Polyurethanes hydrogen bonding

The reaction of di- and polyisocyanates with various polyester and polyether glycols, their co-reactants in many adhesive applications, produces the relatively polar, highly hydrogen-bonded polyurethanes which when wet, intimately contact, and show strong attraction for a variety of surfaces. 

Ambrozic, G., Zigon, M. (2005a), Hydrogen-bonded liquid-crystalline polyurethane complexes with 4-dodecyloxybenzoic acid, Acta Chimica Slovenica, 52,207-14. Ambrozic, G., Zigon, M. (2005b), Hydrogen-bonded polyurethane complexes based on 4-alkoxybenzoic acids as the low molar mass components. Polymer International, 54, 606-13. 

The high melting point of polyamides derives from interchain hydrogen bonding. Polyurethanes are prepared from diisocyanates and diols. 

The elasticity of thermoplastic polyurethane rubbers (which are also known as thermoplastic urethanes or TPUs) is a function of their morphology which comprises hard and soft phases. The hard phases consist of hydrogen bonded clusters of chain segments, which are linked by flexible chain segments that make up the soft phase. The hard blocks, which are the minor phase, exist as separate domains within a continuous matrix of the majority soft phase, as shown schematically in Fig. 25.9. 

Figure 25.10 Example of hydrogen bonding between urethane linkages in a hard block of a thermoplastic polyurethane elastomer...

The nature of the hard domains differs for the various block copolymers. The amorphous polystyrene blocks in the ABA block copolymers are hard because the glass transition temperature (100°C) is considerably above ambient temperature, i.e., the polystyrene blocks are in the glassy state. However, there is some controversy about the nature of the hard domains in the various multiblock copolymers. The polyurethane blocks in the polyester-polyurethane and polyether-polyurethane copolymers have a glass transition temperature above ambient temperature but also derive their hard behavior from hydrogen-bonding and low levels of crystallinity. The aromatic polyester (usually terephthalate) blocks in the polyether-polyester multiblock copolymer appear to derive their hardness entirely from crystallinity. 

Polypropylene ether) polyol is the single most important product from propylene oxide and enjoys a predominant position in polyurethane applications. The ether linkages are very abundant in these polyols and they contribute to the physical and chemical properties in many applications such as surfactant action and hydrogen-bond formation. 

PCL-diol and diphenylmethane-i -diisocyanate (MDI), by R. delemar lipase were examined. These polyurethanes have both the hydrogen bonds among polymer chains and aromatic rings in the polymer molecules. R. delemar lipase could hydrolyze the polyurethanes though the rate of hydrolysis toward polyurethanes decreased as compared to that ward PCL-diol. The rate of hydrolysis decreased with decreasing the Mn of PCL-moiety of polyurethanes (Figure T). 

Thus it was assumed that the rigidity of the polyurethane molecules based on the aromatic rings, rather than the hydrogen bonds among the polyurethane chains, would influenced their biodegradability by R. delemar lipase. 

TGA analysis shows that polymer degradation starts at about 235°C which corresponds to the temperature of decomposition of the cellobiose monomer (m.p. 239°C with decom.). Torsion Braid analysis and differential scanning calorimetry measurements show that this polymer is very rigid and does not exhibit any transition in the range of -100 to +250 C, e.g. the polymer decomposition occurs below any transition temperature. This result is expected since both of the monomers, cellobiose and MDI, have rigid molecules and because cellobiose units of the polymer form intermolecular hydrogen bondings. Cellobiose polyurethanes based on aliphatic diisocyanates, e.g. HMDI, are expected to be more flexible. 

---