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Structure–property relationships for novel wholly aromatic polyamide-hydrazides containing various proportions of para-phenylene and meta-phenylene unitsII. Thermal stability and degradation behaviour
Polymer Degradation and Stability 79 (2003) 61–75 www.elsevier.com/locate/polydegstab
Structure–property relationships for novel wholly aromatic polyamide-hydrazides containing various proportions of para-phenylene and meta-phenylene units II. Thermal stability and degradation behaviour Nadia Ahmed Mohameda,*, Abeer Obaid Hamad Al-Dossaryb a Department of Chemistry, Faculty of Science, Cairo University, Giza, Egypt Department of Chemistry, Faculty of Science, Girls College, Dammam, 838, Saudi Arabia
b
Received 15 April 2002; accepted 3 June 2002
Abstract The influences of controlled structural differences and molecular weight on the thermal stability and degradation behaviour of a series of novel wholly aromatic polyamide-hydrazides have been investigated in nitrogen and in air using differential scanning calorimetry (DSC), thermogravimetric analysis (TG), infrared spectrophotometry (IR) and elemental analysis. The structural variations of the polymers were achieved by varying the content of the meta- and para-substituted phenylene moieties incorporated into their chains. All the polymers were synthesized by a low temperature solution polycondensation reaction of either 4-amino-3hydroxybenzhydrazide [4A3HBH] or 3-amino-4-hydroxybenzhydrazide [3A4HBH] with an equimolar amount of either terephthaloyl chloride [TCl], isophthaloyl chloride [ICl] or mixtures of various molar ratios of TCl and ICl in anhydrous N,N-dimethylacetamide [DMAc] as a solvent at 10 C. The content of para- and meta-phenylene moieties was varied within this series so that the changes in the latter were 10 mol% from polymer to polymer, starting from an overall content of 0–100 mol%. All the polymers have the same structural formula except for the way of linking phenylene units in the polymer chain. The results showed that these polymers have high resistance to elevated temperatures. Their weight loss occurred in three steps. The first was small and was attributed to evaporation of adsorbed surface water. The second was considerable and was assigned to cyclodehydration reactions of the polyamide-hydrazides into the corresponding poly(1,3,4-oxadiazolyl-benzoxazoles) with loss of water. This is not a true degradation, but rather a thermochemical transformation reaction. The third was relatively severe and steep, particularly in air, and corresponded to the decomposition of the polymers. The results clearly indicate that substitution of para-phenylene units for meta-phenylene ones within this polymer series leads to improved polymer stability at elevated temperatures in nitrogen as well as in air. This should be associated with regularity of supermolecular packing within the bulk of the investigated polymers wherein the colinear arrangement of the para-phenylene units should allow for establishment of stronger intermolecular bonds which would be more difficult to break and therefore more resistant to high temperatures. Moreover, polyamide-hydrazides having different molecular weights were also examined. The results clearly reveal that at all temperatures used and in both degradation atmospheres all the investigated samples exhibited similar thermal behaviour regardless of their molecular weights, except in the temperature range 160–200 C where the lower molecular weight samples showed significant weight losses which may be attributed to hydrogen bonded DMAc. This indicates that structural building units of these polymers (which contained characteristic groups, such as: aromatic rings and amide and hydrazide linking bonds in the case of polyamide-hydrazides and aromatic nuclei, 1,3,4-oxadiazolyl rings and benzoxazolyl moieties in the case of poly(1,3,4-oxadiazolyl-benzoxazoles) are responsible for their high thermal stability, rather than the longer chain segments. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Polyamide-hydrazides; Thermal and thermo-oxidative stability; Differential scanning calorimetry; Thermogravimetric analysis, poly(1,3,4-oxadiazolyl-benzoxazoles)
* Corresponding author. E-mail address: [email protected] (N.A. Mohamed). 0141-3910/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0141-3910(02)00239-2
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1. Introduction Wholly aromatic polyamide-hydrazides of the general formula [CO–NH–Ar–CO–NH–NH–CO–Ar0 ]n, where Ar and Ar0 are meta- and/or para-substituted phenylene units, have attracted the attention of many investigators in recent years as high performance materials. The general method adopted to synthesize these polymers is a low temperature solution polycondensation reaction of aromatic aminohydrazides with aromatic dicarboxylic acid dichlorides in an amide solvent [1–6]. Completely ordered polymers were prepared from diamines containing preformed hydrazides with diacid dichlorides [7– 9]. A few polyamide-hydrazides which were obtained from dihydrazides having preformed amide linkages with diacid dichlorides have been reported [10]. A phosphorylation method using aminohydrazides and diacids is also used for synthesis of these polymers [11– 14]. Wholly aromatic polyamide-hydrazides are successfully used in various engineering fields for their ability to form fibres with high mechanical strength and modulus [15–18], excellent salt rejection asymmetric membranes for water desalination [19–25] and efficient semiconductors from their modified metal chelates [26,27]. One of the important and potentially very useful properties of the entire class of wholly aromatic polyamide-hydrazides is certainly their characteristic behaviour at elevated temperatures which leads to materials of pronounced thermal stability in inert as well as in oxidative atmospheres [28–30]. Thermogravimetric analysis and differential scanning calorimetry studies showed that these polymers undergo a thermo-chemical transformation into the corresponding poly(amide1,3,4-oxadiazoles) by loss of water. The resulting 1,3,4oxadiazole containing polyamides could be rightfully classified among the highly thermally stable linear polymers. The reasons responsible for such stability of these polymers are expected to originate primarily from their chemical structure, which is composed of building units generally known to be highly resistant to increased temperatures, such as amide groups, aromatic moieties and 1,3,4-oxadiazolyl rings [31–33]. In addition to this, their high temperature stability is also expected to be further enhanced by their considerable crystallinity [32], which should be promoted by establishment of strong hydrogen bonding between the amide groups of the neighbouring chain segments. The kinetics of the thermal cyclodehydration reaction of hydrazide groups to oxadiazole rings have been investigated under different conditions of thermal treatment by varying the heating rate, degradation atmosphere, morphological state of the polymer samples (films or powder) and the sample’s preparation history. The results indicated a dependence of the kinetics on the morphological state or on the history of the polymer sample [34,35]. Significant differences in conversion
rates were observed between samples prepared by nonsolvent immersion precipitation and by evaporation of solvent. It appeared that contact with the nonsolvent, water, has a considerable influence on the conversion rate. Not only does immersion and washing with water lead to a better removal of the solvent but also the pH of the water determines the rate of conversion. A distinct change in the conversion rate was found after washing at pH 7, basic washing solutions showing lower conversion rates than acid ones. A mechanism explaining these phenomena was proposed [36]. Further, the introduction of aliphatic carbon-carbon linkages into the polymer backbone reduces the temperature at which decomposition of the polyamide-oxadiazoles is observed [37,38]. It is well known that all aromatic polymers containing only benzoxazole [39,40] or 1,3,4-oxadiazole [31–33] moieties as the main structural feature of the polymer chains exhibit excellent thermal stability and other attractive physical properties. Thus, our goal will be the evaluation of the thermal and thermo-oxidative stability of several wholly aromatic polyamide-hydrazides which would be expected to undergo thermo-chemical transformation into linear aromatic polymers with alternating 1,3,4-oxadiazole and benzoxazole structural units within the same polymer backbone in order to classify these materials as a category of heat resistant polymers. The second phase of this work will be extended to study the structure-thermal stability relationships of polyamide-hydrazides which contained different predetermined amounts of para- and meta-substituted phenylene units. Finally, this paper will describe some of recent results obtained from a study of the effect of the molecular weight of these polymers on their degradation behaviour.
