Responsive Polyelectrolyte and Nanodiamond Hybrid Materials

(1)

Department of Chemistry Faculty of Science University of Helsinki

Finland

RESPONSIVE POLYELECTROLYTE AND NANODIAMOND HYBRID MATERIALS

Tony Tiainen

ACADEMIC DISSERTATION

To be presented, with the permission of the Faculty of Science of the University of Helsinki, for public examination in auditorium D101, Department of Physics, on the 11^thof

March 2022, at 12o’clocknoon.

(2)

Supervisors

Docent Sami Hietala and Professor Heikki Tenhu Department of Chemistry

University of Helsinki Finland

Opponent

Professor Seema Agarwal

Faculty of Biology, Chemistry and Earth Sciences, Macromolecular Chemistry II University of Bayreuth

Germany Reviewers

Professor Jukka Seppälä School of Chemical Engineering Aalto University

Finland

&

Professor Minna Hakkarainen

Department of Fibre and Polymer Technology KTH Royal Institute of Technology

Sweden

ISBN 978-951-51-7894-7 (pbk.) ISBN 978-951-51-7895-4 (PDF) Unigrafia

Helsinki

(3)

”Älä anna periksi, voimaa ja nopeutta!”

-Tuntematon

(4)

ABSTRACT

In the face of a climate crisis, ways of making materials more durable, lighter, versatile and sustainable is of great interest. Hybrid nanomaterials comprising of two or more interfaces mixed at the nanoscale can improve or extend the properties and applications of conventional materials.

An integral component of hybrid nanomaterials are polymers, which themselves are materials with nano-sized dimensions and, depending on their structure, are capable to respond to changes in their environment. In combination with different nanoparticles, new and synergistic properties may emerge for these materials.

This thesis examines the synthesis and properties of cationic polyelectrolytes and their hybrid materials made especially with nanodiamonds (ND).

Nanodiamonds (ND) are nanomaterials with intriguing mechanical, thermal and chemical properties. Their chemically modifiable surface makes them suitable for varying applications, for example, as fillers and modifying agents in materials, catalysts and platforms in synthesis and carriers and sensors in biomedicine.

In the first part of the thesis, routes to mitigate ND aggregation via polyelectrolyte complexation is studied. Different poly(dimethyl aminoethyl methacrylate) (PDMAEMA) polymers were complexed with NDs. Depending on the dispersion conditions the colloidal stability of these particles in water and saline water was improved compared to bare NDs.

In the second part, NDs were used as fillers for polyelectrolyte based responsive films. The mechanical properties of the films were improved upon the addition of NDs and the films had stimuli-responsive properties upon change of pH- or temperature.

Finally, poly(aminoethyl methacrylate) (PAEMA) polymers were synthesized and modified with guanidine. They were tested for CO2 adsorption and show promising reversible adsorption of CO2.

(5)

ACKNOWLEDGEMENTS

This work was conducted in the years 2017-2022 at the Department of Chemistry, University of Helsinki and with the financial support from MATRENA doctoral programme and the Department of Chemistry at the University of Helsinki.

This has been long journey, but my legs still have some spring in them and my mind is still sharp-ish, because I had, and luckily still have, the best company a simple man could ask for. During the years poured into this work and everything that goes along with it, I had help from my co-workers, friends and family. Clearly a heartfelt acknowledgement is in place.

Firstly, I would like to thank my Supervisor, Docent Sami Hietala for the guidance, encouragement and patience, without you, this would not have happened, literally. A big thanks goes to Prof. Heikki Tenhu for giving me the opportunity to work in the group. I thank Prof. Sirkka-Liisa Maunu for acting as a support for a young scientist during the first years. Dr. Vladimir Aseyev, Dr. Sami-Pekka Hirvonen and Seija Lemettinen, you made the laboratory a comfortable place to work in. Prof. Seema Agarwal, Prof. Jukka Seppälä and Prof. Minna Hakkarainen are acknowledged for examining this work. I also want to thank my other co- workers and co-authors for all the input, discussions and comradery during these years. Whether it was a scientific problem or a futsal game, you had my back, truly, thank you. A special thanks goes to Joona Kontinen, Dr. Dong Yang, Vikram Baddam, Jere Mannisto, Dr. Teemu Myllymäki, Dr. Kalle Lagerblom, Dr. Arno Parviainen and Dr. Juha Keskiväli, I will cherish the discussions, adventures and laughs we had together and may the office basket forever stay glued to A434!

My friends, you are the ones that have molded me into who I am today. No amount of thanks can describe my gratitude towards you. In addition to multiple close friends, I have two very close groups of friends, one pushes and keeps me going forward and the other helps me to keep level and grounded, sometimes the roles switch, but who knows what happens next with these guys. In the order of appearance, HJ and Pommiviini, a bow and a big hand to you, may our paths never separate!

My family, thank you for being there for me. Mom and dad, Tiina and Jouni, without you I wouldn’t be here, thank you for your patience, care and love. Vili and Tove, bro and sis, thank you for sharing that time with me. Mummoille ja ukeille kiitos huolenpidosta ja vastikkeettomasta luottamuksesta, teiltä olen oppinut, että elämä ei ole niin vakavaa ja että pitää myös opetella luistelemaan takaperin, vaikka onkin hyökkääjä. To my beautiful fiancé Varpu, you were there through the whole journey and more, you truly are the stabilizing surfactant to my life, keeping me whole and stable. Thank you for everything, I love you. Hilla, älä mussuta! You all gave me the tools to succeed and still keep supporting me without asking, I hope that you know that you are the most important thing in my life.

(6)

LIST OF ORIGINAL PUBLICATIONS

This thesis is based on the following publications:

I Tiainen, T.; Myllymäki, T.T.T.; Hatanpää, T.; Tenhu, H.; Hietala, S.

Polyelectrolyte stabilized nanodiamond dispersions.

Diamond and Related Materials. 2019, 95, 185–194.

II Tiainen, T.; Lobanova, M.; Karjalainen, E.; Tenhu, H.; Hietala, S.

Stimuli-Responsive Nanodiamond-Polyelectrolyte Composite Films. Polymers. 2020, 12, 507.

III Tiainen, T.; Mannisto, J. K.; Tenhu, H.; Hietala, S. CO2 Capture and Low Temperature Release by Poly(aminoethyl methacrylate) and Derivatives. Langmuir. 2021.

The publications are referred to in the text by their roman numerals.

The author’s contribution to the publications:

For all publications Tony Tiainen was responsible for majority of planning as well as the synthesis of the materials, conducting the experiments and analysing and visualizing the data. Tiainen wrote the first drafts independently and finalized the writing and editing with the co-authors.

Alongside the thesis, the author has contributed to other publications related to the work:

Kiviaho, J. K.; Linko, V.; Ora, A.; Tiainen, T.; Järvihaavisto, E.; Mikkilä, J.; Tenhu, H.; Nonappa, N.; Kostiainen, M. A. Cationic Polymers for DNA Origami Coating – Examining Their Binding Efficiency and Tuning the Enzymatic Reaction Rates. Nanoscale. 2016, 8 (22), 11674–11680.

