Ultra-high Molar Mass Polyethylene Research Articles

Applying Super High Resolution Fluorescence Microscope and Advanced Smart Cloud Algorithm in Real-Time Quantitative Processing and Analysis of Electroconvection

In an electrochemical cell at high current densities, electroconvection plays a critical role in determining the morphology of electrodeposited metals. The resultant dendrite formation leads to risky battery performance failure. Over the past decades, though intensive theoretical analysis and experimental effort have been done, the detailed structure and dynamics of electroconvection remain unknown, because experimental observations lacked the resolution to expose the fundamental, hierarchical spatial structure of these flows, while the data processing algorithms were not fully established for large sample size. To address this shortcoming, we built an advanced optical electrochemical cell that fits in situ imaging with a super-resolution fluorescence microscope. By observing and recording real-time motions of electrolytes, electroconvection flows have been interrogated at unprecedented, high temporal and spatial resolution. As a complement to the visualization studies, a cloud-computing analysis algorithm was developed by combining a higher resolution Particle Image Velocimetry algorithm and a machine learning model to enable velocity distribution data over the entire optical field of view at nanoscale or microscale spatial resolution. Consequently, detailed velocity maps are obtained across the optical electrochemical cell, allowing us to quantitatively measure and analyze the initiation and evolution of the hierarchical microstructures inside the electroconvection in a unidirectional electric field. With these powerful tools, we further studied the effect of polymer viscoelasticity on electroconvection and electrodeposition. The addition of an ultrahigh molar mass polyethylene oxide to the electrolytes transforms the materials to a viscoelastic fluid state that changes the electroconvection dependence on time and voltage and induces a smooth electrodeposition on the metal anode by unique polymer rheological properties. Especially, the relaxation of long polymer chains in electrolyte provides a promising manner to prevent dendrite growth. We further analyzed these observations using direct numerical simulations for Newtonian and viscoelastic fluid models, and the quantitative analysis of experimental results indicates good consistency with simulation predictions.

Effect of Aging on Unidirectional Composite Laminate Polyethylene for Body Armor.

The construction of ballistic-resistant body armor is experiencing an increasing use of flexible unidirectional (UD) composite laminates that comprise multiple layers. Each UD layer contains hexagonally packed high-performance fibers with a very low modulus matrix (sometimes referred to as binder resins). Laminates are then made from orthogonal stacks of these layers, and these laminate-based armor packages offer significant performance advantages over standard woven materials. When designing any armor system, the long-term reliability of the armor materials is critical, particularly with regard to stability with exposure to temperature and humidity, as these are known causes of degradation in commonly used body armor materials. To better inform future armor designers, this work investigates the tensile behavior of an ultra-high molar mass polyethylene (UHMMPE) flexible UD laminate that was aged for at least 350 d at two accelerated conditions: 70 °C at 76% relative humidity (RH) and 70 °C in a desiccator. Tensile tests were performed at two different loading rates. The mechanical properties of the material after ageing demonstrated less than 10% degradation in tensile strength, indicating high reliability for armor made from this material.

Effects of Thermal Aging on Molar Mass of Ultra-High Molar Mass Polyethylene Fibers.

Ultra-high molar mass polyethylene (UHMMPE) is commonly used for ballistic-resistant body armor applications due to the superior strength of the fibers fabricated from this material combined with its low density. However, polymeric materials are susceptible to thermally induced degradation during storage and use, which can reduce the high strength of these fibers, and, thus, negatively impact their ballistic resistance. The objective of this work is to advance the field of lightweight and soft UHMMPE inserts used in various types of ballistic resistant-body armor via elucidating the mechanisms of chemical degradation and evaluating this chemical degradation, as well as the corresponding physical changes, of the UHMMPE fibers upon thermal aging. This is the first comprehensive study on thermally aged UHMMPE fibers that measures their decrease in the average molar mass via high-temperature size exclusion chromatography (HT-SEC) analysis. The decrease in the molar mass was further supported by the presence of carbon-centered free radicals in the polyethylene that was detected using electron paramagnetic resonance (EPR) spectroscopy. These carbon-centered radicals result from a cascade of thermo-oxidative reactions that ultimately induce C–C ruptures along the backbone of the polymer. Changes in the crystalline morphology of the UHMMPE fibers were also observed through wide-angle X-ray diffraction (WAXS), showing an increase in the amorphous regions, which promotes oxygen diffusion into the material, specifically through these areas. This increase in the amorphous fraction of the highly oriented polyethylene fibers has a synergistic effect with the thermo-oxidative degradation processes and contributes significantly to the decrease in their molar mass.

