Preparation of a P(THF-co-PO)-b-PB-b-P(THF-co-PO) triblock copolymer via cationic ring-opening polymerization and its use as a thermoset polymer

Abstract

The triblock copolymer poly(tetrahydrofuran-co-propylene oxide)-b-polybutadiene-b-poly(tetrahydrofuran-co-propylene oxide) [P(THF-co-PO)-b-PB-b-P(THF-co-PO)] was synthesized via the cationic ring-opening copolymerization of tetrahydrofuran (THF) and propylene oxide (PO) in the presence of hydroxyl-terminated polybutadiene (HTPB). The copolymerization mechanism was studied using Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance (NMR), and size-exclusion chromatography coupled with multi-angle laser light scattering (SEC-MALLS). The results showed both active chain end (ACE) and activated monomer (AM) mechanisms contribute to the chain propagation process to different extents, and the copolyether segments in the triblock copolymer had a gradient microstructure accordingly. Differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and viscosity testing results showed that the triblock copolymer had lower T_g than HTPB, low crystallization tendency and viscosity, and excellent thermal stability. To explore its potential applications, the triblock copolymer was used to prepare elastomer by the thermosetting process. Dynamic thermomechanical analysis (DMA) and tensile test results indicated that the triblock copolymer based elastomer had lower T_g, as well as dramatically higher tan δ value and elongation at break, than HTPB based elastomer because of the introduction of copolyether segments.

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1 Introduction

The typical cationic ring-opening homo- and co-polymerization of cyclic ethers proceeding by the active chain end (ACE) mechanism can usually produce both linear macromolecules and cyclic oligomers. Cyclic oligomers are formed by the back-biting reaction of oxygen atoms in linear units with the active species positioned at the end of growing chain.^1–3 However, if polymerization is carried out in the presence of hydroxyl group containing compounds, such as diols, cyclization can be eliminated or at least be reduced.⁴ For example, Penczek et al. reported in the cationic ring-opening polymerization (CROP) of propylene oxide (PO), the proportion of cyclic oligomer could drop from 50% to 0.95%, and in the case of epichlorohydrin (ECH), polyepichlorohydrin is nearly free of cyclic oligomers.^5,6 These results are attributed to the activated monomer (AM) mechanism. In the CROP proceeding by AM mechanism, the active species is the protonated monomer and at the end of growing chain is the hydroxyl group. Thus, one important advantage of AM polymerization is that the back-biting reaction could be eliminated because of the absence of active species at the end of growing chain.⁷ Until now, AM mechanism has been widely used in the CROP of cyclic ethers, acetals and esters, and corresponding polymers with narrow polydispersity and low cyclic oligomers content are obtained.^8–11 However, most hydroxyl group containing compounds used in these polymerization systems were low-molecular-weight diols, and reports focused on the polymerization in the presence of polymeric diols were rarely published.

Hydroxyl-terminated polybutadiene (HTPB) owns two hydroxyl groups at both ends of chain, and is widely used in polyurethane and adhesives, and especially used as a fuel binder in solid propellants.^12–17 Given that a large number of unsaturation groups exist in the backbone of HTPB, the polymer exhibits some fascinating chemical and physical properties.^18,19 For example, its low-temperature flexibility (the glass transition temperature is nearly −80 °C), low surface energy, and low viscosity (which make the materials be easily processed by thermosetting process). However, the major drawback of HTPB is its immiscible nature with polar components, especially with certain inorganic compounds.^20,21 This phenomenon is due to its weak polarity backbone, where nearly all the groups are nonpolar except for the end groups. To date, several methods have been proposed to increase the polarity of HTPB backbone. The first method was focused on the modification of the double bonds in HTPB main chain, such as the epoxidation of HTPB.²² The other method was to bond some homopolymers that have large number of polar groups such as PCL or PEO to both ends of HTPB.^23,24 Unfortunately, these methods have certain limitations. For example, the first method destroys the main chain structure of HTPB and may lead to the decline of the flexibility of HTPB. As for the second method, the polar group functionalized homopolymers usually have crystallization tendency, which is unfavorable for the processing of the materials.²⁵ Previous study results demonstrated that introduction of side groups into polymer backbone is an efficient approach to inhibit the crystallization of polymers.²⁶ For example, although PTHF, PEO and PPO have the similar backbones, PTHF and PEO are semicrystalline polyols, whereas PPO is amorphous.²⁷ However, despite the fact that PPO has good processability, it is not an ideal material due to its poor mechanical properties.¹⁹ Copolymerization of PO with THF is a promising method to overcome such shortcomings, that is, to enhance the mechanical properties without sacrificing processability.

