Introduction

The incorporation of new additives in polymeric materials provides specific properties. Essential oils (EO) are incorporated into biopolymer films to evaluate their antimicrobial efficiency and antibacterial activity among other properties. Studies of Souza et al. [1] developed bionanocomposites based on chitosan/montmorillonite incorporated with two different EO, rosemary (Rosmarinus officinalis L.) and ginger (Zingiber officinale Rosc), and to evaluate their antimicrobial and antioxidant properties via in vitro assays. Do Evangelho et al. [2] evaluated the morphological, optical, mechanical, and barrier properties and antibacterial activity of corn starch films containing orange EO (Citrus sinensis). Corn starch films were prepared by applying the casting method [2]. Tea tree EO was added in different concentrations to starch/furcellaran/gelatin films. The antioxidant properties proved to be significantly enhanced with the addition of EO into films. These films showed antimicrobial activity against Staphylococcus aureus and Escherichia coli. Results suggested that the films containing EO could be used as an active film which enhances microbial safety and the shelf-life of foods due to its good in vitro antioxidant and antimicrobial properties for food packaging applications [3].

Some paper such as de Flores et al. [4] evaluated the weight gain (swelling) for PCL (by compression), at room temperature (± 23 °C) for 60 days, which had a gain of 9.8% over the initial weight. PLA films obtained by compression molding were evaluated for swelling/solubility in benzyl benzoate and Miglyol 810°R for 13 days. After 48 h at room temperature, complete dissolution of the polymer in benzyl benzoate was observed, while for Miglyol the polymer masses remained unchanged after 13 days, indicating that there was no swelling or dissolution of PLA in this oil [5]. PCL films obtained by compression molding were evaluated for swelling in octyl methoxycinnamate for 13 days (n = 3). The experiment showed that polymer weights remained unchanged after 13 days, indicating no swelling or dissolution [6].

EO are volatile organic compounds. They are a mixture of monoterpenes –C10H16, two isoprene units, and sesquiterpenes –C15H24, three isoprene units and are obtained from aromatic plants. They have been used for thousands of years for producing incenses, food, and medical applications. In addition to its aromatic properties, the EO have numerous properties, such as antimicrobial, antioxidant, antibacterial, antifungal, and anti-inflammatory activities [7,8,9].

According to Food and Drug Administration (FDA), approximately 160 EO are considered Generally Recognized as Safe (GRAS), which means they can be used safely in food, drugs, and cosmetics. Among them are Mentha piperita (MP), Cymbopogon nardus (CN), Mentha arvensis (MA), Syzygium aromaticum (AS), Lavandula hybrida (LH) and Eucalyptus globulus (EG) [10,11,12].

Essential oils are also chemically unstable and practically insoluble in aqueous systems. Additionally, EO are susceptible to losses by volatilization and oxidative deterioration when exposed to oxygen, light, heat, or interactions with other matrix ingredients in foods, cosmetics, and pharmaceutical formulations. These disadvantages limit its applications [13, 14].

The encapsulation of EO can solve the problems mentioned above, since the shell isolates EO from external agents, giving them greater stability due to their lower volatility. Furthermore, depending on the chosen encapsulating agent, the release of bioactive compounds is controlled [15,16,17].

It can be seen remarkable decreases in the amounts of unsaturated terpenes such as γ-terpinene or β-myrcene together with a rise in p-cymene were frequently revealed, especially promoted under light and high temperatures in EO. The storage period and useful life of EO can vary from a few days to years. Also, only a limited number of EO have been subjected to storage experiments [13].

These are some examples of biopolymers (shell materials) used in encapsulation: polysaccharides (gum arabic, modified starches, maltodextrins, alginates, pectin, cellulose derivatives, chitosan, cyclodextrins). Fats and waxes (hydrogenated vegetable oils, lecithin, medium chain triglycerides, glyceryl behenate), proteins (gelatin, whey proteins, sodium caseinate, gluten, caseins, zein), synthetic compounds, or their combinations as polycaprolactone and polylactic acid that are thermoplastics and biodegradables aliphatic polyesters [15, 18]. Furthermore, polymers can act as a versatile wall material to bind or encapsulate a wide variety of bioactive core materials [19].

