Effect of PEW and CS on the Thermal, Mechanical, and Shape Memory Properties of UHMWPE

Modified ultra-high-molecular-weight polyethylene (UHMWPE) with calcium stearate (CS) and polyethylene wax (PEW) is a feasible method to improve the fluidity of materials because of the tense entanglement network formed by the extremely long molecular chains of UHMWPE, and a modified UHMWPE sheet was fabricated by compression molding technology. A Fourier-transform infrared spectroscopy test found that a new chemical bond was generated at 1097 cm−1 in the materials. Besides, further tests on the thermal, thermomechanical, mechanical, and shape memory properties of the samples were also conducted, which indicates that all properties are affected by the dimension and distribution of crystal regions. Moreover, the experimental results indicate that the addition of PEW and CS can effectively improve the mechanical properties. Additionally, the best comprehensive performance of the samples was obtained at the PEW content of 5 wt % and the CS content of 1 wt %. In addition, the effect of temperature on the shape memory properties of the samples was investigated, and the results indicate that the shape fixity ratio (Rf) and the shape recovery ratio (Rr) can reach 100% at 115 °C and 79% at 100 °C, respectively, which can contribute to the development of UHMWPE-based shape memory polymers.

Cross-linked polyethylene is widely used as a shape memory material in developed countries, such as low-density polyethylene (LDPE) and high-density polyethylene (HDPE), but the recovery stress can only reach 3 MPa [28], which cannot meet actual needs. This problem can be solved by using ultra-high-molecular-weight polyethylene (UHMWPE). UHMWPE has many excellent properties due

