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Chapter 8 Polymers with aromatic hydrocarbon backbone
463
CHAPTER 8
Polymers with Aromatic Hydrocarbon Backbone 8.1
POLYHYDROCARBONS WITH AROMATIC RINGS IN THE MAIN CHAIN
- General aspects
A significant number of polymers contain phenylene groups in their backbone. Most of these polymers also contain heteroatoms or heteroatom groups in the backbone and are discussed with other classes of polymers. However, there are some polymers with a carbon backbone that have only aromatic rings, alternating aromatic rings and aliphatic groups, or aromatic rings and unsaturated carbon chain fragments in their backbone. Examples of such polymers are poly(1,4-phenylene), poly(1,4-phenylene-ethylene), poly(1,3-phenylene-methylene), and poly(p-phenylene vinylidene), which are shown below:
CHI ....
.....
C/H/
rl
,,n t
poly(1,4-phenylene)
poly(1,4-p he nylene-ethylene)
poly(1,3-phenylenemethylene)
poly(1,4-phenylene1,2-ethenediyl)
Polymers with more complicated structures, containing triple bonds in addition to the aromatic cycles in the backbone or having substituents on the aromatic ring, also are known and have specific applications. Thermosetting phenolic resins form a separate class of polymers containing aromatic rings and aliphatic carbon groups in the polymeric network. These resins are formed from the reaction of phenol (or substituted phenols) with formaldehyde. The fully crosslinked macromolecule is insoluble and infusible. Other thermosetting resins are known in practice, some derived from the reaction of melamine or of urea with formaldehyde. Because these have a different chemical structure, containing nitrogen, they are included in a different class (see Section 15.3).
- Polyphenylene
and poly(phenylene-alkylenes)
Polyphenylene, CAS# 26008-28-6, is a temperature resistant polymer with the decomposition temperature around 660 ~ C (see Table 3.1.1). For this reason it has applications in heat resistant fabrics, electrical insulators, etc. The thermal decomposition products of the polymers in this group are formed by random scission of the bonds that are not involved in the aromatic rings. This leads in most cases to the formation of oligomers. If side groups are attached to the benzene rings, fragments from these groups typically are seen in the pyrograms. Some reported results for the thermal decomposition of the polymers in this group are summarized in Table 8.1.1 [1].
464
Polymers with aromatic hydrocarbon backbone
TABLE 8.1.1. Summary regarding literature reports on the thermal decomposition of polyphenylenes [1].
- Other polymers with phenylene groups in the backbone
Other polymers containing phenylene groups in a carbon chain backbone include those with some unsaturated groups between the phenylene rings. Some of these polymers still have good stability upon heating, such as poly(1,4-phenylene-1,2-ethenediyl), CAS# 26009-24-5, which decomposes around 400 ~ C [9]. Their use is, however, geared toward their special electricity conducting properties generated by the conjugated unsaturated groups and the aromatic rings. The compounds from this group also may have substituents on the aromatic ring [10]. Among the polymers containing aromatic rings and unsaturated hydrocarbon groups in the backbone are poly(phenylene-ethynylenes) (PPE). This type of polymer has been synthesized by alkyne metathesis of 1,4-dipropynylated benzenes [11] and has applications in optical and electronic industry. A comprehensive study on thermal decomposition of several poly(substituted p-phenylene-ethynylenes) is available [12]. The general formula for this group of polymers is the following:
As shown for the pyrolysis of poly(maleic anhydride-alt-l-octadecene) (see Section 6.8), when a long alkane chain is present in a polymer, the fragment alkanes and alkenes from the side chain generate large peaks in the pyrograms. The same effect is seen for the pyrolysis of PPEs, and a considerable number of peaks in the pyrograms of different substituted PPEs are identical, being generated from the fragmentation of the polymer side chains. However, some peaks are different and they can be used for the differentiation and identification of different polymers from this group.
Polymers with aromatic hydrocarbon backbone
465
References 8.1 1. J. Liggat, in J. Brandrup, E. H. Immergut, E. A. Grulke, eds., PolymerHandbook, J. Wiley, New York, 2000. 2. S. L. Madorsky, S. Straus, J. Res. Natl. Bur. Std., 53 (1954) 361. 3. S. L. Madorsky, S. Straus, J. Res. Natl. Bur. Std., 55 (1955) 223. 4. P. Brandt, V. H. Dibeler, R L. Mohler, J. Res. Nati. Bur. Std., 50 (1953) 201. 5. H. H. G. Jellinek, S. N. Lipovac, J. Polym. Sci. A-l, 8 (1970) 2517. 6. H. H. G. Jellinek, S. H. Ronel, J. Polym. Sci. A-l, 9 (1971) 2605. 7. G. R L. Ehlers, K. R. Fisch, W. R. Powell, J. Polym. Sci. A-l, 7 (1969) 2931. 8. D. G. H. Ballard, A Courtis, I. M. Shirley, S. C. Taylor, Macromolecules, 21 (1988) 294. 9. D. F. Hoeg, D. I. Lusk, E. P. Goldberg, J. Polym. Sci. Polym. Lett. Ed., 2 (1964) 697. 10. H. H. Horhold, J. Opfermann, J. Makromol. Chem., 131 (1970) 105. 11. L. Kloppenburg, Do Song, U. H. F. Bunz, J. Am. Chem. Soc., 120 (1998) 7973. 12. K. W. Sellers, C. M. Towns, C. R. Mubarak, L. Kloppenburg, U. H. F. Bunz, S. L. Morgan, J. Anal. Appl. Pyrol., 64 (2002) 313.
