polymers Article Laser-Induced Microstructuring of Polymers in Gaseous, Liquid and Supercritical Media Alexey Rybaltovskii 1,2 , Nikita Minaev 2, * , Svetlana Tsypina 2 , Svetlana Minaeva 2 and Vladimir Yusupov 2   Citation: Rybaltovskii, A.; Minaev, N.; Tsypina, S.; Minaeva, S.; Yusupov, V. Laser-Induced Microstructuring of Polymers in Gaseous, Liquid and Supercritical Media. Polymers 2021, 13, 3525. https://doi.org/10.3390/ polym13203525 Academic Editors: Xiaoli Zhang and Xia Liao Received: 10 September 2021 Accepted: 12 October 2021 Published: 13 October 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 D.V. Skobeltsyn Institute of Nuclear Physics, M.V. Lomonosov Moscow State University, Vorob’evy gory, 119991 Moscow, Russia; alex19422008@rambler.ru 2 Institute of Photon Technologies, Federal Scientific Research Centre ‘Crystallography and Photonics’, Russian Academy of Sciences, ul. Pionerskaya 2, Troitsk, 108840 Moscow, Russia; tsypina@yandex.ru (S.T.); minaeva.svetlana@gmail.com (S.M.); iouss@yandex.ru (V.Y.) * Correspondence: minaevn@gmail.com; Tel.: +7-915-053-21-03 Abstract: New results are presented for laser formation—in particular, the “drawing” of microstruc- tures in polymer films using continuous-wave (CW) laser radiation λ = 405 nm with an inten- sity of 0.8–3.7 kW/cm 2 . The laser drawing was carried out in the polymer system poly-2,2 ′ -p- oxydiphenylene-5,5 ′ -bis-benzimidazole (OPBI), which consists of two phases: a solid polymer matrix with formic acid (HCOOH) dissolved in it. The formation of microstructures, including the stage of foaming, was carried out in three media: air, water and a supercritical carbon dioxide medium containing dissolved molecules of the silver precursor Ag(hfac)COD. The morphological features of foam-like track structures formed in the near-surface layer of the polymer films by laser “drawing” are considered. A model of processes is presented that explains the appearance of periodic structures. The key point of this model is that it considers the participation of the photoinduced mechanism of explosive boiling of formic acid molecules dissolved in the polymer matrix. Using Raman spec- troscopy, spectra were obtained and interpreted, which relate to different stages in the formation of microstructures in OPBI films. The effects associated with the peculiarities of luminescent microstruc- tures on the surfaces of glasses in close contact with polymer films during laser “painting” in the air have been studied. Keywords: laser microstructuring; laser formation; polybenzimidazole; bubble structures; poly- mer film 1. Introduction Over the past three decades, the formation of various microstructures in polymeric materials has been one of the most demanding problems, since its solution largely dictates the progressive development of such areas as biomedical and chemical technologies, polymer microphotonics and nanoplasmonics [1–8]. In most of the reviews and regular articles presented above, various laser sources are used as a working tool, depending on the goals and materials used. For example, femtosecond laser radiation was used on the surfaces of polymers containing dye impurities [5] to obtain linear luminescent structures, which can be used to address problems associated with polymer microphotonics. In addition, CW CO 2 lasers are widely used to produce microstructured systems in polymer films [6] containing graphene, which is in demand in modern chemical technologies and microelectronics. In our works [9,10], the possibility of the formation of luminescent surface microstruc- tures in polymeric materials based on polybenzimidazoles, namely in films of poly-2,2 ′ - p-oxydiphenylene-5,5 ′ -bis-benzimidazole (OPBI), was considered for the first time. They had a unique combination of properties: high thermal and chemical resistance, as well as good mechanical strength [11]. It was shown that the thermal destruction of OPBI films Polymers 2021, 13, 3525. https://doi.org/10.3390/polym13203525 https://www.mdpi.com/journal/polymers