Stereocomplexed Polylactides

Synonyms

PLA stereocomplex; Stereocomplex formation; Stereocomplexation

Definition

Macromolecules of identical chemical composition but different configuration of repeating units are able to form an intermolecular complex called stereocomplex. Stereocomplex forms in the solid-state crystalline structure melting at higher temperature (T _m) than that of the homochiral components alone. Stereocomplexation or stereocomplex formation of polylactides (PLA) occurs due to non-covalent interactions of enantiomeric chains of poly(l-lactide) (PLLA) and poly(d-lactide) (PDLA).

Introduction

Macromolecules composed of repeating units with complementary attracting sites can form intermolecular complexes. Typical examples are provided by natural macromolecules such as polysaccharides, polypeptides, or nucleic acids. Taking a lesson from biology, synthetic polymer chemistry developed a variety of supramolecular systems in which complementary interactions lead to materials with enhanced physicochemical properties [1–3]. Macromolecules of identical chemical composition but different configuration of repeating units are also able to form intermolecular complexes called stereocomplexes [4]. Macromolecular stereocomplexes usually form crystalline structures melting at higher temperature (T _m) than their homochiral components. Moreover, the PLLA/PDLA equimolar mixture is able to form stereocomplex, both from the melt and from the solution [5].

Microstructure of PLA Stereocomplex

PLAs have an asymmetric carbon atom in each lactate repeating unit, and depending on the monomer used in polymerization (l,l-lactide (l,l-LA), d,d-lactide (d,d-LA), racemic [1:1 (l,l)/(d,d)-lactide], or meso (l,d)-lactide (l,d-LA)), semicrystalline PLLA and PDLA or amorphous PDLLA polymers are obtained as it was shown in Fig. 1.

Stereocomplexed Polylactides, Fig. 1

Structures of stereoisomers of lactide and PLAs

As it was discovered by Ikada and Tsuji in 1987, the intermolecular interactions of enantiomeric PLA chains of the opposite configuration PLLA and PDLA lead to the corresponding stereocomplex (sc-PLA) formation [5]. The minimum chain length for stereocomplex formation is seven lactyl units, whereas individual enantiomeric lactide oligomers crystallize at a degree of polymerization (DP) equal to 11. This difference is due to the different crystal structures of homopolymers and stereocomplexes. It has been reported that the crystals in the homopolymers of PLLA (or PDLA) have a 10₃ helix structure (called also α helix), whereas the stereocomplex forms a more compact 3₁ helix (called also β helix). Obviously, one of the lactyl end groups which is directly attached to initiator is not able to participate in the crystallite formation. As a result, 11 units (not 10) are required to form a 10₃ helix, and consequently, to form the 3₁ helix, seven lactyl residues are needed.

Initially, van der Waals interactions were suspected to be responsible for stereocomplex formation, but finally a weak hydrogen bonding: −CH₃ · · · · · O = C < and ≡ CH · · · · · O = C < between the PLLA/PDLA chains has been found to keep two PLA helical chains with the opposite configuration together (cf. Fig. 2). Assuming multicentric interactions (each C = O is bonding to one −CH₃ and one ≡ CH group), a tentative calculation results in a hydrogen-bonding enthalpy ΔH _f = 5.2 kJ/mol (1.2 kcal/mol). This value is also close to that obtained from ab initio calculations.

Stereocomplexed Polylactides, Fig. 2

PDLA (top) and PLLA (bottom) chains in parallel orientation, as located in the stereocomplex crystal. Hydrogen-bonding angles for the interactions suggested by FTIR are 120° and 150° for CR-H · · · O and CH₃ · · · O bonds, respectively, and hydrogen bond distance is about 2.9 Å (Reprinted with permission from Sarasua et al. [6]. Copyright (2005) American Chemical Society)

It should also be noted that two polylactide chains, because of its asymmetry, can interact in two manners: parallelly and antiparallelly. In the case of the linear sc-PLAs, both abovementioned types of geometry are possible, although according to Brizzolara et al.’s calculations [7], interaction energy is higher for the parallelly oriented PLA helices.

Properties of PLA Stereocomplex

PLA, a biodegradable, biocompatible, nonvolatile, and odorless polymer, is classified as GRAS (generally recognized as safe) by the US Food and Drug Administration. However, in comparison with other commercial thermoplastics, it exhibits a low rate of crystallization and a relatively low melting temperature. Thus, the enhancement of the thermal stability of PLA is important to use this material in a various industrial applications [8]. Stereocomplexation seems to be one of the most important and relatively easy methods for improving the thermal properties of PLA.

