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Item type: Item , Simple and accurate calibration of the Flory-Huggins interaction parameter(American Chemical Society, 2020) Willis, James D.; Beardsley, Tom M.; Matsen, Mark W.This paper improves upon a standard method of determining the Flory-Huggins chi parameter, whereby experimental order-disorder transitions (ODTs) of symmetric diblock polymer melts are fit to the mean-field prediction, (chi N)_ODT=10.495. The improvement is achieved by switching to an accurate prediction of (\chi N)_ODT from Morse and coworkers, supplemented with corrections for the small degrees of polydispersity and compositional asymmetry that inevitably exist in real diblock polymers. The first correction is evaluated by simulating polydisperse diblocks over a wide range of invariant polymerization indices, and the second correction is extracted from analogous simulations for compositionally asymmetric diblocks by Ghasimakbari and Morse. The resulting calibration method is then demonstrated on 19 different chemical pairs, using previously published experimental data. It provides a considerable increase in accuracy, but yet is nearly as simple to apply as the original version.Item type: Item , Surface segregation in athermal polymer blends due to conformational asymmetry(American Chemical Society, 2021) Spencer, Russell K. W.; Matsen, MarkW.Monte Carlo simulations are used to investigate the surfaces of athermal blends of stiff and flexible polymers as the bending modulus of the stiff polymers, kappa, is increased from zero to the point where the bulk undergoes an isotropic-nematic transition. For hard walls characteristic of polymer/solid surfaces, the flexible polymers generally segregate to the surface. However, just prior to the bulk transition, a surface transition occurs where a thin nematic layer rich in stiff polymers forms next to the wall. This appears to be followed by a second surface transition where the degree of order abruptly increases, which in turn causes a rapid thickening of the nematic layer that nucleates the bulk nematic phase. For soft boundaries representative of polymer/air surfaces, a thin layer rich in stiff polymers but without nematic order forms on the outer edge of the surface with a more significant layer rich in flexible polymers beneath. In this case, the surface transitions are lost and the qualitative behavior remains unchanged as kappa increases.Item type: Item , Coexistence of polymeric microemulsion with homopolymer-rich phases(American Chemical Society, 2021) Spencer, Russell K. W.; Matsen, Mark W.Recent field-theoretic simulations of symmetric ternary blends of A- and B-type homopolymers with AB diblock copolymer (Phys. Rev. Lett. 2020, 125, 117801) predicted three-phase coexistence between bicontinuous microemulsion (BmE) and two homopolymer-rich phases. The present study begins by repeating their grand-canonical simulations over longer durations in simulation boxes of different sizes to ensure that the prediction was not an artifact of nonequilibrium effects or finite system sizes. The coexistence is then demonstrated in canonical simulations, where the three phases are explicitly separated by interfaces. From those simulations, we extract the interfacial widths, the interfacial tensions, the domain size in the B$\mu$E phase, and the copolymer concentration in each phase. The latter results are used to improve the accuracy of the previous phase diagram.Item type: Item , Entropic surface segregation from athermal polymer blends of slim and bulky polymers(American Chemical Society, 2022) Matsen, Mark W.Experiments, theory, and simulations have revealed an entropic preference at the surface of athermal blends for the component with the shorter statistical segment length. Here, we examined the resulting surface segregation for blends of slim and bulky polymers of equal molecular volume, using Monte Carlo simulations where the polymers are modeled as chains of impenetrable spheres. Comparing to analogous simulations for blends of stiff and flexible polymers [Spencer and Matsen, Macromolecules 2022, 55, 1120], we find that bulkiness has a considerably stronger effect on the surface segregation than stiffness, consistent with self-consistent field theory (SCFT) predictions for worm-like chains [Matsen, J. Chem. Phys. 2022, 156, 184901]. Furthermore, we demonstrate that, under special conditions, this mismatch in strength can lead to anomalous surface segregation of the component with the longer statistical segment length, as predicted by the SCFT.Item type: Item , Universality of entropic surface segregation from athermal polymer blends due to conformational asymmetry(American Chemical Society, 2022) Spencer, Russell K. W.; Matsen, Mark W.We reexamine off-lattice Monte Carlo simulations for athermal blends of stiff and flexible polymers next to a hard wall. The original simulations [Kumar et al., J. Chem. Phys. 1995, 103, 10332], which were supported by wall-PRISM calculations, predicted a short-range preference for the stiff polymers at the wall, which contradicts experiments as well as alternative calculations. On this basis, Kumar et al. asserted that the true entropic preference in the experiments was masked by enthalpic interactions, and that the other calculations were incorrect because they did not account for local packing effects at the wall. However, with vastly improved equilibration and statistics, we now find that the simulations actually predict a long-range surface preference for the flexible polymers, which resolves a controversy that has existed for over 25 years. Furthermore, we show that the excess of flexible polymer adopts a universal concentration profile, as predicted by mean-field theory [Wu et al., J. Chem. Phys. 1996, 104, 6387].