Synthesis and characterization of novel terpolymers for applications in enhanced oil recovery and carbon nanotubes dispersion

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UNIVERSITÀ DI PISA

Corso di Laurea Magistrale in Chimica Industriale

Curriculum Materiali

Classe: LM 71 (Science e Tecnologie della Chimica Industriale)

“Synthesis and characterization of novel terpolymers for

applications in enhanced oil recovery and carbon nanotubes

dispersion”

Supervisors:

Prof. Dr. Andrea Pucci

Prof. Dr. Francesco Picchioni

Candidate:

Federico Di Sacco

Examiner:

Dr. Giovanni Granucci

Academic Year 2016/2017

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SUMMARY

The everincreasing worldwide request of energy is becoming one of the most concerning issue of our modern age. As renewable resource has already proved unable to single-handedly resolve this problem, investigating new and more efficient way of recovering and producing non-renewable resources is of incumbent importance. Enhancing the recovery of fossil fuel from their sedimentary basins is one of the most promising way to achieve this purpose.

Enhanced oil recovery (EOR) methods has been investigated in the last fifty years and, as widely told by literature, using of chemicals has proved to be a good choice for this kind of applications. Specifically, polymeric water-soluble materials have draw a lot of attentions in the recent years. Macrosurfactans, i.e. polymers composed by block of hydrophobic and hydrophilic monomers, possess the ability to change the rheological proprieties of water making it better at acting as a displacing fluid thus improving the amounts of oil recovered from each process.

Moreover, recent developments in polymerization methods, allows to precisely tune and control the characteristic and the structure of macromolecules which ultimately permit to better investigate the relationship between structure and property.

In this research project, we aimed to synthesize polymer composed by styrene, glycidyl methacrylate and hydrolysable tert-butyl methacrylate in different composition by means of Atom Transfer Radical Polymerization (ATRP), to eventually obtain novel macrosurfactans materials. Characterization and kinetics studies allow to determine the best experimental conditions for the process investigated. Afterwards, functionalization of the glycidyl epoxide moiety has been done with two different amines; notably, pyrene derivative was tested to confer fluorescence features to the prepared polymers. Thus, it would be possible to utilize pyrene as a fluorescent probe to study the polymer’s aggregation in solution or, more interestingly, to evaluate his behaviour during a simulated laboratory EOR process. For this application, rheology of the hydrolysed

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polymer was tested to determine their promising use for EOR applications. At the same time, GMA functionalized by pyrene, allowed to investigate the dispersing behavior of the polymers towards nanostructured conductive fillers such as carbon nanotubes. Solution mixing was utilized for the preparation of the nanocomposites containing different amounts of the graphitic filler to eventually endow electrical condictivity. In this regard, dispersion of pyrene-based polymer and carbon nanotubes were finally tested as resistive sensors to volatile organic compound (VOC).

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INDEX

Summary……….3

1

.

Introduction

………..8

1.1. The renewable and non-renewable resources issue……….…..….8

1.1.2 Present Status of oil productio

……….11

1.1.3 Crude oil: reserves and formation

.………....13

1.2 Extraction techniques for oil recovery……….14

1.2.1 Primary Recovery

……….……….14

1.2.2 Secondary Recovery

……….15

1.2.3 Tertiary recovery (or Enhanced Oil Recovery)

………15

1.3 Factors effecting the recovery………16

1.3.1 Influences on the microscopic displacement efficiency (Eps)

………..17

1.3.2 Influences on macroscopic sweep efficiency (Es)

………..………..18

1.4 Chemical EOR processes……….20

1.4.1 Surfactant flooding

………20

1.4.2 Polymer flooding: polysoaps and macrosurfactans

……….………22

1.5 Example of macrosurfactans for EOR……….…27

1.6 Polymer viscoelasticity……….…28

1.7 Atom Transfer Radical Polymerization (ATRP)………30

1.7.1 Glycidyl methacrylate: a versatile functional monomer

……….…..32

1.8 Composite materials and fillers………..………..33

1.8.1 Carbon materials as conductive filler

……….34

1.9 Carbon Nanotubes………..…35

1.9.1 Synthesis technique of carbon nanotubes

………..…36

1.9.2 Carbon nanotubes proprieties: Mechanical, Thermal, Electrical, Optical

……38

1.9.3 Exfoliation of CNTs bundles

………39

2.

Experimental Section

……….…42

2.1 Materials………...42

2.2 Methods……….43

2.2.1 CuCl, CuBr catalyst regeneration

……….…………43

2.2.2 Synthesis via ATRP of Polystyrene macroinitiator (PS-Br)

……….……44

2.2.3 Synthesis via ATRP of PS-b-(GMA-r-tBMA)

………..45

2.2.4 Kinetic Experiments

………..……47

2.2.5 Functionalization of the PS-b-(GMA-r-tBMA) terpolymer (TP)

………...47

2.2.6 1-Pyrenemethylamine recovery

………..48

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2.2.8 Functionalization with 1-AMP

