Fluorescence Spectroscopy as a Tool for Investigating the Self-Organized Polyelectrolyte Systems - Self Organized Nanostructures of Amphiphilic Block Copolymers - page 210

In contrast, the average rotation correlation time changes more slowly and we
suppose that it better monitors the conformational behavior of PMA and shows grad-
ual changes of chain conformations. A slow growth of

τ c is a result of a compromise
between two effects: (a) the gradual increase in the mass of collapsing and fus-
ing globules and (b) their increasing compactness. We believe that the comparison
of

c versus pH dependences supports the hypothesis that the conforma-
tional change is a gradual transition via the pearl necklace structures with pH. In
the intermediate pH region, small globules grow and fuse with decreasing pH and
simultaneously their density increases. At low pH, when only one compact globular
conformation has been created, further decrease in pH promotes its compactness,
which translates in its decreasing size and faster rotation. Hence the curve, which
first rises with decreasing pH, drops appreciably in the low pH region.
The conclusions that we formulated in early 1990s are, in principle, in agree-
ment with up-to-date knowledge. Water is a poor (but not too bad) solvent for
non-ionized PMA and, hence, the conformational transition is expected to proceed
not as a sharp transition, but via a cascade of pearl necklace structures. In Sect. 4.1 ,
we will show that our recent MD simulations support the above description of the
behavior. We included this almost-forgotten experimental study in the present fea-
ture article mainly because we returned to this problem and studied it theoretically
within the POLYAMPHI network.

τ F and τ

3.3 Solvent Relaxation Study of Self-Assembled Systems

3.3.1 Reversible Aggregation of Block Copolymer Micelles with PVP-PEO

Shells in Acid Aqueous Solutions

Motivation

In a series of recent papers, we studied the copolymers containing PEO blocks
[ 119 - 121 ] . PEO is biocompatible and easily soluble in aqueous media, and there-
fore it has been used as a shell of self-assembled biocompatible nanoparticles
designed for targeted drug delivery [ 122 ] . The behavior of concentrated aqueous
PEO solutions is fairly complex. It is influenced by the presence of a strongly hy-
drophobic (-CH 2 CH 2 -) group and hydrophilic (hydrogen-bonding) oxygen atom in
its monomer unit. The amphiphilic character of PEO results in a strong aggrega-
tion tendency in some solvents. The presence of aggregated PEO chains in solutions
has been proven by different experimental techniques, including light [ 123 - 127 ]
and small-angle neutron scattering [ 128 ] and pulsed-field-gradient nuclear magnetic
resonance (NMR) spectroscopy [ 125 ] . Even though various explanations of the ag-
gregation mechanism (crystallization [ 123 ], inter-chain hydrogen bonding or the
chain-ends effect [ 128 ], and subtle phase separation [ 129 ] ) have been proposed,
the exact origin of PEO aggregation under different conditions (PEO concentration,
temperature, and also specific interactions with solvent molecules and other compo-
nents) remains uncertain.