Phase behavior of confined polymer blends and nanoparticle composites

We have investigated phase behavior in polymer blend films of poly(methyl methacrylate) (PMMA) and poly(styrene-ran-acrylonitrile) (SAN) with 33wt% AN content and their nanoparticle (NP) composites by using the combination of imaging techniques, including atomic force microscopy (AFM), focused-ion beam (FIB), transmission and scanning electron microscopy (TEM and SEM), as well as depth profiling techniques of Rutherford backscattering spectrometry (RBS) and elastic recoil detection (ERD). For neat PMMA:SAN films, we present a novel morphology map based on pattern development mechanisms. Six distinct mechanisms are found for thickness values (d) and bulk compositions between 50-1000 nm and &phis;PMMA = 0.3 to 0.8, respectively. When PMMA is depleted from the mid-layer by preferential wetting at &phis; PMMA = 0.3 (A), stable PMMA/SAN/PMMA trilayer structure is obtained. With increasing &phis;PMMA (0.4 to 0.7), pattern development is driven by phase separation in the mid-layer, which produces circular domains (B), irregular domains (C), and bicontinuous patterns (D). Here, the growth of circular domains can be explained by the coalescence mechanism, which predicts &xgr;∼(σ/η) 1/3d2/3t1/3 , where &xgr;, σ, and η are correlation length between domains, interfacial tension between phases, and viscosity, respectively. In bicontinuous patterns, hydrodynamic pumping mechanism is suppressed with thickness confinement. When SAN composition is lean, &phis;PMMA = 0.8 (E), the SAN phase is minority component in the mid-layer and breaks up into droplets in smooth PMMA film. When film thickness is less than 80 nm at &phis;PMMA = 0.4 or 0.5 (F), films initially display trilayer structure, which then ruptures upon dewetting of the SAN mid-layer. Building upon the understanding of the neat PMMA:SAN blend films, we have performed the first systematic on the effect of NPs in morphology evolution and stability of polymer blend films. Whereas the location of NP impacts morphology evolution, silica NPs with mixed surface of methyl and hydroxyl groups (HM-NP) partition into dPMMA phase upon phase separation. Chlorine terminated PMMA-grafted silica NPs either partition into dPMMA phase or weakly and strongly segregate at the interface between the phases when grafting molecular weight is high (MMA(160K)-NP), intermediate (MMA(21K)-NP), and low (MMA(1.8K)-NP), respectively. Hydrogen terminated low molecular weight NPs (MMA:H(1.8K)-NP) weakly segregate to the interface. When the blend films contain the HM-NP, pattern growth and film roughening slows down with NP loading (2 to 10wt%) due to the increased viscosity of dPMMA phase. In contrast to the HM-NPs, the MMA(1.8K)-NPs pin pattern development and film roughening when they assemble and jam at the interface, resulting in a stable discrete or bicontinuous structure at low (5wt%) and high (10wt%) loading, respectively. A geometric model predicts the shape and size of the stabilized morphology using experimental parameters, including NP loading, NP radius, and film thickness. Film roughening is completely prevented even at very low loading (2wt%). The weakly segregating MMA(21K)-NPs have an intermediate effect on morphology evolution of dPMMA:SAN films compared to HM-NPs and MMA(1.8K)-NPs, which partition into dPMMA and strongly segregating to the interface, respectively. Finally, the mechanism of surface roughening is clearly observed and explained. The internal phase-separated structure of the blends exerts Laplace pressure, resulting in the surface roughening. In summary, we have extensively studied phase behavior in polymer blends and their NP composites and provided various models to explain the mechanisms underlying the morphology evolution and film roughening