Enhanced crystallization kinetics of poly(lactide) with oxalamide compounds as nucleators: effect of spacer length between the oxalamide moieties

Abstract

Poly(lactide), PLA, suffers from low crystallization rate. To speed up the crystallization rate, oxalamide compounds with the formula of C₆H₅NHCOCONH(CH₂)_nNHCOCONHC₆H₅ (n = 2, 4, 6, 8, 12) are synthesized as nucleating agents (NA_(n)). Their thermal properties and nucleation efficiency can be tailored by tuning the aliphatic spacer length, i.e., –(CH₂)_n–. The melting temperatures and crystallization temperatures of the NA_(n) decrease monotonically with increasing n value while the thermal decomposition temperature remains constant. The aliphatic spacer length (n) of NA_(n) influences obviously the crystallization kinetics of PLA, resulting in nucleation efficiency in the sequence of NA₍₂₎ > NA₍₄₎ > NA₍₁₂₎ ≈ NA₍₈₎ > NA₍₆₎. Polarized optical microscopy results show that NA_(n) could self-assemble into needle-like superstructures which subsequently promote the crystallization of PLA macromolecules. The differences in superstructural geometry and the nucleation activity of NA_(n) are responsible for the variation in the nucleation efficiency.

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1. Introduction

Poly(lactide) (PLA) is a biobased and biodegradable polyester that has drawn increasing attention because of the environmental and sustainable issues related to conventional polymeric materials.¹ However, end-use articles of PLA suffer from low heat distortion temperature (HDT) due to its relatively low glass transition temperature (∼60 °C) and low crystallinity after practical processing such as injection molding. Moreover, amorphous PLA possesses poor gas barrier properties, short service life and weak resistance to solvents. Consequently, the application ranges of PLA, notably hot-packaging and durable applications, are restricted. It is thus of importance to achieve a high crystallinity of PLA by speeding up the crystallization.

Many approaches have been carried out to increase the crystallization rate of PLA such as plasticization,² co-polymerization,³ stereocomplexation⁴ and heterogeneous nucleation.^5–8 Addition of heterogeneous nucleating agents is the most applied approach to enhance the crystallization kinetics of polymers by depressing the nucleation energy barrier and subsequently promoting polymer chain folding at high(er) temperatures. Inorganic nucleating agents such as talc, clay, carbon nanotubes and graphene have been applied for PLA, whereas agglomeration was often obtained due to the poor compatibility and insolubility in the PLA melt, leading to uncontrollable physical shape/size and nucleation efficiency.^5–8 On the other hand, organic additives such as N,N-ethylene-bis(12-hydroxylstearamide) (EBH), poly(vinylidene fluoride), orotic acid, nucleobases, substituted-aryl phosphate salts (TMP-5) and N,N′,N′′-tricyclohexyl-1,3,5-benzene-tricarboxylamide (TMC-328), N,N′-bis(benzoyl) hexanedioic acid dihydrazide (TMC306), N₁,N′₁-(ethane-1,2-diyl)bis(N₂-phenyloxalamide) (OXA) were also developed as nucleating agents for PLA.^9–21 Among above nucleating agents, TMC-328, TMC306 and OXA were proven with high activity which can self-organize and precipitate from PLA melt to initiate the fast crystallization of PLA.^18–21 By increasing the crystallinity and controlling ordered morphology using organic nucleating agents, the oxygen permeability coefficient of PLA film was reduced by 2 orders of magnitude.¹⁹ Besides small molecular additives, stereocomplex PLA and high-molecular-weight PLA were also approved as effective nucleating agents. Yang et al. recently reported that a small amount of high-molecular-weight PLA could make the normal grade PLA crystallize even at a cooling rate as high as 100 °C.²²

Recently, our group revealed that oxalamide compounds can serve as effective soluble-type nucleating agents for polyester.^20,21,23 The configuration of the oxalamide compounds is schematically shown in Scheme 1, where the central part contains two oxalamide moieties providing the driving force (H-bonds) for self-organization while the flanking arms (R₁) determine the miscibility of the compounds in the PLA melt and the thermal behavior of the compounds that are crucial to the nucleation efficiency.

Scheme 1 Schematic illustration of chemical configuration of the oxalamide compounds.