8.2
HALOGENATED POLYHYDROCARBONS WITH AROMATIC RINGS IN THE BACKBONE
- General
aspects
The polymers from this group show excellent thermal stability when they do not contain hydrogens, for example, when they are perfluorinated. Compounds having both hydrogen and a halogen (such as fluorine) eliminate the hydrohalogenated acid (HF, HCI, etc.) at lower temperatures. Some literature reports on thermal decomposition for this group of compounds are indicated in Table 8.2.1 [1]. TABLE 8.2.1. Summary regarding reports on the thermal decomposition of halogenated
polyphenylenes [1].
Polymers with aromatic hydrocarbon backbone
466
References 8.2 1. J. Liggat, in J. Brandrup, E. H. Immergut, E. A. Grulke, eds., PolymerHandbook, J. Wiley, New York, 2000. 2. J. L. Cotter, G. J. Knight, W. W. Wright, Br. Polym. J., 7 (1975) 389. 3. J. L. Cotter, G. J. Knight, J. M. Lancaster, W. W. Wright, J. Appl. Polym. Sci., 12 (1968) 2481.
8.3
THERMOSETTING PHENOLIC RESINS AND RELATED POLYMERS
- General aspects
Thermosetting phenolic resins include a number of polymers, the most common being obtained from the condensation of phenol with formaldehyde. The OH group on the benzene ring increases the reactivity in the o- and p- positions leading to three reactive centers for the phenolic component, while formaldehyde acts as having two active centers that can lead to a fully crosslinked polymer. The process may take place in neutral or alkaline conditions when in the first stage of the reaction, compounds known as methylol derivatives are formed. The condensation of phenol with formaldehyde occurs randomly at ortho- or para- position of the phenol, as shown below:
OH + HOH=O
OH OH OH =- {~/CH2OH ~ C[ ~ H 2 HCH=O0 ~ + CH2OH
CH2OH
OH OH H O H 2 C ~ CH2OH HCH--O H O H 2 C ~ ~ cH2OH CH2OH The methylol derivatives under mild heating for a limited time further react forming fusible polymers that are not yet fully crosslinked, as shown below: oH
HOH2C
I~~/
OH
oH
CH2OH + CH2OH
OH
CH2OH CH2OH .... - H20 or - HCH=O ~CCH2OH H2~C ....,~OH _CH2 CH2OH ~ i'Y" "" .....
CH2
In acidic conditions, the reaction between phenol and formaldehyde is believed to occur with the formation of a linear polymer as follows:
Polymers with aromatic hydrocarbon backbone
OH
OH
+ HCH=O
OH
.d
OH
OH
467
OH
......
OH H2C'~CH2,,,,,
Since the fully crosslinked macromolecule is insoluble and cannot be processed by molding, it is common in practice to synthesize first a polymer with a low degree of polymerization that can melt, followed by further crosslinking. This process is known in practice as formation of a two-stage resin (one-stage process is less frequently used). The two-stage resin can be obtained either using basic or acidic conditions. In basic conditions, the first stage must be done at low temperatures and with strictly controlled reaction times. Polymers with relatively low molecular weight and containing numerous -CH2OH groups attached to the benzene rings can be obtained (as previously shown). In the second stage, these polymers known as resols can be further crosslinked by heating, when more condensations are induced by the elimination of water. A different way to synthesize fusible polymers is to conduct the first stage in acidic conditions with the mole ratio of formaldehyde to phenol less than 1 and with mild heating. The linear polymer formed in this way is called novolac (or novolak). The novolac is further transformed into a crosslinked polymer by the addition of more formaldehyde and a basic catalyst. The process can be accomplished by adding during molding the novolac mixed with an appropriate amount of hexamethylene-tetramine. The decomposition of hexamethylenetetramine during heating generates formaldehyde and ammonia, which acts as a basic catalyst. The idealized structure of a crosslinked phenol-formaldehyde resin is shown below:
OH
OH
......... CH2 ......... H2C~
OH2
~
CH2
OH
C
2
.....
H OH2
.....
cH2
In the synthesis of this polymer in the presence of NaOH, long chain or highly branched molecules are obtained, depending on factors such as phenol to formaldehyde mole ratio, phenol to NaOH mole ratio, and the temperature and duration of the heating. Other resins from this group may be obtained with the phenolic component with additional substituents on the benzene ring such as alkyl (methyl, dimethyl, butyl, tert-
Polymers with aromatic hydrocarbon backbone
468
octyl, nonyl), alkenyl (decapentatrienyl when using cardanol), etc. Phenolic resins with two -OH groups present on the benzene ring, like in resorcinol-formaldehyde resin, also are known. Instead of OH groups, ether groups also activate the ortho and para positions on the benzene ring. For example, glycidyl ethers were used as phenolic component, and these groups allow further crosslinking (curing) of the macromolecule. When the attached groups are glycidyl (epoxy propyl), these compounds are known as epoxynovolaks (or epoxynovolac). Typically these polymers also contain some free phenol groups. The idealized structures of two resins containing glycidyl ether attached to the benzene ring are shown below: ........CH2
RO
........C.H2
~ R = H-, / \
@
CH 2_
R
o .........
