Page 35 - Engineered Interfaces in Fiber Reinforced Composites

18 Engineered interfaces in fiber reinforced composites

have presented excellent reviews of most of these techniques, with Ishida (1994)
being particulalry informative for characterization of composite materials.
In addition to surface analytical techniques, microscopy, such as scanning
electron microscopy (SEM), transmission electron microscopy (TEM), scanning
tunneling microscopy (STM) and atomic force microscopy (AFM), also provide
invaluable information regarding the surface morphology, physico-chemical inter-
action at the fiber-matrix interface region, surface depth profile and concentration
of elements. It is beyond the scope of this book to present details of all these
microscopic techniques.

2.3.2. Infrared and Fourier transform infrared spectroscopy

IR spectroscopy, one of the few surface analytical techniques not requiring a
vacuum, provides a large amount of molecular information. The absorption versus
frequency characteristics are obtained when a beam of IR radiation is transmitted
through a specimen. IR is absorbed when a dipole vibrates naturally at the same
frequency as the absorber, and the pattern of vibration is unique for a given
molecule. Therefore, the components or groups of atoms that are absorbed into the
IR at specific frequencies can be determined, allowing identification of the molecular
structure.
The FTIR technique uses a moving mirror in an interferometer to produce an
optical transformation of the IR signal as shown in Fig. 2.6. During this operation,
the source radiation is split into two: one half is reflected into the fixed mirror and
the other half transmitted to the moving mirror. If the mirrors are placed equidistant
from the beam splitter, their beams will be in phase and reinforce each other. In
contrast, the beams that are out of phase interfere destructively. An interferogram is
produced from the equations involving the wavelength of the radiation, and a
Fourier analysis is conducted to determine the relation between the intensity and
frequency. FTIR can be used to analyze gases, liquids and solids with minimal
-
preparation and little time. This technique has been extensively applied to the study

,, \e
Fixed
mirror

Unmodulated
Movable incident Source
mirror-

Splitter

1 Detector
Fig. 2.6. Schematic diagram of an interferometry used in the FTIR spectroscopy. After Lee (1989).