Green synthesis of polymeric microspheres that are monodisperse and superhydrophobic, via quiescent redox-initiated precipitation polymerization

Abstract

By virtue of simple quiescent redox-initiated precipitation polymerization, monodisperse poly(lauryl methacrylate-divinylbenzene) microspheres bearing exceptionally low polydispersity index below 1.01 are obtained with a monomer concentration of 20 vol% in acetic acid at ambient temperature. Superhydrophobic surfaces with water contact angles around 160° are successfully fabricated by dip-coating and deposition of the highly monodisperse microspheres onto glass substrates.

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1. Introduction

Monodisperse microspheres are widely used in numerous fields, including packing materials for chromatography, matrixes for solid phase extraction, building materials for sensors, and the precursors of photonics, among others.^1–4 Since the end of the last century, several decades have witnessed a growing development in synthesis strategies for the fabrication of various monodisperse microspheres. Emulsion polymerization, dispersion polymerization, seeded swelling polymerization and precipitation polymerization have successively been designed and applied to a large number of microsphere preparations.^5–14 Among the developed strategies above, precipitation polymerization is a unique approach to synthesize pure microspheres free from any surfactant or other additives, which is crucially important for biomedical and other bio-related system applications because the surfactant is very difficult to remove completely after polymerization, and thus may interfere interactions between target molecules and biomolecules.

Polymerization at ambient temperature and use of environment-friendly solvent have received great attention with the growing concern about environmental protection and sustainable development in recent years, as is seen in the research field of precipitation polymerization. Irradiation-initiated and photo-initiated precipitation polymerization have proven effective for synthesis of various microspheres. Actually, as early as 1980s, several studies reported on preparation of polymeric microspheres by γ-irradiation initiated precipitation polymerization at room temperature.^15–17 In recent years, photo-initiated precipitation polymerization has attracted more attention due to its simple request for instrument.^18,19 Compared to thermo-initiated precipitation polymerization, UV-initiated precipitation polymerization engendered a more uniformity of poly(divinylbenzene) (PDVB) microspheres with polydispersity index (PDI) below 1.01 when the monomer loading was below 6 vol%.

A few innoxious solvents, such as supercritical carbon dioxide, ethanol and acetic acid, have been adopted in recent investigations of precipitation polymerization. Supercritical carbon dioxide is an appropriate environment-friendly solvent, but an additional organic cosolvent is usually necessary to obtain monodisperse microspheres.^20,21 Ni and Kawaguchi described a method of preparation of poly(acrylamide-methacrylic acid) microspheres in ethanol by precipitation polymerization.²² Recently, Ni et al. further found such microspheres could be also obtained in a quasi-static way.^23,24 They postulated a new mechanism based on particles generated from mini monomer droplets to explain their observations.^22–24 Kong et al. used ethanol and its mixture with water as the solvent for precipitation polymerization of styrene and trihydroxymethyl propane triacrylate.^25,26 The microspheres had a PDI of below 1.05 with a yield of 90% at a monomer loading of 4 wt%. Zhang et al. combined atom transfer controlled polymerization and precipitation polymerization in ethanol to prepare living polymeric microspheres at room temperature.^27,28 Yang et al. replaced acetonitrile with acetic acid as the solvent for preparation of PDVB microspheres by precipitation polymerization.^29,30 Both spherical and pumpkin-like particles were formed with higher particle yield when the DVB loading was below 2 vol%. The uniformity of microspheres, however, was much inferior to that obtained with acetonitrile as the solvent.

Redox-initiated polymerization is a traditional radical polymerization method.^31,32 Especially, it can be conducted at lower temperature. In our previous study, we adopted a pair of redox initiator for synthesis of supermacroporous cryogels at sub-zero temperature.³³ To our best knowledge, there is few literature about redox-initiated precipitation polymerization in organic solvent.

Inspired from nature, superhydrophobic surfaces bearing micrometer-scale roughness are successfully fabricated to mimic the self-clean behaviour of the lotus leave.³⁴ In most cases, such superhydrophobic surfaces can be created by electrospinning, plasma polymerization, etching, moulding, lithography and mechanical treatment, but these methods usually suffer from time-consuming and higher cost.^35,36 Recently, several reports on fluorinated monodisperse microspheres have demonstrated to make superhydrophobic surfaces by a simple dip-coating method.^37–39 Given the cost of the fluorinated microspheres, it would be promising if fluorine-free microspheres could produce such similar biomimic surface.

