Page 61 - Handbook of Properties of Textile and Technical Fibres

42 Handbook of Properties of Textile and Technical Fibres

In the case of polyamide, polyester, and acrylic fibers, it is possible to obtain good-
quality ultrathin sections by the use of an ultramicrotome equipped with a diamond
knife. The fibers are embedded in a suitable resin before sectioning and thicknesses
of around 80 nm can be obtained. Better results are obtained if the fiber is coated
with a layer of gold (by a sputtering technique analogous to the one used for SEM
investigation) prior to embedding; in such circumstances good adhesion is achieved
between fiber and resin and sectioning is easier.
Ultramicrotomy is also easy in the case of preoxidized polyacrylonitrile (PAN)
fibers or cellulosic fibers (previously treated by a chemical “fixative” mixture). At
temperatures below the glass transition temperature (T g ), polyolefin fibers and even
amorphous fibers can be sectioned. This can involve cooling the specimens with
liquid nitrogen. In the case of carbon fibers, longitudinal sections are obtained
without too much difficulty. For glass or SiC fibers and sectioning normal to the lon-
gitudinal axis of carbon fibers, ultrathin sectioning is not feasible. In these cases
another thinning technique such as the one developed by Berger and Bunsell
(1993) must be used. In this technique the fibers are stuck with an adhesive onto a
small rigid sheet of metal hollowed at its center. The fibers must be carefully aligned
and be in contact with each other to avoid the thinning of the fibers’ edges. A 3-mm
external diameter copper or molybdenum ring held with tweezers is put on a drop of
epoxy glue and stuck on the fibers. The ring is then separated from the mount by cut-
ting the outside fibers.
In the case of fibers with diameters of less than 50 mm the as-prepared sample can
be directly thinned by argon ion milling. However, for fibers of larger diameters, the
thinning would take too long, would induce thinning artifacts and the copper ring
would be thinned before the fibers. Prior to this ionic thinning, the thickness of the
sample must be reduced down to 50 mm by mechanical grinding. To ensure the cohe-
sion of the material, only the center of the sample is ground down to 20 mm by concave
grinding. The sample is then put in the ion thinning chamber of a “Gatan dual ion mill
þ
600.” Two guns ionize an argon gas and deliver two focused beams of Ar accelerated
by 6 kW with a 1 mA gun current. The beams sputter the center of the sample with an
incident angle of 15 degrees on each side of the disc. This attack angle of 15 degrees
corresponds to a better sputtering rate without ion implantation or surface structuring.
After around 20 h the attack angle is then reduced to 7 degrees for a final period of 1 h
to obtain larger thin regions for observation. To obtain finer results, particularly with
multiphase structures, finer angles of attack can be used; however, the time to achieve
the required thickness increases. In this way, tapered sections of the fibers can be
obtained and the microstructure studied in the thinnest parts.
Selected area electron diffraction (SAED) is possible on ultrathin sections of single
fibers, if necessary, by the use of low-dose techniques (in the case of electron-sensitive
polymeric organic fibers). This technique can be used to determine crystallinity and
crystal orientation.
For the study of polymeric fibers, dark field imaging is an even more useful tech-
nique than SAED. Dark field microscopy is an imaging technique using some partic-
ular spots of the diffraction patterns; in such circumstances, crystalline domains
(crystallites) appear as bright spots on a black or dark background. The amorphous