Morphology Of Semicrystalline Polymers Research Articles

Property mapping of LDPE during 3D printing: evaluating morphological development with X-ray scattering

Direct digital manufacturing has been identified as one of the key tools of Industry 4.0 and it enables the creation of products directly through digital definition. Commonly known as additive manufacturing, it comprises a set of technologies that are expressively agile in small-scale productions and prototyping, in comparison to conventional mass manufacturing processes, such as injection molding of plastics. It streamlines mass customization, allows the production of highly complex objects, and has been broadly applied in several fields, from medical devices to the aerospace industry. Although a new era of design possibilities and accessibility was unveiled, most developments are focused on shape reproduction precision and the development of new feeding systems and materials. This work is focused on a shift in design for additive manufacturing, where the polymer properties, by means of the adjustment of the process conditions (extrusion rate, the write speed, and the nozzle temperature, among others), constitute a decision-making variable. In order to evaluate the morphology of semicrystalline polymers during extrusion-based 3D printing,in-situtime-resolving small and wide-angle X-ray scattering measurements were performed at the ALBA synchrotron light source in Barcelona (Spain). The goal of this research is to develop a material property mapping methodology during semicrystalline polymer melt extrusion-based 3D printing Some experiments were performed with low-density polyethylene, and we were able to confirm a correlation between the extrusion rate and writing speed of the printing with the level of preferred orientation of the chain folded lamellar crystals in the extrudate.

Probabilistic numerical simulation for predicting spherulitic morphology from calorimetric crystallization conversion curves: An isothermal case

The present work introduces a novel method to estimate the morphology of semicrystalline polymers, namely the average spherulite size, size distribution, and nucleus density based on experimental crystallization conversion curves recorded by differential scanning calorimetry (DSC). A fast and accurate probabilistic numerical simulation method describing random nuclei formation and subsequent growth is introduced to handle the crystallization process of polymers. The developed method was used in a curve-fitting procedure with experimental crystallization curves to obtain structural parameters of neat and nucleated isotactic polypropylene. The simulated morphology was used with success to predict haze of injection-molded samples, proving the reliability and applicability of our new approach. The method presented here can be a useful technique to construct the spherulitic morphology based on crystallization conversion curves and predict properties dependent on the spherulite size, size distribution, and nucleus density, even when microscopic measurement cannot be carried out.

Evolution of iPP/HDPE Morphology under Different Mold Temperatures via Multiflow Vibration Injection Molding: Thermal Field Simulation and Oriented Structures

To date, many advanced polymer processing technologies have been developed to tune the morphology of semicrystalline polymers, such as multiflow vibration injection molding (MFVIM). By adjusting th...

Molecular dynamics simulations of crystal nucleation in entangled polymer melts under start-up shear conditions.

Understanding the flow induced crystallisation process is necessary due to its technological relevance to polymer processing. Polymer crystallisation controls the morphology of semi-crystalline polymers and hence the properties of the end product. We perform molecular dynamics simulations of polymer melts consisting of sufficiently entangled linear chains under shear flow. We determine the Rouse relaxation time (τR) for linear polymer chains using an established rheological model at different temperatures and fit the simulation data with the Arrhenius and Williams-Landel-Ferry equations. We simulate the crystallisation induction times for different values of the Rouse-Weissenberg number (WiR=γ̇τR) at different temperatures. We observe that the level of strain and stretch required to induce crystallisation increases with temperature. We find that the induction times follow a power law in shear rate and observe a more pronounced effect of flow rate for higher temperatures than at lower temperatures. Moreover, we determine that nucleation events occur relatively early in the shear transient and at a stretch value that is smaller than its steady state value. We also report the values of strain at which the occurrence of a nucleation event is most likely to happen.

Controlling Crystal Microstructure To Minimize Loss in Polymer Dielectrics

A model semicrystalline polymer, poly(ethylene naphthalate) (PEN), was used to examine how morphological factors inhibit chain segment relaxations that contribute to dielectric loss. This was achieved by manipulating the extent of crystallization and the crystalline microstructure through a combination of annealing and uniaxial drawing and investigating the effects on dielectric performance. Varying crystallization conditions influenced the dynamic Tg and extent of rigid amorphous fraction formation but had only a moderate effect on loss magnitude. Film orientation, however, greatly reduced loss through strain-induced crystallization and the development of oriented amorphous mesophasic regions. Postdrawing annealing conditions were capable of further refining the crystal microstructure and, in turn, the dielectric properties. These findings demonstrate that the semicrystalline polymer morphology can have a very significant influence on amorphous chain relaxations that contribute to dielectric loss, and understanding how processing conditions affect morphology is critical to the rational design of polymer dielectrics.