Analytical Methods
Fourier-transform infrared (FTIR) spectroscopy of the samples was tested in the form of sheets (except CS and PEW) fabricated by the compression molding technique. The spectrum of all samples was recorded at room temperature over the range 4000-600 cm −1 by an FTIR spectrometer (NEXUS 670, NECO INDUSTRIES INC., Oklahoma, OK, USA) and an attenuated total reflectance (ATR) cell. Besides, the spectra of all samples were averaged over 16 scans with a 4 cm −1 resolution.
The crystallinity and melting behavior of samples were tested by a differential scanning calorimeter (DSC 25, TA Instruments, New Castle, DE, USA). Firstly, the sample was heated from 25 to 200 • C in a nitrogen atmosphere at a heating rate of 10 • C /min, and then cooled to 25 • C at the same rate. Besides, the sample was held at 200 • C for 3 min and at 25 • C for 1 min to eliminate thermal history. Then, the process was repeated again, and the curves were recorded. The degree of crystallinity of the sample (X c ) obtained by DSC was calculated through the following equation: where ∆H m is the melting enthalpy of the samples obtained from the DSC test and ∆H o m is the melting enthalpy of a 100% crystalline sample (for UHMWPE, ∆H o m = 289 J/g [38]). Thermal gravimetric analysis (TGA) was carried out to characterize the relationship between weight loss and temperature, and the decomposition and thermal stability of materials. The test was conducted by a TGA instrument (TGA/DSC1, METTLER TOLEDO INSTRUMENTS CO., LTD., Shanghai, China) in a nitrogen atmosphere (20 mL/min) with the temperature range of 25-700 • C at a heating rate of 10 • C/min.
The Vicat softening temperature (VST) is a parameter to evaluate the heat resistance of samples. During the test, the sample with the size of 10 × 10 × 4 mm 3 was subjected to the force of 10 N with a heating rate of 50 • C/h to obtain a temperature value at which a pressure needle with a size of 1 mm 2 penetrated the sample to a depth of 1 mm. Each data is the average value obtained by testing 4 samples.
Dynamical mechanical analysis (DMA) is used to measure the entanglement density (υ e ) and the average molecular weight (M e ) between the entanglement points [30]. The sample with a size of 35 × 6 × 2 mm 3 was tested by a single cantilever bending mode of a DMA instrument (DMTA-V, Rheometric Scientific, New Castle, DE, USA). The test sample was scanned from 60 to 170 • C with the heating rate of 3 • C/min and the scanning frequency of 1 Hz. The storage modulus (E ) was defined as the rubbery plateau modulus at 160 • C. The M e was calculated using the following equation: where ρ is the density of the materials (for UHMWPE, ρ = 0.935 g/cm 3 ), R is the gas constant, T is the absolute temperature, and υ is the Poisson ratio (for UHMWPE, υ = 0.4 [33]). Additionally, the chain entanglement density (υ e ) was calculated using the following equation: The mechanical properties of samples were tested by a universal testing machine (KXWW, Chengde Taiding Testing Machine Manufacturing Co., Ltd., Chengde, China) with a load cell of 5 kN. All tensile test samples with the size of 150 × 20 × 4 mm 3 were tested at a crosshead speed of 50 mm/min to obtain the tensile strength and elongation at break. Besides, the curve diagrams of the relationship between tensile strength and elongation were also recorded. The flexural test samples with the size of 80 × 10 × 4 mm 3 were tested at a crosshead speed of 10 mm/min to obtain the flexural modulus and flexural strength. Moreover, the notched impact strength of samples was tested with the size of 80 × 10 × 4 mm 3 at room temperature, and the side of each sample had a standard notch with a depth of 2 mm. In addition, all results of the tensile test and the flexural test were the average of at least four samples, and the results of the notched impact strength were the average of at least eight samples.
The bending test was conducted to quantify the shape memory behavior of samples. The schematic diagram of test process is shown in Figure 1. Firstly, a flat sample (θ 0 = 0 • ) with the size of 80 × 10 × 2 mm 3 was heated to the switching temperature (T SW ) in an oil bath, and then bent into a U-shaped structure with 180 • (θ U = 180 • ). The curvature radius in the tip of the U-shape after 180 • bending was 8 mm. Secondly, it was rapidly cooled down to room temperature while the deformation was retained by the external force. Thirdly, after the external force was removed, it was deformed freely in the internal stress field, and then the final fixed angle (θ f ) was measured by an electronic digital angle ruler (Shengtaixin Technology Co., Ltd., Shenzhen, China) with an accuracy of 0.5 • . Finally, it was heated to T SW again without external force, and the final recovery angle (θ r ) of the sample was recorded. Lastly, the shape fixity ratio (R f ), the shape recovery ratio (R r ), and the maximum theoretical stress (σ max ) could be calculated by the following formula: where E is Young's modulus (for UHMWPE, E = 600 MPa [38]), t is the thickness of the sample, and R is the curvature radius of the sample.
Polymers 2020, 12, x FOR PEER REVIEW 5 of 15 where E is Young's modulus (for UHMWPE, E = 600 MPa [38]), t is the thickness of the sample, and R is the curvature radius of the sample.

FTIR Analyses
The FTIR spectra were recorded to characterize the effect of CS and PEW on the molecular structure of UHMWPE, which is shown in Figure 2. It is generally believed that the added PEW and CS played the role of solid lubricants to improve the fluidity of UHMWPE and enhance fusion among materials [42,43]. By comparison, PEW mainly works as the external lubricant, while CS can be used as both external lubricant and internal lubricant. It was found that a new and strong characteristic peak appeared at 1097 cm −1 by adding the solid lubricants, which represented the emergence of new ether bonds (C-O-C) inside the materials, and indicated that the addition of solid lubrications changed the molecular structure of UHMWPE during the compression molding process.

