If thermodynamic fluctuations allow nucleus generation, cell growth is constrained by the balance of pressure forces with surface and viscoelastic forces [37]. Literature gives some data concerning interfacial tension of PMMA/CO2 system, but in foaming conditions differing from that presented in Part 2.2. Interfacial tension of PMMA/CO2 system, at 90 C under a saturation pressure of 100 bar, is 13 mN m 1 [39,40]. However, in these specific conditions, the CO2 uptake of PMMA is about 10 wt%. The increase in solubilised CO2 content until a value of 20 wt% at 90 C causes the decrease of the interfacial tension from 13 to 8 mN m 1 [39]. According to CO2 uptake measurements on PMMA, an interfacial tension of same order of magnitude can be considered in the present study. This corresponds to a surface stress of about 4 105 Pa for a cell radius of 20 nm, which is considered the critical radius rcr in a PMMA/CO2 system with a CO2 concentration lower than 10 wt% [41]. A critical radius of 20 nm corresponds to the minimum cell size required for cell growth: a generated cell with a smaller size does not grow and tends to disappear.