fiber reinforced composites
3.2. Characterization of interlaminar properties of synthetic continuous fiber composites
3.3. Three-point bending characterization
4. Analysis of the microstructure
4.1. Optical microscopy
4.2. Scanning Electron Microscopy (SEM)
5. Hygroscopic conditions and seawater aging
5.1. Specimen storage in various humidity chambers
5.2. Seawater-aging of 3D printed composites
5.3. Moisture sorption measurement
5.4. Hygro-expansion measurement
6. Measurement of the actuation of hygromorphic composites based on continuous carbon fiber and continuous flax fibers, and electro-thermohygromorphic composites based on continuous carbon fiber.
6.1. Description of the environmental conditions for actuation
6.2. Electrical stimulus generation
6.3. Measurement of the actuator curvature
7. Thermal characterization of the composites
7.1. Differential Scanning Calorimetry (DSC)
7.2. Thermogravimetric analysis (TGA)
7.3. Thermal expansion measurement
Chapter 3: Mechanical and hygromechanical properties of continuous carbon and glass fiber composites
1. Microstructure description
1.1. Filament microstructure description
1.2. Cross-section description
1.3. In-plane description
2. Mechanical characterization
2.1. Longitudinal and transverse characterization
2.2. Evaluation of interlaminar properties with Double Cantilever Beam (DCB) tests.
2.2.1. Mode I delamination resistance behavior
2.2.2. Comparison of critical strain energy release rates with literature values
3. Evolution of mechanical performances under exposure to wet environment and accelerated seawater aging
3.1. Sorption behavior and hygroscopic expansion
3.1.1. Moisture uptake and hygroscopic expansion in humid environment
3.1.2. Evolution of water uptake and hygroscopic expansion during seawater aging
3.2. Evolution of tensile behavior under different moisture conditions
3.3. Influence of seawater aging on flexural properties of 3D printed composites
Conclusion
Chapter 4: Bio-inspired reversible electro-thermo-hygro reversible shape-changing materials by 4D printing
1. Concept of 4D printed electro-thermo-hygromorphic structural
composite materials
2. cCF/PA6-I:PA6 bilayers as hygromorphic actuators
3. cCF/PA6-I:PA6 bilayers as electro-thermo-hygromorphic actuators
4. Influence of the print pattern on the electro-thermo-hygro actuation of the cCF/PA6-I:PA6 bilayers
4.1. Influence of the printed pattern on the electro-heating
4.2. Influence of the printed pattern on the electro-thermo-hygromorphic actuation
Conclusion
Chapter 5: Tailoring the mechanical properties of 3D printed continuous flax/PLA biocomposites with slicing parameters 
1. Production of continuous filaments of flax/PLA biocomposites and description of the microstructure
1.1. Production of continuous filament of flax/PLA biocomposites by co-extrusion process
1.2. Microstructure of the continuous filament of flax/PLA biocomposites prior to printing
2. Influence of Layer Height (LH) on the mechanical properties of printed cFF/PLA composites
3. Influence of Trip Number (TN) and Interfilament Distance (ID) on the mechanical properties of printed cFF/PLA composites
4. Influence of Layer Number (LN) on the mechanical properties of printed cFF/PLA composites
5. Influence of raster angle / fiber orientation on the mechanical properties of printed cFF/PLA composites
Conclusion
Chapter 6: 4D printing of hygromorphic biocomposites with continuous flax fiber/PLA: toward compliant mechanism
1. Design of hygromorphic biocomposites
2. Material selection
3. Control of the stiffness with variation of the architecture by controlling the interfilament distance
4. Compliant mechanism with localized bending actuation
Conclusion
General conclusion
Perspectives
References

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