In situ small angle X‐ray scattering study on structural evolution of crosslinked polytetrafluoroethylene during deformation

The structural evolution of virgin and crosslinked polytetrafluoroethylene (PTFE) during stretching was studied by in situ synchrotron small‐angle X‐ray scattering (SAXS). Both yield and tensile stress of crosslinked PTFE increased with increasing crosslinking density. During stretching, for virgin PTFE, amorphous chains gradually turned to tensile direction at early stage, perpendicularly arranged lamellar stacks appeared at high strains (>140%). While for crosslinked PTFE, lamellar structure was observed even at lower strains; with increasing irradiation dose, the lamellar structure became obvious and the long period decreased. Four‐point SAXS patterns were observed only in 3000kGy‐dosed PTFE during deformation, which indicated that an alternately tilted lamella arrangement called herringbone structure was formed. Radiation dose induces crosslinked networks formed, which can carry part of local stress during deformation, resulting in the increase of yield and tensile stress. Crosslinking density is an important factor on structural evolution. In addition, a deformation mechanism of different crosslinked PTFE is proposed. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39883.     

Structural development of natural rubber during uniaxial stretching by in situ wide angle X-ray diffraction using a synchrotron radiation

Structural development of natural rubber during uniaxial stretching was examined by an in situ wide angle X-ray diffraction measurement using a synchrotron. During stretching, the amorphous part showed little change, i.e. an amorphous halo remained clear even at 500% strain. The fraction of induced crystals was very small, though a clear crystalline pattern was observed at 400% strain. Some polymer chains were oriented and crystallized, but most of the chains were not oriented at all in spite of large deformations of the specimen. Only a small amount of polymer chains contributes to the stress and hysteresis loss during elongation.     

Tensile deformation of semi-crystalline polymers by molecular dynamics simulation

In this work, the microstructure evolution of semi-crystalline polymers during tensile deformation is analyzed by molecular dynamics simulation. A perfect semi-crystalline lamellar structure with crystalline/amorphous interface perpendicular to tensile direction is created with the help of coarse-grained (CG) model of poly(vinyl alcohol) (PVA). During the tensile test, two kinds of strain rates are applied to the lamellar stack to determine the stress–strain curves, yield stresses, and crystallinities. Consistent with experimental findings, two yield points were observed in the semi-crystalline sample which was corresponded to the fine and coarse crystallographic slips in the lamellar structure, where the crystal stems gradually rotated into the direction of applied stress during chain slips. After the second yielding point when the crystal stems had been rotated fully into the direction of applied stress, the lamellar structure was destroyed and it resulted in a decrease of crystallinity. In addition, the increase of the strain rate led to the acceleration of destruction of crystal structures. It is worth noting that the stress induced crystallization was observed in the interfacial region, and newly crystallized beads were belonged to the same microcrystalline domain as crystalline region due to memory effects. This work provides direct comparison of structure evolution between crystalline and amorphous region in semi-crystalline polymers during tensile deformation, and it is helpful for the design and mechanical property analysis of semi-crystalline polymers.     

Structural evolution of tensile deformed high-density polyethylene at elevated temperatures: Scanning synchrotron small- and wide-angle X-ray scattering studies

The structural evolution of an ice-quenched high-density polyethylene (HDPE) subjected to uniaxial tensile deformation at elevated temperatures was examined as a function of the imposed strains by means of combined synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) techniques. The data show that when stretching an isotropic sample with the spherulitic structure, intralamellar slipping of crystalline blocks was activated at small deformations, followed by a stress-induced fragmentation and recrystallization process yielding lamellar crystallites with their normal parallel to the stretching direction. Stretching of an isothermally crystallized HDPE sample at 120 °C exhibited changes of the SAXS diagram with strain similar to that observed for quenched HDPE elongated at room temperature, implying that the thermal stability of the crystal blocks composing the lamellae is only dependent on the crystallization temperature. The strain at a characteristic transition point associated with the first indication for the occurrence of a fibrillar structure remains essentially constant in spite of the large changes in drawing temperature and crystalline thickness. In addition, WAXS experiments were used to probe the texture changes accompanying the uniaxial elongation and yield the relationship between the orientational order parameters associated with the crystallites and the amorphous chain segments, and the imposed strain. The results support the existence of intralamellar slip processes from the very beginning of tensile deformation.     

