# The Shape Memory Properties and Actuation Performances of 4D Printing Poly (Ether-Ether-Ketone)

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## Abstract

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## 1. Introduction

_{g}) limits the functionality in harsh environment. There are still some works that remain in creating truly portable or non-contacting SMPs actuators that can match the performances and functions of traditional mechanical structures.

_{g}is 143 °C [16], which is higher than the typical temperature of the Low Earth Orbit, going from −100 °C to 120 °C [17]. Namely, the shape memory effect of PEEK can work in the Low Earth Orbit. Moreover, the unique high-temperature resistance and radiation-resistant properties [18,19] endow PEEK with a more extensive prospect than other SMPs as an aerospace material. Existing research for PEEK mainly aims at enhancing its mechanical properties [20,21,22,23], such as chemical modification [24] and fiber-reinforced integration [25,26]. Though the shape memory effect of PEEK was characterized in a previous study under small strain with tensile mode [27], bending rather than tension is the main actuation method in the applications of SMPs [28,29,30]. Therefore, the shape memory effect and actuated capacities in bending deformation with large strain are considered noteworthy properties for PEEK.

_{f}-temperatures of 120 °C, M

_{s}being below 100 °C [39], and are not suitable for aerospace as mentioned before. Therefore, SMPs play an important role in smart actuators all the time. On the other hand, the stimuli required for shape memory deformation are difficult to provide. Most of SMPs are thermo-responsive and need a high-temperature environment [40,41], which limits the test methods. Four-dimensional printing combined 3D printing with SMPs, bringing a new dimension, time, for 3D printed constructs [42]. Four-dimensional printing induced diversely designed structures for SMPs to gain further application by being responsive to stimuli [43]. We combine materials and stimuli by 4D composite printing in our preview study [44] and replace the indirect heating (hot air or oil) with a contact source embedded in SMPs.

## 2. Materials and Method

#### 2.1. Materials and 4D Printing Method

_{20}Ni

_{80}wire (Juanhui Technology Co., Ltd., Hangzhou, China) with a diameter of 0.05 mm as the continuous wire because of its steady electrothermal effect, which can offer a thermal stimulus for shape memory effect. The properties of the Cr

_{20}Ni

_{80}wire were described in Table S1. The continuous conductive path could be customized by the distribution density or position for the required temperature gradient or deformation areas. Finally, the continuous conductive path was covered with layers of the PEEK base the same as before. These composite samples combined the material and stimulus, enabling the shape memory PEEK to transform and recover anywhere without a thermal container. More details and the principle of 4D printing PEEK composites were explained in our previous study [44]. The printed sandwich structure composite shown in Figure 1c consisted of PEEK bases with five layers at the bottom and top, respectively, and a one-layer wire path in the middle. The pattern of the Cr

_{20}Ni

_{80}wire embedded in the PEEK base was printed with a printing speed of 150 mm/min and a printing height of 0.1 mm. The continuous wire was distributed into the “S” path, and adjacent wires were spaced 2 mm apart. The effective heating area was regarded as 10 mm × 4 mm.

#### 2.2. Design and Experiments

_{F}, which was the angle between the horizontal line and the free side of the sample. Thus, we could calculate the shape fixed ratio R

_{f}according to Equation (1). Then the temporary shape was heated up again and recovered to a permanent shape with a recovery angle θ

_{R}. We could get the shape recovery ratio R

_{f}from Equation (2). These two significant indexes characterized the shape memory effect of the PEEK. The larger fixed ratio meant the more accurate control for temporary shape, and the larger recovery ratio meant the more outstanding capability to recover the original shape.

## 3. Result and Discussion

#### 3.1. The Cycle Shape Memory Properties and Actuation Performances of PEEK

#### 3.2. The Effect of Temperature on Shape Memory Properties and Actuation Performances

^{2}= 0.999, and shown as Equation (3). Additionally, the maximum angular speed increased with current, shown in Figure 3d. When the current increased from 0.21 A to 0.23 A, the recovery ratio increased by 14% (from 50.5% to 57.9%), and the maximum speed increased by 10% (from 2.38 °/s to 2.63 °/s). The relationship between heating temperature and recovery speed (V) was analyzed by linear regression analysis with R

^{2}= 0.997, and shown as Equation (4).

#### 3.3. The Effect of Cooling Speed on Shape Memory Properties and Actuation Performances

_{g}, the long chains in distorted areas were stretched. The elongated chains would spontaneously restore to the tangled state at the high temperature due to the entropic elasticity. However, because of the viscidity, this elastic recovery was a time-dependent process with time series t. Assuming that the initial state of the relaxation process corresponded to t

_{0}and the final state corresponded to t

_{n}, while there were intermediate states corresponding to t

_{1}and t

_{2}. When the sample was cooled down by the forced air, namely short cooling time, the sample recovered to t

_{1}due to the elasticity. However, when the sample was cooled down naturally, namely long cooling time, there was sufficient time for the sample to recover to t

_{2}. Thus, the samples with different cooling strategies would freeze different strain energy. A small cooling speed would offer SMPs more time to evolve towards the equilibrium state, which shifts the onset of the recovery process to a higher temperature [46]. In conclusion, a small cooling speed induced a slower shape recovery behavior and weaker actuated capacity during heating at the given condition. Furthermore, we conducted the one-way ANOVA for shape memory properties and actuation performances under different cooling speed. The F-value and p-value were shown in Table S3. For recovery ratio and actuated speed, there were no significant difference between groups. However, the fixed force and actuated force of forced air cooling were significantly stronger when compared to natural cooling.

