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Transcript
EFFECTS OF MOLECULAR ORIENTATION AND
ANNEALING ON OPTICAL ABSORBTION OF
ORIENTED PET POLYMER
By
Montaser Daraghmeh
1
1.1 Definition Polymers
A polymer is made of large giant molecules or chains built up by
repetition of small chemical units. The repeated units are called
monomers. Polymers are sometimes called "giants" or
"macromolecules".
2
1. Introduction:
1.2 Crystalline and Amorphous Polymers
When the polymer molecules are arranged in an ordered manner, the
polymer is said to be crystalline.
Amorphous polymers are those which show no crystalline order when
examined by X-rays
Lemmellar structure
3
1. Introduction:
1.3 Isotropic and Anisotropic Polymers
The isotropic material
has the same
properties in all
directions. Thus, the
mechanical properties
are independent of
direction. In
anisotropic materials,
the physical properties
change with direction.
Isotropic structure- a two phase model (crystalline
and amorphous).
4
1. Introduction:
X-ray photographs:
a) Isotropic
b) Anisotropic
a
b
Axial drawing
axis
strain
ellipsoid
a) Isotropic polymer
(circle)
b) b) Anisotropic polymer
(strain ellipsoid)
(a)
(b)
5
1. Introduction:
1.5 Orientation of Polymers
When the polymer chains in the crystalline regions are aligned to
some extent in a certain direction, then the polymer is said to be
oriented, and also it becomes anisotropic material.
A schematic diagram of
polymers:
a) Unoriented
b) Oriented
a
b
6
2. Theoretical Background
We need the theoretical background to determine the optical
properties of polyethylene terephthalate which have the following
merits:
2.1 Absorption of light
A brief discussion of the absorption process of light near the absorption
edge will be presented.
This process includes:
(1 ) Interband electron absorption.
( 2) Urbach tails
7
2. Theoretical Background
Light
absorption in a
solid
*
dI ( )   ( ) I ( )dx
Integration of above equation gives:
I ( )  I o ( ) exp(  ( )) x
The transmission of a thin layer of fine thickness "dx" can be defined as the ratio
between the transmitted and incident light intensities i.e,
I ( )
T ( ) 
 e  (  ). X
I ( )
o
8
2. Theoretical Background
general behavior of the absorption coefficient with photon energy is represented
by the curve, which shows three main regions:
1. In the low photon energy region of the curve
the empirical Urbach rule as:
   exp .[(w  E1 ) / E ]
o
2. Intermediate absorption region
3. At high photon energy
General shape of the
absorption spectrum in a
semiconductor
9
2. Theoretical Background
Interband transition can take place via two possible mechanisms
A) Direct Transition:
Light penetrating a sample spends its energy on the excitation of
electrons from the valance band to the conduction band.
This occurs when
*
w  E g
**
k P  K e
Direct transitions
of electrons.
Theoretical calculations of the absorption coefficient for direct transitions
give the following
r
w  (4o / ncE)(w  Eopt. ) 10
2. Theoretical Background
A) Indirect Transition:
If the bottom of the conduction band Ec occupies the position whose wave vector
value of K differs from that for the top of the valence band Ev, then a vertical electronic
transition involving only a photon can't directly connect the two electronic states, a
photon does not possess enough momentum to ensure conservation of momentum for
such transition.
Theoretical calculation of the absorption coefficient for indirect
transition gives :
w ~ [w  ( Eg  E phon )]2
11
3. Experimental Work
3.1 Material and Sample Preparation:
Fig.: Oriented PET samples cut
at angle
from the IDD (yaxis) sheet
IDD
IDD
IDD
20
40
60
Y

IDD
 = 0
IDD
Test specimens were cut from
a sheet of draw ration at
different angle  (0, 20, 40,
60, 75, and 90) from the
initial draw direction (IDD at
=0) as shown in Fig.
The IDD is parallel to the
molecular orientation
direction.
IDD
The material used in this work was oriented polyethylene terephthalate (PET)
sheet 0.29mm thick and has a density of about 1.3g/cm3, glass transition
temperature of about ~69C and melting temperature of about 267C.
