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Preparation of slightly crosslinked monodisperse poly(maleic anhydride-cyclohexyl vinyl ether-divinylbenzene) functional microspheres with anhydride groups via precipitation polymerization
Particuology 19 (2015) 99–106
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Preparation of slightly crosslinked monodisperse poly(maleic anhydride-cyclohexyl vinyl ether-divinylbenzene) functional microspheres with anhydride groups via precipitation polymerization Cen Yin a,b,c , Anhou Xu a,b,c , Li Gong a,b,c , Luqing Zhang a,b,c , Bing Geng a,b,c , Shuxiang Zhang a,b,c,∗ a
Shandong Key Laboratory of Fluorine Chemistry and Chemical Materials, Jinan 250022, China School of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022, China c Shandong Engineering Research Center for Fluorinated Material, Jinan 250022, China b
a r t i c l e
i n f o
Article history: Received 17 June 2013 Received in revised form 24 March 2014 Accepted 3 April 2014 Keywords: Precipitation polymerization Crosslinked functional microspheres Cyclohexyl vinyl ether Maleic anhydride Divinylbenzene
a b s t r a c t Slightly crosslinked monodisperse poly(maleic anhydride-cyclohexyl vinyl ether-divinylbenzene) (MA-CHVE-DVB) microspheres were prepared via precipitation polymerization while using 2,2azobisisobutyronitrile as an initiator in a mixture of methyl ethyl ketone and n-heptane without any stabilizer. The number-average diameter of the resultant poly(MA-CHVE-DVB) microspheres ranged from 0.478 to 1.386 m with a polydispersity index of 1.00 to 1.02 that depended on the feed ratios of the MA/CHVE/DVB monomers. The introduction of one electron donor monomer cyclohexyl vinyl ether strongly affected the yield, size, and morphology of these slightly crosslinked microspheres. Quinolinetype chelating resins were obtained after combining the poly(MA-CHVE-DVB) with 8-hydroxyquinoline; the adsorption properties of these materials were measured through their ability to remove Cu2+ ions from water. The poly(MA-CHVE-DVB) microspheres with low degrees of crosslinking provided more effective functional groups and therefore better ion removal capabilities. These slightly crosslinked microspheres may have applications in water treatment as well as in sensing and drug delivery. © 2014 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
Introduction Crosslinked monodisperse polymer microspheres have attracted much attention across a wide range of applications, including information technology, electric and electronic sciences, and biochemical and biomedical technologies (Covolan, Mei, & Rossi, 1997; Fudouzi & Xia, 2003; McDonald, Bouck, Chaput, & Stevens, 2000; Serpengüzel, Kurt, & Ayaz, 2008; Solorio, Fu, Hernández-Irizarry, & Alsberg, 2010; Ugelstad et al., 1992; Zhao et al., 2012) due to their superior strength, thermal and solvent resistance, and anti-slip properties. Exerting precise control over the properties of these polymer particles has become increasingly important as their applications have expanded (Dai et al., 2014;
∗ Corresponding author at: School of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022, China. Tel.: +86 531 89736365; fax: +86 531 89736365. E-mail address: fhx [email protected] (S. Zhang).
Horák & Shapoval, 2000; Sanders et al., 1984). The crosslinking degree, size, size distribution, functionality of the base polymer, morphology of the polymer beads, among others are the most important controlling properties (Hattori, Sudol, & El-Aasser, 1993; Horák & Shapoval, 2000; Tunc & Ulubayram, 2009; Sanders et al., 1984). Interestingly, a low degree of crosslinking density generates polymer microspheres with good swelling performances, facilitating modification (Ding, Aklonis, & Salovey, 1991; Gupta & Jabrail, 2006). Emulsion, suspension, and dispersion polymerization are wellknown methods used to prepare crosslinked polymer microspheres (Gibanel, Heroguez, & Forcad, 2001; Kawaguchi, Winnik, & Ito, 1996). However, these synthetic processes require an emulsifier or particle stabilizer, such as poly(N-vinyl pyrrolidone) or poly(vinyl alcohol) (Marumoto, Suzuta, Noguchi, & Uchida, 1978; Paine, Luymes, & McNulty, 1990; Uchida, Hosaka, & Murao, 1982). These stabilizers cannot be easily removed from the polymer, often generating adverse consequences in many applications, particularly biological applications. Furthermore, the three synthetic
http://dx.doi.org/10.1016/j.partic.2014.04.018 1674-2001/© 2014 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
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methods described above are usually performed in water, making them inappropriate for monomers containing functional groups that are either reactive or labile in water, such as maleic anhydride (MA). Precipitation polymerization is an alternative approach that produces crosslinked polymer particles with uniform sizes and shapes in the absence of any additional stabilizer or water (Downey, Frank, Li, & Stöver, 1999; Li & Stöver, 1993b). During precipitation polymerization, a homogeneous mixture of a monomer, initiator, and solvent becomes heterogeneous during the reaction as insoluble polymer chains aggregate, forming a separate polymer phase (Bai, Yang, Zhao, & Huang, 2005; Li & Stöver, 1993a; Sosnowski, Gadzinowski, & Slomkowski, 1996). MA, which contains both double bond and anhydride groups, offers dual functionality while providing maximum freedom and flexibility during polymer design (Dispenza, Tripodo, LoPresti, Spadaro, & Giammona, 2009). Polymers microspheres derived from MA are attractive as a reactive starting material when designing compounds with various functional groups (Croll, Stöver, & Hitchcock, 2005; Jeong, Byoun, & Lee, 2002; Qiu, Zhu, & Xu, 2005; Trivedi, Shah, & Indusekhar, 1996; Wehrens & Tomaschewski, 1996). Naka and Yamamoto (1992) reported the preparation of copolymer microspheres of MA with diethylene glycol dimethacrylate (2G) by precipitation polymerization; the yield, size, and the crosslinking degree of microspheres remained unaffected by the concentration of MA. Therefore, the crosslinking degree of the copolymer MA microspheres cannot be decreased by adjusting the amount of MA monomer. While keeping the same number of double bonds in MA and DVB, Croll and Stöver (2003a) obtained copolymer microspheres containing MA and divinylbenzene-55 (DVB-55) via precipitation copolymerization; they proposed that MA copolymerizes with DVB in an alternating pattern. The poly(DVB-MA) microspheres formed spherical tectocapsules that could be used for the controlled release of pharmaceutical and agricultural agents. However due to the high cross-linking density of the microspheres, the links between the hard microspheres were relatively fragile and unstable, restricting the applications of crosslinked microspheres that contain anhydride groups (Croll and Stöver, 2003b). To broaden the applications of these functional microspheres, an electron donor monomer cyclohexyl vinyl ether (CHVE) (Dodgson & Ebdon, 1977; Kharas & Ajbani, 1993; Kokubo, Iwatsuki, & Yamashita, 1968; Kokubo, Iwatsuki, & Yamashita, 1970) was introduced into a MA and DVB copolymer. The purpose of this introduction is to obtain monodisperse microspheres with less crosslinking, more effective anhydride groups and improved swelling properties compared to poly(MA-DVB) microspheres. In this work, slightly crosslinked poly(MA-CHVE-DVB) microspheres were successfully prepared via precipitation polymerization at various monomer concentrations. The effects of the CHVE concentration on the morphology, particle size, size distribution and MA content of the final microspheres were presented and investigated.
