Effects of molecular characteristics and processing conditions on melt‐drawing behavior of ultrahigh molecular weight polyethylene

The effects of molecular characteristics and processing conditions on melt‐drawing behavior of ultrahigh molecular weight polyethylene (UHMW‐PE) are discussed, based on a combination of in situ X‐ray measurement and stress–strain behavior. The sample films of metallocene‐ and Ziegler‐catalyzed UHMW‐PEs with a similar viscosity average MW of ～10⁷ were prepared by compression molding at 180 °C. Stress profiles recorded at 160 °C above the melting temperature of 135 °C exhibited a plateau stress region for both films. The relative change in the intensities of the amorphous scattering recorded on the equator and on the meridian indicated the orientation of amorphous chains along the draw axis with increasing strain. However, there was a substantial difference in the subsequent crystallization into the hexagonal phase, reflecting the molecular characteristics, that is, MW distribution of each sample film. Rapid crystallization into the hexagonal phase occurred at the beginning point of the plateau stress region in melt‐drawing for metallocene‐catalyzed UHMW‐PE film. In contrast, gradual crystallization into the hexagonal phase occurred at the middle point of the plateau stress region for the Ziegler‐catalyzed film, suggesting an ease of chain slippage during drawing. These results demonstrate that the difference in the MW distribution due to the polymerization catalyst system dominates the phase development mechanism during melt‐drawing. The effect of the processing conditions, that is, the including strain rate and drawing temperature, on the melt‐drawing behavior is also discussed. The obtained results indicate that the traditional temperature–strain rate relationship is effective for transient crystallization in to the hexagonal phase during melt‐drawing, as well as for typically oriented crystallization during ultradrawing in the solid state. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2455–2467, 2006     

Development of a simultaneous measurement system of x‐ray diffraction and raman spectra: Application to structural study of crystalline‐phase transitions of chain molecules

The development of a bench‐top‐type system for simultaneous measurement of X‐ray diffraction and Raman spectra has been made to investigate structural changes in the phase transitions of chain molecules such as polyethylene, n‐alkane, and so forth from various viewpoints. For the X‐ray diffraction measurement an imaging plate or a charge‐coupled device camera was used as a two‐dimensional detector. For the Raman spectral measurement a miniature Raman spectrometer was used with optical fibers for the irradiation of incident laser beams and collection of scattered signals. For example, in the heating process of the n‐C₃₀H₆₂ sample, the phase transition from orthorhombic‐to‐hexagonal lattices could be detected clearly by the X‐ray and Raman measurements. By comparing these two different types of data in detail, an intimate relationship between conformational disordering and rotational motion of molecular chains is detected more clearly than before. Also, similar discussion can be made for the orthorhombic‐to‐hexagonal phase transition of the geometrically constrained polyethylene sample occurring immediately below the melting point. Another example concerns the structural change in the photoinduced solid‐state polymerization of cis,cis‐diethylmuconate single crystal. From the shifts in the X‐ray reflection position and Raman frequency characteristic of the produced polymer, it was found that the molecular deformation of the polymer chains and lattice strain was induced in the early stage of the polymerization reaction. For the ferroelectric‐phase transition of vinylidene fluoride copolymer, the simultaneous measurement was made for collecting triple information of small‐angle and wide‐angle X‐ray scatterings and Raman spectra to know the relationship between the structural change in the crystal lattice and the morphological change in the lamellar stacking mode. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 495–506, 2002; DOI 10.1002/polb.10112     

Longitudinal growth of polymer crystals from flowing solutions. VI. Melting behavior of continuous fibrillar polyethylene crystals

