Isothermal Crystallization Time Research Articles

Magnetic polyamide 11 powder for the powder bed fusion process by liquid-liquid phase separation and crystallization

Herein, we demonstrate the preparation of magnetic polyamide 11 feedstock powders for powder bed fusion (PBF) using liquid-liquid phase separation and crystallization. By adding magnetic nanomaterials during the precipitation process, the particulate additive is incorporated into the polymer matrix. This enables the production of filled systems at single particle level, which is advantageous in terms of the adjustable magnetic strength and homogeneity of the powder. Thermogravimetric analysis is used to determine the additive content and onset temperature of decomposition of composite powders, where additive-enhancement of PA11 powders with magnetic iron oxide nanoparticles showed a positive influence on the thermal stability of the feedstock. Successive analysis of particle size distribution, shape, colour, crystal structure, magnetic properties and thermal properties of the composite powders are carried out and their properties are discussed with respect to the PBF process. A decrease in particle size, width of the thermal process window and isothermal crystallization time can be attributed to the nucleating effect of the magnetic additive. The PBF processability of the composite feedstock is demonstrated by the production of magnetic tensile bar specimens from additive-enhanced PA11 powders with 1 wt.-% magnetic iron oxide.

Using an aromatic amide as nucleating agent to enhance the crystallization and dimensional stability of poly(3-hydroxybutyrate-co-3-hydroxyhexanate)

Biosynthesized poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx) has emerged as a promising biodegradable polymer with a great potential to compete with traditional petroleum-based plastics, however, the poor crystallization ability makes it challenge to transform into high-performance products via common melt-processing methods. Herein, we demonstrate that N,N′-dicyclohexyl-2,6-naphthalenedicarboxamide (TMB) can serve as an efficient nucleating agent to significantly enhance the crystallization and resulting storage stability of PHBHHx. The results indicate that PHBHHx with small amounts of TMB (0.3–0.5 wt%) can crystallize completely even under a rapid cooling rate of 100 °C/min and the isothermal crystallization time is greatly reduced. As a result, the crystallinity of the injection-molded PHBHHx products is increased from 24.5 % to 39.5 %, without secondary crystallization after being stored at room temperature for 6 h. The products exhibit superior dimensional stability and the post-shrinkage can be decreased to as low as 0.1 %. Our work offers a feasible method to develop high-performance PHBHHx materials with remarkably enhanced crystallization ability.

Controllable cell structures of poly(ether-block-amide) foams via isothermal melt crystallization-foaming in supercritical CO2

A simple isothermal melt crystallization-foaming method was proposed to regulate the crystallinity (Xc) of PEBA to prepare poly(ether-block-amide) (PEBA) foams with uniform or bimodal cell structures by supercritical CO2. PEBA with different Xc values was obtained by controlling the isothermal crystallization temperature (Tiso) and time (tiso). The rheological and DSC results showed that the viscoelasticity and Xc of the samples increased with decreasing Tiso and increasing tiso. PEBA foams with Xc above 1.1% exhibited a distinct bimodal cell structure. The increased crystals acted as heterogeneous nucleation sites to increase the cell density from 6.8 × 106 cells/cm3 to 3.58 × 1010 cells/cm3. The size of large cells decreased from 206.4 µm to 48.0 µm, and the number ratio of small to large cells increased from 2.34 to 65 times. Finally, a mechanism to explain the cell structure variation in PEBA foams was proposed, providing a new strategy for regulating the cell structure of polymeric foams.