Semenyuk, P.; Tiainen, T.; Hietala, S.; Tenhu, H.; Aseyev, V.; Muronetz, V.

Artificial Chaperones Based on Thermoresponsive Polymers Recognize the Unfolded State of the Protein. International Journal of Biological Macromolecules. 2019, 121, 536–545.

Hörenz, C.; Bertula, K.; Tiainen, T.; Hietala, S.; Hynninen, V.; Ikkala, O. UV- Triggered On-Demand Temperature-Responsive Reversible and Irreversible Gelation of Cellulose Nanocrystals. Biomacromolecules.

2020, 21 (2), 830–838.

(9)

Mannisto, J. K.; Pavlovic, L.; Tiainen, T.; Nieger, M.; Sahari, A.; Hopmann, K. H.;

Repo, T. Mechanistic insights into carbamate formation from CO2 and amines: the role of guanidine-CO2 adducts. Catalysis Science & Technology.

2021, 11, 6877-6886.

(10)

ABBREVIATIONS

AIBN Azobisisobutyronitrile

ATR Attenuated total reflection

BA Butyl acrylate

C-polymer PEO17-PDMAEMA-C12 complexing polymer CPA 4-((((2-Carboxyethyl)thio)carbonothioyl)thio)-4-

cyanopentanoic acid

CTA Chain transfer agent

DLS Dynamic light scattering

DMA Dynamic mechanical analysis

DMAEMA 2-(dimethylamino)ethyl methacrylate

DMF Dimethylformamide

DSC Differential scanning calorimetry

DTG Differential thermogravimetry

EIC Extracted-ion chromatogram

FTIR Fourier-transform infrared (spectroscopy)

GPC Gel permeation chromatography

ND Nanodiamond

NMR Nuclear magnetic resonance (spectroscopy) PAEMA Poly(aminoethyl methacrylate)

PBA Poly(butyl acrylate)

PDMAEMA Poly(dimethyl aminoethyl)methacrylate

PE Polyelectrolyte

PEC Polyelectrolyte complex

PEO Poly(ethyleneoxide)

PEO -PDMAEMA-C12 Poly(ethyleneoxide)-block-poly(dimethylaminoethyl methacrylate)-block-dodecyl

PEO-CTA Poly(ethyleneoxide) methyl ether 2-

(dodecylthiocarbonothioylthio)-2-methylpropionate

PEO-PGEMA Poly(ethyleneoxide)-block-poly(guanidine ethyl methacrylate)

PGEMA Poly(guanidine ethyl methacrylate)

RAFT Reversible addition-fragmentation chain

transfer polymerization

TEM Transmission electron microscopy

Tg Glass-transition

TGA Thermogravimetric analysis

TGA-EGA Thermogravimetric- and evolved gas analysis UV-vis Ultraviolet-visible (spectroscopy)

ZP Zeta Potential

(11)

1 INTRODUCTION

1.1 NANO- AND HYBRID MATERIALS

Nanomaterials are materials where one or more of the components have a dimension in the nanoscale, in other words, in 0-100 nm size range. Due to the vast amount of nanomaterials available, the definitions vary greatly. ¹ Historically, nanomaterials, albeit unknowingly, have been present in human societies for a long time in the form of dyes, coloured glass and forged metals. However, the scientific understanding and purposeful applications of these materials developed later and started at the beginning of 20^th century. The term “nanotechnology” was coined by Norio Taniguchi in 1974 and the time after 1980s began “the golden era of nanotechnology”. ^1–3 Due to the relatively fresh scientific focus on the topic, the field of nanotechnology is still developing and has unanswered questions. However, the potential to impact the materials industry is large because of the wide range of applications the nanoscale opens and has opened up (Figure 1).

Hybrid materials can be regarded as a sub class of nanomaterials and they are defined as a combination of two or more components that have interactions at interfaces of nano- and molecular level while, usually, new properties emerge when the components are mixed. ^4,5 While a nanomaterial has a nanoscale component, it does not necessarily contain interactions. A hybrid material has interactions at a molecular level usually creating new emergent properties not observed in the individual components of the material. Hybrid materials can be classified either via the bond strength of interactions between the components or via functionality into structural- and functional hybrids. ⁵ The benefits of hybrid materials include more homogeneous mixing, tailoring of specific properties or functionalities and optical properties. ⁶ Due to these and more, hybrid materials find a vast range of applications in transportation industry, construction, sports, electronics, biomedicine and more. ⁷

(12)

Figure 1 Schematic of the potential applications of nanomaterials. Reprinted from Ref. ¹ with permission from The Royal Society of Chemistry.

(13)

1.2 NANODIAMONDS

Nanodiamonds (NDs) are a subclass of carbon nanomaterials with a diamond-like inner structure, rich surface chemistry and size in the submicron domain (Figure 2 and Figure 3). ^8–14 They are used in place of and with other carbon nanomaterials in engineering ^15,16 and biomedicine. ^17,18 The useful properties of NDs for these applications include versatile and modifiable surface, ^9,12 high thermal conductivity,

19 good mechanical- and chemical resistance, ¹⁶ biocompatibility ²⁰ and natural non- bleaching fluorescence. ^21,22 The surface, which contains accessible sp² and sp³ carbons, graphite and other forms of hydrophobic carbons plays a big role in its behaviour. The diamond-like properties of the ND cores are combined with the variable surface functionalities accessible by different ND production processes. ²³

Figure 2 Structure of nanodiamond representing the diamond core, impurities such as a nitrogen vacancy (blue) and possible functional groups on the ND surfaces with hydrophobic graphite and amorphous carbon.

(14)

Figure 3 Schematic of NDs from synthesis to applications. Reprinted from Ref. ²⁴ with permission from Elsevier.

1.2.1 SYNTHESIS

A number of different ways to prepare NDs are available, including so-called static high pressure high temperature (HPHT), dynamic detonation methods, laser ablation, microwave plasma, chemical vapor deposition (CVD) and ultrasound cavitation. ¹⁴ The HPHT and detonation methods are commonly used at an industrial level, whereas other methods are more experimental and used in small scale.

Detonation methods are a cost-effective approach for producing NDs on a large scale (Figure 4). ²⁵ Generally, two explosives having a combined oxygen balance under zero (typically trinitrotoluene (TNT) and hexogen (RDX)) are detonated in a closed chamber yielding nanoscale sp³ carbon soot. The soot is collected and purified to yield the end product called detonation nanodiamond. The diamonds produced by detonation usually have very complex mix of surface functionalities.

(15)

This combined with their large surface to volume ratio leads to a tendency of NDs to bind to each other and aggregate. These aggregates are usually observed as tight covalent- and hydrogen bonded core aggregates and more loosely electrostatically bound intermediate aggregates. ²⁶ Aggregation destroys many of the attractive properties stemming from the small size of NDs and makes the production of homogeneous products or stable colloidal dispersions a challenging task. ⁹ For the most efficient use of NDs, deaggregation has to be considered.

Figure 4 Production of detonation nanodiamond from explosion to product. Revisualized from Ref. ⁹ with permission from The Royal Society of Chemistry.

