Università degli studi di Pisa
Dipartimento di Chimica e Chimica Industriale
Corso di Laurea Magistrale in Chimica Industriale, curriculum Materiali
S
YNTHESIS OF AMPHIPHILIC CO
-
AND TERPOLYMERS
FOR ENHANCED OIL RECOVERY APPLICATIONS AND
STUDY OF THEIR SOLUTION PROPERTIES
Supervisors: Candidate:
Prof. Valter Castelvetro
Federico Lo Moro
Prof. Francesco Picchioni
Examiner:
Prof. Luca Bernazzani
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INDICE
1 Introduction ... 4
1.1The renewable and non-renewable resources issue ... 4
1.2 Oil, a limited resource ... 6
1.3 Extraction techniques for oil recovery... 7
1.4 Factors affecting the polymer flooding recovery ... 9
1.4.1 The macroscopic recovery efficiency ... 9
1.4.2 The microscopic recovery efficiency ... 10
1.5 Amphiphilic block copolymers properties and application in EOR ... 11
1.5.1 Structure ... 11
1.5.2 Amphiphilic block copolymers self-assembling ability ... 13
1.5.3 Polymer flooding in EOR with macrosurfactants ... 15
1.6 Synthetic methods used to prepare amphiphilic block copolymers ... 17
1.6.1 Controlled/living radical polymerization (CRP) ... 17
1.6.2 reversible addition-fragmentation chain transfer polymerization (RAFT) .. 18
1.6.3 Synthesis of block copolymers via sequential RAFT polymerization ... 21
2 Aim of the thesis ... 24
3 Results and discussion ... 26
3.1 Synthesis and characterization of the ptBMA, pGMA and p(GMA-r-tBMA) RAFT macroinitiators ... 26
3.2 Synthesis and characterization of the ptBMA-b-pGMA and pGMA-b-ptBMA copolymers ... 34
3.3 Synthesis and characterization of styrene co- and terpolymers ... 37
3.4 Hydrolysis of the polymers ... 41
3.5 Neutralization of the hydrolysed polymers ... 44
3.6 Thermal characterization – TGA ... 45
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3.8 Rheological measurements ... 48
3.9 Solution self-assembly ... 52
3.9.1 Dynamic light scattering (DLS) ... 52
3.9.2 Surface tension measurements ... 54
3.10 Zeta potential ... 56
4 Conclusions ... 62
5 Experimental section ... 65
5.1 Reactants and solvents ... 65
5.2 Synthesis of macro-RAFT agents ... 67
5.2.1 Synthesis of the ptBMA macro-RAFT agent ... 67
5.2.2 Synthesis of the pGMA macro-RAFT agent ... 68
5.2.3 Synthesis of the p(GMA-r-tBMA) macro-RAFT agent ... 69
5.3 Synthesis of diblock copolymers ... 70
5.3.1 Synthesis of the ptBMA-b-pGMA and pGMA-b-ptBMA copolymers ... 70
5.3.2 Synthesis of the ptBMA-b-pSty and pGMA-b-pSty copolymers ... 71
5.4 Synthesis of terpolymers ... 73
5.4.1 Synthesis of the ptBMA-b-pGMA-b-pSty, pGMA-b-ptBMA-b-pSty and p(GMA-r-tBMA)-b-pSty terpolymers ... 73
5.5 Hydrolysis of the polymers ... 75
5.6 Neutralization of the hydrolysed polymers ... 76
5.7 Solution casting of aqueous solution of neutralized pMAA87-b-pSty24 for zeta potential analysis ... 76
5.8 Product characterization ... 77
5.8.1 Proton Nuclear Magnetic Resonance (H1-NMR) ... 77
5.8.2 Gel Permeation chromatography (GPC) ... 77
5.8.3 Fourier Transform Infrared Spectroscopy (FT-IR) ... 77
5.8.4 Rheology measurements ... 78
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5.8.6 Surface tension measurements ... 78
5.8.7 Thermogravimetric Analysis (TGA) ... 79
5.8.8 Zeta potential measurement ... 79
4
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 economies1. While before those years the use of the so called non-renewable energy resources, like coal, oil and natural gas led the global energy markets, in the last 20 years we have assisted to a slow shifting to the “greener” renewable energies (hydropower, solar power, geothermal power, wind power and biomass) due to the growing concern about greenhouse gases (GHG) emissions, which are directly connected to climate change and related environmental problems 2. The agreement negotiated in Paris by the United Nations Framework Convention on Climate Change (UNFCCC) 3 in 2017, aimed at strengthening the global response to this threat by significantly reducing emissions of greenhouse gases (mainly CO2) generated by human activity worldwide in a relatively short time. In addition, it is known that reservoirs of non-renewable energies are finite and thus alternative ways of producing energy power has been intensively investigated 2.
Although it is recognized that shifting the energy production to alternative energy sources is necessary, these have not yet proved to be capable of sustainably meeting the total world energy demand. Generally, renewable energies offer environmental benefits that are still difficult to monetise, are subject to a strong variability and intermittence related to their nature, they fit into a framework of strong, resisting economic and geo-political interests and in addition to be possibly less competitive in terms of the actual extension of land needed to build the related power generation infrastructure 4.
5
Figure 1.1 – Global primary energy consumption, measured in terawatt-hours over the past two century
As shown in figure 1.1, fossil fuel is still the leading resource in worldwide energy usage and it is not possible to proceed with its complete and swift replacement by any alternative energy source. In fact, today fossil fuels supply more than two third of the world’s energy. According to a recent statistical study 5, oil is the most used fuel in the energy mix. Coal is the second one but it is losing share contributing for 27% as of 2018, the lowest value in 15 years. On the other hand, the share of natural gas increased to 24%. The contribution of hydroelectric and nuclear power remained relatively flat in 2018 at 7% and 5%, respectively. A strong growth has pushed the increase in the share of renewable energy to 4%, immediately below nuclear power.
While awaiting the widespread use of clean energy, a mixture of different non-renewable and renewable sources is still widely required and increasing the oil production as opposed to more pollution intensive fossil sources such as coal and shale oil is a way of meeting the needs of an ever-increasing energy demand.
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1.2
Oil, a limited resource
Oil is a limited resource and in the future the supply of oil will be exhausted, when there will be no new reservoirs to be discovered and the existing ones will be no longer exploitable with the current technology. Several theories, the first rigorous one being proposed by geophysicist Hubbert as early as in 1956, have predicted the existence of a “peak oil” production 6,7. According to the theory, the current increase in oil production will reach a maximum, followed from that time on by a steady decline, until full depletion. This follows the evolution of every oil reservoir, which generally experiences a rise, peak, decline and depletion.
Figure 1.2 – Hubbert's hypothesis of peak oil production in the United States, alongside actual oil production trends in the United States, both measured in barrels per year
However, the quantity of petroleum remaining in the Earth’s crust and how soon this resource will begin to run out is a matter of considerable debate and disagreement.
That is mainly due to the following reasons:
• there are no international standards for reporting the total amount of oil resources, that include proven reserves, undiscovered resources and resources which may be potentially recoverable in the future if a suitable technology becomes available;
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• the development of new technologies or improvement of existing ones has led increasing efficiency in the exploitation of conventional oil sources.
About the latter, using older technologies, oil companies generally can only retrieve about 40% of the oil in place; with enhanced technologies companies have increased that amount and with new techniques, still under development, it is believed that it is possible to extract up to 65% of the oil in the field.
