Polylactic acid/carbon nanoparticle composite filaments for sensing

applied sciences Article Polylactic Acid/Carbon Nanoparticle Composite Filaments for Sensing Mariana M. Silva 1, Paulo E. Lopes 1, Yilong Li 2,† , Petra Pötschke 2, Fernando N. Ferreira 3 and Maria C. Paiva 1,*   Citation: Silva, M.M.; Lopes, P.E.; Li, Y.; Pötschke, P.; Ferreira, F.N.; Paiva, M.C. Polylactic Acid/Carbon Nanoparticle Composite Filaments for Sensing. Appl. Sci. 2021,11, 2580. https://doi.org/10.3390/ app11062580 Academic Editor: Alessandro Pegoretti Received: 21 February 2021 Accepted: 12 March 2021 Published: 15 March 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. 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/). 1Institute for Polymers and Composites, University of Minho, 4804-533 Guimarães, Portugal; [email protected] (M.M.S.); [email protected] (P.E.L.) 2Leibniz Institute of Polymer Research Dresden (IPF), Hohe Str. 6, 01069 Dresden, Germany; [email protected] (Y.L.); [email protected] (P.P.) 3Centre for Textile Science and Technology, University of Minho, 4804-533 Guimarães, Portugal; [email protected] *Correspondence: [email protected] † Present Address: College of Material Science and Engineering, Henan University of Technology, Lianhua Str. 100, Zhengzhou 450001, China. Featured Application: Wearable devices. Abstract: Polylactic acid (PLA) is a bio-based, biodegradable polymer that presents high potential for biomedical and sensing applications. Ongoing works reported in the literature concern mainly applications based on 3D printing, while textile applications are hindered by the limited flexibility of PLA and its composite filaments. In the present work, PLA/multiwall carbon nanotube (MWCNT) composite filaments were produced with enhanced flexibility and electrical conductivity, which may be applied on a textile structure. A biodegradable plasticizer was incorporated in the nanocomposites, aiming at improving MWCNT dispersion and increasing the flexibility of the filaments. Filaments were produced with a range of compositions and their morphology was characterized as well as their thermal, thermomechanical, and electrical properties. Selected compositions were tested for sensing activity using saturated acetone vapor, demonstrating a suitable response and potential for the application in fabrics with sensing capacity. Keywords: polylactic acid; multiwall carbon nanotubes; poly(propylene) glycol; vapor sensing 1. Introduction Poly(lactic acid) (PLA) is a biocompatible, biodegradable, melt-processable polymer with good mechanical properties, wicking, breathability, and ability for adsorption of organic compounds, which has been extensively researched as a replacement for noncompostable polymers. PLA and PLA composites have been developed for applications in garment, medical textiles, and disposables [ 1 , 2 ]. Other potential application areas include wearable devices, patch sensors for motion [ 3 ], 3D-printed lithium-ion batteries [ 4 ], flexible pressure sensors [ 5 ], functional coatings for textiles [ 6 ], and piezoresistive sensors [ 7 ]. The brittleness and low mechanical properties of PLA are, however, a limitation for application in wearables, motivating research on plasticization [ 8 ] and reinforcement with nanoparticles [9] and seeking enhanced mechanical performance and flexibility. A plasticizer for PLA should be nontoxic, biocompatible, and should present good miscibility with the polymer, leading to a decrease in the glass transition temperature (T g ) [ 10 ]. A major problem concerns the immiscibility and tendency for migration of plasticizers to the surface of PLA with consequent loss. This effect is most important for low molecular weight (M W ) plasticizers such as lactide monomer and ester-like plasticizers and thus the preference for higher M W plasticizers. Poly(ethylene) glycol (PEG) and