Effect of Emulsifier Type on the Crystallization Kinetics of Oil-in-Water Emulsions Containing a Mixture of Solid and Liquid Droplets

Measurements of the ultrasonic velocity as a function of time (at 6.0°C) have been used to monitor the kinetics of crystallization of oil droplets in n -hexadecane-in-water emulsions containing a mixture of solid and liquid droplets. Crystallization is induced in liquid droplets when solid droplets are present. Induced crystallization is probably caused by crystals, protruding from solid droplets, penetrating into liquid droplets during collisions and thus acting as nucleation sites for crystal growth. The rate of induced crystallization depends strongly on the type of emulsifier used to stabilize the emulsion droplets. For the series of emulsifiers used in this study the rate decreased in the following order: Tween 20 > SDS > β-lactoglobulin > β-casein. Differences in rate can be explained by considering the influence the emulsifiers have on the interactions between droplets.     

On the behaviour of molecules of the quinoline group at the water—Mercury interface: Part III. The kinetics of the isoquinoline transition in the presence of an “inert” electrolyte

The reorientation transition, which involves two distinct monolayers of isoquinoline molecules, has been investigated by using the single-potential step and the double-potential step methods under various experimental conditions.The cathodic transients correspond to a liquid → solid transition. Their morphology and half-times have been measured at various initial and final potentials, and for several isoquinoline concentrations close to the saturation value. At small overvoltages, the reorientation process is determined by heterogeneous (“instantaneous”) nucleation, while progressive nucleation and growth prevail at higher potentials. Recourse to the double-potential step method affords an efficient way of assessing separately the influence of the overvoltage on the rate constants for nucleation and growth. Determination of the critical nucleus size and the activation energy for each of the two processes has been based on an adaptation of the Brandes theory developed for two-dimensional crystallization from a supersaturated vapour phase. The kinetics are markedly dependent on the initial state, which can be easily controlled by the potential and the bulk concentration. When the potential is stepped from the region where there is superadsorption, progressive nucleation is particularly fast, while starting from a partially depleted layer gives much slower transients with extensive tailing due to diffusional hindrance.The anodic transients show consonant characteristics which indicate that the initial film can be considered as a two-dimensional solid, and that boundary defects are acting as nucleation centres, during the solid → liquid transition triggered by the potential step.     

Comparative study of microstructural evolution during melting and crystallization

Molecular dynamics simulations, with the interaction between atoms described by a modified analytic embedded atom method, have been performed to obtain the atomic-scale details of isothermal melting in nanocrystalline Ag and crystallization from supercooled liquid. The radial distribution function and common neighbor analysis provide a visible scenario of structural evolution in the process of phase transition. The results indicate that melting at a fixed temperature in nanocrystalline materials is a continuous process, which originates from the grain boundary network. With the melting developing, the characteristic bond pairs (555), (433), and (544), existing in liquid or liquidlike phase, increase approximately linearly till completely melted. The crystallization from supercooled liquid is characterized by three characteristic stages: nucleation, rapid growth of nucleus, and slow structural relaxation. The homogeneous nucleation occurs at a larger supercooling temperature, which has an important effect on the process of crystallization and the subsequent crystalline texture. The kinetics of transition from liquid to solid is well described by the Johnson-Mehl-Avrami equation.     

Bulk and solution crystallization kinetics of isotactic polybutene-1

The crystallization kinetics of an unfractionated sample of isotactic polybutene-1 have been studied from the melt and from dilute solutions in amyl acetate by the dilatometric method. The kinetics of bulk crystallization followed the Avrami equation for most of the transformation with a deviation towards the end of the crystallization process. The Avrami exponent is found to be temperature dependent with the value of n ≈ 3 at high undercooling (indicating a homogeneous nucleation process) and n ≈ 4 at lower undercooling (indicating a heterogeneous nucleation process). The temperature coefficients of the rate constants indicate a nucleation controlled process of crystallization.     

Heterogeneous nucleation of crystallization of high polymers from the melt. III. Nucleation kinetics and interfacial energies

In the melt crystallization of isotactic polypropylene, poly(ethylene oxide) and poly(butene-1) in contact with substrates, the existence of a fixed number of nucleating sites on the substrate surfaces has been established. When these sites become active successively (the transient in the number of nuclei is long) during crystallization, pseudohomogeneous nucleation on the substrate occurs. Nucleation rates for poly(butene-1) and poly(ethylene oxide) on substrates and in bulk have been measured. These data can be used for comparing the nucleating ability of substrates. Estimates of the variation of bulk nucleation rates from one volume element to another as well as for repeated crystallization within a given volume element have been included. Finally, the temperature coefficients of heterogeneous nucleation rates have been combined with the temperature coefficient of spherulitic growth rate of poly(butene-1), to yield values of the interfacial energy parameters appearing in the theory of heterogeneous nucleation. The quantitative characterization of the nucleating ability of substrates by this method is an improvement over the mere use of nucleation densities or nucleation rates.     

