Bulk synthesis, optimization, and characterization of highly dispersible polypyrrole nanoparticles toward protein separation using nanocomposite membranes

Journal of Colloid and Interface Science 386 (2012) 148–157

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Bulk synthesis, optimization, and characterization of highly dispersible polypyrrole nanoparticles toward protein separation using nanocomposite membranes Yaozu Liao a,b,c,⇑, Xia Wang a,⇑, Wei Qian a, Ying Li a, Xiaoyan Li a, Deng-Guang Yu a a b c

School of Materials Science and Engineering, University of Shanghai for Science and Technology, 516 Jun-Gong Road, Shanghai 200093, China Institute of Materials Chemistry, College of Materials Science and Engineering, Tongji University, 1239 Si-Ping Road, Shanghai 200092, China Department of Chemistry and Biochemistry and California NanoSystems Institute, University of California, Los Angeles, Los Angeles, CA 90095-1569, United States

a r t i c l e

i n f o

Article history: Received 19 May 2012 Accepted 12 July 2012 Available online 27 July 2012 Keywords: Polypyrrole Conducting polymer nanoparticles Nanocomposites Ultrafiltration membranes Carbon nanoparticles

a b s t r a c t A novel initiator-assisted polymerization is used for bulk synthesis of polypyrrole (PPy) nanoparticles by adding a catalytic amount of initiator 2,4-diaminodiphenylamine into pyrrole solution. Through simply modulating reaction parameters such as initiator concentrations, oxidant species, oxidant/monomer molar ratios and acidic media utilized, the chemical structure, nanomorphology, product yield, dispersibility, thermal stability, electrochemical activity, and conductivity of PPy nanoparticles are facilely optimized. The initiator copolymerized with pyrrole in the initial stages of polymerization, acting like bipyrrole and helping to nucleate the PPy main chains. The stronger oxidants and higher oxidant/monomer molar ratios used lead to PPy nanoparticles with higher pp conjugation. Sphere-like PPy nanoparticles with average diameters of 80300 nm show yield and conductivity with values up 73.5% and 102 S/cm, respectively, and are readily dispersible in both water and N-methylpyrrolidone. The PPy nanoparticles are used as effective precursors for fabricating carbon nanoparticles with conductivity of 3.7 S/cm. Nanocomposite membranes consisting of PPy nanoparticles and polysulfone matrix are fabricated by a phase-inversion technique and demonstrate much improved hydrophilicity, water permeability, and bovine serum albumin selectivity against pure polysulfone membranes. Crown Copyright Ó 2012 Published by Elsevier Inc. All rights reserved.

1. Introduction Conducting polymer nanoparticles have been extensively studied since they hold promise for applications involving sensors [1], light-emitting diodes [2], electrochromics [3], solar cells [4], drug delivery systems [5], and bioimaging [6]. These small particles are of fundamental interest, due to their improvements in optical, electrical, and magnetic properties at the nanoscale, and enhanced processing properties and capability of encapsulating bioactive materials [7]. More importantly, advanced nanocomposites can be readily formed by dispersing small amount of conducting polymer nanoparticles into polymer matrix [8]. Polypyrrole (PPy) is the one of most investigated conducting polymers, owing to its facile synthesis, good conductivity, redox properties, long-term environmental stability and biocompatibility as polymer in vivo [9], and has shown promising applications including sensors [10], actuators [11], photovoltaic/solar cells [12,13], electromagnetic interference shielding [14], and corrosion ⇑ Corresponding authors. Address: School of Materials Science and Engineering, University of Shanghai for Science and Technology, 516 Jun-Gong Road, Shanghai 200093, China (Y. Liao and X. Wang). E-mail addresses: [email protected] (Y. Liao), [email protected] (X. Wang).

protection [15]. In addition, PPy nanocomposites have been shown biodegradability and biocompatibility, which are potentially useful in drug delivery and tissue engineering [5,16]. Processability is arguably the biggest problem for utilization of such valuable polymer. In order to improve its processability and widen the applications, considerable effort has been devoted to produce colloidal PPy nanoparticles with controllable size, shape, composition, and surface chemistry. Examples include emulsion [17], interfacial [18], dispersion [19], template [20], and plasma polymerizations [21]. The emulsion, interfacial, and dispersion polymerizations use surfactants or bulky steric stabilizers to prevent nanoparticle aggregation caused by charge interaction. However, pure PPy nanoparticles with a clean surface are hardly obtained due to the heavy contaminations caused by the surfactants or stabilizers. While the template-assisted approach requires scientists to prepare appropriate templates, but they are difficultly removed. Drawbacks to plasma polymerization include PPy nanoparticles with low conductivity, and complex equipment is needed. Therefore, simple and rapid methods to water-dispersible PPy nanoparticles with high quality are clearly desired. In this work, we develop an initiator-assisted polymerization that produces well-defined PPy nanoparticles in the absence of any surfactants and templates. The key to formation and

