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Thermal degradation and flammability characteristics of some polystyrenes and poly(methyl methacrylate)s chemically modified with silicon-containing groups
Polymer Degradation and Stability 83 (2004) 181–185 www.elsevier.com/locate/polydegstab
Thermal degradation and flammability characteristics of some polystyrenes and poly(methyl methacrylate)s chemically modified with silicon-containing groups John R. Ebdon*, Barry J. Hunt, Paul Joseph The Polymer Centre, Department of Chemistry, University of Sheffield, Brook Hill, Sheffield S3 7HF, UK Received 8 May 2003; accepted 7 June 2003
Abstract Several Si-containing methacrylates and acrylates have been free radically copolymerised with methyl methacrylate and with styrene. The copolymers have been examined by DSC to determine glass transition temperatures (Tg), by TGA to determine thermal stabilities, and by measurements of limiting oxygen index (LOI) to determine ignitability/flame retardance. The copolymers are found to have Tgs similar to those of the parent homopolymers suggesting that the mechanical properties are little affected by the incorporation of the Si-containing groups and to have slightly improved flame retardance. LOI and char yields suggest that the mechanism of flame retardance, i.e. whether vapour phase or condensed phase, depends upon the nature of the Si-containing substituent. # 2003 Elsevier Ltd. All rights reserved. Keywords: Methyl methacrylate; Styrene; Silicon-containing monomer; Copolymerisation; Thermal degradation; Flame retardance
1. Introduction Improving the flame retardance of organic polymers is a matter of mounting concern and importance given the increasing use of materials based on such polymers in both domestic and public environments. Flame retardance in polymers is largely achieved currently through the incorporation of flame retardant additives, often requiring high loadings to be effective (typically 10–40 wt.%), and which may result in adverse changes to the physical and mechanical properties of the polymer. An alternative approach is the chemical incorporation of the flame-retardant species via copolymerisation or some other type of chemical modification. The relatively low loadings required to achieve sufficient flame retardance, and the careful selection of the comonomer or other modifying group, may keep detrimental changes to the physical and mechanical properties of the polymer to an acceptable minimum. Furthermore, the flame retardant is then not easily lost from the polymer, eliminating one of the major problems associated with the additive approach [1]. Reactive flame retardants are beginning to be exploited * Corresponding author. Tel.: +44-114-222-9562. E-mail address: j.ebdon@sheffield.ac.uk (J.R. Ebdon). 0141-3910/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0141-3910(03)00261-1
in polymers made by step-reaction processes, e.g. epoxy resins [2–7], polyesters [8–11] and polyurethanes [12–14]; but their use in chain reaction polymers, the types of most interest to us and much of industry, has been little explored. The polymers we have chosen for study are poly(methyl methacrylate) (PMMA) and polystyrene (PSt). Both of these polymers are widely used thermoplastics but are highly flammable owing to the ease with which they thermally degrade (depolymerise), releasing large quantities of highly flammable, volatile, monomeric and oligomeric fragments. Therefore, even small improvements in the fire performances of these polymers would be widely welcomed since they would allow wider and safer applications of materials based on them. Thus far we have concentrated mainly on the use of phosphoruscontaining species [15] as reactive modifiers of chain reaction polymers, with notable success, for example, in improving the flame retardance of PMMA [16–20]. Here we report on more recent work in which we have investigated the efficacy of silicon-containing intermediates. Silicon compounds when present in a polymer, whether as a part of the polymer chain (e.g. as in polysiloxanes) or as an additive (e.g. silica), have a flame retardant effect, arising partly from the property that such compounds have in ‘‘diluting’’ the more combustible
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organic components and partly from the barrier that silicaceous residues can form to an advancing flame [21– 24]. The use of silicon compounds as reactive components in flame-retardant systems seems to have been little explored, despite their relatively low combustibility, although interest has recently been shown in the use of polyhedral oligomeric silsesquioxanes (POSS) as both additives and reactive components in thermoplastics and thermosets [25]. POSS have a notable mechanical reinforcing effect upon polymers in which they are incorporated, even at quite low loadings, and they improve thermal stability. However, POSS are relatively difficult to prepare and thus currently are rather expensive intermediates [26]. It seems to us, therefore, that their use is not appropriate at present in commodity thermoplastics. So we have concentrated in our work on investigating the influence on thermal stability and flammability of some reactive silicon compounds of simpler structure.
