Polymer Durability - American Chemical Society

3

Downloaded by MICHIGAN STATE UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/ba-1996-0249.ch003

Thermolysis of Poly(chloroethyl methacrylates) and Poly(chloroethyl acrylates) Wolfram Schnabel Hahn-Meitner-Institut Berlin GmbH, Bereich C, Glienicker Str. 100, D-14109 Berlin, Germany

Poly (ethyl acrylates) and poly(ethyl

methacrylates),

chlorinated in the

side groups, release chlorine and undergo intermolecular if heated to temperatures

nificant in the cases of the dichloroethyl acrylate

and methacrylate

fragments.

In methacrylate

sists of depolymerization

cross-linking

up to 500 Κ. These processes are most sig polymers

esters. At Τ > 500 Κ both

decompose

into low

molecular

polymers this process predominantly

con

(formation of monomer). As far as the onset

temperature for mass loss is concerned, the chlorinated polymers def initely exhibit a lower stability than the nonchlorinated enhances cross-linking

of the mono- and dichlorethyl

polymers but retards or prevents cross-linking simultaneous

ones. Oxygen methacrylate

in all other cases. The

action of both heat and UV light reduces the

stability. This effect is quite significant in the cases of the polymers that are prone to

thermal

methacrylate

unzipping.

1POLY(CHLOROETHYL METHACRYLATES) readily undergo intermolecular crosslinking during heating to temperatures exceeding about 430 Κ [see Chart I for structures for poly(mono-, di-, and trichloroethyl methacrylates) ( P M C M A , P D C M A , and P T C M A , respectively)]. For this reason, these polymers became interesting resist materials for applications in X-ray and electron-beam lithog raphy. Actually, high-energy radiation induces main-chain degradation of these polymers and thus improves their solubility in appropriate fluid developers; the polymers act as positive tone resists. Thermal cross-linking before expo sure to high-energy radiation significantly improves the development of fine line structures of high contrast. Because little was known of the mechanism

0065-2393/96/0249-0033$12.00/0 © 1996 American Chemical Society In Polymer Durability; Clough, R., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1996.

34

POLYMER DURABILITY

CH 3

CH 3 —CH 2 —h—

—CH 2 —^— 0=i~0-CH2-CH Cl 2

PMCMA

2

PDCMA

0=i-0-CH2-CCl

3

PTCMA

- C H —CH—

2

0=i-0-CH2-CHCl

3

—CH 2 —^—

0=i-0-CH2-CHCl

-CH -CH—


CH

2

2

PDCEA

0=i-0-CH2-CCl

3

PTCEA

Chart I. of the chemical reactions induced thermally in this class of polymers, system atic studies have been performed in our laboratory. These studies were ex tended to the thermolysis of corresponding acrylate polymers [poly(di- and trichloroethyl acrylate) ( P D C E A and P T C E A , respectively); see Chart I]. In this chapter results obtained with these polymers will be reviewed (16). From earlier work (7, 8) it is well known that different mechanisms become operative in the thermal degradation of nonchlorinated methyl and ethyl esters of polyaerylie and polymethaerylic acid; the most significant difference is that the polymethacrylates, in contrast to the polyacrylates, readily undergo depolymerization. This difference was explained in terms of inter- and intra molecular hydrogen abstraction reactions that undergo terminal carboncentered radicals of polyacrylate radicals (8-11). In the case of polymethacrylates, hydrogen abstraction reactions are much less likely, mainly for steric reasons. Therefore, depolymerization (unzipping) is favored. Another difference applies to the capability of unsubstituted poly acrylates such as poly(ethyl acrylate) (PEA) to form cross-finked networks dur ing heating. By contrast, unsubstituted polymethacrylates do not cross-link. Cross-linking of polyacrylates occurs via the combination of lateral macroradicals sited on the backbone or via intermolecular reesterification (10). Principally, the characteristic differences in the thermal behavior of the two families of polymers also exist between chloroethyl acrylate and methacrylate polymers. However, in all cases thermally induced initiation processes in pendant groups were dominant at all temperatures. This phenomenon is due to the readiness at which C - C l bonds are cleaved; interestingly, this cleav age is most pronounced in the case of the dichloroethyl esters. Molecular oxygen plays a peculiar role in cross-linking and depolymerization, which are the major chemical reactions leading to important alterations in the polymers. For instance, gel formation (intermolecular cross-linking) is enhanced in the cases of P M C M A and P D C M A , whereas it is prevented in the case of

In Polymer Durability; Clough, R., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1996.

