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Synthesis of poly(sodium acrylate-co-sodium 1-(acryloyloxy) propan-2-yl phosphate) and comparative study on its swelling properties with poly(sodium acrylate) and poly(sodium acrylate-co-2-hydroxypropyl acrylate).

INTRODUCTION

As a functional polymeric material, superabsorbents (SAPs) can absorb a large amount of water and the absorbed water is hardly removed even under certain pressure due to their excellent characters [1, 2]. They have been widely used in many fields, such as disposable diapers, feminine napkins, soil for agriculture and horticulture, gel actuators, water-blocking tapes, drug delivery systems, and absorbent pads [3-5]. In such applications, pH, ionic type, and ionic strength of swelling medium could significantly influence the water absorbency and the water retention of SAPs. Moreover, the influence of each factor on various types of SAPs is different, so it is important to study the water absorbency and the swelling behavior of various SAPs that contain different kinds of hydrophilic groups. Some researchers had done a lot on this aspect and got some useful results on the development of SAPs [5-12].

As a kind of useful monomer, vinyl phosphate had been widely used in medical appliances, such as artificial organs, blood vessels, contact lenses, cosmetics, etc. [13, 14]. However, these monomers were rarely found in the previous literatures of SAPs. When compared with carboxylate, vinyl phosphate possesses more charges and higher ionizability, so it is more hydrophilic than carboxylate. Besides, the better biodegradation and compatibility with organs and tissues would also make the superabsorbent produced by vinyl phosphate exhibit broader applications.

On the basis of our previous work [15-18], a sort of SAP containing phosphate group, poly(sodium acrylate-co-sodium 1-(acryloyloxy) propan-2-yl phosphate) [P(SA-co-SAPP)], was synthesized and the swelling properties, including the swelling behavior and the saturated water absorbency (SWA), were investigated in different swelling media in comparison with two other kinds of SAPs, i.e., P(SA-co-HPA) and PSA, which were also prepared under the same conditions. The results showed that the influences of cations on swelling properties of P(SA-co-SAPP) were obviously different from those of P(SA-co-HPA) and PSA, and the SWA, the swelling rate, and the hydrogel strength of P(SA-co-SAPP) were all preferable to those of PSA.

EXPERIMENTAL

Materials

2-Hydroxypropyl acrylate (HPA, Beijing Eastern Chemical Works, China) was industrial grade and rectified at reduced pressure (b.p. 76-79 C/0.5 mm Hg). Acenaphthylene (ACE) (Aldrich) was purified by triple recrystallization from ethanol and then by sublimated crystallization. Acrylic acid (AA, Beijing Eastern Chemical Works, China) was of chemical purity and distilled at reduced pressure (b.p. 20-21 C/0.5 mm Hg). Sodium hydroxide (NaOH), hydrogen peroxide ([H.sub.2][O.sub.2]), sodium bicarbonate (NaHC[O.sub.3]), L-ascorbic acid (Vc), and phosphoryl trichloride (PO[Cl.sub.3]), formic acid, hydrochloric acid (HCl), ethyl ether ([Et.sub.2]O), methanol, triethylamine (TEA), and tetrahydrofuran (THF) (all purchased from Xi'an Chemical Reagent Plant, China) were all analytical grade and used as received. Ethylene glycol diglycidyl ether (EGDE, Fluka, Switzerland) was of chemical purity and used directly.

Synthesis Procedures

Synthesis of 1-(Acryloyloxy) Propan-2-yl Phosphoryl Dichloride and 1-(Acryloyloxy) Propan-2-yl Dimethyl Phosphate. The novel monomer APPDC was synthesized and characterized by a more stable derivation, 1-(acryloyloxy) propan-2-yl dimethyl phosphate (APDMP). The synthetic process was illustrated in the following equations.

