Spherulitic Structure Research Articles

Preparation of ECTFE porous membrane with a green diluent TOTM and performance in VMD process

In order to mitigate the harm of solvent volatilization during thermally induced phase separation (TIPS), more and more green solvents/diluents were developed, produced and applied in the membrane field. In this paper, trioctyl trimellitate (TOTM), an environmentally friendly diluent with high boiling point and low vapor pressure, was firstly introduced to prepare porous membranes, especially hydrophobic poly(ethylene-chlorotrifluoroethylene) (ECTFE) membranes by TIPS. Effects of ECTFE content on the membrane properties, including morphology, pore size distribution, porosity, liquid entry pressure of water (LEPw), water contact angle (WCA) and mechanical properties were characterized. Then the prepared ECTFE membranes were tested in vacuum membrane distillation (VMD). Phase diagram of this new ECTFE/TOTM binary system presented the existence of liquid-liquid (L-L) phase separation region when the ECTFE content was below 40%. The characterization results showed that the 15 wt% ECTFE membranes presented the continuous structure without the production of dense layer, but as the further increasing of polymer content, the spherulite structure began to be observed and surface became denser. Significantly, all the prepared ECTFE membranes exhibited excellent hydrophobicity. The liquid entry pressure of water (LEPw) and water contact angle (WCA) of 20 wt% ECTFE membranes reached 0.42 MPa and 139.9°, respectively. Moreover, in VMD, the prepared membranes exhibited high permeability (permeation flux was 23.09 kg/(m2.h)) and separation (salt rejection was 99.9%) performance. The above results show that the prepared ECTFE membrane showed promising stability performance in long-term operation of VMD for concentration of salt solution.

Peculiarities of structure and dielectric relaxation in ferroelectric vinylidene fluoride-tetrafluoroethylene copolymer at different crystallization conditions

Vinylidene fluoride (VDF)/tetrafluoroethylene (TFE) 94/6 copolymer films obtained by low-temperature crystallization from acetone solution and by melt-crystallization under high pressure were studied. As shown in the X-ray diffraction patterns, β- and γ- phases coexist in the sample crystallized from a solution. At melt-crystallization, a number of γ-crystals decrease and spherulitic supramolecular structure is formed. It is accompanied by a decrease of the amorphous phase density in the intra- and inter-spherulite regions, which leads to an increase in the glass transition temperature. Differential scanning calorimetry data indicate a smaller fraction of the amorphous phase for the sample crystallized from the melt. At the same time, the relaxation strength of the mobility in the amorphous phase in this sample is higher. This contradiction is explained by the increased ratio of isomers in planar zigzag conformation, as shown by IR spectroscopy.

Triggering the gas transport in PVdF-HFP membranes via imidazolium ionic liquids

Ionic Liquids (ILs) are attractive additives for developing advanced polymeric membranes owing to their low volatility, high chemical, thermal stability, affinity for specific gases and ability to influence membrane morphology. Self-standing poly(vinylidenefluoride-co-hexafluoropropylene)-based films were prepared by solution casting and solvent evaporation, incorporating selected imidazolium ILs up to 60 wt%. The films were characterized for morphology, thermal properties and stability, and spectral properties by scanning electron microscopy, differential scanning calorimetry, thermogravimetric analysis and Fourier transform infrared spectroscopy.Blend membranes resulted non-porous, with a spherulitic structure. ILs trigger the conversion of the polymer matrix from a predominantly apolar α-phase to polar crystalline phases, as also shown by the FT-IR results. The polymer blends resulted heterogeneous as shown by DSC and TGA analysis. The thermogravimetric analysis showed a reduced onset of degradation for the blends, with no evidence of complexation between polymer and the ILs. The calorimetric analysis suggested a plasticizing action due to the ionic liquids. This behaviour agrees with the experimental and modelling study on gas permeability. Indeed, the blend membranes displayed a significant enhancement of gas permeability (up to three order of magnitude) at the highest ionic liquid content. The ideal CO2/N2 selectivity was simultaneously improved more than 200% upon the addition of the BF4–based IL to the polymer. The observed gas transport properties were correlated to the anion chemistry and to the specific polymer/IL interactions.

