Preparation of energetic polyurethane binders with enhanced properties by nonmigratory reactive monocyclic plasticizers

Journal Pre-proofs Preparation of energetic polyurethane binders with enhanced properties by nonmigratory reactive monocyclic plasticizers Mingyang Ma, Younghwan Kwon PII: DOI: Reference:

S0014-3057(19)31858-0 https://doi.org/10.1016/j.eurpolymj.2019.109414 EPJ 109414

To appear in:

European Polymer Journal

Received Date: Revised Date: Accepted Date:

11 September 2019 29 November 2019 9 December 2019

Please cite this article as: Ma, M., Kwon, Y., Preparation of energetic polyurethane binders with enhanced properties by nonmigratory reactive monocyclic plasticizers, European Polymer Journal (2019), doi: https:// doi.org/10.1016/j.eurpolymj.2019.109414

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Preparation of energetic polyurethane binders with enhanced properties by nonmigratory reactive monocyclic plasticizers

Mingyang Ma a and Younghwan Kwon b,*

aJiangxi

Province Key Laboratory of Polymer Micro/Nano Manufacturing and Devices, East China University of Technology, Nanchang 330013, China

bDepartment

of Chemical Engineering, Daegu University, Gyeongsan, Gyeongbuk 712-714, Korea

*Corresponding author, email: [email protected]

Abstract: A chemical incorporation of alkyne-bearing reactive monocyclic plasticizers (RMCPs) into glycidyl azido polymer (GAP) was performed to mitigate the instability of the cured polymer binders. Cyclopropane and cyclobutane, acted as an easily accessible energy donor derived from ring strain were embedded into the RMCPs which were synthesized via a straightforward esterification and characterized successfully. Those RMCPs were thermodynamically miscible with the GAP prepolymer up to 0.5/0.5 w/w. The rheological profile of the RMCPs/GAP prepolymer mixture was relatively stable without appreciable increase at room temperature for 300 min. The mild and distinct catalyst-free click reactivities of RMCPs toward the GAP prepolymer were feasible to customize appropriate processibility for specific formulation. The experimental determination for the click reactivity was in good agreement with the theoretical prediction of frontier molecular orbital. The chemically incorporation of RMCPs into the GAP-based polyurethane (PU) binders imparted an enhanced tensile properties and impact insensitivity owing to the transformation from azide to triazole group. The positive heat of formation of the resulting RMCP/GAP-based PU binders was obtained indicating a robust contribution from ring strain energy.

Keywords: Reactive plasticizer · Ring strain energy · Catalyst-free click reaction ·Glycidyl azido polymer · Polyurethane

1. Introduction Polyurethanes (PUs), having a wide range of applications in foams, adhesives, elastomers, coatings and so on, are commonly used materials in everyday life [1–4]. The PUs elastomers find versatile applications, such as shoe soles, binders, industrial tyres, etc., by tailoring the structure and content of soft and hard segments [5]. The elastomeric PU binder used extensively in plastic-bonded explosives (PBXs) and solid propellant is capable of lowering the sensitivity to impact, shock and friction to prevent accidental detonation. PBXs and solid propellant requiring to be safely manipulated and formulated, consist of the sensitive materials which may cause the potential hazardous effect on the safety by the labile factor. Therefore, the high requirement for the fabrication, usage, transport and storage of PBXs and solid propellant remains greatly desirable for the polymer binders to mitigate dangers to the safety and environment. Glycidyl azido polymer (GAP) applicable to PBXs and solid propellant is an ideal polymer binder with moderate sensitivity, low smoke, high heat of formation and high density [6–8]. However, GAP-based PUs exposed low elasticity, poor tensile properties and high sensitivity compared to hydroxyl terminated polybutadiene, which is a widely used inert polymer binder [9]. The facile strategies, such as copolymerizing with tetrahydrofuran and ethylene oxide, for improving the mechanical characteristics of GAPbased PUs, impaired inevitably the final energetic performances [10,11]. In recent years, alkyne-terminated crosslinkers are paid more attention in pursuit of isocyanate-free route to GAP-based binders [12–15]. The resulting crosslinked networks with poor elasticity devoid of hydrogen bonding within urethane linkages are limited as the adequate candidates for the ideal workhorse binders. In most cases, inclusion of energetic plasticizers within the formulation of GAP-based PBXs and solid propellant is usually in need to achieve a greater processing ease, improve the mechanical characteristics of base materials to withstand severe stimuli, and provide an enhanced energetic performance [16–19]. On the other hand, the migration of energetic plasticizers may deteriorate the mechanical properties of PBX and expose potential dangers to the circumstances [19]. The safety and environmental concerns stimulated the advent of alkyne-bearing reactive energetic plasticizers (REPs) for use in GAP-based PU binders, migration of which could approach zero via the in situ catalyst-free azide-alkyne 1,3-DPCA reaction [20–26]. Energetic ingredients offering an outstanding performance, but simultaneously subject to safety constraints, were both ascribed to the conventional explosophore groups,

