Characteristics of Polycarbonate Soft Segment-Based Thermoplastic Polyurethane

The pre-experiment of the present study revealed that polyurethane (PU) synthesized using poly (hexamethylene carbonate) glycol (PHC) has high melt viscosity and is difficult to process. Therefore, poly (trimethylene carbonate) glycol (PTC) was employed to synthesize a PU product with low melt viscosity. First, four types of thermoplastic polyurethane (TPU) were formed through one-step solvent-free synthesis. TPU is presented in the format “TPU-X-Y,” with X representing the polyol (PTC or PHC, around 1000 molecular weight) and Y the chain extender (1,3-propane diol [PDO] or 1,4-butane diol [BDO]) used. The TPU was synthesized using a fixed molar ratio of (isocyanate):(polyol):(chain extender) = 2:1:1 and compared. The results indicated that chain entanglement often occurred among the long carbon chains of PHC. The synthesized TPU employed a property of PTC, namely converting polarity into reverse polarity in high temperatures, to resolve the high melt viscosity of TPU of the PHC series, which causes processing difficulties. The synthesized TPU-PTC-PDO exhibited favorable molecular arrangements. Given its polarity, TPU-PTC-PDO has outstanding tensile properties (strength at break: 41.10 ± 10.78 MPa; 100% modulus = 6.73 ± 0.12 MPa), making processing at lower temperatures (180 or 190 °C) feasible. With the inclusion of PTC, the synthesized polycarbonate TPU exhibits the advantages of polycarbonate and is suitable for a wide range of applications.


Introduction
Early polyurethane (PU) synthesis methods involved synthesis in organic solvents. Dimethylformamide (DMF), a frequently used solvent, poses environmental risks, is detrimental to liver function, and reduces fertility. Other common solvents, including methyl ethyl ketone (MEK), toluene, and cyclohexanone, are highly volatile, nonenvironmentally friendly, and harmful to human health. Therefore, scientists have employed novel environmentally friendly synthesis methods such as waterborne polyurethane (WPU) [1,2], which uses cationic, anionic, and nonionic emulsifiers [3][4][5] to disperse PU micelles in water. However, the micelle size used for WPU must be moderate to achieve proper dispersion and prevent deposition in water. Micelle size is directly related to molecular weight; thus, the molecular weight of micelles used for WPU must be of limited size. If excessively large micelles are used for WPU, the mechanical strength of the resulting products will be weaker than that of those created using traditional solvent-based PU synthesis methods [6]. Scientists have also employed solvent-free PU synthesis methods [7,8] that do not require organic solvents and do not involve volatile organic compounds (VOCs) in product enabling the polymer to retain its melt strength in the melt state. However, the resulting poor flowability causes processing difficulties, thereby limiting the polymer's applications. The previous experiment aimed to design a polycarbonate TPU with low melt viscosity to increase the application of polycarbonate TPU. Scholars have noted that trimethylene carbonate (TMC) has a high polar attraction in low-temperature environments. However, its polarity decreases under high temperatures, resulting in reverse polar attraction [22,23]. TMC can be synthesized into poly (trimethylene carbonate) glycol (PTC), comprising repeating units of TMC. Because PTC is not commercially available, we referenced a patented method and used TMC to synthesize PTC [24]. We attempted to design a TPU comprising repeating units of MDI-PTC-PDO and polymer chains with high tacticity. Because the polymer chains have high polar attraction, the TPU will have favorable mechanical strength under room temperature and good flowability in the melted state.
Accordingly, a series of experiments were designed to synthesize four types of TPU using MDI with two types of polycarbonate polyols and two types of chain extender (i.e., PDO or BDO). First, the TPUs were used to test the initial hypothesis. Subsequently, spectral structure analysis, mechanical property analysis, flowability analysis, and thermal property analysis were performed to verify whether chain entanglement reduced the flowability of PHC-synthesized TPU and whether the designed MDI-PTC-PDO has high mechanical properties and low melt viscosity.

Poly (TMC) Glycol Synthesis
TMC was subjected to monomer-based cationic ring-opening polymerization to synthesize PTC, with Nafion NR50 as the catalyst [22]. First, TMC (10 g) was dissolved in CH 2 Cl 2 (20 mL) in a three-necked bottle. Nafion NR50 (2 g) was added, and a condenser was fixed to the bottle to reflux the volatile CH 2 Cl 2 . Mechanical stirring was then performed, and the mixture was left to react in a dry nitrogen environment for 24 h at 25 • C. Figure 1 presents the synthesis process.
Appl. Sci. 2021, 11, x FOR PEER REVIEW 3 of 18 in-depth analysis of this problem has yet to be conducted. The current researchers observed the molecular structures and hypothesized that longer main carbon chain lengths result in chain entanglement, enabling the polymer to retain its melt strength in the melt state. However, the resulting poor flowability causes processing difficulties, thereby limiting the polymer's applications. The previous experiment aimed to design a polycarbonate TPU with low melt viscosity to increase the application of polycarbonate TPU. Scholars have noted that trimethylene carbonate (TMC) has a high polar attraction in lowtemperature environments. However, its polarity decreases under high temperatures, resulting in reverse polar attraction [22,23]. TMC can be synthesized into poly (trimethylene carbonate) glycol (PTC), comprising repeating units of TMC. Because PTC is not commercially available, we referenced a patented method and used TMC to synthesize PTC [24]. We attempted to design a TPU comprising repeating units of MDI-PTC-PDO and polymer chains with high tacticity. Because the polymer chains have high polar attraction, the TPU will have favorable mechanical strength under room temperature and good flowability in the melted state. Accordingly, a series of experiments were designed to synthesize four types of TPU using MDI with two types of polycarbonate polyols and two types of chain extender (i.e., PDO or BDO). First, the TPUs were used to test the initial hypothesis. Subsequently, spectral structure analysis, mechanical property analysis, flowability analysis, and thermal property analysis were performed to verify whether chain entanglement reduced the flowability of PHC-synthesized TPU and whether the designed MDI-PTC-PDO has high mechanical properties and low melt viscosity.

