Figure 2. (a) Tpp and Tss from
IR-DCP and FTIR polarimetry at 0° (top) and 90° (bottom) azimuthal
sample rotation. (b) Corresponding polarized phase-difference spectra
from IR-DCP and FTIR polarimetry.
The band amplitudes in the IR-DCP transmission and phase spectra at 0°
azimuthal angle are slightly larger compared to FTIR. Such differences
could be related to the inhomogeneity of the fiber scaffold and the
different sizes and opening angles of the probe beam in DCP and FTIR.
However, band positions and shapes agree well for both methods. Indeed,
the relative phase spectra exhibit the expected Kramers–Kronig
consistent line shape, thus demonstrating the validity of the method.
The recorded IR-DCP and FTIR polarimetric phases show a constant offset
of approximately 13˚. Its likely origin is the adjustment of incident
power via P1 between the Δpp and Δssmeasurements necessitated by a change in the input polarization. For
future measurements, an invariant P1 is recommended, for example, by
setting P1 to 45˚ and controlling the power through neutral density
filters.
Figure 3 (a) shows images of ss- and sp-polarized transmission and
absolute phase spectra versus sample azimuth in 10° steps. Maximum band
intensities in the Tss images are observed around 90°
azimuth, whereas vanishing band intensities in Tsp are
seen at 0° and 90° azimuth. The average orientation of transition dipole
moments of the related C–O–C vibrational band is therefore aligned
predominantly in the direction of the fibers, in agreement with the
overall predominant alignment of the fibers in the scaffold.