rus
Issue #1/2025
S. O. Liubimovskii, V. S. Novikov, D. D. Vasimov, S. M. Kuznetsov, E. V. Anokhin, A. V. Bakirov, K. T. Kalinin, V. A. Demina, N. G. Sedush, S. N. Chvalun, M. N. Moskovskiy, G. Yu. Nikolaeva
Analysis of Polylactide-Based Materials by Raman Spectroscopy
Analysis of Polylactide-Based Materials by Raman Spectroscopy
DOI: 10.22184/1993-7296.FRos.2025.19.1.50.60
We present Raman study of a number of polylactide-based materials: polylactide stereoisomers, L-lactide oligomers, L-lactide/ε-caprolactone copolymers, and poly(L-lactide)/hydroxyapatite composites. It is established that the composition and crystallinity degree of a wide range of polylactide-based materials can be determined by Raman spectra. The advancement of this technique is crucial for the development of innovative polylactide-based materials used both for diverse medical applications and, for example, in the creation of biodegradable disposable packaging to address environmental pollution problems.
We present Raman study of a number of polylactide-based materials: polylactide stereoisomers, L-lactide oligomers, L-lactide/ε-caprolactone copolymers, and poly(L-lactide)/hydroxyapatite composites. It is established that the composition and crystallinity degree of a wide range of polylactide-based materials can be determined by Raman spectra. The advancement of this technique is crucial for the development of innovative polylactide-based materials used both for diverse medical applications and, for example, in the creation of biodegradable disposable packaging to address environmental pollution problems.
Теги: composites copolymers crystallinity degree laser spectroscopy oligomers polylactide raman scattering raman spectra stereoisomers комбинационное рассеяние света композиты лазерная спектроскопия олигомеры полилактид сополимеры спектры кр степень кристалличности стереоизомеры
Analysis of Polylactide-Based Materials by Raman Spectroscopy
S. O. Liubimovskii1, V. S. Novikov1, D. D. Vasimov1,2, S. M. Kuznetsov1, E. V. Anokhin3, A. V. Bakirov3, K. T. Kalinin3, V. A. Demina3, N. G. Sedush3, S. N. Chvalun3, M. N. Moskovskiy4, G. Yu. Nikolaeva1
Prokhorov General Physics Institute of the RAS, Moscow, Russia
Moscow Institute of Physics and Technology, Moscow Region, Dolgoprudny, Russia
Enikolopov Institute of Synthetic Polymeric Materials of the RAS, Moscow, Russia
Federal Scientific Agronomic and Engineering Center VIM, Moscow, Russia
We present Raman study of a number of polylactide-based materials: polylactide stereoisomers, L-lactide oligomers, L-lactide/ε-caprolactone copolymers, and poly(L-lactide)/hydroxyapatite composites. It is established that the composition and crystallinity degree of a wide range of polylactide-based materials can be determined by Raman spectra. The advancement of this technique is crucial for the development of innovative polylactide-based materials used both for diverse medical applications and, for example, in the creation of biodegradable disposable packaging to address environmental pollution problems.
Keywords: Raman scattering, Raman spectra, laser spectroscopy, polylactide, oligomers, copolymers, composites, stereoisomers, crystallinity degree
Article received: 03.12.2024
Article accepted: 18.01.2025
Introduction
Polylactide (PLA) is a biocompatible, biodegradable and compostable thermoplastic derived from relatively cheap and annually renewable plant materials. Currently, PLA is one of the most promising environmentally friendly polymers; the performance characteristics of PLA-based materials are increasingly approaching those of some bioinert polymers [1]. The world production and consumption of PLA are continuously growing, and its range of applications is constantly expanding. PLA-based materials are already widely used for 3D printing, for the manufacture of various types of food packaging, disposable goods, and for agriculture [2–4]. The most significant area of use of PLA is the manufacturing of medical devices: bioresorbable implants, scaffolds, surgical sutures, fixation devices for surgery and orthopedics [5]. One of the important PLA applications is the development of nanocarriers for targeted drug delivery with prolonged release of the active substances [6, 7].
