- Email: [email protected]
Enhancement of thermal stability of polystyrene and poly(methyl methacrylate) by cyclotriphosphazene derivatives
Polymer Degradation and Stability 84 (2004) 87–93 www.elsevier.com/locate/polydegstab
Enhancement of thermal stability of polystyrene and poly(methyl methacrylate) by cyclotriphosphazene derivatives Takahito Muraki, Masahiro Ueta, Eiji Ihara, Kenzo Inoue* Department of Applied Chemistry, Faculty of Engineering, Venture Business Laboratory, Ehime University, Matsuyama 790-8577 Japan Received 24 July 2003; received in revised form 16 September 2003; accepted 19 September 2003
Abstract The thermal stability and degradation behavior of polystyrene (PSt) and poly(methyl methacrylate) (PMMA) blended with organic cyclotriphosphazenes (N3P3(OR)6) were investigated by thermogravimetric analysis and gel permeation chromatography. The thermal degradation behaviors of polymers are strongly dependent on the organic groups attached to the phosphazene ring (R: S-4,-C6H3(-OCH2O-); S-5,-C6H4CH2OPO(OPh)2; S-6,-C6H4OPO(OPh)2; S-7,-C6H4OCH2OCH3). The onset temperature of decomposition (T0) of PSt increased from 303 C to 351 C in air by the addition of 5 wt.% S-4, whereas S-6 has no ability to increase T0 value of PSt. The GPC traces of PSt/S-4 film heated at 180 C for 30 min in air showed no significant decrease of molecular weight of PSt. A similar enhancement of thermal stability was observed for the PMMA/S-4 system. From the reaction of radical initiators with S-4, it appears that –OCH2O– group in S-4 acts as an effective trapping site of peroxy radical. As expected, the physical loss of S-4 with molecular weight increased by using cyclotriphosphazene core from PSt film was significantly suppressed, i.e., the diffusion coefficient of S-4 was three orders of magnitude smaller than that of 3,4-methylenedioxyphenol (MOP). The enhancement of thermal stability of PSt and PMMA blended with cyclotriphosphazene derivatives were described. # 2004 Elsevier Ltd. All rights reserved. Keywords: Cyclotriphosphazene; Thermal stabilizers; Polystyrene; Poly(methyl methacrylate)
1. Introduction The enhancement of thermal stability of polymers has received attention from academic and practical points of view. Intensive efforts have been made to understand the mechanism of the degradation of polymers and to design stabilizers having high inherent chemical efficiency. The most common antioxidants used are hindered phenols and amines, which are believed to be effective in scavenging the oxidative ability of peroxy radicals [1–4]. The physical persistence and high solubility of stabilizers in polymers are also important practical demands. The stabilizers having low molecular weights function to preserve the properties of polymers under moderate operating conditions but under aggressive conditions the stabilizers may exude from polymers. The physical loss could be minimized by the increase of molecular weight of stabilizers. The solubility of the stabilizers with relatively polar functional groups such * Corresponding author. Tel/Fax: +81-89-927-9918. E-mail address: [email protected] (K. Inoue). 0141-3910/$ - see front matter # 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2003.09.015
as hydroxyl and amino groups is limited for unpolar polymers, and the stabilizers aggregate and distribute unevenly in the polymers. Recently, chemically efficient stabilizers with less polar groups such as b-diketone and benzofuran derivatives carrying labile hydrogens have been reported [5,6]. Cyclic and polyphosphazenes with skeletal –P=Natoms are known to exhibit useful thermal properties such as flame retardancy and self-extinguishability. In addition to thermally favorable properties, the versatility in chemical transformation of P-Cl group of hexachlorocyclotriphosphazene with organic groups is highly convenient to design functional polymers or molecules [7,8]. The incorporation of cyclotriphosphazene groups into the main or side chain improved significantly the thermal behavior of polymers [9–15]. One of characteristic features of these polymers is the formation of a carbonaceous char with a high yield, which could act as a physical barrier of heat-transfer. A somewhat cumbersome process, however, needs for such vinyl polymers containing cyclotriphosphazene units, i.e., synthesis of corresponding monomer and copolymer-
88
T. Muraki et al. / Polymer Degradation and Stability 84 (2004) 87–93
ization procedure. By utilization of the structural features of cyclotriphosphazene with six reactive P-Cl groups, stabilizers with high molecular weight and compatibility with host polymers are easily obtainable by the displacement of organic groups pacifying the oxidative ability of peroxy radicals. In this paper we describe the synthesis of new cyclotriphosphazene-based stabilizers and thermal behaviors of the PSt and PMMA blended with these stabilizers.
2. Experimental 2.1. Materials Hexachlorocyclotriphosphazene (S-1) was kindly supplied by Otsuka Co. and purified by recrystallization from n-hexane. 3,4-Methylenedioxyphenol (MOP), 4hydroxybenzaldhyde, chloromethyl methyl ether, and diphenyl chlorophosphate were used as received. Solvents were distilled from appropriate drying reagents before use. Hexakis(4-hydroxymethylphenoxy)-(S-2) and hexakis(4-hydroxyphenoxy)cyclotriphosphazenes (S-3) were synthesized by procedures reported in the literatures [16,17]. Polystyrene (PSt, Mn=88 000, Mw/ Mn=3.3) and poly(methyl methacryalte)(PMMA, Mn= 105,000, Mw/Mn=3.1 ) were prepared by a radical polymerization initiated by AIBN. 2.1.1. Preparation of hexakis(3,4-methylenedioxypheoxy)cyclotriphosphazene (S-4) A THF solution of hexachlorocyclotriphosphazene (0.87 g, 2.5 mmol) was added to sodium 3,4-methylenedioxyphenoxide in THF, prepared from MOP (3.1 g, 22.5 mmol) and sodium hydride (72% in oil, 0.75 g, 22.5 mmol) at 0 C, and then, the mixture was refluxed for 16 h. After cooling, a precipitate was removed by filtration and the filtrate was concentrated under reduced pressure. The residue was poured into methanol to precipitate the title compound S-4. S-4 was purified by reprecipitation from THF into a large excess of methanol. S-4: Yield 62%. Mp 120 C; IR (KBr) 2885, 1636, 1543, 1447, 1277, 1039, 899, 803, and 729 cm1; 1HNMR (400 MHz, DMSO-d6) d=6.02 (12H, s), 6.40 (6H, d, J=8.0 Hz), 6.45 (6H, s), 6.77 (6H, d, J=8.0 Hz); 31P-NMR (161 MHz, DMSO-d6) d=9.99. 2.1.2. Preparation of hexakis[4-(diphenylphosphoroxymethyl)pheoxy]cyclotri- phosphazene (S-5) The mixture of hexakis(4-hydroxymethylpheoxy)cyclotriphosphazene (S-2, 0.17 g, 0.2 mmol), diphenyl chlorophosphate (0.54 g, 2.0 mmol), 4-dimethylaminopyridine (0.24 g, 2.0 mmol), and pyridine (0.16 g, 2.0 mmol) in THF (30 ml) was stirred for 18 h at rt. Then the reaction mixture was poured into 1 M HCl aq. solution and extracted with ethyl acetate three times,
