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versión On-line ISSN 0717-9707
J. Chil. Chem. Soc. v.53 n.1 Concepción mar. 2008
J. Chil. Chem. Soc, 53, N° 1 (2008)
DEGRADATION OF ACRYLIC ACID-GRAFTED CELLULOSE IN AQUEOUS MEDIUM WITH RADICAL INITIATORS
AÍDA NEIRA1, MIGUEL TARRAGA2 AND ROSA CATALAN2
1Facultad de Ciencias, Universidad del Bío-Bío, Avda. Callao 1202, Concepción, Chile, e-mail: email@example.com
2Facultad de Ciencias Químicas, Universidad de Concepción, Casilla 160-C, Concepción, Chile
The chemical modification of cellulose by grafting reaction with Acrylic Acid (AA) considering different reaction factors was investigated. Natural polymers such as Santa Fe Cellulose, Microcrystalline Cellulose (Avicell) and Secondary Pulp were studied as substrates. pH effect; Ce (IV), Mn(VII) and Fenton as initiators; time and temperature polymerization were investigated. Grafting reachs a maximum, 90 % graft, at pH 7. A medium to weak C=0 band at 1715 cm"1 for carboxylic group in FTIR spectroscopy is presented. Santa Fe and Avicel cellulose grafted whith AA at 17 hours are degraded. Secondary pulp with Fenton at 50-60 and 70 °C at 17 hours are degraded too. The degradation of polymers at long reaction times at 17 hours and 70°C were analized by FTIR spectroscopy and electronic microscopy analysis (SEM).
Keywords: Degradation temperature, Radical polymetization, characterization.
The chemical modification by grafting copolimerization of synthetic or natural polymers, pulp, and paper, to obtain new materials is of increasing interest in macromolecular science and technology [1-5]. It is an important technique to modify polymeric products with hydrophilic and hydrophobic monomers [6,7]. The graft copolimerization modificates the chemical and physical properties of polymers. Polymer degradation gives information about the product natures of polymer chemical oxidation [8-11]. Graft polymerization of insaturated monomers (vinyl monomers) onto cellulose, in water media, is one of the universal, effective and accessible methods of radical polymerization. A large quantity of articles about graft polymerization, properties and process kinetics are reviewed [5,12,13]. It is necessary to take into consideration metal ion interactions with monomers and water in the formation of corresponding radicals. Metal ions of variable valence such as Ce (IV), Fe(II), Mn(VII), Mn(III), V(V), Cr(VI) and Co(III) are investigated in the formation of free radicals. [14,15]. The ability to use Fenton-based systems in dark or light irradiation has been a topic of interest in the field of advanced oxidation technologies involving degradation of polulants. It has been found that illumination of the Fe2+-Fe3+-H202 system increases the degradation rate of many organic substances [16,17]. Photo -Fenton catalityc of an azo - dye, Orange II, has been studied. Prephotocatalytic degradation leads to the observed oxidation of three compounds considered.
To investigate cellulose structure and the mechanism of reaction and degradation , is useful improved_spectroscopy and Microscopy analysis. Rowan et al, have deal with the application of IR method to various aspects of cellulose chemistry . Liang et al considered the relationship between the IR spectra of polymers and those of the monomers of which they are composed. H.G.Higgins et al reports effect of crystallinity on the spectra of sugars, cellulose, influence of hydroxymethyl side chain on cellulose, deuterated cellulose I and II, glucose pentaacetate, effect of substitution in hydroxyl groups and other experiences. An actual graft application to copolimerization of herbicide - containing acrylic monomers conduces to introduce changes in the polymer structure and therefore the ability to control the herbicide - polymer ratio. The free radical copolimerization of AA with a vinyl ester derived of 2,4- dichlorophenoxy acetic acid (2,4- D), a growth regulator herbicidy is reported  Graft copolymerization continues as an important method to obtain polymers of Acrylic Acid onto Polypropylene(PP). The water conservation, water absorption rate and K + exchange capacity were determined on the grafted PP blown nonwovens with a 3- dimentional network. This structure is fabricated by ultrafine fibers, wich gives them small pore, brighporosity, good nitrations and absorption properties .
