Effects of recycling on polystyrene shape memory polymers for in-situ resource utilization

One-way, thermoplastic shape memory polymers (SMPs) used as actuators for self-folding origami are typically single-use materials that would be decommissioned upon completion of mission objectives. As a result, there exists an abundance of unutilized, single-use SMP waste. In-situ resource utilization (ISRU) and recycling offer solutions for the use and integration of sustainable SMP material infrastructures on Earth and for long-duration space missions. Unfortunately, mechanical recycling causes degradation of material properties. Therefore, it is imperative to quantify the effects of recycling on SMP properties. Herein, we utilize a thermo-mechanical recycling method applied to polystyrene SMPs. After recycling, we conduct Fourier transform infrared spectroscopy, differential scanning calorimetry, and dynamic mechanical analysis to investigate changes to the chemical structure, viscoelastic properties, and shape recovery response of the polymer. The results indicate negligible changes to the viscoelastic and shape recovery properties of the recycled material from one to six recycling sequences (extrusions) when compared to the non-recycled material. The most evident form of deterioration occurred in the physical appearance of the material. Otherwise, the shape recovery performance and thermo-mechanical properties remained consistent after recycling. Therefore, the recovery characteristics (recovery ratio, recovery time, and actuation stress) do not change significantly after six recycling sequences, making this material viable for ISRU applications in space environments.


Introduction
Thermally activated shape memory polymers (SMPs) are able to change shape in response to external stimuli such as heat and, indirectly, light [1][2][3][4]. Advantages of SMPs include their low density, low cost, large recoverable deformation, and response to multiple stimuli [4][5][6]. With these benefits in mind, SMPs are highly valuable as actuators for transforming twodimensional sheets into three-dimensional (3D) structures in a process called self-folding origami [3,4]. Self-folding can be controlled through many different methods, making the versatility of SMPs even more evident. One method for self-folding origami involves delineating ink-patterned regions to localize absorption of energy from infrared light [3,7]. Gradients in the heating and shrinking of the material then induce self-folding. Complex 3D shapes can be realized by coupling this folding behavior with traditional origami principles [8]. Alternatively, self-folding can be induced through multistability [9], architected materials [10], cutting theories [11], bio-inspired architectures [12], granular jamming [13], and more [14]. With immense resilience when forming strong and complex structures autonomously, SMPs are regarded as highly valuable for space applications requiring in-situ deployment [5,15].
A major limitation of thermally activated one-way, thermoplastic SMPs is that the shape change is not reversible. In other words, removal of the stimulus does not undo the actuation. Thus, shape memory can only be activated once, thereby rendering the material inoperative outside of its intended function [2]. As a result, these SMPs are typically a source of single-use waste that are disposed of after actuation or decommissioning of the structure. However, SMPs show great potential for use during long-duration space missions as reprogrammable actuators and for other functions [2,5,6]. Rather than acting as waste, the material can be utilized through upcycling or recycling [16]. Upcycling and recycling enable the implementation of renewable and sustainable alternative material infrastructures [17][18][19][20]. The three most common recycling techniques are chemical recycling [21][22][23], energy recycling [24], and mechanical recycling [20,23]. Mechanical recycling is currently the most prevalent method for recycling [20], and involves a cyclical process of collecting, sorting, washing, grinding, and reprocessing of waste material. These steps may occur in various orders and numerous times depending on the material being recycled and the composition of the waste [23][24][25]. Application of this sustainable concept to SMPs, promotes in-situ resource utilization (ISRU) in space exploration by allowing the recycled SMPs to be used repeatedly within a closed-loop environment [26,27]. Unfortunately, mechanical recycling of polymers is known to progressively degrade the material, typically via thermal degradation, mecho-mechanical degradation, and depolymerization mechanisms [28,29]. As materials are continuously utilized and recycled, degradation accumulates leading to reduced performance [30,31]. Material deterioration due to mechanical recycling is expected to influence the thermo-mechanical properties of polymers, which are crucial for the shape memory effect. With enough repeated recycling sequences, the polymer would be rendered useless [30]. Therefore, changes to the thermo-mechanical properties of recycled SMPs must be investigated to determine the ability of the material to be reused as SMP actuators in ISRU applications. An alternative approach would be to use two-way SMPs [14,[32][33][34][35][36], but these specialized materials may not be readily available from waste sources in closed environments, such as space travel.
In this paper, we investigate the effects of recycling on the thermo-mechanical properties and shape memory performance of SMP sheets, as these are the most integral properties for SMP behavior. We recycle SMPs sourced from waste materials using a mechanical desktop polymer recycling process, similar to those adopted for generic polymer recycling in a limited, lab-scale environment. This small-scale process will simulate the conditions available during space travel. Excess polystyrene (PS), a common and easily acquired SMP, was mechanically processed into a recycled polymer resin. The resin was then formed into flat sheets through compression molding. The chemical structure of recycled samples was confirmed using Fourier transform infrared (FTIR) spectroscopy. The glass transition temperature (T g ) was determined using differential scanning calorimetry (DSC). The effects of the recycling process on the thermo-mechanical, i.e., viscoelastic, properties were evaluated using dynamic mechanical analysis (DMA). In addition, we quantified the effects of recycling on the shape recovery effect via free-recovery (zero-force) and constrained-recovery (zero-displacement) DMA experiments. These results aid in quantifying changes to the SMP properties due to recycling and establish limits for use of recycled SMP actuators. Further, these results will support the concept of ISRU for highly functional, recycled SMPs to promote sustainable aerospace practices and applications during long-duration space missions.

