Crystallization Behavior Of Poly Research Articles

Influence of functional group structures in nucleating agents on the crystallization behavior of poly(ethylene terephthalate)

AbstractNucleating agents are crucial for modifying semi‐crystalline polymers, but the impact of different end‐group structures on crystallization behavior remains unclear. This study synthesized succinic dihydrazide nucleating agents with various end groups (methyl, ethyl, and tert‐butyl) for PET composite materials. The tert‐butyl‐terminated nucleating agent showed the best performance, increasing the crystallization peak temperature to 203.8°C, crystallinity to 27.3%, and reducing half‐crystallization time to 2.8 min at 220°C. It also enhanced the impact strength to 47.2 kJ·m−2 and flexural modulus to 2623 MPa, while improving Vicat softening temperature and heat distortion temperature by 41 and 8°C, respectively. The results from FTIR and Raman Spectroscopy indicate that the interactions between the end groups of the nucleating agent and the CH bonds in the PET structure, the π–π interactions between the nucleating agent and the PET molecular structure, as well as the modulation of the PET chain conformation by the nucleating agent, are the primary factors contributing to the enhanced crystallinity of PET. This study provides a new approach for designing efficient nucleating agents for high‐performance materials.

Effects of molecular structure and temperature field on the crystallization behavior of poly(tetrafluoroethylene-co-perfluoroalkylvinyl ether)

Despite the wide industrial application of poly(tetrafluoroethylene-co-perfluoroalkylvinyl ether) (PFA) in semiconductor processing, its crystallization behavior has little been studied. In this work, three PFA resins with similar molecular weight but different comonomer content and distribution were synthetized. Then the non-isothermal crystallization kinetics and crystalline structures were studied by differential scanning calorimetry, polarizing optical microscopy, and X-ray diffraction systematically. It is found that the incorporation of comonomers destroys the chain regularity and reduces the crystallizability of PFA thermodynamically, while the comonomer insertion improves the chain flexibility to accelerate the crystal growth dynamically. When the comonomer content of PFA is low, both the nucleation and growth rates are quite fast, which is favorable for the formation of spherulite. As the comonomer content increases or distribute uniformly in the chain, the nucleation rate declines notably and the side groups distort the crystal cell to increase the spacing of [100] plane. Moreover, two-dimensional growth mode is dominant to form bundle-like structure when crystallizing at slow cooling, where nucleation is suppressed by growth. But three-dimensional symmetrical spherulite can be activated and perfected at high cooling rate due to the initiation of substantial nucleation sites. This unique crystallization behavior is opposite to that of conventional polymers, providing a guidance for the polymerization and processing of PFA.

Synergistic effect of TiO2 nanoparticles and poly (ethylene-co-vinyl acetate) on the morphology and crystallization behavior of polylactic acid

The impact of adding ethylene vinyl acetate copolymer (EVA 80) and 1 wt% TiO2 nanoparticles on the morphology and crystallization behavior of poly(lactic acid) blends was investigated using DSC, SEM, and POM. Thermal analysis revealed the enhancement of crystallinity of PLA in the presence of TiO2 and higher EVA 80 content in the blend. The PLA and EVA 80 components showed compatibility, as evidenced by the shift of the glass transition temperatures of the PLA phase in the blend to lower values compared to neat PLA. The lower temperature shift of the cold crystallization of the PLA and the formation of the small spherulites of the PLA in the blends indicated that the EVA 80 and TiO2 act as a nucleating agent for crystallization. The non-isothermal crystallization parameters of the composites were evaluated using Avrami's modified model, the MO approach, and Friedman’s isoconversional method. The Avrami’s modified rate constant (K) and the effective activation energy values significantly increased with the incorporation of EVA 80 and TiO2 nanoparticles. Furthermore, the thermogravimetric analysis (TGA) showed improved thermal stability of PLA by adding EVA 80 and TiO2.