since most of which were integrated with -ONO2, -NO2, or -N3 groups. The disadvantages of the explosophore groups such as high impact sensitivity, instability at high temperatures and easy oxidation in air have been apparent [27]. Ring strain energy (RSE) derived from cyclic compounds is prone to become a green, alternative, safe and reliable energy source. Notably, the efficacy of RSE is deemed to provide a relatively inactive energy bearing some immunity to impact and shock because the release of which is commonly initiated by ignition [28,29]. High-strained carbocycles mounting onto the skeleton of polymers were of great interest to the development of energetic binders due partly to the high RSE and a low oxidative state [30–32]. Regrettably, such polymers which had conspicuous drawbacks of mechanical instability and low elasticity, were not widely used in modern energetic field. There has been a resurgence of interest in shifting the high-strained carbocycles into being the building blocks for high performance explosives [33,34]. Selection of cycloalkane with an appropriate RSE aiming at improving energetic performance is of great significance because some molecules with an extremely high RSE (e.g., benzvalene) fall short of safety expectation. In this work, cyclopropane and cyclobutane with a moderately high RSE were integrated into alkyne-bearing reactive monocyclic plasticizers (RMCPs) which were synthesized by one-pot esterification of monocyclic carboxylic acids and alkynols (propargyl alcohol and 3-butynyl alcohol). In a previous study, the facile control of Cu(I)-free azide-alkyne 1,3-DPCA reactivity by adjusting methylene spacers (n) between the electron-withdrawing group (EWG) and alkyne group created opportunities for modulating the processing condition [23]. In this paper, the theoretical prediction on frontier molecular orbital (FMO) of the propargyl and 3-butynyl species was correlated with the experimentally determined activation energy (Ea) obtained from differential scanning calorimetry. Mechanical properties, migration resistance and impact insensitivity were jointly promoted by incorporating the RMCPs into the GAP-based PU binders. Additionally, the energetic performance was still feasible to be enhanced with an elevated RSE.

2. Experimental section All materials for the synthesis of RMCPs and PU binders are described in the Supporting Information.

2.1. Synthesis of RMCPs Four types of RMCPs, prop-2-yn-1-yl cyclopropanecarboxylate (PCPC), but-3-yn-1-yl cyclopropanecarboxylate (BCPC), prop-2-yn-1-yl cyclobutanecarboxylate (PCBC) and but-3yn-1-yl cyclobutanecarboxylate (BCBC) were synthesized by a one-pot esterification reaction between monocyclic carboxylic acids and alkynols (Scheme 1). The synthetic procedure of RMCPs is available in Supporting Information. Figure S1 and S2 present the 1H and

13C

NMR spectra of RMCPs, respectively. Figure S3–S6 present the mass spectra of RMCPs.

Scheme 1. Synthetic route to four types of RMCPs. 2.2. Preparation of GAP-based PU binders with RMCPs The preparation of the RMCPs/GAP-based PU was available in Supporting Information. The synthetic route to the RMCPs/GAP-based PU binders was provided in Scheme 2.

Scheme 2. Preparation of the RMCPs/GAP-based PU binders.

2.3. Characterization methods The characterization of the RMCPs and PU binders was performed using various techniques, such as 1H and 13C NMR spectroscopy, mass spectroscopy, differential scanning calorimeter (DSC), rheometer, thermogravimetric analyzer (TGA), texture analyzer, elemental analysis, oxygen bomb calorimeter and impact tester. All conditions of measurements are detailed in the Supporting Information.

3. Results and Discussion 3.1. Predicted RSE of RMCPs The explicit expression of RSE of a cyclic molecule is an excess energy reflected from an increased heat of combustion (c Ho) in comparison with its strain-free isomer [28]. As the release of the RSE offers a driving force for numerous reactions, a good deal of attention has been focused on determining the RSE experimentally and computationally [35]. Cyclopropane and cyclobutane are the most common small-ring compounds with the high RSE. The intrinsic RSE of intact cyclopropane and cyclobutane is available anywhere in both experimental and computational determinations [28,36]. The obtained RSE values of intact cyclic compounds may not be equivalent to those of the substituted ones, to some extent affected by the neighboring substituents which are electron-donating or electron-withdrawing groups [37]. Those cyclic building blocks destined for specific utility are usually embedded with the functional groups to work as a precursor for the subsequent step. However, in most cases the RSE of those substituted cyclic compounds can not be readily determined in an experimental protocol. Calculation of the RSE allowing a reasonable prediction via homodesmotic reaction schemes is of great interest not only to explain the reaction mechanisms but to screen the desired RSE for use as an energetic resource. The homodesmotic schemes designed for computing the RSE of RMCPs were constructed at the B3LYP/6-31G* level of theory maintaining the identical atoms and bond types involved in the two terms of square brackets [37,38]. A typical homodesmotic scheme for prediction of cyclopropane-based RMCPs is shown in Eq. (1):

