The adsorption of block copolymers from a homopolymer melt is studied as a function of matrix molecular weight using neutron reflectivity and low energy forward recoil spectrometry. The block copolymer is poly(deuterated styrene-block-methylmethacrylate) $(d\mathrm{PS}-b-\mathrm{PMMA}),$ which contains short MMA and long $d\mathrm{S}$ blocks. The MMA block adsorbs to the silicon oxide surface, whereas the $d\mathrm{PS}$ extends into the matrix chains. The $d\mathrm{PS}-b-\mathrm{PMMA}$ is blended with a polystyrene matrix of molecular weight $P.\mathrm{}$ Volume fraction profiles and copolymer coverage ${(z}^{*})$ are investigated as a function of $P.\mathrm{}$ We find that $z^{*}$ initially increases rapidly with $P$ and remains almost constant for $P$ larger than ${2N}_B$ ${(N}_B$ is the number of $d\mathrm{S}$ segments). We also observe that the thickness of the adsorbed layer $\mathrm{}\left(L\right)\mathrm{}$ as well as the interfacial width between the brush and the matrix $\mathrm{}\left(w\right)\mathrm{}$ initially decrease rapidly with increasing $P$ and become weakly dependent on $P$ for $P>\mathrm{}{2N}_B.$ By measuring $z^{*},$ $L,$ and $w$ as a function of $P$ we observe a crossover from a stretched to collapsed brush at $P\sim {2N}_B.$ For $P>\mathrm{}{2N}_B$ the matrix chains are driven from the adsorbed layer by entropy. Self-consistent mean field predictions are in qualitative agreement with experimental results and provide an estimate for the MMA-wall interaction energy, $-8kT/$block.

• Received 3 February 1997

DOI:https://doi.org/10.1103/PhysRevE.56.R2383