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Free-radical polymerization of urea, acrylic acid, and glycerol in aqueous solutions.

INTRODUCTION

Urea is the most commonly used fertilizer in agriculture and world production and consumption of urea has increased steadily in recent years. Urea is characterized by its high nitrogen content (46 wt%), low production cost and its high solubility in water. As well as this, urea is noncorrosive and can be easily mixed with other compounds [1-3], The main problem associated with the use of urea as a fertilizer is the high rate of loss to the environment through leaching and volatilization [3-5], Losses can reach 50% of the applied urea fertilizer depending on the climate, soil conditions, and on the application technologies, causing environmental pollution and increasing the costs of crop production [6-10]. A possible alternative to reduce nutrient losses is the development of slow release fertilizers by coating urea granules with materials that present lower water solubilities [5, 9, 10] or by using materials that allow for the slow release of urea [8, 11], The slow release of nitrogen from urea using polymer coatings is promising [7, 9, 12].

Poly(acrylic acid), PAA, is a synthetic polymer that can be obtained through the free radical polymerization of acrylic acid in an aqueous medium. The presence of carboxylic groups in PAA chains confers a polyanionic character to the macromolecule, allowing for ion exchanging and complexation with positively charged ions [13]. PAA presents a number of comparative advantages in various applications including its low cost, biodegradability, biocompatibility, and good water solubility. Because of these properties, PAA is commonly used as a dispersant, emulsifier, or suspending agent in heterogeneous systems and it is used for the formulation of dental cements, pharmaceuticals, cosmetics, paints, and fertilizers [14-17], Furthermore, PAA-based resins frequently present a high capacity for water absorption and water retention and are used for the production of superabsorbent and coatings for slow release fertilizers [18-20].

Glycerol is a polyalcohol that contains three hydroxyl groups. It is used in different applications in many distinct fields including the food industry, medical, and pharmaceutical areas and the textile industry [21-23], In particular, glycerol can act as a chain transfer agent and/or a reticulation agent in free radical and functional polymerizations, leading to the formation of chain branches and increasing the molecular weight of the final product [21, 22], Glycerol is usually obtained through saponification of triglycerides and is a major by-product of the biodiesel production process [23].

Although copolymers of acrylic acid and urea have been little studied, these materials have been described in previous studies. Wang [24] described the production of a crystalline copolymer of acrylic acid and urea prepared by mixing PAA with urea (1:10 in weight) in an aqueous medium at mild temperatures. The product presented low solubility in water and proved to be useful for treatment of uremic animals and for the slow release of urea. More recently, Eritsyan et al. [25] studied the rheological characteristics of aqueous solutions of copolymers of acrylic acid and urea prepared with a similar process. These authors studied some mechanistic aspects of the reaction in dilute aqueous solutions and showed that the produced copolymers were branched and poorly soluble in common organic solvents. However, these studies did not analyze the production of acrylic acid/urea copolymers through the direct reaction of acrylic acid with urea in presence of other chemical constituents, such as glycerol. Therefore, based on these previous studies, it is not possible to infer that the polymerization of acrylic acid can be performed efficiently in the presence of urea and that the resulting product can be useful for subsequent agricultural applications.

Therefore the main objective of the present study was the production of copolymers of urea, acrylic acid, and glycerol through aqueous solution free-radical polymerizations and the analysis of some of the properties of the obtained materials. To do this, aqueous solution copolymerizations were performed in varying reaction conditions and the obtained copolymers were characterized by Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), rheological analysis, thermogravimetric analysis (TGA), gel permeation chromatography (GPC), and rates of urea release in distilled water. The obtained results indicated that all the constituents of the reacting mixture (potassium persulfate, glycerol, urea, and acrylic acid) affect the characteristics of the produced copolymers and that urea can be incorporated into the final copolymer through the proposed reaction scheme.

