i

2015

UMEÅ UNIVERSITET

Fysiska institutionen

Michael Bradley

2015-11-20

Course: Physics Thesis, 15hp (5FY028)

Project:

Spectroscopic Optical Band Gap Properties and Morphological

Study of Ultra-High Molecular Weight Polyethylene Nano

Composites with Mg0.15Ni0.15Zn0.70Fe2O3

Student: Asad Muhammad Azam

Email: asmu0009@student.umu.se

asadpu2003@hotmail.com

Personal no.19830509-5492

Supervisor: Dr.Malik Sajjad Mehmood

Assistant Professor (Physics)

Department of Basic Sciences, University of Engineering and

Technology, 47050, Taxila, Pakistan

Tel.: 0092 51 9047884

Email: malik.mehmood@uettaxila.edu.pk

Examiner: Dr.Ove Andersson

Professor (Physics)

Department of Physics

Umeå University, Sweden

Tel. +46-(0)90-7865034.

Email: ove.andersson@physics.umu.se

mailto:asmu0009@student.umu.semailto:asadpu2003@hotmail.commailto:malik.mehmood@uettaxila.edu.pkmailto:ove.andersson@physics.umu.se

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Spectroscopic optical band gap properties and morphological

study of ultra-high molecular weight polyethylene nano composites with Mg0.15 Ni0.15 Zn0.70Fe2O3

Asad Muhammad Azam

(19830509-5492)

Master’s Thesis 15 ECTS (5FY028)

Department of Physics, Umeå University, Sweden

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Abstract

Ultra high molecular weight polyethylene has been commonly used as a biomaterial for hip and

knee implants. This thesis concerns optical and three phase morphology of nano composites of

ultra high molecular weight polyethylene (UHMWPE) with Mg0.15Ni0.15Zn0.70Fe2O3 using

analytical techniques such as UV-visible spectroscopy and Raman spectroscopy. Muller matrix

spectro-polarimeter has been used to study the absorption behavior over the visible spectral

range i.e. 400-800 nm. The results show significant changes in the absorption behavior of

UHMWPE/Zn nano ferrite samples. To analyze these changes in pure and UHMWPE/Zn-nano

ferrite composites, the Urbach edge method was used for the calculation of optical activation

energy. Moreover, direct and indirect energy band gaps, and the number of carbon atoms in C=C

unsaturation have been evaluated by using the modified Urbach formula and Tauc’s equation,

respectively. The results show that the optical activation energy decreases more with the addition

of 2wt% Mg0.15Ni0.15Zn0.70Fe2O3 than 1wt% of Mg0.15Ni0.15Zn0.70Fe2O3. The values of direct and

indirect energy band gaps are also found to decrease more with 2wt% of Mg0.15Ni0.15Zn0.70Fe2O3.

Furthermore, indirect energy gaps are found to have lower values as compared to direct energy

gaps. The numbers of carbon atoms in clusters determined from modified Tauc’s equation were

approximately the same for 1wt% and 2 wt% of Mg0.15Ni0.15Zn0.70Fe2O3 but differ for pure

UHMWPE. The degree of crystallinity obtained from Raman spectroscopy was 42%, 43%, and

42% for UHMWPE, UHMWPE+1% of Mg0.15Ni0.15Zn0.70Fe2O3, and UHMWPE+2% of

Mg0.15Ni0.15Zn0.70Fe2O3, respectively, i.e. about the same, but the relative amount of interphase

and amorphous regions varied. Optical activation and sub band gap energies can be estimated for

utilization of this composite in applications such as optical sensors, solar cells etc.

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Table of Contents

1. Introduction……………………………………………………………………………………………………………………………………...1

1.2. History of UHMWPE as Biomaterials....................................................................................…...2

1.3. Aims and Objectives………………………………………………………………………………………….………………....3

1.4. Thesis Outlines………………………………….……………………………….……………………………………………..….4

2. Literature Review……………………………………………………………………………………………………………………………...4

2.1. Discarded properties of UHMWPE…………………………………………………………………………....…….….5

2.2. Methods to Improve properties of UHMWPE...………………………………………………………….……….5

2.3. Role of Additives or Fillers…………………………………………………………………………………………………..6

3. Experimental Details……………………………….……………………………………….………………………………...…….……….7

3.1. Introduction……………………………………………………………………………………………………………….….…....7

3.2. Material and Methods………………………………………………….………………………………………………….....7

3.2.1. Preparation of UHMWPE/Zn-nano ferrite composites..........................................................7

3.3. Characterization….....................................................................................................................8

3.3.1. Muller matrix spectro-polarimeter…………………………………………………………….…….…….…….....8

3.3.2. Raman Spectroscopy………………………………………………………………………………………………….…...12

4. Results and Discussion……………………………………………………………………………….…….……………………….………14

4.1. UV-visible Spectroscopy.................................................................................…......................14

