Ind. Eng. Chem. Res. 1997, 36, 2219-2224
2219
Evaluation of Branched Glycidyl Azide Polymer Purified by Solvent Extraction Van Tam Bui,* Elie Ahad,† Dany Rheaume, and Robert Whitehead Department of Chemistry and Chemical Engineering, Royal Military College of Canada, Kingston, Ontario, Canada K7K 5L0
Dimethyl sulfoxide (DMSO), a polar liquid of low volatility, was used as solvent in the synthesis of branched glycidyl azide polymer (GAP) which is a potential energetic binder for rocket propellants. Branched GAP product was purified by an extraction method using dichloromethane to create an organic phase and a mixture of methanol-brine (50:50 in weight) as the extracting solution. The extraction was carried out in four steps, leaving only less than 1% DMSO in the purified branched GAP. The DMSO concentration remaining in the organic phase was deduced from the 1H NMR spectra taken after each extraction step. Finally, the physical performance of branched GAP was evaluated through the tensile properties, the glass transition temperature, and the thermal degradation of the energetic polyurethanes obtained after curing branched GAP with different isocyanate compounds. The effect of residual DMSO in branched GAP on the tensile properties of polyurethanes was also investigated. Introduction Glycidyl azide polymer (GAP) is a potential energetic binder for rocket propellants with reduced smoke and low vulnerability, as well as for insensitive composite explosives and gun propellants. For these reasons, GAP is considered an attractive substitute for the inert hydroxy-terminated polybutadiene conventional binder currently used in many energetic formulations. GAP with a linear structure and a molecular weight between 1000 and 6000 is currently produced in the U.S.A. (Frankel and Flanagan, 1981; Frankel et al., 1992) according to a two-step process which requires two distinct reactions, polymerization and azidation. Recently, a novel degradation process was developed at the Defence Research Establishment at Valcartier (DREV) (Ahad, 1993) for the preparation of GAP with a branched structure and a variable molecular weight (10002 000 000) and a high hydroxyl functionality. This process involves the single-step reaction of a highmolecular-weight solid, rubbery polyepichlorohydrin (PECH) with sodium azide in the presence of a basic cleaving agent (methyllithium), and a polyol (glycerol) in a suitable organic solvent at elevated temperatures (90-120 °C). The solvents used in this process must be able to completely dissolve the rubbery PECH, sodium azide, the cleaving agent, and the polyol, in order to achieve both the degradation and azidation reactions. Details of the synthesis process of branched GAP are found elsewhere (Ahad, 1995; Bui et al., 1996a). The suitable solvents include polar organic solvents such as dimethyl sulfoxide (DMSO), dimethylacetamide (DMA), dimethylformamide (DMF), and N-methyl-2-pyrrolidone (NMP). Mixtures of different solvents can also be used in the synthesis process. Among these solvents, DMSO is preferred over the others due to the quality of the final product (Ahad, 1995). Similarly, these solvents have been widely used for the preparation of polymer membranes by the solvent-casting technique (Hwang et al., 1996). However, it is not easy to evaporate these solvents for recovering the GAP product after the reaction due to * To whom correspondence should be addressed. † Defence Research Establishment at Valcartier, Courcelette, Quebec, Canada. S0888-5885(96)00417-4 CCC: $14.00
their very low volatilities, even using reduced pressure and higher temperatures (limited at 120 °C because of the thermal instability of GAP). In this study, the extraction procedure is proposed for separating branched GAP from DMSO. The purified branched GAP is then cured with various isocyanate compounds and assessed via the analyses on the resulting energetic polyurethanes. Experimental Section Extraction Procedure. Details of the synthesis of branched GAP are found elsewhere (Ahad, 1995). The resulting solution (after filtration of the remaining solid reactants) contains about 20% branched GAP in DMSO. The extraction procedure uses dichloromethane (DCM), twice the volume of the DMSO solution, to form an organic phase with the original solution (Table 1). Then, DMSO has to be extracted from the organic phase by an aqueous phase formed by methanol and brine (50: 50 in weight); the volume of the aqueous phase is twice that of the organic phase. The presence of NaCl salt in