Synthesis and Characterization of Random Block Hydroxyl-Terminated Polyfluoroether-Based Polyurethane Elastomers with Fluorine-Containing Side Chains

Polype ntafluoropropane glycidyl ether (PPFEE), a new random block hydroxyl-terminated polyfluoroether, was synthesized successfully by cationic ring-opening polymerization of 2-(2,2,3,3,3-pentafluoropropoxymethyl) oxirane, and its molecular structure was confirmed by Fourier transform infrared spectroscopy, nuclear magnetic resonance spectrometry, and gel permeation chromatography. The PPFEE-based polyurethane elastomers featuring fluorine in their side chains were prepared using PPFEE as soft segments, polyisocyanate polyaryl polymethylene isocyanate as hard segments, and dibutyltin dilaurate as catalysts under different curing conditions. The microphase separation, mechanical performance, and thermal behavior of the elastomers were investigated by differential scanning calorimetry, uniaxial tensile test, and thermal gravimetric analysis, respectively. Based on the results, the percentage of hard segments dissolved into the soft segments of elastomers was opposite to the change in breaking strength. The PPFEE-based polyurethane elastomer cured with 20 wt% PAPI at the curing temperature of 50 °C displayed the maximum tensile elongation of 2.26 MPa with an elongation at break of nearly 150%. The increased contents of PAPI can effectively strengthen the tensile strength, and the maximum tensile elongation was 3.04 MPa with an elongation at break of nearly 90% when the content of PAPI was 26 wt%. In addition, the PPFEE-based polyurethane elastomers exhibited excellent resistance to thermal decomposition and a sharp weight loss temperature at around 371 °C. All the results demonstrated that the PPFEE may be a potential polymeric binder as one of the ingredients applied to future propellant formulations.


Introduction
Polymeric binders are one of the important components of solid propellants, and they can bind numerous metallic fuels together with oxidizers, plasticizers, and other additives to form integration in solid propellants [1][2][3]. In addition, the integrated structure must be prepared into a certain shape that can withstand a certain degree of stress. In general, the structural integrity is affected by the characteristics of the polymeric binder.
Fluorine is the most electronegative element [4], and fluorinated compounds are commonly used as oxidants. Therefore, the introduction of fluorine to polymeric binders can improve the oxygen balance of solid propellants, resulting in less use of oxidants and increased utilization of energetic materials, which can enhance the energy level of solid propellants [5]. In recent decades, owing to their numerous attractive properties, including high density, low coefficient of friction, long-term chemical stability, broad operating temperature range, and good compatibility with other main ingredients [6][7][8][9], fluoropolymers have emerged as active polymeric binders, and have become one of the most widely applied solid propellants [10][11][12][13][14][15][16][17]. In addition, fluorinated polyurethane elastomers possess unique mechanical properties [18][19][20] by the virtue of the influences of PAPI, whose isocyanate content was 30.87 g/100 g, was purchased from Zhengzhou Keyunlong Chemical Products Co., Ltd. (Zhengzhou, China) and used without further purification. 2,2,3,3,3-Pentafluoropropanol and epichlorohydrin (ECH) were purchased from Aladdin (Shanghai, China). BF 3 ·OEt 2 , phthalic anhydride, DBTDL, sodium hydroxide (NaOH), sodium sulfate (Na 2 SO 4 ), and methylene chloride (CH 2 Cl 2 ) were purchased from Chengdu, Kelong Chemical Reagent Factory (Chengdu, China). BDO was purchased from Tokyo Chemical Industry Co., Ltd. (Shanghai, China). All the reactants were used without further purification.

