Buckling of block copolymer lamellae in supercritical carbon dioxide

Supercritical carbon dioxide (scCO$_2$) swells many kinds of polymers. In particular polymers containing fluorine are highly swollen. Therefore, block copolymers having fluorinated blocks are expected to be swollen selectively in scCO$_2$ due to the higher affinity of scCO$_2$ toward the fluorinated blocks. We studied the phase behavior of fluorinated block copolymers swollen in scCO$_2$ and found multiple order-to-order transitions as a function of pressure. In addition, the swollen structures could be frozen by reducing temperature and subsequently carbon dioxide was removed without disturbing the swollen morphologies. As a result, the volume occupied with carbon dioxide was converted to empty space, and hence a variety of nanoporous structures were successfully formed. In particular we found that swollen lamellae in scCO$_2$ becomes undulated lamellae with a large wavelength, which is similar to ''egg cartoon'' often observed in unbinding membranes of surfactants.