Bulk Crystallization Temperature Research Articles

Direct Observation of Two-Step, Stratified Crystallization and Morphology in Conjugated Polymer Thin Films

A two-step stratified crystallization process has been directly observed during cooling of poly(3-hexylthiophene) (P3HT) thin films of thickness h = 20–250 nm: a thin (<20 nm) layer at the free surface crystallizes ∼25 °C higher than the bulk crystallization temperature (TC,bulk), whereas the rest of the film (i.e., the underlayer) crystallizes near TC,bulk. In situ measurements of films with and without a free surface unequivocally ascribe the high-temperature crystallization to a surface-induced process, which correlates with the formation of large birefringent structures and highly oriented edge-on crystallites at the air–polymer interface. In contrast, crystallization of the bulk-like underlayer leads to mostly edge-on-oriented crystallites in thinner films and becomes increasingly isotropic in thicker films. For h < 20 nm, free-surface effects dominate and only high-temperature crystallization is observed. These results highlight the potential of melt crystallization to tailor morphology and orientation across thin film thickness for specific electronic applications.

Carbon nanotubes interaction with amorphous and semi‐crystalline domains of polypropylene in melt‐mixed composites: Influence of multiwall carbon nanotubes agglomerate and their modifications

AbstractPolypropylene (PP) and multi‐walled carbon nanotubes (MWCNTs) composites were prepared by melt‐mixing with two types of MWCNTs of varying agglomerate size. Agglomerate dominated dispersion was observed in the PP matrix, wherein N‐MWCNTs exhibit a finer dispersion as compared to D‐MWCNTs. Further, MWCNTs were encapsulated by Li‐salt of 6‐amino caproic acid in various proportions along with PP‐g‐maleic anhydride in order to achieve finer MWCNTs dispersion. Thermogravimetric analysis exhibited an increase in onset of degradation temperature and higher residual weight (%) at 500 °C of PP with increased MWCNTs concentration, which suggests the formation of a thicker “interphase.” Glass transition temperature of the PP phase was increased monotonically as a function of unmodified MWCNTs concentration, which suggests a strong interfacial interaction between PP and MWCNTs. An extensive analysis was carried out with modified MWCNTs based PP composites. Moreover, MWCNTs act as a strong hetero‐nucleating agent manifesting in smaller spherulite size of PP, wherein bulk crystallization temperature is increased to ~130 °C of the PP phase at 5 wt% MWCNTs content. The influence of unmodified and modified MWCNTs on the interaction between the PP chains and MWCNTs in the amorphous phase as well as in the crystalline domains are analyzed in PP/MWCNTs composites.

Frustrated Microparticle Morphologies of a Semicrystalline Triblock Terpolymer in 3D Soft Confinement.

Self-assembly of block copolymers (BCPs) in three-dimensional (3D) confinement of emulsion droplets has emerged as a versatile route for the formation of functional micro- and nanoparticles. While the self-assembly of amorphous coil-coil BCPs is fairly well documented, less is known about the behavior of crystalline-coil BCPs. Here, we demonstrate that confining a linear ABC triblock terpolymer with a crystallizable middle block in oil-in-water (O/W) emulsions results in a range of microparticles with frustrated inner structure originating from the conflict between crystallization and curved interfaces. Polystyrene-block-polyethylene-block-poly(methyl methacrylate) (PS-b-PE-b-PMMA, S32E36M3293) in toluene droplets was subjected to different preparation protocols. If evaporation was performed well above the bulk crystallization temperature of the PE block (Tevap > Tc), S32E36M3293 first microphase-separated into microparticles with lamella morphology followed by crystallization into a variety of frustrated morphologies (e.g., bud-like, double staircase, spherocone). By evaporating at significantly lower temperatures that allow the PE block to crystallize from solution (Tevap < Tc), S32E36M3293 underwent crystallization-driven self-assembly into patchy crystalline-core micelles, followed by confinement assembly into lenticular microparticles with compartmentalized hexagonal cylinder lattices. The frequency of these frustrated morphologies depends on polymer concentration and the evaporation protocol. These results provide a preliminary understanding of the morphological behavior of semicrystalline block copolymers in 3D soft confinement and may provide alternative routes to structure multicompartment microparticles from a broader range of polymer properties.

