Introduction

Poly (l-lactic acid) (PLLA), one of the biodegradable plastics, has attracted more and more attention for its superior mechanical and thermal properties compared to other commercial polymers in the past decade [1, 2]. Generally, the mechanical and thermal properties of PLLA are related to its crystal structure and morphology. Up to now, different techniques have been applied to study the crystal structure and morphology of PLLA, such as scanning electronic microscopy, wide-angle X-ray diffraction, Fourier transform infrared spectroscopy (FTIR), and Raman spectra [3–6].

As is well known, FTIR is very sensitive to local molecular environment, and thus it can be used to investigate the structural changes of polymer chains and analyze the variations among the intra- and intermolecular interactions of macromolecules during the melting and crystallization process [7]. The chain conformation and crystallization of PLLA can induce changes in both peak profile and intensity of specific chemical groups in FTIR spectra. Some publications have reported that the methyl (–CH3) and ester (C–O–C) groups of PLLA chains are sensitive groups during the crystallization of PLLA [6, 8–12]. Zhang et al. has demonstrated that CH3 groups in PLLA chains can form close interchain interaction during the induction period and the distorted 103 helix conformation of PLLA chain in α crystals was due to the interchain interactions between the CH3 groups. Furthermore, they proved that the order formation of C–O–C backbone occurred upon the induction period of PLLA crystallization by two-dimensional infrared correlation spectroscopy [9]. Hu [8] investigated the structural changes of PLLA chains during the melt crystallization with FTIR, and found that the CH3 groups and ester groups initially occurred, and then the 103 helix conformation of PLLA chain in α-crystal was formed. But when the carbon nanotube (CNT) was added to induced PLLA crystallization, it was found that PLLA chains absorbed the CNTs through their –CH groups and subsequently PLLA backbones interacted with the pre-adsorbed ones via –(COC+CH3) interactions to form a unique conformation with ordered structure. Additionally, Krikorian [13] has reported that interchain interactions preceded that of intrachain and 103 helix formation in the neat PLLA during the melt crystallization, but in the case of intercalated nanocomposites (PLLA15A10), inter- and intrachain interactions started simultaneously confirmed by investigating normalized peak intensities of FTIR spectroscopy. The above reports are in well accordance with many reports where long-range order has been observed earlier than the formation of crystalline short-range order from a crystallization melt; the interchain interactions (CH3 groups and COC groups) occur prior to the intrachain interactions. Zhang utilized the 2D infrared correlation spectroscopy to determine that the CH3 groups occurred prior to the ester groups during the cold crystallization of PLLA [10].

The chain conformational order during the crystallization of PLLA was well studied by Fourier transform infrared (FTIR) spectroscopy, and the CH3 groups and ester groups of PLLA were highly sensitive to the structural changes taking place during the crystallization of PLLA. To the best of our knowledge, however, there is no report about the chain conformational order sequence between CH3 groups and ester groups during the melt crystallization of PLLA. In this work, the conformation order sequence of CH3 groups and ester groups of PLLA chains would be further investigated by FTIR and 2D infrared correlation analysis during the melt crystallization, and the crystallization mechanism of PLLA would be revealed.

Experiment

Material and sample preparation procedures

The PLLA (Model 4032D) used in this study was a film-grade granular material manufactured by Nature Works LLC. The PLLA film for an IR measurement was prepared by solution casting method. Typically, 1 g of PLLA was dissolved in 100 mL of chloroform and ultrasonically mixed for 30 min. Then the solution was poured onto an aluminum foil and dried overnight at room temperature to evaporate the solvent. The membranes were then placed in a vacuum oven at 40 °C for 24 h to remove the solvent thoroughly.

Time-resolved Fourier transform infrared spectroscopy characterization

In-situ infrared spectra of PLLA film were recorded on a Nicolet IS10. In order to study the melt crystallization process of PLLA, PLLA film was put on the KBr pellets, and then the sample was set on a temperature controller manufactured by Linkman. The sample was heated to 200 °C at 10 °C/min, and held for 5 min to erase the thermal history completely. Then the sample was cooled to 140 °C at the rate of 50 °C/min, and the FTIR spectra were collected at 1 min intervals during the crystallization process, by co-adding 16 scans at a 4 cm−1 resolution. From the IR spectra, it was found that 180 min was enough to finish the crystallization at 140 °C.

Two-dimensional correlation analysis

FTIR spectra were selected in certain wavenumber ranges and the generalized 2D correlation analysis was applied by the 2D-Shige software. (2Dshige (c) Shigeaki Morita, Kwansei-Gakuin University, 2004–2005). The 2D correlation spectrum was shown in the center of the map. In the averaged 1D correlation maps, the red-colored regions were defined as the positive correlation intensities, whereas the blue-colored regions were regarded as the negative correlation intensities.

