Synthesis and Application of a Thermoplastic Plate of Poly(lactide-ε-caprolactone) for Radiation Therapy

In this study, the poly(lactide-ε-caprolactone) (P(LA-CL)) copolymer is synthesized by ring-opening polymerization with glycol used as a molecular weight regulator to adjust the molecular weight of the polymer. The proton nuclear magnetic resonance spectroscopy and gel permeation chromatography (GPC) results demonstrate that the P(LA-CL) copolymer is successfully synthesized, and that the molecular weight can be controlled by the glycol content. The thermoplastic plate is processed with triallyl isocyanurate as a cross-linking agent by a single-screw extruder followed by γ-ray irradiation. Shape memory test results show that the material had the desired shape memory effect, with deformation recovery rates reaching 100%. After secondary stretching of samples, deformation recovery rates are unchanged. The results of mechanical property measurements indicate that with added lactide, the tensile strength is improved and shore hardness is increased by 20%–30%. Data from clinical trials also reveal that the material has good clinical effects in thermoplastic membrane fixation.


Introduction
Poly(ε-caprolactone) (PCL) is a biodegradable polyester obtained by the ring-opening polymerization of ε-caprolactone. This semi-crystalline polymer has a degree of crystallinity around 50%, a low glass transition temperature (approximately −60 • C) and a melting point of approximately 60 • C. The PCL chain is flexible, and exhibits high elongation at break, a low modulus, and low processing temperatures [1,2]. This material has good biocompatibility, and can be used to modify other biopolymer materials. For instance, polylactic acid has been modified to increase the toughness and copolymerization of PCL [3], and poly(L-lactic acid) (PLLA) has modified PLLA mechanical properties for use in orthopedic bone repairs [4]. The three complexes were prepared by the vine-twining polymerization method using poly(tetrahydrofuran) (PTHF), poly(ε-caprolactone) (PCL) and poly(l-lactide) (PLLA) as guest polymers to make a film. [5] Degradable polymers, especially polylactic acid (PLA), polyglycolic acid (PGA) and their copolymers, are the most frequently applied coating materials for the stent [6].
The important properties of PCL are shape memory and low-temperature thermoplasticity at 60 • C. PCL has potential applications in the biomedical field, such as in radiotherapy positioning plates, belts, bandages and appliances. However, pure polycaprolactone materials have been found to be imperfect in strength and shape memory.

Synthesis of Poly(lactide-ε-caprolactone) Copolymer
The opening ring polymerization of different weight ratios of ε-caprolactone (70%, 80%, and 90%) and lactide (with a total monomer weight of 400 g) was performed in a 1000-mL three-necked flask equipped with a magnetic stirrer, using 0.15% (g/g) SnOct as a catalyst. The reactants were filled with high purity argon, degassed in vacuum three times, and then sealed. Glycol as a molecular-weight regulator was added into the reaction system at dosages of 0, 0.0625, and 0.125% (g/g).
Under argon gas protection and a polymerization temperature of up to 180 • C, the reaction time was controlled at 4 h. Following the reaction, the system was cooled to 100 • C. The polymer was poured into a water tank to cool and then pulled into thin strips. The strips were cut into small copolymer pellets by hand. The synthesis mechanisms of copolymers are shown in Figures 1 and 2. To examine the effects of the monomer ratio, we prepared a number of synthetic copolymers from A to I according to the ratio of ε-caprolactone and lactide. The pure poly(ε-caprolactone) (PCL) is identified as Sample J. Specific synthesis conditions are shown in Table 1.
Biomolecules 2019, 9, x FOR PEER REVIEW 3 of 13 with high purity argon, degassed in vacuum three times, and then sealed. Glycol as a molecularweight regulator was added into the reaction system at dosages of 0, 0.0625, and 0.125% (g/g). Under argon gas protection and a polymerization temperature of up to 180 °C, the reaction time was controlled at 4 h. Following the reaction, the system was cooled to 100 °C. The polymer was poured into a water tank to cool and then pulled into thin strips. The strips were cut into small copolymer pellets by hand. The synthesis mechanisms of copolymers are shown in Figures 1 and 2.
To examine the effects of the monomer ratio, we prepared a number of synthetic copolymers from A to I according to the ratio of ε-caprolactone and lactide. The pure poly(ε-caprolactone) (PCL) is identified as Sample J. Specific synthesis conditions are shown in Table 1.