Studies show that it is possible to determine the amount required to solubilize a single component of the EO by using a variety of techniques, such as UV–visible spectroscopy, fluorescence spectroscopy, and chromatography [20,21,22]. A method, named total organic carbon, developed to evaluate the solubility of EO inclusion complexes with cyclodextrins [23]. Similarly to Hansen parameters, they give improved agreement with data but are still not completely accurate for predicting the solution thermodynamics for every system [18].

Therefore, a study with more straightforward, efficient, and low-cost techniques is necessary to determine the best shell option to be used in the process, considering that it is not completely dissolved by the EO.

No manuscript provides more detailed information on these interactions, film production method, temperature, and proper direct telling time with statistical tools. The work is relevant to areas related to processes that use polymers in direct contact with EO, or their major components, such as encapsulation processes and films with the incorporation of EO. This is being a simple, inexpensive, and easy to reproduce methodology, thus allowing a prediction of the possibility of use and compatibility of the materials. Since some polymers can be dissolved by essential oils.

The results show that there is no need for prolonged contact periods of polymer films with essential oils, as well as the importance of polymer resistance in direct contact with essential oils.

This work has reported the effect of two methods to obtain polymer films, which were immersed in different EO to provide an understanding on the swelling of these polymers and the importance of the non-dissolution of the shell material (biopolymer) by the material to be encapsulated (essential oil). The chemical potential of the oil in a swollen system is calculated on a macroscopic scale through the mass transfer rate, including the contributions of the swelling (convection-induced diffusion) of the films over time. The results provide information on the swelling/dissolution process over 112 days.

Materials and methods

Materials

Polycaprolactone (PCL) (Mw = 50.000 g mol−1) was obtained from Perstorp, Brazil (commercial name Capa 6500); poly(lactic acid) (PLA) (Mw = 6.800 g mol−1) was obtained from Natureworks®; gum arabic (GA) was obtained from Neon Comércio, Brazil; sodium alginate (SA), gelatine (GE), and carboxymethyl cellulose (CMC) were obtained from Dinâmica Química Contemporânea, Brazil.

The EO of Mentha piperita (MP) and Cymbopogon nardus (CN) were obtained from Vimontti, Brazil, Mentha arvensis (MA), Syzygium aromaticum (AS), and Lavandula hybrida (LH) from Herbia Cosméticos Orgânicos, Brazil, and Eucalyptus globulus (EG) from Arte dos Aromas, Brazil. Acetone (P.A) and chloroform (P.A) were obtained from Química Moderna, Brazil. All reagents were used as received.

Preparation of polymer films

Figure 1 shows the preparation of polymeric films by both methods (casting and compression molding). In the preparation of the films by casting, 2 g of each polymer was dissolved in a volume of 50 mL of its respective solvent, under continuous stirring (Velp Scientifica) until its complete solubilization [24]. Compression molding (*) was used only for the PCL* and PLA* polymers as described in studies by Guterres et al. [5] and Weiss-Angeli et al. [6]. By using a thermohydraulic press (Bovenau®), the cooling was performed in both, with circulation of water at room temperature. For solvent evaporation in films obtained by casting, all samples were weighed (initial weight = final weight), controlling the absence of solvent traces in the formed films (confirmed by FTIR analysis, Sect. 3.3). The thickness of the films was measured using a manual thickness gauge, model 7301, from the MITUTOYO brand.

Fig. 1
figure 1

Preparation of polymeric films

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Swelling studies

For the study of polymer swelling with EO, 50 mg of each film was placed in contact with each EO. Sufficient quantities of each EO were used to cover the polymer film glass vials in triplicate (n = 3). Afterward, the glass vials were closed and kept at room temperature (23 ± 2 °C) for 112 days. At intervals of 1, 3, 5, 7, 14, 28, 56, and 112 days (ensuring a longer contact time of materials), the films were removed from the EO, dried carefully with absorbent paper, and weighed. After weighing, the films were again placed in contact with EO [6, 24]. Due to their structural relationship within the same chemical group, the components of the essential oil are known to easily convert into each other through different reactions. In the process of degradation of EO, the formation of other chemical compounds can occur [13]. So far, the longest period evaluated did not exceed 60 days. To check for the influence of temperature, the same procedure was repeated at 4 °C (refrigerator) and 35 °C (oven) in 7 days. The percentage of weight variation was calculated by using the following empirical relation described in Eq. 1, in which (WD) is the percentage of liquid absorption, (FW) final weight and (SW) starting weight [25, 26]:

$$ {\text{WD}} \left( \% \right) = \frac{{\left( {{\text{FW}} - {\text{SW}}} \right)}}{\text{SW}} \times 100 $$
(1)

Data from the swelling test, in which samples were kept at room temperature for 112 days, were submitted to ANOVA. The F test was used for statistical significance, with alpha equal to 0.05 (95% confidence). Statistical tests that presented statistical significance (F > Fcritical) were evaluated by multiple comparisons of Duncan’s means.