and then, the organic layer was washed with sat. NaHCO3 aq. solution and brine. Finally, the organic layer was dried over Na2SO4. After removal of the solvent under reduced pressure, the residual oil was purified by the preparative TLC on silica gel using chloroform : methanol (9 : 1) as an eluent to obtain the title compound S-5 in 89% yield. S-5: oil; IR (Neat) 3069, 2957, 1591, 1507, 1488, 1288, 1013, 950, and 827 cm1; 1H-NMR (400 MHz, DMSOd6) d=5.28 (12H, d, J=8.8 Hz), 6.89 (12H, d, J=9.2 Hz), 7.17–7.22 (36H, m), 7.29–7.37 (36H, m); 31P-NMR (161 MHz, DMSO-d6) d=11.41, 8.83. 2.1.3. Preparation of hexakis[4-(diphenylphosphoroxy)pheoxy]cyclotriphosphazene (S-6) The mixture of hexakis(4-hydroxypheoxy)cyclotriphosphazene (S-3, 0.16 g, 0.2 mmol), diphenyl chlorophosphate (0.54 g, 2.0 mmol), 4-dimethylaminopyridine (0.24 g, 2.0 mmol), and pyridine (0.16 g, 2.0 mmol) in THF (10 ml) was refluxed for 18 h. Then the reaction mixture was poured into 1 M HCl aq. solution and extracted with ethyl acetate three times, and then, the organic layer was washed with sat. NaHCO3 aq. solution and brine. Finally, the organic layer was dried over Na2SO4. After removal of the solvent under reduced pressure, the residual oil was purified by the preparative TLC on silica gel using chloroform:methanol (24:1) as an eluent to obtain the compound 6 in 97% yield. 6: oil; IR (Neat) 3071, 1590, 1489, 1300, 1026, 1010, and 846 cm1; 1H-NMR (400 MHz, DMSO-d6) d=6.99 (12H, d, J=10.4 Hz), 7.21-7.25 (48H, m), 7.36– 7.40 (24H, m); 31P-NMR (161 MHz, DMSO-d6) d=16.70, 9.43. 2.1.4. Preparation of hexakis(4-methoxymethoxypheoxy)cyclotriphosphazene (S-7) The mixture of hexakis(4-hydroxypheoxy)cyclotriphosphazene (S-3, 0.32 g, 0.4 mmol), chloromethyl methyl ether (0.64 g, 8.0 mmol) and potassium carbonate (1.66 g, 12 mmol) in acetone (30 ml) was refluxed for 24 h. Then the reaction mixture was filtrated to remove precipitate and the filtrate was concentrated under reduced pressure. After removal of the solvent, the residual oil was purified by the preparative TLC on silica gel using chloroform:methanol (24:1) as an eluent to obtain the compound S-7 in 82% yield. S-7: oil; IR (Neat) 3050, 2954, 1595, 1504, 1443, 1406, 1266, 1102, 1079, 1005, and 838 cm1; 1H-NMR (400 MHz, DMSO-d6) d=3.37 (18H, s), 5.15 (12H, s), 6.76 (12H, d, J=8.0 Hz), 6.89 (12H, d, J=8.0 Hz); 31P-NMR (161 MHz, DMSO-d6) d=10.21. 2.2. Thermal degradation of PSt and PMMA The blend film of polymer and stabilizer was prepared by casting from solution of THF. The thermal
T. Muraki et al. / Polymer Degradation and Stability 84 (2004) 87–93
degradation of PSt and PMMA in air was carried out at 180 and 220 C for 30 min, respectively, in a electric furnace. 2.3. Reaction of 3,4-methylenedioxyphenylacetate (S-8) with benzoyl peroxide A solution of benzoyl peroxide (0.25 g, 1 mmol) and 3,4-methylenedioxyphenylacetate (S-8, 0.36 g, 2 mmol), which was prepared from MOP and acetic anhydride, in toluene was refluxed for 1.5 h under nitrogen atmosphere. The solvent was removed under reduced pressure. Preparative TLC (silica gel) with 60:15:1 hexaneethyl acetate-triethylamine followed by recrystallization from CHCl3 gave 5-acetoxy-benzo[1,3]dioxol-2-yl benzoate (S-9). S-9: Yield, 38%. oil; IR (neat) 3069, 1755, 1731, 1599, 1491, 1375, 1244, 900, 822, and 709 cm1; 1H-NMR (400 MHz, DMSO-d6) d=2.26 (3H, s), 6.79 (1H, dd, J=8.4, 2.4 Hz), 7.12 (1H, d, J=2.4 Hz), 7.22 (1H, d, J=8.4 Hz), 7.55 (2H, t, J=7.6 Hz), 7.72 (1H, t, J=7.6 Hz), 7.96 (2H, d, J=7.6 Hz), 8.20 (1H, s). 2.4. Measurements IR spectra were recorded on a JASCO FT/IR-230 spectrophotometer. UV spectra were recorded on a Shimadzu UV-260 spectrophotometer. 1H and 31P NMR spectra were recorded with a Bruker Avance 400 (1H: 400 MHz; 31P: 161 MHz) spectrometer. Chemical shifts are reported as ppm downfield from TMS in d units. J-Values are given in Hz. Melting points were determined on a Yanaco melting point apparatus MPS3, and uncorrected The thermogravimetric measurements were performed using a Seiko SII TG/DTA 6200
89
under an atmosphere of nitrogen or air at a flow rate of 100 mL/min using a heating rate of 10 C /min. Gel permeation chromatography (GPC) was performed on a Shimadzu LC-10AT equipped with Tosoh TSKgel G3000HHR, G4000HHR, and G6000HHR columns, using THF as the eluent. The columns were calibrated with polystyrene standards. Silica gel 60 F254 (Merck) was used for TLC, and Silica gel 60 PF254 (Merck) was used for PTLC.
3. Results and discussion Organic cyclotriphosphazenes (S-3–S-7) used in this study were prepared according to Scheme 1. 31P NMR spectra were critical for monitoring the degree to which the chlorine atoms in hexachlorocyclotriphosphazene (S-1) had been substituted with organic groups. For the reaction of S-1 with MOP, 31P NMR spectra of the product (S-4) show a sharp singlet peak at d=9.99, indicating the accomplishment of a complete substitution of the chlorine atoms. For the substitution reaction of hexakis(4-hydroxyphenoxy)cyclotriphosphazene (S-3) with diphenyl chlorophosphate, the phosphazene ring remained intact, as evidenced by the presence of sharp singlet around d=9.43 in 31P NMR spectra. This also was supported by IR spectra, the observation of asymmetric stretching vibration around 1250 cm1 attributed to the –P=N- group and the vibrations around 1170 and 945 cm1 ascribed to the P-O-C6H4 groups. The molecular weight, P and N contents (%), and decomposition temperature of the phosphazene compounds (S-2–S-7) are summarized in Table 1. The thermal decomposition of these compounds occurred in the temperature range of 191–
Scheme 1. Preparation of cyclotriphosphazene-based stabilizers.
