Crystallinity and fusion of ethylene oxide/propylene oxide block copolymers: 2. Type PEP copolymers

Abstract

Lamella spacings, specific volumes and melting points have been determined for a series of well characterized poly(ethylene oxide)/poly(propylene oxide) type PEP block copolymers with E-block length 48 chain units and P-block lengths 0 to 7 chain units. These properties are interpreted in terms of a stacked lamella model with alternating amorphous and crystalline layers. Both extended-chain and once-folded-chain crystalline lamellae are found, the former with thickness about 32 E chain units and the latter with thickness about 21 E chain units. Compared with the specific volume of supercooled melt of the same composition the specific volume of the polymer in the amorphous lamellae is lower in the extended-chain polycrystals and higher in the once-folded-chain polycrystals. The melting points of the copolymers are low compared to that of perfectly crystalline poly(ethylene oxide), i.e. 37 to 55°C compared with T 0 m = 76°C. This is due to the large positive free energy of formation from the melt of the crystalline/amorphous end interface ( σ o ) and the amorphous layer ( σ a ). For extended-chain polycrystals we find σ o ∼ 3 kJ/mol and σ a ∼ 3.5 kJ/mol; for once-folded-chain polycrystals we find σ o ∼ 6 kJ/mol and σ a ∼ 2 kJ/mol. We also find σ o, x = 2.5 kJ/mol for a completely extended-chain end interface and σ o, f = 10 kJ/mol for a completely folded-chain end interface.