A 3D phase-field model for simulating the crystal growth of semi-crystalline polymers

In this study a 3D phase-field model, which is a generalization of the 2D phase-field model we ever presented, has been established for investigating the crystal growth of semi-crystalline polymers. By coupling a non-conserved crystal order parameter with a temperature field which is generated by latent heat of crystallization, our developed model can obtain its model parameters from real material parameters. Unlike the model of metals and small molecular compounds, this model has considered the partial crystallization property of semi-crystalline polymers. Moreover, due to the long-chain molecular structure, polymer crystallizations usually exhibit complex anisotropy and have polymorphous nature. In order to account for the various anisotropy of interfacial energy in 3D, three anisotropic functions for describing the anisotropic interfacial growth patterns are deduced phenomenologically which are based on a number of existing experimental facts. Simulation results have preliminarily demonstrated the good performance of our phase-field model in reproducing the complex and diverse morphology of semi-crystalline polymers in 3D. Several kinds of crystal patterns, which have been observed in experiments, can be reproduced.

Tunable Multiscale Nanoparticle Ordering by Polymer Crystallization

While ∼75% of commercially utilized polymers are semicrystalline, the generally low mechanical modulus of these materials, especially for those possessing a glass transition temperature below room temperature, restricts their use for structural applications. Our focus in this paper is to address this deficiency through the controlled, multiscale assembly of nanoparticles (NPs), in particular by leveraging the kinetics of polymer crystallization. This process yields a multiscale NP structure that is templated by the lamellar semicrystalline polymer morphology and spans NPs engulfed by the growing crystals, NPs ordered into layers in the interlamellar zone [spacing of (10–100 nm)], and NPs assembled into fractal objects at the interfibrillar scale, (1–10 μm). The relative fraction of NPs in this hierarchy is readily manipulated by the crystallization speed. Adding NPs usually increases the Young’s modulus of the polymer, but the effects of multiscale ordering are nearly an order of magnitude larger than those for a state where the NPs are not ordered, i.e., randomly dispersed in the matrix. Since the material’s fracture toughness remains practically unaffected in this process, this assembly strategy allows us to create high modulus materials that retain the attractive high toughness and low density of polymers.

Crystalline and Spherulitic Morphology of Polymers Crystallized in Confined Systems

Due to the effects of microphase separation and physical dimensions, confinement widely exists in the multi-component polymer systems (e.g., polymer blends, copolymers) and the polymers having nanoscale dimensions, such as thin films and nanofibers. Semicrystalline polymers usually show different crystallization kinetics, crystalline structure and morphology from the bulk when they are confined in the nanoscale environments; this may dramatically influence the physical performances of the resulting materials. Therefore, investigations on the crystalline and spherulitic morphology of semicrystalline polymers in confined systems are essential from both scientific and technological viewpoints; significant progresses have been achieved in this field in recent years. In this article, we will review the recent research progresses on the crystalline and spherulitic morphology of polymers crystallized in the nanoscale confined environments. According to the types of confined systems, crystalline, spherulitic morphology and morphological evolution of semicrystalline polymers in the ultrathin films, miscible polymer blends and block copolymers will be summarized and reviewed.

Crystallization-Driven Formation of Diversified Assemblies for Supramolecular Poly(lactic acid)s in Solution

Precipitation (or solvent displacement) method has been a simple yet efficient way to prepare the micro- and nanoparticles of polymers. However, control over the particle morphology of semicrystalline polymer is extremely challenging in the precipitation method, due to the interplay of polymer crystallization with liquid–liquid phase separation in solution. This limits the preparation of polymer particles with well-controlled morphology. Herein we report on the preparation of flower-shaped and spherical biodegradable polymer particles by precipitating the ureido-4[1H]-pyrimidione-functionalized supramolecular poly(lactic acid) (PLA) from a good solvent to an antisolvent. Morphology of PLA particles was successfully manipulated by the solution crystallization, molecular weight, and intermolecular interactions of polymer precursors. Homocrystallization of supramolecular poly(l-lactic acid) yielded the flower-shaped particles in precipitation; yet stereocomplex crystallization of PLA supramolecular copolymer...

Understanding chain folding morphology of semicrystalline polymers based on a rod-coil multiblock model.