Based on the above considerations, we explored, herein, a new approach to synthesize monodisperse microspheres by redox-initiated precipitation polymerization at ambient temperature in acetic acid. We chose lauryl methacrylate (LMA) and divinylbenzene (DVB) as monomer and crosslinker, respectively, as they are known hydrophobic vinyl compounds. The obtained suspensions of poly(LMA-DVB) microspheres were further casted on a glass slide to form microsphere assembly, and their hydrophobicities were tested by water contact angle measurements.

2. Experimental section

In a typical reaction, DVB (4.80 g, 36.9 mmol), LMA (9.39 g, 36.9 mmol) and the mixture of benzoyl peroxide (BPO, 1 wt% with respective to the total monomer)/N,N-dimethyl aniline (DMA, 1/1 molar ratio) were dissolved in 64.0 mL of acetic acid. The volume ratio of the total monomers to the solvent was 20%. The monomer mixture was degassed by bubbling dried nitrogen for 15 min and then sealed in a 100 mL capacity of glass bottle, followed by incubation in a water bath with a temperature fixed at 25 °C. After 48 h of polymerization, the resultant particles were separated by centrifuge at 10 000 rpm for 15 min, washed by methanol for three times, and then dried at 50 °C until constant weight. The microsphere array was fabricated by a dip-coating route. The poly(LMA-DVB) microspheres were dispersed in ethanol with a microsphere concentration of 60 mg mL⁻¹. After being ultrasonicated (200 W, 40 kHz) for 20 min, the microsphere suspension was casted on a clean surface of glass slide. The solvent was allowed to evaporate at room temperature, then the microsphere film was further dried under vacuum for 24 h prior to use.

The yields, expressed as monomer conversion into particles, were determined by gravimetry. The detailed information about other characterizations including scanning electron microscopy (SEM), FT-IR, elemental analysis and water contact angle measurement can be found in ESI.†

3. Results and discussion

3.1. Stirring condition

Basically, a gentle agitation is required for maintaining the uniformity of microspheres prepared in traditional precipitation polymerization. In our initial trials, we failed to obtain monodisperse poly(LMA-DVB) microspheres under stirring conditions. Actually, only agglomeration was obtained even when the rotation speed was slower than 20 rpm min⁻¹ (Fig. S1A†). By contrast, monodisperse microspheres with a PDI (U(PDI) = D_w/D_n) value of 1.012 were obtained in a quiescent mode (Fig. S1B†), which is considerably different from that observed in traditional precipitation polymerization. Apparently, compared to the use of stirring apparatus, this quiescent polymerization should be a more economic way for synthesis of microspheres in terms of chemical engineering.^23,24,40,41 Therefore, it was adopted in our subsequent investigations.

It should be also noted that polydisperse poly(LMA-DVB) microspheres with a much higher PDI of 1.121 was obtained under the identical condition except acetonitrile instead of acetic acid used as the solvent (Fig. S1C†), and the corresponding yield of particles decreased from 29.6% in acetic acid to 17.6% in acetonitrile, suggesting that in precipitation polymerization of LMA and DVB, acetic acid was superior to acetonitrile, though the latter is a prior option in traditional precipitation polymerization.

3.2. Monomer loading and composition

A series of experiments were designed to study the effect of total monomer loading amount varied from 2 vol% to 30 vol% in acetic acid at 25 °C with 50/50 molar ratio of LMA to DVB. The size and size distribution of the resultant particles are shown in Table S1† and Fig. 1. The size of microspheres increased from 0.90 μm to 3.49 μm with the monomer loading increased from 2 vol% to 20 vol% (Fig. 1A–E). Meanwhile, the polydispersity index of microspheres decreased from 1.053 to below 1.020. When the monomer loading was increased up to 30 vol%, the microsphere size increased up to 5.64 μm and the polydispersity index of particles was kept at 1.127 (Fig. 1F). Simultaneously, the yield increased gradually from 1.86% to 33.75% with the monomer concentration increased from 2 vol% to 30 vol% (Fig. 1G).

