Hard-Soft Segment Theory

According to the theory [29], soft segments are derived from the polymeric polyol and hard segments from the diisocyanate chain extended with low molecular 

Reaction of isocyanate-terminated prepolymer with water produces urea — 

In addition to hydrogen bonding, secondary-bond forces lead to the aggregation of separate particles into solid and liquid phases they are not of great importance for stable chemical compounds. However, many physical properties such as surface tension and frictional properties, miscibility and solubility are determined to a large extent by intermolecular forces. Three types of forces acting between molecules are recognized, dipole, induction, and dispersion forces. Occasionally, the term van der Waals forces is applied to the dispersion forces alone. 

The usual thermodynamic incompatibility of these two types of segments leads to a phase separation, leading to a structure consisting of hard segments and soft segment domains. This segregation gives rise to a micro-nonuniform 

Tensile and overlap shear strength increases Raises the hardness and modulus (stiffness) 

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In the case of the polycrystalline polyester thermoplastic rubbers the simple domain theory does not seem to apply. With these rubbers it would appear that they contain spherulitic structures consisting of 4GT radial lamellae with inter-radial amorphous regions that are mixtures of PTMEG soft segments and noncrystalline hard segments. 

The authors proposed that the extent of soft segment surface enrichment in a range of polyurethanes could be quantified by dividing the intensity of a soft segment peak (/ss) by the intensity of a hard segment peak (/ns)- This theory was tested by analyzing a series of polyurethanes with increasing amounts of the PPG... 

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