Fluorescence microscopy tracking of dyes, nanoparticles and quantum dots during growth of polymer spherulites

Continuous Spectrum of Morphologies and Phase Behavior across the Contact Zone from Poly(l-lactide) to Poly(d-lactide): Stereocomplex, Homocrystal, and Between

The enantiomeric ratio is a key factor affecting the crystallization behavior and morphology of poly-l-lactide/poly-d-lactide (PLLA/PDLA) blends. Despite a number of studies on crystallization of nonequimolar PLLA/PDLA blends, a full picture of the effect of the L/D ratio is still lacking. Here, we put the two enantiomers in contact and allow interdiffusion above the melting point of the stereocomplex crystal (SC) to prepare samples with a continuously changing L/D ratio from enantiopure PLLA (ratio 0/100) to enantiopure PDLA (100/0). Using polarized optical microscopy, atomic force microscopy, and microbeam X-ray diffraction, the continuous spectrum of morphologies and phase behaviors across the contact zone is investigated. The blend morphology shows clear evidence of "poisoning by purity" of SC crystallization at all blend compositions. The low birefringence of the 50/50 SC is found to be due to the meandering of broken edge-on lamellae. Its further decrease to near zero as L/D deviates further away from 50/50 is explained by transition from radial edge-on lamellae to fully random meandering lamellae, then to mixed flat-on lamellae, and finally to submicron-sized axialites. In comparison with the smooth and straight homocrystal (HC) lamellae of pure enantiomers, the lamellae in the blends often have serrated edges caused by pinning by rejected excess enantiomer acting as an impurity during lamellar growth. A feature of the binary phase diagram is pure enantiomers acting as an impurity to the SC and counter-enantiomer acting as an impurity to homocrystallization of the enantiomers. Crystallization was found to be most suppressed at 99% enantiomeric purity, where the amount of the counter-enantiomer is insufficient for creation of SC nuclei and HC growth is inhibited by the small amount of the enantio-impurity. These and other intriguing results are less likely to be noticed without the continuous composition gradient of the contact sample.

3D Morphology of Different Crystal Forms in β-Nucleated and Fiber-Sheared Polypropylene: α-Teardrops, α-Teeth, and β-Fans

Polymorphism of semicrystalline polymers has significant influence on their physical properties, with each form having its advantages and disadvantages. However, real-life polymer processing often results in different coexisting crystal polymorphs, and it remains a challenge to determine their shape, spatial distribution, and volume fraction. Here, i-polypropylene (i-PP) sheets containing both α- and β-forms were prepared either by adding β-nucleating agent or by fiber pulling-induced crystallization. By adding a compatible dye that is partially rejected from the growing crystalline aggregates (spherulites and cylindrites), we visualize the shape of these objects in 3D using two-photon fluorescence confocal microscopy. To distinguish between crystal forms, we take advantage of the difference in dye-retaining ability of the α- and β-aggregates. Even in 2D, fluorescence microscopy (FM) distinguishes the two crystal forms better than polarized microscopy. In 3D imaging, the volume fraction and spatial distribution of α- and β-forms in different morphological types could be determined quantitatively. Morphologies described as α-teeth, β-fans, and α-teardrops were visualized for the first time in 3D. Furthermore, internal and surface microcracks were seen to be associated predominantly with the β-form and around the fiber. Spatial distribution of α- and β-forms was also determined by scanning with a synchrotron X-ray beam. Good agreement was obtained with 3D microscopy, but XRD could not match the detail obtainable by the tomography. The work demonstrates the ability of the 3D imaging method to distinguish different crystal forms and their specific morphologies.

Real-Time Imaging of Stress in Single Spherulites and Its Relaxation at the Single-Particle Level in Semicrystalline Polymers

Crystallization-induced microscopic stress and its relaxation play a vital role in understanding crystallization behavior and mechanism. However, the real-time measurements for stress and its relaxation seem to be an unachievable task due to difficulties in simultaneous labeling, spatiotemporal discrimination, and continuous quantification. We designed a micron-sized fluorescent probe, whose fluorescence can respond to stress-induced environmental rigidity and whose three-dimensional (3D) flow can respond to stress relaxation. Using the as-prepared fluorescent probe, we established a versatile strategy to realize the real-time 3D imaging of stress and its relaxation in the crystallization process. The rigidity-responsive fluorescence clearly indicated the stress, while the 3D flow movement could quantify the stress relaxation. It is revealed that stress in spherulites increased dramatically as a result of the suppression of stress relaxation in polymer melts. The developed method provides a novel avenue to simultaneously detect stress and its relaxation in various semicrystalline polymers at the single-particle level. This success would achieve the microscopic ways to guide the development of advanced crystallization-dependent materials.

