HYBRID MATERIALS WITH IMPROVED THERMOMECHANICAL PROPERTIES

Transcription

1 Charles University Faculty of Science Department of Physical and Macromolecular Chemistry M.Sc.Eng. Magdalena Ewa Perchacz HYBRID MATERIALS WITH IMPROVED THERMOMECHANICAL PROPERTIES Ph.D. Thesis Supervisor: Hynek Beneš, Ph.D. Institute of Macromolecular Chemistry AS CR Department of Polymer Processing PRAGUE 2017

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3 Univerzita Karlova Přírodovědecká fakulta Katedra fyzikální a makromolekulární chemie Mgr. Ing. Magdalena Ewa Perchacz HYBRIDNÍ MATERIÁLY SE ZLEPŠENÝMI TERMOMECHANICKÝMI VLASTNOSTMI Dizertační práce Školitel: Ing. Hynek Beneš, Ph.D. Ústav makromolekulární chemie AV ČR, v.v.i. Zpracování polymerních materiálů PRAHA 2017

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5 Prohlášení: Prohlašuji, že jsem předkládanou závěrečnou práci vypracovala samostatně a že jsem uvedla všechny použité informační zdroje a literaturu. Předkládaná práce ani její část nebyla předložena k získání jiného nebo stejného akademického titulu. Declaration: Hereby I declare that I have worked out this doctoral thesis independently, under the guidance of Ing. Hynek Beneš Ph.D., and I have fully cited all sources used. This work has not been used in order to gain any other academic degree. Prague, Magdalena Perchacz

6 Acknowledgement I would like to thank to all people without who this Ph.D. thesis could not arise. My special thanks belong to my supervisor Hynek Beneš, Ph.D. for his support, time, relevant advice, effort and valuable comments. I am especially grateful to Ricardo K. Donato, Ph.D. and Libor Matějka, D.Sc. for cooperation, fruitful discussions and their help during my doctoral study. I must express my gratitude to my colleagues, secretary Aleksandra Paruzel, Alessandro Jäger and others, for a wonderful environment in which my work proceeded and a lot of optimism, necessary to survive the worst moments. Finally, my special thanks go to my family and boyfriend Martin for encouraging me to do the Ph.D. studies and their great support.

7 Table of contents List of abbreviations and symbols... 6 Summary Introduction Organic/inorganic hybrid materials Organic phase (polymer matrix) Inorganic phase (filler) Sol-gel synthesis of inorganic silica phase Main routes and parameters Reactants Catalysts Preparation of epoxy-silica hybrid materials Epoxy-silica hybrids from pre-formed silica nanofillers In-situ synthesis of epoxy-silica hybrids Properties of epoxy-silica hybrid materials Ionic liquids and their roles in epoxy-silica hybrids Ionic liquids as catalysts Ionic liquids as coupling agents Aims of the thesis Results and discussion Glassy epoxy-silica hybrid materials Solvent-free sol-gel synthesis of silica-based precursors Investigation on the ionic liquid-driven sol-gel mechanism Morphology of glassy epoxy-silica hybrids Optical properties of glassy epoxy-silica hybrids Thermomechanical properties of glassy epoxy-silica hybrids Dynamic mechanical thermal analysis Tensile properties Thermogravimetric analysis Rubbery epoxy-silica hybrid materials Morphology of rubbery epoxy-silica hybrids Thermomechanical properties of rubbery epoxy-silica hybrids Conclusions References Publications and conference contributions List of publications included in the thesis

8 List of abbreviations and symbols BDMA N,N-dimethylbenzylamine CH 2 CO 2 HMImCl 1-carboxymethyl-3-methylimidazolium chloride C 3 H 6 CO 2 HMImCl 1-carboxypropyl-3-methylimidazolium chloride C 4 MImCl 1-butyl-3-methylimidazolium chloride C 4 MImMeSO 3 1-butyl-3-methylimidazolium methanesulfonate C 7 O 3 MImMeSO 3 1-triethyleneglycol monomethyl ether-3-methylimidazolium methanesulfonate CTE coefficient of thermal expansion d particle diameter D mass fractal dimension DABCO 1,4-diazabicyclo[2.2.2]octane DBTL dibutylbis[1-oxo(dodecyl)oxy]stannane DGEBA diglycidyl ether of bisphenol A GPTMS (3-glycidyloxypropyl)trimethoxysilane HCl hydrochloric acid HCOOH formic acid HF hydrofluoric acid IL ionic liquid Jeffamine D-230 poly(oxypropylene)diamine (with average molecular weight (or Jeffamine D-2000) 230 or 2000) ORMOSIL organically modified silica particles PEI poly(ether imide)s PEK poly(ether ketone)s PES poly(ether sulfone)s PPE poly(phenylene ether) PSU polysulfone POSS polyhedral oligomeric silsesquioxane p-tsa p-toluenesulfonic acid q condensation degree R g TEOS T g (T α ) radius of gyration tetraethoxysilane glass transition temperature (α-transition temperature) 6

9 Summary Epoxy resins have been broadly used in the industry for adhesives, laminates, coatings, composites, encapsulation of electronic devices, printed circuit boards, etc. Despite their excellent adhesion to different materials, heat and chemical resistance, good mechanical properties, they also exhibit few drawbacks like brittleness, high thermal expansion coefficient (CTE), poor resistance to crack initiation and growth. Therefore, the thesis is focused on the preparation of epoxy-silica hybrid materials exhibiting improved thermomechanical properties compared to the neat epoxides, without impairing their beneficial features. The studied synthetic route of epoxy-silica hybrids preparation has been the solgel process of alkoxysilanes, allowing either in-situ formation of high purity and homogeneity silica particles or creation of various siloxane structures in a form of liquid (sol) silica-based precursors. The sol-gel method, on one hand, helps to omit too high viscosity of nanofiller suspension and energy-intensive nanofiller dispergation problems, but on the other hand, is often associated with necessity to use solvents and remove formed volatiles. Therefore, in the first part of the thesis, a simple solvent-free sol-gel procedure, enabling synthesis of pre-condensed epoxy-functionalized silica-based precursors using hydrolytic polycondensation of (3-glycidyloxypropyl)trimethoxysilane (GPTMS), different catalysts (conventional and imidazolium-based ionic liquids (ILs)) and reaction set-ups, was described. The obtained liquid products were homogenous and highly miscible with the epoxy-amine system in which they formed well distributed silica domains, covalently bonded to the epoxy network. The role of conventional catalysts and imidazolium-based ILs on the silica structure evolution and the further impact of the latter on the organic/inorganic interphase was investigated and determined. Furthermore, the mechanism of IL-driven solvent-free sol-gel process of GPTMS, consistent with experimental study and theoretical calculations (simulation), was proposed. The use of appropriate amount of silica-based precursors led to the significant improvement in shear storage modulus in rubbery region, energy to break, elongation at break and thermooxidative stability of the glassy epoxy-amine matrix, without affecting its optical transparency. Moreover, the IL-silica precursors additionally improved the reference matrix s transparency in UV region. 7

10 Also, in the second part of the thesis, the rubbery epoxy-silica hybrids, with in-situ generated silica phase and carboxylic IL coupling agent, were prepared and characterized. It was found that the applied ILs were covalently and/or physically bonded on the fillermatrix interphase, leading to the formation of epoxy-silica hybrids with well-defined morphology and significantly improved thermomechanical properties. 8

