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Synthesis and properties of poly(aryl ether ether ketone) copolymers with pendant methyl groups
European Polymer Journal 40 (2004) 2645–2651
EUROPEAN POLYMER JOURNAL www.elsevier.com/locate/europolj
Synthesis and properties of poly(aryl ether ether ketone) copolymers with pendant methyl groups V.L. Rao
b
a,*
, P.U. Sabeena a, Akanksha Saxena a, C. Gopalakrishnan b, K. Krishnan b, P.V. Ravindran b, K.N. Ninan
a Propellants and Special Chemicals Group, Polymers and Special Chemicals Division, Propellants, Chemicals and Materials Entity, Vikram Sarabhai Space Centre, Thiruvananthapuram, Kerala 695 022, India Analytical and Spectroscopy Division, Propellants and Special Chemicals Group, Polymers and Special Chemicals Division, Propellants, Chemicals and Materials Entity, Vikram Sarabhai Space Centre, Thiruvananthapuram, Kerala 695 022, India
Received 6 February 2003; accepted 1 July 2004 Available online 9 September 2004
Abstract Polyether ether ketone and polyether ether ketone copolymers were prepared by the nucleophilic substitution reaction of 4,4 0 -difluorobenzophenone with hydroquinone and with varying mole proportions of hydroquinone and methyl hydroquinone using sulfolane solvent in the presence of anhydrous K2CO3. The polymers were characterised by different physico-chemical techniques. The crystallinity of the polymers was found to decrease with increase in concentration of the methyl hydroquinone units in the polymer. Thermogravimetric studies showed that all the polymers were stable upto 430 °C with a char yield above 49% at 900 °C in N2 atmosphere. The glass transition temperature was found to increase and the crystalline melting temperature and activation energy were found to decrease with increase in concentration of the methyl hydroquinone units in the polymer. Ó 2004 Published by Elsevier Ltd. Keywords: Activation energy; Wide angle X-ray diffraction; Thermogravimetry; PEEK copolymer; Pendant methyl group; Differential scanning calorimetry
1. Introduction Poly(aryl ether ketone)s (PAEK)S are gaining rapid acceptance as an advanced thermoplastic matrix resins for high performance composites for aerospace structural applications. They possess excellent environmental resistance, good mechanical properties, chemical and *
Corresponding author. Tel.: +91 471 564298/564225; fax: +91 471 415236. E-mail addresses: rao_vl@rediffmail.com, rao_vl@yahoo. co.in (V.L. Rao). 0014-3057/$ - see front matter Ó 2004 Published by Elsevier Ltd. doi:10.1016/j.eurpolymj.2004.07.002
radiation resistance and thermo-oxidative stability. They are semicrystalline polymers having melting temperatures around 340–360 °C and glass transition temperatures around 140–160 °C. Among them poly(ether ether ketone) (PEEK) and polyether ketone (PEK) have achieved significant commercial importance as matrices for carbon fibre composites in aerospace, industrial and automotive applications and as dielectrics in the micro electronics field. However, due to their high crystalline property and low solubility towards organic solvents often restrict the use of these resins for some applications.
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V.L. Rao et al. / European Polymer Journal 40 (2004) 2645–2651
hydroquinone (SRL), methyl hydroquinone (Lancaster) were used as received. Sulfolane (Aldrich) was dried with molecular sieves (type 4A) and distilled over NaOH pellets under reduced pressure. The middle fractions were collected and stored over molecular sieves.
Various structural changes have been introduced in the basic PEEK resin by several researchers and in our laboratory to obtain different properties and for different applications. Structural changes have focussed on the order and ratio of ether–ketone linkage [1,2], on the ratio of meta and para phenyl substitution [3], and the incorporation of –CR2 and sulfone groups [4,5] in the backbone of the polymeric chain. Introduction of pendant alkyl or phenyl groups on to the poly (aryl ether ether ketone) main chain is another important structural change [6]. Ueda and his co-workers prepared PEK having two alkyl substituents per repeating unit [7]. Taguchi et al. [8] reported PEKs with more than two alkyl substituents per repeat unit. The introduction of the bulky substituents onto the PEK backbone could suppress crystallisation to improve its solubility contrast to non-substituted PEK, leading to novel applications of PEK derivatives. However, it appears that poly(aryl ether ether ketone) copolymers containing pendant methyl groups have not been reported in detail in the open literature. In this paper we report the synthesis of poly aryl ether ether ketone and its copolymers derived from 4,4 0 -difluorobenzophenone (DFBP) with hydroquinone (HQ) and methyl hydroquinone (MeHQ). Their structure– property relationships are also discussed.
