Crossover of a block copolymer brush in a polymer melt from a stretched to collapsed conformation

Abstract

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) $(d\mathrm{PS}\ensuremath{-}b\ensuremath{-}\mathrm{PMMA}),$ 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 $d\mathrm{PS}\ensuremath{-}b\ensuremath{-}\mathrm{PMMA}$ is blended with a polystyrene matrix of molecular weight $P.$ Volume fraction profiles and copolymer coverage ${(z}^{*})$ are investigated as a function of $P.$ 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 $(L)$ as well as the interfacial width between the brush and the matrix $(w)$ initially decrease rapidly with increasing $P$ and become weakly dependent on $P$ for $P>{2N}_{B}.$ By measuring ${z}^{*},$ $L,$ and $w$ as a function of $P$ we observe a crossover from a stretched to collapsed brush at $P\ensuremath{\sim}{2N}_{B}.$ For $P>{2N}_{B}$ 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, $\ensuremath{-}8kT/$block.