Temperature Dependence of Photoluminescence Spectra In

TEMPERATURE DEPENDENCE OF PHOTOLUMINESCENCE SPECTRA IN

POLYSTYRENE

A Thesis

Presented to

The Graduate Faculty of The University of Akron

In Partial Fulﬁllment

of the Requirements for the Degree

Master of Science

Kelvin Xorla Tsagli

June, 2021 TEMPERATURE DEPENDENCE OF PHOTOLUMINESCENCE SPECTRA IN

POLYSTYRENE

Kelvin Xorla Tsagli

Thesis

Approved: Accepted:

Advisor Dean of the College Dr. Sasa V. Dordevic Dr Mitchell McKinney

Faculty Reader Dean of the Graduate School Dr. Robert R. Mallik Dr Marnie Saunders

Faculty Reader Date Dr. Ben Yu-Kuang Hu

Department Chair Dr, Christopher Ziegler

ii ABSTRACT

A previous study of low temperature photoluminescence (PL) in several common polymers revealed that photoluminescence intensity was, to a diﬀerent degree, temperature dependent in all of them. Even though polystyrene showed only moderate temperature dependence, it was the only studied polymer in which the wavelength, and therefore the energy of the photoluminescence peak changed with temperature.

In this work I explored that eﬀect in more details. A careful photoluminescence study was done of four polystyrene samples from diﬀerent manufacturers. Samples were labeled PS1 PS2, PS3, and PS4, with measurements done at 77 K, 100 K, 200 K and

292 K.

All samples showed that ﬂuorescence quantum yield increased at cryogenic temperatures, causing the peak intensity to increase at temperatures below room temperature. However, only PS4 sample showed a peak shift, i.e. the energy of the peak changed with temperature. In PS1, PS2 and PS3 samples the peak position did not change with temperature. Therefore, I concluded that the eﬀect was not an intrinsic property of polystyrene. Moreover, the data analysis revealed that the shift of the peak in PS4 was caused by a second photoluminescence peak, which displays a very strong temperature dependence. This second peak was not observed in PS1,

iii PS2 and P3 and therefore is not intrinsic to polystyrene. I speculate that it is due to impurities.

iv ACKNOWLEDGEMENTS

My most gratitude goes to God Almighty, whom I believe is the author and originator of all understanding. I sincerely appreciate the eﬀort of my supervisor Dr. Sasa

V. Dordevic for his time, advice and encouragement all through my study at The

University of Akron. It has been a long time coming but ﬁnally, the moment is here .

I want to express my sincere gratitude and honor to my lovely and distin- guished parents, Torgbi and Mrs Tsagli II for their immeasurable care, love and support. Thank you for being my pillar of strength.

To my most precious, gifted, loving and wonderful siblings, I say thank you for always believing in me and more importantly, standing by me with all that you have.

I would like to thank Dr. Jutta Luettmer-Strathmann for her wealth of knowledge and understanding spirit. For your sake, my transition from Ghana has been a memorable one.

In particular, special thanks to Ebenezer Awadzie and Eric Osei Boateng.

You guys are amazing. Your support is immeasurable . I dedicate this work to you.

v TABLE OF CONTENTS

Page

LIST OF FIGURES ...... viii

CHAPTER

I. INTRODUCTION ...... 1

1.1 Overview ...... 1

1.2 Thesis Outline ...... 4

II. PHOTOLUMINESCENCE ...... 5

2.1 Photoluminescence Spectroscopy ...... 7

III. POLYSTYRENE ...... 16

3.1 Structure and Polymerization of Polystyrene ...... 18

3.2 Properties of Polystyrene ...... 21

IV. EXPERIMENTAL METHOD ...... 24

4.1 Cary Eclipse Fluorescent Spectrophotometer ...... 24

4.2 The Software ...... 28

4.3 Low Temperature Measurements and Experimental Setup ...... 30

4.4 Experimental Procedure ...... 33

4.5 Samples Used In The Thesis ...... 36

V. RESULTS AND EXPERIMENTAL DATA ...... 38

vi 5.1 Previous Photoluminescence Spectra of Polystyrene ...... 40

5.2 2-D Contour Plots of Photoluminescence at Diﬀerent Temperatures . 44

5.3 Temperature Dependence of Photoluminiscence Spectra ...... 49

5.4 Mathematical Functions Used For Fitting ...... 52

5.5 Fitted Spectra ...... 53

5.6 Temperature Dependence of Fitting Parameters ...... 60

VI. CONCLUSIONS ...... 70

vii LIST OF FIGURES

Figure Page

2.1 Schematic Diagram Of A Photoluminescence Phenomenon[9] ...... 6

2.2 A diagram showing ways in which molecules can deexcite from the excited state[9] ...... 7 2.3 Energy Level Diagram that shows the diﬀerence between absorption and emission spectra during photoluminescence[13] ...... 8 2.4 A diagram illustrating the band gap between the valence band and conduction band of a photoluminescence system[2] ...... 9 2.5 A Jablonski diagram that illustrates the processes involved in the photoluminiscence phenomena . The diagram shows ﬂuorescence from an excites singlet state and an excited triplet state.[17] ...... 11

2.6 Illustration of Beer Lambert Attenuation[18] ...... 13

3.1 Diﬀerent Types of Polystyrene ...... 17

3.2 Chemical Structure of Polystyrene[30] ...... 19

3.3 Formation and Polymerization of Polystyrene[21] ...... 20

4.1 The Varian Cary Eclipse Fluorescence Spectrophotometer ...... 25

4.2 Internal component of Cary Eclipse Fluorescent Spectrophotome- ter. The colored beam represents the path of light. The blue light beam is the excitation light emerging out from a Xenon ﬂash lamp, which is ﬁnally transitioned into an orange-colored beam called the emission light[28] ...... 26

4.3 Schematic Diagram of a Fluorescence Spectrometer.[1] ...... 27

4.4 Control Interface of Cary Eclipse Fluorescence Spectrophotometer[28] . 29

viii 4.5 The Full Experimental Setup ...... 30

4.6 Cryostat Used to hold a sample for measurement at low temperatures . 32

4.7 Interface of temperature regulator for roughing pump ...... 32

4.8 Interface of the turbopump controller used to monitor vacuum and the pressure sensor ...... 33 4.9 Adding liquid nitrogen to the cyrostat for cryogenic measurements . . . 35

4.10 Samples Used ...... 36

4.11 Scale Measurements of Samples ...... 37

5.1 A sample 2-D figure obtained with MATLAB showing a plot of excitation against emission wavelength. The Photoluminescence emission peak of the sample is found around an excitation wavelength of 255 and an emission wavelength of 270 nm ...... 40 5.2 The figure shows the PL intensity variation of six different polymers as temperature changes . In exception of PS, all samples showed growth in PL peak as temperature decreased but stayed at the same wavelength however in PS, the peak shifted to a shorter wavelength with temperature decrease [29] ...... 42

