We investigated sol-gel transition of isotactic polypropylene (i-PP) mixed with a small amount of small-molecule gelling agents, PDTS (1, 3:2, 4-p, p′-ditolyliden sorbitol) and DBS (1, 3:2, 4-dibenzylidene sorbitol). The i-PP has viscosity average molecular weight 1.61×10
5 and M
W/M
n =4.4. The sol-gel transitions of PDTS/i-PP and DBS/i-PP systems were studied as a function of concentration of the gelling agents (0, 0.3, 0.5 and 1% by weight) using linear dynamic mechanical analyses.
The results indicated that (i) the two systems underwent thermoreversible sol-gel transition and that (ii) G′ and G″ obtained at a given ω as a function of temperature during the cooling and heating cycles exhibited remarkable hysteresis and two sol-gel transition temperatures T
fg,H and T
fg,C (T
fg,H>T
fg,C). T
fg,H designates the sol-gel transition obtained during the heating cycle and corresponds to the critical temperature for dissolution of the percolation network formed by physical association of PDTS or DBS in the matrix of PP melts. T
fg,C designates the sol-gel transition obtained during the cooling cycle and corresponds to the critical temperature for formation of the PDTS or DBS percolation network. Thus, in this sense, the sol-gel transition is analogous to the first-order phase transition. The master curves of G′ and G″ were obtained by applying the time-temperature superposition separately for the sol and gel states for 0.5% PDTS/i-PP system. The master curve for the sol state was found to be identical with that of the pure i-PP melt. However, the master curve for the gel state was found to be distinctly different from those of the sol state and of the pure i-PP melt. At low reduced frequencies corresponding to the terminal flow region of the sol and pure i-PP, the gel exhibited a plateau in G′, and a significantly high G″, which implies that the real part of the dynamic viscosity goes to infinity at zero frequency limit. These results imply that the PDTS forms a macroscopic percolation network. The loss tangent for the gel was found to be nearly equal to unity overall ωα
T covered in this experiment, implying that the association and dissociation of the PDTS molecules are in dynamic equilibrium, contributing to the large loss tangent.
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