Memory effect on solution crystallization of high molecular weight poly(ethylene oxide)

We investigated the solution crystallization behavior of a high molecular weight (MW) poly(ethylene oxide) (PEO) (PEO400k, MW of 4 × 105 g/mol) in ethanol with the concentrations ranged from dilute to semi-dilute (i.e., from 0.02 to 2 wt %). We followed the dissolving process of PEO400k using differential scanning colorimetry, small-angle X-ray scattering and solution light scattering. The practical solution temperatures (Tsexps) of the solutions, where the PEO400k crystals were fully dissolved, were estimated. In comparison with that of the solution cooled from a dissolving temperature (Td) sufficiently higher than the Tsexp, the crystallization kinetics of the solutions prepared at Tds in the vicinity of Tsexp is much faster. The lower this Td is, the faster the crystallization takes place at a constant isothermal crystallization temperature. Moreover, for the solutions with Td > Tsexp, prolonging the duration time of dissolving can significantly slow down the crystallization kinetics. The experimental results indicate that the solution crystallization of high MW PEO bears some memory effect, similar to that observed in bulk crystallization. As the PEO400k can be completely dissolved at Td > Tsexp, we consider that the solution crystallization can also have a type of memory effect beyond the traditional self-seeding effect. This memory may be related to the relaxation of PEO chains after they are dissolved in solution. Probably, the topological structure of the freshly dissolved chains temporarily retains the less entangled segments, which can promote the subsequent crystallization. For the solution with 2 wt % PEO400k, the erasion time of this memory effect could be several hours long.