the aqueous phase is to reduce the foaming at the interface of the two phases. Due to its strong polar character, DMSO is extracted by the aqueous phase (more specifically by methanol) while branched GAP remains with DCM. There is unavoidably a small amount of DCM, as well as oligomers of branched GAP, passing to the aqueous phase, which makes it slightly yellow. The extraction process is repeated on the resulting organic phase three more times using the same amounts of aqueous solution. For the last extraction, the system is left for several hours to allow the two phases to completely separate. The resulting organic phase was dried using MgSO4 powder. Finally, dichloromethane (DCM) is evaporated using a rotary evaporator, and the final product obtained is the purified branched GAP. Due to the large amount of the aqueous solution used through the four steps of extraction, it is economical to recover the methanol contained in that spent aqueous solution, which consists of methanol, water, DMSO, traces of branched GAP oligomers, DCM, and NaCl. Since methanol is much more volatile than all other components in the spent aqueous solution, its recovery © 1997 American Chemical Society
2220 Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997 Table 1. Weight Percent of DMSO in the Organic Phase as a Function of the Extraction Step peak area extraction step
DMSO
GAP
wt %
initial 1st 2nd 3rd 4th GAP
157.0 132.0 5.2 3.8 2.0 5.0
22.8 128.0 74.2 100.0 131.0 313.0
82.0 40.0 4.4 2.4 0.9 0.9
Table 2. Sources and Characteristics of Different Isocyanate Compounds
name 2,4- and 2,6-tolylene diisocyanate diphenylmethane 4,4′-diisocyanate hexamethylene diisocyanate Desmodur N-100
abbreviation
NCO equivalent weight (g)
source
TDI
81.7
MDI
125.2
HDI
84.1
Aldrich
191.0
Mobay
N-100
Aldrich Eastman Kodak
can be effectively achieved using a single distillation operation. For that purpose, a glass column of 0.5 m height and 0.03 m diameter packed with glass beads was used, and after a period of about 4 h, nearly 80% of the methanol was recovered from the spent solution. This recycled methanol containing a small amount of water and traces of DCM is ready for use in the extraction procedure of the next batch of branched GAP synthesis. Unfortunately, the DMSO recovered from the distillation operation is not pure enough for use as solvent in the next synthesis batch. Residual DMSO Concentrations Determined by NMR Spectra. In order to follow the variation of the DMSO concentration remaining in the extracted solution after each extraction step, a 1H NMR spectrum of the resulting organic phase was acquired using a Varian Gemini 200 NMR spectrometer. A spectrum was also acquired for the original unextracted branched GAP solution. These NMR spectra allow one to check the effectiveness of the extraction process. Curing of Branched GAP. Branched GAP, when used as an energetic binder, should be transformed into a long-chain energetic polyurethane, via a curing reaction using a suitable isocyanate compound. The best way to evaluate the performance of branched GAP is to determine the properties of the resulting energetic polyurethane. The tensile properties, the glass transition temperatures, and the thermal degradation patterns are the most significant ones. The purified branched GAP obtained is a yellowish, viscous liquid having a number-average molecular weight (Mn) and a weight-average molecular weight (Mw) of 10 600 and 18 100, respectively, and a hydroxyl equivalent weight of 2510 g (Bui et al., 1996a). In this work, the following isocyanate compounds were used for the curing of branched GAP: 2,4- and 2,6-tolylene diisocyanate (TDI), diphenylmethane 4,4′-diisocyanate (MDI), hexamethylene diisocyanate (HDI), and Desmodur N-100 (a polyisocyanate based on HDI). The sources and characteristics of these compounds are summarized in Table 2. The curing of branched GAP was performed at 60 °C. The exact amount of branched GAP was weighed in a 50 mL beaker and then introduced into a vacuum oven, preheated at 60 °C, where high vacuum was applied for at least 1 h in order to remove traces of impurities such
