Superheating of the melting kinetics in polymer crystals: a possible nucleation mechanism

Melting kinetics of polymer crystals has been examined experimentally by calorimetric methods utilizing the combination of a conventional differential scanning calorimetry of heat flux type (CDSC-HF) and a temperature-modulated DSC (TMDSC). The superheating effect in the kinetics has been discussed based on a modeling of the melting kinetics. For low-density polyethylene and linear polyethylene, the melting rate showed nearly linear dependence on the degree of superheating, which indicates the kinetics controlled by heat diffusion or by surface kinetics on rough interface. For isotactic polypropylene, poly(ethylene terephthalate) and poly(?-caprolactone), the dependence is non-linear and close to the limiting case of exponential dependence, which indicates nucleation-controlled kinetics of melting. A possible mechanism of the activation process in the melting kinetics has been discussed in consideration of the specific feature of polymer crystals far from its most stable state. The consistency of the results of CDSC-HF and TMDSC has been confirmed by this analysis with a calibration of peak temperature for the instrumental thermal delay in CDSC-HF.     

A nucleation mechanism leading to stacking of lamellar crystals in polymer thin films

Systematic studies based on well‐controllable model systems aim at understanding how crystallization from a melt or solution of randomly coiled polymers leads to the formation of mono‐lamellar crystals. However, besides mono‐lamellar crystals also various other morphologically simple but yet not well understood structures are found. In particular, stacks of correlated lamellar crystals have been observed since the early days of the study of polymer crystallization. Here, we demonstrate that a recently proposed mechanism of self‐induced nucleation within lamellar crystals provides a possibility to explain how in such stacks lamellar crystals can be correlated. Examining various polymer systems, we show that the probability for generating self‐induced nuclei depends on the morphology of an initiating dendritic basal lamellar crystal. In addition, we provide evidence that this self‐induced nucleation mechanism, together with a high rate of transport of molten polymer to the fold surface, may allow the formation of polymer crystals with similar size in all three dimensions, containing a large number of superposed correlated lamellae. © 2019 The Authors. Polymer International published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.     

Volume nucleation of crystals in glass based on blast-furnace slag

The nucleation of crystals in glass obtained by blending metallurgical slag with silicon dioxide has been studied. The type of crystallization (homogeneous or heterogeneous, volume or surface) is revealed for each of nine compositions of synthesized glass. It is shown that the first crystalline phase in a volume-crystallizing glass is perovskite (CaO · SiO₂); in this phase a nucleation of the main phase occurs: melilite (solid solution of gehlinite 2CaO · Al₂O₃ · SiO₂ in akermanite 2CaO · MgO · 2SiO₂). The fundamental characteristics of homogeneous (for a catalizing phase, perovskite) and heterogeneous (for a catalyzed phase, melilite) of crystallization are determined: the steady-state nucleation rate I _st, time of unsteady-state nucleation τ, crystal growth rate U, and activation energy of frictional flow. The temperature dependences of I _st, τ, and U are obtained. Practical recommendations are presented for the use of blast-furnace slag as a raw material for the synthesis of glass and their further utilization.     

乳液聚合中成核机理的研究进展

主要介绍了在乳液聚合反应中,各种成核过程的机理及其对乳液聚合反应和性能的影响;分析了胶束成核、均相成核、液滴成核的产生条件与影响因素;介绍了成核机理的最新研究进展.     

The structure of linear polyethylene crystals formed in dilute solution

Summary Raman LAM and internal mode studies have been made on crystals of linear polyethylene formed in dilute solution. The molecular weight range, 5×10⁴–1.5×10⁶, was much greater than has been studied heretofore. The crystallite core thickness and degree of crystallinity was found to be independent of molecular weight for a given crystallization mode. The internal mode analysis indicates a substantial disordered overlayer, which is primarily isotropic in character.     

溶液结晶中的介尺度成核过程研究进展

成核作为溶液结晶的第一步,是决定晶体产品质量的关键因素。目前,成核理论主要包括经典成核理论和非经典成核理论。相比于仅以原子、离子或分子等均匀稳定结构为单元的经典成核理论,非经典成核理论以纳米级前聚体为单元,这类单元涵盖了聚集体、纳米粒子等介尺度非均匀动态结构,导致形成的非经典成核过程更为复杂,需在传统的化学、化学工程和过程系统工程研究方法的基础上,充分利用介尺度科学研究方法完成其核心规律的探究。为此,总结了二步成核理论、预成核团簇理论、粒子附着晶化理论以及其他新提出的非经典成核理论,分析了其中的介尺度结构及其时空动态行为,并探讨了利用介尺度数学模型对现有成核数学模型的修正和优化的思路,最后对溶液结晶中晶体成核的介尺度研究范式及理论发展进行了展望。     