Characterizations

Gas chromatography/mass spectrometry (GC/MS)

GC analyses for all EO were performed on a Hewlett Packard 6890 Series chromatograph equipped with an HP-Chemstation data processor using an HP-Innowax column (30 m × 320 μm id) with 0.50 μm film thickness (Hewlett Packard, Palo Alto, USA). Column temperature was set at 40 °C (8 min), 180 °C at 3 °C min−1, 180–230 °C at 20 °C/min−1, 230 °C (20 min). The injector temperature was set at 250 °C; split ratio at 1:50, flame ionization detector with a temperature of 250 °C; entrainment gas H2 (34 kPa). The volume injected was 1μL diluted in hexane (1:10). Analyses were performed on a gas chromatograph coupled to a Hewlett Packard 6890/MSD5973 mass selective detector equipped with HP Chemstation software and Wiley 275 spectrophotometer. A HP-Innowax (30 m × 250 μm) fused silica capillary column 0.50 μm film thickness (Hewlett Packard, Palo Alto, USA). The temperature program used was the same as that used in the GC: interface 280 °C; split ratio 1:100; drag gas He (56 kPa); flow rate: 1.0 mL min−1; ionization energy 70 eV; volume injected 1 μL diluted in hexane (1:10). The constituent oils were identified by comparing their mass spectra with the Wiley library (GC/MS) and by comparing the practical linear retention index with literature data (Nist). The linear retention index was calculated from the Van den Dool and Kratz equation using a standard solution of C8 to C25 hydrocarbons [27].

Field emission scanning electron microscopy (SEM-FEG)

The surface morphology of PCL, PLA, GA, SA, CMC, and GE films prepared by casting and compression-molded PCL* and PLA* was investigated by scanning electron microscopy with a Tescan Mira 3 and Shimadzu SSX-550 Superscan microscope at an acceleration of 10 kV. The samples had previously been sputter coated with gold to increase their electric conductivity.

Fourier transform infrared spectroscopy (FTIR)

The Fourier transform infrared spectroscopy (FTIR-Nicolet IS10-Thermo Scientific) analysis was carried out with 32 scans, within the range of 4000 to 400 cm−1, at a resolution of 4 cm−1 by using attenuated total reflectance (ATR) for all polymers as received. The analysis was performed on all polymers after formation the films obtained by both methods and on the films obtained from PCL and PLA by both methods maintained at room temperature for 112 days immersed in MP.

Results and discussion

GC/MS

GC/MS was performed to evaluate the EO components, and the values are shown in Table 1. Several parameters such as ecological condition, harvest time, and extraction techniques can affect the composition of EO [28, 29]. Alcohols, aldehydes, phenylpropanoids, terpenes, and ketones are the main bioactive compounds found in EO and are related to their antioxidant activity [14, 30].

Table 1 Major components of EO
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The table shows that the oils with the highest content of one of its components are: Syzygium aromaticum (AS) and Eucalyptus globulus (EG), 88.09% eugenol, and 83.56% 1.8-cineole, respectively.

Some researchers have found other major components. For instance, EG oil presents eucalyptol (71.51%) and p-cymenene (0.22%) [33], 1,8-cineole (63.6%) and α-terpineol (10.5%) [41], 1,8-cineole 14.55% and α-pinene 1.53% p-cymene 0.49% [42], 1,8-cineole 3.16% and α-pinene 0.13% and p-cymene 18.18% [43]. AS oil presents eugenol 81.9%, isoeugenol 13.1% [41], eugenol 57.12–62.88% [44].