5.3 Previous [29] photoluminescence emission spectra of sample PS4 at 77 K, 100 K, 150 K, 200 K, 250 K and 292 K. The peak intensity measurements are in arbitrary units. The photoluminescence peak intensity increases as temperature decreases; however, the peak intensity shift is only noticed at 77 K and 100 K...... 43 5.4 A 2D contour plot of PS1 at 77 K, 100 K, 200 K and 292 K. The maximum excitation wavelength was found to be 270 nm. The approximate Photoluminescence peak is indicated with an arrow. There is minimal or no shift in intensity peak for this sample as temperature changes; however, it shows a slight increase in peak intensity. 45 5.5 A 2D contour plot of PS2 at 77 K, 100 K, 200 K and 292 K. The maximum excitation wavelength was found to be 260 nm. The approximate Photoluminescence peaks are indicated with an arrow. The Photoluminescence peak does not shift with a decrease in temperature. 46

ix 5.6 A 2D contour plot of PS3 at 77 K, 100 K, 200 K and 292 K. The maximum excitation wavelength was found to be 265 nm. The approximate Photoluminescence peaks are indicated with an arrow. . . 47 5.7 A 2D contour plot of PS4 at 77 K, 100 K, 200 K and 29 2K. The maximum excitation wavelength was found to be 260 nm. The approximate Photoluminescence peaks are indicated with an arrow. This sample showed a peak shift at 77 K and 100 K. On the graph, the peak shift can be seen to vary from 325 nm at 29 2K to 290 nm at 77 K. The PL emission was measured from 240 nm to 380 nm. . . . 48 5.8 Photoluminescence emission spectra of all samples at 77 K, 100 K, 200 K and 292 K. PL luminescence intensity is in arbitrary units. Excitation wavelength for each sample is kept fixed at a value that corresponds to the maximum of the peak for that sample (See Table 5.3). 51 5.9 Photoluminescence emission spectra of PS1 fitted with a Gauss-mod peak function. Good fits were obtained at all four temperatures. . . . . 54 5.10 Photoluminescence emission spectra PS2 fitted with a Gauss-mod peak function...... 56 5.11 Photoluminescence emission spectra PS3 fitted with a Gauss-mod peak analyzer ...... 57

5.12 Figure 5.12 shows Lorentz fits of photoluminescence intensity (in arbitrary units) at 77 (blue line), 100 (green line), 150(magenta line), and 200K (red line). Thick black lines represent the total fits. The yellow and orange modes represent the two Lorentz oscillators. The yellow mode does not change significantly as the temperature decreases. However, the orange mode grows dramatically, its intensity increasing by a factor of five between 150 K and 77 K. This creates the appearance that the PL peak shifts to lower wavelengths. The yellow mode corresponds to the peak observed in the other three samples, whereas the orange mode is most likely due to impurities [30]. 59 5.13 Plot of Area, Center, and Width Vrs Temperature of fitted Gaus- mod curve of PS1 polymer at various temperatures ...... 60 5.14 Plot of Area, Center, and Width Vrs Temperature of fitted Gaus- mod curve of PS2 polymer at various temperatures ...... 62 5.15 Plot of Area, Center, and Width Vrs Temperature of fitted Gaus- mod curve of PS3 polymer at various temperatures...... 64

x 5.16 Plot of Area, Center, and Width Vrs Temperature of fitted Gaus- mod curve of PS4 polymer at various temperatures...... 65 5.17 Plot Comparison of Area Vrs Temperature of fitted Gauss-mod curve of all polymers at various temperatures ...... 67 5.18 Plot Comparison of Center Vrs Temperature of fitted Gaus-mod curve of all polymers at various temperatures ...... 68 5.19 Plot Comparison of Width Vrs Temperature of fitted Gaus-mod curve of all polymers at various temperatures ...... 69

xi CHAPTER I

INTRODUCTION

1.1 Overview

When light is incident on any material, absorption or emission may occur. The absorbed light excites the material in a process called photoexcitation [1]. Pho- toexcitation causes electrons within a material to excite into allowed energy states.

The electrons release excess energy accompanied by the emission of light known as photoluminescence [2]. Photoluminescence can be in the form of fluorescence or phosphorescence. Sir John Fredrick Herchel first discovered the fluorescent phenomena in

1845 after exposing quinine solution to light. He found that UV light can excite a quinine solution to emit blue light [3]. British scientist Sir George G. Stokes further studied this effect. He observed that an object’s fluorescence emission represents a longer wavelength than the UV light that initially excited the object. An essential difference between fluorescence and phosphorescence is the time duration of emitting light. The fluorescence emission process takes less than one microsecond. On the other hand, the phosphorescence emission process spans from a couple of millisec- onds to a second. Fluorescent materials tend to cease glowing almost immediately after radiation stops, with a life span of about (10−8 to 10−4)s.

1 Fluorescence spectroscopy can be used to probe the electronic structure and analyze the temperature dependence of materials, including polymers. In photoluminescence, a spectrophotometer or a spectrofluorometer generates the excitation and emission spectrum. The sample to be analyzed is illuminated by a color of light found to cause some fluorescence. A range of the fluorescent emission is obtained by scanning with the analyzing spectrometer using this fixed illumination color. The analyzer is then set at the brightest emission color and a spectrum of the excitation is obtained by scanning with the illuminating and measuring the emission intensity variation at this fixed wavelength. Although certain substances have broad spectra of excitation and emission, most photoluminescence objects have well-defined excitation and emission bands. The difference in wavelength between the peaks of these bands is referred to as the Stokes shift [4].

In recent years, many polymers have found applications at cryogenic temperatures. Elucidating their properties at those temperatures has become an important research topic. Lowering the temperature was recently argued to be an important way to enhance photoluminescence quantum yield. Alternatively, PL intensity can also be increased by applying mechanical pressure on the studied sample. However, it was shown that in some blue ﬂuorescent proteins lowering the temperature is much more eﬃcient, as it leads to a stronger increase in photoluminescence intensity.

Previous analysis of polystyrene shows the polymer demonstrates some amount of photoluminescence [9]. Mool C. Gupta reported the transfer of electronic energy during this process [8]. The intensity of the ﬂuorescence of polystyrene has been

2 studied to vary as temperature changes. The photoluminescence yield of polystyrene was investigated to increase with crystallinity at 77 K . This was found to be due to the increase in excitation intensity at low temperatures [10]. Fluorescence quenching studies show that at low temperatures, electron interaction increases, which causes higher quantum yield.

In a previous study temperature dependence of PL intensity in six diﬀerent common polymers was examined. PL was measured from room temperature down to 77 K. It was found that PL intensity was temperature dependent in all studied samples. In all polymers examined, except in polystyrene (PS), the peak position was found to be temperature independent. On the other hand, in PS the peak was found to shift by as much as 30 nm: from approximately 325 nm at 292 K to 295 nm at 77 K.

In order to explore this effect in more details, in this work I measure the photoluminescence of four different polystyrene samples from different manufacturers at 77 K, 100 K, 200 K, and 292 K. Particular attention is given to the peak intensity and shifts at low temperatures. I found that the shift of PL peak is not found in all samples. Therefore, the effect is not intrinsic to polystyrene and I hypothesize that it is due to impurities. Moreover, data analysis revealed that the shift of the peak is due to a second photoluminescence peak, which displays a very strong temperature dependence.

