Shape-Memory Effect of 4D-Printed Gamma-Irradiated Low-Density Polyethylene

Abstract

Four-dimensional-printed smart materials have a wide range of applications in areas such as biomedicine, aerospace, and soft robotics. Among 3D printing technologies, fused deposition molding (FDM) is economical, simple, and apply to thermoplastics. Cross-linked polyethylene (XLPE) forms a stable chemical cross-linking structure and shows good shape-memory properties, but the sample is not soluble or fusible, which makes it hard to be applied in FDM printing. Therefore, in this work, a new idea of printing followed by irradiation was developed to prepare 4D-printed XLPE. First, low-density polyethylene (LDPE) was used to print the products using FDM technology and then cross-linked by gamma irradiation was used. The printing parameters were optimized, and the gel content, mechanical properties, and shape-memory behaviors were characterized. After gamma irradiation, the samples showed no new peak in FTIR spectra. And the samples exhibited good shape-memory capabilities. Increasing the irradiation dose increased the cross-linking degree and tensile strength and improved the shape-memory properties. However, it also decreased the elongation at break, and it did not affect the crystallization or melting behaviors of LDPE. With 120 kGy of irradiation, the shape recovery and fixity ratios (R_r and R_f) of the samples were 97.69% and 98.65%, respectively. After eight cycles, R_r and R_f remained at 96.30% and 97.76%, respectively, indicating excellent shape-memory performance.

1. Introduction

Three-dimensional (3D) printing is a manufacturing technology that enables the production of complicated parts in small batches. It obtains a 3D product by building layers of materials in different 2D shapes [1]. This additive manufacturing technology offers a flexible design and is inexpensive in terms of raw material costs. However, 3D printing has been hindered by limited material choices. The prospect of more 3D printing applications relies heavily on the manufacturing of novel materials. As research in 3D-printed smart materials increased, the new concept of 4D printing was introduced in 2013 by Skylar Tibbits, a researcher at the MIT Department of Architecture [2,3,4]. Contrary to static, single, and limited 3D-printed pieces, 4D printing adds a temporal dimension such that the structure and properties of the printed pieces change over time in response to external stimuli [5,6]. Fused deposition modeling (FDM) is one of the most popular 4D printing processes, wherein printing materials are melted and stacked layer by layer based on the printing route, thereby resulting in a finished product [7]. It has a small equipment size and impact on the environment, and it is easy to use [8].

Over the past two decades, shape-memory materials (SMMs) such as shape-memory polymers (SMPs), shape-memory polymer composites (SMPCs), shape-memory alloys (SMAs), shape-memory ceramics (SMCs), and shape-memory hydrogels (SMHs) have been widely developed and researched as smart materials that can reversibly change their structure and properties in response to external stimuli (heat, light, pH, and electricity) [9]. SMPs are widely applied in fields such as biomedicine [10,11,12,13,14,15], aerospace [16], soft robotics [17,18,19], and smart textiles [20,21] owing to their lightweight, low cost, high elastic deformation, ease of processing, and biocompatibility [22]. In addition, the majority of SMPs are heat-responsive, and heat sources are widely available. Therefore, these have been the focus of many recent investigations. The shape-memory change process in SMPs is achieved by the interaction of network points and switching phases, which provide shape fixation and reversible shape recovery, respectively [23]. As the temperature increases to the transition temperature (T_trans), such as the glass transition temperature (T_g) or melting temperature (T_m), the switching phase softens. Then, owing to low stress, the sample can be deformed. By maintaining constant stress, the sample is cooled rapidly below T_trans, wherein the stress is removed, and the sample is fixed in a temporary shape. When the temperature increases above T_trans, the sample returns to its original shape.