Synthesis of PPFEE Prepolymer
PPFEE was obtained via the typical cationic ring-opening polymerization of PFEE as described in Scheme 1b. The reaction steps involved are described below. BDO (256 µL) and BF 3 ·OEt 2 (175 µL) dissolved in methylene chloride (12 mL) were contained in a flask and stirred at room temperature for 0.5 h. After cooling the reaction vessel to 0 • C and replacing the atmosphere with nitrogen, PFEE (30 g) was slowly and dropwise added to the reaction mixture over a period of 12 h. When the addition was completed, the reaction was continued under stirring at room temperature for an additional 12 h. After polymerization, the reaction was terminated through the addition of distilled water. The organic phase containing PFEE was extracted into methylene chloride and washed with distilled water several times until neutral pH was obtained. The washed organic phase was dried over Polymers 2023, 15, 288 3 of 12 sodium sulfate and filtered. Finally, the solvent was evaporated off in a 120 • C vacuum to afford 25 g PPFEE (83% yield). The hydroxyl value was 0.54 mmol g −1 . The weight-average molecular weight (M w ), the number-average molecular weight (M n ), and the polydispersity index (PDI) of PFEE from GPC were 3911 g mol −1 , 3307 g mol −1 , and 1.18, respectively.

Synthesis of PPFEE Prepolymer
PPFEE was obtained via the typical cationic ring-opening polymerization of PFEE as described in Scheme 1b. The reaction steps involved are described below. BDO (256 μL) and BF3·OEt2 (175 μL) dissolved in methylene chloride (12 mL) were contained in a flask and stirred at room temperature for 0.5 h. After cooling the reaction vessel to 0 °C and replacing the atmosphere with nitrogen, PFEE (30 g) was slowly and dropwise added to the reaction mixture over a period of 12 h. When the addition was completed, the reaction was continued under stirring at room temperature for an additional 12 h. After polymerization, the reaction was terminated through the addition of distilled water. The organic phase containing PFEE was extracted into methylene chloride and washed with distilled water several times until neutral pH was obtained. The washed organic phase was dried over sodium sulfate and filtered. Finally, the solvent was evaporated off in a 120 °C vacuum to afford 25 g PPFEE (83% yield). The hydroxyl value was 0.54 mmol g −1 . The weightaverage molecular weight (Mw), the number-average molecular weight (Mn), and the polydispersity index (PDI) of PFEE from GPC were 3911 g mol −1 , 3307 g mol −1 , and 1.18, respectively.

Preparation of PPFEE-Based Polyurethane Elastomers
PPFEE prepolymer was dehydrated in a vacuum oven at 80 °C for 48 h before use. PPFEE-based polyurethane elastomers with various mass percentages of PAPI or at different curing temperatures were prepared using the dried PPFEE prepolymer as soft segments, PAPI as hard segments, and DBTDL as catalysts. The detailed preparation procedure was as follows. The same masses of PPFEE and DBTDL (0.4 wt%) were mixed in four 50 mL beakers. After stirring for 10 min, different contents of PAPI (17, 20, 22, and 26 wt%) were added respectively, and the mixtures were stirred for another 10 min until a uniformly yellow fluid was formed, followed by degassing in a vacuum oven. Afterward, the

Preparation of PPFEE-Based Polyurethane Elastomers
PPFEE prepolymer was dehydrated in a vacuum oven at 80 • C for 48 h before use. PPFEE-based polyurethane elastomers with various mass percentages of PAPI or at different curing temperatures were prepared using the dried PPFEE prepolymer as soft segments, PAPI as hard segments, and DBTDL as catalysts. The detailed preparation procedure was as follows. The same masses of PPFEE and DBTDL (0.4 wt%) were mixed in four 50 mL beakers. After stirring for 10 min, different contents of PAPI (17,20,22, and 26 wt%) were added respectively, and the mixtures were stirred for another 10 min until a uniformly yellow fluid was formed, followed by degassing in a vacuum oven. Afterward, the mixtures were transferred to four round-shaped Teflon-coated molds followed by a reaction for 2 days at 20 • C and then curing for 2 days at 50 • C. Meanwhile, the mixtures of PPFEE and DBTDL with the same mass as before and 20 wt% PAPI were left to react for 2 days at 20 • C and then cured for 2 days at 40 • C, 50 • C, 60 • C, and 70 • C. The dumbbell-shaped specimens of PPFEE-based polyurethane elastomers were prepared to measure the mechanical properties, glass transition temperature, and thermal stability through the universal testing machine, DSC, and TGA, respectively.

