Gelation through phase transformation in synthetic and natural polymers - Physical Gels from Biological and Synthetic Polymers - page 229

Sample 1
Sample 2

30

20

10

0

90 120 150 180

T (°C)

Percentage of rigid PVC determined by 13 C NMR for PVC

DOP gels versus temperature.
Sample 1: M w =60×10 3 g mol − 1 ; sample 2: M w =85×10 3 g mol − 1 . Adapted with permission from
Barendswaard et al.( 1999 ) © 1999 American Chemical Society.

- Figure 8.5

behaviour expected for , since these should not be distributed over

such a large range of temperatures. The shape of the curve (linear decrease) does not
bear the features normally associated with cooperative phenomena, for instance a
sigmoidal or a step-like pro

' small crystallites '

le which would correspond better to melting of crystallites

of a well-de ned average size.

8.2.1 Rheological measurements

A series of papers has dealt with the rheological properties of gels using various
dissolution protocols (te Nijenhuis and Winter, 1989 ; Aoki, 2001 ; Kakiuchi et al.,
2001 ; Watanabe et al., 2001 ). te Nijenhuis and Winter ( 1989 ) dissolved PVC directly
into bis(2-ethylhexyl) phthalate (DOP) and studied gelation by cooling solutions at
80°C and observing ageing effects at various frequencies (for concentrations around
10% w/w). In the other papers, the authors started the PVC dissolution at room
temperature (about 30°C) in a good solvent, 70

90% tetrahydrofuran (THF), which
contained varying amounts of DOP. The evaporation of THF from each solution
proceeded at room temperature and lasted from 1 to 2 weeks depending on the polymer
concentration. After the THF had completely evaporated, transparent liquids or solid

-

DOP were obtained. After about 2 weeks, the weight of each sample
became constant. Reproducibility of rheological measurements was considered to be
an indication of a stable equilibrium state. No traces of THF were left in the solutions.
Li and co-workers (Li and Aoki, 1997 , 1998 ;Liet al., 1997 ) studied stabilized
solutions heated from room temperature to 40°C and measured mechanical spectra
versus frequency for various concentrations and molecular masses of PVC. They applied
the Winter

films of just PVC -

sol transition corresponding to
concentrations at 40°C where the softening transition takes place. At this temperature
they determined power laws with respect to concentration, around the critical PVC
concentration. Unfortunately, these investigations cannot supply any information about

- Chambon criteria ( Chapter 3 ) to identify a gel -