Novel nanocomposites based on epoxy resin/epoxy-functionalized polydimethylsiloxane reinforced with POSS
Introduction
Over the past decade, advanced researches have been focused on the development of polymer based nanocomposites, reinforced with well-defined nanostructured compounds, which are expected to provide high performance materials, with potential application in a wide range of technological areas [1].
Recently, polyhedral oligomeric silsesquioxanes (POSS) nanostructures have gained considerable attention as they possess a synergistic combination of constituent properties of conventional inorganic materials and organic polymers, resulting in a new generation of hybrid materials with superior properties [2], [3], [4], [5]. POSS molecules architecture is based on a well-defined polyhedral cage (1.5 nm), with general formula (RSiO3/2)n, n varying from 6 to 18. These macromers have an inorganic silica-like core with a precise geometry, surrounded by organic ligands (R) covalently bonded to each Si atoms placed at the vertices of the polyhedral cage. R may be hydrogen or any alkyl, alkylene, aryl, arylene groups, or organo-functional derivatives. Unlike silica, silicones and other conventional inorganic fillers, the POSS compounds exhibit advantages like monodisperse size, low density, and synthetically well-controlled functionalities. The functional groups from the organic substituents bring chemical versatility to POSS nanoparticles and make POSS widely compatible with various organic polymers [6], [7], [8].
Epoxy resins, the well-known class of thermosetting polymers, have been widely used as adhesives, electronic encapsulating compounds, and organic phase of composite materials due to their high mechanical strength, high chemical and corrosion resistance, excellent electrical insulation and simplicity in processing [9], [10]. For further improvements of the general properties of epoxy resins, different POSS compounds containing epoxy, hydroxyl and amino reactive functionalities which can generate covalent bonding with the base polymer to reduce the agglomeration effect, have been employed. In general, the obtained results showed that the incorporation of POSS nanounits within epoxy resins improves the performances of the obtained networks [11], [12], [13]. However, even for POSS containing reactive groups, small POSS aggregates can form within the base polymer that lead to lower thermo-mechanical properties [14]. Moreover their highly cross-linked structures exhibit a brittle behavior, poor crack resistance, and low fracture toughness.
One of the most promising approaches used to modify the features of such brittle polymers consists in adding of flexible polymers like polysiloxanes, used as spacers between POSS and polymer matrix during the preparation of POSS/polymer nanocomposites. This method has been demonstrated to be efficient in order to prepare nanocomposites with good dispersion and enhanced thermo-mechanical properties [15], [16], [17], [18], [19]. Polysiloxanes have been extensively used to improve the epoxy resins properties due to their excellent thermal stability [20]. Moreover, these compounds were also used in the preparation of different POSS/polysiloxane composites. Chen and coworkers demonstrated that the incorporation of POSS nanoparticles into polysiloxane elastomers can improve the thermal stability of the obtained composites [21].
Recently, we have reported the synthesis and characterization of simultaneous interpenetrating polymer networks (IPN) based on dimethacrylic/epoxy resins via in situ polymerization reinforced with octafunctional POSS (with methacrylate or epoxy groups) [22]. In this study, we successfully proved that the degree of POSS dispersion within the polymer matrix is a key factor that is strongly influenced by the organic substituents grafted onto the POSS cage. The obtained results show that the incorporation of OEP-POSS within the used epoxy resin lead to a decrease of thermo-mechanical properties (Tg and thermal stability) of the obtained nanocomposites.
Thus, based on our previous experience [23], [24], [25], [26], [27], [28], [29], [30], [31] we intended to design different organic-inorganic hybrid materials with good dispersion and enhanced thermo-mechanical properties by combining diglycidyl ether of bisphenol A (DGEBA) with a small quantity (2 wt.%) of diglycidyl ether terminated poly(dimethylsiloxane), (DG-PDMS). As reinforcing agents, (mono-/octa) epoxy POSS (POSS-(3-Glycidyl) propoxy-Heptaisobutyl substituted (MEP-POSS) and POSS-Octa-(3-glycidyloxypropyl) dimethylsiloxy) (OEP-POSS)) were included into DGEBA/DG-PDMS organic host. To improve the dispersion of POSS during the preparation of POSS/polymer nanocomposites, DG-PDMS was used as a spacer between POSS molecules and DGEBA, enabling a high mobility of the epoxy chains which may contribute to a better dispersion of POSS units within the polymer matrix.
To the best of our knowledge, all previously studies were focused mainly on synthesis of different POSS/epoxy resin and POSS/PDMS nanocomposites, while DGEBA/DG-PDMS nanocomposites reinforced with epoxy POSS compounds remain unexplored [12], [13], [14], [15], [16], [32], [33], [34], [35]. Thus, we focused our research to evaluate the effect of DG-PDMS and epoxy POSS nanostructures on the properties of epoxy network. The curing behavior of DGEBA/DG-PDMS reinforced with epoxy POSS was analyzed by DSC and FTIR spectroscopy. The thermal stability and morphology of the obtained nanocomposites are evaluated by DMA, TGA, SEM/EDX, AFM and contact angle measurements. The hardness of the studied materials was also investigated through the AFM nanoindentation tests.
Section snippets
Materials
The studied networks possess in their structures a mixture of diglycidyl ether of bisphenol A (DGEBA), D.E.R.™ 332 (epoxy equivalent weight of 171–175 g/eq) and diglycidyl ether terminated poly(dimethylsiloxane), (DG-PDMS) (epoxy equivalent weight of 490 g/eq and a Mn ∼ 800 g/mol). Two types of epoxy POSS (POSS-(3-Glycidyl)propoxy-Heptaisobutyl substituted with Mw = 931.63 g/mol (MEP-POSS) and POSS-Octa(3-glycidyloxypropyl)dimethylsiloxy) with Mw = 931.11 g/mol (OEP-POSS)) were used as
The influence of POSS type on the DGEBA/DG-PDMS curing behavior studied by DSC
To investigate the effect of both DG-PDMS and POSS type on the reactivity of epoxy groups during copolymerization reactions of DGEBA/DG-PDMS ± POSS nanocomposites, non-isothermal DSC scans were performed. Fig. 1 shows the DSC curves before curing for DGEBA and hybrids based on DGEBA/DG-PDMS ± POSS (MEP or OEP-POSS).
The curve 1 from Fig. 1 reveals that DGEBA homopolymer exhibits an exothermic peak at 142 °C, assigned to the temperature at which the polymerization enthalpy of epoxy groups is
Conclusions
To prepare DGEBA/DG-PDMS/POSS nanocomposites with good dispersion and improved thermo-mechanical properties, a low amount of epoxy functionalized polydimethylsiloxane (DG-PDMS) together with POSS bearing one or eight epoxy groups (10 wt.%) were covalently introduced within DGEBA epoxy matrix through copolymerization of epoxy groups. The presence of DG-PDMS as a spacer between POSS molecules and DGEBA leads to a nanoscale dispersion of OEP-POSS within the polymer matrix. Thus, for this
Acknowledgment
The work has been funded by the Sectoral Operational Programme Human Resources Development 2007-2013 of the Ministry of European Funds through the Financial Agreement POSDRU/159/1.5/S/132397.
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