Arrangement of Conductive Rod Nanobrushes via Conductive

Dec 10, 2015 - Low-temperature vapour phase polymerized polypyrrole nanobrushes for supercapacitors. Luciano M. Santino , Shinjita Acharya , Julio M. ...
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Arrangement of Conductive Rod Nanobrushes via Conductive− Dielectric−Conductive Sandwiched Single Crystals of Poly(ethylene glycol) and Polyaniline Block Copolymers Maryam Nazari,†,‡ Samira Agbolaghi,†,‡ Saleheh Abbaspoor,†,‡ Homa Gheybi,†,‡ and Farhang Abbasi*,†,‡ †

Institute of Polymeric Materials and ‡Faculty of Polymer Engineering, Sahand University of Technology, Tabriz, Iran S Supporting Information *

ABSTRACT: Conductive rod nanobrushes of polyaniline (PANI) were developed via the growth of conductive−dielectric−conductive sandwiched single crystals obtained from poly(ethylene glycol) (PEG5000)-b-PANIn, PANIn-b-PEG6000-b-PANIn, and PANIn-b-PEG35000b-PANIn block copolymers synthesized by interfacial polymerization fostering two different oxidants (ammonium peroxydisulfate (APS) as a weaker and potassium hydrogen biiodate (PHD) as a stronger oxidant). Based on the dispersity of the diameter of the PANI nanofibers and the various molecular weights of PEG substrates, two distinct morphologies were detected, i.e., matrix−dispersed morphology for PANIn-b-PEG35000b-PANIn and dispersed−dispersed morphology for PANIn-b-PEG6000-bPANIn and PEG5000-b-PANIn single crystals. In matrix−dispersed single crystals, through an elevated crystallization temperature (Tc), a convergence occurred between the heights of matrix (partly stretched PANIs) and disperses (fully stretched PANIs). Because of their higher conductivity (e.g., 3 vs 10−4 S/cm for copolymers and 84 vs 8 × 10−3 S/cm for corresponding homopolymers), the variation in height between the matrix and disperses was lower in PHD-synthesized PANI nanofibers (e.g., height variance of 2 nm for PHD-synthesized PANI180 vs 57 nm for APS-synthesized PANI175 at Tc = 38 °C). The diameter of the dispersed PANI was inversely proportional to the crystallization temperature and was directly proportional to the PANI repeating units. Although in PEG35000-based systems PANI-dispersed diameters of up to 58 nm were detected in PANI109-b-PEG795-b-PANI109 single crystals at Tc = 18 °C due to a scarcity in the provided surface area, the maximum diameters included in PEG6000 and PEG5000 single crystals were 9 and 7 nm, respectively. In dispersed−dispersed morphologies, having extended conformation of PANI brushes on PEG5000 and PEG6000 substrates, their substrate thickness did not vary by the lengthening of the PANI brushes, and the only effect oxidant had in these systems was on the population of grown single crystals; that is, the weaker the oxidant, the larger the population. distributions of diameters.19 The processability and doping properties of PANI could be improved by combining its electron conductivity and the ion conductivity of poly(ethylene glycol) (PEG) via covalent bonding. Furthermore, the selfassembly of respective block copolymers could result in the structural control and functional optimization.20−22 Among the techniques used for developing the polymer brushes,23−30 single crystal growth of block copolymers can accurately control and tune the uniform distribution of tethered brushes.31−36 From the perspective of polymer brushes prepared via single crystal growth, ample categories of the single crystals have already been reported.31,32,37−50 Recently, polymer single crystals (PSCs) have shown numerous applications, for example, the growth of PSC directed programmable assembly of nanoparticles,51 it enabled the nanohybrid shish kebabs to mimic the natural bone

I. INTRODUCTION In the family of π-conjugated polymers, polyaniline (PANI) has prominent properties, for example, diverse structures, good environmental stability, low cost, simple acid/base doping/ dedoping chemistry, etc.1,2 PANI is a promising material with a wide range of applications in different fields, including anticorrosion coatings,3 batteries,4 potentiometric sensors,5 membranes,6 antistatic coatings,7 electromagnetic shields,8 catalyst,8 high-rate supercapacitors,9,10 and fluorescent sensing for nucleic acid detection.11 It can exist as a either salt or base in three isolable oxidation states, that is, leucoemeraldine, emeraldine, and pernigraniline. The emeraldine salt is electrically conductive, while the others are insulators.12−14 Because of the presence of a high interfacial area between PANI and its environment, nanostructured PANI (nanorods/wires/fibers/ tubes) possess enhanced performance; that is, they have found suitability for electronic applications.15,16 Huang et al.17,18 reported a template-free facile chemical route using aqueous/ organic biphase interfacial polymerization. This synthetic method yielded high-quality PANI nanofibers with controlled © XXXX American Chemical Society

