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Aromatic polyisophthalamides with iminobenzoyl pendant groups
Eur. Poh'm. J. Vol. 21, No. 12, pp. 1013-1019, 1985
0014-3057'85 $300 + 0.00 Copyright i ' 1985 Pergamon Press Lid
Printed in Great Britain. All rights reserved
AROMATIC POLYISOPHTHALAMIDES WITH IMINOBENZOYL P E N D A N T GROUPS J. G. DE LA CAMPA, E. GUIJARRO, F. J. SERNA and J. DE ABAJO lnstituto de Plfisticos y Caucho, C.S.I.C., Juan de la Cierva 3, 28006 Madrid, Spain
(Received 19 June 1985) Abstract--Polyisophthalamides were prepared from aromatic diamines and 5-iminobenzoylisophthalic acid by the Yamazaki method of direct polyamidation catalyzed by triphenylphosphite. The properties of the polymers were measured and compared with the analogous unsubstituted polyisophthalamides. The incorporation of one iminobenzoyl pendant group per repeating unit gave rise to better solubility in strongly polar solvents. Higher content of amide groups per repeating unit allowed the modified polymers to absorb moisture to a greater extent than the parent polyisophthalamides. The glass transition temperatures were raised 20-30 ~by the presence of the pendant groups and they ranged from 290 to 317. On the contrary, the substituted polymers showed lower initial decomposition temperatures, as measured by TGA, all of them beginning to decompose at about 410 c. The mechanical properties of polymer films seemed not to be greatly affected by the pendant groups and only small differences were observed between substituted and unsubstituted polymers.
INTRODUCTION
The general properties of these polymers have been
A r o m a t i c polyamides are a m o n g the m o d e r n hight e m p e r a t u r e resistant polymers [1, 2]. N u m e r o u s attempts have been to modify their chemical structure in order to change their properties a n d b e h a v i o u r [3]. The i n c o r p o r a t i o n of substituents as side p e n d a n t groups and the i n t r o d u c t i o n of different units along the b a c k b o n e are general a p p r o a c h e s used to change the chemical structure of polymers a n d they have been m u c h used for a r o m a t i c polyamides [4, 5], We have already reported on the synthesis a n d c h a r a c t e r i z a t i o n of modified p o l y i s o p h t h a l a m i d e s
studied and c o m p a r e d with those o f the h o m o l o g o u s u n s u b s t i t u t e d polyamides. Solubility in organic solvents, thermal transitions, thermal resistance and mechanical strength of films have been the main subjects of o u r study. The effect of the substituent on water a b s o r p t i o n was also investigated.
c o n t a i n i n g various substitutents as p e n d a n t groups a n c h o r e d on the 5-position o f the isophthaloyl moieties [6-8]. The i n c o r p o r a t i o n of oxybenzoyl substitients gave rise to evident a d v a n t a g e s in solubility [8] but the thermal resistance decreased the side ester g r o u p showing m u c h lower thermal stability t h a n the main polyamide chain. As an a t t e m p t to improve these previous results, we have n o w p r e p a r e d new p o l y i s o p h t h a l a m i d e s with b e n z a m i d e (iminobenzoyl) p e n d a n t groups. The modified polymers have been synthesized from a new m o n o m e r derived from the isophthalic acid that acted as the bearer of the p e n d a n t groups. O n condensing it with a r o m a t i c diamines, polyamides with the structural formula:
--HN--At--NHOC-'~f~-~"[----CO-NH CO
were obtained,
EXPERIMENTAL The N,N-dimethylacetamide (DMA), ERTISA. was stored on molecular sieves 4,~ and used without further purification. The N-methyl-2-pyrrolydinone (NMP) was purified by distillation twice on P205 and stored in a dark bottle on molecular sieves. Pyridine was refluxed on KOH for 12 hr, distilled and stored on molecular sieves. Triphenyl phosphite was purified by distillation at reduced pressure. The 5-aminoisophthalic acid was a gift of Dupont de Nemours & Co. Inc. and was used as received.
Monomers The diamines were reagent grade obtained from commercial sources and were purified by recrystallization. Only the 2,2-bis(4-aminophenyl)propane had to be prepared from aniline chlorhydrate and acetone by a published method [9], recrystallized from water, m.p. 133 (lit. 132 [9]). Yield 58%. Isophthaloyl chloride was prepared from isophthalic acid and thionyl chloride by the usual method and recrystallized from n-hexane, m.p. 4 2 . The 5-iminobenzoylisophthalic acid was prepared by reaction of 5-aminoisophthalic acid and benzoyl chloride in stoichiometric amounts in solution in DMA at 0'. The product was isolated by precipitation in an excess of water. It was recrystallized from ethanol-water 50:50, m.p. 350 . Yield 90%. Analysis: Cl~HliNOs(285,27 ). Calc.: %C, 63.16: %H, 3.89; %N, 4.91. Found: C, 63.14; %H. 3.79:° N, 4.53. Polymerization Two methods were used to prepare the polymers. The unsubstituted polyisophthalamides were prepared from isophthaloyl chloride and diamines by the low temperature condensation method, in solution in DMA, according to the procedure described elsewhere [8]. The substituted poly-
1013
J . G . DE LA CAMPA et al.
1014
isophthalamides were prepared by the method developed by Yamazaki et al. [10-12] which consists of the direct polycondensation of aromatic diacids and diamines catalyzed with triphenyl phosphite. In a typical example 6.5897 g (23.10mmol) of 5-iminobenzoylisophthalic acid and 4.5805 g (23.10 mmol) of bis(4-aminophenyl)methane were made to react in a mixture of 16 ml of pyridine, 15.5 ml of triphenyl phosphite (50 mmol) and 62 ml of N M P containing 3.l g of LiCl, at 100" for 4 hr and at 120 ° for 2 hr. After cooling, the reaction mixture was poured into water (800 ml), and the precipitated polymer was filtered, washed with water several times, extracted with refiuxing acetone in a Soxhlet for 12hr and dried in vacuum at 60 ° for 24hr. Yield 98%.
in the T G A curve took place was taken as the degradation temperature (To). Just before the runs, absorbed water was eliminated by heating the sample in the thermobalance oven at 120 ° 15 min, and then the recorder pen was again adjusted to 100% initial weight. The glass transition temperatures of polymers were measured by differential scanning calorimetry on a Perkin Elmer DSC-4 thermal analyzer programmed by means of a System 4 microprocessor and connected to a 3600 Data Station. Usually 10-12 mg powdered polymer sampleswere used, and the measurements were made under N 2 at a heating rate of I0 ° min. TMA, a thermomechanical method, was also used in the determination of Tg. The polymer samples, as discs molded under pressure, 600-700/~m thick, were tested on a Perkin-Elmer TMS-2 device by the expansion method under the same conditions as those for DSC.
