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Rapid crystallization and mesophase formation of poly(L-lactic acid) during precipitation from a solution

  • Muhammad Syazwan and Takashi Sasaki ORCID logo EMAIL logo
Published/Copyright: February 14, 2018
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e-Polymers
From the journal e-Polymers Volume 18 Issue 4

Abstract

Very rapid crystallization behaviors of poly(L-lactic acid) (PLLA) are observed at room temperature when it is precipitated from a chloroform solution into a large amount of alcohols (non-solvents). The resulting crystalline phase contains both a highly ordered (α) and less ordered (α′) modifications, and the fraction of these phases depends on the alcohols used as the non-solvents: methanol tends to produce the highly ordered phase. The degree of crystallinity tends to be high for lower alcohols. When the precipitation occurs in n-hexane, almost no crystalline phase is formed, but a mesomorphic phase is formed as a precursor to the crystalline phase. The results suggest that the hydroxyl group of alcohols tends to promote the crystallization of PLLA. However, it is found that the precipitation in methanol at lower temperatures, such as 0°C, does not yield any crystalline phase. It is suggested that the present rapid crystallization during precipitation originates from the enhanced mobility of PLLA molecules in a metastable (non-equilibrium) liquid state.

Keywords: crystallization; FTIR; mesophase; PLLA; precipitation

1 Introduction

Poly(L-lactic acid) (PLLA) is known as one of the most common biodegradable and biocompatible polymers. For broader applications of PLLA as a multi-purpose plastic material, it is necessary to modify its physical properties. One practical technique for the modification of PLLA is to control the degree of crystallinity. However, it has been revealed that PLLA is rather intractable because of its slow crystallization tendency compared with common crystallizable polymers (1), (2). Efforts have been undertaken to overcome this problem. Nucleation agents and plasticizers are often used as common methods to promote the crystallization (3), (4), (5), (6), (7). Other techniques such as freeze-drying (8), (9), and solvent-induced crystallization (10) have also been reported. In addition, it is worth noting that interface and geometrical nano-confinement significantly affect the crystallization process of PLLA (11), (12), (13), (14).

To promote the polymer crystallization rate, it is important to clarify its fundamental mechanism. It has been reported that a mesomorphic phase (mesophase), which is an intermediately ordered phase, plays an important role in the pathway of polymer crystallization as a precursor of a crystalline state (15), (16), (17), (18), (19), (20). As its existence has been evidenced for some polymers, the issue of the mesophase has evoked new understandings for polymer crystallization kinetics, and there has been much discussion on this issue for various polymers (21), (22), (23), (24), (25). The mesophase for PLLA has been assigned by using Fourier transform infrared (FTIR) spectroscopy (26), (27), (28), (29), and furthermore, it has been evidenced that a rapid crystallization of PLLA is achieved through the mesophase that was formed by supercritical CO2 treatment (30). In addition, a strain-induced mesophase for PLLA was found to occur, which significantly affects the crystalline structure that is formed after a succeeding annealing (31).

In this study, we found that crystallization of PLLA occurs very rapidly at room temperature during the precipitation process from a PLLA/chloroform solution into non-solvents possessing a hydroxyl group (alcohols). On the other hand, precipitation from the PLLA solution into n-hexane does not induce apparent crystallization; however, the formation of the mesophase is observed. The precipitation phenomenon may include two processes, i.e. phase separation and crystallization. The rapid crystallization observed in the precipitation in alcohols concerns the interplay between these two processes. Discussion on this point will be presented.

2 Experimental

PLLA (Mw=210 kDa, 98% L units) supplied from Mitsui Chemicals Co., Tokyo, Japan, was dissolved in chloroform (Nacalai Tesque, Kyoto, Japan) with stirring for 72 h at room temperature to make a 5.0 wt% homogeneous solution. We used n-hexane (Nacalai Tesque, Kyoto, Japan), methanol (Kanto Chemicals, Tokyo, Japan), ethanol (Wako Chemicals, Osaka, Japan), n-propanol (Nacalai Tesque, Kyoto, Japan), and 2-propanol (Wako Chemicals, Osaka, Japan) as non-solvents for PLLA. Precipitation was performed by pouring 2.0 ml of the PLLA/chloroform solution into 60 ml of the non-solvent with constant stirring. The temperature was kept constant at different temperatures (–41°C, 0°C, and 30–32°C). The obtained precipitant was separated via filtration, and it was dried under vacuum at room temperature for 72 h. The obtained samples with the above non-solvents are referred to as PLLA/Hex, PLLA/MeOH, PLLA/EtOH, PLLA/nPrOH, and PLLA/2PrOH, respectively. A reference amorphous PLLA was prepared via press-molding at 100 kg cm−2 at 200°C followed by quenching in water at room temperature.

