Spherulite Deformation Research Articles

Uniaxial tensile‐induced deformation of spherulite: combining experimental and simulation methods

AbstractA combination of experimental and simulation methods is adopted to study the relationship between micro‐stress and micro‐strain of a single spherulite upon uniaxial tensile deformation. To verify the simulation results and establish a suitable spherulite model, large‐sized isotactic polypropylene spherulite is first cultured with isothermal crystallization, whose structural deformation upon application of uniaxial tension is recorded using a polarizing optical microscope. A nanoindenter is used to measure the elastic modulus of the spherulite and amorphous region, preparing the property parameters for numerical simulation. Modeling and meshing of the single spherulite are conducted with the finite element software ABAQUS. The entire deformation of the spherulite is analyzed with fixed strain as the boundary condition, and the microscopic stress and strain distribution inside and outside of the spherulite is obtained. The higher micro‐stress region mainly locates at two poles in the equatorial direction, where might be the origin of craze. Generally, the simulation result is basically consistent with the experimental deformation process, which provides a new idea for the study of microscale structure and performance. © 2024 Society of Chemical Industry.

Preparation of poly(lactic acid) with excellent comprehensive properties via simple deformation or microfibrillation of spherulites

AbstractOur work overcomes the long‐standing challenge to achieve comprehensive improvements in thermo/mechanical properties of poly(lactic acid) (PLA), especially for impact toughness, strength, ductility, stiffness, and heat resistance. Simple deformation or microfibrillation of spherulites can create excellent thermo/mechanical properties making PLA compete against conventional petrochemical‐based polymers. It was performed by pressure‐induced‐flow (PIF) processing, and produced typical deformed spherulite structure and brick‐mud structure. It was found that the critical deformation degree of spherulites was around 2.7, beyond which spherulites would fragment. Through three transition stages of spherulite structures, deformed spherulite structures, and brick‐mud structure, PLA exhibited an enhancement of orientation, storage modulus and glass transition temperature. And a substantial increase was proved in impact strength (105.6 kJ/m2), tensile strength (148.4 MPa) and elongation at break (107.7%), which can be comparable to most engineering plastics. Both the deformed spherulite structures and the brick‐mud structures can enhance the impact toughness of PLA by increasing the fracture path of cracks. Besides, the evolution of the microstructure for a sandwich structure during PIF‐processing was proposed.

Robust, transparent films of propylene−ethylene copolymer through isotropic-orientation transition at low temperature accelerated by adjustment of ethylene contents

Transparently oriented films with high performance are fabricated by uniaxial stretching of propylene−ethylene copolymers (coPPs) at a low processing temperature. The coPP with ethylene content of 4 wt% (coPP4) can reduce processing temperature by approximately 50 °C compared to polypropylene (PP) and reach a tensile strength about 259.2 MPa, which is higher than that of PP about 215.9 MPa. As both crystallinity and orientation of coPP4@7.5 (draw ratio value followed by “@“) is lower than that of PP@4.5, this abnormal phenomenon is ascribed to the fibrillar crystals existing in coPP4@7.5 revealed by small angle X-ray scattering (SAXS) and scanning electronic microscopy (SEM). Structure analysis attests to that the tensile properties of the films are closely related to the hierarchically crystalline morphology especially in the fibrillar crystals. During the stretching process, it is intriguing that an appropriate amount of ethylene contents can accelerate the formation and perfection of fibrillar crystals originated from the deformation of coPPs spherulites, which are favorable for the enhancement of tensile strength. Interestingly, there is a significant linear decrease (R2 = 0.98) of the ultimate molecule orientation of stretched films with the increase of ethylene content. Furthermore, the stretching processing has little effect on crystallinity and long period for coPPs, which is distinctly different from the case of PP stretching. Meanwhile, it is found that ethylene segments lead to formation of lots of γ-form except for coPP16, which tends to transform to α-form during stretching of coPPs.

The effect of microstructure on the toughness of polypropylene random copolymer

ABSTRACTThis article mainly explores the mechanism and deformation of polypropylene random copolymer at different temperatures and the role of microstructure in the toughening process. Firstly, the conventional differential scanning calorimetry, wide-angle X-ray diffraction, and scanning electron microscopy were employed to study different structures of crystalline and amorphous regions. Furthermore, the dynamic thermomechanical analysis was used to study the changes in the molecular mobility in samples. Secondly, the toughness and fracture morphology of the material was analyzed by notched Izod impact test and scanning electron microscopy. Thirdly, samples were stretched and combined 2D-WAXD to analyses the changes in its crystal regions. The thickness and distribution of the lamellae, as well as stress transmitters, work synergistically during polypropylene random copolymer deformation. The yield is due to the spherulite deformation and a small part of lamellae rotation and reorientation when the temperature is 25 C. When near the glass transition temperature, a large number of lamellae are crushed and oriented to form a large number of microfibers at the yield point.The in-situ formed rubber phase in polypropylene random copolymer plays a significant role in the toughening process. When the temperature is 25 C the yield is due to the spherulite deformation and a small part of lamellae rotation and reorientation. While a large number of lamellae are crushed and oriented to form a large number of microfibers at the yield point when the temperature is 0 C.

