Rapid Crystallization of Poly(lactic acid) by Using Tailor-Made

Jul 30, 2014 - Rapid Crystallization of Poly(lactic acid) by Using Tailor-Made Oxalamide Derivatives as Novel Soluble-Type Nucleating Agents. Piming M...
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Rapid Crystallization of Poly(lactic acid) by Using Tailor-Made Oxalamide Derivatives as Novel Soluble-Type Nucleating Agents Piming Ma,†,‡ Yunsheng Xu,† Dawei Wang,† Weifu Dong,† and Mingqing Chen*,† †

The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, 1800 Lihu Road, Wuxi 214122, China ‡ Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, Eindhoven 5612 AZ, The Netherlands S Supporting Information *

ABSTRACT: The crystallization rate and crystallinity of poly(lactic acid) (PLA) was significantly increased by the incorporation of 0.25−1.0 wt % of tailor-made oxalamide derivatives (NAs). The nucleation effect and nucleation mechanisms of the NAs were studied via diferential scanning calorimetry (DSC), polarized optical microscopy (POM), and wide-angle X-ray diffraction (WAXD) techniques. The experimental results convincingly revealed that the NA molecules are soluble in a PLA melt and are capable of self-organizing into fibrils upon cooling. The fibrils as efficient nucleation sites induced rapid growth of α-form PLA crystal along the fibrils, forming shish-kebab-like structures. In isothermal crystallization, very fine PLA sperrulites with high density were obtained in the presence of NAs. The high nucleation efficiency and the simple synthetic routes of the NAs make them promising to be a new generation of nucleating agents for (bio)polymers, e.g., PLA.

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orotic acid,12 nucleobases,13 and N,N′,N″-tricyclohexyl-1,3,5benzene-tricarboxylamide (TMC-328)14 were also applied as nucleating agents for PLA. Among above nucleating agents, TMC-328 was proven with high activity which can self-organize in PLA melt to induce its fast crystallization.14 Consequently, the oxygen permeability coefficient of PLA was reduced by more than 2 orders of magnitude via the addition of TMC-328 and the morphology control.15 Recently, fast epitaxial cold crystallization of PLA was even obtained on highly oriented polyethylene substrate.16 The prime objective of this communication is to briefly report an effective way to achieve rapid crystallization of PLA by using tailor-made oxalamide derivatives (NAs) as novel soluble-type nucleating agents that is free of any prior art reported in the literature. The high nucleation efficiency and the simple synthetic route of the NAs make them promising to be a new generation of nucleating agents for PLA. In principle, it should also work in other crystalline polymers. The authors are confident that this work will generate high impact to the knowledge and technology of crystalline polymers (e.g., PLA). Therefore, the present work is beyond pure academic interests.

INTRODUCTION The well-known bio-based and biodegradable poly(lactic acid) (PLA) has received increasing attention, because of the environmental concerns and the sustainability issues associated with petroleum-based polymers.1 Several favorable properties at room temperature and relatively low cost make PLA the mostpromising substitute for conventional petroleum-based polymers.2 On the other hand, PLA usually exhibits very low crystallinity after practical processing. As a result, PLA articles lose mechanical performances above the glass-transition temperature (50−60 °C). Moreover, amorphous PLA deserves poor gas barrier properties and short service life. Consequently, the application ranges of PLA, notably in the fields of (hot) packaging and durable applications, are restricted. Therefore, it is important to achieve high crystallinity of PLA via fast crystallization. Crystallization of a polymer includes two steps, i.e., nucleation and crystal growth. The inherent short segments and chain stiffness of PLA result in an extremely small crystal growth rate (Gc < 6 μm/min),3 compared to 20−30 μm/min of poly(propylene) and >1000 μm/min of poly(ethylene). Mathematical models and empirical studies suggest that increasing the Gc value for PLA is far more challenging than increasing the nucleation efficiency. To increase the nucleation efficiency, nucleating agents are used to reduce the free energy needed for the formation of a critical nucleus.4 Indeed, the crystallization of PLA was accelerated to a certain extent by inorganic nucleating agents such as talc,5 clay,6 carbon nanotubes,7 and graphene.8 However, agglomeration of inorganic nucleating agents is usually obtained because of their insolubility in the PLA melt, leading to uncontrollable nucleating agent shape/size and nucleation efficiency. On the other hand, organic additives such as N,N-ethylene-bis(12hydroxylstearamide),9 poly(vinylidene fluoride),10 sc-PLA,11 © 2014 American Chemical Society

