Synthesis and Properties of the Novel High-Performance Hydroxyl-Terminated Liquid Fluoroelastomer

Functional liquid fluoroelastomers are in high demand in new energy fields. And these materials have potential applications in high-performance sealing materials and as electrode materials. In this study, a novel high-performance hydroxyl-terminated liquid fluoroelastomer (t-HTLF) with a high fluorine content, temperature resistance, and curing efficiency was synthesised from a terpolymer of vinylidene fluoride (VDF), tetrafluoroethylene (TFE), and hexafluoropylene (HFP). A carboxyl-terminated liquid fluoroelastomer (t-CTLF) with controllable molar mass and end-group content was first prepared from a poly(VDF-ter-TFE-ter-HFP) terpolymer using a unique oxidative degradation method. Subsequently, an efficient “one-step” reduction of the carboxyl groups (COOH) in t-CTLF into hydroxyl groups (OH) was achieved via the functional-group conversion method using lithium aluminium hydride (LiAlH4) as the reductant. Thus, t-HTLF with a controllable molar mass and end-group content and highly active end groups was synthesised. Owing to the efficient curing reaction between OH and isocyanate groups (NCO), the cured t-HTLF exhibits good surface properties, thermal properties, and chemical stability. The thermal decomposition temperature (Td) of the cured t-HTLF reaches 334 °C, and it exhibits hydrophobicity. The oxidative degradation, reduction, and curing reaction mechanisms were also determined. The effects of solvent dosage, reaction temperature, reaction time, and ratio of the reductant to the COOH content on the carboxyl conversion were also systematically investigated. An efficient reduction system comprising LiAlH4 can not only achieve an efficient conversion of the COOH groups in t-CTLF to OH groups but also the in situ hydrogenation and addition reactions of residual double bonds (C=C) groups in the chain, such that the thermal stability and terminal activity of the product are improved while maintaining a high fluorine content.