A temporal graph neural network for cross-scale modelling of polycrystals considering microstructure interaction

Machine learning (ML) based methods have achieved preliminary success in the constitutive modeling for single crystals or homogenized polycrystals with remarkable computational efficiency. However, existing ML-based constitutive models neglect grain-level anisotropy, which limits the accurate analysis of local effects. In the current work, a temporal graph neural network (TGNN) model is proposed to simulate cross-scale deformation behaviors of polycrystals under complex loading conditions, with straightforward consideration of microstructure variation and local interaction. The TGNN-based model, a variant of Linearized Minimal State Cells (LMSCs), extends its scope from macroscopic stress response to the mechanical response and orientation evolution of all grains within the aggregate. Specifically, the polycrystalline microstructure is represented with a graph to incorporate essential features of grains, including the spatial connectivity, crystallographic orientation and deformation state. Graph neural network (GNN) is used to capture the spatial correlation of grains, and the features extracted by the GNN are further processed with LMSCs to account for the history-dependent deformation and microstructure evolution. Moreover, the representative volume element (RVE) simulation with crystal plasticity is performed to provide reliable datasets for model establishment. The proposed model demonstrates high efficiency, accuracy and self-consistency in predicting the strain-stress response and orientation evolution at the scale of both individual grain and the overall aggregate under complex loading cases, such as cyclic loading and arbitrary loading.

Microstructure evolution accounting for crystal plasticity in the context of the multiphase-field method

The role of grain boundaries (GBs) and especially the migration of GBs is of utmost importance in regard of the overall mechanical behavior of polycrystals. By implementing a crystal plasticity (CP) theory in a multiphase-field method, where GBs are considered as diffuse interfaces of finite thickness, numerically costly tracking of migrating GBs, present during phase transformation processes, can be avoided. In this work, the implementation of the constitutive material behavior within the diffuse interface region, considers phase-specific plastic fields and the jump condition approach accounting for CP. Moreover, a coupling is considered in which the phase-field evolution and the balance of linear momentum are solved in each time step. The application of the model is extended to evolving phases and moving interfaces and approaches to strain inheritance are proposed. The impact of driving forces on the phase-field evolution arising from plastic deformation is discussed. To this end, the shape evolution of an inclusion is investigated. The resulting equilibrium shapes depend on the anisotropic plastic deformation and are illustrated and examined. Subsequently, evolving phases are studied in the context of static recrystallization (SRX). The GB migration involved in the growth of nuclei, which are placed in a previously deformed grain structure, is investigated. For this purpose, three approaches to strain inheritance are compared and, subsequently, different grain structures and distributions of nuclei are considered. It is shown, how the revisited method contributes to a simulation of SRX.

Load-bearing capacity under fatigue of bonded-yttria tetragonal zirconia polycrystals and -yttria-stabilized zirconia: Effects of the viscosity of a dual-cured resin cement