These techniques usually go under the collective term of enhanced oil recovery (EOR).
1.3
Extraction techniques for oil recovery
The conventional techniques for extracting oil out of a reservoir consist of primary and secondary methods which recover at most 55% of the original oil in place (OOIP). In practice, the recovery is typically much lower and varies between 20-40% 8.
With primary recovery is intended the process which takes advantage of the natural energy, initially stored in the reservoir itself, to extract the oil. No external use of injection fluid and heat is needed. The natural reservoir energy, such as gasdrive, waterdrive or gravity, move the oil up to the surface, due to the large pressure difference separating the reservoir and the wellbore (known as the point where the drilling is done) which drives the oil toward the surface. The most efficient mechanism is represented by the pressure that is exerted on the oil by the aquifer as the driving force 9.
After primary recovery, only around 10% of the oil in place is effectively recovered in most cases; for this reason, a second process is needed.
Over the lifetime of the well, the pressure decreases, and at some point, there will be insufficient underground pressure to force the oil at the surface. The secondary methods involve the injection of an external fluid (called displacing fluid), such as gas or water, where the increased pressure drives the oil out of the reservoir 9. This phase reaches its limit when the amount of displacing fluid is so high that its use is no longer economical.
Together, primary and secondary recovery allow 20 – 40% of the reservoir's oil to be recovered. Thus, a third step is taken to further increase the recovery 10.
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In the last few decades, many different methods have been developed to increase the oil recovery. These methods belong to the category Improved Oil Recovery (IOR) which also includes operational strategies, such as infill drilling, horizontal wells and intelligent reservoir management 11. A subset of IOR called Enhanced Oil Recovery (EOR) is more specific and implies the injection of special fluids such as chemicals, miscible gases and/or use of thermal energy.
Enhanced Oil recovery refers to any process that try to 12:
• improve oil sweep efficiency by reducing the mobility ratio between injected and in-place fluids;
• eliminate or reduce capillary and interfacial forces and thus improve displacement efficiency;
• act on both phenomena simultaneously.
These aspects will be better discussed in the next paragraph.
EOR processes are traditionally divided into three groups:
1. thermal processes (injection of steam, in situ combustion);
2. processes with gas injection (natural gas, nitrogen, carbon dioxide);
3. chemical processes (injection of aqueous solutions of surfactant, of polymers, of alkaline solutions).
These processes are used in dependence of the type of oil that needs to be extracted 11. For lighter oil (> 31,1°API) polymer flooding (with or without surfactants) or miscible gas injection is preferred. Conversely, for heavy oil (≤ 20° API 13), steam injection, leading to in-situ combustion, is preferred.
For the aim of this work, this introduction will be mainly focused in the EOR by polymer flooding.
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1.4
Factors affecting the polymer flooding recovery
The overall oil recovery efficiency in oil production processes is generally divided in two distinct contributions: macroscopic and microscopic recovery efficiency. The macroscopic recovery efficiency refers to the volume that has been swept by the flooding agents and it is also referred to as volumetric sweep efficiency. The microscopic recovery efficiency is related to the part of oil moved from the rocks pores by the displacing fluid and is a measure of the its effectiveness.
In other words, any mechanism that can improve either macroscale or microscale oil recovery efficiency is beneficial for increasing oil production 14.
The former mechanisms (at the macroscale) are associated with permeability heterogeneities and a viscous ratio called the mobility ratio. The latter mechanisms, the retention mechanisms involve several interfacial effects that are generically associated with the rock interactions that lead to either oil trapping or retention. The competing forces and mechanisms can be summarized by the capillary number.
1.4.1
The macroscopic recovery efficiency
When a viscous fluid such as crude oil is displaced by a less viscous displacing phase, such as water, viscous fingering (see figure 1.3) tends to occur with a consequent reduction in macroscopic sweep efficiency 15. This phenomenon causes the displacing fluid to slip over the interface with the oil, rather than have a proper contact and thus resulting in an efficient transportation.
10
The value of adding polymer to a conventional waterflood can be explained by the mobility ratio M which is defined by the following relationship (Eq. 1):
𝑀 =
𝑘𝑤𝜂𝑤 𝜂𝑜
𝑘𝑜
(Eq. 1)
Where kw and ko are the permeability of the porous media to water and oil and ηw and ηo are
the viscosity of the water and oil, respectively.
The displacing phase should have mobility comparable or lower than the mobility of the oil phase. When the water/oil mobility ratio is 1 or slightly less, the displacement of the oil by the water phase will occur in a piston-like fashion 16. By contrast, if M is greater than 1, the more mobile water phase will finger through the oil, causing a breakthrough and poor recovery. Because the mobility is inversely proportional to the viscosity, the main mechanism of increased oil recovery achieved by polymer flooding is the increase of water viscosity, in order to achieve better mobility control.
1.4.2
The microscopic recovery efficiency
The wettability, or relative ability of one fluid to wet a solid surface (pore surface) in the presence of a second one, determines the arrangement of the fluids in the pore space. If water wets a rock preferentially in the presence of hydrocarbon, then it will tend to sit on the solid surface while the hydrocarbon will sit in the inner portion of the pore space. In theory, this trapping mechanism is responsible for the limited efficiency of water as a displacing agent, which leads to the concept of residual oil saturation or immobile oil fraction in the rock. The higher this saturation value (the more oil that stays trapped in the rock), the lower the recovery of oil 17.
The dimensionless ratio between the viscous-to-capillary forces is called the Capillary number (Ca). This parameter is related to the residual oil saturation in porous media, and thus it is very important for EOR applications.
11 The Capillary number is defined by equation 2:
C
a=
vμγ
(Eq. 2)
Where 𝑣 is the interstitial velocity (or Darcy velocity), μ is the fluid viscosity and γ is the interfacial tension between the displaced and displacing fluid. Other formulations of Ca have been proposed that consider parameters such as porosity and contact angle, but this form is the most used because of its simplicity.
The capillary number reflects the ratio between two competing forces: the viscous drag of the fluid (𝑣μ) over the interfacial contribution given by interfacial tension (γ). 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, Ca is in the order of magnitude of 10-8 - 10-718, so improvement is necessary.
Besides decreasing the mobility ratio, use of polymeric surfactants in polymer flooding should also lower the interfacial tension and therefore tackle the capillary forces problem. Several EOR methods have been devised with the goal of overcoming the capillary forces, which retain a high amount of residual oil in pore network of the reservoir 19.
1.5
Amphiphilic block copolymers properties and application in
EOR
1.5.1 Structure
A copolymer is the result of the polymerisation of two or more different monomers which under certain conditions bind covalently together in the same polymer chain. Depending on the conditions under which the polymerisation reaction takes place, several copolymers may be obtained which differ in their structure; taking into account, for simplicity, the case of the polymerisation of only two monomers, there are three basic types of copolymers:
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• Statistical copolymers, in which the distribution of the two types of structural units along the chain is entirely random
• Alternating copolymers, with alternating regular arrangement of the two structural units
• Block copolymers, in which structural units of the same type form long sequences, from which structural units of the other type are excluded
The main difference between statistical or alternating copolymers and block copolymers is that the former are chemically homogeneous materials, while the latter, because of the incompatibility between the two blocks, are subjected to phase separation at a microscopic level, resulting in two-phase materials: the covalent bond between the blocks effectively prevents separation at a macroscopic level.