as water and DMSO. Then the beaker was placed in an oil bath, preset at 60 °C, before adding the desired quantity of selected isocyanate compound to satisfy a fixed NCO/OH ratio (calculated using the respective NCO and OH equivalent weights of isocyanate and branched GAP). The mixture was stirred for about 3 min and then transferred into the molds made of Teflon according to the ASTM D638 standard for tensile specimens of plastic materials (Storer, 1991). The molds were preheated to 60 °C in order to prevent any decline in temperature in the reaction mixtures. Vacuum was then applied for about 15 min to remove all air bubbles from the specimens. The molds were kept overnight in the oven at 60 °C and then about 1 h at 100 °C to complete the curing reaction. For each value of the NCO/OH ratio, at least three specimens were produced for the tensile tests. In order to assess the effect of residual DMSO in branched GAP on the properties of cured energetic binders, some small amounts (2.0, 4.0, and 6.0% in weight with respect to that of branched GAP) were intentionally added to the curing mixtures of NCO/OH ) 1.20 using TDI as the curing agent. Since DMSO is a nonvolatile solvent, its loss through evaporation during the curing period was negligible; this was checked by weighing the cured samples before and after curing. The cured samples containing 6% DMSO appeared less transparent and much softer than the other samples. This may be attributed to some degree of phase separation caused by the presence of DMSO. Tensile Testing of Energetic Polyurethanes from Branched GAP. The most significant assessment of energetic binders (polyurethanes) from branched GAP is certainly the tensile test to provide the elongation at break and the tensile strength of these binders. This test was performed on cured samples using an Instron tester, Model 4206 fully digitalized, where the force applied to the sample as well as its elongation during the test was automatically recorded by the control board of the system. The tensile load cell has a capacity of 50 N. A suitable grip for elastomeric materials was used for correctly holding the sample in place without slipping. Since the binders are highly elastomeric, a relatively large crosshead speed of 10 cm‚min-1 was used for the tensile tests. Prior to testing, the width and thickness of the gauge region of each cured branched GAP specimen were measured to an accuracy of (0.5 µm using an electronic micrometer. Usually, the gauge dimensions were approximately 25 × 6 × 3 mm. All specimens were drawn at ambient temperature. The mechanical properties of interest are defined as follows (Peacock and Mandelkern, 1990):
elongation at break (ξb) ) (gauge length at break/ initial gauge length) × 100% tensile strength (σu) ) (ultimate tensile force × ξb)/ (original cross-sectional area) These two quantities are directly produced by the computer using the suitable Instron software, and they may be double-checked by the final extension and load recorded on the keyboard of the system. Thermomechanical Analysis (TMA) of Energetic Polyurethanes from Branched GAP. In order to determine the glass transition temperature, Tg, of the energetic binders, TMA was applied to all cured samples from branched GAP. This technique, previously used
Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997 2221
Figure 1. NMR spectrum of the original unextracted branched GAP-DMSO solution.
for the polyurethanes based on castor oil (Liu and Bui, 1995), is based on the principle that there is an abrupt change in the coefficient of expansion of the sample when it passes through the softening temperature or Tg under a constant heating. This manifests itself as that temperature at which the regression line before the break intersects that after the break. TMA was performed using a Mettler TA-4000 with a TMA 40 measuring cell. The approximate sample dimensions were 3 mm thick in the direction of measurement and 6 mm in diameter. Measurements were carried out in the range of -100 to +50 °C. A low-temperature accessory was used with liquid nitrogen as the refrigerant. The heating rate was 10 °C‚min-1, and the constant probe force applied to the sample was 0.02 N. Thermogravimetric Analysis (TGA) of Energetic Polyurethanes from Branched GAP. The thermal stability of the energetic binders was investigated using the same Mettler system, with a TG 50 module. Three samples of binders, between 10 and 12 mg, prepared from branched GAP and TDI with a NCO/OH ratio ) 1.20, were successively scanned from 100 to 450 °C with a heating rate of 10 °C‚min-1 in an inert atmosphere. The weight loss percentage of the samples was recorded as a function of temperature by a thermal analysis data control, TA10. Results and Discussion Purified Branched GAP. The extraction process for removing DMSO from branched GAP using DCM and a MeOH-brine mixture is very efficient. Effectively, the yield