R = H-, ~ . ~ C H
2-
CH3 poly[(phenyl glycidyl ether)-co-formaldehyde]
poly[(o-cresyl glycidyl ether)-co-formaldehyde]
The polymers are initially obtained in linear form, being thermoplastic and soluble in different solvents. The heating of the polymer allows further crosslinking by the opening of the oxirane ring and formation of intermolecular bridges. These types of polymers are used as active components in some epoxy resins. Also, resins containing other aromatic moieties are known. Among these is, for example, naphthalene sulfonate formaldehyde resin, which can be used as a cement additive [1]. Also, thiophenol forms a condensation product with formaldehyde. However, the condensation with thiophenol occurs with a different mechanism than that of phenol, and sulfur atoms are included in the backbone of the polymer. For this reason, thiophenol/formaldehyde resin is discussed in Section 12.1. Phenolic resins have many practical applications. They are widely used to make plywood for furniture and wall paneling when wood dust or wood chips are included as a filler. With paper or fabric phenol-formaldehyde resins form various laminates including those necessary for making boards for electronic circuits. Phenolic resins also are used to bind fiberglass, carbon fiber, and other reinforcing materials applied in aerospace industry. The binding of a filler can be done by different technological procedures, either starting with a solution of novolac that is used to soak the filler followed by heating, or in mixtures of resol or novolac with the filler that is thermoset in the desired shape of the final object. Phenolic resins also have applications in coating industry for high-temperature wire insulation, for making electrical battery separators, for use as structural adhesives, etc. The phenolic foams are used for heat insulators in buildings. Also, phenolic resins can be components of various copolymers and may be used as vulcanizing agents for rubbers. Phenol-formaldehyde resins are relatively resistant to heat. They start decomposing at about 250 ~ C still maintaining some mechanical resistance, the decomposition rate increasing significantly around 300 ~ C. In an inert atmosphere at 750 o C, phenolformaldehyde resins form more than 50% char [2, 3]. The volatile materials consist of xylene (76%), traces of phenol, cresol, and benzene [4]. The heating in air above 300 ~ C leads to the oxidation of the carbonaceous char and complete volatilization of the polymer [5]. More information regarding pyrolysis products of phenol-formaldehyde
Polymers with aromatic hydrocarbon backbone
469
resins can be found in a report on non-flaming burning of phenol-formaldehyde resin foam [6] and in other published materials on the use of pyrolytic techniques for the study of phenol-formaldehyde polycondensates [7-20]. A pyrogram for a crosslinked phenol-formaldehyde resin (made with a basic catalyst) was done similarly to that for other polymers exemplified in this book, at 600 ~ C in He and with the separation on a Carbowax column and mass spectral detection (see Table 4.2.2). The peak identification for the chromatogram shown in Figure 8.3.1 is given in Table 8.3.1.
470
Polymers with aromatic hydrocarbon backbone
TABLE 8.3.1 (continued). Compoundsidentified in the pyrogram of a crosslinked phenolformaldehyde resin sample shown in Figure 8.3.1.
As seen from Figure 8.3.1 and Table 8.3.1, pyrolysis of the phenol-formaldehyde resin generates some formaldehyde probably eliminated from the methylol groups still present in the polymer, and also phenol and various alkyl substituted phenols that result from the breaking of the macromolecule network. Various studies (see e.g. [9]) showed that the decomposition mechanism of the polymer is initiated by bond rupture, which yields free radicals. The stabilization of the free radicals takes place by extraction of hydrogen atoms from the resin structure, leading to a decrease in the hydrogen/carbon ratio. The expected dimers of the methyl-substituted phenols, such as methyl-substituted dihydroxy-diphenylmethanes, which were expected to be formed, are thermally unstable and decompose to methyl-substituted phenols, or through cyclization to form methylsubstituted xanthenes. The cyclization occurs through water removal and not by hydrogen extraction. Some phenol-formaldehyde resins have specific groups attached on the benzene rings with the purpose of generating ion exchange materials. For example, the inert resin can be modified by direct sulfonation into a strong cation exchanger. An anion exchanger also can be obtained after the polymerization process. The polycondensation in the presence of acid catalysts leaves a significant number of free-CH2OH groups. These can be further derivatized with SO2CI, and the -CH2OH groups are changed into -CH2CI groups. Upon treatment with (CH3)3N, the resin can be changed into a strong anion exchange material by the following reactions:
Polymers with aromatic hydrocarbon backbone
471
The derivatization of the already polycondensed resin is not the only possibility to generate either cationic or anionic resins. One common procedure starts with the derivatization of the phenol prior to condensation. Sulfonic resins, for example, can be obtained following the reactions schematically shown below: OH
~~
o~
+ H2SO4 =_[ ~
OH
+ S%H rn,x ure
"',,.,,,,,
OH
OH
I
I
',',., ....
L
I
HCHO .....
The sulfonation products of phenol can be condensed without separation, and a highly cross-linked material can be obtained in appropriate conditions. Other resins can be prepared similarly to sulfonic resins. For example starting with salicylic acid and formaldehyde, a resin with carboxylic groups is obtained. The fixed ionic group also can be generated on the side chain of the resin. For example, condensation of sodium phenolate with Na2SO3 and HCHO leads to the formation of a resin with methylenesulfonic acid groups. Also, phenols such as resorcinol, naphthol, phenoxyacetic acid, and other aldehydes are used in the condensation instead of simple phenol and formaldehyde. For example, the condensation of phenoxyacetic acid and formaldehyde leads to a weak acid resin. Other variations of the condensation reaction are utilized, such as condensation of a phenol, a substituted benzaldehyde, and formaldehyde. Pyrolysis studies on ion exchange resins are common in literature [21-25]. Depending on the nature of the attached group, as well as on the proportion of the polymer that is derivatized, pyrolysis products originating from these groups can be seen in pyrolysates. Another example for the flash pyrolysis results on a sample of a phenol formaldehyde type polymer is given in Figure 8.3.2 for poly[(phenyl glycidyl ether)-co-formaldehyde]. This is in fact a co-polymer since free phenol groups are still present in the material that was used as a sample (CAS# 28064-14-4 and Mn = 345). The pyrolysis was done similarly to that for other polymers exemplified in this book, at 600 ~ C in He at a heating rate of 20 ~ C/ms with 10 s THT. The separation was done on a Carbowax column (60 m, 0.32 mm i.d., 0.32 ~m film thickness) with the GC starting at 40 ~ C with a ramp of 2 ~ C/min. up to 240 ~ C and a final oven time of 20 min. (see Table 4.2.2). The MS was operated in El+ mode. The peak identification for the chromatogram shown in Figure 8.3.2 was done using MS spectral library searches and is given in Table 8.3.2.