3.

SCHNABEL

Thermolysis of PCM A and PCE A

35

P T C M A . F o r this reason, the first part of this chapter is divided into two subsections devoted to processes occurring i n the absence and presence of 0 . The second part deals with the influence of U V light on the thermal degradation of the polymers. 2


Thermolysis in the Absence of Oxygen Poly(chloroethyl methacrylates). Elimination of chlorine, gel for mation (intermolecular cross-linking), and depolymerization (unzipping) are the major chemical processes that determine the fate of the poly(chloroethyl methacrylates) during heating to elevated temperatures. For the initiation of the thermolysis at relatively low temperatures (below about 500 K), chlorine ehmination as the cleavage of C - C l bonds is the major process. With rising temperature scissions of other bonds i n the pendant groups and also i n the backbone of the polymers become more likely. Table I summarizes the major effects observed with P M C M A , P D C M A , P T C M A , and poly(ethyl methacrylate) (ΡΕΜΑ). With respect to the chlorine-containing polymers, gel formation is the major process at temperatures below about 500 K . A t temperature (Γ) > ~500 Κ the polymers decompose readily, mainly via depolymerization. ΡΕΜΑ Table I. Thermolysis of Poly(chloroethyl methacrylates) Elimination Oxygen Effects Depolymerization of CI PMCMA Major process Not occurring at Τ < Minor process Cross-linking en hanced, rate of at Τ < 500 Κ; 500 Κ; major process at all Τ mass loss and unimportant at at Τ > 500 Κ unzipping re Τ > 500 Κ tarded at Τ < 500 Κ PDCMA Major process Not occurring at Τ < Important but Cross-linking en at Τ < 500 Κ; 500 Κ; major process not dominant hanced, rate of mass loss and at all Τ unimportant at at Τ > 500 Κ unzipping re Τ > 500 Κ tarded at Γ < 500 Κ PTCMA Major process Not occurring at Τ < Minor process Cross-linking prevented, rate at Τ < 500 Κ; 500 Κ; major process at all Γ of mass loss and unimportant at at Τ > 500 Κ unzipping re Τ > 500 Κ tarded Rate of mass loss Not occurring Very important at all Not applica ΡΕΜΑ accelerated, ble Τ at all Γ main-chain scission very effective Polymer Cross-linking



36

POLYMER DURABILITY

does not cross-link, but it depolymerizes readily. Notably, P D C M A cross-links with a much higher rate than P M C M A and P T C M A . This result is paralleled by the rate of chlorine eHmination, which is much higher in the case of P D C M A than in the cases of P M C M A and P T C M A . Typical results concern ing gel formation and mass loss are shown in Figures 1 and 2, respectively. Table II shows a typical result of the composition of the condensate of vola tiles; monomer is the major product. The thermal behavior of the methacrylate polymers is compared in Table III. Obviously, ΡΕΜΑ, the nonchlorinated polymer, exhibits the highest thermal stability; and P D C M A , the polymer con taining dichloroethyl groups, is least stable. Generally, the stability decreases in the series ΡΕΜΑ > P M C M A > P T C M A > P D C M A . Poly(chloroethyl acrylates). Table IV summarizes the observations made during heating acrylate polymers. The chlorine-containing polymers do not exhibit a pronounced tendency to undergo depolymerization and, in this way, resemble P E A , the nonchlorinated polymer. However, P T C M A decom poses at Τ > 510 Κ to some extent by unzipping. At Τ > ~500 Κ main-chain scission and decomposition into low-molecular fragments are the major chemical processes. The chlorine-containing polymers P D C E A and P T C E A release chlorine during heating. Interestingly, this process is very important in the case of P D C E A and plays only a minor role in the case of P T C E A . The release of chlorine is paralleled by gel formation and indicates that intermolecular crosslinking is related to the cleavage of C - C l bonds and the subsequent combi nation of lateral radicals generated in this way. Notably, the insolubilization of P D C E A occurs very rapidly. As can be seen from Figure 3, gel formation to ~100% conversion is completed within some minutes after the sample has been warmed up to the set temperature. By contrast, P T C E A cross-links only to a small extent over a heating period of many hours (4). The thermal be havior of the acrylate polymers is compared in Table III. Obviously, P E A