The use of C2H6 can balance the number of methyl fragment which emerged after the cleavage of carbocycle. According to the homodesmotic schemes for cyclopropane and cyclobutane-based RMCPs (see Eq. S1-S3 in Supporting Information), the computed RSEs for the pristine rings and RMCPs are tabulated in Table 1. The computed RSE of cyclopropane and cyclobutane was 121.2 and 110.1 kJ mol-1, respectively, which matched well with the reported ones [28,37]. As the carbocycles were adjoined to an electron-withdrawing group (i.e., carbonyl), the RSE of RMCPs decreased to approximately 100 kJ mol-1 for both types of RMCPs whereas that of cyclopropane-based RMCPs was lower by 20 kJ mol-1 than cyclopropane. Compared to cyclobutane, the higher intrinsic RSE of cyclopropane was more decreased because energy of the more strained ring was highly stabilized by the neighboring carbonyl group [37]. Based on this explanation, it is conceivable that the stabilization magnitude is directly proportional to the intrinsic RSE. In terms of the decreased magnitude of RSE, there were no remarkable differences between n=1 and n=2, indicating that the neighboring substituents attached to carbocycles exerted a significant impact on stabilizing the rings. Table 1. Computed RSE of cyclopropane and cyclobutane with and without substituents. RMCPs

RSE (kJ mol-1) Pristine ring n=1

n=2

121.2

99.8

99.4

110.1

99.8

99.7

3.2. Plasticizing performance of RMCPs As the Tg can readily reflect a miscibility of the plasticizer/polymer binary mixture over a certain composition range [39–41], the Tg of RMCP/GAP prepolymer mixture with three compositions was determined by DSC (see Figure S7 in Supporting Information). Figure S8 shows one Tg in each composition during the entire measurement, whereas Tg appeared in the middle region between the Tgs of the respective compounds. The increase in weight fraction of the RMCPs resulted in a decrease in Tgs of the binary mixtures indicating a mutual miscibility was achieved between the RMCPs and GAP prepolymer. On the other hand, the

magnitude of shifted Tg of binary mixtures was correlated with the weight fraction as well as the structures of RMCPs. A quantitative protocol for investigating the structural effects of RMCPs on decreasing Tg of the GAP prepolymer was utilized, as shown in Eq. (S4) (See Supporting Information) which was applicable to polymer-plasticizer binary mixtures [39,40]. I is interpreted as the interaction parameter. In general, a pronounced ability of plasticizer to decrease the Tg of polymer had a lower I value, indicating that such a mixture was more miscible [39,40]. The I value was calculated by plotting Tg against w2 with a standard deviation. As shown in Figure 1, the I values of BCPC (n=2, –48.1±0.1 K) and BCBC (n=2, –49.6±0.2 K) were lower than those of PCPC (n=1, –45.3±0.1 K) and PCBC (n=1, –41.9±0.1 K) suggesting the higher molecular weight of homologue imparted the GAP prepolymer a better flexibility due to the increased free volume [42,43]. The I values of the commonly used energetic plasticizers, such as the eutectic mixture of bis(2,2dinitropropyl)formal (BDNPF)/bis(2,2-dintropropyl)acetal (BDNPA), butanetriol trinitrate (BTTN) and diethylene glycol dinitrate (DEGDN) were also compared with the RMCPs as presented in Figure 1. Three kinds of conventional energetic plasticizers showed a disadvantage in the miscibility with GAP prepolymer in terms of their relatively higher I values. The higher molecular chain flexibility (i.e., lower Tg) of RMCPs provided a more prominent plasticization effect on the GAP prepolymer.