2.2. Monomer synthesis 4,4 0 -Difluorobenzophenone (DFBP) was synthesised from fluorobenzene and carbon tetrachloride in the presence of AlCl3 as per the procedure described earlier [9]. Yield 55%; m.p. 105–106 °C; Elemental analysis: calculated for C13H8F2O; C, 71.56; H, 3.67; Found: C, 71.46; H, 3.61. 2.3. Polymer synthesis Poly ether ketone copolymers with methyl pendant groups were prepared as shown in Scheme 1 by nucleophilic aromatic substitution reaction of 4,4 0 -difluorobenzophenone (DFBP) with hydroquinone (HQ) and methyl hydroquinone (MeHQ) with slight excess of anhydrous K2CO3 in high boiling solvent sulfolane at 16% solid content under N2 atmosphere as per the procedure described earlier [10]. The product was refluxed with water repeatedly and subjected to soxhlet extraction with acetone, filtered and dried under vacuum at 100 °C for 15 h.
2. Experimental 2.4. Characterisation 2.1. Materials Elemental analysis was performed with Perkin–Elmer Model 2400 CHN analyser. Inherent viscosities were obtained for 0.2% polymer solution in conc. H2SO4 at room temperature using an ubbelhode suspended level viscometer. Glass transition temperature (Tg) and melting temperatures were measured using a Mettler thermal
Fluorobenzene (Navin fluorine) and carbon tetra chloride (BDH) were distilled before use and kept over anhydrous CaCl2 overnight. Anhydrous AlCl3 (Nice) was used as received. Anhydrous K2CO3 (BDH) was dried in vacuum at 100 °C before use. High purity
O F
C
F +
HO
Ar
OH
SULFOLANE K2CO 3
, TOLUENE
O C
O
CH 3 Ar =
, Scheme 1.
Ar
O
V.L. Rao et al. / European Polymer Journal 40 (2004) 2645–2651
analyser model 3000 coupled to a DSC 20, at a heating rate of 10 K min1. The FT-IR spectra of the polymers in KBr pellets (2%) were recorded using a Nicolet FT-IR (510P) spectrophotometer. 13CNMR spectra were recorded at 75.4 MHz using Brucker Avance-300 spectrometer. Methane sulfonic acid and a CDCl3 mixture was used as a solvent and tetramethyl silane (TMS) as an internal standard. Wide angle X-ray scattering data were obtained on pellets (13 mm diameter) of the polymer using a Philips X-ray unit PW1710 diffraction counting unit and PW1729 X-ray generator with nickel filtered Cu Ka radiation at 30 KV and 20 mA. The diffractograms were recorded at room temperature over the range of 10–40°. Thermogravimetric analysis of the polymers were carried out under nitrogen using a Dupont thermal analyser system 2000 in combination with a Dupont 951 thermogravimetric analyser at a heating rate of 20 K min1.
3. Results and discussion Polyether ether ketone copolymers with methyl pendant groups were synthesised by nucleophilic displacement reaction of fluorine from DFBP by HQ and MeHQ. Feed ratios and some of the properties of the polymers are given in Table 1. Inherent viscosity values reveal that high molecular weight polymers were obtained in a shorter reaction time (<1 h) in sulfolane medium for ether ketone type polymers with methyl pendant groups. The methyl pendant group containing polymers possess higher viscosity than the unsubstituted ones which may be due to electron releasing effect (+I effect) of the methyl group which enhances the nucleophilicity of the phenoxy group of MeHQ. The elemental analysis values, i.e. C and H content of copolymers I–V (Table 1) are in close agreement with theoretical values confirming the proposed structure. The FT-IR spectra of all polymers show no absorption in the 3500–3000 cm1 region indicating the absence of –OH groups. All the spectra show characteristic bands at 1240cm1 (S, mC–O) and 1412 cm1 (methyl
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group), except for polymer V where characteristic absorption for methyl group was absent. All polymers show a characteristic band at 1650cm1 (S, mC@O). The 13CNMR spectra of polymers I and V were recorded at 75.4 MHZ in methane sulfonic acid and CDCl3 with TMS as internal standards. The aromatic region of 13 CNMR spectra of the polymers I and V are shown in Figs. 1 and 2 along with assignments. The assignments of the spectrum are listed in Table 2. The calculated chemical shift values for these carbons are also listed [11]. The 13CNMR spectrum of polymer-I (Fig. 1) shows 16 absorptions and polymer-V (Fig. 2) shows seven absorptions corresponding to the sixteen and seven distinguishable carbons as indicated in the structures respectively. The chemical shift assignments of the various carbons were based on the additivity constants for substituted benzenes. In order to confirm the NMR assignment DEPT experiments also were carried out. Wang and co-workers [12] assigned the 13CNMR resonances of C8, C9, C10, C11, C12, C13, C14, C15 and C16 of polymer-I to d (161.52 and 161.58 ppm) 115.65 ppm, 132.34 ppm, 132.24 ppm, 194.15 ppm, (131.70 and 131.77 ppm), 132.24 ppm, 117 and (161.31 and 161.38 ppm) respectively. The split in resonance of C11 is obscured by overlap with signal of C14. But in our study the corresponding resonance of C8, C10, C12, C14 and C16 are shifted to down field of about 5–6 ppm whereas C11 and C13 are shifted to higher field of about 8 ppm. This may be attributed to the interaction of methane sulfonic acid with the carbonyl group of polymer-I, which is used as one of the solvents. This assignment is in agreement with the observations made in the case of PEEK Table 2. Another interesting feature is that Wang and coworkers observed a splitting in polymer-I for resonances of C8 (161.59 and 161.52 ppm), C11 (132.24 ppm), C13 (131.77 and 131.70 ppm) and C16 (161.38 and 161.31 ppm) whereas in our study no splitting was observed for the resonances of C8 (167.7 ppm), C11 (123.6 ppm), C13 (123.5 ppm) and C16 (167.6 ppm) which may be due to the higher magnetic field (400 MHZ) used for the analysis or due to the solvent effect in the former.
