Crystallization Behavior Of Poly Research Articles

Confined crystallization and chain conformational change in electrospun poly(ethylene oxide) nanofibers

This study focused on understanding the confined crystallization of poly(ethylene oxide) (PEO) in electrospun nanofibers. The effect of thermal treatment on PEO crystallization was also studied. An electrospinning process with a rotating drum collector enabled stretching and aligning of polymer chains, which resulted in the formation of a planar zigzag conformation. The confined environment in nanofibers facilitated an ordered crystal arrangement, resulting in an increase in the degree of crystallinity with a decrease in the fiber diameter. By contrast, large fibers extended solvent evaporation, which resulted in large crystallite sizes. The confined geometry and mechanical force provided by a rotating collector induced the preferred crystal orientation parallel to the fiber axis. Upon thermal annealing, the stretched PEO chains relaxed, resulting in a change from the metastable zigzag conformation to a stable helical conformation. Thermal treatment monotonically increased the melting temperature, degree of crystallinity, and crystallite size of PEO nanofibers regardless of the fiber diameter but did not influence the orientation of the organized crystals. The confinement effect on the chain conformation and crystallization behavior of poly(ethylene oxide) (PEO) in electrospun nanofibers was investigated. The PEO chains in electrospun nanofibers were tightly packed and exhibited a zigzag conformation. Increasing the fiber diameter reduced the packing of polymer chains, resulting in increases in the melting temperature and crystallite size. Thermal treatment relaxed the stretched chains, which induced a change from the metastable zigzag conformation to a stable helical conformation. Additionally, the melting temperature, degree of crystallinity, and crystallite size increased with increasing annealing temperature.

A peridynamic model for fracture analysis of polycrystalline BCC-Fe associated with molecular dynamics simulation

In this work, based on the traction-separation (T-S) constitutive relations extracted from molecular dynamics (MD) simulation, a peridynamics (PD) model is proposed to investigate the crack propagation behavior of the polycrystal BBC-Fe under mode I loading condition. MD simulation is carried out to give an insight into the cracking process and fracture mechanism according to the analysis of atomic configuration and stress distribution. The atomic stress at the crack tip with respect to the opening distance is tracked during the steady cracking stage to provide a stable T-S relation. The fracture parameters of single crystal Fe are obtained via MD simulation, based on which the PD parameters are obtained through an energy equivalent method. After that, a PD approach combined with cohesive zone model (CZM) is proposed to study the mode I fracture in polycrystal Fe. A good agreement has been found between the proposed PD model and the classical CZM based on a quasi-static splitting test of a single Fe crystal. Subsequently, PD simulations are performed concerning the dynamic crack propagation in a polycrystal Fe. What’s more, the effect of grain size, the grain boundary strength and the horizon size of PD on the fracture characteristics are examined. It can be concluded that the T-S relation originated from classical cohesive theory can be regarded as an effective bridge between MD and PD. This work provides a new thought to study the fracture behavior of polycrystals from the atomic deformation mechanism to micro-fracture description.

Studies on Enhancement in Crystallization Behavior of Poly (lactic acid) by Silk Fibroin Nanodisc

Studies on Enhancement in Crystallization Behavior of Poly (lactic acid) by Silk Fibroin Nanodisc

Multi-Island Genetic-Algorithm-Based Approach to Uniquely Calibrate Polycrystal Plasticity Models for Magnesium Alloys

The single-crystal yielding and hardening behavior of polycrystals is important for understanding their mechanical behavior. Many parameters (> 10) are usually required to achieve this for magnesium alloys based on physics-based models. However, the efficient and precise determination of these parameters is a very challenging task. An efficient and practical method is proposed herein to determine the parameter set by dividing the parameters into those for yielding and hardening, thereby significantly reducing the time cost of automatic parameter calibration. This method is then applied to calibrate the parameter set used in the viscoplastic self-consistent (VPSC) model to describe the mechanical behavior of the rare-earth magnesium alloy ZEK100 from multiple mechanical test data. The obtained best-fit parameters can be considered realistic and nearly unique, and can successfully reproduce the mechanical behavior and textural evolution. This procedure can be generally applied to calibrate near-unique parameters in other materials and constitutive models.