Introduction
Fluoroelastomers are synthetic polymer elastomers containing fluorine atoms on the carbon atoms of their main or side chains [1]. At present, the commonly used fluoroelastomers are poly(VDF-co-CTFE) copolymer, poly(VDF-co-HFP) copolymer, and poly(VDFter-TFE-ter-HFP) terpolymer [2,3]. There are also some fluoroelastomers with special properties, mainly fluorosilicone rubber and fluorinated phosphazene rubber. Fluorosilicone rubber is an elastomer with trifluoropropyl side chain in the main chain of methyl vinyl siloxane, which is mainly used in aerospace, automobile, chemical industry and other fields. However, the fluorine content of fluorosilicone rubber is lower than that of fluoroelastomer, so its heat resistance and medium resistance are lower than that of fluoroelastomers, which limits its application fields [4]. Fluorinated phosphazene rubber is an elastomer whose methods, which has practical value in the fields of transportation and aerospace. In recent years, the oxidative degradation method has attracted extensive attention for the oxidative degradation of poly(VDF-co-HFP) copolymers because it is simpler and more efficient for preparing liquid fluoroelastomers than polymerisation. Li [24] used a poly(VDF-co-HFP) copolymer as a raw material to prepare carboxyl-terminated liquid fluoroelastomer (c-CTLF) via oxidative degradation method. The effects of various solvents and bases on the molar masses of the products, yield, and end-group conversion were systematically investigated. With the use of acetone as the solvent and potassium hydroxide (KOH) as the alkali, the molar mass of the product was 2.2 × 10 3 g/mol, the yield was up to 98%, and the carboxyl content was up to 2.95 wt%. However, the oxidative degradation mechanism still needs to be thoroughly studied. Li [25] elucidated the primary and secondary relationships of the rules of the elimination reaction for poly(VDF-co-HFP) copolymers under alkaline conditions, the sequence structure and content of C=C, and a detailed reaction mechanism. The poly(VDF-co-HFP) copolymer undergoes the dehydrofluorination reaction mainly via the Zaitsev rule and via the Hofmann rule to a lesser extent. However, because of the complex molecular chain structures of the terpolymers, reports on terpolymer research are limited.
Recently, Li [26] discovered, after an extensive investigation, that the C=C in a chain significantly affects its thermal stability. On this basis, the oxidative degradation of the poly(VDF-ter-TFE-ter-HFP) terpolymer was proposed, and a novel and efficient t-CTLF was prepared. Although t-CTLF can be prepared via the oxidative degradation method with a controlled molar mass and functional-group content [27,28], the nature of the end carboxyl groups determines the defects, such as high curing temperature and poor thermal stability, of the cured products, which affect their overall performance and shorten their service lives. Currently, liquid fluoroelastomers with high fluorine contents, high-temperature resistances, and highly active end groups are being developed. The terminal hydroxyl group exhibits better surface and interface properties and is compatible with metals and inorganic substances. Therefore, the conversion of COOH to OH is of interest [29,30] and is commonly achieved using sodium borohydride (NaBH 4 ) systems [31].
Although NaBH 4 is frequently employed as a reductant [32,33], it typically cannot directly reduce COOH to OH [34] and is commonly used in conjunction with iodine or metal salts. Studies on the efficient conversion of COOH to OH using complicated NaBH 4 systems have recently been reported. By reducing COOH to OH using a NaBH 4 /I 2 reduction system to prepare c-CTLF via oxidative degradation method, Wu and Li [35,36] synthesised a copolymer called hydroxyl-terminated liquid fluoroelastomer (c-HTLF) with a hydroxyl content of 1.0-2.30 wt%. Reaction conversion of 88-100% could be achieved by carefully controlling the reaction conditions. Li [37] conducted a preliminary investigation on the use of a sodium borohydride/rare earth chloride (NaBH 4 /RECl 3 ) system in liquid fluoroelastomers using c-CTLF as the raw material and three reduction systems (NaBH 4 /CeCl 3 , NaBH 4 /LaCl 3 , and NaBH 4 /NdCl 3 ). The highest COOH conversion was achieved when NaBH 4 /CeCl 3 was used. On this basis, Chang [38] synthesised c-HTLF using a one-pot method, with NaBH 4 /SmCl 3 as the reduction system. COOH was effectively reduced with 92% conversion under the optimal reaction conditions. Chang [39] studied the reduction of c-CTLF by sodium borohydride using a metal chloride (NaBH 4 /MCl x ) system and the reduction mechanism. The C=C and COOH groups of c-CTLF could be eliminated, with the rare-earth metal MCl x reduction system exhibiting a more significant reduction effect than the transition-metal MCl x reduction system. Because the LiAlH 4 reaction system does not require the addition of a co-catalyst and can complete the reduction with high efficiency in a single step, it is typically preferred over the NaBH 4 composite system to reduce c-HTLF for meeting the requirement for this material in the new energy field. LiAlH 4 is a stronger reductant than NaBH 4, and it can attribute some spatial selectivity to the reaction [40,41]. Using LiAlH 4 as the reductant, Duan [42] reduced COOH to OH in c-CTLF at a carboxyl concentration as low as 0.12 wt% with a conversion rate as high as 95%. Wen [43] reduced c-CTLF to c-HTLF using two reduction systems: diisobutylaluminum  3 ]/LiAlH 4 . The reducing abilities of the two systems were also evaluated. DIBAl-H/LiAlH 4 was better suited for reducing c-CTLF at 60 • C. The experimental procedure was straightforward when LiAlH 4 was used as the reductant, and the reaction was safer. Additionally, because of the high hydrogen content of the LiAlH 4 molecule, a smaller amount of the reagent is required to provide a greater reduction effect, which significantly reduces manufacturing costs. Consequently, LiAlH 4 has many applications in fine chemicals, medicinal synthesis, and energy production [44][45][46][47][48].
Few reports exist on the preparation methods, reaction mechanisms, structures, properties, and curing of t-HTLF. Thus, to design and synthesise t-HTLF and elucidate the relationship between its molecular chain structure and properties, we used the oxidative degradation and functional-group conversion methods to prepare t-CTLF and t-HTLF, respectively. The poly(VDF-ter-TFE-ter-HFP) terpolymer was selected as the raw material, and fluoroelastomer formation was confirmed via spectral analyses and chemical quantification. Furthermore, property investigations and comparative analyses of the relationship between the structure and properties of the cured t-HTLF were performed.
Solid-state 19 F-NMR was performed using an JEOL JNM ECZ600R spectrometer (Japan), and the specific conditions were frequency of 564 MHz, pulse width of 90 deg and rotating speed of 21 kHz.

Determination of Molar Mass
The molar mass (M n ) and polydispersity index (PD) of the fluoroelastomers was estimated by gel permeation chromatography (GPC) system (PL-GPC50) from Varian, Inc. Company. Polystyrene (PS) was used as the standard sample and chromatographic grade tetrahydrofuran (HPLC) as the mobile phase at a flow rate of 1 mL/min and a test temperature of 30 • C.

Thermal Properties
Differential scanning calorimetry (DSC) measurements were performed using a TA thermal analyser (Newcastle, USA). The test conditions were as follows: nitrogen atmosphere; constant temperature of 40 • C; first ramp up, then ramp down and then ramp up; temperature range of 10-100 • C; and ramp-up rate of 10 • C/min to test the glass transition temperature (T g ) of the products.
The thermogravimetric analysis (TGA) measurements were performed using a TA Instruments-Waters LLC thermal weight-loss analyser with the following test conditions: T d of the product was tested under a nitrogen atmosphere in a temperature range of 30-600 • C with a ramp-up rate of 10 • C/min.

Mechanical Properties
The mechanical properties of the samples were evaluated using the GB/T528-2009 standard and an Instron 3365 universal tensile-testing machine. At least 5 samples were tested at a tensile rate of 500 mm/min at a test temperature of 25 • C. The average value was then calculated.

Surface Properties
The static contact-angle test was performed using a German Dataphysics OCA20 instrument with a test water volume of 5 µL. Points at 5 locations on the sample to be tested were selected and tested, and the average was considered the water contact angle (WCA) of the sample.

