Various kinds of hybrid epoxy-silica networks with in situ formed silica have been synthesized and investigated [19, 24, 25, 28, 45, 141]. Usually, the silicaphase was produced within the network matrix by the hydrolytic polycondensation of TEOS. However, the nonaqueous synthesis of nanosilica in an epoxy matrix has also been reported .
We will describe the formation, structure, and properties of the epoxy-silica interpenetrating network (IPN) DGEBA-D2OOO-TEOS, consisting of the epoxide-amine network DGEBA-D2000 and the silica network in situ generated by the sol-gel process from TEOS [43, 47, 48]. Formation of the silica phase within the organic network depends on the method of preparation (see Scheme 1.1):
(a) The simultaneous polymerization of the organic monomers and TEOS,
(b) The sequential polymerization consisting in polymerization of TEOS within the preformed epoxide network.
simultaneous sequential o o
o organic network organic network o organic monomer « TEOS
Scheme 1.1 Synthesis procedures of the O-I networks with in situ generated silica
Synthesis Procedures of the Hybrid Networks
The network synthesis has been performed by one- or two-step procedures.
The reaction mixture of the monomers (DGEBA, D2000, TEOS) and water was homogenized with the cosolvent isopropanol (IP), and both formation of the DGEBA-D2000 network and sol-gel polymerization of TEOS proceeded simultaneously. Hydrolysis and condensation of TEOS was performed at a molar ratio TEOS:H2O = 1:3(rH = 3) in IP solutions in the volume ratio TEOS:IP = 45:55. The reaction was catalyzed with TSA or DBTDL and the polymer base catalyst D2000. While 2 mol% TSA/ TEOS was used for catalysis, the concentration of the catalytic NH2 groups in D2000 reaches 7-21 mol%/TEOS in different hybrid systems. Hence, an amine excess was applied in the hybrids. The synthesis temperature regime was as follows: T = 20°C 2 h, 90°C 2 days, 130°C 2 days. The one-step polymerized epoxy-TEOS hybrid is indicated as ET-1.
TEOS was prehydrolyzed under acid catalysis at room temperature, and then mixed with the organic components DGEBA-D2000 to start the "simultaneous" formation of both organic and inorganic phases. The hybrid is indicated as ET-2.
The epoxide network was prepared first by reaction of DGEBA with D2000 at 130°C. The cured network was swollen in the mixture TEOS-H2O-IP at room temperature up to equilibrium. The swollen network was then heated in a closed vessel at 90°C for 5 days to polymerize TEOS under TSA or DBTDL catalysis, and to develop the silica phase within the epoxide network. Final curing was performed in vacuum at 130°C for 3 days. The content of silica in the network was controlled by composition of the swelling medium TEOS-IP. The hybrid is indicated as E1-T2.
Formation of the Epoxy-Silica Networks
During the simultaneous procedure of the hybrid IPN synthesis, the two independent reaction mechanisms are simultaneously operative, i.e., the reaction of the epoxy and amine monomers to form the DGEBA-D2000 polymer network, and the hydro-lytic polycondensation of TEOS to form the silica phase. The structure evolution and final morphology of the epoxy-silica network is sensitive to the polymerization procedure and mainly to catalytic conditions. The sol-gel process of TEOS in the DGEBA-D2OOO-TEOS hybrid system proceeds in the presence of both catalysts -TSA and D2000. In this case, the relative concentration of the catalysts is crucial for the sol-gel kinetics, silica structure, and morphology. Evolution of the silica structure by the sol-gel process is much faster under given experimental conditions than formation of the epoxide-amine network. While the silica system gels rapidly at room temperature, gelation of the stoichiometric epoxide network occurs only in 10 h at 80°C. Consequently, during the "simultaneous" polymerization, the silica network is formed first at room temperature, followed by a build-up of the epoxy-amine network at an increased temperature.
The sol-gel process in the one-step polymerization is base-catalyzed because of a molar excess of D2000 content over TSA concentration. The initially homogeneous mixture microphase-separates in the reaction, due to an early formation of high-molecular weight polysiloxane chains typical of base catalysis of TEOS polymerization. Evolution of the silica structures during polymerization is shown in Fig. 1.1a, depicting SAXS profiles of the reaction mixture at an increasing reaction time . The scattered intensity of the SAXS profiles gradually increases during the reaction as the siloxane/silica structures grow. Gelation of the silica structure occurs in 81 min under given conditions at room temperature, according to independent chemorheology measurements.