Characterization
FTIR spectra were recorded in a Nicolet 380 FTIR spectrophotometer (Thermo Fisher Nicolet, Waltham, MA, USA) with a wavenumber resolution of 4 cm −1 in the range from 450 cm −1 to 4000 cm −1 . 1 H NMR and 13 C NMR were measured with AVANCE III 600 MHz Bruker instrument (Switzerland) using CDCl 3 as the solvent. 19 F NMR was conducted on a DSC experiments conducted with DSC Q200 produced by TA Instruments (USA) were used to thermally characterize the samples, which utilized heating ramps of 10 • C min −1 under a dry nitrogen atmosphere (50 mL/min). A second scan from −70 to 20 • C was recorded. Mechanical properties, including tensile strength and elongation at break, of all the polyurethane elastomers gels, were measured on an HS-20KN universal testing machine produced by Yangzhou Huahui Testing Instrument Co., Ltd. (Yangzhou, China) with a tensile rate of 500 mm min −1 . TGA performed on Netzsch STA 409 PC simultaneous thermal analysis instrument (Germany) was used to characterize the thermal decomposition properties of the polyurethane elastomers gels in the temperature range from 50 • C to 800 • C with the heating rate of 10 • C min −1 .

Structure of PPFEE
The PPFEE-based polyurethane elastomers were prepared via a prepolymer process (Scheme 1). The FTIR spectrum provided the first confirmation of the PPFEE structure. As shown in Figure 1, the strongest peak at 3438 cm −1 corresponded to O-H stretching vibration, and the bands around 2921 and 2889 cm −1 were related to -CH 2 -stretching vibrations. The stronger peak at 1198 cm −1 was ascribed to C-O-C, and the bands attributed to C-F stretching vibrations were observed at 1101 cm −1 .
universal testing machine, DSC, and TGA, respectively.

Characterization
FTIR spectra were recorded in a Nicolet 380 FTIR spectrophotometer (Thermo Fisher Nicolet, USA) with a wavenumber resolution of 4 cm −1 in the range from 450 cm −1 to 4000 cm −1 . 1 H NMR and 13 C NMR were measured with AVANCE III 600 MHz Bruker instrument (Switzerland) using CDCl3 as the solvent. 19 F NMR was conducted on a 400 MHz Bruker spectrometer using CDCl3 as solvent. GPC was conducted on a Waters GPC 1515 instrument (USA).
DSC experiments conducted with DSC Q200 produced by TA Instruments (USA) were used to thermally characterize the samples, which utilized heating ramps of 10 °C min −1 under a dry nitrogen atmosphere (50 mL/min). A second scan from −70 to 20 °C was recorded. Mechanical properties, including tensile strength and elongation at break, of all the polyurethane elastomers gels, were measured on an HS-20KN universal testing machine produced by Yangzhou Huahui Testing Instrument Co., Ltd. (Yangzhou, China) with a tensile rate of 500 mm min −1 . TGA performed on Netzsch STA 409 PC simultaneous thermal analysis instrument (Germany) was used to characterize the thermal decomposition properties of the polyurethane elastomers gels in the temperature range from 50 °C to 800 °C with the heating rate of 10 °C min −1 .

