APPLICATION OF POLYMER SYSTEMS TO THE DETECTION AND RETENTION OF EXPLOSIVES

Obtaining, handling, and storing of explosives, especially primaries such as triacetonetriperoxide (TATP), presents significant obstacles to instrument manufacturers and K-9 trainers. Microencapsulation techniques were used to trap TATP in a plastic matrix rendering it safe to handle, store at room temperature, and release by heating. Detection of most explosive vapor is a challenge for current instrumentation. This work provides a study of polymer systems for the preconcentration of explosive vapor for use with portable explosive detection technologies, specifically molecularly imprinted polymers (MIPs).


Introduction
Triacetone triperoxide (TATP) is a primary explosive with a high room temperature vapor pressure (0.052mm Hg) [1]. The high sensitivity and vapor pressure make it impractical for military or industrial use. It is quite easy to synthesize, making it a favored explosive of terrorist organizations and thrill-seeking amateur chemists around the world. The detection of TATP is thus of great interest to military and security agencies. Unfortunately, as a primary explosive, it is highly hazardous to handle. Despite this, two communities require this explosive or at least the explosive scent: bomb sniffing dogs and companies manufacturing trace explosive detection instruments. For the manufacturers, obtaining, handling, and storing any explosive is a significant obstacle; thus, it is our intention that the protocols developed in this study can be transitioned to other energetics materials.
The approach discussed herein is encapsulation of the explosive with sufficient polymer that it is subject to combustion rather than to detonation.
Microencapsulation procedures distinguish between two types of microparticles: 3 microcapsules and microspheres. Microcapsules have a discrete polymer shell which surrounds either pure core material or a microsphere-like matrix of polymer and core material. Microcapsules are capable of higher loadings of core material. A microsphere is a polymer matrix with the desired core material dispersed throughout the polymer (Fig. 1). They can have maximum theoretical loadings of up 50% core material [2].

<Figure 1>
There are numerous physical methods used for microencapsulation [3][4][5][6]. Pan coating is a well-established encapsulation process that is still widely used in the pharmaceutical industry [3,6]. Particles of core material are sprayed with solubilized polymer while they are tumbled in a "pan." The pan is usually heated to facilitate evaporation of organic solvents. The polymer coats the particles as they rotate in the pan and the solvent evaporates, leaving a polymer shell. Particle size is controlled by size of core particles, pan rotation speed, and addition rate of solubilized polymer.
Disadvantages of the pan coating approach include potential for aggregation of particles and adherence of particles and aggregates to walls of the pan as the polymer coating hardens. Because TATP is sensitive to explosive initiation from shock, friction, and heat, tumbling inside a heated container is not prudent.
Fluidized bed coating, also called Wurster coating [3], is similar to pan coating except air jets replace the tumbling pan. Air currents move the core material past a nozzle that sprays them with the solubilized or molten polymer. The spraying nozzle can either be tangential to, above, or below the substrate. The position of the nozzle changes the performance of the coating [3]. As with pan coating, this method applies 4 to solid core material. Particle size is controlled by the original size of the core particles and polymer coating rates by spray conditions. It is prone to the same disadvantages as pan coating.
In spray drying, solid core material is mixed with solubilized polymer in a reservoir and sprayed out a nozzle into a collection chamber [3][4][5]. The chamber is large enough to allow the solvent to evaporate before the particles reach the bottom.
When the solvent evaporates, the core material is left with a solid polymer shell.
Spray cooling is a similar technique [3][4][5] where a molten, rather than a solubilized, polymer is used. The polymer cools and hardens as the droplet surrounding the core material falls into the collection chamber. In both methods the particle size is controlled by the type of nozzle used. For purposes herein, this method would be limited by the number of polymers that melt at temperatures safe for handling TATP.
The solvent evaporation technique uses emulsions and volatile organic solvents to make microspheres rather than microcapsules [2,7,8]. The polymer and the core material are dissolved in a volatile organic solvent which becomes the dispersed phase. All three, the polymer, the core material, and the solvent, must be immiscible with a second liquid phase, which will be used as the continuous phase. Using a surfactant and rapid stirring, an emulsion is used to create droplets which harden into solid microspheres as the dispersed phase solvent evaporates. The surfactants, stirring speed, rate of dispersed phase evaporation, and amount of solvent used in the dispersed and continuous phase all affect particle size [7,8]. Particle size often varies between one to two orders of magnitude inside a batch.