Both lactic acid and lactide exhibit optical activity, existing as two optically active stereoisomers (D- and L-stereoisomers) that are mirror images of each other. In addition, there are two optically inactive isomers of lactide: the D,L-form (a racemic mixture) and the mesoform. This provides an opportunity to regulate the polymer’s structure and properties by varying the contents of L- and D-units. Furthermore, the set of physical and chemical properties of PLA can be adjusted in a task-oriented way and over a wide range, both by altering molecular characteristics and composition (using the variation of molecular weight, modification of end groups, creation of composites, copolymers and blends) and through different types of post-processing (for example, by changing annealing conditions, it is possible to regulate the crystallinity degree of PLA in the range from 0 to 85%).
Currently, the most commonly used methods for analyzing the structure of PLA-based materials include nuclear magnetic resonance (NMR) spectroscopy, X-ray diffraction analysis, and differential scanning calorimetry. Raman spectroscopy is a highly informative and convenient instrument for non-destructive analysis of the structure of molecular substances, including polymeric materials. However, this technique was rarely used in routine studies of PLA-based materials until recently, which was apparently due to the lack of methods for quantitative analysis of the structure of such materials using Raman spectra.
At the same time, the Raman spectroscopy method has a number of significant advantages, above all, its high informativity in the analysis of all levels of the molecular and supramolecular structure of polymers. The chemical structure of the material, the configurational and conformational compositions of the molecules, the presence of different chemical groups, the content and length distribution of stereoregular molecular segments, the phase composition, including the crystallinity degree (CD), can be determined by Raman spectra. The Raman spectroscopy method does not require any sample preparation, it can be used to quickly map the surface of a sample with micron-level spatial resolution and perform diagnostics in real time (in most cases, the Raman spectrum acquisition time of no more than a few seconds is sufficient to obtain a spectrum with a good signal-to-noise ratio). Currently, both Russian and international manufacturers offer a wide selection of Raman spectrometers, including portable models.
Thus, the high informativity, ease of use and simplicity of the Raman spectroscopy method allow for structural diagnostics of materials during development and manufacture, as well as during control of the quality and degree of degradation of the product.
Most of the published works on the study of PLA by the Raman spectroscopy method were carried out over 20 years ago (see the literature review in [8]). Meanwhile, very few publications on the research of oligomers and various stereoisomers of PLA, as well as PLA-based materials such as copolymers, composites, and blends, have been published. Nevertheless, an analysis and compilation of the published Raman spectroscopy data on the study of PLA and PLA-based materials allow us to conclude that the Raman spectra of these materials depend on the crystallinity degree, stereoregularity and type of packing of the chains, the length of sequences of monomeric units, molecular orientation, and contents of comonomers in the case of copolymers [8, 9].
In our recent works [8–10], two Raman methods of the quantitative structural analysis of materials based on poly(L-lactide) (PLLA), the most widely used stereoisomer of PLA, were proposed: a method for determining the crystallinity degree of PLLA (PLLA blocks in copolymers of L-lactide (LLA) and ε-caprolactone (CL)) [8, 9] and a method for identifying the composition of LLA/CL copolymers, as well as blends of PLLA and poly(ε-caprolactone) (PCL) [8, 10].
In this work, we demonstrate the capabilities of Raman spectroscopy in analyzing the structure of a number of PLA-based materials: LLA/CL copolymers, LLA oligomers, PLA stereoisomers, and PLLA and hydroxyapatite (HA) composites.
Materials and Methods
The following samples were studied in the work:
A series of PLLA films with a crystallinity degree (CD) ranging from 0 to 86%, prepared according to the method described in detail in [9].
A series of LLA/CL copolymers with a CL molar content ranging from 10 to 90%, prepared according to the method described in detail in [10].
