Biomedical Engineering Reference
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network structure as a result of a larger degree of polymer aggregation, i.e. a heteroge-
neous network structure.
Scherrenberg et al.
1993
) performed SANS experiments on PVC
tetrahydro-
naphthalene-d
12
(TDN) gels, a solvent considered as a plasticizer for PVC. SANS patterns
exhibit a distinct interference maximum but, surprisingly, no dependence of the interfer-
ence maximum on the volume fraction plasticizer or the polymer tacticity was observed.
This, the authors suggested, could indicate an inhomogeneous distribution of plasticizer in
the gel. Under uniaxial deformation, plasticized PVC with a high syndiotactic content
(0.50
-
scattering
pattern which indicates the formation of concentration inhomogeneities as a consequence
of the random distribution of the crystalline tie-points. Nevertheless, WAXS experiments
on the stretched plasticized PVC samples showed no orientation of the crystalline and/or
non-crystalline structure. The authors concluded that there is a
-
0.58) and a high concentration (50% w/w) shows a distinct
'
butter
y
'
'
superstructural order
'
in
plasticized PVC, directly associated with the network-like mechanical properties.
The question of gel morphology and its implications for the solid-like behaviour
remains dif
cult. Electron microscopy images obtained by critical-point drying of PVC
gels by Cho and Park (
2001
) with mono- or di-ester type solvents (dibutyl phthalate
(DBP) or butyl benzoate (BB)) in both cases showed an extended three-dimensional
network of a
-
50 nm), in contrast with the
assumption of point-like junctions inferred from the amorphous X-ray scattering pat-
terns. In order to elucidate the origin of elasticity of PVC gels, it is interesting to compare
DSC and rheological measurements performed on the same system. The work of Cho and
Park allows this comparison.
Figure 8.2
shows G
0
and tan
fibrillar type (
(fibres with diameters 30
versus temperature in a heating ramp, where the sample
was prepared by dissolution at 160°C and was stabilized at room temperature. First, it is
noteworthy that, after a long ageing period of 7 days, gels containing the diester solvent
DBP show a softening transition near 50°C whereas no such transition is observed in
monoester solvents or for fresh diester gels. The DSC traces corresponding to the diester
and monoester gels are shown in
Figure 8.3
: indeed, at the larger heating rate in DSC,
very likely to shift the transition temperatures compared to the low heating rates used in
Figure 8.2
, there is a small endothermic peak in the temperature range 60
δ
70°C.
In the rheological experiments, the modulus decreased at around 50°C but tan
-
was
very small, meaning that G
00
is very low. Assuming that this peak corresponds to the
melting of the polymer
δ
Gel I transition in
Figure 8.1
(with
dimethyl malonate as solvent), in the next range of temperatures, between 60°C and
130°C, the gel elasticity would result only from crystalline junctions and should be stable
until the crystals themselves melt at high temperatures.
Figure 8.2
shows that the
modulus decreases progressively, while the crystals do not melt before 140°C, corre-
sponding to the conditions for fully dissolving PVC. The value of tan
-
solvent complex or Gel II
-
is low and almost
constant, thus both G
0
and G
00
decrease. The softening in this region is much enhanced;
following the
δ
first wave of softening at low temperatures (60°C) (factor of 2), G
0
is
lowered overall by a factor of 200, i.e. more than two orders of magnitude. It is therefore
necessary to try and understand why the modulus changes. If there is no crystal melting
spread over this very large range of temperatures (at least 70°C), it means that G
0
has a
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