Effects of combined melt stretching and fast cooling fields on crystallization of high-density polyethylene

In many industrial processes, polymer melt undergoes complex flow-cooling environments before being solidified into the final products. Investigating polymer crystallization under processing conditions is a necessary step to predict and customize the structures and properties of semicrystalline polymers. Herein, the crystallization behavior of high-density polyethylene (HDPE) under combined melt stretching and fast cooling fields was studied through a homemade extensional rheometer. A series of HDPE samples were prepared by cooling the stretched melt at different cooling rates (0.02–220 °C/s) immediately after flow, or after waiting for different times of isothermal crystallization at 128 °C. Microstructural characterizations confirm four different structural states related to the evolution of shish-kebabs after melt stretching, namely generation of shish precursors, shish crystal growth, kebab lamellar growth, and perfection of shish-kebabs, which diversify greatly the structures and morphologies of the final solidified samples. By prolonging the isothermal crystallization time at high temperature, the fast cooling-induced lamellae keep decreasing in thickness. This supports the occurrence of molecular segregation caused by shish-kebabs growth at high temperature, which reduces the molecular weight of the remaining un-crystallized melt. In addition, quantitative analysis of crystalline orientation manifests that the lamellar growth during isothermal process takes strictly the shish as a template and adopts a high orientation, while the fast cooling-induced lamellae are only related to the chains conformation of un-crystallized melt prior to cooling. This work would be essential to advance the in-depth understanding on the actual industrial processing of semicrystalline polymeric materials.

Kinetics of cold crystallization in two liquid crystalline fluorinated homologues exhibiting the vitrified smectic CA* phase

Dielectric relaxation processes in the supercooled antiferroelectric smectic CA* phase and crystallization kinetics of two chiral fluorinated 5HF6 and 6HF6 compounds from the same homologous series are investigated. Fragility parameters are determined from the relaxation time of the α-process, including τHN from the Havriliak-Negami formula and τpeak denoting the position of the absorption peak. The coupling coefficient ξ between the characteristic time of isothermal cold crystallization and relaxation time of the α-process is obtained. Despite similar values of the fragility index, the even 6HF6 homologue undergoes cold crystallization much faster than the odd 5HF6 homologue, with significantly different ξ coefficients. Influence of the relaxation time of the PH process (anti-phase phason) in the smectic CA* phase on the crystallization kinetics is presumed.

The bulk enthalpy of melting of α’-crystals of poly (l-lactic acid) determined by fast scanning chip calorimetry

Related to the wide scatter of information in the literature, recently the bulk enthalpy of melting of α-crystals of poly (l-lactic acid) (PLLA) has been re-determined by correlating the crystallinity-dependent heat capacity at a temperature slightly higher than the glass transition temperature and the corresponding enthalpy of melting [Macromol. Rap. Comm. 2022, 43, 2200148], using fast scanning chip calorimetry (FSC). For α-crystals, a value of 104 J/g is suggested, valid at 190 °C. Here, the initial study is expanded such to obtain the bulk enthalpy of melting of PLLA α’-crystals, containing conformational defects and forming at lower crystallization temperatures than the α-phase. Samples containing different amount of α’-crystals were generated by varying the time of isothermal crystallization at 90 °C. Extrapolation of the obtained relationship between the heat capacity at the crystallization temperature, that is, the crystallinity, and the enthalpy of melting on subsequent fast heating, suppressing the transition into α-crystals and simplifying therefore the analysis of the melting process of the formed α’-crystals, toward fully crystallized PLLA yields a bulk enthalpy of melting of 72 J/g at the thermal stability limit of α’-crystals of 150 °C. The used experimental approach to obtain the bulk enthalpy of melting of polymer crystals relies on the presence of a two-phase structure/absence of a glassy rigid amorphous fraction at the temperature of analysis of the crystallinity via the heat capacity (90 °C). Annealing experiments slightly below the analysis temperature support this assumption by absence of enthalpy-recovery peaks on subsequent heating.