Determination of viscosity average molar masses of polyethylene in a wide range using rheological measurements with a harmless solvent

Rheological measurements of polyethylene solutions in a novel nontoxic solvent were used to determine the Mark-Houwink parameters for PE of widely different molecular weights to assess the viscosity average molar mass for PE of several millions of g.mol−1. Solutions of high density polyethylene that had been previously characterized by size exclusion chromatography were prepared in a harmless naphthenic oil. The intrinsic viscosities were obtained by varying the concentrations of the different calibration samples solutions using a rotational rheometer equipped with a Couette cell. The intrinsic viscosities were determined by both Huggins and Kraemer descriptions of the viscosity variations with concentration. Subsequently, a calibration curve of intrinsic viscosities versus the molar mass was plotted, and the parameters of the Mark-Houwink-Sakurada equation were obtained for polyethylene in naphthenic oil. Using this equation and the newly estimated parameters, the viscosity average molar mass of a high molar mass polyethylene and a commercial ultra high molar mass polyethylene were been determined by measuring their respective intrinsic viscosities. Finally, the method has been validated by comparing these results to values obtained by standard viscosimetry in 1,2,4-trichlorobenzene and to the values given by the supplier, obtained by other methods.

Digitally Tuned Multidirectional All-Polyethylene Composites via Controlled 1D Nanostructure Formation during Extrusion-Based 3D Printing

Fused filament fabrication (FFF) of polyethylene (PE) reactor blends containing high amounts of nanophase-separated disentangled ultrahigh molar mass polyethylene (UHMWPE) generates self-reinforced...

Effect of Irradiation and Detection of Long-Lived Polyenyl Radicals in Highly Crystalline Ultra-High Molar Mass Polyethylene (UHMMPE) Fibers.

To improve properties such as thermal conductivity, low temperature thermal strain, and creep resistance of ultra-high molar mass polyethylene (UHMMPE) fibers, several researchers have previously undertaken efforts to crosslink these fibers using radiation. Ionizing radiation is commonly used to crosslink bulk UHMMPE in other applications, such as artificial joints. However, UHMMPE fibers differ from bulk UHMMPE in that they have a higher crystallinity (approximately 85% to 90%) and are very highly oriented during manufacturing in which the fibers are stretched 50 to 100 times their original length. Thus, the amorphous fraction of the UHMMPE fibers is also highly ordered. Several experiments were conducted to crosslink the UHMMPE fibers using both low dose rate (gamma) and high dose rate (electron beam) irradiation, all in the absence of oxygen. In all cases, the tensile strength of the fiber was greatly reduced by the irradiation. The oxidation index was also measured for the irradiated samples, and oxidation was not found to play a major role in the reduction of tensile strength in the fibers after irradiation. While this work did not achieve the desired result of improving the mechanical properties of the UHMMPE fiber, a significant result was found. The electron paramagnetic resonance (EPR) spectrum of the UHMMPE fibers was measured shortly after irradiation, and a mixture of allyl and alkyl radicals were detected. The irradiated samples were stored in dark ambient conditions for at least six years, then reexamined using EPR for free radical characterization. Surprisingly, the gamma-irradiated samples showed clear evidence of long-lived polyenyl radicals present in the material. Free radicals are very reactive species that will typically migrate to the surface of the crystalline domain and decay in a relatively short time through various reactions in the amorphous regions. It is hypothesized herein that due to the high crystallinity and large anisotropy of the highly drawn UHMMPE fiber, the polyenyl radicals were trapped in the crystal phase and were unable to migrate and decay. An experiment was performed to test this hypothesis, by which samples of the irradiated fibers were heated to temperatures above first the alpha relaxation and then melting point of polyethylene, and EPR measurements were taken. Results showed that the polyenyl radical signal persisted below the Tm, but was rapidly eliminated upon melting of the crystals. These experiments support the hypothesis that the long-lived polyenyl radicals are trapped in the crystalline region of the polyethylene fibers.