Previous studies have also detected menthol and isomenthone as the prevalent constituents in MP [37, 45]. The major component in MP was menthol (45.58%), followed by menthone (24.87%), isomenthone (9.48%), eucalyptol (5.65%), menthyl acetate (4.62%), limonene (2.02%), and β-caryophyllene (1.02%) [46].

SEM-FEG

To verify the surface of the formed films, Fig. 2 shows SEM micrographs of all polymer samples. Films obtained by compression molding had a thickness of 24–28 microns, those obtained by casting with 4–6 microns.

Fig. 2
figure 2

Surface micrographs of polymer films

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Small particles with different geometric shapes were noted in the GE film. These objects can be undissolved particles (Fig. 2a). The surfaces of the SA and CMC films (Fig. 2c, d) were more homogeneous and smooth. [47,48,49]. The surface of the GA film (Fig. 2b) showed a weak characteristic and presented cracks in which the oils may have penetrated and been stored, thus increasing their weight gain (Table 2).

Table 2 Weight gain (%) after 112 days immersed in EO
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The surface of the PCL film (Fig. 2e) exhibited crystals which are large and unequal in size from the nucleation and growth process. Between the crystals it is possible to observe openings in the film, which is one of the signs of the greater gain for these samples (Table 2) [50]. PLA (Fig. 2f) also presented particles with different geometric shapes on its surface [51].

The micrographs (Fig. 2h) show that the PLA* sample has a grooved surface, probably due to the film preparation process [52, 53]. PCL* (Fig. 2g) presented the same characteristics.

Observing the PLA obtained by compression molding and by casting (Fig. 2f, h) it is noticeable that in both the polymer surface showed irregularities. In comparison with the PCL obtained by both methods (Fig. 2e, g), its surface has a different morphology. Comparing surface images of the PCL film obtained by casting and compression molding, drastic changes in morphology are observed, probably influenced by the cooling time and thinner film formation (4–6 microns). Changes are greater in the casting process (24 h), which favored the growth of crystallites in recrystallization of the PCL, with the formation of spaces between the crystallites. In the case of PLA films, differences are observed. However, they are less intense, but still, small crystallites were observed in the film obtained by casting. This result could indicate that the recrystallization of PLA is much slower than the PCL's [51].

Some polymer films presented a more robust surface, with the formation of cracks or empty spaces, contributing to the weight gain of these films. Figure 3 shows the micrographs of the films with the greatest weight gain, after 112 days of direct contact with EO of MP.

Fig. 3
figure 3

Surface micrographs of robust polymer films

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The surface showed no changes after 112 days in direct contact with EO at room temperature, indicating that the presence of cracks, voids, and undulations can contribute to weight gain, as EO can penetrate and store in these space (Fig. 3).

FTIR

FTIR analysis was applied to identify characteristic absorption bands and is displayed in Fig. 4. This analysis provides important evidence for the reaction because it is sensitive and effective for structural characterization.

Fig. 4
figure 4

FTIR of samples: a casting polymer films, b, c spectra of PCL and PLA films, respectively, and MP essential oil and 112 days after at room temperature

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Figure 4a shows the IR spectrum of CMC, GA, GE, and SA polymers and their films recorded by the casting method. The characteristic bands remained present for all polymers after film formation, confirming the absence of the solvent. The spectrum of all films exhibits bands of 3668–2998 cm−1 attributed to O–H stretching for the hydroxyl group present in all polymers. The CMC band at 2898 cm−1 is due to C–H stretching vibration. The presence of a strong absorption band at 1588 cm−1 confirms the presence of the COO− group. The bands around 1416 and 1320 cm−1 are assigned to –CH2 scissoring and –OH bending vibration, respectively. The band at 1043 cm−1 is due to OCH–O–CH2 stretching [52, 53], and GA 3321 cm−1 O–H stretching, characteristic of glucosidic ring, 1605 cm−1 COO– symmetric stretching, 1437 cm−1 COO– asymmetric stretching, 1200–900 cm−1 finger print of carbohydrates [54]. The absorption bands for gelatin-based composite films containing CO in the IR spectra are situated in the amide band region. The band at 3276 cm−1 represents N–H stretching coupled with hydrogen bonding at GE; 1626 (amide I) represents C = O stretching/hydrogen bonding coupled with COO; 1522 (amide II) arises from bending vibration of N–H groups and stretching vibrations of C–N groups; 1240 (amide III) is related to the vibrations in plane of C–N and N–H groups of bound amide or vibrations of CH2 groups of glycine [47, 55]. The bands at 1592 and 1406 cm−1 present in the spectrum of SA are assigned to asymmetric and symmetric stretching bands of carboxylate salt groups. In addition, the bands around 1320 cm−1 (C–O stretching), 1130 cm−1 (C–C stretching), 1090 cm−1 (C–O stretching), 1020 cm−1 (C–O–C stretching), and 950 cm−1 (C–O stretching) are attributed to its saccharide structure [56, 57].