Differential scanning calorimetry (DSC) was performed by using a Perkin Elmer (Waltham, MA, USA) Pyris Diamond calorimeter with an Intra-cooler P2 cooling system. The temperature was calibrated by using an indium standard, and all of the measurements were done in a nitrogen atmosphere. We performed step-scan DSC measurements, a kind of temperature-modulated DSC, as well as conventional DSC scan measurements. In the step-scan measurements, we repeated a 2.0 K step of heating at a rate of 5.0 K min−1 followed by a temperature holding period of 1.5 min. Details are described in Ref 8. The conventional DSC scan was done at a rate of 40 K min−1 for both heating and cooling. Wide-angle X-ray diffraction (WAXD) measurements were performed by using a Rigaku (Tokyo, Japan) RINT2100 equipped with a CuKα radiation source (λ=0.1542 nm). FTIR spectroscopy was performed by using a Nicolet 6700 (Thermo Fisher Scientific, Waltham, MA, USA) infrared spectrometer. For transmission measurements of FTIR, the precipitated sample was pressed to yield a translucent sheet. To investigate the crystalline morphology, scanning electron microscopy (SEM) was performed by using a Hitachi (Tokyo, Japan) S-2600 microscope operated at 15 kV. To evaluate the phase diagram for the PLLA/chloroform/non-solvent system, cloud point measurements were performed. Here, the non-solvent was added slowly with stirring to PLLA/chloroform solutions with fixed PLLA content of 2.0 wt% and 5.0 wt% till the solution became turbid, thus the cloud point was determined.

3 Results and Discussion

3.1 Crystallization of PLLAs precipitated in alcohols

Figure 1 shows DSC heating traces for the precipitated PLLAs. There is no exothermic signal between the glass transition temperature Tg and the melting temperature for PLLA/MeOH, PLLA/EtOH, and PLLA/2PrOH. This indicates that crystallization has sufficiently proceeded in these samples so that any additional crystallization does not occur during the DSC heating scan. As for PLLA/nPrOH, only a subtle exotherm is observed, as indicated by an arrow. Figure 2 shows WAXD profiles of the PLLAs precipitated in alcohols. Diffraction peaks that correspond to (200)/(110) and (203) reflections are clearly observed, confirming the crystallinity for these samples. We should also note that for PLLA/MeOH exhibits (210) reflection, which suggests an ordered crystalline structure of PLLA that is often referred to as α modification (32), (33), (34). Indeed, the (210) reflection is very sensitive to the ordered α phase (32), and the appearance of this reflection peak in Figure 2 implies that the α phase is at least partially included in PLLA/MeOH. It has been reported that the ordered α structure occurs when PLLA is crystallized at low supercoolings, i.e. when the crystallization temperature is higher than 120°C. While at temperatures lower than 120°C a less ordered crystalline structure (α′ modification) is usually formed (35). It may be surprising that the present precipitation method produces the ordered α crystals at a temperature below Tg. On the other hand, the (210) reflection was found to be subtle (or absent) for the samples with higher alcohols (alcohols with higher number of carbon atoms). However, the peak position for (200)/(110) shifts only slightly to lower angle as the alcohol becomes higher, which suggests that the present precipitated PLLAs contain both α and α′ phases.

Figure 1: DSC heating traces for precipitated PLLAs in non-solvents at 30°C.The heating rate was 40 K min−1. The arrow indicates a slight exothermic peak observed for PLLA/nPrOH.
Figure 1:

DSC heating traces for precipitated PLLAs in non-solvents at 30°C.

The heating rate was 40 K min−1. The arrow indicates a slight exothermic peak observed for PLLA/nPrOH.

Figure 2: WAXD profiles for precipitated PLLAs in alcohols at 30°C and reference amorphous PLLA.The curves are vertically shifted. The arrow indicates a small diffraction peak observed for PLLA/Hex.
Figure 2:

WAXD profiles for precipitated PLLAs in alcohols at 30°C and reference amorphous PLLA.

The curves are vertically shifted. The arrow indicates a small diffraction peak observed for PLLA/Hex.

