The green preparation of poly N-vinylpyrrole nanoparticles

Abstract

Well-dispersed poly N-vinylpyrrole (PNVPY) nanoparticles have been synthesized by UV-catalytic chemical polymerization with hydrogen peroxide (H₂O₂) as the oxidant and polyvinyl pyrrolidone (PVP) as the stabilizer. The chemical structure of PNVPY was characterized by FTIR and ¹H-NMR. The morphology and size of PNVPY nanoparticles were observed and tested. It was found that the PNVPY particles were spherical and the particle size ranged from 18 to 106 nm with an average value of 25.98 nm. The effect of experimental conditions on the formation process of polymer nanoparticles was investigated. It was found that the polymerization rate of N-vinylpyrrole (NVPY) and the average size of PNVPY increased with the increase of UV irradiation intensity from 0 to 30 W and H₂SO₄ concentration from 2 to 22 g L⁻¹. Meanwhile, with the increase of PVP concentration from 5 to 20 g L⁻¹, the polymerization rate increased while the average size decreased. In addition, it was interesting that PNVPY nanoparticles had good compatibility with polyethylene glycol dimethacrylate due to the introduction of the vinyl group.

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Introduction

Polypyrrole (PPy) has received great attention from researchers in recent years due to its excellent properties, including high conductivity, outstanding stability and good biocompatibility.¹ This enables the development of its technological applications such as sensors,² electromagnetic shielding,³ metal corrosion⁴ and electrically switched ion exchange.⁵ However, PPy has poor solubility in many common solvents due to its rigid molecule structure. As a result, a great effort has been made to improve its solubility in the past several decades.^6,7 In general, there are two main methods to solve this problem: the preparation of soluble PPy derivatives and solvent-dispersed PPy nanoparticles.

In recent years, various researches of soluble PPy derivatives have been reported. Kijima et al. synthesized three sorts of liquid crystalline N-substituted pyrrole from 6-(1-pyrrolyl)hexanol by Mitsunobu reaction. These pyrrole monomers were polymerized by electrochemical and chemical methods, obtaining soluble and fusible polymers.⁸ Additionally, an asymmetric dipyrrole monomer 2-methyl-1,3-di(1H-pyrrol-yl)propan-1-one was prepared and thus the corresponding soluble polymer was synthesized by chemical oxidation polymerization with FeCl₃ as the oxidant.^6,9 Moreover, other researchers obtained PPy derivatives which were soluble in water or common organic solvents as well.^10–13 A polypyrrole derivative (PNVPY) which was soluble in common organic solvents was synthesized by our group. The polymer exhibited a strong optical absorption in the range of visible light, as well as a strong photoluminescence in both organic solution and in its solid state.¹⁴

In addition, a series of new developments have been made recently in both the preparation and application of PPy nanoparticles. Yang et al. sythesized PPy nanoparticles (60–100 nm) using a microemulsion method, where Fe³⁺ acted as the catalyst and PVA as the stabilizer in the aqueous phase. The prepared PPy nanoparticles could be used as photothermal therapy agents of photothermal therapy treatment of cancer due to their excellent photostability and high photothermal conversion efficiency.^15,16 Sang et al. prepared PPy nanoparticles of fixed size and morphology by chemical oxidative polymerization which used myristyl trimethyl ammonium bromide (MTAB) and decyl alcohol as the nanoreactors. The synthesized PPy nanoparticles could be used for a highly efficient counter electrode of dye-sensitized solar cells.¹⁷ Recently, a new temperature and electric field dual-stimulus responsive PPy nanoparticles prepared by emulsion polymerization were used for programmed drug delivery.¹⁸ Yu et al. prepared uniform PPy nanoparticles through a green and effective method which was based on the application of H₂O₂ and UV irradiation. The usage of green oxidant H₂O₂ avoided the residues of metal material in the polymer caused by metal oxidants. Meanwhile, UV irradiation acted as a clean and effective catalyst for promoting the polymerization of pyrrole.¹⁹ Moreover, the polymerization process and the properties of pyrrole could be well-controlled by UV irradiation.^20,21 However, despite its promising application, the combination of UV irradiation and H₂O₂ has not been reported in the preparation of nanoparticles of PPy derivatives so far. The structure of NVPY is symmetrical. Thus, the polymerization of NVPY will result in an increased order and a planarity of polymer backbone. In addition, the effect of double bond of NVPY on polymerization is also worth studying.

