Optics and Photonics Journal, 2017, 7, 109-121
http://www.scirp.org/journal/opj
ISSN Online: 2160-889X
ISSN Print: 2160-8881
Some Structural Properties of Dynamically
Drawn iPP Fibers
Afaf M. Ali1,2
1
Physics Department, Faculty of Applied Science, Umm AL-Qura University, Mecca, KSA
Physics Department, Faculty of Science, Mansoura University, Mansoura, Egypt
2
How to cite this paper: Ali, A.M. (2017)
Some Structural Properties of Dynamically
Drawn iPP Fibers. Optics and Photonics
Journal, 7, 109-121.
https://doi.org/10.4236/opj.2017.76011
Received: May 2, 2017
Accepted: June 12, 2017
Published: June 15, 2017
Copyright © 2017 by author and
Scientific Research Publishing Inc.
This work is licensed under the Creative
Commons Attribution International
License (CC BY 4.0).
http://creativecommons.org/licenses/by/4.0/
Open Access
Abstract
Changes in the different structural parameters of iPP fibers during the dynamically cold drawing process were characterized. Using the dynamic mechanical cold drawing device attached to Fizeau interference system all the
optical and structural properties can be measured. With the aid of this device
the effect of the strain rate on the different structure properties was measured.
The molecular orientations, molecular polarizability, molar reflectivity and
shrinkage stress were measured. Reorientation of the molecules led to a significant variations in the measured structure properties of the drawn iPP fibers during applying the external tension.
Keywords
Drawing, Structural, Orientations and Refractive Indices
1. Introduction
Isotactic polypropylene (iPP) can be used in different applications as automotive
industry, furniture, toys and etc. In the advanced fields of technology and
science, polymers with enhanced mechanical, optical, thermal, and environmental properties are needed [1]. Isotactic polypropylene polymer is a semi
crystalline polymer [2]. Due to the balance in their properties and cost-effectiveness, iPP has averted overall commercial application [3].
The textile characteristics can be improved by drawing process. During the
drawing process an orientation of the chain occur. The degree of axial orientation is often characterized by the birefringence of the fiber. The orientations
formed during the cold drawing process depend on the strain rate and drawing
conditions [4] [5]. Increasing the transverse orientation of the molecules considered as the initial effect of the stretching which resulted from the alignment of
the fibrils [6] [7]. The changes in different properties of fibers as optical, thermal
DOI: 10.4236/opj.2017.76011
June 15, 2017
A. M. Ali
and mechanical can be investigated using the different micro interferometric
techniques.
Tensile test is used to enhance the molecular orientation [8] [9] [10]. The
most common effects of the drawing process on the structural properties are the
phase transitions, crystallization, destruction of crystals, transformation of crystal and drawing conditions [11].
Refractive index can be used as an indicator for the optical, structural and
electrical properties of fibers. The birefringence is another key from the optical
parameter of fiber which can be used to assess the amount of anisotropy and the
amount of orientations [12]. Interferometric techniques are highly accurate
techniques for measuring the optical properties of fibers [13]. Online Video Opto Mechanical device (VOM) [14] was designed to determine the mechanical,
optical and structural properties of fibers at different strain rates. Sokkar et al.,
measured the different optical properties of iPP fibers during the dynamic cold
drawing process [15].
The major objective of this work is to throw light on the effect of mechanical
cold drawing and strain rate on different structural parameters of isotactic polypropylene fibers. The structural properties of iPP fiber were described by measuring the number of chains per unit volume, molecular polarizability, dielectric
constant, dielectric susceptibility, molar reflectivity and Mechanical orientations.
2. Theoretical Consideration
To throw light on the effect of strain rate on the different structural properties of
iPP fibers a mechanical device attached with multiple beam interference technique in transmission can be used [14].