Preparation of Thermoplastic Plates
Thermoplastic plates were prepared by a single-screw extruder (LSJ-20, Shanghai, China). The poly(lactide-ε-caprolactone) (P(LA-CL)) copolymer and triallyl isocyanurate at a ratio of 0.2% copolymer were added to the extruder. A smooth, thermoplastic plate was prepared when the processing temperatures of each section were 70, 70, 80, and 70 °C at a 40 r/min screw speed. At these settings, the melt of the extruded material added by a single screw is pressed out and cooled by a three-roll machine to obtain a plate with a thickness of 2 mm. Finally, we cut, punch and pack according to design requirements.
The packaged sheet is entrusted to a radiation plant to undergo r-ray irradiation, and the irradiation measurement is 5-10 KGY, and the size of the irradiation measurement is determined according to the use requirements of the low temperature thermoplastic board. with high purity argon, degassed in vacuum three times, and then sealed. Glycol as a molecularweight regulator was added into the reaction system at dosages of 0, 0.0625, and 0.125% (g/g). Under argon gas protection and a polymerization temperature of up to 180 °C, the reaction time was controlled at 4 h. Following the reaction, the system was cooled to 100 °C. The polymer was poured into a water tank to cool and then pulled into thin strips. The strips were cut into small copolymer pellets by hand. The synthesis mechanisms of copolymers are shown in Figures 1 and 2.
To examine the effects of the monomer ratio, we prepared a number of synthetic copolymers from A to I according to the ratio of ε-caprolactone and lactide. The pure poly(ε-caprolactone) (PCL) is identified as Sample J. Specific synthesis conditions are shown in Table 1.

Preparation of Thermoplastic Plates
Thermoplastic plates were prepared by a single-screw extruder (LSJ-20, Shanghai, China). The poly(lactide-ε-caprolactone) (P(LA-CL)) copolymer and triallyl isocyanurate at a ratio of 0.2% copolymer were added to the extruder. A smooth, thermoplastic plate was prepared when the processing temperatures of each section were 70, 70, 80, and 70 °C at a 40 r/min screw speed. At these settings, the melt of the extruded material added by a single screw is pressed out and cooled by a three-roll machine to obtain a plate with a thickness of 2 mm. Finally, we cut, punch and pack according to design requirements.
The packaged sheet is entrusted to a radiation plant to undergo r-ray irradiation, and the irradiation measurement is 5-10 KGY, and the size of the irradiation measurement is determined

Preparation of Thermoplastic Plates
Thermoplastic plates were prepared by a single-screw extruder (LSJ-20, Shanghai, China). The poly(lactide-ε-caprolactone) (P(LA-CL)) copolymer and triallyl isocyanurate at a ratio of 0.2% copolymer were added to the extruder. A smooth, thermoplastic plate was prepared when the Biomolecules 2020, 10, 27 4 of 13 processing temperatures of each section were 70, 70, 80, and 70 • C at a 40 r/min screw speed. At these settings, the melt of the extruded material added by a single screw is pressed out and cooled by a three-roll machine to obtain a plate with a thickness of 2 mm. Finally, we cut, punch and pack according to design requirements.
The packaged sheet is entrusted to a radiation plant to undergo r-ray irradiation, and the irradiation measurement is 5-10 KGY, and the size of the irradiation measurement is determined according to the use requirements of the low temperature thermoplastic board.

Measurements 1 H-NMR Measurements
The 1 H spectra of copolymers were recorded with a nuclear magnetic resonator (NMR) (Bruker Avance III 400 MHz, Switzerland). Deuterated chloroform (CDCl 3 ) was used as the solvent at 25 • C and the chemical shifts were given with respect to tetramethylsilane. 1 H spectra were obtained from 16 scans.

GPC Measurements
The molecular weights and molecular distributions of PCL and P(LA-CL) with different ratios were determined by gel permeation chromatography (GPC) with a Waters Associates model ALC/GPC 244 apparatus at 40 • C with a differential refractometer as the detector, tetrahydrofuran (THF) as the solvent, and calibration with polystyrene standards. Three specimens were tested under each condition.

SEM Measurements
The cross-sectional morphology of thermoplastic plate was observed directly by a scanning electron microscope (SEM) (Quanta200, FEI, Hillsboro, USA) without sputter coating, but with a conducting matter. The sample was first frozen in liquid nitrogen and then lyophilized at −47 • C.

