Lamellar orientation on the surface of a polymer determined by ToF-SIMS and AFM

The fold and lateral surfaces of chain-folded lamellae of poly(bisphenol-A-co-etheroctane), containing both aliphatic CH₂ and aromatic segments, were investigated by time-of-flight secondary ion mass spectrometry (ToF-SIMS). At low and high crystallization temperatures, the surfaces of the polymer films were shown to consist mainly of edge-on and flat-on lamellae, respectively. Surfaces with a mixture of edge-on and flat-on lamellae were produced at intermediate temperatures. The edge-on and flat-on lamellae were identified by using ions that recognize the flexible and rigid segments of the polymer. Ion images produced using selected ions that are related to the edge-on or flat-on orientation can be used to identify the location of these lamellae on the polymer surface. Our results indicate that ToF-SIMS can be used to detect different lamellar orientations at the surfaces of semi-crystalline polymers.     

Mechanisms of CVD diamond nucleation and growth on mechanically scratched Si(100) surfaces

The diamond nucleation has been studied on scratched Si(100) both by surface analyses (XPS, AES, ELS) and microstructural probes (AFM, SEM). Two pathways for diamond formation and growth are detected: A seeding pathway occurs by direct growth from part of diamond seeds left by the mechanical pretreatment. Not all of these seeds however are prone to diamond growth as they can be either dissolved or carburized. A nucleation pathway occurs through a stepwise process including the formation of extrinsic (pretreatment) or intrinsic (in situ) nucleation sites, followed by formation of carbon-based precursors. It is believed that nucleation sites could be either grooves of scratching lines or protrusions produced by etching-redeposition. The size of these protrusions is not larger than 100 nm. On top of these protrusions as well as on the bare substrate, a thin layer of silicon carbide rapidly forms and DLC carbon likely. This complex process on top of protrusions may constitute carbon-based embryos for further diamond nucleation. Received 21 December 1998     

Morphologies of diblock copolymer thin films before and after crystallization

Spin-coated thin films of about 100nm of low-molecular-weight hydrogenated poly(butadiene-b- ethyleneoxide) (PB_h-PEO) diblock copolymers have been crystallized at various constant temperatures. Crystallization has been observed in real time by light microscopy. Detailed structural information was obtained by atomic force microscopy, mainly enabled by the large viscoelastic contrast between amorphous and crystalline regions. The behavior in thin films is compared to the bulk properties of the polymer. Crystallization started from an annealed microphase separated melt where optical microscopy indicated a lamellar orientation parallel to the substrate. A small difference in the length of the crystallizable block produced significantly different crystallization behavior, both in the bulk and in thin films. For thin films of the shortest diblock copolymer (45% PEO content) and for an undercooling larger than about 10 degrees, crystallization created vertically oriented lamellae. These vertical lamellae could be preferentially aligned over several micrometers when crystallization occurred close to a three-phase contact line. Annealing at temperatures closer to the melting point or keeping the sample at room temperature for several months allowed the formation of a lamellar structure parallel to the substrate. A tentative interpretation based on kinetically caused chain folding and relaxation within the crystalline state, with implications on general aspects of polymer crystallization, is presented. Received 19 March 1999 and Received in final form 14 December 1999     

Mechanism of spherulitic crystallization as deduced from observations of partially crystallized glassy polystyrene

Thin films of isotactic polystyrene partially crystallized from the glassy state were studied in detail by means of transmission electron microscopy and electron diffraction. Initial nucleation and growth stages of spherulitic fibrils (or lamellae) were illustrated clearly by using novel techniques, such as Au decoration, and novel specimens such as thin films containing holes.

Spherulitic nucleation begins with the crystallization of a liquid-crystal-like nodule or a group of these nodules merging to form a spherulitic center. Fibrils or lamellae grow and fan out from the nucleus by additional incorporation of maturing nodules. Proliferation of fibrils is essentially a space-filling process through the crystallization of uncrystallized nodules or nodules that were left behind by growing fibrils which had initiated earlier. The deduced mechanism of spherulitic crystallization leads directly to the formation of interlamellar links between neighboring fibrils. However, no extended-chain-type interlamellar links were revealed by Au decoration.

