Reinforcement of crosslinked rubbery epoxies by in-situ formed silica
Introduction
For various technological applications, mechanical and thermal properties of polymer systems are commonly improved by addition of an inorganic filler. Material properties of the composites are determined by the respective properties of both physically mixed components and thus depend on the content of filler, its adhesion to polymer matrix, uniformity of dispersion, etc. In the last decade, a variety of elastomers, thermoplastics, linear or crosslinked systems, have been reinforced with inorganic fillers formed in situ [1], [2], [3], [4], [5], [6], [7], [8]. These in-situ formed organic–inorganic (O–I) systems form a new class of composites, frequently described as O–I hybrids. These hybrids are also called as nanocomposites because of the small size of formed inorganic structures; usually of the nanometer size, resulting often in an optically transparent system.
In the hybrids, the inorganic phase is formed within an organic polymer matrix by sol–gel process consisting in hydrolysis and condensation of alkoxy derivatives of metals like Si, Ti, Al [9]. The most common precursor is tetraethoxysilane (TEOS) yielding a glassy silica network which acts as a hard reinforcement in a soft polymer.
Procedures of the O–I hybrid synthesis involve either sequential type, i.e. inorganic phase formation within a polymer matrix, or a simultaneous polymerization of both organic and inorganic monomers. The hybrids with or without a covalent bond of the inorganic component to the polymer matrix may be formed. Organic monomers or polymers modified by alkoxysilane groups are used to provide bonding to the in situ formed inorganic structure. Strong interaction between phases was found [3], [10], [11], [12] to improve the mechanical properties of the hybrid.
As a consequence of the molecular mixing of reacting components in the sol–gel process, the inorganic phase can be dispersed very finely in the organic matrix which is difficult to reach by mechanical mixing in the case of classical composites. The generally accepted Wilkes’ morphological model of O–I hybrids [13] is based on the small-angle X-ray scattering (SAXS) analysis of the system poly(tetramethyleneoxide)-TEOS. According to this model, the hybrid is composed of silica-rich domains dispersed in the organic polymer-rich matrix. Besides, an interphase (intermixed layer) is present as a result of interpenetration of partly condensed siloxane–silica clusters and organic polymer. However, it is well known [9] that there is a possibility of controlling the structure and morphology by reaction conditions of the sol–gel process, which affects the final hybrid properties.
In previous papers [14], [15], we have reported O–I hybrid systems composed of organic rubbery network and inorganic silica structure formed by the sol–gel process from TEOS. The organic part was represented by epoxide–amine network from diglycidyl ether of Bisphenol A (DGEBA) and poly(oxypropylene)diamine, Jeffamine® D2000. Simultaneous and sequential hybrid interpenetrating networks (IPN), DGEBA-D2000-TEOS, were prepared and their formation and structure were studied. The epoxide–silica IPNs were recently investigated by Bauer et al. [7]. During the hybrid network formation, the microphase separation takes place and resulting structure as well as final morphology were shown to be dependent on the synthesis procedure—one-stage or two-stage, the type of catalysis and concentration of the catalyst [7], [14], [15]. The sol–gel reactions were catalyzed by acid, neutral pH and basic catalysts. Grafting between the epoxide and silica networks is also an important factor controlling the hybrid morphology as it promotes miscibility and affects phase separation [7]. The covalent bonds between both networks are formed by the reaction of SiOH groups of hydrolyzed silica–siloxane clusters and C–OH group of the organic network resulting from the epoxide–amine reaction.where ∼∼∼∼ is an epoxide network and ···· is a silica structure.
In this paper, we dealt with the synthesis, phase structure analysis and elastic and thermal properties of the hybrid DGEBA-D2000-TEOS. Reinforcement of the rubbery epoxide–amine network by in situ formed hard silica structure was investigated as a function of factors influencing growth and morphology of the inorganic phase. Different ways of the O–I hybrid synthesis were used and the effects of the network formation, structure, morphology and interphase interaction on mechanical properties were studied. The structure and morphology were analyzed by SAXS and electron microscopy. The interphase interaction between the epoxide network and the silica phase was evaluated by means of dynamic mechanical analysis (DMA). In situ formed hybrid systems were compared with a classical composite, epoxide network filled with a pyrolyzed silica—Aerosil 200.
