Melt-Spun Poly(D,L-Lactic Acid) Monoﬁlaments Containing N,N-Diethyl-3-Methylbenzamide As Mosquito Repellent

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Article Melt-Spun Poly(D,L-lactic acid) Monoﬁlaments Containing N,N-Diethyl-3-methylbenzamide as Mosquito Repellent

Ignatius Ferreira 1,2, Harald Brünig 1, Walter Focke 2 , Regine Boldt 1 , René Androsch 3 and Andreas Leuteritz 1,*

1 Leibniz-Institut für Polymerforschung e. V. Dresden (IPF), Hohe Strasse 6, D-01069 Dresden, Germany; [email protected] (I.F.); [email protected] (H.B.); [email protected] (R.B.) 2 Institute of Applied Materials and Institute for Sustainable Malaria Control, Department of Chemical Engineering, University of Pretoria, Private Bag X20, Hatﬁeld 0028, South Africa; [email protected] 3 Interdisciplinary Center for Transfer-Oriented Research in Natural Sciences, Martin Luther University Halle-Wittenberg, D-06099 Halle (Saale), Germany; [email protected] * Correspondence: [email protected]; Tel.: +49-351-4658-378

Abstract: Malaria is still a major tropical disease, with Africa particularly burdened. It has been proposed that outdoor protection could aid substantially in reducing the malaria incidence rate in rural African communities. Recently, melt-spun polyolefin fibers containing mosquito repellents have been shown to be promising materials to this end. In this study, the incorporation of N,N-Diethyl-3-methylbenzamide (DEET)—a popular and widely available mosquito repellent—in commercially available, amorphous poly(D,L-lactic acid) (PDLLA) is investigated with the aim of producing biodegradable mosquito-repelling filaments with a reduced environmental impact. It is shown to be possible to produce macroscopically stable PDLLA-DEET compounds containing up to

20 wt.-% DEET that can be melt-spun to produce ﬁlaments, albeit at relatively low take-up speeds. A critical DEET content allows for stress-induced crystallization during the spinning of the otherwise

Citation: Ferreira, I.; Brünig, H.; amorphous PDLLA, resulting in the formation of α-crystals. Although the mechanical integrity of Focke, W.; Boldt, R.; Androsch, R.; the filaments is notably impacted by the incorporation of DEET, these filaments show potential as Leuteritz, A. Melt-Spun materials that can be used for Malaria vector control. Poly(D,L-lactic acid) Monofilaments Containing N,N-Diethyl-3- Keywords: poly(lactic acid); DEET; melt spinning; filaments; Malaria; mosquito repellence methylbenzamide as Mosquito Repellent. Materials 2021, 14, 638. https://doi.org/10.3390/ ma14030638 1. Introduction In 2018, there were an estimated 228 million cases of Malaria worldwide with a co- Received: 14 December 2020 occurrence of 405,000 deaths of which children under the age of 5 years accounted for more Accepted: 25 January 2021 Published: 30 January 2021 than 65%. Moreover, the region that is the most adversely affected is Africa with a 93% burden of all Malaria cases. Progress has been made from 2010 until 2018 with a decline

Publisher’s Note: MDPI stays neutral in the worldwide Malaria incidence rate from 71 to 57 cases per 1000 population at risk. with regard to jurisdictional claims in Nevertheless, it is alarming that the incident rate has remained mainly unchanged for the published maps and institutional affil- past four years when Africa even saw a consistent increase in Malaria cases from 2014 to iations. 2018 [1]. In a recent study [2], mosquito-repelling socks were developed as a measure of protection for the ankles and feet of people in rural Africa as they are targeted by the malaria vector mosquito, Anopheles, in the late afternoon while still active outside [3,4]. The socks were successfully produced from melt-spun fibers that consisted of bicomponent sheath- Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. core structured filaments. N,N-Diethyl-3-methylbenzamide (DEET), the most commonly This article is an open access article used topical repellent active [2,5–7], was absorbed in poly(ethylene-co-vinyl-acetate) (EVA) distributed under the terms and that served as the core of the filaments while semi-crystalline high density polyethylene conditions of the Creative Commons (HDPE) served as a release barrier. By incorporating DEET in textiles, the need for frequent Attribution (CC BY) license (https:// reapplication, typical for topical repellents, was eliminated [2,8]. Not only is this more cost creativecommons.org/licenses/by/ effective, but also avoids overuse, reducing the potential skin reactions associated, albeit 4.0/). very rarely, with DEET [8,9].

Materials 2021, 14, 638. https://doi.org/10.3390/ma14030638 https://www.mdpi.com/journal/materials Materials 2021, 14, 638 2 of 14

Although effective, polyolefins are mostly petroleum-based polymers and are non- biodegradable. The concern is that the urban waste management infrastructure in African countries cannot cope with increased urbanization and that waste management infrastructure in rural areas is largely nonexistent [10]. In Nigeria, a country that is notably affected by Malaria, it is estimated that up to 83% of waste is mismanaged [11]. By considering this, there is a need to develop commercially available alternatives for the core and sheath materials to produce fibers that can provide protection against mosquitos with a reduced environmental impact after use. The objective of the present work was to explore the possibility of utilizing poly(lactic acid) (PLA) as an alternative as it is a commercially available, biodegradable, compostable, and biocompatible polyester that is obtained from short-term renewable sources [12–15]. Moreover, the application of PLA ranges from single use packaging to more durable electronic, biomedical, textile, semistructural and automotive applications [16–18]. The monomer of PLA has two optically active forms known as L-lactic acid and D-lactic acid that are caused by an asymmetric carbon atom [19]. This renders the possibility of the synthesis of two homopolymers, namely poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA) as well as D,L-lactic-acid copolymers (PDLLA). Generally, commercially available PLA is often rich in L-isomers containing only a few % D-isomers as defects [19]. Nonetheless, the D-isomer content is of particular significance as the presence of D-isomers causes a notable deterioration of the crystallization behavior, i.e., a reduction in the crystal melting temperature [19,20], the degree of crystallization and crystal growth rate [21]. Moreover, the higher the D-isomer content, the lower the critical cooling rate to suppress ordering of the melt (30 K·min−1 for 0% D-units and 3 K·min−1 for 4% D- units) [21]. Ultimately, at 10–12% D-isomer concentrations, or higher, crystallization is completely suppressed in L-rich PDLLA [19,21,22]. In contrast, the incorporation of DEET in PLLA has been shown to increase the rate of crystallization [23,24] and the maximum degree of crystallinity of PLLA [25]. For example, crystallization of pure PLLA could be suppressed at a cooling rate of 5 K·min−1, but not in a 50 wt.-% DEET solution [26]. This suggests that a higher critical cooling rate is able to suppress ordering of the melt in the presence of DEET. Moreover, as expected according to the Flory equation describing the equilibrium melting point depression of the polymer [27], a decrease in the crystal melting temperature [25] and crystallization temperature of PLLA have been observed with increasing DEET content. Besides the depression of the crystallization temperature, or, alternatively in different words, the solid−liquid (S−L) demixing temperature, the amorphous regions of the DEET- PLLA system exhibit liquid−liquid (L−L) demixing and a subambient upper critical solution temperature (UCST). The DEET content of the system investigated in the study of [27] ranged from 70 to 95 wt.-% and showed a critical temperature of 10 ◦C occurring at 95 wt.-% DEET. As a result, a DEET-rich PLLA phase and a near pure DEET phase formed [27]. Worth noting, a similar investigation of the L−L demixing behavior has been recently performed on the system PLA/ethyl butylacetylaminopropionate (PLA/IR3535), that is, using a different promising mosquito repellent [28]. To the best of our knowledge, the DEET-PLA phase diagram for polymer-rich compo- sitions is unknown with only a single study performed that investigated samples with 3 and 4.5 wt.-% DEET [23]. Moreover, DEET-containing PLA filaments have not been spun yet by means of melt-spinning. This study aims to address the former as a prerequisite to melt-spinning PLA filaments with a DEET content higher than 4.5 wt.-% for a specific noncrystallizable PDLLA grade, as the crystallization effects lead to nonuniform distribution of DEET due to its enrichment in the amorphous regions and possible L−L demixing. Likewise, the DEET enrichment in amorphous regions caused by crystallization effects could increase the release rate of the DEET. It was aimed to address three areas of interest in this study, i.e., the solubility of DEET in commercially available noncrystallizable PDLLA, the general melt-spinnability of DEET-PDLLA solutions/compounds, and the characterization of DEET-containing PDLLA Materials 2021, 14, 638 3 of 14