3 1.2 Thesis Outline

This thesis is organized into six chapters to provide clear and concise details about the research work. A brief overview of what to ﬁnd in each chapter is as follows: Chapter

1 provides an introduction to the project. It discusses photoluminescence and phosphorescence as the main types of fluorescence. It also discusses polystyrene in brief detail. Chapter 2 provides the theory behind photoluminescence and fluorescence. A theoretical background is discussed to provide a comprehensive understanding of the photoluminescence phenomena. Chapter 3 presents synthesis methods of polystyrene, as well as some basic properties of this polymer. Chapter 4 contains the descriptions of instrumentation used for the research. It also explains in detail the experimental procedure and how the low-temperature measurements were performed. Chapter 5 includes the results and experimental data. The analysis of the data, which includes the fitting procedure, are also presented in this chapter. Chapter 6 summarizes the most important results of the thesis. Citations used for the research are included at the end of the thesis.

4 CHAPTER II

PHOTOLUMINESCENCE

Most materials can emit light or radiation solely because of their high temperature[2].

Such materials are said to exhibit incandescence. Any other form of light emission from a material is known as luminescence. In luminescence, electronic states of materials are excited by the external energy, which is released as light. Luminescence may exist in several forms. If the external source of energy is from light, it is termed as photoluminescence . [5]. External energy from a chemical reaction that results in luminescence can be called as chemiluminescence. Other such phenomena include bi- oluminescence when the reactions occur within living organisms, such as glow-worms and fireflies. Photoluminescence can be sub-divided into two main categories: fluorescence and phosphorescence. When an electron absorbs a photon, it may excite from the ground state to a higher state. At the excited state, the electron may interact with other molecules causing it to deexcite to its original state with the release of light (a radiative process) or not (a non-radiative process)[6]. Procedures that may result in electrons’ deexcitation are electron transfer, proton transfer, energy transfer, etc. Fluorescence occurs when the excited electron deexcites from the singlet state to the ground state with the release of radiation; whiles phosphorescence occurs when the electron deexcites from the triplet state to the ground state with the release of

5 Figure 2.1: Schematic Diagram Of A Photoluminescence Phenomenon[9]

radiation; whiles phosphorescence occurs when the electron deexcites from the triplet state to the ground state with the release of radiation after inter-system crossing [7].

In ﬂuorescence, the electron in the excited state is paired by an opposite spin to the second electron in the ground state orbital. This follows the principle of Hund’s rule, which states that: Every orbital in a sublevel is singly occupied before any orbital is doubly occupied [8]. All the electrons in singly occupied orbitals have the same spin

6 Figure 2.2: A diagram showing ways in which molecules can deexcite from the excited state[9]

2.1 Photoluminescence Spectroscopy

2.1.1 Energy Levels and Excitation

Electrons occupy discrete energy levels in an atom. However, they may move to other energy levels due to external influences and internal reactions. Electrons are mostly found in the ground state and stay stuck there until an external energy is introduced. Most materials, including semiconductors, have energy gaps. This is mainly the region between the semiconductor where electrons are absent [10]. To explain the energy gap concept, we consider the electron in the valence band of the atom. Such electrons are not firmly bound to the nucleus of the atom; hence can move randomly. These mobile electrons have different amounts of energies around the particle.Electrons beyond specific energies’ values may be referred to as conducting electrons, while similar electrons with energies below a range of values will be referred to as valence band [11]. Electrons which can move about freely in a solid possess

7 Figure 2.3: Energy Level Diagram that shows the diﬀerence between absorption and emission spectra during photoluminescence[13]

kinetic energy which varies with their velocities, hence because of their motion, they have multiplicity of energies which is referred to as a band of energy. The space between the conduction and valence electron states is known as the energy gap [12].

Under Regular conditions, electrons are forbidden to possess energies in the band gap. If a photon has an energy greater than the band gap energy, then it can be absorbed and thereby raise an electron from the valence band up to the conduction band across the forbidden energy gap.

This process is known as photoexcitation. Photoexcitation causes electrons

8 Figure 2.4: A diagram illustrating the band gap between the valence band and conduction band of a photoluminescence system[2]

within the material to excite into permissible states. When these electrons return to their equilibrium states, excess energy is released accompanied by the emission of light. Photoluminescence associates with diﬀerence in energy levels between the electron states involved in the transition between the equilibrium to the excited state[14].

The diﬀerence in energies can be represented as

Eg = Ec − EV = hf (2.1)

A photoluminescent spectrum is diﬀerent from an absorption spectrum such

9 that an absorption spectrum records the transitions from grounds state to the excited state, but photoluminescent spectrum records the transitions from the excited state to the grounds state. At higher temperatures, non-radiative deexcitation channels are activated, and the PL intensity decreases exponentially [15]. The excited photon has excess energy which it loses before coming to rest at the lowest energy in the conduction band. The period of excitation lasts for about (1x10−15 s)The relaxation period at the excited state lasts for (1x10−14 to 1x10−11 s) [5] . The electron drops to the valence band at this point. As it falls, the energy it loses is converted back into a luminescent photon emitted from the material. This takes about (1x10−15 s).

The relaxation period at the excited state lasts for (1x10−9 to 1x10−7 s) [5]. Hence, the energy of the emitted photon is a direct measure of the energy of the band gap.

When ﬂuorescence occurs, the deexcited photon has energy lower than its initial energy. This tends to make its wavelength longer than that of the excited photon.

The diﬀerence in energies or wavelength between the excited and emitted photon may be termed as stokes shift[16]. Stoke’s shift may also be described as the spectral shift to lower energy between the incident light and emitted light after interaction with a sample.

10 Figure 2.5: A Jablonski diagram that illustrates the processes involved in the photoluminiscence phenomena . The diagram shows ﬂuorescence from an excites singlet state and an excited triplet state.[17]

11 2.1.2 Beer-Lambert Law

When lights pass through a material, attenuation may occur. Beer-Lambert Law relates the attenuation of light to the properties of the material [18]. The amount of absorbed light is directly proportional to the absorbing material’s concentration and the length of the path of light.

A α c (2.2)

A α l (2.3)

At a constant wavelength. The Absorbance A is deﬁned by

I A = log ( o ) (2.4) 10 I

From equation 2.1 and 2.2

A α cl (2.5)

Introducing a constant of proportionality, which is the molar absorption co- eﬃcient,

A = cl (2.6)

Equating equation 3 to equation 4,

12 Figure 2.6: Illustration of Beer Lambert Attenuation[18]

I A = log ( o ) = cl (2.7) 10 I

Where A is the absorbance or attenuation and c in the concentration.

The concentration of light absorbed depends on the properties of the material.

This will indicate the number of interactions that will occur. Materials with high concentration will have very high absorbance due to the excess number of molecules present for interaction. Similarly, a material with less or minimal concentration will absorb less due to the minimal number of molecules available for interaction. From equation 2.2, It can also be noted that the greater the length of a material, the greater the absorption and vice versa.

