Spherulitic Structure Research Articles

RVE Simulations of short fiber reinforced polyamide: Direct and inverse matrix parameter identification in view of the semi-crystalline polymer structure

This paper investigates the modeling capabilities of computational homogenization for the mechanical behavior of short fiber reinforced polyamide. Simulations on a representative volume element (RVE) with elastic fibers and an elastic–plastic matrix are compared to tensile experiments on specimens taken in parallel and transversal direction from injection molded plates. In view of the semi-crystalline polymer structure, focus is put on identifying the matrix parameters through two alternative methods:First, the matrix parameters are identified directly using tensile experiments on a non-nucleated and a nucleated unreinforced polyamide. In the RVE computations based on the non-nucleated grade, the composite stress–strain behavior is somewhat underestimated, and with the nucleated grade, the behavior is slightly overestimated. To explain this, the semi-crystalline polymer structure is studied. Polarized light microscopy images reveal that the non-nucleated grade has a coarser and the nucleated grade a finer spherulite structure, compared to the matrix present in the composite. However, the degree of crystallinity measured by differential scanning calorimetry is in a similar range.As an alternative, the matrix parameters are identified inversely by fitting the RVE model to the composite tensile experiments. As uncertainties with respect to the matrix material as well as possibly remaining simplifications in the micromechanical model are compensated for, this reverse engineering approach allows for a very good fit.

Characterization of polyethylene films drawn by machine direction orientation and consequences on their photooxidation

The effect of uniaxial drawing by Machine Direction Orientation (MDO) on polyethylene films was first studied by complementary analytical methods. XRD and polarized FTIR spectroscopy revealed the transformation of the initial spherulitic structure of PE into microfibrils composed of small crystalline blocks connected by amorphous chains oriented along the drawing direction. No stress-induced crystallization was observed upon MDO processing, but the permeability to oxygen was reduced by 30 % owing to this morphology change. In a second time, the effect of photooxidation on undrawn and MDO-drawn films was studied. There was no difference in the photooxidation kinetics and stoichiometry between them. A slight reorganization of the polymer structure due to physical ageing was observed before any photooxidation, but no change in orientation occurred upon photochemical ageing. Finally, the oxygen permeability of undrawn and MDO-drawn films decreased owing to the changes brought by photooxidation, especially the formation of polar functions within the initially apolar polyethylene and the occurrence of chemicrystallization.

Study on the restoration of glass slides dating back to the 1940s

The Museum of Sun Yat-sen University houses a collection of antique glass slides dating back to the 1940s. These historical artifacts not only serve as a record of the past but also bring history to life. During extended storage, the emulsion layer on glass slides may harden and become brittle, leading to cracking and buckling. This study suggests a method to enhance the physical property of the emulsion layer by using a combination of nonionic surfactant isomeric alcohol ethoxylates eight (TO-8) and waterborne epoxy resin (WER). We investigated the microscopic action mechanism of the two on the emulsion layer of glass slides using scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), water contact angles, and other techniques. Our study revealed that TO-8 can significantly eliminate the spherulitic crystal structure of the emulsion layer, as well as improve its softness and hydrophilicity. The addition of the WER ensures that the cracking and warping of the emulsion layer film is fully corrected, resulting in a flat surface. Additionally, the size of the emulsion layer film remains stable even after wetting. The WER has minimal impact on the image information of glass slides. The emulsion layer of the glass slides, restored using the softening protection solutions developed in this study, showed almost full recovery of image information. This research holds significant theoretical and practical value for repairing cracked and warped emulsion layers on glass slides.