Figure 3 shows FTIR spectra in the region of carbonyl stretching band for PLLAs precipitated in alcohols. We observe a shoulder at 1749 cm−1, which is assigned to the α crystalline structure (36), (37). The shoulder is weakened as the number of carbons in the alcohol becomes higher, thus it is suggested that the ordered crystal phase tends to be formed in lower alcohols such as methanol. This is consistent with the WAXD results. Anyhow, we infer that the precipitated PLLA contains both α and α′ crystals. Figure 4 compares the FTIR spectra of precipitated PLLAs at 30°C, and the amorphous PLLA in the region 840–970 cm−1. The PLLA/MeOH exhibits a crystalline band at 921 cm−1 that has been assigned to a 10/3 helix conformation in the α phase (35). The PLLA/EtOH shows a similar profile to that of PLLA/MeOH.

Figure 3: FTIR spectra in the range of carbonyl stretching band for precipitated PLLAs in alcohols at 30°C and reference amorphous PLLA.
Figure 3:

FTIR spectra in the range of carbonyl stretching band for precipitated PLLAs in alcohols at 30°C and reference amorphous PLLA.

Figure 4: FTIR spectra in the range of 840–970 cm−1 for precipitated PLLAs in alcohols at 30°C and reference amorphous PLLA.
Figure 4:

FTIR spectra in the range of 840–970 cm−1 for precipitated PLLAs in alcohols at 30°C and reference amorphous PLLA.

The degree of crystallinity Xc of the precipitated PLLAs was evaluated as (ΔHm–ΔHc)/ΔHm° from DSC heating traces at 40 K min−1, and the results are shown in Table 1. Here, ΔHm and ΔHc are the observed exothermic heat and endothermic heat in the heating trace, respectively. ΔHm° is the heat of fusion of PLLA crystal, for which we used a literature value 90.9 J g−1 (38). In addition, we evaluated the crystallinity from the FTIR spectra: Xc=A(921 cm−1)/[A(921 cm−1)+A(956 cm−1)] was used (30), where A(921 cm−1) and A(956 cm−1) are absorbance of a crystalline band and an amorphous band, respectively. The observed crystallinity for PLLA/MeOH is rather high and is comparable to that obtained through isothermal crystallization in a bulk state (37). It is noted that Xc decreases as the alcohol becomes higher, being consistent with the results of WAXD.

Table 1:

Crystallinity of the precipitated PLLAs obtained from DSC and FTIR measurements.

Non-solventXc (%)
DSCFTIR
MeOH4347
EtOH4234
nPrOH3928
2PrOH3924

3.2 Mesophase formation of PLLA precipitated in n-hexane

Contrary to the PLLAs precipitated in alcohols, PLLA/Hex exhibits almost no signs of crystallization. The DSC heating trace for PLLA/Hex in Figure 1 shows a clear exothermic peak between the Tg and the melting range, suggesting a much lower crystallinity than those precipitated in alcohols. Figure 4 shows no apparent (or very weak) absorbance peak at 921 cm−1 for PLLA/Hex. However, a band at 918 cm−1 is observed, which has been assign to the mesophase of PLLA crystalline state (29), (30), (39). This peak was not observed for the amorphous PLLA, thus we conclude that for PLLA/Hex, only the mesophase formation occurs, and the crystallization is limited during the precipitation. The FTIR result for PLLA/Hex obtained here is consistent with the conventional DSC heating trace shown in Figure 1, which shows little crystallinity. The WAXD profile for the PLLA/Hex in Figure 2 reveals an almost amorphous state but a very small signal of diffraction is observed at 2θ=16.5°, as indicated by an arrow. This suggests that a small crystalline fraction exists in the PLLA/Hex sample. As for PLLA/nPrOH, a weaker absorbance band at 921 cm−1 than PLLA/MeOH is observed, and an absorbance at 918 cm−1 is discernible as well. This implies that PLLA/nPrOH contains both a crystalline phase and a mesophase, thus exhibits an intermediate crystallization behavior between PLLA/MeOH and PLLA/Hex.

We found that the mesophase formed in PLLA/Hex promotes succeeding crystallization during a heating scan of DSC. Figure 5 shows DSC traces obtained during a step-scan heating measurement for PLLA/Hex as well as for the reference amorphous PLLA. An exotherm appears in the non-reversing heat flow profile for the PLLA/Hex, which indicates that crystallization has not proceeded sufficiently during the precipitation. However, the exotherm is located at a lower temperature for the PLLA/Hex than for the amorphous PLLA. Such a low crystallization temperature on heating suggests that crystallization is promoted in the PLLA/Hex compared with the reference bulk PLLA, and this might be due to the preexisting mesophase.