Shape Memory Process Measurements
The thermoplastic plate was heated in 60 • C water bath for 10 minutes, and then it was stretched by a certain length. The original and stretched lengths were recorded. The stretched sample was heated again with the same time and temperature and the recovering length was measured. After this, the recovered sample was re-placed heated in 60 • C water and repeat the above shape memory experiment Mechanical Property Measurements.
Tensile tests were conducted using a universal testing machine (GMT-400; Shanghai, China) using the national standard GB/T 1040. All samples were cut to 100 mm × 20 mm. The tensile speed was 10 mm/min. All reported results are the averages of at least three test specimens. The elongation at break of samples can be calculated according to equation (1), where Eb(%) is the elongation at break, L (mm) is the distance between the lines when the sample is broken and L 0 (mm) is the original line distance of the sample.

Shore Hardness Measurements
The shore hardness was measured with a digital shore durometer (SAUTER HDD100-1; Germany). Each set of tests was performed in triplicate.

DSC Measurements
Differential scanning calorimetry (DSC) was performed using a TA Instruments DSC-200PC (Netzsch, Germany). An empty pan was used as a reference. After being dried in a vacuum oven at 40 • C for 48 h, circular samples were cut from the plates and accurately weighed (5 mg) into aluminum pans. DSC measurements were carried out under nitrogen flow from 30 • C to 200 • C at a heating rate of 5 • C/min.

Clinical Trials of P(LA-CL) Low-Temperature Thermoplastic Plates
During the clinical testing phase, 60 patients with breast cancer received radiation therapy in the Department of Radiation Oncology of Sichuan Cancer Hospital and were confirmed by pathology. Inclusion criteria included patients aged older than 30 years and weighing more than 38 kg. Of 60 patients, 43 were male and the patients were randomly enrolled into two groups: 30 patients in the vacuum pad fixation group identified as Group 1 and 30 patients in the thermoplastic membrane fixation group identified as Group 2. Vacuum pad fixation and thermoplastic membrane fixation are shown in Figure 3. The vacuum pad fixation group included 15 lung cancer and 15 esophageal cancer patients consisting of 19 men and 11 women. The thermoplastic membrane fixation group included 1 thymic carcinoma, 17 lung cancer and 12 esophageal cancer patients consisting of 15 men and 15 women. All of the patients gave their informed consent before treatment, which was in accordance with the Declaration of Helsinki and also approved by the Ethics Committee of Sichuan Cancer Hospital.  Two groups of patients were treated with intensity-modulated radiation therapy combined with computed tomography (CT) simulation to accurately locate tumors. The image data obtained were used in treatment planning. According to the requirements to determine the positioning center, under the CT simulator will be Move the reference center to the positioning center. Using cone beam CT (CBCT) scans shown in Figure 4, three translational setup errors in the lateral (X), cranial-caudal (Y) and anterior-posterior directions (Z), were obtained. The U, V and W rotation setup errors were obtained by image analysis software to determine the corresponding X, Y and Z values.  Two groups of patients were treated with intensity-modulated radiation therapy combined with computed tomography (CT) simulation to accurately locate tumors. The image data obtained were used in treatment planning. According to the requirements to determine the positioning center, under the CT simulator will be Move the reference center to the positioning center. Using cone beam CT (CBCT) scans shown in Figure 4, three translational setup errors in the lateral (X), cranial-caudal (Y) and anterior-posterior directions (Z), were obtained. The U, V and W rotation setup errors were obtained by image analysis software to determine the corresponding X, Y and Z values.  Two groups of patients were treated with intensity-modulated radiation therapy combined with computed tomography (CT) simulation to accurately locate tumors. The image data obtained were used in treatment planning. According to the requirements to determine the positioning center, under the CT simulator will be Move the reference center to the positioning center. Using cone beam CT (CBCT) scans shown in Figure 4, three translational setup errors in the lateral (X), cranial-caudal (Y) and anterior-posterior directions (Z), were obtained. The U, V and W rotation setup errors were obtained by image analysis software to determine the corresponding X, Y and Z values.

Statistical Analysis
Data were analyzed according to the mean and standard deviation of three test replicates for each sample. The statistical analysis was performed by analysis of variance (ANOVA) using SPSS version 13.0 at a significance level of p < 0.05.