Effects of Isothermal Crystallization on Carrier Transport and Breakdown Characteristics of HDPE/LDPE Insulation

In this paper, the influence of the crystalline morphology of high-density polyethylene (HDPE)/low density polyethylene (LDPE) blend on its electrical conductivity, breakdown performance and trap distribution were studied by using the method of isothermal crystallization. Five HDPE/LDPE blends were prepared by melt blending method, and they were cooled to 118°C for isothermal crystallization treatment for 0, 5, 10, 20 and 40 min. The experimental results show that HDPE/LDPE blend with 10min isothermal crystallization time has the lowest conductivity at 70℃, and its conductivity-temperature dependence is also low. As the isothermal crystallization time increases from 0 to 10min, both the DC and AC breakdown strengths of HDPE/LDPE blend increase, and further increase the isothermal crystallization time to 20 and 40min, the breakdown performance of HDPE/LDPE blend decreases significantly. The aggregate structure of HDPE/LDPE blend is closely related to its electrical properties. When the isothermal crystallization time is 10 min, the HDPE/LDPE blend shows a denser crystalline morphology and introduces many of deep traps measured by the isothermal discharge current (IDC) method. Therefore, space charge accumulation and local electric field distortion in the dielectrics are reduced. Also, the breakdown field strength of the HDPE/LDPE blend is improved.

Structural evolutions of the amorphous fraction of polyethylene terephthalate during the secondary crystallization

We have studied the structural evolution of polyethylene terephthalate (PET) during the isothermal crystallization through the temperature-modulated differential scanning calorimetry (TMDSC) and the positron annihilation lifetime spectroscopy (PALS). By tracing the changes in the amorphous free volume size, and the content of the crystal, the mobile amorphous fraction (MAF), and the rigid amorphous fraction (RAF), we clarified how the RAF participates in the crystallization. It was found that the limiting temperature of the RAF, T* determines the behaviors of the RAF in the crystallization. For isothermal crystallization at temperatures below T*, the RAF is strictly constrained by the lamella such that it cannot participate to the crystallization. For isothermal crystallization at temperatures above T*, chain segments inside the lamella are able to move, and the restrictions exerted on the RAF by the lamella are eased. Then, the activated RAF participates in the crystallization mainly through the lamellar perfection. As a result, instead of a monotonic increase in the RAF content, a non-monotonic change and even a monotonic decrease as a function of the isothermal crystallization time were observed. Moreover, an anomalous increase in the MAF content at the late stage of isothermal crystallization was found. Our results demonstrate that the activated RAF can indeed participate to the crystallization, and reveal how the structural evolutions of the activated RAF and the MAF proceed.

The effect of isothermal crystallization on mechanical properties of poly(ethylene 2,5-furandicarboxylate)

Abstract The effects of isothermal crystallization temperature/time on mechanical properties of bio-based polyester poly(ethylene 2,5-furandicarboxylate) (PEF) were investigated. The intrinsic viscosity, crystallization properties, thermal properties, and microstructure of PEF were characterized using ubbelohde viscometer, X-ray diffraction, polarizing optical microscope, differential scanning calorimetry, and scanning electron microscopy. The PEF sample isothermal crystallized at various temperatures for various times was denoted as PEF-T-t. The results showed that the isothermal crystallization temperature affected the mechanical properties of PEF-T-30 by simultaneously affecting its crystallization properties and intrinsic viscosity. The isothermal crystallization time only affected the crystallization properties of PEF-110-t. The crystallinity of PEF-110-40 was 17.1%. With small crystal size, poor regularity, and α′-crystal, PEF-110-40 can absorb the energy generated in the tensile process to the maximum extent. Therefore, the best mechanical properties can be obtained for PEF-110-40 with the tensile strength of 43.55 MPa, the tensile modulus of 1,296 MPa, and the elongation at a break of 13.36%.