Morphology of Shear-Induced Polymer Cylindrites Revealed by 3D Optical Imaging

Two-photon confocal laser microscopy was used to obtain three-dimensional (3D) images of the morphology of poly(lactic acid) after shear-induced crystallization. The necessary fluorescence contrast was achieved by doping the polymer with Nile Red. The dye gets partially rejected from the growing crystalline aggregates during their formation, thus creating a renderable high-low fluorescence boundary outlining the shape of the aggregates. Parallel-plate melt-shearing and pulling a glass fiber through the melt were used as the two methods to achieve shear-induced crystallization. This study focuses on the shape of the resulting cylindrites, i.e., large-diameter shish-kebabs. The first 3D images of polymer cylindrites show that, if far from boundaries, they are circular cylinders, highly regular after fiber pull, but less so after parallel-plate shear. In the latter case, the cylindrite reveals the trajectory of the foreign particle that had nucleated its growth. Interestingly, lateral growth of the cylindrites was found to accelerate toward the sample surface when approaching it, giving the cylindrite an elliptical cross section. Furthermore and surprisingly, in the case of fiber pull, a row of spherulites is nucleated at the polymer-substrate interface nearest to the fiber, aligned along the fiber axis and appearing ahead of the rest of the space-filling spherulites. Both the phenomena, elliptical cylindrites and row of spherulites, are attributed to negative pressure buildup peaking at the cylindrite growth front and at the nearby film surface caused by crystallization-induced volume contraction. The pressure and flow distribution in the system is confirmed by numerical simulation. The results illustrate the value of 3D imaging of crystalline morphology in polymer science and polymer processing industry.

Fabrication of electrospun polyamide–weathered basalt nano-composite as a non-conventional membrane for basic and acid dye removal

Abstract

In this study, an adsorptive electrospun polyamide membrane (ESPA) and electrospun polyamide–weathered basalt composite membrane (ESPA-WB) were prepared by an electrospinning process at room temperature. Hence, the WB structure was built as a polymeric membrane separation film in combination with the ESPA matrix as a composite nano-filtration membrane. Then, the ESPA and ESPA-WB membranes were characterized using BET surface area analysis, Fourier transform infrared spectroscopy, X-ray diffraction, and scanning electron microscopy (SEM). To avoid cracks forming during the sintering process, the WB should be added in certain percentages. The microstructures of the prepared membranes were investigated to evaluate their efficiency for basic and acidic dyesʼ removal and their permeation flux. Compared with the ESPA, the ESPA-WB membrane combines the characteristics of WB and ESPA, which greatly enhances the performance of both methylene blue (MB) and methyl orange (MO) dyes removal from synthetic wastewater. The outcomes of this study indicated that the dye uptake in the case of ESPA-WB is higher than that of ESPA, and it decreases with an increase in dye concentrations. The obtained membrane ESPA-WB showed both an excellent anti-dye fouling and a good rejection property for both dyes (i.e. 90% rejection for MB and 74% for MO) with no sign of contamination by the applied dyes. It was found that the structure of the ESPA-WB membrane contains a large number of several adsorption sites which leads to an increase in the removal rate of dyes. Hence, this study demonstrated a non-conventional strategy to prepare an effective adsorptive nano-composite membrane that can be applied as a highly recyclable one for the removal of organic dyes.

Graphic abstract

Bowls, vases and goblets-the microcrockery of polymer and nanocomposite morphology revealed by two-photon optical tomography

On the >1 µm scale the morphology of semicrystalline plastics like polyethylene or Nylon features spherulites, "shish-kebabs", cylinddrites and other crystalline aggregates which strongly affect mechanical and other material properties. Current imaging techniques give only a 2D picture of these objects. Here we show how they can be visualized in 3D using fluorescent labels and confocal microscopy. As a result, we see spherulites in 3D, both in neat polymers and their nanocomposites, and observe how unevenly nanoparticles and other additives are distributed in the material. Images of i-polypropylene and biodegradable poly(lactic acid) reveal previously unsuspected morphologies such as "vases" and "goblets", nonspherical "spherulites" and, unexpectedly, "shish-kebabs" grown from quiescent melt. Also surprisingly, in nanocomposite sheets spherulite nucleation is seen to be copied from one surface to another, mediated by crystallization-induced pressure drop and local melt-flow. These first results reveal unfamiliar modes of self-assembly in familiar plastics and open fresh perspectives on polymer microstructure.