This work is concentrated on synthesis and investigation of new core-shell nanocomposites of polystyrene (PS) with doped polyaniline (PANI). The latex containing PS nanoparticles with sizes of 15–30 nm was prepared by microemulsion polymerization of styrene in water media. The PS/PANI nanocomposites were synthesized by chemical oxidative polymerization of aniline in the PS latex media in a presence of lauryl sulfuric acid (LSA), which served as both dopant and plasticizer. The real content of PANI in the synthesized nanocomposites was determined by UV-Vis spectroscopy method. The composition of the nanocomposites and oxidation state of the doped polyaniline were characterized by FTIR spectroscopy. The core-shell morphology of the nanocomposite nanoparticles was proved by transmission and scanning electron microscopy. It was found that conductivity and thermal behavior in air of these nanocomposites not only nonlinearly depended on the doped polyaniline content but also were strongly effected both by plasticizing properties of the acid-dopant and presence of the polyaniline shell. A possibility of application of these nanocomposites as sensor materials has been demonstrated. Keywords: Polystyrene nanoparticles, Polyaniline, Core-shell nanocomposites, Conductivity, Thermal stability, Sensing ability Background polymer (nano)particle and the shell is formed of PANI It is well known that polyaniline (PANI) have a unique [3–10]. To facilitate the formation of the core-shell set of physical and chemical properties, high stability, morphology, aniline is polymerized as its salt, which low price, etc., which allows its multifunctional applica- appears in the latex medium due to aniline interaction tions in different high-tech fields such as micro- and with an added acid-dopant, typically HCl (e.g., [8, 10]), optoelectronics, sensor and electrochromic devices, or as commercial aniline hydrochloride salt (e.g., [4, 7]). batteries, and supercapacitors, etc. [1, 2]. Processability In many cases, stability of the latex polymerization and applicability of PANI can be significantly improved media is additionally supported with non-ionic or ionic if it is used in composites or nanocomposites with stabilizing additives (more often sodium dodecyl sulfate/ soluble or meltable common polymers, which can be lauryl sulfate (SDS/SLS) [3–10]). However, there has readily formed into various articles . Among various been developed an alternative method to prepare such methods of preparation of such materials, the oxidative materials. This method uses a surface active acid (e.g., polymerization of aniline in water-based acidified latexes dodecylbenzenesulfonic acid––DBSA) unifying proper- or dispersions containing nanoparticles or (sub)micron- ties of surfactants, plasticizers, and acid-dopants and sized particles of other polymers (stabilized with differ- therefore allowing to avoid the abovementioned use of ent surfactants or non-ionic polymers) is considered as additional HCl or other acid-dopant [11, 12]. one of the most effective approaches . This approach Mechanistic aspects of the formation of PANI layers allows to obtain multifunctional composites or nano- or shells on the surface of different organic or inorganic composites of core-shell type, where the core is the macroscopic (glass or quartz, polymer films, fibers, etc.) and microscopic (polystyrene latexes, silica or titania, or polymer particles, etc.) substrates (templates) have been * Correspondence: firstname.lastname@example.org; email@example.com 1 discussed in a lot of publications mainly in terms of in Institute of Bioorganic Chemistry and Petrochemistry of NAS of Ukraine, 50 Kharkivske shose, Kyiv 02160, Ukraine situ adsorption polymerization of the positively charged Full list of author information is available at the end of the article © The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Pud et al. Nanoscale Research Letters (2017) 12:493 Page 2 of 11 anilinium cations at surface typically bearing negative 90% at 100 ppm (Fig. 11 in ). This weak response con- charges of preadsorbed/grafted anions/functional groups centration behavior suggests that due to solution prepar- [13–17]. It has been generally accepted that the ation of these blends only a part of the sensitive PANI adsorbed anilinium cations polymerize immediately after clusters is easily accessible to the analyte molecules, and the addition of the oxidant-initiator. Naturally, the non- other part is screened by PS matrix, which imparts some adsorbed anilinium cations are also involved in the diffusion limitations to the sensing materials. Therefore, polymerization process and form positively charged one can deduce that PS/PANI core-shell composites with oligomeric and polymeric molecules, which precipitate/ unscreened PANI surface can have an improved sensing adsorb at the same surface and therefore cause an behavior as compared with the solution prepared blend increase in the PANI shell thickness. materials . In an alternative approach, aniline monomer was first Based on the above discussion, our work was concen- added to the polystyrene (PS) latex and absorbed in a trated mainly on synthesis of new core-shell nanocom- neutral form by PS core particles for 3 days . After posites of PS nanoparticles with PANI doped with lauryl addition of the oxidant-initiator (ammonium persulfate, sulfuric acid (LSA) and on investigation of their practic- APS), the conducting polyaniline membrane is formed at ally important properties (morphology, chemical struc- the particle interface and separate the both reagents. ture, conductivity, thermostability). Their potential Electrons are transferred from the aniline molecules to applicability as sensing materials was also estimated. The the oxidant molecules through polyaniline membrane, LSA choice is the important feature of the nanocompos- and therefore, polyaniline gradually penetrates inside the ites, which is based mainly on three prerequisites: (1) PS latex particle, in contrast to the abovementioned the same lauryl sulfate surface active anion both in the core-shell morphology obtained in classical coating of surfactant SLS used at the stage of the core PS nanopar- latex particles with polyaniline . In another variation ticle synthesis and in the acid-dopant LSA used at the of this approach, after swelling of PS particles with the stage of the PANI shell synthesis, (2) it can perform as neutral aniline monomer for 12 h, APS and then the surface active functionalized protonic acid-dopant hydrochloric acid were added to the reaction medium acidifying the reaction medium [24, 25], and (3) it forms . The HCl addition resulted in transformation of the anilinium salt (i.e., surface active reactive monomer or aniline molecules released from the particles to anili- surfmer) facilitating formation of nanosized PANI shells nium cations, which in turn were polymerized by and structures [25, 26]. chemical oxidation with APS. The clear core-shell struc- ture of the formed PS/PANI composite was confirmed Methods in this case . Materials PS quite frequently serves in such materials as the core Aniline (Merck) and styrene (reagent grade, Ukraine) were polymer component due not only to its good thermal distilled under vacuum and stored under argon at 3–5°C. and chemical stability, mechanical characteristics, The oxidant potassium persulfate (KPS) (Ukraine), anionic biocompatibility, etc.  but probably to its conveni- surfactant sodium lauryl sulfate (SLS, synonymously ence for synthesis of well-shaped nano/submicron/mi- sodium dodecyl sulfate––SDS, Aldrich) were of reagent cron-sized particles being very suitable for the grade and used without further purification. Lauryl (nano)composites with specific applications. For example, sulfuric acid (LSA) was prepared from the SLS via ion- micron/submicron-sized PS particles coated with doped exchange reaction with KU-2-8 resin (Ukraine). PANI were used in electrostatic accelerators, which allowed accelerating the charged particles to hyperveloc- Preparation of PS latexes ities  or in electrorheological fluids , etc. Similar to PS nanoparticulate latexes were prepared by radical other core-shell (nano)composites  PS/PANI ones polymerization of styrene in accord with the method obviously have potential for sensing applications. How- described elsewhere . In short, styrene was polymer- ever, to our knowledge, there is a lack of information on ized in a micellar aqueous solution of SLS with oxidant- using PS/PANI core-shell (nano)composites as sensing initiator KPS as follows: 2 g of styrene were added slowly materials. Nevertheless, recently, it was shown that mixing over a period of 1.5 h to a vigorously stirred solution of PS dissolved in toluene with PANI particles doped with 0.01 g NaH PO , 0.2 g SLS, and 0.01 g KPS in 10 ml of 2 4 camphorsulfonic acid gave dispersions, suitable for water at 70 °C in argon atmosphere. The mixture was formation of the composite films sensitive to ammonia stirred for additional 3 h at 70 °C and then for additional . Interestingly, while these blends showed quite high 1 h at 90 °C. The final polymerization mixture was responses to gaseous ammonia, i.e., (ΔR/R ) × 100 ~ 73% cooled to room temperature and purified by dialysis at 20 ppm, their responses to higher concentrations of through cellulose membrane with MWCO 3500 Da ammonia were not very different and were only up to ~ against distilled water for 48 h. Pud et al. Nanoscale Research Letters (2017) 12:493 Page 3 of 11 Preparation of PS/PANI-LSA nanoparticles nitrogens in PANI namely PANI:LSA = 1:0.5 similarly to The aniline polymerization in the PS latex was carried [28, 29]. In accord with , for simplicity and clarity of out similarly to the method described elsewhere [28, 29] this recalculation, we postulated the complete PANI dop- at next ratios of the reaction mixture components: anil- ing with only LSA. The compositions of the synthesized ine/LSA = 1/1.5 (mol/mol) and aniline/oxidant = 1/1.25 nanocomposites and their notations are given in Table 1. (mol/mol) at 10 °С. The initial weight ratio of aniline to Fourier transform infrared (FTIR) spectra of the PS/ PS nanoparticles in the polymerization mixture was PANI-LSA nanocomposites and pure PANI-LSA sam- predetermined by expected theoretical quantities of ples in pellets with KBr were recorded with a resolution −1 dedoped polyaniline in the ultimate nanocomposites in a of 1 cm with Bruker Vertex 70 spectrometer. range of 1–10 wt%. In short, at the first stage of the Transmission and scanning electron microscopy (TEM preparation of the polymerization mixture, a calculated and SEM) images were obtained with JEOL JEM-1400 quantity of the acid was added to the target PS latex and Hitachi S4800 