2.1. Determination of the Number of Chains per Unit Volume
The number of chains per unit volume Nc affect mainly on the number of crystallites. It can be calculated using the following equation [16]:
=
F (θ )
2
N c DR 2 − DR −1
5
(1)
where DR is the draw ratio. F (θ ) is the orientation function and can be
measured with the aid of the following equation:
F (θ ) =∆n ∆nmax
(2)
∆n is the current double refraction, and ∆nmax is the maximum double refrac-
tion and is given by 0.045 [17]. The birefringence ∆n can be measured using
the calculated refractive indices using the following equation [13].
∆n = n − n ⊥
n ,n
⊥
(3)
are the refractive indices of the fiber. The values of them can be investi-
gated using the following equation [13].
n= nl +
110
Fλ
2bA
(4)
A. M. Ali
where F is the enclosed area under the fringe shift, b is the interfringe spacing, A
is the transverse section area of the fiber. nL is the refractive index of the immersion liquid. λ is the wavelength of monochromatic light used.
2.2. Determination of the Molecular Polarizability of Polymeric
Material
The molecules polarization, can be formed by one of the following methods. By
applying a field that make reoriention of the charge distributions which lead to
the production of induced dipole moment. By applying a field that make orientation up to the initially randomly permanent dipole moments of the molecules.
The molecular Polarizability is given by the following equation [18].
Pm= Pi + Pd2 3K BT
(5)
where Pi is the induced polarizability, Pd is the permanent dipole moment value
and T is the absolute temperature. The molecular polarizability Pm can be measured using the following equation:
pm =
3 n′ 2 − 1
4π n′2 + 2
(6)
where n' mean refractive index. n' can be measured from the following equation
(n
=
n′
+ 2n⊥ ) 3
(7)
2.3. Determination of Dielectric Constant and Dielectric
Susceptibility
The following equation can be used to measure the value of the radically dielectric constant [19].
DE|| =
(
1 − (n
) ( n + 2)
− 1) ( n + 2 )
1 + 2 n||2 − 1
2
||
2
||
2
||
(8)
An analog equation can be used to determine the value of DE⊥. The dielectric
susceptibility (η) can be measured using the obtained values of dielectric constant with the aid of the following equation;
η=
DE − 1
4π
(9)
2.4. Determination of the Molar Reflectivity
For a mole of a substance if the total polarizability values were measured, it's
easy to measure the molar refractivity. The factors affecting the molar refractivity are the temperature, the pressure, and the refractive indices. The refractive indices values with the aid of Lorentz-Lorenz relation the molar refractivity can be
measured [20].
n′ 2 − 1 M
=R
n′ 2 + 2 d
(10)
d is the density and M is molecular weight of the monomer units (42.08 mole
111
A. M. Ali
weight). Using the following equation to measure the density of iPP fibers:
d=
n′ − 0.9374
0.6273
(11)
2.6. Determination of the Mechanical Orientations
P2 ( cos θ ) and P4 ( cos θ ) provide
some understanding of the mechanism of deformation. P2 ( cos θ ) can be
measured using the following equations [16].
The molecular orientation functions
P2 (=
cos (θ ) )
1 2 + U 2 3U cos −1 U
−
32
2 1−U 2
1−U 2
(
(12)
)
where U = DR −3 2 .
P4 ( cos θ ) value can be measured the following equation [21]:
1 35
cos (θ ) )
P4 (=
8 1−U 2
(
)
2
U 2 3U cos −1 U
30
−
−
1 +
1
2
−
1
U2
−U 2 2
2
1
(
)
U cos −1 U
+ 3 (13)
1 −
1
2
1 − U 2
(
)
3. Experimental Technique
An automated cold drawing device (VOM) [22] connected to multiple beam interferometric technique in transmission is used due to its high accuracy in the
measurements of the optical properties of sample under study. The (VOM) setup used in this work consists of three units.
1) First unit (Interferometric unit); this unit is a multiple-beam Fizeau fringes
in transmission technique.
2) Second unit (Mechanical unit): it used in drawing the fiber under study and
controlling the strain rate. The accuracy in measuring the strain rate is
±0.0149 cm/s [14].
3) Third unit (Computerized unit): this unit used to record the obtained video
of the on line drawing process.
