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Aromatic-aliphatic copolyesters—II. Various polycondensation processes
European Polymer Journol Vol. 17. pp. 275 to 279
0014-3057~81 030275-05502.000
© Pergamon Press Lid 1981 Printed in Great Brilain
AROMATIC-ALIPHATIC COPOLYESTERS--II VARIOUS POLYCONDENSATION PROCESSES P. BMAJ and D. N. KHANNA Fiber Science Laboratories, Indian Institute of Technology, Delhi, New Delhi 110016, India (Received 22 July 1980)
Abstract--Aromatic aliphatic copolyesters, using hydroquinone, resorcinol, 4A'-dihydroxybiphenyl (DHBP) 2,2 bis(4-hydroxyphenyl)propane and 4,4'-dihydroxydiphenyl sulphone (DHDPS) as bispbenols and ethylene glycol as diol, have been synthesized by interfacial, low temperature and high temperature solution condensation. Relative reactivities of these bisphenols and ethylene glycol have been evaluated by various polycondensation methods at a fixed ratio of bisphenol/glycol. Decrease in the extent of polymerization and viscosity was observed by incorporation of aliphatic diol. Viscosity was also influenced by the chemical structure of the bisphenol.
INTRODUCTION Aromatic polyesters have been extensively synthesized by interracial, low temperature and high temperature solution condensation [1-4]. Among aromatic polyesters, the polycarbonate of 2,2-bis(4 hydroxyphenol)propane has been commercially developed because of its attractive properties. Aromatic polycarbonates are well known in the form of plastics and coatings having outstanding properties such as high impact strength, high electrical resistance and acceptable hydrolytic and thermal stability. Their high solubility in many organic solvents is an advantage in processing but a disadvantage in many end uses. The wholly aromatic polyesters are similar to the polycarbonates in general properties [5] but commercial aspects are less clear. Recently Zedlinski et al. [6,7] have synthesized aromatic copolyesters with naphthalene rings in the main chain and studied the reactivity as well as sequence distribution of comonomers in the copolymer chain. However, the synthesis of polyesters with glycols and diacid chlorides has received little attention and only a few patents [8, 9] are available. Entelis et aL [10] attempted to study the kinetics of polycondensation of ethylene glycol with terephthaloyl chloride; little further has been reported. Aromatic polyesters present problems in processing due to their high melting points and high glass transition temperatures by virtue of their rigid structures. Some of the aromatic polyesters of hydroquinone and 4,4'-dihydroxybiphenyl are even insoluble and infusible. To improve the processability, an attempt has been made to synthesize copolyesters by incorporation of aliphatic diol in the aromatic polyester backbone, so decreasing the softening temperature and increasing the solubility of polyester. The present investigation is an extension of earlier work [11]. Its purpose is to find a suitable method for the synthesis of aromati~aliphatic copolyesters. EXPERIMENTAL
Purification of monomers Bisphenols, ethylene glycol, terephthaloyl chloride were purified as reporter earlier [11]. Melting points of other
reactants were as indicated: bisphenol-A-lm.p. 158~), resorcinol (110'5, hydroquinone (169°i, 4,4'-dihydroxydiphenyl sulphone (250~), 4,4'-dihydroxybiphenyl (280 I- Toluene, 1,2 dichloroethane and o-dichlorobenzene were dried and distilled before use. Triethylamine and pyridine were dried over dry KOH pellets before distillation (pyridine b.p. 115°/760 mmHg; triethylamine--b.p. 88°/760 mmHg). Polymerization Interracial polycondensation--the method was reported previously [11]. Low temperature solution condensation 2.28 g (0.01 mol) of bisphenol-A and 2.8 ml of triethylamine were added to 50ml of 1,2 dichloroethane. The mixture was cooled to 10°; 2.03 g (0.01 moll of terephthaloyl chloride in 30 ml dichloroethane was added. The reaction mixture was swirled. The triethylamine hydrochloride precipitated in gelatinous form while the polymer dissolved. The solution slowly became more viscous; after 90min, it was poured into acetone. The polymer was washed with water; finally traces of triethylamine hydrochloride were removed by soxhlation for 12 hr in methanol. High temperature solution condensation In a flask with a stirrer, N z inlet tube, HCI outlet tube and condenser, was placed 2.28 g (0.01 mol) of bisphenol-A and 25 ml of o-dichlorobenzene. This mixture was heated to 210 ° under N2 flow and a solution of 2.03 g terephthaloyl chloride in 25 ml of o-dichlorobenzene was added dropwise. The mixture was heated for about 6 hr until evolution of HCI ceased; some solvent vapour was carried out with N 2, assisting in flushing out the HCI. The polymer was precipitated in n-hexane and subsequently washed with alcohol, 50/50 alcohol/water and water. By this method, pale yellow polymers were produced: in the low temperature method, no colour developed. Characterization The softening points of the samples were determined by the capillary method at a heating rate of 5"~/min. Intrinsic viscosities were determined using an Ubbelohde viscometer using sym. tetrachloroethane/phenol (40/60 by weight). The number-average molecular weights (M,) were determined from end-group analysis as reported earlier [I 1]. The i.r. spectra of copolyesters were recorded on SP-1200 spectrophotometer using discs with dried KBr. The chemical compositions were determined by the method reported earlier [ll].
