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Polymers with pendent functional groups. V. Thermooxidative and thermal behavior of chloromethylated polysulfones
Polymer Degradation and Stability 69 (2000) 175±181
Polymers with pendent functional groups. V. Thermooxidative and thermal behavior of chloromethylated polysulfones Ecaterina Avram a,*, Mihai Adrian Brebu a, Abraham Warshawsky b, Cornelia Vasile a a
``Petru Poni'' Institute of Macromolecular Chemistry, 41A, Gr. Ghica Voda Alley, Ro 6600 Iassy, Romania b The Weizmann Institute of Sciences, Rehovot, Israel Received 7 February 2000; accepted 15 February 2000
Abstract The ®rst step of the thermooxidative and thermal decomposition of some chloromethylated polysulfones (CMPSF) with various degrees of substitution has been studied by thermogravimetry (TG) and thermal volatilization analysis (TVA). A step occurring at lower temperatures 260±370 C is particular for halomethylated polysulfones because the polysulfone (PSF) is thermally stable in this temperature range (its ®rst decomposition step occurs from 400 to 550 C). The higher the degree of substitution, the lower the degradation temperature and a better separation of the processes was found. The thermal decomposition products collected were analyzed by IR spectroscopy and elemental analysis. According to our TG and TVA results and literature data , it can be proposed that ±CH2Cl groups react at low temperatures giving cross-linked polymer and gas or liquid products. The thermal characteristics of this process depend on the chlorine content of the sample and explain undesirable cross-linking that could take place during the synthesis of the chloromethylated polysulfones. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Chloromethylated polysulfone; Chloromethylation reaction; Thermogravimetry; Thermal volatilization analysis
1. Introduction Poly(arylene ether sulfone)s are amorphous polymers which possess excellent thermal stability [1]. This characteristic is not dicult to explain since highly thermally stable diphenylether and diphenylsulfone units are present in the polymer backbone. It is well known that polysulfone gives outstanding polymer membranes with a high glass transition temperature (195 C), good thermal and oxidative stability (decomposition range between 400 and 550 C regardless of environment Ð under air, argon or vacuum), excellent strength and ¯exibility, resistance to extremes pH values and low creep [2,3]. There is an interest in the chemical modi®cation of polysulfones especially in the halomethylation reaction [4±7] which leads to precursors for important functional membranes, coatings, ion exchange resins, ion exchange ®bers, selectively permeable ®lms, etc. In a previous paper [8], the kinetic aspects of the ®rst step of thermooxidative decomposition under dynamic
* Corresponding author. Tel.: +40-32-144909; fax: +40-32-211299. E-mail address: [email protected] (E. Avram).
temperature conditions for bromomethylated and carboxylated polysulfones was studied and it has been established that the chemical modi®cation of polysulfone changes the thermal behaviour. Chemically modi®ed polysulfones decompose in two thermogravimetric steps. The ®rst occurs at low temperatures and corresponds with the elimination and reaction of the functional groups [9]. This paper deals with the thermooxidative and thermal decomposition of the chloromethylated polysulfones, especially with the in¯uence of the degree of substitution (DS) on the thermal characteristics. 2. Experimental 2.1. Materials 2.1.1. Synthesis of the chloromethylated polysulfones and their characterisation Methylene bridges between the macromolecular chains can appear during the chloromethylation reaction and the corresponding cross-linked chloromethylated polysulfones are insoluble. To prevent this phenomenon,
0141-3910/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0141-3910(00)00057-4
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E. Avram et al. / Polymer Degradation and Stability 69 (2000) 175±181
the chloromethylation was performed at high dilution and low amount of catalyst and in the presence of paraformaldehyde. The mixture of commercial paraformaldehyde/chlorotrimethylsilane (ClSiMe3) from Merck as chloromethylation agent and tin tetrachloride (SnCl4) from Fluka as catalyst were used for the chloromethylation reaction of polysulfone. Chloroform puri®ed by the conventional method was used as solvent. Polysulfone (Union Carbide) was a commercial product with the structural unit shown in Scheme 1 and average number molecular weight around 20 000. It was puri®ed by dissolution in chloroform, precipitation with methanol and ®nally drying in vacuum at 40 C for 24 h, before use in the synthesis of chloromethylated polysulfones. The chloromethylation reaction was performed in a glass ¯ask heated in a water bath with controlled temperature and equipped with stirrer and re¯ux condenser. Typically the chloromethylation agent and the catalyst were added to a polymer solution heated at 50±52 C. The increase of the reaction time, polymer structural
unit/chloromethylation agent molar ratio and the use of low catalyst amount increase the content of chloromethylated groups of the functionalized polysulfones. Reaction time was varied from 5 to 72 h in order to obtain dierent degrees of substitution of the chloromethylated products. Once the reaction time was ®nished, the reaction mixture was poured into methanol with stirring. The precipitated polymer was ®ltered, washed well with methanol and ®nally dried in vacuum at 40 C for 24 h. The chloromethylation reaction occurs according to Scheme 1. Substitution takes place on the bisphenol-A units, ®rstly in the position 1* leading to monosubstituted product and then in the position 2* when disubstituted product is obtained [4]. The characteristics of the studied materials are given in Table 1. There is a good concordance between the experimental and calculated values for elemental composition of the samples that proves the validity of the chloromethylation reaction model. The total chlorine content of chloromethylated polysulfones was determined by a modi®ed SchoÈninger method [10]. The degree of substitution DS was calculated using the following equation: DS
MPSF Clt MCl 100 ÿ MCH2 Cl Clt
1
where MPSF MCl MCH2 Cl Clt
= = = =
molecular weight of structural unit of PSF; atomic weight of chlorine; molecular weight of group CH2Cl; chlorine concentration, analytically determined.
