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Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSION
  7. Acknowledgments
  8. REFERENCES

Four novel segmented polyurethanes (PUs) based on4,4′-{oxy-1,4-diphenyl bis(nitromethylidine)}diphenol (ODBNMD) diol with different diisocyanates such as 4,4′-diphenylmethane diisocyanate, toluene 2,4-diisocyanate, isophorone diisocyanate, and hexamethylene diisocyanate have been prepared by solution method. The structures of ODBNMD and PUs have been confirmed by Fourier transform infrared (FTIR), nuclear magnetic resonance (1H-NMR and 13C-NMR), UV-visible, and fluorescence spectroscopies. The segmented PUs were further characterized by thermogravimetry (TGA), differential scanning calorimetry (DSC), and wide-angle X-ray diffraction. FTIR confirmed hydrogen bonding interactions, whereas TGA and DSC suggested that introduction of aromatic/phenyl ring in the main chain considerably increased the thermal stability. POLYM. ENG. SCI., 54:24–32, 2014. © 2013 Society of Plastics Engineers


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSION
  7. Acknowledgments
  8. REFERENCES

Polymers containing conjugated —CH N— bonds on the main backbones have attracted considerable interest over the past decade because of their varied applications in many different areas [1-4]. In particular, the Schiff bases such as azomethines that contain imine groups in the main chain are the potential materials that can display liquid crystal behavior [5, 6], thermal stability [7, 8], and electrical/photoelectrical properties [9, 10] and can act as corrosion inhibitors [11, 12]. In the literature, variety of different types of Schiff base polymers have been synthesized and characterized by spectroscopic and thermal techniques [13, 14]. Polyurethanes (PUs) are among the well-known such materials that have received considerable attention because of their numerous industrial applications because they can be fabricated as fibers and used as binder resins, coatings, and high-performance elastomeric products [15, 16]. The segmented PUs contain multiblock chains of alternating “soft” polyester or polyether segments coupled to the “hard” PU segments that can display microphase separation, thereby exhibiting excellent elastomeric properties [17].

Recently, structural and thermal properties of segmented PUs have been well studied because of their wide ranging applications as potential materials in many engineering disciplines [18-20]. In our earlier efforts [21, 22], the segmented PUs and polyureas developed were mostly insoluble in organic and acidic solvents because of their rigid backbones [21, 22], thus prohibiting their applications as useful materials. To overcome these difficulties and to increase the solubility of hard-segmented PUs, chemical modification is necessary, wherein asymmetric or side chain groups can be incorporated onto the polymer main chain [23-26]. In efforts to resolve this problem, we have introduced ether and imine groups containing diol such as 4,4′-{oxy-1,4-diphenyl bis(nitromethylidine)}diphenol (ODBNMD) onto the PU backbone. The diols were prepared using diaminodipheyl ether condensing with p-hydroxy benzaldehyde. Using this diol, four novel hard segmented PUs have been synthesized from the reaction of ODBNMD with 4,4′-diphenylmethane diisocyanate (MDI), tolylene 2,4-diisocyanate (TDI), isophorone diisocyanate (IPDI), or hexamethylene diisocyanate (HDI). The structures of ODBNMD and the segmented PUs have been confirmed by UV-visible, fluorescence, NMR (1H and 13C), and Fourier transform infrared (FTIR) spectroscopic techniques. The hard-segmented PUs have also been characterized by wide-angle X-ray diffraction, thermogravimetry (TGA), and differential scanning calorimetry (DSC) to understand their morphology and thermal properties.

EXPERIMENTAL

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSION
  7. Acknowledgments
  8. REFERENCES

Materials

Diaminodiphenyl ether, p-hydroxy benzaldehyde, MDI, TDI, IPDI, HDI, and dibutyltin dilaurate (DBT) were all purchased from Aldrich (Milwaukee, USA); these chemicals were used without any purification. Methanol, ethanol, acetone, dimethyl sulfoxide (DMSO), dichloromethane (DCM), dimethylformamide (DMF), chloroform, and tetrahydrofuran (THF) were all purchased from Spectro Chem., Mumbai, India.

