Pyrolysis-GC/MS/IR Analysis of Kraton 1107

Application Note 228-100

Authors Matthew S. Klee and Imogene Chang Hewlett-Packard Co. Avondale Division

Abstract The pyrolysis gas-chromato­graphic characterization of Kraton 1107 (isoprene-sty­rene elastomer) is described. Detection of capillary-col­umn effluent and identifi­cation of pyrolysates was accomplished with an MSD and IRD connect ed in paral­lel. Cryogenic cooling allowed separation of low boiling pyrolysates and focused peaks of higher boiling pyrolysates. Pro­grammable column pressure control allowed fast analysis while maintaining chromato­graphic efficiency.

Introduction Pyrolysis gas chromatogTaphy is a useful tool for characterizing polymers. Polymers which are not volatile enough for standard g-as chromatogTaphic analysis can be thermally cleaved into small, volatile fragments which are.

The r esulting chromatograms yield characteristic fingerprints relating to the composition of the polymer, structural informa­tion relating to branch ing and defects, and information on addi tives.

Since many fragments are pro­duced during the pyrolysis process, the ability to provide high-resolution separation of these fragments is very impor­tant. Capillary column chroma­tography with low dead-volume connections and cryogenic cooling of both the column and injector ensme maximum resolu­tion of pyr olysates. Since the flow rate through a capillary column decreases with increasing oven temperature (the gas is more viscous, causing decreased flow with fixed head pressure), standard pressure-controlled systems suffer both losses in efficiency and increases in analysis time. The HP 5890 SERIES II used in this study had a progTammable pressure controller which maintained column flow rate during the run by adjusting the column head pressure. In addi tion, after pyrolysates had been trapped, the injector was rapidly tempera-

rf/n- HEWLETT ~a PACKARD

Gas Chromatography

July 1989

ture programmed to ensure sharp injection profiles of solute bands.

Selective detection of pyrolysate peaks is very useful for accurate identification of the peaks rela t­ing to the starting polymer, the mechanisms of thermal degrada­tion, and identification of additives and residual solvents. The combination of mass spec­tral and infrared detectors can provide complimentary informa­tion on molecu- Jar structure and orientation and is, therefore, extremely effective for polymer pyrolysis studies.

Kraton 1107 is a copolymer of isoprene (2-methyl-1,3-butadi­ene) and styrene and is a rubber­like elastomer. Kraton 1107 was the subject of an ASTM experiment aimed at developing a "molecular thermometer" based on the ratio of isoprene to dipentene fragments (1), and its pyrolysis characteristics are well understood . Therefore, the pyrolysis-GC analysis of Kraton 1107 was used in this study to demonstrate the effec­tive combination of the HP 5890 SERIES II with MSD and IRD for this type of analysis.

Experimental The instrumentation and condi­tions used for this study are listed in table 1.

Kraton 1107 was acquired from Chemical Data Systems, Oxford, PA. Pyrolyses were carried out at 700°C and 1000°C in quartz tubes using a platinum coil pyrolysis probe. Sample amounts are stated in the caption of figure 1 and were less than 1 mg.

Pyrolyses were carried out in a heated interface assembly (CDS, Oxford, PA) which is supplied for use with HP 5890 GCs. The standard interface needle assem­bly was too large to fit int o the on-column injector and was replaced with a 28-gauge needle via a 118 in.-1/16 in. Swagelok union and graphite!Vespel ferrule. The pyrolyzer-GC interface was used without the standard quartz liner, and was fed with an auxil­iary supply of helium carrier gas. This auxiliary helium was con­nected to an event-controlled solenoid (in the Purge B position) such that it could be turned off and on within a time progTam. As long as the auxiliary pressure supplied to the pyrolyzer-GC interface exceeds that of the progTammable pressure controller CPPC), it overrides the PPC and supplies all of chromatogTaphic system's carrier gas flow through the interface. As soon as the solenoid turns off th e gas flow through the interface, the PPC resumes control of the system head pressure and no more He flows through the pyrolysis-GC interface.

Pyrolysates were separated by an HP-5 (5% phenyl methyl silicone) capillary column and detected by both MSD and IRD connected in parallel. The inlet end of the column was connected to the cool on-column injector. The outlet of the column was connected to one side of a 1/16 in. zero dead­volume Swagelok union. To the

other end of the union, two transfer lin es were connected via a two-holed gTaphite!Vespel ferrule: a short piece of 0.32 mm HP-5 coated capillary column was used for connection to the IRD, and a 2m x 0.10 mm id blank column was used for connection to the MSD.