We employ a rod-coil multiblock molecular chain model to investigate chain folding behavior, which is a significant characteristic in semicrystalline polymers, by using the method of self-consistent field theory (SCFT). Polymer chains with different conformations in crystalline and amorphous regions are described by rigid rod chains and flexible Gaussian chains, respectively. At present, we concentrate on the thermodynamic behaviors of polymer semi-crystals after the formation of the initial lamellar crystals. A new mechanism for lamellar thickening is proposed to realize that the end of lamellar thickening depends on the crystallinity degree. In other words, it is impossible for lamellae to develop into extended-chain crystals by means of lamellar thickening if crystallinity is limited to a certain degree. We further discuss the competition between crystalline and amorphous regions and its influence on crystallization behaviors, such as the formation of double lamellae, chain tilt, the anomalies and adjacent re-entry. The synergistic influences of the driving force of crystallization, interfacial energy and crystallinity degree on chain folding behavior are also investigated when the density anomalies in amorphous regions are excluded. Our model demonstrates advantages in accurately describing the mesoscopic layered structures of semicrystalline polymers based upon a microscopic chain model and provides at least a semi-quantitative thermodynamic picture for chain folding.

A Phase Field Technique for Modeling and Predicting Flow Induced Crystallization Morphology of Semi-Crystalline Polymers.

Flow induced crystallization of semi-crystalline polymers is an important issue in polymer science and engineering because the changes in morphology strongly affect the properties of polymer materials. In this study, a phase field technique considering polymer characteristics was established for modeling and predicting the resulting morphologies. The considered crystallization process can be divided into two stages, which are nucleation upon the flow induced structures and subsequent crystal growth after the cessation of flow. Accordingly, the proposed technique consists of two parts which are a flow induced nucleation model based on the calculated information of molecular orientation and stretch, and a phase field crystal growth model upon the oriented nuclei. Two-dimensional simulations are carried out to predict the crystallization morphology of isotactic polystyrene under an injection molding process. The results of these simulations demonstrate that flow affects crystallization morphology mainly by producing oriented nuclei. Specifically, the typical skin-core structures along the thickness direction can be successfully predicted. More importantly, the results reveal that flow plays a dominant part in generating oriented crystal morphologies compared to other parameters, such as anisotropy strength, crystallization temperature, and physical noise.

Application of electron diffraction in the structure characterization of polymer crystals

The dependence of properties on the structure and morphology of semicrystalline polymers offers an effective way to tailor the properties of these materials through structure control. To this end, establishing the structure and property relationship is of great importance. For a right characterization of the crystal structure, several techniques can be used. Among these techniques, electron diffraction has its advantage for determining the crystal structure related to specific formation condition since it can combine with bright and dark fields observation of the sample. This feature article describes the application of electron diffraction in determining the crystal structure of semicrystalline polymers with elaborately selected examples. We focus on how the electron diffraction can be used to disclose the crystal structure, mutual orientation of different crystals, as well as the disorders included in the polymer crystals.

Confinement and nucleation effects in poly(ethylene oxide) melt-compounded with neat and coated sepiolite nanofibers: Modulation of the structure and semicrystalline morphology

Composites have been prepared by melt compounding poly(ethylene oxide) with neat sepiolite, sepiolite coated with polyethylene glycol and with d-α-tocopherol polyethylene glycol 1000 succinate. It has been found that the nature of the sepiolite surface and its ratio in the composite enable the modulation of interfacial (nucleation) and confinement phenomena, both of which have distinct and opposed effects on the polymer’s crystalline phase. The modulation of both phenomena allows to tune the final morphology and structure of the polymer, this tuning including spherulite size, orientation of the crystalline lamellae, tortuosity of the crystalline phase, chain conformational disorder, glass transition temperature and pre-melting α′relaxation. Some of these modifications are thickness dependent, what is frequently found when the semicrystalline morphology of polymers is altered by its confinement into micrometric domains. Basic properties of the polymer such as elastic modulus and gas permeability are seen to depend very strongly on the morphology and structure modulation.

Surface-Induced Polymer Crystallization and the Resultant Structures and Morphologies

The dependence of properties on the structure and morphology of semicrystalline polymers offers an effective way to tailor the properties of these materials through crystal engineering. For purposeful control of the structure and morphology, and therefore the physical and mechanical properties, a full understanding of the crystallization habits of polymers under different environments and conditions is essential. This has stimulated a mass of research work on polymer crystallization. Considering that these materials are frequently in contact with some kinds of solid surfaces in a variety of applications, surface-induced crystallization of polymers has attracted considerable attention during the past decades. This Perspective provides the context as to how the solid surface influences the crystallization behavior of polymers and what kinds of unique crystal structure and morphology of the polymers can be fabricated. We hope that this will afford useful information for polymer processing in different applic...