90
T. Muraki et al. / Polymer Degradation and Stability 84 (2004) 87–93
Table 1 Characteristics of cyclotriphosphazenes S-2–S-7 Stabilizer
S-2 S-3 S-4 S-5 S-6 S-7 a
MW
874 790 958 2267 2183 1054
mp ( C)
219 242 120 oil oil oil
Contents (%)
Tdec ( C)
Char yield (%)a
P
N
N2
air
N2
air
10.64 11.77 9.70 12.30 12.77 8.82
4.81 5.32 4.39 1.85 1.93 3.99
314 311 376 215 191 369
295 294 306 230 195 322
76 68 73 50 56 58
74 72 78 55 53 70
At 500 C.
376 C, and, as expected, char residues left at 500 C were high under both nitrogen and air atmospheres. For the polymer-stabilizer blend system, high compatibility of stabilizers is an essential requirement for a good physical retention of the additive in a polymer. PSt and PMMA blended with phosphazene compounds gave clear and transparent films except for the PSt/S-3 blend film. The thermal behavior of PSt and PMMA is sensitive to characteristics of carbon–carbon bond in the main chain and the terminal structure of polymer, both of which are dependent on the polymerization procedures. PSt prepared from radical polymerization decomposes by a single step process, where random bond scission and weak bond of PSt chains occur to give styrene monomer, dimer and oigomers. For the decomposition of PMMA prepared from radical polymerization a three-step mechanism is operative. The first decomposition involves bond linkages of tail-to-tail bond in PMMA around 160–190 C. Further increase of temperature to around 220–300 C causes the decomposition initiated by radicals generated at the terminal double bonds which were formed by disproportionation
process. This is followed by random chain cleavage at temperatures between 320 and 350 C. The effects of S-3–S-7 on thermal stability of PSt and PMMA were estimated by thermogravimetric analysis (TGA) in nitrogen and in air. Fig. 1 showed the TGA curves of PSt films blended with 5 wt.% of S-4. The onset decomposition temperature (T0), the temperature of 10% weight loss (T10), and char yield obtained from a single step decomposition curve were summarized in Table 2. In nitrogen PSt/S-5 blend film has a T0 value of 396 C, which was ca. 50 C higher than that of PSt. The thermal stabilizing ability of the phosphazene derivatives on PSt increased in the following order S-5 > S-6 > S-4=MOP > S-7. A significant different behavior, however, was observed in air; S-4 was the most effective to increase T0 and T10 values, i.e., the shift from 303 to 351 C and from 332 to 377 C, respectively. Contrast to this, S-6 and S-7 have no ability to increase T0 value, indicating that they can not suppress the decomposition via oxidation process. The char yield in air was higher than that in nitrogen, suggesting that phosphazene derivatives react with oxy radicals to give non-volatile products. For the slightly opaque blend film of PSt and 5 wt.% of MOP, surprisingly, T0 value of 316 C was observed, which was comparable to that of pure PSt. There are at least two plausible explanations for the thermal behavior of the PSt-MOP blend film. The first is that the combination of cyclotriphosphazene and MOP units is required to improve the heat resistance of PSt. The second is physical loss of MOP with relatively low molecular weight by volatilization. The diffusion coefficient (D) of the stabilizer in the polymer film could be calculated by
D=p/16 I2 Mt/M1=It1/2/l Table 2 Thermal behaviors of PSt blended with cyclotriphosphazene-based stabilizers Stabilizer
– S-4
S-5 S-6 S-7 Fig. 1. TGA curves for PSt blended with 5 wt.% of S-4. (a), PSt in air; (b), PSt in nitrogen; (c), PSt/S-4 blend film in air, and (d), PSt/S-4 blend film in nitrogen.
MOP a
wt.%
– 1 3 5 10 5 5 10 5 10 5
At 500 C.
T0 ( C)
T10 ( C)
Char yield (%)a
N2
Air
N2
Air
N2
Air
345 369 373 377 384 396 386 384 371 357 376
303 330 338 351 353 330 299 292 301 301 316
377 393 396 398 403 413 404 402 392 388 394
332 360 371 377 382 362 332 329 363 361 341
0 0.6 1.9 3.3 6.3 3.2 1.2 2.1 3.3 6.8 0
0 2.1 4.3 6.5 10.1 5.7 3.7 5.7 6.6 11.4 0
91
T. Muraki et al. / Polymer Degradation and Stability 84 (2004) 87–93
Table 3 Thermal behaviors of PMMA blended with cyclotriphosphazene-based stabilizers Stabilizer
– S-3 S-4 S-5 Fig. 2. Plots of Mt/M8 vs t1/2/l for PSt/S-4 () and PSt/MOP (*) blend films at 160 C.
where Mt is the total amount of stabilizer lost from the polymer films at time t, and M1 is the corresponding value after infinite time, which could be assumed to be equal to the initial amount of stabilizer; and l, the film thickness [20]. The polymer films blended with MOP and S-4 were heated at 160 C, and the amount of stabilizer remaining in the film was calculated from UV spectra. As shown in Fig. 2, plots of Mt/M1 vs t1/2/l gave straight lines. The most of MOP was lost in a few minutes and the diffusion coefficient was calculated to be 1.6 107 cm2/min. Contrast to this, the D value of S-4 (4.2 1010 cm2/min) was three orders of magnitude smaller than that of MOP. This is due to the increase of the molecular volume and in the degree of compatibility of S-4. The rapid loss of MOP might be responsible for the poor thermal resistance of PSt/MOP blend film. As shown in Fig. 3, all of phosphazene derivatives could contribute to increase the resistance of thermal
S-6 S-7 MOP a
wt.%
– 5 10 5 10 5 10 5 10 5 10 10
Ts ( C)
T10 ( C)
Char yield (%)a
N2
Air
N2
Air
N2
Air
239 264 272 263 269 258 260 244 244 247 257 271
235 295 299 289 286 282 295 261 265 266 268 303
259 292 309 285 297 276 286 264 261 265 274 290
247 313 316 308 302 307 319 269 274 287 286 319
0 1.9 4.8 1.7 4.5 1.6 3.6 0.6 1.8 2.5 5.3 0
0 2.4 5.5 2.3 5.7 2.1 4.6 0.5 1.6 2.5 5.1 0
At 500 C.
decomposition of PMMA in air. The decomposition pattern of PMMA changes depending on the stabilizers, i.e. a three-step decomposition was observed for PMMA films blended with S-3, S-4, and S-7, but the third-step decomposition was not detected for the PMMA/S-5 blend film. The temperature of decomposition in a second step (Ts), T10, and char yield at 500 C were summarized in Table 3. The effects of oxygen on thermal stability of PMMA are complex; the presence of oxygen brings about the slight increase of stability below 200 C but significant decrease above 230 C [21]. The Ts value of PMMA blended with S-3 was ca. 60 C higher than that of pure PMMA, although the weight loss for the
Fig. 3. TGA curves for PMMA blended with 5 wt.% of phosphazene derivatives in air. (a), without stabilizer; (b), S-3; (c), S-4; (d), S-5; (e), S-6; (f), S-7, and (g), MOP.