Received: October 3, 2015 Revised: December 6, 2015

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Macromolecules nanostructures,52 used in magnetically recyclable catalyst support,32,38,53−55 as templates to synthesize nanoparticle clusters,56,57 as substrates for anisotropic deposition of Au nanoparticles,58 utilizable in semiconductor microelectronics and solid-state,59,60 simplified ultrathin film system to probe the interfacial properties of different substrates,61 and served as the amino-functionalized lamellar poly(L-lactide) single crystals, a delivery system used for human papillomaviruses (HPV16-E7)associated tumors,62−64 nano/micromotors,65,66 etc. In the field of conductive materials, large single rectangular crystals of the regioregular octamer of 3-hexylthiophene (3HT)867,68 welldefined single crystalline nanowires of rigid rod conjugated poly(p-phenylene ethynylene) derivatives with thioacetate end groups (TA-PPE),69 single nanowhiskers of poly(3-hexylthiophene) (P3HT),70 1D microwire single crystals of P3HT (regioregularity = 98.5%),60,71 self-organized P3HT with its supramolecular 2D structure,72,73 poly(3-octylthiophene) (P3OT) single crystals through solvent vapor annealing process,74 etc., were reported. In this work, for the first time, conductive PANI rod nanofibers were grafted on the PEG single crystal substrate, and the effects of oxidant type and the crystalline, as well as PANI block molecular weights, were investigated on the matrix− dispersed and dispersed−dispersed morphologies. Patterned conductive single crystals could be used in the nano/micro electronic devices, as some instances, sensors, thin film transistors, solar cells, etc. Therefore, by controlling the different features of PANI nanofiber-covered single crystalline layer, the conductivity of the developed structure could be easily adjusted.

mol, respectively. The thermal behavior of the block copolymers was also tested by thermogravimetric analysis (TGA). The PANI nanofibers synthesized by interfacial polymerization possessed a diameter dispersity on the scale of some nanometers. The PANI nanofibers synthesized by PHD possessed a more uniform diameter distribution than those synthesized by APS. Figures 1a and 1b illustrate the diameter distribution and typical nanofibers of PANI175 and PANI180 synthesized by weaker (APS) and stronger (PHD) oxidants, respectively.

Figure 1. Diameter distribution of (a) PANI(3) or PANI175 nanofibers synthesized with APS oxidant and (b) PANI(6) or PANI180 nanofibers synthesized with PHD oxidant.

The electrochemical properties of the copolymers were investigated by ultraviolet−visible (UV−vis) spectrometry, using 2-chloroethanol as a good solvent. Figure 2a illustrates

II. EXPERIMENTAL SECTION A. Synthesis of Amine-Terminated Poly(ethylene glycol) Benzoate (ATPEGB). ATPEGBs were synthesized from PEG (5000, 6000, and 350 000 g/mol) and 4-aminobenzoic acid with p-toluenesulfonic acid (PTSA) in catalytic amount, by refluxing it with xylene solvent.20,75 The details are reported in the Supporting Information. 1H NMR spectra for ATPEGB5000, ATPEGB6000, and ATPEGB35000 illustrated a singlet peak at 3.6 ppm relating to −CH2 protons of PEG. The percentages of esterification were 80, 54.5, and 26% for ATPEGB5000, ATPEGB6000, and ATPEGB35000, respectively. B. Synthesis of Block Copolymers. The block copolymers were synthesized by an interfacial polymerization as described by Huang et al.16 and Shadi et al.20 The PANIn-bPEG-b-PANIn triblocks were also synthesized by the method of Yan and Tao.76 Ammonium peroxydisulfate (APS) and potassium hydrogen biiodate (PHD) were dissolved in 1 M sulfuric acid solution as weak and strong oxidants, respectively. The details of synthesis are provided in the Supporting Information. FT-IR spectra of PANI154-b-PEG136-b-PANI154 block copolymers are reported as a type in the Supporting Information (Figure S1), and the characteristic peaks are identified. The molecular weights of the block copolymers were calculated from the integral ratios of protons of PANI benzene ring (7.2−7.4 ppm) and CH2 protons of PEG (3.5 ppm). Table S1 reports all synthesized materials, and the 1H NMR spectra of two samples are represented in Figure S2. The characteristics of synthesized polymers are denoted by the PANI repeating units. PEG35000, PEG6000, and PEG5000 were denoted by PEG795, PEG136, and PEG114, respectively. Likewise, in some instances, PANI34, PANI109, PANI154, and PANI180 represented the molecular weights of 3120, 10 050, 14 200, and 16 520 g/

Figure 2. (a) UV−vis spectra of PANIn-b-PEG136-b-PANIn triblock copolymers indicating the effect of PANI molecular weight and the oxidant type. (b) CV of PEG114-b-PANIn diblock copolymers (filled lines) as well as the mats of respective single crystals (dotted lines) in a common cycle with a sweep rate of 50 mV/s, indicating the influence of the PANI molecular weight and the oxidant type. B

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matrix−dispersed morphology for PANIn-b-PEG795-b-PANIn and dispersed−dispersed morphology for PANIn-b-PEG136-bPANIn as well as PEG114-b-PANIn single crystals. Because of a dispersity in the diameter of grafted PANI nanofibers on the PEG substrate surface, as well as the higher molecular weight of crystalline block in PANIn-b-PEG795-b-PANIn block copolymers, matrix−dispersed morphology occurred in the solutiongrown single crystals. Both matrix and dispersed phases were composed of PANI nanofibers, but with different conformations and diameters. Now, in this case, and on the basis of the strength of the employed oxidant, the matrix−dispersed PANInb-PEG795-b-PANIn single crystals were identified. Figure 3a (top) schematically presents APS-based matrix−dispersed morphology, showing the fully stretched larger diameters of