Oil and Acid Resistances
The swelling degree of cured t-HTLF was tested according to the method described in GB/T 1690-2006 using 3# jet aircraft oil, cyclohexane, 36.50 wt% HCl, and 50.00 wt% H 2 SO 4 as solvents. The cured product (2.00 g) was soaked in the desired solvent for 24.0 or 72.0 h at 25 ± 2 • C, removed, dried with filter paper, and weighed. The swelling degree w% is calculated by Equation (1): In the Equation (1), m 1 is the mass of the sample before soaking, and m 2 is the mass of the sample after soaking, both in g.

Determination of the Carboxyl Content
In 40 mL of acetone, 1 g of t-CTLF was dissolved. To this solution, 0.1 mL of bromothymol blue indicator was added at a concentration of 10 g/L, and the solution was titrated with a KOH/C 2 H 5 OH (0.1 mol/L) solution until it turned green, which was the endpoint of the titration. The carboxyl content was calculated using the following Equation (2): where V is the volume of the KOH/C 2 H 5 OH solution consumed for titration (mL), C is the concentration of the KOH/C 2 H 5 OH standard titration solution (mol/L), and m is the mass of the test sample (mg).

Determination of the Carboxyl Conversion Rate
The carboxyl contents in t-CTLF and t-HTLF were determined via chemical titration; the difference between them was the hydroxyl content, which was calculated using the following Equation (3): Further calculations yielded the carboxyl conversion rate using the following Equation (4): In the equations, w 0 represents the carboxyl content in t-CTLF, and w 1 represents the carboxyl content in t-HTLF.

Carboxyl-Terminated Liquid Fluoroelastomer
Poly(VDF-ter-TFE-ter-HFP) terpolymer (20 g) and acetone (500 mL) were added to a 1000 mL flask. The mixture was stirred at room temperature until the terpolymer had dissolved completely. Under stirring at 0 • C, BTEAC, KOH and 30 wt% H 2 O 2 aqueous solutions were added sequentially, and stirred for 7 h. After the reaction, HCl was used to acidify the mixture, and excess deionised water was added. The resultant product was concentrated using a rotary evaporator and dried at 65 • C under vacuum until constant weight, and a light yellow viscous liquid was obtained.

Hydroxyl-Terminated Liquid Fluoroelastomer
The t-CTLF (10 g, COOH content 3.24 wt%) and THF (100 mL) were added to a 250 mL flask. Until the liquid fluoroelastomer had dissolved completely, LiAlH 4 was slowly added to the mixture at room temperature. The temperature was then increased to 80 • C, and the reaction was allowed to proceed for 4 h. Subsequently, LiAlH 4 was neutralised with HCl, and the mixture was filtered. The product was collected until constant weight after drying at 65 • C under vacuum to obtain a yellow viscous liquid.

Curing
The t-HTLF (10 g, OH content 3.05 wt%) and acetone (10 mL) were added to a beaker, according to the OH/NCO molar ratio of 1.00/1.20 weighed HDI trimer, and then dissolved in acetone. The two were uniformly mixed and dried in an oven at 60 • C to remove the solvent. At a solvent content of 3-5 mL, the solution was injected into a preheated mould at 60 • C, which was then placed in a vacuum drying oven at 60 • C and left there for 8-48 h to remove the solvent. Subsequently, the temperature was increased to 90 • C, which was the curing temperature, for 4 h to obtain a light-yellow curing product.
The synthesis and curing route of hydroxyl-terminated liquid fluoroelastomer is shown in Scheme 1.

FTIR Spectra of the Liquid Fluoroelastomers
As shown in Figure 1, the FTIR spectra exhibit absorption peaks at 870-890 cm −1 , 1155-1180 cm −1 , and 1395-1400 cm −1 , which may be ascribed to the stretching vibrations of -CF-, -CF 2 -, and -CF 3 -, respectively [49]. A comparison of the FTIR spectra of t-CTLF and t-HTLF after the reduction reaction indicate that the peaks at approximately 1763 cm −1 , which is assigned to the COOH, decrease significantly, while a new peak appears at 3405 cm −1 , which is assigned to the OH. Thus, t-HTLF had been successfully synthesised [50].
1155-1180 cm −1 , and 1395-1400 cm −1 , which may be ascribed to the stretching vibrations of -CF-, -CF2-, and -CF3-, respectively [49]. A comparison of the FTIR spectra of t-CTLF and t-HTLF after the reduction reaction indicate that the peaks at approximately 1763 cm −1 , which is assigned to the COOH, decrease significantly, while a new peak appears at 3405 cm −1 , which is assigned to the OH. Thus, t-HTLF had been successfully synthesised [50].  Figure 2 shows the characteristic peak corresponding to the -CH2CF2-structure at 3.51-2.30 ppm for the three curves. The t-CTLF also exhibits distinct peaks corresponding to the C=C structure at δ = 1.55 and 4.68 ppm. At 4.68 ppm, the characteristic structural peaks of C=C disappear in the 1 H-NMR spectra of the t-HTLF after the reaction. The characteristic structural peaks of -CH2OH also appear at δ = 3.63 and 3.73 ppm. This indicated that COOH in the t-CTLF was converted to OH and that C=C underwent hydrogenation and addition reaction. Thus, the LiAlH4 reduction system is effective and versatile.  Figure 2 shows the characteristic peak corresponding to the -CH 2 CF 2 -structure at 3.51-2.30 ppm for the three curves. The t-CTLF also exhibits distinct peaks corresponding to the C=C structure at δ = 1.55 and 4.68 ppm. At 4.68 ppm, the characteristic structural peaks of C=C disappear in the 1 H-NMR spectra of the t-HTLF after the reaction. The characteristic structural peaks of -CH 2 OH also appear at δ = 3.63 and 3.73 ppm. This indicated that COOH in the t-CTLF was converted to OH and that C=C underwent hydrogenation and addition reaction. Thus, the LiAlH 4 reduction system is effective and versatile.