Figure 1. Thermal cross-linking of PDCMA in Ar (A) and 0 (O) at 493 Κ and 473 Κ as plot of gel fraction of residual polymer vs. time of heat treatment. (Reproduced with permission from reference 2. Copyright 1993 Elsevier.) 2


3.

SCHNABEL

37


ioo b ^ - N ^ o

(a)

mîn

50 3* ο 5? 100

t

ι

ι

I

ι

ι

ι

Γ Τ

0

(b)

• .2

Ar


M 50

90 mïn'

> « j « » *» 500 550 Temperature (K)

460

J L.

Figure 2. Thermolysis of PDCMA as isothermal mass loss at 30- (Φ) and 90(A) min heat treatment in 0 (a) and Ar (b). (Reproduced with permission from reference 2. Copyright 1993 Elsevier.) 2

exhibits the highest thermal stability and P D C E A is least stable. Generally, the stability decreases in the series P E A > P T C E A > P D C E A .

Thermolysis in the Presence of Oxygen Poly(chloroethyl methacrylates). The effect of molecular oxygen on the thermal reactions of poly(chloroethyl methacrylates) strongly depends on the degree of chlorination. Most strikingly, in the cases of P M C M A and P D C M A , cross-linking is enhanced by 0 . Typical results obtained in the case of P D C M A are presented in Figure 1. The gel fraction representing the crosslinked portion of the polymer sample increases more rapidly in the presence of 0 than in its absence. By contrast, P T C M A does not cross-link at all in the presence of 0 . As far as mass loss is concerned the thermal stability of all three chlorinated polymers is increased by 0 . Typical P D C M A results (Figure 2) demonstrate a significant shift in the onset temperature of mass loss to higher temperatures and an increase in the rate of mass loss in the presence of 0 . O n the other hand, ΡΕΜΑ exhibits a quite different behavior: 0 accelerates the rate of mass loss, and main-chain scission is more effective in the presence of 0 than in its absence. These results are summarized in the last column of Table I. 2

2

2

2

2

2

2

Poly(chloroethyl acrylates). In the case of P D C E A 0 retards cross-linking, but the release of chlorine is not affected. The rate of mass loss 2


POLYMER DURABILITY

38

Table II. Components of Condensate Collected during 60-min Thermal Treatment of PMCMA at 541 Κ under Argon and at 505 Κ under Q 2

Structure

Ar

0

2

ο CH -C-H

+

+

-

0.3

0.06

0.3

-

0.2 +

+

+

CH-C-C-C1

-

+

CH =cf COOH

-

0.1

-

+

3


ο CICHg-C-H CICH -CH CI 2

2

CICH-CHOH CH =C' COOCH /CHo CH=C ° COOCH=CH 2

2

CH3

2

3

2

2

Ο Ο 3

H3

2

Ο

CI-C-OCHCHCI 2

2

,CH CH=C COOCHCH 3

+

2

2

3

CH =:ci \xx>eHcHei

99.0

95.6

ο ο CH-C>-C-OCHCHC!

-

1.0

J V "

-

2.0

CH3

2

2

3

2

2

2

3

COOCHCHCI 2

2

0.3

CICHg—C I COOCHCHCi N

2

2

NOTE: Values are percent concentration (w/w); + means traces detected, — means not detected. Results forfirststructure ( C H C H O ) were determined in a separate run. 3


3.