Table 1. Pyrolysis GC/MSD/IRD Instrument Parameters

Gas Chromatograph

Cooling

Mass Selective Detector Mass Range

EMV Transfer Line Data System

Infrared Detector Resolution Transfer A,B Cell Data System

Column He Flow Oven Program Injector Program

Pyrolyzer Interface Probe Ramp Interval Pyrolysis Temperature

Auxiliary He Supply Pressure OFF Time

2

Hewlett-Packard 5890 SERIES II Cool On-Column Injector w/ Programmable Pressure and Temperature Control Cryo Blast, C02 Cryogenic (Oven and Injector)

Hewlett-Packard 5970 19-250 amu (0 to 8 min) 37-400 amu (8 min to END) + 0 V relative 325•c Hewlett-Packard 9836C (Rev. 3.1.1)

Hewlett-Packard 5965A 8 cm-1 (4000 to 700 cm-1)

3oo·c 3oo•c Hewlett-Packard 59970C (Rev. 1.0)

30m Series 530 J.l, 2.65 J.lm HP-5 (Part No.19095J-123) 5 psi@ o•c, Constant Flow Mode o• c (1 min)-> 275°C (2 min)@ 15°C!min -1o•c (0.5 min)-> 3oo•c (1 min) @ 12o• cJmin

Chemical Data Systems (Oxford, PA) Model1 22 275 Coil Off (Ballistic) 10 sec 700 and 1000°C

10-12 psi 0.5 min

Results and Discussion Pyrograms of Kraton 1107 at 700 and 1000°C are presented in figures 1 and 2. Figure 1 com­pares the t otal ion chromato­gTams (mass spectrometry), and figure 2 compares the total recon­structed infrared chr omatograms. Chromatographic efficiency was maintained throug·h both detec­tors: peaks are symmetrical and peak widths correlate well be­tween detectors. This is partly due to the low dead-volume connector between the analytical column and the transfer lines, and partly due to the constant-flow operation of the progTammable pressure controller. The column head pr essure was automatically increased (constant fl ow mode) during the run from the starting pressure of 5 psi at 0°C, to about 7.8 psi at the ending temperature of 275°C.

In general, the pyrogTams for the sample pyrolyzed at 1000°C con­tain the same fragments as for the 700°C pyrogTam. The sig·nificant differ ence between the hig·h- and low-temperature pyrolyses is that there are more molecular re­arrangements and smaller molec­ular fragments at higher pyrolysis temperatures.

The identities of the major pyroly­sis fragm ents and several of the minor fragments ar e g·iven in table 2. The identification of peak 11 (about 9.5 min) is not definitive by mass spectrometry, as can be seen in the library search results in figure 3. The library search yielded several possibilities with the the same empirical formula but different structures.

3 KRRTON 1107 :·.;.

14 11

?00 c 12

5 .0E.,.6

1200 c 11 14

10 13 '

· ~~~~ ~-----·- ,-··- 1 -~

2 14 16 !8

Figure 1. Total ion c hromatogram (mass spectral) for the pyrolysis of Kraton 1107. (A) 0.43 mg pyrolyzed at 700°C, (B) 0.51 mg pyrolyzed at 1000°C.

·:·:

" t <. ···o:··•

Q_

14000

12000

10000

8000

6000

.· ~ 14000 ~

12000

KRRTON 1107

3

3

1000 c [::::: 10000 10000

:::: \ ~:::: .;

0

' •••••. ~;::+1. -.J.r_:L..;::::._._il~'!>r>~--..,:t.·~""'<J~---rr..;....;.···;,.!'islt&1

~, ~14

~· :::·eo;!~""· ~··!:::;:::: ....... ~~::: 0 5 10 15 20

· T1:.rne c mt ,., • l

Figure 2. Total reconstructed infrare d chroma togra m s for the same analyses as in figure 1, (A) 700°C, (B) 1000°C.

3

:J ::-a:·.· . .

E

41210

30121

200

·····•··ie0 ·.·. 0 -1----------

·:· ..