Characterization

Melting points of ODBNMD were determined in an open capillary tube. UV-visible spectra (Perkin Elmer, Lambda 35, France) were recorded in the wavelength range of 200–600 nm for dilute PU solution of 5 × 10−4 M prepared in a spectroscopic grade DMF solvent. Fluorescence spectra (Varian, CA, USA) were also recorded in the wavelength range of 200–800 nm on excitation at 265 and 320 nm, respectively, for dilute PU solution of 5 × 10−4 M prepared in DMF. FTIR spectral measurements were scanned between 400 and 4000 cm−1 using Bruker (Alpha T, USA) spectrophotometer to confirm the structures of diol and PUs.

The PU samples for FTIR measurements were prepared using dried KBr applying hydraulic pressure of 400–600 kg/cm2. The 1H- and 13C-NMR of the synthesized PUs were performed in a Bruker-300 MHz instrument taking tetramethylsilane (TMS) as the standard at room temperature by dissolving it in DMSO-d6 solvent.

TGA and DSC tracings were recorded on a Perkin-Elmer Diamond analyzer (Shelton, CT, USA) by taking 8–10 mg of PU sample in a platinum crucible and heated from 25 to 400°C under nitrogen atmosphere with a flow rate of 10 mL/min. DSC spectra were recorded against α-alumina at the heating rate of 10°C/min. For TGA, 10 mg of PU sample was loaded in the open platinum pan and heated from 25 to 800°C under nitrogen atmosphere with a flow rate of 10 mL/min at the constant heating rates of 10°C/min.

Wide-angle X-ray diffractograms of the PUs were recorded using a Bruker D2 Phaser (Germany) equipped with Ni-filtered CuKα radiation (λ = 1.5418 Å). PUs were dried and spread on a sample holder, and diffractograms were recorded in the wide-angle range of 5–55° at the speed of 5°/min.

Synthesis of ODBNMD

The Schiff base diol was prepared by adding ethanol solution of diaminodipheyl ether (4.0 g, 0.02 mol) to ethanol solution of p-hydroxy benzaldehyde (4.8 g, 0.04 mol). The mixture was refluxed for 8 h with constant stirring. After cooling, the resultant product was poured in double-distilled water and filtered. The ODBNMD formed was washed with water to remove any unreacted reactants and recrystallized from ethanol to give a yellow crystalline solid. The solid was dried in a hot air oven at 60°C for 24 h to give an yield of 6.85 g (84%); the melting point was found to be 241–242°C. The formation of ODBNMD is presented in Scheme 1.

image

Scheme 1. Formation of 4,4′-{oxy-1,4-diphenyl bis(nitromethylidine)}diphenol.

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FTIR and NMR spectral assignments are given below.

  • FTIR (KBr): 3456, 3065, 2889, 1617, 1572, 1497, 1440, 1377, 1283, 1238, 1163, 1106, and 822 cm−l.
  • 1H-NMR (DMSO-d6, TMS): δ, 6.86–7.76 (aromatic protons), 8.47 (imine protons), and 10.06 (hydroxyl protons).
  • 13C-NMR (DMSO-d6, TMS): δ, 115.58, 119.12, 122.40, 127.53, 130.50, 147.32, 154.66, 159.28 (aromatic ring carbons), and 160.48 (imine carbons).

Synthesis of PUs

General procedure for the synthesis of segmented PUs involves the reaction in a four-necked 100-mL round bottom flask equipped with a mechanical stirrer, condenser, and a dropping funnel under a nitrogen atmosphere. The 0.005 mol of diol such as ODBNMD was dissolved in 10 mL of dry DMF in the presence of DBT (0.2 mL) as a catalyst under constant stirring. Then, equimolar quantity (0.005 mol) of diisocyanates (MDI, TDI, IPDI, or HDI) with respect to the diphenol taken in 10 mL of dry 4-methylpentanone-2 were added all at once with constant stirring. The reaction mixture was heated for 10 h at 80°C. The PUs obtained were cooled, poured into distilled water, filtered, and recrystallized using DMF. The formation of PUs is displayed in Scheme 2.

image

Scheme 2. Reaction pathway for the formation of PUs based on different diisocyanates.