monoﬁlaments with varying DEET mass fractions. For characterization, the following methods were considered: thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), X-ray diffraction (XRD), light microscopy, and scanning electron microscopy (SEM).

2. Materials and Methods 2.1. Materials A commercially available PLA spinning grade was selected based on the information provided by the manufacturer regarding its crystallizability. To establish the molar mass, size exclusion chromatography (SEC) was employed using chloroform as the mobile phase. The D-unit content was determined by polarimetry where the optical rotation of 1 g dL−1 solutions of PLA were measured at a wavelength of 589 nm. The speciﬁc optical rotation values for pure PLLA and PDLA were determined as −166◦ and 166◦. The properties of the PLA grade used are given in Table1. As the D-unit content of the PLA is more than 10%, it can be regarded as amorphous PDLLA.

Table 1. Molar percentage of D-units (XD), mass-average molar mass (Mw), number-average molar mass (Mn), and polydispersity (D) of PLA used in the present work.

XD (%) Mw (kDa) Mn (kDa) D (-) 11.3 147.0 46.8 3.15

2.2. Preparation of DEET-PDLLA Solutions/Compounds The as-received PDLLA pellets were dried in a vacuum oven at 40 ◦C for a minimum of 8 h to remove moisture and were subsequently kept in a desiccator with silica gel. For the determination of the solubility of DEET in the PDLLA, the pellets and DEET were heated to 130 ◦C in glass vials in a silicon-oil bath on a hot plate to produce samples (≈10 g) with varying DEET content. Three low-DEET-content samples were prepared with 10, 20 and 30 wt.-% DEET as well as four high-DEET-content samples with 70, 80, 90, and 95 wt.-% DEET. For the high-DEET-content samples, a magnetic stirrer at 200 rpm was used as the samples had a low viscosity at 130 ◦C. It was, however, not possible to make use of the magnetic stirrer in the case of the low-DEET-content samples. These samples were easily pliable at 130 ◦C and were occasionally compounded manually with a spatula to ensure a homogenous system. Complete dissolution of the pellets in all instances was achieved after 10 to 20 min. Care was a taken to cover the glass vials to minimize the loss of DEET. The samples were then allowed to cool in the silicon bath to room temperature (21 ◦C) during which they were brieﬂy removed from the silicon bath at different temperatures to be photographed. The cooling time was determined every 5 K to estimate the cooling rate. In the high-temperature interval (where the cooling rate was the fastest) the cooling rate was approximately 3 K·min−1. For the melt-spinning trials, DEET-PDLLA samples of 20–30 g with DEET contents of 10 and 20 wt.-% were prepared in a similar fashion. Unfortunately, spinning pretrials indicated that spinning the 30 wt.-% samples was not possible. However, the glass vials were exchanged for Petri dishes on a hot plate so that circular discs could be obtained upon cooling. These could then be removed and cut into pellets with pliers for further use. The high surface area of the Petri dishes also allowed for more uniform heating of the samples while the samples were covered with glass discs to minimize the loss of DEET.

2.3. Monofilament Spinning Monofilaments were spun with an in-house designed-and-built melt-spinning device at the Leibniz-Institut für Polymerforschung Dresden e.V. (Dresden, Germany). The device consists of a heated cylinder with a piston with which the volumetric flow rate can be controlled. The piston speed during spinning was kept constant at 10 mm·min−1 and by doing so the approximate mass-flow rate was kept constant throughout all spinning trials Materials 2021, 14, 638 4 of 14

at roughly 0.8 g min−1. For the determination of the spinning range, take-up speeds of up to 1500 m·min−1 were investigated. In all instances, the relevant polymer in the cylinder was heated to 190 ◦C for approximately two minutes before spinning commenced. The holding time was kept to a minimum as release of DEET vapor could be observed at this temperature. All experiments were performed in a room at ambient temperature and no forced convection was used for cooling.

2.4. Differential Scanning Calorimetry (DSC) All DSC trials were performed on a TA Instruments (New Castle, DE, USA) Model Q2000 using nitrogen as purge gas at a ﬂow rate of 50 mL min−1. The samples with a mass of about 10 mg, used for the determination of solubility, were transferred to 50-µL aluminum pans and sealed while at 130 ◦C. The high-DEET-content samples were transferred via a syringe while the low-DEET content samples were transferred via a spatula. The samples were held isothermally at 130 ◦C for 5 min, before cooling to −20 ◦C at a rate of 2 K·min−1. For the characterization of the spun monoﬁlaments, samples with a mass of between 1.5 and 3 mg were cut with scissors and sealed in the pans. The samples were held isothermally at −30 ◦C for 5 min, before they were heated to 230 ◦C at a rate of 5 K·min−1. The results were analyzed with the TA Universal Analysis software. All reported values represent the average of two measurements.

2.5. Thermogravimetric Analysis (TGA) All TGA trials were performed on a TA Instruments (New Castle, DE, USA) Model Q5000. All samples were kept isothermally in 50-µL alumina pans at 22 ◦C for 10 min, before heating at 10 K·min−1 to 500 ◦C. The heating chamber was purged with nitrogen at a rate of 25 mL min −1. The results were analyzed with the TA Universal Analysis software.

2.6. X-ray Diffraction (XRD) XRD experiments were performed using a Bruker D8 Advance (Billerica, MA, USA) using CuKα radiation (λ = 0.154018 nm). The 2D-diffraction patterns were recorded at room temperature on a 2048 × 2048 pixel Bruker Vantec 500 (Billerica, MA, USA area detector set at a distance of 145.5 mm from the sample. Bundles of filaments were fixed between two clamps. The measurements were performed in transmission mode with a slit size of 0.5 mm perpendicular to the “melt-spin direction”. For phase identification, the 2D patterns were azimuthally integrated to obtain one-dimensional profiles of the scattered intensity as a function of the scattering angle 2Θ = 2.5◦ to 40◦. The XRD analysis was used as a qualitative tool only, i.e., for phase identification and not for quantification of phase fractions or crystal orientations.