Regarding the latter, the miscibility between the two blocks is strongly influenced by the flexibility (or rigidity) of each block. Two main types of copolymers can be identified:
• Copolymers made of flexible blocks
• Copolymers made of a rigid block and a flexible one
Block copolymers with blocks of different rigidity are very interesting materials in several applications 20 because, for example, they can manifest both shock resistance properties, imparted by the flexible block and tensile strength as well as thermal stability ones, due to the rigid block. More generally, by appropriately modulating the chemical-physical characteristics of the constituent blocks it is possible to create materials that combine in a synergistic way the specifications of the two distinct components.
Amphiphilic is a term that defines a substance with a hydrophilic polar portion (lipophobic) and hydrophobic apolar one (lipophilic), which can self-organize into well-defined aggregates in dilute aqueous solution or interact at the interface between the aqueous and
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non-aqueous phases of a two-phase system. Even if none of the two phases is water, the term amphiphilic can still be used to refer to systems with an organophilic portion and an organophobic portion 21.
Furthermore, amphiphilic block copolymers may lower the surface tension of water depending on the nature of the blocks. Amphiphilic block copolymers can thus be considered as the macromolecular counterparts of small-molecule surfactants and are therefore called macrosurfactants 18, 22.
1.5.2 Amphiphilic block copolymers self-assembling ability
The simultaneous presence of hydrophilic and hydrophobic parts in the same macromolecule provides block copolymers with special properties. Among these properties, of particular interest is the self-assembling ability in micellar structures above a critical concentration value, called critical aggregation concentration (CAC) or, by analogy with low MW surfactants, critical micellar concentration (CMC). This is generally lower than the CMC of low molecular weight surfactants, even by orders of magnitude. When an AB-type amphiphilic diblock copolymer is dissolved in a selective solvent, which is thermodynamically a good solvent for one component and is a bad one for the other, chains can reversibly assemble within each other.
In aqueous solvent, above the CMC, the copolymer assembles in micelles, directing the hydrophilic block outwards, solvated by the aqueous solvent, and the hydrophobic block inwards, a process driven by the decrease of free energy (Figure 1.1) 23.
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Similar to low molecular weight surfactants, amphiphilic copolymers at low concentrations are freely soluble and tend to interpose at the air-water interface. Increasing the concentration both the interface and the bulk get saturated until, at the CMC, a further increase of the concentration leads to the formation of micellar aggregates (figure 1.5).
Figure 1.5 – Schematic representation of micelle formation
Spontaneous aggregation of amphiphilic block copolymers is also possible in organic solvent. In the case of micellization in aqueous environment the enthalpic and entropic contributions are both positive, and therefore spontaneous aggregation is a process driven by entropic driving force (the increase in entropy is due to the destruction of the ordered water layer with strong hydrogen bonds around the hydrophobic chains). In the organic solvent the polymer-solvent interactions are weaker, the entropy variation is generally slightly negative (because the possible conformations of the chains are smaller in the aggregates) and so micellization happens when the enthalpic contribution is sufficiently negative, meaning that the phenomenon is driven by enthalpy.
The introduction of a third block incompatible with the previous two, for example in linear terpolymers ABC 24, further increases the complexity of the possible geometries for the
single micelle (multicompartment micelle, MCM).
MCM can be roughly divided into two categories: those that contain two insoluble blocks 25 and therefore have a compartmentalized core and a homogeneous corona and those in which
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only one block is insoluble 26 and therefore the crown is compartmentalized and the nucleus is homogeneous 24.
Macromolecules, however, are prone to solubilization problems in a selective solvent, especially when they contain very large blocks of the insoluble component or have a high glass transition temperature. This problem can be overcome by the temporary use of a solvent which favours dissolution of the component that eventually will be insoluble in water. Thus, a distinction must be made between the micelles formed by spontaneous
equilibrium and those artificially induced.
In any case, the organization of amphiphilic copolymers in solution is an effective route to produce structures on a nanoscale 27.
1.5.3 Polymer flooding in EOR with macrosurfactants
In general, polymer flooding consists of injecting aqueous polymer solutions into a subterranean oil formation in order to improve the sweep efficiency in the reservoir. The increased viscosity of the water causes a better mobility control between the injected water and the hydrocarbons within the reservoir.
In recent decades amphiphilic block copolymers have received great interest due to their ability to self-assemble in stable micelles or other aggregates in a selective solvent, in most cases water. When the water solubility of at least one block is dependent on external parameters such as temperature or pH, they can also exhibit stimuli-responsive behaviour 28. Their characteristics allow controlling interfacial properties and fluid rheology making them good candidates for applications in several fields including smart materials 28, coatings 29, drug delivery 30,31 and Enhanced Oil Recovery 16,32. In this last case, we have seen that the viscosity of the displacing phase and the interfacial tension between the water phase and the oil influence the additional oil recovery from a reservoir by injection of water. Nowadays, for this reason, a combination of water soluble high molecular weight polymers (thickening agent) and surfactants 33 is often used in EOR. Amphiphilic block copolymers can affect both the rheology and the interfacial properties and therefore are a promising alternative 18.
16
We have seen that an important feature of amphiphilic block copolymers is their ability to self-assemble in water forming micellar aggregates, similarly to what happens with low-molecular weight amphiphilic molecules (a.k.a. surfactants). This behaviour of amphiphilic block copolymers in water result in very interesting interfacial and rheological properties: the former derives from the intrinsic molecular structure of the polymer, in particular the different solubility of the single blocks, while the latter arises from the inter-micellar interactions.
In general, the capability of a polymer of increasing the solution viscosity depends on several parameters such as the concentration, molecular weight and its conformation, also affected by the presence of charged or of hydrophobic groups. The increment of viscosity due to the concentration is attributed to a larger number of chains in a given volume, which interact with each other causing greater friction effects. Instead, according to the Mark-Houwink equation (Eq. 3), valid for dilute solutions, a higher molecular weight results in an increase of the hydrodynamic radius of the polymer coils and subsequently a higher viscosity:
[𝜂] = 𝐾M
α(Eq. 3)
Where [𝜂]is the intrinsic viscosity, M the molecular weight and K and α are parameters that depend on the particular polymer-solvent system. In particular, the value of α can vary from 0.5 for a polymer in a theta solvent, to 0.7 – 0.8 for a polymer in a good solvent, to values higher than one for polymers in extended conformation 9. In water this latter case is typical for polymers bearing charges along the backbones, also known as polyelectrolytes. The introduction of charged groups along the backbone of the polymer leads to electrostatic repulsions and osmotic effects, thus increasing the hydrodynamic volume, which results in a higher solution viscosity 34. This is not the case in the presence of a high concentration of salts (ions), which give a strong electrostatic screening effect and decrease the hydrodynamic radius. An increase in viscosity can also be achieved by the introduction of hydrophobic groups, which give either intra or intermolecular hydrophobic associations 35.
Amphiphilic block copolymers have the ability to form shear-dependent transient association in water, which results in increasing solution viscosity 18.
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1.6
Synthetic methods used to prepare amphiphilic block
copolymers
Amphiphilic block copolymers can be synthesized with a large variety of monomers, but the most studied are copolymers in which the hydrophilic block is constituted by polyethylene oxide (PEO) or polyethylene glycol (PEG), polyacrylic acid (PAA) and poly methacrylic acid (PMAA). In these copolymers the hydrophobic block is mainly constituted by polystyrene (PS) 36.
