Organic solvent absorbents based on linear diol.
Environmental pollution caused by petroleum and petroleum-derived organic solvents, which can be spilled on surface, underground or marine water, damage both human health and ecosystems. These organic-based pollutants must be effectively cleaned-up from water surfaces (1-4). An effective way of removing these contaminants from water surfaces is cleanup with absorbent materials (5). An appropriate absorbent material must have properties such as hydrophobicity or oleophilicity, high absorption capacity, high rate of uptake, good absorption selectivity over water, reusability, and biodegradability of the sorbent (6-9). One way to obtain good petroleum and petroleum-derived organic solvent absorbents is to synthesize water-insoluble cross-linked polymer gels (6), (10). Polymer gels are important materials; they are different from solids and consist of cross-linked polymer networks, which can absorb various solvents. Polymer gels can absorb solvents up to several times their original dry weight. Because of this property, they have found both analytical and industrial applications as absorbent materials and separation agents (11-20).
The swelling behavior of a polymer gel is generally affected by three factors: rubber elasticity, affinity with the solution, and cross-linking density (21). Polymers that have hydrophobic networks, such as aromatic polymers and alkyl acrylate polymers, are used as absorbent materials for removing petroleum and petroleum-derived organic solvents from water surfaces (22-28). Zhang and coworkers prepared poly(1,4-butadiene) gels by the polymerization of 4-tert-butylstyrene and divinylbenzene into unvulcanized butadiene rubber. The effects of the reaction conditions (such as the amount of solvent, the amount of monomer, and the amount of initiator) on the equilibrium swelling ratio were investigated. In this work, the highest oil absorbencies of cross-linked gels were found in xylene (51.35 g/g) and cyclohexane (32.98 g/g) (29). In another study, cross-linked styrene--acrylate copolymers were synthesized and their swelling properties were investigated. It was found that the oil absorbency is mainly influenced by the degree of cross-linking and the hydrophobicities of the copolymer units; the copolymer with a longer alkyl acrylate had higher oil absorbency in the order of stearyl acrylate, lauryl acrylate, and 2-ethylhexyl acrylate (27), (28). Atta and coworkers prepared 7-olefin/maleic anhydride cross-linked copolymers from their linear copolymers, using different glycols as cross-linkers, by condensation reactions. It was reported that the swelling capacity of the cross-linked polymers changed as the type of cc-olefin changed (6).
In our previous studies, we synthesized cross-linked poly(orthocarbonate)s using tetraethyl orthocarbonate (TEOC) and different multihydroxyl monomers (aromatic, aliphatic, cyclic, and linear) by condensation reactions at relatively high temperatures. All the synthesized polymers were insoluble in organic solvents such as tetrahydrofuran (THF), dichloromethane (DCM), dichloroethane, acetone, benzene and other common solvents, but they swelled in these solvents. The synthesized cross-linked poly(orthocarbonate)s were thermally stable, and had rapid and good solvent uptake abilities (30), (31).
In the present study, we used the same strategy to synthesize new types of cross-linked poly(orthocarbonate)s, based on linear aliphatic diols of different lengths, by condensation reactions and investigated the organic solvent absorption abilities of the polymers. Additionally, the effects of increasing the lengths of the linear chains of the aliphatic diols on the swelling behaviors of the cross-linked poly(orthocarbonate)s in various solvents were examined systematically. The cross-linked poly(orthocarbonate)s were characterized using Fourier-transform infrared (FTIR) spectroscopy, solid-state [.sup.13]C CPMAS nuclear magnetic resonance (NMR) spectroscopy, and thermal gravimetric analysis (TGA). All the synthesized cross-linked poly(orthocarbonate)s were insoluble in organic solvents but had high and quick absorption capacities.
1,5-pentanediol, 1,7-heptanediol, 1,8-octanediol, 1,9-nonanediol, 1,10-decanediol, 1,12-dodecanediol, TEOC, and trimethylol propane (TMP) were purchased from Aldrich and used without further purification. DCM, THF, acetone, and benzene were obtained from Aldrich and were used as received.