IDD
75
Tensile
specimen
90
X
12
3. Experimental Work
3.2 Annealing Process:
This experimental work includes two parts: the first part deals with unannealed
(as received) samples of oriented PET, and the second part deals with annealed
samples. For annealing test specimens were cut at different angle  from the
molecular direction (or IDD), and clamped firmly between two smooth plates of
steel and after then we are put them in an oven at a temperature of 120C, for 20
hours, then we decreased the temperature slowly until room temperature (~25C).
This is the annealing procedure followed in research work.
13
3. Experimental Work
3.3 Optical Measurements:
Absorption is expressed in terms of a coefficient (), which is defined as the
relative rate of reduction in light intensity. The optical absorbance (A) is taken at
wavelength () range (200-800nm) using Cary Photospectrometer in the
Chemistry Department of University of Jordan. The absorption coefficient ()
was calculated from the absorbance (A) Spectra. After correcting the reflection,
() was calculated using the relation:
I  I o exp(  ( ) x)
Hence
 ( w) 
2.303
I
2.303
log

A( )
x
Io
x
Where Io and I are the incident and transmitted intensity respectively and x is
the sample thickness.
14
3. Experimental Work
3.4 Estimation of PET Crystallinity:
Sample densities were measured using a very sensitive electron balance.
Volume fractions of crystallinity were calculated using the following
relationship:
Xc 
  a
c  a
Where Xc is the volume fraction crystallinity,  is the density of the sample,
a is the density of 100% amorphous PET, and c is the density of 100%
crystalline PET, the values for a (1.335g/cm3) and c (1.455g/cm3)
(P.Varma and S.A. Jabarin, 1998).
15
Calculation of crystallinity volume fraction
Comparison
of A parameters
Before
Annealing
After
Annealing
Thickness (cm)
0.029
0.031
Length (cm)
5.2
3.4
Width (cm)
1.6
0.08
Volume (cm3)
0.241
0.084
Mass (gm)
0.334
0.117
Density of sample
(gm/cm3)
1.386
1.393
Crystallinity (%)
42.5 10
48.4 10
Table shows the difference in the crystallinity (Xc) for oriented PET
before and after annealing. The increase in crystallinity of the annealed
16
is about 6% as it was expected for annealing polymers.
4. Results and Discussion
In this working, we deal with the effect of annealing on the optical
properties of oriented PET samples. This work is done on two types of
samples: the first is unannealed (as received) samples and the
second annealed samples, with different angle of orientation () of
the initial draw direction (IDD) with respect to the y-axis, the optical
properties of PET are studied through determination of some physical
parameters, such as the optical energy gap,and the energy gap tails.
17
4. Results and Discussion
4.1 Samples Thickness Change:
The geometric dimensions of the examined specimens before and after
annealing were measured. Table includes the specimens geometry:
The geometry of PET specimens with different angles of orientation before
and after annealing at 120C
As received
samples
Annealed
samples
Thickness (mm)
Thickness (mm)
0
0.29
0.31
20
0.29
0.31
40
0.29
0.31
60
0.29
0.31
75
0.29
0.31
90
0.29
0.31
Orientation
Angle  (deg.)
The table shows that
the thickness of
specimens changes
with annealing, this was
taken into consideration
during the results of
optical calculations.
18
4. Results and Discussion
4.1 Optical results :
The relationship between the fundamental absorption and optical energy
gap is given by the relation:
Eopt  h
•
c

At high absorption coefficient levels, where (w)>104 cm-1, the
absorption coefficient for non-crystalline materials has the following
frequency dependence (Tauc, 1966, Davis and Mott, 1979).
 (w)w  B(w  Eopt )r
where B is a factor equals to (4o/ncE),
In the case of lower absorption, the absorption coefficient (w) in range
(1cm-1 to 104 cm-1), is described by Urbach formula (Urbach, 1953)
•
 (w)   o exp( w / E )
19
Optical result
for unannealed
oriented PET
Optical
results for
annealed
oriented PET
Angles of
orientation
 (deg.)