92
T. Muraki et al. / Polymer Degradation and Stability 84 (2004) 87–93 Table 4 Effects of stabilizers on the molecular weights and the degree of bond scssion for thermal decomposition of PSta Stabilizer
wt.%
Mn 103
a 104, b
sc
– S-4
– 1 3 5 10 5 10 5 10 5
24 74 78 87 88 53 35 78 62 77
31.1 2.1 1.6 0.05 0.01 7.8 17.8 1.6 5.1 1.6
2.63 0.18 0.13 0.005 0.001 0.66 1.51 0.13 0.42 0.14
S-6 S-7 Fig. 4. GPC curves of the PSt/S-4 blend films heated at 180 C for 30 min. (a), before heating; (b), PSt/5 wt.% of S-4; (c), PSt/ 10 wt.% of S-4; (d) without stabilizer.
MOP a b c
first-step decomposition could not be suppressed. The Ts and T10 values increased in the following order, S-3 > S-4 > S-5 > S-7 > S-6. The thermal stability of PMMA is known to increase by the incorporation of comonomer units in the chains, which act as block units for unzipping process of the main chains [22]. The facts that Ts values in air are higher than those obtained in nitrogen suggest that the unzipping process is suppressed by the reaction of oxy radicals in the main chain with stabilizers. The degree of thermal decomposition (a) and the number of chain scission (s) were calculated from following equations, respectively.
Table 5 Effects of stabilizers on the molecular weights and the degree of bond scssion for thermal decomposition of PMMAa Stabilizer
wt.%
Mn 103
a 104, b
sc
– S-4
– 5 10 5 10 5 10 5
14 88 94 36 89 89 90 73
62.4 0.7 0.01 17.3 0.6 0.6 0.5 3.1
5.9 0.07 0.001 1.63 0.05 0.06 0.04 0.29
S-6 S-7 MOP
a ¼ 1=DP 1=DP0
a
s ¼ DP0 =DP 1
c
b
where DP0 and DP were the number-average degree of polymerization of original and heated polymers, respectively. Fig. 4 showed the GPC traces of PSt film heated at 180 C for 30 min. The molecular weight of PSt (Mn=88000) reduced to Mn=24000 and s-value was calculated to be 2.63 (Table 4). As expected, no significant shift of the peak was observed when S-4 was added to PSt. Similarly, the decrease of molecular weight of PMMA was suppressed by the addition of S-4 and S-7. The chain scission of PSt was reduced by the addition of even 1 wt.% of S-4 and the decomposition was almost completely suppressed at 10wt.% of S-4, indicating that S-4 acts as an effective inhibitor of chain breaking (Tables 4 and 5). In order to clarify the role of S-4 in the thermal decomposition of polymers, the reaction of model
180 C for 30 min. Degree of thermal decomposition. Number of chain scission.
220 C for 30 min. Degree of thermal decomposition. Number of chain scission.
compound S-8 with benzoyl peroxide (BPO) and with AIBN in toluene was conducted at 120 C in nitrogen. For the reaction of BPO with S-8, benzoyloxymethylene-3,4-dioxyphenylacetate (S-9) was formed as a major product, whereas S-8 was recovered without suffering any reaction of 1-cyano-1-methylethyl radical. These results indicate that the-OCH2O- group in MOP units acts as a trapping site of oxy radicals but not for carbon radicals. For the PSt or PMMA film blended with S-4 carrying six trapping sites, the polymer chain with oxy radical could be recombined through the same S-4 molecule, which implies that the rapid decrease of molecular weight is significantly suppressed (Scheme 2). Organophosphorus compounds are known to act as flame-retardants for polymeric materials, due to the
Scheme 2. Reaction of BPO with S-8.
T. Muraki et al. / Polymer Degradation and Stability 84 (2004) 87–93
formation of a char which plays as a physical barrier to heat transfer from the flame to the polymer. Although the T0, T10 and char yield could not give direct information of the flame retardant properties of blend polymers, these relate the formation of flammable gases from the polymers. Qualitative test of flame retardance of polymers blended with 10 wt.% of phosphazene derivatives showed, unfortunately, no self-extinguishing properties, probably due to low char yield at 500 C. This might be ascribed to relatively low concentration of P and N atoms in the materials. In summary, the thermal stability and degradation behavior of PSt and PMMA blended with organic cyclotriphosphazenes (N3P3(OR)6) were strongly dependent on the organic groups attached to the phosphazene ring. Among them, S-4 that has relatively high molecular weight and compatibility functions as an efficient stabilizer, i.e., the addition of S-4 brings about the increase of the onset temperature of decomposition of polymers and the suppression of the bond scission of main chains. The versatility in chemical transformation of hexachlorocyclotriphosphazene allows preparing various types of stabilizers by the introduction of cosubstituents, which have different properties, in various ratios on the cyclotriphosphazene. Synthesis efforts are currently underway to prepare cyclophosphazenebased stabilizers [18,19].
93
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
Pospisˇ il J. Adv Polym Sci 1991;101:65. Lu S-Y, Hamerton I. Prog Polym Sci 2002;27:1661. Klemchuk PP, Gande ME. Polym Degrad Stab 1988;22:241. Step EN, Turro NJ, Gande ME, Klemchuk PP. Macromolecules 1994;27:2529. Nishinaga T, Shiotsu S, Tsujita H. Polym Degrad Stab 2002;76:435. Pauquet JR. J Macromol Sci Pure Appl Chem 1999;A35:1717. De Jaeger R, Gleria M. Prog Polym Sci 1998;23:179. Inoue K, Itaya T. Bull Chem Soc Jpn 2001;74:1381. Inoue K, Nakano M, Takagi M, Tanigaki T. Macromolecules 1989;22:1530. Miyata K, Muraoka K, Itaya T, Inoue K. Eur Polym, J 1996; 32:1257. Dez I, Henry N, De Jaeger R. Polym Degrad Stab 1999;64:433. Allcock HR, Hartle TJ, Taylor JP, Sunderland NJ. Macromolecules 2001;34:3896. Allcock HR, Laredo WL, deDebus CR, Taylor JP. Macromolecules 1999;32:7719. Allcock HR, Kellam III EC. Macromolecules 2002;35:40. Mathew D, Nair CPR, Ninan KN. Polym Int 2000;49:48. Medici A, Fantin G, Pedrini P, Gleria M, Minto F. Macromolecules 1992;25:2569. Inoue K, Negayama S, Itaya T, Tanigaki T. Macromol Rapid Commun 1997;18:225. McNeill IC, Zulfiqar M, Kousar T. Poly Deg Stab 1990;28:1990. Kashiwagi T, Inaba A, Brown JE, Hatada K, Kitayama T, Masuda E. Macromolecules 1986;19:2160. Masuko T, Sato A, Karasawa M. J Appl Polym Sci 1978;22:1978. Kashiwagi T, Hirata T, Brown JE. Macromolecules 1985;18:1410. McNeill IC. Eur Polym, J 1968;4:21.
