Polyurethane backbone

Scheme 4.7 Primary structure in polyurethanes backbone microstructure.

In this regard, polyurethane technology offers a product designer a particular advantage. An aqueous solution or dispersion can be emulsified conveniently with a hydrophilic prepolymer and thus incorporated into the polyurethane matrix. The incorporation is accomplished by covalently bonding within the polyurethane backbone and by entrapping it within the matrix. Both methods are evident in foams produced by this technique. 

The reactive oligomer can be any low-molar-mass polymer containing at least a couple of double bonds. It can be based on a polyester, polyether, or polyurethane backbone. One mole of a, oo-OH-terminated polyester or polyether is prereacted with two moles of acrylic acid to obtain an a, oo-diacrylate oligomer. For polyurethanes, 1 mole of a, m-diisocyanate oligomer is prereacted with 2 moles of hydroxyethylacrylate (Sec. 2.2.3c). 

Chemistry MDl-based isocyanate prepolymer, polyether and/or polyester polyols, reacts with water to form polyurea-polyurethane backbone Often MDl-based isocyanate prepolymer (some are ahphatic isocyanate based), polyether and/or polyester polyols (some contain epoxy based materials), reaction forms polyurethane backbone MD1-, TDl- and some aliphatic isocyanate-based prepolymers, polyether and/or polyester polyols, reaction forms polyurethane backbone... 

Laminating adhesive - epoxy amine (polyurethane backbone)... 

Many other groups can be included in the polyurethane backbone. An example is the carbonate group that can be found in polyurethane with various structures. The formula of a poly(urethane-co-carbonate) is shown below ... 

Takeichi, T. Ujiie, K. Inoue, K., High performance poly(urethane-imide) prepared by introducing imide blocks into the polyurethane backbone, Polymer 2005, 46, 11225-11231. 

Fig. 7 (a) Picture of a mechanochromic elastomer made by integrating C120H-RG into a thermoplastic polyurethane backbone in the unstretched state, (b) Picture of the same material in the stretched state. Both pictures were taken under illumination with ultraviolet light, (c) Ratio of monomer to excimer emission 7m//e (circles) and tensile stress (solid line) under a triangular strain cycle between 0% and 500% at a frequency of 0.0125 Hz. Adapted with permission from [41]. Copyright 2006 American Chemical Society... 

The role chitin as a material of highly ordered crystalline structure has been reported in the study [96]. X-ray diffraction analysis was carried out in order to find the changes of the crystalline structure upon the substitution reaction with NCO terminated prepolymer. The X-ray diffraction studies showed that crystallinity mainly depends on the concentration of chitin in the polyurethane backbone, crystallinity increased as the concentration of chitin into the final PU increased (Fig. 3.22). The crystallinity of some polymers was clearly observed by optical microscopic studies [114]. The results of X-ray diffraction experiments correlate with optical microscopy findings. A crystalline polymer is distinguished from an amorphous polymer by the presence of sharp X-ray Unes superimposed on an amorphous halo. Under an optical microscope, the presence of polycrystalline aggregates appear as spherulites [114]. The spheruhtes are made of small crystallites and grow Irom a nucleus at their centre. They consist of narrow chain folded lamellae growing radially. Since the fibrous crystals are radial, the chains folded with the lamellae are circumferentially oriented. From the evaluation of the X-ray and optical microscopic studies, it has been observed that the involvement of chitin in the PU formulation and have improved crystallinity of the final polyurethane. 

A rotaxane is a complex consisting of a linear molecule threaded through a cyclic one [57]. Recent studies have found that threading a polyurethane backbone through large (36-60 membered) crown ethers could have remarkable effects on the solubility of the polymer - some of these complexes were even soluble in water [58]. This phenomenon is in effect a phase transfer, and it is conceivable that new chemistry might be performed on the polymer backbone via this induced solubility. 

On the other hand, chitin and chitosan are employed as reinforcement for polymers and in particular for PUs. Saad et al. synthesized PUs with block copolymers of PHB and they reinforced them with chitin nanocrystals. They reported an improvement on the thermal stability due to the addition of nanofillers [128]. Barikani et al. developed hybrids of PU and chitosan and found that the chitosan increases the thermal stability of the PU matrix [129]. Similarly, Zia et al. added chitin to the polyurethane backbone, reporting an enhancement on their thermal properties [130]. 

Polyurethane backbone Commonly used isocyanates include ... 

To obtain the graft copolymer a copolycondensation involving an co-dihydroxylic macromonomer together with a diol (such as butanediol) and a diisocyanate (toluene diisocyanate or hexa-methylene diisocyanate) is carried out. Alternately, an co-dicarboxylic macromonomer is reacted with a diacid e.g. sebacic acid) and a diamine (such as phenylenediamine or hexamethylenediamine) (Scheme 31). " The backbone chain is constructed during the poiycondensation process, and each macromonomer incorporated results in a graft. This method offers interesting possibilities of grafting vinylic or acrylic chains onto a polyurethane backbone or onto a polyamide chain. 

Unilink liquid secondary aromatic diamines are well known for their slow reactivity, low toxicity and ease of use. The resulting urea modification of the polyurethane backbone generally improves polymer strength properties tensile, tear and elongation) to give Unilink diamines a wide range of applications in many polyurethane and polyurea systems. 

Polymeric rotaxanes, consisting of crown ethers threaded onto a polyurethane backbone, were synthesized by allowing the crown ether to equilibrate with tetrakis(ethylene glycol) before its reaction to bis(p-isocyanato-phenyl)methane. The threading efficiency, as measured by xln, the mole ratio of the cyclics per repeat unit, increased linearly with ring size from 36- to 60-membered crown ethers. Up to 63% by mass of the macrocycle was incorporated. The physical properties of the polyrotaxanes differ from those... 

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