PLA stereoisomers: PLLA, poly(D-lactide) (PDLA), and poly(D,L-lactide) (PDLLA) with a 50 : 50 ratio of D- and L-lactide units. All the samples were commercial (Corbion, Netherlands). The CD of the samples was determined using X-ray diffraction analysis, following the technique described in [8]. The values of CD were 86% for PLLA, 80% for PDLA, and 0% for PDLLA. In addition, amorphous PLLA and PDLA films were prepared using the method described in [8].
LLA oligomers, synthesized by the method described in detail in [11]. The average polymerization degree of the oligomers, determined using the NMR spectroscopy method according to the technique described in [12], was 10.7, 40.4, and 104.5 L-lactic acid units respectively (hereinafter referred to as oligomers with 10, 40, and 100 lactic acid units). The oligomer with 10 lactic acid units was in the liquid state. Oligomers with a length of 40 and 100 units were analyzed as amorphized plates.
PLLA/HA composites obtained by melt mixing at a temperature of 190 °C. The HA content was selected as 0, 1, 5, 20 and 30 wt.%. The CD of all these composites was 0%.
The Raman spectra of all the samples were recorded at room temperature using a Senterra II confocal Raman microscope (Bruker Optics, USA). The spectra were recorded at 180° scattering with a spectral resolution of 1.5 cm−1, at an excitation wavelength of 785 nm, and a laser power of 100 mW. The exciting and scattered radiations were focused by a 20x objective (numerical aperture 0.40).
Results and Discussion
The dependence of the Raman spectrum of neat PLLA on the CD is presented in Fig. 1a, in the spectral range that is the most informative for determining this characteristic from the Raman spectra. Hereinafter, the CD indicated in the figures was measured by X-ray diffraction analysis. Several Raman bands of PLLA in the region up to 400 cm−1 strongly depend on the CD, but the analysis of these bands is complicated due to their low intensity, as well as due to the proximity of the wavelengths of the scattered and exciting radiations. The most noticeable differences associated with changes in CD are observed for the doublet of the bands around 400 cm−1. The ratio of the peak intensities of the PLLA Raman bands at 411 and 874 cm−1 linearly depends on the CD (Fig. 1b). This dependence is also linear for LLA/CL copolymers [10].
The range of 2500–3300 cm−1 of the PLLA Raman spectrum depends very weakly on the CD, which provides a unique opportunity to use this region to determine the composition of PLLA-based materials with various CD. As an example, Fig. 2a shows the Raman spectra of the LLA/CL copolymers with various compositions, which were determined using NMR spectroscopy. The ratio of the peak intensity of the PLLA Raman band at 2947 cm−1 to the sum of the peak intensities of this band and the PCL band at 2914 cm−1 is a linear function of the molar content of LLA (Fig.2b). This intensity ratio can also be used to determine the composition of PLLA and PCL blends prepared by melt mixing [10].
One of the ways to change the CD of PLA is to regulate the contents of D- and L-units. Figure 3 shows the Raman spectra of PLLA, PDLA, and PDLLA with a 50:50 ratio of the contents of D- and L-lactide units and a statistical distribution of the units along the chain. As in the case of PLLA, in the spectrum of PDLA with a CD of 80%, there is a doublet of the bands around 400 cm−1 (Fig. 3a) and a triplet of the bands around 1750 cm−1 (Fig. 3b), while for amorphous PDLA and PDLLA such splitting is absent. The spectral range of 2575–3200 cm−1 weakly depends on the CD and the enantiomeric composition of the PLA chains (Fig. 3c). It was found that in the spectrum of amorphous PDLLA additional weak bands are observed at around 475 and 660 cm−1 (marked by arrows in Fig. 3a).
It is important to note that exactly at the same wavenumbers additional (in comparison with the PLLA spectrum) weak bands are observed in the Raman spectra of the LLA oligomers shown in Fig. 4 (the additional bands are marked with arrows in Fig. 4a). The intensity of these additional bands decreases as the length of the oligomer molecule increases. Thus, these additional bands are characteristic only of disordered conformations of the molecules.