Methods
Herein, we describe our technical approach to recycling and characterization of SMPs. The research seeks to recycle discarded PS materials and evaluate the effects of successive recycling on the viscoelastic properties and shape memory performance of the material. Characterization is performed using FTIR spectroscopy, DSC, and DMA.

Materials
Recyclable SMPs were sustainably sourced from discarded PS compact disc (CD) cases. The material is an amorphous PS and is initially clear. Although many polymers exhibit shape memory, PS was selected for investigation due to multiple advantages. It is a widely used packaging material that is frequently discarded after a single use. This aligns with a central goal of the present work to sustainably source waste material and upcycle it for use as SMP actuators. In addition, PS has a high modulus of elasticity (compared to other polymers) at room temperature and a sufficiently high glass transition temperature to prevent accidental actuation. By utilizing a single material feedstock, we attempted to simulate an isolated environment with minimal availability of raw materials as one might experience in space. Results were interpreted to see if ISRU of SMPs is viable in a closed-loop environment to allow for a more sustainable material infrastructure.

Mechanical recycling
We implemented a laboratory-scale process to mechanically recycle the PS CD cases in a simulated space station environment. First, the material was mechanically broken down by hand into smaller pieces. The polymer pieces were then fed into a single-screw Filabot EX2 filament extruder preheated to 180 • C. This temperature is well above the glass transition temperature (T g ) of PS, does not cause noticeable thermal degradation, and allows for uniform extrusion of the material, as determined by an iterative process. At higher extrusion temperatures, degradation becomes more significant resulting in greater impacts on material characteristics. At lower extrusion temperatures, the material will not be broken down and reformed in a uniform manner, increasing the likelihood of surface imperfections that can potentially impact material behavior. The nozzle of the extruder was removed to facilitate a quick extrusion through an opening of 4.5 mm. The extruder screw had an outer diameter of 16 mm and a single speed of approximately 35 rpm, thus resulting in a consistent output advertised at over 0.45 kg h −1 . The limited controls of the extruder prevented optimization of the recycling process, which could be considered in future work. The recycling process produced a polymer resin that was then spiraled by hand to a diameter of approximately 82.5 mm, as seen in figure 1.
The polymer resin spiral was compression molded in a Wabash Genesis hydraulic press. The compression molding conditions (time, temperature, and force) were identified using an iterative process to minimize visible, physical imperfections. With experience, the hot press was set to 120 • C and allowed to heat up until the prescribed temperature was reached. Separately, a single polymer resin spiral was placed between two 305.0 × 305.0 mm aluminum plates. The aluminum plates were outfitted with two aluminum spacers of 305.0 × 25.0 × 2.5 mm to ensure the resulting sheets of recycled polymer had a uniform thickness. The polymer resin spiral and aluminum plates were then loaded into the hot press and pre-heated before compressive forces were applied to mold the material. Once the polymer visibly entered the viscous state through an observed softening and flattening of the material, a force of 2490 N was applied to the material. After 50 min, the material, encased in the aluminum plates, was removed from the hot press and allowed to cool in ambient air. Once the material reached approximately room temperature, it was removed from the plates and inspected for imperfections. Examples of potential imperfections, such as air bubbles and shearing, are represented in figure 2. By tailoring the conditions of the compression molding process, the presence of imperfections was minimized. In general, we observed that samples pressed for shorter times or at lower temperatures  tended to have more air bubbles. Higher compression force tended to cause visible shear bands. It is noted that the use of a vacuum press or other processing conditions could reduce these imperfections and processing time, but optimization of the recycling process to this degree is beyond the scope of this work.