275
276
P. BAJAJand D. N. KHANNA Table 1. Terephthalate copolyesters with resorcinol and hydroquinone made by interfacial polycondensation Bisphenol/ethylene glycol (EG) (mol)
Polymer code
Yield (%)
Softening temperature (°C)
Acid Hydroxyl no. no. (mgKOH/g) (mgKOH/g)
[r/] (dig-l)
Mn
Resorcinol 1.0
RT
80
280
0.92
3.8
8.4
9196
Res/EG
REIT
76
270
0.90
4.0
8.65
8870
Res/EG 0.7/0.3
RE2T
70
265
0.74
5.05
13.40
6081
Res/EG
REaT
58
260
0.40
9.2
16.0
4452
Res/EG 0.2/0.8
RE4T
50
255
Partially soluble
--
--
--
EG (1.0)
ET
15
280
Insoluble
--
--
--
Hydroquinone 1.0
HT
92
> 360
Insoluble
--
--
--
Hyd./EG
HE1T
62
> 360
Insoluble
--
--
--
0.8/0.2
0.5/0.5
0.5/0.5
RESULTS AND DISCUSSION
lnterfacial polycondensation Comparing the data in Tables 1 and 2 and as reported earfier [11], the structure of bisphenol appears to exert a considerable influence on the extent of reaction as well as on the viscosity of the polymer. In identical reaction conditions for interfacial polycondensation, polyesters with various bisphenols and terephthaloyl chloride show differences in viscosity and percentage conversion, perhaps due to differences in reactivities of the bisphenols and also to variation in hydrodynamic volume of the polymer. Polyesters synthesized from bisphenol-A and resorcinol were obtained in high yield and exhibited high viscosity whereas polyesters from hydroquinone and 4,4'-dihydroxybiphenyl were obtained with high yield but were insoluble. On the basis of yield, these poly-
esters are expected to have high molecular weight, following Carother's equation D P = 1/(1 - p) where p = extent of reaction. As shown in Table 2, polyesters with comparatively low viscosities and low yield were obtained from 4,4'-dihydroxydiphenyl sulphone. The decrease in viscosity and yield in comparison with polyesters obtained from bisphenol-A may be attributed to the presence of a negative group, such as sulphone, which does not enter into the polycondensation as readily as the unsubstituted or positively substituted bisphenols. In fact, this electron withdrawing group O
II
(--s--) O
Table 2. Terephthalate copolyesters with DHBP and DHDPS made by interfacial polycondensation Bisphenol/EG (tool)
Polymer code
Yield (%)
Softening It/] temperature (dl g- 1) (°C)
4,4'-Dihydroxydiphenyl sulfone (DHDPS)
DHDPST
78.0
0.70
> 360
DHDPS/EG 0.8/0.2
DHDPSE~T
76.0
0.54
>360
DHDPS/EG 0.7/3.0
DHDPSE2T
70.0
0.37
> 360
DHDPS/EG
DHDPSE3T
55.0
0.33
> 360
DHBPT
92.0
Insoluble
> 36O
DHBPEIT
59.0
Insoluble
>360
0.5/0.5 4,4'Dihydroxybiphenyl DHBP DHBP/EG
0.5/0.5
Aromatic-aliphatic copolyesters
277
II
Table 3. Terephthalate copolyesters made by low temperature and high-temperature solution condensation Bisphenol/ethylene glycol (EG) (mol)
Polymer code
Acid acceptor
Softening Yield [~/] temperature (~,) (dl g- 1) (°C)
Polymerization process
Bisphenol-A tl.O) Bisphenol-A (1.0) Bisphenol-A/EG 0.5/0.5 EG 1.0
BT
Pyridine
Low temperature
75
0.20
> 360
BT
Triethylamine Low temperature
88
0.54
>360
BEaT
Triethylamine Low temperature
68
0.32
>360
PET
Triethylamine Low temperature
15
Partially soluble
Bisphenol-A (1.0) Bisphenol/EG 0.5./0.5 EG 1.0
BT
Nil
High temperature
78
0.34
>360
BE3T
Nil
High temperature
73
0.22
340
PET
Nil
High temperature
24
Partially soluble
220
increases the acidity of bisphenol and thereby decreases the basic strength of phenoxide ion, which is a reactive species in interfacial polycondensation. Similar observation has been made by Eareckson [12]. From these results, the reactivities of various bisphenols are: bisphenol-A ~ 4,4'dihydroxybiphenyl (DHBP) > hydroquinone > resorcinol > 4,4'dihydroxydiphenylsulphone (DHDPS). As reported earlier [11], by incorporation of an aliphatic moiety in the aromatic polyester chain, the viscosity, yield and softening temperature decreases gradually; the final properties are composition dependent.