microscopes, respectively. Samples portion and stirred at room temperature for 30 min. At for TEM measurements were prepared by placing 2 μL the second stage, the calculated quantity of aniline was of the sample water dispersion onto a carbon or formvar added to this acidified PS latex followed by stirring for coated 200 mesh copper grids for 15 min followed by a 1 h to allow complete formation of the anilinium salt at careful removal of the dispersion with a filter paper. The room temperature and then the prepared mixture was samples for SEM measurements were prepared by drop- cooled down to 10 °C for 30 min. At the third stage, the ping 5 μl of the dialyzed water dispersion of pure PS or calculated quantity of the precooled to 10 °C KPS solu- a nanocomposite onto a glass plate. The dried samples tion in distilled water was added dropwise into the reac- were sputter coated with a thin (~ 7 nm) gold layer. tion mixture followed by stirring for 24 h at 10 °C. After Thermal stability of the synthesized materials was stud- the aniline polymerization was completed, the obtained ied by thermogravimetry analysis (TGA) of their samples PS/PANI-LSA latexes were purified by dialysis through in air when using a MOM Q-1500 D (Paulik-Paulik-Erdey) the cellophane membrane against distilled water for Derivatograph system with a heating rate of 10 °C/min. 3 days. The purified nanocomposites were dried at ambi- In order to characterize conductivity properties of the ent conditions to visually dry powder condition followed synthesized nanocomposites, their powders were proc- by drying under vacuum at 60 °C until a constant weight essed into films both by compression molding technique was reached. The reference pure PANI-LSA sample was at 240 °C under 5 MPa (using SPECAC press) for 2 min synthesized under the same conditions in the water solu- and by casting on glass plates from their 3% dispersions tion in the absence of PS nanoparticles. prepared under ultrasonication. To estimate applicability of the synthesized PS/PANI- Characterization LSA nanocomposites as materials which are sensitive to The real PANI contents in the synthesized nanocompos- harmful gases, we used the most conducting nanocom- ites were determined similarly to  by UV-Vis posite NC15 and compared its properties with pure spectroscopy analysis of their solutions in in N-methyl- PANI-LSA synthesized under the same conditions. 2-pyrrolidone (NMP) with the help of spectrophotom- Ammonia-air mixtures with ammonia concentrations in eter Cary 50 (Varian). In short, at the first stage, the dry the range of 19–152 ppm served as analytes. Sensitive nanocomposite was typically dedoped in 0.3 wt% ammo- elements were prepared as follows. A 1 μL volume of nia aqueous solution for 24 h followed by washing with the ultrasonically treated dispersions of the nanocom- distilled water and then drying under vacuum at 60 °C posites in solvent (2% w/v) was drop-cast on the mini- until a constant weight was reached. At the second ature system of gold interdigitated electrodes formed on stage, the fixed portion of the dedoped powder nano- Table 1 Description of the samples composite was dissolved in NMP and mixed with Samples Real PANI base PANI-LSA Notation ascorbic acid solution in NMP to obtain leucoemeral- content, wt% content, wt% dine base (LB, fully reduced form of PANI). At the third PANI-LSA Reference sample stage, the LB concentration was calculated from UV PS/PANI-LSA 0.75 1.84 NC2 absorption of this solution in 1 mm quartz cuvette at 343 nm using the previously prepared calibration curve. PS/PANI-LSA 1.24 3.01 NC3 At the fourth stage, this LB concentration was then PS/PANI-LSA 2.45 5.84 NC6 recalculated for the real dedoped PANI content in the PS/PANI-LSA 4.89 11.27 NC11 nanocomposite. The latter content was then used to PS/PANI-LSA 6.58 14.82 NC15 estimate the doped PANI-LSA content in this PS/PANI- NC is a general abbreviation of the nanocomposite; numerals display the LSA nanocomposite. This final recalculation was based on rounded calculated content of PANI-LSA based on the real dedoped the theoretical stoichiometric ratio of LSA and imine PANI contents Pud et al. Nanoscale Research Letters (2017) 12:493 Page 4 of 11 the glass–ceramic substrate. The formed sensing shaped PS nanoparticles of very small sizes in the range elements were dried at 60 °C for 30 min and then of 15–30 nm. To our best knowledge, these PS nanopar- installed into the airtight testing chamber described ticles are among the lowest PS ones. elsewhere . The prepared ammonia-air mixtures TEM images of the PS/PANI-LSA nanoparticles were injected by syringe in this chamber. Sensor separated after the polymerization show that they have responses (SR) of these elements were recorded at ambi- sizes increasing with PANI-LSA content (Fig. 1b–f). This ent temperature and relative humidity around 50% and effect suggests core-shell morphology of these nanopar- determined as a relative variation of the resistance R of ticles with core of PS nanoparticle and shell of PANI- the sensor exposed to the analyte in accord with the LSA. Nevertheless, despite the increased sizes, in the equation SR = [(R−R )/R ] × 100%, where R is the case of low contents of PANI-LSA in the nanocompos- 0 0 sample resistance, R is the initial resistance value. ites (NC2, NC3, NC6), it is quite difficult to visually distinguish thin