An energetic nano-fiber composite based on polystyrene and 1,3,5-trinitro-1,3,5-triazinane fabricated via electrospinning technique

Mhmoud Adelhfiz ,Ahmed K.Hussein ,Wleed F.Khlil ,Ahmed Eleih ,*

a Military Technical College (MTC),Kobry Elkobbah,Cairo,Egypt

b Safety Fuel Cycle Department,NRSRC,Egyptian Atomic Energy Authority (EAEA),Cairo,Egypt

Keywords:Electrospinning Nano-fibers 1,3,5-Trinitro-1,3,5-triazinane (RDX)Polystyrene (PS)Sensitivity

ABSTRACT Electrospinning is a simple technique used to fabricate polymeric nano-fibrous membranes.These nanofibers have found a wide range of valuable applications in the biomedical field.However,it has not been utilized with solid high explosives yet.Herein,the electrospinning technique has been used to fabricate polystyrene (PS)/1,3,5-trinitro-1,3,5-triazinane (RDX) composite nanofibers.The governed electrospinning parameters,voltage,distance from the collector,flow rate,mandrel rotating speed,time,and solution concentration,that greatly affect the morphology of the obtained nanofibers were optimized.The fabricated PS/RDX nano-fibers were characterized using scanning electron microscopy (SEM),X-ray diffractometer (XRD),and Fourier Transform Infrared (FTIR) spectroscopy.The impact and friction sensitivities of PS/RDX were also measured.The thermal behavior of the prepared composite and the pure materials were studied by the thermal gravimetric analysis technique (TGA).SEM results proved the fabrication of PS/RDX fibers in the nano-size via electrospinning.FTIR spectroscopy confirmed the existence of the characteristic functional groups of both PS and RDX in the composite nano-fibers.XRD sharp peaks showed the conversion of amorphous PS into crystalline shape via electrospinning and also confirmed the formation of PS/RDX composite.The PS fibers absorbed the heat and increased the onset decomposition of the pure RDX from 181.5 to 200.7℃ in the case of PS/RDX fibers.Interestingly,PS/RDX nano-fibers showed the relatively low impact and friction sensitivities of 100 J and 360 N respectively.These results could introduce PS/RDX nanofibrous composite in the field of explosives detection with high levels of safety.

1.Introduction

Electrospinning is a technique that has been exploited recently on a wide range to fabricate composite polymeric nanofibrous membranes simply and straightforwardly [1-4].These polymeric nano-fibers have acquired great attention through their diverse applications as biomaterials,biomedical additives for wounds and burns dressings,drug delivery,and sensors[5-8].In this technique,a polymer is suspended in a suitable solvent fabricating a viscous solution.The viscosity is a crucial factor to perform this process properly,where excess viscosity leads to the formation of particle aggregation followed by the syringe blocking and thus failure of the whole process[9,10].However,the electrospinning of the very low viscosity polymeric solutions forms drips instead of fibrous films[9,10].Polymeric solution with an optimized viscosity is fed through a 5 mL plastic syringe equipped with an 8.0 mm stainless steel needle.

As described in the schematic diagram in Fig.1,the syringe needle is connected directly to a voltage source (10-25 kV).This electric voltage is capable to overcome the surface tension of the polymeric solution,which is one of the most important factors affecting the efficiency of the electrospinning process.As the polymeric solution flows with an accurate flow rate towards a rotating mandrel,the electric voltage converts the polymeric droplets into a homogenous jet.As a result of the stepwise solvent evaporation,this polymeric jet is converted into a polymeric nanofibers which is spontaneously deposited on an aluminium foil fitted on the mandrel as a collector [11].The electrospinning efficiency together with the characteristics of the obtained nano-fibres is highly governed by a set of parameters;operating parameters and solution properties.The operating parameters involve the distance between the needle and collector,the flow rate,speed of the rotating mandrel,time of the electrospinning session,and the practical voltage.On the other hand,the solution properties involve the polymer concentration,average molecular weight,viscosity,and surface tension of the solution [12-16].