Influence of composition and microstructure on transparency and diffusivity in ion‐exchangeable spinel glass‐ceramics

AbstractIon‐exchangeable, transparent spinel glass‐ceramics are presented and discussed here for the first time. To retain transparency with increasing crystallinity, spinel glass‐ceramics must have uniform crystallization of small (~9 nm) crystallites, not large spherulitic structures comprised of small crystallites. To obtain such a uniform microstructure, the amount of total nucleating agents (ZrO2 + TiO2) in the precursor glass composition must be greater than 5 mol%. With small changes in composition and significant differences in microstructure, the demarcation between transparent and opaque glass‐ceramics is distinct as is the decrease in K diffusivity during ion‐exchange from the transparent (14.7 microns2/h) to the opaque (11.2 microns2/h) compositions. Understanding how to retain transparency during ceramming and increase diffusivity during chemical strengthening is critical in designing materials for many real‐world applications.Ion‐exchangeable, transparent spinel glass‐ceramics are presented and discussed here for the first time. To retain transparency with increasing crystallinity, spinel glass‐ceramics must have uniform crystallization of small (~9 nm) crystallites, not large spherulitic structures comprised of small crystallites. To obtain such a uniform microstructure, the amount of total nucleating agents (ZrO2 + TiO2) in the precursor glass composition must be greater than 5 mol%. With small changes in composition and significant differences in microstructure, the demarcation between transparent and opaque glass‐ceramics is distinct as is the decrease in K diffusivity during ion‐exchange from the transparent (14.7 microns2/h) to the opaque (11.2 microns2/h) compositions. Understanding how to retain transparency during ceramming and increase diffusivity during chemical strengthening is critical in designing materials for many real‐world applications.

An Effect of Water Presence on Surface Exfoliation of Polypropylene Film Initiated by Photodegradation

A polypropylene (PP) film photodegradation test was performed in water at 60 °C with a specific photocatalyst under visible light irradiation to identify its fragmentation mechanism. The photodegradation degree reached a maximum at the 144 h photodegradation time and showed the decreases at the 288 h. The surface being uneven was observed at the 288 h in the SEM photograph. These results showed that the surface exfoliation was provoked by the test. The spherulite structure contributed to the surface microcrack location and growth. The microcrack allowed entrance of water into the inner amorphous part. The water caused an internal stress by expanding itself due to radiant heat from the visible light irradiation and induced a microcraze in the interlamellar amorphous region. The microcraze developed into an internal microcrack, and surface microcracks finally coalesced together, producing a planar exfoliation. The major long side length of the exfoliation part was below 100 μm at both the 144 h and 288 h. The weight change ratio and rate of the photodegraded PP film showed multi-stages with the increase of photodegradation time. The increment of positive rate was small, showing that the exfoliation caused suppression of the autoxidation rate. The negative rate was due to the exfoliation behavior and was nonlinear to the photodegradation time.

Recent Applications of Advanced Atomic Force Microscopy in Polymer Science: A Review.

Atomic force microscopy (AFM) has been extensively used for the nanoscale characterization of polymeric materials. The coupling of AFM with infrared spectroscope (AFM-IR) provides another advantage to the chemical analyses and thus helps to shed light upon the study of polymers. This paper reviews some recent progress in the application of AFM and AFM-IR in polymer science. We describe the principle of AFM-IR and the recent improvements to enhance its resolution. We also discuss the latest progress in the use of AFM-IR as a super-resolution correlated scanned-probe infrared spectroscopy for the chemical characterization of polymer materials dealing with polymer composites, polymer blends, multilayers, and biopolymers. To highlight the advantages of AFM-IR, we report several results in studying the crystallization of both miscible and immiscible blends as well as polymer aging. Finally, we demonstrate how this novel technique can be used to determine phase separation, spherulitic structure, and crystallization mechanisms at nanoscales, which has never been achieved before. The review also discusses future trends in the use of AFM-IR in polymer materials, especially in polymer thin film investigation.