The melting behavior of continuous fibrillar crystals of high-molecular-weight polyethylene has been investigated. The macrofibers were grown from dilute solutions in xylene subjected to Couette flow in the temperature range between 103 and 118.5°C. The thermograms, as determined by differential scanning calorimetry, exhibit three melting endotherms with peak temperatures at 141, 150.5, and 159.5°C after extrapolation to zero scan speed. All peaks were found to be strongly superheatable. Reduction of fiber length, in particular by etching with fuming nitric acid, led to the disappearance of the melting peaks at 150.5 and 159.5°C. The remaining peak at 136°C appeared not to be superheatable. The heat of fusion of the fragmented fibers was 69.8 cal/g. Wide-angle x-ray diffractograms taken on a macrofiber while gradually heated at a rate of 0.35°C/min at constant length showed that the triclinic phase present in the fiber disappeared at 130°C and that the orthorhombic cell transformed into the hexagonal modification at 150°C. This hexagonal phase was still observable at 180°C. The retractive force developed on heating at constant length displays first a slight decrease followed by a maximum at 150°C. Beyond the latter temperature the stress decays abruptly corresponding to the temperature at which fracture of the fiber could be observed visually. From all these observations it is inferred that the first melting endotherm in the differential scanning calorimeter (DSC) thermograms arises from the melting of unconstrained fibrillar crystal regions which are able to shrink during fusion. Moreover, the melting of lamellar overgrowths on the elementary fibrils on shish-kebab type may contribute to this endotherm. The second melting endotherm at about 150°C is associated with the transformation of the orthorhombic into the hexagonal lattice in constrained parts of the sample. This latter “rotator” phase allows slippage of the polymer chains past each other, giving rise to stress relaxation. The third endotherm arises from melting of this hexagonal phase and the heat take-up connected with the formation of higher energy gauche states upon randomization of the chains in the melt. Almost smooth, fully constrained fibrillar crystals grown at high temperature absorb more than 15.5 cal/g during this process, indicating that the polymer chains in such fibers must be highly extended.     

Phase Development Mechanism during Drawing from Highly Entangled Polyethylene Melts

Summary: The phase development mechanism during drawing from a highly entangled melt of ultra‐high‐molecular‐weight polyethylene is analyzed by simultaneous measurements of in situ X‐ray diffraction using synchrotron radiation and stress/strain behavior. The stress/strain curve exhibits a plateau region at the initial stage of the draw, and no crystalline reflections appear on a series of in situ X‐ray diffraction patterns. However, as the sample draw proceeds above a critical strain, a metastable hexagonal reflection appears and becomes predominant, where the stress/strain curve still shows a plateau deformation. With a further increase of the strain, the intensity of the hexagonal reflection peak begins to decrease and subsequently that of the usual orthorhombic ones increase. Correspondingly, a rapid increase of draw stress, because of the strain‐hardening behavior, is recorded.

Stacked line profiles extracted from in situ WAXD patterns along the equatorial direction. The red profile was obtained at the critical time of 162.5 s.     

Phase transition in polyamide‐66/montmorillonite nanocomposites on annealing

The crystalline‐phase transition in polyamide‐66/montmorillonite nanocomposites before melting was investigated by in situ X‐ray diffraction and is reported for the first time in this work. The phase‐transition temperature in the nanocomposites was 170 °C, 20 °C lower than that in polyamide‐66. The lower phase‐transition temperature of the nanocomposites could be attributed to the γ‐phase‐favorable environment caused by silicate layers. Meanwhile, the addition of silicate layers changed the crystal structure of the polyamide‐66 matrix and influenced the phase‐transition behavior. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 63–67, 2003     

Processing of a Strong Biodegradable Poly[(R)‐3‐hydroxybutyrate] Fiber and a New Fiber Structure Revealed by Micro‐Beam X‐Ray Diffraction with Synchrotron Radiation

Summary: Biodegradable poly[(R)‐3‐hydroxybutyrate] (P(3HB)) fibers with high tensile strength of 1.32 GPa were processed from ultra‐high‐molecular‐weight P(3HB) by a method combining cold‐drawing and two‐step‐drawing procedures at room temperature. The distribution of molecular structures in a mono‐filament was analyzed by micro‐beam X‐ray diffraction with synchrotron radiation. It was revealed that the P(3HB) fiber has a new core‐sheath structure consistent with two types of molecular conformations: a 2₁ helix conformation in the sheath region and a planar zigzag conformation in the core region.

