Differently-catalyzed silica-based precursors as functional additives for the epoxy-based hybrid materials

Polymer 99 (2016) 434e446

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Differently-catalyzed silica-based precursors as functional additives for the epoxy-based hybrid materials Magdalena Perchacz*, Hynek Bene
s, Alexander Zhigunov, Magdalena Serkis, Ewa Pavlova Institute of Macromolecular Chemistry AS CR, v.v.i., Heyrovsky Sq. 2, 162 06 Prague 6, Czech Republic

a r t i c l e i n f o

a b s t r a c t

Article history: Received 21 March 2016 Received in revised form 18 July 2016 Accepted 19 July 2016 Available online 20 July 2016

Two types of liquid pre-condensed silica-based precursors bearing epoxy groups were synthesized using the solvent-free sol-gel process of (3-glycidyloxypropyl)trimethoxysilane (GPTMS), in the presence of either basic (DABCO) or neutral (DBTL) catalyst, and further applied for modification of epoxy (DGEBA) e amine (Jeffamine™ D-230) glassy network. The prepared epoxy-silica hybrid materials were characterized by a set of methods including UVevis, SAXS, TEM, AFM, DMTA, tensile tests, TGA and XRF. All hybrids were optically transparent regardless to the silica-based precursor type and its content showing an improvement mainly in dynamic shear storage modulus in rubbery region (up to 4.3 times), energy to break (up to ~ 62%), elongation at break (up to ~ 50%) and thermooxidative stability. The developed procedure is particularly suitable for preparation of fully-transparent massive bulk materials in which the presence of water and volatiles from the sol-gel is highly undesirable. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Epoxy-silica hybrid material Solvent-free sol-gel process Silica-based precursor

1. Introduction Recently, an intensive research on the synthesis of organic/ inorganic (O/I) hybrid materials has been carried out [1e5]. They have gained much attention due to outstanding and unique properties possible to obtain by combination of different organic and inorganic components. Usually, the organic phase brings mechanical toughness, flexibility and easy processability. The inorganic phase gives hardness and thermal and chemical stability to the system [1e3]. They have been broadly used in many fields, like electronics, optics, membranes, coatings, mechanics, sensors, biology, catalysis, etc. [1,4,6,7]. The most important advantage of innovative hybrids can be easy processing (low temperatures), optical transparency (due to well dispersed nanofillers), possibility to design the desired function of a material (e.g. smart and porous materials) and improve thermomechanical properties of products. The final properties of O/I hybrids can be regulated by changing the ratio between both phases, physicochemical properties of each phase, specific interactions on the O/I interface, surface energy of a filler, its size, shape, origin,

Abbreviations: DABCO, 1,4-Diazabicyclo[2.2.2]octane; DBTL, Dibutylbis[1-oxo(dodecyl)oxy]stannane; DGEBA, Diglycidylether of bisphenol A, D.E.R.™ 332; GPTMS, (3-Glycidyloxypropyl)trimethoxysilane; D-230, Polyoxypropylenediamine, Jeffamine™ D-230; POSS, Polyhedral oligosilsesquixane. * Corresponding author. E-mail address: [email protected] (M. Perchacz). http://dx.doi.org/10.1016/j.polymer.2016.07.053 0032-3861/© 2016 Elsevier Ltd. All rights reserved.

functionality, etc. Very important is a good molecular dispersion of inorganic precursor/filler (often difficult to reach), as well as its content and adhesion to the organic matrix [8,9]. In order to improve the compatibility between O/I components, some chemical or physical interphase interactions need to be introduced (e.g. covalent bonds, electrostatic interactions, hydrogen bonding, etc.) [2,5,10]. Within the last decade, a high attention gained polymeric resins mixed with organically substituted alkoxide compounds based on Si, Ti, Sn, Al or Zr [10e12]. This choice brings a few benefits, since functional metal alkoxides are commercially available and easy to handle network formers. Most of them, as very reactive species, do not need to be catalyzed. They also give plenty of possibilities for modeling of hybrid material properties, thanks to broad opportunities in combining the structural elements, like type of alkoxy and other organic groups, flexibility of chains or functionality of monomers. The most common are organically modified alkoxysilanes containing organic group covalently connected with the silicon atom by unhydrolysable and highly stable SieC bond [13e16]. Such organic/inorganic compounds might be named as coupling agents able to improve the interphase adhesion and compatibility between organic and inorganic phases [2,17]. They have been broadly used as additives especially in the various epoxy-based systems [18e20]. Glassy epoxy materials are amorphous and highly crosslinked thermosets having beneficial properties, like high modulus, low creep, optical transparency, outstanding adhesion to various

