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Opto-mechanical properties of fibres. 2—Optical anisotropy in polyester fibres as a function of the draw ratio
Polymer Testing 10 (1991) 195-203
Opto-Mechanical Properties of Fibres. 2 Optical Anisotropy in Polyester Fibres as a Function of the Draw Ratio A. A. H a m z a , I. M. F o u d a , K. A . E I - F a r a h a t y & E. A. Seisa Physics Department, Faculty of Science, Mansoura University, Mansoura, Egypt (Received 22 November 1990; accepted 23 December 1990)
ABSTRACT The polarising interference microscope is used with a fibre manipulation device to study, dynamically, the variation of refractive indices and birefringence of polyester fibres with the draw ratio. The experimental values of the birefringence are fitted using lagrangian interpolation. Microinterferograms are given for illustration. INTRODUCTION Synthetic fibres spun from the melt of a certain polymer by extrusion through fine holes are almost isotropic in their physical properties. To produce stronger fibres from these spun fibres, suitable for different purposes, they are mechanically drawn. These fibres in the drawn or extended state show considerable optical and mechanical anisotropy. The degree of anisotropy in the drawn state is related to the amount of extension imposed. Optical anisotropy, measured interferometrically, is a convenient method for determining orientation in polymer fibres. ~ Kuhn and Griin 2 developed a theory to yield a relation between the molecular structure of a uniaxially oriented polymer and its optical anisotropy. Optical anisotropy, developed in synthetic fibres by stretching, gives valuable information for characterisation of these fibres. The drawing process can be adjusted by measuring refractive indices and birefringence of fibres, de Vries 3 gave an analysis of the relationship between 195 Polymer Testing 0142-9418/91/$03-50 (~) 1991 Elsevier Science Publishers Ltd, England. Printed in Northern Ireland
196
A. A. Hamza, I. M. Fouda, K. A. El-Farahaty, E. A. Seisa
the birefringence and the draw ratio of some synthetic fibres. Pinnock and Ward 4 studied a series of polyethylene terephthalate fibres of different draw ratios by measuring the mechanical and optical properties of these fibres. The effect of stretching normal viscose rayon fibres on their refractive indices and birefringence was studied by Barakat and Hindeleh. 5 Multiple-beam Fizeau fringes were used to study the optical anisotropy in polypropylene fibres as a function of the draw ratio. 6 The optical behaviour of some synthetic fibres with different draw ratios have been studied interferometrically using two-beam and multiple-beam interferometric techniques: -~° A fibre manipulation device n was constructed for use with the polarising interference microscope in order to rotate the fibre around its axis, and also stretch and twist this fibre. In this work, the opto-mechanical properties of polyester fibres from an Egyptian manufacturer have been studied, interferometrically, by using the Pluta ~1'~2polarising interference microscope.
Theoretical considerations The totally duplicated images of the fibres, using the Pluta polarising interference microscope ~2'~3 enable the measurement of the refractive indices n~ and n~ for light vibrating parallel and perpendicular to the fibre axis from the following expressions: 14 F"~. n~ = nL + - hA (1) F±~. hA
where nL is the refractive index of the immersion liquid, F" and F ± are the areas enclosed by the fringe shift, for light vibrating parallel and perpendicular to the fibre axis, respectively, h is the interfringe spacing corresponding to the wavelength A, and A is the mean cross-sectional area of the fibre. The birefringence Ana of the fibre is measured directly from the nonduplicated image of the fibre according to the formula: Aria =
AF~ hA
(2)
where AF is the area enclosed under the fringe shift using the nonduplicated image of the fibre.