5
Co-acervation uses emulsions and solubility to make microcapsules. It is most commonly performed using a water/oil mix with the oil being the core material [9].
The polymer is dissolved in water and oil is emulsified into the aqueous solution. A change in conditions (temperature, pH, addition of a salt, addition of an anti-solvent) lowers the solubility of polymer in water causing the polymer to reform [3,10,11].
The reforming polymer collects at the surface of the oil droplets, forming a shell.
Particle size is controlled by stirring time, size of emulsion droplet, and the changing solubility of the polymer in the solvent [3].
Supercritical carbon dioxide is showing considerable promise as a means of promoting microencapsulation. Carbon dioxide acts as an organic solvent and solvent removal is accomplished by simply venting the pressurized chamber. The rapid expansion of supercritical solution (RESS) and the gas anti-solvent (GAS) methods as applied to microencapsulation have been recently reviewed [3]. RESS is similar to spray drying; polymer dissolved in supercritical carbon dioxide is sprayed at atmospheric pressure with the core material forming particles as the carbon dioxide flashes off. Nozzle dimensions determine particle size. The GAS method uses supercritical fluid to co-precipitate the core and shell material from solution. The particles formed in GAS would be similar to microspheres in core/shell material distribution although it is unclear whether actual spheres would form rather than random shapes.
Co-extrusion is a continuous process that encapsulates liquid samples [4]. A syringe pump with two feeds is used, one with coating material and the other with core material. The coating line surrounds the core material line, and the pump is adjusted 6 to form droplets of core material in the center with the coating material surrounding the outside. As with spray cooling, the drops fall from a sufficient height to allow the polymer shell to harden before impact. It may be possible to use molten, rather than solubilized, polymer in this system. Particle size is controlled by the flow rate of the pump and the nozzle dimensions. This method allows for highly repeatable particle sizes.
Lastly, a chemical method called interfacial polymerization is a batch process where the microspheres are created at the interface of an emulsified solution [3,4,12].
A monomer is dissolved in the continuous phase of the emulsion, and a second monomer is dissolved in the dispersed phase along with the core material. The emulsion is stirred to make droplets, and a cross-linker is added to start polymerization. The copolymer forms at the interface of the continuous and dispersed phases, making a shell around the dispersed phase droplet. This method requires that both the shell (i.e. polymer) and the core material (i.e. TATP) be solvated in the dispersed phase. Residual odors associated with unreacted monomers and short chain polymers are of concern. The desired product of this study should be free of odors other than the explosive (i.e. TATP).
After review of the literature, the solvent evaporation technique was selected.
This technique required no special equipment and involved limited heating, a major concern with encapsulation of energetic materials. This technique resulted in microspheres, rather than microcapsules. The resulting lower loading of the core material in microspheres was considered advantageous for reducing the sensitivity of TATP, making the microspheres safer to handle. 7

Experimental Section
A pre-made polymer (0.5 to 1 g) was dissolved in 7 to 10 mL of solvent, usually dichloromethane (DCM 100 mg of microspheres were added to a ~11 mL headspace vial, which was sealed and placed in an oven. The oven was rapidly heated to 150°C, and the vial was allowed to equilibrate at temperature for 1 minute. The vial was removed and 1 mL of vapor manually injected into the GC. Before the syringe was reused, it was cleaned with three rinses of volatile solvent, initially acetone and later pentane. The syringe barrel was then baked at ~90°C for ~10 minutes, while the plunger dried in air.
The method for the most headspace runs on the Agilent system was as follows.
The inlet was set to 110ºC splitless injection with a 20 mL/min purge at 0.5 minutes.
The pressure was 1.5 psi for 3 minutes, ramped 10 mL/min to 2.5 psi and held for 4 minutes, ramped 10 mL/min to 1.5 psi and held for 3 minutes, then maintained at 1.5 psi for a 3 minute post-run. the base, the less the explosive power was judged to be. [13,14].
Detonation tests were performed on a large scale using 3" long and ¾" diameter stainless steel pipes. The microsphere synthesis was scaled up to 5g of polycarbonate (PC) and 2.5g of triacetonetriperoxide (TATP) to make sufficient microspheres for this test. The yield for this scale up was ~5.2g of microspheres with average loading of 13.8% TATP by mass. The pipes were lined with anti-static bags which were cut about 1½" above the top of the pipe and formed to the interior of the pipe using cardboard tubes to tamp the bags down. The threads of the pipe were covered with masking tape and then the exposed anti-static bag was cut and folded down over the threads to prevent loose material from falling into the threads. A 0.31 inch hole was drilled through the bottom end cap of the pipe. The TATP and microspheres were then put into separate pre-weighed plastic pop-top containers and weighed again. This allowed easy filling of the pipes at the range and the mass used could be determined later. At the range the pipes had a detonator inserted through the bottom of the pipe, going through the plastic bag to ensure good contact with the contents of the pipe once filled. The pipes were filled using a paper funnel and ziptied to a wooden stake that was placed inside a cardboard concrete form inside a 55 gallon steel drum. The drum was filled with sand on the bottom and around the concrete form. A wooden dowel was placed on top of the opening in the concrete form and sand bags were placed on top of that. Following the range safety guidelines, the detonator was initiated from a safe distance, and once the all clear was given the remains of the pipe were recovered using magnets to sweep the sand. Three shots were done: TATP (5.26 g), PC+TATP (8.64 g), and sand. The remains of the pipe were recovered using magnets to sweep the sand. proceeded as with polystyrene. Figure 5 shows release of TATP began at approximately 88°C and continued for 100 degrees at the given scan rate. The IR spectrum of the off-gas indicated pure TATP (Fig. 6) (Fig. 7). Unfortunately, the TATP release temperature was close to the decomposition temperature of TATP as observed by DSC ( Fig. 8). This, along with the occurrence of extra peaks in the off-gas IR (Fig. 9 Fig. 11). The IR off-gas from the PEI-TATP microspheres was strong but not pure TATP. As with the PSf-TATP microspheres, the IR spectra appeared to be a mix of TATP and TATP decomposition products. The improved signal strength shows even better correlation between the unknown spectra and that seen in TATP decomposition (Fig. 12 The IR of the off-gas showed that along with the DCM, TATP was released as well.

