Characterization of Styrene-Acrylonitrile Copolymer by Pyrolysis Gas Chromatography

8/13/2019 Characterization of Styrene-Acrylonitrile Copolymer by Pyrolysis Gas Chromatography

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JOURNAL OF POLYMER SCIENCE: PART A-1 VOL. 8, 139-146 1970)

Characterization of Styrene-Acrylonitrile Copolymerby Pyrolysis Gas ChromatographyR. WK OV Ie and V. GNJATOVIC,Research Institute I N A Zagreb Yugoslavia

SynopsisSamples of styrene-acrylonitrile (SAN) copolymer of different compositions, molecular

weights, block copolymers, and a blend of styrene and acrylonitrile homopolymen wereprepared and characterized by the method of pyrolysis g s chromatography. On de-composition of SAN copolymer samples a t 645OC, eleven components were identified,the most important of them being styrene, acrylonitrile, and propionitrile. By exam-ination of the pyrolyzate composition during pyrolysis of the SAN copolymer of dif-ferent compositions, it was established that the propionitrile yield was definitely de-creased when the acrylonitrile concentration in copolymer was about 60 mole-o/& Fur-ther, from the propionitrile yield, we could distinguish random SAN copolymer from thestyrene-acrylonitrile homopolymer blend, and on the basis of propionitrile yield someinformation on the molecular structure of the copolymer could be obtained. The styreneyield depends linearly on the copolymer composition. This permits determination ofcopolymer composition on the basis of the styrene yield. Furthermore, the effects ofdecomposition temperature and of molecular weight on the yields of styrene and acrylo-nitrile were examined.

INTRODUCTIONThere is a relatively small number of publications on the study of styrene-

acrylonitrile( SAN) copolymer by the method of pyrolysis gas chroma-tography.'+ According to Voight,' it was possible to identify SAN co-polymer by this method. Lebe12 reported that this method could success-fully be used to distinguish a homopolymer blend from the random SANcopolymer. Shibasaki and Kambe3-5 determined that on the basis ofstyrene and acrylonitrile yields it was possible to get data on the structuralcomposition of the copolymer. These authors succeeded in identifying,in addition to styrene and acrylonitrile, benzene, a-methylstyrene, andethylbenzene by this method. The latter two components were found intrace amounts only.

We used the method of gas chromatography to separate acetonitrileand propionitrile from acrylonitrile and ethylbenzene, and a-methylstyrenefrom styrene; we identified eleven components which helped us to make adetailed study of the effect of the copolymer composition on the yields ofthe individual components of the pyrolysate. I n addition, the effect of the

139Q 197 by John Wiley Sons, Inc.


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140 VUKOVI C AND GNJATOVICdecomposition temperature and the molecular weight on the yields ofstyrene and acrylonitrile monomers was examined.

EXPERIMENTALMonomersand Initiators

Acrylonitrile (AN), Carlo Erba, purity 99 , was treated with concen-trated NaOH and then washed repeatedly with distilled water. After that itwas dried and distilled over calcium hydride. Styrene (St) was polymergrade (produced by Organsko Kemijska Industrija, Zagreb), with no tracesof polymers, purity 99.96% and containing 12 ppm of p-tert-butyl catechol.

Lauroyl peroxide (Noury Van der Lande) and 2,2-azobis-2-methyl-propionitrile (Eastman Organic Chemicals) were used without furtherpurification.

Polymers and PolymerBlendsSamples of SAN copolymers of different composition were prepared by

bulk polymerization a t 70 C n the presence of lauroyl peroxide. Aftera conversion of 3 4 % was reached, the polymer was precipitated withethanol and then purified by precipitation with ethanol from an acetonesolution. This procedure was repeated three times, after which thepolymer was dried under vacuum to a constant weight. The compositionof the prepared samples is shown in Table I.

TABLE IComposition of SAN Copolymer Samples

AN in copolymer,Sample no. mole- .

12.528.838..547.558.062.077.0

* Calculated from nitrogen content (Kjeldahl).

Okirol N-3 (trade name of polystyrene produced by Organsko KemijskaIndustrija) was purified by repeated precipitation of with methanol from abenzene solution. After three precipitations the polymer was dried undervacuum to constant weight.

