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 quantitativelythrough 1H NMR and FTIR spectroscopic analysis. The solubility of the samples ranges from 22 to 37% forcationic samples and from 4.23 to 53% for thermal samples. The content of the grafted linseed oil calculatedfrom 1H NMR results ranges from 22.9 to 43.0% and from 0 to 10% for cationic and thermal samples. Thegrafted linseed oil contents from FTIR are 18.2-45.4% and 0-10.7% for cationic and thermal samples. Thevalues obtained through quantitative 1H NMR and FTIR spectroscopic analysis methods are consistent andcan be applied to other polymers also.

1. Introduction

The spectroscopic techniques are one of the importantparameters for characterizing different polymers.1-3 Thesetechniques are powerful tools for the characterization of poly-mers both qualitatively4 and quantitatively.5 A lot of researchwork is already published on the quantitative analysis ofdifferent polymers by NMR and Fourier transform infrared(FTIR) spectroscopies.6-9 Hazer et al.10 used FTIR and NMRto calculate poly(ethylene glycol) contents in cross-linkedpoly(ethylene glycol)-block-polybutadiene block copolymers.Natural oils are always a center of attraction as an alternativesource for the production of useful polymers.11,12 Larock etal.13-17 have done remarkable work on the polymerization ofvarious natural oils. They directly polymerized oils with othermonomers. Wool and co-workers18-22 used these oils for theproduction of polymers by modifying the oil moieties, andsubsequently polymerizing them with other monomers. Souceket al.23-29 have used epoxidized linseed and soybean oils forpolymerization. Baki et al.30-34 have done work on poly(hy-droxyalkanoate) (PHA) based on natural oils for the preparationof unsaturated bacterial polyester.

In the present work, linseed oil based polymers have beensynthesized and studied quantitatively by 1H NMR and FTIR.In the literature, Larock et al.13,35-37 have quantitatively studiedthe 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 throughFTIR. There is a lot of published work on the quatitification ofpolymers through FTIR.38-40 The present work reports thecomplete quantitative analysis of linseed oil polymers through1H NMR and FTIR.

2. Experimental Section

2.1. Materials. Linseed oil of commercial grade was pro-cured from a local market (Punjab, India), and conjugatedlinseed oil (87% conjugation) was purchased from Alnor OilCompany, Alnor, NY. Styrene (ST), acrylic acid (AcA),tetrahydrofuran, and boron trifluoride diethyl etherate complexwere purchased from Merck Chemical Co., Germany. Divinyl-benzene (DVB) was purchased from Fluka Chemie.

2.2. Sample Preparation. 2.2.1. Cationic Sample Pre-paration. Polymeric materials have been prepared by heatingdesired concentrations of linseed oil, styrene, and divinyl-benzene in glass molds.41 In these experiments, the styreneto divinylbenzene ratio is kept constant at 2:1, respectively.Desired amounts of styrene and divinylbenzene are added tothe linseed oil, and the mixture is vigorously stirred. Then,the mixture is cooled and the initiator is added with constantstirring at low temperature and the whole mass is transferredto the glass mold. The sealed glass mold is kept at roomtemperature for 12 h and then heated sequentially at differenttemperatures, 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. Thenomenclature used in this work is based on the originalcompositions of the reactants (reported in Table 1).

2.2.2. Thermal Sample Preparation. Polymeric materialshave been prepared by heating desired concentrations ofconjugated linseed oil, acrylic acid, and divinylbenzene in glassvials. Desired amounts of acrylic acid and divinylbenzene areadded to the conjugated linseed oil, and the mixture is vigorouslystirred. The glass vial is heated sequentially at differenttemperatures, such as for 6 h at 80 °C, for 12 h at 100 °C, andfor 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 originalcompositions of the reactants (reported in Table 1).

3. Characterization

3.1. Soxhlet Extraction. The polymeric materials as reportedin Table 1 are Soxhlet extracted for their soluble and insoluble

* To whom correspondence should be addressed. E-mail: ppk923@yahoo.com.