Besides the composition, the molecular architecture of block copolymers is also widely studied. A lot of different molecular architectures of block copolymers such as the AB diblock, ABA and ABC triblock and more complex graft/comb/brush/star structures can be synthesized by controlled radical polymerization and/or cliche chemistry techniques. These synthetic routes have provided a really solid basis to better study the effect of composition and molecular architecture on the properties of amphiphilic copolymers, given the possibility to prepare well-defined structures with almost no limitations in geometry and chemistry. Molecular weights, compositions, architectures and molecular weight distributions can be accurately controlled. This allows in turn systematically to study the structure-properties relationships and ultimately selecting the structures more suitable for a desired application.
1.6.1 Controlled/living radical polymerization (CRP)
Conventional free radical polymerization is no longer satisfactory for the new technological applications of polymeric materials because chemical structure control is poor. In order to obtain results that best fit chemical control requests on the macromolecular structure, it is now possible to use controlled/living radical polymerization (CRP) 37, in addition to the more traditional but also less versatile living ionic polymerization techniques. More generally and
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independently from the ionic or free radical nature of the propagating active center, a "living" 38 polymerization process is defined when:
• the polymerization kinetics is first order versus the monomer;
• the polymerization degree of the final polymer is directly proportional to the monomer/initiator ratio in the feed;
• molecular weight distributions with low polydispersities are obtained;
• it is possible to begin a second stage of polymerization, obtaining an extension of the degree of polymerization or the growth of a second block, starting from a preformed macromolecule with well defined “living” chain end.
For these characteristics, "living" polymerizations allow to obtain monodisperse polymers, with controlled molecular weight, functional groups and architecture. In addition, CRP polymerizations allow applicability to a greater number of monomers and solvents, greater tolerance to impurities and functional groups and less sophisticated experimental procedures than ionic polymerizations.
The development of such preparation methods is an industry-relevant objective 39.
One of the most popular CRP techniques is the reversible addition-fragmentation chain transfer polymerization (RAFT), which will be used in this thesis work.
1.6.2 reversible addition-fragmentation chain transfer polymerization
(RAFT)
The RAFT polymerization can be considered a particular case of degenerative transfer (DT) and it is based on the reversible action of a chain transfer agent (CTA, chain transfer agent), also called RAFT agent in this context, having a general structure of type Z(C=S)SR (figure 1.6), in which the double bond C=X represents the reactive part, R is a leaving group and Z can assume multiple structures.
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Figure 1.6 – General structure of a RAFT agent
The most common RAFT agents are aromatic and aliphatic dithioesters, trithiocarbonates and dithiocarbamates. These compounds are preferred in relation to their low cost and simple preparation.
Figure 1.7 – Equilibria of reversible addition-fragmentation chain transfer (RAFT) polymerization
In a typical RAFT polymerization, the first step is a conventional initiation by means of a free radical initiator that attacks the monomer, generating the propagating radical Pn ● that reversibly adds onto the RAFT agent 1 followed by fragmentation of the intermediate radical, forming a polymeric macroRAFT agent 2 and the expelled RAFT agent-derived radical (R●), which re-initiates polymerization until the next encounter with a RAFT agent, and so on. For the preparation of polymers of low dispersity R must be a good homolytic leaving group with respect to Pn and be able to reinitiate polymerization efficiently to prevent rate retardation. Re-initiation occurs when the radical leaving group R● adds onto monomer, initiating a new propagating radical chain (Pm ●) which then adds onto the macroRAFT agent 2. Rapid equilibrium is achieved between the active propagating radicals
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and the macroRAFT agent through the RAFT intermediate radical 4, ensuring that all the chains have an equal probability of growing, thus resulting in polymers of low dispersity. That means the rate of the activation/deactivation equilibrium (main equilibrium) reactions must be much greater than the rate of the propagation process, so that it can ideally be considered that the chain growth takes place by insertion of one monomer unit at a time. Polymerization stops due to the depletion of monomers, except for possible undesired termination reactions that can occur leading to the formation of dead chains (termination reaction, figure 1.7).
In the optimization of the reaction conditions for a RAFT polymerization, two stoichiometric ratios are the most critical factors:
[M]/[RAFT]: in an ideal controlled polymerization the amount of RAFT agent determines the number of polymer chains formed, thus the molar ratio of monomer to RAFT agent determines the theoretical number average degree of polymerization 𝑋𝑛(calc.) and theoretical number average molar mass 𝑀𝑛(calc.). 𝑀𝑛(calc.) is the theoretical mass that would be obtained in the absence of any undesired reactions, such as termination or irreversible chain transfer.
[RAFT]/[I]: the number of propagating chains that terminate irreversibly is somewhat related to the ratio [RAFT]/[I] and in particular to the concentration of the conventional initiator [I]. It follows that the number of irreversibly terminated chains can be predicted and adjusted by changing the initial concentration of the conventional initiator from which the primary radicals originate; in particular, it can be minimized by working at low ratio.
In general the theoretical molar mass, 𝑀𝑛(calc.), of a RAFT polymer can be calculated using equation 4 40,41:
𝑀𝑛(𝑐𝑎𝑙𝑐. ) =
([M]0−[M]t)
([RAFT]0+𝑑𝑓([I]0(1−𝑒𝑘𝑑𝑡))) 𝑥 𝑀𝑀+ 𝑀𝑅𝐴𝐹𝑇 (Eq. 4)
Where [𝑀]0 and [𝑀]𝑡 are the concentration of monomer at t = 0 and t = t; [𝑅𝐴𝐹𝑇]0 is the initial RAFT agent concentration; [𝐼]0 is the initial initiator concentration; [𝑀]𝑀 is the molar mass of the monomer; [𝑀]𝑅𝐴𝐹𝑇 is the molar mass of the RAFT agent; 𝑑𝑓([I]0(1 − 𝑒𝑘𝑑𝑡))
corresponds to the number of initiator-derived chains produced, where d is the average number of chains formed from each radical–radical termination event (determined by the relative prevalence of termination by combination or disproportionation), f is the initiator
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efficiency (f = rate of initiation of propagating radicals/rate of initiator derived radical formation), t is time in seconds and 𝑘𝑑 is the rate coefficient for initiator decomposition. For
a well-controlled RAFT polymerization, the number of initiator derived chains is low in comparison to [RAFT] and the contribution of the 𝑑𝑓([I]0(1 − 𝑒𝑘𝑑𝑡)) can be neglected,
giving the more commonly used expression for the theoretical molar mass, equation 5:
𝑀𝑛(𝑐𝑎𝑙𝑐. ) ≅
([M]0−[M]t)
([RAFT]0) 𝑥 𝑀𝑀+ 𝑀𝑅𝐴𝐹𝑇 (Eq. 5)
1.6.3 Synthesis of block copolymers via sequential RAFT polymerization
The incorporation of two (or more) monomers through sequential copolymerization is the simplest and most common method for the preparation of block copolymers using the RAFT process, with purification undertaken before each additional polymerization step (figure 1.8).
Figure 1.8 – Equilibria of reversible addition-fragmentation chain transfer (RAFT) polymerization
For the synthesis of a simple AB diblock copolymer the homopolymer of A, formed from an initial RAFT polymerization, acts as a macro-RAFT agent for chain growth with formation of block B in a second polymerization step. This results in the formation of the block copolymer. The control of the polymerization of each monomer that compose the block copolymer depends on the Z-group affecting the reactivity of the initial RAFT agent. If the Z group is either too effective or too ineffective in stabilizing the free radical generated on the RAFT agent or the adduct with either one of the two monomers some control over the properties of the final product will be lost. For example if the polymerization of the first monomer (monomer A) is poorly controlled, the generated macro-RAFT agent will have high molar mass dispersity; similarly if the macro-RAFT agent has low activity in the second polymerization step with monomer B, the second block will show high dispersity and the AB block copolymer will be contaminated with the homopolymer of B, due to inefficient chain transfer.