2.7. Scanning Electron Microscopy The melt-spun monoﬁlaments were embedded in an epoxy resin and then cut at −120 ◦C with a Leica UC 7 ultramicrotome (Wetzlar, Germany). The samples were then sputter-coated with 3 nm platinum and imaged with a Zeiss Ultra Plus (Oberkochen, Germany) scanning electron microscope operated at a voltage of 3 kV.

2.8. Tensile Tests The fineness (linear density) of the filaments was determined by weighing bundles of three filaments of 1 m length. The fineness was taken as the average of three of such measurements. Tensile tests were performed at 21 ◦C on single filaments by using a Zwick Roell Z0,5 Tensile Testing Apparatus (Ulm, Germany). The reported tensile properties represent the average of at least five measurements with a clamp distance of 50 mm and a deformation rate of 200 mm·min−1. Materials 2021, 14, x FOR PEER REVIEW 5 of 15

Materials 2021, 14, 638 5 of 14 measurements. Tensile tests were performed at 21 °C on single filaments by using a Zwick Roell Z0,5 Tensile Testing Apparatus (Ulm, Germany). The reported tensile properties represent the average3. Results of at least five measurements with a clamp distance of 50 mm and a deformation rate3.1. of DEET-PLA200 mm·min− Phase1. Behaviour Figure1 shows the effect of temperature on the phase behavior of PDLLA-DEET 3. Results mixtures of varying DEET content. From a macroscopic view, complete dissolution of the 3.1. DEET-PLA Phase Behaviour PDLLA pellets was observed after about 10 to 20 min at 130 ◦C (see column 2 in Figure1 ). Figure 1 shows the effect of temperature on the phase behavior of PDLLA-DEET mix- This was the case over the entire investigated DEET-content range. The mixture contain- tures of varying DEET content. From a macroscopic view, complete dissolution of the PDLLA pellets wasing observed 10 wt.-% after DEET about remained10 to 20 min clearat 130 when°C (see cooledcolumn 2 down in Figure even 1). to the lowest temperature This was the caseconsidered. over the entire This investigat was alsoed DEET-content true for the range. 20 and The 30 mixture wt.-% contain- DEET samples (not shown here), ing 10 wt.-% DEETsuggesting remained theclear absence when cooled of macroscopic down even to phase the lowest separation. temperature Contrarily, mixtures contain- considered. This ingwas 70also and true95 for wt.-% the 20appeared and 30 wt.-% opaque DEET atsamples 21 ◦C, (not indicative shown here), of phase separation occurring suggesting the absencebetween of macroscopic 30 and 21 phase◦C[27 separation.]. Moreover, Contrarily, bulk mixtures phase separationcontaining was clearly observed for 70 and 95 wt.-% samplesappeared opaque containing at 21 95°C, andindicative 90 wt.-% of phase (not separation shown here)occurring at 21 be-◦C, with the formation of a tween 30 and 21 °C [27]. Moreover, bulk phase separation was clearly observed for samples containing 95clear, and 90 low-viscosity wt.-% (not shown liquid here) phase at 21 (indicated°C, with the formation by the red of arrowsa clear, in Figure1) on top of the low-viscosity liquidviscous phase phase. (indicated by the red arrows in Figure 1) on top of the viscous phase.

Figure 1. DEET-PDLLA samples with 10, 70, and 95 wt.-% DEET at varying temperatures during cooling. Figure 1. DEET-PDLLA samples with 10, 70, and 95 wt.-% DEET at varying temperatures during cooling.

The composition of the low-viscosity liquid phase (separated at 21 °C), was deter- ◦ mined by TGA analysis.The This composition is shown in Figure of the 2 where low-viscosity pure DEET liquid is also phaseincluded (separated as a at 21 C), was deter- reference. It was minedclear that by the TGA low-viscosit analysis.y phase This isat shownthis temperature in Figure approached2 where pure pure DEET is also included as a DEET in nature andreference. this was in It good was clearagreemen thatt with the low-viscosityprevious findings phase [27,29]. at However, this temperature approached pure these results do DEETnot indicate in nature whether and the this phase was separation in good at agreement these high withDEET previous contents ﬁndings [27,29]. However, was due to crystallizationthese results (S−L demixing) do not indicate followed whether by L−L demixing the phase or only separation due to L− atL these high DEET contents demixing. To shedwas light due on to this crystallization matter, the DSC (S− Lcooling demixing) curves followedfor the solutions by L− Land demixing or only due to L−L Materials 2021, 14PDLLA, x FOR PEER without REVIEW DEET (cooling rate 2 K·min−1) are shown in Figure 3a. 6 of 15 demixing. To shed light on this matter, the DSC cooling curves for the solutions and PDLLA without DEET (cooling rate 2 K·min−1) are shown in Figure3a.

FigureFigure 2. 2.ThermogravimetricThermogravimetric analysis analysis (TGA) (TGA)curves of curves pure PDLLA, of pure pure PDLLA, DEET pure and the DEET low andvis- the low viscosity cosityphase phase separated separated at 21at 21◦C. °C.

Figure 3. Differential scanning calorimetry (DSC) cooling curves (2 K·min−1) of the PDLLA-DEET system at different heat- flow rate scales: (a) exhibiting the glass transition of the low-DEET-content samples; (b) exhibiting the demixing peaks of high-DEET samples.

No exothermic crystallization event could be observed for any sample within the investigated temperature range of 130 °C to −20 °C, with particular reference to the range between 30 °C and 21 °C. This was significant since it confirmed that the higher opacity of the samples with 70–90 wt.-% DEET at 21 °C was not a result of S−L demixing, but only L−L demixing. Moreover, it showed that if crystallization is theoretically possible, then it was completely suppressed at a cooling rate of 2 K min-1, regardless of the DEET content. This was a result of the high D-unit content of 11.3% of the present PDLLA as the crystallization of PDLLA with a 4% D-unit content was only suppressed at 3 K·min−1. In theory, even a higher cooling rate would be required to suppress crystallization of PDLLA in the presence of DEET [24]. L−L demixing was further evidenced by the detection of small heats of dissolution for these solutions, shown in Figure 3b where the same cooling curves as in Figure 3a are shown, but with magnified scale. The lower the DEET content, the lower the demixing temperature—in line with previous investigations [27,29]. The demixing temperatures for the 90 wt.-% (18 °C) and 95 wt.-% (19 °C) DEET solutions, were in the near vicinity of 21 °C where bulk phase separation had been observed. However, the demixing temperature of the 70 wt.-% solution (6 °C) was far lower than the 21 °C at which the opacity had already changed. A possible explanation is the difference in cooling rates between the two methods. The cooling rate for the DSC measurement was constant at 2 K min-1 for the entire range, in contrast to the cooling rate for the solutions in Figure 2 that decreased with

Materials 2021, 14, x FOR PEER REVIEW 6 of 15

Figure 2. Thermogravimetric analysis (TGA) curves of pure PDLLA, pure DEET and the low vis- Materials 2021, 14cosity, 638 phase separated at 21 °C. 6 of 14

