Development of comprehensive two-dimensional liquid chromatography for investigating aging of plastic bonded explosives

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Development of comprehensive two-dimensional liquid chromatography for investigating aging of plastic bonded explosives Chris E. Freye a,∗, Christopher J. Rosales a, Darla Graff Thompson a, Geoff W. Brown a, Sheldon A. Larson b a b

Los Alamos National Laboratory, M-7, High Explosives Science and Technology, Los Alamos, NM 87545, USA Los Alamos National Laboratory, W-9, Weapon Systems Surveillance, Los Alamos, NM 87545, USA

a r t i c l e

i n f o

Article history: Received 10 June 2019 Revised 26 September 2019 Accepted 29 September 2019 Available online xxx Keywords: Liquid chromatography Comprehensive two-dimensional PBX 9501 Explosives

a b s t r a c t The feasibility of measuring the aging and degradation of PBX 9501 via online two dimensional liquid chromatography (LC × LC) is investigated, and a preliminary instrumental setup and method is developed. Plastic-Bonded eXplosive (PBX) 9501 is nominally composed of 94.9 wt% HMX, 2.5 wt% Estane® 5703 (poly (ester urethane)), 2.5 wt% BDNPA/F (nitroplasticizer), 0.1 wt% Irganox 1010 and PBNA (Nphenyl-naphthylamine) at low concentrations. When exposed to various environmental conditions, PBX 9501 will degrade through different pathways. Because PBX 9501 is composed of both low molecular weight compounds (BDNPA/F, Irganox 1010, PBNA, and potential degradation products) and high molecular weight compounds (Estane® 5703), analysis is normally performed via two independent analyses. The low molecular weight species are analyzed via high pressure liquid chromatography (HPLC) and the high molecular weight species via size exclusion chromatography (SEC). While these individual techniques yield information about the aging of PBX 9501, the combination of HPLC and SEC (i.e. HPLC × SEC) can simplify and streamline the analyses while also providing additional chemical information. A simplified sample preparation method is proposed for LC × LC analysis. Various SEC columns and HPLC column selection, flow rate, and gradient ramps were investigated for their application of measuring aged PBX 9501. Finally, two LC × LC separations of a library standard of PBX 9501 and a sample of aged PBX 9501 are compared. © 2019 Published by Elsevier B.V.

1. Introduction Understanding the aging and degradation of Plastic-Bonded eXplosive (PBX) 9501 is important in order to mitigate unwanted mechanical and chemical changes that may negatively impact performance. PBX 9501 is composed of 94.9 wt% HMX (1,3,5,7-tetranitro1,3,5,7-tetrazocane), 2.5 wt% Estane® 5703 [Estane – a poly(ester urethane)], 2.5 wt% plasticizer, and 0.1 wt% stabilizer, Irganox 1010. The plasticizer is a 1:1 eutectic mixture of bis(2,2-dinitropropyl) acetal (BDNPA) and bis(2,2-dinitropropyl) formal (BDNPF), often referred to as BDNPA/F [1–4]. N-phenyl-naphthylamine (PBNA) is added as a stabilizer to BDNPA/F during the production and is present at 0.004% in PBX 9501. While extensive research has been performed investigating the aging of PBX 9501 through a variety of different approaches in a wide variety of environments [4–8], gaining further insight into the chemical and mechanical proper-

∗

Corresponding author. E-mail address: [email protected] (C.E. Freye).

ties as PBX 9501 ages is important especially when new technology(s) may provide further insight into the material. Estane® 5703, herein referred to as Estane, the binder of PBX 9501, is a copolymer chain consisting of alternating “soft segments” and “hard segments” [9]. The soft segments are oligomers of the ester of adipic acid with 1,4 butanediol while the hard segments are short polyurethanes made from 4,4 -diphenylmethane diisocyanate (MDI) molecules bonded together by urethane links. Estane has a large number of chemically reactive functionalities which can make it susceptible to numerous undesirable chemical modifications. Although Estane is found at very small amounts in PBX 9501 (2.5 wt%), changes to the molecular weight of Estane can drastically change the mechanical properties [10]. Indeed, a large amount of research has been performed looking at the degradation of Estane in a wide variety of environments [4–7,11–15]. The most extensive aging study of Estane and PBX 9501 was published by Salazar et al. with over 1100 closed-container samples held at temperatures below 64 °C in a diverse set of environmental conditions [4]. The environmental conditions Estane is exposed to will strongly influence the decomposition pathways and kinetics. When

https://doi.org/10.1016/j.chroma.2019.460580 0021-9673/© 2019 Published by Elsevier B.V.