is higher than 90% based on the total amount of the main reactant PECH used for the branched GAP synthesis. The loss is mainly attributed to the decantation operations as well as to the branched GAP oligomers which have passed to the aqueous extracting solutions, leading to a yellowish color of the latter. The high yield of purified branched GAP may be interpreted on the basis of the polarity and solubility parameter concepts. First, DCM is almost insoluble in the aqueous phase due to its nonpolar character. This makes the separation of the two phases very effective. Second, the solubility parameter values (Weast, 1988) of DCM, DMSO, and MeOH are quite different; they are 9.7, 12.0, and 14.5, respectively. The value of DMSO is at about the midpoint between those of DCM and MeOH. How-
ever, DMSO with its strong polar character dissolves preferably in the hydrogen-bonded MeOH-water mixture rather than in the nonpolar phase of DCM. During the extraction process, branched GAP, according to its chemical structure (Ahad, 1995), contains several -CH2groups per one -OH group such that its character is mainly a methylene one, making it preferably soluble in DCM. On the other hand, branched GAP oligomers with a smaller number of methylene groups should have a dominating polar character of hydroxyl groups such that they prefer to dissolve in the aqueous phase. This is a marked advantage of the extraction process since the purified branched GAP has a narrower molecular weight distribution, with a polydispersity index less than 2.0, which is considered very small for a polymer obtained from a polycondensation mechanism (Bui et al., 1996a). The variation of the DMSO concentration remaining in the branched GAP solution was followed by successive NMR spectra illustrated by Figures 1-3. It is found that the characteristic peaks assigned to branched GAP are localized between 3 and 4 ppm, while those of DMSO and DCM are at 2.62 and 5.3 ppm, respectively. The peak at 7.27 ppm is attributed to chloroform which is used as the solvent for NMR analysis. After each extraction step, the relative concentration of branched GAP increased while that of DMSO correspondingly diminished (Figure 2). Table 1 summarizes the peak integration values for branched GAP and DMSO, respectively, deduced from the NMR spectra. The last column in Table 1 presents the weight percentage of residual DMSO in the branched GAP-DCM solution after each extraction. These concentrations are calculated by the following method. According to its chemical structure, one DMSO molecule has 6 hydrogen atoms and a molecular weight of 78. This gives rise to a hydrogen equivalent weight of 13 g. Meanwhile, based on the branched GAP structure (Ahad, 1995), there are 5 hydrogen atoms/glycidyl azide unit, which corresponds to a molecular weight of 99. Therefore, the hydrogen equivalent weight of branched GAP is almost 20 g. Based on the functional principle of the proton NMR technique, it is assumed that the area integrated under each NMR peak is directly proportional to the total of the hydrogen equivalent weight of the concerned compound. For example, after the third extraction, the peak
2222 Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997
Figure 2. NMR spectra of the organic phase taken after the 1st (or wash), 2nd, 3rd, and 4th extractions.
Figure 3. NMR spectrum of a purified branched GAP sample, free of DCM, and containing a trace of DMSO.
integration values for DMSO and branched GAP are 3.80 and 100.0 arbitrary units, respectively. These values lead to a ratio of 49.8 g of DMSO on 2000 g of branched GAP or a weight percentage of 2.5%. It is mentioned that, in these calculations, the contributions from -OH groups and -CH- groups at the branching sites of branched GAP molecules are neglected since the number of hydrogen atoms of those groups is effectively negligible with respect to the total number of hydrogen atoms in the branched GAP molecules. Thermal Properties of Energetic Polyurethanes from Branched GAP. The glass transition temperatures (Tg) of various energetic polyurethane samples prepared from the same branched GAP batch are presented in Table 3, together with their tensile properties. These values are significantly higher than that of pure branched GAP, which is about -47 °C (Ahad, 1995). This increase is mainly attributed to the presence of isocyanate units in the polyurethane structure; these units constitute the hard segments of polyurethanes compared to the soft segments of branched GAP. Among the four different types of isocyanate used for the curing of branched GAP, HDI is the most flexible