Low temperature solution condensation In order to determine the reactivity of bisphenol/ glycol, efforts were directed towards the polycondensation of bisphenol-A/EG with terephthaloyl chloride by the low temperature solution method. The viscosity and yield of copolyesters (with identical feed ratio, as in interfacial polycondensation) were comparatively low (Table 3). The low yield by this method may be attributed to the remarkable difference in the reactivities of bisphenol and glycol. The glycol was found to be less reactive with diacid chloride at low
260
temperature. Secondly, the formation of HC1 hinders the polycondensation. In the present studies, pyridine and triethylamine were used as acid acceptors. It was observed that bisphenol-terephthalate polyester with viscosity 0.20 dig-1 was obtained in the presence of pyridine as an acid acceptor while, in the presence of triethylamine, the viscosity increased to 0.54 dl g- 1. The difference between these viscosities may be attributed to the efficiency, in terms of the basicity of acid acceptor used. The use of tertiary amines with pKb > 7 such as pyridine (pKb = 7.8) led to polyesters with low viscosities; while triethylamine with pKb = 3.1 gave polyesters with high intrinsic viscosity due to its highly basic character. Work reported by Sek [13] supports our observation.
Hiyh temperature solution condensation In order to compare the reactivities of bisphenols and glycol, copolyesters were also synthesized by high temperature solution condensation (see Table 4). 24~o yield was obtained for ethylene glycol with terephthaloyl chloride in comparison to 11 and 15~ yields obtained by low temperature and by interfacial polycondensation respectively. As the high temperature solution condensation involves direct reaction of gly-
Table 4. Comparison of various polycondenssation processes
Bisphenol/EG (tool)
Monomer feed ethylene glycol ~o (EG)
Composition of copolymer Polymerization (EG content) Yiled process (~,,,) (%)
[q] (dl g- 1)
Bisphenol/EG 0.5/0.5 Bisph/EG 0.5/0.5 Bisph/EG 0.5/0.5
50
Interfacial
15.2
80
0.34
50
Low temperature
11.0
68
0.32
50
High temperature
29.0
82
0.22
DHDPS/EG 0.5/0.5
50
Interfacial
20.0
55
0.33
278
P. BAJAJand D. N. KHANNA
/
2(X~O
t,500
17'00
II O0
I000
/~-(a)
900
800
Wavenumber , c m - I
Fig. 1. Infrared spectra of PET. (a) Synthesized by melt condensation. (b) Synthesized by interfacial polycondensation. colic or phenolic---q)H group with diacid chloride with the evolution of HCI, the reactivity of the glycolic ----OH is enhanced and copolyesters in better yield are obtained; the temperature was limited to 210 ° in the ethylene glycol reaction with terephthaloyl chloride, due to the possibility of degradation at higher temperature. Since at high temperature most of the polycondensation reactions are reversible, comparatively low viscosity products are obtained by this method.
Comparative study of interfacial, low temperature and high temperature solution condensation For comparative study of these processes, composition was determined for identical feed ratio (bisphenol/EG, 50/50) by i.r. spectrocsopy; the results are tabulated in Table 4. Due to the poor reactivity of alkoxide ion in comparison to phenoxide ion, the incorporation of afiphatic diol is always less than for bisphenol, as discussed earlier [11-1. In the low temperature solution condensation method, poor reactivity of glycol was again observed in comparison to interfacial polycondensation. This may be because of the HCI evolved, decreasing the rate of polycondensation so that copolyester with low yield (68~o) is obtained in comparison to 80~o and also low viscosity (0.32dlg -1) in comparison to 0.34 dl g- ] by interfacial polycondensation. The incorporation of aliphatic diol was only 11~ in comparison to 15~o by interfacial polycondensation. However, by high temperature solution condensation for identical feed ratio, the incorporation of a aliphatic diol increased to 29~, indicating the enhancement in the reaction rate of glycol with terephthaloyl chloride at higher temperature. On the other hand, the viscosity greatly decreased, because of reversible reactions at higher temperature.