19 F-NMR Spectra of the liquid fluoroelastomers
As seen in Figure 3, compared with the poly(VDF-ter-TFE-ter-HFP) terpolymer, t-CTLF shows a more intense structural characteristic peak corresponding to -CF2CF2COOH at δ = −63.53 ppm. However, the structural characteristic peak corresponding to -CF2CF2COOH in t-HTLF shows a significant weakening at δ= −63.53 ppm, while the characteristic peak corresponding to the -CF2CH2OH structure appears at δ = −104.02 ppm [51]. This indicates that the t-CTLF had been successfully reduced to t-HTLF, which is consistent with the 1 H-NMR results.

19 F-NMR Spectra of the Liquid Fluoroelastomers
As seen in Figure 3, compared with the poly(VDF-ter-TFE-ter-HFP) terpolymer, t-CTLF shows a more intense structural characteristic peak corresponding to -CF 2 CF 2 COOH at δ = −63.53 ppm. However, the structural characteristic peak corresponding to -CF 2 CF 2 COOH in t-HTLF shows a significant weakening at δ = −63.53 ppm, while the characteristic peak corresponding to the -CF 2 CH 2 OH structure appears at δ = −104.02 ppm [51]. This indicates that the t-CTLF had been successfully reduced to t-HTLF, which is consistent with the 1 H-NMR results.
As seen in Figure 3, compared with the poly(VDF-ter-TFE-ter-HFP) terpolymer, t-CTLF shows a more intense structural characteristic peak corresponding to -CF2CF2COOH at δ = −63.53 ppm. However, the structural characteristic peak corresponding to -CF2CF2COOH in t-HTLF shows a significant weakening at δ= −63.53 ppm, while the characteristic peak corresponding to the -CF2CH2OH structure appears at δ = −104.02 ppm [51]. This indicates that the t-CTLF had been successfully reduced to t-HTLF, which is consistent with the 1 H-NMR results. The fluorine contents in the raw material and product molecular chains were further calculated [52], and the specific characteristic peaks are shown in Table 1 18. The monomer contents (X) of t-CTLF before and after the reduction reaction were calculated using the following Equations (5), (6) and (7). The fluorine contents in the raw material and product molecular chains were further calculated [52], and the specific characteristic peaks are shown in  18 . The monomer contents (X) of t-CTLF before and after the reduction reaction were calculated using the following Equations (5)- (7).

GPC of the Liquid Fluoroelastomers
As shown in Figure 4, the M n of the t-CTLF decreased from 93.0 × 10 3 g/mol to 3.8 × 10 3 g/mol after oxidative degradation reaction. However, when COOH was converted to OH, the M n of the t-HTLF increased to 4.2 × 10 3 g/mol owing to the hydrogenation and addition of C=C on the molecular chain. The detailed values were shown in Table 2.

Reaction Mechanisms
The abovementioned results suggest that under the alkaline degradation, poly(VDF-ter-TFE-ter-HFP) is deprotonated at -CF2C sequent fluoride ion expulsion affords C=C bonds. These bonds cleaved to afford t-CTLF (Figure 5a) [42]. Upon the further trea

Reaction Mechanisms
The abovementioned results suggest that under the alkaline conditions of oxidative degradation, poly(VDF-ter-TFE-ter-HFP) is deprotonated at -CF 2 CH 2 -units, and the subsequent fluoride ion expulsion affords C=C bonds. These bonds are further oxidatively cleaved to afford t-CTLF (Figure 5a) [42]. Upon the further treatment with LiAlH 4 , the COOH groups of t-CTLF are converted to the corresponding Li salts, which are attacked by AlH 4 − at the carbonyl carbon. The resulting unstable intermediate expels LiOAlH 3 − to form an aldehyde, which reacts with another equivalent of LiAlH 4 to yield an alcoholate of the RCH 2 OAlH 3 − type and eventually afford the corresponding alcohol after hydrolysis (Figure 5b) [53][54][55]. Notably, all four hydride ions of AlH 4 − can participate in reduction.
sequent fluoride ion expulsion affords C=C bonds. These bonds are further oxidatively cleaved to afford t-CTLF (Figure 5a) [42]. Upon the further treatment with LiAlH4, the COOH groups of t-CTLF are converted to the corresponding Li salts, which are attacked by AlH4 − at the carbonyl carbon. The resulting unstable intermediate expels LiOAlH3 − to form an aldehyde, which reacts with another equivalent of LiAlH4 to yield an alcoholate of the RCH2OAlH3 − type and eventually afford the corresponding alcohol after hydrolysis (Figure 5b) [53][54][55]. Notably, all four hydride ions of AlH4 − can participate in reduction.  The efficient reduction system of LiAlH 4 reported in this study can not only achieve an efficient conversion of COOH in the t-CTLF to OH but also the in situ hydrogenation addition reaction of the residual C=C in the chain. Thus, the t-HTLF not only has a high fluorine content but also an improved activity of its end groups. A "one step" reaction is thus achieved, along with several beneficial effects.