The application of the mechanism of spherulitic crystallization from the glass to that from the melt is also suggested; it is based primarily on recent studies which show remarkable similarities between structures existing in the glassy and the melt states prior to crystallization.     

Scratch direction and threshold force in nanoscale scratching using atomic force microscopes

The nanoscaled tip in an AFM (atomic force microscope) has become an effective scratching tool for material removing in nanofabrication. In this article, the characteristics of using a diamond-coated pyramidal tip to scratch Ni-Fe thin film surfaces was experimentally investigated with the focus on the evaluation of the influence of the scratch or scan direction on the final shape of the scratched geometry as well as the applied scratch force. Results indicated that both the scratched profile and the scratch force were greatly affected by the scratch direction. It has been found that, to minimize the formation of protuberances along the groove sides and to have a better control of the scratched geometry, the tip face should be perpendicular to the scratching direction, which is also known as orthogonal cutting condition. To demonstrate the present findings, three groove patterns have been scratched with the tip face perpendicular to the scratching direction and very little amount of protuberances was observed. The threshold scratch force was also predicted based on the Hertz contact theory. Without considering the surface friction and adhesive forces between the tip and substrate, the threshold force predicted was twice smaller than the measurement value. Finally, recommendations for technical improvement and research focuses are provided.     

Transition of β to α phase in isotactic polypropylene

In samples of isotactic polypropylene, the influence of the nucleation agent chinacridine permanent red E3B on the crystallization rate and content of α and β modifications and on the rate and mechanism of the β → α transition was investigated using x-ray diffraction, DSC microcalorimetry, and electron microscopy. It was found that, though E3B increases the content of the β phase, it is also a very efficient nucleation agent for the α-phase crystallization. The β → α transition occurring above 140°C could be characterized by a gradual melting and annealing of the β phase, accompanied by a surface crystallization of the α phase on lateral faces of β lamellae by a mechanism similar to cross-branching of α lamellae.     

Effect of Solvent Annealing on the Nano- and Micro-Structure of Block Copolymer Thin Film

The development of structure of symmetric polystyrene-b-poly(2 vinyl pyridine) (PS-PVP) thin film annealed under a neutral solvent vapor was investigated by atomic force microscopy and optical microscopy. Initially, spin cast film showed a micellar structure. With subsequent neutral solvent vapor treatment, a lamellar structure gradually developed. It was found that the structural evolution of the thin film was a strong function of the quantity of solvent diffused into the film. In addition, with an increase in swelling, the orientation of the lamellae changed from parallel to perpendicular. During solvent annealing, the solvent also induced dewetting via primary and secondary heterogeneous nucleation processes, depending on the quantity of solvent in the film. Secondary nucleation was independent of primary nucleation for lower solvent concentration. With higher amount of solvent, the solvent expedited secondary nucleation, and it occurred within the dewetted area caused by primary nucleation.     

Miscibility,Crystallization, and Morphology of the Double-Crystalline Blends of Insulating Polyethylene and Semiconducting Poly(3-Butylthiophene)

A series of crystalline semiconducting poly(3-butylthiophene) (P3BT)/crystalline insulating polyethylene (PE) blends were prepared and the miscibility, crystallization, and structure/morphology were investigated. Even though phase separation was observed by transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS), several pieces of evidence indicated that limited miscibility should be present in PE/P3BT blends: small changes in both T_m and crystallinity of PE phase and a small portion of PE being dissolved in P3BT. The study of PE isothermal crystallization kinetics revealed that the introduction of P3BT significantly influenced the nucleation mechanism and growth geometry, i.e., PE was transformed from three-dimensional (3D) spherulitic to two-dimensional (2D) disc crystals. A striking reduction of nucleation density and an obvious ringed morphology of PE spherulites (2D) in PE/P3BT blends were also observed by polarized optical microscopy; it is proposed that the limited miscibility between PE and crystalline P3BT favors the formation of ringed PE spherulite in the blends. Additionally, preferred orientation of PE lamellae, with their b-axis largely constrained to the thin film plane, was observed by X-ray diffraction in PE/P3BT blend films. It is evidenced that the PE orientation was due to the b-axis being the crystal growth direction, which can only be in film plane.     

