Figure 4.

Schematic description of the network formation at a photomask edge in thiol–enes. (a) Stoichiometric formulation case: diffusion of radicals (I) from zone 3 to zone 2 during exposure leads to broadening of the photostructuring (= zones 2 and 3) due to reactions with the available ene and thiol functional groups (II). (b) Off-stoichiometric formulation case: radical diffusion (I) occurs, but in this case, the counter-acting diffusion of thiol (II) and, importantly, ene monomers (III) from the non-illuminated region to the illuminated region leads to the formation of two distinct zones: zone C with a local denser cross-linking and conversion, and zone B with a depletion of the deficient groups. (c) Schematic plot of the functional group concentrations and their ratio before and after photostructuring (d) and (f): Vertical Raman cross-section of thiol groups present in stoichiometric (d) and off-stoichiometric (f) thiol–ene after photostructuring. (e) Cross-section plot recording along the solid and dashed horizontal lines (d) and (f). Stoichiometric and off-stoichiometric concentration profiles show that light spreading causes the following features: in the off-stoichiometric case, a sharp interface (i) is achieved, and broadening under the photomask (ii) is diminished compared with the stoichiometric case, where broadening is already evident on the 5-µm-deep cross-section (iii) and significantly wider at 50 µm of depth (iv).