Linear high molar mass (M _n = 10⁵ g/mol) polylactide stereocomplexes are not able to survive melting to reform the stereocomplex crystallites, and after slow cooling from the melt, a mixture composed of homochiral crystallites and stereocomplex crystallites is formed. However, it has been shown that in the case of star-shaped high-molar-mass (M _n ≥ 10⁵) enantiomeric PLAs with more than six arms, the formation of the PLA stereocomplexes in the melt is complete and perfectly reversible, probably because of the “hardlock fruit”-type interactions (see Fig. 3).

Stereocomplexed Polylactides, Fig. 3

Schematic structure of parallel and antiparallel PLA stereocomplexes of the star-shaped macromolecules ((R)-PLA corresponding to D-PLA and (S)-PLA to L-PLA) (Reprinted with permission from Biela [9]. Copyright (2006) American Chemical Society)

The calculations giving the advantage of parallel interaction of PLLA and PDLA in the linear stereocomplexes, as mentioned above, may not apply to the star-shaped structures. In the star-shaped sc-PLA, chains may predominantly interact in the antiparallel geometry because of the better fitting of the complementary chains in this orientation [9]. The steric hindrance of the star cores crowded together rather excludes the parallel arm interactions as shown in Fig. 3.

However, the successful preparation of thermally stable stereocomplexes based on the linear high-molar-mass polylactides is still a strategic target. Recently, it has been found that covalently modified multi-walled carbon nanotubes (MWCNTs) can be used as initiators in the ring-opening polymerization of l- and d-lactides to induce efficient dispersion of the MWCNTs in the PLA matrix. Moreover, the thermal properties of the prepared PLA/MWCNTs composites and the thermal stability of their stereocomplexes are highly enhanced. Furthermore, Purnama et al. proposed preparation of bio-stereocomplex nanocomposites with cellulose nanowhiskers (CNW). The sc-PLA-CNW materials exhibit excellent stereocomplex memory and are able to reform perfect sc-PLA crystallites after melting [10]. The ability to readily obtain the PLA stereocomplex with improved physicochemical properties such as thermal and mechanical properties is very important for their mass applications [11].

The combined effect of the influence of the nature of polylactide end groups and PLA architecture on the morphology and properties of stereocomplexes has also been extensively investigated. It has been shown that for stereocomplexes of low-molecular-weight enantiomeric PLLA and PDLA, the nature of end groups can affect the morphology of stereocomplex particles precipitating from an organic solvent. Stereocomplex of PLLA/PDLA terminated with imidazolium ionic liquid derivative (PLA-IL) precipitated from 1,4-dioxane in the form of uniform microspheres. Stereocomplexation of star-shaped PLLA and linear PDLA functionalized with imidazolium ionic end groups in 1,4-dioxane leads to the formation of uniform microspheres which crystallize in the form of colloidal crystals. Analysis, using scanning electron microscopy (SEM) and atomic force microscopy (AFM), shows the interesting hierarchical structure of the system, starting from the nanometrical grains composed of few stereocomplex macromolecules that aggregate in uniform microspheres, ending with formation of colloidal crystals of microspheres as it is shown in Fig. 4.

Stereocomplexed Polylactides, Fig. 4

Hierarchical structure of stereocomplex microspheres colloidal crystal (Reprinted with permission from Brzeziński et al. [12]. Copyright (2014) Wiley)

Main Analytical Methods of sc-PLA Analysis

The formation of stereocomplex may be observed using different analytical methods, namely, differential scanning calorimetry (DSC), attenuated total reflectance Fourier transform infrared analysis (ATR FTIR), wide-angle X-ray scattering (WAXS), and cross-polarization/magic-angle spinning ¹³C nuclear magnetic resonance (CP/MAS ¹³CNMR).

The melting temperature of sc-PLA is higher than its enantiomeric components, and therefore, DSC analysis can be used as a method to confirm stereocomplexation. DSC thermograms of the sc-PLA and one of its enantiomeric component are shown in Fig. 5. It should be mentioned that melting temperature of the enantiomeric components (PLLA or PDLA) is ∼50 °C lower than that one of the stereocomplex. In the DSC thermogram of sc-PLA, one melting peak appeared at 223 °C. It means that the crystalline fraction of this material contains stereocomplex crystallites only.