4.2. Raman Spectroscopy………………………………….….……………………………….…………… ……………………20

5. Conclusion………………………………………………………..………………………….……………………………………………………24

6. Future Direction………………………………………………………………………………………………………………………………...25

7. References………………………………….………………………………………………………………………………….……….…….....27

1

1. Introduction

Ultra high molecular weight polyethylene (UHMWPE) is a versatile polymer which belongs to

polyethylene (PE) family having an average molecular weight between three to six million g

mol-1. UHMWPE was introduced as a biomaterial for total joint applications after the failure of

poly-tetra-fluoro-ethylene (PTFE) in 1962 by Sir John Charnley. The substitution of rigid tissues

requires biocompatible as well as bioactive materials having properties which correspond to that

of bone. Micro/nano composites formed by biocompatible polymers such as UHMWPE as well

as bioactive inorganic micro/nano-particles of high-density polyethylene/hydroxyapatite

(HDPE/HA) have engrossed awareness as stable bone replacements due to their excellent

mechanical properties [1]. Currently, natural as well as synthetic source materials like polymers,

metals and ceramics have made improvement in engineering of bone substitutions. During the

most recent years, greater interest has been found in polymeric materials concerning their

physical properties. Electrical and optical properties of fabricated polymeric materials have

achieved significant importance as compared to metals and semiconductors. Conducting

polymers can be very attractive for biomedical engineering purposes. In particular, several

tissues have good response to electric fields and have a preference to be formed from conducting

polymeric materials [2]. For industrial applications, conducting polymeric materials are

considered for research. There are a number of devices like sensors, rechargeable batteries;

capacitors as well as organic light emitting diodes (LED’s) which are based on conducting

polymers [3].

For use of UHMWPE in an application, physical and chemical processes are essential for its

modification, e.g. to manufacture light emitting diodes (LED’s), optical sensors, polymeric

2

optical fibers and antireflective coatings etc. For applications in the field of optics, knowledge

about the optical properties as well as parameters which affect these properties is required [4].

The fabrication of reinforced UHMWPE composites is a difficult task due to its extremely high

viscosity. The high viscosity of UHMWPE makes melt processing extremely difficult or almost

impossible. It is therefore for characterization of UHMWPE composites; researchers are trying to

implement the most appropriate processing techniques to make UHMWPE composites suitable

for medical applications e.g. by incorporating some filler [5]. The interest of researchers concerns

morphology, structural, thermal and optical properties of micro/nano filled UHMWPE

composites.

1.2. History of UHMWPE as a Biomaterial

UHMWPE has become the model standard for hip cups or knee inserts since its introduction as a

biomaterial in 1962 due to its mechanical, chemical and structural properties. Many scientists

have studied its potential as a biomaterial in joints arthoplasty. In 1958, a technique of low

friction arthoplasty was introduced by Charnley while using PTFE as the bearing material

implants. However, after the complete failure of PTFE, he selected chemically sterilized

UHMWPE for low friction arthoplasty (LFA) in 1962 due to the abovementioned properties of

UHMWPE. In 1968-1969 commercial production of gamma sterilized UHMWPE was

introduced because of toxic effects of chemical sterilization. In 1998, clinical introduction of

first-generation highly cross linked and thermally stabilized UHMWPE came on the scene. In

2009, Abdul-Kader studied the modification in optical properties of UHMPWE due to ion

bombardment by using photoluminescence and UV-visible spectroscopy. He focused on the

relationship between ion fluency and optical band gaps and also activation energies of irradiated

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samples. He found that energy band gaps and activation energies of irradiated UHMWPE

samples decreased with an increase in the concentration of ions. In 2011-2012, polyethylene

ether ketones (PEEK) in parallel with vitamin E diffused UHMWPE were introduced by various

companies in the orthopedic community.

In 2012, Raghuvanshi et al., studied the effect of gamma rays on optical properties of

UHMWPE. However, efforts have also been made to stabilize or modify UHMWPE by different

means e.g. post irradiation, thermal treatments (melting or annealing) and by adding a natural

antioxidant such as vitamin E. In addition to this, UHMWPE has been modified by incorporation

of micro/nano particles to improve the structural and thermal stability.

1.3. Aims and Objectives

Among the major objectives of the current study are;

1. To study optical properties and three phase morphology of pure and UHMWPE nano

composites with Mg0.15Ni0.15Zn0.70Fe2O3.

2. To calculate Urbach energy of UHMWPE/Zn-nano ferrites.

3. To calculate optical band gap energies of the samples.

4. To estimate the number of carbon atoms in C=C cluster.

Here in this thesis the absorption as well as morphological behavior of crosslinked UHMWPE

with Zn-nano ferrites have been investigated. Analytical techniques such as UV-visible

spectroscopy and Raman spectroscopy were used to study optical properties and morphology of

UHMWPE nano composites with Mg0.15Ni0.15Zn0.70Fe2O3.