Figure 1. Comparison of RMCPs/GAP prepolymer and energetic plasticizers/GAP

prepolymer with respect to I value and viscosity. The good processibility for an energetic polyurethane binder ensures a ready processing of formulated components. The 1,3-DPCA reaction between GAP prepolymer and RMCPs probably occurring in situ during the processing is of some concern which may cause a continuous increase in viscosity. The viscosity of the GAP prepolymer comprised of 20, 35 and 50 wt. % of RMCPs and 50 wt. % of energetic plasticizers was measured isothermally at 30 oC for 300 min in purpose to simulate preliminarily the fabrication process for the RMCP/GAP-based PU binders (see Figure S9 of Supporting Information). The viscosity of GAP prepolymer was reduced dramatically from 6015 to 341-391 cP with the addition of 20 wt. % of RMCPs while in terms of 35 and 50 wt. % of RMCPs, it was in the range of 126155 cP and 59-70 cP, respectively. On increasing the mixing time up to 300 min, the viscosity of all mixtures increased marginally to 370-404 cP (20 wt. %), 156-171 cP (35 wt. %) and 82-92 cP (50 wt. %), respectively. The grafting level of low content of RMCPs (e.g., 20 wt. %) onto GAP prepolymer was lower than that of higher content of RMCPs which had a crossover point for the RMCPs between n=1 and n=2. As expected, the catalystfree azide-alkyne 1,3-DPCA reactivity of RMCPs and GAP prepolymer at room temperature was relatively low which had a minor impact on the processibility of the binder system. BTTN and DEGDN reduced the viscosity of the GAP prepolymer to 16-39 cP. Compared to energetic plasticizers, RMCPs exhibited a competitive ability in reducing the viscosity of the GAP prepolymer, especially a much lower value than BDNPF/A (1441 cP at 30 oC). 3.3. Catalyst-free click reactivity of RMCPs with GAP prepolymer Catalyst-free click reaction was preferred considering mainly the effect of reactivity on the processibility and safety aspects [12,13,44]. A fast and selective reactivity is favorable for most reactions with a maximum yield. The Cu(I)-catalyzed click reaction can fulfill the expectations of the high reactivity and predominant 1,4-regioisomers with an ideal yield (>90%) [45]. However, those advantages are not applicable to the fabrication process of GAP-based PU formulations, where a specific pot life is necessary because a higher reactivity causing a continuous increase in viscosity may pose a processing problem [12]. In addition, the regioselectivity of click reaction is not dispensable for RMCPs linking to the GAP-based PU binders. Moreover, the sensitive explosives formed between Cu(I) and ammonium dinitramide, an oxidizer for a solid propellant, may give rise to an accident [46]. As the

formulations of GAP-based PU binders comprised mainly GAP prepolymer and RMCPs (ca. 95 wt. %), the other components, such as curing agent and polymerization catalyst were not involved in the kinetic study. The preliminary kinetic study on the catalyst-free click reaction was conducted to compare the relative reactivity between RMCPs with n=1 and n=2 in addition to confirming a complete integration of the RMCPs into GAP matrix within a curing period. The 1H NMR spectra of the products formed between the cyclopropane-based RMCPs and GAP prepolymer were shown in Figure 2, and the cyclobutane-based RMCPs series were presented in the Supporting Information (Figure S10). The peak (1) originated from the methine proton of cyclopropane at 1.67 ppm was standardized internally to trace the extent of the 1,3-DPCA reaction. The signals of the proton resonance of peak (2) and (4) ascribed to the methylenes of propargyl and 3-butynyl moieties decreased continuously after a period of time. The resulting peaks destined for the regioisomers appeared at 5.23 (3’) and 5.10 ppm (3) for propargyl species, and at 4.50 (5’) and 4.22 ppm (5) for 3-butynyl species. Similar to the studies on thermally induced click reaction [23,47], 1,4- and 1,5triazole regioisomers were generated simultaneously because the energy gap of the highest occupied molecular orbital (HOMO) of the dipole (HOMOdipole) with the lowest unoccupied molecular orbital (LUMO) of the dipolarophile (LUMOdipolarophile) is nearly equivalent to that of HOMO of the dipolarophile (HOMOdipolarophile) with the LUMO of the dipole (LUMOdipole). Figure S11 shows the conversion of click reaction calculated from a variation of the integral areas of protons based on the peak (2) and (4). The propargyl species (n=1) exhibited a higher reactivity than the 3-butynyl species (n=2) independent of the cyclic moieties. As reported previously, the distinct reactivity could be explained by the inductive effect which imparted the dipolarophile (i.e., alkynyl species) different LUMO energy levels due to the neighboring EWG [12,48]. Compared with 3-butynyl species with two methylene spacers between the alkynyl and EWG, the propargyl species with one methylene spacer may have a low-lying LUMO energy level resulting in a higher reactivity. Importantly, both types of RMCPs could complete the 1,3-DPCA reaction compatible with the curing time of the PU reaction (5-7 days). Those observations are also in good agreement with a previous study [23].

Figure 2. Comparison of 1H NMR spectra of the catalyst-free click reaction between (a) PCPC (n=1) and (b) BCPC (n=2) toward the GAP prepolymer performed at 60 oC. The relative reactivity is of great interest from a quantitative point of view. Based on the salient feature of Huisgen type click reaction which is extremely exothermic, the activation energy (Ea) of Huisgen type click reaction was calculated from Eq. (S5) (see Supporting Information) measured by DSC [49]. A set of dynamic DSC curves with respect to GAP prepolymer with PCBC (n=1) and BCBC (n=2) at 5 heating rates was shown in

Figure 4. The cyclopropane-based RMCPs series are presented in Supporting Information (Figure S12). The degree to which curves shifted with increasing the heating rate is a reactivity comparison of the methylene spacers. The increased heating rate resulted in the Tmax shifting to a higher exothermic region. As compared with a same heating rate, PCBC (n=1) exhibited a lower Tmax than BCBC (n=2), suggesting a higher 1,3-DPCA reactivity. Eq. (S5) gives the Ea values by plotting ln(β) against T −1 max as shown in Figure S13 of Supporting Information. The Ea values of propargyl-based RMCPs were 77.9 kJ mol-1 and 78.2 kJ mol-1, which were approximately 7 kJ mol-1 lower than the corresponding 3-butynylbased RMCPs. The lower Ea of Huisgen type click reaction with respect to propargyl species renders it a higher reactivity than 3-butynyl species.