On the basis of the considerations above, in the present article, we report the polar functionalization of HTPB by covalently attaching copolyether segments onto the chain end of HTPB via the cationic ring-opening copolymerization of THF and PO in the presence of HTPB as a macroinitiator and Lewis acid as a catalyst. The cationic ring-opening copolymerization mechanism of THF and PO in the presence of polymeric diols was studied in detail. Then, the thermal properties of original HTPB and triblock copolymer poly(tetrahydrofuran-co-propylene oxide)-b-polybutadiene-b-poly(tetrahydrofuran-co-propylene oxide) [P(THF-co-PO)-b-PB-b-P(THF-co-PO)] were compared. To explore their applications, the elastomers based on HTPB and P(THF-co-PO)-b-PB-b-P(THF-co-PO) were prepared by the reaction of corresponding polymer with toluene diisocyanate (TDI). Both dynamic and static mechanical properties of the elastomers were then evaluated by DMA and tensile tests, respectively.

2 Experimental

2.1 Materials

THF (99.9%, Alfa Aesar, USA) was refluxed over sodium wire (Na, 98.0%, Sinopharm Chemical Reagent Co., Ltd, China) in the presence of benzophenone until a blue color presence, and was distilled before use. HTPB (M_n = 2200 g mol⁻¹, M_w/M_n = 1.77, hydroxyl value = 0.95 mmol g⁻¹) was kindly supplied by Qilong Chemical Corp. (China). The HTPB contains 19 mol% 1,2-addition structure and 81 mol% 1,4-addition structure. Before use, HTPB was dried by azeotropic distillation with anhydrous toluene. TDI, an 80 : 20 mixture of 2,4 and 2,6 isomers, was procured from Sinopharm Chemical Reagent Co., Ltd (China). PO (99.5%) and trifluoride etherate (BF₃·OEt₂, 99.5%) were purchased from Sinopharm Chemical Reagent Co., Ltd (China), and were stored in sealed preserving condition. Toluene (99.5%, Sinopharm Chemical Reagent Co., Ltd, China), dichloromethane (99.5%, Sinopharm Chemical Reagent Co., Ltd, China), and sodium phenate (98.0%, Alfa Aesar, USA) were all used as received.

2.2 Synthesis of triblock copolymer

The triblock copolymer P(THF-co-PO)-b-PB-b-P(THF-co-PO) was synthesized by cationic ring-opening copolymerization of THF and PO in the presence of HTPB as the macroinitiator and BF₃·OEt₂ as the catalyst. Prior to polymerization, all the glasswares were carefully dried by heating in vacuum to ensure a water-less system. The typical procedure for the preparation of triblock copolymer having M_n = 4500 g mol⁻¹ is given as follows. First, dried HTPB (8.8 g, 4 mmol) was dissolved in 15 mL anhydrous toluene thoroughly, and was charged into a 100 mL round-bottom flask. Then, BF₃·OEt₂ (0.63 mL, 5 mmol) was added under magnetic stirring by a microsyringe. The mixture was immersed into a low-temperature thermostat bath at 0 °C. THF (16.2 mL, 200 mmol) was introduced using syringe. When the temperature of the system was 0 °C, PO (1.4 mL, 20 mmol) was added dropwise using a spring pump over 1 h. After the polymerization was conducted for a prescribed time, excess distilled water was added to quench the polymerization. The crude product was washed with distilled water to neutralization. Finally, the solvent and residual water were removed by rotary evaporation to obtain a pale yellow viscous liquid product. Yield: 61%–71%.