Development of a biaxial stretching test machine and its applications

Abstract The evaluation method for a biaxially oriented film was developed using in-situ measurement during the stretching process. It can obtain basic data such as stress-strain curves, birefringence, light scattering, three dimensional refractive indexes and birefringence distribution. Stress and strain as functions of stretching speed and stretching temperature, as well as the deformation of spherulite of semi-crystalline polymer can be obtained by measuring the birefringence and light scattering during the biaxial stretching process with a small piece of polymer sample. The experimental results show the stress, retardation and three dimensional molecular orientations behavior during the simultaneous biaxial stretching and the sequential biaxial stretching process. Stretchability, thickness uniformity and spherulite size can be obtained simultaneously. In this paper, advantages and details of the newly developed system will be discussed with some experimental data.

Polymorphic transformation morphology of isotactic poly(1-butene)/poly(propylene-co-1-butene-co-ethylene) blends

The aim of this work was to investigate the polymorphic transformation morphology of isotactic poly(1-butene) (iPB-1) in blends with poly (propylene-co-1-butene-co-ethylene) (terPP). The blends were prepared by solution casting followed by compression-molding and were examined immediately after preparation as well as after aging for over 30 days. Morphological changes were firstly observed by polarized optical microscopy, which showed that the terPP promotes the reduction of the spherulite mean size during the crystallization of iPB-1 from the melt. Digital pulsed force microscopy provided data about the terPP interspersion between the interfibrilar and spherulitic regions of the iPB-1 crystals, causing iPB-1 spherulite deformation. However, a better distribution of the amorphous component on the iPB-1 crystals was observed after phase II to I transition, favored by the rearrangement of iPB-1 lamellae and lamellar stacks during phase transition.

Modeling of spherulite microstructures in semicrystalline polymers

Semicrystalline polymers are composed of crystalline structures together with amorphous polymer chain networks and therefore they exhibit deformation mechanisms of both crystalline materials and amorphous polymers. One of the most common microstructures observed in semicrystalline polymers is the spherulite microstructure in which crystalline lamellae are embedded in a matrix of amorphous material and grow out from a common central nucleus in radial directions. The mechanical behavior of semicrystalline polymers is strongly dependent on the underlying spherulite microstructure. Therefore, characterization of the deformation of spherulites is very important to understand the mechanical behavior of semicrystalline polymers. In this work, we propose a new FEM-based model for semicrystalline polymers which explicitly discretizes the spherulite microstructure consisting of crystalline and amorphous phases. In the model, a viscoplastic crystal plasticity model is employed for the crystalline phase, whereas 8-chain model is employed for the amorphous phase. The model captures the evolution of inhomogeneous plastic deformation activity in a spherulite microstructure, as well as the important features of the spherulite deformation reported in the literature.

Crystallization‐induced toughness of rubber‐modified polylactide: combined effects of biodegradable impact modifier and effective nucleating agent

The co‐addition of rubber‐like poly(ε‐caprolactone‐co‐D, l‐lactide) (P[CL‐co‐LA]) as impact modifier and N,N′‐ethylenebis(stereamide) (EBS) as nucleating agent has here been investigated to tailor both the crystallization and toughness of resulting polylactide (PLA)‐based materials. In this work, 10 wt% of P[CL‐co‐LA] copolymer and different concentrations of EBS were added into PLA by twin‐screw extrusion (in a micro‐compounder) in order to establish the mechanical performances of the resulting blends upon the nucleating agent content. Crystallization behavior and impact toughness of PLA‐based materials were both enhanced and were ascribed to a clear alteration of the matrix crystallinity, spherulitic growth rate, and crystal structure upon the nucleating agent content. As a result, toughness of PLA‐based materials was considerably enhanced up to, at least, 11 times with respect to neat PLA. Such improvement of the fracture resistance upon the extent of PLA crystallinity was related to a change in the toughening mechanism from crazing to shear‐yielding (combined with crazing) through the deformation of spherulites, promoting high energy dissipation pathways as crystallization occurs. Copyright © 2015 John Wiley & Sons, Ltd.

