effect of annealing on the optical and mechanical properties of cold drawn polypropylene fibres

Polymer International 39 (1996) 129-140

Effect of Annealing on the Optical and Mechanical Properties of Cold Drawn

Polypropylene Fibres

A. A. Hamza, I. M. Fouda,* T. 2. N. Sokkar & M. A. El-Bakary

Physics Department, Faculty of Science, Mansoura University, Mansoura, Egypt

(Received 31 July 1995; accepted 16 September 1995)

Abstract: Refractive indices and birefringence changes with strain produced by different stresses in annealed and unannealed polypropylene fibres (4 : 1 draw ratio, 51 5 tex polypropylene from Bolton, UK) were measured interferomet- rically. Calculations were carried out using multiple-beam Fizeau fringes in transmission to determine the Cauchy's constants, dispersive coefficient and the dielectric constant at infinity. The orientation factor and orientation angle of the fibre material were calculated for different strain values. Poisson's ratio and the strain optical coefficient were also determined. An empirical formula is suggested to correlate the orientation factor, orientation angle, area of cross-section and birefringence with the draw ratio, and the constants of this formula were determined. The effect of the draw ratio on the refractive index profile was studied. Microinterferograms and curves are given as illustrations.

Key words: polypropylene fibres, annealing, cold drawing, refractive index, birefringence, orientation.

INTRODUCTION

The study of the optical anisotropy in polymer fibres plays an important role in the knowledge of the molecular arrangement within these fibres.' Optical anisotropy produced in fibres by stretching gives valuable informa- tion for the characterization of these fibres. The drawing process can be adjusted by considering this property.' De Vries has given an analysis of the relationship between the birefringence and the draw ratio of some synthetic fibre^.^

Stretching at room temperature is known as cold drawing and concerns the deformation of pre-existing structure^.^ In some materials the undrawn fibre is non-crystalline and drawing promotes the orientation of the molecules, which is followed by cry~tallization.~ Birefringence gives a measure of the orientation, which is the average of the amorphous and crystalline regions.6

Optical anisotropy in polypropylene fibres as a function of the draw ratio has been studied interferomet- r i~al ly .~. ' Several studies have been reported of the

* To whom correspondence should be addressed.

effect of annealing on the structure of synthetic and natural fibres.g-' '

Annealing may be performed with the ends of the sample free or fixed. In the former case the sample shrinks; in the latter case the sample retains its length but measurable retractive forces are exerted on its fixed ends. Both effects increase with increased annealing temperature.' In the present work, the multiple-beam Fizeau technique in conjunction with a microstrain device was used to study the optical and mechanical parameters produced in polypropylene fibres under different conditions. The variations of refractive indices and birefringence by different stresses were studied. The resulting data were utilized to calculate the strain optical coefficient, orientation factor, orientation angle and Poisson's ratio at different annealing temperatures. Also, multiple-beam Fizeau fringes were used (a) to calculate the constants of Cauchy's equation, the dispersion power of polypropylene fibres and the dielectric constant at infinity, and (b) to determine the refractive index profiles of polypropylene fibres at different draw ratios for different annealing temperatures considering the refraction of light beam through the fibre medium.

129 Polymer International 0959-8103/96/$09.00 0 1996 SCI. Printed in Great Britain

130 A. A. Hamza et al.

(4 Fig. 1. Microinterferograms of multiple-beam Fizeau fringes in transmission for untreated polypropylene fibres at draw ratios of (a) 4.0, (b) 4.6, (c) 5.2 and (d) 5.8, for light vibrating

parallel to the fibre axis.

EXPERlM ENTAL

The method discussed in Ref. 2, for producing multiple- beam Fizeau fringes formed by a silvered liquid wedge crossing a fibre for the polarization plane of light paral-

TABLE 1. Values of the parameters characterizing the deformation process relating the birefringence

with the draw ratio

Annealing temperature m b C of polypropylene

fibres (h2"C)

Room temperature -0.021 0 0-8700 21.987 100 0,0330 0.1 126 1.3906 120 0.0292 0.2785 7.4609

(4 Fig. 2. Microinterferograms of multiple-beam Fizeau fringes in transmission for untreated polypropylene fibres at draw ratios of (a) 4.0, (b) 4.6, (c) 5.2 and (d) 5.8, for light vibrating

perpendicular to the fibre axis.