#### 3.4. The Effect of Idling Time on Shape Memory Properties and Actuation Performances

#### 3.5. The Effect of Structure Parameters on Shape Memory Properties and Actuation Performances

_{f})was analyzed by linear regression analysis with R

^{2}= 0.794, and shown as Equation (5). When the thickness of samples increased from 1.2 mm to 1.6 mm, the recovery force increased by 117%. The relationship between thickness and recovery force was analyzed by linear regression analysis with R

^{2}= 0.856, and shown as Equation (6). The actuated performances of PEEK were improved effectively by increasing the thickness. We conducted the one-way ANOVA for different groups, and the F-value and p-value were shown in Table S6. All shape memory properties and actuation performances had significant difference when the thickness changed.

#### 3.6. Weight Transport and Deployable Structure of Shape Memory PEEK

## 4. Conclusions

## Supplementary Materials

_{20}Ni

_{80}wire; Table S1: The properties of Cr

_{20}Ni

_{80}wire; Table S2: The one-way ANOVA for repeated measures of shape memory properties and actuation performances under different heating temperatures; Table S3: The one-way ANOVA for repeated measures of shape memory properties and actuation performances under different cooling speeds; Table S4: The one-way ANOVA for repeated measures of shape memory properties and actuation performances under different idling times; Table S5: The one-way ANOVA for repeated measures of shape memory properties and actuation performances under different actuated lengths; Table S6: The one-way ANOVA for repeated measures of shape memory properties and actuation performances under different thicknesses; Video S1: The measurement of the force; Video S2: Weight Transport by PEEK; Video S3: Deployable structure of PEEK.

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

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**Figure 1.**Four-dimensional printing and test method of shape memory PEEK. (

**a**) The equipment and construction of 4D printing. (

**b**) The process of 4D printing PEEK composites. (

**c**) The printed composite sample and sizes. (

**d**) The measurement method and definition of angle. θ

_{F}was the fixed angle and θ

_{R}was the recovered angle between the horizontal line and the free side of the sample. (

**e**) The test method and devices of the recovery force.

**Figure 2.**The cycle shape memory properties and actuation performances of PEEK. (

**a**) The variation in angle in cycle shape recovery process. (

**b**) The angular speed in real-time for four cycles. (

**c**) The shape fixed ratio and shape recovery ratio in cycle shape memory behavior. (

**d**) The fixed force in cycle shape fixed process. (

**e**) The recovery force in real-time. F

_{s}was the steady force and F

_{m}was the maximum force. (

**f**) The recovery force in the cycle shape recovery process.

**Figure 3.**The effect of currents on shape memory properties and actuation performances. (

**a**) The relationship between the surface temperature of samples and currents of wire. (

**b**) The variation in angle for different currents in the recovery process. (

**c**) The angular speed for different currents. (

**d**) The shape memory properties for different currents. (

**e**) The fixed force for different currents. (

**f**) The recovery force for different currents.

**Figure 4.**The effect of cooling speeds on shape memory properties and actuation performances. (

**a**) The variation in angle for different cooling speeds. (

**b**) The angular speed for different cooling speeds. (

**c**) The maximum speed for different cooling speeds in the recovery process. (

**d**) The recovery force for different cooling speeds. (

**e**) The maximum recovery force for different cooling speeds. (

**f**) The schematic diagram of effect for different cooling speeds.

**Figure 5.**The effect of idling time on shape memory properties and actuation performances. (

**a**) The variation in angle for the different idling time. (

**b**) The shape memory properties for different idling time. (

**c**) The angular speed for the different idling time. (

**d**) The recovery force for different idling time.

**Figure 6.**The effect of actuated length on shape memory properties and actuation performances. (

**a**) The sketch of different actuated lengths in bending deformation. (

**b**) The thermal imaging and optical photograph of sample with actuated length 18 mm. (

**c**) The variation in angle for different actuated lengths. (

**d**) The shape memory properties for different actuated lengths. (

**e**) The angular speed for different actuated lengths. (

**f**) The recovery force for different actuated lengths.

**Figure 7.**The effect of thickness on shape memory properties and actuation performances. (

**a**) The angular recovery for different thicknesses. (

**b**) The shape memory properties for different thicknesses. (

**c**) The angular speed for different thicknesses. (

**d**) The maximum speed for different thicknesses (

**e**) The fixed force for different thicknesses. (

**f**) The recovery force for different thicknesses.

**Figure 8.**Weight transport by shape memory PEEK. (

**a**) The programming of actuated PEEK sample. (

**b**) The experiment process of weight transport.

**Figure 9.**The deployable structure of shape memory PEEK. (

**a**) An experimental simulation of deployed drag sail. (

**b**) A mimetic catcher with deployed joints. (

**c**) The expanding film capturing and loading the stuff.

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**MDPI and ACS Style**

Zhou, Y.; Ren, L.; Zang, J.; Zhang, Z. The Shape Memory Properties and Actuation Performances of 4D Printing Poly (Ether-Ether-Ketone). *Polymers* **2022**, *14*, 3800.
https://doi.org/10.3390/polym14183800

**AMA Style**

Zhou Y, Ren L, Zang J, Zhang Z. The Shape Memory Properties and Actuation Performances of 4D Printing Poly (Ether-Ether-Ketone). *Polymers*. 2022; 14(18):3800.
https://doi.org/10.3390/polym14183800

**Chicago/Turabian Style**

Zhou, Yuting, Luquan Ren, Jianfeng Zang, and Zhihui Zhang. 2022. "The Shape Memory Properties and Actuation Performances of 4D Printing Poly (Ether-Ether-Ketone)" *Polymers* 14, no. 18: 3800.
https://doi.org/10.3390/polym14183800