Imaging 3D molecular orientation by orthogonal-pair polarization IR microscopy

Abstract

Anisotropic molecular alignment occurs ubiquitously and often heterogeneously in three dimensions (3D). However, conventional imaging approaches based on polarization can map only molecular orientation projected onto the 2D polarization plane. Here, an algorithm converts conventional polarization-controlled infrared (IR) hyperspectral data into images of the 3D angles of molecular orientations. The polarization-analysis algorithm processes a pair of orthogonal IR transition-dipole modes concurrently; in contrast, conventional approaches consider individual IR modes separately. The orthogonal-pair polarization IR (OPPIR) method, introduced theoretically but never demonstrated experimentally, was used to map the 3D orientation angles and the order parameter of the local orientational distribution of polymer chains in a poly(ε-caprolactone) film. The OPPIR results show that polymer chains in the semicrystalline film are aligned azimuthally perpendicular to the radial direction of a spherulite and axially tilted from the film normal direction. This newly available information on the local alignments in continuously distributed molecules helps to understand the molecular-level structure of highly anisotropic and spatially heterogeneous materials.

Three-Dimensional Molecular Orientation Imaging of a Semicrystalline Polymer Film under Shear Deformation

“…Multiple possible origins of the failing OPPIR calculation were discussed in the earlier paper. 28 Among them, non-uniaxial orientation distributions showed consistent results with simulations performed with two coexisting orientation distributions of nonparallel azimuthal angles. In contrast to the domain-structured Q region, there are no pixels with unreal values of θ and ⟨P 2 ⟩ in the S region, which suggests that the shearing deformation makes the molecular orientation more uniform macroscopically.…”

“…27 For the orthogonal two-chain geometry discussed above, α(η) of both primary and secondary modes can be expressed with the shared parameters of the molecular symmetry axis (ψ, θ, and ⟨P 2 ⟩). 27,28 { }…”

“…First, the 3D orientation angles and the order parameter of the Q region in Figure 3a,d,g are reproduced from the earlier results. 28 Briefly, the ψ image of Figure 3a shows that PCL chain projections to the polarization plane are symmetrically aligned tangential to circumferences from the nucleation center of each spherulite. This result of the tangential alignment of the azimuthal angles is similar to the earlier report on 2Dprojected polarization IR images of nonbanded PCL spherulites.…”

“…The polarization-controlled hyperspectral IR microscopy system was described elsewhere. 28 (See Figure 1a for the schematic illustration.) Briefly, the PCL film was imaged with a wide-field QCL-IR microscope (Spero, Daylight Solutions, Inc., San Diego, USA).…”

“…In the past decades, quantum cascade lasers (QCL) have been developed to provide a high-intensity monochromatic mid- and far-IR light and used for high-throughput polarization IR imaging. , Recently, an algorithm was theoretically proposed to determine the 3D orientation angle by concurrently analyzing the 2D-projected polarization anisotropy of a pair of orthogonal IR transition dipoles . Later, the orthogonal-pair polarization IR (OPPIR) method was experimentally demonstrated for imaging the 3D molecular orientations in spherulites of a poly­(ε-caprolactone) film. In this paper, we advanced the OPPIR imaging method to study the structural change occurring upon mechanical deformation of the semicrystalline polymer film.…”