High pressure high temperature devitrification of Fe78B13Si9 metallic glass with simultaneous x-ray structural characterization

Changes in bulk crystallization behavior following devitrification at high pressure are investigated for a Fe78B13Si9 composition metallic glass using in-situ energy dispersive x-ray powder diffraction. Crystallization with time was evaluated for a series of measurements to a maximum pressure of 5.63 ± 0.15 GPa for the Fe78B13Si9 glass. Pressure was found to strongly affect onset bulk crystallization temperature Tx. Crystallization at each pressure was found to progress in two stages. In the first stage, α-Fe precipitates and in the second Fe2B forms while α-Fe continues to crystallize. Complementary high pressure room temperature studies were conducted.

Relation between colour- and phase changes of a leuco dye-based thermochromic composite

Reversible colour change of leuco dye-based composites is in general closely related to their phase change, thus the two phenomena should occur at around the same temperature and should be influenced similarly. However, spatial confinement of the analysed sample affects the change in colour differently compared to its phase transition and the most pronounced effects can be observed during cooling. The bulk composite is coloured while still liquid and the colour hysteresis does not exhibit a loop. In an open-porous medium the colouration coincides well with the crystallization and the colour hysteresis widens to about 4 °C. Microencapsulated composite exhibits two crystallization processes, one of them taking place at the bulk crystallization temperature and the other one at about 20 °C lower. Under such conditions the composite is coloured just before the onset of the second crystallization, i.e. about 15 °C below crystallization in the bulk, and the corresponding colour hysteresis widens to 18 °C. The two crystallization forms are thermally independent and have the same crystalline structure. These effects should be taken into account when designing future applications where the phase-changing materials are implemented.

Synthesis of statistical PET/PEN random block copolymers and their crystallizability in the bulk and at the surface

ABSTRACTA series of PET/PEN copolyesters were synthesized by molten transesterification. The degree of randomness and the sequence length of the copolymers were determined by 1H NMR spectroscopy, and the changes in the bulk glass transition temperature (TgB), bulk crystallization temperature (TcB), and bulk melting temperature (TmB) were observed by DSC. A clear relationship was obtained between the observed enthalpy of melting (ΔHm) and degree of randomness (B), and TmB was suppressed for the midcompositions. As with their homopolymer counterparts, there was significant depression in crystallization temperature at the surface (TcS) of the random/block copolymers compared with the bulk, and so surface‐localized crystallization could be induced in spin‐coated thick films (thickness ranging from ca. 400 to 700 nm) by annealing at a temperature in which the surface region is mobile, but the bulk is not. The formation of these clear surface crystals allows the morphology to be directly imaged by AFM, and we observed that the crystallizability and the lamellar morphology of the surface crystals deviated from those of the original homopolymers, depending on the mixing ratio of PET/PEN and degree of randomness. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46515.

In situ visualization of Ni–Nb bulk metallic glasses phase transition

We report the results of in situ investigation of the structural evolution and crystallization behavior of Ni-based bulk metallic glass. The X-ray diffraction, transmission electron microscopy, nanobeam diffraction, differential scanning calorimetry, radial distribution function and scanning tunneling microscopy (STM)/spectroscopy techniques were applied to analyze the structure and electronic properties of Ni63.5Nb36.5 glasses before and after crystallization. According to our STM measurements, the primary crystallization originally starts with the Ni3Nb phase formation as a leading eutectic phase. It was shown that surface crystallization differs drastically from bulk crystallization due to the possible surface reconstruction. The mechanism of Ni63.5Nb36.5 glass alloy two-dimensional crystallization was suggested, which corresponds to the local metastable (3×3)− Ni(111) surface phase formation. The possibility of different surface nanostructures developing by annealment of the originally glassy alloy in an ultrahigh vacuum at a temperature lower than the bulk crystallization temperature was shown. The increase of the mean square surface roughness parameter Rq while transforming from a glassy to a fully crystallized state can be caused by concurrent growth of Ni3Nb and Ni6Nb7 bulk phases. The simple empirical model for the estimation of Ni63.5Nb36.5 cluster size was suggested, and the value obtained (about 8Å) is in good agreement with the corresponding STM measurements (8–10Å).