Fig.1.Schematic description of the fabrication of submicron-fibrous membranes via electrospinning.

Polystyrene (PS) is a widely known water-insoluble thermoplastic polymer.Since its discovery in 1839 by the German apothecary“Eduard Simon”[17],PS has gained an increasing demand in the industrial field in diverse applications such as injecting/extruded polymeric sheets or foams [18-21].Electrospun PS has found a wide range of valuable applications in the environmental[22-25] and energy fields [26-28].Recently,electrospun polymeric fibers have attracted attention in the field of high explosives in different aspects[29-35].1,3,5-trinitro-1,3,5-triazinane(RDX)is one of the most widely used high explosives[36,37].It can be used as a plastic explosive [38] or plastic bonded explosive with high performance [39].It has several applications in filling of shaped charges [40,41].The sensitivity of RDX is high compared with the advanced energetic explosives and oxidizers[42-45].Handling the dry pure RDX is dangerous and requires high safety regulations[46].In this work,PS has been used to adapt the sensitivity characteristics of RDX.The nano-fiber composite based on PS/RDX has not been prepared or studied yet.PS has been electrospun together with RDX to fabricate PS/RDX nano-fibers.The morphology and the crystallinity nature of the fabricated PS/RDX nano-fibers have been characterized using scanning electron microscopy(SEM),and X-ray diffraction (XRD).However,the characteristic functional groups in either pure PS,pure RDX,or PS/RDX composite nano-fiber have been identified through Fourier transform infrared (FTIR).Moreover,the sensitivities and the thermal behavior of PS/RDX nanofibers were determined.

2.Experimental work

2.1.Materials

PS with a molecular weight of 45,000 g/mol was purchased from Al-Gomhoria Company for medicines and medical supplies,Egypt.Pure RDX crystals were acquired from Eurenco,France.Acetone,dimethylformamide (DMF),dimethyl sulfoxide (DMSO),chloroform,n-hexane,and dichloromethane (DCM) were obtained from Sigma-Aldrich,Germany.All acquired chemicals were used in the experimental work as received without any additional purification.

2.2.Preparation of PS/RDX nano-fiber via electrospinning

A set of experiments have been performed to optimize the parameters that govern the fabrication of PS/RDX submicron-fibers membranes via the electrospinning technique.It was observed that the type and composition of the solvent used in the electrospinning technique have a significant impact on the efficiency of the process.The nature of the solution is greatly connected with the viscosity of the polymeric suspension.As the viscosity exceeds the optimum value,an aggregation of particles occurred followed by blockage of the syringe needle and failure of the fabrication of the targeted submicron-fibers.On the other hand,decreasing the viscosity beyond the optimum conditions leads to the formation of undesirable discontinuous drops of the suspension.Herein,a series of experiments were performed to optimize the viscosity by changing either the nature of the composition of the used solvent.Acetone,DMSO,Chloroform,n-hexane,and DCM have been used individually to dissolve PS and RDX.However,none of these solvents was able to induce the proper viscosity to form a continuous jet of the suspension.Thus,different mixtures were prepared to be used as a solvent mixture;acetone-DMF,acetone DMSO,DMFchloroform,acetone-DMSO,DMF-DCM,n-Hexane-DCM at different concentrations as follows;(1:1),(2:1),(1:2),(3:1),(1:3),(4:1),and (1:4) volumetric ratio individually.

After optimizing the solvent nature and composition,the concentration of RDX,10%,20%,40%,and 60% weight ratio of PS were optimized to obtain the maximum packing density of the RDX explosive without affecting the homogeneity of the suspension.It was found that 40% RDX (concerning PS) is the maximum RDX/PS ratio that could be utilized.Then,the polymeric suspension was filled in the syringe before applying the voltage.Different voltages were applied as follows;5,10,12,14,16,18,20,22 and 24 kV.Changing the applied voltage values was found to have a dramatic effect on the jet formation that exists from the syringe needle.Also,the flow rate was found to be a very important factor that should be adjusted carefully to optimize the product.After controlling the applied voltage,the used flow rates were changed from 2,5,10,20,30,and 50 μL/min.The optimum distance between the syringe needle and the rotating mandrel was studied by changing it(while other parameters were kept constant)as follows;8,10,12,14,16,18 and 20 cm.Finally,the rotating speed and time of the mandrel were also optimized among different utilized values;300,400,500,600 and 700 rpm during 30 min,1,2,3,4,5,and 6 h.The homogeneity of the suspension was found to be a critical parameter that could be handled carefully to achieve an optimized electrospinning process.

