X-ray spectromicroscopy of immiscible polymer blends: polystyrene–poly(methyl methacrylate)

Spun cast thin films of blends of low and high molecular weight mono-disperse polystyrene (PS) and poly(methyl methacrylate) (PMMA) with nominal compositions ranging from 66/33 wt.%/wt.% (w/w) up to 10/90 w/w PS/PMMA have been studied, as-made and after annealing. Two synchrotron-based X-ray microscopies — scanning transmission X-ray microscopy (STXM) and X-ray photoemission electron microscopy (X-PEEM) — as well as several variants of atomic force microscopy (AFM) were used to probe the composition and morphology of the bulk and surface of these blends. The chemical sensitivities and spatial resolutions of these three techniques are compared. All samples are observed to have a PS signal in the C 1s X-ray absorption spectrum of the surface of the PMMA-rich domains as measured in the X-PEEM. A continuous thin PS layer is not expected at a PMMA surface since neither polymer should wet the other at thermodynamic equilibrium. The likely origin of this PS surface signal is from a bimodal distribution of PS domain sizes with the PS signal arising from domains at the surface which are smaller than the ∼200-nm resolution of the X-PEEM. High resolution AFM and STXM provide direct evidence for this explanation.