Thermal and Dynamic-Mechanical Characterization of Uni- and Biaxially Oriented Polypropylene Films

The effect of uni- and biaxial orientation in the morphology of polypropylene films has been investigated by thermal, dynamic-mechanical, X-ray pole figure and diffraction patterns. In uniaxial oriented films the level of damping is roughly three times higher in the MD direction than it is in the TD direction. The stretching always produces crystals of the form independently of the starting type. Fast DSC scans show two melting peaks indicative of two crystalline species. The Fujiyama et al. model for the crystalline structure can be also applied to the uniaxially stretched films. Upon biaxially orienting, the folded lamellae crystals (kebabs) are the ones to support all the force applied, and when their maximum level of stress slippage is reached they deform following the Peterlin's model, forming a new shish structure. These new shishes are aligned to the TD direction and by linking the original shishes in the MD direction produce a planar orthogonal net of linked shish structures. The space among the shishes is filled with small and imperfect folded lamellae with c-axis in the film plane and preferentially oriented in the MD and TD directions, keeping constant crystallinity density throughout.This revised version was published online in November 2005 with corrections to the Cover Date.     

Crystallization of polylactic acid under in situ deformation during nonsolvent‐induced phase separation

In this study, we investigate polylactic acid (PLA) crystallization under in situ biaxial extension in a nonsolvent‐induced phase separation foaming process. Our ternary system consists of PLA, dichloromethane (DCM) as solvent and hexane as nonsolvent. For the first time, the formation of a shish‐kebab crystalline morphology is observed in such a solution‐based foaming process in certain solid–liquid phase separated systems. The formation of shish‐kebabs is described based on the coil‐stretch transition concept. The rapid biaxial deformation caused by macropore growth uniaxially stretches the long chains that are tied with at least two single crystals which eventually leads to the formation of shish structures throughout the polymer‐rich phase. The kebab lamellae then form perpendicularly on the shish cores. The scanning electron microscopy (SEM) observations and our interpretation of the crystallization phenomena are confirmed by differential scanning calorimetry (DSC) analysis. The observation of various crystalline morphologies, particularly shish‐kebabs, and the elucidation of their formation mechanisms contribute to the understanding of phase separation and pore growth as well as crystallization in such polymer–solvent–nonsolvent systems. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1055–1062     

Role of molten fraction on the structural evolution in stretching and cooling of crosslinked low‐density polyethylene: Real‐time mechano‐optical measurements

We investigated the uniaxial deformation behavior of crosslinked low‐density polyethylene in partially and substantially molten states using a real‐time true stress–strain birefringence system. The stress–birefringence behavior exhibits a multiregime behavior during stretching and holding process. The details of this regime behavior are primarily governed by the degree of unmelted crystallinity as it has a dominant role in the long‐range structural connectivity. When the long‐range physical connectivity is present, a three‐regime nonlinear stress–optical behavior was observed. When the long‐range connectivity is substantially eliminated at higher temperatures, the regime I behavior disappears. Structural studies including cooling process reveal that the lower the proportion of molten material during stretching, the higher the concentration of fibrillar structure and the shorter are the lengths of the kebabs that exhibit twisted lamellae after solidification. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1825–1841, 2005     

The influence of molten fraction on the uniaxial deformation behavior of polypropylene: Real time mechano‐optical and atomic force microscopy observations

In this study, we have coupled the real time mechano‐optical measurements with the off‐line structural characterization techniques including AFM, WAXS, and DSC to establish the quantitative relationships between the “true mechano‐optical behavior and developed morphology” as influenced by the fraction of molten phase present in the polypropylene films. Stretching PP in the solid state invariably leads to formation of fibrillar texture. The evolution of surface morphology in partially molten state was found to depend on the fraction of the molten phase present at the start of the deformation. If the samples are strained past the yielding in partially molten state, the birefringence begins a rapid rise. Concurrent with this, the equatorial zones of the spherulites begin to crack while meridional regions remaining intact. This leads to temporary reduction of crystallinity because of destruction of some of the crystals. If held in this strained state, the crystallite thickening was observed while the birefringence increases while the lost crystallinity is recovered. If the films are strained past the strain hardening point, the microfibrillar structure was found to dominate the surface morphology. When the films are stretched in the melting temperature range, they exhibit substantial nodular surface topology. These nodules that were absent in the solid state deformed samples are hard lamellae buried inside amorphous “soft matter”. The tangential lamellae increasingly become dominant as the processing temperature approaches substantially molten state leading to the observation of a* oriented crystallites in the X‐ray analysis. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 925–941, 2006     