Opto-mechanical properties of fibres--2
197
E X P E R I M E N T A L RESULTS A N D DISCUSSION In order to measure, dynamically, both strain and refractive index or birefringence by an interferometric technique, The Pluta 12'~3 polarising interference microscope is used in conjunction with the microstrain device described elsewhere. 11 A parallel beam of light is incident normally on the microscope stage. The fibre was placed on a glass slide and its ends are fixed with two small round clamps in the microstrain device by an adhesive. A drop of a suitable immersion liquid is placed on the slide such that the fibre is immersed in it. To determine the birefringence of the fibre directly, the slit diaphragm is placed parallel to the fibre axis to produce the nonduplicated image of the fibre. Figure 1 shows microinterferograms of the nonduplicated images of polyester fibres with draw ratios 1-0, 1.25, 1-5, 1.75, 2.0 and 2.25, respectively. Monochromatic light of wavelength 546 nm was used. The refractive index of the immersion liquid was 1.5300 at 31-5 °C. The mean value of birefringence Ana is calculated from the area enclosed under the interference fringe shift, the interfringe spacing h and the mean cross-sectional area of the fibre. Figures 2 and 3 give the variation of the mean birefringence Ana with the draw ratio for polyester fibres using white light and monochromatic light of wavelength 546 nm, respectively. The Pluta microscope is used with the microstrain device n to determine the variation of the mean refractive index of polyester fibres with draw ratio. The objective prism of this microscope is rotated to obtain maximum duplication of the two images of the fibre and the slit diaphragm is rotated to make an angle of 135 ° with the fibre axis to obtain the sharpest fringes. The resulting pattern has two parallel images of the fibre, which are perpendicular to the interference fringes. The fringe displacement of the upper image of the fibre is caused by the difference in the refractive index ( n ~ - nL) whereas the fringe displacement in the lower image is due to the refractive index difference (n~ - nL). Figure 4 shows microinterferograms of totally duplicated images of polyester fibres with draw ratios 1.0, 1.75 and 2.25, respectively. Monochromatic light of wavelength 546 nm was used. It is clear that the values of the area enclosed under the fringe shift and the fibre thickness is changed with the draw ratio. These values are used in the calculation of the mean refractive indices, n~ and na~, of the fibres. Figures 5 and 6 give relations between n~ and the draw ratio of polyester fibres using white and monochromatic light of wavelength 546 nm, respectively.
198
A. A. Hamza, L M. Fouda, K. A. EI-Farahaty, E. A. Seisa
(a)
(b)
(c)
(d)
(e)
(t3
Fig. 1. Differentially sheared (nonduplicated) images of polyester fibres with draw ratios (a) 1-0, (b) 1.25, (c) 1.5, (d) 1-75, (e) 2-0 and (f) 2.25 using the Pluta microscope with monochromatic light of wavelength 546 nm.
Opto-mechanicalproperties of fibres--2
199
0.18
0.15
0.12
,~ 0.09
g2
•
0.06
Experimental
O Theoretical 0,03
0.00 1.0
I
l
I
1.2
1.4
I .6
I
1.8 Draw
I
I
2.0
2.2
I
It
2.4
ratio
Fig. 2. Mean birefringence of polyester fibres measured from the nonduplicated images as a function of draw ratio. White light is used.
0.18
0.15
4 0.12
C:
0.09
0.06
0.03
0.00 1.0
1.2
1.4
1.6
1.~,
2.0 Draw
2.2
2.4
ratio
]Fig. 3. Mean birefringencc of polyester tibres measured trom the nonduplicated images as a function of draw ratio. Monochromatic light of wavelength 546 nm is used.
200
A. A. Hamza, I. M. Fouda, K. A. El-Farahaty, E. A. Seisa
(a)
(b)
(c)
Fig. 4. Totally duplicated images of polyester fibres with draw ratios (a) 1.0, (b) 1-75 and (c) 2.25 using the Pluta microscope with monochromatic light of wavelength 546 nm. In Fig. 2 the changes of the mean birefringence (Ana) with the draw ratio (R) is given experimentally at equal intervals in a given range. To compute the birefringence (Ana) at a nontabulated value of draw ratio and fitting the experimental values, lagrangian interpolation 15 was used. Y = P . ( x ) = lo(x)..ff(Xo) + I1(x)~LP(x,) + ' "
I.(x)~(x.)
where I,(x) = ~(x)/~r~(x,) ~(x)
=
(x
-
Xo) • • • (x
-
X,_l)(X
and replacing ), by the birefringence
-
Xi+m) • • • (x
(Ana) and
-
x.)
x by the draw ratio (R).
Opto-mechanical properties of fibres--2
//
17~.