In the present work, well-dispersed PNVNY nanoparticles are prepared by chemical oxidation method which is catalyzed by UV irradiation. H₂O₂ acts as a green oxidant to initiate the polymerization of NVPY. Meanwhile, PVP is used as a stabilizer. Furthermore, the influence of experimental parameters including the irradiation intensity and the concentrations of H₂SO₄, PVP and H₂O₂, on the polymerization of NVPY is discussed.

Experimental

Materials

The monomer of NVPY was synthesized according to the procedures which were reported in the previous work.¹⁴ Poly(N-vinylpyrrolidone) (PVP), with molecular masses of 40 000 was purchased from BASF. The aqueous H₂O₂ solution (30 wt%), sulfuric acid (95%), tetrahydrofuran (THF), chloroform (CLF), N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO) and potassium hydroxide (analytical grade) were purchased from Shanghai Sinopharm Chemical Reagent Co., Ltd and used without further purification.

Synthesis of PNVPY nanoparticles

A certain amount of NVPY was added to 30 mL deionized water with dissolved PVP. Then the aqueous solution mixed with H₂O₂ and H₂SO₄ was introduced to the above solution immediately. The polymerization process was performed at room temperature with continuous stirring under UV irradiation (253.7 nm). In order to investigate the effect of different reaction conditions on polymerization, the irradiation intensity varied in the range of 0–30 W, and the concentrations of H₂SO₄, PVP and H₂O₂ varied in the range of 2–22 g L⁻¹, 5–20 g L⁻¹, 2–20 g L⁻¹, respectively. The reaction was terminated after 5 hours. The PNVPY nanoparticles were obtained using a high speed centrifugation (1200 rpm, 10 minutes).

Characterization

The chemical structures of NVPY monomer and polymer were identified by FTIR spectroscopy (Nicolet AVATAR-IR 360 spectrometer) and ¹H NMR spectroscopy (Bruker Advance 400 MHz spectrometer). The polymerization process of NVPY was monitored by a UV-2550 spectrophotometer. The samples were diluted from 0.2 mL to 3 mL before the test. JSM 7000M scanning electron microscope (SEM) and JEM-2100 transmission electron microscope (TEM) were used to observe the morphologies of samples. The particle size and size distributions were analyzed by the dynamic lighting scattering (DLS) technique using Zetasizer Nanoequipment (Nano-ZS90 Malvern instrument). Electrical conductivity of PNVPY particles was measured by a standard four-probe apparatus (SZ85 digital instrument).

Results and discussion

Preparation of PNVPY nanoparticles

The experimental strategy for the synthesis of the PNVPY nanoparticles is illustrated in Fig. 1. H₂O₂ acts as the preferred oxidant and PVP acts as the stabilizer were added to a NVPY solution in aqueous H₂SO₄ to synthesize the PNVPY nanoparticles. The chemical structure of PNVPY was characterized by FTIR spectra and ¹H NMR spectra. Three curves can be observed in the FTIR spectra (Fig. 2a). The curve PNVPY with PVP shows the product contained PVP. The curve PNVPY without PVP represents the PNVPY polymer. In Fig. 2a for PNVPY, a peak appeared at 781 cm⁻¹ represents α-substituted five-membered heterocyclic ring compound.^22,23 However, the peak of the characteristic absorption of NVPY around 950 cm⁻¹ disappeared, indicating that an α-coupling of pyrrole had obtained. The absorption peaks at 1551 and 1487 cm⁻¹ were the characteristic absorption of pyrrole ring. The three absorption bands at 2976, 2930 and 2880 cm⁻¹ were assigned to saturated alkyl groups, suggesting that vinyl was also involved in the polymerization. However, the characteristic absorption of the double bond at 1660 and 3099 cm⁻¹ could be seen, illustrating that vinyl didn't polymerize completely. The FTIR spectra indicated that no matter with or without PVP, the monomer of NVPY polymerized. Fig. 2b shows the UV-vis absorption spectra of the reaction solution before (0 min) and after (300 min) the experiment. It can be seen that two main absorption peaks appeared after the reaction from the curve PNVPY without PVP, one of the peaks at 289 nm was assigned to the π–π* transition of PNVPY and the other one at 474 nm was attributed to the polarons transition of PNVPY.²⁴ The two absorption bands can also be seen in the curve PNVPY with PVP. ¹H NMR spectra of NVPY and PNVPY were also recorded by dissolving the polymer in deuterated chloroform. As shown in Fig. 2c, the signal at 6.92 and 6.73 ppm corresponding to the α-H and β-H of NVPY shifted to 6.66 and 6.16 ppm in the curve of PNVPY without PVP. The integrated area of α-H is less than the integrated area of β-H, suggesting that polymerization of NVPY occurred mainly in α-H of pyrrole ring. The position of vinyl H of NVPY located in 6.87, 5.16 and 5.12 ppm while slightly changed after polymerization, which likewise indicated that the vinyl of NVPY monomer polymerized to some extent. The ¹H NMR spectrum of PNVPY synthesized with PVP presents similar results as shown in Fig. 2c. It also confirmed the polymerization of NVPY with the presence of PVP.

