Ak. Dikshit et Ak. Nandi, Gelation mechanism of thermoreversible gels of poly(vinylidene fluoride) and its blends with poly(methyl acrylate) in diethyl azelate, LANGMUIR, 17(12), 2001, pp. 3607-3615
Poly(vinylidene fluoride) (PVF2) and its blends with poly(methyl acrylate)
(PMA) produce thermoreversible gels in diethyl azelate. SEM studies indicat
e a fibrillar network structure, and WAXS/FTIR studies indicate the presenc
e of alpha -polymorphic crystals of PVF2 in the gels. Some newer X-ray diff
raction peaks are observed in the dried gels than those in the melt-crystal
lized PVF2 samples, and the intensity ratios (I(hkl)degrees /I(Il0)degrees)
Of the diffraction peaks of the dried gels are also different from those o
f the melt-crystallized sample. The gelation rates of these systems are mea
sured by the test tube tilting method. At a particular isothermal temperatu
re the gelation rates of the blends decrease and the critical gelation conc
entrations (C-t=alpha*,) of the blends increase with an increase in PMA con
centration. In terms of PVF2 concentration the C-t=alpha* values are also h
igher for the blends than that of the pure PVF2 at a given gelation tempera
ture. The (t(gel)(-1)) has been analyzed from the equation t(gel)(-1) infin
ity f(C)f(T). At a constant temperature analysis gelation rate (t(gel)-1 of
the concentration function f(C) indicates three-dimensional percolation is
a suitable model for gelation of both PVF2 and its blends, supporting that
blending does not alter the macroscopic mechanism. The microscopic mechani
sm, determined from f(T), is however affected due to blending. The gelation
process is considered as a two-step process: coil --> TGTG conformer --> f
ibrillar crystallization (gelation). The free energy of activation (DeltaF)
of the conformational ordering increases, and the free energy of formation
of the critical size nucleus (DeltaG*) decreases with increasing PMA conce
ntration of the blend. A possible explanation for this difference has been
offered. A comparison of DeltaF and DeltaG*: of the two processes indicates
that transformation of the coil --> TGTG conformer is the rate-determining
step of the gelation process for all the samples. The lower gelation rate
of the blends may be due to its increased DeltaF value.