4 121 121

3121121

21210

10121

A

B

Q H3C CH2

CD ~a: M '4' r...

CD - "'

"' .. m·-.· . ·m·

4 0 121

3 00

200

4 0 0

300.

200

100

e~· ======~~-r~~~====T=~~~~~~~ 3·0 00 200;;, 1000

WAVENUMBER (cm-1)

Figure 6. Infrared spectra of peaks 14, (A), and 12, (B), of the pyrogram in figure 2(A).

Conclusion The combination of a cryogeni­cally cooled chromatographic system and parallel detection with an MSD and IRD has been shown to be very effective for the pyrolysis-GC analysis of Kraton 1107.

References 1. E. J. Levy and J . Q. Walker, J Chrom Sci, 22 (1984) 49- 55.

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Table 2. Kraton 1107 Pyrogram Peak Identification

Retention Time (min)

1 2 3 4 5 6 7 8 9

10 11 12 13 14

Compound Name

2-methyl-1-propene 2 -methyl-1-outene 2-methyl-1 ,3-butadiene (isoprene) 2-methyl-2-outene 2,3-dimethyl-2-outene 1, 3, 5-hexatriene 3-methyl-1 ,3-pentadiene toluene ethyl benzene a-xylene styrene 1,4-dimethyl-4-vinyl cyclohexene [ethyl methyl benzene co-elutes] 1-propenylbenzene 1-methyl-4-isopropenyl cyclohexene (dipentene)

122 I

(8 CI9CII from OATR;N

!BCI9CIJ from .

!t207Hl l t21:17Hl Chr o mium, tr i ca r bon 1((1,2,3,~.5.6-.eta. fro 1~eee

seee 51

/ A '

78 °' 131:!

/ 156 I

e~~~~~~~~~~~~~~,-~~-r~~+-

40 60 ... ae···· 100 120 140 IS0 Mass / Charge

Figure 3. Results f1·om t h e library search of the mass spectrum of peak 11 of pyrogram l(A).

4

Investigation of the infrared spec­trum of peak 11 allowed unequivo­cal identification as styrene. Fur­ther confirmation was obtained by comparing the infrared spectrum of peak 11 with the styrene peak obtained by pyrolysis of polysty­rene, as shown in figure 4.

The pyrogram of Kraton 1107 shows a separation of two frag­ments, peaks 12 and 14, which have the same molecular weig·ht and similar mass spectra (figure 5), indicating that they are structural isomers. The infrared absorption information shown in figure 6 confirms that they have the same general structure. The shift in absorption wavelengths from 1789 to 1829 em-~ and from 893 to 913 cm- 1 for peaks 14 and 12, respectively, indicate that the terminal vinyl group is disubstituted ( > C = CH2 )

for peak 14, and is monosubsti­tuted (-CH = CH2 ) for peak 12. Peak 14 is 1-methyl-4-isopropenyl cyclo-hexene (dipentene), and peak 12 is probably 1,4-dimethyl-4-vinyl cyclohexene.

:J ([

E

:J ([

E

200

100

··j 70

l

60

50

40

30

2 0

10

0

A

3000 2 0 00 WAVENUMBER Ccm- 1 l

B

3 000 2000 WAVE NUMBER Cc m- l l

"' 01

"' '<

"' ..

l 2H2:0

., <D

"'

1000

F igure 4. Infrared spectra of (A) styrene resulting from the pyrolysis of poly­styrene, and (B) peak 11 from the pyrogram 2(A).

['.... A

2 . 1.lE+6 68

I . SE+6 39 93 I I 79

1.0E+6 5 3 / t I

I 65 .. 1L U P 1 2 1

5 .0E+5

,J , ,I..~ . il l ~. ""'- / 13 6 c

.. I I, 1/5. 1. I u

I. I. c 0.0E+0 • ., c

2 . 0E+S~ B ~ ,Q a: 68

l. 5E+6 /

I . 0E+6 93 39 / / 5 3 107 12 1

5 . 0E+S

1,111

I ""'- 109 / 136

0.0E+0 j ,,,1 ~ ,I. I

' ....J......, ~0 50 6 0 7 0 80 9 0 100 1 10 1 2 0 1 3 0 1 ~0

Hass/ Cha r ao

FigureS. Mass spectra of peaks 14 (A), and peak 12 (B), of the pyrogram in figure 1(A).

5