Unique Morphology in Polylactide/Graphene Oxide Nanocomposites

Polylactide (PLA)/graphene oxide (GO) nanocomposites with different GO loadings were prepared by solution blending using tetrahydrofuran (THF) as solvent. The morphologies of the nanocomposites in the cast films and in subsequently isothermally crystallized samples were investigated separately. According to polarized optical microscopy images of the composites after isothermal crystallization, it was deduced that GO had nucleation effects on the crystallization of PLA and led to the size of spherulites decreasing and the number of spherulites increasing. However, by virtue of SEM, unique microsphere morphologies were found in the cast films of the PLA/GO composites. It was found that the features of this kind of microsphere were not influenced by the compositions but by the evaporation rate of the solvent during the preparation of the cast films. A faster evaporation rate of solvent resulted in smaller size of the microspheres. Based on DSC and FTIR results, it was proposed that the forming of this kind of microsphere was related to hydrogen bonds being formed between PLA and GO. The unique microsphere morphology is suggested to provide a new method to prepare PLA microsphere-based scaffolds.     

NOTE: Polymerization of Cyclic Esters Using Aminoacid Initiators

The low-molecular weight poly(? -caprolactone) and polylactide were obtained by the polymerization of cyclic esters in the presence of amino acid initiators. The polymer structures were elucidated by means of MALDI TOF, NMR and IR studies. Effects of temperature, reaction time and initiator dosage on the polymerization process were examined.     

Use of maleic anhydride compatibilization to improve toughness and other properties of polylactide blended with thermoplastic elastomers

Polylactide (PLA) being a very brittle biopolymer could be toughened by blending with thermoplastic elastomers such as thermoplastic polyurethane elastomer (TPU) and thermoplastic polyester elastomer (TPE); unfortunately, these blends are immiscible forming round domains in the PLA matrix. Therefore, the purpose of this study was to investigate the effects of using maleic anhydride (MA) compatibilization on the toughness and other properties of PLA blended with TPU and TPE. MA grafting on the PLA backbone (PLA‐g‐MA) was prepared separately by reactive extrusion and added during melt blending of PLA/thermoplastic elastomers. IR spectroscopy revealed that MA graft might interact with the functional groups present in the hard segments of TPU and TPE domains via primary chemical reactions, so that higher level of compatibilization could be obtained. SEM studies indicated that PLA‐g‐MA compatibilization also decreased the size of elastomeric domains leading to higher level of surface area for more interfacial interactions. Toughness tests revealed that Charpy impact toughness and fracture toughness (K_IC and G_IC) of inherently brittle PLA increased enormously when the blends were compatibilized with PLA‐g‐MA. For instance, G_IC fracture toughness of PLA increased as much as 166%. It was also observed that PLA‐g‐MA compatibilization resulted in no detrimental effects on the other mechanical and thermal properties of PLA blends. Copyright © 2014 John Wiley & Sons, Ltd.     

Amphiphilic PEG/alkyl‐grafted comb polylactides

Amphiphilic polylactides (PLAs) with well‐defined architectures were synthesized by ring‐opening polymerization of AB monomers (glycolides) substituted with both a long chain alkyl group and a triethylene glycol segment terminated in either a methyl or benzyl group. The resulting amphiphilic PLAs had number average molecular weights >100,000 g/mol. DSC analysis revealed a first‐order phase transition at ～ 20 °C, reflecting the crystalline nature of the linear alkyl side chains. Polymeric micelles were prepared by the solvent displacement method in water. Dynamic light scattering measurements support formation of a mixture of 20‐nm‐diameter unimolecular micelles and 60‐nm particles comprised of an estimated 25 polymer molecules. UV–vis characterization of micelles formed from acetone–water solutions containing azobenzene confirmed encapsulation of the hydrophobic dye, suggesting their potential as new amphiphilic PLAs as drug delivery vehicles. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5227–5236, 2007     

Microcellular foaming of polylactide and poly(butylene adipate‐co‐terphathalate) blends and their CaCO3 reinforced nanocomposites using supercritical carbon dioxide

Foamed polylactide (PLA), PLA–PBAT (poly (butylene adipate‐co‐terphathalate)) blend and their composites with CaCO₃ were prepared in a batch process using supercritical carbon dioxide (CO₂) at 12 MPa and 45°C. The solubility of CO₂ and its diffusion patterns in different PLA samples was investigated. PLA systems had a relatively high CO₂ solubility related to the carboxyl groups. CO₂ desorption behaviors in PLA systems first followed the Fickian diffusion mechanism in short time and then decreased slowly to a plateau. The addition of both PBAT and CaCO₃ into PLA impeded the desorption of CO₂. In the presence of second phase PBAT, nanoparticles CaCO₃ and dissolved CO₂, the PLA crystallization behavior investigated by DSC technique was greatly changed. As the desorption time increased, the gas induced crystallinity slightly decreased in consequence of less CO₂ content in each system and thus less plasticization effect. The cell morphology of foamed PLA and PLA composites showed interesting microstructure patterns. The prepared pure PLA foam exhibits a typical bimodal structure because of the foaming in both the amorphous and crystalline zones. With PBAT and CaCO₃ into PLA, the composite foam presented significant increase in cell uniformity and cell density. With less CO₂ content in each PLA sample, the cell structure showed interesting variation. Pure PLA foam presented transition from bimodal structure to more uniform cell structure with decreased cell density. In contract, PLA–PBAT foam show unfoamed regions because of none CO₂ left in the separated PBAT phase. Copyright © 2016 John Wiley & Sons, Ltd.     

