Surface-induced Crystallization Research Articles

Isothermal Crystallization of Short Polymer Chains Induced by the Oriented Slab and the Stretched Bundle of Polymer: A Molecular Dynamics Simulation

Information on the interfacial interaction is vital in understanding the crystallization of short polymer chains around oriented nuclei. However, this interaction is difficult to observe at the atomic level. Molecular dynamics simulations are performed to investigate the structural formation of polymer chains induced by the highly oriented slab or the stretched bundle of polymer chains. The results show that the surface-induced crystallization of polymer chains is greatly influenced by the foreign surface on the crystal structure and the morphology of the polymers, hence providing molecular-level support for previous experimental observations [Lotz et al. Macromolecules 1993, 26, 5915 and Yan et al. Macromolecules 2009, 42, 9321]. The order parameter S and the configurations show that the ability of the polypropylene (PP) slab to induce the polyethylene (PE) melt crystallization is weaker than that of the PE slab and that the short PE chains display multiple orientations on the PP slab. In addition, the crystallization rate was found to be dependent on the lattice matching between the free chains and the substrates on the contact lattice planes.

The role of nanopore shape in surface-induced crystallization

Crystallization of a molecular liquid from solution often initiates at solid-liquid interfaces, and nucleation rates are generally believed to be enhanced by surface roughness. Here we show that, on a rough surface, the shape of surface nanopores can also alter nucleation kinetics. Using lithographic methods, we patterned polymer films with nanopores of various shapes and found that spherical nanopores 15-120 nm in diameter hindered nucleation of aspirin crystals, whereas angular nanopores of the same size promoted it. We also show that favourable surface-solute interactions are required for angular nanopores to promote nucleation, and propose that pore shape affects nucleation kinetics through the alteration of the orientational order of the crystallizing molecule near the angles of the pores. Our findings have clear technological implications, for instance in the control of pharmaceutical polymorphism and in the design of 'seed' particles for the regulation of crystallization of fine chemicals.

Electrospun-fiber induced transcrystallization of isotactic polypropylene matrix

To reveal the influences of fiber diameter and characteristics on the surface-induced matrix crystallization, four different fibers, i.e. syndiotactic polystyrene (sPS), Nylon 6, and polyhydroxyamide (PHA) and poly(p-phenylene benzobisoxazole) (PBO) fibers, were used to study their nucleating abilities towards isotactic polypropylene (iPP) matrix. Among them, micron-sized PBO fibers were obtained from the supplier, whereas submicron-sized fibers of sPS, Nylon 6 and PHA were prepared by the solution electrospinning process developed in this laboratory. To resolve the observation difficulty due to the fast nucleation rate and crystal growth at high supercooling degrees (>60 °C), a high speed camera was mounted on the polarized optical microscope equipped with a hot stage to successfully snapshot the corresponding processes at various crystallization temperatures ( T c) in the range of 96–120 °C. For all the active fibers, only α-form iPP transcrystallites were observed at the fiber/matrix interface. Two crucial parameters were proposed for characterizing the fiber nucleating ability, i.e. the interfacial free energy difference (Δ σ) based on the heterogeneous nucleation from a thermodynamic point of view, and the maximum temperature for transcrystalline layer observed ( T max) based on a kinetic consideration. Values of Δ σ for different fibers were derived on the basis of the tertiary nucleation taking place in the selective “grooves” at the fiber surface. It was found that the nucleating rate of sPS fibers was scaled with the fiber diameter, and Δ σ showed a negligible diameter dependence, but T max slightly increased with increasing fiber diameter. For all the fibers investigated, an intimate relation between the Δ σ and T max was derived and used to compare their nucleating abilities.

Surface-Induced Polymer Crystallization and the Resultant Structures and Morphologies

The dependence of properties on the structure and morphology of semicrystalline polymers offers an effective way to tailor the properties of these materials through crystal engineering. For purposeful control of the structure and morphology, and therefore the physical and mechanical properties, a full understanding of the crystallization habits of polymers under different environments and conditions is essential. This has stimulated a mass of research work on polymer crystallization. Considering that these materials are frequently in contact with some kinds of solid surfaces in a variety of applications, surface-induced crystallization of polymers has attracted considerable attention during the past decades. This Perspective provides the context as to how the solid surface influences the crystallization behavior of polymers and what kinds of unique crystal structure and morphology of the polymers can be fabricated. We hope that this will afford useful information for polymer processing in different applic...