China). 2,2-Azobisisobutyronitrile (AIBN, analytical grade) was supplied by Shanghai Chemical Agent Fourth Factory (Shanghai, China) and purified via recrystallization from ethanol. Preparation of P(MA-CHVE-DVB) and P(MA-DVB) microspheres During a typical polymerization, MA (1.568 g, 16 mmol) was first dissolved in 16 mL of MEK in a 100 mL three necked flask equipped with a mechanical stirrer, a thermometer, and a reflux condenser. Afterwards, 24 mL of Hp was introduced into the glass reactor (i.e. the three necked flask) before 2.016 g of CHVE (16 mmol), 1.043 g of DVB (8 mmol, 20 mol% relative to the total monomer), and 0.0463 g of AIBN (1 wt% relative to the total monomer) were loaded to the glass reactor. After purging with N2 for 30 min, the reactor was placed into a 70 ◦ C water bath for 6 h. At the end of the reaction, the particles were separated from the reaction medium by filtration, before being washed in a Soxhlet extractor with acetone for 48 h. Afterwards, the material was dried at room temperature under vacuum until a constant weight was achieved. The polymerization yield was determined gravimetrically as the mass ratio of the resulting polymer to the feed monomers. Characterization The Fourier transform infrared (FT-IR) spectra of poly(MACHVE-DVB) and poly(MA-DVB) samples were taken in KBr pellets with a Perkin-Elmer Spectrum One Fourier transform infrared spectrometer (Perkin-Elmer Co., USA). The particle morphologies were observed on a Hitachi S-2500 scanning electron microscope (SEM) under a low vacuum. The particle size and size distributions were measured using the Scion Image Analyzer software. One hundred individual particles would be measured from the SEM microphotographs to calculate the SEM size data by using the following equation:
n
Dn = Dw =
i=1
Di
n
(2)
Di
Dw , Dn
n ε=
(1)
n Di 4 , i=1 n 3 i=1
PDI =
,
i=1
(3)
1/2
(Di − Dn )2 /(n − 1) Dn
,
(4)
where Dn is the number-average diameter, Dw is the weightaverage diameter, Di is the particle diameter of the determined microspheres, PDI is the polydispersity index, n is the total number of the measured particles, and ε is the deviation coefficient. Metal ion adsorption kinetics experiments
Experimental Materials Cyclohexyl vinyl ether (CHVE), divinylbenzene (DVB, 80% divinylbenzene isomers), maleic anhydride (MA), 8hydroxyquinoline (HQ), sodium hydroxide (NaOH), hydrochloric acid (HCl), and phenolphthalein indicator were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). The solvents, i.e., methyl ethyl ketone (MEK), n-heptane (Hp), and tetrahydrofuran (THF), were all analytical grade and used as received from Shanghai Chemical Reagents Co. Ltd. (Shanghai,
Quinoline-type chelating resins were obtained when using the poly(MA-CHVE-DVB) and poly(MA-DVB) microspheres as a reaction substrate. During a typical experiment, HQ (1.773 g) was dissolved in 20 mL of tetrahydrofuran (THF) before the HQ solution was dropped through a burette into a gently shaken poly(MACHVE-DVB) solution; this solution was prepared by soaking 3 g of the poly(MA-CHVE-DVB) microspheres in 60 mL of THF at 30 ◦ C for 12 h with stirring. Next, a mixture of HQ and a poly(MA-CHVEDVB) solution was heated to reflux for 10 h with stirring, forming a quinoline-type chelating resin. After the reaction, the quinolinetype chelating resin was separated by filtration and washed with
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Table 1 The yield, size, and size distribution of the poly(MA-CHVE-DVB) microspheres and poly(MA-DVB) with different degrees of crosslinking.a Entry
MA/CHVE/DVB composition (mol% in feed)
Yield (%)
Dn (m)
Dw (m)
PDI
ε
A1 B1 C1 D1 A2 B2 C2 D2
40/40/20 30/30/40 20/20/60 10/10/80 80/0/20 60/0/40 40/0/60 20/0/80
97.4 91.5 93.9 94.2 41.2 83.6 97.9 39.5
0.692 0.478 0.713 1.386 0.596 0.836 0.965 0.988
0.698 0.486 0.719 1.390 0.608 0.847 0.974 0.997
1.01 1.02 1.01 1.01 1.02 1.01 1.01 1.01
0.0527 0.0735 0.0542 0.0316 0.0514 0.0497 0.0538 0.0349
a
Dn is the number-average diameter; Dw is the weight-average diameter; PDI is the polydispersity index; ε is the coefficient of variation.
acetone in a Soxhlet extractor for 48 h. All of the resins were dried in air and were stored in a dry cabinet. Batch adsorption experiments of Cu2+ onto quinoline-type chelating resins based on poly(MA-CHVE-DVB) and poly(MA-DVB) microspheres were conducted. The samples were equilibrated in 50 mL of an aqueous 900 mg/L copper(II) solution with magnetic stirring at 400 rpm for 2 h. The pH was adjusted to 5 with HNO3 or NaOH at the beginning of each experiment. At defined intervals, aliquots (0.2 mL) were removed from the remaining solution, and the concentrations of the Cu2+ ions were determined at 560 nm through the Bicinchoninate method while using a HACH, model DR2000 spectrophotometer. The amount of copper(II) ion adsorbed per unit mass of quinoline-type chelating resin q was calculated:
n
q=
i=1
(Ci−1 − Ci )(V0 − (i − 1)Vs ) 1000m
,
(5)
where q is the amount of adsorption (mg/g). C0 and Ci are the copper(II) ion concentrations of the initial and ith sample, respectively (mg/L). V0 and Vs are the initial and sampled volumes of the solution (mL), respectively, and m is the mass of dry adsorbent (g). Results and discussion Preparation of slightly crosslinked poly(MA-CHVE-DVB) microspheres To clarify the typical behavior of the terpolymerization system containing MA, CHVE, and DVB, a series of experiments using different monomer feed ratio was conducted via precipitation polymerization in a solvent mixture containing MEK and HP in the absence of a stabilizer. Table 1 shows that the yields of the poly(MACHVE-DVB) microspheres range from 97.4 to 93.9%, exceeding those of the poly(MA-DVB) microspheres (Table 1A2–D2, except for C2, the yields are all below 90%). This difference should be attributed to the reactivity of the MA, CHVE, and DVB. The reactivity ratios for MA (rMA ) and DVB (rDVB ) are 0.09 and 0.01, respectively, indicating that a MA and DVB copolymerization system tends to form alternating copolymers due to the low rMA , rDVB , and rMA rDVB (rMA rDVB 1) values. After one monomer was mostly consumed, the residual monomers resisted homopolymerization, leading to low conversion. Specifically, lowering the crosslinking density is difficult to achieve by decreasing the amount of DVB used to produce the poly (MA-DVB) microspheres. CHVE is a donor monomer that can be copolymerized with MA, generating an alternating copolymer. Therefore the yields of A1, B1, C1, and D1 are higher than those of A2, B2, and D2 without CHVE. Table 1 SEM images of the poly(MA-CHVE-DVB) and poly(MADVB) microspheres prepared with different crosslinking degrees are shown in Fig. 1. In the present work, the crosslinking degree is the mole fraction of DVB crosslinker in the entire monomer
feed. Poly(MA-CHVE-DVB) microspheres prepared by the precipitation polymerization of MA and CHVE at a 1:1 mole ratio with concentrations of DVB ranging from 20 to 80 mol% relative to the monomer are shown (Fig. 1A1–D1). Individually stable particles are successfully generated without coagulation at all concentrations of DVB, and spherical shapes with clean surfaces are exhibited, even at the lowest degree of crosslinking (A1 with 20 mol% DVB in the monomer feed). However, uniform individual microspheres could not be obtained when the DVB fraction was lower than 60 mol% (Fig. 1A2–C2) in the absence of a CHVE monomer. Slightly crosslinked microspheres with 20% crosslinking were prepared through the introduction of a third monomer CHVE during the copolymerization of MA and DVB. Therefore, the CHVE monomer plays a key role during the production of slightly crosslinked microspheres from the terpolymerization system. The size and size distribution of the resultant poly(MACHVE-DVB) and poly(MA-DVB) microspheres with different DVB crosslinking degrees are summarized in Table 1. As shown in Table 1, all of the size distributions were narrowed from 1.02 to 1.01 for poly(MA-CHVE-DVB) and poly(MA-DVB) microspheres with different feed compositions of DVB, while the sizes changed significantly as the proportion of DVB was varied. The numberaverage diameter (Dn ) of the poly(MA-DVB) particle increased from 0.596 to 0.988 m when the feed composition of DVB ranged from 20 to 80 mol%. Meanwhile, the Dn of the poly(MA-CHVE-DVB) particles decreased from 0.692 to 0.478 m when the feed composition of DVB ranged from 20 to 40 mol%, and the largest diameter (1.386 m) was obtained at 80 mol%. The effect of DVB on Dn was consistent with DVB polymerizations performed in previous systems; CHVE markedly affects the morphology of the resultant polymer network by comparing poly(MA-CHVE-DVB) to poly(MADVB) microspheres with the same crosslinking degree. Spectroscopic techniques were employed to characterize the structure of the obtained polymers. All of the products generated satisfactory analysis data that corresponded to their expected structures. The FT-IR spectra of the polymeric P(MA-DVB) (trace a), monomeric CHVE (trace b), and polymeric P(MA-CHVE-DVB) (trace c) are shown in Fig. 2. Here, we emphasized the analysis of the terpolymer P(MA-CHVE-DVB). The characteristic absorption peaks for the C O group were obvious at 1859 and 1785 cm−1 , respectively, because the P(MA-CHVE-DVB) contains succinic anhydride groups. The peaks at 1450 and 1385 cm−1 were the two characteristic bands for C–H in the–CH3 groups on P(MA-CHVE-DVB). In addition, the bands at 828 cm−1 were attributed to the bending vibration of two adjacent hydrogens in a disubstituted benzene ring. Upon comparison to the FT-IR spectrum of the CHVE (trace b), the unambiguous disappearance of the characteristic peak for CH2 at 3080 cm−1 was observed, indicating the completion of the terpolymerization. Upon comparison to the FT-IR spectrum of P(MA-DVB) (trace a), the FT-IR absorption peak at 1090 cm−1 in the P(MA-CHVE-DVB) (trace c) spectrum is larger due to the stretching vibration for O C O in
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Fig. 1. SEM micrographs of the poly(MA-CHVE-DVB) and poly(MA-DVB) copolymer microspheres prepared via precipitation polymerization in MEK/Hp with the following DVB fractions: (A) 20, (B) 40, (C) 60, and (D) 80. Sample molar ratios: (A1) MA/CHVE/DVB = 4/4/2, (B1) MA/CHVE/DVB = 3/3/4, (C1) MA/CHVE/DVB = 2/2/6, and (D1) MA/CHVE/DVB = 1/1/8; (A2) MA/DVB = 8/2, (B2) MA/DVB = 6/4, (C2) MA/DVB = 4/6, and (D2) MA/DVB =2/8.