2.1.3 Fluorescence Quantum Yield and Quenching

The quantum yield is the magnitude relation of the emitted photons to absorbed photons, decreases with increase in temperature[7]. The magnitude relation gives the

13 likelihood of the excited state being deactivated by ﬂuorescence rather than by another, non-radiative mechanism such as internal conversion or vibrational relaxation

(non-radiative loss of energy as heat to the surroundings.[19]. Values of Fluorescence quantum yield is between 0 and 1. The higher the fluorescence quantum yield values, the brighter the fluorophore and stronger the fluorescence signal intensity will be

ne Φf = ( ) (2.8) na

The intensity of ﬂuorescence may be reduced by a process called quenching

[2]. Several molecular interactions may result in this process. These include excited state reactions, molecular rearrangements, energy transfer, ground-state complex formation, and collisional quenching. Fluorescence quenching may exist as static or dynamic. In both cases, there needs to be contact between the ﬂuorophore and the quencher . In the case of collisional quenching, the quencher must diﬀuse to the

fluorophore during the excited state’s lifetime. The fluorophore drops to the ground state when contact occurs without emitting a photon. It can, therefore, be said that quenching doesn’t cause any deformation to molecules . In static quenching a complex is formed between the fluorophore and the quencher, and this complex is non-fluorescent. Quenching has numerous applications due to the requirements of molecular contact for quenching results . For example, quenching measurements can reveal the accessibility of fluorophores to quenchers. Also, the diffusion coefficient of the quencher can be determined using the rate of collisional quenching. A wide vari- ety of substances act as quenchers of fluorescence. Aromatic and aliphatic amines are 14 also efficient quenchers of most unsubstituted aromatic hydrocarbons [14].For example, anthracene fluorescence is effectively quenched by diethylaniline.2 For anthracene and diethylaniline, the mechanism of quenching is the formation of an exciting charge- transfer complex. The excited-state fluorophore accepts an electron from the amine.

Nonpolar solvents ﬂuorescence from the excited charge-transfer complex (exciplex) is frequently observed, and one may regard this process as an excited state reaction rather than quenching[10].

15 CHAPTER III

POLYSTYRENE

Polystyrene (PS) is a synthetic polymer produced from the polymerization of styrene

[20]. It is usually solid or foamy and was discovered in 1839 by German pharma- cist Eduard Simon. Its IUPAC name is Poly (1-phenylethene).[21, 22]. It may exist as general-purpose, high impact, or expanded polystyrene. In this research, general- purpose commercial polystyrene from diﬀerent manufacturers was used. The general- purpose polystyrene is a hard and stiﬀ type of polystyrene which is widely used due to its suitable physical, thermal and chemical properties. These polystyrene types may be used to manufacture appliances, automobile parts, and disposable tableware and electronics. Polystyrene has thermoplastic properties that make it solid at room temperature but melts when subjected to temperatures above 100oC [23].It is very slow to biodegrade, raising issues for concern since it is very abundant in our environ- ment. High Impact Polystyrene is versatile, impact-resistant, and easy to fabricate.

Expanded polystyrene (EPS) is rigid, tough, and foamy. It is mostly white and made of pre-expanded beads of polystyrene. EPS is mainly used for food storage and may serve as an insulation material.

16 Figure 3.1: Diﬀerent Types of Polystyrene

17 3.1 Structure and Polymerization of Polystyrene

Polystyrene has the chemical structure

(C8H8)n

. It is derived when styrene is polymerized. Fig 3.3 depicts benzoyl peroxide as a free radical initiator to polymerize the styrene monomer[24]. The benzoyl acid is divided to become a free peroxy radical. The chain starts when the peroxy radical combines with the styrene monomer, forming an active center on the opposite of the initiator.

This is the initiation stage. The chain continues to grow one styrene at a time until the process is terminated. The termination step occurs when the free radicals extra electron is given to another free radical, ending both chains. The polymer gets isolated, puriﬁed if necessary, characterized, and can be used for material purposes.

The structure of polystyrene and the mechanism of the free radical polymerization of styrene is shown below.

18 Figure 3.2: Chemical Structure of Polystyrene[30]

19 Figure 3.3: Formation and Polymerization of Polystyrene[21]

20 3.2 Properties of Polystyrene

3.2.1 Mechanical Properties

Polystyrene’s mechanical properties include a Young’s Modulus between(3000 - 3600)

MPa, a tensile strength of about (30-60)MPa, and a shear modulus of 1400MPa.

Crystal forms of polystyrene have low impact strength, and the quality of polystyrene gets aﬀected when exposed to extreme sunlight due to photo-oxidation, which aﬀects its mechanical properties. Other properties are a tensile elongation of (1-5)%, a

ﬂexural strength of 76 MPa, and a Flexural Modulus of 3200 MPa [25].

3.2.2 Physical Properties

The density of polystyrene can vary from 1.05 - 1.06 gmL−1. Polystyrene, in its natu- ral state, sparkles, has a glassy nature, and lacks crystallinity. Extruded polystyrene can float in water, making it an ideal option for floating materials. The viscosity of polystyrene depends on the shear rate, which is the shear stress’s magnitude to shear rate. It has a specific gravity of about 1.03 - 1.06 g(cm)−3 with its molecular ranging between 100,000 - 400000 g(mol)−1 .

3.2.3 Thermal Properties

The thermal properties of polystyrene are its characteristics when it is subjected to heat. These may include glass transition temperature and thermal conductivity, etc. Polystyrene is present as a solid or glassy at an average temperature. When heated above its glass transition temperature of about 100oC, it melts into a liquid

21 that flows and can be used for extrusion and molding [23]. It solidifies when it cools down. This property of polystyrene makes it appropriate for casting it into molds with fine detail. It has an actual melting range between 180 and 270 oC. The density is around 1.03 - 1.08 g(ml)−1. Polystyrene is an excellent insulator due to the low heat loss ability. Below its transition temperature, it has medium to high tensile strength (35 - 55 MPa) but low impact strength (15 - 20 J/m)[19]. Typical values of thermal conductivity ranges from 1 - 5 W/(mk) −1 . It has a specific heat capacity of about 30-60 J(KgK) −1 with molar Heat Capacity 123 - 127 J(molK) −1. Within a temperature range of (20 - 100)oC , its thermal expansion is about 1400 .[21].

3.2.4 Chemical Properties

Polystyrene is a thermoplastic that is chemically inert and doesn’t react with most substances. It dissolves in some organic solvents, and It is soluble in solvents that contain acetone like most paints. The transformation of carbon-carbon double bonds into less reactive single bonds in polystyrene is the main reason for its chemical stability[25]. It is highly flammable and burns with an orange, yellow flame, giving off carbon particles or soot, as a characteristic of all aromatic hydrocarbons[23].

Polystyrene, on complete oxidation, produces only carbon dioxide and water vapor.[26].

3.2.5 Electrical Properties

Polystyrene is an optically ﬂexible material with good electrical insulation. This helps prevent the loss of electrons to the surroundings or materials around. The dielectric constant of polystyrene ranges from 2.4 - 3 , which may be due to impurities [23].

22 It has low water absorption and very high resistance to cracking; however, it breaks down when an x-ray passes through it.