† Sant Longowal Institute of Engineering & Technology.‡ Punjabi University.

Table 1. Detailed Feed Compositions of Different Linseed OilPolymers

sampleIDa

linseedoil (%)

styrene(%)

divinylbenzene(%)

acrylicacid (%)

initiator(%)

Lin30 30 46 15.5 8Lin40 40 39 13 8Lin50 50 31.5 10.5 8Lin60 60 24 8 8CLin0 0 10 90Clin10 10 10 80CLin20 20 10 70CLin30 30 10 60CLin40 40 10 50CLin50 50 10 40CLin60 60 10 30

a CLin samples contain 87% conjugated linseed oil.

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10.1021/ie800415z CCC: $40.75 2008 American Chemical SocietyPublished on Web 10/21/2008

contents. A sample ranging from 2 to 3 g of the bulk polymeris extracted for 24 h with 150 mL of refluxing tetrahydrofuranusing a Soxhlet extractor. After extraction, the resulting solutionis concentrated by rotary evaporation and subsequent vacuumdrying. The insoluble solid is dried under vacuum for severalhours before weighing.

3.2. 1H Nuclear Magnetic Resonance (1H NMR). Theextracted soluble part of the polymeric material as well as thelinseed oil, styrene, and divinylbenzene are dissolved in CDCl3.Tetramethylsilane (TMS) is used as reference compound. Thesolution is scanned with a multinuclear FT-NMR spectrometer(Bruker AC-300 F) at 300 MHz. A total of 30 scans are averagedto obtain the final data.

Table 2. Detailed Compositions of Cationically Cross-Linked Linseed Oil, Styrene, and Divinylbenzene Copolymers from Soxhlet Extraction,and 1H NMR and FTIR Spectroscopic Results

sample soluble extractible compositiona Soxhlet results

ID composition wt % oilwt % STand DVB

solubleb

(wt %)insolublec

(wt %)

linseed oilcontent (wt %)

by FTIRd

Lin30 Lin30 + ST46.5 + DVB15.5 + In8 64.68 35.32 22.50 (7.9; 14.6) 77.50 (54.1;23.4) 18.16Lin40 Lin40 + ST39 + DVB13 + In8 89.72 10.28 28.00 (2.9; 25.1) 72.00 (49.1;22.9) 24.56Lin50 Lin50 + ST31.5 + DVB10.5 + In8 78.99 21.01 33.33 (7.00; 26.33) 66.67 (28.00;38.67) 36.89Lin60 Lin 60 + ST24 + DVB8 + In8 87.76 12.24 36.67 (4.49; 32.18) 63.33 (20.27;43.06) 45.39

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 7ppm. b The data in parentheses have been calculated directly from the weight percent oil and weight percent aromatic content in the soluble extract. Thefirst value in the parentheses represents the percent aromatic content, and the second value represents the percent oil content. c The data in theparentheses have been calculated indirectly from the weight percent of the oil and aromatic content in the soluble extract, as the total mass of thesoluble and insoluble parts was held constant. The first value in the parentheses represents the percent aromatic content and the second value representsthe 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 aromaticstretching for styrene-divinylbenzene, respectively.