After pressing into flat sheets, rectangular samples were cut from the material for characterization. After evaluating the recycled material, the samples were returned to the bulk recycled material, which was then mechanically broken down again and the recycling process, as seen in figure 3, was repeated. This process was used to produce material samples ranging from non-recycled samples to samples ranging from one to six recycling sequences (extrusions). After each recycling, the percent yield was approximately 75%-80% by weight. This yield presented a limit to the number of times the material could be recycled and was due in part to the small-scale of material processing occurring in the lab and the manual nature of the process. The recycling process developed could be improved and automated by adopting it at a larger, industrial scale and incorporating an automated filament-spooler device for the polymer resin spiraling.

Material characterization
Characterization of the recycled polymer utilizes FTIR spectroscopy (chemical structure), DSC (T g ), and DMA (shape memory performance).

FTIR spectroscopy.
Prior to assessing the degradation to vital material properties for SMP functioning, verification of the chemical structure of the material was carried out using a Nicolet 6700 FTIR spectrometer. The FTIR test was carried out in the spectrum of 400-4000 cm −1 wavenumbers, at 64 scans per spectrum and spectral resolution of 2 cm −1 .
The tests were carried out at room temperature, and an average of three scans were considered for each sample. By verifying that the recycled SMPs were not contaminated or modified during the recycling process, accurate comparisons can be made related to the changes in the viscoelastic and shape recovery properties.

DSC.
A TA Instruments DSC 25 was utilized to measure the T g of non-recycled and recycled PS samples. Samples between 7 and 10 mg were prepared in T-zero aluminum hermetic DSC pans. A heat-cool-heat test from 40 • C to 160 • C with a 5 • C min −1 ramp was performed for each sample. The TA Instruments Trios software was utilized to perform T g analysis via the midpoint method on the second heating cycle, following the first heating cycle, which removes the thermal history. DSC tests were repeated three times.

DMA.
The primary considerations of this work were changes to viscoelastic properties and shape recovery performance of the recycled, one-way SMPs. The SMP properties were characterized using a TA Instruments HR-20 Hybrid Rheometer/DMA with an environmental test chamber for temperature control. Viscoelastic properties were measured using the torsion test fixture. Samples were subjected to temperature sweep tests from 70 • C to 120 • C in 5 • C increments with a frequency range from 1.0 rad s −1 to 100.0 rad s −1 and 0.1% strain at each temperature. The temperature sweep test is preferred because the temperature is held constant at each temperature interval as the frequency is varied. As a result, the temperature is uniform in the sample as the values for storage and loss modulus equilibrate at each frequency. Temperature sweep tests were repeated at least three times and representative results from each set of tests are presented in this paper. The rectangular sample sizes for these tests were approximately 30.0 × 13.0 × 1.5 mm. Experimental data was shifted according to the time-temperature superposition principle using the TA Instruments Trios software to obtain the viscoelastic master curve at a reference temperature of 93 • C. This reference temperature was selected based on the range of T g determined by DSC using the method described in section 2.3.2 (91.63 • C ⩽ T g ⩽ 93.63 • C).

