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Title: Vertical Phase Segregation Induced by Dipolar Interactions in Planar Polymer Brushes

Abstract

In this paper, we present a generalized theory for studying structural properties of a planar dipolar polymer brush immersed in a polar solvent. We show that an explicit treatment of the dipolar interactions yields a macroscopic concentration dependent effective “chi” (the Flory–Huggins-like interaction) parameter. Furthermore, it is shown that the concentration dependent chi parameter promotes phase segregation in polymer solutions and brushes so that the polymer-poor phase consists of a finite/nonzero polymer concentration. Such a destabilization of the homogeneous phase by the dipolar interactions appears as vertical phase segregation in a planar polymer brush. In a vertically phase segregated polymer brush, the polymer-rich phase near the grafting surface coexists with the polymer-poor phase at the other end. Predictions of the theory are directly compared with prior reported experimental results for dipolar polymers in polar solvents. Excellent agreements with the experimental results are found, hinting that the dipolar interactions play a significant role in vertical phase segregation of planar polymer brushes. We also compare our field theoretical approach with the two-state and other models invoking ad hoc concentration dependence of the chi parameter. Interplay between the short-ranged excluded volume interactions and long-ranged dipolar interactions is shown to play an important rolemore » in affecting the vertical phase separation. Finally, effects of mismatch between the dipole moments of the polymer segments and the solvent molecules are investigated in detail.« less

Authors:
 [1];  [1];  [1]
  1. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States). Computer Science and Mathematics Division. Center for Nanophase Materials Sciences
Publication Date:
Research Org.:
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States). Center for Nanophase Materials Sciences (CNMS). Oak Ridge Leadership Computing Facility (OLCF)
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
1327656
Grant/Contract Number:
AC05-00OR22725
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Macromolecules
Additional Journal Information:
Journal Volume: 49; Journal Issue: 18; Journal ID: ISSN 0024-9297
Publisher:
American Chemical Society
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE

Citation Formats

Mahalik, Jyoti P., Sumpter, Bobby G., and Kumar, Rajeev. Vertical Phase Segregation Induced by Dipolar Interactions in Planar Polymer Brushes. United States: N. p., 2016. Web. doi:10.1021/acs.macromol.6b01138.
Mahalik, Jyoti P., Sumpter, Bobby G., & Kumar, Rajeev. Vertical Phase Segregation Induced by Dipolar Interactions in Planar Polymer Brushes. United States. doi:10.1021/acs.macromol.6b01138.
Mahalik, Jyoti P., Sumpter, Bobby G., and Kumar, Rajeev. 2016. "Vertical Phase Segregation Induced by Dipolar Interactions in Planar Polymer Brushes". United States. doi:10.1021/acs.macromol.6b01138. https://www.osti.gov/servlets/purl/1327656.
@article{osti_1327656,
title = {Vertical Phase Segregation Induced by Dipolar Interactions in Planar Polymer Brushes},
author = {Mahalik, Jyoti P. and Sumpter, Bobby G. and Kumar, Rajeev},
abstractNote = {In this paper, we present a generalized theory for studying structural properties of a planar dipolar polymer brush immersed in a polar solvent. We show that an explicit treatment of the dipolar interactions yields a macroscopic concentration dependent effective “chi” (the Flory–Huggins-like interaction) parameter. Furthermore, it is shown that the concentration dependent chi parameter promotes phase segregation in polymer solutions and brushes so that the polymer-poor phase consists of a finite/nonzero polymer concentration. Such a destabilization of the homogeneous phase by the dipolar interactions appears as vertical phase segregation in a planar polymer brush. In a vertically phase segregated polymer brush, the polymer-rich phase near the grafting surface coexists with the polymer-poor phase at the other end. Predictions of the theory are directly compared with prior reported experimental results for dipolar polymers in polar solvents. Excellent agreements with the experimental results are found, hinting that the dipolar interactions play a significant role in vertical phase segregation of planar polymer brushes. We also compare our field theoretical approach with the two-state and other models invoking ad hoc concentration dependence of the chi parameter. Interplay between the short-ranged excluded volume interactions and long-ranged dipolar interactions is shown to play an important role in affecting the vertical phase separation. Finally, effects of mismatch between the dipole moments of the polymer segments and the solvent molecules are investigated in detail.},
doi = {10.1021/acs.macromol.6b01138},
journal = {Macromolecules},
number = 18,
volume = 49,
place = {United States},
year = 2016,
month = 9
}

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  • We present a generalized theory for studying phase separation in polymer blends containing dipoles on their backbone. The theory is used to construct co-existence curves and study the effects of dipolar interactions on interfacial tension for a planar interface between the coexisting phases. It is shown that mismatch in monomeric dipole moments leads to destabilization of homogeneous phase. Corrections to the Flory-Huggins phase diagram are predicted using the theory. Furthermore, it is found that interfacial tension increases with an increase in the mismatch. Density profiles and interfacial tensions are constructed for diffuse and sharp polymer-polymer interfaces by extending Cahn-Hilliard andmore » Helfand-Tagami-Sapse s treatment, respectively. Correlating dipole moments with the dielectric constant of pure phases, it is demonstrated that effects of mismatch between the dipole moments of the two monomers is equivalent to the dielectric mismatch between the polymers.« less
  • In this paper, we use a field theory approach to study the effects of permanent dipoles on interpenetration and free energy changes as a function of distance between two identical planar polymer brushes. Melts (i.e., solvent-free) and solvated brushes made up of polymers grafted on nonadsorbing substrates are studied. In particular, the weak coupling limit of the dipolar interactions is considered, which leads to concentration-dependent pairwise interactions, and the effects of orientational order are neglected. It is predicted that a gradual increase in the dipole moment of the polymer segments can lead to attractive interactions between the brushes at intermediatemore » separation distances. Finally, because classical theory of polymer brushes based on the strong stretching limit (SSL) and the standard self-consistent field theory (SCFT) simulations using the Flory’s χ parameter always predicts repulsive interactions at all separations, our work highlights the importance of dipolar interactions in tailoring and accurately predicting forces between polar polymeric interfaces in contact with each other.« less
  • Brief 160 °C annealing treatments dramatically enhanced the performance of bulk heterojunction inverted polymer solar cells with an ITO/ZnO/P3HT:PCBM/MoO₃/Ag structure. The influence of such treatments on cell performance has been correlated to vertical phase segregation and crystallization within the photoactive layer of such cells. The photoactive layer, comprised of a mixture of P3HT and PCBM deposited on ZnO, was annealed for 10–30 min at 160 °C. Depth profiling with X-ray photoelectron spectroscopy (XPS) revealed that such annealing resulted in enrichment of the P3HT concentration near the ZnO layer, particularly after 20 and 30 min of annealing. Crystallization of P3HT wasmore » detected by X-ray diffraction (XRD) analyses after 10 to 30 min of such annealing, with little difference in the extent of crystallization detected over this time frame. It was found that vertical segregation does not seem to play a role as significant as that of crystallization on cell performance.« less
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