Preparation and some properties of diamine-diacid type alternating copolyamides

European Polymer Journal. Vol. 15. pp. 69 to 73 © Pergamon Press Ltd 1979. Printed in Great Britain

0014-3057/79 0101-0069502.00/I)

PREPARATION A N D SOME PROPERTIES OF DIAMINE-DIACID TYPE ALTERNATING COPOLYAMIDES E. DJODEYRE, F. CARRIERE and H. SEKIGUCHI Laboratoire de Chimie Macromol6culaire, Laboratoire Associ6 au C.N.R.S. No. 24, E.S.P.C.I., 10 rue Vauquelin, 75231 Paris, France

(Received 24 April 1978) Abstract--The polycondensations of N,N'-bis-(6-aminohexyl)oxamide, N,N'-bis-(6-aminohexyl) succinimide and N,N'-bis(6-aminohexyl)-sebacamide, with p-nitrophenyl adipate in 1,2,4-trichlorobenzene solution, using relatively low temperature (100 °) to avoid the amide exchange reactions, gave alternating copolyamides corresponding to (6-6) (6-2), (6-6) (6-4) and (6-6) (6-10) nylons. These copolymers have melting points between those of their corresponding homopolyamides, whereas the random copolymers melt lower than either of the homopolymers. The microstructure of the copolymers seems to be modified by heat treatment below the melting points. Amide exchange reactions occur when they are heated to melting. C 13 spectra allowed differentiation between the CO groups of the two constituent units of the copolymers, but not between the alternating and the random copolymers.

INTRODUCTION The polycondensation of nylon salts or to-aminoacids under relatively mild conditions is free from the amide exchange reactions which generally occur when the polymer is heated above its melting points [1, 2]. These conditions can be realized by the activation of either acid or amine groups. Thus, polyamides have been prepared by the polycondensation of various adipic acid esters [3-5]. Some alternating copolyamides have also been prepared by the activation of aliphatic and a r o m a t i c o)-aminoacids in this way [6-8]. We now describe the synthesis of alternating copolyamides from p-nitrophenyl adipate and some diamide-diamines. The properties of these copolyamides were c o m p a r e d with those of the corresponding r a n d o m copolymers and the homopolymers.

N,N'-bis-(6-aminohexyl)oxamide (BAHOxA): H2N-(CH2) 6NHCOCONH(CH2)6NH2 This compound was prepared from ethyl oxalate and hexamethylenediamine (HMDA). A xylene solution of ethyl oxalate (1 tool) was slowly added with stirring to a xylene solution of a large excess of HMDA (12 tool) at 20°. The mixture was refluxed several hours under nitrogen. After cooling the resulting diamide-diamine was filtered off and then extracted with benzene in a soxhlet (yield 80%). The purification of the diamide-diamine being difficult, we converted it into the hydrochloride. The latter was crystallized several times from methanol. It was then neutralized by dilute aqueous caustic soda. The diamidediamine precipitated and was filtered off and dried (m.p.: found 114'~, Ref. [9]: 114-115~). N N'-b is-(6-aminohe x yl)succinamide( B A H Su A ) : H2N(CH2)6NHCO(CH 2)2CONH(CH2)6NH2 This diamide-diamine was prepared by reacting either succinyl chloride or ethyl succinate with HMDA. Ca) From ethyl succinate. The preparation was as for BAHOxA. (b) From succinyl chloride. Succinyl chloride (1 mol) was slowly added to a stirred chloroformic solution of an excess (6 mol) of HMDA at 0:. The stirring was continued for a few hours at 20: under nitrogen. The crude diamidediamine was washed with water to eliminate HMDA and its hydrochloride (yield 60~o; m.p. 143~) This compound was purified as hydrochloride (see BAHOxA synthesis). Calc(~o): C, 49.60: H, 9.36: N, 14.47: O, 8.26; CI, 17.96. Found (%): C, 49.37; H 9.07: N 15.00; O 8.90:C1 18.30.