FTIR Analyses
The FTIR spectra were recorded to characterize the effect of CS and PEW on the molecular structure of UHMWPE, which is shown in Figure 2. It is generally believed that the added PEW and CS played the role of solid lubricants to improve the fluidity of UHMWPE and enhance fusion among materials [42,43]. By comparison, PEW mainly works as the external lubricant, while CS can be used as both external lubricant and internal lubricant. It was found that a new and strong characteristic peak appeared at 1097 cm −1 by adding the solid lubricants, which represented the emergence of new ether bonds (C-O-C) inside the materials, and indicated that the addition of solid lubrications changed the molecular structure of UHMWPE during the compression molding process.
Polymers 2020, 12, x FOR PEER REVIEW 5 of 15 where E is Young's modulus (for UHMWPE, E = 600 MPa [38]), t is the thickness of the sample, and R is the curvature radius of the sample.

FTIR Analyses
The FTIR spectra were recorded to characterize the effect of CS and PEW on the molecular structure of UHMWPE, which is shown in Figure 2. It is generally believed that the added PEW and CS played the role of solid lubricants to improve the fluidity of UHMWPE and enhance fusion among materials [42,43]. By comparison, PEW mainly works as the external lubricant, while CS can be used as both external lubricant and internal lubricant. It was found that a new and strong characteristic peak appeared at 1097 cm −1 by adding the solid lubricants, which represented the emergence of new ether bonds (C-O-C) inside the materials, and indicated that the addition of solid lubrications changed the molecular structure of UHMWPE during the compression molding process.

DSC Analyses
The DSC test of UHMWPE with different contents of PEW and CS was conducted to characterize the crystallization behavior and the initial value (T m on ), maximum (T m max ), and end value (T m off ) of the melting peak of the materials [31]. The DSC curves recorded during the second melting process are shown in Figure 3a, while the degree of crystallinity and the parameter related to the melting peak are listed in Table 2. According to the data in Table 2, the crystallinity of the materials increases with the PEW content and decreases with the CS content, which indicates that the addition of PEW contributes to the growth of the crystal region, while the addition of CS plays the opposite role. Besides, the increased crystallinity of the materials makes the amorphous region smaller, which contributes to the decrease of T m on , because the melting process starts in the amorphous region. Moreover, the T m on of 13PEW5CS is 2.6 • C higher than that of 13PEW. According to the previous studies [38], T m max is affected by the micro-defects inside the samples. It can be seen from Table 2 that the T m max of the samples gradually increases with the content of PEW and CS, because PEW and CS can penetrate into the gaps between the molecular chains of UHMWPE, reduce the intermolecular force, improve the fluidity of UHMWPE, and reduce the number of micro-defects [43]. However, the addition of PEW with a lower melting point melts first and advances the position of the melting peak, which results in the downward trend of T m max from 5PEW5CS to 13PEW5CS.  Figure 4 shows the effect of PEW and CS on the VST of the samples. The VST curve is closely related to the Tm of the materials. According to the curves in Figure 4, the addition of CS shows little effect on the VST curves of the samples, while the effect of PEW on the VST curves is more and more significant with the increase of PEW content. By comparison, the VST changes little at 5PEW, but decreases from 132.4 to 127.4 °C with the further increase of PEW content, which is mainly because PEW with a lower Tm melts first and destroys the structure of the samples. In addition, comparing 13PEW5CS with 13PEW, it can be found that the former curve has an obvious plateau area below 90 °C, while the latter does not.   Figure 3b shows the TGA curves of UHMWPE with different contents of PEW and CS, and the parameters such as initial degradation temperature (T 1 ) representing 10% weight loss and mid-point degradation temperature (T 50 ) representing 50% weight loss are summarized in Table 2. It can be seen that the thermal stability of UHMWPE modified by PEW was improved, while that of CS was declined. Specifically, the T 1 of materials with the addition of PEW increases from 464.3 to 465.9 • C, while decreasing from 464.3 to 461.9 • C with the addition of CS, which is because the molecular chains' movement of the amorphous region is affected by the crystalline region below the T m , and thus results in the hysteresis phenomenon of the degradation process. That means that the larger the crystalline region, the less likely the movement of molecule chains. However, when PEW and CS are applied to modify UHMWPE simultaneously, the thermal stability of materials declines. According to the above analysis, it can be seen that there is physical cross-linking between the molecular chains in the amorphous region, but it cannot fundamentally hinder the movement of the molecular chains, while the crystal regions can hinder the movement seriously. Figure 4 shows the effect of PEW and CS on the VST of the samples. The VST curve is closely related to the T m of the materials. According to the curves in Figure 4, the addition of CS shows little effect on the VST curves of the samples, while the effect of PEW on the VST curves is more and more significant with the increase of PEW content. By comparison, the VST changes little at 5PEW, but decreases from 132.4 to 127.4 • C with the further increase of PEW content, which is mainly because PEW with a lower T m melts first and destroys the structure of the samples. In addition, comparing 13PEW5CS with 13PEW, it can be found that the former curve has an obvious plateau area below 90 • C, while the latter does not.   Figure 4 shows the effect of PEW and CS on the VST of the samples. The VST curve is closely related to the Tm of the materials. According to the curves in Figure 4, the addition of CS shows little effect on the VST curves of the samples, while the effect of PEW on the VST curves is more and more significant with the increase of PEW content. By comparison, the VST changes little at 5PEW, but decreases from 132.4 to 127.4 °C with the further increase of PEW content, which is mainly because PEW with a lower Tm melts first and destroys the structure of the samples. In addition, comparing 13PEW5CS with 13PEW, it can be found that the former curve has an obvious plateau area below 90 °C, while the latter does not.