3.2.6 Optical Properties

Polystyrene has so many critical optical properties. It has a refractive index of about

1.59, making it very suitable for making optical glasses [26] .Glasses made from polystyrene possess minimal spherical and chromatic aberrations, which causes minimal distortions hence creates sharp images. Most polystyrene is radiation-resistant but may exhibit radioluminescence under peculiar circumstances. The optical density of polystyrene increases in the region of 290-420nm. The non-crystal nature of polystyrene makes it optically clear [21]. The transmission and visibility of light is roughly around (80 - 90)nm

23 CHAPTER IV

EXPERIMENTAL METHOD

4.1 Cary Eclipse Fluorescent Spectrophotometer

The photoluminescence spectra obtained in this research was acquired by the Varian

Cary Eclipse Spectrophotometer, manufactured by Agilent Technologies. The instrument measures the intensity of emitted light from samples inserted across a specified wavelength [28]. The device measures the emission wavelength and its intensity at a specified excitation wavelength. This excitation value corresponds to the maximum of the sample. The excitation wavelength is kept constant throughout the measurement. Photoluminescence intensity for samples may vary at changing temperatures and may be used to determine specific molecules’ structures. The instrument is de- signed with the latest technologies, including a corrected spectrum, excitation and emission filters, and an extended range photomultiplier (PMT) detector.

The mode of operation uses a xenon ﬂash lamp coupled with optimized grat- ing angles and coatings. The lamp shines light at a constant excitation wavelength unto the sample, giving a reading of the intensity of emitted light. This ensures that maximum sensitivity is obtained across the entire wavelength being scanned.

The instrument can compile data that can be used by any programming language to

24 Figure 4.1: The Varian Cary Eclipse Fluorescence Spectrophotometer

plot diagrams. In this work, data obtained from the spectrophotometer is plotted in

MATLAB to obtain a 2-D ﬁgure, which gives a pictorial view of samples’ excitation peak. Further details are discussed in other sections of this work.

25 Figure 4.2: Internal component of Cary Eclipse Fluorescent Spectrophotometer. The colored beam represents the path of light. The blue light beam is the excitation light emerging out from a Xenon ﬂash lamp, which is ﬁnally transitioned into an orange-colored beam called the emission light[28]

26 The device measures the intensity of samples as a function of excitation wavelength. These intensity measurements are arbitrary units, which is the voltage devel- oped by emitted light-induced in the detector optics. The spectrophotometer can scan and take measurements ranging from the ultraviolet range to the infrared spectrum range of light.

Figure 4.3: Schematic Diagram of a Fluorescence Spectrometer.[1]

The manufacturer describes the collection of detector optics as Schwarzschild collection optics [28]. This collection is stated to produce the best sensitivity with the lowest signal noise possible. The optics ensure that relevant data is obtained even in specular reﬂection or other stray light sources from the room and environments.

The instrument has a signal to noise ratio that regulates the amount of noise in the photoluminescent spectra.

27 4.2 The Software

The Cary Eclipse ﬂuorescent software is embedded with so many functions; however, we limit ourselves to what was needed for this work. To launch the software, the setup icon is double-clicked on the desktop after installation. The setup window then pops up with several tabs, which are used for analysis. In the Cary tab, we choose the

ﬂuorescence option under the data mode. The emission tab under the scan section is opened to set the spectra details. An initial excitation wavelength is then given. We specify or emission wavelength range (200nm – 380nm). The emission and excitation

ﬁlter slits are then adjusted to control the amount of light directed at the sample.

In this research, the excitation and emission slits were varied between 5 and 10 nm.

The excitation increments are also set to indicate the intervals at which the scans data should be recorded. Many details are provided in the experimental procedure section. The 3-D mode is selected to give the cross-section contour graphs at the excitation stop of 450 nm. The scan control runs at medium speed. After the scan, the emission intensity is plotted against the emission wavelength in arbitrary units for a speciﬁed excitation wavelength. Depending on the settings, the software scans for other excitation wavelengths after completing the initial one. Data recorded can be exported into a Microsoft Excel document and used to plot 2-D diagrams using any programming software.

28 Figure 4.4: Control Interface of Cary Eclipse Fluorescence Spectrophotometer[28]

29 4.3 Low Temperature Measurements and Experimental Setup

The photoluminescence spectra of four diﬀerent polystyrene samples are measured over a temperature range from 77K to 292K. The experimental setup consists of two vacuum pumps coupled with a temperature regulator, computer, cryostat, liquid nitrogen, and a spectrophotometer. The vacuum pump is used to minimize contami- nation in the air and helps other sources of ﬂuorescence.

Figure 4.5: The Full Experimental Setup

It is essential to perform the experiment under such conditions to achieve maximum results. Using a vacuum pump, helps minimize specular reﬂection and

30 possible stray emissions for molecules and atoms present in the air. In this experiment, two vacuum pumps were used. A roughing pump (Varian DS 102) and a turbopump.

Both pumps can extract air from the experiment’s surroundings; however, they diﬀer in the extent of vacuuming; hence, both are used to complement each other. The roughing pump (Varian DS 102) initially removed the air and created a vacuum to about 1mBars, while the turbopump further removed air and created a stronger vacuum. The roughing pump (Varian DS 102) only operated with an and oﬀ button.

Still, the turbopump has a broad interface to help navigate some of its controls. The interface of the turbopump is used to control the status and health of the pump. The two vacuum pumps are connected to the cryostat through a metal hose and coupling.

A pressure sensor was also connected to the setup to measure the pressure in the cryostat. Advanced Research Systems Helitrans LT3 cryostat is used to hold the test sample under cryogenic temperatures for the measurements to be taken.

It holds the sample in its chamber and is placed in the spectrophotometer.

The cryostat operates like a thermostat but can store liquid gases at low temperatures for some time. The cryostat uses internal refrigeration methods to maintain low temperatures. Liquid nitrogen is used to obtain the low temperatures for the measurements. It has a copper plate called the cryonic ﬁnger, which is its most essential component that holds the sample together. The ﬁnger has a heating coil wrapped around it that allows the liquid nitrogen to heat and reach ambient temperature. The

ﬁnger has two other sensors;one near the sample and the other close to where the liquid nitrogen is poured. The sample is aligned for emission to take place.

31 Figure 4.6: Cryostat Used to hold a sample for measurement at low temperatures

Figure 4.7: Interface of temperature regulator for roughing pump

This interface interprets the surrounding temperature and reading can be recorded on the temperature regulator, which also controls the heater in the cryostat assembly. For other cooler temperatures, liquid helium can be used.

32 Figure 4.8: Interface of the turbopump controller used to monitor vacuum and the pressure sensor

4.4 Experimental Procedure

A full scan of the samples was done using the Cary eclipse ﬂuorescent spectrophotometer at room temperature 292K. The peak intensity is recorded as a function of emission wavelength. From this data, MATLAB was used to plot a 2-D contour graph between the emission wavelength and the excitation wavelength. The speciﬁc excitation wavelength and the range of emission wavelength for each polystyrene type are determined from the contour graph. A second measurement is done, but in this case, the excitation wavelength is kept constant, and the photoluminescent peak is measured. The photoluminescent peaks are measured at 77K, 100K, 200K, and 292K.