Characterization o f polymers i.r. Spectra were recorded on a Perkin-Elmer 457 Spectrophotometer using thin polymer films. ~H-NMR spectra were recorded at 60 ° on a Varian EM-390 (90MHz) apparatus using dimethylsulphoxide-d 6 as solvent and TMS as internal reference, The inherent viscosities of all the polymers were determined on 0.5% (w/v) N M P solutions at 25 + 0.1 ° in an Ostwald viscometer, 0.4mm i.d. A Perkin-Elmer TGS-2 thermobalance controlled by a System 4 microprocessor was used for thermogravimetric analysis. Powdered polymer samples (2-3 mg) were tested at 10° rain under N 2. The temperature at which the first onset
Films Polymers films were obtained by evaporation of cast solutions in DMF, D M A and NMP or mixtures of them. Usually a concentration of 10% in weight of polymer was used, and the solvent was eliminated by heating at 120 ° for 2 hr in an air-circulating oven and then at 120 140 ° for 3~4 hr under vacuum (1 mmHg). For determination of tensile properties of polymers, strips (15 mm width and 80 mm length) were cut from polymer films and tested on an I N S T R O N dynamometer at 20 ~'.
Table 1. Aromatic polyisophthalamides
--HN --Ar --NHOC~ ~ -
CO--
R Solubility Code
AR
R
Flinh
(dl/g)
DMF
Cresol CH
I
~
H
1.12
++
II
~
H
1.10
-
-
H
1.55
++
-
-
H
1.06
++
++
+
H
1.36
++
-
"~O~]----
-- NHOC~
0.76
++
+
+_
-
~
- - NHOC~ " ~
1.13
+
-
-
-
0.65
++
+
_+
-
lIl
CHz-~~
@
IV @ C ( C H 3 ) V
@
O
VI VII
2~ @
-
TFA
-
-
++ +
1
X
-
~
~
O
- ~ ~
-- NHOC-~
CH = cyclohexanone; D M F - N,N-dimethylformamide; TFA = trifluoroacetic acid. (+ +) Soluble; ( + ) soluble in hot; ( + ) partially soluble or swollen; ( - ) insoluble.
Polyisophthalamides with iminobenzoyl pendant groups Pneumatic clamps were used and an extension rate of 2 mm/min was applied using a gauge length of 10 mm. At least six samples were tested for each polymer and the data averaged. To determine water absorption, samples of about 200 mg were placed in a desiccator where 65% r.h. was maintained by means of an oversaturated solution of NaNO2 in water at 20. The samples had been previously dried at 140 in vacuum for 24hr. RESULTS AND DISCUSSIONS
Synthesis and characterization of polymers The method of low temperature solution polycondensation in DMA was used to prepare the series of polyisophthalamides (polymers I-V) from isophthaloyl chloride and five aromatic diamines, viz. p-phenylenediamine, m-phenylenediamine, bis(4-aminophenyl)methane, 2,2-bis(4-aminophenyl)propane and bis(4-aminophenyl)oxide. This method gave rise to essentially quantitative yields of polyisophthalamides of high molecular weight (inherent viscosities over 1.0 dl/g), Unfortunately this procedure at low temperature could not be used in the synthesis of iminobenzoyl-containing polyisophthalamides; attempts to prepare the 5-iminobenzoylisophthaloyl chloride by reacting 5-iminobenzoylisophthalic acid with the common agents, SOCI 2 or PCls, were unsuccessful. In every case the chlorinating agent not only reacted with the carboxyl groups but also with the amide groups, more probably through the Von Braun reaction [13], to yield 5-chloroisophthaloyl chloride as final product. Therefore the modified polyisophthalamides were prepared by the method described by Yamazaki et al. [10 12], consisting in the direct polyamidation from aromatic diacids and diamines in solution at higher temperature catalyzed by triphenyl phosphite in the presence of pyridine and an inorganic salt. The overall reaction may be taken as:
HOOC ~lf/~-~"~I~COOH
-~
H2N - - A r
1015
It has been recently claimed that high molecular weight polyterephthalamides and polyisophthalamides may be obtained by the Yamazaki method [14]. Nevertheless, the factors affecting polyamide formation via the phosphorylation reaction are not yet understood in detail and the molecular weights that can be currently obtained are not very high. The polyisophthalamides, substituted and unsubstituted, prepared in this work are shown in Table 1. The inherent viscosities may be accepted as corresponding to high molecular weight polyamides although some dispersion in the substituted series is to be seen. This heterogeneity in the molecular size of the polyamides with iminobenzoyl pendant groups must be attributed to the method of synthesis, which was not optimized. We could however obtain films with good mechanical properties from all the polymers. The main tools used for characterization of the polymers were i.r. and ~H-NMR. In the i.r. spectra, the side amide groups were indistinguishable from the amide groups on the main chain, the greater difference being observed in the region of aromatic substitution. Two absorptions at about 71(1 and 740-750cm ~ corresponding to monosubstituted benzene rings appeared in the spectra of all the substituted polymers. Clear differences could be observed between the ~H-NMR spectra of the two series. The unsubstituted polyisophthalamides showed a unique signal of amidic protons at about 10.2ppm and a simple system due to aromatic protons in the range 7 8.5 ppm. The substituted polymers showed two peaks corresponding to the two different amidc groups. As expected, the peak corresponding to the side amidic hydrogen integrated one half and appeared at lower field than the peak of the two amidic protons oll the main chain. As an example the ~H-NMR spectra of polymers V and X, measured in DMSO-d~, solution, are
(Ph--OJ3P -t Pyridine --NH 2 .................. ~" NMP t LiCI
NH CO --HN
---Ar--NHOC-~CO--
NH CO
1016
J.G. DE LA CAMPAet al.
reproduced in Fig. I. In spite of the complicated pattern in the region of aromatic protons, where the signals of various systems are superimposed, the assignment of the most characteristic peaks was not difficult. No contribution of end-groups could be observed by i.r. or ~H-NMR.
