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Ind. Eng. Chem. Res. 2008, 47, 8566–8571
Spectroscopic Characterization of Linseed Oil Based Polymers Vinay Sharma,†,‡ J. S. Banait,‡ and P. P. Kundu*,† Department of Chemical Technology, Sant Longowal Institute of Engineering & Technology, Sangrur, Punjab 148106, India, and Faculty of Physical Sciences, Punjabi UniVersity, Patiala, Punjab 147002, India
Linseed oil based polymers from cationic and thermal polymerizations have been investigated quantitatively through 1H NMR and FTIR spectroscopic analysis. The solubility of the samples ranges from 22 to 37% for cationic samples and from 4.23 to 53% for thermal samples. The content of the grafted linseed oil calculated from 1H NMR results ranges from 22.9 to 43.0% and from 0 to 10% for cationic and thermal samples. The grafted linseed oil contents from FTIR are 18.2-45.4% and 0-10.7% for cationic and thermal samples. The values obtained through quantitative 1H NMR and FTIR spectroscopic analysis methods are consistent and can be applied to other polymers also. 1. Introduction The spectroscopic techniques are one of the important parameters for characterizing different polymers.1-3 These techniques are powerful tools for the characterization of polymers both qualitatively4 and quantitatively.5 A lot of research work is already published on the quantitative analysis of different polymers by NMR and Fourier transform infrared (FTIR) spectroscopies.6-9 Hazer et al.10 used FTIR and NMR to calculate poly(ethylene glycol) contents in cross-linked poly(ethylene glycol)-block-polybutadiene block copolymers. Natural oils are always a center of attraction as an alternative source for the production of useful polymers.11,12 Larock et al.13-17 have done remarkable work on the polymerization of various natural oils. They directly polymerized oils with other monomers. Wool and co-workers18-22 used these oils for the production of polymers by modifying the oil moieties, and subsequently polymerizing them with other monomers. Soucek et al.23-29 have used epoxidized linseed and soybean oils for polymerization. Baki et al.30-34 have done work on poly(hydroxyalkanoate) (PHA) based on natural oils for the preparation of unsaturated bacterial polyester. In the present work, linseed oil based polymers have been synthesized and studied quantitatively by 1H NMR and FTIR. In the literature, Larock et al.13,35-37 have quantitatively studied the soluble portions of the oil based polymers through 1H NMR, but the insoluble portions have not been analyzed. In this work, the insoluble portion is also quantitatively analyzed through FTIR. There is a lot of published work on the quatitification of polymers through FTIR.38-40 The present work reports the complete quantitative analysis of linseed oil polymers through 1 H NMR and FTIR. 2. Experimental Section 2.1. Materials. Linseed oil of commercial grade was procured from a local market (Punjab, India), and conjugated linseed oil (87% conjugation) was purchased from Alnor Oil Company, Alnor, NY. Styrene (ST), acrylic acid (AcA), tetrahydrofuran, and boron trifluoride diethyl etherate complex were purchased from Merck Chemical Co., Germany. Divinylbenzene (DVB) was purchased from Fluka Chemie. * To whom correspondence should be addressed. E-mail: ppk923@ yahoo.com. † Sant Longowal Institute of Engineering & Technology. ‡ Punjabi University.