………..49

2.2.9 Hydrolysis of PS-b-(GMA-r-tBMA) and TP-AMP

………..51

2.2.10 Hydrolysis with TFA in DCM solvent

……….51

2.2.11 Hydrolysis with HCl in 1,4-Dioxane solvent

………...52

2.2.12 Neutralization of the hydrolysed polymers

………53

2.2.13 Dissolution and neutralization

………53

2.2.14 Dialysis

……….54

2.2.15 Preparation of the hydrolysed polymers solution for rheology measurements

………55

2.2.16 Multi walled carbon nanotubes dispersion in AMP-functionalized polymers by non-covalent interactions

……….55

2.2.17 Mixing of AMP-functionalized terpolymer with MWCNT C-150-P in organic solvent

………..55

2.2.18 Recovery and centrifugation

………56

2.2.19 Solution casting of the MWCNT-AMPTP dispersion

………..57

2.2.20 Experimental setup for electrical resistance variation

………..57

2.2.22 Experimental setup for VOC exposition test

……….58

2.2.23 VOC exposition: kinetic measurement

……….59

2.2.24 VOC exposition: concentration measurement

……….59

2.3 Instruments………..60

2.3.1 Proton Nuclear Magnetic Resonance (1_H-NMR)

_{……….…60}

2.3.2 Fourier Transform Infrared Spectroscopy (FT-IR)

………..60

2.3.3 UV-Vis Absorbance Spectroscopy

……….60

2.3.4 Fluorescence spectroscopy

……….61

2.3.5 Thermogravimetric Analysis (TGA)

………..…61

2.3.6 Differential Scanning Calometry (DSC)

………..62

2.3.7 Rheology Measurement

……….……….62

2.3.8 Gel Permeation chromatography (GPC)

……….…..63

2.3.9 Scanning Electron Microscopy (SEM) for dispersion analysis

………63

3. Results and discussion

……….64

3.1 Poly(styrene) macroinitiator: synthesis via ATRP and

characterization………...64

3.2 PS-b-(tBMA-r-GMA): synthesis and characterization………67

3.2.1 Kinetic Measurement

………...75

3.3 Functionalization with 2-phenylethylamine (PEA)………..80

3.4 Functionalization with 1-AMP………..….83

3.4.1 Characterization of AMP-functionalized polymer by fluorescence emission

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spectroscopy in solution

………..87

3.4.2 Quantum Yield calculation

………..……88

3.5 Hydrolysis of TP and AMP-functionalized polymers………..88

3.5.1 Neutralization with Na2CO3 and dialysis

………..….92

3.6 Rheological measurement………..………....93

3.6.1 Viscosity versus Shear rate

……….…93

3.6.2 Oscillation Frequency sweep

……….…97

3.6.3 Viscosity versus Temperature

……….100

3.7 Thermal characterization – TGA and DSC……….……..101

3.7.1 Thermogravimetric analysis

………..………….101

3.7.2 Differential Scanning Calorimetry analysis

………102

3.8 Carbon nanotubes dispersion……….……….104

3.8.1 MWCNTs dispersion by solution mixing

……….………104

3.8.2 Determination of CNTs intake in the dispersion

……….………...107

3.8.3 Scanning electron microscopy (SEM)

……….……..…..110

3.8.4 Percolation threshold calculation

……….…111

3.8.5 Volatile organic compounds (VOCs) exposure experiments

………..…112

4. Conclusion

……….…120

5. Biography

………..123

6. Acknowledgments

……….…………..132

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1. Introduction

1.1. The renewable and non-renewable resources issue

Since 1950 the worldwide energy consumption has grown substantially following the development of new technologies and the overall prosperity increase, coming mainly from the developing economies (1). While before those years the global energy markets were driven by use of the so called non-renewable energy resources, like coal, oil and natural gas, we assisted to a slow shifting to the “greener” renewable energies (hydropower, solar power, wind power, geothermal power and biomass) due to the growing concern about greenhouse gases (GHG) emissions, which are directly connected to climate change and environmental problems (2). This eventually leads to the 2017 Paris Agreement (COP 21), aimed at strengthening the global response to this threat by bringing all nation under this common cause. In addition, is it known that reservoirs of non-renewable energies are finite and thus alternative way of producing energy has been intensively investigated (2) (3). Notwithstanding it is recognized that shifting the energy production to alternative energy sources is necessary, these have not

Figure 1.1.1: World energy consumption as million tonnes of oil equivalent over the past 25 years. Oil and coal are, by a large margin, still the most consumed fuels worldwide (from ref. (1))

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yet proved to be capable of meeting the total world energy demand. For this reason, a mixture of different non-renewable and renewable sources is still widely required (4). Generally renewable energies are not adaptable to every single nation or community because of the distribution of natural resources, that has dependency on geographical locations, and the actual extension of land needed to build the related infrastructure (5). As shown in Figure 1.1.1, fossil fuel is still the leading resource in worldwide energy usage and, to date, is not possible to proceed with his complete replacement by any alternative energy. In fact, today fossil fuel supply more than one third of the world’s energy and this value is estimated to grow up in near future. According to a recent statistical study (1), total global reserves of fossil fuel are about now: Coal – 1,2 billion

tonnes, Natural Gas – 187 trillion cubic meters, Crude Oil – 1,7 billion barrels. At today’s

rate of extraction, is estimated that the proved reserves will be exhausted as shown in Figure 1.1.2. Thus, is of mandatory importance to keep developing novels technologies and techniques to improve the efficiency of recovery and production of these fuels (1).

Figure 1.1.2. Estimated years of extraction remaining for crude oil, natural gas and coal (from ref. (1))

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It is also important to remember that in the past many attempted to predict when we will run out of fossil fuel, with the most well-known example in the Hubbert’s Peak Theory (and the so call Hubbert Curve, shown in Figure 1.1.3), without success (6).

Figure 1.1.3: The Hubbert Curve (red line) and the actual U.S crude oil production as millions of barrels per day in the last century. In the last 15 years, the U.S oil production is rapidly growing, clearly not following the Hubbert's prediction of constant decrease. (from ref. (7))

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1.1.2 Present status of oil production

Global oil production is nearly equally distributed across the world. The Middle East is the world’s largest oil producer, with almost 35% of the total. Europe, Eurasia and North America consistently remained the same over the last 50 years, accounting 20% of total production, and Asia, Africa e Latin America all producing between 8-9 percent. By country, the United States is the largest oil producer, leading with almost one-fifth of the global production, closely followed by Saudi Arabia and Russia (7). On the per capita basis, the energy consumption has declined in the early 1970s due to the “oil crisis”, with his peak in 1973 with the oil embargo of the Organization of Petroleum Exporting Countries (OPEC) and then rose from 1980s to 2000s, led by the increasing energy use of new developing economies. Then, from 2000s to nowadays, the per capita consumption moved to a decreasing trend, mainly thanks to the technological improvements (3). Moving to current days, in 2015 the oil market shown a strong growth in the OPEC production which outweighed both the demand and the non-OPEC production, bringing to lower prices. On the other hands, in 2016 the oil demand grown again solidly, with the production growing one-quarter to what seen the year before. This grew in demand in almost entirely due to importers like India and China, and notably the oil demand was more pronounced for consumer fuels, for example gasoline, in contrast with more industrial used fuel. like diesel, which show a slowdown trend for the first time since 2009 (1). As shown in Figure 1.1.2.1, the thousand barrels per day (KBPD) of crude oil of United States, are keep growing since the early 2000s with a rough 10’000 KBPD in 2016. Worldwide we are producing around 87’000 KPBD or 32’000 KPBD per years. That means each year the industry must find about two times the volume of oil stored in the North Sea (usually used as a benchmark for pricing) just to replace the drained reserves. Keeping in mind that the industry cannot guarantee that new reserves would be discovered each year, a solution lies in sustaining production from existing fields (7) (8). For this reason, most oil companies are focusing on maximizing the “recovery factor” (the recoverable amount of hydrocarbon initially in place (9)) instead of searching for new oilfields, which is becoming of increasingly difficult due to different

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reasons. First the sedimentary basins that could contain the oil are usually already being discovered and exploited. Secondly the yet undiscovered basins are in remote and unfriendly world’s areas. In addition, the push for unconventional hydrocarbons (like viscous oil, oil shales and gas hydrates) suffers from technological and environmental problems that are not ready solvable in near future (10) (11). The average recovery factor in a matures oilfields is usually somewhere in between 20% and 40%, which is far lower from a typical gas fields (80-90%). Improving the amount of initial oil recoverable from a single field could more than double the time for which the oil is available.

Figure 1.1.2.1: U.S production of crude oi as KBPDl over the past 130 years: a great increase in production took place since the early 2000s. (from ref: (6))

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1.1.3 Crude oil: reserves and formation

The term crude oil refers to a mixture of hydrocarbons existing as liquid in underground geologic formations which remains in the liquid state when brought to the surface. From the processing of crude oil, petroleum products are obtained. Anyway, petroleum is commonly intended as a broad category that includes both crude oil and petroleum

products. Crude oils are found in large underground deposits (called pools) in

sedimentary basins (the largest in the world is in Saudi Arabia, in the Ghawar region, with around 250 km long, 30 km wide and 90 m thick (12). Because of his liquid state nature, the oil occupies the tiny pore space within the rocks strata, which are typically

sandstones or carbonates.

