Surface-induced Crystallization Research Articles

Phosphate Recovery Mechanism from Low P-Containing Wastewaters via CaP Crystallization Using Apatite as Seed: Seed Adsorption, Surface-Induced Crystallization, or Ion Clusters Aggregation?

Low P-containing wastewaters (LPWs) exhibit huge P recovery potential, considering their larger volume. P recovery via CaP crystallization using apatite as seed is documented as being potentially well suited for LPWs. However, its responsible mechanisms remain a subject for debate. Taking hydroxyapatite (HAP) as the seed of LPWs, this paper conducted HAP adsorption/dissolution experiments, titration experiments, and P recovery experiments to distinguish the primary responsible mechanism. Results showed that it was HAP dissolution, not P adsorption, that occurred when the initial P concentration was no higher than 5 mg/L, ruling out adsorption mechanism of P recovery from LPWs using HAP as the seed. Significant OH− consumption and rapid P recovery occurred simultaneously within the first 60 s in titration experiments, suggesting CaP crystallization should be responsible for P recovery. Moreover, the continuous increase in P recovery efficiency with seed dosages observed in P recovery experiments seemed to follow well the mechanism of pre-nucleation ion clusters (PNCs) aggregation. During PNCs aggregation, P aggregates with Ca2+ quickly, generating CaP PNCs; then, CaP PNCs aggregate with seed particles, followed by CaP PNCs fusion, and ultimately transform into fines attached to the seed surface. PNCs’ aggregation mechanism was further supported by a comparison of seed SEM images before and after P recovery, since denser and smaller rod-shaped fines were observed on the seed surface after P recovery. This study suggests that PNCs’ aggregation is the dominant mechanism responsible for the recovery of P from LPWs via CaP crystallization using HAP as the seed.

Hierarchically Biomimetic Scaffolds with Anisotropic Micropores and Nanotopological Patterns to Promote Bone Regeneration via Geometric Modulation.

Structural engineering is an appealing means to modulate osteogenesis without the intervention of exogenous cells or therapeutic agents. In this work, a novel 3D scaffold with anisotropic micropores and nanotopographical patterns is developed. Scaffolds with oriented pores are fabricated via the selective extraction of water-soluble polyethylene oxide from its poly(ε-caprolactone) co-continuous mixture and uniaxial stretching. The plate apatite-like lamellae are subsequently hatched on the pore walls through surface-induced epitaxial crystallization. Such a unique geometric architecture yields a synergistic effect on the osteogenic capability. The prepared scaffold leads to a 19.2% and 128.0% increase in the alkaline phosphatase activity of rat bone mesenchymal stem cells compared to that of the scaffolds with only oriented pores and only nanotopographical patterns, respectively. It also induces the greatest upregulation of osteogenic-related gene expression in vitro. The cranial defect repair results demonstrate that the prepared scaffold effectively promotes new bone regeneration, as indicated by a 350% increase in collagen I expression in vivo compared to the isotropic porous scaffold without surface nanotopology after implantation for 14weeks. Overall, this work provides geometric motifs for the transduction of biophysical cues in 3D porous scaffolds, which is a promising option for tissue engineering applications.

Selecting for Surface-Induced vs Bulk Crystallization in P3HT Thin Films: Distinct Morphology, Orientation, and Linear Growth Rates

Selecting for Surface-Induced vs Bulk Crystallization in P3HT Thin Films: Distinct Morphology, Orientation, and Linear Growth Rates

Surface-induced oxidation of Mn(II) and crystallization of manganese (hydr)oxides on clay minerals