In the Raman spectra of the oligomers, in addition to the already mentioned bands around 475 and 660 cm−1, Raman bands of toluene (Fig. 4a and 4b) as well as the bands of 1,12‑dodecanediol (Fig. 4c) are observed. Toluene was used as a solvent for introducing the catalyst and is partially remained in the sample. 1,12-Dodecanediol was used as an activator in polymerization [11] and remains as the central part of the oligomer molecules. For comparison, Fig. 4c shows the spectrum of normal alkane C12H26 (n-dodecane).
The intensity of the Raman bands of toluene increases with increasing oligomer length. This indicates that the content of toluene residues is higher in oligomer samples with a higher molecular weight. The relative intensity of the bands of the parts of 1,12‑dodecanediol molecules incorporated into the oligomer molecules decreases as the oligomer length increases. Thus, a sufficiently intense Raman spectrum of 1,12‑dodecanediol in the spectral range of 2 575–3 200 cm−1 (Fig. 4c) potentially allows one to estimate the length of the LLA segments for short oligomers.
Another effective way to regulate the structure and properties of PLLA-based materials is to introduce a filler. For such composites, the Raman spectroscopy method allows one to evaluate not only the CD of PLA but also the content of the filler. As an example, Fig. 5a presents the Raman spectra of the PLLA/HA composites with various compositions. Since the HA Raman band at 962 cm−1 is sharp and intense, the dependence of the ratio of the peak intensities of this band and the PLLA Raman band at 874 cm−1 allows one to evaluate the HA content up to 1 wt.% (Fig. 5b).
Conclusions
The work identifies the Raman bands, the ratio of peak intensities of which linearly depends on the crystallinity degree of PLLA, on the content of monomeric units of PLLA in LLA/CL copolymers, and on the filler content in PLLA/HA composites. It has been demonstrated that in the Raman spectra of PDLLA and LLA oligomers, additional bands are observed at around 475 and 660 cm−1, which are characteristic only of disordered conformations of the molecules.
Thus, the work shows that Raman spectroscopy is a powerful method for studying PLA-based materials and allows determining the crystallinity degree and composition of a wide range of PLA-based materials.
Funding
This study was supported by the Russian Science Foundation under the grant № 23-22-00347, https://rscf.ru/en/project/23-22-00347/.
ABOUT AUTHORS
S. O. Liubimovskii, e-mail: liubimovskii@kapella.gpi.ru, ORCID 0000-0002-9332-4359
V. S. Novikov, Cand.of Sciences (Phys.-Math.), ORCID 0000-0002-3304-1568
D. D. Vasimov, ORCID 0009-0002-8105-0124
S. M. Kuznetsov, Cand.of Sciences (Phys.-Math.), ORCID 0000-0002-7669-1106
E. V. Anokhin, ORCID 0009-0005-2392-6994
A. V. Bakirov, Cand. of Sciences (Phys.-Math.), ORCID 0000-0003-0798-2791
K. T. Kalinin, ORCID 0000-0001-8838-5520
V. A. Demina, Cand.of Sciences (Phys.-Math.), ORCID 0009-0003-7302-1048
N. G. Sedush, Cand.of Sciences (Phys.-Math.), ORCID 0000-0002-6744-7662
S. N. Chvalun, Dr. of Sciences (Chem.), Corresponding Member of the RAS,
ORCID 0000-0001-9405-4509
M. N. Moskovskii, Dr. of Sciences (Tech.), prof. of the RAS,
ORCID 0000-0001-5727-8706
G. Yu. Nikolaeva, Cand.of Sciences (Phys.-Math.), e-mail: nikolaeva@kapella.gpi.ru,
ORCID 0000-0001-5979-9126
Author contributions
S. O. Liubimovskii, registration and analysis of Raman spectra;
V. S. Novikov, analysis of Raman spectra, preparation of the text of the article;
D. D. Vasimov, analysis of Raman spectra, comparison with data from quantum chemical calculations;
S. M. Kuznetsov, registration and analysis of Raman spectra;
E. V. Anokhin, synthesis of L-lactide and ε-caprolactone copolymers, NMR spectroscopy study;
A. V. Bakirov, X-ray diffraction analysis of samples, preparation of poly(L-lactide) samples with varying crystallinity degree;
K. T. Kalinin, synthesis of L-lactide oligomers, study by NMR spectroscopy;
V. A. Demina, preparation of poly(L-lactide) and hydroxyapatite composites;
N. G. Sedush, synthesis and preparation of all samples, preparation of the text of the article;
S. N. Chvalun, managing the synthesis and preparation of samples, X-ray diffraction analysis and NMR spectroscopy studies;
M. N. Moskovskii, registration and analysis of Raman spectra, preparation of the text of the article;
G. Y. Nikolaeva, analysis of literary data, preparation of the text of the article
Competing interests
The authors contributed equally to this article. The authors declare that they have no conflicts of interest.
