Stereocomplex Formation Research Articles

Crystallization Behavior of Asymmetric PLLA/PDLA Blends

The effects of the addition of poly(D-lactide) (PDLA) on the crystallization behavior of poly(L-lactide)(PLLA) were investigated by means of differential scanning calorimetry (DSC) and temperature-dependent X-ray diffraction(XRD). When the blends were cooled from different temperatures (250, 240, and 190 °C) at the rate of cooling of 5 °C/min, stereocomplex (sc) crystallites could stay at diverse states. Accordingly, the stereocomplexes acted as a nucleation agent exerting distinct effects on PLLA crystallization. The speculated mechanisms of the stereocomplex formation and the effectiveness as a nucleating agent are schematically described. Moreover, temperature-dependent XRD was carried out to further investigate the melt-crystallization behavior of PLLA/PDLA blends in real time. Temperature-dependent XRD results indicated that even at 240 °C the stereocomplex crystallites in all blend samples existed clearly, which could not be detected by DSC. These XRD results further suggest that the onset T(c) values for the PLLA α-form crystals formation were 160, 120, 140, and 160 °C, respectively, for neat PLLA, PLLA/PDLA 95/5, 90/10, and 80/20 as well as 70/30 samples.

Biosynthesis of a lactate (LA)-based polyester with a 96 mol% LA fraction and its application to stereocomplex formation

A poly(lactic acid) (PLA)-like terpolyester consisting of 96mol% lactate (LA), 1mol% 3-hydroxybutyrate and 3mol% 3-hydroxyvalerate was produced in recombinant Escherichia coli LS5218 expressing LA-polymerizing enzyme (LPE). The strain was grown on glucose with a feeding of valerate as the monomer precursor. The glass transition and melting temperatures of the terpolyester were close to those of chemically synthesized poly(L-LA)s (PLLAs) having similar molecular weights. Additionally, a blend of the terpolyester, which was composed entirely of (R)-LA (D-LA) due to the strict enantiospecificity of LPE, with PLLA formed a stereocomplex with higher melting temperature (201.9 °C). These results indicate that the biological PLA-like polyester produced via this one-step microbial process has comparable thermal properties to chemically synthesized PLAs.

The control of surface segregation of blend films using stereocomplex formation between enantiomeric polylactide chains

AbstractThis study investigated the stereocomplex‐induced surface structure of enantiomeric poly(lactide) (PLA) blend films by time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS). The design of the blend systems was based on the principle of surface segregation of a modified component with a low surface energy, fluorine‐end‐capped PLLA (F‐PLLA). Analysis of F‐PLLA/PDLA and F‐PLLA/PLLA blends, yielded a difference in the ion beam‐induced behavior between fluorine and lactide groups. In addition, a completely phase‐separated sample of PLLA with the fluorine end‐capping agent cast onto the surface was used for a real analysis of peak intensities from the two components, simulating bulk compositions. The two blend systems, uncomplexed (F‐PLLA/PLLA) and complexed (F‐PLLA/PDLA) blends, showed quite different surface structures: namely, a higher fluorine concentration was observed in the uncomplexed blends while the complexed blends showed a wider range of fluorine concentrations, depending on the degree of complexation. This indicates that the complexed blends offer a greater level of control over the surface fluorine concentration because this concentration changes more significantly with the blend composition (or the degree of complexation). Copyright © 2010 John Wiley & Sons, Ltd.

Transformation of peptide nanotubes into a vesicle via fusion driven by stereo-complex formation

Two types of peptide nanotubes, one is prepared from an amphiphilic peptide having a right-handed helix segment and the other from that having a left-handed helix segment, are shown to transform the morphology into a vesicle by membrane fusion due to stereo-complex formation between these helical segments.

Stereocomplexes formation from enantiomeric star-shaped block copolymers of ε-caprolactone and lactide

AbstractEnantiomeric star-shape block copolymers of ε-caprolactone and lactide with different molar masses and compositions were synthesized using pentaerythritol/tin(II) 2-ethylhexanoate as initiating system. Stereocomplex formation between pair of enantiomeric blocks of star arms was investigated. Thermal properties of both copolymers and obtained stereocomplexes were examined. A rise of above 50 °C of melting temperature of polylactide phase was observed as a consequence of stereocomplex creation.