EXPERIMENTAL

The following products were prepared: p-nitrophenyl adipate (PNPhA in the present paper): phenyl adipate (PhA): NN'-bis-(6-aminohexyl) oxamide (BAHOxA); NN'bis-(6-aminohexyDsuccinamide (BAHSuA); N,N'-bis-(6aminohexyl) sebacamide (BAHSeA). These last three compounds are called diamide-diamines in this paper. The melting points of these products were determined by DTA. The heating rate was fixed at 20°/min. The indicated temperatures refer to the starting of melting (further denoted as T,,).

p-Nitrophenyl adipate (PNPhA) Adipyl chloride (1 mol) was added slowly to a stirred pyridine solution of p-nitrophenol at 5c. The mixture was then stirred for a few hours at 5~ and then for 2 h r at 90° under nitrogen flow. After cooling, the mixture was poured into iced water. PNPhA precipitated and was filtered off. The crude ester was washed with water to eliminate pyridine and its hydrochloride. Then the crude ester was recrystallized twice in ethyl alcohol hydrated to 25 volume O/~o(yield 75%, m.p. 125°). Calc. (~o): C, 55.67; H, 4.12: H, 7.18; O, 32.93. Found (%): C, 55.90: H, 4.32: N, 7.21: O. 32.99.

NN'-bis-(6-aminohexyl)sebacamide (BAHSeA): H2N(CH2)6NHCO(CH2)sCONH(CH2)6NH2 (a) From sebacyl chloride. This preparation was as for BAHSuA. (b) From sebacic acid. Sebacic acid (1 mol) or 6-10 nylon salt (1 mol) was bulk heated under nitrogen up to 180 with a large excess of HMDA (-~ 12 mol). After cooling, the mixture was washed with water. Then the crude product was dissolved in 95% ethanol which dissolved only the BAHSeA at 20c: this treatment allowed separation of the higher compounds. The alcohol was distilled off under 69

70

E. DJODEYRE, F. CARRIERE and H. SEKIGUCHI

vacuum and the resulting diamide-diamine was then purified as hydrochloride. (m.p.: Found 135" Ref. [10] 135'). Calc. (o;): C, 56.05: H, 10.19: N, 11.89: CI, 15.07: O, 6.79. Found ('!,;): C, 56.05: H, 10.12: N, 11.56: CI, 15.29: O, 7.13.

Polycondensat ion 1. I mol of PNPhA or of PhA was polycondensed [11] in solution with 1 mol of HMDA under nitrogen at 100~'C. The concentration of the solution was 0.5 mol/I in various solvents: N-methylpyrrolidone (NMP), dimethylformamide (DMF), hexamethylphosphorotriamide {HMPT), benzyl alcohol (BA), o-dichlorobenzene (ODCB) and 1,2,4-trichlorobenzene (TCB). After cooling, the resulting polymer was filtered off, and the phenol was extracted for a few hours by acetone in a soxhlet. The polymer was then dried under vacuum. Solvents, NMP, DMF and HMPT, were twice distilled before use and others solvents once. DMF and HMPT were kept overnight on Call2 between the two distillations. 2. The diamide-diamines were polycondensed with PNPhA under nitrogen at 100" in a 0.5 mol/l TCB solution. The resulting polymers were purified as indicated above. 3. Random copolymers (reference copolymers). (a) Equimolar mixtures of 6-6 and 6-10 or 6-6 and 6-4 nylon salts were bulk-polycondensed under nitrogen at 270: for 6 h r for the 6-6 and 6-10 mixture and at 245 for 4 h r for the 6-6 and 6-4 mixture. The products were pulverized and dried in vacuum. (b) Since decarboxylation of oxalic acid starts at 166, preparation of its copolymer was carried out at relatively low temperature. A xylene solution of PhA (1 mol), ethyl oxalate (I mol) and HMDA {2 mol) was refluxed for 6 hrs. Then alcohol and xylene were distilled off and the crude polymer was heated for 12 hr at 60 ~ under 20 Tort. Phenol was extracted with acetone in a soxhlet. 4. Viscosities. Inherent viscosities of the polymers were measured in 1 g/100ml m-cresol solutions at 25 + 0.1L

Di~,rential thermal analysis (DT A) All measurements were performed with a Du Pont 990 apparatus. The indicated temperatures were corrected by means of the table supplied for chromel-alumel thermocouples. Temperature calibration of the instrument was achieved for 20~/min by the melting of an indium standard and was confirmed to be independent of the rate. 1. Meltin 9 points. The polymers (3-5 mg) were carefully introduced in the pan and placed under nitrogen flow of 50 cm3/min. The heating rate was fixed at 2ff/min for all the experiments. On the registered curves, the starting temperature of melting, denoted as T~,, was defined by the intersection of the tangent drawn at the point of the greatest slope on the leading edge of the peak with the extrapolated baseline. This point was determined graphically. The peak corresponds to complete melting and it is denoted TM. AT sensitivity of measurements was 0.5~/min. Before the measurements, we provided three types of thermal history the polymers. (a) The polymers were obtained as indicated above (polycondensation). The polycondensation mixture was allowed to cool to room temperature and the polymers were kept several days at room temperature. These polymers are further called crude polymers. (b) The polymers were heated 12 hr under nitrogen up to temperatures close to their melting points, i.e.: (6-6) (6-2): 255 and 265 ~, (6-6) (~4): 245 °, (6-6) (6-10): 212 and 23ff. (c) We applied to the samples a heating-cooling pulse technique 1-12, 13] in two temperature ranges (except for (6-6) (6-4) copolymer).

