Investigations of polymers by field desorption and fast atom bombardment mass spectrometry

Journal of Analytical and Applied Pyrolysis, 8 (1985) 109-121 Elsevier Science Publishers B.V.. Amsterdam - Printed in The Netherlands

INVESTIGATIONS OF POLYMERS BY FIELD DESORPTION FAST ATOM BOMBARDMENT MASS SPECTROMETRY

MONK4

DOERR,

INGO

LUEDERWALD

Fachhochschule Fresenius, Dambachtal20,

* and HANS-ROLF

109

AND

SCHULTEN

D - 6200 Wiesbaden (FIR. G.)

SUMMARY Field desorption (FD) and fast atom bombardment (FAB) mass spectrometry (MS), which do not necessarily require the evaporation of volatile pyrolysis products before ionization and registration, have been applied to synthetic polymers. The resulting mass spectra show that, in accordance with other techniques such as pyrolysis-electron impact and pyrolysis-field ionization MS, ions leave the matrix on the emitter or target only after a thermal degradation step. In contrast to the gas-phase ionization techniques, the volatility of the primarily formed pyrolysis products is of less importance, leading to comparably larger ions and probably ion clusters. Our first results show that FAB-MS can be applied to synthetic polymers in addition to the FD technique. In general, these investigations with polyesters in the positive and negative ion modes yield characteristic ions with a thermally formed end-group and charge localization at one ionic end-group.

INTRODUCTION

The identification and structural determination of polymers by pyrolysis-mass spectrometry (Py-MS) has become a recognized and established method [l]. Using electron impact (EI), MS, field ionization (FI) MS or chemical ionization (CI) MS, polymeric samples are heated in the high vacuum of the ion source until thermal degradation reactions are initiated and the volatile pyrolysis products transferred into the gas phase. After subsequent ionization by either EI, FI or CI, the pyrolysates are partially further fragmented and detected by the usual registration methods. The characteristic features of these Py-MS methods are that macromolecules are thermally degraded in a clearly defined first step and the pyrolysis products are then immediately removed from the hot region. Further, the thermal excitation of the pyrolysis products normally causes fragmentation reactions after or during ionization. Whereas Py-EI mass spectra of synthetic polymers usually do not show “molecular ions”, Py-CI and Py-FI mass spectra exhibit increasing amounts of parent ions in addition to the fragments. The 01652370/85/$03.30

0 1985 Elsevier Science Publishers

B.V.

110

advantages of the above techniques are that secondary chemical reactions are avoided by the fast removal of the pyrolysis products into the gas phase and that fragmentation reactions are diagnostic of functional groups and can help to identify polymers or copolymers. On the other hand, the molecular masses of the detectable pyrolysis products depend on their volatilities and the fragmentation additionally decreases the size of the registered ion signals. The first experiments with synthetic polymers were carried out with matrix ionization methods such as field desorption (FD) [2,3] and fast atom bombardment (FAB) [4]. Using these techniques, the samples are dissolved or dispersed in the appropriate mixtures of organic solvents and electrolytes and placed on the emitter or target, respectively. The ionization takes place by interaction with a strong electric field (in FD) or with high-energy heavy atoms, e.g., xenon (in FAB). Additionally, FD emitters are heated by a heating current or by radiation with a laser beam [5]. Theoretically, macromolecules on an emitter or target could be ionized before thermal degradation, which, however, should not yield a mass spectrum because these macromolecules are outside the mass range of the commercial mass spectrometer used. On the other hand, signals of volatile products formed by two ionic fragmentation steps should be rare. Therefore, a non-ionic degradation process should be involved in the reactions occurring in macromolecules on FD emitters or FAB targets. In this paper, we descibe the investigation of aliphatic polyesters of adipic and succinic acids with various diols by FD and FAB-MS.

EXPERIMENTAL

The polyesters

investigated

were

CO-_(CH,),-CO-0-_(CH,$,-0

X

Y

name

4 4 4 4 2

2 3 4 5 3

poly(ethylene adipate) poly(propylene adipate) poly(butylene adipate) poly(pentylene adipate) poly(propylene succinate)

I II III IV V

Field desorption mass spectra were obtained with a Finnigan MAT 731 double-focusing instrument with a combined EI/FD/FAB ion source. Polymer samples were dissolved in tetrahydrofuran to give a lpg/pl solution. A few microlitres of this solution were applied to the emitter by a syringe under control of a stereomicroscope. The pressure in the ion source was about lo-’ Pa and the temperature was 50°C. The sample was thermally

111

degraded on the emitter surface and field desorbed by raising the emitter heating current from 0 to 40 mA (from 50 to SOO’C) within 10 min. The thermal degradation products were ionized in the high electric field at an emitter potential of + 8 and - 3 kV for the counter electrode. Spectra were recorded electrically and represent the integrated ion signals (SS 300 data system) of approximately 30 scans. Fast atom bombardment spectra were obtained with the same instrument equipped with a FAB pushrod and a saddle field ion gun and power supply (Ion Tech., Teddington, Great Britain). High-purity xenon (99.99 vol.%) was used as the collision gas. The sample was applied to the copper target (2.2 mm diameter) in tetrahydrofuran solution (concentration 10 pg/pl) and a 1 : l’ mixture with glycerol. Sodium chloride was added as an electrolyte at a concentration of ca. 2%. The conditions for electrical registration were similar to those described for FD-MS.