Rational design of peptide nanotubes for varying diameters and lengths

Amphiphilic helical peptides (Sar)(m) -b-(L-Leu-Aib)(n) (m = 22-25; n = 7, 8, 10) with a hydrophobic block as a right-handed helix were synthesized and their mixtures with (Sar)(25) -b-(D-Leu-Aib)(6) containing the hydrophobic block as a left-handed helix were examined in their molecular assembly formation. The single component (Sar)(25) -b-(D-Leu-Aib)(6) forms peptide nanotubes of 70 nm diameter and 200 nm length. The two-component mixtures of (Sar)(25) -b-(D-Leu-Aib)(6) with (Sar)(24) -b-(L-Leu-Aib)(7) , (Sar)(22) -b-(L-Leu-Aib)(8) , and (Sar)(25) -b-(L-Leu-Aib)(10) yield peptide nanotubes of varying dimensions with 200 nm diameter and 400 nm length, 70 nm diameter and several micrometer length (maximum 30 µm), and 70 nm diameter and 100-600 nm length, respectively. The right-handed and the left-handed helix were thus found to be molecularly mixed due to the stereo-complex formation and to generate nanotubes of different sizes. When the mismatch of the hydrophobic helical length between the two components was of four residues, the longest nanotube was generated. Correspondingly, the hydrophobic helical segments have to interdigitate with an anti-parallel orientation at the hydrophobic core region of the nanotube.

Stereocomplexed Materials Based on Poly(3-hexylthiophene)-b-poly(lactide) Block Copolymers: Synthesis by Organic Catalysis, Thermal Properties, and Microscopic Morphology

A new approach to control the supramolecular organization of conjugated polymers is proposed, based on stereocomplex formation. It is illustrated by mixing polymer solutions of block copolymers, i.e., poly(3-hexylthiophene)-b-poly(l-lactide) and poly(3-hexylthiophene)-b-poly(d-lactide). The block copolymers were successfully synthesized via a three-step procedure including the use of metal-free organic catalysts in the ring-opening polymerization of l- and d-lactide. The thermal properties and microscopic morphology of sterecomplexed diblock copolymers show that the stereocomplexation of the polylactide blocks is able to prevent the crystallization of the regioregular poly(3-hexylthiophene) block into long-range ordered parallel fibrillar structures. Stereocomplexation is also observed when mixing poly(d-lactide) homopolymer and poly(3-hexylthiophene)-b-poly(l-lactide) block copolymer.

Efficient formation of stereocomplexes of poly(L‐lactide) and poly(D‐lactide) by terminal Diels–Alder coupling

AbstractPoly(L‐lactide) (PLLA) and poly(D‐lactide) (PDLA) having anthracene and maleimide terminals were prepared and mixed together to induce a terminal Diels–Alder reaction providing PLLA–PDLA diblock copolymers. The resultant Diels–Alder adducts showed a sole melting peak of stereocomplex crystals around 220 °C without homochiral crystallization. In addition to their highly improved stereocomplex formation, the adducts showed no decomposition due to the retro Diels–Alder reaction below the melting temperature of the stereocomplex crystals. Copyright © 2010 Society of Chemical Industry

Uniform poly(methyl methacrylate) stereostars: synthesis, separation and stereocomplex formation

Isotactic (it-) and syndiotactic (st-) uniform poly(methyl methacrylate)s (PMMAs) with hydroxyl end groups were isolated by supercritical fluid chromatography (SFC) and reacted with benzene-1, 3, 5-tricarbonyl trichloride. Two types of uniform three-arm star-PMMAs with different stereoregular arms, [it/it/st]- and [it/st/st]-3-star-PMMAs, that is, ‘stereostar PMMAs’, were isolated by SFC from the product mixture (According to IUPAC nomenclature for nonlinear polymers, star PMMA with three arms is named 3-star-PMMA. In this paper, the stereoregularity of the three arms is designated as [it/it/st], for example, and thus, the polymer is named [it/it/st]-3-star-PMMA. By analogy to ‘stereoblock’ PMMAs, these star PMMAs are termed ‘stereostar’ PMMAs). The formation of stereocomplexes by these stereostar PMMAs was examined in acetone. Although both stereostar PMMAs formed intramolecular stereocomplexes, the [it/st/st]-3-star-PMMA formed two types of intramolecular stereocomplexes with different hydrodynamic volumes because of the different compositions of the it- and st-PMMA arms involved in the stereocomplexes.

