Development of Stereocomplex Crystal of Polylactide in High-Speed Melt Spinning and Subsequent Drawing and Annealing Processes

High-speed melt spinning of racemate polylactide (r-PLA), which is a blend of equal amounts of poly(l-lactide) and poly(d-lactide) molecules, was performed up to the take-up velocity of 7.5 km/min. In the fiber structure analysis, particular attention was paid to the formation of stereocomplex crystals, because this crystal form has a melting temperature about 60° higher than the homocrystals. It was found that highly oriented and highly crystallized fibers containing the α-form and stereocomplex crystals were obtained when the take-up velocity exceeded about 4 km/min. The amount of stereocomplex crystal was higher under the spinning conditions of higher take-up velocity, lower throughput rate, and lower extrusion temperature. Under these conditions, higher tensile stress can be applied to the spinning line, and therefore, the orientation-induced crystallization is promoted. Annealing of the fibers obtained at high-take-up velocities, such as 6 km/min, which already have the crystalline structure with a certain amount of stereocomplex crystal, at a temperature between the melting temperatures of α-form and stereocomplex crystals, yielded the fiber structure mainly consisting of highly oriented stereocomplex crystal. The annealed fibers showed fairly high mechanical properties and good thermal stability.     

High-Speed Melt Spinning of Polylactide/Poly(Butyleneterephthalate) Bicomponent Fibers: Mechanism of Fiber Structure Development and Dyeing Behavior

Bicomponent fibers consisting of polylactide (PLA) as the sheath and poly(butylene terephthalate) (PBT) as the core were produced by high-speed spinning to obtain materials suitable for medical clothing. The higher-order structure of the PLA fiber component appeared to exhibit simple, alternately stacked, uniaxially oriented amorphous and crystalline regions. Therefore, fairly large tanδ peaks were observed for single-component PLA fibers, even when the orientation-induced crystallization was achieved by high-speed spinning. By conjugating PLA with PBT, although limited mutual interference with the crystallization of each component occurred, both the PLA (Mw?=?170,000, L-lactide content?=?98.7%) and PBT (intrinsic viscosity?=?0.835-0.865 dL/g) could crystallize on a high-speed spinning line, and the proposed formation of a shish-kebab-like structure in the PBT component enhanced the thermal stability of the bicomponent fibers, particularly resulting in shrink-proof properties. The bicomponent fibers developed herein could be deeply dyed at 98?°C, with results comparable to those of industrial polyester, and peeling of the PLA skin layer was rarely observed, even when the dyed fibers were flattened by a rubbing force.     

Thermal analysis applied to the characterization of degradation in soil of polylactide: II. On the thermal stability and thermal decomposition kinetics

The disposal stage of polylactide (PLA) was assessed by burying it in active soil following an international standard. Degradation in soil promotes physical and chemical changes in the polylactide properties. The characterization of the extent of degradation underwent by PLA was carried out by using Thermal Analysis techniques. In this paper, studies on the thermal stability and the thermal decomposition kinetics were performed in order to assess the degradation process of a commercial PLA submitted to an accelerated soil burial test by means of multi-linear-non-isothermal thermogravimetric analyses. Results have been correlated to changes in molecular weight, showing the same evolution as that described by the parameters of thermal stability temperatures and apparent activation energies. The decomposition reactions can be described by two competitive different mechanisms: Nucleation model (A2) and Reaction Contracting Volume model (R3). The changes in the kinetic parameters and kinetic models are in agreement with the calorimetric and dynamic-mechanical-thermal results, presented in the Part I of the study [1].     

Effects of transesterification and degradation on properties and structure of polycaprolactone-polylactide copolymers

Ester bonds of biodegradable polymers, such as poly (α-hydroxy esters), synthesized by the coordinated anionic ring opening polymerization (CAROP), are subject to transesterification during synthesis. In this work, a series of poly(?-caprolactone)-poly(l-lactide) (PCL-PLA) block and random copolymers with targeted molar mass of 10,000 Da was synthesized to study the mechanism of transesterification reactions via NMR and MALDI-TOF.Polylactide segments are more vulnerable to transesterification compared to polycaprolactone. As a result, the actual quantity of l-lactide in the polymers was less than the target for all studied copolymers because of the decarboxylation and consequent CO elimination from fragments of macromolecules after transesterification. The presence of decarboxylation during transesterification was confirmed analytically and was reflected in the MALDI-TOF and ¹³C NMR spectra. An analysis of the polymer structure pointed to dehydration reactions that led to the formation of cyclic structures and double bonds with possible crosslinking.     

Crystallographic characterisation of novel Zn(II) silsesquioxane complexes and their application as initiators for the production of polylactide

Herein, the solid state structures of the products from the reaction of the silsesquioxane triol (iso-C₄H₉)₇Si₇O₁₂(OH)₃ (1) with two equivalents of ZnMe₂ in both THF and toluene are reported. In both cases tetrametallic Zn(II) complexes were isolated, with toluene [(iso-C₄H₉)₇Si₇O₁₂]₂Zn₄Me₂ (2) was prepared while performing the reaction in THF the analogous complex [(iso-C₄H₉)₇Si₇O₁₂]₂Zn₄Me₂(THF)₂ (3) was formed. Both species have also been characterised via¹H, ¹³C{¹H} and ²⁹Si{¹H} NMR spectroscopy, which confirm the solid state structures are maintained in solution. Both 2 and 3 show modest activities for the polymerisation of rac-lactide and a heterogeneous catalyst has also been prepared.     

