Mechanisms And Preparation Of Polymer-Network/Single-Crystal Composites

An increasing number of synthetic crystals have been reported to have organic and/or inorganic gel-networks incorporated while still maintain their single crystallinity just like varied biogenic single crystals with biomacromolecules occluded. It contradicts the general concept that single crystal must be of great purity and provides us with a method to prepare polymer/single-crystal composites which have novel and/or promoted functionalities. However, mechanisms underneath this intriguing phenomenon are still poorly understood. In our work, a variety of crystals (NaF, NaCl, KBr, potassium dihydrogen phosphate (KDP), AgCl, AgI, CuCl, pyrene, anthracene, fluorene, naphthalene) have been grown in agarose and silica gels to unravel the possible factors that influence the gel-incorporation. Phase composition of the obtained crystals was analyzed by powder X-Ray diffraction (PXRD) and the single crystallinity of the crystals was proved by single crystal X-Ray diffraction or polarized light. The gel-networks incorporated were observed by dissolution of the crystals and further characterized by thermogravimetric analysis (TGA), inductively coupled plasma atomic emission spectrometer (ICP-AES) and/or Energy Dispersive X-Ray Spectroscopy (EDX). The results indicate that for a gel with weak gel-network like silica gels, strong mutual interaction such as hydrogen bonding between crystal and gel-network is essential for the gel-network to be incorporated into the crystals (NaF, KDP) and the gel-network will be pushed aside (NaCl, KBr) without the strong gel-crystal interaction. For crystals (AgCl, AgI, CuCl) grown by decomplexation in silica gels, the comprehensive effect of the coordination interaction and emission of by-products results in no silica gel-network being incorporated by the crystals. As to aromatic crystals (pyrene, anthracene, fluorene, naphthalene) grown in silica gels, even for naphthalene which has the slightest pi-pi interaction between the crystals, the silica gel-network cannot be incorporated by the growing crystals. So, the aromatic crystals cannot incorporate silica gel-network, though the molecular aromatic crystals favor gel-network incorporation more energetically than ionic or covalent crystals. In addition, if a gel has strong gel-network like agarose gel with nano-fibrous gel-network, the growing crystals (NaF, NaCl, KBr, KDP, AgCl) cannot push the gel-network away but incorporate it. In the cases where the agarose gel-network was degraded during crystallization, the crystals can only partially incorporate (Agl) agarose gel-network or even cannot incorporate any (CuCl). This work may help to understand the incorporation phenomena and to prepare single crystal composites with desired internal structures and properties.