Figure 3. Dynamic DSC curves for (a) PCBC/GAP and (b) BCBC/GAP systems at an elevated heating rate from 2 to 20 oC min-1 (exothermal direction: up). The Tmax in Eq. (S3) was determined from the exotherm temperature maxima. The theoretical explanation underlaying the above correlation between n and reactivity elicited great interest with the advent of the expected results of both experiments. The focus of interest shifted to FMO energy levels giving an explicit description, whether a

low-lying LUMO of RMCP exerted a major impact on enhancing the click reactivity. The FMO energies of the azido model compound (i.e., dipole), 1-azido-3-methoxy-2methylpropane which has the same structure of the repeating unit of GAP was predicted in a previous study [50]. The prediction of the FMO energies of RMCPs (i.e., dipolarophile) was performed at B3LYP/6-31G* level of theory [38]. As detected in the 1,3-DPCA reaction traced by 1H NMR spectra, 1,4- and 1,5-triazole regioisomers were produced due to the nearly equivalent energy gap in a two-way interaction of HOMOdipole – LUMOdipolarophile and HOMOdipolarophile – LUMOdipole. As a result, the energy gaps of HOMOdipole – LUMOdipolarophile and HOMOdipolarophile – LUMOdipole signified as E1, and E2, respectively, were calculated and listed in Table 2. A typical schematic diagram of E1 and E2 for PCBC (n=1) and BCBC (n=2) was shown in Figure 4a. The difference in E2 (0.010 eV) appeared to be relatively trivial compared to the difference in E1 (0.071 eV). Likewise, in the series of PCPC (n=1) and BCPC (n=2), the difference in E1 (0.064 eV) became appreciable compared to the difference in E2 (0 eV). As expected, the low-lying LUMO of RMCP based upon the significant difference in E1 was the predominant factor influencing the 1,3-DPCA reactivity. Importantly, the E1 of propargyl-based RMCPs was approximately 0.06-0.07 eV lower than that of 3-butynyl-based RMCPs whereas the E2 of both type of RMCPs was similar. This observation verified that a low-lying LUMO of RMCP having a lower Ea for the Huisgen 1,3DPCA reaction was in favor of exciting the electron jump of HOMOdipole pairing with LUMOdipolarophile.

Figure 4. (a) Schematic diagram of E1 and E2 for PCBC (n=1) and BCBC (n=2). Compared

to BCBC, a low-lying LUMO of PCBC indicated a higher reactivity, as manifested by a narrower E1. The blue and yellow surfaces are the positive and negative phases of FMOs, respectively, along with an isovalue of 0.02. (b) Relation between Ea and the degree of deshielding of the alkynyl protons. The lower Eas of propargyl species were correlated with the more deshielded alkynyl protons compared with those of 3-butynyl species, suggesting the more electron-deficient alkynes affected by the more neighboring EWG. As shown in Figure 4b, the difference in the 1,3-DPCA reactivity (i.e., different n) with the quantitative Ea values was correlated pertinently with the chemical shift of the alkynyl proton of RMCPs which was interpreted as the magnitude of the deshielding. The signals of alkynyl proton in propargyl-based RMCPs located at approximately 2.50 ppm which was more downfield than those in 3-butynyl-based RMCPs at around 2.00 ppm. The strong deshielding effect of alkynyl proton in propargyl-based RMCPs facilitated the Huisgen 1,3-DPCA reaction with a lower Ea due to the decreased electron density of alkyne by the vicinal EWG. Interestingly, an unexpected result was observed when the sum of the energy gap difference was compared with the difference in Ea between n=2 and n=1. As listed in Table 2, E1, n=1, E1, n=2, E2, n=1 and E2, n=2, were denoted as the E1 and E2 of RMCPs with n=1 and n=2, respectively. E3, representing a sum of the energy gap difference between the 3butynyl and propargyl species, was predicted to be 0.064 and 0.081 eV close to the measured E4, which was the difference in Ea between the n=2 and n=1 with a value of 0.070 eV (~0.6 kJ mol-1 deviation with E3) for cyclopropane-based RMCPs and 0.073 eV (~0.8 kJ mol-1 deviation with E3) for cyclobutane-based RMCPs. Notably, the experimental approaches can be well-matched with the theoretical prediction. Table 2. Comparison of the predicted energy gap difference for Huisgen type click reaction. Dipolarophile

Energy gap (eV) E1

PCPC, n=1 6.933 BCPC, n=2 6.997 PCBC, n=1 6.795 BCBC, n=2 6.866 a E = (E 3 1, n=2 – E1, n=1) + (E2, n=2 – E2, n=1). bE = 4

(Ea, n=2 – Ea, n=1).