Please cite this article as: C.E. Freye, C.J. Rosales and D.G. Thompson et al., Development of comprehensive two-dimensional liquid chromatography for investigating aging of plastic bonded explosives, Journal of Chromatography A, https://doi.org/10.1016/j.chroma.2019. 460580

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Estane is exposed to water, the primary mechanism responsible for the degradation is hydrolysis [9,11,12,14–16]. Estane may also degrade through oxidation. Oxidation is more complex than hydrolysis as multiple parts of the polymer chain may participate in oxidation reactions resulting in increasing or decreasing molecular weight changes depending on the concentration and reactivity of initial oxidizing species [4–9,14]. The aging of Estane is normally examined using size exclusion chromatography (SEC), which can indicate the molecular weight and polydispersity (homogeneity of the sample) [4,7,10,11,17], but can also be examined using infrared spectroscopy [5,6,18], matrix assisted laser desorption ionization mass spectrometry (MALDI-MS) [13], and nuclear magnetic resonance (NMR) [16]. BDNPA/F is added in the production of PBX 9501 as a plasticizer at 2.5 wt%. Like many plasticizers, BNDPA/F has a tendency to diffuse out of the binder domain and can decompose into reactive byproducts [19–22]. Several groups have shown that BDNPA/F can decompose at elevated temperatures and can interact with the polymer, Estane, changing the chemical properties and mechanical properties [4,14,19]. There has been limited research investigating the degradation of BDNPA/F [2,3]. In the presence of air or N2 , BDNPA/F will decompose into trans-nitroso alcohol isomers as well as small molecules (H2 O, HNO3 , formaldehyde, ethanal (acetaldehyde), CO2 ). The presence of water causes the environment to be more acidic which alters the decomposition pathways of BDNPA/F resulting in the formation of poly-OH function groups. Analysis of BDNPA/F is normally performed using liquid chromatography [22], but may also be done using infrared spectrometry or NMR [2,3,21,22]. Irganox 1010 is found at concentration of 0.1 wt% as a stabilizer. Salazar et al. showed that Irganox has minimal effect in protecting against moderate cross-linking but does prevent against depolymerization [7]. Irganox is known to decrease linearly with time when exposed to air but is also susceptible to hydrolysis [23]. In addition, N-phenyl-2-napthylamine (PBNA) is present at 0.004 wt% from the addition of the plasticizer (BDNPA/F). PBNA is added in the production of BDNPA/F as a stabilizer. In the presence of O2 , PBNA may be consumed, but in the presence of an inert gas may still be present [24]. Diphenylamine, similar to PBNA, can react with NOx radicals to form various nitrated products [25–27]. Limited research has been performed evaluating the decomposition of Irganox and PBNA. Comprehensive two-dimensional liquid chromatography (LC × LC) is an apt technique to study aging of PBX 9501. In practice, LC × LC is the coupling of two columns of different separation mechanisms (i.e. SEC, RPLC, Ion exchange (IEX), etc.) [28–33] or columns of different chemistries (i.e. NPLCCyano × RPLC-C18) [34–37]. Several comprehensive reviews on the different column selections for LC × LC have been published [38–41]. The strategy for LC × LC of low molecular mass species is different than that for high molecular mass. Indeed, LC × LC of low molecular mass species often involves the latter type of implementation of LC × LC where different column chemistries are implemented. For analysis of high molecular mass species (i.e. polymers), often interaction chromatography (IC) is combined with SEC, taking advantage of the high resolution of IC in the first dimension and the speed and simplicity of SEC in the second dimension [28–30,42–47]. Furthermore, IC × SEC (as opposed to SEC × IC) is often implemented for several other reasons. In IC, the solvent strength is weak in order to induce adequate interaction strength of the solutes with the stationary phase. If the injection solvent (eluent from the first dimension) is stronger than the eluent from the second dimension, breakthrough of the sample often occurs [28,48]. SEC eluents are usually thermodynamically good, further reducing solvent incompatibility problems. There are many more detectors available for IC × SEC than for SEC × IC