Effect of core‐shell morphology evolution on the rheology, crystallization, and mechanical properties of PA6/EPDM‐g‐MA/HDPE ternary blend

AbstractIn this article, polyamide 6 (PA6), maleic anhydride grafted ethylene‐propylene‐diene monomer (EPDM‐g‐MA), high‐density polyethylene (HDPE) were simultaneously added into an internal mixer to melt‐mixing for different periods. The relationship between morphology and rheological behaviors, crystallization, mechanical properties of PA6/EPDM‐g‐MA/HDPE blends were studied. The phase morphology observation revealed that PA6/EPDM‐g‐MA/HDPE (70/15/15 wt %) blend is constituted from PA6 matrix in which is dispersed core‐shell droplets of HDPE core encapsulated by EPDM‐g‐MA phase and indicated that the mixing time played a crucial role on the evolution of the core‐shell morphology. Rheological measurement manifested that the complex viscosity and storage modulus of ternary blends were notable higher than the pure polymer blends and binary blends which ascribed different phase morphology. Moreover, the maximum notched impact strength of PA6/EPDM‐g‐MA/HDPE blend was 80.7 KJ/m2 and this value was 10–11 times higher than that of pure PA6. Particularly, differential scanning calorimetry results indicated that the bulk crystallization temperature of HDPE (114.6°C) was partly weakened and a new crystallization peak appeared at a lower temperature of around 102.2°C as a result of co‐crystal of HDPE and EPDM‐g‐MA. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Micellization of Poly(ethylene glycol)-block-Poly(caprolactone) in Compressible Near Critical Solvents

Micelles of hydrophilic−hydrophobic block copolymers, such as poly(ethylene glycol)-block-poly(caprolactone) (PEG-b-PCL), are useful for delivery of hydrophobic drugs. Such micelles can be formed by liquid solvent displacement or dialysis. A more recent approach is to use supercritical fluids as solvents, but the selection criteria for solvents are not well understood. The compressible solvents studied in this work can induce pressure-tunable micellization of PEG-b-PCL. Their capacity and selectivity, and hence their ability to form micelles, depends on their density, polarity, and hydrogen bonding potential. By mixing two solvent components, such as dimethyl ether (good solvent) and trifluoromethane (selective antisolvent), one can control not only the micellization temperature and pressure, but also the bulk separation pressure (cloud pressure), crystallization temperature, and melting temperature. This can be utilized to develop efficient ways to prepare micellar precursors for drug-loaded nanoparticles.

Stress Limited Scaling of Ge2Sb2Te5

AbstractThe influence of stress on the phase change behaviour of Ge2Sb2Te5 encapsulated by ZnS-SiO2 and TiN is investigated using temperature dependent Extended X-ray Asbsorption Fines Structure and Ellipsometry to determine the crystallisation temperature. The encapsulation material surrounding the Ge2Sb2Te5 has an increasingly dominant effect on the material's ability to change phase and can cause a profound increase in its crystallization temperature. We have experimentally shown that the increased crystallization temperature originates from compressive stress exerted from the encapsulation material. By minimizing the stress we have maintained the bulk crystallization temperature in Ge2Sb2Te5 films just 2 nm thick.

Toward the Ultimate Limit of Phase Change in Ge2Sb2Te5

The limit to which the phase change memory material Ge(2)Sb(2)Te(5) can be scaled toward the smallest possible memory cell is investigated using structural and optical methodologies. The encapsulation material surrounding the Ge(2)Sb(2)Te(5) has an increasingly dominant effect on the material's ability to change phase, and a profound increase in the crystallization temperature is observed when the Ge(2)Sb(2)Te(5) layer is less than 6 nm thick. We have found that the increased crystallization temperature originates from compressive stress exerted from the encapsulation material. By minimizing the stress, we have maintained the bulk crystallization temperature in Ge(2)Sb(2)Te(5) films just 2 nm thick.