In a previous study, we reported the effect of flanking arms (cyclohexyl, benzyl, and phenyl, respectively) of the oxalamide compounds on the crystallization behavior of PLA while keeping the aliphatic spacer length constant (n = 2).²⁰ The thermal behavior of the compounds (e.g., crystallization and melting temperatures) are strongly dependent on the end-group configuration and the nucleation activity (A_n) that following the sequence of A_n-phenyl > A_n-benzyl > A_n-cyclohexyl. However, the effect of aliphatic spacer length between the two oxalamide moieties is not clear yet.

In the present work, five model compounds with fixed flanking arms (phenyl groups) but different aliphatic spacer length between the two oxalamide moieties were tailor-made (n = 2, 4, 6, 8 and 12, respectively). The effect of the aliphatic spacer length on the thermal properties of the compounds and the crystallization of PLA matrix was well studied. The present work not only provides an insight in the nucleation/crystallization kinetics of PLA with novel self-assembly-type nucleating agents but also shows great industrial interest.

2. Experimental section

2.1. Materials

The PLA 4032D (2% of D-LA, M_w = 220 kDa, PDI = 2.1) was provided by Natureworks LLC., USA. 1,2-diaminoethane were purchased from Sinopharm Chemical Reagent Co., Ltd, China. Ethyl-2-oxo-2-(phenylamino)acetate, 1,4-butanediamine, 1,6-hexylenediamine, 1,8-octanediamine, 1,12-dodecanediamine and chloroform were purchased from J&K Chemical Ltd., Shanghai, China. The chemicals were used as received.

2.2. Synthesis

Five model oxalamide compounds (NA_(n)) were synthesized and studied in this work, i.e., N₁,N′₁-(ethane-1,2-diyl)bis(N₂-phenyl oxalamide) (NA₍₂₎), N₁,N′₁-(butane-1,2-diyl)bis(N₂-phenyl oxalamide) (NA₍₄₎), N₁,N′₁-(hexane-1,2-diyl)bis(N₂-phenyl oxalamide) (NA₍₆₎), N₁,N′₁-(octane-1,2-diyl)bis(N₂-phenyl oxalamide) (NA₍₈₎), N₁,N′₁-(dodecane-1,2-diyl)bis(N₂-phenyl oxalamide) (NA₍₁₂₎). The synthesized oxalamide compounds were abbreviated as NA_(n) where n is the number of –CH₂– units of the aliphatic spacer. Taking NA₍₄₎ as an example, it was synthesized by a one-step reaction as follows: ethyl 2-oxo-2-(phenyl amino)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 1,2-diaminoethane (1.1 g, 12.9 mmol). The mixture was stirred under reflux for 48 hours. The resulted precipitates were filtered, washed and dried overnight at 80 °C in a vacuum oven. The final product was obtained as white powder. The other four NA_(n) were prepared by using the same synthetic route. The chemical structures and corresponding NMR spectra of the model compounds are shown in Fig. 1.

Fig. 1 The chemical structures and ¹H-NMR spectra of the phenyl-capped oxalamide compounds with different central spacer lengths: (a) NA₍₄₎, (b) NA₍₆₎, (c) NA₍₈₎ and (d) NA₍₁₂₎. The chemical structure and ¹H-NMR of the NA₍₂₎ were referred to ref. 20.

2.3. Sample preparation

PLA and the NA_(n) in the form of white powders were dried at 60 °C in a vacuum oven for 12 hours before use. The PLA/NA_(n) blends were prepared in a Haake mixer (Polylab OS, Germany) at 180 °C for 4 min. The rotor speed was fixed at 50 rpm. The samples are denoted as PLA/NA_(n). For comparison, neat PLA was undergone the same processing conditions and the NA_(n) concentration in the PLA/NA_(n) blends was fixed at 0.75 wt%.

2.4. Characterization

Nuclear magnetic resonance (NMR). ¹H NMR (400 MHz) spectra of the NAs were recorded in TFA-d (δ 11.63) solutions using a Bruker Model Avance 400 spectrometer.

Thermal gravimetric analysis (TGA). TGA (1100SF, Mettler-Toledo International Trade Co., Ltd. Switzerland) was used to evaluate the thermal decomposition behavior of the NA_(n). The TGA measurements were performed from 20 to 600 °C at 10 °C min⁻¹ in a nitrogen atmosphere.

