Ashish Chauhan* and Balbir Kaith
Department of Chemistry, Dr. B.R.A. National Institute of Technology, Jalandhar, India
Received date: July 27, 2012; Accepted date: September 27, 2012; Published date: October 03, 2012
Citation: Chauhan A, Kaith B (2012) XRD Elaborates the Metamorphosis in Graft Copolymers. J Anal Bioanal Tech 3:148. doi: 10.4172/2155-9872.1000148
Copyright: © 2012 Chauhan A, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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Sereni is present in abundance through-out the world. Its stem fiber has high tensile strength and low density that promotes it to serve as a backbone for graft-copolymerization. This cellulosic fiber was graft copolymerized by binary vinyl monomeric mixture using ceric ammonium nitrate-nitric acid as initiator system. The graft co-polymers were characterized by SEM, FTIR and XRD. The comparative evaluation of graft copolymers by x-ray diffraction studies revealed the change in morphology and nature of the fiber. The results obtained by XRD were supported by the physcio-chemico-thermal transition in the properties and behavior of the fiber. These modified graft copolymer could be used in numerous scientific and bio-medical applications.
Graft copolymerization; Methyl acrylate; Butyl acrylate; Cellulose
Natural fibers are assets to the man on the earth. They have been useful for the basic needs of the mankind since long. As they are renewable resource so they can also be utilized for various secondary needs. Many attempts have been made to explore their fullest potential yet a lot remains undiscovered. The natural fibers suffer from various draw-backs such as low weather-stability, fast-decaying and low chemical resistance. Amongst various methods prevalent to improve these graft copolymerization of vinyl monomers onto natural backbones has been an interesting means to reduce the ageing in textiles, enhancing the mechanical properties and modifying the texture. Graft copolymerization of vinyl monomers onto cellulose and other macromolecules has been extensively studied with various monomers and methods [1-5]. Numerous scientists have explored the potential of this chemical technique on different natural fibers and used latest advanced means of characterization and evaluated the properties for their variable applications [5-15].
It redefines the properties of polymer back-bone by incorporating the desired features without drastically affecting the basic traits of the substrate. X-ray diffraction is an important technique to characterize the polymers and screen the differences brought about by graft copolymerization. Since, the plant does not possess any significant biomedical use but the low density, non-abrasive, high specific mechanical strength in the stem fiber motivated to use it as backbone for graft copolymerization to explore the virgin Sereni (Hibiscus sabdariffa) fiber by using the rare binary vinyl monomeric mixture of methyl acrylate (MA) as principal monomer with acrylic acid (AA), butyl acrylate (BA), acrylamide (Aam), acrylonitrile (AN), vinyl acetate (VA), 4-vinyl pyridine (4-VP) as secondary monomers that has never been studied before. Many attempts have been made before to utilize the fiber existing as waste bio-mass [16-24], but this paper is an attempt to present a comparative evaluation of change in the graft yield of the fiber after copolymerization and its depiction and correlation with X-ray diffraction studies, that still remains unexplored. These results were further supported by the inference obtained from the characterization by FTIR, SEM and physcio-chemico-thermal evaluation of the graft copolymers.
H. sabdariffa fiber was obtained from the Department of Agronomy, Chaudhary Sarwan Kumar Himachal Krishi Vishwavidyalaya, Palampur (H.P.) India. Monomers (Merck) and Ceric ammonium nitrate (s. d. fine-Chem, Pvt. Ltd, Mumbai, India) were used as received. H. sabdariffa fiber was purified through soxhlet extraction in acetone for 72 hours.