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substrates, good electrical insulating properties, etc. [21e24]. The large field of epoxides applications resulted in their broader investigation in order to improve common drawbacks e low mechanical properties, like brittleness, high thermal expansion coefficient (CTE), poor resistance to crack initiation and growth [18e20,25e34]. Therefore, many efforts have been focused on toughness improvement of the epoxy-based materials [35]. The incorporation of rubber particles increases the overall system viscosity, reduces the strength and glass transition temperature (Tg) of the cured epoxy. The undesirable reduction of stiffness and modulus of the rubber-filled epoxy could be omitted by addition of inorganic (mainly silica) particles or amorphous thermoplastics with high Tg (such as poly(ether sulfone)s, poly(ether ketone)s and poly(ether imide)s) [36e38]. Nevertheless, their incorporations had drawbacks as reduction of material optical transparency and viscosity increase limiting resin processability [39,40]. Recently, the rigid nanosilica particles were introduced via a solegel technique into the epoxy matrix, which led to homogenous particle distribution, improvement in toughness and modulus, and no significant changes in Tg or viscosity [41]. The sol-gel process is based on hydrolytic polycondensation reactions of low viscous monomers (silicon chlorides, alkoxides or organosilicon compounds). Nevertheless, the most broadly used are functionalized alkoxysilanes bearing e.g. amine or epoxy groups, able to covalently connect with the organic phase as well as create strong SieOeSi bonds. It is commonly known that various silica structures might be formed in the sol-gel process of these monomers [13,14,42e52], which enables to tune the final material properties. The widely developed in-situ methods allow obtaining silica particles by omitting general problems connected with high mixture viscosity and energy-intensive dispersion of the filler [2,3,6,11,12,17,18,52e63]. However, several solvent-free sol-gel procedures were also explored [31e33,42,43,64e66], what has a significant meaning especially in the production of bulky materials, where the evaporation of solvent is an undesirable side-effect causing the creation of pores and other material defects [52,67]. Previously, we reported the solvent-free sol-gel process of (3glycidyloxypropyl)trimethoxysilane (GPTMS) leading to formation of either cage-like silica structures (POSS), when a ternary amine catalyst (1,4-diazabicyclo[2.2.2]octane - DABCO) was used, or ladderlike structures under a tin-based catalysis (dibutylbis[1oxo(dodecyl)oxy]stannane - DBTL) [42]. The obtained liquid products were partially hydrolyzed-condensed and almost volatile-free sols, miscible with common epoxy resins as well as amine hardeners. In this paper, we test their potential applicability as the epoxyfunctionalized silica-based precursors for preparation of bulky epoxy-silica hybrids. The two epoxy-functionalized silica precursors have been synthesized using the solvent-free sol-gel process of GPTMS monomer and base (DABCO) or neutral (DBTL) catalysts. The main goal was to study the effect of silica-based precursor addition on morphology and properties of glassy epoxy-amine matrix, based on diglycidyl ether of bisphenol A resin (DGEBA) and poly(oxypropylene)diamine (D-230). The prepared hybrids were characterized by small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM), atomic force microscopy (AFM), dynamic mechanical and thermal analysis (DMTA), tensile test and thermogravimetric analysis (TGA). 2. Experimental 2.1. Synthetic procedures 2.1.1. Synthesis of silica-based precursors The two silica-based precursors bearing epoxy groups were synthesized using the two-stage closed sol-gel system, based on

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the pre-hydrolysis of GPTMS at room temperature, in the presence of water (H2O/SieOCH3 ¼ 0.42) and DABCO or DBTL catalyst (1 wt% from the mass of GPTMS), and further polycondensation under reflux at 40  C or 80  C for DABCO and DBTL, respectively [42]. GPTMS (98%; ABCR), 1,4-diazabicyclo[2.2.2]octane (DABCO; 98%; Sigma-Aldrich) and dibutylbis[1-oxo(dodecyl)oxy]stannane (DBTL; 95%; Sigma-Aldrich) were used as received. The main goal of synthesis was to obtain homogenous, highly condensed and storage stable silica-based precursors, easily miscible and compatible with epoxy system. Therefore, the two-stage closed sol-gel system was used, which led us to prepare the most viscous products [42] shortly characterized in Table 1. The DABCO-catalyzed precursor exhibited much higher condensation degree of alkoxysilane bonds (around 94%) and contained mostly partially-opened cage-like structures (POSS) with a tendency to interconnect and form the larger arrangements. In contrast, the DBTL-catalyzed precursor was mainly composed of ladder-like and cyclic silica structures due to much higher content of linear structures (T2 species) influencing the lower condensation degree of alkoxysilane bonds (ca 66%). 2.1.2. Preparation of epoxy-silica hybrid materials The silica-based precursors (in the amount of 0.5, 0.9, 1.8, 3.6, 6.8, 12.5 and 22.0 wt%) were mixed with the DGEBA resin (D.E.R.™ 332; The Dow Chemical Company; epoxy equivalent weight EE ¼ 171 g/mol) and homogenized for 20 min at 600 rpm. Then, the poly(oxypropylene)diamine hardener (D-230; Jeffamine™ D-230; Huntsman) was added into the system and stirred (1000 rpm) at room temperature under vacuum (30 min at 100 mbar and 30 min at 3 mbar). Finally, the prepared formulation was poured into teflon molds, cured for 2 h at 90  C and 20 h at 130  C, followed by 12 h post-curing at 180  C under vacuum. The glycido groups from the silica-based precursors were taken into consideration for the calculation of the epoxy/NH molar ratio and the stoichiometric amount (1/1 mol) was used. The reference epoxy-amine network was also prepared at stoichiometric molar ratio of DGEBA and D-230 (epoxy/NH groups ¼ 1/ 1 mol), homogenized and cured at the same temperature regime as in the case of epoxy-silica hybrid materials. The post-curing (180  C; vacuum) was reduced to 5 h. In order to simplify the results interpretation, the studied hybrid systems were denoted according to the sol-gel catalyst type (DABCO, DBTL) and the amount of silica precursor added (0.5, 0.9, 1.8, 3.6, 6.8, 12.5 and 22.0 wt%). For example DGEBA-D-230DABCO(6.8) corresponds to the epoxy-silica hybrid material based on the DGEBA resin, the D-230 hardener and 6.8 wt% of the silicabased precursor prepared under catalysis of DABCO. 2.2. Methods 2.2.1. UVevis spectroscopy A UVeVis spectrophotometer (Lambda 950, Perkin Elmer) with the 60 mm integrating sphere accessory was used to measure the scattering transmittance of the epoxy-silica hybrid materials in the visible light region using the scanning speed of 150 nm min1 and the measuring wavelength range of 250e800 nm. The size of each square sample was approximately 15  15  1 mm. 2.2.2. Small angle X-ray scattering (SAXS) The SAXS experiments were performed using pinhole camera (Molecular Metrology SAXS System) attached to a microfocused Xray beam generator (Osmic Micro-Max 002) operating at 45 kV and 0.66 mA (30 W). The camera was equipped with a multi-wire gasfilled detector with an active area diameter of 20 cm (Gabriel design). Two experimental setups were used to cover the q range of 0.04e11 nm1. Scattering vector q ¼ (4p/l)sinq, where