Thermal Properties of the Liquid Fluoroelastomers
Based on the structural analysis, the thermal properties of the t-HTLF were tested and compared with those of the poly(VDF-ter-TFE-ter-HFP) terpolymer and t-CTLF; the results are shown in Figure 6. The T g of both liquid end-group functionalised fluoroelastomers prepared from the poly(VDF-ter-TFE-ter-HFP) terpolymer are significantly lower than that of the raw material, which decreases from −14 • C to −25 • C and −23 • C, respectively. This is because, after the oxidative degradation reaction, the molar mass of t-CTLF is 3.8 × 10 3 g/mol, lower than poly(VDF-ter-TFE-ter-HFP) terpolymer and the existence of isolated double bonds in the molecular chain. These factors will increase the flexibility of the molecular chain, so the T g of t-CTLF decreases. After the reduction reaction, the C=C in the molecular chain are reduced to single bonds, which reduces the number of isolated C=C and flexibility of the molecular chain, thereby increasing the T g of the t-HTLF [56]. Simultaneously, this maintains a good fluidity at low temperatures.
The TGA results for the poly(VDF-ter-TFE-ter-HFP) terpolymer, t-CTLF, and t-HTLF are shown in Figure 7. The heat resistance of the two liquid fluoroelastomers prepared from the poly(VDF-ter-TFE-ter-HFP) terpolymer is significantly reduced, with T d decreasing from 460 • C to 245 • C for the t-CTLF and 261 • C for the t-HTLF, respectively. This may also be attributed to the reduction of C=C to single bonds, which causes a significant decrease in T d . In addition to the molar mass and monomer composition of the liquid fluoroelastomers, C=C in the molecular chains also influences the thermal characteristics. the molecular chain, so the Tg of t-CTLF decreases. After the redu in the molecular chain are reduced to single bonds, which reduce C=C and flexibility of the molecular chain, thereby increasing th Simultaneously, this maintains a good fluidity at low temperatur The TGA results for the poly(VDF-ter-TFE-ter-HFP) terpolym are shown in Figure 7. The heat resistance of the two liquid flu from the poly(VDF-ter-TFE-ter-HFP) terpolymer is significantly re ing from 460 °C to 245 °C for the t-CTLF and 261 °C for the t-HTLF also be attributed to the reduction of C=C to single bonds, which crease in Td. In addition to the molar mass and monomer compo roelastomers, C=C in the molecular chains also influences the the   The TGA results for the poly(VDF-ter-TFE-ter-HFP) terpolym are shown in Figure 7. The heat resistance of the two liquid fluo from the poly(VDF-ter-TFE-ter-HFP) terpolymer is significantly re ing from 460 °C to 245 °C for the t-CTLF and 261 °C for the t-HTLF also be attributed to the reduction of C=C to single bonds, which crease in Td. In addition to the molar mass and monomer compos roelastomers, C=C in the molecular chains also influences the ther

Factors Influencing the Reaction
Because LiAlH 4 is slightly soluble in THF, the effects of the solvent dosage, reaction time, reaction temperature, and reductant dosage on the experimental results were systematically investigated.

Solvent Dosage
The reaction condition was 80 • C × 4 h, the molar ratio of COOH/LiAlH 4 was 1.00/4.00, and the amount of solvent was changed, No. 1,No. 2,No. 3,No. 4,No. 5. and No. 6 corresponded to THF dosage of 30 mL, 50 mL, 70 mL, 90 mL, 100 mL and 120 mL. The effect of solvent dosage on the product properties was studied, and the results were shown in Table 3. At a solvent dosage of 100 mL, the carboxyl conversion was high, reaching 94%. Therefore, the optimal solvent dosage was 100 mL of THF.

Reaction Temperature
The effects of the reaction temperature on the attributes of the products were presented in Table 4, where the molar ratio of COOH/LiAlH 4 was 1.00/4.00, the volume of the solvent was 100 mL, the reaction time was 4 h, and reaction temperature was changed, No. 7,No. 8,No. 9,No. 10 and No. 11. corresponded to reaction temperature of 70 • C, 80 • C, 90 • C, 100 • C and 120 • C. As shown in Table 4, the carboxyl conversion first increased with the reaction temperature before decreasing. At 80 • C, the carboxyl conversion was 94%. The t-HTLF colour gradually deepened and the molar mass increased with the reaction temperature, indicating the occurrence of side reactions. Therefore, 80 • C was chosen as the optimal reaction temperature.

