Effect of diacylhydrazine as chain extender on microphase separation and performance of energetic polyurethane elastomer

Abstract It is low cost and feasible to improve the mechanical properties of polyurethane by using the chain extender with hydrogen bonding function to improve the degree of microphase separation. In this article, hydrazine hydrate was used to react with ethylene carbonate and propylene carbonate, respectively, to synthesize diacylhydrazines as the polyurethane chain extender with amide bonds, which were characterized by 1H nuclear magnetic resonance. Polyurethane with different contents of hard segment were prepared from poly-3,3-bis(azidomethyl)oxetane-tetrahydrofuran as the polyol and 4,4′-diphenylmethane diisocyanate as the isocyanate components. Fourier transform infrared spectroscopy showed that with the increase of the hard segment content, the proportion of hydrogen-bonded ordered carbonyl group increased to 94%, proving that diacylhydrazines could improve the degree of ordered hydrogen bonding, which led to clear microphase separation observed by field emission scanning electron microscopy and higher storage modulus of the polyurethane. Differential scanning calorimetry and dynamic mechanical analysis showed that polyurethane with higher hard segment content is likely to exhibit multiple thermal transitions caused by microphase separation. When the hard segment content was 40%, compared with polyurethane with 1,4-butanediol as the chain extender, the tensile strengths of polyurethanes with diacylhydrazines also improved by 30% and 76%, respectively. Graphical abstract As a chain extender with amide bonds, dihydrazide improves the ratio of the ordered hydrogen bond, the degree of microphase separation and mechanical properties of polyurethane elastomer.


Introduction
Polyurethane elastomer is composed of hard segment with high glass transition temperature (T g ) and soft segment with low T g (1). In the study of the multiblock polyurethane, Cooper and Tobolsky proposed that polyurethane has the bulk structure of microphase separation (2). Due to the thermodynamic incompatibility between the soft segment and the hard segment, microphase separation occurs in polyurethane. Paiksung et al. suggested that the microphase separation in polyurethane is incomplete (3), while the degree of microphase separation has an important impact on mechanical performance of polyurethane elastomer. Mixing of hard and soft segments into the homogenous structure will reduce the application range of materials in low temperature and lead to the decline of heat resistance of polyurethane.
Due to the high directivity and bonding strength, hydrogen bond is conducive to microphase separation and the formation of independent micro regions in soft segments and hard segments. In Hu and coworker's work, a novel supramolecular network with the excellent shape-memory effect was synthesized through the reaction of N,N-bis(2-hydroxylethyl)isonicotinamine and hexamethylene diisocyanate (4)(5)(6). Differential scanning calorimetry (DSC) showed that the microphase separation structure was formed through intermolecular hydrogen bonding. If the content of hydrogen bond can be increased, the microphase separation can be further promoted, and the mechanical performance of the system can be improved (7)(8)(9).
Energetic polyurethane and composite materials have been used as solid propellant binders in the past few decades (10)(11)(12). Composite solid rocket propellant is composed of metal fuel, solid oxidant, binder and other additives (13,14). The polyurethane binder can fix these solid components into ideal shape and complete structure. Its properties have an important impact on the final process properties, energy level and mechanical performance of the propellant (14)(15)(16). The introduction of the azide group into energetic polyurethane can increase the heat release and reduce the oxygen consumption and the molecular weight of decomposition products, so it has a high application potential (17). Typical compounds that introduce azides include glycidyl azide polymer (GAP) (18), 3,3-bis-azido methyl oxetane (BAMO) (16,19) and 3-azidomethyl-3-methyl oxetane (AMMO) (20).
GAP-based material can be utilized as a propellant binder (21,22), but its mechanical performance is not ideal. In the study by Zhang et al., the tensile strength of energetic polyurethane with GAP is less than 5 MPa, and the elongation at break is only 50% (18). Optimization of GAP-based polyurethane as a propellant binder is realized by modifying GAP with polycaprolactone (23), tetrahydrofuran (24), nitrocellulose filler (25) and propargyl-terminated polyether (26).
Compared with GAP, azide groups are added to the side chain of BAMO, which improves the energy and the symmetry of the system. However, BAMO homopolymer (PBAMO) is a rigid solid at room temperature, which makes it difficult to mix with other components. So it is necessary to modify PBAMO (15,27). In addition to the introduction of AMMO to modify PBAMO (28), many studies have also introduced tetrahydrofuran (THF) and other groups to modify PBAMO through copolymerization (29)(30)(31)(32). The influence of two large volume symmetrical azide groups on the lack of secondary crosslinking is eliminated, and the mechanical performance is improved. Currently, the energetic polyurethane synthesized from BAMO/THF copolymer (poly-3,3-bis(azidomethyl)oxetane-tetrahydrofuran [PBAMO-THF]) has good mechanical performance at low temperature, but it is not ideal at high temperature.
The understanding of the relationship among hydrogen bond, microphase separation and the thermomechanical performance is the key to expand the application of energetic polyurethane as a propellant binder. It is a low-cost, easy-processing and feasible way to introduce the group, which can form hydrogen bond into the chain extender. However, there are few studies on chain extenders that contribute to the formation of hydrogen bonds. In our previous work, we have employed investigations for energetic poly (urethane-urea) copolymers (33). With the increase of hard segment, the proportion of hydrogen-bonded ordered part, the degree of microphase separation and the tensile strength all increased. In this article, diacylhydrazine was synthesized and characterized by 1 H Nuclear magnetic resonance ( 1 H NMR). Then, it was introduced into energetic polyurethane elastomer as a chain extender. A series of energetic polyurthane were prepared with PBAMO-THF as the polyol and 4,4′diphenylmethane diisocyanate (MDI) as the isocyanate. Diacylhydrazine contains amide bonds with latent hydrogen bonding function. Compared with traditional chain extender 1,4-butanediol (BDO), the effects of diacylhydrazine and hard segment content on degree of hydrogen bond ordering, microphase separation, morphology and thermomechanical properties were explored by Fourier transform infrared spectroscopy (FTIR), Field emission scanning electron microscopy (FESEM), DSC and dynamic mechanical analysis (DMA). Furthermore, the effects on the mechanical performance of energetic polyurethane elastomers were studied as well.