Number average molecular weight and polydispersity Mw =Mn of the chloromethylated polysulfones was
Scheme 1.
Table 1 Characteristics of the studied chloromethylated polysulfonesa Sample
Elemental composition (%) C
PSF CMPSF-0.48 CMPSF-0.49 CMPSF-0.87 CMPSF-1.25 CMPSF-1.74 CMPSF-1.84 a b
H
S
O
DS
MSU b
Mn
Mw =Mn
0.00 0.48 0.49 0.87 1.25 1.74 1.84
442.00 465.27 465.93 483.97 502.60 526.46 531.95
18 619 17 350 17 556 18 766 19 840 20 523 21 502
2.636 2.742 2.734 2.874 2.951 2.863 2.960
Cl
Calc.b
Found
Calc.b
Found
Calc.b
Found
Calc.b
Found
Found
73.22 70.07 70.00 67.66 64.96 61.73 61.05
73.07 70.81 70.98 68.18 65.37 62.14 61.35
4.97 4.76 4.75 4.59 4.41 4.19 4.15
5.12 4.98 4.87 4.88 4.65 4.35 4.43
7.23 6.62 6.91 6.68 6.42 6.10 6.03
7.47 7.01 6.98 6.72 6.52 6.27 6.27
14.30 15.09 15.09 15.37 15.98 16.51 16.65
14.06 14.04 13.92 14.52 15.23 15.77 15.85
0.18 3.16 3.25 5.70 8.23 11.47 12.12
DS=substitution degree; MSU =molecular weight of structural unit; Mn =number molecular weight average; Mw =Mn =molecular weight distribution. Calculated on the basis of %Cl found in the sample composition.
E. Avram et al. / Polymer Degradation and Stability 69 (2000) 175±181
determined by GPC by means of an instrument GPCPL-EMD.950 from Polymer Laboratories using polystyrene standards. A slight increase of the molecular weight with the degree of substitution can be observed; therefore no chain scission took place during the substitution reaction. The degree of substitution for one structural unit varies from 0.48 to 1.84, therefore the obtained CMPSF have from less than one ±CH2Cl group for one ether sulfone structural unit to approximately two groups for one structural unit and the DS increased with reaction time. The reaction time was 5 to 72 h. The properties of these products are expected to vary in large degree. The samples are denoted for each degree of substitution as CMPSF-0.48 to CMPSF-1.84. All products were insoluble in water, methyl, ethyl or butyl alcohol or acetone and totally soluble in chloroform, dichloroethane, N,N-dimethylacetamide, dimethyl-sulfoxide, N,N-dimethylformamide, dioxane and benzene, even for high degree of substitution (Table 2). The introduction of the ±CH2Cl groups is also evidenced in IR spectra (Fig. 1) by the presence of the C±Cl bands at 760 cmÿ1 and of the ±CH2Cl signal at d=4.45 ppm (Fig. 2) in 1H-NMR spectra (CDCl3 solvent).
177
on the installation were given elsewhere [9]. Gas and liquid products were collected in traps at liquid nitrogen temperature, ``cold'' fraction was deposited near the outlet of the oven on the cold wall of the reaction vessel and residue remained in the sample pan. Gas, liquid, ``cold'' fraction and residue result as thermal decomposition products. IR spectra of the polymer samples, the ``cold'' fraction and residue resulted from the TVA experiments were recorded on a Specord M 80 IR spectrometer, with the samples deposed in thin layer on KBr tablets. 1 H NMR spectra were recorded on a JEOL spectrometer at 80 MHz with CDCl3 as solvent. 3. Results and discussion The characteristic TG DTG and TVA curves are given in Figs. 3 and 4, respectively.