For the high temperature pulses, the maximum temperature of the cycle was fixed lower than the melting temperature (TM,) of the first melting, peak of the crude polymer, then it was gradually raised but it was always maintained below the melting temperature of the second melting peak of the crude polymer. For the low temperature cycles, the maximum temperature of the cycle was lower by about IY' than the starting temperature of the first melting of the crude polymer (T,,,). In the two cases the samples were heated to the maximum temperature and then cooled for 40 ~' and reheated, with heating and cooling rates of 20c'C/min. This procedure was repeated 8 times for alternating copolyamides (6-6) (6-2) and (6-6) (6-10) and 4 times for (6-6) (6-4). The maximum and minimum temperatures are indicated in Table 5. 2. Transitions. 10-15mg of samples were melted, and then quenched in liquid nitrogen. The quenched material was kept under cold nitrogen flow, while the apparatus was cooled to - 2 0 ° : at this temperature the sample was inserted into the apparatus filled with nitrogen gas to avoid water condensation. Then the apparatus was cooled to - 9 0 ~ and the heating started at this temperature. The heating rate was 20°/min and ATsensitivity 0.2C'/min. RESULTS AND DISCUSSION

1. Polycondensation of H M D A with PhA and PNPhA We studied first the polycondensation of adipic ester ( P N P h A ) with H M D A (Table 1) in order to determine the conditions for obtaining well defined copolymers by polycondensation of these esters with the diamide-diamines. F o r this purpose, we carried out solution polycondensations in various solvents [11]. The reaction was homogeneous in all the cases at least at the outset because the m o n o m e r s are all soluble in the solvents' at the chosen temperatures (100°). Moreover, some of them were good enough as solvents for the polymer or swelled it sufficiently for the reaction to be complete. This swelling of the polyamide, due to rupture of hydrogen bonds, led to high yields of polymers. Yields and inherent viscosities of the resulting polycondensates are given in Table 1. The yields represent those of the recovered polymer, the rest being lost during purification and corresponding to low molecular weight fractions. This is the reason why the yield increases in parallel with the inherent viscosity of the polymer. Table 1. Polycondensation of PNPhA with HMDA in 0.5 mol/1 solution in various solvents--T = 10ft Solvent BA BA DMF DMF NMP NMP HMPT HMPT ODCB ODCB TCB TCB

Time (hr)

Inherent viscosity (dl/g)

Yield (o~,)

4.0 6.0 4.0 6.0 4.0 6.0 4.0 6.0 4.0 6.0 4.0 6.0

0.26 0.26 0.30 0.34 0.47 0.47 0.66 0.80 0.93 1.00 1.02 1.10

78 85 70 72 80 82 82 82 87 89 88 92

Diamine-diacid type alternating copolyamides Table 3. Polycondensation of PNPhA with BAHSeA in 0.5 mol/l TCB solution. T = 57, 80, 100, 123, 140" t = 2 hr

Table 2. Polycondensation of PNPhA with BAHOxA and BAHSuA in 0.5 mol/I TCB solution, T = 100 Diamide-diamine

Time (hrl

Inherent viscosity (dl/gl

Yield (%1

BAHOxA BAHOxA BAHSuA BAHSuA

2.0 6.0 2.0 6.0

0.80 0.89 0.75 0.91

71 76 90 91

For a particular ester, inherent viscosities and molecular weights depend markedly on solvent. The solvent influences the polycondensation of PNPhA with HMDA. According to inherent viscosity, the influence decreases in the following order: TCB > ODCB > H M P T > N M P > D M F > BA. The polycondensation reaction with PNPhA is rather fast because there is very little difference between inherent viscosities of polymers for 4 and 6 hr of polycondensation. The reaction seems however to stop at a certain degree of polymerization depending on the nature of the solvent.