Structure of PPFEE
The PPFEE-based polyurethane elastomers were prepared via a prepolymer process (Scheme 1). The FTIR spectrum provided the first confirmation of the PPFEE structure. As shown in Figure 1, the strongest peak at 3438 cm −1 corresponded to O-H stretching vibration, and the bands around 2921 and 2889 cm −1 were related to -CH2-stretching vibrations. The stronger peak at 1198 cm −1 was ascribed to C-O-C, and the bands attributed to C-F stretching vibrations were observed at 1101 cm −1 . The PPFEE structure was further confirmed using NMR analysis. The 1 H NMR spectrum of PPFEE in Figure 2a shows the triple peak observed at δ: 3.95, 3.92, and 3.88 ppm, which were attributed to the methylene protons (d) of the side chains, signals at 3.81-3.45 ppm related to methylene protons of the main (a and b) and side (c) chains, and the multiple peaks at 1.19-1.16 ppm ascribed to the initiator in the main chain, respectively. According to the 13 C NMR of PPFEE shown in Figure 2b, the multiple peaks observed at 123.28-114.07 and 116.11-109.94 ppm were ascribed to the carbons in -CF 3 (f) and in -CF 2 (e), respectively. The signal observed at 78.61-78.38 ppm was attributed to CH in the main chain (b). The signal detected at 73.61-69.27 ppm was related to CH 2 in the main (a) and side (c) chains. The triple peaks identified at 68.14, 67.68, and 67.62 ppm were assigned to -CH 2 -in the side chain (d). Figure 2c shows the 19 F NMR spectrum of PPFEE. The signals observed at −84.80 and −124.00 ppm were due to -CF 2 (a) and -CF 3 (b), respectively. Both 123.28-114.07 and 116.11-109.94 ppm were ascribed to the carbons in -CF3 (f) and in -CF2 (e), respectively. The signal observed at 78.61-78.38 ppm was attributed to CH in the main chain (b). The signal detected at 73.61-69.27 ppm was related to CH2 in the main (a) and side (c) chains. The triple peaks identified at 68.14, 67.68, and 67.62 ppm were assigned to -CH2-in the side chain (d). Figure 2c shows the 19 F NMR spectrum of PPFEE. The signals observed at −84.80 and −124.00 ppm were due to -CF2 (a) and -CF3 (b), respectively. Both signal positions observed in the FTIR and NMR spectra confirmed the successful synthesis of PPFEE.

Glass Transition Temperature (T g ) Measurement
The T g is one of the most important parameters of polymeric binders because it has a decisive influence on mechanical properties at low temperatures [22]. Figure 3 shows the second-heating DSC curves of the PAPI, PPFEE, and PPFEE/PAPI mixture. The PPFEE exhibited a low T g of −64.45 • C, which was beneficial for low-temperature properties. The PPFEE/PAPI mixture showed two T g indicating their incompatibility.

Mechanical Properties of Polyurethane Elastomers
The microphase-separated microstructures in the PPFEE-based elastomers were expected to affect their mechanical properties. In essence, the microphase separation is a phase separation equilibrium process (Scheme 2) [20]. According to the microphase separation theory, a certain number of hard and soft segments can be dissolved into each other, and these mutual solubility results can change the T g of the two phases based on the mode of the copolymer equation. Thus, the T g of the soft link phase will increase, and that of hard segments will decrease. In general, the proportion of soft segments dissolved into the microdomain of hard segments is relatively small, and only the case where hard segments are dissolved into the soft segments is considered. Therefore, by studying the T g of soft segments, the phase separation degree of elastomers can be characterized. This change can be described by the Gordon-Taylor equation (Formula (1)) [23].
where T g1 and T g2 represent the T g of pure soft and hard segments, respectively. W 1 and W 2 denote the percentage of the soft segments and hard segments dissolved into the soft segment phase in the soft segment phase, respectively, W 1 + W 2 = 1. T g was measured through DSC (Figure 4), and it was considered the value of the soft segment phase giving an estimate of the extent of hard segments dissolved in soft segment domains.

Glass Transition Temperature (Tg) Measurement
The Tg is one of the most important parameters of polymeric binders b decisive influence on mechanical properties at low temperatures [22]. Figu second-heating DSC curves of the PAPI, PPFEE, and PPFEE/PAPI mixtu exhibited a low Tg of −64.45 °C , which was beneficial for low-temperature p PPFEE/PAPI mixture showed two Tg indicating their incompatibility.

