Isothermal Crystallization Process Research Articles

Monitoring of Isothermal Crystallization and Time–Temperature Transformation of Amorphous Felodipine: The Time-Domain Nuclear Magnetic Resonance Method

The isothermal crystallization process of felodipine has been investigated using the time-domain Nuclear Magnetic Resonance (NMR) method for amorphous bulk and ground samples. The obtained induction and crystallization times were then used to construct the time–temperature-transformation (TTT) diagram, both above and below the glass transition temperature (Tg). The Nose temperature was found equal to 363 K. Furthermore, the dynamics of crystalline and amorphous felodipine were compared across varying temperatures. Molecular dynamics simulations were also employed to explore the hydrogen-bond interactions and dynamic properties of both systems.Graphical

Composition design and crystallization behavior of Zr–Cu–Ni–Al bulk metallic glasses

The Zr–Cu–Ni–Al system was explored in this study, and seven new compositions of Zr-based bulk metallic glasses (BMGs) were designed by combining different proportions of binary eutectic phases Zr38Cu62, Zr76Ni24 and Zr51Al49, and rod-like shape samples were prepared with different diameters using the copper mold casting method. Modifying the proportion coefficient of these eutectic units, the width of the supercooled liquid region was increased from 79 to 95 K, and the thermal stability of the prepared metallic glass was improved. Differential scanning calorimetry was used to investigate the non-isothermal and isothermal crystallization behavior of the prepared samples. Noticeably, there was a small low-temperature exothermic peak before the glass transition temperature Tg in the non-isothermal crystallization process, which was related to the structural relaxation. The crystallization transition activation energy fitted by the Kissinger equation had the same variation trend as that obtained by the Arrhenius equation. Among the four metallic glasses studied, the glassy phase of Zr55·5Cu23·25Ni9Al12.25 was more stable, showing higher glass transition activation energy and longer incubation time. In addition, the isothermal crystallization processes of four metallic glasses were studied according to the Johnson-Mehl-Avrami (JMA) model. The calculated average Avrami exponents were >2.5, indicating that the nucleation rate increased during the isothermal crystallization process.

The effect of tetra‐needle‐like zinc oxide whisker as additive on the crystallization kinetics of polybutylene adipate terephthalate/polylactic acid blend

AbstractHerein, we investigate the effect of the addition of tetra‐needle‐like zinc oxide whisker (T‐ZnOw), a novel reinforcing additive, on the melt crystallization behavior of biodegradable polybutylene adipate terephthalate (PBAT)/polylactic acid (PLA) polymer blends. PBAT and PLA were blended in a Haake mixer at a fixed weight ratio of 70/30, and the contents of T‐ZnOw varied from 0 to 7 wt%. The melt thermal properties and crystallization behavior of PBAT/PLA blends and PBAT/PLA/T‐ZnOw composites were studied using differential scanning calorimetry. The kinetics of melt crystallization have been quantitatively explained using a range of mathematical formalisms. The Avrami equation is adopted to analyze the isothermal crystallization process and kinetics of PBAT/PLA/T‐ZnOw composites. For its nonisothermal crystallization process and kinetics, the Ozawa equation cannot describe the entire process, while the Jeziomy‐modified Avrami equation can only describe the initial stage of its nonisothermal crystallization. Using Mo‐analysis, the impact of T‐ZnOw contents on the composite's nonisothermal crystallization kinetic has been thoroughly examined. Given that the PBAT/PLA blend system's crystal formation may be inhibited by T‐ZnOw, the observation demonstrates that T‐ZnOw only marginally alters this process. The work is of significant scientific value and adds a new concept to the development of sustainable thin film for packaging.Highlights T‐ZnOw is used as a novel additive for PBAT/PLA polymer blends. The crystallization of PBAT/PLA/T‐ZnOw composite systems is thoroughly examined. T‐ZnOw marginally reduces the kinetics of the crystallization process of PBAT/PLA blends. PBAT/PLA/T‐ZnOw composites follow volume nucleation with mixed 2D/1D growth. The nonisothermal crystallization of PBAT/PLA/T‐ZnOw composites is validated by Mo‐analysis.

