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UNIVERSITY OF WATERLOO
Waterloo Institute for Nanotechnology (WIN);
Department of Chemical Engineering; Department of Physics & Astronomy
Prof. Mark W. Matsen
Entropic segregation at polymer surfaces
According to a clever argument by Silberberg, the surface of a polymer melt can be treated as a reflecting boundary. However, there are various effects that are neglected by this simplified boundary condition. For instance, simulations have found an entropic enrichment of chain ends at the surface, whereas a reflecting boundary gives a uniform bulk concentration right up to the surface. Furthermore, experiments have observed a molecular-weight dependence in the surface tension, whereas the Silberberg argument predicts none. Naturally, these effects are due to violations in the assumptions used by Silberberg, namely the presumptions of a step-like concentration profile and the absence of an energy penalty for folding polymer chains.
Our initial work [1,2] examined the segregation resulting from the use of a discrete-chain model. This work showed that the amplitude of the effect was model dependent, but that certain aspects were universal in nature. Nevertheless, the predictions are virtually impossible to verify by experiment. However, there is a measurable effect in polydisperse melts, where the entropic segregation causes a shift in the molecular-weight distribution at the surface [3]. We participated in a collaboration [4], where the measured effect was compared directly to experiment. There was reasonable agreement, but experiments found that the effect was significantly larger than predicted. We are now exploring possible explanations for this.
References
[1] M. W. Matsen and P. Mahmoudi, Segregation of chain ends to the surface of a polymer melt, Eur. Phys. J. E 37, 78 (2014). pdf
[2] P. Mahmoudi and M. W. Matsen, Segregation of chain ends to the surface of a polymer melt: Effect of surface profile versus chain discreteness, Eur. Phys. J. E 39, 78 (2016). pdf
[2] P. Mahmoudi and M. W. Matsen, Entropic segregation of short polymers to the surface of a polydisperse melt, Eur. Phys. J. E 40, 85 (2017). pdf
[4] J.A. Hill, K.J. Endres, P. Mahmoudi, M.W. Matsen, C. Wesdemiotis and M.D. Foster Detection of surface enrichment driven by molecular weight disparity in virtually monodisperse polymers, ACS Macro Lett. 7 , 487 (2018).
Updated: November 2018