Solubility
One of the aims of this work was to evaluate the effect of the incorporation of bulky pendant groups on the solubility of aromatic polyamides. For this reason, a comparative study of the solubilities in typical organic solvents was done.
I-oc- ,
1
6 2
3
(o)
4
OC
CONH
0
5
NH 3 5
2
NH
i
I
CO
(bl 9
1
1
1
I
I
I
I
11
10
9
8
7
6
B ( ppm )
Fig. 1. IH-NMR spectra of polymers V and X.
Polyisophthalamides with iminobenzoyl pendant groups All the polymers were soluble in cone. H2SO4. With the exception of polyamide II, from p-phenylenediamine and isophthaloyl chloride, all were soluble in highly polar organic solvents such as NMP, D M F and DMSO. In general the presence of iminobenzoyl pendant groups enhanced the solubility in organic solvents relative to the corresponding unsubstituted polyisophthalamides; thus the substituted polyamide VlI, with p-phenylenediamine moieties, is soluble in hot organic solvents. While the unsubstituted structures are insoluble in m-cresol and cyclohexanone, substituted polyisophthalamides show various degrees of solubility in these solvents. The effect is more pronounced with m-cresol because the increasing of amidic groups per repeating unit favours the amide-phenol interactions by hydrogen-bonding, thus favouring solubility. This explanation may also be valid for cyclohexanone, where the ketonic groups of the solvent may interact with more polar groups in the polymer. However, this simple explanation does not account for the poorer solubility of polymer IX in cyclohexanone relative to polymer IV, so that other structural factors must influence and condition the solubility of the substituted polymers, The polymers derived from 2,2-bis(4-aminophenyl)propane show better solubility than the other members of the two series, confirming previous results [8]. They are soluble at room temperature in trifluoroacetic acid (TFA) while the other polymers, substituted and unsubstituted, are insoluble in TFA. Perhaps it is noteworthy to comment on behaviour showed by polymer V in TFA. At room temperature this unsubstituted polyamide is only swollen by TFA; on heating it dissolves and does not precipitate or crystallize from solution on cooling. If the solution is heated again it becomes turbid and eventually segregation of polymer occurs. Unexpectedly the polymer dissolves completely again on cooling. The phenomenon occurs with total reproducibility after any heating--cooling cycle. The only feasible explanation for this behaviour is the complexing of TFA (pKa = 0.23) molecules with the oxygen of the aromatic ether linkage, bringing about the solubilization, On heating, the complex is destroyed and the polymer can no longer be solubilized by TFA. This is typical behaviour of ethers with Br6nsted and Lewis acids [15] and it seems that the conditions are easily achieved in our polymers only in this case of polymer V and TFA. We think that this observation was possible because the temperature at which the complex decomposes occurs in the range 2(~72 (room temperature-boiling point of TFA). Attempts with other organic acids (formic a n d acetic) or with m-cresol were not successful, probably due to the higher pKa of these solvents, 3.75, 4.75 and 10.01 respectively. Filrns Flexible strong films could be obtained by evaporation of polymer cast solutions in DMA, D M F or mixtures of them with NMP. The films were transparent and colourless to slightly yellow. The tensile strengths of these films are listed in Table 2. They may be considered as acceptable for films made on a laboratory scale and compare well with values earlier
1017
Table 2. Properties of polyisophthalamides T~(DSC} Tg(TMA) Ta Tensile strength Polymer (C) (C) (C) (kg/cm ~) I 276 275 455 1200 n 295 300 470 1150 III 264 266 430 1050 Iv 279 2"77 42o ]050 V
267
267
445
I 100
Vl Vll Vlll ix x
298 317 285 302 291
302 325 285 305 298
415 410 405 410 410
900 1050 1050 I000 1200
reported for aromatic polyamides. The values for substituted andunsubstitutedpolyamidesaresimilar, so that the incorporation ofsideiminobenzoyl groups seems not to influence substantially the mechanical strength of polyisophthalamides. The lower viscosity of polymer VI relative to polymer I could account for the significant loss of mechanical resistance, but the gain observed for polymer X must be attributed to the structural changes as the viscosity of this substituted polymer is much lower than its parent polyisophthalamide, polymer V. Thermal properties Glass transition temperatures. A calorimetric method (DSC) and a thermomechanical method (TMA) were used to determine the Tgs of all the polyisophthalamides. The results are shown in Table 2. The data obtained by the two methods closely coincide for the same conditions. The midpoint of the onset corresponding to the transition was taken as T~ (Fig. 2). The data in Table 2 demonstrate that the incorporation of iminobenzoyl groups as substituents of polyisophthalamides, or which is the same, the total change of isophthaloyl moieties by 5-iminobenzoylisophthaloyl units in aromatic polyisophthalamides causes an increase in T~ estimated as about 20 . That means that the effect of asymmetry and irregularity introduced with the pendant groups, which should give rise to a decrease of T~, is well overcome by the great volume and the polar nature of the iminobenzoyl group, which on the other hand contributes to a higher density of hydrogen bridges. I--
@ HN
~ - NHOC
--1 CO
NH I co g_ ~ --
I ~oo
I I zoo 3oo Temperature (*C) Fig. 2. TMA (expansion) trace of polymer Vll.
J
1018
J.G. DE LA CAMPAet al.
The polyamides II and VII with p-phenylene moieties in the diamine portion show the highest Tg as corresponding to the more rigid structures. The other polyamides show TgS in the range 264-280 ° for unsubstituted structures and 285-302 ° for the substituted polymers. For both series, the trend is related to the diamine residue. Thus, Tg decreases in the order p-phenylene > isopropylidenediphenylene > mphenylene > oxydiphenylene > methylenediphenylene,
Table 3. Moistureabsorption of polyisophthalamides Percentage mol H20/rep. mol H20/eq. Polymer water absorp, unit amide I 9.33 1.23 0.62 It 8.58 1.14 0.57 III 5.44 0.99 0.50 Iv 5.34 1.06 0.53
Thermal stability Dynamic thermogravimetric analysis (TGA) was used to evaluate the thermal stability in N2. The initial decomposition temperatures, Td, are shown in Table 2. The Td values of polymers I-V follow a trend that agrees with data of other authors on the thermal resistance of aromatic polymers [16]. Thus, the thermal resistance, taking as criterion the initial decomposition temperature, diminishes in the series: p-phenylene (470 °) > m-phenylene > oxydiphenylene > methylenediphenylene > isopropylidenediphenylene (420°). The first onset at about 410-420 ° for all the substituted polymers in dynamic T G A should be associated to the break-down of amide side-groups, The weight loss of this step is lower than the theoretical value for complete loss of pendant groups, because cross-linking takes place at the same time, so preventing the bulk side benzamide residues to escape. Figure 3 reproduces the thermogram of polymet VIII as an illustration of what happens in this series. Analogous behaviour has been observed for other ring substituted aromatic polymers [8, 1 7 ] .
