nanomaterials

Article A (SDIB)/Polybenzoxazines (PBz) Polymer Blend for Electrospinning of Nanofibers

Ronaldo P. Parreño Jr. 1,2 , Ying-Ling Liu 3 and Arnel B. Beltran 1,4,* 1 Department of Chemical Engineering, De La Salle University, 2401 Taft Avenue, Manila 1004, Philippines; [email protected] 2 Chemicals and Energy Division, Industrial Technology Development Institute (ITDI), Department of Science and Technology (DOST), Taguig 1631, Philippines 3 Department of Chemical Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan; [email protected] 4 Center for Engineering and Sustainable Development Research, De La Salle University, 2401 Taft Ave, Manila 1004, Philippines * Correspondence: [email protected]

 Received: 28 September 2019; Accepted: 23 October 2019; Published: 26 October 2019 

Abstract: This study demonstrated the processability of sulfur copolymers (SDIB) into polymer blend with polybenzoxazines (PBz) and their compatibility with the electrospinning process. Synthesis of SDIB was conducted via inverse vulcanization using elemental sulfur (S8). Polymer blends produced by simply mixing with varying concentration of SDIB (5 and 10 wt%) and fixed concentration of PBz (10 wt%) exhibited homogeneity and a single-phase structure capable of forming nanofibers. Nanofiber mats were characterized to determine the blending effect on the microstructure and final properties. Fiber diameter increased and exhibited non-uniform, broader fiber diameter distribution with increased SDIB. Microstructures of mats based on SEM images showed the occurrence of partial aggregation and conglutination with each fiber. Incorporation of SDIB were confirmed from EDX which was in agreement with the amount of SDIB relative to the sulfur peak in the spectra. Spectroscopy further confirmed that SDIB did not affect the chemistry of PBz but the presence of special interaction benefited miscibility. Two distinct glass transition temperatures of 97 ◦C and 280 ◦C indicated that new material was produced from the blend while the water contact angle of the fibers was reduced from 130◦ to 82◦ which became quite hydrophilic. Blending of SDIB with component polymer proved that its processability can be further explored for optimal spinnability of nanofibers for desired applications.

Keywords: sulfur copolymers; polybenzoxazines; electrospinning; polymer blend; nanofibers; inverse vulcanization

1. Introduction

Elemental sulfur (S8) is a largely available untapped resource with an estimated amount of more than 60 million tons generated annually from the hydrodesulfurization process of natural gas and oil to reduce sulfur dioxide (SO2) emissions from the combustion of these fossil fuels [1,2]. Sulfur as a by-product of the refining process causes the “excess sulfur problem” because of rising global petroleum demand [2,3]. Sulfur is mostly used for the production of sulfuric acid and phosphates for fertilizers, synthetic rubber and cosmetics but still has limited demand with an estimated surplus of 7 million tons annually [1]. The abundant amount of excess sulfur coupled with its unique properties has created renewed interest in sulfur as an alternative feedstock for sulfur-based materials [4]. Previous studies were carried out to overcome the challenges of processing elemental sulfur into new materials such as reinforcing filler for composites [5], chemically modified sulfur for

Nanomaterials 2019, 9, 1526; doi:10.3390/nano9111526 www.mdpi.com/journal/nanomaterials Nanomaterials 2019, 9, 1526 2 of 14 sustainable construction material [6], and as feedstock for reacting with another polymer to obtain network structured and soluble sulfur-rich copolymers [7]. However, the processability of sulfur still remains the main challenge for its direct utilization. Early research works have already proven the ability of S8 to copolymerize with different polymers such as propylene sulfide [8,9], styrene [10], diynes [11] and cyclic disulfides [12] but the polymeric materials formed have limited processability and tunability of properties [1]. Until recently, polymeric material from direct utilization of sulfur was achieved via a solvent free copolymerization process known as “inverse vulcanization” [1,2]. It is the reversal of established vulcanization process where sulfur served as the backbone of the polymeric material while the carbon aromatic comonomer, 1,3-diisopropenyl benzene (DIB) bind the sulfur chains to form more stable sulfur copolymers (SDIB) [5]. The potential of SDIB as processable polymers with its distinctive properties from the high sulfur content and improved chemical and processing characteristics opened opportunities for more research efforts on potential applications. Sulfur copolymers produced using different crosslinkers via inverse vulcanization were evaluated for mercury capture application [3] while another study used inverse vulcanization in a reaction of sulfur with natural to produce flexible sulfur films [13]. Taking advantage of the good electrochemical properties, SDIB became a new addition to the emerging class of electroactive polymers used as active cathode material for performance improvement of lithium sulfur (Li-S) batteries [14]. This was so far the most promising application of SDIB and it continues to constitute the majority of recent advances in SDIB application [14]. Further advances and breakthroughs in SDIB would require looking at its potential applications in considerations of the polymer composition and processing characteristics. Sulfur copolymers with comonomer ratio of 50 wt% DIB were found to be completely soluble in non-polar organic solvents [1]. They are amorphous in nature and classified as uncrosslinked copolymers with low molar mass and high polydispersity [1,2]. These attractive features provided the processing options that could be explored through this work by trying out diverse fabrication techniques and robust modification methods to produce desired structures and preferred surface properties. One of the commonly used polymer processing methods is polymer blending to exploit the properties of synergy formed with other polymers. This involved mixing two polymers with properties that can be manipulated by the component polymers [15]. It was demonstrated previously that polymer blend of SDIB with component polymer showed compatibility in electrospinning process to produce electrospun fiber mats [16]. This study is, to the best of our knowledge, the only successful polymer blending that has used SDIB from inverse vulcanization to further process it into nanofiber via electrospinning technique. Electrospinning as promising fabrication method have shown beneficial effects on property enhancement brought about by the morphology of the nanofibers [17]. It is considered to be a versatile process for producing the desired fiber structures, porosity, orientations and dimensions [18] because of its controllable parameters such as solution, process and ambient conditions [19–21]. Moreover, electrospinning can be applied for different types of materials such as natural and synthetic polymers, polymer blends, composite with metal or ceramic particles, and nanocomposites [20]. Because of these merits, electrospinning became known for producing high-surface area materials and recognized as the method that can provide wide range of applications, including catalysis [22], energy storage [23], biosensing [24], drug delivery [25] and membranes [26]. Hence, SDIB as a component polymer in the blending process has great potential for delivering enhanced materials through the application of electrospinning. In this study, we focus on using sulfur copolymers synthesized via inverse vulcanization of S8 to investigate its processability with a polymer given its processing characteristics and structure. Polybenzoxazines (PBz), a thermoset material, was selected as component polymer because of its unique properties such as chemical inertness, high thermal stability, flexural strength, as well as near zero volumetric change upon curing [27–29]. Another advantage of PBz is the easy thermal curing without hardeners or catalysts resulting to high mechanical and thermal properties [28,29]. This is due to crosslinking of PBz that occurs via thermally activated ring-opening polymerization (ROP) of Nanomaterials 2019, 9, 1526 3 of 14 the oxazine ring in the main polymer chain [28,30]. These remarkable properties led to exploring the potential synergy created by polymer blending of PBz with SDIB. Herein, the combination of SDIB with thermoset material PBz processed into polymer blends were examined for the physical behavior and phase structure prior to subsequent processing. Further processing of the polymer blend was conducted to determine what material can be produced when its compatibility to form nanofibers was investigated. The versatility of electrospinning allowed the control of process parameters that govern the spinning process to accommodate the polymer properties specific for SDIB/PBz as polymer blend. Electrospun nanofiber mats were characterized to evaluate the effect of copolymers-polymer blending on resulting fiber morphology and structure, thermal and structural properties, surface property and solubility. This work offered a different insight on process option that could be further studied to find new applications of SDIB as an emerging class of polymeric material. The main contribution of this study is to demonstrate that processability of SDIB as component in polymer blend and its spinnability to produce electrospun nanofibers via electrospinning has the potential for further optimization of the desired properties of nanofibers.