Poly (TMC) Glycol Synthesis
TMC was subjected to monomer-based cationic ring-opening polymerization to synthesize PTC, with Nafion NR50 as the catalyst [22]. First, TMC (10 g) was dissolved in CH2Cl2 (20 mL) in a three-necked bottle. Nafion NR50 (2 g) was added, and a condenser was fixed to the bottle to reflux the volatile CH2Cl2. Mechanical stirring was then performed, and the mixture was left to react in a dry nitrogen environment for 24 h at 25 °C. Figure 1 presents the synthesis process. After the reaction was completed, the product was instilled into a methanol/deionized water mixture (v/v = 9:1) for polarity separation. PTC sediment was collected from the bottom of the mixture and placed in a vacuum oven to remove excess water and solvent. PTC was subsequently subjected to nuclear magnetic resonance, and an automatic After the reaction was completed, the product was instilled into a methanol/deionized water mixture (v/v = 9:1) for polarity separation. PTC sediment was collected from the bottom of the mixture and placed in a vacuum oven to remove excess water and solvent. PTC was subsequently subjected to nuclear magnetic resonance, and an automatic titrator was employed to determine the hydroxyl value (OHV). The hydroxyl value was then inputted into an equation to calculate a molecular weight (ASTM E 1899-02). Each batch of PTC had an OHV of 88.5-96.1 mg KOH/g.

TPU Synthesis
Polyol (PTC or PHC) and a chain extender (PDO or BDO) were placed in a 100 • C vacuum oven for 4 h to remove water. MDI was placed in a 60 • C convection oven for 2 h for melting. As presented in Table 1, TPU was synthesized using isocyanate, polyol, and a chain extender at a 2:1:1 mole ratio. Polyol and the chain extender were then extracted, evenly mixed utilizing a mechanical stirrer, and placed in an oven at 100 • C. The products were removed, added with MDI and T12 (catalyst), and subjected to machine mixing. After being mixed to a sticky state, the product was placed on an iron Teflon plate and placed in a 120 • C oven for 3 h. The baked lumpy TPU was subjected to tests, one of which involved using redissolved TPU as a coating material for mechanical property testing and dynamic mechanical analysis (DMA). The TPU was named in the TPU-X-Y format, with X and Y representing the polyol and chain extender used. For example, in TPU-PTCD-PDO, PTCD, and PDO represent the used polyol and chain extender, respectively. Figure 2 [25][26][27][28][29] displays the synthesis process. However, TPU copolymers in Figure 2 were ideal chemical structures for one unit cycle due to each lengthy polymer structure. Therefore, Figure 2 cannot show the random structure completely. Because the TPU used one-step synthesis in the research, MDI should react with BDO and polyols under an irregular state, and therefore the TPUs had the random hard segment repeat units. Scheme 1b in the manuscript was a suitable expression [26].

TPU Synthesis
Polyol (PTC or PHC) and a chain extender (PDO or BDO) were placed in a 100 °C vacuum oven for 4 h to remove water. MDI was placed in a 60 °C convection oven for 2 h for melting. As presented in Table 1, TPU was synthesized using isocyanate, polyol, and a chain extender at a 2:1:1 mole ratio. Polyol and the chain extender were then extracted, evenly mixed utilizing a mechanical stirrer, and placed in an oven at 100 °C. The products were removed, added with MDI and T12 (catalyst), and subjected to machine mixing. After being mixed to a sticky state, the product was placed on an iron Teflon plate and placed in a 120 °C oven for 3 h. The baked lumpy TPU was subjected to tests, one of which involved using redissolved TPU as a coating material for mechanical property testing and dynamic mechanical analysis (DMA). The TPU was named in the TPU-X-Y format, with X and Y representing the polyol and chain extender used. For example, in TPU-PTCD-PDO, PTCD, and PDO represent the used polyol and chain extender, respectively. Figure  2 [25][26][27][28][29] displays the synthesis process. However, TPU copolymers in Figure 2 were ideal chemical structures for one unit cycle due to each lengthy polymer structure. Therefore, Figure 2 cannot show the random structure completely. Because the TPU used one-step synthesis in the research, MDI should react with BDO and polyols under an irregular state, and therefore the TPUs had the random hard segment repeat units. Scheme 1b in the manuscript was a suitable expression [26].

Fourier Transform Infrared Spectroscopy
The Perkin Elmer Spectrum One Fourier transform infrared (FT-IR) (Waltham, MA, US) spectrometer was set to the attenuated total reflectance mode to compare the TPU and polyol FT-IR spectrum and elucidate their chemical structures and verify the synthesized TPU. The testing conditions were set as 16 scans, the testing range 4000-650 cm −1 , and the resolution 16 cm −1 .

Fourier Transform Infrared Spectroscopy
The Perkin Elmer Spectrum One Fourier transform infrared (FT-IR) (Waltham, MA, USA) spectrometer was set to the attenuated total reflectance mode to compare the TPU and polyol FT-IR spectrum and elucidate their chemical structures and verify the synthesized TPU. The testing conditions were set as 16 scans, the testing range 4000-650 cm −1 , and the resolution 16 cm −1 .

1 H Nuclear Magnetic Resonance
1 H nuclear magnetic resonance ( 1 H NMR) was performed using the Bruker AVANCE-III 300 MHz spectrometer (Billerica, MA, USA) to elucidate the chemical structure of the polyol and TPU and to verify the successful synthesis of the PTC. The polyol and TPU samples were dissolved in deuterated chloroform and deuterated dimethyl sulfoxide, respectively, and scanned 32 times at room temperature.

Gel Permeation Chromatography
The Malvern Panalytical Viscotek GPCmax VE-2001(Malvern, WOR, UK) was used to test the number-average molecular weight (Mn), weight-average molecular weight (Mw), and polydispersity index (PDI) of the TPU. The testing conditions were set as the DMF mobile phase, mobile speed of 1 mL/min, and a 300 mm × 810 mm column dimension.