S. O. Liubimovskii1, V. S. Novikov1, D. D. Vasimov1,2, S. M. Kuznetsov1, E. V. Anokhin3, A. V. Bakirov3, K. T. Kalinin3, V. A. Demina3, N. G. Sedush3, S. N. Chvalun3, M. N. Moskovskiy4, G. Yu. Nikolaeva1
Prokhorov General Physics Institute of the RAS, Moscow, Russia
Moscow Institute of Physics and Technology, Moscow Region, Dolgoprudny, Russia
Enikolopov Institute of Synthetic Polymeric Materials of the RAS, Moscow, Russia
Federal Scientific Agronomic and Engineering Center VIM, Moscow, Russia
We present Raman study of a number of polylactide-based materials: polylactide stereoisomers, L-lactide oligomers, L-lactide/ε-caprolactone copolymers, and poly(L-lactide)/hydroxyapatite composites. It is established that the composition and crystallinity degree of a wide range of polylactide-based materials can be determined by Raman spectra. The advancement of this technique is crucial for the development of innovative polylactide-based materials used both for diverse medical applications and, for example, in the creation of biodegradable disposable packaging to address environmental pollution problems.
Keywords: Raman scattering, Raman spectra, laser spectroscopy, polylactide, oligomers, copolymers, composites, stereoisomers, crystallinity degree
Article received: 03.12.2024
Article accepted: 18.01.2025
Introduction
Polylactide (PLA) is a biocompatible, biodegradable and compostable thermoplastic derived from relatively cheap and annually renewable plant materials. Currently, PLA is one of the most promising environmentally friendly polymers; the performance characteristics of PLA-based materials are increasingly approaching those of some bioinert polymers [1]. The world production and consumption of PLA are continuously growing, and its range of applications is constantly expanding. PLA-based materials are already widely used for 3D printing, for the manufacture of various types of food packaging, disposable goods, and for agriculture [2–4]. The most significant area of use of PLA is the manufacturing of medical devices: bioresorbable implants, scaffolds, surgical sutures, fixation devices for surgery and orthopedics [5]. One of the important PLA applications is the development of nanocarriers for targeted drug delivery with prolonged release of the active substances [6, 7].
Both lactic acid and lactide exhibit optical activity, existing as two optically active stereoisomers (D- and L-stereoisomers) that are mirror images of each other. In addition, there are two optically inactive isomers of lactide: the D,L-form (a racemic mixture) and the mesoform. This provides an opportunity to regulate the polymer’s structure and properties by varying the contents of L- and D-units. Furthermore, the set of physical and chemical properties of PLA can be adjusted in a task-oriented way and over a wide range, both by altering molecular characteristics and composition (using the variation of molecular weight, modification of end groups, creation of composites, copolymers and blends) and through different types of post-processing (for example, by changing annealing conditions, it is possible to regulate the crystallinity degree of PLA in the range from 0 to 85%).
