1 EFFECTS OF MOLECULAR ORIENTATION AND ANNEALING ON OPTICAL ABSORBTION OF ORIENTED PET POLYMER By...

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EFFECTS OF MOLECULAR ORIENTATION AND EFFECTS OF MOLECULAR ORIENTATION AND ANNEALING ON OPTICAL ABSORBTION OF ANNEALING ON OPTICAL ABSORBTION OF

ORIENTED PET POLYMERORIENTED PET POLYMER

ByBy

Montaser DaraghmehMontaser Daraghmeh

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1.1 Definition Polymers1.1 Definition Polymers

A polymer is made of large giant molecules or chains built up by A polymer is made of large giant molecules or chains built up by repetition of small chemical units. The repeated units are called repetition of small chemical units. The repeated units are called monomers. Polymers are sometimes called "giants" or monomers. Polymers are sometimes called "giants" or "macromolecules". "macromolecules".

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1.2 Crystalline and Amorphous Polymers1.2 Crystalline and Amorphous Polymers

1. Introduction:1. Introduction:

When the polymer molecules are arranged in an ordered manner, When the polymer molecules are arranged in an ordered manner, the polymer is said to be crystalline. the polymer is said to be crystalline.

Amorphous polymers are those which show no crystalline order Amorphous polymers are those which show no crystalline order when examined by X-rays when examined by X-rays

Lemmellar structureLemmellar structure

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1.3 Isotropic and Anisotropic Polymers1.3 Isotropic and Anisotropic Polymers

The isotropic material The isotropic material has the same has the same properties in all properties in all directions. Thus, the directions. Thus, the mechanical properties mechanical properties are independent of are independent of direction. In direction. In anisotropic materials, anisotropic materials, the physical properties the physical properties change with direction.change with direction.

Isotropic structure- a two -Isotropic structure- a two -phase model (crystalline phase model (crystalline and amorphous).and amorphous).

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Axial drawing axis

(a) (b)

strain ellipsoid

X-ray photographs: X-ray photographs: a) Isotropica) Isotropicb) Anisotropicb) Anisotropic

a)a) Isotropic polymer Isotropic polymer (circle)(circle)

b)b) b) Anisotropic polymer b) Anisotropic polymer (strain ellipsoid)(strain ellipsoid)

a b


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1.5 Orientation of Polymers1.5 Orientation of Polymers When the polymer chains in the crystalline regions are aligned to When the polymer chains in the crystalline regions are aligned to some extent in a certain direction, then the polymer is said to be some extent in a certain direction, then the polymer is said to be oriented, and also it becomes anisotropic material.oriented, and also it becomes anisotropic material.

aa bb

A schematic diagram of A schematic diagram of polymers: polymers: a) Unorienteda) Unorientedb) Orientedb) Oriented

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2. Theoretical Background 2. Theoretical Background We need the theoretical background to determine the optical We need the theoretical background to determine the optical properties of polyethylene terephthalate which have the following properties of polyethylene terephthalate which have the following merits:merits:

2.1 Absorption of light2.1 Absorption of lightA brief discussion of the absorption process of light near the absorption A brief discussion of the absorption process of light near the absorption edge will be presented. edge will be presented. This process includes:This process includes:

)1 ( Interband electron absorption.)1 ( Interband electron absorption. ) 2( Urbach tails ) 2( Urbach tails

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2. Theoretical Background 2. Theoretical Background

Light Light absorption in a absorption in a solidsolid

dxIdI )()()(

Integration of above equation gives:Integration of above equation gives: xII o ))(exp()()(

X

o

eI

IT ).(

)(

)()(

The transmission of a thin layer of fine thickness "dx" can be defined as the ratio 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, between the transmitted and incident light intensities i.e,

**

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general behavior of the absorption coefficient with photon energy is represented general behavior of the absorption coefficient with photon energy is represented by the curve, which shows three main regions:by the curve, which shows three main regions:

1.1. In the low photon energy region of the curve In the low photon energy region of the curve

the empirical Urbach rule as:the empirical Urbach rule as:

2.2. Intermediate absorption regionIntermediate absorption region

3.3. At high photon energy At high photon energy

]/).[(exp 1 EEwo

General shape of the General shape of the absorption spectrum in a absorption spectrum in a semiconductor semiconductor

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2. Theoretical Background 2. Theoretical Background Interband transition can take place via two possible mechanisms Interband transition can take place via two possible mechanisms

A) Direct Transition: A) Direct Transition:

Light penetrating a sample spends its energy on the excitation of electrons from the valance band to the conduction band.

gEw

eP Kk

This occurs when

*

**

Theoretical calculations of the absorption coefficient for direct transitions give the following r

opto EwEncw ))(/4( .

