In most polymers, aromatic diisocyanates (MDI) are used to form hard segments. While they may be easier to extrude and mould, polymers based on cycloaliphatic analogues, HMDIs typically have poor long-term biostability, reduced heat and solvent resistance, and lower physico-mechanical properties at the same hard segment concentration. On the other hand, biodegradable polyurethanes typically use aliphatic or cycloaliphatic diisocyanates to avoid aromatic degradation products from their expected decomposition in the body.

01. Chain Extenders

Commonly used chain extenders include butylene glycol (BD) and ethylenediamine (ED). Even numbered chain extenders are often preferred because they improve the hydrogen bonding efficiency and the resulting physico-mechanical properties.

The effect of chain extender length was investigated using three linear chain extenders: ethylene glycol, 1,4-butanediol and 1,6-hexanediol. Short chain extenders produced phase separation, higher crystallinity and adequate rheological properties without affecting the surface properties. By reducing the chain extender length, better crystallinity and a high degree of phase separation were produced in PUs. Furthermore, the dynamic mechanical behaviour of PUs with different types of linear and non-linear chain extenders was compared, showing that the Tg of the soft segment matrix varied with the type of chain extender.

The effect of butanediol (BD) or 2,20-(methylimino)diethanol (MIDE) as chain extender on the degradation, mechanical properties, hydrophilicity and cytophilicity of polyurethanes containing (MDI) and polycaprolactone diol (PCL-diol) was evaluated. In vitro degradation studies showed that PUs containing MIDE had higher degradation rates than PUs synthesised using BD. The researchers used silicone chain extenders and showed improved compatibility of the soft chain segment phase of PDMS with the hard chain segments in mixed macrodiols.

Various perfluorinated chain extenders were used in the manufacture of polyurethanes. The perfluorinated chain extender polymers were found to have more soft chain segments on the surface of the material than polymers with the same amount of hard chain segments. Reducing the molecular weight of the soft chain segments increased the fluorine content or hard chain segment concentration in the surface region. The same fluorine content was found to be uniformly distributed throughout the surface region and the body. Symmetrical fluorine substitution in the chain extender region usually enhances the mixing of hard and soft chain segments.

Aromatic diamines have been used as chain extenders in reactive injection moulding (RIM) of polyurethane elastomers. Heterocyclic polyurethanes synthesised by the extension reaction of a macrodiisocyanate with a heterocyclic diamine have been shown to increase the decomposition temperature of polyurethanes. The nature of the heterocyclic unit determines whether the manufactured material is elastic (based on polyethylene glycol adipate) or brittle (low molecular weight diols).