Influence of sub-Tg annealing on microstructure and crystallization behavior of TiZr-based bulk metallic glass

Herein, the effect of sub-Tg annealing on microstructure, glass transition kinetics and crystallization behavior of Ti33Zr30Cu9Ni5.5Be22.5 bulk metallic glass (BMG) is systematically studied by high-temperature differential scanning calorimetry. The results reveal that sub-Tg annealing can improve the compactness of Ti33Zr30Cu9Ni5.5Be22.5 BMG and expand the supercooled liquid region. Also, the non-isothermal crystallization behavior indicates that the glass transition activation energy (Eg) of Ti33Zr30Cu9Ni5.5Be22.5 BMG decreases and crystallization activation energy (EP1, EP2) increases after sub-Tg annealing. The changes in Eg, EP1 and EP2 exhibit sensitivity to sub-Tg annealing temperature. It is demonstrated that the isothermal crystallization time of Ti33Zr30Cu9Ni5.5Be22.5 BMG is extended by ≈1 min after 593 K annealing, indicating the inhibition of crystallization process. The Avrami indices of as-cast and sub-Tg-annealed Ti33Zr30Cu9Ni5.5Be22.5 BMGs, calculated by the Johnson-Mehl-Avrami equation, range from 2.08 to 2.30 and indicate that the three-dimensional growth process is mainly controlled by the diffusion, which is not influenced by sub-Tg annealing.

Isothermal Crystallization Monitoring and Time-Temperature-Transformation of Amorphous GDC-0276: Differential Scanning Calorimetric and Rheological Measurements.

Cold crystallization of amorphous pharmaceuticals is an important aspect in the search to stabilize amorphous or glassy compounds used as amorphous pharmaceutical ingredients (APIs). In the present work, we report results for the isothermal crystallization of the compound GDC-0276 based on differential scanning calorimetric and rheometric measurements. The kinetics of isothermal crystallization from the induction time to the completion of crystallization can be described by the classic Johnson-Mehl-Avrami (JMA) equation. The time-temperature-transformation (TTT) diagrams were constructed for two time points-that of induction and that of completion of crystallization. The results show that the rheological measurement for GDC-0276 has a better overall sensitivity in detection of the early stage nucleation and, consequently, detects the onset of crystallization sooner than does the differential scanning calorimetry. Rheological measurements were also used to obtain the temperature dependence of the viscosity of GDC-0276 and the relevant parameters were used in a modified form of the JMA model to describe the temperature dependence of the crystal induction and completion times, that is, the TTT diagram for the material. In the modification, we assumed that the kinetics followed the viscosity to the 0.75 power as suggested by the recent work of Huang et al. (Huang, C., et al., J. Chem. Phys.2018,149, 054503). The relationship and the possible impact on crystallization kinetics of the break-down of the Stokes-Einstein relation in glass-forming liquids are discussed. From the crystallization kinetics modeling, the solid-liquid interfacial surface tension σSL was obtained for GDC-0276 and was compared with that obtained from the melting point depression measurements of the material confined in nanoporous glasses. The differences between the values from the two methods are discussed.

Theoretical investigations on melting/crystallization kinetics in overheated/overcooled aluminum at high pressures

In this study, we developed a theoretical model that explores material melting and crystallization kinetics at high pressures by combining the Kolmogorov-Johnson-Mehl-Avrami kinetics model with equations pulled from hydrodynamic and thermodynamic theories. Our model only requires two inputs: an equilibrium melting curve and an equation of state, both of which can be derived empirically through experiments. We use our model to investigate the melting and crystallization behaviors of aluminum at high pressures. At a constant homologous temperature, our model predicts that melting rates will increase, crystallization growth rates will decrease, and the minimum isothermal crystallization time will increase as pressure increases. Additionally, higher pressures also reduce the critical homologous temperature at which the isothermal crystallization time is at the minimum.

Processing LDS-Circuit Boards by Selective Laser Sintering

Abstract As the demand for individualized products rises, the development and need for additive manufacturing (AM) techniques such as selective laser sintering (SLS) has strongly increased. The industrial use of these procedures for prototypes or small-scale production lines has grown due to their specific characteristics like the high achievable complexity. With the increasing demand for electrification and functionalization, the combination of AM with laser-direct structuring (LDS) gains interest. Therefore, the powder used for the investigation is dry coated with a LDS-additive, which enables laser activation and a metallization of the activated sections in a metallization bath. To characterize the influence of the LDS-additive on the process, the powder properties were investigated for unfilled and successive increased additive content. The thermal process window was identified by standard and process adapted isothermal differential scanning calorimetry. This showed a decrease of the isothermal crystallization time due to nucleation effects of the additive. Subsequently, parts were produced with a parameter study and showed a demand for a higher energy density. The resulting parts were then metallized with a parameter variation and characterized by stereomicroscopy. To investigate the influence of the different parameter sets and the LDS content, the mechanical properties were determined.