Direct transitions Direct transitions of electrons.of electrons.

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A) Indirect Transition: 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.

2)]([~ phong EEww

Theoretical calculation of the absorption coefficient for indirect Theoretical calculation of the absorption coefficient for indirect transition gives :transition gives :

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3. Experimental Work3. Experimental Work

3.1 Material and Sample Preparation: 3.1 Material and Sample Preparation:

The material used in this work was oriented polyethylene terephthalate )PET( 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 sheet 0.29mm thick and has a density of about 1.3g/cm3, glass transition temperature of about ~69temperature of about ~69C and melting temperature of about 267C and melting temperature of about 267C.C.

= 0 20

IDD 40 60 75

Tensile specimen 90

IDD

IDD

IDD

IDD

IDD

IDD

Y

X

Test specimens were cut from Test specimens were cut from a sheet of draw ration at a sheet of draw ration at different angle different angle )0 )0, 20, 20, 40, 40, , 6060, 75, 75, and 90, and 90( from the ( from the initial draw direction )IDD at initial draw direction )IDD at =0=0( as shown in Fig. The ( as shown in Fig. The IDD is parallel to the IDD is parallel to the molecular orientation molecular orientation direction.direction.

Fig.: Oriented PET samples cut Fig.: Oriented PET samples cut at angle from the IDD (y-at angle from the IDD (y-axis) sheetaxis) sheet

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3.2 Annealing Process:3.2 Annealing Process:

This experimental work includes two parts: the first part deals with unannealed 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 )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 samples. For annealing test specimens were cut at different angle from the molecular direction )or IDD(, and clamped firmly between two smooth plates of 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 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(. hours, then we decreased the temperature slowly until room temperature )~25C(. This is the annealing procedure followed in research work. This is the annealing procedure followed in research work.

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Absorption is expressed in terms of a coefficient Absorption is expressed in terms of a coefficient ))(, which is defined as the (, which is defined as the relative rate of reduction in light intensity. The optical absorbance )A( is taken at relative rate of reduction in light intensity. The optical absorbance )A( is taken at wavelength )wavelength )( range )200-800nm( using Cary Photospectrometer in the ( range )200-800nm( using Cary Photospectrometer in the Chemistry Department of University of Jordan. The absorption coefficient Chemistry Department of University of Jordan. The absorption coefficient ))( ( was calculated from the absorbance )A( Spectra. After correcting the reflection, was calculated from the absorbance )A( Spectra. After correcting the reflection, ))( was calculated using the relation:( was calculated using the relation:

))(exp( xII o

)(303.2

log303.2

)( AxI

I

xw

o

HenceHence

WhereWhere IIoo and and II are the incident and transmitted intensity respectively and x is are the incident and transmitted intensity respectively and x is

the sample thickness. the sample thickness.

3.3 Optical Measurements:3.3 Optical Measurements:

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3.4 Estimation of PET Crystallinity:3.4 Estimation of PET Crystallinity:

Sample densities were measured using a very sensitive electron balance. Sample densities were measured using a very sensitive electron balance. Volume fractions of crystallinity were calculated using the following Volume fractions of crystallinity were calculated using the following relationship: relationship:

ac

acX

Where Where XXcc is the volume fraction crystallinity, is the volume fraction crystallinity, is the density of the sample, is the density of the sample,

aa is the density of 100% amorphous PET, and is the density of 100% amorphous PET, and cc is the density of 100% is the density of 100%

crystalline PET, the values for crystalline PET, the values for aa )1.335g/cm3( and )1.335g/cm3( and cc )1.455g/cm3( )1.455g/cm3(

)P.Varma and S.A. Jabarin, 1998(. )P.Varma and S.A. Jabarin, 1998(.