Figure 3. Differential scanning calorimetry (DSC) cooling curves (2 K·min−1) of the PDLLA-DEET system at different heat-flow rate scales: (a) exhibiting the glass transition of the low-DEET-content samples; (b) exhibiting the demixing peaks Figure 3. Differentialof high-DEETscanning calorimetry samples. (DSC) cooling curves (2 K·min−1) of the PDLLA-DEET system at different heat- flow rate scales: (a) exhibiting the glass transition of the low-DEET-content samples; (b) exhibiting the demixing peaks of No exothermic crystallization event could be observed for any sample within the high-DEET samples. investigated temperature range of 130 ◦C to −20 ◦C, with particular reference to the range ◦ ◦ No exothermicbetween crystallization 30 C and event 21 C.could This be was observed significant for any since sample it confirmed within the that in- the higher opacity ◦ − vestigated temperatureof the samplesrange of with130 °C 70–90 to −20 wt.-% °C, DEETwith particular at 21 C wasreference not a to result the range of S L demixing, but only L−L demixing. Moreover, it showed that if crystallization is theoretically possible, between 30 °C and 21 °C. This was significant since it confirmed that the higher-1 opacity of the samples withthen 70–90 it was wt.-% completely DEET at suppressed 21 °C was not at aa coolingresult of rate S−L of demixing, 2 K min but, regardless only of the DEET content. This was a result of the high D-unit content of 11.3% of the present PDLLA as the L−L demixing. Moreover, it showed that if crystallization is theoretically possible, then it crystallization of PDLLA with a 4% D-unit content was only suppressed at 3 K·min−1. In was completely suppressed at a cooling rate of 2 K min-1, regardless of the DEET content. theory, even a higher cooling rate would be required to suppress crystallization of PDLLA This was a result of the high D-unit content of 11.3% of the present PDLLA as the crystal- in the presence of DEET [24]. lization of PDLLA with a 4% D-unit content was only suppressed at 3 K·min−1. In theory, L−L demixing was further evidenced by the detection of small heats of dissolution even a higher cooling rate would be required to suppress crystallization of PDLLA in the for these solutions, shown in Figure3b where the same cooling curves as in Figure3a are presence of DEET [24]. shown, but with magnified scale. The lower the DEET content, the lower the demixing L−L demixing was further evidenced by the detection of small heats of dissolution temperature—in line with previous investigations [27,29]. The demixing temperatures for for these solutions, shown in Figure 3b where the same cooling curves as in Figure 3a are the 90 wt.-% (18 ◦C) and 95 wt.-% (19 ◦C) DEET solutions, were in the near vicinity of 21 ◦C shown, but with magnified scale. The lower the DEET content, the lower the demixing where bulk phase separation had been observed. However, the demixing temperature temperature—in ofline the with 70 previous wt.-% solution investigatio (6 ◦C)ns was [27,29]. far lowerThe demixi thanng the temperatures 21 ◦C at which for the opacity had the 90 wt.-% (18 °C)already and changed.95 wt.-% (19 A possible °C) DEET explanation solutions, is were the difference in the near in vicinity cooling of rates 21 between the two °C where bulk phasemethods. separation The coolinghad been rate observed. for the DSCHowever, measurement the demixing was temperature constant at 2 K min-1 for the of the 70 wt.-% solutionentire range, (6 °C) in contrastwas far tolower the coolingthan the rate 21 for°C theat which solutions the in opacity Figure2 had that decreased with already changed.decreasing A possible temperature.explanation is In the the difference latter case, in thecooling average rates cooling between rate the between two 30 and 21 ◦C -1 methods. The coolingof the rate solutions for the could DSC be measurement approximated was as 0.3constant K·min −at1 .2 K min for the entire range, in contrastMoreover, to the cooling glass rate transitions for the weresolutions only in observable Figure 2 that for thedecreased PDLLA with without DEET (onset 58 ◦C) and the solutions with 10 wt.-% (onset 33 ◦C), 20 wt.-% (onset 16 ◦C) and 30 wt.- % DEET (onset −9 ◦C). The only slight broadening of the glass transition temperature (Tg) with DEET, together with the continuous decrease of the Tg with increasing DEET confirmed that DEET and PLA are miscible at 10, 20 and 30 wt.-% [23,30,31]. Nevertheless, some microscopic heterogeneities may exist. In Figure4, the Tgs (blue diamond symbols) and demixing temperatures (green triangle symbols) are shown as well as the composition of the low viscosity phase at 21 ◦C (yellow square symbol). The predicted theoretical Tgs, by fitting the Gordon−Taylor Equation, are given by the solid red line. The phase diagram is in good agreement with the phase diagram proposed in reference [27]. The amorphous PLA exhibited a UCST at approximately 21 ◦C and 95 wt.-% DEET with the formation of a DEET-rich PDLLA phase and near pure DEET phase. It should, however, be noted that the critical temperature was notably higher than the reported 10 ◦C. Nonetheless, of significance for the purposes of melt spinning DEET- containing PDLLA filaments, it has been shown that a macroscopically homogenous Materials 2021, 14, x FOR PEER REVIEW 7 of 15

decreasing temperature. In the latter case, the average cooling rate between 30 and 21 °C of the solutions could be approximated as 0.3 K·min−1. Moreover, glass transitions were only observable for the PDLLA without DEET (onset 58 °C) and the solutions with 10 wt.-% (onset 33 °C), 20 wt.-% (onset 16 °C) and 30 wt.- % DEET (onset −9 °C). The only slight broadening of the glass transition temperature (Tg) with DEET, together with the continuous decrease of the Tg with increasing DEET confirmed that DEET and PLA are miscible at 10, 20 and 30 wt.-% [23,30,31]. Nevertheless, Materials 2021, 14, 638 some microscopic heterogeneities may exist. 7 of 14 In Figure 4, the Tgs (blue diamond symbols) and demixing temperatures (green triangle symbols) are shown as well as the composition of the low viscosity phase at 21 °C (yellowPDLLA-DEET square symbol). phase The exists predicted at DEET theoretical contents Tgs, of up by to fitting at least the 20 Gordon wt.-% where−Taylor the Equa- DEET has tion, area signiﬁcant given by impactthe solid on red the line. glass transition temperature.

Figure 4. Phase diagram of the polymer/solvent system PDLLA/DEET containing information Figure 4. Phase diagram of the polymer/solvent system PDLLA/DEET containing information about aboutglass transition glass transition temperatures temperatures in low-D inEET-content low-DEET-content samples samples (blue diamond (blue diamond symbols) symbols) and and temperaturestemperatures of demixing of demixing of high-DEET-content of high-DEET-content samples samples (green (green triangle triangle symbols). symbols). 3.2. Spinnability of PDLLA Monoﬁlaments with Varying DEET Content The phase diagram is in good agreement with the phase diagram proposed in refer- In Figure5, the attainable take-up speeds of the PDLLA monoﬁlaments with varying Materials 2021, 14, x FOR PEER REVIEWence [27]. The amorphous PLA exhibited a UCST at approximately 21 °C and 958 wt.-% of 15 DEETDEET with contentthe formation are shown of a DEET-rich together with PDLLA the approximated phase and near associated pure DEET draw-down phase. It ratio · −1 should,(DDR however,). Note thatbe noted the maximum that the critical take-up temperature speed investigated was notably did not higher exceed than 1500 the m re-min . ported 10 °C. Nonetheless, of significance for the purposes of melt spinning DEET-containing PDLLA filaments, it has been shown that a macroscopically homogenous PDLLA- DEET phase exists at DEET contents of up to at least 20 wt.-% where the DEET has a significant impact on the glass transition temperature.