1.70
=¢--
201
1.68
1.66
1.6/*
1.62
1.60
1.58
1.0
I
I
I
=
1.2
1.4
1.6 Draw
Fig. 5.
1.8
I
2.0
I
2.2
2.4
ratio
Relation between the mean refractive index n~ of polyester fibres, measured from totally duplicated images, and the draw ratio. White light is used. CONCLUSIONS
The two-beam interference microscope, with the fibre manipulation device (described earlier by Hamza et al. n), gives a clear understanding of the mechanism of fibre stretching and its effect on the molecular orientation with the fibre. From the measurements carried out in the present work, relating the change of optical properties due to external strain in polyester fibre (from an Egyptian manufacturer), the following conclusions may be drawn: 1. The application of a microstrain device with an interference microscope (e.g. Pluta microscope), provides an easy and quick method for studying the opto-mechanical properties of fibres. It is very useful when fibres with high refractive indices are measured, e.g. polyester and Kevlar fibres or when using immersion liquid of refractive index differs noticeably from that of the fibre.
202
A. A. Hamza, I. M. Fouda, K. A. EI-Farahaty, E. A. Seisa 1.72 B 1.70
1.68
1,66 Ill 1.6/*
1.62
1.60
1,58 1.0
'
'
'
'
1.2
1,t,
1.6
1.8 D raw
'
'
'
2.0
2.2
2.4
ratio
Fig. 6. Relation between the mean refractive index hi,I of polyester fibres, measured from the totally duplicated images, and the draw ratio. Monochromatic light of wavelength 546 nm is used.
2. Drawing the fibrous structure, however, only slightly affects the transport properties. 3. Drawing the polyester fibres tends to reorient the molecules parallel to the fibre axis and these are expected due to the increase of n] with increasing the draw ratio. 4. The microinterferograms are clear to identify differences in optical path variations due to different draw ratios. 5. The higher the birefringence, the more mutually parallel the molecules, and the smaller the average angle formed by them within the fibre axis. 6. As the draw ratio increases, the double refraction increases. This means that the inherent optical anisotropy of the chain-like macro-molecules from preferred axial orientation of the molecular chains that constitute the fibre also increases. One can conclude from the above results and considerations that the used manipulation device enables the fibre producer to study the
Opto-mechanical properties of fibres--2
203
opto-mechanical parameters for the usefulness of commercial synthetic fibres which are commonly cold-drawn continuously during manufacture.
ACKNOWLEDGEMENT The authors would like to thank Dr M. A. Madkour, Physics Department, Faculty of Science, Mansoura University, for his useful discussions.
REFERENCES 1. Barakat, N. & Hamza, A. A., Interferometry of Fibrous Materials. Adam Hilger, Bristol, 1990. 2. Kuhn, W. & Griin, F., Kolloid Zschr., 101 (1942) 248. 3. de Vries, H., J. Polym. Sci., 34 (1959) 761. 4. Pinnock, P. R. & Ward, I. M., Br. J. Appl. Phys., 15 (1964) 1559. 5. Barakat, N. & Hindeleh, A. M., Textile Res. J., 34 (1964) 581. 6. Hamza, A. A. & Kabeel, M. A., J. Phys. D: Appl. Phys., 20 (1987) 963. 7. Hamza, A. A., Fouda, I. M., EI-Farahaty, K. A. & Helaly, S. A., Polymer Testing, 7 (1987) 329. 8. Hamza, A. A., Fouda, I. M., Kabeel, M. A. & Shabana, H. M., Polymer Testing, $ (1989) 201. 9. Hamza, A. A., Fouda, I. M., EI-Farahaty, K. A. & Seisa, E. A., Polymer Testing, 10(2) (1991) 83. 10. Fouda, I. M., EI-Tonsy, M. M. & Oraby, A. H., J. Materials Sci., 25 (1990) 1416. 11. Hamza, A. A., EI-Farahaty, K. A. & Helaly, S. A., Optica Applicata, 18 (1988) 133. 12. Pluta, M., Optica Acta, 18 (1971) 661. 13. Pluta, M., J. Microsc., 96 (1972) 309. 14. Hamza, A. A., Textile Res. J., 50 (1980) 731. 15. Fox, L. & Mayers, D. F., Computing Methods for Scientists and Engineers. Oxford University Press, Oxford, 1980, p. 145.
