Poly(ethylene oxide) Crystal Orientation Change under 1D Nanoscale Confinement using Polystyrene-block-poly(ethylene oxide) Copolymers: Confined Dimension and Reduced Tethering Density Effects

In this study, we systematically investigated the effects of confined dimension (the thickness of PEO layer) and reduced tethering density (σ̃PEO, defined by σπRg2, where σ is the tethered chain density and Rg is the radius of gyration of the tethered PEO chain in its end-free state in the melt) on the PEO crystal orientation change within a 1D nanoscale confinement. We realized this confinement by utilizing two symmetric polystyrene-block-poly(ethylene oxide) (PS-b-PEO) diblock copolymer single crystals grown at different crystallization temperatures (Tx) in dilute solution via the self-seeding technique. After these single crystals were collected as mats, the thicknesses of both the PEO single crystal (dPEO) and PS glassy layer were measured using small-angle X-ray scattering (SAXS) coupled to the 1D correlation function. Because TgPS is higher than TmPEO in those PS-b-PEO single crystal mats, 1D confinements having different dPEO and σ̃PEO values can be prepared by changing Tx values. The PEO block single crystals were melted and recrystallized at different temperatures (Trx), and the c-axis orientation of the recrystallized PEO crystals with respect to glassy PS layers at different Trx values can be monitored by simultaneous 2D SAXS and WAXD techniques. Two observations of the PEO crystal orientation were found in these PS-b-PEO mats. First, three different c-axis orientations of the PEO crystals with respect to the PS layer normal were observed: random, homogeneous, and approximately homeotropic. Second, a dramatic change from homogeneous to homeotropic takes place within a few degrees Celsius. The onset temperature of this orientation change (Tonset) decreases with increasing dPEO and σ̃PEO values. The Tonset values were found to lie within a temperature range of nearly 20 °C (from 0 to −19 °C). Mechanisms of the confined dimension and reduced tethering density effects on determining those Tonset values are proposed.