UV−vis spectra of PANIn-b-PEG136-b-PANIn in 2-chloroethanol upon the addition of camphorsulfonic acid for proton doping. The UV−vis spectra of all copolymers displayed three absorbance peaks at about 372, 437, and 625 nm. Regarding UV−vis spectra, the absorbance peaks of block copolymers synthesized by the stronger oxidant (PHD) were considerably higher than those synthesized by the weaker oxidant (APS). This in turn proved significantly higher conductivity of PHDsynthesized block copolymers. As an instance, the intensities of the peaks of 372 nm for the block copolymers, having somewhat the same molecular weight of PANI nanofibers synthesized by APS and PHD, were 3.20 and 3.80, respectively. By the molecular weight of PANI nanofibers for a given oxidant (PHD or APS), the intensity of absorbance peaks was enhanced. This implied that an increase in the molecular weight of PANI resulted in an elevation in the overall conductivity of block copolymers. Recorded cyclic voltammograms (CV) of the block copolymers at scan rates between −0.4 and 1.4 V vs saturated calomel electrode (SCE) indicated their electroactivity. Figure 2b depicts the CV graphs of PEG114-b-PANIn block copolymers as well as the mats of corresponding single crystals. The bulk polymer and the mats of corresponding single crystals demonstrated to some extent similar electroactivity. Hence, CV graphs of the bulk polymer of single crystal building blocks (filled lines) and those of the mats of single crystals (dotted lines) exhibited a significant consistency. This phenomenon could be assigned to the neutrality of the growth environment for the single crystals. More details about CV measurements are explained in the Supporting Information. The two oxidation peaks at 0.27 and 0.625 V, as well as the two reduction peaks at 0.36 and 0.50 V, were detected from CV. The first pair of redox peaks was attributed to the reversible redox process from the leucoemeraldine to the emeraldine form. At higher potentials, the second pair of redox peaks was assigned to the oxidation/ reduction of the emeraldine form to the pernigraniline state.77 The copolymers synthesized by the stronger oxidant (PHD) resulted in higher current peaks, in comparison to those synthesized by the weaker one (APS). Therefore, the former block copolymers possessed higher conductivity. Apart from this, by an enhancement in the molecular weight of PANI blocks (i.e., samples 1−3 for APS and 4−6 for PHD), the oxidation peaks appeared at higher currents, which was related to the higher conductivity of the longer nanofibers. C. Single Crystal Growth. The self-seeding procedure was applied in order to grow the single crystals covered by conductive rod-shaped PANI brushes. Solution crystallization was carried by diluting a concentration of 0.009 wt % in amyl acetate (Merck, >98%). The diblock and triblock copolymers of PEG-b-PANIn and PANIn-b-PEG-b-PANIn were put into a cell tube. Then, the cell tube was purged by high pure nitrogen, sealed, and kept at the dissolution temperature (Td = 70 °C) for 30 min for the elimination of the thermal history. The cell tube was then immersed into primary fast crystallization (at 0 °C for 5 h) and self-seeding (Ts = 41 °C for 20 min) oil bathes. Hereafter, the cell tube was quickly transferred into an isothermal oil bath of the desired crystallization temperature (Tc) and maintained for 3 days.

Figure 3. Identification of the matrix−dispersed surface morphology of PANIn-b-PEG35000-b-PANIn single crystals. The schematic side view of APS-based PANI-covered conductive single crystal (top); the top view of APS-based PANI175-b-PEG795-b-PANI175 single crystal grown at Tc = 35 °C detected by AFM accompanied by the corresponding large zoomed surface morphology (inset) and AFM height profile (bottom) (a); the schematic side view of PHD-based PANI-covered conductive single crystal (top); the top view of PHD-based PANI180-bPEG795-b-PANI180 single crystal grown at Tc = 35 °C detected by AFM accompanied by the corresponding large zoomed surface morphology (inset) and AFM height profile (bottom) (b).

III. RESULTS AND DISCUSSION In general, by fostering diblock and triblock copolymers to grow conductive−dielectric−conductive sandwiched single crystals, two kinds of morphologies were developed, that is, C

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surface was covered by PANI nanofibers. These types of single crystals were completely homogeneous in phase images of AFM as well. Details of formation of different morphologies, the effect of oxidants, crystallization temperature, molecular weights, etc., will be discussed in upcoming sections. A. Matrix−Dispersed Morphologies in PANIn-bPEG795-b-PANIn Single Crystals. The PANI nanofibers synthesized by interfacial polymerization had a diameter dispersity on the scale of several nanometers (Figure 1). The grown single crystals in these systems themselves also had a diameter distribution for grafted PANI brushes on the PEG crystallization substrate. The main reason for possessing the matrix−dispersed surface morphology by the PANIn-b-PEG795b-PANIn single crystals was assigned to the dispersity of PANI nanofibers diameter. Figure 5 illustrates AFM images of

the PANI nanofibers. These are entitled as PANI-disperses. The smaller diameters of PANI nanofibers whose conformation is not that much extended create the matrix phase. The top view of APS-based PANI175-b-PEG795-b-PANI175 single crystal grown at Tc = 35 °C and the respective matrix−dispersed surface morphology (inset) are depicted in Figure 3a (bottom). Based on the lower conductivity of PANI nanofibers, the height variance between fully stretched PANI disperses and PANI brush-covered matrix phase was considerable in APS-based single crystals (AFM height profile of Figure 3a, bottom)). The PHD-based PANIn-b-PEG795-b-PANIn single crystals possessed similar matrix−dispersed morphology, but with more stretched PANI nanofibers in the matrix phase (scheme of Figure 3b, top) and AFM height image of PANI180-b-PEG795-b-PANI180 single crystal grown at Tc = 35 °C in Figure 3b (bottom). This was responsible for the significantly lower height variance between PANI nanobrushes in matrix and dispersed phases in comparison to that of APS-based single crystals (Figure 3b, bottom). On the contrary, the strength of the oxidant did not affect the dispersed−dispersed morphology of the single crystals, which possessed shorter crystalline blocks (i.e., PANIn-b-PEG136-b-PANIn as well as PEG114-b-PANIn). In this morphology, each PANI nanofiber, which was allowed to be incorporated into the single crystal structure, fabricated a PANI-disperse. A colony of these PANI-disperses led to dispersed−dispersed morphology. The scheme of Figure (top)

