Foils of Polymethyl methacrylate (PMMA) 2 mm thick were studied by measuring the total Secondary Electron Emission yield $\sigma $ (SEE yield) in a dedicated Scanning Electron Microscope especially equipped to study the fundamental aspects of the charge transport and trapping in insulating materials. The intrinsic SEE yield $\sigma _{0}$, (yield of the uncharged material) and the charging kinetics were studied under low current density $J=10^{+5 }$ pA/cm2. The curve of the primary beam energy variation of $\sigma _{0}$ exhibits a maximum intrinsic yield $\sigma _{\rm 0max}= 2.2$ at 370 ± 20 eV and two crossover energies $E_{I}=84$± 20 eV and $E_{II }=1465$± 20 eV for which $\sigma _{0} = 1$. For $\sigma _{0} >$ 1 PMMA is positively charged and negatively for $\sigma _{0} <$ 1. As electron injection is proceeding under the low current density used, the SEE yield varies from $\sigma _{0}$ to the steady value $\sigma _{st}=1$. This value that expresses the equality between the average number of emitted and injected electrons, characterizes the steady charge regime called “Self-Regulated Regime”. The evolution of $\sigma $ during the injection process is due to the internal field that blocks or enhances the secondary electron emission, according to the positive or negative nature of the trapped charges. A current density effect, characterized by a steady SEE yield slightly higher than unity, $\sigma _{st }=1.03$, instead of one, is observed at high energy (for example 4000 eV) for a strong current density $J>10^{+6 }$ pA/cm2. It is interpreted by a field ionisation (Poole-Frenkel type) that enhances the secondary electron emission.