0021-9797/$ - see front matter Crown Copyright Ó 2012 Published by Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2012.07.039

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the PPy nanoparticles as methanol dispersions were recorded on an HP 8453 spectrometer at the room temperature. The FT-IR samples were prepared as pellets by mixing with KBr, and the spectra were taken with a JASCO FT/IR-420 spectrophotometer. XRD spectra were scanned on a Philips X’pert Pro powder diffractometer by using copper-monochromatized Cu Ka radiation (k = 1.54178 Å). The size of PPy nanoparticles as methanol dispersions was determined by a Coulter Beckman N4 Plus dynamic light scattering (DLS) analyzer. The morphologies of PPy particles and PPy/PSf nanocomposite membranes formed were imaged using a JEOL JSM-6700 field emission SEM, a PHILIPS CM120 TEM, and a Nanoscope IIIa AFM under ambient conditions. Note that the AFM measurements were performed for five different sample spots and the roughness parameters was obtained by averaging all the values using the off-line processing software, as previously reported [22].

stabilization of PPy nanoparticles is chemical oxidative polymerization of pyrrole in the presence of a catalytic amount of initiator 2,4-diaminodiphenylamine. The effects of reaction parameters including initiator concentration, oxidant species, oxidant/monomer ratios, and acidic media on the nanomorphology, product yield, dispersibility, thermal stability, electrochemical activity, and conductivity of the PPy nanoparticles are systematically studied. The relationship between chemical structure and multifunctionality of the PPy nanoparticles are emphasized. 2. Experimental 2.1. Synthesis of PPy nanoparticles All chemicals were of analytical grade and used as received. In a typical synthesis, 250 mg pyrrole and 75 mg initiator 2,4-diaminodiphenylamine (10 mol% relative to pyrrole) were dissolved in 150 mL methanol, 600 mg FeCl3 as an oxidant was dissolved in 150 mL 1.0 mol/L camphorsulfonic acid (CSA). The two solutions were cooled to 0 °C and rapidly mixed. The reaction (Scheme 1) was vigorously shaken for 10 s and then left undisturbed overnight. Doped PPy nanoparticles were obtained by centrifuging the reaction solution at a speed of 4500 rpm/min using deionized (DI) water/methanol (90/10) at 15 °C until the top liquid became colorless. To monitor the polymerization of pyrrole, open-circuit potentials (OCPs) of the reaction solutions in the presence and absence of the initiator were measured as a function of time on a single component two-electrode cell: Pt|reaction solution||reference electrode. The changes in temperature of the reaction solutions were monitored by Keithley2004. If not specifically mentioned, all experiments were performed using 50 mol% FeCl3 as the oxidant and 10 mol% 2,4-diaminodiphenylamine as the initiator (relative to pyrrole).

2.4. Property measurements The thermal stability of the PPy products was recorded on TGA/ DTGA (Perkin Elmer TGA Pyris 1) from room temperature to 800 °C in N2 at rate of 3 °C/min. The chars, that is, as-fabricated carbon nanoparticles as ethanol dispersions were further characterized by SEM and TEM. The conductivities of the PPy nanoparticles and carbon nanoparticles were obtained by using two-probe technique previously reported [23]. The electrochemical activity of PPy nanoparticles was estimated by depositing thin films on indium tin oxide (ITO) slides and determined by cyclic voltammetry (CV) scanning the films. All CV scans were carried out on a classical three-electrode setup consisting of a saturated calomel reference electrode (SCE), a platinum counter-electrode, and an ITO working electrode. All potentials were measured vs. SCE on a Princeton VersaStat 3 (Princeton Applied Research) by CV scanning between potential limits of 0.2 and 1.4 V at scan rates of 10, 25, 50, and 100 mV/s, respectively. The electrolyte solution used was 1.0 mol/L HCl. The hydrophilicity of all the PPy/PSf nanocomposite membranes was obtained by a captive bubble technique using a DSA10 Krüss goniometer. At least seven contact angle measurements were performed across the membrane coupon at equally spaced intervals. The permeability was determined for each membrane by measuring water-flux at a fixed pressure of 10 psi. Bovine serum albumin (BSA, 66 kDa) with an average diameter of 6 nm was used to evaluate the rejection performance (i.e., selectivity) of the nanocomposite membranes. BSA concentrations in the permeate streams (Cp) and feed streams (Cf) were determined by a HP 8453 UV–vis technique. Solute particle rejection (r) was calculated by:

2.2. Preparation of PPy/polysulfone (PSf) nanocomposite membranes Asymmetric ultrafiltration membranes of PPy/PSf nanocomposite were prepared by a phase-inversion technique using DI water as coagulation bath. Typically, 0.06 g PPy nanoparticles were firstly dispersed in 6.83 g N-methylpyrrolidone (NMP) and then 1.44 g PSf beads were added. The mixtures consisting of 1.5 g polymers and 6.83 g NMP were stirred overnight at 50 °C to produce casting solution with weight concentration of 18%. The solution was cast on a commercial nonwoven polyester support fabric and then immersed in DI water at room temperature. Solvent (NMP) and non-solvent (DI water) exchanges induce the polymers precipitating forming porous nanocomposite membranes. 2.3. Chemical structure and morphology characterization

r ¼ ð1  C p =C f Þ  100%

The chemical structure of PPy nanoparticles was fully characterized by UV–vis, FT-IR, and XRD spectroscopy. The UV–vis spectra of

ð1Þ

Averaged permeability and rejection for each membrane were measured by five times.