PANI-LSA shells (Fig. 1b–d). This prob- Results and discussion lem can be probably explained by a polymer nature of Morphology of the synthesized PS/PANI-LSA the both components and loose structure of these shells. nanocomposites The latter, in turn, can be caused by the large size of the As one can see from the TEM image (Fig. 1a), the used dopant anions hindering the formation of compact synthetic approach allowed to synthesize spherically PANI-LSA shells. However, at higher PANI-LSA Fig. 1 TEM (a–f) and SEM (g–o) images of pure PS and PS/PANI-LSA nanocomposites: a, g-purePS; b, h-NC2; c, i-NC3; d, m-NC6; e, n-NC11and f, o-NC15 Pud et al. Nanoscale Research Letters (2017) 12:493 Page 5 of 11 contents in NC11 and especially in NC15 the irregular In general, the TEM and SEM measurements show shells can be distinguished (Fig. 1e, f). that although pure PS nanoparticles after cleaning tend In spite of quite wide size distributions of the nano- to agglomerate, the low contents of PANI-LSA in the composite particles (Fig. 1b–f), we can roughly estimate nanocomposites NC2 and NC3 suppress this agglomer- their shell thicknesses. In particular, while NC2 with the ation. This effect can be assigned to the surface activity lowest content of PANI-LSA (Table 1) contains nanopar- of charge compensating large LS¯ anions which localize ticles with sizes around 15 nm similar to those of parent around positively charged PANI shells on PS cores and PS, one can find in the TEM image (Fig. 1b) nanoparti- therefore separate the nanoparticles. However, the situ- cles with sizes up to 40 nm that probably indicates pres- ation is reversed at the moderate (NC6) and especially at ence of PANI-LSA shells with thicknesses up to 10 nm the high (NC11 and NC15) PANI-LSA contents, which on their surfaces. apparently facilitate formation of quite thick PANI-LSA In the case of NC3 15 nm nanoparticles are not ob- shells around PS cores. As a result, number of charge served but the number of 30–40 nm nanoparticles with compensating LS anions both around and inside of shell thicknesses up to 10 nm increased significantly positively charged PANI shells becomes higher as com- (Fig. 1c). This tendency is enhanced in NC6 nanoparti- pared with NC2 and NC3 cases. Inevitably, these cles (Fig. 1d). TEM images of NC11 and especially of amphiphylic anions with long dodecyl tails can enhance NC15 display nanoparticles with increased sizes in the existing in the system intermolecular interactions. These range of about 25–50 nm (Fig. 1e, f). A presence of interactions are probably stronger that the abovemen- some spots of irregular shape suggests appearance of a tioned tendency in NC2 and NC3 and in turn can cause separate phase of PANI-LSA in these nanocomposites the observed agglomeration of NC6, NC11, and NC15 due to its higher contents. Moreover, the NC15 image nanoparticles. allows to clearly distinguish irregular PANI shells with thicknesses of 10–20 nm. FTIR measurements After cleaning and preparation of the parent PS latex The structures of the synthesized polymers are charac- for SEM imaging (see “Characterization” section), PS terized by means of their FTIR spectra. In particular, as nanoparticles formed agglomerates with sizes in the one can see in Fig. 2, FTIR spectrum of PS contains five range of 30–150 nm or more, which presumably characteristic peaks of aromatic C–H stretching vibra- −1 included 2–5 or more initial nanoparticles (Fig. 1g). tions with the maximal peak at 3025 cm . Peaks of The aniline polymerization in the latex medium in the C–H stretching vibration of methylene groups occur at −1 presence of surface active LSA changed the situation 2920 and 2850 cm . Four bands of aromatic C=C (Fig. 1 h–o). Thus, at the lowest PANI-LSA content stretching vibrations are observed at 1601, 1583, 1492, −1 (1.84 wt%), one can see in the NC2 image large irregu- and 1452 cm . The very strong bands at 756 and −1 lar entities with sizes of about 400–500 nm which have 697 cm can be assigned to the CH out-of-plane a quite smooth surface (Fig. 1h). In the case of NC3 with increased PANI-LSA content (3.01 wt%), the entities have a tendency to less sizes in the range of about 100–300 nm. This tendency is strongly enhanced at higher PANI-LSA content in NC6 (5.85 wt%). In particular, its SEM image shows not only a small number of entities with sizes up to 150 nm but also irregular agglomerates with sizes in the range of 40– 100 nm (Fig. 1m). SEM images of NC11 and NC15 (Fig. 1n, o) demonstrate further development of the samples morphology, namely, qualitative and quantita- tive changes in these nanocomposites due to the highest PANI-LSA contents 11.27 and 14.82 wt%, respectively. Specifically, one can see quite densely packed agglomerates with sizes mainly in the range of about 25–50 nm on the flat NC11 sample surface, while in the case of the NC15 sample well distinguishable 25–50 nm agglomerates arranged in “bunch of grapes” Fig. 2 FTIR spectra of PS (1), PANI (2), and PS/PANI-LSA composites: NC3 (3), NC3 (4), NC3.5 (5), NC11 (6), NC15 (7). Main characteristic peaks of PS morphology are observed (Fig. 1n, o). This morphology and PANI-LSA are marked with dashed red and blue lines, respectively. All suggests a higher specific surface of NC15 compared marks correspond to the frequencies discussed in the text with other nanocomposites. Pud et al. Nanoscale Research Letters (2017) 12:493 Page 6 of 11 vibration and the ring out-of-plane deformation, respect- contributions. In particular, the PS contribution is the ively, . These bands confirm the presence of a mono- PS spectrum normalized to the band height of NC15 at −1 substituted aromatic group. 