  In recent experiments involving PNIPAM copolymers it has been observed that
stable spherical nanoparticles are being formed by association of several
chains in poor aqueous solution instead of aggregation. This type of mesoscopic
structures called mesoglobules has an extremely monodispersed size
distribution. Previously we have studied theoretically formation of clusters
consisting of several chains in dilute solutions of amphiphilic heteropolymers.
We have seen that the mesoglobules often possess an essentially micellar
structure. In the current work we argue that formation of mesoglobules is
strongly sequence and concentration dependent. In particular, by means of
lattice Monte Carlo simulation we consider structures formed by a number of
tri-block sequences and their certain mutations. For sequences consisting of
two hydrophilic ends with a hydrophobic middle the size distribution of the
resulting particles is rather narrow, i.e. mesoglobules are being formed.
However, in the case of sequences with two hydrophobic ends and a hydrophilic
middle a new type of structures seems to be prevalent. These consist of small
mesoglobules interconnected by hydrophilic bridges. Clearly, on increasing the
concentration these networked structures would play an important role resulting
in a rapid onset of a gel-like behaviour.