lel and perpendicular to the fibre axis, was used to measure the principal optical parameters. Due to the deformation effects occurring during the annealing and drawing processes, we used the following formula' to overcome any irregularity of the fringe shift of the fibre:

where nA1 is the mean refractive index of the fibre material for plane polarized light vibrating parallel to

TABLE 2. Values of the parameters characterizing the deformation process relating the cross-

sectional area with the draw ratio

Annealing temperature U B Y of polypropylene

fibres (*2"C)

Room temperature -40.982 -1 -1 975 0.01 16 100 -41 .I 77 -2.1 01 0 0.01 47 120 65.679 -1.51 57 0.01 07

POLYMER INTERNATIONAL VOL. 39, NO. 2. 1996

Cold drawn polypropylene j b r e s 131

(4 Fig. 4. Microinterferograms of multiple-beam Fizeau fringes for polypropylene fibres annealed at constant temperature (120°C) for 2 h with draw ratios of (a) 4.0, (b) 4.4 and (c) 5.0,

for light vibrating parallel to the fibre axis.

(d)

Fig. 3. Microinterferograms of multiple-beam Fizeau fringes for polypropylene fibres annealed at constant temperature (100°C) for 2 h with draw ratios of (a) 4.0; (b) 4.8, (c) 5.2 and

(d) 5.8, for light vibrating parallel to the fibre axis.

the fibre axis, nL is the refractive index of the immersion liquid, FII is the total area enclosed under the fringe shift, h is the interfringe spacing in the liquid region, A is the fibre cross-sectional area, and I is the wavelength of monochromatic light used. The above equation has an analogous form for light vibrating perpendicular to the fibre axis for the determination of n,'. The birefringence is then given by the following formula:

IAF Ana = -

2hA

where AF = FII - F'.

RESULTS AND DISCUSSION

Changes of the refractive indices due to the drawing (C)

Fig. 5. Microinterferograms of multiple-beam Fizeau fringes for polypropylene fibres annealed at constant temperature (120°C) for 2 h with draw ratios of (a) 4.0, (b) 4.4 and (c) 5.0,

for light vibrating perpendicular to the fibre axis.

process before and after annealing

The multiple-beam technique with a microstrain device, described previously,' was used to determine the mean

POLYMER INTERNATIONAL VOL. 39, NO. 2, 1996

132

5.60

c 4.60 -

0

E E - 0

x m 3

3.60 -

- - 1

2.60 3.5

A. A. Hamza et al.

-2. room temp. *t_f- 100 'C t L 9 - U b 120 'c

--, t 1 , t 7 1 t 8 , I 8 1 , - 7- 7 1 I I I I I , I 1 I - I I 1 , - m 4.0 4.5 5.0 5.5 6

- 1.540 - - t

x W P

I

.E 1.538

._ ? I

E 2 1.536 -

1.532 1 I I , 1 1 , , I , , , I I I -, , , , , I , , , , , , , ~ , , , , , , , , , , ,A 3.5 4.0 4.5 5.0 5.5 6.0

Draw ratio

1.510 (b)

1.508

- 1.506 C

X al 0

W

I

.E 1.504

._ c E 1 1.502 U

ULL. 120 .C

1 498 3-n'"'"' 4.0 4.5 5.0 5.5 ''4 6.0

Draw ratio

Fig. 6. Relationship between refractive indices of polypropylene fibres, before and after annealing at 100 and

120 f 2°C for 2 h, and draw ratio. (a) nil ; (b) n,".

I 0.044

+ ! * 100 " C *-o 120 * C

0 032 3.5 4 0 4 5 5 0 5 5 6 0

Draw ratio

Fig. 7. Relationship between the birefringence, Ana, of polypropylene fibres, before and after annealing at 100 and

120 f 2°C for 2 h, and draw ratio.

3

A, of polypropylene fibres before and after annealing at 100 and

120 f 2°C for 2 h, and draw ratio.

refractive indices, nil and ni, of drawn polypropylene fibres before (at room temperature) and after annealing at 100 and 120 f 2°C at constant annealing time (2 h) with different draw ratios.

Figures 1 and 2 show microinterferograms of multiple-beam Fizeau fringes in transmission for untreated samples of polypropylene fibres at different draw ratios, using plane polarized monochromatic light of wavelength 546.1 nm vibrating parallel and perpendicular to the fibre axis, respectively. The refractive index of the immersion liquid was 1.5282 at 31 k 0.5"C and 1.498 at 30.5 f 0.5"C, respectively.

