**1. Introduction**

Electrospinning is a widely used technique to produce polymer fibers with different diameters and morphology. The nonwovens can be obtained via melt electrospinning or solution electrospinning [1]. The solution electrospinning does not require high temperature. Diameters and morphology of the electrospun fibers are determined by the type and molar mass of polymers, dispersity, and processing parameters, including applied voltage, tip-collector distance, type of solvent, feed rate, solution concentration, viscosity and conductivity. A wide range of electrospun polymer nonwovens has been obtained and studied for application in diverse fields: air [2] and water filtration [3], food packaging [4], tissue engineering [5] and reconstructive medicine [6], drug delivery systems [7], and antibacterial materials [8,9]. The drawback hindering the practical use of electrospun mats in some applications, for instance, filtration, is their poor tensile strength, as compared to woven fabrics. Fibers in nonwovens are loosely packed and weakly connected. The improvement of the mechanical performance of nonwovens can be achieved by modification of the fibers. Owing to their outstanding properties, including extremely high tensile strength and elastic modulus [10–12], carbon nanotubes (CNT) are attractive nanofillers for polymers [13]. The enhancement of properties of electrospun nonwovens by the introduction of CNT to electrospun polymer solutions was reported for different polymers, including poly(ethylene terephthalate), poly(vinylpyrrolidone), polyacrylonitrile/poly(vinyl chloride) blend, polyamide 6, and polylactide (PLA), among others [3,14–21]. Modification of electrospun nonwovens can be also achieved by incorporating other nanoparticles, for example, graphene oxide [14], cellulose nanocrystals [22], or hydroxyapatite nanoparticles [23].

PLA, a biodegradable polyester having good mechanical properties, is the most promising biobased polymer for replacement of conventional thermoplastics [24]. PLA's ability to crystallize depends on its enantiomeric composition; with the increasing content of units of different chirality, the ability to crystallize worsens. Slowly crystallizing PLAs can be cooled to the glassy state and crystallized during subsequent heating via

"cold crystallization". The effective way to accelerate the crystallization of PLA is the addition of nucleating agents [25]. Additionally, shearing of PLA melt can enhance its crystallization [26,27]. To broaden its applications, PLA's properties are modified by various routes, including copolymerization, chain extension, plasticization, blending with other polymers, fillers, nanofillers, and fibers [28–37]. PLA modification with oligomeric linear ladder poly(silsesquioxane)s (LPSQ-R) with different sides substituents, including methoxycarbonyl (R = COOMe) groups was described recently by Herc et al. [38]. LPSQ-R are well-defined macromolecules with a double chain siloxane backbone, which makes them more rigid than typical polysiloxanes and limits their coiling in solutions. Interestingly, a significant increase of elongation at break was achieved for PLA blends with 5 wt.% of LPSQ-COOMe, with only a minor decrease of the yield strength, as compared to neat PLA. LPSQ-COOMe described in this report is a viscous amorphous substance of good thermal stability, well soluble in organic solvents. Herc et al. [39] recently demonstrated that LPSQ-COOMe enhanced the thermal stability of PLA.

Electrospinning allowed to obtain PLA fibers with diameters in the nanometer [40,41] and micrometer range [42–44]. To obtain modified PLA nonwovens, electrospinning of the polymer solutions with added plasticizers, nanofillers, or antibacterial substances, among others, was carried out, although post-electrospinning modification of fibers was also reported. For example, antibacterial activity of PLA nonwovens was achieved by introducing antibacterial agents to the solutions [8,9,42]. Plasticization of PLA/poly(hydroxybutyrate) blend nonwovens with oligomeric lactic acid (OLA) and acetyl(tributyl citrate) was described by Arriata et al. [45,46]. Leones et al. [47] used OLA to plasticize PLA nonwovens. In turn, the improvement of tensile strength of electrospun PLA mats was achieved by addition of nanofillers; for example, by cellulose nanocrystals [48], hydroxyapatite nanorods and graphene oxide [49]. PLA composite nonwovens with 5 wt.% of siliceous sponge spicules exhibited a tensile strength four times higher than that of neat PLA material [50].

The addition of nanocarbon materials to electrospun PLA solutions can result in an improvement of the tensile strength and elastic modulus [14,47,48,51]. A fourfold increase of the tensile strength and elastic modulus was reported for PLA nonwovens with 3 wt.% of multiwall CNT (MWCNT) relative to those of neat PLA nonwovens, although accompanied by a significant decrease of elongation at break [48]. Similar enhancement of the tensile strength of PLA nonwovens was achieved through the addition of 1 wt.% of functionalized MWCNT [51].

In this study, LPSQ-COOMe and MWCNT were used for modification of poly(Llactide) (PLLA) electrospun nonwovens. PLLA grafted on MWCNT was prepared and used for electrospinning; MWCNT content was 0.1 wt.%. For comparison, nonwovens of commercial PLLA with the same content of MWCNT were also obtained. In addition, fibers of PLLA with 5 and 10 wt.% of LPSQ-COOMe were prepared, and also fibers with the same contents of LPSQ-COOMe and 0.1 wt.% of MWCNT with respect to PLLA. LPSQ-COOMe is not biodegradable, but like other silsesquioxanes [52] it is non-toxic and biocompatible. The influence of the additives on the morphology and diameters of the nonwovens as well as on their thermal and mechanical properties was studied. It was demonstrated that the modification with a small amount of MWCNT and 5–10 wt.% of LPSQ-COOMe significantly improved the tensile strength of the nonwovens. It is also worth noting that LPSQ-COOMe also improved thermal stability of the studied nonwovens.

## **2. Materials and Methods**