Structural development of ultra‐high strength polyethylene fibers: Transformation from kebabs to shishs through hot‐drawing process of gel‐spun fibers

Structural development of ultra‐high strength polyethylene fibers via hot‐drawing processes of as‐spun gel fibers was investigated by means of transmission electron microscopy. It is found that the shish‐kebabs developed in both the as‐spun and drawn fibers can be transformed continuously into the micro‐fibril structure composed mostly of the shish structure through the hot‐drawing process. The structure transformation involves a drastic decrease in diameter of the kebab plus the shish but almost no change in the shish diameter. This result suggests that the chains in the kebabs are incorporated into the shishs and consumed to extend the longitudinal dimension of the shishs during the drawing process. The proposed new deformation model well explains the relationship between the fiber morphology and their mechanical properties: the tensile strength and modulus of the fibers can be determined by the number of the shish in the fiber and the macroscopic diameter of the fiber, which are apriori determined at the spinning process. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1861–1872, 2010     

Study on the improvement of crystallization of high density polyethylene through dynamic packing injection molding

To better understand the effect of high‐density polyethylene on crystal morphology under the shear stress field, the dynamic packing injection molding was used to prepare the oriented pure polyethylene. In this present work, dynamic packing injection molding can exert a successive shear field on the melt in the mold cavity during packing stage. The structure of the samples was characterized through differential scanning calorimetry, wide‐angle X‐ray diffraction, and scanning electron microscopy. The oriented lamellae perpendicular to the flow direction can be formed. Oscillating stress between the lower and upper critical values can induce shish forming. The shish kebabs can be generated through sufficient relaxation led by slow cooling and suitable shear. We roughly discuss the relationship between the shear rate and crystal shape. Our results set up a method to reinforce polymer parts by regulating morphology and structure which can be used to do practical processing condition. Copyright © 2016 John Wiley & Sons, Ltd.     

Crystal and phase morphology of dynamic-packing-injection-molded high-density polyethylene/ethylene vinyl acetate blends

The effect of shear stress, provided by so-called dynamic-packing injection molding, on crystal morphology and phase behavior was investigated for high-density polyethylene (HDPE) in blends with ethylene vinyl acetate (EVA) of various viscosities and vinyl acetate (VA) contents, with the aid of differential scanning calorimetry, two-dimensional small-angle X-ray scattering (2D SAXS), and scanning electron microscopy (SEM). A shish-kebab pattern was found in the oriented zones of dynamic samples, and the ratio of shish to kebab increased as a function of the EVA content in the blends up to 20 wt %, regardless of the VA content. This showed that molecules of HDPE could easily be stretched to form a shish structure in the presence of EVA. Moreover, a large increase in the long spacing, characterized by 2D SAXS measurements, was achieved because of the presence of EVA. The SEM results showed an obvious decrease in the domain size of the EVA phase under the effect of shear stress. All these results suggested shear-induced mixing between HDPE and EVA, in that ethylene segments of EVA molecules could be forged in the shish structure during shear and the other fractions of EVA were located in the amorphous regions between the adjacent lamellae of HDPE. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1831–1840, 2004     

Morphological evolution and orientation development of stretched iPP films: Influence of draw ratio

Varying the processing conditions of semicrystalline polymers can produce different morphologies of crystallization, which leads to different properties. There have been extensive studies of flow‐induced crystallization on isotactic polypropylene (iPP) using predominantly shear flow. A stretching method, deduced from extrusion, was introduced to study the morphological evolution of elongation‐induced shish‐kebab crystallization. Different morphologies of the resultant samples with different draw ratios (DRs) were carefully investigated and characterized via differential scanning calorimetry, polarizing light microscopy, scanning electron microscopy, atomic force microscopy, and 2D small‐angle X‐ray scattering. In addition, the degree of orientation of the samples with different DRs was also investigated using the 2D wide‐angle X‐ray scattering technique. The results indicate that the elongation‐induced morphology is strongly dependent on the effective stretching flow expressed in terms of the DR, which is defined as the ratio of rates between take‐up and the extrusion. The spherulite is dominant at low DRs, but it starts to deform along the stretching direction with increasing DR. The shish‐kebab structure in the stretched film, composed of stretched chains (shish) and layered crystalline lamellae (kebabs), increases gradually with an increase in the DR, whereas the spherulites gradually decreased. Furthermore, the overall orientation of α‐phase crystals, expressed by the Hermans orientation parameter, is also found to increase dramatically with the DR, and the rate of increase strongly depends on the DR. The different crystal morphologies are attributed to crystallization induced by different elongations of the stretched iPP films. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1223–1234, 2010     