Currently, most studies on 4D-printed SMP focus on thermoplastic polymers, such as polylactic acid (PLA), poly (ε-caprolactone) (PCL), and thermoplastic polyurethane (TPU). Aroea et al. [24] prepared 4D-printed PLA/graphite composites and found that the shape fixity ratio (R_f) and shape recovery (R_r) of the samples with 1 wt% graphite addition could reach 98.18% and 94.44%, respectively. Wang et al. [25] prepared 3D-printed PCL/TPU shape-memory samples when the ratio of PCL to TPU was 7:3. Here, the composite achieved the best shape-memory effect, with a stable R_f and a R_r of 100% and 81%, respectively. However, because thermoplastic SMPs are physically cross-linked, and the bonding between molecular chains is not sufficiently stable, the shape of the printed sample is hardly restored. Carrillo et al., [26] prepared ternary blends of PCL, TPU, and PLA by fused filament fabrication (FFF) and found that the shape-memory index of samples printed with longitudinal grating patterns was higher (98.8 ± 0.6%). Nevertheless, the sample recovery process results in shrinkage, which the authors attribute to the polymer chains aligning to the tensile direction and then kinking as crystallization occurs during the recovery process. Meanwhile, thermoplastic SMPs show a decrease in the R_r due to intermolecular chain instability with multiple shape-memory cycles. Ojha et al. [27] prepared 4D-printed poly ether ketone ketone (PEKK), which showed a significant decrease in R_r from 100% to 80.47% after eight cycles. On the contrary, thermoset SMPs have chemically cross-linked structures and show a superior shape-memory property. The structure between the molecular chains is stable, and it can still maintain a high R_r even after repeated cycling. The hyperbranched epoxy resin prepared by Lu et al. [28] can maintain an R_r of 97.24% after seven shape-memory cycles, indicating excellent cycling stability. But owing to the insoluble and infusible properties of the thermoset polymers, thermoset SMP samples cannot be obtained directly via FDM printing.

Polyethylene (PE) is an important thermoplastic that is widely used due to its advantages of good chemical stability, lightweight, and ductility. At the same time, PE can be cross-linked to obtain a network structure by chemical [29] and physical [30,31,32] cross-linking methods, thus becoming a thermosetting plastic. By cross-linking modified PE, some special properties can also be obtained [33]. Most of the current work on cross-linked PE has focused on exploring its heat resistance or its use in high-voltage cables [34]. Hu et al. [35] irradiated high-density polyethylene (HDPE) with gamma rays to obtain cross-linked polyethylene (XLPE), making it a solid–solid phase change material with high latent heat. However, it should not be overlooked that XLPE has excellent shape-memory properties, which can be used in intelligent applications such as robotics. Li et al. [36] prepared low-density polyethylene (LDPE) samples using the silane cross-linking method and found that the sample had shape-memory properties, except for the fact that the best shape-memory performance was achieved at the initiator benzoyl peroxide content of 0.4 phr, and the R_r could reach 87%. As an SMP, PE was also found to exhibit excellent shape-memory properties over multiple cycles. Silane-cross-linked ultra-high-molecular-weight polyethylene (UHMWPE) carrying 1.0 phr of the water-carrying agent exhibited more than 98% R_f for nine cycles prepared by Chen et al. [37]. This suggests that the chemical entanglement of the molecular chains in the cross-linked polyethylene can maintain the stability of its permanent shape during cyclic recovery. Although PE has a very high potential for application, very few studies have been conducted on PE as a raw material for FDM printing. This is because the higher crystallinity of PE makes it prone to warping due to temperature changes during the printing process, leading to the deformation of the product. It is worth noting that research has gradually begun to solve the problem of printing on PE. Schirmeister et al. [38] successfully printed HDPE products on the surface of a poly (styrene-block-ethene-co-butene-block-styrene) thermoplastic elastomer plate instead of a printing plate. Olesik et al. [39] added glass powder as a reinforcing material to LDPE to enable it to be used for FDM printing. Jing et al. [40] successfully solved the warping problem by adjusting the printing parameters to obtain FDM-printed linear low-density polyethylene (LLDPE) samples and found that the temperature of the build plate affects the quality of printing.