Opto-Mechanical Properties of Fibres. 2 Optical Anisotropy in Polyester Fibres as a Function of the Draw Ratio A. A. H a m z a , I. M. F o u d a , K. A . E I - F a r a h a t y & E. A. Seisa Physics Department, Faculty of Science, Mansoura University, Mansoura, Egypt (Received 22 November 1990; accepted 23 December 1990)
ABSTRACT The polarising interference microscope is used with a fibre manipulation device to study, dynamically, the variation of refractive indices and birefringence of polyester fibres with the draw ratio. The experimental values of the birefringence are fitted using lagrangian interpolation. Microinterferograms are given for illustration. INTRODUCTION Synthetic fibres spun from the melt of a certain polymer by extrusion through fine holes are almost isotropic in their physical properties. To produce stronger fibres from these spun fibres, suitable for different purposes, they are mechanically drawn. These fibres in the drawn or extended state show considerable optical and mechanical anisotropy. The degree of anisotropy in the drawn state is related to the amount of extension imposed. Optical anisotropy, measured interferometrically, is a convenient method for determining orientation in polymer fibres. ~ Kuhn and Griin 2 developed a theory to yield a relation between the molecular structure of a uniaxially oriented polymer and its optical anisotropy. Optical anisotropy, developed in synthetic fibres by stretching, gives valuable information for characterisation of these fibres. The drawing process can be adjusted by measuring refractive indices and birefringence of fibres, de Vries 3 gave an analysis of the relationship between 195 Polymer Testing 0142-9418/91/$03-50 (~) 1991 Elsevier Science Publishers Ltd, England. Printed in Northern Ireland
196
A. A. Hamza, I. M. Fouda, K. A. El-Farahaty, E. A. Seisa
the birefringence and the draw ratio of some synthetic fibres. Pinnock and Ward 4 studied a series of polyethylene terephthalate fibres of different draw ratios by measuring the mechanical and optical properties of these fibres. The effect of stretching normal viscose rayon fibres on their refractive indices and birefringence was studied by Barakat and Hindeleh. 5 Multiple-beam Fizeau fringes were used to study the optical anisotropy in polypropylene fibres as a function of the draw ratio. 6 The optical behaviour of some synthetic fibres with different draw ratios have been studied interferometrically using two-beam and multiple-beam interferometric techniques: -~° A fibre manipulation device n was constructed for use with the polarising interference microscope in order to rotate the fibre around its axis, and also stretch and twist this fibre. In this work, the opto-mechanical properties of polyester fibres from an Egyptian manufacturer have been studied, interferometrically, by using the Pluta ~1'~2polarising interference microscope.
Theoretical considerations The totally duplicated images of the fibres, using the Pluta polarising interference microscope ~2'~3 enable the measurement of the refractive indices n~ and n~ for light vibrating parallel and perpendicular to the fibre axis from the following expressions: 14 F"~. n~ = nL + - hA (1) F±~. hA
where nL is the refractive index of the immersion liquid, F" and F ± are the areas enclosed by the fringe shift, for light vibrating parallel and perpendicular to the fibre axis, respectively, h is the interfringe spacing corresponding to the wavelength A, and A is the mean cross-sectional area of the fibre. The birefringence Ana of the fibre is measured directly from the nonduplicated image of the fibre according to the formula: Aria =
AF~ hA
(2)
where AF is the area enclosed under the fringe shift using the nonduplicated image of the fibre.
