l,

2.

«opol~~~er Fiber strength, cN tex

Standard copolymer wlth IC 22--25

Copolymer with MAS 22--24

Elongation, % Loop strength, L

30--35 27--35 30--35 26--35

LITERATURE CITED

O. A. Nikitina et al., Khim. Volokna, No. 5, 21 (1979).

Notes

Spinning process stable

Reduced ability to undergo plasticized stretch; breaks in elementary fibres in this stage.

S. S. Voyutskii, A Course in Colloid Chemistry [in Russian], Khimiya, Moscow (1964), 574 pp.

BEHAVIOR OF AROMATIC POLYAMIDES IN DILUTE SULFURIC ACID SOLUTION8

A. I. Koretskaya, M. P. Babushkina, G. I. Kudryavtsev, and L. V. Mileshkina

UDC [678.675:536.495]- 404o5.01

Polymers of the para-structure, in particular poly-p-phenyleneterephthalamide (PPTA) and copolyamides with a predominant content of p-phenyleneterephthalamide units are the most promising ones for the synthesls of aromatic polyamides and the preparation of high-strength, high-modulus fibres based on them. These polymers, which are synthesized by the method of low-temperature polycondensation in solution in aprotic solvents, differ not only in molecular weight (inherent or intrinsic viscosity), but also in molecular weight distribution, con- formation of the macromolecules, solubility, and ability to be processed into articles.

The properties of dilute solutions can give useful information about the quality of a polymer. Interest in the behavior of dilute solutions of rigid-chain aromatic polyamides, PPTA for example, has been displayed only in the first 4-5 years and has been directed mainly toward ascertaining their conformational characteristics [1-5]. We have investigated the properties of dilute solutions of PPTA in sulfuric acid with the objective of ascertaining more fully the conditions for determining the inherent, hin, and intrinsic [~] viscosities, establishing the specific dependence between them, and also ascertaining the possibility of using the Huggins viscosimetric constant, kH, for a relative evaluation of polymer quality.

The objects of study were unfractionated specimens of PPTA having hin of 3.6-8.5 dl/g, synthesized by the low-temperature polycondensation method in a solution N,N-dimethylacetamide in the presence of LiCI.* As solvents for the preparation of dilute solutions of PPTA we used 96% and 99.3-99.6% chemical pure H2SO«. The concentration of the H2SO« was determined by neutralization with 0.i N NaOH solution, with an accuracy of ± 0.1%. The hin was cal- culated from the relative viscosity, nrel, measured in a dilute solution having a PPTA con- tent of 0.2 and 0.5 g per i00 ml of solvent; [nj was determined by the method of extrapolat- ing the reduced viscosity, nsp/C, for 6 or 7 concentrations to zero concentration. Mea- surements weremade in a Ubbelohde viscosimeter with a pendent level at temperatures of 20, 25, and 30°C; the temperature drop was ±0.05°C.

In world practice, the molecular weight of PPTA is indirectly determined from the values of Dsp, qin, or [q], the first two being often determined in solutions containing 0.5 g PPTA

*The PPTA was synthesized in the All-Union Scientific-Research Institute of Synthetic Resins (in Vladimir).

Translated from Khimicheskie Volokna, No. i, pp. 16-17, January-February, 1982. Original article submitted January 20, 1981.

20 0015-0541/82/1401-0020507.50 © 1982 Plenum Publishing Corporation

~p/~dZ/g

*8

5 9 .

gl ~2 g~ 6* Cp~ A, g/dl

Fig. i. Dependence of reduced viscosity, ~~p/ C, on concentration for specimens of PPTA solution in 96% H2SOù with nsp of 41.6 (i), 31.2 (2), 12 (3), and 4.7 (4).

~ 8

h Q

I I ]

Z :+. • F «

~m o.5' dFg Fig. 2. Dependence between values of hin of solutions having a concentration of 0.2 (hin 0.2) and 0.5 (hin 0.5) g of PPTA in 96% H2SO~.

in i00 ml of H2SO«. In solutions of this concentration, in the case of high-molecular- weight speclmens of PPTA, obviously a strong interaction takes place between the polymer macromolecules, whieh leads to distorted results.