Preparation of nanoporous poly (L-lactic acid) microspheres with controllable morphology via thermally-induced phase separation

This article describes a facile method to prepare nanoporous poly (l-lactic acid) microspheres via a thermally-induced phase separation method with controllable diameters. It was found that the morphology and diameter of PLLA microspheres were easily controlled by adjusting solvent ratio, quenching temperature, and PLLA concentrations. The results revealed that by using a ternary solvent system consisting of 1,3-dioxolane as a good solvent, 2-butanone as mid-solvent, and water as a poor solvent, the diameter of PLLA microspheres can be easily controlled by changing the ratio of 1,3-dioxolane and water. The diameter of PLLA microspheres prepared ranged from 28.8 ± 5.9 μm to 63.8 ± 12.5 μm. Moreover, the evolution of PLLA spherulitic structures grown from polymer solution was confirmed as well through varying solvent ratios and PLLA concentrations. The presence of water as a non-solvent governed the phase separation mechanism of the phase separation process. At the same time, the diameter of the spherulites is affected by the ratio of 1,3-dioxolane controlling the polymer crystallization process in the polymer-rich region. Interestingly, the surface of these spherulites contains nanopores due to the stacking of lamellar crystals of PLLA during polymer crystallization which contributed to its superhydrophobicity and superoleophilicity. Hence, the prepared PLLA spherulites were utilized as PLLA microspheres with nanoporous surfaces for the adsorption and removal of oil in oil/water and oil-in-water emulsion. This facile method provides a novel pathway to prepare nanoporous PLLA microspheres with controllable diameters for eco-friendly oil/water and oil-in-water separation applications.

Insights and perspectives on PVDF/MgO NCs films: Structural and optical properties for optoelectronic device applications

In the current investigation, pure polyvinylidene fluoride (PVDF) and PVDF-MgO nanocomposite (NC) thin films are fabricated via solvent casting method using PVDF as a polymer matrix with different proportions of magnesium oxide (MgO) (0.5–2 wt%). The structural, morphological and optical properties of the prepared polymer NCs are studied by using XRD, FTIR, SEM, Raman and UV–Vis spectroscopy techniques. The α and β phases are detected in XRD measurements. MgO powder peaks are in good agreement with the reported literatures. The average crystallite size, lattice strain, and dislocation density of the prepared thin films are estimted by using Williamson-Hall (W-H) plot. SEM micrographs reveals the fluorine-containing carbon chain network that forms the spherulite and fiber-like structure in the PVDF/MgO NCs films. The Raman and FTIR spectra of PVDF/MgO and pure PVDF films are also evaluated. UV–Vis spectroscopy is used to examine the absorbance, transmittance, and reflectance spectra of pure PVDF and PVDF/MgO NCs thin films. The dipole polarization interaction causes the absorption coefficient, direct band gap, indirect band gap, optical activation energy, and skin depth of pure PVDF and PVDF/MgO thin films to decrease, whereas increasing the dielectric constant at high frequency value, refractive index, loss, optical conductivity, and dielectric constant per effective mass of NCs films. Optical dispersion parameters are estimated by the single oscillator model. The obtained results can be employed in the possible applications in supercapacitors. The method used for the preparation of thin films are inexpensive and eco-friendly.

Fatigue analysis of spherulitic semi‐crystalline polymers: Unveiling the effects of microstructure and defect

AbstractA micromechanical model considering the spherulite structure of semi‐crystalline polymers was established in this study. The micro stress–strain histories were captured by combining the constitutive equations and multi‐axial fatigue criterion. The continuous damage theory was employed to describe the degradation of material properties during cycle loading. Based on the proposed model, the effects of microstructure features, such as grain anisotropy, defects, and crystallinity, on the fatigue performance was examined under multi‐axial loading condition. The local material degradation and damage accumulation were then focused on to understand the underlying fatigue mechanisms with various microstructures. Meanwhile, the crack initiation site was precisely predicted and discussed. This research provides theoretical support for understanding the failure mechanisms of spherulitic semi‐crystalline polymers, deepening the understanding of associated microstructural characteristics and strengthening the anti‐fatigue design of semi‐crystalline polymers.