RESULTS

AND DISCUSSION

The FD and (positive and negative mode) FAB mass spectra of poly(ethylene adipate) (I) have recently been reported [4]. They mainly exhibit peaks of ions with obviously one neutral, thermally formed end-group and a second one formed by ionic degradation. The favoured non-ion degradation step is the cleavage of the ester bond and formation of ketene and hydroxyend-groups (eqn. 1).

-CO-CH2--CH2--CH2-CH

,

-CH2

-O-

0)

0

4 -CO-CH2-CHZ-CH2-CH=C=O+HO-CH2-CH2

0 -0

-

ions” with one and/or two hydroxy endSurprisingly, no “molecular groups are found in the FD and negative or positive mode FAB mass spectra [4]. The most intense cations appearing in the mass spectra result from the formation of carboxonium ions, as already known from EI mass spectra (eqn. 2).

A corresponding series of oligomeric cations is found at m/z (173 + n - 172). Carboxonium ions with a thermally formed ketene end-group are not found, which might be due to the reactivity of ketenes with reactants available in the solid or liquid matrices.

112 0 H

O-KHZ

& -0-CO-_(CH,

l4 -CO

0-_(CH2)2

-O-CO-_(CH2)4

-C=OI

-+ x

m/z (x) = 173 (0), 345 (l), 517 (2), 689 (3), 861 (4), 1033 (5) and 1205 (6). A second favoured fragmentation process is the rearrangement shown schematically in eqn. 3.

(3) Signals for these ionic fragments 172).

are recorded

intensely

at m/z

(217 + n .

@O-H H+O-_(CH2),-0-CO-tCH,l,-ld+O-CH=CH, Y (y) = 217 (l), 389 (2), 561 (3), 733 (4), 905 (5), 1077 (6) and 1249 (7). The FAB mass spectrum of I shows the same degradation and fragmentation steps, but with another distribution of oligomeric fragments. In particular, the ions in the upper mass range are less intense. The FAB spectrum of anions of I provides additional information and confirms the cleavage of the ester bond (eqn. 1) as the favoured thermal degradation reaction. Corresponding carboxylic anions are found at m/z (189 + n - 172).

m/z

H -t

O-KHZ

lz -0-CO-_(CH,),

Gl”

-CO +

x

m/z (x) = 189 (l), 361 (2), 533 (3), 705 (4) and 877 (5). In addition, a series of anions appears at m/z (145 + n - 172), which confirms the pyrolysis mechanism shown in eqn. 4. R-0-CH

+ II CHz

+e -

Ho-k

CO-_(CH2)4-CO-O-_(CH&

CO-_(CH$,

-0 +

HO O;C-R’

-Cc0

Y

0’ e

145 (0), 317 (l), 489 (2), 661 (3) and 833;). The FD mass spectrum of poly(propylene adipate) (II) is shown in Fig. 1. Again, the cleavage of the ester bond to give hydroxy and ketene end-groups is shown to be the favoured pyrolysis mechanism (eqn. 2), which yields the oligomeric series of carboxonium ions at m/z (187 + n .186) and m/z (295 + n - 186). m/z

H

(y)

=

e 0-_(CH2

)j-O-CO-_(CH2)4

-CO

0-!CH2)3-O-CO-_(CHZ)4-CC=01 -I- x

m/z

(x)

=

187 (0) and 373 (1).

113

O=C=CH-KHz),

CO-0-_(CH2J3

c ,“ol

-O-CO-(CH,), -I-- Y

m/z (y) = 295 (l), 481 (2), 667 (3), 853 (4), 1039 (5) and 1225 (6). Surprisingly, the latter fragments with a reactive ketene end-group exhibit strong intensities and are not found in the Py-EI mass spectra. In parallel, ,

I

‘OO-

. 80-

al

z

90,

60-

:: c 3 s al .2 ;;

LO-

it

20-

07-

7' I 0-L 50

‘00

150

200

250

300

350

LOO

450

500

I

550

l

m/z

‘OO-

EO-

:

2 'D c z u O .? c) 0 5 =

60-

LO-

853 I

20-

0 550

577 I I 600

650

750

800

850

'000

'050

mlz

Fig. 1. Field desorption

mass spectrum

of poly(propylene

'200 .

adipate).

