Segmental motion, polyurethane

However, an increase in both Tt min and in the temperature at which it occurs, observed in crosslinked polyurethanes with increasing crosslink density, does not links which provide more space for segmental motion 68,69). 

The Arrhenius plot of these polyurethanes is representative of thermoplastics with phase-separated structure (Fig. 6.25).The data for the DC conductivity show a typicd VTFH-type behavior [Eq. (6.24)] (Tuncer et al. 2005), consistent with the coupling of the conductivity mechanism with cooperative segmental motions usually observed in linear polyurethanes and several other thermoplastics. The glass transition temperatures determined by DEA (Tg diei), DSC (TgDsc), and thermally stimulated current (Tg xsc) show very good agreement. In addition, the majority of published works on polymers [e.g., see... 

The melt temperature of a polyurethane is important for processibiUty. Melting should occur well below the decomposition temperature. Below the glass-transition temperature the molecular motion is frozen, and the material is only able to undergo small-scale elastic deformations. For amorphous polyurethane elastomers, the T of the soft segment is ca —50 to —60 " C, whereas for the amorphous hard segment, T is in the 20—100°C range. The T and T of the mote common macrodiols used in the manufacture of TPU are Hsted in Table 2. 

Komfield J.A., Spiess H.W., Nefzger H., Hayen H., and Eisenbach C.D. Deuteron NMR measurement of order and mobility in the hard segments of a model polyurethane, Macromolecules, 24, 4787, 1991. Meltzer A.D., Spiess H.W., Eisenbach C.D., and Hayen H. Motional behaviour within the hard domain of segmented polyurethane A NMR study of a triblock model system. Macromolecules, 25, 993, 1992. 

Photochromic processes in a solid matrix such as polymers generally proceed with considerable deviations from solution. This phenomenon is ascribed to particular interactions between the pho-toreactive molecule and the surrounding matrix (J, 2) on a molecular scale 3) as will be shortly illustrated below. It can be infered from these findings that photochromic molecules, on the other hand, should be suitable probes to detect, e. g., particular motions in solid polymers and changes of the overall chain segmental mobility. This is a challenging technique to be applied in the field of polyurethane elastomers, since quite a few questions are still unsolved in this area. 

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