Characterization of Copolymers
The conversion and yield of a polymer is an important indicator of whether the polymer can be mass produced. As shown in Table 2, the conversion and yield of nine copolymers were more than 90%. Comparison with similar literature indicates that yields and conversion rates have been greatly improved [14], and that the yields obtained in our study are able to meet production requirements. To confirm their structures, copolymers were characterized with 1 H-NMR. Figures 5 and 6 show 1 H-NMR spectra of the poly(LA-CL) copolymer without glycol and poly(LA-CL) copolymer with glycol, respectively. In Figures 5 and 6, the 1 H-NMR spectrum of lactide shows peaks at 1.44 and 5.17 ppm, which are assigned to the methyl and methine protons of the PLA chains, respectively. The 1 H-NMR spectrum of caprolactone shows peaks at 1.52, 2.35, and 4.12 ppm, which are assigned to methylene protons from (CH 2 ) 3 , CO-CH 2 , and CH 2 -O groups of the PCL chains, respectively. These results are consistent with those previously reported [15,16]. 90%. Comparison with similar literature indicates that yields and conversion rates have been greatly improved [14], and that the yields obtained in our study are able to meet production requirements.
To confirm their structures, copolymers were characterized with 1 H-NMR. Figures 5 and 6 show 1 H-NMR spectra of the poly(LA-CL) copolymer without glycol and poly(LA-CL) copolymer with glycol, respectively. In Figures 5 and 6, the 1 H-NMR spectrum of lactide shows peaks at 1.44 and 5.17 ppm, which are assigned to the methyl and methine protons of the PLA chains, respectively. The 1 H-NMR spectrum of caprolactone shows peaks at 1.52, 2.35, and 4.12 ppm, which are assigned to methylene protons from (CH2)3, CO-CH2，and CH2-O groups of the PCL chains, respectively. These results are consistent with those previously reported [15,16].      Figure 7 shows that the gel permeation chromatography (GPC) curves of the copolymer samples each consist of only a single molecular weight distribution peak. These results indicate that the target  Figure 7 shows that the gel permeation chromatography (GPC) curves of the copolymer samples each consist of only a single molecular weight distribution peak. These results indicate that the target product is a copolymer rather than a homopolymer blend. It can be seen in Table 3 that molecular weights increased with a decline in glycol content. The effect of glycol on molecular weight is significant; the molecular weight reached approximately 20 kDa in the absence of glycol and was significantly reduced with its addition. Because the molecular weight can affect the properties and use of polymers, selecting a polymer with an appropriate molecular weight is also a purpose of our study. product is a copolymer rather than a homopolymer blend. It can be seen in Table 3 that molecular weights increased with a decline in glycol content. The effect of glycol on molecular weight is significant; the molecular weight reached approximately 20 kDa in the absence of glycol and was significantly reduced with its addition. Because the molecular weight can affect the properties and use of polymers, selecting a polymer with an appropriate molecular weight is also a purpose of our study.

Characterization of Thermoplastic Plates
In polymer material processing and modification, triallyl isocyanurate (TAIC) is one of the most commonly used cross-linking agents in industrial processes. TAIC has been reported in literature as an agent for cross-linking PLA in the preparation of cross-linked composites. However, the preparation of PCL composite materials using TAIC as a cross-linking agent is typically not reported. We have thus used TAIC to cross-link P(LA-CL) copolymers and prepare composite materials in our study. Figure 8 is the 1 H-NMR spectrum of TAIC and Figure 9a, b are the 1 H-NMR spectra of

Characterization of Thermoplastic Plates
In polymer material processing and modification, triallyl isocyanurate (TAIC) is one of the most commonly used cross-linking agents in industrial processes. TAIC has been reported in literature as an agent for cross-linking PLA in the preparation of cross-linked composites. However, the preparation of PCL composite materials using TAIC as a cross-linking agent is typically not reported. We have thus used TAIC to cross-link P(LA-CL) copolymers and prepare composite materials in our study. Figure 8 is the 1 H-NMR spectrum of TAIC and Figure 9a,b are the 1 H-NMR spectra of thermoplastic plates without irradiation and with irradiation, respectively. Comparison of Figure 9a,b indicate an absence of TAIC characteristic chemical shifts after irradiation. This suggests that the TAIC has reacted with P(LA-CL). To confirm that TAIC acted as a cross-linking agent, the properties of composite materials are examined in the following subsection.