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Fig. 2. FT-IR spectra of (a) P(MA-DVB), (b) CHVE, and (c) P(MA-CHVE-DVB).
the ether bond of CHVE. This FT-IR result confirmed that the slightly crosslinked microsphere P(MA-CHVE-DVB) has been prepared successfully. Effect of the MA/CHVE/DVB feed ratio on the poly(MA-CHVE-DVB) microspheres MA is an acceptor monomer (e = 1.5), while CHVE is donor monomer (e = −1.55). Therefore their copolymerization generated an alternating copolymer. Moreover, neither monomer should homopolymerize because they copolymerize very rapidly. In addition, DVB is an electron-donating monomer that can copolymerize with MA in an alternating mode. Therefore, the monomer feed ratios will greatly affect the terpolymerization as well as the yield, size, and morphologies of the resulting poly(MA-CHVE-DVB) microspheres. To evaluate the effect of the CHVE on the MA/CHVE/DVB terpolymerization, a series of experiments with varied feed ratios of CHVE and DVB were first conducted at a fixed MA concentration (20 mol% for Table 2 A1–A3, 40 mol% for Table 2 B1–B3, 60 mol% for Table 2C1–C3). As shown in Table 2, increasing the feed ratio of CHVE and DVB generated yields of poly(MA-CHVEDVB) microspheres, ranging from 93.88 to 55.51%, 97.4 to 68.55%, and 77.06 to 86.97% when the MA concentration was fixed at 20, 40, and 60 mol%, respectively. Evidently, when the amount of CHVE equaled that of the MA, the yield could exceed 90% (Table 2A1 and B2). This phenomenon occurs because neither the MA nor the CHVE monomers can homopolymerize easily, but their copolymerization rate is rapid. When either MA or CHVE was added in excess, the residual monomers could not homopolymerize easily
103
after one monomer was consumed, leading to low conversions. CHVE and MA copolymerized well, achieving a high conversion only when their feed ratios were equal. In addition, the diameter of the poly(MA-CHVE-DVB) microspheres was related to the feed ratio of CHVE and DVB; a smaller diameter was observed when the feed ratio of CHVE to MA equaled 1 and the MA feed concentration was 20 or 40 mol%. (Table 2A1 Dn = 0.713 and Table 2B2 Dn = 0.692). The morphology of the poly(MA-CHVE-DVB) varied when changing the feed ratio of CHVE to DVB at different proportions of MA. As shown in Fig. 3A1–A3, the SEM micrographs revealed that these particles are spherical and narrowly dispersed while exhibiting a smooth surface morphology. When the MA feed composition increased to 40 and 60 mol%, the size distribution of the poly(MA-CHVE-DVB) microspheres broadened, except for the case shown in Fig. 3B2 and C3. Spherical particles (Fig. 3A3) with cupped and deformable surfaces were observed when the CHVE fractions were higher due to soft surface of the polymer network and the lower crosslinking degree (20 mol%). Table 2 shows that at the same crosslinking degree (feed composition of DVB is 20 mol%, Table 2A3, B2, and C2), the yield increased from 62 to 97% when the feed ratio of MA/CHVE ranged from 2/6 to 4/4. Afterwards, the yield fell to 87% when the feed ratio of MA/CHVE was 6/2. This change occurs because the copolymerization of MA and CHVE only proceeds in an alternating fashion, indicating that adding a DVB monomer may exert little effect on the alternating copolymerization of MA and CHVE. Specifically, the ratio of CHVE and MA strongly affected the morphology, size, and size distribution of the resultant polymer microspheres. The coagulation of the growing poly(MACHVE-DVB) microspheres can be prevented when the ratio of CHVE and MA exceeds 1. In addition, microspheres with a smooth surface and narrowed size distribution are observed with a conversion above 90% when the ratio of CHVE and MA is close to 1. Interestingly, uniform polymer microspheres can be obtained over a wide feed range, while the size of the microspheres increases with the CHVE content over a certain range. This result provides a new method for controlling the diameter of the polymer microspheres. The CHVE as a typical monovinyl electron donor monomer can easily copolymerize with MA obtaining an alternative copolymer (Kokubo et al., 1968, 1970), as a result, monodiperse microspheres would stilled be prepared even when the fraction of the crosslinker decreased. Effect of monomer feed ratio on MA content in poly(MA-CHVE-DVB) The chemical composition of poly(MA-CHVE-DVB), particularly the MA content, is critical for applications in the electric and electronic sciences, as well as in biochemical and biomedical technologies. Concretely, the existence of reactive succinic anhydride groups may favor the functionalization of the particle surface, revealing numerous potential applications.
Table 2 The yield, size, and size distribution of poly(MA-CHVE-DVB) microspheres with different MA/CHVE/DVB monomer feed compositions.a Entry
MA/CHVE/DVB composition (mol% in feed)
Yield (%)
Dn (m)
Dw (m)
PDI
ε
A1 A2 A3 B1 B2 B3 C1 C2 C3
2/2/6 2/4/4 2/6/2 4/2/4 4/4/2 4/5/1 6/1/3 6/2/2 6/3/1
93.9 55.5 61.9 68.6 97.4 78.1 79.8 86.9 77.1
0.713 0.807 0.882 0.774 0.692 0.799 0.789 0.692 0.607
0.719 0.810 0.888 0.780 0.698 0.808 0.793 0.696 0.612
1.01 1.00 1.01 1.01 1.01 1.01 1.01 1.01 1.01
0.0542 0.0327 0.0478 0.0517 0.0527 0.0584 0.0416 0.0445 0.0515
a
Dn is the number-average diameter; Dw is the weight-average diameter; PDI is the polydispersity index; ε is the coefficient of variation.
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Fig. 3. SEM micrographs of the poly(MA-CHVE-DVB) copolymer microspheres prepared via precipitation polymerization in MEK/Hp with the following MA fractions: (A) 20, (B) 40, and (C) 60. Sample molar ratios: (A1) MA/CHVE/DVB = 2/2/6, (A2) MA/CHVE/DVB = 2/4/4, and (A3) MA/CHVE/DVB = 2/6/2; (B1) MA/CHVE/DVB = 4/2/4, (B2) MA/CHVE/DVB = 4/4/2, and (B3) MA/CHVE/DVB = 4/5/1; (C1) MA/CHVE/DVB = 6/1/3, (C2) MA/CHVE/DVB = 6/2/2, and (C3) MA/CHVE/DVB = 6/3/1.