HN HN

HN HN HN Pyrrole

NH2 HN

Oxidant + acids H N

H

NH2

H2N 2,4-diaminodiphenylamine (DDPA, Initiator)

N H

NH

H y

NH

x Polypyrrole (x

Scheme 1. The reaction proposed for polypyrrole nanoparticles.

y)

Y. Liao et al. / Journal of Colloid and Interface Science 386 (2012) 148–157

c d e f g

OCPmaxb (V)

Tic (°C)

Tmaxd (°C)

DTe (°C)

tMTf (s)

tRg (s)

0 2 100

0.4 0.2 –

0.58 0.55 –

24.5 24.6 24.7

33.6 33.7 34.5

9.3 9.1 9.9

99.7 89.3 86.6

5.0 8.0 17.1

OCPi = initial open-circuit potential. OCPmax = maximum open-circuit potential. Ti = initial temperature of polymerization. Tmax = maximum temperature of polymerization. DT = increased temperature from Ti to Tmax. tMT = time at the maximum temperature. tR = retention time at maximum temperature.

3.1. Synthesis of the PPy nanoparticles 3.1.1. Effect of the initiator concentrations Open-circuit potential (OCP) measurements indicated that the addition of a catalytic amount of 2,4-diaminodiphenylamine in the polymerization of pyrrole, leaded to a much lower starting potential (0.2 V) than the reaction carried out in the absence of the initiator (0.4 V) (Table 1). The temperature improved 9 °C by 89.3 s, as opposed to 99.7 s needed for reaction performed in the absence of the initiator (Fig. 1). Additionally, the polymerization sustained at the maximum temperature longer than the reaction carried out in the absence of the initiator (8.0 vs. 5.0 s). All the information implies that the initiator accelerates the reaction rate. This result is consistent with previous suggestions for PPy nanofibers that the initiator likely polymerizes with pyrrole in the initial stages of polymerization, leading to homogeneous nucleation of PPy chains [23]. Remarkable dependences of the product yield and particle size on the initiator concentrations are shown in Fig. 2. With addition of only 2 mol% initiator, the yield of doped PPy increased dramatically to 26.5% compared with 2.7% for polymerization performed in the absence of the initiator. Further increasing initiator concentration from 2 to 10 mol%, the product yield demonstrated a maximum value of 42.6%. As determined by DLS, the

Temperature (oC)

Initiator concentrations (mol%) 0

32

2 100

30

28

26

24 0

10

20

30

10

-3

30 10

20 10

10

-4

-5

0

3. Results and discussion

34

40

40

50

Polymerization time (min) Fig. 1. Reaction temperature measurements of the polymerization of pyrrole at room temperature in the presence of following concentrations of initiator: 0, 2, and 100 mol% (poly(2,4-diaminodiphenylamine)).

1000

1200

900

1000

800

800

700

600

600

400

Particle size (nm)

b

OCPia (V)

UV-vis λ max (nm)

a

Initiator content (mol%)

50

Conductivity (S/cm)

Table 1 Open-circuit potentials and thermal dynamic parameters of the polymerization of pyrrole at room temperature with addition of following concentrations of initiator: 0, 2 and 100 mol%.

Product yield (%)

150

200

500 0

2

4

6

8

10

Initiator concentration (mol%) Fig. 2. Effect of initiator concentration on the product yield, conductivity, kmax and size of doped PPy nanoparticles synthesized using FeCl3 as oxidant in the absence of acidic medium.