P(3HB) fiber processed by cold‐drawing in ice water and two‐step drawing at room temperature, and subsequently annealing at 50 °C.     

Drawing of ultrahigh molecular weight polyethylene fibers in the presence of supercritical carbon dioxide

The drawing behavior of ultrahigh molecular weight polyethylene fibers in supercritical carbon dioxide (scCO₂) is compared to that in air at different temperatures. The temperature substantially influences the drawing properties in air, whereas in scCO₂, a constant draw stress and tensile strength are observed. Differential scanning calorimetry shows an apparent development of a hexagonal phase along with a significant increase in the crystallinity of air‐drawn samples with increasing temperature. The existence of this phase is not confirmed by wide‐angle X‐ray scattering, which instead shows that air‐drawn samples crystallize in an internally constrained manner. In contrast, scCO₂ allows crystals to grow without constraints through a possible crystal–crystal transformation, increasing the processing temperature to 110 °C. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1375–1383, 2003     

Pathway‐Dependent Melting in a Low‐Molecular‐Weight Polyethylene‐block‐Poly(ethylene oxide) Diblock Copolymer

Summary: In a low‐molecular‐weight polyethylene‐block‐poly(ethylene oxide) (PE‐b‐PEO) diblock copolymer, two pathway‐dependent melting processes were observed: Upon slow heating, the PE lamellar crystals melted at ≈97 °C into a disordered state. However, when the temperature rapidly jumped to above the melting point (e.g., 100 °C), the PE lamellar crystals transformed directly into an ordered lamellar melt, followed by an isothermal conversion into a disordered melt. This isothermal order‐to‐disorder transition was explained by superheating of the PE crystals using a G‐T diagram.

A schematic G‐T diagram explaining the pathway‐dependent double melting for a crystalline polyethylene‐block‐poly(ethylene oxide) copolymer.     

Amphiphilic Comb‐Like Poly(oxyethylene) and Its Complexes with LiClO4: Synthesis and Mesomorphic Behavior

Summary: We prepared an amphiphilic, comb‐like poly(oxyethylene) containing decyl‐tri(oxyethylene) amphiphiles in the side chain using a polymer analogous reaction to obtain a novel nonionic amphiphilic polymeric system with high molecular weight. The amphiphilic comb‐like poly(oxyethylene) itself only showed a side‐chain crystalline phase below its melting temperature of −31 °C. When the polymer was mixed with lithium perchlorate, a smectic liquid‐crystalline phase appeared. The ordered phases of the polymer and the polymer mixture were studied by differential scanning calorimetry, polarized optical microscopy, and X‐ray diffraction.

POM image (200 X) of D3OTP1 at room temperature.     

Direct observation of phase transitions of polyethylene under high pressure by a PSPC x-ray system

A position-sensitive proportional counter (PSPC) x-ray measuring system is employed to observe directly phase transition processes of polyethylene at high temperature and high pressure. X-ray diffraction measurements reveal important new experimental data. First, an irreversible crystal transition from the hexagonal to the orthorhombic structures occurs in the critical region where the hexagonal structure begins to appear at a pressure of 350 MPa. That is, the (100) hexagonal reflection is observed only on cooling at 350 MPa. At pressures above about 400 MPa, however, the hexagonal phase is stable and the phase transitions melt ? hexagonal ? orthorhombic occur reversibly. Second, during cooling at pressures above 400 MPa, the (100) hexagonal reflection can be observed at temperatures below the hexagonal ? orthorhombic transition temperature. This behavior suggests that all the crystal morphologies of polyethylene, from “highly-extended-chain” crystals to crystals with a low melting point, are formed by the transitions melt → hexagonal → orthorhombic. Third, in heating at elevated pressures above 500 MPa, a shoulder in the peak intensity versus temperature plot for the (100) hexagonal reflection is observed at a higher temperature than the large maximum which occurs immediately after the crystal transition. This behavior indicates melting in two stages of hexagonal structures with different thermal stabilities, and the shoulder at higher temperature may be due to the fusion of the hexagonal phase annealed either below or above the transition point.     

