Thermotropic and lyotropic behavior of a PEO-PPO-PEO block copolymer

The phase behavior of the (EO)(19)(PO)(43)(EO)(19)/p-xylene system (where EO is ethylene oxide and PO is propylene oxide) with temperature is discussed. In this block copolymer the end blocks are crystallizable, and the middle block is noncrystallizable. Several techniques were used to delineate the phase boundaries (SAXS, WAXS, H-1 NMR, H-2 NMR, and DSC). An anisotropic region with a lamellar structure with semicrystalline PEO domains and amorphous PPO domains is formed at low temperatures and high copolymer concentrations. The lamellar structure has a one-dimensional swelling with increasing p-xylene concentration. The driving force for forming the anisotropic region is that the PEO crystallize at low temperatures. The amount of crystallinity in the system and the interfacial area per PEO block in the ordered region depend on the temperature and the sample composition. At high temperatures and low copolymer concentrations the system contains an isotopic solution phase.