Mechanical Properties of Polyurethane Elastomers
The microphase-separated microstructures in the PPFEE-based elasto pected to affect their mechanical properties. In essence, the microphase s phase separation equilibrium process (Scheme 2) [20]. According to the mi ration theory, a certain number of hard and soft segments can be dissolved i and these mutual solubility results can change the Tg of the two phases base of the copolymer equation. Thus, the Tg of the soft link phase will increa hard segments will decrease. In general, the proportion of soft segments the microdomain of hard segments is relatively small, and only the case w ments are dissolved into the soft segments is considered. Therefore, by stu soft segments, the phase separation degree of elastomers can be chara change can be described by the Gordon-Taylor equation (Formula (1)) [23] : hard segment : soft segment The Tg is one of the most important parameters of polymeric binders because it has a decisive influence on mechanical properties at low temperatures [22]. Figure 3 shows the second-heating DSC curves of the PAPI, PPFEE, and PPFEE/PAPI mixture. The PPFEE exhibited a low Tg of −64.45 °C , which was beneficial for low-temperature properties. The PPFEE/PAPI mixture showed two Tg indicating their incompatibility.

Mechanical Properties of Polyurethane Elastomers
The microphase-separated microstructures in the PPFEE-based elastomers were expected to affect their mechanical properties. In essence, the microphase separation is a phase separation equilibrium process (Scheme 2) [20]. According to the microphase separation theory, a certain number of hard and soft segments can be dissolved into each other, and these mutual solubility results can change the Tg of the two phases based on the mode of the copolymer equation. Thus, the Tg of the soft link phase will increase, and that of hard segments will decrease. In general, the proportion of soft segments dissolved into the microdomain of hard segments is relatively small, and only the case where hard segments are dissolved into the soft segments is considered. Therefore, by studying the Tg of soft segments, the phase separation degree of elastomers can be characterized. This change can be described by the Gordon-Taylor equation (Formula (1)) [23].
where Tg1 and Tg2 represent the Tg of pure soft and hard segments, respectively. W1 and W2 denote the percentage of the soft segments and hard segments dissolved into the soft segment phase in the soft segment phase, respectively, W1 + W2 = 1. Tg was measured through DSC (Figure 4), and it was considered the value of the soft segment phase giving an estimate of the extent of hard segments dissolved in soft segment domains. Glass transitions are associated with the movement of polymer chain segments due to intermolecular interactions. The interaction between hard and soft segments can be indicated through the analysis of changes in the Tg of samples prepared under different conditions. Figure 4a shows that Tg increased first and then decreased with the increase in the curing temperature. The Tg at the curing temperature of 50 °C was the maximum owing to the central arrangement of hard segments, which was consistent with the results for the tensile strength. As shown in Figure 4b, the Tg of PPFEE-based polyurethane elastomers increased slightly with the increased contents of hard segments due to the increase in the percentage of hard segments dissolved in the soft segment domains.
In addition, -NH-in the hard segments can create a hydrogen bond with O in the soft segments, which may greatly limit the movement of soft segments and thus increase their Tg [24][25][26]. Therefore, considering the influence of hard segments on the Tg of soft segments after the dissolution of hard segments into the soft segment phase, the effect of physical crosslinking was also accounted for. The effects can be described by the Dibenedetto and Dimarzio equations (Formula (2)).
where constant k is 0.1; Xc is the mole fraction of soft segment structural units forming hydrogen bonds.
where M1 and M2 are the relative molar masses of soft and hard segment structural units, respectively. We obtained the following after the transformation of Formula (1): Glass transitions are associated with the movement of polymer chain segments due to intermolecular interactions. The interaction between hard and soft segments can be indicated through the analysis of changes in the T g of samples prepared under different conditions. Figure 4a shows that T g increased first and then decreased with the increase in the curing temperature. The T g at the curing temperature of 50 • C was the maximum owing to the central arrangement of hard segments, which was consistent with the results for the tensile strength. As shown in Figure 4b, the T g of PPFEE-based polyurethane elastomers increased slightly with the increased contents of hard segments due to the increase in the percentage of hard segments dissolved in the soft segment domains.
In addition, -NH-in the hard segments can create a hydrogen bond with O in the soft segments, which may greatly limit the movement of soft segments and thus increase their T g [24][25][26]. Therefore, considering the influence of hard segments on the T g of soft segments after the dissolution of hard segments into the soft segment phase, the effect of physical crosslinking was also accounted for. The effects can be described by the Dibenedetto and Dimarzio equations (Formula (2)).