Synthesis of cyclic poly(l‐lactide) catalyzed by Bismuth Salicylates—A combination of two drugs

l ‐lactide was polymerized in bulk at 160 or 180°C with mixtures of bismuth subsalicylate (BiSub) and salicylic (SA) as catalysts. The SA/Bi ratio and the monomer/Bi ratio were varied. The highest molecular weights (weight average, Mw) were achieved at a SA/Bi ratio of 1/1 (Mw up to 92 000 g mol^?1). l ‐Lactide was also polymerized with combinations of BiSub and silylated SA, and Mw values up to 120 000 g mol^?1 were achieved at 180°C. MALDI‐TOF mass spectrometry and Mark‐Houwink‐Sakurada measurements proved that under optimized reaction conditions the resulting polylactides consist of cycles. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 2056–2063     

Synthesis and X-ray crystal structure of dichloro[S-1-phenyl-N-(S-pyrrolidin-2-ylmethyl)ethanamine]zinc(II) and its catalytic application to rac-lactide polymerization

New dichloro zinc(II) complex ligated by the homochiral bidentate ligand S-1-phenyl-N-(S-pyrrolidin-2-ylmethyl)ethanamine (PPMA) was synthesized and characterized by X-ray crystallography. The geometry of the (PPMA)ZnCl₂ is a distorted tetrahedron comprising of zinc metal as a center linked with two N atoms of the PPMA in a bidentate coordination mode along with two chloro ligands. The catalytic capacity of the complex was evaluated in ring opening polymerization (ROP) of rac-lactide. The active catalyst species was generated in situ by treating MeLi to complex (PPMA)ZnCl₂. The dimethyl derivative of the (PPMA)ZnCl₂ showed highly activity in ROP of rac-lactide and gave preference to heterotactic polylactide.     

Synthesis and characterization of polylactide functionalized polyacetylenes

The paper deals with the synthesis and polymerization of novel poly(l-lactide)-derived acetylene monomers and the analysis of the thermal properties of the formed graft copolymers. Poly(l-lactide) macromonomers with different acetylene end groups were prepared using stannous octanoate as a catalyst in the presence of various hydroxyacetylenes. Next, the well-characterized macromonomers were subjected to polymerization using [{RhCl(nbd)}₂]/Et₃N and [RuCl₂(CH–o–OiPrC₆H₄)(IMesH₂)] to obtain graft copolymers. Investigation of these graft copolymers by GPC and NMR spectroscopy revealed the presence of some poly(l-lactide) formed as a side product during the ring opening polymerization of l-lactide. The thermal stability of the polymeric materials has been studied as a function of the polyacetylene backbone substituents and the length of poly(l-lactide) side chains. Introducing polyacetylene into polyester increased the polymer stability. The thermal degradation behavior of the synthesized materials depends on the length of poly(l-lactide) chains and also on l-lactide homopolymer impurities in the graft copolymers.     

Micro-molding with ultrasonic vibration energy: New method to disperse nanoclays in polymer matrices

Ultrasound technology was proved as an efficient processing technique to obtain micro-molded specimens of polylactide (PLA) and polybutylene succinate (PBS), which were selected as examples of biodegradable polyesters widely employed in commodity and specialty applications. Operational parameters such as amplitude, molding force and processing time were successfully optimized to prepare samples with a decrease in the number average molecular weight lower than 6%.Ultrasonic waves also seemed an ideal energy source to provide effective disaggregation of clay silicate layers, and therefore exfoliated nanocomposites. X-ray diffraction patterns of nanocomposites prepared by direct micro-molding of PLA or PBS powder mixtures with natural montmorillonite or different organo-modified clays showed the disappearance of the 0 0 1 silicate reflection for specimens having up to 6 wt.% clay content. All electron micrographs revealed relatively homogeneous dispersion and sheet nanostructures oriented in the direction of the melt flow.Incorporation of clay particles during processing had practically no influence on PLA characteristics but enhanced PBS degradation when an organo-modifier was employed. This was in agreement with thermal stability data deduced from thermogravimetric analysis. Cold crystallization experiments directly performed on micro-molded PLA specimens pointed to a complex influence of clay particles reflected by the increase or decrease of the overall non-isothermal crystallization rate when compared to the neat polymer. In all cases, the addition of clay led to a clear decrease in the Avrami exponent.     

Molecular dynamics and crystallization precursors in polylactide and poly(lactide)/CNT biocomposites in the insulating state

The early stages of the cold crystallization process and the formation of a rigid amorphous phase as seen by the dielectric response of polylactide, PLA, and composites polylactide/carbon nanotubes, PLA/CNT, are studied here by broadband dielectric spectroscopy for CNT concentrations below percolation. The presence of precursors during the nucleation and crystallization process is demonstrated by the existence of a time shift between the decline in the number of mobile segments and the growth of a 3D ordered phase as seen by variable temperature wide angle X-ray scattering measurements. Also, the loss of the mobile amorphous phase is not justified by the slow lamellar growth in identical conditions. The variation of the molecular dynamics, either for short range reorientations or cooperative motions, is followed in both amorphous and semicrystalline states. The changes observed in the composites PLA/CNT show a moderate heterogeneous nucleating effect of the nanofiller and a sensitivity of the three subglass transitions to the chain ordering. The relaxation parameters of the segmental relaxation are not very sensitive to the presence either of lamellae or of the nanofiller.