Graft copolymerization of the monomer onto H. sabdariffa was carried-out for optimization of different reaction conditions like reaction time, reaction temperature, monomer concentration, concentration of initiator system and pH in order to obtain maximum graft yield. The fiber (0.5 g) was activated by swelling in 100 ml of the distilled water for 24 hrs. Mixture of Ceric Ammonium Nitrate (CAN) and Conc. HNO3 mixture was slowly added to the reaction medium with continuous stirring followed by the drop by drop addition of a definite ratio of binary vinyl monomeric mixture. The reaction was carried-out at a definite temperature for a particular time interval. On completion of the reaction, poly(MA), poly(MA-co-BA), poly(MAco- AA), poly(MA-co-AN), poly(MA-co-Aam), and poly(4-VP) were removed on extraction with acetone, di-methyl formamide, alcohol, chloroform, methanol and water. The graft co-polymer was dried at 50°C, till a constant weight was obtained. The percent grafting (Pg) was calculated as per the reported method [1-3,25]:
where, Wf = final weight of the fiber, Wi = initial weight of the fiber.
Moisture absorption study: Moisture absorbance studies were carried-out at various relative humidity levels. Moisture absorbance percentage was found by placing a known weight (Wi) of dry grafted and ungrafted samples in a humidity chamber for about twelve hours and then the final weight (Wf) of the samples exposed to different relative humidities ranging from 30-90% were taken at ambient temperature. The moisture absorbance was calculated by the Eq. 2 [1-3,25]:
Acid and base resistance: Acid and base resistance studies were carried-out as shown in Eq. 3. A known weight (Wi) of dry grafted and ungrafted samples in fixed volume of 1N HCl and 1N NaOH was kept and the final weights (Wf) of the samples was observed after 72 hours, under ambient conditions [1-3,25].
Swelling behavior in different solvents: 250 mg of grafted and raw sample was immersed in a definite volume (100 ml) of water, methanol, n-butanol and dimethyl-formamide under ambient conditions for a period of 24 hours. Samples were removed from the solvent and excess solvent was removed with filter paper. Final weight of the sample was taken and the percentage of swelling was calculated as follows [1-3,25]:
where, W1 and W2 are the initial and final weights of samples, respectively.
Dye uptake behavior: 0.1% Gentian violet solution was prepared in distilled water. 10% NaCl solution and a few drops of acetic acid were added to this solution. Dye uptake of the raw fiber and its graft copolymers were carried-out by immersing the known weight of the sample in 100 ml of Gentian violet dye. Optical densities of the test solutions were observed using Digital Photo Colorimeter for seven consecutive hours and the concentrations of test solution were calculated as [1-3,25]:
where, Io, It, and Co are optical density of standard solution, optical density of test solution and concentration of standard solution, respectively.
IR spectra of the H. sabdariffa and its graft copolymers were recorded with Perkin Elmer Fourier Transform Infrared (FTIR) spectrophotometer using KBr pellets (Sigma Aldrich). Scanning Electron Micrographs (SEM) of H. sabdariffa and its graft copolymers were obtained by using Electron Microscopy Machine (LEO 435-25-20). X-ray diffraction studies were performed on X-ray diffractometer (Bruker D8 Advance) under ambient conditions using Cu Kα (1.5418°A) radiation, Ni-filter and scintillation counter as detector at 40 KV and 40 mA on rotation between 13° to 25° at 2θ scale at 1 sec. step size and increment of 0.01° with 0.5° or 1.0 mm of divergent and anti-scattering slit. Corundum was the reference sample that verifies and calibrates the instrument.
The continuous scans were taken and different d-spacings (Å) and relative intensities (I) were obtained. Crystallinity index (C.I.) which measures the orientation of the cellulose crystals with respect to fiber axis was determined by using the wide angle X-ray diffraction counts at 2θ-scale close to 22.68° and 15°. The counter reading at the peak intensity at 22.68° represent the crystalline material and the peak intensity at 15° corresponds to the amorphous material in cellulose. Percentage Crystallinity (%Cr) and Crystallinity index (C.I.) were calculated as follows [25,26]:
% Cr = [I22.68/ (I22.68 + I15)] × 100 EQ(6)
C.I. = [(I22.68 – I15) / I22.68] EQ(7)
where, I22.68 and I15 are the crystalline and amorphous intensities at 2θ-scale close to 22.68° and 15°, respectively.