Mechanism of formation of shish kebab structures

To describe the characteristic crystalline structure of polyolefins, Pennings first proposed a model consisting of a combination of an extended chain crystal (a “shish”) and folded chain crystals (“kebabs”). In Pennings' model the “shish” forms first during a crystallization process under stress and then later the “kebabs” overgrow this “shish” structure epitaxially. Because we had some doubts about such a mechanism, we undertook a series of experimental studies on linear polyethylene, particularly in regard to the crystallization process from a solution under shear. Our conclusion is that the crystals grow first by a screw dislocation mechanism, like whiskers, and then later these are deformed by the shear stress to form the shish kebab structures.     

Effects of carbon nanotubes on polymer physics

A single carbon nanotube has many similarities with an individual polymer chain including the fact that the end‐to‐end length of both are often about the same and the diameter of the chain is about the same (for single‐walled nanotubes) or only ～10 to 20 times larger (for multiwalled nanotubes). The combination of the solid surface and the similarity of the two materials means that polymer physics are altered in manners not seen with any other type of commonly used filler. The purpose of this review is to update a chapter that appears in a recent tome by Grady (2011) and describe how polymer physics is altered in composites that contain carbon nanotubes. Subjects that will be discussed include chain configuration, glass transition, polymer diffusion, unit cells and crystalline superstructure (lamellae, spherulites and shish‐kebabs), and crystallization kinetics. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

HDPE solution crystallization induced by electrospun PA66 nanofiber

Nanohybrid shish?Ckebab (NHSK), induced by polyamide 66 (PA66) nanofiber, was successfully fabricated in high-density polyethylene (HDPE)/xylene solution via isothermal crystallization. The crystalline morphological features of NHSK were observed by scanning electron microscopy. In the structure of NHSK, PA66 nanofiber serves as shish and HDPE lamellae act as kebabs periodically surrounding the nanofiber. Additionally, it reveals that both HDPE solution concentration and crystallization time have significant effects on the size of HDPE kebab. That is, as the concentration and crystallization time increase, the diameter of the kebab increases. Moreover, when crystallization time further increases, the crystals decorated on PA66 nanofiber exhibit a three-dimensional growth (i.e., aggregate of crystallites) rather than a two-dimensional one (i.e., disk-like lamellae normal to the axis of nanofiber).     

On the origin of the “core‐free” morphology in microinjection‐molded HDPE

This study investigates the morphology of a high‐density polyethylene processed with microinjection molding. Previous work pointed out that a “core‐free” morphology exists for a micropart (150‐μm thick), contrasting with the well‐known “skin‐core” morphology of a conventional part (1.5‐mm thick). Local analyses are now conducted in every structural layer of these samples. Transmission electron microscopy observations reveal highly oriented crystalline lamellae perpendicular to the flow direction in the micropart. Image analysis also shows that lamellae are thinner. Wide‐angle X‐ray diffraction measurements using a microfocused beam highlight that highly oriented shish–kebab morphologies are found through the micropart thickness, with corresponding orientation function close to 0.8. For the macropart, quiescent crystallized morphologies are found with few oriented structures. Finally, the morphology within the micropart is more homogeneous, but the crystalline structures created are disturbed due to the combined effects of flow‐induced crystallization and thermal crystallization during processing. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 1470–1478, 2011     

Confirmation of a nanohybrid shish‐kebab (NHSK) structure in composites of PET and MWCNTs

Here, the confirmation of an oriented nanohybrid shish‐kebab (NHSK) crystalline structure in a series of composites of poly(ethylene terephthalate) (PET) and multiwall carbon nanotubes (MWCNTs) is reported. The combined use of small‐ and wide‐angle X‐ray scattering (SAXS/WAXS) and thermal analysis has been used to investigate the morphology development in PET‐MWCNT nanocomposites under hot isothermal crystallization conditions. The MWCNTs act as both heterogeneous nucleating agents and surfaces (oriented shish structures) for the epitaxial growth of PET crystallites (kebabs) giving an oriented crystalline morphology. In contrast, the PET homopolymer does not show any residual oriented crystalline morphology during isothermal crystallization but gave a sporadic nucleation of a classic unoriented lamellar structure with slower crystallization kinetics. The results provide a valuable insight into the role of MWCNTs as nanoparticulate fillers in the morphology development and subsequent modification of physical properties in engineering polymers. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 132–137     

Ring‐banded spherulites in poly(pentamethylene terephthalate): A model of waving and spiraling lamellae

