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Article Cite This: ACS Omega 2019, 4, 10376−10387

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Improving Processing, Crystallization, and Performance of Poly‑L‑lactide with an Amide-Based Organic Compound as Both Plasticizer and Nucleating Agent Nils Leoné,† Manta Roy,† Sarah Saidi,‡,§ Gijs de Kort,† Daniel Hermida-Merino,§ and Carolus H. R. M. Wilsens*,†

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†

Aachen-Maastricht Institute of Biobased Materials (AMIBM), Maastricht University, PO Box 616, 6200MD Maastricht, The Netherlands ‡ LMOPS, Université de Lorraine, CentraleSupelec Metz, EA 4423, 2 rue Edouard Belin, Metz F-57070, France § Netherlands Organisation for Scientific Research (NWO), DUBBLE@ESRF BP CS40220, 38043 Grenoble, France S Supporting Information *

ABSTRACT: In this work, we report on a novel hydrogenbonding compound, N,N′-bis(2-hydroxyethyl)terephthalamide (BHET), and its potential as additive in poly-L-lactide (PLA). Although the hydroxyl groups of BHET can participate in transesterification with the PLA matrix, we demonstrate through gel permeation chromatography that extrusion at 200 °C does not result in a drastic decrease in molecular weight. When dissolved in the PLA matrix, BHET facilitates a plasticizing effect, indicated by a suppression in both melt viscosity and glass-transition temperature. Additionally, BHET can crystallize from the PLA melt during cooling, where the generated BHET crystals facilitate heterogeneous nucleation of the PLA matrix. In general, BHET crystallization is favored at high undercooling/superstaturation, as this yields BHET crystals with a high surface area-to-volume ratio. However, rapid cooling to temperatures below the glass-transition temperature prevents the crystallization of the BHET, effectively yielding samples where only the plasticizing effect of BHET is evident. A suppression in yield point is observed during mechanical analysis of samples with increasing BHET concentration, a characteristic feature for samples with decreasing glass-transition temperatures. In contrast, when allowed to crystallize during processing, BHET can be used to generate PLA crystals oriented along the flow direction, effectively enhancing the tensile modulus. Overall, the combined plasticizing and nucleating effect of BHET makes it a verstaile additive for controlled processing and performance of PLA.

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INTRODUCTION Poly-L-lactide (PLA) is an aliphatic thermoplastic polyester that can be derived through fermentation of renewable resources and has gained much attention as it is biodegradable when exposed to elevated temperatures and in the presence of bacteria.1,2 Despite its renewable origin, amorphous PLA generally suffers from brittle failure behavior, which is generally related to its high glass-transition temperature (Tg) around 60 °C and thus its high yield point.3 Although PLA can be semicrystalline with a melting temperature of up to 180 °C, it suffers from a low crystal growth rate.4 Furthermore, the melting and crystallization behavior of PLA is dependent on the presence of the D-enantiomer, as random incorporation of this enantiomer in the polymer backbone decreases the crystallinity, until a fully amorphous material is obtained.5 With respect to processing, PLA is susceptible to hydrolysis, transesterification, and several degradation reactions at temperatures above 200 °C, potentially decreasing the molecular weight and thus the mechanical performance.6 Despite these limitations, PLA has proven to be a promising material in, for example, packaging,7 laminates,8 composites,9,10 and three-dimensional (3D) printing.11 © 2019 American Chemical Society

One route to counter the low PLA crystal growth rate is through enhancement of the nucleation process: the introduction of a (foreign) organic or inorganic component can facilitate heterogeneous nucleation, thus acting as a nucleating agent (NA). One particularly interesting class of NAs includes melt-soluble hydrogen-bonding organic compounds.12,13 These compounds are designed to dissolve in the polymer matrix during processing and crystallize during cooling to generate surface for heterogeneous nucleation of the polymer matrix. This approach has successfully been employed to develop nucleating agents for PLA,14−19 polyhydroxybutyrate,20−22 isotactic polypropylene (iPP),23−27 and poly(1,4-butylene adipate).28 Although these organic nucleating agents exhibit promising nucleating ability, their observed nucleating efficiency is largely determined by their surface area-to-volume ratio after crystallization. Crystallization of these melt-soluble organic compounds from the polymer melt proceeds itself through a Received: March 27, 2019 Accepted: June 3, 2019 Published: June 14, 2019 10376