Currently, the most commonly used methods for analyzing the structure of PLA-based materials include nuclear magnetic resonance (NMR) spectroscopy, X-ray diffraction analysis, and differential scanning calorimetry. Raman spectroscopy is a highly informative and convenient instrument for non-destructive analysis of the structure of molecular substances, including polymeric materials. However, this technique was rarely used in routine studies of PLA-based materials until recently, which was apparently due to the lack of methods for quantitative analysis of the structure of such materials using Raman spectra.
At the same time, the Raman spectroscopy method has a number of significant advantages, above all, its high informativity in the analysis of all levels of the molecular and supramolecular structure of polymers. The chemical structure of the material, the configurational and conformational compositions of the molecules, the presence of different chemical groups, the content and length distribution of stereoregular molecular segments, the phase composition, including the crystallinity degree (CD), can be determined by Raman spectra. The Raman spectroscopy method does not require any sample preparation, it can be used to quickly map the surface of a sample with micron-level spatial resolution and perform diagnostics in real time (in most cases, the Raman spectrum acquisition time of no more than a few seconds is sufficient to obtain a spectrum with a good signal-to-noise ratio). Currently, both Russian and international manufacturers offer a wide selection of Raman spectrometers, including portable models.
Thus, the high informativity, ease of use and simplicity of the Raman spectroscopy method allow for structural diagnostics of materials during development and manufacture, as well as during control of the quality and degree of degradation of the product.
Most of the published works on the study of PLA by the Raman spectroscopy method were carried out over 20 years ago (see the literature review in [8]). Meanwhile, very few publications on the research of oligomers and various stereoisomers of PLA, as well as PLA-based materials such as copolymers, composites, and blends, have been published. Nevertheless, an analysis and compilation of the published Raman spectroscopy data on the study of PLA and PLA-based materials allow us to conclude that the Raman spectra of these materials depend on the crystallinity degree, stereoregularity and type of packing of the chains, the length of sequences of monomeric units, molecular orientation, and contents of comonomers in the case of copolymers [8, 9].
In our recent works [8–10], two Raman methods of the quantitative structural analysis of materials based on poly(L-lactide) (PLLA), the most widely used stereoisomer of PLA, were proposed: a method for determining the crystallinity degree of PLLA (PLLA blocks in copolymers of L-lactide (LLA) and ε-caprolactone (CL)) [8, 9] and a method for identifying the composition of LLA/CL copolymers, as well as blends of PLLA and poly(ε-caprolactone) (PCL) [8, 10].
In this work, we demonstrate the capabilities of Raman spectroscopy in analyzing the structure of a number of PLA-based materials: LLA/CL copolymers, LLA oligomers, PLA stereoisomers, and PLLA and hydroxyapatite (HA) composites.
Materials and Methods
The following samples were studied in the work:
A series of PLLA films with a crystallinity degree (CD) ranging from 0 to 86%, prepared according to the method described in detail in [9].
A series of LLA/CL copolymers with a CL molar content ranging from 10 to 90%, prepared according to the method described in detail in [10].
PLA stereoisomers: PLLA, poly(D-lactide) (PDLA), and poly(D,L-lactide) (PDLLA) with a 50 : 50 ratio of D- and L-lactide units. All the samples were commercial (Corbion, Netherlands). The CD of the samples was determined using X-ray diffraction analysis, following the technique described in [8]. The values of CD were 86% for PLLA, 80% for PDLA, and 0% for PDLLA. In addition, amorphous PLLA and PDLA films were prepared using the method described in [8].
LLA oligomers, synthesized by the method described in detail in [11]. The average polymerization degree of the oligomers, determined using the NMR spectroscopy method according to the technique described in [12], was 10.7, 40.4, and 104.5 L-lactic acid units respectively (hereinafter referred to as oligomers with 10, 40, and 100 lactic acid units). The oligomer with 10 lactic acid units was in the liquid state. Oligomers with a length of 40 and 100 units were analyzed as amorphized plates.
PLLA/HA composites obtained by melt mixing at a temperature of 190 °C. The HA content was selected as 0, 1, 5, 20 and 30 wt.%. The CD of all these composites was 0%.