Polyacrylonitrile was prepared by slurry polymerization as described byWilkinson.7

The styrene- and acrylonitrile block copolymer was prepared accordingto the method of Frankel and et a1.8 with n-butyllithium as initiator.


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STYRENEACRYLONITRILE COPOLYMER 141Samples of azeotropic SAN copolymer of different molecular weights

were prepared by suspension polymerization at 70 1C in the presenceof 2,2 -azobis-2-methylpropionitrile of different concentrations. TableI1 shows the characteristics of these samples.

TABLE I1Molecular Weights of SAN Copolymer of Aseol,ropic Coiiiposit ori

0.5041.0201.1781.418

142 000370000466 700623 700

a Calculated according to the data of Shimura et al.? [ v ] = 36 X M P 2 .Intrinsic viscosities were measured in a Cannon-Fenske capillary vis-

cometer, No. 100, in methyl ethyl ketone at 30 0.01 C.Pyrolysis Apparatus, Gas Chromatograph, andExperimental Conditions

The pyrolysis apparatus used in this study has been described else-where. o The analysis of the decomposition products was carried out withthe Model 800 Perkin-Elmer chromatograph, which was connected to thepyrolyzer by a short precut glass column. The analysis was carried outunder the following conditions: column, Chromosorb P 30-60 mesh, 7 X0.6 cm; analytical column, 20% poly(propy1ene glycol) UC oil LB-550-Xon Chromosorb P 30-60 mesh, 200 X 0.6 cm, temperature 25-200C;carrier gas, Nz; sample used, 1-5 mg.

RESULTS AND DISCUSSIONDetermination of the Pyrolyzate Composition

On decomposition of SAN copolymers a t a temperature of 645C (Fig.1 the following components were identified by calibration against puresubstances: acetonitrile, acrylonitrile, propionitrile, benzene, allylcyanide,toluene, ethylbenzene, styrene, a-methylstyrene, diethylbenzene, and ethyl-vinylbenzene.

Quantitative analysis of the decomposition products was performed bymeasuring the peak area and using a calibration curve obtained for purecomponents. The quantitative yeilds of the individual components permilligram of sample is shown in Figure 2.

Figure 2 shows that, in addition to styrene and acrylonitrile the identi-fied components obtained in the largest amounts are propionitrile, toluene,benzene and a-methylstyrene. The yield of acetonitrile from copolymerscontaining 31-63 wt-% of acrylonitrile amounts to 2-4 wt- . When theAN content is below 31 wt-yo, acetonitrile is fourid in coiicentration below


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S T Y R E N E A C R Y L O N IT R IL E CO PO LY M ER 143tween the random copolymer and the mixture of homopolymers a t all thecompositions and the important effect of copolymer composition on thepropionitrile yield when the copolymer contains 60 mole-yo or more of AN.It is apparent that at a higher acrylonitrile content the relative propio-

STYRENEACRYLONITRILEPROPI0Nl~RlL.Ex

TOLUENEBLNZENE~C-MLTHYLSTYRLNE

M 2 30 M x 60 10 80 w mCOPOLYMER COi'lPOSITION IAN Wt. I

Content of various componentsof the pyrolyzate of random SAN copolymer ofdifferent compositions calculated on the sample weight. Pyrolysis temperature 645OC,in Nz stream.

Fig. 2 .

PROPfONiTRlLE FOR COPOLYMU?lu

O 90 80 7 60 50 10 30 2 {O 0COPOLYMER C(WPOSIT/ONAN mole%/

Fig. 3. Propionitrile yields on decompositionof SAN copolymerof different composi-tions and of the homopolymer mixture versus the acrylonitrile content in the sample.Pyrolysis temperature 645 C, in NL tream.


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144 VUKOVIC AND GNJATOVIC

t 20 30 M 50 6 70 do 90 KWCOPDLYH MtPOJfTlON Mmol.;c /

Fig. 4. Calibration curve for the determination of the copolymer composition.Styrene content of the pyrolyaate during the decompositionof random SAN copolymersof different compositions, calculated on the total sample weight. Pyrolysis temperature645OC, in Nzstream.