DOI: 10.1021/acsomega.9b00848 ACS Omega 2019, 4, 10376−10387

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BHET in the PLA matrix under shear conditions. Upon consecutive cooling, the BHET crystallizes from the PLA matrix, where we observe that crystallization of BHET proceeds at decreasing temperatures with decreasing BHET concentration. For example, during cooling at a rate of 10 °C/ min, we observe from optical microscopy that BHET crystallizes from the PLA melt around 190, 170, and 140 °C in the PLA having 2.0, 1.5, and 1.0 wt % of BHET, respectively. An example of such crystallization behavior and the resulting crystal growth of PLA on the BHET surface is shown in Figure 1 for PLA having 1.0 wt % BHET. Overall, given that the pure BHET melts at 350 °C, we can conclude from this phase behavior that, at least at these low concentrations, BHET is molecularly miscible with PLA. Although BHET proves to facilitate heterogeneous nucleation of PLA, as can be deduced from Figure 1, we realize that the two hydroxyl groups can undergo a transesterification reaction with PLA at elevated temperatures. Such transesterification facilitates a decrease in molecular weight of the PLA matrix, which could affect the mechanical performance. Therefore, to identify the effect of processing of PLA in the presence of BHET, GPC analysis was performed on the samples obtained after extrusion (for 3 min at 200 °C). As can be observed from the GPC traces shown in Figure 2A and the molecular weights shown in Table 1, BHET indeed facilitates a slight suppression in Mw during processing. The pure PLA sample shows an Mw of 138 kg/mol, whereas the presence of 2 wt % BHET yields PLA with an Mw of 129 kg/mol. Although these findings confirm that BHET can facilitate a molecular weight decrease of roughly 3−7% during processing (depending on the employed BHET concentration), we do not consider such decrease significant enough to affect the mechanical properties of the PLA. To identify how BHET affects the viscoelastic behavior of the PLA matrix during prolonged heating, we have loaded all samples in the rheometer (plate−plate geometry) at 200 °C and monitored the evolution of the complex viscosity over time (Figure 2B). Note that in this protocol, we start to monitor the exposure time directly after loading of the sample, although the equilibration time is not considered in the time sweeps presented in Figure 2B. We can observe that the pure PLA is stable under the evaluated conditions, indicated by the rather constant complex viscosity over time. In contrast, the complex viscosity of PLA in the presence of BHET decreases over time. Furthermore, the complex viscosity decreases more rapidly over time as the BHET concentration increases. Given the increasing concentration of alcohol groups in the system,

nucleation and growth mechanism, making the crystallite morphology highly dependent on the applied cooling rates, concentrations, and supersaturation. In a previous work, we reported that melt-soluble nucleating agents for iPP favor high cooling rates, as this results in a homogeneous distribution of submicrometer-sized NA crystallites and a high nucleating efficiency upon cooling.23,24 Following this approach, in this work, we report on a novel melt-soluble terephthalic acidbased NA (BHET, Scheme 1) and its nucleation efficiency for Scheme 1. Chemical Structure of Compound N,N′-Bis(2hydroxyethyl)terephthalamide (BHET) Used in This Study

PLA. BHET facilitates an increasing nucleating efficiency when it is allowed to crystallize under increasing cooling rates. However, subjecting PLA samples with BHET to too high cooling rates prevents crystallization of both components, as the temperature rapidly drops below the Tg of PLA. Instead, amorphous PLA is obtained, where the dissolved BHET maintains its plasticizing effect.

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RESULTS AND DISCUSSION Molecular Weight of PLA in the Presence of BHET. The BHET compound has been designed to dissolve in PLA during processing at elevated conditions. When designing the BHET molecule, we have taken the following two parameters into account. (1) The compound should exhibit affinity with the PLA to ensure solubility at elevated temperature and (2) the melting temperature of the nucleating agent should be sufficiently high, as its crystallization from the PLA melt is required during cooling to provide a surface for heterogeneous nucleation of the PLA matrix. In this study, we made use of the polar ethanol amine compound, which, after reaction with terephthalic acid, yields both alcohol and amide functionalities that can interact with the PLA chains through hydrogen bonding. Additionally, the hydrogen bonding of the terephthalamide moieties ensures a high melting temperature and provides the driving force for crystallization upon cooling. Indeed, the molecular miscibility of BHET can be identified by the suppression in both melting and crystallization temperatures when dissolved in PLA. During extrusion at 200 °C, we observe that we can successfully dissolve up to 2.0 wt % of

Figure 1. Optical micrographs recorded between crossed polars and 530 nm λ-wave plate of the sample having 1.0 wt % BHET. (Left) Homogeneous solution of BHET in PLA, (middle) crystallization of treelike BHET crystallites from the PLA melt, and (right) PLA crystals growing on the surface of the BHET crystallites. Images were taken during cooling at a rate of 10 °C/min, after heating the sample to 200 °C. 10377


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Figure 2. (A) GPC traces of the PLA after processing with various concentrations of BHET. (B) Dependency of the complex viscosity over time for PLA with various concentrations of BHET.

temperatures above 220 °C under quiescent conditions to fully melt the BHET crystallites. Such a high temperature is likely to accelerate both chain degradation and transesterification of PLA by BHET molecules. Therefore, samples containing 2.0 wt % BHET are excluded from DSC experiments. For the other samples, the DSC traces are shown in Figure 3, where the characteristic transitions are provided in Table 2. During the first heating, the pure PLA exhibits a distinct glass-transition temperature (Tg) around 64 °C. Note that the large overshoot in heat capacity during the Tg transition in pure PLA is likely resulting from residual stresses imposed on the sample during processing and the resulting enthalpy relaxation, although these are not present in PLA containing BHET. PLA samples with BHET exhibit a decreased Tg between 56 and 59 °C. Such suppression in Tg supports the hypothesis that BHET acts as a plasticizer. However, one characteristic feature of plasticizers is that the use of increasing concentration results in an increasing suppression of the Tg.5 As is visible from Table 2, this is clearly not the case for PLA in the presence of BHET: instead, we observe that the Tg increases slightly with increasing BHET concentration. Additionally, as is highlighted by the dotted lines in Figure 3B, a second Tg is observed in the presence of BHET. The presence of two Tg’s is characteristic for a phase-separated system, suggesting that BHET crystallized from the PLA matrix during cooling after processing. As mentioned earlier, the BHET crystallization proceeds through a nucleation and growth process: we expect that the BHET solubility in PLA decreases rapidly upon cooling, effectively generating a supersaturated solution.29−31 Driven by the supersaturation, nucleation of the BHET proceeds followed by crystal growth, effectively decreasing the BHET concentration in the melt. Furthermore, as crystal growth is dependent on the BHET diffusion through the PLA matrix, it is expected that the BHET concentration decreases considerably close to a BHET crystal surface, effectively generating a spatial gradient in BHET concentration in the PLA melt. A schematic overview of this hypothesis is depicted in Figure 4. One can expect two PLA “phases” resulting from this crystallization mechanism of BHET: the first phase involves the PLA melt containing a low concentration of BHET molecules that are not (yet) taken up in the BHET crystals. The presence of such dissolved BHET facilitates a plasticizing