472
Polymers with aromatic hydrocarbon backbone
Polymers with aromatic hydrocarbon backbone
473
TABLE 8.3.2 (continued). Compounds identified in the pyrogram of poly[(phenyl glycidyl ether)-co-formaldehyde] shown in Figure 8. 3.2.
FIGURE 8.3.3. Pyrogram of a poly[(o-cresyl glycidyl ether)-co-formaldehyde] sample with Mn = 1,080. Pyrolysis done on O.4 mg material at 6000 C in He, with the separation on a Carbowax type column.
474
Polymers with aromatic hydrocarbon backbone
TABLE 8.3.3. Compounds identified in the pyrogram of poly[(o-cresyl glycidyl ether)-coformaldehyde] shown in Figure 8.3.3.
The identification of some of the peaks in the pyrograms of poly[(o-cresyl glycidyl ether)co-formaldehyde] was done only tentatively. For example, the mass spectrum of [(2-
Polymers with aromatic hydrocarbon backbone
4/b
(methylphenoxy)methyl]oxirane is available in commercial mass spectral libraries and is shown in Figure 8.3.4. 164
100% 108 91
51 10
20
30
40
50
133 60
70
80
90
100
110
120
130
140
150
160
170
180
FIGURE 8.3.4. Spectrum of [(2-(methylphenoxy)methyl]oxirane available in NIST'98 mass spectral library. On the other hand, the spectrum of [(2-methylphenoxy)-1,2-dimethyl]oxirane is not available in common mass spectral libraries. The mass spectrum of the peak eluting at 84.07 min. in the pyrogram shown in Figure 8.3.3 is given in Figure 8.3.5 and was assigned to this compound by comparison with its next lower homolog. 178 122
100%
107 77 39
.. ...... ,.Ij .... ,., 10
20
30
40
91
65
60
70
80
I,
.......... I.I..._,__I.Ii5 ! .... ..,!.,.... .,.:......... .....
:,;,.I :,,: .... ,1!1 50
I
,.,,,,. .... 90
100 110 120 130 140 150 160 170 180 190
FIGURE 8.3.5. Spectrum assigned to [(2-(methylphenoxy)-l,2-dimethyl]oxirane. As seen from Tables 8.3.2 and 8.3.3, the pyrolysis products of poly[(phenol glycidyl ether)-co-formaldehyde] and of poly[(o-cresyl glycidyl ether)-co-formaldehyde] give similar compounds. Some of these compounds are the result of the breaking of the polymeric network, some are the result of the elimination of the side groups to the benzene rings, and most compounds result from both processes, attached groups elimination and polymeric network cleavage.
References 8.3
1. A. A. Jeknavorian, Md. A. Mabud, E. F. Barry, J. J. Litzau, J. Anal. Appl. Pyrol., 46 (1998) 85. 2. P. H. R. B. Lemon, Polym. Paint Colour J., (suppl.), 1988, 103. 3. J. B. Henderson, W. D. Emmerich, Thermochim. Acta, 131 (1988) 7. 4. N. Grassie, D. H. Mackerron, Polym. Degrad. Stab., 5(1983)43. 5. R. W. Hall, Rep. Progr. Appl. Chem., 48 (1963) 223. 6. A. Alajbeg, J. Anal. Appl. Pyrol., 9 (1986) 255.
476
Polymers with aromatic hydrocarbon backbone
7. M. Blazs6, T. Toth, J. Anal. Appl. Pyrol., 19 (1991) 251. 8. C. A. Lytle, W. Bertsch, M. McKinley, J. Anal. Appl. Pyrol., 45 (1998) 121. 9. Y. Cohen, Z. Aizenshtat, J. Anal. Appl. Pyrol., 22 (1992) 153. 10. E. G. Leisegang, A. M. Stephen, J. C. Paterson-Jones, J. Appl. Polym. Sci., 14 (1970) 1961. 11. J. Martinez, G. Guiochon, J. Gas Chromatogr., 5 (1967) 146. 12. C. Landault, G. Guiochon, Anal. Chem., 39 (1967) 713. 13. W. Sassenberger, K. Wrabetz, Fresenius Z. Anal. Chem., 184 (1961) 423. 14. J. Zulaica G. Guiochon, J. Polym. Sci. Polym. Let., B 4 (1966) 567. 15. W. M. Jackson, R. T. Conley, J. Appl. Polym. Sci., 8 (1964) 2163. 16. E. Hagen, Plaste Kautsch., 15 (1968) 711. 17. D. H. Ahlstrom, S. A. Liebman, K. B. Abbas, J. Polym. Sci. Chem. Ed., 14 (1976) 2479. 18. T. Saito, Anal. Chem. Acta, 276 (1993) 295. 19. M. Tsuge, T. Tanuka, S. Tanaka, Jap. Analyst, 18 (1969) 47. 20. K. S. Annakutty, K. Kishore, Makromol. Chem., 192 (1991) 11. 21. J. R. Parrish. Anal. Chem., 45 (1973) 1659. 22. J. R. Parrish. Anal. Chem., 47 (1975) 1999. 23. E. Blasius, H. Hausler, Z. Anal. Chem., 277 (1974) 9. 24. C. N. Cascaval, G. Mocanu, A. Carpov, J. Thermal. Anal., 28 (1983) 32. 25. B. -G. Brodda, S. Dix, J. Fachinger, Sep. Sci. Tech., 28 (1993) 653.