Mechanical Property Analysis of Thermoplastic Plates
The tensile strength and elongation at break describe how the mechanical properties of materials are related to their chemical structure [11]. The tensile strength and elongation at break of low thermoplastic plates pre-and post-irradiation are presented in Figure 10a, b. All reported results are the averages of at least three test specimens. Lactide is a rigid segment. As the content of lactide increases, the stiffness of the copolymer is increased, so the tensile strength is increased. When the lactide content was more than 20%, the tensile strength of the composite samples was higher than that observed for pure PCL. Comparison of pre-and post-irradiation samples also indicated that tensile strength was enhanced by irradiation. This is consistent with the change in morphological structure upon irradiation as proven by scanning electron microscopy (SEM). In contrast, the elongation at break decreased with an increase in lactide content.

Mechanical Property Analysis of Thermoplastic Plates
The tensile strength and elongation at break describe how the mechanical properties of materials are related to their chemical structure [11]. The tensile strength and elongation at break of low thermoplastic plates pre-and post-irradiation are presented in Figure 10a, b. All reported results are the averages of at least three test specimens. Lactide is a rigid segment. As the content of lactide increases, the stiffness of the copolymer is increased, so the tensile strength is increased. When the lactide content was more than 20%, the tensile strength of the composite samples was higher than that observed for pure PCL. Comparison of pre-and post-irradiation samples also indicated that tensile strength was enhanced by irradiation. This is consistent with the change in morphological structure upon irradiation as proven by scanning electron microscopy (SEM). In contrast, the elongation at break decreased with an increase in lactide content.

Mechanical Property Analysis of Thermoplastic Plates
The tensile strength and elongation at break describe how the mechanical properties of materials are related to their chemical structure [11]. The tensile strength and elongation at break of low thermoplastic plates pre-and post-irradiation are presented in Figure 10a,b. All reported results are the averages of at least three test specimens. Lactide is a rigid segment. As the content of lactide increases, the stiffness of the copolymer is increased, so the tensile strength is increased. When the lactide content was more than 20%, the tensile strength of the composite samples was higher than that observed for pure PCL. Comparison of pre-and post-irradiation samples also indicated that tensile strength was enhanced by irradiation. This is consistent with the change in morphological structure upon irradiation as proven by scanning electron microscopy (SEM). In contrast, the elongation at break decreased with an increase in lactide content. For the shape memory of a low-temperature thermoplastic plate prepared with PCL composite material, the shore hardness is a more intuitive index for practical applications. According to clinical expert feedback, the shore hardness of low-temperature thermoplastic plates in the current market can reach up to 40 D. With the clinical requirements, normally, it needs to be stretched 1-3 times in local or overall during the use. After stretching, the material becomes longer and thinner, so the model after molding becomes very soft, and the supporting force is very weak, which seriously affects the use effect. The hardness of low thermoplastic plates is hoped to improve to about 50 D [27,28], and Figure 11 shows the shore hardness of P(LA-CL) thermoplastic plates after irradiation. The shore hardness improved with an increase in lactide content and molecular weight; at an Mw of 10.5 kDa, the shore hardness reached 48.4 D. The data shows that the hardness of the three groups A, B and C, is less than 40 D, while the hardness of the three groups G, H and I is too large, and the deformation temperature is too high. It is not conducive to clinical application when the shore hardness is too high. Therefore, Samples D, E and F are considered ideal thermoplastic plate materials.