Table 1. Weight-Average Molecular Weight (Mw) and Polydispersity Index (PDI) of the PLA after Processing with Various Concentrations of NA (BHET), According to HFIP-GPC Analysis Referenced against PMMA Standards sample PLA PLA PLA PLA PLA

pure + 0.5 + 1.0 + 1.5 + 2.0

wt% wt% wt% wt%

BHET BHET BHET BHET

Mw (g/mol)

PDI (−)

138.000 134.000 138.000 130.000 129.000

2.16 2.17 2.18 2.13 2.11

we consider it plausible that this decrease in complex viscosity is the result of the transesterification reaction of BHET with the PLA matrix. This decrease in Mw is rather significant; after roughly 3000 s, the molecular weight of the PLA has decreased with roughly 60% in the presence of 2 wt % BHET compared to pure PLA, indicated by the factor 20 difference in complex viscosity between the samples (and the use of the relation η0 ∼ Mw3.4 for linear entangled polymer chains). When looking closely the values for the complex viscosity of the various samples, one can deduce that the samples with BHET do not start at the same complex viscosity as the pure PLA sample. For example, the complex viscosity upon loading of PLA having 2.0 wt % BHET is estimated to be a factor 3 lower than that of the pure PLA. This difference in complex viscosity is significantly larger than the expected 20%, based on the 7% difference in Mw between the two samples as observed from GPC analysis and the relation η0 ∼ Mw3.4. Given that BHET is dissolved in PLA at these temperatures, we consider it likely that it increases the molecular relaxation of the PLA chains and thereby decreases the melt viscosity. These findings suggest that hydrolysis of PLA in the presence of BHET might not be as dramatic as outlined earlier, as part of the decrease in viscosity originates from BHET acting as a plasticizer for PLA. Nucleating Efficiency of PLA in the Presence of BHET. From the previous section, we can conclude that BHET can readily be used as an additive for PLA under processing conditions, although continued exposure to elevated temperatures should be minimized to limit hydrolysis. To evaluate how the BHET affects the nucleating efficiency of PLA, the extruded samples were subjected to DSC analysis. Note that the sample having 2.0 wt % of BHET required heating to 10378


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Figure 3. Differential scanning calorimetry traces of PLA samples having varying concentrations of BHET. (A) First heating trace, (B) first heating trace, focusing on the region between 54 and 72 °C, (C) first cooling trace, and (D) second heating trace. Note that all traces are acquired at heating and cooling rates of 10 °C/min.

Table 2. Characteristic Thermal Transitions Observed during the First Heating and Cooling Runs in DSC Analysis on the PLA Samples Having Varying Concentrations of BHET first heating (10 °C/min) sample name PLA PLA PLA PLA

pure + 0.5 BHET + 1.0 BHET + 1.5 BHET

Tg (°C) 63.5 56.7/63.6 58.2/65.4 58.5/65.3

Tcc_onsa

(°C)

91.0 93.1 90.8 88.6

first cooling (10 °C/min)

Tcc (°C)

ΔHcc (J/g)

Tm (°C)

ΔHm (J/g)

95.6 97.3 97.1 96.1

29.2 32.0 28.8 29.8

176.5 176.6 177.6 175.4

44.9 45.8 46.7 45.9

a

Tc_onsb

(°C)

108.1 107.8 124.9 133.5

Tcb (°C)

ΔHc (J/g)

NE (%)c

100.2 98.8 117.6 120.5

34.0 30.7 46.3 48.3

0 −2 31 36

a

Tcc_ons denotes the onset temperature for cold crystallization, while Tcc denotes the peak value of the cold crystallization exotherm. bTc_ons denotes the onset temperature for crystallization during cooling, while Tc denotes the peak value of the crystallization exotherm. cNE reflects the nucleating efficiency, calculated using the equation NE = ((Tc − Tcmin)/(Tcmax − Tcmin)) × 100%.

effect on the PLA, where the effective Tg suppression would be determined by actual BHET concentration (blue area in Figure 4). The presence of such a phase would explain the first Tg observed around 56−59 °C in PLA samples containing BHET. Apparently, the use of increasing BHET concentrations in PLA results in a more efficient BHET crystallization: this is evident from increasing Tg with increasing BHET concentration (Table 2), suggesting a decrease in residual BHET concentration dissolved in the PLA phase. Such behavior can be expected as the use of higher BHET concentrations, i.e., imposing a higher supersaturation, allows for BHET crystal

growth already at higher temperatures. As a result, the dissolved BHET molecules have more time to diffuse through the sample and to be taken up in BHET crystals during cooling. The second phase involves the PLA melt close to the BHET crystal surface (white area in Figure 4), effectively having a very low BHET concentration and thus exhibiting a Tg close to that of pure PLA. Indeed, the presence of such a phase would explain the second Tg observed in PLA samples containing BHET. Note that the exact value of this Tg could be influenced by wetting effects of the PLA on the BHET crystal surface.32 10379