size of doped PPy particles obviously decreased from 1142 to 282 nm when 10 mol% initiator was added. This indicates that PPy nanoparticles were created by the initiator-assisted polymerization route, as opposed to microparticles were formed by conventional method. 3.1.2. Effect of the oxidant species Several representative oxidant species with different standard oxidation–reduction potentials (ORPs), including NaClO (1.63 V), H2O2 (1.77 V), H2O2/FeCl2 (2.80 V), (NH4)2S2O8 (2.01 V), FeCl3 (0.77 V), and K2Cr2O7 (0.11 V), were selected to optimize the polymerization of pyrrole. All the control reactions were conducted in the presence of 10 mol% initiator and in the absence of acidic media. Color-change and solid product were not observed when H2O2 was used as oxidant. Brownish solids were hardly obtained when the monomer was oxidized by NaClO. Brownish product with yield of 13.2% was obtained when polymerization was carried out using K2Cr2O7 as oxidant. Either the pyrrole oxidized by NaClO or K2Cr2O7, however, the products demonstrated negligible UV–vis absorbance at >400 nm, signifying that these oxidants hardly initiated pyrrole to form polymers. Another three oxidant species, such as H2O2/FeCl2, (NH4)2S2O8 and FeCl3, produced dark precipitates with the yield of 1173.5%, as shown in Table 2, first panel. When (NH4)2S2O8 and FeCl3 were used, the products obtained exhibited yields of 73.5% and 42.6%, the conductivities of 3.7  105 and 4.0  106 S/cm, and UV–vis absorbance maxima (kmax) at 585 and 890 nm, respectively. It appears that (NH4)2S2O8 and FeCl3 are better oxidants for preparing PPy because of their effective oxidizability toward pyrrole monomers. While applying H2O2/ FeCl2 as oxidant with the highest ORP (2.80 V) leaded to relative lower yield (11.1%), less conjugation (kmax = 510 nm), and much lower conductivity (<1010 S/cm). The reason may be that the products formed were soluble oligomers due to the over-oxidation caused by H2O2/FeCl2. When H2O2/FeCl2, (NH4)2S2O8, FeCl3 and K2Cr2O7 were employed to polymerization, the PPy nanoparticles exhibited sizes of 162, 580, 282, and 235 nm, respectively. Upon enhancing the ORPs (i.e., oxidizability) of oxidant, the size of the

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Table 2 Effect of the oxidant species and acidic media on the size, polymerization yield, UV–vis absorbance, and conductivity of doped PPy nanoparticles synthesized in the presence of 10 mol% concentration of initiator. Oxidant species or acidic media

Particle Size determined by DLS (nm)

UV–vis kmax (nm)

Sheet resistance (KX/sq.)

Bulk conductivity (S/cm)

NaClO H2O2 (NH4)2S2O8 K2Cr2O7 H2O2/FeCl2 FeCl3

Very low yield, no big conjugated UV–vis absorbance No product 580 73.5 235 13.2 162 11.1 282 42.6

Product yield (%)

585 402 510 890

160 >106 >106 1800

3.7  105 <1010 <1010 4.0  106

H2O CSA HClO4 HNO3 HCl

282 220 200 110 85

890 893 894 899 917

1800 736 743 78.2 202.6

4.0  106 8.5  106 1.4  105 7.2  105 3.0  105

42.6 65.1 54.3 53.8 45.2

PPy nanoparticles first increased and then decreased. This suggests that oxidants with either deficient reactivity or over oxidizability are not good for growth of PPy chains. Thus, we believe FeCl3 is the best oxidant to produce PPy nanoparticles with high conductivity, good product yield, and dispersibility. 3.1.3. Effect of the FeCl3/pyrrole molar ratios The dependences of product yield, size, conjugation, and conductivity of the PPy nanoparticles synthesized in the presence of 5 mol% initiator, on the FeCl3/pyrrole molar ratios, are displayed in Fig. 3. When increased the FeCl3/pyrrole molar ratio from 0.5/1 to 5/1, the product yield and particle size increased monotonously from 19.2% to 67.8% and from 205 to 505 nm, respectively. The PPy nanoparticles showed a maximum conductivity of 1.1  102 S/cm at FeCl3/pyrrole molar ratio of 5. The reason may be that more oxidants were added, pyrrole monomers would be better oxidized, and then more chain initiation and propagation occurred, resulting in polymers with higher conjugation and conductivity [24]. In contrast, less oxidants generally produced soluble oligomers with lower nanosize and conductivity [25].

3.1.4. Effect of the acidic media Significant effect of acidic media on the polymerization of pyrrole conducted in the presence of 10 mol% initiator is shown in Table 2, second panel. For comparison, the polymerization performed in the absence of acid medium is also presented. After introduction of 1.0 M HCl, HNO3, HClO4 or CSA into the polymerization, the yield of doped PPy nanoparticles were found to be 45.2%, 53.8%, 54.3% and 65.1%, respectively, as compared to 42.6% of polymerization conducted in the absence of acid. Interestingly, upon introducing small dopants HCl or HNO3 into the reaction, the PPy nanoparticles formed demonstrated low sizes around 85 and 110 nm, respectively, as determined by DLS. When large dopants CSA and HClO4 were used, PPy nanoparticles with size of 200220 nm were created. The smaller acid was used; the tighter and smaller PPy nanoparticles were produced. Different acids changed solubility of the initiator and pyrrole oligomers, which affected the size of PPy nanoparticles. Moreover, the conductivity improved by 18 times (7.2  105 vs. 4.0  106 S/cm) after the acids were introduced. The improvement in doping level probably leads to formation of positive charge colloids and ultimately, more dispersible and stable nanoparticle suspensions, according to Derjaguin–Landau–Verwey–Overbeek (DLVO) theory [26].