Incidence of nanoscale heterogeneity on the nanoindentation of a semicrystalline polymer: Experiments and modeling

The nanoindentation of semicrystalline polyamide 6 (PA6) specimens was evaluated by atomic force microscopy with an extremely sharp diamond tip whose apex was nominally less than 10 nm in radius. Nanoindentation was performed under dry and wet conditions on PA6 samples injection-molded at different melt temperatures. The bulk Young’s modulus of dry PA6 was found to be in good agreement with the literature, thereby confirming the experimental approach developed in this study. A major finding is the observation of sudden increases in force during the loading portion of the experimental load–displacement curves, which occurred regardless of water absorption. We also found some irregularity in the pile-up morphology around the indents, linked to the heterogeneous nature of the deformation in PA6. Finite element analysis was used to elucidate the phenomenon of force discontinuity by independently considering the variations in elasticity, yield stress and friction coefficient of the crystalline lamellar aggregates in PA6. It is shown that force discontinuities in nanoindented PA6 result from local differences in yield phenomena at the lamellar level. The present investigation sheds some light on the importance of mechanical heterogeneity emerging from contact interactions between an extremely sharp tip and the nanoscale morphology of semicrystalline polymers.

Atomic force microscopy analysis of morphology of low density polyethylene influenced by Al nano‐ and microparticles

AbstractSpherulites are important morphological forms in semicrystalline polymers. Polarized optical microscope (POM) is a powerful tool for observing the spherulite morphology of semicrystalline polymers and their composites, but it can not resolve spherulites with average size smaller than 5 μm and also not be used to study the opaque nanocomposites film samples. Atomic force microscopy (AFM) is an excellent tool for observing spherulites comparatively small in size and not restricted by the transparency of the samples. In this work, AFM was used to investigate the impact of Al nano‐ and microparticles on spherulitic textures, lamellar thickness, and thickness distribution of low density polyethylene (LDPE). It was found that the spherulitic texture was seriously disordered by the nanoparticles, and that the LDPE in nanocomposites was found only to have poorly developed spherulite structure. The incorporation of microparticles can facilitate the growth of the lamellae but decrease the size of the spherulites. Furthermore, the lamellar thickness and its distribution did not alter so much with the addition of nano‐ and microparticles. Wide‐angle X‐ray diffraction (WXRD) was used to determine the crystal structure of LDPE, and the results show that both Al nanoparticles and microparticles do not change the unit cell parameters. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Nonisothermal Crystallization Kinetics of Poly(butylene succinate) and Poly(ethylene succinate)

Much more attention has been directed to biodegradable polymers due to their potential applications in the fields related to environmental protection in the last two decades. According to the difference in preparation methods, biodegradable polymers can be classified into two types. One is the biosynthetic polymer, such as bacterial polyhydroxyalkanoates (PHAs). Among them poly(hydroxybutyrate) (PHB) is probably the most extensively studied biodegradable thermoplastic polymer. Ha et al. recently reviewed the miscibility, properties and biodegradability of blends containing either PHB or poly(3-hydroxybutyrate-cohydroxyvalerate). The other is the chemosynthetic polymer, such as the aliphatic polyesters. Poly(butylene succinate) (PBSU) and poly(ethylene succinate) (PES) are just two of them. The chemical structures of PBSU and PES are (–OCH2CH2CH2CH2O2CCH2CH2CO–)n and (–OCH2CH2O2CCH2CH2CO–)n, respectively. The crystal structure, crystallization and melting behaviour of PBSU have been reported in literature. Polymer blending is often performed in order to improve the physical properties and extend the application fields of PBSU. PBSU was found to be miscible with poly(vinylidene fluoride), poly(vinylidene chloride-co-vinyl chloride), poly(ethylene oxide) (PEO) and poly(vinyl phenol). On the other hand, PBSU was found to show no miscibility with PHB, poly(3hydroxybutyrate-co-hydroxyvalerate) and poly(-caprolactone). We also studied the subsequent melting behavior of PBSU crystallized nonisothermally from the melt. The crystal structure, crystallization behaviour and melting behaviour of PES have been reported in literature. The crystallization and morphology of PES in binary miscible blends of two crystalline polymers have also been reported recently, such as in PES/PHB and PES/PEO blends. We also studied the subsequent melting behavior of PES crystallized nonisothermally from the melt and the crystallization kinetics as well as subsequent melting behaviour of PES from the amorphous glassy state. Crystallinity is known to play an important role in the physical properties and biodegradability of biodegradable polymers. Meanwhile, the crystalline structure and morphology of semicrystalline polymers are also influenced greatly by the thermal history. Therefore, much more attention should be paid to the crystallization kinetics study since it affects not only the crystalline structure and morphology of semicrystalline polymers but also the final physical properties and biodegradability for the biodegradable polymers. However, to the best of our knowledge, the overall crystallization kinetics studies, especially the nonisothermal crystallization kinetics studies from the melt, of PBSU and PES have not been reported so far in literature. But it is essential to study the nonisothermal crystallization kinetics from the viewpoint of practical application because most polymer processing operations are carried out under nonisothermal conditions. In this note we reported our results on the nonisothermal crystallization kinetics of PBSU and PES from the melt by differential scanning calorimetry (DSC). It is expected that the results will be helpful for a better understanding of the relationship between structure and properties of PBSU and PES from the viewpoint of practical process.

