Testing the Universality of Entropic Segregation at Polymer Surfaces

Abstract

This study addresses entropic segregation effects at the surfaces of monodisperse and bidisperse melts. For the monodisperse melts, we focus on the segregation of chain ends to the surface, and for the bidisperse melts, we examine the segregation of short polymers to the surface. Universal shapes have been predicted for their concentration profiles, but the derivations rely on the mean-field approximation, which only treats the excluded-volume interactions in an approximate manner. To test whether or not the predictions hold up when the polymers are rigorously prevented from overlapping, we compare mean-field calculations with Monte Carlo simulations performed on the exact same model. Apart from a significant increase in the statistical segment length, the rigorous enforcement of excluded-volume interactions has a relatively small effect on the mean-field predictions. In particular, the universal profiles predicted by mean-field theory are found to be accurate.

Interfacial width and phase equilibrium in polymer-fullerene thin-films

“…However, characterisation and performance measurements of working devices alone are necessary but not sufficient for providing full understanding of these structurally complex blends (made from a wide-range of different material systems). The typically broad mass distributions in conjugated polymers has also resulted in the precise effects of Mw remaining elusive; polydispersity can affect both the thermodynamics 27,28 and the (phase separation) kinetics of mixtures 29 , but the most significant impact may be at interfaces where preferential segregation of lower mass polymer fractions (and hence the accumulation of chain ends) is possible 30,31 .…”

Liquid–liquid equilibrium in polymer–fullerene mixtures; an in situ neutron reflectivity study

“…In the current study the increase in the mobility of the fullerene with decreasing PS Mw occurs as a result of the Mw-dependent mobility of the PS, while in previous work we discovered that the film-thickness dependent mobility of PS has a strong influence on fullerene crystallisation kinetics within PCBM/PS bilayers. 29 Both of these factors have important implications for morphological stability in polymer/small-molecule OPVs under operation, due to the thin-film nature of these devices (and also the nanoscale dimensions of the polymer-rich domains within the active layer 61 ) and the broad Mw distributions in conjugated polymer systems; the Mw-dependent plasticisation of the fullerene reported herein, is of particular significance owing to the potential for preferential segregation of low-Mw polymer fractions to interfaces, 39,40 and the potential evolution of domain composition and morphology 3,6,7,15 as a result. The implications for OPV performance are clear.…”

“…7,36,37 The molecular mobility of the polymer, as a function of Mw, 38 is likely to be of key significance here. This is because of the potential for preferential segregation of low-Mw fractions to interfaces 39,40 within devices, given the typically broad Mw-distributions of conjugated polymers. Measurements using model PS/fullerene systems with narrow Mw-distribution PS allow us to examine this behaviour in well-controlled polymer/small-molecule thin-film systems.…”

Effect of chain stiffness on the entropic segregation of chain ends to the surface of a polymer melt

“…21,22 Excluded-volume interactions will also affect the energy of folding a polymer configuration and could, therefore, contribute to entropic segregation. However, simulations 23 have shown that this effect is relatively minor, The Journal of Chemical Physics ARTICLE scitation.org/journal/jcp presumably because hard-core interactions are, to a good approximation, screened in polymer melts. 24 Because the number of chain ends has to be conserved on the molecular length scale (i.e., two per molecule), the excess at the surface is balanced by a depletion of equivalent magnitude extending into the melt to a distance of order aN 1/2 , the end-to-end length of a polymer.…”

“…26,27 Nevertheless, the segregation has also been detected in polydisperse melts, 32 where it causes a shift in the molecularweight distribution towards smaller N n at the surface relative to the bulk, due to the fact that shorter polymers have more ends per unit volume. 21,23,[28][29][30][31] In this case, the experiments measured the shift in N n using MALDI time-of-flight spectrometry, which does not require any labeling. Although the shift was in qualitative agreement with SCFT, the effect was considerably stronger than predicted.…”