Structural Evolution of UHMWPE Fibers during Prestretching Far and Near Melting Temperature: An In Situ Synchrotron Radiation Small‐ and Wide‐Angle X‐Ray Scattering Study

Combining a homemade extension apparatus and the in situ synchrotron radiation small‐ and wide‐angle X‐ray scattering methods for measurement, the structural evolutions of gel‐spun ultrahigh molecular weight polyethylene (UHMWPE) fibers during prestretching at temperatures of 25 and 100 °C are investigated, respectively. Lamellar rotation toward the stretching direction occurs before strain hardening, while the folded‐chain crystal destruction and extended‐chain fibril formation processes occur in the strain hardening zone at 25 °C. While at 100 °C, stretching induced crystal melting before the stress plateau region and formation of fibrous crystals at the onset of the stress plateau are observed. Further stretching results in shear displacement of crystal blocks and, finally, destruction of the folded‐chain crystals and formation of extended‐chain fibrils. Prestretching UHMWPE fibers at 100 °C within a certain strain range can produce highly oriented fibrous crystals, which may provide an ideal precursor structure for the poststretching process.     

Role of stearic acid in the strain-induced crystallization of crosslinked natural rubber and synthetic cis-1,4-polyisoprene

Strain-induced crystallization of crosslinked natural rubber (NR) and its synthetic analogue, cis-1,4-polyisoprene (IR), both mixed with various amounts of stearic acid (SA), were investigated by time-resolved X-ray diffraction using a powerful synchrotron radiation source and simultaneous mechanical (tensile) measurement. No acceleration or retardation was observed on NR in spite of the increase of SA amount. Even the SA-free IR crystallized upon stretching, and the overall crystallization behavior of IR shifted to the larger strain ratio with increasing SA content. No difference due to the SA was detected in the deformation of crystal lattice by stress for both NR and IR. These results suggested that the extended network chains are effective for the initiation of crystallization upon stretching, while the role of SA is trivial. These behaviors are much different from their crystallization at low temperature by standing, where SA acts as a nucleating agent.     

BOPP纵拉过程的应力应变行为

采用高温下挤出厚片纵拉过程的模拟反映出来的应力应变行为，对厚片高温纵拉过程的形变特征进行分析研究，结果表明，EB在加工过程中具有更大的形态能力和更宽范围拉伸速度的适应性，在实验范围内，不同温度下的拉伸屈服应力与对数应变速率均呈线性关系，同时也受到相对分子质量的影响：高温下拉伸比的变化不会改变厚片形变的基本特征，但拉伸比提高后，应力应变曲线后端的硬化现象比较明显；深入理解BOPP拉伸温区结构变化现理，对于提高挤出厚片的形变能力，降低拉伸过程中薄膜的破损，提高生产效率将起到重要作用。     

Stress induced lamellar thickening in poly(ethylene succinate)

The crystalline structure evolution of poly(ethylene succinate) during tensile deformation was investigated by in-situ synchrotron X-ray scattering. Crystal phase transition from α to β form was confirmed to be fully reversible upon stress loading and unloading. An increase of long period was observed during the α–β crystal transition, which was attributed to the increase of both amorphous layer thickness and crystalline layer thickness (lamellar thickness). The crystalline layer thickness was scaled with the fraction of β crystal, and the change of which was in well agreement with the difference in the repeating length along the crystallographic c axis. Both the crystalline layer thickness and the amorphous layer thickness were fully recoverable. A possible molecular model was proposed to visualize the mechanism for the crystal transition and lamellar thickening.     