However, PE SMP requires cross-linking as a prerequisite, but the XLPE is insoluble without melting, which makes it difficult to reapply in FDM printing. Therefore, in this paper, taking advantage of the fact that PE can be cross-linked by irradiation, a new idea of printing and then cross-linking was developed, using LDPE as the raw material, which successfully prepared shape-memory LDPE by 4D printing. First, we explored the optimal range of printing parameters for LDPE by adjusting the printing parameters. Subsequently, gamma rays were used to irradiate LDPE-printed products. After irradiation with gamma rays, the tertiary hydrogen atoms in the LDPE molecular chain were easily removed, generating stable tertiary carbon radicals, which later underwent a series of chemical reactions, resulting in the formation of a three-dimensional network structure. At this time, a 4D-printed cross-linked LDPE sample was obtained. Subsequently, their shape-memory properties were explored.

2. Experimental Section

2.1. Material

LDPE particles (XJ700) were purchased from PT LOTTE CHEMICAL TITAN Tbk (Seoul, Republic of Korea), with a specific gravity, melting temperature, and melting flow index of 0.914 g/cm³, 103 °C, and 22.0 g/min, respectively.

2.2. Preparation of Printed Samples

Figure 1 shows the preparation process of the 4D printing shape-memory-cross-linked LDPE sample, which was divided into three steps: the preparation of the printing filament, FDM printing, and gamma irradiation.

2.2.1. Fabrication of Filaments

A single-screw extruder (TP-07, Dongguan Song Hu Plastic Machinery Co., Ltd., Dongguan, China) was used to prepare the LDPE filaments. The temperature of each section of the single-screw extruder was set to 135, 140, and 130 °C. The main engine speed of the single-screw extruder was adjusted to 800 r/min, and the tractor speed was set to 215 r/min, such that the filament diameter was controlled within the range of 2.85 ± 0.05 mm. The filaments were dried in a blast (DHG-9073B5-III, Shanghai Xinmiao Medical Equipment Manufacturing Co., Ltd., Shanghai, China) oven at 60 °C for 2 h.

2.2.2. FDM Printing of Filaments

The FDM printer used in this study was Ultimaker 3 (Ultimaker, Utrecht, The Netherlands). The structural models of the printed pieces were designed using AutoCAD, and the model file was imported into Ultimaker Cura 4.12.0. The printed dumbbell-shaped sample was the size of a 1BA-type sample according to ISO 527-2:2012 [41]. The print orientation of the dumbbell-shaped sample was flat, and the deposition path is shown in Figure 2b. Other structural samples were printed to investigate the shape-memory properties of complex structures. The printing parameters are presented in

Table 1. Parameters of FDM printing.

Parameters	0.8 mm Nozzle
Printing temperature	140–220 °C
Build plate temperature	90 °C
Flow	80–120%
Printing speed	5–25 mm−1
Filling degree	100%
Layer thickness	0.1–0.2 mm

. The greatest challenge in FDM printing of LDPE is warpage. Past research has shown that raising the build plate temperature appropriately can slow the cooling rate. After testing, it was found that when the build plate temperature was 90 °C, the LDPE product did not show warpage. This is basically in line with the optimal printing platform temperature of 90–100 °C for LLDPE reported in the literature [35]. Therefore, the printing platform temperature in this study was chosen to be 90 °C.

2.2.3. Gamma Irradiation

The printed samples were encapsulated in sealed plastic bags and irradiated with gamma rays at doses of 40, 60, 80, 100, or 120 kGy.

2.3. Characterization

2.3.1. Fourier Transform Infrared (FTIR) Spectroscopy

The LDPE samples before and after cross-linking were characterized by Fourier Transform Infrared Spectroscopy (FTIR, Thermal Fisher Scientific Nicolet iS20, Waltham, MA, USA) in the beam range of 400–4000 cm⁻¹. The irradiated samples were LDPE-irradiated at 120 kGy.

2.3.2. Scanning Electron Microscopy (SEM)

The printed LDPE sample was dipped in liquid nitrogen, where it quickly underwent brittle fracture. Gold was then sprayed onto the cross-section, and SEM (JSM-5510LV, NEC, Tokyo, Japan) was used to observe the cross-section of the sample with an acceleration voltage of 20 kV.

2.3.3. Mechanical Properties

The mechanical properties of the printed dumbbell-shaped samples were tested according to ISO 527-1:2012 [42]. The tensile strength and elongation at the break of the printed LDPE samples were measured using a tensile machine (TCS-20000, Taiwan High-Speed Rail Tension Machine Co., Ltd., Taiwan, China) at a speed of 50 mm/s. The stretching direction is along the machine. A total of 5 samples were tested for each group and the average, and standard deviation were calculated.

