Characterization of chain alignment at buried interfaces using Mueller matrix spectroscopy
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Research Letter
Characterization of chain alignment at buried interfaces using Mueller matrix spectroscopy Bryan H. Smith, Renxuan Xie, and Wonho Lee, Department of Chemical Engineering, The Pennsylvania State University, University Park, PA 16802, USA Dipendra Adhikari and Nikolas J. Podraza, Department of Physics and Astronomy, The University of Toledo, Toledo, OH 43606, USA; Wright Center for Photovoltaics Innovation and Commercialization, The University of Toledo, Toledo, OH 43606, USA Enrique D. Gomez, Department of Chemical Engineering, The Pennsylvania State University, University Park, PA 16802, USA; Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802, USA; Materials Research Institute, The Pennsylvania State University, University Park, PA 16802, USA Address all correspondence to Enrique D. Gomez at [email protected] (Received 25 September 2019; accepted 16 March 2020)
Abstract The stiffness of conjugated polymers should lead to chain alignment near buried interfaces, even if the polymer film is nominally amorphous. Although simulations predict that this alignment layer is approximately 1.5 times the persistence length, chain alignment at buried interfaces of amorphous polymers has not been experimentally measured. Using Mueller matrix spectroscopy, the optical response of regiorandom poly (3-hexylthiophene-2,5-diyl) (P3HT) was modeled in order to extract the aligned layer thickness. By approximating the optical properties of the aligned layer as that of regioregular P3HT, the data can be effectively modeled. When the film is thicker than 150 nm, optical properties are best described with a 4-nm aligned layer, which is quantitatively consistent with previous predictions.
Chain alignment near buried interfaces may improve adhesion,[1] affect the transport of small molecules in membranes,[2] and enhance charge mobilities in organic thin-film transistors.[3,4] Polymers with stiff conformations are predicted to locally align near impenetrable surfaces, such that chain backbones are parallel to the interface.[3,5–7] Simulations of beadspring semiflexible chains predict that the thickness of the backbone alignment layer is approximately 1.5 times the persistence length (lp), even when the polymer is isotropic in the bulk.[3,8] This uniaxial nematic order (alignment in one axis) near interfaces can also promote the spontaneous formation of biaxial alignment where nematic coupling leads to domains with locally aligned chains.[3,9] The presence of order at buried interfaces has been detected in semicrystalline polymers using sum-frequency generated spectroscopy[10] and grazing incidence X-ray diffraction (GIXRD).[4] Near-edge X-ray absorption spectroscopy, which is sensitive to molecular alignment at surfaces, has demonstrated alignment at air interfaces or buried interfaces when this surface is exposed through delamination.[11–14] Alignment in films of molecular glasses, where the molecule has a rod-like or disk-like shape, has also been demonstrated through GIXRD
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