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complexity of these mesophases, nor on the physical mechanisms regulating
their self - assembly behavior.
1.3 ANALOGIES AND DIFFERENCES BETWEEN LIPIDS AND
BLOCK COPOLYMERS
Alternative approaches can be taken in order to develop a physical under-
standing of lyotropic liquid crystals. A good starting point is to draw analogies
and differences with another important organic self-assembling system, that
is, block copolymers.
Block copolymers bear many similarities to surfactants. As for lipids, they
are also formed by two blocks attached into the same amphiphilic molecule.
If the two blocks differ enough in chemical composition, they will induce a
microphase segregation into nanostructured morphologies spanning a few
nanometers in periodicity (Bates and Fredrickson, 1999). The types of struc-
tures found are also very similar: lamellar, columnar hexagonal, and bicontinu-
ous cubic (Ia3d) phases are present in both classes of materials. The Pn3m
bicontinuous cubic phase, which is missing for a pure diblock copolymer phase
diagram, can be recovered when additional components are added. Indeed,
when comparing the morphologies from the two classes of materials, one must
realize that since the water-lipid system is a two-component system, for direct
comparison with block copolymers one should take the case of a block copo-
lymer plus a second component compatible with one of the two constitutive
blocks of the copolymer. This second component, be it a solvent or a homo-
polymer, has the role to mimic water in the lipidic mesophases. Figure 1.3
shows the phase diagram of a hypothetical diblock copolymer, with a graphic
of the morphologies found.
Beside this encouraging start, some important differences should be men-
tioned. Let us take a classical diblock copolymer system: Onset of microphase
separation occurs when the segregation power,
is the Flory-
Huggins parameter and
N
the polymerization degree of the entire block
copolymer, is of the order of
χ
N
, where
χ
10 or more (Leibler, 1980). By adding the third
block mimicking the role of water, small perturbations on this threshold cri-
terion can be expected, without, however, changing it signifi cantly. Because
typical polymer pairs typically have an unfavorable
≈
0.1 or less at room
temperature, this indicates that in order to observe microphase separation at
standard temperatures the polymerization degree must be 100 or more.
In the case of lipids, the scenario is very different. The Flory-Huggins
parameter of the polar versus unipolar species can be estimated by measur-
ing the activity of alkanes partitioning in water at room temperature, which
indicates values of the order of 3 for
χ
of
≈
(Mezzenga et al., 2005a). This represents
such a high unfavorable mixing enthalpy that microphase segregation between
the polar head (plus water) and the hydrophobic tail occurs at much lower
polymerization degrees.
χ
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