Crystallization Behavior of Poly(Tetramethylene Oxide) Influenced by the Crystallization Condition of Poly(Butylene Succinate) in Their Copolymers

The effect of crystallization conditions of poly(butylene succinate) (PBS) component on the crystallization of poly(tetramethylene oxide) (PTMO) component in their segment block copolymer, with a higher PTMO content (PTMO mass fraction is 67%), was investigated by DSC and temperature-dependent FTIR. It is found that the isothermal crystallization time (tIC) of PBS has an effect on the crystallization behavior of PTMO component. Perturbation correlation move-window two-dimensional (PCMW2D) correlation analysis and generalized 2D correlation analysis (2DIR) were performed to explore the origin of this phenomenon. The PCMW2D and 2DIR results show that the correlation intensity peak observed at around 20 °C for PTMO is due to the PTMO chains movements forced by the PBS chains folded movements. If tIC of PBS at temperature of 20 °C is prolonged, more PTMO components are incorporated in the region between PBS lamellae and the peak at -7.6 °C (belonging to less-constricted PTMO chains) changes smaller and even disappears, while the peak at -16.3 °C belonging to more-constricted PTMO chains gets bigger. A crystallization model was also established in this study. The results of tensile testing showed that tensile strength slightly increased and elongation at break decreased with increasing heat treatment time at 40 °C.

Effect of nano calcium carbonate and attapulgite on isothermal crystallization time of polypropylene under static and dynamic conditions

The isothermal crystallization time of polypropylene/nano calcium (PP/CaCO3) and polypropylene/attapulgite (PP/ATP) under static and dynamic conditions were investigated by using differential scanning calorimeter and Rheological experiments. Relatively, in static conditions, the calcium carbonate was easier to accelerate the crystallization process due to its shape. In the shear field the attapulgite is more advantageous to shorten crystallization time indicating that the attapulgite dispersed in the PP matrices confined the PP chains and hindered the movement of the PP chains.

Effect of Film Thickness on Induction Time of Isothermal Crystallization from the Melt of Poly(3-hydroxybutyrate) Investigated by Time-resolved Grazing-incidence X-ray Scattering Measurements Using Synchrotron Radiation

Effect of Film Thickness on Induction Time of Isothermal Crystallization from the Melt of Poly(3-hydroxybutyrate) Investigated by Time-resolved Grazing-incidence X-ray Scattering Measurements Using Synchrotron Radiation

Epitaxial growth of solution derived (Ce0.8Gd0.2)1-xMnxO2-δ films

(Ce0.8Gd0.2)1-xMnxO2-δ (CGMO) films were deposited on Ni-5at%W (NiW) substrates by chemical solution process. The epitaxial phase and surface morphology of ceria films were evaluated by means of X-ray diffraction and atomic force microscopy. The phase and texture evolution of the CGMO film were investigated with varying the different content of manganese additives. And the morphologies evolution of CGMO films in isothermal crystallization time was also analyzed. Manganese doping might contribute to the improvement of the texture and surface of crystallized films. The full-width at half maximum (FWHM) values of phi scans decreased with the increase of manganese doping. Highly epitaxial oriented films with smooth surface could be acquired with an optimal content of doping. The (Ce0.8Gd0.2)0.9Mn0.1O2-δ film might be a significant candidate buffer layer.

Isothermal Crystallization Kinetics and Time-Temperature-Transformation of the Conjugated Polymer: Poly(3-(2'-ethyl)hexylthiophene).