FTIR spectra were obtained with a Perkin Elmer Spectrum 100 FTIR Spectrometer. [13.sup.C] CPMAS solid-state NMR was observed with a 500 MHz Varian Innova Spectrometer. TGA was performed under a nitrogen atmosphere at a heating rate of 10[degrees]C/min from 25 to 900[degrees]C using a Mettler Toledo model TGA/SDTA 851. Elemental analysis were obtained using a LECO, CHNS-932 analyzer. Innova 2000/platform shaker was used in order to increase polymer--solvent interaction.
Synthesis of Polymers
Various cross-linked polymers were synthesized from the polymerization of the different lengths of linear aliphatic diol monomers with TEOC at partially high temperature under the inert argon atmosphere using a Pyrex (75 mL) pressure vessel (Chemglass, Vineland, NJ).
Synthesis of Poly 1. 1,5-pentanediol (0.5 g, 4.8 mmol), TEOC (0.65 mL, 3.12 mmol), and TMP (0.12 g, 0.89 mmol) (10%) as an additive cross-linker were reacted in a Pyrex pressure vessel at 160[degrees]C for 2 days. The resulting product washed with water, ethanol, and ether; and after being dried under vacuum at room temperature, 0.45 g of transparent-colorless-glassy cross-linked polymer was obtained. FTIR: 3200-3620, 2809-2997, 951-1284, 1749 [cm.sup.-1]. [13.sup.C] CPMAS NMR: 21.37, 53.71, 110.88 ppm. Elemental analysis [[[C.sub.12][H.sub.2][10.sup.6]].sub.n]: experimental: C 56.45%, H 8.24%. Theoretical: C 55.16, H 8.10%.
Synthesis of Poly 2. 1,7-heptanediol (0.55 g, 4 mmol) and TEOC (0.43 mL, 2 mmol) were reacted in a Pyrex pressure vessel at 160[degrees]C for 1 day. The resulting product washed with water, ethanol, acetone, and ether; and after being dried under vacuum at room temperature, 0.41 g of transparent-colorless-glassy cross-linked polymer was obtained. FTIR: 3200-3600, 2856-2932, 1054-1258, 1744 [cm.sup.-1]. [.sup.13]C CPMAS NMR: 21.15, 54.38, 111 ppm. Elemental analysis [[[C.sub.15][H.sub.28[O.sub.4]].sub.n]: experimental: C 63.27%, H 10.61%. Theoretical: C 66.14, H 10.36%.
Synthesis of Poly 3. 1,8-octanediol (0.51 g, 3.48 mmol) and TEOC (0.36 mL, 1.74 mmol) were reacted in a Pyrex pressure vessel at 160[degrees]C for 1 day. The resulting product washed with water, ethanol, acetone, and ether; and after being dried under vacuum at room temperature, 0.41 g of transparent-colorless-glassy cross-linked polymer was obtained. FTIR: 3144-3617, 2821-2965, 1052-1279, 1745 [cm.sup.-1]. [.sup.13]C CPMAS NMR: 21.35, 53.56, 110.7 ppm. Elemental analysis [[[C.sub.17][H.sub.32][O.sub.4]].sub.n]: experimental: C 64.58%, H 10.75%. Theoretical: C 67.96%, H 10.73%.
Synthesis of Poly 4. 1,9-nonanediol (0.58 g, 3.22 mmol) and TEOC (0.39 mL, 1.68 mmol) were reacted in a Pyrex pressure vessel at 160[degrees] C for 1 day. The resulting product washed with water, ethanol, acetone, and ether and after being dried under vacuum at room temperature, 0.49 g of transparent-colorless-glassy cross-linked polymer was obtained. FTIR: 3164-3624, 2808-2972, 950-1200, 1745 [cm.sup.-1]. [.sup.13]C CPMAS NMR: 21.35, 53.56, 111.12 ppm. Elemental analysis [[[C.sub.19][H.sub.36][O.sub.4]].sub.n]: experimental: C 65.59%, H 10.80%. Theoretical: C 69.47, H 11.04%.