Results and Discussion
Furthermore, as soon as the temperature was ramped (1°C/min) after the baking period, TATP loss resumed (Fig. 13). To determine if this low-temperature release of TATP began even lower than the "bake out" temperature of 60°C, the PLGA-TATP microspheres were heated from 40°C to 250°C at a constant rate of 5°C/min. TATP release was first observed at ~46°C. This release could be a result of TATP on the surface of the microsphere prior to release of TATP inside the microspheres.
Nevertheless, PLGA was discarded as a potential shell material because the polymer was not stable at room temperature, requiring refrigerated storage. exhibited the same decomposition problems seen with PEM-TATP: polymer decomposition overlapped with TATP release. A peak unique to TATP in the IR was used to roughly determine the release temperature ( Fig. 21), but PMMA was discarded as a shell material due to its decomposition overlapping with TATP release.

<Figure 21>
<Figure 22> As shown in Table 2, the temperature of initial TATP release was determined by ramping samples slowly (2°C/min) to 300-320°C. The release "max" temperature was determined as the peak of the first derivative of TGA trace. Some polymer microspheres had IR signatures for off-gas indicating pure TATP, while others suggested TATP plus TATP decomposition products. In polystyrene the release of TATP started at 77°C. The release in polystyrene was near to the glass transition temperature (Tg) of polystyrene (109°C) and the melting point of TATP (95°C) [15].
The release of TATP vapor was assumed to be related to one or both of these temperatures. Supporting glass transition temperature was release observed with PVBVAVA-TATP microspheres. The TATP vapor appeared substantially below its melting point. However, with polycarbonate, TATP release began at 88°C, which was far below the glass transition temperature. Furthermore, with polysulfone and polyetherimide, both with high glass transition temperatures, (190°C and 220°C, respectively) TATP release was not observed until 139-140°C, well above the melting point of TATP but below the Tg of either polymer. The IR of the TGA off-gas and independent DSC runs of PSf-TATP and PEI-TATP shed light on the release mechanism. In both cases the off-gas was clearly a mixture of TATP and TATP decomposition products. The DSC of the microspheres showed the "max" release of TATP was at approximately the same temperature as the temperature of the DSC exothermic maximum.
To examine the factors governing TATP release, in addition to changing the polymer matrix, the core material was also changed. Diacetone diperoxide (DADP) was encapsulated in PC and PSf. The six-membered ring with diperoxide functionalities is chemically very similar to the nine-membered TATP ring with triperoxide functionalities. It is a possible side product of TATP synthesis with a melting point of 133°C [15]. DADP was encapsulated using polycarbonate and polysulfone and the release temperature determined by TGA. The results are shown in Table 2.