1.2.2 DEAGGREGATION

Studies of breaking the different aggregates of NDs have been reported by many groups. Mechanical methods such as grinding and milling, ^26–28 sonicating ^26,29,30 or annealing, ³¹ have been shown to break down the larger intermediate aggregates easily and producing a colloidal dispersion of small ND particles ranging from sub 10 nm to a couple of hundred nanometers. Breaking the tight core aggregates may need a more aggressive methods such as annealing under a reactive atmosphere. ^26–

33 However the long term stability and redispersibility remain questionable as the dispersions reaggregate over time. ³⁴ Further processing is usually needed for effective usage and aggregation prevention. To achieve this, the surface of NDs needs to be modified.

1.2.3 SURFACE MODIFICATION

The surface functionalities of detonation NDs can be modified in multiple ways, ¹²

(16)

Most of the approaches involve covalent functionalization of NDs with surface groups that enable longer term colloidal stability or bring additional functionality.

35 Azide-alkyne cycloaddition reactions on azide functionalized NDs have been shown to be a viable method to attach various moieties. ^36,37

By attaching polymers, such as poly(ethylene oxide) (PEO) to NDs, enhanced dispersibility in water has been observed. ³⁸

The properties of NDs can also be enhanced by physical adsorption to the surface. For example, chemotherapy drug doxorubicin has been complexed with negatively charged NDs in order to bind and release a drug upon varying the salt concentration. ³⁹ Covalent and non-covalent approaches have also been combined interestingly by attaching proteins by electrostatic interactions to functionalized NDs followed by covalent protein immobilization via crosslinkers. ⁴⁰ Proteins bovine- and human serum albumin have been shown to complex with fluorescent NDs and to prevent their aggregation in aqueous dispersions, as well as being used for enhanced intracellular delivery. ⁴¹ Similarly block copolymers such as poly(ethylene glycol)-block-poly(dimethylaminoethyl methacrylate-co- butylmethacrylate) can also be used for this purpose. Hybrid ND:poly(ethylene imine) polyelectrolyte complexes have been made as vectors for siRNA delivery. ⁴² As described previously, polymeric moieties can often be utilized to prevent colloidal aggregation and instability. They function as a protective layer providing reduced particle-particle attraction and/or enhanced solvent-particle interaction.

PEO has been used to provide steric stabilization especially in the biomedical field.

43,44 Polyelectrolytes, on the other hand, are capable of providing electrostatic stabilization and may also enable charge inversion when oppositely charged moieties are used for complexation. Poly(dimethyl aminoethyl methacrylate) (PDMAEMA) is a weak polycation that has been used with carboxylated NDs to enhance their stability in biological fluids. ⁴¹ In addition, aqueous solutions of PDMAEMA have pH- and temperature-sensitive properties. ^45–48

(17)

Figure 5 Pathways to modify ND surface. Revisualized from Ref. ^49,50 with permission from Elsevier and Taylor & Francis.

(18)

1.2.4 NANODIAMOND MATERIALS

In composites nanofillers often have an advantage over microfillers because of their higher surface area. Consequently, interfacial adhesion is increased, which enhances the mechanical properties of the composites. ^51–55

Owing to the large surface-area-to-volume ratio of spherical NDs, much of the surface is in contact with the surrounding environment, enabling interactions between ND and its surroundings. ⁵⁶ This, combined with the ND surface chemistry and diamond-like structure of the core, make NDs a very potent nanofiller.

The use of NDs as fillers of polymer composites has been studied extensively for reinforcement and thermal conduction enhancement. 13,15,17,32,50,56–62 The studies show that the addition of NDs into these materials generally improves their mechanical properties, namely, Young’s modulus, strength and hardness. Thermal properties, including thermal conductivity, thermal stability and heat capacity are also changed in different polymer systems. However, problems may exist related to the uneven distribution- and aggregation of NDs. These lower the active surface area of NDs causing the physical reinforcing effect and effect on thermal properties to deteriorate. ⁶¹ Consequently, efficient dispersion is required through the manufacturing process, which can be achieved by tailoring an appropriate ND surface to accommodate them within a specific matrix. ^63,64

1.3 POLYELECTROLYTES

Polyelectrolytes are a class of natural-, semi-synthetic- and synthetic polymers with functional groups such as bases, acids and salts (Figure 6). ⁶⁵ They have the combined properties of polymers, such as dependence on molecular weight, and electrolytes such as electric conductivity in solutions. The extent of these properties and other properties of polyelectrolytes are determined by the amount of charges in the polymer and the strength of the charges, in other words, the polymers charge density and ionic strenght. Because of the charges, polyelectrolytes are generally soluble in polar solvents, especially water. When solubilized, polyelectrolytes have partly (weak polyelectrolyte) or completely charged (strong polyelectrolyte) side groups or chain leading to strong interactions with the surrounding environment intra- and intermolecularily via electrostatic interactions.These interactions make polyelectrolytes react to environmental changes, such as, heat, pH or ionic strength of the solvent. Shifting the balance of the interactions induces changes in the morphology of the polymer leading to responsive behaviour. ^66,67 For example, these responses can be seen at the macroscale as precipitation, colour changes and formation of a dispersion. Due to these characteristics, polyelectrolytes are used in multiple fields such as filtration, ⁶⁸ electronics ⁶⁹ and functional materials. ^67,70,71

(19)

Figure 6 Schematic representation of different polyelectrolytes and the charges induced by dissociation in appropriate conditions.

1.3.1 SYNTHESIS OF POLYELECTROLYTES VIA RAFT

A challenge of polymerizing charged and reactive monomers is to achieve good yields, high enough molar masses and defined chain structures in an environment where there is a mixture of side reactions and various interactions. While polyelectrolytes can be prepared by click chemistry, post-modification or various controlled free radical methods, reversible addition fragmentation chain transfer polymerization (RAFT) is a versatile method for controlled polyelectrolyte synthesis, especially in aqueous conditions. ⁷² RAFT is a living polymerization method where the propagation and termination kinetics are controlled by the use of a thiocarbonylthio- or trithiocarbonate-based chain transfer agent (CTA) (Figure 7). ^73,74 Due to the controlled kinetics and low tendency for the chain growth termination, narrow dispersities and targeted chain lengths can be achieved. In addition, due to the controlled nature of the method, a wide range of polymer architectures are achievable. Due to CTA’s good tolerance of water, RAFT has also been increasingly applied in aqueous conditions opening up ways to prepare polyelectrolytes in a more sustainable way. ^75–77

(20)

Figure 7 Mechanism of RAFT polymerization.