Thermogravimetric Analysis
The Hitachi STA 7200 (Tokyo, Japan) was adopted for the pyrolysis analysis of polyol and TPU under a nitrogen environment at 35-600 • C, with the temperature increasing at 10 • C/min. The test results were analyzed to determine the initial decomposition temperature (T di ) and the 5% weight loss temperature (T d 5wt% ), and the differential thermogravimetry (DTG) curve was employed to label the maximum decomposition rate of temperature in the highest peak (T max ).

Differential Scanning Calorimetry
Differential scanning calorimetry (DSC) was performed on the polyol and TPU using the Hitachi SIINT SII X-DSC7000 (Tokyo, Japan). The resulting Tm and Tg curves were then analyzed. The polyol and TPU testing temperatures ranged from −80 to 150 • C and −80 to 215 • C, respectively, both of which were increased or decreased at 10 • C/min. Two heating-cooling cycles were performed, and the results of the second heating were adopted for DSC.

Dynamic Mechanical Analysis (DMA)
DMA was performed using the Hitachi DMS 6100 (Tokyo, Japan) in the tension mode. The specimens were 20 mm × 10 mm in size; the range of the testing temperature was −100 to 100 • C, increasing at 5 • C/min; and the analysis was conducted at 1 Hz and deformation was 0.1 mm to collect the storage modulus (E ) and loss tangent (Tan δ).

Tensile Testing Machine
The tensile testing machine used was the Cometech A2 Universal (Taichung, Taiwan) with 500 kg load cell. The test was no preload. Dumbbell samples (size: length 33 mm, width 6 mm and thickness 0.32 mm) were employed as the specimen scale for ASTM D412-C tensile testing, and the gauge length was 33 mm. Each formulation was tested three times from the same batch. The stretching speed was 500 mm/min, and testing was performed under room temperature to obtain tensile strength, elongation at break, and modulus at 100% and 300% deformation.

Parallel-Plate Rheometer
Rheological property testing was performed using the Anton Paar Physica MCR-301 (Graz, Austria) under the oscillation testing mode. A cone plate (diameter: 25 mm; cone angle: 1 • ) served as the upper plate. The testing height was 55 microns; the deformation was 1%; temperatures 180, 190, and 200 • C; and angular frequencies 1-500 rad/s. The data of 40 locations were collected. The experimental results were used to determine the complex viscosity and discuss the processing properties of the TPU.

FT-IR Structure Elucidation
The FT-IR spectrums of polyols and TPU were compared, indicating a successful synthesis ( Figure 3). First, the stretching vibration peak of -OH, -CH, -C=O, and -C-Owas observed at 3500, 2940, 1740, and 1250 cm −1 , respectively. In Figure 3a, because PTC repeating units consist of more carbonate functional groups, the peak absorbance of the carbonate characteristic is higher. In Figure 3b, because the PHC repeating unit consists of more -CH groups, the peak absorbance of the -CH characteristic is higher [30,31].

FT-IR Structure Elucidation
The FT-IR spectrums of polyols and TPU were compared, indicating a successful synthesis ( Figure 3). First, the stretching vibration peak of -OH, -CH, -C=O, and -C-O-was observed at 3500, 2940, 1740, and 1250 cm −1 , respectively. In Figure 3a, because PTC repeating units consist of more carbonate functional groups, the peak absorbance of the carbonate characteristic is higher. In Figure 3b, because the PHC repeating unit consists of more -CH groups, the peak absorbance of the -CH characteristic is higher [30,31].  The FT-IR spectrum of the synthesized TPU revealed a sharp -NH stretching vibration peak at 3300 cm −1 , a curved -NH stretching vibration peak at 1530 cm −1 , and no -NCO stretching vibration peak at 2300 cm −1 . This suggests a successful reaction between the -OH functional groups of polyols and the chain extender with the -NCO functional group of diisocyanate, creating urethane functional groups. Furthermore, the stretching vibration peak of the MDI aromatic ring was observed at 1600 cm −1 , confirming synthesis success [32].
In the TPU spectrum, free carbonyl groups and H-bonded carbonyl groups were observed at 1740 and 1700 cm −1 , respectively [33,34]. Figure 4 reveals the number of H-bonds; Figure 4b,d and Table 2 indicate that the PHC-synthesized TPU had more H-bonds because the repeating units in PHC consist of more -CH groups. Accordingly, the soft chain segments of the PHC-synthesized TPU were easily turned, enabling facile interaction, and H-bonds are commonly observed between or inside the structures of molecular chains [35]. By contrast, PTC repeating units comprise more carbonate functional groups, which are rigid and not easily turned, resulting in fewer interactions with hard chain segments. A comparison between the performance of TPU-PTC-PDO and TPU-PTC-BDO in Figure  4a,c and Table 2 revealed that the TPU-PTC-PDO contained more H-bonds, indicating more favorable molecular chain orientation and suggesting an increase in interaction force between molecular chains. Because the TPU was synthesized using one-step synthesis, the molecular chains were randomly arranged. The main structures of PTC and PDO in TPU-PTC-PDO both consist of three C atoms, which increase the regularity of molecular chains and facilitates the formation of H-and polar bonds. The BDO structure of TPU-PTC-BDO contains four C atoms, whereas the polyol contains three C atoms, resulting in the molecular chains formed by randomly arranged TPU monomers having less regularity in arrangement, thus preventing the occurrence of intermolecular interactions (H-bonds The FT-IR spectrum of the synthesized TPU revealed a sharp -NH stretching vibration peak at 3300 cm −1 , a curved -NH stretching vibration peak at 1530 cm −1 , and no -NCO stretching vibration peak at 2300 cm −1 . This suggests a successful reaction between the -OH functional groups of polyols and the chain extender with the -NCO functional group of diisocyanate, creating urethane functional groups. Furthermore, the stretching vibration peak of the MDI aromatic ring was observed at 1600 cm −1 , confirming synthesis success [32]. In the TPU spectrum, free carbonyl groups and H-bonded carbonyl groups were observed at 1740 and 1700 cm −1 , respectively [33,34].  Table 2 indicate that the PHC-synthesized TPU had more H-bonds because the repeating units in PHC consist of more -CH groups. Accordingly, the soft chain segments of the PHC-synthesized TPU were easily turned, enabling facile interaction, and H-bonds are commonly observed between or inside the structures of molecular chains [35]. By contrast, PTC repeating units comprise more carbonate functional groups, which are rigid and not easily turned, resulting in fewer interactions with hard chain segments. A comparison between the performance of TPU-PTC-PDO and TPU-PTC-BDO in Figure 4a,c and Table 2 revealed that the TPU-PTC-PDO contained more H-bonds, indicating more favorable molecular chain orientation and suggesting an increase in interaction force between molecular chains. Because the TPU was synthesized using one-step synthesis, the molecular chains were randomly arranged. The main structures of PTC and PDO in TPU-PTC-PDO both consist of three C atoms, which increase the regularity of molecular chains and facilitates the formation of H-and polar bonds. The BDO structure of TPU-PTC-BDO contains four C atoms, whereas the polyol contains three C atoms, resulting in the molecular chains formed by randomly arranged TPU monomers having less regularity in arrangement, thus preventing the occurrence of intermolecular interactions (H-bonds and polar bonds).