Based on the preceding optimizing trials results,PS particles were dissolved in a solvent mixture composed of DMF and chloroform with a vol/vol ratio of 3:1.The solution was kept under continuous stirring for 6 h at room temperature.Then,40% w/w RDX particles concerning PS were added to the mixture and stirred for another 6 h followed by ultrasonication for 15 min before the electrospinning step.Afterward,the PS/RDX mixture was injected into a 5 mL syringe equipped with a 0.8 mm stainless steel needle.A fixed voltage of 22 kV was applied via the syringe needle which was set at a distance of 14 cm from the rotating mandrel.The rotating mandrel was covered with aluminium foil which acts as a collector for the final submicron-fiber product.The rotation speed of the mandrel was 500 rpm for 3 h continuously,while the feed flow rate was 20 μL/min.The applied voltage induces a Columbia force that works together with the evaporation action of the solvent producing a continuous jet that forms a PS/RDX submicron-fibers on the collector.This desired continuous jet is formed only when a“Taylor cone”shape,as shown in Fig.1,was formed attached to the syringe needle.

2.3.Scanning electron microscopy (SEM)

The morphology of the PS/RDX nano-fibers obtained via the electrospinning technique was investigated through SEM (ZEISS,EVO 10 MA),adjusted at 16 kV,connected with an energydispersive X-ray spectrometer (XRD,Bruker,Quantax 200).The average diameter of the fabricated fibers was determined through the obtained SEM images.

2.4.Fourier Transform Infrared (FTIR) spectra

Infrared spectra of the pure PS,pure RDX samples,and the synthesized PS/RDX nano-fibers were recorded using (Shimadzu,8000 series FTIR spectrometer).Herein,2 mg Potassium bromide(KBr) was ground with 350 mg of pure RDX and then pressed into light-permeable RDX/KBr disks.These disks were then placed in the FTIR instrument for analysis at the wavenumber range of 500-4000 cm-1,a resolution of 8.0 cm-1,and for an average of 500 scans.

2.5.X-ray diffraction pattern (XRD)

X-ray diffraction analysis was carried out for pure PS (raw material),nano-fibrous PS,and nano-fibrous PS/RDX nano-fibers using(Panalytical Empyrean,X-ray diffractometer).A copper (Cu) anode target tube was provided together with the XRD device as a radiation source.All tested samples were subjected to a 1.5405 Å irradiation with Cu(Kα).Finally,XRD diffraction patterns were obtained via recording over the range of 2θ at a temperature range from 15 to 65℃.Nano-fibrous samples produced via the electrospinning technique were collected on aluminum foils.The applied voltage of the XRD apparatus was 40 kV,while a current of 30 mA was utilized.

2.6.Impact and friction sensitivity measurements

The minimal amount of impact energy required for the initiation of 50 mm3of either pure RDX or the fabricated PS/RDX nano-fiber was determined.This was carried out using a drop weight with several adjustable weights,2,5,and 10 kg at several adjustable heights via the Bruceton analysis method (BAM) utilizing the impact sensitivity instrument(fall hammer,R+P MESPRO GmbH)[47].The probit analysis method was utilized to anticipate the possible initiation levels based on the results of the experiment[48].Each position that causes at least 50% initiation for individual samples could be counted as an acceptable level of initiation.The sensitivity to friction of pure RDX and the fabricated PS/RDX nanofibers was measured using the BAM friction test device (C620H friction/Peel Tester,Labthink) together with the probit analysis method [47].In this measurement,0.01 g of each sample was located cautiously on a plate made of porcelain.As the loads change,the resultant force that is acting between the pistil and the porcelain plate also changes.The test is considered as a counted initiation once any of the characteristic actions;smoke,smell,or sound occurs in 50% of experiments [48].