There are comparatively few data in the literature on the determlnation of [~] for PPTA [5-7]. As is well known, to evaluate [nj it is necessary to select correctly the dilution range, for at too great a dilution the appearance of a polyelectrolyte effect is posslble, and a solution containing 0.5 g of PPTA per I00 ml of solvent can hardly be considered dilute.

For four PPTA specimens, the nsp/C = f(C) dependence (Fig. i) eonsists of two stralght- line sections which intersect in the polymer content region 0.15-0.25 g/dl. If we determine [~] from the nsp/C segment intersected by the line in the polymer content range of 0.5-0.25 g/dl, absurd figures are obtained: Regardless of the molecular weight, the value of [n] is found in the range 1 1-3 dl/g, this being lower the higher the molecular weight of the PPTA. Consequently) in determining [~] the initial PPTA content should not exceed 0.20-0.25 g/dl. At a low PPTA content (dilution to C = 0.025 g/dl) no anomalous change in viscoslty is noted.

The preparation of dilute solutions of PPTA in H2SO« at room temperature is a lengthy process. It is possible to accelerate it by raising the solution temperature. However, at the elevated temperature degradation of the PPTA takes place [8, 9]. We have studled the stability of solutions containing 0.2 and 0,5 g of PPTA in i00 ml of 96% H2SOù as a func- tion of the duration and temperature of the solution process. The solution proeess was carried out at about 20, 40, 60, and 70°C .

21

[~], dl/g

8

6

= '~ ~ ; "in-m/g Fig. 3, Dependence bet-~een [~] and Hin a t the i n d i c a t e d contents of PPTA in a s o l u t i o n in 96% H=SO«: 0.Z (1) ; 0.2 (2} ; a n d 0 . 5 g / d i (3 ) .

r

B - - «

õ

Z

O T ! ! T

Fig. 4. Kinetics of moisture sorp- tion by PPTA having hin of 6.18 (i), 6.96 (2), and 5.12 (3).

The results of the viscosity measurements permit one to consider that at 40°C and a heating time of 7 h, hydrolysis of the PPTA is practically absent. The ratio of r]in of solutions prepared by dissolving PPTA at 40°C to the hin of solutions prepared at 20°C fluc- tuates between 0.93 and 1.06 and between 0.99 and 1.03 at polymer contents of 0°2 and 0.5 g per i00 ml of solvent, respectively. At 70°C, with increase in the duration of heating over 3 h, the viscosity of the PPTA is reduced, the reduction being more intense in solutions containing 0.5 g of polymer per i00 ml of solvent than in solutions containing 0.2 g of poly- mer per i00 ml of solvent. Solution at 60°C does not lead to a marked decrease i~ polymer viscosity.

It is to be noted that even a short-term heating at elevated temperatures cuts down the time for preparation of dilute PPTA solutions, the time for solution depending not so much on the polymer content of the solution within the investigated limits as on temperature. It was found that solutions of PPTA are stable at 60°C for 20 h, and this time is sufficient for the solution of even very high-molecular-weight specimens of PPTA.

Comparing the valueo of hin for solutions containing 0.2 and 0.5 g of PPTA in 96% H=SO«, we obtained the specific form of the direct dependence between them (Fig. 2). The study of the dependences of [~] on Hin showed that for each concentration there is a straight-line dependence between these figures, wherein, with decrease in concentration, the [n]/nin ratio approaches unity (Fig. 3). Thus by using the [n] = f(nin) graphical dependence, it is pos- sible from a single point (onemeasurement) to determine the value of [n] with reasonably high accuracyo

The degree of degradation is also determined to a considerable extent by the moisture content of the polymer. The studies of [9, i0] have been devoted to the stability of dilute solutions of PPTA (0.5 and 0.i g/dl) as a function of the H=SO« concentration and temperature. Data on the hydrolytic stability of high-molecular-weight PPTA specimens in dilute solutions in H=SO« as a function of the moisture content of the polymer are missing from the literature. Our studies have shown that, with increase in the moisture content of the PPTA the degree of hydrolysis rises. While at a moisture content of about 1.8%, Hin is reduced approximately by an equal extent, that is, by 2.3-2.7%, at the equilibrium moisture content (about 4.5%) the degree of hydrolysis is somewhat larger for the PPTA specimen of lower molecular weighto This is explained, most probably, by the physical state of the investigated products. The lower molecular weight specimen of PPTA (Hin 5.12) was granular; therefore it absorbs moisture (Fig. 4) and dissolves more slowly thanthehighermolecularweight (hin 6.18 and 6.96), but powdery specimenso

