Melting behavior of ethylene oxide–propylene oxide (sym‐PEP) block copolymers

Melting points and lamellar thicknesses have been measured for ethylene oxide–propylene oxide block copolymers (sym‐PEP) with central poly(ethylene oxide) block lengths of 70–100 chain units and end poly(propylene oxide) block lengths of 0–30 chain units. Melting points of the block copolymers are lower than those of the corresponding poly(ethylene oxide) homopolymer by an amount (up to 15°C) which increases as the poly(propylene oxide) block length increases. Most samples have more than one melting transition, which can be assigned to variously folded chain crystals. End interfacial free energies σe for the various crystals have been estimated by use of Flory's theory of melting of block copolymers. For a given crystal type (e.g., once‐folded‐chain) σe is higher the longer the chain length of the end poly(propylene oxide) blocks. For a given copolymer σe is lower, the more highly folded the poly(ethylene oxide) chain.