Shape memory effect characterization.
The shape memory effect results from a pre-straining, or programming, process applied to the SMP. Pre-straining removes any previous thermal and mechanical history associated with the initial polymer processing steps and imparts a new, temporary shape. We use a thin film tension fixture in the DMA to pre-strain recycled PS samples of approximately 30.0 × 3.5 × 2.0 mm. The thin film test fixture can be used to perform the axial prestraining and recovery procedures. A representative temperature and displacement history for the pre-strain sequence is shown in figure 4. The initial length of the sample between the fixture grips was 10 mm. Samples were pre-strained by heating to 110 • C, stretched by 4000.0 µm (40% strain) at a constant linear rate of 20 µm s −1 (strain rate of 0.002 s −1 ), and cooled to 90 • C at 5 • C min −1 . Upon release of the polymer, a residual strain, or pre-strain, is imparted onto the material [4,[37][38][39]. The stored pre-strain is influenced by several parameters and is significantly impacted by process temperature and strain rate. For example, pre-straining at a range of temperatures spanning T g , significantly impacts the efficiency of the shape recovery [40,41]. It is not the intent of the present work to optimize the pre-strain process.
The stored pre-strain can be recovered upon reheating. Recovery is initiated near or above T g , and because T g = 93 • C, as measured by DSC, we utilized a recovery Representative pre-strain process. The material initiates at an elevated temperature, which remains constant as the material is deformed. The sample is then cooled below Tg, and the displacement can then be released with the temporary shape retained. The blue curve represents temperature, and the green curve represents displacement. See online for color version.
temperature T r = 110 • C. This value of T r guarantees that the temperature during recovery remains consistently above T g . After pre-straining, samples were subjected to either a free-recovery (zero-force) or constrained-recovery (zerodisplacement) test in the DMA by applying the appropriate boundary conditions and reheating the material to 110 • C at a rate of 5 • C min −1 . The zero-force boundary condition for the free-recovery test involved a constant axial force of 0.0 N (±0.1 N), and the displacement of the crosshead was recorded as the sample shrinks toward its initial shape. The zero-displacement boundary conditions for the constrainedrecovery test involved maintaining a grip displacement of 4000 µm, and the contracting force of the sample was recorded throughout the test. It is also noted that due to the presence of a thermal history in the e = 0, non-recycled, samples, an annealing process was carried out prior to constrainedrecovery testing of just these samples. The temperature in both tests was held for the maximum time allowed by the equipment while tracking either force (constrained-recovery) or displacement (free-recovery). Shape recovery is significantly affected by temperature, where the higher the recovery temperature is above T g , the more rapidly recovery will occur [4,42]. It is not the intent of the present work to optimize the pre-straining process for the material, but rather to investigate the effects of recycling on shape recovery. Therefore, we only consider a recovery temperature of 110 • C.
The shape memory effect can be explained at the molecular level by the motion of polymer chains. By heating the material at the beginning of the pre-straining sequence, the polymer chains more readily change conformation, elongate under mechanical loading, and slide past each other. A mechanical pre-strain elongates these chains in the straining direction. After aligning the chains, the material is cooled below its T g , which reduces polymer chain mobility and prevents shape recovery. In the recovery step, the polymer is reheated, which increases chain mobility, and the chains tend to return toward their unstrained, lower energy state. In free-recovery, without an external constraint, the bulk polymer shrinks toward its original shape. Alternatively, if a fixed displacement boundary condition is applied, molecular motion of the chains during recovery generates an actuation stress that decays with time. Significant changes to the molecular weight or chemical structure would lead to changes in T g and affect the mechanical and shape memory performance of the polymer.

Results and discussion
Our presentation of results begins with characterization of the chemical structure of the polymer. Characterization of T g is reported next. We then characterize the viscoelastic properties, which significantly impact the shape recovery. Finally, we characterize the shape recovery and actuation stress of the SMP during actuation.