0.5

room temp. 100 'C 120 ' C

I

0.0 o.oi0 ' ' ' 0.05; ' ' ' e.b'i'i ' ' 0.056 ' ' ' ;.Gi ' * 0.1 Birefringence (An)

Fig. 9. Relationship between the birefringence, Ana, of polypropylene fibres, before and after annealing at 100 and

120 f 2°C for 2 h, and Poisson's ratio.


Cold drawn polypropylene f ibres 133

(C)

Fig. 11. Microinterferograms of multiple-beam Fizeau fringes of unannealed polypropylene fibres with draw ratio 4.0 at wavelength (a) 589.3, (b) 546.1 and (c) 436nm, for light vibrat-

ing perpendicular to the fibre axis.

(4 Fig. 10. Microinterferograms of multiple-beam Fizeau fringes of unannealed polypropylene fibres with draw ratio 4.0 at wavelength (a) 589.3, (b) 578, (c) 546.1 and (d) 436nm, for light

vibrating parallel to the fibre axis.

Figure 3 shows microinterferograms of multiple-beam Fizeau fringes of polypropylene fibres annealed at 100 k 2°C for 2 h with various draw ratios, using monochromatic light of wavelength 546- 1 nm vibrating parallel to the fibre axis. The refractive index of the immersion liquid was 1.531 at 31 f 0.5"C.

Figures 4 and 5 show microinterferograms of multiple-beam Fizeau fringes of polypropylene fibres annealed at 120 k 2°C for 2 h with different draw ratios, using monochromatic light of wavelength 546.1 nm vibrating parallel and perpendicular to the fibre axis, respectively. The refractive index of the immersion liquid was 1-536 at 19 k 0.5"C and 1-506 at 19 f 0.5"C, respectively.

Figure 6 shows the behaviour of the refractive indices, n;' and n t , of polypropylene fibre before and after annealing as a function of draw ratio. It is clear that the parallel refractive index, n! , for all samples increases

with increasing draw ratio while the perpendicular refractive index, n:, decreases slightly.

Figure 7 shows the variation of the birefringence, Ana, of the same samples as a function of draw ratio, before and after annealing. It is clear that the birefringence of polypropylene fibres increases with the draw

1.54

nli

1.53 4 e- /------

/II/_/// 1.52

m

m D

.$ 1.51 E

1.50

1.49

1.48 2.0 2.5 3.0 3.5 4.0 45 5.C 5

1/12 10-6 (nm-2)

1.48 1 2.0 2.5 3.0 3.5 4.0 45 5.C 5

1/12 10-6 (nm-2)

Fig. 12. Relationship between the refractive indices, n! and nf, of polypropylene fibres and ( l /A2) .


134 A . A . Hamza et al.

- o,0698 -=* room-temp. -.---.

w 100 c 3 -...a 120 *C

1 0.744

and dispersive power constants of polypropylene fibres

0 . 7 4 2 - - y d , 2.0 2.5 3.0 3.5 9.0 4.5 5.0 5..

t/h* XIO-6 (nm-2)

Fig. 13. Relationship between { l/(n2 - l)} of polypropylene fibres and (1/A2).

Annealing Draw P ct temperature ratio

(*2"C)

Room temperature 4.2 0.41 77 0.022 4.4 0.21 38 0.006 4.6 0.1 460 0.008 4.8 0.1 123 0.01 4 5.0 0.0921 0.01 8 5.2 0.0788 0.005 5.4 0.0693 0.005 5.6 0.0623 0,004

5 5.8 0.0569 0.004

0.0746 1 0.0746

0.0744

TABLE 3. Poisson's ratio, p, and strain optical coefficient, C,, of polypropylene fibres at different

draw ratios and annealing temperatures

100 4.2 0.3756 0.01 2 4.4 0.1 91 8 0.01 0 4.6 0.1 307 0.01 2 4.8 0.1 002 0.008 5.0 0.0820 0.01 2 5.2 0.0698 0.008 5.4 0.061 3 0.004