as the radical initiator in all polymerisations, and was purified by recrystallisation from methanol. The solvents, used as diluents for the polymerisations, were purified by standard literature procedures. 2.2. General procedure for radical polymerisations in solution Both small-scale ( 2–5 g) and larger scale ( 20–50 g, see Tables 1 and 2) polymerisations were performed in de-aerated solutions of the monomers at ca. 60 C using AIBN as initiator. After the required reaction periods, the polymers were recovered by precipitating the reaction mixtures in an excess of a non-solvent (methanol or hexane). The polymers were collected by filtration, vacuum dried, and purified by reprecipitation (from solution in dichloromethane to the non-solvent) and finally dried in a vacuum oven. 2.3. Characterisation of the copolymers
2. Experimental 2.1. Materials The Si-containing monomers employed in the present study are acrylates and methacrylates, i.e., typical acceptor-type monomers. They are trimethylsilylmethacrylate (TMSM), trimethylsilylmethylmethacrylate (TMSMM), 3-[tris(trimethylsilyloxy)silyl]propylmethacrylate (TTSSPM), poly(dimethylsiloxane)monomethacrylate (PDMSMM), and poly(dimethylsiloxane)monoacrylate (PDMSMA). All these monomers except TMSM are hydrolytically stable. All monomers, except PDMSMA (Dow Corning), other reagents and solvents were obtained from the Aldrich Chemical Company. Silicon-containing monomers were used as received whilst styrene (St) and methyl methacrylate (MMA) were freed from the inhibitors by washing with 5% NaOH solution followed by deionised water until neutral, dried over anhydrous MgSO4, and finally distilled under reduced pressure prior to use. 2,20 -Azoisobutyronitrile (AIBN) was used
1
H NMR (250 MHz) spectra of solutions of polymers in CDC13 were recorded on a Bruker AC250 spectrometer at ambient probe temperature. Infra-red spectra were obtained on a Perkin-Elmer 1 720X FT-IR instrument on thin films cast from solutions of the polymers in CH2C12. Thermogravimetric analyses (TGA) were carried out on ca. 7–15 mg samples using a PerkinElmer 7 Series thermal analysis system, both in air and in nitrogen, at a heating rate of 20 C min1. Differential scanning calorimetric (DSC) analyses were performed on preweighed samples (ca. 5–7 mg) using a Perkin-Elmer 7 Series instrument, under a nitrogen atmosphere. A heating rate of 10 C min1 was employed and the samples were reheated to identify any irreversible phase transitions. The gel permeation chromatography (GPC) set-up, used for measurements of molecular weight and molecular weight distribution, was comprised of a Waters 515 pump coupled via a Gilson 234 autoinjector to Polymer Laboratories columns. A Waters 410 refractometer was used to detect the fractions eluting from the columns. Calibrations of the columns were carried out using polystyrene or
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poly(methyl methacrylate) standards, as appropriate. All samples were run in THF at a concentration of ca. 0.3% w/v at a flow rate of 1 cm3 min1. Limiting oxygen indices (LOI-ASTM-D-2863) were measured on a Stanton-Redcroft flammability unit on hot-pressed plaques measuring 100.60.3 cm. The volatile species emanating from some of the polymers upon thermal degradation, were investigated through Curie-point pyrolysis/GC–MS and also by laser-pyrolysis/time-of-flight mass spectrometry (LP/ToF-MS). Pyrolysis GC–MS analyses of ca. 1 mg samples were carried out on a Fisher Model 1040 PSC Curie-point pyrolyser attached to a Perkin-Elmer Auto-system XL gas chromatograph, equipped with a PE-5MS column and using He as carrier gas at a flow rate of 1 cm3 min1. The gas chromatograph was connected to a Perkin-Elmer Turbomass quadrupole mass spectrometer with electron impact (El) source to enable identification of the separated, volatile, products of pyrolysis. The LP/ToF-MS technique has been described elsewhere in detail [27,28]. Degradation is initiated by exposing the polymer surface to a single 500 ms duration pulse of ca. 1 mm width with a total energy that can be varied between 0.1 and 1.0 J. The temperature rise at the polymer surface has been estimated to be within the range 650–1000 C. The volatiles escaping from this reaction zone are ionised on entry to an ion source and are then analysed by the ToF-MS every 25 ms.