1 H NMR Resonance Structure Elucidation
For the two polyols, 1 H NHR structural elucidation was performed. In Figure 5a, the peaks at PTC δ = 4.24 and 3.74 ppm represent the methyl protons next to the carbonate and alcohol groups, respectively; the peaks at δ = 2.06 and 1.92 ppm represent the remaining methyl protons. Similarly, the peaks at PHC δ = 4.09 and 3.61 respectively represent the methyl protons next to the carbonate and alcohol groups; the peaks at δ = 1.65, 1.54, and 1.38 ppm represent the remaining methyl protons [36,37].

1 H NMR Resonance Structure Elucidation
For the two polyols, 1 H NHR structural elucidation was performed. In Figure 5a, the peaks at PTC δ = 4.24 and 3.74 ppm represent the methyl protons next to the carbonate and alcohol groups, respectively; the peaks at δ = 2.06 and 1.92 ppm represent the remaining methyl protons. Similarly, the peaks at PHC δ = 4.09 and 3.61 respectively represent the methyl protons next to the carbonate and alcohol groups; the peaks at δ = 1.65, 1.54, and 1.38 ppm represent the remaining methyl protons [36,37]. 1 H NMR structural elucidation was performed to verify the chemical structure of the TPU. In Figure 5b, the peak at δ = 9.5 ppm represents the N-H bond in the urethane group, the peaks at δ = 7.3 and 7.05 ppm represent the H-atom on the MDI benzene ring, and the peak at δ = 3.75 ppm represents the methylene group (Ar-CH2-Ar) between the two MDI benzene rings. In the TPU-PTC-PDO spectrum, the peak at δ = 4.11 ppm represents the methyl group next to the urethane group′s carbonate group; the peak at δ = 1.92 ppm represents the remaining methyl group. In the TPU-PHC-PDO spectrum, the peak at δ = 4.01 ppm represents the methyl group next to the carbonate group and the urethane group; the peak at δ = 1.94 ppm represents the methyl group in the middle of the PDO, and the peaks at δ = 1.54 and 1.27 ppm represent the remaining methyl groups on the PHC. In the TPU-PTC-BDO spectrum, the peak at δ = 4.11 ppm represents the methyl group next to the carbonate group and the urethane group; the peak at δ = 1.92 ppm represents the methyl group in the middle of the PTC, and the peak at δ = 1.68 ppm represents the remaining methyl groups on the BDO. Finally, in the TPU-PHC-BDO spectrum, the peak at δ = 4.01 ppm represents the methyl group next to the carbonate group and the urethane group, and the peaks at δ = 1.55 and 1.28 ppm represent the remaining methyl groups. In sum, the characteristic peaks indicate the successful synthesis of TPU [14,38,39].

Gel Permeation Chromatography Analysis
The TPU copolymers using the same chain extender had similar molecular weights ( Table 3). The number-average molecular weights of TPU-PTC-BDO and TPU-PHC-BDO were comparable (between 75,000 and 78,000 g/mole). The number-average molecular weights of TPU-PTC-PDO and TPU-PHC-PDO were also similar, both between 107,000 and 121,000 g/mole. Moreover, the PDI values of the TPU samples were lower than 2. These results suggest that the TPU PDI values are in a reasonable range, with a synthesis quality sufficient and suitable for processing. Moreover, the TPU using PDO as a chain extender had a higher molecular weight than the TPU using BDO as a chain extender during synthesis, reflecting that BDO has a more considerable steric hindrance than does PDO.  1 H NMR structural elucidation was performed to verify the chemical structure of the TPU. In Figure 5b, the peak at δ = 9.5 ppm represents the N-H bond in the urethane group, the peaks at δ = 7.3 and 7.05 ppm represent the H-atom on the MDI benzene ring, and the peak at δ = 3.75 ppm represents the methylene group (Ar-CH 2 -Ar) between the two MDI benzene rings. In the TPU-PTC-PDO spectrum, the peak at δ = 4.11 ppm represents the methyl group next to the urethane group s carbonate group; the peak at δ = 1.92 ppm represents the remaining methyl group. In the TPU-PHC-PDO spectrum, the peak at δ = 4.01 ppm represents the methyl group next to the carbonate group and the urethane group; the peak at δ = 1.94 ppm represents the methyl group in the middle of the PDO, and the peaks at δ = 1.54 and 1.27 ppm represent the remaining methyl groups on the PHC. In the TPU-PTC-BDO spectrum, the peak at δ = 4.11 ppm represents the methyl group next to the carbonate group and the urethane group; the peak at δ = 1.92 ppm represents the methyl group in the middle of the PTC, and the peak at δ = 1.68 ppm represents the remaining methyl groups on the BDO. Finally, in the TPU-PHC-BDO spectrum, the peak at δ = 4.01 ppm represents the methyl group next to the carbonate group and the urethane group, and the peaks at δ = 1.55 and 1.28 ppm represent the remaining methyl groups. In sum, the characteristic peaks indicate the successful synthesis of TPU [14,38,39].

