Examples of DMA Characterization for Thermoplastics

In amorphous polymers the glass transition and the relaxation behavior associated with it are very sensitive to the addition of small amounts of diluents. As the diluent is added, the relaxation is shifted to a lower temperature at constant frequency or higher frequency at constant temperature. The usual explanation is that since the diluent molecules are small and mobile, they act to effectively increase the available free volume for segmental motion and hence speed it up. Similar plasticizing effects on the glass-rubber transition in semicrystalline polymers are observed. One such example is the case of poly(vinyl alcohol), which is water-soluble (Takayanagi 1965). Other examples of semicrystalline polymers where the effects of moisture are observed are aliphatic polyamides such as Nylon 6-6 (Starkweather 1980), Nylon 6-10 (Boyd 1959 Woodward et al. I960), and Nylon 12 (Varlet et al. 1990). 

The sorption of water by nylon has a major effect on the mechanical properties of engineering importance. For example, in molded Nylon 6-6 at room temperature, as the water content is increased from dryness to saturation, the modulus decreases by a factor of 5, the yield stress decreases by more than half, and there are major increases in the tensile elongation and energy to break (Starkweather 1973a,b). These changes are associated with the glass 

The dependence of the relaxation temperatures on the level of absorbed water in Nylon 6-6 is known from dynamic mechanical studies (Starkweather 1980,1973a Prevorsek et al. 1971) as well as dielectric studies (Starkweather and Barkley 1981). The temperature variations with sorbed moisture of the loss modulus peaks for the three relaxations are shown in Fig. 5.29 (Starkweather 1980). The test frequency for the three relaxations varies slightly but is around 1 Hz. The data indicate that the temperature of the a relaxation at a given frequency decreases by about 100 °C (to below ambient) between dryness and saturation. The P relaxation is also shifted to lower temperatures and higher frequencies by absorbed water, while the temperature of the y relaxation is only slightly affected, shifting to somewhat lower temperatures and higher frequencies. 

In addition, it was found that moisture increases the intensity of the p relaxation and reduces its activation energy while decreasing the peak height of the y relaxation with little change in activation energy. The decrease in y intensity has been referred to as an antiplasticization effect, since it is associated with an increase in modulus and decrease in impact strength. This is 

The effect of moisture on the p relaxations in amorphous phenylene polymers is well documented (Allen et al. 1971 Lim et al. 1973 Chung and Sauer 1971). For example, Allen et al. (1971) determined that both the mechanical and dielectric P relaxations in polysulfone, polycarbonate, poly(phenylene oxide), and poly (ether sulfone) were dependent on the water content of the samples as illustrated for polysulfone in Fig. 5.31. In addition, the amount of water absorbed depended on the polarity of the molecule. The results indicate that the absorbed water hydrogen bonds to polar groups along the polymer chain and, as such, takes part in the molecular processes that give rise to the P relaxation. This is consistent with the other examples cited previously. 

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