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During the synthesis of block copolymers by RAFT polymerization it is quite possible that a small number of defective macromolecular structures be formed. These include dead end homopolymer, dead end copolymer, initiator-derived homopolymer and block copolymer as illustrated in figure 1.9.
Figure 1.9 – The various polymer species formed during synthesis of block copolymers via sequential RAFT polymerization
Their presence can be minimized with careful selection of the appropriate RAFT agent and reaction conditions so that they do not affect the performance of the final product.
In any case, since the RAFT agent will be the same for both monomers in a block copolymer, in the preparation of well-defined block copolymers the synthetic strategy may be critical in addition to the appropriate RAFT agent selection 40:
1. the order of monomer addition 2. the effect of initiator concentration.
The order in which the monomers are incorporated into a block copolymer is essential as the first block serves as a macro-RAFT group during the polymerization of the second monomer. The macro-R group must be a good homolytic leaving group with respect to the propagating radical of the second monomer and must also efficiently reinitiate the polymerization of the second monomer. Due to steric stabilization, and hence better leaving group ability, monomers that produce highly stabilized tertiary propagating radicals (i.e. methacrylates, methacrylamides) should be polymerized prior to those that produce less stabilized
23
secondary propagating radicals (i.e. styrenes, acrylates, acrylamides). These should in turn be introduced before monomers which possess more reactive secondary propagating radicals (vinyl esters, vinylamides). As such it is evident that there are strict limitations in the range of block copolymers that can be prepared by sequential RAFT polymerization. The most effective sequence of comonomer addition for the successful synthesis of well-defined block copolymers (macro-R group selection) is illustrated in figure 1.10.
Figure 1.10 – Guidelines for selection of macro-R group for the preparation of block copolymers. Dashed lines indicate partial control over polymerization
The free radical initiator used to start the RAFT polymerization gives rise to two main types of defects that may be incorporated in the block copolymer structure obtained upon chain-extension of the macro-RAFT agent 42. The initiator concentration heavily influences the prevalence of these defects. In particular, a low concentration of free radicals from the initiator with respect to RAFT agent concentration reduces the incidence of these defects; however, a sufficiently high concentration of initiator is required to obtain acceptable rates of polymerization. The suitable range of appropriate initiator concentration depends on the nature of the monomer. The first of these defects is the generation of initiator-derived chains, rather than those with the R group from the RAFT agent in one of the polymer chain ends. The second type of defect arising from a too high concentration of the initiator is the formation of dead polymer dead chain through irreversible radical termination by either combination or disproportionation (see figure 1.9). Dead chains are unable to undergo chain extension and thus remain as impurities in the block copolymer. These usually represent 5-10% by weight of the final reaction product. In order to minimize their presence, the polymerizations are typically performed using relatively high [RAFT]/[initiator] ratios, between 5 and 10.
24
2 Aim of the thesis
The growing global energy needs is becoming one of the most concerning issue of our modern age. As renewable resources are not able to meet the worldwide request of energy yet, investigating new and more efficient ways of recovering and producing non-renewable resources is of incumbent importance. Enhancing the recovery of fossil fuel from those reserves that were partly already exploited is a way to achieve this purpose. Enhanced oil recovery (EOR) methods have received much attention in the last fifty years and using of chemicals has proved to be a good option for this kind of application. Specifically, macrosurfactans, i.e. polymers composed by hydrophilic and hydrophobic sequences, represent very attractive systems for these techniques, because they can increase water viscosity and decrease interfacial tension, both related to the efficiency of the process. Moreover, recent developments in polymerization methods allow to control the characteristic and the structure of such amphiphilic macromolecules, which permits to better investigate the relationship between structure and property.
The amphiphilic block co- and terpolymers examined in this thesis work then will consist of poly(tertbutyl methacrylate), poly(glycidyl methacrylate) and poly(styrene). They will be prepared by reversible addition-fragmentation chain transfer polymerization (RAFT) which allows tailoring of the polymer architecture such as the sequence and the relative lengths of the blocks, i.e the overall chemical composition of the polymer.
Subsequently, the polymers will be hydrolysed and neutralised (in order to obtain charged groups) with the ultimate goal of evaluating their behaviour during a simulated laboratory EOR process. In particular, rheology measurements will be performed to determine the polymer ability to change the rheological proprieties of water, making it a better displacing fluid, thus improving its potential as an auxiliary in EOR.
Regarding the solution properties, the ability of amphiphilic polymers to self-assemble in aqueous solution will be assessed by dynamic light scattering measurements (DLS) and by surface tension measurements.
Finally, zeta potential measurements will be made to investigate the interaction between the polymers and a model surfaces, using sandstones and carbonate as reference rocks. This
25
study may provide information on the possible partition between aqueous phase and adsorbed layer on the porous surfaces of the rock formation of the oil field, that may result in ineffective consumption of the additive and thus in economic loss and environmental impact.
26
3 Results and discussion
This work is divided in four parts: in the first, the synthesis of co- and terpolymers containing tert-butyl methacrylate, glycidyl methacrylate and styrene monomers with a different architecture was performed via RAFT polymerization. The synthesized polymers were characterized via GPC and H1-NMR. In the second, the co- and terpolymers were hydrolysed and neutralized to obtain charged amphiphilic/macrosurfactans. In the third part, solubility tests and rheological measurements were performed to understand which of the neutralized polymers could be suitable for enhanced oil recovery applications. In the fourth and last part, solution properties and zeta potential measurements were carried out on the only one of the neutralized polymers that was found to be fully soluble in water.
3.1 Synthesis and characterization of the ptBMA, pGMA and
p(GMA-r-tBMA) RAFT macroinitiators
Controlled radical polymerisation, in particular RAFT, was adopted for the synthesis of copolymers to achieve control over the structure and composition of the copolymers. As chain transfer agent, 2-cyano-2-propyl benzodithioate (CPDB) was chosen, since this CTA is known to allow a good control of the polymerization of highly activated monomers such as methacrylates 41. In addition, this CTA has an aromatic substituent easily detectable by H1-NMR and therefore very useful for the determination of the molecular weight of the copolymer and thus of the number of repeating units of each repeat unit type.
Copolymerisation reactions were conducted using azo-bis(isobutylnitrile) (AIBN) as a conventional initiator.
In the first stage, RAFT macroinitiators were prepared via RAFT polymerization starting from tertbutyl methacrylate or glycidyl methacrylate. These were subsequently used in the second stage of the procedure for growing the second blocks to produce the copolymers ptBMAx-b-pGMAy and pGMAx-b-ptBMAy, where x and y represent the number-average degrees of polymerization of the poly(tertbutyl methacrylate) and poly(glycidyl methacrylate) blocks.
27
Specifically, three different macroinitiators, one ptBMA and two pGMAs, were synthesized using for all the synthesis a 5:1 molar ratio between the RAFT agent and the initiator. The polymerizations were performed in ethyl acetate solution at 70 °C and anisole solution at 60 °C respectively (figures 3.1 and 3.2, table 3.1). The adopted reaction conditions for the synthesis of the macroinitiators were the same as in previous works 43,44,45.