Around 22 out of 32 billion of barrels produced each year comes from sandstone reservoirs. For this reason, those fields are expected to end the production in around 20 years from now. While the probable amount left in the carbonates reserves has 80 or more years of production left (13). Totally combined, sandstone and carbonates reservoirs have around 3000 billion barrels of remaining oil, and further analysis shown that, of the 60% of the world’s total oil left, a substantial 40% is hold in carbonates fields. Carbonate hydrocarbons typically form by precipitation of calcium carbonate from seawater. As the water is removed due to evaporation, the carbonate is formed and start depositing on the seabed. Remains of marine life (like fish or shellfish) start to be trapped inside this deposit and accumulate over long time in layers which grow at a rate of around half millimetre per year. Under the increasing weight of the carbonates, the layers are constantly forces to go deeper in the earth’s crust, eventually reaching the hotter parts where the carbonates are “cooked” and the oil formed. Unluckily the carbonates brittleness causes major problems to recovery, in fact, under tectonic pressure, fractures are formed and consequent corridors for oil make extremely difficult to locate their position (13).

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1.2 Extraction techniques for oil recovery

Depending on the producing life of a reservoir, the oil recovery can be divided in 3 main phases: primary, secondary and tertiary (8).

1.2.1 Primary recovery

With primary recovery is intended the process which takes advantage of the natural energy, initially stored in the reservoir, to extract the oil itself. No external use or injection fluid and heat is needed. The natural reservoir energy, such gasdrive, waterdrive or gravity, move the oil up to the surface, this is due to the large pressure difference separating the reservoir and the wellbore (known as the point where the drilling is done) which drive the oil toward the surface. Going on with production lead to a reduction in the pressure differential and thus a less efficient recovery. To rise back the differential to high value, an artificial lift (usually a rod pump) system is used as it reduces the wellbore pression. The use of these artificial lift is considered a primary recovery. This technique become not affordable when the pressure differential is too low, so too much energy is required for the process.

After primary recovery, only around 10% of the oil in place is effectively recovered, for this reason a second process is needed.

1.2.2 Secondary recovery

Secondary recovery is recovery by injection of external fluid (call displacing fluid), such as water and/or gas, into the reservoir through some injection wells in the rock

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connected with the production well. The purpose here is to maintain the pressure up and pushing the hydrocarbons toward the production well. Most common secondary recovery techniques are gas e water injection (or waterflooding), both are used to force the oil to sweep from the reservoir. This phase reaches his limit when the amount of fluid displaced is so high that is no longer economical.

This second process allow to extract around 15-40% of the original oil in place. Hence a third step is taken to further increase the recovery (14).

1.2.3 Tertiary recovery (or Enhanced Oil Recovery)

This third stage is characterized by the injection of special fluids such as chemical, miscible gases and/or use of thermal energy. Even if this process usually follows the first and second recovery, nowadays the term “tertiary”, or Enhanced Oil recovery (EOR), is not restricted to any phase of the production (15). Enhanced Oil recovery refers to any process that try to change the in-situ interaction existing between the rocks and the oil. Here, some examples of EOR processes are reported:

• Thermal recovery: hot fluid injection, hot water or steam drive, cyclic steam injection/flood (16).

• Miscible flooding: CO2, N2, flue gas, hydrocarbon and solvent flooding • Microbial

• Chemical flooding: polymer, surfactant, alkaline, emulsion, foam flooding, and the combination of those.

These processes are used in dependence of the type of oil that need to be extracted. For lighter oil polymer flooding (with or without surfactants), miscible gas injection is preferred. Conversely, for heavy oils steam or water injection, leading to in-situ combustion is preferred. Several processes can surely be combined: for example, it is possible to add chemicals to thermal or miscible EOR (17). For the aim of this work, this introduction will be mainly focused in the EOR by chemical flooding.

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1.3 Factors effecting the recovery

As stated before, the purpose of additional recovery operations is to get the highest possible amount of oil out of the fields via increasing the RF (recovery factor) value. Currently water flooding (secondary recovery) is the cheapest and favourite way to improve the recovery and, for those reasons, is used as a reference for theoretical analysis. The factors affecting RF for water flooding follow this relationship (18):

𝑅𝐹 = 𝐸𝑝𝑠 𝑥 𝐸𝑠 𝑥 𝐸𝑝 𝑥 𝐸𝑐 (𝐸𝑞. 1)

where RF is the recovery factor, defined as the volume of oil recovered over the volume of oil initially in place (OIIP). Eps is the microscopic displacement efficiency, which is related to the part of oil moved from the rocks pores by the displacing fluid. Es is the

macroscopic sweep efficiency or the part of reservoir volume that has been swept by the

displacing fluid (mainly function of the rocks morphology/heterogeneity). Ed is the

connected volume factor, showing the part of total reservoir volume connected to the

extraction wells (bad infrastructure sealings or permeability barriers may cause some part of the oil to not be in communication with the rest of the reservoir, lowering the efficiency). Ec is the economic efficiency factor represent the commercial component of the fields, like capacity, stored reservoir energy and lifetime.

It is important to note that each of these factors need to be the closest possible to 100% to hopefully achieve a high RF value, in fact just a drop to 80% by each factor would cause a substantial fall of RF to around 40%. EOR methods aim to maximise the Eps and

Es factors, while Ep and Ec are usually improve by the so called Improved Oil Recovery

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1.3.1 Influences on the microscopic displacement efficiency (Eps)

Oil usually become trapped inside the pores by capillary effects and the relative mobility of the oil-water phase through these pores is driven by the permeability of the rocks. The capillary numbers summaries those effects, as we see in Eq.2:

𝐶𝑎 =

𝑣𝜇

𝛾

(𝐸𝑞. 2)

Where

v

is the interstitial velocity (or Darcy velocity),

μ

is the fluid viscosity and

γ

is the interfacial tension (IFT) between the displaced and displacing fluid. When Ca is very low (10-5_{) flow is greatly affected by capillary forces and trapping of the oil in the pores}

is likely to happen. For a typical water flooding experiment, the Ca is nearly 2.5x10-7 _so improvement must be made. While is generally not possible to apply a very high-pressure gradient to increase the interstitial velocity, is viable to reduce the IFT or increase the viscosity.

Both capillary and permeability effects are considerably affected by the wettability of the rocks in which the oil is placed. If the rocks are “water wet” there is often a higher residual oil saturation as the water covers the rock’s surface, pushing small oil droplets inside the pores. This residual oil is accounted to be permanently trapped inside the pores. Otherwise, if the rocks are “oil wet” the residual oil is much lower because the surface is continuously covered by the oil, letting the water flow over it and thus avoiding droplets trapping.