EXPERIMENTAL

Materials

Urea (CO[(N[H.sub.2]).sub.2], minimum purity of 99.5%, 46.4 wt% of nitrogen, [M.sub.w] = 60.07 g mol ') was provided in the form of granules (average particle size of 1.84 mm) by PETROBRAS (Camacari, Brazil). Acrylic acid ([C.sub.3] [H.sub.4] [O.sub.2], minimum purity of 99.0%, [M.sub.w] = 12.02 g [mol.sup.-1]), glycerol ([C.sub.3] [H.sub.8] [O.sub.3], minimum purity of 99.5%, [M.sub.w] = 92.1 g [mo.sup.-1]), potassium persulfate ([K.sub.2] [S.sub.2] [O.sub.8], minimum purity of 99.0%, [M.sub.w] = 270.32 g [mol.sup.-1]), potassium phosphate (K[H.sub.2] P[O.sub.4], minimum purity of 99.0%, [M.sub.w] = 136.09 g [mol.sup.-1]) and sodium hydroxide (NaOH, minimum purity of 99.0%, [M.sub.w] = 39.99 g [mol.sup.-1]) were acquired from VETEC (Duque de Caxias, Brazil). The enzyme urease and other reagents used to hydrolyze urea and determine the urea content of the analyzed samples were provided by DOLES (Goiania, Brazil) in the enzymatic kit "Urea 500." All the reagents and solvents used for polymer characterization were purchased as analytical grades from VETEC (Duque de Caxias, Brazil). All the chemicals were used as received.

Copolymerization Reactions

Solution copolymerization reactions were carried out in distilled water using different feed compositions, as described in Table 1. In all cases, the total mass of the reacting medium was equal to 90 g, obtained after adding 30 g of an aqueous solution of potassium persulfate (initiator) to 60 g of an aqueous solution of the remaining reagents with the desired compositions. Reactions were performed in jacketed glass flasks of 200 ml equipped with a temperature controller, an automatic agitator and a condenser. The stirring speed was kept equal to 300 rpm in all reactions, while the reaction temperature was maintained at 80[degrees]C and the reaction time was always equal to 2 h. Copolymerization conditions were determined in accordance with Wang (1974), Spychaj (1989), Eritsyan et al. (2006) and Pinto et al. (2012) [16, 24-26]. Although the initial solutions were always homogeneous, it is important to highlight that the final product was biphasic, containing a lighter and less viscous liquid phase and a heavier viscous liquid phase, which contained the copolymer. The two phases could be easily separated from each other through decantation after turning off of the stirrer. Both phases were dried in vacuum oven at 50[degrees]C until constant weight, but the solids content of the lighter phase was negligible and the residual solid material did not indicate the presence of polymer. The dry solid product of the heavier liquid phase was used for characterization. The heavier liquid phase was also characterized through rheological analyses prior to drying.

Fourier Transform Infrared Spectroscopy (FTIR)

Copolymer samples were analyzed by FTIR with a Mid-IR FTIR Spectrophotometer (model IRPrestige-21, manufactured by Shimadzu) in the wavelength range of 4000 to 600 [cm.sup.-1] in order to verify the chemical composition of the polymer chains and evaluate the incorporation of the comonomers in the final products. Analyses were performed with spectroscopic grade KBr tablets at 16 [+ or -] 1[degrees]C in the ATR (Attenuated Total Reflectance) mode, accumulating 50 readings with a resolution of 2 [cm.sup.-1]. Background signals were collected with pure KBr tablets.

Differential Scanning Calorimetry (DSC)

DSC analyses were performed with a calorimeter (model DSC50, manufactured by Shimadzu) to determine the characteristic transition temperatures of the produced copolymers. Analyses were carried out under an inert nitrogen atmosphere with samples of 10 mg in sealed aluminum capsules in the temperature range from -20 to 300[degrees]C, using heating rates of 10[degrees]C/min. As usual, the characteristic transition temperatures were recorded during the second heating scan in order to erase the thermal history of the sample. The first heating scan was performed until 100[degrees]C to prevent the degradation of the material.

Rheological Analysis

The rheological behavior of polymer solutions was analyzed for samples withdrawn directly from the heavier liquid fraction obtained at the end of the reaction run and prior to drying in the vacuum oven. Rheological analyses were performed with a parallel-plate rheometer (Rheometer AR G2, manufactured by TA Instruments) assisted by the software Rheology Advantage Instrument Control AR. The diameter of the plates was equal to 60 mm and the plate spacing was equal to 0.050 mm. Characterizations were performed at room temperature under compressed air atmosphere, with shear rates ranging from 0.01 to 500 [s.sup.-1].

Thermogravimetric Analysis (TGA)

TGA thermograms were obtained with the help of a thermogravimetric analyzer (model STA 6000, manufactured by Perkin Elmer), assisted by Pyris software. Analyses were performed under an inert nitrogen atmosphere (20 ml/min) using dried polymer samples and heating rates of 10[degrees]C/min in the temperature range from 35 to 550[degrees]C.