Nanocomposites of ultrahigh molar mass polyethylene and modified carbon nanotubes

ABSTRACTIn this work, the use of a laboratory twin‐screw extruder was evaluated to process ultrahigh molar mass polyethylene and composites with carbon nanotubes (CNTs). Commercial polymer samples with lubricant (1%) and different percentages (0.01%, 0.05%, and 0.1%) of pure, oxidized, and chemically surface treated multi‐walled carbon nanotubes (MWCNTs) were evaluated. The results showed that polymer melting and crystallization temperatures were not affected by CNTs, although an increase in the degree of crystallinity in all nanocomposites was observed along with a decrease in crystal size. Therefore, CNTs behaved as nucleating agents. All ultrahigh molar mass polyethylene (UHMWPE)/CNT samples showed increased initial degradation temperature, although this was not very great when introducing acetylated and stearic acid modified CNTs. Both oxidized CNTs and stearic acid CNTs did not markedly improve the composites' mechanical properties. Therefore, the nanocomposites containing pure CNTs and most of those with acetylated CNTs resulted in higher reinforcement for UHMWPE. The addition of the lubricant allowed the polymer matrix to be processed in the extruder, whereas the increase in CNT content in UHMWPE improved the stiffness of the material. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47459

Oxidation reactions in kink banded regions of UHMMPE fiber-based laminates used in body armor: A mechanistic study

This work demonstrates the synergy between the thermo-mechanical and humidity induced degradation as well as the oxidation reactions in the kink-banded areas of ultra-high molar mass polyethylene (UHMMPE) fiber-based laminates used in body armor. For aged materials, the energy-dispersive X-ray spectroscopy (EDS) and Fourier transform infrared spectroscopy (FTIR) results reveal high concentrations of oxygen containing products, and the EPR results demonstrate the presence of the peroxyl radicals (RO2•) in the kink-banded areas. After one year of dark ambient storage, very long-lived RO2• radicals were observed primarily in the samples exposed to ageing conditions of elevated temperatures, humidity, and mechanical stress. The total percentage of crystallinity, as measured by differential scanning calorimetry, of the kink-banded fibers was unchanged, indicating that the degradation occurs primarily in the amorphous region, and may also involve recrystallization processes of the degraded chains. However, the most abundant orthorhombic crystalline phase decreases from 77% to 70%. This decrease in the orthorhombic structure leads to more diffusion of oxygen into the kink-banded region, enhancing the oxidation processes. No changes are observed in the monoclinic phase of the kinked fibers, which remained constant and constituted ∼2% of the total crystallinity.

Processing–Nanostructure–Property Relationships of All‐Polyethylene Composites Reinforced by Flow‐Induced Oriented Crystallization of UHMWPE

AbstractAll‐polyethylene composites exhibiting substantially improved toughness/stiffness balance are readily produced during conventional injection molding of high density polyethylene (HDPE) in the presence of bimodal polyethylene reactor blends (RB40) containing 40 wt% ultrahigh molar mass polyethylene (UHMWPE) dispersed in HDPE wax. Scanning electron microscopy (SEM) and differential scanning calorimetry (DSC) analyses shows that flow‐induced crystallization affords extended‐chain UHMWPE nanofibers forming shish which nucleates HDPE crystallization producing shish‐kebab structures as reinforcing phases. This is unparalleled by melt compounding micron‐sized UHMWPE. Injection molding of HDPE with 30 wt% RB40 at 165 °C affords thermoplastic all‐PE composites (12 wt% UHMWPE), improved Young's modulus of 3400 MPa, tensile strength of 140 MPa, and impact resistance of 22.0 kJ/m2. According to fracture surface analysis, the formation of skin‐intermediate‐core structures accounts for significantly improved impact resistance. At constant RB40 content both morphology and mechanical properties strongly depend upon processing temperature. Upon increasing processing temperature from 165 °C to 250 °C the average shish‐kebab diameter increases from the nanometer to micron range, paralleled by massive loss of self‐reinforcement above 200 °C. The absence of shish‐kebab structure at 250 °C is attributed to relaxation of polymer chains and stretch‐coil transition impairing shish formation.