A biomass phosphonamide enables biodegradable polylactide with favorable flame retardancy and rapid crystallization

PLA is widely known as biodegradable plastics whose further application in fields such as automotive and architectural is still constrained by its flammability and unsatisfactory crystallization properties. To address the aforementioned concerns, a novel biomass phosphonamide PDPA was synthesized with chemical structure confirmed by FTIR, NMR and elemental analysis tests. Immediately thereafter, PLA/PDPA composites were prepared by melting blending, with a focus on flame retardancy, crystallization properties and flame-retardant mechanism. As expected, PDPA efficiently enhanced both the flame retardancy and crystallization properties of PLA. Specifically, the PLA/4.0PDPA obtained UL-94 V-0 grade and the LOI value increased to 28.6 % with only 4 wt% PDPA added, which comes down to the superior free radical capture and dilution effect of PDPA in the vapor phase and the melting droplet effect. More appealingly, the crystallinity of PLA/4.0PDPA was significantly enhanced to 43.4 % from 2.5 % of PLA, and the shortest t1/2 was 4 mins in the isothermal crystallization process due to the excellent heterogeneous nucleation of PDPA. Moreover, PLA/PDPA composites maintain almost the same mechanical performance as pure PLA. In brief, this work provides a green strategy for the preparation of PLA composites with excellent comprehensive performance and shows great potential in engineering materials.

Impact of aromatic sulfonate derivative on the crystallization, thermal resistance, and hydrolytic properties of poly(lactic acid)

AbstractThe thermal, crystallization kinetics and hydrolysis properties of poly(lactic acid)‐based blends containing different concentrations of 5‐dimethylthioisosulfate salts (LAK) were systematically investigated and compared with those of poly(lactic acid) (PLA). PLA/LAK blends were prepared by melt blending method‐using LAK as a nucleating agent of PLA. During the isothermal crystallization process, the morphology of PLA crystal in the neat PLA and PLA/LAK blends showed spherical crystals. With an increase in the concentration of LAK, the nucleating density of the PLA spherulites increased and the size of the PLA spherulites decreased. Differential scanning calorimetry indicates that after the addition of LAK, both the crystallinity and crystallization rate of PLA have been enhanced. In situ X‐ray diffraction analysis also shows that the sample exhibits the same crystal structure as neat PLA and continues to strengthen, indicating that LAK enhances the crystallization properties of PLA. Moreover, the crystallinity of PLA increased and crystal size of PLA decreased with the increase in the LAK content. The Vicat softening temperatures of PLA/LAK blends increased, so the heat resistance improved. Through hydrolysis testing under identical conditions, the PLA/LAK blends exhibits better hydrolytic stability.

Isothermal and nonisothermal crystallization kinetics of modified hydroxyapatite-enhanced poly(3-hydroxybutyrate-co-4-hydroxybutyrate)

Poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (P34HB) is a biodegradable material, and efforts have been devoted to enhancing its mechanical properties and biocompatibility for biomedical applications. However, there is a lack of research confirming the influence of nucleating agents on its crystallization behavior. In this study, the isothermal crystallization process was described using the Avrami model. The non-isothermal crystallization process was investigated using the Avrami-Jeziorny and Mo models. The effects of different calcium hydroxyapatite (CA-HA) contents on the nucleation activity, crystallization activation energy, and equilibrium melting point of P34HB/CA-HA composites were compared. The impact of CA-HA on the crystallization rate, nucleation rate, and crystal growth rate of the polymer matrix was discussed. The results indicate that with an increase in CA-HA content, the crystallization rate initially decreases and then increases. The minimum crystallization rate is observed at a CA-HA content of 9 wt%. Moreover, as the CA-HA content increases, nucleation activity is enhanced, suggesting that CA-HA acts as a nucleating agent, facilitating the nucleation process. When the CA-HA content exceeds 9 wt%, the increase in crystallization rate may be attributed to the further enlargement of molecular clusters formed by CA-HA, leading to a reduced dispersion of CA-HA in the polymer matrix. Furthermore, when the CA-HA content surpasses 9%, a significant decrease in crystallization activation energy is observed, indicating a reduction in the mobility of polymer chain segments.

Chain-Level Analysis of Reinforced Polyethylene through Stretch-Induced Crystallization.