SCHNABEL

39

Thermolysis of PCMA and PCE A

Table III. Anoxic Thermolysis of Poly(chloroethyl aerylates) and Poly(chloroethyl methacrylates)


T(onset) Polymer (30 min) PEA PDCEA PTCEA ΡΕΜΑ PMCMA PDCMA PTCMA

580 505 520 490 470 460 470

Ί(50%) (30 min) 640 550 575 600 590 530 540

Ύ(50%) (90 min) 620 530 565 590 570 510 520

NOTE: Values are decomposition temperature (K) for onset of mass loss and 50% mass loss after 30and 90-min heat treatment.

Table IV. Thermolysis of Poly(chloroethyl aerylates) Elimination Oxygen Effects Depolymerization of CI Important at Cross-linking reMajor process Negligible at all Τ all Γ tarded, rate of at Τ < 500 Κ; mass loss en unimportant at hanced, and re Τ > 500 Κ lease of Cl not affected Minor process Not occurring at Γ < Minor process Cross-linking retarded, rate of at Τ < 500 Κ; 500 Κ; important but at all Γ mass loss en polymer chars not dominant at Τ > hanced at Τ > 500 Κ 500 Κ Minor process Not occurring at all Τ Not at all Γ applicable

Polymer Cross-linking PDCEA

PTCEA

PEA

Heatîng-up Period

Figure 3. Thermal cross-linking of PDCEA in Ar (O) and 0 (Φ) at 494 Κ as plot of gel fraction of residual polymer vs. time of heat treatment. (Reproduced with permission from reference 4. Copyright 1994 Elsevier.) 2

Time (min)


POLYMER DURABILITY

40

is enhanced compared with that observed in the absence of 0 . Similar oxygen effects were found in the case of P T C E A . 2


Mechanistic Aspects The thermal decomposition of pendant groups essentially determines the in itiation of chemical processes occurring at Τ < 500 Κ. Among the various types of bond breakage that can be envisaged, the cleavage of C - C l bonds seems to be an important elementary reaction. A n initiation mechanism based on this reaction is discussed below for the case of P D C M A (reaction 1). CH

CH3

_CH -i-

3

JL^-CHu-i-

2

0=è-O-CHr-CHCl

+C1«

0=£-0-CHr-CHCl

2

(I·) Radicals of type I* undergo different reactions: cross-linking according to reaction 2 and decomposition according to reactions 3 and 4. CH

CH

3

— > — C H 2—^—

3

——CHy—

1

H

(2)

i

H

0=C^^H2-C--C-CHr-0-C=0 CI CI CH 3

I.

1

> - C H 2—i— + e O - C H r C H C l (-> 0=C-CH C1) 2

0=i.

CH

(3)

n

3

>-CH -i-

+

2

CH =CHC1

( 4 )

2

The intermediate product II rearranges to chloroacetaldehyde. In the presence of 0 most of the free radicals generated by the ther molysis of the polymers are prone to react with 0 to form peroxyl radicals. A portion of these free radicals will be converted into oxyl radicals according to the general mechanism described by reactions 5 and 6. 2

2

R* + 0

2

> R-0-Ο· -

ROO±> 2 R O * + R

ROOH

>ROm +

H

0

2

(5)

> ROOH + R«

·ΟΗ

(6)

Oxyl radicals appear to play a prominent role in the enhancement of cross-


3.

SCHNABEL

41


linking by molecular oxygen as was observed for P M C M A and P D C M A . At elevated temperatures lateral oxyl radicals (RO") probably undergo a kind of transesterification that results in the immediate formation of intermolecular linkages and the release of low-molecular oxyl radicals. The reaction of oxyl radicals with ester groups is depicted by reaction 7. Note that RO* denotes a macroradical. CH 3

CH 3


RO* 4* —CH 2~i—

> —CH 2—i— + 0 ·—CH2CHCI2

0=i-0-CH CHCl 2

2

(7)

0= -CH --CH -i-

+ HCl

2

^

0=i-O-CH=CHCl

Notably, the processes according to reactions 7-9 are insensitive toward 0 . Evidence for the occurrence of reaction 7 in the presence of 0 is provided by the formation of 2-chloroethanol and 2,2-dichloroethanol in the cases of P M C M A and P D C M A , respectively. The alcohols are formed when alkoxyl radicals abstract hydrogens from surrounding molecules. This reaction is ex emplified by reaction 10 for the case of P D C M A . 2