This study aimed to evaluate the effect of low and high viscosities of dual-cured resin cement on the mechanical fatigue behavior of yttria tetragonal zirconia polycrystals (3Y-TZP) and yttria-stabilized zirconia (4YSZ) adhesively luted to a dentin analogue (glass fiber-reinforced epoxy resin). Ceramic discs were randomly divided into four groups (n = 20) based on the following study factors: dual-cured resin cement viscosities (low and high) and zirconia microstructure (3Y-TZP and 4YSZ). The discs were treated by air abrasion with aluminum oxide particles (50 μm), followed by the application of primer, and then luted with high or low viscosity resin cement to the dentin analogue. Subsequently, the luted sets underwent a step-stress fatigue test, which involved an initial load of 200 N, step increments of 100 N, 10,000 cycles per step, and a frequency of 20 Hz. Data on fatigue failure load (FFL) and the number of cycles for failure (CFF) were collected and analyzed using survival tests, including Kaplan-Meier and Mantel-Cox analyses, as well as Weibull analysis. Additionally, topography analysis, fractographic features, bonding interface analysis, and Raman spectroscopy were performed. The results revealed that 3Y-TZP exhibited superior fatigue behavior compared to 4YSZ, regardless of the viscosity of the resin cement used for luting. Among all groups, 3Y-Low exhibited the best fatigue performance, while 4YSZ luted with low or high viscosity resin cements yielded the lowest fatigue behavior (FFL). There were no significant differences in Weibull modulus among the groups. After air abrasion, both ceramics showed similar topography. Raman analysis indicated that the surface of 3Y-TZP ceramics prior to sintering had a monoclinic phase, which transitioned predominantly to tetragonal phase peaks after sintering. A similar transition was observed in 4YSZ ceramics. In summary, 3Y-TZP exhibited superior mechanical fatigue behavior compared to 4YSZ. The influence of resin cement viscosity on fatigue behavior was more pronounced in 3Y-TZP, with low-viscosity resin cement enhancing its performance. However, the mechanical fatigue behavior of 4YSZ was less affected by the viscosity of the dual-cured resin cement, showing similar results with both low and high viscosities. In conclusion, 3Y-TZP demonstrated superior mechanical fatigue behavior compared to 4YSZ. The impact of resin cement viscosity on fatigue behavior was more pronounced in 3Y-TZP, with low-viscosity resin cement enhancing its performance. Conversely, the mechanical fatigue behavior of 4YSZ was less sensitive to the viscosity of the dual-cured resin cement, resulting in similar outcomes with both low and high viscosities.

The coupled effects of grain boundary strengthening and Orowan strengthening examined by dislocation dynamics simulations

DD simulations are performed to unravel the coupled effects between grain boundary strengthening and Orowan strengthening. With the simulations of a plastically-deformed grain embedded in elastic matrix, the internal stresses associated with inter- and intragranular obstacles are quantified. First, plastic yielding is controlled by the length of dislocation sources. Then, in plastic deformation, the stress contribution of bypassing mechanism increases significantly with the size and number of precipitates, while the stress contribution of geometrically necessary dislocations at grain boundaries slightly decreases. Compared with regular spatial distribution of precipitates, random distribution induces a higher strengthening effect by reducing the dislocation mobility. Simulations of four-grain aggregates involving grain refinement and precipitation are systematically investigated. The volume fraction of precipitates is the key factor controlling the Orowan strengthening in addition to the grain size effect. A generalized Hall–Petch equation based on the mean free path of dislocations is proposed. At low strain, a relatively uniform internal stress field is found inside the aggregates with weak stress concentrations around grain boundaries and precipitates. The underlying mechanisms identified in this work are essential for understanding the plastic behavior of precipitate-strengthened polycrystals.

Different Crystallization Behaviors of Poly(vinylidene fluoride) Blended with or Coated with Cetyltrimethylammonium Bromide

Different Crystallization Behaviors of Poly(vinylidene fluoride) Blended with or Coated with Cetyltrimethylammonium Bromide

Crystal Plasticity simulations of in situ tensile tests: A two-step inverse method for identification of CP parameters, and assessment of CPFEM capabilities

As the computational capability of modern computers increases, the Crystal Plasticity Finite Element Method (CPFEM) becomes more and more popular in materials science to model the mechanical behaviour of polycrystals. Indeed, such analysis provides extensive information about local mechanical fields (such as plastic strain and stress), which can be useful for understanding the behaviour of bulk materials. However, estimating the parameters of the CP constitutive laws is still challenging because they are not directly related to the macroscopic behaviour of the polycrystalline aggregates. Thus, one way to identify such parameters is by inverse analysis from CPFEM simulations. However, such approach is usually extremely time consuming. This paper proposes a two-step optimization scheme to determine these coefficients. The first step is based on a simple model, similar to that proposed by Sachs back in 1928. The second step is based on CPFEM simulations, to be compared with experimental data acquired by an in situ tensile test and full-field measurements made by High-Resolution Digital Image Correlation (HRDIC). The uniqueness of the solution found by inverse analysis is studied and ways to solve the local minima issues are provided. Finally, the ability of CPFEM to replicate an in situ tensile test is assessed.