The theoretical molar mass, 𝑀𝑛(calc.), was calculated using equation 5 (paragraph 1.6.2).
Figure 3.1 – Reaction scheme for RAFT synthesis of ptBMA macroinitiator
Figure 3.2 – Reaction scheme for RAFT synthesis of pGMA macroinitiator
Table 3.1 - Experimental conditions and chemical-physical properties of ptBMA and pGMA macroinitiators
Macroinitiator RAFT : I a) tr b) (h) Yield c) (%) Mn d) (g/mol) Mw/Mn e) MNMR f) (g/mol) ptBMA85h) 5:1 6 72 12330 1,08 12738 pGMA51g) 5:1 6 82 7499 1,25 12220 pGMA50g) 5:1 6 86 7433 1,18 9811
a) RAFT agent (CPDB) : initiator (AIBN) molar ratio.
b) Reaction time.
28
d) Number Average Molecular Weight obtained with GPC.
e) Polydispersity index from GPC.
f) Number Average Molecular weight obtained with H1-NMR
g) Polymerization in Anisole solution.
h) Polymerization in Ethyl Acetate solution.
The degree of polymerization was calculated according by means of GPC analysis. Figures 3.3 and 3.4 show the chromatograms associated with ptBMA85 and pGMA50. The nearly symmetric Poisson-like curves are those expected for polymerizations proceeding in a controlled fashion, with only a minor shoulder at high MW in the case of pGMA50 (figure 3.4) that could indicate some irreversible termination by radical coupling. In accordance with the controlled character of RAFT polymerization, the degrees of dispersion of the molar masses were rather narrow: 1,08 for ptBMA85 and 1,18 for pGMA50.
Figure 3.3 – GPC curve of ptBMA85
5000 10000 15000 20000 25000 0,0 0,2 0,4 0,6 0,8 1,0 R.I
Molar mass (g/mol)
29 0 5000 10000 15000 20000 0,0 0,2 0,4 0,6 0,8 1,0 R.I
Molar mass (g/mol)
pGMA50
Figure 3.4 – GPC curve of pGMA50
A number average molecular weight of around 12330 g/mol was obtained for ptBMA85 and of 7430 for pGMA50, giving a degree of polymerization of 85 and 50, respectively. All reported GPC Mn values are relative to a polystyrene standard calibration curve.
Figures 3.5 and 3.6 illustrate the H1-NMR spectra in CDCl3 for the ptBMA85 and pGMA50 macro RAFT, respectively. The characteristic peaks associated to tertbutyl and glycidyl groups occur from 1,2 ppm to 1,5 ppm and from 2,5 ppm to 4,5 ppm, respectively; the two broad peaks at 0,8-1,2 ppm and 1,7-2,2 ppm are relative to the methyl and methylene protons of the methacrylic polymer backbone, respectively. The molecular weight could also be calculated by considering the small peaks from 7,3 to 7,8 ppm (figure 3.7) representing the aromatic hydrogens of CPDB, which presence as a reversible chain termination confirms the living character of the synthesized polymer.
30
Figure 3.5 – H1-NMR spectrum of ptBMA 85
Figure 3.6 – H1-NMR spectrum of pGMA 50
31
Figure 3.7 – Magnification of the CPDB area
The number of repeating units of the polymers blocks were calculated using the number average molecular weight obtained according by means of GPC analysis, as not for all the polymers, the H1-NMR peaks related to the RAFT terminal were clearly visible and therefore not useful.
A random p(GMA-r-tBMA) macroinitiator was also synthesised, using a 4 : 1 molar ratio tBMA : GMA as the feed mixture and performing the reaction in anisole solution at 60 °C for 6 hours under inert atmosphere.
32
Table 3.2 - Experimental conditions and chemical-physical properties of p(GMA-r-tBMA) macroinitiator
Macroinitiator RAFT : I a) tr b) (h) Yield c) (%) Mn d) (g/mol) Mw/Mn e) MNMR f) (g/mol) p(GMA16-b-tBMA72) 5:1 6 35 12500 1,12 12513
a) RAFT agent (CPDB) : initiator (AIBN) molar ratio.
b) Reaction time.
c) Yield (%) [g final polymer/(g RAFT agent + g Monomer)]*100.
d) Number Average Molecular Weight obtained with GPC.
e) Polydispersity index from GPC.
f) Number Average Molecular weight obtained with H1-NMR
The H1-NMR spectrum was recorded on the purified product so that the molar ratio between monomer units in the copolymer could be calculated (figure 3.9).
Figure 3.9 – H1-NMR spectrum of p(GMA
16-r-tBMA72)
The signals considered are the following:
• the three signals between 7,3 – 7,8 ppm relative to the 5 aromatic protons present in the CTA (C6H5CSS)
33
• the signal between 1,2 - 1,6 ppm relative to the 9 protons of the tertbutyl group in tBMA
• the signal between 3,1 - 3,2 ppm relative to the proton of the methine group (b) in GMA
Since there is only one CTA molecule present for each polymer chain, the value of the integral for CTA aromatic protons was normalised to 5. By virtue of this, it was possible to calculate the average number of repeating units of the two monomers within the polymer.
In the case of tBMA repeating units we have (Eq. 6):
𝑛 𝑡𝐵𝑀𝐴 =𝐴 𝑡𝐵𝑀𝐴
9 (Eq. 6)
where A tBMA represents the value of the integral of the signal at 1,2 – 1,6 ppm.
Likewise, the value of the average units of GMA in the macroinitiator was calculated (Eq. 7):
𝑛 𝐺𝑀𝐴 = 𝐴 𝐺𝑀𝐴
1 (Eq. 7)
where A GMA represents the value of the integral of the signal at 3,1 – 3,2 ppm.
The number of repeating units for GMA of 16, while for tBMA was of 72.
This product was also characterized by GPC. Figure 3.10 shows the GPC chromatogram of p(GMA-r-tBMA) that also presents a Poisson-like distribution of molecular weights, with an acceptable polydispersity index of 1,12.
34 0 5000 10000 15000 20000 25000 30000 0,0 0,2 0,4 0,6 0,8 1,0 R.I
Molar mass (g/mol)
p(GMA16-r-tBMA72)
Figure 3.10 – GPC curve of p(GMA16-r-tBMA72)
Because the narrow polydispersities of the obtained macro RAFT being indicative of a well-controlled polymerization, all the three RAFT macroinitiators were used for the subsequent chain extension steps to prepare the corresponding block copolymers.
3.2 Synthesis and characterization of the ptBMA-b-pGMA and
pGMA-b-ptBMA copolymers
Chain extension reactions were carried out using ptBMA85 and pGMA51 as RAFT agents, with anisole as solvent. The syntheses were carried out using the same reaction condition as those for the preparation of the macroRAFT initiators (ch. 1).
35
Figure 3.11 – Reaction scheme for RAFT synthesis of ptBMA-b-pGMA diblock copolymer
Figure 3.12 – Reaction scheme for RAFT synthesis of pGMA-b-ptBMA diblock copolymer
Table 3.3 - Experimental conditions and chemical-physical properties of ptBMA-b-pGMA and pGMA-b-ptBMA diblock copolymers
Diblock Copolymers RAFT:I a) tr b) (h) Yield c) (%) Mn d) (g/mol) Mw/Mn e) MNMR f) (g/mol) ptBMA85-b-pGMA25 5:1 6 78 15840 1,09 ptBMA 13041 pGMA 7108 pGMA51-b-ptBMA70 5:1 6 27 17400 1,22 pGMA 9436 ptBMA 12182
a) RAFT agent (CPDB) : initiator (AIBN) molar ratio.
b) Reaction time.
c) Yield (%) [g final polymer/(g RAFT agent + g Monomer)]*100.
d) Number Average Molecular Weight obtained with GPC.
e) Polydispersity index from GPC.