[FORMULA NOT REPRODUCIBLE IN ASCII] (APPDC)

[FORMULA NOT REPRODUCIBLE IN ASCII] (APDMP)

The synthetic reaction was conducted, under [N.sub.2] atmosphere, in an oven-dried 200-mL three-necked flask equipped with a magnetic stir bar, a [N.sub.2] line (attached to a bubbler), an addition funnel with a pressure-equalizing side arm, and a thermometer. The flask, further dried with a heat-gun under positive [N.sub.2] flow, was charged with PO[Cl.sub.3] (5 mL, 0.055 mol), THF (8 mL), and [Et.sub.2]O (6 mL). To the stirred colorless solution at -5[degrees]C (which was controlled by a salt-ice bath) was added dropwise, a solution of HPA (7.3 mL, 0.055 mol), TEA (4.5 mL, 0.055 mol), THF (8 mL), and [Et.sub.2]O (6 mL), with the drop rate of two drops per second. The resulting bright yellow solution was allowed to stir at 0[degrees]C for 2 h, then at ambient temperature for 2 h, and subsequently, the reaction mixture was filtered to remove the triethylamine hydrochloride. The resultant mixture was distilled to separate the solvent. Thereafter, the target monomer, APPDC, was obtained. To prepare APDMP, a third mixture-containing methanol (0.11 mol), TEA (9 mL, 0.11 mol), [Et.sub.2]O (10 mL), and THF (14 mL) was added dropwise to the above-mentioned reaction mixture before the purification of APPDC. The bright yellow mixture was allowed to stir at room temperature for another 2 h, and then refluxed at 40[degrees]C for 8 h. The reaction mixture was quenched by the dropwise addition of 100 mL deionized water. The two phases were separated and the [Et.sub.2]O layer was washed successively by using water (2 x 200 mL), 5% aqueous HCl (2 x 200 mL), saturated aqueous NaHC[O.sub.3] (200 mL), and brine (200 mL). After that, the solution was dried by anhydrous [Na.sub.2]S[O.sub.4] and then concentrated to get the APDMP. The crude product was stored in a freezer, which could be further purified by preparative HPLC on a 25 x 2 [cm.sup.2] Hypersil ODS, 52 [micro]m column using an 80/20 mixture of methanol/water containing 0.01% formic acid. The purified APDMP was characterized by [.sup.1.H] NMR after the methanol and water were eliminated.

Preparation of Copolymers. The copolymer of P(SA-co-SAPP) (SAP1) was synthesized by solution polymerization. The reaction was conducted, under [N.sub.2], in a 200-mL three-necked flask equipped with a magnetic stir bar, an [N.sub.2] line (attached to a bubbler), and a thermometer. The flask was charged with a mixture of about 0.12 mol of AA and 0.011 mol of APPDC. The mixture was neutralized carefully with 10 mL of 10.8 mol [L.sup.-1] NaOH aqueous solution. To obtain a sodium acrylate (SA) solution with a neutralization degree of about 65-70% and avoid self-polymerization, the system was controlled under 40[degrees]C by a cooling water bath. After that, the crosslinker, EDGE, was added in, and the molar ratios of EGDE/AA were kept at 2.5 x [10.sup.-4]. The mixture was cooled to 20[degrees]C and then [H.sub.2][O.sub.2] (0.10 mol [L.sup.-1]) and Vc (0.028 mol [L.sup.-1]) in equal volumes were introduced into the SA solution, in which the molar ratio of [H.sub.2][O.sub.2] to AA was 2.8 x [10.sup.-4]. The reactant was then controlled at 45 [+ or -] 1[degrees]C for 1.5 h. Finally, the product was dried at 100[degrees]C, ground and milled through 26-90-mesh screen. Thus SAP1 was obtained.

The copolymer of P(SA-co-HPA) and homopolymer of PSA were prepared using different monomers under similar conditions as that of SAP1, which were named as SAP2 and SAP3, respectively.

Synthesis of PAA/ACE. PAA/ACE was synthesized by free radical polymerization in methanol solution initiated with Vc-[H.sub.2][O.sub.2]. The process was similar to that of PSA except that NaOH was substituted by ACE with the molar ratio of [n.sub.AA]/[n.sub.ACE] = 100/2. The polymerization was lasted for 1.5 h at 45 [+ or -] 1[degrees]C. The mixture was dissolved in water, precipitated by methanol, and then vacuum dried under 0.75 mm Hg at 25[degrees]C for 2 days. These treatment processes were repeated for five times. The residues were dried at 100[degrees]C and the final product was obtained.

