Polyureas hydrogen bonding

The isocyanates are believed to aid wetting of metals by interacting with the metal oxides and hydroxides (M-OH) present on the surface. The isocyanates can then react with water to form a rigid polyurea network which interacts at the metal surface through hydrogen bonding. The isocyanates also react to form hard segments , which will be explained shortly. 

As polymer molecular weight grows, the polyurea hard segment reaches a percolation point, where the driving force for interchain hydrogen bonding exceeds... 

On the other hand, the group of No wick [29] developed polyurea templates with the general structure 35 in order to nucleate parallel /1-sheet structures. They reported that in a model compound (36) the urethane carbonyls are hydrogen bonding to the adjacent NH and that the orientation of the bonding interactions is controlled by the size of the end substituent, in this case a phenyl group. They also recently prepared 37 and showed by NMR that it adopts mostly the proposed parallel /1-sheet conformation in chloroform [30]. 

Fig. 6.3 shows the empirical Yg values of the different series of condensation polymers characterised by their functional groups as a function of the number of the CH2 groups in the structural unit (Nch2)- It is evident that for all series, except the polyamides, polyurethanes and polyurea s, the slope of the lines is constant, viz. 2.7, the increment of the CH2 group (Fig. 6.3a). Only for the polymers mentioned - all containing hydrogen bonding groups - the slope is 4.3. Networks of inter-molecular hydrogen bonds obviously cause an apparent increase of the CH2 increment from 2.7 to 4.3 (Fig. 6.3b).

Isocyanates also react with primary and secondary amine compounds. Tertiary amines cannot react with isocyanates because they do not contain active hydrogen atoms, but they are powerful catalysts for many other isocyanate reactions. Diamines are frequently used as chain extenders and curing agents in PU manufacture. The addition of a diamine to the reaction mixture increases the overall reactivity during polymerization. The reaction between an isocyanate group and an amine results in the formation of a urea bond. The polyurea segments present in the finished PU serve to increase the potential for both covalent and hydrogen bond crosslinks within the polymer. 

The sites for hydrogen bonding and chemical reaction are significant and polyureas are generally easy to bond. Being more oxidation resistant and hydrolysis resistant than the urethane group is significant. 

S. Iksaka, K. Ohisbi. H. S. Nalwa, T. Watanabe, and S. Miyata. Ferroelectric polarization reversal stabilized by hydrogen bonding in N-phenylated aromatic polyurea. Polymer J.. 26 505 (1994V... 

Solution. The polyurethane chains contain the O- swivel, thus they are the most flexible, have the largest AS , and have the lowest T . Hydrogen bonding, and thus AH , is roughly comparable in the polyurethanes and polyamides. The polyureas and polyamides should have chains of comparable flexibility (no... 

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