Spherulite Size Research Articles

Novel insight into evolution mechanism of second liquid-liquid phase separation in metastable immiscible Cu-Fe alloy

The metastable immiscible Cu80Fe20 alloys with different diameters were systematically investigated under conventional solidification. Experimental results indicated that the primary and second liquid-liquid phase separation simultaneously occur during solidification. The average size of primary phase-separated Fe-rich spherulites and the interior morphological pattern of minority Cu-rich phase can be greatly influenced by cooling rate due to the dynamic coupling between thermodynamic and kinetic effects. Moreover, various morphological patterns of minority Cu-rich phase in Fe-rich spherulites were observed, which discloses the dynamic evolution process during self-driven second liquid-liquid phase separation. The Marangoni migration, coalescence and coagulation, and Ostwald ripening are the dynamic mechanisms mainly responsible for various morphological patterns of minority Cu-rich phase after phase separation. Such a clear experimental observation of dynamic microstructure evolution for minority Cu-rich phase provides a strong and visualized evidence for the asynchronous crystallization behavior of primary phase-separated Fe-rich spherulites. Phase field simulation was also performed to reveal the dynamic evolution of minority Cu-rich phase during liquid-liquid phase separation. Besides, the experimental Cu80Fe20 alloys exhibit soft ferromagnetic characteristics possessing relatively low coercivity and high saturated magnetization. This present study provides a new strategy to design immiscible alloy with anticipated microstructure possessing tailored properties and desired functionalities.

Polypropylene Blends with m-EPR Copolymers: Mechanical and Rheological Properties.

The effects of two metallocene ethylene-propylene-based elastomers (m-EPR1 and m-EPR2) differing in molecular mass and viscosity on mechanical, rheological and interfacial properties were compared. The m-EPR elastomers were added to iPP in 2.5, 5, 10, 15, and 20 vol.%. Torque values, elongation at break and impact strength measured of the iPP/m-EPR1 blends were higher than the iPP/m-EPR2 blends due to higher molten viscosity of m-EPR1 than m-EPR2 copolymer. Slight differences in Young moduli as well as in tensile strength at yield and at break might indicate that tensile properties of iPP/m-EPR blends were not significantly affected by difference in viscosity or molecular mass, miscibility and spherulite size. Optimization diagrams indicated the metallocene m-EPR copolymers are efficient impact modifiers for polypropylene and showed good balancing of mechanical properties in iPP/m-EPR blends.

Poly(vinylidene fluoride-hexafluoropropylene) based blend film for ultrahigh energy density capacitor applications

In this paper, we investigated the blend film of poly(vinylidene fluoride-trifluoroethylene)/poly(vinylidene fluoride-hexafluoropropylene) (P(VDF-TrFE)/P(VDF-HFP)) with various compositions for high energy density applications. After characterization using x-ray diffraction and transmission electron microscopy, it was found that the crystallinity and spherulite size of the material decreased with the increasing PVDF-HFP contents in the blend film. Furthermore, the blend film showed a maximum breakdown field of 820 MV m-1 and an energy density of 23.8 J cm-3 for the blend film with a 5:5 composition ratio (P(VDF-TrFE)/P(VDF-HFP)). Moreover, the mechanisms of the breakdown field associated with the spherulite size and the crystallinity of the blend film were investigated for the first time. It was revealed that for the materials with similar Young’s moduli, the breakdown field is greatly improved with the reduced spherulite size and decreased crystallinity of the P(VDF-TrFE)/P(VDF-HFP) blend film. This investigation shows an effective approach for improving the breakdown field of dielectric materials in high energy density capacitor applications.