1350

114

18 a.m.u. below these fragments at m/z (295 + n - 186), a series at m/z (277 + n - 186) appears, which so far cannot be explained and seems to be non-specific for the adipic acid or propanediol subunits.

r

A

(?J_tO-KH,),

-0-CO-_(CH,),

1

-COTO-_(CH,),

-0-CO-KH,),

-C=_Ol

277 (0), 463 (l), 649 (2) and 835 (3). The FD mass spectra of poly(butylene adipate) (III) and poly(pentylene adipate) (IV) are given in Figs. 2 and 3. They confirm the thermal cleavage of the ester bond even under the conditions of this matrix ionization technique, although they only show carboxonium ions with ketene end-groups and, in parallel, ions 18 a.m.u. below at m/z (91 + n * MW,“i,,).

m/z

(x)

=

CO-C-_(CH,),-0-CO-_(CH,

O=CZCH-_(CH~)~

C&I

I4 +

-E-

x

= 4: m/z (y) = 309 (l), 509 (2), 709 (3), 909 (4), 1109 (5) and 1309 (6). x = 5: m/z (y) = 323 (l), 537 (2), 751 (3), 965 (4) and 1179 (5).

x

e 91

0-_(CH,),-0-CO-_(CH,),

0-E

-CO

0-_(CH2), +

-O-CO-_(CH2)4-C~OI

Y

= 4: m/z (y) = 91 (- l), 291 (0), 491 (l), 691 (2), 891 (3) and 1091 (4). x=5: m/z(y)=91 (-1). The FAB mass spectra of III and IV (Figs. 4 and 5) show carboxonium ions with hydroxy end-groups and high relative abundance.

x

H

0-_(CC(2),-O-CO-_(CH2)4-C0 -E-

0-c

CH&

-O-CO-

(CH, j4 -C&i

-!I- Y

= 4: m/z (y) = 201 (0), 401 (l), 601 (2), 801 (3), 1001 (4) and 1201 (5). x = 5: m/z (y) = 215 (0), 429 (l), 643 (2), 857 (3) and 1071 (4). The polyesters III and IV also show the unexplained series of ions at m/z (91+ n - MW”nit) at m/z 91, 291/305,491/519,691/733 and 891/-. In the FD mass spectrum of poly(propylene succinate) (V) the most abundant fragments appear at m/z (77 + n. MWU,il). Again, the structure of these fragments cannot be explained and appear 18 a.m.u. below a second series of ions at m/z 253, 411, 569, 727 and 885.

x

@+om/z

@ (CH2)3-0-CO-(CH2)2

0-(CH,),-0-CO-(CH,),

-CO

+

-C-O1

x

(x) = 235 (0), 393 (l), 551 (2), 709 (3) and 867 (4). Carboxonium ions with a thermally formed hydroxy end-group [m/z (159 + n. 158)] are less intense in the FD spectrum of V but appear with

115

higher abundances

in the corresponding

FAB spectrum

(Fig. 6). $

H •E

0-_(CH,),-0-CO-_(CH2),

m/z

-CO

-+ x

0-_(CH2~,-O-CO-_(CH2)2

--CO1

(x) = 159 (0), 317 (l), 475 (2), 633 (3) and 791 (4).

5

2

LO-

Q .?

91

2

z

90' 7,'

20-

309

55 '73 ,93

50

2Q1 I 150

100

I 200

327 I I

I 300

I 250

I 400

350

I

ml2

:

r

i

G 550

60

909

5

.1

500

*

-i :

I

I 150

40

I

I I,711

6?' I

2oj 56,

,,$,~

\I 01 ' 550

i

E7,,Jfi

‘I

;. I' l 600

*_I 650

Fig. 2. Field desorption

700

mass spectrum

750

900

m/t of poly(butylene

,I- ,‘:r

44

!350

1100

1150

. adipate).

1300

lF: 1350

116

The FAB spectrum of V exhibits, in addition to the above-discussed series of oligomeric ions at m/z (159 + n * MWUnit) and m/z (77 + n - MW,,,,), additional fragments at m/z (91 + n. 158), which are apparently not diagnostic of the adipic acid unit (compare the spectra of III and IV in Figs. 2 and 3, respectively).

90

191 I

92 323

0,7 :93

537

129

5337'

9': 110 I I 50

219

I , II1. 100 150

.I 200

292,

11 .I , 250

"'B',,, /

II

5.4 /

A,

300

I .400

350

loo-

00-

: '

60-

-a 5 s LOf .; 5 LL: 20-

Fig. 3. Field desorption

751 965

mass spectrum

of poly(pentylene

adipate).

I L50

I 500

I 550

117

m/z (y) = 249 (0), 407 (l), 565 (2), 723 (3), 881 (4) and 1039 (5). The results presented show that matrix ionization techniques such as FD and FAB can be applied to the identification of synthetic polymers and to

0 100

150

200

250

300

350

LOO

450

500

550

600

loo0.8

f301

EO0.6

I302 / 819

1

I!