FT-IR (cm⁻¹): 1113 (C–O–C), 1370 (–CH₃), 966 (C=C in trans 1,4 structure of PB block), 911 (C=C in 1,2 structure of PB block), 724 (C=C in cis 1,4 structure of PB block). ¹H NMR (in CDCl₃, δ): 3.41–3.50 (–OCH₂– and –CHCH₃O– of copolyether block), 1.62 (–CH₂– of copolyether block), 1.14 (–CH₃ of copolyether block), 5.38–5.66 (–CH=CH– in 1,4 structure and –CH=CH₂ in 1,2 structure of PB block), 4.84–5.04 (–CH=CH₂ in 1,2 structure of PB block), 1.75–2.25 (–CH₂– in 1,4 structure and –CH– in 1,2 structure of PB block), 1.28–1.43 (–CH₂– in 1,2 structure of PB block), 3.90–4.18 (–CH₂– and –CH– at junction of PB block and polyether block). ¹³C NMR (in CDCl₃, δ): 70.11 (α-carbon in THF and PO units of copolyether block), 26.03 (β-carbon in THF units of copolyether block), 16.34 (–CH₃ in PO units of copolyether block), 113.90–142.55 (olifinic carbons of PB block), 24.42–43.08 (aliphatic carbons of PB block).

2.3 Preparation of elastomers

The elastomers based on HTPB and triblock copolymer were prepared by the reaction of corresponding polymer with TDI at stoichiometry ratio (NCO/OH) of 1.1. First, polymer was dried at 80 °C under vacuum using rotary flash evaporator to remove residual moisture, and then mixed with calculated amount of the TDI. The mixture was vigorously stirred by hand for several minutes and degassed under vacuum to remove air bubbles. The degassed mixture was poured into a glass mould, and cured at 70 °C for 168 h. Light yellow elastomer with a thickness of 3 mm was obtained and subjected to evaluations of dynamic and static mechanical properties.

2.4 Measurements

FT-IR spectra were recorded on a Nicolet iS10 FT-IR instrument (Nicolet Instrument Corporation, USA), and the polymers were coated on KBr disk to form the film. The NMR spectra were recorded at room temperature on a Bruker 400 MHz NMR spectrometer (Bruker Corporation, Germany) in CDCl₃ with tetramethylsilane as internal reference. The number-averaged molecular weight and polydispersity indexes of polymers were determined on a Wyatt DAWN EOS SEC-MALLS equipped with a highly cross-linked styrene/divinylbenzene gel columns (500 Å, 5 μm). HPLC grade THF was used as the eluent with a flow rate of 0.5 mL min⁻¹ at 25 °C. Hydroxyl values for polymers were determined through acetylation method using acetic anhydride. The glass transition and melting behaviors of polymers were investigated by means of DSC using a DSC200PC thermal analysis system (Netzsch instruments, Germany). Samples weighing approximately 10 mg were scanned at a rate of 10 °C min⁻¹ from −110 °C to 100 °C. To remove thermal history, all the samples were first heated to 150 °C, and then cooled down to −110 °C and held at this temperature for 10 min before the scan. The thermal stabilities of polymers were studied on a Q50 TGA (TA Instruments, USA). The TGA experiment was conducted as a function of temperature, over the range of 25 °C to 600 °C at a heating rate of 20 °C min⁻¹ under N₂ atmosphere. The viscosities of the polymers were determined using a CAP 2000+ Brookfield viscometer (Brookfield Corporation, USA).

The dynamic mechanical properties of elastomers were measured by a Q800 DMA (TA Instruments, USA). The samples were cut into 10 mm wide by 3 mm thick strips and placed in a 17.5 mm length single cantilever. The temperature dependence of the samples were performed at temperature ranging from −100 °C to 0 °C at a constant frequency of 1 Hz and a heating rate of 3 °C min⁻¹.

The static mechanical properties of elastomers, such as tensile strength and elongation at break, were determined with an XLD-20C Universal Testing Machine (JJ-Test, China) using dumbbell-shaped specimens at 25 °C. The gauge length was 25 mm and the stretching rate was 100 mm min⁻¹. The values were obtained from the average of five samples.