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Figure 4. Schematic overview of the spatial gradient in BHET concentration after BHET crystallization in the PLA melt. The red area reflects a BHET crystallite, whereas the blue area represents the dissolved BHET molecules that did not participate in crystallization. Diffusion of dissolved BHET molecules is required for BHET crystal growth, effectively resulting in a very low BHET concentration close to the BHET crystal surface, as is highlighted by the white area. Overall, the PLA phase in the white area contains a low BHET concentration and is therefore expected to exhibit a Tg close to that of pure PLA (i.e., ∼64 °C), whereas the dissolved BHET molecules in the white area will effectively plasticize the PLA and lower the Tg, depending on the residual BHET concentration. Note that the placement of the BHET molecules in the crystal is merely used for illustrational purposes and does not need to reflect BHET arrangement in real crystallites.

Figure 5. Optical micrographs recorded between crossed polars and 530 nm λ-wave plate of the sample having 1.5 wt % of BHET. (Left) BHET crystallite grown during a 5 min isotherm at 180 °C; (middle) growth of treelike BHET crystallites generated after a 1 min isotherm at 160 °C; and (right) PLA crystals growing on the surface of the BHET crystallites after a 60 s isotherm at 140 °C. Note that the applied cooling rate in the cooling steps was 30 °C/min.

large BHET crystallite grown at 180 °C; Their growth is halted at roughly 20−30 μm distance. Upon further cooling to 140 °C, PLA crystal growth can be observed on the BHET surface, although no further BHET crystal growth is observed close to the large BHET crystal grown at 180 °C. This observation suggests that the BHET concentration close to a BHET crystal is indeed very low, preventing the nucleation of new BHET crystallites and limiting the crystal growth of approximate BHET crystallites. Therefore, we consider the spatial gradient in BHET concentration schematically represented in Figure 4 a plausible explanation for the presence of a double glasstransition temperature in PLA samples containing BHET. One can imagine that this phase behavior can become very complex as BHET nucleation, crystal habit, and crystal growth rates are all dependent on the imposed supersaturation. Indeed, such features have been observed for other amide-based nucleating agents, which have shown to crystallize into pointlike, needlelike, and treelike crystal morphologies, depending on the imposed thermal history of the sample.26,33,34 We will elaborate further on this topic in a later part of this work when discussing the BHET morphology as a function of cooling rate. To continue on the thermal behavior, as is visible from Figure 3A, heating the samples beyond 80 °C in DSC results in extensive cold crystallization. Upon continued heating, a small exotherm is observed around 160 °C, which is thought to result from the rearrangement of disordered α′ crystals formed during cold crystallization into the thermodynamically stable α crystallites.35,36 Further heating eventually results in melting of

When taking the effective amount of this phase into account, i.e., the height of the Tg transition, one can observe that an increasing concentration of BHET results in a decreasing fraction of this second phase. It is likely that the increased BHET bulk concentration, and thus the increased concentration close to the BHET crystals, yields thinner depletion layers. Additionally, it would be possible that the increasing BHET concentrations allow growth of BHET larger crystals with a decreasing surface area-to-volume ratio, effectively having a decreased surface interacting with the PLA phase. Indeed, as is shown in Figure S1, we observe that increasing BHET concentrations result in the growth of crystallites with increasing size. We have conducted a polarization optical microscopy experiment to verify whether the spatial concentration difference of BHET in PLA can be visualized. First, PLA containing 1.5 wt % BHET was heated to 200 °C, after which it was cooled to 180 °C at a rate of 30 °C/min. At the temperature of 180 °C, the BHET nucleation proceeds rather slowly, allowing for the growth of a small number of large BHET crystallites over time (Figure 5, left). After a 5 min isotherm at 180 °C, the sample was cooled to 160 °C at a rate of 30 °C/min. The decrease in temperature enhances the supersaturation of the BHET, thereby enhancing the nucleation rate of BHET. Indeed, as is visible from the middle image of Figure 5, a characteristic treelike BHET morphology develops in the sample under these conditions. However, the newly grown treelike BHET crystals do not reach up to the 10380


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Figure 6. Optical micrographs taken between crossed polars and a 530 nm λ-wave plate of pure PLA and PLA + 1.5 wt % BHET. The samples were first heated to 200 °C and cooled at a specific rate of 3, 10, or 30 °C/min to 140 °C, after which the sample was allowed to crystallize under isothermal conditions. Note that for the sample cooled at 3 °C/min, nucleation proceeds already upon cooling to 140 °C due to the low cooling rate and the presence of the BHET crystallites. The scale bar, provided in the top left image, is the same for all micrographs.

