Any plasticizers incorporated into the polymers used in the shoe could diffuse out over time which would change the mechanical properties of the material. This process will happen more quickly with plasticizers that are volatile, and if the shoe material has bubbles in it which would increase the surface area and promote plasticizer evaporation. Increased surface area due to bubbles would also increase oxygen and water transport into the shoe materials and therefore increase the rate of oxidative degradation and other types of chemical degradation. Increasing the temperature will make all of these effects happen more quickly, and light will also degrade the materials. So, the best way to keep shoes fresh over time is to store them in total darkness at high pressures and low temperatures in an environment of dry, nonreactive gas, such as nitrogen. Then your shoes will stay fresh.