The spectrum of the MP oil is shown in Fig. 4b, c. The absorption at 3414 cm−1 is due to the stretching of hydroxyl groups that are present in the extract (corresponding to an alcohol group) one of its major compounds 37.94%. The band at 2969 and 2918 cm−1 is due to the C–H asymmetric and symmetric stretching of saturated (sp3) carbon, respectively, 2873 cm−1 ascribed to a methyl group. The band at 1695 cm−1 is assigned to the bending mode of absorbed water since plant extracts are known to have a strong affinity with water. The band at 1453 cm−1 is due to C = C stretching associated with the aromatic skeletal model of the extracts (1027, 1040 cm−1) attributed to the (C–O) bond. The weak bands at 1470 and 968 cm−1 are assigned to C–H bending and C–O skeletal vibrations, respectively. The band at 1358 cm−1 corresponds to the isopropyl group, all of which confirm the purity of the isolated material [58,59,60].

The PCL FTIR (Fig. 4b) showed characteristic bands at 2956–2848 (stretching C–H), 1727 (C = O), and 1167–1460 cm−1 (deformation CH2) [61]. As seen earlier (Fig. 2e), the PCL exhibited large and unequal crystals that led to the appearance of voids between these crystals, which must be filled with EO. When ATR FTIR was performed for PCL casting-MP, the reading possibly occurred on a surface that still contained retained EO, thus increasing their weight gain (Table 2).

PLA (Fig. 4c) –CH stretch 2912 cm−1 (asymmetric), 2848 cm−1 (symmetric), C = O carbonyl stretch 1746 cm−1, –CH3 bend 1447 cm−1, –CH deformation include symmetric and a symmetric band at 1358 cm−1, –C–O– stretch 1179 cm−1, –OH band 1078 cm−1, –C–C– stretch 874 cm−1 [62, 63].

Swelling studies

At a storage temperature of 4 °C for 7 days, the polymers showed similar values to those obtained at room temperature for 112 days (Table 2).

PCL and PLA obtained by both methods, immersed in AS oil, had complete dissolution. These results indicate that the AS oil dissolved the polymer forming micellar colloidal dispersions. The complete solubilization of these polymers is associated with their chemical composition that presents more than 88% eugenol, a chemical structure with aromatic ring. PCL and PLA are soluble in benzene [18].

Table 2 presents the percentage values of the acquired weight after 112 days of immersion. The values found for PCL* (by compression) are close to those mentioned by Flores et al. [4], with EO of Melaleuca alternifolia, which had a gain of 9.8% over the initial weight.

Two-way ANOVA was performed to compare the averages of the EO. Regardless of the polymer (E), the interaction between LH-CN and MA-EG is NS. Comparing the averages of polymers, independent of the EO (D), the interaction between PLA*-CMC is NS.

By comparing each polymer with the different EO (B), differences were not significant between PCL compared to the MA-CN EO. The same was true for the GE polymer. No significant differences were present when compared to the EO from LH-AS for SA, CN-MP for PLA*, and AS-MA for GA. Comparing each EO with the different polymers (C), no significant differences were noticed among CN samples, compared to the GE-CMC polymers and EG EO for these same polymers.

The PLA-CN presented a final weight gain of only 1.533% ± 0.373. PLA* showed gains which did not exceed 3% in the final weight. The final mass gain (%) for MA is NS. According to Guterres et al. [5], the PLA films obtained by compression molding were evaluated for swelling/solubility in benzyl benzoate and Miglyol 810°R for 13 days. After 48 h at room temperature, complete dissolution of the polymer in benzyl benzoate was observed, while for Miglyol the polymer masses remained unchanged after 13 days, indicating that there was no swelling or dissolution of PLA in this oil [5].