J/g) and 133 °C (ΔHc = 48.3 J/g), respectively. The increase in onset of crystallization can be converted to the nucleation efficiency using the equation NE = ((Tc − Tcmin)/(Tcmax − Tcmin)) × 100% described by Lotz and co-workers,37 where Tc is the peak crystallization temperature, Tcmax is the maximum crystallization peak temperature of self-nucleated PLA (156.3 °C for this PLA grade38), and Tcmin is the minimum crystallization peak temperature occurring in the absence of heterogeneous nucleation sites, hence as it proceeds in pure PLA. Indeed, one can observe that the use of 0.5 wt % BHET results in a negative nucleating efficiency (Table 2), confirming that the BHET delays the nucleation process of PLA when it resides in the dissolved state. In contrast, upon increasing the BHET concentration, the NE increases to values above 30%. As is evident from Figure S1, this enhancement in nucleation efficiency (NE) is directly related to the crystallization of BHET during cooling, followed by nucleation and crystal

the α-crystallites as is evident from the melting endotherms at 175−177 °C depicted in Figure 3A. After melting of the PLA crystallites, all samples are cooled to 40 °C at a rate of 10 °C/min. During cooling (Figure 3C), crystallization of the pure PLA is observed with an onset of crystallization at 108.1 °C and an enthalpy of crystallization (ΔHc) of 34.0 J/g. Interestingly, in the presence of 0.5 wt % BHET, crystallization is delayed slightly and starts at 107.8 °C (ΔHc = 30.7 J/g). From polarization optical microscopy (provided in Figure S1 of the Supporting Information), we observe that at this concentration, BHET does not crystallize from the PLA melt prior to the growth of PLA crystallites. In other words, the BHET remains dissolved in the PLA melt, thereby delaying the nucleation process and crystallization of the PLA, which is a common feature of a plasticizer.5 In contrast, increasing the concentration of BHET to 1 and 1.5 wt % increases the onset of crystallization to 125 °C (ΔHc = 46.3 10381


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Figure 7. DSC thermograms observed during crystallization of PLA (left) and PLA containing 1.5 wt % BHET (right) under isothermal conditions at 140 °C. Prior to maintaining isothermal conditions, the samples were heated to 200 °C at a rate of 10 °C/min, after which they were cooled with specified cooling rates of 1, 3, 10, 30, and 100 °C/min to 140 °C.

growth of large PLA spherulites over time (top right image in Figure 6). In contrast, the formation of “treelike” BHET crystallites is detected around 170 °C when subjecting PLA with 1.5 wt % BHET to the same experimental protocol. PLA crystal growth proceeds rapidly on the surface of the BHET network when reaching 140 °C, effectively covering the whole view area in microscopy with crystallites within 60 s. Upon subjecting the PLA with 1.5 wt % BHET to decreasing cooling rates, we observe a change in the BHET crystal morphology/ habit;29 Cooling from 200 to 140 °C at a rate of 10 °C/min favors the growth of thick and short BHET fibrils, which becomes even more pronounced during cooling at a rate of 3 °C/min. In other words, the surface area-to-volume ratio of the BHET crystallites decreases dramatically when they are allowed to crystallize more slowly. Such a decrease in available surface for nucleation of the PLA matrix decreases the nucleation efficiency, as is clearly observed in the optical micrographs taken during isothermal crystallization at 140 °C. The same features are observed from DSC analysis during isothermal crystallization of the samples at 140 °C after cooling them at various rates from 200 to 140 °C. As is evident from Figure 7, the higher the applied cooling rate, the faster the increase in crystalline fraction of PLA during isothermal crystallization at 140 °C, undoubtedly resulting from the increased surface area-to-volume ratio of the BHET crystallites and thus the enhanced nucleation density of PLA. Note that the isothermal DSC traces of the samples having other BHET concentrations are provided in Figure S2 of the Supporting Information. In line with earlier observations, a decrease in nucleating efficiency is observed in the sample containing 0.5 wt % BHET, which is resulting from the inability of BHET to crystallize from the PLA melt under the given conditions. The crystallization morphology of PLA as a function of cooling rate was monitored using wide-angle X-ray diffraction. Again, the samples were heated to 200 °C and cooled at different rates of 10, 30, and 70 °C/min, the highest cooling rate achievable in the used setup, to 140 °C. Upon reaching 140 °C, the samples were subjected to a 3 min isotherm, after which they were cooled at 10 °C/min to 80 °C. WAXD diffractograms were collected throughout the protocol. Figure

growth of PLA on the BHET crystal surface. To reflect, the nucleating efficiency of BHET is higher than that of talc (NE = 28.7%), but lower compared to other melt-soluble oxalamideor hydrazide-based nucleating agents (NE = 49.7 and 53.6%, respectively).38 However, as will be elaborated in the sections below, the nucleating efficiency of BHET can be increased by applying higher cooling rates prior to crystallization. During the second heating run, i.e., after crystallizing the samples during cooling at a rate of 10 °C/min, a difference in melting behavior is observed with increasing BHET concentration. Both the pure PLA and the PLA + 0.5 wt % BHET display the previously discussed α′ to α crystal rearrangement prior to melting, whereas the samples containing 1.0 and 1.5 wt % BHET display double-melting behavior (Figure 3D). As is reported by Tashiro and co-workers,35 such double-melting behavior is characteristic for PLA directly crystallized into the α-phase, which occurs at temperatures of 120 °C and higher. Effect of Cooling Rate on BHET Morphology and Nucleating Efficiency of PLA. In the previous section, we have observed that BHET only acts as a nucleating agent for PLA when it crystallizes from the PLA melt during cooling. As the crystallization of BHET from the PLA melt itself proceeds via a nucleation and growth process that is driven by the supersaturation, we expect that the morphology of the BHET crystallites generated during cooling will strongly depend on the thermal history and BHET concentration. To evaluate the effect of cooling rate on the BHET crystal morphology generated during cooling and its resulting effect on the nucleation of PLA, we have heated the samples to 200 °C, cooled them to 140 °C at a rate of 1, 3, 10, 30, or 100 °C/min, and monitored the crystallization process under isothermal conditions. These experiments have been performed both in a polarized optical microscpe mounted with a Linkam Hotstage and in DSC analysis. Figure 6 displays the characteristic BHET morphology and the ensuing PLA crystallization for pure PLA and for PLA with 1.5 wt % BHET, as observed in polarized optical microscopy. In general, we observe that the nucleation density in PLA during isothermal crystallization at 140 °C is very low, which, combined with the low crystal growth rate, results in the 10382