2. Experimental procedures 2.1. Materials 2.1.1. Reagents 4-Amino-3-hydroxybenzoic acid and 3-amino-4hydroxybenzoic acid (Fluka, Germany); concentrated sulphuric acid and sodium carbonate (BDH, UK); hydrazine hydrate, 80% (Riedal-De-Haen, Germany); terephthaloyl dichloride [TCl], isophthaloyl dichloride [ICl] and calcium hydride (Acros, Germany) were of analytical grade and were used as received. 2.1.2. Solvents Methanol, 99.8% and ethanol, 99.9% (Hayman, UK) were extra pure reagents and were used as received without further purification. N,N-Dimethylacetamide [DMAc] (Aldrich, Germany) was guaranteed reagent, dried over calcium hydride for 24 h, and followed by
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distillation under reduced pressure. The fraction which boiled at 40–42 C/2 mm Hg, was collected and stored over molecular sieves until use. 2.2. Monomer preparation 4-Amino-3-hydroxybenzhydrazide [4A3HBH] and 3amino-4-hydroxybenzhydrazide [3A4HBH] were synthesized from the corresponding acid by two-step procedure in which the acid was esterified to its methyl ester followed by reaction of the product with hydrazine hydrate. A detailed description of this procedure is given elsewhere [41]. The crude materials were recrystallized twice from aqueous ethanol. Their purity was checked by elemental analysis and melting point. Elemental analysis: Calcd. for C7H9N3O2: C, 50.30%; H, 5.39%; N, 25.15%; O, 19.16%. Found for [4A3HBH] : C, 50.35%; H, 5.40%; N, 25.12%; O, 19.13%. Found for [3A4HBH]: C, 50.37%; H, 5.38%; N, 25.09%; O, 19.19%. Melting point for [4A3HBH], 223–224 C and for [3A4HBH], 227 C. 2.3. Polymer synthesis The first series of wholly aromatic polyamide-hydrazides evaluated in this study was synthesized by a low temperature (10 C) solution (in anhydrous DMAc) polycondensation reaction of either 4A3HBH or 3A4HBH with an equimolar amount of either TCl, ICl or mixtures of various molar ratios of TCl and ICl. The preparation was carried out by gradual addition of solid acid chloride to a cooled DMAc solution of aminohydroxybenzhydrazide. A detailed description of this polymerization technique was given elsewhere [41]. Twelve polymers containing various proportions of para- to meta-substituted phenylene moieties were obtained as shown in Table 1. The content of para- and meta-phen-
ylene moieties was varied within this series so that the changes in the latter were 10 mol% from polymer to polymer, starting from an overall content of 0–100 mol%. All of these polymers are novel and prepared for the first time in our laboratory [41] except poly[4-(terephthaloyl amino)-3-hydroxybenzoic acid hydrazide] and poly[3-(terephthaloylamino)-4-hydroxybenzoic acid hydrazide], or polymers I and VII in this work, which had already been reported previously [42,43]. All the polymers were produced in a quantitative yield which ranged between 98.12 and 100%. Although the intrinsic viscosity values (Table 1) are generally higher for polymers having greater para-phenylene units content, it is expected that the molecular weight of these polymers are generally comparable [41]. The higher intrinsic viscosity value of completely para-oriented phenylene type polymer is attributed to the higher rigidity and much interchain hydrogen-bonding as a result of its higher chain symmetry and packing efficiency. The resulting samples were characterized by elemental analysis and IR spectrophotometry. The results obtained agreed well with data reported previously [41] and they confirmed the expected polymer structures, which are illustrated in Table 2. The second series of polymers investigated included three samples of perfectly para-oriented phenylene polymer with different molecular weights (intrinsic viscosity) in addition to polymer sample I. They were prepared by polymerizing 4A3HBH with TCl at various molar ratios, again in DMAc as a solvent and at the same temperature indicated above, as shown in Table 3. 2.4. Polymer characterization 2.4.1. Viscosity Intrinsic viscosity measurements were carried out on 0.5% solutions of the polymers in DMAc at 30 C using a suspended-level Ubbelohde viscometer with negligible
Table 1 Wholly aromatic polyamide-hydrazides having various para-/meta- phenylene units contents Polymer code I II III IV V VI VII VIII IX X XI XII a
Molar monomers compositions in feed 4A3HBH
3A4HBH
TCl
ICl
Expected molar ratio of para-/meta-phenylene rings in polymers
1.0 1.0 1.0 1.0 1.0 1.0 – – – – – –
– – – – – – 1.0 1.0 1.0 1.0 1.0 1.0
1.0 0.8 0.6 0.4 0.2 – 1.0 0.8 0.6 0.4 0.2 –
– 0.2 0.4 0.6 0.8 1.0 – 0.2 0.4 0.6 0.8 1.0
1.0/0.0 0.9/0.1 0.8/0.2 0.7/0.3 0.6/0.4 0.5/0.5 0.5/0.5 0.4/0.6 0.3/0.7 0.2/0.8 0.1/0.9 0.0/1.0
Determined in DMAc at 30 C; polymer concentration was 0.5 g dl1.
Yield (%)
Intrinsic viscositya (dl g1)
99.66 99.00 99.83 99.66 99.32 98.12 99.10 100.00 99.32 99.72 98.80 99.40
4.83 3.95 3.49 2.97 2.58 2.10 2.03 1.77 1.53 1.28 1.01 0.73
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Table 2 Repeating units of the wholly aromatic polyamide-hydrazides Polymer code
Repeat unit
I
II
III
IV
V
VI
VII
VIII
IX
X
XI
XII
Table 3 Synthesis of completely para-oriented phenylene polymers having different molecular weights Polymer code
I XIII XIV XV a
Intrinsic viscositya (dl g1)
Molar monomers compositions in feed 4A3HBH
TCl
1.0 1.0 1.0 1.0
1.0 0.98 0.96 0.94
Determined in DMAc at 30 C; polymer concentration was 0.5 g dl1.
4.83 3.97 3.31 1.83
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kinetic energy correction and which allowed dilution in the viscometer. Flow times were measured at five different concentrations of the polymer sample. All the plots obtained were linear. Intrinsic viscosity was determined by the usual extrapolation of sp/C to zero concentration and are expressed in dl g1. 2.4.2. Differential scanning calorimetry (DSC) A Shimadzu differential scanning calorimeter, Model DSC-50 was used. The temperature interval between 30 and 600 C was investigated, using a constant heating rate of 10 C min1. The samples analysed were 3 0.2 mg in all cases. All DSC measurements were carried out in nitrogen with a flow rate of 30 ml min1. 2.4.3. Thermogravimetric analysis (TG) All samples were subjected to thermal degradation in air and in nitrogen atmospheres. A Shimadzu thermogravimetric Analyzer (TGA-50H) was used. The sample weights ranged from 3 to 5 mg and they were heated at constant rate of 10 C min1 in the temperature range of 30–800 C. Gas flow rate was in all experiments the same, at 30 ml min1.
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Since the areas under the curves are almost the same (allowing for differences in sample size), the extent of the reaction or degradation of the polymers is probably comparable. On the other hand, there is a gradual shift of the cyclodehydration peak to lower temperature as the content of meta-oriented phenylene units in the polymer increased. Moreover, the decomposition temperature of these polymers was found to depend upon the nature and amount of arylene groups incorporated into the polymer chains and increased with increase in the content of para-substituted phenylene rings in the polymer. The better thermal stability of the perfectly p-oriented type of polymer, I, relative to the other polymers, may be ascribed to its greater chain symmetry, which is responsible for its close packing, rodlike structure and much interchain hydrogen bonding. In order to give further evidence for these conclusions, the following experiments were carried out:
2.4.4. Infrared spectrophotometry A Perkin-Elmer Infrared Spectrophotometer (FTIR 1650) was used to record the IR spectra of the polymer samples. Thin films of similar thickness of around 3 mm were individually positioned between two anhydrous KBr pellets. All the spectra were recorded in the wave number range of 4000–600 cm1 at 25 C.
3. Results and discussion 3.1. Effect of the structural variations of the polymers on their thermal and thermo-oxidative stability 3.1.1. Differential scanning calorimetry (DSC) DSC was used to investigate the thermal stability and degradation behaviours of several wholly aromatic polyamide-hydrazides containing various proportions of para-/meta-phenylene moieties in their backbones. Fig. 1 illustrates the typical DSC thermograms of the evaluated polymers in nitrogen. DSC curves indicate a common thermal behaviour of these polymers which all exhibit two endotherms. The first is small and characteristic of evaporation of adsorbed surface water. The second is large and broad due to the thermally induced cyclodehydration reaction of the polymers into the corresponding poly(1,3,4-oxadiazolyl-benzoxazoles) by the loss of water [29,30,42,43] as represented in Scheme 1. Table 4 shows the structure of the resulting poly(1,3,4oxadiazolyl-benzoxazoles). Finally, oxidative degradation of the poly(1,3,4-oxadiazolyl-benzoxazoles) themselves was observed.
Fig. 1. DSC thermograms of wholly aromatic polyamide-hydrazides containing various proportions of para-/meta-phenylene moieties in nitrogen atmosphere at a heating rate of 10 C min1 and a gas flow rate of 30 ml min1.
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Table 4 Repeating units of the poly(1,3,4-oxadiazolyl-benzoxazoles) Polymer code
Repeat unit
I0
II0
III0
IV0
V0
VI0
VII0
VIII0
IX0
X0
XI0
XII0
(1) When polymer samples I, IV, IX, and XII were heated to 150 C in nitrogen and maintained for 30 min at this temperature, then cooled to room temperature and rescanned, the first endotherm disappeared, as shown in Fig. 2(a–d), curves (ii) indicating that liberated moisture was taken away by the purge-gas stream, so that it could not condense again on the samples surface, and completely dry samples were consequently obtained. Experimental proof of the loss of the adsor-
bed surface water can be seen in the elemental analysis values of these samples, which seem to be very close to those calculated for the polymer repeat units, whereas those of the same samples before heating showed appreciable differences as demonstrated in Table 5. (2) The cyclodehydration endotherm disappeared when the polymer samples I, IV, IX, and XII were first heated to 500, 430, 350 and 300 C, respectively, then quickly cooled back to room temperature and scanned
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Scheme 1.
Fig. 2. DSC thermograms of various wholly aromatic polyamide-hydrazides in nitrogen atmosphere at a heating rate of 10 C min1 and a gas flow rate of 30 ml min1: (a) polymer I: (i) full scan; (ii) rescan after preheating at 150 C; (iii) rescan after preheating to 500 C; (b) polymer IV: (i) full scan; (ii) rescan after preheating at 150 C; (iii) rescan after preheating to 430 C; (c) polymer IX: (i) full scan; (ii) rescan after preheating at 150 C; (iii) rescan after preheating to 350 C ; (d) polymer XII: (i) full scan; (ii) rescan after preheating at 150 C; (iii) rescan after preheating to 300 C.
