The secrets of sedimenting star polymers

Star polymers are named as such because they consist of linear polymer arms attached to a common center, like a star. They are also called ultra-soft colloids, because their interactions can be tuned by varying their arm number: If a star polymer has fewer arms, it acts more like a polymer; if it has more, it acts more like a colloid. Star polymers are used in the oil industry and may also find applications in drug delivery.

Reported in The Journal of Chemical Physics, researchers conducted large-scale computer simulations on graphics processing units (GPUs) to study the structure and transport of star polymers during steady-state sedimentation. Sedimentation processes, like centrifugation, allow characterization and separation of macromolecules from mixtures.

The authors discovered that a star polymer’s structure and movement during sedimentation has a non-monotonic dependence on the external field strength, related to gravitation and density differences.

The sedimentation coefficient first increases with increasing field strength, reaches a maximum, and then decreases. This non-monotonic behavior is caused by inhomogeneous hydrodynamic interactions within the star polymer. In a weak external field, the structure of a star polymer is basically unchanged. In an intermediate field, the radius of gyration decreases transversely to the sedimentation direction. In a high field, the radius of gyration increases because polymer arms pull out from the core, forming a trailing tail.

The location and value of the sedimentation coefficient maximum depends on how many arms the star polymer has, which means that researchers may be able use external fields for the separation of various macromolecular mixtures as needed for a specific application.

Source: “Steady state sedimentation of ultrasoft colloids,” by Sunil P. Singh, Gerhard Gompper, and Roland G. Winkler, The Journal of Chemical Physics (2018). The article can be accessed at https://doi.org/10.1063/1.5001886 .