Herein, we propose a large-scale simulation approach to perform the stretch-induced crystallization of entangled polyethylene (PE) melts. Sufficiently long (1000 ns) united-atom molecular dynamics (UAMD) simulations for 16000 chains of 1000 consecutive CH2 united-atom particles under periodic boundary conditions were performed to achieve the crystallinity observed in experiments. Before the isothermal crystallization process, we applied uniaxial stretching as pre-elongation to the embedded strain memory on the entangled PE melts. We confirmed significant differences in the morphologies of crystal domains and scattering patterns for pre-elongation ratios of 400% and 800%. The obtained scattering patterns were consistent with the experimental results. Uniaxial stretching MD simulations revealed that the elastic modulus at 800% pre-elongation was stronger than that at 400% pre-elongation. From this observation, we can derive the structure-property relationship, wherein the magnitude of the pre-elongation governs the crystal domain structures and mechanical properties.

Effect of a Self-Assembled Nucleating Agent on the Crystallization Behavior and Spherulitic Morphology of Poly(lactic acid).

Herein, decanedioic acid dibenzoylhydrazide (DDBH) was used as a nucleating agent to improve the crystallization of poly(lactic acid) (PLA). The formation of DDBH assemblies in PLA melts at different concentrations was systematically investigated. The DDBH (0.5-0.9 wt %) recrystallized as dendrite-like structures during the isothermal crystallization process, and the crystal morphology of PLA underwent a morphological change from spherical form to a similar dendritic crystal form. Differential scanning calorimetry and in situ wide-angle X-ray diffraction analysis results showed that crystallizability and overall crystallization rate of PLA were enhanced by the addition of DDBH. The half-crystallization time at 120 °C reduced to 0.28 min compared to pure PLA (6.12 min), after adding 0.9 wt % DDBH. Moreover, the crystallinity and lamellar thickness of crystalline PLA increased, while the size of the microcrystal of PLA decreased with an increase in DDBH content. The heat deflection temperatures of PLA/DDBH blends increased and hence heat resistance improved.

Effects of Soaking Solvents by an Isothermal Crystallization Process on Physical Properties and Structures in One-Step-Drawing Films of Biodegradable P(3HB) Copolymers

Effects of Soaking Solvents by an Isothermal Crystallization Process on Physical Properties and Structures in One-Step-Drawing Films of Biodegradable P(3HB) Copolymers

Effect of the SiO2/Na2O ratios on the structure and crystallization properties of photo-thermo-refractive glass

Photo-thermo-refractive (PTR) glass is a type of functional optical glass material with multi-component and multivalent particles, which is a substrate material for volume Bragg gratings (VBGs) and widely used in high power laser, holography and other applications. Recording VBGs in PTR glass involves photo-thermo-induced crystallization (UV exposure and subsequent heat treatment), while spontaneous crystallization in the unexposed area is one of the important reasons for the light scattering loss of PTR glass, which can also lead to a decrease in the refractive index modulation and absolute diffraction efficiency of the prepared VBGs. In order to optimize spontaneous crystallization of PTR glass, we analyzed the effects of variations in the SiO2/Na2O ratio on the structure and crystallization behaviors of SiO2–Na2O–Al2O3–ZnO–NaF PTR glass using Raman spectroscopy, FTIR spectroscopy, DSC, XRD, and SEM. As the molar ratio (Rm) of SiO2/Na2O increases from 3.37 to 5.38, the polymerization of the silica network structure is significantly enhanced, the crystallization activation energy of original PTR glass increases from 93.52 kJ/mol to 285.23 kJ/mol, and the crystal growth index decreases from 3.57 to 1.49. In the isothermal crystallization process, we observed that increasing the SiO2/Na2O ratio significantly decreases the grain density and homogenizes the grain distribution of sodium fluoride (NaF) crystals under long-time crystallization conditions, which contributes to maintaining the high transmittance of the crystallized PTR glass in both visible and near-infrared regions.