Effect of 1,2,3-triazolium-functionalized PEG-b-PCL block copolymer on crystallization behavior of poly(L‑lactic acid) as nucleation agent and mobility promoter

The effect of 1,2,3-triazolium-functionalized PCL-b-PEG block copolymer (PCL-b-PEG(IL)) on crystallization behavior and kinetics of PLA was systematically studied by DSC, POM and XRD. A small amount of micelle of aggregated PCL-b-PEG(IL) containing ionic clusters and a large number of disaggregated PCL-b-PEG(IL) can significantly improve the chain segment mobility of PLA. The glass transition temperature and cold crystallization temperature of PLA gradually decreased with the increase in PCL-b-PEG(IL) or PCL-b-PEG content. Compared with PCL-b-PEG, PCL-b-PEG(IL) decreases the Tg of PLA more obviously because of the strong plasticizing effect of PCL-b-PEG(IL). More importantly, the isothermal crystallization kinetics results manifested that the synergistic effect of heterogeneous nucleation agent of ionic clusters of PCL-b-PEG(IL) and chain mobility promoter of PEG chain segment can enhance the crystallization ability of PLA at all crystallization temperature. The 5 mass% PCL-b-PEG(IL) can induce high amount of stable α-form crystal of PLA after annealing treatment.

Effect of cellulose nanocrystals derived from Dunaliella tertiolecta marine green algae residue on crystallization behaviour of poly(lactic acid)

Marine green algae biomass residue (ABR), a waste by-product of Dunaliella tertiolecta, left behind after the extraction of oil from the algal biomass, was utilized for the fabrication of cellulose nanocrystals (CNCs). The fabricated sulphuric acid hydrolysed CNCs had needle-like morphology, with dominant cellulose type I polymorph and a high crystallinity index of 89 %. ICP-MS elemental analysis confirmed the presence of a variety of minerals in the ABR. Washed ABR (WABR)/PLA and CNC/PLA bio-composite films were developed via solvent casting technique with varying bio-filler loadings for comparing their effectiveness on the crystallization behaviour of PLA. FESEM, FTIR, XRD and TGA were used to characterize the bio-fillers. The nucleating and crystallization behaviour of the bio-composite films were confirmed using DSC, SAXS and POM analysis which indicated better effectiveness of CNCs with a significant reduction in cold crystallization temperature, and noteworthy increment in crystallinity and spherulite growth rate.

Deformation behavior of HfNbTaTiZr high entropy alloy singe crystals and polycrystals

Abstract Deformation behavior of equiatomic HfNbTaTiZr high entropy alloy single crystals and polycrystals were investigated. The single crystalline specimens could be obtained from the coarse-grained polycrystals annealed just below the melting point. 1/2 screw dislocations played an important role in the deformation behavior, similar to body centered cubic (bcc) metals. Moreover, a yield stress anomaly was found to appear in the alloys solutionized at 1473 K. The yield stress decreased rapidly with increasing temperature up to 673 K, while the stress increased at 873 K. Further increase in temperature resulted in a decrease in yield stress. It is also noted that the microstructure of the alloys depended strongly on annealing temperature. At 773–1073 K, phase separation into two bcc phase took place, while the hexagonal close-packed (hcp) phase was precipitated at 773–973 K. The ω phase which is typical in β-titanium alloys was also observed at 673 K. The strain-rate sensitivity of the deformation behavior and micro-Vickers hardness after the heat treatment suggest that the dynamic precipitation of the hcp phase is responsible for the yield stress anomaly.

