Welding Colloidal Crystals with Carbon Dioxide

The effect of CO 2 on the welding kinetics for a PS colloidal crystal was determined by in-situ measurement of Bragg diffraction and by ex-situ SEM microscopy. The welds grow tangentially from the points of contact of the spheres in all directions transforming the spheres into a continuous film. The dissolved CO 2 provides the mobility for the polymer chains at the interface needed for welding, which removes the energetically unfavorable interfacial area between the polymer and CO 2 . At high pressures where CO 2 is relatively incompressible, the relaxation time for welding increased by 6 orders of magnitude with an increase in thermal energy, and a glass transition temperature was observed. At constant temperature, an anomalous increase in the welding rate by 1−2 orders of magnitude with a decrease in pressure was observed in the region where CO 2 is highly compressible. It is likely that this increase in welding dynamics is influenced by the excess CO 2 in the interfacial region.