Blended soft segments

Degradable polyurethanes developed with blended soft segments... 

Polyurethane-based FTPEs are produced by reacting fluorinated polyether diols with aromatic disocyanates. The resulting block copolymers contain fluorinated polyether soft segments.68 Another possible method of preparation of fluorinated TPE is dynamic vulcanization. Examples are a blend of a perfluoroplastic and a perfluoroelastomer containing curing sites or a combination of VDF-based fluo-roelastomers and thermoplastics, such as polyamides, polybutylene terephtalate, and polyphenylene sulhde.69 70... 

The data shows the shift in soft segment T from -48 C (100 percent 1,4-BDO) to 6 C (100 percent dMPD) as expected due to phase mixing. As anticipated, hard segment crystallization occurs when the DMPD content is less than 50 percent. The shear modulus data (Figure 9) show a steady decrease in the rubbery modulus as the DMPD content of the blend increases because the degree of hard segment crystallinity is decreasing, and these results... 

It is worth noting that the H values for the homo-PBT (Fakirov et al, 1998) are at least twice as high as for its copolymers (Apostolov et al., 1998) and blends (Boneva et al., 1998). This observation is related to the specific role played by the soft-segment phase during deformation. This subject is discussed in detail in Sections 6.1 and 6.2 and is beyond the scope of the discussion here where interest... 

As such, these amorphous (glassy) hard segments and amorphous (rubbery) soft segments follow classical rules of mixing. For example, the Fox equation1—for the Tg of a compatible blend is given by (1),... 

Thus, if two polymers are compatible - SINGLE PHASE - there will be one Tg given by equation (1). If two polymers are in-coapatible - TWO PHASES - there will be two Tg s - those of the individual polymers. These concepts are demonstrated in Figure 1 which shows blends of the hard segment (MDI-DPG) in soft segments (MDI/Poly G 53-56 and MDI/Poly G 55-56 ). 

Figure 1 shows that the hard segment MDI-DPG is essentially completely conpatible in the soft segment MDI/Poly G-55-56, a 45% B0 polyol and essentially completely incompatible in the soft segment MDI/Poly G-53-56, an 11% E0 polyol. The theoretical line derived from Equation 1 is in good agreement with the compatible blend. 

Figure 1. DSC results of physical blending of hard and soft segments. Key If, MDl-DPG/MDl-Poly G (55-56 J O, 0, and A, MDl-DPG/MDl-Poly G...

Olefinic thermoplastic elastomers are block copolymers or blends of polyolefins — commonly, polypropylene, which forms the hard crystalline block, and another olefin block, most commonly ethylene or EPDM. Some less common soft segments include natural rubber, nitrile rubber, and EVA. Olefinic thermoplastic elastomers exhibit better processability than neoprene and have excellent resistance to oils. Therefore, they offer attractive replacements for neoprene in oil-resistant wire and cable insulation. 

Binary blends containing a multiblock copolymer as one component or both components in the blend, in which a certain block of the copolymer is miscible with the other polymer, were also reported to be an effective way to fabricate SMP with desired properties. This type of SMP blend include the segmented thermoplastic polyurethane (TPU)/phenoxy resin blend and TPU/poly vinyl chloride (PVC) blend [26,27]. In the blends, the soft segment of the TPU is miscible with the phenoxy resin or PVC, and Tg of this miscible... 

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. 

Block copolymers can often offer properties that are unattainable with simple blends or random copolymers. This led to many efforts to combine dissimilar materials, like hydrophilic with hydrophobic, or hard with soft segments, as was shown earlier. A recent paper" describes the formation of block copolymers containing helical polyisocyanide and an elastomeric polybutadiene. Compound [(7 -C3H5>-Ni(OC(0)CF3)]2 was used to carry out living polymerization of butadiene and then followed by polymerization of rm-butyl isocyanide to a helical polymer. 

---