DSC Analysis of Thermoplastic Plates
Based on the tensile test, shore hardness test, morphology test and molecular weight study, samples synthesized with a CL:LA ratio of 8:2 were chosen for studies with DSC. Figure 12 shows the DSC curves of P(LA-CL) composite materials with and without irradiation. As shown in Figure  12 and Table 4, the melting temperature was improved by irradiation. The thermal stability also For the shape memory of a low-temperature thermoplastic plate prepared with PCL composite material, the shore hardness is a more intuitive index for practical applications. According to clinical expert feedback, the shore hardness of low-temperature thermoplastic plates in the current market can reach up to 40 D. With the clinical requirements, normally, it needs to be stretched 1-3 times in local or overall during the use. After stretching, the material becomes longer and thinner, so the model after molding becomes very soft, and the supporting force is very weak, which seriously affects the use effect. The hardness of low thermoplastic plates is hoped to improve to about 50 D [27,28], and Figure 11 shows the shore hardness of P(LA-CL) thermoplastic plates after irradiation. The shore hardness improved with an increase in lactide content and molecular weight; at an Mw of 10.5 kDa, the shore hardness reached 48.4 D. The data shows that the hardness of the three groups A, B and C, is less than 40 D, while the hardness of the three groups G, H and I is too large, and the deformation temperature is too high. It is not conducive to clinical application when the shore hardness is too high. Therefore, Samples D, E and F are considered ideal thermoplastic plate materials. For the shape memory of a low-temperature thermoplastic plate prepared with PCL composite material, the shore hardness is a more intuitive index for practical applications. According to clinical expert feedback, the shore hardness of low-temperature thermoplastic plates in the current market can reach up to 40 D. With the clinical requirements, normally, it needs to be stretched 1-3 times in local or overall during the use. After stretching, the material becomes longer and thinner, so the model after molding becomes very soft, and the supporting force is very weak, which seriously affects the use effect. The hardness of low thermoplastic plates is hoped to improve to about 50 D [27,28], and Figure 11 shows the shore hardness of P(LA-CL) thermoplastic plates after irradiation. The shore hardness improved with an increase in lactide content and molecular weight; at an Mw of 10.5 kDa, the shore hardness reached 48.4 D. The data shows that the hardness of the three groups A, B and C, is less than 40 D, while the hardness of the three groups G, H and I is too large, and the deformation temperature is too high. It is not conducive to clinical application when the shore hardness is too high. Therefore, Samples D, E and F are considered ideal thermoplastic plate materials.

DSC Analysis of Thermoplastic Plates
Based on the tensile test, shore hardness test, morphology test and molecular weight study, samples synthesized with a CL:LA ratio of 8:2 were chosen for studies with DSC. Figure 12 shows the DSC curves of P(LA-CL) composite materials with and without irradiation. As shown in Figure  12 and Table 4, the melting temperature was improved by irradiation. The thermal stability also (a) (b) Figure 11. Shore hardness of composite materials.

DSC Analysis of Thermoplastic Plates
Based on the tensile test, shore hardness test, morphology test and molecular weight study, samples synthesized with a CL:LA ratio of 8:2 were chosen for studies with DSC. Figure 12 shows the DSC curves of P(LA-CL) composite materials with and without irradiation. As shown in Figure 12 and Table 4, the melting temperature was improved by irradiation. The thermal stability also increased as the molecular weight increased. At molecular weights above 11.1 kDa, the Tm showed little increase with further increases in molecular weight. The following clinical experiments were thus conducted using the composite material from Sample E. Since the radiation dose we use is very low, it does not alter crystallization behavior [29,30].
Biomolecules 2019, 9, x FOR PEER REVIEW 10 of 13 increased as the molecular weight increased. At molecular weights above 11.1 kDa, the Tm showed little increase with further increases in molecular weight. The following clinical experiments were thus conducted using the composite material from Sample E. Since the radiation dose we use is very low, it does not alter crystallization behavior [29,30].

SEM Analysis of Thermoplastic Plates
Based on the results of the tensile test, shore hardness test, morphology test and molecular weight test, Figure 13 shows the cross-sectional morphologies of pre-and post-irradiation samples. The pre-irradiation sample consisted of a small block structure that exhibited structural relaxation. However, the flexible structure of the thermoplastic plate disappeared upon exposure to radiation, and its structure became dense and rigid. This change in structure may be explained by the effect of radiation in inducing reactions with the cross-linking agent that could affect the material properties.

Shape Memory Process Analysis of Thermoplastic Plates
Based on the results of the tensile test, shore hardness test, morphology test and molecular weight test, we choose group E for the shape memory test. The material shown in Figure 14 exhibited

SEM Analysis of Thermoplastic Plates
Based on the results of the tensile test, shore hardness test, morphology test and molecular weight test, Figure 13 shows the cross-sectional morphologies of pre-and post-irradiation samples. The pre-irradiation sample consisted of a small block structure that exhibited structural relaxation. However, the flexible structure of the thermoplastic plate disappeared upon exposure to radiation, and its structure became dense and rigid. This change in structure may be explained by the effect of radiation in inducing reactions with the cross-linking agent that could affect the material properties. increased as the molecular weight increased. At molecular weights above 11.1 kDa, the Tm showed little increase with further increases in molecular weight. The following clinical experiments were thus conducted using the composite material from Sample E. Since the radiation dose we use is very low, it does not alter crystallization behavior [29,30].

