Gelation through phase transformation in synthetic and natural polymers - Physical Gels from Biological and Synthetic Polymers - page 227

10 4

0.7

0.6

0.5

10 3

0.4

0.3

10 2

0.2

0.1

0

10 1

20 40 60 80 100 120 140

Temperature ( ° C)

Storage modulus and tan δ versus temperature for aged ( ● ) and fresh ( ○ ) gels of PVC - DBP, c =20
wt%, with a heating rate of 1°C min ‒ 1 . Reproduced from Cho and Park ( 2001 ) with permission
from John Wiley & Sons.

Figure 8.2

endo

(a)

(b)

40 60 80 100 120

Temperature ( ° C)

140 160 180

DSC thermograms of aged gels at c = 20 wt% for (a) PVC BB, with a heating rate

of 20°C min ‒ 1 . Reproduced from Cho and Park ( 2001 ) with permission from John Wiley & Sons.

- DBP and (b) PVC -

Figure 8.3

different contribution from crystalline junctions in this range. At the end of the ramp,
tan

= 0.5, so the system still has an important elastic component. Examining EM images
of the dried gels, for gels prepared at room temperature an important structuring of the
solution appears in which the

δ

fibrillar structure extends throughout the whole volume, as

discussed before.
Electron micrographs were prepared by critical-point drying techniques ( Figure 8.4 )
using CO 2 , which, according to the authors, allows drying of the gels without distortion
or alteration of their structure. Electron micrographs show that the gels

' morphology

consists of a three-dimensional fibrous network in DBP or BB solvent. The mesh size is

about 200 50 nm. The gel morphology deduced

from the EM images is similar for both solvent types and with ageing; thus fresh and aged
gels have similar structures. Here BB is considered as a good solvent for PVC.
It is not clear which mechanism of aggregation

- 300 nm and fibres have diameters of 30 -

-

crystal or solvent complex forma-

-

tion can be associated with this particular structure. It could also indicate an incipient

phase separation after rapid quenching of the solution from high temperature (160°C) to