Dynamic coupling drives conformational evolution of branched polymers in solutions

The critical overlap concentration of polymer solutions, denoted c*, is one of the most important characteristic values of a polymer solution. This geometrically defined parameter is used to identify concentration regimes with different conformational characteristics. Because previous experiments showed that the average size of polymers remains constant below c*, it was generally accepted that within this dilute regime, the conformation of a single polymer remains invariant. Our findings contradict this understanding and demonstrate the importance of incorporating dynamical effect in defining the threshold of diluteness for this important class of soft matter.

Using neutron scattering, we demonstrate that the competition between the inter-polymer translational diffusion and intra-polymer internal dynamics leads to significant conformational evolution below c*. Substantial structural dehydration and slowing-down of internal dynamics are both observed before physical overlap develops. Well below c*, a new threshold of diluteness, c_D*, emerges as the crossover between the characteristic times associated with these two relaxation processes. Below this dynamically defined c_D*, the two relaxation processes are essentially uncoupled and the conformation of polymers remains unperturbed. This finding demonstrates conformational changes are triggered by inter-polymer collisions, instead of the simple overlap between neighboring polymers as generally believed.  

X. Li, L. E. Sánchez-Diáz, B. Wu, W. A. Hamilton, P. Falus, L. Porcar, Y. Liu, C. Do, A. Faraone, G. S. Smith, T. Egami, and W-R. Chen, “Dynamical Threshold of Diluteness of Soft Colloids,” ACS Macro Lett. 3, 1271-1275 (2014).       (http://pubs.acs.org/doi/abs/10.1021/mz500500c)

 

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