Surface-induced breakout crystallization in cylinder-forming P(I-b-EO) diblock copolymer thin films

We have found very slow crystallization in thin films of cylinder-forming poly(isoprene-b-ethyleneoxide) (P(I-b -EO)) diblock copolymers with PEO being the minority block. The film was crystallized at room temperature after melting at 62 °C. Imaging methods were combined with X-ray reflectometry and grazing-incidence small-angle X-ray scattering and diffraction. Initially, hexagonally packed, amorphous PEO cylinders lie in the film plane. After 148 days, crystallized, finger-like terraces were observed over the entire film surface. The terrace height is 20% higher than the repeat distance in the as-prepared film. Thus, at the film surface, the cylinders have been destroyed by crystalline lamellae lying in the film plane. The PEO chain stems are perpendicular to the substrate surface and are once-folded and fully interdigitated. The substrate-near layers still consist of the hexagonally packed, amorphous PEO cylinders within the PI matrix.

Surface-induced crystallization in supercooled tetrahedral liquids

Surfaces have long been known to have an intricate role in solid-liquid phase transformations. Whereas melting is often observed to originate at surfaces, freezing usually starts in the bulk, and only a few systems have been reported to exhibit signatures of surface-induced crystallization. These include assembly of chain-like molecules, some liquid metals and alloys and silicate glasses. Here, we report direct computational evidence of surface-induced nucleation in supercooled liquid silicon and germanium, and we illustrate the crucial role of free surfaces in the freezing process of tetrahedral liquids exhibiting a negative slope of their melting lines (dT/dP|coexist<0). Our molecular dynamics simulations show that the presence of free surfaces may enhance the nucleation rates by several orders of magnitude with respect to those found in the bulk. Our findings provide insight, at the atomistic level, into the nucleation mechanism of widely used semiconductors, and support the hypothesis of surface-induced crystallization in other tetrahedrally coordinated systems, in particular water in the atmosphere.

Dispensing and surface-induced crystallization of zeptolitre liquid metal-alloy drops

The controlled delivery of fluids is a key process in nature and in many areas of science and technology, where pipettes or related devices are used for dispensing well-defined fluid volumes. Existing pipettes are capable of delivering fluids with attolitre (10-18 l) accuracy at best. Studies on phase transformations of nanoscale objects would benefit from the controlled dispensing and manipulation of much smaller droplets. In contrast to nanoparticle melting whose fundamental pathway was studied extensively, experiments on crystallization, testing classical nucleation theory, are hindered by the influence of support interfaces. Experiments on free-standing fluid drops are extremely challenging. Here, we demonstrate the operation of a pipette, which, observed by transmission electron microscopy, delivers a metal-alloy melt with zeptolitre (10-21 l) resolution. We use this exquisite control to produce nearly free-standing Au72Ge28 drops suspended by an atomic-scale meniscus at the pipette tip, and to image their phase transformations with near-atomic resolution. Our observations of the liquid-solid transition challenge classical nucleation theory by providing experimental evidence for an intrinsic crystallization pathway of nanometre-sized fluid drops that avoids nucleation in the interior, but instead proceeds via liquid-state surface faceting as a precursor to surface-induced crystallization.

Epitaxy growth and directed crystallization of high-density polyethylene in the oriented blends with isotactic polypropylene

Various lamellar orientations of high-density polyethylene (HDPE), due to competition between bulk nucleation and interfacial nucleation, have been realized in its melt drawn blends with isotactic polypropylene (iPP) upon cooling after subjected to 160 °C for 30 min. Directed crystallization, with heterogeneous nucleation in the bulk (within domains), is defined as lamellar growth along boundary of anisotropic domains and is favored in larger domains at higher temperature (slow cooling), since overgrowth of lamellae can feel the interface rather than impingement with neighbor ones as a result of scare nuclei at higher temperature. Moreover, lamellar growth caused by directed crystallization is dependent of dimension of confinement. Due to 2D confinement of cylindrical domains, lamellae can only grow along the axis of cylinder and thus b-axis orientation is formed. While in the layered domains with 1D confinement, however, lamellae grow with the normal of (110) plane along the melt drawn direction. On the other hand, epitaxial growth of HDPE chains onto iPP lamellae is related to the surface-induced crystallization and dominated by the interfacial nucleation. Only interfacial nucleation is preferred can epitaxial growth occur. Therefore, retarded crystallization, realized by either strong confinement in finer domains or rapid cooling or both, is favorable for it.

Growth mechanisms of crystallites in the mixed-phase silicon films deposited by low-pressure chemical vapor deposition

The microstructure and growth mechanisms of mixed-phase silicon films deposited on glass substrates by infra-low pressure chemical vapor deposition and low pressure chemical vapor deposition using 20% Si 2H 6/He gas have been investigated by X-ray diffraction and cross-section transmission electron microscopy. The deposition temperature and pressure were in the range of 560–600°C and below 10 mtorr, respectively. In mixed-phase films, there were two kinds of crystallites: initially deposited pre-existing small irregular 〈311〉-preferred crystallites and conventionally incubated 〈111〉-preferred large elliptic bulk-induced crystallites. The behavior of the former changed depending on the deposition conditions of temperature and pressure. Just above the deposition rate below which a 〈110〉 polysilicon film formed, 〈311〉-preferred quasi-columnar growth occurred by surface-induced crystallization. This quasi-columnar growth preserved or strengthened the initial 〈311〉 preference and grew with an activation energy of 3.1 eV. A further increase of deposition rate made it difficult to form quasi-columnar growth due to a smaller nucleation density and because surface-induced crystallization was suppressed. Then conventional bulk-induced crystallization became dominant to form a 〈111〉 preferred mixed-phase films.

