In situ observation of low molecular weight poly(ethylene oxide) crystal melting, recrystallization

A low-molecular-weight poly(ethylene oxide) (PEO) fraction with a number-average molecular weight of 4250 g/mol was used to study the melting and recrystallization behaviors during heating. After complete crystallization at 46 °C, the PEO possessed almost exclusively once-folded chain [IF(n=1)] crystals and very few extended chain IF(n=0) crystals. Comparing differential scanning calorimetry heating experiments of the PEO bulk samples after crystallized at 46 °C with those further step-annealed at temperatures (Tas) in the melting range of the IF(n=1) crystals showed that the crystal melting and recrystallization occur to form the IF(n=0) crystals. In situ melting and recrystallization of the PEO crystals after crystallized at 46 °C were also investigated using both small angle X-ray scattering (SAXS) experiments in reciprocal space and atomic force microscopy (AFM) coupled with a hot stage in real space. It was found that in the SAXS measurements, the lamellar long period shifted from the IF(n=1) crystals towards the IF(n=0) crystals. This transformation process involved melting and recrystallization. During this process, more than one lamellar long period could exist. At Ta=58 °C, the long period reached the thickness of the IF(n=0) crystals. In the in situ AFM observations, the IF(n=1) crystals was melted starting from inner parts of the surface lamellae, whereas the lamellar edges were quickly transformed to the IF(n=0) crystals with higher thermodynamic stability. The remaining lamellar edges serve as nucleation sites of the PEO recrystallization. The IF(n=0) crystal growth rate measured by the in situ AFM was slower than that measured in the single crystals of the isothermal crystallization at the same temperature. The fast overall recrystallization of the IF(n=0) crystals was mainly deduced by the fact that the high nucleation density provided by the survived nucleation sits such as the lamellar edges.