80

60

10

40

10 10

-1

-2

-3

-4

20 10

-5

Conductivity (S/cm)

Product yield (%)

10

0

400 800 300 700

Particle size (nm)

UV-vis λ max (nm)

500 900

200

600 0

1

2

3

4

5

FeCl3/pyrrole molar ratio Fig. 3. Effect of FeCl3/pyrrole molar ratio on the product yield, conductivity, kmax and size of doped PPy nanoparticles synthesized using 5 mol% concentration of initiator in the absence of acidic medium.

3.2. Chemical structure of the PPy nanoparticles 3.2.1. FT-IR spectra The FT-IR spectra of doped PPy nanoparticles synthesized with different concentrations of initiator using FeCl3 as oxidant are shown in Fig. 4. Spectrum of poly(2,4-diaminodiphenylamine) is also presented for comparison. The FT-IR spectra of doped PPy nanoparticles and poly(2,4-diaminodiphenylamine) showed significant differences. Poly(2,4-diaminodiphenylamine) exhibited characteristic bands at 740, 1032, 1480, and 1580 cm1 (Fig. 4e). While PPy nanoparticles demonstrated that stretching bands at 1472 and 1550 cm1 due to the pyrrole ring [27], a shoulder band at 1690 cm1 assigned to part of the oxidized pyrrole ring [28], two bands at 1032 and 1301 cm1 attributed to CN stretching bands [29], and stretching bands at 923 and 1202 cm1 generated by doped PPy [30]. Note that a small shoulder band at 740 cm1 appeared in spectra of PPy nanoparticles, indicating that a small amount of 2,4-diaminodiphenylamine has copolymerized with pyrrole, as shown in Scheme 1. Upon altering the initiator concentrations, all four pyrrole polymers exhibited nearly same FT-IR absorption characteristics. With increasing the initiator concentrations, the shoulder peak of PPy nanoparticles at 740 cm1 became more intensive but the peaks at 923 and 1202 cm1 became weaker, suggesting that more initiators have copolymerized with pyrrole.

Y. Liao et al. / Journal of Colloid and Interface Science 386 (2012) 148–157

Absorbance (a.u.)

152

1550 1472

d c c

1690

b a a 3000

2400

1800

1301 1202 1032 923

740

e e e

1200

600

Wavenumber (cm-1) Fig. 4. FT-IR spectra of (ad) PPy synthesized with the following initiator concentrations: (a) 0, (b) 2, (c) 5, (d) 10, and (e) 100 mol% (poly(2,4diaminodiphenylamine)).

3.2.2. XRD spectra XRD spectra of PPy particles synthesized with different concentrations of initiators using FeCl3 as oxidant are shown in Fig. 5. For comparison, XRD spectrum of polymerization product of the initiator 2,4-diaminodiphenylamine (poly(2,4-diaminodiphenylamine)) is also presented. All the XRD spectra of polymers showed only broad peaks between 15° and 30°. This might result from the nanoparticles geometry or the typical characters of the amorphous polymers. Poly(2,4-diaminodiphenylamine) showed a peak centered at 2h = 20.1° should be attributed to periodicity parallel and perpendicular to the semi-crystalline polyaniline-like main chains [31,32]. PPy synthesized in the absence of the initiator demonstrated a peak centered at 2h = 21.5° assigned to amorphous structure of chloride anion doped PPy chains [33]. PPy synthesized in the presence of the initiator displayed much broader peak, which did not center at 21.5° and 22.3°, indicating new polymeric structures, that is, copolymers consisting of pyrrole and initiator have been created.

d c

3.4. Properties of the PPy nanoparticles

Intensity (a.u.)

e

b a 20

30

40

50

3.3. Size and morphology of PPy nanoparticles Doped PPy nanoparticles synthesized by initiator-assisted polymerization exhibited size of 80500 nm, in contrast to several micrometers of PPy particles obtained by conventional method, as determined by DLS. Figs. 7 and 8 show SEM and TEM morphologies for PPy nanoparticles synthesized in the presence of 10 mol% initiator and acidic media such as HClO4 or CSA using FeCl3 as oxidant. The SEM morphology for PPy microparticles obtained by conventional method using FeCl3 as oxidant is also presented. For reactions performed in the absence of the initiator, PPy microparticles synthesized using FeCl3 exhibited irregular agglomerates along with some big sheets measured several micrometers (Fig. 7a). The microparticles completely precipitated within 5 h. The PPy products synthesized by initiator-assisted polymerization in the presence of HClO4 or CSA, in contrast, exhibited sphere-like nanoparticles with average diameters of about 200 and 250 nm, respectively, as determined by SEM (Fig. 7b and c). This can be further confirmed by TEM observations (Fig. 8a). In addition, PPy nanoparticles synthesized using K2Cr2O7 as oxidant exhibited average diameters as low as 150 nm (Fig. 7d). PPy nanoparticles synthesized using initiator-assisted route did not aggregate or settle out from their dispersions in water, ethanol, or NMP even after standing for 24 h, attesting their excellent processability.