3025 cm (where the PANI-LSA absorption is very In turn, FTIR spectrum of PANI-LSA agrees well with weak), and PANI contribution is the PANI spectrum −1 published data [32–34]. It contains typical bands at normalized to the band height of NC15 at 1560 cm −1 1565, 1492, 1294, 1133, and 818 cm assigned to (where the PS absorption is absent). It is known that −1 stretching vibrations of quinoid rings, benzenoid rings, doped PANI bands at about 1580 and 1490 cm have a C–N stretch in a secondary aromatic amine, vibrational major contribution from the quinoid and benzenoid mode of a B–NH = Q structure, C–H out-of-plane rings, respectively, [32–34]. The intensity ratio of these bending of 1.4-rings, respectively. Some features, such as bands is sensitive to the chemical structure of the PANI, a very weak NH stretching vibrations in the region and therefore, the dominance of quinoid rings over the −1 3l00–3500 cm , indicate that PANI is in the doped benzenoid units in the spectrum of NC15 compared state. However, the B–NH = Q band intensity at with the model spectrum testifies that the oxidation de- −1 1133 cm is quite weak that suggests a quite low gree of PANI-LSA phase in the nanocomposite is higher doping level of this PANI-LSA . than that of the pure PANI. One can also see that the PS −1 A distinct band at about 1180 cm (Figs. 2 and 3b, bands of the aromatic C=C stretching vibrations at 1601 −1 curve 1) originating from S=O stretching vibration  and 1583 cm are broadened in the NC15 spectrum shows that synthesized PS nanoparticles contain lauryl and slightly shifted to lower wavelengths. This shift sulfate anions, which are present apparently due to syn- probably indicates π–π interaction between PANI and −1 thesis conditions of the PS nanoparticles (see Methods PS. The intensity of the NC15 band at 1133 cm is part). Moreover, an excess of these anions is evidently appreciably higher than that of the model spectrum, observed in the final PS/PANI-LSA composites. Thus, as indicating the higher conductivity of PANI phase in this can be seen from Fig. 3a (curve 2), C–H stretching vi- nanocomposite as compared with the pure PANI. brations of aromatic rings and methylene groups of PANI-LSA are very weak. Therefore, the intense bands Thermal stability of C–H stretching vibrations of methylene groups, which Recently, it has been shown for micron-sized particulate are revealed due to subtraction of the PS spectrum from core-shell polymer-polymer composites of polycarbonate the NC15 one (after normalization by height of the band (PC) with quite low contents of PANI (~ 2 wt% of PANI −1 at 3025 cm ) (Fig. 3a, curve 4), can be obviously base or 3.5–5.0 wt% if doped by different aromatic sul- assigned to the separate SLS phase. fonic acid-dopants) that presence of the dopant strongly To evaluate the state of PANI in the nanocomposite, affects their thermal stability . Depending on the we compared the spectrum of NC15 with the model alkyl substituent in aromatic ring of the dopants, the spectrum (Fig. 3b, curves 3 and 4 accordingly). The last composites demonstrated more (long substituent) or less is the sum of the PS and PANI-LSA spectral (short substituent) decrease of their thermal stability as Fig. 3 FTIR spectra of PS nanoparticles (1), PANI-LSA (2), and NC15 (3): a spectrum 4 is the result of subtraction of the normalized PS spectrum −1 from the NC15 one, b spectrum 4 is the sum of the PS spectrum (normalized to the band height of NC15 at 3025 cm ), and the PANI-LSA −1 spectrum (normalized to the band height of NC15 at 1560 cm ) Pud et al. Nanoscale Research Letters (2017) 12:493 Page 7 of 11 compared with pure PC due to specific intermolecular As one can see in Fig. 4, TG curves of the nanocom- interactions of the plasticizing large dopant anions and/ posites have a shape similarity with that of PS and, or thermally released dopant molecules with PC chains moreover, demonstrate similar small mass losses at tem- . But at temperature above 500 °C, at which PANI in peratures up to 120 °C, which typically can be assigned the shell is typically in the dedoped (base) state, the to water evaporation . At higher temperatures, one composites stability was higher than that of PC. This can see significant differences in thermal stability of the effect was assigned to a specific state of PANI located as samples with low and high loading of PANI-LSA, which, a shell at the surface of the core material in the core- in general, give complementary information about the shell composites and to a possible stabilization of the PC thermal behavior specificity of the known core-shell core particle by the PANI shell [35, 36]. Based on this PANI-containing composites. In particular, three of the possibility and the nanocomposites morphology, we nanocomposites (NC2, NC3, and NC11) with PANI-LSA suggest that the PANI shell stabilizing effect can also content ≤ 11.27 wt% display similar to PS high thermal take place for different polymer-PANI core-shell nano- stability up to 208 °C (Fig. 4, Table 2). However, NC15 composites. The suggestion agrees