11 1. Introduction 1.1. Organic/inorganic hybrid materials Organic/inorganic (O/I) hybrid materials consist of two or more intimately mixed organic and inorganic components on the nanometer or molecular level. Consequently, the final properties of O/I hybrids can be tailored, as they are strongly dependent on the ratio between both phases (amount), the nature of interactions on the O/I interphase, specific physicochemical properties of each introduced phase, e.g. surface energies and morphologies (size and shape of inorganic domains), etc. All these factors enable formation of new compex and multifunctional systems. [1-5] The most widely used classification of hybrid materials is based on the nature of interactions between organic and inorganic components and can be divided into two classes. The 1 st class includes all systems characterized by non-covalent interactions between organic and inorganic phases (H-bonding, van der Waals and electrostatic forces, π-π interactions). Typical examples are: polymer blends, interpenetrating networks, ionogels, conductive polymers, organically intercalated inorganic clays, layered hydroxides, etc. [2-4, 6-7] In contrast, the 2 nd class contains materials in which covalent bonds link organic and inorganic components. Thus, hybrid network is formed as the result of a specific chemical reaction between organic polymer and functionalized inorganic building blocks, as well as between two polymers containing reactive groups. The best known are hybrid materials synthesized using the sol-gel process of functional organosiloxanes as well as the organically-functionalized lamellar inorganic hydroxides. [2-3, 6, 8-12] The most common types of O/I hybrids are shown in Figure 1. Figure 1. Main types of hybrid materials. 9

12 The combination of organic and inorganic phases often brings outstanding properties of the final material, impossible or hard to obtain in the case of typical composites. Usually, the organic phase brings mechanical toughness, flexibility and easy processability to the system, which contrasts with hardness, thermal and chemical stability ensured by the inorganic phase. The most frequent case is that the matrix is an organic polymer and the inorganic phase is in the form of dispersed inclusions Organic phase (polymer matrix) The most broadly used organic phases (polymeric matrices) in O/I hybrids are epoxy resins, polysaccharides, polyacrylates, polyesters, polyurethanes, polyamides, polyolefins, etc. [13-18] Among them the epoxy materials are the most versatile ones, possessing several advantages such as low creep and cure shrinkage, optical transparency, outstanding adhesion, good electrical insulating properties, excellent fatigue and chemical resistance, high strength, lack of volatile products emission, high thermal stability, as well as applicability in a wide range of temperatures, depending on the type of curing agent. Therefore, the epoxides are the key materials in the production of adhesives, coatings, laminates, castings, structural foams, etc. [19-21] The three-dimensional epoxy network can be formed in step-growth or chaingrowth polymerization, or combination of both mechanisms. [20-21] The most commonly used epoxy resin prepolymers are ethers containing two or more epoxide groups per molecule, such as diglycidyl ether of bisphenol A, diglycidyl ether of bisphenol F, triglycidyl ether of tris(4-hydroxyphenyl)methane, triglycidyl ether of p-aminophenol, etc. Depending on the type of crosslinking agent (amines, amides, anhydrides, isocyanates, carboxylic acids, phenols, mercaptans, thiols, imidazoles, etc.), which under curing transforms the low-molecular-weight epoxy monomer/oligomer into crosslinked network, different physico-chemical properties of the final epoxy-based material can be obtained. [19, 21, 22-24] The most versatile hardeners (curatives) of epoxy resins are amines, which can be used at room as well as at elevated temperatures. [23] In general, the curing mechanism of an epoxy-amine system is based on the epoxy ring-opening caused by the addition of active hydrogens from primary or secondary amine (Figure 2). 10

13 Figure 2. Curing mechanism of an epoxy resin with primary (1) or secondary amine (2). The curing may be performed with the use of external heat or other source of energy, such as UV radiation, electron beam, etc. In some cases the reaction can also [19-20, 22, 25-26] proceed at a slower rate at room temperature Inorganic phase (filler) Inorganic fillers are the most broadly used additives for polymers, as they possess better mechanical and thermal properties compared to the organic matrix, and therefore, can improve the strength, hardness and thermal stability of the final material. In the last years, the biggest attraction have gained the inorganic nanofillers, due to an extremely large surface area and therefore, also large interfacial area when well-dispersed in the polymer matrix. Moreover, the nanosized additives have become very helpful in designing specific properties of the organic materials. In general, nanofillers are solid, liquid or gaseous additives, with at least one geometric size lower than 100 nm. The smaller filler particles, the higher transparency of final nanocomposites, caused by the decrease in intensity of scattered light. However, if both components have the same refractive indices, the scattering intensity is zero and does not depend on the filler size. Nevertheless, the high specific surface area of nanofillers, their frequent hydrophilicity and mutual interactions, might cause undesirable aggregation in the hydrophobic matrix. Therefore, a very important step is to stabilize the filler by chemical or physical functionalization of its surface (e.g. by silanization, incorporation of anchor groups) and transfer into the organic matrix in the form of filler solution. This technique helps to improve the interfacial adhesion between organic and inorganic phases, which might [6, 27-29] result in the inhibition of agglomeration process. The most broadly used nanofillers are nanosilica or nanorubber particles, polyhedral oligomeric silsesquioxanes (POSS), nanoclays, layered silicates, metal oxides, stanoxane clusters, carbon and titanium nanotubes or nanofibers, graphene and graphene 11

14 oxide sheets, fullerene, cellulose nanofibers and nanoparticles, metal-organic frameworks [11, 15, 30-46] (MOFs), etc. The nanofiller preparation methods can be divided into two main groups: (1) insitu, ex-situ and (2) bottom-up, top-down. The first two processes reflect the direct nanocomposites preparation. In the in-situ method, the nanofiller is synthesized inside the polymer matrix, simultaneously to the formation of nanocomposite structure. In contrast, the ex-situ process is based on the addition of preliminary pre-formed nanofiller with designed properties into the polymeric matrix. The nanofillers are usually synthesized using the bottom-up approach, based on the reactions in the liquid phase. In this method the filler synthesis starts at atomic or molecular level, with further self-assembly process leading to the formation of nanostructures. Typical examples are the sol-gel process, intercalation of clays or hydrothermal reactions. More expensive, slow and not suitable for large-scale production is the top-down approach, commonly used in photolithography or for mechanical exfoliation of graphite. Herein, the nanofillers are formed from the [35, 47-51] larger macroscopic structures, which are further reduced to nanoscale Sol-gel synthesis of inorganic silica phase The sol-gel process is a well-known technique for preparation of gels, ceramic powders, glasses, films, fibers as well as bulk materials. This method has many advantages, such as the usage of low reaction temperatures (compared to those required in conventional techniques), the high purity and homogeneity of a product, the possibility of synthesis in solvent-free conditions and the ability to design the special material s properties. The sol-gel method can be described as the formation of a network through the hydrolytic polycondensation reaction of low viscous component(s) in a solution. The mechanism of the sol-gel process can be divided into two stages (Figure 3), which often go simultaneously: (a) the formation of silanols (Si-OH) by the nucleophilic attack of oxygen from water molecule on the silicon compound with the proton transfer to the leaving alkoxy group (-OR) and further release of alcohol; (b) the formation of siloxane (Si-O-Si) oligomers/polymers in the water- or alcohol-producing polycondensation reactions. 12