In polysulphone copolyester with dihydroxydiphenyl sulphone/EG (50/50) in the initial monomer feed, a very interesting feature was observed in chemical composition (Table 4). The incorporation of ethylene glycol moiety was considerably higher than for the copolyetster obtained using bisphenol-A in identical reaction conditions of interfacial polycondensation. The reason for this behaviour could possibly be the low reactivity of the phenoxide ion of polysulphone because of its less basic character (as discussed in an earlier section) in comparison to other phenoxide ion, leading to low incorporation of aromatic component and hence raising its aliphatic ratio.
Effect of bisphenols on softening temperature and solubility of copolyesters Resorcinol terephthalate has a softening temperature 280 ° and is soluble in m-cresol phenol/tetrachloroethane and o-chlorophenol, whereas hydroquinoneterephthalate is infusible and insoluble in any of these solvents. Low softening temperature is attributed to the disturbed symmetry of resorcinoi having meta substitution. Similarly, DHBP-terephthalate is infusible and insoluble in comparison to bisphenolterephthalate and related copolyesters. The better solubility and decrease in softening temperature of bisphenol-terephthalate is due to the presence of the flexible isopropylidene link
CH3
I
(---C--).
I
CHa Replacing the isopropylidene link by the sulphone
Aromatic-aliphatic copolyesters group O
!i {--S--) !l O caused a decrease in solubility and an increase in the softening temperature. Therefore, D H D P S copolyesters softened at > 360 ° even after the incorporation of a flexible aliphatic moiety (Table 2). Effect of polycondensation procedure on the softening temperature and solubility t f polyethylene terephthalate (PET) It is well known that P E T synthesized by melt condensation is soluble in m-cresol, o-chlorophenol and many other solvents, while P E T synthesized by interfacial method was totally insoluble and had a softening temperature 280 °. Similarly PET synthesized by the low temperature method was partially soluble. The insolubility of PET may be correlated with its crystalline nature as indicated by its i.r. spectrum (Fig. l). Comparison of the i.r. spectra of PET (synthesized by melt condensation) and PET (synthesized by interfacial condensation) shows that many of the absorptions are affected. A simple explanation of these i.r. results may be associated with two possible configurations viz. trans in the crystalline material and gauche in the amorphous. In Fig. l(b) one set of bands at
279
II
1818, 1580, 1505, 1200, 870cm -~ (increasing in intensity) can be assigned to trans vibrations of the glycol moiety and the other set (decreasing in intensity) to gauche vibrations. The trans vibration of the glycol moiety are responsible for the high crystallinity of PET. Ward [14] has reported similar observations on amorphous and crystalline PET. REFERENCES
1. E. L. Wittbeker and P. W. Morgan, J. Polym. Sci. 40, 289 (1959). 2. P. W. Morgan and S. L. Kwolek, J. Polym. Sci. 40, 314 t1959). 3. V. V. Korshak, S. V. Vinogradova and M. A. lskenderov, Vysokomolek. Soedin. 4, 345 (1962). 4. P. W. Morgan, J. Polym. Sci. A2, 437 (1964). 5. A. J. Conix, Ind. En(tng Chem. 51, 147 (1959). 6. Z. Zedlinski and D. Sek, J. Polym. Sci., A I 7, 2587 (1969). 7. Z. Zedlinski, D. Sek and B. Dziwiecka, Eur. Polym. J. 13, 871 (1977). 8. S. M. Livengood, U.S. Pat. 2,567,076 (1951). 9. P. J. Flory and F. S. Leutner, U.S. Pat., L589,688 (1952); Chem. Abstr. 46, 5861 (1952). 10. S. G. Entelis, G. P. Kondratova and N. M. Chirkov, Vysokomolek. Soedin. 3, 1045 (1961). 11. P. Bajaj, D. N. Khanna and G. N. Babu, Eur. Polym. J. 15, 1083 (1979). 12. W. M. Eareckson, J. Polym. Sci. 40, 399 (1959). 13. D. Sek, Eur. Polym. d. 13, 967 (1977). 14. I. M. Ward, Text. Res. J. 31,650 (1961).
