Structural Characterization of the “Knitting Pattern” in Polystyrene-block-poly(ethylene-co-butylene)-block-poly(methyl methacrylate) Triblock Copolymers

Abstract

In a polystyrene-block-polybutadiene-block-poly(methyl methacrylate) triblock copolymer (SBM) a morphological transition from a lamellar (ll) morphology (with sequence ABCB) to the knitting pattern (kp) morphology occurs upon hydrogenation of the center PB block. Structural information of the nonhydrogenated and the hydrogenated material is provided from small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). For the nonhydrogenated SBM sample the SAXS profile correlates well with a simple periodic lamellar structure. The hydrogenated sample forming the kp morphology displays a more complex SAXS pattern. From the TEM images of the knitting pattern the two-dimensional space group has been determined to be c2mm. This morphology represents a first example of a planar morphology for which the two sides of the unit cell are unequal (a ≠ b). The determination of the space group from TEM allows the satisfactory assignment of the SAXS pattern. Furthermore this new kp morphology is the first example in block copolymers providing a highly nonconstant mean curvature (NCMC) intermaterial dividing surface. Varying the casting solvent for this material (from CHCl3 to toluene) results in a lamellar morphology as demonstrated by TEM and SAXS. This morphological change is explained as the consequence of the borderline situation of the kp morphology as an intermediate between the ll and lc morphology where B cylinders of the center block are located at the lamellar A/C interface.