Morphological Changes in Flat-on and Edge-on Lamellae of Poly(Ethylene Succinate) Crystallized from Molten Thin Films

Corrugated crystal morphologies of poly(ethylene succinate) (PESU) with a bird-wing shape grew from the thin-film melt with the film thickness less than 1 μm. The center of the bird-wing-shaped crystal consisted of a lozenge crystal. Corrugated steps were observed in the direction of the short axis of the lozenge crystal (b-axis). Fibrous textures extended in the direction of the long axis of the lozenge crystal (a-axis), and they wrapped around the corrugated crystals. The corrugated crystals and the fibrous textures consisted of flat-on and edge-on lamellae, respectively. The wing-shaped morphology was constructed by combining the crystal growth with the flat-on and edge-on lamellae.     

变温退火制备铝诱导大晶粒多晶硅薄膜的机理研究

为了缩短铝诱导法制备大晶粒多晶硅薄膜的退火时间,用射频磁控溅射法在玻璃衬底上沉积了a-Si/SiO2/Al叠层膜,并用两种方法进行变温退火.分析了变温退火工艺对铝诱导晶化过程的影响,着重讨论了退火过程中温度由低温升到高温时不形成小晶粒的机理和条件.研究表明,当退火温度升高时,是否形成小晶粒取决于晶粒半径、耗尽层厚度和相邻晶粒间距三者之间的关系.     

In situ study of nanotemplate‐induced growth of lysozyme microcrystals by submicrometer GISAXS

Ultrasmall lysozyme microcrystals are grown by classical hanging‐drop vapor diffusion and by its modification using a homologous protein thin‐film template displaying long‐range order. The nucleation and growth mechanisms of lysozyme microcrystals are studied at the thin lysozyme film surface using a new in situµGISAXS (microbeam grazing‐incidence small‐angle X‐ray scattering) technique recently developed at the microfocus beamline of the ESRF in Grenoble, France. New insight on the nucleation and crystallization processes appear to emerge.     

Melting and Crystallization Behaviors of Poly(Lactic Acid) Modified with Graphene Acting as a Nucleating Agent

Graphene (GN)-filled polylactic acid (PLA) nanocomposites were prepared through a solution blending method with GN weight percent ranging from 0.5 to 2?wt%. Rheological, melting and crystallization behaviors of the prepared PLA/GN nanocomposites were investigated by means of dynamic rheological measurements and differential scanning calorimetry (DSC). The shear viscosities of the PLA/GN nanocomposites decreased with increasing GN content, which was remarkably different from previous reports on the modifications using traditional nanofillers (e.g., clay, carbon nanotubes, etc.). The nonisothermal melt crystallization kinetic analysis suggested that GN served as a nucleating agent and could considerably promote the PLA’s crystallization through heterogeneous nucleation. Our findings suggested that at relatively low cooling rates (??≤?10?°C/min) even a small amount of GN promoted the nucleation and considerably increased the crystallization rate. However, the crystallinity began to decrease at higher cooling rates (e.g., ??≥?20?°C/min), especially when the GN content was high (e.g., 2?wt%), possibly owing to the GN aggregation effect considering PLA is a slowly crystallizing polymer.     

