Wavenumber Dependence of FT-IR Image of Molecular Orientation in Banded Spherulites of Poly(3-hydroxybutyrate) and Poly(l-lactic acid)

Abstract

The chemical and the molecular chain orientation image of a banded spherulite of crystalline polymers, poly(l-lactic acid) (PLLA) and poly(3-hydroxybutyrate) (PHB), are visualized using FT-IR imaging with a newly proposed multipolarization calculation method. The vector representation of FT-IR image at different absorption spectral peaks visualizes the magnitude and direction of molecular chain orientation directly in comparison with the retardation and the slow-axis azimuthal images obtained by birefringence imaging. We found for the first time that the wavenumber dependence of in-plane orientation function in the band structure of a PHB spherulite, which is known as a double-band structure originated from a biaxial refractive index ellipsoid. The 15 absorption peaks in the IR spectrum of PHB can be classified into three groups by a different appearance of a band structure in view of the magnitude of molecular chain orientation. The azimuthal image of birefringence also shows the double-band structure; however, the FT-IR azimuthal images of orientation axis indicate the completely different distribution in which there is no band structure at any IR absorption peaks. By contrast, a good correspondence between the azimuthal image of the slow axis of birefringence and the orientation axis of FT-IR is observed in the spherulite of PLLA, which has a uniaxial refractive index ellipsoid. The advanced FT-IR imaging method clarifies the wavenumber-dependent two-types single-band structures of PHB, indicating the local molecular chain orientation along the crystallographic axes in a unit cell.

Macromolecular Orientation in Biological Tissues Using a Four-Polarization Method in FT-IR Imaging

“… 29 Depending on the mechanism of optical interaction, different measurement techniques can determine a different number of Lagrange polynomial coefficients. 29 FT-IR can provide a second-order coefficient, 29 which is also called Herman’s orientation function or in-plane orientation function, calculated as 11 , 13 where D is the dichroic ratio and α is the angle between the transition dipole moment (of a specific absorption band) and the main molecular chain axis. The α angle needs to be defined individually for each absorption band; nonetheless, an approximation can be applied.…”

“… 6 − 8 A more quantitative approach in the form of Herman’s function with a two-polarization approach was used to determine the level of molecular ordering. 5 , 6 , 9 , 10 Later, a four-polarization method was proposed, 11 − 13 which gave a great tool for further polymer analysis, 14 , 15 especially when combined with imaging modalities. 12 , 13 , 16 A variety of other techniques may be used to determine molecular orientation: nuclear magnetic resonance, X-ray diffraction, polarized visible microscopy, fluorescence microscopy, second-harmonic generation (SHG), or Raman spectroscopy (RS).…”

“… 5 , 6 , 9 , 10 Later, a four-polarization method was proposed, 11 − 13 which gave a great tool for further polymer analysis, 14 , 15 especially when combined with imaging modalities. 12 , 13 , 16 A variety of other techniques may be used to determine molecular orientation: nuclear magnetic resonance, X-ray diffraction, polarized visible microscopy, fluorescence microscopy, second-harmonic generation (SHG), or Raman spectroscopy (RS). 17 However, each has specific requirements or drawbacks, such as a specific sample form (liquid or crystallized), lack of biochemical information, a complex instrumentation setup, or the need for labeling.…”

Organization of Twisting Lamellar Crystals in Birefringent Banded Polymer Spherulites: A Mini-Review

“…Micro-focus X-ray diffractograms of polyethylene [28], PLLA [29], PHB [30], poly(ε-caprolactone) (PCL)/poly(vinyl butyral) (PVB) blend [31], PHBV blending with amorphous poly(vinyl acetate) (PVAc) [32], and poly(trimethylene terephthalate) (PTT) [33,34] provide additionally direct and firm support that the lamellar crystals twist in the banded spherulites. Micro-FTIR imaging [35][36][37][38], nanoindentation [39], and AFM force-volume images [40] can all demonstrate the different orientations of lamellar crystals in the banded spherulites.…”

Microscale spectroscopic thermal imaging of n-alkanes

“…The observation of spectrum distribution in IR range contributes much to the investigation of chemical distribution in the composite material or chemical reaction system [1][2][3][4][5]. There are mainly two ways to measure spectrum distribution.…”