2.2. Methods of investigation The TG and the DTG curves were recorded on a Paulik±Paulik±Erdey type Derivatograph MOM Budapest in the following conditions: heating rate 12 C minÿ1, temperature range 20±600 C, sample weight 50 mg, in air ¯ow of 30 cm3 minÿ1. Several isothermal experiments were also performed at 250 C. The TVA experiments were performed in a continuously evacuated system, in the following conditions: heating rate 5 minÿ1, temperature range 20±500 C, sample weight 0.2 g, initial pressure 10ÿ3±10ÿ4 torr. The details
Table 2 The solubilities of chloromethylated polysulfones CMPSF-1.25 Solubilitya Experiment
Solvent
CMPSF undegraded
CMPSF degraded
1 2 3 4 5 6 7 8 9 10 11 12
Water Methyl alcohol Ethyl alcohol Butyl alcohol Acetone Chloroform Dichloromethane N,N-Dimethylformamide N,N-Dimethylacetamide Dimethylsulfoxide Dioxane Benzene
ÿ ÿ ÿ ÿ ÿ + + + + + + +
ÿ ÿ ÿ ÿ ÿ ÿ ÿ ÿ ÿ ÿ ÿ ÿ
a
(+) soluble; (ÿ) insoluble.
Fig. 1. IR spectra of the CMPSF with various degrees of substitution: (- - - -) CMPSF-1.84; (.......) CMPSF-1.25; ( Ð ) CMPSF-0.49.
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E. Avram et al. / Polymer Degradation and Stability 69 (2000) 175±181
Fig. 2. 1H NMR spectra in CDCl3 of CMPSF-1.25.
Fig. 4. TVA curves of unmodi®ed and chloromethylated polysulfones.
Fig. 3. TG/DTG curves of unmodi®ed and chloromethylated polysulfones.
It can easily be remarked from all three types of curves that with increasing substitution degree, the chloromethylated polysulfones exhibit a particular behavior in decomposition in respect with that of polysulfone.
A new thermo-oxidative or thermal decomposition step is developing at low temperatures that becomes well de®ned with increasing substitution. This should mean that if for the CMPSF with low degree of substitution (<0.5) the decomposition mechanism of PSF was perturbed or changed, for higher degree of substitution a thermal instability is evident and a particular way of degradation is developed. The thermal characteristics of this ®rst thermogravimetric step are signi®cantly changed for DS>1.6. For low substitution degree, the steps occurring from 200 to 400 C are not very well separated. The variation of the characteristic temperature and weight loss with the degree of substitution can be established on the basis of the data of Table 3 and Figs. 5 and 6.
E. Avram et al. / Polymer Degradation and Stability 69 (2000) 175±181
179
Table 3 Thermogravimetric data for PSF and CMPSFa Sample
PSF CMPSF-0.49 CMPSF-0.87 CMPSF-1.25 CMPSF-1.74 CMPSF-1.84
Temperature range 340±440 C
Temperature range 250±340 C
Temperature range 440±550 C
Ti ( C)
T5% ( C)
Tm ( C)
Tf ( C)
w (%)
Ti ( C)
Tm ( C)
Tf ( C)
w (%)
Ti ( C)
Tm ( C)
Tf ( C)
w
± ± 267 279 280 237
± ± 276 306 293 264
± ± 308 336 329 322
± ± 329 367 346 343
± ± 7.6 19.6 15.2 8.3
± 362 329 367 346 343
± 451 412 ± 361 422
± 471 437 432 379 448
± 28.0 34.4 24.6 11.3 18.3
403 471 ± ± ± ±
518 506 ± ± ± ±
554 537 ± ± ± ±
51 42.5 ± ± ± ±
W450 C (%)
250±440 C w (%) (I+II)
CH2Cl (%)
1 18.5 51.4 54.5 60 24
± 28 42.0 44.2 26.5 26.6
± 5.21 8.89 12.31 16.36 17.14
a Ti T5%, Tm, Tf, temperature corresponding to the onset, 5% weight loss, the maximum rate of weight loss and the end of the process, respectively; w, percentage of weight loss; W450 C, weight loss at 45 C; w (%) I+II: weight loss corresponding to the two decomposition steps occurring in 250±440 C temperature range; %CH2Cl: percentage of CH2Cl groups in the structural unit of the chloromethylated polysulfones.