15 Initiation - Hydrolysis: Si OR + H 2 O Si OH + ROH Propagation - Condensation: Si OH Si OR + HO Si Si O Si + H 2 O + HO Si Si O Si + ROH Figure 3. Main reactions of the sol-gel process of alkoxysilanes (OR alkoxy group) Main routes and parameters The main routes of the sol-gel process are: (1) formation of colloidal sol, (2) reaching the gel point, (3) gelation, (4) ageing, (5) drying and (6) densification (Figure 4). At first, the colloidal solution (sol) is formed, which, in a further polycondensation step, creates a three-dimensional continuous solid network in a liquid phase (wet gel). The gel point is reached when an infinite polymer or aggregate appears and the further branching leads to the fast disappearance of sol phase and formation of finite network structure. In general, a gel is formed due to the covalent interactions (irreversible gel) or physical bonding (causing the reversibility of the gelation process). All the physical and chemical changes taking place after gelation, like further condensation, reprecipitation, dissolution or phase transformation, are called aging and they occur always before complete drying of the material. The drying process might be carried out in two ways. When it is done at ambient pressure, the shrinkage of material occurs (due to the removal of solvent) and the xerogel structure is formed. In contrast, when the drying proceeds under hypercritical [3, 35, 47- conditions, the network does not shrink and the highly-porous aerogel is produced. 48, 52-57] A high variability of the sol-gel process permits to control the size and morphology of formed silica-based products, which is strictly dependent on selected reaction conditions, functionality of monomer, water/alkoxysilane/solvent molar ratio, ph of solution and type of used catalyst. Polymerization in the acid catalyzed system (ph = 2 4) consists of the faster hydrolysis than condensation step, conversely to the base catalyzed sol-gel (ph = 6 9). [47, 58-60] The optimization of mentioned parameters is essential in designing the final silica structure, which can be in the form of threedimensional network, spherical particles, 3D cubic silica species or two-dimensional ladder-like, cyclic, oligo- and polymeric structures (clusters), depending on the 13

16 mechanism of silica structure growth (Figure 5). [47, 56, 59-72] Therefore, the precise knowledge of sol-gel conditions is the key factor enabling formation of the silica-based product with desired morphology influencing the properties and target application of the final hybrid material. Figure 4. Main routes of the sol-gel process. Figure 5. Mechanism of structure growth in the ph-driven sol-gel process. Adapted from ref. [72] 14

17 Reactants Commonly used and commercially available reactants are organically modified precursors (chlorosilanes, alkoxysilanes, etc.) containing the functional groups able to covalently bond and/or physically interact with the organic matrix. Frequently used are di-, tri- and tetraalkoxysilanes with/without functionalized organic side-chains [6, 47, 57, 65, 73-75] (containing e.g. epoxy, amine, carboxyl or vinyl groups) Catalysts The most commonly used sol-gel catalysts are inorganic and organic acids (e.g. hydrochloric acid HCl, p-toluenesulfonic acid p-tsa, formic acid HCOOH) accelerating the hydrolytic reaction. The products are usually in a form of precipitated nanoparticles or porous gels, with more homogenous distribution and larger pores than in the case of materials base-catalyzed (e.g. N,N-dimethylbenzylamine BDMA, 1,4- diazabicyclo[2.2.2]octane DABCO, etc.). [76] The sol-gel process might be accelerated by the addition of ammonium salts as catalytic agents. Nevertheless, only the salts build of a weak acid and a weak base are expected to act as acid-base pair catalysts, leading to the faster polycondensation reaction. [77-79] Very popular became also the neutral catalysts, due to their medium effect on the rate of sol-gel process and tendency to reduce the distribution of formed species, when compared to acid catalysts. For instance, the dibutylbis[1-oxo(dodecyl)oxy]stannane (DBTL)-catalyzed polycondensation of alkoxysilane goes faster than in the system acid-catalyzed, but slower than in the system base-catalyzed. [61] It is important that the neutral and basic catalysis usually causes an intensive cyclization or even formation of cubic cage-like species. [61-62] 1.3. Preparation of epoxy-silica hybrid materials The epoxy-silica hybrid materials can be prepared using two main routes: by physical incorporation of pre-formed silica nanofillers: well-defined POSS building blocks, admixed silica nanoparticles or pre-condensed silica-based precursors in-situ, using the sol-gel process of alkoxysilanes Epoxy-silica hybrids from pre-formed silica nanofillers We can distinguish three preparation methods of epoxy-silica hybrid materials based on the physical incorporation into the epoxy-based system of: 15

18 [30-32, 34, 80- specifically shaped inorganic clusters (e.g. POSS) bearing functional groups. 81] They are frequently synthesized using the sol-gel process of various alkoxysilanes, which are resistant to hydrolysis due to the the presence of Si-C covalent bonds. Nevertheless, most of the building blocks are not thermodynamically stable because of their high surface energy, which often leads to the coalescence and formation of larger aggregates. Therefore, in order to prevent this process, some surface modification (e.g. its functionalization) of the building blocks should be carried out. Usually it can be obtained by: formation of inorganic clusters in the presence of functional organic molecules, grafting of functional groups to the inorganic building blocks (post-synthesis method), e.g. the silica-based building blocks covered by OH groups, can react with various silane coupling agents or surfactants (acting as compatibilizing agents), synthesis of inorganic clusters in the presence of capping agents or ligands, usually using the sol-gel approach. For instance, the POSS or ladder-like silica structures can be prepared from trialkoxy- or trichlorosilanes containing [6, 11-12, 35] functional organic groups. fumed or precipitated silica nanoparticles. [82-85] Nevertheless, it is often difficult to obtain a good molecular dispersion of filler resulting in the formation of aggregates and heterogeneities in the final hybrid material. In order to avoid this problem, the surface functionalization is often necessary. The rapidly growing method is based on the synthesis of organically modified silica particles (ORMOSIL) bearing covalently linked IL as well as on the grafting of silane agents on silica using ILs as activators. [86-87] pre-condensed silica-based precursors synthesized using the sol-gel process of alkoxysilanes bearing functional groups (e.g. epoxide, amino or vinyl groups). Depending on the reaction conditions and type of alkoxysilane, different silica structures can be obtained, including ladder-like, cage-like, cyclic, linear or branched species, in the form of liquid (sol) precursors easily homogenized with the epoxy [65, 73-74, 88] system. 16

19 In-situ synthesis of epoxy-silica hybrids The preparation of epoxy-silica hybrid materials using the in-situ sol-gel method is based on the simultaneous formation of epoxy network and silica structures by independent reaction mechanisms and can be divided into two groups: in-situ formation of epoxy-silica hybrids using the hydrolytic sol-gel process of alkoxysilanes, with formation of two interpenetrating networks (independent networks, which, in general, do not chemically interact with each other), interconnected networks (covalently bonded with each other) or silica domains (physically or covalently bonded [1, 6, 35, 73-74, 88-93] with the epoxy network). in-situ epoxy-silica network formation using the non-hydrolytic sol-gel process of alkoxysilanes. [94-96] The method is based on the sol-gel synthesis of nanosilica directly in the epoxy resin using alkoxysilanes and BF 3 MEA catalyst, which helps to obtain a fine silica morphology in the epoxy matrix. Such precursor is further cured with an epoxy hardener (e.g. amine). Nevertheless, except few advantages of the nonhydrolytic sol-gel process, e.g. non-toxicity and simplicity of synthesis, it has also some disadvantages, e.g. complexity of the reaction mechanism and necessity to use a reatively high amount of expensive catalyst. Moreover, due to the slow non-hydrolytic sol-gel process the epoxy network forms faster and hinder the silica growth. Therefore, appropriate conditions must be assured, i.e. optimum content of BF 3 MEA and coupling agent (e.g. GPTMS), high amine basicity and relatively high curing temperature Properties of epoxy-silica hybrid materials The highest attention gained the glassy epoxy materials due to their outstanding physicochemical properties. Nevertheless, they exhibit also few common drawbacks such as brittleness, low toughness, high thermal expansion coefficient (CTE), poor resistance to crack initiation and growth, which are mostly caused by the high crosslinking density and internal stresses induced during epoxy curing. [15, 22, 25-26, 65, 97-99] In order to decrease CTE, the micro-sized silica has been usually incorporated into the epoxy system, which, nevertheless, increases viscosity of epoxy resin disabling processability and makes the final material more britle with reduced optical transparency. [ ] The surface of such silica filler is covered by silanol and/or siloxane groups able to form H-bonding and cause particles aggregation, which contributes to the decrease in fracture toughness of the 17