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1.3.2 Influences on macroscopic sweep efficiency (Es)

Macroscopic sweep efficiency is principally affected by the morphology and heterogeneity of the rocks in which the oil is displaced. Permeability is the most important factor with dependence over size, number, distribution and thus connectivity of the pores networks inside the rocks. Apparently, this factor is directly a function of the type of sedimentary materials which form the rock itself and of the chemical modifications and cementation followed over long periods of time. For those reasons sedimentary rocks are very often characterized by differences in permeability that scales from millimetres to kilometres. In a very heterogeneous system (Figure 1.3.2.1), water will flow preferentially through high permeability layers, entirely bypassing the oil trapped in the lower permeability zones. This would result in very low RF values. Unluckily, predictions over the “distribution” of permeability are still very complex and only uncertain statistics approaches are possible (19).

The negative effect of this heterogeneity is enhanced if the displacing fluid has a much lower viscosity than the oil (and even more if the fluid is a gas) (20). This effect is taken in consideration by use of the mobility factor M, which allows to compare the mobility of the displacing and displaced fluid inside the porous media. An equation (Eq. 3), derived from the Darcy’s one, is used to describe this behaviour (21):

𝑀 =

𝜇𝑆𝑘𝑟𝐷(𝑆𝑜𝑟)

𝜇𝐷𝑘𝑟𝐷𝑆(𝑆𝑤𝑐)

(𝐸𝑞. 3)

Figure 1.3.2.1: Examples of types of geological heterogeneities encountered in a reservoir. Each colour represents a zone of different permeability. Is important to note the extension of these zones, covering more than two hundred meters in length and around 6 meters in deepness. (from ref. (10))

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Where

𝑘𝑟𝐷(𝑆𝑜𝑟)

is the relative permeability of the porous medium to the displacing phase at the residual oil saturation

𝑆𝑜𝑟,

while 𝑘𝑟𝐷𝑆(𝑆𝑤𝑐) is the relative permeability of the oil to the displacing phase at the immovable water saturation

𝑆𝑤𝑐.

Finally,

μ

dis the

viscosity of the displacing fluid which is usually the ruling factor inside the equation. If M is way higher than 1, so the displacing fluid viscosity is quite low, an unstable viscous fingering is likely to happen (14), as shown in Figure 1.3.2.2.

Figure 1.3.2.2: An illustration of the fingering effect for an inefficient oil recovery process: water alone possess lower viscosity than oil, then it moves over the oil surface instead of displacing it with his movement. (from ref. (22))

This phenomenon causes the displacing fluid to slip over the interface with the oil, rather than have a proper contact and thus transportation (23). To avoid this effect a reduction of mobility difference between the two fluids is needed. The displacing phase must have a mobility equal or lower than the oil’s one, giving a M value equal to 1 or slightly less. When M is around these value, the movement of the oil will happen with a piston-like way (24) hence improving the recovery. As reported above, mobility is inversely proportional to viscosity so modify this propriety in the injected fluid is likely to boost the oil recovery.

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1.4 Chemical EOR processes

Chemical flooding refers to the addition of chemicals to water. This has been an option for EOR since the 60s, and the early processes using polymers alone were quickly implemented by adding surfactants. Alkalis were also added afterwards to reduce the absorption of the chemical by the rocks (25) (26). Depending on the process, the addition of chemicals may have two different purposes: reducing the IFT between displacing fluid and oil or, as stated before, to increase the displacing fluid viscosity to modify the mobility ratio.

1.4.1 Surfactant flooding

To fit the IFT reduction, usually surfactants alone or mixed with alkali are used (8) (23) . Surfactants are composed by a long hydrophobic hydrocarbon chain and a polar hydrophilic group, allowing this compound to exhibit solubility in both water and organic solvents. They concentrate at the separation surface between two immiscible (or partially miscible) fluids and change the surface proprieties, especially the IFT. According to the chemical nature of the hydrophobic group, surfactants are classified in four groups: anionic, cationic, non-ionic and zwitterionic (27). Different types of surfactants are used according to the nature of the rocks in the basin to minimize their absorption over the rock’s surface. Anionic surfactants find great usage in sandstone rocks, which have a negative charged surface. On the other hands, cationic surfactants are generally preferred for carbonate rocks deposits. Non-ionic surfactants are not employed alone because they have lower ability to reduce IFT compared to anionic or cationic, but can be mixed to improve the overall effect (in fact, they exhibit better resistance to high salinity level (28)). Zwitterionic surfactants possess two active groups which can be

non-21

ionic-anionic, non-ionic-cationic and anionic-cationic, allowing good resistance to high temperature and salinity, but they are still expensive to develop substantial use in EOR (29). Most common surfactants used in EOR are sulfonated

hydrocarbons (Figure 1.4.1.2). They show thermal stability above 200°C and tolerate to low value of pH, but they are sensitive to divalent ions presence, which can cause degradation (28). On the other hand, the corresponding sulfate derivatives possess greater tolerance to divalent ions (mainly Mg2+ _{and Ca}2+_{, which can cause precipitation and} thus pores blocking) but cannot resist temperature higher than 100°C and experience hydrolysis phenomena at acidic pH, and for these reasons sulfonated surfactants are preferred (8). Recent studies (30) state that adding

ethylene oxide/propylene oxide units to the chemical structure of the surfactant can lower the IFT value via formation of hydrogen bonding with the water. Other research (31) showed that adding ethoxy unit into the surfactants structures can lead to increased tolerance to divalent ions and salts in general.

Surfactants are characterized by the hydrophile-lipophile balance or HLB. This number indicated the tendency of the compound to solubilize in oil or water and so the ability to form water-in-oil or oil-in-water emulsions. A low HLB value is assigned to surfactants more soluble in oil then likely to form water-in-oil emulsion, otherwise high HLB value indicate a more hydrophilic behaviour causing oil-in-water emulsion. Normally HLB values can shift between 0 (corresponding to a completely hydrophobic molecule) and 20 (corresponding, instead, to a molecule fully made by hydrophilic parts), this can be used to predict the surfactants proprieties thus making quicker to select the proper compound for every application. The value itself is calculated by a simple equation which takes in consideration the number of hydrophilic/hydrophobic groups and their “strongness” (directly related to a specific number) (32).

During the flooding, surfactant retention is the most important factor concerning the quality of the recovery. Retention can happen by precipitation, phase trapping or

Figure 1.4.1.2: Example of alkylarylsulfonates

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absorption of the surfactants but, because these are difficult to be accounted separately, they are usually reported as total surfactants loss without specifying from which mechanisms the losses come from. Precipitation is caused by divalent ions such Ca2+ _{and Mg}2+ _{that forms divalent complex in solution with low solubility in water.} Chelating agents, like phosphates, are used to limit the precipitation. Phase trapping mechanisms in complex but can be summarised as a mechanical trapping of the surfactants molecules due to formation of immiscible microemulsion inside the oil-water main emulsion (33) . Adsorption of the surfactants depends on number and volume of pores, surface area of the rocks, molecular weight of the surfactant. Higher molecular weight generally causes higher adsorption, considering all other parameters the same (concentration, type of rocks, temperature, pH and so on) (34). As stated before, anionic surfactants are good candidates for applications in sandstone fields, as they show less adsorption compared to non-ionic ones which, on the other hand, are better for carbonate fields.