2.7.Thermal study

The Thermogravimetric Analysis (Mettler Toledo,TGA 55) was employed to record the thermal decomposition of PS/RDX nanofibers,PS nano-fibers,and the pure RDX.Calibration for the TG mass was carried out with calibration weights and empty beams to ensure the accuracy and reliability of the obtained data.About 2 mg of the test sample was weighed and placed in a high-temperature Platinum crucible.Then,the crucible was transferred to the TGA sample holder assembly,which had been set at room temperature(around 25℃).The samples were tested in a temperature range of 40-500℃,and the whole thermal decomposition process was carried out in an inert atmosphere (N2,40 mL/min).The experimental data were obtained at a data-collecting rate of 20 points per Kelvin.The running experiments with heating rates(β),10 ℃/min.

3.Results and discussion

3.1.PS/RDX nano-fiber morphology

The morphology of the fabricated PS/RDX nano-fibers is presented in the SEM images at different magnifications;2.00 KX,and 5.00 KX,as given in Figs.2(a) and 2(b) respectively.SEM micrographs confirmed the fabrication of the PS/RDX composite nanofibers via the electrospinning of the PS/RDX solution.The fabricated composite was continuous fine nano-fibers without any grips.The particle size distribution was determined by the Lazer diffraction particle size analyzer (Anton Paar,PSA 1190).The diameter of PS/RDX fibers ranges between 400 and 700 nm which represents 79% of the total particle sizes,and the calculated average particle size was 546 nm as shown in Fig.3.According to Yang et al.[35],nitrocellulose(NC)was used properly to encapsulate the RDX crystals forming composite nanofibers.The average diameter of the NC/RDX fibers was higher than the fibers of the pure nitrocellulose.In this study,the results approved that the electrospinning technique was capable of grafting RDX crystals through the PS frame.

3.2.XRD patterns and FTIR spectra

Fig.4 illustrates the XRD patterns as a comparison between amorphous PS crystals and pure RDX crystals before applying the electrospinning against both the electrospun PS and PS/RDX nanofibrous composites.PS raw material wide peaks at 2θ=19.06°indicated its basic amorphous nature.On the other hand,electrospun PS fibers indicated a strong sharp peak at 2θ=16.76°,ascribed to the dramatic change from the amorphous structure into the crystalline form.This indicates that the electrospinning technique is responsible for the significant change in the PS particles structure as it converts the amorphous particles into PS crystals with a high ordering value,indicated by the intensity of the peak of 539.Also,it can be observed that the peaks of electrospun nano-fibrous PS were shifted when compared with the raw material samples.This is supported by the increase that happened in the d-spacing from 4.6 A (PS) to 5.28 A°(nano-fibrous PS).These results agree with the findings in the literature given in references [49,50].The XRD patterns of nano-fibrous PS and nano-fibrous PS composite after loading RDX crystals via electrospinning were observed to be similar at 2θ=16.76°;however,the peak intensities were reduced from 539 to 389 according to the RDX content.This logical observation could be attributed to the decrease in the crystallinity % when adding RDX particles to PS crystals.Also,Yang et al.[35]stated that the presence of RDX with the nitrocellulose fibers caused a non-smooth baseline of the XRD but the main peaks are not drifted.This result proved that the NC/RDX composite fibers were properly fabricated by the electrospinning technique.

Fig.4.X-ray Diffraction Patterns of PS crystals: (a) PS/RDX Nanofiber;(b) Pure RDX crystals;(c) Pure PS nano fiber,and (d) pure PS crystals.