The crystallization induced by foreign surfaces is crucial for various geological and geochemical processes on Earth’s surface, but the underlying mechanisms remain poorly understood. In this study, we investigated the oxidation of Mn(II) and crystallization of manganese (hydr)oxides (MnOx) on kaolinite (Kln) and montmorillonite (Mnt), two prevalent clay minerals in the nature with different structures and properties. Our results reveal that both Kln and Mnt surfaces can catalyze the heterogeneous oxidation of Mn(II) and crystallization of MnOx, but the crystallization pathways, as well as the morphologies and phases of the products, are distinct. The Mn(II) adsorbed by Kln’s outer surfaces were oxidized to Mn(III) and crystallized into MnOx, which were identified as hausmannite and manganite nanoaggregates. In contrast, the adsorption of Mn(II) and its oxidation to Mn(III) occurred both on the outer surfaces and interlayer spaces of Mnt, while the crystallization of MnOx only occurred on the outer surfaces, indicating that the confined volume of Mnt’s interlayer spaces can inhibit the crystallization of Mn(II/III) cations. Hausmannite emerged as the first MnOx product on Mnt, followed by groutite and manganite, all of which were well-dispersed fine nanoparticles. Additionally, the Si(IV) ions dissolved from Mnt participated in the formation of rhodonite with Mn(II), but this mineral phase was absent on Kln. Notably, MnOx nanoaggregates on Kln were assembled by a group of MnOx nanoparticles with slight misorientations among them, and the misorientations of these nanoparticles can gradually recover over time, indicating the contribution of crystallization by particle attachment (CPA). On the other hand, the strong electrostatic interactions between MnOx nanoparticles and Mnt surfaces reduced the mobility of nanoparticles, inhibiting their growth via CPA. Above results demonstrate the significant influence of the confinement effect and surface interactions on the surface-induced crystallization process of MnOx. These findings can also advance our knowledge about the role of clay minerals in Mn(II) oxidation and MnOx formation, and offer a possible explanation for the presence of various MnOx with diverse structures in the nature.

Effects of annealing in vacuum on the microstructure of silicon-containing calcium phosphate films deposited on a ZrNb alloy by radio-frequency magnetron sputtering

Coatings were deposited on the surface of a ZrNb alloy by radio-frequency magnetron sputtering using targets consisting of either pure crystalline hydroxyapatite (HA) or silicon-containing HA (Si-HA). The coated ZrNb samples were heated in a vacuum furnace at a pressure of 5 × 10−5 Torr and temperature 700 °C for 2 h. Precursor powders of silicate-substituted HA [Ca10(PO4)6−x(SiO4)x(OH)2−x, x = 0, 0.5, or 1.72] were used to fabricate the targets for sputtering. The aim of this study was to analyze the effect of Si addition to HA on crystallization behavior and structure of the resulting films using transmission electron microscopy and electron diffraction patterns of selected sites. The average thickness of the films was in the range of 175–345 nm. The Si substitution in HA played a major role in the mechanisms of coating formation and crystallization behavior, which influenced the structure and crystallinity of the layers deposited by radio-frequency magnetron sputtering. Moreover, the results indicate that surface-induced crystallization of the deposited thin films is more pronounced than internal crystallization. A possible mechanism underlying the observed decrease in the crystallization rate with the increasing silicate content in the HA-based films is discussed.

Tunable β-crystals formation from transcrystallinity to cylindrites at PP/PE interface via using melt penetration engineering

Transcrytallinity plays a critical role in the interfacial crystalline morphology. However, it is rather rare to observe transcrytallinity grown from completely molten surface without using nucleating agent. Herein, we utilize melt penetration engineering to provide a sheared polymeric melt interface in order to tailor interfacial polypropylene (PP) crystalline morphology from β-transcrystals (β-TC) to β-cylindrites. The investigated system containing polymorphic PP as penetrating phase and polyethylene (PE) as penetrated layer were selected. Interestingly, we can clearly observe a continuous and ordered β-transcrystals (β-TC) at PP/PE interface and morphological transition from β-TC to β-cylindrites upon using higher molecular weight PE as the penetrated melt. Foreign PE surface plays an indispensable role in transcrystallinity formation since only α-spherulites were developed during identical PP penetrating PP process. The effects of surface-induced crystallization as well as coupling flow and temperature field have been discussed, in order to offer an insight on interfacial morphological transition. Consequently, melt penetration engineering can provide a facile and scalable route to achieve designed interfacial morphology from β-TC to β-cylindrites, which opens a new perspective to develop interfacial crystalline morphology. • Melt penetration to design interface. • Tunable structure from transcrystal to cylindrite. • β-interfacial morphology is developed.