Appl. Sci. 2021,11, 2580. https://doi.org/10.3390/app11062580 https://www.mdpi.com/journal/applsci Appl. Sci. 2021,11, 2580 2 of 17 poly(propylene) glycol (PPG) are available in a range of Mw values and are attractive as plasticizers [ 11 ]. PPG is a viscous liquid with low T g ( − 60 ◦ C to − 75 ◦ C) unable to crystallize [ 12 ]. Studies of the miscibility of PPG with amorphous PLA were first performed by Kulinski et al. [ 12 ], comparing the effect of different concentrations of PPG with M W of 425 g · mol −1 and PPG with M W of 1000 g · mol −1 . It was concluded that the ability for plastic deformation of PLA was increased for lower M W PPG, increasing with PPG content, with phase separation occurring only for the incorporation of 12.5 wt.% of PPG with higher M W . Semicrystalline PLA was more effectively plasticized by PPG as compared to PEG with similar MW[13]. Plasticizers can benefit PLA nanocomposites by increasing their processability, aiding the dispersion of fillers, and improving their mechanical properties [ 14 – 16 ]. Table 1shows a summary of three-component PLA composites described in the literature. Table 1. Polylactic acid (PLA) composite systems with different plasticizers and fillers. Plasticizer Filler Processing Method Reference Tributyl 2-acetylcitrate Bi0·5Sb1·5Te3 MWCNT Melt extrusion [17] PEG (MW200 g·mol−1)Reduced graphene oxide Batch mixing [18] Epoxidized soybean oil MWCNT Solvent casting [14] PEG (MW950–1050 g·mol−1) Acid treated MWCNT Two-roll mill [19] PEG (MW200 g·mol−1) Graphene nanoplatelets Batch mixing and compression molding [20] PEG (MW2000 g·mol−1)SWCNT Melt extrusion and compression molding [15] Triacetin Flax fiber Melt extrusion [21] Poly(butylene adipate-co-terephthalate) Rami fiber Melt extrusion and compression molding [22] Epoxidized linseed oil Hazelnut shell flour Melt extrusion [16] PEG (MW6000 g·mol−1) CaCO3Halloysite nanotubes LAK 301 Micro-sized talc Melt extrusion [23] Glycerol Bamboo (different forms) Batch mixing [24] PEG (MW1500 g·mol−1) ZnO Clove essential oil Solvent cast [25] Acetyl triethyl citrate Montmorillonite layered silicate Melt extrusion [26] Diglycerine tetraacetate and PEG (MW1000 g·mol−1) Sodium montmorillonite and organoclays Milling and injection molding [27] PEG (MW6000 g·mol−1)Montmorillonite Melt extrusion [28] PEG (MW1000 g·mol−1)Montmorillonite Batch mixing and compression molding [29] PEG (MW6000 g·mol−1)Organoclay Melt extrusion and microinjection molding [30] PEG (M W 400 g · mol −1 ) and Tributyl Citrate Nanosized CaCO3Melt extrusion [31] Glycerol Halloysite nanoclay Solvent casting [32] Methoxy polyethylene glycol Silica Batch mixing [33] Appl. Sci. 2021,11, 2580 3 of 17 PLA nanocomposites produced with carbon nanotubes (CNT) showed improved dispersion of CNT upon addition of a plasticizer, enabling the preparation of electrically conductive, flexible multifunctional nanostructured composites with lower T g compared to PLA, finding applications as shape-memory polymers [14,15]. In the present work, we report the melt extrusion of PLA with multiwall CNT (MWCNT), comparing composites with the neat polymer and PPG as a plasticizer. The influence of PPG on the dispersion of MWCNT in PLA and its plasticizing effect is studied. Electrical conductivity and sensing ability under acetone vapor are studied for composites with different MWCNT content. Filaments were produced with different diameters foreseeing an application in textile wearable devices. 2. Materials and Methods 2.1. MWCNT Dispersion in PPG MWCNT (NC 7000, Nanocyl, Sambreville, Belgium) were dispersed in ethanol (EtOH) in an ultrasonic bath for 5 min. The dispersion was added to diluted PPG (M W 425 g · mol −1 , Sigma-Aldrich, Sintra, Portugal) in EtOH (1:20 v/v) and stirred for 1 h 30 min with a homogenizer IKA ® ULTRA-TURRAX ® T 18, with a dispersing element S 18 N 10 G (IKA ® - Werke GmbH & Co. KG, Staufen, Germany). The resulting suspensions were then used to coat the PLA pellets. 