Memory foam derives its viscoelastic properties from several effects, due to the materials internal structure. The network effect is the force working to restore the foams structure when it is deformed. This effect is generated by the deformed porous material pushing outwards to restore its structure against an applied pressure. Three effects work against the network effect, slowing the regeneration of the foam's original structure:
* The pneumatic effect, caused by the time it takes air to flow into the foam's porous structure.
* The adhesive effect, or adhesion, caused by the stickiness of the surfaces within the foam, which work against decompression as the internal pores within the foam are pressed together
* The relaxation effect (the strongest of the three forces working against expansion), caused by the foam's material being near its glass transition temperature—limiting its mobility, forcing any change to be gradual, and slowing the expansion of the foam once the applied pressure has been removed
The effects are temperature-dependent, so the temperature range at which memory foam retains its properties is limited. If it is too cold, it hardens. If it is too hot, it acts like conventional foams, quickly springing back to its original shape. The underlying physics of this process can be described by polymeric creep.
The pneumatic and adhesive effects are strongly correlated with the size of the pores within memory foam. Smaller pores lead to higher internal surface area and reduced air flow, increasing the adhesion and pneumatic effects. Thus the foams properties can be controlled by changing its cell structure and porosity. Its glass transition temperature can also be modulated by using additives in the foams material.
Memory foam's mechanical properties can affect the comfort of mattresses produced with it. There is also a trade-off between comfort and durability. Certain memory foams may have a more rigid cell structure, leading to a weaker distribution of weight, but better recovery of the original structure, leading to improved cyclability and durability. Denser cell structure can also resist the penetration of water vapor, leading to reduced weathering and better durability and overall appearance.