Table 3. Detailed Compositions of Thermally Cross-Linked Conjugated Linseed Oil, Acrylic Acid, And Divinylbenzene Copolymers fromSoxhlet 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 CLin0 + AcA90 + DVB10 0 100 4.23 (4.23; 0) 95.77 (95.77; 0) 0CLin10 CLin10 + AcA80 + DVB10 91.75 8.25 9.47 (0.78; 8.69) 90.53 (89.22; 1.31) 1.75CLin20 CLin20 + AcA70 + DVB10 93.94 6.06 19.49 (1.18; 18.31) 80.51 (78.82; 1.69) 1.83CLin30 CLin30 + AcA60 + DVB10 96.74 3.26 30.03 (0.98; 29.05) 69.97 (69.02; 0.95) 1.70CLin40 CLin40 + AcA50 + DVB10 96.86 3.14 38.95 (1.22; 37.73) 61.05 (58.78; 2.27) 2.87CLin50 CLin50 + AcA40 + DVB10 95.82 4.18 44.85 (1.87; 42.98) 55.15 (48.13; 7.02) 6.84CLin60 CLin60 + AcA30 + DVB10 95.12 4.88 52.53 (2.56; 49.97) 47.47 (37.44; 10.03) 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 9.8ppm, 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-DVBcontent in the soluble extract. The first value in the parentheses represents the percent acrylic-DVB content, and the second value represents the percentoil content. c The data in the parentheses have been calculated indirectly from the weight percent of the oil and acrylic-DVB content in the solubleextract, as the total mass of the soluble and insoluble parts was held constant. The first value in the parentheses represents the percent acrylic-DVBcontent, and the second value represents the percent oil content. d The data are calculated from absorbance peaks at 1744, 1711, and 1530 cm-1 forcarbonyl 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. Figure 2. 1H NMR spectra of divinylbenzene, extract of the sample Lin50

(Lin50-ST29-DVB13-In8), linseed oil, styrene, and solvent for extractionand solvent collected from extracts.

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3.3. Fourier Transform Infrared Spectroscopy. The driedinsoluble part after Soxhlet extraction is analyzed by FTIRspectrometry. The samples are analyzed by a Perkin-Elmer RX-Ispectrophotometer. Samples are prepared by mixing a weighedamount of polymer with KBr. Specifically, 1 mg of finelypowdered polymer sample is mixed with 100 mg of KBr powderin a mortar and pestle. The mixture is then pressed in a die atabout 100 MPa for 3 min to get a transparent disk. This disk isthen placed in a sample holder, and the peak is recorded inabsorbance. A total of 32 scans at 4 cm-1 resolution arecollected to get average spectra.

4. Results and Discussion

4.1. Soxhlet Extraction. Polymeric samples were extractedfor their insoluble contents, and these results are reported inTables 2 and 3. For the cationic samples, with an increase inlinseed 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 plotbetween the content of the linseed oil and soluble contents ofthe copolymer sample is shown in Figure 1. These resultsindicate that, with the increase of linseed oil content, the cross-

linking densities of the polymeric samples decrease. In thethermal samples, an increase in the conjugated linseed oilcontent results in an increase in the soluble portion of copolymer.It is observed that the soluble portion increases from 4.23 to52.53% with an increase in the oil content from 0 to 60. Figure1 also shows that the cationic samples have higher cross-linkdensities than the thermal samples, as cationic samples haveless soluble portion in tetrahydrofuran. This is why the cationicsamples are superior to the thermal samples, because cationicsamples were prepared by using boron trifluoride diethyl etherate(BFE).

4.2. 1H NMR of Cationic and Thermal Polymers. The 1HNMR spectra of ST, DVB, linseed oil, solvent collected fromextractable 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 arerepresentative of the soluble extracts obtained from all othercationic samples (Lin30 to Lin60). The peaks at 2.8 ppm aredue to the methylene (CH_ 2) protons present between the twounsaturated CdC double bonds of the fatty acid chain. Thepresence of a similar peak in DVB is due to the presence ofmethylene protons in the ethylvinylbenzene, which is presentto the extent of about 20% in the DVB. The peaks for the vinylic(CdCsH_) protons of the linseed oil, ST, and DVB are present

Figure 3. 1H NMR spectra of divinylbenzene, extract of the sample CLin50(CLin50-AcA40-DVB10), conjugated linseed oil, acrylic acid, and solventfor extraction and solvent collected from extracts.

Scheme 1. Intermolecular Hydrogen Bonding Cleavage in theAcrylic Acid Dimer during Polymerization Reaction

Figure 4. FTIR absorbance spectra of the cationic samples Lin30 and Lin40.

