Segmented polyurethane elastomers

The unit that gives the polyurethanes their name stems from the basic urethane structure  

However, the overall structure of the polymers as normally prepared is much more complicated. Two basic types of polyurethanes exist the polyether-urethanes (Harrell, 1969,1970), 

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Seymour, R.W., G.M. Estes, and S.L. Cooper. Infrared Studies of Segmented Polyurethane Elastomers. I. Hydrogen Bonding. Macromolecules 3 5 (1970) 579-583. 

Several studies have been published in recent years on the properties of RIM polyurethanes and polyurethane-ureas (1-7). Unlike most studies, however, we have restricted our attention to simplified linear systems in order to establish the effect of reaction rates and mold temperatures on the phase separation and molecular weight of segmented polyurethane elastomers produced by RIM(6). 

SEGMENTED POLYURETHANE ELASTOMERS SCHEME OF THE CHAIN STRUCTURE ... 

Figure 5. Scheme of primary chain structure of segmented polyurethane elastomers and chain segment organization. 

There are two important types of polymer in which an elastomeric and a glassy or non-crystalline rigid component are copolymerised. The simpler thermoplastic elastomers are usually triblock ABA copolymers with B as the elastomeric component and A as the glassy component, whereas the segmented polyurethane elastomers are multiblock copolymers in which alternate blocks are hard , i.e. relatively inflexible, and soft , i.e. relatively flexible. 

Segmented polyurethane elastomers, prepared from diisocyanates, macrodiols and chain extenders, are frequently used in the construction of implantable medical devices, such as cardiac pacemakers, heart valves, catheters, and heart assist devices, because of their excellent mechanical properties and haemocompatibility. Polyether macrodiols, such as poly(tetramethylene oxide) (PTMO), are used to prepare polyurethanes for implant, since they offer an increased resistance to enzymatic hydrolysis compared with polyester-based polyurethanes. 

Estes, G.M., Seymour, R.W., and Cooper, S.L. (1971) Infrared studies of segmented polyurethane elastomers. II. Infrared dichroism. Macromolecules, 4, 452-457. 

Fig. 3.20. Strain-induced elongation crystallization of polyether soft segments in a segmented polyurethane elastomer by elongating it to 200% extension (Allport Janes, 1973).

Bagdi, K. Molnar, K. Pukanszky, B. Jr. Pukanszky, B. Thermal analysis of the structure of segmented polyurethane elastomers. Journal of Thermal Analysis and Calorimetry, 2009, 98, 825-832. 

Reaction polymerization reactions of isocyanates with suitable monomers can he performed in an extruder or in a RIM machine. In the latter reaction thermosets (cross-hnked polymers) are produced. In an extruder usually linear polymers are manufactured. For example from methylene di-p-phenylene isocyanate (MDI), with some macroglycols and 1,4-hutanediol as extenders, segmented polyurethane elastomers are produced in an extruder (6). However, linear condensation polymers are also produced in a vented extruder. For example from MDI, with macrodicarboxylic acids and dicarboxyhc acids as extenders thermoplastic block copolyamide elastomers are produced. The by-product of the condensation reaction, carbon dioxide, is removed in the vented extruder. The polycondensation process can also be performed in solution. For example, MDI can be added to a solution of dicarboxyhc acids in tetramethylene sulfone, with simultaneous removal of the carbon dioxide. Tetramethylene sulfone is the solvent of choice for solution polymerization of isocyanates (7). In addition to dicarboxyhc acids trimellitic acid anhydride and benzophenonetetracarboxylic acid dianhydride (BTDA) are utilized as monomers for condensation polymers. With these monomers poly(amide imides) and poly(imides) are produced. The diisocyanate-derived commercial polycondensation products are listed in Table 1. 

The symmetrical MDI is more suitable for the preparation of segmented polyurethane elastomers having excellent physical properties. Segmented polyurethanes are also obtained from 2,6-TDI, but an economically attractive separation process for the TDI isomers has yet to be developed. 

Kavlock KD, Whang K, Guelcher SA, Goldstein AS. Degradable segmented polyurethane elastomers for bone tissue engineering effect of polycaprolactone content J Biomater Sci Polym Ed 2013 24(l) 77-93. 

Scaffolds synthesized from HDI-derived segmented polyurethane elastomers have been reported to support bone healing in an ovine iliac crest defect model [46,47]. Chain extenders comprising 1,4-butanediol, 2-amino-l-butanol, 2-mercaptoethyl ether, and isosorbide diol [48-50] were investigated, as well as poly(ethylene oxide) (PEO), poly(ethylene-fc-propylene-Z -ethylene oxide) (PEO-PPO-PEO) block copolymers, and poly(caprolactone) macrodiols. Using a salt leaching/phase inversion technique, porous scaffolds with up to 90% porosity were fabricated from the HDI-derived... 

Koberstein, J. T., Rs, S. (1983), Small-angle X-ray scattering measurements of diffuse phase-boundary thickness in segmented polyurethane elastomers, J. Polym. ScL, Polym. Phys.Ed.,2, 2m-lQ0. 

Based on various experimental studies, one can schematically represent the morphology of segmented polyurethane (elastomer or flexible foam polymer) on the nano- and micro-scale as shown in Figm-e 2.1. For the range of hard segments volume fraction less than 50%, much of the space is occupied by the soft phase matrix. Microphase-separated nano-domains of the hard phase are dispersed in this matrix they can be individual islands or can form percolated networks. Finally, there could also be some larger (micron-sized) macrophase-separated domains of hard phase, where hard phase domains are ordered at the macro-scale (this is especially true in the case of flexible foams). The relative amounts of all these elements depend on the formulation and processing history. 

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