Differential scanning calorimetry (DSC). The crystallization and melting behavior of samples were characterized by using DSC analyzer (DSC 8000, Perkin Elmer, USA). For non-isothermal crystallization of PLA/NA_(n) samples, the samples were first heated to 190 °C for 3 min, then cooled to 0 °C and re-heated to 190 °C at 10 °C min⁻¹. For isothermal crystallization, samples were quenched (100 °C min⁻¹) to the desired crystallization temperatures after prior heating at 190 °C for 3 min. Films with a weight of 3–4 mg were used for DSC characterization.

DSC conditions for neat NA_(n) are as follows: samples were first heated to 370 °C for 3 min, then cooled to 0 °C, both of heating and cooling rates are 10 °C min⁻¹. A linear baseline was subtracted from the raw data before data analysis as a function of the temperature. The crystallinity (X_c) of PLA is calculated via X_c = ΔH_c/(ω × ΔH⁰_m), where ω is the weight fraction of PLA in the PLA/NA_(n) blends and ΔH⁰_m = 93 J g⁻¹ is the melting enthalpy of 100% crystalline PLA.²⁴

Polarized optical microscope (POM). The evolution in crystal morphology during isothermal crystallization were monitored with a POM (Axio Scope 1, Zeiss, Germany) in combination with a Linkam THMS600 hot-stage. The film samples of PLA/NA_(n) sandwiched between two carefully cleaned glass slides, were first melted at 190 °C for 3 min to remove thermal history, and then cooled to designed temperatures at 50 °C min⁻¹ for isothermal crystallization.

Scanning electron microscope (SEM). The dispersion of NAs in the as-prepared PLA/NA_(n) sheets were investigated by using a scanning electron microscope (S-4800, HITACHI, Japan) at an accelerating voltage of 2 kV. The cross sections of the as-prepared sheets were cryogenically fractured in liquid nitrogen and subsequently coated with a thin layer of gold before observation.

Fourier transform infrared (FT-IR). The PLA and PLA/NA_(n) samples were analyzed on FT-IR spectrometers (Nicolet 6700, USA) in an attenuated total reflection mode (ATR). The final spectrum of each sample was an average of 16 scans at a resolution of 2 cm⁻¹ in the wavenumber range of 4000–500 cm⁻¹.

Wide angle X-ray diffraction (WAXD). 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° and a scan rate of 1° min⁻¹.

3. Results and discussion

A series of oxalamide compounds with fixed flanking arms (phenyl groups) were synthesized. The aliphatic spacer was designed as –(CH₂)_n– where n is 2, 4, 6, 8 and 12, respectively. The chemical structures of the oxalamide compounds are confirmed by ¹H-NMR, as shown in Fig. 1. In order to demonstrate the effect of aliphatic spacer length on their nucleation efficiency, the thermal properties of the oxalamide compounds and their assembly behavior in the PLA melt are first investigated as a function of n values. Then the crystallization behavior of PLA with the oxalamide compounds was studied.

3.1. Self-assembly of the oxalamide compounds with different spacer length

The heating and cooling DSC curves of the oxalamide compounds are shown in Fig. 2. The thermal parameters such as melting temperatures (T_m), crystallization temperatures (T_c), crystallization enthalpy (ΔH_c) and thermal degradation temperature (T_d, obtained from TGA) are listed in Table 1.

Fig. 2 DSC heating (h) and subsequent cooling (c) curves of oxalamide compounds (NA_(n)) with different spacer lengths. The temperature scan rate is 10 °C min⁻¹.

Table 1 Thermal parameters of the oxalamide compounds

NA(n)	Tm (°C)	Tc (°C)	ΔHc (J g^−1)	Td (°C)
NA(2)	338	328	120	353
NA(4)	299	287	178	361
NA(6)	274	255	164	333
NA(8)	244	234	177	351
NA(12)	225	206	174	330

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The decomposition temperature (T_d) of the oxalamide compounds is around 350 °C which is far above the melting temperatures of PLA. The melting and crystallization (self-organization) behaviors of the oxalamide compounds are strongly affected by the spacer length (n value), as shown in Fig. 2. The T_m and T_c gradually decreased from 338 °C and 328 °C to 225 °C and 206 °C, respectively, with increasing the n values from 2 to 12. Moreover, all the examined oxalamide compounds showed small supercoolings (ΔT = T_m − T_c), typically 10 °C, suggesting a strong interaction and rapid self-organization process among the compounds. In our previous study, the self-assembly mechanism of oxalamide compounds was revealed by means of solid-NMR and confirmed that the driving force for self-organization is hydrogen bonding between oxalamide moieties.^21,23 These results indicate that the oxalamide compounds are capable of organizing into ample heterogeneous nuclei within a short time span prior to the crystallization of PLA matrix (100–140 °C).