Enhancement of thermal stability of polystyrene and poly(methyl methacrylate) by cyclotriphosphazene derivatives Takahito Muraki, Masahiro Ueta, Eiji Ihara, Kenzo Inoue* Department of Applied Chemistry, Faculty of Engineering, Venture Business Laboratory, Ehime University, Matsuyama 790-8577 Japan Received 24 July 2003; received in revised form 16 September 2003; accepted 19 September 2003
Abstract The thermal stability and degradation behavior of polystyrene (PSt) and poly(methyl methacrylate) (PMMA) blended with organic cyclotriphosphazenes (N3P3(OR)6) were investigated by thermogravimetric analysis and gel permeation chromatography. The thermal degradation behaviors of polymers are strongly dependent on the organic groups attached to the phosphazene ring (R: S-4,-C6H3(-OCH2O-); S-5,-C6H4CH2OPO(OPh)2; S-6,-C6H4OPO(OPh)2; S-7,-C6H4OCH2OCH3). The onset temperature of decomposition (T0) of PSt increased from 303 C to 351 C in air by the addition of 5 wt.% S-4, whereas S-6 has no ability to increase T0 value of PSt. The GPC traces of PSt/S-4 film heated at 180 C for 30 min in air showed no significant decrease of molecular weight of PSt. A similar enhancement of thermal stability was observed for the PMMA/S-4 system. From the reaction of radical initiators with S-4, it appears that –OCH2O– group in S-4 acts as an effective trapping site of peroxy radical. As expected, the physical loss of S-4 with molecular weight increased by using cyclotriphosphazene core from PSt film was significantly suppressed, i.e., the diffusion coefficient of S-4 was three orders of magnitude smaller than that of 3,4-methylenedioxyphenol (MOP). The enhancement of thermal stability of PSt and PMMA blended with cyclotriphosphazene derivatives were described. # 2004 Elsevier Ltd. All rights reserved. Keywords: Cyclotriphosphazene; Thermal stabilizers; Polystyrene; Poly(methyl methacrylate)
1. Introduction The enhancement of thermal stability of polymers has received attention from academic and practical points of view. Intensive efforts have been made to understand the mechanism of the degradation of polymers and to design stabilizers having high inherent chemical efficiency. The most common antioxidants used are hindered phenols and amines, which are believed to be effective in scavenging the oxidative ability of peroxy radicals [1–4]. The physical persistence and high solubility of stabilizers in polymers are also important practical demands. The stabilizers having low molecular weights function to preserve the properties of polymers under moderate operating conditions but under aggressive conditions the stabilizers may exude from polymers. The physical loss could be minimized by the increase of molecular weight of stabilizers. The solubility of the stabilizers with relatively polar functional groups such * Corresponding author. Tel/Fax: +81-89-927-9918. E-mail address: [email protected] (K. Inoue). 0141-3910/$ - see front matter # 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2003.09.015
as hydroxyl and amino groups is limited for unpolar polymers, and the stabilizers aggregate and distribute unevenly in the polymers. Recently, chemically efficient stabilizers with less polar groups such as b-diketone and benzofuran derivatives carrying labile hydrogens have been reported [5,6]. Cyclic and polyphosphazenes with skeletal –P=Natoms are known to exhibit useful thermal properties such as flame retardancy and self-extinguishability. In addition to thermally favorable properties, the versatility in chemical transformation of P-Cl group of hexachlorocyclotriphosphazene with organic groups is highly convenient to design functional polymers or molecules [7,8]. The incorporation of cyclotriphosphazene groups into the main or side chain improved significantly the thermal behavior of polymers [9–15]. One of characteristic features of these polymers is the formation of a carbonaceous char with a high yield, which could act as a physical barrier of heat-transfer. A somewhat cumbersome process, however, needs for such vinyl polymers containing cyclotriphosphazene units, i.e., synthesis of corresponding monomer and copolymer-
88
T. Muraki et al. / Polymer Degradation and Stability 84 (2004) 87–93
ization procedure. By utilization of the structural features of cyclotriphosphazene with six reactive P-Cl groups, stabilizers with high molecular weight and compatibility with host polymers are easily obtainable by the displacement of organic groups pacifying the oxidative ability of peroxy radicals. In this paper we describe the synthesis of new cyclotriphosphazene-based stabilizers and thermal behaviors of the PSt and PMMA blended with these stabilizers.
2. Experimental 2.1. Materials Hexachlorocyclotriphosphazene (S-1) was kindly supplied by Otsuka Co. and purified by recrystallization from n-hexane. 3,4-Methylenedioxyphenol (MOP), 4hydroxybenzaldhyde, chloromethyl methyl ether, and diphenyl chlorophosphate were used as received. Solvents were distilled from appropriate drying reagents before use. Hexakis(4-hydroxymethylphenoxy)-(S-2) and hexakis(4-hydroxyphenoxy)cyclotriphosphazenes (S-3) were synthesized by procedures reported in the literatures [16,17]. Polystyrene (PSt, Mn=88 000, Mw/ Mn=3.3) and poly(methyl methacryalte)(PMMA, Mn= 105,000, Mw/Mn=3.1 ) were prepared by a radical polymerization initiated by AIBN. 2.1.1. Preparation of hexakis(3,4-methylenedioxypheoxy)cyclotriphosphazene (S-4) A THF solution of hexachlorocyclotriphosphazene (0.87 g, 2.5 mmol) was added to sodium 3,4-methylenedioxyphenoxide in THF, prepared from MOP (3.1 g, 22.5 mmol) and sodium hydride (72% in oil, 0.75 g, 22.5 mmol) at 0 C, and then, the mixture was refluxed for 16 h. After cooling, a precipitate was removed by filtration and the filtrate was concentrated under reduced pressure. The residue was poured into methanol to precipitate the title compound S-4. S-4 was purified by reprecipitation from THF into a large excess of methanol. S-4: Yield 62%. Mp 120 C; IR (KBr) 2885, 1636, 1543, 1447, 1277, 1039, 899, 803, and 729 cm1; 1HNMR (400 MHz, DMSO-d6) d=6.02 (12H, s), 6.40 (6H, d, J=8.0 Hz), 6.45 (6H, s), 6.77 (6H, d, J=8.0 Hz); 31P-NMR (161 MHz, DMSO-d6) d=9.99. 2.1.2. Preparation of hexakis[4-(diphenylphosphoroxymethyl)pheoxy]cyclotri- phosphazene (S-5) The mixture of hexakis(4-hydroxymethylpheoxy)cyclotriphosphazene (S-2, 0.17 g, 0.2 mmol), diphenyl chlorophosphate (0.54 g, 2.0 mmol), 4-dimethylaminopyridine (0.24 g, 2.0 mmol), and pyridine (0.16 g, 2.0 mmol) in THF (30 ml) was stirred for 18 h at rt. Then the reaction mixture was poured into 1 M HCl aq. solution and extracted with ethyl acetate three times,
