Preparation of thermoset/clay nanocomposites by thiol-epoxy click chemistry

The use of nanocomposite materials is increasing every day due to their excellent optical, thermal and mechanical properties. Polymer/clay nanocomposites are one kind of composite materials containing of nanometer-sized inorganic nanoparticles, typically in the range of 1–100 nm, which are uniformly dispersed in and fixed to a polymer matrix. The inclusion of inorganic nanoparticles into a polymer matrix not only combines their properties but also brings advanced new functions that find applications in many industrial fields. Polymer/clay nanocomposites find a number of different industrial applications including automotive, coatings, biomedical and packaging industries. Polymer/clay nanocomposite materials, in which nano-sized silicate plates of clay are uniformly dispersed in the polymer matrix, exhibit superior physical properties such as high dimensional stability, gas barrier performance, flame retardancy, and mechanical strength that cannot be achieved by pure polymer or conventional composites (micro- and macro composites). However, the dispersion of clay as individual platelets throughout the polymer is difficult to achieve due to strong van der Waals forces holding platelets together in conjunction with the incompatibility of the hydrophilic clay with the organophilic (hydrophobic) polymer matrix, giving way to clay agglomeration. Thus, the surface of the clays is commonly modified with a cation exchange technique to expand basal spacing and make the layered silicate compatible with polymer matrixes. Currently polymer/clay nanocomposites can be prepared by three ways such as solution mixing, melt blending, and in-situ polymerization. In the solution mixing method, the polymer is dissolved in an organic solvent, then the clay is dispersed in the obtained solution and subsequently either the solvent is evaporated or the polymer precipitated. However, the large quantities of volatile solvent necessary for this approach make it less attractive as an industrial process. Melt blending is a solvent-free method to enable mixing of the layered silicate with the polymer matrix in the molten state. However, very careful attention has to be paid to finely tune the processing conditions to increase the compatibility of clay layer surfaces with the polymer matrix. In the in-situ polymerization technique, the monomer, together with the initiator and/or catalyst, is intercalated within the silicate layers and the polymerization is initiated by external stimulation such as thermal, photochemical or chemical activation. The chain growth in the clay galleries triggers the clay exfoliation and hence the nanocomposite formation. Recently, a highly efficient copper (I) catalyzed azide/alkyne cycloaddition "click" reaction, in which exfoliation is rooted in the functional groups of the intercalant that readily react with the antagonist groups of the preformed polymers has been developed. The quantitative efficiency of click reaction coupled with tolerance to a wide variety of functional groups and reaction conditions make this process highly attractive for the nanocomposite preparation. Another important example of click chemistry reaction is thiol-epoxy coupling reaction, which can be conducted from commercial available and cheap epoxide and thiol compounds under ambient conditions. This chemistry is an efficient and simple tool for the synthesis of various polymers and produces a reactive secondary hydroxyl functionality that can be further utilized in another reactions. Here, we report the synthesis of thermoset/clay nanocomposites by thiol-epoxy click reaction. This approach is conceived to greatly extend the synthetic capabilities of polymer/clay nanocomposites by careful choices of organic clays and polymers, and the optimization of the synthesizing process to deliver the biggest benefit.