Effect of proton-donors on the structure and properties ...

7.

L. V. Lashina, Investigation of the Thermal Transformations of PVA- and PVC-Fibres in the Preparation of Carbon Fibres [in Russian], Author's Abstract of Candidate's Dissertation in Technical Sciences, Leningrad: S. M. Kirov LITLP (1979).

8.

R. M. Levit, Electrically Conducting Man-Made Fibres [in Russian], Khimiya, Moscow (1986). R. M. Levit, Fourth Intl. Symposium on Man-Made Fibrcs, Kalinin, VNIISV (1986) Preprints, Vol. 5, pp. 17-22. L. A. v o r f and R. M. Levit, Low-Tonnage Man-Made Fibres for Technical and Medicinal Purposes [in Russian], Lensovet LTI, Leningrad (1980).

9. 10. 11.

l~vlon Carbon Fibre [in Russian], LenTsNTI (1982) (Informational Sheet on Scientific and Technical Attainments), No. 72-82. Ser. 61-67.

EFFECT OF PROTON-DONORS

ON THE STRUCTURE AND

PROPERTIES OF POLYBENZIMIDAZOLETEREPHTHALAMIDE FIBRES

O. S. Karchmarchik, L. Yu. Direnko, M. N. Shuster, V. I. Bobylev, T. E. Petrova, and E. A. Egorov

UDC 677.037.678.01

Fibres based on polybenzimidazoleterephthalamide (PBITA) [1-3] are prepared by spinning from a solution of the aromatic heterocyclic polymer in an amide--salt solvent. In the course of synthesizing the starting polymers, hydrogen chloride (HCI) is liberated, which is present in the situation in the form of the hydrochloride of PBITA. The presence of HCI in highstrength, high-modulus PBITA fibres leads to a decrease in the thcrmo-oxidative and climatic resistance of these fibres; therelore, studies associated with the preparation of neutralized PBITA fibres in the stage of spinning, which would not contain HC1, while preserving their high physico-mechanical properties acquire particular importance. Changes in the properties of a number of polymers which contain acid--base groups (proteins or polypeptides) under the effect of acidity of the medium are connected with changes in stiffness, degree of coiling of the macromolecules, or rearrangement of the system of intra- and intermolecular hydrogen bonds, and consequently, with changes in molecular mobility. In view of this, we have used the method of wide-line proton magnetic resonance (PMR) which is sensitive to changes in the dynamics of polymer chains [4] to observe the processes which take place on changing the HCI content of PBITA fibres. Increasing the mobility of polymer chains leads to a narrowing of the wide component of the PMR spectra (and, conversely, limitation in mobility leads to a broadening of the spectra); therefore, we used the width of the broad component AH as a measure to evaluate the molecular mobility of the polymer chains. Studies by the PMR method were carried out on a wide-line spectrometer developed and made-up in the Strength Physics Laboratory of the A. F. Ioffe Physico-Technical Institute. The working frequency was 30 MHz, and the modulation amplitude was 1.10--4 TI. The specimens were bundles of fibres laid down parallel, oriented in the magnet gap perpendicularly to the direction of the magnetic field. The fibres were heated directly in the spectrometer with a current of dry nitrogen. There are two characteristic contraction regions on the AH temperature dependences of the PMR spectra of fibres (Fig. 1): In the temperature range 200-400 K and above 500 K. Below 200 K, AH does not depend on temperature. This means that at low temperatures, the PBITA macromolecules may bc considered as rigid. At 200 K and above, local forms of molecular mobility arise and develop in the fibre. Specially performed experimenls on recording the PMR spectra of specimens oriented at various angles with respect to the magnetic field have shown that the orientation dependences of the mean-squared spectrum width (second moment) at the temperature of well-developed local mobility (300 K) and in the absence of molecular motion (100 K) are similar (they are equally distant from one another). Hence, one may conclude that the form of the observable local mobility is due to the twisting vibrations of the phenylene rings, which cause contraction of the spectrum due to a decrease in the intermolecular contribution, which may be considered isotropic [5].

Translated from Khimicheskie Volokna, No. 6, pp. 18-20, November-December, 1990. 374

0015-0541/90/2206-0374512.50

'~1991 Plenum Publishing Corporation

eN

8 7

x

l

6

2 5

3

I

I .....

100

ZOO

I

f

YO0

#00

i

500 T,K

Fig. 1. Temperature dependences of the PMR line widths of the spectra of PBITA fibres containing 4% by wt. HCI (1) and of fibres not containing HCI (2).

The sharp decrease in line width above 500 K is caused by the development of segmental chain mobility and may be considered an a-relaxation transition [6, p. 225]. Differences are observed in the region of local forms of motion (sce Fig. 1) in the temperature dependences of the of spectra of fibres containing and not containing HCI. From the change in width (or the second moment) of the spectrum one can estimate the mean amplitude of vibrations. Let us employ an approximate formula from the work of [5]: h 1 h'l:,/M,,= t--3A~/'2. where M2h and M2t are the mean-squared widths of the spectra at low (M2t) temperature where motion is retarded and at high (Mzh) temperature which corresponds to complete realization of the form of vibrational motion which interests us; and A is the amplitude of the vibrations. In conformity with the data of [7], instead of the second moments, one can use the squares of the spectral widths. The absolute systematic errors associated with using the approximate formula do not affect the correctness of the conclusion. Results obtained were the following: at 400 K (in the region of well-developed local mobility), for fibres containing 4% by wt. HCI, A = 29°; for fibres containing no HCI, A = 34°, i.e., the presence of the HCI limits the amplitude of the twisting vibrations of the para-phenylene rings. The limitation in amplitude of vibrations of the para-phenylene rings in PBITA may be associated with the proton-donor properties of HC1. Protonation of the basic imidazole rings leads to the formation of symmetrical imidazole cations [8]. Thereupon, in the para-substituted benzene rings, a redistribution of elcctron density takes place, as a result of which rotation around the C---C bond, which connects the aromatic ring with the hcterocycle is retarded (the bond becomes partially a double bond), and the degree of conjugation of the para-phenylene rings with the heterocycle will be increased:

NH

H÷ i ~ N i + ~ ~