CHAPTER 8
Polymers with Aromatic Hydrocarbon Backbone 8.1
POLYHYDROCARBONS WITH AROMATIC RINGS IN THE MAIN CHAIN
- General aspects
A significant number of polymers contain phenylene groups in their backbone. Most of these polymers also contain heteroatoms or heteroatom groups in the backbone and are discussed with other classes of polymers. However, there are some polymers with a carbon backbone that have only aromatic rings, alternating aromatic rings and aliphatic groups, or aromatic rings and unsaturated carbon chain fragments in their backbone. Examples of such polymers are poly(1,4-phenylene), poly(1,4-phenylene-ethylene), poly(1,3-phenylene-methylene), and poly(p-phenylene vinylidene), which are shown below:
CHI ....
.....
C/H/
rl
,,n t
poly(1,4-phenylene)
poly(1,4-p he nylene-ethylene)
poly(1,3-phenylenemethylene)
poly(1,4-phenylene1,2-ethenediyl)
Polymers with more complicated structures, containing triple bonds in addition to the aromatic cycles in the backbone or having substituents on the aromatic ring, also are known and have specific applications. Thermosetting phenolic resins form a separate class of polymers containing aromatic rings and aliphatic carbon groups in the polymeric network. These resins are formed from the reaction of phenol (or substituted phenols) with formaldehyde. The fully crosslinked macromolecule is insoluble and infusible. Other thermosetting resins are known in practice, some derived from the reaction of melamine or of urea with formaldehyde. Because these have a different chemical structure, containing nitrogen, they are included in a different class (see Section 15.3).
- Polyphenylene
and poly(phenylene-alkylenes)
Polyphenylene, CAS# 26008-28-6, is a temperature resistant polymer with the decomposition temperature around 660 ~ C (see Table 3.1.1). For this reason it has applications in heat resistant fabrics, electrical insulators, etc. The thermal decomposition products of the polymers in this group are formed by random scission of the bonds that are not involved in the aromatic rings. This leads in most cases to the formation of oligomers. If side groups are attached to the benzene rings, fragments from these groups typically are seen in the pyrograms. Some reported results for the thermal decomposition of the polymers in this group are summarized in Table 8.1.1 [1].
464
Polymers with aromatic hydrocarbon backbone
TABLE 8.1.1. Summary regarding literature reports on the thermal decomposition of polyphenylenes [1].
- Other polymers with phenylene groups in the backbone
Other polymers containing phenylene groups in a carbon chain backbone include those with some unsaturated groups between the phenylene rings. Some of these polymers still have good stability upon heating, such as poly(1,4-phenylene-1,2-ethenediyl), CAS# 26009-24-5, which decomposes around 400 ~ C [9]. Their use is, however, geared toward their special electricity conducting properties generated by the conjugated unsaturated groups and the aromatic rings. The compounds from this group also may have substituents on the aromatic ring [10]. Among the polymers containing aromatic rings and unsaturated hydrocarbon groups in the backbone are poly(phenylene-ethynylenes) (PPE). This type of polymer has been synthesized by alkyne metathesis of 1,4-dipropynylated benzenes [11] and has applications in optical and electronic industry. A comprehensive study on thermal decomposition of several poly(substituted p-phenylene-ethynylenes) is available [12]. The general formula for this group of polymers is the following:
As shown for the pyrolysis of poly(maleic anhydride-alt-l-octadecene) (see Section 6.8), when a long alkane chain is present in a polymer, the fragment alkanes and alkenes from the side chain generate large peaks in the pyrograms. The same effect is seen for the pyrolysis of PPEs, and a considerable number of peaks in the pyrograms of different substituted PPEs are identical, being generated from the fragmentation of the polymer side chains. However, some peaks are different and they can be used for the differentiation and identification of different polymers from this group.
Polymers with aromatic hydrocarbon backbone
465
References 8.1 1. J. Liggat, in J. Brandrup, E. H. Immergut, E. A. Grulke, eds., PolymerHandbook, J. Wiley, New York, 2000. 2. S. L. Madorsky, S. Straus, J. Res. Natl. Bur. Std., 53 (1954) 361. 3. S. L. Madorsky, S. Straus, J. Res. Natl. Bur. Std., 55 (1955) 223. 4. P. Brandt, V. H. Dibeler, R L. Mohler, J. Res. Nati. Bur. Std., 50 (1953) 201. 5. H. H. G. Jellinek, S. N. Lipovac, J. Polym. Sci. A-l, 8 (1970) 2517. 6. H. H. G. Jellinek, S. H. Ronel, J. Polym. Sci. A-l, 9 (1971) 2605. 7. G. R L. Ehlers, K. R. Fisch, W. R. Powell, J. Polym. Sci. A-l, 7 (1969) 2931. 8. D. G. H. Ballard, A Courtis, I. M. Shirley, S. C. Taylor, Macromolecules, 21 (1988) 294. 9. D. F. Hoeg, D. I. Lusk, E. P. Goldberg, J. Polym. Sci. Polym. Lett. Ed., 2 (1964) 697. 10. H. H. Horhold, J. Opfermann, J. Makromol. Chem., 131 (1970) 105. 11. L. Kloppenburg, Do Song, U. H. F. Bunz, J. Am. Chem. Soc., 120 (1998) 7973. 12. K. W. Sellers, C. M. Towns, C. R. Mubarak, L. Kloppenburg, U. H. F. Bunz, S. L. Morgan, J. Anal. Appl. Pyrol., 64 (2002) 313.