Results and discussion

As mentioned above, methyl (CH3) and ester (C–O–C) groups were highly sensitive to the structural changes of PLLA chains. The absorption bands of CH3 bending vibration and C–O–C stretching vibration are in the region of 1500–1000 cm−1, and 103 helix formation is related to the region of 970–850 cm−1. Specifically, IR bands at 921, 1091, 1208 and 1455 cm−1, corresponding to respectively intrachain 103 helix structure, C–O–C interchain interaction, the interchain interaction between C–O–C and –CH3, and CH3 interchain interaction, were chosen to analyze the chain conformation order of PLLA during the isothermal melt crystallization. Figure 1 shows representative time-resolved FTIR spectra in the region of 1500–850 cm−1 collected during the isothermal crystallization of PLLA at 140 °C within 180 min. For clear observation, bands from 970 to 850 cm−1 are amplified as shown in Fig. 2.

Fig. 1
figure 1

Time-dependent IR spectra in the range of 1500–850 cm−1 collected during the melt crystallization of PLLA at 140 °C (the spectra were recorded with 20 min intervals)

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Fig. 2
figure 2

Time-dependent IR spectra in the range of 970–850 cm−1 collected during the melt crystallization of PLLA at 140 °C (the spectra were recorded with 20 min intervals)

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The isothermal crystallization of PLLA

It’s well-known that PLLA forms α-crystal when PLLA is purified from solutions or under annealing crystallization. Kister et al. [6] and Zhang et al. [9] reported that the absorption band at 921 cm−1 was recognized as α-crystal of PLLA, while the absorption band at 908 cm−1 was characteristic of β-crystal [6, 14, 15]. Figure 2 shows representative time-resolved FTIR spectra of PLLA during the isothermal crystallization at 140 °C in the range of 970 to 850 cm−1. It can be seen that there was no absorption band at 908 cm−1, but the absorption band at 921 cm−1 turned up and its intensity increased gradually with the annealing time, indicating the formation of α-crystal with distorted 103 helix conformation during isothermal crystallization. It was also found that the absorption at 871 cm−1 increased while that at 955 cm−1 decreased with annealing time. As known, the intensity of bands corresponding to crystalline would increase while the one of amorphous state decrease during polymer crystallization [7]. In addition, Zhang et al. [9] proposed the 871 cm−1 band is sensitive to the 103 helix conformation of PLLA crystal, and the 955 cm−1 bands are related to the amorphous state of PLLA. Thus, with the reference of previous study [9, 15, 16, 25], here we ascribed 871 and 955 cm−1 to the characteristic peaks of crystalline and amorphous state of PLLA, respectively.

Difference spectra can distinguish the variation of absorption peak when the polymer is undergoing annealing crystallization. The absorption band that appeared or with increased intensity is above the base line of difference spectra, and the spectra below the base line is representative of the disappeared absorption band or the decreased absorption band. Difference spectra obtained by the subtraction of the initial spectrum from the spectra in Fig. 2 is displayed in Fig. 3.

Fig. 3
figure 3

Difference spectra in the range of 970–850 cm−1 obtained by subtraction of the initial spectrum from time-dependent IR spectra of PLLA at 140 °C (the spectra were arranged with a 20-min interval)

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As shown in Fig. 3, the intensity of absorption peaks at 921 and 871 cm−1 increased with the annealing time, which were contributed to the appearance of crystalline structure during the crystallization process of PLLA. On the other hand, the intensity of absorption bands at 955 and 860 cm−1 decreased with the annealing time, indicating the gradually disappeared amorphous state of PLLA during the melt crystallization. The differential spectra further confirmed the transition from amorphous state to crystalline structure when PLLA underwent isothermal melt crystallization at 140 °C.

The conformation order of methyl groups and ester groups of PLLA chains by FTIR during the isothermal melt crystallization

The fingerprint regions of FTIR spectra can supply enough information about the intra- or intermolecular interaction of the functional groups when PLLA are undergoing isothermal crystallization. CH3 and C–H bending vibration and C–O–C stretching vibration, which are distributed in the region of 1500–1000 cm−1 [8, 14, 15], are sensitive to the crystallization of PLLA. Figure 4 shows the spectra of PLLA in the region of 1500–1420 cm−1 during the isothermal crystallization. Some researchers [3, 6, 14, 15, 17] have reported that the absorption peaks at 1455 and 1385 cm−1 were respectively CH3 asymmetric and symmetric bending vibration. Usually the band around 1455 cm−1 assigned to the CH3 asymmetry bending vibration was taken as the internal standard [6, 10]. As shown in Fig. 4, the intensity of peak at 1455 cm−1 increased gradually and shifted to the high wavenumber with the annealing time, indicating that intermolecular interaction took place during the melt crystallization. To further investigate the bands changes, the difference spectra obtained by the subtraction of the initial spectrum from the spectra in Fig. 1 is displayed in Fig. 5. The peak at 1455 cm−1 assigned to CH3 asymmetric bending vibration was split into two peaks at 1458 and 1438 cm−1 during the melt crystallization of PLLA. Nikimin et al. stated in their study that the fundamental modes of a regular polymer chain structure are split into various spectral components in the crystal if the intermolecular forces between polymer chains are sufficient [18]. Further, Zhang et al. [9] has proven the intermolecular interaction between methyl groups in PLLA chains during the melt crystallization and when the interaction is strong enough, it can lead to the splitting of original absorption peak into different peak components. Accordingly, it is plausible that the splitting of the adsorption peak of methyl during the melt crystallization at 140 °C is caused by the methyl-induced intermolecular interaction between the PLLA chains.