2. Materials and Methods

2.1. Materials

Sulfur (S8) (powder, sublimed, Alfa Aesar 99.5%), 1,3-diisopropenylbenzene (DIB) (TCI Chemicals, >97.0%) were used as received without any purification in the synthesis of sulfur copolymers. Polybenzoxazines (PBz) was prepared in the lab as reported in the work of Liu et al. [30]. Dimethylsulfoxide (DMSO) (ACS grade, Echo, 99.9%), and Tetrahydrofuran (THF) (inhibitor free high purity, Tedia, 99.8%) were used as received without any purification.

2.2. Synthesis of SDIB via Inverse Vulcanization Process The synthesis of sulfur copolymers (SDIB) was undertaken using the procedure described in the work of Chung et al. [1]. The comonomer ratio of sulfur to DIB used in the synthesis was 50 wt%. Synthesis of SDIB was undertaken by adding the sulfur powder in a 25 mL glass vial equipped with a magnetic stir bar and then, heated to 185 ◦C in a thermostated oil bath for 5 min until a clear orange colored molten phase was formed. DIB was then directly added to the molten sulfur in the glass vial with the molten sulfur. Then the resulting mixture was continuously stirred for 8–10 min at 185 ◦C until vitrification was complete. Lastly, the product from the reaction after complete vitrification of the solution was cooled to room temperature.

2.3. Preparation of Polymer Blend Polymer blending process of SDIB and PBz was carried out using simple mixing technique by dissolving the polymers in separate mixture of solvents. Three polymer blends of SDIB and PBz were prepared according to concentration of PBz (fixed at 10 wt%) and SDIB (0, 5 and 10 wt%) in the polymer solution with final blends coded as SDIB/PBz-Y where Y (0, 5 and 10) denotes the varying compositions of component polymer SDIB. For SDIB/PBz-0 or pure PBz, electrospinning solution was previously prepared in a mixture of DMSO/THF with a ratio of 1:3 (v/v) which was used as basis for blending of SDIB/PBz-5 and SDIB/PBz-10. Initially, two separate solutions of SDIB and PBz in DMSO/THF were prepared. After continuous stirring of the two solutions for at least 24 h at room temperature when both SDIB and PBz were fully dissolved, the two solutions were mixed in one solution. Then, the blend of two solutions was stirred for at least 1 h at room temperature to fully mix the polymers into one another to form the final polymer blend.

2.4. Electrospinning of Polymer Blend The electrospun nanofiber mat from the polymer blend was produced using vertically-aligned electrospinning apparatus composed of a 10 mL syringe with needle (ID = 0.8 mm) connected to Nanomaterials 2019, 9, 1526 4 of 14

Nanomaterials 2019, 9, x FOR PEER REVIEW 4 of 14 a syringe pump, a ground electrode, and a high voltage supply (Falco Enterprise Co., Ltd., Taipei, Taiwan).prepared Three and coded electrospun as electrospun nanofibers (ES) having nanofibers polymer of SDIB concentration and PBz (SDIB/PBz) of 0, 5 and 10or wt%ES‐SDIB/PBz SDIB were‐X, preparedwhere X (0, and 5 and coded 10) as identifies electrospun the amount (ES) nanofibers of SDIB while of SDIB PBz and was PBz fixed (SDIB to polymer/PBz) or concentration ES-SDIB/PBz-X, of where10 wt%. X (0,The 5 andneedle 10) was identifies connected the amount to the high of SDIB voltage while supply, PBz was which fixed generates to polymer positive concentration DC voltages of 10 wt%.up to The40 kV. needle Polymer was blend connected of SDIB/PBz to the high was voltageplaced in supply, a 10 mL which syringe generates which was positive ejected DC through voltages a upneedle to 40 spinneret kV. Polymer by a blendsyringe of pump SDIB/ PBz(Falco was Enterprise placed in Co.) a 10 with mL syringea mass flow which rate was of ejected1.00 mL through h−1 and 1 atip needle‐to‐collector spinneret distance by a syringe(TCD) of pump 15 cm. (Falco Applied Enterprise voltage Co.) was with varied a mass from flow 10 ratekV to of 20 1.00 kV mL in hthe− andelectrospinning tip-to-collector process distance depending (TCD) of on 15 the cm. change Applied in concentrations voltage was varied of polymer from 10 blends kV to 20(0, kV 5 and in the 10 electrospinningwt% SDIB concentrations) process depending while other on the process change parameters in concentrations were not of changed. polymer Electrospinning blends (0, 5 and process 10 wt% SDIBwas carried concentrations) out at ambient while other conditions. process After parameters electrospinning, were not changed. the electrospun Electrospinning nanofibers process were was set carriedaside for out at at least ambient 24 h conditions.to vaporize After remaining electrospinning, solvent prior the electrospunto thermal treatment. nanofibers For were pure set asidePBz, the for atcuring least was 24 h conducted to vaporize at remaining temperatures solvent of 80 prior °C, 100 to thermal °C, 160 °C, treatment. 200 °C and For 240 pure °C, PBz, each the for curing 1 h inside was conductedthe oven (Deng at temperatures Yng, DH 400) of 80 to ◦thermallyC, 100 ◦C, crosslinked 160 ◦C, 200 the◦C andPBz. 240 Then,◦C, thermally each for 1 treated h inside mats the ovenwere (Dengcool down Yng, to DH room 400) temperature to thermally crosslinkedinside the oven the PBz. and Then,then, removed thermally from treated the mats collector were plate. cool down For the to roomelectrospun temperature nanofibers, inside ES the‐SDIB/PBz oven and‐5 then, and removedES‐SDIB/PBz from‐10, the the collector thermal plate. treatment For the was electrospun initially nanofibers,tested based ES-SDIB on the /curingPBz-5 andconditions ES-SDIB used/PBz-10, for ES the‐SDIB/PBz thermal treatment‐0 and adjusted was initially to new tested thermal based curing on theconditions curing conditionsusing only usedtwo‐step for ES-SDIB temperature/PBz-0 of and 160 adjusted°C and 240 to new°C each thermal for 1 h. curing conditions using only two-step temperature of 160 ◦C and 240 ◦C each for 1 h. 2.5. Characterization of Electrospun Nanofiber Mat 2.5. Characterization of Electrospun Nanofiber Mat Characterization of electrospun nanofibers mats were conducted to evaluate the resulting Characterization of electrospun nanofibers mats were conducted to evaluate the resulting properties from blending of SDIB with PBz and to understand the effect of the physical structure and properties from blending of SDIB with PBz and to understand the effect of the physical structure and behavior of blend on the spinnability of blend for nanofiber formation. Fourier‐transform infrared behavior of blend on the spinnability of blend for nanofiber formation. Fourier-transform infrared spectroscopy (FTIR) spectra were obtained using a Perkin Elmer Spectrometer at the wavenumber spectroscopy (FTIR) spectra were obtained using a Perkin Elmer Spectrometer at the wavenumber between 400 cm−1 to 4000 cm−1. SEM‐EDX analysis was carried out with a Scanning Electron between 400 cm 1 to 4000 cm 1. SEM-EDX analysis was carried out with a Scanning Electron Microscope (SEM)− (FEI Helioz −Nanolab 600i, Eindhoven, The Netherlands) with Energy Dispersive Microscope (SEM) (FEI Helioz Nanolab 600i, Eindhoven, The Netherlands) with Energy Dispersive X‐ray Spectroscopy (EDX) (Oxford Instrument X‐Max, Abingdon, UK.) at the Advanced Device and X-ray Spectroscopy (EDX) (Oxford Instrument X-Max, Abingdon, UK.) at the Advanced Device and Materials Testing Laboratory (ADMATEL) of DOST. Thermal property was measured by differential Materials Testing Laboratory (ADMATEL) of DOST. Thermal property was measured by differential scanning calorimetry (DSC) using a Q20, TA instrument at heating rate of 10 °C/min in the scanning calorimetry (DSC) using a Q20, TA instrument at heating rate of 10 C/min in the temperature temperature range of 50–450 °C under nitrogen environment. Water contact◦ angle (WCA) was range of 50–450 C under nitrogen environment. Water contact angle (WCA) was measured using an measured using◦ an instrument from First Ten Angstroms (FTA, Model: FTA 1000 B) to evaluate the instrument from First Ten Angstroms (FTA, Model: FTA 1000 B) to evaluate the surface properties surface properties of the electrospun nanofiber mats. The contact angle data were obtained from the of the electrospun nanofiber mats. The contact angle data were obtained from the average of three average of three replicates from five measurements of samples. Solubility tests of the electrospun replicates from five measurements of samples. Solubility tests of the electrospun nanofibers were nanofibers were undertaken by soaking the samples for at least 24 h in common solvents to determine undertaken by soaking the samples for at least 24 h in common solvents to determine its solvent its solvent resistance after SDIB was incorporated. resistance after SDIB was incorporated. 3. Results and Discussion 3. Results and Discussion