Gel Permeation Chromatography Analysis
The TPU copolymers using the same chain extender had similar molecular weights ( Table 3). The number-average molecular weights of TPU-PTC-BDO and TPU-PHC-BDO were comparable (between 75,000 and 78,000 g/mole). The number-average molecular weights of TPU-PTC-PDO and TPU-PHC-PDO were also similar, both between 107,000 and 121,000 g/mole. Moreover, the PDI values of the TPU samples were lower than 2. These results suggest that the TPU PDI values are in a reasonable range, with a synthesis quality sufficient and suitable for processing. Moreover, the TPU using PDO as a chain extender had a higher molecular weight than the TPU using BDO as a chain extender during synthesis, reflecting that BDO has a more considerable steric hindrance than does PDO.

Decomposition Properties
A typical TPU is thermally unstable. Thus, thermogravimetric analysis was used to measure the decomposition characteristics of the copolymers. Figure 6a,b displays the thermogravimetry (TG) and differential thermogravimetry (DTG) curves for the TPU copolymers. The TG curves of TPU-PTC-PDO and TPU-PTC-BDO revealed three distinct regions of weight loss, which are reflected in three peaks on the DTG curve. This result implies that both copolymers possess at least three significant stages of decomposition. However, the TPU-PHC-PDO and TPU-PHC-BDO presented only two significant stages of decomposition. Typical TPU is composed of hard and soft segments, which usually represent two stages of decomposition. The first stage is related to the hard segments, and the second to the soft segments. The temperatures of initial decomposition (T di ) for the TPU copolymers were approximately 220 to 260 • C (Table 4), possibly due to the decomposition of urethane segments caused by MDI and a chain extender, or by MDI and a polyol [40]. Subsequently, the decompositions of the TPU exhibited obvious differences at 5 wt% of weight loss (T d 5wt% ). The temperatures of T d 5wt% for TPU-PHC-PDO and TPU-PHC-BDO herein were higher than those of TPU-PTC-PDO and TPU-PTC-BDO. This result may be due to the effect of the decomposition of soft segments. To confirm the decomposition properties of polyol monomers, their decomposition temperatures were also examined (Figure 6c,d). As may be observed from Figure 6c, the temperature regions of decomposition for the PTC and PHC were respectively from 180 to 325 • C and from 200 to 400 • C. Furthermore, the decomposition temperature of the maximum rate (T max ) for the PHC was 367.1 • C, obviously exceeding that of the PTC. Therefore, the T d 5wt% of the TPU-PHC copolymers were higher than those of the TPU-PTC copolymers because the decomposition temperature of PHC is higher than that of PTC [41,42]. The TPU-PTC copolymers (i.e., TPU-PTC-PDO and TPU-PTC-BDO) on the TG curve presented their first and second shoulder peak from 270 to 375 • C and from 375 to 470 • C, which are reflected in the two peaks on the DTG curve, as shown in Figure 6a,b. The decomposition of the region was possibly related to the decomposition of soft polycarbonate segments (PTC), including the decomposition, cyclization, and re-decomposition for the cyclic compounds [43]. The decomposition of the TPU-PHC copolymers (i.e., TPU-PHC-PDO and TPU-PHC-BDO) presented only one shoulder peak between 400 and 500 • C, which was related to the decomposition of soft polycarbonate segments (PHC), which also involves cyclization and re-decomposition of the cyclic compounds. The PHC polyol on the DTG curves displays a T max peak at 300 to 400 • C; therefore, the TPU-PHC presented only one shoulder peak. The T d 5wt% of the TPU-PTC copolymers was lower than that of the TPU-PHC copolymers, but their decomposition properties were sufficient to withstand the processing temperature. The TPU-PHC samples also had favorable processability, which is investigated in the section on rheological behavior.

Decomposition Properties
A typical TPU is thermally unstable. Thus, thermogravimetric analysis was used to measure the decomposition characteristics of the copolymers. Figure 6a,b displays the thermogravimetry (TG) and differential thermogravimetry (DTG) curves for the TPU copolymers. The TG curves of TPU-PTC-PDO and TPU-PTC-BDO revealed three distinct regions of weight loss, which are reflected in three peaks on the DTG curve. This result implies that both copolymers possess at least three significant stages of decomposition. However, the TPU-PHC-PDO and TPU-PHC-BDO presented only two significant stages of decomposition. Typical TPU is composed of hard and soft segments, which usually represent two stages of decomposition. The first stage is related to the hard segments, and the second to the soft segments. The temperatures of initial decomposition (Tdi) for the TPU copolymers were approximately 220 to 260 °C (Table 4), possibly due to the decomposition of urethane segments caused by MDI and a chain extender, or by MDI and a polyol [40]. Subsequently, the decompositions of the TPU exhibited obvious differences at 5 wt% of weight loss (Td 5wt%). The temperatures of Td 5wt% for TPU-PHC-PDO and TPU-PHC-BDO herein were higher than those of TPU-PTC-PDO and TPU-PTC-BDO. This result may be due to the effect of the decomposition of soft segments. To confirm the decomposition properties of polyol monomers, their decomposition temperatures were also examined (Figure 6c,d). As may be observed from Figure 6c, the temperature regions of decomposition for the PTC and PHC were respectively from 180 to 325 °C and from 200 to 400 °C. Furthermore, the decomposition temperature of the maximum rate (Tmax) for the PHC was 367.1 °C, obviously exceeding that of the PTC. Therefore, the Td 5wt% of the TPU-PHC copolymers were higher than those of the TPU-PTC copolymers because the decomposition temperature of PHC is higher than that of PTC [41,42]. The TPU-PTC copolymers (i.e., TPU-PTC-PDO and TPU-PTC-BDO) on the TG curve presented their first and second shoulder peak from 270 to 375 °C and from 375 to 470 °C, which are reflected in the two peaks on the DTG curve, as shown in Figure 6a,b. The decomposition of the region was possibly related to the decomposition of soft polycarbonate segments (PTC), including the decomposition, cyclization, and re-decomposition for the cyclic compounds [43]. The decomposition of the TPU-PHC copolymers (i.e., TPU-PHC-PDO and TPU-PHC-BDO) presented only one shoulder peak between 400 and 500 °C, which was related to the decomposition of soft polycarbonate segments (PHC), which also involves cyclization and re-decomposition of the cyclic compounds. The PHC polyol on the DTG curves displays a Tmax peak at 300 to 400 °C; therefore, the TPU-PHC presented only one shoulder peak. The Td 5wt% of the TPU-PTC copolymers was lower than that of the TPU-PHC copolymers, but their decomposition properties were sufficient to withstand the processing temperature. The TPU-PHC samples also had favorable processability, which is investigated in the section on rheological behavior.

