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Globules of amphiphilic macromolecules

We consider the theoretical model of an amphiphilic macromolecule with a complex structure of hydrophobic/hydrophilic monomer units. Each unit consists of a hydrophobic group (H) in the backbone and a side hydrophilic group (P). The units are able to orient in the density gradient at the surface layer of a globule. First, we use the density functional method to obtain the surface tension at a flat surface. We obtain that the effect of orientation decreases the surface tension of an amphiphilic globule in comparison with the surface tension of a homopolymer globule of the same density. Therefore, the amphiphilic globule is more stable with respect to the transition to a coil conformation. Then, macromolecules with strong orientational ability of amphiphilic units are considered. The free energy of spherical, bead-like, disc-like and torus-like globules is analysed for flexible and rigid macromolecules. For very long macromolecules in poor solvent, it is predicted that a disc-like globule for flexible chains should be formed. It is shown that the coil–globule transition in amphiphilic macromolecules is in most cases accompanied by a disintegration of the initially formed globule into several ‘bead globules’. Upon further increase of the attraction of hydrophobic units, these beads merge with each other with the formation of a disc-like or torus-like globule, depending on the chain stiffness.