Cell nucleation plays a pivotal role in polymer foaming because it significantly affects the foam's cellular structure and properties. One of the major drawbacks of classical cell-nucleation theory is that it ignores elastic strain energy's effect on cell nucleation. Hereby, we conducted an experimental study to clarify elastic strain energy's role in cell nucleation. We found that not only gas super-saturation but also elastic strain energy can offer a driving force for cell nucleation. Meanwhile, not only interfacial energy barrier but also material's elastic energy barrier can act as a cell nucleation resistance. The uniaxial stretching assisted foaming process and the uniaxial compressing-assisted foaming process were explored and conducted to investigate the stretching strain energy's effect and the compressive strain energy's effect on cell nucleation, respectively. We demonstrated that both stretching and compressing can promote cell nucleation, but stretching is much more effective than compressing. The larger the elastic strain energy, the higher the cell nucleation density and the more uniform the cellular morphology. Whereas no foam structure was developed in the regular foaming process, sub-microcellular thermoplastic polyurethane (TPU) microfilms were achieved by applying the uniaxial or biaxial stretching-assisted foaming process. The fine sub-microcellular TPU microfilm with an average cell size of 382.6 nm has a void fraction of 0.64, which is to the best of our knowledge the largest void fraction of the porous polymer microfilm. Thus, we reported a promising and versatile way to control cell nucleation and to develop new techniques by which to produce novel multi-functional porous polymers. (c) 2017 Elsevier Ltd. All rights reserved.