Reaction Time
The effects of reaction time on the characteristics of the products were examined at a reaction temperature of 80 • C, molar ratio of COOH/LiAlH 4 of 1.00/4.00, and 100 mL of the solvent, and reaction time was changed, No. 12,No.13,No.14,No.15 and No.16. corresponded to reaction time of 2 h, 4 h, 6 h, 8 h and 10 h. The results obtained under these conditions are listed in Table 5. The reduction efficiency increased with the reaction period, and the carboxyl conversion was the most significant at a reaction time of 4 h. The carboxyl conversion remained constant when the reaction time was increased to 6 h. Thus, 4 h was the ideal reaction time.

Reductant Ratio
The influence of the COOH/LiAlH 4 reduction system on the product properties was examined, and the results were presented in Table 6. The reaction conditions were 80 • C, 4 h, and 100 mL of solvent dosage, with other parameters remaining the same. Table 6 showed that the carboxyl content steadily decreased when the reductant dosage was increased. The carboxyl conversion was the maximum at 94% at a COOH/LiAlH 4 molar ratio of 1.00/4.00, and it stabilized with a further increase in the reduction system dosage. The ideal COOH/LiAlH 4 molar ratio for the reductant dosage was 1.00/4.00. In summary, Figure 8 shows the effect of different factors on product conversion. The influence of the COOH/LiAlH4 reduction system on the product p examined, and the results were presented in Table 6. The reaction conditio 4 h, and 100 mL of solvent dosage, with other parameters remaining the showed that the carboxyl content steadily decreased when the reductant d creased. The carboxyl conversion was the maximum at 94% at a COOH/LiAl of 1.00/4.00, and it stabilized with a further increase in the reduction system ideal COOH/LiAlH4 molar ratio for the reductant dosage was 1.00/4.00. In s ure 8 shows the effect of different factors on product conversion. 1.00/5.00 3.9 1.61 9

Structural Characterisation
Structural characterisation of the t-HTLF, HDI trimers, and cured prod formed using FTIR, and the results are shown in Figure 9. In the FTIR spect

Structural Characterisation
Structural characterisation of the t-HTLF, HDI trimers, and cured products was performed using FTIR, and the results are shown in Figure 9. In the FTIR spectra, the stretching vibration peaks corresponding to imino (-N-H-) and carbonyl group (C=O) in the carbamate (-NHCOO-) structure appear at 3355 and 1688 cm −1 , respectively. This indicates that t-HTLF undergoes a curing cross-linking reaction with the HDI trimers.
The 19 F-NMR spectrum of the cured t-HTLF is shown in Figure 10. The peak centred at −75 ppm is assigned to -CF 3 groups from the HFP sequences; the multiplets in the range of −83 to −165 ppm is attributed to the -CF 2 -groups from the HFP, VDF, and TFE sequences; and the peak centred at −185 ppm is assigned to the -CF-groups from the HFP sequences. Moreover, the characteristic peak corresponding to the -CF 2 CH 2 OH structure at −104.02 ppm is significantly weaker, indicating the reaction of the OH in the t-HTLF with the NCO in the HDI trimer to form -NHCOO-, concluding the cross-linking reaction and curing the liquid fluoroelastomer. These results are consistent with the FTIR results. The result was further confirmed by the solid-state 19 F-NMR spectrum, and the characteristic peak area corresponding to the VDF sequence structure was ∑CF 2 = I −63.42 + I −89.28 + I −102.16 + I −110.34 + I −117.45 , the distinct peak area corresponding to the HFP sequence structure was ∑CF 3 = I −70.17 + 2(I −74.79 ), and the distinct peak area corresponding to the TFE sequence structure was ∑CF 2 = I −124.80 . According to the Equations (5)- (8), it can be calculated that the contents of VDF, HFP and TFE in t-CTLF were 73%, 10%, 12%, respectively. The fluorine content in the cured t-HTLF was calculated to decrease after the curing process, from 67% to 65%. olymers 2023, 15, x ing vibration peaks corresponding to imino (-N-H-) and carbonyl group (C bamate (-NHCOO-) structure appear at 3355 and 1688 cm −1 , respectively. that t-HTLF undergoes a curing cross-linking reaction with the HDI trimers The 19 F-NMR spectrum of the cured t-HTLF is shown in Figure 10. The at −75 ppm is assigned to -CF3 groups from the HFP sequences; the multiple of −83 to −165 ppm is attributed to the -CF2groups from the HFP, VDF quences; and the peak centred at −185 ppm is assigned to the -CF-groups sequences. Moreover, the characteristic peak corresponding to the -CF2CH at −104.02 ppm is significantly weaker, indicating the reaction of the OH with the NCO in the HDI trimer to form -NHCOO-, concluding the cross-lin and curing the liquid fluoroelastomer. These results are consistent with the The result was further confirmed by the solid-state 19 F-NMR spectrum, and istic peak area corresponding to the VDF sequence structure was ∑CF2 = I−63.4 + I−110.34 + I−117.45, the distinct peak area corresponding to the HFP sequence ∑CF3 = I−70.17 + 2(I−74.79), and the distinct peak area corresponding to the TFE se ture was ∑CF2 = I−124.80. According to the equation (5), (6), (7) and (8), it can that the contents of VDF, HFP and TFE in t-CTLF were 73%, 10%, 12%, res fluorine content in the cured t-HTLF was calculated to decrease after the c from 67% to 65%.  The 19 F-NMR spectrum of the cured t-HTLF is shown in Figu at −75 ppm is assigned to -CF3 groups from the HFP sequences; th of −83 to −165 ppm is attributed to the -CF2groups from the H quences; and the peak centred at −185 ppm is assigned to the -CF sequences. Moreover, the characteristic peak corresponding to th at −104.02 ppm is significantly weaker, indicating the reaction o with the NCO in the HDI trimer to form -NHCOO-, concluding th and curing the liquid fluoroelastomer. These results are consisten The result was further confirmed by the solid-state 19 F-NMR spec istic peak area corresponding to the VDF sequence structure was ∑ + I−110.34 + I−117.45, the distinct peak area corresponding to the HFP ∑CF3 = I−70.17 + 2(I−74.79), and the distinct peak area corresponding to ture was ∑CF2 = I−124.80. According to the equation (5), (6), (7) and that the contents of VDF, HFP and TFE in t-CTLF were 73%, 10% fluorine content in the cured t-HTLF was calculated to decrease from 67% to 65%.