2

RH + ·0-0Η0Η01 —> 2

2

R# + H0-CHCHC1 2

2

() 10

Notably, 2,2-dichloroethanol and 2-chloroethanol are formed only in traces in the absence of 0 . In connection with the effect of 0 on the thermal cross-Mnking of P M C M A and P D C M A , the quite different thermolysis course observed in the case of P T C M A should be pointed out. The thermal crosslinking of P T C M A is completely prevented by 0 . This apparently contradic tory behavior can be understood by considering that reaction 7 cannot take 2

2

2


POLYMER DURABILITY

42

place in the case of P T C M A because steric hindrance significantiy reduces the reactivity of radical R 0 \


2

Thermolysis under the Influence of UV Light Under the influence of U V light the thermal degradation of poly(chloroethyl methacrylates) occurs overwhelmingly by depolymerization and commences at lower temperatures than in the dark (350 K) (3). At ambient temperature irradiation at λ = 253.7 nm of P T C M A results in the formation of an insoluble gel due to cross-Unking, whereas P M C M A and P D C M A undergo predomi nantly main-chain scission. Cross-linking of P T C M A occurs because CC1 groups, in contrast to C H C l and C H C l groups, absorb light relatively strongly at X = 253.7 nm. In this way, R-CC1 * radicals capable of forming cross-links by combination are generated. U V irradiation of the three chlorinated poly(ethyl methacrylates) at temperatures between 430 and 470 Κ induces main-chain scission. Terminal radicals thus formed initiate the depolymerization; that is, under the influence of U V irradiation the onset temperature for mass loss is significantly lowered. The results can be interpreted on the basis of reactions 11 and 12, which show that the chemical deactivation of excited carbonyl groups involves two different chemical routes. 3

2

2

2

—CH 2

-CH

+

•OR

(ID

2

o.i i :»R

OR

+

•i=o

(12)

Radical III' can either abstract hydrogen inter- or intramolecularly ac cording to reaction 13 or decompose according to reaction 14.


3.

SCHNABEL

43


CH

3

RH -> —CH ,2—C—CH2— 4-

(13)

4- R»

0=C-H CH

3


-> -CH2-è~CHr-

+ CO

(14)

(IV.) Notably, reaction 13 results in the formation of aldehyde groups. These groups were identified (13) in photolyzed poly(acrylates) and poly(methacrylates) via the conversion with 2,4-nitrophenylhydrazine and the separation of the polymer containing hydrazone groups from the reagent mix ture by gel permeation chromatography (13). The new absorption band formed during the irradiation (Figure 4) is attributed to aldehyde groups. Reaction 14 yields carbon monoxide and radical IV*. Decomposition of IV* according to reaction 15 results in the scission of a bond in the polymer backbone. CH

3

CHj

CH

:-CH -i-

-CH;

- C H —i/—CH 22 —0—CH2— 2

(IV.)

o.A

CH

3

3

2-i=< CH

2

4-

ti-CHr-

(15)

R In this way photolysis gives rise to the formation of terminal macroradicals capable of initiating unzipping. Thus, an explanation is provided for the sig-

M

1

I I I I

350 250 Wavelength (nm)

I

I I I I

I

I

M

350

Figure 4. Change in optical absorption spectrum of PDCMA during irradiation at λ = 253.7 nm in Ar (a) and 0 (b) at 297 Κ· I = 6 X 1() photons/(cm s). (Reproduced with permission from reference 3. Copyright 1994 Elsevier.) 2

14


2

44

POLYMER DURABILITY

nificarit lowering of the onset temperature for mass loss i f the polymers are subjected to the simultaneous impact of heat and U V light.