In surfactants flooding two separate mechanisms of displacement can happen, called dilute flooding and micellar flooding (35). In both mechanisms the residual oil droplets inside the rocks are basically emulsified by the solution, because of the low IFT, and pushed through out the pores. Along the way they eventually coalesce to form an oil bank on top of the surfactant flow. Other specific details will not be treated here.

1.4.2 Polymer flooding: polysoaps and polymeric surfactants

A displacing fluid with viscosity near to the oil one can achieve a better mobility ratio thus improving the recovery. Using high molecular, water soluble compound to viscosity the water has been highly investigated in the past twenty years. When dissolved in

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water, the solution has a viscosity that depends on the polymer concentration, polymer molecular weight, temperature and salinity. Compared to low molecular weight surfactants, polymeric ones offer great structural complexity that allow to achieve very different and modulable proprieties. Polymeric surfactants can be divided in macrosurfactans and polysoaps (as shown in Figure 1.4.2.1), in dependence of their structure.

Polysoaps are defined as polymer with intrinsically amphiphilic monomer or with a random copolymer of hydrophilic and hydrophobic monomer. They are characterized by intramolecular iterations in solution. On the contrary, macrosurfactans are composed by hydrophilic and hydrophobic blocks which are spatially separated from each other. This allow them to exhibit intermolecular iterations in solution (36).

In polysoaps, intramolecular associations are due to number of surfactant-like hydrophobic interactions randomly distributed along the polymer chains, then forming unimeric micelles at low concentration. This similarity with conventional low molecular weight surfactants give rise to two important features: high solubilization capacity for hydrophobic molecules and low viscosity of aqueous solutions due to hydrophobic aggregation (leading to a very small hydrodynamic radius) (37). Again, like low molecular weight surfactants, polysoaps often have charged functional groups along the chain causing them to be called polyelectrolytes (if negative or positive charge are present in Figure 1.4.2.1: A schematic summary of polymeric surfacants classification: polysoaps and

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the monomers), polyampholytes (if both type of charges are present simultaneously in different monomers composing the chain) or polyzwitterions (when the same monomer hold both anionic and cationic charge) (38). The molecular structure of a polysoaps can be described as a combination of a polymer backbone to which a surfactants structure is attached. The overall structure, and thus proprieties, can be varied by modifying

different part of the molecule like the nature and geometry of the polymer backbone and the distance and type of surfactants used. For example, the surfactants can be attached to the backbone from the hydrophilic head or from the hydrophobic tail (Figure 1.4.2.2) or it can be fully included into the polymer backbone. By varying the chemical composition of the polymer is possible to achieve different density and position of the polysoaps side chains, leading to different behaviour in solution (37). In addition, side chains (called spacer) can be link between surfactants and backbone to modify their distance, causing increased mobility of the overall molecule (39). Despite of the architecture, almost all polysoaps are characterized by low viscosity in water solution (a key feature that differs them from macrosurfactans). Moreover, the viscosity of polysoaps solutions is subjected to ageing and temperature effects (40). Surface activity possess substantial difference compare to low molecular weight surfactants: while for those a CMC (critical micellar concentration, a critical value of concentration after which all micelles are formed and further surfactants is just added inside the micelles) is defined and always present as the concentration of surfactants rises, polysoaps can act differently. In some case, no IFT reduction is shown as well as a CMC, while in other occasion the IFT decreases with rising concentration but no sign of CMC is resembled.

Figure 1.4.2.2: Geometries of polysoaps: the black circle is the hydrophilic head while the segmented line is the hydrophobic chain. Various types of connection to the polymer's backbone are shown (from ref. (37) )

25

Even if the missing of an actual CMC is in accordance with the intramolecular hydrophobic aggregation behaviour, still no report can clearly explain the mechanisms behind this phenomenon (41). Usually polysoaps are synthesized via well establish free radical polymerization or polycondensation methods (37).

Besides polysoaps, water-soluble polymer can be used in application for EOR. They differentiate from macrosurfactans as they interaction with water usually do not involve formation of “classical” micellas but follow

intermolecular interaction between nerby chains. The most studied polymer in this regard is polyacrylamide (PAM) or one of his derivate, the partially hydrolysed polyacrylamide (HPAM). PAM is often used as a reference material for chemical modifications when a EOR process is tested (36), indeed many authors modify the chemical structure of PAM trying to achieve better proprieties, as temperature resistance, shear resistance along with better compatibility with water (42). Although both systems show high shear and but low temperature resistance (below 100°C), they can show shear thinning or shear thickening behaviour as a function of different shear rates (24).

Another polymer studied for EOR is Xanthan gum, a polysaccharide produced by bacteria fermentation of glucose and fructose (43). His thickening capability is mainly due to his high molecular weight (more than 106 _{g/mol). Xanthan has ability to resist} high shear forces and, instead of PAM/HPAM, show resistance to higher temperature. On the other hand, this polymer is susceptible to bacterial degradation.

Macrosurfactans can be linear, grafted, star or branched blocks copolymers (Figure 1.4.2.3) where a distinct separation between anionic and cationic charges is permitted. In fact, even in condition of dilution, these polymers produce intermolecular aggregation in water solution causing some major change in the solution rheology and

26

surface activity. Recently,

amphiphilic blocks copolymers have received great interest in many fields of applications (44) also thanks to the tuneable architectures that is possible to achieve with novels controlled polymerization techniques, like

Atom Transfer Radical

Polymerisation (ATRP) that will be

illustrate in further chapter of this work (36) (24). These materials exhibit in water a particular rheological behaviour (45) when the hydrophilic blocks are polyelectrolytes: they aggregate in solutions forming a rigid hydrophobic core which is surrounded by a very stretched hydrophilic “corona”. This is caused by a combination of Columbic interactions and osmotic effects that causes the formation of dense gel in water, showing non-Newtonian behaviour. The gel formation is explained considering that, in dilute solutions, all micelles are isolated and not interacting (called “frozen” micelles). With increasing concentration of polymers, micellas start to interact with each other and finally an “overlapping” concentration is reached. At this point, is supposed that stretched coronas shrinks as the overall volume between their chains rapidly diminish. Eventually, after a critical volume fraction is passed a sol-gel transition occurs, with the expulsion of internal water and formation of the gelified network (Figure 1.4.2.4) (46).

Figure 1.4.2.4: Micellas overlapping phenomenon: As the micellas concentration rises (from left to right sqare in the image), finally a critical volume fraction is passed (last square) and a sol-gel transition takes place. (from ref: (36))

Figure 1.4.2.3: Examples of different molecular architecture: the white dots are the hydrophilic monomers and black ones are hydrophobic monomers. (from ref: (36))

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Despite the high numbers of architectures that can be synthesized (some examples are shown in Figure 9), only a few types have been studied (47). In the next section some of the most used polymers for EOR will be illustrated along with their proprieties.

