Preparation of crosslinked porous polyurea microspheres in one-step precipitation polymerization and its application for water treatment

Abstract

Porous polyurea microspheres (PPUMs) were prepared in one-step by the precipitation polymerization of isophorone diisocyanate (IPDI) in a water–acetone solvent. Monodisperse SiO₂ particles were added before polymerization and etched by a crosslinking agent triethylenetetramine (TETA), which led to the arrangement of the pores on the crosslinked polyurea microspheres' surface. Another method was also tested for the synthesis of PPUMs, in which non-crosslinked polyurea/SiO₂ composite microspheres were synthesized first, and the SiO₂ nanoparticles were then etched away. Compared to the second way, the one-step method is simple and efficient. The crosslinked PPUMs showed excellent hydrophilicity. The influence of the amounts of SiO₂ nanoparticles, TETA, and the size of SiO₂ nanoparticles were studied. The adsorption tests for iodine and methyl orange (MO) dyes in water were carried out, which showed that the as-prepared crosslinked PPUMs possessed great adsorption performance.

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1. Introduction

With the advances in the pigment and the correlation industry, a significant quantity of coloured wastewater has become one of the major contamination sources for the environment.^1,2 Dyes are extensively utilized particularly in the colouring of textiles, paper, rubber, plastic, concrete, medicine etc.^3,4 Azo dyes are the major productions due to their better staining power, simple formation and other properties. Colouring agents existing in water that are released from industrial waste cause local contamination issues, which cause serious damage to humankind.⁵ A variety of strategies have been utilized for the removal of colour from wastewater. The adsorption procedure is one of the principle systems for the decolourization of dyes.^6–13 Porous materials have pulled various scientists to consider their potential in the expulsion of colours from wastewater with the advantages of large particular surface area, and higher adsorption capacity.^14,15 Porous polymeric materials, particularly microspheres, have attracted incredible interest in dye adsorption. To date, various reports have been adopted to fabricate microspheres for the adsorption of dye.^16–19 Zhou and colleagues synthesized rattle-type carbon-alumina core shell particles with a high surface area and large holes that show magnificent adsorption for the removal of organic dyes from wastewater.²⁰ The fabrication of permeable attractive microspheres for the expulsion of cationic dyes from wastewater was researched by Liu et al.²¹

The fabrication of these materials frequently needs to undergo polymerization and modification. Traditional polymerization includes scattering, suspension and emulsion techniques.²² Stabilizers or porogens should be included to fabricate the porous materials. However, the remaining stabilizer or organic porogen in the product could influence its performance and the procedure is complex and time consuming, which not only discharges waste gas and water, but also utilizes more energy.^23–26 Nevertheless, functionalized porous materials generally require multi-step complicated chemical reactions, which frequently lead to a loss of surface area.²⁷ Recently, waterborne polyurethane (WPU) has attracted research interest from various academic groups because of their significant advantages, such as brilliant biocompatibility, adaptability, wide application temperature range, qualities of transportation assurance, eco-friendly qualities and low secretions of volatile organic compounds.^28,29 The fabrication of WPUs was studied as a green methodology that is free of any solvents.^30–33 Several researchers have concentrated on WPUs for its potential applications in adhesives, coatings, fabrics, therapeutic devices, covers, sealants, and leather finishing etc.^34–41 Similarly, polyurethane, polyurea (PU) is also a microphase-isolated structure comprised of “soft” and “hard” segments, which are synthesized by reaction of an isocyanate and an amine component.^42–45 It has been applied in a number of fields with its high effect resistance, great adaptability, climate resistance, and thermal shock and abrasion resistance.⁴⁶ A novel PU bioconjugate particle was synthesized by Breucker and co-workers, and was demonstrated to be a type of stimuli-responsive structure.⁴⁷ Kong and co-workers fabricated empty PU microspheres by the addition of TETA and treating PU microspheres with acetic acid–acetone in a three-stage chemical process.⁴⁸

In this study, a novel type of polymer particle, PU microspheres with porous and crosslinked structure, were prepared by adding crosslinking agent TETA and SiO₂ nanoparticles directly when the precipitation polymerization of IPDI was carried out in the mixed solvents of acetone and water. The adsorption properties for dyes of the cross-linked PPUMs were investigated. The results demonstrated that the crosslinked PPUMs showed a high capability of absorbing both iodine and MO dyes in water. The influence of the reaction conditions on the one-step synthesis of PPUMs was studied and discussed preliminarily.