The Effect of Orientation Relaxation on Polymer Melt Crystallization Studied by Monte Carlo Simulations

We use dynamic Monte Carlo simulations to study the athermal relaxation of bulk extended chains and the isothermal crystallization in intermediately relaxed melts. It is found that the memory of chain orientations in the melt can significantly enhance the crystallization rates. The crystal orientation and lamellar thickness essentially depend on the orientational relaxation. Moreover, there is a transition of the nucleation mechanism during the isothermal crystallization from the intermediately relaxed melts. These results explain the mechanism of the self-nucleation by orientation and suggest that in flow-induced polymer crystallization, the orientational relaxation of chains decides the crystal orientation.     

Crystal growth of small-molecule organic semiconductors with nucleation additive

In this study, we employ a nucleation additive 4-octylbenzoic acid (OBA) with an optimized solvent evaporation method to regulate crystal orientation and grain width of small-molecule organic semiconductors. When 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) was utilized as a benchmark material to mix with the additive, a self-assembled OBA interfacial layer was formed and promoted uniform deposition of nucleation seeds. As a result, the TIPS pentacene/OBA blend crystalline film exhibited crystal alignment in long range order, attributing to a 11-fold reduction of the crystal misorientation angle and a 4-fold increase of the grain width. We further discussed the important correlation between the effective hole mobility, grain boundaries, grain width and length, and nucleation sites. Organic thin film transistors were fabricated to test charge transport, yielding a hole mobility of up to 0.17 cm²/V. This work provides a new pathway to modulate the nucleation and crystallization events of organic semiconductors, and can potentially be applied to optimize the thin film morphology and electrical performance of organic semiconducting materials in general.     

Nanophase-separated synchronizing structure with parallel double periodicity from an undecablock terpolymer

A new nanophase-separated structure with parallel double periodicity has been identified for an undecablock terpolymer in bulk. The polymer includes two long poly(2-vinypyridine) (P) chains on each end, with five short polyisoprene (I) and four short polystyrene (S) chains at the center. This polymer exhibits a hierarchical lamellar structure with two crystallographic periods: 88 nm and 16 nm. The 88 nm period includes one thick P lamella and five thin I-S-I-S-I lamellae, of extremely high orientation.     

Morphology,Growth Rate,and Lamellar Thickness of Polymer Crystals

At high supercoolings, isotactic polystyrene and polybutene-1 have a rounded crystal shape, suggesting kinetic roughening. Still, the growth rates of these polymer crystals show the supercooling dependence derived for nucleation controlled growth. On the other hand, isotactic poly-4-methylpentene-1 1,4 trans-polybutadiene at higher crystallization temperatures and polyethylene at high pressures show a rounded crystal shape: thermal roughening. Again, the growth rate is described by the nucleation theory. On the basis of these observations, we propose a crystallization kinetics taking account of the entropic barrier that was originally proposed by Sadler.     

Nature of Shear-Induced Primary Nuclei in iPP Melt

Although observations of molecular processes in the formation of primary nuclei prior to actual crystallization are beyond the detection limits of current instrumentation, we attempted to probe the nature of primary nuclei in sheared isotactic polypropylene (iPP) polymer melt. In situ rheo-SAXS (small-angle X-ray scattering) and -WAXD (wide angle X-ray diffraction) experiments using synchrotron radiation were carried out to evaluate the effects of an addition of a high molecular weight atactic polypropylene (aPP) (5 wt%), which is compatible with the iPP matrix but does not crystallize, on the evolution of oriented structures in the sheared iPP melt and its crystallization kinetics. It is unlikely that the aPP chain segments can be incorporated into iPP nuclei or crystal; hence, its addition effects, if any, would be seen only in the amorphous melt prior to crystallization. The results showed stonger orientation and improved crystallization kinetics in the iPP/aPP blend compared to pure iPP. Observations that the presence of long chains of an amorphous polymer aid in nucleation and crystallization kinetics of iPP, combined with our previous synchrotron results of sheared iPP melts at high temperature (165°C), lead us to conclude that primary nuclei in iPP most likely consist of liquid-crystalline or mesomorphic bundles of aligned chain segments prior to the formation of crystals.     