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Table 5 Elemental analysis data of some polymers before and after thermal treatment Polymer code
Elemental analyses After thermal treatmenta
Before thermal treatment
Calculated for the expected repeating unit, C15H11N3O4 Found for I Found for IV Found for IX Found for XII a b
%C
%H
%N
% Ob
%C
%H
%N
% Ob
60.61 59.05 58.94 58.79 58.85
3.70 3.89 3.91 4.00 3.95
14.14 13.65 13.85 13.79 13.81
21.55 23.41 23.30 23.42 23.39
– 60.59 60.60 60.58 60.56
– 3.72 3.69 3.69 3.71
– 14.13 14.14 14.09 14.11
– 21.56 21.57 21.64 21.62
Polymer samples were thermally treated at 150 C for 30 min, in nitrogen atmosphere. Calculated as % O=100(% C+% H+% N).
again [Fig. 2(a–d) curves (iii)]. In addition, the cyclodehydration reaction was demonstrated experimentally by following the IR spectra of sample, I, which had been thermally treated at 500 C in nitrogen for various times. The heating periods ranged from 30 to 120 min. Fig. 3 illustrates the changes of the IR spectra of this sample as a function of thermal treatment time. The gradual reduction of the intensity of the absorption
Fig. 3. Changes of the IR spectra of sample I, which had been thermally treated at 500 C in nitrogen atmosphere for various time intervals: (a) 0 min; (b) 30 min; (c) 60 min; (d) 90 min; (e) 120 min.
peak at 3600–3100 cm1, which corresponds to groups, indicates almost complete cleavage of the hydrogen bonds and deprotonation of this group. The gradual disappearance of the carbonyl vibration band (at 1670–1650 cm1) suggested loss of carbonyl double bond character. Moreover, IR spectra showed the appearance of a new characteristic absorption band at 1620 cm1, corresponding to a C¼N stretching vibration of benzoxazole and 1,3,4-oxadiazole rings [28,42], in addition to the appearance of two reasonably strong bands at 1020 and 960 cm1, which are assigned to the ¼C–O–C¼ group [28,32,42]. These bands indicate the transformation of the polyamidehydrazides into poly(1,3,4-oxadiazolyl-benzoxazoles) by thermal treatment. This reaction was further proved experimentally by identifying the nature of the residual products formed in the later stages of thermal treatment. For this purpose, elemental analyses of the above samples, which had been thermally heated at the above-mentioned temperatures for 120 minutes were carried out. Results are presented in Table 6. It may be observed that the elemental analysis data are in excellent agreement with those calculated for the expected poly(1,3,4-oxadiazolylbenzoxazole) repeat units (Table 4). The results of both IR and elemental analyses are strong proof that weight loss in the second stage is due to the cyclodehydration reaction and are in accordance with the proposed structure for the poly(1,3,4-oxadiazolyl-benzoxazoles). Finally, it should be noted that it was not possible to observe any melting transitions of the evaluated polymers during the DSC measurements.
3.1.2. Thermogravimetric analysis (TG) Thermal stability and degradation behaviour of several wholly aromatic polyamide-hydrazides containing various proportions of para-/meta-phenylene rings incorporated into their chains were also investigated by TG measurements. All these measurements were per-
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Table 6 Elemental analysis data of some poly(1,3,4-oxadiazolyl-benzoxazoles)a Polymer code
Calculated for the expected repeating unit, C15H7N3O2 Found for I0 Found for IV0 Found for IX0 Found for XII0
Elemental analyses %C
%H
%N
% Ob
68.97 68.99 68.95 68.97 69.01
2.68 2.65 2.69 2.66 2.70
16.09 16.11 16.04 16.08 16.03
12.26 12.25 12.32 12.29 12.26
a Poly(1,3,4-oxadiazolyl-benzoxazoles), I0 , IV0 , IX0 and XII0 were prepared by the cyclodehydration reaction of the corresponding polyamidehydrazides, I, IV, IX, and XII at 500, 430, 350 and 300 C respectively, for 120 min in nitrogen atmosphere. b Calculated as % O=100(% C+% H+% N).
formed at 10 min1 heating rate, under the constant streams of prepurified nitrogen and air (the flow rates of which were in all cases 30 ml min1) and the results obtained are shown in Figs. 4 and 5, respectively. These results are in fairly good agreement with those of DSC. It can be seen from these results that in both degradation atmospheres all polymers showed a characteristic similar thermal behaviour which consisted of three distinct steps in which appreciable weight losses were observed. During the first weight-loss step, which occurred in both investigated atmospheres between 90 and 130 C, all of the samples exhibited relatively small losses of only about 1–3% of their original weights, as shown in Figs. 4 and 5. These weight losses were clearly
attributable to evaporation of adsorbed moisture from the surface of the polymer samples. The second step in which all of the investigated samples showed considerable losses, occurred in different temperature ranges for various polymers in nitrogen and in air atmospheres as listed in Tables 7 and 8, respectively. This step reflected the occurrence of the thermally induced cyclodehydration reaction of the polymers into the corresponding poly(1,3,4-oxadiazolyl-benzoxazoles) by losing water (Scheme 1). The amount of water evolved during the cyclodehydration reaction was 11–13.5 wt.% (based on the weight of the perfectly dried polymer samples), which seems to be in good agreement with the theoretical value (12 wt.%) calculated for the expected
Fig. 4. Typical TG thermograms patterns of polyamide-hydrazides containing various proportions of para-/meta-phenylene moieties. All the thermograms were recorded in nitrogen atmosphere at a heating rate of 10 C min1 and a gas flow rate of 30 ml min1.
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Fig. 5. Typical TG thermograms patterns of polyamide-hydrazides containing various proportions of para-/meta-phenylene moieties. All the thermograms were recorded in air atmosphere at a heating rate of 10 C min1. Table 7 Thermogravimetric analyses of wholly aromatic polyamide-hydrazides, in nitrogen atmosphere Polymer code I II III IV V VI VII VIII IX X XI XII
Onset of cyclodehydration reaction ( C)
End of cyclodehydration reaction ( C)
Weight loss at 470 C (%)
Onset of degradation ( C)
Percentage weight loss at 600 C
700 C
750–800 C
350 340 325 310 290 280 275 255 240 230 215 200
470 450 435 415 395 380 360 350 340 325 310 300
11.0 11.5 12.0 12.5 12.0 13.5 14.0 14.5 15.5 16.5 18.5 20.5
550 520 500 475 455 430 425 400 380 365 345 330
3.5 5.0 6.5 8.0 10.0 12.0 12.5 15.5 18.0 21.0 26.5 31.0
19.5 22.5 24.5 27.5 29.0 30.5 31.0 34.0 35.5 37.0 39.0 41.0
28.0 29.5 30.5 32.5 34.0 35.0 35.5 38.0 39.0 40.5 41.5 43.5
poly(1,3,4-oxadiazolyl-benzoxazoles) repeating units (Table 4). The third weight loss step is steep and indicated the decomposition of the polymers containing 1,3,4-oxadiazole and benzoxazole rings which were formed in the second step. As can be seen from Figs. 4 and 5, in both degradation atmosphere improved resistance to high temperatures
was always associated with increased content of paraphenylene moieties of the investigated polymer. Thus, at all used temperatures and in both degradation atmospheres, the wholly para-oriented phenylene polymer, I, showed the highest stability, relative to that of the other polymers, as judged by the lowest weight losses and by the highest initial decomposition temperature. On the
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N.A. Mohamed, A.O. Hamad Al-Dossary / Polymer Degradation and Stability 79 (2003) 61–75 Table 8 Thermogravimetric analyses of wholly aromatic polyamide-hydrazides in air Polymer code I II III IV V VI VII VIII IX X XI XII
Onset of cyclodehydration reaction ( C)
End of cyclodehydration reaction ( C)
Weight loss at 465 C (%)
350 335 315 305 285 280 275 250 235 220 210 200
465 445 435 410 390 375 350 340 330 320 305 300
11.0 11.5 12.0 12.5 13.0 14.5 15.5 18.0 19.5 22.5 26.5 30.0
other hand, the completely meta-oriented phenylene polymer, XII, exhibited the lowest thermal stability. The other polymers of this series aligned themselves in between these two extreme cases so that with respect to their weight remaining at any particular temperature their order of stability was I> II > III > IV > V > VI > VII > VIII > IX > X > XI > XII. Thus, substitution of para-phenylene rings for meta-phenylene units leads to improved polymer stability at high temperature in purely thermal as well as in thermo-oxidative conditions. This should also be associated with regularity of supermolecular packing within the bulk of the investigated polymers wherein the collinear arrangement of the para-phenylene units should allow for establishment of stronger intermolecular hydrogen bonds which would be more difficult to break and therefore more resistance to elevated temperatures. In both degradation atmospheres, the resulting poly(1,3,4-oxadiazolyl-benzoxazoles) start degradation in the temperature range above 550–330 C in nitrogen and 540–320 C in air without weight loss at lower temperatures as shown in Tables 7 and 8, respectively . They lost 28.0–43.5% of their original weights at 800 C in nitrogen. On the other hand, no residues were observed at 800 C in air. These results suggest that the degradation rate of the poly(1,3,4-oxadiazolyl-benzoxazoles) in air is faster than in nitrogen, and the former atmosphere appeared more destructive as confirmed by the weight loss of the samples at 800 C as well as at particular temperatures (Table 8). This high thermal stability may be attributed to the chemical structure of the polymer which possesses an aromatic, a benzoxazole and a 1,3,4-oxadiazole rings in its repeating unit. These groups are known to be highly resistant to elevated temperatures. This indicates that polyamide-hydrazides can be used as precursors for the preparation of thermally stable 1,3,4-oxadiazole-benzoxazole polymers.
