TRANSIENT SURFACE ROUGHENING OF THIN-FILMS OF PHASE-SEPARATING POLYMER MIXTURES

The surface morphology of thin molten films of phase separating mixtur es of poly(ethylenepropylene) (PEP) and perdeuterated poly(ethylenepro pylene) (dPEP) of the critical composition were investigated using sca nning force microscopy. Volume fraction versus depth profiles were als o obtained using time-of-flight forward recoil spectrometry (TOF-FRES) . The free surface of a film with a nominal thickness of about d = 200 nm is initially smooth but develops a regular roughness pattern, with a wavelength about 500 nm and an amplitude about 2.5 nm, after a cert ain annealing time, which corresponds to the transition from a four-la yer (dPEP/PEP/dPEP/PEP) to a two-layer (dPEP/PEP) phase-separated doma in structure in the direction normal to the film thickness. At much lo nger times the two-layer films become smooth again. We attribute these fine surface patterns to transient pressure differences accompanying mass transport by hydrodynamic flow, where this transport occurs thoug h perforations in the PEP-rich layered domain just below the dPEP-rich surface layer. This flow is driven by the pressure beneath the highly curved lateral interfaces of the intermediate layers. In keeping with this hypothesis, thicker films (d > 280 nm), which do not develop bre aks in the PEP-rich layered domain and for which diffusional transport is necessary for coarsening of the layer structure to occur, do not s how this transient roughening.