Gel Permeation Chromatography (GPC)

Analyses of gel permeation chromatography were performed with dried polymer samples dissolved in an aqueous solution (2 mg/ml) that also contained potassium phosphate (1 mg/l) and sodium hydroxide (used to adjust the pH to 7.2), used as the mobile phase. Analyses were carried out at 40[degrees]C, flow rate of 0.5 ml/min and pressure of 250 psi. Polymer solutions were filtrated with Teflon membranes with pores of 0.45 pm, using injection volumes of 200 [micro]l. GPC analyses were carried out on a Phenomenex Chromatograph, model TS-430, equipped with 3 Shodex OH-PAK columns and a Viscotek refractometric detector, model VE3580. The equipment was calibrated with linear polystyrene sulfonate standards with average molecular weights ranging from 31 x [10.sup.3] to 2260 x [10.sup.3] g/gmol. As the analysis copolymer is a branched and ionic terpolymer, the average molecular weights are only semi-quantitative.

Urea Release in Water

Release analyses were performed to determine the urea release profiles of the produced copolymers in distilled water and observe whether urea might be present in the free state or incorporated into the copolymer chains. Masses of copolymer samples were defined in order to provide 0.80 mg of urea/ml of water. Tests were always performed with 25 ml of distilled water at 30[degrees]C. The aqueous solution containing the copolymer sample was kept under magnetic stirring at 100 rpm. Samples of 10 pi of the copolymer solution were withdrawn for analysis after time intervals of 1 min, 5 min, 10 min, 20 min, 30 min, 60 min, and 90 min. The system volume was kept constant through the addition of distilled water immediately after sampling. The kit "Urea 500" allowed for the enzymatic determination of the urea concentration in accordance with Eq. 1. Bernhard et al. (2012) and Todorova et al. (2010) [27, 28] showed that ammonia could also be formed by the hydrolysis of urea in the presence of Ti[O.sub.2]. The ammonia concentrations (and, therefore, the urea concentrations) were obtained at room temperature with help of a calibration model, using a UV--Visible spectrophotometer (Model Lambda 35, manufactured by Perkin Elmer) in the wavelength range of 570 to 720 nm.

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RESULTS AND DISCUSSION

Fourier Transform Infrared Spectroscopy (FTIR)

Figure 1 shows the FTIR spectra of the obtained products. Figure la summarizes the FTIR spectra of the comonomers. It can be noticed that urea has peaks at 3430-3337 [cm.sup.-1] corresponding to the symmetric stretching of N-H. The peak at 1675 [cm.sup.-1] corresponds to the elongation of the double bond C=0; the peak at 1590 [cm.sup.-1] is related to the stretching of the bonds in NH or N-[H.sub.2]; the peak at 1462 [cm.sup.-1] corresponds to the shortening of the C-N bond; the peak at 1150 [cm.sup.-1] is due to the symmetric stretching of N-H; and the peak at 715 [cm.sup.-1] corresponds to the bending of the N-H bond. These results have been described in previous studies [29, 30].

Acrylic acid presents a wide band at 2988 [cm.sup.-1] that corresponds to the stretching of the hydroxyl O-H bond. The peak at 1699 [cm.sup.-1] corresponds to the stretching of the C=0 double bond; the peak at 1615 [cm.sup.-1] corresponds to the stretching of the C=C double bond; the peak at 1433 [cm.sup.-1] corresponds to the stretching of the C[H.sub.2] bonds; the peaks at 1296 and 1240 [cm.sup.-1] correspond to the vibrations of the OH bond in phase and out of phase, respectively; the peaks at 1045 and 982 [cm.sup.-1] correspond to vibrations of the C[H.sub.2] bonds in-phase and in torsion, respectively; the peaks at 925 and 863 [cm.sup.-1] also represent the vibrations of the OH bond in phase and out of phase, respectively; the peak at 816 [cm.sup.-1] corresponds to the stretching of the CH bond; and the peak at 649 [cm.sup.-1] corresponds to the deformation out of phase of the C[O.sub.2] bond. These results have been described in previous studies [31].

The main wide band of glycerol is located at 3310 [cm.sup.-1] and is related to the stretching of the OH group. The peaks at 2940, 2880, and 1410 [cm.sup.-1] represent the vibrations of the CH bond. The peaks at 1109 and 1033 [cm.sup.-1] correspond to the stretching of the CO bond. These results have been described in previous studies [32-35],

Figure 1b shows the FTIR spectra of the copolymers developed in the absence of at least one of the comonomers. The copolymer of acrylic acid with urea (1/1/1/0) presented an FTIR spectrum that was very similar to the one obtained and reported by Eritsyan et al. [25]. According to these authors, the most expected reaction between acrylic acid and urea is the formation of a salt as the result of the interaction between the carboxylic group of AA with the amino group of urea. This reaction can occur through both intramolecular and intermolecular interactions, as shown in Eq. 2.