Curing Reaction Mechanism
Because of the higher electron-cloud density and electroneg atoms in NCO, the C atom has a correspondingly higher positiv

Curing Reaction Mechanism
Because of the higher electron-cloud density and electronegativity of the N and O atoms in NCO, the C atom has a correspondingly higher positive charge. It is more vulnerable to the attack of a nucleophilic reagent. The process proceeds with further expansion of the molecular chain to form a mesh structure and complete the solidification reaction, as the active H atoms in the t-HTLF attack the C atoms of NCO to produce -NHCOO-via a nucleophilic addition reaction [36,[57][58][59]. The curing reaction mechanism is shown in Figure 11.
nerable to the attack of a nucleophilic reagent. The process proceeds with further sion of the molecular chain to form a mesh structure and complete the solidificatio tion, as the active H atoms in the t-HTLF attack the C atoms of NCO to produce -NH via a nucleophilic addition reaction [36,[57][58][59]. The curing reaction mechanism is in Figure 11.

Mechanical Properties of the Cured Hydroxyl-Terminated Liquid Fluoroelas
By keeping the reaction conditions and amount of curing agent constant, the m ical properties of the cured t-HTLF were investigated and compared with those cured c-HTLF under the same conditions. The results are shown in Table 7. The strength of the cured t-HTLF is 2.15 MPa after curing, and the elongation at break is which is better than that of the cured c-HTLF [42]. This indicates that hydroxyl c and monomer composition affect the mechanical properties of cured products. Bec HTLF contains TFE, its molecular chain exhibits higher polarity and rigidity; the the tensile strength of its cured products is higher. The thermal stability of the cured t-HTLF was tested using TGA for a t-HTL OH/NCO molar ratio of 1.00/1.20, and the results are shown in Figure 12. Before an the curing of the t-HTLF, Td increases from 261 °C to 334 °C, and the residual carbo tent at 600 °C increased from 3.82% to 35.96%. This indicates that the presence of the t-HTLF further enhanced the thermal stability of the cured t-HTLF [60,61].

Mechanical Properties of the Cured Hydroxyl-Terminated Liquid Fluoroelastomer
By keeping the reaction conditions and amount of curing agent constant, the mechanical properties of the cured t-HTLF were investigated and compared with those of the cured c-HTLF under the same conditions. The results are shown in Table 7. The tensile strength of the cured t-HTLF is 2.15 MPa after curing, and the elongation at break is 200%, which is better than that of the cured c-HTLF [42]. This indicates that hydroxyl content and monomer composition affect the mechanical properties of cured products. Because t-HTLF contains TFE, its molecular chain exhibits higher polarity and rigidity; therefore, the tensile strength of its cured products is higher.

Thermal Stability
The thermal stability of the cured t-HTLF was tested using TGA for a t-HTLF and OH/NCO molar ratio of 1.00/1.20, and the results are shown in Figure 12. Before and after the curing of the t-HTLF, T d increases from 261 • C to 334 • C, and the residual carbon content at 600 • C increased from 3.82% to 35.96%. This indicates that the presence of TFE in the t-HTLF further enhanced the thermal stability of the cured t-HTLF [60,61].

Surface Properties
Since the OH activity in the t-HTLF is higher than that of COOH, its surface energy is lower and it exhibits hydrophobicity [62]; therefore, the contact angle of the cured t-HTLF is higher, reaching 90 • . In comparison, the contact angle of the cured t-CTLF is 85 • , cured t-HTLF exhibits stronger hydrophobicity than the cured t-CTLF. The specific results are shown in Figure 13.