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1.4. Thesis outline

The thesis has a major focus on the investigation of optical as well as structural properties of

UHMWPE/Zn-nano ferrites using Muller matrix spectro-polarimeter and Raman spectroscopy.

Thesis provides a brief literature review, detail of materials, sample preparations along with the

experimental techniques used during the study. Optical properties and morphological changes of

UHMWPE/Zn-nano ferrites are discussed in the results and discussion chapter. At the end,

conclusions along with proposed future extensions and directions are discussed as well.

2. Literature Review

UHMWPE is an amorphous matrix at nano scale made of extremely long chains of polyethylene

with crystalline lamellae. UHMWPE can be fabricated at micro/nano scale by joining together

micro/nano particles resin with polymer powder before unification. UHMWPE fibers are helpful

in fabricating composites because sheets or plies made from UHMWPE fibers can be made into

multi-dimensional structures. UHMWPE can be taken as matrix/fiber in multi choice of

composite materials by keeping in view the application. UHMWPE as a polymer and composite

materials play a vital role in armed forces, industry, and devour applications. Porous UHMWPE

materials have application in water filters while UHMWPE fibers are serving as an insulative

barrier in lead automotive batteries. UHMWPE fibers have applications in armed and personal

shield equipment like cut-resistant ornament for kitchen use and body protection [6]. These non-

medical applications of UHMWPE fiber and matrix opened a door for researchers to consider

UHMWPE composites as a candidate for biomaterials. Currently, the improvements of

UHMWPE composites by irradiation and thermal techniques may provide a low cost and

beneficial material for biomedical applications. In order to check the feasibility of using

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UHMWPE for abovementioned applications, research is on going to investigate the optical as

well as structural properties of pure and irradiated UHMWPE composites.

2.1. Inadequate Properties of UHMWPE

Scientists and researchers have put great attention on ultra high molecular weight polyethylene

(UHMWPE) because of its wide use in biomedical engineering, medicine, electrical insulation,

orthopedic, food industry and microelectronics engineering applications etc. Regrettably, special

use of UHMWPE is inadequate due to discarded properties of the surface like bad conductor,

hydrophobic and high shock resistance. Two major defects like wear as well as delamination

bounded the life of UHMWPE prosthesis and these defects are known to be caused during

chemical oxidation process of polymers [7]. Wear debris of polyethylene was considered as a

primary factor in the reduction of longevity of total lap substitutions [8]. However, due to its

higher molecular weight, strong chain and crystalline lamellae, it does not flow beyond its

melting temperature (Tm). Hence, conventional processes like screw extrusion and insertion

molding are considered impracticable to process UHMWPE.

2.2. Methods to Improve Properties of UHMWPE

In order to make UHMWPE suitable for biomedical as well as non-medical applications,

research is ongoing to improve processability of UHMWPE. Radiation method makes use of

natural as well as human made sources of high dose radiation at industrial level. The basis of

radiation is to provide high dose radiation in order to form reactive anions, cations as well as free

radicals in samples. In the case of polymers, radiation may induce grafting, polymerization,

degradation and crosslinking. Radiation method as compared to conventional chemical

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techniques is more competent because it does not require any solvent or any chemical mediator.

The effect of gamma ray irradiation on UHMWPE’s properties has been a hot subject since late

1970’s for the reason of sterilization as well as crosslinking in medical applications [9]. In these

situations, factor of oxidative degradation of UHMWPE is ignored and researchers try to keep

away from it. Gamma ray irradiation method was used to degrade UHMWPE in order to improve

volatility to an appropriate degree [10]. Therefore, demand of new techniques is a necessary task

to enhance physical as well as chemical properties of UHMWPE in a controlled manner. Several

techniques are used to change the physico/chemical properties of UHMWPE composites like

electron beam, gamma ray irradiations, plasma, ultraviolet visible-light and ion bombardment

[11]. Polymeric materials modified by irradiation with electrons, gamma rays and ions can form

new polymeric materials with greater potential in technology applications.

2.3. Role of Additives or Fillers

Nano composites with carbon nano-tubes can show much improved properties, e.g. optical

properties, thermal, mechanical behavior as well as electrical conductivities [12]. The nano

materials reported in powder form may be employed as additives in order to improve UHMWPE

for industrial as well as for biomedical applications. Surface free energy is an important factor of

biocompatible polymers which has great influence on surface properties like adhesion, wetting as

well as biocompatibility [13].