8.2
HALOGENATED POLYHYDROCARBONS WITH AROMATIC RINGS IN THE BACKBONE
- General
aspects
The polymers from this group show excellent thermal stability when they do not contain hydrogens, for example, when they are perfluorinated. Compounds having both hydrogen and a halogen (such as fluorine) eliminate the hydrohalogenated acid (HF, HCI, etc.) at lower temperatures. Some literature reports on thermal decomposition for this group of compounds are indicated in Table 8.2.1 [1]. TABLE 8.2.1. Summary regarding reports on the thermal decomposition of halogenated
polyphenylenes [1].
Polymers with aromatic hydrocarbon backbone
466
References 8.2 1. J. Liggat, in J. Brandrup, E. H. Immergut, E. A. Grulke, eds., PolymerHandbook, J. Wiley, New York, 2000. 2. J. L. Cotter, G. J. Knight, W. W. Wright, Br. Polym. J., 7 (1975) 389. 3. J. L. Cotter, G. J. Knight, J. M. Lancaster, W. W. Wright, J. Appl. Polym. Sci., 12 (1968) 2481.
8.3
THERMOSETTING PHENOLIC RESINS AND RELATED POLYMERS
- General aspects
Thermosetting phenolic resins include a number of polymers, the most common being obtained from the condensation of phenol with formaldehyde. The OH group on the benzene ring increases the reactivity in the o- and p- positions leading to three reactive centers for the phenolic component, while formaldehyde acts as having two active centers that can lead to a fully crosslinked polymer. The process may take place in neutral or alkaline conditions when in the first stage of the reaction, compounds known as methylol derivatives are formed. The condensation of phenol with formaldehyde occurs randomly at ortho- or para- position of the phenol, as shown below:
OH + HOH=O
OH OH OH =- {~/CH2OH ~ C[ ~ H 2 HCH=O0 ~ + CH2OH
CH2OH
OH OH H O H 2 C ~ CH2OH HCH--O H O H 2 C ~ ~ cH2OH CH2OH The methylol derivatives under mild heating for a limited time further react forming fusible polymers that are not yet fully crosslinked, as shown below: oH
HOH2C
I~~/
OH
oH
CH2OH + CH2OH
OH
CH2OH CH2OH .... - H20 or - HCH=O ~CCH2OH H2~C ....,~OH _CH2 CH2OH ~ i'Y" "" .....
CH2
In acidic conditions, the reaction between phenol and formaldehyde is believed to occur with the formation of a linear polymer as follows:
Polymers with aromatic hydrocarbon backbone
OH
OH
+ HCH=O
OH
.d
OH
OH
467
OH
......
OH H2C'~CH2,,,,,
Since the fully crosslinked macromolecule is insoluble and cannot be processed by molding, it is common in practice to synthesize first a polymer with a low degree of polymerization that can melt, followed by further crosslinking. This process is known in practice as formation of a two-stage resin (one-stage process is less frequently used). The two-stage resin can be obtained either using basic or acidic conditions. In basic conditions, the first stage must be done at low temperatures and with strictly controlled reaction times. Polymers with relatively low molecular weight and containing numerous -CH2OH groups attached to the benzene rings can be obtained (as previously shown). In the second stage, these polymers known as resols can be further crosslinked by heating, when more condensations are induced by the elimination of water. A different way to synthesize fusible polymers is to conduct the first stage in acidic conditions with the mole ratio of formaldehyde to phenol less than 1 and with mild heating. The linear polymer formed in this way is called novolac (or novolak). The novolac is further transformed into a crosslinked polymer by the addition of more formaldehyde and a basic catalyst. The process can be accomplished by adding during molding the novolac mixed with an appropriate amount of hexamethylene-tetramine. The decomposition of hexamethylenetetramine during heating generates formaldehyde and ammonia, which acts as a basic catalyst. The idealized structure of a crosslinked phenol-formaldehyde resin is shown below:
OH
OH
......... CH2 ......... H2C~
OH2
~
CH2
OH
C
2
.....
H OH2
.....
cH2
In the synthesis of this polymer in the presence of NaOH, long chain or highly branched molecules are obtained, depending on factors such as phenol to formaldehyde mole ratio, phenol to NaOH mole ratio, and the temperature and duration of the heating. Other resins from this group may be obtained with the phenolic component with additional substituents on the benzene ring such as alkyl (methyl, dimethyl, butyl, tert-
Polymers with aromatic hydrocarbon backbone
468
octyl, nonyl), alkenyl (decapentatrienyl when using cardanol), etc. Phenolic resins with two -OH groups present on the benzene ring, like in resorcinol-formaldehyde resin, also are known. Instead of OH groups, ether groups also activate the ortho and para positions on the benzene ring. For example, glycidyl ethers were used as phenolic component, and these groups allow further crosslinking (curing) of the macromolecule. When the attached groups are glycidyl (epoxy propyl), these compounds are known as epoxynovolaks (or epoxynovolac). Typically these polymers also contain some free phenol groups. The idealized structures of two resins containing glycidyl ether attached to the benzene ring are shown below: ........CH2
RO
........C.H2
~ R = H-, / \
@
CH 2_
R
o .........