3.2. Self-organization of oxalamide compounds in PLA melt as a function of the spacer length

The self-organization behavior of the oxalamide compounds in PLA matrix would be influenced by the neighboring macromolecules. Fig. 3 is the POM images demonstrating the self-organization process of the different oxalamide compounds in the PLA melt upon cooling at 10 °C min⁻¹. The oxalamide compounds could dissolve completely in the PLA matrix at 240 °C as evidenced by a homogeneous mixture of the blend (Fig. 3, left column). Interestingly, the dissolved molecules of oxalamide compounds gradually self-organized into “shishi-like” superstructures upon cooling, however the “shishi” length decreased while the number increased with increasing the spacer length. It was also noticed the self-organization process of the oxalamide compounds is fast, completed within 5 °C (∼0.5 min), which is in agreement with the self-organization behavior of the bulk oxalamide compounds (Fig. 2).

Fig. 3 POM images of the PLA/NA_(n) blends upon cooling from 240 °C at 10 °C min⁻¹ as a function of temperature under cooling.

The T_m and T_c of the oxalamide compounds in the PLA melt are plotted and compared with those in their bulk materials, as shown in Fig. 4. It can be seen (i) both T_m and T_c decreased in the PLA melt in comparison with that in the bulk materials, and (ii) both T_m and T_c in the PLA melt showed the same variation trend as a function of spacer length in comparison with that in the bulk state. Taking the NA₍₄₎ for example, the T_m and T_c decrease by 81 °C and 93 °C, respectively, while the other NA_(n) have similar behavior. Following the Flory–Huggins miscibility concepts, the suppression in T_m and T_c of the oxalamide compounds at low concentrations demonstrates a certain extent of miscibility between the oxalamide compounds and the PLA matrix where the PLA matrix behaves as a solvent of the oxalamide molecules. Such a depression in T_m and T_c is characteristic for efficient melt-soluble nucleating agents, e.g., 1,3:2,4-bis(3,4-dimethylbenzylidene)sorbitol and Irgaclear® in poly(propylene).^25,26 It was also observed that the shorter spacer the larger depression in T_m and T_c. The larger suppression indicates a stronger interaction (hydrogen bonds) between the oxalamide groups and the PLA molecules which may eventually results in better compatibility. It has to be addressed that the crystallization of NA_(n) in the PLA melt is not detectable at low concentrations, thus all the T_c of the NA_(n) in the PLA melt was obtained from POM for comparison.

Fig. 4 (a) crystallization temperatures and (b) melting temperatures of the NA_(n) in the PLA/NA_(n) blends in comparison with those of the NA_(n) bulk materials.

3.3. Non-isothermal crystallization of the PLA and PLA/NA_(n) blends

The crystallization and melting DSC curves of the PLA and PLA/NA_(n) blends are shown in Fig. 5, and the corresponding thermal parameters are listed in Table 2. The PLA hardly crystallized upon cooling (Fig. 5a), consequently, a pronounced cold crystallization peak (T_cc = 100 °C, ΔH_cc = 32 J g⁻¹) was detected in the subsequent heating process (Fig. 5b). The poor crystallization ability of the PLA resulted from the intrinsically low mobility of PLA chains due to the hard and short repeating units and the lacking of sufficient nucleators.^27,28 A high(er) crystallization temperature and a sharp(er) crystallization peak upon cooling usually indicate a rapid(er) crystallization rate. The DSC data in Fig. 5a show that the oxalamide compounds with different spacer lengths could initiate the crystallization of PLA in the cooling process, however their nucleation efficiencies are quite different. The T_c of the PLA occurred at 108–118 °C when the spacer length varied between –(CH₂–)₂ and –(CH₂)₁₂–. Meanwhile, the crystallinity, obtained from cooling process, was increased up to 38.1%. Nam et al. reported a T_c of 110 °C for PLA with EBH as nucleating agent (2 °C min⁻¹ cooling rate)¹⁰ while Huneault et al. reported a T_c of 103.2 °C for talc nucleated PLA (10 °C min⁻¹ cooling rate).⁵ Compared with literatures, the oxalamides compounds can be good nucleating agents of PLA by tailoring the molecular motif.