[FORMULA NOT REPRODUCIBLE IN ASCII] (2)

According to Eritsyan et al. [25], Eq. 2 can explain the peak positioned at 1622 [cm.sup.-1], related to the interaction between N[H.sub.3.sup.+] and HCO[O.sup.-] groups. The peak placed at 1704 [cm.sup.-1] suggests the existence of the amide CONHR group, which can also be formed through reaction of -COOH and -N[H.sub.2], as shown in Eq. 3 [25]:

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Eritsyan et al. [25] also observed and reported the existence of peaks located in the regions of 1200 to 1280 [cm.sup.-1] (-C-O-C-) and of 1710 to 1730 [cm.sup.-1] (COOH), which could not be observed in the present work. However, it is important to mention that the peaks located at 3413, 2955, 1718, 1450, 1417, 1248, and 1174 [cm.sup.-1] are characteristic of PAA samples [16, 36], while the peaks at 1622 and 1704 [cm.sup.-1] indicate the presence of urea in the solid material, suggesting the incorporation of urea into the copolymer structure.

The analysis of the polymer sample obtained in presence of acrylic acid and glycerol (1/0/1/1) indicated the formation of typical PAA structures, as the characteristic broad glycerol hydroxyl band at 3310 [cm.sup.-1] could not be detected. The band positioned at 1700 [cm.sup.-1] is related to the stretching of the C=0 bond, while the bands located at 1162 and 1045 [cm.sup.-1] are related to the asymmetric and symmetric stretching of the C-O-C bond, respectively. Additionally, as already discussed, the peaks at 1450 and 1398 [cm.sup.-1] are characteristic of PA A.

According to Morita [37], the occurrence of esterification reactions between acrylic acid and glycerol molecules can produce a mixture of monoacylglycerol, diacylglycerol, and triacylglycerol, depending on the number of hydroxyl groups of glycerol involved in the reaction, as illustrated in Eq. 4.

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This can explain the disappearance of typical hydroxyl peaks from the FT1R spectra of polymer samples, although FTIR analyses can also indicate that glycerol molecules were not incorporated in significant amounts into the copolymer structure. It is important to mention, though, that Pinto et al. [16] detected the typical glycerol hydroxyl bands at PAA samples prepared in presence of glycerol, which can indicate that the reaction conditions can significantly affect the copolymer composition and molecular structure of PAA-based resins prepared in the presence of glycerol.

The analysis of the solid material obtained when urea and glycerol (1/1/0/1) were submitted to the reaction conditions revealed the typical FTIR spectrum of urea. However, three additional bands could also be detected at 1102, 1052, and 787 [cm.sup.-1], which characterize the CO bond of glycerol. Reactions between glycerol and urea have already been studied in the open literature [38-40], with the identification of glycerol carbonate and ammonia as products, as shown in Eq. 5.

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The formation of significant amounts of glycerol carbonate should not be expected in the present case, as the characteristic NH bond peaks positioned at 3431 and 3340 [cm.sup.-1] could be observed. Besides, ammonia could not be detected in the FTIR spectra of the reacting mixture (and was not present in the gaseous effluent). Moreover, the FTIR spectrum of glycerol carbonate has been studied in detail and its characteristic peaks located at 1791, 1713, 1186, and 778 [cm.sup.-1] [40] could not be detected in the FTIR spectrum of the product (1/1/0/1). For these reasons, the characteristic glycerol peaks observed in the solid product probably indicate the contamination of the urea powder with glycerol, also indicating that urea and glycerol do not react at significant rates at the analyzed reaction conditions.

Figure 1c shows the FTIR spectra of copolymer samples prepared with different amounts of initiator. It can be observed that the spectra are very similar, although the relative intensity of certain peaks changed, possibly indicating changes in the copolymer composition. The largest observed shift was related to the band at 1047 [cm.sup.-1], a characteristic glycerol band which can indicate the incorporation of different amounts of glycerol and different degrees of crosslinking, induced by the incorporation of glycerol. However, as already observed, glycerol was not incorporated in large amounts as the FTIR spectra of copolymers produced in the presence of all recipe constituents (1/1/1/1) were very similar to the ones prepared in absence of glycerol (1/1/1/0).