Study on structure and property relations of α‐iPP during uniaxial deformation via in situ synchrotron SAXS/WAXS and POM investigations

Micro‐ and meso‐scale structure changes of α‐form isotactic polypropylene (α‐iPP) during uniaxial stretching is studied by time‐resolved synchrotron small‐angle X‐ray scattering (SAXS) and wide‐angle X‐ray scattering (WAXS). The structure/property relations are investigated at different temperatures, and the effects of isothermal crystallization are also studied with POM. The X‐ray scattering results show that the long period increased and the lamellar oriented along the stretching direction in the elastic deformation stage. The lamellar and crystals start destructing after yielding. And from it POM images it can be seen that with higher crystallization temperature the spherulites connected to form a crystalline network, on which the stress is mainly loaded. It turns out different environment temperatures affect mostly the amorphous domains. And samples exhibit different yielding mechanisms with different thermal histories. A hypothetical structural mechanism is proposed based to explain the observed relationship between the processing parameters, thermal history and the structure/property relations of α‐iPP. POLYM. ENG. SCI., 58:160–169, 2018. © 2017 Society of Plastics Engineers     

Structure and properties of high-speed melt-spun filaments of poly(butylene terephthalate)

Filaments of poly(butylene terephthalate) were prepared by melt spinning with take-up velocities in the range 1000–5600 m/min. Two polymers with different molecular weights were used (intrinsic viscosities of 0.75 and 1.0 dL/g). The filaments were characterized using measurements of density, birefringence, shrinkage, thermal properties (differential scanning calorimetry), crystal size, crystalline orientation and phases present (wide angle X-ray diffraction), and tensile mechanical properties. Filaments spun from the 0.75 IV polymer with a mass throughput of 6 g/min at 1000 m/min have essentially amorphous structures, while higher take-up velocities result in α-form crystals or, at the highest take-up velocity, a mixture of α-form and β-form crystals. Only α-form crystals were detected in the higher IV polymer. Crystal size varied with crystallographic direction but generally increased as take-up velocity increased. At the lowest take-up velocities the filaments increased in length during thermal shrinkage measurements. With increasing take-up velocity the shrinkage became positive and continued to increase until reaching a maximum in the range of the highest sprinning speeds. This behavior correlates with the variation of the orientation factors of the amorphous phase. A plateau was observed in stress versus strain curves corresponding to strain-induced transformation from α-form to β-form crystals. The length of this plateau increased with increase of take-up velocity and the α-form crystal content in the sample. Both morphology and physical properties varied with polymer molecular weight and melt spinning conditions.     

Strain‐induced deformation mechanisms of polylactide plasticized with acrylated poly(ethylene glycol) obtained by reactive extrusion

This work aimed at identifying the tensile deformation mechanisms of an original grade of plasticized polylactide (pPLA) obtained by reactive extrusion. This material had a glass transition temperature of 32.6 °C and consisted of a polylactide (PLA) matrix grafted with poly(acryl‐poly(ethylene glycol)) (poly(Acryl‐PEG)) inclusions. pPLA behaved like a rubber‐toughened amorphous polymer at 20 °C, and its tensile behavior evolved toward a rubbery semicrystalline polymer with increasing temperature. The drawing of pPLA involved orientation of amorphous and crystalline chains, crystallization, and destruction of crystals. It was found that crystal formation and crystal destruction were in competition below 50 °C, resulting in a constant or slightly decreasing crystallinity with strain. Increasing temperature enhanced crystal formation and limited crystal destruction, resulting in an increased crystallinity with the strain level. Drawing yielded a transformation of the initial spherical poly(Acryl‐PEG) inclusions into ellipsoids oriented in the tensile direction. This mechanism may engender the formation of nanovoids within the inclusions due to a decreased density, assumed to be responsible for the whitening of the specimen. © 2015 Society of Chemical Industry     

Evaluation of oriented amorphous regions in polymer films during uniaxial deformation; structural characterization of a poly(vinyl alcohol) film during stretching in boric acid aqueous solutions