where constant k is 0.1; X c is the mole fraction of soft segment structural units forming hydrogen bonds.
where M 1 and M 2 are the relative molar masses of soft and hard segment structural units, respectively. We obtained the following after the transformation of Formula (1): From Formula (3), we arrived at the equation below: Incorporating Formula (5) into Formula (4), we obtained the following: Assuming that the effects of copolymerization and crosslinking on the T g can be added linearly, we combined Equations (2) and (6) [27]: Given that polyether polyurethane elastomer has a good degree of microphase separation, the ratio of the hard segment phase dissolved into the soft segment phase (W 2 ) is generally small, and the corresponding X c is also small. To simplify the calculation, ignoring the high item (Formula (7)), we obtained the equation (Formula (8)) that affects the T g of the soft segment phase as follows, and X c was obtained through the equation: Let W be the percentage of hard segments dissolved into the soft segments in the elastomers, and H is the percentage of hard segments in the elastomers. Then, we obtained the following: Substituting Formula (9) into Formula (5) and after mathematical transformation, the following equation was solved: The calculated results are listed in Tables 1 and 2. The percentage of hard segments dissolved into soft segments (W) was proportional to the T g , and the minimum values were observed at 50 • C at 26% PAPI, indicating that the extent of microphase separation reached the maximum which had a great influence on the mechanical properties. The nature of binder systems plays a decisive role in the mechanical properties of composite propellants [28]. Figure 5a depicts the stress-strain curves of PPFEE cured with 20 wt% PAPI at different temperatures. The PPFEE-based polyurethane elastomers gave the maximum tensile strength of 2.26 MPa with an elongation at break of nearly 150% at the curing temperature of 50 • C, and this finding was attributed to the possible reason for the poor compatibility between soft and hard segments, resulting in the maximum extent of microphase separation. Thus, the mechanical properties of cured splines were greatly affected by the degree of microphase separation. However, the hard segment content may play a critical role when the curing temperature is constant. The stress-strain curves (Figure 5c) for four test series of PPFEE cured with various contents of PAPI at 50 • C were depicted. As anticipated, the tensile strength increased, and elongation at Polymers 2023, 15, 288 9 of 12 break correspondingly decreased along with the increase in the hard segment content. The maximum tensile strength was 3.04 MPa, that is, an elongation at break of nearly 90%. Moreover, as shown in Figure 5b,d, the most reproducible data with a standard deviation of tensile strength and elongation at break were provided. The tensile strength with the largest dispersion degree reached the maximum at 50 • C (seen in Figure 5b), which was still in the acceptable range. The result indicated that with the increase in temperature, the tensile strength increased and then decreased. The tensile strength reached the maximum when the content of PAPI was 26 wt% (seen in Figure 5d). This result indicated that with the increase in the content of PAPI, the tensile strength increased in direct proportion. In addition to these, the variation in the tensile strength of the splines was opposite the trend of W representing the ratio of hard segments dissolved into the soft segment phase. The reason may be that the elastomers, whose hard segments were dispersed into the soft segment matrix, had no obvious hindrance to the deformation under external stress [29].
the poor compatibility between soft and hard segments, resulting in the maximum extent of microphase separation. Thus, the mechanical properties of cured splines were greatly affected by the degree of microphase separation. However, the hard segment content may play a critical role when the curing temperature is constant. The stress-strain curves (Figure 5c) for four test series of PPFEE cured with various contents of PAPI at 50 °C were depicted. As anticipated, the tensile strength increased, and elongation at break correspondingly decreased along with the increase in the hard segment content. The maximum tensile strength was 3.04 MPa, that is, an elongation at break of nearly 90%. Moreover, as shown in Figure 5b,d, the most reproducible data with a standard deviation of tensile strength and elongation at break were provided. The tensile strength with the largest dispersion degree reached the maximum at 50 °C (seen in Figure 5b), which was still in the acceptable range. The result indicated that with the increase in temperature, the tensile strength increased and then decreased. The tensile strength reached the maximum when the content of PAPI was 26 wt% (seen in Figure 5d). This result indicated that with the increase in the content of PAPI, the tensile strength increased in direct proportion. In addition to these, the variation in the tensile strength of the splines was opposite the trend of W representing the ratio of hard segments dissolved into the soft segment phase. The reason may be that the elastomers, whose hard segments were dispersed into the soft segment matrix, had no obvious hindrance to the deformation under external stress. [29]