Electrical and self‐heating properties of UHMWPE‐EMMA‐NiCF composite films

Conductive polymer composites (CPC) containing nickel‐coated carbon fiber (NiCF) as filler were prepared using ultra‐high molecular weight polyethylene (UHMWPE) or its mixture with ethylene‐methyl methacrylate (EMMA) as matrix by gelation/crystallization from dilute solution. The electrical conductivity, its temperature dependence, and self‐heating properties of the CPC films were investigated as a function of NiCF content and composition of matrix in details. This article reported the first successful result for getting a good positive temperature coefficient (PTC) effect with 9–10 orders of magnitude of PTC intensity for UHMWPE filled with NiCF fillers where the pure UHMWPE was used as matrix. At the same time, it was found that the drastic increase of resistivity occurred in temperature range of 120–200 °C, especially in the range of 180–200 °C, for the specimens with matrix ratio of UHMWPE and EMMA (UHMWPE/EMMA) of 1/0 and 1/1 (NiCF = 10 vol %). The SEM observation revealed to the difference between the surfaces of NiCF heated at 180 and 200 °C. Researches on the self‐heating properties of the composites indicated a very high heat transfer for this kind of CPCs. For the 1/1 composite film with 10 vol % NiCF, surface temperature (T_s) reached 125 °C within 40 s under direct electric field where the supplied voltage was only 2 V corresponding to the supplied power as 0.9 W. When the supplied voltage was enough high to make T_s beyond the melting point of UHMWPE component, the T_s and its stability of CPC films were greatly influenced by the PTC effect. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1253–1266, 2009     

Preferential orientation of crystallites spatially confined in lamellar microdomains of polyethylene‐block‐[atactic poly(propylene)]

Synchrotron small angle X‐ray scattering (SAXS), wide angle X‐ray scattering (WAXS), and transmission electron microscopy were carried out for an oriented polyethylene‐block‐[atactic poly(propylene)] with a molecular weight of 1.13×10⁵ and a volume fraction of polyethylene of 0.5. Isothermal crystallization at 93°C did not destroy the pre‐formed microdomain, however, with a higher crystallization temperature, the microdomain was more heavily deformed and more crystalline lamella grew. In WAXS profiles, preferential orientation of (020) reflection peak was observed, indicating that the crystalline lamella grew in parallel with the micro domain interface.     

Structural transformation from shish‐kebab crystals to micro‐fibrils through hot stretching process of gel‐spun ultra‐high molecular weight polyethylene fibers with high concentration solution

Structural evolution of gel‐spun ultra‐high molecular weight polyethylene fibers with high concentration solution via hot stretching process was investigated by in situ small‐angle X‐ray scattering, in situ wide‐angle X‐ray diffraction measurements, scanning electron microscopy, and differential scanning calorimetry. With the increase of stretching strain, the long period continuously increases at relative lower stretching temperature, while it first increases and then decreases rapidly at relative higher stretching temperature. The kebab thickness almost keeps constant during the whole hot‐stretching process and the kebab diameter continually decreases for all stretching temperatures. Moreover, the length of shish decreases slightly and the shish quantity increases although there is almost no change in the diameter of shish crystals during the hot stretching process. The degree of crystal orientation at different temperatures is as high as above 0.9 during the whole stretching process. These results indicate that the shish‐kebab crystals in ultra‐high molecular weight polyethylene fibers can transform continuously into the micro‐fibril structure composed mostly of shish crystals through the hot stretching process. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 225–238     