Stereocomplexed Polylactides, Fig. 5

Typical DSC curves of PLLA (dotted line) and sc-PLA (solid line) (Reprinted with permission from Fukushima and Kimura [13]. Copyright (2014) Wiley)

A very useful and fast method to observe the stereocomplexation is also infrared analysis (ATR-FTIR). The significant changes in the conformation of PLA chains have been directly observed during this process. The α helix (wave number 921 cm⁻¹) which is characteristic for both PLLA and PDLA enantiomers is transformed into the more compact β helix (wave number 908 cm⁻¹) of the stereocomplex, and a new band at 1,748 cm⁻¹ also appears. In Fig. 6, the ATR-FTIR spectrum of the PLA stereocomplex compared with the spectrum of the PLLA enantiomer is shown. Regions of wavelength in which characteristic changes were observed at 1,700–1,800 cm⁻¹ and at 970–850 cm⁻¹ (stretching vibration of C = O group and skeletal stretching vibration of α and β helixes, respectively) are enlarged.

Stereocomplexed Polylactides, Fig. 6

The ATR-FTIR spectra of enantiomeric PLLA (dashed line) and stereocomplex sc-PLA (solid line) (Reprinted with permission from Brzeziński et al. [12]. Copyright (2014) Wiley)

High-resolution solid-state CP/MAS ¹³C NMR spectroscopy is also an effective method for tracing PLA stereocomplexation. The range of chemical shifts characteristic for the carbonyl carbon atom (165–175 ppm δ) is particularly diagnostic. In ¹³C NMR spectra (Fig. 7), the rigid stereocomplex crystalline component (C and D in line I) should give a signal at 173.3 ppm, because line I is not observed for the crystallized nonblended PDLA and PLLA precipitates. It can be assumed that the disordered stereocomplex crystalline component may also partly contribute to this peak. The amorphous component should give a peak at 169.7 ppm, whereas if the homocrystalline component B corresponds to line II (i.e., crystallites of individual enantiomeric macromolecules) is present, it should give a signal at 172 ppm because it has a very similar chemical shift to that one of precipitates of the nonblended PDLA or PLLA. In the spectrum of PLA stereocomplex, the shoulder in the 169–172 ppm region (component A in line III) is clearly visible, indicating the contribution of the amorphous phase. Those results could be confirmed with the results from DSC analysis.

Stereocomplexed Polylactides, Fig. 7

Analysis of components A–D in resonance lines I–III for the total ¹³C NMR spectrum of carbonyl carbon in the stereocomplex precipitate: A noncrystalline (amorphous) component (169.7 ppm); B homocrystalline component (172.0 ppm); C rigid stereocomplex crystalline component (173.3 ppm); D disordered stereocomplex crystalline component (173.3 ppm) (Reprinted with permission from Tsuji [5]. Copyright (2014) Wiley)

The supplementary examination confirming the formation of a stereocomplex is the WAXS analysis, which, like ATR-FTIR method, is clear and useful but time-consuming.

In the Fig. 8, the WAXS profiles of the PLA stereocomplex and one of its enantiomeric components are shown. The main peaks of PLLA and PDLA film that appear at 2θ values of 15, 17, and 19° are related to the α form of both enantiomeric PLAs, crystallized in a pseudo-orthorhombic unit cell of dimensions: a = 1.07 nm, b = 0.595 nm, and c = 2.78 nm, which contains two 10₃ helices. The most intense peaks of equimolar blend of PLLA/PDLA are observed at 2θ values of 12, 21, and 24°. These peaks are ascribed to the PLA stereocomplex crystallized in a triclinic unit cell of dimensions – a = 0.916 nm, b = 0.916 nm, and c = 0.870 nm – in which l-lactyl and d-lactyl unit sequences are packed parallel taking 3₁ helical conformation (called also β helix). The lattice containing a PLLA or PDLA chain with a 3₁ helical conformation has the shape of an equilateral triangle, which is expected to form equilateral-triangle-shaped single crystals of the PLA stereocomplex.

Stereocomplexed Polylactides, Fig. 8

WAXS profiles of equimolar blend of PLLA/PDLA and pure PLLA, respectively (Reprinted with permission from Fukushima and Kimura [13]. Copyright (2014) Wiley)

Furthermore, Brizzolara et al. [7] compared the WAXS profiles from actual stereocomplexed specimens with a force-field simulated stereocomplex. They also proposed the growth mechanism of the stereocomplex equilateral-triangle-shaped single crystal.

Application of PLA Stereocomplex

PLA-based materials have been used mostly for short-time applications from drug carriers and implants to packaging and textiles [14]. However, poor mechanical and thermal properties of PLA exclude its use as an ordinary structural material. Stereocomplexation improves these properties and thus broaden the possible application of PLA-based materials.