Eopt. (eV)
B (eV-1.cm-1)
E (eV)
(Eopt. + E) eV
0
3.770.32
27.83
0.090.01
3.86
20
3.850.4
26.33
0.080.01
3.93
40
3.80.46
27.92
0.090.01
3.89
60
3.760.2
28.23
0.090.01
3.85
75
3.760.25
28.48
0.10.01
3.86
90
3.780.31
29.6
0.10.01
3.88
Angles of
orientation
 (deg.)
E opt. (eV)
B (eV-1.cm-1)
E (eV)
(Eopt. + E) eV
0
3.770.11
22.21
0.070.01
3.84
20
3.790.21
22.20
0.070.01
3.86
40
3.790.31
22.94
0.070.01
3.86
60
3.790.41
23.12
0.080.01
3.87
75
3.770.31
23.29
0.090.01
3.86
90
3.740.13
24.52
0.090.01
3.83
20
Optical results of isotropic PET
E opt. (eV)
B (eV-1.cm-1)
E (eV)
(Eopt. + E) eV
3.750.2
28.7
0.10.01
3.85
21
4
 = 0 oriented PET
Absorbance
3
2
1
0
300
400
500
600
700
800
wavelength(nm)
Optical absorbance for  = 0 of oriented PET before annealing
22
4
Absorbance
3
 = 0 oriented PET
2
1
0
300
400
500
600
700
800
wavelength(nm)
Optical absorbance for  = 0 of oriented PET after annealing
23
3.0
isotropic PET
2.5
Absorbance
2.0
1.5
1.0
0.5
0.0
200
300
400
500
600
700
800
wavelength
Optical absorbance for isotropic PET
24
8.0
 = 60 oriented PET
7.5
7.0
ln
6.5
6.0
5.5
5.0
4.5
2
3
4
h(eV)
Natural logarithm of ( α ) versus the incident photon energy for Ө = 60 of
25
oriented PET before annealing
 = 60 oriented PET
8.0
7.5
7.0
ln
6.5
6.0
5.5
5.0
4.5
2
3
4
h(eV)
Natural logarithm of ( α ) versus the incident photon energy for Ө = 60
26
of oriented PET after annealing
7
ln
isotropic PET
6
5
1
2
3
4
5
h(eV)
Natural logarithm of ( α ) versus the incident photon energy
for isotropic PET
27
10000
 = 90 oriented PET
8000
(*h)
1/2
6000
4000
2000
0
-2000
1
2
3
4
h(eV)
(αћω)^1/2 versus the incident photon energy for Ө = 90 of oriented PET
before annealing
28
10000
 = 90 oriented PET
8000
(*h)
1/2
6000
4000
2000
0
-2000
1
2
3
4
h(eV)
(αћω)^1/2 versus the incident photon energy for Ө = 90 of oriented PET
after annealing
29
10000
isotropic PET
8000
(h)
1/2
6000
4000
2000
0
1
2
3
4
5
6
7
h(eV)
(αћω)^1/2 versus the incident photon energy for isotropic PET
30
Y

Molecular direction
Orientation
ellipsoid
UV-propagation direction
X
where  is molecular angle
rotation with
respect to the vertical
direction (y-axis)
Molecular direction with respect to UV-propagation direction.
31
5. Conclusions
5.1 Conclusions
From the analysis of the results obtained, one can conclude the followings:
1. The optical behavior of the given oriented PET polymer depend on angle 
measured from the IDD.
2. No great changes, before and after annealing, appeared in the observed
optical properties Eg and E approximately.
3. The PET samples showed small increase in their thickness through to
annealing at 120C, which means that a degree of recrystalization took place
in received samples by annealing.
4. The analysis of optical results indicates that the transition energy for electrons
is indirect in k-space.
5. The crystallinity was calculated from density measurements.
6. The observed slight changes in the measured optical properties of oriented
PET are attributed to some structural changes as crystallinity took place by
annealing.
32
33