3. Results and discussion PSt and PMMA containing the Si-based comonomers were successfully made by radical copolymerisations. Various characterisation data for these copolymers are collected in Tables 3 and 4. The compositions of the copolymers were deduced from their 1H NMR spectra by comparing the integral areas of appropriately assigned signals. In the case of St/TMSM and MMA/TMSM, there is substantial hydrolysis of the trimethylsilyloxy groups to give methacrylic acid units. The GPC chromatograms of the homopolymers and the copolymers generally showed unimodal distributions of molecular weight. The Mn values ranged from 13,000 to 60,000 with polydispersity indices between 1.6 and 2.5. Introduction of the Si-containing monomers into PSt and PMMA appeared to reduce the Tg’s of the polymers only very slightly from the values for the homopolymers (105 and 120 C, respectively). However, unambiguous assignment of the phase transition corresponding to the glass transition was not possible for the St and MMA copolymers containing the polydimethylsiloxane macromonomers, PDMSMM and PDMSMA. Generally, the copolymers did not show any marked increases in thermal or thermo-oxidative stability or in amounts of char produced in TGA experiments when
Table 1 Typical preparative data for copolymers of St and Si-containing monomers Expt. no.
St (g)
Comonomer (g)
Solvent (g)
AIBN (mg)
Temp ( C)
Time (h)
Conversion (%w/w)
1 2 3 4 5 6 7
60.0 39.0 39.0 21.8 19.4 25.8 25.8
– TMSM, 19.8 TMSMM,21.5 TTSSPM,9.8 TTSSPM, 19.7 PDMSMM, 33.6 PDMSMA, 25.1
Toluene, 208 Toluene, 173 Toluene, 217 Toluene, 173 Toluene, 173 Chlorobenzene, 221 Chlorobenzene, 221
516 523 530 534 512 508 500
60 60 60 60 60 60 60
72 216 191 161 161 122 144
67 65 73 70 70 87 94
Table 2 Typical preparative data for copolymers of MMA and Si-containing monomers Expt. no.
MMA (g)
Comonomer (g)
Solvent (g)
AIBN (mg)
Temp ( C)
Time (h)
Conversion (%w/w)
8 9 10 11 12 13 14
60.0 35.0 35.0 21.0 18.6 24.7 24.8
– TMSM, 23.5 TMSMM,25.8 TTSSPM, 10.1 TTSSPM, 19.7 PDMSMM, 34.0 PDMSMA, 25.0
Toluene, 208 Toluene, 173 Toluene, 260 Toluene, 173 Toluene, 173 Chlorobenzene, 221 Chlorobenzene, 221
503 522 553 523 515 499 506
60 60 60 60 60 60 60
23 45 93 145 144 122 143
90 92 91 77 52 93 92
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compared with unmodified PSt and PMMA. The only noticeable exceptions were the copolymers with PDMSMM and PDMSMA. These copolymers produced significant amounts of char, both in air and in nitrogen, in TGA experiments. From the LOI values obtained for the Si-modified polymers, it can be seen that introduction of Si-containing groups results in an increase in flame retardance in both St and MMA copolymers. However, such increases can only be considered to be marginal given the relatively high Si loadings. Also, there is no obvious correlation between LOI values, the char yields and the Si contents of these copolymers. In the case of MMA/ TMSM, there is a significant increase in LOI (23.5) compared with that of PMMA (17.9) and it also forms char during the TGA runs both in air (4.2% w/w) and nitrogen (7.0% w/w) even though the Si content is only 0.7% w/w. However, this may be a consequence of the formation of intra- and/or intermolecular anhydride crosslinks between the methacrylic acid units arising from hydrolysis of the trimethylsilyloxy groups. Such anhydride links could, in turn, interrupt the thermally induced chain depolymerisation of MMA sequences leading to evolution of less MMA monomer (the principal fuel in any fire involving PMMA). Eventually, the anhydride sequences will decarboxylate, losing CO and CO2, resulting in the formation of unsaturated precursors to the aromatic components of char [18]. In the case of copolymers of St and MMA with the Si-containing macromonomers, PDMSMM and PDMSMA,