Preparation of compact spherulite of 2,2′,4,4′,6,6′-hexanitrostilbene (HNS) via specific adsorption of antisolvent combined with polymer

The application value of micro-nano hierarchical structure has been widely recognized in many fields, as it can greatly improve the surface performance of particles. Furthermore, the advantages of micro-nano hierarchical structure also bring new opportunities for the performance regulation of energetic materials, as an important material for both civilian and military applications, whose performance directly affects the application areas. This work developed a novel strategy for controlling the construction of compact spherulite of 2,2′,4,4′,6,6′-hexanitrostilbene(HNS) by utilizing antisolvent specific adsorption to generate noncrystallographic branching combined with the polymer additive to inhibit diffusion. Computational simulations revealed that toluene (TOL) exhibits specific adsorption on the (100) face of HNS through π-π interactions, while the polymer additive (Poly(acrylic acid), PAA) strongly interacts with each crystal face of HNS through hydrogen bonding, effectively inhibiting diffusion. Based on these findings, compact spherulites of HNS were successfully prepared by controlling supersaturation. The HNS spherulites exhibited superior flowability, specific surface area (11.84 m2/g for SP-I, 6.43 m2/g for SP-II), porosity (1.94 % for SP-I and 2.14 % for SP-II), and impact sensitivity (impact energy that can withstand increased from 4 J to 32.5 J for SP-I and to 11 J for SP-II). This work not only validates the effectiveness of antisolvent-induced branching in constructing compact spherulite structures, but also provides valuable insights into controlling crystal morphology. Furthermore, this research offers a promising approach to enhancing the safety and performance of energetic materials.

Influence of starch spherulites with different allomorphs and morphologies on reducing gastrointestinal digestibility in bread

This study aimed to enhance the resistance of bread to gastrointestinal digestion by partially substituting wheat flour with starch spherulites. Three types of starch spherulites, specifically A-type (exhibiting an A-type crystalline pattern with mostly positive birefringence), B(−)-type (B-type crystalline with negative birefringence), and B(+)-type (B-type crystalline with positive birefringence), were investigated. The A-, B(−)-, and B(+)-type spherulites showed significantly higher resistant starch contents of 63.5, 63.8, and 89.2 %, respectively, compared to the control wheat flour (7.4 %). The melting temperatures of A-type and B(+)-type spherulites were notably higher than those of the control wheat flour, suggesting the potential preservation of certain enzyme-resistant starch during the baking process. The partial substitution of wheat flour with spherulites resulted in a denser crumb structure, increased bread hardness and chewiness, and a pale brown color in the case of B(+)-type spherulite. However, these variations in physicochemical properties did not significantly impact consumer acceptability. Remarkably, in bread containing A- or B(+)-type spherulite, residual ordered spherulite structures were present after baking, as confirmed by differential scanning calorimetry. This resulted in significantly lower digestibility during in vitro gastrointestinal digestion. These findings are useful for the rational design of bread with sustained glucose release during gastrointestinal digestion.

Effects of crystallization temperature on structure and digestibility of spherulites formed from debranched high-amylose maize starch

High-amylose maize starch (69.3 % amylose) was debranched to increase the level of linear molecules and enhance the formation of spherulites. Debranched high-amylose maize starch (25 %, w/w) was heated to 180 °C in a Parr reactor followed by crystallization at different temperatures between 25 and 150 °C. The objectives of this study were to investigate the effects of crystallization temperature on the yield, morphology, structure, crystallinity, and digestibility of the spherulites formed. When the crystallization temperature was 150 °C, spherulites with negative birefringent sign were formed. High crystallization temperature caused molecular degradation and the degree of degradation was severe at 150 °C, resulting in relatively short chain amylose (DP < 150). When crystallized at 25 to 120 °C, spherulites with strong positive birefringence were produced. The long chain amylose was attributed to the positive birefringence. All spherulites had a predominant B-type crystalline structure. The spherulites with negative birefringence showed a lower degree of crystallinity and lower resistance to enzyme digestion, but all the spherulites with positive birefringence had a high resistant starch content (89–94 %). α-Amylase was not able to penetrate inside the spherulites as revealed by the confocal laser scanning microscopic images.

Ultrastructure of the Jurassic serpulid tubes-phylogenetic and paleoecological implications.