We have improved the conventional analytical method of the scattering data obtained with in‐situ synchrotron X‐ray scattering experiments, which is applicable to the structural characterization of the film during stretching. Four components of molecular chains in the film are divided from two‐dimensional wide‐angle X‐ray diffraction patterns. These components are the oriented crystals, the unoriented ones, the oriented amorphous chains, and the unoriented ones, respectively. This method allows to evaluate directly the degree of orientation of the amorphous chains and the amount of the oriented amorphous fraction in addition to each evaluation for the crystalline regions. This method is applied to the structural characterization of the poly(vinyl alcohol) (PVA) film during stretching in boric acid aqueous solutions, suggesting that boric acid accelerates orientation of molecular chains in the amorphous regions, and increases the oriented amorphous fraction by producing the cross‐links between the PVA chains and hindering strain‐induced crystallization. At a film break on stretching in the 3 wt% boric acid solution, the oriented amorphous fraction increases up to 70%, which is much higher compared to that in the film stretched in water, 45%. POLYM. ENG. SCI., 55:513–522, 2015. © 2014 Society of Plastics Engineers     

聚乙烯薄膜拉伸过程中结构变化的原位研究

使用差示扫描量热分析(DSC)研究了其拉伸前后结晶动力学与热力学行为的区别,使用同步加速器小角X射线散射(SAXS)和广角X射线衍射(WAXD)研究了薄膜拉伸过程中晶体结构、尺寸和长周期的变化。结果表明,拉伸使薄膜结晶度提高;拉伸过程中,二维散射图案发生明显变化,片晶结构被破坏后高应力诱导下重新结晶,拉伸使薄膜晶体取向度和晶体尺寸明显增大;随拉伸进行,纵向(拉伸方向)和45°方向的长周期先增大后减小,而横向一直呈下降趋势。聚乙烯工业缠绕膜通常在拉伸应变1.5倍附近下使用,此时薄膜纵向上已经高度取向并形成了拉伸诱导结晶,从而在纵向上有较高的力学强度。而横向上片晶结构尚未完全破坏,仍保持着一定的力学性能,因此综合使用性能较好。     

Structure evolution of polyamide (11)’s crystalline phase under uniaxial stretching and increasing temperature

Thermal processing of polyamide influences the internal crystalline structure and thereafter the post product mechanical performance. In this article, the crystalline transition of polyamide-11 (PA11) plate under uniaxial stretching and increasing temperature was investigated systematically using in-situ synchrotron X-ray technique. It was discovered that the lamellar slippage, fragmentation and recrystallization occurred in sequence under increasing temperature. In detail, the crystal of PA11 plate was stretched with a transition from triclinic α-form to mesomorphic phase at 25 °C. For the thermally activated γ-form crystals, crystal transition was inhibited when temperature was increased up to 160 °C. The melt-recrystallization was inclined to take place at large tensile strains. This work enhances the research significance of the thermal processing of polyamide and provides a theoretical method to improve the high performance of polyamide products.     

Evolution of phase behavior and orientation in uniaxially deformed polylactic acid films

The development of crystalline structure and orientation during uniaxial stretching of cast amorphous linear and branched lactic acid films were investigated in the rubbery temperature ranges that spans between glass transition temperature and cold crystallization temperature. This material exhibited almost ideal stress‐strain behavior in the temperature range 65–80°C. Because of its strain crystallizability, films with uniform thickness can be obtained at high deformation levels as a result of self‐leveling. Branching was found to retard this self‐leveling through its slightly detrimental effect on the strain hardening. Upon stretching the material undergoes rapid orientation in the amorphous state and beyond a critical level very sharp and highly oriented β crystalline form chains with ?3/1 helix. If the temperature is at or below Tg, with additional stretching, the films were found to revert to a highly oriented amorphous state through the destruction of the crystalline domains. At higher temperatures, further stretching results in continuation of improvement in crystalline order.     

Tensile deformation of nylon 6 fibers

Nylon 6 fibers which had been relaxed to different extents by annealing were examined at fixed strains by small angle and wide angle X-ray techniques. It was found that the strain of the long period of the semicrystalline microfibrils is identical to the macroscopic fiber strain. Approximately 1/3 of the tensile deformation results from molecular shear of imperfectly oriented crystalline chains. Virtually no evidence for intercrystalline slip is found; the orientation of the intercrystalline amorphous regions results in a low compliance for the shear of crystals past one another. The majority of the microfibril deformation occurs by stretching these intercrystalline amorphous regions, accompanied by the flow of extrafibrillar amorphous material to maintain constant volume. In highly annealed fibers this “filling” mechanism is less efficient, as the amount of extrafibrillar material has been reduced during shrinkage. This effect leads to a decrease in Poisson's ratio after increasingly severe annealing. A related result of annealing is the dehomogenization of the microstructure, leading to the presence of more stress-induced “microcracks” during the stretching of annealed fibers.     