Preparation of polyurethane elastomer
PBAMO-THF was heated at 120°C, dehydrated by vacuum agitation for 30 min and cooled to room temperature. Then, MDI was added under the protection of nitrogen. The system was stirred and heated to 70°C. After reaction for 4 h, the   quantity, 5-10 mg; the inner diameter of tube used in the test, 5 mm; the volume, 0.5 mL; scanning time, 4; temperature, 25°C.

FTIR
FTIR was used to track the curing process of polyurethane with Nicolet 6700 (Thermofisher, USA). The chain extenders were fully mixed with prepolymers and coated on KBr salt tablets. The sample was cured at 60°C in 4 h. The fully cured polyurethane elastomers were characterized by FTIR as well. Test conditions were as follows: mid-infrared 4,000-400 cm −1 , 64 scanning times, resolution 4 cm −1 .

FESEM
The fully cured polyurethane was fractured in liquid nitrogen. The fracture surface was coated with a layer of gold of 5-7 nm thick by an ion sputtering instrument. Then, the surface morphology of polyurethane elastomers was observed by Ultra 55FESEM (Zeiss, Germany). The test voltage was set to 5 kV. The magnification is 1,000× and 5,000×.

DSC
The thermal transition of polyurethane elastomer was measured by Q2000 (TA, USA) with the nonisothermal DSC method. The sample quantity was 5-10 mg each time, and the measurement was carried out in a solid sealed sample disk in the atmosphere of nitrogen. The heating range was from −80°C to 200°C with a heating rate of 10°C/min.

DMA
The storage modulus and loss factor (tan δ) of polyurethane elastomers were measured by SDTA861e (Mettler Toledo, Switzerland). The sample size was 10 × 6 × 5 mm 3 . Test conditions were as follows: tensile mode, 1 Hz frequency; 1 N dynamic force; and 100 μm displacement. The atmosphere was air. The heating range was from −80°C to 200°C with a heating speed 3°C/min.