2.2. Sample Preparation. 2.2.1. Cationic Sample Preparation. Polymeric materials have been prepared by heating desired concentrations of linseed oil, styrene, and divinylbenzene in glass molds.41 In these experiments, the styrene to divinylbenzene ratio is kept constant at 2:1, respectively. Desired amounts of styrene and divinylbenzene are added to the linseed oil, and the mixture is vigorously stirred. Then, the mixture is cooled and the initiator is added with constant stirring at low temperature and the whole mass is transferred to the glass mold. The sealed glass mold is kept at room temperature for 12 h and then heated sequentially at different temperatures, such as for 12 h at 60 °C and for 24 h at 110 °C, and is finally postcured at 120 °C for 3 h. The nomenclature used in this work is based on the original compositions of the reactants (reported in Table 1). 2.2.2. Thermal Sample Preparation. Polymeric materials have been prepared by heating desired concentrations of conjugated linseed oil, acrylic acid, and divinylbenzene in glass vials. Desired amounts of acrylic acid and divinylbenzene are added to the conjugated linseed oil, and the mixture is vigorously stirred. The glass vial is heated sequentially at different temperatures, such as for 6 h at 80 °C, for 12 h at 100 °C, and for 12 h at 120 °C, and is finally postcured at 140 °C for 12 h. The nomenclature used in this work is based on the original compositions of the reactants (reported in Table 1). 3. Characterization 3.1. Soxhlet Extraction. The polymeric materials as reported in Table 1 are Soxhlet extracted for their soluble and insoluble Table 1. Detailed Feed Compositions of Different Linseed Oil Polymers sample IDa
linseed oil (%)
styrene (%)
divinylbenzene (%)
Lin30 Lin40 Lin50 Lin60 CLin0 Clin10 CLin20 CLin30 CLin40 CLin50 CLin60
30 40 50 60 0 10 20 30 40 50 60
46 39 31.5 24
15.5 13 10.5 8 10 10 10 10 10 10 10
a
acrylic acid (%)
CLin samples contain 87% conjugated linseed oil.
10.1021/ie800415z CCC: $40.75 2008 American Chemical Society Published on Web 10/21/2008
initiator (%) 8 8 8 8
90 80 70 60 50 40 30
Ind. Eng. Chem. Res., Vol. 47, No. 22, 2008 8567 Table 2. Detailed Compositions of Cationically Cross-Linked Linseed Oil, Styrene, and Divinylbenzene Copolymers from Soxhlet Extraction, and 1H NMR and FTIR Spectroscopic Results soluble extractible compositiona
sample
ID
composition
wt % oil
wt % ST and DVB
Lin30 Lin40 Lin50 Lin60
Lin30 + ST46.5 + DVB15.5 + In8 Lin40 + ST39 + DVB13 + In8 Lin50 + ST31.5 + DVB10.5 + In8 Lin 60 + ST24 + DVB8 + In8
64.68 89.72 78.99 87.76
35.32 10.28 21.01 12.24
Soxhlet results insolublec (wt %)
linseed oil content (wt %) by FTIRd
77.50 (54.1;23.4) 72.00 (49.1;22.9) 66.67 (28.00;38.67) 63.33 (20.27;43.06)
18.16 24.56 36.89 45.39
solubleb (wt %) 22.50 28.00 33.33 36.67
(7.9; 14.6) (2.9; 25.1) (7.00; 26.33) (4.49; 32.18)
a Microcomposition of the extracted soluble materials calculated from the 1H NMR integrals of the glyceride peak at 4.1 ppm and aryl CH peak at 7 ppm. b The data in parentheses have been calculated directly from the weight percent oil and weight percent aromatic content in the soluble extract. The first value in the parentheses represents the percent aromatic content, and the second value represents the percent oil content. c The data in the parentheses have been calculated indirectly from the weight percent of the oil and aromatic content in the soluble extract, as the total mass of the soluble and insoluble parts was held constant. The first value in the parentheses represents the percent aromatic content and the second value represents the percent oil content. d The data are calculated from absorbance peaks at 1744 and 1601 cm-1 for carbonyl stretching of ester in oil and aromatic stretching for styrene-divinylbenzene, respectively.
