ASSESSING HOMOPOLYMER DISTRIBUTION IN ABC TRIBLOCK COPOLYMER HOMOPOLYMER BLENDS THROUGH A TRANSITION IN INTERFACIAL GEOMETRY/

Intermaterial dividing surfaces (IMDS) in strongly segregated block co polymer systems are typically of constant mean curvature (CMC). This o bservation largely derives from the interplay of the thermodynamics of chain deformation and interfacial tension. Recently, a compositionall y symmetric, linear poly(2-vinylpyridine)-b-polyisoprene-b-polystyrene (P2VP-PI-PS) triblock copolymer with each block having a M-w similar to 15 000, was observed to order in a hexagonal lattice of P2VP cylind ers, surrounded by a PI annulus within a PS matrix. The PI/PS IMDS was of non-CMC, assuming a pseudohexagonal cross section. This striking m orphological behavior originates from the nonuniform degree of chain d eformation occurring in the microstructure and indicates that the PI/P S IMDS is strongly coupled to the shape of the Wigner-Seitz cell of th e microdomain lattice. We further probe this phenomenon by attempting to alter the curvature of the PI/PS interface through incorporation of homopolystyrene (h-PS) into the PS matrix by preparing P2VP-PI-PS/h-P S (PIS/S) blends having a composition of 90/10 (v/v) and (M-w)(h-PS) s imilar to 4000-100 000. Using TEM and SAXS, we observe a transition in the PI/PS IMDS as a function of the molecular weight of h-PS. When (M -w)(h-PS) < 50 000, the tethered PS blocks behave as ''wet brushes''. In this regime, the non-CMC character of the PI/PS IMDS is preserved a nd a slight contraction in the domain spacing is observed due to a nea rly uniform distribution of h-PS in the matrix. When (M-w)(h-PS) = 50 000, behavior characteristic of the ''dry brush'' regime is seen. The PI/PS IMIDS appears nearly circular in cross section with an accompany ing increase in the domain spacing relative to that of the neat triblo ck. This behavior originates from a nonuniform distribution of h-PS in the matrix, with preferential segregation to the corners of the Wigne r-Seitz cell. As the molecular weight of h-PS is further increased, th e homopolymer is ejected from the microdomain structure and macrophase separates from the tethered PS blocks.