Surface Nucleation of Dispersed Polyethylene Droplets in Immiscible Blends Revealed by Polypropylene Matrix Self-Nucleation

Usually, the nature of surface-induced nucleation in polymer blends is not easily disclosed. A novel approach for studying surface-induced crystallization in blends of semicrystalline polymers is proposed here. It consists of detecting variations in the crystallization kinetics of the dispersed phase with changing the crystalline state of the matrix through self-nucleation. It can be used only when the dispersed phase has a lower melting temperature than the matrix phase. As a case study, the crystallization behavior of dispersed polyethylene droplets in a polypropylene matrix was investigated. An enhancement of crystallization kinetics of polyethylene was achieved when the lamellar thickness of polypropylene increased, and it was proved by the formation of a transcrystalline layer of polyethylene at the interface, as observed by scanning electron microscopy. Compared to the self-nucleated neat polyethylene, the efficiency of the nucleating effect of polypropylene toward polyethylene was estimated around 140%. This result together with a very low value for the interfacial free energy difference as obtained from isothermal crystallization measurements is evidence that such surface-induced nucleation occurs through epitaxial growth. Moreover, a mechanism of polyethylene nuclei formation through epitaxy, which was proposed in the literature, was proved to be valid for blends of the two polymers through small- and wide-angle X-ray scattering structural analysis. While epitaxy between polyethylene and polypropylene was previously shown only for ideal systems such as thin-layered films, it is hereby reported for common melt mixed blends of the two polyolefins.

Surface-Induced Crystallization of Sodium Dodecyl Sulfate (SDS) Micellar Solutions in Confinement.

We investigate the role of confinement on the onset of crystallization in subcooled micellar solutions of sodium dodecyl sulfate (SDS), examining the impact of sample volume, substrate surface energy, and surface roughness. Using small angle neutron scattering (SANS) and dynamic light scattering (DLS), we measure the crystallization temperature upon cooling and the metastable zone width (MSZW) for bulk 10-30 wt% SDS solutions. We then introduce a microdroplet approach to quantify the impact of surface free energy (18-65 mN/m) and substrate roughness (Rα ≃ 0-60 μm) on the kinetics of surface-induced crystallization through measurements of induction time (ti) under isothermal conditions. While ti is found to decrease exponentially with decreasing temperature (increasing subcooling) for all tested surfaces, increasing the surface energy could cause a significant further reduction of up to ∼40 fold. For substrates with the lowest surface energy and longest ti, microscale surface roughness is found to enhance crystal nucleation, in particular for Rα ≥ 10 μm. Finally, we demonstrate that tuning the surface energy and microscopic roughness can be effective routes to promote or delay nucleation in bulk-like volumes, thus greatly impacting the stability of surfactant solutions at lower temperatures.

Crystallization Kinetics Analysis of the Amorphouse Mg72Zn24Ca4 Alloy at the Isothermal Annealing Temperature of 507 K.

This paper presents tests of metallic glass based on Mg72Zn24Ca4 alloy. Metallic glass was made using induction melting and further injection on a rotating copper wheel. A differential scanning calorimeter (DSC) was used to investigate the phase transformation of an amorphous ribbon. The tests were carried out at an isothermal annealing temperature of 507 K. The Kolmogorov-Johnson-Mahl-Avrami-Evans model was used to analyze the crystallization kinetics of the amorphous Mg72Zn24Ca4 alloy. In this model, both Avrami’s exponent n and transformation rate constant K were analyzed. Both of these kinetic parameters were examined as a function of time and the solid fraction. The Avrami exponent n value at the beginning of the crystallization process has value n = 1.9 and at the end of the crystallization process has value n = 3.6. The kinetic constant K values change in the opposite way as the exponent n. At the beginning of the crystallization process the constant K has value K = 9.19 × 10−7 s−n (ln(K) = −13.9) and at the end of the crystallization process has the value K = 6.19 × 10−9 s−n (ln(K) = −18.9). These parameters behave similarly, analyzing them as a function of the duration of the isothermal transformation. The exponent n increases and the constant K decreases with the duration of the crystallization process. With such a change of the Avrami exponent n and the transformation rate constant K, the crystallization process is controlled by the 3D growth on predetermined nuclei. Because each metallic glass has a place for heterogeneous nucleation, so called pre-existing nuclei, in which nucleation is strengthened and the energy barrier is lowered. These nuclei along with possible surface-induced crystallization, lead to rapid nucleation at the beginning of the process, and therefore a larger transformed fraction than expected for purely uniform nucleation. These sites are used and saturated with time, followed mainly by homogeneous nucleation. In addition, such a high value of the Avrami exponent n at the end of the crystallization process can cause the impingement effect, heterogeneous distribution of nuclei and the diffusion-controlled grain growth in the Mg72Zn24Ca4 metallic glassy alloy.