E2 6.443 6.443 6.512 6.522

E3a

E4b

0.064

0.070

0.081

0.073

3.4. Properties of the RMCP/GAP-based PU binders The binder which accounted for a small proportion (10-15 wt.%) of the PBXs formulations, played a significant role in enhancing the mechanical and safety performance by wetting the energetic particles to offer a void-free matrix. Moreover, the flowable formulation with improved processibility was allowed to cast into the large size and irregular shape of cases. There are two approaches available for the preparation of energetic PUs, plasticized GAP/curing or GAP-plasticizer/curing. In terms of the plasticized GAP/curing approach, the pre-grafted GAP prepolymer was found to be an unflowable and viscous liquid at room temperature which will be a problematic issue during processing. In contrast, the post-grafted approach improves processability due to the presence of the unreacted plasticizers. The RMCPs content in the formulation of RMCP/GAP-based PU binders was controlled with the molar ratio of [alkyne]/[azide] = 0.1/1 and 0.3/1. According to a synchronous linking/curing protocol adopted, the RMCP/GAP-based PU binders were obtained typically within 5-7 days at 50-60 oC via Huisgen 1,3-DPCA reaction and polyurethane reaction [44]. The completion of PU reaction was determined by a FT-IR spectrometer as shown in Figure S14 (see Supporting Information). The characteristic –OH (~3,420 cm-1) and –NCO (2,274 cm-1) peaks are virtually absent, implying the PU reaction is complete in all cases. The end of mix (EoM) and end of cast (EoC) viscosities determine the pot life of PBXs and solid propellant. The EoM and EoC viscosities of the formulated mixture including RMCPs, GAP prepolymer, isophorone diisocyanate (IPDI), 3,5-dinitro salicylic acid (DNSA) solution and triphenyl bismuth (TPB) solution were monitored at the interval of 20 min for 1 hr which complied with the mixing time of the RMCPs/GAP/IPDI/TPB/DNSA formulations. As shown in Table 3, the EoM viscosity of mixture was 513-539 cP and 138-163 cP at the molar ratio of [alkyne]/[azide] = 0.1/1 and 0.3/1, respectively. The viscosity suddenly increased at the first interval of 20 min due to the loss of benzene which evaporated under vacuum condition, followed by a linearly increasing trend (see Figure S15a Supporting Information). The similar phenomenon was observed with respect to the molar ratio of [alkyne]/[azide] = 0.3/1 as shown in Figure S15b (see Supporting Information). Based on the theoretical molar ratio of added RMCPs, the weight fraction of RMCPs will be ranked as BCBC > PCBC = BCPC > PCPC. It is plausible that more addition of plasticizers resulted in a lower EoM viscosity. The EoC viscosity of mixture was 578-605 cP and 190-219 cP at the molar ratio of [alkyne]/[azide] = 0.1/1 and 0.3/1, respectively. As distinct from the viscosity variation of the RMCPs/GAP mixtures, the formulated mixture comprising the curing agent

and catalysts exhibited an increased viscosity at 1 hr mixing interval indicating the isocyanates were undergoing the PU reaction. The marginal increase in EoC viscosity suggested the low PU reaction rate at room temperature and may still have potential to allow for ease of processing when solid fuels are added. Table 3. End of mix (EoM) and end of cast (EoC) viscosity of the formulated mixtures. Formulation [C≡C]/[N3] = 0.1/1 PCPC/GAP/IPDI/TPB/DNSA BCPC/GAP/IPDI/TPB/DNSA PCBC/GAP/IPDI/TPB/DNSA BCBC/GAP/IPDI/TPB/DNSA [C≡C]/[N3] = 0.3/1 PCPC/GAP/IPDI/TPB/DNSA BCPC/GAP/IPDI/TPB/DNSA PCBC/GAP/IPDI/TPB/DNSA BCBC/GAP/IPDI/TPB/DNSA