Characterization

Characterization of Monomers. The structure of monomers, APPDC and APDMP, were characterized by [.sup.1.H] NMR (Aspect 300 MHz, Bruker, Germany), fast atom bombardment MS (FAB, ZAB-HS, VG, UK), and gas chromatography/mass spectrometer (GC/MS, GC 2000 Trace MS, Finnigan Mass, UK). [.sup.1.H] NMR and FAB (m-nitrobenzyl alcohol was used as a matrix) were used to determine the APDMP, while the GC/MS was used to confirm the content of APPDC after its preparation finished. The yield was about 82%.

Figure 1 showed the [.sup.1.H] NMR characteristic peaks (CD[Cl.sub.3], [[delta].sub.H], ppm). It could be observed: 6.41 (q, 1H, [H.sub.c]), 6.15 (t, 1H, [H.sub.a]), 5.90 (t, 1H, [H.sub.b]), 4.22 (m, 1H, [H.sub.f]), 4.15 (dd, 2H, [H.sub.d]), 3.75 (s, 6H, [H.sub.c]), 1.40 (d, 3H, [H.sub.g]). All these data indicated that the product was APDMP. Besides, the peak of 239 in FAB could be assigned as the M+1 peak of APDMP, which also showed the product was APDMP.

Characterization of SAPs. To eliminate the influence of soluble molecules, each SAP was repeatedly swollen in distilled water to equilibrium, then filtered, and dried at 100[degrees]C to a xerogel for five times. Finally, the xerogel was extracted for 48 h in Soxhlet extraction apparatus using THF, and the purified samples for characterization were obtained. FTIR spectra of these types of SAPs were conducted by using NEXUS 670 FTIR spectrometer (Nicolet, USA). The content of phosphorus in SAP1 was tested by ICP instrument (IRIS ER/S. Thermo Electro, USA), while carbon and hydrogen were tested by the element analysis instrument (1106, Elemental Vario EL Corporation, Germany).

The FTIR spectra of SAP1, SAP2, and SAP3 were shown in Fig. 2, in which the peaks could be assigned as follows: -OH ([v.sub.O-H]), 3433 [cm.sup.-1], -C[H.sub.3], -C[H.sub.2], and -CH ([v.sub.C-H]), 2926 [cm.sup.-1], -POH([v.sub.O-H]) and dimeric acid. 2532 [cm.sup.-1], -O-C=O ([v.sub.C-O]), 1723 [cm.sup.-1], -O-C=O ([v.sub.C=O]), 1621 [cm.sup.-1], -C- (C[H.sub.2]-CH)[.sub.n] -C-(n [greater than or equal to] 4), 1456 [cm.sup.-1], -C[H.sub.2]-C(O) -OH, 1410 [cm.sup.-1], RO-P=O and C-O-C, 1241 [cm.sup.-1], P-O-C and C-O-C, 1061-1106 [cm.sup.-1], P-O-C, 821 [cm.sup.-1]. Because all the main characteristic bands of SAP1 were overlapped with those of SAP2 and SAP3, it was difficult to ensure that SAP1 was the copolymer of SA and sodium l-(acryloyloxy) propan-2-yl phosphate by IR spectroscopy. Therefore, the composition of SAP1 was analyzed by the elemental analysis, and the results were shown as follows: C (27.12%), H (5.10%), and P (2.6%). This confirmed that the monomer SAPP was copolymerized on the chain of SAP1.

Determination of ACE Group. The ACE content on the polymer was obtained by using UV absorption spectroscopy in methanol, with a series of ACE solution in methanol ([[epsilon].sub.max] = 321 nm) as references. The result showed that the content of incorporated ACE in PAA/ ACE was 1.84 mol% and the molar ratio of [n.sub.AA]/[n.sub.ACE] in the resultant polymer was 68/1.

Swelling Kinetics of SAP. The used swelling media were 0.9 wt% of NaCl aqueous solutions with various pH, metal chloride aqueous solutions with the same ionic strength and pH, and sodium salt solutions with various anions at the same ionic strength and pH. The swelling kinetics of SAPs in three types of swelling medium were investigated as follows: 0.200 g of SAPs enclosed in a sealed tea bag (40 x 50 [mm.sup.2]) were placed in 200 mL of swelling mediums, and then removed the tea bag in set intervals and weighed until the weight of hydrogels was constant. The water absorbency was calculated by the following equation.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

WA = [M/[M.sub.0]] - 1 (1)

where WA, M, and [M.sub.0] denoted the water absorbency, the weights of the swollen hydrogel, and SAPs, respectively. The water absorbency of SAPs swelled to equilibrium was defined as SWA.