Tensile test
Tensile strength and elongation at break of polyurethane elastomer were measured by Instron 5966 (American Instron,   To verify the reaction of hydrazine hydrate with ethylene carbonate or propylene carbonate, the synthesized products were characterized by 1 H NMR, as shown in Figure 1. The peak at a chemical shift of δ 4.80 is the solvent D 2 O peak. In the 1 H NMR spectrum of HCAHE, the peaks of δ 3.71 and δ 4.15 are hydrogen atoms from methylene groups. Affected by adjacent methylene, the proton absorption peak of methylene splits into triplet peak. In the 1 H NMR spectrum of HCAHP, the peaks of δ 1.12 and δ 1.37 are methyl hydrogen. The peak splitting indicates that there is a proton on the adjacent carbon. Due to the three possible structures of HCAHP, the broad peaks of δ 3.50-4.90 are different hydrogen atoms forming methylene and methyne. The proportion range of the three structures can be calculated according to the ratio of the area of -CH 3 proton peak at δ 1.12 and δ 1.37 since the methylene proton peak, the methyne proton peak of HCAHP and the solvent D 2 O peak overlap due to the similar δ values and multiplicities, which makes it difficult to calculate the ratio quantitatively. The proportion of structure A is 48.2-74.1%, that of structure B is 0-51.8% and that of structure C is 0-25.9% (structures A/B/C are shown in Figure 1). It can be seen that the 1 H NMR spectrum of the product is consistent with its molecular structure. The chain extenders HCAHE and HCAHP containing amide bonds were synthesized successfully.

Analysis of microphase separation
The synthesized chain extenders HCAHE and HCAHP were introduced into the energetic polyurethane elastomer with PBAMO-THF as a soft segment and MDI as diisocyanate. Figures 2 and 3 show the in situ curing FTIR spectra of HCAHE-20% and HCAHP-20% in different wavenumber ranges at 60°C. The absorption peak near 2,270 cm −1 corresponds to the stretching vibration of the unsaturated bond in -NCO and the typical peak of an NCO group. The peak area decreases significantly with the curing process, which represents the consumption of isocyanate groups. HCAHE is solid at room temperature and insoluble in prepolymer, so it needs to be dissolved by DMF to participate in reaction. There will be an obvious C═O stretching vibration absorption peak at 1,680 cm −1 , which corresponds to DMF and gradually decreases with the curing process. HCAHP is liquid at room temperature, and it can be directly mixed with prepolymer. With the reaction of -NCO and -OH, there are obvious increases in the half peak width of the N-H stretching vibration absorption peak (3,100-3,600 cm −1 ). Taking 3,300 cm −1 as the peak center, the N-H bonds at the later stage of curing reaction contains more N-H bonds with low wave number. Since the N-H bonds form more ordered hydrogen bonds and make the N-H absorption peak move to low wave number. The peak intensity from 1,640 to 1,775 cm −1 , which corresponds to C═O stretching vibration and N-H bending vibration also increases with the curing time, which reveals the formation of carbamate. The change of the C═O region to low wavenumber region reflects the process of hydrogen bond ordering.
To study the relationship among the chain extender, hard segment content and the degree of hydrogen bond ordering, the fully cured polyurethane was characterized by FTIR, as shown in Figure 4. The proportion of the carbonyl forming hydrogen bond and the degree of hydrogen bond ordering can be quantitatively calculated by studying the absorption peak of carbonyl in   polyurethane. In the study by Yilgor et al., peak fitting was used to determine the peak area of a specific C═O. Then, the area was divided by the total area to get the percentage. Among the C═O stretching vibration absorption peaks, 1,733, 1,708 and 1,696 cm −1 are considered to be free carbonyl, hydrogen-bonded disordered carbonyl and ordered carbonyl (34). The corresponding peak position of our work was defined, as shown in Figure 5. Peak fitting by origin was used to get the area of the corresponding peak, as shown in Figure 6. Substitute the peak area into Eq. 1 to obtain the percentage of free, hydrogen-bond disordered and ordered C═O groups in polyurethane.
By the quantitative analysis, the percentage of different C═O in polyurethane samples is presented in Table 2. Compared with BDO-cured polyurethanes, the diacylhydrazine-cured polyurethanes obviously contain more carbonyls with lower wave number. For samples containing the same diacylhydrazine, the percentage of hydrogen-bonded carbonyl increased with the hard segment content, while the free carbonyl decreased. The content of the ordered hydrogen bond formed by carbonyl also increased. For instance, in the hydrogenbonded ordered carbonyl absorption peak of 1,682 cm −1 , the relative area was 31.97% for HCAHE-20%, 49.46% for HCAHE-30% and 94.64% for HCAHE-40%. With the increase of the hard segment content, more HCAHP and HCAHE were introduced into the main chain of polyurethane elastomer. The amide hydrogen in the chain extender can form hydrogen bond with carbamate carbonyl or ether in soft segment, thus increasing the content of the hydrogen bond and the degree of ordered hydrogen bonding. Since HCAHE is more symmetrical than HCAHP in the molecular structure, the polyurethane with HCAHE formed more ordered hydrogen bonds than the polyurethane with HCAHP. Figure 7 shows the FTIR spectra of the fully cured HCAHE-40%. The peak at 2,100 cm −1 corresponded to the stretching vibration absorption peak of unsaturated bond of nitrogen and nitrogen in azido groups. The -N 3 peak with highest intensity shown in Figure 7 proved that the series of polyurethane in our study was energetic polyurethane, which could meet the requirements of high-energy propellant. Due to the easy and environmentally friendly processing, high energy based on energetic -N 3 groups and contribution to the low signature, polyurethanes studied in this study have potential application as binders in solid rocket propellants.
Observation of the surface morphology of the fully cured polyurethane is helpful to judge the microphase separation directly. Morphology is also closely related to the final properties of the product (35). In the study by Saha et al., the morphological analysis of drug-nanoclay for controlled drug delivery was well studied (36). Figure 8 shows the FESEM images of polyurethane containing HCAHE/HCAHP. It can be clearly observed that with the increase of the hard segment content, the number of dispersed phase particles from microphase separation increased rapidly. The hard segments tend to be separated from the soft matrix instead of mixing with soft segments. The change of morphology and structure will obviously affect the mechanical properties and dynamic mechanical performance of polyurethane elastomer. With the increase of hard segment particles in the dispersed phase, it can be predicted that the tensile strength of the material will increase significantly, while elongation at break and other properties will depend on the degree of microphase separation.