Crystallization Behaviors of Poly (Lactic Acid) Modified with ST-NAB3 and Its Improved Mechanical and Thermal Properties

Crystallization Behaviors of Poly (Lactic Acid) Modified with ST-NAB3 and Its Improved Mechanical and Thermal Properties

Phase-Field Crystal Studies on Grain Boundary Migration, Dislocation Behaviors, and Topological Transition under Tension of Square Polycrystals

In this paper, the tensile deformation behaviors of polycrystals after relaxation were studied using the phase-field-crystal (PFC) method. Here, the free energy density map characterized the 2D energy distribution of atomic configuration effectively. The application of the Read–Shockley equation distinguished high-energy grain boundary (HEGB) and low-energy grain boundary (LEGB) in large-angle grain boundary (LAGB), and they demonstrated different migration behaviors at the early and later stages. The behaviors of small-angle grain boundary (SAGB), including its migration and grains’ rotation, were also studied. Two different mechanisms of dislocation emission and absorption were explored, which demonstrates the possibility of dislocation elevating interfacial energy. The simulated results on the topological transition of grain boundaries prompted us to propose the thinking about the applications of the Neumann–Mullins law and Euler formula.

Effect of gauge length on the mechanical responses and crystallization behavior of poly(glycolide-co-caprolactone) (PGCL) suture

Effect of gauge length on the mechanical responses and crystallization behavior of poly(glycolide-co-caprolactone) (PGCL) suture

Deformation-induced grain rotation and grain boundary formation achieved through dislocation-disclination reactions in polycrystalline hexagonal close-packed metals

Grain boundary mediated plasticity (e.g., grain rotation and grain boundary sliding) plays a critical role in determining the deformation behavior of polycrystals, which is especially important for nanocrystalline metals or plastic deformation at elevated temperatures. However, due to the lack of a theoretical framework, the mechanism of deformation-induced grain rotation has not been well understood. By introducing a disclination-based description, we show that crystal rotation can be captured conveniently through the characteristic rotational vector of a disclination (i.e., the Frank vector). Under the framework of topological defect theory, grain boundary mediated plasticity can be treated as topological reactions between dislocations and disclinations, leading to rigorous predictions of grain/subgrain rotation through a pseudo-inverse solution for the reformulated Frank-Bilby equation. Systematic classifications of grain/subgrain rotation and grain boundary formation are raised for hexagonal close-packed (HCP) metals, which has been validated by Electron-Backscattered Diffraction characterizations in Mg alloys. Our work not only suggests a theoretical approach to investigate grain/subgrain rotation based on the topological correlation between dislocations and disclinations, but also provides a new insight into the plastic behaviors of polycrystalline HCP metals.

Deformation, dislocation evolution and the non-Schmid effect in body-centered-cubic single- and polycrystal tantalum

A physically-informed continuum crystal plasticity model is presented to elucidate deformation mechanisms, dislocation evolution and the non-Schmid effect in body-centered-cubic (bcc) tantalum widely used as a key structural material for mechanical and thermal extremes. We show the unified structural modeling framework informed by mesoscopic dislocation dynamics simulations is capable of capturing salient features of the large inelastic behavior of tantalum at quasi-static (10−3 s−1) to extreme strain rates (5000 s−1) and at low (77 K) to high temperatures (873 K) at both single- and polycrystal levels. We also present predictive capabilities of the model for microstructural evolution in the material. To this end, we investigate the effects of dislocation interactions on slip activities, instability and the non-Schmid behavior at the single crystal level. Furthermore, ex situ measurements on crystallographic texture evolution and dislocation density growth are carried out for polycrystal tantalum specimens at increasing strains. Numerical simulation results also support that the modeling framework is capable of capturing the main features of the polycrystal behavior over a wide range of strains, strain rates and temperatures. The theoretical, experimental and numerical results at both single- and polycrystal levels provide critical insight into the underlying physical pictures for micro- and macroscopic responses and their relations in this important class of refractory bcc materials undergoing large inelastic deformations.