1.3.2 PDMAEMA

PDMAEMA is a weak polycation with a basic tertiary amine group, meaning that its amine group will dissociate in neutral and acidic aqueous conditions (pKb ~8.0, ^78,79 Figure 8). Close to and above the pKb, aqueous PDMAEMA solutions show lower critical solution temperature (LCST) -type behaviour where the polymer conformation changes abruptly with temperature. The temperature of the phase transition is related to the concetration and charge density of the polymer, in other words, the extent of dissociation and pH. In case of PDMAEMA, LCST behaviour is lost in neutral or acidic conditions due to the high amount of charges in the chain repulsing each other and increasing polymer solubility in water. In basic conditions, the LCST behaviour manifests when the increased energy from heating breaks the dipole-dipole bonds between non-charged PDMAEMA and water molecules. This causes the chain to collapse in on itself, precipitating the polymer out of the water phase. By changing the pH, temperature or both, stimuli responsiveness can be obtained from PDMAEMA-based materials.⁴⁵

Thus, this property of PDMAEMA have been extensively studied for stimuli- responsive purposes. For example, PDMAEMA has been combined with graphene to form colloidally stable multilayer structures sensitive to pH, ⁸⁰ to form pH- triggered antibiotic release systems when complexed with gelatin ⁸¹ and has also been used with carboxylated NDs to enhance their stability in biological fluids. ⁴¹ Various studies regarding responsiveness have also been conducted in the presence of acids, ⁴⁶ in nanoparticle form ⁸² and in copolymer form. ^48,83

(21)

Figure 8 Chemical structure of PDMAEMA.

1.3.3 PAEMA

PAEMA is a weak polyelectrolyte with a primary amine functionality instead of a tertiary one (Figure 9). A primary amine is usually more reactive, although less basic, than secondary or tertiary amines. ^84–86 Primary amines are prone to imine- and enamine formation as well as amidation. ^87,88 This reactivity can be a problem during synthesis and storage and thus PAEMA is usually stored in a protected form such as a salt. However, this increased reactivity is also a great asset when modifying the polymer, making PAEMA a more attractive target for post-modification compared to PDMAEMA. Although less studied than PDMAEMA, PAEMA has been studied for uses in biomedical applications such as DNA vaccine delivery ⁸⁹ and protein resistant coatings. ⁹⁰ Also, guanidine modified PAEMA shows biocompatibility and good functional group conversion. ⁹¹

Figure 9 Chemical structure of PAEMA.

(22)

1.3.4 POLYELECTROLYTE COMPLEXES

A polyelectrolyte complex is a system containing polyelectrolytes that are combined, or complexed, with polymers and particles of the opposite charge (Figure 10). The factors that influence the formation of polyelectrolyte complexes are, for example, mixing ratios, charge density of polyelectrolytes, charge distribution, ionic strength, polyelectrolyte-chain properties and mixing conditions. ^65,92,93

By adsorbing at the surface of the oppositely charged polymer/particle, polyelectrolytes can provide a protective layer that reduces intra- and intermolecular attraction and/or enhances solvent interaction. These interactions make a multitude of functionalities and morphologies possible via tuning of the preparation parameters mentioned previously. Complexation with polyelectrolytes is commonly used to improve biocompatibility and solubility of drugs, ⁹⁴ as protective coatings, ⁹⁰ to increase the stability of particles, ⁴¹ optimize DNA vaccines

89 and prepare solid electrostatically linked polyelectrolyte materials. ⁷⁰

Figure 10 Schematic illustration of different polyelectrolyte complex formations at different charge ratios K. Adopted from Ref. ⁹² with the permission from Taylor & Francis.

(23)

1.3.5 APPLICATIONS OF POLYELECTROLYTES

Materials that incorporate polyelectrolytes ⁹⁵ are used in ion exchange, ⁹⁶ gas separation, ⁹⁷ flocculants, ⁹⁸ cosmetics, ⁹⁹ water purification membranes, ¹⁰⁰ drug delivery ⁹⁴ and many more (Figure 11). ^65,92 This is due to the strong electrostatic interactions present in the polyelectrolyte containing materials.

PDMAEMA has been studied for these applications, especially as it has stimuli- responsivity that can be utilized in functional material applications.

Correspondingly, PDMAEMA containing copolymer films have been studied ^101,102 and utilized as a CO2 separation membrane, exhibiting high permeability and gas selectivity, ^103,104 and as an anion exchange membrane with improved cation-gating properties. ¹⁰⁵ The stimuli responsivess of PDMAEMA has inspired studies of PDMAEMA membranes for their ability to variate the flux of water through them,

106,107 their use as oil/water separating membranes with the capability of using temperature or pH as a trigger, ¹⁰⁸ and for actuation purposes. ¹⁰⁹

Figure 11 Applications of polyelectrolytes.

(24)

1.3.6 CO2 ADSORPTION WITH SOLID POLYMERIC MATERIALS

CO2 is considered to be one of the major contributing factors to the warming of the climate via the greenhouse effect. ^110–114 In addition, CO2 is used in various industrial processes and efficient capture and storage of CO2 and its use as carbon source is a very important topic to research (Figure 12). 113,115–119

Figure 12 Schematic representation of a possible carbon capture and utilization (CCU) cycle.

It is not surprising that recent times have seen a rise in interest regarding CO2

capture and separation. ^119–134 Various physical and chemical methods of adsorption of CO2 into solids 121,122,124,135–139 or absorption into liquids have been explored. ^140–

142 Liquid technologies are already well established and applied in industry, yet it has been suggested that further development is needed. 125,143,144 The main issues are the energy consumption of regeneration, running costs coming from evaporation and environmental impact of the solvents used. ^119,141 Solid adsorbents have the potential to be less energy demanding and more efficient compared to liquids. ¹²⁵

Solid monomeric, oligomeric or polymeric adsorbents, 124,137,145,146 polymer membranes ^135,138 and hybrid systems 104,120,123,147 are being developed to solve the issues with conventional liquid systems to allow lower regeneration temperatures, avoiding solvents and increase the active surface area for more sustainable and efficient systems.

Among polymeric materials, various nitrogen-containing polymers have been investigated, ¹²⁵ especially polymers with amine functionalities. ¹⁴⁸ By using a polymer backbone with suitable side groups, CO2 adsorption kinetics, release temperatures and material properties can be tailored. For example, Nie et al.

prepared polyethyleneimide impregnated polyacrylamide composite beads for CO2

capture and managed to achieve CO2 capacity of 2.64 mmol/g and a CO2 uptake of 90% in less than 10 minutes at temperatures between 50 and 125 °C. ¹²³ Whereas Goeppert et al. used polyethyleneimine on fumed silica to achieve 1.74 mmol/g capacity under similar humid conditions. ¹³⁷ Yue et al. prepared poly(N-isopropyl acrylamide) microgel-based films having functional N-[3-(dimethylamino)-propyl]

methacrylamide repeating units with large reversible CO2 capture from water- saturated gas. ¹³⁸

(25)

In these examples, a high amount of amine groups incorporated on supports with suitable morphologies were used to achieve fast adsorption, high CO2 capacity and low desorption temperatures. Kortunov et al. have shown that steric hindrance decreases the affinity of amine nitrogen to CO2 and thus morphology directly affects the adsorption and desorption performance. ¹⁴⁹ However, amine functionality is the key in interacting with the acidic CO2,especially if dry gas adsorption is preferable.