Measuring the orientation of chains in polypropylene welds by infrared microscopy: A tool to understand the impact of thermo-mechanical treatment and processing

The superstructures developed in the welds between injection molded plates made from α-nucleated and non-nucleated isotactic polypropylene (iPP) were analyzed by infrared (IR) microscopy, polarized light microscopy (PLM) and differential scanning calorimetry (DSC). Layers of distinct structures and size formed during welding in the welds were identified by PLM. Deformation of spherulites occurred in the weld seams due to the sheared melt flow. The orientation of the polymer chains in the welds, which is the result of melting and recrystallization, was quantified by IR microscopy for the first time: In the injection molded plates the chains were oriented along machine direction (MD), whereas this pattern changed to normal direction (ND) at the weld seams. Two positions at the welds were inspected to study the orientation of the chains: In one of these the PLM image depicts two well defined weld seams, while at the other, the weld seams were absent. DSC analysis of microtomed sections revealed the presence of both iPP polymorphs in the injection molded plates, whereas in the weld seam and weld core the α- and β-phase, respectively, were found. The degree of crystallinity (Xc) of the welds of iPP calculated using DSC and IR microscopy showed a homogeneous profile across the welds.The antioxidants (AOs) and the nucleating agent were profiled across the weld using IR-microscopy. A significant consumption of AOs during welding could be shown, and, using the principle of IR-dichroism, thermo-oxidative degradation of iPP chains was observed, even in the presence of AOs. The nucleating agent was homogeneously distributed across the weld.

Phase behaviour and morphology of composite comprising of poly(ethylene oxide), polyacrylate and lithium perchlorate

Thin films of poly(ethylene oxide) (PEO) and polyacrylate (PAc) blend and the composite PEO/PAc/lithium perchlorate (LiClO4) are prepared via solution casting method. Thermal analysis using differential scanning calorimetry depicts that both the PEO/PAc blend and the PEO/PAc/LiClO4 composite form single phase systems, as evident from the presence of single composition-dependant glass transition temperature (Tg). Polarized optical microscopy (POM) was applied to study the spherulite growth rate and morphology of PEO in both the PEO/PAc blend and PEO/PAc/LiClO4 composite. Homogeneity in the phase behaviour of the PEO/PAc blend and the PEO/PAc/LiClO4 composite is reaffirmed by the continuous suppression of the growth rate and the severe deformation and coarseness of the PEO spherulite with ascending PAc content in the blend and increasing LiClO4 concentration in the composite. Additionally, the amount of salt that causes macro-phase separation in the neat polymers and the PEO/PAc blends is governed by the coordination ability of the polymer, especially PEO, to the Li+ ion. Results from both thermal analysis and POM reflect the higher preference of LiClO4 for PEO than PAc. Therefore, PEO/PAc 85/15 blend is observed to exhibit the highest capability to interact with the Li+ ion.

In-situ SAXS study of the mesoscale deformation of polyethylene in the pre-yield strain domain: Influence of microstructure and temperature

The deformation of polyethylene spherulites at the scale of the lamellar stackings is investigated by in-situ SAXS as a function of draw temperature in comparison with the macroscopic strain. Local elongation strain in equatorial region and local compressive transverse strain in polar region both increase in absolute value and follow linear relations versus macroscopic strain in the visco-elastic range. However, local strains turn out heterogeneous over the whole spherulite volume. This is attributed to the different type of mechanical coupling between the crystalline lamellae and the amorphous layers in the polar and equatorial regions. Increasing the temperature in the range 50–100 °C gradually promotes homogeneous distribution of strain at the local scale. The origin of this phenomenon is discussed in terms of activation of the chain mobility in the crystalline phase that modifies the mechanical coupling between the two phases. This ratio between local and macroscopic deformation increases with temperature up to the macroscopic value.

Markedly improving mechanical properties for isotactic polypropylene with large-size spherulites by pressure-induced flow processing

Mechanical property of semicrystalline polymer with large-size spherulites is rather poor, and needs to be improved by adding nucleating agent to make spherulites smaller. However, the nucleating agent causes many problems limiting its applications. In this paper, we report a method to simultaneously improve the toughness and tensile strength of isotactic polypropylene (iPP) with large-size spherulites by deformation of spherulites and orientation of lamellaes through pressure-induced flow processing (PIF-processing) at solid state. The initial spherulite sizes of iPP samples with the same crystallinity and crystal form were controlled by adding nucleating agent (NA, Talc, nano-scaled), and then the samples were subjected to PIF-processing. The resulting mechanical performance of the bulk materials, including impact and tensile strength, was simultaneously enhanced. In particular, the impact strength increases upto 8 folds higher than those obtained by conventional processing for the same bulk materials, attributing to the synergistic effect of both the orientation and the large-size spherulites. The measurements of SEM, XRD and SAXS revealed the spherulites deformation and lamellae orientation during PIF-processing, as the basis of discussing the mechanism.