Thermomechanical Properties
The DMA test is considered to be an effective method to measure the molecular weight between physically effective cross-linking points, including physical entanglement and chemical cross-linking [30]. The DMA curves of UHMWPE with different contents of PEW and CS are shown in Figure 5, and the platform modulus (E′), the average molecular weight between entanglement points (Me), and the entanglement density (υe) are summarized in Table 3. The platform modulus, which is also known

Thermomechanical Properties
The DMA test is considered to be an effective method to measure the molecular weight between physically effective cross-linking points, including physical entanglement and chemical cross-linking [30]. The DMA curves of UHMWPE with different contents of PEW and CS are shown in Figure 5, and the platform modulus (E ), the average molecular weight between entanglement points (M e ), and the entanglement density (υ e ) are summarized in Table 3. The platform modulus, which is also known as the storage modulus in the rubbery plateau, is a function of entanglement and cross-linking. Larger storage modulus generally results in greater entanglement density. storage modulus generally results in greater entanglement density. It is not difficult to see that E′ of UHMWPE modified by either PEW or CS has a little change, while the E′ of UHMWPE modified by PEW and CS increases first and then decreases with the increase of content. According to the υe calculated by the E′ in Table 3, the υe of UHMWPE that is modified by CS can reach the maximum of 499 mol/m 3 , while the PEW is only about 469 mol/m 3 , which indicates that the effect of CS on physical entanglement of UHMWPE molecular chains is more significant than that of PEW. Meanwhile, the excessive content of PEW results in the decrease of the υe. Obviously, the physical entanglement is more significantly enhanced when UHMWPE is modified by PEW and CS simultaneously, and the maximum υe can reach 583 mol/m 3 of 5PEW1CS. Both PEW and CS can improve the movement capacity of UHMWPE chains without external force, which results in the more disordered the molecular chain, the more physical entanglement points and the greater the entropy. However, PEW with a smaller molecular weight can enter the gaps among the molecular chains of UHMWPE during the compression molding process, and the excessive content of PEW occupies the movement space of UHMWPE chains and hinders the movement of UHMWPE chains, which reduces the υe of materials.