Fluorescence Experiment and Analytical Methods. All fluorescence spectra were recorded on a PerkinElmer LS 55 luminescence spectrometer. The measurements of the PAA/ACE solution (6.35 x [10.sup.-5] wt%) were performed using a quartz cuvette with path length of 1 cm at [lambda] = 340 nm. The pH and concentrations of tested solutions were changed at the range of 1-9 and [10.sup.-9][-10.sup.1] mol/L, respectively. Measurements were carried out automatically and the results reported herein were the average value of at least 20 times tests.

RESULTS AND DISCUSSION

Effect of pH on SWA

Figure 3 showed the relationships between SWA of SAPs and pH in 0.9 wt% NaCl aqueous solution. The pH was adjusted by 11.5 M aqueous solution of HCI or NaOH to eliminate the influence of the ionic strength [19] on SWA of SAPs, which was controlled with a pH meter (PHS-3C, People's Republic of China). It could be seen that the influence of pH between 4 and 12 on SWA of SAP2 was rather little, which was similar to the investigation results of Lee and Wang [5, 10-12]. It was caused by the introduction of nonionic comonomer (HPA), a weaker chelate reagent, which resulted in the dilution effect of acid radical (RCO[O.sup.-]) in the polymer chain, and decreased the electrostatic action of acid radical and the chelation among [Na.sup.+] and RCO[O.sup.-]. It is further confirmed that SAPs containing nonionic comonomers could resist salt obviously, which have been presented in previous literatures [17, 18]. Scheme 1 showed the sketch map of the structure of SAP2.

[FIGURE 3 OMITTED]

However, SWA of SAP1 and SAP3 were obviously affected by pH. SWA for both SAP1 and SAP3 initially increased to a maximum, and then decreased to a minimum with the increase of pH. The pH related to the maximum of SAP1 (pH 6.00) was higher than that of SAP3 (pH 5.61), while the minimum exhibited at about pH = 7.4, which has not been reported in previous literatures. This could be attributed to the chelation between [Na.sup.+] and acid radicals fixed on the polymer chains. The thought of chelation was based on the Na-EDTA and Na-crown ethers coordination because the structure of SAP1 and SAP3 had similar near-range structure and long-range structure with that of EDTA and crown ethers, respectively. And the coordination number of Na-SAP would reach to six or more [20]. When SAPs of PSA series swelled in aqueous solution of 0.9 wt% NaCl with various pH adjusted by HCl or NaOH, the exchange of cations would take place between [Na.sup.+] and [H.sup.+] in vitro and in vivo of hydrogel, and the entry of [Na.sup.+] would increase neutralization degree and chelation between cations and radical groups bonded to the networks, which further affected SWA of SAPs.

[GRAPHIC OMITTED]

For SAP1, a SAP of PSA series with neutralization degree of 65-70%, by rough estimate, if pH of swelling medium was about 4.55, the exchange would not take place. When pH of swelling medium was less than 4.25, [H.sup.+] in swelling medium would exchange with [Na.sup.+] in hydrogel and decrease the neutralization degree, so the quantity of radical groups was smaller than that of acid. The chelation was weak and could be ignored, and the penetration pressure and repulsion force between radical groups of SAPs were also small, and SWA of SAPs was small too. With the increase of pH, the exchange would reverse, the quantity of radical groups would increase, and the penetration pressure and repulsion force between radical groups of SAPs would increase too. Although chelation also increased under this environment, the quantity of ligands (acid radical) was still small, and the influence of chelation was minor. So, SWA of SAPs increased. With the further increase of pH (pH [greater than or equal to] 6.0), because large amount of [Na.sup.+] bonded to hydrogel networks and decreased the penetration pressure and repulsion force [21], the influence of the penetration pressure and repulsion force decayed to the minor factor, while the chaletion rose to the main factor, which meant more amount of [Na.sup.+] existing in the hydrogel network and lead to the greater chelate degree and lower SWA. However, overmuch amount of [Na.sup.+] diffusing into SAP would decrease the ratio of oxygen and sodium on polymer chains [22], which would cause the change of coordination methods (inter- and intrachains) and coordination numbers and further make the hydrogel network stretch. As a result, SWA of SAP1 showed a maximum at pH = 6.0 and a minimum at pH = 7.4.