3.1.3.1. Preparation of Polymer BlendBlend

The SDIB used for blending process was synthesized via inverse vulcanization of S8 with The SDIB used for blending process was synthesized via inverse vulcanization of S8 with comonomercomonomer DIB DIB as as shown shown in Figure in Figure1. SDIB 1. hasSDIB polysulfide has polysulfide backbone backbone with aromatic with carbon aromatic monomers carbon boundmonomers to sulfur bound chains to sulfur to form chains stable to form copolymers. stable copolymers.

FigureFigure 1.1. SchematicSchematic representationrepresentation of of sulfur sulfur copolymers copolymers (SDIB) (SDIB) produced produced via via inverse inverse vulcanization vulcanization of

Sof8 withS8 with 1,3-diisopropenyl 1,3‐diisopropenyl benzene benzene (DIB). (DIB).

The processability of SDIB in polymer blending process was conducted using the common and simple technique of dissolving the polymers in common mixture of solvents. It is known that

Nanomaterials 2019, 9, x FOR PEER REVIEW 5 of 14 Nanomaterials 2019, 9, 1526 5 of 14 blendingNanomaterials of 2019 polymers, 9, x FOR having PEER REVIEW each individual characteristic is considered an effective technique5 of to14 developThe new processability material with of SDIB desired in polymer properties blending [31,32]. process However, was employing conducted the using appropriate the common method and issimpleblending critical technique ofin polymersachieving of dissolving havinga homogeneous theeach polymers individual system in commoncharacteristic of polymer mixture mixtures. is considered of solvents. Moreover, an It is effective known polymer thattechnique blending blending to processofdevelop polymers are new primarily having material each influencedwith individual desired by properties characteristicthe nature [31,32]. of is the considered copolymersHowever, an employing‐ epolymerffective technique mixture the appropriate and to develop the methodratio new of thematerialis critical component with in achieving desired polymers properties a homogeneouswhich significantly [31,32]. system However, affect of polymer employingthe behavior mixtures. the and appropriate phaseMoreover, structure method polymer of is the criticalblending blends in [33].achievingprocess In mixingare a primarily homogeneous SDIB and influenced PBz system which by of therepresents polymer nature mixtures.ofcopolymers the copolymers Moreover,‐polymer‐polymer polymersystem mixture with blending different and processthe amount ratio are of ofprimarilythe SDIB, component different influenced polymers approaches by the which nature of significantly ofblending the copolymers-polymer were affect carried the behavior out mixture initially and andphase to thefind structure ratio out of the the of appropriate componentthe blends strategypolymers[33]. In mixingto which produce SDIB significantly homogenous and PBz awhichff ectblend the represents instead behavior of copolymers formation and phase of‐polymer structure gel. It is systemimportant of the blendswith to differentobtain [33]. a In miscible amount mixing blendSDIBof SDIB, andas propertydifferent PBz which mixapproaches represents depends of copolymers-polymeron blendingthe degree were of miscibility carried system out withof initiallythe di twofferent polymersto amountfind out at of themolecular SDIB, appropriate diff erentlevel [33,34].approachesstrategy to produce of blending homogenous were carried blend out instead initially of formation to find out of gel. the It appropriate is important strategy to obtain to a producemiscible homogenousblendIn as this property work, blend mixblending instead depends of was formation on achieved the degree of gel.by dissolvingof It ismiscibility important separately of to the obtain two the apolymers PBz miscible and at blendSDIB molecular in as propertyseparate level mixturesmix[33,34]. depends of DMSO/THF on the degree solvent of miscibility to form two of the polymer two polymers solutions at and molecular later on level mixed [33 ,together34]. into the final Inpolymer this work, blend. blending This approach was achieved resulted by to dissolving miscible blends separately with the SDIB PBz and and PBz SDIB mixing in separate in the solutionsmixtures ofto produce DMSODMSO/THF/THF homogeneous, solvent to form transparent two polymer and single solutions‐phase and structure later on as mixed shown together in Figure into 2. theAs observedfinalfinal polymer in the blend. polymer This This blend, approach approach the homogeneity resulted resulted to to misciblewas very blends evident with with SDIB the absence and PBz of mixing separation in the of interfacesolutions when to to produce produce two phases homogeneous, homogeneous, appear in transparent the transparent blend [15,35]. and and single single-phase‐phase structure structure as asshown shown in Figure in Figure 2. As2. Asobserved observed in the in the polymer polymer blend, blend, the the homogeneity homogeneity was was very very evident evident with with the the absence absence of separation of separation of ofinterface interface when when two two phases phases appear appear in inthe the blend blend [15,35]. [15,35 ].