Opto-mechanical properties of fibres--2
197
E X P E R I M E N T A L RESULTS A N D DISCUSSION In order to measure, dynamically, both strain and refractive index or birefringence by an interferometric technique, The Pluta 12'~3 polarising interference microscope is used in conjunction with the microstrain device described elsewhere. 11 A parallel beam of light is incident normally on the microscope stage. The fibre was placed on a glass slide and its ends are fixed with two small round clamps in the microstrain device by an adhesive. A drop of a suitable immersion liquid is placed on the slide such that the fibre is immersed in it. To determine the birefringence of the fibre directly, the slit diaphragm is placed parallel to the fibre axis to produce the nonduplicated image of the fibre. Figure 1 shows microinterferograms of the nonduplicated images of polyester fibres with draw ratios 1-0, 1.25, 1-5, 1.75, 2.0 and 2.25, respectively. Monochromatic light of wavelength 546 nm was used. The refractive index of the immersion liquid was 1.5300 at 31-5 °C. The mean value of birefringence Ana is calculated from the area enclosed under the interference fringe shift, the interfringe spacing h and the mean cross-sectional area of the fibre. Figures 2 and 3 give the variation of the mean birefringence Ana with the draw ratio for polyester fibres using white light and monochromatic light of wavelength 546 nm, respectively. The Pluta microscope is used with the microstrain device n to determine the variation of the mean refractive index of polyester fibres with draw ratio. The objective prism of this microscope is rotated to obtain maximum duplication of the two images of the fibre and the slit diaphragm is rotated to make an angle of 135 ° with the fibre axis to obtain the sharpest fringes. The resulting pattern has two parallel images of the fibre, which are perpendicular to the interference fringes. The fringe displacement of the upper image of the fibre is caused by the difference in the refractive index ( n ~ - nL) whereas the fringe displacement in the lower image is due to the refractive index difference (n~ - nL). Figure 4 shows microinterferograms of totally duplicated images of polyester fibres with draw ratios 1.0, 1.75 and 2.25, respectively. Monochromatic light of wavelength 546 nm was used. It is clear that the values of the area enclosed under the fringe shift and the fibre thickness is changed with the draw ratio. These values are used in the calculation of the mean refractive indices, n~ and na~, of the fibres. Figures 5 and 6 give relations between n~ and the draw ratio of polyester fibres using white and monochromatic light of wavelength 546 nm, respectively.
198
A. A. Hamza, L M. Fouda, K. A. EI-Farahaty, E. A. Seisa
(a)
(b)
(c)
(d)
(e)
(t3
Fig. 1. Differentially sheared (nonduplicated) images of polyester fibres with draw ratios (a) 1-0, (b) 1.25, (c) 1.5, (d) 1-75, (e) 2-0 and (f) 2.25 using the Pluta microscope with monochromatic light of wavelength 546 nm.
Opto-mechanicalproperties of fibres--2
199
0.18
0.15
0.12
,~ 0.09
g2
•
0.06
Experimental
O Theoretical 0,03
0.00 1.0
I
l
I
1.2
1.4
I .6
I
1.8 Draw
I
I
2.0
2.2
I
It
2.4
ratio
Fig. 2. Mean birefringence of polyester fibres measured from the nonduplicated images as a function of draw ratio. White light is used.
0.18
0.15
4 0.12
C:
0.09
0.06
0.03
0.00 1.0
1.2
1.4
1.6
1.~,
2.0 Draw
2.2
2.4
ratio
]Fig. 3. Mean birefringencc of polyester tibres measured trom the nonduplicated images as a function of draw ratio. Monochromatic light of wavelength 546 nm is used.
200
A. A. Hamza, I. M. Fouda, K. A. El-Farahaty, E. A. Seisa
(a)
(b)
(c)
Fig. 4. Totally duplicated images of polyester fibres with draw ratios (a) 1.0, (b) 1-75 and (c) 2.25 using the Pluta microscope with monochromatic light of wavelength 546 nm. In Fig. 2 the changes of the mean birefringence (Ana) with the draw ratio (R) is given experimentally at equal intervals in a given range. To compute the birefringence (Ana) at a nontabulated value of draw ratio and fitting the experimental values, lagrangian interpolation 15 was used. Y = P . ( x ) = lo(x)..ff(Xo) + I1(x)~LP(x,) + ' "
I.(x)~(x.)
where I,(x) = ~(x)/~r~(x,) ~(x)
=
(x
-
Xo) • • • (x
-
X,_l)(X
and replacing ), by the birefringence
-
Xi+m) • • • (x
(Ana) and
-
x.)
x by the draw ratio (R).
Opto-mechanical properties of fibres--2
//
17~.
1.70
=¢--
201
1.68
1.66
1.6/*
1.62
1.60
1.58
1.0
I
I
I
=
1.2
1.4
1.6 Draw
Fig. 5.