Fig. 5 DSC curves of the PLA and PLA/NA_(n) blends: (a) cooling from the melt and (b) subsequent heating processes. The concentration of NA_(n) in the PLA/NA_(n) blends was fixed at 0.75 wt%. The temperature ramp is 10 °C min⁻¹.

Table 2 Non-isothermal crystallization parameters of the PLA and PLA/NA_(n) blends

Samples	Tc (°C)	Tcc (°C)	Xc (%)	Tm (°C)
PLA	—	110.3	0.00	170.9
PLA/NA(2)	118.3	—	38.1	168.0
PLA/NA(4)	110.2	—	36.6	169.9
PLA/NA(6)	108.1	106.0	21.6	170.2
PLA/NA(8)	112.2	84.4	36.4	170.1
PLA/NA(12)	112.6	92.4	31.2	170.1

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Interestingly, the nucleation effect of the oxalamides compounds first decreased and then increased with increasing the spacer length from –(CH₂)₂– to –(CH₂)₁₂– although the thermal properties of the compounds (T_c and T_m) varied monotonically. The PLA with NA₍₆₎ shows a notable cold crystallization peak (T_cc) at 105 °C (Fig. 5b) which is 5 °C lower than that of the neat PLA. Moreover, the PLA with NA₍₆₎ has a crystallinity of 21.6% after cooling that is higher than that of neat PLA (X_c ≈ 0) but lower than that of the PLA with other NA_(n) (X_c = 31.2–38.1%). In contrast, the T_cc of PLA was disappeared or indistinctive in the presence of other NA_(n). Considering the overall crystallization characters of the PLA/NA_(n) samples (T_c, X_c and sharpness of the crystallization peaks), the nucleation efficiency of the NA_(n) in the non-isothermal crystallization of PLA is concluded in the sequence of NA₍₂₎ > NA₍₄₎ > NA₍₁₂₎ ≈ NA₍₈₎ > NA₍₆₎.

The non-isothermal crystallization of the PLA and PLA/NA_(n) samples were also studied at different cooling rates. To further evaluate the nucleation ability of the NA_(n) with varied spacer length, the nucleation activity (A_n) of the NA_(n) in the PLA melt was analyzed via Dobreva and Gutzow's method.²⁹ According to this method, the cooling rate (Q) for homogeneous nucleation can be described as a function of supercooling degrees (ΔT_P = T_m − T_c), i.e.,

where A and B are constants that determined by linear fitting of the relationship of Q and ΔT_P. In the case of heterogeneous nucleation, B has to be replaced by B*. The nucleation activity (A_n) is then defined as the ratio of B* to B. The A_n value is between 0 and 1, and the smaller of the A_n value the more active the heterogeneous nucleator. The log Q versus 10⁴/ΔT_P² for PLA and PLA/NA_(n) blends are plotted in Fig. 6 with an inset of the A_n values. The small A_n values (0.3–0.4) indicate good nucleation activities of the oxalamide compounds. Moreover, the nucleation activity sequence of the examined NA_(n) is NA₍₂₎ > NA₍₄₎ > NA₍₁₂₎ ≈ NA₍₈₎ > NA₍₆₎, which is in agreement with the above DSC results (Q = 10 °C min⁻¹, Fig. 5). Thus, the nucleation efficiency of the NA_(n) in the PLA melt has positive correlation with the nucleation activity.

Fig. 6 Plots of log Q versus 10⁴/ΔT_P² for PLA and PLA/NA_(n) blends.

The FT-IR was performed on the PLA and PLA/NA_(n) blends with 0.75 wt% NA_(n) to illustrate the interaction between the PLA and the NA, see ESI (Fig. S1 and Table S1†). Since the NA concentration is low (0.75 wt%), the PLA and PLA/NA_(n) blends show very similar FT-IR spectra. However, it can still be seen that the C=O stretching band of the PLA chains decreased slightly from 1748.1 to 1747.6 cm⁻¹ after addition of the NA_(n). In the meanwhile, the C–H stretching band of the PLA chains (∼3000 cm⁻¹) shifted slightly to higher wavenumbers. These changes can be regarded as an evidence of the interaction, i.e., hydrogen bonding, between N–H groups of the NA and C=O groups of the PLA.

