Preparation and Characterization of Extruded PLA Films Coated with Polyaniline or Polypyrrole by In Situ Chemical Polymerization

Conductive polymers, such as polypyrrole and polyaniline, have been extensively studied for their notable intrinsic electronic and ionic conductivities, rendering them suitable for a range of diverse applications. In this study, in situ chemical polymerization was employed to coat extruded PLA films with PPy and PANi. Morphological analysis reveals a uniform and compact deposition of both polyaniline and polypyrrole after polymerization periods of 3 and 1 h, respectively. Furthermore, the PLA–PANi-3h and PLA–PPy-1h composites exhibited the highest electrical conductivity, with values of 0.042 and 0.022 S cm–1, respectively. These findings were in agreement with the XPS results, as the polyaniline-coated film showed a higher proportion of charge carriers compared to the polypyrrole composite. The elastic modulus of the coated films showed an increase compared with that of pure PLA films. Additionally, the inflection temperatures for the PLA–PANi-3h and PLA–PPy-1h composites were 368.7 and 367.2 °C, respectively, while for pure PLA, it reached 341.47 °C. This improvement in mechanical and thermal properties revealed the effective interfacial adhesion between the PLA matrix and the conducting polymer. Therefore, this work demonstrates that coating biopolymeric matrices with PANi or PPy enables the production of functional and environmentally friendly conductive materials suitable for potential use in the removal of heavy metals in water treatment.


INTRODUCTION
Intrinsically conducting polymers (ICPs) with conjugated πelectron systems, such as polyaniline (PANi), polythiophene, and polypyrrole (PPy), have been extensively researched in recent decades. 1 PANi and PPy have garnered significant attention due to their ease of polymerization, high intrinsic conductivity (ranging from 0.01 to 500 S cm −1 in the doped state), redox reversibility, and environmental stability. 2−6 In addition, owing to their excellent ion exchange behavior, considerable attention has been given in recent years to the capacity for removing toxic heavy metal ions such as mercury, nickel, cadmium, and chromium. 7,8Nevertheless, their limited surface area, poor mechanical properties, and challenges in conventional processing methods have posed obstacles to fully harnessing their potential. 9,10Luckily, the limitations of PANi and PPy can be overcome by creating composite materials that synergistically integrate the desirable mechanical properties of the insulating host matrix with the electrical properties of conductive polymers. 11Coating the polymer matrix with ICPs is a promising alternative for obtaining such systems.−15 On the other hand, there is growing interest in the use of biopolymeric and/or natural matrix templates.−19 Several researchers have utilized these biopolymers as matrices in composite materials.In a notable study, El-Nahrawy et al. (2020) successfully created a nanocomposite using cellulose nanowhiskers (CNW) through the in situ emulsion polymerization reaction of pyrrole, with the favorable electrical properties dependent on temperature. 20Similarly, Reis et al.  (2021) developed a conductive biocomposite based on chitosan and polypyrrole, capable of removing chromium from aqueous solutions. 21Finally, Imgharn et al. (2023)  achieved the synthesis of a highly porous polyaniline-based hydroxyapatite-montmorillonite composite through in situ chemical polymerization. 22oly(lactic acid) (PLA) is a biopolymer that has garnered significant attention recently due to its remarkable biodegradability, biocompatibility, and exceptional mechanical properties.In addition, it is economically polymerized from lactic acid obtained from corn and sugar beet and can be easily processed using the same techniques as that for conventional polymers. 23icciani et al. (2010) developed conducting electrospun fiber mats based on PLA and PANi.This study achieved homogeneous nanoscale fibers with excellent interfacial adhesion between the electroconductive polymer and the matrix. 24Similarly, Leng et al. (2018) prepared core−shell microfibers of PLA−PPy using the electrospinning technique.The results demonstrated the achievement of a highly conductive system with electrical conductivity values of 0.5 S cm −1 , rendering it suitable for stimulus-responsive applications. 25On the other hand, some authors have also reported the preparation of blends based on PLA and conductive polymers.Wong et al. (2020) fabricated a biodegradable material of PLA−PANi through ex situ polymerization using the solution casting method.The results revealed a highly compact and porous material with properties suitable for antistatic packaging applications. 26Likewise, Monleoń Pradas et al. (2020) fabricated PLA cast membranes coated with polypyrrole; the findings unveiled a uniform and continuous PPy coating, accompanied by a high electrical conductivity, making them candidates for applications such as solid polymer electrolytes. 27Although both electrospinning and casting techniques are among the most reported methods for the preparation of PLA−PPy and PLA−PANi systems, to date, there are no reports in the literature of systems based on extruded PLA films coated with PANi or PPy.Therefore, the objective of our work was to develop a viable approach to prepare electroconductive films based on PLA using simple, economically feasible methods and, above all, scalable to an industrial level, such as extrusion molding.
In this regard, in the current work, the coating of extruded PLA films with PANi or PPy was carried out through an in situ chemical oxidation method.The morphological, mechanical, and thermal properties of the films were studied.Additionally, the conductive nature of the materials was demonstrated by assessing their electrical properties, and the chemical composition of the coated films was obtained by FT-IR and XPS analyses.For the coating of PLA films with polypyrrole (PLA−PPy), a solution of 0.5 M pyrrole in HCl (1 M) was prepared.The PLA films were cut into strips (1.5 × 1 cm) to facilitate their coating and were immersed in 5 mL of pyrrole solution for 15 min.Subsequently, to start the polymerization reaction, 5 mL of 0.5 M hexahydrate ferric chloride was added under magnetic stirring at 200 rpm, and five polymerization times were studied corresponding to 0.5, 1, 2, 3, and 4 h.For the coating of the films with polyaniline (PLA−PANi), a similar procedure described for the coating with polypyrrole was used, but 0.5 M ammonium persulfate and 0.5 M aniline solutions with equal contact time were used instead.The coated films were washed with deionized water and dried at 25 °C for 24 h.