Thermal annealing strongly impacts the nano- and microstructure of conjugated polymers. Despite the fundamental importance for the resulting optoelectronic behavior of this class of materials, the underlying crystallization processes have not received the same attention that is encountered in other disciplines of materials science. The question arises whether classical treatment of nucleation and growth phenomena is truly applicable to conjugated polymers? Here, the isothermal crystallization behavior of the conjugated polymer poly(3-(2′-ethyl)hexylthiophene) (P3EHT) is monitored with differential scanning calorimetry (DSC). Avrami analysis reveals growth- and nucleation-limited temperature regimes that are separated by the maximum rate of crystallization. The molecular weight of the polymer is found to strongly influence the absolute rate of crystallization at the same degree of undercooling relative to the melting temperature. A combination of optical microscopy and grazing-incidence wide-angle X-ray scattering (GIWAXS) confirms that the resulting nano- and microstructure strongly correlate with the selected isothermal annealing temperature. Hence, this work establishes that classical nucleation and growth theory can be applied to describe the solidification behavior of the semicrystalline conjugated polymer P3EHT.

Crystallization behaviours of bacterially synthesized poly(hydroxyalkanoate)s in the presence of oxalamide compounds with different configurations

Bacterially synthesized poly(hydroxyalkanoate)s (PHAs) suffers from low crystallization rate which is enhanced by using tailor-made oxalamide compounds as nucleators. The influence of nucleator configurations on the crystallization behaviour of the PHAs was investigated using differential scanning calorimetry (DSC), polarized optical microscopy (POM) and X-ray diffraction (XRD). The oxalamide compounds with ringy terminal structures (cyclohexyl and phenyl), notably the phenyl group, show higher nucleation efficiency and a better compatibility in the PHAs matrix, while the linear terminal structure (n-hexane) has poor nucleation effect. The crystallization temperature (Tc) and the crystallinity (Xc) of the PHAs are increased from 58°C to 71°C and from 5% to 48%, respectively, after addition of 0.75wt% of the nucleator (phenyl group) upon cooling from the melt. Meanwhile, the half-life isothermal crystallization time (t0.5) of the PHAs at 110°C is decreased by 70%. The oxalamide compounds increases the nuclei density of the PHAs accompanied with a reduction in spherulitic size. In addition, the crystal form and crystallization mechanism of the PHAs are not altered obviously after addition of the nulceators as confirmed by the POM, XRD and Avrami analysis.

Transition in Crystal Morphology for Flow-Induced Crystallization of Isotactic Polypropylene

Deformation applied to a semicrystalline polymer melt prior to quenching can dramatically increase the concentration of nuclei and even transform the final crystalline morphology, phenomena collectively known as flow-induced crystallization (FIC). Using polarized optical microscopy and atomic force microscopy, we image the changes in morphology of an isotactic polypropylene (iPP) sample previously sheared in a rotational rheometer. Sufficiently large deformations promote the formation of “rice grain” shaped crystalline domains. These anisotropic structures are randomly oriented and are about 1.5–3.0 μm long (depending on applied work), with an aspect ratio of about 2:1. Well below the critical specific work threshold Wc, the crystalline morphology of our iPP sample is predominately composed of spherulites, with a fraction of rice grains that increases as the applied work increases. Above Wc, rice grains predominate, with no visible spherulites. The size of spherulites appears to be independent of applied work. In contrast, the size of rice grains decreases with increasing applied work, up to a saturation specific work Wsat, remaining constant thereafter. Similarly, the isothermal crystallization time decreases with increasing applied work up to Wsat and is constant thereafter. The effects of flow-induced precursors persist after repeated melting and recrystallizations, observable by elevated freezing temperatures in DSC experiments. Here, we probe directly the robustness of these precursors in optical microscopy, in which we repeatedly melt, anneal, and recrystallize a thin slice of an FIC sample. We observe rice grain domains appearing over and over, with the first domains to crystallize tending to reappear near the same locations.