Polycrystal thermo-elasticity revisited: theory and applications

The self-consistent (SC) theory is the most commonly used mean-field homogenization method to estimate the mechanical response behavior of polycrystals based on the knowledge of the properties and orientation distribution of constituent single-crystal grains. The original elastic SC method can be extended to thermo-elasticity by adding a stress-free strain to an elastic constitutive relation that expresses stress as a linear function of strain. With the addition of this independent term, the problem remains linear. Although the thermo-elastic self-consistent (TESC) model has important theoretical implications for the development of self-consistent homogenization of non-linear polycrystals, in this paper, we focus on TESC applications to actual thermo-elastic problems involving non-cubic (i.e. thermally anisotropic) materials. To achieve this aim, we provide a thorough description of the TESC theory, which is followed by illustrative examples involving cooling of polycrystalline non-cubic metals. The TESC model allows studying the effect of crystallographic texture and single-crystal elastic and thermal anisotropy on the effective thermo-elastic response of the aggregate and on the internal stresses that develop at the local level.

Effect of Storage Conditions on the Thermal Stability and Crystallization Behaviors of Poly(L-Lactide)/Poly(D-Lactide).

Polylactide (PLA) is a biodegradable thermoplastic aliphatic polyester. The thermal stability and crystallization behavior of PLA are extremely sensitive to storage, processing, and usage conditions. This work systematically studied the thermal stability and crystallization behavior of poly(L-lactide) (PLLA), poly(D-lactide) (PDLA), and a PLLA/PDLA (LD) blend, which were stored under two sets of laboratory storage conditions: (1) stored in a vacuum-free desiccator and (2) stored in vacuum-sealed bags. Both were stored at room temperature for 3 years. Gel permeation chromatography results revealed that the PLLA, PDLA, and LD samples hydrolyzed slowly when stored in vacuum-sealed bags and degraded significantly when stored in a vacuum-free desiccator; this process significantly reduced the thermal stability of the samples stored in the vacuum-free desiccator. Owing to hydrolysis, the levorotation and dextrorotation (L- and D-) molecular chains were shortened; consequently, more nuclei were formed, and this caused the melting points of the PLLA, PDLA, and LD samples to decrease and the melting enthalpy of the crystals in these samples to increase. Wide-angle X-ray diffraction analysis revealed that when the L- and D- molecular chains were packed side by side to form stereocomplex crystals and the randomly arranged L- and D- molecular chains were easy hydrolyzed and degraded, this interfered with the formation of homocrystals in LD. When PLLA, PDLA, and LD samples are stored in a vacuum-free desiccator, they will be significantly hydrolyzed, resulting in the formation of only stereocomplex crystals, and no homocrystals are observed.

Poly(propylene vanillate): A Sustainable Lignin-Based Semicrystalline Engineering Polyester

Vanillic acid that can be prepared from lignin through vanillin is a relatively new and most promising aromatic monomer for the synthesis of biobased polyesters. Due to its structural similarity to terephthalic acid, vanillic acid-containing polymers have been reported to exhibit comparable properties to poly(ethylene terephthalate). Herein, the nanomechanical and thermal properties and crystallization behavior of poly(propylene vanillate) (PPV), a new alipharomatic biobased polyester synthesized from 4-(2-hydroxypropoxy)-3-methoxybenzoic acid, are reported. As the phase transitions dominate the processing of polymers and determine the processing temperature window, the melting, crystallization, and glass transition temperatures were determined, and the thermodynamics and kinetics of phase changes were studied. The enthalpy of fusion was estimated (ΔHm0 = 162 J/g), and the equilibrium melting temperature (Tm0 = 220.5 °C) was calculated using the Hoffman–Weeks method. The multiple melting behavior of PPV was interpreted based on the melting-recrystallization model. Isothermal crystallization was studied. Since processing is always dynamic in industries, the non-isothermal crystallization profiles from the melt or from glass were investigated; differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and polarized light microscopy (PLM) were elaborated. Finally, as the mechanical properties of aromatic polymers are of crucial importance for applications, the mechanical behavior of amorphous and annealed PPV samples was examined via nanoindentation.