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Poly[4,4′-{oxy-1,4-diphenyl bis[nitrilomethylylidene]}diphenyl, 4,4′-methylene diphenyl diurethane] (PU1).

PU1 was prepared using 1.25 g of MDI and 2.04 g of ODBNMD. The yield of the compound was 94.71% (3.47 g). FTIR and NMR analysis results are given below.

  • FTIR (KBr): 3317, 3034, 2920, 2850, 1652, 1619, 1518, 1410, 1307, 1236, 1161, 1101, and 836 cm−l.
  • 1H-NMR (DMSO-d6, TMS): δ, 3.33 (methylene protons merged with DMSO-CH3 protons), 6.80–7.79 (aromatic protons), 8.50 (imine protons), and 10.09 (urethane protons).
  • 13C-NMR (DMSO-d6, TMS): δ, TMS): δ, 40.13 (methylene carbon merged with DMSO-CH3 carbons), 114.74–159.29 (aromatic ring carbons), 160.35 (imine carbons), and 160.49 (urethane carbons).
Poly[4,4′-{oxy-1,4-diphenyl bis[nitrilomethylylidene]}diphenyl, tolylene 2,4-diurethane] (PU2).

PU2 sample was prepared using 0.88 g of TDI and 2.04 g of ODBNMD. The yield of the compound was 95.53% (3.05 g). FTIR and NMR analysis results are given below.

  • FTIR (KBr): 3234, 3043, 2964, 2926, 2851, 1643, 1601, 1547, 1494, 1222, 1153, and 834 cm−l.
  • 1H-NMR (DMSO-d6, TMS): δ, 2.20 (methyl protons), 6.84–7.95 (aromatic ring protons), 8.47 (imine protons), and 10.06 (urethanes protons).
  • 13C-NMR (DMSO-d6, TMS): δ, 17.33 (methyl carbons), 114.73–152.71 (aromatic ring carbons), 159.29 (imine carbons), and 160.48 (urethane carbons).
Poly[2,2′-{4,4′-{ oxy-1,4-diphenyl bis[nitrilomethylylidene]}diphenyl, isophorone diurethane] (PU3).

PU-3 was prepared by taking 1.12 g of IPDI and 2.04 g of ODBNMD. The yield of the compound was 94.60% (3.34 g). FTIR and NMR analysis are given below.

  • FTIR (KBr): 3231, 2532, 2950, 1675, 1620, 1533, 1491, 1301, 1238, 1157, and 841 cm−l.
  • 1H-NMR (DMSO-d6, TMS): δ, 0.84–0.98 (methyl protons), 1.04–2.77 (isophorone ring protons), 6.77–7.76 (aromatic protons), 8.47 (imine protons), and 9.77 (urethane protons).
  • 13C-NMR (DMSO-d6, TMS): δ, 27.04–29.39 (methyl carbons), 31.36–47.03 (isophorone ring carbons), 114.72–158.51 (aromatic ring carbons), 159.28 (imine carbons), and 162.49 (urethane carbons).
Poly[4,4′-{oxy-1,4-diphenyl bis[nitrilomethylylidene]}diphenyl, hexamethylene diurethane] (PU4).

PU4 was prepared by using 0.85 g of HDI and 2.04 g of ODBNMD. The yield of the compound was obtained as 96.46% (2.30 g). FTIR and NMR analysis results are given below.