EXPERIMENTAL METHODS
2.3.Characterization.2.3.1.Morphological Analysis.The morphology of coated films was examined using a JEOL JSM-5410LV scanning electron microscope coupled with an energy-dispersive X-ray spectroscope, operated at a voltage of 20 kV.The samples were gold-sputtered before the SEM examination.

Electrical Properties.
The electrical conductivity was determined from the I−V curves of the materials and was carried out by using the two-point method.Measurements were made by using a Keithley 2400 A Tektronix Company semiconductor characterization system at room temperature.The measurements were performed under a linear sweep mode from −5 to 5 V and a 2.4 mm separation between the tips.

Fourier Transform Infrared Spectroscopy Analysis.
Fourier transform infrared (FTIR) spectra of the samples were obtained with a Thermo Scientific Nicolet iS50 spectrophotometer.The spectra were recorded employing the attenuated total reflectance (ATR) technique, scanning from 4000 to 400 cm −1 .An average of 32 scans were recorded.

Tensile Strength Test.
The mechanical properties of the films were determined by using a United SSTM-5kN universal testing machine equipped with a 5 kN load cell.A speed of testing of 1 mm/min and a distance between the ends of the gripping surfaces of 20 mm were used.The thickness of the films was determined with a Mitutoyo micrometer, and the dimensions were maintained in the range of 0.3−0.6 mm thickness and a width of 5.2 mm.The samples were conditioned to room temperature and humidity before and during testing.An average of 10 specimens for each test is reported.

Thermogravimetric Analysis.
To analyze the coated film samples' thermal stability, TGA was performed using a Pyris 1 instrument from PerkinElmer.About 4 mg of the materials was placed in a porcelain sample holder; the samples were subject to a temperature increase rate of 10 °C/min from room temperature to 600 °C under a nitrogen atmosphere.