Preparation of a novel dual-layer polyvinylidene fluoride hollow fiber composite membrane with hydrophobic inner layer for carbon dioxide absorption in a membrane contactor

In order to improve the polyvinylidene fluoride (PVDF) hollow fiber membrane performance for carbon dioxide (CO2) absorption in membrane contactors, dual-layer PVDF composite membranes combining the advantages of high gas permeability and high hydrophobicity of the inner layer were designed and prepared. The membranes were prepared through a homocentric triple-hole spinneret via the dry-jet wet-spinning phase inversion method, and the membrane structure and properties were optimized. The outer layer of the optimized membrane exhibits a large finger-like porous structure, while the inner layer is composed of PVDF and hydrophobic modified nano-silicon dioxide (SiO2), and shows a thin but dense spherulite stacking structure. The results of membrane gas absorption experiment demonstrated that the optimized membrane exhibited excellent CO2 mass transfer flux and long-term operation stability. CO2 absorption flux of the membrane contactor obtained 2.23 × 10−3 mol·m−2·s−1 under the conditions of the inlet gas (CO2/N2 = 19/81, v/v) flowing on the shell side at a flowrate of 100 mL/min and the absorbent liquid (1 mol/L DEA) flowing counter-currently on the tube side at a flowrate of 50 mL/min. This CO2 absorption flux decreased by 18% after 160 h of operation.

Controlling spherulitic structures at surface and sub-layer of hollow fiber membranes prepared using nucleation agents via triple-orifice spinneret in TIPS process

In this study, the growth of a spherulitic structure at the surface and sub-layer of the polyvinylidene fluoride (PVDF) hollow fiber membrane was controlled by co-extruding nucleation agent (NA) solutions as the outermost layer of a triple-orifice spinneret in the thermally induced phase separation (TIPS) process. Using this method, the spherulites were tailored to be a variety of morphologies having different combinations with the bi-continuous network at the membrane surface and sub-layer, while the bulk membrane structure remained almost unchanged. These typically tailored spherulitic structures dramatically enhanced the surface roughness, hydrophobicity, liquid entry pressure, and permeation stability of the TIPS-prepared PVDF hollow fiber membranes. The experimental results from the pressure-driven filtration and the direct contact membrane distillation demonstrated great potential of the tailored membranes to achieve water treatment and seawater desalination. The electrostatic interaction between NA and PVDF played a key role in tailoring the special membrane spherulite structures, providing new possibilities for engineering TIPS-prepared hollow fiber membranes with desirable structures and performances to address current and future water shortages.

Modelling of morphology development towards spherulites and shish–kebabs: Application to isothermal flow-induced crystallization experiments on isotactic polypropylene

The final properties of a plastic object are related to the structures developed during the process adopted for part production. The structures development is induced by the complex thermomechanical history experienced by the macromolecules during the process. In particular, for isotactic polypropylene it has been observed that flow field influences the spherulitic crystallization and also induces the formation, when strong flow fields are present, of fibrillar structures composed of a core (the shishes) oriented along the flow direction, and lamellae that grow in the transverse direction (the kebabs). In this work, a model able to describe the morphology development during the polymer crystallization is proposed. This model accounts for the morphologies, shish–kebabs and spherulites, which can develop during the crystallization of the α-phase of the considered iPP, the effect of flow on crystallization kinetics and the dependence of the relevant material properties on the crystallinity. Two competing kinetics are considered, one for spherulites and another one for the shish-kebabs. In order to validate the model predictions, isothermal flow induced crystallization experiments were conducted. The analysis of the flow induced crystallization tests adopting the proposed model correctly reproduces the formation of fibrillar and spherulitic structures and detects the shearing condition at which fibrillar morphology overcomes the spherulitic morphology.

Nonlinear Optical Diagnostics of Thin Polycrystalline Lead Zirconate Titanate Films

Nonlinear optical microscopy techniques have been used to study thin films of single-phase (perovskite) and two-phase (perovskite–pyrochlore) lead zirconate titanate (PZT) deposited on Pt/TiO2/SiO2/Si by means of radio-frequency magnetron sputtering at various target–substrate distances (D = 30–70 mm). Results revealed inhomogeneous distribution of second-harmonic generation in spherulitic perovskite islands, including increased signal intensity at perovskite/pyrochlore interface, which may be related to a nonuniform distribution of mechanical stresses. Factors responsible for the observed strong variation in second-harmonic signal are discussed, including the mutual relationship between changes in the character of spherulitic structure and conditions of PZT film deposition depending on the target–substrate distance.