As shown in Table 3, the MA content in poly(MA-CHVE-DVB) ranged from 20.38 to 39.12% when the MA feed ratio increased from 20 to 80 mol%. However, when the proportion of MA was fixed, the MA content increased disproportionally when the CHVE feed ratio increased. Therefore, CHVE performed better during
Table 3 Effect of the monomer feed composition on the MA content in poly(MA-CHVE-DVB). MA/CHVE/DVB composition (mol% in feed)
MA content in polymer (wt%)
Yield (wt%)
Conversion of MA monomer (wt%)
2/2/6 2/4/4 2/6/2 4/2/4 4/4/2 4/5/1 6/1/3 6/2/2 6/3/1 8/1/1
20.38 25.56 28.23 25.54 33.52 36.33 30.72 33.25 38.91 39.12
93.88 55.51 61.99 68.55 97.4 78.02 79.79 86.97 77.06 60.40
99.87 88.43 95.28 52.06 96.34 83.30 46.07 54.13 55.89 31.34
copolymerization when more MA was used. In addition, the conversion and content of MA in the poly(MA-DVB) (shown in Table 4) are generally lower than those of the corresponding terpolymer (poly(MA-CHVE-DVB)). These results agree with previous reports, indicating that MA (an electron acceptor) and CHVE (an electron donor) tend to form an alternating polymer through a charge-transfer-complex (CTC) mechanism (Croll & Stöver, 2003a). In addition, the MA content further increased slowly when much more MA was introduced. The MA content of the poly(MA-CHVE-DVB) only reached
Table 4 Effect of the monomer feed composition on the MA content of the poly(MA-DVB). MA/CHVE/DVB composition (mol% in feed)
MA content in polymer (wt%)
Yield (wt%)
Conversion of MA monomer (wt%)
MA/DVB = 2/8 MA/DVB = 4/6 MA/DVB = 6/4 MA/DVB = 8/2
20.26 22.94 29.24 32.98
39.46 97.90 83.60 41.20
50.52 67.29 46.18 18.09
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Conclusions
Fig. 4. Capacity for Cu2+ adsorption by the quinoline-type chelating resin beads with different crosslinking densities. The reaction substrates are resins from MA/CHVE/DVB = 4/4/2 and from MA/DVB = 4/6.
39.12 wt% when the MA concentration was 80 mol% and the conversion fell to 31.34 wt%. After CHVE and MA were mostly consumed, the residual MA was difficult to homopolymerize, lowering its conversion.
Metal ion adsorption of quinoline-type chelating resins 8-Hydroxy quinoline and its derivatives are frequently used as complexing agents during various applications due to their good complex forming ability (Faltynski & Jezorek, 1986; Janák & Janák, 1986). Quinoline-type chelating resins are highly selective toward the exchangeable adsorption of heavy metal ions (Abd El-Rehim, Hegazy, & El-Hag Ali, 2000). Specifically, the chelating ion-exchange resin containing 8-HQ could be used to enrich heavy metals in water and facilitate their analysis. Cu2+ ions are present in industrial waste effluent; excess Cu2+ in the human body can cause stomach and intestinal distress as well as serious cellular or organ damage. In this study, we utilized Cu2+ ions to evaluate the ability of the quinoline-type chelating resins from P(MA-CHVE-DVB) and P(MA-DVB) microspheres to adsorb metal ions from water. The results are shown in Fig. 4. For quinoline-type chelating resins from P(MA-CHVE-DVB), the amount of adsorbed ions increased sharply during the initial 90 min before increasing slowly to 69.3 mg/g. The quinoline-type chelating resins from poly(MA-DVB) adsorbed 8.7 mg/g of Cu2+ after 60 min; this amount increased slowly to 9.7 mg/g by the end of the experiment. The quinoline-type chelating resin from poly(MA-CHVE-DVB) exhibits a much higher adsorption capacity for Cu2+ than from P(MA-DVB) due to the lower degree of crosslinking exhibited by the poly(MA-CHVE-DVB) resin. Kaliyappan, Raman, and Kannan (1999) and Zhu, Liu, and Cheng (2000) studied the interaction of heavy metal ions and chelating ion exchange resins containing 8-HQ; they suggest that metals are coordinated through the oxygen atom of the ester carbonyl and the nitrogen atom. Increasing the crosslinking density may consume the nitrogen binding site on the chelating resins, decreasing the free volume of the microsphere matrix during the diffusion of metal ions. Therefore, the adsorption capacity for metal ions may depend on the presence of a pendant quinolinyl group and the microstructure of the material, including the crosslinking degree, the free volume and the spatial location of quinolinyl group.
Monodisperse and slightly crosslinked microspheres of poly(MA-CHVE-DVB) were successfully synthesized via precipitation polymerization while utilizing AIBN as an initiator and DVB as a crosslinking agent in MEK/Hp. These microspheres have clean surfaces due to the omission of stabilizer and can be formed over a wide monomer feed range. Introducing CHVE, which is an electron donating monomer in poly(MA-CHVE-DVB), promoted the formation of monodisperse microspheres and reduced the concentration of crosslinker to 10 mol% during the precipitation terpolymerization. The effects of CHVE on the yield, size, and morphology of these slightly crosslinked microspheres were clarified. The adsorption capacity of quinoline-type chelating resins from poly(MA-CHVE-DVB) toward copper(II) ions was much higher than that from poly(MA-DVB), broadening the range of applications for functional microspheres with anhydride groups. In addition to the removal of heavy metal ions from water, these slightly crosslinked microspheres with a pendant quinolinyl group may be used in other applications, such as water treatment, sensing and drug delivery. The present study provides a facile approach toward the synthesis of slightly crosslinked functional microspheres with an anhydride group, facilitating their application in water treatment, sensing and drug delivery. Acknowledgments The financial supports from the National Science Foundation of China (Grant No. 20774037 and 21304037) and Shandong Excellent Young Scientist Research Award Fund (No. BS2013CL039) are gratefully acknowledged. References Abd El-Rehim, H. A., Hegazy, E. A., & El-Hag Ali, A. (2000). Selective removal of some heavy metal ions from aqueous solution using treated polyethyleneg-styrene/maleic anhydride membranes. Reactive & Functional Polymers, 43, 105–116. Bai, F., Yang, X., Zhao, Y., & Huang, W. (2005). Synthesis of core-shell microspheres with active hydroxyl groups by two-stage precipitation polymerization. Polymer International, 54, 168–174. Covolan, V. L., Mei, L. H. I., & Rossi, C. L. (1997). Chemical modifications on polystyrene latex: Preparation and characterization for use in immunological applications. Polymers for Advanced Technologies, 8, 44–50. Croll, L. M., & Stöver, H. D. H. (2003a). Formation of tectocapsules by assembly and cross-linking of poly(divinylbenzene-alt-maleic anhydride) spheres at the oil–water interface. Langmuir, 19, 5918–5922. Croll, L. M., & Stöver, H. D. H. (2003b). Mechanism of self-assembly and rupture of cross-linked microspheres and microgels at the oil–water interface. Langmuir, 19, 10077–10080. Croll, L. M., Stöver, H. D. H., & Hitchcock, A. P. (2005). Composite tectocapsules containing porous polymer microspheres as release gates. Micromolecules, 38, 2903–2910. Dai, J. D., Zou, Y. L., Zhou, Z. P., Dai, X. H., Pan, J. M., Yu, P., et al. (2014). Narrowly dispersed imprinted microspheres with hydrophilic polymer brushes for the selective removal of sulfamethazine. RSC Advances, 4, 1965–1973. Ding, Z. Y., Aklonis, J. J., & Salovey, R. (1991). Model filled polymers. VI. Determination of the crosslink density of polymeric beads by swelling. Journal of Polymer Science Part B: Polymer Physics, 29, 1035–1038. Dispenza, C., Tripodo, G., LoPresti, C., Spadaro, G., & Giammona, G. (2009). Synthesis, characterisation and properties of ␣,-poly(N-2-hydroxyethyl)-dlaspartamide-graft-maleic anhydride precursors and their stimuli-responsive hydrogels. Reactive & Functional Polymers, 69, 565–575. Dodgson, K., & Ebdon, J. R. (1977). On the role of monomer–monomer donor–acceptor complexes in the free-radical copolymerisation of styrene and maleic anhydride. European Polymer Journal, 13, 791–797. Downey, J. S., Frank, R. S., Li, W. H., & Stöver, H. D. H. (1999). Growth mechanism of poly(divinylbenzene) microspheres in precipitation polymerization. Macromolecules, 32, 2838–2844. Faltynski, K. H., & Jezorek, J. R. (1986). Liquid chromatographic separation of metal ions on several silica-bound chelating-agent stationary phases. Chromatographia, 22, 5–12. Fudouzi, H., & Xia, Y. (2003). Photonic papers and inks: Color writing with colorless materials. Advanced Materials, 15, 892–896.
