## 1 Introduction  
Due to its interesting properties, such as high mechanical strength, chemical resistance, high abrasion, spinnability, and low thermal expansion, polyamide 6 is one of the most important thermoplastic polymers, widely used as a constructive material [1; 2]. The main limitation of polyamide 6 (PA6) applications is its flammability. Among different methods to reduce PA6 flammability, the application of nanoparticles, such as layered silicates, has been reported as an efficient route toward flame-retarded polyamide composites [3]. The use of nanoparticles is also beneficial from the viewpoint of enhanced mechanical, thermal, and barrier properties [3; 4]. In order to improve the flame retardancy of polyamides, novel compounds are used in composite materials. [PERSON] et al. obtained composites of polyamide 6 and aluminum diisobutyl phosphinate (APBA), combined with organically modified layered montmorillonite (OMMT) by melt compounding. The results showed synergy between these two compounds in terms of flame retardancy; the highest value of limiting oxygen index (LOI)--36%, was obtained for the sample with a ratio for APBA and OMMT of 5:1 [3; 5]. [PERSON] et al. investigated the influence of caged bicyclic pentaerythnitol phosphate alcohol (PEPA) on the flame-retardant properties of PA6. The results showed that the LOI values of composites increased with increasing PEPA content [6]. Other compounds which have been recently investigated to reduce the flammability of PA6 were phosphorous-nitrogen flame-retardant and platelet-shaped hexagonal boron nitride. The incorporation of those compounds into polyamide allows the application of the final product in electronic and electrical devices [7]. Moreover, char sulfonic acid and ammonium polyphosphate, metal oxalates, brominated polystyrene/antimony trioxide, or red phosphorus/magnesium hydroxide systems have been applied to reduce the flammability of polyamides [8; 9; 10; 11; 12; 13]. The layer-by-layer (LbL) deposition technique offers a direct way to form flame-retardant coatings on the polymers' surface [14; 15]. LbL involves applying layers via the immersion of the substrate, alternately in solutions containing positively and negatively charged moieties. The absorption of layers occurs mainly through electrostatic interactions, though other interactions, i.e., the formation of covalent bonds, hydrogen bonding, donor/acceptor interactions, and the formation of stereo-complexes are also possible [16; 17]. Considering the traditional approach to incorporating flame retardants in polymer matrices, the application of the LbL technique may prove advantageous. Firstly, it diminishes problems related to the introduction of flame retardants into the matrix. Secondly, as the flame affects the exterior part of the substrate, the presence of layers containing flame retardants may directly impact the combustion process [16]. Recent reports on PA6 flame retardancy improvements using LbL include the coating of polyamide with poly(allylamine) and montmorillonite [15], poly(allylamine) with sodium polyphosphate and titanium(IV) oxide [18; 19], anionic sodium phosphate and cationic polysiloxanes [20], chitosan, phytic acid and oxidized sodium alginate [21] with additional cross-linking between the layers by the introduction of sodium tetraborate [22] or chitosan and montmorillonite [23]. In other work [24], membranes with a polyamide thin-film active layer were used in reverse osmosis-based water desalination applications. The incorporation of graphene oxide nanoplatelets in the polyamide layer can alter the surface characteristics, permeability, selectivity, and can enhance the chlorine resistance of these membranes. Polyamide layers with graphene oxide nanoplatelets were synthesized in various sequences. The incorporation of graphene oxide nanoplatelets resulted in an increase in surface hydrophilicity as captured by the change of the water contact angle. Water flux and salt rejection properties of the synthesized membranes have been investigated by using a dead-end cell.