Figure 5: Step-scan DSC heating traces for PLLA/Hex and amorphous PLLA.The solid curves indicate reversing heat capacity, and the dotted curves indicate non-reversing heat flow. The arrows indicate cold crystallization signal during the step-scan heating measurement.
Figure 5:

Step-scan DSC heating traces for PLLA/Hex and amorphous PLLA.

The solid curves indicate reversing heat capacity, and the dotted curves indicate non-reversing heat flow. The arrows indicate cold crystallization signal during the step-scan heating measurement.

3.3 Morphology

We checked the crystalline morphology of the precipitated PLLA samples via SEM, and found that no spherulites were formed. Figure 6 shows typical images observed for PLLA/MeOH. The rather random texture on a scale of several tens of μm shown in the left panel of Figure 6 was probably formed during the phase separation from the solution state, and the size of crystallites may be much smaller. It has been reported that crystallization of PLLA from solutions on cooling results in characteristic porous spherulites or crystallites, depending on the solvents (40). On the contrary, the morphology of the present precipitated PLLA is less porous. It is likely that the rapid crystallization of the present precipitation process induces a high rate of primary nucleation, leading to a large number of small crystallites. It has been reported that very small rod-like crystallites are formed via a CO2 induced mesophase, but no spherulites are formed (30). In this case, the crystallization of PLLA occurs very rapidly, and the crystallization behavior may be similar to that of the present precipitation-mediated system.

Figure 6: SEM images for the PLLA/MeOH sample precipitated at 30°C.
Figure 6:

SEM images for the PLLA/MeOH sample precipitated at 30°C.

3.4 Crystallization mechanism during precipitation

The above results show that the alcohols tend to promote the crystallization of PLLA during the precipitation from the chloroform solution. It may be reasonably understood that the rates of mesophase formation and crystallization from a mobile phase of the PLLA/chloroform solution are higher than that from the much less mobile phase of a bulk state, from which the crystallization process takes a much longer time. Also, the tendency of promoting crystallization is stronger for lower alcohols such as methanol. This suggests that the hydroxyl group of alcohols plays a role in accelerating the crystallization. Rapid crystallization has been reported to occur for blends of PLLA/polyurethane during the formation of porous materials under the presence of ethanol (41). Also, it has been found that liquid alcohols induce the crystallization of a bulk PLLA when it is immersed in methanol and ethanol (10). These results suggest specific molecular interactions between PLLA and the hydroxyl group of alcohols, which might raise the mobility of PLLA chain. Table 2 shows the results of the cloud point measurements at 25°C. The value R indicates the mass ratio of the non-solvent to the PLLA/chloroform solution. As expected, n-hexane exhibits a lower value of R than alcohols, indicating its lower compatibility with PLLA. However, the R value increases as the number of carbons in the alcohol becomes high. This may be inconsistent with the observed crystallization behaviors. The reason for this is unknown, but it is suggested that the interplay between the phase separation and crystallization kinetics is not determined only by the quench depth (and thus supercooling) into the bi-phase region.

Table 2:

Cloud point for PLLA/chloroform/non-solvent system at 25°C with 5.0 wt% PLLA/chloroform solution.

Non-solventRa
MeOH0.577
EtOH0.625
nPrOH0.803
2PrOH0.863
Hex0.471

  1. aR=(mass of non-solvent)/(mass of PLLA/chloroform solution).

We found that the crystallization behavior depends strongly on temperature of precipitation. Figure 7 shows the FTIR spectra for the samples precipitated in methanol at different temperatures. At 0°C and –41°C, no signs of crystallization were observed, even for PLLA/MeOH. Instead, a weak signal at 918 cm−1 is observed, which indicates formation of the mesophase. The temperature dependence observed here suggests that the crystallization rate is governed by the diffusion process in the present temperature range, i.e. the lower the mobility, the lower the crystallization rate. This is consistent with the idea that the higher crystallization rate during precipitation originates mainly from the high molecular mobility in the liquid phase.

Figure 7: FTIR spectra in the range of 840–970 cm−1 for precipitated PLLAs in methanol at different temperatures.
Figure 7:

FTIR spectra in the range of 840–970 cm−1 for precipitated PLLAs in methanol at different temperatures.

