Page 54 - Handbook of Properties of Textile and Technical Fibres

Testing and characterization of fibers 35

such as the deposition of spin finish at the final stages of fiber production (hence called
spin finish) or the effect of oxygen plasma treatments to be revealed. The spin finishes
are an added value for the fiber uses and its integration into complex structures are
designed to respond to several purposes, from fiber protection and handling, to the
enhancement of fiber activity in adhesion-related problems. Note that inverse gas chro-
matography is also often used to probe the effect of spin finish on the fiber surface
properties (Nardin et al., 1990; Montes-Moran et al., 2002) by determining the
enthalpy and entropy of sorption, the surface energy, and can give insights on the sur-
face heterogeneities (Nardin and Papirer, 2006).

2.3.4 Raman spectroscopy
Raman spectroscopy yields complementary information to IR spectroscopy. In the
case of Raman spectroscopy, the incident light used is coherent monochromatic, usu-
ally from a laser in the visible range. The interaction of the light with matter is mostly
elastic Rayleigh scattering. This means that it is scattered possessing the same
frequency, n 0 ; however, a very small proportion of the scattered intensity, less than
one-thousandth of the incident light, interacts inelastically with the matter, i.e.,
involving a frequency change. This is the Raman effect. This lightematter interaction
depends on the change in polarizability of the electric dipole of a molecule, or part of a
molecule, and can be expressed as a polarizability tensor (in contrast to the dipole
moment vector obtained by IR). Basically, the Raman spectra contains two symmetric
components on both sides of the incident frequency, n 0 , with Stokes Raman and anti-
Stokes Raman scattering, respectively at n 0 n V and n 0 þ n V , n V being the Raman
scattering. Conventionally, spectra are recorded on the Stokes side where intensities
are higher and independent of temperature and defined with respect to the Raman shift
1
n in wave numbers (cm ):
n 0 n V
n ¼ (2.10)
c c
In Eq. (2.10), n is the wave number shift and c is the speed of light.
Both types of scattering give the same frequency information, and the ratio of the
two types of scattering depends on the temperature of the material. The spectra for both
Stokes and anti-Stokes scattering using laser light polarized parallel and perpendicular
to the polyamide 66 (PA66) fiber axis are shown in Fig. 2.9.
For the study of fibers the exciting incident light is concentrated (using a light
microscope close to confocal conditions) to a spot size of around 1.5 mm for the
micro-Raman measurements in backscattering configuration. The excitation power
2
is kept to a few milliwatts/mm measured on the sample, to avoid inducing any thermal
effects in the fiber structure. As illustrated in Fig. 2.9, depending on the frequencies
investigated in the Raman spectrum, different parts of a macromolecular structure
1
can be investigated. Low wave numbers (w100 cm ) reveal information on macro-
molecular skeletal movements and the amorphous and crystalline domains in the fiber,
1
whereas higher wave numbers (w1600 cm ) can be used to investigate particular
molecular species or bonds.