Surface-induced enantiomorphic crystallization of achiral fullerene derivatives in thin films.

The chirality of organic semiconductors is important for various applications in optoelectronics and spintronics. Here, we propose a new strategy to induce structural chirality in achiral organic semiconductors in thin films. Enantiomeric fullerene derivatives (S)-pSi and (R)-pSi, which have oligo(dimethylsiloxane) as a low-surface-energy moiety, were synthesized and used as surface-segregated monolayers (SSMs) in spin-coated films of several achiral fullerene derivatives. Upon thermal annealing, the presence of the chiral SSMs led to the crystallization of the fullerenes in the films as an SSM-induced crystal phase at lower temperatures. The crystallized films showed circular dichroism ascribed to the fullerene absorption, the sign and the intensity of which depended on the handedness of the SSM molecules and the film thickness, respectively. These results indicate that the achiral fullerene derivatives in the films were induced by the SSMs to crystallize into enantiomorphic crystals. Our approach to inducing chirality in organic thin films is compatible with many device applications.

Structure and High Performance of Lead-Free (K0.5Na0.5)NbO3 Piezoelectric Nanofibers with Surface-Induced Crystallization at Lowered Temperature.

Lead-free potassium and sodium niobate (KNN) nanofiber webs with random and aligned configurations were prepared by the electrospinning process from polymer-modified chemical solution. The crystallization process, structure, composition, dielectric, ferroelectric, and piezoelectric properties of the nanofibers and nanofiber webs were investigated. Theoretical analysis and experimental results showed that the surface-induced heterogeneous nucleation resulted in the remarkable lower crystallization temperature for the KNN nanofibers with the {100} orientation of the perovskite phase in contrast to the bulk KNN gel and thus well-controlled chemical stoichiometry. Low dielectric loss, large electric polarization, and high piezoelectric performance were obtained in the nanofiber webs. In particular, the aligned nanofiber web exhibited further improved piezoelectric strain and voltage coefficients and higher FOM than their thin film counterparts and is promising for high-performance electromechanical sensor and transducer applications.

Real-Time Atomic Scale Observation of Surface-Induced Crystallization of a Bismuth Nanodroplet by Stepwise Ordering Mechanism

Surface-induced crystallization of an isotropic Bi nanodroplet in three distinct steps was recorded at atomic resolution by in situ high resolution transmission electron microscopy. First, formation of a crystalline nucleus at the edge of the nanodroplet is induced by the liquid-state surface faceting, which initiates the formation of a prefreezing state with a distorted structure. Then, a periodic line pattern of the atomic columns with a periodicity of 0.49 nm that is relative to the interlayer spacing of the Bi crystal along the ⟨112⟩* direction (one-dimensional ordered structure) was formed by structure relaxation of the prefreezing nanoparticle, followed by a concerted growth and ordering process. Finally, the crystallization from the periodic structure was carried out by atom/atom interactions within and between the atomic layers to form the covalent Bi–Bi bonds that have the same arrangement as in the Bi crystal (crystallization along the ⟨110⟩ direction). Motivated by the experimental observation...