Fig. 1 SEM images of poly(LMA-DVB) (50/50 molar ratio) microspheres at various monomer concentrations of (A) 2 vol%, (B) 5 vol%, (C) 10 vol%, (D) 15 vol%, (E) 20 vol%, (F) 30 vol%. (G) Effect of monomer concentration on particle yield and diameter of poly(LMA-DVB) microspheres.

These results above indicated that poly(LMA-DVB) microspheres could maintain the monodispersity until the monomer loading was as high as 20 vol%, which is the highest monomer loading, to our best knowledge, in the research field of precipitation polymerization at present. In general, the microspheres have good uniformity only when the monomer loading amount is lower than 2 vol% to avoid coagulation in traditional precipitation polymerization. Based on the reported studies,^29,42 when acetic acid was used as the solvent, the monomer loading had to be decreased to 1 vol% in order to maintain the uniformity of PS-DVB microspheres in a thermal precipitation polymerization, whereas it was elevated up to 4 vol% in a UV-initiated precipitation polymerization. In other similar protocols proceeding at ambient temperature, the photo-initiated precipitation polymerization increased the monomer loading up to 6 vol% without sacrificing any uniformity of PDVB microspheres prepared in acetonitrile,¹⁹ and iniferter-induced precipitation polymerization achieved the monomer loading of 18 vol% in the synthesis of poly(4-vinylpridine-ethlyene glycol dimethacrylate) microspheres in ethanol.⁴³ Together with our observation, a relative low temperature may be a positive effect on increasing monomer loading in precipitation polymerization. This was possibly due to the decrease in particle collision frequency at lower temperature, which hindered particles aggregation. On the other hand, an increase in monomer loading accelerates particle collision and elevates the compatibility between the primary nuclei and the reaction medium.⁴¹ At lower monomer loading range, it would be helpful for the primary nuclei to capture monomers and oligomers, and thus possibly leading to improvement on the microsphere uniformity; at higher monomer loading range, owing to stronger solvency, it may weaken the auto-steric stabilization of the primary nuclei, leading to broader size distribution (Fig. 1G).

A series of experiments were further designed to investigate the effect of the monomer composition on the polymerization with different molar ratios of LMA to DVB (Table S1† and Fig. 2). Polymerization of DVB alone gave an aggregation composed of amorphous submicron-sized particles (Fig. 2A), while polymerization of LMA alone obtained a viscous transparent liquid. All copolymers varying the ratio of DVB to LMA from 20 mol% to 80 mol% were beaded particles, and both the yield of particles and the diameter of microspheres present a slight increase from 31.45% to 35.21%, and 3.11 μm to 3.77 μm, respectively. Noticeably, when the LMA amount or the DVB amount was lower than 20 mol%, a quantity of fragments appeared in the final polymers (Fig. 2B and E). By contrast, smooth monodisperse microspheres were obtained in the range of 40/60 to 60/40 molar ratio of LMA to DVB. Based on the above observations, LMA and DVB seemed to have different impact on determining the morphologies of microspheres. LMA possibly gave an additional stabilization for the forming particles, while DVB was a dominant factor governing whether the stable nuclei can precipitate out from the solvent. Cooperation of LMA and DVB played the important role in facilitating the formation of monodisperse microspheres.

Fig. 2 SEM images of poly(LMA-DVB) particles with various ratios of LMA to DVB, (A) 0/100, (B) 20/80, (C) 40/60, (D) 60/40, (E) 80/20 at 20 vol% monomer concentration. (F) Effect of DVB concentration on particle yield and diameter of poly(LMA-DVB) particles, not considering irregular fragments when calculating U and particle diameter for (B) and (E).