The ultrastructural diversity of the Middle and Late Jurassic serpulid tubes from the Polish Basin has been investigated. The inspection of 12 taxa representing the two major serpulid clades allowed for the identification of three ultrastructure types-irregularly oriented prismatic structure (IOP), spherulitic prismatic structure (SPHP), and simple prismatic structure (SP). Six of the studied species are single-layered and six species possess two distinct layers. Ultrastructural diversity corresponds to certain serpulid clades. The members of Filograninae have single-layered tube walls composed of possibly plesiomorphic, irregularly oriented prismatic structure (IOP). Two-layered tubes occur solely within the clade Serpulinae, where the external, denser layer is built of either the ordered spherulitic (SPHP) or simple prismatic microstructure (SP), and the internal layer is composed of irregularly oriented prismatic structure (IOP). Apart from phylogenetic signals provided by the tube ultrastructure, it can be used in analyzing paleoecological aspects of tube-dwelling polychaetes. Compared to the more primitive, irregularly oriented microstructures of Filograninae, the regularly oriented microstructures of Serpulinae need a higher level of biological control over biomineralization. The advent of the dense outer protective layer (DOL) in serpulids, as well as the general increase in ultrastructure diversity, was likely a result of the evolutionary importance of the tubes for serpulids. Such diversity of the tube ultrastructural fabrics allowed for maximizing functionality by utilizing a variety of morphogenetic programs. The biomineralization system of serpulids remains more complex compared to other tube-dwelling polychaetes. Physiologically more expensive tube formation allows for mechanical strengthening of the tube by building robust, strongly ornamented tubes and firm attachment to the substrate. Contrary to sabellids, which perform a fugitive strategy, an increased tube durability allows serpulids a competitive advantage over other encrusters.

Melt-crystallization and self-nucleation of UHMWPE/bi-HDPE blends: The combined role of composition and molecular weight

Blends of bi-modal high-density polyethylene (bi-HDPE) and ultra-high molecular weight polyethylene (UHMWPE) with varying content (0–30 wt%.) and different molecular weights (MWs) (2.2, 3.5, and 4.5 M g/mol) of UHMWPE were prepared. The effect of UHMWPE content, MW, and their interplay on the microstructure, nucleation, and crystallization behavior of bi-HDPE was examined through rheology, scanning electron microscopy, and differential scanning calorimetry. The results revealed that UHMWPE serves as a partially miscible nucleating agent, effectively dispersing in the bi-HDPE matrix up to 5 wt%, thereby promoting the transition from randomly dispersed lamellae to a spherulitic morphology. The nucleating effect saturated when the UHMWPE content reached 10 wt%, leading to a diminished spherulitic structure. Beyond 10 wt% of UHMWPE, an anti-nucleation effect with segregated crystalline regions emerged. Isothermal kinetics analysis also revealed the highest crystallization rate at the optimum UHMWPE content of 5 wt%, where the critical nucleation energy barrier and fold-surface free energy were minimized. Apart from the UHMWPE content, signs of nucleation enhancement were slightly more pronounced in the bi-HDPE blend with the lowest molecular weight of UHMWPE. This phenomenon results from increased concentration fluctuations due to the larger interfacial area, as postulated from entanglement density measurements. For the first time, self-nucleation (SN) analysis revealed that the incorporation of 5 wt% of UHMWPE into bi-HDPE resulted in a wider Domain II width, indicating an increased memory effect. Notably, the crystalline memory was more pronounced in the blend containing UHMWPE with the MW of 2.2 M g/mol, exhibiting the super-nucleating effect with the highest nucleation efficiency of 218 %.