3.2. Spinnability of PDLLA Monofilaments with Varying DEET Content In Figure 5, the attainable take-up speeds of the PDLLA monofilaments with varying DEET content are shown together with the approximated associated draw-down ratio (DDR). Note that the maximum take-up speed investigated did not exceed 1500 m·min−1.

Figure 5.Figure Attainable 5. Attainable take-up take-upspeeds for speeds melt-spun for melt-spun DEET-containing DEET-containing PDLLA monofilaments PDLLA monoﬁlaments as a as a functionfunction of DEET of content. DEET content.

The pureThe PDLLA pure PDLLA monofilaments monofilaments could be could spun beat the spun maximum at the maximum take-up speed take-up in- speed investigated, i.e., 1500 m·min−1. Conversely, the addition of DEET resulted in a notable vestigated, i.e., 1500 m·min−1. Conversely, the addition of DEET resulted in a notable de- decrease of the attainable take-up speed to 500 m·min−1 and 400 m·min−1 for 10 wt.-% crease of the attainable take-up speed to 500 m·min−1 and 400 m·min−1 for 10 wt.-% and 20 and 20 wt.-% DEET, respectively. This shows that DEET-containing PDLLA monofilaments wt.-% DEET, respectively. This shows that DEET-containing PDLLA monofilaments can can indeed be produced, although only at relatively low take-up speeds compared to the indeed be produced, although only at relatively low take-up speeds compared to the gen- general standard of at least 1000 m·min−1 [32]. This is likely due to the marked decrease in eral standard of at least 1000 m·min−1 [32]. This is likely due to the marked decrease in viscosity due to DEET addition. For comparison with samples containing 20 wt.-% PDLLA, viscosity due to DEET addition. For comparison with samples containing 20 wt.-% monofilaments were spun at 500 m·min−1 in all other instances for further analyses. PDLLA, monofilaments were spun at 500 m·min−1 in all other instances for further analyses. 3.3. Semicrystalline Morphology DSC heating scans, recorded at a rate of 5 K·min−1, of PDLLA monofilaments with 3.3. Semicrystalline Morphology varying DEET content are shown in Figure6. As reference, the heating scan of as-received DSC heating scans, recorded at a rate of 5 K·min−1, of PDLLA monofilaments with varying DEET content are shown in Figure 6. As reference, the heating scan of as-received PDLLA granulate without DEET is also included. Note that the slopes were adjusted manually to fit the general trend of increasing heat capacity (Cp) with increasing temperature.

Figure 6. DSC heating scans, recorded at a heating rate of 5 K·min−1, of PDLLA monofilaments containing different amounts of DEET.

The PDLLA granulate and the PDLLA monofilaments without DEET exhibited only a single glass transition temperature at 55 °C that was superimposed by an endothermic enthalpy-recovery peak, which was expectedly greater for the PDLLA granulate [25,33]. The absence of cold crystallization and melting upon heating implies that neither crystals

Materials 2021, 14, x FOR PEER REVIEW 8 of 15

Figure 5. Attainable take-up speeds for melt-spun DEET-containing PDLLA monofilaments as a function of DEET content.

The pure PDLLA monofilaments could be spun at the maximum take-up speed investigated, i.e., 1500 m·min−1. Conversely, the addition of DEET resulted in a notable decrease of the attainable take-up speed to 500 m·min−1 and 400 m·min−1 for 10 wt.-% and 20 wt.-% DEET, respectively. This shows that DEET-containing PDLLA monofilaments can indeed be produced, although only at relatively low take-up speeds compared to the general standard of at least 1000 m·min−1 [32]. This is likely due to the marked decrease in viscosity due to DEET addition. For comparison with samples containing 20 wt.-% PDLLA, monofilaments were spun at 500 m·min−1 in all other instances for further analyses.

Materials 2021, 14, 638 3.3. Semicrystalline Morphology 8 of 14 DSC heating scans, recorded at a rate of 5 K·min−1, of PDLLA monofilaments with varying DEET content are shown in Figure 6. As reference, the heating scan of as-received PDLLAPDLLA granulate granulate without without DEET DEET is also is included. also included. Note Notethat the that slopes the slopes were wereadjusted adjusted man- man- ually to ﬁt the general trend of increasing heat capacity (Cp) with increasing temperature. ually to fit the general trend of increasing heat capacity (Cp) with increasing temperature.

−1 FigureFigure 6. DSC 6. heatingDSC heating scans, scans,recorded recorded at a heating at a heating rate of 5 rate K·min of 5−1,K of·min PDLLA, of monofilaments PDLLA monoﬁlaments containingcontaining differen differentt amounts amounts of DEET. of DEET.

The PDLLA granulate and the PDLLA monofilaments without DEET exhibited only The PDLLA granulate and the PDLLA monofilaments◦ without DEET exhibited only a singlea single glass glasstransition transition temperature temperature at 55 at°C 55 thatC was that superimposed was superimposed by an by endothermic an endothermic enthalpy-recoveryenthalpy-recovery peak, peak, which which was expectedly was expectedly greater greater for the for PDLLA the PDLLA granulate granulate [25,33]. [25 ,33]. The absence of cold crystallization and melting upon heating implies that neither crystals The absence of cold crystallization and melting upon heating implies that neither crystals nor nuclei formed during the spinning process. By considering the high cooling rates that are typical of the melt-spinning process [34], it is in line with the findings in Section 3.1 where a cooling rate as low as 2 K·min−1 had already completely suppressed crystallization. However, the melt-spinning process is typically characterized by high elongational strain rates [35] that have been shown to notably increase the degree of crystallization by stress- induced crystallization [36]. Nonetheless, it seems that the high D-unit content prevented not only thermally induced crystallization but also stress-induced crystallization in the pure PDLLA filaments and when DEET was incorporated at a concentration of 10 wt.-%. In contrast, at 20 wt.-% DEET, an exothermic cold-crystallization peak at 68 ◦C and endothermic melting peak at 157 ◦C, were observed. The heat of melting exceeded the heat of cold crystallization as shown in Table2, indicating that minor crystallization took place during the spinning process regardless of the high cooling rate. Upon heating, additional nuclei continued to grow during the cold-crystallization event. Note that no crystallization was observed for the 20 wt.-% DEET solution cooled at 2 K·min−1 in Section 3.1. This implies that the high elongational strain rate during the spinning process plays a determinant role in the crystallization process of these filaments. A possible explanation is that the DEET allows for a greater mobility of PDLLA chain segments, thus allowing their transition to an ordered state more easily when drawn during the melt-spinning process. This is in agreement with the study of [25] that reported that no crystallization had occurred during electrospinning of pure PLLA, but the incorporation of DEET markedly increased crystallization.

Table 2. Summary of thermal events for pure PDLLA granulate and PDLLA monoﬁlaments with varying DEET content.