Improved interfacial properties of carbon fiber/UHMWPE composites through surface coating on carbon fiber surface

Carbon fibers were coated in an attempt to improve the interfacial properties between carbon fibers and ultra‐high molecular weight polyethylene resin matrix. Atomic force microscopy, scanning electron microscopy, and X‐ray photoelectron spectroscopy were performed to characterize the changes of carbon fiber surface. Atomic force microscopy results show that the coating of carbon fiber significantly increased the carbon fiber surface roughness. X‐ray photoelectron spectroscopy indicates that silicon containing functional groups obviously increased after modification. Interlaminar shear strength was used to characterize the interfacial properties of the composites.     

Stretching temperature and direction dependency of uniaxial deformation mechanism in overstretched polyethylene

In order to elucidate microscopic deformation behavior at different locations in isotropic semicrystalline polymers, the structural evolution of a preoriented high‐density polyethylene sample during tensile deformation at different temperatures and along different directions with respect to the preorientation was investigated by means of combined in situ synchrotron small‐angle X‐ray scattering (SAXS) and wide‐angle X‐ray diffraction (WAXD) techniques. For samples stretched along preorientation, two situations were found: (1) at 30 °C, the sample broke after a moderate deformation, which is accomplished by the slippage of the microfibrils; (2) at 80 and 100 °C, fragmentation of original lamellae followed by recrystallization process was observed resulting in new lamellar crystals of different thickness depending on stretching temperature. For samples stretched perpendicular or 45° with respect to the preorientation, the samples always end up with a new oriented lamellar structure with the normal along the stretching direction via a stress‐induced fragmentation and recrystallization route. The thickness of the final achieved lamellae depends only on stretching temperature in this case. Compared to samples stretched along the preorientation direction, samples stretched perpendicular and 45° with respect to the preorientation direction showed at least several times of maxima achievable stress before macroscopic failure possibly due to the favorable occurrence and development of microdefects in those lamellar stacks with their normal parallel to the stretching direction. This result might have significant consequence in designing optimal procedure to produce high performance polyethylene products from solid state. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 716–726     

A new cross‐linked system of silicone rubber based on silicone‐polyurea block copolymer

A new cross‐linked system of silicone rubber (SR) was obtained from silicone‐polyurea block copolymers that was synthesized with aminopropyl terminated polydimethylsiloxane and (4‐isocyanatocyclohexyl)‐methane. SR possessed self‐reinforced and physical cross‐linked structure. It had better mechanical properties that the hardness, the tensile strength, and the elongation at break could reach 65 Shore A, 3.78 MPa, and 458% with the polyurea segment content ranging from 2.01% to 9.13% by weight . The hydrogen bond that led to the physical cross‐linked structure was proved byFourier transform infrared spectroscopy. The microphase separated structure that caused the self‐reinforcement was illustrated by scanning electron microscopy, X‐ray diffraction analysis, and dynamic mechanical analysis. Fourier transform infrared spectroscopy results showed the hydrogen bond formation between the polyurea units. Scanning electron microscopy, dynamic mechanical analysis, and X‐ray diffraction analysis results proved the microphase separation existed between polyurea units and ―Si―O―Si― chains. The increase of polyurea contents enhanced the binding of hydrogen bond and improved the extent of microphase separation. Accordingly, it decreased the thermal properties and lowered the glass transition temperature (T_g) from −108°C to −114°C. Also, the increase of polyurea contents increased the hydrophobicity of SR that the surface free energy could reach to −24.81 mN/m.     