Specific recognition of syndiotactic poly(methacrylic acid) in porous isotactic poly(methyl methacrylate) thin films based on the effects of stereoregularity, temperature, and solvent

AbstractThe effects of stereoregularity, temperature, and solvent on the specific recognition of syndiotactic (st)‐poly(methacrylic acid) (PMAA) in macromolecularly porous isotactic (it)‐poly(methyl methacrylate) films were investigated to give important insights into the regularity and stability of nanospaces in the it‐PMMA films as well as template polymerization. The porous it‐PMMA films were fabricated on quartz crystal microbalance (QCM) substrates via the layer‐by‐layer (LbL) assembly of it‐PMMA/st‐PMAA, plus the st‐PMAA extraction from the assembly. QCM analysis and infrared spectroscopy revealed the first case of stereocomplex formation using st‐PMAA with lower stereoregularity (rr = 73%) in the LbL films, while st‐PMAA obtained with conventional free radical polymerization (rr = 62%) was barely incorporated into the porous it‐PMMA films. The maximum st‐PMAA incorporation increased from 25 to 40 °C, but there were almost no difference between 40 and 55 °C, indicating that the it‐PMMA crystallization would also be accelerated with increasing temperature. The studies on st‐PMAA incorporation with various complexing solvents revealed that the host it‐PMMA in the porous films could only form the original stereocomplex with 2/1 unit‐molar stoichiometry (st‐PMAA/it‐PMMA) in acetonitrile/water or ethanol/water. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3651–3657, 2010

Strong Compression Rate Dependence of Phase Separation and Stereocomplexation between Isotactic and Syndiotactic Poly(methyl methacrylate)s in a Langmuir Monolayer Observed by Atomic Force Microscopy

The stereocomplex formation between isotactic and syndiotactic poly(methyl methacrylate) (it-PMMA, st-PMMA) in a Langmuir monolayer was studied by surface pressure-area isotherms and atomic force microscopy (AFM). We found that the stereocomplex formation was highly sensitive to the compression rate of the monolayer. At a normal compression rate of 0.5 mm/s provided by the moving barrier, the blend monolayer formed a clear phase separation of the it- and st-PMMA domains at 1 mN/m. Further compression to 15 mN/m resulted in a limited degree of stereocomplexation, mainly at the interface between the two domains. However, at a 1/50 slower compression rate of 0.01 mm/s, the blend did not form a clear phase separation at 1 mN/m and quantitatively formed a stereocomplex at 15 mN/m. This apparent immiscibility observed at the faster compression rate was found to be kinetically induced as a result of the rapid compression of the phase-separated mixture at the dilute state because it-PMMA and st-PMMA form expanded and condensed monolayer, respectively. On the other hand, at the slower compression rate, the blend formed a thermodynamically miscible phase, and as a result, the stereocomplex was quantitatively formed. This apparent phase separation of a mixed monolayer composed of an expanded and a condensed monolayer should be a common phenomenon for similar systems and might have caused misjudgment of the miscibility in such cases. The compression rate dependence should be carefully evaluated in order to determine the precise miscibility of blended monolayers in similar systems.

A DFT study on poly(lactic acid) polymorphs

Poly(lactic acid) (PLA) can crystallize in α-, β-, γ- and stereocomplex (sc)- forms. It has been shown that the formation of stereocomplex between poly(l-lactic acid) and poly(d-lactic acid) significantly improve thermal stability and mechanical properties. However the mechanisms of enhancements are still unclear. In this study, we investigate the PLA polymorphs from the first-principles theoretical perspective in order to understand the intermolecular interaction in the crystals. Density functional theory at the level of Perdew-Wang generalized-gradient approximation was applied to optimize PLA crystal unit cells. A comparison of energies in the various unit cells reveals that sc-form is the most energetically favorable form among the four PLA polymorphs. The order of thermodynamically relative stability is that sc-form is 0.3, 1.1, and 1.3kcal/mol more stable than α-form, β-form, and γ-form, respectively (when using the ultrasoft pseudopotential and a plane-wave basis set with an energy cutoff of 380eV) or 0.4, 1.1, and 1.3kcal/mol more stable than α-form, β-form, and γ-form, respectively (when employing the density functional semi-core pseudopotentials and the double numerical plus polarization orbital basis set with a global orbital cutoff of 3.7Å). In addition to the energetic properties, structural and electronic properties were calculated as well. The theoretical predicted stability rank is in agreement with some reported observations. Such as, sc-form has higher melting point and larger heat of fusion than those of α-form. The enhanced thermal stability of the sc-form compared to the other three homopolymer forms may be attributed to the unique intermolecular non-conventional hydrogen bonding C–H⋯O(C) network in the stereocomplex.