Differential Scanning
Calorimetry.DSC measurements were carried out on a PerkinElmer DSC 8500 equipment; samples of 7 mg each were placed in high-purity aluminum sample holders.The temperature was raised from 25 to 200 °C at a rate of 10 °C min −1 under a nitrogen atmosphere.After cooling, the samples were heated again by employing the same heating rate.performed to evaluate the different contributions associated with the various types of nitrogen (N 1s) and (Cl 2p) bonds present on the material.

SEM Image Analysis.
Figure 1 shows the SEM micrographs of PLA films coated with polyaniline (PLA− PANi) at different polymerization times.At the polymerization times of 0.5 and 1 h, as depicted in Figure 1b,c, respectively, the coating showed limited effectiveness, as polyaniline did not fully cover the PLA film surface, leaving significant areas without the conductive polymer.Possibly, the short reaction time did not allow the polymerization of sufficient aniline monomers, resulting in the deposition of a small amount of polyaniline particles. 28,29As the reaction time increased, a greater amount of polyaniline was deposited on the surface of  the PLA film, resulting in a complete coating at 3 h of polymerization (Figure 1e).In this coating, the PANi particles were uniformly distributed and exhibited a high degree of compaction.For the final polymerization time, corresponding to the 4 h reaction (Figure 1f), agglomeration of the electroconductive polymer and a nonhomogeneous coating on the surface of pure PLA were observed.These findings suggested that a prolonged polymerization time hindered the attainment of a homogeneous film.
Figure 2 presents the SEM micrographs of PLA films coated with polypyrrole (PLA−PPy) at different polymerization times.Unlike the films coated with polyaniline, 1 h of polymerization was sufficient for polypyrrole to uniformly coat the PLA film (Figure 2c).This can be observed more clearly in the cross-sectional SEM micrographs (Figure 3b), where a thin, homogeneous layer of the electroconductive polymer is distributed on the surface of the PLA matrix.Additionally, starting from 2 h of reaction (Figure 2d), agglomeration of the electroconductive polymer was observed.This agglomeration progressively increased as the reaction time was extended.This behavior became more pronounced after 4 h of polymerization (Figure 2f).
The SEM−EDX analysis of neat PLA films (a) and coated PLA films with PANi at 3 h of polymerization (b) and PPy at 1 h of polymerization (c) is depicted in Figure 4.The EDX analysis of the neat PLA film reveals the presence of carbon (C) and oxygen (O) atoms, along with gold (Au), which is a result of the prior coating before analysis.Furthermore, both coated films show discernible peaks corresponding to C and N, confirming the incorporation of PANi and PPy particles onto the PLA film. 30Finally, sulfur and iron are observed for the polyaniline-coated and polypyrrole-coated films, respectively.This suggests the potential existence of residues from the oxidizing agents used in the reaction. 31.2.Electrical Properties.Polypyrrole and polyaniline are among the most studied conductive polymers due to their long π-conjugated length, unique electrical properties, reversible doping/dedoping process, and controllable chemical and electrochemical properties.When these electroactive polymers are homogeneously deposited on the surface of a film, their electroconductive capacity can be transferred to the composite material and even enhanced. 13,32n Figure 5a,b, the I−V curves of PLA−PANi and PLA− PPy, respectively, at different polymerization times are illustrated.It is evident that the pristine PLA film exhibits an insulating behavior with no measurable conductivity.On the contrary, from the initial 0.5 h of in situ polymerization of pyrrole and aniline, the composite films became electrically conductive.Specifically, for the PLA−PANi film, the electrical conductivity exhibited an increase as the reaction time prolonged (Figure 5a).With the increase in the reaction time, a higher number of polyaniline particles were deposited onto the PLA film, facilitating electron transfer within the composite film and resulting in a gradual increase in electrical conductivity. 33In particular, at 3 h of polymerization, the PLA−PANi film presented the highest electrical conductivity with a value of 0.042 S cm −1 (Table 1), which is attributed to the highly compact and homogeneous distribution of PANi particles observed through scanning electron microscopy (Figure 1e). 34Additionally, at 4 h of polymerization, the PLA−PANi film presented a decrease in electrical conductivity as it exhibited a conductivity of 0.0172 S cm −1 .This behavior could be related to the overoxidation of the PANi chains. 34,35 the other hand, the PLA−PPy film showed its highest electrical conductivity during the first hour of polymerization, with a value of 0.0220 S cm −1 .Similarly, for the PLA−PANi film, this was attributed to the high compaction of the polypyrrole particles and their homogeneous distribution (Figure 2c), which facilitates proper charge percolation. 36,37fter the first hour of polymerization, the electrical conductivity of the PLA−PPy films gradually decreased (Figure 5b).This could be related to phase separation present in the composite film due to the formation of polypyrrole agglomerates on the surface of the PLA film. 38,39.3.FTIR Spectroscopy Analysis.Figure 6a shows the FT-IR spectra of the PLA−PANi films at different polymerization times.For each reaction time, the characteristic bands of PANi are present, thereby confirming the presence of polyaniline on the PLA film.The main peaks of polyaniline were located around 1536, 1446, 1292, and 1255 cm −1 , which were characteristic of the −C�N quinoid ring, −C�C− benzenoid structure, C−N + protonated aromatic amine, and C−N deprotonated amine.40,41 Additionally, the peak at 673 cm −1 was assigned to the out-of-plane bending vibration of the symmetric �C−H bonds in the benzene ring.42 The intensity  and position of the peaks associated with the quinoid and benzenoid rings are strongly related to the conducting states of polyaniline in the coated films.It can be observed that the position