Micromechanical Analysis for Iron-Based Composites Under Large Deformations

A micromechanical analysis under large deformations for iron-based composites is introduced to predict the deformation behavior of iron (Fe)-copper (Cu) composites. In particular, pure Fe, pure Cu, Fe-17 wt. $$\%$$ Cu, and Fe-83 wt. $$\%$$ Cu are studied. In Fe-Cu composites, Cu is a soft-phase material. Hence, it undergoes a large deformation. Fe being the hard phase material shows elevated stresses. Because of the heterogeneity in the microstructure of Fe-Cu composites, the micromechanical model enables the capture of the flow behavior of Fe-Cu polycrystals. Both Fe and Cu constituents are rate-dependent elasto-viscoplastic materials. Hence, they are formulated in the framework of large deformations. The reliability of the prediction of flow behavior is investigated by comparing the numerical model (Taylor approach) and experiments.

Macroscopic response and microstructure evolution in viscoplastic polycrystals with pressurized pores

This paper presents a finite-strain homogenization model for the macroscopic behavior of porous polycrystals containing pressurized pores that are randomly distributed in a polycrystalline matrix. The porous polycrystal is modeled as a three-scale composite, where the pore size is taken to be much larger than the grain size, and the grains are described by single-crystal viscoplasticity. The instantaneous macroscopic response and corresponding field statistics in the material are determined using a generalization of the recently developed iterated second-order homogenization method, which employs the effective behavior of a linear comparison composite to estimate that of the nonlinear composite by means of a suitably designed variational approximation. Moreover, consistent evolution laws are derived for the pore pressure, pore geometry, and the underlying texture for the polycrystalline matrix. The model is then used to investigate porous ice polycrystals under a wide range of loading conditions. It is found that the pore pressure evolution has a strong effect on the material’s response under compressive loadings. More specifically, the macroscopic response of the porous polycrystals can be categorized into three different regimes: (i) a texture-controlled regime at low triaxialities, where the materials behave like solid polycrystals; (ii) a porosity-controlled regime at high triaxialities, where the materials behave like porous untextured materials; and (iii) a transition regime at intermediate triaxialities, where the materials exhibit a more complex behavior. This work highlights the importance of accounting for the interplay between porosity and matrix texture evolution in describing the constitutive response of porous polycrystals undergoing finite deformations.

Mechanical behavior and microstructural characterization of different zirconia polycrystals in different thicknesses.

PURPOSETo characterize the microstructure of three yttria partially stabilized zirconia ceramics and to compare their hardness, indentation fracture resistance (IFR), biaxial flexural strength (BFS), and fatigue flexural strength.MATERIALS AND METHODSDisc-shaped specimens were obtained from 3Y-TZP (Vita YZ HT), 4Y-PSZ (Vita YZ ST) and 5Y-PSZ (Vita YZ XT), following the ISO 6872/2015 guidelines for BFS testing (final dimensions of 12 mm in diameter, 0.7 and 1.2 ± 0.1 mm in thicknesses). Energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses were performed, and mechanical properties were assessed by Vickers hardness, IFR, quasi-static BFS and fatigue tests.RESULTSAll ceramics showed similar chemical compositions, but mainly differed in the amount of yttria, which was higher as the amount of cubic phase in the diffractogram (5Y-PSZ > 4Y-PSZ > 3Y-TZP). The 4Y- and 5Y-PSZ specimens showed surface defects under SEM, while 3Y-TZP exhibited greater grain uniformity on the surface. 5Y-PSZ and 3Y-TZP presented the highest hardness values, while 3Y-TZP was higher than 4Y- and 5Y-PSZ with regard to the IFR. The 5Y-PSZ specimen (0.7 and 1.2 mm) showed the worst mechanical performance (fatigue BFS and cycles until failure), while 3Y-TZP and 4Y-PSZ presented statistically similar values, higher than 5Y-PSZ for both thicknesses (0.7 and 1.2 mm). Moreover, 3Y-TZP showed the highest (1.2 mm group) and the lowest (0.7 mm group) degradation percentage, and 5Y-PSZ had higher strength degradation than 4Y-PSZ group.CONCLUSIONDespite the microstructural differences, 4Y-PSZ and 3Y-TZP had similar fatigue behavior regardless of thickness. 5Y-PSZ had the lowest mechanical performance.