Table 3. Tensile Properties and Glass Transition Temperature of Polyurethanes from Branched GAP as a Function of the NCO/OH Ratio and Type of Isocyanate
NCO/OH
isocyanate
elongation (%)
0.80 1.05 1.20 0.85 1.10 1.25 1.20 1.10 1.25
TDI TDI TDI MDI MDI MDI HDI HDI + N-100 (65:35) HDI + N-100 (65:35)
240 230 190 180 160 170 210 220 200
tensile strength (MPa)
Tg (°C)
0.21 0.24 0.25 0.32 0.38 0.42 0.14 0.21 0.23
-25 -23 -23 -20 -18 -19 -32 -27 -28
segment due to its aliphatic character, so that polyurethanes obtained from HDI have the lowest Tg with respect to those obtained from HDI-N100 mixture, TDI, and MDI. Results from thermogravimetric analysis show a twostage degradation pattern for energetic polyurethanes. The first stage of about 80% weight loss between 220 and 260 °C, with a maximum degradation rate at 236 ( 2 °C, may be assigned to the spontaneous detachment
Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997 2223 Table 4. Effect of Residual DMSO on the Properties of Polyurethanes Cured with TDI at NCO/OH ) 1.20 DMSO (wt %)
elongation (%)
tensile strength (MPa)
Tg (°C)
2.0 4.0 6.0
220 240 180
0.22 0.12 0.05
-25.0 -30.0 -38.0
of the azide groups (-N3). This major thermochemical process is highly energetic such that the large amount of heat evolved during a very short period should have to instantaneously decompose most of the glycidyl groups in the overall structure of energetic binders. The last stage with about 20% weight loss corresponds to a slower thermal degradation of isocyanate units. This highly thermal resistance of isocyanate units has also been confirmed by the thermogravimetric analyses of several nonenergetic polyurethanes obtained from castor oil and different types of isocyanate compounds (Bui et al., 1996b). Tensile Properties of Energetic Polyurethanes from Branched GAP. The tensile properties of various polyurethane binders prepared from the same branched GAP sample, using different isocyanate compounds at various NCO/OH ratios, are presented in Table 3, together with their corresponding Tg values. In general, the elongation at break decreases with increasing NCO/OH ratio, while the tensile strength increases. This may be attributed to the following causes: the isocyanate molecules constitute the hard segments of the polyurethane chains with respect to the branched GAP molecules which act as their soft counterparts such that the increasing NCO/OH ratio corresponds to the dominant effect of the hard segments over the soft ones; a larger NCO/OH ratio may increase the cross-linking density between polyurethane chains, which makes them stronger but more rigid; finally, the excess of isocyanate compound may produce some polyisocyanate, named biuret, which is a rigid polymer. These findings are also supported by the increasing values of Tg with increasing NCO/OH ratio. Except for HDI yielded binders with low tensile properties, all other isocyanate compounds give rise to samples with acceptable properties. However, the mixture of N-100 with HDI seemed to increase both elongation at break and the tensile strength of the resulting polyurethanes. This is likely the consequence of an increase in the crosslinking degree due to the polyisocyanate functionality of N-100, such as observed in the case of polyurethanes based on castor oil (Liu and Bui, 1995). In particular, TDI appears to be the best one to yield samples with very high elongation at break, which is the most significant quality of energetic binders, while MDI with its two aromatic rings gives rise to strong but rigid polyurethanes. All isocyanate compounds except HDI yield energetic binders with a tensile strength high enough for their use as an elastomeric matrix in the formulation of propellant composite systems (Ahad et al., 1995). However, the elongation at break of these binders could be improved by means of incorporating the ethylene oxide groups, -CH2CH2O-, to branched GAP units via a copolymerization process (Bui et al., 1996a). Finally, Table 4 lists the glass transition temperature, Tg, and tensile properties of the cured samples containing various additional DMSO amounts. It is found that the presence of 2% DMSO in the curing mixture does not significantly alter the properties of the binders