Table 1 Conditions and some properties of polymers Polymer
I II III IV V a b
Feed ratio
Elemental analysis, %
DFBP
HQ
MeHQ
C
H
0.5 0.5 0.5 0.5 0.5
– 0.25 0.375 0.125 0.5
0.5 0.25 0.125 0.375 –
78.56 (79.47) 78.21 (79.32) 78.31 (79.24) 78.72 (79.39) 78.6 (79.16)
4.52 4.28 4.13 4.41 3.95
(4.63) (4.40) (4.28) (4.5) (4.16)
Inherenta viscosity (dl/g)
Tmb °C
1.029 1.389 1.26 0.92 0.85
230 280 310 240 330
Inherent viscosity in 98% H2SO4 for 0.2% polymer solution at RT (dissolved the samples at 60 °C). Melting point (Tm) by DSC method.
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V.L. Rao et al. / European Polymer Journal 40 (2004) 2645–2651
Fig. 1. 13 CNMR spectra of polymer-I.
Fig. 2. 13 CNMR spectra of polymer-V.
13
CNMR spectra could not be recorded for methyl substituted ether ketone polymers prepared from sulfolane as reaction medium due to their poor solubility. The detailed studies of microstructure for methyl substituted ether ketone copolymers obtained from NMP as reaction medium are under progress.
4. Solubility Ether ether ketone polymers with methyl pendant groups obtained from sulfolane as reaction medium are not soluble in dipolar aprotic solvents and H2SO4 at room temperature, which may be due to its very high
V.L. Rao et al. / European Polymer Journal 40 (2004) 2645–2651 Table 2 Assignments of Carbon
13
CNMR signals of polymers I and V Polymer-I
Polymer-V a
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
148.8 (149.1) 119.9 (114.0)a 122.9 (116.9)a 150.8 (149.9)a 133.0 (126.2)a 123.9 (117.7)a 16.1 167.7 (159.8)a 116.9 (117.0)a 138.6 (129.8)a 123.6(130.9)a 199.1 (199.8)a 123.5 (130.9)a 138.4 (129.8)a 117.5 (117.0)a 167.6 (159.8)a a
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199.3 123.7 138.5 117.6 167.7 150.8 122.7 – – – – – – – – –
(199.8)a (130.9)a (129.8)a (117)a (159.8)a (149.2)a (117.0)a
Calculated value.
molecular weight. Polymers obtained from NMP as reaction medium are soluble in concentrated H2SO4 at room temperature and this solubility increases with increase in methyl content in feed. 5. Thermal transition The Tgs of the copolymers were determined by DSC and the results are given in Table 3. Samples were heated up to 350 °C at the rate of 10 °C min1 in N2, quenched to 50 °C and rerun to obtain Tg. The Tg values of the copolymers increases with increase in concentration of MeHQ in the polymer, however, the increase is marginal. This gradual increase in Tg may be due to the gradual increase in concentration of methyl pendant groups which hinders the free rotation of aromatic ring. X-ray diffraction data (Table 3, Fig. 3) reveal that % crystallinity decreases with increase in MeHQ concentration in the copolymers. Similarly, melt crystallisation temperature of the copolymers decreases with increase in the concentration of MeHQ, which further confirms the tendency towards the amorphous nature. 6. Thermogravimetric analysis Thermal degradation behaviour of polymers I–V was studied by thermogravimetry (TG) in an N2 atmosphere at a heating rate of 20 K min1. From TG trace, the initial decomposition temperature (IDT) and the char residue at 500, 700 and 900 °C were determined and values are given in Table 4. All polymers are stable upto 430 °C and gave a char residue of 49–60% at 900 °C. The relative thermal stabilities of the copolymers can be assessed by their integral procedural decomposition temperature
Fig. 3. X-ray analysis of polymers I–V. Table 3 Glass transition temperature Tg and percentage of crystallinity of the polymers Polymer
% Crystallinity
Tg, °C
I II III IV V
2 10 14 8 47
154 152 150 152 145
(IPDT) proposed by Doyle [13]. IPDT values indicate that the thermal stability of the polymers increases with increase in HQ units as expected. The overall activation energy (Ea) for decomposition was calculated using the Coats and Redfern equation [14] assuming the order of the reaction is 1. ln½ lnð1 aÞ=T 2 ¼ ln½AR=Uð1 2RT =Ea ÞEa =RT where a is the fraction decomposed at temperature T, U is the heating rate, R is the gas constant, and A is the Arrhenius frequency factor. Ea was calculated from the plot of ln[ ln(1 a)/T2] versus 1/T. The results obtained are given in Table 5 and indicate that the activation energy increases with increase in the MeHQ units in the copolymer. The Coats–Redfern plot indicates a two
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V.L. Rao et al. / European Polymer Journal 40 (2004) 2645–2651
value is 179 kJ mol1 for polymer-V which contains only hydroquinone units. A plausible explanation for this behaviour is that the initial cleavage of the B-CH3 bond results in the modification of the polymer backbone as indicated in Fig. 4. Thus the initial cleavage of the B-CH3 bond may result in the formation of cross-linked structures. The DEa values (DEa = 60 ± 15 kJ mol1) for the second stage of decomposition suggests that it is a physical process which involves the volatilisation of the products formed during the first stage of decomposition.