Tensile Test Analyses
The representative stress-elongation curves obtained in the tensile experiments are shown in Figure 6a, and the tensile strength and the elongation at break of the samples with different contents of PEW and CS are shown in Figure 6b,c, respectively. The tensile strength listed in Figure 6b refers to the maximum tensile strength during the stretching process, including the yield strength for 13PEW or the fracture strength. It can be found from Figure 6 that PEW and CS can improve the elongation at break of the materials, and make the samples have an obvious plastic deformation process in the tensile test. Obviously, pure UHMWPE shows deformation of high elasticity and no plastic deformation, which indicates that pure UHMWPE chains may only have segment movements of the molecular chains during the stretching process. However, significant slippage emerges in  It is not difficult to see that E of UHMWPE modified by either PEW or CS has a little change, while the E of UHMWPE modified by PEW and CS increases first and then decreases with the increase of content. According to the υ e calculated by the E in Table 3, the υ e of UHMWPE that is modified by CS can reach the maximum of 499 mol/m 3 , while the PEW is only about 469 mol/m 3 , which indicates that the effect of CS on physical entanglement of UHMWPE molecular chains is more significant than that of PEW. Meanwhile, the excessive content of PEW results in the decrease of the υ e . Obviously, the physical entanglement is more significantly enhanced when UHMWPE is modified by PEW and CS simultaneously, and the maximum υ e can reach 583 mol/m 3 of 5PEW1CS. Both PEW and CS can improve the movement capacity of UHMWPE chains without external force, which results in the more disordered the molecular chain, the more physical entanglement points and the greater the entropy. However, PEW with a smaller molecular weight can enter the gaps among the molecular chains of UHMWPE during the compression molding process, and the excessive content of PEW occupies the movement space of UHMWPE chains and hinders the movement of UHMWPE chains, which reduces the υ e of materials.

Tensile Test Analyses
The representative stress-elongation curves obtained in the tensile experiments are shown in Figure 6a, and the tensile strength and the elongation at break of the samples with different contents of PEW and CS are shown in Figure 6b,c, respectively. The tensile strength listed in Figure 6b refers to the maximum tensile strength during the stretching process, including the yield strength for 13PEW or the fracture strength. It can be found from Figure 6 that PEW and CS can improve the elongation at break of the materials, and make the samples have an obvious plastic deformation process in the tensile test. Obviously, pure UHMWPE shows deformation of high elasticity and no plastic deformation, which indicates that pure UHMWPE chains may only have segment movements of the molecular chains during the stretching process. However, significant slippage emerges in molecular chains or crystal regions due to the lubrication of PEW and CS, especially for the samples simultaneously modified by PEW and CS. Besides, the elongation at break of the samples modified by PEW is smaller than that of the samples modified by CS, which is because the former mainly emerges as a slippage between crystal regions, while the latter mainly emerges as a slippage between molecular chains. In addition, the slippage caused by the external force results in the orientation of the molecular chains, which improves the tensile strength from 21.6 MPa of UHMWPE to 23.5 MPa of 1CS, as shown in Figure 6b, and the break caused by the slippage of the crystal regions makes the fracture strength less than the yield strength. Furthermore, the tensile strength decreases from 23.23 MPa of 1CS to 20.1 MPa of 5CS because the increased size of calcium ionic clusters may hinder the slippage of the molecular chains [49]. The schematic diagram describing the increased size of the calcium ionic clusters is shown in Figure 7. In addition, the simultaneous addition of PEW and CS makes the samples show better plastic deformation and higher mechanical properties, such as 5PEW1CS with the elongation of 344% and tensile strength of 23.6 MPa. However, the excessive content of PEW and CS can decrease the elongation at break and tensile strength to 299% and 18.2 MPa, respectively, such as 13PEW5CS, which is mainly due to the excessive slippage of molecular chains caused by the solid lubricants, resulting in the absence of molecular chain orientation.
Polymers 2020, 12, x FOR PEER REVIEW 9 of 15 molecular chains or crystal regions due to the lubrication of PEW and CS, especially for the samples simultaneously modified by PEW and CS. Besides, the elongation at break of the samples modified by PEW is smaller than that of the samples modified by CS, which is because the former mainly emerges as a slippage between crystal regions, while the latter mainly emerges as a slippage between molecular chains. In addition, the slippage caused by the external force results in the orientation of the molecular chains, which improves the tensile strength from 21.6 MPa of UHMWPE to 23.5 MPa of 1CS, as shown in Figure 6b, and the break caused by the slippage of the crystal regions makes the fracture strength less than the yield strength. Furthermore, the tensile strength decreases from 23.23 MPa of 1CS to 20.1 MPa of 5CS because the increased size of calcium ionic clusters may hinder the slippage of the molecular chains [49]. The schematic diagram describing the increased size of the calcium ionic clusters is shown in Figure 7. In addition, the simultaneous addition of PEW and CS makes the samples show better plastic deformation and higher mechanical properties, such as 5PEW1CS with the elongation of 344% and tensile strength of 23.6 MPa. However, the excessive content of PEW and CS can decrease the elongation at break and tensile strength to 299% and 18.2 MPa, respectively, such as 13PEW5CS, which is mainly due to the excessive slippage of molecular chains caused by the solid lubricants, resulting in the absence of molecular chain orientation.