SAP3 showed the same trends except the relevant pH of maximum SWA was 5.61. The reason was that its neutralization degree was 90% during its preparation, and [H.sup.+] in the medium would exchange with [Na.sup.+] in SAP3 to make the neutralization degree of SAP3 decrease in acidic medium. According to rough estimate, if the neutralization degree of SAP3 decreased from 90 to 65-70%, the pH of swelling medium with pH = 5.61 would changed to about 6.00-6.31. The determined pH of the above swelling medium (using PHS-3C) showed pH of 6.00-6.25 after SAP3 swelling to equilibrium, and this was accordance with the result that the maximum SWA of SAPs was always shown as the neutralization degree of 65-70% in aqueous solution of 0.9 wt% NaCl without adjusting pH (pH = 6.25) [15-18, 23, 24].

High SWA indicated the polymer chains was a stretched-out structure, while low SWA was a curled structure, which was related to the coil and globular structure of linear polymer chains in extremely dilute solution, respectively. The alteration of the above-mentioned two conformations caused by the chelation between [Na.sup.+] and polymer chains in 0.9 wt% NaCl under different pH could be further investigated by fluorescence technique using linear PAA/ACE [25]. Figure 4 showed the fluorescence intensity fluctuated manifestly with the increase of pH. At pH < 4, the fluorescence intensity increased with the increase of pH, which was due to globular to coil translation of liner polymers caused by the increase of the neutralization degree. At pH = 4-6.5, the fluorescence intensity decreased to the minimum, which was caused by coil to globular translation resulted from the chelation between polymer chains acid radical and [Na.sup.+]. At pH > 6.5, the polymer chain stretched out, which was caused by the decrease of coordination numbers of [Na.sup.+], and the fluorescence intensity increased. Because SAPs were lightly crosslinked polymer networks, the response to pH stimulation would be delayed, and the pH lag of the minimum SWA value was reasonable. To further verify the chelation of [Na.sup.+], the relationship between the fluorescence intensity and concentrations of [Na.sup.+] was investigated. It could be found from Fig. 5 that the fluorescence intensity waved with the concentration of [Na.sup.+], in which peaks could be assigned as the transition state between two stable complexes with different coordination numbers, while the trough could be assigned as a state of stable complex with single coordination number. According to the above-mentioned analysis, the chelation model between acid radical of PSA and [Na.sup.+] in the solution of 0.9% NaCl at different pH was shown in Scheme 2. These indicated that the chelation was the main reason of the minimum SWA showed at pH = 7.4.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

Figure 3 also showed the SWA of SAP1 was higher than that of SAP3 under most of pH conditions, which could be attributed to the higher affinity of RO-P[O.sub.3.sup.2] for water.

Effect of Cations on SWA

Table 1 showed the effect of cations existed in swelling medium on SWA. The results were obtained under conditions of pH = 5.11 and ionic strength I = 0.1539 mol [L.sup.-1]. It could be found that the order of SWA for various SAPs with different cations as follows: SAPI. [K.sup.+] > [Na.sup.+] > N[H.sub.4.sup.+] > [Mg.sup.2+] > [Zn.sup.2+] [approximately equal to] [Ca.sup.2+] > [Cu.sup.2+] > [Ba.sup.2+]; SAP2, N[H.sub.4.sup.+] > [K.sup.+] > [Na.sup.+] > [Zn.sup.2+] > [Cu.sup.2+] > [Ba.sup.2+] [approximately equal to] [Mg.sup.2+] > [Ca.sup.2+]; SAP3, [Na.sup.+] > N[H.sub.4.sup.+] > [K.sup.+] > [Mg.sup.2+] [approximately equal to] [Ca.sup.2+] > [Ba.sup.2+] [approximately equal to] [Zn.sup.2+] > [Cu.sup.2+]. The significant difference of SWA order with various cations might be because of the chelation and affinities between cations and functional groups.