Some oxidants have been successfully usedto initiate graft copolimerization onto cellulosic substrate. Toyo Rayon Co  carried out graft copolimerization of acrylonitrile onto pre- treated rayon fibers using KMn04 - H2 S04 system as a catalyst. El-Rafie et al  used dimethyl anilina- Copper Sulphate (Cu2+) as an initiator system to graft polimerize methyl metacrylate onto viscose stable fibers. Acidic Potassium Permanganate is an efficient initiator for grafting the acrylic monomers onto starch, linters and dissolved wood pulp. Polymerization at room temperature gave copolymers with 98% grafting efficiency ( 2% homopolymer ) . If the polimerization medium is sufficiently acidic to
dissolve the Manganese dioxide produced by the interaction of the monomer with the permanganate, polymerization can be affected. In adition to suitable reducing agents, especially organic acids, such as, H2C204 oxalic acid, permanganate provides efficient redox systems. Some such pairs are : KMn04 / tartaric acid  , KMnO/ malic acid , KMn04 / H2C204 , KMnO /glyceric acid , Mn3+ / ethoxyaceticacid redox systems  were studied in the aqueous polymerization of AAm
Ceric ions are capable of forming radicals at the backbone of the cellulose in the presence of acid . The grafting of monomers mixture on cellulose is carried out in aqueous medium. Thus, ethylmethacrylate with antimicrobial properties is grafting onto cellulose with Ceric ammonium nitrate, CAN, in combination with Acrylonitrile with an alternate arrangement of monomer blocks in the grafted copolymer chain . The Ceric ion method has been used to graft Acrylic acid onto cellulose with a minimum amount of homopolymer. Mc Dowall, Gupta and Stannett  have reported yield up to 60% and less than 5% homopolymer. Vitta, Stahel and Stannett prepared a solution dissolving a known amount of CAN in 0.1 N nitric acid and mixing with deionised water. Copper powder was added to inhibit homopolymer formation.
Continuing with our grafting copolymerization studies , this paper reports grafting copolymerization and oxidative degradation by radical initiators of cellulose pulp, Santa Fe cellulose and microcrystalline cellulose with acrylic acid. Fe(II) / H202, Ce(IV), Mn(VII) are using as radical initiators at differents pHs, reaction time and reaction temperatures. Mn(II) and Cu(II) salts are using as catalyst systems.
Preparation of cellulose and acrylic acid solutions to graft: Acrylic acid was obtained from the Merck and purified by distilling twice under vacuum in the presence of copper to prevent polymerization. The reaction medium was first degassed by passing nitrogen through the aqueous solution for at least 30 minutes. An exact mass of cellulose fiber (secondary pulp, Santa Fe cellulose or Avicel) is agitated in the corresponding aqueous initiator: Fenton, Mn(VII) or Ce(IV) solutions. Nitrogen gas continued to be bubbled through the medium during the wole reaction period. After the reaction time was completed, the grafted cellulose product was removed and washed thoroughly to remove the soluble contents. Finally, the grafted polymer was extracted with the solvent appropriate for the homopolymer, methanol for polyacrylic acid. Twenty-four hours was found to lead to constant weights.
Preparation of secondary cellulose reacting with Fenton. The method and the reaction system is the same described for grafting cellulose with Acrylic acid and Fenton reagent.
Infrared Spectroscopy. Infrared spectra was obtained by a Nicolet Magna 550. The samples powdered were prepared in KBr pellets.
Electron Microscopy. The morphology of the polymers was studied with a SEM instrument, an AUTOSCAN U-1. The samples were treated with gold previously.
RESULTS AND DISCUSSION
The chemical modification of the grafted cellulose is affected by factors such as: Fe2+ concentration, H202 concentration, cellulose-comonomer relation, pH, temperature, and polymerization time. In Figures 1 and 2, it can been observed that the graft percentage for acrylic acid in the cellulose fiber increased with increased pH, reaching a maximum at pH 7. An abrupt diminishment of this percentage is produced at higher pHs. Basically, this can be due to an increase of the charge density on the carbonyl as a result of acid deprotonation, the reactivity of the vinyl group is diminished. At a higher iron concentration, a significant graft increase also exists.
The capacity of chitosan to absorb iron (III) ion from an Jectofer, an Iron (III) -sorbitol - citric acid complex solution, was investigated. This, to mimic the composition of excess iron (III) as may be found in biological systems . _
The amount of iron (III) adsorbed onto chitosan increases with the contact time during 3 hours at 37°C. Higher pH values in the range 2 to 7, also resulted in an increase in iron(III). Chitosan samples were analized by FTIR spectroscopy before and after contact with the Jectofer solution. The amount of iron(III) adsorbed increases in time, reaching equilibrium within 3 hours.
It can been observed in Figure la, an graft increase with pH values, reaching a maximum at pH 7. The C=0 group increases in intensity as the pH increases. Figure 2 presents grafted cellulose FTIR spectras. They inclue the typical C=0 group band at 1718 cm"1. This band increases in intensity as the pH increases, reaching a maximum at pH 7.
Figure 2: FTIR spectrum for Polyacrylic Acid and Cel-g-AA at different pH.
a) Polyacrylic Acid (PAA)
b) Cel-g-AA, pH = 2.5
c) Cel-g-AA, pH = 5
d) Cel-g-AA, pH = 6
e) Cel-g-AA, pH = 7
The surface morphology of the cellulose fiber grafted with acrylic acid at different graft percentages was examined using scanning electronic microscopy (SEM) according to the micrograph sequence presented in Figure 3. Small deposits on the irregular fiber grow around it, covering it completely are observed at pH 5 in Figure 3. At pH 6, a greater quantity of deposited material is observed. The maximum graft percentage occurs at pH 7.
