Composition fluctuation and domain spacing of low molar weight PEO–PPO–PEO triblock copolymers in the melt, during crystallization and in the solid state

Abstract

We have studied a series of PEO–PPO–PEO triblock copolymers (Pluronics) in their melt and solid state mainly using static and time-resolved small-angle X-ray scattering (SAXS). In the melt state, composition fluctuations were observed. Their temperature variation was in accordance with mean-field theory. A crossover from the mean-field regime to the fluctuation regime was observed for samples with high molar mass. To check the overall conformation of molecules in the disordered state, composition fluctuations during crystallization were investigated by time-resolved SAXS. Detailed analysis on the time dependent intensity and peak position indicate that molecules remaining in the disordered state adopt a stretched overall conformation. In the solid state, crystallization of PEO blocks induced phase separation, resulting in an alternating crystalline/amorphous lamellar structure. Samples with short PEO block formed a simple lamellar structure with extended-chain conformation. The domain spacing increased with crystallization temperature due to the swelling of the amorphous domain by uncrystallized molecules. Samples with long PEO block formed a mixed lamellar structure. Structures with once-folded and extended PEO block coexisted in a large temperature range and their relative fraction changed with crystallization temperature. This mixed structure was reduced to a simple lamellar structure with once-folded crystalline structure at low crystallization temperatures.