Physical appearance
Samples ranging from non-recycled material and recycled material between one and six recycling sequences were produced using the in-house, mechanical recycling process described earlier. Recycled samples were compared visually for signs of physical degradation. In this case, physical degradation was defined as changes to the physical appearance, i.e., color, texture, and opacity of samples. Figure 5 illustrates the changes observed to the physical appearance of the samples as seen in untested excess material after recycling. Degradation resulting from recycling is increasingly obvious when working with a transparent and colorless material like PS [43,44]. In fact, physical appearance was the most obvious sign of material deterioration for the up to six times recycled SMPs. With each successive recycling sequence, the material became increasingly cloudy and yellow. These changes in the physical appearance of the material signify that the material is breaking down at the atomic level, i.e., variations in length of polymer chains. Furthermore, the increasingly recycled samples tended to be more viscous during processing and thicker in dimension after cooling compared to the non-recycled material as determined by visible analysis. This observation agrees with published data revealing slight decreases in the melt flow rate of multi-recycled high impact PS (HIPS) with each recycling process, which is indicative of atomic level modifications to the polymer, i.e. chain scission phenomena and decreased molecular weight [45]. Further, previous studies have found a method to combat the viscosity increase in multi-recycled polymers. To reduce the melt flow rate and revive other desired characteristics, the material can be mixed with additional non-recycled material during processing [46]. With this information, methods could be adapted in small-scale laboratory environments to further control the shape memory properties of recycled polymers and SMPs.

FTIR spectroscopy
FTIR spectroscopy was used to confirm the chemical structure of the recycled SMPs, between non-recycled samples and samples from one to six recycling sequences. As seen in figure 6, the absorbance peaks for all SMP samples occur at roughly the same wavenumber and with similar intensity. This means that the structure of the PS samples remains largely unchanged and uncontaminated throughout the recycling process. The results confirm that the materials used were in fact PS due to the presence of characteristic absorbance peaks for PS. Specifically, the largest peak occurring at approximately 700 cm −1 for all samples is typical for PS which corresponds to out of plane C-H bending vibrations of aromatic rings [47]. Furthermore, the variation in absorbance peak intensity between the non-recycled sample and the recycled samples becomes more evident between 1620 and 1780 cm −1 , which is the carbonyl region of PS. This difference can be attributed to degradation from recycling [48]. Therefore, direct comparisons can accurately be made to determine the nature of the material degradation with increased recycling.

DSC
DSC was utilized to measure the T g of non-recycled and recycled PS samples. The midpoint method on the second heating run was used to calculate the T g of the samples. The second heating cycles for non-recycled samples and recycled samples are shown in figure 7. Average values and standard deviations of three experiments for each number of extrusions are presented in table 1. Overall, the T g remained consistently around 93 • C with increased recycling. In addition, one additional e = 1 recycled material was tested in the DSC to a temperature of 220 • C (not shown), and no other thermal transitions were observed. This further confirms that our materials was amorphous PS.