Thermal-mechanical properties
The thermal transition of the polyurethane samples was investigated by DSC, as shown in Figure 9. For all polyurethanes, the glass transition of the soft segment can be clearly observed, and the corresponding T g was presented in Table 1. Pure hard segment was the product of the corresponding chain extender and MDI. In the polyurethane elastomer containing hydrogen bonding chain extender, the thermal transition of dissociation can be observed in the short-range and long-range ordered hard segments. For example, in the DSC curve of HCAHE-40%, three kinds of thermal transitions can be observed: glass transition of soft segment near −50°C, dissociation of short-range ordered hard segment near 60°C and dissociation of long-range ordered hard segment near 110°C. In the DSC curve of BDO-40%, there are two thermal transitions, which are the glass transition of the soft segment near −50°C and the dissociation of the corresponding short-range ordered hard segment near 60°C, but there is no obvious thermal transition of the long-range ordered hard segment dissociation. It is observed by comparison that when the content of the hard segment is 40%, HCAHE samples have more long-range ordered hard segment structure than BDO samples. For polyurethane elastomers with the same kind of chain extender containing hydrogen bond, with the increase of the hard segment content (i.e., the increase of the amount of the chain extender), it is easier to observe the thermal transformation near 110°C, corresponding to the dissociation of long-range ordered hard segments. Due to the amide bond in HCAHE/ HCAHP, the introduction of hydrogen bond into polyurethane elastomer is conducive to the formation of ordered hydrogen bond.
The storage modulus of polyurethane elastomer was monitored by DMA, as shown in Figure 10. The storage modulus at −50°C and 25°C was presented in Table 3. The glass transitions of the soft and hard segment were observed in HCAHP-30% and HCAHP-40% at around −36°C and 140°C. However, thermal transition of hard segment was unavailable in HCAHP-20%, which reflected inadequate microphase separation. The storage modulus after glass transition of soft segment reflects the physical crosslinking strength of polyurethane. Storage modulus of HCAHE/HCAHP-cured polyurethane at 25°C was generally larger than that of BDO-cured polyurethane. The modulus after T g of soft segment increased with the hard segment content. Since the connectivity of the hard segment domain increased, a continuous hard micro domain structure formed, which contained both hydrogen-bonded disordered part and ordered part. With the increase of the hard segment content, the hydrogen bonds formed physical crosslinks with higher intensity to reinforce the structure of polyurethane.
As presented in Table 1, there is no significant difference in the T g of the soft segment of all the polyurethane elastomers, which are around −36°C. The thermal transitions of polyurethane elastomers were compared by tan δ curves, as shown in Figure 11. For HCAHE-40% and HCAHP-40%, there is a steep peak that represents glass transition in the hard segment near 140°C. For BDO-cured polyurethane, there is only a very gentle peak in the range of 100-150°C, which means the glass transition of the hard segment is not clear. For polyurethane containing HCAHE/HCAHP, with the increase of the hard segment content, the maximum tanδ is getting higher at T g of the hard segment. Therefore, the introduction of HCAHP and HCAHE is conducive to microphase separation and the formation of independent soft and hard segments, which leads to clear glass transition of hard segments.
T g shift was closely related to the degree of microphase seperation. The closer the T g value of the hard segment is to that of the pure hard polymer, the fewer the hard segment is dissolved in the soft segment. Taking HCAHE-cured polyurethane as an example, the T g values of the hard segment of HCAHE-30% and HCAHE-40% are 136°C and 145°C, respectively, while the T g of the pure hard polymer (HCAHE + MDI) is 146°C. With the increase of the hard segment content, hard segment tends to self-associate to form hard domain rather than mixing in the soft segment, which contributes to microphase separation and closer T g of hard segment. Finally, the mechanical performance of polyurethane was monitored by tensile test, as listed in Table 3. Among the three chain extender systems, the elongation at break of polyurethane elastomer with the same chain extender reaches the maximum value at 30% hard segment content. When the hard segment content is 40%, the elasticity is greatly reduced. The elasticity of polyurethane is mainly provided by the soft segment, which is rarely affected by the percentage of the hard segment. In contrast, the tensile strength of the polyurethane elastomer increased with the hard segment content. For instance, the tensile strengths of HCAHE-20%, -30%, -40% were 4.7, 11.4 and 19.4 MPa, respectively. For polyurethane elastomer with 30% and 40% hard segment content, the tensile strength of the polyurethane elastomer with HCAHE and HCAHP is higher than that of polyurethane with the BDO chain extender, since more HCAHE/HCAHP with amide bond is introduced into the molecular structure of polyurethane. The intermolecular interaction was enhanced, and more  ordered hydrogen bonds were formed, which led to more physical crosslinks and higher tensile strength.
The mechanical performance is consistent with the previous test results. The introduction of N-H bond into the main chain of polyurethane is conducive for improving the degree of hydrogen bonding in the material, further promoting the microphase separation to form a hard segment crystal area and improving its mechanical performance. Because of the better symmetry of HCAHE than HCAHP, the tensile strength of HCAHE-cured elastomers is larger than that of HCAHPcured elastomers.
For the application of polyurethane in binders, the mechanical property of the binder plays a decisive role compared to that of the propellants since all the components in the composite propellant are dispersed in the binder. The propellant should have good mechanical properties at low temperature and good thermal stability at high temperature (10,16,21). Compared with BDO-cured polyurethane, HCAHE/HCAHPcured polyurethane has higher storage modulus at both low and high temperatures. The polyurethane cured with HCAHE/HCAHP can maintain its stability at 170°C without decomposition, and the tensile strength also improved. All these factors make HCAHE/HCAHP have a certain application potential in the field of propellant binders.