An In situ Study on the Orderly Crystal Growth of Pluronic F127 Block Copolymer Blended with and without Ionic Liquid during Isothermal Crystallization

Isothermal crystallization behavior of Pluronic F127 blended with and without an ionic liquid (IL) was investigated by in situ polarized optical microscopy (POM) and Fourier transform infrared spectroscopy (FTIR). For the pure F127, the POM and FTIR results showed that the spherulite size and crystallinity of F127 increased with the melting temperature increasing to 60, 80, and 135°C. This could be explained by the flexibility of the polymer chain at high melting temperatures. For the F127 blended with IL, the POM results showed that the morphology of F127 evolved from spherulite to dendritic segregation and fibrous crystal with the increasing IL content. FTIR results indicated that hydrogen bonds were formed between F127 and IL, and the intensity of the hydrogen bonds became strengthened gradually with increasing IL content. The effect of hydrogen bonds on the morphology evolution of F127/IL is discussed.

Preparation and characterization of thermoplastic poly(urethane‐urea) solid–solid phase‐change materials for thermal energy storage

AbstractThree kinds of thermoplastic poly(urethane‐urea) PCMs were synthesized by two‐step condensation polymerization for thermal energy storage. Polyethylene glycol (PEG) was employed as the phase‐change functional chain, and three kinds of diamine compounds were individually adopted as chain extender. The chemical structure, crystalline properties, phase‐change behaviors, thermal stabilities, and reliabilities were studied by Fourier transform infrared spectroscopy, X‐ray diffraction, polarizing optical microscopy, and thermogravimetric, respectively. The crystalline structure of synthesized thermoplastic PCMs is identical to pure PEG, but the spherulite size decreases to some extent due to the confined crystalline region. The synthesized thermoplastic PCMs have proper phase‐change temperature with high phase‐change enthalpy of about 88 J/g. The synthesized thermoplastic PCMs are stable at their processing and working temperature with the onset degradation temperature higher than 250°C, and tensile strength reaches 20.59 MPa. Thus, the synthesized thermoplastic PCMs can be processed into desirable shape for the special applications in energy storage.

Initiation, Development and Stabilization of Cavities during Tensile Deformation of Semicrystalline Polymers

By using polybutene-1 as a typical example, we illustrate the initiation, development and stabilization of cavities in the sample during tensile deformation. Samples with the same crystallinity, long spacing and crystalline lamellar thickness but very different sizes of spherulites were prepared via changing the melt history. Dimension of cavities during stretching the samples was determined by in situ ultra small angle X-ray scattering techniques. It turned out that the size of the cavities was bigger in the sample with larger spherulites than the one with smaller spherulites. The results show clear evidence of initiating cavities within crystalline phase at the grain-boundary of crystalline blocks, growing of cavities passing through parallel stacked lamellar crystals and amorphous layers and finally stablized by tilted lamellae at both ends of the plate-like cavities within the spherulites.

Development of Crystalline Morphology and Its Relationship with Mechanical Properties of PP/PET Microfibrillar Composites Containing POE and POE-g-MA.

The main goal of this research is to study the development of crystalline morphology and compare it to various mechanical properties of microfibrillar composites (MFCs) based on polypropylene (PP) and poly(ethylene terephthalate) (PET), by adding a functional compatibilizer and a non-functional rubber in two different steps in the processing sequence. The MFCs were prepared at a weight ratio of 80/20 PP/PET by twin screw extrusion followed by cold drawing and injection moulding. The non-functionalized polyolefin-based elastomer (POE) and the functional compatibilizer (i.e., POE grafted with maleic anhydride (POE-g-MA)) were added in a fixed weight percentage at two stages: during extrusion or during injection moulding. The morphology observations showed differences in crystalline structure, and the PP spherulite size was reduced in all MFCs due to the presence of PET fibrils. Their relationship with the mechanical performances of the composite was studied by tensile and impact tests. Adding the functional compatibilizer during extrusions showed better mechanical properties compared to MFCs. Overall, a clear relationship was identified between processing, structure and properties.