121,126,134,150,151 Tertiary amines usually do not work well in dry adsorption, whereas secondary- and primary amines are capable of forming ammonium carbamates with CO2 under anhydrous conditions increasing dry adsorption performance with primary amines having higher heat of adsorption and thermal stability. 126,144,152

Another aspect to consider in CO2 adsorption is that a more basic group than amine could have stronger interactions with the acidic CO2. Superbases have been shown to interact with CO2 strongly. 133,153,154 Guanidine, a type of superbase, has been used in an application to capture CO2 in solvated systems by Seipp et al. ¹⁴⁰ Guanidines have also been shown to increase CO2 uptake per mole in mixed base systems, where a nucleophilic amine is mixed with a non-nucleophilic Brønsted-base, guanidine in this case. ¹³³

Polyacrylates, such as PDMAEMA, are well known for their versatility in materials applications as well as in gas separation membranes. 58,104,107,130,155,156

However, PDMAEMA has interactions with acidic moieties, such as CO2, only in moist or wet conditions due to the chemical nature of the tertiary amine. ^144,152 PAEMA, which has a reactive primary amine, is biocompatible and easily modifiable and could be a more potent dry CO2 adsorbent than PDMAEMA. ^91,157,158 Poly(ethylene oxide) (PEO) is a widely used biocompatible polymer, that has been shown to enhance gas separation and adsorption of CO2 in membrane applications.

117,127,159 By tailoring the polymers and functional groups, potent CO2 adsorbing polymer materials are possible to use in various conditions.

(26)

2 OBJECTIVES OF THE STUDY

In this thesis polyelectrolytes were used to complex nanodiamonds and to study CO2

capture by polymers.

More specifically, this thesis studies:

1. Synthesis of polyelectrolytes PDMAEMA and PAEMA and their block copolymers and thorough characterization of the polymers. ^{I, III}

2. Modification of PAEMA with guanidine side groups and the use of the polymers as a solid CO2 adsorbent. ^III

3. Complexation of carboxyl NDs with PDMAEMA and their interactions, dispersibility and colloidal stability. ^I

4. PDMAEMA films impregnated with NDs and their mechanical- and stimuli- responsive properties. ^II

(27)

3 EXPERIMENTAL

This section describes the characterization and synthesis procedures used in the studies. Full experimental details are found in the corresponding publications.

3.1 CHARACTERIZATION

Asymmetric Flow Field Flow Fractionation (AF4)

AF4 was measured with Wyatt Eclipse AF4-system running aqueous 50 mM NaNO3

+ 0.05 mM NaN3 through a Frit Inlet (FI) channel. Wyatt DAWN HELEOS II MALS- and Wyatt REX RI-detectors were used to obtain raw data. Raw data was processed with Astra version 6.1.7. Samples were prepared with a concentration of 1 mg/mL and filtered with 0.45 μm PVDF filters. The refractive index increment, dn/dc, was measured from a series of concentrations of PAEMA in 50 mM NaNO3

+ 0.05 mM NaN3 using Wyatt REX RI-detector.

Dynamic Light Scattering (DLS) and Zeta Potential (ZP)

DLS and ZP measurements of complexes were made with Malvern Zetasizer Nano DS instrument using a HeNe laser operating at 633 nm in a backscattering configuration at room temperature. The Smoluchowski model was used in analysis of the electrophoretic mobility and zeta potential. The stability of the dispersions was measured from the samples stored in zeta cells at different periods of time.

Dynamic Mechanical analysis (DMA)

The mechanical analyses of the films were made with TA-Instruments Q800 DMTA.

Stress–strain measurements were made at room temperature from rectangular samples by ramping up the stress with 3 N/min.

Differential Scanning Calorimetry (DSC)

DSC measurements were made with TA-Instruments DSC Q2000 in aluminium pans. Samples saturated with CO2 were prepared into hermetic aluminium pans.

Fourier-Transform Infrared (FTIR)

FTIR-spectra were collected with PerkinElmer Spectrum One using an attenuated total reflection (ATR) setup for polymers and films and transmission setup for ND- and complex samples at room temperature.

(28)

Gel Permeation Chromatography (GPC)

The molar masses of the polymers were determined with a Waters Acquity APC - system equipped with Acquity APC XT 200Å, 450Å columns and UV- and RI- detectors. DMF + 1 mg/mL LiBr or THF 1 mg/mL TBAB was used as an eluent and molar masses were compared to PMMA standards.

Samples were prepared to 2 mg/mL concentration and filtered through 0.4 μm PTFE filters before measurement.

Nuclear Magnetic Resonance (NMR)

NMR-spectra were collected either with Bruker Avance III, Bruker Neo 400 MHz or 500 MHz spectrometers. NMR samples were prepared into D2O at 2-50 mg/mL concentration.

NMR titration experiments ^I

In the titration experiments a polymer sample in D2O (2 mg/mL) was titrated by ND dispersion in D2O (10 mg/mL), stirred and left to stabilize overnight. The stabilized sample was shaken by hand and sonicated in a bath for 30 min before measuring. The results were corrected to sample dilution.

CO2 NMR experiments ^III

Samples for measurements under CO2 atmosphere were prepared by first flushing them in a Schlenk line under CO2 for 10 min and left stirring for an hour to saturate.

Saturated samples were transferred to sealable Young’s NMR-tubes and measured.

After the measurements, the tubes were opened to air and placed into a pre-heated 60 °C oil bath for an hour. After cooling, the samples were measured again.

pH

pH were measured at room temperature with a VWR Phenomenal IS2100L using WTW or WVR electrodes calibrated within a day of the measurement with pH 4, 7 and 10 buffers from VWR.

Scanning Electron Microscopy (SEM)

SEM measurements of the films were carried out using a Hitachi S-4800 FESEM.

The film samples were cut by hand using a scalpel, placed on a SEM stand with two- sided carbon tape, and dusted with pressurized air.

Colloidal stability studies ^I

DLS and ZP samples in deionized water were stored in folded capillary zeta cells at room temperature in a dark environment and measured between set time intervals up to 5 weeks of storage. For measurements under saline conditions, samples in deionized water were added to salt solutions to desired ionic strength ([NaCl] = 0.01-0.154 mol/L) and solid content of 0.1 mg/mL. Each sample in saline was measured separately within 2 h of preparation.

(29)

Swelling capacity ^II

The swelling capacity of the films was determined from uniformly shaped samples by immersing the samples into deionized water for 24 h. The wet mass was determined by carefully drying the surface by blotting and weighing (mwet). The dry mass (mdry) was taken after drying the samples for 24 h in a vacuum desiccator.

The swelling capacity (SW) was calculated as follows:

SW = (mwet-mdry)/mwet × 100.

Thermogravimetric- and Evolved Gas Analysis (TGA and EGA)

TGA analyses of polymers, ND and complexes and CO2 adsorption/desorption studies were done using a Netzsch STA 449 F3 Jupiter simultaneous thermal analyser connected to JAS-Agilent GC-MS (7890B/MSD5977A). For EGA of ND and complexes, half of the gas flow containing the evolved gases was led through a heated transfer line to JAS valve box, from where the flow was continuously sampled through a 60 cm long inert GC capillary acting as a pressure restriction to the MS detector (MSD5977A).