2.2. Coating of PLA Pellets Semicrystalline PLA pellets (Luminy ® L175, Total Corbion PLA, 4203 NS Gorinchem, The Netherlands) were coated with MWCNT dispersions in PPG by stirring with a common kitchen mixer until EtOH evaporation. Coated pellets were further dried for 2 h at 60 ◦ C and then for 8 h at 100 ◦C prior to melt extrusion. 2.3. Melt Extrusion of PLA/MWCNT/PPG Composites Melt compounding of PLA nanocomposites was carried out in a co-rotating intermeshing twin-screw extruder (Rondol Microlab, France) with 10 mm diameter screws composed of three mixing sections (Figure 1) and a length-to-diameter ratio (L/D) of 25. A temperature profile was defined for the feeding zone (FZ), the three different zones in the barrel (Z1, Z2, and Z3) and die, as follows: 120 ◦ C/160 ◦ C/175 ◦ C/180 ◦ C/175 ◦ C. A volumetric feeder (Piovan MDP1, Italy) supplied coated and noncoated PLA pellets at FZ. For composites without prior coating with PPG and MWCNT, MWCNTs were added manually at FZ. The pellets were dried for 2 h at 60 ◦ C and then for 8 h at 100 ◦ C before processing. The extruded rod presented a diameter of approximately 1.5 mm after the die exit. A filament puller with controlled speed was used to collect the extruded filament and set the diameter at approximately 0.5 mm. Filaments were collected with larger diameter (collected directly from the die exit) and with smaller diameter (collected with diameter control). Appl. Sci. 2021, 11, x FOR PEER REVIEW 4 of 17 Figure 1. Extrusion layout: the screws are composed of an initial transport zone at feeding zone (FZ), followed by the first set of mixing blocks at the beginning of Z1, a transport zone, plus an intensive mixing zone at Z2; at Z3, there is a small mixing zone in between transport ones. Table 2. Melt-extruded PLA/multiwall carbon nanotube (MWCNT)/ poly(propylene) glycol (PPG) composites. Sample Designation Composition Average Diameter (mm) PPG (wt.%) MWCNT (wt.%) PLA 0 0 1.17 PLA/20PPG 20 0 0.75 PLA/2MW/20PPG 20 2 1.64 PLA/3MW/20PPG 20 3 1.69 PLA/2MW_s * 0 2 0.50 PLA/3MW_s * 0 3 0.54 PLA/2MW/20PPG_s * 20 2 0.44 PLA/3MW/20PPG_s * 20 3 0.39 * The “s” indicates small-diameter filaments. 2.4. Characterization of PLA composite Filaments 2.4.1. Optical Microscopy Optical microscopy (OM) images were obtained with a Leica DM2500 PLA microscope equipped with a digital camera Leica DMC2900. Transversal sections of 1.5–2 µm thickness were cut with an ultramicrotome (Leica UC6). LAS X software was used to record the images and ImageJ software to analyze total and particle areas. Contrast differences were employed and the agglomerate areas larger than 5 µm2 were measured. The analysis was performed on micrographs collected from at least three different sections per composite. 2.4.2. Scanning Electron Microscopy Scanning electron microscopy (SEM) images were obtained with a Nano SEM–FEI Nova 200 (FEG/SEM) (FEI Europe Company, Hillsboro, OR, USA) at high vacuum (resolution of 1.0 nm at 15 kV). Samples were cryofractured in liquid nitrogen, and the resulting surfaces were coated with Au/Pd to observe the transversal section of filaments. The images were obtained in secondary electron mode. 2.4.3. Thermogravimetric Analysis Thermogravimetric analysis (TGA) measurements were performed on a TGA Q500 from T.A. Instruments® (New Castle, DE, USA) (weighting precision of +/− 0.01%, 0.1 µg Figure 1. Extrusion layout: the screws are composed of an initial transport zone at feeding zone (FZ), followed by the first set of mixing blocks at the beginning of Z1, a transport zone, plus an intensive mixing zone at Z2; at Z3, there is a small mixing zone in between transport ones. Appl. Sci. 2021,11, 2580 4 of 17 The compositions prepared are listed in Table 2. Table 2. Melt-extruded PLA/multiwall carbon nanotube (MWCNT)/ poly(propylene) glycol (PPG) composites. Sample Designation Composition Average Diameter (mm) PPG (wt.