Phase Transition Regions
The morphology of TPU presents a two-phase microstructure because the two dissimilar segment types are incompatible. Therefore, DSC thermograms of TPU are likely to display four transition regions as both phases crystallize. However, the structure and concentration of the hard and soft segments and the molecular weights and types of soft segments all influence the phase transition regions in DSC thermograms [44][45][46]. Therefore, not all phase transition regions are displayed on DSC thermograms. Figure 7a indicates the DSC thermograms of TPU-PTC and TPU-PHC with PDO and BDO, respectively.

Phase Transition Regions
The morphology of TPU presents a two-phase microstructure because the two dissimilar segment types are incompatible. Therefore, DSC thermograms of TPU are likely to display four transition regions as both phases crystallize. However, the structure and concentration of the hard and soft segments and the molecular weights and types of soft segments all influence the phase transition regions in DSC thermograms [44][45][46]. Therefore, not all phase transition regions are displayed on DSC thermograms. Figure 7a indicates the DSC thermograms of TPU-PTC and TPU-PHC with PDO and BDO, respectively.

Phase Transition Regions
The morphology of TPU presents a two-phase microstructure because the two dissimilar segment types are incompatible. Therefore, DSC thermograms of TPU are likely to display four transition regions as both phases crystallize. However, the structure and concentration of the hard and soft segments and the molecular weights and types of soft segments all influence the phase transition regions in DSC thermograms [44][45][46]. Therefore, not all phase transition regions are displayed on DSC thermograms. Figure 7a indicates the DSC thermograms of TPU-PTC and TPU-PHC with PDO and BDO, respectively.  TPU-PHC copolymers had a phase transition region at 0-30 • C, which corresponds to the glass transition temperature of soft segments (T gs ). On the other hand, the glass transition temperatures of hard segments (T gh ) for the TPU-PTC and TPU-PHC copolymers were 100-160 • C. Thus, none of the samples exhibited a significant soft segment crystallization because of the effect of copolymer synthesis, though the PHC presented a melting temperature at approximately 42.2 • C (Figure 7b). Furthermore, the glass transition temperature of hard segments (T gh ) and the melting temperature were not apparent on the thermograms because of the low content of hard segments. These results indicate that the TPU-PTC and TPU-PHC copolymers are amorphous copolymers [47,48].
Dynamic mechanical analysis (i.e., DMA) can clearly determine the phase transition temperature and modulus of polymers. Chain extenders and polyols affect the T gs of TPU. As Figure 8a indicates, the TPU storage modulus dropped by two to three orders between −25 and 50 • C, indicating the T gs . In addition, the peak of the loss tangent (Figure 8b) is affected by the T gs of the TPU. The TPU copolymers using the BDO chain extender presented lower T gs than did the TPU with PDO (Table 5). In addition, the TPU copolymers with PHC soft segments displayed lower T gs than did the TPU with PTC. These results are attributed to the soft segments and chain extenders of long carbon chains having more conformations. Furthermore, the TPU-PTC-BDO had the lowest value of Tan δ, indicating it has low levels of damping behavior under the action of external force. By contrast, the TPU-PHC-PDO had the highest energy loss. This result suggests that the TPU-PTC-BDO is suitable for resisting external forces, whereas the TPU-PHC-PDO is suitable for recoil-resistant materials.
ppl. Sci. 2021, 11, x FOR PEER REVIEW 11 of 18 copolymers were 100-160 °C. Thus, none of the samples exhibited a significant soft segment crystallization because of the effect of copolymer synthesis, though the PHC presented a melting temperature at approximately 42.2 °C (Figure 7b). Furthermore, the glass transition temperature of hard segments (Tgh) and the melting temperature were not apparent on the thermograms because of the low content of hard segments. These results indicate that the TPU-PTC and TPU-PHC copolymers are amorphous copolymers [47,48]. Dynamic mechanical analysis (i.e., DMA) can clearly determine the phase transition temperature and modulus of polymers. Chain extenders and polyols affect the Tgs of TPU. As Figure 8a indicates, the TPU storage modulus dropped by two to three orders between −25 and 50 °C, indicating the Tgs. In addition, the peak of the loss tangent (Figure 8b) is affected by the Tgs of the TPU. The TPU copolymers using the BDO chain extender presented lower Tgs than did the TPU with PDO (Table 5). In addition, the TPU copolymers with PHC soft segments displayed lower Tgs than did the TPU with PTC. These results are attributed to the soft segments and chain extenders of long carbon chains having more conformations. Furthermore, the TPU-PTC-BDO had the lowest value of Tan δ, indicating it has low levels of damping behavior under the action of external force. By contrast, the TPU-PHC-PDO had the highest energy loss. This result suggests that the TPU-PTC-BDO is suitable for resisting external forces, whereas the TPU-PHC-PDO is suitable for recoilresistant materials.   Figure 8a shows that the storage modulus (E′) of TPU-PTC-BDO decreased rapidly between 50-75 °C. However, this decrease was not observed for TPU-PHC-BDO. The length of the plateau was associated with molecular mass, chain entanglement, and degree of crosslinking [49]. Because TPU-PTC-BDO and TPU-PHC-BDO have a similar molecular weight, the difference should probably be attributed to PHC molecular structure units having longer carbon chain lengths. That is, the long carbon chains entangled, enabling TPU-PHC-BDO to retain its main structure strength at high temperatures. Furthermore, FT-IR results suggested weaker intermolecular force in TPU-PTC-BDO, which causes its main structure to break up at temperatures between 50 and 75 °C, resulting in the intermolecular forces collapsing and the storage modulus significantly decreasing. This result is presented in Figure 8b, indicating that the Tan δ of TPU-PTC-BDO increased   Figure 8a shows that the storage modulus (E ) of TPU-PTC-BDO decreased rapidly between 50-75 • C. However, this decrease was not observed for TPU-PHC-BDO. The length of the plateau was associated with molecular mass, chain entanglement, and degree of crosslinking [49]. Because TPU-PTC-BDO and TPU-PHC-BDO have a similar molecular weight, the difference should probably be attributed to PHC molecular structure units having longer carbon chain lengths. That is, the long carbon chains entangled, enabling TPU-PHC-BDO to retain its main structure strength at high temperatures. Furthermore, FT-IR results suggested weaker intermolecular force in TPU-PTC-BDO, which causes its main structure to break up at temperatures between 50 and 75 • C, resulting in the intermolecular forces collapsing and the storage modulus significantly decreasing. This result is presented in Figure 8b, indicating that the Tan δ of TPU-PTC-BDO increased dramatically between 50 and 75 • C. In a glassy state, the TPU-PTC copolymers possess more carbonate groups so that the polymer chains are restricted in mobility, contrary to the TPU-PHC copolymers that own the long carbon chains [50].