except a small increase in elongation at break due to the plasticizing effect. However, larger amounts of DMSO result in a large weakening effect, leading to a much smaller tensile strength, while the elongation at break is not significantly improved. Conclusion The branched glycidyl azide polymer product obtained from the synthesis using DMSO as solvent could be effectively purified by means of the extraction procedure using DCM for the organic phase and a mixture of methanol + brine for the aqueous phase. After four steps of extraction, the residual DMSO concentration was found to be less than 1% using the NMR technique, which seems to be a rapid and suitable method to follow the variation of the DMSO concentration. The total yield of the purification method is higher than 90%, and the purified branched GAP has a narrow molecular weight distribution due to the removal of GAP oligomers. The curing of purified branched GAP using different isocyanate compounds except HDI at various NCO/OH ratios yielded energetic binders (polyurethanes) with relatively high tensile properties. However, the elongation at break of the binders could be improved for better performance in the practical energetic composite formulations. The thermogravimetric analyses performed on the binders show a fast thermal decomposition of the glycidyl azide groups, which is characteristic of energetic materials. The presence of more than 2% of residual DMSO in branched GAP results in large effects of plasticizing, and eventually phase separation, on the properties of the energetic binders. Acknowledgment The authors are thankful to the Defence Research Establishment at Valcartier, Quebec, Canada, for financial support through a FE research contract. Nomenclature ASTM ) American Society for Testing and Materials DCM ) dichloromethane DMA ) dimethylacetamide DMF ) dimethylformamide DMSO ) dimethyl sulfoxide DREV ) Defence Research Establishment at Valcartier GAP ) glycidyl azide polymer HDI ) hexamethylene diisocyanate Mw ) weight-average molecular weight MDI ) diphenylmethane 4,4′-diisocyanate MeOH ) methanol MgSO4 ) magnesium sulfate MPa ) mega or million pascals, 106 kg m-1 s-2 N ) Newton, force unit, kg m s-2 N-100 ) polyisocyanate based on HDI NaCl ) sodium chloride NCO/OH ) ratio between isocyanate equivalents and hydroxyl equivalents NMP ) N-methyl-2-pyrrolidone NMR ) nuclear magnetic resonance technique 1H NMR ) proton nuclear magnetic resonance PECH ) polyepichlorohydrin Tg ) glass transition temperature, °C TDI ) 2,4- and 2,6-tolylene diisocyanate TGA ) thermogravimetric analysis TMA ) thermomechanical analysis Greek Letters
2224 Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997 ξb ) elongation at break, % σu ) ultimate strength or tensile strength, MPa
Literature Cited Ahad, E. Improved Branched Energetic Azido Polymers. U.S. Patent 08/130,129, Oct 4, 1993; Canadian Patent 2133507, Oct 3, 1994; European Patent 94307164.7, Sept 30, 1994. Ahad, E. Improved Process for the Synthesis of Second Generation Branched GAP. DREV Report 9414, 1995, unclassified. Ahad, E.; Lavigne, J.; Lessard, P.; Dubois, C. Effect of Branched GAP Synthesis Parameters on Mechanical Properties of Rocket Propellants. DREV Report 9513, 1995, unclassified. Bui, V. T.; Ahad, E.; Rheaume, D.; Raymond, M.-P. Energetic Polyurethanes from Branched Glycidyl Azide Polymer and Copolymer. J. Appl. Polym. Sci. 1996a, 62, 27-32. Bui, V. T.; Liu, T. M.; Legault, J. F. Characterization of FullInterpenetrating Polymer Networks based on Castor Oil and Methyl Methacrylate. Int. J. Polym. Anal. Charact. 1996b, 3, 1-15. Frankel, M. B.; Flanagan, J. E. Energetic Hydroxy-Terminated Azido Polymer. U.S. Patent 4,268,450, May 19, 1981. Frankel, M. B.; Grant, L. R.; Flanagan, J. E. Historical Development of GAP. J. Propul. Power 1992, 3, 560.
Hwang, J. R.; Koo, S.-H.; Kim, J.-H.; Higuchi, A.; Tak, T.-M. Effects of Casting Solution Composition on Performance of Poly(ether sulfone) Membrane. J. Appl. Polym. Sci. 1996, 60, 1343-1348. Liu, T. M.; Bui, V. T. Instrumented Impact Testing of Castor OilBased Polyurethanes. J. Appl. Polym. Sci. 1995, 56, 345-354. Peacock, A. J.; Mandelkern, L. Mechanical Properties of Copolymers. J. Polym. Sci., Part B: Polym. Phys. 1990, 28, 19171941. Storer, R. A. Standard Test Method for Tensile Properties of Plastics. American Society for Testing and Materials; Committee D-20 of Plastics, 1991; Designation D638-41. Weast, R. C. Handbook of Chemistry and Physics; CRC Press: Boca Raton, FL, 1988; pp C-680-682.
Received for review July 19, 1996 Revised manuscript received October 14, 1996 Accepted March 8, 1997X IE9604177
X Abstract published in Advance ACS Abstracts, May 1, 1997.