TGA measurements of the films were made with Mettler-Toledo TGA850.

Transmission Electron Microscopy (TEM)

TEM images of complexes were taken with a Jeol JEM-2800. Samples with 0.1 mg/mL concentration in water were sonicated in a water bath for 90 min and blotted on an ultrathin CF300H-Cu-UL carbon support grid and dried at ambient atmosphere.

TEM imaging of films were carried out using a Jeol JEM-1400 transmission electron microscope from cross-sectioned samples prepared with a Leica EM Ultracut UC6i ultramicrotome.

Ultraviolet-visible (UV-vis)

The UV-spectra of polymers were measured with a Shimadzu UV-1601PC UV-Vis spectrometer.

The transmittance of the films was measured with a Jasco V-750 UV spectrophotometer with a Jasco CTU-100 temperature control unit.

(30)

3.2 SYNTHESIS

PEO-PDMAEMA-C12 block copolymers ^{I, II}

Poly(ethyleneoxide)-block-poly(dimethylaminoethyl methacrylate)-block-dodecyl (PEO-PDMAEMA-C12) were synthesized by RAFT polymerization reported earlier (Figure 13, step I). ¹⁶⁰ Generally, 1 g of DMAEMA (6.35 mmol) and 46.2 mg of PEO- CTA (0.042 mmol) were weighed into a 25 mL round bottom flask followed by addition of 1 mL AIBN/dioxane solution (AIBN 0.0042 mmol/mL, 0.0042 mmol) and 4 mL of dioxane. Dissolved reagents were freeze-thawed three times under 3 mbar vacuum to remove oxygen followed by addition of nitrogen atmosphere to the flask. The polymerization was initiated by immersing the flask into an 80 °C oil bath with magnetic stirring. The reaction was left to stir in the bath for 6 h and was stopped by placing the opened flask into an ice-water bath for 10 min. The product was dialyzed in a MWCO 3500 g/mol cellulose membrane against deionized water for 2 days changing dialysis water at least 2 times a day. After dialysis the product was freeze dried and stored in a freezer until used.

This method was also used to prepare the PEO17-PDMAEMA-C12 used in film preparations.

End-group removal ^I

The removal of the dodecyl end-group was performed with a modified version of the method by Perrier et al. (Figure 13, step II). ¹⁶¹ 0.1 g of PEO-PDMAEMA-C12 (end groups 0.0016 g, 0.00624 mmol) and 0.0154 g of AIBN (0.0936 mmol) was weighed into a 25 mL round bottom flask. The flask was sealed and purged with argon for 5 minutes under stirring. 5 mL of dried dioxane was added through a rubber septum and the mixture was left to stir with argon bubbling for 35 min at room temperature.

The reaction was started by immersing the flask into an 80 °C oil bath with magnetic stirring. The flask was left to stir for 18 h and the reaction was stopped by placing the opened flask into an ice-water bath for 10 min The product was purified by precipitation to cold n-hexane three times and dried in a vacuum. After purification, the dried product was dissolved in dioxane, freeze dried and stored in a freezer until used.

(31)

Figure 13 PEO-PDMAEMA-C12and PEO-PDMAEMA syntheses.

PAEMA and PEO-PAEMA ^III

The synthesis of PAEMA and PEO-PAEMA (Figure 14) was conducted with a modified version of a synthesis by Alidedeogly et al. ⁷⁶ Generally 14.1 mg of CPA (0.0503 mmol) or 300.9 mg of PEO-CTA (0.0503 mmol) and 4.1 mg of VA-044 (0.0126 mmol) were dissolved into 50 mL of AEMA dissolved in acetate buffer (30.190 mmol, 0.1 g/mL) in a 50 mL round bottom flask fitted with a septum. This solution was bubbled with argon under stirring for 1 h and immersed into a 60°C oil bath to start the polymerization. The argon needle was lifted above the liquid surface 30 min after immersion and was completely removed after 15 min. The flask was left to stir for 24 h. The polymerization was quenched by opening the flask to air and immersing it into an ice bath for 15 min. The crude product was a mixed ammonium salt of acetate and chloride, which was converted to chloride salt form by adding concentrated HCl and stirring until the reaction mixture was strongly acidic.

(32)

Figure 14 PAEMA and PEO-PAEMA syntheses.

Regeneration of PAEMA and PEO-PAEMA ^III

To convert the amine group from a less reactive salt into a free base, 500 mg of PAEMA or PEO-PAEMA were dissolved under stirring into 30 mL of 0.1M NaOH for 2h. The products were purified by dialysis against water in 12 MWCO cellulose membrane and freeze dried (Figure 15). This was not necessary for poly(guanidine ethyl methacrylate) (PGEMA) or poly(ethyleneoxide)-block-poly(guanidine ethyl methacrylate) (PEO-PGEMA) as the synthesis was made in basic conditions leading to free base form products. To test for differences, a regenerated PGEMA was also prepared and tested, however, no clear difference was observed. Regenerated polymers are denoted as “REG”.

Figure 15 Regeneration of PAEMA.

(33)

Guanidinylation of PAEMA ^III

PGEMA (Figure 16A) and PEO-PGEMA (Figure 16B) were prepared using a modified version of synthesis by Cheng et al. ⁹¹ Generally 200 mg (1.207 mmol) of PAEMA or PEO-PAEMA was dissolved into 10 mL of 0.1M carbonate buffer. At the same time 0.2235 mg (1.207 mmol) of 2-ethyl-2-thiopseudourea hydrobromide was dissolved into 5 mL of buffer. Both solutions were combined and mixed at room temperature for 25h. Conversion samples were taken at time intervals to determine the rate of reaction and degree of modification. The product was purified by dialysis against water in 3.5 MWCO cellulose membrane and freeze dried. Polymers with different amounts of guanidine were prepared by changing reaction time and by using either the salt form or free base form of the polymer.

Figure 16 PGEMA and PEO-PGEMA syntheses.

3.3 ND-POLYELECTROLYTE PREPARATION

A Hielscher UP400S Ultrasonic Processor was used to disperse NDs, complexes and ND-prepolymer mixtures using a H3 sonotrode at 400W and a 0.5 duty cycle at 100% amplitude unless otherwise mentioned. The samples were placed into an ice- water-salt bath (IWS, ~0 °C) during sonication to prevent heating of the solutions.

Stock dispersions of ND were prepared by weighing the as-received ND powder into a glass vial, adding deionized water, mixing by hand vigorously and sonicated for 30 min. All samples containing ND were refrigerated at +4 °C when not used and sonicated for 10 min prior to use. The polymer solutions in water were made by

(34)

ND-Polyelectrolyte complexation ^I

For the complexation, a Watson-Marlow solvent pump was employed to obtain a constant addition rate of polymer solvent (0.64 mL/min). Purification and fractionation of the complexes was performed with Sigma 2K15C centrifuge at 5000 RPM (3773 g) at 20 °C.