Figure 3: SEM Micrographs a) Cel-g-AA; pH 5 (400 X) b) Cel-g-AA, pH 6 (400 X) c) Cel-g-AA, pH 7 (400 X) d) Cel-g-AA, pH 7 (16000 X)
It is important to observe that as the reaction time increases, new bands appear in the IR spectrum that could indicate a polymer degradation process (figure 4). The increase of the band 2963 cm-1 signals a vibration of the C-H stretching of new methylene groups in the polymeric unit. The band at 1260 cm-1 corresponds to a vibration of C-O stretching and the -CH - rocking vibration at 803 cml corresponds to the methylene groups. These absorption bands are very intense at 17 hours of reaction .
Santa Fe cellulose grafted with the system AA - Mn(VII)/ H2C204- Mn2+ and Microcrystalline cellulose grafted with AA and Ce (IV), Mn(VII) /H2C204-Cu2+ and Fenton , presents similar spectral data (Figure 4 and Figure 5).
Another significant aspect in this reaction is the absence of the carboxylate band at 1580 cm-1, which demonstrates the low availability to form a metallic complex with this group.
he addition of Cu2+ or Mn2+ catalyst as sulphates would raise both, the initial rate and maximum conversion. A disproportionation reantion occurs between Mn2+ md Mn4+ ions, producing the highly active Mn3+ ions in the system at a rapid rate. The Mn3+reacts rapidly with the organic substrate, oxalic acid, prodining free radicals. These radicals propagate the polymerization at a comparatively faster rate.
To determine the efficacy of the different metals used as radical initiators, the graft of acrylic acid in microcrystalline cellulose with the ions Ce (IV), Mn(VII) and Fenton were studied at 17 hours.(Figure 5).
The principal band at 1715 cm-1, characteristic of carboxylic groups is diminished after 17 hours of reaction with 3 different initiators: Ce(IV), Mn(VII) and Fenton. A broad
band with two peaks near 1098 and 1025 cm"1, due to the structure stretching of C-COOH, is increased. Methylene rocking at 803 cm4, increases with the time too. The polymer degradation is supported by this data.
The micrographs in Figure 6 present cellulose materials whose surface was attacked and clearly degraded.
The cellulose material micrographs are presented in Figure 6. It shows the Fe2+ and Mn7+ action on grafted Avicel at 70°C and 17 hours as reaction time. The initial crystallinity changes with the time and temperature. Grafted Avicel becoming an amourphous and smooth material with Fenton and KMn04 reagents at large time and temperature.
The secondary pulp is modified by Fenton's reactive at different times and reaction temperatures (Figure 7).
The secondary cellulose is modified by Fenton during 3 and 17 hours, at 50, 60 and 70°C as reaction temperature. The wide band at 3420 cm"1 , characteristic of hydroxyl group is very diminished (Figure 7b). Other important bands at 1260, 1094,1025, 803 cm"1 are shown too. They increase in absorption as the time increases. These features are evidence for the microcrystalline cellulose degradation with different times and temperatures reaction.
The cellulose material of smooth fiber was modified with the reaction time. It becoming an agglutinated and amorphous material with granulated surfaces, indicating fiber degradation (Figure 8).
It can been proposed that in acidic medium the oxidation of primary hydroxyl in the C-6 conduces to the formation of the carboxylic acid. Then the iron complex is formed and finally the decarbaxylation is produced (Diagram I).
The radical grafting reaction variables such as pH, initiator system, cellulose nature and reaction time, were studied. By physical and chemical modifications the degradation polymer was investigated.
The grafting by Fenton-system of AA onto cellulose reaching a maximum at pH 7 , 90%, and then the percentage is reduced to 30% . FTIR spectrums and SEM micrographs evidence these changes. The carbonyl group at 1718 cm"1 and small new deposits covering the irregular fibers are presented.
Santa Fe cellulose is grafted with AA and system Mn(VII) - Mn(II) as co-initiator, at 17 hours as reaction time. This cellulose is degraded at 70°C and 17 hours, as time reaction. This is corroborated by FTIR spectrums.
Avicel grafted with AA and three different initiator systems: Ce(IV) / HN03, KMn04/ H2C204 - Cu2+ , and Fe(II) / H2 02); at 70°C and 17 hours is degraded. FTIR spectrums and SEM micrographs evidence these changes . The principal band at 1715 cm"1, carboxylate group, is diminished and grafted Avicel becoming an amourphous material.
For grafting AA onto cellulose fiber, Ce(IV) is the best initiator. It was observed a minor monomer incorporation with Fenton and Mn(VII)/ Cu(II)initiator. By analysis of the band intensity of carboxyl group at 1715 cm" 1 in the FTIR spectra, this was performed qualitatively
Secondary pulp in reaction with Fenton reagent at 70°C and 17 hours is degraded. FTIR and SEM confirm the new amorphous material
Thanks are given to the Proyect N° 020404 3/R , Research Direction, University of BíoBío of Concepción.
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(Received: 2 November 2006 - Accepted: 21 March 2007)