Effect of cavitation on the plastic deformation and failure of isotactic polypropylene

To clarify the effect of cavitation, which is mostly induced by crystalline phase, on the plastic deformation and failure of isotactic polypropylene, solid‐state annealing at 160°C for 1.5 h is adopted to change the crystalline phase only while the amorphous phase keeps nearly intact. With aid of a special video setup, the relation of true stress and strain as well as the evolution of volume strain with axial strain has been derived. Enhancing crystalline phase due to annealing increases the yield stress and volume strain simultaneously. Moreover, the strain corresponding to steep increasing of volume strain is comparable with that related to yield, indicating that cavitation at early stage is accompanied with process of yield. With knowledge of toughness derived from impact tensile stretching and essential work of fracture (EWF), respectively, the relationship between cavitation and toughness has been correlated to some degree. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Mechanical properties of Bombyx mori silk yarns studied with tensile testing method

The mechanical properties of Bombyx mori silk yarns and baves were investigated with tensile testing method. After silk yarns were pre‐extended at different strain levels and fixed for a while followed by recovery process, the tensile characteristics were examined in detail. It was commonly observed that low preliminary extensions up to 2–3% do not cause the changes of the mechanical properties and stress‐strain curves because they result in small structural changes and distortions, which were recovered within relatively short time (～ 1 min) in recovery process. However, pre‐extension values >3% strain lead to great changes of the mechanical properties and fibre structure, i.e., the changes of the shape of stress‐strain curve where additional transition point was observed, increase in the rigidity and stress at rupture, but decrease in extensibility as a result of orientation and destruction of the fibre structure especially in the amorphous region. It was stated that silk fibre consists of two distinct deformation regions, namely first linear region extending up to 2–3% strain and the second region beyond 2–3% strain where the main reorganization processes of the fibre structure, that is, the straining of macromolecular chains especially in the amorphous regions, the orientation of structural units such as β‐sheet microcrystals in stretching direction, and the destruction of macromolecules take place. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Crystal structure of uniaxially stretched bio-based polyamide 510 films

The crystal structure changes of PA510 films during uniaxially stretching at 80°C, 110°C, 140°C and 170°C had been investigated as a function of stretching ratio and stretching rate. The stress–strain relationship curves showed that the stress of the PA510 films gradually increased when the stretching ratio increased. The wide-angle X-ray diffraction results verified that only one distinct equator reflection of stretched films was clearly identified at 80°C, 110°C and 140°C, namely γ(100) at 2θ = 20.6°. However, when the stretching temperature reached 170°C, the γ(004), γ(006) and γ(008) crystal form appeared in the meridional direction at λ = 12. Combined with differential scanning calorimeter analysis, it was found that the Xc increased from 7% to 40% as a result of the strain induced crystallization phenomenon and the stretching promoted the appearance of γ crystal form. In addition, the increase in the crystallinity and the molecular chain orientation increased the strength of the PA510 films in the tensile direction. And it also found that the microcracks occurred in the stretched films at high stretching ratio (λ = 12).     

Mesomorphic phase in oriented poly(pentamethylene 2,6-naphthalate)

The mesomorphic structure of poly(pentamethylene 2,6-naphthalate) (PPN) was investigated using a synchrotron X-ray scattering. The PPN fibers cold-drawn from the super-cooled amorphous state showed a smectic mesomorphic structure and further a crystalline phase at high strain. Based on the experimental evidence showing the split of amorphous halo up and down the equator and the conformational constraint suggested by the crystal structure refinement and computation, we suggested the smectic phase as S_CA where the mesogens are tilted against the layer surface normal and the tile direction is opposite between the neighboring layers.