Table 3. Detailed Compositions of Thermally Cross-Linked Conjugated Linseed Oil, Acrylic Acid, And Divinylbenzene Copolymers from Soxhlet Extraction, and 1H NMR and FTIR Spectroscopic Results sample
soluble extractible compositiona
Soxhlet results
ID
composition
wt % oil
wt % AcA and DVB
solubleb (wt %)
insolublec (wt %)
linseed oil content (wt %) by FTIRd
CLin0 CLin10 CLin20 CLin30 CLin40 CLin50 CLin60
CLin0 + AcA90 + DVB10 CLin10 + AcA80 + DVB10 CLin20 + AcA70 + DVB10 CLin30 + AcA60 + DVB10 CLin40 + AcA50 + DVB10 CLin50 + AcA40 + DVB10 CLin60 + AcA30 + DVB10
0 91.75 93.94 96.74 96.86 95.82 95.12
100 8.25 6.06 3.26 3.14 4.18 4.88
4.23 (4.23; 0) 9.47 (0.78; 8.69) 19.49 (1.18; 18.31) 30.03 (0.98; 29.05) 38.95 (1.22; 37.73) 44.85 (1.87; 42.98) 52.53 (2.56; 49.97)
95.77 (95.77; 0) 90.53 (89.22; 1.31) 80.51 (78.82; 1.69) 69.97 (69.02; 0.95) 61.05 (58.78; 2.27) 55.15 (48.13; 7.02) 47.47 (37.44; 10.03)
0 1.75 1.83 1.70 2.87 6.84 10.73
a Microcomposition of the extracted soluble materials calculated from the 1H NMR integrals of the glyceride peak at 4.1 ppm, acrylic OH peak at 9.8 ppm, and aryl CH peak at 7 ppm. b The data in parentheses have been calculated directly from the weight percent oil and weight percent acrylic-DVB content in the soluble extract. The first value in the parentheses represents the percent acrylic-DVB content, and the second value represents the percent oil content. c The data in the parentheses have been calculated indirectly from the weight percent of the oil and acrylic-DVB content in the soluble extract, as the total mass of the soluble and insoluble parts was held constant. The first value in the parentheses represents the percent acrylic-DVB content, and the second value represents the percent oil content. d The data are calculated from absorbance peaks at 1744, 1711, and 1530 cm-1 for carbonyl stretching of ester in the oil, carbonyl stretching of acrylic acid, and aromatic stretching for divinylbenzene, respectively.
Figure 1. Variation in soluble contents of the linseed oil polymers (wt %) with increasing linseed oil content (wt %) for cationic and thermal samples.
contents. A sample ranging from 2 to 3 g of the bulk polymer is extracted for 24 h with 150 mL of refluxing tetrahydrofuran using a Soxhlet extractor. After extraction, the resulting solution is concentrated by rotary evaporation and subsequent vacuum drying. The insoluble solid is dried under vacuum for several hours before weighing.
Figure 2. 1H NMR spectra of divinylbenzene, extract of the sample Lin50 (Lin50-ST29-DVB13-In8), linseed oil, styrene, and solvent for extraction and solvent collected from extracts.
3.2. 1H Nuclear Magnetic Resonance (1H NMR). The extracted soluble part of the polymeric material as well as the linseed oil, styrene, and divinylbenzene are dissolved in CDCl3. Tetramethylsilane (TMS) is used as reference compound. The solution is scanned with a multinuclear FT-NMR spectrometer (Bruker AC-300 F) at 300 MHz. A total of 30 scans are averaged to obtain the final data.
8568 Ind. Eng. Chem. Res., Vol. 47, No. 22, 2008
Figure 4. FTIR absorbance spectra of the cationic samples Lin30 and Lin40. Figure 3. 1H NMR spectra of divinylbenzene, extract of the sample CLin50 (CLin50-AcA40-DVB10), conjugated linseed oil, acrylic acid, and solvent for extraction and solvent collected from extracts.
Scheme 1. Intermolecular Hydrogen Bonding Cleavage in the Acrylic Acid Dimer during Polymerization Reaction
Figure 5. FTIR absorbance spectra of the cationic samples Lin50 and Lin60.