Spun PDLLA with Spun PDLLA with Pure PDLLA Granulate Pure Spun PDLLA 10 wt.-% DEET 20 wt.-% DEET Tg onset (◦C) 53.6 ± 0.6 55.1 ± 3.9 27.4 ± 1.6 18.5 ± 0.1 CC peak (◦C) - - - 67.5 ± 0.1 Melting peak (◦C) - - - 156.4 ± 0.5 ∆Hm (J/g) - - - 11.4 ± 0.1 ∆Hcc (J/g) - - - 2.3 ± 0.4

XRD measurements, shown in Figure7, were performed to conﬁrm the presence of Materials 2021, 14, 638 9 of 14

crystals and to establish the crystal form of the monofilaments with 20 wt.-% DEET. To replicate the effect of cold crystallization during heating, the filaments were annealed in an oven for 1 h at 80 ◦C. The XRD scans of the filaments after spinning (Position A in Figure6) Materials 2021, 14, x FOR PEER REVIEW 10 of 15 are shown in Figure7a whereas the scans of the annealed filaments are shown in Figure7b (Position B in Figure6). Crystallizable PLLA annealed at 80 ◦C is also included as reference in both instances.

Figure 7. XRD-scans of PDLLA monofilaments with varying DEET content (a) after spinning and (b) after additional Figure 7. XRD-scans of PDLLA monofilaments with varying DEET content (a) after spinning and (b) after additional sub- ◦ subsequent annealingsequent at annealing 80 C for 1at h. 80 °C for 1 h. Crystallizable PLLA forms α’-crystals at temperatures below 120 ◦C. This morphology Crystallizable PLLA forms α’-crystals at temperatures below 120 °C. This morphol- is a less stable crystal structure with a looser chain packing compared to the stable α-form ogy is a less stable crystal structure with a looser chain packing compared to the stable α- that forms at temperatures exceeding 100 ◦C[19,37–41]. The presence of α’-crystals was form that forms at temperatures exceeding 100 °C [19,37–41]. The presence of α’-crystals confirmed by the intense peaks at 16.6◦ 2θ (planes (110)/(200)) and 19.1◦ (planes(203)/(113)) was confirmed by the intense peaks at 16.6° 2θ (planes (110)/(200)) and 19.1° as well as the absence of the (211) peak at 22.4◦ [42,43]. (planes(203)/(113)) as well as the absence of the (211) peak at 22.4° [42,43]. From Figure7a, a halo is visible for the PDLLA monofilaments containing 0 and 10 wt.- From Figure 7a, a halo is visible for the PDLLA monofilaments containing 0 and 10 % DEET. This confirms that these filaments did at most undergo miniscule crystallization wt.-% DEET. This confirms that these filaments did at most undergo miniscule crystalli- during the spinning process, being in line with the DSC results. Conversely, the PDLLA zation during the spinning process, being in line with the DSC results. Conversely, the monofilaments with 20 wt.-% DEET showed intense peaks at 16.8◦ (planes (110)/(200)) and 19.1◦ (planesPDLLA (203)/(113)), monofilaments i.e., a general with shift20 wt.-% to a higher DEET 2θ showedcompared intense to the reference.peaks at 16.8° (planes This, and a peak(110)/(200)) at 22.4◦ (plane and 19.1° (211)) (planes confirmed (203)/(113)), the presence i.e., a of general the α-crystal shift to form a higher [42,43 2].θ compared to In Figure7theb,the reference. annealed This, PDLLA and monofilamentsa peak at 22.4° (plane containing (211)) 20 confirmed wt.-% DEET the exhibited presence of the α-crys- an intensificationtal form of the [42,43]. existing peaks. This implies that the cold-crystallization event observed during heatingIn Figure leads 7b, to the the annealed continued PDLLA formation monofi andlaments growth containing of α-crystals. 20 wt.-% Sig- DEET exhib- nificantly, the emergenceited an intensification of the (110)/(200) of the existing peak at peaks. 16.8◦ 2Thisθ in implies the case that of 10 the wt.-% cold-crystallization DEET event filaments was alsoobserved observed during when heating annealed. leads Since to th noe continued cold crystallization formation was and observed growth of for α-crystals. Sig- the same filamentsnificantly, during the the emergence DSC heating of th scan,e (110)/(200) it can be concludedpeak at 16.8° that 2θ the in DSCthe case heating of 10 wt.-% DEET rate had also playedfilaments a determinant was also observed role. when annealed. Since no cold crystallization was observed By establishingfor the thesame nature filaments of the during crystals the that DSC formed heatin ing thescan, 20 it wt-% can be DEET, concluded it was that the DSC possible to estimateheating the rate degree had ofalso crystallinity played a determinant at points A androle. B in Figure6. The bulk heat of melting (∆H◦m_αBy) ofestablishingα-crystals is the 135 nature J g−1 and of the 91 Jcrystals g−1 at 157 that◦C formed and 68 ◦inC the respectively, 20 wt-% DEET, it was as estimated frompossible [43]. to This estimate resulted the indegree crystallinities of crystallinity at points at points A and A B and (in FigureB in Figure6) of 66. The bulk heat and 8%, respectively,of melting for the(∆H°m_ filamentsα) of α with-crystals 20 wt.-% is 135 DEET. J g−1 and 91 J g−1 at 157 °C and 68 °C respectively, In summary,as estimated for the given from DEET-PDLLA[43]. This resulted compounds in crystallinities containing at points 10 and A and 20 wt.-% B (in Figure 6) of 6 DEET, crystallizationand 8%, wasrespectively, not observed for the when filaments cooling with at 20 2 wt.-% K·min DEET.−1. However, if the 20 wt.-% solution wasIn spun,summary,α-crystals for the formed. given ThisDEET-PDLLA occurred regardless compounds of the containing high cooling 10 and 20 wt.-% rate during theDEET, melt-spinning crystallization process was and not suggested observed that when the cooling high elongational at 2 K·min−1 strain. However, rate if the 20 wt.- of the process was% solution a decisive was factor, spun, promoting α-crystals stress-inducedformed. This occurred crystallization. regardless Nevertheless, of the high cooling rate this was not observedduring the for melt-spinning lower DEET process contents, and implying suggested that that a critical the high DEET elongational content strain rate of allows for increasedthe process segmental was a decisive mobility factor, for the promoting chains to st assembleress-induced in an crystalli orderedzation. state Nevertheless, during the spinningthis was process. not observed for lower DEET contents, implying that a critical DEET content allows for increased segmental mobility for the chains to assemble in an ordered state during the spinning process.

3.4. DEET Content and Distribution Derivative TGA-mass-loss curves of DEET and the PDLLA filaments are given in Figure 8a–d. From Figure 8a, where no DEET was present in the filaments, it was observed that the PDLLA filaments have a thermal degradation peak at 344 °C, with completion

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3.4. DEET Content and Distribution Materials 2021, 14, x FOR PEER REVIEW Derivative TGA-mass-loss curves of DEET and the PDLLA11 of ﬁlaments15 are given in

Figure8a–d. From Figure8a, where no DEET was present in the ﬁlaments, it was observed that the PDLLA ﬁlaments have a thermal degradation peak at 344 ◦C, with completion near 380 ◦C. The onset of mass loss for pure DEET was at around 95 ◦C with a peak at near 380 °C. The onset of◦ mass loss for pure DEET was◦ at around 95 °C with a peak at 176 °C and completion by176 190C °C. and completion by 190 C.

Figure 8. TGA derivative-mass-lossFigure 8. TGA curves derivative-mass-loss of DEET and PDLLA curves monofilaments of DEET and PDLLA (a) without monofilaments DEET, (b) (PDLLA-DEETa) without DEET, (b) PDLLA-DEET compounds with varyingcompounds DEET content, with varying (c) PDLLA DEET monofilaments content, (c) PDLLA with varying monofilaments DEET content with varyingand (d) a DEET representative content and (d) a representative deconvolution curve ofdeconvolution 20 wt.-% DEET curve monofilaments. of 20 wt.-% DEET monofilaments.