and then, the organic layer was washed with sat. NaHCO3 aq. solution and brine. Finally, the organic layer was dried over Na2SO4. After removal of the solvent under reduced pressure, the residual oil was purified by the preparative TLC on silica gel using chloroform : methanol (9 : 1) as an eluent to obtain the title compound S-5 in 89% yield. S-5: oil; IR (Neat) 3069, 2957, 1591, 1507, 1488, 1288, 1013, 950, and 827 cm1; 1H-NMR (400 MHz, DMSOd6) d=5.28 (12H, d, J=8.8 Hz), 6.89 (12H, d, J=9.2 Hz), 7.17–7.22 (36H, m), 7.29–7.37 (36H, m); 31P-NMR (161 MHz, DMSO-d6) d=11.41, 8.83. 2.1.3. Preparation of hexakis[4-(diphenylphosphoroxy)pheoxy]cyclotriphosphazene (S-6) The mixture of hexakis(4-hydroxypheoxy)cyclotriphosphazene (S-3, 0.16 g, 0.2 mmol), diphenyl chlorophosphate (0.54 g, 2.0 mmol), 4-dimethylaminopyridine (0.24 g, 2.0 mmol), and pyridine (0.16 g, 2.0 mmol) in THF (10 ml) was refluxed for 18 h. Then the reaction mixture was poured into 1 M HCl aq. solution and extracted with ethyl acetate three times, and then, the organic layer was washed with sat. NaHCO3 aq. solution and brine. Finally, the organic layer was dried over Na2SO4. After removal of the solvent under reduced pressure, the residual oil was purified by the preparative TLC on silica gel using chloroform:methanol (24:1) as an eluent to obtain the compound 6 in 97% yield. 6: oil; IR (Neat) 3071, 1590, 1489, 1300, 1026, 1010, and 846 cm1; 1H-NMR (400 MHz, DMSO-d6) d=6.99 (12H, d, J=10.4 Hz), 7.21-7.25 (48H, m), 7.36– 7.40 (24H, m); 31P-NMR (161 MHz, DMSO-d6) d=16.70, 9.43. 2.1.4. Preparation of hexakis(4-methoxymethoxypheoxy)cyclotriphosphazene (S-7) The mixture of hexakis(4-hydroxypheoxy)cyclotriphosphazene (S-3, 0.32 g, 0.4 mmol), chloromethyl methyl ether (0.64 g, 8.0 mmol) and potassium carbonate (1.66 g, 12 mmol) in acetone (30 ml) was refluxed for 24 h. Then the reaction mixture was filtrated to remove precipitate and the filtrate was concentrated under reduced pressure. After removal of the solvent, the residual oil was purified by the preparative TLC on silica gel using chloroform:methanol (24:1) as an eluent to obtain the compound S-7 in 82% yield. S-7: oil; IR (Neat) 3050, 2954, 1595, 1504, 1443, 1406, 1266, 1102, 1079, 1005, and 838 cm1; 1H-NMR (400 MHz, DMSO-d6) d=3.37 (18H, s), 5.15 (12H, s), 6.76 (12H, d, J=8.0 Hz), 6.89 (12H, d, J=8.0 Hz); 31P-NMR (161 MHz, DMSO-d6) d=10.21. 2.2. Thermal degradation of PSt and PMMA The blend film of polymer and stabilizer was prepared by casting from solution of THF. The thermal
T. Muraki et al. / Polymer Degradation and Stability 84 (2004) 87–93
degradation of PSt and PMMA in air was carried out at 180 and 220 C for 30 min, respectively, in a electric furnace. 2.3. Reaction of 3,4-methylenedioxyphenylacetate (S-8) with benzoyl peroxide A solution of benzoyl peroxide (0.25 g, 1 mmol) and 3,4-methylenedioxyphenylacetate (S-8, 0.36 g, 2 mmol), which was prepared from MOP and acetic anhydride, in toluene was refluxed for 1.5 h under nitrogen atmosphere. The solvent was removed under reduced pressure. Preparative TLC (silica gel) with 60:15:1 hexaneethyl acetate-triethylamine followed by recrystallization from CHCl3 gave 5-acetoxy-benzo[1,3]dioxol-2-yl benzoate (S-9). S-9: Yield, 38%. oil; IR (neat) 3069, 1755, 1731, 1599, 1491, 1375, 1244, 900, 822, and 709 cm1; 1H-NMR (400 MHz, DMSO-d6) d=2.26 (3H, s), 6.79 (1H, dd, J=8.4, 2.4 Hz), 7.12 (1H, d, J=2.4 Hz), 7.22 (1H, d, J=8.4 Hz), 7.55 (2H, t, J=7.6 Hz), 7.72 (1H, t, J=7.6 Hz), 7.96 (2H, d, J=7.6 Hz), 8.20 (1H, s). 2.4. Measurements IR spectra were recorded on a JASCO FT/IR-230 spectrophotometer. UV spectra were recorded on a Shimadzu UV-260 spectrophotometer. 1H and 31P NMR spectra were recorded with a Bruker Avance 400 (1H: 400 MHz; 31P: 161 MHz) spectrometer. Chemical shifts are reported as ppm downfield from TMS in d units. J-Values are given in Hz. Melting points were determined on a Yanaco melting point apparatus MPS3, and uncorrected The thermogravimetric measurements were performed using a Seiko SII TG/DTA 6200
89
under an atmosphere of nitrogen or air at a flow rate of 100 mL/min using a heating rate of 10 C /min. Gel permeation chromatography (GPC) was performed on a Shimadzu LC-10AT equipped with Tosoh TSKgel G3000HHR, G4000HHR, and G6000HHR columns, using THF as the eluent. The columns were calibrated with polystyrene standards. Silica gel 60 F254 (Merck) was used for TLC, and Silica gel 60 PF254 (Merck) was used for PTLC.
3. Results and discussion Organic cyclotriphosphazenes (S-3–S-7) used in this study were prepared according to Scheme 1. 31P NMR spectra were critical for monitoring the degree to which the chlorine atoms in hexachlorocyclotriphosphazene (S-1) had been substituted with organic groups. For the reaction of S-1 with MOP, 31P NMR spectra of the product (S-4) show a sharp singlet peak at d=9.99, indicating the accomplishment of a complete substitution of the chlorine atoms. For the substitution reaction of hexakis(4-hydroxyphenoxy)cyclotriphosphazene (S-3) with diphenyl chlorophosphate, the phosphazene ring remained intact, as evidenced by the presence of sharp singlet around d=9.43 in 31P NMR spectra. This also was supported by IR spectra, the observation of asymmetric stretching vibration around 1250 cm1 attributed to the –P=N- group and the vibrations around 1170 and 945 cm1 ascribed to the P-O-C6H4 groups. The molecular weight, P and N contents (%), and decomposition temperature of the phosphazene compounds (S-2–S-7) are summarized in Table 1. The thermal decomposition of these compounds occurred in the temperature range of 191–
Scheme 1. Preparation of cyclotriphosphazene-based stabilizers.