The size of the forming siloxane polymers evaluated as the Guinier radius, RG from SAXS analysis is concentration-dependent, and is larger in the diluted solutions, Rg ~ 15 nm. This fact implies that in the reaction mixture an overlap of the forming polysiloxane clusters occurs from the beginning of the reaction. The Guinier analysis provides a spatial correlation length, within the overlapped polysiloxane clusters where intermolecular interferences contribute to the scattering profile. At the gel point, Guinier radius reaches the value of the spatial correlation length in the gel, X ~ 10 nm. A cluster overlap was also reported by Schaefer and Keefer  during the first step of the two-step acid-acid catalysis with sub-stoichiometric water content.
In the dilute system, the individual clusters are separated and their true size can be determined. SAXS intensity profiles of the reaction system after dilution, as shown in Fig. 1.1b, characterize the inner structure of the polysiloxane cluster. In contrast to the smooth scattering curves of the bulk reaction mixture, an increase in intensity at low q values as well as the break on the curve at q ~ 0.1 Á-1appear in the diluted solutions. The shape of this profile is interpreted by formation of small domains with a diameter d ~ 3 nm (according to the Guinier analysis) of a higher branching density within a large "heterogeneous" polysiloxane cluster (RG ~ 15 nm). These domains are formed by nonrandom branching under base catalysis. The inner siloxane groups in the chain (-O-)2 Si (-OC2H5)2-n (-OH)n and the branched groups (-O-)3 Si-OH are more reactive than the terminal ones -O-Si(-OC2H5)3-n (-OH)n , which results in formation of more branched and compact domains
within the "heterogeneous" chain. On the contrary, under acid catalysis, the terminal groups react preferably and the chain grows at the end to form linear sequences. The size of the "branched" subunits in the base-catalyzed system increases during the polymerization and their number grows, as revealed by the shift of the break to lower q and by an increasing scattered intensity. The branched parts of the cluster become gradually interconnected, and finally fill in the whole cluster before the gel point and the break on the SAXS curve disappears. In bulk systems, the inner structure of the clusters is screened by their overlap, and hence no break on SAXS profiles corresponding to subunits is observed.
The forming structures show fractal behavior, as revealed from linearity of the intensity curves of the reaction mixture . The change in the inner structure of the chain during the polymerization is shown by the gradual growth of the fractal dimension Dm of the polysiloxanes in the reaction mixture, as illustrated in Fig. 1.2. The high fractal dimension reaching the value Dm > 2.5 after the gel point corresponds to a relatively compact structure. The large value of the fractal dimension can be explained by the participation of the reaction-limited monomer-cluster type reaction mechanism. This is the result of the presence of the monomer in the reaction mixture even at a late reaction stage, due to slow hydrolysis under base catalytic conditions.
Two-step acid-base polymerization is an optimum method for a fast formation of the inorganic structure. The procedure consists of prehydrolysis of TEOS in an acid medium in the first stage, followed by the build-up of a network in the presence of nucleophilic D2000 in the second stage. The formation of silanol groups in the acid medium serves as an initiation step for the subsequent condensation under base catalysis. Gelation of TEOS at the D2000-catalyzed reaction is significantly accelerated by the acid prehydrolysis. The dependence of gelation time tgel on the time interval of the hydrolysis in the first step is shown in Fig. 1.3 . Only 5 min of the
acid prehydrolysis, corresponding to a conversion of more than 50% of TEOS, results in the dramatic acceleration of gelation in the second base-catalyzed step from ~100 to approximately 2 min. The prehydrolysis of TEOS also prevents precipitation of silica or microgel formation in the basic medium. The transparent gels are built-up under these conditions.
Evolution of the structure during the two-step polymerization of TEOS in the hybrid system DGEBA-D2000-TEOS significantly differs from that in the one-step process. In the first, acid-catalyzed step, a fast formation of the small particles with a size of ~2 nm takes place. The SAXS profiles in Fig. 1.4 illustrate the corresponding structure build-up at large scattering angles. These siloxane structures are low-molecular weight small cyclics formed by intramolecular condensation, in agreement with cage-like structures determined by Himmel et al. (1990) and Ng et al. (1995). Acid catalysis thus encourages not only fast hydrolysis but also condensation, to form small condensed structures in the early reaction stage.
The mixing of the prehydrolyzed TEOS with the system components DGEBA and D2000 results in a very fast polycondensation and gelation within 1-2 min. Figure 1.4 depicts an increase in the scattered intensity at low angles as the sizes of heterogeneities of the system grow. This second sol-gel step is catalyzed with
D2000, which leads to increased ionization of the unreacted SiOH groups and acceleration of the polycondensation. The primary particles formed in the acid medium immediately grow by aggregation to form large clusters and the system gels. Fast gelation results in a chemical quenching and slowing down of the diffusion, thus leading to diffusion control of the reaction. The monomer is consumed during the first fast hydrolysis acid step, and only clusters are available for the polycondensation in the second step, thus allowing only the cluster-cluster reaction. The clusters show a more open structure, compared to the one-stage process. The fractal dimension is low, Dm = 1.7, and does not change during polymerization (see Fig. 1.5) . Such a low value of the fractal dimension is consistent with the model of diffusion-limited cluster-cluster reaction, which can be effective in fast polymerization processes  such as this one. The polymer chains grow; however, their inner structure remains unchanged (no change of fractal dimension). This is in contrast to gradual structure densification in the case of monomer-cluster aggregation in one-step polymerization.
