Polyurethane chemical structure

Glycolysis of PETP leads to oligomers that are polycondensed with eaprolaetone. The obtained diols are extended with hexamethylene diisoeyanate. In eertain conditions the polyurethanes are totally miseible with PVC, leading to acceptable meehanieal eharaeteristies for the blend. A relation between the strueture of the polyurethane and miscibility with PVC is described. The mechanical characteristics of the blend depends on the polyurethane chemical structure. 34 refs. 

Polyurethanes (PUs, PURs), 197-258 ammonolysis and aminolysis of, 556 analytical techniques for, 241-246 applications, properties, and processing methods for, 198, 202-205 application testing of, 244-245 blood contact applications for, 207 chemical structure of, 5 chemistry and catalysis of, 222-236, 546... 

Given the wide variety of chemical structures within the polyurethane family, it should come as no surprise that polymer scientists can tailor compositions and structures to meet a correspondingly broad range of end use requirements. In order to better understand the nature of these relationships, we have divided this section into subsections based on some of the more characteristic forms in which we encounter these materials. 

Statistical and block copolymers based on ethylene oxide (EO) and propylene oxide (PO) are important precursors of polyurethanes. Their detailed chemical structure, that is, the chemical composition, block length, and molar mass of the individual blocks may be decisive for the properties of the final product. For triblock copolymers HO (EO) (PO)m(EO) OH, the detailed analysis relates to the determination of the total molar mass and the degrees of polymerization of the inner PPO block (m) and the outer PEO blocks (n). 

Telechelic polymers rank among the oldest designed precursors. The position of reactive groups at the ends of a sequence of repeating units makes it possible to incorporate various chemical structures into the network (polyether, polyester, polyamide, aliphatic, cycloaliphatic or aromatic hydrocarbon, etc.). The cross-linking density can be controlled by the length of precursor chain and functionality of the crosslinker, by molar ratio of functional groups, or by addition of a monofunctional component. Formation of elastically inactive loops is usually weak. Typical polyurethane systems composed of a macromolecular triol and a diisocyanate are statistically simple and when different theories listed above are... 

The effects of chemical structure of diisocyanate component on the hydrolysis of polyurethanes by R. delemar lipase were examined (Figure 8). The rates of hydrolysis of the polyurethanes containing MDI or tolylene-2,U-diisocyanate (TDI) were smaller than that of the polyurethane containing 1,6-hexamethylene-diisocyanate (HDI). 

Figure 8. Effects of chemical structure of diisocyanate component on the hydrolysis of polyurethanes by R. delemar lipase.

To avoid the obviously incorrect classification of polyurethanes as well as of some other polymers as addition polymers, polymers have also been classified from a consideration of the chemical structure of the groups present in the polymer chains. Condensation polymers have been defined as those polymers whose repeating units are joined together by functional... 

The chemical structures of thermosets are generally much more diverse than the commodity thermoplastics. The most common types of thermosets are the phenol-formaldehydes (PF), urea-formaldehydes (UF), melamine-formaldehydes (MF), epoxies (EP), polyurethanes (PU), and polyimides (PI). Appendix 2 shows the chemical structure of these important thermosetting polymers. 

Most polyurethanes are different from other elastomers in that they are cast. Two components are mixed together. One component is a prepolymer which consists of two major chemical structures. One... 

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. 

The chemical structure of the polyurethanes make them relatively non-conductive and, with suitable modification, semiconductive, allowing for the controlled discharge of static. Polyurethanes will not produce a spark when they are struck by another object. 

The rate of reversion, or hydrolytic instability, depends on the chemical structure of the base polymer, its degree of crosslinking, and the permeability of the adhesive or sealant. Certain chemical linkages such as ester, urethane, amide, and urea can be hydrolyzed. The rate of attack is fastest for ester-based linkages. Ester linkages are present in certain types of polyurethanes and anhydride cured epoxies. Generally, amine cured epoxies offer better hydrolytic stability than anhydride cured types. 

The variation of Tg of the soft matrix in segmented polyurethanes as a function of composition or segmental chemical structure has been monitored and used as an indicator of the degree of microphase separation. Factors influencing the phase-separation process in these MDI-based polyurethanes have been summarized by Aitken and Jeffs (69) as follows (a) crystallization of either component, (b) the steric hin-... 

For the work presented here, the polymers considered are in the general class of materials known as polyurethanes. Polyurethanes are particularly attractive for a study of the effect of chemical structure on damping since it is possible to change their Tg s over a wide range of temperatures (>100"C). This corresponds to a damping peak location that spans more than 10 decades of frequency. In addition, changes in polyurethane structure can be used to produce a transition that can vary from narrow to broad. To take advantage of these desirable properties, one must understand the dependence of Tg on the chemical structure of polyurethane polymers. 

CFCs (chlorofluorocarbons) - As applied to polyurethane foams, blowing agents having chlorine and fluorine in their chemical structure. CFCs are gradually being replaced by other blowing agents because of concern about their adverse effect on the atmospheric ozone layer. 

The chemical structure of polyurethanes can be further complicated. The presence of water, for example, can lead to the formation of urea linkages between two chains that have isocyanate end groups. The reaction may take place as follows ... 

In the current study, samples of impact polystyrene that contained a combination of light stabilizers LS I and LS II and samples that contained no light stabilizers were weathered outdoors in Florida and were monitored for changes in physical appearance and for changes in the chemical structure of the surface by use of a multiple internal reflectance IR spectrophotometer. Also, samples of a thermoplastic polyurethane that was stabilized with just LS I and samples that contained no light stabilizers were weathered and monitored in the same way as the impact polystyrene. We hope to establish at what point... 

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