Section snippets
Organic phase
A rubbery epoxide material was prepared by curing the diglycidyl ether of Bisphenol A (DGEBA) with poly(oxypropylene)diamine, Jeffamine® D2000 (Huntsman Int. Trading) (MW=1970). Long flexible polyether chain makes the cured material rubbery and moreover, it solubilizes siloxane structures formed in the sol–gel process and makes a hybrid transparent. The equivalent of functional groups for the epoxy groups in DGEBA and NH groups in D2000 were respectively, EE=171 g/mol ENH=492 g/mol.
Inorganic phase components
Network structure and morphology
The three polymerization procedures lead to different structures and morphologies of the O–I hybrids [14] as shown in micrographs in Fig. 1 and SAXS graphs in Fig. 2. The morphology ranges from silica domains of various sizes dispersed within an organic matrix to co-continuous epoxy–silica phases. Siloxane–silica clusters are incompletely reacted and the condensation conversion α determined by 29Si CP/MAS NMR reaches the value α=0.79–0.85. Nevertheless, the silica regions are in glassy state as
Dynamic mechanical properties and interphase formation
Valuable information on morphology and mainly on interface of the microphase-separated hybrid systems was obtained from DMA.
Fig. 3 shows shear storage modulus G′(T) and loss factor tan δ (=G″/G) of studied systems as functions of temperature. One can see a significant reinforcement of some “in situ” prepared hybrid networks (Fig. 3(a)) with respect to neat DGEBA-D2000 network (curve 7). Storage modulus of the hybrids rises with increasing content of the inorganic component and this reinforcing
Discussion
Morphology of O–I hybrids is usually described by Wilkes’ model corresponding to a particulate composite with silica domains dispersed in an organic matrix. However, some authors [7], [26], [27], [28], [29], [30], [31] assume a co-continuous morphology. In order to understand the morphology of the O–I hybrid systems better, we will compare our experimental results on the moduli of studied hybrids with the prediction of existing models assuming either particulate or bicontinuous phase structure.
Model of a particulate composite with filler dispersed in matrix
Modulus of a heterogeneous two-phase system depend on the moduli and volume fractions of components as well as on morphology. Of existing models, the Kerner model modified by Nielsen [19] (Eq. (4)) is frequently used:where GC, GM, Gf are moduli of the composite, matrix and filler, respectively, νM the Poisson ratio of the matrix, vf the volume fraction of the filler and vmax the maximum packing fraction of the
Bicontinuous model
We suppose that high moduli of the heterogeneous systems under study can be explained by assuming that the O–I hybrid consists of two co-continuous phases, i.e. of the organic and inorganic networks and of dispersed particles of the inorganic sol. Bicontinuous morphology of O–I hybrids was already evidenced by SAXS measurements [27], [28]. On the basis of the simulation of SAXS models, Landry et al. [28] concluded that silica forms a continuous phase in the organic polymer matrix as a result of
Crosslinking of the matrix with filler
Interphase crosslinking is sometimes considered [39], [40] as alternative explanation of the excessive reinforcement. The filler dispersed in the matrix is believed to act as a giant multifunctional crosslink [41] if there is a good adhesion between phases or a covalent bond to the matrix exists. The plateau modulus above Tg in the linear PMMA–TEOS system [3] was interpreted as a result of crosslinking between the polymer matrix and silica clusters formed from TEOS. Physical and chemical
Conclusions
The rubbery epoxide network DGEBA-D2000 was reinforced with silica formed in situ by sol–gel process. Increase in the modulus by two orders of magnitude at low contents of the silica phase (<10 vol%) was achieved. Three different polymerization procedures were used to prepare microphase-separated simultaneous and sequential IPNs of the epoxy–silica hybrid DGEBA-D2000-TEOS. The results show very close relations among polymerization mechanism, interphase interaction, morphology and dynamic
Acknowledgements
The authors wish to thank the Grant Agency of the Czech Republic for support of the work (project no. 203/98/0884). The third author J.K. is greatly indebted to the Grant Agency of the Academy of Sciences of the Czech Republic for financial support of this work (project no. A4050706).
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