Poly (butylene succinate): Low-temperature nucleation and crystallization, complex morphology and absence of lamellar thickening

Melt-crystallization of poly (butylene succinate) (PBS) at largely different melt-supercooling was analyzed by X-ray scattering techniques, direct imaging using microscopy, and by fast scanning chip calorimetry, with the latter employed to achieve high melt-supercooling. Crystallization at 100 and 20 °C, representing structure formation at low and high melt-supercooling, respectively, yields lamellae with thicknesses of around 8 and 4 nm. In both cases, lamellar thickening is excluded as main mechanism of isothermal secondary crystallization, suggesting that absence of intracrystalline chain-sliding diffusion is not dependent on the absolute value of the thickness of lamellae. The wide-angle X-ray pattern of PBS crystallized at low temperature shows less and broader peaks than in case of PBS crystallized at high temperature, pointing to presence of crystal defects, and causing an enlargement of the unit cell. Secondary crystallization leads to an increase of the melting temperature, being distinctly larger (per unit annealing time) for low-temperature crystallized PBS. Since lamellar thickening is absent, and imperfection of the bulk crystal structure persists even on long-term annealing, merging of crystalline blocks and lateral crystal growth at the crystallization temperature, for explaining the observed increases of the crystallinity and melting temperature, are suggested. Though there is detected a large rigid amorphous fraction in PBS crystallized at high supercooling, vitrification only occurs continuously on cooling, after the isothermal crystallization process.

A Relatively Simple Look at the Rather Complex Crystallization Kinetics of PLLA.

This work demonstrates that, despite the existence of a significant number of works on PLA crystallization, there is still a relatively simple way, different from those already described, in which its complex kinetics can be observed. The X-ray diffraction (XRD) results presented here confirm that the PLLA under study crystallizes mostly in the α and α' forms. An interesting observation is that at any temperature in the studied range of the patterns, the X-ray reflections stabilize with a given shape and at a given angle, different for each temperature. That means that both α and α' forms coexist and are stable at the same temperatures so that the shape of each pattern results from both structures. However, the patterns obtained at each temperature are different because the predominance of one crystal form over the other depends on temperature. Thus, a two-component kinetic model is proposed to account for both crystal forms. The method involves the deconvolution of the exothermic DSC peaks using two logistic derivative functions. The existence of the rigid amorphous fraction (RAF) in addition to the two crystal forms increases the complexity of the whole crystallization process. However, the results presented here show that a two-component kinetic model can reproduce the overall crystallization process fairly well over a broad range of temperatures. The method used here for PLLA may be useful for describing the isothermal crystallization processes of other polymers.

Isothermal crystallization process and thermal analysis of polylactic acid/cotton fiber composites during thermal degradation

ABSTRACT The effects of reactive and non-reactive surface treatment agents on the isothermal crystallization rate and subsequent melting behavior of polylactic acid/cotton fiber (PLA/COF) composites before and after accelerated degradation testing at 60°C for 2000 h were demonstrated. The results showed that the crystallization rate of PLA was rapidly accelerated by the incorporated COF, while the use of epoxy-based reactive surface treatment agents inhibited the movement of PLA molecular chains and reduced the crystallization rate and melting point of the PLA/COF. These results suggest that the reactivity of the agent affects the crystallization behavior of the composite during long-term use.

Molecular Dynamics of Topological Barriers on the Crystallization Behavior of Ring Polyethylene Melts with Trefoil Knots

Topological barriers in ring polyethylene (PE) melts of trefoil knots inhibit crystallization due to self-entanglement, as confirmed by united atom molecular dynamics (UAMD) simulations. In this study, we clarified the decrease in the topological barriers of self-entangled knots with increasing polymer chain length (N) and the localization of the knotted segments in the noncrystallized region. In the UAMD simulations, isothermal crystallization processes were performed at T = 300 K for the trefoil knots using a crystallization time tIC = 200 ns. Here, the trefoil knot gives the simplest nontrivial topology with nonzero crossing number. Crystallization was not observed in trefoil PE knots for N ≤ 120; however, crystallization did take place in the ring PE melt of the trivial knot with trivial topology and zero minimal crossing number. For N = 140, suppression of the growth of a crystalline phase (i.e., the polycrystalline phase) was observed in the trefoil PE melts, whereas such suppression was not detected in the trivial PE melts. In contrast, no significant differences in the crystallization behaviors of the trefoil and trivial PE melts were observed at N = 200. These results indicated that the topological barriers decreased with increasing N. Furthermore, to investigate the relationship between the chain conformation and degree of local crystallization, we developed a new mathematical method for searching the knotted segment of a trefoil knot in a crystallized trefoil PE melt. We found that trefoil knots with large subloops (“large-leaves”) exhibited localization of the knotted segment. In addition, the large leaves of the localized knots dominated in the crystallized region, and the knotted segments of the localized knots were located mainly in the noncrystallized region.

