Poly(ethylene oxide) crystallization within a one-dimensional defect-free confinement on the nanoscale

A new approach was designed to study polymer crystallization in a one-dimensional (ID), defect-free, nanoscale confinement utilizing single crystals of poly(ethylene oxide)-block-polystyrene (PEO-b-PS) diblock copolymers as templates. The single crystals grown in dilute solution consisted of a PEO single crystal layer sandwiched between two PS layers formed by the tethered PS blocks on the PEO single crystal top and bottom basal surfaces. Transition behaviors of PEO-b-PS single crystals were investigated using differential scanning calorimetry, transmission electron microscopy, and atomic force microscopy. It was observed that the glass transition temperature of PS blocks in a thin film with a thickness of 7.2 nm is 71 degrees C, which is higher than the melting temperature of a PEO single crystal with a thickness of 7.9 mn (57 degrees C). Therefore, the PEO single crystal could be melted, and the PEO blocks could recrystallize in between these two vitrified PS layers when the temperature is brought down to generate sufficient supercooling. This recrystallization process of the PEO blocks was thus carried out in a ID lamellar confinement without any defects. After the recrystallization, the crystal orientation change of the PEO crystals at different recrystallization temperatures (T-rx) was monitored using electron diffraction (ED). First, it was found that the PEO blocks could not recrystallize at T-rx > -5 degrees C, indicating absence of heterogeneous nucleation in the PEO blocks. This T-rx of -5 degrees C corresponds to the starting temperature for homogeneous nucleation of PEO as reported previously. Second, crystallographic analyses of the ED obtained from the recrystallized PEO blocks show that for T-rx < -20 degrees C the c-axis, and thus the PEO small crystals, possess a random orientation. When -20 degrees C <= T-rx < -5 degrees C, the c-axis of the PEO crystals is parallel to the glassy PS basal plane and aligned with the a*- or b-axis of the original PEO-b-PS single crystal grown in dilute solution. The self-seeding technique was used for T-rx > -5 degrees C to create heterogeneous nucleation. Here, the c-axis of PEO crystals was inclined with respect to the vitrified PS lamellar normal at an angle of about 26 degrees.