Thermal dynamics affected formation and dislocation of PDLA morphology

The morphologies and structures of crystalline polymers depend on the crystallization condition, such as the quenching rate, crystallization temperature, pressure, and molecular weight, tacticity, branching, topology and so on. The morphologies and growth of poly (d-lactic acid) (PDLA) spherulites were observed by POM when the samples isothermally crystallized at various temperatures after cooling from 200 °C to 250 °C. The crystallization behavior of PDLA was characterized by DSC measurements at various temperatures and it is not consistent with the kinetics of spherulite growth, which is due to the difference between the characterized partial and total crystallization behavior. It was found that the formation of the banded spherulites of PDLA is not only dependent on the crystallization temperatures, but also on the melting temperatures. Light fluctuation of the POM images was investigated during heating and it was found that the dark bands or black bands disappeared earlier than the bright ones during heating of the banded spherulites. The observed melting phenomenon implies mutiple lamellar stacking model inside the spherulites. The formation condition of the banded spherulites morphology was discussed based on the crystallization rate and diffusion rate. The proposed mechanism would be helpful to understand the structure and formation of PDLA ring-banded spherulites.

Crystal-to-crystal transitions in nylon M5T

Crystal-to-crystal transitions of a high-temperature nylon, poly(2-methtyl pentamethylene terephthalamide, nylon M5T), were studied by conventional and modulated temperature DSC (DSC and MT-DSC, respectively), as well as modulated thermomechanical analysis (MT-TMA), wide-angle X-ray scattering (WAXS), dynamic mechanical analysis (DMA) and hot stage microscopy. This polymer can easily be quenched to the completely amorphous phase at a cooling rate as low as 80 °C min−1. This polymer has three different crystal forms plus a smectic phase, and these crystal forms change into each other during a continuous heating of the amorphous polymer as evidenced by MT-DSC, MT-TMA, DMA and WAXS. The crystal-to-crystal transitions of two vastly different molecular masses were investigated: One was prepared by polycondensation followed by chain extension (see Menczel et al. J Thermal Anal 46:753, 1996), and the other sample was made by simple polycondensation. Tg of the amorphous polymer (including the as-spun fiber) is 143 °C, which is about 25 °C higher for the drawn fiber. The crystal-to-crystal transitions take place between 170 and 290 °C. MT-DSC and DMA measurements (the reversing heat flow signal and the tensile storage modulus of the as-spun fiber) proved that these crystal-to-crystal transitions take place through melting. Efficient nucleating agents (carbon nanofibers) do affect the temperature of these transitions as well as the melt-to-crystal transition during cooling of the melt and also during heating of the amorphous polymer. Polarization optical microscopy observations suggest that the highest temperature crystal form obtainable in high-temperature melt isothermal crystallization is probably a smectic, and this phase changes to some spherulitic structure during further cooling. Non-isothermal and isothermal crystallization measurements let us conclude that the two high-temperature crystal forms (smectic and crystal form C) are thermodynamically stable (enantiotropic), while the two low-temperature crystal forms are metastable (monotropic). The Hoffman–Weeks method gave 339 °C for the equilibrium melting point of this polymer. DMA experiments gave three secondary transitions. A transition at ~ − 120 °C was assigned to a local mode relaxation of the methylene groups. The transition at − 65 °C corresponds to the onset of rotation of the amide groups, while another secondary transition at 63 °C is the onset of rotation of the phenylene groups.

Glycine-Chitosan-Based Flexible Biodegradable Piezoelectric Pressure Sensor.