Crystallization behavior of poly(lactic acid) and its blends

Poly(lactic acid) (PLA) is one of the most interesting bio-based biodegradable alternatives, with great promise to replace petroleum-based plastics. However, the intrinsic slow crystallization ability of PLA restricts its extensive applications. In this perspective, we highlight some recent advances in PLA crystallization, including nucleation by self-assembly nucleating agents, self-nucleation (or memory effect), conformational ordering in the very early stage of crystallization, crystal nucleation investigated by fast scanning calorimetry (Flash DSC), crystallization under flow, and pressure fields, and crystallization of enantiomeric blends. Finally, major challenges and future perspectives are proposed.

Isothermal Crystallization Behavior of Poly(vinylidene fluoride) Based Membrane with Graphene Oxide

Abstract Graphene oxide (GO) sheets were obtained via an improved Hummer's method, and the Poly(vinylidene fluoride) (PVDF)/GO membranes employing GO as nucleating agent were prepared by solution casting. The crystalline behaviors and the crystallization kinetics of PVDF/GO membranes with varying GO concentration were investigated after isothermal crystallization at different temperatures. The results showed that the addition of GO induced the β-phase formation of PVDF. The spherulite diameter reached the peak value when the content of GO was 0.1 wt%, then reduced to the minimum value at 1 wt% GO within the tested temperature range of isothermal crystallization. It is worth mentioning that the relative fraction of β phase F(β) was dramatically improved to 0.85 for the PVDF/GO membrane with 0.5 wt% GO at 155°C. Furthermore, an enhancement of the crystallization rate was observed as the contents of GO increased in the system.

Crystallization behaviors of poly(ethylene terephthalate) (PET) with monosilane isobutyl-polyhedral oligomeric silsesquioxanes (POSS)

The crystallization behaviors of poly(ethylene terephthalate) (PET) with monosilane isobutyl-polyhedral oligomeric silsesquioxanes (POSS) were investigated in detail. It is observed that POSS acts as an effective nucleating agent under appropriate concentration in the PET system. Polarized optical microscopy (POM) results showed that POSS improved the nucleation density of PET. Besides, the transmission electron microscopy (TEM) observation demonstrated POSS dispersed mostly at the nanometer scale throughout the PET matrix. The crystal structures of POSS and PET/POSS were analyzed using wide-angle X-ray diffraction (WAXD). Moreover, non-isothermal crystallization kinetics of PET and PET/POSS were studied using Mo’s method. The PET/POSS system demonstrated the lower value of F(T) than pure PET, indicating that the lower cooling rate was required to attain a specified relative crystallinity for the system. The comparison of the crystallization activation energy (ΔE) showed that the POSS nanoparticles improved the crystallization of PET through an acceleration of the nucleation process. Furthermore, the isothermal crystallization behaviors were also investigated using classic Avrami method. Overall, POSS can act as a nucleating agent and greatly improve the crystallization of PET through accelerating the nucleation process.

