Overview

The structure - property relationships in polyurethanes (PU) have been excellently presented in the general monographs on PU [1-11]. As a consequence, this chapter will not be very long and it will present only the specific general effects of the oligo-polyol s structure on the resulting polyurethane properties.

Several general properties, characteristic to classical macromolecular chemistry, are strongly linked to the polyurethane structure, as a direct consequence of the oligo-polyol structure - these are [1, 2, 5, 9, 11]:

1. Molecular weight (MW),

2. Intermolecular forces,

3. Stiffness of chain,

4. Crystallinity, and

5. Crosslinking.

21.1 Molecular Weight

As a general rule, for linear polymers all the properties, such as tensile strength, elongation, elasticity, melting points, glass transition temperature (T _g), modulus and increase of the MW, increase up to a limited value, where all the properties remain practically constant. This behaviour is valuable for linear polymers, in our particular case in linear polyurethanes (PU elastomers, spandex fibres, etc).

For crosslinked polymers (in this category they are the majority of polyurethanes, for example flexible, semiflexible and rigid PU foams, etc.), which have a MW that is practically infinite [12], the molecular weight between the branching points (M _c) is considered. The value of M _c depends strongly on the oligo-polyol structure.