%) MWCNT (wt.%) PLA 0 0 1.17 PLA/20PPG 20 0 0.75 PLA/2MW/20PPG 20 2 1.64 PLA/3MW/20PPG 20 3 1.69 PLA/2MW_s * 0 2 0.50 PLA/3MW_s * 0 3 0.54 PLA/2MW/20PPG_s * 20 2 0.44 PLA/3MW/20PPG_s * 20 3 0.39 * The “s” indicates small-diameter filaments. 2.4. Characterization of PLA Composite Filaments 2.4.1. Optical Microscopy Optical microscopy (OM) images were obtained with a Leica DM2500 PLA microscope equipped with a digital camera Leica DMC2900. Transversal sections of 1.5–2 µ m thickness were cut with an ultramicrotome (Leica UC6). LAS X software was used to record the images and ImageJ software to analyze total and particle areas. Contrast differences were employed and the agglomerate areas larger than 5 µ m 2 were measured. The analysis was performed on micrographs collected from at least three different sections per composite. 2.4.2. Scanning Electron Microscopy Scanning electron microscopy (SEM) images were obtained with a Nano SEM–FEI Nova 200 (FEG/SEM) (FEI Europe Company, Hillsboro, OR, USA) at high vacuum (resolution of 1.0 nm at 15 kV). Samples were cryofractured in liquid nitrogen, and the resulting surfaces were coated with Au/Pd to observe the transversal section of filaments. The images were obtained in secondary electron mode. 2.4.3. Thermogravimetric Analysis Thermogravimetric analysis (TGA) measurements were performed on a TGA Q500 from T.A. Instruments ® (New Castle, DE, USA) (weighting precision of +/ − 0.01%, 0.1 µ g sensitivity) at a heating rate of 50 ◦ C · min −1 from 40 ◦ C to 500 ◦ C followed by 10 ◦ C · min −1 from 500 ◦C to 700 ◦C, under N2atmosphere in a platinum crucible. 2.4.4. Differential Scanning Calorimetry An ultra-microbalance PERKIN-ELMER AD-4 Autobalance was used to weigh the samples in Al crucibles (Ø 6 mm, Netzsch-Gerätebau GmbH, Selb, Germany). Differential scanning calorimetry (DSC) scans were performed in a Netzsch DSC 200 F3 Maia ® (Netzsch-Gerätebau GmbH, Selb, Germany), equipped with a cooling system Netzsch Intracooler 40 (Netzsch-Gerätebau GmbH, Selb, Germany) at a heating rate of 10 ◦ C · min −1 , from − 10 ◦ C to 190 ◦ C, under nitrogen atmosphere. An error of 0.5 ◦ C is associated with all temperature measurements. Peak areas and relevant transition temperatures were determined with Netzsch Proteus®software. 2.4.5. Raman Spectroscopy Raman spectra were obtained using a HORIBA LabRAM HR Evolution confocal Raman microscope spectrometer (Horiba Scientific, Piscataway, NJ, USA) using Horiba Scientific’s Labspec 6 Spectroscopy Suite Software for instrument control, data acquisition, and processing. The Raman spectrometer was equipped with a 532 nm laser, a Syncerity CCD Camera detector (Horiba) and a microscope equipped with a 10 × and 100 × magnifi- Appl. Sci. 2021,11, 2580 5 of 17 cation objectives. A grating of 600 gr · mm −1 was used. For each composite, three spectra were collected from distinct zones of the filament and averaged, and peak positions were determined by applying a baseline (in LabSpec 6) and fitting with a Lorentzian function (in OriginPro 2016). 2.4.6. Dynamic Mechanical Analysis Dynamic mechanical analysis (DMA) was carried out in a TT DMA (Triton Technology, Grantham, UK) equipped with a 1L Dewar and a TTDMA AutoCryo Type 2 for cooling with liquid nitrogen. The analysis was performed in tension mode with a 10 mm gauge length. The measurements were carried out at a frequency of 1 Hz with a displacement amplitude of 0.001 mm, from − 50 ◦ C to 120 ◦ C at a heating rate of 2 ◦ C · min −1 . Storage and loss moduli (E 0 and E”, respectively) reported at 0 ◦ C were obtained by computing a linear fit in the temperature range from −40 C to 10 ◦C. 2.4.7. Resistivity Measurements Electrical resistivity measurements were carried out