Low temperature measurements are done using the cryostat. Liquid nitrogen is being poured into the cryostat, and temperature readings are recorded from the temperature regulator. Liquid nitrogen has a temperature of 77K. After pouring the liquid

33 nitrogen, we wait a while to achieve equilibrium temperature. The temperature regulator is regulated whiles monitoring the pouring of liquid nitrogen to achieve the desired temperature. We allow about 10 minutes intervals between measurements for samples to reach thermal equilibrium. Data is extracted and exported to Origin Pro.

A plot of the peak intensity in arbitrary units is plotted as a function of emission wavelength to observe the peak shift. The temperature dependence of the photoluminescence peak of samples of polystyrenes is analyzed using origin pro mathematical software. Origin pro has inbuilt mathematical functions that help ﬁt out curves.

Gauss-mod mathematical function was used to ﬁt all samples. Values from the ﬁt are extracted and plotted as a function of temperature to see how the photoluminescence peak changes or shifts with varying temperatures.

34 Figure 4.9: Adding liquid nitrogen to the cyrostat for cryogenic measurements

35 Figure 4.10: Samples Used

4.5 Samples Used In The Thesis

Four types of polystyrene from diﬀerent manufacturers were used for this research work. These samples are commercial polymers and are not necessarily made for experimental analysis. Their speciﬁc chemical composition is unknown due to manufacturers unwillingness to disclose such information at the time of purchase . The samples were cut into pieces of approximate dimensions (1cm x 1cm x 1cm). Samples are labeled as PS1, PS2, PS3, PS4 for this work.

36 PS 1 PS 2 PS 3 PS 4

Figure 4.11: Scale Measurements of Samples

37 CHAPTER V

RESULTS AND EXPERIMENTAL DATA

Previous studies of the PL intensity/peak variation of several polymers at low temperatures showed significant temperature dependence. In this work, I present the results and data obtained from the analysis of the photoluminiscence spectra of four samples of polystyrene (PS) from different manufacturers. The investigation is done at 77K, 100K, 200K, and 292K. This chapter is further divided into five sections. The

first section presents previous analysis on six different polymers. All polymers showed growth in PL peak as temperature decreased. It was realized that only polystyrene showed a peak shift as temperature decreased. All other samples had the PL intensity grow, but the wavelength of the peak did not change [29]. In order to explored that effect we obtaining three other samples of polystyrene, and check if the shift in the peak was intrinsic to this particular sample or a general characteristic of polystyrene.

The second section shows 2D contour plots of samples of polystyrene at 77 K, 100 K,

200 K and 292 K. Bright spots in these plots indicate the photoluminescence peak intensity. From these plots we extracted excitation wavelength of the peak for each sample. The third section focuses on the Photoluminescence spectra of the samples analyzed at temperatures 77 K, 100 K, 200 K and 292 K. The fourth section discusses mathematically functions used for ﬁtting the spectra: Lorentz and GaussMod func-

38 tions. Section five presents the best fits of PL spectra using Lorentz and GaussMod functions. Finally, in section six I discuss the temperature dependence of the fitted parameters extracted from the fits. The following parameters were discussed: the peak wavelength (position), peak strength (intensity) and peak width.

As an example, in the ﬁgure below, we show a 2-D plot of PL intensity obtained with MATLAB. The horizontal axis indicates the wavelength at which the sample emits light during photoluminescence. The vertical axis represents the excitation wavelength of the sample. Both are measured in nanometers. The diagonal region comprising several colors is specular reﬂection that may arise. This region is of no interest for our study. The bright region indicated with an arrow shows the photoluminiscence intensity peak, which is the main focus of this work. We observe the peak shift or changes with temperature variation.

39 Figure 5.1: A sample 2-D ﬁgure obtained with MATLAB showing a plot of excitation against emission wavelength. The Photoluminescence emission peak of the sample is found around an excitation wavelength of 255 and an emission wavelength of 270 nm

5.1 Previous Photoluminescence Spectra of Polystyrene

The ﬁgure below shows the PL intensity spectra of six diﬀerent polymers at 77 K, 100

K, 150 K, 200 K, 250 K and 300 K. The ﬁgure is taken from the previous study from our group [29]. A clear indication of PL intensity growth is observed in all samples as temperature decreases. However only polystyrene showed a peak shift at lower wavelengths as temperature decreased.

Temperature dependence of PL intensity in polystyrene is shown in more details in the next Figure. It can be seen that the intensity of the peak grows as temperature increases, but at the two lowest temperatures (77 K and 100 K) the peak also shifts to lower wavelengths.

40 Table 5.1: List of Polymers and Acronym

Name and Type of Polymer Acronym

Low Density Polyethylene LDPE

Polycarbonate PC

Poly (methyl methacrylate) PMMA

(Acrylic)

Polypropylene PP

Polystyrene PS

Polyurethane PU

41 Figure 5.2: The ﬁgure shows the PL intensity variation of six diﬀerent polymers as temperature changes . In exception of PS, all samples showed growth in PL peak as temperature decreased but stayed at the same wavelength however in PS, the peak shifted to a shorter wavelength with temperature decrease [29]

42 Figure 5.3: Previous [29] photoluminescence emission spectra of sample PS4 at 77 K, 100 K, 150 K, 200 K, 250 K and 292 K. The peak intensity measurements are in arbitrary units. The photoluminescence peak intensity increases as temperature decreases; however, the peak intensity shift is only noticed at 77 K and 100 K.

43 5.2 2-D Contour Plots of Photoluminescence at Diﬀerent Temperatures

In order to determine the position of the PL peak in each of the four samples, we performed detailed temperature dependence study. Each sample’s maximum excitation wavelength is determined by scanning across a range of wavelength between (230 nm and 280 nm) and plotted against a range of emission wavelength (240 nm - 380 nm).

The results were plotted using MATLAB as 2D contour graphs. The bright spot on each graph indicates the photoluminescence peak. The color ranges from blue to red, representing the magnitude of the intensity measured in arbitrary units. The next four ﬁgures show the 2D contour plots all four samples (PS1, PS2, PS3 and PS4) at four diﬀerent temperatures: 77 K, 100 K, 200 K and 292 K.

44 Figure 5.4: A 2D contour plot of PS1 at 77 K, 100 K, 200 K and 292 K. The maximum excitation wavelength was found to be 270 nm. The approximate Photoluminescence peak is indicated with an arrow. There is minimal or no shift in intensity peak for this sample as temperature changes; however, it shows a slight increase in peak intensity.

45 Figure 5.5: A 2D contour plot of PS2 at 77 K, 100 K, 200 K and 292 K. The maximum excitation wavelength was found to be 260 nm. The approximate Photoluminescence peaks are indicated with an arrow. The Photoluminescence peak does not shift with a decrease in temperature.

46 Figure 5.6: A 2D contour plot of PS3 at 77 K, 100 K, 200 K and 292 K. The maximum excitation wavelength was found to be 265 nm. The approximate Photoluminescence peaks are indicated with an arrow.