All-polyethylene composites reinforced via extended-chain UHMWPE nanostructure formation during melt processing

Nanophase separation during injection molding of high density polyethylene (HDPE) together with polyethylene reactor blend (RB) additives that have ultrabroad bimodal molar mass distribution and a high content of ultrahigh molar mass polyethylene (UHMWPE), produces thermoplastic all-polyethylene composites reinforced with extended-chain UHMWPE nanostructures formed in situ. Neither alien fibers, inorganic fillers, hazardous nanoparticles nor modified single- and multi-step molding processes are required to convert HDPE into higher performance engineering plastics. The RB40 additive is readily tailored by ethylene polymerization on silica-supported chromium two-site catalysts and contains 40 wt% UHMWPE (Mw = 1.5 × 106 g∙mol−1) dispersed in 50 wt% HDPE wax (Mw = 1.1 × 103 g∙mol−1). The presence of disentangled nanophase-separated UHMWPE together with HDPE wax serving as a processing aid enables injection molding of all-PE composites with high UHMWPE content of up to 24 wt% without changing processing parameters typical for HDPE. As verified by both scanning electron microscopic analyses of samples etched with hot xylene and thermal analysis, the HDPE matrix is efficiently reinforced by in situ formed polyethylene shish-kebab fibers where flow-induced crystallization yields extended-chain UHMWPE nanostructures as shish nucleating the crystallization of HDPE kebab. For the first time, by etching quenched samples, it was possible to completely suppress kebab formation and to image in situ extended-chain UHMWPE shish with an average diameter of 80 nm. Upon increasing the UHMPE content the average total diameter of shish-kebab fiber-like structures drastically decreases from micron to nanometer range, forming non-woven-like architectures. This self-reinforcement simultaneously improves toughness, stiffness and strength parameters unparalleled by conventional melt-blending HDPE with micron-sized UHMWPE and HDPE wax. Compared to HDPE, the addition of 60 wt% RB40 increases the Young's modulus to 4.2 GPa, tensile strength to 160 MPa and impact strength to 20 kJ/m2. This sustainable route to self-reinforced all-PE composite composites preserves the high energy, resource, cost and eco-efficiencies typical for pure hydrocarbon resins. Here, we examine the impact of RB40 addition on in situ nanostructure formation and its correlation with thermal and mechanical properties of all-PE composites.

Tailored Nanostructured HDPE Wax/UHMWPE Reactor Blends as Additives for Melt-Processable All-Polyethylene Composites and in Situ UHMWPE Fiber Reinforcement

Tailored polyethylene reactor blend additives (RB) with ultrabroad bimodal molar mass distributions comprise nanophase-separated ultrahigh molar mass polyethylene (UHMWPE) uniformly dispersed in polyethylene wax. During injection molding of high-density polyethylene (HDPE) together with variable amounts of the nanophase-separated HDPE wax/UHMWPE (70/30) additive (RB30) flow-induced oriented crystallization affords shish-kebab fiber-like UHMWPE nanostructures accounting for efficient HDPE self-reinforcement. RB additives are readily prepared by ethylene polymerization on silica-supported two-site chromium catalysts which simultaneously produce HDPE wax together with disentangled nanoplatelet-like UHMWPE. The presence of HDPE wax is essential for lowering melt viscosity at high UHMWPE content. Since HDPE wax crystallizes onto extended-chain UHMWPE shish to form kebab structures, high HDPE wax content is tolerated without encountering emission problems and impairing mechanical properties as observed in the a...

Segmental mobility in the noncrystalline regions of nascent polyethylene synthesized using two different catalytic systems with implications on solid-state deformation

Recently, it has been shown that using a single-site catalytic system it is possible to tailor entanglement density in the amorphous region of a semi-crystalline polymer, such as polyethylene, with a molar mass greater than 1 million g/mol. The synthesized polymer can be processed in its solid-state, along uniaxial and biaxial directions. It also shows strong heating rate dependence on melting invoking kinetics in randomization of chains from crystal to amorphous phase. With the help of solid-state NMR the distinction between amorphous regions of the synthesized and commercially available ultra-high molar mass polyethylene is made. For example, the polymer synthesized under the controlled conditions, using a homogeneous single-site catalytic system, shows faster motion of methylene units from the noncrystalline to the crystalline regions compared to commercially available nascent polyethylene synthesized using a Ziegler–Natta (Z–N) catalyst. The differences in chain diffusion, and the resultant 13C polarization transfer from the noncrystalline to the crystalline region, are attributed to the difference in entanglement density arising from the polymerization method employed. The two polyethylene samples investigated are of similar molar mass and crystallinity. Conformational changes caused by deformation of the two samples have been characterized and co-relationship with the entanglement density has been established.