of these peaks shifts from 1536 to 1527 cm −1 and from 1446 to 1440 cm −1 , respectively, as the polymerization time increases from 0.5 to 3 h.This shift is accompanied by an increase in the intensity of the 1527 cm −1 band and a decrease in the intensity of the 1440 cm −1 band (Table 2).These results suggest the formation of highly oxidized protonated states that can be attributed to the improvement in the conjugation and conductivity states of PANi with the increasing polymerization time.33 For the film obtained at 4 h of polymerization, the position of these bands shifted to a higher wavenumber, and the intensity of the 1450 cm −1 band increased, which may be attributed to the overoxidation of the coated film.33,43 These results are consistent with the electrical conductivity analysis (Table 1).However, variations were observed in the FTIR spectra of the PLA−PANi coated films compared with pure PANi.Specifically, in the PLA−PANi-3h film (Figure 6b), it was noticed that the peaks originally located at 1564 and 1482 cm −1 shifted to 1527 and 1440 cm −1 , respectively.These changes suggest an interaction between polyaniline and the polymeric structure of PLA, possibly due to the formation of hydrogen bonds.44,45 The FT-IR spectra of the PLA−PPy films at different polymerization times are presented in Figure 6c.For each polymerization time, the main peaks of polypyrrole are displayed.The peaks at 1575 and 1432 cm −1 are attributed to the symmetric and antisymmetric vibrations of C�C−N in the plane of the benzene ring, and peaks at 1210 and 1078 cm −1 correspond to the C−N in-plane deformation and the vibration of the N−H bond, respectively.46,47 The peak observed at 959 cm −1 corresponds to the stretching vibration of C�N + −C.This particular peak at 959 cm −1 confirms that PPy was effectively doped with FeCl 3 , resulting in positively charged units that act as charge carriers, i.e., polarons/ bipolarons.48,49 It is worth noting that the position of this peak shifts to higher wavenumbers as the polymerization time increases from 1 to 4 h. To complement these findings, we determined the effective "conjugation length" of PPy in the films.The degree of electron delocalization is directly proportional to the ratio of peak areas at 1432 and 1575 cm −1 , and conductivity correlates directly with the A 1432 /A 1575 ratio.51 In general, a decrease in  this ratio was observed as the polymerization time increased.Specifically, the film obtained at 1 h exhibited the lowest value of 0.10.These findings align with the electrical conductivity values as the PLA−PPy-1h film proved to be the most conductive.Finally, the peaks initially observed at 1542 and 1044 cm −1 for pristine PPy shifted to 1575 and 1078 cm −1 for the PLA−PPy-1h films Figure 6d, indicating the intermolecular interaction between the PLA matrix and polypyrrole. 46 3.. Mechnical Properties.The stress−strain curves of the pristine films and the films coated with electroconductive polymers are depicted in Figure 7.As observed, the pure PLA film exhibited a tensile strength of 44.27 MPa and an elongation at break of 26.48%.Furthermore, the modulus of elasticity for this film was 1075.39MPa. Tese findings are consistent with those reported by several authors in the literature.52,53 Nevertheless, the addition of electroconductive polymers to the PLA matrix resulted in a series of variations in the material's properties.First, a slight decrease in tensile strength was observed for both composite materials, presenting values of 43.20 and 43.90 MPa for the PLA−PPy-1h and PLA−PANi-3h films, respectively.,55 Contrarily, the composite films presented an increase in their modulus of elasticity as compared to the pure PLA film.This behavior became more evident in the film coated with polyaniline (PLA−PANi-3h), since it presented Young's modulus of 1609.63 MPa.,57 Finally, the elongation values of the PLA−PPy-1h and PLA−PANi-3h films showed a reduction of approximately 80% compared to that of neat PLA, confirming the increase in rigidity upon the incorporation of PPy and PANi particles.58 3.5. Thrmal Properties.TGA and derivative thermogravimetry (DTG) curves of neat PLA, PLA−PPy-1h, and PLA−PANi-3h films are presented in Figure 8.The neat PLA film shows a single degradation step, characterized by a weight loss onset at approximately 308.42 °C, corresponding to the degradation of polyester chains.Additionally, the film exhibits an inflection temperature (T max ) of 341.47 °C (as confirmed by the DTG signal).These results are in agreement with previous literature reports. 59−61On the other hand, the coated films exhibited higher thermal stability compared to the pure PLA films, as evidenced by their higher inflection temperatures of 367.2 and 368.7 °C for the PLA−PPy-1h and PLA−PANi-3h films, respectively.This enhancement in thermal stability can be attributed to the effective interfacial adhesion between the PLA and the electroconductive polymers.62,63 Finally, the coated films, like the pure PLA film, demonstrated degradation in a single step, indicating a continuous and homogeneous PPy and PANi coating on the PLA films.64 The DSC curves up to 200 °C obtained for pure PLA, PLA− PPy-1h, and PLA−PANi-3h are presented in Figure 9.It is evident that the incorporation of PANi and PPy increases the thermal properties of the composite films.Regarding neat PLA, the glass-transition temperature (T g ) was observed at 62.65 °C, while for the composites, it increased slightly to 64.85 and 64.57°C for PLA−PPy-1h and PLA−PANi-3h, respectively.This shift in T g can be attributed to the intermolecular interactions between PLA and conductive polymers, as observed by FT-IR analysis.65,66 Similarly, the addition of PANi and PPy increases the crystallization temperature (T c ) and the melting temperature (T m ).Specifically, the melting peaks are found at 151.87 and 150.90 °C for PLA−PPy-1h and   PLA−PANi-3h, respectively, while for neat PLA, it was at 148.75 °C.This enhancement in temperature values is attributed to the nucleating action of the PPy and PANi particles onto the PLA matrix.67,68 Finally, the degree of crystallinity (X c ) of the neat PLA and coated films was calculated by the following equation 69