Tensile Strength
The molecular weight of a polymer is a key factor affecting its tensile strength. As mentioned in Section 3.3, the TPU copolymers using the same chain extender had comparable molecular weights. Thus, the molecular weight can be ignored in the analysis of tensile strength. As Table 6 indicates, the TPU had high elongations at break (above 460%), and both the TPU-PHC-PDO and TPU-PHC-BDO had moderate strain at break. However, the TPU-PHC-BDO and TPU-PTC-PDO differed considerably in terms of the strain at break. The results are possibly affected by hydrogen bonding. According to the FT-IR results in Section 3.1 (Scheme 1a), the TPU-PHC-BDO and TPU-PHC-PDO had strong intermolecular hydrogen bonds in the TPU-PTC-BDO; because the regularity of the molecular chains was poor (atactic) and the intermolecular attraction weak, poor strength resulted. Notably, TPU-PTC-PDO uses an odd-numbered polyol and chain extender to cause regular molecular chains (isotactic), stacking well, and more secondary bonds (such as hydrogen bonds, benzene π−π stacking, and dipole-dipole interactions) (Scheme 1b). Both of the copolymers had the same number of the hydrogen bonds per unit in Scheme 1b, and whereas we can see that Scheme 1b focuses on the blue and red squares in the soft segments, TPU-PTC-PDO could have possible dipole-dipole force (δ + to δ − ) and TPU-PTC-BDO does not (δ − to δ − ). More dipole interactions for TPU copolymers can drive better mechanical strength. According to the literature, the storage modulus increases with dipole interactions under a glassy state [51]. We also saw this phenomenon in this study, and Figure 8a in the manuscript revealed that the storage modulus of TPU-PTC-PDO was higher than TPU-PTC-BDO., It can be compared to Scheme 1b to prove that TPU-PTC-PDO has more dipole interactions than TPU-PTC-BDO. Thus, it has a high tensile strength close to TPU-PHC-BDO and TPU-PHC-PDO ( Table 6). The regularity of molecular chains also affected the stretching process. As is shown in Figure 9a,b, the stress of the TPU samples presented a rapid rise at approximately 300% of elongation, except in the case of TPU-PTC-BDO. This was because TPU-PTC-BDO is an odd-numbered carbon chain (a long chain of three carbons) corresponding to an even-numbered carbon chain (a short chain of four carbons), and it cannot be arranged regularly after stress as is expected in straininduced crystallization [52][53][54][55]. Therefore, the intermolecular hydrogen bonding effect for the TPU-PTC-BDO was lower than for other samples. Accordingly, the TPU-PTC-BDO presented the lowest stress but the highest elongation.

Rheology Testing of Complex Viscosity
The complex viscosity of the TPU using the PTC polyol was much lower than that of the TPU using PHC (Figure 10). This result is attributed to the considerable reduction in the interaction of the polarity of PTC polyol at high temperatures, and intermolecular attraction is lost [22,23]. The master curves obtained from time-temperature superposition also revealed the same trend ( Figure 11). Remarkably, the storage modulus of the TPU using the PTC polyol plummeted in the high-temperature region (at a lower angular frequency). However, the soft segments of the PHC resulted in chain entanglement, with more interactions existing in the hard chain segments, which were extremely attractive. Therefore, it can reliably support the main structure and maintain the melt strength at high temperatures; for this reason, it will be slightly challenging to process and require higher processing temperatures. However, the urethane group is prone to decomposition under a long-term high-temperature environment. Thus, attention should be paid to the processing time of TPU-PHC samples. Furthermore, the viscosity of TPU-PTC-PDO at 180 • C was slightly lower than that of the TPU-PHC samples at 200 • C. Therefore, the low viscosity of TPU-PTC-PDO facilitates processing. From an energy usage perspective, the suitable processing temperature of TPU-PTC-PDO is lower than that of TPU-PHC samples. Accordingly, the tensile strength of TPU-PTC-PDO was similar to that of the TPU-PHC samples, and it had favorable fluidity at high temperatures. These results indicate that TPU-PTC-PDO can be applied in coating or precision injection, which may expand the future usage of polycarbonate TPU elastomer.