The effect of inorganic and organic nucleating agents on the electrical breakdown strength of polyethylene

ABSTRACTAltering the morphology of polyethylene affects physical and electrical properties with reduced spherulite size correlating with higher electrical breakdown strength. Nucleating agents in polyethylene influence the final crystal morphology by increasing the number of spherulites and reducing spherulite size. Few studies are available that relate the nucleating activity to improved electrical breakdown strength. Although nanosilica is known to improve electrical breakdown strength of polyethylene in addition to serving as a nucleating agent, previous studies have not fully addressed the relationship between the improved breakdown strength and nucleating activity. In this article, direct current electrical breakdown strength and nucleation effects on morphology are assessed on a single set of controlled polyethylene compositions containing two types of surface treated nanosilica particles. The results are compared to composites with two types of organic nucleating agents 1,3:2,4‐bis(3,4‐dimethylbenzylidene) sorbitol or calcium 1,2‐cyclohexanedicarboxylate (CDA). CDA was the most effective organic nucleating agent and the hexamethyldisilizane treated nanosilica was the most effective inorganic nucleating agent in reducing spherulite sizes in low density polyethylene (LDPE). Reduced spherulite sizes in nucleated samples correlated with increased breakdown strength and lower conduction current compared to the neat LDPE. The LDPE sample with CDA also had the highest increase in crystallization temperature indicating stronger nucleating agent performance than the nanosilica and 1,3:2,4‐bis(3,4‐dimethylbenzylidene) sorbitol composite samples. The addition of these inorganic and organic nucleating agents all resulted in improvements in electrical breakdown strength. The results show that nucleation deserves more attention as a potential cause for improved breakdown strength observed with silica and organic nucleating agents. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46325.

Fatigue damage mechanisms of short fiber reinforced PA66 as observed by in-situ synchrotron X-ray microtomography

Abstract The understanding of fatigue damage mechanisms of short fiber reinforced thermoplastics are a key issue in order to optimize material processing and propose physically based multiscale fatigue damage models. The presented work aims at a fine description of 3D damage development as observed by synchrotron X-ray microtomography. Damage processes at the micro and mesoscale are fully described in order to extract the elementary damage mechanisms, their sequence and kinetics. The effects of local fiber configuration and orientation are particularly detailed. From observations it is clearly evidenced that cavitation plays a major role in the fatigue damage process as it triggers all elementary damage mechanisms observed at the microscale. It is also shown that a characteristic length appears in the fatigue damage development. This internal length is in the order of magnitude of the spherulite size, suggesting a strong impact of the spherulite size on the fatigue damage development. Finally the effect of local fiber orientation on the micro and meso crack orientation is presented.

Toughness and crystallization enhancement in wood fiber-reinforced polypropylene composite through controlling matrix nucleation

Wood fiber/polypropylene (WF/PP) composites are commonly used to efficiently utilize natural resources and reduce environmental problems, but the low impact toughness has been a major disadvantage of this composite. Most studies are focused on fiber modification or adding coupling agents to overcome this problem, but few of them paid attention to the microstructure of the matrix and its effect on the interface adhesion. Isotactic polypropylene (PP) is polymorphic with different morphological forms: α-, β- and γ-forms. α-PP is predominated form, while β-PP has received considerable attention due to its promising toughness behavior. This study investigated the effect of adding a highly active β-nucleating agent (TMB) for PP on the mechanical and crystallization properties of PP/WF composite. Specifically, the impact strength, the nucleating effect, content of β-crystal, non-isothermal crystallization behavior and mechanical properties of the composite were investigated. Results showed that the content of the β-form crystal of nucleated PP (K value) reached 87% with the addition of 0.3 wt% TMB. Composites with TMB increased the crystallization peak temperature, accelerated the crystallization process and improved its impact strength by 53.12% with the addition of 0.3 wt% TMB nucleating agent, though tensile and flexural properties were dropped by about 10–20%. Also, the spherulite size of nucleated PP in the composite was dramatically decreased.