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3. Experimental Details

3.1. Introduction

In this study, absorption as well as structural behavior of UHMWPE with Mg, Zn, Ni and Fe2O3

nano-ferrites was analyzed using UV-visible spectroscopy and Raman spectroscopy. The optical

properties and morphology of these composites were assessed with the help of Muller matrix

spectro-polarimeter and Raman spectroscopy. The main focus of this study was to analyze the

structural and absorption changes at molecular and atomic levels. Moreover, the effect of these

changes on the optical characteristics was evaluated for its potential industrial applications. After

discussing the material and method used in this study, the basic principle of the experimental

techniques along with used experimental setting, equations etc. are discussed briefly in this

chapter.

3.2. Material and Methods

3.2.1. Preparation of UHMWPE/Zn-Nano Ferrite Composites

The appropriate concentrations (1.0% & 2.0 %) of nano-scale Mg0.15Ni0.15Zn0.70Fe2O3 were

dispersed in acetone (100 mL) for approximately 45 to 50 min. The blends of UHMWPE and

dispersed Zn-nano ferrite (in acetone) were prepared by shaking the admixture. The acetone was

then evaporated from the slurry by heating the mixture at 60-70 °C for overnight in an oven.

Afterward, ball milling of samples was performed for 2 hours at 200 rpm using the ball milling

facility available at Pakistan Institute of Engineering and Applied Sciences, 45650, Islamabad

Pakistan. Subsequently, the homogenized UHMWPE/ Zn-nano ferrite composites were pressed

by using a hot press (Gibitre instrument srl or Gibitre instruments laboratory press) and molded

into sheets of micron size.

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The compression molding of composites was carried out in three systematic steps given below:

Step-1: The composites in powder form were preheated at 150°C for 5 minutes. It is worth to

mention here that during this preheated period the pressure was kept as low as possible.

Step-2: The pressure and temperature were gradually raised to 200 bars and 190 °C, and on

achieving the required pressure and temperature samples were kept at 190 °C for 15 minutes.

Step-3: After combined heating and pressing treatment, samples were cooled to room

temperature (i.e. 25 °C) under the pressure of 40 MPa.

After the preparation of UHMPWE/Zn-nano ferrite composite sheets, all the samples were

labeled with various codes for their identification as far as concentration of Zn-nano ferrites was

concerned. The samples with codes are given in table3.2.1.

Table3.2.1: Samples codes used for identification within the text of this thesis.

3.3. Characterization

3.3.1. Muller matrix spectro-polarimeter

In this experiment, samples have been tested by using an automated Muller matrix spectro-

polarimeter which is controlled using AxoScan TM (manufactured by Axometrics, 2006). The

schematic figure of the spectral Muller matrix polarimeter used during experiment is illustrated

9

in figure 3.3.1. Comprehensive details of spectro-polarimeter can be found in the literature [14].

Briefly, spectro-polarimeter setup consists of

A low-noise xenon lamp of 150 W power rating as an input light source.

A built in diffraction grating monochromator for required wavelengths i.e. from 400 nm

to 800 nm.

A polarization state generator which is used to pass polarized light from source to light

up sample via fiber optic cable with required polarization states.

After passing through the sample, the output light is then detected. The output from spectro-

polarimeter manipulates transmittance data analogous to each wavelength along with other

polarization properties. This setup has more sensitivity and accuracy than usual UV-visible

setup. The following variations like sample placement, laboratory environment, built-in optics

alignment, source power etc may change results including transmittance from sample, therefore,

four to five readings were taken additionally.

.

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Figure 3.3.1: Schematic representation of Muller matrix spectro-polarimetery.

Main interest of the thesis will be in finding the optical properties of UHMWPE/Zn-nano ferrites

and morphological analysis of these composites. The optical properties include absorption

coefficient, direct and indirect band gap energies, Urbach energy and number of carbon atoms in

C=C clusters. Optical absorption coefficient a(v) was determined from absorbance A by using

the following formula [15].

a(ʋ) = A/L………………………………………………...…………………………………….(1)

Where, L is the sample thickness in μm and A is absorbance and defined as A = log (1/T) in

which T represents transmittance.

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Absorption coefficient a(ʋ) has an exponential dependence on the photon energy (hυ) near the

band edge for amorphous materials and is well described by the Urbach formula [16].

a(ʋ) = ao exp(hʋ/Eu)………………………………………………..…………………………...(2)

Where a0 is a constant, ʋ is frequency of radiation, h is Planck’s constant and Eu is the Urbach

energy known as optical activation energy.

Optical activation energy originated from thermal vibrations in crystal lattice is basically the

width of tail of localized states in forbidden energy band gap [17] [18]. The value of the optical

activation energy is obtained by taking the reciprocal of the linear portion of the slope in the

region of lower photon energy of each absorbance curve.