2. Polycondensation of diamide-diamines activated esters

with the

71

T (°C)

Inherent viscosity {dl/g)

Yield (%)

57 80 100 123 140

0.34 0.57 0.75 1.52 1.92

19.7 83.7 89.0 93.5 98.4

number giving the number of carbon atoms of diamine and the second that of diacid), we obtained the alternating (6-6) (6-2), (6-6) (6-4) and (6-6) (6-10) copolyamides, corresponding to the general formula:

H[NH(CH2)6NHCO(CH2)4-CO-NH(CH2)6 NHCO(CH2),_ zCO]~-OH 6-6 unit 6-n unit where n = 2: (6-6) (6-2), n = 4: (6-6) (6-4t, n = 10: (6-6) (6-10).

3. Properties of the alternatiny copolyamides 3-1. Meltin 9 points

The high reactivity of adipic ester (PNPhA) allowed us to prepare alternating copolyamides from diamidediamines at relatively low temperatures, avoiding the amide exchange reactions (Tables 2 and 3). It is well known [1, 2] that these reactions occur when a mixture of homopolyamides is heated above the melting point. Our diamide-diamines having amide links, we verified that at our polycondensation temperature (100 °) there were no amide exchange reactions. Thus we heated for 4 hr a mixture of N,N'-di-n-butyladipamide and N,N'-diacetylhexane-diamide in 0.5 mol/l solution in 1,2,4-trichlorobenzene at various temperatures (77, 120, 140 and 180°). The reaction products were then analysed by paper chromatography [14, 15] and by D.T.A. for polyhexamethyleneadipamide which would appear if reaction occurred. We found that no amide exchange occurs for any of the four considered temperatures. Such a result could be expected since it was reported that this reaction is effective only above 180 ° [16] (see also discussion 3-3). We chose 1,2,4-trichlorobenzene (TCB) as solvent because it gave the best results in the polycondensation of H M D A with PNPhA (Table 2). These results are quite comparable with those of the previous polycondensations. The diamide-diamines would therefore be as reactive as H M D A towards PNPhA. With the usual polyamide notation (the first

(a) Crude polymers. The melting curves of the random and the alternating copolymers show two endothermal melting peaks, usual for polyamides [17] (Table 4). These results show that values of T,, for the alternating copolyamides lie between those of the corresponding homopolyamides. On the other hand, the random copolymers melt below either of homopolyamides. It seems therefore that the alternating copolyamides have a new crystalline structure; in particular, the hydrogen bonds form a regular network. (b) Annealin 9 at fixed temperature. The alternating copolymers were heated at a temperature close to their melting point. The annealing temperature has a considerable importance. If it is lower than T,,,, the sample crystallizes more or less into a low melting structure. On the other hand, for annealing between TM, and TM2, the samples crystallize into the high melting structtire and the curves show only one melting peak. The crystallization is important because the observed temperatures are raised (see reference temperatures Table 4). These results are comparable to those of cycle annealing but the observed temperatures are a little lower. For example: copolyamide (6-6) (6-2). Annealing temperature 25Y- Tin, = 266' and Tu, = 27Y. Annealing temperature 26Y T,,2 = 281: and TM, = 287 °. (Cycle annealing temperature see Table 5).

Table 4. Melting temperatures of the random and the alternating copolyamides and corresponding homopolyamides. Heating rate 20:/min. AT sensitivity 0.5"/in. Copolyamide Polyamide 6-6 TM: 255

Polyamide Copolyamide 6-n Tu_~ ( 6 - 6 1 (6-n)

T.,,

Tu,

T.._

Tu.

T,.,

Tu,

T~:

TM:

6-2 320 6-4 290 6-10 225

260 248 221

266 260 229

272 264 240

280 269 248

211 223 190 "201 191 195

227 211 196

237 219 199

(6-6) {6-6) (6-6t

(6-2) (6-4t (6-10)

Alternating

Random

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ESMAILDJODEYRE,FRANt,'OISCARRIe.REand HIKARUSEKIGUCHI Table 5. Melting temperatures of the alternating copolyamides after heatingcooling pulses. Heating rate 20'/rain, 8 cycles. ATsensitivity 0.5"/in Copolyamides (6-6) (6-n)

Annealing temperatures (~'C)

T~,,

Tu,

T.,.

Tu."