Role of the Stereocomplex Crystallites in the PLLA/PDLA Mixed Spinning Dope on the Stereocomplex Formation in the Wet-spun Fibers

Wet-spinning of the PLA fibers was carried out from the mixed solution of PLLA and PDLA in CHCl3 held for various periods of time after preparation. The solution immediately after preparation indicated a low viscosity and a good spinnability. The viscosity and the storage modulus increased and the spinnability deteriorated with increasing holding time indicating a partial network formation with stereocomplex crystallites as crosslinking points. The as-spun fiber obtained from the solution held for a long time contained both the homo-crystal and the stereocomplex crystal as revealed by WAXD measurements. Although the HC crystal in the fiber spun from the solution held for short time was not eliminated, the SC crystal turned to be a major crystalline form in the fiber after drawing and annealing processes at an elevated temperature, when the fibers were spun from the solution held for long time. These results indicate that the stereocomplex crystallites in the mixed solution of PLLA and PDLA promote the stereocomplexation during the coagulation process. Furthermore, the stereocomplex crystallites dispersed in the fiber also promote the stereocomplexation in the fiber during the annealing process acting as a nucleating agent for the stereocomplex crystal.

Stereocomplex Formation of High-Molecular-Weight Polylactide Using Supercritical Fluid

Stereocomplex polylactide has been known as one of the choices to enhance the properties of polylactides. Numerous studies have been carried out on stereocomplex formation and characterizations, but these have a limitation in the case of high-molecular-weight polylactides. The stereocomplex formation of high-molecular-weight polylactides was observed by supercritical CO2 in the presence of solvent. Furthermore, the addition of solvent increased the degree of stereocomplex formation. We evaluated the degree of stereocomplex formation as a function of pressures, solvent concentration, temperature, and times. The high degree of stereocomplex formation was achieved at 30% of dichloromethane, 350 bar, and 65 °C. Compared with the solution casting method, stereocomplex formation by supercritical CO2-solvent had a higher degree of stereocomplex formation, and dry product was obtained in situ. Significant improvements of thermal and mechanical properties have been assessed for the stereocomplex obtained by superc...

Morphological changes of isotactic poly(methyl methacrylate) thin films via self-organization and stereocomplex formation

Morphological changes of macromolecularly porous thin films during isotactic (it) poly(methyl methacrylate) (PMMA) crystallization and the subsequent syndiotactic (st) poly(methacrylic acid) (PMAA) incorporation are described herein. Porous it-PMMA thin films were prepared on a gold substrate by the selective extraction of st-PMAA from stereocomplex films composed of it-PMMA/st-PMAA using layer-by-layer assembly. it-PMMA crystallization occurred when the porous it-PMMA films were immersed in a mixed solvent of acetonitrile/water (4/6, v/v). Surface analyses of films were performed by scanning electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy (XPS) and static contact angles. Dotted aggregates of it-PMMA appeared on crystallized films, whereas networks composed of crystallized it-PMMA and the stereocomplex were observed on st-PMAA incorporated films. Gold substrate peaks were observed not on porous it-PMMA films, but on crystallized it-PMMA films by XPS, indicating that homogeneously broadened it-PMMA in porous films was localized because of its crystallization. The static contact angle of crystallized it-PMMA films was 36.5±1.6°, which was smaller than that of porous it-PMMA films (49.2±1.9°). This difference resulted from the surface morphology of the thin films. Dotted it-PMMA aggregates were also observed when spin-coated films were immersed in the mixed solvent. Morphological changes of macromolecularly porous thin films in it-PMMA crystallization and subsequent st-PMAA incorporation

Polylactide Stereocomplex-Based Electrospun Materials Possessing Surface with Antibacterial and Hemostatic Properties

Novel fibrous materials of stereocomplex between high-molecular-weight poly(d- or l-)lactide (HMPDLA or HMPLLA) and diblock copolymers consisting of poly(l- or d-)lactide and poly(N,N-dimethylamino-2-ethyl methacrylate) blocks, respectively (PLLA-block-PDMAEMA or PDLA-block-PDMAEMA), were prepared by solution electrospinning. Fibers with mean diameters ranging from 1400 to 1700 nm were obtained. The stereocomplex formation was evidenced by differential scanning calorimetry (DSC) and X-ray diffraction (XRD) analyses. Annealing at 100 degrees C for 8 h resulted in the appearance of crystalline peaks at 2theta values of 12, 21, and 24 degrees for PLA stereocomplex. X-ray photoelectron spectroscopy (XPS) analyses revealed the gradient composition of the fibers with a surface enriched in tertiary amino groups from PDMAEMA blocks. The availability of tertiary amino groups imparts hemostatic and antibacterial properties to the stereocomplex fibrous materials, as indicated by the performed tests on blood cells and on pathogenic microorganisms.