The weight loss values are dicult to discuss as they correspond in fact to two overlapping processes whose weight losses are approximately 27±30%. The variation of the overall apparent activation energy and pre-exponential factor with the degree of substitution is presented in Fig. 5. The shape of the curves is similar, both values decrease with increasing DS. The CMPSF-1.84 exhibits the most important decrease. The reaction order values of the ®rst decomposition step range from 0.5 to 0.9. For the sample with the degree of substitution of 1.74, the step occurring at low temperature is well separated and the corresponding weight loss is 15.2% being close to the weight loss and the calculated value of 16.3% for the loss of the CH2Cl groups. This could suggest that
Fig. 5. Overall activation energy (Coats±Redfern method) [11] and the pre-exponential factor, corresponding to the ®rst decomposition step of chloromethylated polysulfones.
for certain degree of substitution a dechloromethylation reaction occurs opposite to the synthesis pathway, the CH2Cl groups being eliminated ®rst from the disubstituted structural units, leading to monosubstituted chloromethylated product that decomposes at higher temperatures. For all chloromethylated polysulfones, the activation energy is constant in the conversion degree interval of 0.2±0.6 (Fig. 6), therefore the process involved in this ®rst thermogravimetric step exhibits a mechanism that is maintained over the whole studied temperature range. Variation of the ERL with the degree of substitution is similar to the variation of the other kinetic parameters, namely it decreases with increasing DS. The material balance of the thermal decomposition by TVA method is presented in Table 4. The quantities of the ``cold'' fraction and residue increase with the chloromethylation degree at the expense of the gas yield. Therefore the chloromethyl
Fig. 6. Variation of the activation energy (Reich±Levy method [12,13]) with the degree of substitution.
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E. Avram et al. / Polymer Degradation and Stability 69 (2000) 175±181
groups facilitate large fragment formation in the ``cold'' fraction. At the same time, the decomposition residue has a high percentage of carbon and hydrogen and low content of oxygen. A part of the chlorine remained in the carbonaceous residue, because of the short time of decomposition. Some information about the decomposition pathway has been obtained by the analysis of the reaction products resulted from TVA experiments. From the IR spectra of the polymers, the ``cold'' fraction and residue resulting from the thermal degradation (Figs. 7 and 8) one can observe the diminishing and vanishing of the bands assigned to C±Cl groups (760 cmÿ1), therefore the elimination of functional groups from both products is possible. This observation is in accordance with elemental analysis results (Table 5) of the sample isothermally degraded at 250 C. After 1 h degradation, no chlorine was identi®ed in isothermally degraded CMPSF at 250 C. To explain these results, we have to note that the energies of the bonds Caliphat.±Cl and Ar±CH2±Cl are very close at 332 and 336 kJ/mol, respectively; therefore, their scission is equally probable with slight preference for the ®rst [14]. A possible free-radical
mechanism for the ®rst thermogravimetric step (250± 340 C temperature range) has been proposed (Scheme 2). The Ph±CH2±Ph methylene links are formed between macromolecular chains, resulting in a crosslinked product with very low solubility even at short reaction time (5 min), the remained material being totally insoluble.
Table 4 Material balance of the TVA experiments Sample
PSF CMPSF-0.84 CMPSF-1.13r
Decomposition products (%) Cold fraction
Residue
Gas+losses
26.36 29.11 33.72
47.36 52.29 53.27
26.28 18.60 13.01
Fig. 8. IR spectra of the CMPSF-1.25 isothermally degraded at 250 C for various time periods.
Table 5 The chlorine content of the sample CMPSF-1.25 degraded at 250 C various time periods Sample
Fig. 7. IR spectra of CMPSF-0.48 and the corresponding ``cold'' fraction obtained from thermal decomposition.
1 2 3 4 5
Conditions of degradation Time (min)
Temperature ( C)
± 5 10 15 60
± 250 250 250 250
Cl (%)
8.23 7.21 6.44 4.25 ±
E. Avram et al. / Polymer Degradation and Stability 69 (2000) 175±181
181
It can be supposed that the particular decomposition step of chloromethylated polysulfones occurs both by the elimination of functional groups and a cross-linking reaction. References
Scheme 2.
4. Conclusion In order to prevent the cross-linking reaction and consequently insolubility of the chloromethylated polysulfones, the conditions of the chloromethylation reaction have been modi®ed. Chloromethylated polysulphones exhibit a supplementary thermal and thermo-oxidative decomposition step, occurring at low temperatures in comparison with unmodi®ed polysulfone. Weight losses, characteristic temperatures and overall kinetic parameters have a particular variation with the increase of the substitution degree.