[ILLUSTRATION OMITTED]

According to the theory of hard and soft acids and bases (HSAB) [26-29], the hardness order of cations investigated in our study was [Mg.sup.2+] > [Ca.sup.2+] > [Ba.sup.2+] > [Na.sup.2+] > [Na.sup.+] [greater than or equal to] N[H.sub.4.sup.+] [greater than or equal to] [K.sup.+] > [Zn.sup.2+] > [Cu.sup.2+], and [Cu.sup.2+] was the intermediate acid [29]. For anions, the calculating method of the group's electronegativity was used to scale the hardness of ROH, RO-P[O.sub.3.sup.2-], and RCO[O.sup.-] [30]. The resulted order was ROH > RO-P[O.sub.3.sup.2] > RCO[O.sup.-], and RCOO was the intermediate base.

[FIGURE 6 OMITTED]

For RCO[O.sup.-], according to the principle of HSAB, the stability order of the complex compounds should be RCO[O.sup.-]-[Cu.sup.2+], RCO[O.sup.-]-[Zn.sup.2+], RCOO-[K.sup.+], RCO[O.sup.+]-N[H.sub.4.sup.+] and RCO[O.sup.-]-[Na.sup.+], RCOO-[Ba.sup.2+], RCO[O.sup.-]-[Ca.sup.2+], and RCO[O.sup.-]-[Mg.sup.2+]. Considering the chelation [22], the stability of [Ba.sup.2+], [Ca.sup.2+], and [Mg.sup.2+] would exceed [K.sup.+], [Na.sup.+], and N[H.sub.4.sup.+]. This order was also be proved by the solubility of the product constant ([K.sub.sp]) of corresponding carbonates [31]. Obviously, SWA order of SAP3 was approximately consistent with the stability order of complex compounds.

For RO-P[O.sub.3.sup.2], the best-suited acid-base pair should be found out by comparing the [K.sub.sp] of the corresponding phosphate. The [K.sub.sp] of [Cu.sub.3](P[O.sub.4])[.sub.2] was the least and then [Ca.sub.3](P[O.sub.4])[.sub.2]. Because RO-P[O.sub.3.sup.2] was a hard base, while [Cu.sup.2+] was an intermediate acid, the best-suited acid-base pair should be P[O.sub.4.sup.3-]-[Ca.sup.2+]. Thus, the stability order of the complex compounds should be P[O.sub.4.sup.3] -[Ca.sup.2+], P[O.sub.4.sup.3-]-[Ba.sup.2+], P[O.sub.4.sup.3-]-[Mg.sup.2+], P[O.sub.4.sup.3] -[Na.sup.+], P[O.sub.4.sup.3-] -N[H.sub.4.sup.+], P[O.sub.4.sup.3] -[K.sup.+], P[O.sub.4.sup.3-]-[Zn.sup.2+], and P[O.sub.4.sup.3-]-[Cu.sup.2+]. However, because R of RO-P[O.sub.3.sup.2-] denotes the hydrogel network and was very large, RO-P[O.sub.3.sup.2] could shift toward softness, and the stability of the complex compounds should be RO-P[O.sub.3.sup.2] -[Ba.sup.2+], RO-P[O.sub.3.sup.2-]-[Ca.sup.2+], RO-P[O.sub.3.sup.2-] -[K.sup.+], RO-P[O.sub.3.sup.2-]-[Na.sup.+], RO-P[O.sub.3.sup.2-]-N[H.sub.4.sup.+], RO-P[O.sub.3.sup.2-]-[Zn.sup.2+], RO-P[O.sub.3.sup.2-] -[Mg.sup.2+], RO-P[O.sub.3.sup.2-] -[Cu.sup.2+]. Considering the chelation, the finally stable order of RO-P[O.sub.3.sup.2-]-M should be RO-P[O.sub.3.sup.2]-[Ba.sup.2+], RO-P[O.sub.3.sup.2]-[Ca.sup.2+], RO-P[O.sub.3.sup.2-]-[Zn.sup.2+], RO-P[O.sub.3.sup.2-]-[Mg.sup.2+], RO-P[O.sub.3.sup.2+]-[Cu.sup.2+], RO-P[O.sub.3.sup.2-]-[Na.sup.+], RO -P[O.sub.3.sup.2-]-N[H.sub.4.sup.+], and RO-P[O.sub.3.sup.2-]-[K.sup.+]. Because the network of SAP1 contained the functional group of RCO[O.sup.-] and RO-P[O.sub.3.sup.2], and RCO[O.sup.-]-[Cu.sup.2+] was the most stable acid-base pair, the influence of [Cu.sup.2+] on SWA of SAP1 exceeded that of [Mg.sup.2+], [Zn.sup.2+], and [Ca.sup.2+] under the consideration of the coordination effect among these groups. The above stability order of complex compounds was similar to SWA order for SAP1.