The copolymerization of LMA and DVB was verified by FT-IR spectra as shown in Fig. S2.† The characteristic peaks at 1730 cm⁻¹ and 1190 cm⁻¹ should be attributed to stretching vibration of C=O and stretching vibration of C–O in the structure of LMA, respectively. The characteristic peaks at 1600 cm⁻¹ and 1450 cm⁻¹ should be attributed to stretching vibration of C=C in benzene ring of DVB. As the amount of LMA was increased from null to 80 mol% in the feed, the signals corresponding to LMA (i.e. 1730 cm⁻¹ and 1190 cm⁻¹) distinctively increased. By contrast, those related to DVB (i.e. 1600 cm⁻¹ and 1450 cm⁻¹) decreased in accordance. The copolymerization of LMA and DVB was further confirmed by elemental analysis (Table S2†). The measured content of LMA in copolymer increased from 15.9 mol% to 64.1 mol% with LMA loading content in feed increased from 20 mol% to 80 mol%, but was distinctively lower than the corresponding one in feed. These results indicated that DVB monomer possibly possesses higher polymerization reactivity than LMA monomer.

3.3. Initiator concentration

Fig. S3† depicted the effect of initiator concentration on particle size and its distribution. Different from that observed in thermal precipitation polymerization, the initiator concentration in redox-initiated precipitation polymerization displayed a great influence on the morphologies of the microspheres. The microspheres uniformity apparently decreased with the redox initiator concentration increased from 1 wt% to 10 wt%. Only below 2 wt% of initiator, the monodisperse microspheres with a PDI value of below 1.02 were obtained, whereas a large quantity of floatation even an aggregation of smaller particles was observed as the initiator concentration was higher than 3 wt%. Another distinctive variant lay the microsphere size significantly increasing from 3.11 μm to 6.84 μm with initiator concentration increased from 1 wt% to 5 wt% if not taking the irregular particles at higher initiator concentrations into consideration. Noticeably, with the initiator concentration increased from 1 wt% to 10 wt%, the particle yield initially increased from 29.6% to 36.6%, then rapidly decreased to 12.3 wt%. Higher initiator concentration would lead to higher radical concentration, thus resulting in higher amount of oligomers. On one hand, such short oligomers are capable of greater mobility and easily aggregate to form bigger particles. On the other hand, more oligomers increase the solubility of the total medium for monomers and their polymers, thus prolonging the formation of the stable nuclei. This solubility increase possibly gave the decrease in the particle yield, and the formation of irregular particles as a consequence of aggregation of smaller particles.

3.4. Microsphere formation

Based on the above observations, a possible mechanism of microsphere formation is illustrated in Scheme 1. The monomer (LMA) and the crosslinker (DVB) are initially dissolved in acetic acid to form a homogenous solution based on their solubility parameters (Table S3†). Following introduction of the pair of redox initiator, the polymerization is initiated immediately at room temperature without any stirring and agitation. The oligomers that are characteristic of short polymeric chains precipitate out from the solvent due to crosslinking in the presence of DVB, then aggregate and form a primary nuclei. The primary nuclei are stabilized by self-steric protection that is possibly attributed to both pendent double bonds from DVB and lauryl groups from LMA. The primary nuclei further adsorb monomers, crosslinkers, and oligomers onto their surfaces and grow to bigger particles until the adsorption equilibrium is achieved. Owing to polymerization initiated at ambient temperature, such adsorption equilibrium means a relative higher amount of unreactive monomers and oligomers existing in the solvent, which accounts for the lower particle yields by this method. Additionally, as the density of acetic acid, compared to that of acetonitrile, is closer to those of monomers, the forming polymer particles possibly suspend in the reaction medium, partially avoiding coagulation of smaller particles.

Scheme 1 A possible mechanism of poly(LMA-DVB) microsphere formation in quiescent redox-initiated precipitation polymerization at ambient temperature.

3.5. Superhydrophobic behavior of microspheres

The superhydrophobic surface with micrometer-scale roughness was produced by casting the suspensions of microspheres onto the surface of glass, followed by evaporation of the solvent. The water contact angles distinctively increased from 145.2° ± 0.6° to 159.2° ± 1.9° with LMA content in the polymerization feed increased from null to 60 mol%. When the LMA content further increased to 80 mol% in the feed, the as-prepared surface displayed a lower hydrophobicity with a water contact angle of 149.6° ± 4.7°. As well-known, superhydrophobic surfaces are generally fabricated through controlling the surface chemistry and surface roughness. Fig. 3(B and C) depicted rough surfaces produced by the obtained microsphere arrays, where the monodisperse microspheres formed close colloidsomes. Such dip-coating and deposition of microspheres onto substrate demonstrates to be an efficient method for making superhydrophobic surfaces with simple and rapid process.^37–39

Fig. 3 Dependent of water contact angles on different poly(LMA-DVB) microspheres obtained with different molar ratios of LMA to DVB (A). SEM images of poly(LMA-DVB) microspheres obtained with 60/40, (B) and 80/20 (C) molar ratio of LMA to DVB casting on a clean glass.