0014-3057~81 030275-05502.000
© Pergamon Press Lid 1981 Printed in Great Brilain
AROMATIC-ALIPHATIC COPOLYESTERS--II VARIOUS POLYCONDENSATION PROCESSES P. BMAJ and D. N. KHANNA Fiber Science Laboratories, Indian Institute of Technology, Delhi, New Delhi 110016, India (Received 22 July 1980)
Abstract--Aromatic aliphatic copolyesters, using hydroquinone, resorcinol, 4A'-dihydroxybiphenyl (DHBP) 2,2 bis(4-hydroxyphenyl)propane and 4,4'-dihydroxydiphenyl sulphone (DHDPS) as bispbenols and ethylene glycol as diol, have been synthesized by interfacial, low temperature and high temperature solution condensation. Relative reactivities of these bisphenols and ethylene glycol have been evaluated by various polycondensation methods at a fixed ratio of bisphenol/glycol. Decrease in the extent of polymerization and viscosity was observed by incorporation of aliphatic diol. Viscosity was also influenced by the chemical structure of the bisphenol.
INTRODUCTION Aromatic polyesters have been extensively synthesized by interracial, low temperature and high temperature solution condensation [1-4]. Among aromatic polyesters, the polycarbonate of 2,2-bis(4 hydroxyphenol)propane has been commercially developed because of its attractive properties. Aromatic polycarbonates are well known in the form of plastics and coatings having outstanding properties such as high impact strength, high electrical resistance and acceptable hydrolytic and thermal stability. Their high solubility in many organic solvents is an advantage in processing but a disadvantage in many end uses. The wholly aromatic polyesters are similar to the polycarbonates in general properties [5] but commercial aspects are less clear. Recently Zedlinski et al. [6,7] have synthesized aromatic copolyesters with naphthalene rings in the main chain and studied the reactivity as well as sequence distribution of comonomers in the copolymer chain. However, the synthesis of polyesters with glycols and diacid chlorides has received little attention and only a few patents [8, 9] are available. Entelis et aL [10] attempted to study the kinetics of polycondensation of ethylene glycol with terephthaloyl chloride; little further has been reported. Aromatic polyesters present problems in processing due to their high melting points and high glass transition temperatures by virtue of their rigid structures. Some of the aromatic polyesters of hydroquinone and 4,4'-dihydroxybiphenyl are even insoluble and infusible. To improve the processability, an attempt has been made to synthesize copolyesters by incorporation of aliphatic diol in the aromatic polyester backbone, so decreasing the softening temperature and increasing the solubility of polyester. The present investigation is an extension of earlier work [11]. Its purpose is to find a suitable method for the synthesis of aromati~aliphatic copolyesters. EXPERIMENTAL
Purification of monomers Bisphenols, ethylene glycol, terephthaloyl chloride were purified as reporter earlier [11]. Melting points of other
reactants were as indicated: bisphenol-A-lm.p. 158~), resorcinol (110'5, hydroquinone (169°i, 4,4'-dihydroxydiphenyl sulphone (250~), 4,4'-dihydroxybiphenyl (280 I- Toluene, 1,2 dichloroethane and o-dichlorobenzene were dried and distilled before use. Triethylamine and pyridine were dried over dry KOH pellets before distillation (pyridine b.p. 115°/760 mmHg; triethylamine--b.p. 88°/760 mmHg). Polymerization Interracial polycondensation--the method was reported previously [11]. Low temperature solution condensation 2.28 g (0.01 mol) of bisphenol-A and 2.8 ml of triethylamine were added to 50ml of 1,2 dichloroethane. The mixture was cooled to 10°; 2.03 g (0.01 moll of terephthaloyl chloride in 30 ml dichloroethane was added. The reaction mixture was swirled. The triethylamine hydrochloride precipitated in gelatinous form while the polymer dissolved. The solution slowly became more viscous; after 90min, it was poured into acetone. The polymer was washed with water; finally traces of triethylamine hydrochloride were removed by soxhlation for 12 hr in methanol. High temperature solution condensation In a flask with a stirrer, N z inlet tube, HCI outlet tube and condenser, was placed 2.28 g (0.01 mol) of bisphenol-A and 25 ml of o-dichlorobenzene. This mixture was heated to 210 ° under N2 flow and a solution of 2.03 g terephthaloyl chloride in 25 ml of o-dichlorobenzene was added dropwise. The mixture was heated for about 6 hr until evolution of HCI ceased; some solvent vapour was carried out with N 2, assisting in flushing out the HCI. The polymer was precipitated in n-hexane and subsequently washed with alcohol, 50/50 alcohol/water and water. By this method, pale yellow polymers were produced: in the low temperature method, no colour developed. Characterization The softening points of the samples were determined by the capillary method at a heating rate of 5"~/min. Intrinsic viscosities were determined using an Ubbelohde viscometer using sym. tetrachloroethane/phenol (40/60 by weight). The number-average molecular weights (M,) were determined from end-group analysis as reported earlier [I 1]. The i.r. spectra of copolyesters were recorded on SP-1200 spectrophotometer using discs with dried KBr. The chemical compositions were determined by the method reported earlier [ll].