-601

OFL 600

, 650

I

I

700

750

800

I

I

I

850

900

950

m/z

Fig. 4. Fast atom bombardment

mass spectrum

I 1000

/

Y&i+.

of poly(butylene

adipate).

1300

118

the investigation of their thermal degradation behaviour. In comparison with the EI and FI gas-phase ionization techniques, high-mass fragments exhibit higher intensities in the reported FD and FAB spectra, probably as a consequence of the smaller influence of volatility in these matrix techniques. As no parent ions of the polymer subunits were found, it is assumed that

215

80-

126 '1' t

147

325 I 305 I I

450

.

m/z

t

500

loo1 80-

857 Q

2

60-

:: c 3 z

LO-

: .CI z

20-

0 600

m/z Fig. 5. Fast atom bombardment

mass spectrum

. of poly(pentylene

adipate).

550

600

119

the thermal excitation on the FD emitter or FAB target exceeds the low activation energies required for ionic fragmentation processes of esters. Still unexplained are probable aromatization reactions on the emitter or target, leading to intense fragments with end-groups of 91 and 77 mass units and require further combined FD/FAB-MS investigations for clarification.

II I 100

2

159

80

loo-

i

80-

aI "r 60Jz c 1 2 40aa .c 0 5 =

1.5

1.0

0.5

200.0 0 600

613 c

f 650

7?'r723 ,,I_ I. 1 t 700 750

i 800

i 850

ml2

Fig. 6. Fast atom bombardment

mass spectrum of poly(propylene

*

succinate).

120 CRITICAL

EVALUATION

AND CONCLUSION

As mass spectra provide only mass numbers, whereas the above discussions are based on definite structures, some critical evaluations of the results should be added. Macromolecules with molecular ions above 5000 daltons are unable to pass the magnetic sector field employed in this study, independent of any calculation concerning their ability to reach the gas phase in high vacuum. Therefore, the registration of ions from polymer samples requires a prior degradation either by thermally induced processes or by ionic fragmentation reactions. For any detected linear fragment, two end-groups have to be explained, which theoretically may derive from two ionic or two thermal degradation processes, or one thermal and one ionic process. Ions with two ionically formed end-groups would be doubly charged and registered at m/2z (and identified by their isotopic peaks), whereas degradation products with two thermally formed end-groups (parent ions) should be partially or completely further fragmented, yielding species with one ionitally formed end-group. As all the mass spectra discussed exhibit sets of peaks separated by exactly the molecular masses of the structural subunit, they may assumed to be oligomeric ions with identical end-groups. Thus, subtracting n times the molecular mass of the structure unit from those sets of peaks, e.g., m/z (x+ n*M.,it)orb + n . M unit), the molecular masses of these groups x or y can be calculated. Knowing additionally the sequence of structural building blocks of these degradation products from the original polymer, a concrete structure can be proposed for most of the detected masses. Further, as every proposed thermal degradation mechanism yields “pieces” with two different end-groups, the corresponding series of oligomeric ions, e.g., with ketene and hydroxy end-groups (eqn. l), can be predicted and are indeed found. Using all this information derived from pyrolysis-mass spectra, it seems to be acceptable and useful in correlating the detected masses with proposed structures.

ACKNOWLEDGEMENT

Financial supports from the Deutsche Forschungsgemeinschaft 416/l-7 and Lue 237/4-l) is gratefully acknowledged.

(Schu

REFERENCES 1 H.-R. Schulten and R.P. Lattimer, Spectrom. Rev., 3 (1984) 231.

Applications

of mass spectrometry

to polymers,

Mass

121 2 H.-R. Schulten and H.J. Duessel, Pyrolysis field desorption mass spectrometry of polymers. II. Pyrolysis field ionization and field desorption mass spectrometry of aliphatic and aromatic poly(4,4’-dipiperidylamides), J. Anal. Appl. Pyrol., 2 (1980/1981) 293. 3 U. Bahr, I. Luederwald, R. Mueller and H.-R. Schulten, Pyrolysis field desorption mass spectrometry. III. Aliphatic polyamides, Angew. Makromol. Chem., 120 (1984) 163. 4 M. Doerr, I. Luederwald and H.-R. Schulten, Characterization of polymers by field desorption and fast atom bombardment mass spectrometry, Fresenius Z. Anal. Chem., 318 (1984) 339. 5 H.-R. Schulten, T. Komori, K. Fujita, A. Shinoda, T. Imoto and T. Kawasaki, Laser-assisted field desorption mass spectrometry of cyclomalto-hexaose and heptaose and some 6-alkylthio derivatives, Carbohydr. Res., 107 (1982) 177, and references cited therein.