Exciton dynamics in hybrid polymer/QD blends

Abstract The prospect of exploiting quantum dots (QDs) properties (tunable absorption spectrum, multiple exciton generation) while maintaining the flexible structure of polymer systems opens new possibilities in the photovoltaic field. Although charge transport dynamics in pristine polymer and QDs systems have been quite well established lately, a complete understanding of the charge transfer process between QDs and polymers when they are in blends is still lacking. In this work we used static and ultrafast fluorescence spectroscopy together with Atomic force Microscopy (AFM) to study the exciton dynamics in polymer/QDs films. Specifically we used poly(3-hexylthiophene) (P3HT) as the hole conducting donor material and the core shell CdSe(ZnS) QDs as the electron acceptor material. The QDs surface has been treated with two different capping ligands treatments: one based on the use of pyridine and the other one on hexanoic acid. The influence of the two different methods on the exciton dynamics and on the morphology will also be discussed. Blends containing differently treated P3HT/CdSe(ZnS) wt% ratios have been prepared producing films having uniform morphology and good intermixing, as proved by AFM measurements. Ultrafast fluorescence decays allowed us to compare the exciton dynamics in the polymer pristine respect to the treated P3HT/CdSe(ZnS) films. Efficient fluorescence quenching has been shown by both kind of blends respect to the pure polymer

Tube Expansion Deformation Enables In Situ Synchrotron X-ray Scattering Measurements during Extensional Flow-Induced Crystallization of Poly l-Lactide Near the Glass Transition

Coronary Heart Disease (CHD) is one of the leading causes of death worldwide, claiming over seven million lives each year. Permanent metal stents, the current standard of care for CHD, inhibit arterial vasomotion and induce serious complications such as late stent thrombosis. Bioresorbable vascular scaffolds (BVSs) made from poly l-lactide (PLLA) overcome these complications by supporting the occluded artery for 3–6 months and then being completely resorbed in 2–3 years, leaving behind a healthy artery. The BVS that recently received clinical approval is, however, relatively thick (~150 µm, approximately twice as thick as metal stents ~80 µm). Thinner scaffolds would facilitate implantation and enable treatment of smaller arteries. The key to a thinner scaffold is careful control of the PLLA microstructure during processing to confer greater strength in a thinner profile. However, the rapid time scales of processing (~1 s) defy prediction due to a lack of structural information. Here, we present a custom-designed instrument that connects the strain-field imposed on PLLA during processing to in situ development of microstructure observed using synchrotron X-ray scattering. The connection between deformation, structure and strength enables processing–structure–property relationships to guide the design of thinner yet stronger BVS

Effect of tungsten disulfide (WS_2) nanotubes on structural, morphological and mechanical properties of poly(L-lactide) (PLLA) films

Poly(L-lactide) (PLLA) is a semicrystalline, biocompatible and biodegradable polymer widely employed in many applications (food packaging, biomedical devices, drug delivery systems). This work deals with nanocomposites of PLLA and tungsten disulfide (WS_2) nanotubes (NTs) as a novel material to obtain thinner and stronger bioresorbable vascular scaffolds. We studied the influence of WS_2 NTs on the mechanical properties of PLLA-WS_2 films. Polarized optical microscopy reveals a high degree of orientation of the polymer molecules in stretched films that further increases with a post-stretching annealing treatment. At the same time, X-ray diffraction (XRD) and Raman spectroscopy confirm enhancement of the crystallinity induced by the WS_2 NTs

Effect of tungsten disulfide (WS_2) nanotubes on structural, morphological and mechanical properties of poly(L-lactide) (PLLA) films

Poly(L-lactide) (PLLA) is a semicrystalline, biocompatible and biodegradable polymer widely employed in many applications (food packaging, biomedical devices, drug delivery systems). This work deals with nanocomposites of PLLA and tungsten disulfide (WS_2) nanotubes (NTs) as a novel material to obtain thinner and stronger bioresorbable vascular scaffolds. We studied the influence of WS_2 NTs on the mechanical properties of PLLA-WS_2 films. Polarized optical microscopy reveals a high degree of orientation of the polymer molecules in stretched films that further increases with a post-stretching annealing treatment. At the same time, X-ray diffraction (XRD) and Raman spectroscopy confirm enhancement of the crystallinity induced by the WS_2 NTs

Tube Expansion Deformation Enables In Situ Synchrotron X-ray Scattering Measurements during Extensional Flow-Induced Crystallization of Poly l-Lactide Near the Glass Transition