2. Experimental

2.1 Materials

IPDI and TETA were acquired from Aldrich. Acetone was obtained from Tianjin Fuyu Fine Chemicals. Hydrofluoric corrosive (HF) was acquired from Qingdao Huadong Chemicals. Iodine (99.8%) and MO (98%) were acquired from Shanghai Yindian Company. Deionized water was purified by a Millipore framework (Milli-Q, Millipore). All chemicals were used as received unless stated otherwise. Monodisperse SiO₂ particles of different sizes were synthesized by the Stöber method.⁴⁹

2.2 Preparation of non-crosslinked PPUMs

A mixture of 58.92 g of acetone, 39.28 g of H₂O, 1.2 g of SiO₂ of the size of 375 nm was taken in an oven dried 250 mL round bottom flask, and the flask was subjected to ultrasonic treatment to homogenize the mixture. After adding 1.8 g IPDI, the flask was shaken manually for 1 min to make it uniform and placed in a water bath for 2 h at 30 °C without shaking. After completing this polymerization, samples were centrifuged for 5 min at 2500 rpm. The precipitate was filtered, washed with acetone/H₂O (m/m = 7/3) three times and dried under vacuum for 12 h at 80 °C; the composite microspheres were obtained. A specific amount of composite microspheres was immersed in HF for 24 h to etch away the SiO₂ particles and then washed and dried.

2.3 Preparation of crosslinked PPUMs

The fabrication strategy of the crosslinked porous microspheres is similar to that of the non-crosslinked microspheres. To a 250 mL flask was added 66.5 g of acetone and 28.5 g of H₂O and 1.2 g of SiO₂ of the size of 375 nm, and the flask was subjected to ultrasonic treatment to make the mixture uniform; 5 g of IPDI was then added to the mixture. After shaking manually for 1 min, the flask was placed into a water bath at 30 °C without shaking. After 1 h, 1.644 g of TETA was dropped within 1 min into the reaction mixture. The mixture solution was stirred at 30 rpm when dropping the TETA into it. After the complete addition of TETA, the polymerization was continued for 1 h. To isolate the microspheres from the solvent, the reaction mixture was centrifuged for 5 min at 2500 rpm. The polymer spheres were washed three times with acetone/H₂O (m/m = 7/3), and dried under vacuum at 80 °C for 12 h. Keeping in mind the end goal to study the impact of the components on sizes of the microspheres and size distributions, the reaction was revised by transforming one component and holding the others constant.

2.4 Water treatment experiments

Adsorption tests of iodine and MO were carried out at room temperature. Each crosslinked PPUM sample was added to an aqueous solution of iodine or MO with a concentration of 245 mg L⁻¹ or 10 mg L⁻¹, followed by shaking to reach equilibrium. After 30 minutes of inactivity, the microspheres were precipitated in the bottle, the dye concentration in the supernatant was determined by absorption measurements using a UV spectrophotometer. The following equation can be used to calculate the adsorption efficiency (η):

η = (C₀ − C_e) × 100%/C₀

where C₀ is the initial concentration of the dye solution (mg L⁻¹) and C_e is the equilibrium concentration of dye (mg L⁻¹).

Further adsorption tests were conducted, a 15 mL plastic tube was used as the reaction vessel. 10 mL of MO (10 mg L⁻¹) or iodine (245 mg L⁻¹) solution was added to the tube, and a set amount of IPDI particles was added to the dye solution. After adsorption for a certain time, the sample was obtained by centrifugation. The liquid solution was then ready for the UV/Vis measurements.

2.5 Characterization

The surface morphologies of PPUMs were characterized by scanning electron microscopy (SEM, JEOL JSM-6309LV, Japan), at a voltage of 20 kV. The FTIR spectra were measured in Bruker alpha type infrared spectrometer. The contact angles of water on the PPUMs were described with a mechanized contact point goniometer (Dingsheng JY-82). The amount of the absorbed dye was examined using a UV-Vis spectrometer (Puxi TU-1810) at wavelengths of 465 nm and 203 nm, respectively.

3. Results and discussion

The strategy for preparation of PPUM is shown in Scheme 1. The fabrication of PPUM could be processed in two ways. One comprised of two stages: initially, the non-crosslinked PU/SiO₂ composite microspheres were fabricated by adding SiO₂ monodisperse particles in IPDI precipitation polymerization,^50,51 and etching by HF was performed to remove the SiO₂ particles. The other process comprises only one-step. To prepare PPUM in one-step polymerization, TETA is needed. TETA has two capacities. First, it acts as crosslinking agent of PPUM. Second, it acts as a strong basic etchant for SiO₂ particles to ensure that the pores are formed on the surface of the microspheres during the one-step polymerization.

