Annealing-Induced Oriented Crystallization and Its Influence on the Mechanical Responses in the Melt-Spun Monofilament of Poly(l-lactide)

Regulating mechanical performance of poly (l-lactide acid) stent by the combined effects of heat and aqueous media

Annealing process has been applied to the development of thermoforming polymer braided stent and treating its basic constitute monofilaments, especially for Poly (l-lactide acid) (PLLA) condensed by lactic acid monomer made from the plant starch. In this work, high performance monofilaments were produced by melting spun and solid-state drawing methods. Inspired by the effects of water plasticization on semi-crystal polymer, PLLA monofilaments were annealed with and without constraint in vacuum and aqueous media. Then, the co-effects of water infestation and heat on the micro-structure and mechanical properties of these filaments were characterized. Furtherly, mechanical performance of PLLA braided stents shaped by different annealing methods was also compared. Results showed that annealing in aqueous media generated more obvious structure change of PLLA filaments. Interestingly, the combined effects of aqueous phase and thermal effectively increased the crystallinity, and decreased the molecular weight and orientation of PLLA filaments. Therefore, higher modulus, smaller strength, and elongation at the break for filaments could be obtained, which could furtherly realize better radial compression resistance of the braided stent. This annealing strategy could provide new perspectives between anneal and material properties of PLLA monofilaments, and provide more suitable manufacturing technics for polymer braided stent.

Modeling and Analysis of Morphology of Injection Molding Polypropylene Parts Induced by In-Mold Annealing

It is generally recognized that high-temperature treatments, namely annealing, influence the microstructure and the morphology, which, in turn, determine the mechanical properties of polymeric parts. Therefore, annealing can be adopted to control the mechanical performance of the molded parts. This work aims to assess the effect of annealing on the morphology developed in isotactic polypropylene (iPP) injection-molded parts. In particular, a two-step annealing is adopted: the polymer is injected in a mold at a high temperature (413 or 433 K), which is kept for 5 min (first annealing step); afterward, the mold temperature is cooled down at 403 K and held at that temperature for a time compatible with the crystallization half-time at that temperature (second annealing step). The characterization of morphology is carried out by optical and electronic scanning microscopy. The temperature of the first annealing step does not influence the thickness of the fibrillar skin layer; however, such a layer is thinner than that found in the molded parts obtained without any annealing steps. The second annealing step does not influence the thickness of the fibrillar skin layer. The dimension of spherulites found in the core is strongly influenced by both annealing steps: the spherulite dimensions enlarge by the effect of annealing steps. A model that considers spherulite and fibril evolutions is adopted to describe the effect of molding conditions on the final morphology distribution along the part thickness. The model, which adopts as input the thermo-mechanical histories calculated by commercial software for injection molding simulation, consistently predicts the main effects of the molding conditions on the morphology distributions.

Fabrication of piezoelectric poly(L-lactic acid)/BaTiO3 fibre by the melt-spinning process

Poly(l-lactic acid) (PLLA) based piezoelectric polymers are gradually becoming the substitute for the conventional piezoelectric ceramic and polymeric materials due to their low cost and biodegradable, non-toxic, piezoelectric and non-pyroelectric nature. To improve the piezoelectric properties of melt-spun poly(l-lactic acid) (PLLA)/BaTiO₃, we optimized the post-processing conditions to increase the proportion of the β crystalline phase. The α → β phase transition behaviour was determined by two-dimensional wide-angle x-ray diffraction and differential scanning calorimetry. The piezoelectric properties of PLLA/BaTiO₃ fibres were characterised in their yarn and textile form through a tapping method. From these results, we confirmed that the crystalline phase transition of PLLA/BaTiO₃ fibres was significantly enhanced under the optimised post-processing conditions at a draw ratio of 3 and temperature of 120 °C during the melt-spinning process. The results indicated that PLLA/BaTiO₃ fibres could be a one of the material for organic-based piezoelectric sensors for application in textile-based wearable piezoelectric devices.

Mucoadhesive Electrospun Fibre-Based Technologies for Oral Medicine

Oral disease greatly affects quality of life, as the mouth is required for a wide range of activities including speech, food and liquid consumption. Treatment of oral disease is greatly limited by the dose forms that are currently available, which suffer from short contact times, poor site specificity, and sensitivity to mechanical stimulation. Mucoadhesive devices prepared using electrospinning offer the potential to address these challenges by allowing unidirectional site-specific drug delivery through intimate contact with the mucosa and with high surface areas to facilitate drug release. This review will discuss the range of electrospun mucoadhesive devices that have recently been reported to address oral inflammatory diseases, pain relief, and infections, as well as new treatments that are likely to be enabled by this technology in the future.