there are only very small improvements in LOI, even though there are substantial amounts of Si present in the form of polydimethylsiloxane pendent groups and even though they produce significant amounts of char during the TGA runs. However, on rapid heating such as that encountered during flaming combustion, the pendent polydimethylsiloxane chains are likely to depolymerise to produce a mixture of flammable low molecular weight cyclic oligomers and, to a lesser extent, low molecular weight hydrocarbons, such as methane [29– 31]. These could act as fuels to the flame in addition to the principal contributors from the monomeric and oligomeric components from the base polymers themselves. Thus rapid pyrolysis of these copolymers actually results in enhanced fuel production with little or no formation of char, as was evident from the LOI experiments. LP/ToF-MS spectra of St/PDMSMM, MMA/ PDMSMM and MMA/PDMSMA showed Si-containing mass fragments with m/z values of 73, 147 and 221 corresponding to species of the type —(OSi(Me)2)n— with n=1, 2 and 3, respectively. Rapid pyrolyses at 590 C in the Curie point/GC–MS experiments showed the parent monomer (St or MMA) and the comonomeric species to be the major species evolved indicating that the predominant pyrolysis mechanism operating under these conditions is chain depolymerisation. A more detailed examination of the GC–MS spectra of St/PDMSMA, St/PDMSMM, MMA/PDMSMA and MMA/PDMSMM showed various fragments incorporating the butyl end group of the polydimethylsiloxane chain in addition to
Table 3 Charactensation data for copolymers of St and Si-containing monomers Expt. No. Polymer
Mol. fract. comonomer Si-content (wt.%) TGAa residue in air (wt.%) TGAa residue in N2 (wt.%) LOI (%O2 v/v)
1 2 3 4 5 6 7
– 0.06 (80b) 0.25 0.11 0.2 0.014 0.014
PSt St/TMSM St/TMSMM St/TTSSPM St/TTSSPM St/PDMSMA St/PDMSMM a b
– 1.6 7.4 8.9 13.3 24.3 21.5
0 0 0 0 0 40.0 25.0
0 0 0 0 0 56.4 50.3
18.0 19.8 20.5 25.0 25.3 20.0 19.0
Char residue at 500 C. Percentage of these comonomer units not adventitiously hydrolysed to methacrylic acid units.
Table 4 Characterisation data for copolymers of MMA and Si-containing monomers Expt. No. Polymer
Mol. fract. comonomer Si-content (wt.%) TGAa residue in air (wt.%) TGAa residue in N2 (wt.%) LOI (%O2 v/v)
8 9 10 11 12 13 14
– 0.025 (75b) 0.27 0.10 0.18 0.01 0.02
a b
PMMA MMA/TMSM MMA/TMSMM MMA/TMSSPM MMA/TMSSPM MMA/PDMSMA MMA/PDMSMM
– 0.7 6.3 8.5 12.7 21.7 25
0 4.2 0 0 3.0 43.8 25.0
Char residue at 500 C. Percentage of these comonomer units not adventitiously hydrolysed to methacrymic acid units.
0 7.0 0 0 2.6 55.8 7.4
17.9 23.5 18.6 18.1 18.4 19.4 19.0
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dimethylsiloxane monomeric units. These include butylended monomeric (m/z=131), dimeric (m/z=207), trimeric (m/z=281), tetrameric (m/z=355) and pentameric (m/z=429) fragments.
4. Conclusions Copolymers of St and of MMA with some Si-containing acrylates and methacrylates show marginally improved flame retardance compared with those of the parent homopolymers. The flame retardance seems to arise mainly from a vapour phase mechanism except in the case of copolymers with monomers containing oligodimethylsiloxane side chains in which case significant char yields suggest also a condensed phase component. In the case of these latter copolymers, however, pyrolysis data suggest that rapid depolymerization of the oligodimethylsiloxane side chains may actually result in additional fuel being produced, thus tending to counteract the advantage to flame retardancy arising from the production of any char. Overall it seems that Si-containing monomers alone when incorporated into acrylic and styrenic polymers are unlikely to offer sufficient improvement in flame retardancy to warrant much further investigation for this purpose.
Acknowledgements The authors thank the EPSRC and MoD (grant no. GR/L 85886) for financial support. Technical collaboration with the ICI STG, Wilton and Dow Corning, Barry are also gratefully acknowledged.