استكشاف مفصل لتصنيع وتوصيف كوبولميرات متعددة الكتل من استر- سيلوكسان

This research paper presents a comprehensive study on the synthesis and characterization of Ester-Siloxane multi block copolymers. Two different synthesis methods, namely the one-prepolymer method and the two-prepolymer method, were employed to prepare the copolymers. The distribution of the Ester segment within the copolymers was analyzed using refractive index and ultraviolet detectors coupled with size exclusion chromatography (SEC). SEC-FTIR spectroscopy was utilized to investigate the distribution and composition of the Siloxane segment. Gel permeation chromatography (GEC) was employed to assess copolymer purity based on Siloxane content. Atomic force microscopy (AFM) was utilized to visualize the morphology of Siloxane domains in the copolymers. The results revealed the presence of well-defined Siloxane-rich domains in the form of spheres. Moreover, the occurrence of spherulitic crystal structures was observed in random copolymers with low Siloxane content and small Siloxane segment length. This research provides valuable insights into the manufacturing process, characterization, and structural aspects of multi-block ester-siloxane polymers, and contributes to the understanding of polymer design, the relationship between composition and molecular structure, and the material properties. Key words: Copolymer, -Siloxane multiblock, One-prepolymer, Two-prepolymer, Polycondensate.

Thermally induced phase separation PVDF membrane fabricated by using NaCl coagulation bath: Relation of membrane surface morphology and permeation performance

Efforts have been made in thermally induced phase separation (TIPS) process to modify the morphology of polyvinylidene fluoride (PVDF) membranes in order to improve their permeation performance and mechanical properties. Nevertheless, many methods not only altered the outer surface but also impacted the overall membrane structure, resulting in a trade-off between permeability and mechanical properties. In this study, we utilized a modified TIPS process to refine the outer surface morphology without altering the bulk structure. This was achieved by introducing NaCl in the coagulation bath. The PVDF membranes were fabricated using a dope with water insoluble diluent dimethyl phthalate (DMP) as main solvent and water-soluble additives polyethylene glycol 400 (PEG400)/triethylene glycol (TEG) as pore formers. The inclusion of NaCl in the coagulation bath serves to decrease the solubility of PEG within this medium, owing to the salting-out effect. Consequently, the NaCl concentration in the coagulation bath emerges as a crucial factor in regulating the migration of PEG400 toward the membrane surface. This control mechanism facilitates the precise adjustment of the outer surface morphology in the fabrication of membranes. As the NaCl concentration increases in the coagulation bath, the outer surface of the fabricated membrane transited from a mesh-like structure to a spherulite structure. As 0.5 mol L−1 NaCl was added to the coagulation bath, the membranes displayed a pure water permeability of 1073.9 L m−2 h−1 bar−1 while maintaining a narrow pore size distribution. Compared to the membranes fabricated without NaCl addition, the increment of the PWP contributed to the slight increase in mean pore size from 65 nm to 84 nm. Meanwhile, the water-insoluble diluent DMP was not affected by the addition of NaCl, which means that the bulk structure of the membrane could be maintained. Consequently, the increase in permeability did not compromise the mechanical properties of the membranes. All the membranes fabricated in this study maintained a reasonable tensile strength of approximately 3 MPa. This study introduces a simple and environmental method to increase the permeability effectively and fine-tune the pore size of the TIPS membranes while having little effect on the bulk structure. Furthermore, the study provides valuable insights into how changes in outer surface morphology can impact the pore size and permeability of TIPS PVDF membranes.

ТЕРМОТРОПНЫЕ И МЕЗОМОРФНЫЕ СВОЙСТВА СОПОЛИЭФИРА НА ОСНОВЕ ПОЛИЭТИЛЕНТЕРЕФТАЛАТА И 4-ГИДРОКСИБЕНЗОЙНОЙ КИСЛОТЫ

Polyethylene terephthalate is an affordable, harmless and easily processed multi-tonnage polymer, but its usage as a structural material is limited by temperature and strength properties. It was predicted by calculation that the introduction of monomers such as 4-hydroxybenzoic acid makes it possible to impart asymmetry to macromolecules, leading to an increase in the Kuhn segment and the manifestation of mesomorphic properties due to a more rigid and regular structure of the polyester, which should also lead to an increase in its thermal stability. The high degree of order of liquid crystalline polymers gives them the ability to self-reinforce, which makes it possible to obtain ultra-strong fibers. This determines the relevance of the development of this type of materials with the possibility of their use in the rapidly developing additive methods of manufacturing products by Fused Deposition Modeling (FDP) technology. As part of the work, copolyesters based on polyethylene terephthalate and 4-hydroxybenzoic acid with different molar ratios of the initial reagents were obtained and studied. A catalyst based on titanium compounds was also used for the synthesis, which also improves the thermal stability properties. Thermal stability of the resulting compounds was analyzed by thermogravimetric analysis, which showed an increase in the temperature of the onset of destruction and a 10% weight loss of the samples relative to the unmodified polyethylene terephthalate sample. Also, based on experimental data, we calculated the activation energies of thermal destruction for the samples we obtained. The study of phase transitions showed the presence of two endothermic melting peaks, the interval between which corresponds to the liquid crystalline state, which is confirmed by the formation of optically active spherulite structures visible in a polarizing microscope.