4. Experimental Results and Discussions
Fixing of iPP fibers with the gear boxes. A drop of liquid with refractive index nL
= 1.5001 at T = 30˚C close to the parallel refractive index of iPP fiber was used.
Light of monochromatic wavelength 546.1 nm was used. The strain rate was
controlled by controlling the speed of the stretching device. The stepper motor
velocity was adjusted using the software program installed in the computer system. The CCD camera was adjusted to record the video output images from the
microscope field. The CCD camera adjusted to record 25 frames/s during the
drawing process. The obtained video film was cut into separate images to deal
with each frame separately. The draw ratio calculated using the VOM calibration curve [14]. The images of the cut frames were enhanced and the noises
were removed by using Fourier transform method. The obtained contour lines
were analyzed for the determination of fiber refractive index. To change the
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A. M. Ali
speed of drawing or the strain rate value, the frequency of the stepper motor was
changed. The draw ratios values were selected in the range from 2 to 6. Figure 1
gives some examples of the enhanced contour line from the obtained microinterferograms for iPP at different strain rate at constant DR = 4. The direction of
the vibrating light is parallel to the fiber axis. It is clear from this figure that the
enclosed area under the fringe shift change during the drawing process. Which
led to the change in the refractive indices with the draw ratio.
In case of perpendicular direction, a filament of the fiber was immersed in a
liquid with refractive index nL = 1.492 at temperature T = 30˚C. The same steps
described above were repeated carefully. Figure 2 gives some of the obtained
contour line from the microinterferograms at different strain rate values. Figure
3(a), Figure 3(b) shows the variation of the refractive indices with the draw ratio using different strain rate, a) for parallel refractive index and b) for perpendicular refractive index. It is clear from the obtained data, the refractive index
for light vibrating parallel to the fiber axis increases by increasing the strain rate
and the draw ratio which mean that the chain of fiber become more oriented in
this direction and a large improvement in the axial packing. The perpendicular
refractive index decreases by increasing the draw ratio but its values increase
with increasing the strain rate. The perpendicular refractive index values decrease by increasing the draw ratio due to a slight decrease in the radial direction. The accuracy in the refractive index measurements is ±0.0007 [23].
Figure 1. Gives some examples of the enhanced contour line from the obtained microinterferograms for iPP at different strain rate at constant DR = 4.
113
A. M. Ali
Figure 2. Gives some of the obtained contour line from the microinterferograms at different strain rate values.
(a)
(b)
Figure 3. Shows the variation of the refractive indices with the draw ratio using different
strain rate, (a) for parallel refractive index and (b) for perpendicular refractive index.
114
A. M. Ali
Figure 4 shows the variation of the birefringence of iPP fibers with the draw
ratio using different strain rates. It is clear that the birefringence increases with
increasing the draw ratio and the strain rate. iPP fiber is a semi crystalline polymer so its birefringence is considered as an indicator of the amorphous and
crystalline regions. The recorded increase in the birefringence values means the
constituting molecules were aligned in parallel direction more than the perpendicular direction as a result of the on line cold drawing process [23] [24]. The
alignment of molecules increases with increasing the strain rate.
The effect of the draw ratio and the strain rate on the different structural
properties were investigated through the calculation of the number of chains per
unit volume. Figure 5 represents the variation of the number of chains per unit
volume with the draw ratio at different strain rates. From the obtained data, it is
clear that as the draw ratio increases the number of chains per unit volume decreases for the same strain rate. The elastic behavior of a molecular network
during the drawing process is predicted to depend only on the number of molecular chains. The number of chains per unit volume decreases as the draw ratio
increases that due to the crosslink density depending mainly on the draw ratio.
In this case the crystallites work as a physical crosslink point.
The molecular polarizability of iPP fibers was measured in terms of the obtained values of the refractive indices using Equations (6), Equations (7). Figure
6 shows the variation of the molecular polarizability with the draw ratio at different strain rate. From calculated data, it is clear that the molecular polarizability increases by increasing the draw ratio and the strain rate.