Miscibility, crystallization and properties of bio-based polyamide 56/polyamide 6 blends

The miscibility, crystallization behaviors, and morphology of polyamide 56/polyamide 6 (PA56/PA6) blends were investigated. The blends had only one Tg, which confirmed that PA56 and PA6 are miscible in the blends. The melting point, crystallization temperature, crystallinity of the blends depressed due to the interaction of PA56 and PA6 in the blends. The crystalline structure of both PA56 and PA6 in the blends did not change and crystallized separately in both isothermal and non-isothermal crystallization process. PA56 in the blends crystallized in all blending ratio, while PA6 only crystallized with PA6 content φ6 > 0.2. The crystals of PA56 and PA6 grew in the same crystal skeleton but in different layers. Mechanical properties of the blends indicated that the elongation at break could be improved significantly without scarifying the strength in the case of adding a small amount of PA6 in PA56, which provides a promising approach for fabricating polyamide materials with desired properties.

Role of the Branched PEG-b-PLLA Block Chain in Stereocomplex Crystallization and Crystallization Kinetics for PDLA/MPEG-b-PLLA-g-glucose Blends with Different Architectures.

The isothermal crystallization behavior and corresponding morphology evolution of poly(d-lactic acid) (PDLA) blends with PLLA6.7k or MPEG-b-PLLA6.7k-g-glucose with different architectures and different PLLA-grafted copolymer contents were investigated. The formation of stereocomplexes (SCs) in between the chain branched structure of MPEG-b-PLLA6.7k-g-glucose and PDLA chains acting as the physical crosslinking points slows down the motion of PDLA chains, but the SCs could act as a heterogeneous nucleating agent for the late formation of homocrystals (HCs) in the blend system, accelerating the crystallization kinetics of HCs through enhancing the nucleation density. For PDLA/MPEG-b-PLLA6.7k-g-glucose blends, the mobility of SCs in the blend system and the nucleation density of SCs in the blends exhibit oppositional behavior during the isothermal crystallization at a Tc of 130 °C. The evolution of the crystal growth and structure during the isothermal crystallization process by rheometry has revealed the interesting role of the branched chains of MPEG-b-PLLA6.7k-g-glucose in the mechanism of the crystallization in PDLA blends.

Sustained antioxidant properties of epigallocatechin gallate loaded halloysite for PLA as potentially durable materials

AbstractHalloysite nanotube (HNT) was etched by hydrochloric acid, and epigallocatechin gallate (EGCG) was loaded into the etched HNT (HH) to prepare loaded antioxidant (HH‐E) for polylactic acid (PLA) with improved thermal‐oxidative stability. The loading process was confirmed by TGA and FT‐IR, and the loading capacity was 16.15%. The presence of EGCG on the surface of etched HNT was further confirmed by nitrogen adsorption and TEM. The effect of loaded EGCG on thermal‐oxidative stability of PLA was characterized by the change in OOT values of PLA after the aging process under 140°C. The results showed that HH‐E increased OOT values of PLA from 231.2°C to 293.6°C at first, indicating that HH‐E provided PLA with excellent thermal‐oxidative stability. Furthermore, as can be seen from the trend of OOT changing over aging time, during the 14 days aging process, OOT values of PLA composites with HH‐E decreased more slowly than that of PLA composites with EGCG alone, endowing PLA with sustainable thermal‐oxidative stability. Further kinetic analysis showed that the apparent activation energy of the degradation process (Ea) of PLA composites with HH‐E changed less than that of PLA composites with EGCG alone, which showed that the loading system provided PLA better sustainable stability. Moreover, the isothermal crystallization process of modified PLA was characterized by DSC and POM. The results showed that HH‐E provided heterogeneous crystal nucleus for PLA, reducing the half crystallization time (t0.5) and improvement of crystallization rate for PLA. These results prove that HH‐E exhibited excellent performance in improving the thermal stability and crystallization property of PLA, showing multifunctional characterization.