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Preparation of slightly crosslinked monodisperse poly(maleic anhydride-cyclohexyl vinyl ether-divinylbenzene) functional microspheres with anhydride groups via precipitation polymerization Cen Yin a,b,c , Anhou Xu a,b,c , Li Gong a,b,c , Luqing Zhang a,b,c , Bing Geng a,b,c , Shuxiang Zhang a,b,c,∗ a
Shandong Key Laboratory of Fluorine Chemistry and Chemical Materials, Jinan 250022, China School of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022, China c Shandong Engineering Research Center for Fluorinated Material, Jinan 250022, China b
a r t i c l e
i n f o
Article history: Received 17 June 2013 Received in revised form 24 March 2014 Accepted 3 April 2014 Keywords: Precipitation polymerization Crosslinked functional microspheres Cyclohexyl vinyl ether Maleic anhydride Divinylbenzene
a b s t r a c t Slightly crosslinked monodisperse poly(maleic anhydride-cyclohexyl vinyl ether-divinylbenzene) (MA-CHVE-DVB) microspheres were prepared via precipitation polymerization while using 2,2azobisisobutyronitrile as an initiator in a mixture of methyl ethyl ketone and n-heptane without any stabilizer. The number-average diameter of the resultant poly(MA-CHVE-DVB) microspheres ranged from 0.478 to 1.386 m with a polydispersity index of 1.00 to 1.02 that depended on the feed ratios of the MA/CHVE/DVB monomers. The introduction of one electron donor monomer cyclohexyl vinyl ether strongly affected the yield, size, and morphology of these slightly crosslinked microspheres. Quinolinetype chelating resins were obtained after combining the poly(MA-CHVE-DVB) with 8-hydroxyquinoline; the adsorption properties of these materials were measured through their ability to remove Cu2+ ions from water. The poly(MA-CHVE-DVB) microspheres with low degrees of crosslinking provided more effective functional groups and therefore better ion removal capabilities. These slightly crosslinked microspheres may have applications in water treatment as well as in sensing and drug delivery. © 2014 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
Introduction Crosslinked monodisperse polymer microspheres have attracted much attention across a wide range of applications, including information technology, electric and electronic sciences, and biochemical and biomedical technologies (Covolan, Mei, & Rossi, 1997; Fudouzi & Xia, 2003; McDonald, Bouck, Chaput, & Stevens, 2000; Serpengüzel, Kurt, & Ayaz, 2008; Solorio, Fu, Hernández-Irizarry, & Alsberg, 2010; Ugelstad et al., 1992; Zhao et al., 2012) due to their superior strength, thermal and solvent resistance, and anti-slip properties. Exerting precise control over the properties of these polymer particles has become increasingly important as their applications have expanded (Dai et al., 2014;
∗ Corresponding author at: School of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022, China. Tel.: +86 531 89736365; fax: +86 531 89736365. E-mail address: fhx [email protected] (S. Zhang).
Horák & Shapoval, 2000; Sanders et al., 1984). The crosslinking degree, size, size distribution, functionality of the base polymer, morphology of the polymer beads, among others are the most important controlling properties (Hattori, Sudol, & El-Aasser, 1993; Horák & Shapoval, 2000; Tunc & Ulubayram, 2009; Sanders et al., 1984). Interestingly, a low degree of crosslinking density generates polymer microspheres with good swelling performances, facilitating modification (Ding, Aklonis, & Salovey, 1991; Gupta & Jabrail, 2006). Emulsion, suspension, and dispersion polymerization are wellknown methods used to prepare crosslinked polymer microspheres (Gibanel, Heroguez, & Forcad, 2001; Kawaguchi, Winnik, & Ito, 1996). However, these synthetic processes require an emulsifier or particle stabilizer, such as poly(N-vinyl pyrrolidone) or poly(vinyl alcohol) (Marumoto, Suzuta, Noguchi, & Uchida, 1978; Paine, Luymes, & McNulty, 1990; Uchida, Hosaka, & Murao, 1982). These stabilizers cannot be easily removed from the polymer, often generating adverse consequences in many applications, particularly biological applications. Furthermore, the three synthetic
http://dx.doi.org/10.1016/j.partic.2014.04.018 1674-2001/© 2014 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
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methods described above are usually performed in water, making them inappropriate for monomers containing functional groups that are either reactive or labile in water, such as maleic anhydride (MA). Precipitation polymerization is an alternative approach that produces crosslinked polymer particles with uniform sizes and shapes in the absence of any additional stabilizer or water (Downey, Frank, Li, & Stöver, 1999; Li & Stöver, 1993b). During precipitation polymerization, a homogeneous mixture of a monomer, initiator, and solvent becomes heterogeneous during the reaction as insoluble polymer chains aggregate, forming a separate polymer phase (Bai, Yang, Zhao, & Huang, 2005; Li & Stöver, 1993a; Sosnowski, Gadzinowski, & Slomkowski, 1996). MA, which contains both double bond and anhydride groups, offers dual functionality while providing maximum freedom and flexibility during polymer design (Dispenza, Tripodo, LoPresti, Spadaro, & Giammona, 2009). Polymers microspheres derived from MA are attractive as a reactive starting material when designing compounds with various functional groups (Croll, Stöver, & Hitchcock, 2005; Jeong, Byoun, & Lee, 2002; Qiu, Zhu, & Xu, 2005; Trivedi, Shah, & Indusekhar, 1996; Wehrens & Tomaschewski, 1996). Naka and Yamamoto (1992) reported the preparation of copolymer microspheres of MA with diethylene glycol dimethacrylate (2G) by precipitation polymerization; the yield, size, and the crosslinking degree of microspheres remained unaffected by the concentration of MA. Therefore, the crosslinking degree of the copolymer MA microspheres cannot be decreased by adjusting the amount of MA monomer. While keeping the same number of double bonds in MA and DVB, Croll and Stöver (2003a) obtained copolymer microspheres containing MA and divinylbenzene-55 (DVB-55) via precipitation copolymerization; they proposed that MA copolymerizes with DVB in an alternating pattern. The poly(DVB-MA) microspheres formed spherical tectocapsules that could be used for the controlled release of pharmaceutical and agricultural agents. However due to the high cross-linking density of the microspheres, the links between the hard microspheres were relatively fragile and unstable, restricting the applications of crosslinked microspheres that contain anhydride groups (Croll and Stöver, 2003b). To broaden the applications of these functional microspheres, an electron donor monomer cyclohexyl vinyl ether (CHVE) (Dodgson & Ebdon, 1977; Kharas & Ajbani, 1993; Kokubo, Iwatsuki, & Yamashita, 1968; Kokubo, Iwatsuki, & Yamashita, 1970) was introduced into a MA and DVB copolymer. The purpose of this introduction is to obtain monodisperse microspheres with less crosslinking, more effective anhydride groups and improved swelling properties compared to poly(MA-DVB) microspheres. In this work, slightly crosslinked poly(MA-CHVE-DVB) microspheres were successfully prepared via precipitation polymerization at various monomer concentrations. The effects of the CHVE concentration on the morphology, particle size, size distribution and MA content of the final microspheres were presented and investigated.