Experimental study of nucleation and quality of CVD diamond adopting two-step deposition approach using MPECVD

Effects of the deposition conditions on quality and nucleation density of CVD diamond were investigated using a microwave plasma enhanced chemical vapor deposition (MPECVD) method with methane-hydrogen gas mixtures. Diamond films were deposited at pressures of 665–4000 Pa, temperatures of 660–950 °C, and methane concentrations of 0.5–5 vol.%. Deposited diamond films were characterized by scanning electron microscopy, field emission scanning electron microscopy, micro-Raman spectroscopy, and X-ray diffraction. Diamond quality and nucleation density significantly affected by the deposition pressure, substrate temperature, and methane concentration. The findings of this work were discussed in terms of the effects of deposition conditions on the plasma composition. A two-step deposition approach was applied to improve nucleation density and quality of CVD diamond films. Polycrystalline diamond films were grown using the two-step deposition process changing a combination of parameters in the two steps. Growth and quality of the deposited diamond films were improved altering the deposition pressure and substrate temperature in the two steps.     

The influence of fast nucleation reactions in the solution phase on electrodes with dissolution-precipitation reactions

The influence on electrode polarization from fast nucleation reactions in the solution phase within an electrode surface were analysed for dissolving-precipitation reactions. For electrode reactions where sparingly soluble reaction products are formed it is predicted that a polarization maximum can occur in galvanostatic transients. The maximum effect is explained from a fast build up of dissolved species which are in a reversible exchange with the electrode and then followed by a nucleation reaction in the oversaturated solution which consumes the reversible species which give a decreasing concentration polarization.     

Crystal-oriented mechanism of dynamic recrystallization nucleation in cubic boron nitride

Structural transformations occurring at 8 GPa and 2400 °C in twinned cubic boron nitride (cBN) with a triplex texture [112] have been explored by electron microscopy using the thin foil procedure. The following reconstructions have been established: destruction of the texture that occurs by rotation about the [220] axis, fragmentation of twinned grains, detwinning of large fragments by rotation about the [110] axis and formation of nuclei of a primary recrystallization from fragments with no twins.     

Primary nucleation of polyethylene: Embryogenesis from a semidilute solution

Using “realistic” molecular dynamics simulation extended up to 100 ns, we have investigated the evolution of cluster size, intrachain vs. interchain potential energies and pair correlations of polyethylene (PE) in a semidilute (ca. 28 wt%) 1,2,4-trichlorobenzene solution at 300 K. Results indicate that the embryonic development begins with the aggregation of trans-rich sequences of characteristic length l_o ≈ 2 nm, forming clusters of short stems. This is immediately followed by reorganization/thickening via intracluster axial translation and reeling-in of segments from the surrounding matrix in dynamic competition with neighboring embryos. Up to this stage, the embryonic clusters are loosely packed, retaining largely the conformer populations in the solution state but with gauche conformers enriched in the loose fold loops. After reaching a critical size with l^∗ ≈ 4 nm, the intracluster order starts to significantly improve via a “solidification” process with sigmoidal decreases of valence and nonbonding energies, while axial diffusion dramatically slows down and intracluster torsions become fully adjusted to trans conformation by annihilation of gauche conformers. In these “solidified” embryos, although molecular packing remains deviated from the orthorhombic structure (as reflected in significant differences in pair correlations) while reminiscent of the mesomorphic “rotator” or hexagonal phase, the decrease in potential energy is already significant (corresponding to about half of the heat of crystallization) as the intrachain valence contribution is fully realized.     

Thermal properties of solution flow-induced PVA crystals

Poly(vinyl alcohol) was crystallized from its aqueous solution under a steady-state flow with different conditions. Using a differential scanning calorimeter (DSC), the thermal properties of PVA precipitates obtained by the flow-induced crystallization were measured, showing a single melting peak in the DSC curve. The melting temperature of PVA crystallized depended on the rate of stirring, crystallization temperature, concentration of PVA, yield of precipitates, and morphology of the crystals. Particularly, there was a closer relationship between the melting temperature and the yield of precipitates, i.e., it was through the effect on the yield of precipitates that crystallization conditions influenced the melting temperature. Generally, with certain conditions, the higher the yield of precipitates, the lower the melting temperature. Moreover, using electron microscopy, it was observed that there were some structural differences between the inner and outer sides of a PVA crystal mat round stirrer and between film-shaped and block-shaped crystals, leading to different melting temperatures. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 1337–1344, 1997     