[FIGURE 7 OMITTED]

For ROH, using the same criterion as that of for RO-P[O.sub.3.sup.2-], we obtained the finally stability order of ROH-M: ROH-[Ca.sup.2+], ROH -[Ba.sup.2+], ROH-[Mg.sup.2+], ROH-[Na.sup.+], ROH-[K.sup.+], ROH-N[H.sub.4.sup.+], ROH-[Zn.sup.2+], and ROH-[Cu.sup.2+]. Considering the chelation and the coordination effect of ROH and RCO[O.sup.-], the stability order of complex compounds should be [Ca.sup.2+] > [Mg.sup.2+] [approximately equal to] [Ba.sup.2+] > [Cu.sup.2+] > [Zn.sup.2+] > [Na.sup.+] [greater than or equal to] [K.sup.+] [greater than or equal to] N[H.sub.4.sup.+], which was similar to SWA order for SAP2.

Effect of pH and Monomer on the Swelling Rate

Figs. 6 and 7 showed the swelling behavior of various SAPs in aqueous solutions of 0.9 wt% NaCl with pH = 4.16 and pH = 11.18, respectively. It was found that under different pH conditions, the order of SR for three types of SAPs was SAP2 > SAP1 > SAP3. Because SR was dominated mainly by the capability of acquiring and carrying water of functional groups bonded to SAPs networks. For SAP2, relatively larger amount of hydroxyl could form more hydrogen bonds with water, thus it could acquire and carry water more easily [32]. For SAP1 and SAP3, because the affinity of RO-P[O.sub.3.sup.2-] for water was stronger than that of RCO[O.sup.-], SR of SAP1 was higher than that of SAP3. Certainly, more hydroxyl was another main reason for larger SR of SAP1 relative to SAP3.

Effect of Cations on the Swelling Behavior

Figs. 8 and 9 showed the swelling behavior of various SAPs in aqueous solutions of N[H.sub.4]C1 and Cu[Cl.sub.2] at pH = 5.11 and ionic strength I = 0.1539 mol [L.sup.-1]. In fact, the univalent cations and bivalent cations showed the similar swelling trend as that of N[H.sub.4]Cl and Cu[Cl.sub.2], respectively. From these figures, we could observe that (1) the order of SR of SAP was SAP2 > SAP1 > SAP3, which was relative to the capability of acquiring and carrying water of functional groups bonded to SAPs networks; (2) WA showed the maximum for bivalent cations in the swelling process, and the swelling process obviously displayed the exclusive volume effect. This might be accounted for the chelation of the bivalent ion. During the swelling process, the solution entered into the outer region of SAPs and made the range swell [9]. With the accompany of the swelling, the entered cations would chelate with functional groups bonded to networks of SAPs, and lead to the constriction of the network, thus, the water was released and the deswelling happened. With the swelling region gradually expanded towards the center, more cations entered into the hydrogel, and more water was released from the hydrogel. As the rate of the swelling was equal to that of the deswelling, WA reached its maximum. Because complexes of bivalent cations were stabler than those of univalent cations, WA showed a maximum before SAP swelling to its equilibrium in bivalent cations solutions; (3) the time attained to the swelling equilibrium in bivalent cations solutions was longer than in univalent ones. This was accounted for the formation of stable complexes [22], which would lead to the relatively larger constriction of the network and decrease the osmotic pressure and the capacity of building hydrogen bonds.