on a Keithley SMU 2635B SourceMeter ® (Keithley Instruments Inc., Cleveland, OH, USA). Intensity/voltage (I/V) curves were acquired under direct current (DC) on a source range of 20 V (from − 10 V to 10 V with 0.5 V steps) at room temperature. A linear fit was applied, in Microsoft Excel, to calculate the resistance (R) as the inverse of the I/V curve slope. Bulk conductivity ( σDC ) was calculated according to Equation (1), where L is the length of the conductive path (distance between electrodes) and A is the sample cross-section area. σDC = L A×R(1) 2.4.8. Vapor Sensing Measurements The vapor sensing ability of the composite filaments was investigated by in situ monitoring of their electrical resistance response upon alternating exposure to acetone vapor and dry air for four successive cycles. The exposure time to organic vapor and the drying time in dry air were 100 s and 150 s, respectively. Prior to the exposure to the saturated acetone vapor, the filaments were set in the measuring chamber under dry-air flow for 50 s to reach a stable initial resistance (R i ). The filaments for the vapor sensing test were coated with silver ink at the end sides to ensure good contact between the specimen and the electrodes. All samples were tested at 25 ◦ C. The set-up of the sensing device is a self-made equipment described in previous works [ 34 ]. The sample resistance R was measured using a Keithley 2001 digital multimeter (Keithley Instruments Inc., Cleveland, OH, USA), and the experimental results are presented as relative resistance (R rel ). R rel is the ratio of the resistance variation ( ∆ R = R − R i ) to the initial resistance. A bubbler evaporation system was used to deliver the desired organic vapor to the detection chamber using dry air as carrier. In the drying run, pure dry air was passed through the chamber. The total gas flow rate was set to 30 L·h−1using a mass flow controller. 2.5. Statistical Analysis and Data Visualization Statistics and data visualization were performed in RStudio (1.3.959); dplyr [ 35 ], tidyverse [ 36 ], and plyr [ 37 ] packages were used to treat data, and plots were created with ggplot2 [38], cowplot [39], and ggridges [40] packages. 3. Results 3.1. Morphology of Melt-Extruded Filaments OM images of the composite filament cross sections with different loads of MWCNT, with and without PPG, are displayed in Figure 2. OM analysis provides a 2D description of the MWCNT fraction that did not disperse in the polymer during melt mixing, remaining as agglomerates. The micrographs show that filaments with 3 wt.% MWCNT present a larger number of agglomerates compared to 2 wt.% MWCNT. They also illustrate the effect Appl. Sci. 2021,11, 2580 6 of 17 of the predispersion of MWCNT in PPG, resulting in improved dispersion of MWCNT with consequent decrease in the number of agglomerates present as well as formation of smaller agglomerates. Appl. Sci. 2021, 11, x FOR PEER REVIEW 7 of 17 Figure 2. Optical microscopy (OM) (transmission mode) images of filament composites with different diameters, filler load, and PPG wt.%. Table 3. Quartiles (Median, Q1, and Q3) of the areas measured per sample, area ratio, and count of agglomerates per mm2. Sample Median (µm2) Q1 (0.25) (µm2) Q3 (0.75) (µm2) Area Ratio (%) Number of Agglomerates (mm−2) PLA/2MWCNT_s 18.05 8.25 39.96 1.74 356 PLA/3MWCNT_s 23.30 10.83 53.60 5.83 789 PLA/2MWCNT/20PPG_s 6.44 5.52 7.91 1.38 303 PLA/3MWCNT/20PPG_s 6.91 5.73 9.42 1.93 648 PLA/2MWCNT/20PPG 6.91 5.02 8.80 0.37 178 PLA/3MWCNT/20PPG 6.82 5.68 9.09 1.88 336 Figure 2. Optical microscopy (OM) (transmission mode) images of filament composites with different diameters, filler load, and PPG wt.