Surface Properties
Since the OH activity in the t-HTLF is higher than that of COOH is lower and it exhibits hydrophobicity [62]; therefore, the contact a HTLF is higher, reaching 90°. In comparison, the contact angle of the cured t-HTLF exhibits stronger hydrophobicity than the cured t-CTLF are shown in Figure 13.

Oil and Acid Resistances
The oil and acid resistances of the cured t-HTLF were tested a those of cured t-CTLF, and the results are shown in Figure 14. The rat of the cured t-HTLF in these media after 24 and 72 h is also less than 5. the cured t-HTLF had better oil and acid resistances.

Surface Properties
Since the OH activity in the t-HTLF is higher than that of COOH, its surface energy is lower and it exhibits hydrophobicity [62]; therefore, the contact angle of the cured t-HTLF is higher, reaching 90°. In comparison, the contact angle of the cured t-CTLF is 85°, cured t-HTLF exhibits stronger hydrophobicity than the cured t-CTLF. The specific results are shown in Figure 13.

Oil and Acid Resistances
The oil and acid resistances of the cured t-HTLF were tested and compared with those of cured t-CTLF, and the results are shown in Figure 14. The rate of change of mass of the cured t-HTLF in these media after 24 and 72 h is also less than 5.00%, indicating that the cured t-HTLF had better oil and acid resistances.

Conclusions
In summary, the molar mass of a poly(VDF-ter-TFE-ter-HFP) terpolymer decreases from 93.0 × 10 3 g/mol to 3.8 × 10 3 g/mol after the oxidative degradation reaction. The t-CTLF can be prepared via the oxidative degradation method, and there is a small amount

Oil and Acid Resistances
The oil and acid resistances of the cured t-HTLF were tested and compared with those of cured t-CTLF, and the results are shown in Figure 14. The rate of change of mass of the cured t-HTLF in these media after 24 and 72 h is also less than 5.00%, indicating that the cured t-HTLF had better oil and acid resistances.

Surface Properties
Since the OH activity in the t-HTLF is higher than that of COOH is lower and it exhibits hydrophobicity [62]; therefore, the contact an HTLF is higher, reaching 90°. In comparison, the contact angle of the c cured t-HTLF exhibits stronger hydrophobicity than the cured t-CTLF. are shown in Figure 13.

Oil and Acid Resistances
The oil and acid resistances of the cured t-HTLF were tested a those of cured t-CTLF, and the results are shown in Figure 14. The rate of the cured t-HTLF in these media after 24 and 72 h is also less than 5.0 the cured t-HTLF had better oil and acid resistances.

Conclusions
In summary, the molar mass of a poly(VDF-ter-TFE-ter-HFP) ter from 93.0 × 10 3 g/mol to 3.8 × 10 3 g/mol after the oxidative degradati CTLF can be prepared via the oxidative degradation method, and ther

Conclusions
In summary, the molar mass of a poly(VDF-ter-TFE-ter-HFP) terpolymer decreases from 93.0 × 10 3 g/mol to 3.8 × 10 3 g/mol after the oxidative degradation reaction. The t-CTLF can be prepared via the oxidative degradation method, and there is a small amount of C=C in the chains of t-CTLF that is not entirely degraded via oxidative. This affects the chemical and thermal stability of the t-CTLF and even causes side reactions in the subsequent functional reaction of the liquid fluoroelastomers. The efficient reduction system of LiAlH 4 developed in this study can not only achieve an efficient conversion of COOH in the t-CTLF to OH but also the in situ hydrogenation addition reaction of the residual C=C in the chain. Thus, the thermal stability and end-group activity of the product are improved while maintaining a high fluorine content.
Furthermore, when the reaction conditions are 80 • C, 4 h, and a COOH/LiAlH 4 molar ratio of 1.00/4.00, t-CTLF is reduced to t-HTLF via LiAlH 4 . The end-group conversion rate can reach 94%. The molar mass increases from 3.8 × 10 3 g/mol to 4.2 × 10 3 g/mol owing to the hydrogenation and addition reaction of C=C to the molecular chain. The T g of t-HTLF is −23 • C, which is 2 • C higher than that of t-CTLF. Thus, it exhibits good fluidity at low temperatures. The T d of t-HTLF is 261 • C, which exhibits a good thermal stability.
After curing, various end groups of the liquid fluoroelastomers exhibit distinct curing efficiencies and properties. Because the activity of OH is higher than that of COOH, OH can be cured efficiently in a shorter time. The T d of cured t-HTLF can be as high as 334 • C, which is approximately 73 • C higher than that of the cured t-CTLF. In addition, its tensile strength is 2.15 MPa and elongation at break is 200%. Moreover, both fluoroelastomers exhibit hydrophobicity, and the WCA of the cured t-HTLF is 90 • . The higher the activity of an end group, the stronger its hydrophobicity. The cured t-HTLF possesses high oil and acid resistances, and the fluorine content in the cured t-HTLF is well kept above 65%, and decreases from 67% to 65% after curing.