Three-Point Bending Test Analyses
Flexural strength refers to the ability of a material to resist bending. Flexural strength and flexural modulus of the samples are shown in Figure 8a,b, respectively. The flexural strength is significantly affected by the crystallite size and crystallinity of the samples. The flexural strength and flexural modulus increase with the PEW content, and first increase and then decrease with the CS content, while the flexural strength of UHMWPE modified by PEW and CS decreases significantly. It was found that the flexural strength from 28.7 to 31.6 MPa and the flexural modulus from 919.1 to 1079.9 MPa of the samples modified by PEW increase with the increased crystallinity, while the samples modified by CS show the opposite trend. Compared with 1CS, the higher flexural strength and flexural modulus of 5CS attributes to the fact that the increased size of calcium ionic clusters hinders the movement of molecular chains, as shown in Figure 7. Besides, when PEW and CS are applied to modify UHMWPE simultaneously, the added CS could decline the flexural performance of the samples. For example, although 13PEW and 13PEW5CS have almost the same crystallinity, the flexural strength and flexural modulus of the former are higher than those of the latter.

Three-Point Bending Test Analyses
Flexural strength refers to the ability of a material to resist bending. Flexural strength and flexural modulus of the samples are shown in Figure 8a,b, respectively. The flexural strength is significantly affected by the crystallite size and crystallinity of the samples. The flexural strength and flexural modulus increase with the PEW content, and first increase and then decrease with the CS content, while the flexural strength of UHMWPE modified by PEW and CS decreases significantly. It was found that the flexural strength from 28.7 to 31.6 MPa and the flexural modulus from 919.1 to 1079.9 MPa of the samples modified by PEW increase with the increased crystallinity, while the samples modified by CS show the opposite trend. Compared with 1CS, the higher flexural strength and flexural modulus of 5CS attributes to the fact that the increased size of calcium ionic clusters hinders the movement of molecular chains, as shown in Figure 7. Besides, when PEW and CS are applied to modify UHMWPE simultaneously, the added CS could decline the flexural performance of the samples. For example, although 13PEW and 13PEW5CS have almost the same crystallinity, the flexural strength and flexural modulus of the former are higher than those of the latter.

Notched Impact Test Analyses
The notched impact strength of the samples with different contents of PEW and CS is shown in Figure 8c. Compared with pure UHMWPE, the notched impact strength of samples with the addition of PEW and CS decreases from 96.5 to 74.8 kJ/m 2 , which is mainly due to the fact that PEW and CS can penetrate into the gaps between the molecular chains of UHMWPE and change the molecular weight distribution.