For the optimized procedure, 5 mL of refrigerated 10 mg/mL ND dispersion was measured into a 25 mL round-bottom flask with a large magnetic stirrer. The dispersion was sonicated in an IWS bath for 10 min at full duty cycle. Sonicated dispersion was moved into another IWS bath with magnetic stirring at 750 RPM. 15 mL of refrigerated 1 mg/mL polymer solution was added to the ND dispersion dropwise with a solvent pump. The mixture was left to stir for 24 h without replacing the IWS bath.

After stirring, the dispersion was subjected to 120 min of sonication (Figure 17).

For IR and TGA analysis, the complex was washed twice by centrifuging for 30 min, removing supernatant fraction and dispersing in fresh deionized water. The washed complex was dried in a 50 °C oven and stored in a desiccator if not used.

Figure 17 Schematic representation of complexation.

ND-Polyelectrolyte films ^II

The prepolymerization mixtures for the films were prepared by weighing a predetermined ratio of the monomers bytylacrylate (BA) and DMAEMA, and adding 2.5 wt% cross-linker BuDMA into a 20 mL glass vial and shaking until homogenous. For the samples prepared with a PEO17-PDMAEMA-C12 complexing polymer (C-polymer), the polymer was added to the mixture with the monomers and the cross-linker, and the mixing was continued until the polymer was dissolved.

In the case of the ND-polyelectrolyte complex films, NDs were added to the polymerization mixture and the mixture was sonicated for 15 min. ND–

polyelectrolyte complex films with the C-polymer were prepared by adding the NDs into the polymerization mixture with all of the other components, followed by

(35)

Before casting, the initiator 2-hydroxy-2-methylpropiophenone was added and the premixture shaken by hand (Figure 18). The prepared dispersion was placed on a Teflon-coated Petri dish covered with a lid coated with semitransparent Teflon tape.

The mixtures were cured under four 365 nm 9 W UV-lights for 2 h, followed by overnight washing in acetonitrile to remove the excess monomers and the initiator.

The washed films were dried under ambient conditions for 24 h, followed by a 2 h drying in a vacuum desiccator at 50 °C. The prepared films were stored in a dark, ventilated space in a container with moisture-absorbing silica beads.

Figure 18 Schematic representation of film preparation.

(36)

4 RESULTS AND DISCUSSION

This section consists of three parts. The first part discusses the synthesis and properties of ND-Polyelectrolyte complex dispersions. This is followed by an investigation into ND-Polyelectrolyte films prepared from the same building blocks focusing on mechanical and stimuli responsive properties. In the third part, the CO2

adsorption properties and synthesis of CO2adsorbent materials are presented.

4.1 ND-POLYELECTROLYTE DISPERSIONS

^I

The PEO-b-PDMAEMA-C12polymers were synthesized via RAFT polymerization and subsequent post-modification (Sections 3.2.1 and 3.2.2). The three polymers were synthesized to have the same PDMAEMA-block lengths, while the PEO-block length and presence of the hydrophobic C12-dodecyl end-group were varied. In order to investigate the interactions between NDs and polymers, the complexation process and the resulting complexes were first studied. Then complexation variables such as temperature, polymer to ND ratio and use of sonication were varied and their effect on the size distribution, pH and zeta potential of the complexes were monitored. Complexes with 20:1 (polymer:ND) molar ratio, in other words, polymer chains per nanodiamond, were chosen to be further investigated to assess their size, surface charge and colloidal stability.

Prepared polymers and complexes.

Sample Label

PEO17-b-PDMAEMA-C12 a1 PEO105-b-PDMAEMA-C12 a2

PEO17-PDMAEMA b

PEO17-PDMAEMA-C12+ ND C-a1 PEO105-PDMAEMA-C12+ ND C-a2 PEO17-PDMAEMA + ND C-b

ND -

(37)

4.1.1 COMPLEXATION OF PDMAEMA AND ND

The polymer:ND complexation was studied by ¹H-NMR by titrating polymer solution with ND dispersion. As the surface of NDs comprise mostly of carboxylic acids, the amine containing PDMAEMA-based polymers are expected to interact with their surface in aqueous conditions. This would lead to lowered mobility of the interacting polymer segments and consequently to suppressed NMR signal intensity. The titration of polymers a1 and a2 by ND dispersion (Figure 19) shows a decrease of signal intensities with an increasing amount of ND added. By observing the change in signal intensities of block copolymer a1 (Figure 19A), it can be concluded that all parts of the polymer lose mobility with the addition of NDs. When polymer a2 with longer PEO is studied (Figure 19B) the PDMAEMA signal decreases similarly as with polymer a1, but the PEO and C12 signals are less suppressed. This shows that the electrostatic interaction between the ND carboxyls and PDMAEMA plays the main role in the complexation process. The less suppressed signal of PEO of a2 compared to a1 indicates that longer PEO-chains of a2 extend as dangling chains from the ND surface towards the solvent. The situation is different for a1, where the short PEOs are embedded in the ND-polymer interface. Analysis of the C12 signals in the case of a1 indicates that the dodecyl end-group is in a very restricted environment due to the presence of hydrophobic interactions with the ND surface. For polymer a2, the signal from C12 suppresses less during titration, which can be related to the overall more hydrophilic nature of the a2 polymer due to the longer PEO-blocks.

(38)

Figure 19 ¹H-NMR signal suppression from titration of polymers a1 (A) and a2 (B) with ND-D2O dispersion. Signal intensities of different polymer blocks are normalized with respect to dilution.

The IR spectrum of the complexes (Figure 20) shows the combined signals of polymer and ND. Compared to polymer and ND separately, the absorbances are shifted slightly indicating the binding of polymers onto the ND surfaces. The clearest indication of the interaction is the attenuation of the N-(CH3)2 signal at 2750 cm⁻¹, where the absorption band is nearly undetectable in the complex sample, as has been observed earlier for PDMAEMA complexed with anionic moieties. ^94,162

(39)

Figure 20 FTIR-spectra of as received ND, polymer a1 and complex C-a1.

The thermal stability of the complexes was studied with TGA under He- atmosphere (Figure 21). The thermogram shows a combination of the degradation patterns of pure ND and polymer with three distinguishable steps. Nearly all polymer degrades prior to the onset of ND degradation, enabling the estimation of the amount of polymer in the complex. The polymer mass of the purified complex according to TGA is around 20 wt%, which corresponds well with the mass ratio of the polymer in the feed and proves efficient complex formation.

(40)

Figure 21 TG-curves of ND, Polymer a1 and complex C-a1.

DLS, Zeta Potential and pH of complexes.

Label Polymer pH Zeta Potential (mV)

Diameter (nm)

PDI

C-a1 PEO17-PDMAEMA-C12 7.3 51.1 â154.2,^b27.8 0.224 C-a2 PEO105-PDMAEMA-C12 7.4 44.3 â190.6, ^b50.0 0.310 C-b PEO17-PDMAEMA 7.5 40.4 â155.5, ^b79.7 0.225

ND - 3.8 -74.1 ^a149.5, ^b30.5 0.240

aZ-Average size.

bNumber mean size.