1.5 Example of macrosurfactans for EOR

The use of macrosurfactans for enhanced oil recovery can be attractive because, differently from low molecular weight surfactants and polysoaps, both IFT reduction and viscosity increase can be achieved simultaneously using a single component, instead of mixing two or more.

The most common hydrophobic blocks for such systems are based on polystyrene, polyacrylates, polyolefins and non-water-soluble polyethers. Otherwise hydrophilic blocks can be made by positively or negatively charged monomers, usually modified acrylates bearing carboxylic, sulfonated or amino groups. In addition, neutral blocks can be used such as poly(ethylene glycol) or 2-hydroxyethyl methacrylate or PEGylated acrylic monomers. According to literature, acrylic and methacrylic acid blocks are preferred as hydrophilic portion in macrosurfactans thanks to the high reactivity of the carboxylic groups, that strongly modify they hydrophilicity as the degree of protonation (see pH) change in solution, in addition to their pronounced thickening capability in water. In general, the higher degree of hydrolysis is reached along the chains, the more pronounce is the thickening behaviour (48). However, acrylic and methacrylic acid polymerization using controlled polymerization (49) like ATRP is still very challenging due to the reactivity of the acid charge with the catalytic systems employed. Hence the best approach is to utilize the corresponding ester as a monomer (usually tert-butyl is preferred) and proceed with a post-polymerization hydrolysis, eventually yielding the free acid group. Typical conditions are acidic, with hydrochloric acid in 1,4-dioxane or trifluoroacetic acid in dichloromethane (50).

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1.6 Polymer viscoelasticity

Viscoelasticity is the property of a material to exhibit both viscous and elastic characteristics when undergoing some type of deformation. For a simple viscous fluid, the viscous behaviour is described by the followingequation (called Newton’s law) (Eq. 4):

𝜏 = 𝜇𝛾 (𝐸𝑞. 4)

Where τ is the stress, μ is the viscosity and γ̇ is the shear rate. When viscosity is not a constant a non-Newtonian fluid is present (in comparison with Newtonian fluid, which show constant viscosity with applied stress). However, when an elastic material is subjected to a stress it tends to return to his original configuration. Then if shear stress is applied to an ideal solid, Hooke’s law is valid (Eq. 5):

𝜏 = 𝐺

′

𝛾 (𝐸𝑞. 5)

Where G’ is the elastic modulus and γ is the registered strain. Eq.4 and Eq.5 are each other analogous for viscous and elastic material respectively. For a Newtonian fluid, the shear stress is proportional to the shear rate, while in a Hookean solid, it is proportional to the strain. Therefore, a fluid that shows both viscous and elastic behaviour must be described by an equation including both Newton’s and Hooke’s law, then the following relation (Eq. 6) is adopted:

𝜏

𝜇

+

1

𝐺

′

(

𝑑𝜏

𝑑𝑡

) = 𝛾 (𝐸𝑞. 6)

This relationship is known as Maxwell model and assume that a viscous damper (that follows Eq.4 model) and a spring (instead following Eq.5 model) are connected in series. Here a limiting behaviour is present: when the applied shear is constant over time, 𝑑𝜏

𝑑𝑡 tends to zero, then resembling the Newton’s law for a simple viscous fluid. Otherwise,

29

when

𝜏

is changed quickly over time, the second factor in the equation is negligible, leading to the Hooke’s law of a solid.

The principle of a shear viscosity measurement is to let a fluid to flow between two parallel plates, which one of them is moving at a certain constant rate. Knowing some parameters (like velocity of the moving plate, distance between them and the direction of the force to which the stationary plate is subjected) is possible to calculate the shear rate and then the shear viscosity of the fluid. This type of measurement is known as steady shear flow.

Anyway, for EOR application, more information can be found in a non-steady shear flow measurement (or dynamic oscillatory rheological measurement): here a sinusoidal movement is imposed to the moving plate, producing a non-continuous, multidirectional motion. According to previous Eq.4 and Eq.5 a viscoelastic fluid has two components depending on applied stress, γ̇ and γ respectively, that yield a two components stress response with different phase, meaning that some part of the energy absorbed is stored elastically, while another part is dissipated and lost (these two phenomena resemble what happens to energy in an elastic solid and a viscous fluid respectively (51). Then two moduli are defined: a storage modulus G’ and a loss modulus G’’. The first defines information about the fluid’s elasticity structure, while the second gives information about the viscous property. Then two more equations are specified (52), taking in consideration that one stress response is in-phase with the mechanical movement (defining G’) while the other is out-of-phase with the movement (defining G’’). Eq. 7 and Eq. 8 show the relationship between the viscosities of the elastic and viscous “portion” of the viscoelastic material:

𝜇′ = 𝐺′⁄ (𝐸𝑞. 7) _𝜔 𝜇′′ _{= 𝐺′}′

𝜔

⁄ (𝐸𝑞. 8)

Polymer solutions usually exhibit viscous behaviour when going through, for examples, small tubes of constant diameter, but when moving inside media with changing size and

30

shape, like pores in rocks, pulling and contractions of polymer chains can lead to an elastic behaviour thus increasing viscosity (53).

1.7 Atom Transfer Radical Polymerization (ATRP)

ATRP represents today the most robust and commercially available methods to conduct a controlled radical polymerization. The possibility to build up polymers in a controlled fashion (or piece-by-piece fashion) (54) allows to synthesize a large variety of polymeric structures with very fine tuning of molecular weight value and distribution. Before the discovery of ATRP, it was difficult to provide specific and uniform properties to macromolecules because of the fast (then “uncontrolled”) way monomers tends to build up onto the growing chains, like during typical radical polymerization methods. ATRP

fortunes lies in his catalyst action mechanisms that adds one monomers at a time to a growing polymer chain. This slower, thus controlled process, can also be shut down or re-started at will changing reaction parameters, like temperature. For all these reasons, ATRP is almost every time the choose polymerization technique for producing

Figure 1.7.1: A scheme of the ATRP reaction equilibrium: growing/dorment are rapidly activated/deactivated during the process (font. Matyjaszewski Polymer Group, website:.cmu.edu/maty/chem/fundamentals-atrp)

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macrosurfactans and, in general, for all materials that need a very well define structure (55).

ATRP mechanisms is principally based on a dynamic equilibrium between a propagating specie and a dormant specie (Figure 1.7.1), which is in the form of initiating alkyl halides/macromolecular specie (R-X). The halide atom is transferred by the catalyst complex, here the transition metal atom is subjected to a redox cycle thus adding/removing one atom of halide each reactive step. Periodically the dormant specie reacts with the rate constant of activation (keact) with the transition metal complexes on their lower oxidation state (acting as activator), Mtm_{/L (in the above figure) activating} the dormant specie to growing radicals (R•). Otherwise, the higher transition metal complexes act as deactivators of the growing chain by transferring back the halide atom, by reduction to the lower oxidation state. This reverse process is guided by a deactivation constant (Kdeact). If this equilibrium is shifted to the dormant species, so a high value of Kdeact is present compared to keact, a growth via “intermittent activation of dormant species” is conceivable, thus all chains will grow with the same rate and in the same moment, yielding to very low polydispersity index and allowing a fine control on the molecular weight (55).