Ceric ion forms complex with the cellulose through C-2 and C-3 hydroxyl groups of the anhydro glucose unit. Transfer of the electron from the cellulose molecule to Ce (IV) would follow, leading to its reduction to Ce (III), breakage of –OH bonds at C-2 and C-3 and the formation of the free radical sites where the monomeric chains get grafted. Graft yield and homo-polymer formation have been found to be the functions of both the monomer and initiator concentration .
During the graft copolymerization of MA onto H. sabdariffa fiber, the different optimized reaction parameters to obtain the maximum graft yield (60.24%) were: temperature (°C), 35; time (minute), 120; CAN (mol L-1), 1.49×10-4; HNO3 (mol L-1), 3.36×10-3; MA (mol L-1), 2.21×10-3 and pH, 7.0. Since, it is hard to justify the results in Table 1, merely on the basis of concentration of binary vinyl mixtures, therefore, it is important to consider the reactivity, nature and behavior of monomer species. It is clear that the reactivity of the acrylate monomers in reference to styrene radicals is normally higher. Since, the carboalkoxy group is highly electronegative; therefore, it causes the acrylate double bond to be positive in character. Methyl acrylate radical has hyper conjugative resonance with methyl hydrogen as well as with the carboxy group. The polar character of the acrylates results in the penultimate group effects in both propagation and the termination reactions. Methyl acrylate as a principal monomer has relative high reactivity, suitable chemical properties (Kt, Kp /Kt, CM) needed for grafting onto H. sabdariffa backbone that results in Pg: 60.24.
|Sample||Binary mixture (×10-3mole L-1)||Mean Pg||± SD||± SE|
|2.21+3.03||68.08||± 7.18||± 4.15|
|2.21+3.77||100.12||± 7.13||± 4.12|
|2.21+4.56||100.64*||± 4.54||± 2.62|
|2.21+5.28||57.06||± 6.26||± 3.61|
|2.21+6.07||52.77||± 3.55||± 2.05|
|2.21+0.72||18.00||± 4.47||± 2.58|
|2.21+1.45||74.14||± 2.68||± 1.54|
|2.21+2.17||42.35||± 3.53||± 2.04|
|2.21+2.91||10.28||± 4.40||± 2.55|
|2.21+3.62||10.04||± 3.45||± 2.00|
|2.21+0.70||20.22||± 1.78||± 1.02|
|2.21+1.05||23.75||± 4.42||± 2.55|
|2.21+1.40||15.54||± 3.44||± 1.98|
|2.21+1.75||14.00||± 3.57||± 2.06|
|2.21+2.10||12.38||± 1.96||± 1.13|
|2.21+2.16||113.40||± 5.36||± 3.09|
|2.21+2.70||120.00||± 7.15||± 4.13|
|2.21+3.24||138.52*||± 5.59||± 3.23|
|2.21+3.78||96.16||± 5.36||± 3.09|
|2.21+4.32||61.04||± 1.78||± 1.02|
|2.21+0.35||29.80||± 3.57||± 2.06|
|2.21+0.70||47.14*||± 2.68||± 1.54|
|2.21+1.05||43.62||± 4.44||± 2.56|
|2.21+1.40||29.08||± 3.45||± 1.99|
|2.21+1.75||28.02||± 5.38||± 3.10|
|2.21+0.92||11.50||± 0.98||± 0.50|
|2.21+1.38||11.30||± 1.87||± 1.08|
|2.21+1.85||7.70||± 1.70||± 0.98|
|2.21+2.30||5.00||± 1.71||± 0.98|
where,* refers to the effective Pg.
Table 1: Effect of the binary mixtures on Pg using MA as a principal monomer.