The variation in the obtained optical parameters measured before may be
considered as a result of adjustment in the electrical properties. Dielectric constant and dielectric susceptibility were measured using Equations (8), (9). Figure
7(a), Figure 7(b) represents the variation of the dielectric constant with the
Figure 4. Shows the variation of the birefringence of iPP fibers with the draw ratio using
different strain rates.
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A. M. Ali
Figure 5. Represents the variation of the number of chains per unit volume with the draw
ratio at different strain rates.
Figure 6. Shows the variation of the molecular polarizability with the draw ratio at different strain rate.
draw ratio using different strain rate, a) for parallel direction and b) for perpendicular direction. It is clear that the parallel dielectric constant increase with increasing the draw ratio and the strain rate. While the perpendicular (decrease
with increasing the strain rate and the draw ratio. Figure 8(a), Figure 8(b) gives
the variation of dielectric susceptibility with the draw ratio at different strain
rates where, a) for parallel direction and b) for perpendicular direction. From
the calculated values for dielectric susceptibility, it is clear that it follow the same
behavior as the dielectric constants. Space changes and the residual electric field
in the polymers after drawing at different strain rate may be considered as the
most common factors affecting the variation in the values of the dielectric con116
A. M. Ali
(a)
(b)
Figure 7. (a) (b) represents the variation of the dielectric constant with the draw ratio
using different strain rate, a) for parallel direction and b) for perpendicular direction.
stants and the dielectric susceptibility [25].
Molar reflectivity can be considered as an indicator of the total polarizability
of a mole of a substance. The molar reflectivity measured using equations (10,
11). Figure 9 gives the variation of the molar reflectivity with the draw ratio at
different strain rate. It is clear the molar reflectivity that the molar reflectivity
increase by increasing the draw ratio and the strain rate.
The mechanical orientation factors P2 ( cos (θ ) ) and
P4 ( cos (θ ) ) are
only mechanically dependent as shown in Equations (12, 13). Figure 10 shows
the variation of the calculated P2 ( cos (θ ) ) with the draw ratio and Figure 11
gives the variation of P4 ( cos (θ ) ) with the draw ratio. It is clear that these
117
A. M. Ali
(a)
(b)
Figure 8. (a) (b) gives the variation of dielectric susceptibility with the draw ratio at different strain rates where, a) for parallel direction and b) for perpendicular direction.
mechanical orientation factors increase by increasing draw ratio and the values
of P4 ( cos (θ ) ) are always comparatively small. It is clear that these mechanical orientation function depends mainly on the draw ratio. So most of the opto-mechanical device can be used to investigate the variation on molecular
orientations.
5. Conclusions
Isotactic polypropylene iPP fiber is of the common semi-crystalline polymer.
The results of this study prove that dynamic cold drawing process has a significant effect on the optical and structural properties of iPP fibers. The draw ratio
118
A. M. Ali
Figure 9. Gives the variation of the molar reflectivity with the draw ratio at different
strain rate.
Figure 10. Shows the variation of the calculated <P2(cos(θ))> with the draw ratio.
effect on the structural properties of iPP fibers is greater than the effect of strain
rate on that physical properties. From the calculated data the following conclusions may be drawn:
1- The number of chains per unit volume decrease by increasing the draw ratio but increase with increasing the strain rate.
2- The molecular polarizability decrease by increasing the draw ratio and
strain rate.
3- Both of the dielectric constants and dielectric susceptibility have the same
119
A. M. Ali
Figure 11. Gives the variation of <P4(cos(θ))> with the draw ratio.
behaviors.
4- Increasing the molar reflectivity by increasing the draw ratio and the strain
rate.
Acknowledgements
The authors would like to thank the Deanship of Scientific Research at Umm
Al-Qura University for the continuous support. This work was supported financially by the Deanship of Scientific Research at Umm Al-Qura University to DR
Afaf M Ali. (Grant Code: 15-SCI-3-3-0011).
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