Figure 5. AFM NanoscopeIII image of matrix−dispersed single crystals. (a) The matrix−dispersed surface morphology of PANI109-bPEG795-b-PANI109 single crystals (grown at Tc = 23 °C with the oxidant of APS); left: height image (the maximum z-scale is 5 nm), diameter of PANI disperses: 50 nm; right: phase image (the maximum z-scale is 5°). (b) Matrix−dispersed surface morphology of PANI111-bPEG795-b-PANI111 single crystals (grown at Tc = 18 °C with PHD as an oxidant); left: height image (the maximum z-scale is 5 nm), diameter of PANI disperses: 44 nm; right: height profile (matrix overall thickness = 148.2 nm and disperses overall thickness = 194.2 nm).

Figure 4. Identification of the dispersed−dispersed surface morphology of PEG5000-b-PANIn and PANIn-b-PEG6000-b-PANIn single crystals. The schematic side view of PANI-covered conductive single crystal (top); the top view of PHD-based PANI38-b-PEG136-b-PANI38 single crystal grown at Tc = 28 °C detected by AFM accompanied by the respective large zoomed surface morphology (bottom).

matrix−dispersed surface morphology of PANI109-b-PEG795-bPANI109 single crystals grown at Tc = 23 °C with APS (Figure 5a) and matrix-dispersed surface morphology of PANI111-bPEG795-b-PANI111 single crystals grown at Tc = 18 °C with PHD as an oxidant (Figure 5b). As an instance, in APS-based PANI109-b-PEG795-b-PANI109 single crystals at Tc = 23 °C, the diameter of fully stretched PANI-disperses or critical diameter was 50 nm. Here, the PANI nanofibers in the diameter range of 6−50 nm could be fluently incorporated into the matrix phase. Because of the critical preparation of the cross section of PANIs with the diameter of 50 nm, they were forced to be fully stretched and resulted in the development of the PANIdisperses. In this sample, diameters above 50 nm were definitely excluded from single crystal structure. In general, the conformation of PANI nanofibers synthesized by the stronger

and AFM image of PANI38-b-PEG136-b-PANI38 single crystal grown at Tc = 28 °C, accompanied by the respective dispersed−dispersed surface morphology in Figure (bottom), represent this type of morphology. In the AFM phase image of PANI, in either matrix−dispersed morphologies of PANIn-bPEG795-b-PANIn single crystals or dispersed−dispersed morphologies of PEG114-b-PANIn and PANIn-b-PEG136-b-PANIn single crystals, the whole surface of the single crystal was in a united phase. In addition, in matrix−dispersed single crystals, the matrix and disperses constituted the same PANI nanobrushes; that is, the two regions depicted the same phase. Likewise, in dispersed−dispersed single crystals, the entire D

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brushes (i.e., polystyrene (PS) and poly(methyl methacrylate) (PMMA)), the laterals were smooth, even in extreme states.41,46−48 As an instance, PHD-based PANI180-b-PEG795b-PANI180 single crystals grown at Tc = 38 °C possessed completely needle-like lateral habits (Figure 6a, left). However, APS-based PANI175-b-PEG795-b-PANI175 single crystals (Figure 6b), as well as PHD-based PANI38-b-PEG136-b-PANI38 single crystals (Figure 6c), both grown at Tc = 23 °C, appeared in rippled lateral habits. Furthermore, in the polyethylene (PE) decoration, the PE crystal rods on PHD-based PANI180-bPEG795-b-PANI180 single crystals surface at Tc = 38 °C were randomly oriented (Figure 6a, right). The key message delivered here was that the surface of PEG substrate was covered by PANI nanobrushes. On the contrary, the PEdecorated homo-PEG single crystals possessed the oriented PE oligomer extended-chain crystals on the surface. B. Oxidant Influence on Matrix−Dispersed Morphologies. In PANIn-b-PEG795-b-PANIn single crystals, the PANI brushes thickness vs the crystallization temperature (Tc) exhibited an increasing trend for matrix phase regions. However, this trend was not observed for PANI-disperses. The reason for the enhancement in the height of PANI brushes in the matrix phase in parallel with Tc elevation was due to an increase in the thickness of PEG substrate with Tc and, subsequently, a decrease in the chain foldings in the crystalline substrate. A decrease in the fold number of the PEG substrate resulted in a lower surface of PANI brushes supported by PEG substrate. This resulted in more stretched conformations by the PANI brushes. On the other hand, the thickness of PANIdisperses was not affected by Tc because even at the lower crystallization temperatures, these dispersed PANI fibers demonstrated their ultimate stretched conformations. Figure 7 represents these trends by the graphs of the thickness of PANI brushes vs crystallization temperature for matrix and dispersed phase regions of PANIn-b-PEG795-b-PANIn single crystals in the temperature range of 18−38 °C. Because of a higher conductivity, and consequently more extended conformation, the matrix and disperses height variance in the graphs of PHD-based PANI180 (hollow green circles and pink dashes for matrix and disperses, respectively) was less than that in the respective graphs of APS-based PANI175 (maroon multiple and filled purple diamond for matrix and disperses, respectively). The infinitesimal fluctuations (on the scale of some nanometers) in the thickness of dispersed PANI brushes could be assigned to the variance in length of the in-synthesized PANI nanofibers by interfacial polymerization. The explained trends for matrix and dispersed phase regions of PANI nanofibers also satisfied the condition of all other PEG795based single crystals. For quantitative explanation, in PHDbased PANI111-b-PEG795-b-PANI111 single crystals, PANI regions at Tc = 18, 23, 28, and 35 °C for disperses were 95, 94, 94, and 98 nm, respectively, and for matrix phases were 72, 79, 85, and 91 nm, respectively. Therefore, an increase in Tc resulted in the thickness of PANI matrix phase regions gradually approaching the height of the respective PANIdisperses. The height variance of matrix and dispersed regions for the same sample reached from 23 to 7 nm in the temperature range of 18−35 °C. In samples based on the weaker APS oxidant, like PANI175-b-PEG795-b-PANI175 at Tc = 23, 28, 32, and 38 °C, the thickness of PANI brushes in PANIdisperses were 142, 141, 140, and 142 nm, respectively, and in matrix they were 70, 75, 80, and 85 nm, respectively. Because of

