Chemistry Reference
In-Depth Information
Chart 2.2 Structure of block copolymer containing amylase and PEO.
with MO-containing water. The extraction was confirmed by a peak at 428 nm
observed in the UV-vis spectra of the chloroform layer. The 36-nm blueshift
observed for the encapsulated MO was attributed to the microenvironment provided
by the polymeric assembly.
Most of the discussion above has focused on block copolymers in which the
hydrophilic block is made of neutral functionalities, but it is also interesting to
consider polymers in which one of the blocks is ionic. The first part of the discussion
here will be about block copolymers, where the ionic segment will end up as the core
component of an assembly. In order to encapsulate charged molecules in the core
of block copolymer micelles, amphiphilic ionic block copolymers consisting of an
ionic block and a PEO block have been developed. Because both PEO and polyionic
blocks are water soluble, these are also called double-hydrophilic block copolymers
(Colfen 2001). Even though the ionic block is water soluble in this type of polymer, it
can be converted into a relatively hydrophobic one either by varying the pH of the
solution or by the formation of a polyion complex (PIC). The micellization of
PEO-b-poly(2-vinylpyrine) (P2VP) block copolymer (PEO as shell and P2VP as
core) was pH responsive: micellization was observed when increasing the pH of
the solution. In contrast, PEO-b-poly(methacrylic acid) block copolymers were
able to form micelles by decreasing the pH. At high pH, the resultant carboxylate
anions of these polymers destabilize the micelles because of the electrostatic
repulsion between them (Fig. 2.5; Kabanov et al. 1996).
In ionic block copolymers, the micellar assemblies can also be induced by com-
plexation with oppositely charged molecules, forming a PIC at the core. The electro-
static interaction between PEO-polycation and PEO-polyanion block copolymers
is the driving force for the formation of micellar assemblies in which the complex
Figure 2.5 Formation of micellar assemblies induced by pH complexation in ionic block
copolymers.