Onset of degradation ( C)
Percentage weight loss at 600 C
700 C
750–800 C
540 515 495 470 445 430 425 395 370 360 340 320
22.5 26.5 30.0 36.5 41.5 46.0 47.0 50.0 52.5 55.5 58.0 60.0
53.0 60.0 65.0 68.5 71.0 73.0 73.0 75.0 76.5 78.0 79.0 80.0
73.5–100 80.0–100 82.5–100 85.0–100 86.0–100 87.0–100 87.0–100 87.5–100 88.5–100 89.5–100 89.5–100 90.0–100
3.2. Effect of the molecular weight on the thermal and thermo-oxidative stability of the polymer 3.2.1. Differential scanning calorimetry (DSC) In order to evaluate whether the molecular weight of the polymers has any effect on their thermal degradation behaviour, DSC measurements for three samples of perfectly para-substituted phenylene type polymers having different molecular weights (intrinsic viscosity, Table 3) were performed. Polymer I, of the highest molecular weight, which was investigated in the previous section of this study, is also included for comparison. Typical DSC thermograms obtained for the investigated samples are shown in Fig. 6. All these thermograms were recorded in nitrogen at a heating rate of 10 C min1. Major differences between the DSC behaviour of the polymer sample of highest molecular weight (I) and those of lower molecular weights (XIII, XIV and XV) were observed in the temperature range between 160 and 200 C, at which the lower molecular weight samples showed an additional endotherm. This endotherm disappeared if the samples XIII, XIV and XV were heated to 220 C, then cooled back to ambient temperature and rescanned again as shown in Fig. 7 (a– c) curves (ii). In order to identity the nature of the volatile products given off by samples XIII, XIV and XV under the above-mentioned range of temperature, elemental analyses and IR spectrophotometry for their residues left over after heating to 150 C and after 220 C were compared, and the volatile products were trapped in methanol cooled at 0 C. After removal of methanol, these products were analyzed by gas chromatography. Table 9 shows the elemental analysis data for samples XIII, XIV and XV before and after thermal treatment. It can be seen that elemental compositions of these samples thermally treated at 220 C are quite similar to
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Table 9 Elemental analysis data of perfectly para-oriented phenylene polymers having different molecular weights before and after thermal treatment in nitrogen atmosphere for 30 min Polymer code
Thermal treatment temperature ( C)
Elemental analysis %C
%H
%N
% Oa
Calculated for the expected repeating unit, C15H11N3O4 Found for XIII
– – 150 220
60.61 56.80 58.29 60.60
3.70 5.29 5.52 3.71
14.14 13.91 14.76 14.15
21.55 24.00 21.43 21.54
Found for XIV
– 150 220
56.95 58.32 60.59
5.11 5.44 3.68
13.86 14.83 14.12
24.08 21.41 21.61
Found for XV
– 150 220
57.01 58.49 60.63
5.10 5.61 3.72
13.45 14.69 14.16
24.44 21.21 21.49
a
Calculated as % O=100(% C+% H+% N).
Fig. 6. DSC thermograms of perfectly p-oriented phenylene type polymers of different molecular weights in nitrogen at a heating rate of 10 C min1 and a gas flow rate of 30 ml min1.
those calculated for the polymers themselves, whereas those of both the as-prepared and the samples thermally treated at 150 C showed appreciable differences. At the same time, no significant changes in IR spectra of the residues, left after thermal treatment at 220 C, relative to that of polymer sample I were observed. These results indicated that volatile products given off by the lower molecular weight samples could not be products of their true degradation, but rather some compounds which had been bonded to them by some secondary association forces probably stronger than simple adsorption forces but at the same time weaker than the covalent bonds. This suggested that the volatile product could have been DMAc, the polycondensation medium, left in the samples even after their vacuum drying to constant
Fig. 7. DSC thermograms of completely p-oriented phenylene type polymers of different molecular weights in nitrogen at a heating rate of 10 C min1 and a gas flow rate of 30 ml min1: (a) sample XIII; (b) sample XIV; (c) sample XV (i) full scan; (ii) rescan after preheating to 220 C.
N.A. Mohamed, A.O. Hamad Al-Dossary / Polymer Degradation and Stability 79 (2003) 61–75
73
weight. DMAc may be associated with the polar groups of the polymer by hydrogen bonds and could not be removed by vacuum drying to constant weight at 75 C (the used drying condition of the polymers). In order to verify this assumption, volatized products which were trapped during the heating of the lower molecular weight samples between 160 and 220 C were analyzed by gas chromatography. The results obtained correlated well with the DMAc standard. These results clearly support the observation which had been reported previously [29,30]. Thus, it can be concluded from Fig. 6 that with the exception of the endotherm at 160–200 C, all of the investigated samples showed similar behaviour regardless to their molecular weights. In addition, their other endotherms showed the same starting, maxima, ending and flushing temperatures. It is, however, not completely clear why DMAc associated only with the lower molecular weight samples, although the same drying treatment was used for all of these samples prior to their DSC runs.
perature range between evaporation of the adsorbed surface water and the cyclodehydration reaction for the high molecular weight sample, I, while a considerable weight loss (8–10%) could be detected for those of lower molecular weights, XIII, XIV and XV, in the temperature range of 160–200 C. This weight loss is attributed to the liberation of DMAc as previously confirmed by gas chromatography of the volatile product. All the investigated samples exhibited similar decomposition temperatures, which indicates that the high thermal stability is independent of the molecular weight (length of the macromolecular chains), but dependent upon the chemical structure of the repeating units. Further attempts to understand the reason for DMAc association with the lower molecular weight polymer, but not the high molecular weight one are in progress.
3.2.2. Thermogravimetric analysis (TG) TG measurements were also used to evaluate the influence of the molecular weight of the polymers on their degradation behaviour. For this purpose, polymer samples listed in Table 3 were used. TG results for these samples, obtained in nitrogen and in air, are illustrated in Figs. 8 and 9, respectively. In both degradation atmospheres, no weight loss was observed in the tem-
The thermal and thermo-oxidative stabilities of the polymers investigated are affected by the nature and amount of arylene groups incorporated into their chains, being higher for polymers with greater content of para-oriented phenylene rings, which permits more interchain hydrogen bonds as a results of greater chain symmetry and packing efficiency. Increasing the content of para-oriented phenylene rings leads to a strong
4. Conclusions
Fig. 8. Typical TG thermograms patterns of completely p-oriented phenylene type of polymers of different molecular weights in nitrogen at a heating rate of 10 C min1 and a gas flow rate of 30 ml min1.
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Fig. 9. Typical TG thermograms patterns of completely p-oriented phenylene type of polymers of different molecular weights in air at a heating rate of 10 C min1.
improvement in both the initial decomposition temperature as well as in the residual weight at a particular temperature. The major difference between the degradation behaviour under the purely thermal and under the thermo-oxidative conditions was observed at high temperatures. Air seemed much more destructive than nitrogen atmosphere, so that it led to complete annihilation of the investigated samples at 800 C, as compared to the residues of 72–56.5% which remained at the same temperature when nitrogen atmosphere was used. The stability of the polymers was found to be independent of their molecular weights. No significant differences could be observed between the degradation behaviour of the low molecular oligomers and the corresponding high molecular weight polymers at temperature above 200 C in nitrogen as well as in air. However, differences existed in both atmospheres at lower temperatures. Thus, while all of the investigated samples clearly showed a visible tendency towards absorption of moisture, which was liberated between 90 and 130 C during TG measurements, only lower molecular weight samples exhibited yet another pronounced weight loss (about 8–10% wt) at temperature range between 160 and 200 C. This could possibly be associated with liberation of DMAc, the polymerization solvent used in their preparation. The reasons for such behaviour do not seem clear on the basis of these results only. This confirms that high thermal stability is not a
polymer property which would depend upon the length of its macromolecular chains, but rather upon its chemical structure in which all and every atomic group (aromatic ring, 1,3,4-oxadiazole and benzoxazole rings) contributes by its own thermal stability to the macroscopic properties of the whole polymer.
References [1] Dvornic PR. J Appl Polym Sci 1983;28:2729. [2] Mohamed NA, Nakamae K. J Appl Polym Sci Appl Polym Symp 1993;52:297. [3] Mohamed NA. Eur Polym J 1995;31:859. [4] Shukla JS, Dwivedi H. J Polym Sci Polym Chem Ed 1979; 17:4129. [5] Nanjan MJ, Jayaprakash D. J Polym Sci Polym Chem Ed 1983; 21:1227. [6] Thanuja J, Srinivasan M. J Macromol Sci—Chem 1989;26:1007. [7] Mandal R, Maiti S. Die Angew Makromol Chem 1999;267:10. [8] Mandal R, Maiti S. Polym Int 1997;44:428. [9] Mandal R, Maiti S. J Appl Polym Sci 1997;64:1211. [10] Shukla JS, Dwivedi H. J Macromol Sci—Chem 1981;16:681. [11] Hsiao S-H, Chang L-M. J Polym Sci Polym Chem 2000;38:1599. [12] Hsiao S-H, Yu C-H. J Polym Sci Polym Chem 1998;36:1847. [13] Joseph KA, Srinivasan M. Polym Int 1993;32:257. [14] Hsiao S-H, Dai LR, He M-H. J Polym Sci Polym Chem 1999; 37:1169. [15] Preston J, Black WB, Hofferbert Jr WL. J Macromol Sci—Chem 1973;7:45.