Scheme 1 Preparation process of PPUM.

Crosslinked and non-crosslinked PPUMs were both introduced to FTIR (Fig. 1). The characteristic absorption peaks of polyurethane can be seen from the infrared spectra. Two absorption peaks at 3368 cm⁻¹ and 1545 cm⁻¹ confirmed the N–H stretching and N–H plane-banding vibration. The presence of two absorption bands at 2855 cm⁻¹ and 2955 cm⁻¹ was attributed to stretching vibration of methyl and methylene group. The small peak at 2264 cm⁻¹ was attributed to the stretching vibration of residual NCO from IPDI. Compared to non-crosslinked PPUM, the Si–O–Si absorption peaks at 1104 and 475 cm⁻¹ were reduced in crosslinked PPUM, indicating that the silica component was reduced significantly after one-step synthesis.

Fig. 1 FTIR spectra of PPUM materials.

The SEM photos of the crosslinked and non-crosslinked PPUMs are given in Fig. 2. Fig. 2a shows an image of the crosslinked PPUM. Fig. 2b and c present the images of the non-crosslinked polyurea/SiO₂ composite microspheres before and after HF etching, respectively. As shown from the pictures, SiO₂ particles play a vital role in the formation of pores on the PU particles, and using TETA is the key to obtain high porosity PU microspheres facilely. From the SEM images, it is clear that in crosslinked PPUM, the residual silica is less than that in non-crosslinked PPUM, and the arrangement of pores in the former surface is more uniform.

Fig. 2 SEM images of the microspheres prepared by two different methods: (a) crosslinked PPUM; (b) and (c) non-crosslinked PPUM before and after HF etching, respectively.

The distribution and size of the pores in the surface of PPUM are tuned by controlling the amount and size of SiO₂ particles. The two factors had a significant influence on the structure of as-synthesized PPUM. Fig. 3 shows SEM images of crosslinked PPUM synthesized with different amounts of SiO₂ particles. According to the measurements, the mean sizes of these particles were 3.14 μm, 4.51 μm, 4.74 μm, and 4.07 μm, with PDI of 10.16%, 8.42%, 6.34% and 6.22% respectively. When the amount of SiO₂ particles increased, the pores density increased (Fig. 3a–c). However, excess SiO₂ particles cause less porous structures because they cannot be removed completely by a certain amount of TETA in the polymerization (Fig. 3d).

Fig. 3 SEM images of the microspheres prepared with different amount of SiO₂ particles: (a) 0.5 g; (b) 1.0 g; (c) 1.2 g; (d) 1.5 g.

Fig. 4 shows SEM images of crosslinked PPUM with varying pore size synthesized by changing the size of the SiO₂ particles. According to measurements, the mean sizes of these particles were 5.48 μm, 4.74 μm, 5.40 μm, and 5.58 μm, with PDI of 4.31%, 6.34%, 14.32% and 14.18% respectively. The larger the size of the SiO₂ particles, the harder it was to remove them with a certain amount of TETA in the one-step polymerization. Therefore, the SiO₂ particle size suitable for crosslinked PPUM is less than 400 nm.

Fig. 4 SEM images of the microspheres prepared with different sizes of SiO₂ particles: (a) 200 nm; (b) 375 nm; (c) 450 nm; (d) 1200 nm.

In crosslinked PPUMs, TETA acts as a crosslinking agent for polyurea and strong base for SiO₂ etching. TETA is added to the reaction mixture following one hour of precipitation polymerization, as has been mentioned in the experimental procedure. The addition of TETA in the reaction solution is very important; it should be neither too early nor too late because early addition may etch away SiO₂ particles before their adhesion to the surface of microspheres and the pores will not be formed well, whereas too late addition will lead to poor etching and residual silica on the surface of the PPUMs. The amount of TETA plays a major role in effectively etching SiO₂ particles and improving the uniform distribution of the pores. As shown in Fig. 5a, a small amount of TETA could not remove SiO₂ particles fully. When 1.644 g TETA was added to the solution, almost all of the SiO₂ particles were etched away, and we obtained PPUM with a uniform porous surface (Fig. 5b). However, the surface became rough when the amount of TETA increased (Fig. 5c and d). This is possibly due to the excessive crosslinking on the PPUM surfaces. According to the measurements, the mean sizes of these particles were 3.93 μm, 4.74 μm, 4.17 μm, and 4.71 μm, with PDI of 8.73%, 6.34%, 7.04% and 10.42% respectively.