Self-assembling conditions of 4C10Sn nanoclusters in Ge:(C, Sn)

Abstract Phase diagram of Ge:(C, Sn) describing the self-assembling conditions of 4C10Sn nanoclusters is represented. The smaller lattice strains caused by the isoelectronic impurity nanoclusters in comparison with the strains produced by isolated impurities are a reason of self-assembling. Occurrence of nanoclusters should be a result of continuous phase transition. At the carbon and Sn contents equal to 0.01 and 0.025, respectively, the fractions of Sn atoms situated in nanoclusters are 0.85 and 0.32 at 500 and 800 °C, respectively. At the bulk crystallization temperature of Ge self-assembling is insignificant.

Morphology and thermal properties of a PC/PE blend with reactive compatibilization

AbstractReactive compatibilization of immiscible polymers is becoming increasingly important and hence a representative study of a polycarbonate/high density polyethylene (PC/HDPE) system is the focus of this paper. A grafted copolymer PC‐graft‐ethylene‐co‐acrylic acid (PC‐graft‐EAA) was generated as a compatibilizer in situ during processing operation by ester and acid reaction between PC and ethylene‐acrylic acid (EAA) in the presence of the catalyst dibutyl tin oxide (DBTO). As the polyethylene (PE) matrix does not play any part during the synthesis of the copolymer and since PC and EAA are also immiscible, to simplify the system, the influence of this copolymer formation at the interface between PC and EAA on rheological properties, phase morphology, and crystallization behavior for EAA/PC binary blends was first studied. The equilibrium torque increased with the DBTO content increasing in EAA/PC blends on Haake torque rheometer, indicating the in situ formation of the graft copolymer. Scanning electron microscopy (SEM) studies of cryogenically fractured surfaces showed a significant change at the distribution and dispersion of the dispersed phase in the presence of DBTO, compared with the EAA/PC blend without the catalyst. Differential scanning calorimetry (DSC) studies suggested that the heat of fusion of the EAA phase in PC/EAA blends with or without DBTO reduced with the formation of the copolymer compared with pure EAA. Then morphological studies and crystallization behavior of the uncompatibilized and compatibilized blends of PC/PE were studied as functions of EAA phase concentration and DBTO content. Morphological observations in PC/PE blends also revealed that on increasing the EAA content or adding the catalyst DBTO, the number of microvoids was reduced and the interface was intensive as compared to the uncompatibilized PC/PE blends. Crystallization studies indicated that PE crystallized at its bulk crystallization temperature. The degree of crystallinity of PE phase in PC/PE/EAA blends was also reduced with the addition of EAA and DBTO compared to the uncompatibilized blends of PC/PE, indicating the decrease in the degree of crystallinity was more in the presence of PC‐graft‐EAA. Copyright © 2007 John Wiley &amp; Sons, Ltd.

Co-continuous and encapsulated three phase morphologies in uncompatibilized and reactively compatibilized polyamide 6/polypropylene/polystyrene ternary blends using two reactive precursors

Phase morphology development in ternary uncompatibilized and reactively compatibilized blends based on polyamide 6 (PA6), polypropylene (PP) and polystyrene (PS) has been investigated. Reactive compatibilization of the blends has been performed using two reactive precursors; maleic anhydride grafted polypropylene (PP- g-MA) and styrene maleic anhydride copolymer (SMA) for PA6/PP and PA6/PS pairs, respectively. For comparison purposes, uncompatibilized and reactively compatibilized PA6/PP and PA6/PS binary blends, were first investigated. All the blends were melt-blended using a co-rotating twin-screw extruder. The phase morphology investigated using scanning electron microscope (SEM) and selective solvent extraction tests revealed that PA6/PP/PS blends having a weight percent composition of 70/15/15 is constituted from polyamide 6 matrix in which are dispersed composite droplets of PP core encapsulated by PS phase. Whereas, a co-continuous three-phase morphology was formed in the blends having a composition of 40/30/30. This morphology has been significantly affected by the reactive compatibilization. In the compatibilized PA6/(PP/PP–MA)/(PS/SMA) blends, PA6 phase was no more continuous but gets finely dispersed in the PS continuous phase. The DSC measurements confirmed the dispersed character of the PA6 phase. Indeed, in the compatibilized PA6/(PP/PP–MA)/(PS/SMA) blends where the PA6 particle size was smaller than 1 μm, the bulk crystallization temperature of PA6 (188 °C) was completely suppressed and a new crystallization peak emerges at a lower temperature of 93 °C as a result of homogeneous nucleation of PA6.