Figure 2. Polymer blend preparation from: (a) SDIB solution; (b) PBz solution; (c ) Polymer Blend of SDIB/PBz (5/10 wt%); and (d)Polymer blend of SDIB/PBz (10/10 wt%). Figure 2. Polymer blend preparation from: ( a)) SDIB solution; ( b) PBz solution; (c) Polymer Blend of SDIB/PBz (5/10 wt%); and (d) Polymer blend of SDIB/PBz (10/10 wt%). MixingSDIB/PBz of (5/10 polymers wt%); andresults (d)Polymer in a miscible blend ofblend SDIB/PBz due to (10/10 the specialwt%). interactions that exist between the component polymers which could be due to similarity in structure [36]. In this case, the miscibility Mixing of polymers results in a miscible blend due to the special interactions that exist between the of SDIBMixing and ofPBz polymers solutions results indicated in a miscible the presence blend dueof special to the specialinteraction interactions between that the exist copolymers between‐ component polymers which could be due to similarity in structure [36]. In this case, the miscibility of polymerthe component components. polymers This which interaction could becould due be to similarityattributed into structure the nature [36]. of theIn this copolymers, case, the miscibility SDIB and SDIB and PBz solutions indicated the presence of special interaction between the copolymers-polymer componentof SDIB and polymer, PBz solutions PBz which indicated both the are presence chemically of specialaromatic interaction in their betweenstructures. the The copolymers chemical‐ components. This interaction could be attributed to the nature of the copolymers, SDIB and component structurespolymer components. of SDIB and ThisPBz interactionused in the couldblending be attributed process both to the having nature aromatic of the copolymers, structures are SDIB shown and polymer, PBz which both are chemically aromatic in their structures. The chemical structures of SDIB incomponent Figure 3. polymer,Polymers PBzwith which the presence both are of chemically phenyl groups aromatic tend in to their have structures. special attractions The chemical and and PBz used in the blending process both having aromatic structures are shown in Figure3. Polymers associatestructures with of SDIB each and other PBz during used inmixing the blending allowing process the polymers both having to mix aromatic and formed structures miscible are blendsshown with the presence of phenyl groups tend to have special attractions and associate with each other [33].in Figure 3. Polymers with the presence of phenyl groups tend to have special attractions and during mixing allowing the polymers to mix and formed miscible blends [33]. associate with each other during mixing allowing the polymers to mix and formed miscible blends [33].

Figure 3. Polymer blending of SDIB and PBz with chemically aromatic structures. Figure 3. Polymer blending of SDIB and PBz with chemically aromatic structures. Another important consideration for mixing polymers is the amount of components in the polymer Another Figureimportant 3. Polymer consideration blending offor SDIB mixing and PBz polymers with chemically is the amountaromatic structures.of components in the blend. A lower amount of one polymer compared to the other component promotes a discontinuous polymer blend. A lower amount of one polymer compared to the other component promotes a phase in the polymer blend while a continuous phase occurs when there is an equal amount of the two discontinuousAnother importantphase in the consideration polymer blend for whilemixing a continuouspolymers is phase the amount occurs whenof components there is an inequal the polymers in the polymer blend [33]. For SDIB/PBz-10, the equal amount of polymer also contributed amountpolymer ofblend. the two A lower polymers amount in theof onepolymer polymer blend compared [33]. For to SDIB/PBz the other‐10, component the equal promotesamount of a discontinuous phase in the polymer blend while a continuous phase occurs when there is an equal amount of the two polymers in the polymer blend [33]. For SDIB/PBz‐10, the equal amount of

Nanomaterials 2019, 9, 1526 6 of 14 to a stronger interaction between SDIB and PBz because of the formation of continuous phases in the two polymers which resulted to miscible blend. For unequal amounts of polymer blend SDIB/PBz-5, the miscibility of the polymer blend was not affected, exhibiting a similar homogeneous system. The interaction present due to the nature of the two polymers could be responsible for the degree of miscibility from the minimal change in the amount of SDIB in the polymer blend which also produced a miscible blend. The miscibility and single-phase structure of the SDIB/PBz polymer blend were integral in the compatibility on electrospinning.

3.2. Electrospinning of Polymer Blend In the polymer blend, the amount of polymer components of SDIB (0, 5 and 10 wt%) and PBz (10 wt%) represents the polymer concentration which have direct effects on spinnability. Polymer concentration is considered as a primary parameter of the electrospinning process which is the main consideration for other process parameters. For SDIB/PBz polymer blends with varying concentrations of components, spinning process was govern by adjusting only the applied voltage to accommodate the change in polymer concentration while other parameters such as flow rate and TCD were not changed. Electrospinning of SDIB/PBz-0 was conducted with an applied voltage of 10 kV that produced nanofibers. First run of SDIB/PBz-5 polymer blend was conducted at the same applied voltage of 10 kV but showed unsatisfying results with polymer dopes accumulating at the needle tip which produced mostly electrosprayed droplets instead of fibers. The polymer concentration effect of the polymer blend is directly related to the viscosity and surface tension to obtain nanofibers [20]. By applying high voltage in electrospinning process, the polymer solution gets charged and Taylor cone is formed [19]. However, when the Taylor cone is applied with sufficient strength of electric field which overcomes the surface tension of the droplet and polymer solution feed at the tip of needle becomes electrically charged, the charged polymer solution is ejected as a jet to form nanofibers [37]. Addition of SDIB in the polymer concentration caused insufficient applied voltage which was not enough to overcome the surface tension. This led to disruptions of jet formation due to weaker electrically charged polymer solution. In subsequent runs, the spinnability of polymer blend to form nanofibers was achieved with higher applied voltage. It was realized at this voltage level through the formation of nanofiber from the SDIB/PBz blend but optimization with other process parameters is still needed to obtain desired morphology and properties of nanofibers. The applied voltage used for the polymer blends to obtain nanofibers are shown in Table1.

Table 1. Variation in Applied Voltage used for Electrospinning of SDIB/PBz.

Applied Voltage, kV Polymer Blend 10 12 14 16 18 20 SDIB/PBz-0 ⁄ 1 SDIB/PBz-5 x 2 x x ⁄ SDIB/PBz-10 x x x x x ⁄ 1 fibers were formed; 2 fibers were not formed.

The electrospun nanofiber mats produced from polymer blend were thermally cured to remove residual solvent and enhance mechanical and thermal properties from the PBz component polymer. The thermally cured mats of SDIB/PBz are shown in Figure4. An observable di fference in physical appearance was evident in the surface and color of the nanofiber mats which could be directly related to the SDIB added. ES-SDIB/PBz-0 has smoother and uniformed colored light brown surface while for ES-SDIB/PBz-5 and ES-SDIB/PBz-10, a similar smooth surface was observed but with some presence of areas with uneven surface that has darker shades of yellowish brown. The color of the nanofiber mats with the addition of SDIB changed from light brown to light yellowish brown for ES-SDIB/PBz-5 and darker yellowish brown for ES-SDIB/PBz-10. The changed in surface and uneven color tone was attributed to the presence of sulfur in the surface of the nanofibers. Nanomaterials 2019, 9, 1526 7 of 14 Nanomaterials 2019, 9, x FOR PEER REVIEW 7 of 14

