Structural evolution of a colloidal crystal fiber during heating and annealing studied by in situ synchrotron small angle X-ray scattering.

The structural evolution of a colloidal crystal fiber during heating and annealing was followed by in situ synchrotron small-angle X-ray scattering. The polymer dispersion (with a particle size of 118 nm) from which the fibers were formed by directed drying contained emulsifier and salt. A cellular structure formed upon drying in which the percolating phase (the "membrane phase") is composed from these components; this membrane phase gives rise to the scattering contrast on which the present observations build. Changes of the lattice constant of the colloidal crystallites and the intensity evolution of the scattering from the crystalline and the amorphous phases during heating and annealing indicate characteristic temperatures where the system exhibits pronounced structural changes. The first characteristic temperature was identified as 125 degrees C above which residue water in the membrane material was evaporated leading to shrinkage of the colloidal crystalline lattice. At a temperature above about 140 degrees C the membrane material was expelled out of the crystalline domains. This effect is accompanied by the progressive interdiffusion of polymer chains between adjacent latex particles and leads to further thermal shrinkage of the colloidal crystals. The second characteristic temperature is defined by a rapid increase in isotropic scattering. This effect is attributed to the formation of increasingly large domains of the membrane material and the concomitant disappearance of the membrane phase from the former crystal domains.