Nanoscale study of polymer dynamics

The thermal motion of polymer chains in a crowded environment is anisotropic and highly confined. Whereas the dynamics are theoretically and experimentally quite well understood, typically only indirect and bulk information on polymer dynamics was obtained from studies such as light scattering, NMR and rheology.

We studied the thermal motion of an individual “reporter” polymer molecule, a relatively stiff and several micrometers long, fluorescently labeled polyisocyanide, surrounded and entangled by a concentrated solution of unlabeled but otherwise very comparable polymers. The combination of the extreme length, stiffness and fluorescence makes it possible to follow the shape and the motion of the reporter in time using wide-field fluorescence microscopy. The movies thus obtained immediately show the restricted motion of the polymers. They appear to move back and forth along their long axis, and barely move sideways; as if it is moving in a tube. The notion of an imaginary tube was postulated decades ago in the reptation model by De Gennes, Edwards and Doi, and is now directly observed.

The thermal motion of an individual reporter polymer molecule recorded in time. Each movie frame (inset) was analysed in order to extract the molecule's coordinates as a function of time indicated by the colour bar.

Using image analysis techniques similar to those used in single-molecule based localization techniques, we have been able to determine the polymer trajectories at sub-diffraction resolution. Using this, a wealth of information became available, such as the tube diameter, characteristic times (including the Rouse time), as a function of the contour length of the probe chain. Interestingly, the scaling laws we obtained match extremely well with those from bulk rheology. This implies that our local, nanoscale technique can be used for studying much more complex systems, such as biological cells, where molecular individualism, crowdedness and heterogeneity are important but hinder more traditional, bulk studies.

Reference:
Nanoscale study of polymer dynamics
M. Keshavarz, H. Engelkamp, J. Xu, E. Braeken, M. B. J. Otten, H. Uji-i, E. Schwartz, M. Koepf, A. Vananroye, J. Vermant, R. J. M. Nolte, F. De Schryver, J. C. Maan, J. Hofkens, P. C. M. Christianen, A. E. Rowan.
ACS Nano 10, 1434-1441 (2016)
DOI: 10.1021/acsnano.5b06931