聚对苯二甲酸乙二酯／热致液晶聚合物共混体系中液晶组分的诱导结晶效应

用差示扫描量热法对聚对苯二甲酸乙二酯（ＰＥＴ）／热致液晶高分子（ＬＣＰ）共混体系的等温和非等温结晶行为进行了研究．结果表明，由于液晶组分的加入，共混体系中ＰＥＴ的结晶速率和结晶度均得到提高．说明ＬＣＰ具有ＰＥＴ结晶成核剂的作用．在较低的ＬＣＰ组分含量下（～２ｗｔ％），这一效果最为明显，说明ＬＣＰ是以很小的微区或某些ＬＣＰ分子链介晶微束的形式对ＰＥＴ的结晶起成核剂的作用．     

Interpretation of the nonisothermal crystallization kinetics of polypropylene using a power law nucleation rate function

A nucleation rate function is proposed for use in analyzing the overall crystallization kinetics of polymers. This function allows for the possibility that the nucleation rate varies substantially during the crystallization. This feature is particularly useful in analyzing nonisothermal crystallization, but it can be used to analyze isothermal crystallization as well. The nucleation rate function was used in the derivation of a modified transformation kinetics equation of the Avrami type. The modified Avrami equation was found to be suitable for kinetics analysis for the data obtained from nonisothermal crystallization at rapid cooling rates. Kinetics parameters used to describe nonisothermal crystallization under rapid cooling rates are presented and discussed. These include crystallization induction time, plateau (crystallization) temperature, crystallization half-time, crystallization rate constant, Avrami index, and newly defined quantities called nucleation index, geometric index, and nucleation rate constant. The procedure used to obtain the nucleation rate constant and nucleation index for the nucleation rate function is described and illustrated by application to the analysis of the crystallization kinetics of polypropylene. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 1077–1093, 1997     

Spinodal patterns indicating unstable regime of polymer crystallization

It has been considered that crystallization of polymer from melt proceeds via the coexistence of molten matrix and growing crystals that have once overcome a nucleation barrier to a critical size. The nucleation process has often been explained analogously with so-called nucleation and growth (NG) behavior of the phase separation of a binary mixture in metastable conditions, although the crystallization in one-component polymer is not a real component separation but a phase transition. Among the mechanisms of polymer crystallization, the topic is whether a liquid–liquid transition between states of different densities within one-component polymers takes place before the aforementioned nucleation process. The liquid–liquid transition between states, which is probably driven by chain orientation, is also categorized into NG and the controversial spinodal decomposition (SD) type processes depending on the quenching depth. This article provides the optical microscopic observations that favor the occurrence of the SD-like process when a one-component polymer melt is very rapidly quenched below a stability limit, including a drastic morphological change from a spherulitic to a spinodal pattern at the critical (or spinodal) temperature. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1817–1822, 2004     

Computer simulation of crystallization kinetics and morphology in fiber-reinforced thermoplastic composites. III. Thermal nucleation

A computer simulation has been used to predict crystallization kinetics and crystalline morphology in composite materials based on thermally nucleated crystallizable matrices. As demonstrated for athermally nucleated composites, the presence of reinforcing fibers increases the complexity of the system. Fibers are shown to have a dual effect on the spherulitic crystallization process. The influence that fibers have depends on the interplay between the enhancing effects that fibers have on nucleation and the depressing effects that fibers have on spherulitic growth. Fibers that do not provide additional nuclei to the system depress the rate of crystallization relative to an unreinforced polymer, while fibers that add nuclei to the system increase the rate of crystallization. The transcrystalline morphologies that develop in thermally nucleated fiber-reinforced polymers are controlled primarily by the relative numbers of bulk and fiber nuclei. The extent of transcrystalline regions can be suppressed either by increasing the rate of bulk nucleation, or by decreasing the rate of fiber nucleation. Finally, the qualitative appearance of the morphology in the transcrystalline region was found to be indicative of the mode of fiber nucleation. © 1995 John Wiley & Sons, Inc.     