36
Figure 3.13 shows the GPC traces of the two macroinitiators and of the corresponding diblock copolymers ptBMA85-b-pGMA25 and pGMA51-b-ptBMA70, whose monomodal shape confirmed the block macromolecular structure.
0 5000 10000 15000 20000 25000 30000 35000 0,0 0,2 0,4 0,6 0,8 1,0 R.I
Molar mass (g/mol)
ptBMA85 ptBMA85-b-pGMA25 0 10000 20000 30000 40000 50000 0,0 0,2 0,4 0,6 0,8 1,0 R.I
Molar mass (g/mol) pGMA51 pGMA51-b-ptBMA70
Figure 3.13 – Comparison of GPC curves of ptBMA85-b-pGMA25 with its respective ptBMA85 macroinitiator
(left) and of pGMA51-b-ptBMA70 with its respective pGMA51 macroinitiator
Based on the macroinitiators molecular weights obtained by GPC, we calculated the numerical average molecular weights and the number of repeating units of the added blocks, shown in table 3.3.
In general, GPC data of molecular weight dispersions (Mw/Mn = 1,09 and 1,22) suggest that the copolymerizations followed a controlled mechanism, without significant irreversible chain transfer and or chain termination processes during the growth phase.
The H1-NMR spectra, recorded after purification (see paragraph 5.3.1), also confirmed the simultaneous presence of the two blocks in the copolymers, as highlighted by their characteristic signals (see paragraph 3.1).
37
Figure 3.14 – H1-NMR spectrum of pGMA
51-b-ptBMA70
3.3 Synthesis and characterization of styrene co- and terpolymers
The addition of a poly styrene block via RAFT polymerization was carried out using ptBMA85 and pGMA50 as macroRAFT agents for the synthesis of the copolymers, while ptBMA85-b-pGMA25, pGMA51-b-ptBMA70 and p(GMA16-r-tBMA72) were used as the macro RAFT precursors for the synthesis of the terpolymers. A molar ratio between monomer and RAFT agent of 40:1 for the copolymers and of 30:1 for the terpolymers was used to target a low degree of polymerization, fitting with our purpose of adding small hydrophobic chains as to achieve water solubility upon decomposition of the tert-butyl groups of ptBMA, as discussed later.38
39
Table 3.4 - Experimental conditions and chemical-physical properties of styrene co- and terpolymers
Sty polymers RAFT:I a) tr
b) (h) Yield c) (%) Mn d) (g/mol) Mw/Mn e) MNMR f) (g/mol)
p(GMA16-r-tBMA72)-b-pSty28 5:1 24 67 15410 1,15 1706
ptBMA85-b-pGMA25-b-pSty24 5:1 24 60 18380 1,09 1040
ptBMA85-b-pSty24 5:1 24 69 14780 1,16 2283
pGMA51-b-ptBMA70-b-pSty17 5:1 24 58 19160 1,29 2898
pGMA50-b-pSty34 5:1 24 61 10940 1,22 1874
a) RAFT agent (CPDB) : initiator (AIBN) molar ratio.
b) Reaction time.
c) Yield (%) [g final polymer/(g RAFT agent + g Monomer)]*100.
d) Number Average Molecular Weight obtained with GPC.
e) Polydispersity index from GPC.
f) pSty block Number Average Molecular Weight obtained with H1-NMR
All polymers were subjected to GPC analysis to assess their M.W. dispersion (PDI), a parameter that gives information on the effective control during the polymerization process. By comparing the normalized elution curves (figure 3.16) of the polymers obtained with those of the mono and/or diblock macroinitiators, we observe a translation of the maximum value of intensity to higher molecular weights (in agreement with the targeted ones), confirming the formation of the styrene blocks. On the other hand, it can be observed that there is a loss of monomodality. This was attributed to the formation of dead end styrene homopolymer chains formed by radical coupling reactions, probably due to a high concentration of initiator used. As a result, after hydrolysis and neutralisation (see paragraphs 3.4 and 3.5) of the polymers a further purification was needed.
40 0 10000 20000 30000 40000 50000 0,0 0,2 0,4 0,6 0,8 1,0 R.I
Molar mass (g/mol) ptBMA85 ptBMA85-b-pGMA25 ptBMA85-b-pGMA25-b-pSty24 0 10000 20000 30000 40000 50000 60000 0,0 0,2 0,4 0,6 0,8 1,0 R.I
Molar mass (g/mol) pGMA51-R(2) pGMA51-b-ptBMA70 pGMA51-b-ptBMA70-b-pSty17 0 10000 20000 30000 40000 0,0 0,2 0,4 0,6 0,8 1,0 R.I
Molar mass (g/mol)
p(GMA16-r-tBMA72) p(GMA16-r-tBMA72)-b-pSty28 0 5000 10000 15000 20000 25000 30000 35000 40000 0,0 0,2 0,4 0,6 0,8 1,0 R.I
Molar mass (g/mol)
ptBMA85 ptBMA85-b-pSty24 0 10000 20000 30000 40000 50000 0,0 0,2 0,4 0,6 0,8 1,0 R.I
Molar mass (g/mol)
pGMA50 pGMA50-b-pSty34
Figure 3.16 – Comparison of GPC curves of styrene co- and terpolymers with their respective mono/diblock macroinitiators
These products were also characterized by H1-NMR. As all the spectra are very similar, only the one of pGMA51-b-ptBMA70-b-pSty17 spectra will be discussed as a representative one. The spectrum in figure 3.17 shows, in addition to the typical sharp peaks of the epoxide ring protons and tertbutyl groups mentioned in the previous paragraphs, the presence of the
41
characteristic peaks associated to benzene rings from 6,25 ppm to 7,2 ppm, due to the styrene block.
Figure 3.17 – H1-NMR spectrum of pGMA
51-b-ptBMA70-b-pSty17
3.4 Hydrolysis of the polymers
Figure 3.18 – Reaction scheme for hydrolysis of the polymers: example of the pMAA-b-p[GM(OH)2]-b-pSty
42
The hydrolysis of the tert-butyl and/or glycidyl groups of the synthesised polymers was performed according to literature procedures 36,48,49, as reported in the experimental part (see paragraph 5.5).
The amounts used in each experimental run are listed in table 3.5.
Table 3.5 - Experimental conditions of hydrolysis reactions
Polymers Amount of polymer g (10-6 mol) Amount of 37 wt% HCl ml (mmol) Reaction time at 100ºC
p(GMA16-r-tBMA72)-b-pSty28 1,50 (97,5) 3,55 (42,9) 4 h 30 min
p(GMA16-r-tBMA72) 1,20 (96,2) 3,50 (42,3) 4 h 30 min
ptBMA85-b-pGMA25-b-pSty24 1,11 (60,4) 2,80 (33,8) 4 h 30 min
ptBMA85-b-pGMA25 0,75 (47,3) 2,20 (26,5) 4 h 30 min
ptBMA85-b-pSty24 2,20 (148,8) 5,40 (64,7) 4 h 30 min
pGMA51-b-ptBMA70-b-pSty17 0,99 (51,9) 2,64 (31,9) 4 h 30 min
pGMA50-b-pSty34 2,25 (205,4) 4,42 (53,4) 4 h 30 min
The hydrolysed polymers are identified as p([GM(OH)2]16-r-MAA72)-b-pSty28, p([GM(OH)2]16-r-MAA72), pMAA85-b-p[GM(OH)2]25-b-pSty24, pMAA85-b-p[GM(OH)2]25, pMAA85-b-pSty24,p[GM(OH)2]51-b-pMAA70-b-pSty17 and p[GM(OH)2]50-b-pSty34.