Revealing non-crystalline polymer superstructures within electrospun fibers through solvent-induced phase rearrangements

The design of nanofibers for biomedical applications requires a deep understanding of the fiber formation process and the resulting internal structure. In this regard, non-crystalline, mesomorphic structures play a central role in the processing of many polymers as precursors in the formation of crystalline superstructures (e.g. shish-kebab) and influence strongly the physical properties of polymers with a low degree of crystallinity. Yet, our ability to probe these relevant features is often greatly limited by their low contrast differences with the amorphous phase. We present an approach to reveal the organization of the mesomorphic superstructures within such polymeric materials, on the example of electrospun poly(l-lactide) nanofibers. Based on solvent-induced crystallization, this method employs fine-tuned solvent/non-solvent systems to enhance the contrast of these structural features by selectively triggering and controlling reorganization of the phases. Hereby, the mesomorphic regions are transformed into an α-crystalline phase, while the nanoscale spatial arrangement of the underlying superstructures is preserved. Combined with X-ray analytical techniques and electron microscopy, our approach provides detailed insights into the nanofiber's inner architecture, allowing for its direct visualization. Thereby, the influence of electrospinning parameters on the fiber formation process is explained as well as the impact of the resulting non-crystalline superstructures on single fiber mechanical properties. The method can be applied to comparable polymers for the development of materials with controlled, tailored properties.

Influence of poly(lactide) stereocomplexes as nucleating agents on the crystallization behavior of poly(lactide)s

The influence of the addition of linear and four-arm poly(lactide) stereocomplexes on the crystallization behavior of poly(l-lactide) and poly(d-lactide) from the molten state was investigated.

The crystallization behaviors and mechanical properties of poly(l-lactic acid)/magnesium oxide nanoparticle composites

Nanocomposites of biodegradable PLLA and magnesium oxide composite (PLLA/MgO-NPs) and surface modified magnesium oxide composite (PLLA/m-MgO-NPs) were prepared using a solution casting method.

Unusual structural evolution of poly(lactic acid) upon annealing in the presence of an initially oriented mesophase

Uniaxial deformations of amorphous poly(lactic acid) (PLA) films were performed at two different temperatures, 70 and 80 °C, at various draw strains. The samples deformed at 70 °C showed a strain-induced mesophase, and the structural ordering and thermal stability increased as the draw strain increased. Further annealing was performed in situ at constant length at the drawing temperature of 70 °C for the films drawn to strains of 100% and 230%. Unusually, we found that after annealing, the crystal structure of the film at lower strain was more ordered than the one at higher strain. Further investigations revealed that upon annealing the structural evolution followed a distinct molecular mechanism for the samples stretched to the two draw strains. For the sample drawn to 100%, the mesophase melted very quickly upon annealing, resulting in chain randomization and the release of the constraints on the thermodynamic relaxation of the oriented amorphous chains. The chain relaxation motions had a beneficial effect on the occurrence of the conformational rearrangements that are necessary for crystalline ordering. By contrast, for the 230% sample, the melting of the mesophase was slow and most of the chain orientations were preserved upon annealing. As a result, a less ordered crystal structure was formed since the local relaxation motions that are necessary for promoting crystalline order via conformational rearrangements were hindered.

Crystalline structure changes in preoriented metallocene-based isotactic polypropylene upon annealing

Partially melted metallocene-based isotactic polypropylene (m-iPP), which was preoriented with a high degree of molecular orientation and a shish-kebab structure, was annealed at various temperatures and isothermally crystallized at 130 °C. The melting and crystallization process was examined using synchrotron wide-angle X-ray diffraction, small-angle X-ray scattering, and differential scanning calorimetry. For the m-iPP samples annealed at relatively low temperatures, lamellar thickening, lateral growth, and a decrease in the γ-crystal fraction occurred. Because of parallel evolution of α- and γ-crystal growth in the limited crystallizable melt volume, the fraction of γ-crystals was very low. Furthermore, topological constraints in the melt dominate the chain flux in crystal evolution; the chains are consumed by the thickening lamellae and lateral growth, forming α-crystals with parallel chains in the unit cell. For the m-iPP samples isothermally annealed at medium annealing temperatures, the increase in the amount of crystallizable melt caused the γ-crystal fraction to increase. A shish-kebab (α-crystals) structure with high thermal stability and a newly formed macro-unoriented structure coexisted in the final sample. After annealing at high temperatures, at which no crystals survived, γ-crystal formation was greatly favored; this was attributed to the nature of m-iPP molecules and their dynamic behavior at 130 °C. Because of the lack of oriented nuclei, randomly oriented lamellae were formed. On the basis of the structural cooperative changes at different scales, the morphological features at different annealing temperatures were proposed.