Surface composition profiles in some polymer mixtures

Abstract

The surface composition of selected polymer mixtures has been studied to a depth of circa 4000Å with a resolution of up to l0Å using neutron reflectometry (NR) and nuclear reaction analysis (NRA). The effective interaction parameters, X, of several blends have been measured as a function of both composition and temperature, using Small Angle Neutron Scattering (SANS) and the incompressible random phase approximation, in order to understand the surface segregation behaviour of the polymer blends. No surface segregation was observed in annealed blends of syndiotactic poly (methyl methacrylate) (h-PMMA) with perdeuterated poly (methyl methylacrylate) (d-PMMA), where the h-PMMA was the majority component with a high molecular weight and the d-PMMA had lower molecular weights. Values of X for these blends showed a chain length disparity effect, higher disparity led to a small negative X. Increases in X were observed at low volume fractions of d-PMMA. Surface segregation of perdeuterated poly (ethylene oxide) (d-PEO) to the polymer - silicon oxide interface of an annealed d-PEO/h-PMMA blend was observed, where the bulk volume fraction of the d-PEO was <0.30. The surface composition profile could not be described by current theory. Measured X values were small and negative and there was a change in X on changing the locus of deuteration from PEO to PMMA in a PEO/PMMA blend. These blends exhibited a decrease in X at low volume fractions of PEO. Polymer brushes were found at the air - polymer interface of a blend of low molecular weight polystyrene (h- PS) with perdeuterated polystyrene with a single perfluorohexane end group (d-PS(F)) or two perfluorohexane end groups (d-PS(F2)). These results were in good agreement with a self consistent field theory. Similar blends of high molecular h-PS / d-PS(F) showed enhanced surface segregation, compared to blends with no perfluorohexane end groups. NR data showed that the surface of a blend of polystyrene with perdeuterated dibutyl phthalate (d-DBP) (a model additive) was enriched with d-DBP over a 30Ålength scale. The loss of d-DBP from a thin film (~ 8OO Å thick) was observed using NR and attenuated total reflection (ATR) infra red spectroscopy.