N.A. Mohamed, A.O. Hamad Al-Dossary / Polymer Degradation and Stability 79 (2003) 61–75 [16] Preston J, Black WB, Hofferbert Jr WL. J Macromol Sci—Chem 1973;7:67. [17] Black WB, Preston J, Morgan HS, Raumann G, Lilyquist MR. J Macromol Sci—Chem 1973;7:137. [18] Black WB, Preston J. High-modulus wholly aromatic fibers. New York: Marcel Dekker; 1973. [19] McKinney Jr R, Rhodes JH. Macromolecules 1971;4:633. [20] Dvornic PR. J Polym Sci Polym Chem 1986;24:1133. [21] Satre MD, Ghatge ND, Ramani MPS. J Appl Polym Sci 1990; 41:697. [22] Dvornic PR. J Appl Polym Sci 1991;42:957. [23] Satre MD, Ghatge ND. Desalination 1993;91:121. [24] Nakamae K, Mohamed NA. J Appl Polym Sci Appl Polym Symp 1993;52:307. [25] Mohamed NA. Polymer 1997;38:4705. [26] Mohamed NA, Nakamae K. Polymer 1993;34:3940. [27] Mohamed NA. Eur Polym J 1998;34:387. [28] Culbertson BM, Murphy R. J Polym Sci Polym Lett 1967;5:807. [29] Dvornic PR. Bull Soc Chim Beograd 1984;49:589.
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[30] Mohamed NA. Polym Degrad Stab 1994;44:33. [31] Sauer J, Huisgen R, Sturm J. Tetrahedron 1960;11:214. [32] Frazer AH, Sweeny W, Wallenberger FT. J Polym Sci 1964; A2:1157. [33] Abshire CJ, Marvel CS. Makromol Chem 1961;44–46:388. [34] Gebben B, Mulder MHV, Smolders CA. J Polym Sci Polym Chem Ed 1988;26:1743. [35] Gebben B, Mulder MHV, Smolders CA. J Polym Sci Polym Chem Ed 1988;26:1757. [36] Gebben B, Rolevink E, Mulder MHV, Smolders CA. J Polym Sci Polym Chem Ed 1989;27:4129. [37] Frazer AH, Wallenberger FT. J Polym Sci 1964;A2:1137. [38] Nakamae K, Mohamed NA. Polymer 1993;34:1310. [39] Kubota T, Nakanishi R. J Polym Sci 1964;2:655. [40] Moyer WW, Cole Jr C., Anyos T. J Polym Sci 1965;A3:2107. [41] Mohamed NA, Al-dossary AO. Eur Polym J [in press]. [42] Culbertson BM, Murphy R. J Polym Sci Polym Lett 1966; 4:249. [43] Gervasi PG, Petris SD, Lupinacci D. Eur Polym J 1975;11:233.
Structure–property relationships for novel wholly aromatic polyamide-hydrazides containing various proportions of para-phenylene and meta-phenylene units II. Thermal stability and degradation behaviour Nadia Ahmed Mohameda,*, Abeer Obaid Hamad Al-Dossaryb a Department of Chemistry, Faculty of Science, Cairo University, Giza, Egypt Department of Chemistry, Faculty of Science, Girls College, Dammam, 838, Saudi Arabia
b
Received 15 April 2002; accepted 3 June 2002
Abstract The influences of controlled structural differences and molecular weight on the thermal stability and degradation behaviour of a series of novel wholly aromatic polyamide-hydrazides have been investigated in nitrogen and in air using differential scanning calorimetry (DSC), thermogravimetric analysis (TG), infrared spectrophotometry (IR) and elemental analysis. The structural variations of the polymers were achieved by varying the content of the meta- and para-substituted phenylene moieties incorporated into their chains. All the polymers were synthesized by a low temperature solution polycondensation reaction of either 4-amino-3hydroxybenzhydrazide [4A3HBH] or 3-amino-4-hydroxybenzhydrazide [3A4HBH] with an equimolar amount of either terephthaloyl chloride [TCl], isophthaloyl chloride [ICl] or mixtures of various molar ratios of TCl and ICl in anhydrous N,N-dimethylacetamide [DMAc] as a solvent at 10 C. The content of para- and meta-phenylene moieties was varied within this series so that the changes in the latter were 10 mol% from polymer to polymer, starting from an overall content of 0–100 mol%. All the polymers have the same structural formula except for the way of linking phenylene units in the polymer chain. The results showed that these polymers have high resistance to elevated temperatures. Their weight loss occurred in three steps. The first was small and was attributed to evaporation of adsorbed surface water. The second was considerable and was assigned to cyclodehydration reactions of the polyamide-hydrazides into the corresponding poly(1,3,4-oxadiazolyl-benzoxazoles) with loss of water. This is not a true degradation, but rather a thermochemical transformation reaction. The third was relatively severe and steep, particularly in air, and corresponded to the decomposition of the polymers. The results clearly indicate that substitution of para-phenylene units for meta-phenylene ones within this polymer series leads to improved polymer stability at elevated temperatures in nitrogen as well as in air. This should be associated with regularity of supermolecular packing within the bulk of the investigated polymers wherein the colinear arrangement of the para-phenylene units should allow for establishment of stronger intermolecular bonds which would be more difficult to break and therefore more resistant to high temperatures. Moreover, polyamide-hydrazides having different molecular weights were also examined. The results clearly reveal that at all temperatures used and in both degradation atmospheres all the investigated samples exhibited similar thermal behaviour regardless of their molecular weights, except in the temperature range 160–200 C where the lower molecular weight samples showed significant weight losses which may be attributed to hydrogen bonded DMAc. This indicates that structural building units of these polymers (which contained characteristic groups, such as: aromatic rings and amide and hydrazide linking bonds in the case of polyamide-hydrazides and aromatic nuclei, 1,3,4-oxadiazolyl rings and benzoxazolyl moieties in the case of poly(1,3,4-oxadiazolyl-benzoxazoles) are responsible for their high thermal stability, rather than the longer chain segments. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Polyamide-hydrazides; Thermal and thermo-oxidative stability; Differential scanning calorimetry; Thermogravimetric analysis, poly(1,3,4-oxadiazolyl-benzoxazoles)
* Corresponding author. E-mail address: [email protected] (N.A. Mohamed). 0141-3910/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0141-3910(02)00239-2
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1. Introduction Wholly aromatic polyamide-hydrazides of the general formula [CO–NH–Ar–CO–NH–NH–CO–Ar0 ]n, where Ar and Ar0 are meta- and/or para-substituted phenylene units, have attracted the attention of many investigators in recent years as high performance materials. The general method adopted to synthesize these polymers is a low temperature solution polycondensation reaction of aromatic aminohydrazides with aromatic dicarboxylic acid dichlorides in an amide solvent [1–6]. Completely ordered polymers were prepared from diamines containing preformed hydrazides with diacid dichlorides [7– 9]. A few polyamide-hydrazides which were obtained from dihydrazides having preformed amide linkages with diacid dichlorides have been reported [10]. A phosphorylation method using aminohydrazides and diacids is also used for synthesis of these polymers [11– 14]. Wholly aromatic polyamide-hydrazides are successfully used in various engineering fields for their ability to form fibres with high mechanical strength and modulus [15–18], excellent salt rejection asymmetric membranes for water desalination [19–25] and efficient semiconductors from their modified metal chelates [26,27]. One of the important and potentially very useful properties of the entire class of wholly aromatic polyamide-hydrazides is certainly their characteristic behaviour at elevated temperatures which leads to materials of pronounced thermal stability in inert as well as in oxidative atmospheres [28–30]. Thermogravimetric analysis and differential scanning calorimetry studies showed that these polymers undergo a thermo-chemical transformation into the corresponding poly(amide1,3,4-oxadiazoles) by loss of water. The resulting 1,3,4oxadiazole containing polyamides could be rightfully classified among the highly thermally stable linear polymers. The reasons responsible for such stability of these polymers are expected to originate primarily from their chemical structure, which is composed of building units generally known to be highly resistant to increased temperatures, such as amide groups, aromatic moieties and 1,3,4-oxadiazolyl rings [31–33]. In addition to this, their high temperature stability is also expected to be further enhanced by their considerable crystallinity [32], which should be promoted by establishment of strong hydrogen bonding between the amide groups of the neighbouring chain segments. The kinetics of the thermal cyclodehydration reaction of hydrazide groups to oxadiazole rings have been investigated under different conditions of thermal treatment by varying the heating rate, degradation atmosphere, morphological state of the polymer samples (films or powder) and the sample’s preparation history. The results indicated a dependence of the kinetics on the morphological state or on the history of the polymer sample [34,35]. Significant differences in conversion
rates were observed between samples prepared by nonsolvent immersion precipitation and by evaporation of solvent. It appeared that contact with the nonsolvent, water, has a considerable influence on the conversion rate. Not only does immersion and washing with water lead to a better removal of the solvent but also the pH of the water determines the rate of conversion. A distinct change in the conversion rate was found after washing at pH 7, basic washing solutions showing lower conversion rates than acid ones. A mechanism explaining these phenomena was proposed [36]. Further, the introduction of aliphatic carbon-carbon linkages into the polymer backbone reduces the temperature at which decomposition of the polyamide-oxadiazoles is observed [37,38]. It is well known that all aromatic polymers containing only benzoxazole [39,40] or 1,3,4-oxadiazole [31–33] moieties as the main structural feature of the polymer chains exhibit excellent thermal stability and other attractive physical properties. Thus, our goal will be the evaluation of the thermal and thermo-oxidative stability of several wholly aromatic polyamide-hydrazides which would be expected to undergo thermo-chemical transformation into linear aromatic polymers with alternating 1,3,4-oxadiazole and benzoxazole structural units within the same polymer backbone in order to classify these materials as a category of heat resistant polymers. The second phase of this work will be extended to study the structure-thermal stability relationships of polyamide-hydrazides which contained different predetermined amounts of para- and meta-substituted phenylene units. Finally, this paper will describe some of recent results obtained from a study of the effect of the molecular weight of these polymers on their degradation behaviour.
