Scrolled Polymer Single Crystals Driven by Unbalanced Surface Stresses: Rational Design and Experimental Evidence

Abstract

To understand the formation mechanism of nonflat polymer single crystals, two types of triblock copolymers with a middle crystalline block and two amorphous, immiscible end-blocks were designed and synthesized. Specifically, polystyrene-block-poly(ethylene oxide)-block-poly(1-butene oxide) and polystyrene-block-poly(ethylene oxide)-block-polydimethylsiloxane were examined. When the end-blocks possess different volumes and are microphase separated onto the opposite sides of the single crystal lamella formed by the middle crystalline block, unbalanced surface stress can be generated. As a result, large scrolled single crystals (∼80 μm) were grown from dilute solution using the self-seeding procedure at low supercoolings. The scrolling direction was identified to be along the (120) planes based on transmission electron microscopy (TEM) observations of the sedimented scrolled single crystals, which is in line with the fact that the scrolling occurs along the planes with the highest coefficient of thermal expansion. Using high-resolution TEM at high tilting angles, three layers of distinct chemical compositions can be clearly identified from the edges of the single crystals after RuO4 staining. It suggests the formation of microphase separated domains of the amorphous end-blocks on the opposite sides of PEO single crystals. Although the tethering densities of these amorphous end-blocks are identical, their reduced tethering densities are different, resulting in dissimilar volumes and surface crowdedness on the opposite sides of PEO single crystal. The unbalanced surface stress is thus generated to scroll the lamellar single crystal. Macroscopically, based on the observed curvature and the assumption of a solid plate cylinder, the strain energy for each individual single crystal with lateral size of 80 μm was estimated to be ∼3 × 10–9 erg, which, though small, is sufficient to maintain the scrolling of single crystal in solution at room temperature (the thermal energy is approximately kT ∼ 4 × 10–14 erg). Microscopically, the difference of the reduced surface free energy of the tethered blocks at the opposite sides of the PEO lamellar single crystal is analyzed and understood to be the driving force of the scrolling.