Table 4 Thermal analysis of polymers I–V Polymer
IDT, °C
Char residue, % 500 °C
700 °C
900 °C
I II III IV V
430 460 490 440 550
83 85 93 88 96
52 53 55 64 62
49 50 50 60 58
IPDT
620 629 641 651 657
IDT = initial degradation temperature. IPDT = integral procedural decomposition temperature.
7. Conclusions
Table 5 Kinetic parameters of polymers I–V Polymer
T (°C)
/-Range
DEa (kJ mol1)
Correlation coefficient
I
405 485 500 675
0.002–0.333 0.401–0.944
256.5 46.1
0.9865 0.9901
II
400–485 490–675
0.0004–0.191 0.225–0.977
321.1 72.1
0.9991 0.9982
III
405–495 500–690
0.0014–0.0013 0.0012–0.9890
252.1 94.8
0.9972 0.9967
IV
405–495 505–695
0.0014–0.2828 0.656–0.9905
237.0 62.5
0.9816 0.9936
V
470–560 590 780
0.007–0.485 0.628–0.981
179.7 32.2
0.9896 0.9946
The important conclusions that can be drawn from the present study are as follows: 1. High molecular weight polyether ether ketone polymers with pendant methyl groups can be synthesised by nucleophilic displacement reaction of DFBP with HQ and/or MeHQ in sulfolane medium in a shorter reaction time (< 1 h). 2. Incorporation of MeHQ into the copolymers increases the glass transition temperature. 3. Incorporation of MeHQ into the copolymers decreases the percentage crystallinity and melt crystallisation temperatures which confirms the tendency towards the amorphous nature. 4. The overall activation energy for thermal decomposition of MeHQ based polymerI is 256 kJ mol1 and is much higher than that of HQ based polymer-V. This was attributed to the initial cleavage of /-CH3 bond result in the formation of cross-linked structures.
stage decomposition. The overall activation energy (DEa) for polymer-I, which contains only methyl hydroquinone units is 249 kJ mol1 whereas corresponding
CH 3 O
.
O
.
C
O O
C
CH 3
.
O O
C
O O
C O
O
Fig. 4. Mode of decomposition of polymer I.
C
V.L. Rao et al. / European Polymer Journal 40 (2004) 2645–2651
Acknowledgments The authors thank the authorities of Vikram Sarabhai Space Centre for giving the permission to publish the article. P.U. Sabeena and Akanksha Saxena thank the Indian Space Research Organisation for awarding a Research Fellowship. Thanks are also due to the colleagues of Analytical and Spectroscopy Division of VSSC and Sree Chitra Thirunal Institute for providing analytical support. References [1] Rose JB. NATO ASI Ser SerC 1987;215:207. [2] Hergenrother PM, Jensen BJ, Havens SJ. Polymer 1988;29:358. [3] Gardner KH, Hsiao BS, Matheson RR, Wood AA. Polymer 1992;33:2483.
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[4] Kricheldorf HR, Delius U, Tonnes KU. New Polym Mater 1988;1:127. [5] Kricheldorf HR, Bier G. Polymer 1984;25:1151. [6] Risse W, Sogah DY. Macromolecules 1990;23:4029. [7] Ueda M, Sugiyama J, Yokota H. Polym Prep Jpn 1992;41:362. [8] Taguchi Y, Uyama H, Kobayashi S. J Polym Sci Part A: Polym Chem Ed 1996;34:561. [9] Rao VL, Sivadasan P. Euro Polym J 1994;30:1381. [10] Rao MR, Rao VL, Radhakrishnan TS, Ramachandran S. Polymer 1992;33:2334. [11] Pretsch E, Buhlmann P, Affolter C. Structure determination of organic compounds. 3rd ed. Berlin: SpringerVerlag; 2000. [12] Wang J, Roovers J, Toporwski PM. Macromolecules 1993;26:3826. [13] Doyle CD. Analyt Chem 1961;33:77. [14] Einhorn I. Thermal degradation and flammability characteristics of polymeric materials. In: Polymer Conference Series, University of Utah; June 15 1970.