Figure 5. FTIR absorbance spectra of the cationic samples Lin50 and Lin60.

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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 inlinseed oil are due to the methylene protons (CH_ 2) of theglyceride unit. This is a particularly characteristic peak for thelinseed oil. It is used in calculating the oil content in the soluble

extract of the polymeric material. The aromatic protons of ST,DVB, and the oligomeric portion of these materials are observedbetween 7.1 and 7.9 ppm. These aromatic peaks are distinctiveand are used to calculate the ST and DVB contents in the solubleextracts. The initiator used in the copolymerization of linseedoil with other monomers is modified by linseed oil, and thecontents of initiator mixture are taken as linseed oil content.However, the peak due to the solvent CDCl3 that occurs in thesame region at 7.26 ppm has been excluded from all calcula-tions. The solvent collected from the soluble portion by vacuumevaporation is checked for the presence of any oligomer. It isfound 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 oilcontent (weight percent) in the soluble extract varies from 64to 90%, whereas the aromatic contents (ST-DVB) vary from36 to 10%. The values in parentheses in the Soxhlet results ofTable 2 indicate the detailed microcomposition of the polymericsamples. The soluble portion present in the samples helps inplasticization of the cross-linked insoluble materials. Thus, theinsoluble materials mainly determine the properties of thepolymeric material. For the samples Lin30 to Lin60 in Table2, the amount of oil increases in the insoluble fraction. Thisfact is consistent with our hypothesis that the linseed is graftedinto the polymer chain of the ST and DVB copolymer duringprolonged heating. This is calculated from the FTIR studies ofthe insoluble extract in the next section.

In the thermal polymerization of conjugated linseed oil,acrylic acid, and divinylbenzene, the maximum amount oflinseed oil is present in the soluble fraction. In comparison tothe cationic samples, there is improvement in the quantity oflinseed oil in the insoluble portion also due to the conjugationin 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 areshown in Figure 3. The extracts from CLin50 are representativeof the soluble extracts obtained from all other samples (CLin0to CLin60). The peaks at 2.8 ppm are due to the methylene(CH_2) protons present between the two unsaturated CdC double

Figure 6. FTIR absorbance spectra of the thermal samples CLin0 andCLin60.

Figure 7. Regression calibration curves for cationic and thermal samples.

Figure 8. Variation of insoluble contents of linseed oil (wt %) versus linseedoil contents (wt %) in polymer sample obtained through 1H NMR and FTIRanalysis.

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bonds of the fatty acid chain. The presence of a similar peak inDVB is due to the presence of methylene protons in ethylvi-nylbenzene, 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 at4.1-4.5 ppm in the soluble extract (CLin0 to CLin60) (sampleCLin50 is shown in Figure 3) and in conjugated linseed oil aredue to the methylene protons (CH_ 2) of the glyceride unit. Thisis a characteristic peak for the linseed oil. It is used in calculatingthe oil content in the soluble extract of the polymeric material.The aromatic protons of the DVB and the oligomeric portionof the material are observed between 7.1 and 7.9 ppm. Thesearomatic peaks are distinctive and are used to calculate the DVBcontent in the soluble extracts. However, the solvent (CDCl3)peak, which occurs in the same region at 7.26 ppm, has beenexcluded from all calculations. The peak at 11.9 ppm in acrylicacid is due to the -OH_ of the carboxylic group present in theacid, which shifted downward to 9.8 ppm in the polymericsamples. The acids generally exist in dimeric form due to thepresence of intermolecular hydrogen bonding (Scheme 1),42 andwhen polymerization occurs, the dimeric form becomes non-existent. The cleavage of intermolecular hydrogen bonding leadsto the shifting of the signal downfield. This peak is thecharacteristic peak of acrylic acid. The solvent removed fromthe soluble portion by vacuum evaporation is free from anyoligomers. and the peak is shown in Figure 3. The peak is thesame as for pure solvent.