PDLLA granulate-DEETPDLLA compounds granulate-DEET with vary compoundsing fractions with of varyingDEET infractions Figure 8b, of DEET in Figure8b, showed two distinct mass-lossshowed two steps. distinct The high mass-losser mass-loss steps. event The corresponded higher mass-loss to the event ther- corresponded to the ◦ mal degradation of thethermal PDLLA. degradation The onset ofof thethe PDLLA.lower mass-loss The onset event of the was lower at 145 mass-loss °C with event was at 145 C ◦ a peak at 215 °C. Although,with a peak at a atsignificantly 215 C. Although, higher temperature, at a significantly this event higher scaled temperature, with this event scaled the DEET content. Thiswith could the DEETbe beneficial content. for This the couldprospect be of beneficial using the for filaments the prospect for the of using the filaments controlled release of DEETfor the as controlled it suggests release delayed of evaporation DEET as it suggests of the DEET delayed when evaporation dissolved of the DEET when in the PDLLA. dissolved in the PDLLA. Spun PDLLA-DEET Spuncompounds PDLLA-DEET with varying compounds fractions with of varying DEET in fractions Figure of8c DEETexhib- in Figure8c exhibited ited the same two mass-lossthe same events. two mass-loss Notably, events. in the Notably,case of 20 in wt.-% the case DEET of 20 filaments, wt.-% DEET a filaments, a third ◦ third peak at 145 °Cpeak was atobserved 145 C that was corre observedsponded that to corresponded the DEET-mass-loss to the DEET-mass-loss peak that peak that was was reported in [25]. reportedIt appears in that [25 ].the It lower-temperature appears that the lower-temperature peak was not directly peak depend- was not directly dependent ent on the mass fractionon theof DEET, mass fractionbut rather of on DEET, the presence but rather of ona crystalline the presence phase of as a crystallinethis phase as this was the only case wherewas thea crystalline only case phase where was a crystalline detected phasein the wasPDLLA detected filaments. in the This PDLLA filaments. This phenomenon is, however,phenomenon not yet fully is, however, understood. not yet fully understood. A representative curveA representativeof the deconvoluted curve peaks of the to deconvoluted estimate the amount peaks to of estimate DEET in the amount of DEET the filaments is shownin in the Figure filaments 8d, and is shown estimated in Figure DEET8 massd, and fractions estimated are DEETgiven in mass Table fractions are given in Table3. 3. The experimentally determined DEET mass fractions scale with the intended theo-

retical mass fractions. The amount of DEET lost during the spinning process, however, cannot be accurately estimated from the current data. Nevertheless, it has been proven

Materials 2021, 14, x FOR PEER REVIEW 12 of 15

Materials 2021, 14, 638 Table 3. Estimated DEET content of PDLLA monofilaments with varying DEET content. 11 of 14 PDLLA with 10 wt.-% PDLLA with 20 wt.-% Description DEET DEET that DEET-containing filaments can be melt-spun while retaining most of the initial DEET Experimentalduring the spinning DEET content process. 12.8 17.8 Theoretical DEET content 10.0 20.0 Table 3. Estimated DEET content of PDLLA monofilaments with varying DEET content. The experimentally determined DEET mass fractions scale with the intended theoretical massDescription fractions. The amount PDLLA of DEET with 10 lo wt.-%st during DEET the PDLLAspinning with process, 20 wt.-% however, DEET cannotExperimental be accurately DEET contentestimated from the curre 12.8nt data. Nevertheless, it has 17.8 been proven thatTheoretical DEET-containing DEET content filaments can be melt-spun 10.0 while retaining most of 20.0the initial DEET during the spinning process. 3.5. Filament Morphology and Size 3.5. Filament Morphology and Size In Figure9, SEM images of the PDLLA filaments with varying DEET content are shown.In Figure Circular 9, SEM filaments images within of the the PDLLA micrometer-range filaments with were varying observed DEET with content an outer are shown.boundary Circular ring that filaments was due wi tothin the preparationthe micrometer-range method and were should observed not be confused with an withouter a boundarysheath boundary. ring that In was some due instances, to the preparatio where DEETn method was incorporated,and should no globulart be confused structures with a(indicated sheath boundary. by the red In arrow) some instances, were also observed.where DEET This was is possiblyincorporated, due to globular the evaporation structures of (indicatedthe DEET duringby the thered spinningarrow) were process also at observed. 190 ◦C as This the TGA is possibly results due indicated to the that evaporation the onset ofDEET the DEET mass lossduring in PDLLA the spinning had occurred process alreadyat 190 °C at aas temperature the TGA results as low indicated as 145 ◦ C.that the onset DEET mass loss in PDLLA had occurred already at a temperature as low as 145 °C.

FigureFigure 9. SEMSEM images images of of PDLLA PDLLA monof monoﬁlamentsilaments with no DEET ( a), 10 wt.-% DEET ( b) and 20 wt.-% DEET (c).

TheThe average average filament filament diameter diameter ( (ØØ),), determined determined by by optical optical microscopy, microscopy, and the the linear linear densitydensity (F) ofof thethe filamentsfilaments are are given given in in Table Table4. There4. There was was a definite a definite increase increase in the in filamentthe fila- mentdiameter diameter with increasingwith increasing DEET DEET content content that could that becould attributed be attributed to the greater to the freegreater volume. free volume.It should It be should kept inbe mind, kept in though, mind, thatthough, the PDLLAthat the filamentsPDLLA filaments with 20 wt.-%with 20 DEET wt.-% had DEET also hadbeen also spun been at a spun slightly at a lower slightly take-up lower speed. take-up Furthermore, speed. Furthermore, the incorporation the incorporation of DEET led of DEETto a significant led to a increasesignificant in increase the linear in density the linear of the density filaments. of the This filaments. marked This higher marked linear higherdensity linear of the density PDLLA of filaments the PDLLA with filaments 20 wt.-% with DEET 20 wt.-% may also DEET be may due toalso the be existence due to the of existencethe crystalline of the phase. crystalline phase.