2.3.4. Determination of Gel Content

First, 0.2 g of each gamma-irradiated sample was cut and placed in 120 stainless steel wire mesh. The sample and mesh were weighted and placed in a reflux device for 8 h with boiling xylene, after which the sample and mesh assembly were removed and dried in a vacuum-drying oven (DZF, Beijing Yongguangming Medical Instrument Co., Ltd., Beijing, China) until a constant weight was reached. The was repeated 3 times for each group, and the average of the results was taken.

The gel content is expressed as follows:

$$Gelratio={\displaystyle \frac{\left[m_0-\left(m_1-m_2\right)\right]}{m_0}}\times 100\%$$

(1)

where m₀ is the initial weight of the sample (g); m₁ is the total weight of the sample and the mesh assemblies wrapping the sample (g); and m₂ is the total weight of the sample and the stainless-steel wire mesh after extraction (g).

2.3.5. Differential Scanning Calorimetry (DSC)

The crystallization behavior and melt enthalpy change in the irradiated printed samples were characterized using DSC (DSC 200 F3, Netzsch, Selb, Germany). Using nitrogen as a protective gas, 5–8 g samples were heated from 25 to 200 °C at a rate of 20.0 °C/min and held for 5 min to eliminate thermal history. The temperature was later reduced to 25 at 20.0 °C/min to obtain the first cooling curve, after which the same ramp-up steps were repeated to obtain a second heating curve. The crystallization and melting temperatures were obtained from the DSC curves based on the first temperature reduction and second temperature increase. The enthalpy of crystallization and melting and degree of crystallization are expressed as follows:

$$X_c={\displaystyle \frac{\Delta H_m}{\Delta H_0}}\times 100\%$$

(2)

where ΔH_m and ΔH₀ are the actual and theoretical melting enthalpy, respectively. The corresponding enthalpy of melting LDPE at 100% crystallization was 293.6 J/g [36].

2.3.6. Shape-Memory Properties

The 3D-printed dumbbell-shaped samples were selected to characterize the shape-memory properties. First, the printed sample was pre-heated to eliminate thermal history. And then, a 30 mm length in the middle of the dumbbell-shaped sample was marked as ε₀. The samples were later placed in glycerine at 120 °C for 2 min, and then, the middle section of the sample was stretched to three times ε₀. This is recorded as ε₁. The external force on the tensile sample was maintained while the sample was placed in room-temperature water for rapid cooling. At this point, the length marked in the middle of the cooled sample was measured as ε₂. The cooled sample was placed again in glycerine at 120 °C. After the shape of the sample was recovered, the middle section was recorded as ε₃. The R_r and R_f were calculated using the following Equations (3) and (4):

$$R_r={\displaystyle \frac{{\epsilon }_2-{\epsilon }_3}{{\epsilon }_2-{\epsilon }_0}}\times 100\%$$

(3)

$$R_f={\displaystyle \frac{{\epsilon }_2-{\epsilon }_0}{{\epsilon }_1-{\epsilon }_0}}\times 100\%$$

(4)

3. Results

3.1. Exploration of Optimal Printing Parameter Ranges

The samples utilized to explore the optimal range of printing parameter ranges were all dumbbell-shaped samples with a thickness of 2 mm, and the SEM images show the cross-sections perpendicular to the print paths of dumbbell-shaped samples.

Figure 3a shows dumbbell-shaped LDPE samples at different printing flow rates. At an under 100% flow rate, the sample exhibited clear print alignments. When the flow rate exceeded 120%, the sample was deformed. Furthermore, the SEM images of samples with printing flow rates of 80% and 110%, respectively, are compared in Figure 3b,c. Here, the cross-section of the 80% sample shows multiple dense pores, whereas the 110% sample is smooth. This is because when the flow is very low, less melt is extruded from the nozzle, and the print alignment is thinner, thereby resulting in poor adhesion between the alignments and the layers. This causes voids inside the sample, thereby reducing the quality and properties. On the contrary, when the flow rate is very high, the amount of melt-extruded from the nozzle is too large, which could easily deform the sample. Hence, the optimum print flow rate was in the range of 100–120%.