oxidant was more extended in comparison with the weaker oxidant, but even with stronger oxidant (i.e., PHD) the PANI nanofibers did not possess extremely stretched conformation. This was in accordance with literature reports.78 In PANIn-bPEG795-b-PANIn single crystals, PANI brushes did not have fully stretched morphology; however, in especial conditions, they reached fully extended conformations, that is, in PANIdisperses. This was attributed to the longer PEG795 chains in the crystalline substrate and thus their capability of being folded. In the present growth systems, the substrate was PEG and the growth condition was in dilute solution (0.009 wt %). Therefore, the lateral habit of the grown single crystals was square, including the growth prisms of (120). In addition to (120), the growth fronts of (040), (200), (110), and (020) appeared in the electron diffraction (ED) of TEM (the insets of Figure 6). The grown single crystals with PANI grafted brushes

Figure 6. TEM images of (a) PANI180-b-PEG795-b-PANI180 single crystals grown at Tc = 38 °C with PHD (left), PE-decorated surface (right), ED pattern (inset); (b) PANI175-b-PEG795-b-PANI175 single crystals grown at Tc = 23 °C with APS, ED pattern (inset); and (c) PANI38-b-PEG136-b-PANI38 single crystals grown at Tc = 23 °C with PHD, ED pattern (inset).

possessed rippled edges, and in the extreme state, the respective edges transformed to needle-like. In single crystals with higher molecular weights of PANI brushes, in the samples based on the stronger oxidant, and at higher crystallization temperature, the shapes of the grown single crystals were transformed from rippled to the needle-like lateral habits; this was because in the mentioned intensive conditions the repulsive interactions between tethered nanofibers on the single crystal substrate surface increased. It could in turn squeeze the nanobrushes located in the vicinity of each other to the edges of single crystal, and consequently, the brushes on the edges were inclined in outward direction of the single crystal in order to release the respective force. Hence, the lateral habits became nonsmooth. On the contrary, in the single crystals having coily E

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Figure 7. Thickness of PANI brushes vs crystallization temperature for matrix and dispersed phase regions of PANIn-b-PEG795-b-PANIn single crystals at Tc = 18−38 °C.

Figure 8. IDF of SAXS of PANI175-b-PEG795-b-PANI175 single crystals grown at Tc = 35 and 38 °C with APS (red dashed and light green dot-dashed lines, respectively) as well as PANI180-b-PEG795-b-PANI180 single crystals at Tc = 35 and 38 °C with PHD (blue dotted and purple long dashed lines, respectively), indicating the effect of two different oxidants at two distinct crystallization temperatures.

areas covered by PANI nanobrushes having different conformations proved that the dominant thermodynamic effect was determined by matrix phase. In details, this homogeneous substrate provided an extra surface area for smaller diameters of PANI nanofibers to be deviated from fully stretched state. However, the mentioned substrate was capable of preparing only a critical surface area equivalent with the cross section for larger diameters; that is, they possessed the tendency to be fully stretched. On the contrary, in leopard skin-like matrix (PS)− dispersed (PMMA) mixed brush single crystals, each phase region possessed its own substrate thicknesses.47 This discrepancy had root in different PS and PAMMA brushes in matrix and dispersed areas; but here, both phase regions were composed of the PANI rod nanobrushes. In Figure 8 at Tc = 35 °C, APS-based PANI175-b-PEG795-b-PANI175 and PHD-based PANI180-b-PEG795-b-PANI180 single crystals represent the three peaks of 3.76, 164, 278 nm (Figure 3a shows the overall

their higher conductivity, for example, 3 S/cm for PANI180-bPEG795-b-PANI180 (by PHD) vs 10−4 S/cm for PANI175-bPEG795-b-PANI175 (by APS) block copolymers and 84 vs 8 × 10−3 S/cm for corresponding homopolymers, the ultimate matrix and disperse height variance was significantly lower in PHD synthesized PANI nanofibers (for example, 147 and 149 nm vs 85 and 142 nm for mentioned single crystals at Tc = 38 °C). In the interface distribution function (IDF) of SAXS, the first peak stood for the thickness of crystalline substrate, and the second and third peaks indicated twice the thickness of PANI brushes in matrix and dispersed phase regions, respectively. Figure 8 demonstrates IDF of PANI175-b-PEG795-b-PANI175 and PANI180-b-PEG795-b-PANI180 single crystals prepared at Tc = 35 and 38 °C with weak and strong oxidants. In these graphs, the approach of the matrix and disperse heights could be easily followed. A unique substrate thickness for matrix and dispersed F

