Biomedical Engineering Reference
In-Depth Information
(a)
(b)
0.05
0.04
0.03
0.02
0.05
0.04
0.03
0.02
Dry
100
o
C
Dry
RT
Wet
Wet
0.01
0.00
0.01
0.00
0.0
0.1
0.2
q
(1/nm)
0.3
0.4
0.5
0.0
0.1
0.2
q
(1/nm)
0.3
0.4
0.5
(d)
(c)
0.12
0.10
0.08
0.06
D
~ 25 nm
RT
Dry
Wet
0.04
0.02
0.00
5
10
15
20
25
q
(1/nm)
2.15
X-ray scattering patterns in PLLA dry and wet samples. (a) SAXS
at room temperature; (b) SAXS pattern at 100°C; (c) a model for the
lamellar structure and the diffusion of water; (d) WAXS pattern.
chains. These are called lamellae even in instances where these crystals are
about the same size in all directions (Plate Ic). These lamellae also give rise
to meridional peaks, peaks closer to the draw direction (Plate Ib), because
of the way lamellae are arranged in the polymer, also shown in the fi gure.
The changes in these lamellar refl ections, the spacing, shape and the size of
these refl ections, with heat, deformation and solvent-induced swelling, can
be analyzed in detail to understand the effect of processing on polymer.
Figure 2.15a and 2.15b show SAXS peak from PLLA. This peak is
the signature of the formation of folded chain lamellae that are orga-
nized in the form of lamellar stacks. The position of the peak shows that
the lamellae are separated by ~25 nm within the stack (Fig. 2.15c). The
spaces between the lamellae and between the stacks are fi lled amorphous
segments. Figure 2.15a shows that at room temperature, the water mol-
ecules swell the interlamellar space from ~24 to 28 nm without changing
the crystalline nature as can be seen in the WAXS pattern in Fig. 2.15d.
This behavior is common to semicrystalline polymers.
46,47
At 100°C (
T
m
of PLLA = 173-178°C), there was still no change in the WAXS pattern,
while the SAXS pattern (Fig. 2.15b) shows a large increase in the order-
ing of the lamellar stacks; the water absorption increases the diffuse scat-
tering but does not affect the ordered structure (the peak) due to the
lamellar stacks.
