Elasticity of urethane networks

Various polymer networks have been prepared based on poly(propylene oxide) which has been linked either by urethane groups or ester groups. These networks have been examined by stress-strain measurements to find the degree of deviation from ideal rubber elasticity (as specified by the statistical theory) and by neutron scattering to find the mobility of the poly(propylene oxide) chain. The stress-strain results showed the urethane-linked networks to be identical to the ester-linked networks as long as there were no short chain polyols in the formulation. Networks containing short chain polyols have a greater deviation from ideal rubber elasticity. The neutron scattering results showed the ester-linked poly(propylene oxide) to have the same mobility as an uncrosslinked poly(propylene oxide), whereas urethane-linked networks have a lower mobility, similar to an analogous urethane-linked polymer. A small effect of crosslinking was observed. The results are explained in terms of some phase separation occurring in the polyurethanes only when short chain polyols are present. In the simple urethane-linked networks the poly(propylene oxide) chain mobility is reduced by hydrogen bonding to the urethane groups, but this has no effect on the equilibrium stress-strain properties.