The GE film had a low interaction with all EO, not exceeding 1.3% weight gain by the end of the 112-day period. This low interaction may be related to the crosslinking of polyphenols with proteins (gelatine) under oxidation [64]. Gelatin is insoluble in less polar or nonpolar organic solvents. Amino acids are bound in gelatin by peptide bonds [65].

GA, CMC, and SA are polysaccharides and presented a low interaction with the EO. The highest weight gains are attributed to GA because of its cracked surface structure (Fig. 2b) [66,67,68]. SA swelling mechanism can be explained by the ionic exchange of monovalent Na+ ions from the saline solution with the Ca2+ ions present in the alginate chain polymanuronate and polyguluronate blocks. For this study, its maximum gain was 3.69% in LH [27]. CMC contains a hydrophobic polysaccharide backbone and many hydrophilic carboxyl groups, which show an amphiphilic characteristic, resulting in its low interaction with EO [52].

Figure 5 shows the weight gain (%) over time for all polymers with different EO over 112 days (1, 3, 5, 7, 14, 28, 56, and 112 days) at room temperature. In some cases the variation of the triplicate is so small, and it is not possible to notice differences in the error bars and does not appear in the graph (Fig. 5).

Fig. 5
figure 5

Weight gain (%) for all polymers with different EO over 112 days at room temperature

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Some polymers in different EO remained unchanged over time (112 days), such as GE-MP, CMC-LH/MA, SA-MP/MA, and PLA-LH/EG/MP/MA. The other polymers reached balance after 3 days of contact with the EO. According to Weiss-Angeli et al. [6] PCL films obtained by compression molding were evaluated for swelling in octyl methoxycinnamate for 13 days (n = 3). The experiment showed that polymer weights remained unchanged after 13 days, indicating no swelling or dissolution.

The sample that was stored at a temperature of 35 °C for 7 days showed values similar to those presented in Table 2. There was complete dissolution for PCL/PCL* in all EO. GA had a decrease in weight of approximately 6% for all EO. This may be related to the softening temperature of the polymers associated with the chemical composition of the EO. As chemical reactions in EO accelerate by increasing heat, van Hoff’s law states that a temperature increase of 10 °C approximately doubles the rates of a chemical reaction increases the amount of several unidentified polymerization products formed from myrcene [48, 69].

Also, oil oxidation accelerates with the concentration of dissolved oxygen, which, in turn, depends largely on oxygen partial pressure in the headspace as well as room temperature [70]. Resulting from this conversion (oxidation, isomerization, dehydrogenation reactions, or cyclization, triggered either chemically or enzymatically) they give rise to a range of stable oxidized secondary products such as mono- to polyvalent alcohols, aldehydes, ketones, epoxides, peroxides, or acids as well as highly viscous, often oxygen-bearing polymers [71,72,73].

These results indicate, except for PCL/PLA-AS, that the other EO and polymers have low macroscopic scale interactions, so much so that the degradation products of the EO did not interact with the polymers after a long period of contact. Therefore, it is likely that these polymers could serve as wall material in a process of encapsulating EO, which would delay their degradation.

Conclusion

This study identified the need for a prior compatibility test (dissolution/swelling) of polymeric materials to be used as core and shell in encapsulation processes. Depending on the processing method for obtaining polymeric films, their surface morphology may change, which may alter the macroscopic interaction results between the materials involved in a swelling process. The weight gain values at a temperature of 4 °C and 35 °C were similar to room temperature, except for PCL/PCL* completely dissolved for all EO, showing the need to perform this simple process, but temperature conditions in which the material will be exposed. Therefore, this work was limited to comparing 6 polymers, with a weight gain ratio PCL > GA > PCL* > AS > CMC > PLA > GE > PLA*, with 6 EO. Weight gain balance was obtained at 7 days, without the need for longer. It is important to mention that, in general, the results of this study showed that this simple and inexpensive step before processing the production of nano-/microcapsule or films with the incorporation of EO is valuable in ensuring process stability as seen in PCL/PCL* and PLA/PLA*, which completely dissolved in AS within 24 h, for all temperatures. Depending on the temperature and the EO used, all of the polymers studied here have the potential for use in EO encapsulation processes and/or films with the incorporation of EO. PCL and PLA become more advantageous, due to their large-scale production, making their cost less compared to others.