90
T. Muraki et al. / Polymer Degradation and Stability 84 (2004) 87–93
Table 1 Characteristics of cyclotriphosphazenes S-2–S-7 Stabilizer
S-2 S-3 S-4 S-5 S-6 S-7 a
MW
874 790 958 2267 2183 1054
mp ( C)
219 242 120 oil oil oil
Contents (%)
Tdec ( C)
Char yield (%)a
P
N
N2
air
N2
air
10.64 11.77 9.70 12.30 12.77 8.82
4.81 5.32 4.39 1.85 1.93 3.99
314 311 376 215 191 369
295 294 306 230 195 322
76 68 73 50 56 58
74 72 78 55 53 70
At 500 C.
376 C, and, as expected, char residues left at 500 C were high under both nitrogen and air atmospheres. For the polymer-stabilizer blend system, high compatibility of stabilizers is an essential requirement for a good physical retention of the additive in a polymer. PSt and PMMA blended with phosphazene compounds gave clear and transparent films except for the PSt/S-3 blend film. The thermal behavior of PSt and PMMA is sensitive to characteristics of carbon–carbon bond in the main chain and the terminal structure of polymer, both of which are dependent on the polymerization procedures. PSt prepared from radical polymerization decomposes by a single step process, where random bond scission and weak bond of PSt chains occur to give styrene monomer, dimer and oigomers. For the decomposition of PMMA prepared from radical polymerization a three-step mechanism is operative. The first decomposition involves bond linkages of tail-to-tail bond in PMMA around 160–190 C. Further increase of temperature to around 220–300 C causes the decomposition initiated by radicals generated at the terminal double bonds which were formed by disproportionation
process. This is followed by random chain cleavage at temperatures between 320 and 350 C. The effects of S-3–S-7 on thermal stability of PSt and PMMA were estimated by thermogravimetric analysis (TGA) in nitrogen and in air. Fig. 1 showed the TGA curves of PSt films blended with 5 wt.% of S-4. The onset decomposition temperature (T0), the temperature of 10% weight loss (T10), and char yield obtained from a single step decomposition curve were summarized in Table 2. In nitrogen PSt/S-5 blend film has a T0 value of 396 C, which was ca. 50 C higher than that of PSt. The thermal stabilizing ability of the phosphazene derivatives on PSt increased in the following order S-5 > S-6 > S-4=MOP > S-7. A significant different behavior, however, was observed in air; S-4 was the most effective to increase T0 and T10 values, i.e., the shift from 303 to 351 C and from 332 to 377 C, respectively. Contrast to this, S-6 and S-7 have no ability to increase T0 value, indicating that they can not suppress the decomposition via oxidation process. The char yield in air was higher than that in nitrogen, suggesting that phosphazene derivatives react with oxy radicals to give non-volatile products. For the slightly opaque blend film of PSt and 5 wt.% of MOP, surprisingly, T0 value of 316 C was observed, which was comparable to that of pure PSt. There are at least two plausible explanations for the thermal behavior of the PSt-MOP blend film. The first is that the combination of cyclotriphosphazene and MOP units is required to improve the heat resistance of PSt. The second is physical loss of MOP with relatively low molecular weight by volatilization. The diffusion coefficient (D) of the stabilizer in the polymer film could be calculated by
D=p/16 I2 Mt/M1=It1/2/l Table 2 Thermal behaviors of PSt blended with cyclotriphosphazene-based stabilizers Stabilizer
– S-4
S-5 S-6 S-7 Fig. 1. TGA curves for PSt blended with 5 wt.% of S-4. (a), PSt in air; (b), PSt in nitrogen; (c), PSt/S-4 blend film in air, and (d), PSt/S-4 blend film in nitrogen.
MOP a
wt.%
– 1 3 5 10 5 5 10 5 10 5
At 500 C.
T0 ( C)
T10 ( C)
Char yield (%)a
N2
Air
N2
Air
N2
Air
345 369 373 377 384 396 386 384 371 357 376
303 330 338 351 353 330 299 292 301 301 316
377 393 396 398 403 413 404 402 392 388 394
332 360 371 377 382 362 332 329 363 361 341
0 0.6 1.9 3.3 6.3 3.2 1.2 2.1 3.3 6.8 0
0 2.1 4.3 6.5 10.1 5.7 3.7 5.7 6.6 11.4 0
91
T. Muraki et al. / Polymer Degradation and Stability 84 (2004) 87–93
Table 3 Thermal behaviors of PMMA blended with cyclotriphosphazene-based stabilizers Stabilizer
– S-3 S-4 S-5 Fig. 2. Plots of Mt/M8 vs t1/2/l for PSt/S-4 () and PSt/MOP (*) blend films at 160 C.
where Mt is the total amount of stabilizer lost from the polymer films at time t, and M1 is the corresponding value after infinite time, which could be assumed to be equal to the initial amount of stabilizer; and l, the film thickness [20]. The polymer films blended with MOP and S-4 were heated at 160 C, and the amount of stabilizer remaining in the film was calculated from UV spectra. As shown in Fig. 2, plots of Mt/M1 vs t1/2/l gave straight lines. The most of MOP was lost in a few minutes and the diffusion coefficient was calculated to be 1.6 107 cm2/min. Contrast to this, the D value of S-4 (4.2 1010 cm2/min) was three orders of magnitude smaller than that of MOP. This is due to the increase of the molecular volume and in the degree of compatibility of S-4. The rapid loss of MOP might be responsible for the poor thermal resistance of PSt/MOP blend film. As shown in Fig. 3, all of phosphazene derivatives could contribute to increase the resistance of thermal
S-6 S-7 MOP a
wt.%
– 5 10 5 10 5 10 5 10 5 10 10
Ts ( C)
T10 ( C)
Char yield (%)a
N2
Air
N2
Air
N2
Air
239 264 272 263 269 258 260 244 244 247 257 271
235 295 299 289 286 282 295 261 265 266 268 303
259 292 309 285 297 276 286 264 261 265 274 290
247 313 316 308 302 307 319 269 274 287 286 319
0 1.9 4.8 1.7 4.5 1.6 3.6 0.6 1.8 2.5 5.3 0
0 2.4 5.5 2.3 5.7 2.1 4.6 0.5 1.6 2.5 5.1 0
At 500 C.
decomposition of PMMA in air. The decomposition pattern of PMMA changes depending on the stabilizers, i.e. a three-step decomposition was observed for PMMA films blended with S-3, S-4, and S-7, but the third-step decomposition was not detected for the PMMA/S-5 blend film. The temperature of decomposition in a second step (Ts), T10, and char yield at 500 C were summarized in Table 3. The effects of oxygen on thermal stability of PMMA are complex; the presence of oxygen brings about the slight increase of stability below 200 C but significant decrease above 230 C [21]. The Ts value of PMMA blended with S-3 was ca. 60 C higher than that of pure PMMA, although the weight loss for the
Fig. 3. TGA curves for PMMA blended with 5 wt.% of phosphazene derivatives in air. (a), without stabilizer; (b), S-3; (c), S-4; (d), S-5; (e), S-6; (f), S-7, and (g), MOP.
