Solid State 13C CP-MAS NMR Studies of the Crystallinity and Morphology of Poly(l-lactide)

Stereochemical Heterogeneity Analysis of Polylactides by Multidimensional Liquid Chromatography

A new and robust high-performance liquid chromatography (HPLC) method that separates poly(lactic acid) (PLA) according to its stereochemical composition is presented. Using this method, poly(l-lactide) incorporating trace amounts of meso-lactide resulting from the racemization is separated from the pristine polymer. To prove this aspect in more detail, a representative poly(l-lactic acid) standard, assumed to be highly homogeneous, was separated using this method. The result showed that this was not the case as a fraction incorporating meso-lactide due to racemization occurring during the synthesis is separated. Employing two-dimensional liquid chromatography (2D-LC), the molar mass differences of the separated species were investigated, and fractions with similar molecular sizes were detected, confirming that the LC separation is solely based on stereochemical heterogeneity. The sample was further fractionated by preparative HPLC, followed by an in-depth analysis of the fractions using homonuclear decoupling in proton nuclear magnetic resonance (1H NMR). Convincing results that unveiled significant differences in the stereochemistry of the isolated PLA fractions were obtained. Subsequent analysis by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS) also confirmed oligomer series with different end group structures, indicating that the applied HPLC method is very sensitive to minor variations in stereochemistry and end groups. This integrated approach offers detailed insight into the structural characteristics of PLA polymers, contributing to a better understanding of their composition and potential applications.

s-Tetrazine-Bridged Photochromic Aromatic Framework Material

Integrating fluorescent chromophores in aromatic frameworks could not only prevent aggregation-induced quenching caused by the π-π stacking interaction between the chromophore components but also confer new fluorescence properties. Herein, we report the fabrication of s-tetrazine-bridged aromatic frameworks TzAF by the incorporation of the smallest aromatic fluorophore, s-tetrazine (Tz), into the skeleton of a tetrahedrally connected lattice of aromatic frameworks. The thin films of TzAF coated on silica gel plates were found to exhibit reversible photoswitching fluorescence characteristics under alternate UV and visible-light irradiations with excellent fluorescence stability and high on/off contrast. The repeatable "on/off"fluorescence photoswitchability of the TzAF thin films was mechanistically attributed to light-induced reversible transformation between TzAF's neutral and radical states.

Nucleating Agents to Enhance Poly(l-Lactide) Fiber Crystallization during Industrial-Scale Melt Spinning

The nucleating agent N,N′-bis(2-hydroxyethyl)-terephthalamide (BHET) has promising effects on poly(l-lactide) (PLA) under quiescent conditions and for injection molding applications, but its suitability for industrial-scale fiber melt spinning is unclear. We therefore determined the effects of 1% and 2% (w/w) BHET on the crystallinity, tenacity, and elongation at break of PLA fibers compared to pure PLA and PLA plus talc as a reference nucleating agent. Fibers were spun at take-up velocities of 800, 1400 and 2000 m/min and at drawing at ratios of 1.1–4.0, reaching a final winding speed of 3600 m/min. The fibers were analyzed by differential scanning calorimetry, wide-angle X-ray diffraction, gel permeation chromatography and tensile testing. Statistical analysis of variance was used to determine the combined effects of the spin-line parameters on the material properties. We found that the fiber draw ratio and take-up velocity were the most important factors affecting tenacity and elongation, but the addition of BHET reduced the mechanical performance of the fibers. The self-organizing properties of BHET were not expressed due to the rapid quenching of the fibers, leading to the formation of α′-crystals. Understanding the behavior of BHET in the PLA matrix provides information on the performance of nucleation agents during high-speed processing that will allow processing improvements in the future.

Synthesis and characterisation of an enantiomerically pure scandium pentadienyl complex and its application in the polymerisation of rac-lactide

The alkyl-functionalised scandium complex [(pdl*SiMe₂N^tBu)Sc(thf)(CH₂SiMe₃)] (2) was synthesised in enantiomerically pure form and characterised by NMR spectroscopy and X-ray diffraction analysis. Complex 2 features a chiral constrained geometry ligand derived from the natural compound (1R)-(-)-myrtenal, in which the pentadienyl (pdl*) fragment coordinates in η³:η²-allyl-en fashion to the scandium atom. Compound 2 catalyses the polymerisation of rac-lactide at 30 °C and 50 °C yielding amorphous poly(lactide) with slightly heterotactic enchainment (P_m = 0.36 and 0.37). In agreement with the data obtained from GPC and DSC measurements, a chain-end control mechanism is proposed with fast chain propagation relative to the initiation, which leads to broad molecular weight distributions (Đ ≈ 1.80) and higher than expected molecular weights. Furthermore, chain transfer processes are observed, but only small amounts of transesterification and racemisation occur. Kinetic studies reveal a second-order dependence in rac-lactide (monomer) concentration and a first-order dependence in the concentration of catalyst 2.

