Nylon 6, segmental mobility

In both nylon 66, studied by Starkweather [25], and high-performance epoxy resin, described by Moy and Karasz [30], the water is believed to hydrogen bond between polar sites in the polymers, acting as crosslinks at low temperatures but plasticizing at elevated temperatures because of the greater thermal mobility of water compared to segments of the polymer chain. 

Figure 2.39 shows the response of the modulus to stress. The nylon fibers are characterized by an initial modulus at elongation —> 0 that is proportional to the ratio at which the fibers were drawn. This initial modulus is related to the glass transition temperature of the amorphous region and consequently to the mobility of the chain segments in these regions. It depends therefore on factors such as temperature and water content, which affect the mobility of the chain segments. 

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). 

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