Conclusion
The studies on microphase separation and a new chain extender for energetic polyurethane are of great significance to scientific research and engineering application. In this article, chain extenders HCAHE/HCAHP with the diacylhydrazine group were synthesized, characterized and introduced into energetic polyurethane elastomer. FTIR showed that with the increase of the hard segment content, the proportion of the hydrogen-bonded ordered carbonyl group increased, proving that HCAHE/ HCAHP could improve the degree of ordered hydrogen bonding. Thermal transition T g and storage modulus were further studied by DSC and DMA. With the increase of the HCAHE/HCAHP content, hard segment tended to self-associate to form independent hard domain from the soft segment, which contributed to microphase separation and clear glass transition of the hard segment in polyurethane. Compared with the traditional chain extender BDO, the tensile strength of polyurethane with HCAHE was also significantly improved.
Both HCAHE and HCAHP are diacylhydrazine with amide hydrogen, which can form hydrogen bond with carbamate carbonyl, increase the percentage of ordered hydrogen bond and lead to microphase separation and physical crosslinks, thus improving the mechanical performance of energetic polyurethane. HCAHE/ HCAHP-cured polyurethane was proved to be energetic polyurethane with improved mechanical properties at high and low temperatures. By optimization of the structure of diacylhydrazine with amide bonds, HCAHE and HCAHP are expected to be potential chain extenders for polyurethane as propellant binders.