Viscoelastic properties
Viscoelastic master curves obtained from DMA results were compared for non-recycled samples and samples ranging from one to six recycling sequences. Representative results for a once recycled material are shown in figure 8, and the results for the other, iteratively recycled samples can be found in the Supplementary Material, figure S1 through figure S6. Figure 8 has been annotated to show a variety of parameters which were compared for each dataset. In the results, the storage modulus (G ′ , blue curve), loss modulus (G ′′ , green curve), and phase angle (tan δ, magenta curve) agree with expected trends for viscoelastic polymers [45,49]. At high frequencies, the material is in a glassy state and the viscoelastic behavior is dominated by the storage modulus. As the frequency is reduced, equivalent to increasing temperature or time, the storage modulus drops three orders of magnitude, which is associated with glass transition. At approximately 10 • Hz, the phase angle reaches a maximum, which is indicative of T g of the sample and corresponds to the highest damping of the  material. This transition is important to the actuation or selffolding response of SMPs. When analyzing the viscoelastic master curves, changes in the tan δ curve were analyzed specifically as this property is a ratio of the loss modulus to the storage modulus at a given frequency. Therefore, changes to both properties are accounted for in this singular analysis. Specifically, we assessed changes in the bandwidth of the peak with tan δ ⩾ 1, area under the tan δ curve, and magnitude and frequency of the tan δ peak. We also report the glassy storage modulus, G ∞ . A summary of the results for all six recycling sequences is shown in table 2. These results were averaged from three repetitions of the experiment. Plots illustrate a representative result from one experimental run.
By assessing the changes to the storage modulus, the loss modulus, and the tan δ of the increasingly recycled SMPs, we can evaluate changes to material properties due to recyclinginduced degradation. These changes reveal how the recycled material should perform as SMP actuators.
The bandwidth of the tan δ peak was considered as the range of frequencies at which the loss modulus exceeds the storage modulus. Behaviorally, this area is where energy dissipation is dominant for the material and where a shape change will occur for SMPs. This is due to the dynamic heterogeneity of chain relaxation that can suppress vibrations that the material may experience [50]. Observing all the viscoelastic master curves, this characteristic saw large amounts of variation, ranging from 0.041 to 0.49 rad s −1 , but the bandwidth was observed to increase and decrease indeterminately. This means that energy dissipation varies with recycling but in an unpredictable manner.
The area underneath the peak in tan δ is indicative of the damping abilities of the material. The larger the area under the curve, the more efficient the material is at damping experienced vibrations. Again, this glass transition region is where the viscoelastic behavior of polymers is most evident. Polymer chains are able to relax in dynamic manners that can more or less effectively damp experienced conditions. Comparing non-recycled samples and samples from one to six recycling sequences, the area under the tan δ curve fluctuated between values of 0.069 and 0.64. This value was lowest for the nonrecycled material. The fluctuation in values indicates that material damping does not change in a predictable way with increased recycling for the number of iterations considered. In fact, the recycled SMP samples illustrated the highest damping abilities after being recycled three times. Random chain relaxation patterns allowed the three times recycled material to dissipate energetic environmental impacts. Because the nature of this relaxation is complex and difficult to map, the changes in damping abilities between recycling sequences is best described as random or inconsequential variation due to minimal recycling degradation.
The magnitude of the tan δ peak reveals the peak damping effect. Observing the viscoelastic master curves of the recycled SMP samples, this peak often occurs near an average value of 4.21. However, the non-recycled material and the twice extruded material had a peak of 3.62 and 3.88, respectively. These two values may be the result of an accumulation of errors in the test procedure, the test fixture, and the test samples. However, it is concluded that the tan δ peak does not vary by large amounts between non-recycled samples and samples from one to six recycling sequences. Overall, the molecular behavior of the recycled SMP samples remains consistent as the optimal range in which chain relaxation results in dampened experienced vibrations occurs most commonly around material damping values of 4.21.
The frequency for the peak in tan δ is one measure of the glass transition temperature of the material, and for the viscoelastic master curves shifted to a reference temperature of T g , this should occur near a frequency of 1 rad s −1 . A consistent frequency for the peak in tan δ indicates that T g did not vary with recycling for the material. The frequencies at which the tan δ peak occurs varied around 23 rad s −1 . Since this value is approximately near 1 rad s −1 and remains fairly consistent between non-recycled and increasingly recycled samples, it is likely that the T g remains consistent. This result agrees with previously published data concerning T g values of recycled polymers [51] and unchanged T g values of recycled HIPS [45] as well as DSC results for determining T g .
Finally, the glassy modulus of the material ranges from 993.44 MPa to 2055.90 MPa, decreasing and increasing invariably with the number of recycling sequences. Molecular mobility in this region is limited, and a stiffer, more crystalline structure of the polymer allows for higher storage modulus values. The stiff polymer structure allows the material to retain its characteristics and chain conformation more readily. Degradation to the material at the molecular level would lead to a less stable glassy region and lower glassy modulus value. However, since these results indicate that there is no clear trend to the manner in which the glassy storage modulus of recycled SMPs are changing, minimal overall degradation is a likely conclusion. Published data on the characteristics of recycled HIPS supports this observation as the elastic modulus, which represents similar behavior as the storage modulus in the glassy plateau, generally decreases with subsequent recycling [52]. However, with the seemingly random variation, the modulus remains virtually the same throughout multiple reprocessing cycles [45]. Therefore, degradation is minimal and does not prevent recycled SMPs from functioning properly in aerospace applications. It is noted that the fluctuation of results may well be due to stochastic material variations and defects attributed with the test specimens.