By determining [n] in a "poor" (96% H=SO«) and a "good" (99.4% H2SO4) solvent, we cal- culated the Huggins contants, kH, for a reasonably large number of PPTA specimens. From Table 1 it follows that there is no definite dependence between the values of [nj and k H. The intrinsic viscosity, as a rule, is reduced or does not change on increasing the measure- ment temperature of ~sp from 20 to 30°C. The change in kH, apparently, is connected not so

22

TABLE i. Values of [hi and k H of PPTA in H=SO~ solutions

96% Hg_SO 4

(~]

6,25/5,73 6,30/6,8O 6,3O/6,2O 6,32/5,6O 6,40/5,80 6,45/6,0O 6,70/6,16 6,70t6,20

k H

0,50/0,57 0,65/0,71 0,48/0,39 0,59/0,79 0,47/0,52 0,47/0,50 0,59/0,63 0,45/0,47

99,~.%~S0 4

[~)

7,15/7,30 7,70/7,15 6,30/6,25 7,80/7,30 6,85/6,35 6,75/6,65 8,00/7,55 7,40/6,87

0,4210,26 0,3510,37 0,4710,46 0,3810,40 0,5110,55 0,54/0,48 0,4310,43 0,37/0,41

Note° In the numerator we give data obtained at 20°; in the denominator, data obtained at 30°C.

much with temperature as with the composition of the PPTA, specifically, with a different molecular weight distribution, and by the presence of cross-links in a polymer containing impurities. The lower values of k H in 99.4% H=SO, confirmed the improvement in the thermo- dynamic quality of the solvent with respect to PPTA. This is also indlcated by the values of In], which are appreciably higher in 99.4% H=SO~.

The data obtained are difficult to interpret unequivocally, but it may be said that k H characterizes the quality of the PPTA to some extent. If, at an identical [n], polymers differ in kH, probably the higher value of ~ indicates a lower quality of the PPTA. The 96% H=S04 is more sensitive to this index; in solution in it, the molecular weight distribu- tion, cross-links, branching of molecular chains, and other anomalies are not subjected to appreciable changes.

Thus, this set of data on the viscosity of dilute solutions of PPTA in H=SO, of various concentrations indicate the need to measure nre I, and, correspondingly, nin of PPTA is a solution containingO.2 g of polymer in i00 ml df 96% H=SO,, and the possiblity determining [q] from a single measurement.

LITERATURE CITED

I. J. R. Schaefgen et al., Am, Chem. Soc., Polym. Preprints, 17, 69 (1976). 2. V. N. Tsvetkov et al., Dokl. Akad. Nauk SSSR, 231, 1373 (1976). 3. M. Arpin and C. Strazielle, Polymer, 18, 591 (1977). 4. D. G. Baird and J. K. Smith, J. Polym. Sci., Polym. Chem. Ed., 16, 61 (1978). 5. B. Milland and C. Strazielle, Makromol. Chem., 179, 1261 (1978). 6. T. S. Sokolova et al., Khim. Volokna, No. i, 26 (i974). 7. M. M. lovleva, S. G. Efimova, and G. E. Prozorova, in: Theory of Spinning Man-made

Fibres [in Russian], VNIIV, Mytishchi (1975), p. 27. 8. T. S. Sokolova et al., Khim, Volokna, No. i, 29 (1976). 9. A. V. Krasnovperova, V. M. Golubev, and V. M. Savinov, Vysokomol. Soedin., 2!i, 923

(1979). i0. F. M. Silver, F. Dobinson, and J. S. Ridgeway, J. Polym. Scio, Polym. Chem. Ed., I_66,

2151 (1978.

23