4. Conclusions

An effective and green route to synthesis of superhydrophobic and monodisperse polymeric microspheres was proposed. Monodisperse poly(LMA-DVB) microspheres with the monomer loading as high as 20 vol% were obtained in acetic acid by redox initiated precipitation polymerization at ambient temperature without any shaking or stirring. Compared to acetonitrile that was usually used as the solvent in traditional precipitation polymerization, acetic acid demonstrated to be a more appropriate alternative for fabrication of poly(LMA-DVB) microspheres from a point view of particle polydispersity. With the molar ratio of LMA to DVB in the range of 40/60 to 60/40, the exceptionally monodisperse poly(LMA-DVB) microspheres were obtained with PDI below 1.02, along with the particle yields reaching 29–37%. The initiator concentration in redox-initiated precipitation polymerization largely influenced the morphologies of the microspheres. Superhydrophobic surfaces with water contact angles from 150.1° to 159° were obtained by casting the highly monodisperse microspheres onto glass slides.

Acknowledgements

The authors gratefully thank startup funds from University of Jinan, and Natural Science Foundation of Shandong Province (No. ZR2013BM011) for financial support.

Notes and references

1. G. L. Li, H. Möhwald and D. G. Shchukin, Chem. Soc. Rev., 2013, 42, 3628–3646.
2. M. Lee, E. Y. Lee, D. Lee and B. J. Park, Soft Matter, 2015, 11, 2067–2079.
3. J. Liu, N. P. Wickramaratne, S. Z. Qiao and M. Jaroniec, Nat. Mater., 2015, 14, 763–774.
4. S. L. Wang, W. Zhao, J. Song, S. Cheng and L. J. Fan, Macromol. Rapid Commun., 2013, 34, 102–108.
5. K. Li, H. D. H. Stöver and J. Polym, J. Polym. Sci., Part A: Polym. Chem., 1993, 31, 3257–3263.
6. H. Y. Cao, Q. Wang, M. Li and Z. Y. Chen, Colloid Polym. Sci., 2015, 293, 441–451.
7. W. H. Li and H. D. H. Stöver, J. Polym. Sci., Part A: Polym. Chem., 1999, 37, 2899–2907.
8. P. P. Lv, W. Wei, F. L. Gong, Y. L. Zhang, H. Y. Zhao, J. D. Lei, L. Y. Wang and G. H. Ma, Ind. Eng. Chem. Res., 2009, 48, 8819–8828.
9. M. S. Manga, O. J. Cayre, R. A. Williams, S. Biggs and D. W. York, Soft Matter, 2012, 8, 1532–1538.
10. M. Adamczak, A. Kupiec, E. Jarek, K. Szczepanowicz and P. Warszyński, Colloids Surf., A, 2014, 462, 147–152.
11. D. Dendukuri, T. A. Hatton and P. S. Doyle, Langmuir, 2007, 23, 4669–4674.
12. D. Spencer and H. Morgan, Lab Chip, 2011, 11, 1234–1239.
13. V. Skowronek, R. W. Rambach, L. Schmid, K. Haase and T. Franke, Anal. Chem., 2013, 85, 9955–9959.
14. R. Shabnam, K. Tauer, H. Minami and H. Ahmad, J. Appl. Polym. Sci., 2015, 132, 41881–41888.
15. M. Yoshida, M. Asano, I. Kaetsu and Y. Morita, Radiat. Phys. Chem., 1987, 30, 39–45.
16. Y. Naka, I. Kaetsu, Y. Yamamoto and K. Hayashi, J. Polym. Sci., Part A: Polym. Chem., 1991, 29, 1197–1202.