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Thermal degradation and flammability characteristics of some polystyrenes and poly(methyl methacrylate)s chemically modified with silicon-containing groups John R. Ebdon*, Barry J. Hunt, Paul Joseph The Polymer Centre, Department of Chemistry, University of Sheffield, Brook Hill, Sheffield S3 7HF, UK Received 8 May 2003; accepted 7 June 2003
Abstract Several Si-containing methacrylates and acrylates have been free radically copolymerised with methyl methacrylate and with styrene. The copolymers have been examined by DSC to determine glass transition temperatures (Tg), by TGA to determine thermal stabilities, and by measurements of limiting oxygen index (LOI) to determine ignitability/flame retardance. The copolymers are found to have Tgs similar to those of the parent homopolymers suggesting that the mechanical properties are little affected by the incorporation of the Si-containing groups and to have slightly improved flame retardance. LOI and char yields suggest that the mechanism of flame retardance, i.e. whether vapour phase or condensed phase, depends upon the nature of the Si-containing substituent. # 2003 Elsevier Ltd. All rights reserved. Keywords: Methyl methacrylate; Styrene; Silicon-containing monomer; Copolymerisation; Thermal degradation; Flame retardance
1. Introduction Improving the flame retardance of organic polymers is a matter of mounting concern and importance given the increasing use of materials based on such polymers in both domestic and public environments. Flame retardance in polymers is largely achieved currently through the incorporation of flame retardant additives, often requiring high loadings to be effective (typically 10–40 wt.%), and which may result in adverse changes to the physical and mechanical properties of the polymer. An alternative approach is the chemical incorporation of the flame-retardant species via copolymerisation or some other type of chemical modification. The relatively low loadings required to achieve sufficient flame retardance, and the careful selection of the comonomer or other modifying group, may keep detrimental changes to the physical and mechanical properties of the polymer to an acceptable minimum. Furthermore, the flame retardant is then not easily lost from the polymer, eliminating one of the major problems associated with the additive approach [1]. Reactive flame retardants are beginning to be exploited * Corresponding author. Tel.: +44-114-222-9562. E-mail address: j.ebdon@sheffield.ac.uk (J.R. Ebdon). 0141-3910/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0141-3910(03)00261-1
in polymers made by step-reaction processes, e.g. epoxy resins [2–7], polyesters [8–11] and polyurethanes [12–14]; but their use in chain reaction polymers, the types of most interest to us and much of industry, has been little explored. The polymers we have chosen for study are poly(methyl methacrylate) (PMMA) and polystyrene (PSt). Both of these polymers are widely used thermoplastics but are highly flammable owing to the ease with which they thermally degrade (depolymerise), releasing large quantities of highly flammable, volatile, monomeric and oligomeric fragments. Therefore, even small improvements in the fire performances of these polymers would be widely welcomed since they would allow wider and safer applications of materials based on them. Thus far we have concentrated mainly on the use of phosphoruscontaining species [15] as reactive modifiers of chain reaction polymers, with notable success, for example, in improving the flame retardance of PMMA [16–20]. Here we report on more recent work in which we have investigated the efficacy of silicon-containing intermediates. Silicon compounds when present in a polymer, whether as a part of the polymer chain (e.g. as in polysiloxanes) or as an additive (e.g. silica), have a flame retardant effect, arising partly from the property that such compounds have in ‘‘diluting’’ the more combustible
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organic components and partly from the barrier that silicaceous residues can form to an advancing flame [21– 24]. The use of silicon compounds as reactive components in flame-retardant systems seems to have been little explored, despite their relatively low combustibility, although interest has recently been shown in the use of polyhedral oligomeric silsesquioxanes (POSS) as both additives and reactive components in thermoplastics and thermosets [25]. POSS have a notable mechanical reinforcing effect upon polymers in which they are incorporated, even at quite low loadings, and they improve thermal stability. However, POSS are relatively difficult to prepare and thus currently are rather expensive intermediates [26]. It seems to us, therefore, that their use is not appropriate at present in commodity thermoplastics. So we have concentrated in our work on investigating the influence on thermal stability and flammability of some reactive silicon compounds of simpler structure.