3.2.3. UV–vis spectra All the products synthesized in the presence of initiator exhibited four typical UV–vis absorbance (Fig. 6), at 300 and 715 nm

10

attributed to p–p and n–p transitions of conjugated polymers [34], at 460 and 900 nm assigned to the transitions from the valence band to the anti-bonding and bonding polaron state of doped PPy, respectively. With increasing concentrations of the initiator, the absorbance shifted to lower wavelength (Fig. 6a), since more initiators have been copolymerized with pyrrole. The effect of oxidant species on the UV–vis spectrum of PPy nanoparticles is shown in Fig. 6b. Except the product obtained using K2Cr2O7 exhibited a shoulder band at 400 nm, the products obtained using FeCl3, (NH4)2S2O8, and FeCl2/H2O2 exhibited a band at >600 nm and a shoulder band at >800 nm. In particular, the two peaks distinctly appeared in the product obtained using FeCl3 as oxidant, implying that FeCl3 was the best oxidant for the formation of PPy with longest conjugation length. With increased in FeCl3/pyrrole molar ratio from 0.5 to 5, kmax of product continuously shifted to higher wavelength (891 nm vs. 950 nm) (Fig. 6c). The increased absorbance at large wavelengths and a shift of the kmax toward larger wavelengths are due to the increased conjugation length for PPy in the nanoparticles, as previously reported for nanostructured PPy thin films [35]. This also explain that the PPy nanoparticles exhibited an increased conductivity (3.0  106 S/cm vs. 1.1  102 S/cm). Fig. 6d shows UV–vis spectra of PPy nanoparticles synthesized in the presence of 1.0 M acidic media such as HCl, HNO3, HClO4, or CSA. As opposed to the PPy nanoparticles obtained in the absence of acid, the products obtained by the introduction of acidic media displayed much stronger band at 900 nm due to the higher doping level.

60

70

80

90

100

2Theta (Degrees) Fig. 5. Powder XRD patterns of (ad) PPy synthesized with the following initiator concentrations: (a) 0, (b) 2, (c) 5, (d) 10, and (e) 100 mol% (poly(2,4diaminodiphenylamine)).

3.4.1. Thermal stability and capacity into carbon nanoparticles Simultaneous TGADTGADSC scans for doped PPy nanoparticles synthesized in the presence of initiator are shown in Fig. 9. For comparison, the scans for poly(2,4-diaminodiphenylamine) and PPy microparticles synthesized in the absence of initiator are also presented. The PPy nanoparticles displayed three thermal decomposition stages: first stage (25150 °C) with weight loss of 5% should be attributed to water vapor, second stage (150350 °C) is associated with diffused dopants from PPy macromolecules, and third step

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(a)

Initiator concentr. (mol%)

1.6

(c)

0.6

5/1

2 3/1 5

0.8

Absorbance

Absorbance

1.2

2/1

0.4

10

1/1 0.2

0.5/1 FeCl3/pyrrole molar ratios

100

0.4

0.0 250

500

750

1000

250

Wavelength (nm)

(b) 0.8

1000

Acidic media

FeCl3 0.4

APS

500

HClO4 HNO3 HCl

0.4

H2O

0.0

Fe(II)/H2O2 250

CSA

0.8

K2Cr2O7

Absorbance

Absorbance

750

(d) Oxidant species

0.0

500

Wavelength (nm)

750

Wavelength (nm)

1000

250

500

750

1000

Wavelength (nm)

Fig. 6. UV–vis spectra of methanol dispersions of PPy nanoparticles synthesized with following (a) initiator concentrations: 2, 5, 10, and 100 mol% (poly(2,4diaminodiphenylamine)); (b) oxidant species: FeCl2/H2O2, K2Cr2O7, (NH4)2S2O8, and FeCl3 in the absence of acid medium using 10 mol% initiator; (c) FeCl3/pyrrole molar ratios: 0.5/1, 1/1, 2/1, 3/1, and 5/1 in the absence of acid medium using 5 mol% initiator; and (d) acidic medium: H2O, HCl, HNO3, HClO4, or CSA using 10 mol% initiator.

Fig. 7. Field-emission SEM images of PPy particles synthesized (a) in the absence of initiator and acid medium, (bc) in the presence of 10 mol% concentration of initiator, and 1.0 M acid medium; (b) HClO4 or (c) CSA using FeCl3 as oxidant and (d) in the absence of acidic medium and in the presence of 10 mol% concentration of initiator using K2Cr2O7 as oxidant; all the oxidant/pyrrole molar ratios were fixed at 1/1.