Table 1, depicts the effect of binary vinyl monomeric mixtures on Pg. Some binary vinyl monomeric mixtures yielded an effective graft yield whereas the other did not. The monomer reactivity ratio (r1:r2) between MA and VA is 0.8:0.23 therefore, it is quite evident that Pg (138.8) should be higher in its binary mixture, which is further supported by high value of CM (1.45×104, 2.4×104, 10.7×104), high Kp/ Kt value [37.2×106 (25°C), 13.4×106 (35°C), 22.6×106 (50°C)]. In case of MA+AN binary mixture, the monomeric reactivity ratio (r1:r2) is (1.25 ± 0.15: 0.86 ± 0.05 at 30°C) and the value of Kp is (28000×10-6 l/mol.s (25°C), 14500×10-6 l/mol.s (25°C), 32500×10-6 l/mol.s (30°C)] which again supports the high graft yield (Pg: 100.64). However, in case of MA+AA binary mixture, r1: r2 ratio (0.98 ± 0.02: 0.42 ± 0.02 at 45°C) is quite satisfactory but due to more association of AA with water, low free radical transfer to the backbone (due to low CM) results in low Pg (74.14). A low grafting (Pg: 47.14) has been found in case of MA+BA binary mixture which is due to the fact that since BA and MA both have relatively less affinity towards reaction medium and ultimately results in low transfer of the active chain propagating species towards the active backbone. In case of acrylamide, a low graft yield (23.75%) was observed as in this case Kp value is higher but CM value is low which results in more homo-polymerization.
However, many other factors also determine the graft yield like the type of fiber, swelling, number of active sites, the nature and amount of the solvent and temperature of polymerization strongly influence the reactivity ratios. In absence of monomer rich phase, the diluents will compete with the monomers for adsorption sites. The amount of adsorption will depend upon the total amount of surface area present and this in turn, is dependent upon the rate of stirring. Physical factors like mixing efficiency determines the melt temperature, the pressure, the rheological properties, solubility of the initiator and the monomer. Elevated temperature favors the degradation, reduces the initiator half life, modifies the rate or specificity of the reaction, influences the solubility and rheological parameters may affect the graft yield [25,27-30].
Fourier Transformer Infrared Spectroscopy (FTIR): H. sabdariffa fiber showed a broad peak at 3422 cm-1 (bound -OH groups) and at 2924.8, 1454 and 1055 cm-1 (-CH2, C-C and C-O stretching, respectively). In case of methyl acrylate as a principal monomer, additional peaks due to MA (carbonyl stretch) appeared in all the cases and other incorporated functional groups of secondary monomers were witnessed at 1635.0 cm-1 (vinyl group) in Hs-g-poly(MA-co-VA); 2236.1 cm-1 (-CN group) in Hs-g-poly(MA-co-AN); 1639.4 cm-1 (vinyl group) in Hs-g-poly(MA-co-4VP); 3369.7 cm-1 (N-H stretch) of amide in Hs-g-poly(MA-co-AAm); 1649.7 cm-1 (>C=O of acid) in Hs-g-poly(MA-co-AA) and 1741.5 cm-1 (>C=O stretch) in Hs-g-poly(MA-co-BA).
Scanning Electron Microscopy (SEM): In SEM, the cellulose was gold plated to have an impact. It is quite evident from the figures 1-3 that there has been a sufficient deposition of polyvinyl monomers onto fiber. Comparison of the scanning electron micrographs of the raw H. sabdariffa fiber with the highest grafted graft copolymer reveals the distinction between the ungrafted and grafted samples on the basis of the accumulation of poly vinyl monomer onto the fiber, depending upon the Pg.