47 Figure 5.7: A 2D contour plot of PS4 at 77 K, 100 K, 200 K and 29 2K. The maximum excitation wavelength was found to be 260 nm. The approximate Photoluminescence peaks are indicated with an arrow. This sample showed a peak shift at 77 K and 100 K. On the graph, the peak shift can be seen to vary from 325 nm at 29 2K to 290 nm at 77 K. The PL emission was measured from 240 nm to 380 nm.

48 Table 5.2 summarizes PL peak position (emission wavelength vs. excitation wavelength) for all four samples at 292 K. For all PL studies we used the excitation wavelength that corresponds to the position of the peak, and the emission spectra were taken over the appropriate wavelength range to include the whole PL peak.

Table 5.2: Excitation and emission wavelengths of the peak in all four samples.

Sample Excitation Wavelength Emission Wavelength

PS1 257 326

PS2 254 328

PS3 240 313

PS4 262 326

5.3 Temperature Dependence of Photoluminiscence Spectra

The Photoluminescence emission spectra that corresponds to the maximum excitation wavelength Table 5.2 . are shown in the figure below for all four samples. The peak intensity is in arbitrary units. A look at the peak of sample PS1 shows minimal peak shift. The shape of the curve remains constant with temperature variation, but the intensity peak reduces when the temperature increases. Between 77 K and 100 K, the intensity peak is about the same; however, 200 K and 292 K temperature curves show a significant difference in the intensity peak. The maximum intensity peak occurs at an emission around 318 nm at 77 K. For Sample PS2, the peak intensity at 77

49 K, 100 K and 200 K is very similar. However, at 292 K, the sample shows an increase in the intensity peak. The shape of the curve also does not change with temperature. The maximum intensity peak occurs at an emission of 330 nm. No noticeable peak shift was recorded. PS3 showed a relatively constant peak wavelength at all temperatures. At a temperature of 292 K, there was a decrease in the peak intensity but no substantial shift. The maximum intensity peak occurred at an emission of

337 nm. Sample PS4 was the only sample that showed signiﬁcant peak shift. The sample showed a large peak shift from 325 nm emission wavelength at 292 K to 290 nm at 77 K. The growth in PL intensity of all samples as temperature decreases can be attributed to the thermal suppression of fast nonradiative relaxation process [30].

50 Figure 5.8: Photoluminescence emission spectra of all samples at 77 K, 100 K, 200 K and 292 K. PL luminescence intensity is in arbitrary units. Excitation wavelength for each sample is kept ﬁxed at a value that corresponds to the maximum of the peak for that sample (See Table 5.3).

51 5.4 Mathematical Functions Used For Fitting

The results presented clearly indicates that the behavior of PS4 is anomalous and requires further investigation [30]. In order to gain deeper insight, I fitted the PL spectra with different mathematical functions. I used several different functions, but the best results were obtained with Lorentz and the so-called Gauss-mod functions.

Gauss-mod function is exponentially modiﬁed Gaussian peak function, frequently used for ﬁtting spectra.

Gauss-mod mathematical function was used to model samples PS1, PS2 and

PS3. On the other had in PS4 we tried both Lorentz and a Gauss-mod functions.

Essential parameters like the Area, Width, and Center (peak position) were extracted from the ﬁts, and plotted as a function of temperature in section 6 below. In Samples

PS1, PS2 and PS3 we were able to obtain good ﬁts of the spectra suing a single mode of the mathematical function used. However PS4 required a sum of two oscillators due to the asymmetric nature of the peak at 100 K and 77 K [30].

The Lorentz function is given as:

wp IPL(w) = (5.1) 2 2 (w − wo) + Γ where wp is the peak strength, w0 is the peak position, and Γ is the peak width.

The Gauss-mod equation is given as:

52 2 A 1 w − x−xc z 1 y 2 ( to ) to R √ − 2 f(x) = yo + (f1 + f2)(x) = yo + t e −α e dy o 2π (5.2) 2 x (x−xc) A − 1 − 2 x−xc w Where f1(x) = e to , f2(x) = √ e 2w , z = − to 2πw w to where y0 is the oﬀset, A is the Area, W is the width, x is the initial position, xc is the center and, t0 the retention time .

In order to quantify the temperature change of the PL peak we used the following equation: I (77K) P ercentageIncrease% = PL (5.3) IPL(100K)

5.5 Fitted Spectra

This section presents the ﬁtted spectra of all samples. The red lines is the ﬁt line from the Gauss-Mod function whiles the black line represents the actual spectra of the sample .

Figure 5.9 shows the Photoluminescence emission spectra of PS1 ﬁtted with a

Gauss-mod peak function. Good ﬁts are obtained at all four temperatures. Analysis of the Area, Width, and Center as the temperature varied is presented in the next section. In this sample we found the percentage increase in peak intensity to be around 16.8% [30]. This was found by the ratio of the least intensity peak at 292 K to 77 K using equation 5.3.

53 Figure 5.9: Photoluminescence emission spectra of PS1 ﬁtted with a Gauss-mod peak function. Good ﬁts were obtained at all four temperatures.

54 Figure 5.10 shows the Photoluminescence emission spectra of PS2 ﬁtted with a Gauss-mod peak function. Good ﬁts were obtained at all temperatures. Analysis of the Area, Width, and Center as the temperature varied is presented in the next section. In this sample we found the percentage increase in peak intensity to be around 26.6% [30].

55 Figure 5.10: Photoluminescence emission spectra PS2 ﬁtted with a Gauss-mod peak function.

Figure 5.11 shows the Photoluminescence emission spectra of PS3 ﬁtted with a Gauss-mod peak function. The PL peak of the sample was very sharp and the quality of the ﬁts are not as good as in samples PS1 and PS2. Analysis of the Area,

Width, and Center as the temperature is varied is presented in the next section. In this sample we found the percentage increase in peak intensity to be around 11.8%

[30].

56 Figure 5.11: Photoluminescence emission spectra PS3 ﬁtted with a Gauss-mod peak analyzer

Figures 5.12 shows the Photoluminescence emission spectra of PS4 modeled with a Lorentz peak function. It is the only sample with a signiﬁcant peak shift. A single mode was insuﬃcient to reproduce the PL intensity curves at all temperatures.

Satisfactory ﬁts with a single oscillator could only be achieved only at 292 and 200

K. At lower temperatures, the quality of fits deteriorates and then get progressively worse as the temperature decreases. Most notably, the fits fail because of the highly asymmetric shape of the PL peak. Therefore, to improve the quality of the fits and to account for the asymmetric shape of the peak, we used a sum of two oscillators. The

57 results of the best fits are shown in figure 5.12. It can be seen that satisfactory fits with two oscillators can be achieved at all temperatures.The fits are shown with thick black lines. They capture the most important feature of the data at all temperatures.