This paper presents flexible pressure sensors based on free-standing and biodegradable glycine–chitosan piezoelectric films. Fabricated by the self-assembly of biological molecules of glycine within a water-based chitosan solution, the piezoelectric films consist of a stable spherulite structure of β-glycine (size varying from a few millimeters to 1 cm) embedded in an amorphous chitosan polymer. The polymorphic phase of glycine crystals in chitosan, evaluated by X-ray diffraction, confirms formation of a pure ferroelectric phase of glycine (β-phase). Our results show that a simple solvent-casting method can be used to prepare a biodegradable β-glycine/chitosan-based piezoelectric film with sensitivity (∼2.82 ± 0.2 mV kPa–1) comparable to those of nondegradable commercial piezoelectric materials. The measured capacitance of the β-glycine/chitosan film is in the range from 0.26 to 0.12 nF at a frequency range from 100 Hz to 1 MHz, and its dielectric constant and loss factor are 7.7 and 0.18, respectively, in the high impedance range under ambient conditions. The results suggest that the glycine–chitosan composite is a promising new biobased piezoelectric material for biodegradable sensors for applications in wearable biomedical diagnostics.

Fabrication of poly(vinylidene fluoride) membrane via thermally induced phase separation using ionic liquid as green diluent

Ionic liquid (IL), 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM]PF6) as a new and environmentally friendly diluent was introduced to prepare poly(vinylidene fluoride) (PVDF) membranes via thermally induced phase separation (TIPS). Phase diagram of PVDF/[BMIM]PF6 was measured. The effects of polymer concentration and quenching temperature on the morphologies, properties, and performances of the PVDF membranes were investigated. When the polymer concentration was 15 wt%, the pure water flux of the fabricated membrane was up to nearly 2000 L·m −2 ·h −1 , along with adequate mechanical strength. With the increasing of PVDF concentration and quenching temperature, mean pore size and water permeability of the membrane decreased. SEM results showed that PVDF membranes manufactured by ionic liquid (BMImPF6) presented spherulite structure. And the PVDF membranes were represented as β phase by XRD and FTIR characterization. It provides a new way to prepare PVDF membranes with piezoelectric properties.

Rheo-Raman spectroscopic study of plasticity and elasticity transformation in poly(ether-block-amide) thermoplastic elastomers

The microscopic deformation behavior of polyamide (PA) elastomers (PAEs) with various PA contents has been investigated by rheo-Raman spectroscopy. The PAEs with relatively high PA content show a uniaxial molecular orientation and the stretching load is isotropically applied on the crystalline chains, similar to typical semi-crystalline polymers. Conversely, the crystalline chains of the PAEs with relatively low PA content show a biaxial molecular orientation up to the highly strained region. Moreover, the stretching load is anisotropically applied on the crystalline chains located in the equatorial region of the spherulites. The elasticity of the PAEs with relatively low PA content is ascribed to the affine-like deformation of the spherulitic structure owing to the existence of soft interlamellar layers.

Crystal aggregation into periodically grating-banded assemblies in phthalic acid modulated by molten poly(ethylene oxide)

A small-molecule compound, phthalic acid (PA), crystallized in the presence of poly(ethylene oxide) (PEO) with various compositions was utilized as a model to investigate the morphology and crystal assembly of periodically ordered structures in banded spherulites.

Нелинейно-оптическая диагностика поликристаллических тонких пленок цирконата-титаната свинца

Using nonlinear optical microscopy, single-phase (perovskite) and double-phase (perovskite+pyrochlore) thin films of lead zirconate titanate deposited on Pt/TiO2/SiO2/Si substrates by the method of high-frequency magnetron sputtering at different distances of the substrate targets (D=30-70 mm) were studied. An inhomogeneous distribution of the second harmonic signal in spherulite perovskite islands was detected, including an increase in the signal at the perovskite/pyrochlore boundary, which may be due to an inhomogeneous distribution of mechanical stresses. The reasons for strong changes in the signal of the second harmonic, its relationship to changes in the character of the spherulite structure and conditions of film deposition when the distance of the target-substrate varies are discussed.

Spherulitic Crystallization in Langmuir-Blodgett Films of the Ditetradecyldimethylammonium-Au(dmit)2 Salt.

