SEGREGATION STUDY OF A BAB TRIBLOCK COPOLYMER AT THE A/B HOMOPOLYMER INTERFACE

The segregation of a poly[(2-vinylpyridine)-b-styrene-d8-b-(2-vinylpyridine)] triblock copolymer (PVP-dPS-PVP) to a planar interface between the homopolymers PS and PVP is determined by forward recoil spectrometry (FRES). Compared to a diblock copolymer of dPS-PVP with the same mole fraction of PVP (approximately 0.15) and total degree of polymerization (approximately 500), we find that the triblock copolymer interfacial excess z(i)* at saturation is roughly half that of the diblock copolymer. Self-consistent mean field theory produces a segregation isotherm in fair agreement with the experimental results for the triblock copolymer using the same value of the Flory parameter chi deduced for the diblock copolymer. Both triblock and diblock copolymers show an increase in z(i)* beyond the saturation plateau which is due to the condensation of block copolymer micelles adjacent to the interface in the PS phase. This increase for the triblock copolymer occurs at a copolymer volume fraction phi(infinity) = 0.15 in the PS phase, whereas for the diblock copolymer it occurs at a much lower volume fraction, 0.05. This result indicates that the triblock copolymer has a larger critical micelle concentration (CMC) than the diblock copolymer. While the condensation of diblock copolymer micelles at the surface occurs above the CMC, no such preferential segregation is observed for the triblock copolymer. Transmission electron microscopy was used to examine the structure of the triblock copolymer micelles at and near the interface.