The crucial issue for stereocomplex PLA-based materials is production costs of both l- and d-lactide acid, the main raw materials for the production of LA monomers. The l-lactide acid has been widely used as a food additive, and it is produced in the industrial scale. The discovery of sc-PLA caused the increase attention in the manufacturing technology of the d-lactide acid. Therefore, the reduction of d-lactide acid costs will solve, at least partially, the problem of producing sc-PLA materials on large scale and serve as an alternative for commercial polymeric materials.

Sc-PLA-based materials are already used in biomedicine as drug delivery systems (DDS) and hydrogels. Biodegradable stereocomplex-based microspheres and micelles are proposed for the transportation and delivery of biologically active agents. Stereocomplex self-assemblies exhibit strong thermodynamic and kinetic stability and slower drug release. Hydrogels are prepared from block and graft copolymers with hydrophilic segments and l- and d-lactide unit sequences.

Stereocomplex crystallites can act as nucleation agents for pure PLA crystallization. Moreover, narrowly dispersed stereocomplex nanocrystals are proposed for the all-poly(lactide acid) nanocomposites. Small size and high aspect ratio of nano-reinforcing agents will dramatically improve thermal and mechanical properties of PLA nanocomposite material. The same composition of the nanofiller and the matrix prevents from their incompatibility and increases their potential recyclability.

Sc-PLA could find higher applications in automotive, communication, and electronic industries. Sc-PLA is suitable for making fibers, films, rods, and other processed plastics [4]. Thermally stable sc-PLA fibers could be used for manufacturing woven and knitted products. On the other hand, sc-PLA films have found the application as optically transparent heat-resistant insulators. Moreover, PLA stereocomplex materials could have potential in single-use microwave applications. In the near future, sc-PLA will provide many applications as bio-based sustainable materials, for example, as “green” notebooks, cell phones, or panels for DVD drivers.

Nowadays, three companies produce bioplastics based on sc-PLA: Corbion Purac, Teijin, and Hycail. Teijin and Hyacil produce sc-PLA on the industrial scale and begin to apply sc-PLA for packaging, civil engineering and construction, and oil fields (the drilling phase in shale gas extraction, etc.). Corbion Purac prepares highly heat-resistant sc-PLA materials that can withstand temperatures up to 180 °C/ 356 °F. PURALACT® is ideal for bio-based beverage cups for hot drinks such as coffee, soup, hot chocolate, tea, etc.

Conclusions and Perspectives

Crystallization of the polylactide stereocomplex crystalline structure resulted in an increase of melting temperature of about 50 °C with respect to that obtained in homocrystalline samples as it was detected in DSC analysis. The WAXS profiles of stereocomplexed PLA revealed a more compact crystalline structure, and FTIR results provided new bands at 1,748 cm⁻¹ (stretching vibration of C = O) and 908 cm⁻¹ (skeletal stretching vibration), attributed to crystalline polylactide chains in the 3₁ helical conformation (called β helix). In addition, the spectral bands attributed to the interlamellar material indicate a truly amorphous phase, in contrast to the semi-ordered interphase found during crystallization of PLLA. Hence, crystallization mechanisms of PLLA and the stereocomplex revealed differences.

The thermal and mechanical properties of sc-PLA are similar to other bio-based polymers as it was shown in Table 1. The sc-PLA exhibits better thermal and mechanical properties even for not fully stereocomplexed materials (containing a certain amount of homocrystallites) formed using high-molecular-mass enantiomeric polymers. However, the usage of PLA nanocomposites paves the way for future stereocomplex products with excellent stereocomplex memory. Along with its heat resistance, sc-PLA has many other properties, including crystallinity, chemical resistance, transparency, and gas permeability, making it suitable for use as an engineering plastic material. The application of sc-PLA products has just started to replace conventional oil-based polymers and should have high potential as sustainable structural materials to make fibers, films, rods, and other processed plastics. Particularly, sc-PLA fibers show high thermal stability and can be subjected to dyeing and hot pressing by ordinary techniques. It has already been confirmed that a knit of sc-PLA withstands laundering above 180 °C. Because of its incombustible nature, sc-PLA could be potentially applied in housing of various electric appliances and automobile parts. Since both PLLA and PDLA are already commercially available and their market prices are gradually falling, stereocomplex manufacturing in the near future can be profitable. It is likely that soon, wide industrial applications of these materials will be possible.

Stereocomplexed Polylactides, Table 1 Mechanical and thermal properties of the representative bio-based polymers (Copyright (2010) Wiley)