<Table 2>
In selecting the preferred polymer for encapsulation of the explosive, ideal candidates were subjected to three criteria as follows: 1) the polymer/explosive combination must meet solubility constraints amenable to our preparation method; 2) the desired polymer must have long-term shelf-life; 3) the release of the core material (explosive) must be pure (type A release), not contaminated by release of polymer or polymer decomposition products (type B) or by explosive decomposition products (type C) ( Table 2). The solvent evaporation method requires that both the polymer and the core material (the explosive) be soluble in readily removable solvent, i.e. dichloromethane or chloroform, and be insoluble in a second solvent which is immiscible with the first, i.e. water. For example, using toluene or ethyl acetate produced no useful spheres with polystyrene due to difficulty removing the solvent at room temperature. While heating can be used to aid in solvent removal, it has been shown to decrease encapsulation efficiency [7,8]. For polyvinylchloride, Nylon 6/6, and polyurethane, no suitable solvent system could be found. However, this does not rule out the possibility that suitable microspheres could be made by one of the other methods reviewed. Polymer instability at room temperature ruled out PLGA. The presence of polymer decomposition products in the microsphere off-gases ruled out the use of PMMA as shell materials, while the evidence of TATP decomposition in the microspheres of PSf and PEI discouraged use of those polymers. After the above exclusions, the acceptable polymers were polystyrene, poly-4-methylstyrene, and polycarbonate.
The purity of the evolved vapor released by the microspheres was confirmed by headspace analysis using GC/MS. Headspace was examined first and led to several changes to the microsphere production process. The headspace of PS microspheres was compared to that of a headspace vial that was crimped shut with nothing inside but air. This air sample control revealed background from the syringe and vial associated with the heating cycle used for the microspheres. Contaminants (Fig. 23) consisted of ethylbenzene, styrene, and tetramethylbutanedinitrile. As discussed in the experimental section, the source of the tetramethylbutanedinitrile was the PVA. A new source of PVA lacking the contaminant was found. The styrene and ethylbenzene were thought to be residual in the polystyrene. To remove these contaminants, "baking" of empty polystyrene microspheres was required (see experimental section).
This approach successfully removed the remaining contaminant peaks from PS microspheres, but the process more than doubled the effort required to make the spheres.

<Figure 23>
In contrast to polystyrene, polycarbonate was devoid of contaminants in the normal headspace. Polycarbonate was considered a more desirable polymer than polystyrene because with little effort pure TATP vapor was achieved. Note that all microspheres were subject to gentle heating to remove DCM or any surface TATP.
The TATP release temperature near 90°C was sufficiently high to allow bake off of contaminating solvents from the microspheres while not triggering the release of 20 TATP. Nearly all TATP was released from the PC-TATP by 170°C (Fig. 25) which was just below where TATP decomposition began between 170°C and 180°C.

<Figure 24>
<Figure 25> As shown in Figure 25, once TATP released from the microspheres reached a maximum at a given temperature, the weight loss gradually declined. The trace ( Fig.   25) of PC-TATP microspheres suggests that this release behavior can be manipulated.
Heating to 90°C releases a certain amount of TATP, but the release slows considerably over an hour. A new rapid release can be obtained if the temperature is raised to 100°C. For the application desired, TATP generation, Figure 25 shows that the microspheres could be used serially. Sufficient TATP for training could be released at a given temperature and at a later time more TATP could be released by heating to a higher temperature.
Repeatability of the microcapsule loading was tested by comparing ten replicate batches of PS and PC microspheres. The results showed little difference in loading between PC and PS with slightly less loss in mass from the blank of polycarbonate. These microspheres were baked at 80°C for 24 hours; later GC/MS studies indicated 48 hours was required for complete removal of DCM.

<Table 3>
Using TGA-IR to check loadings of the microspheres shelf-stability was investigated. Samples were stored at room temperature after initial experiments revealed that TATP was retained in the microspheres until released by heating.
Samples of polystyrene, polysulfone, and polycarbonate spheres were left at room 21 temperature for one to two years. The loss of TATP from the microspheres over time at room temperature was negligible as the data in Table 4 shows.

<Table 4>
The SSED and pipe tests showed similar results, with PC-TATP microspheres performing similarly to a blank. In the SSED test 1g of 12.9% by weight PC-TATP microspheres failed to damage the shell casing aside from opening a hole in the side.
This damage was similar to that seen in an empty shell with only the detonator inside.

Introduction
Molecularly imprinted polymers (MIPs) are copolymer systems designed to bind specific analytes, akin to a man-made antibody. Specific binding sites are created by coordinating the target analyte (termed "template") to a functional group on a monomer (termed "functional monomer"). Once the functional monomer and the template are bound or coordinated in solution, the monomer is polymerized using a second monomer ("structural monomer") to bridge between functional monomers.  [4].
A way to selectively pre-concentrate explosive vapors from a shipping container or a room may dramatically improve the ability of current instruments to detect trace amounts of explosive vapors. In addition, possibilities for novel explosive detection techniques utilizing MIPs are wide-ranging.