(6-6) (6-2)

200-240 240-279 210-260 160-206 205-247

268 -258 230 --

275 -264 238 --

-282 265 242 247

-289 268 248 254

16-6) (6-4) (6-6) (6- I0)

(c) Cycle annealing. When the maximum temperature of the cycles lies between TM, and TM: of the crude copolymer, the copolyamides crystallize into a high melting structure and only one melting peak is observed (case of (6-6) (6-2) and (6-6) (6-10) copolymers). When the maximum temperature of the cycles is lower than T,,,, of the crude copolymer, the crystallization is not so well defined. Copolyamide (6-6) (6-10) still gives two melting peaks but the upper peak is much reduced as compared to the other. The heatingcooling pulses provide a very efficient technique for crystallizing polymers[12, 131, (6-6) (6-10) copolyamide for example presepts melting temperatures T,,,., and TM,.,after 8 high temperature pulses totalling only 45 min, higher than those obtained after 12 hr of annealing at fixed temperature. 3-2. Transitions We measured the glass transition temperatures of the alternating and the random copolyamides. The glass transition is followed by a peak of cold crystallization in the case of (6-6) (6--10) copolyamides. The first transition was observed towards - 1 0 ° in all the samples studied. It would be the same as that found dilatometrically in 6-6 nylon at - 4 ° [18] and corresponds to that predicted by Boyer and Spencer [19] for the hydrocarbon portion of the nylon chain. In the higher temperature range, the following transition temperatures were found for the alternating and the random copolyamide respectively: (6-6) (6-2): 70 and 50°; (6-6) (6-4): 49 and 40°; (6-6) (6-10): 39 and 26 °. The observed temperatures for the random copolyamides are lower than those of the alternating copolymers.

3-3. Amide exchange reactions (6-6) (6-10) Alternating copolyamide was heated for 1, 5 and 12 hr under nitrogen at (Table 6). It is likely that the amide exchange tions are fast at this temperature and that the

bulk 275 ° reacalter-

Table 6. Melting temperature of (6-6) (6-10) alternating copolyamide heated at 275° for 1, 5 and 12 hrs. Heating rate 20C/min, AT sensitivity 0.5-~/in. Time (hr)

Tm

TM

T.,

T~t,

Tin,_

Tu:

0 1 5 12

-153 140 --

-167 165 --

221 182 185 172

229 198 199 198

240 225 213 207

248 249 233 217

191

195

196

199

Random

Melting temperatures

Melting temperatures

nating copolymer is gradually transformed into random copolymer. These results show that the first melting peak (Tin,, TM,) of the alternating copolyamides disappears quickly. There is very little difference between TM, temperatures after 1, 5 and 12 hr of heating and TM, of the random copolymers. On the other hand the upper peak (Tin,, TM,_)disappears gradually but even after 12 hr of heating we do not find Tin2 and TM~ of the random copolymer. However it is possible to state that the alternating copolyamide gradually changes into the random copolymers when it is bulk heated for a few hours, but after 12 hr the transformation is still not complete.

3--4. Thermal degradation (6-6) (6-10) Random and alternating copolyamides as well as the corresponding homopolyamides were heated to complete degradation. It is known [20] that nylon 6-6 starts to degrade in a very irregular mode much before nylon 6-10. For the alternating or the random copolymers, the observed degradation temperatures are the same but the forms of the curves are very different. The alternating copolymer has characteristic behaviour whereas the random one could rather be compared to a mixture of the two homopolymers. Moreover the relative fragility of nylon 6-6 has disappeared for both the alternating and the random copolymers.

3-5. Discussion We studied the properties of copolyamides by three techniques: DTA, infra-red spectroscopy, ~aC NMR. By laC NMR it was not possible to find any difference in properties between the random and the alternating copolyamides. However every unit could be distinctly observed. Proton NMR did not allow this separation. The differences between the infra-red spectra of the alternating and the random copolymers are very small [21, 22,1. This lack of difference in proton NMR and IR spectroscopy is due to the similarity of the chemical constitution of these copolymers (see .~2). On the contrary, the physical environments of these copolymers and the corresponding homopolymers allowed distinction between the random and the alternating copolymers. This is the case with the DTA technique which gave the best results in characterizing the products. CONCLIrSION The polycondensation of diamides-diamines such as N,N'-bis-(6-aminohcxyl)oxamide, N,N'-bis-(6aminohcxyl)succinamide, N,N'-bis-(6-aminohexyl)

Diamine-diacid type alternating copolyamides sebacamide, with p-nitrophenyl adipate leads to alternating copolyamides: (6-6) (6-2), (6-6) (6-4), (6-6) (6-10). Comparison of the properties of these copolyamides with those of the corresponding random copolyamides, especially by DTA, confirms the preparation of a new class of copolymers with characteristic properties. REFERENCES

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