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(>350 °C) accompanying tremendous weight-loss and exothermicity can be attributed to rapid decomposition of polymer backbones. Also, the PPy nanoparticles exhibited char yield with value up to 42.2% at 800 °C (Fig. 9a, first panel). In contrast, the PPy microparticles and poly(2,4-diaminodiphenylamine) demonstrated char yields with values down to 7.7% and 27.9% at 800 °C, respectively. PPy nanoparticles exhibited much higher char yields than conventional PPy [36]. Decomposition of the conventional PPy main chains occurred at 580 °C and temperature corresponding to maximal weight-loss rate achieved 7%/min (Fig. 9a, second panel). The decomposition temperature of PPy nanoparticles dramatically improved to 670 °C, while the maximum weight-loss rate evidently decreased to 2.6%/min (Fig. 9d, second panel). Additionally, the burning enthalpies of PPy nanoparticles were much smaller than that of conventional PPy (2.6 vs. 7.3 lV/mg Fig. 9, third panel). This information further confirms that the PPy obtained by the initiator-assisted polymerization possesses better thermal stability than those obtained by conventional methods. Fig. 8bd shows morphology of carbon products fabricated by pyrolyzing CSA-doped PPy nanoparticles. Upon pyrolyzing the PPy nanoparticles at 800 °C, the carbon products showed foam-like morphologies (Fig. 8b). Interestingly, the carbon foam formed can be readily dispersed in ethanol by the help of sonication and exhibited morphology of individual sphere-like nanoparticles as same as polymeric

precursors; except that their average diameters were decreased by 2030 nm due to dehydrogenation and aromatization shrink of the precursors, as indicated by both SEM and TEM images (Fig. 8a, c, and d). Surprisingly, the carbon nanoparticles fabricated showed conductivity with values up to 3.7 S/cm. Therefore, pyrolyzing PPy nanoparticles provides a facile route to carbon nanoparticles with well-defined shape, high conductivity, and good dispersibility. 3.4.2. Electrochemical activity The electrochemical activity of PPy nanoparticles was estimated by depositing thin films on ITO and determined by cyclic voltammetry (CV) scanning the films (Fig. 10). With increased in scan rates from 10 to 25, to 50 and to 100 mV/s, PPy nanoparticle films demonstrated highest currents at 10 mV/s. High scan rates leaded to low electrochemical activity since over-oxidation destroyed conjugation of PPy. This is consistent with that polymers were less conjugated when PPy nanoparticles were synthesized using strong oxidants with high ORPs, as discussed for Fig. 6b. 3.4.3. Electrical conductivity Bulk conductivity of PPy nanoparticles was readily tuned by controlling polymerization conditions such as the initiator concentrations, oxidant species used, FeCl3/pyrrole molar ratios, and

Fig. 8. (a) TEM image of PPy nanoparticles synthesized in the presence of CSA and 10 mol% concentration of the initiator at FeCl3/pyrrole molar ratio of 1.0; (b) SEM image of carbon nanoparticles obtained at 800 °C by pyrolyzing PPy nanoparticles; (c) SEM; and (d) TEM image of the as-fabricated carbon nanoparticles.

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Weight (%)

90

TGA 60

d c e

30

a

b

Exo Endo

Heat flow (µV/mg)

Weight-loss rate (%/min)

0

6

a

DTGA

b

3

e

c d

0

a

6

b

DSC 3

e

c d

0

100

200

300

400

500

600

700

Temperature (oC) Fig. 9. Simultaneous TGA–DTGA–DSC scans of doped PPy particles synthesized in the presence of following concentrations of initiator: (a) 0, (b) 2, (c) 5, (d) 10, and (e) 100 mol% (poly(2,4-diaminodiphenylamine)).

acidic media utilized. As shown in Fig. 2, with increasing the initiator concentrations from 0 to 2 to 5 and to 10 mol%, PPy synthesized in the absence of acidic medium displayed decreased conductivity from 3.5  103 to 3.1  103 to 1.0  103 and to 4.0  103 S/cm, respectively, probably due to the initiator copolymerized with pyrrole lowered the regularity of PPy main chains. The conductivity can be largely improved through enhancing

0.2

0.1

Current (mA)

FeCl3/pyrrole molar ratios. For example, PPy nanoparticles obtained at FeCl3/pyrrole molar ratio of 5 exhibited good conductivity with values up to 1.1  102 S/cm. This is comparable to that of PPy nanofibers reported previously [37]. The conductivity of PPy nanoparticles can be readily improved by 218 times when the initiator-assisted polymerization conducted in the presence of acidic media (Table 2, second panel). The tunable conductivities should be attributed to the variable redox state, doping state, molecular weight, and p-conjugation length of the PPy dramatically controlled by the polymerization conditions.