When the as-received NDs are dispersed in water by stirring, the particles remain as large aggregates. Sonication leads to narrower size distribution and decreased size. However, the dispersions sediment quickly due to re-formation of the larger structures. By adding an oppositely charged polymer and optimizing the mixing ratio with the NDs, complexation temperatures and sonication times, colloidally stable complexes can be prepared. Increasing the amount of polymer increases the size of the particles as well as their zeta potential.

Initially negatively charged particles show positive zeta potential when enough polymer is added, but aggregate when the zeta potential is close to zero.

(41)

Lowering the complexation temperature to 0 °C from room temperature results in smaller complexes and increases the zeta potential to positive value at lower polymer feeds. The pH of complexes are generally slightly basic due to the basic polymer layer. Sonication up to 120 min decreased the size of the complexes considerably. Complexes made with a molar mixing ratio of 20:1 (polymer:ND), mixing temperature of 0 °C and 120 min of sonication had the most suitable properties for stable and small dispersions.

4.1.2 SIZE AND ZETA POTENTIAL OF COMPLEXES

According to DLS, the smallest complexes are obtained by using polymer a1 with short PEO-block and C12-group (Table 2, Figure 22A) showing particle sizes comparable to the plain ND dispersions after sonication. The particle size increases for complexes with polymer a2 with the longer PEO-block, which is caused by the extended PEO-chains from the complex surface as shown by NMR (Section 4.1.1).

Complexes prepared from the polymer without the hydrophobic C12-group, C-b, show the largest size, which is also demonstrated in a morphological change in the TEM images discussed later in this section (Figure 23). To achieve small particle sizes, the hydrophobic-hydrophilic balance is important in a similar manner as the charge ratio. Too strong hydrophobic interactions will lead to loss of stability and without the interactions loose aggregates with dangling chains and bigger size are formed. Similar effects have been noticed when using polymers with hydrophobic groups as dispersants. ^163,164

Upon stoichiometric complexation, the acidic NDs and basic polymers should neutralize each other. Acidic pH < 7 indicates free ND and basic pH > 8 free polymer. Complexes prepared with polymers containing the hydrophobic C12-group are neutral even without sonication (Figure 22B). The length of PEO-block does not affect this. However, complexes prepared with the polymer without C12 are basic, suggesting incomplete complexation without sonication. After sonication, the pH is neutral. This shows that the binding of polymers to the ND surface is aided by the hydrophobic C12 end-group. The pH of the processed complex dispersions is neutral, which is preferable in biological applications.

All studied stable complexes have a ZP above +30 mV (Figure 22C), in contrast to the pure ND dispersion having a ZP below −60 mV. The complexation thus induces a charge inversion of the particles. Sonication increases the ZP if the value is positive and decreases ZP if the value is negative. This is due to the breaking down of the aggregates thus increasing the electrophoretic mobility of the particles being measured. The surface zeta potential>+40mV of the sonicated complexes provides efficient electrostatic stabilization to the dispersed particles.

(42)

Figure 22 Size distributions of complexes and ND by number (A) with pH- (B) and zeta potential (C) values of complexes and NDs before and after sonication. Error bars represent instrument maximum error (5%) in pH data and measured deviation in ZP data.

TEM was used to visualize the NDs and their complexes (Figure 23). The images confirm large and dense aggregates of pure NDs, while the complexed Nds are more finely and evenly dispersed at all magnifications. The complex C-a1 with the C12- chain is smaller and clearly more dispersed than the complex C-b without C12, highlighting the important role of the dodecyl end-group in the complexation process via interactions with the hydrophobic groups on the ND surface.

(43)

Figure 23 TEM images of ND (A), C-a1 (B) and C-b (C). Note the increase in magnification from left to right 10,000 nm→1000 nm→100 nm.

4.1.3 COLLOIDAL STABILITY

The colloidal stability of the ND and complex dispersions in deionized water was followed using samples stored at room temperature (Figure 24). The pure ND dispersion aggregated considerably after 1 week of storage. The average diameter nearly doubled (Figure 24A) and the ZP halved (Figure 24B) on top of visual observation of sedimentation. As the size of the aggregate increases, less surface is exposed causing smaller ZPs. The complexed NDs showed small increase in size and no significant changes in ZP over 5 weeks of observation time, apart from a small fraction of sediment seen by eye. This shows clearly how complexing NDs enhance the stability of the formed dispersion significantly and the composition of polymer

(44)

Figure 24 Stability measurements conducted over a time span of 5 weeks. Size (A) and ZP (B) with standard deviation.

The stability of NDs and complexes C-a1 and C-a2 in saline were measured up to similar salt concentrations as in human blood. Samples prepared in different NaCl solutions were measured by DLS (Figure 24A) and ZP (Figure 24B) within 2 h of preparation. The pure ND dispersion began to precipitate already at a concentration of 0.05 mol/L and was unmeasurable by DLS at higher salt concentrations due to complete sedimentation. Complexes remained in solution upon increasing ionic strength. The size of C-a1 complexes increased when the salt concentration was above 0.05 mol/L. Complexes of C-a2 with the long PEO retained their size relatively unchanged over the whole concentration range studied, indicating that the increased length of the PEO-block improves the stability. ZP of both complexes decreased due to the screening of charges. The results show that the steric stabilization provided by PEO-chains promotes the colloidal stability even though the charges are screened through increased ionic strength.

Dried complexes are redispersable and the flakes are easier and safer to work with compared to the powdery ND that is easily airborne.

(45)

Figure 25 Stability measurements at increasing ionic strenght.

4.2 ND-POLYELECTROLYTE FILMS

^II

Films with a thickness of around 200 μm were made by photopolymerization of BA and DMAEMA P(BA-DMAEMA) in the presence of 2.5 wt % of the cross-linker BuDMA. In order to study the effects of the NDs and the complexing polymer PEO17- PDMAEMA-C12 (C-polymer), a series of films were prepared by varying the ND content and the presence of C-polymer (Table 3). The NDs and PEO-PDMAEMA were added either separately or at the same time (Figure 18). IR-spectra and a single Tg transition observed in DSC measurements confirmed the statistical copolymerization of the monomers during curing. TGA of the films showed two distinct steps corresponding to PDMAEMA and poly(butyl acrylate) (PBA) degradation (Figure 26). The addition of NDs with the C-polymer did not considerably change the thermal decomposition profile, but resulted in an increase of the residue after the heating. The increase of the residue corresponded to the amount of ND added to the polymer mixtures before curing.

(46)

Figure 26 Thermogravimetric analysis (TGA)-curves of films with different filler contents.

Data of prepared films.

Film Label BA (wt%)

DMAEMA (wt%)

ND (wt%)

PEO17-PDMAEMA-C12

(wt%)

F1 20 80 0 0

F1/0.1ND 20 80 0.1 0

F1/0.5ND 20 80 0.5 0

F1/1ND 20 80 1 0

F1/2ND 20 80 2 0

F1/C 20 77.5 0 2.5

F1/C/0.1ND 20 77.5 0.1 2.5

F1/C/0.5ND 20 77.5 0.5 2.5

F1/C/1ND 20 77.5 1 2.5

F1/C/2ND 20 77.5 2 2.5

Responsive Polyelectrolyte and Nanodiamond Hybrid Materials