X-ray diffraction studies of graft copolymers (XRD): Hibiscus sabdariffa is rich in cellulose content (73.9%). Cellulosic fiber possesses both crystalline and amorphous regions. The X-ray pattern of the crystalline polymer shows sharp peaks associated with the region of three dimensional order and the diffused features are characteristics of the molecularly disordered substances coexisting within itself. Lower crystallinity means higher amorphous content that may be more accessible to chemicals (vinyl monomers) and water. Crystallinity is correlated to the strength of the fiber. On grafting, the crystal lattice of the polymer is disrupted but the strength of the monomers incorporated reinforces the strength in the structure. However, if crystallinity is not affected on grafting then continuous increase in strength can be obtained with increase in Pg.
The XRD result obtained in figures 4, 5 and Table 2, clearly shows that the percentage crystallinity and crystallinity index was found to decrease with increase in percentage grafting. The X-ray spectrum of raw fiber is more complex than that of graft copolymers. In case of raw Hibiscus sabdariffa fiber the % Cr (77.20) and C.I (0.70) are higher and the incorporation of the monomer chains to the back-bone has impaired the crystallinity. There was significant conversion of the sharp peaks in crystalline raw form to the hallo pattern in the highest grafted Hs-g-poly (MA-co-VA) due to the accumulation of the amorphous content. For Hs-g-poly (MA-co-VA) percentage crystallinity (60.0) and crystallinity index (0.33) decreased significantly as compared to the raw fiber. Therefore, on grafting there was reduction in its stiffness and hardness. Crystallinity index gives a quantitative measure of the orientation of the cellulose crystals in the fibers. A lower crystalline index in case of graft co-polymers means poor order of arrangement of cellulose crystals in the fiber that is due to dis-orientation of the cellulose crystalline lattice to the fiber axis during grafting. The individual diffraction pattern of the raw H. sabdariffa fiber, Hs-g-poly (MA) and highest grafted fiber Hs-g-poly(MA-co-VA) depicts the d-values and further elaborates the differences between the two on the basis of the peak type and peak intensity. However, the diffractogram of the unmodified Hibiscus sabdariffa fiber and the treated samples confirmed that cellulose I form remained unchanged after graft copolymerization under the optimized reaction conditions. This change in morphology due to accumulation of amorphous content will alter the nature and properties of the fiber [25,26].
|H. sabdariffa Hs-g-poly(MA)||-
Table 2: The % Cr and C. I. of the H. sabdariffa fiber and its graft copolymers.
XRD can easily elaborate the difference in nature, morphology and properties of the raw and the grafted fiber by overlaying the scan in diffractogram. The result obtained in Table 1 ,2 and figures 1-5 are further supported by the results obtained in tables 3-5. It is because of the accumulation of the amorphous content with increase in Pg.
|Sample||Pg||% Chemical resistance
% wt. loss after 72 hours
|% Moisture absorbance
at different RH after 12 hours
|Hs-g-poly-||In 1N HCl||In 1N NaOH||30-35%||50-55%||60-65%||85-90%|
Table 3: Chemical resistance and moisture absorbance behavior of graft copolymers vis-à-vis back bone.
Table 4: Effect of solvents on swelling behavior of graft copolymers vis-à-vis backbone.
|Pg||Dye concentration of the test solution at different time intervals (×10-4 mol L-1)|
|1 h||2 h||3 h||4 h||5 h||6 h||7 h|
Table 5: Dye uptake studies of the graft copolymers vis-à-vis back bone.
Physico-chemico evaluation: The morphological transformations brought about by the graft copolymerization, modified the nature, properties and behavior of the fiber. These results obtained by XRD studies are supported by various physico-chemical tests. It was observed that when native fiber was compared with Hs-g-poly(MA) (Pg:60.24) and the highest grafted Hs-g-poly(MA-co-VA) (Pg:138.52) the properties of the fiber changed after graft copolymerization with the increase in Pg.