0.004 0.0550 5.6 5.8 0.0500 0.002

120 4.2 0.2525 0,020 4.4 0.1 251 0.01 0 4.6 0,0847 0.01 6 4.8 0.0645 0.01 2 5.0 0.0524 0.01 4

ratio, R, following the De Vries empirical r e l a t i ~ n : ~

(3) -- - m + bAn dAn

0.0736 3.5 4.0 4.5 5.0 5.5 6.0 d In R

Draw ratio

Integrating eqn (3) gives 0.0708

0.0706

0.0704 4- b 1. 0.0702

.ij

._ s L

m - a" 0.0700

where m, b and c are parameters which characterize the deformation process. The values calculated for polypropylene fibre before and after annealing are shown in Table 1. rn indicates the initial slope of the birefringence

TABLE 4. Values obtained of the Cauchy's formula

Direction A B x lo3 (nm-2) dnldA 0.0696 1 1 3 , 8 I 1 0 7 7 of the

3.5 4.0 4.5 5.0 5.5 6.0 Draw ratio vibrating light

2.905 x 1 0-6 Fig. 14. Polarizability per unit volume, of polypropylene Parallel 1.5226 1.449 fibres, before and after annealing at 100 and 120 _+ 2°C for 2 h, 10.81 9 Perpendicular 1.461 2 I ,503 x 1 o - ~

as a function of draw ratio. (a) PI1; (b) P'.


Cold drawn polypropylene jibres 135

J

1.51 4

1.512

s 1 5 1 0 1 . -.

....a 120 *c

1.506 3.5 4.0 4.5 5.0 5.5 t

Draw ratio

Fig. 15. Relationship between the isotropic refractive index, niso, of polypropylene fibres, before and after annealing at 100

and 120 f 2°C for 2 h, and draw ratio.

Draw ratio

Fig. 16. Relationship between (a) the orientation factor and (b) the orientation angle of polypropylene fibres, before and after annealing at 100 and 120 f 2°C for 2 h, and draw ratio.

0.20 350 400 450 500 550 E

Wavelength (nm)

2oq 15 350 400 450 500 550 6

Wavelength (nm)

Fig. 17. Relationship between (a) the orientation factor and (b) the orientation angle of polypropylene fibres, using differ-

ent wavelengths.

natural extension curve; if drawing starts from the isotropic state, m can assume positive or negative values according to the birefringence sign in the fibre. The different values of b express3 the difference in interaction between the chain elements; accordingly they become progressively better oriented with respect to the fibre axis.

Figure 8 shows the mean cross-sectional area of the polypropylene fibres, before and after annealing as a function of draw ratio. It is clear that the cross-sectional area, A, of the fibres decreases with increasing draw ratio, R, according to the following suggested empirical formula :

where u, j? and y are parameters which characterize the deformation due to the drawing process. The values of



*-* room temp. * . 100 c 0- 120 C

0 1.20 1.30 1 .40 1.50 1.60 1.70 1.80

In R

Fig. 18. Linear relationship between [F(O)An/AO] and In R of polypropylene fibres, before and after annealing at 1 0 0 and

120 f 2°C for 2 h.

the constants a, /3 and y are given in Table 2 for polypropylene fibres before and after annealing.

The differential form of eqn (5) is

dA d In R -- - r - P

On straining, the fibre becomes thinner and this change in radius, r, can be related to the change in length, L, by Poisson's ratio, p,14 defined by

dr dL r (7) _ - - -PI

The values of p at different temperatures are given in Table 3. The strain optical coefficient, C, , is defined as

dAn c =- de

where E is the strain. C, was determined at different strain ranges and different annealing temperatures, and the results are also given in Table 3. Figure 9 shows the relationship between the birefringence of polypropylene fibre, before and after annealing, and Poisson's ratio. It is clear that Poisson's ratio decreases with increasing birefringence and also decreases with increasing annealing temperature.

Application of multiple -beam Fizeao fringes to determine the constants of Cauch y's dispersion formula and the dispersive coefficient for polypropylene fibres

Figures 10 and 11 show microinterferograms of multiple-beam Fizeau fringes crossing polypropylene fibres with draw ratio 4 at different wavelengths for

TABLE 5. Variation of the constants of eqn (16) with annealing temperature of polypropylene fibres

Annealing C K temperature (*2"C)

Room temperature 39,4966 -44.41 65 29.6924 -32.4771 100

120 22.4855 -23.8894

light vibrating parallel and perpendicular to the fibre axis, respectively.