Experimental


Monomers. Chlorine-substituted ethyl methacrylates and aerylates were synthesized by esterification of methaerylic acid and acrylic acid with chlorinesubstituted ethanols. The compounds were obtained from either E . Merck or Aldrich. Polymers. The polymers were prepared by free-radical polymerization of the monomers to a conversion not exceeding 15% by using 2,2-azo-bis(2-methylpropionitrile) as initiator. After several reprecipitations from solution the polymers were dried in vacuo for several days. Appropriate solvent-precipitant systems were methyl ethyl ketone-η-hexane or toluene-methanol for polymethacrylates and ac etone-water for polyacrylates. Gel permeation chromatography measurements based on P M M A calibration yielded the following weight average molar masses: 9 X 10 (PMCMA), 3.2 Χ 10 (PDCMA), 3.4 X 10 (PTCMA), 7.2 Χ 10 (PDCEA), and 5.5 X 10 (PTCEA). 5

5

s

5

5

Thermal Degradation Experiments. The isothermal mass loss of polymer samples was determined at temperatures between 373 Κ and 573 Κ by using 60mg samples in glass tubes placed in a small furnace. During heating the tubes containing the polymer were flushed constantly either with oxygen or argon at a rate of 50 mL/min. In addition, differential scanning ealorimetric tests were per formed with 3-4-mg polymer samples by using a Perkin-Elmer apparatus (model DSC 2C). The scan rate was varied from 1 to 5 K/min at a gas now rate ( N or 0 ) of 30 mL/min. 2

2

Analysis of Condensable Volatile Products and Residual Poly mer. Volatile products generated in the presence and absence of 0 were con densed in a cold trap at -78 °C. For the determination of H C l the gas stream was conducted from the cold trap into an aqueous solution of A g N 0 / H N 0 . The amount of H C l formed was calculated on the basis of precipitated AgCl. Similarly, acetaldehyde was determined by conducting the gas stream into an aqueous 2 Ν H C l solution of 2,4-nitrohydrazine. Gas chromatography-mass spectrometry tech niques were applied for volatile product analysis. Details were described elsewhere (I). The chlorine content of the residual polymer was determined by elemental analysis by Mikroanalytisches Labor Pascher. The gel content of cross-linked sam ples was determined by extraction with methyl ethyl ketone with the aid of a Soxhlet extractor. 2

3

3

References 1. Popovic, I.; Song, J.; Fischer, Ch.-H.; Katsikas, L.; Hohne, G.; Velickovic, I.; Schnabel, W. Polym. Degrad. Stab. 1991, 32, 265-283. 2. Song, J. Fischer, Ch.-H.; Schnabel, W. Polym. Degrad. Stab. 1993, 42, 141-147. 3. Song, J. Fritz, P. M., Fischer, Ch.-H.; Schnabel, W. Polym. Degrad. Stab. 1994, 43, 177-185. 4. Song, J. Schnabel, W. Polym. Degrad.Stab.1994, 43, 335-342. ;

;

;



3.

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45

5. Popovic, I.; Katsikas, L.; Voloschuk, Κ. Α.; Velickovic, I.; Schnabel, W.J.Therm. Anal. 1992, 38, 267-275. 6. Popovic, I.; Katsikas, L.; Velickovic, I.; Schnabel, W. In The Thermal Degradation of Poly(2-mono, 2,2-di- and 2,2,2-trichloroethyl methacrylate): Kinetics and Mec anism; Scientific Series of the International Bureau; Forschungszentrum Julich GmbH, Julich, Vol. 8. 7. Grassie, N.; Scott, G. Polymer Degradation and Stabilization; Cambridge Univer sity Press: 1985. 8. Madorski, S. L. Thermal Degradation of Organic Polymers; Interscience: New York, 1964. 9. Cameron, G. G.; Kane, D. R. Makromol. Chem. 1967,109, 194. 10. Cameron, G. G.; Kane, D. R. J. Polym. Sci., Polym. Lett. 1964, 2, 693. 11. Grassie, N.; Speakman, J.G.J.Polym. Sci. A-I, 1971, 9, 919. 12. Cameron,G.G.,Davie, F. Chem. Zvesti. 1972, 26, 200. 13. Fritz, P. M.; Zhu, Q. Q. Schnabel, W. Eur. Polym.J.1994, 30, 1335-1338. ;

RECEIVED for review January 26, 1994. ACCEPTED revised manuscript September 23, 1994.


Polymer Durability - American Chemical Society

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