Spherulitic crystallization in Langmuir-Blodgett (LB) films of the ditetradecyldimethylammonium-Au(dmit)2 [2C14N+Me2-Au(dmit)2] salt has been characterized by polarized light microscopy, Fourier-transform infrared (FT-IR) spectroscopy, and X-ray diffraction (XRD) analyses. Analyses by differential scanning calorimetry (DSC) for the bulk crystals indicate that annealing in the temperature range of 58-100 °C may be appropriate to improve the order in the LB film. The polarized light microscopy measurement further revealed that a spherulite structure was formed after the film was annealed at 80 °C for 60 min. FT-IR spectroscopy measurements demonstrated that the order of the principal hydrocarbon chains was improved and that the rotation of CH3 groups was hindered by the annealing. Out-of-plane XRD analyses revealed that the d-spacing of the 2C14N+Me2-Au(dmit)2 LB film changed from 3.1 to 2.5 nm upon annealing. We hypothesize that a layered structure with interdigitated hydrocarbon chains, which is equivalent to or close to that of the corresponding bulk crystal, has been realized in the LB film by annealing. We consider that two different kinds of one-dimensional (1D) interactions along the a-axis are the driving forces to realize the spherulite structure in the LB system; one is an extended 1D contact due to the pronounced interdigitation of the alkyl chains of the ammonium ion, which plays a role similar to that of the folding of chains in the lamellar structures of polymers, and the other is a 1D extended sulfur-sulfur contact between Au(dmit)2 dimer pairs. So far, spherulite formation in LB films has been reported almost exclusively for polymerized materials. Here, we demonstrate that a spherulite texture can also be formed in LB films based on nonpolymerized materials via the interdigitation of hydrocarbon chains, leading to a new well-ordered state.

Effect of mixed diluents during thermally induced phase separation process on structures and performances of hollow fiber membranes prepared using triple-orifice spinneret

In this study, polyvinylidene fluoride hollow fiber membranes were prepared using a triple-orifice spinneret in thermally induced phase separation. A solvent, propylene carbonate (PC), was extruded through the outermost layer of the spinneret. Through the use of this method, all the membranes were developed an entirely porous surface with similar pore size and porosity. Using mixed diluents with different concentrations of diphenyl carbonate and PC, the membrane bulk structures were controlled from an entirely bicontinuous network, a combination of bicontinuous and spherulitic structures, to an entirely spherulitic structure. The tailored bulk structures showed significant effects on the permeability, rejection, and mechanical strength of the membrane. Furthermore, the membrane sub-layer structure was modified to form different types from the composite-like structure of the combination of the spherulites and bicontinuous network to entirely spherulites, which controlled the membrane permeation stability within an extensive range of 6% to nearly 100%. A change in the polymeric dope solution phase diagram, as well as the interfacial behaviors of the polymeric dope solution and extruded PC, played important roles in appropriately tailoring membrane structures and performances. These novel and solid findings regarding the connections between structures and performances provide an advisable venue for the design of hollow fiber membranes with satisfactory performances.

Influence of the crystalline structure on the fragmentation of weathered polyolefines

Blown-extruded low density polyethylene (LDPE) and polypropylene (PP) films were submitted to accelerated weathering in water in order to identify their fragmentation mechanism and to compare the fragments number, size and shapes generated for long irradiation time. The study of the chemical modification during weathering was performed by following the progressive oxidation (FTIR, contact angle), increase of crystallinity (DSC) and surface rigidity (AFM) for the two polymers. It was demonstrated that although the kinetics of degradation is faster for PP than for LDPE films, the same fracture mechanism namely slow crack propagation is observed for both polymers. The analysis of the cracks structures on the surface of the films by AFM indicates that the crack initiation and propagation strongly depends on the crystalline morphology of the polymer. Indeed, for both material cracks propagation mainly occurs in the direction perpendicular to the extrusion direction explaining the elongated shape observed for the LDPE fragments. However, for PP film, the presence of spherulite structures induced the initiation of cracks in other directions (between the lamellae) leading to the formation of an increased number of fragments significantly smaller and with non-elongated shape compared to LDPE. To our knowledge this study is the first to directly link the size, shape distributions of plastics fragments to the crystalline morphology of the weathered polymers. These results point out the importance of taking into account the crystalline morphology to predict plastic fate in the environment.