Influence of interfacial constraints on the microdomain morphology of block copolymers

Both an external surface constraint and a thin film constraint were found to strongly influence the microdomain morphology and macromolecular conformations in linear and star poly(styrene-b-butadiene) (SB) and poly(styrene-b-isoprene) (SI), and linear poly(styrene-b-isoprene-b-2-vinylpyridine) (P2VP) block copolymers. Transmission electron microscopy (TEM) of cryo-ultramicrotomed cross-sections of the external surface and of thin films was employed to determine the interfacial microdomain morphology. Assuming sharp interfaces of constant mean curvature between phases, the mean curvature (H) of, the area per junction ($\sigma\sb{\rm j}$) along, and the respective block chain extensions away from the intermaterial dividing surface (IMDS) were determined. Under near-equilibrium processing conditions, segregation of the lowest critical surface tension ($\gamma\sb{\rm c}$) polydiene blocks to the external surface and to the interface with the substrate was observed for all of the block copolymer architectures and compositions investigated. In diblock copolymers, the thickness (t) of the external surface layer was noted to depend upon the molecular weight of the lower $\gamma\sb{\rm c}$ block, the composition of the diblock, and in star diblocks the position of the lower $\gamma\sb{\rm c}$ block in the arm. TEM measurements indicated that t can decrease by up to 800% for certain microdomain types and surface orientations. Both decreases by up to 50%, and increases by up to 200% in $\sigma\sb{\rm j}$ were found. These large variations in t and $\sigma\sb{\rm j}$ indicated that the chain conformations of block copolymers are highly adaptable in the presence of an external surface constraint. Preferred surface orientations of the microdomains were observed. For body-centered cubic packed spherical microdomains, the closest-packed $\{$110$\}$ planes and occasionally the $\{$100$\}$ planes were parallel to the external surface. For cylindrical microdomains, the $\{$100$\}$ planes were parallel to the external surface. The ordered bicontinuous double-diamond (obdd) structure was found to undergo a surface-induced morphological transition to cylindrical microdomains, except when the surface orientation was with the $\{$110$\}$ planes of the obdd morphology parallel to the external surface. At equilibrium, parallel surface orientations of lamellar microdomains are preferred. In all cases, the preferred orientations facilitate the formation of interfacial layers of the lower $\gamma\sb{\rm c}$ block. The results of this work provide new insight for the basic understanding of the physics which govern the microdomain morphology of block copolymers