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Table 1 Properties of silica-based precursors synthesized using DABCO and DBTL catalysts. Catalyst type

DABCO

DBTL

Main silica structures

partially-opened and interconnected cages [42], e.g.:

ladder-like, cyclic and branched structures [42], e.g.:

Dynamic viscosity, 25  C [mPa.s]a Condensation degree [%]b Storage stabilityc

9600 93.5 1 day

3000 66.1 4 days

a b c

Viscosity measured after removing of residual volatiles (vacuum oven, 70  C, 2 h). Calculated from solid state 29Si-NMR measurements. Time to gelation during storage in refrigerator at 8  C.

l ¼ 0.154 nm, q is the angle between the incident X-ray beam and the detector measuring the scattered intensity. Scattering intensities were put on absolute scale using a Glassy Carbon standard. The size of heterogeneities was estimated using generalized ! Guinier approximation: IðqÞ ¼ Aexp



R2g q2 3

, where the pre-factor 2

A describes the excess differential cross-section per unit mass (cm / g) of a particle and Rg is the radius of gyration. Fractal dimensions of silica structures present in the final epoxy-silica hybrid materials were obtained from the slopes of a linear part of the scattering curves in the lower-q region (log I  log q) following the equation: I(q) z q-D [51,68].

modulus (G0 R) was determined in the rubbery plateau region at T ¼ 150  C. The temperature of main (alfa) transition (Ta) was evaluated as the maximum of tan delta peak. The typical precision of the measurements was Ta ± 2  C and G0 R ± 5%. 2.2.6. Tensile test Tensile test was carried out on Instron 6025 instrument (High Wycombe) at room temperature and at test speed 1 mm min1. At least nine “dog-bone” shaped specimens (EN ISO 527-2) with the dimension of 89  5  (1.5 ± 0.2) mm3 were tested from each system and the final value was an average from at least five measurements. The Young's modulus, tensile strength, energy to break and elongation at break were evaluated.

2.2.3. Transmission electron microscopy (TEM) The samples for transmission electron microscopy (TEM) were prepared by ultramicrotomy (Ultrotome III (LKB), Sweden) at room temperature. The ultrathin sections were placed to copper grids, transferred to a transmission electron microscope (Tecnai G2 Spirit Twin 12, FEI) and observed in bright field mode at accelerating voltage of 120 kV.

2.2.7. Thermogravimetric analysis (TGA) The thermal stability of hybrids was determined from thermogravimetric analysis (TGA) performed on a Pyris 1 TGA (Perkin Elmer). The weight loss was measured under oxidizing atmosphere (air) in a platinum pan using a heating rate of 10  C min1 up to 910  C. The total amount of solid residues from each sample was obtained after further isothermal heating at 910  C for 30 min.

2.2.4. Atomic force microscopy (AFM) Morphology of epoxy/silica hybrid materials and dispersion of admixed silica species was investigated by atomic force microscopy (Dimension Icon, Bruker) equipped with the SSS-NCLR-20 probe (Super Sharp Silicon™ e SPM-Sensor from NanoSensors™ Switzerland; spring constant: 35 N m1, resonant frequency: z170 kHz, tip radius: 2 nm) using tapping mode AFM technique. The samples of hybrids were freeze e fractured in liquid nitrogen in order to avoid the covering of silica particles by the polymer layer. The silica particle size was measured by NanoScope Analysis Software via section analysis.

2.2.8. X-ray fluorescence (XRF) The samples were analyzed using X-ray fluorescence spectrometer SPECTRO XEPOS (SPECTRO Analytical Instruments, Germany) equipped with silicon drift detector and the excitation system with a 50 W Pd anode X-ray tube and three polarization secondary targets. HOPG crystal as BRAGG polarizer was used for determination of Si content. The 1 mm thick layer of the sample was deposited on an aluminum foil (0.03 mm thick), placed into measuring chamber and directly irradiated. The chamber was flushed with He during the sample measurement (time: 600 s). The Spectro Xepos software (TurboQuant method) was used for data analysis.