17. M. Kumakura, Eur. Polym. J., 1995, 31, 1095–1098.
18. R. Joso, E. H. Pan, M. H. Stenzel, T. P. Davis, C. Barner-Kowollik and L. Barner, J. Polym. Sci., Part A: Polym. Chem., 2007, 45, 3482–3487.
19. F. Limé and K. Irgum, Macromolecules, 2007, 40, 1962–1968.
20. T. J. Romack, E. E. Maury and J. M. DeSimone, Macromolecules, 1995, 28, 912–915.
21. A. I. Cooper, W. P. Hems and A. B. Holmes, Macromol. Rapid Commun., 1998, 19, 353–357.
22. H. M. Ni and H. Kawaguchi, J. Polym. Sci., Part A: Polym. Chem., 2004, 42, 2823–2832.
23. H. M. Ni, M. Wu, M. Li, H. L. Wang and Y. M. Sun, Polym. Chem., 2010, 1, 899–907.
24. H. M. Ni, H. Zhang, G. X. Chen, J. X. Liu, Y. D. Yang, L. J. Zhang, M. Wu, K. Zhan and Y. L. Chen, Chin. J. Polym. Sci., 2014, 32, 1400–1412.
25. X. Z. Kong, X. L. Gu, X. L. Zhu and L. N. Zhang, Macromol. Rapid Commun., 2009, 30, 909–914.
26. X. L. Gu, H. W. Sun, X. Z. Kong, C. H. Fu, H. Yu, J. Li and J. H. Wang, Colloid Polym. Sci., 2013, 291, 1771–1779.
27. J. S. Jiang, Y. Zhang, X. Z. Guo and H. Q. Zhang, Macromolecules, 2011, 44, 5893–5904.
28. J. S. Jiang, Y. Zhang, X. Z. Guo and H. Q. Zhang, RSC Adv., 2012, 2, 5651–5662.
29. Q. Yan, Y. W. Bai, Z. Meng and W. T. Yang, J. Phys. Chem. B, 2008, 112, 6914–6922.
30. Q. Yan, T. Y. Zhao, Y. W. Bai, F. Zhang and W. T. Yang, J. Phys. Chem. B, 2009, 113, 3008–3014.
31. K. Kempe, P. A. de Jongh, A. Anastasaki, P. Wilson and D. M. Haddleton, Chem. Commun., 2015, 51, 16213–16216.
32. B. Strachota, L. Matějka, A. Zhigunov, R. Konefal, J. Spěváček, J. Dybal and R. Puffr, Soft Matter, 2015, 11, 9291–9306.
33. Z. Y. Chen, L. Xu, Y. Liang, J. B. Wang, M. P. Zhao and Y. Z. Li, J. Chromatogr. A, 2008, 1182, 128–131.
34. X. F. Gao and L. Jiang, Nature, 2004, 432, 36.
35. M. L. Ma, Y. Mao, M. Gupta, K. K. Gleason and G. C. Rutledge, Macromolecules, 2005, 38, 9742–9748.
36. M. F. Wang, N. Raghunathan and B. Ziaie, Langmuir, 2007, 23, 2300–2303.
37. W. Wei, X. B. Huang, X. L. Zhao, P. Zhang and X. Z. Tang, Chem. Commun., 2010, 46, 487–489.
38. A. Muñoz-Bonilla, A. Bousquet, E. Ibarboure, E. Papon, C. Labrugère and J. Rodriguez-Hernández, Langmuir, 2010, 26, 16775–16781.
39. D. W. Zhang, J. W. Liu, T. W. Liu and X. L. Yang, Colloid Polym. Sci., 2015, 293, 1799–1807.
40. X. B. Jiang, X. L. Zhu and X. Z. Kong, Chem. Eng. J., 2012, 213, 212–217.
41. H. Q. Huang, Y. Y. Ding, X. L. Chen, Z. Y. Chen and X. Z. Kong, Chem. Eng. J., 2016, 289, 135–141.
42. Q. Yan, PhD thesis, Beijing University of Chemical Technology, 2009.
43. M. Zhao, H. T. Zhang, F. N. Ma, Y. Zhang, X. Z. Guo and H. Q. Zhang, J. Polym. Sci., Part A: Polym. Chem., 2013, 51, 1983–1998.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra28153a

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