Fig. 5 SEM images of the microspheres prepared with different amounts of TETA: (a) 0.822 g; (b) 1.644 g; (c) 3.288 g; (d) 6.576 g.

To determine if PPUM will be dispersed in water, the water contact angles (CA) of the crosslinked and non-crosslinked PPUM are measured. The CA photographs of the particles are shown in Fig. 6. From the photographs, the CA value of the crosslinked particles was significantly lower than that of the non-crosslinked, which indicates strong hydrophilicity of the crosslinked PPUMs. This difference could be due to a large number of hydrophilic amino groups on the surface of the crosslinked PPUMs, which are generated by the reaction of IPDI with TETA, as confirmed by FTIR analysis.

Fig. 6 CA values of PPUMs: (a) non-crosslinked PPUM; (b) crosslinked PPUM.

The dye adsorption performance of the crosslinked PPUM was confirmed, as shown in Fig. 7 and Table 1. Crosslinked PPUMs prepared with an IPDI/TETA/SiO₂ mass ratio of 5.0/1.644/1.2 were used in the experiments. For iodine and MO dyes, PPUMs showed excellent adsorption properties, particularly for iodine adsorption. From Fig. 7a, we can see that iodine was adsorbed and the PPUMs precipitated naturally in the bottom of the bottle. This is primarily due to the chemical adsorption of iodine at the amino groups of PPUMs.⁵² In contrast, the MO adsorbed in the PPUMs is mainly due to the physical adsorption in the porous structure (Fig. 7b).

Fig. 7 Photographs of the dye adsorption experiments in water using crosslinked PPUMs: (a) iodine solution before (left) and after (right) adsorption; (b) MO solution before (left) and after (right) adsorption.

Table 1 Adsorption capacity parameters of crosslinked PPUMs

Dye	C0 (mg L^−1)	Ce (mg L^−1)	η (%)
Iodine	245	0	100
MO	10	0.47	95.3

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Further adsorption tests were conducted by changing the contact time and adsorbent dosage. The time-dependent adsorption efficiency is shown in Fig. 8. The adsorption efficiency for MO increased gradually with contact time and ultimately reached 95% after 120 min; however for iodine solution, the adsorption efficiency reached 100% within 5 min. The effect of the IPDI dosage on the adsorption efficiency is shown in Fig. 9. The adsorption efficiency for MO increased from 24.7% to 78.8% as the amount of IPDI particles increased from 1 g L⁻¹ to 6 g L⁻¹; however for iodine, the adsorption efficiency increased from 90.7% to 100% as the amount of IPDI particles increased from 0.3 g L⁻¹ to 2 g L⁻¹.

Fig. 8 Effect of contact time on the adsorption of MO and iodine by 30 mg of IPDI particles with an initial MO concentration of 10 mg L⁻¹ and iodine concentration of 245 mg L⁻¹.

Fig. 9 Effect of the IPDI adsorbent dosage on the adsorption of MO and iodine with contact time of 30 min, initial MO concentration of 10 mg L⁻¹ and iodine concentration of 245 mg L⁻¹.

4. Conclusion

A new crosslinked PPUM was synthesized in one-step by the precipitation polymerization of IPDI in water and acetone solvents with the addition of a crosslinking agent TETA and monodisperse SiO₂ particles. TETA, as a type of strong base, could remove SiO₂ particles in the synthesis of PPUMs. Highly uniform pores were formed in the PPUMs with the addition of monodisperse SiO₂ particles, which greatly increased the surface areas. The distribution and size of the pores in the surface of PPUM were tuned by controlling the amount and size of SiO₂ particles. The amount of TETA played a major role in effectively etching SiO₂ particles and improving the uniform distribution of the pores. The most uniform pores circulation with most elevated level of silica etching was observed for PPUM prepared with IPDI/TETA/SiO₂ at a mass ratio of 5.0/1.644/1.2. The crosslinked PPUM exhibited excellent hydrophilic performance. The adsorption of dye had been tested and the PPUM exhibited excellent adsorption properties, which demonstrated that crosslinked PPUM could remove the dyes effectively from the wastewater and showed great application prospects.

Acknowledgements

This study is financially supported by the Natural Science Foundation of China (21375069, 21404065, 21574072, 21675091), the Natural Science Foundation for Distinguished Young Scientists of Shandong Province (JQ201403), the Project of Shandong Province Higher Educational Science and Technology Program (J15LC20), the Graduate Education Innovation Project of Shandong Province (SDYY14028), the Scientific Research Foundation for the Returned Overseas Chinese Scholars of State Education Ministry (20111568), and the Postdoctoral Scientific Research Foundation of Qingdao.

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