R = H-, ~ . ~ C H
2-
CH3 poly[(phenyl glycidyl ether)-co-formaldehyde]
poly[(o-cresyl glycidyl ether)-co-formaldehyde]
The polymers are initially obtained in linear form, being thermoplastic and soluble in different solvents. The heating of the polymer allows further crosslinking by the opening of the oxirane ring and formation of intermolecular bridges. These types of polymers are used as active components in some epoxy resins. Also, resins containing other aromatic moieties are known. Among these is, for example, naphthalene sulfonate formaldehyde resin, which can be used as a cement additive [1]. Also, thiophenol forms a condensation product with formaldehyde. However, the condensation with thiophenol occurs with a different mechanism than that of phenol, and sulfur atoms are included in the backbone of the polymer. For this reason, thiophenol/formaldehyde resin is discussed in Section 12.1. Phenolic resins have many practical applications. They are widely used to make plywood for furniture and wall paneling when wood dust or wood chips are included as a filler. With paper or fabric phenol-formaldehyde resins form various laminates including those necessary for making boards for electronic circuits. Phenolic resins also are used to bind fiberglass, carbon fiber, and other reinforcing materials applied in aerospace industry. The binding of a filler can be done by different technological procedures, either starting with a solution of novolac that is used to soak the filler followed by heating, or in mixtures of resol or novolac with the filler that is thermoset in the desired shape of the final object. Phenolic resins also have applications in coating industry for high-temperature wire insulation, for making electrical battery separators, for use as structural adhesives, etc. The phenolic foams are used for heat insulators in buildings. Also, phenolic resins can be components of various copolymers and may be used as vulcanizing agents for rubbers. Phenol-formaldehyde resins are relatively resistant to heat. They start decomposing at about 250 ~ C still maintaining some mechanical resistance, the decomposition rate increasing significantly around 300 ~ C. In an inert atmosphere at 750 o C, phenolformaldehyde resins form more than 50% char [2, 3]. The volatile materials consist of xylene (76%), traces of phenol, cresol, and benzene [4]. The heating in air above 300 ~ C leads to the oxidation of the carbonaceous char and complete volatilization of the polymer [5]. More information regarding pyrolysis products of phenol-formaldehyde
Polymers with aromatic hydrocarbon backbone
469
resins can be found in a report on non-flaming burning of phenol-formaldehyde resin foam [6] and in other published materials on the use of pyrolytic techniques for the study of phenol-formaldehyde polycondensates [7-20]. A pyrogram for a crosslinked phenol-formaldehyde resin (made with a basic catalyst) was done similarly to that for other polymers exemplified in this book, at 600 ~ C in He and with the separation on a Carbowax column and mass spectral detection (see Table 4.2.2). The peak identification for the chromatogram shown in Figure 8.3.1 is given in Table 8.3.1.
470
Polymers with aromatic hydrocarbon backbone
TABLE 8.3.1 (continued). Compoundsidentified in the pyrogram of a crosslinked phenolformaldehyde resin sample shown in Figure 8.3.1.
As seen from Figure 8.3.1 and Table 8.3.1, pyrolysis of the phenol-formaldehyde resin generates some formaldehyde probably eliminated from the methylol groups still present in the polymer, and also phenol and various alkyl substituted phenols that result from the breaking of the macromolecule network. Various studies (see e.g. [9]) showed that the decomposition mechanism of the polymer is initiated by bond rupture, which yields free radicals. The stabilization of the free radicals takes place by extraction of hydrogen atoms from the resin structure, leading to a decrease in the hydrogen/carbon ratio. The expected dimers of the methyl-substituted phenols, such as methyl-substituted dihydroxy-diphenylmethanes, which were expected to be formed, are thermally unstable and decompose to methyl-substituted phenols, or through cyclization to form methylsubstituted xanthenes. The cyclization occurs through water removal and not by hydrogen extraction. Some phenol-formaldehyde resins have specific groups attached on the benzene rings with the purpose of generating ion exchange materials. For example, the inert resin can be modified by direct sulfonation into a strong cation exchanger. An anion exchanger also can be obtained after the polymerization process. The polycondensation in the presence of acid catalysts leaves a significant number of free-CH2OH groups. These can be further derivatized with SO2CI, and the -CH2OH groups are changed into -CH2CI groups. Upon treatment with (CH3)3N, the resin can be changed into a strong anion exchange material by the following reactions:
Polymers with aromatic hydrocarbon backbone
471
The derivatization of the already polycondensed resin is not the only possibility to generate either cationic or anionic resins. One common procedure starts with the derivatization of the phenol prior to condensation. Sulfonic resins, for example, can be obtained following the reactions schematically shown below: OH
~~
o~
+ H2SO4 =_[ ~
OH
+ S%H rn,x ure
"',,.,,,,,
OH
OH
I
I
',',., ....
L
I
HCHO .....
The sulfonation products of phenol can be condensed without separation, and a highly cross-linked material can be obtained in appropriate conditions. Other resins can be prepared similarly to sulfonic resins. For example starting with salicylic acid and formaldehyde, a resin with carboxylic groups is obtained. The fixed ionic group also can be generated on the side chain of the resin. For example, condensation of sodium phenolate with Na2SO3 and HCHO leads to the formation of a resin with methylenesulfonic acid groups. Also, phenols such as resorcinol, naphthol, phenoxyacetic acid, and other aldehydes are used in the condensation instead of simple phenol and formaldehyde. For example, the condensation of phenoxyacetic acid and formaldehyde leads to a weak acid resin. Other variations of the condensation reaction are utilized, such as condensation of a phenol, a substituted benzaldehyde, and formaldehyde. Pyrolysis studies on ion exchange resins are common in literature [21-25]. Depending on the nature of the attached group, as well as on the proportion of the polymer that is derivatized, pyrolysis products originating from these groups can be seen in pyrolysates. Another example for the flash pyrolysis results on a sample of a phenol formaldehyde type polymer is given in Figure 8.3.2 for poly[(phenyl glycidyl ether)-co-formaldehyde]. This is in fact a co-polymer since free phenol groups are still present in the material that was used as a sample (CAS# 28064-14-4 and Mn = 345). The pyrolysis was done similarly to that for other polymers exemplified in this book, at 600 ~ C in He at a heating rate of 20 ~ C/ms with 10 s THT. The separation was done on a Carbowax column (60 m, 0.32 mm i.d., 0.32 ~m film thickness) with the GC starting at 40 ~ C with a ramp of 2 ~ C/min. up to 240 ~ C and a final oven time of 20 min. (see Table 4.2.2). The MS was operated in El+ mode. The peak identification for the chromatogram shown in Figure 8.3.2 was done using MS spectral library searches and is given in Table 8.3.2.