Nanoscratch behavior of multi-layered films using molecular dynamics

Molecular dynamics simulations are performed to study the plastic deformation, stress and chip formation of scratched multi-layered films. The results showed that stick–slip and work-hardening behaviors were observed during the scratching process. There was a pile-up of amorphous disordered debris atoms and shear rupture ahead of the probe and a clear side-flow on the lateral sides of the probe when the probe moved forward. Both the plastic energy and the adhesion increased with an increase in the scratching depth. The glide band of the interface was on the {111}〈110〉 slip system with a maximum width of the glide band of about 1 nm. The strain energy stored in the deformed structure caused a higher stress region in the material in front of the tool edge, with a maximum stress of about 10 GPa. In addition, the mechanical response and thermal softness phenomenon are discussed. PACS 02.70.Ns; 46.55.+d; 47.11.Mn; 91.55.Ax; 62.40.+i     

A comparison study of scratch and wear properties using atomic force microscopy

The patterning technique that uses an AFM (atomic force microscopy) tip as a scratch tool, also known as AFM scratching, has been a vital technique for nanofabrication because of its low cost and potential to reach a resolution into the sub-nanometer domain. The AFM scratching technique was first used to study the scratch characteristics of silicon, with an emphasis on establishing its scratchability or the nanoscale machinability. The effects of the scratch parameters, including the applied tip force and number of scratches, on the size of the scratched geometry were specifically evaluated. The primary property that measures the scratchability was identified and assessed. To illustrate its suitability and reliability, the value of the scratchability, based on the present Si scratching experiments, was compared with the values based on the data available in the literatures for different scratching conditions or for materials other than Si. Since AFM scratching is in some aspects similar to the nanoscale wear test, the scratchability property identified is also compared with two major wear resistance indicators, wear coefficient and hardness. All comparison results indicate that the scratchability property identified, the scratch ratio, is an appropriate manufacturability indicator for measuring the degree of the ease or difficulty of a material scratched by an AFM tip and more suitable than the wear coefficient and hardness to gauge the nanoscale AFM scratchability.     

Investigation of the Melt‐Crystallization of Polypropylene by a Temperature Slope Method

Abstract

To investigate the in‐situ ordering process of isotactic polypropylene (iPP) from a melt state, a stationary growth front was prepared by the temperature slope crystallization (TSC) method. During the melt‐crystallization, iPP was crystallized into the α‐phase or β‐phase depending on the crystallizing conditions. The mechanism of the melt‐crystallization at the growth front was precisely observed by wide‐angle and small‐angle x‐ray scattering (WAXS and SAXS) using a strong synchrotron beam. In the TSC apparatus, the sample was crystallized in between a heater, controlled to 220°C, and a cooler, cooled by water to 25°C. We define the z‐axis parallel to the temperature gradient. A‐lamellae and B‐lamellae are also defined as those whose lamellar normal are perpendicular and parallel to the z‐axis, respectively. In a sample‐stop (SS) stage before the TSC, the original α‐phase lamellae became thicker, approaching to the melt‐solid boundary by annealing. The annealing process showed that the α‐phase B‐lamellae remained and the SAXS reflection was stronger on the meridian near the melt‐solid boundary in the SS stage. In the beginning of the TSC, the α‐phase B‐lamellae developed as a primary crystallization. During secondary crystallization under high supercooling, the SAXS cross pattern appeared showing that the α‐phase developed both A‐ and B‐lamellae. As the growth direction of A‐lamellae is parallel to the z‐axis, A‐lamellae grow faster than B‐lamellae. By the self‐epitaxial mechanism on the side surface of the A‐lamellae, the B‐lamellae grow on the base of the A‐lamellae. Following appearance of a spontaneous β‐nucleus, the β‐phase lamellae grew preferentially, excluding the α‐phase, and occupied the whole area of the sample. In this case also, A‐lamellae are advantageous to grow because of the growth direction parallel to the z‐axis. As a result, the SAXS β‐phase reflection appeared on the equator.