Frey and Lang focused on the dynamics of semi-flexible to stiff polymers – the group to which biopolymers including DNA, or actin filaments and microtubules that are a major component of the cytoskeleton belong. All polymers are made up of repeating subunits that are linked together to form long macromolecular chains. In solution, these macromolecules are intricately entangled with each other, like the fibers in clumps of fluff. In the 1970s, a model was developed to describe their dynamics. In this reptation model, each polymer molecule is viewed as being confined within a flexible tube through which it moves in an undulatory manner, like the proverbial snake in the grass (hence the name). The walls of these tubes are themselves defined by all the other polymer molecules in the medium.
Dense solutions of polymers are viscoelastic: while they respond like a fluid to low-frequency stresses, they act like a cross-linked elastic network at high frequencies. These intriguing material properties are attributed to the extended structure of polymers, which makes topological interactions particularly important. Polymers can effortlessly slide past each other but are not allowed to cross each other’s path. These ‘entanglements’ mutually restrict the accessible configuration space, turning the dynamics of polymer solutions into a difficult many-body problem. Efforts to incorporate these features into a single-polymer mean-field model led to the famous tube model. In this model, the dynamic topological constraints on the motion of a given polymer are represented as a static, confining tube. By this means, the single-polymer dynamics in an entangled solution is reduced to the curvilinear Brownian motion of its center of mass along the long axis of the tube termed ‘reptation’. For flexible polymers, reptation theory is well established and also fairly predictive, though there are still many interesting open questions.
Read more at: Collective disentanglement of entangled polymers
thumbnail courtesy of phys.org