[1] Johnson RN, Farnham AG, Clendinning RA, Hale WF, Merriam CN. J Polym Sci A-1 1967;5:2375. [2] Moshay A, Rabeson LM. J Appl Polym Sci 1976;20:1885. [3] Ritter H, editor. Macromol Eng, Munmaya Mishra. New York: Plenum Press, 1995. p. 291. [4] Warshawsky A, Kahana N, Deshe A, Gottlieb HE, Arad-Yellin R. J Polym Sci Part A Polym Chem 1990;28:2885. [5] Warshawsky A, Deshe A, Gutman R. Brit Polym J 1984;16:234. [6] Daly WH. J Macromol Sci Chem 1985;A22:713. [7] Avram E, Butuc E, Luca C, Druta I. J Macromol Sci Pure and Appl Chem 1997;A34:1701. [8] Vasile C, Costea E, Pascu MC, Warshawsky A. J Therm Anal 1998;52:569. [9] Vasile C, Calugaru ME, Stoleriu A, Mihai E, Sabliovschi M. Thermal behaviour of polymers. Bucharest: Acad. Rom, 1983. [10] Haslan J, Hamilton B, Squirell DM. Analyst 1960;85:556. [11] Coats AW, Redfen JP. Nature (London) 1964;68:201. [12] Reich RI, Levi DW. Makromol Chem 1963;66:66:102. [13] Reich RI, Levi DW. J Polym Sci Letters 1964;2:621. [14] Daly WH, Wu SJ. In: Culbertson BV, editor. Modi®cation of polymers. Washington DC: Amer. Chem. Soc., 1983.
Polymers with pendent functional groups. V. Thermooxidative and thermal behavior of chloromethylated polysulfones Ecaterina Avram a,*, Mihai Adrian Brebu a, Abraham Warshawsky b, Cornelia Vasile a a
``Petru Poni'' Institute of Macromolecular Chemistry, 41A, Gr. Ghica Voda Alley, Ro 6600 Iassy, Romania b The Weizmann Institute of Sciences, Rehovot, Israel Received 7 February 2000; accepted 15 February 2000
Abstract The ®rst step of the thermooxidative and thermal decomposition of some chloromethylated polysulfones (CMPSF) with various degrees of substitution has been studied by thermogravimetry (TG) and thermal volatilization analysis (TVA). A step occurring at lower temperatures 260±370 C is particular for halomethylated polysulfones because the polysulfone (PSF) is thermally stable in this temperature range (its ®rst decomposition step occurs from 400 to 550 C). The higher the degree of substitution, the lower the degradation temperature and a better separation of the processes was found. The thermal decomposition products collected were analyzed by IR spectroscopy and elemental analysis. According to our TG and TVA results and literature data , it can be proposed that ±CH2Cl groups react at low temperatures giving cross-linked polymer and gas or liquid products. The thermal characteristics of this process depend on the chlorine content of the sample and explain undesirable cross-linking that could take place during the synthesis of the chloromethylated polysulfones. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Chloromethylated polysulfone; Chloromethylation reaction; Thermogravimetry; Thermal volatilization analysis
1. Introduction Poly(arylene ether sulfone)s are amorphous polymers which possess excellent thermal stability [1]. This characteristic is not dicult to explain since highly thermally stable diphenylether and diphenylsulfone units are present in the polymer backbone. It is well known that polysulfone gives outstanding polymer membranes with a high glass transition temperature (195 C), good thermal and oxidative stability (decomposition range between 400 and 550 C regardless of environment Ð under air, argon or vacuum), excellent strength and ¯exibility, resistance to extremes pH values and low creep [2,3]. There is an interest in the chemical modi®cation of polysulfones especially in the halomethylation reaction [4±7] which leads to precursors for important functional membranes, coatings, ion exchange resins, ion exchange ®bers, selectively permeable ®lms, etc. In a previous paper [8], the kinetic aspects of the ®rst step of thermooxidative decomposition under dynamic
* Corresponding author. Tel.: +40-32-144909; fax: +40-32-211299. E-mail address: [email protected] (E. Avram).
