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

subsequently, of their thermodynamic stability by modification of some factors such as block
copolymer molecular weight, the relatively length of both hydrophobic and hydrophilic blocks,
the chemical nature of the different monomer units and the architecture of the polymeric
backbone. Firstly, this can be achieved by means of the block copolymer synthetic processes.
However, the synthesis of block copolymers is time-consuming and expensive, disadvantages
that have to be taken into account. A simpler alternative is the possibility of manipulating the
block copolymer solution properties through the change of external parameters such as
temperature, salinity, pH, addition of cosolvents… Under this context, it is possible to obtain
different aggregate sizes or morphologies using the same block copolymer or the same family
of copolymers. The morphology of the structured aggregates can be determined by a balance
of different factors such as the solvent properties, the conformation of the hydrophobic chains
into the micelle core, the repulsion between hydrophilic blocks into the micelle shell, and the
surface tension between the micelle cores and the bulk solution (77)(78).
3.4.- Block copolymers of polyoxyalkylenes in aqueous solution.
The block copolymers of polyoxyalkylenes are composed of a hydrophilic block constituted by
poly(ethylene oxide) (OCH2CH2 ethylene oxide unit, E) and by a second hydrophobic block that
can be made of, for example, poly(propylene oxide) (P), poly(styrene oxide) (S), or
poly(butylene oxide) (B), amongst others. In aqueous medium, polyoxyalkylene amphiphilic
block copolymer contains both the hydrophilic and hydrophobic blocks that can associate to
usually form spherical micelles in dilute aqueous solution. Increasing the concentration leads
to the formation of lyotropic crystal mesophases (gels) where the structural objects are
micelles, usually efficiently packed in a body-centered or face-centered cubic structure as a
result of their interaction as hard spheres (80)(81).
For poly(oxyalkylenes) block copolymers, values of the cmc are affected by the variation of
hydrophilic polyoxyethylene and hydrophobic block lengths. It is known that the dependence
of the cmc on the number of E units in a copolymer is small unless the hydrophobic-block
length is short. Indeed, when comparing results for two copolymers of nominally the same
hydrophobic-block length, the effect of E-block length may well be less than the variation
caused by the uncertainty in hydrophobic-block length. However, within a series of copolymers
it is desirable to account for variation in E-block length. Taking as an example PE-PP-PE
Pluronic block copolymers, micelle formation of this class of polymers in water is believed to
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