Effect of concentration reduction due to adsorption on the reduced viscosity of polymer solution in extremely dilute concentration regime

The phenomena of reduced viscosity bending up and sometimes appearing a maximum in extremely dilute concentration regime of polymer solution are resulted from the concentration reduction due to adsorption of polymer onto the glass capillary wall. A quantitative theoretical description of this effect based on Langmuir adsorption isotherm is given. Applying the deduced formula to fit Ohrn's precisely measured reduced viscosity data of polystyrene, linear and branched polyvinyl acetate solutions for three viscometers with different capillary radius gave satisfactory results. The radius dependence of the effective adsorption layer thickness of these polymers indicates it consists of two parts, that is a true adsorption layer and a repulsion layer between anchored chains on the capillary surface and flowing chains in solution. The reduced viscosities of an ultrahigh molar mass polyethylene oxide sample in aqueous solution were measured both by an ordinary glass viscometer and by a viscometer constructed with a polytetrafluoroethylene capillary. The former exhibits a maximum due to adsorption and the later possesses the typical behavior of slipping flow. After correction for solute adsorption and slippage, respectively, the two sets of data coincide well and gave a common linear reduced viscosity plot down to extremely dilute concentration regime.

Reversible melting of gel-spun fibers of polyethylene

The melting and crystallization of gel-spun, ultrahigh molar mass polyethylene (UHMM-PE) fiber is analyzed with both, standard differential scanning calorimetry (DSC) and temperature-modulated DSC (TMDSC) to identify the differences to extended-chain crystals of polyethylene. The analyzed samples were studied before with respect to their molecular mobility, morphology, and crystal and phase structure. The UHMM-PE contains a mobile, oriented mesophase and only few folded chains. The truly amorphous fraction is about 3%. Reversing melting and crystallization is observed for these fibers. It increases in amount on recrystallization after melting. Difficulties in measuring the original fibers arose from the shrinkage of the fibers and a resulting deformation of the sample pan. The new data are compared to literature information on other polyethylenes and linked to reversible melting of decoupled segments of the polymer chains contained in small, nanometer-size phases within the crystalline phase and the oriented-intermediate fractions.

Influence of intermolecular entanglements on crystallization behavior of ultra‐high molar mass polyethylene

AbstractUltra high molar mass polyethylene (UHPE) was melted at 160°C for various times or at various temperatures for 5 min and then the crystallization of the UHPE was carried out on cooling. It was found that the crystallization temperature decreased as the heating time or heating temperature increased. During the melting process, thermal motion of the chains leads to a change of chain conformation from parallel‐extended chains to interpenetrated random coils, accompanied by the occurrence of entanglements. As a result, the crystallization temperature shifts to lower temperature. On the other hand, samples of UHPE with less entanglement were prepared from a dilute solution by a freeze‐drying procedure. It was observed that the crystallization temperature of the freeze‐dried samples from the melt depressed with dereasing solution concentration. UHPE would produce small crystals in the freezing process, thus leading to a reduction in melting point, and a sifting of crystallization temperature to lower temperature. Based on the melting point, the average volume of small crystals was estimated; it is even smaller than that of one single chain of the UHPE.

Melting Behavior and Morphology of Freeze-Dried Ultra-high Molar Mass Polyethylene

Freeze-dried ultra-high molar mass polyethylene (UHPE) was prepared from dilute solutions, and the resulting samples are a mixture of single-, few-, and multichain particles, each containing several or a number of small crystals. Thermal analysis of the freeze-dried samples by DSC indicated that the melting point decreased as the solution concentration decreased. The depression in melting point is most likely due to the decreasing crystal size. The effect of annealing the freeze-dried samples and the samples obtained after melting and recrystallization of the nascent UHPE at 122°C are discussed. Wide-angle x-ray diffraction (WAXD) measurements of average crystal dimensions normal to (110) and (210) and crystallinity were calculated for the freeze-dried samples. The crystallinity measured by WAXD is in agreement with that by DSC. By means of polarized optical microscopy (POM), morphology of the freeze-dried samples was studied. The melt of the freeze-dried samples became optically isotropic after melting at a temperature beyond the equilibrium melting point () of polyethylene, while the nascent UHPE retained birefringence even if the melting temperature exceeded 180°C. It is assumed that the freeze-dried samples have a structure of folded-chain lamellae, which melt completely at the temperature above However, the nascent UHPE was proposed to be extended-chain crystals, and the parallel chain conformation remains in the UHPE samples even if the melting temperature is beyond therefore, birefringence can still be observed under POM at higher temperature.