Shape recovery properties
Shape recovery tests were utilized to determine the extent to which the material retains its shape memory properties after recycling. Herein, we considered free-recovery and constrained-recovery tests. These tests represent extremes of possible use cases. Both tests begin by pre-straining the material, which is performed as described in the Methods section. After cooling, a zero-force (free-recovery) or zerodisplacement (constrained-recovery) boundary condition is applied as the material is reheated to 110 • C at a rate of 5 • C min −1 . Three repetitions of each experiment were conducted, and the average values are reported. Plots illustrate representative results from one experimental run.

Free-recovery.
Free-recovery tests use zero-force conditions during the recovery process to provide a measure of the maximum shape recovery [53]. In this test procedure, the SMP samples were initially subjected to a pre-straining process so that shape recovery could occur. The material was then reheated and allowed to recover fully within the thin film test fixture of the DMA. In other words, the sample was free to recover its original shape, unimpeded by an external, axial force. The results, shown in figure 9, illustrate the recovery ratio (R r ) experienced during shape recovery when subjected to zero axial force. R r is described by where ϵ m is the maximum strain during pre-straining and ϵ p (t) is the instantaneous strain [38,39,54]. The shape fixity ratio (R f ) was also calculated using equation (2) to provide information on the fixation of the deformed shape where ϵ u is the strain of the unloaded sample [38,39,54].
where ϵ u is the unloaded length of the SMP. These R r and R f are crucial parameters for quantifying the performance of SMPs, wherein R r = 1 signifies full shape recovery of the SMP and R f = 1 signifies that all of the deformation during pre-strain is stored in the temporary shape. The changing value of the recovery ratios throughout the DMA tests reveal the extent of shape recovery in comparison to its maximum strained length. By observing the changes that occurred in the free-recovery results for non-recycled samples (e = 0) and samples ranging from one to six recycling sequences (e = 1 through e = 6), it is evident that recycling had a limited effect on the free-recovery behavior, i.e., there is no observable pattern to which the percentage of shape recovery decreased. As samples were increasingly recycled, the results indicate that the recycled material was able to recover approximately the same amount as the non-recycled material. In fact, from non-recycled samples to samples from one to six recycling sequences, there was a maximum decrease in shape recovery of approximately 8% for the e = 6 recycled material relative to the non-recycled material. According to published data, a novel, semi-aromatic polyamide was able to achieve an R r of 98%-99% throughout 20 consecutive shape changes [55]. With the acknowledgment that polyamides and PS should behave similarly as SMPs, it is clear that degradation to the shape memory capabilities of multi-recycled material is negligible [56]. Differences between our results can be attributed to the unique characteristics of the semiaromatic polyamide and that the polyamide is reprocessed by the solution or melt processing methods [55]. A summary of the key results for the free-recovery tests can be found in table 3. The shape fixity ratio (R f ) was also calculated for the constrained-recovery process using equation (2). Further, we have not sought to optimize the pre-straining conditions, which has led to lower maximum recovery ratios for all samples.