EUROPEAN POLYMER JOURNAL www.elsevier.com/locate/europolj
Synthesis and properties of poly(aryl ether ether ketone) copolymers with pendant methyl groups V.L. Rao
b
a,*
, P.U. Sabeena a, Akanksha Saxena a, C. Gopalakrishnan b, K. Krishnan b, P.V. Ravindran b, K.N. Ninan
a Propellants and Special Chemicals Group, Polymers and Special Chemicals Division, Propellants, Chemicals and Materials Entity, Vikram Sarabhai Space Centre, Thiruvananthapuram, Kerala 695 022, India Analytical and Spectroscopy Division, Propellants and Special Chemicals Group, Polymers and Special Chemicals Division, Propellants, Chemicals and Materials Entity, Vikram Sarabhai Space Centre, Thiruvananthapuram, Kerala 695 022, India
Received 6 February 2003; accepted 1 July 2004 Available online 9 September 2004
Abstract Polyether ether ketone and polyether ether ketone copolymers were prepared by the nucleophilic substitution reaction of 4,4 0 -difluorobenzophenone with hydroquinone and with varying mole proportions of hydroquinone and methyl hydroquinone using sulfolane solvent in the presence of anhydrous K2CO3. The polymers were characterised by different physico-chemical techniques. The crystallinity of the polymers was found to decrease with increase in concentration of the methyl hydroquinone units in the polymer. Thermogravimetric studies showed that all the polymers were stable upto 430 °C with a char yield above 49% at 900 °C in N2 atmosphere. The glass transition temperature was found to increase and the crystalline melting temperature and activation energy were found to decrease with increase in concentration of the methyl hydroquinone units in the polymer. Ó 2004 Published by Elsevier Ltd. Keywords: Activation energy; Wide angle X-ray diffraction; Thermogravimetry; PEEK copolymer; Pendant methyl group; Differential scanning calorimetry
1. Introduction Poly(aryl ether ketone)s (PAEK)S are gaining rapid acceptance as an advanced thermoplastic matrix resins for high performance composites for aerospace structural applications. They possess excellent environmental resistance, good mechanical properties, chemical and *
Corresponding author. Tel.: +91 471 564298/564225; fax: +91 471 415236. E-mail addresses: rao_vl@rediffmail.com, rao_vl@yahoo. co.in (V.L. Rao). 0014-3057/$ - see front matter Ó 2004 Published by Elsevier Ltd. doi:10.1016/j.eurpolymj.2004.07.002
radiation resistance and thermo-oxidative stability. They are semicrystalline polymers having melting temperatures around 340–360 °C and glass transition temperatures around 140–160 °C. Among them poly(ether ether ketone) (PEEK) and polyether ketone (PEK) have achieved significant commercial importance as matrices for carbon fibre composites in aerospace, industrial and automotive applications and as dielectrics in the micro electronics field. However, due to their high crystalline property and low solubility towards organic solvents often restrict the use of these resins for some applications.
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V.L. Rao et al. / European Polymer Journal 40 (2004) 2645–2651
hydroquinone (SRL), methyl hydroquinone (Lancaster) were used as received. Sulfolane (Aldrich) was dried with molecular sieves (type 4A) and distilled over NaOH pellets under reduced pressure. The middle fractions were collected and stored over molecular sieves.
Various structural changes have been introduced in the basic PEEK resin by several researchers and in our laboratory to obtain different properties and for different applications. Structural changes have focussed on the order and ratio of ether–ketone linkage [1,2], on the ratio of meta and para phenyl substitution [3], and the incorporation of –CR2 and sulfone groups [4,5] in the backbone of the polymeric chain. Introduction of pendant alkyl or phenyl groups on to the poly (aryl ether ether ketone) main chain is another important structural change [6]. Ueda and his co-workers prepared PEK having two alkyl substituents per repeating unit [7]. Taguchi et al. [8] reported PEKs with more than two alkyl substituents per repeat unit. The introduction of the bulky substituents onto the PEK backbone could suppress crystallisation to improve its solubility contrast to non-substituted PEK, leading to novel applications of PEK derivatives. However, it appears that poly(aryl ether ether ketone) copolymers containing pendant methyl groups have not been reported in detail in the open literature. In this paper we report the synthesis of poly aryl ether ether ketone and its copolymers derived from 4,4 0 -difluorobenzophenone (DFBP) with hydroquinone (HQ) and methyl hydroquinone (MeHQ). Their structure– property relationships are also discussed.