Confocal scanning laser microscopy investigation of crystallization behavior of hot blast furnace slag

The isothermal and non-isothermal crystallization behavior of blast furnace (BF) slag with varying CaO/SiO2 ratios was in-situ observed through a confocal scanning laser microscopy (CLSM). The crystallization kinetics was analyzed and precipitation of crystalline phases was discussed by theoretical analysis from FactSage 8.0 and X-ray Diffraction (XRD). The results showed that, with the increasing CaO/SiO2 ratio from 1.0 to 1.3, the crystals morphology changed first from rectangles and squares to small-size and irregular, and further to long strip shape at the cooling rate of 100 °C/min. The onset crystallization temperature in CTT diagram was higher at a larger CaO/SiO2 ratio due to the decrease of polymerization degree of the slag. For the isothermal crystallization process at 1250°C, the crystals morphology changed gradually from lamellar to long-trip or columnar, and the incubation period in TTT diagram became shorter with the increase of CaO/SiO2 ratio. Avrami index n decreased from appropriately 2.3 to 1.5, and the growth mode of crystals converted from two-dimensional to one- dimensional. XRD analysis showed that only merwinite existed in four slags at 1380°C. Akermanite and gehlenite were generated at 1350°C, and their amount first increased greatly and then decreased slightly with a continuous increase of CaO/SiO2 ratio. At the low temperature, the number of akermanite and gehlenite reduced, while that of merwinite increased obviously in the slags when CaO/SiO2 ratio was 1.2 and 1.3, indicating that increasing CaO/SiO2 ratio promoted the generation of merwinite.

Investigation on the crystallization properties of BaO-bearing blast furnace slag through a confocal laser scanning microscope

• The crystallization ability of blast furnace slag is weakened with an increase in BaO content. • One-dimension growth of crystals at low temperature is identified. • A high BaO content induces a phase transformation from melilite to merwinite. Investigating the crystallization behavior of BaO-bearing blast furnace slag (BFs) is of great significance for the dry granulation technology to guide the production of glassy slag with high hydration activity, and simultaneously collect the waste heat of hot slag. In this study, the influence of BaO content on the crystallization behavior was investigated through in-situ observation using a confocal laser scanning microscope (CLSM). The theoretical calculations by FactSage 8.0 and X-ray diffraction (XRD) analysis of the quenched slag were carried out for the crystalline phases characterization. The results showed that the incubation time increased in the isothermal crystallization process and the initial crystallization temperature decreased in the continuous cooling process with the increasing BaO concentration, indicating that the crystallization ability of the slag was weakened. BaO existence had little effect on the crystal morphology during the isothermal crystallization process at 1200℃. Furthermore, the crystallization kinetics analysis of the JMA model showed the Avrami index n close to 1.5, indicating the one-dimension growth of crystals at low temperature. The akermanite (Ca 2 MgSi 2 O 7 ) and gehlenite (Ca 2 AlSi 2 O 7 ) were the main crystalline phases, and their precipitation was restrained to a certain extent as the BaO content increased.

Isothermal and Non-Isothermal Crystallization Kinetics of Poly(ethylene chlorotrifluoroethylene).

The isothermal (IT) and non-isothermal (NIT) crystallization kinetics, morphology, and structure of poly(ethylene chlorotrifluoroethylene) (ECTFE) were investigated via differential scanning calorimetry (DSC), polarized optical microscopy (POM), and wide-angle X-ray diffraction (XRD). The Avrami equation could well describe the overall IT crystallization process of ECTFE, and, furthermore, the overall crystallization rate decreased at higher crystallization temperatures (Tc). The equilibrium melting point for ECTFE was found to be 238.66 °C. The activation energies for IT and NIT crystallization were determined as −137.68 and −120.54 kJ/mol, respectively. The Jeziorny model fitted well with the initial stages of NIT melt crystallization, while deviations from linearity in the later stages of the process were due to the collisions of spherulites. Spherulites of ECTFE organized in a hexagonal crystal system were found. The relative crystalline degree of ECTFE under NIT conditions was about 54.55%, and this decreased with the increase in cooling rate. Moreover, the Ozawa and Mo models were suitable for modeling the overall NIT crystallization process of ECTFE.