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

temperatures, the chains are mixed homogeneously, as in any polymer melt. As the
temperature is reduced the tendency for the blocks to segregate is enhanced, i.e. the enthalpic
process of demixing is favoured. However, this is necessary accompanied by a reduction in the
entropy as the chain configuration becomes more constrained. The extent of segregation of
the copolymer may be expressed using the reduced parameter N. Here, is the Flory-Huggins
interaction parameter, which contains a significant enthalpic contribution, and is governed by
the incompatibility of the monomers; and N is the copolymer degree of polymerization,
reflecting the N-dependent translational and configurational entropy. The transition from
homogeneous melt of chains to heterogeneous melt of ordered microphase-separated
domains occurs at a critical value of N, depending on the composition of the copolymer (65).
In bulk, the minority block is segregated from the majority block forming regularly shaped and
uniformly-spaced nanodomains. The shape of the segregated domains in a diblock is governed
by the volume fraction of the minority block, f, and block incompability. Figure 3.1 shows the
equilibrium morphologies documented for diblock copolymers as an example. At a volume
fraction of ≈ 20 %, the minority block forms a body-centred cubic spherical phase in the matrix
of the majority block. It changes to hexagonally packed cylinders at a volume fraction ≈ 30 %.
Alternating lamellae are formed at approximately equal volume fractions for the two blocks. At
a volume fraction of ≈ 38 %, the minority block forms bicontinuous structures at moderate and
high incompatibility, respectively (64)(65)(69).
A Block B Block
Sphere Cylindric Bicontinuous Lamellar
Figure 3.1. Regular structures in block copolymer leaded by theirs self-assembling.
The physical properties of copolymeric materials have been applied in a wide range of fields.
For example, by decades copolymeric materials in solid state have been used as thermoplastic
and elastomeric materials, foams, and glues owing to their high compatibility, sensitivity, and
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