92
T. Muraki et al. / Polymer Degradation and Stability 84 (2004) 87–93 Table 4 Effects of stabilizers on the molecular weights and the degree of bond scssion for thermal decomposition of PSta Stabilizer
wt.%
Mn 103
a 104, b
sc
– S-4
– 1 3 5 10 5 10 5 10 5
24 74 78 87 88 53 35 78 62 77
31.1 2.1 1.6 0.05 0.01 7.8 17.8 1.6 5.1 1.6
2.63 0.18 0.13 0.005 0.001 0.66 1.51 0.13 0.42 0.14
S-6 S-7 Fig. 4. GPC curves of the PSt/S-4 blend films heated at 180 C for 30 min. (a), before heating; (b), PSt/5 wt.% of S-4; (c), PSt/ 10 wt.% of S-4; (d) without stabilizer.
MOP a b c
first-step decomposition could not be suppressed. The Ts and T10 values increased in the following order, S-3 > S-4 > S-5 > S-7 > S-6. The thermal stability of PMMA is known to increase by the incorporation of comonomer units in the chains, which act as block units for unzipping process of the main chains [22]. The facts that Ts values in air are higher than those obtained in nitrogen suggest that the unzipping process is suppressed by the reaction of oxy radicals in the main chain with stabilizers. The degree of thermal decomposition (a) and the number of chain scission (s) were calculated from following equations, respectively.
Table 5 Effects of stabilizers on the molecular weights and the degree of bond scssion for thermal decomposition of PMMAa Stabilizer
wt.%
Mn 103
a 104, b
sc
– S-4
– 5 10 5 10 5 10 5
14 88 94 36 89 89 90 73
62.4 0.7 0.01 17.3 0.6 0.6 0.5 3.1
5.9 0.07 0.001 1.63 0.05 0.06 0.04 0.29
S-6 S-7 MOP
a ¼ 1=DP 1=DP0
a
s ¼ DP0 =DP 1
c
b
where DP0 and DP were the number-average degree of polymerization of original and heated polymers, respectively. Fig. 4 showed the GPC traces of PSt film heated at 180 C for 30 min. The molecular weight of PSt (Mn=88000) reduced to Mn=24000 and s-value was calculated to be 2.63 (Table 4). As expected, no significant shift of the peak was observed when S-4 was added to PSt. Similarly, the decrease of molecular weight of PMMA was suppressed by the addition of S-4 and S-7. The chain scission of PSt was reduced by the addition of even 1 wt.% of S-4 and the decomposition was almost completely suppressed at 10wt.% of S-4, indicating that S-4 acts as an effective inhibitor of chain breaking (Tables 4 and 5). In order to clarify the role of S-4 in the thermal decomposition of polymers, the reaction of model
180 C for 30 min. Degree of thermal decomposition. Number of chain scission.
220 C for 30 min. Degree of thermal decomposition. Number of chain scission.
compound S-8 with benzoyl peroxide (BPO) and with AIBN in toluene was conducted at 120 C in nitrogen. For the reaction of BPO with S-8, benzoyloxymethylene-3,4-dioxyphenylacetate (S-9) was formed as a major product, whereas S-8 was recovered without suffering any reaction of 1-cyano-1-methylethyl radical. These results indicate that the-OCH2O- group in MOP units acts as a trapping site of oxy radicals but not for carbon radicals. For the PSt or PMMA film blended with S-4 carrying six trapping sites, the polymer chain with oxy radical could be recombined through the same S-4 molecule, which implies that the rapid decrease of molecular weight is significantly suppressed (Scheme 2). Organophosphorus compounds are known to act as flame-retardants for polymeric materials, due to the
Scheme 2. Reaction of BPO with S-8.
T. Muraki et al. / Polymer Degradation and Stability 84 (2004) 87–93
formation of a char which plays as a physical barrier to heat transfer from the flame to the polymer. Although the T0, T10 and char yield could not give direct information of the flame retardant properties of blend polymers, these relate the formation of flammable gases from the polymers. Qualitative test of flame retardance of polymers blended with 10 wt.% of phosphazene derivatives showed, unfortunately, no self-extinguishing properties, probably due to low char yield at 500 C. This might be ascribed to relatively low concentration of P and N atoms in the materials. In summary, the thermal stability and degradation behavior of PSt and PMMA blended with organic cyclotriphosphazenes (N3P3(OR)6) were strongly dependent on the organic groups attached to the phosphazene ring. Among them, S-4 that has relatively high molecular weight and compatibility functions as an efficient stabilizer, i.e., the addition of S-4 brings about the increase of the onset temperature of decomposition of polymers and the suppression of the bond scission of main chains. The versatility in chemical transformation of hexachlorocyclotriphosphazene allows preparing various types of stabilizers by the introduction of cosubstituents, which have different properties, in various ratios on the cyclotriphosphazene. Synthesis efforts are currently underway to prepare cyclophosphazenebased stabilizers [18,19].
93
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
Pospisˇ il J. Adv Polym Sci 1991;101:65. Lu S-Y, Hamerton I. Prog Polym Sci 2002;27:1661. Klemchuk PP, Gande ME. Polym Degrad Stab 1988;22:241. Step EN, Turro NJ, Gande ME, Klemchuk PP. Macromolecules 1994;27:2529. Nishinaga T, Shiotsu S, Tsujita H. Polym Degrad Stab 2002;76:435. Pauquet JR. J Macromol Sci Pure Appl Chem 1999;A35:1717. De Jaeger R, Gleria M. Prog Polym Sci 1998;23:179. Inoue K, Itaya T. Bull Chem Soc Jpn 2001;74:1381. Inoue K, Nakano M, Takagi M, Tanigaki T. Macromolecules 1989;22:1530. Miyata K, Muraoka K, Itaya T, Inoue K. Eur Polym, J 1996; 32:1257. Dez I, Henry N, De Jaeger R. Polym Degrad Stab 1999;64:433. Allcock HR, Hartle TJ, Taylor JP, Sunderland NJ. Macromolecules 2001;34:3896. Allcock HR, Laredo WL, deDebus CR, Taylor JP. Macromolecules 1999;32:7719. Allcock HR, Kellam III EC. Macromolecules 2002;35:40. Mathew D, Nair CPR, Ninan KN. Polym Int 2000;49:48. Medici A, Fantin G, Pedrini P, Gleria M, Minto F. Macromolecules 1992;25:2569. Inoue K, Negayama S, Itaya T, Tanigaki T. Macromol Rapid Commun 1997;18:225. McNeill IC, Zulfiqar M, Kousar T. Poly Deg Stab 1990;28:1990. Kashiwagi T, Inaba A, Brown JE, Hatada K, Kitayama T, Masuda E. Macromolecules 1986;19:2160. Masuko T, Sato A, Karasawa M. J Appl Polym Sci 1978;22:1978. Kashiwagi T, Hirata T, Brown JE. Macromolecules 1985;18:1410. McNeill IC. Eur Polym, J 1968;4:21.