Table 1. Electrical Conductivity of Films Coated After Different Polymerization Times
where ΔH m refers to the melting enthalpy of the films, ΔH cc represents crystallization enthalpy, and ΔH m 100% represents the melting enthalpy of 100% crystalline PLA (93.6 J/g).As indicated in Table 3, the calculated X c values were 42.04, 28.64, and 33.62% for neat PLA, PLA−PPy-1h, and PLA− PANi-3h, respectively.These results are consistent with the obtained melting enthalpies (ΔH m ), showing that the enthalpies of the coated films decrease compared with those of pure PLA. 70.6.XPS Analysis.The XPS patterns were employed to validate the elemental composition and chemical state of the samples.Figure 10a shows the XPS survey spectrum of the obtained films.Comparison with PLA, PLA−PPy-1h, and PLA−PANi-3h films exhibits an additional peak corresponding to N 1s, indicating the successful coating of PPy and PANi, respectively. 71Additionally, both coated films exhibited binding energy peaks at approximately 284.50 and 531.50 eV, which were attributed to C 1s and O 1s, respectively. 72urthermore, the PLA−PPy-1h film exhibited a peak related to Fe 2p located at 710.10 eV, possibly corresponding to ferric chloride residues present in the coated film. 73Moreover, the high-resolution spectra of Cl 2p presented in Figure 10b revealed that Cl in PLA−PPy and PLA−PANi existed primarily in three forms: chloride anion (Cl − ) located at 197.14 eV, (Cl − ) resulting from the charge transfer with PPy and PANi polarons at the binding energy of 198.10 eV, and covalently bonded chlorine located at 200.11 eV. 74,75he deconvoluted N 1s spectra of PLA−PPY-1h and PLA− PANi-3h composite films are presented in Figure 10c.The deconvoluted N 1s spectrum of the coated films showed four peaks located at 397.88, 399.18, 400.63, and 402.23 eV.The peaks at 397.88 and 399.18 eV could be attributed to the nitrogen atoms in the amine (−NH−) and imine (−C�N) structures, respectively, while the peaks at 400.63 and 402.23 eV confirm the presence of the polaron (C−N + ) and the bipolaron (C�N + ) structures, respectively. 75Furthermore, when comparing the different amounts of N + in the coated films, it was observed that in the PLA−PANi-3h film, the proportion of N + (27.49%) was higher compared to that in PLA−PPy-1h (17.03%).This finding was coherent with the doping level (N + /N) of the films, as the polyaniline-coated film exhibited a value of 0.27, while the polypyrrole-coated film had a value of 0.17. 75,76These results were consistent with the higher conductivity of the polyaniline composite compared with that of polypyrrole (Table 1).