Autonomic healing of a pinhole in polyethylene and photografted polyethylene‐g‐poly(hexyl methacrylate) films

The structural changes of polyethylene (PE) and photografted polyethylene‐g‐poly(hexyl methacrylate) (PE‐g‐PHMA) with the mechanical formation of pinholes were evaluated with differential scanning calorimetry, wide‐angle X‐ray scanning, and small‐angle X‐ray scanning. The crystallinity and the long period of the lamella increased with pricking under extremely high compression stress. The partial transformation of an orthorhombic crystal into a monoclinic one was also detected. The autonomic healing of pinholes in PE and PE‐g‐PHMA was studied in detail. The degree of healing increased with an increase in the grafting ratio of poly(hexyl methacrylate) (PHMA). Three mechanisms for the healing were investigated and related to the molecular motions of PE and PHMA grafted chains, which were evaluated with dynamic mechanical analyses. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1705–1714, 2004     

Melting behaviour of gelspun/drawn polyolefins

The melting behaviour of gelspun/drawn UHMW-PE and UHMW-PP fibres was investigated. Unconstrained UHMW-PE and UHMW-PP fibres melt at 142°C and 170°C, respectively. Upon constraining, by holding the fibres at a fixed length or by embedding the fibres in a matrix, an increase in the melting temperature of both fibres is observed. In the case of UHMW-PE fibres a solid-solid phase transition in polyethylene at 155°C from the orthorombic to the hexagonal crystal structure occurs. Above 155°C, the fibres can not sustain any load. This solid-solid phase transition at 155°C sets an upper limit to both the maximum curing and continuous use temperature of PE-fibre reinforced composites. In gelspun/drawn UHMW-PP, such a detrimental solid-solid phase transition is absent, and therefore the increase in melting temperature can be utilized effectively. For example, heating of UHMW-PP fibres for 30 minutes at 200°C does hardly affect the room temperature Young's modulus and tensile strength if the fibre is constrained during heating.     

Optimization of UHMWPE/graphene nanocomposite preparation by single‐supported Ziegler‐Natta catalytic system via RSM

In this study, the preparation of ultra‐high molecular weight polyethylene/graphene nanocomposite was carried out using single‐supported Ziegler‐Natta catalyst, and the operational conditions were optimized via response surface methodology. For this purpose, the effect of 3 parameters, monomer pressure, temperature, and molar ratio of [Al] respect to [Ti] on the catalyst productivity and molecular weight of the synthesized nanocomposite polymer, was investigated using the Box‐Behnken experimental design at 3 levels. Monomer pressure, temperature, and molar ratio of [Al] respect to [Ti] were considered as independent variables and catalyst productivity and molecular weight as dependent variables. The highest catalyst productivity and molecular weight were equal to 923 (grPE/mmolTi.h) and 2.04 (million gr/mol), respectively, which were obtained under optimal reaction conditions: temperature of 60°C, pressure of 8 bar, and molar ratio of 185. Finally, in order to investigate the morphology and nanoparticle dispersion in polymer matrix, scanning electron microscope and X‐ray diffraction were used. The results indicate the homogenous dispersion of graphene nanoparticles in polymer matrix.     

Melt Behaviour of Poly(p-xylylene) (PPX) Coated Gel-spun UHMW-PE Fibers As Revealed by Temperature Modulated DSC

The method of temperature modulated DSC has been applied to obtain additional information about the effect of constraints on the melting behaviour of gel-spun ultra high molecular mass polyethylene (UHMW-PE) fibers coated with a high temperature stable poly(p-xylylene) (PPX) polymer. The underlying signal, corresponding to the normal DSC signal, reveals two endothermic peaks for the coated PE fibers. A shift in the underlying and magnitude signal from 142 to 145°C at 0.1 K min^–1 , a relative small magnitude signal, together with a vanishing step-like change in the phase signal with increasing PPX coating layer thickness characterize the constraints in terms of a hindrance of the melting of the unconstrained orthorhombic crystal fraction. The time constant of the melting process can be estimated as larger than the reciprocal angular frequency 1/ω=5 s of the modulation. This revised version was published online in July 2006 with corrections to the Cover Date.