Experimental Section
MIP Syntheses: Preliminary imprinting work was done following the work of Ellen Holthoff [5]. The functional monomer was aminopropyltriethoxysilane (APTES) and the structural monomer was methyltriethoxysilane (C1 TriEOS). These along with 1M hydrochloric acid were mixed and stirred. A solution of TNT was added and the whole mixture vortexed for 30 seconds. The resulting red solution was then spin-coated onto surfaceenhanced Raman spectroscopy substrates. Later, a similar formulation, as outlined in the work of Xie, was tested [6]. The only major differences were the ratio of reactants, the use of sodium acetate as the catalyst rather than hydrochloric acid, and the use of various substrates instead of spin-coating. In early formulations, the MIP was coated on various substrates: glass wool, sand, silica gel, and steel wool. While the coating was eventually successful on glass and steel wool, the synthesis was changed to bulk, also called block, polymerization to allow easier template extraction and analysis. were similar. Tetraethoxysilane (TEOS) was used as a structural monomer for all three. PTMS was selected based on the work of Lordel et al. [7]. TMOTFS was selected because of its use as a stationary phase in chromatography of explosives and TEOTES was also a good medium for explosives.
Solid TNT or hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) was weighed into a test tube; the monomers were added by syringe in the appropriate ratios along with a stir bar. Initially minimal solvent (1 mL of acetonitrile and isopropanol or methanol) was added to dissolve the explosive. The reaction was then mixed by vortexing. The ammonium hydroxide catalyst was then added by syringe and mixed again by vortexing. The homogeneous mixture was put in a water bath at 60°C with magnetic stirring. It was later discovered that the Meisenheimer complex of TNT which formed on the addition of the ammonium hydroxide was miscible in the monomers after sufficient reaction time, so vortex mixing was not required or helpful. Thereafter, the test tube with only the monomers, catalyst, and TNT was placed in a 60°C water bath with magnetic stirring. In either case the sol gel was allowed to cure until hardened.
This was usually overnight but could take up to a week under some reaction conditions. Initial reactions were allowed to react at room temperature, but reaction times were unacceptably long. Once samples were determined to be solid by visual inspection and probing with a spatula, they were removed from the bath and placed in an oven overnight at 120°C. Controls and MIPs were baked separately, and the oven was purged for an hour at 350°C to ensure that controls were not exposed to TNT vapor. The block polymers were then ground into a powder with a pill crusher before extraction.
Some PTMS based MIPs were instead coated onto steel wool. To do this the above procedure was changed to allow for dip coating of steel wool. To each reaction test tube 3 mL of methanol and acetonitrile were added before the ammonium hydroxide catalyst. After addition of the catalyst, the reaction was mixed using vortexing. A pre-weighed sample of steel wool, sometimes pre-treated with a UV ozone generator, was then submerged in the now homogenous reaction solution for 30 minutes. The wool was then removed and placed into a metal tin in a dark room to cure for several weeks. The coating appeared to cure completely; however, pooling of the coating towards the bottom of the wool and on the surface of the tin indicated faster curing was necessary for uniform coating on the surface of the wool.
The most successful imprinting followed the work of Bunte [8]. The success of imprinting was judged by sorption experiments using UV-Vis.
A solvent system of 90% water:10% methanol was adopted for the UV-Vis

Results and Discussion
A summary of MIP performance in terms of percent TNT uptake by the MIP relative to the untemplated polymer (the control) are shown in Table 1

70
Ethanol has also previously been used as a porogen. In this work, it was added to the TEOTES reactions to slow suspected self-polymerization and encourage copolymerizing with TEOS.
Simple NMR titration experiments were performed to attempt to identify the best functional monomer and to probe its interaction with the analyte [10,11] In an effort to eliminate some of the complexities of the NMR titration study, the titration of nitromethane with PTMS was performed. This appeared to be a good starting point because of the simple nature of the nitromethane molecule and the prevalence of the -NO 2 functionality in explosives. The resulting plot of the Δδ of the methyl protons on nitromethane appears very similar to that seen for the TNT protons.

Conclusions
The  [12,13], much more involved than the simple equilibria assumed in the experiments performed. At a minimum, the interaction of the monomer with itself must be accounted for with self-titration. The complex nature of the equilibria must also be taken into consideration and volume effects compensated for or eliminated. In addition, modeling work is needed to examine possible interactions between the monomers and templates that overcome dimerization and sensitivity in the NMR.
Further development of general screening methods to evaluate potential functional monomers is needed; ideally it would give insight as to best ratios of template to monomer and easing design novel MIPs for both explosives and other compounds.