Schematic illustration of the MT phases observed. For <0.4% (wt/wt) 20 k PEO (corresponding to an osmotic pressure of 600 Pa), the MTs are undistorted and form a nematic. Above this concentration of 20 k PEO, the MTs buckle to a noncircular cross section and form bundles with rectangular symmetry. The MTs distort further as the osmotic pressure increases. At 5% 20 k PEO (25,000 Pa), approximately c*, the polymer overlap concentration, the PEO is forced inside the lumen of the MTs, and the MTs convert to undistorted MTs in hexagonal bundles. At 18% 20 k PEO (400,000 Pa), all the MTs are in the hexagonal bundle phase.