3 Results and discussion

3.1 Synthesis mechanism and characterization of the triblock copolymer P(THF-co-PO)-b-PB-b-P(THF-co-PO)

The previous study in the copolymerization of THF with PO (or EO) in the presence of low-molecular-weight diols prompted us to explore its copolymerization system in the presence of HTPB to synthesis the new block copolymer.^28–32 The ring-opening copolymerization of THF with PO in the presence of HTPB was realized by slowly adding PO to the reaction system containing THF, toluene, and Lewis acid catalyst at 0 °C. Scheme 1 illustrates the mechanism of copolymerization. Given the low ring strain of the five-membered ring, the reactivity of protonated THF (secondary oxonium ions) was very low. However, when the three-membered ring monomer with high ring strain was introduced, because of the high reactivity of secondary oxonium ions in three-membered ring, the copolymerization reaction was initiated. Thus, PO in this system acted as a comonomer and as promoters of THF polymerization. THF–PO segments were formed as long as PO was added, and once PO was consumed, the reaction was stopped. The chain propagation process included the reaction of protonated PO with hydroxyl group containing compounds (known as AM mechanism; reaction 4 in Scheme 1), and the reaction of protonated PO with THF to form tertiary oxonium ions ended copolymer segments (known as ACE mechanism), which subsequently reacted with hydroxyl group containing compounds and formed block copolymer (reactions 3 and 5 in Scheme 1). Hydroxyl groups are reproduced after each propagation step, and participate in next propagation reaction.

Scheme 1 The mechanism for the ring-opening copolymerization of THF with PO in the presence of HTPB.

The question we addressed first was whether all the oxonium ions locating at the end of copolyether segments (4 and 5 in Scheme 1) were quenched with hydroxyl groups in the system and formed block copolymers at the end of polymerization. To clarify this issue, excess sodium phenoxide was added to the system at different reaction stages, and if the system exist THF–PO segments that were terminated with oxonium ions, then phenyl ether groups could be incorporated by the reaction of the oxonium ions with sodium phenoxide.^33,34 FT-IR was used to confirm the structure of the resulting copolymers. The FT-IR results are shown in Fig. 1. As the reaction proceeded, the characteristic absorption peak of phenyl at 1600 cm⁻¹ was found to disappear gradually, which meant that the content of THF–PO segments which ended with oxonium ion decreased. When the copolymerization was carried out for 2 h, no absorption peak of phenyl was observed, indicating that the reaction was terminated at this time, and the pure triblock copolymer could be obtained without any individual THF–PO copolymers. In addition, resulting triblock copolymers can be dissolved in n-hexane thoroughly. This result also proved that no THF–PO copolymers (insoluble in n-hexane) exist in the resulting triblock copolymers.

Fig. 1 FT-IR spectra of copolymers quenched by sodium phenolate at different reaction stages.

Fig. 2 displays ¹H NMR spectra of HTPB and triblock copolymer. In the ¹H NMR spectrum for triblock copolymer, the peaks at 3.41 and 1.62 ppm were attributed to the methylene group protons (–CH̲₂CH̲₂OCH̲₂CH̲₂–) of THF units. Furthermore, the resonance signal at 1.14 ppm for the methyl groups (–OCH₂CHCH̲₃–) of PO units was also detected.¹³C NMR spectra were used to check the structure of the triblock copolymer, as shown in Fig. 3. In the ¹³C NMR spectrum of triblock copolymer, the resonance signals at 63 and 65 ppm for carbon of methylene-ended group in HTPB disappeared completely, revealing that all the HTPB were linked with the P(THF-co-PO) segments successfully.³⁵ Furthermore, typical SEC traces in Fig. 4 clearly indicated a monomodal distribution without any trace of homopolymer and unreacted HTPB. The elution peak of triblock copolymer was located at the higher molecular weight side compared with the trace of HTPB. All the above results demonstrated that the triblock copolymer was successfully prepared.

Fig. 2 ¹H NMR spectra of (a) triblock copolymer and (b) HTPB.

Fig. 3 ¹³C NMR spectra of (a) triblock copolymer and (b) HTPB.

Fig. 4 Typical SEC curves of (a) triblock copolymers and (b) HTPB.