[FIGURE 8 OMITTED]

[FIGURE 9 OMITTED]

It was noteworthy that a very interesting phenomenon was observed during the swelling process, i.e., WA of SAP2 changed sharper in divalent cations solutions than that of SAP1 and SAP3. The possible reason might be related to the strong chelate capability of divalent cations, which would strongly polarize the hydroxyl and further build coordination bonds. During the process of the swelling, divalent cations that diffused into networks combined with RCO[O.sup.-] on polymer chains first, and the constriction of the network was smaller, which resulted in the preferred SR of SAP2. Besides, the good capacities of acquiring and carrying water would also result in the large SR of SAP2 at the beginning of the swelling. As a result, WA of SAP2 increased rapidly at the earlier swelling process. With the increase of divalent cations on hydrogel networks, the hydroxyl began to chelate with divalent cations, which would decrease the pores of the hydrogel networks and lead to more significant exclusive volume effect relative to SAP1 and SAP3. Thus, WA of SAP2 always changed sharply in divalent cations solutions.

Effect of Anions on the Swelling Behavior

Figs. 10 and 11 showed the swelling behavior of SAPs in aqueous solutions of [Na.sub.3]P[O.sub.4] and a mixture solution containing NaAc, NaN[O.sub.3], [Na.sub.2]S[O.sub.4], NaCl, NaBr, Nal, and [Na.sub.3]P[O.sub.4], respectively. All these results were obtained under the conditions of pH = 11.94 and I = 0.1539 mol [L.sup.-1]. When compared with the effect of cations on SAPs, SWA and SR were hardly affected by anions. The reason might be that all SAPs were polyanionic electrolyte and could not chelate with anions, which meant SWA and SR were equivalent under the same conditions of the ionic strength and pH.

[FIGURE 10 OMITTED]

[FIGURE 11 OMITTED]

CONCLUSIONS

This work focused on the preparation of the superab-sorbent containing the monomer of APPDC (SAP1) and the comparative investigation of swelling properties of SAP1, SAP2, and SAP3 under the same conditions. The results showed that the introduction of l-(acryloyloxy) propan-2-yl phosphate could increase the water absorbency effectively. The water absorbency of SAP2 was almost independent with pH of swelling medium, whereas SAP1 and SAP3 were obviously affected by pH. Cations, multivalent cations in particular, have significant and different effect on SWA of various SAPs. However, anions showed very little effect on the process of the swelling and SWA of all SAPs. The above-mentioned results could be explained by the chelation. The order of the swelling rate was SAP2 > SAP1 > SAP3, which was not relevant to the swelling environment but to the essence of SAPs.

ABBREVIATIONS
AA acrylic acid
ACE acenaphthylene
APDMP l-(acryloyloxy) propan-2-yl dimethyl phosphate
APPDC l-(acryloyloxy) propan-2-yl phosphoryl dichloride
EGDE ethylene glycol diglycidyl ether
[Et.sub.2]O ethylether
HPA 2-hydroxypropyl acrylate
PSA poly(sodium acrylate)
P(SA-co-HPA) poly(sodium acrylate-co-2-hydroxypropyl acrylate
P(SA-co-SAPP) poly(sodium acrylate-co-sodium l-(acryloyloxy) propan-2-
 yl phosphate)
SAPs superabsorbents
SAPP sodium l-(acryloyloxy) propan-2-yl phosphate
TEA triethylamine
THF tetrahydrofuran
Vc L-ascorbic acid


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Zhenbin Chen, (1,2) Mingzhu Liu, (1) Xiaohua Qi, (1) Zhen Liu (1)

(1) Department of Chemistry and State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, People's Republic of China

(2) Department of Chemistry, Hexi University, Zhangye 734000, People's Republic of China

Correspondence to: Mingzhu Liu; e-mail: m-zliu@163.com

Contract grant sponsor: Special Doctorial Program Funds of the Ministry of Education of China; contract grant number: 20030730013.
TABLE 1. The effect of cation types existed in swelling medium on SWA.

 SWA (g [g.sup.-1])
Cation types SAP1 SAP2 SAP3

[Na.sup.+] 71 49 91
[K.sup.+] 76 52 54
N[H.sub.4.sup.+] 63 54 77
[Cu.sup.2+] 13 9 10
[Ba.sup.2+] 7 4 12
[Ca.sup.2+] 14 3 13
[Zn.sup.2+] 14 12 12
[Mg.sup.2+] 24 4 13
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Author:Chen, Zhenbin; Liu, Mingzhu; Qi, Xiaohua; Liu, Zhen
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Date:May 1, 2007
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