3.3. Fourier Transform Infrared Spectroscopy. The dried insoluble part after Soxhlet extraction is analyzed by FTIR spectrometry. The samples are analyzed by a Perkin-Elmer RX-I spectrophotometer. Samples are prepared by mixing a weighed amount of polymer with KBr. Specifically, 1 mg of finely powdered polymer sample is mixed with 100 mg of KBr powder in a mortar and pestle. The mixture is then pressed in a die at about 100 MPa for 3 min to get a transparent disk. This disk is then placed in a sample holder, and the peak is recorded in absorbance. A total of 32 scans at 4 cm-1 resolution are collected to get average spectra. 4. Results and Discussion 4.1. Soxhlet Extraction. Polymeric samples were extracted for their insoluble contents, and these results are reported in Tables 2 and 3. For the cationic samples, with an increase in linseed oil content in the polymeric samples from 30 to 40%, the insoluble contents in the samples decreased from 78 to 63%, while the soluble part increased from 22 to 37%. The plot between the content of the linseed oil and soluble contents of the copolymer sample is shown in Figure 1. These results indicate that, with the increase of linseed oil content, the cross-
linking densities of the polymeric samples decrease. In the thermal samples, an increase in the conjugated linseed oil content results in an increase in the soluble portion of copolymer. It is observed that the soluble portion increases from 4.23 to 52.53% with an increase in the oil content from 0 to 60. Figure 1 also shows that the cationic samples have higher cross-link densities than the thermal samples, as cationic samples have less soluble portion in tetrahydrofuran. This is why the cationic samples are superior to the thermal samples, because cationic samples were prepared by using boron trifluoride diethyl etherate (BFE). 4.2. 1H NMR of Cationic and Thermal Polymers. The 1H NMR spectra of ST, DVB, linseed oil, solvent collected from extractable portion of the polymer (vacuum rotary evaporator), and the soluble extract from the cationic polymeric sample (Lin50) are shown in Figure 2. The extracts from Lin50 are representative of the soluble extracts obtained from all other cationic samples (Lin30 to Lin60). The peaks at 2.8 ppm are due to the methylene (CH _ 2) protons present between the two unsaturated CdC double bonds of the fatty acid chain. The presence of a similar peak in DVB is due to the presence of methylene protons in the ethylvinylbenzene, which is present to the extent of about 20% in the DVB. The peaks for the vinylic (CdCsH _ ) protons of the linseed oil, ST, and DVB are present
Ind. Eng. Chem. Res., Vol. 47, No. 22, 2008 8569
Figure 6. FTIR absorbance spectra of the thermal samples CLin0 and CLin60.
Figure 7. Regression calibration curves for cationic and thermal samples.
at 5.1-5.8 ppm. The peaks at 4.1-4.5 ppm in the soluble extract (Lin30 to Lin60) (sample Lin50 is shown in Figure 2) and in linseed oil are due to the methylene protons (CH _ 2) of the glyceride unit. This is a particularly characteristic peak for the linseed oil. It is used in calculating the oil content in the soluble
Figure 8. Variation of insoluble contents of linseed oil (wt %) versus linseed oil contents (wt %) in polymer sample obtained through 1H NMR and FTIR analysis.
extract of the polymeric material. The aromatic protons of ST, DVB, and the oligomeric portion of these materials are observed between 7.1 and 7.9 ppm. These aromatic peaks are distinctive and are used to calculate the ST and DVB contents in the soluble extracts. The initiator used in the copolymerization of linseed oil with other monomers is modified by linseed oil, and the contents of initiator mixture are taken as linseed oil content. However, the peak due to the solvent CDCl3 that occurs in the same region at 7.26 ppm has been excluded from all calculations. The solvent collected from the soluble portion by vacuum evaporation is checked for the presence of any oligomer. It is found that the peak appears at the same point as for pure solvent (peak shown in Figure 2). The contents of oil and aromatic components (weight percent) of the different samples are reported in Table 2. The linseed oil content (weight percent) in the soluble extract varies from 64 to 90%, whereas the aromatic contents (ST-DVB) vary from 36 to 10%. The values in parentheses in the Soxhlet results of Table 2 indicate the detailed microcomposition of the polymeric samples. The soluble portion present in the samples helps in