Figure 4. Electrospun Nanofibers Nanofibers of: ( (aa)) Electrospun SDIB/PBz SDIB/PBz with 0 % SDIB (ES (ES-SDIB‐SDIB/PBz/PBz-0);‐0); ( b) ES-SDIBES‐SDIB/PBz/PBz-5;‐5; and (c)) ES-SDIBES‐SDIB/PBz/PBz-10.‐10. 3.3. SEM-EDX of Electrospun Nanofibers 3.3. SEM‐EDX of Electrospun Nanofibers The SEM images were used to evaluate the morphology of the electrospun nanofibers from The SEM images were used to evaluate the morphology of the electrospun nanofibers from the the polymer blending of SDIB with PBz. For ES-SDIB/PBz-0, analysis of SEM image in Figure5(a1) polymer blending of SDIB with PBz. For ES‐SDIB/PBz‐0, analysis of SEM image in Figure 5(a1) showed a uniform fiber morphology with an average fiber diameter of 2.668 0.987 µm and an showed a uniform fiber morphology with an average fiber diameter of 2.668 ± 0.987± μm and an evenly evenly distributed fiber diameter between 1.3–3.3 µm (Figure5(a2)). Comparing the ES-SDIB /PBz-5 in distributed fiber diameter between 1.3–3.3 μm (Figure 5(a2)). Comparing the ES‐SDIB/PBz‐5 in Figure Figure5(b1,b2), the average fiber diameter of 2.578 1.041 µm was almost the same with ES-SDIB/PBz-0 5(b1,b2), the average fiber diameter of 2.578 ± 1.041± μm was almost the same with ES‐SDIB/PBz‐0 but but the fiber diameter was slightly non-uniform as indicated by the skewed distribution. This means the fiber diameter was slightly non‐uniform as indicated by the skewed distribution. This means that that more fibers have diameter less than the average fiber diameter and have broader dispersion more fibers have diameter less than the average fiber diameter and have broader dispersion concentrated between 1.3–4.6 µm. The increase in concentration from SDIB (5 wt%) in the polymer concentrated between 1.3–4.6 μm. The increase in concentration from SDIB (5 wt%) in the polymer blend had no significant effect on the fiber diameter but a more broader fiber distribution. In the case blend had no significant effect on the fiber diameter but a more broader fiber distribution. In the case of ES-SDIB/PBz-10 as shown in Figure5(c1,c2), the fiber diameter slightly increased by 0.5 um with of ES‐SDIB/PBz‐10 as shown in Figure 5(c1,c2), the fiber diameter slightly increased by 0.5 um with average fiber diameter of 3.073 1.350 µm resulting to more dispersed distribution between 1.3–6.5 average fiber diameter of 3.073 ± 1.350 μm resulting to more dispersed distribution between 1.3–6.5 µm indicating increased non-uniformity in fiber diameter as compared to both ES-SDIB/PBz-0 and μm indicating increased non‐uniformity in fiber diameter as compared to both ES‐SDIB/PBz‐0 and ES-SDIB/PBz-5. These results were attributed to the increasing SDIB concentration which influenced ES‐SDIB/PBz‐5. These results were attributed to the increasing SDIB concentration which influenced directly the polymer concentration resulting to increased surface tension of the polymer mixture. Higher directly the polymer concentration resulting to increased surface tension of the polymer mixture. applied voltage was required to overcome the increased in surface tension to produce interactions Higher applied voltage was required to overcome the increased in surface tension to produce between electrically charged polymer blend and external electric field to reach Taylor cone formation. interactions between electrically charged polymer blend and external electric field to reach Taylor However, the higher electric field from applied voltage resulted in increased fiber diameter greater cone formation. However, the higher electric field from applied voltage resulted in increased fiber than the desired size for nanofibers which are classified as microfibers as well as a change in fiber diameter greater than the desired size for nanofibers which are classified as microfibers as well as a morphology. The electrospinning process can be further optimized for SDIB/PBz blend by adjusting change in fiber morphology. The electrospinning process can be further optimized for SDIB/PBz other process parameters aside from voltage to produce the desired nanofiber size and morphology. blend by adjusting other process parameters aside from voltage to produce the desired nanofiber size Further analysis of SEM images of all electrospun nanofibers in Figure6(a1–c2) and morphology. exhibited microstructures of randomly-oriented continuous, interconnected nanofibrous structures. ES-SDIB/PBz-0 in Figure6(a1,a2) showed fibrous structure with highly uniform and smooth fibers with no occurrence of beads. Formation of randomly-oriented continuous nanofibers were also seen in both ES-SDIB/PBz-5 and ES-SDIB/PBz-10 in Figure6(b1–c2) but not as uniform and as smooth as ES-SDIB/PBz-0. Based on these results, the polymer blending allowed the spinnability of SDIB/PBz by attaining the required molar mass to produce nanofibers. Previous study on electrospinning of pure sulfur copolymers resulted in unsatisfying results which produced broad fiber diameter with heavy beads and melting of fibers at the contact points [16]. It was explained that a polymer below the required molar mass may be blended with another polymer to produce a homogeneous blend capable of forming nanofibers [38]. SDIB has low molar mass with number-average molecular weight (Mn) of 1 1 1262 g-mol− [2] while PBz has high molar mass with Mn of 8000–9000 g-mol− [28]. It was found out that, generally, higher molecular weight of the polymer results in desirable viscosity aside from the effect of concentration for fiber formation [20]. Although nanofibers were formed, the presence of few beads-on-string in ES-SDIB/PBz-10 were observed which could be due to the high voltage application. Increasing the applied voltage in electrospinning causes changes in the morphology and structure of the fibers [19] and results in the formation of beads [39]. Another observable change in the microstructures of the ES-SDIB-5 and ES-SDIB/PBz-10 were the occurrence of partial aggregation and conglutination with each fibers (Figure6(b2,c2)). The fine tuning of nanofiber morphology and

Nanomaterials 2019, 9, x FOR PEER REVIEW 7 of 14

Figure 4. Electrospun Nanofibers of: (a) Electrospun SDIB/PBz with 0 % SDIB (ES‐SDIB/PBz‐0); (b) ES‐SDIB/PBz‐5; and (c) ES‐SDIB/PBz‐10.

3.3. SEM‐EDX of Electrospun Nanofibers The SEM images were used to evaluate the morphology of the electrospun nanofibers from the polymer blending of SDIB with PBz. For ES‐SDIB/PBz‐0, analysis of SEM image in Figure 5(a1) showed a uniform fiber morphology with an average fiber diameter of 2.668 ± 0.987 μm and an evenly distributed fiber diameter between 1.3–3.3 μm (Figure 5(a2)). Comparing the ES‐SDIB/PBz‐5 in Figure 5(b1,b2), the average fiber diameter of 2.578 ± 1.041 μm was almost the same with ES‐SDIB/PBz‐0 but the fiber diameter was slightly non‐uniform as indicated by the skewed distribution. This means that more fibers have diameter less than the average fiber diameter and have broader dispersion concentrated between 1.3–4.6 μm. The increase in concentration from SDIB (5 wt%) in the polymer blend had no significant effect on the fiber diameter but a more broader fiber distribution. In the case of ES‐SDIB/PBz‐10 as shown in Figure 5(c1,c2), the fiber diameter slightly increased by 0.5 um with average fiber diameter of 3.073 ± 1.350 μm resulting to more dispersed distribution between 1.3–6.5 μm indicating increased non‐uniformity in fiber diameter as compared to both ES‐SDIB/PBz‐0 and ES‐SDIB/PBz‐5. These results were attributed to the increasing SDIB concentration which influenced directly the polymer concentration resulting to increased surface tension of the polymer mixture. Higher applied voltage was required to overcome the increased in surface tension to produce interactions between electrically charged polymer blend and external electric field to reach Taylor cone formation. However, the higher electric field from applied voltage resulted in increased fiber Nanomaterials 2019,, 9,, 1526x FOR PEER REVIEW 88 of of 14 diameter greater than the desired size for nanofibers which are classified as microfibers as well as a changeFigure in fiber5. Fiber morphology. diameter distribution The electrospinning based on SEM process micrographs can of:be (furthera1,a2) ES optimized‐SDIB/PBz‐ 0;for (b1 SDIB/PBz,b2) structureblendES by‐SDIB/PBz byadjusting increasing‐5; andother ( thec1 process,c2 amount) ES‐SDIB/PBz parameters of SDIB‐10 in (Magnification: aside the polymer from voltage 1000× blend toand could produce scale be bar: manipulated the 50 desired μm). nanofiber by adjusting size otherand morphology. parameters such as feed rate, TCD and needle size to suit the polymer concentration. Further analysis of SEM images of all electrospun nanofibers in Figure 6(a1–c2) exhibited microstructures of randomly‐oriented continuous, interconnected nanofibrous structures. ES‐ SDIB/PBz‐0 in Figure 6(a1,a2) showed fibrous structure with highly uniform and smooth fibers with no occurrence of beads. Formation of randomly‐oriented continuous nanofibers were also seen in both ES‐SDIB/PBz‐5 and ES‐SDIB/PBz‐10 in Figure 6(b1–c2) but not as uniform and as smooth as ES‐ SDIB/PBz‐0. Based on these results, the polymer blending allowed the spinnability of SDIB/PBz by attaining the required molar mass to produce nanofibers. Previous study on electrospinning of pure sulfur copolymers resulted in unsatisfying results which produced broad fiber diameter with heavy beads and melting of fibers at the contact points [16]. It was explained that a polymer below the required molar mass may be blended with another polymer to produce a homogeneous blend capable of forming nanofibers [38]. SDIB has low molar mass with number‐average molecular weight (Mn) of 1262 g‐mol−1 [2] while PBz has high molar mass with Mn of 8000–9000 g‐mol−1 [28]. It was found out that, generally, higher molecular weight of the polymer results in desirable viscosity aside from the effect of concentration for fiber formation [20]. Although nanofibers were formed, the presence of few beads‐on‐string in ES‐SDIB/PBz‐10 were observed which could be due to the high voltage application. Increasing the applied voltage in electrospinning causes changes in the morphology and structure of the fibers [19] and results in the formation of beads [39]. Another observable change in the microstructures of the ES‐SDIB‐5 and ES‐SDIB/PBz‐10 were the occurrence of partial aggregation and conglutination with each fibers (Figure 6(b2,c2)). The fine tuning of nanofiber morphology and structure by increasing the amount of SDIB in the polymer blend could be manipulated by adjusting Figure 5. Fiber diameter distribution based on SEM micrographs of: (a1,a2) ES-SDIB/PBz-0; (b1,b2) otherES-SDIB parameters/PBz-5; such and as (c1 ,feedc2) ES-SDIB rate, TCD/PBz-10 and (Magnification: needle size to 1000 suit theand polymer scale bar: concentration. 50 µm). ×