2. Experimental procedures 2.1. Materials 2.1.1. Reagents 4-Amino-3-hydroxybenzoic acid and 3-amino-4hydroxybenzoic acid (Fluka, Germany); concentrated sulphuric acid and sodium carbonate (BDH, UK); hydrazine hydrate, 80% (Riedal-De-Haen, Germany); terephthaloyl dichloride [TCl], isophthaloyl dichloride [ICl] and calcium hydride (Acros, Germany) were of analytical grade and were used as received. 2.1.2. Solvents Methanol, 99.8% and ethanol, 99.9% (Hayman, UK) were extra pure reagents and were used as received without further purification. N,N-Dimethylacetamide [DMAc] (Aldrich, Germany) was guaranteed reagent, dried over calcium hydride for 24 h, and followed by
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distillation under reduced pressure. The fraction which boiled at 40–42 C/2 mm Hg, was collected and stored over molecular sieves until use. 2.2. Monomer preparation 4-Amino-3-hydroxybenzhydrazide [4A3HBH] and 3amino-4-hydroxybenzhydrazide [3A4HBH] were synthesized from the corresponding acid by two-step procedure in which the acid was esterified to its methyl ester followed by reaction of the product with hydrazine hydrate. A detailed description of this procedure is given elsewhere [41]. The crude materials were recrystallized twice from aqueous ethanol. Their purity was checked by elemental analysis and melting point. Elemental analysis: Calcd. for C7H9N3O2: C, 50.30%; H, 5.39%; N, 25.15%; O, 19.16%. Found for [4A3HBH] : C, 50.35%; H, 5.40%; N, 25.12%; O, 19.13%. Found for [3A4HBH]: C, 50.37%; H, 5.38%; N, 25.09%; O, 19.19%. Melting point for [4A3HBH], 223–224 C and for [3A4HBH], 227 C. 2.3. Polymer synthesis The first series of wholly aromatic polyamide-hydrazides evaluated in this study was synthesized by a low temperature (10 C) solution (in anhydrous DMAc) polycondensation reaction of either 4A3HBH or 3A4HBH with an equimolar amount of either TCl, ICl or mixtures of various molar ratios of TCl and ICl. The preparation was carried out by gradual addition of solid acid chloride to a cooled DMAc solution of aminohydroxybenzhydrazide. A detailed description of this polymerization technique was given elsewhere [41]. Twelve polymers containing various proportions of para- to meta-substituted phenylene moieties were obtained as shown in Table 1. The content of para- and meta-phen-
ylene moieties was varied within this series so that the changes in the latter were 10 mol% from polymer to polymer, starting from an overall content of 0–100 mol%. All of these polymers are novel and prepared for the first time in our laboratory [41] except poly[4-(terephthaloyl amino)-3-hydroxybenzoic acid hydrazide] and poly[3-(terephthaloylamino)-4-hydroxybenzoic acid hydrazide], or polymers I and VII in this work, which had already been reported previously [42,43]. All the polymers were produced in a quantitative yield which ranged between 98.12 and 100%. Although the intrinsic viscosity values (Table 1) are generally higher for polymers having greater para-phenylene units content, it is expected that the molecular weight of these polymers are generally comparable [41]. The higher intrinsic viscosity value of completely para-oriented phenylene type polymer is attributed to the higher rigidity and much interchain hydrogen-bonding as a result of its higher chain symmetry and packing efficiency. The resulting samples were characterized by elemental analysis and IR spectrophotometry. The results obtained agreed well with data reported previously [41] and they confirmed the expected polymer structures, which are illustrated in Table 2. The second series of polymers investigated included three samples of perfectly para-oriented phenylene polymer with different molecular weights (intrinsic viscosity) in addition to polymer sample I. They were prepared by polymerizing 4A3HBH with TCl at various molar ratios, again in DMAc as a solvent and at the same temperature indicated above, as shown in Table 3. 2.4. Polymer characterization 2.4.1. Viscosity Intrinsic viscosity measurements were carried out on 0.5% solutions of the polymers in DMAc at 30 C using a suspended-level Ubbelohde viscometer with negligible
Table 1 Wholly aromatic polyamide-hydrazides having various para-/meta- phenylene units contents Polymer code I II III IV V VI VII VIII IX X XI XII a
Molar monomers compositions in feed 4A3HBH
3A4HBH
TCl
ICl
Expected molar ratio of para-/meta-phenylene rings in polymers
1.0 1.0 1.0 1.0 1.0 1.0 – – – – – –
– – – – – – 1.0 1.0 1.0 1.0 1.0 1.0
1.0 0.8 0.6 0.4 0.2 – 1.0 0.8 0.6 0.4 0.2 –
– 0.2 0.4 0.6 0.8 1.0 – 0.2 0.4 0.6 0.8 1.0
1.0/0.0 0.9/0.1 0.8/0.2 0.7/0.3 0.6/0.4 0.5/0.5 0.5/0.5 0.4/0.6 0.3/0.7 0.2/0.8 0.1/0.9 0.0/1.0
Determined in DMAc at 30 C; polymer concentration was 0.5 g dl1.
Yield (%)
Intrinsic viscositya (dl g1)
99.66 99.00 99.83 99.66 99.32 98.12 99.10 100.00 99.32 99.72 98.80 99.40
4.83 3.95 3.49 2.97 2.58 2.10 2.03 1.77 1.53 1.28 1.01 0.73
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Table 2 Repeating units of the wholly aromatic polyamide-hydrazides Polymer code
Repeat unit
I
II
III
IV
V
VI
VII
VIII
IX
X
XI
XII
Table 3 Synthesis of completely para-oriented phenylene polymers having different molecular weights Polymer code
I XIII XIV XV a
Intrinsic viscositya (dl g1)
Molar monomers compositions in feed 4A3HBH
TCl
1.0 1.0 1.0 1.0
1.0 0.98 0.96 0.94
Determined in DMAc at 30 C; polymer concentration was 0.5 g dl1.
4.83 3.97 3.31 1.83
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kinetic energy correction and which allowed dilution in the viscometer. Flow times were measured at five different concentrations of the polymer sample. All the plots obtained were linear. Intrinsic viscosity was determined by the usual extrapolation of sp/C to zero concentration and are expressed in dl g1. 2.4.2. Differential scanning calorimetry (DSC) A Shimadzu differential scanning calorimeter, Model DSC-50 was used. The temperature interval between 30 and 600 C was investigated, using a constant heating rate of 10 C min1. The samples analysed were 3 0.2 mg in all cases. All DSC measurements were carried out in nitrogen with a flow rate of 30 ml min1. 2.4.3. Thermogravimetric analysis (TG) All samples were subjected to thermal degradation in air and in nitrogen atmospheres. A Shimadzu thermogravimetric Analyzer (TGA-50H) was used. The sample weights ranged from 3 to 5 mg and they were heated at constant rate of 10 C min1 in the temperature range of 30–800 C. Gas flow rate was in all experiments the same, at 30 ml min1.
65
Since the areas under the curves are almost the same (allowing for differences in sample size), the extent of the reaction or degradation of the polymers is probably comparable. On the other hand, there is a gradual shift of the cyclodehydration peak to lower temperature as the content of meta-oriented phenylene units in the polymer increased. Moreover, the decomposition temperature of these polymers was found to depend upon the nature and amount of arylene groups incorporated into the polymer chains and increased with increase in the content of para-substituted phenylene rings in the polymer. The better thermal stability of the perfectly p-oriented type of polymer, I, relative to the other polymers, may be ascribed to its greater chain symmetry, which is responsible for its close packing, rodlike structure and much interchain hydrogen bonding. In order to give further evidence for these conclusions, the following experiments were carried out:
2.4.4. Infrared spectrophotometry A Perkin-Elmer Infrared Spectrophotometer (FTIR 1650) was used to record the IR spectra of the polymer samples. Thin films of similar thickness of around 3 mm were individually positioned between two anhydrous KBr pellets. All the spectra were recorded in the wave number range of 4000–600 cm1 at 25 C.
3. Results and discussion 3.1. Effect of the structural variations of the polymers on their thermal and thermo-oxidative stability 3.1.1. Differential scanning calorimetry (DSC) DSC was used to investigate the thermal stability and degradation behaviours of several wholly aromatic polyamide-hydrazides containing various proportions of para-/meta-phenylene moieties in their backbones. Fig. 1 illustrates the typical DSC thermograms of the evaluated polymers in nitrogen. DSC curves indicate a common thermal behaviour of these polymers which all exhibit two endotherms. The first is small and characteristic of evaporation of adsorbed surface water. The second is large and broad due to the thermally induced cyclodehydration reaction of the polymers into the corresponding poly(1,3,4-oxadiazolyl-benzoxazoles) by the loss of water [29,30,42,43] as represented in Scheme 1. Table 4 shows the structure of the resulting poly(1,3,4oxadiazolyl-benzoxazoles). Finally, oxidative degradation of the poly(1,3,4-oxadiazolyl-benzoxazoles) themselves was observed.
Fig. 1. DSC thermograms of wholly aromatic polyamide-hydrazides containing various proportions of para-/meta-phenylene moieties in nitrogen atmosphere at a heating rate of 10 C min1 and a gas flow rate of 30 ml min1.