well with thermal with the highest PANI-LSA content (14.82 wt%) is less behavior of the synthesized PS nanoparticles and nano- stable than PS even at 120 °C (Fig. 4, curves 1 and 5; composites illustrated in Fig. 4. Table 2) that probably can be assigned also to evapor- Indeed, the synthesized PS nanoparticles demonstrate ation of not only moisture but also probably to the thermal stability differing in some extent from that of unbound dopant and/or unreact monomer/oligomer im- the bulk PS (compare curve 1 in Fig. 4 and Figure 1 in purities . ). In particular, while the latter degrades in air In the temperature range of 208–262 °C, all nanocom- primarily in a single step from 200 to 450 °C , the posites show weight losses, which are higher than LSA thermogravimetric (TG) curve of the former shows contents but significantly less than weight losses of PS roughly three stages: week weight losses (~ 1.9 wt%) (Fig. 4, curves 1, 4, and 5, Table 2). However, in the case of from beginning to 262 °C, the second one in the range NC2 and NC3, these losses are even higher than contents of 262–330 °C and the third one in the range of 330– of PANI-LSA. Based on the high thermal stability of PANI 505 °C. This difference can be probably explained by base  and thermal behavior of the PS nanoparticles specificity of PS nanoparticle synthesis resulted in inevit- (Fig. 4, curve 1), we probably may assign the nanocompos- able presence in their composition of SLS impurity ite losses not only to evaporation and degradation of the which in turn changed the PS thermal behavior. This dopant but also to degradation of the PS component. suggestion agrees well with the fact that the final degrad- Moreover, whereas weight losses of NC2 and NC3 at 262 ° ation temperature of SLS is very close to the beginning C (Table 2) exceed sums of their LSA contents and PS loss (330 °C) of the third (main) stage of the PS nanoparticles (3.02 and 3.7, respectively), one may assume that some en- degradation (Fig. 4, curves 1 and 6). hancement of the thermooxidative degradation of the PS core component of the nanocomposites can be caused by degradation products of the dopant. Although the nanocomposites losses typically increase at higher temperatures, at 290 °C TG curves of NC2 and NC3 (unlike those of NC11 and NC15) intersect with TG curve of PS in the point of 5.58 wt% (Fig. 4, Table. 2). This behavior, in general, suggests a complete loss of the dopant [35, 37, 38] and transformation of the PANI- LSA component in the dedoped PANI. Above this, temperature NC2 and NC3 are more stable than PS up to the end of the heating process (Fig. 4, curves 1–3). As a consequence, the position of the TG trace of PS nano- particles along the temperature axis in the range of 262– 430 °C roughly separates positions of the nanocompos- ites with low and high contents of PANI-LSA (Fig. 4). This fact confirms a difference which is probably inher- ent to these two sets of nanocomposites. Indeed, one can see strongly different course of the Fig. 4 Thermogravimetric curves of the PS/PANI-LSA nanocomposites thermal degradation of these nanocomposites both in with different PANI-LSA contents (wt%): 1 PS, 2 1.84 (NC2), 3 3.01 the range of 262–430 °C and above 430 °C. Whereas all (NC3), 4 11.27 (NC11), 5 14.82 (NC15), 6 SLS these nanocomposites have the core-shell morphology, it Pud et al. Nanoscale Research Letters (2017) 12:493 Page 8 of 11 Table 2 Weight losses (in wt%) of the PS/PANI-LSA nanocomposites (see Table 1), control PS nanoparticles, and SLS samples at different temperatures. LSA contents are given in brackets Temperature, °C NC2 (1.09) NC3 (1.77) NC11 (6.38) NC15 (8.24) PS SLS 120 0.12 0.12 0.18 0.9 ~0 0.61 180 0.96 0.64 0.85 2.12 0.8 1.29 208 1.33 1.33 1.33 2.93 1.33 3.92 262 3.45 4.59 7.64 8.78 1.93 39.98 290 5.68 5.68 8.74 10.27 5.68 58.06 330 7.04 6.65 12.01 12.01 9.33 70.50 350 7.83 7.06 15.86 15.86 12.80 71.00 430 69.42 50.68 83.56 80.50 82.04 72.86 505 99.42 98.65 91.01 83.36 97.5 73.04 is unlikely that only this morphological factor can ex- is probably their ability to withstand conditions of com- plain their specific thermal behavior. However, if to take mon treatments, which are typically applied to produce into account the presence in their composition of the different articles. Therefore, a lot of studies have been LSA dopant, which contains the long dodecyl tail with performed to estimate changes in properties of these plasticizing ability , we can at least partially under- materials after treatments by melting or solution stand such difference as a result of intermolecular inter- techniques [3, 38, 39]. Based on these studies and the actions (causing a plasticizing effect ) of the dopant thermally induced weight losses of the synthesized nano- anion with the polymer components of the nanocom- composites (Table 2, Fig. 4), one might expect that such posites. Naturally, in the case of low or high contents of important property of doped PANI as conductivity could the PANI-LSA component, its influence on thermal be- be changed under these treatments. Indeed, as one can havior of the nanocomposites will be less (NC2, NC3) or see from Fig. 5, the values of conductivity of the cast and more (NC11, NC15) significant. In the latter case, the compression molded PS/PANI-LSA films strongly differ. plasticization effect is so strong that the thermogramms To quantify