All of them were characterized by H1-NMR analysis in DMSO-d
6. Figure 3.19 shows representative example of the H1-NMR spectrum of pGMA
51-b-ptBMA70-b-pSty17. Confirming the achievement of quantitative hydrolysis, the strong signal at 1,2 – 1,6 ppm, from the tert-butyl moiety, has completely disappeared 50. Signals from the acidic proton of the methacrylic acid (MAA) unit are not present in the typical 12-13 ppm range, probably due to the presence of moisture in the sample, resulting in fast proton exchange. The peak at 5,5 ppm is assigned to hydrogens of the newly formed -OH groups of 2,3-dihydroxypropyl methacrylate (GM(OH)2).
43
Figure 3.19 – H1-NMR spectrum of p[GM(OH)
2]51-b-pMAA70-b-pSty17
The hydrolysed polymers were also investigated with FT-IR and compared to their precursors to confirm that hydrolysis had occurred quantitatively and selectively on the tert-butyl and glycidyl moieties without further modification in the polymer structure. In the representative example of figure 3.20, the appearance of the wide band corresponding to the -OH stretching vibration in the 3000- 3500 cm-1 range and the disappearance of the sharp band at 848 cm-1 corresponding to the C-O-C stretching of the oxirane group are further proofs that the polymer underwent effectively the expected hydrolytic reactions 51.
44 500 1000 1500 2000 2500 3000 3500 4000 0 10 20 30 40 50 60 70 80 90 100 T (%) Wavelenght (cm-1) pGMA51-b-pMAA70-b-pSty17 p[GM(OH)2]51-b-pMAA70-b-pSty17 2500 - 3500 cm-1 848 cm-1
Figure 3.20 – Comparison of FT-IR spectra of pGMA51-b-ptBMA70-b-pSty17 and of
p[GM(OH)2]51-b-pMAA70-b-pSty17
3.5 Neutralization of the hydrolysed polymers
To achieve better solubility of polymers in water and to improve the thickening property of the polymers towards aqueous solutions 36,50,16, the newly formed methacrylic acid blocks were neutralized with triethylamine (Et3N).
Figure 3.21 – Reaction scheme for neutralization of the hydrolysed polymers: example of the neutralised pGM(OH)2-b-pMAA-b-pSty terpolymer
Neutralization of the carboxyl groups of the hydrolysed polymers was done according to literature works 52 and to the procedure reported in the experimental part (see paragraph 5.6).
45
3.6 Thermal characterization – TGA
TGA analyses were carried out in the temperature range from 0°C to 600°C under nitrogen flux and from 600 °C to 700 °C under air as a reactive gas.
The thermogram of the terpolymer pGMA51-b-ptBMA70-b-pSty17 is reported as a representative example, along with the thermograms of the parent GMA and tBMA homopolymers (figure 3.22).
Figure 3.22 – TGA curves of pGMA51-b-ptBMA70-b-pSty17 terpolymer and corresponding homopolymers
ptBMA85 and pGMA50
The terpolymer (black line) presents a 3-stage degradation process; their respective onset temperatures of weight loss (Ton) are in accordance with those typical of the corresponding
homopolymers, as reported in the literature 53,54,55. The decomposition of the polyglycidyl block starts at about 200 °C, while that of the polytert-butyl methacrylate one at about 240
46
°C, corresponding to the loss of the tertbutyl groups and formation of carboxyl groups. The second degradation step of the tert-butyl methacrylate block (at about 410 °C in the homopolymer), during which poly(methacrylic anhydride) is formed, overlaps with the decomposition step of the polystyrene block, that begins at 320-340 °C.
Figure 3.23 – TGA curves of pGMA51-b-ptBMA70-b-pSty17 terpolymer and its hydrolysed derivate
Figure 3.23 shows a comparison between the terpolymer and its hydrolysed derivative. The disappearance of the decomposition step related to the loss of tert-butyl groups confirms that hydrolysis occurred. Furthermore, the thermogram of p[GM(OH)2]51-b-pMAA70-b-pSty17 shows the two thermal degradation steps typical of methacrylate polymers containing OH groups on the side chains 49,56. The first degradation step, which occurs at relatively low temperatures, involves decomposition via depolymerization mechanism to produce mainly monomer units. The second degradation step at around 350-400 °C involves intramolecular side-chain reactions such as dehydration and cyclization and takes place simultaneously with the onset of depolymerization.
47
3.7 Solubility test
Solubility tests were carried out to assess the possibility of using the neutralised block co/ter polymers in EOR applications.
In particular, water/organic solvent mixtures (acetone, tetrahydrofuran, methanol, ethanol, dioxane, dimethylsulfoxide and dimethylformamide) were prepared at different v/v % (100/0, 50/50, 60/40, 70/30, 80/20, 90/10, 95/5) and approximately 10 mg of neutralised polymer (p([GM(OH)2]16-r-MAA72)-b-pSty28, p([GM(OH)2]16-r-MAA72), pMAA85 -b-p[GM(OH)2]25-b-pSty24, pMAA85-b-p[GM(OH)2]25, pMAA85-b-pSty24, p[GM(OH)2]51 -b-pMAA70-b-pSty17 and p[GM(OH)2]50-b-pSty34) were added to each of them. After initial sonication, polymer mixtures were left stirring overnight and checked the following day. All the polymers, except pMAA85-b-pSty24, were found to be insoluble in water: p([GM(OH)2]16-r-MAA72)-b-pSty28, p([GM(OH)2]16-r-MAA72), pMAA85-b-p[GM(OH)2]25 -b-pSty24, pMAA85-b-p[GM(OH)2]25 and pMAA85-b-pSty24 provided a swollen (gel-like) precipitate, while p[GM(OH)2]51-b-pMAA70-b-pSty17 and p[GM(OH)2]50-b-pSty34 were retrieved as powdery precipitates.
The same result was obtained for water/organic solvent mixtures, with the only exception of the copolymer p[GM(OH)2]50-b-pSty34 that was soluble in water/organic solvent.
The occurrence of cross-linking reactions was excluded since all polymers, after hydrolysis, remained soluble in dioxane; furthermore, after purification, they were soluble in DMSO-d6, the solvent used to H1-NMR characterization.
Therefore, on the hypothesis that the insolubility be due to the presence of strong intra and intermolecular hydrogen bonds between hydroxyl groups of the 2,3-dihydroxypropyl methacrylate and the carboxyl groups of pMAA, LiCl was added to each aqueous polymer solution, as it is known to interfere with hydrogen bonds 57,58. The mixtures of water, polymer and LiCl were sonicated and then stirred overnight, without any change concerning the solubility.
A further attempt to solubilize the polymers, also unsuccessful was carried out, by replacing the counterion of the charged carboxyl groups. For this purpose the polymeric aqueous