as the radical initiator in all polymerisations, and was purified by recrystallisation from methanol. The solvents, used as diluents for the polymerisations, were purified by standard literature procedures. 2.2. General procedure for radical polymerisations in solution Both small-scale ( 2–5 g) and larger scale ( 20–50 g, see Tables 1 and 2) polymerisations were performed in de-aerated solutions of the monomers at ca. 60 C using AIBN as initiator. After the required reaction periods, the polymers were recovered by precipitating the reaction mixtures in an excess of a non-solvent (methanol or hexane). The polymers were collected by filtration, vacuum dried, and purified by reprecipitation (from solution in dichloromethane to the non-solvent) and finally dried in a vacuum oven. 2.3. Characterisation of the copolymers
2. Experimental 2.1. Materials The Si-containing monomers employed in the present study are acrylates and methacrylates, i.e., typical acceptor-type monomers. They are trimethylsilylmethacrylate (TMSM), trimethylsilylmethylmethacrylate (TMSMM), 3-[tris(trimethylsilyloxy)silyl]propylmethacrylate (TTSSPM), poly(dimethylsiloxane)monomethacrylate (PDMSMM), and poly(dimethylsiloxane)monoacrylate (PDMSMA). All these monomers except TMSM are hydrolytically stable. All monomers, except PDMSMA (Dow Corning), other reagents and solvents were obtained from the Aldrich Chemical Company. Silicon-containing monomers were used as received whilst styrene (St) and methyl methacrylate (MMA) were freed from the inhibitors by washing with 5% NaOH solution followed by deionised water until neutral, dried over anhydrous MgSO4, and finally distilled under reduced pressure prior to use. 2,20 -Azoisobutyronitrile (AIBN) was used
1
H NMR (250 MHz) spectra of solutions of polymers in CDC13 were recorded on a Bruker AC250 spectrometer at ambient probe temperature. Infra-red spectra were obtained on a Perkin-Elmer 1 720X FT-IR instrument on thin films cast from solutions of the polymers in CH2C12. Thermogravimetric analyses (TGA) were carried out on ca. 7–15 mg samples using a PerkinElmer 7 Series thermal analysis system, both in air and in nitrogen, at a heating rate of 20 C min1. Differential scanning calorimetric (DSC) analyses were performed on preweighed samples (ca. 5–7 mg) using a Perkin-Elmer 7 Series instrument, under a nitrogen atmosphere. A heating rate of 10 C min1 was employed and the samples were reheated to identify any irreversible phase transitions. The gel permeation chromatography (GPC) set-up, used for measurements of molecular weight and molecular weight distribution, was comprised of a Waters 515 pump coupled via a Gilson 234 autoinjector to Polymer Laboratories columns. A Waters 410 refractometer was used to detect the fractions eluting from the columns. Calibrations of the columns were carried out using polystyrene or
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poly(methyl methacrylate) standards, as appropriate. All samples were run in THF at a concentration of ca. 0.3% w/v at a flow rate of 1 cm3 min1. Limiting oxygen indices (LOI-ASTM-D-2863) were measured on a Stanton-Redcroft flammability unit on hot-pressed plaques measuring 100.60.3 cm. The volatile species emanating from some of the polymers upon thermal degradation, were investigated through Curie-point pyrolysis/GC–MS and also by laser-pyrolysis/time-of-flight mass spectrometry (LP/ToF-MS). Pyrolysis GC–MS analyses of ca. 1 mg samples were carried out on a Fisher Model 1040 PSC Curie-point pyrolyser attached to a Perkin-Elmer Auto-system XL gas chromatograph, equipped with a PE-5MS column and using He as carrier gas at a flow rate of 1 cm3 min1. The gas chromatograph was connected to a Perkin-Elmer Turbomass quadrupole mass spectrometer with electron impact (El) source to enable identification of the separated, volatile, products of pyrolysis. The LP/ToF-MS technique has been described elsewhere in detail [27,28]. Degradation is initiated by exposing the polymer surface to a single 500 ms duration pulse of ca. 1 mm width with a total energy that can be varied between 0.1 and 1.0 J. The temperature rise at the polymer surface has been estimated to be within the range 650–1000 C. The volatiles escaping from this reaction zone are ionised on entry to an ion source and are then analysed by the ToF-MS every 25 ms.