20 glassy epoxy-based materials. Therefore, the silica surface functionalization (chemical or physical) method has been broadly used, which helps to avoid the particles aggregation, phase separation and improve the final material properties, strongly depending on the mutual interphase interactions. Accordingly, the homogenous particle distribution can be obtained if the polymer-filler interactions are stronger than the filler-filler ones and the [47, 102] appropriate mixing is applied. Epoxy resins are toughened significantly by a dispersion of rubber particles. [103] However, it is well-known that their incorporation increases the overall viscosity of epoxy resin and reduces dramatically the mechanical strength and glass transition temperature (T g ) of a cured epoxy matrix. The undesirable reduction in stiffness of the rubber-filled epoxy system could be omitted by addition of inorganic (mainly silica) nanoparticles or amorphous thermoplastics with high T g (such as poly(ether sulfone)s PES, poly(ether ketone)s PEK and poly(ether imide)s PEI, polysulfone PSU and poly(phenylene ether) PPE). [ ] According to the latter, the incorporation of high T g glassy thermoplastics into the epoxy system can improve its toughness, when the cocontinous or phase-inverted morphology is achieved. [112] Nevertheless, such macroscopic phase separation of thermoplastic domains limits the utilization of this method only to non-transparent materials. Another approach uses rigid nanosilica particles introduced via sol-gel technique into the epoxy matrix, which causes an improvement in toughness and modulus, with/without significant changes in viscosity of reactive mixture, optical transparency and T g of fully cured materials. [113] Also, the pre-condensed silica-based precursors, synthesized using the solventfree sol-gel process of alkoxysilanes, were successfully applied into an epoxy-amine network and led to the improvement in thermomechanical properties of the final hybrid [73-74, 88] materials. Further, the non-aqueous sol-gel process of alkoxysilanes was developed leading to the epoxy-silica hybrid materials with fine morphology and improved mechanical properties. [94-96] The epoxy-silica hybrids have been recently prepared using addition of different [92-93, 114] imidazolium-based ILs leading to the improvement of interphase interactions. Similarly, a synergistic effect of phosphonium-based ILs and PPE on toughness of epoxy material has been reported. [115] From this point of view, ILs and their combinations with nanosilica phases might be considered as novel multifunctional additives for epoxy materials. 18

21 1.5. Ionic liquids and their roles in epoxy-silica hybrids ILs are salts with melting temperatures below 100 C, negligible vapor pressures, non-flammability, moderate polarities, wide range of conductivities, high thermal stability, etc. [ ] Therefore, they are widely used as selective and reaction-rate- [92-93, ] enhancing catalysts, coupling agents, low volatile solvents, electrolytes, etc. The variety of physicochemical properties of ILs results from the type of cation and anion, and their mutual interactions. It was evidenced that the melting temperature of IL increases with increasing size and asymmetry of the cation, and branching of the alkyl side-chain, as well as strongly depends on the nature of an anion. The choice of suitable anion allows designing the viscosity and density of molten salts, as well as the IL solubility. [ ] The major classes of ILs and their functions during the polymer formation are shown in Figure 6. Nevertheless, the most widely used are ILs containing cationic N- [117, 123] heterocycles (imidazolium, ammonium, pyridinium). The IL s anion may be organic (CF 3 COO -, CF 3 SO 2 N -, RSO 3 -, etc.) as well as inorganic (PF 6 -, AlCl 4 -, Cl -, BF 4 -, etc.). Figure 6. Classification of ILs depending on cation s type (a) and their final functions in the polymer formation processes (b). 19

22 Nowadays, the well-known are imidazolium-based ILs, which found applications [117, 120, ] in batteries, catalysis, fuel cells, ionogels, lubricants, electrochemistry, etc. The unique properties of each IL depend on dominant Coulombic ion-ion interactions, characteristic H-bonding, π-π stacking of the rings and van der Waals forces (Figure 7). It was evidenced that ILs form nanostructural supramolecular networks, based on cations and anions connected by hydrogen bonds. Moreover, ILs in solvent solutions create complex and concentration dependent structures, e.g. supramolecular aggregates, contact [121, ] ion pairs, triple ions, etc. It is important that even the hydrophobic ILs absorb water from the air, which forms H-bonded water-anion complex. In the water-il solutions the cation-anion interactions present in ILs are replaced by the H-bonding and therefore, the internal organization of IL changes resulting in a looser packing of the imidazolium rings. The addition of small amount of water into the IL causes isolation of water molecules from each other. In contrast, the water-il system in which the water content prevails, is less defined with non-selective solvation, due to an increase in the water-il s proton interactions. [130] Multiple H-bonding occurs in the vicinity of imidazolium ring as well as alkyl side-chains and is partially responsible for the anion localization. Usually, the anion forms the strongest H-bonding with the C 2 -H, C 4 -H and C 5 -H hydrogens. The medium strength H- bonding interactions, between C 7 -H or C 6 -H and the anion, are dependent on the alkyl chain conformation, which also influences the anion displacement. The hydrogens localized on the further chain carbons (C 8 -C 10 ) can form weak interactions with the larger anions. [131] Figure 7. Possible anion association sites in the imidazolium-based IL with n-butyl sidechain. Adapted from ref. [131] 20

23 Ionic liquids as catalysts The imidazolium-based ILs have been recently applied as catalysts of the sol-gel process. [ ] It is known that the hydrogens of imidazolium ring possess acid nature and can protonate alkoxysilanes leading to their faster hydrolysis. Also, the H-bonding between water molecules and IL s anions (Lewis bases), can facilitate hydrolysis and accelerate the polycondensation of formed silianols. [121, ] According to the literature, [129, 133, 139, ] some ILs can additionally form other catalytic species (such as hydrofluoric acid HF) as a result of IL s anion decomposition. Nevertheless, the more detailed investigation on the mechanism of IL s catalysis in the sol-gel process has not been carried out yet Ionic liquids as coupling agents Interactions between organic/inorganic components at phase boundaries determine the final thermomechanical properties and application of hybrids. In order to improve the affinity and adhesion between inorganic filler/precursor and organic matrix, various coupling agents have been broadly applied, as they provide chemical or physical bonding on the O/I interphase. [144] Recently, it has been found [87, 92-93] that the IL s structure enables formation of the chemical and physical bonding between hydrophobic and hydrophilic phases, what has been broadly used especially in the preparation of epoxy-silica hybrid networks with improved interphase compatibility (Figure 8). Accordingly, the addition of small amount of IL into the epoxy-silica system allowed controlling the silica morphology and interphase O/I interactions, which led to the improved dispersion of inorganic filler and the increased thermomechanical properties of the final hybrid material. [92-93, 145] Such ILs behaviour suggested their role as a filler-matrix coupling agents. Figure 8. ILs interactions on the O/I interphase. 21