Surface-induced crystallization of syndiotactic polystyrene on high modulus carbon fibers

Measurements have been made using a polarized optical microscope equipped with hot stages to investigate the surface-induced crystallization of syndiotactic polystyrene(s-PS) on high modulus(HM) carbon fibers. Both the induction times and crystal growth rates at various crystallization temperatures were measured. Based on the theory of heterogeneous nucleation, the interfacial free energy difference function Δσ of s-PS on HM carbon fibers was determined to be 0.61±0.02 erg/cm2. No difference in the crystal growth rate of s-PS has been found in either spherulites in the bulk or transcrystalline layers at the interface. From the morphology studies, it has been found that the thickness of the transcrystalline layer increases with crystallization temperatures, from 5 to 13 μm in the temperature range of 247–269°C. The efficiency of HM carbon fibers to induce the transcrystalline layer is found better in s-PS matrix than that in i-PP matrix based on the surface energies of the constituents. (Keywords: transcrystalline layer · syndiotactic polystyrene · high modulus carbon fibers)

Crystallization of Cu50 Ti50 Glasses and Undercooled Melts

Crystallization of melt-spun Cu 50 Ti 50 ribbons was studied in detail by optical and electron microscopy. At rather low annealing temperatures surface-induced polymorphic crystallization of γ-CuTi is finished prior to any crystallization in the bulk. Only a minor influence of the glass transition on the temperature dependence of crystal growth has been observed; the nucleation rate, however, is strongly affected by the glass transition as known from metal-metalloid glasses. In this temperature range an Arrhenius equation for nucleation assuming diffusion-controlled atomic transport would lead to rather unrealistic activation energies. Above the glass transition temperature, i.e. in undercooled melts, the atomic transport necessary for crystallization should be better described by viscous flow

Nanorheology of Confined Polymer Melts. 1. Linear Shear Response at Strongly Adsorbing Surfaces

The linear-response effective shear moduli of polymer melts confined between strongly adsorbing surfaces (parallel plates of mica) was studied as a function of the excitation frequency. Linear response (achieved with shear amplitudes of e2 A) implies that measurements did not perturb the film structure. The measurements employed a surface forces apparatus modified for dynamic mechanical shear oscillation. The polymers (atactic poly(phenylmethylsiloxane), PPMS, chain length from 31 to 153 skeletal bonds) were selected to be glass-forming with a low glass transition temperature, in order to avoid issues of possible surface-induced crystallization which have been much discussed. Three principal conclusions emerged. First, in a comparison of polymers of different molecular weight, both the shear moduli (measured at fxed film thickness) and the static normal forces (to compress the polymers to this film thickness) scaled approximately with the estimated unperturbed radius of gyration of the chain (RG). Second, a transition from fluid- to solidlike response occurred when the film thickness was less than 5-6 RG: the longest relaxation time slowed to the point that the storage shear modulus (G) exceeded the loss shear modulus (G) over the entire frequency spectrum. This contrasts strongly with the behavior of the bulk samples, for which Rouse dynamics would be expected. Third, the magnitude of G under conditions of strong confinement was characteristic of rubberlike elasticity, approximately lo5 N mF2 for the range of chain length that was studied. This indicates enhanced entanglement interactions in thin polymer films. Two accompanying papers analyze the nonlinear rheology and the respective influences of surface adsorption and of geometrical confinement.

Effects of thermal annealing on the optoelectronic properties of hydrogenated amorphous germanium

Abstract In this work we investigate the effect of thermal annealing above the deposition temperature (200°C) on the optoelectronic properties of hydrogenated amorphous Ge thin films. For this purpose, visible and infrared transmission spectroscopy, photothermal deflection spectroscopy (PDS), Raman scattering, dark conductivity and electron spin resonance (ESR) spectroscopy have been used. For thermal treatments below 350°C, the defect density increases with increasing anneal temperature Ta, but the sample remains amorphous. In this case the absorption coefficient at 0.7 eV determined by PDS can be used as a measure of the dangling-bond spin density (obtained by ESR). For 350°C <; Ta <; 420°C, crystallites are formed and grow, up to T a=430°C, when the sample becomes microcrystalline. The crystallization process is best described by a surface-induced crystallization with nucleation occurring at the film-substrate interface.