plasticization of the cross-linked insoluble materials. Thus, the insoluble materials mainly determine the properties of the polymeric material. For the samples Lin30 to Lin60 in Table 2, the amount of oil increases in the insoluble fraction. This fact is consistent with our hypothesis that the linseed is grafted into the polymer chain of the ST and DVB copolymer during prolonged heating. This is calculated from the FTIR studies of the insoluble extract in the next section. In the thermal polymerization of conjugated linseed oil, acrylic acid, and divinylbenzene, the maximum amount of linseed oil is present in the soluble fraction. In comparison to the cationic samples, there is improvement in the quantity of linseed oil in the insoluble portion also due to the conjugation in the linseed oil. The 1H NMR spectra of AcA, DVB, CLin, solvent collected on vacuum evaporation of the soluble extract, and the soluble extract from the polymeric sample CLin50 are shown in Figure 3. The extracts from CLin50 are representative of the soluble extracts obtained from all other samples (CLin0 to CLin60). The peaks at 2.8 ppm are due to the methylene (CH _ 2) protons present between the two unsaturated CdC double
8570 Ind. Eng. Chem. Res., Vol. 47, No. 22, 2008
bonds of the fatty acid chain. The presence of a similar peak in DVB is due to the presence of methylene protons in ethylvinylbenzene, which is present to the extent of about 20% in DVB. The peaks for the vinylic (CdCsH _ ) protons of the linseed oil, AcA, and DVB are present at 5.1-6.8 ppm. The peaks at 4.1-4.5 ppm in the soluble extract (CLin0 to CLin60) (sample CLin50 is shown in Figure 3) and in conjugated linseed oil are due to the methylene protons (CH _ 2) of the glyceride unit. This is a characteristic peak for the linseed oil. It is used in calculating the oil content in the soluble extract of the polymeric material. The aromatic protons of the DVB and the oligomeric portion of the material are observed between 7.1 and 7.9 ppm. These aromatic peaks are distinctive and are used to calculate the DVB content in the soluble extracts. However, the solvent (CDCl3) peak, which occurs in the same region at 7.26 ppm, has been excluded from all calculations. The peak at 11.9 ppm in acrylic acid is due to the -OH _ of the carboxylic group present in the acid, which shifted downward to 9.8 ppm in the polymeric samples. The acids generally exist in dimeric form due to the presence of intermolecular hydrogen bonding (Scheme 1),42 and when polymerization occurs, the dimeric form becomes nonexistent. The cleavage of intermolecular hydrogen bonding leads to the shifting of the signal downfield. This peak is the characteristic peak of acrylic acid. The solvent removed from the soluble portion by vacuum evaporation is free from any oligomers. and the peak is shown in Figure 3. The peak is the same as for pure solvent. The contents of the conjugated linseed oil, acrylic acid, and divinylbenzene (weight percent) for different polymeric samples are reported in Table 3. The content of the linseed oil (weight percent) in the soluble extract varies from 0 to 97%, and the content of acrylic-DVB components varies from 100 to 3%. The content of conjugated linseed oil in the soluble extract and insoluble portion increases with an increase in the oil content in the samples. The linseed oil used for thermal polymerization is 87% conjugated and is more reactive. Therefore, more linseed oil is grafted to the matrix. When the reactivity of conjugated linseed oil is compared with other monomers, it is less reactive. By conjugating the carbon-carbon double bonds, the reactivity of the oils can be improved.43 The peaks at 2.76 ppm, which are due to CH2 groups present between two C-C double bonds, disappear. The decrease in the rigid part in the polymer with increasing oil content results in the higher solubility of the polymer in the solvent. From the results it is observed that the reactivity of monomers has a greater impact on the properties of resulting polymers. Although conjugated linseed oil is reactive, the branching in the oil and the low mobility of the oil results in the low oil content in the cross-linked polymer. 4.3. FTIR Analysis of Linseed Polymers. The quantitative analysis of a component in solution can be successfully carried out, provided there is a suitable characteristic band in the spectrum of the component of the interest. The simple solid mixture can be easily analyzed quantitatively, but a component in a complex mixture presents special problems. The mixture of linseed oil, styrene, and divinylbenzene is also a complex mixture. The selection of the characteristic peaks solves the complexity of this copolymer. To quantify the unknown amount of linseed oil grafted to styrene-divinylbenzene copolymer and the percentage of grafting, two absorbance peaks at 1744 and 1601 cm-1 for the cationic samples and three absorbance peaks at 1744, 1711, and 1530 cm-1 for the thermal samples are selected. In cationic samples, the peak at 1742 cm-1 is for the ester linkage in oil and the second at 1601 cm-1 is for the aromatic -CdC- linkage. These peaks are the characteristic