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Macromolecules thicknesses of 167.8 and 281.8 nm for matrix and disperse regions) and 4.18, 288, 292 nm (Figure 3b shows the overall thicknesses of 292.2 and 296.2 nm for matrix and disperse regions), respectively. AFM and SAXS data exhibited a high consistency with each other. Likewise, at Tc = 38 °C, the mentioned single crystals depicted the three peaks of 3.81, 170, 284 nm and 4.24, 294, 298 nm, respectively. The distance between the two amorphous peaks in IDF of SAXS was dependent on two parameters. First, in the stronger PHD oxidant, the PANI nanofibers which created the matrix phase exhibited more extended conformation. Therefore, the PANI brushes which developed the matrix and the dispersed regions were closer together, in comparison to the PANI brushes which exhibited less stretched conformations in the matrix phase regions. On the basis of these explanations, the difference between the two amorphous peaks in IDF of SAXS should be larger for weaker oxidant. The second parameter was the crystalline temperature, whose increase did not affect the height of fully stretched PANI-disperses, although it enhanced the thickness of PANI nanobrushes in the matrix regions. Hence, via Tc elevation, the difference between PANI brushes in matrix and dispersed phases decreased; consequently, the two amorphous peaks in IDF of SAXS converged together. At the highest crystallization temperature, that is, at Tc = 38 °C, the height variance between matrix and dispersed phase regions for PANI180-b-PEG795-b-PANI180 single crystals was 2 nm (149 minus 147 nm). This height variance between matrix and dispersed regions in APS and PHD oxidants was significantly different (57 nm for APS and 2 nm for PHD). The conductivity of APS-based homo-PANIs with the molecular weight of 17 100 g/mol was 7 × 10−3 S/cm, while that of PHD-based homo-PANIs, having the molecular weight of 18 800 g/mol, was 71 S/cm. Via the enhancement of the number of repeating units for PANI nanofibers, the thickness of PANI brushes in either matrix or dispersed phase regions increased. Figure 9a illustrates the variation of PANI brushes thickness vs the number of PANI repeating units. This was related to the lengthening of the PANI nanobrushes in parallel to an increase in PANI repeating units. This trend continued for all single crystals at the whole range of crystallization temperature (18− 38 °C). At Tc = 35 °C, by increasing the repeating units of PANIs from 42 to 109 and to 175, APS-based PANI brush thicknesses in matrix regions reached 22, 51, and 82 nm and in PANI-disperses it varied from 34 to 85 and 139 nm, respectively. With the same trend for PHD as a stronger oxidant, by increasing the repeating units of PANI from 44 to 111 and to 180, the thicknesses of PANI brushes were 33, 91, and 144 nm in matrix and 37, 98, and 146 nm in the dispersed regions, respectively. As another numeric example, at Tc = 18 °C for PANI111- and PANI180-covered single crystals (synthesized by PHD oxidant), the thickness of matrix and disperses were 72 and 95 nm and 127 and 148 nm, respectively. An increase in the repeating units of PANI resulted in an increase in the diameter of PANI-disperses (Figure 9b). Because via the enhancement of PANI nanofibers repeating units, PANI nanobrushes were lengthened, and this caused the PEG substrate to be thinner and have more foldings (due to increase of exerted osmotic pressure on it). Hence, in this manner, the demanded surface area would be provided for larger diameters of PANI-disperses. Besides, under the same growth conditions, the weaker oxidant led to larger diameters of fully stretched PANI-disperses. For example, at Tc = 35 °C, for

Figure 9. Variation of (a) thickness of PANI brushes vs the repeating units of PANI at Tc = 23 and 35 °C for matrix and the dispersed regions of PANIn-b-PEG795-b-PANIn single crystals; (b) the diameter of the PANI disperses vs the repeating units of PANI at Tc = 28, 32, and 38 °C; and (c) the diameter of the PANI nanofibers in disperses vs crystallization temperature for various molecular weights of PANI nanofibers and for two oxidants of APS and PHD.

PHD-based PANI44, PANI111, and PANI180, the PANIdispersed diameters were 12, 21, and 30 nm. However, for similar samples with the weaker APS oxidant and at the same crystallization temperature, the diameters of PANI-disperses G

