Biomedical Engineering Reference
In-Depth Information
E
SAXS
ρ
c
, PEO
ρ
c
, PEO + PMMA
b
a
, PEO + PMMA
E
SANS
b
c
, PEO
b
a
, PEO
Distance
2.20
A schematic showing the miscibility of PEO and PMMA in the
amorphous phase, and the exclusion of PMMA from the crystalline
phase and signifi cant part of the amorphous phase.
one can determine the miscibility of two polymers via the interaction
parameter
χ
. This has been done for many polymer systems.
χ
has been
found to be small and concentration dependent. In some pairs of polymers,
χ changes at small concentrations, indicating a transformation from a misci-
ble to an immiscible system.
The use of SANS to study blends will be illustrated with an example of
a blend of a crystallizable polymer PEO (unlabeled) and an amorphous
polymer PMMA (deuterium labeled).
59
It was found that the polymers are
miscible in the amorphous phase, and that PMMA is excluded from the
crystalline phase PEO. It was also observed that as the PEO chains gradu-
ally transition from their ordered arrangement within the crystalline lamel-
lae into the amorphous phase outside the lamellae, PMMA is excluded from
for about 15 Å from the lamellar surface until suffi cient crystalline order of
the PEO segments is dissipated. PMAA then begins to mix with the amor-
phous PEO over a short distance ~ 5 Å before entering into the segregated
PMMA phase. The total width of the amorphous phase of PEO is 20 Å. The
scattering length density profi le (for SANS) and the electron density profi le
(for SAXS) derived from these data are shown in Fig. 2.20.
2.7.3 Diffusion of solvents
SANS techniques discussed thus far require synthesizing a deuterated
version of the polymer. However, in studies dealing with the diffusion of
