Elastic modulus and surface tension of a polyurethane rubber in nanometer thick films

Estane is a polyurethane with thermodynamically incompatible hard and soft segments and behaves like a thermoplastic elastomer. In the present work the mechanical properties of Estane at the nano- and macro-scales have been characterized. The macroscopic viscoelastic properties were found to follow time-temperature superposition. Also we used a nano bubble inflation method to obtain the compliance of the Estane ultra thin films over thicknesses ranging from 220 nm to 22 nm. The results have been analyzed using both a direct stress-strain analysis and an energy balance method to separate the rubbery plateau regime modulus and the surface tension of the ultra thin films. It is found that the rubbery regime modulus stiffens significantly as thickness decreases, while the surface tension is independent of film thickness. The rubbery stiffening dependence on film thickness is found to be consistent with a new analysis that suggests that it is related to the shape of the macroscopic segmental relaxation.