temperature conditions for bromomethylated and carboxylated polysulfones was studied and it has been established that the chemical modi®cation of polysulfone changes the thermal behaviour. Chemically modi®ed polysulfones decompose in two thermogravimetric steps. The ®rst occurs at low temperatures and corresponds with the elimination and reaction of the functional groups [9]. This paper deals with the thermooxidative and thermal decomposition of the chloromethylated polysulfones, especially with the in¯uence of the degree of substitution (DS) on the thermal characteristics. 2. Experimental 2.1. Materials 2.1.1. Synthesis of the chloromethylated polysulfones and their characterisation Methylene bridges between the macromolecular chains can appear during the chloromethylation reaction and the corresponding cross-linked chloromethylated polysulfones are insoluble. To prevent this phenomenon,
0141-3910/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0141-3910(00)00057-4
176
E. Avram et al. / Polymer Degradation and Stability 69 (2000) 175±181
the chloromethylation was performed at high dilution and low amount of catalyst and in the presence of paraformaldehyde. The mixture of commercial paraformaldehyde/chlorotrimethylsilane (ClSiMe3) from Merck as chloromethylation agent and tin tetrachloride (SnCl4) from Fluka as catalyst were used for the chloromethylation reaction of polysulfone. Chloroform puri®ed by the conventional method was used as solvent. Polysulfone (Union Carbide) was a commercial product with the structural unit shown in Scheme 1 and average number molecular weight around 20 000. It was puri®ed by dissolution in chloroform, precipitation with methanol and ®nally drying in vacuum at 40 C for 24 h, before use in the synthesis of chloromethylated polysulfones. The chloromethylation reaction was performed in a glass ¯ask heated in a water bath with controlled temperature and equipped with stirrer and re¯ux condenser. Typically the chloromethylation agent and the catalyst were added to a polymer solution heated at 50±52 C. The increase of the reaction time, polymer structural
unit/chloromethylation agent molar ratio and the use of low catalyst amount increase the content of chloromethylated groups of the functionalized polysulfones. Reaction time was varied from 5 to 72 h in order to obtain dierent degrees of substitution of the chloromethylated products. Once the reaction time was ®nished, the reaction mixture was poured into methanol with stirring. The precipitated polymer was ®ltered, washed well with methanol and ®nally dried in vacuum at 40 C for 24 h. The chloromethylation reaction occurs according to Scheme 1. Substitution takes place on the bisphenol-A units, ®rstly in the position 1* leading to monosubstituted product and then in the position 2* when disubstituted product is obtained [4]. The characteristics of the studied materials are given in Table 1. There is a good concordance between the experimental and calculated values for elemental composition of the samples that proves the validity of the chloromethylation reaction model. The total chlorine content of chloromethylated polysulfones was determined by a modi®ed SchoÈninger method [10]. The degree of substitution DS was calculated using the following equation: DS
MPSF Clt MCl 100 ÿ MCH2 Cl Clt
1
where MPSF MCl MCH2 Cl Clt
= = = =
molecular weight of structural unit of PSF; atomic weight of chlorine; molecular weight of group CH2Cl; chlorine concentration, analytically determined.
Number average molecular weight and polydispersity Mw =Mn of the chloromethylated polysulfones was
Scheme 1.
Table 1 Characteristics of the studied chloromethylated polysulfonesa Sample
Elemental composition (%) C
PSF CMPSF-0.48 CMPSF-0.49 CMPSF-0.87 CMPSF-1.25 CMPSF-1.74 CMPSF-1.84 a b
H
S
O
DS
MSU b
Mn
Mw =Mn
0.00 0.48 0.49 0.87 1.25 1.74 1.84
442.00 465.27 465.93 483.97 502.60 526.46 531.95
18 619 17 350 17 556 18 766 19 840 20 523 21 502
2.636 2.742 2.734 2.874 2.951 2.863 2.960
Cl
Calc.b
Found
Calc.b
Found
Calc.b
Found
Calc.b
Found
Found
73.22 70.07 70.00 67.66 64.96 61.73 61.05
73.07 70.81 70.98 68.18 65.37 62.14 61.35
4.97 4.76 4.75 4.59 4.41 4.19 4.15
5.12 4.98 4.87 4.88 4.65 4.35 4.43
7.23 6.62 6.91 6.68 6.42 6.10 6.03
7.47 7.01 6.98 6.72 6.52 6.27 6.27
14.30 15.09 15.09 15.37 15.98 16.51 16.65
14.06 14.04 13.92 14.52 15.23 15.77 15.85
0.18 3.16 3.25 5.70 8.23 11.47 12.12
DS=substitution degree; MSU =molecular weight of structural unit; Mn =number molecular weight average; Mw =Mn =molecular weight distribution. Calculated on the basis of %Cl found in the sample composition.
E. Avram et al. / Polymer Degradation and Stability 69 (2000) 175±181
determined by GPC by means of an instrument GPCPL-EMD.950 from Polymer Laboratories using polystyrene standards. A slight increase of the molecular weight with the degree of substitution can be observed; therefore no chain scission took place during the substitution reaction. The degree of substitution for one structural unit varies from 0.48 to 1.84, therefore the obtained CMPSF have from less than one ±CH2Cl group for one ether sulfone structural unit to approximately two groups for one structural unit and the DS increased with reaction time. The reaction time was 5 to 72 h. The properties of these products are expected to vary in large degree. The samples are denoted for each degree of substitution as CMPSF-0.48 to CMPSF-1.84. All products were insoluble in water, methyl, ethyl or butyl alcohol or acetone and totally soluble in chloroform, dichloroethane, N,N-dimethylacetamide, dimethyl-sulfoxide, N,N-dimethylformamide, dioxane and benzene, even for high degree of substitution (Table 2). The introduction of the ±CH2Cl groups is also evidenced in IR spectra (Fig. 1) by the presence of the C±Cl bands at 760 cmÿ1 and of the ±CH2Cl signal at d=4.45 ppm (Fig. 2) in 1H-NMR spectra (CDCl3 solvent).