Molecular dynamics simulations of polymer crystallization under confinement: Entanglement effect

We carried out molecular dynamics simulations to study the crystallization of polymer melts subjected to confinement between two parallel walls. Two types of walls, bare and grafted walls, are studied. The bare walls are slippery for chain motions, and the interactions between polymer chains and the walls are moreover chosen either attractive or repulsive. The crystallization in the case of bare walls generally consists of surface-induced processes close to the walls followed by homogeneous nucleation in both the boundary and middle regions. In the case of grafted walls, parts of polymer chains residing close to the walls are adhesive to the surfaces and become permanent graft points. We find that the surface-induced crystallization is increasingly suppressed with increasing grafting density. At high grafting densities, only crystallization in the middle regions is observed. We calculated the spatial distribution of entanglement lengths and related it to the crystallization behavior. The entanglement length close to the walls is found to decrease with increasing grafting density, as the adhesion points act as effective entanglement knots. In light of our recent results that less entangled polymer melts lead to faster crystallization and higher crystallization order, we now show that this conclusion stands also for the case of confined polymer melts. Our results suggest entanglements to be an universal factor towards the understanding of polymer crystallization under different situations, in particular at supercooling.

Heterogeneous-surface-mediated crystallization control

The crystallization of matter at interfaces has long been a significant issue in science and technology, but surface-directed crystallization with controlled kinetics remains a matter of challenge. Here, we demonstrate a conceptual novel mechanism to steer liquid–solid phase transformation at interfaces by tailoring the chemical and structural inhomogeneity of a glass substrate through self-limited nanocrystallization of the glassy phase. Importantly, this approach enables large-scale development of metastable crystallization products, such as nanowire membranes. The thorough studies of the intermediate stages of crystallization reveal a unique cooperative mechanism in which the intricate interplays between inherent nanoscale forces and unique heterogeneous surfaces contribute to the mesoscale structural transformation from isolated units to superstructures. We further show that the constructed superstructures offer unprecedented opportuntities for the development of functional membrane systems possessing the combination of robust trace-detection performance and molecular trapping function. These findings not only present clear technological implications, but also provide an improved understanding of the fundamental mechanisms of surface-induced geological and biological processes. A glassy surface embedded with zones of crystalline order can spur materials to form nanowires and molecule-trapping membranes on demand. Surface-induced crystallization plays important roles in processes ranging from bone biomineralization to transistor fabrication. Shifeng Zhou from South China University of Technology in Guangzhou and co-workers have reported a great discovery that glass-ceramics surface can help to control liquid-to-solid transformations at interfaces without resorting to chemical treatments or specialized templates. Using thermal treatments, the team precipitated titanium dioxide nanocrystals into an otherwise homogenous inorganic oxide glass. The resulting non-uniform interface created periodic potential field that enabled strict control over crystal nucleation and aggregation. Additional charge imbalances between tiny aggregations guided materials to grow into remarkable superstructures. As a practical example, microporous silver membranes that detect trace amounts of hazardous aromatic vapours were demonstrated. A new surface-directed crystallization mechanism enabled by the chemical and structural inhomogeneity of the glass substrate through self-limited nanocrystallization of glassy phase was proposed and demonstrated. Benefiting from the intricate interplays between nanoscale force and heterogeneous surface, the strategy offers new opportunities in the large-scale development of various metastable crystallization products and also serves practical purpose.

Crystallization of linear low density polyethylene on an in situ oriented isotactic polypropylene substrate manipulated by an extensional flow field

In this work, we demonstrate that utilization of extensional flow with different intensities can regulate the flow-induced crystallization and epitaxially surface-induced crystallization simultaneously in crystalline–crystalline immiscible blends, leading to improved interfacial adhesion and thus enhanced mechanical properties, which provides a versatile methodology to industrially achieve polymer blends with advanced performance. An accessible methodology, i.e., “extrusion–hot stretching–quenching”, was applied to fulfill the scalable achievement of an epitaxial interface for a linear low density polyethylene (LLDPE)/isotactic polypropylene (iPP) blend, where LLDPE could epitaxially grow on an oriented iPP substrate but greatly influenced by the flow field, with its chains and lamellae aligned abnormally off the flow direction revealed by wide angle X-ray diffraction and small angle X-ray scattering, respectively. Depending on the intensity of flow, the above effect of flow can be divided into two types: under a strong flow field, the LLDPE chains prefer to align along the flow direction, inducing the formation of a shish-kebab structure. For another type, i.e., under a weak flow field, the pre-oriented LLDPE chains can relax quickly and epitaxially nucleate on the surface of the oriented iPP substrate. During further growth, the epitaxial LLDPE lamellae deform and reorient along the flow direction under the mechanism of flow-induced block slips, fragmentation and reorientation. Moreover, it is believed that incomplete lamellar twist also occurs under flow. Mechanical property tests demonstrate that an epitaxial structure significantly improves the interfacial adhesion between LLDPE and iPP, showing remarkable enhancements in both strength and toughness.