Influence of nanosized confinements on 4-n-decyl-4′-cyanobiphenyl (10CB): A broadband dielectric study

Real (epsilon') and imaginary (epsilon") parts of the complex dielectric permittivity (epsilon*) of the liquid crystal (LC) 4-n-decyl-4'-cyanobiphenyl (10CB) embedded in Anopore membranes and Vycor porous glass, as well as dispersed with hydrophilic aerosils, have been studied by means of broadband dielectric spectroscopy in the frequency range from 10(-2) Hz to 1 GHz . In bulk 10CB, which has a direct transition from an isotropic to a smectic- A phase, there exists one main relaxation process for the parallel orientation of the director with respect to the probing field and a faster one for the perpendicular orientation. All molecular relaxation processes in 10CB are of Debye type and have Arrhenius like temperature dependence. For 10CB embedded in untreated and surface treated cylindrical pores of Anopore membranes the dielectric spectra are similar to the bulk with the exception that both the rotation around the short axis and the libration motion are faster in the pores. In the case of 10CB dispersed with two different concentrations of hydrophilic aerosils an emergence of a slow relaxation process, which is stronger for the higher concentration, is additionally observed along with the bulklike processes. The slow process in the LC-hydrophilic aerosil system is attributed to the relaxation of the molecules that are homeotropically aligned close to the surfaces of the aerosil particles. This process also has an Arrhenius type of temperature dependence. For 10CB embedded in narrow channels of Vycor porous glass three relaxation processes are observed. Two of these processes are bulklike and are due to the librational motion of molecules and the rotation of molecules around their short axes. The slowest process seems to be a surface process, similar in origin to that observed for 10CB dispersed with hydrophilic aerosils, and is prominent amongst all. The material in the Vycor porous glass could be supercooled by at least 185 degrees below bulk crystallization temperature. The slow process has a Vogel-Fulcher-Tamman (VFT) type of temperature dependence typical for glass formers in this wide temperature range. In addition, the bulklike processes are found to be strongly modified and also have a VFT like temperature dependence from measurements done in a wide temperature range. This behavior is in contrast to other reports of glassy behavior in confined LC, where the glassy behavior as concluded from a slow relaxation process observed in a narrow temperature range.

Non-Debye Relaxation and Glass-Like Behavior of Confined Liquid Crystals

We report on non-Debye relaxation of fluctuations of director orientations observed in dynamic light scattering experiments for 5CB confined to random pores with average pore sizes of 100Å and in 5CB filled with Aerosil particles. The relaxation due to reorientations of molecules around short axes investigated by means of dielectric spectroscopy is also found to be of non-Debye type in confined 5CB. Deep supercooling of 5CB below the bulk crystallization temperature was observed in pores. The relaxation rate of the process due to the molecular rotation in deeply supercooled state is slower than at the temperatures corresponding to nematic phase by many orders of magnitude and shows glass-like temperature dependence.

