VUV grafting: an efficient method for 3D bulk patterning of polymer sheets

Numerous fields such as electronics, biotechnology, optics, energy, etc. require polymer films with 2D or increasingly 3D structuration and with a combination of different physicochemical properties. For this purpose, methods usually used are based on: (i) phase separation of block copolymers which leads to polymer blend films with an isotropic structure or (ii) radiation-induced graft polymerization to obtain isotropic architecture by an electron beam or gamma-ray radiation or an anisotropic structure by swift heavy ion irradiation or by X-rays or Extreme UltraViolet (EUV) light radiation. However, these processes present some limitations such as block copolymer and film synthesis, drastic safety procedures or the high cost of ionising sources. To overcome these drawbacks, we propose here an innovative process, based on VUV irradiation, which proves efficient not only for surface modification, but also for the bulk modification of industrially relevant polymers such as beta-polyvinylidene fluoride (beta-PVDF), polyethylene (PE) or fluorinated ethylene propylene (FEP). Contrary to common expectations, we assume that VUV irradiation of a PVDF film can induce radical active species at depths up to 50 micrometers, as demonstrated by ESR. Those active species are able to initiate the radical polymerization of a vinylic or acrylate monomer as acrylic acid through the polymer film, as confirmed by the EDX profile of the film thickness. Similar results are obtained on PE and FEP films, while aromatic polymers such as PET strongly absorb VUV energy and dissipate it along other pathways. By mixing this process and photolithographic masks, 3D structuration of commercial polymer films is obtained.