Salt-Induced Polymer Gelation and Formation of Nanocrystals in a Polymer−Salt System

Abstract

Salt-induced polymer gelation and nanocrystal fabrication in a polymer−salt system of Pluronic surfactant L35 (E11P16E11) and saturated CdCl2 aqueous solution were studied by means of small-angle X-ray scattering (SAXS), wide-angle X-ray diffraction (WAXD), and transmission electron microscopy (TEM). The symbols E11 and P16 denote the ethylene oxide end block with 11 segments and the propylene oxide middle block with 16 segments, respectively. The L35 surfactant started to form a gel-like polymer network with only 2 wt % of the saturated CdCl2 solution (weight ratio of CdCl2:H2O = 50:50) in the system. When the amount of salt solution was further increased to 5 wt %, nanocrystals composed mainly of CdCl2 were formed in the gel matrix. In this system, the minimized water content was not the driving force for polymer gelation and microphase separation. The poly(ethylene oxide) (PEO) block in the EPE surfactant could not crystallize by itself, but it could be strongly bonded with salt. The formation of uniform nanocrystals with 3-dimensional ordered packing could be achieved in the presence of soft polymer networks. On the basis of the TEM results, the nanocrystals were found to undergo morphological changes from cubic to rodlike structures, depending on the amount of saturated salt solution.