2.2.5. Dynamical mechanical and thermal analysis (DMTA) Dynamical mechanical and thermal properties were tested on ARES G2 rheometer (TA Instruments). The temperature dependence of the complex shear modulus of rectangular samples (dimension: 20  10  1.5 mm3) was measured by oscillatory shear deformation at a frequency of 1 Hz and a heating rate of 3  C min1 in a temperature range of 25e180  C. The rubbery storage shear

3. Results and discussion 3.1. Structure of epoxy-silica hybrid materials 3.1.1. Optical transparency All prepared bulky samples of epoxy-silica hybrids were optically transparent and homogeneous (Fig. 1) indicating lack of

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Fig. 1. Optical transparency of the neat epoxy-amine matrix (a) and the epoxy-silica hybrids containing 22.0 wt% of DBTL- (b) and DABCO-catalyzed (c) silica-based precursors; Sample thickness: 1.5 mm.

macroscopic silica agglomerates and a good dispersion of small silica structures which do not affect the intensity of scattered light. Optical transmission measurements in the UVevis region showed that the incorporation of DABCO- as well as DBTL-catalyzed silica-based precursors in the amounts up to 22.0 wt% did not change the optical properties of epoxy-amine matrix (Fig. 2).

3.1.2. Small angle X-ray scattering (SAXS) Depending on the catalyst type and the amount of silica-based precursor, various SAXS profiles were obtained (Fig. 3). The scattering curves of epoxy-silica hybrids containing 6.8 wt% of DABCO or DBTL catalyzed silica-based precursors showed the characteristic peak at q ¼ 4.5 nm1, originating from partial

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ordering of the network chains (repeating distance of approximately 1.4 nm) of DGEBA-D230 matrix (Fig. 3(a)). Fitting by Lorentzian function showed the identical position and width of this peak confirming the same peak origin for all samples. Bene
s et al. [43] found similar maximum at around 4.1e4.6 nm1 corresponding to the regular structure of organic epoxy-amine network present in the neat DGEBA-D-230 matrix as well as in epoxy-silica hybrids prepared using the sol-gel process of alkoxysilanes bearing NH2 group. In our study, the higher precursor content (22.0 wt%) led to the peak overlapping by the increased scattering intensity in the middle-q range (0.12e1.3 nm1) proportional to the siloxane clusters content (Fig. 3(b)). The SAXS curves of all samples displayed similar quick fall of scattered intensity in the region of low qvalues, which is characteristic for organic networks (Fig. 3(a)) [69]. The differences between the epoxy-silica hybrids appeared in the middle-q SAXS region and were dependent on the type of used silica-based precursor. The scattering curve of DGEBA-D-230DABCO(6.8) showed a bump of intensity at q ¼ 0.12e1.3 nm1 corresponding to inorganic silica inhomogenities which possess higher electron density (Fig. 3(a)). Applying Guinier approximation we could assume that these inhomogenities were objects with Rg of ca. 3 nm and corresponded to the silsesquioxane clusters. According to the literature [70e72], the diameter of polyhedral oligosilsesquixane (POSS) is approximately 1e3 nm, including inorganic core and organic side-chains. This confirms our previous results about the formation of cage-like silica structures (POSS) during DABCO-catalyzed sol-gel of GPTMS [42] and is in agreement with a reaction-limited monomer-cluster silica growth mechanism in the base-catalyzed system (Eden growth model), based on the fast condensation of monomers with growing clusters [51,73]. The condensation between clusters (cluster-cluster type) would require

Fig. 2. UVeVis spectra of the neat epoxy-amine matrix and the epoxy-silica hybrids containing different amounts of DABCO- (a) or DBTL-catalyzed (b) silica-based precursors.

Fig. 3. SAXS patterns of the neat epoxy matrix (curves no. 1) and the epoxy-silica hybrids with 6.8 wt% (a) and 22.0 wt% (b) of DBTL- (curves no. 2) and DABCO-catalyzed (curves no. 3) silica-based precursors.

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inversion of configuration at one of Si atom involved in the reaction, thus such mechanism is unfavorable. It was evidenced, that the hydrolysis is the rate-determining step under basic conditions, therefore the silica species are more rapidly consumed in the condensation reaction leading to formation of compact smooth clusters and big three-dimensional (3D) particles [51,73]. In contrast to the DABCO-catalyzed system, the SAXS curve of DGEBA-D-230-DBTL(6.8) (Fig. 3(a)) exhibited no maximum in the middle-q region, which indicated negligible phase separation between organic and inorganic phases. Apparently, the DBTLcatalyzed 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 the less mobile branched structures [42]. It is known that DBTL promotes the cluster-cluster hydrolytic polycondensation of GPTMS, which could lead to interconnection of the large branched and the smaller highly mobile structures together. Such silica growth mechanism is consistent with the reaction-limited cluster-cluster model describing fast formation of small oligomers (clusters) due to predominant hydrolysis step and their further condensation to more branched and open fractal structures. In such process, the final silica structures might be formed by condensation between clusters (cluster-cluster model) as well as clusters with remaining monomers [51]. The calculation of intensity slope in the low-q region led us to gain information about the topology of the epoxy-silica hybrid networks. In general, the slopes (D) between - 3 and - 4 characterize the volumetric objects with the fractally rough surfaces therefore the neat epoxy-amine matrix, exhibiting the steep slope of - 3.36 (Fig. 3(a); curve 1), might be ascribed as the threedimensional network with non-uniform surface. The slope values between - 2 and - 3 corresponds to the mass fractal objects such as branched gel systems or networks and they can be compared with the mass fractal dimension [51]. The closer slope value to - 3, the more compact the object is. Accordingly, the DGEBA-D-230DABCO(6.8) hybrid material showed a scattering curve with the slope of - 2.7 indicating the presence of quite compact silica clusters in the system (Fig. 3(a); curve 3). In contrast, the SAXS curve of the DGEBA-D-230-DBTL(6.8) hybrid revealed lower slope in the same region (D ¼ - 2.15) due to more open silica clusters exhibiting lower fractal dimensions (Fig. 3(a); curve 2). The SAXS curves of hybrids with the highest content of silicabased precursors (22.0 wt%) were slightly different. We did not observe the peak corresponding to the matrix regularities at q ¼ 4.5 nm1, but it could happen that it was just overlapped by the background and thus became almost invisible (Fig. 3(b)). In the case of DGEBA-D-230-DBTL(22.0) an additional peak (q ¼ 2.6 nm1) 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 neat epoxy matrix (1.4 nm) (Fig. 3(b)). In contrast, no new peak was observed on the SAXS curve of DGEBA-D-230-DABCO(22.0) (Fig. 3(b)). Moreover, the scattering slope in the low-q region was more steep in the case of DGEBA-D-230-DBTL(22.0) (D ¼ 3.1) indicating formation of more compact (by behavior at q ~ 0.4 nm1) silica structures than in the DGEBA-D-230-DABCO(22.0) system. In the latter, the slope was equal to D ¼ 2.7 (Fig. 3(b), curve 2 and 3). 3.1.3. Microscopic evaluation (TEM, AFM) TEM and AFM images of silica-epoxy hybrids with the low content of silica-based precursors (0.5e3.6 wt%) did not display the presence of any silica species. It seemed that the size of silica domains approached the resolution limits of AFM and TEM techniques [43,74] indicating homogeneous dispersion of small silica structures in the epoxy matrix.

