Structure of Polyurethane

Using controlled reaction conditions such as the temperature profile and rate and time of the addition of polyols, more uniform materials can be produced. The correct spacing of the hard segments required to produce the physical properties can be obtained. The controlled conditions will also help prevent the formation of undesirable side products such as allophanate, biuret, and trimers. These reactions will give branching of the polymer chains. 

The reaction rates of the various ingredients must be understood to follow the probable course of the reaction. Certain polyols may have secondary hydroxyl groups that are very much slower in reaction than the primary hydroxyl groups. The preparation of a prepolymer will allow the secondary hydroxyls to fully react. The reaction of a 100-percent 2,4 TDI is much faster than the standard 80 20 TDI isomer blend. The reaction rate between the terminal NCO and chain extender can be adjusted to allow the prepolymer mix to fill the mold and allow entrapped air to escape before the material gels. 

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Molar mass boosting, 89 Molecular composites, polyimide, 269 Molecular structure, of polyurethanes, 209-217... 

This new material is, of course, the reticulated polyurethane foam discussed in the earlier example. Later in this book, we will expand on this example to show how we and others used the chemistry and physical structure of polyurethane to remediate environmental pollution and also as a system to produce fine chemicals and proteins including enzymes for industrial and medical uses. 

FIGURE 7.4 Hepatic spheroid in structure of polyurethane foam. (From Yamashita, Y. et ah, High Metabolic Function of Primary Human and Porcine Hepatocytes, Cell Transplantation, Vol. 11, pp. 379-384, 2002.)... 

Blokland, R. Elasticity and structure of polyurethane networks. Rotterdam Universitaire Pers 1968. 

There are a number of instrumental techniques that can be used to elucidate the structure of polyurethanes. These are often expensive and need specialized operation and interpretation of results. 

Blokland R, "Elasticity and Structure of Polyurethane Networks", Thesis, Delft University of Technology, 1968. Blokland R and Prins W, J Polym Sci (A2) 7 (1969) 1595. 

The unique two-phase structures of polyurethane that offers the elasticity of rubber combined with the toughness and durability of metal make them one of the most extensively studied and frequently used materials in carbon nanotube related nanocomposites. The main difficulty in developing CNT based polyurethane nanocomposites was how to achieve uniform and homogeneous CNT dispersion. Further investigations on the interactions between carbon nanotubes and two-phase structures are critical for the wider applications of carbon nanotube/polyurethane composites. 

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. 

Shutov, F. A., Chaikin, 1.1. Micro- and macrocellular structure of polyurethane foams. Proizvodstvo i Pererabotka Plasticheskikh Mass i Sinteticheskikh Smol 1979, No. 4,14 (in Russian)... 

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 ... 

The reaction between isocyanates and water leads to production of gaseous carbon dioxide and an urea group. This reaction is a very convenient source of a gas necessary to generate the cellular structure of polyurethane foams [1-26] ... 

A partial esterification reaction takes places (19.5). This side reaction does not have a negative effect on rigid PU foam fabrication because it gives a very convenient tetrafunctional compound, which participates, together with the amidic polyol, to build the crosslinked structure of polyurethane. These compounds containing ester groups are present in low concentrations, maximum 5-7% [1,2]. 

Through the synthesis of poly(urethane-imide) films and their carbonization, carbon films were obtained whose macropore structure could be controlled by changing the molecular structure of polyurethane prepolymer [164-166]. Poly(urethane-imide) films were prepared by blending poly(amide acid), which was synthesized from pyromellitic dianhydride (PMDA) and 4,4 -oxydianiline (ODA), and phenol-terminated polyurethane pjrejwlymers, which were synthesized through the reaction of polyester polyol with either hexamethylene diisocyanate (HDI), tolylene-2,4-diisocyanate (TDI) or 4,4 -diphenyknethane-diisocyanate (MDI). The reaction schemes of two components, poly(imide) (PI) and poly(urethane) (PU), are shown in Fig. 46a). 

Table 9. Representation of some typical structures of polyurethanes.

FIGURE 53.7 Chemical structure of polyurethane, where n stand for whole numbers signifying the number of monomers present in the polymeric form. 

Swelling of polyurethane networks. Proceeding from the general conception of the role of surfactants in the formation of the secondary structure of polyurethanes, studies of the influence of KEP-2 on the distribution of hydrogen bonds in polyurethane networks were required. The literature on the thermodynamic properties of polymer solutions and gels was searched and, in particular, the work of the authors of [117] is of direct interest in this context. This work... 

Fig. 3. General chemical structure of polyurethane prepolymer (modified from Hepburn, 1982).

The two-phase structure of polyurethane elastomers bestows unusual abrasion resistance (Baumann, 1969a,h). In fact, were it not for their high cost, polyurethanes would be excellent candidates for tire tread materials. For instance, under conditions described in ASTM test C-501 (Taber... 

V.N. Vatulev, S.V. Laptii, and Yu.Yu. Kercha, Infrared Spectra and Structure of Polyurethanes,... 

Vatulev V N, Laptii S P, Kercha Yu Yu, Infiared Spectra and Structure of Polyurethanes, Naukova Dumka, Kiev, 1987. 

Figure 19. Basic chemical structure of polyurethane prepolymers [8].

The basic chemical structure of polyurethane prepolymers is illustrated in Fig. 19. A good overview on polyurethane adhesive chemistry is given by Habenicht [8]. 

Table 6.19 Properties of films made from hybrid dispersions differing only in the structure of polyurethane-urea part of the hybrid ...

The crystalline structure of polyurethanes is controlled by the formation of hydrogen bonds between NH and C=0 groups of the urethane linkages [28, 29]. From the IR analysis, the hydrogen bonded C=0 present in the crystalline domain can be identified by the peak absorbance at 1699-1706 cm and the non-bonded or free C=0 group shows peaks at 1735 cm and 1748 cm. ... 

NMR spectroscopy is employed largely to gain understanding of the actual structure of polyurethane along with its precursors (polyol, isocyanate, etc.). An idea may also be obtained of the degree of curing of the polymer hence the extent of the reaction can be measured by observing the resolution of the NMR spectrum. 

Table 4.17 Structure of polyurethane ureas obtained vwth diamines TDA, DAB and MDA...

Segmented poly etherurethanes contain no water-soluble additives and are characterized by a high hydrolytic resistance, good processability, and good mechanical properties in comparison with otho elastomers. Therefore, polyurethanes of this type are widely used in medicine, for example as aortic balloon pumps, catheters, and housings for artificial hearts Thus, a corrdation betweaa blood compatibiUty and the chemical structure of polyurethanes has often been reported... 

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