Estimation of the nucleation and crystal growth contributions to the overall crystallization energy barrier

Classical kinetic theories of polymer crystallization were applied to isothermal crystallization kinetics data obtained by polarized optical microscopy (PLOM) and differential scanning calorimetry (DSC). The fitted parameters that were proportional to the energy barriers obtained allow us to quantitatively estimate the nucleation and crystal growth contributions to the overall energy barrier associated to the crystallization process. It was shown that the spherulitic growth rate energy barrier found by fitting PLOM data is almost identical to that obtained by fitting the isothermal DSC crystallization data of previously self‐nucleated samples. Therefore, we demonstrated that by self‐nucleating the material at the ideal self‐nucleation (SN) temperature, the primary nucleation step can be entirely completed and the data obtained after subsequent isothermal crystallization by DSC contains only contributions from crystal growth or secondary nucleation. In this way, by employing SN followed by isothermal crystallization, we propose a simple method to obtain separate contributions of energy barriers for primary nucleation and for crystal growth, even in the case of polymers where PLOM data are very difficult to obtain (because they exhibit very small spherulites). Comparing the results obtained with poly(p‐dioxanone), poly(ε‐caprolactone), and a high 1,4 model hydrogenated polybutadiene, we have interpreted the differences in primary nucleation energy barriers as arising from differences in nuclei density. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1478–1487, 2008     

Localized Crystallization of Enantiomeric Organic Compounds on Chiral Micro‐patterns from Various Organic Solutions

The controlled crystallization of enantiomers of an organic compound (a cyclic phosphoric acid derivative) on templated micro‐patterned functionalised surfaces is demonstrated. Areas where a complementary chiral thiol has been located were effective heterogeneous nucleation centres when a solution of the compound is evaporated slowly. Various organic solvents were employed, which present a challenge with respect to other examples when water is used. The solvent and the crystallization method have an important influence on the crystal growth of these compounds. When chloroform was employed, well‐defined crystals grow away from the surface, whereas crystals grow in the plane from solutions in isopropanol. In both cases, nucleation is confined to the polar patterned regions of the surface, and for isopropanol growth is largely limited within the pattern, which shows the importance of surface chemistry for nucleation and growth. The apparent dependence on the enantiomer used in the latter case could imply stereo‐differentiation as a result of short‐range interactions (the templating monolayer is disordered, even at the nanometre scale). The size of the pattern of chiral monolayer also determines the outcome of the crystallization; 5 μm dots are most effective. Despite the low surface tension of the samples (relative to the high surface tension of water), differential solvation of the polar and hydrophobic layers of the solvents allows crystallization in the polar regions of the monolayer, therefore the polarity of the regions in which heterogeneous nucleation takes place is indeed very important. Despite the complex nature of the crystallization process, these results are an important step towards to the use of patterned surfaces for heterogeneous selective nucleation of enantiomers.     

Fluctuation‐assisted crystallization of an iPP/PEOc polymer alloy prepared on a single multicatalyst reactor granule

The new fluctuation‐assisted mechanism for nucleation and crystallization in the isotactic polypropylene/poly(ethylene‐co‐octene) alloy has been studied. We found that the liquid–liquid phase separation (LLPS) had a dominant influence on the crystallization kinetics through the nucleation process. After LLPS, the nucleation of crystallization mainly occurred at the interface of the phase‐separated domains. It is because that the concentration fluctuations of the LLPS induced the motion of polymer chains and possibly some segmental alignment and/or orientation in the concentration gradient regions through interdiffusion, which could assist the formation of nuclei for crystallization. In other words, the usual nucleation energy barrier could be overcome (or at least partially) by the concentration fluctuation growth of LLPS in the unstable regions. This could be viewed as a new kind of heterogeneous nucleation and could be an addition to the regular nucleation and growth mechanism for crystallization. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 166–172, 2009     

聚丙烯-g-聚氨酯共聚物的非等温结晶动力学研究

用DSC法研究了聚丙烯 (PP)和聚丙烯接枝聚氨酯的共聚物 (PP g PU)在不同冷却速率下的非等温结晶动力学 .用Avrami方程和莫志深改进法对DSC测定结果进行了处理 ,结果表明 ,PP g PU的动力学参数能很好的符合Avrami方程和莫志深改进方程 .PP接枝了聚氨酯支链后 ,结晶速率增大 ,球晶的生长和成核机制也相应发生改变 ,而其变化规律与接枝物的组成和结构密切相关     

Molecular dynamics simulation of cavitation in a lead melt at negative pressures

The molecular dynamics method was used to simulate cavitation in a metastable lead melt and determine the stability limits. States at temperatures below critical (T < 0.5T _c) and large negative pressures were considered. Interatomic interactions were described by the realistic embedded atom potential. The kinetic boundary of liquid phase stability was shown to be different from the spinodal. The kinetics and dynamics of cavitation were studied. The pressure dependences of cavitation frequencies were obtained over the temperature range 700–2700 K. The results of molecular dynamics calculations were compared with estimates based on classical nucleation theory.     