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EXPERIMENTAL SECTION Materials. The PLA 4032D (2% D-lactic acid (D-LA), Mw = 220 kDa, PDI = 2.1) was provided by Natureworks LLC, USA. 1,2-Diaminoethane and 1,6-hexamethylenediamine were purchased from Sinopharm Chemical Reagent Co., Ltd., China. Ethyl 2-oxo-2-(phenylamino)acetate was purchased from J&K Chemical Ltd., Shanghai, China. The chemicals were used as received. Received: Revised: Accepted: Published: 12888

May 30, 2014 July 21, 2014 July 25, 2014 July 30, 2014 dx.doi.org/10.1021/ie502211j | Ind. Eng. Chem. Res. 2014, 53, 12888−12892

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Synthesis. Two of the oxalamide derivatives, i.e., N1,N1′(hexane-1,6-diyl)bis(N2-phenyloxalamide) (abbreviated as NA1) and N1,N1′-(ethane-1,2-diyl)bis(N2-phenyloxalamide) (abbreviated as NA2) were synthesized by a one-step reaction. The NA 1 was synthesized as follows: ethyl 2-oxo-2(phenylamino)acetate (5 g, 25.8 mmol) was first dissolved in 200 mL of chloroform in a three-necked round-bottom flask, followed by a slow feeding of 1,6-hexamethylenediamine (1.5 g, 12.9 mmol). The mixture was stirred under reflux for 48 h. The resulted precipitates were filtered, washed, and dried overnight at 80 °C in a vacuum oven. The final product was obtained as white powder. NA2 was obtained using the same synthetic route. Sample Preparation. PLA was dried at 60 °C in a vacuum oven for 12 h before use. The PLA/NAs blends were prepared in a Haake mixer at 180 °C for 4 min. The rotation speed was fixed at 50 rpm. The samples are denoted as PLA-xNAi, where x and i represent the loading and the type of NAs, respectively. For comparison, neat PLA was prepared by using the same procedures. Characterization. The differential scanning calorimetry (DSC) characterizations were performed on a Model DSC 8000 analyzer (Perkin−Elmer, USA). For nonisothermal crystallization study, samples were first heated to 190 °C for 3 min, then cooled to 0 °C and reheated to 190 °C at 10 °C/ min. For isothermal crystallization, samples were quenched (100 °C/min) to desired temperatures after isothermal at 190 °C for 3 min and then held until the crystallization is complete. Thin films with a weight of 3−4 mg cut directly from Haake mixed samples were used for DSC characterization. The evolution in crystal morphology and phase morphology during isothermal and nonisothermal crystallization were observed via polarized optical microscopy (POM) (Axio Scope 1, Zeiss, Germany) equipment, in combination with a Linkam THMS600 hot stage. Wide-angle X-ray diffraction (WAXD) measurements were carried out by using an X-ray diffractometer (Bruker AXS D8, Germany) equipped with a Ni-filtered Cu Kα radiation source with a wavelength of 1.542 Å. The measurements were operated at 40 kV and 40 mA with scan angles from 5° to 40° at a scan rate of 1°/min.

Figure 1. (a) DSC cooling and subsequent heating curves of the PLA and the PLA/NAs blends with 0.75 wt % of different NAs, and (b) the t1/2 of the PLA and the PLA/NAs blends as a function of isothermal crystallization temperature (Tc‑iso). The heat flow of the samples as a function of isothermal crystallization time (t) at 135 °C is shown as an inset in panel b. Curves (1,1′), (2,2′), and (3,3′) correspond to the samples PLA, PLA−0.75 wt %NA1, and PLA−0.75 wt %NA2, respectively.

indicate a poor crystalline ability of the PLA. Interestingly, the crystallization of PLA was dramatically promoted by the addition of NA2. A sharp crystallization peak (Tc) and a high crystallinity (Xc = 37%) were obtained during cooling (see curve 3 in Figure 1a). The PLA completed crystallization already upon cooling (10 °C/min), since no Tcc was visible in the subsequent heating curve (see curve 3′ in Figure 1a). The crystallization of PLA was also accelerated by NA1, which is not as efficient as NA2. It is associated with the differences in thermal parameters (Table 1) that affect the self-assembly of the NAs. More details about the self-assembly behavior of NAs and the effect on PLA crystallization will be discussed in a forthcoming paper. Half-life crystallization time (t1/2) obtained from isothermal crystallization is an important parameter to evaluate the overall crystallization rate. The t1/2 value, as a function of the isothermal crystallization temperature (Tc‑iso), is plotted in Figure 1b. Obviously, the t1/2 value of the PLA is remarkably decreased after the incorporation of NAs. The decrease is more pronounced at small supercoolings where homogeneous nucleation is hardly possible.4 It can be seen from the exothermal traces at 135 °C that PLA, in the presence of NAs, completed crystallization within several minutes (insert in Figure 1b). In contrast, it took around 90 min for neat PLA to complete crystallization at the same temperature. These