Electrospun poly(butylene terephthalate) fibers: Entanglement density effect on fiber diameter and fiber nucleating ability towards isotactic polypropylene

Electrospun poly(butylene terephthalate) (PBT) fibers were prepared from trifluoroacetic acid (TFA)-based solvent. Rheological studies revealed the concentration (ϕ, volume fraction) dependence of zero-shear viscosity (ηo) to be ηo ∼ ϕ3.87 in the entangled solution regime, and the entanglement concentration (ϕe) was 7.84 vol.%. Effects of ηo on the straight jet-end diameter (dj) and fiber diameter (df) were investigated; two scaling laws existed for PBT/TFA solutions, i.e., dj ∼ ηo0.06 and df ∼ ηo0.73. Moreover, df can be scaled with entanglement density and solution plateau modulus (GN) through df ∼ (ϕ/ϕe)2.6 ∼ GN. Results of differential scanning calorimetry and wide-angle X-ray diffraction revealed that as-spun PBT fibers were not amorphous but contained imperfect small α crystallites. During stepwise annealing, crystal perfection gradually occurred, and well-ordered, α-form crystal was developed at temperatures above 160 °C. When electrospun PBT fibers were embedded in isotactic polypropylene (iPP) matrix, fiber surface-induced crystallization readily occurred to develop a transcrsytalline layer of iPP monoclinic crystals at the interface. Similar effects were found for electrospun fibers of poly(ethylene terephthalate) (PET) and poly(trimethylene terephthalate) (PTT). The conventional method of counting iPP nuclei occurring on the fiber surface to determine fiber nucleation rate was infeasible because of the fine fiber diameter. A new approach to characterize the nucleating ability of polyester fibers with submicron diameter was developed. By measuring the intensity of depolarized light during cooling from the melt, the initial temperature (Ti) for iPP transcrystallization was determined and compared. Based on the derived Ti, the nucleating ability of electrospun fibers toward iPP followed the order PBT > PET ∼ PTT. This finding was consistent with isothermal crystallization results. Moreover, the maximum temperature for PBT fiber to induce crystallization of iPP was ca. 157 °C, which was 20 °C higher than those of PET and PTT fibers. The remarkable nucleating ability of PBT fibers was likely associated with epitaxial crystallization and the large substrate crystal dimension in the matching direction, as revealed by atomic force microscopy.

Effect of surface texture on freezing in nanopores: surface-induced versus homogeneous crystallization.

Freezing of argon in ordered and disordered carbon pores of a similar diameter D ∼ 2.4 nm is investigated using extensive molecular simulations with large system sizes up to 10(4) atoms. While crystallization in the atomistically smooth pore consists in a surface-induced phase transition occurring at a temperature larger than the bulk, crystallization in the disordered pores, which is only partial as it is spatially restricted to the pore center, occurs through homogeneous crystallization. These results shed light on solidification in pores by showing that there is a crossover between surface-induced and homogeneous crystallization upon increasing the surface disorder of the host material. In the latter case, the Gibbs-Thomson equation, in which crystallization is assumed to occur when the crystal size equals the pore size corrected for the thickness of the unfreezable layer at the pore surface, is in reasonable agreement with the observed freezing temperature.