China). 2,2-Azobisisobutyronitrile (AIBN, analytical grade) was supplied by Shanghai Chemical Agent Fourth Factory (Shanghai, China) and purified via recrystallization from ethanol. Preparation of P(MA-CHVE-DVB) and P(MA-DVB) microspheres During a typical polymerization, MA (1.568 g, 16 mmol) was first dissolved in 16 mL of MEK in a 100 mL three necked flask equipped with a mechanical stirrer, a thermometer, and a reflux condenser. Afterwards, 24 mL of Hp was introduced into the glass reactor (i.e. the three necked flask) before 2.016 g of CHVE (16 mmol), 1.043 g of DVB (8 mmol, 20 mol% relative to the total monomer), and 0.0463 g of AIBN (1 wt% relative to the total monomer) were loaded to the glass reactor. After purging with N2 for 30 min, the reactor was placed into a 70 ◦ C water bath for 6 h. At the end of the reaction, the particles were separated from the reaction medium by filtration, before being washed in a Soxhlet extractor with acetone for 48 h. Afterwards, the material was dried at room temperature under vacuum until a constant weight was achieved. The polymerization yield was determined gravimetrically as the mass ratio of the resulting polymer to the feed monomers. Characterization The Fourier transform infrared (FT-IR) spectra of poly(MACHVE-DVB) and poly(MA-DVB) samples were taken in KBr pellets with a Perkin-Elmer Spectrum One Fourier transform infrared spectrometer (Perkin-Elmer Co., USA). The particle morphologies were observed on a Hitachi S-2500 scanning electron microscope (SEM) under a low vacuum. The particle size and size distributions were measured using the Scion Image Analyzer software. One hundred individual particles would be measured from the SEM microphotographs to calculate the SEM size data by using the following equation:
n
Dn = Dw =
i=1
Di
n
(2)
Di
Dw , Dn
n ε=
(1)
n Di 4 , i=1 n 3 i=1
PDI =
,
i=1
(3)
1/2
(Di − Dn )2 /(n − 1) Dn
,
(4)
where Dn is the number-average diameter, Dw is the weightaverage diameter, Di is the particle diameter of the determined microspheres, PDI is the polydispersity index, n is the total number of the measured particles, and ε is the deviation coefficient. Metal ion adsorption kinetics experiments
Experimental Materials Cyclohexyl vinyl ether (CHVE), divinylbenzene (DVB, 80% divinylbenzene isomers), maleic anhydride (MA), 8hydroxyquinoline (HQ), sodium hydroxide (NaOH), hydrochloric acid (HCl), and phenolphthalein indicator were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). The solvents, i.e., methyl ethyl ketone (MEK), n-heptane (Hp), and tetrahydrofuran (THF), were all analytical grade and used as received from Shanghai Chemical Reagents Co. Ltd. (Shanghai,
Quinoline-type chelating resins were obtained when using the poly(MA-CHVE-DVB) and poly(MA-DVB) microspheres as a reaction substrate. During a typical experiment, HQ (1.773 g) was dissolved in 20 mL of tetrahydrofuran (THF) before the HQ solution was dropped through a burette into a gently shaken poly(MACHVE-DVB) solution; this solution was prepared by soaking 3 g of the poly(MA-CHVE-DVB) microspheres in 60 mL of THF at 30 ◦ C for 12 h with stirring. Next, a mixture of HQ and a poly(MA-CHVEDVB) solution was heated to reflux for 10 h with stirring, forming a quinoline-type chelating resin. After the reaction, the quinolinetype chelating resin was separated by filtration and washed with
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Table 1 The yield, size, and size distribution of the poly(MA-CHVE-DVB) microspheres and poly(MA-DVB) with different degrees of crosslinking.a Entry
MA/CHVE/DVB composition (mol% in feed)
Yield (%)
Dn (m)
Dw (m)
PDI
ε
A1 B1 C1 D1 A2 B2 C2 D2
40/40/20 30/30/40 20/20/60 10/10/80 80/0/20 60/0/40 40/0/60 20/0/80
97.4 91.5 93.9 94.2 41.2 83.6 97.9 39.5
0.692 0.478 0.713 1.386 0.596 0.836 0.965 0.988
0.698 0.486 0.719 1.390 0.608 0.847 0.974 0.997
1.01 1.02 1.01 1.01 1.02 1.01 1.01 1.01
0.0527 0.0735 0.0542 0.0316 0.0514 0.0497 0.0538 0.0349
a
Dn is the number-average diameter; Dw is the weight-average diameter; PDI is the polydispersity index; ε is the coefficient of variation.
acetone in a Soxhlet extractor for 48 h. All of the resins were dried in air and were stored in a dry cabinet. Batch adsorption experiments of Cu2+ onto quinoline-type chelating resins based on poly(MA-CHVE-DVB) and poly(MA-DVB) microspheres were conducted. The samples were equilibrated in 50 mL of an aqueous 900 mg/L copper(II) solution with magnetic stirring at 400 rpm for 2 h. The pH was adjusted to 5 with HNO3 or NaOH at the beginning of each experiment. At defined intervals, aliquots (0.2 mL) were removed from the remaining solution, and the concentrations of the Cu2+ ions were determined at 560 nm through the Bicinchoninate method while using a HACH, model DR2000 spectrophotometer. The amount of copper(II) ion adsorbed per unit mass of quinoline-type chelating resin q was calculated:
n
q=
i=1
(Ci−1 − Ci )(V0 − (i − 1)Vs ) 1000m
,
(5)
where q is the amount of adsorption (mg/g). C0 and Ci are the copper(II) ion concentrations of the initial and ith sample, respectively (mg/L). V0 and Vs are the initial and sampled volumes of the solution (mL), respectively, and m is the mass of dry adsorbent (g). Results and discussion Preparation of slightly crosslinked poly(MA-CHVE-DVB) microspheres To clarify the typical behavior of the terpolymerization system containing MA, CHVE, and DVB, a series of experiments using different monomer feed ratio was conducted via precipitation polymerization in a solvent mixture containing MEK and HP in the absence of a stabilizer. Table 1 shows that the yields of the poly(MACHVE-DVB) microspheres range from 97.4 to 93.9%, exceeding those of the poly(MA-DVB) microspheres (Table 1A2–D2, except for C2, the yields are all below 90%). This difference should be attributed to the reactivity of the MA, CHVE, and DVB. The reactivity ratios for MA (rMA ) and DVB (rDVB ) are 0.09 and 0.01, respectively, indicating that a MA and DVB copolymerization system tends to form alternating copolymers due to the low rMA , rDVB , and rMA rDVB (rMA rDVB 1) values. After one monomer was mostly consumed, the residual monomers resisted homopolymerization, leading to low conversion. Specifically, lowering the crosslinking density is difficult to achieve by decreasing the amount of DVB used to produce the poly (MA-DVB) microspheres. CHVE is a donor monomer that can be copolymerized with MA, generating an alternating copolymer. Therefore the yields of A1, B1, C1, and D1 are higher than those of A2, B2, and D2 without CHVE. Table 1 SEM images of the poly(MA-CHVE-DVB) and poly(MADVB) microspheres prepared with different crosslinking degrees are shown in Fig. 1. In the present work, the crosslinking degree is the mole fraction of DVB crosslinker in the entire monomer
feed. Poly(MA-CHVE-DVB) microspheres prepared by the precipitation polymerization of MA and CHVE at a 1:1 mole ratio with concentrations of DVB ranging from 20 to 80 mol% relative to the monomer are shown (Fig. 1A1–D1). Individually stable particles are successfully generated without coagulation at all concentrations of DVB, and spherical shapes with clean surfaces are exhibited, even at the lowest degree of crosslinking (A1 with 20 mol% DVB in the monomer feed). However, uniform individual microspheres could not be obtained when the DVB fraction was lower than 60 mol% (Fig. 1A2–C2) in the absence of a CHVE monomer. Slightly crosslinked microspheres with 20% crosslinking were prepared through the introduction of a third monomer CHVE during the copolymerization of MA and DVB. Therefore, the CHVE monomer plays a key role during the production of slightly crosslinked microspheres from the terpolymerization system. The size and size distribution of the resultant poly(MACHVE-DVB) and poly(MA-DVB) microspheres with different DVB crosslinking degrees are summarized in Table 1. As shown in Table 1, all of the size distributions were narrowed from 1.02 to 1.01 for poly(MA-CHVE-DVB) and poly(MA-DVB) microspheres with different feed compositions of DVB, while the sizes changed significantly as the proportion of DVB was varied. The numberaverage diameter (Dn ) of the poly(MA-DVB) particle increased from 0.596 to 0.988 m when the feed composition of DVB ranged from 20 to 80 mol%. Meanwhile, the Dn of the poly(MA-CHVE-DVB) particles decreased from 0.692 to 0.478 m when the feed composition of DVB ranged from 20 to 40 mol%, and the largest diameter (1.386 m) was obtained at 80 mol%. The effect of DVB on Dn was consistent with DVB polymerizations performed in previous systems; CHVE markedly affects the morphology of the resultant polymer network by comparing poly(MA-CHVE-DVB) to poly(MADVB) microspheres with the same crosslinking degree. Spectroscopic techniques were employed to characterize the structure of the obtained polymers. All of the products generated satisfactory analysis data that corresponded to their expected structures. The FT-IR spectra of the polymeric P(MA-DVB) (trace a), monomeric CHVE (trace b), and polymeric P(MA-CHVE-DVB) (trace c) are shown in Fig. 2. Here, we emphasized the analysis of the terpolymer P(MA-CHVE-DVB). The characteristic absorption peaks for the C O group were obvious at 1859 and 1785 cm−1 , respectively, because the P(MA-CHVE-DVB) contains succinic anhydride groups. The peaks at 1450 and 1385 cm−1 were the two characteristic bands for C–H in the–CH3 groups on P(MA-CHVE-DVB). In addition, the bands at 828 cm−1 were attributed to the bending vibration of two adjacent hydrogens in a disubstituted benzene ring. Upon comparison to the FT-IR spectrum of the CHVE (trace b), the unambiguous disappearance of the characteristic peak for CH2 at 3080 cm−1 was observed, indicating the completion of the terpolymerization. Upon comparison to the FT-IR spectrum of P(MA-DVB) (trace a), the FT-IR absorption peak at 1090 cm−1 in the P(MA-CHVE-DVB) (trace c) spectrum is larger due to the stretching vibration for O C O in
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Fig. 1. SEM micrographs of the poly(MA-CHVE-DVB) and poly(MA-DVB) copolymer microspheres prepared via precipitation polymerization in MEK/Hp with the following DVB fractions: (A) 20, (B) 40, (C) 60, and (D) 80. Sample molar ratios: (A1) MA/CHVE/DVB = 4/4/2, (B1) MA/CHVE/DVB = 3/3/4, (C1) MA/CHVE/DVB = 2/2/6, and (D1) MA/CHVE/DVB = 1/1/8; (A2) MA/DVB = 8/2, (B2) MA/DVB = 6/4, (C2) MA/DVB = 4/6, and (D2) MA/DVB =2/8.