In addition to these total fits,the figure also displays the two individual Lorentz oscillators used for each temperature: they are shown as yellow and orange modes. The yellow mode is at 320 nm and is almost temperature independent, both in terms of its wavelength and its strength. On the other hand, the orange mode shifts slightly (less than 3 nm) to a lower wavelength as temperature decreases. But, most importantly, its strength increases dramatically as the temperature decreases from to 77 K. At temperatures above 150 K, the orange mode is very weak and does not contribute to the overall strength. Below 150 K, the orange mode begins to gain intensity, and by 77 K, it is five times stronger than it was at 150 K. Below 100 K, the orange mode becomes comparable in strength to the yellow mode, and it is this mode that causes the broadening and asymmetric shape of the overall PL peak. The combined strength of the two modes shifts the position of the peak to lower wavelengths and creates the appearance of a highly asymmetric mode. The wavelength of the yellow mode is close to the wavelength of the peak in the other three samples(approximately

325 nm). Based on this, we argue that it is intrinsic to PS4. On the other hand, the orange mode is not observed in any other PS sample studied here, and we hypothesize that this mode is due to impurities, which are commonly added to commercial PS samples [30].

58 Figure 5.12: Figure 5.12 shows Lorentz fits of photoluminescence intensity (in arbitrary units) at 77 (blue line), 100 (green line), 150(magenta line), and 200K (red line). Thick black lines represent the total fits. The yellow and orange modes represent the two Lorentz oscillators. The yellow mode does not change significantly as the temperature decreases. However, the orange mode grows dramatically, its intensity increasing by a factor of five between 150 K and 77 K. This creates the appearance that the PL peak shifts to lower wavelengths. The yellow mode corresponds to the peak observed in the other three samples, whereas the orange mode is most likely due to impurities [30].

59 5.6 Temperature Dependence of Fitting Parameters

This section presents the temperature dependence of the parameters obtained from

ﬁtting the ﬂuorescence emission spectra. In the following graphs I plot the Area (peak intensity), Width and Center (peak position) against a varying temperature.

Figure 5.13: Plot of Area, Center, and Width Vrs Temperature of ﬁtted Gaus-mod curve of PS1 polymer at various temperatures

Figure 5.13 shows the variation of the center, area, and width in sample PS1 as temperature changes from 77 K to 292 K. The center represents the peak position 60 of the spectra. It can be seen that the center of the sample increases as temperature increases. The variation, however, is not large large. There is a very insigniﬁcant peak shift in this sample. The area represents the entire region of the curve and increases with decrease in temperature. The width of the spectra does not change much.

Table 5.3: Plotted Parameters of Gauss-mod ﬁtted spectra of PS1

TEMPERATURE AREA WIDTH CENTER

77K 5796 25 313

100K 5658 25 314

200K 5398 23 316

292K 4832 18 320

61 Figure 5.14 shows the variation of the center, area, and width for sample

PS2 as temperature changes from 77K, 100K, 200K, and 292K of PS2. The center represents the peak shift of the spectra. It can be seen that the center of the sample increases as temperature increases. The variation, however, is not that large. There is a very minimal peak shift in this sample. The area represents the entire region of the curve. The width of the spectra does not change much as there is minimal peak shift.

Figure 5.14: Plot of Area, Center, and Width Vrs Temperature of ﬁtted Gaus-mod curve of PS2 polymer at various temperatures

62 Table 5.4: Plotted Parameters of Gauss-mod ﬁtted spectra of PS2

TEMPERATURE AREA WIDTH CENTER

77K 14661 21 323

100K 14746 22 325

200K 13432 18 324

292K 16798 23 327

Figure 5.15 shows the variation of the center, area, and width for sample PS3 as temperature changes from 77K, 100K, 200K, and 292K of PS3. The sample shows a smaller decrease in intensity and a very insigniﬁcant peak shift. The width of the sample is relatively constant.

Table 5.5: Plotted Parameters of Gauss-mod ﬁtted spectra of PS3

TEMPERATURE AREA WIDTH CENTER

77K 30130 10 313

100K 30305 10 312

200K 28978 9 311

292K 21432 8 311

63 Figure 5.15: Plot of Area, Center, and Width Vrs Temperature of ﬁtted Gaus-mod curve of PS3 polymer at various temperatures.

Figure 5.16 shows the center, area, and width variation for sample PS4 as temperature changes from 77K, 100K, 200K, and 292K of PS4. This is the only sample that shows a peak shift. The results correspond to previous results obtained .

Due to the asymmetric nature of the spectrum, the width showed an non-monotonic temperature dependence.

64 Figure 5.16: Plot of Area, Center, and Width Vrs Temperature of ﬁtted Gaus-mod curve of PS4 polymer at various temperatures.

65 Table 5.6: Plotted Parameters of Gauss-mod ﬁtted spectra of PS4

TEMPERATURE AREA WIDTH CENTER

77K 2038 14.9 295

100K 1799 14.8 310

200K 1601 22.1 321

292K 975 18.8 326

66 In the next three plots we compare the variation of the parameters extracted from the Gauss-mod ﬁt of all four samples. The color legends indicate the speciﬁc samples.

Figure 5.17: Plot Comparison of Area Vrs Temperature of ﬁtted Gauss-mod curve of all polymers at various temperatures

67 Figure 5.18: Plot Comparison of Center Vrs Temperature of ﬁtted Gaus-mod curve of all polymers at various temperatures

68 Figure 5.19: Plot Comparison of Width Vrs Temperature of ﬁtted Gaus-mod curve of all polymers at various temperatures

69 CHAPTER VI

CONCLUSIONS

A previous study from our group showed that the PL intensity of six different polymers was temperature dependent [29]. However, polystyrene was the only polymer that showed PL peak shift. In this work, I studied this effect in details. We acquired three other samples of polystyrene from different manufactures to analyze and probe if the PL spectra peak shift was intrinsic to polystyrene. Measurements were done using a Cary Eclipse spectrophotometer at 77 K, 100 K, 200 K and 292 K. The low temperature measurements were done by filling a Cyrostat with liquid nitrogen. The cryostat held the sample in its finger and was placed in the spectrophotometer for measurements to be taken. Measurements were performed at the fixed excitation wavelength, which corresponded to the maximum of the PL peak for each sample.

PL intensity was measured for a range of emission wavelength values, in order to determine the peak position and its shift with temperature. Samples PS1, PS2 and

PS3 showed insigniﬁcant peak shift with temperature. On the other hand, the photoluminescence peak of PS4 shifted by as much as 30 nm: from approximately 325 nm at 292 K to 295 nm at 77 K. I concluded that previously observed shift in the peak position was not intrinsic to PS and is most likely due to impurities.

In order to gain deeper understanding of this eﬀect, I ﬁtted the PL spectra

70 with Lorentz and Gaussian peak function. In samples PS1, PS2 and PS3 good fits were obtained using a single Gaussian peak. The fitting parameters (peak position, peak strength and peak width) were extracted from the fits. On the other hand, in

PS4 a single peak was not suﬃcient to achieve a good ﬁt. Instead, I used two peaks

(either Lorentz or Gaussian) in order to achieve good ﬁts. My analysis revealed that peculiar temperature dependent shift of the PL peak can be explained as originating from the superposition of two independent peaks: one that corresponds to the PL peak observed in the other PS samples (PS1, PS2 and PS3), and the second peak which is most likely due to impurities. The intensity of this second peak increases dramatically at low temperatures, which creates the appearance that the position of the peak shifts to lower wavelengths.

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