The silica domains begun to be visible on TEM images at the higher precursor loadings (>6.8 wt%). In the DGEBA-D-230DABCO(6.8) hybrid system (Fig. 4(a)) the formation of compact spherical aggregates (10e60 nm) was observed. Similar behavior revealed the DGEBA-D-230-DABCO(22.0) system (Fig. 4)(b) in which larger silica aggregates, reaching sizes up to 90 nm (but typically from 30 to 50 nm), were detected. Such morphology of epoxy-silica hybrids driven by the DABCO-catalyzed silica-based precursor might correspond to the monomer-cluster silica growth mechanism leading to formation of more compact 3D silica structures [51,73]. In contrast, the hybrids with 6.8e22.0 wt% of DBTLcatalyzed silica-based precursor showed tendency to form interconnected branched and fractal silica structures in the epoxyamine network, independent of the silica content (Fig. 4(d)e(f)), which was consistent with the cluster-cluster theory of silica growth mechanism [51] and the SAXS results (Fig. 3). TEM images also displayed that the interface between organic matrix and inorganic silica species was rather diffused signifying interconnection between the both phases. Only in the case of large aggregates (present in minority) quite sharp interface was detected (e.g. Fig. 4(a) and (b)). The AFM phase images of size 2 mm  2 mm (Fig. 5, Fig. S1 and Fig. 6: a-c1) showed the differences between the neat epoxy-amine matrix and the epoxy-silica hybrid materials containing 6.8, 12.5 and 22.0 wt% of DABCO- or DBTL-catalyzed silica-based precursors. The dark field represented the organic matrix and the light spots corresponded to the inorganic silica phase. The 3D phase images of DGEBA-D-230-DABCO and DGEBA-D230-DBTL hybrids containing 6.8 or 12.5 wt% of precursors showed the same size range (15e60 nm) of silica particles (Fig. 5(b), (c) and Fig. S1(b) and (c)). Nevertheless, it was visible that with increasing content of silica-based precursor the amount of large silica structures prevailed (compare Fig. 5 and Fig. S1) and only the height diagrams of hybrids with 12.5 wt% of precursors showed the silica species slightly protruded above the matrix (Fig. S2). The highest aggregation tendency was detected in the system with the DABCOcatalyzed precursor, which might be explained by the high condensation degree (~93.5%) of silica-based product containing the interconnected POSS structures [42]. In contrast, the DGEBA-D230-DBTL hybrids contained mainly smaller, spherical silica particles (Fig. 5(c) and Fig. S1(c)). In the case of DGEBA-D-230-DABCO(22.0), the particle size distribution was in the range of 20e80 nm (the average: 55 nm) (Fig. 6: b1). The surface morphology and the regular particle shape in all directions were evidenced from the 3D height image displaying particles protruded above the matrix surface (see circled species in Fig. 6: b2 and the surface profile in Fig. 7(a)). The structure, regularity and dimension of observed silica particles were assigned to the aggregated POSS cages. This correlates well with other studies reporting the tendency of cubic silica species (POSS) to form well-ordered aggregates with lateral sizes of 50e60 nm. Nevertheless, the aggregates distribution and their sizes strongly depend on the intermolecular interactions and silica concentration [70e72]. In this study, the presence of larger aggregates (up to 80 nm) might be explained by the POSS tendency to be physically or covalently interconnected [42]. In contrast, the DGEBA-D-230-DBTL(22.0) hybrid contained more planar silica structures (Fig. 7(b)) with lateral dimensions of 15e60 nm (Fig. 6: c1). It was impossible to estimate the average particle size due to the high variability of silica shapes. The embedded inorganic particles created relatively flat pits on the matrix surface (Fig. 6: c2 and Fig. 7(b)). This type of silica aggregation was also observed in the TEM images (Fig. 4(e) and (f)) and caused slight deterioration of tensile properties of the final epoxysilica hybrid material (see Section 3.2b).