A new aryl polyester, poly(pentamethylene terephthalate) (PPT) with five methylene groups in the repeat unit, was synthesized. Its multiple‐melting behavior and crystal structure were analyzed with differential scanning calorimetry and wide‐angle X‐ray diffraction. In addition, the spherulitic/lamellar morphology of melt‐crystallized PPT was investigated. Typical Maltese‐cross spherulites (with no rings) were seen in melt‐crystallized PPT at low temperatures (70–90 °C), but ring patterns were seen in PPT crystallized only at temperatures ranging from 100 to 115 °C, whereas rings disappeared with crystallization above 120 °C. The mechanisms of the rings in PPT were explained with several coordinated directional changes (wavy changes, twisting changes, and combinations) in the lamellae during growth. Scanning electron microscopy, in combination with atomic force microscopy, further proved that the ringed spherulites originated from the aggregation of sufficient numbers of edge‐on lamellar crystals; the radial‐growth edge‐on/flat‐on lamellae could be twisted and/or waved to form realistic band patterns. A postulated model properly described a possible origin of the ring bands through combined mechanisms of waving (zigzagging) and twisting (spiraling) of the lamellae during crystallization. Superimposed twisting and/or wavy models during crystallization were examined as examples. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4421–4432, 2004     

Observation of shish crystal growth into nondeformed melts

Thin films of isotactic polystyrene (iPS) were deformed on a carbon supported TEM grid at temperatures between 210 °C and 240 °C. The elongation of the supercooled polymer melt films is localized in a deformation zone due to the formation of cracks within the brittle carbon support film, whereas the adjoining areas stay in a relaxed state. Since the nucleation of fibrous (“shish”) crystals needs an orientation mechanism to align the molecular chains, their origin is located in the deformation zone. It is shown that the subsequent growth of the shish crystal can propagate into the relaxed melt although it has to surmount the adhesion to the carbon substrate. From these results the conclusion is drawn that the shish crystal growth can be an autocatalytic process, which induces a self‐orientation of the molecules in the growth front of the crystal tip and does not necessarily need an external flow field. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1183–1187, 2000     

Effects of several lanthanide trichloride hydrates on the melting behavior and spherulitic superstructure of poly(ethylene oxide)

Binary mixtures of poly(ethylene oxide) (PEO) with the trichloride hydrates of lanthanum, cerium, europium, terbium, and ytterbium have been studied with calorimetry, polarized optical microscopy, and infrared spectroscopy. Melting‐point depression of the PEO‐rich phase occurs in all cases. At sufficiently high concentrations of the low molecular weight lanthanide complex, crystallization of the polymer is absent. The lighter lanthanides with larger ionic radii, such as lanthanum and cerium, are more effective in suppressing PEO crystallization from solution or the molten state because they are more oxophilic. The spherulitic superstructure of PEO disappears at rather low concentrations of the lanthanide salts, between 2 and 8 mol % Ln³⁺. Lanthanum and terbium are most efficient at disrupting the formation of PEO spherulites, and europium is least efficient. Infrared spectroscopy identifies twisting and wagging vibrational absorptions of CH₂ groups in the polymer that are sensitive to the morphologies of these mixtures. Modifications of the PEO infrared absorbances in the presence of these five lanthanide salts correlate more closely with the presence or absence of major PEO melting, not the formation of a spherulitic superstructure. The phase behavior is rather simple, with no evidence of eutectic solidification upon cooling from the molten state. Multiple melting endotherms are observed in the differential scanning calorimetry heating traces of binary mixtures containing 8 mol % Yb³⁺ and between 10 and 20 mol % Eu³⁺, but the concentration dependence of these first‐order endothermic transitions is not characteristic of eutectic phase behavior. The presence of trivalent cations, such as Eu³⁺ or Yb³⁺, in these complexes perturbs the crystallization kinetics of PEO upon cooling from the molten state, as well as the melting behavior upon heating. Ion–dipole or electrostatic interactions between the lanthanide cation and the ether oxygen of PEO might alter the surface free energy at the periphery of the crystalline lamellae and perturb the chain‐folding characteristics of PEO. Consequently, coupling between the amorphous matrix and the PEO crystallites is strengthened, and this provides stability for the existence of multiple‐chain‐folded crystals composed of rather thin lamellae that could be responsible for multiple melting behavior. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2200–2213, 2003     

The influence of short chain branch on formation of shish‐kebab crystals in bimodal polyethylene under shear at high temperatures