The influence of nano‐PS particle on structure evolution and electrical properties of PP/PS

ABSTRACTThe polypropylene‐g‐polystyrene (PP‐g‐PS) copolymers with different grafting ratios are used as compatibilizers to control the size of polystyrene (PS) particles at nanometer scale in polypropylene (PP) matrix. Then the PP/PS insulating nanocomposites (containing 10 wt % PS calculated from PS and PP‐g‐PS) are manufactured. With the increase in grafting ratio of PP‐g‐PS, the size of PS particle is reduced and the interfacial adhesion is enhanced. Meanwhile, the dielectric properties, DC breakdown strength and volume resistivity are increased with the decreasing of PS particle size. The spherulite size of PP is decreased and the boundary between crystals and amorphous regions is blurred or even disappears due to the presence of PS nanoparticles. This evolution of PP structure is attributed to the serious entanglements of PP and PS molecular chains. Finally, the correlation between morphological structure and electrical properties is ultimately established based on the in‐depth understanding of the molecular chain movement, crystal structure, and phase morphology. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 706–717

Preparation and crystallization of isotactic polypropylene composites filled by titanium dioxide-supported montmorillonite with a β-nucleating surface

To obtain multi-functional β-iPP composites, β-iPP composites are always filled by multiple fillers with α-nucleating ability. To prepare β-iPP/TiO2/MMT composites, TiO2-supported MMT with a β-nucleating surface was prepared through hydrolysis reaction of Tetra-n-butyl titanate and chemical reaction between pimelic acid and calcium ion. X-ray diffraction and scanning electron micrograph confirmed the formation of TiO2 on the surface of MMT. The differential scanning calorimeter and X-ray diffraction illustrated that the β-iPP composites filled by TiO2 and MMT had lower relative β-phase contents than β-iPP. It is observed by polarized optical microscope that addition of TiO2-supported MMT with a β-nucleating surface into iPP can increase the spherulite nuclei density and decrease the spherulite size significantly and obtain the β-iPP/TiO2/MMT composites with high relative β-phase content.

Development of a biaxial stretching test machine and its applications

Abstract The evaluation method for a biaxially oriented film was developed using in-situ measurement during the stretching process. It can obtain basic data such as stress-strain curves, birefringence, light scattering, three dimensional refractive indexes and birefringence distribution. Stress and strain as functions of stretching speed and stretching temperature, as well as the deformation of spherulite of semi-crystalline polymer can be obtained by measuring the birefringence and light scattering during the biaxial stretching process with a small piece of polymer sample. The experimental results show the stress, retardation and three dimensional molecular orientations behavior during the simultaneous biaxial stretching and the sequential biaxial stretching process. Stretchability, thickness uniformity and spherulite size can be obtained simultaneously. In this paper, advantages and details of the newly developed system will be discussed with some experimental data.

Crystallization/Melting Kinetics and Morphological Analysis of Polyphenylene Sulfide

AbstractCrystallization and melting kinetics of polyphenylene sulfide (PPS) are determined using fast scanning calorimetry. The temperature dependence of half‐time crystallization of isothermally melt‐crystallized PPS shows a downward convex curve with a minimum at 160 °C. The minimum crystallization half‐time is about 3 s. The analysis of heating rate dependence of the melting temperature reveals a zero‐entropy‐production melting temperature of the sample. The microstructure of the sample, which is prepared by fast scanning calorimetry, is investigated by small‐angle X‐ray scattering and polarizing optical microscopy. The crystallinity and lamellar thickness of the sample annealed for 400 s decrease with decreasing crystallization temperature. The size of spherulites becomes small as the isothermal temperature is decreased. Transcrystalline morphology is observed near the surface between the sample and nitrogen gas at a crystallization temperature of 120 °C. The thickness of the transcrystalline layer disappears as the isothermal temperature is increased.

From Cellulose Nanospheres, Nanorods to Nanofibers: Various Aspect Ratio Induced Nucleation/Reinforcing Effects on Polylactic Acid for Robust-Barrier Food Packaging.