including differential-refractive-index (DRI), light-scattering, and viscometry detectors which are especially useful for polymers. IC for polymers can be accomplished via changing the eluent composition (gradient elution), [49–51] changing the column temperature (temperature gradient elution, TGIC), [52,53] or liquid chromatography at critical conditions (LCCC). [54] PBX 9501 contains both low molecular weight compounds (BDNPA/F, PBNA, Irganox 1010, and potential decomposition products) and high molecular weight compounds (Estane). RPLC with gradient-elution in combination with SEC is implemented as gradient-elution LC is most applicable for separation of both low and high molecular compounds. Herein we investigate the feasibility of LC × LC for studying the aging of PBX 9501 and potentially other plastic bonded explosives. To the best of the authors’ knowledge, this is the first time PBX 9501 has been analyzed via LC × LC. Experimental parameters such as sample preparation, solvent selection for gradient-elution LC, solvent gradient program, LC flow rate, and SEC column type and flow rate are investigated. Through use of RPLC × SEC, we investigate degradation of small molecules present in PBX 9501, and explore changes in the chemical composition distribution (CCD) and molecular weight distribution (MWD) for the polymer, Estane. 2. Experimental 2.1. Chemicals Methanol (HPLC Grade) and 1,2-dichloroethane (HPLC Grade) were purchased from Sigma-Aldrich. Acetonitrile (LC-MS Grade) was purchased from Honeywell. Non-stabilized tetrahydrofuran (HPLC Grade) was purchased from Fisher Chemical. Polystyrene standards were obtained from Polymer Laboratories (Agilent Technologies, Palo Alto, CA, USA). N-phenyl-2-napthylamine (PBNA) and Irganox 1010 were purchased from Sigma-Aldrich. Bis(2,2dinitropropyl) acetal (BDNPA) and Bis(2,2-dinitropropyl) formal (BDNPF) were manufactured at the Naval Propellant Plant (Indian Head, Maryland). The BDNPA/F used was manufactured in 1992. Two samples of PBX 9501, one reference material, and one sample which has been oxidatively aged were used to investigate the viability of LC × LC for studying aging in PBX samples. NOTE: PBX 9501 is a high explosive and should only be handled in an explosives rated facility. 2.2. Sample preparation For SEC optimization, a polystyrene standard composed of 7110 kDa, 10 0 0 kDa, 218 kDa, 30 kDa, and 580 Da was prepared at a concentration of 5 mg/mL in THF. A standard for the LC-PDA and LC × LC-PDA analysis composed of Irganox 1010, PBNA, and BDNPA/F was prepared at a nominal concentration of 1 mg/mL in THF. 2.3. Instrumentation summary The SEC optimization study was carried out on a Malvern OMNISEC (Westborough, MA, USA) equipped with quad detection: refractive index, UV/Vis PDA, multi-angle light scattering, and viscometer. Only the UV/Vis data was used to calculate column/column set efficiencies, but in principle any of the detectors could be used to calculate the efficiency. For all SEC analyses (optimization and LC × LC), non-stabilized THF was used as the mobile phase. LC–UV/Vis and LC × LC–UV/Vis were performed on a Prominence HPLC system (Shimadzu, Kyoto, Japan), consisting of a CBM-20A controller, three LC-20AD pumps, a DGU-20A5 R degasser, two CTO-20AC column ovens, a SIL-20AC autosampler, and a

Please cite this article as: C.E. Freye, C.J. Rosales and D.G. Thompson et al., Development of comprehensive two-dimensional liquid chromatography for investigating aging of plastic bonded explosives, Journal of Chromatography A, https://doi.org/10.1016/j.chroma.2019. 460580

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Fig. 1. Solubility of PBX 9501 in various solvent/solvent systems was compared using SEC and HPLC. (A) SEC separation of PBX 9501 for the various solvent/solvent systems. DCE, DCE in combination with another solvent, and THF were found to sufficiently dissolve the Estane. (B) HPLC separation of PBX 9501 for the various solvent/solvent systems. (C) Enhanced view of (B) at the elution window of BDNPA/F. (D) Enhanced view of (B) at the elution window of PBNA. (E) Enhanced view of (B) at the elution window of Irganox.