Figure 12 shows the relationship between refractive indices, nil and n,l, and ( l /A2)) , which is constructed to evaluate the constants A and B of the well-known Cauchy's formula:

(9)

with an analogous formula for n:, where A and B are Cauchy's dispersion constants characterizing the dispersion activity of the material. The dispersive power is

n! = A + B/A2

TABLE 6. Orientation factor, orientation angle, cross-sectional area and birefringence of polypropylene fibres at different draw ratios for differ-

ent annealing temperatures ~ ~~~ ~~

Annealing In R ~ ~ 1 0 - 3 e F(e) temperature (mrn') (rad)

(i2"C)

Room 1.3863 temperature 1.4351

1.481 6 1,5261 1.5682 1.6094 1.6487 1.6864 1.7228 1,7579

100 1.3863 1.4351 1.481 6 1.5261 1.5682 1,6094 1.6487 1.6864 1.7228 1.7579

120 1.3863 1,4351 1.481 6 1,5261 1.5682 1.6094

0.0342 0.0353 0.0356 0.0360 0.0367 0,0376 0.0379 0.0381 0.0379 0.0381

0,0327 0.0333 0.0338 0.0342 0.0346 0,0352 0.0356 0.0358 0.0360 0.0361

0.031 0 0.0320 0.0325 0.0333 0,0339 0.0346

5.468 5,331 5.064 4.81 9 4.555 4.351 4.1 27 3,890 3,693 3.558

5.400 5.060 4.820 4-643 4.442 4.338 4,039 3.854 3,652 3.481

5.491 5.342 5,261 5.1 12 4.858 4.789

0.4330 0.7359 0.41 25 0.7589 0,4068 0,7651 0.3989 0.7736 0.3851 0.7882 0.3675 0.8062 0.3672 0.8066 0.3558 0.81 78 0.3605 0.81 34 0,3583 0.81 55

0.461 9 0.7020 0,451 3 0.71 47 0.4422 0.7253 0.4349 0.7337 0.4274 0.7421 0.41 62 0.7548 0.4085 0,7633 0.4045 0.7676 0,4005 0.7720 0,3984 0.7741

0.4965 0.6596 0.4794 0.6808 0.4705 0.691 6 0.4562 0.7089 0.4450 0.7220 0.431 7 0,7373


Cold drawn polypropylene Jibres

given by

dnldl = -2B/A3 (10) Values of the constants A, B and dnldl are given in Table 4.

Since the index of refraction is given by the square root of the dielectric constant E by the e q ~ a t i o n ' ~

n = & or E , = n ' (11) by plotting {l/(n2 - l)} versus (112') (Fig. 13), a linear relationship is obtained from which we can calculate E ,

(equal to 2.2003).

Mean polarizability per unit volume and isotropic refractive index

The equation'

gives the mean polarizability of polypropylene fibres; an analogous formula is used for the perpendicular direction. Figure 14 shows the polarizability per unit volume of polypropylene fibres before and after annealing as a function of draw ratio.

The isotropic refractive index, niso, can be calculated from the following equation:'

for these fibres before and after treatment. Figure 15 shows the variation of the isotropic refractive index with the draw ratio of polypropylene fibre before and after annealing.

1.545

1.540

1.535

1.530

1.525

546.1 nm - 593 nrn - 436 nrn - 578 nrn

1.520

1.515 -40 -20 0 20

Distance across the fibre diameter

-593 nm - 546.1 nrn - 436 nrn

1.490

1.480 -40 -20 0 20


137

Fig. 19. Refractive index profiles at different wavelengths of polypropylene fibres with draw ratio 4.0. (a) d ( r ) ; (b) n'(r).

Application of multiple -beam Fizeau fringes to determine the degree of orientation

The optical orientation angle, 8, can be found using Hermans orientation factor, F(8), from the following equation :

(14) 3 . 2

F(e ) = 1 - - sin2 8

where F(8) is the optical orientation factor which can be calculated from'

where rill and n' and An = (nit - n*) are the mean measured refractive indices in the parallel and perpendicular directions and An is the birefringence, respectively. The values of n,, n, and Ano are taken to be 1.521, 1-476 and 0.045, respectively."

Figure 16 shows the orientation factor and the orientation angle as functions of the draw ratio of poly-

propylene fibres before and after annealing. The orientation factor increases with increasing draw ratio while the orientation angle decreases with increasing draw ratio.

Figure 17 shows the relationship between the orientation factor and orientation angle with wavelength.