Fig. 1 Schematic illustrating the formation of PNVPY nanoparticles.

Fig. 2 (a) FTIR spectra of NVPY and PNVPY (b) UV-vis absorption spectra of the reaction solution before reaction and after reaction. (c) ¹H NMR spectra of NVPY and PNVPY. (d and e) SEM images of PNVPY. The inset of image (e) shows the size distribution of PNVPY nanoparticles measured by DLS. (f) TEM image of PNVPY nanoparticles. The radiation power was 30 W, and the concentration of PVP, H₂O₂ and H₂SO₄ were 10 g L⁻¹, 10 g L⁻¹ and 15 g L⁻¹, respectively.

The morphologies of PNVPY samples were shown in Fig. 2d–f. It can be seen that PNVPY synthesized without PVP shows irregular cauliflower-like structure (Fig. 2d), which was similar the typical morphology of PPy powders.²⁵ Distinctly, the polymer was granular when PVP participated in the polymerization of NVPY as shown in Fig. 2e. PVP molecules covered on the surface of the particles and provided a steric stability for the growth of polymer nanoparticles during the polymerization process of NVPY.²⁶ In addition, the particle size ranged from 18 to 106 nm with an average size of 25.98 nm (the inset of Fig. 2e). As shown in Fig. 2f, the size of PNVPY particles was about 60 nm, which almost matched the data from DLS. The conductivity of the film of PNVPY particles was around 0.58 S m⁻¹ at room temperature (Table S1†), which was similar to the conductivity of PVNPY prepared by the method of ferric chloride oxidation.²⁷

The reaction process is investigated by UV-vis spectrum. Fig. 3a shows the UV absorption spectra of the reaction solution varied in the range of 0–300 min. It is clear that the absorption intensity increased as reaction progress (Fig. 3b), indicating the products formed gradually. When it proceeded at 300 minutes, the absorption intensity of PNVPY had a downward trend. It indicated that polymerization of NVPY monomers tend to complete. Under UV irradiation, the H₂O₂ decomposed into hydroxyl radicals which induced the polymerization of NVPY monomer directly.¹⁹ The molecular chain of PNVPY will reunite nucleation due to the hydrophobicity of PNVPY molecules. PVP could cover the small PNVPY particles to reduce the surface energy owing to its amphiphilic molecular structure. Then the core continued to grow until sufficient PVP were added, forming stable PNVPY particles.

Fig. 3 (a) UV-vis absorption spectra of aqueous solution at different reaction time. (b) The absorption intensity at different wavelengths. The radiation power was 30 W, and the concentration of PVP, H₂O₂ and H₂SO₄ were 5 g L⁻¹, 10 g L⁻¹ and 15 g L⁻¹, respectively.

Effect of irradiation intensity on the synthesis of PNVPY

The influence of radiation intensity (0–30 W) on the polymerization of NVPY was investigated as shown in Fig. 4a. It can be seen that the solution has almost no absorption peak without UV illumination. On the contrary, the solution with UV irradiation exhibited two main absorption peaks at 289 and 474 nm after the reaction, which implied the formation of PNVPY. The peak around 289 nm was attributed to the π–π* transition of PNVPY and the other one around 474 nm corresponded to the polarons transition of polymer. In addition, the absorption intensity increased significantly with the increase of light intensity from 0 to 30 W. Therefore, it indicated that the process of polymerization of NVPY initiated by H₂O₂ was slow. Yet UV irradiation promoted the polymerization and the increase of illumination intensity gave rise to the increase of polymerization rate. It could be ascribed to that UV irradiation not only accelerated the decomposition of H₂O₂ to form hydroxyl radicals but also activated monomers.^20,28 As discussed above, the polymerization rate of monomer was improved with the increase of illumination power in the range of 0–30 W.

Fig. 4 (a) UV-vis absorption spectra of the reaction systems under different UV irradiation intensity. (b) The size distribution of PNVPY nanoparticles with different UV irradiation. The inset of (b) represents the number-average size of PNVPY nanoparticles. The concentration of PVP, H₂O₂ and H₂SO₄ were 5 g L⁻¹, 10 g L⁻¹ and 15 g L⁻¹, respectively.