Fig. 4
figure 4

Time-dependent IR spectra in the range of 1500–1420 cm−1 collected during the melt crystallization of PLLA at 140 °C (the spectra were recorded with a 20-min interval)

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Fig. 5
figure 5

Difference spectra in the range of 1500–1000 cm−1 obtained by subtraction of the initial spectrum from time-dependent IR spectra of PLLA at 140 °C (the spectra were arranged with a 20-min interval)

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The band in the region of 1215–1180 cm−1 is highly sensitive not only to the structural adjustment of the C–O–C backbone but also to the structural order of CH3 group during the annealing crystallization of PLLA [10]. The peak at 1091 cm−1 is related to the C–O–C structural order during the melt crystallization of PLLA [10]. According to the previous study [10], the band at 1181 cm−1 assigned to ν asCOC + r asCH3 was related to the amorphous phase, while the band at 1208 cm−1 assigned to ν asCOC + r asCH3 corresponded to the crystalline phase during the melt crystallization of PLLA. When a polymer transforms from amorphous phase to crystalline structure, the intensity of crystalline bands will increase and the intensity of amorphous bands will decrease [7]. Accordingly, as shown in Figs. 1 and 5, absorption peak at band of 1208 cm−1 was not obvious at the beginning of crystallization, but its intensity remarkably increased with the annealing time. It was also found that the band gradually sharpened during crystallization, suggesting a transition to a well-defined order [12]. In addition, the intensity of bands at 1184 and 1091 cm−1 decreased during the melt crystallization. Therefore, the band at 1208 cm−1 was sensitive to the structure order of ν asCOC + r asCH3 in the crystalline phase during the melt crystallization of PLLA, while the band at 1184 cm−1 was related to the structure order of ν asCOC + r asCH3 in the amorphous phase during the melt crystallization of PLLA. The band at 1091 cm−1 was related to structure adjustment of C–O–C backbone during the melt crystallization of PLLA. In the previous reports [8, 13, 15], the bands at 1208 cm−1 (ν asCOC + r asCH3) and 1091 cm−1 (ν sCOC) were taken as the internal standard to study the structure order sequence of PLLA chains during the isothermal crystallization.

According to the above analysis, the band at 1455, 1208 and 1091 cm−1 which were assigned to δasCH3, ν asCOC + r asCH3 and ν sCOC, respectively, were sensitive to structure changes of PLLA chains during the melt crystallization. To the best of our knowledge, however, there is no report about the chain conformational order between CH3 groups and ester groups (C–O–C) during the melt crystallization of PLLA. In order to probe the crystallization mechanism of PLLA, the sequence of conformation order of δasCH3, ν asCOC + r asCH3 and ν sCOC is investigated during the melt crystallization.

The conformation order of CH3 groups and ester groups

Generalized two-dimensional (2D) correlation spectroscopy, which is based on the perturbation of samples with time, temperature, concentration, reaction time and so forth, is proposed by Noda [19–22]. 2D correlation spectroscopy is a powerful tool to explore subtle spectral changes induced by an external perturbation. In the generalized 2D correlation spectrum, according to the Noda’s rules [22, 23], the cross peak observed in a synchronous 2D correlation spectrum Φ(ν 1, ν 2) represents the simultaneous or coincidental changes at corresponding variables ν 1 and ν 2, as an external perturbation is applied. On one hand, the sign of synchronous cross peaks becomes positive if the spectra intensities at corresponding wavenumbers are either increasing or decreasing together as the perturbation is changed during the observation time. On the other hand, the negative sign of cross peaks indicates that one of the spectra intensity at ν 1 is increasing, while the other is decreasing during the observation period. The asynchronous spectrum of dynamic spectral intensity variation represents sequential or unsynchronized at corresponding variables ν 1 and ν 2, an asynchronous cross peak shows that the intensities of two dynamic spectral intensities (ν 1, ν 2) vary out of phase with each other. 2D correlation spectrum can be a visible way to study the sequence of structure order of PLLA chains during the isothermal crystallization. The sign of asynchronous cross peaks Ψ(ν 1, ν 2) can be either positive or negative. If the intensity change at ν 1 occurs prior to that at ν 2, the sign of the asynchronous cross peak becomes positive; on the other hand, the intensity changes at ν 1 occurs after ν 2, the sign of the asynchronous cross peak is negative. The rule, however, is reversed if the sign of the synchronous cross peak Φ(ν 1, ν 2) is negative. In the following work, the 2D correlation spectrum in the region of 1500–1000 cm−1 was analyzed and to study the sequence of structure order of CH3 group and COC group in the PLLA chains.