the difference, we treated these data (Fig. 5) of NC11 and NC15 (Fig. 4, TG curves 4 and 5) take by the scaling law based on the percolation theory  in positions below the PS one up to 430 °C even after a accordance with the known methodology of processing complete removal of the dopant (above ~ 290 °C) the conductivity behavior of polyaniline networks in because of weakened interactions between PS macro- composites [42, 43]: molecules. Slowing down the degradation rate of the nanocomposites with high content of PANI base at tem- t σ ¼ σðÞ f −f ð1Þ peratures above 430 °C can be probably explained by cross-linking of its chains  and possible enhance- ment of the stabilizing role of the PANI shell. In the case of NC2 and NC3, the situation is obviously opposite to NC11 and NC15. In particular, contents of PANI-LSA are quite small, and therefore, quantities of the plasticizing dopant LSA are not enough to signifi- cantly weaken interactions between PS macromolecules. As a consequence, once the dopant is eliminated com- pletely, the nanocomposites display thermostability which is higher than that of PS nanoparticles (Fig. 4, curves 2 and 3, intersection point at 290 °C). In spite of the low content of PANI-LSA and, therefore, of its thin shell, these NC2 and NC3 behaviors match well with the suggestion about stabilizing effect of the PANI base shell. Conductivity and sensing properties of the synthesized Fig. 5 Dependencies of DC conductivity of the cast (1)and compression nanocomposites molded (2) PS/PANI-LSA nanocomposite films on the volume fraction One of most important features of polymer-polymer of PANI-LSA composites, in particular of PANI-containing composites, Pud et al. Nanoscale Research Letters (2017) 12:493 Page 9 of 11 where σ is the constant displaying conductivity of the consequence, the conductivities of the cast nanocompos- PANI conducting phase, f is the volume fraction of ite films are more than three orders of magnitude higher PANI, f is the percolation threshold, and t is the critical than those of the compression molded ones at low vol- exponent. Volume fractions of PANI-LSA in the nano- ume fractions (contents) of PANI-LSA (Fig. 5). composites were calculated on the basis of densities of Nevertheless, despite the significant difference of the cast PS and PANI-LSA, i.e., 1.04  and 1.18 g/cm , and compression molded films, one can deduce that the respectively. conductivity level is enough to apply the both materials for The power-law dependence was determined with vari- antistatic applications. On the other hand, the obtained ous trial values of f by applying a linear regression conductivity values of the synthesized nanocomposites are analysis to the plot of log σ versus log (f − f ). The solid significantly lower (by 2–3 orders of magnitude) than in lines represent best fits to the data with the correlation the case of the similar core-shell submicron/micron-sized coefficients of 0.996 and 0.993 for the cast and compres- PS/PANI composites [4, 7, 9, 14]. To understand this sion molded nanocomposite films, respectively. difference and to improve the conductivity of these new The observed nonlinear dependences (Fig. 5) are PS/PANI-LSA nanocomposites, new studies are planned. obviously the result of formation of the phase-separated However, one can suggest that non-optimal conditions of conducting percolation network of PANI-LSA in the preparation of these new materials are at least partial bulk of the nanocomposite films. It is interesting to note explanation of this low conductivity level. that the percolation thresholds are quite low (f = 1.26%) Based on better conductivity properties of the cast PS/ and independent on the used processing techniques. PANI-LSA films and known high sensing ability of This f value is significantly lower than the theoretical doped PANI , we estimated their potential as sensing model suggests for a random lattice of spheres (from 15 materials to determine concentrations of ammonia in its to 30% depending on the sphere diameter) . gaseous mixtures with air. The measurements were However, the conductivity of the PANI conducting phase performed on the example of the films of NC15 and (σ ) in the cast nanocomposite films is more than two pure PANI-LSA cast on electrodes of the transducer (see and a half times higher than that of the compression “Methods” chapter). −4 −5 molded ones (2.3 × 10 and 8.9 × 10 S/cm, respect- Both films demonstrate quite high sensitivity to ammo- ively). Obviously, the lower conductivity of the conduct- nia in the range of 19–152 ppm (Fig. 6). However, while ing phase in the compression molded film is caused by NC15 is more sensitive to ammonia than pure PANI-LSA the partial thermal degradation of PANI-LSA under the in the concentration range of 19–114 ppm, at higher melting treatment temperature (240 °C). The values of concentrations, the situation becomes opposite. the critical exponent t for the cast and compression The better efficiency of NC15 in this narrowed ammo- molded films are 1.14 and 2.62 accordingly. Such in- nia concentration range can be probably assigned to equality in the critical exponent indicates a strong differ- core-shell morphology of the nanoparticles constituting ence in the spatial structure of the percolation cluster, the cast nanocomposite film. This morphology typically which results in the different