177
on the installation were given elsewhere [9]. Gas and liquid products were collected in traps at liquid nitrogen temperature, ``cold'' fraction was deposited near the outlet of the oven on the cold wall of the reaction vessel and residue remained in the sample pan. Gas, liquid, ``cold'' fraction and residue result as thermal decomposition products. IR spectra of the polymer samples, the ``cold'' fraction and residue resulted from the TVA experiments were recorded on a Specord M 80 IR spectrometer, with the samples deposed in thin layer on KBr tablets. 1 H NMR spectra were recorded on a JEOL spectrometer at 80 MHz with CDCl3 as solvent. 3. Results and discussion The characteristic TG DTG and TVA curves are given in Figs. 3 and 4, respectively.
2.2. Methods of investigation The TG and the DTG curves were recorded on a Paulik±Paulik±Erdey type Derivatograph MOM Budapest in the following conditions: heating rate 12 C minÿ1, temperature range 20±600 C, sample weight 50 mg, in air ¯ow of 30 cm3 minÿ1. Several isothermal experiments were also performed at 250 C. The TVA experiments were performed in a continuously evacuated system, in the following conditions: heating rate 5 minÿ1, temperature range 20±500 C, sample weight 0.2 g, initial pressure 10ÿ3±10ÿ4 torr. The details
Table 2 The solubilities of chloromethylated polysulfones CMPSF-1.25 Solubilitya Experiment
Solvent
CMPSF undegraded
CMPSF degraded
1 2 3 4 5 6 7 8 9 10 11 12
Water Methyl alcohol Ethyl alcohol Butyl alcohol Acetone Chloroform Dichloromethane N,N-Dimethylformamide N,N-Dimethylacetamide Dimethylsulfoxide Dioxane Benzene
ÿ ÿ ÿ ÿ ÿ + + + + + + +
ÿ ÿ ÿ ÿ ÿ ÿ ÿ ÿ ÿ ÿ ÿ ÿ
a
(+) soluble; (ÿ) insoluble.
Fig. 1. IR spectra of the CMPSF with various degrees of substitution: (- - - -) CMPSF-1.84; (.......) CMPSF-1.25; ( Ð ) CMPSF-0.49.
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E. Avram et al. / Polymer Degradation and Stability 69 (2000) 175±181
Fig. 2. 1H NMR spectra in CDCl3 of CMPSF-1.25.
Fig. 4. TVA curves of unmodi®ed and chloromethylated polysulfones.
Fig. 3. TG/DTG curves of unmodi®ed and chloromethylated polysulfones.
It can easily be remarked from all three types of curves that with increasing substitution degree, the chloromethylated polysulfones exhibit a particular behavior in decomposition in respect with that of polysulfone.
A new thermo-oxidative or thermal decomposition step is developing at low temperatures that becomes well de®ned with increasing substitution. This should mean that if for the CMPSF with low degree of substitution (<0.5) the decomposition mechanism of PSF was perturbed or changed, for higher degree of substitution a thermal instability is evident and a particular way of degradation is developed. The thermal characteristics of this ®rst thermogravimetric step are signi®cantly changed for DS>1.6. For low substitution degree, the steps occurring from 200 to 400 C are not very well separated. The variation of the characteristic temperature and weight loss with the degree of substitution can be established on the basis of the data of Table 3 and Figs. 5 and 6.
E. Avram et al. / Polymer Degradation and Stability 69 (2000) 175±181
179
Table 3 Thermogravimetric data for PSF and CMPSFa Sample
PSF CMPSF-0.49 CMPSF-0.87 CMPSF-1.25 CMPSF-1.74 CMPSF-1.84
Temperature range 340±440 C
Temperature range 250±340 C
Temperature range 440±550 C
Ti ( C)
T5% ( C)
Tm ( C)
Tf ( C)
w (%)
Ti ( C)
Tm ( C)
Tf ( C)
w (%)
Ti ( C)
Tm ( C)
Tf ( C)
w
± ± 267 279 280 237
± ± 276 306 293 264
± ± 308 336 329 322
± ± 329 367 346 343
± ± 7.6 19.6 15.2 8.3
± 362 329 367 346 343
± 451 412 ± 361 422
± 471 437 432 379 448
± 28.0 34.4 24.6 11.3 18.3
403 471 ± ± ± ±
518 506 ± ± ± ±
554 537 ± ± ± ±
51 42.5 ± ± ± ±
W450 C (%)
250±440 C w (%) (I+II)
CH2Cl (%)
1 18.5 51.4 54.5 60 24
± 28 42.0 44.2 26.5 26.6
± 5.21 8.89 12.31 16.36 17.14
a Ti T5%, Tm, Tf, temperature corresponding to the onset, 5% weight loss, the maximum rate of weight loss and the end of the process, respectively; w, percentage of weight loss; W450 C, weight loss at 45 C; w (%) I+II: weight loss corresponding to the two decomposition steps occurring in 250±440 C temperature range; %CH2Cl: percentage of CH2Cl groups in the structural unit of the chloromethylated polysulfones.
