Sw. An et al., NEUTRON REFLECTIVITY OF ADSORBED WATER-SOLUBLE BLOCK-COPOLYMERS AT THE AIR WATER INTERFACE - THE EFFECTS OF COMPOSITION AND MOLECULAR-WEIGHT/, Macromolecules, 31(22), 1998, pp. 7877-7885
We have used neutron reflection to follow the effects of composition a
nd molecular weight on the structure of layers of poly(2-(dimethylamin
o)ethyl methacrylate-block-methyl methacrylate) copolymer (poly(DMAEMA
-b-MMA)) adsorbed at the air/water interface. We had previously shown
that for a 70% DMAEMA copolymer of M-n about 10K at a pH of 7.5 there
is a surface phase transition from a layer about 20 Angstrom thick to
one about 40 Angstrom thick and that the adsorbed layer at the higher
concentration has a layered cross-sectional structure with the most hy
drophobic part of the polymer forming a central layer rather than the
outer layer in contact with air, as would be expected. The present wor
k shows that copolymers containing 80% DMAEMA (about the same M-n) and
70% DMAEMA (M-n about 20K) form the same layered structure at the sur
face. The 80% DMAEMA copolymer is more surface active than the 70% cop
olymer, and this parallels the tendency to aggregate into micelles in
bulk solution. The increase in molecular weight from 10K to 20K decrea
ses the surface activity of the 70% DMAEMA copolymer. Addition of elec
trolyte enhances the formation of the layered structure at the interfa
ce but tends to equalize the surface activities of the three copolymer
s. Other surprising features are the increase of surface activity with
increasing polyelectrolyte block size when the block is the larger co
mponent and the apparent suppression of ionization in the surface regi
on..