275
276
P. BAJAJand D. N. KHANNA Table 1. Terephthalate copolyesters with resorcinol and hydroquinone made by interfacial polycondensation Bisphenol/ethylene glycol (EG) (mol)
Polymer code
Yield (%)
Softening temperature (°C)
Acid Hydroxyl no. no. (mgKOH/g) (mgKOH/g)
[r/] (dig-l)
Mn
Resorcinol 1.0
RT
80
280
0.92
3.8
8.4
9196
Res/EG
REIT
76
270
0.90
4.0
8.65
8870
Res/EG 0.7/0.3
RE2T
70
265
0.74
5.05
13.40
6081
Res/EG
REaT
58
260
0.40
9.2
16.0
4452
Res/EG 0.2/0.8
RE4T
50
255
Partially soluble
--
--
--
EG (1.0)
ET
15
280
Insoluble
--
--
--
Hydroquinone 1.0
HT
92
> 360
Insoluble
--
--
--
Hyd./EG
HE1T
62
> 360
Insoluble
--
--
--
0.8/0.2
0.5/0.5
0.5/0.5
RESULTS AND DISCUSSION
lnterfacial polycondensation Comparing the data in Tables 1 and 2 and as reported earfier [11], the structure of bisphenol appears to exert a considerable influence on the extent of reaction as well as on the viscosity of the polymer. In identical reaction conditions for interfacial polycondensation, polyesters with various bisphenols and terephthaloyl chloride show differences in viscosity and percentage conversion, perhaps due to differences in reactivities of the bisphenols and also to variation in hydrodynamic volume of the polymer. Polyesters synthesized from bisphenol-A and resorcinol were obtained in high yield and exhibited high viscosity whereas polyesters from hydroquinone and 4,4'-dihydroxybiphenyl were obtained with high yield but were insoluble. On the basis of yield, these poly-
esters are expected to have high molecular weight, following Carother's equation D P = 1/(1 - p) where p = extent of reaction. As shown in Table 2, polyesters with comparatively low viscosities and low yield were obtained from 4,4'-dihydroxydiphenyl sulphone. The decrease in viscosity and yield in comparison with polyesters obtained from bisphenol-A may be attributed to the presence of a negative group, such as sulphone, which does not enter into the polycondensation as readily as the unsubstituted or positively substituted bisphenols. In fact, this electron withdrawing group O
II
(--s--) O
Table 2. Terephthalate copolyesters with DHBP and DHDPS made by interfacial polycondensation Bisphenol/EG (tool)
Polymer code
Yield (%)
Softening It/] temperature (dl g- 1) (°C)
4,4'-Dihydroxydiphenyl sulfone (DHDPS)
DHDPST
78.0
0.70
> 360
DHDPS/EG 0.8/0.2
DHDPSE~T
76.0
0.54
>360
DHDPS/EG 0.7/3.0
DHDPSE2T
70.0
0.37
> 360
DHDPS/EG
DHDPSE3T
55.0
0.33
> 360
DHBPT
92.0
Insoluble
> 36O
DHBPEIT
59.0
Insoluble
>360
0.5/0.5 4,4'Dihydroxybiphenyl DHBP DHBP/EG
0.5/0.5
Aromatic-aliphatic copolyesters
277
II
Table 3. Terephthalate copolyesters made by low temperature and high-temperature solution condensation Bisphenol/ethylene glycol (EG) (mol)
Polymer code
Acid acceptor
Softening Yield [~/] temperature (~,) (dl g- 1) (°C)
Polymerization process
Bisphenol-A tl.O) Bisphenol-A (1.0) Bisphenol-A/EG 0.5/0.5 EG 1.0
BT
Pyridine
Low temperature
75
0.20
> 360
BT
Triethylamine Low temperature
88
0.54
>360
BEaT
Triethylamine Low temperature
68
0.32
>360
PET
Triethylamine Low temperature
15
Partially soluble
Bisphenol-A (1.0) Bisphenol/EG 0.5./0.5 EG 1.0
BT
Nil
High temperature
78
0.34
>360
BE3T
Nil
High temperature
73
0.22
340
PET
Nil
High temperature
24
Partially soluble
220
increases the acidity of bisphenol and thereby decreases the basic strength of phenoxide ion, which is a reactive species in interfacial polycondensation. Similar observation has been made by Eareckson [12]. From these results, the reactivities of various bisphenols are: bisphenol-A ~ 4,4'dihydroxybiphenyl (DHBP) > hydroquinone > resorcinol > 4,4'dihydroxydiphenylsulphone (DHDPS). As reported earlier [11], by incorporation of an aliphatic moiety in the aromatic polyester chain, the viscosity, yield and softening temperature decreases gradually; the final properties are composition dependent.