Figure 6. SEM micrographs of electrospun nanofibersnanofibers of of ( (a1a1,a2,a2)) ES-SDIB ES‐SDIB/PBz/PBz-0;‐0; ( b1(b1,b2,b2)) ES-SDIB ES‐SDIB/PBz/PBz-5;‐ and5; and (c1 (,c1c2,)c2 ES-SDIB) ES‐SDIB/PBz/PBz-10;‐10; (Magnification (Magnification for fora1, a1b1, andb1 andc1: 500c1: 500×;; a2, a2b2, andb2 andc2: c2 5000: 5000×;; scale scale bar: bar: 10 × × and10 and 100 100µm). μm).

The EDX spectra and map were analyzed to determine the overall chemical composition and the distribution of the chemical elements of interestinterest inin thethe electrospunelectrospun nanofibers.nanofibers. As observed in the EDX map and spectra of ES-SDIBES‐SDIB/PBz/PBz-0‐0 in Figure7 7a–d,a–d, as as expected, expected, sulfur sulfur waswas not not present present in in the the nanofibersnanofibers where a carbon element predominantly appeared in blue color with oxygen and nitrogen while thethe EDXEDX spectraspectra showedshowed onlyonly peakspeaks ofof carbon,carbon, nitrogennitrogen andand oxygen.oxygen. The presence of sulfur (green(green color)color) inin the the EDX EDX map map of of ES-SDIB ES‐SDIB/PBz/PBz-5‐ and5 and ES-SDIB ES‐SDIB/PBz/PBz-10‐10 in Figurein Figure8(a1–b2) 8(a1–b2) confirmed confirmed the incorporationthe incorporation of SDIB of SDIB in thein the nanofibers. nanofibers. Based Based on on the the EDX EDX spectra spectra for for both both electrospun electrospun nanofibersnanofibers with SDIB,SDIB, sulfur sulfur peaks peaks were were observed observed alongside alongside with with peaks peaks assigned assigned to carbon, to carbon, nitrogen nitrogen and oxygen. and oxygen. In addition, the relative peak heights of the chemical elements present in the polymer blends

Nanomaterials 2019, 9, 1526 9 of 14

Nanomaterials 2019, 9, x FOR PEER REVIEW 9 of 14 In addition,Nanomaterials the 2019 relative, 9, x FOR peak PEER heightsREVIEW of the chemical elements present in the polymer blends9 of 14 were clearlywere correlated clearly correlated with the with respective the respective amount amount present present in the in nanofibers. the nanofibers. Applying Applying this this to to the the sulfur were clearly correlated with the respective amount present in the nanofibers. Applying this to the peaksulfur observed peak inobserved the EDX in the spectra, EDX spectra, the relatively the relatively shorter shorter sulfur sulfur peak peak in the in the nanofibers nanofibers with with lower sulfur peak observed in the EDX spectra, the relatively shorter sulfur peak in the nanofibers with lower concentration of SDIB (5 wt%) compared to the sulfur peak with higher concentration of SDIB concentrationlower concentration of SDIB (5 of wt%) SDIB compared (5 wt%) compared to the sulfur to the peak sulfur with peak higher with higher concentration concentration of SDIB of SDIB (10 wt%) (10 wt%) showed the quantitative amount of SDIB in the two electrospun nanofibers. This confirmed showed(10 thewt%) quantitative showed the amountquantitative of SDIB amount in the of SDIB two electrospunin the two electrospun nanofibers. nanofibers. This confirmed This confirmed that greater that greater concentration of S was present in the nanofibers with 10 wt% SDIB (darker green color) concentrationthat greater of concentration S was present of in S thewas nanofibers present in the with nanofibers 10 wt% SDIBwith 10 (darker wt% SDIB green (darker color) green as compared color) to as compared to the nanofibers with 5 wt% SDIB (lighter green color). the nanofibersas compared with to the 5 wt% nanofibers SDIB (lighterwith 5 wt% green SDIB color). (lighter green color).

Figure 7. Energy Dispersive X‐ray Spectroscopy (EDX) map and spectra of ES‐SDIB/PBz‐0 showing FigureFigure 7. Energy 7. Energy Dispersive Dispersive X-ray X‐ray Spectroscopy Spectroscopy (EDX) (EDX) map map and and spectra spectra ofof ES-SDIBES‐SDIB/PBz/PBz-0‐0 showing showing the the elemental compositions: (a) carbon; (b) oxygen; (c) nitrogen and (d) spectra without sulfur (S) elementalthe elemental compositions: compositions: (a) carbon; (a) carbon; (b) oxygen; (b) oxygen; (c) nitrogen (c) nitrogen and and (d) ( spectrad) spectra without without sulfur sulfur (S) (S) peak; peak; (scale bar = 5 μm); Other peaks without label in the spectra are Au used as coating material. (scalepeak; bar = (scale5 µm); bar Other = 5 μm); peaks Other without peaks without label in label the in spectra the spectra are Au are used Au used as coating as coating material. material.

Figure 8. EDX Map and spectra of (a1,a2) ES-SDIB/PBz-5 with the presence of sulfur (S) (green color) Figure 8. EDX Map and spectra of (a1,a2) ES‐SDIB/PBz‐5 with the presence of sulfur (S) (green color) Figure 8. EDX Map and spectra of (a1,a2) ES‐SDIB/PBz‐5 with the presence of sulfur (S) (green color) and sulfurand sulfur peak; peak; and and (b1 ,b2(b1),b2 ES-SDIB) ES‐SDIB/PBz/PBz-10‐10 showing showing the the presence presence of of sulfur sulfur (S)(S) (dark(dark green color) color) and and sulfur peak; and (b1,b2) ES‐SDIB/PBz‐10 showing the presence of sulfur (S) (dark green color) sulfurand peak; sulfur (scale peak; bar (scale= 5 µ barm); = Other 5 μm); peaks Other without peaks without label in label the in spectra the spectra are Au are used Au used as coating as coating material. and sulfur peak; (scale bar = 5 μm); Other peaks without label in the spectra are Au used as coating material. material.