The nucleation efficiency of the NA_(n) may also be affected by the dispersion of NAs in PLA matrix which was investigated by using SEM, as shown in Fig. 7. It was observed that the NA₍₂₎ and NA₍₄₎ self-assembled into needle-like superstructures whereas the NA₍₂₎ has a better self-organization than NA₍₄₎. The NA₍₆₎, NA₍₈₎ and NA₍₁₂₎ formed particles rather than needle-like superstructures, and the NA₍₆₎ showed larger particle size than the NA₍₆₎ and NA₍₈₎ indicating a poorer dispersion in the PLA matrix. Moreover, it can be seen from Fig. 7 that the NA₍₂₎ needle-like superstructures and NA₍₁₂₎ particles have better adhesion to the PLA matrix than the other NA superstructures. Therefore, the geometry and dispersion of the NA as well as their adhesion to the PLA matrix could be another reason for the above NEs sequence.

Fig. 7 SEM imagines of cross sections of the sheet samples showing the dispersion of NA_(n) in the PLA matrix: (a/a′) PLA/NA₍₂₎, (b/b′) PLA/NA₍₄₎, (c/c′) PLA/NA₍₆₎, (d/d′) PLA/NA₍₈₎ and (e/e′) PLA/NA₍₁₂₎. The arrows point to the NA superstructures.

3.4. Isothermal crystallization of the PLA and PLA/NA_(n) blends

The completion of isothermal crystallization inside the mold over a short cycle period is important for the injection molding of high performance PLA articles. In this regard, the isothermal crystallization of PLA containing NA_(n) was explored in the temperature range of 120–135 °C. Taking 135 °C for example, the exothermal DSC curves of the PLA and PLA/NA_(n) blends are shown in Fig. 8a. Obviously, the crystallization of PLA is slow at such a high temperature leading to a wide but indistinctive exothermic peak. With addition of 0.75 wt% NA_(n), the exothermal peak became sharper and larger in amplitude. In addition, the peak position of PLA/NA_(n) shifted to right side (i.e., longer time) with increasing the n values from 2 to 6 and then shifted to left side (i.e., shorter time) with further increasing the n values to 12. These results are consistent with their crystallization half-life time (t_1/2) (Fig. 8b) which is determined directly from the relative crystallinity (X_t) versus crystallization time (t) curves. Crystallization half-life time is defined as the time required complete 50% of the final crystallinity. Thus the larger t_1/2 values the lower overall crystallization rate. Due to the difficulty in homogeneous nucleation, the t_1/2 of neat PLA in the temperature range of 120–135 °C is as large as 10–40 min and increased obviously with temperature (inset of Fig. 8b). On the other hand, the t_1/2 of PLA/NA₍₂₎ is below 0.9–3 min and less sensitive with temperature. In addition, the most commercialized PLA nucleator, i.e., talc (0.75 wt%) was applied as well in the PLA matrix for comparison purpose, which lead to a t_1/2 of 21 min at 135 °C (see ESI Fig. S2†). These results further proven that the oxalamide compounds are can be efficient nucleating agent for PLA at different temperatures while the efficient can be tuned by controlling the configuration of the compounds such as the spacer length.

Fig. 8 (a) DSC exothermal curves and (b) crystallization half-life time (t_1/2) of the PLA and PLA/NA_(n) blends during isothermal crystallization at 135 °C. The DSC exothermal curves in the first 10 min is enlarged and inset in (a) while the correlation of t_1/2 versus isothermal crystallization temperature for the PLA and PLA/NA₍₂₎ blend is inset in (b).

Fig. 9 shows the polarized optical microscopy (POM) images of the PLA and PLA/NA_(n) blends during isothermal crystallization at 135 °C. Several spherulites were observed in the PLA melt after an induction period of 10 min (Fig. 9a). Classical maltese cross pattern are observed in neat PLA and the spherulites reached 100 μm after 30 min and kept on growing. In contrast, smaller spherulites with much larger numbers are observed in the PLA/NA_(n) blends (Fig. 9b–f). The individual spherulites are too small to be distinguished. It is noticed that some fine crystals were already formed prior to the crystallization of PLA (left column of Fig. 9) which is only associated with the NA_(n) crystals. Then a significant amount of PLA crystal filled in the field of vision within a short time span (typically 2–6 min) which is in agreement with the above DSC results. The POM results demonstrated that the superstructures of the oxalamide compounds as nucleation sites initiated and accelerated the crystallization of PLA matrix.