The size distribution of PNVPY particles at different irradiation watts is measured by DLS with the results shown in Fig. 4b. It is found that the size of PNVPY particles was substantially smaller than 100 nm. When the irradiation intensity increased from 0 to 30 W, the average size of PNVPY nanoparticles increased from 7.74 to 25.98 nm (the inset of Fig. 4b). It could be attributed to that the ultraviolet radiation promoted the polymerization of monomers. The polymerization of NVPY initiated by H₂O₂ was a slow reaction in the absence of ultraviolet radiation. PVP can minimize agglomeration of PNVPY nanoparticles, resulting in the smaller size and narrower size distribution. The polymerization rate of NVPY was accelerated under UV irradiation, which facilitated the growth and agglomeration of PNVPY.¹⁹ Therefore, both the average size and size distribution of the nanoparticles increased as the irradiation intensity became higher.

Effect of PVP concentration on the synthesis of PNVPY

PVP as an important capping agent could be adsorbed preferentially on certain polymer surface, which prevent the PNVPY nanoparticles from aggregation, and promotes the synthesis of PNVPY nanoparticles. The effect of PVP concentration on the formation of PNVPY particles is shown in Fig. 5a. It can be seen that the absorption intensity of the solution increased with the increase of PVP concentration in the range of 5–20 g L⁻¹. It indicated that the increase of PVP concentration gave rise to the increase of polymerization rate of NVPY. Xia et al. reported that the lactam groups of PVP molecule could adsorb cationic radical of monomer to form the active sites of polymerization of monomer.²⁹ Therefore, the active sites increased with the increase of PVP concentration, accelerating the rate of polymerization.

Fig. 5 (a) UV-vis absorption spectra of the reaction systems with different PVP concentrations. (b) The size distribution of PNVPY nanoparticles with different PVP concentrations. The inset of (b) represents the number-average size of PNVPY nanoparticles measured by DLS. The radiation power was 30 W, and the concentration of H₂O₂ and H₂SO₄ were 10 g L⁻¹ and 15 g L⁻¹.

The size distribution of PNVPY particles at different PVP concentrations is shown in Fig. 5b. In Fig. 5b, the size distribution of PNVPY particles gradually narrowed with the increase of PVP concentration in the range of 5–20 g L⁻¹, and the average size of particles reduced from 221.95 to 8.47 nm (the inset in Fig. 5b). The PNVPY particles might be given sufficient stability at the early stage of their growth when a large amount of PVP was added. Consequently, the transmission and accumulation of the small particles were hindered. Therefore, both average size and distribution width of particles decreased with the increase of PVP concentration.

Effect of H₂O₂ concentration on the synthesis of PNVPY

The H₂O₂ acted as the oxidizing agent for chemical oxidation polymerization of NVPY monomer. As shown in Fig. 6a, the effect of H₂O₂ concentration (2–20 g L⁻¹) on the synthesis of PNVPY is also discussed. It confirms that no product generated in a blank experiment without H₂O₂, indicating H₂O₂ is essential to the formation of PNVPY in this experiment. The polymerization rate of NVPY increased sharply with the increase of H₂O₂ concentration from 2 g L⁻¹ to 4 g L⁻¹, but the polymerization rate decreased when the concentration of H₂O₂ was 10 g L⁻¹. Then the polymerization rate raised again when the concentration of H₂O₂ was 20 g L⁻¹. Hydroxyl radical produced by photolysis of H₂O₂ initiated polymerization of monomers. The amount of hydroxyl radical increased with the increase of H₂O₂ concentration in the low concentration range. However, the reaction would be inhibited when the concentration of H₂O₂ was high enough due to the role of H₂O₂ as the scavenger of hydroxyl radical.³⁰ Therefore, the polymerization rate of NVPY didn't increase regularly with the increase of H₂O₂ concentration from 2 g L⁻¹ to 20 g L⁻¹. The results also indicated that the polymerization of NVPY was initiated by hydroxyl radical instead of H₂O₂.

Fig. 6 (a) UV-vis absorption spectra of the reaction systems with different H₂O₂ concentrations. (b) The size distribution of PNVPY nanoparticles with different H₂O₂ concentrations. The inset of (b) represents the number-average size of PNVPY nanoparticles. The radiation power was 30 W, and the concentration of PVP and H₂SO₄ were 5 g L⁻¹ and 15 g L⁻¹.