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Table 4 Repeating units of the poly(1,3,4-oxadiazolyl-benzoxazoles) Polymer code
Repeat unit
I0
II0
III0
IV0
V0
VI0
VII0
VIII0
IX0
X0
XI0
XII0
(1) When polymer samples I, IV, IX, and XII were heated to 150 C in nitrogen and maintained for 30 min at this temperature, then cooled to room temperature and rescanned, the first endotherm disappeared, as shown in Fig. 2(a–d), curves (ii) indicating that liberated moisture was taken away by the purge-gas stream, so that it could not condense again on the samples surface, and completely dry samples were consequently obtained. Experimental proof of the loss of the adsor-
bed surface water can be seen in the elemental analysis values of these samples, which seem to be very close to those calculated for the polymer repeat units, whereas those of the same samples before heating showed appreciable differences as demonstrated in Table 5. (2) The cyclodehydration endotherm disappeared when the polymer samples I, IV, IX, and XII were first heated to 500, 430, 350 and 300 C, respectively, then quickly cooled back to room temperature and scanned
N.A. Mohamed, A.O. Hamad Al-Dossary / Polymer Degradation and Stability 79 (2003) 61–75
67
Scheme 1.
Fig. 2. DSC thermograms of various wholly aromatic polyamide-hydrazides in nitrogen atmosphere at a heating rate of 10 C min1 and a gas flow rate of 30 ml min1: (a) polymer I: (i) full scan; (ii) rescan after preheating at 150 C; (iii) rescan after preheating to 500 C; (b) polymer IV: (i) full scan; (ii) rescan after preheating at 150 C; (iii) rescan after preheating to 430 C; (c) polymer IX: (i) full scan; (ii) rescan after preheating at 150 C; (iii) rescan after preheating to 350 C ; (d) polymer XII: (i) full scan; (ii) rescan after preheating at 150 C; (iii) rescan after preheating to 300 C.
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Table 5 Elemental analysis data of some polymers before and after thermal treatment Polymer code
Elemental analyses After thermal treatmenta
Before thermal treatment
Calculated for the expected repeating unit, C15H11N3O4 Found for I Found for IV Found for IX Found for XII a b
%C
%H
%N
% Ob
%C
%H
%N
% Ob
60.61 59.05 58.94 58.79 58.85
3.70 3.89 3.91 4.00 3.95
14.14 13.65 13.85 13.79 13.81
21.55 23.41 23.30 23.42 23.39
– 60.59 60.60 60.58 60.56
– 3.72 3.69 3.69 3.71
– 14.13 14.14 14.09 14.11
– 21.56 21.57 21.64 21.62
Polymer samples were thermally treated at 150 C for 30 min, in nitrogen atmosphere. Calculated as % O=100(% C+% H+% N).
again [Fig. 2(a–d) curves (iii)]. In addition, the cyclodehydration reaction was demonstrated experimentally by following the IR spectra of sample, I, which had been thermally treated at 500 C in nitrogen for various times. The heating periods ranged from 30 to 120 min. Fig. 3 illustrates the changes of the IR spectra of this sample as a function of thermal treatment time. The gradual reduction of the intensity of the absorption
Fig. 3. Changes of the IR spectra of sample I, which had been thermally treated at 500 C in nitrogen atmosphere for various time intervals: (a) 0 min; (b) 30 min; (c) 60 min; (d) 90 min; (e) 120 min.
peak at 3600–3100 cm1, which corresponds to groups, indicates almost complete cleavage of the hydrogen bonds and deprotonation of this group. The gradual disappearance of the carbonyl vibration band (at 1670–1650 cm1) suggested loss of carbonyl double bond character. Moreover, IR spectra showed the appearance of a new characteristic absorption band at 1620 cm1, corresponding to a C¼N stretching vibration of benzoxazole and 1,3,4-oxadiazole rings [28,42], in addition to the appearance of two reasonably strong bands at 1020 and 960 cm1, which are assigned to the ¼C–O–C¼ group [28,32,42]. These bands indicate the transformation of the polyamidehydrazides into poly(1,3,4-oxadiazolyl-benzoxazoles) by thermal treatment. This reaction was further proved experimentally by identifying the nature of the residual products formed in the later stages of thermal treatment. For this purpose, elemental analyses of the above samples, which had been thermally heated at the above-mentioned temperatures for 120 minutes were carried out. Results are presented in Table 6. It may be observed that the elemental analysis data are in excellent agreement with those calculated for the expected poly(1,3,4-oxadiazolylbenzoxazole) repeat units (Table 4). The results of both IR and elemental analyses are strong proof that weight loss in the second stage is due to the cyclodehydration reaction and are in accordance with the proposed structure for the poly(1,3,4-oxadiazolyl-benzoxazoles). Finally, it should be noted that it was not possible to observe any melting transitions of the evaluated polymers during the DSC measurements.
3.1.2. Thermogravimetric analysis (TG) Thermal stability and degradation behaviour of several wholly aromatic polyamide-hydrazides containing various proportions of para-/meta-phenylene rings incorporated into their chains were also investigated by TG measurements. All these measurements were per-
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Table 6 Elemental analysis data of some poly(1,3,4-oxadiazolyl-benzoxazoles)a Polymer code
Calculated for the expected repeating unit, C15H7N3O2 Found for I0 Found for IV0 Found for IX0 Found for XII0
Elemental analyses %C
%H
%N
% Ob
68.97 68.99 68.95 68.97 69.01
2.68 2.65 2.69 2.66 2.70
16.09 16.11 16.04 16.08 16.03
12.26 12.25 12.32 12.29 12.26
a Poly(1,3,4-oxadiazolyl-benzoxazoles), I0 , IV0 , IX0 and XII0 were prepared by the cyclodehydration reaction of the corresponding polyamidehydrazides, I, IV, IX, and XII at 500, 430, 350 and 300 C respectively, for 120 min in nitrogen atmosphere. b Calculated as % O=100(% C+% H+% N).
formed at 10 min1 heating rate, under the constant streams of prepurified nitrogen and air (the flow rates of which were in all cases 30 ml min1) and the results obtained are shown in Figs. 4 and 5, respectively. These results are in fairly good agreement with those of DSC. It can be seen from these results that in both degradation atmospheres all polymers showed a characteristic similar thermal behaviour which consisted of three distinct steps in which appreciable weight losses were observed. During the first weight-loss step, which occurred in both investigated atmospheres between 90 and 130 C, all of the samples exhibited relatively small losses of only about 1–3% of their original weights, as shown in Figs. 4 and 5. These weight losses were clearly
attributable to evaporation of adsorbed moisture from the surface of the polymer samples. The second step in which all of the investigated samples showed considerable losses, occurred in different temperature ranges for various polymers in nitrogen and in air atmospheres as listed in Tables 7 and 8, respectively. This step reflected the occurrence of the thermally induced cyclodehydration reaction of the polymers into the corresponding poly(1,3,4-oxadiazolyl-benzoxazoles) by losing water (Scheme 1). The amount of water evolved during the cyclodehydration reaction was 11–13.5 wt.% (based on the weight of the perfectly dried polymer samples), which seems to be in good agreement with the theoretical value (12 wt.%) calculated for the expected
Fig. 4. Typical TG thermograms patterns of polyamide-hydrazides containing various proportions of para-/meta-phenylene moieties. All the thermograms were recorded in nitrogen atmosphere at a heating rate of 10 C min1 and a gas flow rate of 30 ml min1.
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Fig. 5. Typical TG thermograms patterns of polyamide-hydrazides containing various proportions of para-/meta-phenylene moieties. All the thermograms were recorded in air atmosphere at a heating rate of 10 C min1. Table 7 Thermogravimetric analyses of wholly aromatic polyamide-hydrazides, in nitrogen atmosphere Polymer code I II III IV V VI VII VIII IX X XI XII
Onset of cyclodehydration reaction ( C)
End of cyclodehydration reaction ( C)
Weight loss at 470 C (%)
Onset of degradation ( C)
Percentage weight loss at 600 C
700 C
750–800 C
350 340 325 310 290 280 275 255 240 230 215 200
470 450 435 415 395 380 360 350 340 325 310 300
11.0 11.5 12.0 12.5 12.0 13.5 14.0 14.5 15.5 16.5 18.5 20.5
550 520 500 475 455 430 425 400 380 365 345 330
3.5 5.0 6.5 8.0 10.0 12.0 12.5 15.5 18.0 21.0 26.5 31.0
19.5 22.5 24.5 27.5 29.0 30.5 31.0 34.0 35.5 37.0 39.0 41.0
28.0 29.5 30.5 32.5 34.0 35.0 35.5 38.0 39.0 40.5 41.5 43.5
poly(1,3,4-oxadiazolyl-benzoxazoles) repeating units (Table 4). The third weight loss step is steep and indicated the decomposition of the polymers containing 1,3,4-oxadiazole and benzoxazole rings which were formed in the second step. As can be seen from Figs. 4 and 5, in both degradation atmosphere improved resistance to high temperatures
was always associated with increased content of paraphenylene moieties of the investigated polymer. Thus, at all used temperatures and in both degradation atmospheres, the wholly para-oriented phenylene polymer, I, showed the highest stability, relative to that of the other polymers, as judged by the lowest weight losses and by the highest initial decomposition temperature. On the
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N.A. Mohamed, A.O. Hamad Al-Dossary / Polymer Degradation and Stability 79 (2003) 61–75 Table 8 Thermogravimetric analyses of wholly aromatic polyamide-hydrazides in air Polymer code I II III IV V VI VII VIII IX X XI XII
Onset of cyclodehydration reaction ( C)
End of cyclodehydration reaction ( C)
Weight loss at 465 C (%)
350 335 315 305 285 280 275 250 235 220 210 200
465 445 435 410 390 375 350 340 330 320 305 300
11.0 11.5 12.0 12.5 13.0 14.5 15.5 18.0 19.5 22.5 26.5 30.0
other hand, the completely meta-oriented phenylene polymer, XII, exhibited the lowest thermal stability. The other polymers of this series aligned themselves in between these two extreme cases so that with respect to their weight remaining at any particular temperature their order of stability was I> II > III > IV > V > VI > VII > VIII > IX > X > XI > XII. Thus, substitution of para-phenylene rings for meta-phenylene units leads to improved polymer stability at high temperature in purely thermal as well as in thermo-oxidative conditions. This should also be associated with regularity of supermolecular packing within the bulk of the investigated polymers wherein the collinear arrangement of the para-phenylene units should allow for establishment of stronger intermolecular hydrogen bonds which would be more difficult to break and therefore more resistance to elevated temperatures. In both degradation atmospheres, the resulting poly(1,3,4-oxadiazolyl-benzoxazoles) start degradation in the temperature range above 550–330 C in nitrogen and 540–320 C in air without weight loss at lower temperatures as shown in Tables 7 and 8, respectively . They lost 28.0–43.5% of their original weights at 800 C in nitrogen. On the other hand, no residues were observed at 800 C in air. These results suggest that the degradation rate of the poly(1,3,4-oxadiazolyl-benzoxazoles) in air is faster than in nitrogen, and the former atmosphere appeared more destructive as confirmed by the weight loss of the samples at 800 C as well as at particular temperatures (Table 8). This high thermal stability may be attributed to the chemical structure of the polymer which possesses an aromatic, a benzoxazole and a 1,3,4-oxadiazole rings in its repeating unit. These groups are known to be highly resistant to elevated temperatures. This indicates that polyamide-hydrazides can be used as precursors for the preparation of thermally stable 1,3,4-oxadiazole-benzoxazole polymers.