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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

Calculation of crystallinity volume fractionCalculation of crystallinity volume fraction

Table shows the difference in the crystallinity )Table shows the difference in the crystallinity )XXcc( for oriented PET ( for oriented PET

before and after annealing. The increase in crystallinity of the annealed before and after annealing. The increase in crystallinity of the annealed is about 6% as it was expected for annealing polymers.is about 6% as it was expected for annealing polymers.

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4. Results and Discussion4. Results and Discussion

In this working, we deal with the effect of annealing on the optical 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 properties of oriented PET samples. This work is done on two types of samples: the first is unannealed )as received( samples and the of samples: the first is unannealed )as received( samples and the second annealed samples, with different angle of orientation )second annealed samples, with different angle of orientation )( of ( of the initial draw direction )IDD( with respect to the y-axis, the optical the initial draw direction )IDD( with respect to the y-axis, the optical properties of PET are studied through determination of some physical properties of PET are studied through determination of some physical parameters, such as the optical energy gap,and the energy gap tails.parameters, such as the optical energy gap,and the energy gap tails.

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4.1 Samples Thickness Change:4.1 Samples Thickness Change: The geometric dimensions of the examined specimens before and after The geometric dimensions of the examined specimens before and after annealing were measured. Table includes the specimens geometry: annealing were measured. Table includes the specimens geometry:

The geometry of PET specimens with different angles of orientation before The geometry of PET specimens with different angles of orientation before and after annealing at 120and after annealing at 120CC

As received samples

Annealed samples Orientation

Angle (deg.) 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

The table shows that The table shows that the thickness of the thickness of specimens changes with specimens changes with annealing, this was annealing, this was taken into consideration taken into consideration during the results of during the results of optical calculations. optical calculations.

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4.1 Optical results : 4.1 Optical results : The relationship between the fundamental absorption and optical energy The relationship between the fundamental absorption and optical energy gap is given by the relation:gap is given by the relation:

c

hEopt

• At high absorption coefficient levels, where At high absorption coefficient levels, where )w(>10)w(>1044 cm cm-1-1, the , the absorption coefficient for non-crystalline materials has the following absorption coefficient for non-crystalline materials has the following frequency dependence )Tauc, 1966, Davis and Mott, 1979(.frequency dependence )Tauc, 1966, Davis and Mott, 1979(.

roptEwBww )()(

where where BB is a factor equals to )4 is a factor equals to )4oo/nc/ncE(, E(,

•In the case of lower absorption, the absorption coefficient In the case of lower absorption, the absorption coefficient )w( in range )w( in range )1cm)1cm-1-1 to 10 to 1044 cm cm-1-1(, is described by Urbach formula )Urbach, 1953((, is described by Urbach formula )Urbach, 1953(

)/exp()( Eww o

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Angles of orientation

(deg.) Eopt. (eV) B (eV-1.cm-1) E (eV) (Eopt. + E) eV

0 3.770.32 27.83 0.090.01 3.86

20 3.850.4 26.33 0.080.01 3.93

40 3.80.46 27.92 0.090.01 3.89

60 3.760.2 28.23 0.090.01 3.85

75 3.760.25 28.48 0.10.01 3.86

90 3.780.31 29.6 0.10.01 3.88

Optical result Optical result for unannealed for unannealed oriented PEToriented PET

Angles of orientation

(deg.) E opt. (eV) B (eV-1.cm-1) E (eV) (Eopt. + E) eV

0 3.770.11 22.21 0.070.01 3.84

20 3.790.21 22.20 0.070.01 3.86

40 3.790.31 22.94 0.070.01 3.86

60 3.790.41 23.12 0.080.01 3.87

75 3.770.31 23.29 0.090.01 3.86

90 3.740.13 24.52 0.090.01 3.83

Optical Optical results for results for annealed annealed oriented PEToriented PET

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E opt. (eV) B (eV-1.cm-1) E (eV) (Eopt. + E) eV

3.750.2 28.7 0.10.01 3.85

Optical results of isotropic PETOptical results of isotropic PET

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300 400 500 600 700 800

0

1

2

3

4

Ab

sorb

an

ce

wavelength)nm(

= 0 oriented PET

Optical absorbance for Optical absorbance for = 0 = 0 of oriented PET before annealing of oriented PET before annealing