The printing speed had a similar effect on the LDPE samples as the printing flow. Figure 4a shows the LDPE dumbbell-shaped samples printed at different speeds. At a speed of 5 mm/s, the nozzle extruded too much material, thereby resulting in wider and thicker samples and a longer time of consumption. However, as evidenced by the SEM images (Figure 4b,c), the extruded melt became thinner owing to the quick traction of the nozzle at a speed of 35 mm/s, which finally created gaps. Hence, the printing speed must be set between 15 and 25 mm/s.

The printing temperature plays an important role in molding. This is because it affects the amount of extruded melt. Figure 5a shows dumbbell-shaped LDPE samples printed at different temperatures. The samples with printing temperatures below 180 °C exhibit evident holes and weaknesses, whereas those with printing temperatures beyond 240 °C exhibit deformation and high thickness. Figure 5b,c show the SEM morphology observation of the samples printed at 140 and 200 °C. The cross-section of the sample printed at 140 °C has several holes, whereas that printed at 200 °C has none. This result is similar to the effects of the two previous print parameters. At a lower temperature, less LDPE flowed out of the nozzle, thereby resulting in thinner extruded filaments and smaller wire-to-wire and layer-to-layer contact surfaces. In this case, the quality of the printed product was poor. When the printing temperature was too high, excess melt came out of the nozzle, and it did not allow rapid cooling, thereby deforming the sample and causing faults. Thus, a higher-quality sample can be generated when printing at temperatures between 180 and 220 °C.

Hence, the printing parameters of the LDPE dumbbell-shaped printed samples were selected to be at a printing temperature, speed, and flow rate of 200°C, 15 mm/s, and 110%, respectively.

3.2. Characterization of Cross-Linking LDPE

The FTIR spectra of LDPE before and after cross-linking are shown in Figure 6. It can be observed that the two samples have the same characteristic peaks, in which 720 cm⁻¹ is the bending vibration of C-C, 1465 cm⁻¹ is the bending vibration of -CH₂-, and 2847 and 2914 cm⁻¹ are the antisymmetric and symmetric stretching vibrations, respectively. In addition, no new peaks appeared, indicating that irradiation did not cause new valence bonds to be created in LDPE. If irradiated LDPE showed peaks at 1700⁻¹ that increased with the dose, it would indicate that carboxyl groups formed during irradiation and that the sample underwent oxidative degradation. However, in this work, no significant peaks were observed at around 1700 cm⁻¹, which may be attributed to the use of bags to seal off the effects of oxygen during irradiation, and oxidation was not clear.

3.3. Effect of Irradiation Dose on LDPE-Printed Samples

3.3.1. Effect of Irradiation Dose on Gel Content of LDPE-Printed Samples

The average value and standard deviation of the results of the three gel tests are shown in

Table 2. Effect of the irradiation dose on the gel content of the LDPE-printed samples.

Dose (kGy)	0	40	60	80	100	120
Gel content (%)	0	24.15	33.17	45.29	53.84	62.54
Standard deviation value (%)	0	0.48	0.46	0.66	0.80	0.48

. As shown in Table 2, the gel content increased with an increase in the irradiation dose. The gel content was 24.15% at an irradiation dose of 40 kGy. When the irradiation dose increased to 120 kGy, the gel content reached a maximum of 62.54%. The gel content of the sample represents its degree of cross-linking. Hence, the degree of cross-linking increased with the increase in the irradiation dose.