24 2. Aims of the thesis The main aim of the thesis is to prepare epoxy-silica hybrid materials with improved thermomechanical properties, which can be designed as bulky fully transparent samples with potential industrial application for casting and encapsulation of electronic devices, optical elements and and light-emitting diodes (LED). The specific objectives of the thesis comprise: synthesis and characterization of pre-condensed silica-based liquid precursors via the solvent-free ex-situ sol-gel process of GPTMS, optimization of the sol-gel conditions and silica structure evolution determination, synthesis of silica nanoparticles with controlled morphology using the in-situ sol-gel process of TEOS, preparation and characterization of brittle epoxy-silica hybrids based on DGEBA resin and Jeffamine D-230 curing agent, preparation and characterization of rubbery epoxy-silica hybrids based on DGEBA resin and Jeffamine D-2000 curing agent, investigation of the effect of imidazolium ILs on preparation and properties of epoxysilica hybrids. 22

25 3. Results and discussion Two preparation routes of bulk epoxy-silica hybrid materials were investigated. The first one uses the two-step process based on the ex-situ sol-gel synthesis of precondensed silica-based precursors bearing epoxide groups and their further chemical crosslinking with the epoxy-amine system. This approach enabled preparation of the glassy epoxy-silica hybrid materials [Appendices: 1-3]. The second route comprises simultaneous crosslinking of epoxy-amine system and formation of silica nanoparticles using the in-situ sol-gel. This method was applied for the preparation of rubbery epoxysilica hybrids crosslinked by a less reactive amine hardener [Appendix 4]. The used epoxy matrices were composed of diglycidyl ether of bisphenol A-based epoxy resin (DGEBA) and two poly(oxypropylene)diamine hardeners Jeffamine D-230 (glassy system) and Jeffamine D-2000 (rubbery system) (Figure 9). Figure 9. Components of epoxy-amine network diglycidyl ether of bisphenol A-based resin (DGEBA) and poly(oxypropylene)diamines (Jeffamine D-230 and Jeffamine D- 2000) Glassy epoxy-silica hybrid materials In the first three papers [Appendices: 1-3] we described the ex-situ sol-gel process of (3-glycidyloxypropyl)trimethoxysilane (GPTMS) leading to the pre-condensed silicabased precursors containing reactive epoxide groups able to covalently bond with the organic epoxy-amine network. In order to obtain the O/I hybrid material with high SiO 2 content, GPTMS was pre-condensed using differently-catalyzed solvent-free sol-gel process, until homogenous and storage stable liquid silica-based precursor (sol) was obtained. The influence of different reaction conditions, set-ups and catalysts on the silica 23

26 structures evolution and further, on the thermomechanical properties of synthesized glassy epoxy-silica hybrids was investigated Solvent-free sol-gel synthesis of silica-based precursors Two conventional catalysts tin- (DBTL) and amine-based (DABCO) were compared with novel, imidazolium-based ILs: 1-butyl-3-methylimidazolium chloride (C 4 MImCl) and 1-butyl-3-methylimidazolium methanesulfonate (C 4 MImMeSO 3 ) (Figure 10). Figure 10. Structures of solvent-free sol-gel components. In the first paper [Appendix 1], we described the synthesis of silica-based precursors using various reaction conditions, two conventional catalysts DABCO and DBTL and different sol-gel set-ups the open, the two-stage open and the two-stage closed systems, which were tested and optimized (Figure 11): the open system the sol-gel components (GPTMS, catalyst) were placed in a flask immersed in an oil bath and heated at 80 C. The water necessary for hydrolysis was continously introduced into the system from the bubbler in a form of vapor-saturated nitrogen and the side product methanol, was collected in a separated flask. the two-stage open system consisted of preliminary hydrolysis at room temperature and further polycondensation with water vapor saturation system. At first, the GPTMS, catalyst and distilled water (H 2 O / Si-OCH 3 = 0.42) were mixed for 1 h in a closed flask. Subsequently, the reaction temperature was increased to 80 C and the water necessary for the process was introduced from the bubbler (as described in the open system). 24

27 the two-stage closed system consisted of preliminary hydrolysis at room temperature (as in the two-stage open system) and further polycondensation at 80 C (for DBTL) or 40 C (for DABCO) under reflux. Figure 11. Sol-gel set-ups used in the preparation of pre-condesed silica-based precursors. 25

28 The two-stage closed sol-gel process was the most optimal as it allowed preparation of the highly-condensed and homogenous products exhibiting good storage stability, miscibility and compatibility with the epoxy-amine system (Table 1) [Appendix 1]. Therefore, also the IL-catalyzed silica-based precursors were synthesized using this approach (Table 1) [Appendix 3]. Table 1. Experimental conditions and properties of liquid precursors (sols) prepared in the open, the two-stage open and the two-stage closed sol-gel systems. Catalyst type Conditions of sol-gel H 2 O / Si-OCH 3 (mol/mol) Catalyst amount 1 (wt %) Reaction time (h) Open system Condensation degree (q) (%) Precursors properties Dynamic viscosity (mpa.s) 2 Storage stability 3 DABCO days DBTL months Two-stage open system DABCO h DBTL days DABCO Two-stage closed system day DBTL days C 4 MImCl days C 4 MImMeSO days 1 wt% calculated from the weight of GPTMS. 2 at 25 C; In the case of the two-stage closed system the viscosity was measured after removal of volatiles (2h, 70 C, vacuum). 3 gel point obtained after sample storage at 8 C. 4 temperature of the polycondensation step: 40 C (DABCO) or 80 C (DBTL, C 4 MImCl, C 4 MImMeSO 3 ). We observed that the DABCO catalyst as well as C 4 MImCl led to the faster polycondensation reaction and promoted predominant formation of three-dimensional cage-like silica structures with tendency to interconnect and form larger arrangements (Figure 12) [Appendices 1 and 3]. The final products revealed the highest (> 90%) condensation degrees (q) (Table 1). The C 4 MImMeSO 3 caused formation of similar silica structures as in the precursor containing C 4 MImCl, but they exhibited higher size and shape distribution and therefore, the final product showed lower condensation degree of alkoxysilane bonds ( 74%). In contrast, the DBTL-catalyzed silica-based precursor was 26

29 mainly composed of ladder-like, cyclic and clustered silica structures, influencing the final condensation degree ( 66%) (Figure 12 and Table 1). It is important that the ILcatalyzed silica-based precursors were more storage stable, which might be the result of physical interactions between IL and silica species slowing down the further polycondensation (Table 1). Figure 12. Main silica structures formed in the differently-catalyzed sol-gel process of GPTMS (R glycidyloxypropyl side-chain, R further siloxane chain, hydroxy or methoxy group) Investigation on the ionic liquid-driven sol-gel mechanism In order to better understand the IL s role in the sol-gel process, FTIR and 1 H NMR spectra were collected during the synthesis of IL-silica precursors. The obtained experimental results were compared with theoretical ones, calculated using density functional theory (DFT) simulation (Figure 13). We showed that the type of IL s anion has a significant influence on the rate of the whole sol-gel process. Accordingly, the 1 H NMR spectra revealed that the addition of C 4 MImCl caused faster hydrolysis of -OCH 3 ( 4 h of reaction), compared to C 4 MImMeSO 3 (the -OCH 3 groups were not fully hydrolysed even after 15 h of sol-gel) [Appendix 3]. Also, after 1 h of hydrolysis (at room temperature) in the C 4 MImCl-driven sol-gel system, two different types of interactions were present, compared to the C 4 MImMeSO 3 -driven process. Based on 1 H NMR of neat ILs and DFT simulations of the most energetically favourable species formed in both IL-silica sol-gel processes (Figure 13), we assumed that the Cl anion possess the ability to form multiple H-bonding with the imidazolium ring s hydrogens and the -OH groups (from silanol, water or methanol) and is more mobile than the MeSO 3 anion. The latter could be caused by the bigger distance between imidazolium ring and the Cl anion and weaker H-bonding with imidazolium ring s hydrogens and formed silanol compared to the MeSO 3 anion (Figure 13). Such 27