The compact structures similar to those prepared in the one-stage process are formed when the neutral DBTDL catalyst was used instead of TSA in the first step.
The higher fractal dimension of these gels may refer to a low efficiency of DBTDL to catalyze the hydrolysis. As a result, a high content of the unreacted monomer, TEOS, is present in the second reaction stage. The participation of the reaction between monomers and large clusters (monomer-cluster growth) is likely, leading to formation of the compact structures.
Both structure evolution and final morphology of the O-I networks synthesized by the simultaneous polymerization are determined by the early reaction stages. Nucleophilic catalysis of the sol-gel process in the beginning stage brings about a gradual densification of the silica clusters during polymerization and formation of a more compact structure. Acid catalysis in the early stage prevents change of the inner chain structure and development of the fractal dimension (densification) during the reaction, despite the second step being base-catalyzed.
The silica structures grow within the preformed epoxide network. During swelling of the network with a TEOS-H2O-IP mixture at room temperature, the hydrolysis of TEOS takes place while increasing the degree of swelling. The sol-gel polymerization of TEOS within the network is catalyzed with the acid or DBTDL, because the polyamine D2000 already incorporated in the epoxide-amine network is not efficient as a base catalyst. Hence, under acid catalysis, the hydrolysis is very fast, and the structure evolution resulting in small particles corresponds to the first acid stage in the two-stage "simultaneous" process. However, the polycondensation proceeds only at an increased temperature. Nevertheless, diffusion of TEOS into the sample is slow, compared to the rate of formation of small clusters, and therefore a gradient of swelling degree and silica content throughout the sample appear.
In most cases, the generated silica phase percolates through the system and the epoxy-silica interpenetrating network with bicontinuous phase structure is formed. Heterogeneous microphase-separated hybrid IPNs DGEBA-D2000-TEOS are optically transparent because of the small size of the silica domains and the solubilizing effect of the poly(oxypropylene) chain of D2000. Only BDMA-catalyzed systems are opaque. Three polymerization procedures used to prepare the networks lead to different morphologies of the O-I network characterized by SEM (Fig. 1.6), and to different structures of the silica determined by SAXS (Fig. 1.7).
The morphology of the network synthesized by the one-step base-catalyzed simultaneous polymerization is the most heterogeneous one. The hybrid involves large siloxane-silica aggregates with a size of ~100-300 nm, composed of smaller particles/clusters of 20-70 nm in diameter (see SEM micrograph in Fig. 1.6a). The silica structure is very compact, as is obvious from the steep intensity curve of the SAXS profile in Fig. 1.7. The fractal dimension in the dry hybrid reaches the value of D = 2.7.
The networks prepared by the two-step acid-base polymerization show smaller silica structures. The silica domains are of size 50-100 nm in Fig. 1.6b. The very fast polymerization and gelation of the siloxane phase in this case result in quenching of microphase separation in the early reaction stage, and in formation of a fine structure. The relative rates of polymerization and microphase separation play a crucial role for the final morphology. The dried system exhibits the two-length scale structure revealed from two linear parts in a double logarithmic plot of the SAXS profile shown in Fig. 1.7. The structure corresponds to large, loose polysi-loxane aggregates of low fractal dimension, (D ) = 2.0, composed of smaller, oo o m aggregate 7 + '
r r 'v m particle
The finest morphology of the O-I network is created by the sequential polymerization with the preformed epoxide network. The small inorganic domains with a size of ~10-20 nm are formed, and no larger aggregates are observed in the SEM micrographs (see Fig. 1.6c). The content of SiO2 in the hybrids increases with the fraction of TEOS in the TEOS-IP mixture; however, the size of the siloxane structures formed within the epoxide network does not grow with the silica content. The distribution of the inorganic phase is not homogeneous throughout the sample, due to a nonhomogeneous swelling of the epoxide network. The surface skin appears with a higher SiO2 concentration, compared to that in the inner part.
Fig. 1.6 SEM micrographs of the hybrid DGEBA-D2OOO-TEOS prepared by (a) one-step polymerization, (b) two-step polymerization, including acid prehydrolysis of TEOS, (c) sequential polymerization with preformed epoxide network
Was this article helpful?