Synthesis of Poly 5. 1,10-decanediol (0.5 g, 2.87 mmol), TEOC (0.38 mL, 1.83 mmol), and TMP (0.07 g, 0.52 mmol) (10%) as an additive cross-linker were reacted in a Pyrex pressure vessel at 160[degrees]C for 2 days. The resulting product washed with water, ethanol, acetone, and ether; and after being dried under vacuum at room temperature, 0.39 g of transparent-colorless-glassy cross-linked polymer was obtained. FTIR: 3200-3600, 2830-2990, 1040-1200, 1740 [cm.sup.-1]. [13.sup.C] CPMAS NMR: 30.0, 63.43, 120.60 ppm. Elemental analysis [[[C.sub.17][H.sub.31][O.sub.6]].sub.n]: experimental: C 65.65%, H 9.79%. Theoretical: C 61.60%, H 9.42%.
Synthesis of Poly 6. 1,12-dodecanediol (0.5 g, 2.5 mmol), TEOC (0.33 mL, 1.6 mmol), and TMP (0.06 g, 0.45 mmol) (10%) were reacted in a Pyrex pressure vessel at 160 C for 4 days. The resulting product washed with water, ethanol, acetone, and ether; and dried under vacuum at room temperature, 0.26 g of very light yellow-transparent-glassy cross-linked polymer was obtained. FUR: 3200-3600, 2800-2996, 1032-1200, 1747 [cm.sup.-1]. [13.sup.C] CPMAS NMR: 30.33, 63.43. 120.18 ppm. Elemental analysis [[[C.sub.19][H.sub.35][O.sub.6]].sub.n]: experimental: C 68.31, H 10.18%. Theoretical: C 63.48%, H 9.81%.
Soluble Fraction. A weighed quantity of polymer was put in a solvent and the soluble fraction (SF) was extracted at room temperature for 48 h using DCM. After the extraction, the swelled polymer was dried under vacuum at room temperature. The SF was calculated according to the following Eq. (1) (32).
SF(%) = ([[W.sub.0] - W]/[W.sub.0] x 100 (1)
where [W.sub.0] and W are the weights of the polymers before and after the extraction, respectively.
Swelling Tests. After elimination of the SF, swelling tests on the cross-linked polymers were carried out using the following method. Bags prepared from filter paper were used to determine the swelling properties of the cross-linked polymers. First, the bags were immersed in the solvent and blotted quickly with absorbent paper; this was followed by addition of dried polymer samples of known weight to the bags. All swelling measurements, same size, and same shape polymers were used as possible. The filled bags were immersed in the solvent. All swelling experiments were conducted at room temperature and swelling test was performed for 24 h. The bags were removed and their surfaces were dried gently by blotting, and then weighed in a stoppered weighing bottle. Solvent uptake percentages were calculated using the Eq. 2 (33).
Swelling ratio(%) = ([W.sub.s] - [W.sub.d])/[W.sub.d]) x 100 (2)
where [W.sub.d] and [W.sub.s] are the weights of the dry and swollen polymers, respectively.
Swelling Kinetics. Swelling kinetics measurements were conducted using the procedure described above, using DCM as the solvent. After removing the bags from the solvent after various time intervals, they were blotted quickly to remove DCM attached to the surface and weighed.
Desorption Kinetics. The DCM retention of the polymer in air was determined by weighing the swollen polymer in air as a function of time.