Reactively compatibilised polymer blends: a case study on PA6/EVA blend system

Reactive compatibilisation of immiscible polymers is becoming increasingly important and a representative study with PA6/EVA system is the focus of this paper. Morphological studies and crystallization behaviour of uncompatibilised and compatibilised blends of PA6/EVA were studied as functions of dispersed phase concentration (EVA or EVA-g-MA) and maleic anhydride (MA) content. Impact properties of the compatibilised PA6/EVA blends were studied as a function of MA content at fixed dispersed phase concentration. SEM studies of cryogenically fractured surfaces showed an increase in average domain size with increase in EVA level. On contrary, the average domain size and the domain size distributions reduced significantly in the presence of EVA-g-MA. This observation is found consistent with increase in EVA-g-MA concentration at a fixed MA level and also at a fixed concentration of dispersed phase with different level of MA in binary and ternary compositions. Morphological observations also revealed that the phenomenon of coalescence is slower in the presence of EVA-g-MA indicating the formation of in situ graft co-polymer at the interface. Impact strength of the compatibilised PA6/EVA blends increased significantly as compared to uncompatibilised PA6/EVA blends. Crystallization studies indicate that PA6 and EVA (or EVA-g-MA) crystallize separately at their bulk crystallization temperature. The degree of crystallinity is reduced marginally with increase in EVA level, whereas, the decrease in crystallinity is more in the presence of EVA-g-MA. SAXS studies indicate the superposition of PA6 and EVA lamellar scattering and the possible mode of insertion is random in nature. In case of reactive systems, SAXS studies also revealed the hindered crystal growth of PA6 and EVA due to the interfacial reaction.

Glass-Like Relaxation in Confined Liquid Crystals

We have investigated the dynamics of reorientations of molecules due to their rotation around short axes and dynamics of director fluctuations (collective mode) in 5CB confined in porous matrices with randomly oriented, interconnected pores of average size 100 Å. The confinement strongly influences the dynamics of molecular and collective modes and has resulted in qualitative changes in 5CB properties. Deep supercooling of 5CB in pores up to 150 degrees below the bulk crystallization temperature was observed. The relaxation rate of the process due to the molecular rotation in deeply supercooled state is slower than at the temperatures corresponding to nematic phase by many orders of magnitude. This slowing down is accompanied by anomalous broadening of the spectra. The temperature dependencies of relaxation times of both processes obey the Vogel-Fulcher law suggesting that confined 5CB has dynamic features typical for glass-like behavior although in bulk it is a non glass former.

Non-equilibrium crystallization diagram for the Co75.26−xFe4.74(BSi)20+x amorphous metal alloys

A non-equilibrium crystallization diagram for Co sub-surface layers of the Co75.26−xFe4.74(BSi)20+x amorphous alloy for x > 1 during magnetic field annealing is determined using Transmission Electron Microscopy (TEM). The diagram shows that, depending on the composition and temperature, the Co sub-layer exhibits five distinctively different microstructures when the metallic glass is heat treated below its bulk crystallization temperature. At high boron content, the structure is dominated by the FCC structure with a high degree of oxygen impurity faulting regardless of the annealing temperature. At low concentration, the Co sub-layer exhibits FCC or HCP structures, depending on the temperature. Also found was a two-phase region in which FCC and HCP Co co-existed. Such a diagram serves as a useful guide to obtaining the desired properties as the microstructure is closely linked to the magnetic properties of the material.

Dielectric Relaxation in a Deeply Supercooled Liquid Crystals Confined in Random Porous Media

AbstractLiquid crystals (LCs)—pentylcyanobiphenyl (5CB) and octylcyanobiphenyl (8CB)—were confined in random porous media with narrow pores of mean size equal to 100 Å and investigated by means of broad band dielectric spectroscopy in deeply supercooled state. In liquid crystalline phases, bulk 5CB and 8CB have two dielectrically active modes. The main mode with the relaxation time τ ∼ 10”8s is due to the rotation of molecules about their short axis, and the secondary mode is due to the tumbling motion of molecules with the relaxation time ツ ∼ 10”10s. Bulk 5CB and 8CB are nonglass formers and they crystallize at cooling. The confinement strongly influences the dynamical behavior of LCs and is resulted in qualitative changes in their properties. Deep supercooling of LCs in pores up to ∼ 150 degrees below the bulk crystallization temperature was observed. The relaxation rate of the process due to the molecular rotation in deeply supercooled state is slower than at the temperatures corresponding to nematic phase by many orders of magnitude. This slowing down is accompanied by anomalous broadening of the dielectric spectra.Other new properties observed in confined LCs are two low frequency relaxation processes absent in bulk LCs. One of these processes is due to the molecular relaxation in the surface layers at liquid crystal-solid pore wall interface. The second process is probably a collective mode due to the relaxation of the surface induced polarization. The collective process due to surface polarization and the surface molecular mode show features typical for glass formers.