In the 1970s, Chen Xinfang-Liu Kejing-Tang Aoqing modified the Charlesby–Pinner equation [43,44] based on the theory of the solution and gel distribution of Schulz and the results of irradiated cross-linked PE tests and obtained the Chen-Liu-Tang equation, which more closely matches the actual results when analyzing the relationship between the solute fraction and irradiation dose:

$$R\left(s+s^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}\right)={\displaystyle \frac{1}{q_0{\overline{u}}_0}}+\frac{{{\alpha }_0}^{\prime }}{q_0}{R}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}$$

(5)

where s is the soluble fraction, R is the dose, q₀ is the cross-linking density, α₀^′ is a constant, and ${\overline{u}}_0$ represents the initial values of number-average polymerization. After processing the data through Equation (5), the relationship between R(s + s^1/2) and R^1/2 was obtained, as shown in Figure 7. It can be seen that R(s + s^1/2) and R^1/2 are basically linearly correlated at irradiation doses of 40–120 kGy. The slope, intercept, and coefficient of determination R² of these equations were calculated to be 11.59 ± 1.272, −3.927 ± 11.38, and 0.9651, respectively.

3.3.2. Crystallization Behavior

The shape-memory behavior of LDPE is achieved through the interaction of the cross-linked mesh structure with the melting and formation of crystals. The melting and formation of the crystals enabled shape-shifting, thereby allowing the printed samples to bend and recover. The effect of the irradiation dose on the melting and crystallization behaviors and thermal properties of the LDPE was investigated using DSC.

Figure 8 shows the DSC curves of the first cooling crystallization and second heating and melting of the printed samples irradiated with different doses. The melting temperatures (T_m) and crystallization temperatures (T_c) of the irradiated LDPE samples at each irradiation dose were similar, thereby indicating that cross-link grafting was conducted in the amorphous domain (

Table 3. Parameters determined from the first cooling and second heating curves.

Dose (kGy)	Tm (°C)	Tc (°C)	ΔHm (J/g)	ΔHc (J/g)	Xc (%)
0	112.0	95.8	100.4	−79.2	34.2
40	109.9	94.4	94.7	−80.9	32.3
60	109.3	93.6	103.8	−83.4	35.4
80	108.6	92.5	104.4	−81.8	35.6
100	109.2	90.7	97.3	−74.9	33.1
120	107.1	91.8	102.5	−81.0	34.9

). This is similar to the findings of Girard-Perier et al. [45], who discovered that the melting peaks of the gamma-irradiated PE composite multilayer film did not change compared with pre-irradiation, indicating that irradiation has little effect on thermal properties. The melting peaks of the samples widened considerably as the irradiation dose increased, which might be the result of cross-linking reducing the homogeneity of the molecular structure. Increased cross-linking reduces the orderliness of crystallization and the uniformity of the molecular structure, thereby causing defective crystalline regions, while the peaks of crystallization and melting shift slightly toward lower temperatures. Furthermore, no new peaks were observed, thereby indicating the absence of a significant phase separation in LDPE after irradiation. The crystallinity (X_c) obtained using Equation (2) show that no significant changes occurred in X_c as the degree of cross-linking increased. Mouchache et al. [46] mentioned that there was no change in the X_c and T_m of the irradiated samples, which is similar to the results of this study, suggesting that irradiation does not have a significant effect on crystallization. However, the results of Alsabbagh et al. [30] indicated an increase in HDPE X_c with an increasing irradiation dose. This was attributed to the reorganization of short chains towards the crystalline order caused by chain scission from oxidation.

3.3.3. Mechanical Properties

The effects of the irradiation dose on the tensile strength and elongation at the break of the LDPE-printed samples are shown in Figure 9. Here, the tensile strength of the samples increased with an increase in the irradiation dose. When the irradiation dose was 120 kGy, the tensile strength of the printed samples reached a maximum of 13.98 MPa, which is an increase of approximately 51% compared with that of the unirradiated sample. After irradiation, the LDPE sample formed a network structure inside, which restricted the movement of the molecular chains. The higher the degree of cross-linking, the stronger the resistance of the molecular chains to external forces. The tensile strength increased with an increase in the irradiation dose, thereby indicating that the higher the irradiation dose, the greater the degree of cross-linking in the samples, which corresponds with the gel content results. The elongation at break of the LDPE-printed samples showed a decrease in this trend with an increase in the irradiation dose. The elongation at break of the non-irradiated samples was 543.53% (the length of the sample increased by more than 6.4 times compared to the undeformed state). At 120 kGy, the elongation at break reached a minimum of 378.05% (the length of the sample increased by 4.8 times compared to the undeformed state), which is a decrease of 30% compared with that of the unirradiated sample. Since the effect of oxygen is excluded during irradiation, the decrease in elongation at break can be attributed to the inhibition of the molecular chain movement by the increased degree of cross-linking. Hence, the elongation at break tended to decrease.