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300 400 500 600 700 800

0

1

2

3

4

Ab

sorb

an

ce

wavelength)nm(

= 0 oriented PET

Optical absorbance for Optical absorbance for = 0 = 0 of oriented PET after annealing of oriented PET after annealing

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200 300 400 500 600 700 800

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Ab

sorb

an

ce

wavelength

isotropic PET

Optical absorbance for isotropic PETOptical absorbance for isotropic PET

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2 3 4

4.5

5.0

5.5

6.0

6.5

7.0

7.5

8.0

ln

h)eV(

= 60 oriented PET

Natural logarithm of ( α ) versus the incident photon energy for Ө = 60Natural logarithm of ( α ) versus the incident photon energy for Ө = 60 of of oriented PET before annealing oriented PET before annealing

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2 3 4

4.5

5.0

5.5

6.0

6.5

7.0

7.5

8.0

ln

h)eV(

= 60 oriented PET

Natural logarithm of ( α ) versus the incident photon energy for Ө = 60Natural logarithm of ( α ) versus the incident photon energy for Ө = 60 of of oriented PET after annealing oriented PET after annealing

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1 2 3 4 55

6

7

ln

h)eV(

isotropic PET

Natural logarithm of ( α ) versus the incident photon energy Natural logarithm of ( α ) versus the incident photon energy for isotropic PETfor isotropic PET

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(αћω)^1/2 versus the incident photon energy for Ө = 90(αћω)^1/2 versus the incident photon energy for Ө = 90 of oriented PET of oriented PET before annealing before annealing

1 2 3 4-2000

0

2000

4000

6000

8000

10000

)*h(

1/2

h)eV(

= 90 oriented PET

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1 2 3 4-2000

0

2000

4000

6000

8000

10000

)*h(

1/2

h)eV(

= 90 oriented PET

(αћω)^1/2 versus the incident photon energy for Ө = 90(αћω)^1/2 versus the incident photon energy for Ө = 90 of oriented PET of oriented PET after annealing after annealing

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1 2 3 4 5 6 7

0

2000

4000

6000

8000

10000

(h)

1/2

h(eV)

isotropic PET

(αћω)^1/2 versus the incident photon energy for isotropic PET(αћω)^1/2 versus the incident photon energy for isotropic PET

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Molecular direction

Y

X

UV-propagation direction

Orientation ellipsoid

Molecular direction with respect to UV-propagation directionMolecular direction with respect to UV-propagation direction . .

where where is molecular angle is molecular angle rotation with rotation with respect to the vertical respect to the vertical direction (y-axis) direction (y-axis)

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5. Conclusions5. Conclusions

5.1 Conclusions5.1 Conclusions

From the analysis of the results obtained, one can conclude the From the analysis of the results obtained, one can conclude the followings:followings:1.1. The optical behavior of the given oriented PET polymer depend on angle The optical behavior of the given oriented PET polymer depend on angle

measured from the IDD.measured from the IDD.

2.2. No great changes, before and after annealing, appeared in the observed No great changes, before and after annealing, appeared in the observed optical properties Eoptical properties Eg g and and E approximately.E approximately.

3.3. The PET samples showed small increase in their thickness through to The PET samples showed small increase in their thickness through to annealing at 120annealing at 120C, which means that a degree of recrystalization took place in C, which means that a degree of recrystalization took place in received samples by annealing.received samples by annealing.

4.4. The analysis of optical results indicates that the transition energy for electrons The analysis of optical results indicates that the transition energy for electrons is indirect in k-space.is indirect in k-space.

5.5. The crystallinity was calculated from density measurements.The crystallinity was calculated from density measurements.

6.6. The observed slight changes in the measured optical properties of oriented The observed slight changes in the measured optical properties of oriented PET are attributed to some structural changes as crystallinity took place by PET are attributed to some structural changes as crystallinity took place by annealing. annealing.

1 EFFECTS OF MOLECULAR ORIENTATION AND ANNEALING ON OPTICAL ABSORBTION OF ORIENTED PET POLYMER By...

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