2.2. Monomer synthesis 4,4 0 -Difluorobenzophenone (DFBP) was synthesised from fluorobenzene and carbon tetrachloride in the presence of AlCl3 as per the procedure described earlier [9]. Yield 55%; m.p. 105–106 °C; Elemental analysis: calculated for C13H8F2O; C, 71.56; H, 3.67; Found: C, 71.46; H, 3.61. 2.3. Polymer synthesis Poly ether ketone copolymers with methyl pendant groups were prepared as shown in Scheme 1 by nucleophilic aromatic substitution reaction of 4,4 0 -difluorobenzophenone (DFBP) with hydroquinone (HQ) and methyl hydroquinone (MeHQ) with slight excess of anhydrous K2CO3 in high boiling solvent sulfolane at 16% solid content under N2 atmosphere as per the procedure described earlier [10]. The product was refluxed with water repeatedly and subjected to soxhlet extraction with acetone, filtered and dried under vacuum at 100 °C for 15 h.
2. Experimental 2.4. Characterisation 2.1. Materials Elemental analysis was performed with Perkin–Elmer Model 2400 CHN analyser. Inherent viscosities were obtained for 0.2% polymer solution in conc. H2SO4 at room temperature using an ubbelhode suspended level viscometer. Glass transition temperature (Tg) and melting temperatures were measured using a Mettler thermal
Fluorobenzene (Navin fluorine) and carbon tetra chloride (BDH) were distilled before use and kept over anhydrous CaCl2 overnight. Anhydrous AlCl3 (Nice) was used as received. Anhydrous K2CO3 (BDH) was dried in vacuum at 100 °C before use. High purity
O F
C
F +
HO
Ar
OH
SULFOLANE K2CO 3
, TOLUENE
O C
O
CH 3 Ar =
, Scheme 1.
Ar
O
V.L. Rao et al. / European Polymer Journal 40 (2004) 2645–2651
analyser model 3000 coupled to a DSC 20, at a heating rate of 10 K min1. The FT-IR spectra of the polymers in KBr pellets (2%) were recorded using a Nicolet FT-IR (510P) spectrophotometer. 13CNMR spectra were recorded at 75.4 MHz using Brucker Avance-300 spectrometer. Methane sulfonic acid and a CDCl3 mixture was used as a solvent and tetramethyl silane (TMS) as an internal standard. Wide angle X-ray scattering data were obtained on pellets (13 mm diameter) of the polymer using a Philips X-ray unit PW1710 diffraction counting unit and PW1729 X-ray generator with nickel filtered Cu Ka radiation at 30 KV and 20 mA. The diffractograms were recorded at room temperature over the range of 10–40°. Thermogravimetric analysis of the polymers were carried out under nitrogen using a Dupont thermal analyser system 2000 in combination with a Dupont 951 thermogravimetric analyser at a heating rate of 20 K min1.
3. Results and discussion Polyether ether ketone copolymers with methyl pendant groups were synthesised by nucleophilic displacement reaction of fluorine from DFBP by HQ and MeHQ. Feed ratios and some of the properties of the polymers are given in Table 1. Inherent viscosity values reveal that high molecular weight polymers were obtained in a shorter reaction time (<1 h) in sulfolane medium for ether ketone type polymers with methyl pendant groups. The methyl pendant group containing polymers possess higher viscosity than the unsubstituted ones which may be due to electron releasing effect (+I effect) of the methyl group which enhances the nucleophilicity of the phenoxy group of MeHQ. The elemental analysis values, i.e. C and H content of copolymers I–V (Table 1) are in close agreement with theoretical values confirming the proposed structure. The FT-IR spectra of all polymers show no absorption in the 3500–3000 cm1 region indicating the absence of –OH groups. All the spectra show characteristic bands at 1240cm1 (S, mC–O) and 1412 cm1 (methyl
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group), except for polymer V where characteristic absorption for methyl group was absent. All polymers show a characteristic band at 1650cm1 (S, mC@O). The 13CNMR spectra of polymers I and V were recorded at 75.4 MHZ in methane sulfonic acid and CDCl3 with TMS as internal standards. The aromatic region of 13 CNMR spectra of the polymers I and V are shown in Figs. 1 and 2 along with assignments. The assignments of the spectrum are listed in Table 2. The calculated chemical shift values for these carbons are also listed [11]. The 13CNMR spectrum of polymer-I (Fig. 1) shows 16 absorptions and polymer-V (Fig. 2) shows seven absorptions corresponding to the sixteen and seven distinguishable carbons as indicated in the structures respectively. The chemical shift assignments of the various carbons were based on the additivity constants for substituted benzenes. In order to confirm the NMR assignment DEPT experiments also were carried out. Wang and co-workers [12] assigned the 13CNMR resonances of C8, C9, C10, C11, C12, C13, C14, C15 and C16 of polymer-I to d (161.52 and 161.58 ppm) 115.65 ppm, 132.34 ppm, 132.24 ppm, 194.15 ppm, (131.70 and 131.77 ppm), 132.24 ppm, 117 and (161.31 and 161.38 ppm) respectively. The split in resonance of C11 is obscured by overlap with signal of C14. But in our study the corresponding resonance of C8, C10, C12, C14 and C16 are shifted to down field of about 5–6 ppm whereas C11 and C13 are shifted to higher field of about 8 ppm. This may be attributed to the interaction of methane sulfonic acid with the carbonyl group of polymer-I, which is used as one of the solvents. This assignment is in agreement with the observations made in the case of PEEK Table 2. Another interesting feature is that Wang and coworkers observed a splitting in polymer-I for resonances of C8 (161.59 and 161.52 ppm), C11 (132.24 ppm), C13 (131.77 and 131.70 ppm) and C16 (161.38 and 161.31 ppm) whereas in our study no splitting was observed for the resonances of C8 (167.7 ppm), C11 (123.6 ppm), C13 (123.5 ppm) and C16 (167.6 ppm) which may be due to the higher magnetic field (400 MHZ) used for the analysis or due to the solvent effect in the former.