Decomposition Factor Analysis Based on Virtual Experiments throughout Bayesian Optimization for Compost-Degradable Polymers

Bio-based polymers have been considered as an alternative to oil-based materials for their “carbon-neutral” environmentally degrative features. However, degradation is a complex system in which environmental factors and preparation conditions are involved, and the relationship between degradation and these factors/conditions has not yet been clarified. Moreover, an efficient system that addresses multiple degradation factors has not been developed for practical use. Thus, we constructed a decomposition degree predictive model to explore degradation factors based on analytical data and experimental conditions. The predictive model was constructed by machine learning using a dataset. The objective variable was the molecular weight, and the explanatory variables were the moisture content in a compost environment, degradation period, degree of crystallinity pre-experiment, and features of solid-state nuclear magnetic resonance spectra. The good accuracy of this predictive model was confirmed by statistical variables. The moisture content in the compost environment was a critical factor for considering initial degradation; specific scores revealed the contribution of degradation factors. Furthermore, the optimum decomposition degree, various analytical values, and experimental conditions were predictable when this predictive model was combined with Bayesian optimization. Information obtained from virtual experiments is expected to promote the material design and development of bio-based plastics.

Poly(l-Lactic Acid)/Pine Wood Bio-Based Composites

Bio-based composites made of poly(l-lactic acid) (PLLA) and pine wood were prepared by melt extrusion. The composites were compatibilized by impregnation of wood with γ-aminopropyltriethoxysilane (APE). Comparison with non-compatibilized formulation revealed that APE is an efficient compatibilizer for PLLA/wood composites. Pine wood particles dispersed within PLLA act as nucleating agents able to start the growth of PLLA crystals, resulting in a faster crystallization rate and increased crystal fraction. Moreover, the composites have a slightly lower thermal stability compared to PLLA, proportional to filler content, due to the lower thermal stability of wood. Molecular dynamics was investigated using the solid-state ¹H NMR technique, which revealed restrictions in the mobility of polymer chains upon the addition of wood, as well as enhanced interfacial adhesion between the filler and matrix in the composites compatibilized with APE. The enhanced interfacial adhesion in silane-treated composites was also proved by scanning electron microscopy and resulted in slightly improved deformability and impact resistance of the composites.

Elucidation of the hierarchical structure of natural eumelanins

Eumelanin is one of the most ubiquitous pigments in living organisms and plays an important role in coloration and UV protection. Because eumelanin is highly cross-linked and insoluble in solvents, the chemical structure is still not completely known. In this study, we used atomic force microscopy, X-ray photoelectron spectroscopy and solid-state nuclear magnetic resonance (NMR) to compare intact eumelanosomes (pigment granules mostly made of eumelanin) from four phylogentically distant species: cuttlefish (Sepia officinalis) inks, black fish crow (Corvus ossifragus) feathers, iridescent wild turkey (Melleagris gallopavo) feathers and black human hair. We found that eumelanosomes from all four species are composed of subunit nanoparticles with a length of 10-60 nm, consistent with earlier observations in eumelanosomes from the sepia ink and human hair. The solid-state NMR results indicate the presence of quinone methide tautomers in all four eumelanins. We also found clear differences in the UV absorbance, the ratio of 5,6-dihydroxyindole-2-carboxylic acid/5,6-dihydroxyindole and protonated aryl carbon ratios in sepia eumelanin relative to the other three. This comparison of natural eumelanin across a phylogenetically broad group of organisms provides insights into the change in the eumelanin structure over the evolutionary history and enables the production of synthetic eumelanin with properties that are similar to natural eumelanin.

Nucleating and Plasticization Effects in Drawn Poly(Lactic Acid) Fiber during Accelerated Weathering Degradation

Changes in the polymer properties of poly(lactic acid) (PLA) fibers during drawing and degradation processes were analyzed using solid-state NMR, with the goal of elucidating morphological changes that influence fiber tensile properties. Combination of X-ray diffraction (XRD) and differential scanning calorimeter (DSC) indicated that the drawn PLA fibers consisted of different proportions of α crystalline and amorphous forms. ¹³C CP-MAS NMR spectra showed amorphous-like broad singlet signals, of which the full width at half maximum (FWHM) decreased with increasing crystallinity and crystal orientation. The T₁H value decreased by interaction with additives and increased with increasing crystal orientation. The interaction with additives also reduced T₁C values, which increased with increasing crystallinity. Use of organic clay enhanced the crystallization of high draw-ratio PLA fibers due to nucleation, which increased tensile strength; this effect gradually decreased with time during accelerated weathering. In contrast, the plasticization due to the addition of flexible polymers increased fiber elongation, which rapidly dropped during the degradation. Changes of FWHM, T₁H, and T₁C values indicated that the degradation occurred at sites within the amorphous portions of the PLA fibers containing organic clay, while the flexible polymers were preferentially degraded if they were present in the PLA fibers.