Nanomaterials 2019, 9, 1526 10 of 14 Nanomaterials 2019, 9, x FOR PEER REVIEW 10 of 14

3.4.3.4. FTIR Spectroscopy ToTo confirmconfirm thethe eeffectffect ofof blendingblending ofof SDIB,SDIB, electrospunelectrospun nanofibersnanofibers werewere characterizedcharacterized byby usingusing FourierFourier transformtransform infrared (FTIR).(FTIR). TheThe FTIRFTIR spectraspectra ofof ES-SDIBES‐SDIB/PBz/PBz-5‐5 andand ES-SDIBES‐SDIB/PBz/PBz-10‐10 werewere comparedcompared toto thethe absorptionabsorption characteristiccharacteristic bandsbands ofof unblendedunblended SDIB and ES-SDIBES‐SDIB/PBz/PBz-0‐0 asas shownshown inin FigureFigure9 .9. The The spectrum spectrum of ES-SDIBof ES‐SDIB/PBz/PBz nanofibers nanofibers contained contained all the all characterization the characterization absorption absorption bands 1 ofbands ES-SDIB of ES/PBz-0‐SDIB/PBz assigned‐0 assigned to benzoxazine to benzoxazine structure structure at 1260–1220 at 1260–1220 cm− due tocm asymmetric−1 due to asymmetric stretching 1 1 ofstretching C-O-C bonds, of C‐O 1390–1350‐C bonds, cm1390–1350− due to cm CH−1 2duewagging to CH and2 wagging at 1520–1450 and at cm 1520–1450− for the cm tri-substituted−1 for the tri‐ 1 benzenesubstituted ring benzene as well asring C= asC bondwell atas 1680–1590C=C bond cm at− 1680–1590. The spectra cm of−1. electrospunThe spectra nanofibers of electrospun with SDIBnanofibers showed with no SDIB difference showed to thatno difference of electrospun to that PBz of withoutelectrospun SDIB. PBz There without was noSDIB. absorption There was band no 1 associatedabsorption with band C-S associated bond at 620 with cm C−‐S(dashed bond at yellow 620 cm line−1 (dashed in Figure yellow9) that line would in Figure indicate 9) the that e ffwouldect of SDIBindicate in the the structural effect of SDIB group in of the the structural electrospun group nanofibers. of the electrospun This confirmed nanofibers. that the This SDIB confirmed did not have that anythe SDIB effect did on thenot chemistryhave any effect of PBz on and the no chemistry specific interactionsof PBz and no occurred specific during interactions the polymer occurred blending. during Moreover,the polymer SDIB blending. and PBz Moreover, did not undergo SDIB and any PBz chemical did not reaction undergo during any thechemical blending reaction process during which the is desirableblending forprocess blending which of is materials. desirable for blending of materials.

FigureFigure 9. FourierFourier transform transform infrared infrared (FTIR) (FTIR) spectra spectra of ofpure pure SDIB SDIB and and ES‐ ES-SDIBSDIB/PBz/PBz (0, (0,5 and 5 and 10) 10)nanofibers. nanofibers. 3.5. Thermal Property 3.5. Thermal Property Pure PBz polymer exhibits a high glass transition temperature (Tg)[40] while SDIB (10–50 wt% S Pure PBz polymer exhibits a high glass transition temperature (Tg) [40] while SDIB (10–50 wt% ratio) has a very low glass transition temperature that ranged from 14 C to 28 C[1]. Based on the S ratio) has a very low glass transition temperature that ranged from− −14◦ °C to 28◦ °C [1]. Based on the DSC measurements as shown in Figure 10a,b, the electrospun nanofibers of pure PBz has a Tg of 383 C DSC measurements as shown in Figure 10(a,b), the electrospun nanofibers of pure PBz has a Tg◦ of while pure SDIB has a Tg of 22 C. Both ES- SDIB/PBz-5 and ES-SDIB/PBz-10 exhibited two new Tg’s of 383 °C while pure SDIB has a◦ Tg of 22 °C. Both ES‐ SDIB/PBz‐5 and ES‐SDIB/PBz‐10 exhibited two 97 C and 280 C (Figure 10c) which are between the Tg values (22–383 C) of each component polymers. new◦ Tg’s of 97◦ °C and 280 °C (Figure 10c) which are between the◦ Tg values (22–383 °C) of each This result indicated that the degree of miscibility of the polymer blend during electrospinning process component polymers. This result indicated that the degree of miscibility of the polymer blend during changed after formation of fibers made of blend of two polymers. A polymer blend would exhibit electrospinning process changed after formation of fibers made of blend of two polymers. A polymer two distinct Tg’s when the polymer blend is partially miscible where the Tg value of each polymer blend would exhibit two distinct Tg’s when the polymer blend is partially miscible where the Tg component was affected by the other one [36]. This was evident with distinctly different Tg’s but value of each polymer component was affected by the other one [36]. This was evident with distinctly between the values of the pure polymers which showed the effect of interaction of SDIB and PBz in the different Tg’s but between the values of the pure polymers which showed the effect of interaction of blend when formed into nanofibers. The same Tg’s for both polymer concentrations of 5 wt% and SDIB and PBz in the blend when formed into nanofibers. The same Tg’s for both polymer 10 wt% SDIB showed that the amount of SDIB have no significant effect on the thermal property of concentrations of 5 wt% and 10 wt% SDIB showed that the amount of SDIB have no significant effect the electrospun nanofibers. Although based on observation of the polymer blend a high degree of on the thermal property of the electrospun nanofibers. Although based on observation of the polymer miscibility is observed prior to electrospinning due to the nature of the copolymers-polymer interaction, blend a high degree of miscibility is observed prior to electrospinning due to the nature of the the application of the electric field during electrospinning may have caused repulsive forces on the copolymers‐polymer interaction, the application of the electric field during electrospinning may have surface’s jet of the polymer blend resulting the occurrence of phase-separation for the blend of SDIB caused repulsive forces on the surface’s jet of the polymer blend resulting the occurrence of phase‐ and PBz while fibers were formed. The special interactions that existed in the miscible blend of SDIB separation for the blend of SDIB and PBz while fibers were formed. The special interactions that and PBz were not sufficient to retain miscibility during the electrospinning process. For polymer existed in the miscible blend of SDIB and PBz were not sufficient to retain miscibility during the blends, miscibility is often attained with the occurrence of attractive specific interactions between the electrospinning process. For polymer blends, miscibility is often attained with the occurrence of component polymers [15]. The new Tg’s of the electrospun SDIB/PBz are lower than the PBz polymer attractive specific interactions between the component polymers [15]. The new Tg’s of the electrospun SDIB/PBz are lower than the PBz polymer but higher than SDIB copolymers which indicate that a new material with properties different from SDIB and PBz was produced.

