  The phase behaviour of amphiphilic multiblock copolymers with a large number
of blocks in semidilute solutions is studied by lattice Monte Carlo
simulations. The influence on the resulting structures of the concentration,
the solvent quality and the ratio of hydrophobic to hydrophilic monomers in the
chains has been assessed explicitely. Several distinct regimes are put in
evidence. For poorly substituted (mainly hydrophilic) copolymers formation of
micelles is observed, either isolated or connected by the hydrophilic moieties,
depending on concentration and chain length. For more highly substituted chains
larger tubular hydrophobic structures appear which, at higher concentration,
join to form extended hydrophobic cores. For both substitution ratios gelation
is observed, but with a very different gel network structure. For the poorly
substituted chains the gel consists of micelles cross-linked by hydrophilic
blocks whereas for the highly substituted copolymers the extended hydrophobic
cores form the gelling network. The interplay between gelation and phase
separation clearly appears in the phase diagram. In particular, for poorly
substituted copolymers and in a narrow concentration range, we observe a
sol-gel transition followed by an inverse gel-sol transition when increasing
the interaction energy. The simulation results are discussed in the context of
the experimentally observed phase properties of methylcellulose, a
hydrophobically substituted polysaccharide.