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Fig. 4. TEM images of the epoxy-silica hybrid materials containing 6.8 wt% (a, d) and 22.0 wt% (bec, eef) of DABCO- (aec) and DBTL-catalyzed (def) silica-based precursors.

Fig. 5. 3D Phase diagrams of DGEBA-D-230 matrix (a), DGEBA-D-230-DABCO(12.5) (b) and DGEBA-D-230-DBTL(12.5) (c).

3.2. Thermomechanical properties of hybrid nanomaterials 3.2.1. Dynamic mechanical and thermal analysis (DMTA) The influence of the type and amount of silica-based precursor on thermomechanical properties of epoxy-amine network was followed by DMTA (Table 2 and Fig. S3). All hybrids exhibited an increase in G0 R compared to the neat epoxy-amine matrix due to the nanofiller reinforcing effect and the increased crosslinking density. The latter originated from the chemical bonding between organic matrix and glycidyloxypropyl side-chains of inorganic phase (Table 2, Fig. S3a) and c)). The low contents (up to 0.9 wt%) of the DABCO- and DBTL-catalyzed silicabased precursors slightly increased Ta of the hybrid materials (Table 2) probably due to a loss in the segmental mobility of epoxyamine network resulting from interactions between homogenously dispersed small silica species and organic matrix. The higher precursors loadings (1.8 wt%) led to a slight decrease in Ta (Table 2), which, nevertheless, was more evident in the system containing DABCO-catalyzed silica-based precursor. Such behavior might be

explained by the formation of larger silica particles and aggregates resulting in the increased free network volume, especially in the case of DGEBA-D-230-DABCO (22.0) (see AFM results) [70e72,75]. In contrast, a milder reduction of Ta observed in the DGEBA-D-230DBTL(22.0) hybrid system (Table 2, Fig. S3d)), might indicate the formation of larger silica clusters (Fig. 4(e) and (f)), which only slightly increase free volume of the epoxy-amine network due to their better compatibility with the glassy system (see SAXS results). DMTA results also showed (Fig. S3b) and d)), that the height of the loss factor (tan delta) peak decreased with increasing silica content, which might indicate the restriction effect of segmental mobility, strong reduction of the effective polymer chains volume and the micro-Brownian motion of the epoxy network due to hybridization with silica phase. The epoxy-amine matrix could be immobilized by silica domains and do not contribute in the relaxation process [75e77]. 3.2.2. Tensile properties Tensile properties of the epoxy-amine matrix and epoxy-silica

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Fig. 6. Phase (a1ec1) and height (a2ec2) diagrams of DGEBA-D-230 matrix (a1-2), DGEBA-D-230-DABCO(22.0) (b1-2) and DGEBA-D-230-DBTL(22.0) (c1-2).

Fig. 7. Surface profiles of hybrid materials containing a) DABCO- and b) DBTL-catalyzed silica precursors.

hybrids containing different amounts of DABCO- or DBTL-catalyzed silica-based precursors were tested in a glassy state (room temperature) in order to evaluate the stiffness (Young's modulus), tensile strength, toughness (energy to break) and elongation at break of the prepared materials (Fig. 8 and Fig. S4). The neat DGEBA-D230 matrix showed a poor toughness (~4.3 MJ/m3) and high Young's modulus (~2.2 GPa) typical for

brittle epoxy materials. The incorporation of different silica-based precursor loadings did not influenced the Young's modulus of hybrid materials, which remained comparable with the neat epoxyamine matrix (Fig. 8 and Fig. S4). Such behavior signified that the used precursor amounts were not sufficient to increase stiffness of the glassy system. However, the addition of low amount of silicabased precursors (up to 3.6 wt%) increased the energy to break

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Table 2 DMTA results of the neat epoxy-amine matrix and the epoxy-silica hybrids containing various amounts of DABCO- and DBTL-catalyzed silica-based precursors. Sample name

Silica-based precursor (wt%)

Ta ( C)

G0 R at 150  C (MPa)

DGEBA-D-230 DGEBA-D-230-DABCO

e 0.5 0.9 1.8 3.6 6.8 12.5 22.0 0.5 0.9 1.8 3.6 6.8 12.5 22.0

95 99 98 94 95 93 90 87 98 97 94 94 95 93 91

12.9 25.5 28.0 24.2 24.8 26.5 27.6 55.5 27.6 28.3 24.7 26.5 29.2 32.1 52.6

DGEBA-D-230-DBTL

Fig. 8. Tensile properties of the epoxy-amine matrix and the epoxy-silica hybrid materials containing different amounts of the DABCO- (“red circle” symbol - ) and DBTL- (“black star” symbol - +) catalyzed silica-based precursors. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