Figure 1d shows the FTIR spectra of copolymers prepared in the presence of different amounts of glycerol. It can be observed that the obtained spectra were very similar, although some changes in the peak at 1398 and 1232 [cm.sup.-1] could be detected. These changes could indicate the modification of the COC bond concentration due to the incorporation of small amounts of glycerol through a chain transfer reaction to glycerol. The peak at 1705 [cm.sup.-1] also became more intense in presence of larger glycerol concentrations, suggesting the formation of CONHR groups due to the chemical interaction of acrylic acid or glycerol with urea.

Figure 1e shows the FTIR spectra of copolymer samples prepared with different urea/acrylic acid molar ratios. It can be observed that the FTIR spectra of the obtained copolymers are similar, although the relative intensities of the different peaks change with the feed composition, as expected. Therefore, modifications to the feed composition affected the composition of the solid material obtained.

Differential Scanning Calorimetry (DSC)

Figure 2 shows DSC curves for some of the analyzed materials. It can be noticed that all the analyses revealed the occurrence of endothermic events that correspond mainly to melting and degradation of the analyzed material, although secondary events related to sublimation, vaporization, condensation, and degradation could also be detected, as discussed below.

Figure 2a shows the DSC curves for urea and some of the copolymers. The urea curve presents three endothermic events related to the formation of biuret, formation of cyanuric acid and product decomposition [41, 42]. Stradella and Argentero [43] observed that DSC and TG curves of urea presented similar patterns, although they were able to observe a fourth endothermic event in the DSC curve when the solid material was heated up to 300[degrees]C. The copolymer material prepared in the absence of urea (1/0/1/1) presented the highest thermal stability and a single DSC endothermic event above 250[degrees]C, which characterizes the decomposition of poly(acrylic acid). Therefore, the DSC curves of the copolymers prepared in the presence of urea were strongly affected by this chemical. The solid material prepared in the absence of acrylic acid (1/1/0/1) also showed a single endothermic thermal event during the DSC analysis, although product decomposition began at 50[degrees]C, indicating that urea and glycerol can lead to the formation of mixtures that are more unstable than urea and that urea and glycerol react in the analyzed reaction conditions, as the FTIR spectra of the solid product also indicate. The DSC curves of the obtained copolymers (1/1/1/0) and (1/1/1/1) revealed two similar and well-defined endothermic thermal events indicating that the reaction between urea and acrylic acid strongly influences the characteristics of the final product. This is reinforced by the fact that the FTIR spectra of these copolymers were also very similar, as already discussed.

Figure 2b shows the DSC curves for copolymers prepared in the presence of different amounts of initiator. It can be observed that the obtained thermal profiles were quite different, although constrained to the same temperature range of 120-250[degrees]C. The DSC curve of copolymer (1/1/1/1) presented two well-defined endothermic thermal events, while the DSC curve of copolymer (0.33/1/1/1), prepared with a smaller amount of initiator, presented several endothermic peaks between 130 and 175[degrees]C, possibly indicating the occurrence of vaporization, sublimation, condensation, and degradation of unreacted material or byproducts. On the other hand, the DSC curve of the copolymer prepared with a higher amount of initiator (2/1/1/1) showed four distinct endothermal transitions, clearly indicating that the initiator concentration affects the copolymer structure and that the copolymer structure (composition and average molecular weight) can affect the thermal stability of the obtained copolymer.

Figure 2c shows DSC curves of copolymers prepared in the presence of different amounts of glycerol. It can be observed that all the DSC curves were very similar, although the characteristic thermal events begin at different temperatures. It must be pointed out that the DSC curve of the copolymer prepared in the absence of glycerol (1/1/1/0) revealed the occurrence of higher heat transitions and the existence of a characteristic glass transition temperature before the polymer melted. The glass transition temperature could not be detected for copolymer materials prepared in the presence of glycerol, as expected for crosslinked polymer products and observed previously for PAA produced in the presence of glycerol [16]. Therefore, although glycerol is not incorporated in large amounts during polymerization, it seems clear that the presence of glycerol does affect the copolymer structure and the thermal properties of the obtained material.

Figure 2d shows the DSC curves for copolymers prepared with different urea/acrylic acid molar ratios. As observed previously, the DSC curves were very similar for all copolymers, showing two well-defined endothermic thermal events and showing that any modifications to the thermal properties were not related to a modification in the final urea content of the copolymer, but to a modification in the copolymer structure. However, as one might expect, the relative intensities of the distinct thermal transitions change with the urea content of the obtained material. In particular, thermal stability increased with the acrylic acid content of the copolymer.