Moisture absorbance study: It is clear from Table 3, that graft co-polymerization of poly(MA) and other poly(vinyl) monomer chain onto H. sabdariffa had a great impact on the moisture absorbance behavior of the fiber. It has been observed that with increase in graft yield, there was significant decrease in the percentage of moisture absorbed in the graft copolymers. It was due to blocking of the sites vulnerable for moisture absorbance with hydrophobic poly (MA), poly (vinyl) monomer chains, thereby, transforming the fiber less sensitive to moisture [1-3,25].
Acid-base resistance study: The results obtained in Table 3 depict that acid-base resistance of the fiber increased with the increase in percent grafting. It could be accounted by the fact that poly (MA) and poly (vinyl) monomer chains grafted and covalently bound onto H. sabdariffa fiber (Hs-g-poly(MA-co-VA) had no weight loss on reactivity with 1 N HCl and 1N NaOH as compared to the free hydroxyl groups present in the ungrafted fiber with a weight loss of 55% and 43%, respectively. Therefore, the resistance in the fiber for strong acid and base was found to increase by the incorporation of poly (vinyl) chains on the active sites of the backbone after graft copolymerization [1-3,25].
Swelling behavior study: The swelling behavior studies were carried-out in different solvents like Water, MeOH, BuOH and DMF. It has been observed that H. sabdariffa fiber showed maximum swelling in Water (59%) followed by MeOH (46%), BuOH (38%) and DMF (30%). However, the swelling behavior of the graft copolymers followed the pattern: DMF > BuOH > Water > MeOH and the trend obtained had a direct correlation with the solubility parameters like solvent basicity, the molar ratio, hydrogen bond formation and the percentage grafting. The differential swelling behavior of the fibers could be justified depending upon the chemical nature and the property of the pendent groups (-COCH3, -COC4H9, -COOH,-CONH2 and -CN) that had different interactions with the solvent. Higher swelling in DMF and BuOH in grafted copolymers could be due to better interaction with the pendent groups that increases with increase in Pg. However, a reverse trend was found in the case of raw H. sabdariffa. Since, the cellulose is semi crystalline, polar polymer and miscible with in polar solvent due to hydrogen bond formation and imbibitions therefore, the raw fiber had higher swelling in water and MeOH followed by DMF and BuOH. Moreover, other factors like the fiber size, steric hindrance and temperature also influence the percentage of swelling [1-3,25] (Table 4).
Dye-uptake behavior: The dye uptake behavior of the graft copolymers vis-à-vis raw fiber was studied for seven consecutive hours to find out the effect of grafting on dye uptake. It is clear from Table 5 that the graft copolymers exhibited less dye uptake as compared to the raw fiber as it was found to be a function of Pg. It was observed that dye uptake decreases with increase in Pg. Cellulose is semi crystalline polymer that easily swells due to competitive processes of adsorption through hydrogen bonding and the scission of the internal hydrogen bonds between the amorphous molecules. Presence of free reactive sites like –OH and –CH2OH in raw fiber helped in the absorption of the dye. But, these sites get occupied with poly(MA) chains and poly(MA-co-VA) chains in the backbone that restrain dye uptake depending upon the Pg [1-3,25].
Grafting of poly (MA) and poly (MA-co-vinyl monomers) onto H. sabdariffa fiber in presence of Ceric ion initiator yielded novel regenerated graft copolymers. It is evident from the characterization by SEM, FTIR and XRD that the fiber properties were modified due to accumulation of the comonomers on the cellulosic backbone. Grafting of MA in binary mixture with VA yielded the highest graft yield (Pg:138.52). XRD confirmed the reduction in % Cr and C.I. with increase in Pg and it was supported by the results obtained by the evaluation of the properties of the grafted fiber. The grafted fibers were physico-chemico-thermally resistant and incorporated the features like hydrophobicity, miscibility in organic solvents that were desired for commercially and industrial applications. The use of this renewable waste bio-mass to obtain novel graft copolymers could be useful in insulator, packaging, aerospace and transportation. XRD is a simple advanced technique that could present the changes in nature, morphology of the fiber after graft copolymerization very easily.
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