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Fig. 2. FT-IR spectra of (a) P(MA-DVB), (b) CHVE, and (c) P(MA-CHVE-DVB).
the ether bond of CHVE. This FT-IR result confirmed that the slightly crosslinked microsphere P(MA-CHVE-DVB) has been prepared successfully. Effect of the MA/CHVE/DVB feed ratio on the poly(MA-CHVE-DVB) microspheres MA is an acceptor monomer (e = 1.5), while CHVE is donor monomer (e = −1.55). Therefore their copolymerization generated an alternating copolymer. Moreover, neither monomer should homopolymerize because they copolymerize very rapidly. In addition, DVB is an electron-donating monomer that can copolymerize with MA in an alternating mode. Therefore, the monomer feed ratios will greatly affect the terpolymerization as well as the yield, size, and morphologies of the resulting poly(MA-CHVE-DVB) microspheres. To evaluate the effect of the CHVE on the MA/CHVE/DVB terpolymerization, a series of experiments with varied feed ratios of CHVE and DVB were first conducted at a fixed MA concentration (20 mol% for Table 2 A1–A3, 40 mol% for Table 2 B1–B3, 60 mol% for Table 2C1–C3). As shown in Table 2, increasing the feed ratio of CHVE and DVB generated yields of poly(MA-CHVEDVB) microspheres, ranging from 93.88 to 55.51%, 97.4 to 68.55%, and 77.06 to 86.97% when the MA concentration was fixed at 20, 40, and 60 mol%, respectively. Evidently, when the amount of CHVE equaled that of the MA, the yield could exceed 90% (Table 2A1 and B2). This phenomenon occurs because neither the MA nor the CHVE monomers can homopolymerize easily, but their copolymerization rate is rapid. When either MA or CHVE was added in excess, the residual monomers could not homopolymerize easily
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after one monomer was consumed, leading to low conversions. CHVE and MA copolymerized well, achieving a high conversion only when their feed ratios were equal. In addition, the diameter of the poly(MA-CHVE-DVB) microspheres was related to the feed ratio of CHVE and DVB; a smaller diameter was observed when the feed ratio of CHVE to MA equaled 1 and the MA feed concentration was 20 or 40 mol%. (Table 2A1 Dn = 0.713 and Table 2B2 Dn = 0.692). The morphology of the poly(MA-CHVE-DVB) varied when changing the feed ratio of CHVE to DVB at different proportions of MA. As shown in Fig. 3A1–A3, the SEM micrographs revealed that these particles are spherical and narrowly dispersed while exhibiting a smooth surface morphology. When the MA feed composition increased to 40 and 60 mol%, the size distribution of the poly(MA-CHVE-DVB) microspheres broadened, except for the case shown in Fig. 3B2 and C3. Spherical particles (Fig. 3A3) with cupped and deformable surfaces were observed when the CHVE fractions were higher due to soft surface of the polymer network and the lower crosslinking degree (20 mol%). Table 2 shows that at the same crosslinking degree (feed composition of DVB is 20 mol%, Table 2A3, B2, and C2), the yield increased from 62 to 97% when the feed ratio of MA/CHVE ranged from 2/6 to 4/4. Afterwards, the yield fell to 87% when the feed ratio of MA/CHVE was 6/2. This change occurs because the copolymerization of MA and CHVE only proceeds in an alternating fashion, indicating that adding a DVB monomer may exert little effect on the alternating copolymerization of MA and CHVE. Specifically, the ratio of CHVE and MA strongly affected the morphology, size, and size distribution of the resultant polymer microspheres. The coagulation of the growing poly(MACHVE-DVB) microspheres can be prevented when the ratio of CHVE and MA exceeds 1. In addition, microspheres with a smooth surface and narrowed size distribution are observed with a conversion above 90% when the ratio of CHVE and MA is close to 1. Interestingly, uniform polymer microspheres can be obtained over a wide feed range, while the size of the microspheres increases with the CHVE content over a certain range. This result provides a new method for controlling the diameter of the polymer microspheres. The CHVE as a typical monovinyl electron donor monomer can easily copolymerize with MA obtaining an alternative copolymer (Kokubo et al., 1968, 1970), as a result, monodiperse microspheres would stilled be prepared even when the fraction of the crosslinker decreased. Effect of monomer feed ratio on MA content in poly(MA-CHVE-DVB) The chemical composition of poly(MA-CHVE-DVB), particularly the MA content, is critical for applications in the electric and electronic sciences, as well as in biochemical and biomedical technologies. Concretely, the existence of reactive succinic anhydride groups may favor the functionalization of the particle surface, revealing numerous potential applications.
Table 2 The yield, size, and size distribution of poly(MA-CHVE-DVB) microspheres with different MA/CHVE/DVB monomer feed compositions.a Entry
MA/CHVE/DVB composition (mol% in feed)
Yield (%)
Dn (m)
Dw (m)
PDI
ε
A1 A2 A3 B1 B2 B3 C1 C2 C3
2/2/6 2/4/4 2/6/2 4/2/4 4/4/2 4/5/1 6/1/3 6/2/2 6/3/1
93.9 55.5 61.9 68.6 97.4 78.1 79.8 86.9 77.1
0.713 0.807 0.882 0.774 0.692 0.799 0.789 0.692 0.607
0.719 0.810 0.888 0.780 0.698 0.808 0.793 0.696 0.612
1.01 1.00 1.01 1.01 1.01 1.01 1.01 1.01 1.01
0.0542 0.0327 0.0478 0.0517 0.0527 0.0584 0.0416 0.0445 0.0515
a
Dn is the number-average diameter; Dw is the weight-average diameter; PDI is the polydispersity index; ε is the coefficient of variation.
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Fig. 3. SEM micrographs of the poly(MA-CHVE-DVB) copolymer microspheres prepared via precipitation polymerization in MEK/Hp with the following MA fractions: (A) 20, (B) 40, and (C) 60. Sample molar ratios: (A1) MA/CHVE/DVB = 2/2/6, (A2) MA/CHVE/DVB = 2/4/4, and (A3) MA/CHVE/DVB = 2/6/2; (B1) MA/CHVE/DVB = 4/2/4, (B2) MA/CHVE/DVB = 4/4/2, and (B3) MA/CHVE/DVB = 4/5/1; (C1) MA/CHVE/DVB = 6/1/3, (C2) MA/CHVE/DVB = 6/2/2, and (C3) MA/CHVE/DVB = 6/3/1.