Coronary Heart Disease (CHD) is one of the leading causes of death worldwide, claiming over seven million lives each year. Permanent metal stents, the current standard of care for CHD, inhibit arterial vasomotion and induce serious complications such as late stent thrombosis. Bioresorbable vascular scaffolds (BVSs) made from poly l-lactide (PLLA) overcome these complications by supporting the occluded artery for 3–6 months and then being completely resorbed in 2–3 years, leaving behind a healthy artery. The BVS that recently received clinical approval is, however, relatively thick (~150 µm, approximately twice as thick as metal stents ~80 µm). Thinner scaffolds would facilitate implantation and enable treatment of smaller arteries. The key to a thinner scaffold is careful control of the PLLA microstructure during processing to confer greater strength in a thinner profile. However, the rapid time scales of processing (~1 s) defy prediction due to a lack of structural information. Here, we present a custom-designed instrument that connects the strain-field imposed on PLLA during processing to in situ development of microstructure observed using synchrotron X-ray scattering. The connection between deformation, structure and strength enables processing–structure–property relationships to guide the design of thinner yet stronger BVS

Crimping-induced structural gradients explain the lasting strength of poly L-lactide bioresorbable vascular scaffolds during hydrolysis

Biodegradable polymers open the way to treatment of heart disease using transient implants (bioresorbable vascular scaffolds, BVSs) that overcome the most serious complication associated with permanent metal stents—late stent thrombosis. Here, we address the long-standing paradox that the clinically approved BVS maintains its radial strength even after 9 mo of hydrolysis, which induces a ∼40% decrease in the poly L-lactide molecular weight (Mn). X-ray microdiffraction evidence of nonuniform hydrolysis in the scaffold reveals that regions subjected to tensile stress during crimping develop a microstructure that provides strength and resists hydrolysis. These beneficial morphological changes occur where they are needed most—where stress is localized when a radial load is placed on the scaffold. We hypothesize that the observed decrease in Mn reflects the majority of the material, which is undeformed during crimping. Thus, the global measures of degradation may be decoupled from the localized, degradation-resistant regions that confer the ability to support the artery for the first several months after implantation

Crimping-induced structural gradients explain the lasting strength of poly L-lactide bioresorbable vascular scaffolds during hydrolysis

Biodegradable polymers open the way to treatment of heart disease using transient implants (bioresorbable vascular scaffolds, BVSs) that overcome the most serious complication associated with permanent metal stents—late stent thrombosis. Here, we address the long-standing paradox that the clinically approved BVS maintains its radial strength even after 9 mo of hydrolysis, which induces a ∼40% decrease in the poly L-lactide molecular weight (Mn). X-ray microdiffraction evidence of nonuniform hydrolysis in the scaffold reveals that regions subjected to tensile stress during crimping develop a microstructure that provides strength and resists hydrolysis. These beneficial morphological changes occur where they are needed most—where stress is localized when a radial load is placed on the scaffold. We hypothesize that the observed decrease in Mn reflects the majority of the material, which is undeformed during crimping. Thus, the global measures of degradation may be decoupled from the localized, degradation-resistant regions that confer the ability to support the artery for the first several months after implantation

WS2 nanotubes as a 1D functional filler for melt mixing with poly(lactic acid) : implications for composites manufacture

Multi-walled WS2 nanotubes (NTs) with lengths ranging from 2 to 65 μm and widths from 50 to 110 nm were synthesized in a horizontal quartz-made reactor by a process yielding NTs with aspect ratios (ARs) between ∼40 and >1000. The NTs obtained were thermally stable in air up to 400 °C but were oxidized within the temperature range 400–550 °C to produce yellow WO3 particles. Critically, 400 °C is well above the temperature used to mix additives with the majority of melt-processable polymers. The hydrophilic WS2 NTs were easily dispersed in poly(lactic) acid (PLA) using a twin-screw extruder, but the shear stresses applied during melt mixing resulted in chopping of the NTs such that the AR decreased by >95% and the tensile mechanical properties of the PLA were unchanged. Although the as-extruded unfilled PLA was >99% amorphous, the much-shortened WS2 NTs had a significant effect on the crystallization behavior of PLA, inducing heterogeneous nucleation, increasing the crystallization temperature (Tc) by ∼3 °C and the crystalline content by 15%, and significantly increasing the rate of PLA crystallization, producing smaller and more densely packed spherulites. The reduction in the AR and the nucleating effect of WS2 NTs for PLA are critical considerations in the preparation, by melt mixing, of composites of rigid 1D NTs and polymers, irrespective of the target application, including bone tissue engineering and bioresorbable vascular scaffolds

Charge Transfer Properties of Surface-treated WS2 Nanotubes and Fullerene-like Nanoparticles

We studied the effect of incorporation of inorganic fullerene like nanoparticles (IF) and inorganic nanotubes (INT) of WS2 into device structures. In order to disperse in a uniform fashion the semiconducting INT/IF WS2 nanoparticles were functionalized with dodecyltrichlorosilane (DTS). Poly-3(hexylthiophene) (P3HT) has been used along with WS2 nanoparticles as an active layer for the solution processable material in the proposed OLED structure of the type: ITO/WS2/P3HT/LiF-Al to test the electrical effect of the WS2 and to obtain information on its energy levels. Based on the obtained results, we discuss the possibility to use the WS2 nanoparticles in organic electronic devices