Self-Assembly of Synthetic and Biological Polymeric Systems of ...

In aqueous media, the micelle structure consists of a dense hydrophobic core surrounded by a
hydrophilic shell which is exposed toward the bulk solvent phase in a spherical configuration;
the micelles can change their size from 5 to some hundreds nanometers and can possess
different geometries (spheres, worn-like, and so on) (74)-(78).
The various reported micellar-like morphologies are primarily a result of the inherent
molecular curvature and how this influences the packing of the copolymer chains: specific self-
assembled nanostructures can be targeted according to a dimensionless packing parameter, ,
which is defined as
, where is the volume of the hydrophobic chains, is the
optimal area of the head group, and, is the legth of the hydrophobic tail, as occurred for
classical surfactants. Therefore, the packing parameter of a given molecule usually dictates its
most likely self-assembled morphology. As a general rule, spherical polymeric micelles are
favored when
, cylindrical micelles when
structures (vesicles, also known as polymersomes) when
, and enclosed membrane
Despite the structured aggregates from block copolymers formed in aqueous solution are
similar to those produced by the self-assembly of small surfactant molecules such as lipids and
detergents, the polymeric nature of the amphiphilic macromolecules provides new special
properties to these structural complexes, as stability and functionality. In this way, their
potential applications in different technological areas are largely increasing, including
materials science, bioengineering, medicine, pharmaceutics industry, and ink and paint
industry (70)(79).
3.3.- Mechanism of morphologic control and thermodynamic stability.
The conformation of the block copolymer chains depends on the interaction between chain
segments and solvent molecules. In aqueous solution, the structure of the aggregates is
governed by the balance of the interaction forces between the block copolymer molecules
with water molecules, and by the intrachain interactions between the blocks which constitute
the copolymer molecule (where the hydrophilic chain expands, avoiding unfavourable
segment-segment interactions and the hydrophobic part contract to minimize the contact with
solvent molecules).
It is possible to achieve a precise control of the size and shape of polymeric aggregates, and
.
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