Effect of acetone and ethanol on the physicomechanical properties of ...

Fibre Chemistt% Vol. 31, No. 6, 1999

EFFECT OF A C E T O N E A N D ETHANOL ON THE P H Y S I C O M E C H A N I C A L PROPERTIES OF ARAMID FIBRES

I. M. Zakharova, A. E. Zavadskii, and Z. N. Zhukova

UDC [677.494.675:536.495].017.2/7

A specific change in the initial modulus q[eIasticiO~ andstress relaxation is the most important consequence o/ the structural reorganizations of the aramid fibres Armos, VMN. and SVM caused by dissohttion of acetone and ethanol in the bulk o f the polymez:

As part of research to investigate the physicochemical characteristics of the reaction ofaramid fibres with low-molecularweight liquids, we examined the effect of sorption of acetone and ethanol on the strength and deformation properties of these materials. It was previously found in [1] that the processes that take place in the bulk ofaramids in the reaction with acetone and ethanol causes structural changes in the matrix of the fibre. Dissolution of these liquids in SVM and VMN fibres is accompanied by an increase in the packing density of the macromolecules in regions accessible to the solvent. A tendency for the intermolecular distances to increase and the packing density to decrease is characteristic of Arums fibre. It is necessary to note that this group of aramids, which is liquid crystalline in nature, is characterized by rigorous longitudinal ordering related to a simultaneous shift in adjacent maeromolecules and the fomlation of distinctive layers as a result [2]. We can hypothesize that a change in its physicomechanical indexes is the consequence of the structural reorganizations of the polymer [3]. We investigated the effect of the structural changes observed on such practically important indexes of aramid fibres as the initial modulus of elasticity, strength, and elongation with breaking strains, and stress relaxation. The aramid fibres Arlnos, Kevlm; VMN, and SVM treated with acetone and a ethanol used as solvents of curable composites. The fibres were treated with the solvents using tbe method in [1]. The fibre strength indexes were determined on a RT-250 tensile-testing machine with the deformation rate of 3.3-10-2 m/sec and a sample working length of 50 mm, while the stress relaxation curves were recorded on a modernized [4] PMR-1 relaxometer with a sample clamping of 17 mm and initial stress equal to 1% of the breaking stress. The area of the cross section of the fibres was calculated by detemfining the diameter and number of monofibres in a complex fibre in the MBI-1 microscope using a MOV-1 attachment. Stress relaxation in aramid fibres was studied in the following samples: initial fibre -+ fibre--solvent complex ~ fibre free of solvent. The experimental data describing the effect of treatment in acetone and the ethanol on stress relaxation in aramid fibres are reported in Table 1 and Fig. 1. The data in Table 1 show that in the entire group of aramids investigated, the residual stresses of the fibre--solvent complex are lower than in the initial fibre. The segmental mobility of the macromolecules caused by the low-molecular-weight liquid dissolved in the bulk of the fibre is responsible for such low values of the residual stresses in the complex on one hand. On the other hand, the increase in the interchain distances with preservation of the rigorous longitudinalordering &the structure of the polymer causes this, as the decrease in the relative intensity of the meridional x-ray reflection due to the formation of a fibre--solvent complex indicates [ 1]. As for the fibres from which the solvent was removed, the residual stress is lower in Arums, while it is higher in SVM and VMN than in the initial fibre, as the data in Table 1 show. The observed change in the residual stress totally corresponds to the character of the structural reorganizations taking place in aramids as a result of dissolution of acetone and ethanol. The increase in the residual stress in SVM and VMN fibres

Ivanovo State University of Chemical Engineering. Translated from Khimicheskie Volokna, No. 6, pp. 31-33, November--December, 1999. 452

0015-0541/99/3106-0452522.00 :~ 1999 Kluwer Academic/Plenum Publishers

T A B L E 1. Effect o f Treatment in Ethanol and Acetone o n Stress Relaxation in A r a m i d Fibres at 293 K Residual stress,* in %, in fibres

Fibre loading conditions Kevlar

I

Ammos

[

SVM

I

VMN

Initial fibre in air

90.7

89. l

88.2

89.4

In acetone

89.5

-

76.0

87.19

In air after eliminationof acetone

91.1

88.6

92.9

In ethanol

88.8

70.6

64.1

85.0

In air after elimination of ethanol

92.5

85.1

90.0

93.4

*The initial stress was 1% o f the tensile stress for the untreated fibre in all cases; the duration of the experiment was 5 h.