Textural characterization, major and volatile element quantification and Ar–Ar systematics of spherulites in the Rocche Rosse obsidian flow, Lipari, Aeolian Islands: a temperature continuum growth model

Spherulitic textures in the Rocche Rosse obsidian flow (Lipari, Aeolian Islands, Italy) have been characterized through petrographic, crystal size distribution (CSD) and in situ major and volatile elemental analyses to assess the mode, temperature and timescales of spherulite formation. Bulk glass chemistry and spherulite chemistry analyzed along transects across the spherulite growth front/glass boundary reveal major-oxide and volatile (H2O, CO2, F, Cl and S) chemical variations and heterogeneities at a ≤5 μm scale. Numerous bulk volatile data in non-vesicular glass (spatially removed from spherulitic textures) reveal homogenous distributions of volatile concentrations: H2O (0.089 ± 0.012 wt%), F (950 ± 40 ppm) and Cl (4,100 ± 330 ppm), with CO2 and S consistently below detection limits suggesting either complete degassing of these volatiles or an originally volatile-poor melt. Volatile concentrations across the spherulite boundary and within the spherulitic textures are highly variable. These observations are consistent with diffusive expulsion of volatiles into melt, leaving a volatile-poor rim advancing ahead of anhydrous crystallite growth, which is envisaged to have had a pronounced effect on spherulite crystallization dynamics. Argon concentrations dissolved in the glass and spherulites differ by a factor of ~20, with Ar sequestered preferentially in the glass phase. Petrographic observation, CSD analysis, volatile and Ar data as well as diffusion modeling support continuous spherulite nucleation and growth starting at magmatic (emplacement) temperatures of ~790–825 °C and progressing through the glass transition temperature range (T g ~ 750–620 °C), being further modified in the solid state. We propose that nucleation and growth rate are isothermally constant, but vary between differing stages of spherulite growth with continued cooling from magmatic temperatures, such that there is an evolution from a high to a low rate of crystallization and low to high crystal nucleation. Based on the diffusion of H2O across these temperature ranges (~800–300 °C), timescales of spherulite crystallization occur on a timescale of ~4 days with further modification up to ~400 years (growth is prohibitively slow <400 °C and would become diffusion reliant). Selective deformation of spherulites supports a down-temperature continuum of spherulite formation in the Rocche Rosse obsidian; indeed, petrographic evidence suggests that high-strain zones may have catalyzed progressive nucleation and growth of further generations of spherulites during syn- and post-emplacement cooling.

Novel Method to Toughen Polypropylene by Uniaxial Compression Processing

Based on the orientation of molecule chain in fibers, it will urge a rapid increase in mechanical properties. Nevertheless, block materials mainly gain rise of mechanical properties by blend or composite today, it is hardly proposed on self-toughening materials by optimizing the microstructure because of a lack of relations between the performance and the microstructure in block materials. In this paper, a toughening PP was gained through small deformation of spherulite, processed by uniaxial compression. A series of measurements were applied, including impact test and polarizing optical microscope (POM). The results show that the impact properties of polypropylene (PP) have a rapid rise. POM suggests that the spherulite has deformed into ellipsoid even belt along the orientation of sample of PP, which plays an important role in toughening.

Study on Structural Deformation and Performance of Polypropylene Spherulite Prepared by Uniaxial Compression Processing

Mechanical properties have close relation with microstructure. Based on orientation of the molecule chain in fiber, they show a rapid increase in tensile properties. Nevertheless, block materials mainly gain rise of mechanical properties by blend or composite, it is hardly proposed on the self-reinforced materials by optimizing the microstructure. In this paper, a strengthening of PP was gained by uniaxial compression through small deformation of spherulite. A series of measurements were applied, including tensile test, polarizing optical microscope (POM) and differential scanning calorimeter (DSC). The results show that the tensile properties of polypropylene (PP) have a rapid rise. POM suggestes that the spherulite has deformed into ellipsoid even belt along the orientation of sample of PP, which plays an important role in self-reinforcing of PP. And the initial melt point displayed by DSC slightly decreases because of the partial ruptures of crystal phase.