Molecular Characterization of Ultrahigh Molar Mass and Soluble Fractions of Partially Cross-Linked Polyethylenes

A reliable method is described for analyzing ultrahigh molar mass polyethylene (UHMW-PE) with a weight-average molar mass of 5700 kg/mol and molar mass fractions up to 50,000 kg/mol by size exclusion chromatography (SEC) without molecular degradation as was proved by comparison of on-line and off-line viscometry. The use of low-bleed SEC columns (PLgel 20 µm Mixed-A LS), especially designed for on-line light-scattering detectors, allowed SEC to be coupled to a low-angle laser light-scattering detector without a filter ahead of the light-scattering cell. With this procedure, molded specimens of UHMW-PE, which are used for medical implants, such as hip endoprostheses, containing various amounts of f -tocopherol as a biocompatible stabilizer, were investigated. Furthermore, the effect of sterilization of the samples by n -irradiation ( 60 Co, 1.33 MeV, absorbed dose: 25 kGy) was studied with respect to the content of insoluble cross-linked material and the molar mass distribution of the soluble fraction.

Another approach to the Gibbs–Thomson equation and the melting point of polymers and oligomers

The common Gibbs–Thomson equation, widely used to explain the melting temperature of lamella crystals, is based on a given heat of fusion and a given surface free energy and the size (thickness) of the crystal. With this equation it is not possible to explain the, compared to the thickness of the crystals, very high melting temperature of cyclic alkanes and ultra-high molar mass polyethylene (UHMMPE). Another thermodynamic approach to the Gibbs–Thomson equation, starting from an incremental composition of enthalpy and entropy of the chain molecule, is presented. This describes the melting temperature of (lamella) crystals of linear, folded and cyclic alkanes as well as UHMMPE, all forming crystals of the same lattice type, with only one set of parameters. The essential variable turns out to be the number of CH 2-groups of the respective molecule, incorporated into the crystallite, rather than its thickness. This may be explained if we assume the melting process caused by conformation dynamics which are more restricting the greater number of CH 2-groups that are involved in the chain movement. In a lamella crystal of a certain thickness, a cyclic alkane ‘feels’ longer than an n-alkane, as well as a linear molecule with adjacent or tight folds feels longer than one with randomly distributed chains and large loops in the amorphous. This approach helps to understand the melting behavior of polymers forming folded-chain crystals. It enables the cyclic and folded ultra-long alkanes to serve as model substances for the folded-chain crystals of polyethylene without further assumptions concerning the surface energy and fits all findings smoothly into one picture.

The effect of electron beam radiation on the molecular structure of ultra-high molar mass polyethylene used for medical implants

Systematic investigations on the influence of electron beam radiation of different dosage on molecular degradation as well as crosslinking of ultra-high molar mass polyethylene containing various amounts of α-tocopherol as stabilizer were carried out in air and an inert nitrogen atmosphere. The samples were analyzed with respect to their content of insoluble crosslinked fraction and the molar mass distribution of the soluble fraction. The extent of crosslinking and degradation is dependend on the absorbed dosage of the radiation. The crosslinking process is favored in nitrogen atmosphere whereas in air molecular degradation is increasing. The extent of crosslinking as well as degradation can be reduced by the addition of α-tocopherol both, in air and nitrogen.

Melting and heat capacity of gel-spun, ultra-high molar mass polyethylene fibers

The melting behaviors of gel-spun, ultra-high molar mass polyethylene fibers have been investigated using standard differential scanning calorimetry and wide-angle X-ray diffraction. For laterally constrained fibers the standard DSC exhibits the usually reported two melting peaks (one at about 415K and one at about 423K), a glass transition (at about 275K) and an unusually large and reversible heat capacity as one approaches the melting peaks (starting at about 350–370K). Earlier observations that sometimes only the lower melting endotherm is retained, are proven to be of fibers free of longitudinal and lateral constraints. Up to the beginning of major melting, all fibers are mainly orthorhombic. The orthorhombic–hexagonal transition is proven to occur only for the constrained fibers, implying that the hexagonal phase is a non-equilibrium phase, usually not present in the initial fibers. An oriented, intermediate phase, discovered by solid state NMR and full-pattern X-ray analysis, gives rise to a substantially broadened glass transition that reaches to about 300K, compared to the glass transition of the amorphous polyethylene with a midpoint at 237K and end at 250K.