The catalytic process can be mediated by many redox-active transition metal complexes: most used metal is Cu (50), but also Ru, Fe, Mo, Os has been intensively studied (56). Ligands are usually alkylated mono-, di- or tri- amine exhibiting different chelating nature. For a common Cu complex, the activity in ATRP is the highest for a tetradentate ligand, and diminishes going to a tridentate and then bidentate. Also, the complex geometry is important: cyclic-bridged and branched always have better activity than a linear geometry. Among the most active there are tris(2-pyridylmethyl)amine (Me6TREN) and tris(2-pyridylmethyl)amine (TPMA), while the less active are pyridineimine and 2,2’-bipyridine. In general, alkyl amine forms stronger complex with Cu2+ _{leading to higher reactivity (57). In this work} N,N,N′,N′′,N′′-pentamethyldiethylenetriamine (PMDETA) was used as ligand as several studies demonstrate its effectiveness in ATRP of acrylic and methacrylic monomers (45) (58) (54). The ATRP reactive process is always initialized by alkyl halides, working as an

32

initiator. This alkyl halides possess a reactive halogen atom that undergoes homolytic bond cleavage, inducing formation of a radical into a monomer thus producing the very first growing chain. Tertiary alkyl halides are the most reactive ones and radical stabilization by phenyl or ester groups enhance the overall reactivity. Ethyl α-bromophenylacetate is considered the most active initiator, followed by 1-phenylethyl bromide (PEBr) and methyl 2-bromopropionate (BMP). Lastly, alkyl iodides are more reactive than bromides, that are more reactive than chlorides as well. Is important to remember that the most reactive catalyst complex and initiator are not always the best options for an ATRP synthesis, for example when low degrees of polymerization are aimed. Finally, solvent polarity usually greatly increases the rate of ATRP, by stabilization of the more polar M2+ _species.

1.7.1 Glycidyl methacrylate: a versatile functional monomer

Thanks to the development of improved polymerization methods such as ATRP, a wide range of new monomers can be efficiently used, one of those is Glycidyl Methacrylate (GMA) (Figure 12). GMA has recently drawn researcher attention in the fields of polymer science and

biochemistry (59). Various homopolymers and copolymers of GMA can be synthesized using controlled polymerization methods (ATRP, RAFT), in addiction easy pre- or post- modification of the monomer are possible. Different studies have focused on post-modification via ring opening reaction with thiols, alcohols (aliphatic or aromatic) and amines (60) (61). In addition, fluorophores like pyrene, coumarin, or rhodamine B have been successfully used to provide GMA with photoluminescent proprieties (62). This variety of functionalization possibilities allows GMA-containing polymers to exhibit highly tunable structures. Recent works, has demonstrate the possible use of GMA derivatives as carriers for drugs and biomolecules delivery.

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To our knowledge, the use of GMA in polymeric surfactants for EOR has not been investigated yet. Via post-polymerization modification of the epoxide functionalities new proprieties can be conferred to the macrosurfactans with possible improvements in in oil recovery applications. In this work, a GMA-based macrosurfactans has been modified via post-polymerization ring opening reaction with two aromatic amines, including 1-pyrenemethylamine, trying to obtain a novel “emitting” displacing fluid.

1.8 Composite materials and fillers

Polymers proprieties can be modified by either changing their chemical structure, via different synthetic routes or functionalization, or by adding low molecular weight compounds called additives (63). These compounds can sort different effects: colorant, for example, can achieve new optical proprieties, while stabilizer can improve resistance to temperature and light irradiation. More interestingly, fillers refer to a class of additives defined by their low cost and by some notable characteristics. Fillers are usually added to polymers to achieve better mechanical proprieties and, noteworthy, to allow electrical and thermal conductivity (64). Here, the main issue is that a homogeneous dispersion of the fillers is always needed to obtain a good performance improvement.

The mechanisms on which conduction is allowed into a polymeric material, is called percolation. A polymeric composite material is made by an insulating host (called matrix, the polymer itself) in which a dispersion of micro- or nano- sized particles is present. Increasing the intake of dispersed particles into the matrix eventually reach a critical value, called percolation threshold, where a continuous path for the current’s passage is permitted. Here is supposed that a dense, connected network of fillers particles is present, that grants conductivity by letting electrons flow from particle to particle throughout the matrix bulk. After this critical point, the conductivity tends to increase as more filler is added into the matrix but finally a saturation level is pass, and

34

conductivity do not increase anymore (65). The percolation threshold value, reported as % of filler intake, depends on the shape and dimension: usually spherical fillers (such as carbon black) gives around a 10% value, while for tubular/needle-like fillers like carbon nanotubes lower values (around 1-3%) are expected. In general, the higher the shape ratio (ratio between two dimensions of a filler) the lower the percolation threshold. The percolation network can be altered if, after some sorts of stimuli, the fillers increase or diminish the numbers of conductive pathways along the polymeric matrix. In this work the effects of volatile organic compounds (VOC) on the conductive behaviour of carbon nanotubes composite has been tested.

1.8.1 Carbon materials as conductive filler

Carbon materials are found in variety of forms such as carbon fibres, graphite, diamond, fullerenes and carbon nanotubes (66), thanks to the many types of hybridization that carbon atom can undergoes. Thanks to their ability to improve mechanical properties, carbon fibres have been widely studied since the early 19th _{century. Important progress} in carbon fibres science follows the discovery of a controlled process to synthesize growing carbon fibres with specific characteristics and low cost (67). Then, after fullerene discovery (68), helical microtubes of graphitic carbon were discovered (69), leading to an exponential interest in carbon material science in years to come.

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1.9 Carbon Nanotubes

Carbon nanotubes are cylindrical

carbon molecules with very

interesting properties. Thanks to their thermal, mechanical and electrical properties, they are the most utilized fillers in polymer science (70), also for the realization of smart plastic materials such as sensors.

They present a long, hollow structure composed by a circular

folded, one atom thick, sheet of graphitic carbon. Different orientation in the folding of the circular graphitic sheet change his chirality. Then a chiral vector is defined, its two components (m,n) and the angle between them cause the nanotubes to assume distinct conformations called armchair, zig-zag and chiral. Different chirality (Figure 1.9.2) shows different electrical behaviour (armchair similar to metal, while zig-zag and chiral similar Figure 1.9.1: Types of CNT's: single walled are composed by a single wrapped graphitic sheet. Multi walled, on the other hand, are made by multiple concentric sheet folded one inside the other (from ref: (118))

36

to semiconductors) (71). Another carbon nanotubes classification depends on the number of sheets involved in the formation of tubular structure. Then Single-, Double- and Multi- Walled Carbon Nanotubes are present (known as SWCNTs, DWCNTs and MWCNTS respectively). Most common structures are MWCNTs and SWCNTs. The first class is composed by multiple concentric graphene sheet (Figure 1.9.1) held together by van der Walls forces, induced by π-stacking interaction. Their diameters range from 0.6 to around 50 nm. In addition, MWCNTs can show or not their terminal ends capped by carbon hexagons or pentagons (71). SWCNTs are made by only one wrapped graphene sheet (Figure 13) with a diameter of 1.2-1.5 nm. Both classes exhibit one dimension exceeding the nanometric scale: MWCNTs length is around 1-10 µm, whereas for SWCNTs is around 5-25 µm.