[ __HN~CH2.__~NHO c - _ _ __~ o
e
-
-
s]
6.12
1.12
0.56
vl
I 1.I0
2.20
0.73
x
7.76
1.94
0.65
VII VIII IX
10.52 7.21 6.39
2.09 1.80 1.69
0.70 0.60 0.56
For the less heat-resistant polyisophthalamides, viz. polymers III and IV, the decrease of thermal stability caused by the introduction of pendant groups is very low; for the fully aromatic polyamides, I and II, and the polyoxydiphenyleneisophthalamide, polymer V, the incorporation of the side iminobenzoyl groups brought about a substantial lowering of Td by about 60°; thus for polymer II Td is 470 °, whereas for polymer VII Td is 410 °. For polymer IV, the less heat-resistant polyisophthalamide, the incorporation of iminobenzoyl pendant groups reduced Td by only 10°. Water absorption
The ability to absorb water is an important characteristic of polyamides. Generally moisture absorption is proportional to the frequency of amide groups. Therefore isothermal sorption of water by the present polyisophthalamides was measured and related to the proportion of amide groups in each polymer. The values obtained are listed in Table 3. The absorptions are rather high, as corresponds to structures with s amorphous. high e density n t ofi polar a amide l l groups Y cando The unsubstituted fully aromatic polyisophthalamides (I and II) absorbed more water than those with aliphatic linkages (III and IV), which in turn showed a lower ability to take up water than polymer V with an ether linkage per repeating unit, thus confirming that the oxygen linkages increase the moisture absorption [18]. The substituted polyisophthalamides
NH
400
V
I
20 -
40-
60--
~
C -•
BO
loo
~
I 100
I 200
I 300
I 400
I 500
Temperoture (°C)
Fig. 3. TGA curve of polymer VIII.
I 600
I
HN ~
NHOC
4~-
CO NH
I 10
I 20
I 30
I 40
I 50
60
1 70
t (hr)
Fig. 4. Moisture absorption of polymers l and VI.
Polyisophthalamides with iminobenzoyl pendant groups also obey this trend but they expectedly a b s o r b e d more water t h a n the c o r r e s p o n d i n g u n s u b s t i t u t e d polymers. This b e h a v i o u r is illustrated in Fig. 4 where the isothermal moisture a b s o r p t i o n for a pair o f polymers has been plotted. F o r each pair of polymers, s u b s t i t u t e d / u n s u b s t i t u t e d , the increase observed in water a b s o r p t i o n falls in the range 2 0 - 3 0 % in weight. But if the a b s o r p t i o n is expressed in moles of water per mole of repeating unit the increase is m u c h higher, from 60% for the pair from 2 , 2 - b i s ( 4 - a m i n o p h e n y l ) p r o p a n e (polymer IX against polymer IV) to 84% for the pair from m p h e n y l e n e d i a m i n e (polymer VIII against polymer II), significantly more t h a n the difference of 50% expected on passing from two to three amide groups per repeating unit. The most plausible e x p l a n a t i o n is to suppose that the i n t r o d u c t i o n of side bulky substituents affects the degree of disorder with a n increase in the accessibility of water, a result that agrees with a better solubility. Acknowledgement The financial support provided by the "Comisi6n Asesora de lnvestigaci6n Cientifica y T6cnica'" is gratefully acknowledged.
REFERENCES 1. J. Preston and J. Economy (Eds), High Temperature and Flame-Resistant Fibers. Interscience, New York (1973). 2. E. Cassidy, Thermally Stable Polymers. Syntheses and Properties. Dekker, New York (1980).
1019
3. P. W. Morgan, Condensation Polrmers: By Inter/acial and Solution Methods. Interscience, New York (1965). 4. V. Guidotti and N. J. Johnston, Polvm. Prepr. 15, 570 (1974). 5. A. K. Chauduri, B. Y. Min and E. M. Pearce. J. Polvm. Sci., Polym. Chem. Ed. 18, 2949 (1980). 6. J. de Abajo and E. de Santos, Angew. Makromolek. Chem. I l l , 17 (1983). 7. E. Guijarro, J. G. de la Campa and J. de Abajo, ,L Polym. Sci., Polym. Chem. Ed. 22, 1531 (1984). 8. J. de Abajo, J. G. de la Campa, E. Guijarro and F. Serna, J. Po(vm. Sci.. Po(vm. Chem. Ed. In press. 9. Anon, Br. Pat. 204722 (1923), Chem. ,4bstr. 18, 839 (1921). 10. N. Yamazaki, F. Higashi and J. Kawabata, J. Po!vm. Sci., Polym. Chem. Ed. 12, 2149 (1974). 11. N. Yamazaki, M. Matsumoto and F. Higashi, J. Po(vm. Sci., Polym. Chem. Ed. 13, 1373 (1975). 12. J. Asrar, J. Preston and W. R. Krigbaum, J. Polvm. Sei., Polym. Chem. Ed. 20, 79 (1982). 13. J. F. Bieron and F. J. Dinan, The Chemistry o/'Amides (Edited by J. Zabicky), Chap. 4. Interscience, London (1970). 14. F. Higashi, S. Ogata and Y. Aoki, J. Poh'm. Sci., Polvm. Chem. Ed. 20, 2081 (1982). 15. S. Searles Jr and M. Tamres, The Chemistry ~/'the Ether Linkage (Edited by S. Patay), Chap. 6. lnterscience. London (1967). 16. W. W. Wright. Degradation and Stabilization (~[ Polvmers (Edited by G. Geuskens), Chap. 3. Applied Science, London (1975). 17. J. M. Augl, J. V. Duffy and S. E. Wentworth, J. Polvm. Sci., Polym. Chem. Ed. 12, 1023 (1974). 18. F. B. Cramer and R. G. Beaman, J. Polvm. Sci. 21,237 (1956).