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Macromolecules were higher (= 25, 32, and 40 nm for PANI42, PANI109, and PANI175, respectively). The major reason for the larger diameters of PANI brushes synthesized by a weaker oxidant (that is, APS) was that the conformations of PANI nanofibers synthesized by APS were less stretched. Hence, the osmotic pressure which they inserted onto the PEG795 substrate surface was more conspicuous. That was responsible for the respective thinner substrate and, consequently, higher number of foldings. Therefore, the mentioned substrate was capable of providing the required surface area for larger diameters. As an instance, at Tc = 28 °C the diameter of APS-based PANI175-disperses was 49 nm and the corresponding diameter of PANI180-disperses with a stronger oxidant of PHD was 40 nm. This trend was also detected for all other crystallization temperatures and for all samples comprising different molecular weights of PANI. Furthermore, in the graph of PANI-disperses diameter vs Tc (Figure 9c), by elevation of crystallization temperature, the diameter of fully stretched PANI-disperses decreased because via increasing Tc, the thickness of PEG substrate increased, and subsequently the foldings of PEG chains in the crystalline substrate decreased. Hence, the nanofibers whose required surface areas were initially prepared by the substrate in higher foldings (i.e., at lower Tc) resulted in the matrix phase. However, now in lower foldings (i.e., at higher Tc), they were caused to create fully stretched PANI-disperses. In simple words, the maximum diameter with a demanded surface area being supported by the substrate, decreased at elevated crystallization temperatures. In all matrix-dispersed single crystals, the diameters of all fully stretched PANI-disperses were exactly the same. Therefore, PANI-disperses probably consisted of a single nanofiber. With respect to the graph of the diameter of PANI-disperses vs Tc for PANI109 with the oxidant of APS, moving from the crystallization temperatures of 18, 23, 28, 32, 35, and 38 °C, the diameter of PANI-disperses gradually decreased (= 58, 50, 43, 37, 32, and 28 nm, respectively). Likewise, for the single crystals with PANI111 and the oxidant of PHD, at the same Tc, the diameter of PANI-disperses passed from 44, 38, 31, 25, 21, and 16 nm, respectively. This trend was also observed for all other PANIn-b-PEG795-b-PANIn single crystals. Table S2 reports the data of PANIn-b-PEG795-b-PANIn single crystals. C. Dispersed-Dispersed Morphologies in PANIn-bPEG136-b-PANIn and PEG114-b-PANIn Single Crystals. In PEG114-b-PANIn and PANIn-b-PEG136-b-PANIn single crystal systems, all surface morphologies were of the closely dispersed−dispersed type. Figure 10 shows AFM images of dispersed−dispersed surface morphology of PHD-based PANI98-b-PEG136-b-PANI98 single crystals grown at Tc = 18 °C (Figure 10a) and dispersed-dispersed surface morphology of APS-based PEG114-b-PANI45 single crystals grown at Tc = 23 °C (Figure 10b). In these systems which were similar to PANInb-PEG795-b-PANIn single crystals, via elevation of Tc, the thickness of substrate increased and consequently the foldings decreased. Therefore, the required surface area of lower diameters was provided. Unlike PANIn-b-PEG795-b-PANIn single crystals, here an increase in the molecular weight of PANI did not vary the thickness of substrate. This was due to the extended conformation of PANI brushes on the PEG114 and PEG136 substrates. In this way, by increasing the length of the PANI nanofibers, their exerted osmotic pressure onto the substrate surface did not change. That is to say, in these two systems, in order to obtain short PEG chains (equivalent length of 27 nm for 5000 g/mol and 32 nm for 6000 g/mol79) from

Figure 10. AFM NanoscopeIII image of dispersed-dispersed single crystals. (a) The dispersed−dispersed surface morphology of PANI98b-PEG136-b-PANI98 single crystals (grown at Tc = 18 °C with the oxidant of PHD), left: height image (the maximum z-scale is 5 nm); right: phase image (the maximum z-scale is 5°). (b) The dispersed− dispersed surface morphology of PEG114-b-PANI45 single crystals (grown at Tc = 23 °C with the oxidant of APS); left: height image (the maximum z-scale is 5 nm); right: height profile (disperses overall thickness = 82.80 nm).

the beginning, only PANI nanofibers with very little diameters were allowed to be incorporated into the single crystals. Although in PEG795-based systems, the diameters of sPANIdispersed were detected up to 58 nm (in PANI109-b-PEG795-bPANI109 single crystals at Tc = 18 °C), due to a shortage of the provided surface area; the maximum diameters included in PEG136 and PEG114 single crystals were 9 and 7 nm, respectively. Therefore, the diversity of the diameters which entered into the PEG114- and PEG136-based single crystals was significantly lower than PANIn-b-PEG795-b-PANIn single crystals. In PANIn-b-PEG795-b-PANIn single crystals, in addition to the cross-sectional area of PANI nanofibers having diameters less than critical diameters (that is, fully stretched PANIdisperses), an extra surface was prepared by the substrate in order to develop more free conformations (i.e., these PANI nanofibers were not enforced to be extremely stretched). That is the reason why the majority of PANI nanofibers developed matrix phase regions. However, in PEG114-b-PANIn and PANInb-PEG136-b-PANIn single crystals, from the beginning, there was insufficient surface area for the free morphology of PANI nanobrushes; that is, all the nanofibers possessed extensive nanofibers and led to the closely dispersed−dispersed morphology. The principal reason for the difference in the surface morphologies of the matrix-dispersed PANIn-b-PEG795b-PANIn single crystals and the surface morphologies of the dispersed−dispersed PEG114-b-PANIn and PANIn-b-PEG136-bPANIn single crystals was assigned to the higher equivalent length of PEG795 (189 nm79). In single crystals with matrixdispersed surface morphologies, the diversity of PANI nanofibers diameters was considerably higher than PEG114-b-PANIn and PANIn-b-PEG136-b-PANIn single crystals, and the lower diameters possessed enough distance from critical dispersed H

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these systems was on the population of grown single crystals. In the systems with a stronger PHD oxidant, the single crystals appeared in less population, in comparison with the systems based on a weaker APS oxidant. Figure S3 proves this claim by scanning transmission electron microscopy (STEM) images. Since the amount and the condition of dropped single crystals with two different oxidants were exactly the same, this was attributed to the wider dispersity of PANI nanofibers synthesized by the weaker APS oxidant in comparison with the stronger PHD oxidant. Therefore, in weaker oxidant-based systems, the quantity of nanofibers with low diameters, that is, in the range of 6−9 nm, was considerably high. In PEG114-bPANIn and PANIn-b-PEG136-b-PANIn single crystals, only PANI nanofibers which possessed the mentioned range of diameter were capable of attaining the single crystal structure. Hence, the population of single crystals grown from APS synthesized polymer chains was significantly larger. Figures S3a and S3b illustrate the growth systems of APS-based PANI154-b-PEG136b-PANI154 and PHD-based PANI158-b-PEG136-b-PANI158 single crystals, respectively, at Tc = 28 °C. All other samples also proved this phenomenon. In the PEG114-b-PANIn and PANIn-b-PEG136-b-PANIn single crystals having dispersed−dispersed surface morphologies, IDF of SAXS graphs depicted only two distinct peaks. The first peak stood for the PEG crystalline substrate thickness, and the second one was for twice the height of the grafted PANI brushes on the substrate surface. Figure 12 shows IDF of SAXS