Changes in Crystallization Behaviors of Poly(Vinyl Alcohol) Induced by Water Content

AbstractWater as the low‐cost, environmentally friendly, easily obtained plasticizer was used widely in the preparation of poly(vinyl alcohol) (PVA) products. However, few researches were done to investigate the effect of water content on the crystallization behaviors of PVA. The water content in various PVA films was controlled by drying in a vacuum oven at different time and was determined by thermogravimetric analysis. The changes in crystallization behaviors of PVA affected by water content were investigated using differential scanning calorimeter, Fourier transform infrared spectra, and X‐ray diffraction. The wafer thickness and nonisothermal crystallization kinetics of various PVA films were also studied. The corresponding results showed that the crystallinity of various PVA films increased and then stabilized as the drying time increased. The distribution of wafer thickness in PVA films became narrow, and their wafer thickness value became small. The decrease of water content in PVA films promoted the crystallization rate, because the movement ability of PVA molecular chains was enhanced by the decrement in hydration hydrogen bonds in various PVA films.

A semi-analytical scale bridging approach towards polycrystalline ferroelectrics with mutual nonlinear caloric-electromechanical couplings

Various caloric aspects of polycrystalline ferroelecrics are investigated, based on an efficient modelling approach. In this context, both linear reversible effects, leading to heating or cooling of the material, and nonlinear dissipation heating, being associated with ferroelectric domain switching, are considered. The semi-analytical approach focusses on the constitutive behavior of a polycrystalline representative volume element (RVE) under the impact of combined electromechanical loadings, incorporating mutual interactions of caloric and electromechanical fields within a framework of non-equilibrium thermodynamics. Temperature dependence of material coefficients is further taken into account. The so-called condensed method (CM) is employed to bridge the scales of domain wall motion within grains and grain interaction in the RVE. Results are compared to those of finite element simulations and experiments taken from literature, illustrating hysteresis loops and dissipation heating due to cyclic electrical loading. Additional findings illuminate the role of mechanical compression during electric cycling, behaviors of single vs. polycrystals, and separate the impacts of linear and nonlinear caloric effects.

New insights into the memory effect on the crystallization behavior of poly(phenylene sulfide)

In this article, the melt and crystallization kinetics of poly(phenylene sulfide) (PPS) powder was studied by differential scanning calorimetry (DSC), fourier transform infrared spectroscopy (FTIR) and polarized optical microscopy (POM). It is found that the folded-chain crystals of PPS (melting point at 281 °C) could be erased at about 350 °C, while the equilibrium melting point of extended-chain crystals (melting point at 295 °C) is 370 °C. Although the residual nuclei could be eliminated completely at 370 °C, the spherulite growth rate of PPS during crystallization keeps accelerating with the increasing melting relaxation time, resulting from the transformation of the lamellae and the amorphous cluster to an ordered structure with low entanglement density. Furthermore, due to the restricted chain mobility for the semi-flexible PPS, this unique memory effect would survive in the whole manufacturing process, resulting in the varying results from different researches and the significant influence on the processability and performance of PPS products.

Synthesis, characterization, and crystallization behaviors of poly(D-lactic acid)-based triblock copolymer

Poly(D-lactic acid) (PDLA) with different polyethylene glycol (PEG) segment synthesized PDLA-PEG-PDLA triblock copolymer through the ring-opening reaction of D-LA and PEG will be used as a toughening modifier. The microstructure, crystal structures and crystallization behaviors of this triblock copolymer were investigated by Fourier transform infrared (FTIR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, X-ray diffraction (XRD), differential scanning calorimetry (DSC) and polarized optical microscopy (POM). The triblock copolymer is synthesized from the appearance of CH2 stretching vibration peak at 2910 cm−1 and C-O stretching vibration peak at 1200 cm−1 from PEG in FTIR spectra. Moreover, the chemical shift that is about 3.6 ppm in 1H NMR and 68.8ppm in 13C NMR proves this matter. The results of XRD and DSC reveal that PDLA and PEG are crystallized separately, and are not fully compatible, and microphase separation has occurred in this triblock copolymer. PEG can induce the triblock copolymer to accelerate the rate of crystallization, allowing it to crystallize more completely in the same amount of time. When the molecular weight of PEG is 6000 or the ratio of D-LA/PEG is 1/1, the crystallizability of PDLA-PEG-PDLA triblock copolymer is the best.