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Figure 8. Left: Evolution of the WAXD patterns during isothermal crystallization for 3 min of pure PLA (A), and PLA containing 1.5 wt % BHET (B), after cooling from 200 to 140 °C at a rate of 100 °C/min. Right: Evolution of the WAXD patterns during consecutive cooling at a rate of 10 °C/min, observed directly after the 3 min isotherm at 140 °C.

8 depicts the results obtained for the pure PLA sample and the PLA containing 1.5 wt % BHET, after cooling from 200 to 140 °C at a rate of 70 °C/min. In general, we observe that PLA only crystallizes around 100 °C in this protocol, corresponding well with the DSC data presented in Figure 3. The crystallization is detected by the rise in diffraction signal around 12 nm−1 (200/110 reflection) and 13.7 nm−1 (203 reflection), which are characteristic for the α′ phase.35 In contrast, when subjecting PLA containing 1.5 wt % BHET to the same experimental protocol, a rapid rise in diffraction signals at 12 nm−1 (200/110 reflections), 13.7 nm−1 (203 reflection), 8.8 nm−1 (103 reflection), 10.5 nm−1 (010 reflection), and 16 nm−1 (210 reflection), corresponding to the growth of α-crystals, is already observed after 90 s at 140 °C. In line with previous observations, decreasing the cooling rate between 200 and 140 °C delays crystallization in the presence of BHET, as is shown in Figures S3 and S4 of the Supporting Information. Overall, these observations are in line with the DSC data shown in Figure 3: In the presence of BHET, the onset of PLA crystallization takes place already above 120 °C, a temperature window which allows for growth of α crystals. Effect of BHET on the Thermomechanical Performance of PLA. Based on our findings from POM, DSC, and WAXD, we can conclude that the application of high cooling rates (i.e., facilitating a high BHET supersaturation) is preferred for efficient PLA nucleation, as this allows for the generation of treelike BHET crystals with high surface area-tovolume ratio. Although the application of high cooling rates is

desired to obtain an optimal BHET crystal morphology, it also prevents PLA crystal growth when the temperatures rapidly decrease to values below the Tg of PLA. Indeed, this is likely the scenario when injection-molding the samples into a cold mold placed at room temperature. To evaluate the effect of the presence of dissolved BHET in such amorphous PLA, we have evaluated the thermomechanical performance of the injectionmolded products using tensile testing and torsional DMTA analysis. As can be seen from Figures 9B and S5, and in line with previous observations, PLA samples containing BHET display a lowered Tg than the pure PLA sample, detected by the downward shift of the peak value in the viscous modulus (G″). In contrast to previous observations, it seems that the use of increasing BHET concentration does yield a gradually decreasing Tg. This confirms that the quenching step during injection molding prevents BHET crystallization, thereby maintaining its plasticizing effect. This feature can also be detected from the characteristic stress−strain curves of PLA with various concentrations of BHET depicted in Figure 9C: Although the modulus of the samples remains constant, the yield point of the PLA decreases with increasing BHET concentration, corresponding well with the decrease in Tg. The values and standard deviations for the tensile modulus and yield points extracted from the tensile tests are provided in Table S1 of the Supporting Information. As discussed earlier, heterogeneous nucleation of PLA only proceeds when the BHET resides in the crystalline phase. Given that BHET remains dissolved in the PLA after injection molding, we would expect no effect of BHET on the cold 10383


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Figure 9. (A) Elastic modulus and (B) viscous modulus of injection-molded PLA samples with various concentrations of BHET, as measured in torsional DMTA. (C) Characteristic stress−strain curves for the same injection-molded samples. (D) Stress−strain curves for PLA samples injection-molded in a mold at 110 °C for 1 min. Note that the inset of (D) displays the 2D-SAXS patterns obtained for the same tensile bars tested in (D), where the arrow denotes the flow direction during injection molding.

injection molding is expected to align the BHET crystals in the flow direction, allowing for the PLA crystal growth perpendicular to the flow direction (and hence having their lamella oriented along the flow direction). Figure 9D shows the characteristic stress−strain curves of the resulting specimens. In general, we observe that the tensile modulus of the pure PLA had dropped from 3.0 to 2.5 GPa, likely resulting from the relaxation of stresses in the PLA during the isotherm in the mold. In contrast, the PLA sample containing 2.0 wt % BHET exhibits an enhanced tensile modulus of 3.8 GPa, suggesting the presence of crystallites oriented along the flow direction. Indeed, 2D-SAXS analysis confirms the presence of oriented crystal lamella characteristic of a shish kebab or transcrystallization morphology, as is detected by the presence of vertical lobes in scattering intensity shown in the inset of Figure 9D. Note that the sudden change in signal around 1% strain is thought to result from the detachment of the generated shear layer and the core layers during elongation. Finally, as is shown in Figure S6 of the Supporting Information, the sample containing 2.0 wt % BHET does not exhibit a suppressed Tg anymore. Instead, the glass-transition temperature is higher than the pure PLA reference. This is likely a result of the PLA crystals providing restrictions to the