0014-3057'85 $300 + 0.00 Copyright i ' 1985 Pergamon Press Lid
Printed in Great Britain. All rights reserved
AROMATIC POLYISOPHTHALAMIDES WITH IMINOBENZOYL P E N D A N T GROUPS J. G. DE LA CAMPA, E. GUIJARRO, F. J. SERNA and J. DE ABAJO lnstituto de Plfisticos y Caucho, C.S.I.C., Juan de la Cierva 3, 28006 Madrid, Spain
(Received 19 June 1985) Abstract--Polyisophthalamides were prepared from aromatic diamines and 5-iminobenzoylisophthalic acid by the Yamazaki method of direct polyamidation catalyzed by triphenylphosphite. The properties of the polymers were measured and compared with the analogous unsubstituted polyisophthalamides. The incorporation of one iminobenzoyl pendant group per repeating unit gave rise to better solubility in strongly polar solvents. Higher content of amide groups per repeating unit allowed the modified polymers to absorb moisture to a greater extent than the parent polyisophthalamides. The glass transition temperatures were raised 20-30 ~by the presence of the pendant groups and they ranged from 290 to 317. On the contrary, the substituted polymers showed lower initial decomposition temperatures, as measured by TGA, all of them beginning to decompose at about 410 c. The mechanical properties of polymer films seemed not to be greatly affected by the pendant groups and only small differences were observed between substituted and unsubstituted polymers.
INTRODUCTION
The general properties of these polymers have been
A r o m a t i c polyamides are a m o n g the m o d e r n hight e m p e r a t u r e resistant polymers [1, 2]. N u m e r o u s attempts have been to modify their chemical structure in order to change their properties a n d b e h a v i o u r [3]. The i n c o r p o r a t i o n of substituents as side p e n d a n t groups and the i n t r o d u c t i o n of different units along the b a c k b o n e are general a p p r o a c h e s used to change the chemical structure of polymers a n d they have been m u c h used for a r o m a t i c polyamides [4, 5], We have already reported on the synthesis a n d c h a r a c t e r i z a t i o n of modified p o l y i s o p h t h a l a m i d e s
studied and c o m p a r e d with those o f the h o m o l o g o u s u n s u b s t i t u t e d polyamides. Solubility in organic solvents, thermal transitions, thermal resistance and mechanical strength of films have been the main subjects of o u r study. The effect of the substituent on water a b s o r p t i o n was also investigated.
c o n t a i n i n g various substitutents as p e n d a n t groups a n c h o r e d on the 5-position o f the isophthaloyl moieties [6-8]. The i n c o r p o r a t i o n of oxybenzoyl substitients gave rise to evident a d v a n t a g e s in solubility [8] but the thermal resistance decreased the side ester g r o u p showing m u c h lower thermal stability t h a n the main polyamide chain. As an a t t e m p t to improve these previous results, we have n o w p r e p a r e d new p o l y i s o p h t h a l a m i d e s with b e n z a m i d e (iminobenzoyl) p e n d a n t groups. The modified polymers have been synthesized from a new m o n o m e r derived from the isophthalic acid that acted as the bearer of the p e n d a n t groups. O n condensing it with a r o m a t i c diamines, polyamides with the structural formula:
--HN--At--NHOC-'~f~-~"[----CO-NH CO
were obtained,
EXPERIMENTAL The N,N-dimethylacetamide (DMA), ERTISA. was stored on molecular sieves 4,~ and used without further purification. The N-methyl-2-pyrrolydinone (NMP) was purified by distillation twice on P205 and stored in a dark bottle on molecular sieves. Pyridine was refluxed on KOH for 12 hr, distilled and stored on molecular sieves. Triphenyl phosphite was purified by distillation at reduced pressure. The 5-aminoisophthalic acid was a gift of Dupont de Nemours & Co. Inc. and was used as received.
Monomers The diamines were reagent grade obtained from commercial sources and were purified by recrystallization. Only the 2,2-bis(4-aminophenyl)propane had to be prepared from aniline chlorhydrate and acetone by a published method [9], recrystallized from water, m.p. 133 (lit. 132 [9]). Yield 58%. Isophthaloyl chloride was prepared from isophthalic acid and thionyl chloride by the usual method and recrystallized from n-hexane, m.p. 4 2 . The 5-iminobenzoylisophthalic acid was prepared by reaction of 5-aminoisophthalic acid and benzoyl chloride in stoichiometric amounts in solution in DMA at 0'. The product was isolated by precipitation in an excess of water. It was recrystallized from ethanol-water 50:50, m.p. 350 . Yield 90%. Analysis: Cl~HliNOs(285,27 ). Calc.: %C, 63.16: %H, 3.89; %N, 4.91. Found: C, 63.14; %H. 3.79:° N, 4.53. Polymerization Two methods were used to prepare the polymers. The unsubstituted polyisophthalamides were prepared from isophthaloyl chloride and diamines by the low temperature condensation method, in solution in DMA, according to the procedure described elsewhere [8]. The substituted poly-
1013
J . G . DE LA CAMPA et al.
1014
isophthalamides were prepared by the method developed by Yamazaki et al. [10-12] which consists of the direct polycondensation of aromatic diacids and diamines catalyzed with triphenyl phosphite. In a typical example 6.5897 g (23.10mmol) of 5-iminobenzoylisophthalic acid and 4.5805 g (23.10 mmol) of bis(4-aminophenyl)methane were made to react in a mixture of 16 ml of pyridine, 15.5 ml of triphenyl phosphite (50 mmol) and 62 ml of N M P containing 3.l g of LiCl, at 100" for 4 hr and at 120 ° for 2 hr. After cooling, the reaction mixture was poured into water (800 ml), and the precipitated polymer was filtered, washed with water several times, extracted with refiuxing acetone in a Soxhlet for 12hr and dried in vacuum at 60 ° for 24hr. Yield 98%.
in the T G A curve took place was taken as the degradation temperature (To). Just before the runs, absorbed water was eliminated by heating the sample in the thermobalance oven at 120 ° 15 min, and then the recorder pen was again adjusted to 100% initial weight. The glass transition temperatures of polymers were measured by differential scanning calorimetry on a Perkin Elmer DSC-4 thermal analyzer programmed by means of a System 4 microprocessor and connected to a 3600 Data Station. Usually 10-12 mg powdered polymer sampleswere used, and the measurements were made under N 2 at a heating rate of I0 ° min. TMA, a thermomechanical method, was also used in the determination of Tg. The polymer samples, as discs molded under pressure, 600-700/~m thick, were tested on a Perkin-Elmer TMS-2 device by the expansion method under the same conditions as those for DSC.
