Two-Dimensional Phase Separation of a Poly(methyl methacrylate)/Poly(l-lactide) Mixed Langmuir Monolayer via a Spinodal Decomposition Mechanism

We have found the first evidence that a polymer blend Langmuir monolayer can phase-separate via spinodal decomposition (SD) mechanism. The system was a poly(methyl methacrylate)/poly(L-lactide) mixture. It phase-separated immediately after compression on a water surface and formed a spinodal-like morphology, as observed by atomic force microscopy (AFM). The fast Fourier transform of the AFM images showed a clear spinodal ring with a maximum intensity at a wavenumber of q(m). At the small quench depth at a surface pressure of 2 mN/m, q(m) did not change, but the concentration difference between the domains (ΔΦ) grew with time, corresponding to the early-stage SD. At larger quench depths at 4 and 5 mN/m, q(m) significantly decreased, but ΔΦ was constant with time; this behavior corresponded well to the late-stage SD. Thus, the 2D phase separation in the Langmuir monolayer was basically explained by the SD mechanism well-known in 3D systems. In the late stage SD of the monolayer, q(m) scaled with time much faster than that expected by theories for the 2D state. Phase separation via a SD mechanism is a promising new way to control the lateral morphology of Langmuir monolayers, one of the main issues in nanotechnology that remains difficult to attain even today.