Table 1 Conditions and some properties of polymers Polymer
I II III IV V a b
Feed ratio
Elemental analysis, %
DFBP
HQ
MeHQ
C
H
0.5 0.5 0.5 0.5 0.5
– 0.25 0.375 0.125 0.5
0.5 0.25 0.125 0.375 –
78.56 (79.47) 78.21 (79.32) 78.31 (79.24) 78.72 (79.39) 78.6 (79.16)
4.52 4.28 4.13 4.41 3.95
(4.63) (4.40) (4.28) (4.5) (4.16)
Inherenta viscosity (dl/g)
Tmb °C
1.029 1.389 1.26 0.92 0.85
230 280 310 240 330
Inherent viscosity in 98% H2SO4 for 0.2% polymer solution at RT (dissolved the samples at 60 °C). Melting point (Tm) by DSC method.
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V.L. Rao et al. / European Polymer Journal 40 (2004) 2645–2651
Fig. 1. 13 CNMR spectra of polymer-I.
Fig. 2. 13 CNMR spectra of polymer-V.
13
CNMR spectra could not be recorded for methyl substituted ether ketone polymers prepared from sulfolane as reaction medium due to their poor solubility. The detailed studies of microstructure for methyl substituted ether ketone copolymers obtained from NMP as reaction medium are under progress.
4. Solubility Ether ether ketone polymers with methyl pendant groups obtained from sulfolane as reaction medium are not soluble in dipolar aprotic solvents and H2SO4 at room temperature, which may be due to its very high
V.L. Rao et al. / European Polymer Journal 40 (2004) 2645–2651 Table 2 Assignments of Carbon
13
CNMR signals of polymers I and V Polymer-I
Polymer-V a
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
148.8 (149.1) 119.9 (114.0)a 122.9 (116.9)a 150.8 (149.9)a 133.0 (126.2)a 123.9 (117.7)a 16.1 167.7 (159.8)a 116.9 (117.0)a 138.6 (129.8)a 123.6(130.9)a 199.1 (199.8)a 123.5 (130.9)a 138.4 (129.8)a 117.5 (117.0)a 167.6 (159.8)a a
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199.3 123.7 138.5 117.6 167.7 150.8 122.7 – – – – – – – – –
(199.8)a (130.9)a (129.8)a (117)a (159.8)a (149.2)a (117.0)a
Calculated value.
molecular weight. Polymers obtained from NMP as reaction medium are soluble in concentrated H2SO4 at room temperature and this solubility increases with increase in methyl content in feed. 5. Thermal transition The Tgs of the copolymers were determined by DSC and the results are given in Table 3. Samples were heated up to 350 °C at the rate of 10 °C min1 in N2, quenched to 50 °C and rerun to obtain Tg. The Tg values of the copolymers increases with increase in concentration of MeHQ in the polymer, however, the increase is marginal. This gradual increase in Tg may be due to the gradual increase in concentration of methyl pendant groups which hinders the free rotation of aromatic ring. X-ray diffraction data (Table 3, Fig. 3) reveal that % crystallinity decreases with increase in MeHQ concentration in the copolymers. Similarly, melt crystallisation temperature of the copolymers decreases with increase in the concentration of MeHQ, which further confirms the tendency towards the amorphous nature. 6. Thermogravimetric analysis Thermal degradation behaviour of polymers I–V was studied by thermogravimetry (TG) in an N2 atmosphere at a heating rate of 20 K min1. From TG trace, the initial decomposition temperature (IDT) and the char residue at 500, 700 and 900 °C were determined and values are given in Table 4. All polymers are stable upto 430 °C and gave a char residue of 49–60% at 900 °C. The relative thermal stabilities of the copolymers can be assessed by their integral procedural decomposition temperature
Fig. 3. X-ray analysis of polymers I–V. Table 3 Glass transition temperature Tg and percentage of crystallinity of the polymers Polymer
% Crystallinity
Tg, °C
I II III IV V
2 10 14 8 47
154 152 150 152 145
(IPDT) proposed by Doyle [13]. IPDT values indicate that the thermal stability of the polymers increases with increase in HQ units as expected. The overall activation energy (Ea) for decomposition was calculated using the Coats and Redfern equation [14] assuming the order of the reaction is 1. ln½ lnð1 aÞ=T 2 ¼ ln½AR=Uð1 2RT =Ea ÞEa =RT where a is the fraction decomposed at temperature T, U is the heating rate, R is the gas constant, and A is the Arrhenius frequency factor. Ea was calculated from the plot of ln[ ln(1 a)/T2] versus 1/T. The results obtained are given in Table 5 and indicate that the activation energy increases with increase in the MeHQ units in the copolymer. The Coats–Redfern plot indicates a two
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V.L. Rao et al. / European Polymer Journal 40 (2004) 2645–2651
value is 179 kJ mol1 for polymer-V which contains only hydroquinone units. A plausible explanation for this behaviour is that the initial cleavage of the B-CH3 bond results in the modification of the polymer backbone as indicated in Fig. 4. Thus the initial cleavage of the B-CH3 bond may result in the formation of cross-linked structures. The DEa values (DEa = 60 ± 15 kJ mol1) for the second stage of decomposition suggests that it is a physical process which involves the volatilisation of the products formed during the first stage of decomposition.