%. Statistical analysis confirms that melt-extruded filaments of neat PLA and MWCNT present agglomerates with larger area (Table 3) with medians ranging from 18 to 23 µ m 2 . Nanocomposites incorporating PPG present agglomerates with smaller median areas, within a narrow range from 6 to 7 µ m 2 . OM images depict prominent agglomerates of MWCNT for composites without PPG and smaller MWCNT agglomerates for composites prepared with PPG. Appl. Sci. 2021,11, 2580 7 of 17 Table 3. Quartiles (Median, Q1 , and Q3 ) of the areas measured per sample, area ratio, and count of agglomerates per mm 2 . Sample Median (µm2) Q1(0.25) (µm2) Q3(0.75) (µm2)Area Ratio (%) Number of Agglomerates (mm−2) PLA/2MWCNT_s 18.05 8.25 39.96 1.74 356 PLA/3MWCNT_s 23.30 10.83 53.60 5.83 789 PLA/2MWCNT/20PPG_s 6.44 5.52 7.91 1.38 303 PLA/3MWCNT/20PPG_s 6.91 5.73 9.42 1.93 648 PLA/2MWCNT/20PPG 6.91 5.02 8.80 0.37 178 PLA/3MWCNT/20PPG 6.82 5.68 9.09 1.88 336 Density plots (Figure 3) and the cumulative distribution (Figure S1) of agglomerate areas show that composites with PPG present smaller agglomerates with a narrower area range (75% of agglomerates < 10 µ m 2 , Table 3), whereas composites without PPG present a wider dispersion in agglomerate areas. A Kruskal–Wallis statistic for alpha = 5% shows that both the addition of PPG (for H (1) = 5.39, p-value = 0.02) and MWCNT concentration (for H (1) = 7.79, p-value = 0.005) have a significant effect in the agglomerate area ratio. It may be concluded that (i) the agglomerate area ratio is significantly lower for composites containing PPG and that (ii) increasing MWCNT concentration increases the agglomerates area ratio. The latter conclusion is expected and reported in Table 3. Comparison of filaments with similar composition shows that the composites without PPG display the highest agglomerate area ratio values, with a larger area covered by MWCNT agglomerates.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     PLA/2MWCNT_s PLA/2MWCNT/20PPG PLA/2MWCNT/20PPG_s PLA/3MWCNT_s PLA/3MWCNT/20PPG PLA/3MWCNT/20PPG_s 0 50 100 150 Area (µm2) Figure 3. Density plot of MWCNT agglomerate areas. Red lines indicate the quartiles. SEM images of the cross sections of melt-extruded composite filaments (Figure 4) depict a homogenous matrix, indicating good miscibility of PPG in PLA at the prepared compositions. The dispersed fraction of MWCNT is well distributed across the composite. Appl. Sci. 2021,11, 2580 8 of 17 Appl. Sci. 2021, 11, x FOR PEER REVIEW 8 of 17 Figure 3. Density plot of MWCNT agglomerate areas. Red lines indicate the quartiles. Figure 4. SEM images (secondary electron mode) of composite filaments. Magnification indicated by the scale bar. Figure 4. SEM images (secondary electron mode) of composite filaments. Magnification indicated by the scale bar. 3.2. Raman Spectroscopy Raman spectra of neat-PLA were recorded and averaged, and the spectrum is presented in the supplementary materials (Figure S2) with highlighted bands corresponding to the vibrational modes of each bond. Bands of the vibrational modes of C = O, C–O–C, and C–H bonds are common to the Raman spectra of all composites. The asymmetrical stretch of CH 3 bond, with a Raman shift ~ 1450 cm −1 , is observed between the D and G bands of MWCNT (~ 1338 cm −1 and ~ 1577 cm −1 , respectively) as presented in Figure 5. The slight upshift of D, G, and 2D (~ 2672 cm −1 ) bands for the composites relative to the MWCNT in powder form (Figure 5) may be attributed to the dispersion and disentanglement of MWCNT and a slight compression effect of the surrounding polymer [41,42]. Appl. Sci. 2021,11, 2580 9 of 17 PLA PLA/2MWCNT_s 1000 2000 3000 Raman shift (cm-1) Intensity (a.u.) MWCNT PLA/2MWCNT/20PPG PLA/2MWCNT/20PPG_s PLA/3MWCNT_s PLA/3MWCNT/20PPG PLA/3MWCNT/20PPG_s Figure 5. Raman spectra of neat PLA, composites, and pristine MWCNT. Dashed lines indicate MWCNT’s D, G, and 2D band positions. 