The effect of the polymerization condition on the molecular weight of triblock copolymer was also investigated. The relationship between the feed ratios of the reactants and number-averaged molecular weight (M_n) of resulting triblock copolymers is summarized in Table 1. The influence of the feed amounts of BF₃·OEt₂ and PO on the M_n of the triblock copolymer was insignificant (entry 2–6). This result was due to the recyclability of protons in polymerization system provided by BF₃·OEt₂, as shown in reaction 5 in Scheme 1. In addition, the main effect of PO is to form active species at the initial stage of reaction. The M_n of the triblock copolymer mainly depended on the feed ratios of THF to HTPB (entry 6–8). It was understandable that the extent of THF incorporated into the copolymer highly depended on the ratio of the rates of the chain propagation through the reaction of THF monomer with active species (reaction 3 in Scheme 1) and the temporary chain deactivation through the reaction of active species with HO– groups (reaction 5 in Scheme 1). Considering both reactions possessed the fixed rate constant, the amount of THF incorporated into the copolymer should closely depend on the relative concentrations of THF and HO– groups.

Table 1 Triblock copolymers' molecular weights and molecular weight distribution at different polymerization condition

Sample^a	[BF3]/[HTPB]	[PO]/[THF]	[THF]/[HTPB]	Mn^b/g^−1 mol	Mw/Mn^b	dn/dc^b
a The copolymerization was carried out in toluene at 0 °C for 2 hours.b Determined by SEC-MALLS.
1	2.5	0	50	2200	1.77	0.14
2	2.5	0.05	50	4400	1.71	0.11
3	1.25	0.1	50	4500	1.66	0.11
4	1.5	0.1	50	4600	1.61	0.11
5	2	0.1	50	4500	1.51	0.11
6	2.5	0.1	50	4500	1.45	0.11
7	2.5	0.1	40	3700	1.79	0.11
8	2.5	0.1	30	2900	1.61	0.12

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3.2 Composition of the P(THF-co-PO) segments in the triblock copolymer

To investigate the polymerization process and the final composition of copolyether segments in detail, the samples at different reaction stages were withdraw and analyzed by NMR. As shown in Fig. 5, peaks at 1.62 and 1.14 ppm were attributed to protons of methylene groups (–CH̲₂CH₂OCH₂CH̲₂–) in THF units and protons of methyl groups (–OCH₂CHCH̲₃–) in PO units, respectively. Thus, the overall compositions of copolyether segments can be calculated from the integration of both peaks. Apparently, as the polymerization proceeded, THF units in copolyether segments declined linearly. This phenomenon further confirmed that the copolymerization mechanism is indeed as discussed earlier. In the copolymerization system, the ACE mechanism and AM mechanism are coexisting and competitive. At the early stage, as the concentration of THF was considerably high, THF monomer may be more effectively incorporated into copolyether segments by the reaction of THF with protonated PO (reactions 2 and 3 in Scheme 1). However, with the consumption of the THF monomer, the concentration of THF deceased and became insufficient to ensure effective incorporation of THF into copolyether segments. At this stage, protonated PO introduced was quenched with the hydroxyl group terminated macromolecule instantly (reaction 4 in Scheme 1). In particular, at the early stage of the copolymerization, reactions were dominated by ACE mechanism, leading more THF monomers to be incorporated into copolyether segments. However, at later stage, reactions controlled by AM mechanism may become significant, and more PO monomers were incorporated into copolyether segments. Therefore, copolyether segments in the resulting triblock copolymer possess a gradient microstructure, as illustrated in Scheme 2, as well as possess more THF units in the middle of the chain and enriched PO units at the end of the chain.²⁸

Fig. 5 Overall compositions of copolyether segments as a function of reaction time calculated based on ¹H NMR. Copolymerization conditions: [BF₃]/[HTPB] = 2.5, [PO]/[THF] = 0.1, [THF]/[HTPB] = 50, at 0 °C.

Scheme 2 Schematic illustration of obtained triblock copolymer with gradient copolyether segments.