Deformation of elastomeric polyolefin spherulites

AbstractThe ethylene‐octene block copolymers in this study consist of long crystallizable sequences with low comonomer content alternating with rubbery amorphous blocks with high comonomer content. The crystallizable blocks form lamellae that organize into space‐filling spherulites even when the fraction of crystallizable block is so low that the crystallinity is only 7%. These unusual spherulites are highly elastic and recover from strains as high as 300%. This new class of thermoplastic elastomers is fundamentally different from conventional elastomeric olefin copolymers that depend on isolated, fringed micellar‐like crystals to provide the junctions for the elastomeric network. The elastomeric block copolymers are shown to be unique in that a hierarchical organization of space‐filling lamellar spherulites provides the junctions for the elastomeric network. The deformation of the elastic spherulites is readily studied with small angle light scattering, wide angle X‐ray diffractograms, and atomic force microscopy. At strains in excess of 300%, the spherulites break up into a fibrillar structure following lamellar deformation processes that are similar to those established for high density ethylenic polymers. The crystalline transformation produces a stiffer elastomer that exhibits complete recovery on subsequent loadings. Similar experiments on elastomeric random ethylene‐octene copolymers where fringed micellar crystals provide the physical crosslinks that connect the rubbery, amorphous chain segments reveal significant differences. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1313–1330, 2009

Self-Reinforced Isotactic Polypropylene Prepared by Melt Vibration Injection Molding

Polypropylene (PP) has a good combination of properties, but at low temperatures it is friable and its impact ductility is very low. To improve impact strength, a vibration injection molding (VIM) technology was used to investigate the mechanical properties of polypropylene. Yield strength is upgraded with an increment in vibration frequency and a peak stands at a special frequency for VIM; the elongation at break and impact strength are also enhanced by increased vibration frequency. The wide-angle x-ray diffraction (WAXD) curves and the scanning electronic microscopy (SEM) micrographs have shown that, in the vibration field, the enhancement of mechanical properties is attributed to the occurrence of γ-phase crystals and more pronounced spherulite deformation than those seen in conventional injection moldings (CIM), and the smaller spherulites with the existence of β-phase crystals are favored for improving toughness. With the application of vibration injection molding, the mechanical properties of isotactic PP are improved. To prepare self-reinforcing and self-toughening polypropylene molded parts it has been concluded that high vibration frequency is required. Increasing vibration pressure amplitude obviously significantly improves the yield strength and impact strength.

Ｘ線と中性子をプローブとした結晶性ポリプロピレンの外場下での変形挙動の研究

The understanding of crystal and chain structure evolution during the processing is one of the most essential information for the design of polymer with better processability and physical properties. For this purpose, X-ray and neutron scatterings are very powerful tools. In this paper, focusing on the deformation behavior of isotactic polypropylene (iPP) under external field, we introduce 1) application of microbeam SAXS-WAXS to the observation of iPP spherulite deformation process and 2) application of SANS for clarifying the role of each molecular weight component (low, medium and high molecular weight component) on shish-kebab structure formation of iPP.

Studies of single spherulite deformation. I

In this work a mechanical model of deformation of two-dimensional spherulites is proposed taking into consideration its two phase structure (amorphous and crystalline phase) as well as the internal radial correlation of orientation of crystalline elements. On the basis of the proposed model, assuming specific properties of amorphous and crystalline phases, the changes in shape and in internal orientation within the spherulites, related to a given strain, have been calculated. Using the derived relations the distribution of the intensity in low angle light scattering patterns on deformed spherulites has been obtained. Theoretical results were compared with these obtained from experimental low angle light scattering and optical microscope studies on deformed spherulites in polyoxymethylene.

Structure development and biodegradability of uniaxially stretched poly( l-lactide)

The effects of uniaxial drawing conditions on the orientation of various poly( l-lactide) (PLLA) samples were investigated using Fourier transform infrared dichroism and thermal analysis. The results showed that the orientation in uniaxially stretched PLLA was determined by the drawing rate, drawing temperature, and draw ratio. For the amorphous sample, a higher orientation was obtained at a lower drawing temperature due to the slower chain relaxation. On the other hand, a higher orientation was achieved for the precrystallized samples at a higher drawing temperature due to the larger deformation of spherulites. The hydrolysis tests of PLLA films showed that the rate of hydrolysis was dependent on the degree of orientation as well as the percent crystallinity. After preferential hydrolysis of the amorphous regions, the surface of oriented PLLA samples might be covered by a layer of relatively inactive crystal lamellae, causing a reduction in the rate of hydrolysis.