Mechanism of PVDF Membrane Formation by NIPS Revisited: Effect of Precipitation Bath Nature and Polymer-Solvent Affinity.

A new interpretation of the mechanism of the polyvinylidene fluoride (PVDF) membrane formation using the nonsolvent-induced phase separation (NIPS) method based on an analysis of the complete experimental phase diagram for the three-component mixture PVDF-dimethyl acetamide (DMAc)-water is proposed. The effects of the precipitation bath's harshness and thermodynamic affinity of the polymer's solvent on the morphology, crystalline structure, transport and physical-mechanical properties of the membranes are investigated. These characteristics were studied via scanning electron microscopy, wide-angle X-ray scattering, liquid-liquid porosimetry and standard methods of physico-mechanical analysis. It is established that an increase in DMAc concentration in the precipitation bath results in the growth of mean pore size from ~60 to ~150 nm and an increase in permeance from ~2.8 to ~8 L m-2 h-1 bar-1. It was observed that pore size transformations are accompanied by changes in the tensile strength of membranes from ~9 to ~11 and to 6 MPa, which were explained by the degeneration of finger-like pores and appearance of spherulitic structures in the samples. The addition of water to the dope solution decreased both the transport (mean pore size changed from ~55 to ~25 nm and permeance reduced from ~2.8 to ~0.5 L m-2 h-1 bar-1) and mechanical properties of the membranes (tensile strength decreased from ~9 to ~6 MPa). It is possible to conclude that the best membrane quality may be reached using pure DMAc as a solvent and a precipitation bath containing 10-30% wt. of DMAc, in addition to water.

Investigating the thermal effects of plasma surface treatment on crystallinity and mechanical behavior of PEEK

This study investigates the influence of plasma surface treatment on the local crystallinity and its impact on the local and global mechanical properties of PEEK samples. Local degree of crystallinity is assessed using Raman spectroscopy, while nanoindentation measurements are employed to examine the nanoscale mechanical properties for commercial, raw injected, annealed, and plasma-treated PEEK samples. The results reveal a significant correlation between the local degree of crystallinity and the nanoscale mechanical properties within the PEEK cross-section. Scanning Electron Microscopy (SEM) observations further support these findings by illustrating the density and evolution of spherulitic structures within the PEEK cross-section. Tensile mechanical tests are conducted to evaluate the influence of crystallinity variation on overall behavior. The results demonstrate that even localized variations in crystallinity can affect the overall mechanical response of PEEK samples. These findings enhance our understanding of the relationship between crystallinity, mechanical properties, and surface treatment on PEEK, providing valuable insights for optimizing its performance in various applications.

Yielding behavior of isotactic polypropylene at elevated temperature understood at the spherulite level

It is of great significance to study the deformation mechanism of isotactic polypropylene (iPP) which differs considerably from that of traditional polymers in consideration of its unique cross-hatched structure of α crystal. In this work, the yielding mechanism of three iPP precursor films prepared via crystallizing in different supercooling during stretching under elevating temperature had been investigated systematically. The results show that three mechanisms of deformation had been aroused in the change of yielding stress with stretching temperature named as inter-spherulitic, intra-spherulitic and mix deformation, respectively. Moreover, the transition temperature (60 °C) is in close correlation with the α2 relaxation of iPP, where the chain diffusion within the crystal blocks is activated. The competition between inter-spherulitic deformation and intra-spherulitic deformation is not only determined by the spherulite structures, but also the stretching temperatures. Further studies indicate that, larger spherulitic size, more daughter lamellae and lower stretching temperature are dominant issues to induce inter-spherulitic deformation. This work is expected to provide a guideline for the preparation of iPP films with desired structures and functionalities.