The contents of the conjugated linseed oil, acrylic acid, anddivinylbenzene (weight percent) for different polymeric samplesare reported in Table 3. The content of the linseed oil (weightpercent) in the soluble extract varies from 0 to 97%, and thecontent of acrylic-DVB components varies from 100 to 3%.The content of conjugated linseed oil in the soluble extract andinsoluble portion increases with an increase in the oil contentin the samples. The linseed oil used for thermal polymerizationis 87% conjugated and is more reactive. Therefore, more linseedoil is grafted to the matrix. When the reactivity of conjugatedlinseed oil is compared with other monomers, it is less reactive.By conjugating the carbon-carbon double bonds, the reactivityof the oils can be improved.43 The peaks at 2.76 ppm, whichare due to CH2 groups present between two C-C double bonds,disappear. The decrease in the rigid part in the polymer withincreasing oil content results in the higher solubility of thepolymer in the solvent. From the results it is observed that thereactivity of monomers has a greater impact on the propertiesof resulting polymers. Although conjugated linseed oil isreactive, the branching in the oil and the low mobility of theoil results in the low oil content in the cross-linked polymer.

4.3. FTIR Analysis of Linseed Polymers. The quantitativeanalysis of a component in solution can be successfully carriedout, provided there is a suitable characteristic band in thespectrum of the component of the interest. The simple solidmixture can be easily analyzed quantitatively, but a componentin a complex mixture presents special problems. The mixtureof linseed oil, styrene, and divinylbenzene is also a complexmixture. The selection of the characteristic peaks solves thecomplexity of this copolymer. To quantify the unknown amountof linseed oil grafted to styrene-divinylbenzene copolymer andthe percentage of grafting, two absorbance peaks at 1744 and1601 cm-1 for the cationic samples and three absorbance peaksat 1744, 1711, and 1530 cm-1 for the thermal samples areselected. In cationic samples, the peak at 1742 cm-1 is for theester linkage in oil and the second at 1601 cm-1 is for thearomatic -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 esterlinkage in the oil, the second at 1711 cm-1 is for the carbonylstretch in the acrylic acid, and the third at 1530 cm-1 is foraromatic -CdC- linkage in divinylbenzene. The selectedabsorbance peaks are shown in Figures 4, 5, and 6. Theregression calibration curves of various contents (weight percent)against absorbance are obtained from the absorbance of thesamples at different peaks (Figure 7). The data from thecalibration curve are used to calculate the linseed oil content(weight percent) in the insoluble portion and are reported inTables 2 and 3. The content of linseed oil (weight percent)through 1H NMR and FTIR analysis is shown in Figure 8. It isobserved from Figure 8 that the content (weight percent) oflinseed oil in the polymer is almost equal for both 1H NMRand FTIR analysis. It is also observed that the cationic polymersamples have higher contents of bound linseed oil than thethermal samples. The cationic polymer samples contain 3-5times more polymerized linseed oil than the thermal samples.

Conjugated linseed oil is also studied for cationic polymer-ization by using boron trifluoride diethyl etherate. The catalystused is very reactive, and it is not possible to control the reactioneven at low temperature. The addition of the boron trifluoridediethyl etherate into the reaction mixture results in phase-separated agglomerates in the mixture immediately.

5. Conclusions

Linseed oil based polymers from cationic and thermalpolymerizations have been investigated quantitatively through1H NMR and FTIR spectroscopic analysis. The solubilities ofthe 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 through1H NMR ranges from 22.9 to 43.0% and from 0 to 10% forcationic and thermal samples, and that from FTIR is 18.2-45.4%and 0-10.7% for cationic and thermal samples. The valuesobtained through quantitative 1H NMR and FTIR spectroscopicanalysis methods are consistent and can be applied to otherpolymers also.

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ReceiVed for reView March 12, 2008ReVised manuscript receiVed July 10, 2008

Accepted September 1, 2008

IE800415Z

Ind. Eng. Chem. Res., Vol. 47, No. 22, 2008 8571