Page 56 - Plastics News February 2018

TECHNOLOGY

Collective disentanglement of entangled polymers

U researchers have disproven the conventional theory other, like the fibers in clumps of fluff. In the 1970s,
Mused to explain the dynamics of polymer solutions. a model was developed to describe their dynamics. In
They show that for biopolymers collective effects facilitate this reptation model, each polymer molecule is viewed
chain mobility, which is reminiscent of the behavior of as being confined within a flexible tube through which
glass-like materials. What do silk threads, plastics and it moves in an undulatory manner, like the proverbial
DNA have in common? They are all made up of polymeric snake in the grass (hence the name). The walls of these
constituents and are representative of what scientists call tubes are themselves defined by all the other polymer
'soft matter'. Unlike hard condensed matter, these materials molecules in the medium. In this way, the model captures
how the mobility of each individual macromolecule is
restricted by the spatial distribution of all the others.
In this picture, the only way to disentangle such nest of
vipers is to pull individual fibers out of their confining
tubes, because motion orthogonal to the tube walls is not
possible. "Our extensive computer simulations, however,
suggest a very different type of polymer dynamics for
biopolymers," says Frey. "We do not observe convoluted
motions of individual polymers. Instead, we find
relatively rapid, collective reorganization of the tubes,
which results in the disentanglement of polymer chains."
are intrinsically flexible ('soft'), but can nevertheless form According to the authors, the dynamics resembles that of
stable structures under ambient conditions. Hence, soft glass-like materials. This kind of behavior is not based on
matter cannot be unequivocally classified as either solid or the independent motions of individual polymer molecules,
liquid, because its material properties are very sensitive to but derives from interactions between polymers on much
environmental parameters. – Changes in temperature can larger scales. This leads to collective motion of all the
have a marked effect on their susceptibility to deformation, polymer chains in a local neighborhood, such that the
for instance. Indeed, their structural dynamics is the key to whole jumble begins to sort itself out, tangled chains
their behavior and the major determinant of their functions are unraveled, the dense ball loosens up, and new paths
and applications.
through the maze are created. "We have developed a new
In a new study, LMU physicists Professor Erwin Frey and Dr. theoretical concept that can account for the collective
Philipp Lang have refuted the conventional theory hitherto dynamics and reproduces the results obtained in our
used to account for the rheological characteristics (i.e. simulations," says Frey. "Our findings will fundamentally
their response to external mechanical stresses) of polymer change current assumptions concerning the relevance
solutions, and developed an alternative model. The new of collective effects not only in biopolymer solutions
study appears in the online journal Nature Communications. but potentially also in other soft-matter systems." The
Frey and Lang focused on the dynamics of semi-flexible to theoretical predictions made by the new model should
stiff polymers – the group to which biopolymers including be quite easy to test experimentally in systems involving
DNA, or actin filaments and microtubules that are a major either biopolymers or carbon nanotubes. Indeed, Lang
component of the cytoskeleton belong. All polymers are and Frey have already shown that the data derived
made up of repeating subunits that are linked together from their simulations are in complete agreement with
to form long macromolecular chains. In solution, these results of a study on carbon nanotubes published by a
macromolecules are intricately entangled with each Dutch-American research group.

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