0.0

-0.1

10 25 50

-0.2

100

3.4.4. Nanocomposite membranes of the PPy nanoparticles Special engineering plastics like PSf (Scheme 2) is promising ultrafiltration (UF) membrane material due to high mechanical property, easy processability, good thermal resistance, and chemical stability [38]. However, UF membranes made from above materials have surfaces with bad wettability, causing serious membrane fouling because of the solute–membrane hydrophobic interactions. Meanwhile, these UF membranes are either quite permeable but not very selective, or selective but not that permeable. The concept of nanocomposite membranes is believed to offer advanced membranes with improved mechanical property, chemical stability, porosity, hydrophilicity,, and anti-fouling property. This is expected for PPy/PSf nanocomposites membranes. Fig. 11 shows surface and cross-section morphologies of PPy/PSf nanocomposite membrane with 4% composition of PPy nanoparticles (Fig. 11a, inset). The nanocomposite membrane exhibited highly porous surface with pore size of 210 nm, as determined by SEM observations (Fig. 11a); and average roughness (Ra) of 5.1 ± 3.5 nm, root-mean-square (RMS) roughness of 7.0 ± 5.1 nm and maximal roughness (Rmax) of 64.6 ± 22.7 nm, as determined by AFM (Fig. 11b). Note that the PPy/PSf nanocomposite membranes displayed surface area difference (SAD) as low as 1.6%, evidencing that homogenous and smooth membranes were created. The cross-section of the nanocomposite membranes exhibited typical finger-like morphologies consisting of a dense top-layer (12 lm thick) with nanopores and a sub-layer (100120 lm thick) with microvoids (Fig. 11c). It can be seen that the PPy nanoparticles were well dispersed in the PSf matrix (Fig. 11d, arrows). By adding PPy nanoparticles into the PSf matrix, hydrophilicity of nanocomposite membranes distinctly improved as compared to pure PSf membranes (42° vs. 65°), as shown in Fig. 12. The influence of weight-fraction of PPy nanoparticles on initial water permeability and bovine serum albumin (BSA) rejection (i.e., selectivity) of PSf/PPy nanocomposite membranes is also shown in Fig. 12. Generally, adding higher concentrations of PPy nanoparticles into PSf matrix produced greater membrane permeability, translating into >10 times water flux than that of pure PSf membranes (9.2 vs. 97.6 gfd/psi). The nanocomposite membranes maintained high BSA rejections (85.797.5%) even 20% PPy nanoparticles were used. In particular, a synergy of permeability (9.2 vs. 41.3 gfd/psi) and rejection (90.2 vs. 96.3%) was created using 24% PPy nanoparticles as fillers. As compared to pure PSf, the formation of PPy/PSf nanocomposites therefore provided an effective pathway to improve the membrane properties such as porosity, hydrophilicity, permeability, selectivity, and anti-fouling property.

-0.3

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

E (V vs. SCE) Fig. 10. Cyclic voltammetry (CV) scans of PPy nanoparticles synthesized in the presence of 10 mol% concentration of initiator at following scan rates: 10, 25, 50, and 100 mV/s.

O

CH3 C CH3

O

O S O

Scheme 2. The chemical structure of polysulfone (PSf).

n

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Fig. 11. (a) SEM and (b) AFM surface morphologies of (a, inset) a typical PPy/PSf nanocomposite membrane made by addition of 10% PPy nanoparticles; (c and d) SEM crosssection morphologies of the nanocomposite membrane at (c) low and (d) high magnifications; inset arrows in (d) show the PPy nanoparticles dispersed in the PSf matrix.

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optimized by simply altering initiator concentrations, oxidant/ monomer ratios, oxidant species used, and acidic media employed. PPy nanoparticles provided effective precursors for fabricating carbon nanoparticles with conductivity of 3.7 S/cm. By blending PPy nanoparticles with PSf provided an effective pathway to ultrafiltration membranes with high-performance including excellent hydrophilicity, water permeability, and BSA selectivity.

Contact angle (o) Permeability (gfd/psi)

120

BSA rejection (%)

Unit

100

80

Acknowledgments 60

40

20

0 0

2

4

10

20

PPy percentages (wt%) Fig. 12. Contact angle, permeability, and BSA rejection of PPy/PSf nanocomposite membranes made by addition of PPy nanoparticles at the following weight percentages: 0%, 2%, 4%, 10%, and 20%.

4. Conclusions In summary, highly dispersible PPy nanoparticles were synthesized by a facile chemical oxidative polymerization in the presence of a catalytic amount of 2,4-diaminodiphenylamine and acidic medium. The initiators copolymerized with pyrrole in the initial stages of polymerization, acting like bipyrrole and helping to nucleate the PPy main chains. The morphology, nanosize, product yield, conductivity, electrochemical activity, thermal stability, and processability of the PPy nanoparticles obtained can be

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