Rheological Analysis

Figure 3 shows the rheological behavior of aqueous solutions prepared with the obtained copolymers. It can be observed that the rheological behavior of the solutions were very sensitive to modifications in the concentrations of all the chemical constituents, demonstrating that the analyzed reaction conditions significantly affected the properties of the final material. The solutions showed a characteristic non-Newtonian pseudoplastic behavior, with a decrease in the apparent solution viscosity with an increase in the shear rate.

It is important to mention that the viscosity of some of the prepared solutions were so high that it was not possible to perform reliable rheological analyses for these samples. These solutions were obtained with 2% of initiator (2/1/1/1) and in the absence of glycerol (1/1/1/0). The second result reinforces the idea that glycerol acts as a chain transfer agent, leading to the production of polymer materials with lower average molecular weights. However, as the solution prepared with 2% glycerol (1/ 1/1/2) presented higher viscosity than the solution prepared with 1% glycerol (1/1/1/1), it seems clear that glycerol also promotes chain growth and crosslinking, as glycerol is a multifunctional chain transfer agent. This probably explains the first result, as the viscosity of the reaction medium increases with the increase in the initiator concentration in the presence of glycerol.

The solution prepared in the absence of acrylic acid (1/1/0/1) presented the lowest viscosity, as the radical polymerization reaction did not take place. In the absence of urea (1/0/1/1) the viscosity of the solution was also low, reinforcing the chain transfer role of glycerol and showing that urea promotes cross-linking of polymer chains through a reaction between the carboxylic groups of the acrylic acid and the amino groups of urea. It must also be observed that the viscosity of the solution increases with the increase in acrylic acid in the feed due to the increasing amounts of polymer produced by the radical mechanism.

Thermogravimetric Analysis (TGA)

Figure 4 shows the TGA curves for the analyzed materials. It can be observed that the obtained copolymers presented two distinct degradation steps: the first step in the range from 100 to 250[degrees]C and the second step in the range from 300 to 450[degrees]C. This TGA behavior was similar to the TGA behavior of urea, indicating the incorporation of urea into the final copolymer structure. The main difference among the TGA curves was related to the mass losses observed at each degradation step, which can be related to the urea content of the analyzed material.

Figure 4a shows that the TGA trace of glycerol presents a single and well defined range of mass loss between 150 and 280[degrees]C, which can be explained in terms of vaporization and degradation over the hot metal surface, as also observed in the Refs. [34, 44], The TGA trace of acrylic acid presented two distinct stages: the first is related to acrylic acid vaporization and the second is related to the degradation of the PAA formed by spontaneous thermal polymerization over the hot metal surface. The TGA trace of urea presented three stages as well documented in the open Refs. [41-43, 45, 46]. The first stage, between 135 and 250[degrees]C, it is associated with urea vaporization and urea degradation, which leads mainly to production of hydrocyanic acid and biuret NH[(CO).sub.2] [(N[H.sub.2]).sub.2], among many other degradation products such as cyanuric acid [(CNOH).sub.3] and 6-amino-2,4-dihydroxy-1,3,5-triazine, [(CN).sub.3] [(OH).sub.2]N [H.sub.2]. The second stage, between 250 and 360[degrees]C, is related to sublimation and secondary decomposition of urea degradation products. Finally, the third stage, above 450[degrees]C, is related to the final decomposition and elimination of carbonaceous products.

In the absence of acrylic acid (1/1/0/1), the TGA curve of the obtained material was very similar to the TGA trace of pure urea, indicating once more that the chemical interaction between urea and glycerol was weak at best. In the absence of glycerol (1/1/1/0), the TGA curve of the obtained material was very similar to the TGA curves of copolymer samples, as might be expected. In the absence of urea (1/0/1/1), the TGA curves indicated the attainment of the highest thermal stability, showing that the incorporation of urea changed the characteristic TGA behavior of the PAA sample significantly.

When the amount of initiator was varied (Fig. 4c), the TGA curves became very similar, indicating that the initiator exerted a secondary effect on the thermal stability of obtained products, as thermal stability seemed to be related to the urea content of the copolymer. When the amount of glycerol was varied (Fig. 4d), the obtained TGA curves became sensitive to the initial feed composition, indicating that glycerol exerted a positive effect on the thermal stability, probably due to crosslinking induced by this multifunctional chain transfer agent. This positive glycerol effect on thermal stability of PAA-based resins has also been observed by other authors [16]. As expected, variation of the urea/acrylic acid feed ratio (Fig. 4e) showed that the increase in the acrylic acid content caused the increase in the thermal stability of the produced copolymer.