Formation of shish‐kebab crystals due to the coil–stretch transition under shear in the molten state using a bimodal polyethylene system with high molecular weight (HMW) fraction having different branch content was investigated. In specific, in situ small‐angle X‐ray scattering (SAXS) and wide‐angle X‐ray diffraction (WAXD) techniques were used to study the structure evolution of shish‐kebab crystals at high temperatures under simple shear. The SAXS results revealed that with the increase of branch content, shish‐kebab crystals became more stable at high temperatures (e.g., 139 °C). However, the shish length of the bimodal PE containing 0.11% branch was shorter than that with no branch. The WAXD results showed that the degree of crystallization for bimodal PE with HMW fraction having 0.11% branch increased with time but reached a plateau value of 1%, while that with no branch increased continuously till 11%. Furthermore, the crystal orientation of bimodal PE with HMW fraction having 0.11% branch was above 0.9 and maintained at a constant value, while that with no branch decreased from 0.9 to 0.1 upon relaxation. This study indicates that even though the crystallizability of the HMW fraction with branch content decreased, they could effectively stabilize the shear‐induced crystalline structure with shorter shish‐kebab crystals. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 786–794     

Classification of homogeneous copolymers of propylene and 1‐octene based on comonomer content

The solid‐state structure and properties of homogeneous copolymers of propylene and 1‐octene were examined. Based on the combined observations from melting behavior, dynamic mechanical response, morphology with primarily atomic force microscopy, X‐ray diffraction, and tensile deformation, a classification scheme with four distinct categories is proposed. The homopolymer constitutes Type IV. It is characterized by large α‐positive spherulites with thick lamellae, good lamellar organization, and considerable secondary crystallization. Copolymers with up to 5 mol % octene, with at least 28 wt % crystallinity, are classified as Type III. Like the homopolymer, these copolymers crystallize as α‐positive spherulites, however, they have smaller spherulites and thinner lamellae. Both Type IV and Type III materials exhibit thermoplastic behavior characterized by yielding with formation of a sharp neck, cold drawing, strong strain hardening, and small recovery. Copolymers classified as Type II have between 5 and 10 mol % octene with crystallinity in the range of 15–28%. Type II materials have smaller impinging spherulites and thinner lamellae than Type III copolymers. Moreover, the spherulites are α‐negative, meaning that they exhibit very little crystallographic branching. These copolymers also contain predominately α‐phase crystallinity. The materials in this category have plastomeric behavior. They form a diffuse neck upon yielding and exhibit some recovery. Type I copolymers have more than 10 mol % octene and less than 15% crystallinity. They exhibit a granular texture with the granules often assembled into beaded strings that resemble poorly developed lamellae. Type I copolymers crystallize predominantly in the mesophase. Materials belonging to this class deform with a very diffuse neck and also exhibit some recovery. They are identified as elastoplastomers. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4357–4370, 2004     

Polymer decoration on carbon nanotubes via physical vapor deposition

The polymer decoration technique has been widely used to study the chain folding behavior of polymer single crystals. In this article, we demonstrate that this method can be successfully adopted to pattern a variety of polymers on carbon nanotubes (CNTs). The resulting structure is a two-dimensional nanohybrid shish kebab (2D NHSK), wherein the CNT forms the shish and the polymer crystals form the kebabs. 2D NHSKs consisting of CNTs and polymers such as polyethylene, nylon 66, polyvinylidene fluoride and poly(L-lysine) have been achieved. Transmission electron microscopy and atomic force microscopy were used to study the nanoscale morphology of these hybrid materials. Relatively periodic decoration of polymers on both single-walled and multi-walled CNTs was observed. It is envisaged that this unique method offers a facile means to achieve patterned CNTs for nanodevice applications.     

Morphology control of polypropylene by crystallization under carbon dioxide

We investigated the crystalline morphology of isotactic polypropylene obtained by melt crystallization under carbon dioxide (CO₂) at various pressures. Spherulites consisting of regularly arranged fibrils without subsidiary lamellae were obtained by crystallization under CO₂ below 2 MPa, whereas large spherulites consisting of irregularly arranged fibrils with subsidiary lamellae were obtained under ambient pressure. Distorted domain crystals with uniform optical anisotropy consisting of α‐form were found to be obtained under CO₂ above 2 MPa, and needle crystals consisting of γ‐form were obtained above 12 MPa. Transmission electron micrographs showed that straight and thick lamellae are regularly arranged in both the distorted domain crystals and the needle crystals. The uniformly thick lamellae were confirmed by differential scanning calorimetry thermograms; that is, the melting temperature is higher and the melting peak is sharper than those obtained under ambient pressure. Such characteristic crystalline morphologies obtained under CO₂ may be attributed to local ordering in the melt state. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2738–2746, 2004