Fabrication of Hydrophobic Poly(vinylidene fluoride) Membranes with Manipulated Micromorphology and Enhanced Strength

In this work, an ethanol/water/glycerol ternary coagulation bath system was used to fabricate a hydrophobic PVDF membrane, which will have a considerable impact on the micromorphology. This change will further affect the performance of the membrane. After introducing glycerol into the coagulation bath, the precipitation process was finely regulated. The obtained results implied that glycerol could inhibit solid-liquid separation and promoted liquid-liquid separation. A pleasant discovery was that the mechanical properties of the membrane were improved because of the more fibrous polymers formed by liquid-liquid separation. Besides, a more uniform pore size can be achieved. A fascinating symmetrical interconnected fibrous and spherulitic structure was depicted by membranes fabricated with a coagulation bath consisting of 6% water, 34% ethanol, and 60% glycerol. This membrane had a high water contact angle of 146.6° and a small mean pore size of 0.46 μm. Enhanced tensile strength and elongation at break evidenced that the membrane enjoyed good robustness and flexibility. This facile approach provided the possibility to prepare membranes with tailored pore size and the required strength.

Hydrate Growth over a Sessile Drop of Water in Cyclopentane

Liquid cyclopentane is frequently used in hydrate formation studies as an analogue of natural gas because cyclopentane hydrates are stable above the ice melting point at ambient pressure. In this study, hydrate growth was established on a sessile water drop of 11 mm in diameter and 4.5 mm in height (volume of 0.25 mL) immersed in liquid cyclopentane. The hydrate formation mechanism and growth processes were observed optically over an extended range of subcooling temperatures from 5.1 to 15.2 °C, with the cyclopentane bulk temperature maintained in different experimental runs between 2.6 and −7.5 °C. Qualitative and quantitative comparisons were performed to confirm the absence of ice freezing during hydrate formation, and thus, the lack of contamination of the latter from the former in the experiments. Different transformations in the hydrate film morphology were registered from macroscopic observation over the considered range of subcooling temperatures, with the hydrate crystals composing the film taking the form of polyhedral, dendritic, or spherulitic structures. It was also found that the hydrate growth rate varied depending on the subcooling temperature, with the variation of the growth rate as a function of this temperature changing from a power to an (approximately) linear law with an increase in the degree of subcooling. We postulate that hydrate film growth can be governed by different mechanisms, whose roles change over the range of explored subcooling temperatures.

Microbialite Textures and Their Geochemical Characteristics of Middle Triassic Dolomites, Sichuan Basin, China

Microbialite textures, such as microbial mats and biofilms, were observed in the Middle Triassic dolomite in the Sichuan Basin, western China, using core examination, thin section petrography, scanning electron microscopy (SEM), and geochemical analyses. The dolomite texture, consisting of fibrous and spherulitic structures, is similar in morphology and size distribution to those observed in microbial culture experiments. Extracellular polymeric substances (EPS) were identified based on the occurrence of fibers forming a reticular pattern and nanometer-sized spheroids. The rare earth element (REE) and stable isotope (C, O, and Sr) compositions of the Middle Triassic dolomite were measured to determine their geochemical characteristics. Using seawater as a standard, the dolomitic microbialites (MD) exhibited significantly positive La and Eu anomalies and higher REE concentrations and (Nd/Yb)sn values than associated limestones, and these patterns are inferred to be related to initial complexation on organic ligands in the biofilm, as proposed by previous researchers. The ambient temperature during dolomite precipitation was estimated to be within the 23 °C to 50 °C range, as indicated by the δ18O values of the dolomite. This study suggests that various microbial effects can significantly affect diagenetic processes in the Middle Triassic dolomite.