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A First-Principles Analysis of the Synthesis of Organic/Inorganic Hybrid Nanoscale Systems: Epoxy Amine Addition for Polysilsesquioxanes

Vamsi M. Vadhri and Matthew Neurock. Chemical Engineering, University of Virginia, 102 Engineers' Way, PO Box 400741, Charlottesville, VA 22904

Organic/inorganic hybrid nanocomposites display novel properties and offer significant advantages over conventional composites in terms of mechanical strength, thermal endurance and chemical stability. Polyhedral oligomeric silsesquioxane (POSS) based composites are an example of such hybrid materials. POSS molecules have a polyhedral core which is composed of a rigid silicon-oxygen framework that can be independently functionalized at each corner with different organic or inorganic groups. This arrangement facilitates the formation of nanocomposites with a uniform distribution of the silica cores via crosslinking of the functional groups. Significant property changes have been observed by varying the tether length and cross-linking chemistry at the atomic level. For instance, the macroscopic properties like glass transition temperature, fracture toughness and tensile modulus of polymer networks resulting from the reactions between epoxy-based POSS molecules and diamines have been found to strongly depend on the stoichiometry and reaction chemistry. Consequently, extensive studies have been performed to assess the properties of the resulting network by controlled variation of the organic architecture and efforts have been made to obtain composites having the desired properties.

Despite elaborate experimental studies on the subject, however, little is known about the reaction chemistry and the factors which affect the morphology of network formation. Knowledge of the curing reaction mechanisms can provide valuable insights about the relationships between the reaction kinetics and polymer structure. This work seeks to elucidate the mechanism of the addition reactions between epoxy-POSS molecules and diamines and understand some of the factors which affect the kinetics and energetics.

Density Functional Theory (DFT) calculations were carried out for the addition of epoxide to different amines. An explicit solvation model was used to simulate the solvent environment. Both primary and secondary amines are considered to understand substituent effects. The reaction was found to proceed by the typical SN2 mechanism. The reaction is initiated by the backside attack by the nucleophilic amine on the epoxide group to form an intermediate zwitterion. This is followed by a concerted proton transfer by the intermediate amines to form the products. Hydrogen bonding in the solution was observed to be crucial for the reaction to proceed. First, the addition reactions between primary and secondary methylamine and epoxide were examined followed by those between primary and secondary aniline and epoxide. Reactions involving amines with aromatic substituents were found to have a higher energy of activation compared to reactions involving aliphatic amines which is attributed their reduced nucleophilicity. No significant steric effect was observed in the reactions thus making the primary and secondary amines almost equally reactive towards epoxides. For instance, the activation energy of the reaction of primary methylamine and epoxide is 102 kJ mol-1 while that of secondary methylamine with epoxide is 107 kJ mol-1. On the other hand, the activation energy of the reaction of primary aniline and epoxide is 123 kJ mol-1 while that of secondary aniline with epoxide is 129 kJ mol-1.

This study was subsequently extended to the reaction between diaminodiphenylmethane (DDM) and epoxy-based Polyhedral Oligomeric Silsesquioxanes (POSS) molecules. It was observed that the inorganic core of the POSS molecule was not involved in bonding with the solvent. Steric effects however are more pronounced in this case implying that the secondary amino hydrogen is less reactive than the primary one.