3.3. Thermal Characterization The thermal characterization of the extruded filaments was performed by TGA and DSC analysis. Thermal degradation curves, and the corresponding derivatives (DTG), of PLA and composites are presented in Figure 6. Neat PLA displays one degradation step with first derivative peak maximum occurring at 394 ◦ C. Two weight loss steps are observed for PLA/20PPG: one with a first derivative peak maximum at 255 ◦ C that may be attributed to PPG vaporization and the second near 396 ◦ C, corresponding to PLA degradation ( Figure 6 , Table 4). Appl. Sci. 2021,11, 2580 16 of 17 10. Mascia, L.; Xanthos, M. An overview of additives and modifiers for polymer blends: Facts, deductions, and uncertainties. Adv. Polym. Technol. J. Polym. Process. Inst. 1992,11, 237–248. [CrossRef] 11. Farah, S.; Anderson, D.G.; Langer, R. Physical and mechanical properties of PLA, and their functions in widespread applications— A comprehensive review. Adv. Drug Deliv. Rev. 2016,107, 367–392. [CrossRef] 12. Kulinski, Z.; Piorkowska, E.; Gadzinowska, K.; Stasiak, M. Plasticization of poly(L-lactide) with poly(propylene glycol). Biomacromolecules 2006,7, 2128–2135. [CrossRef] [PubMed] 13. Piorkowska, E.; Kulinski, Z.; Galeski, A.; Masirek, R. Plasticization of semicrystalline poly(L-lactide) with poly(propylene glycol). Polymer 2006,47, 7178–7188. [CrossRef] 14. Mohan, R.; Subha, J.; Alam, J. Influence of Multiwalled Carbon Nanotubes on Biodegradable Poly(lactic acid) Nanocomposites for Electroactive Shape Memory Actuator. Adv. Polym. Technol. 2018,37. [CrossRef] 15. Sun, Y.C.; Chu, M.; Huang, M.; Hegazi, O.; Naguib, H.E. Hybrid Electroactive Shape Memory Polymer Composites with Room Temperature Deformability. Macromol. Mater. Eng. 2019,304. [CrossRef] 16. Balart, J.F.; Fombuena, V.; Fenollar, O.; Boronat, T.; Sanchez-Nacher, L. Processing and characterization of high environmental efficiency composites based on PLA and hazelnut shell flour (HSF) with biobased plasticizers derived from epoxidized linseed oil (ELO). Compos. Part B Eng. 2016,86, 168–177. [CrossRef] 17. Wang, J.Z.; Li, H.Z.; Liu, R.X.; Li, L.L.; Lin, Y.H.; Nan, C.W. Thermoelectric and mechanical properties of PLA/Bi0.5Sb1.5Te3 composite wires used for 3D printing. Compos. Sci. Technol. 2018,157, 1–9. [CrossRef] 18. Ahmad, A.F.; Ab Aziz, S.; Abbas, Z.; Obaiys, S.J.; Matori, K.A.; Zaid, M.H.M.; Raad, H.K.; Aliyu, U.S. Chemically Reduced Graphene Oxide-Reinforced Poly(Lactic Acid)/Poly(Ethylene Glycol) Nanocomposites: Preparation, Characterization, and Applications in Electromagnetic Interference Shielding. Polymers 2019,11, 661. [CrossRef] [PubMed] 19. Norazlina, H.; Hadi, A.A.; Qurni, A.U.; Amri, M.; Mashelmie, S.; Kamal, Y. Effects of multi-walled carbon nanotubes (MWCNTs) on the degradation behavior of plasticized PLA nanocomposites. Polym. Bull. 2019,76, 1453–1469. [CrossRef] 20. Chieng, B.W.; Ibrahim, N.A.; Yunus, W.; Hussein, M.Z. Poly(lactic acid)/Poly(ethylene glycol) Polymer Nanocomposites: Effects of Graphene Nanoplatelets. Polymers 2014,6, 93–104. [CrossRef] 21. Oksman, K.; Skrifvars, M.; Selin, J.F. Natural fibres as reinforcement in polylactic acid (PLA) composites. Compos. Sci. Technol. 2003,63, 1317–1324. [CrossRef] 22. Chen, D.K.; Li, J.; Ren, J. Study on sound absorption property of ramie fiber reinforced poly(L-lactic acid) composites: Morphology and properties. Compos. Part A Appl. Sci. Manuf. 2010,41, 1012–1018. [CrossRef] 23. Shi, X.T.; Zhang, G.C.; Phuong, T.V.; Lazzeri, A. Synergistic Effects of Nucleating Agents and Plasticizers on the Crystallization Behavior of Poly(lactic acid). Molecules 2015,20, 1579–1593. [CrossRef] 24. Posada, J.C.; Jaramillo, L.Y.; Cadena, E.M.; Garcia, L.A. Bio-based composites from agricultural wastes: Polylactic acid and bamboo Guadua angustifolia. J. Compos. Mater. 