crystallization process during heating. However, as is visible from Figure 9A, the samples containing BHET undergo cold crystallization already at lowered temperature than the pure PLA, indicated by the increase in elastic modulus (G′) above 80 °C. This would suggest that heating to temperatures above the Tg enables sufficient mobility of the PLA chains to enable diffusion, nucleation, and BHET crystal growth. In turn, PLA crystals can grow either on the generated BHET crystal surface or as a result of the stresses imposed on the PLA matrix resulting from the BHET crystal growth. Retrospectively, such enhanced cold crystallization in the presence of BHET is also evident from the DSC data displayed in Figure 3A, in particular when using a BHET concentration of 1.5 wt %. Finally, to identify whether BHET can be used to generate an oriented PLA crystal structure for mechanical reinforcement purposes, we have (1) extruded the pure PLA and the PLA containing 2.0 wt % BHET, (2) transferred it into a barrel heated to 160 °C, and (3) injection-molded the melt into a mold at a temperature of 110 °C. The samples were subjected to a 3 min isotherm at 110 °C, prior to removal from the mold. The temperature of 160 °C was deliberately chosen to ensure the generation of the treelike BHET crystal morphology prior to injection molding; the intense shear field during the 10384


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Weight-average molecular weight (Mw) and polydispersity (PDI) of PLA blends after processing were determined from gel permeation chromatography (GPC) on a PSS SECcurity GPC system using Agilent 1260 Infinity instrument technology. The GPC system is equipped with two PFG combination medium micro-columns with 7 μm particle size (4.6 × 250 mm, separation range 100−1 000 000 Da), a PFG combination medium pre-column with 7 μm particle size (4.6 × 30 mm), and a refractive index detector (RI). Distilled 1,1,1,3,3,3hexafluoroisopropanol (HFIP) containing 0.019% sodium trifluoroacetate was used as mobile phase at 40 °C, with a 0.3 mL/min flow rate. The GPC apparatus was calibrated with poly(methyl methacrylate) standards obtained from PSS. GPC samples were prepared by dissolving 5 mg of PLA in 1.5 mL of HFIP overnight, after which the samples were filtered over a 0.2 μm PTFE syringe filter prior to injection. Differential scanning calorimetry (DSC) was performed on the samples using a TA Instruments Q2000 DSC. All samples were measured at heating and cooling rates of 1, 3, 10, 30, or 100 °C/min up to a maximum temperature of 200 °C, unless mentioned otherwise. The glass-transition temperature (Tg) and peak melting temperature (Tm) were determined from heating runs. The crystallization exotherm observed during the first cooling run was used to define peak crystallization temperature (Tc) and onset of crystallization temperature (Tons). The peak crystallization temperature is considered the peak value of the crystallization exotherm, whereas the intersection of the tangents of the baseline and the crystallization curve is considered to be the onset temperature for crystallization. Polarization optical micrographs were taken (between crosspolarizers and using a 530 nm λ-wave plate) on an Olympus BX53 microscope mounted with an Olympus DP26 camera and a Linkam Hotstage. The samples were heated to the desired temperature (generally 200 °C) at a rate of 30 °C/min. After leaving the samples for 3 min under isothermal conditions, the samples were cooled at a specified cooling rate (generally being 3, 10, or 30 °C/min) to the desired temperature, where crystallization was monitored for 30 min. Optical micrographs depicting the morphological changes were recorded both during cooling and under isothermal conditions. The evolution of the complex viscosity of the PLA blends was followed over time at 200 °C in an Anton Paar 702 twindrive rheometer. Parallel-plate geometry, having a diameter of 15 mm and gap of 1000 μm, was used to study the time dependence of the complex viscosity. The strain (1%) and angular frequency (10 rad/s) were chosen such that the experiments were performed in the linear viscoelastic regime. Torsional DMTA experiments on the inner parts of the PLA tensile bars were performed on an Anton Paar 702 twin-drive rheometer equipped with DMTA clamps. The samples were heated from 40 to 140 °C and exposed to a strain of 1%. Wide-angle X-ray diffraction (WAXD) measurements (λ = 0.104 nm), as a function of temperature, were carried out at the European Synchrotron Radiation Facility (ESRF), the DUBBLE beamline (BM26B, Grenoble, France) and the NCD station of Alba synchrotron in Cerdanyola del Valles (Spain). The DUBBLE beamline is particularly optimized for polymer science as is reported by Bras et al.39 and Portale et al.40 WAXD patterns, with an exposure time of 15 s, were collected using a Pilatus 300 K detector (1472 × 195 pixels of 172 μm × 172 μm) placed at a distance of 0.28 m from the sample (DUBBLE beamline) and a Rayonix Lx 255-HS detector with

mobility of the amorphous components of the PLA, effectively overruling the plasticizing effect of BHET.

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CONCLUSIONS In this work, we report on the effect of the BHET molecule on processing, crystallization, and thermomechanical behavior of PLA. We demonstrate that BHET dissolves in PLA during processing and acts as plasticizer for the PLA matrix. However, the presence of alcohol groups facilitates hydrolysis of the PLA matrix over time. During cooling, BHET proves to be an excellent nucleating agent for PLA, but only when it resides in the crystalline form. BHET crystallization favors high cooling rates, as this yields crystallites with a high surface area-tovolume ratio. However, too high cooling rates effectively quench the sample and prevent crystallization of the BHET, yielding amorphous samples. Under these conditions, the BHET maintains its role as a plasticizer, evidenced by a suppressed Tg and decreased yield point. However, heating such quenched samples above their Tg allows the BHET to crystallize and effectively enhance the cold crystallization process. Overall, this makes the BHET molecule a versatile additive that can be used both as plasticizer for processing purposes and as nucleating agent during cooling.