CONCLUSIONS
Novel conductive biomaterials were obtained by coating extruded PLA films with polyaniline and polypyrrole.FT-IR and EDS analyses confirmed the successful incorporation of PANi and PPy particles onto the PLA film surface.The composites obtained after 3 and 1 h of polymerization for PANi and PPy, respectively, exhibited a homogeneous coating.The PLA−PANi-3h composite exhibited higher electrical conductivity compared to PLA−PPy-1h, which is attributed to the greater percentage of charge carriers (polarons and bipolarons) in the polyaniline-based film, as revealed by XPS.The addition of PANi and PPy improved the thermal stability of the material.Additionally, the composite films showed an increase in glass-transition temperatures (T g ) and fusion temperatures (T m ) compared to neat PLA.These findings  are attributed to the effective interfacial adhesion between the conductive polymers and polylactic acid (PLA).Finally, the incorporation of conductive particles increased the elastic modulus of the composite material.In conclusion, the appropriate physicochemical properties and electroactive behavior of these conductive films make them suitable for potential use in the removal of heavy metals in water treatment.

2. 3 . 7 .
XPS Analysis.XPS spectra were measured on a PerkinElmer vacuum Products model PHI 5100 photoelectron spectrometer with Mg Kα exciting radiation at 15 kV and 10 mA; the base pressure was approximately 10−9 Torr.The survey scans were in the range of 0−1100 eV.To compensate for the surface charging effects, all binding energies were referenced to the C (1s) peak at 284.6 eV.A Gaussian deconvolution curve fitting on a Shirley background was

Figure 6 .
Figure 6.FTIR spectra of coated films obtained at different polymerization times: (a) PANi-coated and (c) PPy-coated films, respectively.Subfigures (b,d) correspond to pristine polyaniline and polypyrrole, as well as their respective composite films.

Table 2 .
Changes in the Absorption Bands in the Spectra of PLA−PANi Films at Different Polymerization Times

Table 3 .
Crystallinity Percentages of the Pure and Coated Films