The traditional approach toward improving the crystallization rate as well as the mechanical and barrier properties of poly(lactic acid) (PLA) is the incorporation of nanocelluloses (NCs). Unfortunately, little study has been focused on the influence of the differences in NC morphology and dimensions on the PLA property enhancement. Here, by HCOOH/HCl hydrolysis of lyocell fibers, microcrystalline cellulose (MCC), and ginger fibers, we unveil the preparation of cellulose nanospheres (CNS), rod-like cellulose nanocrystals (CNC), and cellulose nanofibers (CNF) with different aspect ratios, respectively. All the NC surfaces were chemically modified by Fischer esterification with hydrophobic formate groups to improve the NC dispersion in the PLA matrix. This study systematically compared CNS, CNC, and CNF as reinforcing agents to induce different kinds of heterogeneous nucleation and reinforce the effects on the properties of PLA. The incorporation of three NCs can greatly improve the PLA crystallization ability, thermal stability, and mechanical strength of nanocomposites. At the same NC loading level, the PLA/CNS showed the highest crystallinity (19.8 ± 0.4%) with a smaller spherulite size (33 ± 1.5 μm), indicating that CNS, with its high specific surface area, can induce a stronger heterogeneous nucleation effect on the PLA crystallization than CNC or CNF. Instead, compared to PLA, the PLA/CNF nanocomposites gave the largest Young's modulus increase of 350 %, due to the larger aspect ratio/rigidity of CNF and their interlocking or percolation network caused by filler-matrix interfacial bonds. Furthermore, taking these factors of hydrogen bonding interaction, increased crystallinity, and interfacial tortuosity into account, the PLA/CNC nanocomposite films showed the best barrier property against water vapor and lowest migration levels in two liquid food simulates (well below 60 mg kg-1 for required overall migration in packaging) than CNS- and CNF-based films. This comparative study was very beneficial for selecting reasonable nanocelluloses as nucleation/reinforcing agents in robust-barrier packaging biomaterials with outstanding mechanical and thermal performance.

Mechanisms contributing to mechanical property changes in composites of polypropylene reinforced with spray-dried cellulose nanofibrils

This study revealed the effects of spray-dried cellulose nanofibril (SDCNF) addition (3,10 and 30 wt%) and maleic anhydride polypropylene (MAPP) coupling agent (2 wt%) on the mechanical properties of polypropylene (PP). Results indicated that the elastic moduli of the PP composites increased as the SDCNF content increased above 10 wt%. The addition of MAPP into the SDCNF/PP composites did not improve the elastic moduli. Flexural strength of PP was improved when the SDCNF content increased above 10 wt%, while the tensile strength of PP decreased as the SDCNF content increased. The addition of MAPP into the SDCNF/PP composites increased the strength of the composites when the SDCNF content was above 10 wt%. Without the addition of MAPP, the composite’s impact strength did not exhibit a significant increase among the pure PP and SDCNF/PP composites. No significant differences in crystallinity or crystal forms were found in the pure PP and PP/MAPP/SDCNF composites. The spherulitic size of PP was reduced after adding SDCNF into the PP.

Functionalized graphene nanoplatelets/poly(ethylene oxide) nanocomposites: Correlation between crystallization behavior and mechanical performance

Poly(ethylene oxide) (PEO) nanocomposites containing pristine or functionalized graphene nanoplatelets (FGnP) prepared via solution blending and thermal and mechanical properties of nanocomposites were investigated. Chemical functionalization of graphene nanoplatelets was conducted through an amidation reaction between carboxylic acid groups of acidified graphene and hydroxyl groups of polyethylene glycol (PEG). An interfacial linkage and a good dispersion of FGnP was observed via scanning electron microscope (SEM). Differential scanning calorimetry (DSC) analysis revealed that the degree of crystallinity of samples decreased by addition of graphene nanoplatelets, while the size of spherulites increased as indicated by polarized optical microscope (POM). A lower degree of crystallinity and larger spherulites were detected in the case of FGnP. Filler/matrix interfacial adhesion was also remarkably influenced the mechanical properties of PEO as an effective reinforcement of matrix obtained upon the addition of FGnP nanosheets compared to untreated graphene.

