Block copolymer adsorption from a homopolymer melt to silicon oxide: Effects of nonadsorbing block length and anchoring block-substrate interaction

The adsorption of block copolymers from a homopolymer matrix is studied as a function of nonadsorbing block length and adsorbing block type. The copolymers are poly(deuterated styrene-blockmethyl methacrylate), dPS-b-PMMA, and poly(deuterated styrene-block-(dimethylamino)ethyl methacrylate), dPS-b-PDMAEMA, where the PMMA and PDMAEMA blocks adsorb to silicon oxide, SiOx, and the dPS block extends into a polystyrene, PS, matrix. Using neutron reflectivity and low-energy forward recoil spectrometry, the volume fraction profile, phi(z), copolymer interfacial excess, z*, interfacial width, w, and adsorbed layer thickness, h, are investigated as a function of the degree of polymerization of the dPS block, N-dPS. For N-dPS < 200, the distance between grafted copolymer chains, D, is greater than the dPS radius of gyration, R-g, whereas for N-dPS > 200, D is less than Rg. Thus, a crossover from a collapsed to a stretched conformation occurs near the N-dPS for entanglements in PS. Correspondingly, z*, w, and h strongly increase as N-dPS increases. Using an adsorbing block-substrate interaction parameter that depends on the adsorbing block length, self-consistent mean-field predictions of phi(z), z*, w, and h are in very good agreement with experimental results. By considering the fraction of adsorbing block segments in direct contact with the wall, the segmental interaction energies between MMA and SiOx and DMAEMA and SiOx are about -0.8 k(B)T and -1.1 k(B)T, respectively, consistent with a stronger affinity of DMAEMA to the oxide.