According to these data, MWCNTs have a shape ratio ranging from 100 to 1000, while for SWCNTs it ranges from 10’000 to 100’000. As stated before, shape ratio is one of the most important feature leading to improved electrical and mechanical behaviour.

1.9.1 Synthesis technique of carbon nanotubes

Most common techniques for CNTs synthesis are (72): electric arc, laser vaporization and chemical vapour deposition (CVD) (Figure 1.9.1.1a, 1c, 1b, respectively). For all techniques, a source of carbon is needed, while a catalyst can be use or not depending of the types of nanotubes needed (72). The electric arc vaporizes the carbon source (usually a graphite block working as positive electrode) at around 3000°C, forming a plasma that deposit as a carbon layer on the negative electrode. The operation take place at low pressure and an inert atmosphere of He. For this particular technique, a catalyst is needed only for the SWCNTs synthesis, usually Co, Fe, Ni, Cu, Y and Gd are used in metallic or oxide form (73). Laser vaporization basically works with analogous

37

mechanisms (72). Anyway, both processes produce very inhomogeneous nanotubes with low yield but at low cost.

Today, chemical vapour deposition is the leading synthetic procedure. A carbon volatile source (methane or other low boiling hydrocarbons) is pyrolyzed and then deposit over nanoparticles of a supported metal catalyst which promote a homogeneous growth of carbon nanotubes. Deposition for MWCTNs take place at 700°C, while higher temperature of 1100°C is needed for SWCNTs. The catalyst is usually Fe supported over alumina or silicon, but also MgO and CaCO3 have been recently investigated as supporting materials (73). The success of CVD method lies in its versatility, which permits different carbon sources to be used, and its continuous nature allowing a much better fit for industrial applications.

After the synthetic process, purification is required to remove metallic or carbonaceous impurities that could persist from the synthetic procedure (74). Notably, physical or chemical approaches are possible. Chemical approaches involve gas phase (air or O2) oxidation reaction at temperature ranging from 225°C to 760°C, followed by chemical oxidation with strong acid (H2SO4 and HNO3) at 100°C (to pushing the removal of metallic particle, resistant to the first oxidation). Drawback of this second procedure, is that can cause destroying of the CNTs, with cutting and openings. Physical approaches are based on microfiltration with solvent, aided by ultrasonication, that help separating

38

compounds with low aspect ratio (such as fullerenes or polyaromatic carbons) from CNTs (72).

1.9.2 Carbon nanotubes proprieties: Mechanical, Thermal, Electrical,

Optical

Carbon nanotube possess excellent mechanical properties, for this reason they are the most used reinforcement filler for polymeric materials. Their covalent sp2 _bonds between carbons cause them to display the highest elastic modulus and tensile strength ever measured in a material (75). This behaviour is possibly due to weak, but several, interaction in the radial direction between adjacent tubes, leading to a great reduction in tensile strength value (76).

Electrical properties depend on the chirality of the nanotubes, as stated before in the chapter. The nanotubes exhibit metallic behaviour when in armchair conformation (so the two components defining the chiral vector are equal, n = m). It is semiconducting in the zig-zag conformation (when n – m is multiple of 3), showing the characteristic band gap. Finally, chiral nanotubes show a semiconductor behaviour (72). CNTs conduction happens thanks to the electron tunnelling effect. When the length of the conductors is smaller than the mean free path of the electrons, the conduction process becomes ballistic. In a normal conductor the existence of phonons, that represent vibrational modes in electrons, lead to decreasing conductivity as the temperature rises. This is caused by the increasing phonons vibration that extend the electrons scatter over the conductor itself thus heating the material. In CNTs, the ballistic transport allows electrons to flow without any interactions during their movement, causing no heating in the material. This is caused by the non-interaction with the phonons, avoiding vibrations and then heating.

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Carbon nanotubes can be readily characterised by spectroscopy technique, allowing to determine their quality in terms of chirality, dimension and, most important, structural defects that can greatly influence the electrical and mechanical behaviour. On the application side, the optical proprieties of CNSs are still of little use for industry, compared to the well-known mechanical and electrical features. Recent studies show that light-emitting diode (LED) can be produce using SWCNTs (77) or even memory storage devices still based on a single CTNs sheet (78).

Carbon nanotubes are excellent anisotropic thermal conductors (79) thanks to their nanoscale size and shape ratio. They show great heat conduction along the tubes direction, but they act as thermal insulator on their transversal axis.

1.9.3 Exfoliation of CNTs bundles

Figure 1.9.3.1: Covalent and non-covalent modification of CNTs: the first method provides the formation of a chemical bond with the nanotubes surface; the second, instead, use only weak force to interact (thus keeping intact the nanotubes surface (from font: (80))

In the nanoscale dimension, attractive forces between single tubes ere predominant, causing presence of strongly aggregated tubes, called bundles (Figure 1.9.3.2) (70). This phenomenon in of mandatory importance when a composite material is produced. As stated before, the homogeneous dispersion of the fillers within the polymeric matrix allows to maximise the property enhancement that CNTs can provide. For example, if

40

electrical conductivity is achieved by adding CNTs, the percolation threshold could be high if the selected intake of nanotubes is not properly exfoliated, being single tubes sticked together in a poorly percolative and unconnected pathways. Moreover, bundles can become “fragile” point into the composite structure, eventually worsening mechanical performances.

A necessary step is the separation (or exfoliation) of nanotubes bundles by physical (kinetic) methods. Most used is ultrasonication, which permits to destroy the π-π stacking interactions between aromatics rings hence separating the tubes. Of course, this separation is not permanent, so a proper (thermodynamic) stabilization is needed. Two ways exist to stabilize thermodynamically CNTs: the first one is a non-covalent stabilization that adopt low energy interaction to keep separated the tubes from each other, like others π- π stacking interaction or using surfactants or specific polymers (81) (82). Notably, in this work, pyrene dye moieties have been used to test the ability of the synthesized polymers to stabilize dispersion of carbon nanotubes, as already tested in literature on various other systems (81) (83) (82) (84). Second way to stabilize the exfoliated carbon nanotubes is the covalent modification of the π bond of the C=C repeating units in the CNT structure. Many types of covalent modification have been investigated through the years including, electrophilic and radical addition,

Figure 1.9.3.2: CNTs image at SEM: (a) a bundles of nanotubes, (b) efficently exfoliated nanotubes (font. (121))

41

cycloaddition and metal-based reduction (85). However, non-covalent

stabilization/functionalization is more attractive because no modification of the sp2 structure are induced thus leaving intact the exceptional proprieties discussed in the previous chapters.

.