Abstract
In this study, we systematically investigated the effects of confined dimension (the thickness of PEO layer) and reduced tethering density (σ̃ PEO , defined by σπR g 2 , where a is the tethered chain density and R g 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 ID 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 (T x ) in dilute solution via the self-seeding technique. After these single crystals were collected as mats, the thicknesses of both the PEO single crystal (d PEO ) and PS glassy layer were measured usingsmall-angle X-ray scattering (SAXS) coupled to the ID correlation function. Because T g PS is higher than T m PEO in those PS-b-PEO single crystal mats, ID confinements having different d PEo and σ̃ PEO values can be prepared by changing T x values. The PEO block single crystals were melted and recrystallized at different temperatures (T rx ), and the c-axis orientation of the recrystallized PEO crystals with respect to glassy PS layers at different T rx 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 (ronSet) decreases with increasing d PEo and σ̃ PEO values. The T onset values were found to lie within a temperature range of nearly 20 °C (from O to -19 °C). Mechanisms of the confined dimension and reduced tethering density effects on determining those T onset values are proposed. © 2009 American Chemical Society.