The gel melting temperatures of poly(vinylidene fluoride)
(PVF2) gels in acetophenone (ACTP), ethyl
benzoate (EB), and glyceryl tributyrate (GTB) are measured by
differential scanning calorimetry (DSC)
at high PVF2 concentration
(W
PVF
2
≥ 0.1) and
visually for very low PVF2 concentration
(WPVF
2
≤ 0.1).
The
gelation temperatures are also measured in DSC by dynamic cooling
method. The enthalpy of gelation
and enthalpy of gel fusion vs weight fraction of PVF2
(W
PVF
2
) plots exhibit
different nature in the three
solvents. It is linear for the PVF2/ACTP gels but the
PVF2 gels in the other two solvents exhibit
positive
deviation from linearity. A thermodynamic analysis of the enthalpy
values in PVF2/EB and PVF2/GTB
systems indicates the polymer−solvent complex formation in both the
systems. The stoichiometry of the
PVF2−EB compound is 1:1 molar ratio and that of
PVF2−GTB compound is 3:1 molar ratio with
respect
to PVF2 monomeric unit and the solvent molecule,
respectively. The phase diagram of the
PVF2/ACTP
system is linear, that of PVF2/EB system exhibits compound
formation with incongruent melting point,
and that of PVF2/GTB system exhibits compound formation
with singular point. The spheroidal morphology
of the PVF2/ACTP gels has been attributed to the chain
folding process, whereas the fibrillar morphology
of PVF2/GTB gels is due to polymer−solvent complex
formation. The reason for the mixed morphology
of PVF2/EB gels is not clear and is probably due to the
comparable rates of the polymer−solvent compound
formation and the chain-folding processes.
The gelation rate of poly(vinylidene fluoride) (PVF2)/glyceryl tributyrate (GTB) system has been measured. It has been analysed with the help of an equation which contains φn and f(T) term where φ is a reduced overlapping concentration and n is analogous to the percolation exponent β in a three‐dimensional lattice. f(T) is related to the temperature function of the coil‐to ‐helix transition. Analysis of the gelation rates supports that the three‐dimensional percolation is a suitable mechanism in this gelation process and it also indicates that the gelation is caused by coil‐to‐helix transition followed by their association.
SUMMARY: The morphology of thermoreversible gels of three commercial poly(vinylidene fluoride) (PVF 2 ) samples in the three solvents acetophenone (ACTP), ethyl benzoate (EB), and glyceryl tributyrate (GTB) are studied by means of scanning electron microscopy (SEM). The PVF 2 /ACTP gels have a spheroidal morphology, the PVF 2 /GTB gels have a fibrillar morphology, and the PVF 2 /EB gels have a mixed morphology containing both the fibrillar and the spheroidal texture. The fibrillar and spheroidal radii increase with increasing gelation temperature and polymer concentration. At identical gelation condition the radii decrease with increasing head to head (H1H) defect structure in the polymer chain and also with increasing molecular weight. These results are explained with the nucleation and growth mechanism of polymer crystals in solution. It is concluded from this study that though a polymer/solvent compound formation is responsible for the fibrillar morphology it does not affect the nucleation and growth process controlling the radii of spheroids and fibrils.
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