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Nanomaterials 2019, 9, x FOR PEER REVIEW 11 of 14 but higher than SDIB copolymers which indicate that a new material with properties different from SDIBNanomaterials and PBz 2019 was, 9, x produced.FOR PEER REVIEW 11 of 14

Figure 10. Differential scanning calorimetry (DSC) curves of (a) ES‐SDIB/PBz‐0 (pure PBz); (b) pure FigureSDIB; and 10. Di(c)ff ESerential‐SDIB/PBz scanning‐5 and calorimetry ES‐SDIB/PBz (DSC)‐10 with curves the ofnew (a) glass ES-SDIB transition/PBz-0 temperatures. (pure PBz); (b ) pure Figure 10. Differential scanning calorimetry (DSC) curves of (a) ES‐SDIB/PBz‐0 (pure PBz); (b) pure SDIB; and (c) ES-SDIB/PBz-5 and ES-SDIB/PBz-10 with the new glass transition temperatures. 3.6. WaterSDIB; Contactand (c) ES Angle‐SDIB/PBz ‐5 and ES‐SDIB/PBz‐10 with the new glass transition temperatures. 3.6. Water Contact Angle 3.6. WaterThe effect Contact of Angleblending SDIB with PBz on the hydrophobicity of the electrospun nanofibers’ surfaceThe was effect determined of blending by measuring SDIB with the PBz water on the contact hydrophobicity angle of the of sample the electrospun nanofibers. nanofibers’ Pure PBz surfaceexhibitedThe was effecta water determined of contactblending by angle measuringSDIB of with130.03 thePBz ± water0.27° on theas contact shown hydrophobicity angle in Figure of the 11.of sample theThe electrospunaddition nanofibers. of SDIBnanofibers’ Pure in PBz the exhibitedsurface was a water determined contact by angle measuring of 130.03 the water0.27 contactas shown angle in Figureof the sample11. The nanofibers. addition of Pure SDIB PBz in electrospun nanofibers enhanced the hydrophilicity± ◦ which is inversely correlated with SDIB thecomposition.exhibited electrospun a water At nanofibersconcentration contact angle enhanced ofof 5130.03 wt% the SDIB,± hydrophilicity0.27° the as shownwater whichcontact in Figure is angle inversely 11. Thewas addition110.64 correlated ± of1.49° SDIB with while in SDIB the at composition.electrospun nanofibers At concentration enhanced of 5 the wt% hydrophilicity SDIB, the water which contact is angleinversely was 110.64correlated1.49 withwhile SDIB at concentration of 10 wt% SDIB, the electrospun nanofibers became quite hydrophilic± with◦ a water concentrationcontactcomposition. angle Atof 82.29concentration 10 wt% ± 1.25°. SDIB, Thisof the 5 electrospunwt%reduction SDIB, in the nanofibers water water contact contact became angle angle quite of wasthe hydrophilic electrospun110.64 ± 1.49° with nanofibers while a water at contactconcentration angle ofof 82.2910 wt%1.25 SDIB,. This the electrospun reduction in nanofibers water contact became angle quite of the hydrophilic electrospun with nanofibers a water provided surface properties± modification◦ which could presumably be caused by the extra hydrogen providedcontactbonding angle between surface of 82.29 properties the water ± 1.25°. and modification This the hydroxylreduction which groups in could water from presumably contact the addition angle be causedof of the SDIB. electrospun by the extra nanofibers hydrogen bondingprovided between surface properties the water and modification the hydroxyl which groups could from presumably the addition be caused of SDIB. by the extra hydrogen bonding between the water and the hydroxyl groups from the addition of SDIB.

Figure 11. Water contact angles of ES-SDIB/PBz-0, ES-SDIB/PBz-5 and ES-SDIB/PBz-10. Values plotted are means standard deviations with three replicates taken per data point. Figure 11.± Water contact angles of ES‐SDIB/PBz‐0, ES‐SDIB/PBz‐5 and ES‐SDIB/PBz‐ 10. Values plotted are means ± standard deviations with three replicates taken per data point. Figure 11. Water contact angles of ES‐SDIB/PBz‐0, ES‐SDIB/PBz‐5 and ES‐SDIB/PBz‐10. Values 3.7. Solubilityplotted are means ± standard deviations with three replicates taken per data point.

3.7. Solubility

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3.7. Solubility Pure electrospun PBz is insoluble in most commonly used organic solvents such as dimethlyformamide (DMF), dimethyl sulfoxide (DMSO), dimethylacetamide (DMAc) and ethanol (EtOH). The solubility of electrospun nanofibers with SDIB were tested in the same set of common solvents to determine the retention of solvent resistance of the nanofibers with the incorporation of SDIB to PBz. The results in Table2 showed that electrospun nanofibers were all insoluble in the DMF, DMSO, DMAc and ethanol. This is another way of confirming that SDIB did not have any effect on the chemical structure of the PBz after blending and electrospinning process.

Table 2. Solubility of Electrospun SDIB/PBz Nanofibers in Common Solvents.

Sample Solvent Solubility ES-SDIB/PBz-0 DMF, DMSO, DMAc, Ethanol Insoluble ES-SDIB/PBz-5 DMF, DMSO, DMAc, Ethanol Insoluble ES-SDIB/PBz-10 DMF, DMSO, DMAc, Ethanol Insoluble

4. Conclusions This work has demonstrated the processability of sulfur copolymers from inverse vulcanization of elemental sulfur blended with polybenzoxazines via electrospinning to produce electrospun nanofibers. Miscible polymer blend was attained by simple mixing of the component polymers which could be mostly attributed to the copolymers-polymer nature resulting from the interactions between the SDIB and PBz. The polymer blends have shown their compatibility to electrospinning and their capability of producing nanofibers with the resulting fiber structure and morphology. However, the effect of polymer properties and composition of SDIB/PBz could be studied further to optimize process conditions of electrospinning for the desired nanofiber morphology and microstructures. The control of other process parameters, particularly the feed rate, could be explored in electrospinning of SDIB. Effective incorporation of SDIB in polymer blend produced a new material which also modified the surface properties but was able to retain some of the desirable properties from the synergy created by blending polymers. Increasing the amount of SDIB in polymer blend could be studied for the resulting property mix of polymer blend using other component polymers. This new processing option for SDIB was able to produce new material but additional research on the optimized amount of SDIB as well as the process conditions of electrospinning process is needed to improve the properties and structures of nanofibers. Nanofibers are important one-dimensional (1D) nanostructures having unique functional properties such as high specific surface areas, porosities, aspect ratio and better pore connectivity. They could have potential applications in adsorption and separation processes as well as in fuel cell membranes.

Author Contributions: Conceptualization, R.P.P.J. and A.B.B.; methodology, R.P.P.J., Y.-L.L. and A.B.B.; formal analysis, R.P.P.J., Y.-L.L. and A.B.B.; investigation, R.P.P.J., Y.-L.L. and A.B.B.; data curation, R.P.P.J.; resources, Y.-L.L.; writing—original draft preparation, R.P.P.J.; writing—review and editing, R.P.P.J. and A.B.B.; supervision, Y.-L.L. and A.B.B.; funding acquisition, R.P.P.J. and A.B.B. Funding: This research was funded by Department of Science and Technology (DOST) under the sandwich program of the DOST Human Resource Development Program (DOST-HRDP) Scholarship, DOST AO No. 008 s 2018. Acknowledgments: This work was supported through a research exchange program between the National Tsing Hua University (NTHU), Taiwan and De La Salle University (DLSU), Manila, Philippines. The authors thank the Special Polymer and Chemical Lab (SPCL) of NTHU for the resources and technical assistance provided during the conduct of experiments, Maricar B. Carandang of CED, ITDI-DOST and ADMATEL for the conduct of additional tests on the samples and lastly, Ronaldo Parreño III for editing the photos and images. Conflicts of Interest: The authors declare no conflict of interest. Nanomaterials 2019, 9, 1526 13 of 14

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