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RESULTS AND DISCUSSION Two oxalamide derivatives, i.e., N1,N1′-(hexane-1,6-diyl)bis(N2phenyloxalamide) (NA1) and N1,N1′- (ethane-1,2-diyl)bis(N2phenyloxalamide) (NA2) were synthesized by a one-step reaction. Each of the two NAs comprises a core motif with two oxalamide motifs, flanked by two aromatic arms. Their chemical structures are clarified by 1H NMR and 13C NMR characterizations, as shown in Figure S1 in the Supporting Information). The two NAs were applied in this work to demonstrate their nucleation effect on the PLA. Similar to DMDBS, the oxalamide derivatives are capable of selforganizing into fibrillar superstructures through intermolecular hydrogen bonding, which could significantly accelerate the crystallization of PLA. The crystallization of PLA/NAs blends was first investigated by using DSC. The DSC curves and some corresponding parameters are shown in Figure 1 and Table 1, respectively. As expected, no trace of PLA crystallization was detected upon cooling, followed by a notable cold crystallization (Tcc) and a pronounced glass transition (Tg) in the subsequent heating process (see curves 1 and 1′ in Figure 1a). These results 12889

dx.doi.org/10.1021/ie502211j | Ind. Eng. Chem. Res. 2014, 53, 12888−12892

Industrial & Engineering Chemistry Research

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Table 1. Thermal Parameters of the PLA, PLA/NAs Blends, and NAs Obtained from DSC Cooling and Subsequent Heating Scans sample PLA PLA/ NA1 PLA/ NA2 NA1 NA2

crystallization peak temperature, Tc (°C)

crystallization enthalpy, ΔHc (J/G)

cold crystallization peak temperature, Tcc (°C)

crystallinity, Xc (%)a

melting peak temperature, TM (°C)

108

0 20

111 106

0.0 22

171 170

118

34

37

168

255 328

175 86

274 342

a Calculated using the equation Xc = ΔHc/(ω × ΔHm ° ), where ω is the weight percentage of PLA in the blends and ΔHm ° = 93 J/g is the melting enthalpy of 100% crystalline PLA.19 The content of NAs in the PLA/NAs blends is 0.75 wt %.

isothermal data demonstrate the enhanced overall crystallization rate by the presence of NAs, which is more pronounced at high temperatures. It must be remarked that the nucleation effect of NAs is much better than that of talc and PDLA, the results of which are not shown here. Double melting peaks of PLA are observed due to a melt/ recrystallization/remelt mechanism.17,18 The lower temperature melting peak (or shoulder) corresponds to primary crystals, which weakens after addition of the NAs. This phenomenon indicates that the perfection of PLA crystals is improved after the addition of the NAs. The crystal morphology of the PLA and the PLA/NA2 blends is studied by using POM, as shown in Figure 2. Several Figure 3. WAXD patterns of the PLA and the PLA/NA2 blends as a function of NA2 contents: (a) 0.00 wt %, (b) 0.25 wt %, (c) 0.50 wt %, (d) 0.75 wt %, and (e) 1.00 wt %. The samples were quenched from melt to 135 °C and annealed for 3 min at 135 °C before WAXD characterizations.

The α- and α′-forms with a 103 helical chain conformation is grown from melt or cold crystallization.20−22 The β-form with a left-handed 31 helical conformation was formed from highly drawing at high temperatures,23 whereas the γ-form was only obtained through epitaxial crystallization.24 The PLA in the absence of NA is amorphous as evidenced by a broad X-ray diffraction (XRD) peak (Figure 3a). This peak gradually disappeared as the NA content increased up to 0.75−1.0 wt %. Meanwhile, three sharp diffraction peaks located at 2θ = 14.7°, 16.6°, and 19.1° are observed after addition of the NAs. The three sharp peaks are correlated to the 010, 200/110, and 203 planes of the PLA α-crystals, respectively.20 It can be concluded from the WAXD results that the NAs significantly enhanced the formation of α-form PLA crystals. Although the nucleation efficiency of the oxalamide derivatives has been convincingly approved by above DSC, POM, and WAXD results, the nucleation mechanism is still unknown. It is found that the oxalamide derivatives are capable of self-assembly in the PLA melt through intermolecular interactions in the form of fibrils. The fibrillar superstructures significantly accelerate the crystallization of PLA. Such behavior is similar as the role of DMDBS in poly(propylene), which has been studied and understood.25 The self-assembly of the NAs in PLA melt and the crystallization of PLA in the presence of the NAs are schematically illustrated in Figures 4a−c. The illustrations are proposed based on the study by using POM (see insets in Figures 4a−c), in combination with the phase diagram of the PLA/NA2 binary system (see Figure 4d).