Fig. 9 Polarized optical microscopy images (scale bar = 400 μm, the same magnification) of the PLA and PLA/NA_(n) blends during isothermal crystallization at 135 °C as a function of time: (a) PLA, (b) PLA/NA₍₂₎, (c) PLA/NA₍₄₎, (d) PLA/NA₍₆₎, (e) PLA/NA₍₈₎ and (f) PLA/NA_(12). The samples were first melted at 190 °C for 3 min and then quenched to 135 °C for observation. A 1/4λ optical filler was applied to make a better color contrast in the images.

The effect of NA_(n) on the crystalline modifications of the PLA was investigated by using wide-angle X-ray diffraction (WAXD), as shown in Fig. 10. Three crystalline modifications of PLA were reported in literature, i.e., α/δ, β and γ forms. The α and δ forms with 10₃ helical chain conformation is typically formed from melt or cold crystallization.^30–33 The β form with left-handed 3₁ helical conformation is usually formed by drawing at high temperatures, whereas γ form was only obtained via epitaxial crystallization.³⁴ The PLA after isothermal crystallization from the melt for 3 min at 135 °C was almost amorphous as evidenced by a broad X-ray diffraction peak (Fig. 10). The broad peak disappeared after addition of 0.75 wt% NA_(n). Meanwhile, three diffraction peaks located at 2θ = 14.7°, 16.5° and 18.9° are observed, which are correlated to the 010, 200/110 and 203 planes of PLA α crystals, respectively.³⁰ Similar WAXD patterns (not shown) were obtained as well for these samples after cooling (10 °C min⁻¹) from their melt. Hence, the NA_(n) mainly enhanced the formation of α form PLA crystals because the examined temperatures are in the domain of preferential PLA crystallization in alpha form. In addition, the three diffraction peaks of PLA/NA₍₆₎ are less pronounced and smaller in area compared with the other PLA/NA_(n) samples indicating again that the nucleation effect of NA_(2,4,8,12) is better than NA₍₆₎.

Fig. 10 1D-WAXD patterns of the PLA and PLA/NA_(n) blends. The samples were annealed at 135 °C for 3 min before being subjected to WAXD characterization.

4. Conclusion

Oxalamide compounds with the chemical formula of C₆H₅NHCOCONH(CH₂)_nNHCOCONHC₆H₅ (n = 2, 4, 6, 8, 12) were synthesized as nucleating agents (NA_(n)) for poly(lactide) (PLA). The effect of the spacer length, i.e., n values, on their thermal properties and nucleation efficiency was investigated. Both the melting temperature (T_m) and the crystallization temperature (T_c) of the NA_(n) decreased with increasing the spacer length which is the soft segment between the two stiff moieties (−NHCOCONH−). Moreover, the T_c and T_m of the compounds decreased considerably after compounding with the PLA indicating a good miscibility or strong interaction between the NA_(n) and the PLA. The NA_(n) could assemble into needle-like superstructures in the PLA melt via hydrogen bonds, while the morphology and dispersion of the superstructures are strongly associated with the spacer length and the assembly conditions. DSC and POM results indicate that the nucleation efficiency of the NA_(n) reduced with increasing the spacer length up to n = 6 and then increased. The differences in the morphology of superstructures and the nucleation activity of the NA_(n) with different spacer lengths are responsible for the varied nucleation efficiency of the NA_(n) which is in the sequence of NA₍₂₎ > NA₍₄₎ > NA₍₁₂₎ ≈ NA₍₈₎ > NA₍₆₎. This work conclusively demonstrated that highly efficient nucleating agents of polymeric materials such as PLA can be designed by tailoring the chemical configuration of the oxalamide compounds.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (51573074, 51303067), Natural Science Foundation of Jiangsu Province (BK20130147) and the Fundamental Research Funds for the Central Universities (JUSRP51624A).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra04050k

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