As it is illustrated in Fig.5(a),the peak at 3070 cm-1could be assigned to C-H asymmetric stretching vibration in the aromatic hexagonal ring of RDX,the peak at 1600 cm-1could be attributed to the asymmetric stretching vibration of C-NO2bonding and the peak detected at 854.31 cm-1could be regarding the stretching vibration of the C-N of the RDX ring.Furthermore,the peak that appeared at 1272 cm-1could be attributed to the intense band of the nitramines including RDX,and the strong peak detected at 1033 cm-1could be a characteristic peak assigned to the crystalline α-RDX structure.On the other hand,Fig.5(c) represents the FTIR spectrum for the pure PS where a strong peak appeared at 3041 cm-1that could be attributed to the C-H aromatic ring,and the peak at 2919 cm-1represents the CH2asymmetric tension.Also,the peaks at 1947,1452,and 1022 cm-1are corresponding to aromatic ring mono-substitution,deformation of CH2,C=C of the aromatic ring,and flexion of the C-H in the place respectively.These concluded FTIR results either for pure RDX or for pure PS coincided with Refs.[51,52] respectively.In Fig.5(b),it was observed a dramatic decrease in the intensity of the characteristic peaks of RDX:3070,1272,and 1033 cm-1,while the peaks at 1600 and 854 cm-1were completely disappeared.These observations could be attributed to the full-encapsulation of the RDX crystals by the PS fibers which are confirmed by the XRD findings together with the SEM spectrum.

Fig.5.FTIR spectra of (a) Pure RDX,(b)PS-RDX nano-fibrous Membrane,and (c) Pure PS.

3.3.PS/RDX nano-fiber sensitivity to external stimuli

Interestingly,PS/RDX fibers have no impact sensitivity under the influence of the maximum load (100 J impact energy),also they have no sensitivity to friction with the maximum load of 360 N.To clarify the influence of PS fibers on the sensitivity of the pure RDX,the impact and friction sensitivities of several traditional explosive compositions were studied.Composition C-4 (comp C-4) [53],Semtex 1H based on RDX with pentaerythritol tetranitrate (PETN)[54] and RDX-sylgard [55] as plastic explosives,Composition B(comp B)based on RDX with trinitro-toluene(TNT)[56]as melt cast explosives,RDX-GAP based on glycidyl azide polymer (GAP) [57],and RDX-HTPB based on hydroxyl-terminated polybutadiene(HTPB) [57] as cast cured plastic bonded explosives (PBXs),and RDX-Viton based on a copolymer of vinylidene fluoride and hexafluoropropylene[53]as pressed plastic bonded explosives(PBXs)in addition to the pure RDX [53] were studied.The sensitivities of these compositions are reported in Table 1.

Table 1 Impact and friction sensitivities of different RDX compositions.

Interestingly,a dramatic decrease in the impact sensitivity of the PS/RDX nano-fibers was determined.The cast and pressed PBXs samples have high impact sensitivity with low friction sensitivity while the plastic explosives have lower impact sensitivity with slightly higher friction sensitivity.By comparing the results,it is obvious that PS/RDX fibers have very low impact and friction sensitivities and seem to be quite safe during the manipulation.These dramatic results of PS/RDX fibrous sensitivities in comparison to the traditional RDX compositions encourage introducing the fabricated PS/RDX nano-fibers to be used widely in the field of explosives detection with a high safety level.

3.4.Thermal behaviors for PS/RDX nano-fiber and their raw materials

The TGA and derivative thermogravimetry (DTG) curves of PS/RDX nano-fiber,PS nano-fiber,PS Polymer,and pure RDX at a heating rate of 10 K/min in a nitrogen environment are presented in Fig.6.In addition,the decomposition data of the studied samples are presented in Table 2.The DTG shows a single thermal decomposition peak at 381.2℃ and 220.2℃ for PS nano-fiber and pure RDX respectively.For PS/RDX nano-fiber,it is obvious from(Fig.6(a)) that it has two thermal decomposition stages at the temperature range between about 200 and 235℃,in which the mass-loss rate was about 39.6%.This thermal decomposition represents the complete decomposition of the RDX filler and represents the actual weight of the RDX in the studied PS/RDX nanofibers.

Table 2 Results of non-isothermal TG/DTG of the PS/RDX nano-fibers,PS polymer,PS nanofibers,and pure RDX at different heating rates.