Respective Contribution Research of Soft Component and Macroinitiator on Synthesis and Performance of MCPA‐PEA Materials

The monomer casting polyamide‐polyether amine (MCPA‐PEA) materials initiated by TDI‐PEA macroinitiator presented an obvious toughness characteristic which was contributed by the combination effect of PEA‐TDI macroinitiator and soft PEA component itself. The MCPA‐PEA materials with different content of PEA‐TDI macroinitiator or PEA component were prepared and then the respective contribution of PEA‐TDI macroinitiator and soft PEA component on the synthesis and performance were researched. It was found that the PEA‐TDI macroinitiator presented the dominant contribution and main influence on the polymerization process with higher activation energy and reaction order than PEA component. The PEA molecular chains did not participated in the crystallization process and made no influence on crystal structure of MCPA material whatever the way of copolymerization or blending. The PEA‐TDI macroinitiator also showed the dominant contribution on the decreasing effect of crystallization properties than PEA component with lower crystallinity and smaller spherulites size. The fibrous‐structure for MCPA‐0.3%PEA matrix was more obvious with notable deformation and high impact strength which further indicated the dominant contribution of PEA‐TDI macroinitiator on toughness. The toughening mechanism was deduced as the vast energy consumption by intense extension of fibrous‐structure and the highly orientation of molecular chains. POLYM. ENG. SCI., 58:1353–1361, 2018. © 2017 Society of Plastics Engineers

The evaluation of stretchability and its applications for biaxially oriented polypropylene film

AbstractIn recent years, plastic film is essential in our daily lives. Among them, polypropylene has good physical properties, for example, tensile strength, impact strength, and transparency. Recently, it was found that the stretchability of polypropylene (PP) is improved by blending a small amount of low modulus polypropylene (LMPP) which is a low tacticity polypropylene. The purpose of this research was to investigate the effect of blending LMPP, which has different characteristic of resin such as tacticity and molecular weight, on PP and to study the mechanism of the structure formation in a biaxial stretching process. The blending of LMPP to PP reduces the spherulite size and increases the thickness of the amorphous phase in the lamella without changing the thickness of the crystal phase, namely the melting temperature. LMPP blends also reduced the yield stress during the stretching process and increased the stress buildup ratio. For these reasons, it improved film thickness uniformity of biaxially stretched film and reduced film break during the high production line. Furthermore, as the stretchable temperature window was widened to the low‐temperature side, the thickness uniformity was improved in the wide stretchable temperature. As a result, improving the stretchability and the film thickness uniformity of PP were achieved by blending LMPP to PP.

MORPHOLOGICAL, THERMAL, MECHANICAL AND SOLVENT UPTAKE OF CLAY/NYLON 6,6 COMPOSITES

Clay/nylon 6,6 nanocomposites were prepared via solution casting technique. Kaolin clay was used as filler in polymer matrix. The morphological study presented that clay particles were dispersed and embedded well in nylon 6,6 matrix. The thermal stability of the clay/nylon 6,6 nanocomposites (20-50 oC) were higher than neat nylon 6,6 polymer. The polarized optical microscopy (POM) analyses showed that the spherulites size of clay/nylon 6,6 nanocomposite reduced highly as compare to neat nylon 6,6. The decrease in size of spherulites after loading the clay is might due to nucleation effect of kaolin clay in the nylon 6,6 polymer. The mechanical properties of nanocomposite were decreased as increased kaolin amount. The solvent uptake study presented that the nanocomposite uptake was low as compared to neat nylon 6,6. It was also found that the solvent uptake decreased as increased the amount of kaolin in nylon 6,6.