Constrained recovery.
Constrained-recovery tests use a fixed displacement condition during recovery to provide a measure of the maximum recovery stress generated by the material [40]. Herein, the fixed displacement boundary condition prevented shape recovery, and instead, an actuation stress was generated, which decayed as the polymer chains relaxed. This information reveals the effectiveness of the material shape recovery when subjected to an applied load [57]. After the pre-strain sequence, the axial force is set to 0 N ± 0.1 N, and then the sample is reheated to T r = 110 • C at a rate of 5 • C min −1 with a fixed displacement value. Results for constrained-recovery tests for all six recycling sequences are presented in figure 10. In these results, a compressive stress is generated during the first 50 s due to thermal expansion of the material. At this time, the change in stress reverses as the shape recovery process initiates. A maximum tensile stress is observed around 150-180 s for all samples which is approximately at a temperature between 102.5 • C-105 • C. At long times, the recovery stress approaches a value of just below 0.05 MPa. For a thermoplastic polymer, this would theoretically approach 0 MPa for infinitely long times. However, in our experiments, the equipment limited maximum duration for the recovery step is 3200 s. Further, the non-zero recovery stress at long times relates to physical entanglements in the polymer. Unrecycled CD cases were compression molded in the hot press in the same manner that all recycled samples were compression molded, as described in the Methods section. This compression molding step erased all prior thermal history of the unrecycled material and produced a material with a similar thermal history as the recycled samples. In figure 10, it is observed that the most significant change in recovery stress occurs between the fourth and fifth recycling sequence but there is minimal variation for all prior and subsequent recycling sequences. This is likely due to the unpredictable accumulation of degradation from repeated recycling sequences. From non-recycled samples to samples ranging from one to six recycling sequences, the changes to the maximum tensile stress occur in no particular pattern. The general concept of these results is supported by the stresses produced during constrained-displacement recovery tests conducted on polyurethane SMPs where the recovery stress produced decreases in an almost linear fashion with increasing temperature and time [53]. A summary of key results from the constrained-recovery tests is presented in table 4. In tables 3 and 4, it is observed that the ability of the material to recover, whether constrained or not, is minimally degraded by recycling as revealed through constrained-and free-recovery tests. This is consistent with the viscoelastic property results, which indicated minimal, indeterminant changes upon subsequent recycling. Further, this indicates that minimal damage occurs to the polymer chains, and the shape recovery performance is minimally affected by recycling. Therefore, the microscopic alterations to the polymer chain do not significantly affect the recovery behavior of the increasingly recycled material. Characteristic recovery and relaxation times were also analyzed for all free-and constrained-recovery tests revealing information about the speed of the recovery process. For free-recovery tests, characteristic recovery times varied between 29.47 and 50.69 s. Constrained-recovery characteristic time values ranged between 346.18 and 803.88 s. Five times recycled samples typically recovered the quickest while four times recycled samples on average recovered the slowest. We note that the unrecycled material recovered the quickest when constrained but the slowest when allowed to freely recover. There is no particular pattern in which these changes occur, so these results can be attributed to the random and minimal degradation that results from mechanical recycling.

Conclusion
In this paper, we investigated the effects of recycling on the thermo-mechanical properties and shape memory performance of recycled PS SMP sheets to develop a sustainable method for small-scale production of specialized materials. It is imperative to quantify the recycling induced degradation of shape memory properties. We implemented a desktop recycling process to extrude recycled PS polymer resin. This resin was then molded into uniform sheets through compression molding. By developing a laboratory scale, mechanical recycling process, waste material, such as unused PS, CD cases, can be recycled multiple times to form flat SMP sheets suitable for various aerospace applications, including as actuators for deployable structures.
It was demonstrated that recycled, one-way, thermoplastic SMPs sourced from limited, small-scale environments do not experience significant degradation as evidenced by negligible variation to the molecular structure and recovery characteristics (recovery amount, time, and stress) after six recycling sequences. This was confirmed through the use of FTIR, DSC, and DMA. Degradation occurred most obviously in the physical appearance of the material as discerned through visual observations. The material became increasingly cloudy and yellow with each successive recycling sequence. Slight changes did occur to the storage modulus, loss modulus, and shape recovery abilities of the material, but in an indeterminant manner. Therefore, recycled PS SMPs are likely to retain their shape memory and viscoelastic properties for continued use throughout the six tested recycling sequences. The material may be useful after additional recycling sequences, but the processes would need to be scaled to ensure sufficient material by the final recycling.

Data availability statement
All data that support the findings of this study are included within the article (and any supplementary files).