3. Results and discussion PSt and PMMA containing the Si-based comonomers were successfully made by radical copolymerisations. Various characterisation data for these copolymers are collected in Tables 3 and 4. The compositions of the copolymers were deduced from their 1H NMR spectra by comparing the integral areas of appropriately assigned signals. In the case of St/TMSM and MMA/TMSM, there is substantial hydrolysis of the trimethylsilyloxy groups to give methacrylic acid units. The GPC chromatograms of the homopolymers and the copolymers generally showed unimodal distributions of molecular weight. The Mn values ranged from 13,000 to 60,000 with polydispersity indices between 1.6 and 2.5. Introduction of the Si-containing monomers into PSt and PMMA appeared to reduce the Tg’s of the polymers only very slightly from the values for the homopolymers (105 and 120 C, respectively). However, unambiguous assignment of the phase transition corresponding to the glass transition was not possible for the St and MMA copolymers containing the polydimethylsiloxane macromonomers, PDMSMM and PDMSMA. Generally, the copolymers did not show any marked increases in thermal or thermo-oxidative stability or in amounts of char produced in TGA experiments when
Table 1 Typical preparative data for copolymers of St and Si-containing monomers Expt. no.
St (g)
Comonomer (g)
Solvent (g)
AIBN (mg)
Temp ( C)
Time (h)
Conversion (%w/w)
1 2 3 4 5 6 7
60.0 39.0 39.0 21.8 19.4 25.8 25.8
– TMSM, 19.8 TMSMM,21.5 TTSSPM,9.8 TTSSPM, 19.7 PDMSMM, 33.6 PDMSMA, 25.1
Toluene, 208 Toluene, 173 Toluene, 217 Toluene, 173 Toluene, 173 Chlorobenzene, 221 Chlorobenzene, 221
516 523 530 534 512 508 500
60 60 60 60 60 60 60
72 216 191 161 161 122 144
67 65 73 70 70 87 94
Table 2 Typical preparative data for copolymers of MMA and Si-containing monomers Expt. no.
MMA (g)
Comonomer (g)
Solvent (g)
AIBN (mg)
Temp ( C)
Time (h)
Conversion (%w/w)
8 9 10 11 12 13 14
60.0 35.0 35.0 21.0 18.6 24.7 24.8
– TMSM, 23.5 TMSMM,25.8 TTSSPM, 10.1 TTSSPM, 19.7 PDMSMM, 34.0 PDMSMA, 25.0
Toluene, 208 Toluene, 173 Toluene, 260 Toluene, 173 Toluene, 173 Chlorobenzene, 221 Chlorobenzene, 221
503 522 553 523 515 499 506
60 60 60 60 60 60 60
23 45 93 145 144 122 143
90 92 91 77 52 93 92
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compared with unmodified PSt and PMMA. The only noticeable exceptions were the copolymers with PDMSMM and PDMSMA. These copolymers produced significant amounts of char, both in air and in nitrogen, in TGA experiments. From the LOI values obtained for the Si-modified polymers, it can be seen that introduction of Si-containing groups results in an increase in flame retardance in both St and MMA copolymers. However, such increases can only be considered to be marginal given the relatively high Si loadings. Also, there is no obvious correlation between LOI values, the char yields and the Si contents of these copolymers. In the case of MMA/ TMSM, there is a significant increase in LOI (23.5) compared with that of PMMA (17.9) and it also forms char during the TGA runs both in air (4.2% w/w) and nitrogen (7.0% w/w) even though the Si content is only 0.7% w/w. However, this may be a consequence of the formation of intra- and/or intermolecular anhydride crosslinks between the methacrylic acid units arising from hydrolysis of the trimethylsilyloxy groups. Such anhydride links could, in turn, interrupt the thermally induced chain depolymerisation of MMA sequences leading to evolution of less MMA monomer (the principal fuel in any fire involving PMMA). Eventually, the anhydride sequences will decarboxylate, losing CO and CO2, resulting in the formation of unsaturated precursors to the aromatic components of char [18]. In the case of copolymers of St and MMA with the Si-containing macromonomers, PDMSMM and PDMSMA,
