Liquid–liquid equilibrium in polymer–fullerene mixtures; anin situneutron reflectivity study

Abstract

The fullerene diffusion and interfacial broadening onset in bilayers is strongly correlated with the Mw-dependent molecular mobility of the polymer.

Equilibration and thermal reversibility in mixtures of model OPV small-molecules and polymers

“…Our aim was to heat the samples up to a sufficiently high temperature, so that the components became mobile, and the structure of the films evolved towards equilibrium. In samples with pure PS top layers, mixing of fullerenes into these layers occurs rapidly (within a few minutes), 20,21 once you approach the reported glass transition temperature ( T g ) of the fullerene. 31 Using our previous knowledge of PS/bis-PCBM systems, 20,21 we mostly chose top layer blend compositions that were expected to be in the two-phase region of phase space.…”

“…The annealing protocol that was employed was dependent on the category of sample and the batch. Batch 1 samples with pure PS top layers were first heated to 80 1C, at which there is little change in bilayer composition profile (layer thickness, composition or interface/sample-surface roughness) in comparison to room temperature, 20,21 and then measured using NR. They were then heated straight to the maximum temperature used (tens of degrees above the reported T g of the fullerene), and, after a suitable waiting time, a NR measurement was taken.…”

“…These samples have not been annealed using a protocol similar to that set out in Fig. 3a, but have just been heated from 99 1C (a temperature at which there is no discernible mixing of constituents between layers) 21 straight to temperatures of approximately 150-160 1C. Fig.…”

“…34 To probe the behaviour of the system further we also fabricated two bilayer samples using 2k PS (samples H and I, both with pure PS top layers). These were annealed using a similar protocol to the 5k and 300k PS samples, except that the investigation was shifted to slightly lower temperatures (due to evidence that the onset of mixing in these PS/fullerene systems is influenced by the MW-dependent molecular-mobility of the PS 21,43 ). NR data, bilayer fits, SLD profiles and fit parameters for these samples are shown in Fig.…”

“…In recent work, we investigated the capability of FH theory to describe model amorphous-polymer/small-molecule systems, consisting of narrow molecular-weight-distribution polystyrene (PS) and two different fullerenes, phenyl-C60-butyric acid methyl ester (PCBM) and bis-adduct phenyl-C60-butyric acid methyl ester (bis-PCBM). [20][21][22] We found that FH theory can indeed be used to quantitatively predict co-existing phase composition as a function of polymer molecular weight (MW) in these systems. In addition, self-consistent-field-theory (SCFT) was used in a semi-quantitative way to interpret increased interfacial width at low MW in PS/PCBM.…”