slopes of the curves. As a specifies higher surface of the film as compared with Fig. 6 Sensor responses (calibration curves) of the cast pure PANI-LSA (1)and NC15 (2) films to different concentrations of ammonia in the mixtures with air Pud et al. Nanoscale Research Letters (2017) 12:493 Page 10 of 11 pure PANI-LSA and improves sensitivity of sensing Based on FTIR and conductivity studies of the synthe- materials [25, 28]. The enhancement of the sensing sized nanocomposites, we proved that oxidation state responses of pure PANI-LSA at ammonia concentrations and conductivity of the PANI phase are appreciably above 114 ppm (Fig. 6, curve 2) can be probably higher than those of pure PANI-LSA. Moreover, we assigned to additional involving in the sensing process of demonstrate here that thermal behavior of these nano- the PANI-LSA clusters located under the surface of the composites in air is strongly different for low and high film. Naturally, the quantity of these clusters in the pure PANI-LSA loadings that probably stems both from the doped PANI film is much higher than in the case of the plasticizing ability of the LSA dopant and stabilizing ef- thin PANI-LSA shells on the core particles constituting fect of the PANI shell. This fact, in general, gives the the nanocomposite film. Therefore, their involvement in complementary information about the thermal behavior the sensing process inevitably increases sensor responses specificity of the known PANI containing core-shell of the pure doped PANI film as compared with the composites. NC15 one. At the same time, based on thermal stability, conduct- ivity and sensor studies, we conclude that properties of Conclusions the synthesized PS/PANI-LSA nanocomposites testify to The new PS/PANI-LSA nanocomposites have been their potential applicability as materials for antistatic synthesized with the core-shell nanoparticle sizes ~ 25– and sensing applications. 50 nm, which to our knowledge are the lowest ones Acknowledgements among the similar composites published elsewhere. The This work is partially supported in the frames of the project “The formation, use of LSA as acidifying agent for the aniline containing properties and interactions of nanocomposites of conducting polymers and PS latex medium and addition of the oxidant resulted in bioactive compounds in heterophase systems” of the NASU program of fundamental research and the complex scientific-technical program “Sensing the precipitation of the thin PANI-LSA shell (~ 10– devices for medical–ecological and industrial–technological problems: 20 nm) on the surface of the PS nanoparticles (synthe- metrology support and trial operation” of National Academy of Sciences of sized in the presence of SLS). As a consequence, both Ukraine for a partial financial support. The authors acknowledge the Centre of collective usage of the Institute of Microbiology and Virology of NAS of the shell and PS core contained the same lauryl sulfate Ukraine for the TEM measurements. surface active anion unlike the known core-shell PS/ PANI composites synthesized with a PS latex surfactant- Authors’ contributions stabilizer and PANI dopant of different nature. AAP planned, supervised this work, and participated in paper writing. OAN We have found that although the synthesized very and LOV adjusted, planned, performed, and optimized syntheses of PS latexes and their analyses. YVN adjusted and performed syntheses of core- small PS nanoparticles (15–30 nm) after cleaning tend shell PS/PANI-LSA latexes, determined their composition, and performed TG to agglomerate in the dry state, the low contents of studies. NAO analyzed FTIR spectra and conductivity percolation behavior, PANI-LSA in the nanocomposites suppress this agglom- found state of PANI in the nanocomposites, and participated in paper writ- ing. OSK found conditions and carried out sensing studies. EAF performed eration probably due to the surface activity of charge and analyzed FTIR studies. All authors took part in the discussion of the compensating large LS¯ anions which localize around results. All authors read and approved the final manuscript. positively charged PANI shells on PS cores and separate the nanoparticles. However, the situation becomes op- Competing interests The authors declare that they have no competing interests. posite at the moderate and especially at the high PANI- LSA contents, which apparently facilitate formation of quite thick PANI-LSA shells around PS cores. In this Publisher’sNote case, a number of charge compensating LS anions both Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. around and inside of the positively charged PANI shells is higher as compared with the low contents of PANI- Author details LSA in the nanocomposites. As a consequence, these Institute of Bioorganic Chemistry and Petrochemistry of NAS of Ukraine, 50 Kharkivske shose, Kyiv 02160, Ukraine. Taras Shevchenko National University amphiphylic anions with long dodecyl tails can enhance of Kyiv, 64/13 Volodymyrska street, Kyiv 01601, Ukraine. intermolecular interactions in the system and lead to the agglomeration of the nanoparticles with high contents of Received: 30 December 2016 Accepted: 3 August 2017 PANI-LSA. A possibility of such agglomeration effects should References be taken into account when using similar nanocom- 1. 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