  • FTIR (KBr): 3390, 2944, 2826, 1719, 1621, 1574, 1491, 1371, 1246, 1157, 1100, and 841 cm−l.
  • 1H-NMR (DMSO-d6, TMS): δ, 1.22–2.87 (methylene protons), 6.86–8.47 (aromatic ring protons), 9.77 (imines protons), and 10.06 (urethanes protons).
  • 13C-NMR (DMSO-d6, TMS): δ, 26.03–40.12 (methylene groups carbons some of them merged with DMSO CH3 peaks), 115.58–158.93 (aromatic ring carbons), 159.27 (imine carbons), and 160.48 (urethane carbons).

RESULTS AND DISCUSSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSION
  7. Acknowledgments
  8. REFERENCES

Spectral Data

Hydrogen bonding plays a major role in determining phase separation in PUs, which controls the crystallization and packing and morphological setup in PUs. The phase separation in PUs can be characterized by measuring the intensity and position of both bonded N—H and C O stretching vibrations. In general, PUs containing N—H groups free from hydrogen bonding have a stretching vibration at 3450 cm−1, and C O groups free from hydrogen bonding have a stretching vibration at 1720 cm−1. FTIR spectra show several characteristic stretching/bending vibration modes due to N—H, C O, CH N, OH, and C—H bonds. However, the PUs indicated the absence of hydroxyl (no absorption at 3456 cm−l) or isocyanate (no absorption at 2275 cm−l) groups, indicating complete utilization of both isocyanate and hydroxyl groups during the polymerization reaction. The peak assignments of all PUs, that is, PU1 to PU4 are shown in Fig. 1.

image

Figure 1. Representative FTIR of PU samples based on different diisocyantes.

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PU1 and PU4 show broad bands between 3317 and 3390 cm−l that are due to the presence of non/free hydrogen bonded N—H group, whereas PU2 and PU3 display stretching bands appearing between 3234 and 3231 cm−l because of the presence of hydrogen-bonded N—H group. Similarly, PU1 and PU4 show broad bands between 1692 and 1719 cm−l that are attributed to the presence of non/free hydrogen bonded C O group, whereas PU2 and PU3 display stretching bands appearing between 1643 and 1675 cm−l because of the presence of hydrogen bonded C O group [27, 28]. Because of the presence of aromatic/phenyl ring, PU1 and PU2 show non-hydrogen bonding in nature compared with PU3 and PU4 that display hydrogen bonding. In PU3 and PU4 structures, because of the presence of methyl and methylene groups, hydrogen bond exists. The imine bands (CH N) have appeared in the region from 1601 to1621 cm−1 [29].

NMR is useful to identify the structural properties such as chemical shifts, relaxation time, and line widths that depend on the local structure and dynamics. NMR indicates the disappearance of hydroxyl and isocyanate groups, suggesting the formation of new PUs. The 1H-NMR spectra of PUs have characteristic signals shown in Fig. 2, of which the resonance peaks in the region 0.84–3.33 ppm correspond to methyl, methylene, and isophorone ring protons in the ODBNMD and in the PUs. The resonance peaks of urethane (—NH—COO—) protons of all the PUs occur around 9.77–10.09 ppm, whereas those of imine (—CH N—) protons appear in the region 8.86–9.02 ppm. The aromatic ring protons occur in the region 6.30–8.60 ppm [30].

image

Figure 2. 1H-NMR spectra of diol and PUs based on different diisocyantes.

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The 13C-NMR spectra of all the PUs and ODBNMD have shown characteristic signals as shown in Fig. 3. For instance, the ppm range between δ of 17.33 and 47.03 is due to carbon atoms of aliphatic and isophorone ring. Resonance in the region between δ of 114.72 and 159.29 ppm is due to the aromatic carbons. Peaks observed in the regions 159.27–159.29 and 160.48–162.49 ppm are due to imines and urethane carbonyl carbons, respectively [30].

image

Figure 3. 13C-NMR spectra of diol and PUs based on different diisocyantes.

