Strong compression rate dependence of phase separation and stereocomplexation between isotactic and syndiotactic poly(methyl methacrylate)s in a Langmuir monolayer observed by atomic force microscopy.

The stereocomplex formation between isotactic and syndiotactic poly(methyl methacrylate) (it-PMMA, st-PMMA) in a Langmuir monolayer was studied by surface pressure-area isotherms and atomic force microscopy (AFM). We found that the stereocomplex formation was highly sensitive to the compression rate of the monolayer. At a normal compression rate of 0.5 mm/s provided by the moving barrier, the blend monolayer formed a clear phase separation of the it- and st-PMMA domains at 1 mN/m. Further compression to 15 mN/m resulted in a limited degree of stereocomplexation, mainly at the interface between the two domains. However, at a 1/50 slower compression rate of 0.01 mm/s, the blend did not form a clear phase separation at 1 mN/m and quantitatively formed a stereocomplex at 15 mN/m. This apparent immiscibility observed at the faster compression rate was found to be kinetically induced as a result of the rapid compression of the phase-separated mixture at the dilute state because it-PMMA and st-PMMA form expanded and condensed monolayer, respectively. On the other hand, at the slower compression rate, the blend formed a thermodynamically miscible phase, and as a result, the stereocomplex was quantitatively formed. This apparent phase separation of a mixed monolayer composed of an expanded and a condensed monolayer should be a common phenomenon for similar systems and might have caused misjudgment of the miscibility in such cases. The compression rate dependence should be carefully evaluated in order to determine the precise miscibility of blended monolayers in similar systems.