Relaxation soft-segment

Also displayed in Table II are spin-lattice relaxation data for liquidlike (CH2) groups that were observable in DPMAS experiments. Both the dependence on temperature and the particular Ti values suggested rapid segmental motions within long runs of methylene groups, quite similar to the dynamic behavior reported for soft-segment CH2 s in synthetic polyesters (19). 

Extensive dynamic mechanical property studies have been carried out on hydrogen-bonded (81) and nonhydrogen-bonded (60,82) polyurethanes. Several secondary relaxations were found in addition to the major hard- and soft-segment transitions. Molecular mechanisms could... 

Table I. Relaxations of Hard- and Soft-Segment Polymers...

Hard- and Soft-Segment Polymers. The structure of the diisocyanate component has a significant influence on dynamic mechanical properties of both hard- and soft-segment polymers. Both 2,4- and 2,6-TDI/ BD a relaxations occur at about the same temperature and have a comparable activation energy. However, the symmetrical structure of... 

Above the -relaxation process, the 2,4-TDI/PTMO polymer displayed a short rubbery plateau at a storage modulus of about 5 MPa while 2,6-TDI/PTMO was capable of crystallization, as evidenced by the ac-loss process. This difference in dynamic mechanical properties demonstrates the effect of a symmetric diisocyanate structure upon soft-segment properties. As previously discussed, single urethane links can sometimes be incorporated into the soft-segment phase. The introduction of only one of these diisocyanate molecules between two long PTMO chains inhibits crystallization if the diisocyanate is asymmetric. In the case of a symmetric diisocyanate, soft-segment crystallization above Tg can readily occur. The crystals formed were found to melt about 30°C below the reported melting point for PTMO homopolymer, 37°-43°C (19), possibly because of disruption of the crystal structure by the bulky diisocyanate units. 

Only the 2,6-T-2P polyurethanes exhibited a relaxation associated with crystallization and later melting of the soft segments. The position of the crystallization maximum was estimated from the location of tan(8)m lx and was somewhat difficult to characterize accurately because of extreme overlapping of the a9 and ac relaxations, as evident in Figure 10. Melting occurred over the region 2°-7°C in all samples. Phase segregation in 2,6-T-2P samples was well developed to the point that the... 

Here we consider a series of new poly(ester ether carbonate) (PEEC) multiblock terpolymers with varying amount of ether and carbonate soft-segment content. Dielectric relaxation experiments on the same PEECs revealed the existence of two relaxation processes (Roslaniec et al, 1995). The dielectric loss values show the existence of a relaxation maximum appearing at about 0 °C for 10 kHz relaxation) accompanied by a lower temperature relaxation (y relaxation) which appears at about —50 °C. 

MDI. Three low temperature transition maxima are found in the loss modulus (G") of the 73-series Halthanes (see Fig. 1). Two low temperature secondary relaxations below the glass transition of the soft segment are arbitrarily labeled Tp (-100°C) and Ty (-155°C). These relaxations are probably associated with molecular motions in the urethane (9) and polyether (10) components of the soft segment, respectively. The glass transition of the soft segment occurs at about -50°C and is responsible for the drop in the storage modulus G1 by two orders of magnitude. 

Figure L The low-temperature dynamic mechanical spectrum of Halthane 73-14 is typical of the 73-series polyurethane adhesives. Two secondary relaxations, Tp and Ty, are shown as peaks in the loss modulus at —100° and —150°C. The soft segment glass transition, Tg(SS), occurs at about —50°C. The frequency of oscillation was held constant during the measurement at 0.1 Hz.

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