Electrospun isotactic polypropylene fibers: Self-similar morphology and microstructure

Macroscopically aligned isotactic polypropylene (iPP) fibers with different diameters ranging from 0.7 to 13.2 μm were successfully fabricated by high-temperature solution electrospinning using an infrared (IR) heating system. Polarized optical microscope (POM), scanning electron microscope (SEM), wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC) and polarized Fourier transform infrared (FTIR) were employed to characterize the surface morphology and microstructure of the electrospun fibers. From SEM images, three macroscopically structural models have been found: (1) cylindrical fibers, (2) twisted fibers and (3) beaded fibers. Interestingly, regardless of fiber's shape and size, they (especially for cylindrical fibers) exhibit well-defined self-similar morphology. That is, the tertiary, secondary and primary morphology are similar with each other. Thermally induced phase separation (TIPS) is the most pertinent origin to explain the formation of self-similar morphology. WAXD and DSC results show that α-form crystal prevails in the electrospun iPP fibers. For comparison, the solution-cast film was also prepared under the same conditions as those adopted in electrospinning except that the voltage was 0 kV. Compared with the solution-cast film, lower crystallinity, smaller average crystallite size and higher melting point are achieved in the electrospun iPP fibers. The results of polarized FTIR spectra demonstrate that the macroscopically aligned electrospun fibers are highly oriented along the fiber's longitudinal axis at molecular level. Furthermore, when the electrospun iPP fibers were embed in iPP matrix, surface-induced crystallization is significantly enhanced and finally develops into transcrystalline layer which can be even revived after recrystallization. As a result, it is logical that long-lived thermodynamically stable structure has been formed in the electrospun iPP fiber. Combined with FTIR result, the α-form crystal formed in the fiber is conjectured to be a kind of oriented structure with thermodynamically long-lived stability which is similar to "shish-kebab" structure.

Solution-Electrospun Poly(ethylene terephthalate) Fibers: Processing and Characterization

Electrospun poly(ethylene terephthalate) (PET) fibers were prepared from a trifluoroacetic acid (TFA)-based solvent. Rheological studies revealed the concentration (ϕ) dependence of the specific viscosity (ηsp) to be ηsp ∼ ϕ3.7 for PET/TFA solutions in the entangled regime. The determined entanglement concentration (ϕe) was higher using a lower-molecular-weight PET. To obtain bead-free fibers, the minimum concentration for the electrospinning was 0.8–1.0ϕe owing to the high volatility of TFA solvent, which significantly enhanced the chain network strength during jet whipping. The double-logarithmic plots of the jet (dj) and fiber (df) diameters versus the zero-shear viscosity (η0) revealed that two scaling laws existed for the present solutions, i.e., dj ∼ η00.06 and df ∼ η00.77. The microstructural evolution of the electrospun PET fibers from stepwise annealing to crystal melting was investigated by simultaneous small-angle X-ray scattering (SAXS)/wide-angle X-ray diffraction (WAXD) measurements using synchrotron radiation sources. The conformer transformation from gauche to trans was monitored by in-situ Fourier transform infrared spectral measurement. In the absence of any WAXD reflection, the as-spun PET fibers possessed a SAXS scattering peak, indicating the presence of a mesomorphic phase with an interdomain distance of 6.8 nm. At annealing temperatures (Ta) higher than 100 °C, the mesomorphic phase gradually transformed into imperfect triclinic crystals and reached its saturation at 130 °C. Further increased Ta perfected the triclinic structure without altering fiber crystallinity until the initial crystal melting at 218 °C, at which a significantly increased long period was detected. When the electrospun PET fibers were embedded in an isotactic polypropylene (iPP) matrix, surface-induced crystallization occurred to develop a transcrsytalline layer of iPP monoclinic crystals at the interface.

Effects of Temperature and Template Surface on Crystallization of Syndiotactic Polystyrene in Cylindrical Nanopores

Crystallization of syndiotactic polystyrene (sPS) in anodized aluminum oxide (AAO) templates is investigated by FTIR. Nanopore surfaces of AAO templates are modified with n-hexyltrimethoxysilane to produce templates with alkyl surfaces, and effects of crystallization temperature and surface property of the template on the degree of crystallinity of sPS in nanopores of various sizes are studied. When isothermally crystallized at 245–260 °C, while the crystallinity in the bulk barely varies, in pristine and modified nanopores it increases substantially with degree of supercooling, which may be attributed to more surface-induced crystallization at lower temperatures. For all crystallization temperatures and pore sizes studied, sPS exhibits much lower crystallinity in the surface-modified templates, likely due to suppression of surface nucleation by the lower energy surfaces. This work reveals the significant role surface plays in polymer crystallization in nanotemplates and may aid the design and fabrication...