Figure 2. POM images of (a, a′, a″) PLA and (b, b′, b″) PLA−0.75% NA2 blend as a function of isothermal crystallization time at 135 °C. The same magnification was applied for all images.

spherulites are visible in the PLA melt (135 °C) after an induction period of ∼10 min (Figure 2a). The spherulites with typical Maltese cross-optical extinction patterns grow up slowly, leaving a large area of the PLA still in the melt state (or amorphous), even after 30 min (Figure 2a″). Moreover, hardly any new spherulites arose after the induction period. On the other hand, massive dotlike spherulites are observed in the PLA/NA2 blend, as shown in Figures 2b and 2b′. The crystallization of PLA with NA2 was already completed within 5 min, indicating a high nuclei density and a fast crystallization rate. The POM images are well in agreement with above DSC results and offer direct insight into the high nucleation efficiency of the NA in the PLA melt. The effect of NAs on the crystalline modifications of PLA was further investigated by using WAXD, as shown in Figure 3. Until now, four crystalline modifications (α, α′, β, γ) of PLA have been identified, depending on the preparation conditions. 12890

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bottleneck of the current PLA technology and broaden its application range.

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ASSOCIATED CONTENT

S Supporting Information *

Additional information about chemical structures and 1HNMR, 13C-NMR spectra of the NA1 and NA2 (Figure S1). This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 510 85917019. Fax: +86 510 8591 7763. E-mail: [email protected].

Figure 4. (a−c) Schematic illustrations of the self-assembly of the NAs and the crystallization of PLA in the presence of NA; (d) the temperature/composition phase diagram of the PLA/NA2 binary system constructed from the POM experiments; (e) the proposed intermolecular interaction among the NA molecules. The POM images on crystal morphology of the PLA−0.75 wt %NA2 blend upon cooling are inserted in the top right corner of panels a−c (all shown at the same magnification, scale bar = 20 μm). “LN” and “LP” refer to liquid (or melt) NA2 and PLA, respectively, while “SN” and “SP” refer to solid NA2 and PLA, respectively.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (No. 51303067) and Natural Science Foundation of Jiangsu Province (No. BK20130147). The authors would like to acknowledge Prof. Sanjay Rastogi, Prof. Piet J. Lemstra, and Dr. Yogesh Deshmukh (Eindhoven University of Technology, The Netherlands) for valuable discussions in the early stage of this work.

The NA2 possesses a Tm value of 342 °C; however, it is soluble in the PLA melt at relative low temperatures and forms a homogeneous system (Figure 4a). Upon cooling, the NAs self-assemble into fibrillar structures via intermolecular hydrogen bonding prior to the solidification of PLA chains (Figure 4b). The fibrillation temperature is also called crystallization temperature or gelation temperature of the NA (i.e., Tc‑NA in Figure 4d). Subsequently, the fibrils serve as nucleation sites (shish) and induce the fast growth of PLA-bundled lamellae (kebab). Consequently, a charming macroscopic shish-kebablike structure is created, where the dish-shaped kebab-like structure is uniformly packed along the shish axis (Figure 4c). Such an evolution of morphology is clearly observed by POM and demonstrated as the insets in Figures 4a−c. The temperature/composition diagram of the PLA/NAs systems exhibits a binary monotectic behavior, which is typical for mixtures of two species showing limited liquid miscibility and high solid immiscibility. In the studied NAs range (i.e., 0.0−5.0 wt %), no liquid−liquid phase separation was observed. The gelation temperature of the NAs (Tc‑NA) and the solidification temperature of the PLA (Tc‑PLA) are strongly dependent on the NAs concentration, as shown in Figure 4d. Consequently, the NA superstructures and the PLA crystal morphology also exhibit composition-dependent characteristics, which will be systematically discussed in a forthcoming paper.

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CONCLUSION Two oxalamide derivatives (NAs) were successfully synthesized by a one-step reaction, which showed high nucleation efficiency in the poly(lactic acid) (PLA) matrix. The NAs are capable of self-organizing in the PLA melt in the form of fibrils that, as nucleation sites, significantly accelerate the crystallization of PLA. The fibrils could induce fast growth of PLA bundled lamellae forming shish-kebab-like structures. Only α-form crystal was obtained in neat PLA which was not modified by the addition of the NAs under the studied conditions. In addition, the simple synthetic routes of the NAs make them promising to be a new generation of nucleating agents for polymers, e.g., PLA. Thus, the discovery of oxalamide derivatives as nucleating agents may break through the 12891

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dx.doi.org/10.1021/ie502211j | Ind. Eng. Chem. Res. 2014, 53, 12888−12892