Crystal morphology and transcrystallization mechanism of isotactic polypropylene induced by fibres: interface nucleation versus bulk nucleation

Using polarized optical microscopy (POM) equipped with a hot stage, morphological investigations of an isotactic polypropylene (iPP) matrix, induced by a homogeneous iPP fibre and heterogeneous pure/modified nylon 6 fibres, were carried out. With respect to transcrystallization related to heterogeneous nucleation on the surface of the fibre, the nucleation mode was found to be different for iPP fibres and nylon 6 fibres. An iPP fibre can serve as a macroscopic linear nucleus, similarly to the shish‐type structure formed in stress‐induced crystallization, to induce kebab‐like growth of lamellae, whereas numerous closely packed spherulites along nylon 6 fibres resulted in macroscopic transverse growth to form a transcrystallite owing to the limitation along the fibre axis. The difference in nature between these two transcrystallites can be further demonstrated by their optical characters related to the lamellar arrangement inside the transcrystallite. As for homogeneous iPP composites, the formation of transcrystallites results from lattice matching, in consideration of the same chemical structure and lattice parameters between fibre and matrix. The incorporation of calcium chloride into a nylon 6 fibre—to destroy its crystal structure—confirmed the role of lattice matching between nylon 6 fibre and iPP matrix. The addition of atactic polypropylene (aPP) in order to enhance the nucleation ability of the iPP matrix also greatly weakened transcrystallization. Our work demonstrates that transcrystallization is just a matter of competition between interface nucleation and bulk nucleation, namely, if interface nucleation is faster than bulk nucleation, transcrystallization will develop. If not, it will be suppressed. Copyright © 2006 Society of Chemical Industry     

Crystallisation of high density polyethylene from dilute solution: Heterogeneous nucleation

The self-seeding crystallisation technique is applied to a dilute solution of polyethylene in xylene containing, in suspension, foreign particles; data concerning some pigments usually employed for colouring the plastics are reported. It is shown that this method allows easy demonstration of the interactions existing between the dissolved polymer and the pigment; at the same time, it is possible to study the morphology which is evolved as a consequence of these interactions under particularly simple conditions as only a small amount of polymer crystallises on the pigment particles. The behaviour of several pigments is consistent with the results obtained by crystallising the bulk polyethylene coloured with the same pigments. Some unusual morphologies are made evident.     

Annealing behavior of solution grown polyethylene single crystals

The morphology evolution of solution grown polyethylene single crystals has been studied upon annealing below their melting temperature by using atomic force microscopy (AFM). AFM investigations have been performed ex situ, which means AFM investigations at room temperature after the annealing treatment, and in situ at real temperature. Beside the well-known Swiss-cheese and picture frame appearance a novel morphological feature has been observed: the formation of saw-tooth-like patterns at the edges of the crystals, which become more and more pronounced with increasing annealing temperature. Controlled dissolution experiments have resulted in similar saw-tooth-like edge patterns of the single crystals, which indicate that the initial organization of the crystals influences the reorganization. This reorganization behavior upon annealing can be related to the presence of nano-sized fold defects, which have been monitored using the technique of n-alkane surface decoration.     

Surface reaction-controlled dissolution of crystals in a solvent or solution

It is shown that surface reaction-controlled dissolution of crystals in a solvent or solution is very unlikely.     

The growth kinetics of hexamethylene tetramine crystals from a water/acetone solution

A brief summary of the theory of solvent effects in crystal growth from solution is presented. Based on solution thermodynamic properties, some qualitative predictions are made relating to the mechanism and kinetics of the crystallisation of hexamethylene tetramine from a water/acetone solution. These predictions are confronted with the experimentally determined surface integration kinetics and a good agreement is found between theory and experiment.     

The spin-coating process mechanism related to polymer solution properties

Coating the substrate with an amorphous polymer layer is a very common process in the manufacturing of integrated circuits, but the fundamental spin-coating mechanism has not yet been precisely determined. This study examines the influence of the macromolecular characteristics on the spin-coating process both experimentally and from a phenomenological point of view. It is concluded that the weight-average molecular weight of the spun-on solution is a pertinent parameter, and that chain entanglements in polymer solutions may be considered as the basic phenomenon responsible for the formation of the solid polymer layer.