In general, the precipitation phenomenon from a solution in a non-solvent is considered to include both phase separation and crystallization process, and the resulting structure depends on the interplay between these two processes. The former process is essentially a liquid-liquid phase separation in a tri-component system, but actually it may be regarded as a nearly solid-liquid separation (PLLA-chloroform/methanol), because both PLLA/chloroform and chloroform/methanol are completely miscible. If the phase separation completed without crystallization, PLLA would not crystallize, because crystallization does not occur in a nearly bulk state at the precipitation temperature that is below Tg. Thus it is likely that the crystallization occurs in the liquid state before or during the phase separation process. Such a process may be regarded as crystallization at high supercoolings from a metastable solution prior to or on the pathway to the phase separation. Considering the value of 5.0 wt% for the PLLA content of the initial solution, we assume that the phase separation would occur from a metastable solution via nucleation and growth mechanism.

As it has been reported that the primary nucleation of PLLA crystallization occurs even at temperatures lower than Tg in a bulk state (42), (43), (44), (45), the present crystallization of PLLA occurring below Tg may be conceivable. We should further note that no crystallization was observed at temperatures below 0°C when precipitated even in methanol. This implies that the diffusion of polymer chain plays a role in the crystallization process, and thus the present crystallization occurs at rather high supercoolings. From the above discussion, we speculate that the main origin of the present rapid crystallization during precipitation is the higher mobility of the PLLA molecules in the liquid state.

Alcohols such as methanol have compatibility with PLLA, and this leads to a stabilization of the metastable solution state and a delay of phase separation, giving a sufficient time for the crystallization. As the present results show that lower alcohols tend to promote the crystallization of PLLA, we therefore infer that the hydroxyl group is responsible for the compatibility with PLLA. As for the PLLA/Hex system, on the other hand, the compatibility between PLLA and n-hexane is lower, leading to faster phase separation. In this case, the crystallization process is interrupted and only a mesophase prior to crystalline state is formed.

4 Conclusions

In this study, we found that rapid crystallization of PLLA occurs when it is precipitated from a chloroform solution in alcohols. The rapid crystallization during the precipitation in alcohols occurs from a metastable solution state, where the mobility of PLLA molecules is expected to be high enough for the crystallization. As for the PLLA/Hex, crystallization does not occur, and just a mesophase is formed. In this case, repulsive interaction between PLLA and n-hexane is rather strong, inducing a faster phase separation prior to crystallization. It is likely that the hydroxyl group of alcohols raises the compatibility with PLLA. Lower alcohols that have relatively high content of hydroxyl group tend to suppress the phase separation rate, and as a result, promote crystallization. The low crystallinity observed at lower temperatures suggests that the present crystallization process is governed by the mobility of PLLA molecules. The rapid crystallization of PLLA during precipitation revealed in this study is a remarkable finding, which can contribute to the development of process technology of bio-based polymer materials like PLLA.

Acknowledgments

This work was partially supported by JSPS KAKENHI Grant Number JP16K05907 from the Ministry of Education, Culture, Sports, Science and Technology of Japan. The authors thank Sora Demura for help with the cloud point measurements.

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Received: 2017-11-24
Accepted: 2018-01-13
Published Online: 2018-02-14
Published in Print: 2018-07-26

©2018 Walter de Gruyter GmbH, Berlin/Boston

This article is distributed under the terms of the Creative Commons Attribution Non-Commercial License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

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https://doi.org/10.1515/epoly-2017-0247
Keywords for this article
crystallization; FTIR; mesophase; PLLA; precipitation
Creative Commons
BY-NC-ND 3.0

Articles in the same Issue

  1. Frontmatter
  2. In this Issue
  3. Full length articles
  4. Synthesis, characterization and application of polypyrrole-cellulose nanocomposite for efficient Ni(II) removal from aqueous solution: Box-Behnken design optimization
  5. Biodegradable glucose and glucosamine grafted polyacrylamide/graphite composites for the removal of acid violet 17 from an aqueous solution
  6. Evaluation of activated composite membranes for the facilitated transport of phenol
  7. The effects of nanoparticles on morphology and thermal properties of erythritol/polyvinyl alcohol phase change composite fibers
  8. Rapid crystallization and mesophase formation of poly(L-lactic acid) during precipitation from a solution
  9. Structural deformation of PVDF nanoweb due to electrospinning behavior affected by solvent ratio
  10. Poly(vinyl amine) as a matrix for a new class of polymers
  11. Review
  12. Polymeric advanced delivery systems for antineoplasic drugs: doxorubicin and 5-fluorouracil
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