(up to ~ 62%) and the elongation at break (up to ~ 50%), what can be explained by homogenous distribution of nano-sized silica particles, which could not be detected by TEM and AFM microscopy. In general, when interactions between nanofiller and organic matrix are weak, the final material is more brittle at high strains, exhibits low elongation at break and strength [78]. Therefore, the quality of O/I interface plays a crucial role in elastic deformation and transfer of stresses from the matrix to the filler. Herein, the low silica-based precursor amounts (0.5e3.6 wt%) led to slight increase in the tensile strength and energy to break of synthesized hybrid materials, what might be explained by improved interfacial

adhesion resulting from covalent bonding of the small silica-based structures with the epoxy-amine matrix. Such inorganic particles tend to hinder crack propagation by absorption of stresses accumulated in the organic matrix as well as by collection of externally applied stress which is transmitted through the matrix [8]. The optimum silica-based precursor amount sufficient to enhance the tensile properties of epoxy-amine matrix was around 1.8e3.6 wt% (Fig. 8). Only in the case of hybrids containing 12.5e22.0 wt% of precursors, the insignificant decrease in tensile strength was observed (Fig. 8), which might be explained by formation of larger silica aggregates or branched structures (AFM and TEM results). The

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Fig. 9. The fracture surfaces of the neat matrix and hybrid materials containing 1.8, 6.8 and 22.0 wt% of the DABCO-catalyzed silica-based precursor. Three regions of brittle fracture mechanism were marked as: A e mirror zone, B e mist zone and C e hackle zone. Photographs were taken from both parts of the „dog-bone” samples after tensile testing.

DGEBA-D-230-DABCO hybrids exhibited the higher elongation at break, which remained at constant value at high precursor loadings (6.8e22.0 wt%) indirectly evidencing the formation of silica aggregates. In contrast, the hybrids with 12.5 and 22.0 wt% of DBTLcatalyzed silica-based precursor formed highly crosslinked O/I mixed phase with reduced extensibility (Fig. 8). The fracture surface of the epoxy-amine matrix and the prepared hybrids displayed a crack initiation site and three zones of crack propagation: mirror, mist and hackle (marked as A, B, C, respectively, in Fig. 9 and Fig. S5). All samples exhibited the fracture behavior consisting of (i) the slow crack growth zone (mirror zone), which was smooth and featureless due to in plane crack propagation, (ii) the mist zone, in which the surface roughness increased due to faster in plane crack growth, and (iii) the rough hackle zone characterized by irregular bifurcate 3D facets and marks [79e82]. It was visible that the tensile fractures of all the samples started with the initiation center localized on the edge of the fracture surface and contained lines or striations radiating from the crack origin. We observed that the small amount of silica-based precursor (1.8 wt%)

improved the brittle sample fracture by making it more ductile (Fig. 9, Fig. S4). In general, less brittle materials have larger mirror zones [83] what fits well with our results showing that the ductility is partially increased with the decreasing silica content in the order: DGEBA-D-230-DABCO(1.8) > DGEBA-D-230-DABCO(6.8) > DGEBAD-230-DABCO(22.0). Similar fracture behavior was found for the hybrid materials containing DBTL-catalyzed silica-based precursor (Fig. S5). The only exception was the DGEBA-D-230-DBTL(22.0) system which exhibited the most brittle fracture surface in the hackle zone (Fig. S5) as well as the lowest toughness and elongation at break (Fig. 8) compared to the neat matrix. Such decrease in tensile properties might be explained by the formation of inorganic network mixed with the organic epoxy-amine matrix (SAXS result and Fig. 4(e) and (f)). 3.2.3. Thermal stability and silica content All the prepared hybrid materials exhibited an improved thermal stability at the beginning of thermooxidative degradation since no visible weight loss was observed bellow 390  C (Fig. 10). The

Fig. 10. Thermal properties of organic-inorganic hybrid materials containing: a) DABCO and b) DBTL-catalyzed silica-based precursor.

M. Perchacz et al. / Polymer 99 (2016) 434e446

443

Table 3 Thermal properties obtained by TGA in air and the Si-content determined using XRF of the epoxy-silica hybrid materials. Sample name

Silica-based precursor (wt%)

T10% ( C)a

Solid residue (wt%) after TGA in air

Si (wt%) obtained by XRF

SiO2 (wt%)c

DGEBA-D-230 DGEBA-D-230-DABCO

e 0.5 0.9 1.8 3.6 6.8 12.5 22.0 0.5 0.9 1.8 3.6 6.8 12.5 22.0

366 392 392 393 392 387 386 383 396 394 396 391 390 384 390

0.0 0.2 0.3 1.0 1.4 2.5 3.6 6.2 0.3 0.4 1.2 1.7 2.6 3.7 7.2b

0.0 0.1 0.2 0.4 0.9 1.8 2.9 5.2 0.2 0.3 0.6 1.1 2.2 3.6 6.5

0.0 0.2 0.4 0.9 1.9 3.9 6.2 11.1 0.4 0.6 1.3 2.4 4.7 7.7 13.9

DGEBA-D-230-DBTL

a b c

T10% e the temperature of 10% weight loss. Value obtained at 960  C. The equivalent of SiO2 calculated from the Si-content (wt%) obtained by XRF.

Scheme 1. Proposed structure of epoxy-silica hybrids based on: a) DABCO- and b) DBTL- catalyzed silica-based precursors.