Urbach formula in equation (2) was initially formulated for alkali halide crystals and found to

work for many amorphous materials as well. However, a more generalized form of Urbach

formula was suggested by Mott and Davies [19] [18].

a(hʋ) = B(hʋ-Eg)n/hʋ……………………………………………...…………………………….(3)

Where hʋ is the energy of incident photons, Eg is the value of optical energy band gap between

valence and conduction bands, B is a constant which depends on transition probability within the

optical frequency range, n represents the electronic transition mode i.e. whether it is direct or

indirect under absorption process in K-space. The values of n are: 2, 3, 1/2, 3/2, for indirect

allowed and indirect forbidden, direct allowed and direct forbidden transitions, respectively.

Number of carbon atoms N in a cluster of each sample is associated with the optical energy band

gaps Eg and was evaluated by using the modified Tauc’s equation [19] [18].

N = 2βπ/Eg….…………………………………………...……………………..………………. (4)

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Where 2β is the band structure energy of a pair of adjacent π sites and for π to π* optical

transitions in –C=C–structure, β has a value of approximately 2.9 eV.

3.3.2. Raman Spectroscopy

The basis of Raman spectroscopy deals with the formation of inelastic scattered photons from

high frequency laser excitations i.e. vibrational modes of molecules. It is generally used to find

the frequencies of longitudinal and transversal modes.

Raman spectroscopic setup consists of a laser with suitable wavelength for excitation and a

detector which detects scattered light from the sample under examination. A sample is irradiated

with a strong monochromatic light source i.e. a laser. The reflection of excited laser signal from

sample is avoided by proper setting of the sample. Most of the radiation scatters from the sample

with the same wavelength as that of the original laser radiation which is known as Rayleigh

scattering. It is known that a very small amount, approximately one photon out of a million, will

scatter from the sample at a wavelength which varies from the original laser wavelength.

In crystals, the general inversion symmetry rule is used to define whether a particular photon is

infrared active or Raman active. The vibrational modes have either odd or even parity. The odd

parity has infrared active mode while even parity has Raman active mode. The first excited

vibrational level results in a Stokes-Raman shift. This Stokes-Raman shift has lower energy i.e.

longer wavelength compared to that of the laser light. Most of the Raman spectroscopic setup has

a small population initially in an excited vibrational state. When the Raman process starts from

the excited vibrational level, stay at the ground state is possible, producing scatter of higher

energy i.e. shorter wavelength compared to that of the laser light. This type of scattering is

known as anti-Stokes Raman shift.

13

Figure 3.3.2: Schematic representation of Raman spectrometer.

The Raman signal intensity has a magnitude much weaker than the elastic scattering intensity i.e.

Raman scattering signal is more than 1000 times weaker than the Rayleigh signal, hence stray

light can be regarded as a considerable issue. Spectrometers are commonly used to separate the

elastic scattering and Raman scattering signals, the major difference in scattering intensity can

help the elastically scattered light to control the Raman light with the help of stray light. Notch

filters or edge filters serve as an obstacle to remove the elastically scattered light before it enters

the spectrometer. The efficiency of Raman scattering is proportional to 1/λ4, here “λ” represents

wavelength, and therefore it is an improvement because the excitation laser wavelength becomes

shorter.

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In order to observe the Raman spectrum, the collected Raman scattered light is separated into

individual wavelengths. In dispersive Raman instruments, this task is functioned by focusing the

Raman signal on a grating, which separates Raman signal into the different wavelengths. A

detector is used to detect Raman signal after grating. The Raman active bands of particular

interest in present study are 1080 cm-1, 1460 cm-1, 1295 cm-1, and 1395 cm-1, respectively.

An Ez Ramman-M Raman Analyzer by Enwave Optronics Inc. was used to collect the Raman

spectra from 1000 to 1600 cm-1. The percentage fractions of crystalline and amorphous regions

were calculated from the intensity bands at 1416 cm-1 and 1080 cm-1. The sum of the intensities

of 1295 cm-1 and 1303 cm-1 was used as internal standard. The values of inter-phase region were

obtained as the balance after the calculation of the crystalline and amorphous contents [20] [21] [22].

4. Results and Discussion

4.1. UV-Visible Spectroscopy

The optical absorption method can be used for analysis of the optically induced transitions and is

helpful in studying optical properties such as energy band gap structure of crystalline as well as

amorphous materials [18] [19]. The absorption spectra taken from Muller matrix spectro-

polarimeter for UHMWPE (P-0) and its composites with 1wt % of Mg0.15Ni0.15Zn0.70Fe2O3 (PF1-

0) and with 2 wt% of Mg0.15Ni0.15Zn0.70Fe2O3 (PF2-0) are shown in figure 4.1.

15

Figure 4.1: UV–Visible spectra of UHMWPE (P-0) and its composites with 1wt% of

Mg0.15Ni0.15Zn0.70Fe2O3 (PF1-0) and with 2wt % of Mg0.15Ni0.15Zn0.70Fe2O3 (PF2-0).