The combination of fluctuation-assisted crystallization and interface-assisted crystallization in a crystalline/crystalline blend of poly(ethylene oxide) and poly(ε-caprolactone)

The nucleation and crystallization of poly(ethylene oxide) (PEO) and poly(ε-caprolactone) (PCL) in the PEO/PCL blends have been investigated by means of optical microscopy (OM) and differential scanning calorimetry (DSC). During the isothermal or nonisothermal crystallization process, when the adjacent PEO is in the molten state, PCL nucleation preferentially occurs at the PEO and PCL interface; after the crystallization of the adjacent PEO, much more PCL nuclei form on the surface of the PEO crystal. However, PEO crystallizes normally and no interfacial nucleation occurs in the blend. The concentration fluctuation caused by liquid–liquid phase separation (LLPS) induces the motion of PEO and PCL chains through interdiffusion and possible orientation of chain segments. The oriented PEO chain segments can assist PCL nucleation, and the heterogeneous nucleation ability of PEO increases with the orientation of PEO chains. Oriented PCL chain segments have no heterogeneous nucleation ability on PEO. It is postulated that the interfacial nucleation of PCL in the PEO/PCL blend follows the combination of “fluctuation-assisted crystallization” and “interface-assisted crystallization” mechanisms.

Molecular dynamics studies of the kinetics of phase changes in clusters III: structures, properties, and crystal nucleation of iron nanoparticle Fe331

Molecular dynamics (MD) computer simulations have been carried out to study the structures, properties, and crystal nucleation of iron nanoparticles with 331 Fe atoms or with diameter around 2 nm. Structure information for the nanoparticles was analyzed from the MD simulations. Three crystalline phases and one amorphous phase were obtained by cooling the nanoparticles from their molten droplets at different cooling rates or with different lengths of cooling time periods. Molten droplets froze into three different solid phases and a solid-solid transition from a disordered body-centered cubic (BCC) phase to an ordered BCC phase were observed during the slow cooling and the quenching processes. Properties of nanoparticle Fe₃₃₁, such as melting point, freezing temperature, heat capacity, heat of fusion, heat of crystallization, molar volume, thermal expansion coefficient, and diffusion coefficient, have been estimated. Nucleation rates of crystallization to two solid phases for Fe₃₃₁ at temperatures of 750, 800, and 850 K are presented. Both classical nucleation theory and diffuse interface theory are used to interpret our observed nucleation results. The interfacial free energy and the diffuse interface thickness between the liquid phase and two different solid phases are estimated from these nucleation theories.     

Small-angle x-ray scattering and crystallization kinetics of poly(ethylene terephthalate) crystallized in a liquid environment

By small-angle x-ray scattering, a systematic investigation was performed of the long spacing of poly(ethylene terephthalate) (PET) crystallized in a liquid environment. The results indicated that the measured long spacings were temperature dependent and apparently relatively insensitive to liquid type under the conditions studied. The kinetic nucleation model of polymer crystallization was found to adequately explain this dependence. The differences in the long spacings between thermal and liquid-induced crystallization were in part rationalized in terms of the suspected supercoolings involved in the respective processes. Calculation of the spherulite growth rates for liquid-induced crystallization was made on the basis of the kinetic nucleation model and the classic theory of polymer–diluent crystallization. The results were shown to agree with inferential experimental observations of these growth rates and to elucidate the physics underlying liquid–induced crystallization. Finally, use of this growth rate theory in conjunction with a previous model for overall crystallization kinetics was shown to adequately describe and predict the diffusion-limited kinetics observed experimentally for most liquid-induced crystallization situations.     

Fluctuation‐Assisted Crystallization: In a Simultaneous Phase Separation and Crystallization Polyolefin Blend System

Summary: The liquid‐liquid phase separation (LLPS) is often coupled with other ordering processes such as crystallization. In a polyolefin blend system, overwhelming changes in crystallization kinetics due to concentration fluctuation caused by spontaneous spinodal LLPS have been observed. Consequently, we are proposing a new mechanism of “fluctuation‐assisted crystallization”. In this process, the usual nucleation barrier could be overcome (or at least partially) by the spontaneous fluctuation growth of LLPS in the spinodal region.

Time‐resolved polarized optical micrographs for poly(ethylene‐co‐hexene) (PEH)/poly(ethylene‐co‐butene) (PEB) = 40:60 isothermally crystallized at 117 °C for 2 min after LLPS at 135 °C for the times shown and the nucleation rates at 117 °C as a function of LLPS time at 135 °C.