Shape Memory Behaviors
Most SMPs contain two parts inside, including the "hard phase" and the "soft phase". The hard phase mainly plays a fixity role to maintain a permanent shape, while the soft phase mainly plays a deformation role to provide the SMPs with a temporary shape [14]. As shown in Table 2, the crystalline region could not melt at T SW in this study. Therefore, the crystalline regions in UHMWPE mainly acted as the "hard phase", while the amorphous regions acted as the "soft phase". According to Equation (6), the maximum theoretical stress is 75 MPa. Figure 9 shows the representative shape recovery process of 13PEW5CS over time at different T SW , which indicates that the SME has a strong temperature dependence [31]. Figure 10 shows the R f and the R r of UHMWPE modified by PEW and CS within two minutes (little shape recovery at more than two minutes) at different T SW . It can be seen that the R f remains basically constant at the same T SW , but increases with the temperature because of more movement of the chain segments in the amorphous region, such as 90% at 85 • C, 95% at 100 • C, and 100% at 115 • C, which is consistent with the research of Wu et al. [50]. In particular, the R f of 13PEW5CS reaches 99% at 100 • C. On one hand, a continuous "hard phase" cannot be formed to resist deformation; on the other hand, the deformation of amorphous phases among the crystal regions also drives the deformation of crystal regions.
flexural modulus increase with the PEW content, and first increase and then decrease with the CS content, while the flexural strength of UHMWPE modified by PEW and CS decreases significantly. It was found that the flexural strength from 28.7 to 31.6 MPa and the flexural modulus from 919.1 to 1079.9 MPa of the samples modified by PEW increase with the increased crystallinity, while the samples modified by CS show the opposite trend. Compared with 1CS, the higher flexural strength and flexural modulus of 5CS attributes to the fact that the increased size of calcium ionic clusters hinders the movement of molecular chains, as shown in Figure 7. Besides, when PEW and CS are applied to modify UHMWPE simultaneously, the added CS could decline the flexural performance of the samples. For example, although 13PEW and 13PEW5CS have almost the same crystallinity, the flexural strength and flexural modulus of the former are higher than those of the latter.

Notched Impact Test Analyses
The notched impact strength of the samples with different contents of PEW and CS is shown in Figure 8c. Compared with pure UHMWPE, the notched impact strength of samples with the addition of PEW and CS decreases from 96.5 to 74.8 kJ/m 2 , which is mainly due to the fact that PEW and CS can penetrate into the gaps between the molecular chains of UHMWPE and change the molecular weight distribution.

Shape Memory Behaviors
Most SMPs contain two parts inside, including the "hard phase" and the "soft phase". The hard phase mainly plays a fixity role to maintain a permanent shape, while the soft phase mainly plays a deformation role to provide the SMPs with a temporary shape [14]. As shown in Table 2, the crystalline region could not melt at TSW in this study. Therefore, the crystalline regions in UHMWPE mainly acted as the "hard phase", while the amorphous regions acted as the "soft phase". According to Equation (6), the maximum theoretical stress is 75 MPa. Figure 9 shows the representative shape recovery process of 13PEW5CS over time at different TSW, which indicates that the SME has a strong temperature dependence [31]. Figure 10 shows the Rf and the Rr of UHMWPE modified by PEW and CS within two minutes (little shape recovery at more than two minutes) at different TSW. It can be seen that the Rf remains basically constant at the same TSW, but increases with the temperature because of more movement of the chain segments in the amorphous region, such as 90% at 85 °C, 95% at 100 °C, and 100% at 115 °C, which is consistent with the research of Wu et al. [50]. In particular, The shape recovery process is the process of releasing the energy stored during the deformation at T SW [51]. Compared with the maximum value (approximately 77%) of R r of UHMWPE at 100 • C, it can be seen from Figure 10b that the R r of each group sample ranges from 48% of 13PEW5CS at 115 • C to 79% of 13PEW at 100 • C, which indicates that the addition of PEW and CS does not significantly improve the R r of UHMWPE. In addition, the R r of the samples increases first and then decreases with the increase of the T SW , which may be attributed to the fact that the energy has already released as the external force is removed at low temperature, and the movement of the chain segments dissipates part of the energy at high temperature. It is also found that the R r increases with the increased crystallinity due to the greater recovery stress that emerges when the external force coerces the "hard phase" deformation. However, there is no such relationship between R r and crystallinity. The shape recovery process is the process of releasing the energy stored during the deformation at TSW [51]. Compared with the maximum value (approximately 77%) of Rr of UHMWPE at 100 °C, it can be seen from Figure 10b that the Rr of each group sample ranges from 48% of 13PEW5CS at 115 °C to 79% of 13PEW at 100 °C, which indicates that the addition of PEW and CS does not significantly improve the Rr of UHMWPE. In addition, the Rr of the samples increases first and then decreases with the increase of the TSW, which may be attributed to the fact that the energy has already released as the external force is removed at low temperature, and the movement of the chain segments dissipates part of the energy at high temperature. It is also found that the Rr increases with the increased crystallinity due to the greater recovery stress that emerges when the external force coerces the "hard phase" deformation. However, there is no such relationship between Rr and crystallinity.