diameter. The mentioned PANI nanofibers intended to create matrix phase. However, in PEG114-b-PANIn and PANIn-bPEG136-b-PANIn single crystals, even the lowest diameters were considered as the critical dispersed diameter, and subsequently, there was no opportunity to fabricate the matrix regions. In the graph of thickness of PANI brushes vs the repeating units of PANI for PEG114 and PEG136 similar to PANIn-bPEG795-b-PANIn single crystals, an increase in the PANI repeating units resulted in an increased height of PANI nanobrushes. This completely obvious trend satisfied the condition for all other samples in the whole temperature ranges (i.e., 18−28 °C). Neither PEG114-b-PANIn nor PANIn-bPEG136-b-PANIn single crystals having various molecular weights of PANI brushes grew above 28 °C because above 28 °C, due to the enhancement of PEG substrate thickness, and consequently reduction of foldings, the demanded surface area of PANI nanofibers, having the minimum diameter of 6 nm for PEG114 systems and 6−7 nm for PEG136 ones, was not provided anymore by the substrate. On the other hand, PANI nanofibers with the diameters below 6 nm did not exist in the growth systems. Hence, PEG114-b-PANIn and PANIn-b-PEG136-bPANIn single crystals did not appear above Tc = 28 °C. Similar to PANIn-b-PEG795-b-PANIn single crystals, in dispersed−dispersed PEG136 single crystals, as a type at Tc = 28 °C, by increasing the PANI repeating units from 34 to 95 and to 154 with the oxidant of APS, the thicknesses of PANIdispersed brushes were 28, 83, and 127 nm, respectively. At the same Tc, with PHD and repeating units of 38, 98, and 158, the corresponding thicknesses were 33, 80, and 133 nm, respectively. Likewise, a similar trend was observed for PEG114-b-PANIn single crystals. To qualitatively explain this, for APS-based PEG114-b-PANI85 and PEG114-b-PANI140 single crystals grown at Tc = 18 °C, PANI-disperse thicknesses were 70 and 117 nm, respectively. Figure 11 depicts the thickness of

Figure 12. IDF of SAXS of PANI95-b-PEG136-b-PANI95 with APS (purple dotted line), PANI154-b-PEG136-b-PANI154 with APS (green dashed line), PANI98-b-PEG136-b-PANI98 with PHD (orange double dot-dashed line), and PANI158-b-PEG136-b-PANI158 with PHD (red dot-dashed line) single crystals at Tc = 23 °C in order to investigate the effects of the molecular weight of PANI as well as the type of oxidant on the single crystals features.

of APS-based PANI95-b-PEG136-b-PANI95 (3.24 and 162 nm), APS-based PANI154-b-PEG136-b-PANI154 (3.24 and 258 nm), PHD-based PANI98-b-PEG136-b-PANI98 (3.25 and 164 nm), and PHD-based PANI158-b-PEG136-b-PANI158 (3.23 and 264 nm) single crystals at Tc = 23 °C to clearly view the effect of PANI molecular weight and oxidant on the single crystals features. The rest of the graphs depict the iteration of the respective peaks. Tables S3 and S4 report the data of PEG114-bPANIn and PANIn-b-PEG136-b-PANIn single crystals, respectively.

Figure 11. Thickness of PANI brushes vs the repeating units of PANI for PANIn-b-PEG136-b-PANIn and PEG114-b-PANIn single crystals grown at Tc = 23 and 28 °C.

PANI brushes vs repeating units of PANI nanofibers for PANInb-PEG136-b-PANIn and PEG114-b-PANIn single crystals grown at Tc = 23 and 28 °C with two distinct oxidants (that is, APS and PHD). Furthermore, in PEG114-b-PANIn and PANIn-b-PEG136-bPANIn single crystals, unlike PANIn-b-PEG795-b-PANIn ones, the strength of the oxidant had no effect on the morphology and thickness of the brush. The only effect of the oxidant in

IV. CONCLUSIONS Conductive rod-shaped PANI nanobrushes were developed on PEG lamellar single crystals. In general, the matrix−dispersed I

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morphologies for PANIn-b-PEG795-b-PANIn single crystals and dispersed−dispersed morphologies for PANIn-b-PEG136-bPANIn, as well as PEG114-b-PANIn single crystals, were detected. In matrix−dispersed PANIn-b-PEG795-b-PANIn single crystals, an increase in the crystallization temperature resulted in an increase in the height of the PANI brushes in the matrix regions, although they were constant in fully stretched disperses. Although in the weaker oxidant (i.e., APS), exactly like the stronger oxidant (i.e., PHD) via Tc enhancement, there was a convergence between the heights of matrix and disperses; a conspicuous height variance was observed even at elevated temperatures. This was associated with the lower conductivity of PANI nanofibers synthesized by APS. Furthermore, the diameter of fully stretched PANI disperses was inversely proportional and directly proportional to the crystallization temperature and repeating units of PANI, respectively. Having extended conformation of PANI brushes on PEG114 and PEG136 substrates, their substrate thickness did not vary by increasing the PANI molecular weight. The only effect of oxidant in these systems was on the population of grown single crystals; that is, the weaker the oxidant, the larger population.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b02179. Figures S1−S3 and Tables S1−S3 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (F.A.). Notes

The authors declare no competing financial interest.

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REFERENCES

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