The Raman spectra of all the samples were recorded at room temperature using a Senterra II confocal Raman microscope (Bruker Optics, USA). The spectra were recorded at 180° scattering with a spectral resolution of 1.5 cm−1, at an excitation wavelength of 785 nm, and a laser power of 100 mW. The exciting and scattered radiations were focused by a 20x objective (numerical aperture 0.40).
Results and Discussion
The dependence of the Raman spectrum of neat PLLA on the CD is presented in Fig. 1a, in the spectral range that is the most informative for determining this characteristic from the Raman spectra. Hereinafter, the CD indicated in the figures was measured by X-ray diffraction analysis. Several Raman bands of PLLA in the region up to 400 cm−1 strongly depend on the CD, but the analysis of these bands is complicated due to their low intensity, as well as due to the proximity of the wavelengths of the scattered and exciting radiations. The most noticeable differences associated with changes in CD are observed for the doublet of the bands around 400 cm−1. The ratio of the peak intensities of the PLLA Raman bands at 411 and 874 cm−1 linearly depends on the CD (Fig. 1b). This dependence is also linear for LLA/CL copolymers [10].
The range of 2500–3300 cm−1 of the PLLA Raman spectrum depends very weakly on the CD, which provides a unique opportunity to use this region to determine the composition of PLLA-based materials with various CD. As an example, Fig. 2a shows the Raman spectra of the LLA/CL copolymers with various compositions, which were determined using NMR spectroscopy. The ratio of the peak intensity of the PLLA Raman band at 2947 cm−1 to the sum of the peak intensities of this band and the PCL band at 2914 cm−1 is a linear function of the molar content of LLA (Fig.2b). This intensity ratio can also be used to determine the composition of PLLA and PCL blends prepared by melt mixing [10].
One of the ways to change the CD of PLA is to regulate the contents of D- and L-units. Figure 3 shows the Raman spectra of PLLA, PDLA, and PDLLA with a 50:50 ratio of the contents of D- and L-lactide units and a statistical distribution of the units along the chain. As in the case of PLLA, in the spectrum of PDLA with a CD of 80%, there is a doublet of the bands around 400 cm−1 (Fig. 3a) and a triplet of the bands around 1750 cm−1 (Fig. 3b), while for amorphous PDLA and PDLLA such splitting is absent. The spectral range of 2575–3200 cm−1 weakly depends on the CD and the enantiomeric composition of the PLA chains (Fig. 3c). It was found that in the spectrum of amorphous PDLLA additional weak bands are observed at around 475 and 660 cm−1 (marked by arrows in Fig. 3a).
It is important to note that exactly at the same wavenumbers additional (in comparison with the PLLA spectrum) weak bands are observed in the Raman spectra of the LLA oligomers shown in Fig. 4 (the additional bands are marked with arrows in Fig. 4a). The intensity of these additional bands decreases as the length of the oligomer molecule increases. Thus, these additional bands are characteristic only of disordered conformations of the molecules.
In the Raman spectra of the oligomers, in addition to the already mentioned bands around 475 and 660 cm−1, Raman bands of toluene (Fig. 4a and 4b) as well as the bands of 1,12‑dodecanediol (Fig. 4c) are observed. Toluene was used as a solvent for introducing the catalyst and is partially remained in the sample. 1,12-Dodecanediol was used as an activator in polymerization [11] and remains as the central part of the oligomer molecules. For comparison, Fig. 4c shows the spectrum of normal alkane C12H26 (n-dodecane).
The intensity of the Raman bands of toluene increases with increasing oligomer length. This indicates that the content of toluene residues is higher in oligomer samples with a higher molecular weight. The relative intensity of the bands of the parts of 1,12‑dodecanediol molecules incorporated into the oligomer molecules decreases as the oligomer length increases. Thus, a sufficiently intense Raman spectrum of 1,12‑dodecanediol in the spectral range of 2 575–3 200 cm−1 (Fig. 4c) potentially allows one to estimate the length of the LLA segments for short oligomers.