TableTable 4. Dimensions of the PDLLA monofilaments monoﬁlaments with varying DEET mass fractions. PDLLA with PDLLA with PDLLA with Property PDLLA with 0 PDLLA with 10 PDLLA with 20 Property 0 wt.-% DEET 10 wt.-% DEET 20 wt.-% DEET wt.-% DEET wt.-% DEET wt.-% DEET ØØ(µ (m)μm) 30.730.7±0.8 ±0.8 38.9 38.9± ±2.8 2.8 49.0 49.0± ± 7.57.5 F (dTex) 6.3 12.7 26.3 F (dTex) 6.3 12.7 26.3

3.6.3.6. Filament Filament Mechanical Mechanical Integrity Integrity RepresentativeRepresentative stress stress−−strainstrain curves curves of ofthe the PDLLA PDLLA filaments filaments with with varying varying DEET DEET con- tentcontent are arepresented presented in Figure in Figure 10. 10 In. Inall all case cases,s, a aclear clear yield yield point point was was observed observed that that was followedfollowed by notablenotable plasticplastic deformation deformation and and strain strain hardening hardening before before failure. failure. The The prominent promi- nentplastic plastic deformation deformation region region was was expected, expected, as the as filamentsthe filaments were were spun spun at a relativelyat a relatively low take-up speed of 400 m·min−1 (20 wt.-% DEET) or 500 m·min−1. A lengthening of the low take-up speed of 400 m·min−1 (20 wt.-% DEET) or 500 m·min−1. A lengthening of the plastic deformation region, as well as an increase in the elongation at break, was observed for the PDLLA filaments with 10 wt.-% compared to the DEET-free PDLLA filaments. This suggested the DEET enabled a higher degree of chain orientation during the mechanical

Materials 2021, 14, x FOR PEER REVIEW 13 of 15

Materials 2021, 14, 638 plastic deformation region, as well as an increase in the elongation at break, was observed12 of 14 for the PDLLA filaments with 10 wt.-% compared to the DEET-free PDLLA filaments. This suggested the DEET enabled a higher degree of chain orientation during the mechanical drawing,drawing, attributed attributed to more to more freedom freedom for the for mo thelecular molecular chains chains to rearrange. to rearrange. However, However, this this was wasnot true not truefor the for PDLLA the PDLLA filaments filaments with with 20 wt.-% 20 wt.-% DEET. DEET. Substantially, Substantially, this thiswas wasalso also the casethe casewhen when there there was wascrystallization crystallization in th ine presence the presence of DEET. of DEET. Therefore, Therefore, there there are two are two opposingopposing factors, factors, i.e., an i.e., increase an increase in ductility in ductility due dueto the to increased the increased chain chain mobility mobility in the in the amorphousamorphous phase phase vs. the vs. rigidity the rigidity of the of crystallin the crystallinee phase. phase. From From Table Table 5, it5 can, it canbe seen be seen that that the additionthe addition of DEET of DEET led to led a tosignificant a significant decrease decrease in the in yield the yield strength, strength, which which was wasin line in line withwith the theincreased increased mobility mobility of of the the PLA PLA chains, asas wellwell as as a reductiona reduction of theof ultimatethe ultimate strength strengthand theand elastic the elastic modulus. modulus. Considering Considering the significant the significant impact impact that the that incorporation the incorpora- of DEET tion hasof DEET on the has mechanical on the mechanical integrity integrity of the filaments, of the filaments, it is proposed it is proposed to introduce to introduce a sheath polymer that could potentially allow higher attainable take-up speeds and simultaneously a sheath polymer that could potentially allow higher attainable take-up speeds and sim- act as a tailorable release barrier to the DEET. Moreover, the postdrawing of the filaments ultaneously act as a tailorable release barrier to the DEET. Moreover, the postdrawing of could also aid in improving the knittability of these filaments. the filaments could also aid in improving the knittability of these filaments.

Figure 10. Representative stress−strain curves of PDLLA ﬁlaments with varying DEET content. Figure 10. Representative stress−strain curves of PDLLA filaments with varying DEET content.

Table 5. Tensile properties PDLLA ﬁlaments with varying DEET content. Table 5. Tensile properties PDLLA filaments with varying DEET content.

Tensile Property PDLLA with 0 wt.-% DEETPDLLA PDLLA with with 0 10 wt.-%PDLLA DEET with 10 PDLLA PDLLA with 20 with wt.-% 20 DEET Tensile Property Elastic modulus (cN/Tex) 628.5 +/− 53.2wt.-% DEET 368.8 +/− 9.1wt.-% DEET 129.8wt.-% +/ −DEET29.2 − − − Ultimate strength (cN/Tex)Elastic modulus 24.7 (cN/Tex) +/ 0.6 628.5 +/− 53.2 9.3 +/ 0.3 368.8 +/− 9.1 129.8 3.6 +/ +/−0.7 29.2 Elongation at break (%) 181.6 +/− 8.9 376.0 +/− 13.2 148.1 +/− 20.1 Ultimate strength (cN/Tex) 24.7 +/− 0.6 9.3 +/− 0.3 3.6 +/− 0.7 Elongation at break (%) 181.6 +/− 8.9 376.0 +/− 13.2 148.1 +/− 20.1 4. Conclusions 4. ConclusionsDEET can be incorporated into commercially available noncrystallizable PDLLA to produce compounds containing 10 wt.-% and 20 wt.-% DEET that are macroscopically DEET can be incorporated into commercially available noncrystallizable PDLLA to stable. It has been shown that it is possible to produce DEET-containing PDLLA monofila- produce compounds containing 10 wt.-% and 20 wt.-% DEET that are macroscopically ments in the micrometer-range from these compounds by melt spinning. The DEET content, stable. It has been shown that it is possible to produce DEET-containing PDLLA monofil- however, greatly limits the attainable take-up speed. Moreover, DEET plays a critical role amentsin enabling in the micrometer-range stress-induced crystallization from these compounds of α-crystals by melt during spinning. the spinning The DEET process con- that tent,is however, amorphous greatly under limits the the same attainable conditions take-up without speed. the Moreover, presence DEET of DEET. plays The a critical degree of role crystallinityin enabling stress-induced of 6% is albeit relativelycrystallization low inof theα-crystals case of during 20 wt -%the DEET spinning monofilaments. process that Theseis amorphous results show under promise the same that conditions it is possible without to produce the presence biodegradable of DEET. mosquito-repelling The degree of crystallinityfilaments from of 6% a is commercially albeit relatively available low in PDLLA the case grade of 20 thatwt -% can DEET serve monofilaments. as an environmen- Thesetally results alternative show promise to existing that petroleum-based it is possible to produce filaments, biodegradable however their mosquito-repel- effectiveness at the ling currentfilaments DEET from loading a commercially still needs available to be determined. PDLLA grade Moreover, that can it is serve proposed as an to environ- introduce a mentallysheath alternative polymer to existing improve petroleum-ba the mechanicalsed integrity filaments, of however the filaments their thateffectiveness can potentially at the currentsimultaneously DEET loading aid in tailoringstill needs the to releasebe determined. rate of the Moreover, DEET. it is proposed to introduce a sheath polymer to improve the mechanical integrity of the filaments that can potentially simultaneously aid in tailoring the release rate of the DEET.

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Author Contributions: Conceptualization, I.F. and A.L.; data curation, I.F.; formal analysis, I.F., R.B. and A.L.; funding acquisition, W.F., R.A. and A.L.; investigation, I.F. and R.B.; methodology, I.F., H.B., R.B. and A.L.; project administration, W.F., R.A. and A.L.; resources, H.B., W.F., R.A. and A.L.; supervision, W.F. and A.L.; validation, I.F., H.B., W.F., R.B., R.A. and A.L.; visualization, I.F.; writing—original draft, I.F.; writing—review and editing, I.F., H.B., W.F., R.B., R.A. and A.L. All authors have read and agreed to the published version of the manuscript. Funding: This work was funded, in part, by the Deutsche Forschungsgemeinschaft (DFG), grant number LE 4165/1-2. W.F. and R.A. are grateful for financial support by the Deutsche Forschungsge- meinschaft (DFG), grant number AN 212/22. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Acknowledgments: The authors would also like to express their gratitude to the employees of the Department Processing at the Leibniz-Institut für Polymerforschung e.V. Dresden for making available their equipment and expertise. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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