Low temperature solution condensation In order to determine the reactivity of bisphenol/ glycol, efforts were directed towards the polycondensation of bisphenol-A/EG with terephthaloyl chloride by the low temperature solution method. The viscosity and yield of copolyesters (with identical feed ratio, as in interfacial polycondensation) were comparatively low (Table 3). The low yield by this method may be attributed to the remarkable difference in the reactivities of bisphenol and glycol. The glycol was found to be less reactive with diacid chloride at low
260
temperature. Secondly, the formation of HC1 hinders the polycondensation. In the present studies, pyridine and triethylamine were used as acid acceptors. It was observed that bisphenol-terephthalate polyester with viscosity 0.20 dig-1 was obtained in the presence of pyridine as an acid acceptor while, in the presence of triethylamine, the viscosity increased to 0.54 dl g- 1. The difference between these viscosities may be attributed to the efficiency, in terms of the basicity of acid acceptor used. The use of tertiary amines with pKb > 7 such as pyridine (pKb = 7.8) led to polyesters with low viscosities; while triethylamine with pKb = 3.1 gave polyesters with high intrinsic viscosity due to its highly basic character. Work reported by Sek [13] supports our observation.
Hiyh temperature solution condensation In order to compare the reactivities of bisphenols and glycol, copolyesters were also synthesized by high temperature solution condensation (see Table 4). 24~o yield was obtained for ethylene glycol with terephthaloyl chloride in comparison to 11 and 15~ yields obtained by low temperature and by interfacial polycondensation respectively. As the high temperature solution condensation involves direct reaction of gly-
Table 4. Comparison of various polycondenssation processes
Bisphenol/EG (tool)
Monomer feed ethylene glycol ~o (EG)
Composition of copolymer Polymerization (EG content) Yiled process (~,,,) (%)
[q] (dl g- 1)
Bisphenol/EG 0.5/0.5 Bisph/EG 0.5/0.5 Bisph/EG 0.5/0.5
50
Interfacial
15.2
80
0.34
50
Low temperature
11.0
68
0.32
50
High temperature
29.0
82
0.22
DHDPS/EG 0.5/0.5
50
Interfacial
20.0
55
0.33
278
P. BAJAJand D. N. KHANNA
/
2(X~O
t,500
17'00
II O0
I000
/~-(a)
900
800
Wavenumber , c m - I
Fig. 1. Infrared spectra of PET. (a) Synthesized by melt condensation. (b) Synthesized by interfacial polycondensation. colic or phenolic---q)H group with diacid chloride with the evolution of HCI, the reactivity of the glycolic ----OH is enhanced and copolyesters in better yield are obtained; the temperature was limited to 210 ° in the ethylene glycol reaction with terephthaloyl chloride, due to the possibility of degradation at higher temperature. Since at high temperature most of the polycondensation reactions are reversible, comparatively low viscosity products are obtained by this method.
Comparative study of interfacial, low temperature and high temperature solution condensation For comparative study of these processes, composition was determined for identical feed ratio (bisphenol/EG, 50/50) by i.r. spectrocsopy; the results are tabulated in Table 4. Due to the poor reactivity of alkoxide ion in comparison to phenoxide ion, the incorporation of afiphatic diol is always less than for bisphenol, as discussed earlier [11-1. In the low temperature solution condensation method, poor reactivity of glycol was again observed in comparison to interfacial polycondensation. This may be because of the HCI evolved, decreasing the rate of polycondensation so that copolyester with low yield (68~o) is obtained in comparison to 80~o and also low viscosity (0.32dlg -1) in comparison to 0.34 dl g- ] by interfacial polycondensation. The incorporation of aliphatic diol was only 11~ in comparison to 15~o by interfacial polycondensation. However, by high temperature solution condensation for identical feed ratio, the incorporation of a aliphatic diol increased to 29~, indicating the enhancement in the reaction rate of glycol with terephthaloyl chloride at higher temperature. On the other hand, the viscosity greatly decreased, because of reversible reactions at higher temperature.