30 behavior would explain a faster polycondensation step at 80 ºC in the case of C 4 MImCldriven sol-gel process and formation of silica structures with well-defined morphology. It is important that during C 4 MImMeSO 3 -driven sol-gel process we observed a visible low field shift of signals corresponding to the imidazolium ring s hydrogens ( 1 H NMR results Appendix 3), which could signify the formation of more stable silicate anion (Figure 13) together with the weak and low reactivity methanesulfonic acid (HMeSO 3 ). Figure 13. Proposed mechanism of IL-driven sol-gel process of GPTMS showing the pre-hydrolysis step (a) and the formation of most stable siloxane structures in the presence of C 4 MImCl (b) and C 4 MImMeSO 3 (b) (Legend of atoms: C grey, H white, N blue, S yellow, O red). 28

31 Morphology of glassy epoxy-silica hybrids The epoxy-silica hybrids were prepared from the DGEBA-based resin, Jeffamine D-230 hardener (Figure 9) and the pre-condensed silica-based precursors admixed in the range of 0 22 wt%. All types of precursors led to the improvement in thermomechanical properties of the glassy epoxy-amine matrix, nevertheless only the hybrids containing the silica-based precursors prepared using the two-stage closed sol-gel system were studied in detail [Appendices 2 and 3], as they revealed the best physicochemical properties (Table 1). The different phase structures of epoxy-silica hybrids with conventional catalysts (DABCO, DBTL), dependent on the amount and type of introduced silica species, were obtained from the SAXS measurements (Figure 14) [Appendix 2]. The DGEBA-D-230- DABCO(6.8) system contained silica structures with R g of ca. 3 nm, corresponding to the silsesquioxane clusters (bump of intensity at q = nm -1 on Figure 14a). This partially confirmed our previous results [Appendix 1], showing the formation of cage-like silica structures (POSS) during DABCO-catalyzed sol-gel of GPTMS and was in agreement with a reaction-limited monomer-cluster silica growth mechanism in the basecatalyzed system (Eden growth model), based on the fast condensation of monomers with growing clusters. The addition of 6.8 wt% and 22.0 wt% of the DABCO-catalyzed silicabased precursor into the glassy epoxy-amine system led to the formation of quite compact silica clusters, characterized by the mass fractal dimensions D = and - 3.1, respectively. In contrast, a negligible phase separation between organic and inorganic phases was observed in the DGEBA-D-230-DBTL(6.8) hybrid system (Figure 14a). Apparently, the DBTL-catalyzed precursor was more compatible with the epoxy-amine matrix, probably due to the presence of highly mobile monomeric GPTMS and not fully condensed short oligomers, besides less mobile branched structures [Appendix 1]. The silica growth mechanism in the DBTL-catalyzed sol-gel process was consistent with the reaction-limited cluster-cluster model describing fast formation of small oligomers (clusters), due to the predominant hydrolysis step, and their further condensation to more branched and open fractal structures. The incorporation of 6.8 wt% and 22.0 wt% of DBTL-catalyzed silica-based precursor into the epoxy-amine network caused formation of more open silica clusters, exhibiting lower fractal dimensions than in the previous hybrid system D = and - 2.7, respectively. 29

32 The hybrids containing 6.8 wt% of silica-based precursors revealed a peak at q = 4.5 nm -1 characteristic for partial ordering of DGEBA-D-230 network chains (Figure 14a). Nevertheless, at higher precursors loadings (22.0 wt%) the mentioned peak was overlapped by the increased scattering intensity in the middle-q range, proportional to the siloxane clusters content (Figure 14b). Moreover, the scattering slope in the low-q region of DGEBA-D-230-DBTL(22.0) was more steep (D = - 3.1) indicating formation of more compact silica structures than in the DGEBA-D-230-DABCO(22.0) system (D = - 2.7). Also, in the case of DGEBA-D-230-DBTL(22.0) an additional peak (q = 2.6 nm -1 ) appeared probably due to the higher silica content, forming the mixed epoxy-silica phase with increased distance (~2.4 nm) between network segments, compared to the DGEBA- D-230 matrix (1.4 nm) (Figure 14b). The application of C 4 MImCl- and C 4 MImMeSO 3 -silica precursors into the epoxy matrix led to the hybrid materials with improved homogeneity, as evidenced by their traditional crosslinked networks behavior (Figure 14c) and the lack of other features in the scattering curve. The incorporation of IL-silica precursors led to the increase in distance between network crosslinks, as the scattering curve shifted to lower angles (0.04 nm -1 < q < 0.2 nm -1 ). Moreover, the addition of 22.0 wt% of silica-based precursors did not change the regularities in the epoxy-amine network, as the peak at q = 4.5 nm -1 corresponding to the packing of matrix chains remained unchanged (in contrast to the hybrid with conventionally-catalyzed silica-based precursors). The morphology of silica structures could not be established, as the calculation of form factor was impossible. Only in the case of C 4 MImCl-silica precursor some correlation distance of 118 nm was detected, which was in agreement with AFM results (well-ordered silica aggregates with sizes ranging from 50 to 125 nm). 30

33 Figure 14. SAXS patterns of the epoxy-silica hybrids containing (a) 6.8 wt% and (b) 22.0 wt% of conventionally-catalyzed as well as (c) 22.0 wt% of IL-silica precursors. Graph a- b: the reference DGEBA-D-230 matrix (curve no. 1), the hybrids with DBTL-catalyzed (curve no. 2) and DABCO-catalyzed (curve no. 3) silica-based precursor. Graph c: the reference DGEBA-D-230 matrix (curve no. 1), the hybrids with C 4 MImCl-silica (curve no. 2) and C 4 MImMeSO 3 -silica (curve no. 3) precursors. The most significant influence on the final morphology of conventionallycatalyzed epoxy-silica hybrids exerted the chemical and physical bonding between silica structures and epoxy-amine matrix. In the case of hybrids containing up to 3.6 wt% of DABCO- and DBTL-catalyzed silica-based precursors, any silica species could not be detected by TEM and AFM microscopy, indicating the presence of small silica structures. Only at higher precursor loadings (> 6.8 wt%) the silica domains were detectable. In the DGEBA-D-230-DABCO hybrid systems the formation of compact spherical aggregates, with sizes ranging from 10 to 90 nm, was observed (Figure 15a-c). In contrast, the hybrids with wt% of DBTL-catalyzed silica-based precursor showed tendency to form interconnected, branched and fractal silica structures in the epoxy-amine network (Figure 15d-f). TEM images also displayed that the interface between organic matrix and inorganic silica species was rather diffused, which indicated 31