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EXPERIMENTAL SECTION

Synthesis of N,N′-Bis(2-hydroxyethyl)terephthalamide (BHET). The synthesis of N,N′-bis(2hydroxyethyl)terephthalamide (henceforth abbreviated as BHET) was achieved through reaction of terephthalic acid (10 g, 0.06 mol) and ethanol amine (8 ml, 0.132 mol) in dimethylformamide at 150 °C for 18 h. Next, the ensuing mixture was cooled and the formed crystallites were filtered, washed with methanol, and dried. The product was recrystallized from methanol and dried overnight in vacuo at 60 °C prior to use. BHET was isolated with a yield of 78% and a purity ≥98%; according to NMR analysis: 1H NMR (CDCl3, 300 MHz): δ 8.55 (m, 2H), 7.92 (s, 2H), 4.74 (m, 2H), 3.5 (m, 4H) and 3.35 (m, 4H). BHET did not display stable melting behavior; instead, melting is observed in polarized optical microscopy mounted with a Linkam Hotstage at 350 °C, followed by immediate degradation. Material Preparation and Blending Process. PLA (L130 grade, Corbion) and BHET were dried for 24 h at 60 °C in vacuo prior to use. In this study, we extruded PLA samples with BHET concentrations of 0 wt % (pure PLA), 0.5, 1.0, 1.5, and 2.0 wt %. All samples were mixed in the desired ratio prior to blending in a DSM Explore twin-screw microextruder with a barrel size of 5 mL. The samples were processed at 200 °C for 3 min (100 rpm), under constant nitrogen flow. After processing, the extrudate was either cooled to room temperature or directly injection-molded into tensile bars (2 mm × 4 mm × 70 mm, with a gage length of 25 mm) using a DSM Xplore IM 5.5 micro injection mold with (1) a barrel temperature of 200 °C and a mold temperature of 25 °C or (2) a barrel temperature of 160 °C and a mold temperature of 110 °C. Characterization Methods. 1H NMR spectra were recorded with a Bruker Ultrashield 300 spectrometer (300 MHz magnetic field). NMR samples were prepared by dissolving ca. 10 mg of sample in 0.5 mL of deuterated chloroform (CDCl 3). Spectra were referenced against tetramethylsilane (TMS). 10385


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a pixel size of 44 × 44 μm and an active area of 85 × 255 mm2 (NCD station, ALBA). The data were normalized for synchrotron beam fluctuations using an ionization chamber placed before the sample. Furthermore, a correction for the sample absorption was performed using a photodiode located at the beamstop before the background contribution was subtracted. The wavenumber q = 4π sin θ/λ, with θ being half of the scattering angle for WAXD experiments scale calibration, has been achieved by α-Al 2O 3 (alumina) (DUBBLE beamline, ESRF) and Cr2O3 (NCD station, ALBA). Heating and cooling of the samples were performed using a Linkam DSC 600 cell placed in the X-ray beam. To probe the effect of cooling rate on the BHET crystal morphology and its effect on the PLA nucleation, the samples were subjected to varying cooling rates of 10, 30, and 70 °C/ min (maximum achievable rate with the Linkam DSC cell) while cooling from 200 and 140 °C. After a 3 min isotherm to equilibrate the samples, the samples were cooled to 80 °C at a rate of 10 °C/min. The evolution of the WAXD pattern reflecting the PLA crystallization was monitored on-line throughout the cooling process. Finally, the melting behavior of the samples was monitored during heating at a rate of 10 °C/min. Data reduction was conducted using bubble software.41 Two-dimensional (2D) small-angle X-ray scattering (SAXS) analysis on the injection-molded bars was performed using a SAXSLAB Ganesha diffractometer, with a sample-to-detector distance of 1076.3 mm using Cu Kα radiation (λ = 1.5406 Å) and silver behenate (d001 = 58.380 Å) as calibrant. Tensile testing on the injection-molded tensile bars was conducted using a Zwick 100 tensile instrument equipped with a 10 kN load cell. Samples were tested at room temperature under a constant deformation rate of 5 mm/s.

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are acknowledged for supporting the X-ray experiments. Enabling Technologies B.V. is acknowledged for providing access to equipment for SAXS analysis. This project has received funding from the INTERREG V program FlandersNetherlands (Puur Natuur: 100% biobased), the cross-border collaboration program financially supported by the European fund for regional development.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.9b00848.

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REFERENCES

Additional information on BHET morphology and its effect on PLA crystallization as a function of concentration (Figure S1), isothermal crystallization after cooling at various rates (DSC) (Figure S2), evolution of the WAXD patterns during isothermal crystallization after cooling at various rates (Figures S3 and S4), and thermomechanical behavior after processing (Table S1, Figures S5 and S6) (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Carolus H. R. M. Wilsens: 0000-0003-3063-9510 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS NWO (Nederlandse Organisatie voor Wetenschappelijk Onderzoek) is acknowledged for providing beam time at the ESRF. The staff of both the DUBLLE (Dutch Belgian beamline, ESRF) and the NCD station (Alba synchrotron) 10386


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