Onset of degradation ( C)
Percentage weight loss at 600 C
700 C
750–800 C
540 515 495 470 445 430 425 395 370 360 340 320
22.5 26.5 30.0 36.5 41.5 46.0 47.0 50.0 52.5 55.5 58.0 60.0
53.0 60.0 65.0 68.5 71.0 73.0 73.0 75.0 76.5 78.0 79.0 80.0
73.5–100 80.0–100 82.5–100 85.0–100 86.0–100 87.0–100 87.0–100 87.5–100 88.5–100 89.5–100 89.5–100 90.0–100
3.2. Effect of the molecular weight on the thermal and thermo-oxidative stability of the polymer 3.2.1. Differential scanning calorimetry (DSC) In order to evaluate whether the molecular weight of the polymers has any effect on their thermal degradation behaviour, DSC measurements for three samples of perfectly para-substituted phenylene type polymers having different molecular weights (intrinsic viscosity, Table 3) were performed. Polymer I, of the highest molecular weight, which was investigated in the previous section of this study, is also included for comparison. Typical DSC thermograms obtained for the investigated samples are shown in Fig. 6. All these thermograms were recorded in nitrogen at a heating rate of 10 C min1. Major differences between the DSC behaviour of the polymer sample of highest molecular weight (I) and those of lower molecular weights (XIII, XIV and XV) were observed in the temperature range between 160 and 200 C, at which the lower molecular weight samples showed an additional endotherm. This endotherm disappeared if the samples XIII, XIV and XV were heated to 220 C, then cooled back to ambient temperature and rescanned again as shown in Fig. 7 (a– c) curves (ii). In order to identity the nature of the volatile products given off by samples XIII, XIV and XV under the above-mentioned range of temperature, elemental analyses and IR spectrophotometry for their residues left over after heating to 150 C and after 220 C were compared, and the volatile products were trapped in methanol cooled at 0 C. After removal of methanol, these products were analyzed by gas chromatography. Table 9 shows the elemental analysis data for samples XIII, XIV and XV before and after thermal treatment. It can be seen that elemental compositions of these samples thermally treated at 220 C are quite similar to
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Table 9 Elemental analysis data of perfectly para-oriented phenylene polymers having different molecular weights before and after thermal treatment in nitrogen atmosphere for 30 min Polymer code
Thermal treatment temperature ( C)
Elemental analysis %C
%H
%N
% Oa
Calculated for the expected repeating unit, C15H11N3O4 Found for XIII
– – 150 220
60.61 56.80 58.29 60.60
3.70 5.29 5.52 3.71
14.14 13.91 14.76 14.15
21.55 24.00 21.43 21.54
Found for XIV
– 150 220
56.95 58.32 60.59
5.11 5.44 3.68
13.86 14.83 14.12
24.08 21.41 21.61
Found for XV
– 150 220
57.01 58.49 60.63
5.10 5.61 3.72
13.45 14.69 14.16
24.44 21.21 21.49
a
Calculated as % O=100(% C+% H+% N).
Fig. 6. DSC thermograms of perfectly p-oriented phenylene type polymers of different molecular weights in nitrogen at a heating rate of 10 C min1 and a gas flow rate of 30 ml min1.
those calculated for the polymers themselves, whereas those of both the as-prepared and the samples thermally treated at 150 C showed appreciable differences. At the same time, no significant changes in IR spectra of the residues, left after thermal treatment at 220 C, relative to that of polymer sample I were observed. These results indicated that volatile products given off by the lower molecular weight samples could not be products of their true degradation, but rather some compounds which had been bonded to them by some secondary association forces probably stronger than simple adsorption forces but at the same time weaker than the covalent bonds. This suggested that the volatile product could have been DMAc, the polycondensation medium, left in the samples even after their vacuum drying to constant
Fig. 7. DSC thermograms of completely p-oriented phenylene type polymers of different molecular weights in nitrogen at a heating rate of 10 C min1 and a gas flow rate of 30 ml min1: (a) sample XIII; (b) sample XIV; (c) sample XV (i) full scan; (ii) rescan after preheating to 220 C.
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weight. DMAc may be associated with the polar groups of the polymer by hydrogen bonds and could not be removed by vacuum drying to constant weight at 75 C (the used drying condition of the polymers). In order to verify this assumption, volatized products which were trapped during the heating of the lower molecular weight samples between 160 and 220 C were analyzed by gas chromatography. The results obtained correlated well with the DMAc standard. These results clearly support the observation which had been reported previously [29,30]. Thus, it can be concluded from Fig. 6 that with the exception of the endotherm at 160–200 C, all of the investigated samples showed similar behaviour regardless to their molecular weights. In addition, their other endotherms showed the same starting, maxima, ending and flushing temperatures. It is, however, not completely clear why DMAc associated only with the lower molecular weight samples, although the same drying treatment was used for all of these samples prior to their DSC runs.
perature range between evaporation of the adsorbed surface water and the cyclodehydration reaction for the high molecular weight sample, I, while a considerable weight loss (8–10%) could be detected for those of lower molecular weights, XIII, XIV and XV, in the temperature range of 160–200 C. This weight loss is attributed to the liberation of DMAc as previously confirmed by gas chromatography of the volatile product. All the investigated samples exhibited similar decomposition temperatures, which indicates that the high thermal stability is independent of the molecular weight (length of the macromolecular chains), but dependent upon the chemical structure of the repeating units. Further attempts to understand the reason for DMAc association with the lower molecular weight polymer, but not the high molecular weight one are in progress.
3.2.2. Thermogravimetric analysis (TG) TG measurements were also used to evaluate the influence of the molecular weight of the polymers on their degradation behaviour. For this purpose, polymer samples listed in Table 3 were used. TG results for these samples, obtained in nitrogen and in air, are illustrated in Figs. 8 and 9, respectively. In both degradation atmospheres, no weight loss was observed in the tem-
The thermal and thermo-oxidative stabilities of the polymers investigated are affected by the nature and amount of arylene groups incorporated into their chains, being higher for polymers with greater content of para-oriented phenylene rings, which permits more interchain hydrogen bonds as a results of greater chain symmetry and packing efficiency. Increasing the content of para-oriented phenylene rings leads to a strong
4. Conclusions
Fig. 8. Typical TG thermograms patterns of completely p-oriented phenylene type of polymers of different molecular weights in nitrogen at a heating rate of 10 C min1 and a gas flow rate of 30 ml min1.
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Fig. 9. Typical TG thermograms patterns of completely p-oriented phenylene type of polymers of different molecular weights in air at a heating rate of 10 C min1.
improvement in both the initial decomposition temperature as well as in the residual weight at a particular temperature. The major difference between the degradation behaviour under the purely thermal and under the thermo-oxidative conditions was observed at high temperatures. Air seemed much more destructive than nitrogen atmosphere, so that it led to complete annihilation of the investigated samples at 800 C, as compared to the residues of 72–56.5% which remained at the same temperature when nitrogen atmosphere was used. The stability of the polymers was found to be independent of their molecular weights. No significant differences could be observed between the degradation behaviour of the low molecular oligomers and the corresponding high molecular weight polymers at temperature above 200 C in nitrogen as well as in air. However, differences existed in both atmospheres at lower temperatures. Thus, while all of the investigated samples clearly showed a visible tendency towards absorption of moisture, which was liberated between 90 and 130 C during TG measurements, only lower molecular weight samples exhibited yet another pronounced weight loss (about 8–10% wt) at temperature range between 160 and 200 C. This could possibly be associated with liberation of DMAc, the polymerization solvent used in their preparation. The reasons for such behaviour do not seem clear on the basis of these results only. This confirms that high thermal stability is not a
polymer property which would depend upon the length of its macromolecular chains, but rather upon its chemical structure in which all and every atomic group (aromatic ring, 1,3,4-oxadiazole and benzoxazole rings) contributes by its own thermal stability to the macroscopic properties of the whole polymer.
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