An empirical formula is suggested to evaluate the relationship between orientation factor, orientation angle, area of cross-section and birefringence with the draw ratio, as follows:

-- F( 8)An - C l n R + K

A8

where C and K are constants characterizing the pro- portionality between [F(8)An/A8] and In R. The values of C and K vary with the annealing temperature. Figure 18 shows a linear relationship between [F(B)An/AB] and In R; the values of the constants are given in Table 5. The values of the orientation factor F_(8), orientation


138 A . A . Hamza et al.

1.520,

+H+t106 *C -120 *c

1.495 l " N m bc)


1.515 c I

C

8 1.510 .-


1.500 4

1.495


1.520-

-room blrp. -100.c -120 'C

Distance across fhe fibre diameter

Fig. 20. Refractive index profiles, nil(r), at draw ratio (a) 4.2, (b) 4.4, (c) 4.6 and (d) 5.0 for polypropylene fibres, before and after annealing at 100 and 120 f 2°C for 2 h.

angle 0, birefringence Ana, cross-sectional area A and In R are given in Table 6.

Determination of the refractive index profiles in annealed polypropylene fibres with different draw ratios by multiple-beam interference method taking into account the refraction of light beam through the fibre medium

The variations of the refractive indices across the diameter of the fibre were determined, taking into account the actual refracted beam inside the fibre, using the following formula for the multiple-beam technique :' 9*20

(17) where IZ(r)I is the absolute value of Z(r) , and Z(r) is the distance under the fringe shift along the radius, r is the distance along the radius of the fibre, fi, = nL - 2(n, - nf), m = nf/n, where nL is the liquid refractive index and n, is the mean refractive index of the fibre.

Figure 19 shows the refractive index profiles, nll(r) and n'(r), of unannealed polypropylene fibres with draw ratio 4 at different wavelengths. Figure 20 and 21 show the refractive index profiles, n " ( I ) and n'(r), respectively, at different draw ratios for polypropylene fibres before and after annealing.

The refractive index profiles of polypropylene at different draw ratios throw light on the changes in orientation of molecules along the fibre axis. This helps in controlling the dyeing of these fibres and related processes.

CONCLUSIONS

The following conclusions may be drawn from the present investigation :

( 1 ) An empirical formula is suggested to relate the variation of the cross-sectional area, angle of orientation, the orientation factor and the birefringence of polypropylene fibres with the draw ratio.


Cold drawn polypropylene Jibres 139


1545 1 (b’

- -*tmp. - i w c -120 ‘C


1 (*’ 1.545

: - c 1.540 1-

I 20.’00 40.00


- room tamp.

-120 .C


Refractive index profiles, n’(r), at draw ratio (a) 4.2, (b) 4.4, (c) 4.6 and (d) 5.0 of polypropylene fibres, before and after annealing at 100 and 120 f 2°C for 2 h.

The dimensional changes of the fibres are a function of draw ratio. The higher the orientation, the more mutually parallel the molecules and the smaller the average angle formed by them with the fibre axis. The orientation angle decreases with increasing wavelength in the visible range of the spectrum while the orientation factor increases. Poisson’s ratio is reduced by the strain effect. The study of the rate of change of nil and nf with respect to the draw ratio clarifies that the mechanical properties of the structure in a direction perpendicular to the fibre axis differ from those in an axial direction, which is expected for an anisotropic medium. Drawing the annealed fibrous structure affects the transport properties due to changes in the cohesive forces between adjacent molecules, which needs further study in future. Changes in the isotropic refractive index of annealed polypropylene fibres due to the drawing process indicate a change in density

and mass redistribution of the molecular structure.

(9) The results of the refractive index profiles of undrawn and drawn fibres before and after annealing throw light on the structural variation of optical properties across the diameter.

(10) The microinterferograms clearly identify differ- ences in unannealed-undrawn and annealed- drawn polypropylene fibres.

From the above results and considerations, we conclude that these measurements provide acceptable results for optical, thermal and mechanical parameters. Since Ana, F(B), 8, A, C, and Poisson’s ratio are consequences of the material stretched after annealing, so orientational strengthening of polymers may occur not only during fabrication but during deformation and cold drawing to achieve some improvements for the fibrous material.

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POLYMER INTERNATIONAL VOL. 39, NO. 2. 1996

effect of annealing on the optical and mechanical properties of cold drawn polypropylene fibres

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