The production of PEO polymer brushes via Langmuir-Blodgett and Langmuir-Schaeffer methods: Incomplete transfer and its consequences

Using fixed-angle ellipsometry, we investigate the degree of mass transfer upon vertically dipping a polystyrene surface through a layer of a polystyrene-poly(ethylene oxide) (PS-PEO) block copolymer at the air water interface (Langmuir-Blodgett or LB transfer). The transferred mass is proportional to the PS-PEO grafting density at the air-water interface, but the transferred mass is not equal to the mass at the air-water interface. We find that depending on the chain length of the PEO block only a certain fraction of the polymers at the air-water interface is transferred to the solid surface. For the shortest PEO chain length (PS₃₆-PEO₁₄₈), the mass transfer amounts to 94%, while for longer chain lengths (PS₃₆- PEO₃₇O and PS₃₈-PEO₇₇₀), a transfer of, respectively 57% and 19%, is obtained. We attribute this reduced mass transfer to a competition for the PS surface between the PEO block and the PS block. Atomic force microscopy shows that after transfer the material is evenly spread over the surface. However, upon a short heating of these transferred layers (95 °C, 5 min) a dewetting of the PS-PEO layer takes place. These results have a significant impact on the interpretation of the results in a number of papers in which the above-described transfer method was used to prod uce PEO polymer brushes, in a few cases in combination with heating. We briefly review these papers and discuss their main results in light of this new information. Furthermore, we show that, by using Langmuir-Schaeffer (LS, horizontal) dipping, much higher mass transfers can be reached than with the LB method. When the LB or LS methods are carefully applied, it is a very powerful technique to produce PEO brushes, as it gives full control over both the grafting density and the chain length.