PVA Dielectric Relaxations

A plasticizing effect on CS a-relaxation is observed by DS. For moisture contents less than 0.05 wt%, the glass transition is difficult to observe due to the superposition of two relaxation process. 

Impedance spectroscopy is a powerful tool to monitor the plasticizing effect of water on CS glass transition. 

The well-known secondary a-relaxation often associated with proton mobility is also observed in CS (neutralized and nonneutralized) from 80 °C to the onset of degradation. On minimum moisture content conditions, this relaxation process could be noticed in the whole temperature range before the onset of thermal degradation. It is strongly affected by moisture content for dry samples by water effects, the activation energy shifts to lower values when compared to dry annealed samples. The nonneutralized CS showed an easier mobility in this ion motion process. This relaxation process exhibits a normal Arrhenius-type temperature dependence with activation energy of 80-90 kJ/mol. 

Finally, the high frequency secondary -relaxation is also observed with Arrhenius activation energy of 

In the above context, Bhargav et al. [89] reported a single Arrhenius behavior for the dependence of conductivity in the 30-100°C temperature range for pure PVA, but their activation energy is almost two times higher than that of Hanafy [30]. On the other hand, Agrawal et al. [90] also suggested a VFT behavior of conductivity in pure PVA and PVA with ammonium salt films, but no further discussion is provided on the nature of the relaxations. 

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The first mention of the a(x) dependence was in experimental work [265], The dielectric relaxation data of water in mixtures of seven water-soluble polymers was presented there. It was found that in all these solutions, relaxation of water obeys the CC law, while the bulk water exhibits the well-known Debye-like pattern [270,271], Another observation was that a is dependent not only on the concentration of solute but also on the hydrophilic (or hydrophobic) properties of the polymer. The seven polymers were poly(vinylpyrrolidone) (PVP weight average molecular weight (MW) = 10,000), poly (ethylene glycol) (PEG MW = 8000), poly(ethylene imine) (PEI MW = 500,000), poly(acrylic acid) (PAA MW = 5000), poly(vinyl methyl ether) (PVME MW = 90,000), poly(allylamine) (PA1A MW = 10,000), and poly(vinyl alcohol) (PVA MW = 77,000). These polymers were mixed with different ratios (up to 50% of polymer in solution) to water and measured at a constant room temperature (25°C) [265]. 

Figure 2.2 Relaxation time (r) versus lOOO/r for PVA disclosing the a-relaxation obtained by dielectric spectroscopy. The solid line is the VFTH fitting of the dielectric data. Source Reproduced with permission from Gonzalez-Campos JB, Garcia-Carvajal ZY, Prokhorov E, Luna-Barcenas JG, Mendoza-Duarte ME, Lara-Romero J, Del Rio RE, Sanchez IC. J Appl Polym Sci 2012 125 4082 [7]. Copyright 2012 John Wiley and Sons, Inc.

The two groups of relaxatory modes which lead in mechanical relaxation experiments to the a-transition and the final viscous flow also emerge in the dielectric response. Figure 5.17 presents, as a first example, the frequency dependencies of the real and imaginary part of the dielectric constant, obtained for poly(vinylacetate) (PVA) at the indicated temperatures. One observes a strong relaxation process. 

Fig. 5.23. Temperature dependence of the relaxation strength Aca of the dielectric a-process in PVA. Data from Ishida et al.[56]...

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