Conclusions
In this article, the UHMWPE sheets with the shape memory property were prepared by compression molding technology. The FTIR spectra show the generation of chemical bond C-O-C The shape recovery process is the process of releasing the energy stored during the deformation at TSW [51]. Compared with the maximum value (approximately 77%) of Rr of UHMWPE at 100 °C, it can be seen from Figure 10b that the Rr of each group sample ranges from 48% of 13PEW5CS at 115 °C to 79% of 13PEW at 100 °C, which indicates that the addition of PEW and CS does not significantly improve the Rr of UHMWPE. In addition, the Rr of the samples increases first and then decreases with the increase of the TSW, which may be attributed to the fact that the energy has already released as the external force is removed at low temperature, and the movement of the chain segments dissipates part of the energy at high temperature. It is also found that the Rr increases with the increased crystallinity due to the greater recovery stress that emerges when the external force coerces the "hard phase" deformation. However, there is no such relationship between Rr and crystallinity.

Conclusions
In this article, the UHMWPE sheets with the shape memory property were prepared by compression molding technology. The FTIR spectra show the generation of chemical bond C-O-C Figure 10. Shape memory properties of the samples: (a) shape fixity ratio; (b) shape recovery ratio.

Conclusions
In this article, the UHMWPE sheets with the shape memory property were prepared by compression molding technology. The FTIR spectra show the generation of chemical bond C-O-C domains at 1097 cm −1 in the materials. Further research on the thermal properties of the samples found that the addition of PEW can improve the crystallinity of UHMWPE from 49.6% to 60.3%, while the addition of CS decreases the crystallinity to 43.5%. Besides, the addition of PEW or CS shows the reverse effect on the thermal stability performance because of the effect of crystallization on UHMWPE. However, it should be noted that excessive CS will reduce T 1 from 464.3 • C of UHMWPE to 460.1 • C of 13PEW5CS with the increased crystallinity. The chain entanglement density can be significantly improved from 453 mol/m 3 of UHMWPE to 583 mol/m 3 of 5PEW1CS. Due to the increased degree of slippage of the UHMWPE molecular chains by the addition of PEW and CS, modified UHMWPE exhibits obvious plastic deformation, which further improves the tensile strength from 21.7 MPa of UHMWPE to 23.6 MPa of 5PEW1CS and elongation at break from 161.6% of UHMWPE to 344.4% of 5PEW1CS. The temperature dependence of shape memory was characterized and found that the R f of modified UHMWPE increases with the temperature and reaches 100% at 115 • C, but the value of R r is generally low, and the maximum is just 79%, therefore further research is required to be focused on the improvement of R r .
Funding: This research received no external funding.