It is clear from figure 4.1 that shifting of absorption edge proceeds towards the longer

wavelengths having a significant increase in absorption for the spectra of PF1-0 and PF2-0

samples. It can be seen that absorption edge shifting is approximately a linear trend for all

samples. Moreover, for all composites, same trend of peak shifting is noted with respect to

concentration of Mg0.15Ni0.15Zn0.70Fe2O3 (see figure 4.1).

The shifting of absorption edge towards a longer wavelength is mainly due to formation of

chains of double bonds; i.e. C=C unsaturation. This shifting of absorption edge and increased

absorption show that there is an increase in the number of conjugated bonds within the

composites. The reason behind absorption edge shifting is that the π-electrons can be excited

16

with smaller amount of energy i.e. π→π* transitions which is supposed to occur at longer

wavelengths at the C=C unsaturation position of PE.

The unusual trend for 1 wt% and 2 wt% ferrite (i.e. higher peak absorption as compared to P-0

sample) might be due to the fact that oxidation reaction plays its role in PE crosslinking with

ferrites leaving C=C unsaturation bonds [25].

In order to calculate the Urbach energy, Urbach formula equation (2) was used. Briefly,

absorption coefficients were plotted on a logarithmic scale as a function of photon energy (hυ)

and reciprocal of the slopes of linear regions that belong to lower photon energies (see the figure

4.1.2) was used to calculate the values of Urbach energies of UHMWPE (P-0) and its composites

with 1 wt% of Mg0.15Ni0.15Zn0.70Fe2O3 (PF1-0) and 2 wt% of Mg0.15Ni0.15Zn0.70Fe2O3 (PF2-0). The

values for UHMWPE (P-0) and its composites with 1 wt% of Mg0.15Ni0.15Zn0.70Fe2O3 (PF1-0) and

with 2 wt% of Mg0.15Ni0.15Zn0.70Fe2O3 (PF2-0) are presented in Table 4.1.

Figure 4.1.2: The natural logarithm of absorption coefficient plots as a function of photon energy (hν) for the

calculation of Urbach Energy

17

Table 4.1: Urbach energy, energy band gap (direct and indirect) and carbon atoms in a cluster as estimated

from methods mentioned in the experimental part.

Sample

code

Urbach

energy (meV)

Energy band gaps (eV) Carbon atoms (N) in a cluster

Direct Indirect Direct Indirect

P-0 103 2.14 1.54 8.51 11.8

PF1-0 119 2.13 1.40 8.55 13.00

PF2-0 81 2.08 1.38 8.75 13.19

It can be seen that optical activation energy decreases with the addition of 2 wt% of

Mg0.15Ni0.15Zn0.70Fe2O3 than 1 wt% of Mg0.15Ni0.15Zn0.70Fe2O3 (PF1-0). Decrease in optical

activation energy in PF2-0 sample is mainly due to increase in the concentration of ferrites and

decreased value of crystallinity. Researchers [18] [25] have attributed this decrease in Urbach

energy with decrease in crystallinity i.e. decrease in optical activation energy is due to the

decrease in the degree of crystallinity of PE. The lower value of optical excitation energy for

PF2-0 sample (as compared to P-0 and PF1-0 samples) might be due to the lower value of

oxidation index (O.I) and higher concentration of ferrites.

In order to find the values of the direct and indirect energy band gaps for UHMWPE (P-0) and its

composites with 1wt% of Mg0.15Ni0.15Zn0.70Fe2O3 (PF1-0) and with 2wt% of

Mg0.15Ni0.15Zn0.70Fe2O3 (PF2-0); (αhυ)2 and (αhυ)1/2 were plotted as a function of hυ [23] [19] in

figure 4.1.3 & 4.1.4, respectively. Individual intercepting of linear portion of absorption edge on

photon energy axis, i.e. x-axis in figure 4.1.3 & 4.1.4, values of direct and indirect energy band

18

gaps were found and are listed in table 4.1. These energy band gaps and absorption edge have an

inverse relationship i.e. increase in the absorption edge shifting will cause the band gap energy to

decrease, independent of their type.

Figure 4.1.3: Plots for direct energy band gaps (eV) in UHMWPE (P-0) and its composites with 1 wt% of

Mg0.15Ni0.15Zn0.70Fe2O3 (PF1-0) and with 2wt% of Mg0.15Ni0.15Zn0.70Fe2O3 (PF2-0).

19

Figure 4.1.4: Plots for indirect energy band gaps (eV) in UHMWPE (P-0) and its composites with 1wt %

of Mg0.15Ni0.15Zn0.70Fe2O3 (PF1-0) and with 2 wt% of Mg0.15Ni0.15Zn0.70Fe2O3 (PF2-0).