Gel Permeation Chromatography (GPC)

Table 2 shows the weight-average molecular weights ([M.sub.w]) and polydispersities (PDI) of the produced copolymers. It can be observed that the copolymer produced in the absence of urea (1/0/1/1) presented relatively low [M.sub.w] due to chain transfer to glycerol. The increase in the glycerol composition led to a significant increase in [M.sub.w] values due to crosslinking induced by the multifunctional character of glycerol. Samples prepared without acrylic acid (1/1/0/1) did not provide significant [M.sub.w] values because the polymer product was not formed. In the presence of urea, [M.sub.w] values were not very sensitive to modification of the initial glycerol composition, probably because the reaction with urea constitutes the main route for chain crosslinking. The increase in [M.sub.w] with the initial urea content seemed to confirm this effect. Finally, the copolymer produced with the smallest initiator composition (0.33/1/1/1) presented the highest [M.sub.w] value, as expected in conventional radical reactions.

Urea Release Profile in Distilled Water

Figure 5 shows the urea release profiles of the analyzed materials. It can be observed in Fig. 5a that urea dissolved extremely fast in water, as expected. The material prepared in the absence of acrylic acid (1/1/0/1) led to urea release profiles that were similar to the ones observed for pure urea, as expected. However, the observed rates of urea release were smaller in this case, indicating once more that glycerol and urea interacted to some extent. The copolymer prepared with urea and acrylic acid (1/1/1/0) produced much slower rates of urea release, leading to complete urea release after 30 min in water. This is a very important experimental observation, as the chemical bonds that are formed between acrylic acid and urea molecules can apparently be destroyed in water through hydrolysis, allowing the recovery of the original urea content of the reacting mixture. When glycerol was added to the reacting mixture, the obtained copolymer (l/l/l/l) produced even slower rates of urea release, meaning that reticulation induced by glycerol can somehow encapsulate urea molecules and hampering the release of urea. In the absence of urea, rates of urea release from the obtained product (1/0/1/1) were null, indicating the appropriateness of the proposed experimental technique.

Figure 5b shows that rates of urea release from products prepared with different amounts of initiator were also different, although always much slower than rates of urea release obtained for urea granules. The slowest rates of urea release were obtained for the product prepared with the highest amount of initiator (2/1/1/1) due to the extremely large [M.sub.w] values of the sample, caused by chain crosslinking. Despite this, the rates of urea release were also very small for copolymer samples prepared with low initiator compositions (0.33/1/1/1), probably because of the high [M.sub.w] value of the obtained material. These results indicate that the rate of urea release can probably be modulated by the [M.sub.w] value of the obtained copolymer sample, for similar urea contents, as urea molecules can also provoke chain crosslinking through a reaction with the carboxylic groups of the acrylic acid monomer. When the feed glycerol composition was changed, Fig. 5c shows that the rates of urea release decreased with the glycerol composition, reinforcing our hypothesis. It is also interesting to observe that the rates of urea release decreased with the increase in the initial acrylic acid content, as shown in Fig. 5d, which can be explained in terms of the most effective reaction between the carboxylic groups of acrylic acid and the amino groups of urea.

On the basis of the obtained results, we suggest that urea can be released from the reaction products in two ways:

--hydrolysis of the bond CONHR formed between urea and acrylic acid units.

--diffusion of unreacted urea molecules from the PAA-based copolymer material prepared through the proposed reaction scheme.

In both cases, the weight-average molecular weights and reticulated nature of the copolymer chains can significantly affect the rates of urea release in water, which can be beneficial for a large number of applications in the field of agriculture.