Figure 6 shows the synchronous and asynchronous correlation spectra of PLLA in the region of 1500–1000 cm−1, providing further information on the crystallization process of PLLA with the annealing time. The sign of synchronous cross peaks was Φ(1458, 1208) > 0, Φ(1458, 1085) < 0, and the sign of asynchronous cross peaks showed that Ψ(1458, 1208) < 0, Ψ(1458, 1085) > 0. According to the Noda’s rule [22, 23], the intensity at 1208 and 1085 cm−1 changed prior to that at 1458 cm−1, and the bands at 1458, 1208 and 1085 cm−1 were assigned to asCH3, ν asCOC + r asCH3 and ν sCOC, which were highly sensitive to structural changes of PLLA chains and taken as the internal standard for the sequence of structure order of PLLA chains. Consequently, the –(COC + CH3) conformational changes and the structure adjustment of COC groups were prior to the CH3 groups. Compared to the above analysis, the structural adjustment of the CH3 group preceded that of the COC groups during the cold crystallization of PLLA at 78 °C [10]. Thus the chain conformation order of PLLA was different under melt crystallization and cold crystallization.

Fig. 6
figure 6

Synchronous (a) and asynchronous (b) correlation spectra of PLLA in the region of 1500–1000 cm−1 calculated from the spectra obtained during the isothermal crystallization at 140 °C

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In order to obtain the detailed information about the structural changes of PLLA chains during the melt crystallization, the 2D correlation spectra in the region of 1300–1000 cm−1 was shown in Fig. 7. The band at 1135 cm−1 was assigned to the CH3 rocking bands [15], and the bands at 1208 and 1085 cm−1 were assigned to ν asCOC + r asCH3 and ν sCOC, respectively. The sign of synchronous cross peaks was Φ(1208, 1135) > 0, Φ(1208, 1085) < 0 and Φ(1135, 1085) < 0 (Fig. 7a), and the sign of asynchronous cross peaks was Ψ(1208, 1135) = 0, Ψ(1208, 1085) > 0, Ψ(1135, 1085) < 0 (Fig. 7b). According to the Noda’s rule [22, 23], the peak of 1085 cm−1 varied prior to the peaks of 1208 and 1135 cm−1. Thus, the above three bands varied in the sequence order as 1085 > 1208 cm−1, 1085 > 1135 cm−1. Therefore the intensity changes of ester groups (COC) occurred prior to that of CH3 groups and the –(COC+CH3) conformational changes. Based on the previous analysis, the conformation changes of ester groups preceded that of –(COC+CH3), and the –(COC+CH3) conformational changes occurred prior to the changes of –CH3 counterparts. In conclusion, the order of conformation changes is –COC, –(COC + CH3) and –CH3.

Fig. 7
figure 7

Synchronous (a) and asynchronous (b) correlation spectra of PLLA in the region of 1300–1000 cm−1 calculated from the spectra obtained during the isothermal crystallization at 140 °C

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The isothermal crystallization process of PLLA is summarized in Scheme 1. First, the conformation ordering begun with the ester groups labeled as a blue line. Second, the formation of the initial helix chain conformation was interfered by the interchain interactions between the CH3 groups labeled as a red line, and then the initial helix chains incorporated and propagated with the annealing time [24]. Eventually, when the length of helix chains exceeded the critical value, the helix chains folded into the crystal lattice and the α-crystal with the 103 helix conformation was formed.

Scheme 1
scheme 1

Schematic diagrams of conformational order in isothermal crystallization of PLLA

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Conclusion

The melt isothermal crystallization behavior of PLLA at 140 °C was investigated by FTIR spectroscopy and 2D correlation spectroscopy. It was found that PLLA formed α-crystal during the isothermal crystallization because the intensity of spectra band at 921 cm−1, which was recognized as α-crystal structure of PLLA, increased with the annealing time. In order to explore the chain conformation order of PLLA chains during the isothermal crystallization, the ester groups stretching vibration and CH3 rocking vibration bands in the region of 1500–1000 cm−1 was analyzed by 2D correlation analysis. The results showed that the conformation changes of C–O–C groups occurred prior to that of CH3 groups during the isothermal crystallization, and then the interchain interactions between the CH3 groups formed the initial helix chain conformation. Finally, α-crystal with the distorted 103 helix conformation was formed.