Stereocomplex Formation between Enantiomeric Substituted Poly(lactide)s: Blends of Poly[(S)-2-hydroxybutyrate] and Poly[(R)-2-hydroxybutyrate

Stereocomplex Formation between Enantiomeric Substituted Poly(lactide)s: Blends of Poly[(<i>S</i>)-2-hydroxybutyrate] and Poly[(<i>R</i>)-2-hydroxybutyrate

Stereoblock Polylactides as High-Performance Bio-Based Polymers

Mixing of poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA) affords stereocomplex-type PLA (sc-PLA) having a high melting temperature of 230°C, and its use provides higher thermal properties for PLA materials. The stereocomplex formation, however, is always accompanied by homochiral crystallization of PLLA and PDLA, causing deterioration of properties, when their molecular weight is higher than 100 kDa. Therefore, stereoblock-type PLAs (sb-PLA) consisting of PLLA and PDLA stereosequences are developed by utilizing new synthetic methodologies. The sb-PLAs exclusively generate the stereocomplex because of the better mixing state of their enantiomeric sequences and can be utilized as high-performance bio-based materials.

Amphiphilic Poly(d- or l-lactide)-b-poly(N,N-dimethylamino-2-ethyl methacrylate) Block Copolymers: Controlled Synthesis, Characterization, and Stereocomplex Formation

Novel well-defined amphiphilic poly(D-lactide)-b-poly(N,N-dimethylamino-2-ethyl methacrylate) (PDLA-b-PDMAEMA) and poly(L-lactide)-b-poly(N,N-dimethylamino-2-ethyl methacrylate) (PLLA-b-PDMAEMA) copolymers were obtained. The synthesis strategy consisted of a three-step procedure: (i) controlled ring-opening polymerization (ROP) of (D- or L-)lactide initiated by Al(O(i)Pr)(3), followed by (ii) quantitative conversion of the polylactide (PLA) hydroxyl end-groups with bromoisobutyryl bromide and (iii) atom transfer radical polymerization (ATRP) of DMAEMA. The PLA block molecular weight was kept below 5000 g/mol. The macromolecular parameters of the (co)polymers were determined by (1)H NMR spectroscopy and size exclusion chromatography (SEC). The stereocomplexes of PDLA-b-PDMAEMA/PLLA-b-PDMAEMA diblock copolymers were prepared via solvent casting. The stereocomplex formation was evidenced by differential scanning calorimetry (DSC) and X-ray diffraction (XRD) analyses. The obtained stereocomplexes had melting temperature of about 65 degrees C above that of the individual copolymers and exhibited diffraction patterns assigned to the stereocomplex crystallites. In addition, for the first time it was shown that the replacement of one of the PLA partners with high molecular weight PLLA or PDLA did not hamper the stereocomplex formation. The presence of PDMAEMA blocks proved to impart hydrophilicity of the synthesized copolymers and related stereocomplexes, as determined by static water contact angle measurements.

Effect of a Miscible Polymeric Diluent on Complex Formation between Isotactic and Syndiotactic Poly(methyl methacrylate)

Formation of a stereocomplex structure in a blend of isotactic and syndiotactic poly(methyl methacrylate)s (iPMMA/sPMMA) was found to be significantly enhanced by introducing a mutual miscible diluent of poly(ethylene oxide) (PEO) in the blend, even when prepared using a kinetically hindered noncomplexing CHCl3 solvent. Mechanisms and influencing factors, such as thermal treatment, annealing temperature, annealing time, or PEO content in blends, on structures of stereocomplexes in the ternary iPMMA/sPMMA/PEO blends are discussed. WAXD result showed that addition of PEO in the iPMMA/sPMMA blend did not alter the crystal cells in the complexes. Melting peaks associated with the complexes were analyzed for discerning possible structural mechanisms. When annealed at specific temperatures, multiple endothermic peaks of the complexes increased in intensity as PEO contents increased in the ternary blends. Analysis shows that the lower melting peak (P1) represents shorter stereocomplex sequences or stereocomplex detached from PEO or connected with partial PEO, but the higher melting peak (P3) is related to the stereocomplex physically interconnected in the presence of PEO. The PEO molecules are thought to more easily interact with the stereocomplexes in blends by acting as a plasticizer, which decreases Tg of the more rigid PMMA chain segments. Physical interaction or miscibility-enhanced chain entanglement probably exists between PEO and the stereocomplex of tactic PMMAs, leading to stereocomplex formation in the iPMMA/sPMMA/PEO ternary blend. FT-IR characterization provided evidence of weak interactions between PEO and the stereocomplex of PMMA. IR peaks sensitive to complex structures with respect to temperature were analyzed for discerning effects by adding PEO. The stereocomplex formation of ternary blends with PEO is due to the weak interaction of ester groups and methoxy groups between PMMA and PEO.