Reaction Product

On the basis of all the results presented previously, a product structure and a mechanistic reaction route can be described as follows. It is assumed that acrylic acid polymerizes through the well-known radical mechanism and that glycerol acts as a multifunctional chain transfer agent, allowing for chain crosslinking when its composition is sufficiently high. The amino groups of urea can also react with the carboxylic groups of acrylic acid, leading to chain crosslinking and the incorporation of urea into the copolymer structure. The CONH bonds can be hydrolyzed in water, releasing urea molecules at slower release rates. Therefore, the schematic representation of the obtained product can be represented as:

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CONCLUSION

Copolymers of urea, acrylic acid, and glycerol were prepared through aqueous solution free-radical polymerizations in different reaction conditions and the obtained copolymers were characterized by Fourier transform infrared spectroscopy, differential scanning calorimetry, rheological analyses, thermogravimetry, gel permeation chromatography, and rates of urea release in distilled water. The obtained results indicated that all the constituents of the reacting mixture (potassium persulfate, glycerol, urea, and acrylic acid) affected the characteristics of the produced copolymers. More specifically, it was shown that urea can be incorporated into the final copolymer through the proposed reaction scheme, that glycerol promotes crosslinking of polymer chains and that the obtained copolymer materials can be used in agricultural applications.

More specifically, it was shown that glycerol can act as a multifunctional chain transfer agent, allowing for chain crosslinking when its composition is sufficiently high. Furthermore, it was also shown that the amino groups of urea probably react with the carboxylic groups of acrylic acid, leading to chain crosslinking and the incorporation of urea into the copolymer structure. The urea molecules can be released slowly from the obtained copolymer materials in water through a combination of hydrolysis of the formed CONH bonds and diffusion through the polymer mass, so that the rates of urea release are sensitive to the copolymer composition, structure, and average molecular weights.

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Bruno S. Fernandes, (1) Jose Carlos Pinto, (2) Elaine C.M. Cabral-Albuquerque, (1) Rosana L. Fialho (1)

(1) Programa de Pos-Graduaqao em Engenharia Industrial, Escola Politecnica, Universidade Federal da Bahia, Rua Aristides Novis, no. 02, Federacao, Salvador, 40.210-630 BA, Brazil

(2) Programa de Engenharia Quimica/COPPE, Universidade Federal do Rio de Janeiro, Cidade Universitaria, CP: 68502, Rio de Janeiro, 21.941-972 RJ, Brazil

Correspondence to: Rosana Lopes Lima Fialho; e-mail: rosanafialho@ufba.br Contract grant sponsors: CAPES (Coordenacao de Aperfeicoamento de Pessoal de Nivel Superior), CNPq (Conselho Nacional de Pesquisa e Desenvolvimento Tecnologico), FAPERJ (Fundacao Carlos Chagas Filho de Apoio a Pesquisa do Estado do Rio de Janeiro).

DOI 10.1002/pen.24081

Published online in Wiley Online Library (wileyonlinelibrary.com).

TABLE 1. Feed compositions of the copolymerization reactions.

Reaction runs
[persulfate/
urea/            Persulfate          Urea/AA           Glycerol
AA/glyceroll     {% weight)       (molar ratio)       (% weight)

1/0/1/1         1 (0.90 g)     0/1 (0.00 g/19.10 g)   1 (0.90 g)
1/1/0/1         1 (0.90 g)     1/0 (19.10 g/0.00 g)   1 (0.90 g)
1/1/1/0         1 (0.90 g)     1/1 (9.09 g/10.91 g)   0 (0.00 g)
1/1/1/1         1 (0.90 g)     1/1 (8.68 g/10.42 g)   1 (0.90 g)
1/1/1/2         1 (0.90 g)     1/1 (8.27 g/9.93 g)    2 (1.80 g)
1/2/1/1         1 (0.90 g)     2/1 (11.94 g/7.16 g)   1 (0.90 g)
1/1/2/1         1 (0.90 g)     1/2 (5.62 g/13.48 g)   1 (0.90 g)
2/1/1/1         2 (1.80 g)     1/1 (8.68 g/10.42 g)   1 (0.90 g)
0.33/1/1/1      0.33 (0.3 g)   1/1 (8.68 g/10.42 g)   1 (0.90 g)

TABLE 2. Weight-average molecular weights ([M.sub.w]) and
polydispersities (PDI) of obtained materials.

Copolymers [persulfate/
urea/AA/glycerol]         [M.sub.w] (kDa)    PDI

1/0/1/1                       57.914        1.387
1/1/0/1                          0            0
1/1/1/0                       435.451       3.845
1/1/1/1                       548.021       3.619
1/1/1/2                       464.459       2.725
1/2/1/1                       678.369       3.520
1/1/2/1                       284.754       2.059
2/1/1/1                       545.521       3.580
0.33/1/1/1                    888.568       3.379
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Author:Fernandes, Bruno S.; Pinto, Jose Carlos; Cabral-Albuquerque, Elaine C.M.; Fialho, Rosana L.
Publication:Polymer Engineering and Science
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Date:Jun 1, 2015
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