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UV-visible electronic and florescence spectra of diphenol diol and hard-segmented PUs have been recorded in DMF solvent at ambient temperature. The electronic absorption and emission spectral data of both ODBNMD and hard-segmented PUs are given in Table 1. In the electronic spectra of Schiff base hard segment PUs, the aromatic bands at 255 and 264 nm are attributed to benzene π–π* transitions, whereas those observed at 315 and 321 nm are assigned to —CH N— π–π* transitions. The emission spectra from these diols and PUs have appeared around 364–375 nm and 462–485 nm, respectively [29]. From the absorption and emission spectra, it can be concluded that there is no significant difference in ODBNMD and hard-segmented PUs and that the observed absorption and emission spectral data are in close agreement with the previous reports [29, 30].

Table 1. Absorption and emission peaks for ODBNMD and the PUs
Sample codePeak λmax (nm)
AbsorptionEmission
ODBNMD260–315365–470
PU1255–318370–462
PU2263–320372–485
PU3264–321371–481
PU4258–315375–467

Solubility Properties

Hydrogen bonding plays an important role in segmented PUs and such PUs are insoluble in polar organic and acidic solvents [21, 22]. However, because of the presence of ether and Schiff base moieties in the main backbone, such PUs are soluble in polar organic solvents like N-methyl-2-pyrrolidone, DMSO, DMF, and DCM. All the PUs are insoluble in both room temperature and on heating in ethanol, acetone, methanol, chloroform, diethyl ether, THF, and hexane because of more lyophobic groups in the PU backbone.

Thermal Properties

DSC

Thermal properties of all the PUs were evaluated using DSC and TGA techniques. From the DSC data presented in Table 2 and the curves displayed in Fig. 4, the hard-segmented PUs showed the multiple peaks. Such existence of multiple endotherm peaks was reported before while investigating the thermal behavior of segmented PU block copolymers [31-33]. Koberstein and Galambos [34] suggested that the origin of multiple endotherms peaks in PUs is dependent on the specimen preparation procedure. Martin et al. [35] indicated that four to five endotherms are possibly due to the melting of various hard segment length populations; van Bogart et al. [36] have identified three endothermic transitions associated with the ordering of MDI/1,4-butane diol hard segments in PUs subjected to a third thermal cycle. Blackwell and Lee [37] studied the multiple melting in MDI-based PUs that were thermally annealed. In the light of the above reports, it is obvious that the melting behavior of PUs is highly dependent on the procedure adopted for their preparations. Thus, the origin of multiple melting may be inherently different for PUs prepared under varying conditions.

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Figure 4. DSC thermograms of PU based on different diisocyanates.

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Table 2. Different melting DSC endotherms for segmented PUs based on different diisocyanates
Sample codeT1 (°C)T2 (°C)T3 (°C)
  1. T1, the lowest temperature endotherm; T2, the intermediate temperature endotherm; T3, the melting temperature endotherm.

PU1212320370
PU2205376
PU3210331
PU4167269339

In this work, we have observed multiple melting endotherm phenomena in identical PUs prepared from only hard segments in the main backbone. The DSC data of PU1 and PU4 have shown three endotherm peaks, whereas in PU2 and PU3, the DSC curves display two endotherm peaks. In PU1, the lowest endotherm (T1) observed at 212°C is due to local restructuring of hard-segment units within the hard microdomains. The intermediate temperature endotherm (T2) observed at 320°C is related to melting of microcrystalline regions within the hard microdomains. Higher melting temperature (T3) was observed at 370°C. In PU2, the lowest endotherm (T1) has displayed peaks at 205°C. The melting of microcrystalline regions within the hard microdomains (T3) is also observed at 376°C. In PU3, the lowest temperature endotherm (T1) observed at 167°C is related to a local restructuring of hard-segment unit with the hard microdomains, whereas higher melting temperature (T3) was observed peak at 331°C. In PU4, the lowest endotherms (T1) observed at 167°C is due to a local restructuring of hard-segment units within the hard microdomains. The intermediate temperature endotherm (T2) was shown at 296°C. The melting of microcrystalline regions within the hard microdomains (T3) is also observed at 339°C. Overall, in the current study, DSC data are in good agreement with our earlier reports [21, 22, 29, 30].