The size distribution of PNVPY particles at different H₂O₂ concentrations is shown in Fig. 6b. In the Fig. 6b, the size of PNVPY particles prepared with H₂O₂ concentrations of 2 g L⁻¹ and 4 g L⁻¹ was almost larger than 100 nm, the average size was 129.18 and 155.09 nm (the inset in Fig. 6b), respectively. However, the size of PNVPY particles became smaller when the H₂O₂ concentrations were 10 g L⁻¹ and 20 g L⁻¹, the average size were 25.98 and 4.48 nm (the inset in Fig. 6b). The variation of the size of PNVPY particles wasn't consistent with the polymerization rate of NVPY. It indicated that the polymerization rate wasn't the only factor that effected the size of PNVPY nanoparticles.

Effect of H₂SO₄ concentration on the synthesis of PNVPY

The H₂SO₄ provided an acidic environment to the polymerization of NVPY. The effect of acidity on the fabrication of PNVPY particles was also investigated. As shown in Fig. 7a, it is clear that the reaction system without H₂SO₄ (alkaline or neutral) had no significant absorption, implies that almost no PNVPY particles formed. It is probably due to that the oxidation of the pyrrole ring was vulnerable to be attacked by nucleophilic hydroxyl radical, accordingly the polymerization process of the pyrrole ring in neutral or alkaline environment would be strongly suppressed.³¹ However, the polymerization of NVPY was promoted dramatically with the addition of H₂SO₄. The increase of H₂SO₄ concentration from 2 g L⁻¹ to 22 g L⁻¹ led to the increase of polymerization rate of monomer. As discussed above, it is concluded that an acidic environment was more conducive to the polymerization of NVPY monomer.³²

Fig. 7 (a) UV-vis absorption spectra of the reaction systems with different H₂SO₄ concentrations. (b) The size distribution of PNVPY nanoparticles with different H₂SO₄ concentrations. The inset of (b) represents the number-average size of PNVPY nanoparticles. The radiation power was 30 W, and the concentration of PVP and H₂O₂ were 5 g L⁻¹ and 10 g L⁻¹.

The increase of H₂SO₄ concentration not only improved the polymerization rate of monomers, but also had a significant impact on the size of PNVPY particles. The size distribution of PNVPY particles at different H₂SO₄ concentrations is shown in Fig. 7b. The size increased with the increase of H₂SO₄ concentration from 2 g L⁻¹ to 22 g L⁻¹. The average size of H₂SO₄ concentrations corresponding to 2 g L⁻¹, 10 g L⁻¹, 15 g L⁻¹, 22 g L⁻¹ were 6.16, 9.84, 25.98 and 125.73 nm, respectively. When polymerization rate was high enough, PVP molecular couldn't coat PNVPY particles completely, which facilitated the growth and coalescence of the particles. Therefore, the size and distribution of particles increased.

The compatibility of PNVPY nanoparticles with polyethylene glycol dimethacrylate

The PNVPY nanoparticles can be stably dispersed in aqueous solution. The SEM images of the section of the curing film were gradually enlarged as shown in Fig. 8. It can be seen that nanoparticles were dispersed uniformly in the matrix resin, illustrating that PNVPY nanoparticles had good compatibility with polyethylene glycol dimethacrylate. It could be ascribed to the introduction of vinyl group.

Fig. 8 The cross-sectional SEM photographs of the cured film of polyethylene glycol dimethacrylate filled with PNVPY nanoparticles.

Conclusions

PNVPY nanoparticles have been synthesized by UV-catalytic polymerization with H₂O₂ as an oxidant and PVP as a stabilizer. PNVPY nanoparticles were spherical and presented the size ranged from 18 to 106 nm. The reaction mechanism was inferred that the hydroxyl radicals produced from the photolysis of H₂O₂ initiated the oxidant polymerization of NVPY. It is found that the polymerization rate of NVPY increased with the increase of UV irradiation intensity, PVP and H₂SO₄ concentration in the range from 0–30 W, 5–20 g L⁻¹ and 2–22 g L⁻¹, respectively. In addition, it is found that the average size of PNVPY nanoparticles increased with the raise of UV radiation intensity and H₂SO₄ concentration, but decreased with the PVP concentration. It is notable that the PNVPY nanoparticles not only can be stably dispersed in aqueous solution, but also had good compatibility with polyethylene glycol dimethacrylate. At last, it is interesting that the existence of free vinyl group of polymer convenient for some other reactions, such as graft polymerization and composite reaction.

Acknowledgements

The research was financially supported by NSFC under Grant No. 51473133 and by the International Cooperation project of Shaanxi province of 2015KW-016.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra18234h

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