peaks for the oil and styrene-divinylbenzene in the polymer. In thermal samples, the selected peak at 1744 cm-1 is for ester linkage in the oil, the second at 1711 cm-1 is for the carbonyl stretch in the acrylic acid, and the third at 1530 cm-1 is for aromatic -CdC- linkage in divinylbenzene. The selected absorbance peaks are shown in Figures 4, 5, and 6. The regression calibration curves of various contents (weight percent) against absorbance are obtained from the absorbance of the samples at different peaks (Figure 7). The data from the calibration curve are used to calculate the linseed oil content (weight percent) in the insoluble portion and are reported in Tables 2 and 3. The content of linseed oil (weight percent) through 1H NMR and FTIR analysis is shown in Figure 8. It is observed from Figure 8 that the content (weight percent) of linseed oil in the polymer is almost equal for both 1H NMR and FTIR analysis. It is also observed that the cationic polymer samples have higher contents of bound linseed oil than the thermal samples. The cationic polymer samples contain 3-5 times more polymerized linseed oil than the thermal samples. Conjugated linseed oil is also studied for cationic polymerization by using boron trifluoride diethyl etherate. The catalyst used is very reactive, and it is not possible to control the reaction even at low temperature. The addition of the boron trifluoride diethyl etherate into the reaction mixture results in phaseseparated agglomerates in the mixture immediately. 5. Conclusions Linseed oil based polymers from cationic and thermal polymerizations have been investigated quantitatively through 1 H NMR and FTIR spectroscopic analysis. The solubilities of the samples through Soxhlet extraction range from 22 to 37% and from 4.23 to 53% for cationic and thermal samples, respectively. The content of grafted linseed oil obtained through 1 H NMR ranges from 22.9 to 43.0% and from 0 to 10% for cationic and thermal samples, and that from FTIR is 18.2-45.4% and 0-10.7% for cationic and thermal samples. The values obtained through quantitative 1H NMR and FTIR spectroscopic analysis methods are consistent and can be applied to other polymers also. Literature Cited (1) Koenig, J. L. Spectroscopy of Polymers, 2nd ed.; Elsevier: New York, 1999. (2) Eidelman, N.; Simon, C. G., Jr. Characterization of combinatorial polymer blend composition gradients by FTIR microspectroscopy. J. Res. Natl. Inst. Stand. Technol. 2004, 109, 219–231. (3) Everall, N.; Griffiths, P. R.; Chalmers, J. M. Vibrational Spectroscopy of Polymers: Principles and Practice; Wiley: New York, 2007. (4) Boccaleri, E.; Arrais, A.; Frache, A.; Gianelli, W.; Fino, P.; Camino, G. Comprehensive spectral and instrumental approaches for the easy monitoring of features and purity of different carbon nanostructures for nanocomposite applications. Mater. Sci. Eng., B 2006, 131, 72–82. (5) Smith, B. C. Multiple components II: Chemometric methods and factor analysis in QuantitatiVe spectroscopy: Theory and practice. Academic Press: San Diego, CA, 2002. (6) Henrichs, P. M.; Hewitt, J. M.; Schwartz, L. J.; Bailey, D. B. Quantitative NMR measurements in copolymers of vinylidene chloride and acrylonitrile: The effect of saturation and varying nuclear Overhauser enhancements. J. Polym. Sci., Part A: Polym. Chem. 1982, 20, 775–782. (7) Cheng, H. N. NMR characterization of copolymers that exhibit nonsymmetric compositional heterogeneity. Macromolecules 1997, 30, 4117–4125. (8) Jamaludin, S. M. S.; Nor Azlan, M. R.; Fuad, M. Y. A.; Ishak, Z. A. M.; Ishiaku, U. S. Quantitative analysis on the grafting of an aromatic group on polypropylene in melt by FTIR technique. Polym. Test. 2000, 19, 635–642.
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ReceiVed for reView March 12, 2008 ReVised manuscript receiVed July 10, 2008 Accepted September 1, 2008 IE800415Z