Overshoots as polymers adsorb

An overshoot in the time-dependent mass adsorbed from dilute solution was observed for poly(dimethylsiloxane) and cis-polyisoprene adsorbed onto single oxide surfaces. However, no overshoot was observed for poly(methyl methacrylate) and polystyrene, suggesting the possibility that the overshoot reflects transient surface-induced crystallization.

Crystallization Kinetics In A Ni24Zr76 Amorphous Alloy

ABSTRACTIsothermal crystallization kinetics of Ni24Zr76 amorphous alloy has been studied by differential scanning calorimetry (DSC). Theoretically estimated transient times are found to be consistent with the experimentally observed incubation periods. The crystallization kinetics has been analyzed in terms of Kolmogorov-Johnson-Mehl-Avrami (KJMA) Model and significant departure from the linear KJMA behavior was observed. Such non-linearity is due to the surface-induced crystallization, anisotropie growth of the crystals, impingement effects and variation of the nucleation rate during crystallization etc. By applying high-resolution electron Microscopy (HREM), it has been shown that the nucleation and growth of the crystals do not take place at a constant state of disorder.

Isothermal crystallization kinetics of Ni 24Zr 76 and Ni 24(ZrX) 76 amorphous alloys

Isothermal devitrification kinetics of the melt-spun Ni 24Zr 76 and Ni 24(ZrX) 76 amorphous alloys has been studied by means of DSC, optical microscopy, TEM and X-ray diffraction. These amorphous alloys exhibit eutectic crystallization. Isothermal devitrification kinetics, as obtained by DSC, has been analyzed in terms of Kolmogorov-Johnson-Mehl-Avrami (KJMA) model. The classical KJMA plots exhibit significant non-linearity from the beginning of the transformation. Such non-linearity has been attributed to a number of factors such as surface-induced crystallization, anisotropic growth of crystals, impingement effects (hard and soft) and variation of the nucleation rate during devitrification etc. In these alloys, the steady state nucleation frequency is established after a well-defined transient period. A quantitative analysis of the transient period has been performed in terms of non-steady state nucleation theory and the theoretically estimated transient times are found to be consistent with the experimentally observed incubation periods. By applying classical theory of homogeneous nucleation, it is found that the density of quenched-in nuclei will be very small in the absence of heterogeneous nucleation.

The influence of grain size on the overall kinetics of surface-induced glass crystallization

A simple kinetic model of surface-induced glass crystallization is proposed. The grain size of glass powders, a constant density of surface nuclei and a steadily increasing temperature throughout the reaction are taken into account. The crystal growth rate and the density of surface nuclei can be estimated easily from overall kinetic curves (e.g. DTA curves) of powder of different grain size.

Studies of Crystallization Processes of Amorphous Chalcogenide Thin Films with Electrical Measurements

Electrical measurements have been carried out to elucidate the crystallization processes of amorphous Te:Se thin films, which have been applied to information storage media. A model has been proposed to explain the conductivity changes of the material with time at various temperatures, in which surface-induced crystallization plays a key role. The surface-induced crystallization velocity and activation energy are determined to be 4.2×102 nm/s at 338 K and 1.75 eV, respectively. The influence of aging on the crystallization is also examined.

Surface crystallization of amorphous alloy Metglas 2826

A study of the surface region of the amorphous alloy Metglas 2826 was made using reflection high-energy electron diffraction and Auger electron spectroscopy. Changes in the near surface structure and composition as a result of heat treatment were followed and compared with observations made with transmission electron microscopy (TEM) and X-ray diffraction. Treatment of the surface in an ultraviolet radiation/ozone environment at room temperature to remove absorbed carbon results in surface oxidation, a subsurface crystalline layer, and a varying composition in the near-surface region. Samples thinned to the point where the two surfaces were in close proximity exhibited explosive crystallization behavior when observed in the TEM. The fact that surface-induced crystallization has been shown to occur in this alloy at low and even room temperatures suggests that environmental and mechanical treatments are of some consequence in the practical utilization of this type of material.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Write-Erase Characteristics of Phase Change Optical Recording in Ga–Se–Te Systems

Reversible optical changes in a thin-film system of parylene/Tez(GaxSe1-x)1-z film (0.6≤z≤0.9, 0≤x≤0.3)/parylene structure has been studied by measuring the time required to crystalline from the amorphous state (the erasing time) and the number of reversible reflectance changes occurring between amorphous (written) and crystalline (erased) states. For a specimen with a Ga composition of x=0.15, the erasing time is about 1 µs. From a discussion of the Ga–Se–Te phase diagram, it becomes clear that the optimum composition showing write-erase characteristics is obtained at an eutectic point, in which the segregation is not caused and many cycle operations are possible. The SIC (Surface Induced Crystallization) model seems to be suitable for explaining the erasing process in the chalcogenide thin films.