Biomedical Engineering Reference
In-Depth Information
crystalline and amorphous, although there can be structures of vary-
ing degrees of order from completely amorphous to perfectly crystalline
states. In oriented polymers, an oriented or rigid amorphous phase is often
regarded as a third phase.
12-17
The fractions of these phases infl uence the
properties of these materials. Crystallinity in a polymer makes the polymer
stronger, less permeable and in general resistant to solvents. Crystallinity
also increases the dimensional stability of the polymer with respect to tem-
perature and solvent exposure. The orientation of the amorphous phase
affects these same properties as well. Thus, a determination of various
phases is important. X-ray diffraction (WAXS and SAXS) is used to unam-
biguously determine the phase composition, characterize the nature of
these phases and study the nature of the transitions between the phases.
The fraction of the chains that form crystals is typically expressed in terms
of degree of crystallinity. In general, XRD scans are used to determine the
weight fraction of the various phases in the material by comparing the area
under the diffraction peaks of the respective phases. In a two-phase sys-
tem with crystalline and amorphous segments, the total crystallinity can be
determined by comparing the area under the crystalline peaks to the total
scattering area. Typically, scan over a limited angular range (2
θ
= 10−50°,
where 2
θ
is the scattering angle) is used and the resulting value is called
the crystalline index (CI) rather than crystallinity. A diffractometer scan is
resolved into amorphous and crystalline peaks (Fig. 2.8). The areas of these
peaks,
A
a
and
A
c
of the amorphous and the crystalline peaks, respectively,
are used to calculate the CI using
A
CI
100
∗
[2.13]
a
=
(
)
+
Selection of the crystalline peaks (especially when they are broad and
weak), choice of a proper baseline, and the shape of the amorphous halo
are the three factors that affect the accuracy and reliability of the results,
and need to be carefully considered. The most versatile method is to obtain
an amorphous template from fully amorphous polymer or to extract it from
a well crystallized specimen.
18-20
Figure 2.8 shows two examples of the determination of crystallinity in
polymers using WAXS. Figure 2.8a demonstrates the determination of
the fraction and the crystallinities of two separate polymers polyethylene
(PE) and polypropylene (PP) that are present in a blend.
21
In this fi gure,
the scan is resolved into crystalline and amorphous peaks of PE, and into
crystalline and amorphous peaks of PP. The ratio of the crystalline and
amorphous peaks of each of polymers gives the respective crystallinities
of the two polymers. The ratio of the total area under the PE and PP
peaks gives the relative amounts of PE and PP present in the polymer.
