CROSSOVER OF A BLOCK-COPOLYMER BRUSH IN A POLYMER MELT FROM A STRETCHED TO COLLAPSED CONFORMATION

The adsorption of block copolymers from a homopolymer melt is studied as a function of matrix molecular weight using neutron reflectivity an d low energy forward recoil spectrometry. The block copolymer is poly( deuterated styrene-block-methylmethacrylate) (dPS-b-PMMA), which conta ins short MMA and long dS blocks. The MMA block adsorbs to the silicon oxide surface, whereas the dPS extends into the matrix chains. The dP S-b-PMMA is blended with a polystyrene matrix of molecular weight P. V olume fraction profiles and copolymer coverage (z) are investigated a s a function of P. We find that z initially increases rapidly with P and remains almost constant for P larger than 2N(B) (Ng is the number of dS segments). We also observe that the thickness of the adsorbed la yer (L) as well as the interfacial width between the brush and the mat rix (w) initially decrease rapidly with increasing P and become weakly dependent on P for P>N-2(B). By measuring z, L, and w as a function of P we observe a crossover from a stretched to collapsed brush at P s imilar to 2N(B). For P>2N(B) the matrix chains are driven from the ads orbed layer by entropy. Self-consistent mean field predictions are in qualitative agreement with experimental results and provide an estimat e for the MMA-wall interaction energy, -8kT/block.