SPD-M20A detector. For all HPLC analyses water and non-stabilized THF were used as the mobile phase. 5 μL of sample were injected for both the LC and LC × LC analyses. The first and second dimension columns were connected by one two-position, tenport switching valves with micro-electric actuator VICI model C2H20 0 0EUH (Valco Instruments Company Inc., Houston, TX, USA) fitted with two 100 μL stainless steel loops. For the LC-PDA and LC × LC-PDA, wavelengths 190–450 nm were collected at rate of 6.25 Hz. The LC-PDA, LC × LC-PDA, and switching valve were controlled by the Shimadzu Labsolution software (Version 5.7.3). Data analysis and visualization was performed in MATLAB R2018b (The Mathworks, Inc., Natick, MA, USA). 3. Results and discussion 3.1. Sample preparation Traditionally, analysis of aged PBX 9501 is carried out using two (or more) analytical methods. As previously mentioned, SEC is the most common methodology for measuring changes to the polymer Estane [4,7,10,11,17], and HPLC for analysis of the small molecules

[22]. Historically, two different preparation methods have been used, one for the SEC analysis and another for the HPLC analysis. As part of developing a LC × LC methodology for analyzing aging of PBX 9501, a single preparation method was developed which would be applicable for both SEC and HPLC analyses. Five different solvent or solvent combinations were investigated. ∼250 mg of PBX 9501 was dissolved in 3 mL of solvent/solvent system. When two solvents were used, equal volumes of each were used (i.e. 1.5 mL of each solvent). Fig. 1A shows the SEC analysis of PBX 9501 for Estane. DCE, a combination of DCE and another solvent, or THF were found to sufficiently dissolve the Estane. The solvent or solvent combinations that yielded a measureable amount of Estane were then analyzed via HPLC (see Fig. 1B). Peak areas of BDNPA/F, PBNA, and Irganox were compared for all the different solvent/solvent combinations (see Fig. 1C–E). All the solvent/solvent combinations that were tested on the HPLC yielded roughly equal amounts of BDNPA/F, PBNA, and Irganox and thus were deemed acceptable solvents/solvent combinations for preparation of PBX 9501. THF was chosen as the preferable solvent as it has a lower UV cutoff and the use of THF requires only two solvents to be implemented for the LC × LC system (water and THF).

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Table 1 Summary of the 12 SEC columns studied for the second dimension of an LC × LC separation of PBX 9501. The columns are listed in order of their separation efficiency as a function of time (Rspec /min). The PLGel Mixed C had the highest Rspec /min with the second shortest separation time and thus was chosen as the best column. Column/Column Configurations

Column Dimensions

Exclusion limit/Penetration limit

Separation Conditions

Separation Time (min)

Rspec

Rspec /min

1 × PLGel Mixed C 1 × T6000M 1 × PLGel Rapid M 1 × PLGel MiniMix A 2 × T6000M 1 × PLGel 5 μm 103 A˚ 1 × PLGel 5 μm 105 A˚ 1 × PLGel Mixed A 1 × PLGel Polypore 1 × Styragel HT 6E 1 × Acquity 1.7 μm 200 A˚ 1 × Acquity 2.5 μm 450 A˚ 1 × PLGel MiniMix D 1 × PLGel MiniMix C

300 mm × 7.5 300 mm × 8.0 150 mm × 7.5 250 mm × 4.6 600 mm × 8.0 600 mm × 7.5

mm mm mm mm mm mm

2000,000/200 20,000,000/∼1000 2000,000/200 40,000,000/2000 20,000,000/∼1000 1700,000/500