In polysulphone copolyester with dihydroxydiphenyl sulphone/EG (50/50) in the initial monomer feed, a very interesting feature was observed in chemical composition (Table 4). The incorporation of ethylene glycol moiety was considerably higher than for the copolyetster obtained using bisphenol-A in identical reaction conditions of interfacial polycondensation. The reason for this behaviour could possibly be the low reactivity of the phenoxide ion of polysulphone because of its less basic character (as discussed in an earlier section) in comparison to other phenoxide ion, leading to low incorporation of aromatic component and hence raising its aliphatic ratio.
Effect of bisphenols on softening temperature and solubility of copolyesters Resorcinol terephthalate has a softening temperature 280 ° and is soluble in m-cresol phenol/tetrachloroethane and o-chlorophenol, whereas hydroquinoneterephthalate is infusible and insoluble in any of these solvents. Low softening temperature is attributed to the disturbed symmetry of resorcinoi having meta substitution. Similarly, DHBP-terephthalate is infusible and insoluble in comparison to bisphenolterephthalate and related copolyesters. The better solubility and decrease in softening temperature of bisphenol-terephthalate is due to the presence of the flexible isopropylidene link
CH3
I
(---C--).
I
CHa Replacing the isopropylidene link by the sulphone
Aromatic-aliphatic copolyesters group O
!i {--S--) !l O caused a decrease in solubility and an increase in the softening temperature. Therefore, D H D P S copolyesters softened at > 360 ° even after the incorporation of a flexible aliphatic moiety (Table 2). Effect of polycondensation procedure on the softening temperature and solubility t f polyethylene terephthalate (PET) It is well known that P E T synthesized by melt condensation is soluble in m-cresol, o-chlorophenol and many other solvents, while P E T synthesized by interfacial method was totally insoluble and had a softening temperature 280 °. Similarly PET synthesized by the low temperature method was partially soluble. The insolubility of PET may be correlated with its crystalline nature as indicated by its i.r. spectrum (Fig. l). Comparison of the i.r. spectra of PET (synthesized by melt condensation) and PET (synthesized by interfacial condensation) shows that many of the absorptions are affected. A simple explanation of these i.r. results may be associated with two possible configurations viz. trans in the crystalline material and gauche in the amorphous. In Fig. l(b) one set of bands at
279
II
1818, 1580, 1505, 1200, 870cm -~ (increasing in intensity) can be assigned to trans vibrations of the glycol moiety and the other set (decreasing in intensity) to gauche vibrations. The trans vibration of the glycol moiety are responsible for the high crystallinity of PET. Ward [14] has reported similar observations on amorphous and crystalline PET. REFERENCES
1. E. L. Wittbeker and P. W. Morgan, J. Polym. Sci. 40, 289 (1959). 2. P. W. Morgan and S. L. Kwolek, J. Polym. Sci. 40, 314 t1959). 3. V. V. Korshak, S. V. Vinogradova and M. A. lskenderov, Vysokomolek. Soedin. 4, 345 (1962). 4. P. W. Morgan, J. Polym. Sci. A2, 437 (1964). 5. A. J. Conix, Ind. En(tng Chem. 51, 147 (1959). 6. Z. Zedlinski and D. Sek, J. Polym. Sci., A I 7, 2587 (1969). 7. Z. Zedlinski, D. Sek and B. Dziwiecka, Eur. Polym. J. 13, 871 (1977). 8. S. M. Livengood, U.S. Pat. 2,567,076 (1951). 9. P. J. Flory and F. S. Leutner, U.S. Pat., L589,688 (1952); Chem. Abstr. 46, 5861 (1952). 10. S. G. Entelis, G. P. Kondratova and N. M. Chirkov, Vysokomolek. Soedin. 3, 1045 (1961). 11. P. Bajaj, D. N. Khanna and G. N. Babu, Eur. Polym. J. 15, 1083 (1979). 12. W. M. Eareckson, J. Polym. Sci. 40, 399 (1959). 13. D. Sek, Eur. Polym. d. 13, 967 (1977). 14. I. M. Ward, Text. Res. J. 31,650 (1961).