Cast polyurethanes Soft segments

Polyurethane. [This polymer was supplied by R. A. Auerbach, Lord Corporation.] The polyurethane was composed of a soft segment of poly(tetramethylene glycol) with a molecular weight of 1000 and a hard segment of p-diphenylmethane 4,4 -diisocyanate (MDI), and was chain extended with ethylenediamine (7). A solution of 7 wt % polyurethane in N,N-dimethylacetamide was used to cast the films. The phase-separated samples (ANN) were prepared by annealing the film at 135°C for 90 min in a helium atmosphere, and the mixed, nonphase-separated samples (NEQ)... 

Proton spin-temperature equilibration between the hard- and soft-segment-rich domains of the polyurethane elastomer on the order of 10-100 ms might be considered fast relative to a macroscopically phase-separated blend [26] or copolymer, but slow relative to a strongly interacting mixture [25]. This is reasonable for a microphase-separated material whose solid state morphology has been the subject of considerable theoretical and experimental research. Under fortuitous circumstances, intimate (near-neighbor) contact between dissimilar molecules in a mixture can be studied via direct measurement of proton spin diffusion in a two-dimensional application of the 1H-CRAM PS experiment (Combined Rotation And Multiple Pulse Spectroscopy). Belfiore et al. [17,25,31] have detected intermolecular dipolar communication in a hydrogen-bonded cocrystallized solid solution of poly(ethylene oxide) and resorcinol on the f00-/xs time scale, whereas Ernst and coworkers [26] report the absence of proton spin diffusion on the 100-ms time scale for an immiscible blend of polystyrene and poly(vinyl methyl ether), cast from chloroform. 

Table 8.4 Distribution of hard and soft segments in cast polyurethanes ... 

In cross-linked polyurethanes, there are actual chemical bonds formed in a three-dimensional manner. The main difference is that with the cast-able polyurethanes the actual chemical structure consists of two major zones, a hard zone and a soft zone. This is not as pronounced in the cross-linked polyurethanes. A typical example is the addition of TMP (trimethy-lol propane) to polyurethane to make it softer. This curative will make the material much softer as it breaks the hard segment zoning up to a certain degree. They do, however, give a material with improved compression set properties. 

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