Relationships between microstructure, fracture-surface morphology, and mechanical properties in ethylene and propylene polymers and copolymers

The fracturing of four different polyolefin materials (a polypropylene homopolymer, a propylene–ethylene copolymer, a polyethylene homopolymer, and an ethylene–hexene copolymer) was studied with the objective of developing a better understanding of the relationships between the morphology of semicrystalline polymers, the morphology and growth kinetics of their fracture surfaces, and their mechanical properties. A scanning electron microscope and an optical microscope were used to obtain images of the fracture surfaces. The samples were injection-molded or hot-pressed to generate different microstructures. Fracture experiments were performed at 23, 0, and −20°C to generate fracture surfaces with different morphologies from the same supermolecular structure. It appears that the fracture propagates through the spherulites in a brittle manner. The macroscopic aspect of the fracture surfaces is temperature-independent and changes are visible only at the microscopic scale. Over the range of temperatures studied, the rms roughness [root mean square roughness decreased by only about 20%, while the fracture energy of all but one of the materials (a high-density ethylene–hexene copolymer) decreased by about 60% as the temperature was reduced. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2370–2382, 2000

Use of SAXS and linear correlation functions for the determination of the crystallinity and morphology of semi-crystalline polymers. Application to linear polyethylene

The use of correlation functions to obtain the morphological parameters of crystalline-amorphous two-phase lamellar systems is critically reviewed and extended. It is shown that processing of the experimental SAXS-patterns only significantly affects the curvature of the autocorrelation triangle and that the parameters of the corresponding ideal two-phase structure can be determined independently of the data processing procedure. The methods to be used depend on the normalization of the correlation function. The validity of the formulation is illustrated for a sample of linear polyethylene, cooled and heated at 10°C per min. Crystallite thickening during crystallization and surface melting during heating are observed. The overall crystallinity and the fraction of semi-crystalline stacks during crystallization and melting are determined quantitatively as a function of temperature using the total scattering power of the corresponding ideal two-phase structure, correlation functions, and a scaling procedure. Absolute intensities are not required. The SAXS results are confirmed by independent techniques (DSC, WAXD, and SALLS). During crystallization, amorphous regions are present outside the semi-crystalline regions because growing spherulites do not fill space completely. During melting, larger amorphous regions develop in the spherulites because of the complete melting of stacks. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1715–1738, 1999

Crystallization behavior and mechanical properties of a nylon-6, -6/6, and -12 terpolyamide

Composition, tacticity, and processing history can affect the morphology of semicrystalline polymers. Although homopolyamides are a family of polymers well known for semicrystalline character, through copolymerization or multicomponent copolymerization significant changes in materials' crystalline and thermal properties can occur. Due to chain irregularities introduced by terpolymerization, differential scanning calorimetry shows RDG 114T, a commercial polyamide of nylon-6, -6/6, and 12, to have an atypically low Tm and exhibit interesting recrystallization behavior. Specifically, the polyamide is wholly amorphous upon cooling from the melt, and since its Tg is about 20°C (due to the presence of plasticizers), chain ordering is found to occur over time at room temperature. Since the polyamide's morphology is time-dependent, the tensile properties of the polymer are also found to vary with ambient aging. For instance, Young's moduli for an amorphous and 7-day room temperature-annealed sample are 1.3×102 and 2.8×102 MPa, respectively. © 1996 John Wiley & Sons, Inc.