Another effective way to regulate the structure and properties of PLLA-based materials is to introduce a filler. For such composites, the Raman spectroscopy method allows one to evaluate not only the CD of PLA but also the content of the filler. As an example, Fig. 5a presents the Raman spectra of the PLLA/HA composites with various compositions. Since the HA Raman band at 962 cm−1 is sharp and intense, the dependence of the ratio of the peak intensities of this band and the PLLA Raman band at 874 cm−1 allows one to evaluate the HA content up to 1 wt.% (Fig. 5b).
Conclusions
The work identifies the Raman bands, the ratio of peak intensities of which linearly depends on the crystallinity degree of PLLA, on the content of monomeric units of PLLA in LLA/CL copolymers, and on the filler content in PLLA/HA composites. It has been demonstrated that in the Raman spectra of PDLLA and LLA oligomers, additional bands are observed at around 475 and 660 cm−1, which are characteristic only of disordered conformations of the molecules.
Thus, the work shows that Raman spectroscopy is a powerful method for studying PLA-based materials and allows determining the crystallinity degree and composition of a wide range of PLA-based materials.
Funding
This study was supported by the Russian Science Foundation under the grant № 23-22-00347, https://rscf.ru/en/project/23-22-00347/.
ABOUT AUTHORS
S. O. Liubimovskii, e-mail: liubimovskii@kapella.gpi.ru, ORCID 0000-0002-9332-4359
V. S. Novikov, Cand.of Sciences (Phys.-Math.), ORCID 0000-0002-3304-1568
D. D. Vasimov, ORCID 0009-0002-8105-0124
S. M. Kuznetsov, Cand.of Sciences (Phys.-Math.), ORCID 0000-0002-7669-1106
E. V. Anokhin, ORCID 0009-0005-2392-6994
A. V. Bakirov, Cand. of Sciences (Phys.-Math.), ORCID 0000-0003-0798-2791
K. T. Kalinin, ORCID 0000-0001-8838-5520
V. A. Demina, Cand.of Sciences (Phys.-Math.), ORCID 0009-0003-7302-1048
N. G. Sedush, Cand.of Sciences (Phys.-Math.), ORCID 0000-0002-6744-7662
S. N. Chvalun, Dr. of Sciences (Chem.), Corresponding Member of the RAS,
ORCID 0000-0001-9405-4509
M. N. Moskovskii, Dr. of Sciences (Tech.), prof. of the RAS,
ORCID 0000-0001-5727-8706
G. Yu. Nikolaeva, Cand.of Sciences (Phys.-Math.), e-mail: nikolaeva@kapella.gpi.ru,
ORCID 0000-0001-5979-9126
Author contributions
S. O. Liubimovskii, registration and analysis of Raman spectra;
V. S. Novikov, analysis of Raman spectra, preparation of the text of the article;
D. D. Vasimov, analysis of Raman spectra, comparison with data from quantum chemical calculations;
S. M. Kuznetsov, registration and analysis of Raman spectra;
E. V. Anokhin, synthesis of L-lactide and ε-caprolactone copolymers, NMR spectroscopy study;
A. V. Bakirov, X-ray diffraction analysis of samples, preparation of poly(L-lactide) samples with varying crystallinity degree;
K. T. Kalinin, synthesis of L-lactide oligomers, study by NMR spectroscopy;
V. A. Demina, preparation of poly(L-lactide) and hydroxyapatite composites;
N. G. Sedush, synthesis and preparation of all samples, preparation of the text of the article;
S. N. Chvalun, managing the synthesis and preparation of samples, X-ray diffraction analysis and NMR spectroscopy studies;
M. N. Moskovskii, registration and analysis of Raman spectra, preparation of the text of the article;
G. Y. Nikolaeva, analysis of literary data, preparation of the text of the article
Competing interests
The authors contributed equally to this article. The authors declare that they have no conflicts of interest.
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