Characterization o f polymers i.r. Spectra were recorded on a Perkin-Elmer 457 Spectrophotometer using thin polymer films. ~H-NMR spectra were recorded at 60 ° on a Varian EM-390 (90MHz) apparatus using dimethylsulphoxide-d 6 as solvent and TMS as internal reference, The inherent viscosities of all the polymers were determined on 0.5% (w/v) N M P solutions at 25 + 0.1 ° in an Ostwald viscometer, 0.4mm i.d. A Perkin-Elmer TGS-2 thermobalance controlled by a System 4 microprocessor was used for thermogravimetric analysis. Powdered polymer samples (2-3 mg) were tested at 10° rain under N 2. The temperature at which the first onset
Films Polymers films were obtained by evaporation of cast solutions in DMF, D M A and NMP or mixtures of them. Usually a concentration of 10% in weight of polymer was used, and the solvent was eliminated by heating at 120 ° for 2 hr in an air-circulating oven and then at 120 140 ° for 3~4 hr under vacuum (1 mmHg). For determination of tensile properties of polymers, strips (15 mm width and 80 mm length) were cut from polymer films and tested on an I N S T R O N dynamometer at 20 ~'.
Table 1. Aromatic polyisophthalamides
--HN --Ar --NHOC~ ~ -
CO--
R Solubility Code
AR
R
Flinh
(dl/g)
DMF
Cresol CH
I
~
H
1.12
++
II
~
H
1.10
-
-
H
1.55
++
-
-
H
1.06
++
++
+
H
1.36
++
-
"~O~]----
-- NHOC~
0.76
++
+
+_
-
~
- - NHOC~ " ~
1.13
+
-
-
-
0.65
++
+
_+
-
lIl
CHz-~~
@
IV @ C ( C H 3 ) V
@
O
VI VII
2~ @
-
TFA
-
-
++ +
1
X
-
~
~
O
- ~ ~
-- NHOC-~
CH = cyclohexanone; D M F - N,N-dimethylformamide; TFA = trifluoroacetic acid. (+ +) Soluble; ( + ) soluble in hot; ( + ) partially soluble or swollen; ( - ) insoluble.
Polyisophthalamides with iminobenzoyl pendant groups Pneumatic clamps were used and an extension rate of 2 mm/min was applied using a gauge length of 10 mm. At least six samples were tested for each polymer and the data averaged. To determine water absorption, samples of about 200 mg were placed in a desiccator where 65% r.h. was maintained by means of an oversaturated solution of NaNO2 in water at 20. The samples had been previously dried at 140 in vacuum for 24hr. RESULTS AND DISCUSSIONS
Synthesis and characterization of polymers The method of low temperature solution polycondensation in DMA was used to prepare the series of polyisophthalamides (polymers I-V) from isophthaloyl chloride and five aromatic diamines, viz. p-phenylenediamine, m-phenylenediamine, bis(4-aminophenyl)methane, 2,2-bis(4-aminophenyl)propane and bis(4-aminophenyl)oxide. This method gave rise to essentially quantitative yields of polyisophthalamides of high molecular weight (inherent viscosities over 1.0 dl/g), Unfortunately this procedure at low temperature could not be used in the synthesis of iminobenzoyl-containing polyisophthalamides; attempts to prepare the 5-iminobenzoylisophthaloyl chloride by reacting 5-iminobenzoylisophthalic acid with the common agents, SOCI 2 or PCls, were unsuccessful. In every case the chlorinating agent not only reacted with the carboxyl groups but also with the amide groups, more probably through the Von Braun reaction [13], to yield 5-chloroisophthaloyl chloride as final product. Therefore the modified polyisophthalamides were prepared by the method described by Yamazaki et al. [10 12], consisting in the direct polyamidation from aromatic diacids and diamines in solution at higher temperature catalyzed by triphenyl phosphite in the presence of pyridine and an inorganic salt. The overall reaction may be taken as:
HOOC ~lf/~-~"~I~COOH
-~
H2N - - A r
1015
It has been recently claimed that high molecular weight polyterephthalamides and polyisophthalamides may be obtained by the Yamazaki method [14]. Nevertheless, the factors affecting polyamide formation via the phosphorylation reaction are not yet understood in detail and the molecular weights that can be currently obtained are not very high. The polyisophthalamides, substituted and unsubstituted, prepared in this work are shown in Table 1. The inherent viscosities may be accepted as corresponding to high molecular weight polyamides although some dispersion in the substituted series is to be seen. This heterogeneity in the molecular size of the polyamides with iminobenzoyl pendant groups must be attributed to the method of synthesis, which was not optimized. We could however obtain films with good mechanical properties from all the polymers. The main tools used for characterization of the polymers were i.r. and ~H-NMR. In the i.r. spectra, the side amide groups were indistinguishable from the amide groups on the main chain, the greater difference being observed in the region of aromatic substitution. Two absorptions at about 71(1 and 740-750cm ~ corresponding to monosubstituted benzene rings appeared in the spectra of all the substituted polymers. Clear differences could be observed between the ~H-NMR spectra of the two series. The unsubstituted polyisophthalamides showed a unique signal of amidic protons at about 10.2ppm and a simple system due to aromatic protons in the range 7 8.5 ppm. The substituted polymers showed two peaks corresponding to the two different amidc groups. As expected, the peak corresponding to the side amidic hydrogen integrated one half and appeared at lower field than the peak of the two amidic protons oll the main chain. As an example the ~H-NMR spectra of polymers V and X, measured in DMSO-d~, solution, are
(Ph--OJ3P -t Pyridine --NH 2 .................. ~" NMP t LiCI
NH CO --HN
---Ar--NHOC-~CO--
NH CO
1016
J.G. DE LA CAMPAet al.
reproduced in Fig. I. In spite of the complicated pattern in the region of aromatic protons, where the signals of various systems are superimposed, the assignment of the most characteristic peaks was not difficult. No contribution of end-groups could be observed by i.r. or ~H-NMR.