Fig.6.Non-isothermal TG/DTG of (a) PS/RDX nano-fiber,(b) PS nano-fiber,(c) Pure RDX,and (d) polystyrene Polymer at a heating rate of 10 K/min.

Another interesting observation,the presence of RDX in the PS/RDX nano-fiber has an obvious effect on the thermal behavior where the onset decomposition temperatureTowas increased by about 19.2℃ while the maximum peak temperature was decreased in comparison with the pure RDX.The increase of the onset decomposition peak might be due to the presence of PS fibers which absorbs energy and inhibit the starting of the RDX decomposition at a lower temperature while the decrease of the maximum decomposition peak is due to the presence of PS fiber enclosed in the melted RDX with its decomposition products which prevent the escaping of the decomposition species and accelerated the complete decomposition of the RDX as discussed in details in Refs.[58,59].However,an increase in the thermal stability of RDX crystals might be due to the inert oil compensates from the intermolecular interactions at failure points on the crystal surface as described in Ref.[60].On the other side,Yang et al.observed that increasing the percentage of RDX in the nitrocellulose nanofibers caused an increase in the decomposition temperature of the NC/RDX fibers [35].This result is due to the presence of nitrocellulose as an energetic polymer with a lower exothermic decomposition peak than the pure RDX.The reverse was observed in this study,the PS fibers acted as an insulator and prevented the escaping of the decomposition species,so it accelerates the decomposition of PS/RDX.

The second stage of the thermal decomposition was observed at the temperature range between about 238.2℃ and 500℃,which was the main decomposition stage of PS nano-fibers with a mass loss of 60.4% with the whole thermal decomposition process of the PS/RDX nano-fibers membrane.The thermal decomposition of PS/RDX nano-fiber ended when the temperature was over 500℃,and the corresponding maximum mass loss rate was about 99.9%.RDX has also a significant effect on the PS nano-fiber by increasing the onset temperature from 335.1 to 368.2℃ and increasing the maximum peak decomposition temperature by 16.0℃.This result might be due to the perfect coating of the RDX crystal surfaces with PS in which the PS polymer contains π-electrons in the benzene units in the macromolecule should be stabilize more intense due to π-π stacking with the π-electron system of the nitro groups.For PS polymer DTG,it has one stage of thermal decomposition as that in the PS nano-fiber however there is a certain significant shift in the decomposition data by 42.8,24.0,and 5.0℃ for the onset temperature,the maximum peak temperature,and the end peak temperature respectively.As it was mentioned in Subsection 3.2,the PS nano-fiber was changed from the amorphous PS polymer to the crystalline phase after using the electrospinning technique which has a noticeable change in the thermal behavior data.

4.Conclusions

The PS/RDX nano-fibers were prepared using a solvent mixture of dimethylformamide and chloroform in a volumetric ratio of 3:1 and by applying the optimum conditions as discussed.The SEM images confirmed the complete coverage of RDX crystals by PS fibers in the nano-sizes and the average particle size was 546 nm.Also,the XRD showed a dramatic change in the morphology of the PS nano-fibers membrane from the amorphous to the crystalline phase.Electrospun nano-fiber is considered a technology that significantly reduces the sensitivity of the RDX to a safe limit for the applications of explosives detection with high security(impact and friction sensitivities were lower than 100 J and 360 N respectively).The thermal study of the PS/RDX nano-fibers showed a higher onset decomposition temperature than the pure RDX.The PS fibers absorbed the heat and increased the onset decomposition of the RDX in the PS nano-fibers in comparison with the pure RDX by 19.2℃.Consequently,RDX has a significant effect on the thermal behavior of PS nano-fiber and caused increasing of the maximum peak decomposition temperature by 16.0℃.The decomposition data for the PS polymer was changed after the spun of the pure PS nano-fiber which confirmed the change of the amorphous PS polymer to a crystalline PS/RDX nano-fiber.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors are very grateful to the members of the Chemical Engineering Branch,Military Technical College,Cairo,Egypt for their encouragement,support,and help.