3.3 Thermal properties of triblock copolymer

The glass transition temperature and crystalline behavior of triblock copolymer (M_n = 4500) were studied by DSC. The DSC results are shown in Fig. 6. Given that the crystalline behaviors of two blocks in the triblock copolymer are different, PTHF and HTPB with nearly equal molecular weights to the corresponding blocks in triblock copolymer were also examined for comparison. To remove thermal history, all samples were first heated to 150 °C, and then cooled down to −110 °C and held at this temperature for 10 min before scanning. The crystalline PTHF exhibited a distinct melting peak at 23 °C without any cold crystallization peak, suggesting that the crystallization was completed during the cooling process. Since HTPB is a non-crystalline polymer, its DSC curve exhibited only a glass transition peak at −79.8 °C. As expected, the DSC curve of triblock copolymer exhibited a glass transition, a crystallization, and a melting peak. The DSC curve of triblock copolymer displayed only one glass transition at −81.7 °C, which was slightly lower than that of HTPB. This result indicated that the introduction of copolyether segments did not affect the cryogenic properties of HTPB. Meanwhile, the triblock copolymer curve showed a weak exothermal peak at −26.8 °C and an endothermal peak at −7.5 °C which can be ascribed to the cold crystallization and melting of copolyether segments, respectively. Interestingly, T_m and enthalpy of fusion (ΔH_f) for copolyether segments were obviously lower than those of PTHF, indicating that the crystallization ability of copolyether segments in triblock copolymers was weak. The following two reasons may explain this result. First, the methyl side groups in PO units could decrease the regularity of the backbone, and prevented the alignment and nucleation of copolyether segments. Second, the crystallization of copolyether segments could be suppressed by the amorphous PB segments.³⁶ Furthermore, the triblock copolymer is a viscous liquid at room temperature. The viscosities of triblock copolymer and HTPB at different temperatures were compared. The comparison results are shown in Fig. 7. Clearly, the viscosity of triblock copolymer was much lower than HTPB with the same M_n. In general, the low crystallization tendency and low viscosity properties ensured that the triblock copolymer can be processed by thermosetting process conveniently.

Fig. 6 DSC thermograms of PTHF (M_n = 2000), HTPB (M_n = 2200) and triblock copolymer (M_n = 4500).

Fig. 7 Viscosities of HTPB (M_n = 4500) and triblock copolymer (M_n = 4500) as a function of temperature.

The thermal stabilities of HTPB and triblock copolymer were also investigated by TGA measurements, and results are presented in Fig. 8. As previously reported, the thermal degradation of HTPB underwent a two-step process.^15,37,38 In our study, the first step appeared around 359 °C and the second around 461 °C. However, the triblock copolymer only showed one thermal degradation process during the heating, this result was due to fact that the main weight loss process of copolyether segments in triblock copolymers occurred at the temperature near the first decomposition step of HTPB.^39,40 Moreover, at weight loss of 5%, the degradation temperature of HTPB and triblock copolymer are 344 °C and 347 °C, and the final decomposition temperature of two samples are 509 °C and 495 °C, respectively. The above results indicated that the triblock copolymer exhibits an excellent thermal stability similar to that of HTPB.

Fig. 8 TGA curves of (a) HTPB and (b) triblock copolymer under nitrogen.

3.4 Dynamic and static mechanical properties of triblock copolymer based elastomer

As mentioned earlier, HTPB is widely used in the field of polyurethane elastomer. However, the weak polarity of main chain of HTPB leads to poor compatibility with other polar components, and limits its applications. In this study, we attempted to bond two copolyether segments onto both end of HTPB to overcome such inherent shortcoming. The physical properties of triblock copolymer can satisfactorily meet the processing requirements for a thermoset polymer according to the study results. Considering the mechanical properties is one of the most important properties of these elastomers in practical application and in order to clarify the effect of adding copolyether segments on the mechanical properties, elastomers based on original HTPB and triblock copolymer were prepared by curing reaction of corresponding polymer with TDI at stoichiometry ratio (NCO/OH) of 1.1. The reaction scheme is presented in Scheme 3. Because the isocyanate is used in excess of the stoichiometry requirement (NCO/OH = 1), the excess NCO groups are likely to undergo reaction with urethane to form allophanate. Visually, allophanate served as potential crosslinks in the network of elastomers.^41,42 The parameters of HTPB and triblock copolymer, which were used to prepare elastomers, are summarized in Table 2. Fig. 9 shows the attenuated total reflectance FTIT (ATR-FTIR) spectra of two elastomers, where the appearance of the stretching vibration for the amine groups at 3300 cm⁻¹, C–N bonds at 1530 cm⁻¹, and carboxyl groups at 1730 cm⁻¹, together with the disappearance of the stretching vibration peaks for isocyanate groups at 2270 cm⁻¹ and hydroxyl groups at 3400 cm⁻¹, suggested that the curing reaction has proceeded completely.^12,43

Scheme 3 Curing reaction between the hydroxyl telechelic triblock copolymer and TDI.

