Whether interfacial tension between two molten polymers can be reduced significantly by
the formation of copolymer at the interface by chemical reaction of functionalized chains is the question
addressed by this paper. To answer it, model experiments are carried out by grafting of benzylamine
end-functionalized deuterated polystyrene (dPS−NH2) onto poly(styrene-r-maleic anhydride) (PSMA)
random copolymer at an initially planar melt interface between polystyrene (PS) and PSMA. Various
volume fractions of dPS−NH2 with polymerization indices N = 33, 55, and 270 were mixed with PS and
then reacted with PSMA above the T
g of PS and PSMA. The interfacial excess, z*, of the dPS portion of
the graft copolymer formed at the boundary was measured using forward recoil spectrometry. The values
of normalized grafting density z*/R
g, where R
g is the radius of gyration of dPS−NH2, are observed to be
as large as 40, 9, and 4 for the N = 33, 55, and 270 dPS−NH2 chains, respectively. These large values
signal the formation of a layer of microemulsion which occurs when the interfacial tension of the flat
interface is driven negative by the increasing graft copolymer excess at the interface. The interfacial
instability is followed by monitoring the positions of Au particles deposited on the original (flat) interface
using cross-sectional transmission electron microscopy (TEM). Evidence of the interfacial corrugation
induced by the instability is also available from scanning force microscopy (SFM) of the exposed PSMA
interface after selective removal of PS using a solvent wash. The length scale of the corrugation is around
200 nm, which is the same magnitude as the size of the emulsion droplets shown by TEM near the interface.
The onset of the interface instability occurs at critical values of z*/R
g of about 1.8 for N = 55 and z*/R
g
of about 2.5 for N = 270 dPS−NH2 chains. These values are predicted qualitatively by self-consistent
mean field theory.
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