Thermal Decomposition
The thermal stability of polymeric binders is a critical property for their application in solid propellants [30][31][32]. Thus, TG and DTG were used to study the thermal decomposition behavior of polyurethane elastomers ( Figure 6). The TG curve showed one distinct region of weight loss in the thermal decomposition of the PPFEE-based polyurethane elastomer. The stage presented a slow weight loss starting at nearly 200 • C, and this result corresponded to the decomposition of the end groups and a small number of side chains. Then, the main chain decomposed, and the weight loss rate gradually increased. The weight loss temperature was observed at 371 • C with a sharp weight loss. After the complete thermal decomposition, the remaining residue was approximately 18%. In any case, the TG/DTG results confirmed that PPFEE-based polyurethane elastomers had satisfactory thermal stability. sition behavior of polyurethane elastomers ( Figure 6). The TG curve showed one distinct region of weight loss in the thermal decomposition of the PPFEE-based polyurethane elastomer. The stage presented a slow weight loss starting at nearly 200 °C , and this result corresponded to the decomposition of the end groups and a small number of side chains. Then, the main chain decomposed, and the weight loss rate gradually increased. The weight loss temperature was observed at 371 °C with a sharp weight loss. After the complete thermal decomposition, the remaining residue was approximately 18%. In any case, the TG/DTG results confirmed that PPFEE-based polyurethane elastomers had satisfactory thermal stability.

Conclusions
A novel random block hydroxyl-terminated polyfluoroether binder, PPFEE, was synthesized through typical cationic ring-opening polymerization. From the FTIR, NMR, and GPC results, PPFEE was synthesized successfully via the synthesis route. The DSC curves indicated that PPFEE had a low Tg of −64.45 °C . Through DSC research, the percentage of hard segments dissolved into the soft segments of elastomers exerted important influences on the mechanical properties. The PPFEE-based polyurethane elastomer cured with 20 wt% PAPI at 50 °C displayed the maximum tensile strength of 2.26 MPa with elongation at break of nearly 150%. The increased contents of PAPI can effectively enhance the tensile strength, and the maximum tensile strength was 3.04 MPa with an elongation at break of nearly 90% when the content of PAPI was 26 wt%. The TG/DTG curves displayed adequate resistance to thermal decomposition up to 200 °C , and the weight loss temperature was observed at 371 °C with a sharp weight loss. All of these results indicated that PPFEE may be a potential polymeric binder as one of the ingredients applied to future propellant formulations.

Conclusions
A novel random block hydroxyl-terminated polyfluoroether binder, PPFEE, was synthesized through typical cationic ring-opening polymerization. From the FTIR, NMR, and GPC results, PPFEE was synthesized successfully via the synthesis route. The DSC curves indicated that PPFEE had a low T g of −64.45 • C. Through DSC research, the percentage of hard segments dissolved into the soft segments of elastomers exerted important influences on the mechanical properties. The PPFEE-based polyurethane elastomer cured with 20 wt% PAPI at 50 • C displayed the maximum tensile strength of 2.26 MPa with elongation at break of nearly 150%. The increased contents of PAPI can effectively enhance the tensile strength, and the maximum tensile strength was 3.04 MPa with an elongation at break of nearly 90% when the content of PAPI was 26 wt%. The TG/DTG curves displayed adequate resistance to thermal decomposition up to 200 • C, and the weight loss temperature was observed at 371 • C with a sharp weight loss. All of these results indicated that PPFEE may be a potential polymeric binder as one of the ingredients applied to future propellant formulations.  Institutional Review Board Statement: Not applicable. This study did not involve humans or animals.

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