1.0 mL/min 1.0 mL/min 1.0 mL/min 0.3 mL/min 1.0 mL/min 1.0 mL/min

@ @ @ @ @ @

60 °C 60 °C 60 °C 60 °C 60 °C 60 °C

3.7 4.7 2.2 4.0 9.3 11.9

0.0497 0.0619 0.0256 0.0423 0.0980 0.0664

0.0135 0.0131 0.0119 0.0109 0.0105 0.0100

300 mm × 7.5 300 mm × 7.5 300 mm × 7.8 300 mm × 4.6

mm mm mm mm

40,000,000/2000 2000,000/200 10,000,000/5000 1500,000/10,000

1.0 mL/min 1.0 mL/min 1.0 mL/min 0.5 mL/min

@ @ @ @

60 °C 60 °C 60 °C 60 °C

3.7 4.6 3.3 3.9

0.0356 0.0405 0.0238 0.0193

0.00958 0.00876 0.00726 0.00500

400,000/200 2000,000/200

0.3 mL/min @ 60 °C 0.3 mL/min @ 60 °C

4.2 5.3

0.0130 0.0150

0.00309 0.00213

250 mm × 4.6 mm 250 mm × 4.6 mm

Table 2 The influence on molecular weight (Mw ) on coefficient of diffusion (D) and subsequently optimum linear flow velocity (u) are shown. Optimum volumetric flow is shown for various column internal diameters. For chosen column, a flow rate of 0.05 mL/min was implemented. Molecular Weight (MW)

D (m2 /s)

100 500 1000 10,000 100,000 1,000,000

1.5 7.0 5.2 2.1 9.3 4.2

× × × × × ×

10−9 10−10 10−10 10−10 10−11 10−11

u (mm/s)

F (mL/min) 3.0 mm I.D.

F (mL/min) 2.1 mm I.D.

F (mL/min) 1.0 mm I.D.

2.0 0.93 0.69 0.28 0.12 0.056

0.68 0.32 0.23 0.094 0.043 0.019

0.33 0.16 0.12 0.047 0.021 0.009

0.076 0.035 0.026 0.010 0.005 0.002

3.2. SEC analyses: second dimension Following the guidelines Schoenmakers established for designing an LC × LC system [55], it was elected to focus on the second dimension first. Twelve different SEC column/column configurations were compared using a polystyrene standard (see Experimental). A small injection volume, 5 μL, was implemented to ensure that injection band broadening would not significantly influence measured results as the columns had various internal diameters. All columns were run at their optimum flow rate with an oven temperature of 60 °C (maximum of instrument). Numerous researchers have shown that SEC at elevated temperatures can decrease runtimes while keeping resolution constant [43,56–58]. To characterize the performance of each column/column configuration, several different metrics were calculated: separation time (in minutes), specific resolution, Rspec , and specific resolution per unit time (Rspec /min). The separation time is defined as the time between the elution of the first and last peak. Specific resolution is defined as:

Rspec =

0.25

|Slope of calibration curve| ∗ Wb /4

where Wb is width at base of a specific peak. The slope of the calibration curve was measured only using the polystyrene standards that fell within the linear portion of the calibration curve (i.e. between the exclusion limit and penetration limit of the column). Table 1 shows 12 column/column configurations that were deemed most applicable as a second dimension for LC × LC. The Rspec in Table 1 was the average Rspec of all the peaks that fell within the linear portion of the calibration curve. An unexpected result was observed with the Acquity column set as the smaller particles sizes should have improved specific resolution relative to the other column/column sets tested. It is theorized that the polystyrene standards may have interacted with the unbonded stationary phase resulting in unwanted adsorption [44]. An Agilent PLGel Mixed C (300 mm × 7.5 mm) was chosen as the best option

for the second dimension for LC × LC. It had the highest Rspec per unit time, a short separation time, and a MW separation range (exclusion and penetration limit) which was most appropriate for the application of analyzing aged PBX 9501. Furthermore, the large diameter of the column allows for larger injection volumes from the modulator without a significant loss in resolution; [55] however, a negative side effect is a larger amount of solvent used as compared to a smaller internal diameter column. 3.3. HPLC analyses: first dimension While selection of the second dimension column was relatively straightforward mainly due to the lack of different column combinations and simplicity of SEC, evaluation of HPLC was not as straightforward. As previously mentioned, separation of high molecular weight species in LC × LC is normally accomplished using a different methodology than low molecular weight species. Herein, it was chosen to perform gradient-elution LC as this is the most applicable separation mode for both high and low molecular weight compounds. The first consideration for the HPLC analyses is column selection (i.e. diameter, length, particle size) and flow rate. The chosen column was an Agilent Zorbax C18 – Extended 2.1 mm × 150 mm, 3.5 μm with 300 A˚ pores. A C18 phase was chosen based on previous work done by the group and a large pore column was implemented in order to allow the polymer, Estane, to access the stationary phase and prevent any discrimination due to molecule size. While flow rate on the first dimension for LC × LC is normally selected based on the loop volume and second dimension runtime [38,55,59], further consideration for PBX 9501 analyses is warranted due to it containing both low and high molecular weight compounds. Table 2 summarizes how molecular weight and subsequently coefficient of diffusion (Dm ) affects optimum linear flow velocity and volumetric flow rate. Low molecular weight compounds have a coefficient of diffusion around ∼1.5 × 10−9 m2 /s which translates to an optimum linear flow

