# Development and Assessment of a 4D Printing Technique for Space Applications
## 2 Materials and Methods  
### Materials: PEEK  
The SMP used in this work is polyetheretherketon (PEEK). PEEK is a semi crystalline polymer [13; 14] of a material class, which is often referred to as high performance or engineering polymers [15]. These materials are called engineering polymers as they exhibit exceptional mechanical and often also chemical properties, which allows them to be used in demanding applications that were traditionally reserved for metal, ceramic, or composite materials [16; 17]. PEEK, for example, has a relatively high ultimate tensile strength of around 90 MPa [13; 15] with a pronounced elastic range and it is resistant to most chemicals and radiation [15; 16; 17]. PEEK is gaining more and more attention in lightweight engineering due to its high strength-to-density ratio, which is comparable to that of steel [17]. In this context, PEEK has also already been tested on the International Space Station (ISS) for potential applications in space [18] and was deemed valid for use in spacecrafts due to its high radiation resistance [18].  
Historically, PEEK parts were mainly produced through methods like injection molding or CNC milling. However, there is a growing trend towards using 3D printing, specifically fused filament fabrication (FFF), for manufacturing PEEK components. This is significant because PEEK is a high-cost material. FFF's appeal, especially when compared with CNC milling, lies in its material usage approach. FFF precisely deposits the material where needed, as opposed to milling, which subtracts the material. This offers the potential for substantial cost savings when working with PEEK.  
Concerning its shape memory characteristics, PEEK has already been researched by scientists because of its heightened mechanical and chemical properties compared with traditional SMPs [17]. The shape memory properties of PEEK can be traced to its semi crystalline nature, where the crystalline phase acts as the permanent network and the amorphous regions act as the thermally activated switching or transition segments [17; 19]. The \(T_{\text{trans}}\) for PEEK is traditionally the \(T_{g}\), which is around 150 \({}^{\circ}\)C. However, it is worth noting that the observed shape memory properties of PEEK have been relatively small [17].  
In the scope of this study, PEEK was chosen as the SMP due to its prior testing on ISS and NASA's approval for deployment in spaceflight missions [18]. Given that the primary aims of this research is to create hinges suitable for space environments, the selection of a material capable of addressing the specific challenges inherent in such conditions is crucial. The specific PEEK filament material used in this work for the 4D printing procedure was kindly provided by Evonik Industries AG (www.corporate.evonik.com, (accessed on 20 September 2023)) and is called \"Evonik Infnam PEEK 9359 F\". This PEEK filament was specifically developed for industrial and aerospace applications; the material characteristics from the data sheet can be seen in Table 1.  
Aside from PEEK, PLA was also used in the scope of this work. PLA is a biopolymer that is widely used in the 3D printing scene, especially for manufacturing prototypes.  
\begin{table}
\begin{tabular}{l l} \hline \hline
**Description** & **Value** \\ \hline Tensile Modulus & 3600 MPa \\ \hline Yield stress & 90 MPa \\ \hline Yield strain & 5\% \\ \hline Melting temperature & 340 \({}^{\circ}\)C \\ \hline Glass transition temperature & 152 \({}^{\circ}\)C \\ \hline Density & 1300 kg m\({}^{-3}\) \\ \hline Filament Diameter & 1.75 mm \\ \hline \hline \end{tabular}
\end{table}
Table 1: Material characteristics of Evonik Infnam PEEK 9359 F filament, data from the datasheet provided by Evonik [14].  
In this work, the material was only used for preliminary investigations and for form fitting purposes. For further information on the material, refer to [20; 21].