Page 39 - Engineered Interfaces in Fiber Reinforced Composites

22 Engineered interfaces in jiber reinforced composites

it causes the light of slightly longer or shorter wavelengths to be scattered
inelastically. The inelastic proportion of the photons imparts energy to the
molecules, which are collected for analysis. An interesting feature of the Raman
spectroscopy is that certain functional groups or elements scatter incident radiation
at characteristic frequency shifts. The vibrational frequency of the group or element
is the amount of shift from the exciting radiation. Functional groups with high
polarizability on vibration can be best analyzed with Raman spectroscopy.
Raman and IR spectroscopies are complementary to each other because of their
different selection rules. Raman scattering occurs when the electric field of light
induces a dipole moment by changing the polarizability of the molecules. In Raman
spectroscopy the intensity of a band is linearly related to the concentration of the
species. IR spectroscopy, on the other hand, requires an intrinsic dipole moment to
exist for charge with molecular vibration. The concentration of the absorbing
species is proportional to the logarithm of the ratio of the incident and transmitted
intensities in the latter technique.
As the laser beam can be focused to a small diameter, the Raman technique can
be used to analyze materials as small as one micron in diameter. This technique
has been often used with high performance fibers for composite applications in
recent years. This technique is proven to be a powerful tool to probe the
deformation behavior of high molecular polymer fibers (e.g. aramid and
polyphenylene benzobisthiazole (PBT) fibers) at the molecular level (Robinson
et al., 1986; Day et al., 1987). This work stems from the principle established
earlier by Tuinstra and Koenig (1970) that the peak frequencies of the Raman-
active bands of certain fibers are sensitive to the level of applied stress or strain.
The rate of frequency shift is found to be proportional to the fiber modulus, which
is a direct reflection of the high degree of stress experienced by the longitudinally
oriented polymer chains in the stiff fibers.
In the case of carbon fibers, two bands are obtained: a strong band at about
1580 cm-' and a weak band at about 1360 cm-', which correspond to the Ezs and
AI, modes of graphite (Tuinstra and Koenig, 1970). The intensity of the Raman-
active band, AI^ mode, increases with decreasing crystalline size (Robinson
et al., 1987), indicating that the strain-induced shifts are due to the deformation
of crystallites close to the surfaces of the fibers. The ratio of the intensities of the two
modes, Z(Alg)/Z(Ezg), has been used to give an indirect measure of the crystalline
size in carbon fibers (Tuinstra and Koenig, 1970). Table 2.5 gives these ratios and
the corresponding average crystal diameter, La, in the graphite plane, as determined
by X-ray techniques. Typical examples of strain dependence of the Raman
frequencies is shown in Fig. 2.8 for two different carbon fibers, and the
corresponding plots of the shifted Raman frequency are plotted as a function of
the applied strain in Fig. 2.9.
Enabled by the high resolution of spectra, which is enhanced by the use of spatial
filter assembly having a small (200 pm) pin hole, the principle of the strain-induced
band shift in Raman spectra has been further extended to the measurement of
residual thermal shrinkage stresses in model composites (Young et al., 1989; Filiou
et al., 1992). The strain mapping technique within the fibers is employed to study the