Number (N) of carbon atoms in a C=C cluster which are obtained while using the modified

Tauc’s equation are useful to describe the dependence of energy band gaps [19] and are listed in

table 4.1. There is an indirect relationship between energy band gaps and the number of carbon

atoms in the C=C cluster (i.e. the higher the number of carbon atoms in a C=C cluster, the lower

the energy band gap), and higher values of carbon atoms in cluster of 1 wt% and 2 wt% ferrites

increases the C=C unsaturation within their structure.

20

4.2. Raman Spectroscopy

In order to evaluate the incorporation effect of Mg0.15Ni0.15Zn0.70Fe2O3 on three phase

morphology of UHMWPE, Raman spectroscopy has been carried out to determine % values of

crystalline and amorphous regions in each sample since this technique has been used by others in

past [20] [22] to estimate the % age of interphase region (which is the transition zone between the

crystalline and amorphous regions) of UHMWPE.

Figure 4.2 represents Raman spectra of pure UHMWPE and its composites with 1 wt% and 2

wt% of Mg0.15Ni0.15Zn0.70Fe2O3 along with the Raman spectra of Mg0.15Ni0.15Zn0.70Fe2O3 within

the region of interest i.e. from 1000 cm-1 to 1600 cm-1. All the spectra are de-convoluted in the

region of 1000-1600 cm-1 (for details please see the figures 4.2.1-3) and the maximum intensity

values of the peak centered at or around 1416 cm-1 have been used to obtain the degree of

crystallinity at the position CH2 units in orthorhombic packing. The maximum values of intensity

at 1080 cm-1 has been used to obtain the % amorphous regions while the peak intensity values at

the band centered at or around 1295 cm-1 have used as internal standard (see Fig.4.2). The

approach of using peak intensity values is adopted because of a strong Raman active band of

polyethylene at 1440 and 1460 cm-1. The % values of crystalline, amorphous and interphase

regions were obtained using the following formulas and tabulated in table 4.2 [24] [27].

% Crystalline = area under peak at 1416 cm-1 × 100………………………….… (1)

area under peak at 1295 cm-1×0.493

% Amorphous = area under peak at 1080 cm-1 × 100……………………...…………...… (2)

area under peak at 1295 cm-1

Interphase (%) = 100- {Crystalline (%) + Amorphous (%)}………………………………(3)

21

Figure 4.2: Raman Spectra of Pure UHMWPE (P-O), UHMWPE+1wt% Mg0.15Ni0.15Zn0.70Fe2O3,

UHMWPE+2wt% Mg0.15Ni0.15Zn0.70Fe2O3 and Mg0.15Ni0.15Zn0.70Fe2O3 powder

22

Figure 4.2.1: De-convolution of Raman Spectra of Pure UHMWPE (P-O)

Wave number (cm-1)

Figure 4.2.2: De-convolution of Raman Spectra of UHMWPE/Zn-nano ferrite composites containing 1wt% of

Mg0.15Ni0.15Zn0.70Fe2O3 (PF1-O)

1000.01693943Pk=Gauss Amp 19 Peaks

r^2=0.996628 SE=10.1668 F=617.44

1021.5

1058.1

1086.7

1122.5

1158

1179.41206.1

1231.5

1256.8

1287.6

1313.31355

1409

1433.5

1483.21509.3

1532.71552.5

1455

1000 1200 1400 1600-100

0

100

200

300

400

500

-1000

100

200

300

400

500

0

100

200

300

400

500600

0

100

200

300

400

500600

1000.01693895Pk=Gauss Amp 24 Peaks

r^2=0.993931 SE=11.9727 F=636.631

1002.11024.1

1057.4

1086.9

1121.1

1158.31189.4

1219

1237.1

1256.9

1288.4

1313.61338.3

1359.9

1381.8

1411.6

1432.9

1480.11503.7

1525.5

1543.5

1568.61590.6

1452.9

1000 1200 1400 1600-100

0

100

200

300

400

500

-1000

100

200

300

400

500

0

100

200

300

400

500600

0

100

200

300

400

500600

Wave number (cm-1)

23

Wave number (cm-1)

Figure 4.2.3: De-convolution of Raman Spectra of UHMWPE/Zn-nano ferrite composites containing 2wt %

of Mg0.15Ni0.15Zn0.70Fe2O3 (PF2-O)

Table 4.2: Calculated % ages of crystalline, amorphous, and inter-phase regions from Raman spectroscopy

data

Table 4.2 shows the % age crystalline, % amorphous and % interphase regions of pure