Please cite this article as: C.E. Freye, C.J. Rosales and D.G. Thompson et al., Development of comprehensive two-dimensional liquid chromatography for investigating aging of plastic bonded explosives, Journal of Chromatography A, https://doi.org/10.1016/j.chroma.2019. 460580

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Fig. 2. (A) Gradient program and flow rate for constant flow rate regime. (B) Gradient program and flow rate for flow ramp regime.

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velocity of ∼2.0 mm/s. However, Estane has a molecular weight average of ∼140 kDa which translates to an optimum linear flow velocity of ∼0.10 mm/s. Two different flow regimes were chosen. First, a constant flow rate of 0.05 mL/min which corresponds to a linear flow rate of ∼ 0.3 mm/s for a 2.1 mm inner diameter column was implemented. The constant flow rate of 0.05 mL/min was chosen as balance between minimizing longitudinal diffusion caused by too slow of flow for small molecules and minimizing mass transfer caused by too fast of flow for the polymer. The second flow regime was a flow rate ramp which started at 0.3 mL/min and decreased to 0.05 mL/min (∼2.0 mm/s → ∼0.3 mm/s). This is possible for the separation of PBX 9501 because most of the small molecules elute (i.e. BDNPA/F, PBNA, Irganox) before the polymer, thus it is possible to optimize for the two different areas of the chromatogram. For both flow rate regimes, a two-step gradient was implemented as this allowed for better separation of Estane and the oven temperature was set to 40 °C. Fig. 2 shows the flow regimes and gradient ramps for the two different separation regimes. A potential issue when using a strong solvent to dissolve PBX 9501 (i.e. THF) in conjunction with a gradient ramp that starts with a low organic modifier (see Fig. 2), is breakthrough [48]. However, this can become overcome by using small injection volumes (i.e. 5 μL) and lower column temperatures as is implemented herein. Fig. 3A shows the HPLC separation for a library sample of PBX 9501 at a constant flow rate and Fig. 3B shows an enhanced view of the separation. The average width at base, Wb , is ∼2.17 min. Fig. 3C shows the HPLC separation of a library standard of PBX 9501 using the flow ramp and Fig. 3D shows an enhanced view of the separation. The average Wb is significantly lower at ∼1.33 min. It was elected to use the constant flow rate because it produced peaks that were wider which would minimize under sampling and maintain resolution when implemented as the first dimension in

Fig. 3. (A) RPLC separation of a library standard of PBX 9501 using the constant flow rate regime. (B) Enhanced view of (A) to show separation of the small molecules. (C) RPLC separation of a library sample of PBX 9501 using the flow ramp regime. (D) Enhanced view of (C) to show separation of the small molecules.

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Fig. 4. (A) RPLC × SEC separation of a library standard of PBX 9501. (B) RPLC × SEC separation of an aged sample of PBX 9501. The major constituents are label in each chromatogram.

an LC × LC separation [60]. However, an analyst could use the flow ramp regime and place an additional detector at the end of the first dimension before the valve in order to measure the narrower peaks produced.

technique can be implemented as a quality control technique for acceptance on new PBX 9501 batches.

3.4. LC × LC: coupling RPLC to SEC

The feasibility of performing online LC × LC on plastic bonded explosives has been explored by investigating the aging of PBX 9501. The method incorporates gradient RPLC on the first dimension followed by SEC on the second dimension. A variety of SEC columns were investigated for their application of evaluating the MW changes in Estane. Because PBX 9501 has both small molecules (BDNPA/F, PBNA, Irganox, and potential degradation products) and large molecules (Estane), it can be difficult to optimize RPLC conditions. Column selection and flow rate for RPLC were discussed. Finally, LC × LC analysis was performed on a library standard of PBX 9501 and an aged sample of PBX 9501. Further refinement of this technique is warranted, and a comprehensive study is needed in order to determine the relationship between the chemical information measured by LC × LC and the mechanical properties of PBX 9501.