However, the gamma-irradiated thermo-compression-molded HDPE studied by Alsabbagh et al. [30] was prepared in an oxygen atmosphere, and it was found that at lower doses, the elongation at break showed an increasing trend up to ~1300%. While the samples at higher doses exhibited an increase in brittleness caused by the degradation of the chains due to the additional trapping of free radicals from the crystalline region during irradiation. Girard-Perier et al. [47] compared the elongation at break of PE composite multilayer films before and after gamma irradiation in the literature. No significant difference was found, and this was attributed to the fact that the chain scission and cross-linking that occurred during irradiation did not affect the mechanical properties of the whole multilayer film.

3.3.4. Shape-Memory Behavior

The shape-memory properties of the LDPE-printed samples with different irradiation doses were evaluated. As shown in Figure 10, R_r showed an increasing trend with the increasing irradiation dose. When the irradiation dose was 40 kGy, R_r reached 93.63% and increased to 97.69% at 120 kGy. However, it had little effect on R_f as the dose increased. This suggests that shape-memory properties improve at a higher dose. Since LDPE’s shape-memory effect is primarily achieved through the cross-linking network and the melting and generation of crystals, the shape-memory properties are closely related to the degree of cross-linking. This result indirectly indicates that the degree of cross-linking increases with the irradiation dose, which is consistent with the previous gel content test results. Based on these results, the best shape-memory properties of the LDPE were achieved at an irradiation dose of 120 kGy.

A dumbbell-shaped sample with an irradiation dose of 120 kGy was subjected to multiple shape-memory cycling tests, and the test results are shown in Figure 11. The R_r and R_f of the sample were maintained at 96.30% and 97.76%, although there was a decrease. This is due to the fact that above T_trans, the chemical cross-linking network provides shape recovery stress. As the number of cycles increases, the shape recovery stress relaxes, and it eventually converges to a constant value. It is evident that after eight cycles, the sample retains high R_f and R_r, demonstrating its strong shape-memory cycling capability. The 4D-printed carbon fiber/PEKK composite prints prepared by Kumar et al. [48] reduced the recovery rate to 89% after ten cycles, with a significant loss of 7% compared to the first cycle. The excellent shape-memory cycling ability of γ-irradiated LDPE with a chemical cross-linking structure was demonstrated by comparison.

3.3.5. Shape-Memory Behavior of 4D-Printed Physical Objects

Figure 12 shows the original shape, temporary shape, and shape recovery process of the 4D-printed flower. Here, the curled flower stretched into its original shape at 22 s in glycerine at 120 °C. This demonstrates the fast response and high shape recovery of the 4D-printed-irradiated LDPE.

4. Conclusions

In this study, an LDPE SMP was successfully prepared by utilizing FDM printing, and its performance was evaluated. The printing parameters considerably impacted the printed samples. The quality of the printed sample is best when the temperature, flow rate, and speed are 180–220 °C, 90–110%, and 15–25 mm/s, respectively. The FTIR results indicated that no new peaks were generated in the samples before and after irradiation, indicating that the effect of oxidation was not significant. The irradiation dose was directly proportional to the gel content (the degree of cross-linking) in the range measured in this study. As observed from the DSC results, there was no significant effect on the crystallization and melting behavior of the LDPE samples when the irradiation dose was increased. The tensile strength increased steadily, and the elongation at break decreased with an increase in the irradiation dose. The R_r of the samples increased from 93.63% to 97.69% as the irradiation dose increased. Simultaneously, R_f stabilized at about 98%. Furthermore, after the shape-memory cycle test, the R_r and R_f of the sample remained at 96.3% and 97.76%, respectively. The 4D-printed gamma-irradiated LDPE produced in this study is simple to fabricate and has good shape-memory qualities. Successfully applied LDPE to 4D printing provides more choices for the materials for 4D printing. Future research should focus on the structural design to widen its applicability in the biomedical and soft robot domains.