34 the interconnection between both phases. Only in the case of large aggregates (present in minority) quite sharp interface was detected (e.g. Figure 15b). Figure 15. TEM images of the epoxy-silica hybrids containing 6.8 wt% (a, d) and 22.0 wt% (b-c, e-f) of DABCO- (a-c) and DBTL-catalyzed silica-based precursors (d-f). The detailed study on the hybrids using AFM microscopy allowed estimating the size of silica particles and their compatibility with the epoxy-amine system. In the case of hybrids containing 6.8 and 12.5 wt% of DABCO- and DBTL-catalyzed silica-based precursors the silica structures formed the particles in the size range of nm. Moreover, it was visible, that with the increasing precursor s content the amount of large silica structures prevailed [Appendix 2]. The visible difference in morphology was observed for the systems containing 22.0 wt% of silica-based precursors. The hybrid with the DABCO-catalyzed precursor exhibited the particle size distribution of silica in the range of nm (the average: 55 nm), having regular shape in all directions and protruding above the matrix (Figure 16: a1-a2). The structure, regularity and dimension of observed silica particles were assigned to the aggregated cage-like structures (POSS) [Appendix 1]. In contrast, the hybrid with 22.0 wt% of DBTL-catalyzed silica-based precursor contained more planar silica structures, with lateral dimensions of nm. It was impossible to estimate the 32

35 average particle size, due to the high variability of silica shapes [Appendices: 1 and 2]. The embedded inorganic particles created relatively flat pits on the matrix surface (Figure 16: b1-b2). Figure 16. AFM phase (a1, b1) and 3D height (a2, b2) diagrams of the DGEBA-D-230- DABCO(22.0) (a1, a2) and DGEBA-D-230-DBTL(22.0) (b1, b2) (silica aggregates were highlighted with a black circle). In the case of all hybrids containing IL-silica precursors [Appendix 3], no contrast between organic and inorganic components was observed in the AFM phase profiles (Figure 17c-d), signifying improved interphase interactions and final adhesion between silica and the epoxy-amine matrix. Such results were obtained due to the presence of ILs, which acted as sol-gel catalysts as well as coupling agents. In contrast to the hybrids containing the DABCO- and DBTL-catalyzed silica-based precursors, herein spherical inclusions were formed independently on the silica structures present in the admixed precursors (cages, ladders, cycles, etc.). It was clearly visible, that the hybrids with 6.8 wt% of C 4 MImCl- and C 4 MImMeSO 3 -silica precursors contained well dispersed and regularly shaped silica particles, with the average size of 30 nm (Figure 17a-b). In contrast, the hybrids with 22.0 wt% of IL-silica precursors contained larger well-ordered aggregates, due to the higher silica content able to physically or covalently interconnect [Appendix 3]. They had lateral sizes ranging from 50 to 125 nm and 30 to 80 33

36 nm in the case of hybrids containing C 4 MImCl- and C 4 MImMeSO 3 -silica precursors, respectively. Figure 17. 3D AFM height (a-b) and phase (c-d) diagrams of the DGEBA-D-230- C 4 MImCl(6.8) (a, c) and DGEBA-D-230-C 4 MImMeSO 3 (6.8) (b, d) (silica aggregates were highlighted with a white circle) Optical properties of glassy epoxy-silica hybrids All prepared epoxy-silica hybrid materials exhibited the optical transparency and homogeneity, indicating lack of macroscopic silica agglomerates and a good dispersion of small silica structures, which do not affect the intensity of scattered light. Moreover, the application of IL-silica precursors into the epoxy-amine network, led to the reduction of UV absorption ability (at 350 nm) of the final hybrid (to 25%) compared to the reference matrix ( 38%). 34

37 Thermomechanical properties of glassy epoxy-silica hybrids In order to compare the influence of conventionally- and IL-catalyzed silicabased precursors on the final thermomechanical properties of epoxy-silica hybrids, the same amounts of precursors (ranging from 0.5 to 22.0 wt%) were applied into the glassy DGEBA-D-230 system [Appendices: 2 and 3] Dynamic mechanical thermal analysis The addition of low amounts ( 0.9 wt%) of the DABCO- and DBTL-catalyzed silica-based precursors into the epoxy-amine system caused a slight increase in α- transition temperature (T α ), which might be explained by a loss in the segmental mobility of epoxy-amine network resulting from interactions between homogenously dispersed small silica species and organic matrix (Figure 18b, d). The higher precursor loadings ( 1.8 wt%) slightly decreased T α, which could be due to the formation of larger silica domains increasing a free network volume. According to the latter, in the DGEBA-D- 230-DBTL(22.0) hybrid system the milder T α reduction was observed, which suggested formation of silica structures more compatible with the glassy epoxy-amine system. In contrast, the addition of C 4 MImCl- and C 4 MImMeSO 3 -catalyzed precursors, in the amount 6.8 wt%, did not influence T α of the final hybrid materials, which remained comparable with the reference matrix suggesting lack of significant changes in a free network volume. The latter could be caused by the presence of much smaller silica domains compared to the hybrids with DABCO- and DBTL-catalyzed precursors (Figure 19c-d). The higher loadings ( 12.5 wt%) of both IL-silica precursors caused an insignificant decrease in T α, which could be influenced by the presence of IL and bigger silica aggregates (Figure 19c-d). Moreover, the drop in the loss factor (tan delta) peak intensity was the highest in the IL-driven hybrid systems indicating the lowest contribution of epoxy-amine network chains in the relaxation process (compare Figure 18c-d and Figure 19c-d). It is important that the small amounts ( 6.8 wt%) of IL-silica precursors significantly reduced the segmental mobility, compared to the DABCO- and DBTL-catalyzed precursors, which might suggest the presence of much smaller silica domains in the hybrids with ILs. Moreover, the drop in the tan delta was similar for all hybrids containing 12.5 wt% of 35

38 IL-silica precursors (Figure 19c-d), which might be due to the increasing amount of added precursor and consequently, increasing content of IL, and its stronger impact on the fillermatrix interphase leading to the hybrid system immobilization. Figure 18. DMTA results of the reference DGEBA-D-230 matrix and the epoxy-silica hybrids containing wt% of the DABCO- (a, c) and DBTL-catalyzed (b, d) silica-based precursors. All hybrids exhibited an increase in shear storage modulus in rubbery region compared to the reference matrix, which can be explained by the nanofiller reinforcing effect and an increase in crosslinking density originating from the chemical bonding between organic matrix and glycidyloxypropyl side-chains of silica structures (Figure 18a-b, Figure 19a-b). Moreover, the dependence between amount of added silica-based precursors and the changes in rubber modulus were more visible in the case of hybrids containing the IL-silica precursors, which might be influenced by the presence of smaller silica domains, well dispersed and more compatible with the epoxy matrix (as confirmed by AFM). 36

39 Figure 19. DMTA results of the reference DGEBA-D-230 matrix and the epoxy-silica hybrids containing wt% of the C 4 MImCl- (a, c) and C 4 MImMeSO 3 -silica (b, d) precursors Tensile properties The incorporation of conventionally-catalyzed and IL-silica precursors into the epoxy-amine network significantly improved the tensile properties of hybrids compared to the reference epoxy-amine matrix (Table 2). Accordingly, the addition of low silicabased precursors amounts ( wt%) caused an increase in the tensile strength and energy to break of hybrids, which could be explained by the improved O/I interphase interactions resulting from the covalent bonding of glycidyloxypropyl-functionalized silica structures with the epoxy-amine network as well as, in the case of IL-driven systems, from the combination of both, chemical and physical bonding on the epoxysilica interphase. Only higher precursor loadings ( 12.5 wt%) caused insignificant decrease in tensile strength, which could be explained by the formation of larger silica aggregates/domains. 37