T A B L E 2. Effect of Treatment in Ethanol and Acetone on the Physicomechanical Indexes o f Aramid Fibres Fibre Index Linear density, tex

168

I

100

I

29

I

34

Initial modulus, GPa - starting - after treatment with ethanol after treatment with acetone

115-+8.1 114_+8.0 112+_7.8

160_+11.2 157_+11.0 145_+10.2

135-+9.2 141+9.9 145-+10.2

136_+9.5 148_+10.4 155_+10.9

Breaking stress, MPa - starting after treatment with ethanol - after treatment with acetone

2967_+208 2949+206 2931 -+205

4184_+293 4061_+284 3784_+265

3102_+217 3384_+237 3626_+254

3523_+247 3857_+267 4357+304

Relative elongation, % - starting after treatment with ethanol - after treatment with acetone

1.3_+0.1 1.4+0.1 1.8+0.1

-

-

-

2.9_+0.2 4.3_+0.3 4.9-+0.4

3.1_+0.3 3.4_+0.3 3.6_+0.3

3.5_+0.3 3.2+0.3 3.2_+0.3

is determined by the greater ordering o f the structure, indicated by the increase in the packing density of the macromolecules. In the case o f Armos fibre, the decrease in the residual stress not only causes a decrease in the packing density, but also a tendency toward an increase in the i n t e n n o l e c u l a r distances [1]. With respect to Kevlar, based on the data in Table 1, we can conclude that treatment o f this fibre in acetone and ethanol has almost no effect on the residual stress. The change in the residual stress in the group o f aramids investigated also indicates the variable free volume o f the polymer [5]. If we assume that the free volume is correlated with the transverse ordering o f the structure of the potymel, then this quantity will vary in the sequence V M N > Armos > S V M for the initial aralnids [2]. The decrease in the residual stress in Armos fibre is caused by an increase in its free volume. As a consequence, after treatment with the solvents, the fibres can be placed in the following order as a function o f the values o f their free volume: Armos > V M N > SVM. The curves in Fig. 1 characterize the change in stress relaxation in V M N fibre. N o t e that after the testing time expires (5 h), stress relaxation attains a quasi-equilibrium state. The quasi-equilibrium state in this case can be considered as the state in which only some o f the macromolecules or their segments attained confonnational equilibrium, and the number is smaller in S V M and V M N fibres and larger in A r m o s fibre than in the initial fibre. Further infinitely small confomlational transformations whose recording is restricted by the capabilities of the instrument should bring the value o f the residual stress to zero. 453

o. 8-

'"

0

t

t I

2

0

s

z,

Fig. 1. Stress relaxation curves for VMN aramid fibre: 1) fibre after desorption of ethanol; 2) fibre after desorption of acetone; 3) initial, untreated fibre; 4) fibre in acetone medium; 5) fibre in medium of ethanol. The fundamental physicomechanical indexes of the aramid fibres - - both the initial samples and the fibres freed of solvent - - were investigated. The results of these studies are reported in Table 2. As we can see, the initial modulus of elasticity of SVM and VMN fibres after treatment with acetone and ethanol increases markedly, while it decreases for Kevlar and Armos. The change in such a structurally sensitive characteristic as the initial modulus of elasticity takes place in total agreement with the character of the structural reorganizations. Ordering of the structure of the polymer causes an increase in the packing density of the macromolecules in the SVM and VMN fibres and consequently an increase in the initial modulus [6]. A major difference is characteristic of Armos fibre: there is a tendency toward an increase in the intermolecular distances and a decrease in the packing density, which indicates some disordering of the structure and in the final analysis, unfavorably affects the values of the initial modulus. The change in the strength and defomaation indexes of this group of aramids is not directly a function of the structural changes. In Kevtar fibre, these characteristics are almost independent of treatment with the solvents. Armos fibre becomes more elastic with an insignificant change in the strength indexes, and an increase in the strength indexes with the relative elongation almost unchanged is characteristic of SVM and VMN fibres. The data presented thus demonstrate that dissolution of even small amounts (2-8 vol. %) [1] of acetone and ethanol in rigid-chain polymeric fibres of the aramid type alters their strength and deformation properties. REFERENCES 1, 2. 3. 4. 5. 6.

454

L. N. Mizerovskii, I. M. Zakharova, et al., Khim. Volo~vm,No. 5, 20-23 (1998). A. E. Zavadskii, I. M. Zakharova, and Z. N. Zhukova, Khim. Volokna, No. 1, 7-11 (1998). J. Rebenfeld, E J. Makarewicz, et al., J. Macromol. Sci. Pt., 15, No. 2, 279-393 (1976). K. V. Pochivalov, Yu. A. Anikeev, and L. N. Mizerovskii, Plast. Massy, No. 1, 56 (1993). Lawrence E. Nielsen, Mechanical Properties of Polymers and Composites, Marcel Dekker, New York (1974). S. E Papkov, Polymeric Fibre Materials [in Russian], Khilniya, Moscow (1986).