2016,50, 3229–3237. [CrossRef] 25. Ahmed, J.; Mulla, M.; Jacob, H.; Luciano, G.; Bini, T.B.; Almusallam, A. Polylactide/poly(epsilon-caprolactone)/zinc oxide/clove essential oil composite antimicrobial films for scrambled egg packaging. Food Packag. Shelf Life 2019,21. [CrossRef] 26. Thellen, C.; Orroth, C.; Froio, D.; Ziegler, D.; Lucciarini, J.; Farrell, R.; D’Souza, N.A.; Ratto, J.A. Influence of montmorillonite layered silicate on plasticized poly(L-lactide) blown films. Polymer 2005,46, 11716–11727. [CrossRef] 27. Shibata, M.; Someya, Y.; Orihara, M.; Miyoshi, M. Thermal and mechanical properties of plasticized poly(L-lactide) nanocomposites with organo-modified montmorillonites. J. Appl. Polym. Sci. 2006,99, 2594–2602. [CrossRef] 28. Zenkiewicz, M.; Richert, J.; Rozanski, A. Effect of blow moulding ratio on barrier properties of polylactide nanocomposite films. Polym. Test. 2010,29, 251–257. [CrossRef] 29. Gorrasi, G.; Tammaro, L.; Vittoria, V.; Paul, M.A.; Alexandre, M.; Dubois, P. Transport properties of water vapor in polylactide/montmorillonite nanocomposites. J. Macromol. Sci. Phys. 2004,B43, 565–575. [CrossRef] 30. Mohapatra, A.K.; Mohanty, S.; Nayak, S.K. Properties and characterization of biodegradable poly(lactic acid) (PLA)/poly(ethylene glycol) (PEG) and PLA/PEG/organoclay: A study of crystallization kinetics, rheology, and compostability. J. Thermoplast. Compos. Mater. 2016,29, 443–463. [CrossRef] 31. Nekhamanurak, B.; Patanathabutr, P.; Hongsriphan, N. Thermal-mechanical property and fracture behaviour of plasticised PLA-CaCO3nanocomposite. Plast. Rubber Compos. 2012,41, 175–179. [CrossRef] 32. Risyon, N.P.; Othman, S.H.; Basha, R.K.; Talib, R.A. Effect of Halloysite Nanoclay Concentration and Addition of Glycerol on Mechanical Properties of Bionanocomposite Films. Polym. Polym. Compos. 2016,24, 795–801. [CrossRef] 33. Lai, S.M.; Chen, J.R.; Han, J.L.; Yu, Y.F.; Lai, H.Y. Preparation and properties of melt-blended polylactic acid/polyethylene glycol-modified silica nanocomposites. J. Appl. Polym. Sci. 2013,130, 496–503. [CrossRef] 34. Li, Y.L.; Pötschke, P.; Pionteck, J.; Voit, B. Electrical and vapor sensing behaviors of polycarbonate composites containing hybrid carbon fillers. Eur. Polym. J. 2018,108, 461–471. [CrossRef] 35. Wickham, H.; François, R.; Henry, L.; Müller, K. dplyr: A Grammar of Data Manipulation. R Package Version 1.0.2. Available online: https://CRAN.R-project.org/package=dplyr (accessed on 9 December 2020). 36. Wickham, H.; Averick, M.; Bryan, J.; Chang, W.; D’Agostino McGowan, L.; François, R.; Grolemund, G.; Hayes, A.; Henry, L.; Hester, J.; et al. Welcome to the {tidyverse}. J. Open Source Softw. 2019,4, 1686. [CrossRef] 37. Wickham, H. The Split-Apply-Combine Strategy for Data Analysis. J. Stat. Softw. 2011,40, 1–29. [CrossRef] 38. Wickham, H. ggplot2: Elegant Graphics for Data Analysis; Springer: New York, NY, USA, 2016. Appl. Sci. 2021,11, 2580 17 of 17 39. Wilke, C.O. cowplot: Streamlined Plot Theme and Plot Annotations for ‘ggplot2’. Available online: https://CRAN.R-project.org/ package=cowplot (accessed on 12 January 2021). 40. Wilke, C.O. ggridges: Ridgeline Plots in ‘ggplot2’. Available online: https://CRAN.R-project.org/package=ggridges (accessed on 9 December 2020). 41. McNally, T.; Pötschke, P.; Halley, P.; Murphy, M.; Martin, D.; Bell, S.E.J.; Brennan, G.P.; Bein, D.; Lemoine, P.; Quinn, J.P. Polyethylene multiwalled carbon nanotube composites. Polymer 2005,46, 8222–8232. [CrossRef] 42. Batakliev, T.; Petrova-Doycheva, I.; Angelov, V.; Georgiev, V.; Ivanov, E.; Kotsilkova, R.; Casa, M.; Cirillo, C.; Adami, R.; Sarno, M.; et al. Effects of Graphene Nanoplatelets and Multiwall Carbon Nanotubes on the Structure and Mechanical Properties of Poly(lactic acid) Composites: A Comparative Study. Appl. Sci. 2019,9, 469. [CrossRef] 43. Bautista-Quijano, J.R.; Pötschke, P.; Brünig, H.; Heinrich, G. Strain sensing, electrical and mechanical properties of polycarbonate/multiwall carbon nanotube monofilament fibers fabricated by melt spinning. Polymer 2016,82, 181–189. [CrossRef]