TGA

Weight loss data from TGA for all the PUs shown in Fig. 5 and the numerical values presented in Table 3 suggest that 10% of weight loss occurred from 175 to 233°C, whereas the curves show a major weight loss in the region 290 to 454°C. The residual weight remaining at 700°C was about 28–49%, and this variation in weight loss is due to the differences in the structure of diisocyanates in PUs. In PU1, the 10% weight loss begins at 233°C, but the major weight loss occurred from 248 to 454°C. In PU2, the 10% weight loss began at 224°C, and the major weight loss occurred from 240 to 437°C. In PU3, 10% weight loss began at 213°C and major weight loss occurred from 238° to 409°C. In PU4, the 10 % weight loss occurred at 175°C, with the major weight loss occurring from 236 to 398°C. TGA data indicate that PU1 exhibits good thermal stability compared with other PUs because of an increase in the phenyl rings of MDI [[38, 39][.

image

Figure 5. TGA tracings of PUs based on different diisocyanates.

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Table 3. Thermal properties of PUs based on different diisocyanates determined by TGA
 Decomposition temperature (°C)   
PolymerT10T50Major weight loss transition (°C)Residual weight loss at 700°C (%)On set temp (°C)
  1. T10, temperature at which 10% of weight loss observed by TGA; T50, temperature at which 50% of weight loss occurred in TGA; (%), residual weight observed by TGA at 700°C in N2.

PU1233357248–45449230
PU2224360240–43747229
PU3213290238–40928226
PU4175305236–39830196
Wide-Angle X-Ray Diffraction Data

The wide angle X-ray diffraction curves of PUs are shown in Fig. 6. The crystalline nature of the polymers depends on their structures and on the crystallization conditions. PU1 and PU4 show semicrystalline nature, whereas PU2 and PU3 show amorphous behavior; this may be due to the distance of the urethane linkage between repeating unit. The semicrystalline/amorphous nature of PUs decrease in the order: PU1 < PU4 < PU2 < PU3 because of the variations in diisocyanates structural units of Schiff base/ether backbone of the main PU chain [29, 30]. By comparing the DSC and X-ray diffraction data, PU1 and PU4 are found to be semicrystalline in nature having three melting endothermic peaks, whereas PU2 and PU3 showed the amorphous nature with two melting endothermic peaks.

image

Figure 6. X-ray diffractograms of PUs based on different diisocyanates.

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CONCLUSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSION
  7. Acknowledgments
  8. REFERENCES

Novel type of hard-segmented PUs containing both ether and Schiff base moieties like ODBNMD diol in the main backbone were prepared using MDI, 2,4-TDI, IPDI, and HDI via polyaddition reaction in quantitative yields up to 96.46%. All the PUs are soluble in polar aprotic organic solvents. The structures of ODBNMD and PUs have been confirmed by FTIR, 1H- and 13C-NMR, UV-visible, and fluorescence spectroscopic results. DSC data indicated that MDI-based PU1 and TDI-based PU2 show high melting endotherm peaks at 370 and 376°C, respectively. FTIR indicated that reaction was completed with full utilization of both hydroxyl and isocyanate units and confirmed the formation of hydrogen bonding in segmented PUs. TGA confirmed that all the PUs are stable up to 155°C because the curves showed major weight losses between 160 and 460°C. DSC displayed two to three multiple endotherms because of the presence of alternative hard segments. The semicrystalline and amorphous nature of the PUs were also confirmed by wide angle X-ray diffraction.

Acknowledgments

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSION
  7. Acknowledgments
  8. REFERENCES

Mr. D.P. Suhas thanks the Poornaprajna Institute of Scientific Research (PPISR) for the award of a junior research fellowship. Dr. A.V. Raghu thanks to VGST, GOK for awarding seed money for young Scientist.

REFERENCES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSION
  7. Acknowledgments
  8. REFERENCES