Table 4 Thermal analysis of polymers I–V Polymer
IDT, °C
Char residue, % 500 °C
700 °C
900 °C
I II III IV V
430 460 490 440 550
83 85 93 88 96
52 53 55 64 62
49 50 50 60 58
IPDT
620 629 641 651 657
IDT = initial degradation temperature. IPDT = integral procedural decomposition temperature.
7. Conclusions
Table 5 Kinetic parameters of polymers I–V Polymer
T (°C)
/-Range
DEa (kJ mol1)
Correlation coefficient
I
405 485 500 675
0.002–0.333 0.401–0.944
256.5 46.1
0.9865 0.9901
II
400–485 490–675
0.0004–0.191 0.225–0.977
321.1 72.1
0.9991 0.9982
III
405–495 500–690
0.0014–0.0013 0.0012–0.9890
252.1 94.8
0.9972 0.9967
IV
405–495 505–695
0.0014–0.2828 0.656–0.9905
237.0 62.5
0.9816 0.9936
V
470–560 590 780
0.007–0.485 0.628–0.981
179.7 32.2
0.9896 0.9946
The important conclusions that can be drawn from the present study are as follows: 1. High molecular weight polyether ether ketone polymers with pendant methyl groups can be synthesised by nucleophilic displacement reaction of DFBP with HQ and/or MeHQ in sulfolane medium in a shorter reaction time (< 1 h). 2. Incorporation of MeHQ into the copolymers increases the glass transition temperature. 3. Incorporation of MeHQ into the copolymers decreases the percentage crystallinity and melt crystallisation temperatures which confirms the tendency towards the amorphous nature. 4. The overall activation energy for thermal decomposition of MeHQ based polymerI is 256 kJ mol1 and is much higher than that of HQ based polymer-V. This was attributed to the initial cleavage of /-CH3 bond result in the formation of cross-linked structures.
stage decomposition. The overall activation energy (DEa) for polymer-I, which contains only methyl hydroquinone units is 249 kJ mol1 whereas corresponding
CH 3 O
.
O
.
C
O O
C
CH 3
.
O O
C
O O
C O
O
Fig. 4. Mode of decomposition of polymer I.
C
V.L. Rao et al. / European Polymer Journal 40 (2004) 2645–2651
Acknowledgments The authors thank the authorities of Vikram Sarabhai Space Centre for giving the permission to publish the article. P.U. Sabeena and Akanksha Saxena thank the Indian Space Research Organisation for awarding a Research Fellowship. Thanks are also due to the colleagues of Analytical and Spectroscopy Division of VSSC and Sree Chitra Thirunal Institute for providing analytical support. References [1] Rose JB. NATO ASI Ser SerC 1987;215:207. [2] Hergenrother PM, Jensen BJ, Havens SJ. Polymer 1988;29:358. [3] Gardner KH, Hsiao BS, Matheson RR, Wood AA. Polymer 1992;33:2483.
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[4] Kricheldorf HR, Delius U, Tonnes KU. New Polym Mater 1988;1:127. [5] Kricheldorf HR, Bier G. Polymer 1984;25:1151. [6] Risse W, Sogah DY. Macromolecules 1990;23:4029. [7] Ueda M, Sugiyama J, Yokota H. Polym Prep Jpn 1992;41:362. [8] Taguchi Y, Uyama H, Kobayashi S. J Polym Sci Part A: Polym Chem Ed 1996;34:561. [9] Rao VL, Sivadasan P. Euro Polym J 1994;30:1381. [10] Rao MR, Rao VL, Radhakrishnan TS, Ramachandran S. Polymer 1992;33:2334. [11] Pretsch E, Buhlmann P, Affolter C. Structure determination of organic compounds. 3rd ed. Berlin: SpringerVerlag; 2000. [12] Wang J, Roovers J, Toporwski PM. Macromolecules 1993;26:3826. [13] Doyle CD. Analyt Chem 1961;33:77. [14] Einhorn I. Thermal degradation and flammability characteristics of polymeric materials. In: Polymer Conference Series, University of Utah; June 15 1970.