Supercritical CO2-assisted impregnation of PLA films with R-carvone. I: Effect of processing on thermal and crystallinity properties

MIRANDA VILLA, PATRICIA P.; GAÑAN, NICOLÁS A. ; MARTINI, RAQUEL E.; GOÑI, MARÍA L.

Los Cocos

Congreso; VI Iberoamerican Conference on Supercritical Fluids (PROSCIBA 2023); 2023

IPQA-UNC-CONICET; PLAPIQUI-UNS-CONICET

Polylactic acid (PLA) is a biodegradable and biocompatible material that has become of great interest for biomedical, pharmaceutical, and food packaging applications. The incorporation of active compounds enhances its attractivity, due to it can be used as a release device for varied applications such as drug delivery or food preservation. In this context, the use of supercritical impregnation technology for the development of this kind of material appears as a good strategy. Supercritical CO2 (scCO2) treatment of polymers can promote changes in the polymer structure and/or morphology, due to swelling and plasticizing effects induced by scCO2. These changes depend on the compound-polymer system and also on the operating conditions and can affect not only the amount of active compound that can be incorporated but also the release kinetics and the final properties of the polymer. In this sense, in the framework of a project concerning the development of PLA-based active materials, the supercritical impregnation of PLA films with carvone, a naturally occurring compound with antimicrobial and insecticide activity, was investigated. The influence of process variables on the impregnation yield and release kinetics was previously studied and reported [1]. In this work, a systematic study of the effect of the supercritical processing and the incorporation of R-(–)-carvone on the thermal behavior and crystallinity properties of PLA films was assessed. Commercial films of PLA were kindly provided by Converflex (ARCOR Group, Argentina). R-(–)-carvone (98% purity) was purchased from Sigma-Aldrich (Germany), and industrial extra-dry carbon dioxide (water content ≤ 10 ppm v/v) was from Linde (Argentina). The supercritical processing of PLA was performed in batch mode, using a lab-scale high-pressure system described before [1], and three variables were evaluated: scCO2 density (278–683 kg m-3), temperature (40–60 °C), and depressurization rate (0.6–6.0 MPa min-1). All runs were performed in duplicate, with a contact time of 2 h and a carvone/PLA mass ratio of 0 or 0.5.Thermal behavior was assessed by differential scanning calorimetry (DSC) using a Discovery DSC 250 equipment (TA Instruments, USA), heating film samples from -10 °C to 200 °C at 10 °C min-1. Main transition phenomena were observed, recording the glass transition (Tg), cold crystallization (Tcc), and melting (Tm) temperatures, as well as the transition enthalpies (∆H_cc,∆H_m). The crystallinity degree (XC%) was also calculated [1]. Neat films were nearly amorphous (XC% = 3.5 %) and showed the typical behavior of PLA (Tg ~58°C, Tcc ~102°C, and a Tm ~149 °C). Treated films with and without the addition of carvone showed an increase in the XC% that varied from 23 to 32 %, depending on the process conditions. This increase in the crystalline region can be explained by the plasticizing effect of scCO2, which promotes polymer chain mobility, allowing their rearrangement into different crystalline structures. Furthermore, a decrease in the cold crystallization phenomenon was also observed for all treated samples, with values of ∆H_cc in a range of 1–3 J/g, compared to a value of 19 J/g for neat samples. These results are in accordance with the increase in XC%, mainly due to polymer chains that are able to crystallize in neat PLA during the DSC heating, in treated samples are mainly reorganized during the supercritical process and only a small fraction is rearranged.Finally, impregnated samples showed a dependency of some transition temperatures with the content of carvone, which varied from ~6 to ~30 wt% [1]. In particular, a decrease in Tm and Tg was observed as the amount of carvone incorporated in the films increased, which is related to the compatibility and interaction between carvone and PLA, and this behavior may be described by thermodynamic models. On one hand, Flory-Huggings relation [2] showed a good prediction of the melting point depression with the increase of carvone content, observing a less pronounced decrease of Tm for carvone contents higher than 18 wt%. On the other hand, Fox equation [3] was used to model the Tg variation. In this case, the decrease was only observed for small amounts of carvone content (6–10 wt%), being Tg nearly constant for higher values (18–30 wt%). Therefore, experimental data for Tg behavior was only described by the Fox equation for carvone contents up to 10 wt%. Taking into account that the Fox equation describes the behavior of miscible blends, and the observed behavior of the melting point depression, these results may suggest that the miscibility limit of carvone in PLA was reached at around 10–18 wt%.