Millisecond Self-Assembly of Stable Nanodispersed Drug Formulations

We report the development of a new spray-drying and nanoparticle assembly process (SNAP) that enables the formation of stable, yet rapidly dissolving, sub-200 nm nanocrystalline particles within a high T_g glassy matrix. SNAP expands the class of drugs that spray-dried dispersion (SDD) processing can address to encompass highly crystalline, but modestly hydrophobic, drugs that are difficult to process by conventional SDD. The process integrates rapid precipitation and spray-drying within a custom designed nozzle to produce high supersaturations and precipitation of the drug and high T_g glassy polymer. Keeping the time between precipitation and drying to tens of milliseconds allows for kinetic trapping of drug nanocrystals in the polymer matrix. Powder X-ray diffraction, solid state 2D NMR, and SEM imaging shows that adding an amphiphilic block copolymer (BCP) to the solvent gives essentially complete crystallization of the active pharmaceutical ingredient (API) with sub-200 nm domains. In contrast, the absence of the block copolymer results in the API being partially dispersed in the matrix as an amorphous phase, which can be sensitive to changes in bioavailability over time. Quantification of the API-excipient interactions by 2D ¹³C-¹H NMR correlation spectroscopy shows that the mechanism of enhanced nanocrystal formation is not due to interactions between the drug and the BCP, but rather the BCP masks interactions between the drug and hydrophobic regions of the matrix polymers. BCP-facilitated SNAP samples show improved stability during aging studies and rapid dissolution and release of API in vitro.

Polylactides-Methods of synthesis and characterization

Polylactides with various molar masses, microstructures and crystallinities are used as degradable and biocompatible polymers suitable for preparation of drug carriers and temporary medical implants. This paper presents state of current knowledge on synthesis of lactic acids, high purity lactide monomers and their polymerization. Syntheses of high molar mass polylactides by polycondensation of lactic acid and by ring-opening polymerization of lactides are described and their advantages and disadvantages are discussed. Mechanisms of lactide polymerization initiated by metal alkoxides are described. There are presented also results of more recent studies of polymerization initiated with the so-called "no metal" organocatalysts; both anionic and cationic. Presented are advantages and limitations of synthesis of PLA by all the major polymerization processes until now. Some properties of PLA and most important methods used for PLA characterization are also described.

Effect of composition ratio on the thermal and physical properties of semicrystalline PLA/PHB-HHx composites

In this study, composites of semicrystalline, biodegradable polylactide (PLA) and poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHB-HHx) were prepared by direct melt compounding. The physical and thermal properties of the composites were investigated as a function of the composition ratio. Differential scanning calorimetry analysis indicated that PLA and PHB-HHx formed immiscible composites over the observed range of composition. The crystallization of PLA was gradually suppressed by increasing proportions of PHB-HHx. Dynamic mechanical analysis results confirmed that the innate ductility of PHB-HHX and its inhibiting effect on PLA crystallization improved the stiffness of the composite compared to those of neat PLA. The infrared spectra of the immiscible PLA/PHB-HHx composites at two crystallization temperatures (30 °C, 130 °C) were obtained and presented. At 30 °C, PHB-HHx existed as crystalline domains in the PLA matrix, while, amorphous phase of molten PHB-HHx was diffused within the crystalline phase of PLA at 130 °C. The interaction between PHB-HHX and PLA could not be elucidated from the temperature data. Mechanical tests showed that the addition of PHB-HHx improves ductility of PLA/PHB-HHx composite. Morphological analysis revealed that small proportions of PHB-HHx exhibited less tendency to aggregate, which resulted in greater plastic deformation and improved toughness. From this study, PLA blended with small portions of PHB-HHx may further expand the use of bio-friendly resources in a variety of applications such as flexible films, food packaging and something like that.

Thermal and rheological properties of L-polylactide/polyethylene glycol/silicate nanocomposites films

The melt rheology and thermal properties of polylactide (PLA)-based nanocomposite films that were prepared by solvent casting method with L-PLA, polyethylene glycol (PEG), and montmorillonite clay were studied. The neat PLA showed predominantly solid-like behavior (G' > G″) and the complex viscosity (η*) decreased systematically as the temperature increased from 184 to 196 °C. The elastic modulus (G') of PLA/clay blend showed a significant improvement in the magnitude in the melt, while clay concentration was at 6% wt or higher. At similar condition, PEG dramatically reduced dynamic modulii and complex viscosity of PLA/PEG blend as function of concentration. A nanocomposite blend of PLA/PEG/clay (74/20/6) when compared to the neat polymer and PLA/PEG blend exhibited intermediate values of elastic modulus (G') and complex viscosity (η*) with excellent flexibility. Thermal analysis of different clay loading blends indicated that the melting temperature (T(m)) and glass transition temperature (T(g)) remained unaffected irrespective of clay concentration due to immobilization of polymer chain in the clay nanocomposite. PEG incorporation reduced the T(g) and the T(m) of the blends (PLA/PEG and PLA/PEG/clay) significantly, however, crystallinity increased in the similar condition. The transmission electron microscopy (TEM) image of nanocomposite films indicated good compatibility between PLA and PEG, whereas clay was not thoroughly distributed in the PLA matrix and remained as clusters. The percent crystallinity obtained by X-ray was significantly higher than that of differential scanning calorimeter (DSC) data for PLA.