The HPLC separation and SEC separation were coupled together in order to investigate the advantage and feasibility of performing these separations in tandem. The first dimension RPLC separation was performed using the constant flow rate regime. No modifications were made to the method. The SEC separation flow rate was increased to 4 mL/min from 1 mL/min (optimum flow rate) in order to allow for a modulation period of 1.6 min. A flow rate of 4 mL/min was selected as this reduced the separation time to ∼1.5 min (from 3.7 min at 1 mL/min). However, by increasing the flow rate the efficiency of the column is reduced by ∼30% which matches the theoretical predictions of the van Deemter equation for a 5 μm particle column. The SEC column was housed in a separate oven which was held at 60 °C (same as the SEC studies). Fig. 4 shows the RPLC × SEC analyses of a library standard of PBX 9501 and an aged sample of PBX 9501. The chromatograms have been registered on the second dimension to remove the dead time. Minor fluctuation in flow caused by the valve actuating can be seen at 0 and 1.6 min on the second dimension. The concentrations (weight percent) for the compounds BDNPA/F, Irganox and PBNA were calculated for both samples. The aged PBX 9501 sample showed a decrease in the concentration of all three analytes with Irganox and PBNA decreasing to below the limit of detection for the aged PBX 9501 sample. Further comparison of the small molecules present from degradation could be performed, but are outside the scope of this work presented and are excluded for brevity. We now turn our attention to address the observed changes in the polymer, Estane. For the library standard, most of the concentration of the polymer is located at ∼4.8 h on the first dimension and has a molecular weight (Mw ) of ∼72 kDa. The “tail” located between 3.5–4.5 h on the first dimension has an average molecular weight of ∼50 kDa. It is believed that this tail is due to the library standard having aged but more work needs to be done to confirm this theory. The aged PBX 9501 polymer appears at 2.8–3.8 h and has an average molecular weight of 37 kDa. Molecular weight of Estane is known to strongly influence the mechanical properties of PBX 9501, but to the best of authors’ knowledge, there exists no model which describes the mechanical properties of Estane degraded via different mechanisms (e.g. hydrolysis and/or oxidation) [10]. The chemical composition of PBX 9501 gleaned from LC × LC should provide comprehensive information allowing for the creation of a complete model. Furthermore this

4. Conclusion

Declaration of Competing Interest None. Acknowledgments This work was supported by the US Department of Energy through the Los Alamos National Laboratory. Los Alamos National Laboratory is operated by Triad, LLC, for the National Nuclear Security Administration of U.S. Department of Energy (Contract 89233218CNA0 0 0 0 01). The authors wish to thank Robert J. Houlton and Adelaida C. Valdez for the aged PBX 9501 sample. References [1] A. Provatas, Energetic polymers and plasticisers for explosive formulations – a review of recent advances, (20 0 0). [2] D. Yang, R. Pacheco, S. Edwards, K. Henderson, R. Wu, A. Labouriau, P. Stark, Thermal stability of a eutectic mixture of bis(2,2-dinitropropyl) acetal and formal: Part A. Degradation mechanisms in air and under nitrogen atmosphere, Polym. Degrad. Stab. 129 (2016) 380–398, doi:10.1016/j.polymdegradstab.2016. 05.017. [3] D. Yang, R. Pacheco, S. Edwards, J. Torres, K. Henderson, M. Sykora, P. Stark, S. Larson, Thermal stability of a eutectic mixture of bis(2,2-dinitropropyl) acetal and formal: part B. Degradation mechanisms under water and high humidity environments, Polym. Degrad. Stab. 130 (2016) 338–347, doi:10.1016/j. polymdegradstab.2016.06.007. [4] M.R. Salazar, J.D. Kress, J.M. Lightfoot, B.G. Russel, W.A. Rodin, L. Woods, Experimental study of the oxidative degradation of PBX 9501 and its components, Propellants Explos. Pyrotech. 33 (2008) 182–202, doi:10.10 02/prep.20 070 0272.

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