Solubility
One of the aims of this work was to evaluate the effect of the incorporation of bulky pendant groups on the solubility of aromatic polyamides. For this reason, a comparative study of the solubilities in typical organic solvents was done.
I-oc- ,
1
6 2
3
(o)
4
OC
CONH
0
5
NH 3 5
2
NH
i
I
CO
(bl 9
1
1
1
I
I
I
I
11
10
9
8
7
6
B ( ppm )
Fig. 1. IH-NMR spectra of polymers V and X.
Polyisophthalamides with iminobenzoyl pendant groups All the polymers were soluble in cone. H2SO4. With the exception of polyamide II, from p-phenylenediamine and isophthaloyl chloride, all were soluble in highly polar organic solvents such as NMP, D M F and DMSO. In general the presence of iminobenzoyl pendant groups enhanced the solubility in organic solvents relative to the corresponding unsubstituted polyisophthalamides; thus the substituted polyamide VlI, with p-phenylenediamine moieties, is soluble in hot organic solvents. While the unsubstituted structures are insoluble in m-cresol and cyclohexanone, substituted polyisophthalamides show various degrees of solubility in these solvents. The effect is more pronounced with m-cresol because the increasing of amidic groups per repeating unit favours the amide-phenol interactions by hydrogen-bonding, thus favouring solubility. This explanation may also be valid for cyclohexanone, where the ketonic groups of the solvent may interact with more polar groups in the polymer. However, this simple explanation does not account for the poorer solubility of polymer IX in cyclohexanone relative to polymer IV, so that other structural factors must influence and condition the solubility of the substituted polymers, The polymers derived from 2,2-bis(4-aminophenyl)propane show better solubility than the other members of the two series, confirming previous results [8]. They are soluble at room temperature in trifluoroacetic acid (TFA) while the other polymers, substituted and unsubstituted, are insoluble in TFA. Perhaps it is noteworthy to comment on behaviour showed by polymer V in TFA. At room temperature this unsubstituted polyamide is only swollen by TFA; on heating it dissolves and does not precipitate or crystallize from solution on cooling. If the solution is heated again it becomes turbid and eventually segregation of polymer occurs. Unexpectedly the polymer dissolves completely again on cooling. The phenomenon occurs with total reproducibility after any heating--cooling cycle. The only feasible explanation for this behaviour is the complexing of TFA (pKa = 0.23) molecules with the oxygen of the aromatic ether linkage, bringing about the solubilization, On heating, the complex is destroyed and the polymer can no longer be solubilized by TFA. This is typical behaviour of ethers with Br6nsted and Lewis acids [15] and it seems that the conditions are easily achieved in our polymers only in this case of polymer V and TFA. We think that this observation was possible because the temperature at which the complex decomposes occurs in the range 2(~72 (room temperature-boiling point of TFA). Attempts with other organic acids (formic a n d acetic) or with m-cresol were not successful, probably due to the higher pKa of these solvents, 3.75, 4.75 and 10.01 respectively. Filrns Flexible strong films could be obtained by evaporation of polymer cast solutions in DMA, D M F or mixtures of them with NMP. The films were transparent and colourless to slightly yellow. The tensile strengths of these films are listed in Table 2. They may be considered as acceptable for films made on a laboratory scale and compare well with values earlier
1017
Table 2. Properties of polyisophthalamides T~(DSC} Tg(TMA) Ta Tensile strength Polymer (C) (C) (C) (kg/cm ~) I 276 275 455 1200 n 295 300 470 1150 III 264 266 430 1050 Iv 279 2"77 42o ]050 V
267
267
445
I 100
Vl Vll Vlll ix x
298 317 285 302 291
302 325 285 305 298
415 410 405 410 410
900 1050 1050 I000 1200
reported for aromatic polyamides. The values for substituted andunsubstitutedpolyamidesaresimilar, so that the incorporation ofsideiminobenzoyl groups seems not to influence substantially the mechanical strength of polyisophthalamides. The lower viscosity of polymer VI relative to polymer I could account for the significant loss of mechanical resistance, but the gain observed for polymer X must be attributed to the structural changes as the viscosity of this substituted polymer is much lower than its parent polyisophthalamide, polymer V. Thermal properties Glass transition temperatures. A calorimetric method (DSC) and a thermomechanical method (TMA) were used to determine the Tgs of all the polyisophthalamides. The results are shown in Table 2. The data obtained by the two methods closely coincide for the same conditions. The midpoint of the onset corresponding to the transition was taken as T~ (Fig. 2). The data in Table 2 demonstrate that the incorporation of iminobenzoyl groups as substituents of polyisophthalamides, or which is the same, the total change of isophthaloyl moieties by 5-iminobenzoylisophthaloyl units in aromatic polyisophthalamides causes an increase in T~ estimated as about 20 . That means that the effect of asymmetry and irregularity introduced with the pendant groups, which should give rise to a decrease of T~, is well overcome by the great volume and the polar nature of the iminobenzoyl group, which on the other hand contributes to a higher density of hydrogen bridges. I--
@ HN
~ - NHOC
--1 CO
NH I co g_ ~ --
I ~oo
I I zoo 3oo Temperature (*C) Fig. 2. TMA (expansion) trace of polymer Vll.
J
1018
J.G. DE LA CAMPAet al.
The polyamides II and VII with p-phenylene moieties in the diamine portion show the highest Tg as corresponding to the more rigid structures. The other polyamides show TgS in the range 264-280 ° for unsubstituted structures and 285-302 ° for the substituted polymers. For both series, the trend is related to the diamine residue. Thus, Tg decreases in the order p-phenylene > isopropylidenediphenylene > mphenylene > oxydiphenylene > methylenediphenylene,
Table 3. Moistureabsorption of polyisophthalamides Percentage mol H20/rep. mol H20/eq. Polymer water absorp, unit amide I 9.33 1.23 0.62 It 8.58 1.14 0.57 III 5.44 0.99 0.50 Iv 5.34 1.06 0.53
Thermal stability Dynamic thermogravimetric analysis (TGA) was used to evaluate the thermal stability in N2. The initial decomposition temperatures, Td, are shown in Table 2. The Td values of polymers I-V follow a trend that agrees with data of other authors on the thermal resistance of aromatic polymers [16]. Thus, the thermal resistance, taking as criterion the initial decomposition temperature, diminishes in the series: p-phenylene (470 °) > m-phenylene > oxydiphenylene > methylenediphenylene > isopropylidenediphenylene (420°). The first onset at about 410-420 ° for all the substituted polymers in dynamic T G A should be associated to the break-down of amide side-groups, The weight loss of this step is lower than the theoretical value for complete loss of pendant groups, because cross-linking takes place at the same time, so preventing the bulk side benzamide residues to escape. Figure 3 reproduces the thermogram of polymet VIII as an illustration of what happens in this series. Analogous behaviour has been observed for other ring substituted aromatic polymers [8, 1 7 ] .