SEM Analysis of Thermoplastic Plates
Based on the results of the tensile test, shore hardness test, morphology test and molecular weight test, Figure 13 shows the cross-sectional morphologies of pre-and post-irradiation samples. The pre-irradiation sample consisted of a small block structure that exhibited structural relaxation. However, the flexible structure of the thermoplastic plate disappeared upon exposure to radiation, and its structure became dense and rigid. This change in structure may be explained by the effect of radiation in inducing reactions with the cross-linking agent that could affect the material properties.

Shape Memory Process Analysis of Thermoplastic Plates
Based on the results of the tensile test, shore hardness test, morphology test and molecular weight test, we choose group E for the shape memory test. The material shown in Figure 14 exhibited

Shape Memory Process Analysis of Thermoplastic Plates
Based on the results of the tensile test, shore hardness test, morphology test and molecular weight test, we choose group E for the shape memory test. The material shown in Figure 14 exhibited the desired shape memory effect where the deformation recovery rate had substantially reached 100%. The deformation recovery rate was essentially unaffected by secondary stretching. Despite a secondary stretching length of 1.5 times the first stretch, the thermoplastic plate returned to its initial length upon heating. Thus, a composite material with shape memory has been achieved in this study.
Biomolecules 2019, 9, x FOR PEER REVIEW 11 of 13 the desired shape memory effect where the deformation recovery rate had substantially reached 100%. The deformation recovery rate was essentially unaffected by secondary stretching. Despite a secondary stretching length of 1.5 times the first stretch, the thermoplastic plate returned to its initial length upon heating. Thus, a composite material with shape memory has been achieved in this study. Figure 14. Shape memory process analyses of thermoplastic plates.

Clinic Data Analysis of Thermoplastic Plates
Clinical tests were processed in the Sichuan Cancer Hospital in Sichuan Province. Groups 1 and 2 are trialed vacuum pad fixation and thermoplastic membrane fixation, respectively. The setup errors are shown in Table 5. The results revealed that setup errors with thermoplastic membrane fixation were less than errors for vacuum pad fixation in all directions with P values lower than 0.05.

Conclusions
In our study, PCL was modified with biocompatible and biodegradable PLA, and glycol was used as a molecular weight regulator to adjust the molecular weight of the polymer. The results demonstrated that the P(LA-CL) copolymer was successfully synthesized with the conversion and yield of nine copolymers at more than 90% using our synthesis method. To get a better shape memory function, we used TAIC with three terminal alkenyl functional groups as a cross-linking agent to prepare the composite material. The thermoplastic plate was processed by a single-screw extruder, and then placed under γ-ray irradiation. Shape memory and performance test results showed that the material has the desired shape memory effect with a deformation recovery rate reaching 100%. After the secondary stretching of samples, the deformation recovery rate remained unchanged. With the added lactide, the tensile strength was improved and shore hardness increased by 20%-30%. Data from clinical trials reveal that the material has good clinical effects in thermoplastic membrane

Clinic Data Analysis of Thermoplastic Plates
Clinical tests were processed in the Sichuan Cancer Hospital in Sichuan Province. Groups 1 and 2 are trialed vacuum pad fixation and thermoplastic membrane fixation, respectively. The setup errors are shown in Table 5. The results revealed that setup errors with thermoplastic membrane fixation were less than errors for vacuum pad fixation in all directions with P values lower than 0.05.

Conclusions
In our study, PCL was modified with biocompatible and biodegradable PLA, and glycol was used as a molecular weight regulator to adjust the molecular weight of the polymer. The results demonstrated that the P(LA-CL) copolymer was successfully synthesized with the conversion and yield of nine copolymers at more than 90% using our synthesis method. To get a better shape memory function, we used TAIC with three terminal alkenyl functional groups as a cross-linking agent to prepare the composite material. The thermoplastic plate was processed by a single-screw extruder, and then placed under γ-ray irradiation. Shape memory and performance test results showed that the material has the desired shape memory effect with a deformation recovery rate reaching 100%. After the secondary stretching of samples, the deformation recovery rate remained unchanged. With the added lactide, the tensile strength was improved and shore hardness increased by 20-30%. Data from clinical trials reveal that the material has good clinical effects in thermoplastic membrane fixation. In summary, the low thermoplastic plate of the P(LA-CL) copolymer has very good application prospects in radiotherapy positioning.