S W w 6 m a m 7 M I W mPYRCX YSIS IP RAlURE TI

Fig. 5. Effect of decomposition temperature on the yields of styrene and acrylonitrileYields were cal-n the decomposition of aaeotropic SAN copolymer in a stream of Nz.

culated on the total sample weight.

STYRENEACRYLONfTR/LE

-.

1 2 3 4 5 8MOLECULARWEIOHT / f i v x a - 5 /

Fig. 6 Effect of the molecular weight of azeotropic SAN copolymer on the styrene andCalculations based on the total sample weight. Pyrolysis tempera-cryloriitrile yields.

t u r e 645 C, in Ne stream.


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STYRENEACRYLONITRILE COPOLYMER 145nitrile yield is smaller as a consequenceof the random SAN copolymer struc-ture.

For different polystyrene and polyacrylonitrile mixtures, the propioni-trile, styrene, and acrylonitrile yields remain constant at all homopolymerratios. The block copolymer shows the same relative yield of propio-nitrile, acrylonitrile, and styrene as the corresponding mixtures and it isimpossible to distinguish it from the homopolymer blend. The styreneyield expressed in weight per cent depends linearly on the copolymer com-position expressed in mole per cent AN, so that the copolymer'compositioncan easily be determined (Fig. 4).

Styrene and acrylonitrile yields of the decomposition of SAN coploymersof different compositions obtained in this study are different from theyield percentages reported by Shibasaki and Kambe.3

We suppose that this difference may be atrributable to a difference in theapparatus used3r6and of a difference in the separation of the componentsidentified in the pyrolyzate (Fig. 1). Figure 1 shows that the acetonitrileand propionitrile peaks appear before and behind the acrylonitrile peak.Figure 2 shows that the concentration of these may have a significanteffect on the differences in results. The same applies to the styrene con-tent of the copolymer, because the a-methylstyrene and ethylbenzenepeaks are completely separated from this of styrene.

Effect of Decomposition TemperatureThe effect of the decomposition temperature in the range 500-8GOO C on

the yields of styrene and acrylonitrile was studied. Figure 5 shows thatthe acrylonitrile yield continues to increase up to 700C. Above this tem-perature the decomposition of acrylonitrile units begins, resulting in anincrease of the content of more volatile components and decrease of theacrylonitrile content. The same figure shows that the increase of thetemperature from 500 to 700C has no influence on the styrene yield.

At above 700C the styrene yield also decreases as a consequence ofdecomposition of the styrene units.

Effect of Molecular WeightIn order to examine the effect of molecular weight on the styrene and

acrylonitrile yields, samples of azeotropic S A N copolymers of differentmolecular weights were pyrolyzed at 645C.

Figure 6 shows the yields of styrene and acrylonitrile monomers depend-ing on the change of the molecular weight.It is apparent that the styrene yield is decreased by an insignificantamount and the acrylonitrile yield remains constant with increasingmolecular weights.

The authors wish to express their thanks to Dr. 11. FleJ and Dr. D. Deur-Siftar forhelpful discussionsand nterest in this work, and to Mi-. Z. SliepEevi6 for microanalysis.


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146 VUKOVIC AND GNJATOVIC

References1. J. Voight,Kunststofe 51,18 (1961).2. P. Lebel Rubber Plastics Age, 46,677 (1965).3. J. Shibasaki and H. Kambe, KohnshiKagaku 21,71 (1964).4. J. Shibasaki,Kohnshi Kagalcu 21,125 (1964).5 . J. Shibasaki,J. Polym. Sci.,A-I 5, 21 (1967).6. F. A. Lehman and G. M. Brauer, Anal. C h m . , 3 3 ,8 7 3 (1961).7. W . K. Wilkinson in Mac r ml e c uZar Syntheses, Vol. 11,J. lt Elliott, Ed., Wiley,8. M. Frankel, A. Otolenghi, M. Alabeck, and A. Zilkha, J . Chem.SOC , 959,5858.9. J. Shimura,J. Mita, and H. Kttmbe, J. Polym. Sci.B , 2,403 (1964).

New York, 1966, p. 78.

10. D. Deur-Siftar, T. Bistrirki, and T. Tandi, J . Chromatog. 24,404 (1966).Received April 4 969

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Characterization of Styrene-Acrylonitrile Copolymer by Pyrolysis Gas Chromatography