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decomposition of organic epoxy-amine matrix and glycidyloxypropyl side-chains of silica structures corresponded to the first (main) weight loss in the temperature range of 300e500  C [43,84,85]. The hybrids containing 0.5e3.6 wt% of the silica-based precursors showed the highest thermal stability in the first part (bellow ca 450  C) of the main weight loss (Fig. 10; Table 3). This might signify that the small silica amounts were better incorporated and connected with the organic matrix than higher precursor amounts (6.8e22.0 wt%) forming silica aggregates. In contrast, different behavior was observed in the second weight loss corresponding to the oxidative degradation of products formed during the first step of thermal decomposition and was significantly shifted to the region of higher temperatures with increasing silica content (Fig. 10a) and b)). The improved thermal stability of all hybrid materials might be attributed to the silica phase dispersed in the epoxy matrix which acted as a barrier for the heat and oxygen transport [85e87]. In this study, TGA in air of the DGEBA-D-230-DABCO hybrids produced the lower amount of solid residues compared to the DGEBA-D-230-DBTL systems (Table 3). This discrepancy originated from the different condensation degrees of the DABCO- and DBTLcatalyzed silica-based precursors, as mentioned previously (Table 1). The slightly higher amount of silica was incorporated into the final hybrids with the DBTL-catalyzed precursor containing less of methanol (the by-product of GPTMS condensation). Therefore, the exact Si-contents in hybrid materials were determined using XRF (Table 3). The results showed that for the same amount of added silica-based precursors, the Si-content was always higher in the DGEBA-D-230-DBTL system (Table 3). The Si wt% values obtained from XRF were then recalculated to theoretical SiO2 content enabling correlation with the amount of solid residues formed during thermooxidative treatment of hybrids (Table 3). At higher precursor loadings (>3.6 wt%), the content of solid residue was always lower than the theoretical SiO2 content from XRF. The reason might be the sublimation of thermally less stable silica structures [88]. 3.3. Proposed structures of epoxy-silica hybrid networks Based on the SAXS, TEM and AFM results as well as our previously published work [42], two structures of the epoxy-silica hybrids containing 6.8e22.0 wt% of the DABCO- or DBTL-catalyzed silica-based precursors were proposed (Scheme 1). According to the model silica structure calculations (Avogadro 1.1.1), the cubic 3D POSS has diameter of 1.67 nm including the inorganic SieOeSi core (0.3 nm) and the organic glycidyloxypropyl substituents (Scheme 1a). The calculated Rg of 3D objects present in the DGEBA-D-230-DABCO hybrid system (Fig. 3) was higher (3 nm) than the value from modeling, probably due to the formation of interconnected cages (detected previously in Ref. [42]) or physical interactions between silica structures. Such 3D objects exhibited aggregation tendency in the epoxy-amine matrix, especially at the higher silica-based precursor amounts (6.8e22.0 wt%). The formed spherical inorganic domains were covalently connected with the organic matrix and had diameter of 10e90 nm (AFM and TEM) depending on the silica-based precursor content. The DGEBA-D-230-DBTL hybrids containing 6.8e22.0 wt% of silica-based precursor consisted of the large branched silica clusters showing tendency to interconnect and form the co-condensed structure as indicated by SAXS (the epoxy-silica mixed phase structuration), AFM and TEM (Scheme 1b)). The aggregation process was suppressed and the increased GR0 proved the nanofiller reinforcing effect and the presence of covalent connections at the epoxy-silica interface (higher crosslinking density). Such behavior can be explained by the major presence (in the admixed DBTL-

catalyzed silica-based precursor) of ladder-like oligomers (Mn < 3500 g mol1, i.e. up to tridecamers) and not fully condensed branched and cyclic structures bearing OH groups (the total condensation degree: 66% - Table 1) [42]. Typical steric arrangement and inorganic chain conformation of ladders was demonstrated on the model pentacyclic structure with total length of 2.6 nm (Scheme 1b)). We observed that with increasing number of tetrameric cycles in the ladder structure, the total structure length does not proportionally depend on the SieOeSi bond length due to higher number of chain conformations. For instance, the modeled decacyclic ladder structure exhibited total length of 4.01 nm (not 3.73 nm as could be calculated from the length of two pentacyclic structures shown in Scheme 1b). 4. Conclusions The highly condensed silica-based precursors synthesized in the solvent-free sol-gel process were used for preparation of glassy hybrid materials based on epoxy-amine network. The morphological study showed that the incorporation of the DABCO-catalyzed silica-based precursor into the epoxy-based system caused the formation of spherical aggregates of cage-like POSS structures according to the monomer-cluster growth mechanism. The DGEBA-D-230-DABCO(1.8) hybrid exhibited the highest increase in energy to break (by ~62%), elongation at break (by ~50%) and slight improvement in tensile strength, while maintaining Young's modulus of the neat epoxy matrix. Addition of higher precursor contents led to significant increase in the shear storage modulus in the rubbery region (GR0 ), even though the large spherical aggregates were formed (10e90 nm). In contrast, the DBTL-catalyzed silica-based precursor led to formation of the inorganic silica network covalently interconnected with the organic epoxy-amine matrix according to the clustercluster silica growth model. Moreover, the silica structures present in this type of precursor (mainly ladders and cycles) exhibited higher compatibility with the neat matrix. The incorporation of only 1.8 wt% of DBTL-catalyzed precursor into the epoxy-amine network led to significant improvement in tensile properties (energy to break increased by ~60% and elongation at break - by ~50%). Nevertheless, the higher content of silica-based product (22.0 wt%) caused formation of mixed epoxy-silica phase and decrease in mechanical properties of the final material. All hybrids exhibited optical transparency and enhanced resistance against thermooxidative degradation due to the barrier effect of inorganic silica phase. Acknowledgements This work was supported by the Czech Science Foundation [project 14-05146S]; the Ministry of Education, Youth and Sports of CR [project POLYMAT LO1507]. The authors thank RNDr. Ji
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