Microstructural Model of Magnetic and Deformation Behavior of Single Crystals and Polycrystals of Ferromagnetic Shape Memory Alloy

In this article, a microstructural model of the Heusler alloy with the shape memory effect caused by the application of an external magnetic field is constructed. The dynamics of the magnetization process are described using the Landau–Lifshitz–Gilbert equation. For the numerical implementation of the model using the finite element method, the variational equations corresponding to the differential formulation of the magnetic problem are used. Such an approach makes it possible to reduce (weaken) the requirements for the smoothness of the sought solution. The problem of magnetization of single crystals of the Ni2MnGa alloy, which has a “herringbone”-type martensitic structure (a twinned variant of martensite), is considered. In each element of the twin, the magnetic domains with walls of a certain thickness are formed. The motion and interaction of these walls and the rotation of magnetization vector in the walls and domains under the action of the external differently directed magnetic fields are studied. These processes in the Heusler alloy are also accompanied by the detwinning process. A condition for the detwinning of a ferromagnetic shape memory alloy in a magnetic field is proposed, and the effect of the reorientation (detwinning) of martensitic variants forming a twin on the magnetization of the material and the occurrence of structural (detwinning) deformation in it are taken into account. First, the processes of magnetization and structural deformation in a single grain are considered at different angles between the anisotropy axes of twinned variants and the external magnetic field. For these cases, the magnetization curves are constructed, and the deformed states are identified. The model described such experimental facts as the detwinning process and the jump in magnetization on these curves as a result of this process. It was shown that the jump occurred at a certain magnitude of the strength of the applied external magnetic field and a certain direction of its action relative to the twinning system. Then, based on the obtained results, deformed states arising due to the detwinning process were determined for various (isotropic and texture-oriented) polycrystalline samples, and magnetization curves taking into account this process were constructed for these materials.

Impact of point defects on the creep behavior of MgY Polycrystal:A molecular dynamics study

The points defects offer greater fluctuations in the material features of many materials. In this study, the impact of point defects on the compressive creep performance of monolithic MgY bimetallic polycrystal compound has been examined under various creep circumstances through a molecular dynamics method and found that the creep circumstance has invoked greater impact on the MgY related to the point defects, owed to the existence of greater diffusion and grain rotational behavior. The existence of point defects offers a lowermost impact on the creep performance and its feature, owing to the existence of moderate resistance against its deformation as a result of creep deformation time and the trend is irrespective of the applied pressure and temperature.

Mechanical behavior of polycrystals: Coupled in situ DIC-EBSD analysis of pure copper under tensile test

Understanding the mechanisms at the microstructure scale is of great importance for modeling the behavior of materials at different scales. To this end, digital image correlation (DIC) is an effective measurement method for evaluating the strains generated by various loading conditions. The objective of this paper is to describe the experimental setup and the use of high resolution digital image correlation (HRDIC) during in situ Scanning Electron Microscope (SEM) tests in order to provide a coupling between polycrystalline modeling and experiment in the near future. The HRDIC technique is used to evaluate the tensile behavior of a pure copper polycrystal at room temperature. Several magnitudes are investigated in order to discuss the representativeness of the results with respect to the macroscopic scale. The selected image correlation parameters are discussed regarding the ability of the technique to define inter- and intra- granular strain heterogeneities. Finally, based on EBSD analyzes, the impact of grain orientation on the mechanical behavior is discussed. The Schmid factor, calculated from a macroscopic stress, appears to be the determining factor concerning the orientation of the location bands. On the other hand, it is not sufficient to define the mean strains in the grains.