The weight loss values are dicult to discuss as they correspond in fact to two overlapping processes whose weight losses are approximately 27±30%. The variation of the overall apparent activation energy and pre-exponential factor with the degree of substitution is presented in Fig. 5. The shape of the curves is similar, both values decrease with increasing DS. The CMPSF-1.84 exhibits the most important decrease. The reaction order values of the ®rst decomposition step range from 0.5 to 0.9. For the sample with the degree of substitution of 1.74, the step occurring at low temperature is well separated and the corresponding weight loss is 15.2% being close to the weight loss and the calculated value of 16.3% for the loss of the CH2Cl groups. This could suggest that
Fig. 5. Overall activation energy (Coats±Redfern method) [11] and the pre-exponential factor, corresponding to the ®rst decomposition step of chloromethylated polysulfones.
for certain degree of substitution a dechloromethylation reaction occurs opposite to the synthesis pathway, the CH2Cl groups being eliminated ®rst from the disubstituted structural units, leading to monosubstituted chloromethylated product that decomposes at higher temperatures. For all chloromethylated polysulfones, the activation energy is constant in the conversion degree interval of 0.2±0.6 (Fig. 6), therefore the process involved in this ®rst thermogravimetric step exhibits a mechanism that is maintained over the whole studied temperature range. Variation of the ERL with the degree of substitution is similar to the variation of the other kinetic parameters, namely it decreases with increasing DS. The material balance of the thermal decomposition by TVA method is presented in Table 4. The quantities of the ``cold'' fraction and residue increase with the chloromethylation degree at the expense of the gas yield. Therefore the chloromethyl
Fig. 6. Variation of the activation energy (Reich±Levy method [12,13]) with the degree of substitution.
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E. Avram et al. / Polymer Degradation and Stability 69 (2000) 175±181
groups facilitate large fragment formation in the ``cold'' fraction. At the same time, the decomposition residue has a high percentage of carbon and hydrogen and low content of oxygen. A part of the chlorine remained in the carbonaceous residue, because of the short time of decomposition. Some information about the decomposition pathway has been obtained by the analysis of the reaction products resulted from TVA experiments. From the IR spectra of the polymers, the ``cold'' fraction and residue resulting from the thermal degradation (Figs. 7 and 8) one can observe the diminishing and vanishing of the bands assigned to C±Cl groups (760 cmÿ1), therefore the elimination of functional groups from both products is possible. This observation is in accordance with elemental analysis results (Table 5) of the sample isothermally degraded at 250 C. After 1 h degradation, no chlorine was identi®ed in isothermally degraded CMPSF at 250 C. To explain these results, we have to note that the energies of the bonds Caliphat.±Cl and Ar±CH2±Cl are very close at 332 and 336 kJ/mol, respectively; therefore, their scission is equally probable with slight preference for the ®rst [14]. A possible free-radical
mechanism for the ®rst thermogravimetric step (250± 340 C temperature range) has been proposed (Scheme 2). The Ph±CH2±Ph methylene links are formed between macromolecular chains, resulting in a crosslinked product with very low solubility even at short reaction time (5 min), the remained material being totally insoluble.
Table 4 Material balance of the TVA experiments Sample
PSF CMPSF-0.84 CMPSF-1.13r
Decomposition products (%) Cold fraction
Residue
Gas+losses
26.36 29.11 33.72
47.36 52.29 53.27
26.28 18.60 13.01
Fig. 8. IR spectra of the CMPSF-1.25 isothermally degraded at 250 C for various time periods.
Table 5 The chlorine content of the sample CMPSF-1.25 degraded at 250 C various time periods Sample
Fig. 7. IR spectra of CMPSF-0.48 and the corresponding ``cold'' fraction obtained from thermal decomposition.
1 2 3 4 5
Conditions of degradation Time (min)
Temperature ( C)
± 5 10 15 60
± 250 250 250 250
Cl (%)
8.23 7.21 6.44 4.25 ±
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181
It can be supposed that the particular decomposition step of chloromethylated polysulfones occurs both by the elimination of functional groups and a cross-linking reaction. References
Scheme 2.
4. Conclusion In order to prevent the cross-linking reaction and consequently insolubility of the chloromethylated polysulfones, the conditions of the chloromethylation reaction have been modi®ed. Chloromethylated polysulphones exhibit a supplementary thermal and thermo-oxidative decomposition step, occurring at low temperatures in comparison with unmodi®ed polysulfone. Weight losses, characteristic temperatures and overall kinetic parameters have a particular variation with the increase of the substitution degree.
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