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

esult from dehydratation of the hydrophobic block with increasing temperature, water being
a selective solvent for PE. This explains why the solution properties of Pluronic polymers are
strongly temperature-dependent. For most PE-PP-PE copolymers, the cmt increases with
decreasing in copolymer concentration. The number of oxypropylene units has a strong effect
on the micellization, copolymers with large hydrophobic PP block length forming micelles at
much lower concentrations. Indeed, the cmc is found to decrease exponentially with the PP
block length. On the other hand, the PE block has a smaller influence on the micellization. An
increase in the number of PE units leads to small increases in the cmc and cmt values. The cmt
and cmc decrease with increasing the total copolymer molecular weight, if comparisons are
made for a constant PP/PE ratio. On the other hand, the large positive enthalpy of
micellization, and the standard free energy of micellization observed on the self-assembly of
PE-PP-PE demonstrate that the association process is entropy driven. Chain architecture has
also a great influence on micelle formation. For example, the tendency for micellization of PP-
PE-PP copolymer is reduced compared to a PE-PP-PE copolymer of the same composition.
3.5.- E12S10, E10S10E10 and E137S18E137 block copolymers
Our research group has been collaborating during last years with research groups of the School
of Pharmacy and Pharmaceutical Sciences and the School of Chemistry of the University of
Manchester in synthesizing block copolymers formed by ethylene oxide and styrene oxide
blocks (81)-(83). In this regard, it has been previously demonstrated that the micellization
process of diblock copolymers of this type has not cmt values. Moreover, they usually have
small cmc values and micellization enthalpies (micH) as a consequence of the high
hydrophobicity of the styrene oxide block. Triblock copolymers show cmc values higher than
diblock ones for the same hydrophobic block length. On the other hand, the cmc values of the
triblocks weakly depend of the hydrophilic block length.
Block copolymers studied in this work were: E12S10, E10S10E10 and E137S18E137, (where E
represents the oxyethylene unit, S represents the oxystyrene unit, and the subscripts
correspond to block lengths). These copolymers were synthesised by sequential anionic
polymerisation of the two monomers, oxyethylene and oxystyrene. Oxyrane compounds are
heterocyclic compounds constructed by two carbon atoms and one oxygen atom into the
cyclic structure (Figure 3.4) (84).
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