UHMWPE, UHMWPE + 1 wt% of Mg0.15Ni0.15Zn0.70Fe2O3 and UHMWPE + 2 wt% of

1000.01693924Pk=Gauss Amp 22 Peaks

r^2=0.99546 SE=9.61175 F=951.351

1024.5

1056.7

1086.9

1120.8

1159.21199.9

1214.61261

1289.5

1338.11360

1382.4

1434.3

1500.61523.4

1545.91587.1

1412.11454.8

1313.71071.2

1000 1200 1400 1600-100

0

100

200

300

400

500

-1000

100

200

300

400

500

0

100

200

300

400

500600

0

100

200

300

400

500600

Sample Code/Name % Crystalline % Amorphous % Inter-phase

P-0 42 % 23 % 35 %

PF1-0 43 % 30 % 27 %

PF2-0 42 % 28 % 30 %

24

Mg0.15Ni0.15Zn0.70Fe2O3, respectively. It can be seen that % age values of crystalline fractions for

all samples are almost constant, while the fraction of % interphase and % amorphous regions has

strong dependence on the Mg0.15Ni0.15Zn0.70Fe2O3 concentration. It has been found that on

incorporation of the nano scale Mg0.15Ni0.15Zn0.70Fe2O3, % amorphous contents increases while a

significant decrease in all trans-interphase regions has been experimentally observed. This is

mainly attributed to the chain scission induced (close to crystalline lamella of UHMWPE) due to

incorporation of nano scale Mg0.15Ni0.15Zn0.70Fe2O3 which suggests the potential use of ferrites

for the oxidation degradation of UHMWPE without disturbing its % crystalline contents [25] [27].

This aforementioned observation is higher for 1 wt% of Mg0.15Ni0.15Zn0.70Fe2O3 as compared to 2

% of Mg0.15Ni0.15Zn0.70Fe2O3. By incorporating higher percentages of Mg0.15Ni0.15Zn0.70Fe2O3

within the UHMWPE matrix may not give clear insight.

5. Conclusion

In this study, spectroscopic and sub band optical properties of UHMWPE/Zn-nano ferrites were

examined by using Muller matrix spectro-polarimeter. The analysis was performed for pure

UHMWPE, UHMWPE+1wt% Mg0.15Ni0.15Zn0.70Fe2O3 and UHMWPE+2wt%

Mg0.15Ni0.15Zn0.70Fe2O3 samples, respectively. An increase in absorption for PF1-0 and PF2-0

samples was found as compared to pure UHMWPE. However, shifting of absorption edge

towards a longer wavelength is the same for all samples. For the assessment of optical activation

energy, energy band gaps and number of carbon atoms in a C=C cluster respectively, modified

Urbach formula and modified Tauc’s equation were used. It was found that spectroscopic and

sub optical band gaps properties show a linear relation for all samples. However, the trend is

different for UHMWPE with 2 wt% Mg0.15Ni0.15Zn0.70Fe2O3. The existing trend for this behavior

25

would be due to the reason that decrease in optical activation energy in PF2-0 sample is mainly

due to increased concentration of ferrites, decreased value of crystallinity and PE crosslinking.

Larger number of carbon atoms in C=C cluster for PF1-0 and PF2-0 samples as compared to P-O

sample clearly indicates that oxidation index for both samples have approximately same role in

UHMWPE crosslinking with ferrites. Results from Raman spectroscopy showed that % age

values of crystalline fractions for all samples are almost constant, while the fraction of %

interphase and % amorphous regions has strong dependence on the Mg0.15Ni0.15Zn0.70Fe2O3

concentration which suggests oxidation degradation of UHMWPE accompanied with use of

ferrites is an improvement without disturbing UHMWPE % crystalline contents.

6. Future direction:

On the basis of investigations presented in this study following future directions are

recommended:

Investigations of the effect of high energy radiations on the incorporation of

Mg0.15Ni0.15Zn0.70Fe2O3 nano particles with UHMWPE in order to further improve

structural and thermal properties.

Examinations of the role of natural, synthetic antioxidant and Mg0.15Ni0.15Zn0.70Fe2O3 to

improve results at lower contents.

Techniques such as scanning electron microscopy can also be used to study surface

properties of UHMWPE/Zn-nano ferrites.

26

Acknowledgment:

First and the foremost, I would like to say '' All praises to Allah'' who gave me the strength and

good health to complete this thesis.

Dr.Malik Sajjad, Supervisor: I am grateful to you who has been served as an excellent

supervisor for the duration of my M.Sc. Physics thesis. Your door has always been open to me

for which I am grateful to you. You are truly an inspiring person with your knowledge, passion,

curiosity, dedication and struggle for Science and Technology.

Dr.Jens Zamanian, Current Study Advisor: I am thankful to you for kind supports in the

achievement of one of the greatest goals of my life that is M.Sc. Physics thesis. I will always be

in debt to you for your educational and administrative help extended to me.

Abdul Rehman Ahmed: I want to dedicate my thesis work to you who always inspired me to

struggle to do more to secure and make my future better. Finally, I acknowledge the constant

help, support and encouragement of my family members especially my mother Mrs. Subedar

ch. Muhammad Khan.

27

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