Crystallization and melting behavior of low molar weight PEO–PPO–PEO triblock copolymers

The crystallization behavior of a series of poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEO–PPO–PEO) triblock copolymers (Pluronics) was investigated using time-resolved small-angle X-ray scattering (SAXS), thermal analysis, and polarized optical microscopy. For comparison, a PEO homopolymer, PEO3K, was also included. Time-resolved SAXS during the crystallization of PEO3K shows a typical “two-step” process, i.e., in the initial stage, a metastable crystal with nonintegral folding (NIF) structure forms first, then, it transforms into integral folding (IF) structures, the IF(0) and the IF(1). In contrast with PEO3K, the PEO–PPO–PEO triblock copolymers show a “one-step” crystallization process, i.e., the PEO blocks crystallize directly into the final state and do not change with time. In thermal analysis, only one major solid–melt transition is observed during isothermal crystallization and subsequent melting for triblock copolymers. In the full temperature range, a linear crystal growth is observed. The crystal growth rates monotonously decrease with crystallization temperatures. Notches or breaks due to the NIF–IF transition as clearly seen for PEO3K cannot be recognized for Pluronics. Based on these results, we conclude that the crystallization of PEO–PPO–PEO triblock copolymers follows a “one-step” process; no metastable structure serving as an intermediate state is formed during the crystallization process within the time scale of the current experiments (<120 min).