Role of dispersion stabilizer with hydroxy groups in preparation of monodisperse polylactide microspheres

Monodisperse poly(D ,L ‐lactide) (PDLLA) microspheres were prepared by dispersion polymerization of D ,L ‐lactide in xylene/heptane (1/2, v/v) with poly[(dodecyl methacrylate)‐co‐(2‐hydroxyethyl methacrylate)] (P(DMA‐co‐HEMA)) as a dispersion stabilizer. P(DMA‐co‐HEMA) contains hydroxy groups, which act as an initiation group for pseudoanionic dispersion polymerization. The best coefficient of variation (CV) values concerning particle diameter distribution and the particle diameter of obtained PDLLA microspheres were 3.7% and 5.3 μm, respectively. The particle diameter decreased with increasing concentration of P(DMA‐co‐HEMA) and HEMA maintained low CV (<10%) values. As a result, monodisperse PDLLA microspheres ranging from 1.3 to 5.3 μm were obtained. In addition, it was found that monodisperse PDLLA microspheres were obtained by sufficient capture of growing polymers and monomers in the particle growth stage. Therefore, the HEMA concentration in P(DMA‐co‐HEMA) strongly affecting the capturing capability is the most important factor. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5230–5240, 2009     

Plasticized polylactide/clay nanocomposites. II. The effect of aging on structure and properties in relation to the filler content and the nature of its organo‐modification

Plasticized polylactide (PLA) – layered silicate nanocomposites were obtained by melt blending PLA with polyethylene glycol as plasticizer (20 wt %) and with different montmorillonite fillers: Cloisite® 20A, Cloisite® 25A, and Cloisite® 30B (from 1 to 10 wt %). Comparative samples of melt‐blended polylactide (without filler) and plasticized PLA with 20 wt % PEG were considered as well. Samples have been aged for 1 and 4 years and their chemical and physical characteristics were compared with not aged reference ones. It was found that molecular weight of the PLA decreased upon melt‐processing and aging, particularly when the Cloisite content increased, without a clear relation to the nature of the organo‐modifier. On the contrary, the PEG plasticizer was practically undegraded upon melt processing and aging. Structural studies revealed that plasticized PLA and plasticized PLA‐based nanocomposites are unstable in time of aging and undergo deplasticization. They showed, after aging, the presence of a thin PEG crystalline layer at the surface of the samples and improved the order in the PLA matrix to a higher extent in plasticized polylactide than in plasticized nanocomposite (due to clay stabilization effect). The amount of PEG diffusing toward sample surface was correlated with aging time, molecular weight of PLA matrix, and Cloisite® type, in clear relation to the extent of intercalation with PLA and PEG. Some modifications of the viscoelastic properties of PLA matrix, induced by the presence of both the nanoparticlate filler and the plasticizer, as well as a deterioration of the mechanical properties upon aging were observed. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 312–325, 2006     

Drug Release Behavior of a Drug-Loaded Polylactide Nanofiber Web Prepared via Laser-Electrospinning

In recent years drug-loaded nanofibers prepared using solution electrospinning methods have been actively studied. However, there are a number of problems connected to their solution electrospinning with respect to medical applications because of the hazards associated with the residual solvents. To avoid the use of solvents in this study we prepared and evaluated drug-loaded polylactide (PLA) fiber webs using a laser-electrospinning (LES) type of a melt electrospinning process. The structures and properties of the obtained drug-loaded PLA fiber webs were evaluated by scanning electron microscopy, fluorescence microscopy, wide-angle X-ray diffraction and UV–vis spectrometry. As shown by the various characterization techniques, we employed LES to prepare PLA nanofiber webs with average fiber diameters of 4.21 and 0.67?μm. Additionally, the webs were loaded with argatroban, a thrombin inhibitor, resulting in amorphous structures for both the argatroban and the PLA matrix. An in-vitro investigation of the drug release behavior of the webs revealed that higher release rates occurred for the fiber samples with the small fiber diameters, particularly in comparison with melt spun fibers with an average diameter of 150?μm. Overall, we expect that the herein described drug-loaded PLA nanofiber webs can be applied as medical materials with drug delivery system functions.     

碳纤维增强羟基磷灰石/聚乳酸复合生物材料的制备和力学性能

为了提高羟基磷灰石粒子和碳纤维与聚乳酸之间的粘接力和相互作用力，对羟基磷灰石和碳纤维分别进行表面改性。羟基磷灰石采用表面接枝聚乳酸的方法，结果在红外谱图中1740cm^-1处出现了羰基的吸收峰，经热失重分析接枝率达到5％。碳纤维则用硝酸进行氧化处理后表面变粗糙，有效地提高了与聚乳酸基体的界面结合力。采用溶液共混-热压方法制备了碳纤维增强羟基磷灰石／聚乳酸三元复合生物材料，研究了其剪切强度随羟基磷灰石含量和体外降解时间的变化关系。复合材料的剪切强度随HA含量的增加存在一个峰值，最大剪切强度为340MPa。在37℃PBS模拟体液中体外降解16周后，剪切强度维持在190MPa左右，有很好的强度保持性，观察复合材料的微观结构发现，聚乳酸降解后碳纤维和基体之间的界面出现空隙。