472
Polymers with aromatic hydrocarbon backbone
Polymers with aromatic hydrocarbon backbone
473
TABLE 8.3.2 (continued). Compounds identified in the pyrogram of poly[(phenyl glycidyl ether)-co-formaldehyde] shown in Figure 8. 3.2.
FIGURE 8.3.3. Pyrogram of a poly[(o-cresyl glycidyl ether)-co-formaldehyde] sample with Mn = 1,080. Pyrolysis done on O.4 mg material at 6000 C in He, with the separation on a Carbowax type column.
474
Polymers with aromatic hydrocarbon backbone
TABLE 8.3.3. Compounds identified in the pyrogram of poly[(o-cresyl glycidyl ether)-coformaldehyde] shown in Figure 8.3.3.
The identification of some of the peaks in the pyrograms of poly[(o-cresyl glycidyl ether)co-formaldehyde] was done only tentatively. For example, the mass spectrum of [(2-
Polymers with aromatic hydrocarbon backbone
4/b
(methylphenoxy)methyl]oxirane is available in commercial mass spectral libraries and is shown in Figure 8.3.4. 164
100% 108 91
51 10
20
30
40
50
133 60
70
80
90
100
110
120
130
140
150
160
170
180
FIGURE 8.3.4. Spectrum of [(2-(methylphenoxy)methyl]oxirane available in NIST'98 mass spectral library. On the other hand, the spectrum of [(2-methylphenoxy)-1,2-dimethyl]oxirane is not available in common mass spectral libraries. The mass spectrum of the peak eluting at 84.07 min. in the pyrogram shown in Figure 8.3.3 is given in Figure 8.3.5 and was assigned to this compound by comparison with its next lower homolog. 178 122
100%
107 77 39
.. ...... ,.Ij .... ,., 10
20
30
40
91
65
60
70
80
I,
.......... I.I..._,__I.Ii5 ! .... ..,!.,.... .,.:......... .....
:,;,.I :,,: .... ,1!1 50
I
,.,,,,. .... 90
100 110 120 130 140 150 160 170 180 190
FIGURE 8.3.5. Spectrum assigned to [(2-(methylphenoxy)-l,2-dimethyl]oxirane. As seen from Tables 8.3.2 and 8.3.3, the pyrolysis products of poly[(phenol glycidyl ether)-co-formaldehyde] and of poly[(o-cresyl glycidyl ether)-co-formaldehyde] give similar compounds. Some of these compounds are the result of the breaking of the polymeric network, some are the result of the elimination of the side groups to the benzene rings, and most compounds result from both processes, attached groups elimination and polymeric network cleavage.
References 8.3
1. A. A. Jeknavorian, Md. A. Mabud, E. F. Barry, J. J. Litzau, J. Anal. Appl. Pyrol., 46 (1998) 85. 2. P. H. R. B. Lemon, Polym. Paint Colour J., (suppl.), 1988, 103. 3. J. B. Henderson, W. D. Emmerich, Thermochim. Acta, 131 (1988) 7. 4. N. Grassie, D. H. Mackerron, Polym. Degrad. Stab., 5(1983)43. 5. R. W. Hall, Rep. Progr. Appl. Chem., 48 (1963) 223. 6. A. Alajbeg, J. Anal. Appl. Pyrol., 9 (1986) 255.
476
Polymers with aromatic hydrocarbon backbone
7. M. Blazs6, T. Toth, J. Anal. Appl. Pyrol., 19 (1991) 251. 8. C. A. Lytle, W. Bertsch, M. McKinley, J. Anal. Appl. Pyrol., 45 (1998) 121. 9. Y. Cohen, Z. Aizenshtat, J. Anal. Appl. Pyrol., 22 (1992) 153. 10. E. G. Leisegang, A. M. Stephen, J. C. Paterson-Jones, J. Appl. Polym. Sci., 14 (1970) 1961. 11. J. Martinez, G. Guiochon, J. Gas Chromatogr., 5 (1967) 146. 12. C. Landault, G. Guiochon, Anal. Chem., 39 (1967) 713. 13. W. Sassenberger, K. Wrabetz, Fresenius Z. Anal. Chem., 184 (1961) 423. 14. J. Zulaica G. Guiochon, J. Polym. Sci. Polym. Let., B 4 (1966) 567. 15. W. M. Jackson, R. T. Conley, J. Appl. Polym. Sci., 8 (1964) 2163. 16. E. Hagen, Plaste Kautsch., 15 (1968) 711. 17. D. H. Ahlstrom, S. A. Liebman, K. B. Abbas, J. Polym. Sci. Chem. Ed., 14 (1976) 2479. 18. T. Saito, Anal. Chem. Acta, 276 (1993) 295. 19. M. Tsuge, T. Tanuka, S. Tanaka, Jap. Analyst, 18 (1969) 47. 20. K. S. Annakutty, K. Kishore, Makromol. Chem., 192 (1991) 11. 21. J. R. Parrish. Anal. Chem., 45 (1973) 1659. 22. J. R. Parrish. Anal. Chem., 47 (1975) 1999. 23. E. Blasius, H. Hausler, Z. Anal. Chem., 277 (1974) 9. 24. C. N. Cascaval, G. Mocanu, A. Carpov, J. Thermal. Anal., 28 (1983) 32. 25. B. -G. Brodda, S. Dix, J. Fachinger, Sep. Sci. Tech., 28 (1993) 653.