Table 2 Polymers used to prepare elastomers

	Mn/g mol^−1	Mw/Mn	Viscosity at 25 °C/Pa s	Hydroxyl value/mmol g^−1
HTPB	2200	1.77	6.0	0.95
Triblock copolymer	4500	1.45	7.5	0.47

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Fig. 9 ATR-FTIR spectra of elastomers based on (a) HTPB and (b) triblock copolymer.

DMA analyses were performed to evaluate the dynamic mechanical properties of both elastomers. From DMA, the temperature of the maximum loss factor (tan δ) is usually defined as the T_g of the elastomers.^44–46 From Fig. 10, the T_gs of two elastomers are −52.90 °C and −47.30 °C, which indicates that introducing copolyether segments can effectively improve the low-temperature properties of HTPB. Two elastomers' T_gs were also measured using DSC, as shown in Fig. 11. The triblock copolymer based elastomer also had lower T_g than HTPB. We inferred that this result was due to the low interior-rotation barrier for the ether bonds. Ether bonds can easily undergo secondary relaxation with increased temperature. The different values of T_gs measured by DMA and DSC can be attributed to the varying underlying property being monitored.^47,48 In addition, the tan δ value of triblock copolymer based elastomer showed in Fig. 10 is significantly higher than that of HTPB based elastomer. This result was due to the enhanced the polarity of polymer backbones and intermolecular interactions after introducing ether bonds into triblock copolymer, resulting in increased internal friction and energy dissipation during the vibration.⁴⁹ Additionally, the effect of molecular weight of polymers should also be considered, since with the increase of the polymer molecular weight, the crosslink density of elastomer decreased, which would results in the increase of tan δ value.⁴²

Fig. 10 Storage modulus, loss modulus and tan δ as a function of temperature for two elastomers.

Fig. 11 DSC thermograms of elastomers based on HTPB and triblock copolymers.

Tensile analyses were conducted to evaluate the static mechanical properties of both elastomers. The results are listed in Table 3. According to Table 3, the triblock copolymer based elastomer exhibits higher elongation at break than HTPB elastomer without sacrificing any strength. This result can be explained by the increasing of the molecular weight between crosslinks in the network of triblock copolymer elastomer, and the increase of intermolecular interactions after introducing ether bonds, as aforementioned.

Table 3 Parameters and static mechanical properties of the elastomers

Sample	Density (kg m^−3)	Tensile strength (MPa)	Stress at break (MPa)	Elongation at break (%)
HTPB	960	1.12	1.08	112
Triblock copolymer	970	1.11	1.09	232

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4 Conclusions

In this study, a novel triblock copolymer P(THF-co-PO)-b-PB-b-P(THF-co-PO) was synthesized by cationic ring-opening copolymerization, and the chain propagation process involved both ACE and AM mechanisms. At the early stage, ACE mechanism was favored, whereas AM mechanism became significant at the later stage. As a result, copolyether segments of the triblock copolymer had a gradient microstructure. The M_n of the triblock copolymer could be tailored by the feed ratio of THF to HTPB. The synthesized triblock copolymer exhibited low T_g and low crystallization tendency, as well as excellent thermal stability. Elastomers based on HTPB and P(THF-co-PO)-b-PB-b-P(THF-co-PO) were prepared, and the introduction of copolyether segments had a significant effect on the dynamic and static mechanical properties of the elastomers. The triblock copolymer based elastomer had lower T_g, higher tan δ value, and elongation at break than HTPB based elastomer. We believe that compared with HTPB, the triblock copolymer synthesized in this work can offer more advantages in various fields, such as polyurethane, adhesives, and damping.

Acknowledgements

This work was supported by the key laboratory project of department of science and technology of shannxi province of China (No. 2013SZS17-Z02).

Notes and references

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