An implicit formulation of a two-grain cluster type homogenization approach for polycrystals

The crystal-plasticity-based finite element method has received considerable attention in the field of sheet-metal forming. When the size of stamped sheets is hundreds to thousands of millimeters, a homogenization approach with a lower computational cost is required. A previous study proposed a two-grain cluster (TGC)-type homogenization method, which allows relaxation from the fully-constrained Taylor model (Yoshida, 2022. An alternative formulation of a two-grain cluster model for homogenization of elastoviscoplastic behavior of polycrystal, International Journal of Plasticity, 156, 103368). In the present study, a fully implicit numerical procedure was developed for the TGC model, and it was applied to a cup drawing simulation. The present implicit TGC model used the fully implicit stress integration method. In addition, the relaxation vector that satisfies the equilibrium and compatibility conditions at the end of increment was determined. Moreover, the algorithmic tangent moduli that are consistent with the Newton–Raphson method were derived analytically. These features facilitate the implementation of the TGC model in the FEM. The developed model was implemented in static implicit FEM software (MSC Marc), and cylindrical cup drawing was simulated. Consequently, the FE simulation in conjunction with the TGC model was found to more accurately predict the cup height and thickness than that in conjunction with the Taylor model. The better prediction accuracy of R-values obtained by the TGC model contributed to the accurate prediction of the drawn cup profile.

Some Issues with Statistical Crystal Plasticity Models: Description of the Effects Triggered in FCC Crystals by Loading with Strain-Path Changes.

The justification of the applicability of constitutive models to exploring technological processes requires a detailed analysis of their performance when they are used to describe loadings including the complex loading mode that is characteristic of these processes. This paper considers the effect of equivalent stress overshooting after the strain-path changes known to occur in metals and alloys. The macrophenomenological and multilevel models, which are based on crystal plasticity, account for this effect by applying anisotropic yield criteria at the macro- and mesolevels, respectively. We introduce a two-level constitutive statistical inelastic deformation model (identified for aluminum) that incorporates the popular simple phenomenological anisotropic hardening law for describing the behavior of FCC polycrystals. The results of the numerical simulation are in satisfactory agreement with existing experimental data. Statistical analysis of the motion of a mesostress in the stress space on the crystallite yield surface is performed. The obtained data are compared with the results found using the isotropic hardening law. The results clarify the simulation details of statistical crystal plasticity models under loading with strain-path changes in materials and demonstrate their suitability for describing the processes under consideration.

Effects of chain microstructure on the thermal, mechanical and crystallization behaviors of poly(ε-caprolactone-co-lactide) copolymers: Processable biomaterials with tunable properties

The current research underlines one-pot melt polymerization process for the copolymerization of L-lactide (LA) and ε-caprolactone (CL), where the composition of the comonomers is so varied as to obtain different chain microstructures of the resulting copolymers. The conversion of monomers to caproyl and lactidyl units is determined from 1H NMR (nuclear magnetic resonance) spectroscopy. The number-average sequence lengths of caproyl and lactidyl units as a function of monomer composition is evaluated using 13C NMR spectroscopy. The blocky chain structure in induced in the copolymers with the increasing LA content by virtue of difference of reactivity of LA and CL monomers, in order to develop thermally processable materials possessing mechanical integrity. The chain microstructure governs the thermal and mechanical properties which are reflected by an increase in melting temperature from 111.8 ℃ to 161.8 ℃, and increase in elongation at break from 2.2% to 812%. The copolymers possess a lower degree of crystallinity, where the crystalline LA units serve as physical crosslink points and contribute to the mechanical strength of the copolymers. Further the synthesized copolymers are scalable and thermally processable by conventional injection-molding-cum-extrusion technique to yield flexible products, where the adhesion and multiplication of fibroblast cells on the copolymers indicate their biocompatibility.