Rheology Testing of Complex Viscosity
The complex viscosity of the TPU using the PTC polyol was much lower than that of the TPU using PHC (Figure 10). This result is attributed to the considerable reduction in the interaction of the polarity of PTC polyol at high temperatures, and intermolecular attraction is lost [22,23]. The master curves obtained from time-temperature superposition also revealed the same trend ( Figure 11). Remarkably, the storage modulus of the TPU using the PTC polyol plummeted in the high-temperature region (at a lower angular frequency). However, the soft segments of the PHC resulted in chain entanglement, with more interactions existing in the hard chain segments, which were extremely attractive. Therefore, it can reliably support the main structure and maintain the melt strength at high temperatures; for this reason, it will be slightly challenging to process and require higher processing temperatures. However, the urethane group is prone to decomposition under a long-term high-temperature environment. Thus, attention should be paid to the processing time of TPU-PHC samples. Furthermore, the viscosity of TPU-PTC-PDO at 180 °C was slightly lower than that of the TPU-PHC samples at 200 °C. Therefore, the low viscosity of TPU-PTC-PDO facilitates processing. From an energy usage perspective, the suitable processing temperature of TPU-PTC-PDO is lower than that of TPU-PHC samples. Accordingly, the tensile strength of TPU-PTC-PDO was similar to that of the TPU-PHC samples, and it had favorable fluidity at high temperatures. These results indicate

Conclusions
The present study synthesized PTC and performed one-step solvent-free synthesis to synthesize two types of polycarbonate TPU (TPU-X-Y, X = PTC or PHC, Y = PDO or BDO) to explore differences in properties between TPU consisting of chains of odd-numbered and even-numbered carbon atoms. The results indicated that pyrolysis occurred in at least one segment in the TPU of PHC polyol. However, such pyrolysis may have been caused by two types of cracking behaviors, given that the pyrolysis temperatures of PHC and TPU hard segments are similar (Td ranges between 275 and 400 °C) and that PHC had adequate heat tolerance (Td 5wt% = 293.5-294.1 °C). However, because PTC consists of more carbonate groups, the pyrolysis temperature of PTC was lower than that of the TPU hard segments (Td = 251.7 °C). Accordingly, three-phase pyrolysis occurs in the TPU of PTC

Conclusions
The present study synthesized PTC and performed one-step solvent-free synthesis to synthesize two types of polycarbonate TPU (TPU-X-Y, X = PTC or PHC, Y = PDO or BDO) to explore differences in properties between TPU consisting of chains of odd-numbered and even-numbered carbon atoms. The results indicated that pyrolysis occurred in at least one segment in the TPU of PHC polyol. However, such pyrolysis may have been caused by two types of cracking behaviors, given that the pyrolysis temperatures of PHC and TPU hard segments are similar (Td ranges between 275 and 400 • C) and that PHC had adequate heat tolerance (T d 5wt% = 293.5-294.1 • C). However, because PTC consists of more carbonate groups, the pyrolysis temperature of PTC was lower than that of the TPU hard segments (T d = 251.7 • C). Accordingly, three-phase pyrolysis occurs in the TPU of PTC polyol (phase one: 220-315 • C; phase two: 295-385 • C, and phase three: 395-485 • C). The heat tolerance of the TPU of the PTC series was lower than that of the TPU of the PHC series (T d 5wt% = 243.5-270.4 • C). Therefore, when processing materials containing TPU of the PTC series, the temperature must be maintained below pyrolysis temperature to prevent pyrolysis and avoid material deterioration. The DSC and DMA results revealed the T g of all the synthesized TPU to be 13.2-16.9 • C. In particular, chain entanglement often occurs in TPU of the PHC series, given the long carbon chain in the PHC group. This increases the TPU of the PHC series to levels closer to that of the TPU of the PTC series while retaining a constant storage modulus at 50-90 • C. Given the occurrence of chain entanglement between molecules, TPU of the PHC series exhibited outstanding tensile properties (tensile strength at break: around 38 MPa). By contrast, the high regularity of molecule arrangements in TPU-PTC-PDO generates immense polarity, thereby substantially enhancing its tensile properties (tensile strength at break: 41.1 ± 5.6 MPa). The rheology experiment revealed that TPU of the PTC series had high melt viscosity (TPU-PHC-PDO: 5.58 × 10 3 Pa·s at 200 • C at 1 rad/s, TPU-PHC-BDO: 3.16 × 10 3 Pa·s at 200 • C at 1 rad/s); this was due to entanglement between TPU molecular chains. In high-temperature environments, the polarity of TPU-PTC-PDO decreases, whereas the flowability increases. In low-temperature environments, the flowability of TPU-PTC-PDO is suitable for processing applications (2.51 × 10 3 Pa·s at 180 • C at 1 rad/s). The low-temperature processing of TPU-PTC-PDO conserves energy and saves costs; the material exhibits adequate flowability and mechanical properties under room temperature, thus boosting the potential for precision injection applications. The experimental results indicate that the synthesized TPU is suitable for a wide range of applications and exhibits the various advantages of carbonate TPU. Table 7 can show clearly the main results of these TPU copolymers to compare their advantages and disadvantages.  Table S1 describes the composite of TPU copolymers, and Table S2 displays Mn, Mw, PDI, and MI of TPU copolymers. Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.

Conflicts of Interest:
The authors declare no conflict of interest.