EoM

Viscosity 20 min 40 min

EoC

539 529 525 513

579 567 562 555

592 581 577 568

605 595 590 578

163 152 150 138

193 185 180 171

205 192 190 180

219 205 202 190

The thermal properties, such as the 5 wt.% weight loss temperature (Td,5wt.%) and thermal degradation temperature of maximum weight loss (Td,max) were determined by TGA in a N2 atmosphere. Compared to the Td,max at 250 oC of the GAP prepolymer [23], the RMCPs having a Td,max ranged from 165 to 178 oC suggested that their relatively lower thermal stabilities may trigger an early thermal degradation below 185 oC if the unreacted RMCP existed. As shown in Figure S16 (see Supporting Information), the RMCPs were reacted completely with GAP-based PU binders because a concomitant decomposition was observed at 220–228 oC without early weight loss of the unbonded RMCPs. It is conceivable that the weight loss of the RMCPs is likely due to the evaporation rather than decomposition. On the other hand, all RMCP/GAP-based PU binders exhibited a congruent decomposition temperature throughout the composition due to the thermal degradation of azide groups [7,51]. As reported, the commercially available energetic plasticizers, such as trimethylolethane

trinitrate

(TMETN),

butanetriol

trinitrate

(BTTN),

bis(2,2-

dintropropyl)acetal (BDNPA), bis(2,2-dinitropropyl)formal (BDNPF) and triethylene glycol dinitrate (TEGDN) had the Td,max ranged from 123 to 260 oC [18,51–56]. Those energetic plasticizers, as distinct from the RMCPs, generally evaporated from the matrix while the Td,max of energetic plasticizers was relatively lower. Moreover, energetic plasticizers having a

higher Td,max usually decomposed concomitantly with the matrix. Mechanical characteristics of the workhorse binders were desired to have sufficient tensile strength and elasticity to protect the bulk materials against the internal and external forces. Figure 5a shows the typical stress-strain curves for the control GAP-based PU and RMCP/GAP-based PUs. The tensile strength, elongation at break and tensile modulus were compared in Figure S17 (see Supporting Information). The GAP-based PUs incorporating 10 mol. % of RMCP exhibited a trivial increase in tensile strength ranging from 0.30 to 0.36 MPa compared to the control GAP-based PU (0.24 MPa), whereas the elongation at break was highly improved from 380% to a range of 624–770 %. The tensile modulus of 0.07 MPa for the control GAP-based PU was maintained at a similar level ranging from 0.04 to 0.05 MPa after incorporation of 10 mol. % of RMCP. Those results indicate that a low feeding of RMCP into the GAP-based PU binders contributes minorly to the enhancement of mechanical properties except elongation at break. Specifically, the resulting triazole group as well as monocyclics which are sterically hindered species may not sufficiently reinforce the mechanical properties with regard to current composition. With the increase in RMCPs content up to 30 mol. %, an appreciable enhancement in tensile strength of 0.49–0.55 MPa and modulus of 0.12–0.23 MPa was observed along with a slightly decrease in elongation at break of 460–600 %. Those corresponding variations leading to a relatively stiffer texture were basically dependent upon the increased amount of triazole and monocyclic groups. The grafted RMCPs in analogy to the flank of macromolecular backbone were acted as an anchor in favor of enhancing the intermolecular force which hindered the orientation and torsion of the polymer skeleton [42]. In contrast, the GAP-based PUs plasticized by energetic plasticizers exhibited a softer manner with increasing plasticizer amount [18,57]. A significant decreases in tensile properties manifested that energetic plasticizers weakened the intermolecular interaction within the polymer chains by plasticization effect.

Figure 5. (a) Typical stress vs. strain curves of RMCPs/GAP-based PUs. An increase in RMCPs feeding resulted in an enhanced tensile strength and modulus ascribed to the more triazole formation. (b) Thermochemical properties of RSPs/GAP-based PUs. The c Hos of the control GAP-based PU and GAP-based PUs with 30 mol. % of RMCPs were measured in an oxygen bomb calorimeter. As shown in Figure 5b, a negative value of the observed c Ho of five PU specimens represented the exothermic process of combustion reaction. The c Ho of the control GAP-based PU was –20.92 kJ g-1 close to the reported one [58], whereas that of RMCPs/GAP-based PUs was ranged from –22.80 to – 23.51 kJ g-1. Cyclobutane-based RMCPs provided a higher c Ho than cyclopropane-based ones ascribed to the higher content of carbon and hydrogen where more carbon dioxide and water were produced during the combustion. The element contents of PU specimens including carbon, hydrogen and nitrogen were tabulated in Table S1. Because the resulting polymer was a not pure compound which had an identical molecular weight and elemental composition, an empirical formula for individual PU specimen was indispensable to obtain the f Ho values based on Eq. (2): b 2

 f Ho (CaHbOcNd, s) = a  f Ho (CO2, g) +  f Ho (H2O, l) –  c Ho (CaHbOcNd, s)

(2)

