Materials 2021, 14, 6286. https://doi.org/10.3390/ma14216286 www.mdpi.com/journal/materials Article New Organophilic Montmorillonites with Lactic Acid Oligomers and Other Environmentally Friendly Compounds and Their Effect on Mechanical Properties of Polylactide (PLA) Katarzyna Rucińska 1, *, Zbigniew Florjańczyk 2 , Maciej Dębowski 2 , Tomasz Gołofit 2 and Rafał Malinowski 1, * 1 Łukasiewicz Research Network—Institute for Engineering of Polymer Materials and Dyes, M. Skłodowska-Curie 55, 87-100 Toruń, Poland 2 Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland; evala@ch.pw.edu.pl (Z.F.); maciej.debowski@pw.edu.pl (M.D.); tomgol@ch.pw.edu.pl (T.G.) * Correspondence: katarzyna.rucinska@impib.lukasiewicz.gov.pl (K.R.); rafal.malinowski@impib.lukasiewicz.gov.pl (R.M.); Tel.: +48-2272-36020 (K.R.); +48-5306-00220 (R.M.) Abstract: New organophilic montmorillonites with oligomers of lactic acid and other compounds such as citric acid, stearic acid, maleic anhydride, pentaerythritol and ε-caprolactone were synthe- sized. They were characterized by Fourier Transform Infrared Spectroscopy (FTIR), X-ray Diffrac- tion (XRD), Thermogravimetric Analysis (TGA), Scanning Electron Microscopy (SEM), elemental analysis and swelling capacity in water. In all tested composites, an increase in the montmorillonite interlayer distance resulting from intercalation of the modifying substance in the montmorillonite was found by means of XRD. Elemental analysis and FTIR showed that all of the tested samples contained an organic segment in the montmorillonite structure. TGA studies revealed that compo- sites modified with lactic acid oligomers, stearic acid or ε-caprolactone had the highest thermal sta- bility. They also exhibited the lowest swelling capacity which was 2–3 times lower than that for unmodified sodium montmorillonite. Some preliminary studies on the mechanical properties of PLA/modified montmorillonite are also presented and discussed. Keywords: montmorillonite; organophilization; lactic acid oligomers; intercalation; X-ray diffraction; PLA 1. Introduction Sodium montmorillonite (Mt) belongs to a group of natural aluminosilicates with a 2:1 layered structure [1]. It is the main component of bentonite, a clay mineral derived from the transformation of volcanic ash. The crystal structure of Mt consists of three layers connected by a common oxygen atom. Two outer layers with a tetrahedral silicon dioxide crystal structure and one internal layer consisting of octahedral crystals of magnesium oxide or aluminum oxide form a package with gap between them called the interlayer spacing or gallery [2]. Isomorphic substitution within the layer (for example Al 3+ replaced by Fe 2+ ) generates negative charges that are counterbalanced by sodium cations located in the gallery. The existence of interlayer cations gives them the ability to absorb water, as a way to increase the distance between the Mt layers. Montmorillonite is characterized by high water absorption, it can absorb five times more water than it weighs, simultaneously increasing its volume up to fifteen times [3,4]. The interlayer spacing can be modified by introducing different cations or molecules into their structure. These capabilities are uti- lized by introducing various inorganic cations between the Mt layers, which are used as catalytic layer precursors [5] or organic cations, most often derived from quaternary am- monium salts [6]. Citation: Rucińska, K.; Florjańczyk, Z.; Dębowski, M.; Gołofit, T.; Malinowski, R. New Organophilic Montmorillonites with Lactic Acid Oligomers and Other Environmentally Friendly Compounds and Their Effect on Mechanical Properties of Polylactide (PLA). Materials 2021, 14, 6286. https://doi.org/10.3390/ma14216286 Academic Editor: Teofil Jesionowski Received: 26 September 2021 Accepted: 18 October 2021 Published: 21 October 2021 Publisher’s Note: MDPI stays neu- tral with regard to jurisdictional claims in published maps and institu- tional affiliations. Copyright: © 2021 by the authors. Li- censee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and con- ditions of the Creative Commons At- tribution (CC BY) license (http://crea- tivecommons.org/licenses/by/4.0/).