As shown in Table 3, the MA content in poly(MA-CHVE-DVB) ranged from 20.38 to 39.12% when the MA feed ratio increased from 20 to 80 mol%. However, when the proportion of MA was fixed, the MA content increased disproportionally when the CHVE feed ratio increased. Therefore, CHVE performed better during
Table 3 Effect of the monomer feed composition on the MA content in poly(MA-CHVE-DVB). MA/CHVE/DVB composition (mol% in feed)
MA content in polymer (wt%)
Yield (wt%)
Conversion of MA monomer (wt%)
2/2/6 2/4/4 2/6/2 4/2/4 4/4/2 4/5/1 6/1/3 6/2/2 6/3/1 8/1/1
20.38 25.56 28.23 25.54 33.52 36.33 30.72 33.25 38.91 39.12
93.88 55.51 61.99 68.55 97.4 78.02 79.79 86.97 77.06 60.40
99.87 88.43 95.28 52.06 96.34 83.30 46.07 54.13 55.89 31.34
copolymerization when more MA was used. In addition, the conversion and content of MA in the poly(MA-DVB) (shown in Table 4) are generally lower than those of the corresponding terpolymer (poly(MA-CHVE-DVB)). These results agree with previous reports, indicating that MA (an electron acceptor) and CHVE (an electron donor) tend to form an alternating polymer through a charge-transfer-complex (CTC) mechanism (Croll & Stöver, 2003a). In addition, the MA content further increased slowly when much more MA was introduced. The MA content of the poly(MA-CHVE-DVB) only reached
Table 4 Effect of the monomer feed composition on the MA content of the poly(MA-DVB). MA/CHVE/DVB composition (mol% in feed)
MA content in polymer (wt%)
Yield (wt%)
Conversion of MA monomer (wt%)
MA/DVB = 2/8 MA/DVB = 4/6 MA/DVB = 6/4 MA/DVB = 8/2
20.26 22.94 29.24 32.98
39.46 97.90 83.60 41.20
50.52 67.29 46.18 18.09
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Conclusions
Fig. 4. Capacity for Cu2+ adsorption by the quinoline-type chelating resin beads with different crosslinking densities. The reaction substrates are resins from MA/CHVE/DVB = 4/4/2 and from MA/DVB = 4/6.
39.12 wt% when the MA concentration was 80 mol% and the conversion fell to 31.34 wt%. After CHVE and MA were mostly consumed, the residual MA was difficult to homopolymerize, lowering its conversion.
Metal ion adsorption of quinoline-type chelating resins 8-Hydroxy quinoline and its derivatives are frequently used as complexing agents during various applications due to their good complex forming ability (Faltynski & Jezorek, 1986; Janák & Janák, 1986). Quinoline-type chelating resins are highly selective toward the exchangeable adsorption of heavy metal ions (Abd El-Rehim, Hegazy, & El-Hag Ali, 2000). Specifically, the chelating ion-exchange resin containing 8-HQ could be used to enrich heavy metals in water and facilitate their analysis. Cu2+ ions are present in industrial waste effluent; excess Cu2+ in the human body can cause stomach and intestinal distress as well as serious cellular or organ damage. In this study, we utilized Cu2+ ions to evaluate the ability of the quinoline-type chelating resins from P(MA-CHVE-DVB) and P(MA-DVB) microspheres to adsorb metal ions from water. The results are shown in Fig. 4. For quinoline-type chelating resins from P(MA-CHVE-DVB), the amount of adsorbed ions increased sharply during the initial 90 min before increasing slowly to 69.3 mg/g. The quinoline-type chelating resins from poly(MA-DVB) adsorbed 8.7 mg/g of Cu2+ after 60 min; this amount increased slowly to 9.7 mg/g by the end of the experiment. The quinoline-type chelating resin from poly(MA-CHVE-DVB) exhibits a much higher adsorption capacity for Cu2+ than from P(MA-DVB) due to the lower degree of crosslinking exhibited by the poly(MA-CHVE-DVB) resin. Kaliyappan, Raman, and Kannan (1999) and Zhu, Liu, and Cheng (2000) studied the interaction of heavy metal ions and chelating ion exchange resins containing 8-HQ; they suggest that metals are coordinated through the oxygen atom of the ester carbonyl and the nitrogen atom. Increasing the crosslinking density may consume the nitrogen binding site on the chelating resins, decreasing the free volume of the microsphere matrix during the diffusion of metal ions. Therefore, the adsorption capacity for metal ions may depend on the presence of a pendant quinolinyl group and the microstructure of the material, including the crosslinking degree, the free volume and the spatial location of quinolinyl group.
Monodisperse and slightly crosslinked microspheres of poly(MA-CHVE-DVB) were successfully synthesized via precipitation polymerization while utilizing AIBN as an initiator and DVB as a crosslinking agent in MEK/Hp. These microspheres have clean surfaces due to the omission of stabilizer and can be formed over a wide monomer feed range. Introducing CHVE, which is an electron donating monomer in poly(MA-CHVE-DVB), promoted the formation of monodisperse microspheres and reduced the concentration of crosslinker to 10 mol% during the precipitation terpolymerization. The effects of CHVE on the yield, size, and morphology of these slightly crosslinked microspheres were clarified. The adsorption capacity of quinoline-type chelating resins from poly(MA-CHVE-DVB) toward copper(II) ions was much higher than that from poly(MA-DVB), broadening the range of applications for functional microspheres with anhydride groups. In addition to the removal of heavy metal ions from water, these slightly crosslinked microspheres with a pendant quinolinyl group may be used in other applications, such as water treatment, sensing and drug delivery. The present study provides a facile approach toward the synthesis of slightly crosslinked functional microspheres with an anhydride group, facilitating their application in water treatment, sensing and drug delivery. Acknowledgments The financial supports from the National Science Foundation of China (Grant No. 20774037 and 21304037) and Shandong Excellent Young Scientist Research Award Fund (No. BS2013CL039) are gratefully acknowledged. References Abd El-Rehim, H. A., Hegazy, E. A., & El-Hag Ali, A. (2000). Selective removal of some heavy metal ions from aqueous solution using treated polyethyleneg-styrene/maleic anhydride membranes. Reactive & Functional Polymers, 43, 105–116. Bai, F., Yang, X., Zhao, Y., & Huang, W. (2005). Synthesis of core-shell microspheres with active hydroxyl groups by two-stage precipitation polymerization. Polymer International, 54, 168–174. Covolan, V. L., Mei, L. H. I., & Rossi, C. L. (1997). Chemical modifications on polystyrene latex: Preparation and characterization for use in immunological applications. Polymers for Advanced Technologies, 8, 44–50. Croll, L. M., & Stöver, H. D. H. (2003a). Formation of tectocapsules by assembly and cross-linking of poly(divinylbenzene-alt-maleic anhydride) spheres at the oil–water interface. Langmuir, 19, 5918–5922. Croll, L. M., & Stöver, H. D. H. (2003b). Mechanism of self-assembly and rupture of cross-linked microspheres and microgels at the oil–water interface. Langmuir, 19, 10077–10080. Croll, L. M., Stöver, H. D. H., & Hitchcock, A. P. (2005). Composite tectocapsules containing porous polymer microspheres as release gates. Micromolecules, 38, 2903–2910. Dai, J. D., Zou, Y. L., Zhou, Z. P., Dai, X. H., Pan, J. M., Yu, P., et al. (2014). Narrowly dispersed imprinted microspheres with hydrophilic polymer brushes for the selective removal of sulfamethazine. RSC Advances, 4, 1965–1973. Ding, Z. Y., Aklonis, J. J., & Salovey, R. (1991). Model filled polymers. VI. Determination of the crosslink density of polymeric beads by swelling. Journal of Polymer Science Part B: Polymer Physics, 29, 1035–1038. Dispenza, C., Tripodo, G., LoPresti, C., Spadaro, G., & Giammona, G. (2009). Synthesis, characterisation and properties of ␣,-poly(N-2-hydroxyethyl)-dlaspartamide-graft-maleic anhydride precursors and their stimuli-responsive hydrogels. Reactive & Functional Polymers, 69, 565–575. Dodgson, K., & Ebdon, J. R. (1977). On the role of monomer–monomer donor–acceptor complexes in the free-radical copolymerisation of styrene and maleic anhydride. European Polymer Journal, 13, 791–797. Downey, J. S., Frank, R. S., Li, W. H., & Stöver, H. D. H. (1999). Growth mechanism of poly(divinylbenzene) microspheres in precipitation polymerization. Macromolecules, 32, 2838–2844. Faltynski, K. H., & Jezorek, J. R. (1986). Liquid chromatographic separation of metal ions on several silica-bound chelating-agent stationary phases. Chromatographia, 22, 5–12. Fudouzi, H., & Xia, Y. (2003). Photonic papers and inks: Color writing with colorless materials. Advanced Materials, 15, 892–896.
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