there are only very small improvements in LOI, even though there are substantial amounts of Si present in the form of polydimethylsiloxane pendent groups and even though they produce significant amounts of char during the TGA runs. However, on rapid heating such as that encountered during flaming combustion, the pendent polydimethylsiloxane chains are likely to depolymerise to produce a mixture of flammable low molecular weight cyclic oligomers and, to a lesser extent, low molecular weight hydrocarbons, such as methane [29– 31]. These could act as fuels to the flame in addition to the principal contributors from the monomeric and oligomeric components from the base polymers themselves. Thus rapid pyrolysis of these copolymers actually results in enhanced fuel production with little or no formation of char, as was evident from the LOI experiments. LP/ToF-MS spectra of St/PDMSMM, MMA/ PDMSMM and MMA/PDMSMA showed Si-containing mass fragments with m/z values of 73, 147 and 221 corresponding to species of the type —(OSi(Me)2)n— with n=1, 2 and 3, respectively. Rapid pyrolyses at 590 C in the Curie point/GC–MS experiments showed the parent monomer (St or MMA) and the comonomeric species to be the major species evolved indicating that the predominant pyrolysis mechanism operating under these conditions is chain depolymerisation. A more detailed examination of the GC–MS spectra of St/PDMSMA, St/PDMSMM, MMA/PDMSMA and MMA/PDMSMM showed various fragments incorporating the butyl end group of the polydimethylsiloxane chain in addition to
Table 3 Charactensation data for copolymers of St and Si-containing monomers Expt. No. Polymer
Mol. fract. comonomer Si-content (wt.%) TGAa residue in air (wt.%) TGAa residue in N2 (wt.%) LOI (%O2 v/v)
1 2 3 4 5 6 7
– 0.06 (80b) 0.25 0.11 0.2 0.014 0.014
PSt St/TMSM St/TMSMM St/TTSSPM St/TTSSPM St/PDMSMA St/PDMSMM a b
– 1.6 7.4 8.9 13.3 24.3 21.5
0 0 0 0 0 40.0 25.0
0 0 0 0 0 56.4 50.3
18.0 19.8 20.5 25.0 25.3 20.0 19.0
Char residue at 500 C. Percentage of these comonomer units not adventitiously hydrolysed to methacrylic acid units.
Table 4 Characterisation data for copolymers of MMA and Si-containing monomers Expt. No. Polymer
Mol. fract. comonomer Si-content (wt.%) TGAa residue in air (wt.%) TGAa residue in N2 (wt.%) LOI (%O2 v/v)
8 9 10 11 12 13 14
– 0.025 (75b) 0.27 0.10 0.18 0.01 0.02
a b
PMMA MMA/TMSM MMA/TMSMM MMA/TMSSPM MMA/TMSSPM MMA/PDMSMA MMA/PDMSMM
– 0.7 6.3 8.5 12.7 21.7 25
0 4.2 0 0 3.0 43.8 25.0
Char residue at 500 C. Percentage of these comonomer units not adventitiously hydrolysed to methacrymic acid units.
0 7.0 0 0 2.6 55.8 7.4
17.9 23.5 18.6 18.1 18.4 19.4 19.0
J.R. Ebdon et al. / Polymer Degradation and Stability 83 (2004) 181–185
dimethylsiloxane monomeric units. These include butylended monomeric (m/z=131), dimeric (m/z=207), trimeric (m/z=281), tetrameric (m/z=355) and pentameric (m/z=429) fragments.
4. Conclusions Copolymers of St and of MMA with some Si-containing acrylates and methacrylates show marginally improved flame retardance compared with those of the parent homopolymers. The flame retardance seems to arise mainly from a vapour phase mechanism except in the case of copolymers with monomers containing oligodimethylsiloxane side chains in which case significant char yields suggest also a condensed phase component. In the case of these latter copolymers, however, pyrolysis data suggest that rapid depolymerization of the oligodimethylsiloxane side chains may actually result in additional fuel being produced, thus tending to counteract the advantage to flame retardancy arising from the production of any char. Overall it seems that Si-containing monomers alone when incorporated into acrylic and styrenic polymers are unlikely to offer sufficient improvement in flame retardancy to warrant much further investigation for this purpose.
Acknowledgements The authors thank the EPSRC and MoD (grant no. GR/L 85886) for financial support. Technical collaboration with the ICI STG, Wilton and Dow Corning, Barry are also gratefully acknowledged.
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