40 Table 2. Tensile and thermal properties of the reference epoxy-amine matrix and the epoxy-silica hybrid materials containing different amounts of silica-based precursors. Sample name Silica precursor (wt%) Tensile strength (MPa) Energy to break (MJ/m 3 ) Elongation at break (%) T 10% ( C) 1 Solid residue (wt%) 2 DGEBA-D ± ± ± ± ± ± ± ± ± ± ± ± DGEBA-D ± ± ± DABCO ± ± ± ± ± ± ± ± ± DGEBA-D-230- DBTL DGEBA-D-230- C 4 MImCl DGEBA-D-230- C 4 MImMeSO ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± temperature of 10% weigt loss; 2 as obtained by TGA (air) at 860 ºC; 3 value obtained at 960 ºC. The optimum amount of the DABCO- and DBTL-catalyzed silica-based precursors, sufficient to enhance the tensile properties of epoxy-amine matrix, was around wt% (Table 2). According to the latter, the homogenous distribution of nanosized silica particles significantly increased the energy to break (up to ~62%), elongation at break (up to ~50%) and tensile strength (up to 7%) compared to the reference matrix. It was observed that the type of silica structures (cages, ladders, cycles, branches, etc.) introduced with the DABCO- and DBTL-catalyzed silica-based precursors into the epoxy-amine system had similar influence on the thermomechanical properties of the final epoxy-silica hybrids. In contrast, the ILs contributed to the better dispersion of silica domains and reduced the amount of aggregates, which allowed to admix higher amounts of IL-silica 38

41 precursors (up to 6.8 wt%) with the epoxy-amine system and improve its tensile properties. The obtained epoxy-silica hybrids showed an increase in energy to break and elongation at break up to 53% and 43%, when wt% of C 4 MImCl- and wt% of C 4 MImMeSO 3 -silica precursor was used, respectively (Table 2). It was concluded that the insignificantly lower tensile properties of hybrids with IL-silica precursors could be caused by the plasticizing tendency of ILs Thermogravimetric analysis The addition of conventionally-catalyzed as well as IL-silica precursors into the epoxy-amine system caused an improvement in thermooxidative stability, due to the presence of well dispersed and hard silica species acting as a barrier for the heat and oxygen transport (Table 2). The highest thermal stability, bellow ca. 450 C, showed hybrids containing wt% of the silica-based precursors, what signified a good distribution and connection of the small silica structures with the epoxy-amine matrix. Higher precursor loadings ( wt%) caused the retardation of degradation process at higher temperatures (T > 450 C) Rubbery epoxy-silica hybrid materials In the fourth paper [Appendix 4] we described the in-situ synthesis of rubbery epoxy-silica hybrids using the hydrolytic sol-gel process of tetraethoxysilane (TEOS). This one-stage hybrids synthesis has not been successfully adapted for the glassy epoxysilica system due to the higher reactivity of used amine hardener (Jeffamine D-230) leading to silica gelation before system homogenization. The use of TEOS instead of GPTMS ensured larger SiO 2 content in the final hybrid and therefore, more effective reinforcing. The formed silica phase did not provide any covalent bonding with the epoxy-amine matrix and therefore, in order to improve the polymer-filler interphase compatibility, few types of novel imidazolium ILs bearing carboxylic and ether groups were added into the system (Figure 20). Their role in the formation of epoxy-silica hybrids was also investigated. The preparation method was based on the two-step procedure: 1 st step pre-hydrolysis of TEOS in the presence of isopropanol, hydrochloric acid (HCl) and carboxy- (CH 2 CO 2 HMImCl or C 3 H 6 CO 2 HMImCl) or ether-functionalized IL (C 7 O 3 MImMeSO 3 ) (Figure 20) 39

42 2 nd step homogenization of obtained mixture with DGEBA-Jeffamine D-2000 system (Figure 9) leading to the simultaneous polycondensation of pre-condensed TEOS and crosslinking of epoxy system by poliaddition reaction. Figure 20. Components of the in-situ sol-gel Morphology of rubbery epoxy-silica hybrids The application of carboxy-functionalized ILs strongly influenced the size of silica domains, which formed opened structures (D = 1.7) and were better homogenized in the epoxy-amine system [Appendix 4]. Accordingly, the IL-free epoxy-silica hybrid contained non-uniform aggregates of particles, with a broad size distribution (Figure 21a). In contrast, the hybrid with carboxylic ILs contained very small, well dispersed and more uniform silica particles, forming loosely packed aggregates (< 50 nm) (Figure 21b-c). It was found that the C 3 H 6 CO 2 HMImCl caused formation of smaller silica particles (d 6 nm) than the CH 2 CO 2 HMImCl IL (d 13 nm). Figure 21. TEM images of epoxy-silica hybrids: (a) without IL, (b) with C 3 H 6 CO 2 HMImCl and (c) with CH 2 CO 2 HMImCl. 40

43 Thermomechanical properties of rubbery epoxy-silica hybrids The influence of carboxylic ILs with different side-chain length (CH 2 CO 2 HMImCl vs. C 3 H 6 CO 2 HMImCl), enabling covalent bonding on the epoxy-silica interphase, and IL bearing ether groups (C 7 O 3 MImMeSO 3 ), allowing physical interactions with the matrix via hydrogen bonding, on the thermomechanical properties of hybrids was determined [Appendix 4]. The most significant improvement in thermomechanical properties was achieved in the case of hybrids containing 7.5 wt% of silica (as SiO 2 ) and 0.2 wt% of C 3 H 6 CO 2 HMImCl IL, able to physically and chemically crosslink on the O/I interphase. Moreover, the higher amounts of carboxylic ILs significantly increased the rate of polycondensation, which confirmed the strong catalytic effect of IL on the sol-gel process and made impossible to prepare homogenous hybrid films. All the prepared hybrids did not reveal any change in the T α compared to the reference matrix, which signified no plasticization by the used ILs (Figure 22a). Moreover, all the samples showed similar tensile strength improvement ( 100% higher than in the case of IL-free hybrid), which was most probably the result of a stronger contribution of silica dispersion and morphology than the interphase bonding (Figure 22b). Figure 22. Thermomechanical properties of the epoxy-silica hybrids obtained from (a) DMTA and (b) tensile measurements. Such results indicated that both carboxylic ILs induced very strong polymer-filler bonding, which was most probably caused by the covalent and physical crosslinking at the interphase (Figure 23). Nevertheless, the best thermomechanical properties were 41

44 obtained in the case of hybrid containing C 3 H 6 CO 2 HMImCl IL. The shear storage modulus in rubbery region was two orders of magnitude higher compared to the reference epoxy-amine matrix and one order of magnitude higher than the IL-free system (Figure 22a). This type of IL also caused an increase in Young s modulus for about 200% and toughness, which was almost one order of magnitude higher than the IL-free hybrid (Figure 22b). Most probably the longer propyl chain of C 3 H 6 CO 2 HMImCl provided some interphase flexibility resulting in the release of part of the stress and higher toughness. Also, better accessibility of carboxylic group of C 3 H 6 CO 2 HMImCl compared to CH 2 CO 2 HMImCl led to its higher reactivity with epoxide groups, what also gave the indirect evidence about IL-matrix covalent bonding formation. Figure 23. Schematic representation of types of interphase interactions induced by carboxylic and ether ILs. 42