RESULTS AND DISCUSSION
Synthesis and Characterization of the Cross-linked Polymers
The reactions between TEOC and linear aliphatic diol monomers producing cross-linked poly(orthocarbonate)s are summarized in Scheme I. All synthesized polymers have swelling abilities in common organic solvents such as benzene, acetone, DCM, and THF, but are insoluble in these solvents. To examine the solvent absorption abilities of the synthesized cross-linked poly(orthocarbonate)s, different types of linear diols were used: in the synthesis of Poly 2, 1,7-heptanediol was condensed with TEOC, in the synthesis of Poly 3, 1,8-octanediol was used, and in the synthesis of Poly 4, 1,9-nonanediol was used as the hydroxyl functional monomer. In the synthesis of the other polymers, to obtain a more stable cross-linked structure. TMP was used as an additional cross-linker. In the absence of TMP, cross-linked polymers can be synthesized, but, in this case, the percentages of SFs increased and the polymers' swelling capacities and the amounts of polymers which could be used as sorbents decreased. In the synthesis of Poly 1, Poly 5, and Poly 6, TEOC and TMP (10%) were used, and the diol monomers used were 1,5-pentanediol, 1,10-decanediol, and 1,12-dodecanediol, respectively.
Linear-aliphatic-diol-based cross-linked poly(orthocarbonate)s were prepared by the condensation reactions of monomers in a solvent-free medium, under an argon atmosphere in a pressure vessel at 160[degrees]C. Depending on the linear diol monomers used, the reaction was completed in 1-4 d. The synthesized polymers were insoluble in common organic solvents, but swelled in these solvents. The effects of the chain lengths of the aliphatic diol monomers on the swelling behaviors of the cross-linked poly(orthocarbonate)s were examined.
FTIR and solid-state [.sup.13]C CPMAS NMR spectroscopies, and TGA were used to analyze the structures of the synthesized polymers. Schematic representation of proposed polymer structure was given in Scheme 2. Figure 1 represents the FTIR spectrums of all reagents that used in the synthesis of Poly 1 and Poly 2 as an example. Aliphatic--[CH.sub.2]--absorption peaks were detected at 2856-2932 [cm.sup.-1] in the FTIR spectra of the cross-linked polymers. The FTIR spectra also revealed a broad absorption peak between 3200 and 3600 [cm.sup.-1], characteristic of the stretching vibrations of the hydroxyl group, which came from the diol monomers. The stretching vibrations at 1054 and 1258 [cm.sup.-1] indicated the existence of --C--O-- bonds, and a carbonyl peak at 1744 [cm.sup.-1] showed that at least one of the end groups was an ester.
A solid-state [.sup.13]C CPMAS NMR spectrum of Poly 4 is given as an example (Fig. 2). The peak at 21.35 ppm indicates --[CH.sub.2]--carbon atoms. The signals at 53.56 ppm and 111.12 ppm confirm the presence of --[CH.sub.2]--0-- and --C[O.sub.4]-- carbon atoms, respectively.
TGA was used to examine the thermal stability of the synthesized cross-linked polymers under a nitrogen atmosphere (Fig. 3). The TGA results indicate that the linear-aliphatic-diol-based cross-linked poly(orthocarbonate)s are thermally stable at temperatures lower than 300[degrees]C, except Poly 4, which is thermally less stable than the others. The order of thermal stability of the polymers is Poly 6 > Poly 5 > Poly 2 > Poly 3 > Poly 1 > Poly 4 (at 300[degrees]C). Among Poly 2, Poly 3, and Poly 4, Poly 2 is the most stable. When the chain length of the diol monomer increased, the stability of the polymer decreased. Using TMP as an additional cross-linker resulted in structures that are more rigid, except in the case of Poly 1.
Swelling Properties of the Cross-linked Polymers
An extraction process was used on the obtained cross-linked polymers to remove the polymer chains and oligomers that were not attached to the polymer network. These SFs might reduce the organic solvent absorption rate of the cross-linked polymers. The negative effects of these SFs must therefore be eliminated or minimized in order to increase the solvent uptake ability of the cross-linked polymers. The SFs were removed using DCM, which does not damage the structure of the polymer network and can dissolve the unwanted polymer SFs. The SF percentages of the cross-linked polymers ranged between 2.5 and 8%. The SFs are affected by monomer properties, monomer concentrations, and cross-linking agents (21).