According to Eq. (2), the f Hos of five PU specimens were calculated and compared in Figure 5b. The control GAP-based PU exhibited a reasonable positive f Ho of 78.9 kJ mol-1 compared to the reported result [58]. Surprisingly, the RMCPs/GAP-based PUs provided a

positive f Ho ranging from 12.1 to 40.0 kJ mol-1 except BCBC/GAP-based PU which was – 12.8 kJ mol-1. The RMCPs/GAP-based PUs with the positive f Hos revealed that the high RSE was of great value in terms of enhancing the energetic performance. On the other hand, the formation of triazole groups may enhance the energetic performance of the GAP-based PU binder because the enthalpy of decomposition of triazole group was 418 kJ mol-1 higher than that of azide of 317 kJ mol-1 [59]. The reported impact sensitivities of some conventional explosives and energetic plasticizers comprising the explosophore groups were basically lower than 15 J, especially in the case of the compounds with -ONO2 group, which were even below 3 J [60]. The impact sensitivity of the control GAP-based PU was previously measured to be 27.5 J [23], lower than the RMCP/GAP-based PUs in the range of 57.1–64.4 J. The incorporation of RMCPs improved the impact insensitivity of GAP-based PUs owing to the formation of the relatively insensitive triazole moiety through the 1,3-DPCA reaction [23]. Moreover, the values of impact sensitivity of RMCP/GAP-based PUs were higher by an average of approximately 40 J than the equivalent composition of energetic plasticizers/GAP-based PU of approximately 20 J [50]. It is conceivable that the strategy converting the azide groups into triazole groups can improve the safety performance of the GAP-based PU binder.

4. Conclusions Cyclopropane and cyclobutane-based RMCPs were synthesized via an efficient one-pot esterification reaction, and characterized by 1H and

13C

NMR spectroscopy. The intrinsic

RSE of RMCPs using homodesmotic schemes was predicted to estimate the energetic contribution derived from the RSE, both of which could contribute ca. 100 kJ mol-1 of RSE. The RMCPs were found to be mutually miscible with the GAP prepolymer over a wide range of composition up to 50/50 w/w. The isothermal measurement of the rheological feature for RMCP/GAP prepolymer mixture illustrated that the viscosity of pristine GAP prepolymer could be reduced from 6,015 to 59-70 cP at ambient temperature followed by a trivial increase after 300 min. A preliminary study on catalyst-free 1,3-DPCA reactivity concerning propargyl versus 3-butynyl traced by 1H NMR spectroscopy verified a higher reactivity of the propargyl species based on the inductive effect as well as the possibility of the simultaneous completion of 1,3-DPCA reaction and PU reaction. The Ea of propargyl species was measured to be in a range of 77.9-78.2 kJ mol-1 whereas that of 3-butynyl species was

measured to be a range of 84.7-85.2 kJ mol-1. Notably, both kinetic studies on the catalystfree Huisgen 1,3-DPCA reaction proved consistently that the reactivity was tunable by controlling the distance between the alkyne group and an adjacent EWG. The evidence of propargyl species having a higher reactivity was readily found in the chemical shift of alkynyl proton of pure RMCPs where the more deshielded alkynyl protons of propargyl moieties suggested the more electron-deficient alkynes. The computed FMO energy gaps of RMCPs were indicative of a reasonable match of theoretical explanation with experimental determination in addition to that a low-lying LUMOdipolarophile of the propargyl species was in favor of electron pairing with HOMOdipole leading to a narrower energy gap. A good agreement of the sum of energy gap difference was reached with the difference in the determined Ea between RMCPs with n=2 and n=1. There was no appreciable loss of the RMCPs before the entire thermal decomposition of RMCPs/GAP-based PUs during TGA measurement indicating the RMCPs coupled completely with the GAP-based PU. The more elevated incorporation of RMCPs improved the tensile properties of GAP-based PU binders ascribed to the more formation of triazole groups, steric hindrance of which imparted a depressed orientation of the polymer chains. Several PUs incorporated with 30 mol. % of RMCPs exhibited the positive f Ho owing to the high RSE. The RMCPs could lower the impact sensitivity of the GAP-based PUs by converting the azide into triazole group. Acknowledgement This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2019R1F1A1057939). References [1]

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Graphical abstract

Nonmigratory reactive monocyclic plasticizers bearing tunable click reactivity were covalently bound to energetic polyurethane binders to achieve enhanced properties.

Highlights: 

Reactive monocyclic plasticizers (RMCPs) with high ring strain energy was synthesized.



Plasticization effect of the RMCPs on glycidyl azido polymer (GAP) was examined.



Click reactivity of the RMCPs toward GAP was performed and predicted.



Nonmigratory and insensitive RMCPs/GAP-based polyurethane binders were developed.

Author statement Younghwan Kwon: Conceptualization, Methodology, Resources, Supervision, Validation, Supervision, Project administration, Writing-Reviewing and Editing, Funding acquisition; Mingyang Ma: Software, Data curation, Formal analysis, Investigation, Visualization, Writing-Original draft preparation.