[ __HN~CH2.__~NHO c - _ _ __~ o
e
-
-
s]
6.12
1.12
0.56
vl
I 1.I0
2.20
0.73
x
7.76
1.94
0.65
VII VIII IX
10.52 7.21 6.39
2.09 1.80 1.69
0.70 0.60 0.56
For the less heat-resistant polyisophthalamides, viz. polymers III and IV, the decrease of thermal stability caused by the introduction of pendant groups is very low; for the fully aromatic polyamides, I and II, and the polyoxydiphenyleneisophthalamide, polymer V, the incorporation of the side iminobenzoyl groups brought about a substantial lowering of Td by about 60°; thus for polymer II Td is 470 °, whereas for polymer VII Td is 410 °. For polymer IV, the less heat-resistant polyisophthalamide, the incorporation of iminobenzoyl pendant groups reduced Td by only 10°. Water absorption
The ability to absorb water is an important characteristic of polyamides. Generally moisture absorption is proportional to the frequency of amide groups. Therefore isothermal sorption of water by the present polyisophthalamides was measured and related to the proportion of amide groups in each polymer. The values obtained are listed in Table 3. The absorptions are rather high, as corresponds to structures with s amorphous. high e density n t ofi polar a amide l l groups Y cando The unsubstituted fully aromatic polyisophthalamides (I and II) absorbed more water than those with aliphatic linkages (III and IV), which in turn showed a lower ability to take up water than polymer V with an ether linkage per repeating unit, thus confirming that the oxygen linkages increase the moisture absorption [18]. The substituted polyisophthalamides
NH
400
V
I
20 -
40-
60--
~
C -•
BO
loo
~
I 100
I 200
I 300
I 400
I 500
Temperoture (°C)
Fig. 3. TGA curve of polymer VIII.
I 600
I
HN ~
NHOC
4~-
CO NH
I 10
I 20
I 30
I 40
I 50
60
1 70
t (hr)
Fig. 4. Moisture absorption of polymers l and VI.
Polyisophthalamides with iminobenzoyl pendant groups also obey this trend but they expectedly a b s o r b e d more water t h a n the c o r r e s p o n d i n g u n s u b s t i t u t e d polymers. This b e h a v i o u r is illustrated in Fig. 4 where the isothermal moisture a b s o r p t i o n for a pair o f polymers has been plotted. F o r each pair of polymers, s u b s t i t u t e d / u n s u b s t i t u t e d , the increase observed in water a b s o r p t i o n falls in the range 2 0 - 3 0 % in weight. But if the a b s o r p t i o n is expressed in moles of water per mole of repeating unit the increase is m u c h higher, from 60% for the pair from 2 , 2 - b i s ( 4 - a m i n o p h e n y l ) p r o p a n e (polymer IX against polymer IV) to 84% for the pair from m p h e n y l e n e d i a m i n e (polymer VIII against polymer II), significantly more t h a n the difference of 50% expected on passing from two to three amide groups per repeating unit. The most plausible e x p l a n a t i o n is to suppose that the i n t r o d u c t i o n of side bulky substituents affects the degree of disorder with a n increase in the accessibility of water, a result that agrees with a better solubility. Acknowledgement The financial support provided by the "Comisi6n Asesora de lnvestigaci6n Cientifica y T6cnica'" is gratefully acknowledged.
REFERENCES 1. J. Preston and J. Economy (Eds), High Temperature and Flame-Resistant Fibers. Interscience, New York (1973). 2. E. Cassidy, Thermally Stable Polymers. Syntheses and Properties. Dekker, New York (1980).
1019
3. P. W. Morgan, Condensation Polrmers: By Inter/acial and Solution Methods. Interscience, New York (1965). 4. V. Guidotti and N. J. Johnston, Polvm. Prepr. 15, 570 (1974). 5. A. K. Chauduri, B. Y. Min and E. M. Pearce. J. Polvm. Sci., Polym. Chem. Ed. 18, 2949 (1980). 6. J. de Abajo and E. de Santos, Angew. Makromolek. Chem. I l l , 17 (1983). 7. E. Guijarro, J. G. de la Campa and J. de Abajo, ,L Polym. Sci., Polym. Chem. Ed. 22, 1531 (1984). 8. J. de Abajo, J. G. de la Campa, E. Guijarro and F. Serna, J. Po(vm. Sci.. Po(vm. Chem. Ed. In press. 9. Anon, Br. Pat. 204722 (1923), Chem. ,4bstr. 18, 839 (1921). 10. N. Yamazaki, F. Higashi and J. Kawabata, J. Po!vm. Sci., Polym. Chem. Ed. 12, 2149 (1974). 11. N. Yamazaki, M. Matsumoto and F. Higashi, J. Po(vm. Sci., Polym. Chem. Ed. 13, 1373 (1975). 12. J. Asrar, J. Preston and W. R. Krigbaum, J. Polvm. Sei., Polym. Chem. Ed. 20, 79 (1982). 13. J. F. Bieron and F. J. Dinan, The Chemistry o/'Amides (Edited by J. Zabicky), Chap. 4. Interscience, London (1970). 14. F. Higashi, S. Ogata and Y. Aoki, J. Poh'm. Sci., Polvm. Chem. Ed. 20, 2081 (1982). 15. S. Searles Jr and M. Tamres, The Chemistry ~/'the Ether Linkage (Edited by S. Patay), Chap. 6. lnterscience. London (1967). 16. W. W. Wright. Degradation and Stabilization (~[ Polvmers (Edited by G. Geuskens), Chap. 3. Applied Science, London (1975). 17. J. M. Augl, J. V. Duffy and S. E. Wentworth, J. Polvm. Sci., Polym. Chem. Ed. 12, 1023 (1974). 18. F. B. Cramer and R. G. Beaman, J. Polvm. Sci. 21,237 (1956).