1.8
I
2.0
I
2.2
2.4
ratio
Relation between the mean refractive index n~ of polyester fibres, measured from totally duplicated images, and the draw ratio. White light is used. CONCLUSIONS
The two-beam interference microscope, with the fibre manipulation device (described earlier by Hamza et al. n), gives a clear understanding of the mechanism of fibre stretching and its effect on the molecular orientation with the fibre. From the measurements carried out in the present work, relating the change of optical properties due to external strain in polyester fibre (from an Egyptian manufacturer), the following conclusions may be drawn: 1. The application of a microstrain device with an interference microscope (e.g. Pluta microscope), provides an easy and quick method for studying the opto-mechanical properties of fibres. It is very useful when fibres with high refractive indices are measured, e.g. polyester and Kevlar fibres or when using immersion liquid of refractive index differs noticeably from that of the fibre.
202
A. A. Hamza, I. M. Fouda, K. A. EI-Farahaty, E. A. Seisa 1.72 B 1.70
1.68
1,66 Ill 1.6/*
1.62
1.60
1,58 1.0
'
'
'
'
1.2
1,t,
1.6
1.8 D raw
'
'
'
2.0
2.2
2.4
ratio
Fig. 6. Relation between the mean refractive index hi,I of polyester fibres, measured from the totally duplicated images, and the draw ratio. Monochromatic light of wavelength 546 nm is used.
2. Drawing the fibrous structure, however, only slightly affects the transport properties. 3. Drawing the polyester fibres tends to reorient the molecules parallel to the fibre axis and these are expected due to the increase of n] with increasing the draw ratio. 4. The microinterferograms are clear to identify differences in optical path variations due to different draw ratios. 5. The higher the birefringence, the more mutually parallel the molecules, and the smaller the average angle formed by them within the fibre axis. 6. As the draw ratio increases, the double refraction increases. This means that the inherent optical anisotropy of the chain-like macro-molecules from preferred axial orientation of the molecular chains that constitute the fibre also increases. One can conclude from the above results and considerations that the used manipulation device enables the fibre producer to study the
Opto-mechanical properties of fibres--2
203
opto-mechanical parameters for the usefulness of commercial synthetic fibres which are commonly cold-drawn continuously during manufacture.
ACKNOWLEDGEMENT The authors would like to thank Dr M. A. Madkour, Physics Department, Faculty of Science, Mansoura University, for his useful discussions.
REFERENCES 1. Barakat, N. & Hamza, A. A., Interferometry of Fibrous Materials. Adam Hilger, Bristol, 1990. 2. Kuhn, W. & Griin, F., Kolloid Zschr., 101 (1942) 248. 3. de Vries, H., J. Polym. Sci., 34 (1959) 761. 4. Pinnock, P. R. & Ward, I. M., Br. J. Appl. Phys., 15 (1964) 1559. 5. Barakat, N. & Hindeleh, A. M., Textile Res. J., 34 (1964) 581. 6. Hamza, A. A. & Kabeel, M. A., J. Phys. D: Appl. Phys., 20 (1987) 963. 7. Hamza, A. A., Fouda, I. M., EI-Farahaty, K. A. & Helaly, S. A., Polymer Testing, 7 (1987) 329. 8. Hamza, A. A., Fouda, I. M., Kabeel, M. A. & Shabana, H. M., Polymer Testing, $ (1989) 201. 9. Hamza, A. A., Fouda, I. M., EI-Farahaty, K. A. & Seisa, E. A., Polymer Testing, 10(2) (1991) 83. 10. Fouda, I. M., EI-Tonsy, M. M. & Oraby, A. H., J. Materials Sci., 25 (1990) 1416. 11. Hamza, A. A., EI-Farahaty, K. A. & Helaly, S. A., Optica Applicata, 18 (1988) 133. 12. Pluta, M., Optica Acta, 18 (1971) 661. 13. Pluta, M., J. Microsc., 96 (1972) 309. 14. Hamza, A. A., Textile Res. J., 50 (1980) 731. 15. Fox, L. & Mayers, D. F., Computing Methods for Scientists and Engineers. Oxford University Press, Oxford, 1980, p. 145.