The resulting synthesized linear-aliphatic-diol-based cross-linked poly(orthocarbonate)s exhibited good solvent absorption abilities, particularly in organic solvents such as DCM, THF, acetone, and benzene. After elimination of the SFs, swelling measurements were performed and the measured organic solvent absorption percentage of the cross-linked polymers was shown in Fig. 4. Poly 4 that was synthesized by the reaction of 1,9-nonanediol and TEOC gave the best result among the synthesized polymers. The maximum solvent uptake rates of all the polymers were observed in DCM. When the length of the linear aliphatic monomer increased, the swelling rate of the cross-linked polymer increased regularly in the cases of Poly 2, Poly 3, and Poly 4. The addition of the crosslinking agent TMP negatively affected the swelling ratios of Poly 1, Poly 5, and Poly 6. This is because addition of a chemical cross-linker causes the formation of a denser polymer network and reduces the chain length between cross-links, resulting in a lower swelling degree. When we compared the swelling values of Poly 1, Poly 5 and Poly 6, the same trend toward smaller values was observed, but a longer chain length increased the solvent affinity. Poly 6, which was synthesized by the reaction of 1,12-dodecanediol, TMP, and TEOC, had the highest swelling capacity, namely 235% in DCM.
When the swelling properties of cross-linked poly(orthocarbonate)s are compared with swelling properties of other cross-linked polymers in the literature (i.e., reversibly cross-linkable polyesters in THF, 3000% (34); polycarbonate in DCM, 148% (35); carbonate polymers of dihydroxyaryl fluorine in DCM, 284% (36); aminopolysiloxane in hexane, 360% (37), butyl rubber in THF, 223% (38); polyether-based polyurethane in benzene, 181% (39), cross-linked poly(orthocarbonate)s in DCM, 320% (30) and 314% (31), cross-linked poly(orthosilicate)s based on cyclohexanediol in THF 165% (40) and based on cyclohexanedimethanol in THF 205% (41); our results for cross-linked poly(orthocarbonate)s are clearly competitive and they can be used as organic solvent absorbent.
The swelling kinetics of the polymers in DCM was investigated to measure the saturation times of the synthesized cross-linked polymers. All the synthesized polymers showed fast solvent absorption abilities. For instance, Poly 4 reached its maximum capacity in 25 min. Short saturation times of the cross-linked polymers are an important factor and are preferred for use as organic solvent absorbents (Fig. 5).
All the polymers showed very short time periods for releasing the absorbed DCM. The DCM retention times of the polymers were determined by analyzing the weight losses of the swollen polymers in air at room temperature. Almost all the absorbed DCM was released by the polymers within 25-30 min (Fig. 6). All the swelling results were obtained by averaging at least four different swelling measurements. In addition, the same polymer sample was used in repeated swelling tests. FTIR spectroscopy was used to determine the reusability of the polymer, based on swelling tests using the same polymer sample. The same polymer sample was used six times for swelling measurements. As can be seen from Fig. 7, there were no changes before ([S.sub.0]) and after the swelling tests as a result of further condensation reactions by unreacted groups, and the synthesized cross-linked polymers were stable in repeated measurements in DCM.
In conclusion, we have synthesized a new type of polymer, poly(orthocarbonate)s, using aliphatic diol monomers of different lengths with TEOC and TMP. Analysis of the effects of the linear chain length on the swelling properties indicated that all the polymers had good and fast organic solvent uptake abilities. Easy desorption of the absorbed solvents and the reusability of the cross-linked polymers showed that they can be used as organic solvent absorbent.
The authors thank the Gebze Institute of Technology for the support of this work through Project: 2009-A-06.
Correspondence to: Hayal Bulbul Sonmez; e-mail: email@example.com
Published online in Wiley Online Library (wileyonlinelibrary.com).
[c] 2013 Society of Plastics Engineers
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Ilker Yati, Gulsah Ozan Aydin, Hayal Bulbul Sonmez
Department of Chemistry, Gebze Institute of Technology, Gebze, Kocaeli, Turkey
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|Author:||Yati, Ilker; Aydin, Gulsah Ozan; Sonmez, Hayal Bulbul|
|Publication:||Polymer Engineering and Science|
|Date:||Oct 1, 2013|
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